Transition metal phosphide catalysts for the reduction of carbon monoxide and carbon dioxide

Abstract

Disclosed is a method for the catalytic reduction of CO2 and / or CO, the method comprising steps of (i) providing an electrolytic cell containing at least one reaction chamber that has at least one anode and at least one cathode and at least one electrolyte between the anode and the cathode, wherein the at least one cathode comprises at least one catalyst comprising at least one transition metal phosphide selected from phosphides of Cr, V, Ti, Hf, Ta, Zr, Nb, Ta; (ii) providing CO2 and / or CO in the electrolytic cell; and (iii) applying electrical potential to the electrolytic cell so that the CO2 and / or CO undergoes a reduction reaction at the cathode. Also disclosed are electrolytic cells and chemical reactors containing the disclosed transition metal phosphide catalysts.

Description

FIELD

[0001] The disclosure relates to the reduction of carbon dioxide and carbon monoxide by electrolysis and new transition metal catalysts therefor.INTRODUCTION

[0002] Since the start of the 20 th< century the world climate has been changing more noticeably. The balance in nature is lost due to dramatic increasing CO 2 emissions by burning of carbonaceous fuels which results in global warming. An urgent effort is needed to overcome this problem to reduce the ratio of CO 2 and turn it into fuel and other useful chemical products. On the other hand, the large-scale use of unsustainable fossil fuels for energy has led to an energy crisis in the world and an increasing amount of CO 2 in the atmosphere. Due to this, the world climate is changing day by day and scientific effort is needed to overcome the emission of CO 2 in the air by converting the already present CO 2 into green energy fuels. Previous experimental and theoretical research concluded that conversion of CO 2 into valuable products can be achievable, but not yet efficient for commercialization. In the whole process of CO 2 reduction reaction (CO 2 RR), the catalyst plays a key role in capturing CO 2 and converting it into valuable chemical products and fuels. CO 2 is a chemically stable molecule and this makes its electrolytic reduction challenging with high reduction overpotential, poor product selectivity and low current efficiency. To address this challenge, researchers and scientists have been exploring novel catalysts which can overcome these issues and speed up the electrolytic process (Ebbesen & Mogesen, J. Power Sources 2009, 193:349; Fu et al, Energy & Environmental Sci 2010, 3:1382).

[0003] In CO 2 RR the catalysts play a key role in the conversion of CO 2 into valuable products. In previous decades many experimental studies have been done for electrolytic CO 2 RR on different metal catalysts. For example, pure Cu, Ag, Au, and Pt catalysts have been studied as catalysts for the CO 2 reduction reaction. However, their application for the commercial purposes is not enough due to poor product selectivity, low faradic efficiency and high overpotential that is needed. It is well known that the Cu is a widely used catalyst for the reduction of CO 2 to CH 4 but its overpotential is high (~0.9 V) and moreover it produces 15 different carbon containing products which require large amount of energy for their separation. In addition to pure Cu, polycrystalline Cu was also used for the application of CO 2 RR and it produces amounts of C 2 and C 3 aldehydes as well as ethylene.

[0004] In addition to pure metal electrocatalysts, metal oxide catalysts i.e. TiO 2 have been studied by DFT methods and considered impressive and more efficient catalysts for CO 2 conversion to CH 3 OH and CH 4 products (Ramesha, ACS Catalysis, 2014, 4:3249). In another study RhO 2 is predicted to be a novel candidate for the formation of formic acid at -0.20 V onset potential. However, stability and electron conductivity of oxides as cathode materials are a challenge that limits the application of oxides for CO 2 RR. There are numerous research studies that have been conducted to explore novel catalyst materials for the CO 2 RR. Materials studied by DFT (density functional theory) include graphene-based materials, metals, metal oxides, zeolites, sulfides, and metal organic frameworks. The utilization of catalysts for large scale production is however still a challenge and improved methods are required to overcome this problem.SUMMARY

[0005] The present disclosure provides methods to overcome the above deficiencies of the prior art. The need to explore alternative materials for catalysis of CO 2 RR and CORR arises from the imperative to develop efficient and sustainable methods for converting carbon dioxide and carbon monoxide into valuable products. Traditional catalysts may have limitations in terms of activity, selectivity, efficiency, or cost. By searching for other materials, the aim is to identify catalysts that can enhance the performance of CO 2 RR and CORR processes, making them more economically viable and environmentally friendly.

[0006] In particular, in light of the challenge of finding good electrocatalysts for CO 2 and CO reduction, the disclosure provides certain transition metal phosphides (TMPs) that are useful catalysts for CO 2 RR and CORR.

[0007] An aspect of the disclosure relates to a method for catalytic electrolytic reduction of CO 2 and / or CO, the method comprising (a) providing an electrolytic cell containing at least one reaction chamber that has at least one anode and at least one cathode and at least one electrolyte between the anode and the cathode, wherein the at least one cathode comprises at least one catalyst comprising at least one phosphide of one or more transition metal; (b) providing CO 2 and / or CO in the electrolytic cell; and (c) applying electrical potential to the electrolytic cell; whereby the CO 2 and / or CO undergoes at least one reduction reaction at the cathode.

[0008] Another aspect of the disclosure relates to an electrolytic cell for the reduction of carbon dioxide and / or carbon monoxide, comprising: an anode; a cathode, the cathode comprising a catalyst comprising at least one transition metal phosphide, and a power supply connected to the anode and the cathode.

[0009] The transition metal phosphide in these applications is preferably a transition metal phosphide of one or more transition metal selected from Cr, V, Ti, Hf, Ta, Zr, Nb and Ta.

[0010] The disclosure further relates to a chemical reactor comprising at least one electrolytic cell as described herein; and a power supply connected to the electrolytic cell.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows a side (a) and top (b) view of TMP surfaces where the larger spheres indicate the metal atoms and smaller spheres indicate phosphorous atoms. FIG. 2 shows a schematic diagram of results of screening of TMPs for CO 2 RR. FIG. 3 shows a schematic diagram of adsorption of all species on a clean surface in CO 2 RR. FIG. 4 shows a free energy diagram for HER on-metal side of TMPs. FIG. 5 shows a energy diagram for CrP (a) and VP (b) in CO 2 RR, where *MH indicates proton binding on the metal side, *PH indicates proton binding on the phosphorous side, *COOH indicates COOH binding on the metal side, *OCHO indicates binding of OCHO on metal side and HCOOH (aq) is formic acid in aqueous form. FIG. 6 shows free energy diagram for CO 2 RR on TiP catalyst. FIG. 7 shows a schematic representation of onset potential required for formic acid, methanediol, and methanol formation in CO 2 RR. FIG. 8 shows a free energy diagram of CO 2 RR for (a) TaP, (b) ZrP, (c) HfP. FIG. 9 shows a theoretical volcano for the formation of formic acid via CO 2 RR. FIG. 10 shows a comparison of adsorption free energies of different CORR adsorbates on the catalyst surfaces. FIG. 11 shows adsorption free energies of proton on the surface metal site as an indication of HER in the CORR. FIG. 12 shows possible reaction pathways to produce CH 3 OH (aq), CH 2 O (g) and CH 4(g) on catalyst surfaces in the CORR. FIG. 13 shows free energy diagrams (FED) for CORR in the production of CH 3 OH (aq) CH 2 O (g) and CH 4(g) for various catalyst surfaces; HfP (a), TiP (b), TaP (c), ZrP (d), NbP (e). FIG. 14 shows necessary onset potential to generate products via the CORR. FIG. 15 shows an illustration of the most favorable pathway to create the CH 3 OH (aq), CH 2 O (g) and CH 4(g)) via the conventional mechanism in the CORR for VP. DESCRIPTION

[0012] In the following, exemplary embodiments of the invention will be described, referring to the figures. These examples are provided to provide further understanding of the invention, without limiting its scope.

[0013] In the following description, a series of steps are described. The skilled person will appreciate that unless required by the context, the order of steps is not critical for the resulting configuration and its effect. Further, it will be apparent to the skilled person that irrespective of the order of steps, the presence or absence of time delay between steps, can be present between some or all of the described steps.

[0014] As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0015] Throughout the description and claims, the terms "comprise", "including", "having", and "contain" and their variations should be understood as meaning "including but not limited to" and are not intended to exclude other components.

[0016] The present invention also covers the exact terms, features, values and ranges etc. in case these terms, features, values and ranges etc. are used in conjunction with terms such as about, around, generally, substantially, essentially, at least etc. (i.e., "about 3" shall also cover exactly or "substantially constant" shall also cover exactly constant).

[0017] The term "at least one" should be understood as meaning "one or more", and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with "at least one" have the same meaning, both when the feature is referred to as "the" and "the at least one".

[0018] The present disclosure is based on the surprising finding that transition metal phosphides, when used as a catalyst in the electrolytic reduction of CO 2 and CO, allow for efficient reduction to produce valuable small compounds at low temperatures and pressure and using low applied potential. Given the ever-increasing levels of CO 2 in the global atmosphere and concomitant need to find ways to achieve a carbon neutral energy technology, these catalysts can be used to facilitate the conversion of CO 2 and CO into valuable carbon-containing fuels.

[0019] In general, the catalysts can be used to reduce CO 2 and / or CO to produce hydrocarbons or oxygenates having from 1 to 3 carbon atoms. Exemplary products include methane, methanol, methanediol, formaldehyde, formic acid, ethanol, ethylene glycol, ethanediol, propane and propanol.

[0020] Accordingly, the disclosure provides means in the form of methods and systems for electrolytic conversion of CO 2 and / or CO at low temperature and pressure. Compounds such as short-chain alkanes and oxygenates (e.g. alcohols and acids) having from 1 to 3 carbon atoms can be produced using the disclosed transition metal phosphides. Through selection of appropriate catalysts, catalyst surface and appropriate applied voltage, the reduction reaction can be geared to produce compounds of particular interest.

[0021] In an aspect, the present disclosure is based on the investigation of the capability of transition metal phosphides (TMPs) to catalyze the conversion of CO 2 and CO into valuable organic compounds that can for example be used as valuable fuel. The results are based on studies that delve into the electronic intricacies of TMPs, unraveling their potential to drive the transformation of CO 2 and CO.CO 2 Reduction Reaction (CO 2 RR)

[0022] Density Functional Theory (DFT) was used to explore the reactivity of the reduction of CO 2 in the presence of 29 different transition metal phosphide surfaces. The goal was to get an understanding of the performance and competence of these materials for CO 2 RR. After a screening for most suitable TMPs, a total of 6 TMPs were considered most promising and studied further.

[0023] DFT is a quantum mechanical methodology that accounts for the interaction between electrons and the nucleus. The input is the atomic structure of the molecular system of interest and the output is the ground state energy of the system. The reaction mechanism of CO 2 reduction to CH 4 on a stepped Cu(211) surface was first proposed by Peterson et al (Energy Environ Sci 2010, 3:1311). The free energy of adsorbed intermediates on the catalyst surface was calculated using a thermochemical model (TCM). The same approach has been successfully applied on several other electrolytic systems, including water oxidation on transition metal oxides, N 2 electroreduction on transition metal surfaces, transition metal nitrides and transition metal oxides, hydrogen evolution reaction (HER) on transition metal nitrides and CO 2 RR on transition metals and transition metal oxides.

[0024] The need to explore alternative materials for the CO 2 RR arises from the imperative to develop efficient and sustainable methods for converting carbon dioxide into valuable products. Traditional catalysts may have limitations in terms of activity, selectivity, efficiency, or cost. By searching for other materials, researchers aim to identify catalysts that can enhance the performance of CO 2 RR processes, making them more economically viable and environmentally friendly. The surface chemistry of TMPs is comparable to precious Pt-group metals which was already described by Levy and Boudart (Science 1973, 181:185). In previous studies these TMPs have been used for a wide range of reactions such as CO oxidation, hydrogenation, methane dry reforming, desulfurization, conversation of methane to gas and water gas shift reaction. Hence these materials have received great attention for application in catalysis.

[0025] Utilizing advanced quantum mechanical simulations and computational models, the present disclosure is based on an exploration of the binding energies and reaction pathways of 29 TMPs with a focus on their suitability for catalyzing CO 2 reduction. In this research a complete and comprehensive analysis for above mentioned TMPs on (100) facets were conducted. A schematic illustration of the considered TMP surfaces is illustrated in Fig. 1.

[0026] In the following Example 1 it is described how DFT calculations can be used to describe the interaction of CO 2 with TMP surfaces and predict the reaction pathway for CO 2 RR on TMPs. As shown by the results illustrated in Example 1, the conventional surface mechanism was investigated to study the CO 2 RR towards methane and methanol production on TMP surfaces. The analysis reveals that several TMPs exhibit remarkable catalytic activity for CO 2 reduction, surpassing the performance of conventional catalysts. The electronic structure of these TMPs plays a pivotal role in facilitating the conversion of CO 2 into valuable chemical products.

[0027] The present disclosure is based on a complete analysis of transition metal phosphides (TMPs) with (100) facets by using density functional calculations. In comparison to transition metal oxides, transition metal nitrides (TMNs) and transition metal dichalcogenides (TMDs), transition metal phosphides (TMPs) exhibited superior electrocatalytic conductivity and stability. The electrocatalytic activities of TMPs have comparable properties with Pt-based materials due to their tunable structures, multifunctional sides, and excellent stability. The TMPs are composed of metals and phosphorous atoms.

[0028] The presence of P atoms decreases the metal dissolution thermodynamically and improves the stability of transition metal-based catalysts for the application of electrocatalysis. It has for example been reported that the stability of MoP is greater than that of Mo 2 N and Mo 2 C.

[0029] Hence TMPs are an excellent choice for the application of electrocatalysis due to desirable properties of being cost-effective, stable, and showing good stability and activity. The present invention is based on a study of 29 TMPs, assessing their suitability for catalyzing CO 2 RR. The results show that certain TMPs (CrP, HfP, TiP, TaP, VC, and ZrP) are especially beneficial for electrocatalytic activity. The results, exemplified by the data shown in Example 1, reveal that the TMPs are more promising for CO 2 RR than previously studied transition metal-based catalysts.CO reduction reaction (CORR)

[0030] The disclosure also shows that transition metal phosphides (TMPs) are capable of catalyzing the conversion of CO into valuable organic compounds via CORR.

[0031] Protonation along the reaction pathway from CO to CH 4 and CH 3 OH on 12 transition metal phosphides have been calculated, including VP, WP, YP, ZrP, CrP, HfP, NbP, MoP, CoP, TaP, TiP, and ScP with the (100) facets. These facets of TMPs are considered the most stable structure having the 1:1 TM:C:P ratio and cubic crystallography.

[0032] The conventional mechanism was investigated to study the CORR towards methane and methanol production on TMPs surfaces. In the conventional surface mechanism the interaction of CO gas molecule on the surface of TMPs with different sites is studied. All aspects of each mechanism are calculated explicitly, and free energy diagrams are drawn for each adsorbed species on the surface of catalyst. Thermochemical model was implemented to obtain the trends of CORR on each TMPs surface.

[0033] Employing Density Functional Theory (DFT) of the 12 TMPs were thus explored for the conversion of CO to methane (CH 4 ), methanediol (CH 2 O) and methanol (CH 3 OH).

[0034] Results of the study are presented in the following Example 2. In the study, CrP, HfP, NbP, TaP, TiP, VP and ZrP were found to be particularly good candidates for the CORR. It was furthermore found that hydrogen does not bind exergonically to the explored surfaces, thus being less favorable than CO adsorption on the surfaces (see Fig. 11). As a consequence, there is no risk of competing hydrogen evolution via the HER reaction.

[0035] All of these surfaces having the most promising catalyst activity were found to require a potential less than -1.0V to produce any product. The most promising catalysts were found to be VP and NbP, requiring onset potential of less than -0.7V, although TaP, TiP and HfP also show great promise with onset potential in the range of -0.8V to -0.9V.

[0036] Overall, the findings indicate due to CO spontaneously adsorption, less poisoning, less over potential and high product selectivity TMPs are excellent choice for CORR to produce valuable products such as methane, methanediol, formic acid and methanol.Electrolytic cells

[0037] In general, an electrolytic cell is a chemical reactor where chemical reactions take place, driven by an applied electric potential. An electrolytic cell in the present context is therefore an electrolytic cell that undergoes a redox reaction when electrical potential is applied to the cell.

[0038] Design of electrolytic cells is known in the art. Examples of suitable electrolytic cells include H-type cells, polymer electrolyte membrane (PEM) flow cells, microfluidic flow cells, solid oxide electrolytic cells and DEMS cells. It should be appreciated that the transition metal phosphides disclosed herein can be adapted for use in any type of electrolytic cell. Examples of suitable electrolytic cells are described in Liang et al. (Journal of CO2 Utilization, 2020, 35:90-105).

[0039] In an H-type cell, the cathodic and anodic compartments are typically separated by an ion-exchange membrane that allows ions to flow across while preventing reaction products from being reoxidized at the anode. There can be a reference electrode present in the cathodic compartment. In these cells, there is a liquid electrolyte solution at the cathode and anode, and CO 2 gas is typically streamed into the cathodic compartment.

[0040] In PEM flow cells, CO and / or CO 2 can be streamed directly at the cathode or it can be provided by dissolved bicarbonate. There can be either a liquid phase at both electrodes, a gas phase at the cathode and liquid at the anode or a humified gas phase at both electrodes. Typically, there is a gas diffusion electrode at the cathode and possibly also at the anode.

[0041] In microfluidic flow cells, an electrolyte channel is provided between a cathode and an anode, with a stream of CO and / or CO 2 gas delivered at the cathode. Instead of utilizing a membrane, the cathode and anode are separated by the diffusion of products.

[0042] Solid state electrolysis cells can improve the kinetics of reactions and achieve high efficiency. In these cells, there is a solid electrolyte to facilitate transfer of ions, with a membrane separating the cathode and anode. The cells can operate at high temperatures if needed, e.g. at or above 600°C, such as high as 700°C, 800°C or 900°C, even as high as 1000°C.

[0043] The transition metal phosphide catalysts disclosed herein can be provided in any suitable form in the electrolytic cell of interest, depending on its design and intended use. Thus, the catalysts can be provided as a coating, film, as a powder, dopant etc., and in forms such as microparticles, nanoparticles such as zero- dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three- dimensional (3D) nanoparticles, nanotubes, nanosheets, heterostructures, a single atom catalyst, a dual-atom catalyst and a triple-atom catalyst.

[0044] Transition metal phosphides can have the general 1:1 ratio (TM:P) stoichiometry, e.g. VP, MoP, ZrP, TiP etc. Other stoichiometries are however also possible.

[0045] Thus, the skilled person will appreciate that the phosphides may have an alternative stoichiometry and such alternative stoichiometries are also within scope of the present disclosure. Further, chemical compounds as described herein are provided by their chemical formula irrespective of their phase or state. In particular, compounds that are present in their gaseous state when present in a pure and isolated form at room temperature (such as CO or CO 2 ) are herein described by their chemical formula. For example, carbon monoxide and carbon dioxide is herein described as CO and CO 2 , whether present as a gas, as individual molecules, in clusters, bound to surfaces or present as a solute, and the same applies to other molecular species described herein.

[0046] A mixture of CO and CO 2 can be provided in the electrolytic cell. During such operation of the cell, both CORR and CO 2 RR reactions will take place with products formed depending on the type of catalyst used (i.e., which TMP) and the applied voltage.

[0047] Alternatively, only CO or only CO 2 can be provided in the cell. This may be preferable, as it will be easier to control the respective reaction (CORR or CO 2 RR) for the type of TMP being used in the cell.

[0048] The proton donor in the electrolytic reactions can be any suitable substance that is capable of donating protons in the electrolytic cell. The proton donor can for example be water or it can be in the form of an acid, such as any suitable organic or inorganic acid. The proton donor can be provided in an acidic, neutral or alkaline aqueous solutions. The proton donor can also, or alternatively, be provided by H 2 oxidation at the anode. In other words, hydrogen can be considered as a source of protons: H 2 <=> 2(H +< + e -< )

[0049] The electrolytic cell can in general comprise at least three general parts or components, a cathode electrode, an anode electrode and an electrolyte.

[0050] The electrolytic reduction of carbon dioxide is the conversion of carbon dioxide to more reduced chemical species using electrical energy.

[0051] The different parts or components can be provided in separate containers, or they can be provided in a single container. The electrolyte can be a solid or an aqueous solution in which ions are provided, for example in a dissolved form in an aqueous solution when present. When provided as an aqueous solution, the aqueous solution can be a neutral, an alkaline or an acidic solution.Sources of CO 2 and CO

[0052] Carbon dioxide (CO 2 ) can be provided by a source such as any one of bicarbonate (HCO 3 -< ), carbonate (CO 3 -2< ) and / or carbonic acid (H 2 CO 3 ). For example, bicarbonate and carbonate can be provided as bicarbonate or carbonate salts, either in pure form as a solid or in solution, or in a mixture into a solution, that can preferably be an aqueous solution. A mixture of bicarbonate, carbonate and carbonic acid will reach equilibrium in solution. Therefore, the relative concentration of these species will depend on pH of an aqueous solution.

[0053] An alternate source of CO and CO 2 is the gaseous form of the compounds, CO(g) and CO 2 (g). Gaseous CO and CO 2 can be provided as a sole source of CO and / or CO 2 , or it can be provided as a supplement to other sources of CO or CO 2 in the cell. In the case of CO 2 , this includes the previously mentioned bicarbonate, carbonate and carbonic acid.

[0054] Other or additional sources of CO and CO 2 are possible. For example, direct injection of CO 2 gas is possible by bubbling CO 2 directly into an electrolyte solution.

[0055] Alternatively, dissolved carbonate salts (e.g., sodium carbonate, Na 2 CO 3 ) in water can be used to provide a source of CO 2 .

[0056] An alternative source can be provided from industrial CO 2 Emissions. Capturing CO 2 from industrial processes such as cement production, steel manufacturing, or chemical production can also form a source of CO 2 . Further, chemical reactions that release CO 2 can be a source of CO 2 .

[0057] Certain microbial processes, such as fermentation, produce CO 2 as a byproduct and can serve as a source of CO 2 . The CO 2 can also be provided through photocatalytic production.

[0058] Another source is represented by CO 2 captured from fossil fuel-based power plants (coal, natural gas, oil) which produce large amounts of CO 2 .

[0059] Yet another source can be CO 2 generated from burning biomass such as agricultural waste, forestry residues, or bioenergy crops. Further, CO 2 emitted from the incineration of municipal solid waste can be captured as a source of CO 2 .

[0060] Through advanced capture technologies, CO 2 can be captured directly from the ambient air.

[0061] Carbon monoxide can be provided e.g. from microbial production, decomposition of metal carbonyls, thermal decomposition of formic acid, reforming reactions, water-gas shift reaction or gasification of biomass or coal.Catalysts

[0062] In general terms, the catalyst on the electrode surface should ideally have the following characteristics: It should (a) be chemically stable, it should (b) not become reduced or otherwise consumed during the electrolytic process, it should facilitate the formation of carbon-containing products, and (d) use of the catalyst should lead to the production of minimal amount of hydrogen gas. The catalysts disclosed herein fulfil these characteristics.

[0063] A catalyst can comprise one or more stabiliser that serves the role of preventing degradation of the catalyst. Suitable stabilisers should be more stable to degradation than the metal phosphide(s) being employed, but otherwise are inert with respect to the catalytic reactions taking place on the electrode surface. Exemplary stabilisers include, but is not limited to, metal oxyphosphides, metal oxynitrides, bimetallic oxides and the like.

[0064] The transition metal phosphides can be provided as a film, i.e. as a thin layer (e.g., as few layers or as a monolayer) on a stable and conductive surface.

[0065] The catalysts disclosed herein are transition metal phosphide catalysts. The catalysts can generally contain one or more transition metal phosphide, or any mixture of transition metal phosphides.

[0066] The transition metal phosphide can preferably be selected from the group consisting of phosphides of Cr, V, Ti, Hf, Ta, Zr, Nb, Ta. The preferable catalyst composition can be varied depending on the intended use, i.e. the reactant being provided (CO or CO 2 ) and the desired product(s) formed in the electrolytic cell.

[0067] For the catalytic reduction of CO 2 , the suitable catalyst can comprise one or more phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta and Zr, more preferably a phosphide of a transition metal selected from Cr and V.

[0068] For the catalytic reduction of CO, the suitable catalyst can comprise one or more phosphide of a transition metal selected from Nb, V, Hf, Ta, Ti, and Zr, more preferably a phosphide of a transition metal selected from Nb and V.

[0069] For formation of methanediol from CO 2 , the suitable catalyst(s) can comprise one or more phosphide of a transition metal selected from Ti, Hf, Ta and Zr.

[0070] For formation of formic acid from CO 2 , the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from Cr, V and Ti, more preferably Cr and V.

[0071] For formation of methanol from CO 2 , the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from Ti, Hf, Ta and Zr.

[0072] For formation of methane from CO, the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from V, Hf, Nb, Ta, Ti and Zr, more preferably a transition metal selected from Nb and V.

[0073] For formation of methanol from CO, the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from V, Hf, Nb, Ta, Ti and Zr, more preferably a transition metal selected from Nb and V.

[0074] For formation of methanediol from CO, the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from V, Nb, Ta and Ti.

[0075] An advantage of the present invention is that the process can be suitably operated using suitable electrolytes in solid or liquid form. An electrolyte solution can be non-aqueous or aqueous. For example, an electrolyte solution can comprise, or consist of an aqueous solutions containing dissolved electrolytes (salts). Thus, in certain embodiments of the process and system, the electrolytic cell comprises one or more aqueous electrolytic solutions, in one or more cell compartments. Individual cell compartments can be separated by suitable barriers, such as membranes that allow electrolytes to pass through.

[0076] Solid and liquid electrolytes may comprise any of various typical inorganic or organic salts such as but limited to soluble salts of e.g. chloride, nitrate, chlorate bromide, etc. e.g. sodium chloride, potassium chloride, calcium chloride, ammonium chloride, and other suitable salts. The electrolyte may also comprise any one or a combination of, alkali or alkaline earth metal oxides, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, rubidium hydroxide and cesium hydroxide.

[0077] An aqueous electrolyte solution can preferably comprise carbonate, bicarbonate or carbonic acid as a source of CO 2 . Such an aqueous electrolyte solution can also further comprise one or more organic or inorganic acids. Inorganic acids can include mineral acids that include but are not limited to, hydrochloric acid, nitric acid, phosphoric acid, sulphuric acid, boric acid, hydrofluoric acid, hydrobromic acid, and perchloric acid. The electrolyte can alternatively be provided as a protic or aprotic, non-aqueous solution. For example, the electrolyte can be provided as an ionic liquid, i.e. as a molten salt, for example a sodium chloride salt.

[0078] Depending on the substance composition of the catalyst, a suitable surface crystal structure may be preferred. Various different crystal structures exist for transition metal phosphides and different structures can be obtained at different growth conditions. It is within scope of the skilled person to select appropriate surface crystal structures.

[0079] Further, it is possible that a catalyst contains more than transition metal phosphide, such as two or more or three or more transition metals. For example, a transition metal phosphide can be doped by a small amount of a second (or more) transition metal. Alternatively, a catalyst can contain one or more layer of a transition metal phosphide that is provided over one or more layers of another transition metal phosphide.

[0080] In certain embodiments of the invention, the catalyst surface is provided as a pure transition metal phosphide, i.e. the catalyst comprises a single transition metal phosphide, i.e. the catalyst for example does not contain a mixture of transition metal phosphides or one transition metal phosphide that is coated by one or several layers of a second (or more) transition metal phosphides.

[0081] Thus, as will be apparent to the skilled person, the catalyst according to the invention can comprise a single transition metal phosphide. The catalyst can alternatively comprise, or consist of, a mixture of two or more transition metal phosphides. Such mixed transition metal phosphides can comprise a single crystal structure or polycrystalline structures, for example a rocksalt structure. The mixed transition metal phosphides can also comprise a mixture of transition metal phosphides that are of different crystal structures and / or phosphide with different catalytic facets. Accordingly, such mixed transition metal phosphides can further comprise a single, or a mixture of, facets. Mixed transition metal phosphide catalysts can be grown or manufactured separately and then assembled into mixed catalysts comprising the different metal phosphides, wherein the phosphides in the mixture have the same or different crystal structures.

[0082] As described in more detail herein, running a current through the electrolytic cell leads to a chemical reaction in which carbon monoxide (CO) and / or carbon dioxide (CO 2 ) is reduced in a series of steps to ultimately form one or more valuable short chain organic products, including for example methane, methanol and formic acid. The running of current is achieved by applying a voltage to the cell. An advantage of the disclosed transition metal phosphide catalysts is the possibility of performing electrolytic production of these valuable products at a low electrode potential under conditions of low temperature and / or gas pressure, which is beneficial in terms of energy efficiency and required equipment.

[0083] An electrolytic cell can be operated at an ambient gas pressure of about 1 atmosphere. The electrolytic cell can also be operated at higher pressure, i.e. pressure that is greater than ambient pressure. For example, the cell can be operated at a pressure of up to 50 atmospheres (atm), up to 40 atmospheres, up to 30 atmospheres, up to 20 atmospheres or up to 10 atmospheres. In some embodiments, the electrolytic cell is operated at a pressure that is in the range of 1 to 30 atmospheres, in the range of 1 to 20 atmospheres, in the range of 1 to 10 atmospheres, in the range of 1 to 5 atmospheres or in the range of 1 to 3 atmospheres. The electrolytic cell can also be operated at a pressure that is in the range of 2 to 20 atmospheres, in the range of 3 to 20 atmospheres, in the range of 4 to 20 atmospheres, or in the range of 5 to 20 atmospheres, such as at about atmospheres, about 6 atmospheres, about 7 atmospheres, about 8 atmospheres, about 9 atmospheres, about 10 atmospheres, about 11 atmospheres, about 12 atmospheres, about 13 atmospheres, about 14 atmospheres, about 15 atmospheres, about 16 atmospheres, about 17 atmospheres, about 18 atmospheres, about 19 atmospheres or about 20 atmospheres.

[0084] The electric potential can be applied as a constant or variable electric potential. Pulsed electric fields generated by pulsed potentials can be varied by adjusting a number of parameters such as electric field intensity, rise time of voltage pulses, number of pulses, frequency of pulses, pulse wave shape, treatment time (i.e. the length of time the pulse sequence is applied, resulting in from a product of the number of pulses and the duration of each pulse).

[0085] In certain useful embodiments, the cell is operated at an electric potential that results in formation of the desired product(s). The electric potential can in different applications be less than (i.e., less negative) than about -2.0V, less than about -1.5V, less than about -1.0 V, less than about -0.9 V, less than about -0.8V, less than about - 0.7 V, less than about -0.6 V, less than about 0.5 V, less than about -0.4 V, less than about -0.3 V or less than about -0.2 V. In some embodiments, the cell is operated at electrode potential in the range of about -1.5 V to about 0.0V or in the range of about -1.0 V to about 0.0 V, such as in the range of about -0.9 V to about 0.0V, such as in the range of about -0.8 V to about 0.0 V, such as in the range of about -0.7V to about 0.0V, about -0.6 V to about 0.0 V, about -0.5 V to about 0.0 V, or in the range of about -0.3 V to about 0.0 V or in the range of about -0.2 V to about 0.0 V. The upper limit (i.e., more negative potential limit) of the range can be about -0.3 V, about -0.4 V, about -0.5 V, about -0.6 V, about -0.7 V, about -1.0V, about -1.5V or about -2.0V. The lower limit (i.e. less negative potential limit) of the range can be as high as about +1.0V, about +0.5V, about 0.0 V, about -0.1 V, about -0.2 V, or about -0.3 V. Any of these electric potentials and ranges of electric potentials are contemplated for the electrolytic methods and electrolytic cells disclosed herein. The appropriate electric potential will be selected based on the chemical composition of the catalyst in question and the desired reaction products.

[0086] A high (more positive) or low (more negative) potential may be applied to the electrolytic cell before and / or after operation of the cell to produce products from CO / CO 2 . For example, it may be beneficial to increase voltage to a more positive potential (e.g., a voltage greater than 0.0V) to free the surface of the catalyst / electrode from adsorbents that may have an adverse effect on its operation.

[0087] After such cleaning, the electrolytic cell can be operated under normal production potential.

[0088] During operation of the cell, the composition of products obtained in the reduction of CO and / or CO 2 can be altered / controlled by selective adjustment of applied potential for any given catalyst surface. Thus, the data provided herein shows that depending on the relative binding energies of adsorbed intermediates the selectivity of the electrolytic cell changes depending on the applied voltage.

[0089] The temperature of the cell may be varied as needed. One advantage of the present catalysts is that the CO 2 RR and CORR reactions can be carried out at low temperature, such as at or near ambient room temperature. However, it will be appreciated that the temperature may be adapted and / or adjusted as needed.

[0090] For example, solid state electrolytic cells can operate at high temperatures of up to or close to 1000°C, and the catalysts described can be used at such high temperatures.

[0091] In general therefore, the temperature of the electrolytic cell, and in particular the temperature at the cathode of the cell, can be in the range of 0°C to 1000°C, in the range of 0°C to 900°C, 0°C to 800°C, 0°C to 700°C, 0°C to 600°C, 0°C to 500°C, 0°C to 400°C, 0°C to 300°C, 0°C to 200°C, 0°C to 100°C or 0°C to 50°C. In some embodiments, the temperature is in the range of 0°C to 50°C, 5°C to 45°C, 10°C to 40°C, 15°C to 35°C, or 20°C to 30°C.

[0092] An advantage of the present invention is the specificity and efficiency of product formation over side-product (such as H 2 ) formation, which is a challenge due to the competing binding energies of hydrogen and oxygen on the catalyst surface. In certain embodiments, less than about 50% moles H 2 are formed compared to moles of the desired product formed, and preferably less than about 40% moles H 2 , less than about 30% moles H 2 , less than about 20% moles H 2 , less than about 10% moles H 2 , less than about 5% moles H 2 , less than about 2% moles H 2 , or less than about 1 % moles H 2 .

[0093] Further, reaction conditions (choice of catalyst, applied potential in particular) can be set so that one reaction product is selectively formed over other reaction products. This means that the reaction predominantly or even completely leads to the formation of one particular reaction product (e.g., methane, methanol, formic acid, methanediol).

[0094] The reaction pathway of CO and CO 2 reduction depends on the relative energies of reaction intermediates. Thus, the pathway can depend on the system within which the reaction takes place, including the chemical composition of the catalysis and the catalytic surface being used in the reaction.

[0095] The active part of an industrial heterogeneous catalyst is most commonly a solid surface, e.g. a transition metal or transition metal phosphide or mixture of transition metal phosphides. The surface offers a favourable energy path from reactants to products, by binding reactants and reaction intermediates. The binding energy of reactants to the surface must be strong enough to produce reaction intermediates, but weak enough to allow products to leave the surface, allowing more reactions to take place on the surface. The consequence is that for a reaction, there will be an optimum binding energy for an intermediate, such that both stronger and weaker binding leads to lower activity. A result of this phenomenon is a reaction relationship called the volcano-shaped relationship, or commonly referred to as a Volcano plot.

[0096] Catalyst activity can in general be modified by altering the local electronic structure by strain, ligand, substitution and / or alloying. These alterations can result in changes in binding energies of reaction intermediates, and thereby a change in the thermodynamics of the overall reaction profile. Thereby it may be possible to adapt the reaction profile to specifically obtain desired reaction products.

[0097] Exemplary embodiments in accordance with the present disclosure include the following: 1. A method for catalytic electrolytic reduction of CO 2 and / or CO, the method comprising: a. providing an electrolytic cell containing at least one reaction chamber that has at least one anode and at least one cathode and at least one electrolyte between the anode and the cathode, wherein the at least one cathode comprises at least one catalyst comprising at least one transition metal phosphide selected from phosphides of Cr, V, Ti, Hf, Ta, Zr, Nb, Ta; b. providing CO 2 and / or CO in the electrolytic cell; and c. applying electrical potential to the electrolytic cell; whereby the CO 2 and / or CO undergoes at least one reduction reaction at the cathode. 2. The method of clause 1, wherein the catalyst is provided as a surface coating, a film, a microparticle, nanotubes, nanosheets, heterostructure, dopant and / or a nanoparticle. 3. The method of any one of the previous clauses, wherein the temperature at the at least one cathode is in the range of 0°C to 300°C, 0°C to 200°C, 0°C to 100°C, 0°C to 50°C 5°C to 45°C, 10°C to 40°C, 15°C to 35°C, or 20°C to 30°C. 4. The method of any one of the previous clauses, wherein the electrolytic cell comprises gas diffusion electrodes at the anode and / or the cathode. 5. The method of any one of the previous clauses, wherein the electrolytic cell further comprises at least one reference electrode. 6. The method of any one of the previous clauses, wherein the electrolytic reduction reaction results in formation of at least one alkane, lower alcohol or acid having from 1 to 3 carbon atoms. 7. The method of any one of the previous clauses, wherein the electrolytic reduction reaction results in formation of at least one product selected from methanol, methane, methanediol, formic acid, formaldehyde, ethanol, ethane, ethylene glycol, ethanediol, propane and propanol. 8. The method of any one of the previous clauses, wherein CO 2 and / or CO are provided in gaseous form into the electrolytic cell, and wherein the gas pressure at the at least one cathode is in the range of about 1 atm to 40 atm, 5 atm to 30 atm, or 10 atm to 20 atm. 9. The method of any one of the previous clauses 1 to 8, wherein the gas pressure at the at least one cathode is ambient pressure of about 1 atm. 10. The method of any one of the previous clauses, wherein the electrical potential applied to the electrolytic cell is in the range of -2.0 V to +1.0 V using a reversible hydrogen electrode (RHE) as a reference. 11. The method of any one of the previous clauses, wherein molar production of hydrogen (H 2 ) compared with CO 2 and / or CO reduction in the electrolytic cell is less than 50%. 12. The method of any one of the previous clauses, wherein the electrolyte solution comprises at least one source of CO 2 selected from carbonate (CO 3 2-< ), carbonate salts, bicarbonate (HCO 3 -< ) and carbonic acid (H 2 CO 3 ). 13. The method of any one of the previous clauses, wherein CO and / or CO 2 is provided in the solution by a stream of gas. 14. The method of any one of the previous clauses, wherein CO 2 is provided in the electrolytic cell, and wherein the transition metal phosphide is a phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta and Zr, more preferably a phosphide of a transition metal selected from Cr and V. 15. The method of any one of the previous clauses, wherein CO is provided in the electrolytic cell, and wherein the transition metal phosphide is a phosphide of a transition metal selected from Nb, V, Hf, Ta, Ti, and Zr, more preferably a phosphide of a transition metal selected from Nb and V. 16. The method of any one of the previous clauses, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of V, Nb, Hf, Ta, Ti, and Zr the catalytic reduction resulting in formation of methane. 17. The method of any one of the previous clauses 1-13, wherein CO 2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of Cr, V and Ti, more preferably Cr and V, the catalytic reduction resulting in the formation of formic acid. 18. The method of any one of the previous clauses 1-13, wherein CO 2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from Ti, Hf, Ta and Zr, the catalytic reduction resulting in formation of methanol. 19. The method of any one of the previous clauses 1-13, wherein CO 2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from Ti, Hf, Ta and Zr, the catalytic reduction resulting in formation of methanediol. 20. The method of any one of the previous clauses 1-13, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Nb, Ta, Hf, Ti and Zr, the catalytic reaction resulting in formation of methanol. 21. The method of the previous clause, wherein the phosphide is a phosphide of a transition metal selected from Nb and V. 22. The method of any one of the previous clauses 1-13, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Nb, Ta, Hf, and Ti, the catalytic reaction resulting in formation of methanediol. 23. The method of any one of the previous clauses 1-13, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Hf, Nb, Ta, Ti and Zr, preferably selected from V and Nb, the catalytic reaction resulting in formation of methane. 24. The method of any one of previous clauses 1-13, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Nb, Ta and Ti, the catalytic reaction resulting in formation of CH2O(g), the catalytic reaction resulting in formation of formaldehyde. 25. An electrolytic cell for the reduction of carbon dioxide and / or carbon monoxide, comprising: an anode; and a cathode, the cathode comprising a catalyst comprising at least one transition metal phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta, Zr, Nb, Ta. 26. The electrolytic cell of the previous clause, wherein the catalyst is provided as a surface coating, a film, a microparticle, nanotubes, nanosheets, nanoarrays, nanoribbons, heterostructure, dopant, a nanoparticle, zero- dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three- dimensional (3D), a single atom catalyst, a dual-atom catalyst and a triple-atom catalyst. 27. The electrolytic cell of any one of the previous two clauses, wherein the catalyst is provided as a pure phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta, Zr, Nb, Ta. 28. The electrolytic cell of any one of the previous four clauses, wherein the electrolytic cell comprises gas diffusion electrodes at the anode and / or the cathode. 29. The electrolytic cell of any one of the previous five clauses, wherein the electrolytic cell further comprises at least one reference electrode. 30. A chemical reactor comprising at least one electrolytic cell as set forth in any one of clauses 25-29; and a power supply connected to the electrolytic cell.

[0098] The invention will now be exemplified by the following non-limiting Examples.Example 1 CO 2 RR on transition metal phosphide surfaces Computational method

[0099] The interaction of CO 2 with TMP surfaces and formation of different intermediates were studied by DFT calculations with Generalized Gradient Approximation (GGA) using RPBE exchange correlation function. The tool used for these calculations was ab initio simulation package (VASP) and cutoff energy was 450 eV with the 4×4×1 Monkhorst-Pack K-point mesh until energy differences are converged within 10 -4< eV. The Projector augmented wave (PAW) method was implemented due to full wavefunction by utilizing computationally efficient pseudopotentials, significantly reducing computation time. The surface of TMPs were considered in the rocksalt (RS) crystallographic structure with the texture orientation of (100). A slab was prepared from the bulk phosphides of 29 different materials. The composition of the slab follows a 1:1 ratio, meaning it consists of 40 atoms in total: 20 atoms of a transition metal, 10 phosphorus atoms. The boundary conditions were periodic with 20 Å vacuum along the z direction to avoid self-interaction of neighboring layers. The bottom two layers were fixed, and the top 3 layers were allowed to fully relax together with any adsorbates on the surface.

[0100] The binding energy of an adsorbate was calculated according to the equation 1: ΔE ads = E adsorbate / TMP slab − E TMP slab − E adsorbate

[0101] Here E (adsorbate / TMP slab) is the total energy of the system with an adsorbate on the TMP slab, E (TMP slab) is the total energy of the pristine TMP slab, and E(adsorbate) is the total energy of the adsorbate. The negative ΔE ads corresponds to the exothermic adsorption phenomenon.

[0102] The Computational hydrogen electrode model as proposed by Norskov et al was used to calculate the free energy diagram for CO 2 RR and / or CORR along different paths at certain applied potential. According to this method at room temperature kinetic barrier is negligible for the proton coupled electron step and the potential limiting step is the most positive energy difference between the two adjacent steps.

[0103] The reaction free energy of an adsorbate for CO 2 RR at any arbitrary potential U vs RHE and any arbitrary pH was calculated by equation: Δ G U RHE = Δ G U RHE = 0 + neU RHE eU RHE = eU RHE + 2.3 k b TpH

[0104] where U RHE is the applied potential referred against the standard hydrogen electrode (SHE), n is number of electrons, e is the elementary charge, k b is the Boltzmann constant and T is the Temperature. By putting eq (2) into eq(3) we will get : Δ G U RHE = Δ G U RHE = 0 + n eU RHE = eU RHE + 2.3 k b TpH

[0105] Generally overpotential are independent of pH of electrolyte so in this study we will consider pH = 0. Hence for each elementary step ΔG (U = 0) is calculated by Δ G U = 0 = ΔE DFT + ΔE ZPE − T Δ S + ΔE 0K → T + ΔE sol

[0106] Where ΔG is the Gibbs free energy, ΔE DFT is the electronic energy calculated by DFT, ΔE ZPE is the zero-point energy correction and TΔS is entropy differences between the gas phase and adsorbed species calculated by vibrational frequencies for the adsorbed species, ΔE Sol is the adsorbate stabilization term due to the solvent and we have not included in this study. ΔE 0K→T is the change in internal energy because of temperature. The value of the gas phase is taken from the thermodynamic tables from textbooks.Results and discussion

[0107] The analysis was based on using the DFT method with the exchange-correlation function of (RPBE). In the analysis, a 4×4×1 Monkhorst-Pack K-point mesh and a cutoff energy of 400 eV was used. All structures of TMPs are rock salt with texture orientation of (100). The atomic structure with different reactants and products was minimized until their atomic forces dropped less than 0.03 eV / A. The TMPs-based structure consists of 5 layers and 40 atoms. In all analyses, the bottom two layers were fixed, and the top 3 layers were allowed to relax and take part in interaction with different species as shown in Figure 1. This figure shows a side and top view of TMP surfaces where the larger spheres indicate the metal atoms and smaller spheres indicate phosphorous atoms. The view in (a) represents the side view of TMP and dotted lines indicate that the bottom two layers were fixed, and the top three layers were allowed to fully relax for optimization. The view in (b) represents a top view of TMPs and dotted lines indicate the unit cell which is repeated along the x and y axis. Boundary conditions along the x and y directions were considered periodic and along the z axis, a vacuum of 20 Å was generated to avoid self-intersection of layers.

[0108] To study the reaction pathways for the reduction of CO 2 into valuable products on TMPs we have used the thermochemical model (TCM) and there are several studies carried out using this method. The phenomena of applied potential were included by using the computational hydrogen electrode (CHE) but the other parts of the electrochemical environment i.e. pH dependency and solvent effects were not considered.

[0109] A total of 29 TMPs in the (100) facets crystallography were optimized and studied for their suitability for CO 2 RR. Out of these 29 TMPs, 6 TMPs (CrP, HfP, TaP, TiP, VP and ZrP) were found to be stable and suitable for CO 2 RR study as shown in Fig 2, and after the optimization the other TMPs were not considered further due to their instability and unsuitability for CO 2 RR. We explored the binding energies and reaction pathways of the TMPs CrP, HfP, TaP, TiP, VP and ZrP further, with a focus on their suitability for catalyzing CO 2 reduction.

[0110] In the first step, various species were adsorbed on clean surfaces of fully relaxed structures of TMPs (CrP, HfP, TaP, TiP, VP, and ZrP) to test favorable side, poisoning and to start the favorable reaction pathway. As can be seen in Fig 3, in some of the cases poisoning of O and OH is exergonic but shows less poisoning behavior. The OH, showing poisoning in the case of HfP, TaP, TiP, ZrP and little for CrP and VP but this can be turned well to be desorbed in form of water and free the surface for other intermediates. In the case of CrP, OH binds less strong than OCHO and thus of no concern. The TaP required -0.61V for OH to desorb in form of water and this value is less than the applied potential for the CO 2 RR (formation of formic acid, methanediol, and methnol) which is -0.8V and hence of no concern. Similarly for TiP, the potential required for OH to turn into water is -0.32V which is less than the value of applied potential which is -0.42 V for the formation of HCOOH, and of no concern. In all the cases water is difficult to adsorb, showing poor poisoning behavior. In our analysis, we have investigated adsorption of each species on the metal, phosphorus, and bridge sites of the TMPs and our study explored that the metal site is the most active and favorable reaction site. The most interesting of our findings conclude that OCHO is showing exergonic behavior at moderate Gibbs free energy as shown in Fig 3 which is favorable towards formic acid, methanethiol, and methanol formation.

[0111] Another important aspect to investigate is the activity between the hydrogen evolution reaction (HER) or CO 2 RR. The first step towards HER is the proton adsorption on the metal site and this is shown in Fig 4, but this adsorption is weaker than the adsorption of OCHO. Thus, OCHO is more dominant and exergonic over the adsorption of H on the clean surface of catalysts. The adsorption of COOH is also weaker than the adsorption of OCHO hence our catalytic activity for CO 2 RR starts from the spontaneous adsorption of OCHO except in the case of VP as shown in Fig 3.

[0112] In this study adsorption of OCHO leads to electrochemical activity towards the formation of formic acid, methanediol, and methane. In the first hydrogenation step, the adsorbed OCHO converts to formic acid in either adsorbed or aqueous form. The reaction followed for the formation of formic acid is as:         *OCHO + H +< + e → *HCOOH or HCOOH (aq)      eq (6)

[0113] It was found that CrP and VP do not bind *HCOOH intermediate on their surface and these catalysts have the potential to produce an aqueous form of formic acid with - 0.25 V and -0.29 V, respectively, as shown in Fig 5. This is an interesting finding on activity and selectivity of CrP and VP to only produce formic acid, while in previous studies catalysts such as Cu showed to produce more than 15 products, and it requires large overpotential and required large amount of energy to separate those products.

[0114] In this study, TiP is the most promising candidate for the formation of formic acid, methanediol, and methanol. The reaction pathway for the formation of formic acid on TiP catalyst is similar as for CrP and VP and the onset potential value is -0.43 V with two electron-proton transfer steps. The methanediol formation has four-step electron transfer in the mechanism of CO 2 RR with onset potential of -0.62 V. The reaction followed by methanediol formation is as: *OCHO + H +< + e -< → *OCH 2 O eq (7) *OCH 2 O + H +< + e -< → *H 2 COOH eq (8) *H 2 COOH + H +< + e -< → *H 2 C(OH) 2 eq (9)

[0115] The binding free energy for *OCHO is -0.1 eV which indicates that the reaction is exergonic, and no additional energy to start the reaction is needed. The binding free energy required for hydrogen adsorption on the metal and phosphorous site were calculated as 0.76 eV and 0.74 eV as shown in Fig 3 which is less than *OCHO binding free energy. The *COOH binding free energy is 1.5 eV. Hence binding of *OCHO is dominant over hydrogen and *COOH so the reaction pathway will start from *OCHO. In the next protonation step, the OCH2O intermediate is formed, and the binding free energy is 0.45 eV, and in the 3rd, protonation step leads to *H2COOH intermediate formation with the binding free energy of 1.06 eV. In the fourth and last protonation step, an aqueous form of methanediol is formed and in the complete reaction for methanediol formation, the highest PDS step calculated is 0.62 eV which is between the OCH2O and H2COOH formation.

[0116] TiP is also promising for the formation of methanol with 6 protonation steps and an onset potential value of -0.62V. The reaction pathways for methanol formation are:         *OCHO + H +< + e -< → *OCH 2 O     eq (10)         *OCH 2 O + H +< + e -< → *H 2 COOH     eq (11)         * H 2 COOH + H +< + e -< → *CH 2 O+ H 2 O     eq (12)         *CH 2 O+ H +< + e -< → *CH 3 O     eq (13)         *CH 3 O+ H +< + e -< → *CH 3 OH (aq)      eq (14)

[0117] After the formation of the H2COOH intermediate, there is also a possibility for this intermediate to split into a CH2O intermediate, and a water molecule as shown in Fig 6 with the CH2O binding free energy of 0.58 eV. In further protonation, CH2O leads towards the formation of CH3O with the binding free energy of -1.0 eV, and in the final 6th protonation step methanol is formed in the aqueous form. The overall highest PDS step is the same as for the methanediol which is between the OCH2O and H2COOH with a value of 0.62 eV.

[0118] ZrP, TaP, and HfP are also good candidates for the formation of formic acid, methanediol, and methanol as shown in Fig 8. As seen, *COOH is difficult to adsorb on HfP, TaP, and ZrP catalysts and the required binding free energies are 0.99 eV, 1.09 eV, and 1.27 eV respectively. Similarly, TiP *OCHO also spontaneously adsorb on HfP, TaP, and ZrP without any additional energy and the binding free energies are -0.65 eV, -0.42 eV, and -0.33 eV respectively which are more favorable than hydrogen and COOH binding. The reaction pathway followed by this catalyst towards the formic acid formation is the same as that followed by CrP, VP, and TiP which is *OCHO → *HCOOH or HCOOH (aq). The analysis revealed that in the second protonation step, an aqueous form of formic acid is formed on HfP, TaP, and ZrP catalysts and the onset potentials required are -1.09, -0.43 and -0.79 V respectively as shown in Fig 8. The adsorbed form of formic acid is difficult in our analysis for the CO 2 RR on HfP, Tap, and ZrP due to high binding energies and we have not considered it in the free energy diagram due to high energy steps.

[0119] In Fig. 7, a summary of the onset potentials required for formation of formic acid, methanediol and methanol for CrP, HfP, TaP, TiP, VP and ZrP is shown.

[0120] The findings reveal that HfP, TaP, and ZrP also show activity towards the formation of methanediol and methanol. In the second protonation step, the proton absorbs more favourably on the metal site as compared to OCH2O and it is difficult to form OCH2O, and in the third step, OCH2O is formed with the additional step of adsorption of the proton on the metal (OCH2O + MH). The binding free energies required for OCH2O on HfP, TaP, and ZrP catalysts with additional protons on the metal side are -0.21, - 0.07, and 0.41 eV respectively. The reaction pathways towards the methanediol and methane are slightly different from the TiP with the additional adsorption of proton on the metal due to its minimum energy. The reaction pathways are as follows:         *OCHO + H +< + e -< → *OCHO +MH     eq (15)         *OCHO +MH + H +< + e -< → *OCH 2 O+MH     eq (16)         *OCH 2 O+MH + H +< + e -< → * H 2 COOH + MH     eq (17)         * H 2 COOH + H +< + e -< → * H 2 C(OH) 2      eq (18)

[0121] After the OCH2O formation, in the next protonation step H2COOH intermediate formed on HfP, TaP, and ZrP catalyst, with binding free energies of 0.73 eV, 0.87, and 0.98 V respectively. In the case of ZrP, H2COOH formation is exergonic and for HfP and TaP, it shows endergonic behaviour. The onset potential required for the formation of methanediol on HfP, TaP, and ZrP catalysts are -1.01, -0.86, and -0.83 V, respectively. The highest PDS steps for TaP and ZrP catalysts are between OCHO*+MH* and OCH2O + MH* and for HfP the highest PDS step is H2COOH *+MH* → H2C(OH)2 +MH.

[0122] After H2COOH formation this intermediate can also split into CH2O, and a water molecule as shown in Fig. 8 and reaction goes toward methanol formation. The reaction pathway toward the methanol formation was:         *OCHO + H +< + e -< → *OCHO +MH     eq (19)         *OCHO +MH + H +< + e -< → *OCH 2 O+MH     eq (20)         *OCH 2 O+MH + H +< + e -< → * H 2 COOH + MH     eq (21)         * H 2 COOH + MH + H +< + e -< → *CH 2 O+ MH + H 2 O     eq (22)         *CH 2 O+ MH + H +< + e -< → *CH 3 O + MH     eq (23)         *CH 3 O+ MH + H +< + e -< → *CH 3 OH+ MH     eq (24)

[0123] The highest PDS step for HfP, TaP, and ZrP for the methanol formation and methanediol is OCHO*+MH* → OCH2O*, and onset potential values are -1.01, -0.86 , and -0.83V respectively.

[0124] For further investigations of the results scaling relation has been drawn to study the trend of formation of formic acid on the studied TMP-based catalyst. To map the multidimensional problem into one variable the adsorption energies of OH were considered as descriptors for other adsorbate-binding free energies. In previous studies, OH was considered a good descriptor, and Bhowmik et al. has used the same for the study of CO 2 RR study on rutile oxide surfaces. Using the scaling relation we have drawn an activity volcano for the formation of formic acid. Our results conclude that CrP and VP are the best candidates for the formation of formic acid as shown on the top of the volcano plot Fig. 9. Each line of the volcano indicates the reaction step. The left leg of the volcano indicates the reaction of OCHO → HCOOH(aq) and the left leg of the volcano OH → H2O.

[0125] The investigation of charge analysis was conducted by using the Bader charge analysis and we have investigated the charge analysis of the intermediate OCHO of each candidate on the most active metal sites. Table 1. Calculated Gibbs energies (ΔG) and Bader charge (Q) of OCHO on the surface of each catalyst. (still calculating for H) CatalystCrPHfPTaPTiPVPZrPΔG-1.04-0.65-0.420.010.2-0.33Q-0.31-0.71-0.66-0.67-0.65-0.71

[0126] The charge analysis results show negative values in all the cases, which means OCHO transfers the charge to the surface of the catalyst, and negative Gibbs free energy energies indicate favourable adsorption of OCHO on the surface of the catalyst.Conclusion

[0127] In our investigations utilising advanced quantum mechanical simulations and computational models, 29 TMPs in the (100) facets of the rocksalt structure were optimized and studied for their suitability for CO 2 RR. Out of 29, only 7 TMPs (CrP, NbP, HfP, TaP, TiP, VP, and ZrP,) were screened out suitable for the CO 2 RR study, and the remaining were not considered due to their instability and unsuitable factor for CO 2 RR after the optimizations. We explored the binding energies, and reaction pathways of the TMPs CrP, HfP, TaP, TiP, VP, and ZrP, with a focus on their suitability for catalysing CO2 reduction. In this research, we have conducted a complete and comprehensive analysis of the above-mentioned TMPs. The CrP and ZrP are found the best candidates to produce formic acid at -0.25 and -0.29 V. The most interesting and novel findings in this research work are that CrP and VP only produce formic acid which is promising and better than commercially used Cu and other materials. In addition, TaP, TiP, and ZrP are also explored as good candidates to produce formic acid with small onset potentials of -0.87, -0.43, and -0.79 V, respectively. The selectivity towards methanol formation in this analysis reveals TiP is the best candidate with -0.62 V onset potential. The studied HfP, TaP, and ZrP are also good candidates for methanol formation with corresponding onset potentials of - 0.76, -0.86, and -0.83 V, respectively. Methanediol is also formed while studying the CO 2 RR mechanism on TMP surfaces and results concluded that TiP is the best candidate for the formation of methanediol with the onset potential value of -0.62 V. These results demonstrated that these TMPs as electrocatalysts are more promising than previously studied and commercially used materials (metals and oxides) for the application of CO 2 RR when comparing the onset potential needed for the reactions to produce different chemicals.Example 2 CORR on transition metal phosphide surfaces Evaluating CO Adsorption Versus Other Species

[0128] The fundamental purpose of this work is to identify the catalyst that can be used to produce a variety of carbon-based products with low onset potential from CO capture and reduction. To do this, we looked at the following twelve metal phosphides: VP, WP, YP, ZrP, CrP, HfP, NbP, MoP, CoP, TaP, TiP, and ScP. Nevertheless, after the adsorption of CO, the CoP and MoP were found to be unstable, and WP could not successfully capture the CO. Further, ScP and YP also showed endergonic processes in forming additional intermediates. Consequently, the remaining seven materials that exergonically captured CO were examined further. Moreover, the next obstacle is the identification of surface poisoning. For example, the active site is exclusively accessible to CO, or it can be occupied by other species. To comprehend this phenomenon, tests with results as shown in Fig 10 were performed. After obtaining extensive information on the most active sites, the adsorption of O, OH, H 2 O, and H was examined, intending to compare their adsorption with that of CO by evaluating their respective free energies. The investigation indicated that the presence of O, OH, H 2 O, or H should not contaminate any surface, since CO adsorption consistently occurred more favorably as shown below.

[0129] The next step is to investigate the competition between CORR and HER, as both reactions have the potential to occur concurrently at the cathode of the electrochemical cell. The reason for this is that the cathode serves as the focal point for the process of reduction, where CO is converted into different substances through the exchange of electrons, and protons undergo transformation to generate hydrogen gas. Thus, a catalyst's performance is determined by its intended purpose, either CORR or HER. Since the emphasis of this research is on CORR, we conducted tests on these materials to establish their effectiveness for CORR or HER. When compared to the potency of CO adsorption, hydrogen adsorption is found to be less efficient, either *MH (proton adsorption on metal site) or *PH (proton adsorption on phosphorus site), as shown in Fig 10. Based on the adsorption energies, it can be deduced that hydrogen does not bind the surface exergonically and it is less favorable than CO adsorption on the surface. For clarity, the relevant free energy information is shown in Fig 11.

[0130] Therefore, it is evident from Fig 10 and Fig 11 that no surface is being poisoned by hydroxyl, water, hydrogen, or oxygen, and that all the active sites are completely accessible for CO capture. Furthermore, there is no potential of HER occurring because this reaction requires more energy than CORR, which is not viable from a thermodynamic standpoint. Consequently, both assessments demonstrate the promise of the catalysts that we have selected for CORR, and this enables them to go on to the subsequent stage of reaction (hydrogenation) to produce different products.CH 3 OH (aq) , CH 2 O (g) and CH 4(g) Formation

[0131] Methanol is one of the most important products during CORR because it is used as a fuel and in the production of various chemicals. During its formation, there are two possible reaction pathways: one starting from CHO and the other from COH, as shown in Fig 12. Since these two intermediates play a fundamental role in advancing the reaction, it is crucial to calculate the potential from CO* to CHO* / COH*. Therefore, we have calculated the free energies for all the discussed catalysts.

[0132] To screen out the most effective catalyst, we selected only those materials for which the PDS from CO to CHO / COH was less than 1 eV. However, in our case, no catalyst met this criterion. For example, the ΔGs from CO* to CHO* were 1.07, 1.02, 1.11, 1.05, 1.11, and 1.12 eV, and the ΔGs from CO* to COH* were 2.57, 1.26, 2.43, 2.24, 2.14, and 2.66 eV for HfP, NbP, TiP, TaP, VP, and ZrP, respectively, as shown in Fig 13. This indicates that these steps require too high a potential to be considered viable. As an alternative, other available sites were considered, such as metal and phosphorus sites, to proceed with the reaction. The potentials for CO* to CO*+PH* were 1.14, 0.46, 0.75, 0.67, 0.54, and 0.96 eV, and for CO* to CO*+MH* were 0.51, 0.50, 0.72, 0.03, 0.63, and 0.76 eV for HfP, NbP, TiP, TaP, VP, and ZrP, respectively (as also shown in Fig 13).

[0133] After this analysis, we selected only the most exergonic pathways to proceed with the reactions for CHO and COH. In some cases, when the reaction stalled due to the high potential for CHO / COH after the CO*+PH* or CO*+MH* steps, we screened other sites and continued this process until we found the most reliable reaction pathway. For instance, the formation of CHO and COH started after the CO*+MH*+MH* steps for HfP and ZrP, while for TiP and VP, it began after CO*+MH*+PH* and CO*+PH*+MH*, respectively, leading to the exergonic creation of CHO / COH. However, TaP required the absorption of more protons (CO*+MH*+MH*+MH*) to initiate the reaction from CHO / COH.

[0134] Fig 13 displays the free energy diagrams (FED) illustrating the production of CH 3 OH (aq), CH 2 O (g) and CH 4(g) for various catalyst surfaces. Based on Fig 13(a), it is evident that the PDS for HfP corresponds to the second protonation step involving CO*+MH*+MH*, resulting in the production of CH 30 H (aq) and CH 4(g) with a ΔG PDS of 0.82 eV. In the context of TiP (b), the PDS (0.90 eV) represents the second stage of protonation in the formation of CO*+MH*+PH*. Similarly, in the case of TaP (c), it was found that the identical ΔG value (0.90 eV) was detected after the fourth protonation, which resulted in the manufacturing of CHO*. These steps led to the subsequent generation of CH 30 H (aq) , CH 2 O (g) and CH 4(g) on TiP and TaP. When considering ZrP (d), the most futile results were found with the second protonation, which led to the production of CO*+MH*+MH* with ΔG PDS for CH 3 OH (aq) and CH 4(g) generation. However, this catalyst exhibited peculiar behavior compared to others, as it followed the R2 and R7 reaction pathways. The catalysts mentioned in the study mainly use the R1 and R4 pathways for CHO* formation. It is worth noting that none of the catalysts adhere to the R3 pathway because of the significant potential needed for COH formation. Also NbP was found to be a particularly promising catalyst for CH 3 OH and CH 4 formation, as shown in Fig. 13 (e).

[0135] To get information about the onset potential, which is crucial for acquiring these three products, onset potential was determined as shown in Fig. 14. From Fig 14, it is evident that most catalysts have similar onset potentials for the formation of all products. However, only four catalyst surfaces are capable of producing CH 2 O(g), formaldehyde, while all the catalysts excel in terms of overall product output. Regarding the activity of catalytic substrates, it is obvious from Fig. 14 that NbP and VP catalysts exhibit higher activity compared to other catalysts in the formation of CH 3 OH (aq), CH 2 O (g) and CH 4(g) at incredibly low onset potentials of -0.58 V and -0.69 V, respectively.

[0136] From Fig. 15 we can see that the ΔG for VP was 0.69 eV after the second hydrogenation that was linked to the CO*+PH*+MH, during the exploration of VP. For NbP, the second protonation was to form the CHO* with PDS of 0.58 eV. Anyhow, these PDSs were further required to produce the CH 30 H (aq) , CH 2 O (g) and CH 4(g) .

[0137] In summary, it can be concluded that several TMPs shows promising results for CORR although NbP and VP are the most promising choices for because of their great activity in producing valuable products with low onset potential. Further, they are not suitable for competing reactions with CORR, such as HER. The surfaces are not being contaminated by other species.

Examples

example 1

Example 1

CO 2 RR on transition metal phosphide surfaces

Computational method

[0099]The interaction of CO 2 with TMP surfaces and formation of different intermediates were studied by DFT calculations with Generalized Gradient Approximation (GGA) using RPBE exchange correlation function. The tool used for these calculations was ab initio simulation package (VASP) and cutoff energy was 450 eV with the 4×4×1 Monkhorst-Pack K-point mesh until energy differences are converged within 10 -4< eV. The Projector augmented wave (PAW) method was implemented due to full wavefunction by utilizing computationally efficient pseudopotentials, significantly reducing computation time. The surface of TMPs were considered in the rocksalt (RS) crystallographic structure with the texture orientation of (100). A slab was prepared from the bulk phosphides of 29 different materials. The composition of the slab follows a 1:1 ratio, meaning it consists of 40 atoms in total: 20 atoms of a transition metal, 10 p...

example 2

Example 2

CORR on transition metal phosphide surfaces

Evaluating CO Adsorption Versus Other Species

[0128]The fundamental purpose of this work is to identify the catalyst that can be used to produce a variety of carbon-based products with low onset potential from CO capture and reduction. To do this, we looked at the following twelve metal phosphides: VP, WP, YP, ZrP, CrP, HfP, NbP, MoP, CoP, TaP, TiP, and ScP. Nevertheless, after the adsorption of CO, the CoP and MoP were found to be unstable, and WP could not successfully capture the CO. Further, ScP and YP also showed endergonic processes in forming additional intermediates. Consequently, the remaining seven materials that exergonically captured CO were examined further. Moreover, the next obstacle is the identification of surface poisoning. For example, the active site is exclusively accessible to CO, or it can be occupied by other species. To comprehend this phenomenon, tests with results as shown in Fig 10 were performed. After...

FIELD

[0001] The disclosure relates to the reduction of carbon dioxide and carbon monoxide by electrolysis and new transition metal catalysts therefor.INTRODUCTION

[0002] Since the start of the 20 th< century the world climate has been changing more noticeably. The balance in nature is lost due to dramatic increasing CO 2 emissions by burning of carbonaceous fuels which results in global warming. An urgent effort is needed to overcome this problem to reduce the ratio of CO 2 and turn it into fuel and other useful chemical products. On the other hand, the large-scale use of unsustainable fossil fuels for energy has led to an energy crisis in the world and an increasing amount of CO 2 in the atmosphere. Due to this, the world climate is changing day by day and scientific effort is needed to overcome the emission of CO 2 in the air by converting the already present CO 2 into green energy fuels. Previous experimental and theoretical research concluded that conversion of CO 2 into valuable products can be achievable, but not yet efficient for commercialization. In the whole process of CO 2 reduction reaction (CO 2 RR), the catalyst plays a key role in capturing CO 2 and converting it into valuable chemical products and fuels. CO 2 is a chemically stable molecule and this makes its electrolytic reduction challenging with high reduction overpotential, poor product selectivity and low current efficiency. To address this challenge, researchers and scientists have been exploring novel catalysts which can overcome these issues and speed up the electrolytic process (Ebbesen & Mogesen, J. Power Sources 2009, 193:349; Fu et al, Energy & Environmental Sci 2010, 3:1382).

[0003] In CO 2 RR the catalysts play a key role in the conversion of CO 2 into valuable products. In previous decades many experimental studies have been done for electrolytic CO 2 RR on different metal catalysts. For example, pure Cu, Ag, Au, and Pt catalysts have been studied as catalysts for the CO 2 reduction reaction. However, their application for the commercial purposes is not enough due to poor product selectivity, low faradic efficiency and high overpotential that is needed. It is well known that the Cu is a widely used catalyst for the reduction of CO 2 to CH 4 but its overpotential is high (~0.9 V) and moreover it produces 15 different carbon containing products which require large amount of energy for their separation. In addition to pure Cu, polycrystalline Cu was also used for the application of CO 2 RR and it produces amounts of C 2 and C 3 aldehydes as well as ethylene.

[0004] In addition to pure metal electrocatalysts, metal oxide catalysts i.e. TiO 2 have been studied by DFT methods and considered impressive and more efficient catalysts for CO 2 conversion to CH 3 OH and CH 4 products (Ramesha, ACS Catalysis, 2014, 4:3249). In another study RhO 2 is predicted to be a novel candidate for the formation of formic acid at -0.20 V onset potential. However, stability and electron conductivity of oxides as cathode materials are a challenge that limits the application of oxides for CO 2 RR. There are numerous research studies that have been conducted to explore novel catalyst materials for the CO 2 RR. Materials studied by DFT (density functional theory) include graphene-based materials, metals, metal oxides, zeolites, sulfides, and metal organic frameworks. The utilization of catalysts for large scale production is however still a challenge and improved methods are required to overcome this problem.SUMMARY

[0005] The present disclosure provides methods to overcome the above deficiencies of the prior art. The need to explore alternative materials for catalysis of CO 2 RR and CORR arises from the imperative to develop efficient and sustainable methods for converting carbon dioxide and carbon monoxide into valuable products. Traditional catalysts may have limitations in terms of activity, selectivity, efficiency, or cost. By searching for other materials, the aim is to identify catalysts that can enhance the performance of CO 2 RR and CORR processes, making them more economically viable and environmentally friendly.

[0006] In particular, in light of the challenge of finding good electrocatalysts for CO 2 and CO reduction, the disclosure provides certain transition metal phosphides (TMPs) that are useful catalysts for CO 2 RR and CORR.

[0007] An aspect of the disclosure relates to a method for catalytic electrolytic reduction of CO 2 and / or CO, the method comprising (a) providing an electrolytic cell containing at least one reaction chamber that has at least one anode and at least one cathode and at least one electrolyte between the anode and the cathode, wherein the at least one cathode comprises at least one catalyst comprising at least one phosphide of one or more transition metal; (b) providing CO 2 and / or CO in the electrolytic cell; and (c) applying electrical potential to the electrolytic cell; whereby the CO 2 and / or CO undergoes at least one reduction reaction at the cathode.

[0008] Another aspect of the disclosure relates to an electrolytic cell for the reduction of carbon dioxide and / or carbon monoxide, comprising: an anode; a cathode, the cathode comprising a catalyst comprising at least one transition metal phosphide, and a power supply connected to the anode and the cathode.

[0009] The transition metal phosphide in these applications is preferably a transition metal phosphide of one or more transition metal selected from Cr, V, Ti, Hf, Ta, Zr, Nb and Ta.

[0010] The disclosure further relates to a chemical reactor comprising at least one electrolytic cell as described herein; and a power supply connected to the electrolytic cell.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows a side (a) and top (b) view of TMP surfaces where the larger spheres indicate the metal atoms and smaller spheres indicate phosphorous atoms. FIG. 2 shows a schematic diagram of results of screening of TMPs for CO 2 RR. FIG. 3 shows a schematic diagram of adsorption of all species on a clean surface in CO 2 RR. FIG. 4 shows a free energy diagram for HER on-metal side of TMPs. FIG. 5 shows a energy diagram for CrP (a) and VP (b) in CO 2 RR, where *MH indicates proton binding on the metal side, *PH indicates proton binding on the phosphorous side, *COOH indicates COOH binding on the metal side, *OCHO indicates binding of OCHO on metal side and HCOOH (aq) is formic acid in aqueous form. FIG. 6 shows free energy diagram for CO 2 RR on TiP catalyst. FIG. 7 shows a schematic representation of onset potential required for formic acid, methanediol, and methanol formation in CO 2 RR. FIG. 8 shows a free energy diagram of CO 2 RR for (a) TaP, (b) ZrP, (c) HfP. FIG. 9 shows a theoretical volcano for the formation of formic acid via CO 2 RR. FIG. 10 shows a comparison of adsorption free energies of different CORR adsorbates on the catalyst surfaces. FIG. 11 shows adsorption free energies of proton on the surface metal site as an indication of HER in the CORR. FIG. 12 shows possible reaction pathways to produce CH 3 OH (aq), CH 2 O (g) and CH 4(g) on catalyst surfaces in the CORR. FIG. 13 shows free energy diagrams (FED) for CORR in the production of CH 3 OH (aq) CH 2 O (g) and CH 4(g) for various catalyst surfaces; HfP (a), TiP (b), TaP (c), ZrP (d), NbP (e). FIG. 14 shows necessary onset potential to generate products via the CORR. FIG. 15 shows an illustration of the most favorable pathway to create the CH 3 OH (aq), CH 2 O (g) and CH 4(g)) via the conventional mechanism in the CORR for VP. DESCRIPTION

[0012] In the following, exemplary embodiments of the invention will be described, referring to the figures. These examples are provided to provide further understanding of the invention, without limiting its scope.

[0013] In the following description, a series of steps are described. The skilled person will appreciate that unless required by the context, the order of steps is not critical for the resulting configuration and its effect. Further, it will be apparent to the skilled person that irrespective of the order of steps, the presence or absence of time delay between steps, can be present between some or all of the described steps.

[0014] As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise.

[0015] Throughout the description and claims, the terms "comprise", "including", "having", and "contain" and their variations should be understood as meaning "including but not limited to" and are not intended to exclude other components.

[0016] The present invention also covers the exact terms, features, values and ranges etc. in case these terms, features, values and ranges etc. are used in conjunction with terms such as about, around, generally, substantially, essentially, at least etc. (i.e., "about 3" shall also cover exactly or "substantially constant" shall also cover exactly constant).

[0017] The term "at least one" should be understood as meaning "one or more", and therefore includes both embodiments that include one or multiple components. Furthermore, dependent claims that refer to independent claims that describe features with "at least one" have the same meaning, both when the feature is referred to as "the" and "the at least one".

[0018] The present disclosure is based on the surprising finding that transition metal phosphides, when used as a catalyst in the electrolytic reduction of CO 2 and CO, allow for efficient reduction to produce valuable small compounds at low temperatures and pressure and using low applied potential. Given the ever-increasing levels of CO 2 in the global atmosphere and concomitant need to find ways to achieve a carbon neutral energy technology, these catalysts can be used to facilitate the conversion of CO 2 and CO into valuable carbon-containing fuels.

[0019] In general, the catalysts can be used to reduce CO 2 and / or CO to produce hydrocarbons or oxygenates having from 1 to 3 carbon atoms. Exemplary products include methane, methanol, methanediol, formaldehyde, formic acid, ethanol, ethylene glycol, ethanediol, propane and propanol.

[0020] Accordingly, the disclosure provides means in the form of methods and systems for electrolytic conversion of CO 2 and / or CO at low temperature and pressure. Compounds such as short-chain alkanes and oxygenates (e.g. alcohols and acids) having from 1 to 3 carbon atoms can be produced using the disclosed transition metal phosphides. Through selection of appropriate catalysts, catalyst surface and appropriate applied voltage, the reduction reaction can be geared to produce compounds of particular interest.

[0021] In an aspect, the present disclosure is based on the investigation of the capability of transition metal phosphides (TMPs) to catalyze the conversion of CO 2 and CO into valuable organic compounds that can for example be used as valuable fuel. The results are based on studies that delve into the electronic intricacies of TMPs, unraveling their potential to drive the transformation of CO 2 and CO.CO 2 Reduction Reaction (CO 2 RR)

[0022] Density Functional Theory (DFT) was used to explore the reactivity of the reduction of CO 2 in the presence of 29 different transition metal phosphide surfaces. The goal was to get an understanding of the performance and competence of these materials for CO 2 RR. After a screening for most suitable TMPs, a total of 6 TMPs were considered most promising and studied further.

[0023] DFT is a quantum mechanical methodology that accounts for the interaction between electrons and the nucleus. The input is the atomic structure of the molecular system of interest and the output is the ground state energy of the system. The reaction mechanism of CO 2 reduction to CH 4 on a stepped Cu(211) surface was first proposed by Peterson et al (Energy Environ Sci 2010, 3:1311). The free energy of adsorbed intermediates on the catalyst surface was calculated using a thermochemical model (TCM). The same approach has been successfully applied on several other electrolytic systems, including water oxidation on transition metal oxides, N 2 electroreduction on transition metal surfaces, transition metal nitrides and transition metal oxides, hydrogen evolution reaction (HER) on transition metal nitrides and CO 2 RR on transition metals and transition metal oxides.

[0024] The need to explore alternative materials for the CO 2 RR arises from the imperative to develop efficient and sustainable methods for converting carbon dioxide into valuable products. Traditional catalysts may have limitations in terms of activity, selectivity, efficiency, or cost. By searching for other materials, researchers aim to identify catalysts that can enhance the performance of CO 2 RR processes, making them more economically viable and environmentally friendly. The surface chemistry of TMPs is comparable to precious Pt-group metals which was already described by Levy and Boudart (Science 1973, 181:185). In previous studies these TMPs have been used for a wide range of reactions such as CO oxidation, hydrogenation, methane dry reforming, desulfurization, conversation of methane to gas and water gas shift reaction. Hence these materials have received great attention for application in catalysis.

[0025] Utilizing advanced quantum mechanical simulations and computational models, the present disclosure is based on an exploration of the binding energies and reaction pathways of 29 TMPs with a focus on their suitability for catalyzing CO 2 reduction. In this research a complete and comprehensive analysis for above mentioned TMPs on (100) facets were conducted. A schematic illustration of the considered TMP surfaces is illustrated in Fig. 1.

[0026] In the following Example 1 it is described how DFT calculations can be used to describe the interaction of CO 2 with TMP surfaces and predict the reaction pathway for CO 2 RR on TMPs. As shown by the results illustrated in Example 1, the conventional surface mechanism was investigated to study the CO 2 RR towards methane and methanol production on TMP surfaces. The analysis reveals that several TMPs exhibit remarkable catalytic activity for CO 2 reduction, surpassing the performance of conventional catalysts. The electronic structure of these TMPs plays a pivotal role in facilitating the conversion of CO 2 into valuable chemical products.

[0027] The present disclosure is based on a complete analysis of transition metal phosphides (TMPs) with (100) facets by using density functional calculations. In comparison to transition metal oxides, transition metal nitrides (TMNs) and transition metal dichalcogenides (TMDs), transition metal phosphides (TMPs) exhibited superior electrocatalytic conductivity and stability. The electrocatalytic activities of TMPs have comparable properties with Pt-based materials due to their tunable structures, multifunctional sides, and excellent stability. The TMPs are composed of metals and phosphorous atoms.

[0028] The presence of P atoms decreases the metal dissolution thermodynamically and improves the stability of transition metal-based catalysts for the application of electrocatalysis. It has for example been reported that the stability of MoP is greater than that of Mo 2 N and Mo 2 C.

[0029] Hence TMPs are an excellent choice for the application of electrocatalysis due to desirable properties of being cost-effective, stable, and showing good stability and activity. The present invention is based on a study of 29 TMPs, assessing their suitability for catalyzing CO 2 RR. The results show that certain TMPs (CrP, HfP, TiP, TaP, VC, and ZrP) are especially beneficial for electrocatalytic activity. The results, exemplified by the data shown in Example 1, reveal that the TMPs are more promising for CO 2 RR than previously studied transition metal-based catalysts.CO reduction reaction (CORR)

[0030] The disclosure also shows that transition metal phosphides (TMPs) are capable of catalyzing the conversion of CO into valuable organic compounds via CORR.

[0031] Protonation along the reaction pathway from CO to CH 4 and CH 3 OH on 12 transition metal phosphides have been calculated, including VP, WP, YP, ZrP, CrP, HfP, NbP, MoP, CoP, TaP, TiP, and ScP with the (100) facets. These facets of TMPs are considered the most stable structure having the 1:1 TM:C:P ratio and cubic crystallography.

[0032] The conventional mechanism was investigated to study the CORR towards methane and methanol production on TMPs surfaces. In the conventional surface mechanism the interaction of CO gas molecule on the surface of TMPs with different sites is studied. All aspects of each mechanism are calculated explicitly, and free energy diagrams are drawn for each adsorbed species on the surface of catalyst. Thermochemical model was implemented to obtain the trends of CORR on each TMPs surface.

[0033] Employing Density Functional Theory (DFT) of the 12 TMPs were thus explored for the conversion of CO to methane (CH 4 ), methanediol (CH 2 O) and methanol (CH 3 OH).

[0034] Results of the study are presented in the following Example 2. In the study, CrP, HfP, NbP, TaP, TiP, VP and ZrP were found to be particularly good candidates for the CORR. It was furthermore found that hydrogen does not bind exergonically to the explored surfaces, thus being less favorable than CO adsorption on the surfaces (see Fig. 11). As a consequence, there is no risk of competing hydrogen evolution via the HER reaction.

[0035] All of these surfaces having the most promising catalyst activity were found to require a potential less than -1.0V to produce any product. The most promising catalysts were found to be VP and NbP, requiring onset potential of less than -0.7V, although TaP, TiP and HfP also show great promise with onset potential in the range of -0.8V to -0.9V.

[0036] Overall, the findings indicate due to CO spontaneously adsorption, less poisoning, less over potential and high product selectivity TMPs are excellent choice for CORR to produce valuable products such as methane, methanediol, formic acid and methanol.Electrolytic cells

[0037] In general, an electrolytic cell is a chemical reactor where chemical reactions take place, driven by an applied electric potential. An electrolytic cell in the present context is therefore an electrolytic cell that undergoes a redox reaction when electrical potential is applied to the cell.

[0038] Design of electrolytic cells is known in the art. Examples of suitable electrolytic cells include H-type cells, polymer electrolyte membrane (PEM) flow cells, microfluidic flow cells, solid oxide electrolytic cells and DEMS cells. It should be appreciated that the transition metal phosphides disclosed herein can be adapted for use in any type of electrolytic cell. Examples of suitable electrolytic cells are described in Liang et al. (Journal of CO2 Utilization, 2020, 35:90-105).

[0039] In an H-type cell, the cathodic and anodic compartments are typically separated by an ion-exchange membrane that allows ions to flow across while preventing reaction products from being reoxidized at the anode. There can be a reference electrode present in the cathodic compartment. In these cells, there is a liquid electrolyte solution at the cathode and anode, and CO 2 gas is typically streamed into the cathodic compartment.

[0040] In PEM flow cells, CO and / or CO 2 can be streamed directly at the cathode or it can be provided by dissolved bicarbonate. There can be either a liquid phase at both electrodes, a gas phase at the cathode and liquid at the anode or a humified gas phase at both electrodes. Typically, there is a gas diffusion electrode at the cathode and possibly also at the anode.

[0041] In microfluidic flow cells, an electrolyte channel is provided between a cathode and an anode, with a stream of CO and / or CO 2 gas delivered at the cathode. Instead of utilizing a membrane, the cathode and anode are separated by the diffusion of products.

[0042] Solid state electrolysis cells can improve the kinetics of reactions and achieve high efficiency. In these cells, there is a solid electrolyte to facilitate transfer of ions, with a membrane separating the cathode and anode. The cells can operate at high temperatures if needed, e.g. at or above 600°C, such as high as 700°C, 800°C or 900°C, even as high as 1000°C.

[0043] The transition metal phosphide catalysts disclosed herein can be provided in any suitable form in the electrolytic cell of interest, depending on its design and intended use. Thus, the catalysts can be provided as a coating, film, as a powder, dopant etc., and in forms such as microparticles, nanoparticles such as zero- dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three- dimensional (3D) nanoparticles, nanotubes, nanosheets, heterostructures, a single atom catalyst, a dual-atom catalyst and a triple-atom catalyst.

[0044] Transition metal phosphides can have the general 1:1 ratio (TM:P) stoichiometry, e.g. VP, MoP, ZrP, TiP etc. Other stoichiometries are however also possible.

[0045] Thus, the skilled person will appreciate that the phosphides may have an alternative stoichiometry and such alternative stoichiometries are also within scope of the present disclosure. Further, chemical compounds as described herein are provided by their chemical formula irrespective of their phase or state. In particular, compounds that are present in their gaseous state when present in a pure and isolated form at room temperature (such as CO or CO 2 ) are herein described by their chemical formula. For example, carbon monoxide and carbon dioxide is herein described as CO and CO 2 , whether present as a gas, as individual molecules, in clusters, bound to surfaces or present as a solute, and the same applies to other molecular species described herein.

[0046] A mixture of CO and CO 2 can be provided in the electrolytic cell. During such operation of the cell, both CORR and CO 2 RR reactions will take place with products formed depending on the type of catalyst used (i.e., which TMP) and the applied voltage.

[0047] Alternatively, only CO or only CO 2 can be provided in the cell. This may be preferable, as it will be easier to control the respective reaction (CORR or CO 2 RR) for the type of TMP being used in the cell.

[0048] The proton donor in the electrolytic reactions can be any suitable substance that is capable of donating protons in the electrolytic cell. The proton donor can for example be water or it can be in the form of an acid, such as any suitable organic or inorganic acid. The proton donor can be provided in an acidic, neutral or alkaline aqueous solutions. The proton donor can also, or alternatively, be provided by H 2 oxidation at the anode. In other words, hydrogen can be considered as a source of protons: H 2 <=> 2(H +< + e -< )

[0049] The electrolytic cell can in general comprise at least three general parts or components, a cathode electrode, an anode electrode and an electrolyte.

[0050] The electrolytic reduction of carbon dioxide is the conversion of carbon dioxide to more reduced chemical species using electrical energy.

[0051] The different parts or components can be provided in separate containers, or they can be provided in a single container. The electrolyte can be a solid or an aqueous solution in which ions are provided, for example in a dissolved form in an aqueous solution when present. When provided as an aqueous solution, the aqueous solution can be a neutral, an alkaline or an acidic solution.Sources of CO 2 and CO

[0052] Carbon dioxide (CO 2 ) can be provided by a source such as any one of bicarbonate (HCO 3 -< ), carbonate (CO 3 -2< ) and / or carbonic acid (H 2 CO 3 ). For example, bicarbonate and carbonate can be provided as bicarbonate or carbonate salts, either in pure form as a solid or in solution, or in a mixture into a solution, that can preferably be an aqueous solution. A mixture of bicarbonate, carbonate and carbonic acid will reach equilibrium in solution. Therefore, the relative concentration of these species will depend on pH of an aqueous solution.

[0053] An alternate source of CO and CO 2 is the gaseous form of the compounds, CO(g) and CO 2 (g). Gaseous CO and CO 2 can be provided as a sole source of CO and / or CO 2 , or it can be provided as a supplement to other sources of CO or CO 2 in the cell. In the case of CO 2 , this includes the previously mentioned bicarbonate, carbonate and carbonic acid.

[0054] Other or additional sources of CO and CO 2 are possible. For example, direct injection of CO 2 gas is possible by bubbling CO 2 directly into an electrolyte solution.

[0055] Alternatively, dissolved carbonate salts (e.g., sodium carbonate, Na 2 CO 3 ) in water can be used to provide a source of CO 2 .

[0056] An alternative source can be provided from industrial CO 2 Emissions. Capturing CO 2 from industrial processes such as cement production, steel manufacturing, or chemical production can also form a source of CO 2 . Further, chemical reactions that release CO 2 can be a source of CO 2 .

[0057] Certain microbial processes, such as fermentation, produce CO 2 as a byproduct and can serve as a source of CO 2 . The CO 2 can also be provided through photocatalytic production.

[0058] Another source is represented by CO 2 captured from fossil fuel-based power plants (coal, natural gas, oil) which produce large amounts of CO 2 .

[0059] Yet another source can be CO 2 generated from burning biomass such as agricultural waste, forestry residues, or bioenergy crops. Further, CO 2 emitted from the incineration of municipal solid waste can be captured as a source of CO 2 .

[0060] Through advanced capture technologies, CO 2 can be captured directly from the ambient air.

[0061] Carbon monoxide can be provided e.g. from microbial production, decomposition of metal carbonyls, thermal decomposition of formic acid, reforming reactions, water-gas shift reaction or gasification of biomass or coal.Catalysts

[0062] In general terms, the catalyst on the electrode surface should ideally have the following characteristics: It should (a) be chemically stable, it should (b) not become reduced or otherwise consumed during the electrolytic process, it should facilitate the formation of carbon-containing products, and (d) use of the catalyst should lead to the production of minimal amount of hydrogen gas. The catalysts disclosed herein fulfil these characteristics.

[0063] A catalyst can comprise one or more stabiliser that serves the role of preventing degradation of the catalyst. Suitable stabilisers should be more stable to degradation than the metal phosphide(s) being employed, but otherwise are inert with respect to the catalytic reactions taking place on the electrode surface. Exemplary stabilisers include, but is not limited to, metal oxyphosphides, metal oxynitrides, bimetallic oxides and the like.

[0064] The transition metal phosphides can be provided as a film, i.e. as a thin layer (e.g., as few layers or as a monolayer) on a stable and conductive surface.

[0065] The catalysts disclosed herein are transition metal phosphide catalysts. The catalysts can generally contain one or more transition metal phosphide, or any mixture of transition metal phosphides.

[0066] The transition metal phosphide can preferably be selected from the group consisting of phosphides of Cr, V, Ti, Hf, Ta, Zr, Nb, Ta. The preferable catalyst composition can be varied depending on the intended use, i.e. the reactant being provided (CO or CO 2 ) and the desired product(s) formed in the electrolytic cell.

[0067] For the catalytic reduction of CO 2 , the suitable catalyst can comprise one or more phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta and Zr, more preferably a phosphide of a transition metal selected from Cr and V.

[0068] For the catalytic reduction of CO, the suitable catalyst can comprise one or more phosphide of a transition metal selected from Nb, V, Hf, Ta, Ti, and Zr, more preferably a phosphide of a transition metal selected from Nb and V.

[0069] For formation of methanediol from CO 2 , the suitable catalyst(s) can comprise one or more phosphide of a transition metal selected from Ti, Hf, Ta and Zr.

[0070] For formation of formic acid from CO 2 , the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from Cr, V and Ti, more preferably Cr and V.

[0071] For formation of methanol from CO 2 , the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from Ti, Hf, Ta and Zr.

[0072] For formation of methane from CO, the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from V, Hf, Nb, Ta, Ti and Zr, more preferably a transition metal selected from Nb and V.

[0073] For formation of methanol from CO, the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from V, Hf, Nb, Ta, Ti and Zr, more preferably a transition metal selected from Nb and V.

[0074] For formation of methanediol from CO, the suitable catalyst can comprise one or more transition metal phosphide of a transition metal selected from V, Nb, Ta and Ti.

[0075] An advantage of the present invention is that the process can be suitably operated using suitable electrolytes in solid or liquid form. An electrolyte solution can be non-aqueous or aqueous. For example, an electrolyte solution can comprise, or consist of an aqueous solutions containing dissolved electrolytes (salts). Thus, in certain embodiments of the process and system, the electrolytic cell comprises one or more aqueous electrolytic solutions, in one or more cell compartments. Individual cell compartments can be separated by suitable barriers, such as membranes that allow electrolytes to pass through.

[0076] Solid and liquid electrolytes may comprise any of various typical inorganic or organic salts such as but limited to soluble salts of e.g. chloride, nitrate, chlorate bromide, etc. e.g. sodium chloride, potassium chloride, calcium chloride, ammonium chloride, and other suitable salts. The electrolyte may also comprise any one or a combination of, alkali or alkaline earth metal oxides, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, rubidium hydroxide and cesium hydroxide.

[0077] An aqueous electrolyte solution can preferably comprise carbonate, bicarbonate or carbonic acid as a source of CO 2 . Such an aqueous electrolyte solution can also further comprise one or more organic or inorganic acids. Inorganic acids can include mineral acids that include but are not limited to, hydrochloric acid, nitric acid, phosphoric acid, sulphuric acid, boric acid, hydrofluoric acid, hydrobromic acid, and perchloric acid. The electrolyte can alternatively be provided as a protic or aprotic, non-aqueous solution. For example, the electrolyte can be provided as an ionic liquid, i.e. as a molten salt, for example a sodium chloride salt.

[0078] Depending on the substance composition of the catalyst, a suitable surface crystal structure may be preferred. Various different crystal structures exist for transition metal phosphides and different structures can be obtained at different growth conditions. It is within scope of the skilled person to select appropriate surface crystal structures.

[0079] Further, it is possible that a catalyst contains more than transition metal phosphide, such as two or more or three or more transition metals. For example, a transition metal phosphide can be doped by a small amount of a second (or more) transition metal. Alternatively, a catalyst can contain one or more layer of a transition metal phosphide that is provided over one or more layers of another transition metal phosphide.

[0080] In certain embodiments of the invention, the catalyst surface is provided as a pure transition metal phosphide, i.e. the catalyst comprises a single transition metal phosphide, i.e. the catalyst for example does not contain a mixture of transition metal phosphides or one transition metal phosphide that is coated by one or several layers of a second (or more) transition metal phosphides.

[0081] Thus, as will be apparent to the skilled person, the catalyst according to the invention can comprise a single transition metal phosphide. The catalyst can alternatively comprise, or consist of, a mixture of two or more transition metal phosphides. Such mixed transition metal phosphides can comprise a single crystal structure or polycrystalline structures, for example a rocksalt structure. The mixed transition metal phosphides can also comprise a mixture of transition metal phosphides that are of different crystal structures and / or phosphide with different catalytic facets. Accordingly, such mixed transition metal phosphides can further comprise a single, or a mixture of, facets. Mixed transition metal phosphide catalysts can be grown or manufactured separately and then assembled into mixed catalysts comprising the different metal phosphides, wherein the phosphides in the mixture have the same or different crystal structures.

[0082] As described in more detail herein, running a current through the electrolytic cell leads to a chemical reaction in which carbon monoxide (CO) and / or carbon dioxide (CO 2 ) is reduced in a series of steps to ultimately form one or more valuable short chain organic products, including for example methane, methanol and formic acid. The running of current is achieved by applying a voltage to the cell. An advantage of the disclosed transition metal phosphide catalysts is the possibility of performing electrolytic production of these valuable products at a low electrode potential under conditions of low temperature and / or gas pressure, which is beneficial in terms of energy efficiency and required equipment.

[0083] An electrolytic cell can be operated at an ambient gas pressure of about 1 atmosphere. The electrolytic cell can also be operated at higher pressure, i.e. pressure that is greater than ambient pressure. For example, the cell can be operated at a pressure of up to 50 atmospheres (atm), up to 40 atmospheres, up to 30 atmospheres, up to 20 atmospheres or up to 10 atmospheres. In some embodiments, the electrolytic cell is operated at a pressure that is in the range of 1 to 30 atmospheres, in the range of 1 to 20 atmospheres, in the range of 1 to 10 atmospheres, in the range of 1 to 5 atmospheres or in the range of 1 to 3 atmospheres. The electrolytic cell can also be operated at a pressure that is in the range of 2 to 20 atmospheres, in the range of 3 to 20 atmospheres, in the range of 4 to 20 atmospheres, or in the range of 5 to 20 atmospheres, such as at about atmospheres, about 6 atmospheres, about 7 atmospheres, about 8 atmospheres, about 9 atmospheres, about 10 atmospheres, about 11 atmospheres, about 12 atmospheres, about 13 atmospheres, about 14 atmospheres, about 15 atmospheres, about 16 atmospheres, about 17 atmospheres, about 18 atmospheres, about 19 atmospheres or about 20 atmospheres.

[0084] The electric potential can be applied as a constant or variable electric potential. Pulsed electric fields generated by pulsed potentials can be varied by adjusting a number of parameters such as electric field intensity, rise time of voltage pulses, number of pulses, frequency of pulses, pulse wave shape, treatment time (i.e. the length of time the pulse sequence is applied, resulting in from a product of the number of pulses and the duration of each pulse).

[0085] In certain useful embodiments, the cell is operated at an electric potential that results in formation of the desired product(s). The electric potential can in different applications be less than (i.e., less negative) than about -2.0V, less than about -1.5V, less than about -1.0 V, less than about -0.9 V, less than about -0.8V, less than about - 0.7 V, less than about -0.6 V, less than about 0.5 V, less than about -0.4 V, less than about -0.3 V or less than about -0.2 V. In some embodiments, the cell is operated at electrode potential in the range of about -1.5 V to about 0.0V or in the range of about -1.0 V to about 0.0 V, such as in the range of about -0.9 V to about 0.0V, such as in the range of about -0.8 V to about 0.0 V, such as in the range of about -0.7V to about 0.0V, about -0.6 V to about 0.0 V, about -0.5 V to about 0.0 V, or in the range of about -0.3 V to about 0.0 V or in the range of about -0.2 V to about 0.0 V. The upper limit (i.e., more negative potential limit) of the range can be about -0.3 V, about -0.4 V, about -0.5 V, about -0.6 V, about -0.7 V, about -1.0V, about -1.5V or about -2.0V. The lower limit (i.e. less negative potential limit) of the range can be as high as about +1.0V, about +0.5V, about 0.0 V, about -0.1 V, about -0.2 V, or about -0.3 V. Any of these electric potentials and ranges of electric potentials are contemplated for the electrolytic methods and electrolytic cells disclosed herein. The appropriate electric potential will be selected based on the chemical composition of the catalyst in question and the desired reaction products.

[0086] A high (more positive) or low (more negative) potential may be applied to the electrolytic cell before and / or after operation of the cell to produce products from CO / CO 2 . For example, it may be beneficial to increase voltage to a more positive potential (e.g., a voltage greater than 0.0V) to free the surface of the catalyst / electrode from adsorbents that may have an adverse effect on its operation.

[0087] After such cleaning, the electrolytic cell can be operated under normal production potential.

[0088] During operation of the cell, the composition of products obtained in the reduction of CO and / or CO 2 can be altered / controlled by selective adjustment of applied potential for any given catalyst surface. Thus, the data provided herein shows that depending on the relative binding energies of adsorbed intermediates the selectivity of the electrolytic cell changes depending on the applied voltage.

[0089] The temperature of the cell may be varied as needed. One advantage of the present catalysts is that the CO 2 RR and CORR reactions can be carried out at low temperature, such as at or near ambient room temperature. However, it will be appreciated that the temperature may be adapted and / or adjusted as needed.

[0090] For example, solid state electrolytic cells can operate at high temperatures of up to or close to 1000°C, and the catalysts described can be used at such high temperatures.

[0091] In general therefore, the temperature of the electrolytic cell, and in particular the temperature at the cathode of the cell, can be in the range of 0°C to 1000°C, in the range of 0°C to 900°C, 0°C to 800°C, 0°C to 700°C, 0°C to 600°C, 0°C to 500°C, 0°C to 400°C, 0°C to 300°C, 0°C to 200°C, 0°C to 100°C or 0°C to 50°C. In some embodiments, the temperature is in the range of 0°C to 50°C, 5°C to 45°C, 10°C to 40°C, 15°C to 35°C, or 20°C to 30°C.

[0092] An advantage of the present invention is the specificity and efficiency of product formation over side-product (such as H 2 ) formation, which is a challenge due to the competing binding energies of hydrogen and oxygen on the catalyst surface. In certain embodiments, less than about 50% moles H 2 are formed compared to moles of the desired product formed, and preferably less than about 40% moles H 2 , less than about 30% moles H 2 , less than about 20% moles H 2 , less than about 10% moles H 2 , less than about 5% moles H 2 , less than about 2% moles H 2 , or less than about 1 % moles H 2 .

[0093] Further, reaction conditions (choice of catalyst, applied potential in particular) can be set so that one reaction product is selectively formed over other reaction products. This means that the reaction predominantly or even completely leads to the formation of one particular reaction product (e.g., methane, methanol, formic acid, methanediol).

[0094] The reaction pathway of CO and CO 2 reduction depends on the relative energies of reaction intermediates. Thus, the pathway can depend on the system within which the reaction takes place, including the chemical composition of the catalysis and the catalytic surface being used in the reaction.

[0095] The active part of an industrial heterogeneous catalyst is most commonly a solid surface, e.g. a transition metal or transition metal phosphide or mixture of transition metal phosphides. The surface offers a favourable energy path from reactants to products, by binding reactants and reaction intermediates. The binding energy of reactants to the surface must be strong enough to produce reaction intermediates, but weak enough to allow products to leave the surface, allowing more reactions to take place on the surface. The consequence is that for a reaction, there will be an optimum binding energy for an intermediate, such that both stronger and weaker binding leads to lower activity. A result of this phenomenon is a reaction relationship called the volcano-shaped relationship, or commonly referred to as a Volcano plot.

[0096] Catalyst activity can in general be modified by altering the local electronic structure by strain, ligand, substitution and / or alloying. These alterations can result in changes in binding energies of reaction intermediates, and thereby a change in the thermodynamics of the overall reaction profile. Thereby it may be possible to adapt the reaction profile to specifically obtain desired reaction products.

[0097] Exemplary embodiments in accordance with the present disclosure include the following: 1. A method for catalytic electrolytic reduction of CO 2 and / or CO, the method comprising: a. providing an electrolytic cell containing at least one reaction chamber that has at least one anode and at least one cathode and at least one electrolyte between the anode and the cathode, wherein the at least one cathode comprises at least one catalyst comprising at least one transition metal phosphide selected from phosphides of Cr, V, Ti, Hf, Ta, Zr, Nb, Ta; b. providing CO 2 and / or CO in the electrolytic cell; and c. applying electrical potential to the electrolytic cell; whereby the CO 2 and / or CO undergoes at least one reduction reaction at the cathode. 2. The method of clause 1, wherein the catalyst is provided as a surface coating, a film, a microparticle, nanotubes, nanosheets, heterostructure, dopant and / or a nanoparticle. 3. The method of any one of the previous clauses, wherein the temperature at the at least one cathode is in the range of 0°C to 300°C, 0°C to 200°C, 0°C to 100°C, 0°C to 50°C 5°C to 45°C, 10°C to 40°C, 15°C to 35°C, or 20°C to 30°C. 4. The method of any one of the previous clauses, wherein the electrolytic cell comprises gas diffusion electrodes at the anode and / or the cathode. 5. The method of any one of the previous clauses, wherein the electrolytic cell further comprises at least one reference electrode. 6. The method of any one of the previous clauses, wherein the electrolytic reduction reaction results in formation of at least one alkane, lower alcohol or acid having from 1 to 3 carbon atoms. 7. The method of any one of the previous clauses, wherein the electrolytic reduction reaction results in formation of at least one product selected from methanol, methane, methanediol, formic acid, formaldehyde, ethanol, ethane, ethylene glycol, ethanediol, propane and propanol. 8. The method of any one of the previous clauses, wherein CO 2 and / or CO are provided in gaseous form into the electrolytic cell, and wherein the gas pressure at the at least one cathode is in the range of about 1 atm to 40 atm, 5 atm to 30 atm, or 10 atm to 20 atm. 9. The method of any one of the previous clauses 1 to 8, wherein the gas pressure at the at least one cathode is ambient pressure of about 1 atm. 10. The method of any one of the previous clauses, wherein the electrical potential applied to the electrolytic cell is in the range of -2.0 V to +1.0 V using a reversible hydrogen electrode (RHE) as a reference. 11. The method of any one of the previous clauses, wherein molar production of hydrogen (H 2 ) compared with CO 2 and / or CO reduction in the electrolytic cell is less than 50%. 12. The method of any one of the previous clauses, wherein the electrolyte solution comprises at least one source of CO 2 selected from carbonate (CO 3 2-< ), carbonate salts, bicarbonate (HCO 3 -< ) and carbonic acid (H 2 CO 3 ). 13. The method of any one of the previous clauses, wherein CO and / or CO 2 is provided in the solution by a stream of gas. 14. The method of any one of the previous clauses, wherein CO 2 is provided in the electrolytic cell, and wherein the transition metal phosphide is a phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta and Zr, more preferably a phosphide of a transition metal selected from Cr and V. 15. The method of any one of the previous clauses, wherein CO is provided in the electrolytic cell, and wherein the transition metal phosphide is a phosphide of a transition metal selected from Nb, V, Hf, Ta, Ti, and Zr, more preferably a phosphide of a transition metal selected from Nb and V. 16. The method of any one of the previous clauses, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of V, Nb, Hf, Ta, Ti, and Zr the catalytic reduction resulting in formation of methane. 17. The method of any one of the previous clauses 1-13, wherein CO 2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of Cr, V and Ti, more preferably Cr and V, the catalytic reduction resulting in the formation of formic acid. 18. The method of any one of the previous clauses 1-13, wherein CO 2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from Ti, Hf, Ta and Zr, the catalytic reduction resulting in formation of methanol. 19. The method of any one of the previous clauses 1-13, wherein CO 2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from Ti, Hf, Ta and Zr, the catalytic reduction resulting in formation of methanediol. 20. The method of any one of the previous clauses 1-13, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Nb, Ta, Hf, Ti and Zr, the catalytic reaction resulting in formation of methanol. 21. The method of the previous clause, wherein the phosphide is a phosphide of a transition metal selected from Nb and V. 22. The method of any one of the previous clauses 1-13, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Nb, Ta, Hf, and Ti, the catalytic reaction resulting in formation of methanediol. 23. The method of any one of the previous clauses 1-13, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Hf, Nb, Ta, Ti and Zr, preferably selected from V and Nb, the catalytic reaction resulting in formation of methane. 24. The method of any one of previous clauses 1-13, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Nb, Ta and Ti, the catalytic reaction resulting in formation of CH2O(g), the catalytic reaction resulting in formation of formaldehyde. 25. An electrolytic cell for the reduction of carbon dioxide and / or carbon monoxide, comprising: an anode; and a cathode, the cathode comprising a catalyst comprising at least one transition metal phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta, Zr, Nb, Ta. 26. The electrolytic cell of the previous clause, wherein the catalyst is provided as a surface coating, a film, a microparticle, nanotubes, nanosheets, nanoarrays, nanoribbons, heterostructure, dopant, a nanoparticle, zero- dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three- dimensional (3D), a single atom catalyst, a dual-atom catalyst and a triple-atom catalyst. 27. The electrolytic cell of any one of the previous two clauses, wherein the catalyst is provided as a pure phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta, Zr, Nb, Ta. 28. The electrolytic cell of any one of the previous four clauses, wherein the electrolytic cell comprises gas diffusion electrodes at the anode and / or the cathode. 29. The electrolytic cell of any one of the previous five clauses, wherein the electrolytic cell further comprises at least one reference electrode. 30. A chemical reactor comprising at least one electrolytic cell as set forth in any one of clauses 25-29; and a power supply connected to the electrolytic cell.

[0098] The invention will now be exemplified by the following non-limiting Examples.Example 1 CO 2 RR on transition metal phosphide surfaces Computational method

[0099] The interaction of CO 2 with TMP surfaces and formation of different intermediates were studied by DFT calculations with Generalized Gradient Approximation (GGA) using RPBE exchange correlation function. The tool used for these calculations was ab initio simulation package (VASP) and cutoff energy was 450 eV with the 4×4×1 Monkhorst-Pack K-point mesh until energy differences are converged within 10 -4< eV. The Projector augmented wave (PAW) method was implemented due to full wavefunction by utilizing computationally efficient pseudopotentials, significantly reducing computation time. The surface of TMPs were considered in the rocksalt (RS) crystallographic structure with the texture orientation of (100). A slab was prepared from the bulk phosphides of 29 different materials. The composition of the slab follows a 1:1 ratio, meaning it consists of 40 atoms in total: 20 atoms of a transition metal, 10 phosphorus atoms. The boundary conditions were periodic with 20 Å vacuum along the z direction to avoid self-interaction of neighboring layers. The bottom two layers were fixed, and the top 3 layers were allowed to fully relax together with any adsorbates on the surface.

[0100] The binding energy of an adsorbate was calculated according to the equation 1: ΔE ads = E adsorbate / TMP slab − E TMP slab − E adsorbate

[0101] Here E (adsorbate / TMP slab) is the total energy of the system with an adsorbate on the TMP slab, E (TMP slab) is the total energy of the pristine TMP slab, and E(adsorbate) is the total energy of the adsorbate. The negative ΔE ads corresponds to the exothermic adsorption phenomenon.

[0102] The Computational hydrogen electrode model as proposed by Norskov et al was used to calculate the free energy diagram for CO 2 RR and / or CORR along different paths at certain applied potential. According to this method at room temperature kinetic barrier is negligible for the proton coupled electron step and the potential limiting step is the most positive energy difference between the two adjacent steps.

[0103] The reaction free energy of an adsorbate for CO 2 RR at any arbitrary potential U vs RHE and any arbitrary pH was calculated by equation: Δ G U RHE = Δ G U RHE = 0 + neU RHE eU RHE = eU RHE + 2.3 k b TpH

[0104] where U RHE is the applied potential referred against the standard hydrogen electrode (SHE), n is number of electrons, e is the elementary charge, k b is the Boltzmann constant and T is the Temperature. By putting eq (2) into eq(3) we will get : Δ G U RHE = Δ G U RHE = 0 + n eU RHE = eU RHE + 2.3 k b TpH

[0105] Generally overpotential are independent of pH of electrolyte so in this study we will consider pH = 0. Hence for each elementary step ΔG (U = 0) is calculated by Δ G U = 0 = ΔE DFT + ΔE ZPE − T Δ S + ΔE 0K → T + ΔE sol

[0106] Where ΔG is the Gibbs free energy, ΔE DFT is the electronic energy calculated by DFT, ΔE ZPE is the zero-point energy correction and TΔS is entropy differences between the gas phase and adsorbed species calculated by vibrational frequencies for the adsorbed species, ΔE Sol is the adsorbate stabilization term due to the solvent and we have not included in this study. ΔE 0K→T is the change in internal energy because of temperature. The value of the gas phase is taken from the thermodynamic tables from textbooks.Results and discussion

[0107] The analysis was based on using the DFT method with the exchange-correlation function of (RPBE). In the analysis, a 4×4×1 Monkhorst-Pack K-point mesh and a cutoff energy of 400 eV was used. All structures of TMPs are rock salt with texture orientation of (100). The atomic structure with different reactants and products was minimized until their atomic forces dropped less than 0.03 eV / A. The TMPs-based structure consists of 5 layers and 40 atoms. In all analyses, the bottom two layers were fixed, and the top 3 layers were allowed to relax and take part in interaction with different species as shown in Figure 1. This figure shows a side and top view of TMP surfaces where the larger spheres indicate the metal atoms and smaller spheres indicate phosphorous atoms. The view in (a) represents the side view of TMP and dotted lines indicate that the bottom two layers were fixed, and the top three layers were allowed to fully relax for optimization. The view in (b) represents a top view of TMPs and dotted lines indicate the unit cell which is repeated along the x and y axis. Boundary conditions along the x and y directions were considered periodic and along the z axis, a vacuum of 20 Å was generated to avoid self-intersection of layers.

[0108] To study the reaction pathways for the reduction of CO 2 into valuable products on TMPs we have used the thermochemical model (TCM) and there are several studies carried out using this method. The phenomena of applied potential were included by using the computational hydrogen electrode (CHE) but the other parts of the electrochemical environment i.e. pH dependency and solvent effects were not considered.

[0109] A total of 29 TMPs in the (100) facets crystallography were optimized and studied for their suitability for CO 2 RR. Out of these 29 TMPs, 6 TMPs (CrP, HfP, TaP, TiP, VP and ZrP) were found to be stable and suitable for CO 2 RR study as shown in Fig 2, and after the optimization the other TMPs were not considered further due to their instability and unsuitability for CO 2 RR. We explored the binding energies and reaction pathways of the TMPs CrP, HfP, TaP, TiP, VP and ZrP further, with a focus on their suitability for catalyzing CO 2 reduction.

[0110] In the first step, various species were adsorbed on clean surfaces of fully relaxed structures of TMPs (CrP, HfP, TaP, TiP, VP, and ZrP) to test favorable side, poisoning and to start the favorable reaction pathway. As can be seen in Fig 3, in some of the cases poisoning of O and OH is exergonic but shows less poisoning behavior. The OH, showing poisoning in the case of HfP, TaP, TiP, ZrP and little for CrP and VP but this can be turned well to be desorbed in form of water and free the surface for other intermediates. In the case of CrP, OH binds less strong than OCHO and thus of no concern. The TaP required -0.61V for OH to desorb in form of water and this value is less than the applied potential for the CO 2 RR (formation of formic acid, methanediol, and methnol) which is -0.8V and hence of no concern. Similarly for TiP, the potential required for OH to turn into water is -0.32V which is less than the value of applied potential which is -0.42 V for the formation of HCOOH, and of no concern. In all the cases water is difficult to adsorb, showing poor poisoning behavior. In our analysis, we have investigated adsorption of each species on the metal, phosphorus, and bridge sites of the TMPs and our study explored that the metal site is the most active and favorable reaction site. The most interesting of our findings conclude that OCHO is showing exergonic behavior at moderate Gibbs free energy as shown in Fig 3 which is favorable towards formic acid, methanethiol, and methanol formation.

[0111] Another important aspect to investigate is the activity between the hydrogen evolution reaction (HER) or CO 2 RR. The first step towards HER is the proton adsorption on the metal site and this is shown in Fig 4, but this adsorption is weaker than the adsorption of OCHO. Thus, OCHO is more dominant and exergonic over the adsorption of H on the clean surface of catalysts. The adsorption of COOH is also weaker than the adsorption of OCHO hence our catalytic activity for CO 2 RR starts from the spontaneous adsorption of OCHO except in the case of VP as shown in Fig 3.

[0112] In this study adsorption of OCHO leads to electrochemical activity towards the formation of formic acid, methanediol, and methane. In the first hydrogenation step, the adsorbed OCHO converts to formic acid in either adsorbed or aqueous form. The reaction followed for the formation of formic acid is as:         *OCHO + H +< + e → *HCOOH or HCOOH (aq)      eq (6)

[0113] It was found that CrP and VP do not bind *HCOOH intermediate on their surface and these catalysts have the potential to produce an aqueous form of formic acid with - 0.25 V and -0.29 V, respectively, as shown in Fig 5. This is an interesting finding on activity and selectivity of CrP and VP to only produce formic acid, while in previous studies catalysts such as Cu showed to produce more than 15 products, and it requires large overpotential and required large amount of energy to separate those products.

[0114] In this study, TiP is the most promising candidate for the formation of formic acid, methanediol, and methanol. The reaction pathway for the formation of formic acid on TiP catalyst is similar as for CrP and VP and the onset potential value is -0.43 V with two electron-proton transfer steps. The methanediol formation has four-step electron transfer in the mechanism of CO 2 RR with onset potential of -0.62 V. The reaction followed by methanediol formation is as: *OCHO + H +< + e -< → *OCH 2 O eq (7) *OCH 2 O + H +< + e -< → *H 2 COOH eq (8) *H 2 COOH + H +< + e -< → *H 2 C(OH) 2 eq (9)

[0115] The binding free energy for *OCHO is -0.1 eV which indicates that the reaction is exergonic, and no additional energy to start the reaction is needed. The binding free energy required for hydrogen adsorption on the metal and phosphorous site were calculated as 0.76 eV and 0.74 eV as shown in Fig 3 which is less than *OCHO binding free energy. The *COOH binding free energy is 1.5 eV. Hence binding of *OCHO is dominant over hydrogen and *COOH so the reaction pathway will start from *OCHO. In the next protonation step, the OCH2O intermediate is formed, and the binding free energy is 0.45 eV, and in the 3rd, protonation step leads to *H2COOH intermediate formation with the binding free energy of 1.06 eV. In the fourth and last protonation step, an aqueous form of methanediol is formed and in the complete reaction for methanediol formation, the highest PDS step calculated is 0.62 eV which is between the OCH2O and H2COOH formation.

[0116] TiP is also promising for the formation of methanol with 6 protonation steps and an onset potential value of -0.62V. The reaction pathways for methanol formation are:         *OCHO + H +< + e -< → *OCH 2 O     eq (10)         *OCH 2 O + H +< + e -< → *H 2 COOH     eq (11)         * H 2 COOH + H +< + e -< → *CH 2 O+ H 2 O     eq (12)         *CH 2 O+ H +< + e -< → *CH 3 O     eq (13)         *CH 3 O+ H +< + e -< → *CH 3 OH (aq)      eq (14)

[0117] After the formation of the H2COOH intermediate, there is also a possibility for this intermediate to split into a CH2O intermediate, and a water molecule as shown in Fig 6 with the CH2O binding free energy of 0.58 eV. In further protonation, CH2O leads towards the formation of CH3O with the binding free energy of -1.0 eV, and in the final 6th protonation step methanol is formed in the aqueous form. The overall highest PDS step is the same as for the methanediol which is between the OCH2O and H2COOH with a value of 0.62 eV.

[0118] ZrP, TaP, and HfP are also good candidates for the formation of formic acid, methanediol, and methanol as shown in Fig 8. As seen, *COOH is difficult to adsorb on HfP, TaP, and ZrP catalysts and the required binding free energies are 0.99 eV, 1.09 eV, and 1.27 eV respectively. Similarly, TiP *OCHO also spontaneously adsorb on HfP, TaP, and ZrP without any additional energy and the binding free energies are -0.65 eV, -0.42 eV, and -0.33 eV respectively which are more favorable than hydrogen and COOH binding. The reaction pathway followed by this catalyst towards the formic acid formation is the same as that followed by CrP, VP, and TiP which is *OCHO → *HCOOH or HCOOH (aq). The analysis revealed that in the second protonation step, an aqueous form of formic acid is formed on HfP, TaP, and ZrP catalysts and the onset potentials required are -1.09, -0.43 and -0.79 V respectively as shown in Fig 8. The adsorbed form of formic acid is difficult in our analysis for the CO 2 RR on HfP, Tap, and ZrP due to high binding energies and we have not considered it in the free energy diagram due to high energy steps.

[0119] In Fig. 7, a summary of the onset potentials required for formation of formic acid, methanediol and methanol for CrP, HfP, TaP, TiP, VP and ZrP is shown.

[0120] The findings reveal that HfP, TaP, and ZrP also show activity towards the formation of methanediol and methanol. In the second protonation step, the proton absorbs more favourably on the metal site as compared to OCH2O and it is difficult to form OCH2O, and in the third step, OCH2O is formed with the additional step of adsorption of the proton on the metal (OCH2O + MH). The binding free energies required for OCH2O on HfP, TaP, and ZrP catalysts with additional protons on the metal side are -0.21, - 0.07, and 0.41 eV respectively. The reaction pathways towards the methanediol and methane are slightly different from the TiP with the additional adsorption of proton on the metal due to its minimum energy. The reaction pathways are as follows:         *OCHO + H +< + e -< → *OCHO +MH     eq (15)         *OCHO +MH + H +< + e -< → *OCH 2 O+MH     eq (16)         *OCH 2 O+MH + H +< + e -< → * H 2 COOH + MH     eq (17)         * H 2 COOH + H +< + e -< → * H 2 C(OH) 2      eq (18)

[0121] After the OCH2O formation, in the next protonation step H2COOH intermediate formed on HfP, TaP, and ZrP catalyst, with binding free energies of 0.73 eV, 0.87, and 0.98 V respectively. In the case of ZrP, H2COOH formation is exergonic and for HfP and TaP, it shows endergonic behaviour. The onset potential required for the formation of methanediol on HfP, TaP, and ZrP catalysts are -1.01, -0.86, and -0.83 V, respectively. The highest PDS steps for TaP and ZrP catalysts are between OCHO*+MH* and OCH2O + MH* and for HfP the highest PDS step is H2COOH *+MH* → H2C(OH)2 +MH.

[0122] After H2COOH formation this intermediate can also split into CH2O, and a water molecule as shown in Fig. 8 and reaction goes toward methanol formation. The reaction pathway toward the methanol formation was:         *OCHO + H +< + e -< → *OCHO +MH     eq (19)         *OCHO +MH + H +< + e -< → *OCH 2 O+MH     eq (20)         *OCH 2 O+MH + H +< + e -< → * H 2 COOH + MH     eq (21)         * H 2 COOH + MH + H +< + e -< → *CH 2 O+ MH + H 2 O     eq (22)         *CH 2 O+ MH + H +< + e -< → *CH 3 O + MH     eq (23)         *CH 3 O+ MH + H +< + e -< → *CH 3 OH+ MH     eq (24)

[0123] The highest PDS step for HfP, TaP, and ZrP for the methanol formation and methanediol is OCHO*+MH* → OCH2O*, and onset potential values are -1.01, -0.86 , and -0.83V respectively.

[0124] For further investigations of the results scaling relation has been drawn to study the trend of formation of formic acid on the studied TMP-based catalyst. To map the multidimensional problem into one variable the adsorption energies of OH were considered as descriptors for other adsorbate-binding free energies. In previous studies, OH was considered a good descriptor, and Bhowmik et al. has used the same for the study of CO 2 RR study on rutile oxide surfaces. Using the scaling relation we have drawn an activity volcano for the formation of formic acid. Our results conclude that CrP and VP are the best candidates for the formation of formic acid as shown on the top of the volcano plot Fig. 9. Each line of the volcano indicates the reaction step. The left leg of the volcano indicates the reaction of OCHO → HCOOH(aq) and the left leg of the volcano OH → H2O.

[0125] The investigation of charge analysis was conducted by using the Bader charge analysis and we have investigated the charge analysis of the intermediate OCHO of each candidate on the most active metal sites. Table 1. Calculated Gibbs energies (ΔG) and Bader charge (Q) of OCHO on the surface of each catalyst. (still calculating for H) CatalystCrPHfPTaPTiPVPZrPΔG-1.04-0.65-0.420.010.2-0.33Q-0.31-0.71-0.66-0.67-0.65-0.71

[0126] The charge analysis results show negative values in all the cases, which means OCHO transfers the charge to the surface of the catalyst, and negative Gibbs free energy energies indicate favourable adsorption of OCHO on the surface of the catalyst.Conclusion

[0127] In our investigations utilising advanced quantum mechanical simulations and computational models, 29 TMPs in the (100) facets of the rocksalt structure were optimized and studied for their suitability for CO 2 RR. Out of 29, only 7 TMPs (CrP, NbP, HfP, TaP, TiP, VP, and ZrP,) were screened out suitable for the CO 2 RR study, and the remaining were not considered due to their instability and unsuitable factor for CO 2 RR after the optimizations. We explored the binding energies, and reaction pathways of the TMPs CrP, HfP, TaP, TiP, VP, and ZrP, with a focus on their suitability for catalysing CO2 reduction. In this research, we have conducted a complete and comprehensive analysis of the above-mentioned TMPs. The CrP and ZrP are found the best candidates to produce formic acid at -0.25 and -0.29 V. The most interesting and novel findings in this research work are that CrP and VP only produce formic acid which is promising and better than commercially used Cu and other materials. In addition, TaP, TiP, and ZrP are also explored as good candidates to produce formic acid with small onset potentials of -0.87, -0.43, and -0.79 V, respectively. The selectivity towards methanol formation in this analysis reveals TiP is the best candidate with -0.62 V onset potential. The studied HfP, TaP, and ZrP are also good candidates for methanol formation with corresponding onset potentials of - 0.76, -0.86, and -0.83 V, respectively. Methanediol is also formed while studying the CO 2 RR mechanism on TMP surfaces and results concluded that TiP is the best candidate for the formation of methanediol with the onset potential value of -0.62 V. These results demonstrated that these TMPs as electrocatalysts are more promising than previously studied and commercially used materials (metals and oxides) for the application of CO 2 RR when comparing the onset potential needed for the reactions to produce different chemicals.Example 2 CORR on transition metal phosphide surfaces Evaluating CO Adsorption Versus Other Species

[0128] The fundamental purpose of this work is to identify the catalyst that can be used to produce a variety of carbon-based products with low onset potential from CO capture and reduction. To do this, we looked at the following twelve metal phosphides: VP, WP, YP, ZrP, CrP, HfP, NbP, MoP, CoP, TaP, TiP, and ScP. Nevertheless, after the adsorption of CO, the CoP and MoP were found to be unstable, and WP could not successfully capture the CO. Further, ScP and YP also showed endergonic processes in forming additional intermediates. Consequently, the remaining seven materials that exergonically captured CO were examined further. Moreover, the next obstacle is the identification of surface poisoning. For example, the active site is exclusively accessible to CO, or it can be occupied by other species. To comprehend this phenomenon, tests with results as shown in Fig 10 were performed. After obtaining extensive information on the most active sites, the adsorption of O, OH, H 2 O, and H was examined, intending to compare their adsorption with that of CO by evaluating their respective free energies. The investigation indicated that the presence of O, OH, H 2 O, or H should not contaminate any surface, since CO adsorption consistently occurred more favorably as shown below.

[0129] The next step is to investigate the competition between CORR and HER, as both reactions have the potential to occur concurrently at the cathode of the electrochemical cell. The reason for this is that the cathode serves as the focal point for the process of reduction, where CO is converted into different substances through the exchange of electrons, and protons undergo transformation to generate hydrogen gas. Thus, a catalyst's performance is determined by its intended purpose, either CORR or HER. Since the emphasis of this research is on CORR, we conducted tests on these materials to establish their effectiveness for CORR or HER. When compared to the potency of CO adsorption, hydrogen adsorption is found to be less efficient, either *MH (proton adsorption on metal site) or *PH (proton adsorption on phosphorus site), as shown in Fig 10. Based on the adsorption energies, it can be deduced that hydrogen does not bind the surface exergonically and it is less favorable than CO adsorption on the surface. For clarity, the relevant free energy information is shown in Fig 11.

[0130] Therefore, it is evident from Fig 10 and Fig 11 that no surface is being poisoned by hydroxyl, water, hydrogen, or oxygen, and that all the active sites are completely accessible for CO capture. Furthermore, there is no potential of HER occurring because this reaction requires more energy than CORR, which is not viable from a thermodynamic standpoint. Consequently, both assessments demonstrate the promise of the catalysts that we have selected for CORR, and this enables them to go on to the subsequent stage of reaction (hydrogenation) to produce different products.CH 3 OH (aq) , CH 2 O (g) and CH 4(g) Formation

[0131] Methanol is one of the most important products during CORR because it is used as a fuel and in the production of various chemicals. During its formation, there are two possible reaction pathways: one starting from CHO and the other from COH, as shown in Fig 12. Since these two intermediates play a fundamental role in advancing the reaction, it is crucial to calculate the potential from CO* to CHO* / COH*. Therefore, we have calculated the free energies for all the discussed catalysts.

[0132] To screen out the most effective catalyst, we selected only those materials for which the PDS from CO to CHO / COH was less than 1 eV. However, in our case, no catalyst met this criterion. For example, the ΔGs from CO* to CHO* were 1.07, 1.02, 1.11, 1.05, 1.11, and 1.12 eV, and the ΔGs from CO* to COH* were 2.57, 1.26, 2.43, 2.24, 2.14, and 2.66 eV for HfP, NbP, TiP, TaP, VP, and ZrP, respectively, as shown in Fig 13. This indicates that these steps require too high a potential to be considered viable. As an alternative, other available sites were considered, such as metal and phosphorus sites, to proceed with the reaction. The potentials for CO* to CO*+PH* were 1.14, 0.46, 0.75, 0.67, 0.54, and 0.96 eV, and for CO* to CO*+MH* were 0.51, 0.50, 0.72, 0.03, 0.63, and 0.76 eV for HfP, NbP, TiP, TaP, VP, and ZrP, respectively (as also shown in Fig 13).

[0133] After this analysis, we selected only the most exergonic pathways to proceed with the reactions for CHO and COH. In some cases, when the reaction stalled due to the high potential for CHO / COH after the CO*+PH* or CO*+MH* steps, we screened other sites and continued this process until we found the most reliable reaction pathway. For instance, the formation of CHO and COH started after the CO*+MH*+MH* steps for HfP and ZrP, while for TiP and VP, it began after CO*+MH*+PH* and CO*+PH*+MH*, respectively, leading to the exergonic creation of CHO / COH. However, TaP required the absorption of more protons (CO*+MH*+MH*+MH*) to initiate the reaction from CHO / COH.

[0134] Fig 13 displays the free energy diagrams (FED) illustrating the production of CH 3 OH (aq), CH 2 O (g) and CH 4(g) for various catalyst surfaces. Based on Fig 13(a), it is evident that the PDS for HfP corresponds to the second protonation step involving CO*+MH*+MH*, resulting in the production of CH 30 H (aq) and CH 4(g) with a ΔG PDS of 0.82 eV. In the context of TiP (b), the PDS (0.90 eV) represents the second stage of protonation in the formation of CO*+MH*+PH*. Similarly, in the case of TaP (c), it was found that the identical ΔG value (0.90 eV) was detected after the fourth protonation, which resulted in the manufacturing of CHO*. These steps led to the subsequent generation of CH 30 H (aq) , CH 2 O (g) and CH 4(g) on TiP and TaP. When considering ZrP (d), the most futile results were found with the second protonation, which led to the production of CO*+MH*+MH* with ΔG PDS for CH 3 OH (aq) and CH 4(g) generation. However, this catalyst exhibited peculiar behavior compared to others, as it followed the R2 and R7 reaction pathways. The catalysts mentioned in the study mainly use the R1 and R4 pathways for CHO* formation. It is worth noting that none of the catalysts adhere to the R3 pathway because of the significant potential needed for COH formation. Also NbP was found to be a particularly promising catalyst for CH 3 OH and CH 4 formation, as shown in Fig. 13 (e).

[0135] To get information about the onset potential, which is crucial for acquiring these three products, onset potential was determined as shown in Fig. 14. From Fig 14, it is evident that most catalysts have similar onset potentials for the formation of all products. However, only four catalyst surfaces are capable of producing CH 2 O(g), formaldehyde, while all the catalysts excel in terms of overall product output. Regarding the activity of catalytic substrates, it is obvious from Fig. 14 that NbP and VP catalysts exhibit higher activity compared to other catalysts in the formation of CH 3 OH (aq), CH 2 O (g) and CH 4(g) at incredibly low onset potentials of -0.58 V and -0.69 V, respectively.

[0136] From Fig. 15 we can see that the ΔG for VP was 0.69 eV after the second hydrogenation that was linked to the CO*+PH*+MH, during the exploration of VP. For NbP, the second protonation was to form the CHO* with PDS of 0.58 eV. Anyhow, these PDSs were further required to produce the CH 30 H (aq) , CH 2 O (g) and CH 4(g) .

[0137] In summary, it can be concluded that several TMPs shows promising results for CORR although NbP and VP are the most promising choices for because of their great activity in producing valuable products with low onset potential. Further, they are not suitable for competing reactions with CORR, such as HER. The surfaces are not being contaminated by other species.

Examples

example 1

Example 1

CO 2 RR on transition metal phosphide surfaces

Computational method

[0099]The interaction of CO 2 with TMP surfaces and formation of different intermediates were studied by DFT calculations with Generalized Gradient Approximation (GGA) using RPBE exchange correlation function. The tool used for these calculations was ab initio simulation package (VASP) and cutoff energy was 450 eV with the 4×4×1 Monkhorst-Pack K-point mesh until energy differences are converged within 10 -4< eV. The Projector augmented wave (PAW) method was implemented due to full wavefunction by utilizing computationally efficient pseudopotentials, significantly reducing computation time. The surface of TMPs were considered in the rocksalt (RS) crystallographic structure with the texture orientation of (100). A slab was prepared from the bulk phosphides of 29 different materials. The composition of the slab follows a 1:1 ratio, meaning it consists of 40 atoms in total: 20 atoms of a transition metal, 10 p...

example 2

Example 2

CORR on transition metal phosphide surfaces

Evaluating CO Adsorption Versus Other Species

[0128]The fundamental purpose of this work is to identify the catalyst that can be used to produce a variety of carbon-based products with low onset potential from CO capture and reduction. To do this, we looked at the following twelve metal phosphides: VP, WP, YP, ZrP, CrP, HfP, NbP, MoP, CoP, TaP, TiP, and ScP. Nevertheless, after the adsorption of CO, the CoP and MoP were found to be unstable, and WP could not successfully capture the CO. Further, ScP and YP also showed endergonic processes in forming additional intermediates. Consequently, the remaining seven materials that exergonically captured CO were examined further. Moreover, the next obstacle is the identification of surface poisoning. For example, the active site is exclusively accessible to CO, or it can be occupied by other species. To comprehend this phenomenon, tests with results as shown in Fig 10 were performed. After...

Claims

1. A method for catalytic electrolytic reduction of CO2 and / or CO, the method comprising: a. providing an electrolytic cell containing at least one reaction chamber that has at least one anode and at least one cathode and at least one electrolyte between the anode and the cathode, wherein the at least one cathode comprises at least one catalyst comprising at least one transition metal phosphide selected from phosphides of Cr, V, Ti, Hf, Ta, Zr, Nb, Ta; b. providing CO2 and / or CO in the electrolytic cell; and c. applying electrical potential to the electrolytic cell; whereby the CO2 and / or CO undergoes at least one reduction reaction at the cathode.

2. The method of claim 1, wherein the electrolytic reduction reaction results in formation of at least one product selected from methanol, methane, methanediol, formic acid, formaldehyde, ethanol, ethane, ethylene glycol, ethanediol, propane and propanol.

3. The method of any one of the previous claims , wherein the gas pressure at the at least one cathode is ambient pressure of about 1 atm.

4. The method of any one of the previous claims, wherein the electrical potential applied to the electrolytic cell is in the range of -2.0 V to +1.0 V using a reversible hydrogen electrode (RHE) as a reference.

5. The method of any one of the previous claims, wherein CO2 is provided in the electrolytic cell, and wherein the transition metal phosphide is a phosphide of a transition metal selected from Cr, V, Ti, Hf, Ta and Zr, more preferably a phosphide of a transition metal selected from Cr and V.

6. The method of any one of the previous claims 1 - 4, wherein CO is provided in the electrolytic cell, and wherein the transition metal phosphide is a phosphide of a transition metal selected from Nb, V, Hf, Ta, Ti, and Zr, more preferably a phosphide of a transition metal selected from Nb and V.

7. The method of any one of the previous claims 1 - 5, wherein CO2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from Ti, Hf, Ta and Zr, the catalytic reduction resulting in formation of methanediol.

8. The method of any one of the previous claims 1 - 5, wherein CO2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of Cr, V and Ti, more preferably selected from Cr and V, the catalytic reduction resulting in the formation of formic acid.

9. The method of any one of the previous claims 1 - 5, wherein CO2 is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from Ti, Hf, Ta and Zr, the catalytic reduction resulting in formation of methanol.

10. The method of any one of claims 1 - 4 or claim 6, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Hf, Nb, Ta, Ti and Zr, preferably a transition metal selected from V and Nb, the catalytic reaction resulting in formation of methane.

11. The method of any one of claims 1 - 4 or claim 6, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Hf, Nb, Ta, Ti and Zr, the catalytic reaction resulting in formation of methanol.

12. The method of any one of claims 1 - 4 or claim 6, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Nb, Ta and Ti, the catalytic reaction resulting in formation of methanediol.

13. The method of any one of claims 1 - 4 or claim 6, wherein CO is provided in the electrolytic cell, and wherein the phosphide is a phosphide of a transition metal selected from the group consisting of V, Nb, Ta and Ti, the catalytic reaction resulting in formation of CH2O(g), the catalytic reaction resulting in formation of formaldehyde.

14. An electrolytic cell for the reduction of carbon dioxide and / or carbon monoxide, comprising: an anode; and a cathode comprising a catalyst comprising at least one phosphide of at least one transition metal selected from Cr, V, Ti, Hf, Ta, Zr, Nb, Ta; wherein the catalyst is provided as a surface coating, a film, a microparticle, nanotubes, nanosheets, nanoarrays, nanoribbons, heterostructure, dopant, a nanoparticle, zero- dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three- dimensional (3D), a single atom catalyst, a dual-atom catalyst or as a triple-atom catalyst.

15. A chemical reactor comprising at least one electrolytic cell as set forth in claim 14; and a power supply connected to the electrolytic cell.

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