Catalyst for the synthesis of methanol from methane and carbon dioxide

Abstract

There is provided a photocatalyst for the direct synthesis of methanol from methane and carbon dioxide. The photocatalyst comprises an AuPd alloy nanoparticles dispersed on a support comprising GaN, InN, AlN, BN, TlN, GaP, InP or a combination thereof. The AuPd alloy nanoparticles are uniformly dispersed on the support such that at least 90% of the AuPd alloy nanoparticles are positioned at a distance of from 1 nm to 15 nm from other AuPd alloy nanoparticles. There is also provided a method of synthesizing methanol from methane and carbon dioxide with a photocatalyst comprising a transition metal dispersed on a support comprising GaN, InN, AlN, BN, TlN, GaP, InP or a combination thereof.

Description

CATALYST FOR THE SYNTHESIS OF METHANOL FROM METHANE AND CARBON DIOXIDECROSS REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims priority from U.S. provisional application number 63 / 593,598 filed on October 27, 2023, and from U.S. provisional application 63 / 653,459 filed on May 30, 2024, both of which are incorporated herein by reference in their entirety.TECHNICAL FIELD

[0002] This disclosure relates to the field of catalysts of methane and carbon dioxide, particularly for the production of methanol.BACKGROUND OF THE ART

[0003] CO2 and CH4 accounted for approximately 91 % of total global gas emissions in 2021 and are therefore the primary greenhouse gases driving the global warming effect. Efforts have been taken to transform CO2 and CH4 into other useful compounds including alkanes and alcohols such as methanol. Recent research into methanol has improved its general utility as an energy source and methanol is now widely used as a biofuel in many different industries. The conversion of CO2 and CH4 into methanol has traditionally relied on first producing a syngas intermediate and then producing methanol (or other carbon products) from the syngas in a subsequent step (e.g. Fischer-Tropsch reactions). The initial formation of syngas from methane requires a high temperature and is therefore both costly and energy intensive. Moreover, when syngas is produced, it is produced in crude form and requires post-processing and cleaning steps before undergoing Fischer-Tropsch reactions. It would therefore be desirable to form methanol from CO2 and CH4 in a more cost and energy efficient way directly without going through intermediate species such as syngas.SUMMARY

[0004] In a first aspect, there is provided a photocatalyst for the direct synthesis of methanol from methane and carbon dioxide, the photocatalyst comprising AuPd alloy nanoparticles, dispersed on a support comprising GaN, InN, AIN, BN, TIN, GaP, InP or a combination thereof, wherein the AuPd alloy nanoparticles are uniformly dispersed on the support such that at least 90% of the AuPd alloy nanoparticles are positioned at a distance of from 1 nm to 15 nm from other AuPd alloy nanoparticles. In some embodiments, at least 60% of the AuPd alloynanoparticles have a diameter of from 1 to 5 nm. In some embodiments, at least 80% of the AuPd alloy nanoparticles have a diameter of from 2 to 4.5 nm. In some embodiments, less than 10%, preferably less than 5 %, more preferably less than 3 % of the AuPd alloy nanoparticles are in aggregates of two or more of the AuPd alloy nanoparticles. In some embodiments, the AuPd alloy nanoparticles have an element ratio of AuPd of from 1 :3 to 3:1 , or of from 1 :1 .25 to 1 .25:1 . In some embodiments, the AuPd alloy nanoparticles are present in a concentration of from 0.75 to 4 % by weight. In some embodiments, the photocatalyst has an X-ray photoelectron spectroscopy spectra comprising the following peaks: a Ga 3d peak between 19 and 21 eV, a N 1s peak between 395 and 398 eV, two Pd 3d peaks between 339 and 341 eV and between 333 and 335 eV, and two Au 4f peaks between 82 and 83 eV and between 86 and 87 eV.

[0005] In a further aspect, there is provided a method of producing methanol directly from methane and carbon dioxide, the method comprising: contacting the methane and the carbon dioxide at a surface of the photocatalyst as defined herein; and exposing the surface of the photocatalyst with a light irradiation. The method can also be performed with a photocatalyst comprising a transition metal dispersed on a support, and wherein the support is selected from GaN, BN, AIN, TIN, InN, InP, GaP and combinations thereof. In some embodiments, the transition metal is selected from Au, Pd, Rh, Pt, Ru, Ir, Cu, Fe, Co, Ni, Bi, Sn, Cr, 2O3 and combinations thereof. In some embodiments, the transition metal is AuPd nanoparticles, Cu, Co, Fe, Ni, Bi and Cr.

[0006] In some embodiments, the light irradiation is a full visible light spectrum irradiation. In some embodiments, the method is performed at a temperature of from 15 to 30 °C. In some embodiments, the method is performed at pressure of 1 atm ±5%. In some embodiments, the methane and the carbon dioxide are provided in a molar ratio of from 1 :1.25 to 1.25:1. In some embodiments, the method is a direct synthesis of methanol in which the carbon of the methane becomes the carbon of the methanol with the addition of the oxygen from the carbon dioxide.

[0007] In yet a further aspect, there is provided a method for making the catalyst as defined herein, the method comprising: immobilizing Au and Pd colloids on the surface of the support to form the AuPd alloy nanoparticles; and etching the surface of the support with an acid or a base. In some embodiments, the Au colloids are provided by a precursor selected from NaAuCU and HAuCU. In some embodiments, the Pd colloids are provided by a NaPdCU precursor. In some embodiments, the immobilizing is performed in darkness and in an argon inert environment.

[0008] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a graph showing the methanol production rate and the CO-free selectivity of methanol using different photocatalysts.

[0010] FIG. 2 is a schematic diagram of the preparation of a GaN photocatalyst on which AuPd nanoparticles were deposited by chemical reduction (labelled AuPd / GaN-cr).

[0011] FIG. 3A is a high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image of AuPd / GaN-cr.

[0012] FIG. 3B is an energy dispersive X-ray spectroscopy (EDX) mapping profile of the photocatalyst of Fig. 3A showing Pd and Au atoms.

[0013] FIG. 3C is an EDX mapping profile of the photocatalyst of Fig. 3A showing N atoms.

[0014] FIG. 3D is an EDX mapping profile of the photocatalyst of Fig. 3A showing Ga atoms.

[0015] FIG. 3E is a graph showing the intensity of the EDX signal in function of the energy for the Au and Pd atoms inside the dotted square of Fig. 3B.

[0016] FIG. 4A is a transmission electron microscopy (TEM) of AuPd / GaN-cr (scale bar 100 nm).

[0017] FIG. 4B is a TEM of AuPd / GaN-cr (scale bar 50 nm).

[0018] FIG. 5A is a schematic diagram of the preparation of the photocatalyst of the present disclosure (labelled AuPd / GaN-ci).

[0019] FIG. 5B is a HAADF-STEM image of AuPd / GaN-ci photocatalyst.

[0020] FIG. 5C is a graph showing the particle diameter distribution based on Fig. 5B.

[0021] FIG. 5D is an EDX mapping profile of the photocatalyst of Fig. 5B showing Au and Pd atoms.

[0022] FIG. 5E is a graph showing the intensity of the EDX signal in function of the energy for the Au and Pd atoms inside the dotted circle of Fig. 5E.

[0023] FIG. 5F is an EDX mapping profile of the photocatalyst of Fig. 5B showing N atoms.

[0024] FIG. 5G is an EDX mapping profile of the photocatalyst of Fig. 5B showing Ga atoms.

[0025] FIG. 5H is a TEM of AuPd / GaN-ci (scale bar 100 nm).

[0026] FIG. 5I is a TEM of AuPd / GaN-ci (scale bar 50 nm).

[0027] FIG. 5J is a X-ray diffraction (XRD) patterns of AuPd / GaN-ci and GaN.

[0028] FIG. 5K is a TEM of AuPd / GaN-ci showing particles of AuPd on the GaN surface (scale bar 10 nm).

[0029] FIG 5L is a close-up of Fig. 5K in the area inside the dotted square.

[0030] FIG. 5M is a ultraviolet-visible (UV-vis) spectrum of GaN, Au / GaN-ci AuPd / GaN-ci and Pd / GaN-ci.

[0031] FIG. 5N is a X-ray photoelectron spectroscopy (XPS) spectrum of Ga 3d for AuPd / GaN-ci and GaN.

[0032] FIG. 50 is a XPS spectrum of N 1s for AuPd / GaN-ci and GaN.

[0033] FIG. 5P is a XPS spectrum of Au 4f for AuPd / GaN-ci.

[0034] FIG. 5Q is a XPS spectrum of Pd 3d for AuPd / GaN-ci.

[0035] FIG. 6A is a graph showing the methanol yield rate (bar graph) and CO-free selectivity(dot plot) for AuPd / GaN-cr and AuPd / GaN-ci (reaction conditions: 2.5 mmol of CP , 2.5 mmol of CO2, 1 mg of catalyst, full-spectrum Xenon lamp, 2 h).

[0036] Fig. 6B is a graph showing the methanol yield rate (bar graph) and CO-free selectivity (dot plot) with AuPd / GaN-ci where the CH4 / CO2 ratio was 1 :3, 1 :1 or 3:1 .

[0037] FIG. 6C is a graph showing the methanol yield rate (bar graph) and CO-free selectivity (dot plot) with AuPd / GaN-ci at 1 bar or 0.2 bar.

[0038] FIG. 6D is a graph showing a performance comparison of AuPd / GaN-ci with other catalysts (Ag / pg-CsNzi, Rh / TNTs and AuTNTs).

[0039] FIG. 7 is a graph showing the reusability of AuPd / GaN-ci for catalyzing methane and CO2 to methanol.

[0040] FIG. 8A is a XPS spectra of Au 4f comparing fresh AuPd / GaN-ci (before undergoing catalysis reaction) and used AuPd / GaN-ci (after 6 catalysis cycles).

[0041] FIG. 8B is a XPS spectra of Pd 3d comparing fresh AuPd / GaN-ci and used AuPd / GaN- ci.

[0042] FIG. 8C is a XPS spectra of Ga 3d comparing fresh AuPd / GaN-ci and used AuPd / GaN-ci.

[0043] FIG. 9A is HAADF-STEM image of used AuPd / GaN-ci after reaction.

[0044] FIG. 9B is HAADF-STEM image of used AuPd / GaN-ci after reaction at a different section than Fig. 9A.

[0045] FIG. 9C is a TEM image of used AuPd / GaN-ci after reaction.

[0046] FIG. 9D is close-up of Fig. 9C.

[0047] FIG. 9E is high-resolution TEM image of used AuPd / GaN-ci after reaction.

[0048] FIG. 9F is a close-up of Fig. 9D.

[0049] FIG. 10A is a graph showing the yield rate of methanol from the photoconversion of 2.5 mmol H2 + 2.5 mmol CO2, 2.5 mmol H2 + 2.5 mmol CO, 2.5 mmol CH4 + 2.5 mmol CO, or 2.5 mmol CH4 + 0.25 mmol O2 over AuPd / GaN-ci.

[0050] FIG. 10B is gas-chromatography mass-spectroscopy (GC-MS) spectra of CO and CH3OH produced from photoconversion of12CH4 +12CO2 or12CH4 +13CO2 with AuPd / GaN-ci.

[0051] FIG. 10C is a13C nuclear magnetic resonance (NMR) spectrum of the liquid products obtained from the photoconversion of12CH4 and13CO2 over AuPd / GaN-ci.

[0052] FIG. 10D is a GC-MS spectrum of CH3OH produced from the photoconversion of12CH4+12CO2or13CH4+12CO2with AuPd / GaN-ci.

[0053] FIG. 10E is a13C NMR spectrum of the liquid products from the photoconversion of13CH4and12CO2with AuPd / GaN-ci.

[0054] FIG. 10F is a bar graph showing the kinetic isotope effects for photoconversion of methane with CO2

[0055] FIG. 11 A is a graph showing the yield rate of methanol over AuPd colloid, AuPd / GaN- ci or GaN catalyst.

[0056] FIG. 11B is a graph showing the yield rate of CO over AuPd colloid, AuPd / GaN-ci or GaN catalyst.

[0057] FIG. 11C is a graph showing the methanol production of the catalysts Au / GaN-ci, AuPd / GaN-ci and Pd / GaN-ci.

[0058] FIG. 11 D is a photoluminescence spectrum of GaN Au / GaN-ci AuPd / GaN-ci and Pd / GaN-ci.

[0059] FIG. 11E is a graph showing the methanol production of AuPd / GaN-ci under light irradiation with different wavelengths of cut-off filters.

[0060] FIG. 11 F is graph showing the yield rate of CO over Au / GaN-ci, AuPd / GaN-ci and Pd / GaN-ci for photoconversion of CH4and CO2.

[0061] FIG. 12A is an in situ thermographic image of the reactor captured when the reaction was conducted for a duration of 1 hour (temperature range 22 - 35.9 °C).

[0062] FIG. 12B is an in situ thermographic image of the reactor captured when the reaction was conducted for a duration of 1 hour (temperature range 21 .9 - 38.7 °C).

[0063] FIG. 13 is an Arrhenius plots of the co-conversion of CH4and CO2for CH3OH photosynthesis over GaN.

[0064] FIG. 14 is a bar graph showing the generation rate of methanol from CH4photooxidation over AuPd / GaN-ci under photo-driven or thermal catalytic conditions.

[0065] FIG. 15 is a UV-Vis spectra of AuPd / GaN-ci, AuPd / InN-ci and AuPd / InN+GaN-ci.

[0066] FIG. 16 is bar graph showing the methanol production rate over GaN, AuPd / GaN-ci, AuPd / InN-ci and AuPd / InN+GaN-ci surfaces under visible-light irradiation.DETAILED DESCRIPTION

[0067] There is provided herein a photocatalyst and methods of using same for converting methane and carbon dioxide directly into methanol. Methane and carbon dioxide are both greenhouse gases polluting the atmosphere. The accumulation of both of these gases in the atmosphere is a concern and their accumulation is projected to increase year over year due, in part, to the various industrial activities around the world. The revalorization of both of these gases into methanol is therefore highly desirable and is a green process that has a positive impact on the environment and global warming. The present disclosure provides a method of converting methane and carbon dioxide directly into methanol which saves time, energy and money when compared to the multiple step syntheses starting from methane that utilize syngas as an intermediate species. Indeed, the present method can advantageously be performed under ambient conditions (e.g. room temperature of from 15 to 30 °C and / or pressure of 1 atm ± 10%, ± 5 % or ±3%) whereas traditional processes that go through the syngas intermediate require heating at high temperature such as above 800 °C to produce the syngas.

[0068] The photocatalyst of the present disclosure is a metal-semiconductor light-activated catalyst comprising a support on which various transition-metals (i.e. metals of groups 3-12 of the periodic table), such as Au, Pd, Rh, Pt, Ru, Ir, Cu, Fe, Co, Ni, Bi, Sn, Cr, ln2Os and combinations thereof, and alloys thereof particularly AuPd particles, are dispersed. The transition metals form a dispersion of particles loaded on the support. The support of the present catalyst is or comprises GaN, InN, AIN, BN, TIN, GaP, InP or combinations thereof. In some embodiments, the combination of InN and GaN is in a weight ratio of about 1 :1 , from 1 :1 .5 to 1 .5:1 , from 1 .25:1 to 1 :1 .25 or from 1 :1 .1 to 1 .1 :1 . A surface hydrogen transfer pathway is created at the photocatalyst surface which breaks the inert bond of methane and promotes the catalysis of methanol. In preferred embodiments where the transition metal comprises AuPd particles, together with the support form an efficient AuPd / nitride or AuPd / phosphide interface that performs a direct methanol photosynthesis starting from CH4 and CO2.

[0069] The nitrides of the present disclosure (GaN, InN, AIN, BN, and TIN) are characterized by a wurtzite crystal structure, which is the thermodynamically stable phase. In this crystalstructure, the exposed surfaces of the nitrides are composed of c-planes and m-planes. Taking GaN as an example, the m-plane is a one dimensional rectangular configuration of the Ga and N atoms and the c-plane is a one dimensional hexagonal configuration of the Ga and N atoms. The overall m-plane of GaN is nonpolar since it is composed of equal numbers of Ga and N atoms which are tetrahedrally coordinated with each other, whereas the polar c-plane comprises only one type of atom (either Ga or N) which exhibits piezoelectric polarization along the c-axis. In some embodiments, the GaN can be a nanoparticle for example having a diameter of between 10 and 1000 nm, between 20 and 900 nm, between 30 and 800 nm, or between 40 and 700 nm. At industrial scale, the GaN is preferably an unsupported or a supported sheet in a reactor, such as a flow reactor. When a continuous flow of methane and carbon dioxide is supplied, the concentration of the transition metals (such as the AuPd nanoparticles) can be modified based on the flow rate of the gases. For example, when the flow rate is high the concentration of the transition metals can be reduced so as to space the transition metals across a longer distance which is covered in a shorter period of time because of the high flow rate. In some embodiments, the catalyst is suspended in a reactor or is deposited onto a reactor bed or surface. In additional embodiments, the catalyst itself may be deposited onto at least a portion of a reaction surface (i.e. support by the reactor).

[0070] The transition metals can collectively form from about 0.75 to about 4 %, from about 1 to about 4 %, from about 0.75 to about 2 %, from about 0.75 to about 1 .5 %, from about 0.75 to about 1 .25 %, or about 1 % by weight of the photocatalyst. In preferred embodiments, the transition metal is AuPd particles and collectively forms from about 0.75 to about 4 %, from about 1 to about 4 %, from about 0.75 to about 2 %, from about 0.75 to about 1 .5 %, from about 0.75 to about 1 .25 %, or about 1 % by weight of the photocatalyst. The term “about” as used herein in the context of a weight percentage is defined as ± 20%, ± 15%, ± 10%, ± 5%, or ± 3%. In some embodiments, the balance forthe weight percent provided herein is the total weight of the catalyst. In other embodiments, the catalysts only consists of the transition metals and the support.

[0071] The AuPd particles are nanoparticles that have an average diameter of from 1 to 5 nm, from 2 to 4.5 nm, or from 3 to 4 nm. In some embodiments, at least 60%, at least 70%, at least 80%, at least 90%, or at least 95% of the AuPd nanoparticles have a diameter of from 1 to 5 nm, from 2 to 4.5 nm, or from 3 to 4 nm. The term “AuPd” particle as used herein refers to an AuPd alloy that forms a nanoparticle. The element ratio (Au : Pd) in the AuPd alloy is for example from 1 :3 to 3:1 , from 1 :2 to 2:1 , from 1 :1 .5 to 1 .5:1 , from 1 :1 .25 to 1 .25:1 , from 1 :1.1 to 1.1 :1 or is 1 :1 . In some embodiments, the AuPd content with respect to the support is from 0.2 to 2 wt. %.

[0072] In the preferred embodiments where the transition metals comprise AuPd nanoparticles, the photocatalyst of the present disclosure has an advantageous uniform distribution of AuPd nanoparticles on the support surface This is achieved by a soft colloidimmobilization process to assemble AuPd alloy with the support to form evenly distributed metal nanoparticles coupled with intact and highly active support (for example GaN, InN, GaN + InN) that is subjected to a surface etching by base or acid. The term "soft" as used herein refers to the colloid immobilization method used to load the transition metal on the support that can keep its surface particularly the area in contact with metal from chemical destruction in terms of morphology, crystal structure and electronic structure. The protocol of this preparation process includes the generation of nanoalloy particle colloids and sequent metal loading with the support. The detailed steps for a GaN support as an example are as follows. Firstly, a certain amount of HAuCU aqueous solution and NaPdCU (the specific amount depends on the loading) solution are dissolved in de-ionized water. Polyvinylpyrrolidone (PVP, e.g. average molecular weight 1 ,300,000) is added as a stabilizer to give the required metal-to-PVP ratio (typically 1 :1 .2). After around 5 min of stirring, freshly prepared 0.1 M sodium borohydride (NaBF ) solution is added such that the molar ratio of NaBF -to-metal was 5. Secondly, this produced a dark brown colloid stirred for 30 minutes and 100 mg of GaN is added to immobilize these single or bimetallic metal nanoparticles for another 30 minutes. Finally, the as-prepared metal-GaN-co catalyst was filtered, washed thoroughly with distilled water and then left to dry at 100 °C for overnight. The utilization of photo deposition strategy is also an alternative method to load AuPd metals uniformly on GaN.

[0073] The uniformity of the dispersion of AuPd nanoparticles on the support surface can be characterized by having at least 90% of the nanoparticles of AuPd being positioned on the support surface at a distance of from 5 nm to 20 nm from other AuPd nanoparticles. This can be measured with microscopy techniques such as electron microscopy or dynamic light scattering. In some embodiments, the uniformity is defined as having at least 90% of the AuPd nanoparticles not being in contact with another AuPd nanoparticles and not forming an agglomerate of AuPd nanoparticles. This can also be determined with microscopy techniques such as electron microscopy or dynamic light scattering. In some embodiments, less than 10%, less than 5%, less than 3%, less than 2% or less than 1 % of the AuPd nanoparticles are in AuPd aggregates or agglomerates of two or more particles. The AuPd aggregate or agglomerate of nanoparticles can be defined as two or more nanoparticles forming a total size of 8 nm or more. In some embodiments, the uniformity can be further characterized by separating the surface area of the support into pixels of equal surface area and comparing the number of AuPd nanoparticles withineach delineated pixel. A uniform distribution can then be defined as having an amount of AuPd nanoparticles that is the same or statistically similar within each pixel. The determination of statistical similarity depends on the size of the pixels selected. In preferred embodiments, the pixel surface area size is selected such that there is from 20 to 100 nanoparticles in a single pixel. A uniform distribution is then defined as having a number of nanoparticles per pixel that varies less than 20%, less than 10%, less than 5% or less than 3% between each pixel.

[0074] In some embodiments, the photocatalyst of the present disclosure has a GaN support and is optionally characterized by peaks of X-ray photoelectron spectroscopy (XPS) spectra, specifically a Ga 3d peak between 19 and 21 eV, a N 1s peak between 395 and 398 eV, two Pd 3d peaks between 339 and 341 eV and between 333 and 335 eV, and two Au 4f peaks between 82 and 83 eV and between 86 and 87 eV.

[0075] The scheme for the direct catalysis of methanol starting from methane and carbon dioxide is shown below. With the catalyst of the present disclosure a selectivity of more than 75 %, more than 80 %, more than 85 % or more than 90 % for methanol can be achieved. Moreover, it was presently found that it is specifically the carbon of the methane compound that becomes carbon of the methanol. The present catalyst breaks one of the C-H bonds of methane to incorporate the hydroxyl group in its place. The O of the hydroxyl group is provided from the carbon dioxide.LightCH4+ CO2- - -CH3OH + coCatalyst

[0076] The enthalpy for the one-step conversion of methane and carbon dioxide into methanol is 167 kJ mol1at 298K. This high positive reaction enthalpy is a limitation that requires harsh reaction conditions in order to overcome it and drive the reaction. The harsh reaction conditions required (e.g. a high-temperature of above 800 °C and a pressure of 10-40 bar) increase the likelihood of producing undesirable by-products (e.g. dimethyl ether) because of the low activation energy of methanol. To break this thermodynamic trade-off bottleneck, the present photocatalytic system was developed, where photoexcited electron-hole pairs offer a solution to make direct conversion of methane with carbon dioxide into methanol. Indeed, it was presently discovered that the present catalyst combining a transition metal and a nitride or phosphide support provides an interface which significantly reduces the activation barrier of methane dehydrogenation and CO2 reduction concurrently, which in turn allows for the production of methanol to occur directly.As demonstrated in the example section below, the productivity of methanol achieved with the present photocatalyst can reach up to 1405 pmol g-1tr1at ambient conditions which overperforms the productivity in dry forming to CO reported in the literature by a factor of 3.8. Moreover, the photocatalyst of the present disclosure can be reused for multiple cycles (e.g. 3, 4, 5 cycles or more) for methanol production as demonstrated in the example section below.

[0077] As previously explained, the catalytic generation of methanol according to the present process can be performed under ambient conditions. In some embodiments, the catalysis is performed without heating at room temperature, for example at a temperature of from 15 to 30 °C or from 20 to 25 °C. In some embodiments, the catalysis is performed in an open environment at a pressure at or around atmospheric pressure (e.g. 1 atm ± 5 %). It should be noted that at industrial scale the temperature or the pressure may be modified to accelerate the process. For large-scale industrial production, this catalytic process is adaptable in terms of temperature and pressure by using a pressurized bench reactor for example, particularly a flow-reactor system. Firstly, the availability and price stability of feedstocks, which are CO2 and CH4, are in large demand based of industrial applications and as waste by-products. Secondly, this catalytic strategy results in state-of-the-art yield and selectivity of liquid products in a very short time under bench condition. Expanding this process to a flow system can lead to a significant qualitative leap in yield. Thirdly, the production of methanol represents a crucial product that aligns with market demand for subsequent manufacturing processes. The methane and carbon dioxide provided are preferably provided in a stoichiometric ratio but the molar ratio can vary for example from 1 :1 .25 to 1 .25:1 , from 1 :1 .1 to 1 .1 :1 , or from 1 :1 .05 to 1 .05:1 . These stoichiometric amounts of methane and carbon dioxide can be achieved at industrial scale by either high-pressure bench reactors or flow reactors. However, the ratio can be optimized in different systems used for large-scale processes.

[0078] The ever-increasing global energy crisis has led to the development of alternative fuels such as methanol biofuel for fuel cells or combustion engines. Methanol has also found a use as a means to store energy (liquid energy storage). The industrial production of methanol has traditionally depended on indirect conversion of syngas from natural gas, oil and biomass materials into methanol. The present disclosure provides a greener alternative compared to the syngas route.EXAMPLE

[0079] A screen of different photocatalysts was carried out by performing a dry-reforming reaction of methane and carbon dioxide in a chamber under light exposure (full spectrum xenon lamp at room temperature and ambient pressure). The photocatalysts used were C3N4, Ga2C>3, TiO2, ZnO, GaN, Cu-GaN, Co-GaN, Ni-GaN, Fe-GaN, Bi-GaN, Cr-GaN, Sn-GaN, WO3-GaN, AI2O3-GaN, ln2O3-GaN, Ir-GaN, Ru-GaN, Rh-GaN Pt-GaN, Pd-GaN, Au-GaN, Au+Pd-GaN, AuPd-GaN, AuiPds-GaN, AusPdi-GaN. The nomenclature of Au+Pd-GaN refers to a mixture of Au-GaN and Pd-GaN (in this example 0.5 mg of each). This same nomenclature is applied to label the combination of other species. The three control conditions were the absence of catalyst, the absence of light, and the absence of carbon dioxide. The photocatalysts C3N4, Ga2O3, TiO2, ZnO, and GaN were obtained commercially and the modifications to the GaN catalysts with metals to obtain the remaining photocatalysts is explained further below. GaN was purchased from Thermo Fisher Scientific Chemicals, ZnO, Ga2O3, and TiO2were purchased from Sigma-Aldrich and C3N4 was purchased from TCI. Unless stated otherwise, the reaction was performed with 1 mg of photocatalyst, 2.5 mmol of CH4, 2.5 mmol of CO2, for a duration of two hours and the methanol production rate as well as the CO-free selectivity of methanol were measured (Fig. 1 and Table 1). Besides noble metals as co-catalysts, transition metals including Cu, Co, Ni, Bi, Cr and Fe also demonstrated enhanced reactivity and selectivity when compared to other metals.

[0080] More specifically, 1 milligram of the catalysts was evenly fixed at the bottom of a 120 mL quartz chamber reactor. The reactor was degassed to remove water and possible gases adsorbed on the catalyst surface at 250 °C. Then the methane and CO2gases were introduced with a ratio of 1 -to-1 into the reactor after Pump-vacuum for three times. The reactor was placed into a double layered beaker with water bath to irradiate under a 300 W Xenon lamp with PE300BF™ light bulb (Excelitas) for 2 h. After the photocatalytic reaction, gas products were identified and quantified as follows.

[0081] The liquid products of CH3OH, CH3OOH and HCOOH were determined and quantified via proton nuclear magnetic resonance (NMR) spectroscopy spectrometer (Bruker AVIIIHD™ 500 MHz). Specifically, 0.4 mL of crude reaction liquid was filtered and mixed with 0.1 mL deuterium oxide containing 0.0167 pL of dimethylsulfoxide (DMSO) as internal standard sample. The calibration curves of CH3OH and HCOOH were carried out by plotting concentrations of standard chemicals with HNMR area ratio of products versus DMSO. In terms of HCHO quantification, colorimetric method was used for its concentration determination. 0.3 mL of crude reaction liquidwas diluted by 2 mL of deionized water. After adding 0.3 mL of reagent solution into diluted crude liquid, the final test liquid was placed in 35 °C of water bath for 1 h. Unless stated otherwise, the reagent solution (100 mL) contained 15 g of ammonium acetate, 0.3 mL of acetic acid and 0.2 mL of pentane-2, 4-dione. The quantification of CO was obtained by gas chromatography (GC, Clarus™ 590) equipped with a methanizer and flame ionization detector. We used GC (Agilent™ 6890N) with thermal conductivity detector (GC-TCD) to quantify the H2 amount. Then we used GC (Agilent 6890N) with flame ionization detection (GC-FID) to identify and quantify hydrocarbon products (ethane).

[0082] Following the liquid product and gas product analyses, 1 mL of water was injected into the reactor to absorb the liquid products, and then the crude liquid was analyzed respectively by hydrogen nuclear magnetic resonance (HNMR) for CHsOH, CH3OOH and HCOOH as well as by a colorimetric method for HCHO. The colorimetric method was as follows. Briefly, 0.3 mL of crude reaction liquid was diluted by 2 mL of deionized water. After adding 0.3 mL of reagent solution into diluted crude liquid, the final test liquid was placed in a 35 °C water bath for 1 h. Typical reagent solution (100 mL) was made of 15 g of ammonium acetate, 0.3 mL of acetic acid and 0.2 mL of pentane-2, 4-dione.Table 1 . Results of the photocatalyst screen for a direct photoconversion of methane and CO2

[0083] The control experiments show that the reaction of CH4 and CO2 at room temperature did not yield detectable methanol without light or photocatalysts. With commercial GaN powder as a catalyst, a higher methanol rate of 41 pmol g-1tr1was obtained compared to the other semiconductors tested that are known for their capacity to perform methane dehydrogenation and CO2 reduction photoconversion. In subsequent testing, air-stable single-metal particles, including Ir, Ru, Pt, Ru, Bi, Sn, Cr, WO3, ln2C>3, AI2O3, Cu, Co, Fe, Ni, Pd, and Au, were in situ depositedon the GaN support via a chemical-reduction method (Fig. 2) to form an active interface, denoted as metal / GaN-cr. The deposited metals form nanoparticles on the support surface, generally a size of 5 to 20 nm diameter. The precursors of the Cu-GaN, Co-GaN, Ni-GaN, Fe-GaN, Ir-GaN, Ru-GaN, Rh-GaN, Pt-GaN, Pd-GaN, Au-GaN, and Bi-GaN, Cr-GaN, and Sn-GaN, catalysts were respectively CuCI2-2H2O, Co(N03)2-6H20, Ni(NO3)2-6H2O, FeCI2, lrCI3, RuCI3, RhCI3, H2PtCl6'6H2O, Na2PdCl4, HAuCl4'4H2O, (CH3CO2)3Bi, CrCI2, and SnCU,. The preparation was performed first dispersing 0.2 wt % metal / GaN-cr photocatalyst, and 100 mg of GaN into 25 mL of aqueous metal solution by ultrasonication (0.5 wt. % and 1 wt. % were used for entries 16b and 16c in Table 1). After stirring for 0.5 h, a certain amount of aqueous NaBF solution (molar ratio of NaBF to metal is 5) was added dropwise into the solution. After further stirring for 0.5 h, the samples were filtered, washed with water three times, and then dried at 100 °C under vacuum overnight. For WO3-GaN, AI2O3-GaN, and ln2O3-GaN, WO3, AI2O3, or ln2O3were obtained commercially and mechanically mixed in a 1 :1 ratio with the commercial GaN.

[0084] As shown in Fig. 1 and Table 1 , among monometallic interfaces with GaN, Au and Pd nanoparticles provided more prolific photocatalytic reactivity enhancement due to more considerable differences in the work function of metal and GaN. Cu, Co, Fe, Ni, Bi and Cr nanoparticles also showed prolific photocatalytic reactivity and selectivity for methanol. Without wishing to be bound by theory, it is believed that this is not only because of the as-formed interface for the photogenerated charge separation efficiency of GaN but also because introducing a metal at the surface of GaN results in a more active surface for CO2and CP pre-adsorption and dissociation. Bimetallic nanoparticles with a typical “1 +1 >2 synthetic” effect have been remarkably applied to improve catalytic performance in conversion and selectivity for traditional thermal and sustainable-energy-driven reactions. In the following screen, a 0.2 wt. % metal loading of Au with Pd of physical mixture was tested. The ratio of Au and Pd was varied, with 1 :3, 1 :1 and 3:1 being tested (respectively AuiPd3-GaN, AuPd-GaN, and Au3Pdi-GaN). The alloyed AuPd-GaN interface with 1 -to-1 atomic ratio demonstrated the best methanol yield rate of all samples. Further increasing metal loading to 0.5 wt. % even 1 wt. % did not increase the CH3OH yield (Table 1 entries 16b and 16c). Besides noble metals as co-catalysts, cheap transition metals including Cu, Co, Fe, Ni, Bi and Cr have also demonstrated considerable potential for enhancing reactivity (see Table 1).

[0085] The metal particle nano-sized distribution of AuPd / GaN-cr catalyst was then characterized. The bright field transmission electron microscopy (TEM) and HAADF-STEM observations were carried out on FEI Tecnai™ G2 F20 S / TEM at accelerating voltage of 200 kV.Transmission electron microscopy (TEM) images of AuPd / GaN-cr demonstrated a poor metal dispersion in bulky GaN support made by in-situ chemical reduction preparation (Figs. 3A-3E and 4A-4B). To improve this apparent drawback of metal aggregation, a soft colloid-immobilization process was used instead to assemble AuPd alloy with GaN to form evenly distributed metal nanoparticles coupled with intact and highly active GaN (AuPd / GaN-ci) that quickly encounters a surface etching by base or acid (Fig. 5A). The metal colloids were prepared according to the protocols of N. Agarwal, et al. Aqueous Au-Pd colloids catalyze selective CP oxidation to CH3OH with O2 under mild conditions. Science 2017, 358, 223-227, and of X. Huang, et al. Au-Pd Separation Enhances Bimetallic Catalysis of Alcohol Oxidation. Nature 2022, 603, 271-275. However, the solution is not acidified to PH1 as per Agarwal et al. and the Pd metal precursor are switched to NaPdCU instead of PdC acid solution. For the present process, the Au precursor can be both NaAuCU and HAuCU. Another difference with the referenced processes is that the immobilization process is done under the protection of Ar in darkness, which can be beneficial for the protection of GaN surfaces. Briefly, a certain amount of HAuCU aqueous solution and Na2PdCk solution were dissolved in 25 mL of de-ionized (DI) water. This certain amount for the typical 0.2 wt. % samples with a metal mole ratio of 1 is 200 mg of NaAuCU 2H2O and 278 mg of Na2PdCU. Polyvinylpyrrolidone (PVP, average molecular weight 1 ,300,000) was added as a stabilizer to give the required metal-to-PVP ratio (typically 1 :1.2). After 5 min of stirring, freshly prepared 0.1 M sodium borohydride (NaBH4) solution was added such that the molar ratio of NaBH4-to-metal was 5. This produced a dark brown colloid stirred for 30 minutes and add 100 mg of GaN to immobilize these single or bimetallic metal nanoparticles for another 30 minutes. The as-prepared metal-GaN-ci catalyst was filtered, washed thoroughly with distilled water and then left to dry under vacuum at 100 °C overnight.

[0086] As can be seen from the HAADF image and its size population analysis of AuPd / GaN- ci (Figs. 5B-5I), the average diameter of 3.59 nm of metal colloid particles has been evenly immobilized on the GaN surface. The well-established bimetallic composition of supported AuPd alloy was also confirmed by the following EDX spectroscopy analysis. More importantly, the composition and crystalline structure of the GaN host did not change markedly even after successive load-wash-dry catalyst processing treatment, which were reflected as well as in the X-ray diffraction results (Fig. 5J). Powder X-ray diffraction patterns (XRD) measurement were collected on a Bruker™ D8 Advance X-ray diffractometer with a Cu-Ka radiation source (A = 1 .5418 A) and operated with an increment of 0.02 °and a counting time of 0.24 s underthe voltage of 40 kV and 40 mA. The TEM and high-resolution TEM (HRTEM) results clearly illustrate thewell-dispersion and formation of active metal / GaN interface, respectively (Figs. 5K-5L). In addition, the enhanced UV-Vis photo-adsorption property (Fig. 5M) at around 510 nm of AuPd / GaN-ci might arise from plasmonic effect on metallic AuPd nanoparticles surface that is also well proved in the results of Au 4f and Pd 3d X-ray photoelectron spectroscopy (XPS) spectra (Figs. 5N-5Q). X-ray photoelectron spectroscopy (XPS) measurement were conducted on a Thermo ESCALAB™ 250 spectrometer equipped with a monochromated Al Ka radiation (hv=1486.6 eV). The energy of spectrometer was calibrated by C 1 s peak at 284.8 eV.

[0087] The AuPd / GaN-ci was selected as the preferred photocatalyst to test its photocatalytic behavior compared to AuPd / GaN-cr under the initial reaction conditions as per Fig. 1 (1 mg of photocatalyst, 2.5 mmol of CP , 2.5 mmol of CO2, for a duration of two hours at room temperature and ambient pressure under full spectrum xenon lamp). It was observed that the methanol yield rate over AuPd / GaN-ci increased to a high value of 1405 pmol g1h1that is 2.6 times more than the rate of AuPd / GaN-cr (Fig. 6A and Table 2).Table 2. Performance of metal-GaN-ci catalyzed direct photoconversion of CH4 and CO2.

[0088] Additionally, the influence of different CH4 / CO2 ratio on methanol production was investigated while maintaining 1 bar of total gas pressure (Fig. 6B and Table 3). It was found that the higher partial pressure of CH4 or CO2 resulted in a lower methanol yield and selectivity, demonstrating that a CH4 / CO2 ratio of 1 is optimal for the present photocatalytic system. Moreover, when submitting this photoconversion to a low pressure of 0.2 atm, 435 pmol g1h1of methanol productivity was still achieved which is comparable with the value for CO production reported in (Tahir, Beenish, Muhammad Tahir, and Nor Aishah Saidina Amin. "Silver loaded protonated graphitic carbon nitride (Ag / pg-C3N4) nanosheets for stimulating CO2 reduction to fuels via photocatalytic bi-reforming of methane." Applied Surface Science 493 (2019): 18-31) (Fig. 6C and Table 4). The yield rate of 1405 pmol g1h1obtained under ambient pressure also surpasses the other tested photocatalytic systems (Fig. 6D and Table 5). Although such active AuPd / GaN surface begins to decrease in methanol yield after long-time reuse (Table 6) possibly due to unavoidable oxidation of metal cocatalyst with O species from CO2 (Figs. 7 and 8A-8C) or slow aggregation of metal (Figs. 9A-9B). In spite of this, the robustness and applicability of the photocatalytic system towards CH3OH production was demonstrated by reusing it at least 5 times with a total CH3OH yield of 13.66 mmol g1and no noticeable carbon depositions (Figs. 9C-9F).Table 3. The gas ratio optimization for AuPd-GaN-ci catalyzed direct photoconversion of CH4 and CO2 (Reaction condition: total pressure of 1 bar, 1 mg of catalyst, full-spectrum Xenon lamp, 2 h)Table 4. The gas pressure optimization for AuPd-GaN-ci catalyzed direct photoconversion of CH4 and CO2 (reaction condition: gas CH4 and CO2 ratio of 1 -to-1 , 1 mg of catalyst, full-spectrum Xenon lamp, 2 h)Table 5. Summary of the results for the photoconversion of CH4and CO2 over the tested catalytic systems.Table 6. The gas pressure optimization for AuPd-GaN-ci catalyzed direct photoconversion of CH4 and CO2 (Reaction condition: total pressure of 1 bar, 1 mg of AuPd / GaN-ci, full-spectrum Xenon lamp, 2 h)

[0089] With the excellent methanol productivity observed, it was decided to move forward and investigate the reaction mechanism under light-driven conditions. In the final gas products analysis, a CO yield was observed but there was no observable amount of H2 or other hydrocarbons detected after 2 h of reaction. Accordingly, it was demonstrated that the present catalytic system eliminated the methane dehydrogenation as a side reaction. Considering the indirect pathway from CH4 (CO2) via intermediates (H2 CO, O2) to methanol (Fig. 10A and Table 7), the reactivity of AuPd / GaN-ci was first examined in control experiments where the reactants were replaced with identical ratio of H2 + CO2, H2 + CO or CH4 + CO. As shown in Fig. 10A, the tested conditions did not display remarkable activity for the contribution of methanol yield, thus directly excluding the possibility that CO2 or CO reacts with H2 possibly generated from methane to form methanol. Secondly, when replacing CH4 + CO2 with a CH4 0-O2 ratio of 10, the excess O2 generated from CO2 to CO (6.6 pmol) still did not help promote the CH4 conversion to yield methanol, suggesting that methane oxidation with O2 possibly in situ derived from CO2 activation is not primarily responsible for the methanol productivity. It was overall deduced that thephotocatalytic methanol mechanism is a direct transformation from CH4 and CO2 through surface catalysis rather than a CO intermediate-mediated indirect route.Table 7. The catalytic performance of control reaction over AuPd-GaN-ci (total pressure of 1 bar, 1 mg of catalyst, full-spectrum Xenon lamp, 2 h)

[0090] Then, a carbon labelling experiment was performed to further confirm that the carbon source of the product after the reaction. Specifically, a13C labelled carbon (13CO2 and13CH4) was used. When a13CO2 instead of12CO2 was used, the characteristic peak of CO at m / z = 28 shifted up to m / z = 29 (Fig. 10B). This demonstrates that the majority of as-formed CO comes from CO2. In the presence of13CO2, there was no detectable13C labelled methanol (m / z = 33) as measured by NMR and GCMS results (Fig. 10C), indicating that the carbon source of methanol was not CO2. In the following isotope labelling13CH4 experiment, the signal of the mass spectrum and NMR for13CH3OH was detected further validating the concert incorporation of CH4 into methanol (Figs. 10D-10E). These results consolidate that CH4 and CO2 make a dominant contribution to methanol and CO in products, respectively. Complementary to this synthesis approach, the determination of a high kinetic isotope effect (KIE) value of 3.79 forthe CH3OH formation revealed that the C-H cleavage in CH4 is a critical component of the rate determining step (Fig. 10F). Based on all these findings, a distinctive transformation involving grafting of oxygen-atom (OAG) from CO2 to CH4 was proposed wherein CH4 mainly contributes to CH3OH and CO2 to CO in the reaction products.

[0091] To better unravel the role of the surface interface of the AuPd / GaN-ci catalyst in the photocatalytic dry reforming of methane, the photocatalytic performance of controlled catalysts of unsupported AuPd colloid and GaN support were tested to compare their performance with that of AuPd / GaN-ci. Compared with the low yield of methanol formed by AuPd colloids and bare GaN, the as-formed AuPd / GaN-ci interface showed a significant increase in methanol productivity under light excitation (Fig. 1 1 A). Such a huge difference of activity towards methanol suggests that the introduction of metal nanoparticles on the GaN support is of great importance in enhancement of methane adsorption and activation to promote methanol generation (Table 8).Table 8. The activity of unsupported metal colloid for photoconversion of CP and CO2 under light irradiation.

[0092] The CO yield rate of AuPd colloid and GaN was also compared with AuPd / GaN-ci to evaluate the contribution of the AuPd-GaN interface in the CO2 activation reaction that produces CO. The fact that the GaN support was found to be active to yield considerable CO confirms the energetically favored adsorption and dissociation of CO2 molecule on the photoexcited GaN surface (Fig. 1 1 B). The further enhanced CO formation rate was clearly observed when the interfacial AuPd-GaN was used which reached almost 2.4 times the rate for bare GaN. Therefore, the AuPd metal nanoparticles act as a cocatalyst that facilitates the CO2-t0-CO activation with sole GaN (Table 9).Table 9. The gas products productivity of unsupported metal colloid for photoconversion of CH4 and CO2 under light irradiation (reaction condition: total pressure of 1 bar, 1 mg of catalyst, fullspectrum Xenon lamp, 2 h)

[0093] Based on the formation of intact metal-GaN interface, photoexcited GaN support exhibits a stronger function of activating loaded AuPd alloy cocatalyst than monometallic ones that improve methane and CO2 catalytic performance simultaneously (Fig. 11 C). To prove this point, the UV-Vis diffraction and photoluminescence (PL) spectra of GaN and Au / GaN-ci AuPd / GaN-ci and Pd / GaN-ci were collected to investigate the effect of photo-absorption on direct methanol photosynthesis (Fig. 11 D). The UV-vis spectrometry was performed by Agilent Cary™ 5000 series UV-vis-NIR spectrometer and the PL measurement was performed with a 213-nm He-Cd laser (Kimmon™ Koha) as excitation source. Introducing metal cocatalyst did not lead to much improvement for light adsorption with pristine GaN regardless of whether it was monometallic or bimetallic. The only PL characteristic peak of all GaN samples with or without metal further demonstrated that the photo-absorption at the metal-GaN interface originates from GaN semiconductor rather than from a metal plasmonic effect. The decreased reactivity of AuPd / GaN-ci under visible light also means that the plasmonic adsorption from AuPd is not strong enough to trigger the adsorptive activation of methane (Fig. 11 E and Table 10). When the GaN support is excited by photoirradiation, the charge transfer occurring on the interface of AuPd alloy and GaN showed better performance in prompting methane and CO2 adsorption and dissociation at the same time that was reflected as an enhanced methanol and CO yield rate over AuPd / GaN- ci (Fig. 11 F). The present observations demonstrate the necessity of photoexcitating the GaNsurface to keep metal cocatalyst active for the photocatalysis. In addition, active interfacial sites located at the interface may lead to improved performance of the catalytic activity. Accordingly, the difference in photocatalytic activity for monometallic and bimetallic metal-GaN interfacial catalysts suggests that alloyed AuPd nanoparticle with random and uniform metal arrangement has apparent reactivity enhancement with the help of GaN photo-promoter.Table 10. Reaction with AuPd-GaN-ci for the photoconversion of CP and CO2 into methanol under light irradiation with different wavelengths of light using wavelength filters (reaction condition: total pressure of 1 bar, 1 mg of catalyst, full-spectrum Xenon lamp, 2 h)

[0094] In addition, thermographic photograph measurements demonstrated that this surface photocatalysis reaction occurred at 25 °C (Figs. 12A-12B). This result confirmed the absence of concomitant photothermal effect upon light irradiation. All these analyses identify the synergy between photoexcited GaN and alloyed AuPd, forming an active photoelectron-transfer interface that potentially enable energy and electron transfer with CP and CO2. Such synergistic photoactive interface thus endows the GAG process with a lower activation energy (60 kj mol-1) than GaN (70 kj mol1) alone (Fig. 13). Moreover, the negligible CH3OH yield under thermally driven conditions implies the AuPd / GaN interface’s effectiveness in overcoming the thermodynamic challenges of traditional thermo-catalysis in OAG conversion (Fig. 14 and Table 11). The successful application of interfacial AuPd / GaN in OAG process introduces allows using photoexcited metal / semiconductor interfaces as a versatile toolbox in advancing sustainable chemical conversions.Table 11. Reactivity comparison of photocatalysis with thermo catalysis over AuPd-GaN-ci catalyzed direct photoconversion of CP and CO2 (Photo reaction condition: gas CP and CO2ratio of 1-to-1 , 1 mg of catalyst, full-spectrum Xenon lamp, 2 h. For the thermal test, irradiation was replaced with heating at 300 °C)

[0095] Lastly, to extend the synthesis for broader applicability in the visible light region, improving the semiconductor component of the interfacial catalyst was investigated by transitioning from GaN to indium nitride (InN) powders. Indium nitride has an enhanced visible- light absorption capabilities with narrow theoretical bandgap. The combination of GaN and InN was also explored (AuPd / InN+GaN-ci).

[0096] The following materials were sourced: Gallium (III) nitride (thermo scientific, 99.99 %), indium (III) nitride (Santa Cruz Biotechnology), indium gallium nitride (Nanochemazone, Canada), indium (III) phosphide (Santa Cruz Biotechnology) and boron nitride (Thermo scientific), chloroplatinic acid hexahydrate (Sigma-Aldrich, ACS reagent), gold (III) chloride trihydrate (Sigma-Aldrich, ACS reagent), rhodium (III) chloride hydrate (Sigma-Aldrich, ACS reagent), iridium (III) chloride trihydrate (Acros organics), NaPdCL (Sigma-Aldrich, 98 %), zinc oxide (Sigma-Aldrich, nanopowder, < 100 nm particle size), titanium (IV) oxide (nanopowder, ~21 nm particle size, 99.5 %), gallium (III) oxide (Sigma-Aldrich, 99.99 %), methane (Air Liquide, 99.99 %), carbon dioxide (Linde),13C labelled methane (Sigma-Aldrich, 99 atom %13C).

[0097] Synthesis of monometallic and bimetallic AuPd / GaN-ci, AuPd / InN-ci, AuPd / InN+GaN- ci, AuPd / BN, and AuPd / InN was performed by a soft colloid-immobilization process. Briefly, specific amounts of HAuCL aqueous and Na2PdCU solutions were dissolved in 25 mL of deionized water. Polyvinylpyrrolidone (PVP), with average molecular weight 1 ,300,000, was added as a stabilizer to give the desired metal-to-PVP ratio (typically set as 1 :1.2). After 5 min of stirring, a freshly prepared 0.15 M sodium borohydride (NaBFL) solution was introduced and maintained a NaBFL-to-metal molar ratio of 5. This step produced a dark brown colloid after stirring for 30minutes. Subsequently, 100 mg of GaN, InN, BN, InP or a mixture of two of them (50 mg of each) was added to immobilize these single or bimetallic metal nanoparticles over another 30 minutes. The as-prepared catalysts were filtered, thoroughly washed with distilled water, and left to dry under vacuum at 100 °C overnight.

[0098] The UV-Vis absorption results confirm InN’s superior visible-light absorption compared to GaN (Fig. 15). The AuPd / InN-ci gave an even higher methanol yield-rate in GAG process than the AuPd / GaN interface. Interestingly, this improvement was further pronounced with AuPd / InN+GaN-ci catalyst, wherein a 1 -to-1 weight ratio of mechanical mixture of InN and GaN was used as photo-active support. The resulting productivity is beyond what AuPd / InN-ci could reach under visible light irradiation (Fig. 16 and Table 12). Overall, these results show the applicability of the present process in CH3OH photosynthesis from CP and CO2 across a wide solar spectrum.Table 12. The methanol yield achieved under visible-light driven condition for co-conversion of CH4 and CO2 (Reaction condition: 2.5 mmol of CP , 2.5 mmol of CO2 (total pressure of 1 bar), 1 mg of catalyst, Xenon lamp with a 435 nm wavelength of cut-off filter, 2 h. For run 5 a negative control, 2.5 mmol of N2 was replaced with CO2. For run 6 a second negative control, 2.5 mmol of N2 was replaced with CP )

[0099] For entries 7-12 of Table 12, the selectivity for ethanol was also measured and determined and presented in Table 13.Table 13. Selectivity for ethanol of entries 7-12 of Table 12.

[0100] In summary, a direct photocatalytic method for methanol synthesis using two major greenhouse gases of CH4 and CO2 over interfacial metal / semiconductor composed of AuPd alloy as cocatalyst and GaN, BN, InP, InN or their combination as photo-redox catalyst at roomtemperature was demonstrated. The intact interface between AuPd nanoparticle and the support allowed for simultaneously enhancing the CH4 dehydrogenation and CO2 reduction on the catalyst surface to undergo direct transformation into methanol rather than go through CO as an intermediate and indirect route.

Claims

WHAT IS CLAIMED IS:1 . A photocatalyst for the direct synthesis of methanol from methane and carbon dioxide, the photocatalyst comprising AuPd alloy nanoparticles dispersed on a support comprising GaN, InN, AIN, BN, TIN, GaP, InP or a combination thereof, wherein the AuPd alloy nanoparticles are uniformly dispersed on the support such that at least 90% of the AuPd alloy nanoparticles are positioned at a distance of from 1 nm to 15 nm from other AuPd alloy nanoparticles.

2. The photocatalyst of claim 1 , wherein at least 60% of the AuPd alloy nanoparticles have a diameter of from 1 to 5 nm.

3. The photocatalyst of claim 1 or 2, wherein at least 80% of the AuPd alloy nanoparticles have a diameter of from 2 to 4.5 nm.

4. The photocatalyst of any one of claims 1 to 3, wherein less than 10% of the AuPd alloy nanoparticles are in aggregates of two or more of the AuPd alloy nanoparticles.

5. The photocatalyst of any one of claims 1 to 4, wherein the AuPd alloy nanoparticles have an element ratio of AuPd of from 1 :3 to 3:1 .

6. The photocatalyst of claim 5, wherein the element ratio is of from 1 :1.25 to 1.25:1.

7. The photocatalyst of any one of claims 1 to 6, wherein the AuPd alloy nanoparticles are present in a concentration of from 0.75 to 4 % by weight.

8. The photocatalyst of any one of claims 1 to 7, wherein the photocatalyst has an X-ray photoelectron spectroscopy spectra comprising the following peaks: a Ga 3d peak between 19 and 21 eV, a N 1s peak between 395 and 398 eV, two Pd 3d peaks between 339 and 341 eV and between 333 and 335 eV, and two Au 4f peaks between 82 and 83 eV and between 86 and 87 eV.

9. A method of producing methanol directly from methane and carbon dioxide, the method comprising: contacting the methane and the carbon dioxide at a surface of the photocatalyst as defined in any one of claims 1 to 8; andexposing the surface of the photocatalyst with a light irradiation.

10. A method of producing methanol directly from methane and carbon dioxide, the method comprising: contacting the methane and the carbon dioxide at a surface of a photocatalyst, the photocatalyst comprising a transition metal dispersed on a support, and wherein the support is selected from GaN, BN, AIN, TIN, InN, InP, GaP and combinations thereof. exposing the surface of the photocatalyst with a light irradiation.

11. The method of claim 9 or 10, wherein the light irradiation is a full visible light spectrum irradiation.

12. The method of any one of claims 9 to 11 , wherein the method is performed at a temperature of from 15 to 30 °C.

13. The method of any one of claims 9 to 12, wherein the method is performed at pressure of 1 atm ±5%.

14. The method of any one of claims 9 to 13, wherein the methane and the carbon dioxide are provided in a molar ratio of from 1 :1 .25 to 1 .25:1 .

15. The method of any one of claims 9 to 14, wherein the method is a direct synthesis of methanol in which the carbon of the methane becomes the carbon of the methanol with the addition of the oxygen from the carbon dioxide.

16. The method of any one of claims 10 to 15, wherein the transition metal is selected from Au, Pd, Rh, Pt, Ru, Ir, Cu, Fe, Co, Ni, Bi, Sn, Cr, 2O3 and combinations thereof.

17. The method of claim 16, wherein the transition metal is AuPd nanoparticles, Cu, Co, Fe, Ni, Bi and Cr.

18. A method for making the catalyst of any one of claims 1 to 8, the method comprising: immobilizing Au and Pd colloids on the surface of the support to form the AuPd alloy nanoparticles; andetching the surface of the support with an acid or a base.

19. The method of claim 18, wherein the Au colloids are provided by a precursor selected from NaAuCU and HAuCU.

20. The method of claim 18 or 19, wherein the Pd colloids are provided by a NaPdCU precursor.

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