Single metal atom enabled photo-reforming of hydrogen vectors

Abstract

A process is provided using a single atom metal or semi-metal catalyst to photoreform abundant small molecule hydrogen vectors to produce hydrogen and a value-added by- product.

Description

SINGLE METAL ATOM ENABLED PHOTO-REFORMING OF HYDROGEN VECTORSField of Invention

[0001] The present invention relates to a process for photo-reforming one or more abundant small molecule hydrogen vectors which may be denoted as MHx where M = C, N, 0, in the presence of a light-absorbing single metal atom enabler. The process generates both hydrogen and a value-added carbon, nitrogen, and oxygen containing co-product. The invention also relates to hydrogen production methods and to catalysts useful in the production of hydrogen and such co-products. Specifically, the invention relates to a process for the photo-reforming of hydrogen vectors using single metal atom catalysts.Background of Invention

[0002] Hydrogen is a cornerstone in the transition from the use of fossil fuels to sustainable energy. Unlike fossil fuels, the combustion of hydrogen generates only water and heat, making hydrogen a highly attractive alternative energy source for achieving goals related to de-carbonization and de-fossilization. However, despite the potential benefits arising from a transition to hydrogen, there are significant challenges associated with the transportation and storage of hydrogen, primarily due to the difficultyin liquifying hydrogen, its high flammability and tendency to leak, and the consequential dangers of its forming explosive mixtures with air.

[0003] To address these issues, hydrogen vectors can be used as a means of storing, transporting and releasing hydrogen. Hydrogen vectors are stable compounds that can store and release hydrogen safely and conveniently. Examples of these vectors include methane (CH4), ethane (C2H6), ammonia (NH3), and water (H2O). These small molecules store hydrogen, and can release hydrogen on demand through catalytic reforming or dissociation processes. The methods used to convert these hydrogen vectors back into hydrogen generally involve thermochemical, electrochemical, or photochemical reactions, which require some form of, and different degrees of, energy input. The required energy can come from non-renewable sources like natural gas or coal, or from renewable sources such as solar or wind power. Regardless of the source, high temperatures and consequently high energy inputs are required for most conventional conversion processes, and, depending on the source and the process, carbon, CO and I or CO2 may be an inconvenient by-product if not properly recovered.

[0004] The present invention is, in one embodiment, a hydrogen production process using photocatalysis.

[0005] Catalysts are crucial for the photocatalytic production of hydrogen from hydrogen vectors. By definition, a catalyst is a substance that can accelerate a chemical reaction without being consumed. In photocatalysis, catalysts harvest and transform photons from light into charge carriers and / or heat, which then excite reactants on the surface-active center to trigger efficient and / or low-energy-barrier chemical transformations. In reported hydrogen production photocatalysis, catalysts arecomposed of a light absorber and a catalytic center attached to a solid support. Typically, the light absorber includes metal nanoparticles, which can strongly absorb photons via the plasmonic effect, such as Au, Cu, Ag. These catalysts help break down hydrogen vectors to release hydrogen gas and other co-products, such as carbon from methane, nitrogen from ammonia, and oxygen from water.

[0006] For example US10766024 discloses a multicomponent photocatalyst that includes a reactive component optically, electronically, or thermally coupled to a plasmonic material, and a method of performing a catalytic reaction that includes loading a multicomponent photocatalyst including a reactive component optically, electronically, or thermally coupled to a plasmonic material into a reaction chamber, introducing molecular reactants into the reaction chamber, and illuminating the reaction chamber with a light source. Plasmonic metals are metals that are incorporated into bulk metal structure which gives them the ability to facilitate plasmons. Single-metal atoms cannot have plasmons.

[0007] Despite their effectiveness, metal nanoparticle photocatalysts of the prior art have several drawbacks. For instance, plasmonic nanoparticles only expose the outmost surface atoms for reactions, leading to inefficiencies such as lower conversion or yield rates, and lower atomic efficiency when considering the number of active sites or atoms of metal used. Furthermore, plasmonic metals are usually expensive and high loading amounts, up to 20% or more, are usually required to achieve satisfactory rates. Additionally, the electronic properties of these metallic nanoparticles can result in less efficient catalytic activity due to collective electron effects that are not optimized for specific reactions.Summary of the Invention

[0008] Unlike metal nanoparticle catalysts, single-atom catalysts (SACs), generally consist of individual metal atoms dispersed on a solid support and offer several advantages over the traditional multi-atom catalysts used in the prior art. SACs permit the maximization of the atomic efficiency of each metal atom, significantly reducing the loading amount of the metal and the cost of the catalytic material and of the conversion process. The unique electronic properties of single metal atoms can be fine-tuned by coordination elements and numbers to optimize specific reactions, making them highly effective for targeted catalytic processes.

[0009] In the context of hydrogen production by the process of the present inventors, single-atom catalysts show great promise for photo-reforming hydrogen vectors by lowering the catalyst cost while maintaining high efficiency for specific reactions. Moreover, single-atom catalysts eliminate the possibility of plasmonic effects, which are collective oscillations of free electrons typically observed in metallic nanoparticles. While plasmonic effects can enhance certain catalytic reactions, they are not always beneficial and can introduce complexities that reduce overall efficiency. The absence of plasmonic effects in single-atom catalysts simplifies the reaction dynamics, resulting in more predictable and controllable catalytic behavior, which can be particularly useful in the photo-reforming of small molecule hydrogen vectors.

[0010] The development of single-atom catalysts for the photo-reforming of hydrogen vectors marks a significant advancement in hydrogen production technology. By addressing the limitations of traditional multi-atom catalysts, single-atom catalysts provide a more efficient, sustainable, and safer method for generating hydrogen. Thisinvention harnesses the unique properties of single metal atoms to enhance catalytic performance, offering an innovative approach to hydrogen production that aligns with global sustainability goals.

[0011] The invention provides advantages of the known prior art by using single metal atoms instead of metal particle catalysts, attached to a solid support to photoreform hydrogen vectors to hydrogen and a value-added co-product. The invention is characterized by the following. The process of the present invention includes the use of single metal atoms, attached to a solid support, in which the single atoms include transition-metal atoms and main-group atoms such as Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, lr, Pt, Au, Sn, Pb, Sb, Bi, N, P, F, and S.

[0012] Use of such single metal atoms and light allow these photo-reforming reactions to proceed enabling the conversion of the hydrogen vector to hydrogen and a value-added carbon, nitrogen, and oxygen co-product.

[0013] The use of the single metal atom catalysts in that enables the hydrogen vector photo-reforming reactions of the present invention instead of the known multiple metal atom particles of the prior art improves (i) atom efficiency, (ii) orbital efficiency, (iii) surface area efficiency, and (iv) cost efficiency.

[0014] Moreover, the localized variance electrons of single metal atoms attached to a solid support does not allow collective electron plasmonic resonances, which are only allowed for with the delocalized electrons found in multiple metal atom particles described in prior art. Therefore, the single metal enablers described herein on a solid support are distinct to all known plasmonic multiple metal atom particles and distinct in their utilization for photo-reforming of hydrogen vectors.

[0015] The photoactivity of these single metal atom enablers is believed to derive from excited state electronic effects associated with localized electrons in single metal atoms. This is distinct from the collective multi-electron effects of delocalized electrons in plasmonic multiple metal atom particles which are utilized for photo-reforming of hydrogen vectors in the prior art.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic representation of the single-atom catalyst enabled photoreforming hydrogen vectors. The single-atom catalyst composed of a single atom located on a solid support, and the described hydrogen vectors include CH4, NH3, H2O, and C2H6.

[0017] Figure 2 shows scanning transmission electron microscope image of a Rui / CARBON1 catalyst, confirming Ru atoms (bright dots) dispersed on the carbon support.

[0018] Figure 3 shows on-stream H2 generation rates over single metal atom (Rui) and nanoparticle (RuNP) carbon supported catalysts under light intensities from 0-48 suns under a 2 seem NH3 flow. Figure 3 confirms that single atom catalysts are highly efficient and stable for photoreforming ammonia into H2. The light intensity was increased from zero (dark) to 48 suns (light) and then gradually decreased to 39, 31 , 26, 21 and 17 suns in a flow catalytic reactor.

[0019] Figure 4 shows comparative H2 generation rates over Rui and nanoparticle (RuNP) carbon supported catalysts under steady state. Figure 4 confirms that single atom catalysts have greater atomic efficiency than nanoparticle (RuNP) catalysts.

[0020] Figure 5 shows on-stream H2 generation rates over single metal atom (Fei) carbon supported catalysts under light intensities from 0-48 suns under a 2 seem NH3 flow. Figure 5 confirms that single iron atoms are highly efficient and stable for photoreforming ammoina into H2. The light intensity was increased from zero (dark) to 48 suns (light) and then gradually decreased to 39, 31 , 26, 21 and 17 suns in a flow catalytic reactor.

[0021] Figure 6 shows the photocatalytic ethane conversion to ethylene production rates over single metal atom (Rui) and particles (RuNP) supported carbon catalysts under light intensity of 48 suns with the flow rate of 2.2 seem.DETAILED DESCRIPTION OF THE INVENTION

[0022] According to the present invention, a process for photo-reforming one or more small molecule hydrogen vectors is achieved by exposing such vectors to a light source in the presence of light-absorbing single metallic or semi-metallic atom catalyst dispersed on a solid support.

[0023] The single metal or semi-metallic atom may be one of the metallic or semimetallic elements of the periodic table. In some embodiments, the single metallic or semimetallic atom catalyst is selected from the group consisting of Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, Au, Sn, Pb, Sb, Bi, N, P, F, S and B. The single metallic or semi-metallic atom catalyst may be a metal. In some embodiments, the single metallic or semi-metallic atom catalyst is a selected from the group consisting of Ru and Fe.

[0024] The solid support may be an insulator or semiconductor. In some embodiments, the solid support may be mixture or a doped form of a metal oxide, nitride, sulfide, phosphide, boride, silicide, carbide, silicon or carbon.

[0025] The single-atom metal or semi-metallic catalyst is prepared by dispersing single atoms of the selected metal or semi-metallic catalyst on the solid support. The catalyst bearing solid support so prepared is used in a reactor into which a small molecule hydrogen vector is introduced. In some embodiments, the small molecule hydrogen vector may be selected from the group consisting of organic alkane, linear or cyclic, alcohol or ether. In other embodiments, the small molecule hydrogen vector may be selected from the group consisting of ethane, hydrogen sulfide, methane, ammonia and water. In further embodiments, the small molecule hydrogen vector is selected from the group consisting of ethane, methane, ammonia and water. In further embedments, the one or more small molecule hydrogen vectors comprises a mixture of one or more of ethane, methane, ammonia and water.

[0026] The surface of the solid support is irradiated with light having an intensity ranging from about 10 to about 50 suns, which may be supplied by sunlight, by a light emitting diode source or by other artificial light sources. In some embodiments, the light intensity may also range from about 1 to about 50 W cm-1 , or from about 10 to about 50 W cm-1 . The intensity of the light may be adjusted optically or electronically to achieve the desired results. The wavelength of the light can range from ultraviolet, through the visible spectrum and into the infrared spectrum.

[0027] The resulting gas flow may be separated by conventional means to separate the hydrogen gas from the co-products of the photo-reforming process.

[0028] In some embodiments, the photo-reforming of the small molecule hydrogen vector causes said hydrogen vector to dissociate into separable gaseous hydrogen and a co-product of the photo-reforming process.

[0029] The process of the present invention may be more easily understood by reference to the following specific embodiments.Synthesis of a Single-Atom Catalyst

[0030] Synthesis of the single-atom catalyst, for example, Rui / CARBON can be carried out using the carbon obtained, for example, by pyrolysis of biomass at temperatures between 500-1800 K with base chemicals under inert atmosphere. The obtained carbon is then washed and dried under 273-473 K, the resulting product termed CARBON1. By mixing CARBON1 with base chemicals under inert atmosphere for pyrolysis again, the obtained carbon was termed CARBON2. To prepare the catalyst sample, 300 mg of carbon samples were dispersed in a 1 % weight pecent tris(acetylacetonato)Ru(lll) solution, with stirring for 24 h, followed by washing with water several times and drying and calcination above 400 °C. The obtained catalysts were identified as Rui / CARBON1 and Rui / CARBON2. Scanning transmission electron microscope imaging confirmed, as shown in Figure 2, that the Ru atoms were atomically dispersed on the carbon support at nanoscale. This is distinct from the reported nanoparticle catalysts used in the prior art.

[0031] Testing of the catalysts so prepared was carried out in a laboratory scale reactor, which may have stainless steel reactor body with one or more quartz glass windows disposed on its circumference, and valves controlling the inflow and outflow of gas to and from the reactor body. The quartz glass windows in the reactor allow light from a Xe lamp, simulating solar light, to irradiate the surface of the carbon support on which the single atom catalyst is dispersed, within the reactor and to promote the photoreforming of the hydrogen vector. The laboratory reactor could be connected to a temperature thermocouple and pressure gauge. In one embodiment, the reactor was used with a Perfect Light™ solar simulator 300WXe lamp. A Newport™ power meter with an 18 mm detector spot diameter could be used. Manually varying the power to the Xe lamp yielded varying light intensities. Adjustment could further vary the diameter of the light spot using focusing lenses for example, light intensities to achieve the intensities used.

[0032] The carbons prepared, such as CARBON 1 and CARBON2, may be set up by placing them into the bed of the reactor.Photo-Reforming of Ammonia

[0033] According to one embodiment of the invention, single Ru atom (Rui) on carbon (CARBON1 and CARBON2) can be used to reform ammonia to hydrogen under ambient pressure at ultra-high conversion rates of 9—5000 mmol H2 / gRu / hour.

[0034] According to this process, the activities of catalysts Rui -CARBON 1 , and Rui- CARBON2 were assessed by placing known quantities of the catalysts in the reactor and flowing a gaseous fluid including ammonia through the reactor at a flow rate of 2 seem under ambient pressures and at varying light intensities for repeated runs. Thelight intensity was increased from zero (dark) to 48 suns, (light) and then gradually decreased to 39, 31 , 26, 21 and 17 suns in the reactor. The process was repeated with catalysts of Ru nanoparticles through the same light intensity range carbon without metal was used as a control.

[0035] The conversion rate as shown in Figure 3 increases with the light intensity in the range of 13—48 suns (kW / m2). Notably, activities of CARBON1 supported singleatom Ru catalysts outperformed those of Ru nanoparticle (RuNP) catalysts in the whole light intensity range and were found to be 5—10 times higher.

[0036] In comparison, activities of CARBON2 supported single-atom Ru catalysts were 2—3 times that of Ru nanoparticle (RuNP) catalysts under 39—48 suns and 17—26 suns, respectively, while Ru nanoparticle catalysts slightly outperformed the single-atom catalysts under intermediate light intensities of 26—31 suns (Figure 4).

[0037] Furthermore, single Ru metal atom catalysts showed better durability, i.e. were better able to withstand the higher light intensities than the nanoparticle catalysts as observed over both CARBON 1 and CARBON2 carbon supported Ru catalysts. To amplify this, under 48 suns RuNP-carbon nanoparticle catalysts became deactivated as the reaction proceeded, while Rui -carbon single atom catalysts showed gradually increased activity that stabilized within 1.5 hours. The highest ammonia conversion yield was around 15%, observed on CARBON2 supported Ru nanoparticle when irradiated under 48 suns. However, the single metal atom catalyst had a much higher atom efficiency than the nanoparticle analogue. As a consequence, the single metal atom catalysts may be scaled and manufactured at much lower cost.Fe Single Atom Catalyst with Ammonia

[0038] The efficacy of other transition-metal atoms and main-group atoms can be demonstrated by the use of iron (Fe) active site. Two samples, Fei-CARBON1 and Fei- CARBON2, were prepared using the method described above for the preparation of the Ru catalyst, except for replacing the tris(acetylacetonato)Ru(lll) with tris(acetylacetonato)Fe(lll).

[0039] Under 48 suns and an ammonia flow rate of 2 seem, hydrogen generation activities of Fei -CARBON 1 and Fei-CARBON2 were both around 45 mmol Fh / gcataiyst / h, which corresponded to 9000 mmol H2 / gFe / h. Under attenuated light intensities, the activity decreased to below 2000, 120, 20, 20, and 20 mmol H2 / gFe / h at 39, 31 , 26, 21 , and 17 suns, respectively. The results, shown in Figure 5, confirm that a single atom iron catalyst is both efficient and stable for reforming ammonia into H2.

[0040] Single metal atoms can enable similar photoreforming reactions for methane and water. The gaseous products in the case of ammonia (nitrogen and hydrogen) and water (hydrogen and oxygen) photoreforming can be separated by a permselective membrane integrated into the photoreactor or by other methods known to the person skilled in the art. As disclosed above, the distinctive feature of single metal atom catalysts compared to nanoparticle metal catalysts is that the plasmonic collective electron character of the latter is not possible in the localized electron properties of the former.Dehydrogenation of Ethane to Ethylene

[0041] Single Ru atom catalyst (Rui ) on a carbon support can be used dehydrogenate ethane to ethylene at ambient pressure under LED lights. Notably, as shown in Figure 6, activities of ethane to ethylene over carbon supported single-atom Ru catalysts at an intensity of 48 suns and a flow rate of 2.2 seem outperformed those of Ru nanoparticle (RuNP) catalysts and showed a 3 fold activity increase. Activity stability was also higher in the single-atom catalyst. Single-atom Ru catalyst showed an increase followed by a decrease in the ethylene production. However, Ru nanoparticle (RuNP) catalyst showed consistently decreasing activity.

Claims

CLAIMS1 . A process for photo-reforming one or more small molecule hydrogen vectors comprising exposing such vectors to a light source in the presence of lightabsorbing single metallic or semi-metallic atom catalyst dispersed on a solid support.

2. The process of claim 1 wherein the small molecule hydrogen vector is selected from the group consisting of organic alkane, linear or cyclic, alcohol or ether.

3. The process of claim 1 wherein the small molecule hydrogen vector is selected from the group consisting of ethane, hydrogen sulfide, methane, ammonia and water.

4. The process of claim 1 wherein the small molecule hydrogen vector is selected from the group consisting of ethane, methane, ammonia and water.

5. The process of claim 1 wherein the one or more small molecule hydrogen vectors comprises a mixture of one or more of ethane, methane, ammonia and water.

6. The process of any one of claims 1 - 3 wherein the photo-reforming of the small molecule hydrogen vector causes said hydrogen vector to dissociate into separable gaseous hydrogen and a co-product.

7. The process of any of one of claims 1 - 3 wherein the single metal or semimetallic atom of Claim 1 can be one of the metallic or semi-metallic elements of the periodic table.

8. The process of claim 1 wherein the single metallic or semi-metallic atom catalyst is selected from the group consisting of Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, Au, Sn, Pb, Sb, Bi, N, P, F, S and B.

9. The process of claim 1 wherein the single metallic or semi-metallic atom catalyst is a metal.

10. The process of claim 1 wherein the single metallic or semi-metallic atom catalyst is a selected from the group consisting of Ru and Fe.

11. The process of any one of claims 1 - 3 wherein the photo-reforming of the small molecule hydrogen vector is carried out under a light intensity ranging from about 1 to about 50 W cm-1.

12. The process of any one of claims 1 - 3 wherein the photo-reforming of the small molecule hydrogen vector is carried out under a light intensity ranging from about 10 to about 50 W cm-1.

13. The process of any one of claims 1 - 3 wherein the light is provided by sunlight, light emitting diodes or by other artificial light sources.

14. The process of any one of claims 1 - 3 wherein the wavelength of the light can range from ultraviolet, through the visible spectrum and into the infrared spectrum.

15. The process of any one of claims 1 - 3 wherein the solid support can be an insulator or semiconductor.

16. The process of any one of claims 1 - 3 wherein the solid support can be mixture or a doped form of a metal oxide, nitride, sulfide, phosphide, boride, silicide, carbide, silicon or carbon.

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