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Title:
METHOD FOR CARBON DIOXIDE METHANATION USING RH PLASMONIC PHOTOCATALYST
Document Type and Number:
WIPO Patent Application WO/2018/140326
Kind Code:
A2
Abstract:
A method for hydrogenating carbon dioxide to methane comprises providing a rhodium photocatalyst having rhodium nanoparticles on a metal oxide support, conditioning the rhodium photocatalyst, exposing the rhodium photocatalyst to hydrogen gas and carbon dioxide gas, and illuminating the rhodium photocatalyst with blue light or ultraviolet light. In some embodiments, the rhodium nanoparticles are in the form of nano-cubes and the intensity of illumination is sufficient to cause the reaction rate of methane to have a super-linear dependence on illumination intensity,

Inventors:
LIU JIE (US)
YANG WEITAO (US)
ZHANG XIAO (US)
LI LUCY XUEQIAN (US)
Application Number:
PCT/US2018/014545
Publication Date:
August 02, 2018
Filing Date:
January 19, 2018
Export Citation:
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Assignee:
UNIV DUKE (US)
International Classes:
C07C1/12
Attorney, Agent or Firm:
BREYER, Wayne S. (LLP100 Matawan Road, Suite 12, Matawan NJ, US)
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Claims:
What is claimed:

1. A method for hydrogenating carbon dioxide to methane, comprising:

providing a rhodium photocatalyst;

conditioning the rhodium photocatalyst;

exposing the rhodium photocatalyst to hydrogen gas and carbon dioxide gas; and illuminating the rhodium photocatalyst with blue light or ultraviolet light.

2. The method of claim 1 wherein an intensity of the blue light or the ultraviolet light is sufficient for causing a reaction rate for the production of methane to have a super-linear dependence on the intensity.

3. The method of claim 1 wherein illuminating an intensity of the blue light or ultraviolet light is in a range of about 1 to 10 Watts per square centimeter.

4. The method of any of claims 1 through 3 wherein a wavelength of the blue light is not longer than 460 nanometers.

5. The method of any of claims 1 through 4 wherein a wavelength of the ultraviolet light is not shorter than 350 nanometers.

6. The method of any of claims 1 through 5 wherein the rhodium photocatalyst comprises rhodium nanoparticles.

7. The method of any of claims 1 through 6 wherein the rhodium nanoparticles are in a form of rhodium na no-cubes.

8. The method of claims 1 through 7 wherein the rhodium nano-cubes have an edge length in a range of about 1 nanometers to 100 nanometers.

9. The method of any of claims 1 through 7 wherein the rhodium nano-cubes have an edge length in a range of about 30 nanometers to about 40 nanometers.

10. The method of any of claims 1 through 7 wherein the rhodium nano-cubes have an edge length in a range of about 45 nanometers to about 55 nanometers.

11. The method of any of claims 1 through 10 wherein providing a rhodium photocatalyst further comprises forming the rhodium nano-cubes via a slow-injection polyol method.

12. The method of any of claims 1-7 wherein providing a rhodium photocatalyst further comprises:

determining a preferred edge length, or preferred range of edge length, for rhodium nano-cubes based on the wavelength of the ultraviolet or blue light that is illuminating the rhodium photocatalyst; and

forming the rhodium nano-cubes to have the preferred length or a length in the preferred range of length.

13. The method of claim 12 wherein forming the rhodium nano-cubes further comprises forming the rhodium nano-cubes via a slow-injection polyol method.

14. The method of any of claims 1 through 13 wherein rhodium nano-cube precursor is injected into a potassium bromide solution.

15. The method of any of claims 1 through 14 wherein the rhodium photocatalyst comprises a metal oxide for supporting the rhodium nanoparticles.

16. The method of any of claims 1 through 15 wherein the metal oxide is selected from the group consisting of aluminum oxide powder and titanium oxide powder.

17. The method of any of claims 1 through 16 wherein the rhodium nanoparticles have a mass loading in the range of about 0.5 to about 2 percent by weight on the metal oxide.

18. The method of any of claims 1 through 17 wherein conditioning the rhodium catalyst further comprises reducing the catalyst and activating the catalyst.

19. The method of any of claims 1 through 18 wherein the method is conducted at a temperature in a range of about 25 °C to about 450 °C.

Description:
METHOD FOR CARBON DIOXIDE M ETHAN ΑΤΊΟΝ

USING RH PLASMONIC PHOTOCATALYST

Cross-Reference to Related Application

[oooi] This application claims the benefit of U.S. Provisional Application Serial Number 62/448,645, filed January 20, 2017, and which is incorporated by reference herein.

Statement Regardlng Federally-Sponsored Research

[0002] This invention was made with Government support under NSF grant CHE- 1565657 and U.S. Army Research Office Award W911NF-15-1-0320. The Government has certain rights in the invention.

Field of the Invention

[0003] The present invention relates to a process and catalyst for methanation of carbon dioxide.

Background of the Invention

[0004] Fossil fuel remains the predominate source of the world's energy.

Producing energy via fossil fuel generates carbon dioxide, much of which finds its way to the atmosphere. In addition to increasing atmospheric carbon dioxide levels, it represents a waste of a large amount of carbon.

[0005] Carbon-dioxide emissions can be captured and converted to

hydrocarbons, such as for use as fuels or other end uses. For example, carbon dioxide can be hydrogenated to form methane. This methanation reaction, also known as the Sabatier reaction, has been known for many years. The reaction is shown below:

[0006] The hydrogenation of carbon dioxide can proceed via two competing pathways. One is via the methanation reaction [1], shown above. The reaction can also proceed via the so-called "reverse water gas shift" reaction:

[0007] To increase the reaction rate of the methanation reaction, a catalyst is used. Nickel- and Ruthenium-based catalysts are highly selective for methanation. Relatively less-reactive metal catalysts, such as palladium, platinum, rhodium, molybdenum, rhenium, and gold, tend to produce methane, carbon monoxide, and methanol. Other metal catalysts, such as copper and silver, tend to produced methanol. [0008] A photocatalyst is a substance that can modify the rate of chemical reaction when exposed to light. Semiconductor-based photocatalysts offer a promising route to room-temperature reactions, but they exhibit limited selectivity and reaction rates that typically scale only as the square root of the light intensity (Rphoto « J 0 - 5 ), making it impractical to increase the reaction rate by increasing light intensity.

[0009] Recently, it has been discovered that plasmonic metal nanoparticles are photocatalytically active, driving chemical reactions with photo-generated hot carriers and exhibiting a super-linear dependence on light intensity (flphoto « J", n> l).

Plasmonic metal nanoparticles are characterized by strong light absorption through excitations of collective free electron oscillations, called localized surface plasmon resonances (LSPRs) that may be spectrally tuned throughout the visible or ultraviolet by choice of metal, size, shape and host medium. The plasmonic effect is a

phenomenon that is specific to nano-sized metal particles; however, not all metal nanoparticles are plasmonic. The most common plasmonic nanoparticles are gold, silver, copper, and, as has recently been discovered, rhodium, due to their high electron mobility and other properties.

[0010] In the prior art, plasmonic photocata lysis has featured intense laser pulses (~kW/sq cm) on nanoparticle clusters to generate high concentrations of hot carriers, or they used alloyed or hybrid nanostructures composed of plasmonic (gold, silver, aluminum) and catalytic (platinum, cobalt, palladium) transition metals. An ideal photocatalyst would be a single metal that simultaneously exhibits good plasmonic and catalytic behaviors to increase the rates and selectivity of the reaction.

[0011] The art would benefit from a photocatalyst for carbon-dioxide methanation that can simultaneously lower reaction temperature and increase reaction rates while being highly selective for methane.

Summary of the Invention

[0012] In accordance with the present teachings, supported rhodium

nanoparticles are used as a highly selective and active photocatalyst for methanation (conversion of COxto methane via hydrogenation). In an illustrative embodiment, the rhodium photocatalyst comprises rhodium na no-cubes impregnated on aluminum oxide (AI2O3) nanoparticles.

[0013] Carbon-dioxide hydrogenation on transition metals at atmospheric pressure can proceed via the two competing pathways mentioned above: reaction [1], CO2 methanation and reaction [2], reverse water gas shift The Inventors discovered, surprisingly, that mild illumination of Rh nanoparticles not only reduces activation energies for carbon dioxide hydrogenation by about 35% below thermal activation energies, it also produces a strong selectivity towards methane over carbon monoxide {i.e., reaction [1] >> reaction [2]).

[0014] In a method for methanation in accordance with the present teachings, the rhodium photocatalyst is exposed to low intensity {i.e., about 1 W/sq. cm.) blue or ultraviolet light at atmospheric pressure and in the presence of hydrogen and carbon dioxide. Under illumination from continuous-wave blue light-emitting diodes (LEDs), the photocata lytic reactions on unheated rhodium nanoparticles generated

methane with selectivity of greater than 86%. Under illumination from continuous- wave ultraviolet LEDs, the photocata lytic reactions on unheated rhodium nanoparticles produced methane with selectivity of greater than 98%.

[0015] This result was quite surprising. First, the high selectivity towards methane is not observed when the rhodium photocatalyst is not illuminated. By way of comparison, for the rhodium photocatalyst, the reaction rate for methane under low intensity UV illumination and at ambient temperature was about 2x that of the

thermocata lytic (dark) reaction rate for methane production at 275 °C. And whereas the selectivity for methane production was well below 50% at 275 °C for the

thermocata lytic reaction, it approached 100% for UV illumination at ambient

temperature. Secondly, the result for rhodium photocatalyst was in marked contrast to that for gold catalyst comprising plasmonic gold nanoparticles. The gold

photocatalyst only catalyzes carbon monoxide production, whether it is illuminated or not.

[0016] It is believed, based on density functional theory (DFT) calculations, that the photo-selectivity of the Rh photocatalyst is attributable to the alignment of the hot electron distribution with the anti-bonding orbital of the critical reaction intermediate, CHO, which activates the CO2 methanation pathway.

[0017] Some embodiments of the invention provide a method for hydrogenating carbon dioxide to methane, comprising providing a rhodium photocatalyst; conditioning the rhodium photocatalyst; exposing the rhodium photocatalyst to hydrogen and carbon dioxide gas; and illuminating the rhodium photocatalyst with blue light or ultraviolet light.

Brief Description of the Drawings

[0018] FIG. 1 depicts TEM images of rhodium photocatalyst in accordance with an illustrative embodiment of the invention.

[0019] FIG. 2 depicts a system for methanation in accordance with an illustrative embodiment of the invention. [0020] FIG. 3A depicts rates of carbon monoxide and methane production for rhodium photocatalyst and bare aluminum oxide support.

[0021] FIG. 3B depicts comparative data, showing rates of carbon monoxide and methane production for gold photocatalyst.

[0022] FXGs. 4A-4D depict product selectivity and reactions rates for rhodium photocatalyst.

[0023] FIG. 5A depicts thermocata lytic reaction rates for rhodium photocatalyst as a function of temperature.

[0024] FIG. 5B depicts photo-reaction rates for rhodium photocatalyst as a function of illumination intensity and temperature.

[0025] FIG. 5C depicts comparative data, showing thermocata lytic reaction rates for gold photocatalyst as a function of temperature.

[0026] FIG. 5D depicts comparative data, showing photoreaction rates for gold photocatalyst as function of illumination intensity and temperature.

[0027] FIG. 6 depicts optimal light (absorption) wavelength as a function of edge length for rhodium photocatalyst.

[0028] FIG. 7 depicts a method for methanation in accordance with the present invention.

[0029] FIG. 8 depicts a method for performing one of the operations of the method of FIG. 7.

[0030] FIG. 9 depicts a method for performing another one of the operations of method of FIG. 7.

Detailed Description

[0031] Catalyst and Catalyst Preparation.

[0032] FIG. 1 depicts TEM images of rhodium photocatalyst in accordance with an illustrative embodiment of the invention. In some embodiments, the catalyst comprises rhodium (Rh) nanoparticles on an alumina support.

[0033] In the illustrative embodiment, the Rh nanoparticles are in the form of nano-cubes (see FIG. 1). The nano-cube form was selected, among other geometries {e.g., tripod, tetrapod, icosahedra, etc.), due to its sharp corners. The sharp corners concentrate electromagnetic fields and liberate hot carriers, the latter of which play a role in product selectivity. [0034] In some embodiments, the rhodium nano-cubes are produced via a modified slow-injection polyol method, such as described in Zhang, X. et al. Size- tunable rhodium na restructures for wavelength-tunable ultraviolet

plasmonics. Nanoscale Horiz. 1, 75-80 (2016), which is incorporated herein by reference. This method has been found to be useful for synthesizing size-tunable, monodisperse Rh nano-cubes with wavelength-tunable local surface plasmon resonances (LSPRs). As discussed later in further detail, the ability to produce Rh nano-cubes having a specific size is very important for embodiments of the invention.

[0035] The slow-injection polyol method uses a two-channel syringe pump to control the addition rate of Rh precursor. Two variants of the method can be used; unseeded and seeded. The latter uses rhodium nano-cubes as "seeds" to start the synthesis.

[0036] For the unseeded slow-injection method, a mixture of potassium bromide and ethylene glycol is heated to a temperature of about 160 °C. Rhodium(III) chloride hydrate (RhCL 3 -xH2 O, 38% Rh) and polyvinylpyrrolidone (the molecular mass of the repeating unit « 55,000) are dissolved, in two separate vials, into ethylene glycol at room temperature. These two solutions are injected simultaneously into the heated potassium-bromide solution by the two-channel syringe pump. After injection, the mixture is held at 160 °C for a brief period of time and then cooled to room temperature. The size of the resulting rhodium nano-cubes are a function of the amount of Rh precursor that is present. Reducing the amount of Rh precursor (while keeping all other conditions identical) results in relatively smaller Rh nano-cubes.

[0037] For the seeded slow-injection method, a seed solution of rhodium nano- cubes is diluted with ethylene glycol and is heated to 160 °C. Rhodium(III) chloride hydrate is dissolved in ethylene glycol and, separately, PVP and potassium bromide were co-dissolved in a different vial of ethylene glycol. These two solutions were injected into the seed solution simultaneously by the syringe pump. After injection, the mixture is held at 160 °C for a brief period and then cooled to room temperature. Rhodium nano- cubes with an edge length between 27 and 59 nm can be obtained by adjusting the ratio of Rh precursor and seeds. High reproducibility is achievable for seeded syntheses when the atomic ratio of rhodium between added Rh precursor and Rh seeds is below 3.5. This atomic ratio corresponds to a size enlargement of about 1.6 for each round of seeded synthesis. That is, 20 nm rhodium na no-cube seeds grow into 32 nm rhodium nano-cubes. Rhodium nano-cubes having an edge length of up to about 120 nm and a narrow size distribution can be synthesized by applying multiple rounds of seeded syntheses. That is, the product (rhodium nano-cubes) of a previous synthesis becomes the seed for a subsequent synthesis, and so forth. [0038] Rhodium nano-cubes, as opposed to any other geometry, are formed due to the presence of bromide during the synthesis. It is believed that the bromide attaches to a specific surface of the rhodium particles, resulting in the formation of cubes. Other factors affecting the size and shape of the rhodium nanoparticles are the reaction temperature, concentration of precursors, and reaction time.

[0039] The size and intrinsic properties of the metal—rhodium for embodiments of the invention- determine the frequency/wavelength of the light that can form localized surface plasmonic resonance on metal nanoparticles. Consequently, the optimal frequency/wavelength for exciting a plasmonic nanoparticle depends on its size.

[0040] FIG. 6 depicts the relationship between the edge length of rhodium nano- cubes and the optimal absorption wavelength. Typical rhodium catalysts have exceedingly small nanoparticles, mostly less than 1 nm. For such small nanoparticles, the optimal absorption frequency/wavelength corresponds to the deep UV region of the spectrum (wavelength < 350 nm). Indeed, extrapolating from FIG. 6, the optimal absorption wavelength for rhodium nanoparticles having a size less than 1 nm will be 200 nm or less. In fact, for conventional rhodium catalyst, which has nanoparticles that are not nano-cubes, the optimal absorption wavelength is likely to be even lower. Although existing light sources having a wavelength less than 350 nm are available, they are very expensive. So, although conventional rhodium catalysts are indeed plasmonic, a light source that can be used to excite them to any substantial degree is too expensive, at least for commercial uses.

[0041] Rhodium photocatalyst comprising rhodium nano-cubes having a size {i.e., edge length) in the range from about 1 nm to about 100 nm can, in principle, be used to hydrogenate carbon dioxide to methane in accordance with embodiments of the invention since the UV spectrum extends from about 10 nm to about 400 nm (see, FIG. 6). However, for a UV light source (LED) having a wavelength of 350 nm, an edge length for the rhodium nano-cubes in the range of 30-40 nm is near optimal. For a UV light source having a wavelength of 365 nm, an edge length for the rhodium nano-cubes in the range of about 45-55 nm is near optimal.

[0042] Rhodium photocatalyst having a mass loading of metal (Rh nano-cubes) on support in the range of about 0.1% to about 20% {i.e., mass loading= 100 x mass of metal/mass of support). More preferably, the mass loading of Rh nano-cubes is in the range of 0.5% to 2%. This latter range provides a desirably dilute dispersion of the catalyst on the substrate to avoid metal aggregation, yet is dense enough to avoid wasting supporting materials. The mass loading will primarily affect the reaction rate per unit of catalyst. It has minimal or no effect on the selectivity of the reaction. [0043] The alumina {i.e., aluminum oxide: AI2O3) support, is in the form of nano- particles. In some other embodiments, other supports are used, such as titanium dioxide, silicon dioxide, cerium oxide, and other metal oxides, etc. In light of this disclosure, those skilled in the art will be able to select a support for the rhodium nanoparticles.

[0044] In some embodiments, the photocatalyst is formed by an impregnation method. For the impregnation, a solution of the rhodium na no-cubes is formed, such as using ethanol, which is mixed with a suspension of the support material. The resulting solid (i.e., rhodium nano-cubes on support) is then calcined.

[0045] Catalyst Pretreatment. Before using the rhodium photocatalyst for methanation, the catalyst is advantageously conditioned. In a first conditioning operation, the rhodium photocatalyst is reduced by exposing it to hydrogen gas, either alone or in combination with an inert gas, such as argon, nitrogen, etc. In some embodiments, the rhodium photocatalyst is reduced by exposing it to hydrogen gas and argon at 350 °C and at atmospheric pressure for 4 hours. In a second optional conditioning operation, the catalyst is "activated," wherein the methanation reaction is conducted for a period time until stable catalytic activity is observed. In some embodiments, activation is conducted by exposing the reduced rhodium photocatalyst to light having an appropriate wavelength {i.e., blue, ultraviolet) and a hydrogen/carbon dioxide atmosphere at 350 °C and atmospheric pressure for 12 hours. For the purposes of testing, as described further below, in some embodiments, the hydrogen/carbon dioxide atmosphere is hydrogen-rich and in some other embodiments, the atmosphere is hydrogen-deficient, relative to the 1:4 C02:H2 Stoichiomety of COz methanation.

[0046] Methanation Reaction. In accordance with the illustrative embodiment, reactants -hydrogen gas and carbon dioxide gas- are fed to a catalyst bed containing conditioned rhodium photocatalyst and illuminated by ultraviolet or (the shortest wavelengths of) blue light. The wavelength of the light is advantageously less than about 460 nm, because longer wavelengths reduce the selectivity of methane production. In some embodiments, the intensity of the illumination is at least about 1 watt per square centimeter when the reaction is conducted in hydrogen-rich conditions. As discussed further below, at this intensity and above (for hydrogen-rich conditions), the photoreaction rate advantageously transitions from a linear to a "super-linear" dependence on light intensity.

[0047] Testing and Comparative Testing. Rhodium photocatalyst was prepared in accordance with the slow-injection polyol method discussed above, and as detailed further below. For comparison purposes, a gold photocatalyst was prepared, as discussed further below. The rhodium and gold photocatalyst, as well as support material without rhodium, were tested in a reactor under a variety of temperature and illumination conditions to evaluate the reaction rate and selectivity of the photocatalysts for the methanation reaction. FIGs. 3A-3B, 4A-4D, and 5A-5D and the accompanying discussion provide the results and implications of the comparative testing.

[0048] Preparation of Rhodium Photocatalvst

[0049] A. Unseeded Method. 0.45 mmol potassium bromide (KBr, Acros, 54 mg) and 2 ml ethylene glycol (EG, J. T. Baker) were added to a 20 ml glass vial (cleaned with detergent and deionized water and dried in a vacuum oven at 80 °C) equipped with a Teflon-coated magnetic stir bar. The colorless solution was heated in an oil bath held at 160 °C for 40 min with a loosely placed cap. Separately, 0.045 mmol rhodium(III) chloride hydrate 38% Rh, Acres, 12 mg) and 0.225 mmol

polyvinylpyrrolidone (the molecular mass of the repeating unit « 55,000, Aldrich, 25 mg) were dissolved in 2 ml EG at room temperature. These two solutions were injected simultaneously into the heated KBr solution by a two-channel syringe pump at a rate of 1 ml/h. After injection, the reaction mixture was held at 160 °C for another 10 min and then cooled to room temperature. The suspension was washed with deionized water/acetone until no CI- and Br- was detected in the supernatant. Rhodium na no- cubes have an edge length of 37 nm were formed using the specified amount of Rh precursor. The solid was dispersed in 20 ml ethanol and impregnated on 90 mg AI2O3 nanoparticles (Degussa, Alu Oxide C, specific surface area 85-115 m 2 /g). The obtained solid was ground into powder and calcined in air at 400 °C for 2 h. The rhodium mass loading (AU/AI2O3) was 1.02%

[0050] B. Seeded Method. 0.4 ml of the reaction mixture formed via the unseeded method (i.e., 37 nm rhodium na no-cubes) were used as the seed solution. The seed solution was diluted with 1.6 ml EG in a cleaned 20 ml glass vial with stirring, and heated in an oil bath that was held at 160 °C for 40 min with a loosely placed cap. 0.045 mmol was dissolved in 2 ml EG. Separately, 0.225 mmol

polyvinylpyrrolidone (25 mg) and 0.45 mmol KBr (54 mg) were co-dissolved in another 2 ml EG. These two solutions were injected into the seed solution simultaneously by the syringe pump at a rate of 1 ml/h. The reaction mixture was held at the reaction temperature (160 °C) for another 10 min and then cooled to room temperature. This resulted in the formation of rhodium nano-cubes having an edge length of 59 nm. The solid was dispersed in 20 ml ethanol and impregnated on 90 mg AI2O3 nanoparticles. The obtained solid was ground into powder and calcined in air at 400 °C for 2 h. The rhodium mass loading (AU/AI2O3) was 1.02%. [0051] Preparation of Gold Photocatalvst

[0052] A deposition-precipitation method was used to prepare highly dispersed small gold nanoparticles on alumina support. The gold nanoparticles had a spherical shape and an average diameter of 2.6 nm. Overall, 100 mg AI2O3 nanoparticles were dispersed in 10 ml deionized water in a 20 mL glass vial by sonication. A total of 16 mg gold(III) chloride trihydrate (HAuCU-xHzO, 99.9+%, Aldrich) was added to the suspension and stirred in an oil bath at 80 °C. The pH was adjusted to about 8 by 1 M sodium hydroxide (NaOH) solution. After 4 h, the suspension was cooled and washed with deionized water/acetone until no Cl ~ was detected in the supernatant. The solid was dried at 100 °C overnight and calcined at 300 °C for 2 h. The gold mass loading (AU/AI2O3) was 1.70%.

[0053] Testing Protocol

[0054] The photocata lytic reaction was carried out in reaction system 100 depicted in FIG. 2. Reaction system 100 includes reaction chamber 102, catalyst bed 104, illumination source 106, light guide 110, window 112, reactant source(s) 114A and 114B, inert gas source 116, mass flow meters 118, and analysis device 120, interrelated as shown.

[0055] Catalyst bed 104 comprises 15 mg of either rhodium photocata lyst, gold photocata lyst, or alumina support. The catalyst bed was retained in a sample cup (not depicted) having a diameter of 6 millimeters and a height of 4 millimeters. A

thermocouple (not depicted), was disposed under catalyst bed 104. The temperature of the catalyst bed was controlled via a PID temperature controller that managed the resistive heating power of the reaction chamber, as well as the flow of cooling water to mitigate heating caused by illumination.

[0056] Carbon dioxide (research grade), hydrogen (research grade), and argon, from respective sources thereof 114A, 114B, and 116, were individually metered to reaction chamber 102 under the control of mass flow controllers 118. The

photocata lysts were first conditioned, via reduction and activation steps, as previously discussed. Specifically, reduction was performed under atmospheric pressure at 350 °C for 4 hours, with the flow of hydrogen gas at 60.1 millimeters/minute

(mm/min) and the flow of Argon gas at 27.6 mm/min. Following reduction, carbon dioxide was introduced to reaction chamber 102. Activation was performed for about 12 hours, at which point stable catalytic activities were obtained. Gas flows during activation and testing for experiments conducted in a hydrogen-rich atmosphere (C02: Hz ratio of 1: 5.5) were: Hz = 60.1 mm/min; CO2 = 10.9 mm/min; argon = 27.6 mm/min. Gas flows during activation and testing for experiments conducted in a hydrogen-deficient atmosphere (C02: H2 ratio of 1 :3.1) were: H2 = 60.1 mm/min; CO2 = 19.5 mm/min; argon = 16.5 mm/min. Gas flows rates are at STP.

[0057] Illumination source 106 was one of three LEDs of different wavelengths: ultraviolet at 365 nm (3.40 eV), blue at 460 nm (2.70 eV), and white 400-800 nm (5700 K). The output power of the LEDs was controlled by a voltage controller (not depicted) and measured with a thermopile power meter (not depicted). The emission spectra of the light sources were measured with a CCD-based spectrometer. Analysis of the reaction product is performed by analysis device 120, which in the illustrative embodiment is a quadrupole mass spectrometer (Hiden, HPR-20), equipped with a Faraday cup detector. The detection limit of the mass spectrometer is about 0.001% conversion of CO 2 . The conversion of CO 2 was maintained <5% to eliminate reactant- transport limitations and ensure that the concentrations of products in the effluent represent the reaction rates.

[0058] For each temperature and light-intensity condition, at least 15 minutes elapsed before reaching steady state and sequential measurements were taken to determine the steady-state concentration of each gas and the associated reaction rates and uncertainties. The 15 atomic mass unit (amu) signal was used to quantify the methane production rate. The 28 amu signal was used to quantify the carbon monoxide production rate, from which the background from carbon dioxide feedstock was subtracted. Deuterium (D2, Sigma Aldrich, 99.8% atom D) was used in place of Hz for the isotopic labelling experiments.

[0059] Test Results

[ 0060] FIG. 3A depicts rates of carbon monoxide and methane production at 623 K in hydrogen-rich conditions for the rhodium photocatalyst (solid lines) and bare aluminum oxide support (triangles and circles), with UV (365 nm) illumination at 3 Watt/sq. cm. and in the dark.

[0061] As depicted in FIG. 3A, methane production was strongly and very selectively enhanced by UV light on the rhodium photocatalyst. More particularly, methane and carbon monoxide were produced at comparable rates in the absence of UV illumination (e.g., 0-8 min). Under UV illumination [e.g., 8-22 min), a better than seven-fold increase in the methane-production rate was observed, while only a slight increase in CO production was detected. No other carbon-containing product was observed above the detection limit of the mass spectrometer. As depicted in FIG. 3A, the reaction rates responded instantly and reversibly to the UV light. Control experiments using pure AI2O3 nanoparticles showed that methane (triangles) and carbon monoxide (circles) are not produced, confirming that methane and carbon monoxide were produced from photocata lytic reactions on the rhodium nano-cubes.

[0062] FIGs. 3B and 3C depict comparative data, showing rates of carbon monoxide and methane production for gold photocata lyst. The results depicted in FIG. 3B were under white light of similar intensity to FIG. 3A. The results depicted in FIG. 3C compare the reaction under UV and white light of the same intensity (1.18 Watts/sq. cm) and at 623 K. As these Figures depict, gold photocatalyst also demonstrates photo- enhanced carbon-dioxide hydrogenation. However, the product selectivity was distinctly different. Carbon monoxide, rather than methane, was the exclusive carbon- containing product produced by the gold photocatalyst in both the presence and absence of light, and for both white light (FIG. 3A) and UV light (FIG. 3B). Thus, wavelength alone cannot account for the different selectivity of the rhodium photocatalyst versus the gold photocatalyst. These results demonstrate that the different selectivity of thermo- and photocata lytic reactions on the rhodium and gold nanoparticles is determined primarily by the properties of metals, specifically the differing metal-adsorbate interactions.

[0063] The plots in FIGs. 4A and 4B depict the different selectivity of thermo- and photo-reactions on the rhodium photocatalyst. In particular, the "circles" in FIG. 4A depict selectivity towards methane as function of overall reaction rates in the dark (i.e., the thermocata lytic reaction). In FIG. 4B, the solid circles depict selectivity towards methane as a function of temperature under hydrogen-rich conditions and the open circles depict that selectivity under hydrogen-deficient conditions, in both cases in the dark. These figures show that the thermocata lytic reaction exhibited mild selectivity, with a methane to carbon monoxide ratio of about 60:40, in the tested range of temperatures and reaction rates.

[0064] The squares in FIG. 4A depict methane selectivity under UV illumination (365 nm, 3 Watt/sq. cm.) as a function of reaction rate. The solid squares in FIG. 4B depict methane selectivity as a function of temperature under hydrogen-rich conditions and the open squares depict that selectivity under hydrogen-deficient conditions, both under UV illumination. These figures demonstrate that under UV illumination, the methane production rate was significantly and selectively enhanced. The photoreactions exhibit >95% selectivity towards methane, and the resulting selectivity towards methane from the overall reaction is >90% under 3 W/sq. cm. UV illumination and fc-rich conditions over the tested temperature range. [Experiments under hh-deficient conditions maintained this high selectivity under UV illumination. But under dark conditions (FIG. 4B, circles), experiments under H2-deficient conditions (open circles) exhibited lower selectivity than under Hz-rich conditions (solid circles). This confirms that illumination, not heat or excess H2 feedstock, is responsible for the highly selective production of methane.

[0065] The photoreactions under UV light (FIG. 4B, squares) show higher selectivity towards methane than under blue light (FIG. 4B, triangles), which are both much higher than that of the thermocata lytic reaction (FIG. 4B, circles).

[0066] FIG. 4C depicts the rates of methane photo-production as a function of UV light intensity at 623K (squares) and 573 K (circles). The photoreaction rates show a transition from linear dependence to super-linear dependence on intensity at around 1 W/sq. cm. for UV light. That is, at or above that intensity under UV illumination, the photocata lytic reaction was proportional to intensity raised to a power greater than 1. In particular:

The inset in FIG. 4C shows the intensity-dependent reaction rates in the linear (lower intensity) region. In the linear region, the slope is significantly higher at 623 K than at 573 K, since heat accelerates the photocata lytic rate.

[0067] FIG. 4D depicts methane production rates under different illumination wavelength and intensity as a function of temperature. In particular, the methane production rate is shown in the dark (circles), blue light (triangles, 2.4 W/sq. cm.), UV light (squares, 3 W/sq. cm.), and 2x the blue photon flux (diamonds, 4.9 W/sq. cm.). The figure shows that UV light is more efficient at enhancing the reaction rates than blue light. The circled points show the unheated steady-state temperatures and reaction rates.

[0068] For the ambient temperature, efficient photocata lytic methane production with high selectivity was demonstrated on rhodium photocatalyst under UV illumination at 3 W/sq. cm. and hh-rich conditions with a reaction rate (circled square in FIG. 4D) comparable to the thermocata lytic reaction rate (circles) at 548 K

(275 °C). The reaction rate at ambient temperature for the blue LED at 4.9 W/sq. cm. (circled diamond in FIG. 4D) was twice as high as the thermocata lytic reaction rate at 623 K (350 °C). It is notable that these high reaction rates, with high selectivity, were achieved using an efficient, low-intensity LED.

[0069] FIGs. 5A-5D depict apparent reaction rate as a function of temperature for rhodium photocatalyst (FIGs. 5A-5B) and gold photocatalyst (FIGs. 5C-5D). FIG. 5A depicts the thermocata lytic reaction rates for methane (squares) and carbon monoxide (circles) production on rhodium photocatalyst as a function of temperature. The apparent activation energy for methane was calculated to be 78.6 +/- 2.0 kJoules/mol and for carbon monoxide was calculated to be 64.7 +/- 6.0 kJoules/mol (0.81 eV and 0.67 eV, respectively). The apparent activation energies were obtained by fitting the results with an Arrhenius equation.

[0070] FIG. 5B depicts photoreaction rates for methane production on rhodium photocatalyst under 1.18 W/sq. cm. (squares), 0.59 W/sq. cm. (circles) and

0.24 W/sq. cm. (triangles) UV illumination as a function of temperature. The photocata lytic reactions show, for all light intensities, the same apparent activation energy, 50.4 +/- 1.8 kJoules (0.52 eV), which is lower than that of thermocata lytic reaction.

[0071] FIG. 5C depicts the thermocata lytic reaction rates of CO production on gold photocatalyst as a function of temperature. The apparent activation energy for carbon monoxide production on the gold photocatalyst was calculated to be 55.8 +/- 0.5 kJoules/mol (0.58 eV). FIG. 5D depicts the photoreaction rates for carbon monoxide production on gold photocatalyst under 1.27 W/sq. cm. (squares) and 0.89 W/sq. cm. (circles) white light illumination as a function of temperature. The photocata lytic reactions show, for both light intensities, the same apparent activation energy, 39.5 +/- 2 kJoules (0.41 eV).

[0072] Thus, a reduction in activation energy was also observed for carbon monoxide production on the gold photocatalyst with visible light, but the rhodium and gold photocatalyst exhibit different selectivity.

[0073] Methods In Accordance with the Present Teachings.

[0074] FIGs. 7 - 9 depict methods for methanation using rhodium

photocatalyst. In accordance with operation S701 of method 700, rhodium photocatalyst is provided. As previously described and as defined below, rhodium photocatalyst suitable for use in conjunction with the present invention comprises rhodium nanoparticles on a metal oxide support. The mass loading of the rhodium on the support is in a range of about 0.1 to 20.0 weight percent, and more preferably in the range of 0.5 to 2.0 weight percent. The nanoparticles are preferably in the form of nano-cubes.

[0075] As previously discussed, rhodium nano-cubes have a preferred edge length as a function of illumination wavelength. As test data reveals (see, e.g., FIG. 4D, etc.), rhodium-nano-cube-based photocatalyst can operate effectively outside of its preferred range. For example, a satisfactory production rate of methane is obtained under low-intensity (4.9 W/sq. cm.) blue (460 nm) illumination with rhodium nano-cubes having an edge length of 37 nm, whereas edge length should more preferably be about twice that wavelength per FIG. 6. As previously indicated, rhodium nano-cubes having an edge length in the range of about 1 to about 100 nm is suitable for use in conjunction with the present invention.

[0076] For practical applications {i.e., readily available, low-cost illumination), the rhodium nano-cubes most preferably have an edge-length as follows:

for 350 nm illumination: 30 to 40 nm

for 365 nm illumination: 45 to 55 nm

[0077] FIG. 8 depicts further detail about operation S701 (provide rhodium photocatalyst). In accordance with operation S801, a preferred edge length, or preferred range of edge length for rhodium nano-cubes is determined based on the illumination wavelength. This information may be obtained from a plot of edge length versus absorption wavelength, as depicted in FIG. 6. Or the desired range of edge length might be known from previous experience.

[0078] If the rhodium nanoparticles have a shape other than a cube, a relation such as shown in FIG. 6, can be developed for such other shape rhodium nanoparticles. In conjunction with this disclosure, it is within the capabilities of those skilled in the art to develop such a relation and express it either in graphical form, in tabular form, or as a discrete expression. Alternatively, FIG. 6 can be used as a starting point for an estimate of a desired range of size, which can be refined through simple experimentation.

[0079] Per operation S802, rhodium nano-cubes are formed to have an edge length in the desired range. This can be done, for example, using the slow-injection polyol methods previously discussed.

[0080] Returning to method 700, the catalyst is conditioned per operation S702. As per FIG. 9. conditioning includes catalyst reduction, at a minimum, in accordance with operation S901. As an option, in some embodiments, the catalyst is also "activated," per operation S902, as the previously discussed and defined below.

[0081] In operation S703 of method 700, the catalyst is exposed to reactants for methanation, including hydrogen gas and carbon dioxide gas, advantageously, but not necessarily, in at least the stoichiometric amounts. Temperature can be ambient, but reaction rate increases with increasing temperature with little effect on selectivity. Temperatures within the range of ambient temperature {i.e., 22 °C) to about 350 °C provide suitable performance, although temperatures in excess of 350 °C are not prohibited. In fact, the catalyst has been tested to 450 °C and provided acceptable performance.

[0082] In operation S704 of method 700, the rhodium photocatalyst is illuminated with blue or UV light. In principle, the wavelength of the illumination can cover the UV spectrum as well as a portion of the "blue" range; in other words, from about 10 nm to about 460 nm. As previously noted, illumination wavelengths less than 350 nm are not practical for use due to their availability/cost. And although illumination with light have a wavelength longer than 460 nm can be used, selectivity toward methane will decrease. Thus, a typical range for the wavelength of illumination will be 350 to 460 nanometers. The intensity of the illumination should be at least about 1 W/sq. cm. to be in a regime in which reaction rate has a super- linear dependence on illumination intensity.

[0083] Definitions. The terms appearing below and their inflected forms are defined for use in the appended claims as follows:

• "Blue" (light or illumination) means electromagnetic radiation having a wavelength in a range of about 455 nm to about 492 nm.

• "UV (light or illumination) means electromagnetic radiation having a wavelength in a range of about 10 nm to about 400 nm.

• "Deep UV (light or illumination) means electromagnetic radiation

having a wavelength in a range of 10 nm to less than 350 nm.

• "Condition" when referring to catalyst treatment, means to "reduce" the catalyst and optionally "activate" the catalyst.

• "Reduce" when referring to catalyst treatment, means to expose the catalyst to hydrogen gas at elevated temperature and for a period of time.

• "Activation" when referring to catalyst treatment, means to expose a catalyst to reactants and conditions suitable for carrying out catalytic reactions, and is continued until such time as catalytic activity is stable.

• "Rhodium Photocatalyst" means a catalyst having supported rhodium nanoparticles, wherein the mass loading of the rhodium on the support is in a range of about 0.1 to 20.0 weight percent. The support can be aluminum oxide, as titanium dioxide, silicon dioxide, cerium oxide, and other metal oxides. The rhodium nanoparticles preferably, but not necessarily, are in the form of a cube. Other preferred forms include nanoparticles having other shapes that are able to concentrate electromagnetic fields and liberate hot carriers to the same extent as the cube.

• "Super-linear dependence" when used to describe a relationship between reaction rate and illumination intensity, means that the reaction rate is proportional to the illumination intensity raised to a power greater than 1 : Reaction Rate « Intensity" , where n >1.

• "About" or "substantially" when used as a term of degree, means within +/- 15% of the stated value.