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US 20150291434A1

as) United States

a2) Patent Application Publication co) Pub. No.: US 2015/0291434 Al

Huberet al. (43) Pub. Date: Oct. 15, 2015

(54) METHOD TO REDUCE CO2 TO CO USING BOIS 23/72 (2006.01)PLASMON-ENHANCED PHOTOCATALYSIS BOLJ 23/66 (2006.01)

BOIS 23/52 (2006.01)(71) Applicant: Wisconsin Alumni Research Bols 21/06 (2006.01)

Foundation, Madison, WI (US) Bold 23/10 (2006.01)

. (52) U.S. CL(72) Inventors: George W. Huber, Middleton, WI (US): CPC vieveeeessseee C01B 31/18 (2013.01); BOLT 21/063

Aniruddha A. Upadhye, Madison, WI (2013.01); BOLJ 21/04 (2013.01); BOLT 23/10(US); Hyung Ju Kim, Madison, WI (2013.01); BOLT 23/72 (2013.01); BOLT 23/66(US); Insoo Ro, Madison, WI (US); M. (2013.01); BOLT 23/52 (2013.01): BOLJ 35/004Isabel Tejedor-Anderson, Madison, WI (2013.01)

(US): (57) ABSTRACT

(73) Assignee: Wisconsin Alumni ResearchFoundation, Madison, WI (US) Described is a method of reducing CO, to CO using visible

radiation and plasmonic photocatalysts. The method includes

(21) Appl. No.: 14/248,729 contacting CO, with a catalyst, in the presence ofH,, whereinthe catalyst has plasmonic photocatalytic reductive activity

(22) Filed: Apr. 9, 2014 whenexposedto radiation having a wavelength between 380

nm and 780 nm. The catalyst, CO, and H, are exposed toPublication Classification non-coherentradiation having a wavelength between 380 nm

and 780 nm suchthat the catalyst undergoes surface plasmon(51) Int. Cl. resonance. The surface plasmon resonance increases the rate

COIB 31/18 (2006.01) of CO, reduction to CO as compared to the rate of CO,BOIS 21/04 (2006.01) reduction to CO without surface plasmon resonance in the

BOLJ 35/00 (2006.01) catalyst.

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US 2015/0291434 Al

METHOD TO REDUCE CO2 TO CO USINGPLASMON-ENHANCED PHOTOCATALYSIS

FEDERAL FUNDING STATEMENT

[0001] This invention was made with governmentsupportawarded under DE-AR0000329 awarded by the US Depart-

ment of Energy. The government has certain rights in the

invention.

BACKGROUND

[0002] There are many sources of renewable energy that

have been explored as possible meansto limit the worldwide

reliance on fossil fuels. Among the more promising renew-able sources are solar energy, wind energy, hydro-generated

energy (e.g., dams, tide-driven generators), geothermalenergy, and biomass. With the exception of solar-derived,

however,all ofthese sources suffer from inherent drawbacks.Hydroelectric energy, for example, requires massive infra-

structure and along with it inevitable habitat destruction. Har-

nessing wind energy likewise requires considerable invest-ment in infrastructure, namely large windmills arranged in

very large arrays. Hydro energy, wind energy, and geothermalenergyare also inherently limitedto suitable geographic loca-

tions on the earth. Biomass, while promising,also brings withit potential interference with the production of food for

humansbecausearable acreage is devoted to energy produc-

tion rather than food production. The most readily availablesource ofrenewable energy, of course, is the sun. Even at the

poles, the sun shines for at least part of the year. Solar energycan be harnessed passively, in the form of black bodies that

are heated in the sun’s rays (e.g, to heat water), or by usinglenses that focus the sun’s light to heat a given area. Solar

energy can also be converted directly into electricity in a

photovoltaic (PV) cell. The simplest ofPV devices is a semi-conductor photodiode. When photonsofsolar light contact

the photodiode, electron-hole pairs (e7/h*) are generated,which are then usedto carry an electric current.

[0003] Generally speaking, “photochemical”reactions are

chemical reactions induced by light, e.g., photosynthesis.Photochemicalreactions do not generate an electric current in

the conventional sense. In contrast, photoelectrochemical(PEC)reactions operationally connect a semiconductor pho-

tovoltaic device with a chemical reaction such the energy of

the photons striking the photovoltaic device are convertedinto electrochemical energy. Theefficient use of natural sun-

light in these reactions has been a long-standing researchfocus because PEC reactions are potentially more energy

efficient than the corresponding reaction using a traditionalcatalyst.

[0004] For example,the idea ofusing a photoelectrochemi-

cal device to split water into H, and O, molecules has beeninvestigated since the 1970’s. In essence, a PEC semiconduc-

tor with appropriate electronic properties is immersed in an

aqueouselectrolyte andirradiated with sunlight. The photonenergy from the sunlight is converted to electrochemical

energy, which then breaks the H—O bondsinthe water oftheaqueouselectrolyte. The fundamental atomic processes are

reasonably well understood: Incoming solar photons ofappropriate energy strike the PV cell and generate conducting

electrons and corresponding holes, i.e, e~/h* pairs. The elec-

trons and holes move in opposite directions through the PVcell. In a simple, two-electrode device, the holes drive an

oxygen evolution reaction at one electrode, and the electrons

Oct. 15, 2015

drive a hydrogen evolution reaction at the counter-electrode.See, for example, Chen, Zhebo; Dinh, Huyen; and Miller,

Eric; “Photoelectrochemical Water Splitting, Standards,Experimental Methods, and Protocols,’© 2013, Springer-

Verlag GmbH, Heidelberg, Germany, ISBN 978-1-4614-8298-7. See also Wenbo Hou and Stephen Cronin (2013) “A

Review of Surface Plasmon Resonance-Enhanced Photoca-

talysis,’ Adv. Funct. Mater. 23:1612-1619.

[0005] Similarly, photocatalysis is the acceleration of a

photochemical reaction in the presence of a catalyst. Several

groups have investigated using heterogeneous photocatalyststo drive industrially important reactions. See, for example,

Phillip Christopher, Hongliang Xin, Andiappan Marimuthuand Suljo Linic (2012) “Singular characteristics and unique

chemical bondactivation mechanismsofphotocatalytic reac-tions on plasmonic nanostructures,’ Nature Materials

11:1044-1050. Here, the authors demonstrate ethylene epoxi-

dation over an Ag/Al1,O, plasmonic metallic nanostructuredcatalyst. The authors conclude that this photocatalytic system

exhibit fundamentally different behavior as compared tosemiconductors. The photocatalytic reaction rates on these

excited plasmonic metallic nanostructures exhibit a super-linear power law dependenceonlight intensity (rate « Inten-

sity”, with n>1), at significantly lower intensity than required

for super-linear behavior on extended metal surfaces. Addi-tionally, in contrast to semiconductor photocatalysts, photo-

catalytic quantum efficiencies on this plasmonic metalliccatalyst increased with light intensity and operating tempera-

ture. See also Andiappan Marimuthu, Jianwen Zhang, andSuljo Linic (29 Mar. 2013) “Tuning Selectivity in Propylene

Epoxidation by Plasmon Mediated Photo-Switching of Cu

Oxidation State,” Science 339(6127):1590-1593.

[0006] Photocatalysis has also been investigated as a means

to convert CO, to hydrocarbon fuels (Cronin et al. (2011),

“Photocatalytic Conversion of CO, to Hydrocarbonfuels viaPlasmon-Enhanced Absorption and Metallic Interband Tran-

sition,” ACS Catal. 1:929-936). Other groups have used vis-ible light plasmonic heating of a gold/zinc oxide catalyst to

drive a reverse water-gas shift reaction coupled to a metha-nation reaction. See Matrangaet al. (2013) “Visible light

plasmonic heating ofAu—ZnOforthe catalytic reduction of

CO,,” Nanoscale 5:6968-6974. Photocatalysis has also beeninvestigated in the context of using the forward water-gas

shift reaction to generate hydrogen at room temperature. SeeGarcia et al. (2013) “Photocatalytic water gas shift using

visible or simulated solar light for efficient, room-tempera-

ture hydrogen generation,’ Energy Environ. Sci. 6:2211-2215.

[0007] In the patent literature, see US 2013/0122396, toLinic & Christopher (published 16 May 2013). The published

patent application describes a plasmon-resonating nanostruc-

ture that catalyzes the reduction of an oxidant via a photo-thermal mechanism. The plasmon-resonating nanostructure

can be a nanoparticle that comprises copper, silver, gold, oralloys these elements. The methodis described as being use-

ful to catalyze the reduction of an oxidant, for example, in acatalytic reactor or in a fuel cell. The only oxidant described,

however, is molecular oxygen, C,. The publication describes

CO oxidation with molecular oxygen as the oxidant (CO+%O,—CO,), as well as NH, oxidation with molecular oxy-

gen as the oxidant (NH,+O,—-N,+N,0+NO+NO,+H,O[non-stoichiometric]).

[0008] Patent publication US2010/0288356, to Linicetal.

(published 18 Nov. 2010), describes a composition compris-

US 2015/0291434 Al

ing a semiconducting photocatalyst and plasmon-resonatingnanoparticles. The plasmon-resonating nanoparticles are

capable ofconcentrating light at a wavelength that is substan-tially the same as the wavelength of light necessary to pro-

mote an electron from a valance bandto a conduction band inthe semiconductor photocatalyst. Thus, the plasmon-resonat-

ing nanoparticles direct light to the band gap ofthe semicon-

ductor at an increased intensity as contrasted to when thenanoparticles are not present.

SUMMARYOF THE INVENTION

[0009] Disclosed herein is a method ofreducing CO, to COusing H, as the reductant. The method comprises contacting

CO, with a catalyst, in the presence of H,, wherein the cata-lyst has plasmonic photocatalytic reductive activity when

exposedto radiation having a wavelength between about 380nm and about 780 nm (i.e., in the visible range). The catalyst,

CO,, and H, are then exposed to non-coherentradiation hav-

ing a wavelength between about 380 nm and about 780 nmsuch that the catalyst undergoes surface plasmon resonance.

The surface plasmon resonancecausesa photocatalytic effectthat increases the rate ofCO, reduction to CO as compared to

the rate of CO, reduction to CO without surface plasmonresonancein the catalyst. In the preferred method, the cata-

lyst, CO,, and H, are exposedto solar radiation.

[0010] The catalysts that may be used in the method gen-erally comprise a metallic element have an average particle

size no greater than 100 nm in combination with a semicon-ductor material. The preferred metallic elements for usein the

method are calcium, copper, europium, gold, lithium, mag-

nesium, palladium, platinum, potassium, silver, sodium,rubidium,andyttrium, and/or combinationsthereof. The pre-

ferred semiconductor materials are oxides of titanium, alu-minum,iron, silicon, zinc, and cerium, and/or combinations

thereof. The mostpreferred pairings are copper, silver, plati-num,or gold nanoparticles in combination with a semicon-

ductor material comprising titania, alumina,orceria.

[0011] Asa general proposition, the surface plasmonreso-nance in the catalyst increasesthe rate ofCO, reduction to CO

by a factor of at least 1.8 as compared to the rate of CO,reduction to CO in the absenceof surface plasmon resonance

in the catalyst, and morepreferablya factor ofat least 3, 4, or

5 as compared to running the reaction in the absence ofplasmonic photocatalysis.

[0012] More specifically disclosed herein is a method ofreducing CO, to CO, wherein the method comprises contact-

ing CO, with a catalyst, in the presence of H,, wherein thecatalyst has plasmonic photocatalytic reductive activity when

exposed to non-coherent radiation having a wavelength

between about 380 nm and about 780 nm, and exposing thecatalyst, CO,, and H,to solar radiation such that the catalyst

undergoes surface plasmon resonance. As noted previously,the surface plasmon resonance increases the rate of CO,

reduction to CO as comparedto the rate of CO, reduction toCO without surface plasmon resonancein the catalyst.

[0013] It is preferred that when the reaction is run using

solar radiation, the plasmonic catalyst achieves a solar lightefficiency of at least about 2%, more preferably at least 3%,

and more preferably still at least 4%. The same catalysts notedabove maybe used. The intensity of the incomingsolar light

may be used “asis,” i.e., at an intensity of 1 sun or air mass

coefficient 1.5 (“AM1.5”) (conventionally taken to be 1kW/m”) or concentrated with optical elements up to an inten-

sity of 100 suns or more. The air mass coefficient (“AM”) is

Oct. 15, 2015

used to characterize the performance of solar cells understandardized conditions, and is often referred to using the

syntax “AM”followed by a number. “AM1.5”is convention-ally used when characterizing terrestrial power-generating

solar panels. The air mass coefficient defines the direct opticalpath length through the Earth’s atmosphere, expressed as a

ratio relative to the path length vertically upwards, i.e. when

the sun is at the zenith. In short, the air mass coefficientcharacterizes the solar spectrum after solar radiation has trav-

eled through the atmosphere.[0014] The methods disclosed herein are preferably con-

ducted at a temperature of from about 100° C. to about 400°C., wherein H, is present ina greater concentration than CO,,

and the H, and CO,are present at a pressure of from atmo-

spheric to about 2000 psi, absolute. (As used herein, psi=psia;thatis the absolute pressure in pounds per square inch.) These

are non-limiting, preferred ranges. Temperature, pressure,and concentration ranges above and below those stated are

explicitly within the scope of the disclosed method.

[0015] Numerical ranges as used herein are intended toinclude every number and subset of numbers contained

within that range, whether specifically disclosed or not. Fur-ther, these numerical ranges should be construed as providing

support for a claim directed to any numberor subset ofnum-bers in that range. For example, a disclosure of from 1 to 10

should be construed as supporting a rangeoffrom 2 to 8, from

3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, and soforth.

[0016] All references to singular characteristics or limita-tionsofthe present invention shall include the corresponding

plural characteristic or limitation, and vice-versa, unless oth-erwise specified or clearly implied to the contrary by the

context in which the reference is made.

[0017] Allcombinations ofmethodor processsteps as usedherein can be performedin any order, unless otherwise speci-

fied or clearly implied to the contrary by the context in whichthe referenced combination is made.

[0018] The methodsofthe present invention can comprise,consist of, or consist essentially ofthe essential elements and

limitations of the method described herein, as well as any

additionalor optional ingredients, components, or limitationsdescribed herein or otherwise useful in synthetic organic

chemistry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a schematic representation of harnessing

solar energy to drive photoelectrochemical (PEC)splitting ofwater with subsequent plasmonic photocatalytic reduction of

CO, to value-added products such as formic acid, syn-gas (a

mixture of CO and H,) and hydrocarbons.

[0020] FIG. 2 is a schematic representation of plasmonic

photocatalysis, illustrating that the plasmonic resonancecauses a numberofbeneficial phenomenathatdrivecatalysis,

including intense scattering of the incomingradiation, elec-tron/hole pair generation, and localized heating, all ofwhich

impactcatalysis.

[0021] FIG. 3 isa histogram showing CO, conversion ratesfor the reverse water gas shift reaction (CO,+H,-CO+H,0;

AHeaction=41 kJ/mol) using different catalysts in “dark”mode(no added light) and “enhanced” mode(in presence of

simulated solar radiation). Dark reaction shownin blackbars,

with rate depicted on theleft-handY-axis; reaction with simu-lated solar radiation shown in open bars, with enhancement

factor depicted on the right-handY-axis. Reaction conditions:

US 2015/0291434 Al

H,:CO,=2:1; total gas flow rate=15 scem, P=110 psi, T=400°C. (The term “sccm” denotes standard cubic centimeters per

minute indicating cc/min at 0° C.at 1 atmosphere ofpressure.This unit is used to calculate the amountofgas or volume of

gas that passes through a given point in a unit time.)

[0022] FIG. 4 is a graph depicting CO, conversionrates forthe reverse water gas shift reaction as a function of tempera-

ture. Light reaction (#); dark reaction (@). Reaction condi-tions: H,:CO,=2:1; total gas flow rate=15 sccm, P=110 psi,

T=100° C.to 400° C.

[0023] FIG.5isa graph depicting enhancementdue to lightas a function of temperature for the reverse water gas shift

reactions whose conversionrates are depicted in FIG. 4. Rate

enhancement (light rate/dark rate) depicted on Y-axis. Asshown in the figure, the enhancement moves inversely to

temperature.

[0024] FIG. 6 is a graph depicting rate due to light versus

temperature for the reverse water gas shift reactions whose

conversion rates are depicted in FIG.4.

[0025] FIG. 7: is a graph depicting light efficiency versus

temperature for the reverse water gas shift reactions whose

conversion rates are depicted in FIG. 4. Again, Reactionconditions: H,:CO,=2:1; total gas flow rate=15 sccm, P=110

psi. Light efficiency is defined as

Lightefficiency (%) =

CO, conversion rate due to lightx AMpeaction x 100% Intensityx Catalyst surface area

[0026] FIG. 8 is a plot depicting In(CO, conversion rate)versus 1/Temp(1/K)x10° for the reverse water gas shift reac-

tions whose conversion rates are depicted in FIG. 4. If theincreased rate for the light reaction werestrictly a localized

heating effect, the activation energy for the light reaction

versus the dark reaction should be the same. However, thedark reaction (@) has anE,, of47.094/-0.27 kJ/mol, while the

light reaction (#) has an E, of 34.934/-0.47 kJ/mol. Thechange in E, indicates that it not solely a localized heating

effect that is responsible for the light-induced enhancementofthe CO, reduction rates.

[0027] FIG. 9 is a graph depicting In(reaction rate) for the

CO, to CO reduction as a function ofthe In(partial pressure ofCO.) for the light reaction (#) versus the dark reaction (@).

[Rate]=Kappo> [PPco2]|”. It was foundforthe light reaction

that m,,.;,;=1.04; for the dark reaction, m,,,,=0.50. Reactionconditions: Total gas flow rate=15 sccm, P=110 psi, T=200°

Cc

[0028] FIG. 10 is a graph similar analogous to FIG.9, butdepicting In(reaction rate) for the CO, to CO reduction as a

function ofthe In(partial pressure ofH,) for the light reaction(#) versus the dark reaction (@). [Rate]=Kapp,,. [PP,,.]”. It

wasfoundforthe light reaction that n,,,,,-0.17; for the darkreaction, 0,,,4—0.07. Reaction conditions were the same as

noted for FIG.9.

[0029] FIG. 11 is a graph depicting the dependenceoflight

efficiency on H,:CO, ratio in plasmon-enhanced water gasshift reaction over Au/TiO, catalyst. Experimental condi-

tions: P=103 psi, T=200° C., Total gas flow rate=15 sccm,catalyst amount=7.9 mg.

[0030] FIG. 12 is a graph depicting the dependenceofrate

enhancement on H,:CO, ratio in plasmon-enhanced water

Oct. 15, 2015

gas shift reaction over Au/TiO, catalyst. Experimental con-ditions: P=103 psi, T=200° C., Total gas flow rate=15 sccm,

catalyst amount=7.9 mg.

DETAILED DESCRIPTION

[0031] Disclosed herein is a method ofreducing CO, to CO

using hydrogen (H,) as the reducing agent, and using plas-monic photocatalysts andvisible light (preferably solar light

to increase the speed of the reaction to unprecedentedrates.The methodincludesthe steps ofcontacting the CO, with the

plasmonic photo catalyst, in the presence of H,. The plas-

monic photocatalytic is then exposed to non-coherent radia-tion having a wavelength between about 380 nm and about

780 nm (that is, in the visible range) so that the catalystundergoes surface plasmon resonance. It has been found that

when using mixed catalysts comprising a nanoparticulatemetal and a semiconductor, the surface plasmon resonance

induced in the catalyst greatly increases the rate of CO,

reduction reaction.

[0032] In particular, a catalyst comprising a noble metal

nanoparticle (preferably gold) is fabricated via the sol-geltechniqueor deposition precipitation technique with an oxide

semiconductor material, preferably a titania or alumina semi-

conductor. The Auw/TiO, (DP), Au/CeO, (DP), Au/Al,O,(DP) were prepared by deposition-precipitation (DP)

method’ *. Degussa P25 TiO, (Sigma-Aldrich, St. Louis,Mo., USA >99.5%), CeO, (Sigma-Aldrich), Al,O, (Strem

Chemicals, Newburyport, Mass., USA) were used as sup-ports, while HAuCl,.3H,O (Sigma-Aldrich) and CuSO,.

5H,O (Sigman-Aldrich) were used as metal precursors for

catalyst synthesis. The Au/TiO, (SG) catalyst was preparedusing, sol-gel chemistry*. The Au/TiO, (SG) solutions were

then dried to obtain powdered Au/TIO, (SG) catalyst.Au/Al,O, (IVO)catalyst was prepared by incipient wetness

impregnation. Cu/TiO,(1) catalyst was prepared by impreg-nation (1) method*-®. The resulting photocatalytic material

can then be used, in conjunction with light in the visible

spectrum,to photocatalytically reduce CO, in the presence ofhydrogenvia the reverse water gas shift reaction. The reverse

watergas shift reaction produces a syn-gas mixture which canthen be further converted to liquid fuels using mature existing

technologies.

[0033] The reverse water gas shift reaction, of course, isendothermic. Thus, the reaction needs to be driven. As

described herein, it has been shown that metallic nano-par-ticles absorb light radiation in the visible range. Thus, by

coupling a suitable plasmonic catalyst comprising one ormore nano-particulate metals that exhibit surface plasmon

resonance (SPR)in responseto light in the visible range of

wavelengths (such as the photon found in solar radiation),solar radiation (a non-coherentradiation) can be usedto drive

the endothermic reverse watergas shift reaction. In this sense,the plasmonic responseofthe catalyst has a two-fold benefit:

it both derives from solar energy the energy required for thereaction, and also catalyzesthe reaction. For the reverse water

gas shift reaction, where CO, is being reduced to CO in the

present of H., the data presented herein show thattherate ofreaction increases up to 13 times under simulated solar radia-

tion as compared to the corresponding dark reaction. Thus,process is highly useful as a meansto usethe visible part of

sunlight to drive chemical reactions.

[0034] As used herein, the term “nanoparticle,” generallyrefers to a particle that exhibits one or more properties not

normally associated with the corresponding bulk material

US 2015/0291434 Al

(e.g., quantum optical effects such as surface plasmon reso-nance). The term also generally refers to materials having an

average particle size no larger than about 100 nm. Nanopar-ticles include particles of any shape or geometry (spheres,

rods, other crystalline and non-crystalline shapes, etc.),including individual nanoparticles and clusters of adhered

nanoparticles. The nanoparticles can have a variety of shapes,

dependent or independent, on their crystalline structure. Thepreferred nanoparticles for use in the process comprise cal-

cium, copper, europium, gold, lithium, magnesium,palla-dium, platinum, potassium, silver, sodium, rubidium, and

yttrium, and/or combinations thereof, mixtures thereof, and/or alloys containing these metals. The size and/or shape of a

nanoparticle can be determined by transmission electron

microscopy.

[0035] Nanoparticles with well-controlled, highly-uniformsizes, and particle geometries can be fabricated using known

techniques. Nanoparticles are widely available commerciallyfrom several worldwide suppliers, such as Sigma-Aldrich, St.

Louis, Mo., USA.Various shapes ofplasmonic nanoparticlescan also be obtained by various methods such as those

described in the U.S. Pat. No. 7,820,840. Some of these

nanoparticles (e.g., metals with free-electron-like valencebands, such as noble metals) exhibit a strong localized surface

plasmon resonance due to the nanometerscale spatial con-finement, and the metal’s inherent electronic structure. For

example, the resonance frequency of silver and gold nano-particles falls in the ultravioletto visible light range, and can

be tuned by changing the geometry andsize oftheparticles.

The intensity of resonant electromagnetic radiation isenhancedby several orders ofmagnitude near the surface of

plasmonic nanoparticles. Thus, the catalysts described hereinare compositions that exploit the ability of plasmonic nano-

particles to create electron-hole pairs, and simultaneouslycatalyze the reduction of CO, to CO.

[0036] Surface plasmon resonance (SPR) or simply plas-

mon resonance is an optical phenomenon arising from thecollective oscillation ofconduction electrons in a metal when

the electrons are disturbed from their equilibrium positions.

Whenelectromagnetic energy (photons)ofthe proper energyimpinge on such a metal, the free electrons of the metal are

driven by the alternating electric field to coherently oscillateat a resonant frequencyrelativeto thelattice ofpositive ions.

The plasmon frequencies for most metals occur in the UV

region of the electromagnetic spectrum. However severalalkali metals and transition metals, including copper, silver,

gold, and others have plasmon frequencies in the visibleregion of the spectrum. A “plasmonic nanoparticle,” there-

fore, is a nanoparticle having conduction electrons that col-lectively oscillate when excited by a stream of photon of the

appropriate energy (i.e., wavelength).

[0037] In the disclosed process, the plasmon resonance ofthe plasmonic catalyst is induced by non-coherent electro-

magnetic energy, preferably solar radiation. The solar radia-

tion may be concentrated by any meansor device now knownor developedin the future. (A hostof solar radiation concen-

trators are knownin the art) The frequency andintensity of aplasmon resonance is generally determined by the intrinsic

dielectric property of a given metal, the dielectric constant ofthe medium in contact with the metal, and the pattern of

surface polarization. Thus, variations in the shape or size of

the nanoparticulate metals in the catalyst can alter the surfacepolarization and cause a change to the plasmon resonance

frequency. This dependence offers the ability to tune the

Oct. 15, 2015

surface plasmon resonance of metal nanoparticles throughshape-controlled synthesis. A suitable shape-control synthe-

sis is described in Lu et al. (2009) Annu. Rev. Phys. Chem.60:167-92.

[0038] The radiation applied comprises incoherent radia-

tion in the visible range, approximately 380 nm to approxi-mately 780 nm). The wavelengths ofthe photons that contact

the catalyst may be full spectrum or otherwise attenuated byfilters, monochromators, andthe like.

[0039] In various embodiments, the plasmon-resonating

nanostructures include at least one of copper, silver, and goldnanoparticles. These nanoparticles may be copper/silver/gold

alloy nanoparticles (e.g., copper-silver nanoparticles, copper-

gold nanoparticles, silver-gold nanoparticles, copper-silver-gold nanoparticles). The nanostructures also may include, for

example, silica as a core onto which the copper, silver and/orgold are deposited. In another variation, the nanostructures

can be particles of substrates, for examplesilica, platinum, orother metal particles, onto which a plasmon-resonating layer

or plasmon-resonating nanoparticle is deposited, e.g., layers

or nanoparticles of Cu, Ag, and/or Au. In one preferredembodiment, the nanostructures include copper. In another

preferred embodiment, the nanostructures include silver. Inyet another preferred embodiment, the nanostructures

include gold.

[0040] There are many advantagesto using plasmonic cata-lysts for driving solar-powered chemical reactions. Notably,

plasmonic catalysts, such as Au/TiO,, operate in the visiblewavelength range ofthe solar radiation spectrum. This is an

important consideration because 48% ofthe solar spectrum of

radiation falls within the visible range, while only 6% fallswithin the ultraviolet range. Photocatalysts that operate only

in the UV rangeare thus incredibly inefficient at convertingsolar energy into chemical energy. Thus, plasmonic catalysts

that operate in the visible range of solar radiation providehigher efficiencies as compared to conventional, heteroge-

neous catalysts, as well as plasmonic catalysts that do not

resonate in responseto visible wavelengths of energy. Addi-tionally, surface plasmon resonanceitselfdepends onboth the

metal substrate selected andits particle size. Theparticle sizedependency of SPR allows for the catalyst to be “tuned” or

optimized overthe visible range ofwavelengths by adjustingthe particle size accordingly. Additionally, there is no Shock-

ley-Queisser limit on SPR. Thatis, the maximum theoretical

efficiency of a p-n junction photovoltaic solar cell (as mod-eled by Shockley and Queisser) is a function of black-body

radiation, e~/h* pair recombination(i.e., the opposite of e7/h*pair generation), and spectrum losses due to the wide range of

wavelengths presentin solar radiation. (That is, a significantportion of solar photonsdo not have the proper wavelength to

generate e7/h* pairs when they strike a PV panel.)

[0041] The catalysts described herein are preferably fabri-cated using the sol-gel technique. This technique is well

knownto those skilled in theart, so it will not be described in

exhaustive detail. Very briefly, in a typical sol-gel process,metal alkoxide and metal chloride precursors are solubilized

to form a solution (sol) and then undergo hydrolysis andpolycondensation reactions to form a colloid system com-

posed of solid particles dispersed in a solvent. These solidparticles continue to coalesce until they define an inorganic

network containing a liquid phase (gel). The gel is then dried

to removethe liquid phase, thereby yielding a highly porousmaterial. Becauseofthe high porosity, catalysts fabricated by

the sol-gel technique typically have very high surface areas.

US 2015/0291434 Al

In effect, solid nanoparticles dispersed in a liquid (a sol)agglomerate together to form a continuousthree-dimensional

network extending throughoutthe liquid (a gel). The liquidphase is then removed. The term “sol-gel” is sometimes

improperly used as a noun to refer to gels made through thesol-gel process. See, for example, Brinker and Scherer, “The

Physics and Chemistry of Sol-Gel Processing,” © 1990, Aca-

demic Press, Inc. San Diego, Calif., USA; ISBN 978-0-12-134970-7.

[0042] Referring now to the figures, FIG. 1 is a schematic

illustration showing how to reduce CO, to CO (and otherdownstream products such as formic acid, syn-gas and hydro-

carbons) using H, produced from solar-powered photoelec-trocatalytic (PEC) hydrolysis ofwater. Starting from theleft-

hand side of the figure, the box labeled “PV electricitygeneration” represents a conventional photovoltaic solar cell

for producingelectricity from sunlight. This electricity is then

introduced into a photoelectrocatalytic reaction of water,along with additional, concentrated sunlight, as shown in the

middle of FIG.1, in the box labeled “PEC H,production.” Inthis reactor, the wateris split into H, and O, using a plamonic

photoelectrochemical catalyst and solar radiation to inducethe plasmonic resonance in the catalyst. Preferred catalysts

for the water-splitting reaction includeAu/TiO, andAg/Ti0,,

as well as Au, Ag, and/or Cu supported on other semiconduc-tors. As noted previously, the preferred semiconductors are

oxides of titantum, aluminum,iron, silicon, zinc, and/orcerium.

[0043] The molecular hydrogen generated by the water-

splitting reaction can then be used to drive the plasmonicphotocatalytic reduction ofCO, (which can be obtained from

a myriad of industrial processes, including any processinvolving the combustion of carbohydrates). This is shown in

the box labeled “CO,+H, conversion” in FIG. 1. As noted

earlier, the plasmonic catalyst preferably comprises a metal-lic element have an average particle size no greater than 100

nm incombinationwith a semiconductor material. The metal-lic element must exhibit surface plasmonresonance, when the

required average particle size range, in response to photonswithin the visible spectrum (about 380 to about 780 nm). The

Oct. 15, 2015

aluminum. The radiation used to drive the plasmonic photo-catalysis is preferably solar radiation.

[0044] FIG. 2 is a schematic representation of the basic

operation of plasmonic photocatalysis. When visible radia-tion induces plasmon resonancein the metallic nanoparticle

(“Metal NP in FIG. 2) a number of quantum and macrophenomena occur, including intense scattering of the incom-

ing radiation, electron/hole pair generation, and localizedheating, all ofwhich impact catalysis and can be harnessed to

drive an endothermic reaction such as the reverse water-gas

shift reaction.

[0045] FIG. 3 is a histogram that demonstrates the consid-

erable reaction rate enhancementthat can be achieved using

the plasmonic photocatalysis method described herein ascomparedto traditional heterogeneous catalysis. FIG. 3 is a

histogram showing CO, reduction rates for the reverse watergas shift reaction (CO,+H,CO+H,O; AH,.cnion=41 kJ/mol)

using different catalysts in “dark” mode (no addedlight) and“enhanced” mode (in presence of simulated solar radiation

that causes plasmon resonancein the catalysts). In the reac-

tion, high-pressure hydrogen and CO, (H,:CO,=2:1, 110 psi)were flowedinto a sealed photocatalytic reaction vessel at 15

sccm and 400° C. Light reactions (using simulated solarradiation) and dark reactions were conducted forsix different

catalyst compositions, as noted in the figure. The results forthe dark reactions are shown in black bars, with the reaction

rate (umol/gm-cat/min) depicted on the left-handY-axis. The

enhancementofthe reaction rate when exposed to simulatedsolar radiation is shown in open bars, with the enhancement

factor depicted on the right-handY-axis. As can be seen fromFIG.3, CeO,, Al,O,, TiO, on their own,inthe dark,is a rather

poorcatalyst (relative to the others) for the reverse water-gasshift reaction. Even so, when run plasmonically, the reaction

rate for TiO, improved by almost 100%. Underthese condi-

tions, a catalyst comprised of nanoparticulate gold and TiO,was a reasonably good catalyst when run in dark mode, and

the dark rate was improvedby almost 200% whenthe reactionwas run plasmonically. For all of the catalyst combinations

depicted in FIG. 3, the plasmonic enhancementof the reac-tion rate was significant. See also Table 1.

TABLE1

CO, Conversion Rates under Various Condition

Catalyst CO, conversion rate

Amount umol/gm-cat/min Enhancement

Sr.No. Catalyst (mg) DARK LIGHT (LIGHT-DARK) (LIGHT-DARK)

1 Au—TiO, (DP) 74 2033.4 2663.4 630.1 1.32 Au—CeO, 12.8 655.9 1416.6 760.8 2.2

(DP)3. Au—TiO, (SG) 12.4 641.2 900.4 259.2 144 Au-Al,0, 16.6 76.5 118.3 41.8 1.5

(DP)5 Au-A1203 13.1 47.3 71.6 24.4 1.5

dwt6 Cu—Tio, () 8.6 19.5 25.1 5.7 1.37 TiO; 12.2 21.2 18.9 -2.3 0.98 CeO, 23.9 21.0 22.0 1.1 1.19 Al203 30.00 67.4 73.8 6.4 1.2

preferred nanoparticles comprise copper, mercury, ruthe-

nium, rhodium, rhenium,palladium, silver, osmium,iridium,

platinum, gold, and/or combinations thereof. The semicon-

ductor is preferably an oxide of titanium and/or an oxide of

[0046] In light of these results, using the reverse water gas

shift reaction over anAu/TiO,catalyst run plasmonically and

in the dark as a meansto reduce CO, with H, was investigated

in greater detail. FIG. 4 is a graph that depicts the rate of the

US 2015/0291434 Al

reduction reaction as a factor of temperature for both theplasmonic reactions (“light”, #) and dark reactions, @. Of

particular note in FIG. 4 is the enhancementofthe plasmonicreaction rates across all temperatures tested. As the tempera-

ture rises, the reaction rate predictably rises. The reaction isendothermic, so its rate would be expectedto rise with rising

temperature. However, the enhancement due to running the

reaction plasmonically is not expected, especially at thehigher end ofthe temperature range. Thatis, at the higher end

of the temperature range, the expectation is that the thermaleffect on catalysis would dominate and the enhancement due

to running the reaction plasmonically would decrease or dis-appear entirely. However, even at the highest temperature

tested, 400° C., FIG. 4 showsthat there is a very significant

enhancementin the reaction rate between the light reactionand the dark reaction.

[0047] FIG. 5 presents the enhancement data between the

light reaction versus the dark reaction in isolation—.e., itis agraph depicting the enhancementin rate due to running the

reaction plasmonically as a function of temperature for thereverse water gas shift reactions described above for FIG.4.

Here, the data show that in a direct comparison, the enhance-mentfactor (1.e., the rate of light reaction/rate of dark reac-

tion) is more pronounced at 100° C. and decreases in a smooth

curve to approximately a factor of 2 at 400° C. Extrapolated,these data indicate a light enhancementofa factor of7; 1.e.,

700%. These same date are presented in FIG. 6 notas a rateenhancement,but ratheras the actualdifference between CO,

reduction rate (umol/gm-cat/min) under light and dark vs.temperature that can be attributed solely to the plasmonic

influence ofthe catalyst (and not temperature). FIG. 6 indi-

cates that the maximum plasmonic-induced enhancement inthe reaction rate as a function of temperature-induced

increases in reaction rate peaks somewhere between about300° C. and about 350° C. in the Au/TiO, system. FIG. 7

corroborates these findings by showing that the light effi-

ciency versus temperature for this same reaction also reachesa peak between about 300° C. and about 350° C.In FIG.7,

light efficiency is defined as

Lightefficiency (%) =

CO, conversion rate due to lightx AMpeaction x 100% Intensityx Catalyst surface area

[0048] The salient point of FIGS. 4 through 7 takentogetheris that the visible light energy that induces plasmonic

activity in the catalyst is the cause of a very marked increasein the reaction rate ofthe reverse water-gas shift reaction. The

enhancementis achieved using simulated, non-coherent solarradiation.

[0049] Now, itcould be possible that the enhancedcatalytic

effect is not a photocatalytic effect, per se, but simply a

thermaleffect due to localized heating caused by the surfaceplasmonresonance.To investigate this possibility, an Arrhe-

nius plot (In(rate) v 1/T) was constructed for the light reac-tions described above for FIGS. 4-7 and the corresponding

dark reactions. The results are shown in FIG.8. Thus, FIG. 8is a plot depicting In(CO, reduction rate) versus 1/Temp(1/

K)x10° for the reverse water gas shift reactions. Theplots for

the light reaction versus the dark reaction clearly show dif-ferent activation energies. If the increased rate for the light

reaction werestrictly a localized heating effect, the activation

Oct. 15, 2015

energy for the light reaction versus the dark reaction shouldbe the same. However, the dark reaction (#) has an E, of

47.09+/—-0.27 kJ/mol, while the light reaction (@) has an E,, of34.93+/-0.47 kJ/mol. The change in E, indicates that it not

solely a localized heating effect that is responsible for thelight-induced enhancementofthe CO, reductionrates.

[0050] FIGS. and 10 are correspondingplots that map the

In(rate of CO, reduction) versus the In(partial pressure ofCO,) (FIG. 9) and In(rate) versus the In(partial pressure of

H,) (FIG. 10) for the light (#) and dark (@) reactions. In bothfigures, the reaction conditions were identical: Total gas flow

rate=15 sccm, P=110 psi, T=200° C., H,:CO,=2:1. In FIG. 9,

the rate equation sets up as [Rate]=Kapp-o> [PPco2|”. Thus,the exponent“m”is the reaction order andits value is depen-

dent upon the mechanism that causes the CO, reduction. InFIG.9, which is the data based on the partial pressure ofCO,,

it was foundforthe light reaction that m,,.,,=1.04; for the dark

reaction, m,,,,=0.50. These data clearly indicate that there isadistinctly different reaction mechanism forthe “light,” plas-

monically catalyzed reaction as comparedto the dark reac-tion.

[0051] The same holds true whenIn(rate) versus the In(par-

tial pressure of H,) is plotted for the light reaction versus thedark reaction. See FIG. 10. Here, the rate equation sets up as

[Rate]=Kapp;,. [PP,.]”. It was found for the light reactionthat n,,,,,-0.17; for the dark reaction, nj,,,=0.07.

[0052] FIGS. 11 and 12is a graph depicting the dependence

of light efficiency and rate enhancement on H,:CO, ratio inplasmon-enhanced water gas shift reaction over Aw/TiO,

catalyst. Experimental conditions: P=103 psi, T=200° C.,Total gas flow rate=15 sccm, catalyst amount=7.9 mg. As can

be seen in FIG.11, lowertheratio ofH,:CO,in the plasmoni-cally catalyzed reaction results in the higherlight efficiency

ofthe reaction. Thatis, at high light efficiencies, the reaction

producedincreased amounts ofH, as compared to CO. FIG.12 showsthat at low H,:CO,ratio, plasmonic rate enhance-

ment up to 1300% can be achieved.

[0053] Suitable catalysts for use in the present method may

be fabricated by the following methods. Note that these meth-

ods are exemplary andare includedsolely to provide a morecomplete disclosure ofthe method claimed herein. The exem-

plary catalysts are not limiting.

Preparation ofAu/T10, (DP) Catalyst:

[0054] The Au/TiO, DP was prepared by deposition-pre-

cipitation with NaOH (1M)'~. Titania Degussa P25 was used

as support (Sigma-Aldrich, >99.5% trace metal basis) andsolid HAuCl,.3H,O (Sigma-Aldrich, >99.9% trace metal

basis) as the precursor. Before the preparation, TiO, was driedin the air at 110° C. overnight. 100 ml of aqueous HAuCl,

solution (4.2*10-* M) washeated to 80° C.and the pH wasadjusted to 8 by drop-wise addition ofNaOH (1M). Then, 1 g

of TiO, was dispersed in the solution, and the pH wasread-

justed to 8 with NaOH. The suspension was thermostated at80° C. wasstirred for 2 h and centrifuged. The solids were

then washed, dried, and calcined at 300° C. underthe flow ofair (30 ml/min) with a heating rate of 2° C./min and main-

tained for 4h.

Preparation ofAu/CeO, DP Catalyst:

[0055] The Au/CeO, (DP)° was prepared by deposition-precipitation with NaOH (1M) which is same with Aw/TiO,

(DP)'?. Cerium (IV) oxide was used as support (Sigma-

US 2015/0291434 Al

Aldrich) and solid HAuCl,.3H,O (Sigma-Aldrich, >99.9%trace metal basis) as the precursor. Before the preparation,

CeO, was dried in the air at 110° C. overnight. 100 ml ofaqueous HAuCl1,solution (4.2*10-3 M) was heatedto 80° C.

and the pH wasadjustedto 8 by drop-wise addition ofNaOH(1M). Then, 1 g ofCeO, was dispersed in the solution, and the

pH was readjusted to 8 with NaOH. The suspension was

thermostatedat 80° C.wasstirred for 2h and centrifuged. Thesolids were then washed, dried, and calcined at 300° C. under

the flow ofair (30 ml/min) witha heating rate of2° C./min andmaintained for 4 h.

Preparation ofAu/Al,O, (DP) Catalyst:

[0056] The Au/Al,O,; (DP)* was prepared by deposition-precipitation with NaOH (1M) which is same with Au/TiO,

(DP)'?. Alumina was usedas support (Strem Chemicals) and

solid HAuCl,.3H,O (Sigma-Aldrich, >99.9% trace metalbasis) as the precursor. Before the preparation, Al,O, was

dried in the air at 110° C. overnight. 100 ml of aqueousHAuCl, solution (4.2*10-* M) washeated to 80° C.and the

pH wasadjusted to 8 by drop-wise addition of NaOH (1M).Then, 1 g ofAl,O, wasdispersed in the solution, and the pH

wasreadjusted to 8 with NaOH.The suspension was thermo-

stated at 80° C. was stirred for 2 h and centrifuged. The solidwas then washed, dried, and calcined at 300° C. under the

flow of air 30 ml/min) with a heating rate of 2° C./min andmaintained for 4 h.

Preparation of Cu/TiO, (1) Catalyst:

[0057] The Cu/TiO, (1) was prepared by impregnating 1 gof titania Degussa P25 (Sigma-Aldrich, >99.5% trace metal

basis) with a solution of 53 mg of CuSO,.5H,O (Sigma-

Aldrich, puriss, meets analytical specification ofPh. Eur., BP,USP, 99-100.5%) in 10 ml of DI water**. The slurry wasstirred for 4 h at room temperature, then all liquid was evapo-rated and the solid was dried at 110° C. overnight. The cata-

lyst was calcined at 300° C. undertheflow ofair (30 ml/min)

with a heating rate of 2° C./min and maintainedfor 4 h.

REFERENCE CITED

[0058] 1.R. Zanella, S. Giorgio, C. H. Shin, C. R. Henry

and C. Louis, J. Catal., 2004, 222, 357-367.

[0059] 2.R. Zanella, S. Giorgio, C. R. Henry and C. Louis,

J. Phys. Chem. B, 2002, 106, 7634-7642.

[0060] 3. D.L. Nguyen, S. Umbarkar, M. K. Dongare, C.Lancelot, J.S. Girardon, C. Dujardin and P. Granger, Catal.

Commun., 2012, 26, 225-230.

[0061] 4. C. Burda, X. Chen, R. Narayanan and M. A.El-Sayed, Chem. Rev., 2005, 105, 1025-1102.

[0062] 5. F. Sastre, M. Oteri, A. Corma and H. Garcia,

Energy Environ. Sci., 2013, 6, 2211-2215.

[0063] 6.F. Boccuzzi, A. Chiorino, G. Martra, M. Gargano,

N. Ravasio and B. Carrozzini, J. Catal., 1997, 165, 129-139.

Whatis claimedis:

1. A method ofreducing CO, to CO, the method compris-

ing:

(a) contacting CO, with a catalyst, in the presence of H,,

wherein the catalyst has plasmonic photocatalytic

reductive activity when exposedto radiation having awavelength between about 380 nm and about 780 nm;

and

Oct. 15, 2015

(b) exposing the catalyst, CO,, and H, to non-coherentradiation having a wavelength between about 380 nm

and about 780 nm suchthat the catalyst undergoes sur-face plasmon resonance, wherein the surface plasmon

resonanceincreasesthe rate of CO, reduction to CO ascompared to the rate of CO, reduction to CO without

surface plasmon resonancein the catalyst.

2. The methodofclaim 1, comprising, in step (b), exposing

the catalyst, CO,, and H, to solar radiation.

3. The method ofclaim 1, wherein the catalyst comprises a

metallic element have an average particle size no greater than

100 nm in combination with a semiconductor material.

4. The method of claim 3, wherein the metallic elementis

selected from the group consisting of calcium, copper,

europium, gold, lithium, magnesium, palladium, platinum,potassium, silver, sodium, rubidium, and yttrium, and com-

binations thereof; and wherein the semiconductor materialisselected from the group consisting of oxides of titanium,

aluminum,iron, silicon, zinc, and cerium, and combinationsthereof.

5. The method of claim 3, wherein the metallic element

comprises copper, silver, platinum,or gold, and the semicon-ductor material comprises titania orceria.

6. The method according to any one of claims 1 to 5,

wherein the surface plasmon resonance in the catalyst

increasesthe rate ofCO, reduction to COby a factorofat least1.8 as compared to the rate of CO, reduction to CO in the

absence of surface plasmon resonancein the catalyst.

7. The method according to any one of claims 1 to 5,wherein the surface plasmon resonance in the catalyst

increasesthe rate ofCO, reduction to COby a factorofat least3 as compared to the rate of CO, reduction to CO in the






wherein the surface plasmon resonance in the catalystincreasesthe rate ofCO, reduction to COby a factorofat least

5 as compared to the rate of CO, reduction to CO in theabsence of surface plasmon resonancein the catalyst.

10. A method ofreducing CO,to CO, the method compris-

ing:

(a) contacting CO, with a catalyst, in the presence of H,,wherein the catalyst has plasmonic photocatalytic

reductive activity when exposed to non-coherentradia-tion having a wavelength between about 380 nm and

about 780 nm; and

(b) exposing the catalyst, CO,, and H, to solar radiationsuch that the catalyst undergoes surface plasmon reso-

nance, wherein the surface plasmon resonanceincreases

the rate of CO, reduction to CO as comparedtothe rateof CO, reduction to CO without surface plasmon reso-

nancein the catalyst.

11. The method of claim 10, wherein upon exposing thecatalyst, CO,, and H,to solar radiation, the catalyst achieves

a light efficiency of at least about 2%.


a solar light efficiency of at least about 3%.

US 2015/0291434 Al


a solar light efficiency of at least about 4%.14. The method ofclaim 10, wherein the catalyst comprises

a metallic element have an average particle size no greaterthan 100 nm in combination with a semiconductor material.

15. The methodofclaim 14, wherein the metallic element

is selected from the group consisting calcium, copper,europium, gold, lithium, magnesium, palladium, platinum,

potassium, silver, sodium, rubidium, and yttrium, and com-binations thereof; and wherein the semiconductor materialis

selected from the group consisting of oxides of titanium,aluminum,iron, silicon, zinc, and cerium, and combinations

thereof.

16. The methodofclaim 14, wherein the metallic elementcomprises copper, silver, platinum,or gold, and the semicon-

ductor material comprises titania or ceria.17. The method according to any one of claims 10 to 16,

wherein the surface plasmon resonance in the catalyst

increasestherate ofCO, reduction to COby a factorofat least1.8 as compared to the rate of CO, reduction to CO in the

absenceof surface plasmon resonance inthe catalyst.

Oct. 15, 2015





increasesthe rate ofCO, reduction to COby a factorofat least



wherein the surface plasmon resonance in the catalystincreasesthe rate ofCO, reduction to COby a factorofat least


21. The method of any one of the preceding claims,

wherein the method is conducted at a temperature of fromabout 100° C. to about 400° C., wherein H, is present in a

greater concentration than CO,, and the H, and CO, arepresent at a pressure of from atmospheric to about 2000psi.

* * * * *

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