mJournal of CO2 Utilization 1 (2013) 1827

Contents lists available at SciVerse ScienceDirect

Journal of CO2

jo ur n al ho m ep ag e: www .eBoxun Hu a, Curtis Guild a, Steven L. Suib a,b,*aDepartment of Chemistry, University of Connecticut, Unit 3060, 55 North Eagleville Road, Storrs, CT 06269-3060, USAb Institute of Materials Science, University of Connecticut, USA

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2. Thermodynamics and kinetics of CO2 conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3. CO2 conversion to fuel and value-added products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1. CO2 conversion to CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.1. CO production by reduction of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1.2. CO production by electrocatalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.1.3. CO production by plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1.4. Synthesis gas production by reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2. CO2 conversion to HCOOH and HCHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3. CO2 conversion to CH3OH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4. CO2 conversion to long chain hydrocarbons and oxygenates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.5. CO2 as building blocks for oxygen-rich compounds and polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4. Prospective in CO2 conversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1. Introduction

Carbon dioxide (CO2) utilization technologies have emerged toreduce CO2 emissions by developing benecial uses of CO2 [14]. Intodays world, two major environmental concerns attributed in

part to buildup of carbon dioxide (CO2), the acidication of theoceans and global warming [5,6]. These issues are consequences inpart to atmospheric CO2 concentrations (ACC) rising around theglobe. Fossil fuel consumption has caused ACC increases from 280parts per million (ppm) in pre-industrial times to 382 ppm in 2006according to the National Oceanic and Atmospheric Administra-tion (NOAA). Currently, the atmospheric CO2 concentration is stillsteadily increasing at a rate of about 1.9 ppm/year. In tandem withthis, the international energy outlook (2011) has projected thatworld energy consumption will increase 53 percent from 2008 to2035 [7]. Scientists from the Intergovernmental Panel on Climate

A R T I C L E I N F O

Article history:

Received 27 December 2012

Received in revised form 14 March 2013

Accepted 15 March 2013

Available online 24 April 2013

Keywords:

Review

Carbon dioxide

Activation

A B S T R A C T

This review compares various alternate fuels and value-added products from conversion of carbon

dioxide such as simple molecules to higher hydrocarbon fuels and polymers. Different methods of

activation are summarized that lead to different products. We summarize the advantages and

disadvantages of different methods of conversion of carbon dioxide. An overall summary is given at the

end of the review that discusses future approaches and promising approaches.

2013 Elsevier Ltd. All rights reserved.

* Corresponding author at: Department of Chemistry, University of Connecticut,

Unit 3060, 55 North Eagleville Road, Storrs, CT 06269-3060, USA.

Tel.: +1 860 486 2797; fax: +1 860 486 2981.

E-mail address: steven.suib@uconn.edu (S.L. Suib).

2212-9820/$ see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jcou.2013.03.004Review

Thermal, electrochemical, and photochevalue-added productsical conversion of CO2 to fuels and

Utilization

l s evier . c om / lo cat e/ jc o u

Mimicking natures photosynthesis process, photoreduction ofCO2 is one of the most alluring methods for CO2 conversion due tothe abundance and free access of sunlight. To meet with globalenergy demands, Lewis and Nocera have proposed to convert andstore solar energy in chemicals (H2, methanol, and hydrocarbons)via the photosynthetic process [21]. Solar radiation varies acrossthe globe from altitude, height, atmospheric conditions, andseason. For example, solar radiation for at-plates facing south at axed tilt in New York City is about 26 kWh/m2/day [22]. A typicalphotoreduction electrode is composed of a semiconductor andphotocatalysts, and many of these are transition metal complexes.Semiconductors absorb photons to make excited electrons transferfrom a valence band to the conducting band, which is thentransferred to a photocatalyst complex, which reduces CO2 to COand other useful organic compounds (Fig. 2). Such photoelec-trocatalytic processes should be distinguished from purelyphotocatalytic routes.

Kubiak and Kumar reported the photo-assisted electrochemicalreduction of CO2 to CO on Re(bipy-Bu

t)(CO)3Cl((bipy-But) = 4,40-di-

tert-butyl-2,2-bipyridine)/p-type silicon with a Faradaic efciency

B. Hu et al. / Journal of CO2 Utilization 1 (2013) 1827 19Change (IPCC) have suggested that 350 ppm CO2 is likely the safeupper limit for atmospheric greenhouse gases [8]. To realize thisgoal, CO2 must be captured and either stored or converted intoglobal warming neutral impact compounds. Utilization of CO2provides an attractive avenue for this objective.

CO2 is extensively used for enhanced oil recovery, urea andpolymer synthesis as a monomer feedstock, the food and beverageindustry as a propellant, and chemical production [9]. However,only less than 1% of global anthropogenic CO2 generated areutilized to these ends. The rest is released to the atmosphere due tolack of economical technologies to convert these C1 sources tocommodity products, and therein lack of demand. CO2 conversionto fuel and value-added products is an ideal route for CO2utilization due to the simultaneous disposal of CO2 and the benetthat many products can be used as alternate transportation fuels.

CO2 is a kinetically and thermodynamically stable molecule,thus CO2 conversion reactions are endothermic and need efcientcatalysts to obtain high yield. Various renewable energies such assolar, wind, and hydroelectric are proposed as energy sources forCO2 conversion. These largely intermittent kinetic energies arestored in the alternate fuels in the stable form of chemical energythat can be transported and used on demand. These synthetic fuelswill sustain new and expanding markets in transportation.Furthermore, the synthetic fuels produced by CO2 utilization arecompatible with current hydrocarbon-based automobiles andtransportation systems. The products of carbon dioxide conversioncan supplement or replace chemical feedstocks in the chemical,pharmaceutical, and polymer industries.

This review summarizes the various alternate fuels and value-added products from CO2 conversion in order of products fromsimple molecules to higher hydrocarbon fuels and polymers. Eachproduct may be produced by several methods. The pros and consof different conversion approaches are compared, with the intentof giving direction to the reader in selecting a proper approach tomeet specic needs. Many good reviews and books [3,10] havedescribed CO2 utilization by photocatalytic synthesis [4,1113],electrochemical reduction [14,15], plasma [16], and othermethods [1,17,18]. We do not attempt to duplicate thesereferences but summarize possible reaction mechanisms, uniquecatalyst and experimental design, structure-activity relation-ships, and energy efciency. These issues provide help inunderstanding the product selectivity and catalytic activity ofeach system, and evaluate the potential practical applications ofthe process.

2. Thermodynamics and kinetics of CO2 conversion

Fig. 1 gives the Gibbs free energy of CO2 and the productsconverted from CO2 (CRC Handbook of Chemistry and Physics)[19]. CO2 molecules have a highly stable linear and centrosym-metric (O55C55O) structure. The difference in Gibbs free energy(DG) between the product and reactants at specic reaction is thedriving force shown in Eq. (1):

DG DH T DS (1)

where DH is the enthalpy change, DS is the entropy change, and T istemperature.

Most conversions of CO2 to these listed products areendothermic reactions like natural photosynthesis as shown inEq. (2):

6CO2g 6H2Ol hn ! C6H12O6s 6O2g (2)

There are several kinds of driving forces for the CO2 conversion.First, highly efcient articial enzymes lower the activation energyrequired for photoreduction of CO2. These exhibit rates faster thanin nature, though the cost of the catalysts is still a concern.Secondly, inorganic nanocatalysts promote conversion to value-added olens in the FischerTropsch synthesis. Third, while notcatalytic, applied potentials serve to facilitate the electrochemicalreduction of CO2. Fourth, CO2 will react with other reactants withhigher Gibbs free energy, such as methane and hydrogen. Last butnot least, CO2 reacts with other monomeric species to formpolymers. These polymerization reactions run at mild conditionswith high turnover numbers. In a practical operation, two or moretypes of principles may function simultaneously and make the CO2conversion more efcient. The net result of these processes is theconversion of different types of energies into chemical energy.

3. CO2 conversion to fuel and value-added products

3.1. CO2 conversion to CO

CO2 conversion to CO (carbon monoxide) looks like the simplestroute for CO2 reduction. CO is a feedstock or intermediate for theproduction of methanol and hydrocarbon fuels via the FischerTropsch synthesis [20]. Several technical routes lead to the CO2conversion to CO, such as photoreduction, electrolysis, plasma,electrocatalysis, dry- and bi-reforming, and (tri-) reforming.Different types of catalysts are involved in these processes.

3.1.1. CO production by reduction of CO2

Fig. 1. Gibbs free energy of CO2 and its related products.

production has been conducted in either liquid electrolytes orsolid oxide electrolytes. Low operation temperature is one of theadvantages of using liquid electrolytes, but low temperature oftenleads to low selectivity and low reaction rates. In aqueous solution,the selectivity for the above carbon species is low due to thecompetition of the hydrogen evolution reaction with CO2reduction. Highly acidic electrolytes exhibit similar behavior forCO2 reduction. Electrocatalysis cells based on solid electrolytes(alumina, yttrium-stabilized zirconia) show high conversion rates

B. Hu et al. / Journal of CO2 Utilization 1 (2013) 182720of 97 3%, and a short-circuit quantum efciency of 61% for light-to-chemical energy conversion, and an overall efciency of about 10% forthe conversion of polychromatic light [23]. Smieja et al. furtherreported that the electron transfer from the electrode to the catalystcan be controlled by modifying the p-Si surface with phenylethylgroups. The interaction experiments of the electrocatalyst with thetargeted catalytic substrate CO2, H2O, and CH3OH show that thereaction with CO2 is about 25 times faster than that with H2O, and 50times faster than that with CH3OH. Calculations based on densityfunctional theory (DFT) show that the nature of the binding of CO2 tothe anion forms a Re(bipy-tBu)(CO)3(CO2)K complex [24].

Kaneco has developed metal-modied p-InP photoelectrodesfor the photoelectrochemical reduction of CO2 in the LiOH/methanol-based electrolyte. Ag, Au, Pd, and Cu deposited p-InPphotoelectrodes show higher selectivity to CO than that to H2.Ag deposited p-InP photoelectrode show maximum currentefciency of carbon monoxide (rf = 80.4%) and Pd deposited p-InP photoelectrode has the highest selectivity to CO (100%) [25].CO is produced via one electron reduction reaction of CO2 andCO2 is an intermediate. The Gibbs energy differences of CO(g)splitting on many different metals change the catalyticselectivities. Compared to electrochemical reduction at thesame metallic electrodes, onset potentials on Pb, Ag, Au, and Nideposited photoelectrodes are lower (about 0.250.95 V less).

The activation energy barrier in the wide-scale applicationof photoreduction is the efciency of the catalyst vs. the cost ofthe materials used for synthesis. If the cost of photocatalystsand photoelectrocatalysts can be lowered and the efcienciesand lifetimes are improved, the photo-assisted electrochemicalreduction of CO2 could have possible practical applications.

Fig. 2. Illustration of semiconductor catalysts for photoreduction of CO2.3.1.2. CO production by electrocatalysis

Electricity is an easily accessible convenient energy, and can bereadily produced by a variety of renewable (wind, hydro, solar)energy sources. The captured kinetic energy can be converted toelectricity, and then stored in the form of chemical fuels byelectrocatalysis, electrochemical reduction, electrolysis, and plas-ma-assisted catalytic reduction. CO2 electrocatalysis for CO

Table 1Comparison of Pt electrocatalysts in CO2 reduction in 0.5 M KHCO3 solution. The percen

products are not counted.

Electrocatalysts Work conditions Rat

2 nm Pt/GDM(E-TEK) Semi-half continuous, 20 8C, 20 mA, 1 h 2 3 nm Pt/GDM 4.5

5.4 nm Pt-C/GDM 7.4

PtIr-C/GDM 80 mA/cm2, 25 8C, 2 V vs. SCE COand selectivity to CO, but require higher operating temperatures(around 8001000 8C) to achieve a high oxygen ion conductivity ofthe electrolyte. Electrode stability based on heat expansion is also aconcern as electrodes may crack or delaminate from the electrolytestructure.

For the aqueous CO2 reduction reaction (Eq. (3)):

CO2ad e ! CO2aq (3)

where CO2* is a radical anion as an intermediate. The equilibriumpotential (E vs. SCE, pH 7.0) is highly negative at 2.14 V. Ionicliquid electrolytes such as 1-ethyl-3-methylimidazolium tetra-uoroborate (EMIM-BF4) lower the potential for formation ofthe CO2

intermediate, most likely by complexation via a weakbonding between CO2 and BF4 anions. Very recently, Rosen et al.[26] reported that CO2 can be reduced at an applied voltage of1.5 V. This electrocatalytic system (Pt/Naon/(Ag, EMIM-BF4))reduces CO2 to carbon monoxide (CO) at very low overpotentials(

carbon supported Pt nanoparticles (3 nm) catalysts have the highestactivity among three different catalysts and supports. The nanome-ter scale of the catalysts signicantly increased the catalytic activity.Hydrocarbons were produced for the rst time using electrocatalyticreduction of CO2, and the hydrocarbon production will be includedin Section 3.4. Newman and Delacourt designed a PEM electrolysis-cell for simultaneous electrochemical reduction of CO2 and H2O tomake syngas CO + H2. A current density of 30 mA/cm

2 has beenachieved using silver-based cathode catalysts in these electrolysiscells [35]. Higher current density (135 mA/cm2) was achieved onsupported Au catalysts [36].

Solid oxide electrolysis cells (SOECs) have been developed forthe CO2 reduction [37]. Mogensen and Ebbesen have studiedreduction of H2O and CO2 in a Ni/YSZ electrode supported SOEC. COwas produced via the reverse water gas shift reaction with highercurrent densities (0.251 A/cm2) [38,39]. A degradation studyusing impedance spectroscopy shows that ve electrode processescontribute to the cell resistance [39]. Other SOECs using Pt pasteelectrodes were much less active compared to nano-size Ptcatalysts, and no quantitative CO products have been reported[40,41].

conditions [52]. There is considerable debate on the meaning ofCO2 reforming with methane, which from an energetic and lifecycle analysis perspective appears to be a questionable approach tolimit CO2 emissions.

3.1.4. Synthesis gas production by reforming

While carbon dioxide is the primary focus of this paper, aspart of the rening process the fractions of CO2 isolated oftencome with a high fraction of methane, which can be a valuableco-reactant with carbon dioxide. Methane itself has a globalwarming potential (GWP) of 30 (GWP of CO2 = 1). Methane isabundant in shale gas, coal gas, nature gas, and landll gas. Fortransportation, methane is not practical as a fuel due to itsstability and requirements for transport, though can be used inindustrial power applications. CH4/CO2 reforming to form syngas(CO/H2) is an important process [23,24] to synthesize alternatetransportation fuels via the FischerTropsch synthesis, whichuses syngas as its feedstock. As an added benet, thereformation of methane to fuel precursors prevents methanefrom escaping into the environment and preserves carbon intothe hydrocarbon production cycle. Considerable work has beenconducted in CH4/CO2 reforming [53,54]. A few good reviews are

SC).

B. Hu et al. / Journal of CO2 Utilization 1 (2013) 1827 213.1.3. CO production by plasma

Plasmas are fast and clean methods to convert CO2 to CO and O2.Numerous works have been performed in the plasma-assisted CO2reduction to CO [4247]. Early work was performed in 1990s underthe international cooperation of The University of Connecticut,Nagasaki University, and Fujitsu Laboratories Limited. Thedecomposition of CO2 in fan-type ac glow discharge plasmareactors and dielectric-barrier discharge (DBD)-plasma reactorswas investigated. CO2 decomposition was promoted by asynergetic effect between plasma excitation in the gas phaseand catalytic actions of the metal (Au, Cu, Pt, Pd, and Rh) coatedelectrode surface. CO was the main carbonaceous product withmoderate conversions at 30.5% and selectivities >80%. The energyefciency (36%) of plasma methods need to be improved [48],and Spencer et al. reported a theoretical energy cost analysismethod to evaluate the effectiveness of plasma systems [46]. Othermetal (Al, Cu, Ti, and Fe) electrodes for plasma activation in CH4/CO2 were tested in Lius group [49,50], and titanium showed thehighest activity, for reasons still under investigation [51]. DBDplasmas have been used in CH4/CO2 reforming for producingalkanes, alkenes, oxygenates, and syngas (CO + H2) at ambient

Fig. 3. Flow chart of the integrated tri-reforming power plant-steam cycle (ITRPP-Source: adapted from Ref. [58].recommended [5557].Power plant CO2 emission mixture has residue oxygen, which is

a poison for most CO2 reduction catalysts by promoting the water-gas shift reaction. Separating the oxygen from the carbonaceousgases adds considerable cost and complexity to CO2 conversionefforts, thus leading to the tri-reforming process that has beendeveloped at Pennsylvania State University. The three-stepreaction process avoids the separation step and has the promiseof being cost-efcient for producing industrially useful synthesisgas. The energy and environmental analysis of integrated tri-reforming power plants (ITRPP) has been reported by Minutillo andPerna [58,59]. The reduction in CO2 emissions has been estimatedat 83% (15.4 vs. 93.4 kg/GJ Fuel input) and 84% (8.9 vs. 56.2 kg/GJFuel input) for the ITRPP-SC (Fig. 3) and ITRPP-CC respectively. Thepower plant efciency is not penalized by using the tri-reformingprocess because the produced syngas can be used to generatechemicals and/or to feed fuel cell-based power plants. Further-more, this conventional chemical process by ammines needs highthermal power in order to regenerate the solvent. With newcapture technology development, the lower energy requirementsfor regeneration will lower the cost.

our global economy. From building blocks for plastics, paints, andorganic solvents to clean fuels applied in fuel cells and combustionengines, 50 million tons of methanol have been consumed in 2011.George Olah (Nobel Prize 1994) has proposed repurposing thehydrocarbon fossil fuel network into a methanol economy inwhich methanol gathered by CO2 reduction and bioconversion isused as a feedstock for transportation and energy storage. Forindustrial production of methanol from synthesis gas (CO, CO2, andH2), the mixture of copper, zinc oxide, and alumina has achievedhigh selectivity to methanol (99.8%) at 250300 8C and 510 MPain the Lurgi MegaMethanol process [79] and ICI processes [80,81].CO2 precipitates in the methanol synthesis via the reverse water-gas shift reaction. Park, Lim, and their coworkers found that anoptimal CO2 fraction can maximize the methanol yield and the CO2fraction also depends on the reaction temperature [82]. This mayindirectly show that CO2 contributes to the formation of methanol.Not all of the equilibria have been considered in this work. Theactive sites of Cu/ZnO/Al2O3 catalysts proposed by Behrens et al.consist of Cu steps decorated with Zn atoms, all stabilized by a

B. Hu et al. / Journal of CO2 Utilization 1 (2013) 182722CO2 reforming with CH4 is an example of CO2 being used as asoft oxidant, where the dioxide is dissociated into CO and surfaceoxygen, and oxygen abstracts hydrogen from methane to formwater via the water-gas shift reaction. This type of oxidativereaction has been extended into the oxidative coupling andoligomerization of CH4 [60,61] and dehydrogenation of alkanes(C2H6, C3H8, etc.) [6266] and alkyl aromatics [67,68]. Theemergence, application and mechanistic pathways of thesesystems have been reviewed by Park and Ansari [1]. Due tothe abundant availability, non-toxic, economic and mild oxidizingproperties of CO2, these green methods are economical and energyefcient and they have potential industrial applications for thedevelopment of various useful chemicals.

Shale gas contains mainly methane (60%) and light alkanes(C2C6). With the rapid increase of shale gas production in the USand other countries, the facile production of ethylene [63],propene [69], butene [70], and hexene[71] from shale gas usingthe method of oxidative coupling of CH4 and dehydrogenation ofalkanes is promised to lower production costs of these com-pounds.

3.2. CO2 conversion to HCOOH and HCHO

Formic acid (HCOOH) and formaldehyde (HCHO) are thesimplest oxygenates produced from the reduction of CO2 withH2O (or proton solvents). Generally, they are resulting productsof CO2

reduction with protons in an electrochemical reaction,and they are intermediates in the formation of methanol andhigher hydrocarbons. The selectivities of HCOOH, HCHO,and CH3OH are largely dependent on the reduction methodsand catalysts used.

Yadav et al. [72] recently reported an efcient articialphotosynthetic production of formic acid from CO2 using agraphene coupled multianthraquinone-substituted porphyrinphotocatalyst. Under 0.5 cm3/min of CO2, quantitative formic acidproducts were measured with a gas chromatograph but noquantum efciency was reported. About 46% of the total solarlight source available on earth is in the visible light range. Visiblelight driven photocatalysts give an alluring prospect in theproduction of fuels using solar energy, although currently mostcatalysts exhibit efciencies too low for practical applications.Graphene exhibits high electron mobility and acts as an electronreservoir; as such a major research effort into grapheneselectrochemical and electrical properties is in effect as of thiswriting. There is great potential for further accelerating thephotoreduction reaction of CO2. A signicant question here iswhether in the HCOOH synthesis by this photochemical approachthe effective mechanism involves an electrode for H2 photo-generation coupled with a catalyst to catalyze the reactionbetween CO2 and H2, probably in the liquid phase. Thesephotocatalysts are apparently not stable.

Another photoreduction system reported by Sato et al. iscomposed of a p-type semiconductor photosensitizer (N-Ta2O5)and a Ru complex reducing catalyst in an acetonitrile/triethano-lamine solution. This system has achieved a selectivity of morethan 75% for HCOOH using visible-light. The highest turnovernumbers (89) have been achieved (Fig. 4) using the N-Ta2O5photosensitizer but the quantum efciency of 1.9% at 405 nm islow [73].

Several different electrocatalytic reduction methods/catalystshave been developed for continuous electrocatalytic reduction ofCO2 to oxygenate products, including HCOOH and HCHO. Inaqueous KHCO3 electrolyte, Pt and less expensive Co and Fenanocatalysts were respectively loaded on carbon nanotubes forelectrocatalytic reduction of CO2, with oxygenates (isopropanol,methanol, ethanol, acetone, and acetaldehyde) isolated as theprimary products; [74] In a microuidic reactor, nanosize Ptelectrocatalysts reduced CO2 to formic acid at high Faradaic (89%)and energetic efciencies (45%) [73].

Solid oxide electrolyzers have attracted strong interest in recentyears due to high efciency in the conversion of electrical energyinto chemical energy [37,38,7577]. Co-electrolysis of CO2 andH2O using Ni/YSZ electrodes reported the formation of CO and H2,attributed to the water-gas shift reaction [38]. The electrocatalysisreactions of CO2 and H2O on Pt and ZnO nanocatalysts form HCHOin the gas headspace, and HCHO further forms paraformaldehydepolymers in condensed water [78]. A high CO2 conversion (8%)was reported in the continuous gas-phase reduction. Compared toelectrocatalytic reduction in liquid electrolyte, product separationin the gas phase reduction is not required.

Recent efforts and opportunities in the heterogeneous electro-chemical conversion of carbon dioxide have been described byWhipple and Kenis [15]. Compared to the production of HCOOHand HCHO by photoreduction of CO2, electrocatalytic reduction ofCO2 has a much higher energy efciency (3345%); the electro-catalytic system is less complicated; also, the operation unit can bemore compact and continuously achieve high time/space yield ofthe reactor. Therefore, large-scale utilization of electrocatalyticprocess is technically possible. Another process relevant here is thereduction of CO2 to HCOOH using biomass methods [139].

3.3. CO2 conversion to CH3OH

Methanol as a key commodity has become an important part of

Fig. 4. Turnover number for HCOOH formation from visible-light-induced selectiveCO2 reduction using Ru complex electrocatalysts.

Source: adapted from Ref. [73].

series of well-dened bulk defects and surface species that need tobe present jointly for the system to work [83].

Methanol can be produced directly from carbon dioxide sourcesby catalytic hydrogenation and photo-assisted electrochemicalreduction. Relevant reactions for hydrogenation from CO2 are asEqs. (4)(6):

CO2g 3H2g $ CH3OHl H2Ol DH298 K 49:9 kJ=mol(4)

CO2g H2g $ COg H2Og DH298 K 41 kJ=mol (5)

Pyridinium and its substituted derivatives exhibit effective andstable homogeneous electrocatalytic performance for the aqueousmultiple-electron/proton reduction of carbon dioxide to productssuch as formic acid, formaldehyde, and methanol [95]. A p-GaPsemiconductor has reduced CO2 in pyridine to methanol with 44%quantum efciency and near 100% selectivity at 0.5 V vs. SCE andunder 365 nm illumination [96]. Organic molecules can reduce CO2to highly reduced species through multiple electron transferswithout the need for a metal-based multi-electron transfer. Thisunique electrocatalyst shows that a single catalyst has the abilityto reduce multiple species. But the separation of methanol andrelated health issues should be considered.

fere

m2

B. Hu et al. / Journal of CO2 Utilization 1 (2013) 1827 23COg 2H2g $ CH3OHl DH298 K 90:8 kJ=mol (6)While the overall reaction of CO2 hydrogenation is exothermic

(DH = 49.9 kJ/mol), the rate determination step is activating CO2in the reverse water-gas shift reaction (Eq. (5)). Transitional metaloxides (Fe3O4, Mn3O4, and Co3O4) and Raney copper are activeRWGS catalysts at temperatures of about 300 8C. The kinetics andmechanisms of the WGS reaction have been extensively studied onFe, Cu, Ni, and Au catalysts [8489]. Formate (HCOO), carbonate(CO3

2), and carboxylate (HOCO) intermediates play an impor-tant role in the synthesis of hydrocarbon fuel [84]. The active siteshave been proposed by density functional theory (DFT) calcula-tions [83,85,86]. More details about the WGS reaction can be found[90].

Copper oxide is a catalyst for the WGS reaction. Cu/Zr/Al2O3catalysts work for both CO and CO2 hydrogenation in the methanolsynthesis. Logically, doping metals, which function as catalysts inthe RWGS reaction, would promote CO2 hydrogenation. DFTcalculations and Kinetic Monte Carlo (KMC) simulations on metal-doped Cu(1 1 1) surfaces have shown that the overall methanolyield increased in the sequence: Au/Cu(1 1 1) < Cu(1 1 1) < Pd/Cu(1 1 1) < Rh/Cu(1 1 1) < Pt/Cu(1 1 1) < Ni/Cu(1 1 1) [91].

A considerable research effort has been made on CO2 activationby visible light photocatalysts due to the natural abundance ofsunlight (Table 2). Due to the high energy requirements, thismethod is often paired with electrochemical methods viaphotoelectrocatalysis to push the reaction. The catalysts tradition-ally used are transition metal complexes, TiO2, ZnO, CdS, andfunctionalized metal surfaces. While TiO2 does not have the idealconducting band energy for dissociating CO2 (both the CB and CO2dissociation energies occur at 0.24 V), its anatase phase presentsa viable candidate for implementation due to its UV active bandgap (3.2 eV), its non-toxicity, and stability. A wide variety of CO2photoreduction has been performed on the surface of TiO2 underUV irradiation [92,93]. Mesoporous zeolite supported Ti-oxidesmainly produced methane and methanol. Addition of Pt increasesCH4 over methanol. The insight into the mechanistic aspects of CO2photoreduction using TiO2/mesoporous materials is included inIndrakantis review [12]. Bi2S3/CdS hetero-junction photocatalystsexhibit a high yield of 613 mmol methanol/g catalysts undervisible light irradiation [94]. Low potentials of conduction bands ofBi2S3 and CdS favor the formation of methanol.

Table 2Photoreduction and electrocatalytic reduction results of CO2 to methanol using dif

Catalytic-system Working conditions

Bi2S3/CdS + CO2+ NaOH/Na2SO3/H2O Catalysts + visible light

p-GaP + 10 mM pyridine + CO2 0.5 V vs. SCE, 365 nm

Pyridinium-KCl-H2O + CO2 Hydrogenated Pd or Pt, 50 mA c

TiO2/FSM16 zeolite CO2+ H2O, 328 K

Pt-TiO2/Y-zeolite CO2+ H2O, 328 K 3.4. CO2 conversion to long chain hydrocarbons and oxygenates

From the thermodynamic and kinetic point of view, CO2conversion to long chain hydrocarbons and oxygenates is morechallenge than to simple CO, CH4, HCOOH, and HCHO. Thesevaluable liquid state fuels have higher energy density and are moreconvenient to store and transport. The worlds current transporta-tion infrastructure and vehicles are designed for consuming theseliquid fuels.

In Section 3.1, various methods for CO2/H2O conversion tosyngas CO and H2 have been introduced. Syngas can be convertedto liquid fuel via FischerTropsch synthesis (Eq. (7)).

xH2g yCOg ! aCnH2n2l=g bCnH2nl=g cCnH2n1OHl dCn1H2n1CHOl eCn1H2n1COOHl zH2Ol (7)

where n is a positive integer, a, b, c, d, e, and z stand for the productfractions.

x na b c d e a c

y na b c d e

z na b c d ea d 2eAlkane products (CnH(2n+2)) are unbranched hydrocarbons,

suitable for diesel fuel and jet fuel. In addition, competingreactions also produce alkenes (CnH2n), as well as oxygenatedhydrocarbons (alcohols (CnH(2n+1)OH), aldehydes Cn1H2n1CHO,and carboxylic acids Cn1H2n1COOH). Various types of FTScatalysts with different catalytic selectivity have been developedfor the selective production of diesel fuel, light olens, and highalcohols. The FTS process has been used for the production ofsynthetic fuels for more than 50 years. A few companies (Sasol,PetroSA, and Shell) currently produce commercial synthetic fuelsand ne chemicals by the FTS process.

Considerable work has been reported in catalytic hydrogena-tion of CO2 to formic acid [97,98], methanol [99], and jet fuels [100]using homogeneous and heterogeneous catalysts. CO2 hydrogena-tion using FischerTropsch catalysts has shown the advantagesover homogeneous catalysts in terms of solvent separation,

nt catalytic systems.

Products and selectivity Energy eff. Refs.

613mmol CH3OH/g cat. NA [94]100% CH3OH Quant. 44%

Farad. 62%

[96]

HCOOH

CH3OH

Farad. 33% [95]

CH4CH3OH

NA [92]

CH4, CH3OH NA [93]

operation cost, and catalytic activity. But different from CO from that expected from the AndersonSchultzFlory (ASF)

Table 3Product selectivity, olen/parafn ratio, and CO2 conversion over three different Fe FTS catalysts.

Catalysts GHSV (h1) Temp. (K) Conv. (%) Selectivity (%) Refs

CO, CH4 C2C6 C55/C

15%Fe/10%K/g-Al2O3 3600 673 51 37.4 62.6 3.6 [117]6%Fe/4%K/28%Mn 3360 593 45 31.3 68.7 2.9 (11.2) [118]

2%Fe/4%Mn/12%K/4% Ce 270 563 50.4 37.7 62.3 4.4 [102]

B. Hu et al. / Journal of CO2 Utilization 1 (2013) 182724hydrogenation, CO2 is thermodynamically stable and is practicallyan inert gas. The reverse water-gas shift (RWGS) reaction (Eq. (5))is the key step for CO2 hydrogenation.

Fe [101104], Co [105,106], and Ru [107,108] catalystssupported on silica, gamma alumina, zeolite, titania, and carbonnanomaterials have been investigated. Addition of potassium (0.53%) can promote the growth of the long chain hydrocarbons[109,110], but higher potassium contents impede the catalyticactivity, and can even kill cobalt catalysts [111]. Manganese oxide[112] and ceria [104] are added as promoters due to theimprovement of RWGS activity [101]. Addition of Cu and Ru iniron catalysts increases the CO2 conversion [108], and Cu sites playthe role of forming hydroxyl groups and produce higher alcoholsdue to the synergetic effect among copper, potassium, and ironcatalytic sites [113]. Except catalysts design, a new concept of FTSreactor design has been proposed by Rahimpour to increasegasoline yield and reduce undesired products [114116].

Several efcient CO2 hydrogenation catalysts have beenrecently reported for higher hydrocarbon (>C2+) synthesis(Table 3). Without Mn promoters for the production of lightolens, the reaction temperature (where t is the absolutetemperature) in a xed bed reactor is higher as 673 K, and alsothe H2/CO2 (1:3) is low [117]. With addition of MnO2 nanobersupports, the reaction temperature in a xed fed reactor decreasedto 593 K, the selectivity to C2+ hydrocarbons increased about 6%.The interfaces of Fe/Mn supports signicantly affect the ratio ofolens/parafns. Fe supported K-OMS-2 nanocatalysts have high-est selectivity to light olens (C55/C = 11.2) [118]. For the ceriamodied Fe/Mn/K catalysts in a stirred tank reactor, addition ofceria increase 22% of CO2 conversion and 5% of increase in olenformation due to an increase of RWGS activity compared to thecatalysts without ceria, less Fe content in catalysts still leads tohigh activity for CO2 conversion due to excellent Fe dispersion[102]. The product distributions of Mn promoted catalysts showdeviated AndersonSchultzFlory distributions due to secondaryreactions and olen adsorption and insertion [119,120].

Long chain hydrocarbons (up to C9) have been synthesized byother CO2 conversion methods, such as electrocatalytic reduc-tion using Pt nanoparticles on carbon-based electrodes [34]. Butthe electrocatalytic reduction experiments showed that thesurface reaction-chain growth was very slow about 20 min forthe conversion of ethane to propane due to low reactiontemperature (298 K) [34]. The product distribution is differentdistribution for FischerTropsch synthesis. This may be due tostrong readsorption of intermediates in nanostructured (porous)carbon. Direct analyses in real time-time of ight-massspectroscopy (DART-TOF-MS) analysis of post-reaction catalystscan examine the adsorbed species [118]. Syngas, light hydro-carbons, and liquid fuels have been synthesized by CO2/CH4reforming using plasmas [52,121]. The conversions and selec-tivities are determined by the CH4/CO2 feed ratio, residencetime, and input power. The hydrocarbon distribution also didnot follow an ASF distribution. Carbon lms were formedwithout zeolite A [121].

Copper electrodes are effective for the formation of hydro-carbons in the CO2 electrochemical reduction in the aqueoussolution (Table 4) [122124]. Ogura et al. reported that ethylenehas been produced with a high selectivity (69%) and a total currentefciency of 97% using copper (I) halide/copper electrodes[124,125]. Copper alloy did not produce ethylene although AuCulowers the over potentials required [124,126]. AuCu catalysts needto work under the same conditions for comparison. A pathway ofCO2 to C2H4 was through the formation of carboxylic acid COOHgroups and adsorbed intermediates like CH2CO, which issupported by the formation of C1C3 carboxylic acids. Li andKanan reported that Cu2O layers formed on annealed Cu foil at500 8C is more active than polycrystalline Cu electrodes [127], butCu2O electrodes are still less selective than copper (I) halide/copperelectrodes. 30% of HCOOH in the products are also produced,indicating that the same pathway is followed. These experimentssuggest that Cu+ ions are the active sites for the selective ethyleneproduction.

3.5. CO2 as building blocks for oxygen-rich compounds and polymers

CO2 can be utilized as a monomeric building block to synthesizevarious value-added oxygen-rich compounds and polymers atmild conditions. Since polycarbonate (Eq. (8)) was synthesized byS. Inoue and co-workers using CO2 and propylene oxide in thepresence of ZnEt2 and H2O in 1969 [128], considerable efforts havebeen put into developing a copolymer synthesis based on CO2[129131], More active and controllable catalysts [132,133] basedon Zn, Co, and Cr complexes show very high activity with aturnover number up to 26,000, as well as excellent control for thecopolymerization of CO2 and cyclohexene (or propylene) oxide(Eq. (9)).(8)

(9)

(10)

Another breakthrough in CO2 utilization was made by Zhangand Yu. Various propiolic acids were synthesized under mild

system at 2080 8C. The reaction mechanism was proposed toinvolve superelectrophilic aluminum chloride activated carbondioxide reacting with aromatics via a typical electrophilicsubstitution.

4. Prospective in CO2 conversion

Table 4Comparison of Cu-containing electrodes with other metal electrodes in CO2 reduction for ethylene production. Alloy 1: monel metal, Ni + Co: 65%, Cu: 33%, Fe: 2%; NA: not

available in the reference.

Electrocatalysts Working cond. Product selectivity CO2 conversion Refs.

C2H4 CH4 CO H2 h

Pure Cu 3 M KBr, 1.8 V vs. Ag/AgCl 51.9 11.4 18.2 21.7 105 7.7 [124]Cu/CuCl 69.4 4.0 7.1 9.4 97.2 9.3 [124]

B. Hu et al. / Journal of CO2 Utilization 1 (2013) 1827 25conditions through copper- and copper-N-heterocyclic carbene(NHC)-catalyzed transformation of CO2 to carboxylic acid by CHbond activation and carboxylation of terminal alkyne (Eq. (11))[136]

The formation of oxazole 2-carboxylic acid in carboxylation ofoxazole (Eq. (12)) has been recently reported by Boogaerts andNolan using [(NHC)AuOH] complexes in THF at 45 8C [137]. Thesignicantly strong Au-OH base species permit the facilefunctionalization of CH bonds without the use of otherorganometallic reagents.

(12)A high selectivity (100%) and efcient process for the synthesisof cyclic carbonates has been developed by Yang et al. using anelectroreduction method in an ionic liquid at room temperature(Eq. (10)) [134,135]. A current efciency as high as 90% wasachieved at a constant potential of 2.4 V. Ethylene carbonate isused as a polar solvent with a molecular dipole moment of 4.9 D.Propylene carbonate is a high permittivity component of electro-lytes in lithium batteries.

Ag/CuCl 64.0 3.8

CuCl only 42.9 9.9

Alloy 1 0.3 5.2

Ag 1.3 4.9

Pt 4 M KBr, 2 V 49.9 8.0 Cu2O/Cu 0.8 V, 0.5 M NaHCO3 5 NA Except for production of CC and CO bonds in the aboveactivation of CO2, great success has made in direct formation ofCN bonds based on CO2 activation through molecular catalysis(Eq. (13)).Catalysts play a key role in the CO2 conversion to fuels andvalued added products. In the past decades, numerous methodsand catalysts have been developed to realize enhanced carbonrecovery. Current catalytic technologies for CO2 conversion have

achieved high energy efciency, high reaction rates, and high valueproducts although these are not achieved simultaneously by asingle method. An integration of several techniques and strategiesmay achieve practical production of high value chemical productsfrom CO2. Nanostructured, porous, and functional materials haveplayed and will continue to play an important role in thesecatalytic conversion processes. With a better understanding of thefundamental structurecompositionactivity relationships ofthese catalytic systems, the recipes, sizes, shapes, and morphol-ogies of the catalysts can be tuned for better catalytic performance.

(11)CN bond formation using CO2 as C1 feedstock for theproduction of various oxygenates, such as oxazolidinones,quinazolines, carbamates, isocyanates, and polyurethanes hasbeen reviewed by Yang et al. [2]. These commodity chemicals havebeen synthesized from green methods and have importantapplications in pharmaceutical and plastic industries.

Other important examples of CO2 utilization include carboxyl-ation of aromatics to arylcarboxylic acids with carbon dioxide[140]. Aromatic carboxylic acids were obtained in excellent yieldby carboxylation of aromatics with a carbon dioxideAl2Cl6/Al

9.3 13.0 98 10.2

6.7 34.8 103 5.8

5.9 93.3 108 1.3

79.6 7.4 98.4 11.6

4.9 28.1 98 9.5 [122]

8 >50 NA NA [127]Modern in situ characterization techniques and theories willpromote the rational design of cost-effective catalysts andprocesses.

This review demonstrated that it is technically possible touse CO2 as a carbon source for the synthesis of commodityproductsfrom simple CO to liquid fuels and high molecular

(13)

B. Hu et al. / Journal of CO2 Utilization 1 (2013) 182726polymers. Unfortunately, these highly endothermic CO2 conver-sions consume lots of energy. Renewable solar, wind, wave,hydropower, geothermal energy, and waste heat in plants are therst consideration. CO2 conversion has attracted more interest dueto simultaneously reducing emissions and creating value to offsetthe cost of disposal of CO2. For sustainable large-scale utilization ofCO2, the commodity products of CO2 conversion processes shouldbe economically viable and are in high demand. For practicalproduction, evaluations of energy balances and economic feasi-bilities of the processes are essential. The conversion processesmust also take into account the life cycle of the process to ensurethat additional CO2 is not produced beyond what is already beingremoved from or going into the atmosphere.

CO2 conversion is one stage of the carbon dioxide cycle. Thecompatibility of these projects and products with currentinfrastructure should be considered rst and is vital for thesuccess of research in this area. To develop successful CO2conversion projects, many technical and commercial barriersneed to be overcome. An interdisciplinary study should consideroverall design of product ow diagrams, economic analyses,processes, and catalyst development and optimization. Universi-ties and research institutions supported by government agenciesand industries mainly contribute to technology development.Industries are commercializing these mature technologies.Government agencies are nancially supporting this researchand development of CO2 conversion technologies, and will alsoregulate and stimulate CO2 conversion technology development.At last, CO2 utilization is a viable solution for repurposing andstoring this greenhouse gas, and international cooperation willhelp mature and apply technologies to achieve the CO2 emissioncontrol target.

The mechanisms of many of these reactions are not wellknown. Various reactions are believed to have differentintermediates and mechanistic steps. One electron transferreactions to CO2 (to form carbon dioxide anion radical) inhibitsfurther conversion, while a different mechanism is necessary toproduce CO via breaking of CO bonds. Identity of the exactsteps in activation of CO2 needs to be done to improveselectivities and yields in these reactions by modifying catalystsbased on such data. Other issues that are critical includestability, productivity, cost, environmental friendliness ofprocesses and other factors.

Acknowledgements

We acknowledge the support of the U. S. Department of Energy,Ofce of Basic Energy Sciences, Division of Chemical, Biological,and Geological Sciences for this work under grant DE-FGO2-86ER13622.A000.

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Thermal, electrochemical, and photochemical conversion of CO2 to fuels and value-added productsIntroductionThermodynamics and kinetics of CO2 conversionCO2 conversion to fuel and value-added productsCO2 conversion to COCO production by reduction of CO2CO production by electrocatalysisCO production by plasmaSynthesis gas production by reforming

CO2 conversion to HCOOH and HCHOCO2 conversion to CH3OHCO2 conversion to long chain hydrocarbons and oxygenatesCO2 as building blocks for oxygen-rich compounds and polymers

Prospective in CO2 conversionAcknowledgementsReferences