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Phil. Trans. R. Soc. A (2010) 368, 3343–3364doi:10.1098/rsta.2010.0119

REVIEW

Turning carbon dioxide into fuelBY Z. JIANG, T. XIAO, V. L. KUZNETSOV AND P. P. EDWARDS*

Department of Chemistry, Inorganic Chemistry Laboratory,University of Oxford, South Parks Road, Oxford OX1 3QR, UK

Our present dependence on fossil fuels means that, as our demand for energy inevitablyincreases, so do emissions of greenhouse gases, most notably carbon dioxide (CO2). Toavoid the obvious consequences on climate change, the concentration of such greenhousegases in the atmosphere must be stabilized. But, as populations grow and economiesdevelop, future demands now ensure that energy will be one of the defining issuesof this century. This unique set of (coupled) challenges also means that science andengineering have a unique opportunity—and a burgeoning challenge—to apply theirunderstanding to provide sustainable energy solutions. Integrated carbon capture andsubsequent sequestration is generally advanced as the most promising option to tacklegreenhouse gases in the short to medium term. Here, we provide a brief overview ofan alternative mid- to long-term option, namely, the capture and conversion of CO2,to produce sustainable, synthetic hydrocarbon or carbonaceous fuels, most notably fortransportation purposes.

Basically, the approach centres on the concept of the large-scale re-use of CO2 releasedby human activity to produce synthetic fuels, and how this challenging approach couldassume an important role in tackling the issue of global CO2 emissions. We highlight threepossible strategies involving CO2 conversion by physico-chemical approaches: sustainable(or renewable) synthetic methanol, syngas production derived from flue gases fromcoal-, gas- or oil-fired electric power stations, and photochemical production of syntheticfuels. The use of CO2 to synthesize commodity chemicals is covered elsewhere (Arakawaet al. 2001 Chem. Rev. 101, 953–996); this review is focused on the possibilities for theconversion of CO2 to fuels. Although these three prototypical areas differ in their ultimateapplications, the underpinning thermodynamic considerations centre on the conversion—and hence the utilization—of CO2. Here, we hope to illustrate that advances in the scienceand engineering of materials are critical for these new energy technologies, and specificexamples are given for all three examples.

With sufficient advances, and institutional and political support, such scientificand technological innovations could help to regulate/stabilize the CO2 levels in theatmosphere and thereby extend the use of fossil-fuel-derived feedstocks.

Keywords: energy materials; CO2 conversion; sustainable methanol; tri-reforming; solar fuel;photocatalyst

*Author for correspondence ([email protected]).

One contribution of 13 to a Discussion Meeting Issue ‘Energy materials to combat climate change’.

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3344 Z. Jiang et al.

1. Introduction

Fossil fuels pose a fundamental dilemma for our human society. On the one hand,their importance cannot be overstated—the combustion of coal, oil and naturalgas supply close to 90 per cent of our current energy needs and makes much ofwhat we do possible. On the other hand, their widespread use comes at a cost—the gases emitted during the burning of fossil fuels are strongly implicated asthe main drivers of climate change. Burning 1 t of carbon in fossil fuels releasesmore than 3.5 t of carbon dioxide (CO2); such cumulative concentrations of CO2now approach 1 Tt in the atmosphere (Mikkelsen et al. 2010). Nevertheless,it will remain difficult to wean our world away from fossil fuels. For all theirobvious faults, they remain relatively inexpensive, widely available and readilyadaptable to applications large and small, simple and complex. Figure 1 is arepresentation of our current global energy scenario based on fossil fuels; here,their intrinsic chemical energy is transformed into heat, electricity or movement(transportation) by combustion processes (Marbán & Valdés-Solís 2007).

There are several reasons why fossil fuels remain so popular (Fulkerson et al.1990). First, they are accessible in one form or another in almost all regions ofthe world. Second, humankind has learned how to use them effectively to provideenergy for a myriad of applications at every scale. Third, they are without equalas fuels for transportation: they are portable and contain a considerable amountof stored chemical energy. The overwhelming energy source for transportationderives from oil stocks (figure 1). Considering all of this, it becomes obvious thatour energy supply for the foreseeable future will surely be based on fossil-derivedhydrocarbon fuels, with the unavoidable production of CO2.

The prediction of our future energy supply of, say, oil has typically been lessaccurate than predictions of demand. In figure 2, we show the so-called humandevelopment index (HDI) plotted against per capita energy consumption, withenergy expressed in units of kilogram equivalents of oil. The HDI is a recognizedmeasure of the ‘quality of life’ developed by the UN development program. It isclear that a universal curve is followed; in our current global energy economy, ahigher quality of standard of living equates with increasing energy (fossil fuel)consumption (Kolasinski 2006).

Shifting the global energy balance in realistic time scales with a balanced useof oil, gas and coal while protecting our environment is now a critical universalchallenge. Until new, totally renewable or sustainable energy sources supplant oil,coal and natural gas, the scientific, technical and socio-economic challenge is clear:extract maximum energy from fossil fuels while attempting to minimize harm toour environment. A recent opinion paper on emissions pathways (Rogelj et al.2010) shows that the scale of the CO2 emissions problem is not being addressedquickly enough. Indeed, current national pledges mean that there is a greaterthan 50 per cent chance that global warming will exceed 3◦C by the year 2100.

Current approaches within the present fossil-fuel energy scenario (figure 1) forthe reduction of CO2 emissions from large-scale fossil energy facilities (e.g. powerstations and cement works) are primarily focusing on carbon capture and storage(CCS), with three generic options being proposed: post-combustion capture, pre-combustion capture and oxy fuel combustion (IPCC 2005, 2007; Nguyen & Wu2008). All three approaches are based on different physical and chemical processesinvolving absorption, adsorption and cryogenic capture of CO2.

Phil. Trans. R. Soc. A (2010)



Review. Turning carbon dioxide into fuel 3345

geo, solar, wind hydroelectric nuclear natural gas coal biomass oil

3.8%11.2%8.2%15.5%12.7%7.4%5.4%5.4%

27.0%electricitynetworks

38.1%lossesduring

distribution

4.3%

4.2%

16.6%

24.1%

industry residential and others transport

20.2%

12.5%

44.3%

11.8%

0.9%

0.3%

0.5%

16.7%

heat and industrynetworks

0.4%0.3%

15.7% totalelectricity,

EU objective(2010) = 22.1%

Figure 1. Current global energy scenario based on fossil fuels. Here, the distribution of currentconsumption of energy sources is noted. The figure shows the primary energy sources in our modernenergy society, and their ultimate secondary energy outlets. (Marbán & Valdés-Solís 2007.)

1000

900China

Poland UK

Italy Germany Finland USA

Canada

Saudi Arabia

United Nations Development Programmelife expectancyadult literacy

school enrolmentGDP per capita

RussiaLithuania

South Africa

India

Ethiopia

Iceland800

700

600

hum

an d

evel

opm

ent i

ndex

500

400

300

0 2 4 6 8energy consumption (kg oil equivalent per capita ×103)

10 12

Figure 2. The human development index (HDI) plotted against the per capita energy consumptionof 103 of the world’s most populous nations (adapted from Kolasinski 2006).





30

25

net v

olum

etri

c en

ergy

den

sity

(M

J l–1

)

net gravimetric energy density (MJ kg–1)

20

15

10

5

05 10 15 20 25 30 35 40batteries

700 bar H2200 bar methane

methanol

ethanol

E85

gasoline

diesel

M85

L H2

Figure 3. Fuel energy densities: net systems volumetric and gravimetric energy densities for variouson-board energy carriers (adapted from Pearson et al. 2009).

Major reductions in emissions from the transportation sector will necessitatea change in vehicle fuels. The three leading alternatives generally advanced atpresent are electric (battery), hydrogen and biofuels; the first two options requirefundamental large-scale changes in our energy infrastructure, while the latterwill not meet the exceptionally high and ever-growing (figure 2) demand fortransportation fuels. Indeed, there are forecasts that there exists a global biomasslimit, which dictates any truly sustainable supply of biofuels to between 20 and30 per cent of the projected requirement for transport fuels. A recent review byInderwildi & King (2009) provides an excellent overview of the potential, and thechallenges facing biofuels (see §2).

In order to take full advantage of the high ‘tank-to-wheel’ efficiency of electricvehicles, critical steps will also be needed to decarbonize the upstream energy(electricity) supply. In addition, batteries are fundamentally limited by their verylow net gravimetric and volumetric energy densities, as shown in figure 3 by arecent comprehensive ‘net systems analysis’ of various on-board energy carriers(Pearson et al. 2009). This figure also clearly illustrates that, while the neton-board density of liquid hydrogen comfortably exceeds that of batteries, itis still extremely low when compared with carbonaceous liquid fuels such asdiesel, gasoline, ethanol and methanol. In addition, the provision of hydrogenproduction, distribution and refuelling facilities will necessitate enormousinvestments for this completely new infrastructure base. This same analysis(Pearson et al. 2009) concludes:

The fundamentals of physics and electrochemistry dictate that the energy density of batteriesand molecular hydrogen is unlikely ever to be competitive with liquid fuels for transportapplications.

A fourth option, the focus of this review, has emerged, namely, the idea ofusing captured, anthropogenically produced CO2 to synthesize liquid renewable orsustainable hydrocarbon and carbonaceous fuels (Arakawa et al. 2001; Olah 2005;Centi & Perathoner 2009). This approach offers the intriguing possibility of usingprimary energy from renewable, carbon-free sources (such as electricity derived





from solar, wind, wave or nuclear) to convert CO2, in association with hydrogen(or indeed methane), into high-density vehicle fuels compatible with our currenttransportation infrastructure. Its real attraction is that this approach offers theprospect of decarbonizing transport without the paradigm shift in infrastructurerequired by electrification of the vehicle fleet or by conversion to a hydrogeneconomy (Pearson et al. 2009).

This increasing interest is focused on the concept of recovered CO2 beingused to synthesize fuels (through, for example, Fischer–Tropsch chemistry),and such an approach could substantially reduce carbon emissions (Arakawaet al. 2001; Sakakura et al. 2007; Centi & Perathoner 2009; Olah et al. 2009;Mikkelsen et al. 2010). This approach, which also encompasses the synthesis ofcommodity chemicals from CO2, we term carbon capture and conversion (CCC).It also has merit in that in the medium term it is expected that fossil-fuel powerplants fitted with CCS technologies may provide a source of high concentrations ofCO2. It is therefore critically important that the CCC technologies outlined hereare developed alongside work on CCS technologies. Research and development inmaterials for this energy technology are of critical importance to support the newenergy technologies to convert CO2 into fuels, and indeed chemicals. There are anincreasing number of excellent reviews on the important aspects of CCC (Song &Pan 2004; Song 2006; Sakakura et al. 2007; York et al. 2007; Centi & Perathoner2009; Indrakanti et al. 2009; Olah et al. 2009; Mikkelsen et al. 2010). Our objectivehere is to produce a brief overview of progress in the area and its potential, andto identify barriers to future progress. We identify three cases to illustrate theseaspects; this commentary interfaces with that of Züttel et al. (2010) in relationto the future development of hydrogen energy as a critical component of the newenergy technology.

2. Substitution of fossil fuels for transport by synthetic renewable fuels

The only way to stabilize the Earth’s climate is to stabilize the concentrationof greenhouse gases in the atmosphere. However, projected energy demands(figure 2) will make this much more difficult than has been previously thought(Friedlingstein 2008). Short-term strategies for the stabilization of CO2 emissionscentre on energy savings and efficient utilization (Boden et al. 2009; OECD/IEA2009). But, a successful long-term local and global strategy must surely be ableto stabilize the atmospheric CO2 levels by substitution of fossil fuels by renewableenergy sources. It is often written that the potential of renewable energy sourcesis higher—by several orders of magnitude—than any estimated world energydemand. Unfortunately, the vast majority of large renewable energy sources arealmost always located far away from the main consumption areas. One possibilityis the production of electricity to join an electric grid. If this is not a realisticpossibility, the renewable energy must be harvested in the form of energy carriers.As well as conventional energy storage for sustainable electricity, the generationof chemical energy carriers is another attractive alternative, with hydrogen andliquid carbonaceous carriers as the primary candidates.

The issues relating to hydrogen have been identified in the previous section. Thegreat value of liquid carbonaceous fuels (e.g. petrol, diesel and others) lies bothin their intrinsic (high) chemical energy content (figure 3) and in the ease with





renewable energy (all known sources)

electricity

sequestered

hydrocarbons + air + water

Fischer–Tropsch

NET production of carbon dioxide is ZERO

+ H2

nCO + 2nH2

CO + H2O

(CH2)n + nH2Ohydrocarbons

electrolysis of water

store energy as hydrocarbons

CO2

sustainablehydrogen

CO2

transport energy as hydrocarbons

use hydrocarbons as liquid fuels

Figure 4. A generic energy cycle using captured or sequestered CO2 and sustainable or renewablehydrogen to yield carbon-neutral or renewable carbonaceous fuels (courtesy of M. L. H. Green).

which they are stored and transported using existing infrastructure. It is of coursepossible to reduce CO2 directly with hydrogen (hydrogenation), or potentiallyelectricity, to synthesize carbonaceous fuels. However, such an approach wouldnot impact positively on the global carbon balance since hydrogen and electricityas produced today are largely derived from fossil fuels, which, themselves, producelarge amounts of CO2.

If, however, renewable sources could be used as the energy vector to transformCO2 into fuels, one has a most attractive route to providing carbonaceous fuelsthat would not contribute to net CO2 emissions.

In figure 4, we show a generic, idealized energy cycle where one transformsCO2 to ‘carbon-neutral’ liquid fuels in which sustainable or renewable electricityis used to produce hydrogen and the resulting Fischer–Tropsch process yieldsliquid hydrocarbon fuels. As noted earlier, the intrinsically high energy densityof these fuels and their transportability make them highly desirable.

Importantly, such synthetic fuels do not contain any sulphur. In addition,methanol (arguably the ‘simplest’ synthetic carbonaceous fuel) is a candidateboth as a hydrogen source for a fuel cell vehicle and indeed as a transportfuel, and dimethyl ether is viewed as a ‘superclean’ diesel fuel (Pan et al. 2007;Centi & Perathoner 2009; Minutillo & Perna 2009).

The energy requirement for the production of such renewable liquid fuelsdepends critically on the method used for the capture of CO2 (invariably fromlarge-scale emitters) and the method used for the production of hydrogen. Aslong as power generation for our present energy economy using fossil-fuel sourcesremain (figure 1), sources of CO2 will be available for use in such proposedrecycling systems. These would capture CO2 from the atmosphere by physical,chemical or biological methods, and any successful cycle would rely on cheap and





electricity

propulsionfood=

synthetic fuels

pollutedexhaust gas

CO2+H2O

biomass

sugar or starch-richbiomass

cellulosicbiomass

bio gascatalytic liquefaction

exhaust gas purification

putrefaction

photosynthesis

Figure 5. A possible carbon cycle for synthetic fuel production from biomass. Reproduced fromInderwildi & King (2009) with permission of the Royal Society of Chemistry.

abundant non-fossil primary energy sources—incidentally, the same requirementsfor any hydrogen economy—to ensure that the proposed synthesis cycle isCO2-neutral (figure 4).

In the biological world, fuels derived directly from photosynthesis (biofuels)could form the underpinning new technologies based on the successful principlesof photosynthesis. As Barber (2009) has noted in an overview of the subject,there are significant possibilities in exploiting solar energy to generate fuels, suchas hydrogen, alcohols and methane, by sustainable routes. The reader is referredto this excellent review for the potential of new photochemical energy technologiesthat mimic our natural system (Barber 2009). Similarly, Inderwildi & King (2009)have outlined an innovative carbon cycle for the production of synthetic fuels frombiomass (figure 5). Here, biomass is converted to biogas, mainly methane, usinganaerobic digestion. The biogas can then be converted into a mixture of CO andH2, the so-called synthesis gas (syngas), using catalytic processes. This syngas isthen converted to liquid synthetic fuel and gaseous hydrocarbons using catalystssuch as Co, Ru or Fe.

3. CO2 conversion to carbonaceous fuels

(a) Thermodynamic considerations

In figure 6, we illustrate a key aspect of the thermodynamics of any possibleCO2 conversion, where the Gibbs free energy of formation of CO2 and relatedsubstances are shown. CO2 is clearly a highly stable molecule and consequentlya substantial input of energy, optimized reaction conditions and (almostinvariably) active catalysts are necessary for any chemical conversion of CO2to a carbonaceous fuel.

However, it is important to note that any chemical reactions (conversions)are driven by differences in the Gibbs free energy between the reactants andproducts of a chemical reaction (under certain conditions), as shown by theGibbs–Helmholtz relationship:

DG0 = DH 0 − TDS0.





50 N2

NH3CH4

CO

H2 O2

CO2

C10H22

C8H18(0.0) (0.0) (0.0) (34.4)

(17.3) C3H8

CH3OH

C2H6(–23.5)(–32.9)

(–16.6)

(–50.7)

(–137.15)(–159.2)

(–394.0)

H2O(–228.4)

0

–50

–100

–150

–200

–250

ΔG0 (

kJ m

ol–1

)

–300

–350

–400

–450

Figure 6. Gibbs free energy of formation for selected chemicals (data compiled and calculated fromNIST database, http://webbook.nist.gov/chemistry/name-ser.html). Here, DG0 for the constituentelements is taken as the reference point.

Any attempt to use CO2 as a ‘chemical feedstock’ therefore must take closeaccount of the relative stability of the ultimate reaction products, as comparedto the reactants.

Both terms (DH 0 and TDS0) of the Gibbs free energy are not favourablein converting CO2 to other molecules (Freund & Roberts 1996). The carbon–oxygen bonds are relatively strong and substantial energy must be input for theircleavage in terms of carbon reduction. Similarly, the entropy term (TDS0) makeslittle or no contribution to the thermodynamic driving force for any reactioninvolving CO2. Importantly, one can then take the enthalpy term DH 0 as a goodinitial guide for assessing thermodynamic stability and feasibility of any CO2conversions.

It was pointed out by Freund & Roberts (1996) in an important contributionon the surface chemistry of CO2 that any progress in the use of CO2 as a usefulreactant (here towards fuel synthesis) will only emerge through judicious useof novel catalytic chemistry. As we will attempt to illustrate, it is in this areathat materials chemistry, physics and engineering can have the greatest potentialimpact. Again, as these authors point out, a positive change in free energy shouldnot by itself be taken as a sufficient reason for not pursuing potentially usefulreactions involving CO2. Thus, DG0 only provides information as to the yieldof products at equilibrium through the relationship DG0 = −RT ln K and thekinetics of such a process might indeed be favourable. Thus, provided that thekinetics is favourable, CO2 reduction to CO (a key step in all conversion reactions)may also be possible at metal surfaces, or some other catalytic material, e.g.nanoscale metal particles encapsulated in nano- and mesoporous hosts (Pan et al.2007; Centi & Perathoner 2009). As noted by Song & Pan (2004) and Song (2006):

There appears to be some perceptions . . . that CO2 conversion would be so endothermic thatits conversion would not be feasible.

Of course, a large number of industrial-scale chemical manufacturing processesare currently operated worldwide on the basis of strongly endothermic chemical





0

FT synthesis

syngas

reactants from fiuegas for tri-reforming

–50

–100

–150

–200

–250

–300

–350

–400

–450

–500

ΔH0 (

kJ m

ol–1

)

H2

CO + 3H2 CO + 2H2

2CO + 2H2

CH4

–73.6(FT DME)

–94.5(FT Methanol)

+206.3(SRM)

–35.6 (POM)

+247.6(DRM)

CO2

CO

H2O(g)CH3OCH3

CH4 + H2O

CH4 + CO2

CH3OH

O2

CH4+ O212

Figure 7. The enthalpy of reaction for syngas production and Fischer–Tropsch (FT) synthesis ofmethanol and dimethyl ether. The various reactions labelled DRM, SRM and POM are listed intable 1 and discussed in §3c.

reactions. The steam reforming of hydrocarbons to yield syngas and hydrogen isa classic example:

CH4 + H2O −→ CO + 3H2 DH 0 = +206.3 kJ mol−1.

It is important to stress that the above, highly endothermic reaction is usedworldwide for the high-volume production of ‘merchant hydrogen’ in the gas,food and fertilizer industries.

The corresponding CO2 reforming of CH4 (so-called ‘dry reforming’) illustratesthe important reaction of CO2 with a hydrocarbon, which will be of centralimportance to our considerations of converting CO2 in flue gases to yield achemical fuel:

CH4 + CO2 −→ 2CO + 2H2 DH 0 = +247.3 kJ mol−1.

The energy input for CO2 reforming of CH4 requires approximately 20 per centmore energy input as compared with steam reforming, but this is certainly not aprohibitive extra energy cost for this chemical reaction.

Importantly, these two reactions give rise to syngas with different H2/COmolar ratios. Both are useful for the formation of syngas, for ultimate liquidfuel production.

In figure 7, we collate the relevant enthalpy contributions, which nicelyillustrate the importance of chemical reactions to convert CO2. That is, it issignificantly easier, thermodynamically, if CO2 is used as a co-reactant typicallywith another substance that has a higher (i.e. less negative) Gibbs free energy,e.g. H2 or CH4. These hydrogen-bearing energy carriers give up their intrinsicchemical energy to promote the conversion of CO2.

Thus, the heats of reaction (the enthalpies of reaction) for CO productionfrom CO2 as the single reactant, and with CO2 energetics as a co-reactant,





are particularly important and illustrative. Compare the energetics of the thermaldissociation of CO2,

CO2 → CO + 12O2 DH 0 = +293 kJ mol−1,

with that of the reaction of CO2 with H2,

CO2 + H2 → CO + H2O DH 0 = +51 kJ mol−1.

This aspect may be further illustrated by the process of ‘oxyforming’, whereby,on deliberately increasing the amount of oxygen into the dry reforming reaction,the enthalpy of reaction is significantly reduced:

3CH4 + O2 + CO2 −→ 4CO + 6H2 DH 0 = +165.8 kJ mol−1,

5CH4 + 2O2 + CO2 −→ 6CO + 10H2 DH 0 = +104.6 kJ mol−1.

(b) Renewable, synthetic methanol—a sustainable organic fuel for transport

Commercially, methanol is produced from syngas using natural gas or coal,mainly containing CO and H2 along with a small amount of CO2. The reactionis usually catalysed by Cu/ZnO-based catalysts that have high reactivity andselectivity (Centi & Perathoner 2009; Fan et al. 2009). The annual, worldwideproduction currently exceeds 40 Mt.

It is also possible to synthesize methanol directly from CO2 by combining itwith hydrogen according to the reaction:

CO2 + 3H2 −→ CH3OH + H2O.

Interestingly, this can also be viewed as a mechanism for ‘liquefying’ hydrogenchemically using CO2!

In the same way that biofuels recycle CO2 biologically, a ‘sustainable’,synthetically artificial cycle can also be envisaged where the carbon in themethanol is recycled by extracting and utilizing CO2 from the atmosphere(figure 8). Overall, such a process could be carbon-neutral. The energy input toelectrolyse water to produce hydrogen and that used for the capture and releaseof CO2, of course, need to be carbon-neutral.

These ideas date back over 30 years, in one form or another. The obviousattraction is that carbon is effectively ‘chemically sequestered’ to yield syntheticfuels, allowing our continued exploitation of fossil fuels without causing the netaccumulation of CO2 in the atmosphere.

Olah and colleagues (Olah 2005; Olah et al. 2009) have pioneered and advancedthe concept, and potential widespread adoption, of the methanol economy,emphasizing the production of CH3OH (or dimethyl ether) by the chemicalrecycling of CO2. A real attraction of such an approach is that one could envisagethe catalytic hydrogenation of CO2 focused at small, delocalized production sitesas an alternative to the current large-scale, localized sites producing methanol bysteam reforming of CH4.

A fundamental materials challenge in this area centres on the fact that,generally, CO2 and H2 will only react at high temperatures in multi-componentheterogeneous catalysts (York et al. 2007; Centi & Perathoner 2009). A recent,important process has been developed for the homogeneous conversion of CO2 to





energy inhydrogen from

electrolysis of water

methanol synthesis

fuel use32

atmospheric CO2

CO2 capture

CO2 from fossilfuels burning

at power plants

synthetichydrocarbonsand products

H2O → H2 + O2

CO2 + 3H2 → CH3OH + H2O

CH3OH + O2 → CO2 + 2H2O

carbon out

carbon in

12

Figure 8. A cycle for sustainable methanol production (adapted from Olah et al. 2009 and usedwith permission).

methanol using non-metal complexes (Ashley et al. 2009; Riduan et al. 2009). Thiscentres on the activation of H2 by so-called frustrated Lewis pairs in specificallydesigned, organometallic complexes (Mömming et al. 2009).

The critical issues for this methanol cycle (figure 8) therefore centre onthe following points:

— The separation (and hence availability) of CO2 at high (volume)concentrations, for example, in large industrial plants.

— Concentrating CO2 directly from the atmosphere—essentially, cost-effective, energy-efficient, high-rate ‘direct air capture’ technologies.

— The development of high-performance, robust and inexpensive catalysts,operating (ideally) at low temperatures (in general, entropy considerationsfor such energy storage reactions are best carried out at low temperaturesto reduce the free energy required).

— The availability and cost of sustainable hydrogen, particularly if therequirement is hydrogen production using renewable energy (figure 4).

A recent, detailed analysis of the ‘well-to-tank’ energetics of methanol synthesisfrom atmospheric CO2 (Pearson et al. 2009) concludes that by far the greatestcomponent of the energy requirement of the process is that to produce thehydrogen (figure 8). This aspect highlights, once again, the close links between themajor research challenges in any future hydrogen economy—in this case, cheaprenewable routes to the production of hydrogen—and any future economy basedon renewable, carbonaceous liquid fuels.

(c) Tri-reforming: using flue gas for CO2 conversion

Currently, around one-third of all anthropogenic CO2 emissions originatefrom fossil-fuel power plants. Adding CCS (typically employing post-combustiontechnologies based on chemical absorption with ammines, at ca 85–90% capture





Table 1. Main reactions for syngas production by tri-reforming of natural gas.

enthalpy,reaction stoichiometry DH 0

298 (kJ mol−1)

CO2 reforming of methane (DRM) CH4 + CO2 ↔ 2CO + 2H2 +247.3 (endothermic)steam reforming of methane (SRM) CH4 + H2O ↔ CO + 3H2 +206.3 (endothermic)

partial oxidation of methane (POM) CH4 + 12O2 ↔ CO + 2H2 −35.6 (exothermic)

catalytic combustion of methane (CCM) CH4 + 2O2 ↔ CO2 + 2H2O −880 (exothermic)

efficiencies) leads to overall efficiency penalties of about 8–10% and increases inenergy costs of some 30–60%. Therefore, it would be highly desirable if flue gasmixtures (typically 8–10% CO2, 18–20% H2O, 2–3% O2 and 60–70% N2) couldbe used directly for CO2 conversion without costly pre-separation of CO2.

Song and colleagues (Song 2000, 2001; Song & Pan 2004) have pioneered anovel process centred on the unique advantages of directly utilising flue gas,rather than pre-separated and purified CO2 from flue gases, for the production ofhydrogen-rich syngas from methane reforming of CO2 (so-called ‘dry reforming’).The overall process, named ‘tri-reforming’, couples the processes of CH4/CO2reforming, steam reforming of CH4, and partial oxidation and complete oxidationof CH4. The reactions involved are itemized in table 1, together with thecorresponding enthalpies of reaction (298 K).

Coupling CO2 and H2O can give syngas with the desired H2/CO ratiosfor methanol and dimethyl ether synthesis and higher-carbon Fischer–Tropschsynthesis of fuels. It also helps to avoid the formation of particulate (solid) carbondeposits arising from reactions such as

CH4 −→ C + 2H2O and 2CO −→ C + CO2.

Experimental studies by Song & Pan (2004) have shown that the introductionof the CO2 tri-reforming reaction may also enhance the durability and lifetimeof metal nanoparticle catalysts owing to the addition of oxygen (and consequentoxidation of carbon deposits).

It is possible to achieve up to 95 per cent methane conversion by this processat equilibrium temperatures in the range 1073–1123 K. To achieve effectiveconversion (of both CO2 and CH4), the flue gas is combined with natural gasand used as chemical feedstocks for the production of syngas (CO + H2) withdesired H2/CO ratios. In addition, the process makes use of ‘waste heat’ inthe power plant and heat generated in situ from partial oxidation of methane(POM) with the O2 present in the flue gas (table 1). In effect, the twoendothermic reactions noted in table 1 are thermally sustained by the wasteheat content of the exhaust gases, and the partial combustion of the primarymethane fuel.

In summary, the advantages of tri-reforming centre on the prevention ofcarbon deposition, controllable H2/CO ratios (for effective syngas production)and a more autothermic reaction enthalpy than simple dry reforming of methanewith CO2.





The growing worldwide interest towards the tri-reforming process is connectedto the attractive possibility of potential integration of this technology intogas-turbine-based electric power cycles, having very low overall CO2 emissions(Halmann 1993; Song 2006; Minutillo & Perna 2009). Detailed experimentalstudies, computational analysis and engineering evaluations are under wayon the tri-reforming process. The great attraction is that the CO2 in powerplant exhausts could now be used directly in catalytic processes to generatea syngas suitable for ultimately delivering energy fuels (and a variety ofchemical products).

In terms of catalyst materials, the majority of studies have focused onthe important CH4–CO2 reforming component of the tri-reforming process(Song & Pan 2004; Song 2006; Cho et al. 2009). Both nanoparticulate Niand Co have frequently been employed as active metal components owing totheir high intrinsic catalytic activities, wide availability and (relatively) lowcosts (Hu 2009). The critical problem for all of these (and other) catalyticmaterials centres on serious carbon deposition in the industrial CO2 reformingof methane. This leads to rapid catalyst deactivation and reaction inhibitionas the supporting structures become blocked by carbon particulate deposits(York et al. 2007; Fan et al. 2009). Comprehensive investigations, however, haverevealed that such carbon deposition was strongly influenced by the precisemode of operation of the chemical conversion process. Importantly, fluidized-bed reforming leads to significant enhancement in the CH4 conversion processand a considerably reduced carbon deposition when compared with the fixed-bed operation process (Wurzel et al. 2000; Tomishige 2004; Hao et al. 2009).Further optimization of the fluidized-bed configuration has taken the form ofinnovative approaches using a fluidized bed assisted by an external, axial magneticfield. Hao et al. (2008) have recently reported studies of CH4–CO2 reforming onaerogel Co/Al2O3 nanoparticulate catalysts in a magnetic fluidized bed. In theirstudy, Co was introduced as the active catalyst component for the reformingprocess; here, they have taken advantage of the high Curie temperature of Co(ca 1120◦C) that makes it ideally suited for the high operating temperatures ofbetween 700 and 1000◦C necessary for the reforming process. In addition, theinfluence of an external magnetic field on the catalytic activity and stabilityof these catalyst systems was investigated in detail and compared with datafor a conventional fluidized bed and a static bed. These impressive studies aresummarized in figure 9, which is a compilation of conversion efficiencies forboth CH4 and CO2. Also shown are images of the operating catalysts thatclearly demonstrate that carbon deposition is considerably reduced throughimproving the gas–solid efficiency by the use of the external magnetic field.For these ferromagnetic particulate catalysts, it is quite clear that magnetic-fieldenhancement of operating process properties may be a most important avenuefor future, major studies.

One should also highlight the very great potential of ‘sustainable methane’ forthe tri-reforming process, and this might alleviate some of the issues discussedby Inderwildi & King (2009) in their recent review of biofuels.

A recent, extensive investigation for the treatment of CO2 from fossil-fuel-firedpower plants, using the tri-reforming process integrated with full electrical powerco-generation, reveals an impressive estimated reduction in CO2 emissions closeto 85 per cent (Minutillo & Perna 2009).




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(d) Photochemical production of synthetic fuels

As noted in §3a, the CO2 reduction process is highly endothermic. Therefore,sustainable CO2 utilization is possible only if renewable energy, such as solar,wind, wave or nuclear, is used as the energy source. Solar photocatalytic reductionof CO2 has the potential to be a means both of CO2 recycling and of storingintermittent solar energy in synthetic carbon-neutral fuels suitable for storage anduse in the residential, industrial and transportation sectors. However, the efficientphotoreduction of CO2 using solar radiation is one of the most challenging tasksof environmental catalysis. In a recent report, the US Department of Energy (Bellet al. 2007) has identified the development of advanced catalysts for the photo-driven conversion of CO2 and water as one of the three priority research directionsfor advanced catalysis science for energy applications.

Inoue et al. (1979) were the first to report the photocatalytic reduction ofCO2 in aqueous solutions to produce a mixture of formaldehyde, formic acid,methanol and methane using various wide-band-gap semiconductors. Since then,intensive efforts have been focused on the photochemical production of fuels byCO2 reduction using a variety of photocatalysts (Halmann 1993; Hwang et al.2005; Indrakanti et al. 2009; Olah et al. 2009).

The primary steps of photocatalytic reduction of CO2 are absorption of lightphotons in a photocatalyst material, and subsequent conversion of these photonsinto electron–hole pairs, which then have to be spatially separated to drivechemical oxidation and reduction half-reactions at the semiconductor–electrolyteinterface (figure 10).

The overall conversion efficiency of photocatalytic production of solar fuelsis currently significantly lower than that of photovoltaic solar systems. Unlikephotovoltaic electricity generation, where absorption of a photon results directlyin the formation of an electron–hole pair, all methods of producing solarfuels must involve the coupling of an absorption process with multi-electron





redox reactions. In the case of CO2 photoreduction using water as a reductant,both multi-electron transfer reactions of CO2 reduction and water oxidationhave to occur simultaneously. As a result, current CO2 photoreduction catalystsare less effective than current solar water-splitting catalysts (Indrakanti et al.2009). Several other important factors limit the solar-to-fuel energy conversionefficiency of photocatalytic systems, including inefficient absorption of solarenergy, fast recombination processes of the photo-excited electron–hole pairs andfacile backward redox reactions.

The properties of potential photocatalysts are determined by the redoxpotentials of the rate-limiting steps of water oxidation and CO2 reduction:

2H2O(l) −→ O2(g) + 4H+(aq) + 4e− E0ox = −1.23 V,

CO2(aq) + e− −→ CO−2 (aq) E0

red = −1.65 V.

These values impose a minimum threshold for the energy of photo-excitedelectrons required to reduce CO2 and also the energy levels of the conduction andvalence bands of a photocatalyst. Wide-band-gap semiconductors are the mostsuitable photocatalysts for CO2 reduction because photo-generated electrons inthe bottom of the conduction band can have sufficient negative redox potentialto drive CO2 reduction, while the photo-generated holes in the valence band canbe sufficiently energetic (positive holes) to act as acceptors and oxidize waterto O2. A significant disadvantage of using wide-band-gap semiconductors is theinability to use visible light efficiently. Quite simply, light with energy smallerthan the semiconductor electronic band gap cannot generate electron–hole pairsand is therefore wasted as heat. As a result, only the UV and near-UV parts of thesolar spectrum can be used with the present photocatalysts such as TiO2 (withan electronic band gap of around 3.2 eV), yielding a low absorption coefficientand, consequently, low conversion efficiency in the visible region.

Although many semiconductors have smaller band gaps and absorb in thevisible range (e.g. CdS and Fe2O3 with band-gap values of 2.4 and 2.3 eV,respectively), just a few of them are catalytically active because the energylevels of either the conduction or valence bands are unsuitable for CO2 reductionand/or water oxidation. This limitation, together with poor photo-corrosionstability of many semiconductors, limits significantly the number of potentialphotocatalyst materials.

Figure 11 displays the quantum efficiency of photocatalytic water splittingplotted against wavelength of a wide range of inorganic materials (Osterloh 2008).Note that this interesting summary represents only the water-splitting componentof the photocatalytic process, but it nevertheless highlights a range of potentialmaterials for the combined H2O/CO2 photochemical process. For any prospectivephotocatalyst to be considered economically viable, it has to display a quantumefficiency of greater than 10 per cent in the visible region of the solar spectrum.To date, no materials have been discovered that satisfy this criterion, as seenin the shaded area in figure 11. Worldwide, active research is now focused onthe search for new photocatalysts efficient in the visible spectrum of sunlight.Several groups of novel efficient photocatalysts have recently been discovered,including oxynitrides (e.g. Ga1−xZnxO1−yNy ; Maeda et al. 2006; Jiang et al. 2008;





100 ZnSLa : NaTaO3

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10

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Figure 11. Quantum efficiency of different materials for photocatalytic water splitting in the UVand visible parts of the spectrum (adapted by D. Payne, Department of Chemistry, University ofOxford, from Osterloh 2008).

biomass

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syngaschemicals

fuels

H2/electrolysis

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combustion

biogasor

CH4

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Figure 12. Natural photosynthesis uses solar energy to recycle CO2 (and H2O) into new plant life(biomass) and ultimately fuels (biofuels). Sustainable, tri-reforming uses CO2, renewable energyand CH4 (or biogas) to yield syngas and, ultimately, synthetic fuels and commodity chemicals.

Luo et al. 2009; Varghese et al. 2009), tantalates (e.g. NaTaO3 and others;Kudo & Miseki 2008), bismuth-based photocatalysts (e.g. BiVO4; Long et al.2006; Shang et al. 2009; Jiang et al. 2010) and some others (Kudo & Miseki 2008).

For a combination of cost, stability and performance, TiO2 still remains oneof the most effective photocatalysts among other semiconductors, and it hasbeen used to convert CO2 to useful compounds, in both gas- and aqueous-phase





photoreactions (Ni et al. 2007; Nguyen & Wu 2008; Indrakanti et al. 2009). Severalapproaches have been employed to enhance the efficiency of photocatalysts.Promising methods to expand the light absorption of TiO2 into the visibleregion are band-structure modification by suitable cation or anion doping andthe use of co-catalysts loaded on the surface as nanoparticles (Jiang et al. 2008;Kudo & Miseki 2008; Luo et al. 2009). Using photocatalyst and co-catalystmaterials in the nanocrystalline form also offers the attractive opportunityto minimize the distances (and thus the times) over which photo-inducedelectron–hole pairs have to survive and be transported to reactant moleculesafter photo-excitation. The formation of heterojunction structures between twonanostructured semiconductors with different band gaps and matching bandpotentials can also extend light absorption, improve charge separation, increasethe lifetime of charge carriers, and enhance the interfacial charge-transferefficiency (Long et al. 2006; Tada et al. 2006; Yang et al. 2009). Depositionof noble metal nanoscale clusters on the photocatalyst surface has also beenshown to enhance the photocatalytic activity owing to increased transfer ratesof photo-generated electrons to the absorbed metal/semiconductor clusters, thusdecreasing markedly the possibility of electron–hole recombination (Baba et al.1985; Zhang et al. 2009).

An alternative approach using the photoelectrocatalytic reduction of CO2has been proposed recently (Centi et al. 2007; Centi & Perathoner 2009). Inthis concept, water dissociation occurs at a photoanode and CO2 reductionat a photocathode, which are attached to both sides of a proton-conductingmembrane. Such a photoelectrocatalytic device shows a strong similarity toproton-exchange membrane fuel cells and can take advantage of significanttechnological developments of that energy technology.

Developments of cheap, reliable and efficient methods for the production ofsolar fuels represent a major challenge in solar energy utilization. The key issuein optimizing photocatalyst materials is to achieve the same conversion efficiencyfor visible-light-induced CO2 reduction as already demonstrated for near-UVirradiation and, at the same time, to significantly increase the chemical stabilityagainst damage from incident light and also attack from highly reactive speciesformed during the photoelectrochemical redox reactions (figure 10).

4. Concluding remarks

In a recent masterly survey of their work over the past 15 years, Olah et al. (2009)expound a vision that reflects the basis of our present review:

Carbon dioxide . . . can be chemically transformed from a detrimental greenhouse gas causingglobal warming into a valuable, renewable and inexhaustible carbon source of the futureallowing environmentally neutral use of carbon fuels and derived hydrocarbon products.

With carbon capture and sequestration (storage) becoming a key element inworldwide efforts to control/minimize emissions, it can be anticipated that largeamounts of CO2 will become available as feedstock for innovative conversions tosynthetic fuels. Of course, ‘whole process’ energy balances and economics remain





a critical issue—as indeed is the case for any overall vision of energy futures. Itis an obvious, but nevertheless a most potent, fact that any new fossil fuels canonly be formed naturally over geological time scales.

The challenge, therefore, is to achieve such transformations from natural andindustrial sources, as well as human activities, via CO2 capture and subsequentconversion to synthetic fuels over, what one might term, less demandingtime scales! The cycle in figure 12 reflects particularly the tri-reforming routeto a sustainable fuel (§3c).

In this work, we have attempted a brief overview of the current scientificchallenges in the area, with an emphasis on the important, underlyingthermodynamic considerations and the urgent agenda for materials researchand development (figure 12). This vision in many ways constitutes humankind’sartificial version of the natural recycling of CO2 to energy fuels by photosynthesisusing sunlight as the primary energy source (Barber 2009).

To bring any, or all, of these innovative approaches into real life, technologieswill require major advances in research and development, as well as significantsocio-political commitments to CO2 capture and utilization. Internationalcollaborative research between developed, and developing countries (figure 2) isalso absolutely critical in such a venture. Our hope is that this present summaryhelps to catalyse such a worthwhile development.

We are most grateful to James Barber, Oliver Inderwildi, Steven Jenkins, David King, RichardPearson, Malcolm Green, David Payne, John Bottomley and Leon Di Marco for many importantdiscussions. This work was supported by EPSRC under their UKSHEC Consortium (SUPERGEN)programme, and we are also grateful for that support. Z. J. is grateful for the John HoughtonResearch Fellowship in Sustainable Energy, Jesus College, Oxford.

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