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Phil. Trans. R. Soc. A (2010) 368, 3343–3364
doi:10.1098/rsta.2010.0119
REVIEW
Turning carbon dioxide into fuel
B Y Z. J IANG, T. X IAO, V. L. K UZNETSOV
AND
P. P. E DWARDS*
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 inevitably
increases, so do emissions of greenhouse gases, most notably carbon dioxide (CO2 ). To
avoid the obvious consequences on climate change, the concentration of such greenhouse
gases in the atmosphere must be stabilized. But, as populations grow and economies
develop, future demands now ensure that energy will be one of the defining issues
of this century. This unique set of (coupled) challenges also means that science and
engineering have a unique opportunity—and a burgeoning challenge—to apply their
understanding to provide sustainable energy solutions. Integrated carbon capture and
subsequent sequestration is generally advanced as the most promising option to tackle
greenhouse gases in the short to medium term. Here, we provide a brief overview of
an alternative mid- to long-term option, namely, the capture and conversion of CO2 ,
to produce sustainable, synthetic hydrocarbon or carbonaceous fuels, most notably for
transportation purposes.
Basically, the approach centres on the concept of the large-scale re-use of CO2 released
by human activity to produce synthetic fuels, and how this challenging approach could
assume an important role in tackling the issue of global CO2 emissions. We highlight three
possible strategies involving CO2 conversion by physico-chemical approaches: sustainable
(or renewable) synthetic methanol, syngas production derived from flue gases from
coal-, gas- or oil-fired electric power stations, and photochemical production of synthetic
fuels. The use of CO2 to synthesize commodity chemicals is covered elsewhere (Arakawa
et al. 2001 Chem. Rev. 101, 953–996); this review is focused on the possibilities for the
conversion of CO2 to fuels. Although these three prototypical areas differ in their ultimate
applications, the underpinning thermodynamic considerations centre on the conversion—
and hence the utilization—of CO2 . Here, we hope to illustrate that advances in the science
and engineering of materials are critical for these new energy technologies, and specific
examples are given for all three examples.
With sufficient advances, and institutional and political support, such scientific
and technological innovations could help to regulate/stabilize the CO2 levels in the
atmosphere 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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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 natural
gas supply close to 90 per cent of our current energy needs and makes much of
what 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 as
the main drivers of climate change. Burning 1 t of carbon in fossil fuels releases
more than 3.5 t of carbon dioxide (CO2 ); such cumulative concentrations of CO2
now 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 their
obvious faults, they remain relatively inexpensive, widely available and readily
adaptable to applications large and small, simple and complex. Figure 1 is a
representation 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 of
the world. Second, humankind has learned how to use them effectively to provide
energy for a myriad of applications at every scale. Third, they are without equal
as fuels for transportation: they are portable and contain a considerable amount
of stored chemical energy. The overwhelming energy source for transportation
derives from oil stocks (figure 1). Considering all of this, it becomes obvious that
our energy supply for the foreseeable future will surely be based on fossil-derived
hydrocarbon fuels, with the unavoidable production of CO2 .
The prediction of our future energy supply of, say, oil has typically been less
accurate than predictions of demand. In figure 2, we show the so-called human
development index (HDI) plotted against per capita energy consumption, with
energy expressed in units of kilogram equivalents of oil. The HDI is a recognized
measure of the ‘quality of life’ developed by the UN development program. It is
clear that a universal curve is followed; in our current global energy economy, a
higher 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 use
of oil, gas and coal while protecting our environment is now a critical universal
challenge. 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 to
our environment. A recent opinion paper on emissions pathways (Rogelj et al.
2010) shows that the scale of the CO2 emissions problem is not being addressed
quickly enough. Indeed, current national pledges mean that there is a greater
than 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) for
the reduction of CO2 emissions from large-scale fossil energy facilities (e.g. power
stations and cement works) are primarily focusing on carbon capture and storage
(CCS), with three generic options being proposed: post-combustion capture, precombustion capture and oxy fuel combustion (IPCC 2005, 2007; Nguyen & Wu
2008). All three approaches are based on different physical and chemical processes
involving absorption, adsorption and cryogenic capture of CO2 .
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3345
Review. Turning carbon dioxide into fuel
geo, solar, wind
hydroelectric
nuclear
0.3%
5.4%
5.4%
0.4%
natural gas
7.4%
12.7%
coal
15.5%
biomass
8.2%
oil
3.8%
11.2%
0.3%
15.7% total
electricity,
EU objective
(2010) = 22.1%
electricity
networks
heat and industry
networks
27.0%
11.8%
4.3%
losses
during
distribution
38.1%
44.3%
0.9%
4.2%
16.7%
0.5%
16.6%
12.5%
24.1%
20.2%
residential and others
industry
transport
Figure 1. Current global energy scenario based on fossil fuels. Here, the distribution of current
consumption of energy sources is noted. The figure shows the primary energy sources in our modern
energy society, and their ultimate secondary energy outlets. (Marbán & Valdés-Solís 2007.)
1000
Canada
Poland UK
900
human development index
China
Italy
800
Germany
700
Lithuania
Russia
600
South Africa
Finland USA
Iceland
Saudi Arabia
India
500
United Nations Development Programme
life expectancy
adult literacy
school enrolment
GDP per capita
400
300
Ethiopia
0
2
4
6
8
10
12
energy consumption (kg oil equivalent per capita ×103)
Figure 2. The human development index (HDI) plotted against the per capita energy consumption
of 103 of the world’s most populous nations (adapted from Kolasinski 2006).
Phil. Trans. R. Soc. A (2010)
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3346
net volumetric energy density (MJ l–1)
Z. Jiang et al.
30
diesel
25
gasoline
E85
20
M85
15
ethanol
methanol
10
L H2
5 700 bar H2
200 bar methane
0
batteries
5
10
15
20
25
30
net gravimetric energy density (MJ kg–1)
35
40
Figure 3. Fuel energy densities: net systems volumetric and gravimetric energy densities for various
on-board energy carriers (adapted from Pearson et al. 2009).
Major reductions in emissions from the transportation sector will necessitate
a change in vehicle fuels. The three leading alternatives generally advanced at
present are electric (battery), hydrogen and biofuels; the first two options require
fundamental large-scale changes in our energy infrastructure, while the latter
will not meet the exceptionally high and ever-growing (figure 2) demand for
transportation fuels. Indeed, there are forecasts that there exists a global biomass
limit, which dictates any truly sustainable supply of biofuels to between 20 and
30 per cent of the projected requirement for transport fuels. A recent review by
Inderwildi & King (2009) provides an excellent overview of the potential, and the
challenges facing biofuels (see §2).
In order to take full advantage of the high ‘tank-to-wheel’ efficiency of electric
vehicles, critical steps will also be needed to decarbonize the upstream energy
(electricity) supply. In addition, batteries are fundamentally limited by their very
low net gravimetric and volumetric energy densities, as shown in figure 3 by a
recent comprehensive ‘net systems analysis’ of various on-board energy carriers
(Pearson et al. 2009). This figure also clearly illustrates that, while the net
on-board density of liquid hydrogen comfortably exceeds that of batteries, it
is still extremely low when compared with carbonaceous liquid fuels such as
diesel, gasoline, ethanol and methanol. In addition, the provision of hydrogen
production, distribution and refuelling facilities will necessitate enormous
investments 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 batteries
and molecular hydrogen is unlikely ever to be competitive with liquid fuels for transport
applications.
A fourth option, the focus of this review, has emerged, namely, the idea of
using captured, anthropogenically produced CO2 to synthesize liquid renewable or
sustainable hydrocarbon and carbonaceous fuels (Arakawa et al. 2001; Olah 2005;
Centi & Perathoner 2009). This approach offers the intriguing possibility of using
primary energy from renewable, carbon-free sources (such as electricity derived
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Review. Turning carbon dioxide into fuel
3347
from solar, wind, wave or nuclear) to convert CO2 , in association with hydrogen
(or indeed methane), into high-density vehicle fuels compatible with our current
transportation infrastructure. Its real attraction is that this approach offers the
prospect of decarbonizing transport without the paradigm shift in infrastructure
required by electrification of the vehicle fleet or by conversion to a hydrogen
economy (Pearson et al. 2009).
This increasing interest is focused on the concept of recovered CO2 being
used to synthesize fuels (through, for example, Fischer–Tropsch chemistry),
and such an approach could substantially reduce carbon emissions (Arakawa
et al. 2001; Sakakura et al. 2007; Centi & Perathoner 2009; Olah et al. 2009;
Mikkelsen et al. 2010). This approach, which also encompasses the synthesis of
commodity 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 power
plants fitted with CCS technologies may provide a source of high concentrations of
CO2 . It is therefore critically important that the CCC technologies outlined here
are developed alongside work on CCS technologies. Research and development in
materials for this energy technology are of critical importance to support the new
energy technologies to convert CO2 into fuels, and indeed chemicals. There are an
increasing number of excellent reviews on the important aspects of CCC (Song &
Pan 2004; Song 2006; Sakakura et al. 2007; York et al. 2007; Centi & Perathoner
2009; Indrakanti et al. 2009; Olah et al. 2009; Mikkelsen et al. 2010). Our objective
here is to produce a brief overview of progress in the area and its potential, and
to identify barriers to future progress. We identify three cases to illustrate these
aspects; this commentary interfaces with that of Züttel et al. (2010) in relation
to the future development of hydrogen energy as a critical component of the new
energy 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 concentration
of 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 emissions
centre on energy savings and efficient utilization (Boden et al. 2009; OECD/IEA
2009). But, a successful long-term local and global strategy must surely be able
to stabilize the atmospheric CO2 levels by substitution of fossil fuels by renewable
energy sources. It is often written that the potential of renewable energy sources
is higher—by several orders of magnitude—than any estimated world energy
demand. Unfortunately, the vast majority of large renewable energy sources are
almost always located far away from the main consumption areas. One possibility
is the production of electricity to join an electric grid. If this is not a realistic
possibility, the renewable energy must be harvested in the form of energy carriers.
As well as conventional energy storage for sustainable electricity, the generation
of chemical energy carriers is another attractive alternative, with hydrogen and
liquid carbonaceous carriers as the primary candidates.
The issues relating to hydrogen have been identified in the previous section. The
great value of liquid carbonaceous fuels (e.g. petrol, diesel and others) lies both
in their intrinsic (high) chemical energy content (figure 3) and in the ease with
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3348
Z. Jiang et al.
renewable energy (all known sources)
electrolysis of water
electricity
sustainable
hydrogen
sequestered
CO2
+ H2
CO + H2O
nCO + 2nH2
(CH2)n + nH2O
hydrocarbons
Fischer–Tropsch
store energy as hydrocarbons
transport energy as hydrocarbons
use hydrocarbons as liquid fuels
hydrocarbons + air
CO2
+ water
NET production of carbon dioxide is ZERO
Figure 4. A generic energy cycle using captured or sequestered CO2 and sustainable or renewable
hydrogen 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 course
possible to reduce CO2 directly with hydrogen (hydrogenation), or potentially
electricity, to synthesize carbonaceous fuels. However, such an approach would
not impact positively on the global carbon balance since hydrogen and electricity
as produced today are largely derived from fossil fuels, which, themselves, produce
large amounts of CO2 .
If, however, renewable sources could be used as the energy vector to transform
CO2 into fuels, one has a most attractive route to providing carbonaceous fuels
that would not contribute to net CO2 emissions.
In figure 4, we show a generic, idealized energy cycle where one transforms
CO2 to ‘carbon-neutral’ liquid fuels in which sustainable or renewable electricity
is used to produce hydrogen and the resulting Fischer–Tropsch process yields
liquid hydrocarbon fuels. As noted earlier, the intrinsically high energy density
of 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 candidate
both as a hydrogen source for a fuel cell vehicle and indeed as a transport
fuel, 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 fuels
depends critically on the method used for the capture of CO2 (invariably from
large-scale emitters) and the method used for the production of hydrogen. As
long as power generation for our present energy economy using fossil-fuel sources
remain (figure 1), sources of CO2 will be available for use in such proposed
recycling systems. These would capture CO2 from the atmosphere by physical,
chemical or biological methods, and any successful cycle would rely on cheap and
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Review. Turning carbon dioxide into fuel
exhaust gas purification
CO2
+H2O
photosynthesis
biomass
polluted
exhaust gas
sugar or starch-rich
biomass
=
food
cellulosic
biomass
electricity
propulsion
synthetic fuels
catalytic liquefaction
3349
bio gas
putrefaction
Figure 5. A possible carbon cycle for synthetic fuel production from biomass. Reproduced from
Inderwildi & King (2009) with permission of the Royal Society of Chemistry.
abundant non-fossil primary energy sources—incidentally, the same requirements
for any hydrogen economy—to ensure that the proposed synthesis cycle is
CO2 -neutral (figure 4).
In the biological world, fuels derived directly from photosynthesis (biofuels)
could form the underpinning new technologies based on the successful principles
of photosynthesis. As Barber (2009) has noted in an overview of the subject,
there are significant possibilities in exploiting solar energy to generate fuels, such
as hydrogen, alcohols and methane, by sustainable routes. The reader is referred
to this excellent review for the potential of new photochemical energy technologies
that mimic our natural system (Barber 2009). Similarly, Inderwildi & King (2009)
have outlined an innovative carbon cycle for the production of synthetic fuels from
biomass (figure 5). Here, biomass is converted to biogas, mainly methane, using
anaerobic digestion. The biogas can then be converted into a mixture of CO and
H2 , the so-called synthesis gas (syngas), using catalytic processes. This syngas is
then converted to liquid synthetic fuel and gaseous hydrocarbons using catalysts
such 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 possible
CO2 conversion, where the Gibbs free energy of formation of CO2 and related
substances are shown. CO2 is clearly a highly stable molecule and consequently
a substantial input of energy, optimized reaction conditions and (almost
invariably) active catalysts are necessary for any chemical conversion of CO2
to 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 and
products of a chemical reaction (under certain conditions), as shown by the
Gibbs–Helmholtz relationship:
DG 0 = DH 0 − T DS 0 .
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3350
Z. Jiang et al.
50
0
–50
ΔG0 (kJ mol–1)
–100
O2
(0.0)
H2
(0.0)
N2
(0.0)
NH3
(–16.6)
C2H6
(–32.9)
CH4
(–50.7)
–150
CO
–200
(–137.15)
–250
–300
C10H22
C8H18
(34.4)
(17.3)
C3H8
(–23.5)
CH3OH
(–159.2)
H2O
(–228.4)
–350
–400
–450
CO2
(–394.0)
Figure 6. Gibbs free energy of formation for selected chemicals (data compiled and calculated from
NIST database, http://webbook.nist.gov/chemistry/name-ser.html). Here, DG 0 for the constituent
elements is taken as the reference point.
Any attempt to use CO2 as a ‘chemical feedstock’ therefore must take close
account of the relative stability of the ultimate reaction products, as compared
to the reactants.
Both terms (DH 0 and T DS 0 ) of the Gibbs free energy are not favourable
in converting CO2 to other molecules (Freund & Roberts 1996). The carbon–
oxygen bonds are relatively strong and substantial energy must be input for their
cleavage in terms of carbon reduction. Similarly, the entropy term (T DS 0 ) makes
little or no contribution to the thermodynamic driving force for any reaction
involving CO2 . Importantly, one can then take the enthalpy term DH 0 as a good
initial guide for assessing thermodynamic stability and feasibility of any CO2
conversions.
It was pointed out by Freund & Roberts (1996) in an important contribution
on the surface chemistry of CO2 that any progress in the use of CO2 as a useful
reactant (here towards fuel synthesis) will only emerge through judicious use
of novel catalytic chemistry. As we will attempt to illustrate, it is in this area
that materials chemistry, physics and engineering can have the greatest potential
impact. Again, as these authors point out, a positive change in free energy should
not by itself be taken as a sufficient reason for not pursuing potentially useful
reactions involving CO2 . Thus, DG 0 only provides information as to the yield
of products at equilibrium through the relationship DG 0 = −RT ln K and the
kinetics of such a process might indeed be favourable. Thus, provided that the
kinetics 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 that
its conversion would not be feasible.
Of course, a large number of industrial-scale chemical manufacturing processes
are currently operated worldwide on the basis of strongly endothermic chemical
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Review. Turning carbon dioxide into fuel
0
O2
–50
–100
ΔH0 (kJ mol–1)
–250
CH4+
CH4
FT synthesis
CO
CH3OCH3
CH3OH
–94.5
(FT Methanol)
syngas
2CO + 2H2
H2O(g)
+206.3
(SRM)
CH4 + H2O
CO2
–400
–450
O2
CO + 3H2 CO + 2H2
–300
–350
1
2
–35.6 (POM)
–73.6
(FT DME)
–150
–200
H2
reactants from fiue
gas for tri-reforming
+247.6
(DRM)
CH4 + CO2
–500
Figure 7. The enthalpy of reaction for syngas production and Fischer–Tropsch (FT) synthesis of
methanol and dimethyl ether. The various reactions labelled DRM, SRM and POM are listed in
table 1 and discussed in §3c.
reactions. The steam reforming of hydrocarbons to yield syngas and hydrogen is
a classic example:
CH4 + H2 O −→ CO + 3H2
DH 0 = +206.3 kJ mol−1 .
It is important to stress that the above, highly endothermic reaction is used
worldwide 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’) illustrates
the important reaction of CO2 with a hydrocarbon, which will be of central
importance to our considerations of converting CO2 in flue gases to yield a
chemical fuel:
CH4 + CO2 −→ 2CO + 2H2
DH 0 = +247.3 kJ mol−1 .
The energy input for CO2 reforming of CH4 requires approximately 20 per cent
more energy input as compared with steam reforming, but this is certainly not a
prohibitive extra energy cost for this chemical reaction.
Importantly, these two reactions give rise to syngas with different H2 /CO
molar ratios. Both are useful for the formation of syngas, for ultimate liquid
fuel production.
In figure 7, we collate the relevant enthalpy contributions, which nicely
illustrate the importance of chemical reactions to convert CO2 . That is, it is
significantly easier, thermodynamically, if CO2 is used as a co-reactant typically
with 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 intrinsic
chemical energy to promote the conversion of CO2 .
Thus, the heats of reaction (the enthalpies of reaction) for CO production
from CO2 as the single reactant, and with CO2 energetics as a co-reactant,
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Z. Jiang et al.
are particularly important and illustrative. Compare the energetics of the thermal
dissociation of CO2 ,
CO2 → CO + 12 O2
DH 0 = +293 kJ mol−1 ,
with that of the reaction of CO2 with H2 ,
CO2 + H2 → CO + H2 O
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 reaction
is usually catalysed by Cu/ZnO-based catalysts that have high reactivity and
selectivity (Centi & Perathoner 2009; Fan et al. 2009). The annual, worldwide
production currently exceeds 40 Mt.
It is also possible to synthesize methanol directly from CO2 by combining it
with hydrogen according to the reaction:
CO2 + 3H2 −→ CH3 OH + H2 O.
Interestingly, this can also be viewed as a mechanism for ‘liquefying’ hydrogen
chemically using CO2 !
In the same way that biofuels recycle CO2 biologically, a ‘sustainable’,
synthetically artificial cycle can also be envisaged where the carbon in the
methanol is recycled by extracting and utilizing CO2 from the atmosphere
(figure 8). Overall, such a process could be carbon-neutral. The energy input to
electrolyse water to produce hydrogen and that used for the capture and release
of CO2 , of course, need to be carbon-neutral.
These ideas date back over 30 years, in one form or another. The obvious
attraction is that carbon is effectively ‘chemically sequestered’ to yield synthetic
fuels, allowing our continued exploitation of fossil fuels without causing the net
accumulation of CO2 in the atmosphere.
Olah and colleagues (Olah 2005; Olah et al. 2009) have pioneered and advanced
the concept, and potential widespread adoption, of the methanol economy,
emphasizing the production of CH3 OH (or dimethyl ether) by the chemical
recycling of CO2 . A real attraction of such an approach is that one could envisage
the catalytic hydrogenation of CO2 focused at small, delocalized production sites
as an alternative to the current large-scale, localized sites producing methanol by
steam 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-component
heterogeneous catalysts (York et al. 2007; Centi & Perathoner 2009). A recent,
important process has been developed for the homogeneous conversion of CO2 to
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3353
Review. Turning carbon dioxide into fuel
hydrogen from
electrolysis of water
H2O → H2 + 12 O2
energy in
carbon out
methanol synthesis
CO2 + 3H2 → CH3OH + H2O
fuel use
CH3OH + 32 O2
→ CO2 + 2H2O
CO2 capture
carbon in
CO2 from fossil
fuels burning
at power plants
synthetic
hydrocarbons
and products
atmospheric CO2
Figure 8. A cycle for sustainable methanol production (adapted from Olah et al. 2009 and used
with permission).
methanol using non-metal complexes (Ashley et al. 2009; Riduan et al. 2009). This
centres on the activation of H2 by so-called frustrated Lewis pairs in specifically
designed, organometallic complexes (Mömming et al. 2009).
The critical issues for this methanol cycle (figure 8) therefore centre on
the 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, costeffective, 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 considerations
for such energy storage reactions are best carried out at low temperatures
to reduce the free energy required).
— The availability and cost of sustainable hydrogen, particularly if the
requirement is hydrogen production using renewable energy (figure 4).
A recent, detailed analysis of the ‘well-to-tank’ energetics of methanol synthesis
from atmospheric CO2 (Pearson et al. 2009) concludes that by far the greatest
component of the energy requirement of the process is that to produce the
hydrogen (figure 8). This aspect highlights, once again, the close links between the
major research challenges in any future hydrogen economy—in this case, cheap
renewable routes to the production of hydrogen—and any future economy based
on renewable, carbonaceous liquid fuels.
(c) Tri-reforming: using flue gas for CO2 conversion
Currently, around one-third of all anthropogenic CO2 emissions originate
from fossil-fuel power plants. Adding CCS (typically employing post-combustion
technologies based on chemical absorption with ammines, at ca 85–90% capture
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Z. Jiang et al.
Table 1. Main reactions for syngas production by tri-reforming of natural gas.
reaction
stoichiometry
enthalpy,
0 (kJ mol−1 )
DH298
CO2 reforming of methane (DRM)
steam reforming of methane (SRM)
CH4 + CO2 ↔ 2CO + 2H2
CH4 + H2 O ↔ CO + 3H2
+247.3 (endothermic)
+206.3 (endothermic)
partial oxidation of methane (POM)
catalytic combustion of methane (CCM)
CH4 + 12 O2 ↔ CO + 2H2
CH4 + 2O2 ↔ CO2 + 2H2 O
−35.6 (exothermic)
−880 (exothermic)
efficiencies) leads to overall efficiency penalties of about 8–10% and increases in
energy costs of some 30–60%. Therefore, it would be highly desirable if flue gas
mixtures (typically 8–10% CO2 , 18–20% H2 O, 2–3% O2 and 60–70% N2 ) could
be used directly for CO2 conversion without costly pre-separation of CO2 .
Song and colleagues (Song 2000, 2001; Song & Pan 2004) have pioneered a
novel process centred on the unique advantages of directly utilising flue gas,
rather than pre-separated and purified CO2 from flue gases, for the production of
hydrogen-rich syngas from methane reforming of CO2 (so-called ‘dry reforming’).
The overall process, named ‘tri-reforming’, couples the processes of CH4 /CO2
reforming, steam reforming of CH4 , and partial oxidation and complete oxidation
of CH4 . The reactions involved are itemized in table 1, together with the
corresponding enthalpies of reaction (298 K).
Coupling CO2 and H2 O can give syngas with the desired H2 /CO ratios
for methanol and dimethyl ether synthesis and higher-carbon Fischer–Tropsch
synthesis of fuels. It also helps to avoid the formation of particulate (solid) carbon
deposits arising from reactions such as
CH4 −→ C + 2H2 O
and
2CO −→ C + CO2 .
Experimental studies by Song & Pan (2004) have shown that the introduction
of the CO2 tri-reforming reaction may also enhance the durability and lifetime
of metal nanoparticle catalysts owing to the addition of oxygen (and consequent
oxidation of carbon deposits).
It is possible to achieve up to 95 per cent methane conversion by this process
at equilibrium temperatures in the range 1073–1123 K. To achieve effective
conversion (of both CO2 and CH4 ), the flue gas is combined with natural gas
and used as chemical feedstocks for the production of syngas (CO + H2 ) with
desired H2 /CO ratios. In addition, the process makes use of ‘waste heat’ in
the 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 two
endothermic reactions noted in table 1 are thermally sustained by the waste
heat content of the exhaust gases, and the partial combustion of the primary
methane fuel.
In summary, the advantages of tri-reforming centre on the prevention of
carbon deposition, controllable H2 /CO ratios (for effective syngas production)
and a more autothermic reaction enthalpy than simple dry reforming of methane
with CO2 .
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3355
The growing worldwide interest towards the tri-reforming process is connected
to the attractive possibility of potential integration of this technology into
gas-turbine-based electric power cycles, having very low overall CO2 emissions
(Halmann 1993; Song 2006; Minutillo & Perna 2009). Detailed experimental
studies, computational analysis and engineering evaluations are under way
on the tri-reforming process. The great attraction is that the CO2 in power
plant exhausts could now be used directly in catalytic processes to generate
a syngas suitable for ultimately delivering energy fuels (and a variety of
chemical products).
In terms of catalyst materials, the majority of studies have focused on
the important CH4 –CO2 reforming component of the tri-reforming process
(Song & Pan 2004; Song 2006; Cho et al. 2009). Both nanoparticulate Ni
and Co have frequently been employed as active metal components owing to
their high intrinsic catalytic activities, wide availability and (relatively) low
costs (Hu 2009). The critical problem for all of these (and other) catalytic
materials centres on serious carbon deposition in the industrial CO2 reforming
of methane. This leads to rapid catalyst deactivation and reaction inhibition
as the supporting structures become blocked by carbon particulate deposits
(York et al. 2007; Fan et al. 2009). Comprehensive investigations, however, have
revealed that such carbon deposition was strongly influenced by the precise
mode of operation of the chemical conversion process. Importantly, fluidizedbed reforming leads to significant enhancement in the CH4 conversion process
and a considerably reduced carbon deposition when compared with the fixedbed operation process (Wurzel et al. 2000; Tomishige 2004; Hao et al. 2009).
Further optimization of the fluidized-bed configuration has taken the form of
innovative approaches using a fluidized bed assisted by an external, axial magnetic
field. Hao et al. (2008) have recently reported studies of CH4 –CO2 reforming on
aerogel Co/Al2 O3 nanoparticulate catalysts in a magnetic fluidized bed. In their
study, Co was introduced as the active catalyst component for the reforming
process; 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 of
between 700 and 1000◦ C necessary for the reforming process. In addition, the
influence of an external magnetic field on the catalytic activity and stability
of these catalyst systems was investigated in detail and compared with data
for a conventional fluidized bed and a static bed. These impressive studies are
summarized in figure 9, which is a compilation of conversion efficiencies for
both CH4 and CO2 . Also shown are images of the operating catalysts that
clearly demonstrate that carbon deposition is considerably reduced through
improving the gas–solid efficiency by the use of the external magnetic field.
For these ferromagnetic particulate catalysts, it is quite clear that magnetic-field
enhancement of operating process properties may be a most important avenue
for future, major studies.
One should also highlight the very great potential of ‘sustainable methane’ for
the tri-reforming process, and this might alleviate some of the issues discussed
by Inderwildi & King (2009) in their recent review of biofuels.
A recent, extensive investigation for the treatment of CO2 from fossil-fuel-fired
power plants, using the tri-reforming process integrated with full electrical power
co-generation, reveals an impressive estimated reduction in CO2 emissions close
to 85 per cent (Minutillo & Perna 2009).
Phil. Trans. R. Soc. A (2010)
Phil. Trans. R. Soc. A (2010)
(ii)
(ii)
0
20
40
60
80
(ii) 100
0
20
40
60
80
0
(iii)
4
8
12
time (h)
16
20
Figure 9. Aerogel Co/Al2 O3 catalysts for CH4 –CO2 reforming. (a) (i) Conventional and (ii) magnetic fluidized bed. (b) Conversions of (i) CH4 and
(ii) CO2 . (c) Microstructure of the catalysts after 20 h operation: (i) magnetic fluidized bed, (ii) fluidized bed and (iii) fixed bed. Note that in the
fluidized-bed operation mode, (i), carbon deposition is mainly of particulates, while in the fixed-bed mode, (iii), we see extensive filamentous, graphitic
carbon, causing deactivation of the catalyst (after Hao et al. 2008). Symbols: (b) (i) filled squares, magnetic fluidized bed; filled triangles, fluidized
bed; filled inverted triangles, fixed bed; dotted line, equilibrium conversion; (ii) open squares, magnetic fluidized bed; open triangles, fluidized bed;
open inverted triangles, fixed bed; dotted line, equilibrium conversion.
(i)
(c)
(i)
(b)
(i) 100
conversion of CH4 (%)
conversion of CH2 (%)
(a)
3356
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Z. Jiang et al.
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3357
CH4/CH3OH
CO2
Pt
conduction band
hv
e–
band gap
h+
valence band
TiO2
O2
H2O
Figure 10. Basic principle of photoreduction of CO2 by water on an effective Pt semiconductor
photocatalyst.
(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 reduction
of CO2 has the potential to be a means both of CO2 recycling and of storing
intermittent solar energy in synthetic carbon-neutral fuels suitable for storage and
use in the residential, industrial and transportation sectors. However, the efficient
photoreduction of CO2 using solar radiation is one of the most challenging tasks
of environmental catalysis. In a recent report, the US Department of Energy (Bell
et al. 2007) has identified the development of advanced catalysts for the photodriven conversion of CO2 and water as one of the three priority research directions
for advanced catalysis science for energy applications.
Inoue et al. (1979) were the first to report the photocatalytic reduction of
CO2 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 by
CO2 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 light
photons in a photocatalyst material, and subsequent conversion of these photons
into electron–hole pairs, which then have to be spatially separated to drive
chemical oxidation and reduction half-reactions at the semiconductor–electrolyte
interface (figure 10).
The overall conversion efficiency of photocatalytic production of solar fuels
is currently significantly lower than that of photovoltaic solar systems. Unlike
photovoltaic electricity generation, where absorption of a photon results directly
in the formation of an electron–hole pair, all methods of producing solar
fuels must involve the coupling of an absorption process with multi-electron
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3358
Z. Jiang et al.
redox reactions. In the case of CO2 photoreduction using water as a reductant,
both multi-electron transfer reactions of CO2 reduction and water oxidation
have to occur simultaneously. As a result, current CO2 photoreduction catalysts
are less effective than current solar water-splitting catalysts (Indrakanti et al.
2009). Several other important factors limit the solar-to-fuel energy conversion
efficiency of photocatalytic systems, including inefficient absorption of solar
energy, fast recombination processes of the photo-excited electron–hole pairs and
facile backward redox reactions.
The properties of potential photocatalysts are determined by the redox
potentials of the rate-limiting steps of water oxidation and CO2 reduction:
2H2 O(l) −→ O2 (g) + 4H+ (aq) + 4e−
CO2 (aq) + e− −→ CO−
2 (aq)
0
Eox
= −1.23 V,
0
Ered
= −1.65 V.
These values impose a minimum threshold for the energy of photo-excited
electrons required to reduce CO2 and also the energy levels of the conduction and
valence bands of a photocatalyst. Wide-band-gap semiconductors are the most
suitable photocatalysts for CO2 reduction because photo-generated electrons in
the bottom of the conduction band can have sufficient negative redox potential
to drive CO2 reduction, while the photo-generated holes in the valence band can
be sufficiently energetic (positive holes) to act as acceptors and oxidize water
to O2 . A significant disadvantage of using wide-band-gap semiconductors is the
inability to use visible light efficiently. Quite simply, light with energy smaller
than the semiconductor electronic band gap cannot generate electron–hole pairs
and is therefore wasted as heat. As a result, only the UV and near-UV parts of the
solar spectrum can be used with the present photocatalysts such as TiO2 (with
an electronic band gap of around 3.2 eV), yielding a low absorption coefficient
and, consequently, low conversion efficiency in the visible region.
Although many semiconductors have smaller band gaps and absorb in the
visible range (e.g. CdS and Fe2 O3 with band-gap values of 2.4 and 2.3 eV,
respectively), just a few of them are catalytically active because the energy
levels of either the conduction or valence bands are unsuitable for CO2 reduction
and/or water oxidation. This limitation, together with poor photo-corrosion
stability of many semiconductors, limits significantly the number of potential
photocatalyst materials.
Figure 11 displays the quantum efficiency of photocatalytic water splitting
plotted against wavelength of a wide range of inorganic materials (Osterloh 2008).
Note that this interesting summary represents only the water-splitting component
of the photocatalytic process, but it nevertheless highlights a range of potential
materials for the combined H2 O/CO2 photochemical process. For any prospective
photocatalyst to be considered economically viable, it has to display a quantum
efficiency 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 seen
in the shaded area in figure 11. Worldwide, active research is now focused on
the 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−x Znx O1−y Ny ; Maeda et al. 2006; Jiang et al. 2008;
Phil. Trans. R. Soc. A (2010)
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Review. Turning carbon dioxide into fuel
quantum efficiency (%)
100
ZnS
La : NaTaO3
TiO2
Nb2O5 : TiO2
La4CaTi5O17
M4Nb6O17 (M = K, Rb)
ZrNaTaO3
La2Ti2O7
Ge3N4
HCa2Nb3O10
KBa2Ta3O10
Zn : La2O3/Ga2O3
MTa2O6 (M = Sr, Ba, Sn)
M2La2Ti3O10
TiO2
GaN : ZnO
(M = K, Rb, Cs)
(anatase)
MZnS (M = Ni, Pb, Cu)
K2Sr1.5Ta3O10
PbBi2Nb2O9
Ni : lnTaO4
Zn : [In(OH)ySz] BiVO
4
Sr2Nb2O7
10
1
UV
0.1
100
CdS
200
Cr/Ta : SrTiO3
300
visible
LaTiO2N
WO3
400
500
600
700
wavelength (nm)
Figure 11. Quantum efficiency of different materials for photocatalytic water splitting in the UV
and visible parts of the spectrum (adapted by D. Payne, Department of Chemistry, University of
Oxford, from Osterloh 2008).
biomass
H2/electrolysis
CO2
fuels
combustion
tri-reforming
renewable
energy
chemicals
syngas
biogas
or
CH4
Figure 12. Natural photosynthesis uses solar energy to recycle CO2 (and H2 O) into new plant life
(biomass) and ultimately fuels (biofuels). Sustainable, tri-reforming uses CO2 , renewable energy
and 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 one
of the most effective photocatalysts among other semiconductors, and it has
been used to convert CO2 to useful compounds, in both gas- and aqueous-phase
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Z. Jiang et al.
photoreactions (Ni et al. 2007; Nguyen & Wu 2008; Indrakanti et al. 2009). Several
approaches have been employed to enhance the efficiency of photocatalysts.
Promising methods to expand the light absorption of TiO2 into the visible
region are band-structure modification by suitable cation or anion doping and
the 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-catalyst
materials in the nanocrystalline form also offers the attractive opportunity
to minimize the distances (and thus the times) over which photo-induced
electron–hole pairs have to survive and be transported to reactant molecules
after photo-excitation. The formation of heterojunction structures between two
nanostructured semiconductors with different band gaps and matching band
potentials can also extend light absorption, improve charge separation, increase
the lifetime of charge carriers, and enhance the interfacial charge-transfer
efficiency (Long et al. 2006; Tada et al. 2006; Yang et al. 2009). Deposition
of noble metal nanoscale clusters on the photocatalyst surface has also been
shown to enhance the photocatalytic activity owing to increased transfer rates
of photo-generated electrons to the absorbed metal/semiconductor clusters, thus
decreasing markedly the possibility of electron–hole recombination (Baba et al.
1985; Zhang et al. 2009).
An alternative approach using the photoelectrocatalytic reduction of CO2
has been proposed recently (Centi et al. 2007; Centi & Perathoner 2009). In
this concept, water dissociation occurs at a photoanode and CO2 reduction
at a photocathode, which are attached to both sides of a proton-conducting
membrane. Such a photoelectrocatalytic device shows a strong similarity to
proton-exchange membrane fuel cells and can take advantage of significant
technological developments of that energy technology.
Developments of cheap, reliable and efficient methods for the production of
solar fuels represent a major challenge in solar energy utilization. The key issue
in optimizing photocatalyst materials is to achieve the same conversion efficiency
for visible-light-induced CO2 reduction as already demonstrated for near-UV
irradiation and, at the same time, to significantly increase the chemical stability
against damage from incident light and also attack from highly reactive species
formed 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 causing
global warming into a valuable, renewable and inexhaustible carbon source of the future
allowing environmentally neutral use of carbon fuels and derived hydrocarbon products.
With carbon capture and sequestration (storage) becoming a key element in
worldwide efforts to control/minimize emissions, it can be anticipated that large
amounts of CO2 will become available as feedstock for innovative conversions to
synthetic fuels. Of course, ‘whole process’ energy balances and economics remain
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3361
a critical issue—as indeed is the case for any overall vision of energy futures. It
is an obvious, but nevertheless a most potent, fact that any new fossil fuels can
only be formed naturally over geological time scales.
The challenge, therefore, is to achieve such transformations from natural and
industrial sources, as well as human activities, via CO2 capture and subsequent
conversion to synthetic fuels over, what one might term, less demanding
time scales! The cycle in figure 12 reflects particularly the tri-reforming route
to a sustainable fuel (§3c).
In this work, we have attempted a brief overview of the current scientific
challenges in the area, with an emphasis on the important, underlying
thermodynamic considerations and the urgent agenda for materials research
and development (figure 12). This vision in many ways constitutes humankind’s
artificial version of the natural recycling of CO2 to energy fuels by photosynthesis
using sunlight as the primary energy source (Barber 2009).
To bring any, or all, of these innovative approaches into real life, technologies
will require major advances in research and development, as well as significant
socio-political commitments to CO2 capture and utilization. International
collaborative research between developed, and developing countries (figure 2) is
also absolutely critical in such a venture. Our hope is that this present summary
helps to catalyse such a worthwhile development.
We are most grateful to James Barber, Oliver Inderwildi, Steven Jenkins, David King, Richard
Pearson, Malcolm Green, David Payne, John Bottomley and Leon Di Marco for many important
discussions. 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 Houghton
Research Fellowship in Sustainable Energy, Jesus College, Oxford.
References
Arakawa, H. et al. 2001 Catalysis research of relevance to carbon management: progress, challenges,
and opportunities. Chem. Rev. 101, 953–996. (doi:10.1021/cr000018s)
Ashley, A. E., Thompson, A. L. & O’Hare, D. 2009 Non-metal-mediated homogeneous
hydrogenation of CO2 to CH3 OH. Angew. Chem. Int. Edn 48, 9839–9843. (doi:10.1002/anie.
200905466)
Baba, R., Nakabayashi, S., Fujishima, A. & Honda, K. 1985 Investigation of the mechanism of
hydrogen evolution during photocatalytic water decomposition on metal-loaded semiconductor
powders. J. Phys. Chem. 89, 1902–1905. (doi:10.1021/j100256a018)
Barber, J. 2009 Photosynthetic energy conversion: natural and artificial. Chem. Soc. Rev. 38,
185–196. (doi:10.1039/b802262n)
Bell, A. T., Gates, B. C. & Ray, D. 2007 Basic research needs: catalysis for energy (PNNL-17214).
US Department of Energy, Report from a Workshop held in Bethesda, MD, 6–8 August. See
http://www.sc.doe.gov/bes/reports/list.html.
Boden, T. A., Marland, G. & Andres, R. J. 2009 Global, regional, and national fossil-fuel CO2
emissions, 2009. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory,
US Department of Energy, Oak Ridge, TN. (doi:10.3334/CDIAC/00001)
Centi, G. & Perathoner, S. 2009 Opportunities and prospects in the chemical recycling of carbon
dioxide to fuels. Catal. Today 148, 191–205. (doi:10.1016/j.cattod.2009.07.075)
Centi, G., Perathoner, S., Wine, G. & Gangeri, M. 2007 Electrocatalytic conversion of CO2 to long
carbon-chain hydrocarbons. Green Chem. 9, 671–678. (doi:10.1039/b615275a)
Cho, W., Song, T., Mitsos, A., McKinnon, J. T., Ko, G. H., Tolsma, J. E., Denholm, D. & Park,
T. 2009 Optimal design and operation of a natural gas tri-reforming reactor for DME synthesis.
Catal. Today 139, 261–267. (doi:10.1016/j.cattod.2008.04.051)
Phil. Trans. R. Soc. A (2010)
Downloaded from http://rsta.royalsocietypublishing.org/ on January 11, 2015
3362
Z. Jiang et al.
Fan, M.-S., Abdullah, A. Z. & Bhatia, S. 2009 Catalytic technology for carbon dioxide
reforming of methane to synthesis gas. ChemCatChem 1, 192–208. (doi:10.1002/cctc.
200900025)
Freund, H. J. & Roberts, M. W. 1996 Surface chemistry of carbon dioxide. Surf. Sci. Rep. 25,
225–273. (doi:10.1016/S0167-5729(96)00007-6)
Friedlingstein, P. 2008 A steep road to climate stabilization. Nature 451, 297–298. (doi:10.1038/
nature06593)
Fulkerson, W., Judkins, R. R. & Sanghui, M. K. 1990 Energy from fossil fuels. Sci. Am. 83–89.
Halmann, M. M. 1993 Chemical fixation of carbon dioxide: methods for recycling CO 2 into useful
products. Boca Raton, FL: CRC Press.
Hao, Z., Zhu, Q., Jiang, Z. & Li, H. 2008 Fluidization characteristics of aerogel Co/Al2 O3 catalyst
in a magnetic fluidized bed and its application to CH4 –CO2 reforming. Powder Technol. 183,
46–52. (doi:10.1016/j.powtec.2007.11.015)
Hao, Z., Zhu, Q., Jiang, Z., Hou, B. & Li, H. 2009 Characterization of aerogel Ni/Al2 O3 catalysts
and investigation on their stability for CH4 –CO2 reforming in a fluidized bed. Fuel Process.
Technol. 90, 113–121. (doi:10.1016/j.fuproc.2008.08.004)
Hu, Y. H. 2009 Solid-solution catalysts for CO2 reforming of methane. Catal. Today 148, 206–211.
(doi:10.1016/j.cattod.2009.07.076)
Hwang, J. S., Chang, J. S., Park, S. E., Ikeue, K. & Anpo, M. 2005 Photoreduction of carbondioxide
on surface functionalized nanoporous catalysts. Top. Catal. 35, 311–319. (doi:10.1007/
s11244-005-3839-8)
Inderwildi, O. R. & King, D. A. 2009 Quo vadis biofuels? Energy Environ. Sci. 2, 343–346.
(doi:10.1039/b822951c)
Indrakanti, V. P., Kubicki, J. D. & Schobert, H. H. 2009 Photoinduced activation of CO2 on
Ti-based heterogeneous catalysts: current state, chemical physics-based insights and outlook.
Energy Environ. Sci. 2, 745–758. (doi:10.1039/b822176f)
Inoue, T., Fujishima, A., Konishi, S. & Honda, K. 1979 Photoelectrocatalytic reduction of carbon
dioxide in aqueous suspensions of semiconductor powders. Nature 277, 637–638. (doi:10.1038/
277637a0)
IPCC. 2005 (Intergovernmental Panel on Climate Change) Carbon dioxide capture and storage
(eds B. Metz, O. Davidson, H. de Coninck, M. Loos & L. Meyer). Cambridge, UK: Cambridge
University Press.
IPCC. 2007 (Intergovernmental Panel on Climate Change) Contribution of Working Group I to the
Fourth Assessment Report, Climate change 2007: The physical science basis (eds S. Solomon,
D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor & H. L. Miller). Cambridge,
UK: Cambridge University Press.
Jiang, Z., Yang, F., Luo, N., Chu, B. T. T., Sun, D., Shi, H., Xiao, T. & Edwards, P. P. 2008
Solvothermal synthesis of N-doped TiO2 nanotubes for visible-light-responsive photocatalysis.
Chem. Commun. 47, 6372–6374. (doi:10.1039/b815430a)
Jiang, Z., Yang, F., Yang, G., Kong, L., Jones, M. O., Xiao, T. & Edwards, P. P. 2010 The
hydrothermal synthesis of BiOBr flakes for visible-light-responsive photocatalytic degradation
of methyl orange. J. Photochem. Photobio. A 212, 8–13. (doi:10.1016/j.jphotochem.2010.03.004)
Kolasinski, W. K. 2006 Frontiers of surface science. Curr. Opin. Solid State Mater. Sci. 10,
129–1312. (doi:10.1016/j.cossms.2007.04.002)
Kudo, A. & Miseki, Y. 2008 Heterogeneous photocatalyst materials for water splitting. Chem. Soc.
Rev. 38, 253–278. (doi:10.1039/b800489g)
Long, M., Cai, W., Cai, J., Zhou, B., Chai, X. & Wu, Y. 2006 Efficient photocatalytic degradation
of phenol over Co3 O4 /BiVO4 composite under visible light irradiation. J. Phys. Chem. B 110,
20 211–20 216. (doi:10.1021/jp063441z)
Luo, N., Jiang, Z., Shi, H., Cao, F., Xiao, T. & Edwards, P. P. 2009 Photo-catalytic conversion of
oxygenated hydrocarbons to hydrogen over heteroatom-doped TiO2 catalysts. Int. J. Hydrogen
Energy 34, 125–129. (doi:10.1016/j.ijhydene.2008.09.097)
Maeda, K., Teramura, K., Lu, D., Takata, T., Saito, N., Inoue, Y. & Domen, K. 2006 Photocatalyst
releasing hydrogen from water. Nature 440, 295–295. (doi:10.1038/440295a)
Phil. Trans. R. Soc. A (2010)
Downloaded from http://rsta.royalsocietypublishing.org/ on January 11, 2015
Review. Turning carbon dioxide into fuel
3363
Marbán, G. & Valdés-Solís, T. 2007 Towards the hydrogen economy? Int. J. Hydrogen Energy 32,
1625–1637. (doi:10.1016/j.ijhydene.2006.12.017)
Mikkelsen, M., Jorgensen, M. & Krebs, F. C. 2010 The teraton challenge. A review of fixation and
transformation of carbon dioxide. Energy Environ. Sci. 3, 43–81. (doi:10.1039/b912904a)
Minutillo, M. & Perna, A. 2009 A novel approach for treatment of CO2 from fossil fired power
plants, Part A: the integrated systems ITRPP. Int. J. Hydrogen Energy 34, 4014–4020.
(doi:10.1016/j.ijhydene.2009.02.069)
Mömming, C. M., Otten, E., Kehr, G., Fröhlich, R., Grimme, S., Stephan, D. W. & Erker, G. 2009
Reversible metal-free carbon dioxide binding by frustrated Lewis pairs. Angew. Chem. Int. Edn
48, 6643–6646. (doi:10.1002/anie.200901636)
Nguyen, T.-V. & Wu, J. C. S. 2008 Photoreduction of CO2 to fuels under sunlight using optical-fiber
reactor. Solar Energy Mater. Solar Cells 92, 864–872. (doi:10.1016/j.solmat.2008.02.010)
Ni, M., Leung, M. K. H., Leung, D. Y. C. & Sumathy, K. 2007 A review and recent developments
in photocatalytic water-splitting using TiO2 for hydrogen production. Renew. Sust. Energy Rev.
11, 401–425. (doi:10.1016/j.rser.2005.01.009)
OECD/IEA. 2009 World energy outlook 2009. Paris, France: OECD/International Energy Agency.
Olah, G. A. 2005 Beyond oil and gas: the methanol economy. Angew. Chem. Int. Edn 44, 2636–2639.
(doi:10.1002/anie.200462121)
Olah, G. A., Goeppert, A. & Surya Prakash, G. K. 2009 Chemical recycling of carbon dioxide
to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon
neutral fuels and synthetic hydrocarbons. J. Org. Chem. 74, 487–498. (doi:10.1021/jo801260f)
Osterloh, F. E. 2008 Inorganic materials as catalysts for photochemical splitting of water. Chem.
Mater. 20, 35–54. (doi:10.1021/cm7024203)
Pan, X., Fan, Z., Chen, W., Ding, Y., Luo, H. & Bao, X. 2007 Enhanced ethanol production
inside carbon-nanotube reactors containing catalytic particles. Nat. Mater. 6, 507–511.
(doi:10.1038/nmat1916)
Pearson, R. J., Turner, J. W. G. & Peck, A. J. 2009 Gasoline–ethanol–methanol tri-fuel vehicle
development and its role in expediting sustainable organic fuels for transport. In Low carbon
vehicles 2009, Institution of Mechanical Engineers, London, 20–21 May 2009, IMechE Conf.
C678. See http://www.grouplotus.com/mediagallery/image/1002548.pdf.
Riduan, S. N., Zhang, Y. & Ying, J. Y. 2009 Conversion of carbon dioxide into methanol with silanes
over N-heterocyclic carbene catalysts. Angew. Chem. Int. Edn 48, 3322–3325. (doi:10.1002/
anie.200806058)
Rogelj, J., Nabel, J., Chen, C., Hare, W., Markmann, K., Meinshausen, M., Schaeffer, M.,
Macey, K. & Höhne, N. 2010 Copenhagen Accord pledges are paltry. Nature 464, 1126–1128.
(doi:10.1038/4641126a)
Sakakura, T., Choi, J.-C. & Yasuda, H. 2007 Transformation of carbon dioxide. Chem. Rev. 107,
2365–2387. (doi:10.1021/cr068357u)
Shang, M., Wang, W., Zhou, L., Sun, S. & Yin, W. 2009 Nanosized BiVO4 with high visible-lightinduced photocatalytic activity: ultrasonic-assisted synthesis and protective effect of surfactant.
J. Hazard. Mater. 172, 338–344. (doi:10.1016/j.jhazmat.2009.07.017)
Song, C. 2000 A proposed concept for CO2 -based tri-generation of chemicals, fuels and electricity.
Prepr., Am. Chem. Soc., Div. Petrol. Chem. 45, 159–163.
Song, C. 2001 Tri-reforming: a new process for reducing CO2 emissions. Chem. Innov. 31,
21–26.
Song, C. 2006 Global challenges and strategies for control, conversion and utilization of CO2 for
sustainable development involving energy, catalysis, adsorption and chemical processing. Catal.
Today 115, 2–32. (doi:10.1016/j.cattod.2006.02.029)
Song, C. & Pan, W. 2004 Tri-reforming of methane: a novel concept for catalytic production
of industrially useful synthesis gas with desired H2 /CO ratios. Catal. Today 98, 463–484.
(doi:10.1016/j.cattod.2004.09.054)
Tada, H., Mitsui, T., Kiyonaga, T., Akita, T. & Tanaka, K. 2006 All-solid-state Z-scheme in
CdS–Au–TiO2 three-component nanojunction system. Nat. Mater. 5, 782–786. (doi:10.1038/
nmat1734)
Phil. Trans. R. Soc. A (2010)
Downloaded from http://rsta.royalsocietypublishing.org/ on January 11, 2015
3364
Z. Jiang et al.
Tomishige, K. 2004 Syngas production from methane reforming with CO2 /H2 O and O2 over NiO–
MgO solid solution catalyst in fluidized bed reactors. Catal. Today 89, 405–418. (doi:10.1016/
j.cattod.2004.01.003)
Varghese, O. K., Paulose, M., LaTempa, T. J. & Grimes, C. A. 2009 High-rate solar photocatalytic
conversion of CO2 and water vapor to hydrocarbon fuels. Nano Lett. 9, 731–737. (doi:10.1021/
nl803258p)
Wurzel, T., Malcus, S. & Mleczko, L. 2000 Reaction engineering investigations of CO2 reforming in
a fluidized-bed reactor. Chem. Eng. Sci. 55, 3955–3966. (doi:10.1016/S0009-2509(99)00444-3)
Yang, C., Wang, W., Shan, Z. & Huang, F. 2009 Preparation and photocatalytic activity of highefficiency visible-light-responsive photocatalyst SnSx /TiO2 . J. Solid State Chem. 182, 807–812.
(doi:10.1016/j.jssc.2008.12.018)
York, A. P. E., Xiao, T.-C., Green, M. L. H. & Claridge, J. B. 2007 Methane oxyforming for
synthesis gas production. Catal. Rev. 49, 511–560. (doi:10.1080/01614940701583315)
Zhang, Q.-H., Han, W.-D., Hong, Y.-J. & Yu, J.-G. 2009 Photocatalytic reduction of CO2 with
H2 O on Pt-loaded TiO2 catalyst. Catal. Today 148, 335–340. (doi:10.1016/j.cattod.2009.07.081)
Züttel, A., Remhof, A., Borgschulte, A. & Friedrichs, O. 2010 Hydrogen: the future energy carrier.
Phil. Trans. R. Soc. A 368, 3329–3342. (doi:10.1098/rsta.2010.0113)
Phil. Trans. R. Soc. A (2010)