RESEARCH NEWS
T hrowing New Light on the Reduction of CO 2
G e offre y A. O zin*
P rof. G. A. Ozin
S olar Fuels Research Group Center for Inorganic and Polymeric Nanomaterials
C hemistry Department
80 St. George Street, University of Toronto T oronto ,O ntario ,C anada ,M 5S 3H6 E-mail: g ozin@chem.utoronto.ca DOI: 10.1002/adma.201500116
that evolved from this enterprise.
[ 2]
After years of practicing the basic science
endeavoring to achieve this objective, often without much success, one may reach the conclusion that “biomimetics” may in
fact be a fruitless distraction on the way
forward to a global solution for climate change and energy security using sunlight to make fuels from H 2 O and CO 2. W hile imitating Nature has proven to be an appropriate way of thinking about
the production of H 2 and O 2 from the solar powered splitting of H 2 O , as judged by the remarkable scienti? c progress reported during this 40 year time frame, one begins to question whether the same praise can be voiced for the CO 2conver-sion side of the photosynthetic cycle.
[ 1–4 ]
O n the other hand, once the large scale production of H 2 from renewable sources
becomes economically advantageous to fossil-based systems, the utilization of H 2
for large scale conversion of CO 2 to solar fuels is an interesting proposition for the creation of a long-term sustainable global
economy with the added bene? ts of greenhouse gas control and energy security. Quantitative evaluation of these aims however requires a life cycle assessment to determine the energy and
economic bene? ts of H 2 O splitting and CO 2 capture and utili-zation on the climate and the environment. [ 5]
T o amplify upon these ideas, the conclusion that can be drawn from the surfeit of papers published on solar fuels made from CO 2
, produced by either aqueous phase photoelectro-chemistry or aqueous phase photocatalysis, employing a rather large composition range and structure library of materials, is that the conversion rates and ef? ciencies reported to date while showing promise must be signi?
cantly improved by at least an order of magnitude to offer a practical value. A technologically signi? cant advance may likely require a gas-phase, light-assisted, heterogeneous CO 2 reduction pro-cess and discovery of a photocatalyst that can enable at least
three orders of magnitude improvement in CO 2conversion
rates and ef? ciencies compared to the state-of-the-art today
(millimole h ?1g cat ?1 to mole h ?1g cat ?1 ). This is what must be
achieved to permit the “solar re? nery of the future” to compete with existing non-renewable technologies and to be operated at a scale that could stabilize the impact of the global emission of greenhouse gas in the troposphere. I n this context, one is drawn to the inescapable conclu-sion that lessons from Nature’s miraculous photosynthesis machinery are not proving to be particularly helpful in ? nding W hile the chemical energy in fossil fuels has enabled the rapid rise of modern civilization, their utilization and accompanying anthropogenic CO 2 emissions is
occurring at a rate that is outpacing nature’s carbon cycle. Its effect is now con-sidered to be irreversible and this could lead to the demise of human society.
This is a complex issue without a single solution, yet from the burgeoning global research activity and development in the ? eld of CO 2
capture and utiliza-tion, there is light at the end of the tunnel. In this article a couple of recent advances are illuminated. Attention is focused on the discovery of gas-phase,
light-assisted heterogeneous catalytic materials and processes for CO 2pho-toreduction that operate at suf? ciently high rates and conversion ef? ciencies, and under mild conditions, to open a new pathway for an energy transition from today’s “fossil fuel economy” to a new and sustainable “CO 2economy”.
Whichever of the competing CO 2 capture and utilization approaches proves to
be the best way forward for the development of a future CO 2 -based solar fuels economy, hopefully this can occur in a period short enough to circumvent the
predicted adverse consequences of greenhouse gas climate change. 1. I ntroduction
T he future is bright for the transformation of sunlight, water and carbon dioxide into renewable solar fuels that can sustain civilization inde? nitely. However, “biomimetics”, the paradigm whereby we learn from and improve upon nature’s photosyn-thetic molecules, materials, processes and devices, may nei-ther be the champion game-changer nor the most competitive player to realize this goal, as we ? rst envisioned. W hile it is a truism that materials researchers have learned much from studying nature’s photosynthetic machinery in algae, bacteria, plants, and trees that utilize H 2 O and CO 2to
sustain life on earth, it is fair to say they have not really mim-icked nature in the laboratory to solve the problem of capturing and converting H 2 O and CO 2 into solar fuels.
A fter reading the science underpinning the 40 year history of how and why we should make solar fuels from H 2 O and CO 2,[ 1] it is clear that many materials researchers have been inspired by the paradigm of a global project on “arti?
cial photosynthesis” D edicated to my wife Linda Ozin on the occasion of our golden wedding anniversary
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a materials solution to the challenging problem of CO 2photo-reduction. Speci? cally, the Calvin cycle that operates in Photosystem I , reducing CO 2 to glucose, involves complex aqueous organic biochemistry, which would be dif? cult to adapt to a large scale solar fuels process.
P erhaps, we would be more productive in this enterprise to invest our creative energies in learning from more than a cen-tury of successful fuel making, by classical thermally powered CO 2 heterogeneous catalysis, and using this know-how to drive the reaction by light rather than fossil sources of energy. This “abiomimetic” strategy may turn out to be a more fruitful quest for achieving the “holy grail” of solar fuel from the sun instead
of continuing to use fossil fuels from the earth. [ 6–9 ]
F acing the dilemma of “which way to go” in attacking this problem, searching for new materials-based heterogeneous catalysis solutions to light-assisted, gas-phase CO 2reduction catalysis, utilizing both the light and heat from the sun, may likely yield the most signi? cant scienti? cally and technologi-cally relevant results in the ? ght against mitigating greenhouse gas induced global warming with the added bene? t of creating a new CO 2 economy and a sustainable future. With global CO 2 emissions around 30–40 gigatons per annum and deployment of CO 2 capture facilities on the rise, it is expected that concentrated sources of CO 2 will become readily available at a competitive cost making it a non-depleting renewable chemical feedstock.
T his account of the recent research, mainly by the University of Toronto solar fuels team, illuminates some new possibilities for solar powered CO 2 reduction by reference to a couple of exciting advances ( F igure 1 ). The implication of this work with respect to the development of a future CO 2 based solar fuels economy is discussed.
2. A n Overview of the Quest
T he focus of the program of research of the University of Toronto solar fuels team over the last few years has been on the discovery of gas-phase, light-assisted heterogeneous photocatalysis materials and processes for CO 2 reduction that would operate at suf? cient ef? ciency to help enable an energy transition from today’s unsustainable “fossil fuel economy”,
with its associated risks of climate change caused by CO 2emis-sions, to a new and sustainable “CO 2economy”.
T hese new materials and processes would use CO 2together with a source of H 2 and sunlight, with the help of a photocata-lyst, for making renewable fuels such as CO, CH 4 or CH 3O H, in one of the photoreactor modules in an envisoned solar
re? nery of the future, [ 10 ] the structure and operation of which is
illustrated in F igure 2 . Within the imagined solar re? nery, key modules contain materials and processes for i) harvesting sun-light, ii) capturing, purifying and releasing CO 2 , and iii) directly converting CO 2 and H 2 O into fuels or iv) indirectly producing H 2 from H 2 O and converting CO 2 into fuels.
T o amplify, the solar utility i) harvests sunlight in the form of heat, light or electricity; a CO 2 facility ii) uses absorption, adsorption, membrane or cryogenic techniques for cap-turing, purifying and releasing CO 2 on demand; and iii,iv) the CO 2 is piped with H 2
O and either directly transformed to fuels using electrochemical, photoelectrochemical, or pho-tocatalytic methods or indirectly converted to CO and H 2by thermochemical, electrochemical, photoelectrochemical, or photocatalytic means. The CO and H 2 formed by these pro-cesses are subsequently made into fuels by well-known indus-trial heterogeneous catalytic processes based on Fischer–Tropsch, methanol synthesis, water gas shift, or reverse water gas shift chemistry.
T he view that emerges from this techno-economic evalua-tion of building and operating a solar re? nery is one of guarded
optimism. [ 10 ] On the subject of energy ef? ciency, it is clear that
solar powered CO 2 reduction is currently lagging far behind that of solar driven H 2
O splitting and more research is needed to improve the activity of photocatalysts and the ef? cacy of pho-toreactors. I n the indirect process of transforming CO 2/H 2O to fuels, it is apparent that if the currently achievable solar H 2O -to-H 2 conversion of greater than 10% can be matched by solar CO 2
/H 2
-to-fuel conversion ef? ciencies, through creative catalyst design and photoreactor engineering, this would represent a promising step towards an energetically viable solar re? nery. For the process that can directly transform CO 2/H 2 O to fuels, improvements in conversion rates and product selectivity are key requirements for
achieving energy ef? ciency in the solar re? nery.
[ 10 ] E conomic ef? ciency is also a key to the success of the solar re? nery of the future. For currently achievable CO 2reduction rates and ef? ciencies, the minimum selling price of a representa-tive fuel, was evaluated by the techno-economic analysis and turned out to be more than three times greater than the indus-trial selling price, even though the cost of the CO 2 reduction step, which is estimated to be quite expensive, was not included in the estimates. I mprovement in the activity of CO 2 reduction photo-catalysts by several orders of magnitude would have a signi? cant impact on the energy and economic costs of operating a solar
re? nery. [ 10 ]
T he solar fuels team saw the need for improved CO 2reduc-tion photocatalysts for making solar fuels as an interesting and exciting challenge for materials chemistry. About three
years ago, a multidisciplinary team of materials chemists, scientists and engineers was assembled to embark on this adventurous and challenging research project. The aim was exploit the boundless energy of the sun in making fuels and
F igure 1. I lluminations on the utilization of CO 2 . The illustration depicts the grand challenge that the global community must confront in the face
of CO 2 induced climate change in order to achieve a sustainable environ-ment, a renewable energy supply and a carbon neutral economy. Graphic
courtesy of Chenxi Qian.
RESEARCH NEWS
chemicals from abundant greenhouse gas CO 2
, rather than continuing to follow the current practice of depleting legacy fossil fuels.
T he ultimate objective of this research was to discover a material that could transform gaseous CO 2 in the presence of H 2 O or H 2 and sunlight to CO, CH 4 or CH 3 O H at suf? ciently high rates, ef? ciencies, and scales, such that the concentration of greenhouse gas emissions in the atmosphere could be stabi-lized at current levels. T his bold goal may be achievable in a closed-carbon loop using only renewable energies. If reduced to practice, it would provide a practical global solution to the intertwined climate change and fossil fuel conundrum facing society today and it would also create a new CO 2 economy. Over the ?
rst two years of operation aiming towards this objec-tive, the ? gure-of-merit for gas-phase CO 2 conversion rates and ef? ciencies achieved was comparable to the state-of-the-art in
the open literature. [1,6,7] However they were
still many orders of magnitude below those required for a scalable process that could meaningfully impact the rate of emission of CO 2 into the atmosphere and ameliorate fossil fuel based climate change and energy concerns.
R ecently, some notable successes were enjoyed in this endeavor and when two dif-ferent working catalyst systems were dis-covered changed this picture from one of guarded pessimism to cautious optimism.
These two systems [ 6,7 ] are the centerpiece of this article, a brief description of which is described below.
2.1. W orking Catalyst System 1: Rational Design of a Single-Component Photocatalyst for Gas-Phase CO 2 Reduction Using Both UV and Visible Light T he University of Toronto solar fuels team recently identi? ed the key attributes of In 2O 3- x (OH) y nanocrystals that demonstrate activity towards the photocatalytic reduction of CO 2 in a H 2envi-ronment using both ultraviolet and visible light. [ 6] It was shown that surface populations of oxygen vacancies and hydroxides work in concert as active sites for CO 2 adsorption and charge transfer under solar simulated irradiation, the overall scheme of which is depicted in the illustration in F igure 3 .
F igure 2. S chematic that illustrates the constitution and operation of a futuristic solar re? nery for making fuels and chemicals from CO 2,H 2O , and
sunlight. Reproduced with permission. [ 10 ]
Copyright 2015, Royal Society of Chemistry.
F igure 3. I llustration of key materials attributes of In 2O 3- x (OH) y nanocrystals that are consid-ered responsible for their activity towards the photocatalytic reduction of CO 2 to CO in a H 2
environment. Reproduced with permission. [ 6]
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I t turns out that the Bixbyite form of In 2O 3- x (OH) y nanocrys-tals contain 25% intrinsic oxygen vacancies [O] v that concentrate on the surface, capture CO 2
, accumulate charge with long-lived photoexcited states, and form sub-gap states that provide visible light absorption. T his know-how has inspired the development of a col-lection of new materials, which share the characteristics of In 2O 3- x (OH) y
but have the bene? t of being earth abundant, low cost, and non-toxic elemental compositions.
S ince this breakthrough, a novel method for supporting In 2O 3- x (OH) y
nanocrystals on a glass ? ber ? lter for the light-assisted gas-phase production of solar fuels, which enable high photocatalyst loadings over a large surface area, has been devel-oped. This small but important change enabled optimization of
CO 2 conversion rates to 10 μmol g cat ?1h ?1
. This optimization is essential to realize the potential of the method for making solar
fuels. [ 6]
E ncouraged by this advance, the use of a capillary ? ow reactor that allowed strict control over the gas phase environ-ment, improved gas-solid contact and operating temperatures, and reduced deleterious reverse reactions between product and reactant molecules was developed. Many very promising results followed. Latest developments show that the photoactivity
of I n 2O 3- x (OH) y nanocrystals can exceed 1 mmol g c at ?1h ?1,[ 6]
which represents a technologically signi? cant conversion rate.
I n this study [ 11 ]I n 2O 3- x (OH) y
served as a model nanostruc-tured catalyst for the photoreduction of gaseous CO 2 by H 2to form CO using ultraviolet and visible light. The knowledge obtained provided much experimental insight into how to select a material’s composition, design a structure, and introduce functional defects to optimize the performance of a photocata-lyst that, with improvements in ef? ciency, might prove to be suitable for large-scale solar fuels production. The motivation for the research stems from the fact that large-scale production of hydrocarbons and methanol currently rely on CO as a feed-stock, hence the development of a photochemical, rather than a thermochemical reverse water gas shift process for making CO from CO 2 and H 2 might ultimately prove to be an energetic, cost effective and clean alternative.
2.2. W orking Catalyst System 2: Photomethanation of Gaseous
CO 2 over Ru/Silicon Nanowire Catalysts with Visible and Near-Infrared Photons
C O 2 photomethanation was recently achieved in a H 2envi-ronment at target rates of the order of 1 mmol g ?1h ?1under ≈15 Suns concentrated solar irradiation over Ru nanoparticles
af? xed to ultrablack black silicon nanowire (SiNW) supports
[ 7] ( F igure 4).
T hese SiNW catalyst supports are ideal for solar powered catalysis because, with a band-gap of 1.1 eV, they can absorb more than 85% of the solar irradiance across the entire ultra-violet to the visible to the near infrared wavelength range. Addi-tionally, the valence and conduction band energy of silicon is preferentially located, such that photoexcited electron–hole pairs activate hydrogen atoms adsorbed on the surface of the support. I t was demonstrated that traditional thermal hetero-geneous catalysts that operate at temperatures around 300 °C and pressures of 60 Bar, such as Ru, can be photothermally driven using the light and heat from the sun around 100 °C and ambient pressures, when af? xed to ultrablack black SiNW supports.
I t is expected that the initial champion rates of 1 mmol g ?1h ?1 achieved over these catalysts
[ 7] will be increased by orders of magnitude by optimizing the size, distribution, and loading of the Ru nanoparticles over the support and integrating these catalysts into ? ow-reactors situated at the focal point of solar concentrators with ×100–1000 solar concentration. The Ru nanoparticles will also be replaced with less expensive metals such as Ni to reduce costs and render the system economically viable for large-scale production.
O n-going research to explore the aforementioned systems in more depth is aimed at i) expanding the materials compo-sition-structure-morphology-hierarchy ? eld, ii) experimental and computational studies of surface chemistry, kinetics, and mechanisms, iii) optimization of the photocatalyst activity, iv) understanding the effect of solar concentration, v) sophis-tication of batch and ? ow photoreactor design, construction, and operation to improve conversion rates and ef? ciencies, and vi) process modeling to assess the energy ef? ciency and eco-nomic feasibility of making solar fuels from CO 2 for best per-forming materials.
I n closing this section it is emphasized that the skeptic judging the practicality of these archetype CO 2photoreductions systems will recognize the truism of the materials dilemma: expensive to discover and understand the science, cheap to develop and implement the technology. It is still early days for
the advances described in this article and original papers,
[ 6,7 ] stay posted to see the materials dilemma in action!
2.3. S igni? cance of Advances
T
o place the most recent results into perspective, a target con-version rate of 10 mole CO 2h ?1g ?1 of catalyst translates into
a conversion rate of 1 Gton CO 2 per year per ton of catalyst when spread over 250 sunny locations around the earth. For a
1 m
2 area solar fuels panel containing a catalyst ? lm of 1 μm thickness made of a MO x nanomaterial with average density of 5 g cm ?
3 , a conversion capacity of 1 Gton CO 2 per year per ton
F igure 4. I llustration of ultrablack silicon nanowires etched into a silicon
wafer functioning as a broad band absorber for solar radiation and a support for metallic and/or semiconductor catalysts that can photother-mally transform, with high ef? ciency, gaseous CO 2 and H 2 into solar fuels.
Reproduced with permission.
[ 7]
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of catalyst will require 2000 panels. Thereby, CO 2 could be recy-cled to produce solar fuels at a globally relevant rate and scale.
I n this context, it is worth noting Ivanpah, the world’s largest solar thermal power station in the Mojave Desert produces high-value electricity and steam for power, petroleum, and industrial-process markets worldwide and deploys 330 000 mir-rors with roughly the same area as those proposed for the envi-sioned CO 2 to fuels and chemical process. [ 12 ] In terms of land
usage, recall that advanced designs, such as the solar chimney,
exist and could address this important and recurring issue. [ 13 ]
W ith an earth abundant, cost effective, and scalable catalyst integrated with the existing chemical and petrochemical indus-trial infrastructure one can begin to appreciate that CO 2conver-sion rates of this magnitude provide a potentially practical, eco-nomical and sustainable alternative to burning and depleting fossil fuels with unacceptable climate change consequences.
2.4. T he 10% Solar Fuels “Silver Bullet” Solution
T
he big question on the minds of climate concerned scientists and engineers, fossil fuel and renewable energy companies, environmentalists, economists, investors, and government policy makers these days is whether unprecedented solar-to-fuel
conversion ef? ciencies of more than 10%,
[ 14 ] is achievable in the next ? ve years, when today’s state-of-the-art, by any known method, is around 1%? The main tools available to realize this challenging ef? ciency goal include innovations in materials discovery and creative design, development and optimization of photoreactors. The former is a nanomaterials innovation problem the latter is a test of engineering ingenuity.
I n the spirit of this article, namely the quest for high-ef? -ciency, gas-phase, light-assisted heterogenous catalytic photo-reduction of CO 2 to solar fuels using photoactive metal oxide based nanostructured materials, some speci? c points where we currently lack the necessary basic understanding required to make progress towards the 10% solution in a reasonable time frame are mentioned here. Some of the outstanding issues include the following: 1) Reproducibly synthesizing nanomate-rials with well-de? ned sizes, shapes, surfaces, defects, porosity, assemblies. 2) Scaling the synthesis of nanomaterials from laboratory amounts to prototype demonstration quantities. 3) Aligning the energies of valence and conduction bands of nanomaterials with the redox potentials of reacting molecules for solid-gas reactions. 4) Doping and aliovalent substitution of nanomaterials without photochemical deactivation. 5) Elimi-nating adventitious carbonaceous residues in nanomaterials originating from the use of organic solvents, templates and ligands used in syntheses. 6) Developing all-inorganic syntheses of nanomaterials to avoid carbon contamination. 7) Under-standing the surface chemistry, kinetics and mechanisms on nanomaterials in the dark and light. 8) Transient absorption and emission measurements to determine the dynamics of photogenerated electron–hole pairs in nanomaterials. 9) Estab-lishing the electrical, optical, thermal, and vibrational proper-ties of nanomaterials in the dark and light under reaction con-ditions. 10) Understanding and controlling the enhancement of conversion rates using cocatalysts plamonics, upconverters, and photonic crystals. 11) Controlling the (photo)chemical,
thermochemical and mechanical stability of nanomaterials under reaction conditions. 12) Applying the most up-to-date dif-fraction, microscopy, spectroscopy, electrical, optical, mechan-ical, and thermal nano tools to elucidate nanoscale structure–property relationships. 13) Measuring conversion rates and ef? -ciencies of CO 2 to solar fuels with respect to mass and energy balance. 14) Evaluating turn over numbers and frequencies and elucidating if photocatalysis is catalytic or stoichiometric. 15) Combining theoretical chemistry with high-performance com-putation to understand ground state and excited state surface chemical reactions on nanomaterials.
I t is apparent that discovering nanomaterials that can use sunlight and water to convert CO 2 to energy rich fuels with 10% ef? ciency would represent a giant leap forward towards the solar re? nery of the future, however from the above list one can see there is still much to do. Nevertheless, solar fuels researchers around the world have taken the ? rst step towards achieving the 10% solution and now the really hard work begins! 2.5. R eality Check
I t is pertinent to appreciate the difference between the solar fuels work described in this article and the most promising technology of concentrated solar thermal production of fuel from H 2 O and CO 2
. This approach uses the thermal content of concentrated sunlight over the full solar spectral range to drive
H 2O –CO 2 thermochemical redox cycles. [ 15 ] The most ef?
cient solar thermal system reported to date involves the production
of H 2 –CO syn-gas using a two-step H 2O -CO 2 redox cycle. [ 16 ]
I t is founded upon non-stoichiometric ceria and involves a high temperature reduction step operating around 1500 °C, CeO 2 → CeO 2- x +
x /2O 2 and low temperature re-oxidation steps occurring around 1000 °C, CeO 2- x + x H 2O → CeO 2 + x H 2and CeO 2- x + x C O 2 → CeO 2 +
x C O. This study utilized a 4-kW solar reactor converting CO 2 to CO with rates of 0.5 mL CO min ?1
g
?1 catalyst, yields of 4 mL of CO g ?1 catalyst per redox cycle, and, most importantly, solar-to-fuel energy conversion ef? cien-cies that approach 4%. One only requires an improvement of 3 to 4 times for this process to become an economic competi-tive and industrially viable technology. [ 16 ]
B y comparison with the low temperature (100–150 °C) photo c hemical and/or photothermal reduction of CO 2 to CO or CH 4 using standard photoreactor con?
gurations used in the research described herein, [ 15,16 ] existing solar thermal processes
operate at very high temperatures (1000–2000 °C) requiring intricately structured porous metal oxide monolithic structures, specialized photoreactors and complex thermal cycling proto-cols of metal oxide redox pairs at two different temperatures to produce H 2 from H 2 O and CO from CO 2
. This high-tempera-ture chemistry has to be cognizant of the thermal, mechanical, photochemical, and chemical stability of the various materials and modules that would need to be integrated in a concen-trated solar thermal re? nery, such as the catalysts and reactors. Clearly the development of comparably ef? cient low tempera-ture photo c hemical and/or photothermal CO 2 reduction pro-cesses of the kind described in this article would be both inter-esting and desirable. [ 6,7 ]
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I t is pertinent to note that in Germany, H 2 from readily available renewable sources is beginning to be used to gen-erate fuels such as CH 4 by the classical Sabatier thermally driven heterogeneous catalytic reduction of CO 2
. n this process, denoted Power-to-Gas (P2G), [17] part of Germany’s
planned transition from old to new energy systems, while the H 2 could come from solar splitting of H 2 O , currently the preferred source is H 2
O electrolysis using electricity from solar and wind. These are known technologies with de? ned energy and economic consequences, where the most resource ef? cient and cost effective one for a particular geographical
locality will determine the method of choice. [18] An added
bene? t of storing electrical energy in the form of a fuel such as CH 4
, which can be readily transported through existing natural gas pipelines is that it reduces the night and day and seasonal challenges of intermittency associated with the gen-eration, storage, transmission, and delivery of wind and solar electricity. I f solar H 2 from H 2
O splitting become an energetic and economic proposition relative to current practice then clearly it will be an attractive alternative feedstock to enable classical thermal catalytic reduction of CO 2 . This approach for making fuels from CO 2 will, however, always require thermal sources either from non-renewable fossil fuels or renewable energy such as solar power, solar heat, or solar photons, which in turn will require ef? cient solar electrochemistry, solar thermochem-istry, or solar photochemistry technologies. Clearly the work described in this article has focused attention on the funda-mental science underpinning heterogeneous catalysis strate-gies using light and heat from the sun to drive CO 2reduction reactions to fuels under mild conditions. The generation of solar H 2 to effect solar CO 2 reduction in a tandem solar photo-reactor is an appealing option for making fuels in an all-solar re? nery of the future, the practicality of which will depend on its easy access to water, carbon dioxide and sunlight and its ef? ciency. On the subject of ef? ciency, it is worth commenting that few articles report solar-to-fuel energy conversion ef? cien-cies, [16] which is a key indicator to economic feasibility. Often, reported ef? ciency values actually refer to yield, i.e., mass bal-ance instead of energy balance, and for technological assess-ment the latter is required.
2.6. A Vision for a Sustainable Future
A n appealing initiative in the outlook for a sustainable future is to make the solar fuels technology of capturing and uti-lizing CO 2 from thin air and in concentrated localized sources
compatible with existing chemical and petrochemical infra-structure around the world. Illustrated in F igure 5 is an envis-aged Air-to-Fuel (A2F) technology that uses concentrated solar-assisted ? ow-based heterogeneous catalysis methods for ef? ciently splitting gaseous water into H 2 and using the H 2 to reduce gaseous CO 2 to CO, CH 4 , and CH 3
O H fuels and chemicals. If A2F can be reduced to practice, the time that it
should take to transition solar fuels laboratory scale science to a global technology could be short enough to circumvent the predicted adverse consequences of greenhouse gas climate change.3. S olar Fuels Prescience and Conclusions
B
ased upon these promising materials chemistry developments for light-assisted, gas-phase photochemical conversion of carbon dioxide to fuels and chemicals, it seems that it will only be a matter of time before globally signi? cant ef? ciencies will be achieved, and the vision of the solar re? nery of the future (Figure 2 ) could be reduced to practice. I n the context of the solar re? nery, it is worth recalling that roughly one hundred years ago, Giacomo Luigi Ciamician, con-sidered by many to have pioneered the ? eld of photochemistry, predicted that humans will one day master the ability to mimic
photosynthesis:
[ 19 ]“ O n the arid lands there will spring up indus-trial colonies w ithout smoke and w ithout smokestacks; forests of glass tubes will extend over the plains and glass buildings will rise everyw here; inside of these w ill take place the photochemical pro-cesses that hitherto have been the guarded secret of the plants, but that w ill have been mastered by human industry w hich w ill know how to make them bear even more abundant fruit than nature, for nature is not in a hurry and mankind is. And if in a distant future the supply of coal becomes completely exhausted, civilization w ill not be checked by that, for life and civilization will continue as long as the sun shines! ”
I n his Science paper, [ 19 ] Giacomo Luigi Ciamician empha-sized that the photochemistry of the future should not be post-poned to distant times and that industry would bene? t by using all the energies nature puts at its disposal. He saw that human civilization was exploiting mainly legacy fossil solar energy and thought it would be much better to make use of radiant energy
from the sun. I t is also worth recalling in a quote about twenty years later Thomas Edison said: [ 20 ]“ I ’d put my money on the sun and solar energy. What a source of power! I hope we don ’t have to wait until
oil and coal run out before we tackle that. I wish I had more years left. ” Roughly thirty years later Nobel Laureate Melvin Calvin
said: [ 21 ]“ I t is time to build an actual arti? cial photosynthetic
system, to learn what works and what doesn ’t work, and thereby set the stage for making it work better.” I n view of the impending climate change and energy secu-rity challenges confronting the human race, the prescience of Giacomo Luigi Ciamician, Thomas Edison, and Melvin Calvin is extraordinarily close to actuality today and materials chem-ists around the world are charged with the mission to make the
F igure 5. E nvisioned sunlight powered Air-to-Fuel (A2F) technology.
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Utopian vision of a sustainable solar fuels earth, in which H 2O and CO 2 are used as chemical feed stocks for producing H 2,CO, CH 4 , and CH 3
O H, a reality–one that’s founded upon the science and technology embedded within the solar re? nery of the future (Figure 2 ).
I n this context, the Global Carbon Capture and Storage Insti-tute has concluded in a recent study that once the technological feasibility of producing hydrocarbon fuels from CO 2 is demon-strated this could accelerate the growth of carbon capture and sequestration and catalyze its commercial exploitation into a
mature industry. [ 22 ]
I t is now obvious that our globalized community is con-fronted with fundamental challenges regarding the way we produce, store, transmit, and use energy, and the way it affects the climate and environment that need to be resolved in the next 10–20 years. I f the science and technology of solar fuels discussed here can deliver on its promises it has the capacity to transition an unsustainable CO 2
-positive fossil fuels economy with its risk of climate change, into a sustainable CO 2-neutral solar fuels economy with its promise of climate stability.
A cknowledgements
G .A.O. is Government of Canada Tier 1 Canada Research Chair in Materials Chemistry and Nanochemistry. Strong and sustained ? nancial
support from the Ontario Ministry of Research and I nnovation (MRI ),
the Ontario Ministry of Economic Development and Innovation (MEDI),
the Natural Sciences and Engineering Council of Canada (NSERC), the Connaught I nnovation Fund and the University of Toronto is deeply appreciated. The creative contributions of the faculty and student members of the University of Toronto Solar Fuels Team to the work described in the A dvance d Scie nce publications upon which this article is based are gratefully acknowledged. The critical reading and insightful feedback on this article from Sir John Thomas, Christopher Munnings, Thomas Bein, Bettina Lotsch, Dwight Seferos, Gregory Scholes, Sebastian Polartz, Linda Nazar, Aldo Steinfeld, Christos Maravelius, Aron Walsh, Frank Markham, André Bardow, Jinlong Gong, Nicolas
Serpone, Mario Pagliari, Robert Whetten, Avelino Corma, Hermenegildo
Garcia, Michele Aresta, Simon Hall, Ludovico Cademartiri and Frank
Osterloh, are deeply appreciated.
Received: J anuary 5, 2015
Published online: February 5, 2015 [1] a ) J . R ongé ,T . B osserez ,D . M artel ,C . N ervi ,L . B oarino ,F . T aulelle ,G . D echer ,S . B ordigac ,J . A. M artens ,C he m. Soc. Re v. 2014,43,7963 ;b ) K . L i ,X . A n ,K . H. P arka ,M . K hraishehb ,J . T anga ,C atal.
Today 2014,224,3;c ) M . Q. Y ang ,N . Z hang ,M . P agliaro ,Y . J. X u , C hem. Soc. Rev. 2014,43,8240 ;d ) M . T ahir ,N . A. S. A min ,R enew.
Sust. Ene rgy Re v. 2013,25,560 ;e ) S . N eat ?u ,J . A. M aciá-Agulló ,H . G arcia ,I nt. J. Mol. Sci. 2014,15,5246 ;f ) A . G oeppert ,M . C zaun ,J . P. J ones ,G . K. S. P rakash ,G . A. O lah ,C he m. Soc. Rev. 2014,43,7995 ;g ) W . T u ,Y . Z hou ,Z . Z ou ,A dv. Mate r. 2014, 26,4607 ;h ) S . P erathoner ,G . C enti ,C hemSusChem. 2014,7,1274 ;i ) P . Z hou ,J . Y u ,M . J aroniec ,A dv. Mater. 2014,26,4920 ;j ) H . S un ,S . W ang ,E ne rgy Fue ls 2014,28,22 ;k ) M . A resta ,A . D ibenedetto ,A . A. A ngelini ,P hil. Trans. R. Soc. A 2013,371,20120111 ;l ) S . D as ,W . D aud ,W . M. A . Wan Daud ,R SC Adv. 2014,4,20856 ;m ) J . M. T homas ,C hemSusChem. 2014,7,1801 .[2] T . A. F aunce ,W . L ubitz ,W . R utherford ,D . M acFarlane ,G . F. M oore ,P . Y ang ,D . G. N ocera ,T . A. M oore ,D . H. G regory ,S . F ukuzumi ,K . B. Y oon ,F . A. A rmstrong ,M . R. W asielewskin ,S . S tyring ,E nergy Environ. Sci. 2013,6,695 .[3] C . A. R odriguez ,M . A. M odestino ,D . P. P saltis ,C . M oser ,E nergy
Environ. Sci. 2014,7,3828 .[4] S . P rotti ,A . A lbini ,N . S erpone ,P hys. Chem. Chem. Phys. 2014,16,19790 .[5] a ) N . v on der Assen ,P . V oll ,M . P eters ,A . B ardow ,C hem. Soc. Rev. 2014,43,7982 ;b ) N . v on der Assen ,J . J ung ,A . B ardow ,E nergy Environ. Sci. 2013,6,2721 .[6] L . B. H och ,T . E. W ood ,P . G. O ’Brien ,K . L iao ,L . M. R eyes ,C . A. M ims ,G . A. O zin ,A dv. Sci. 2014,1,1400013 .[7] P . G. O ’Brien ,A . S andhel ,T . E. W ood ,A . J elle ,L . B. H och ,C . A. M ims ,G . A. O zin ,A dv. Sci. 2014,1,1400001 .[8] X . G. M eng ,T . W ang ,L . Q. L iu ,S . X. O uyang ,P . L i ,H . L. H u ,T . K ako ,H . I wai ,A . T anaka ,J . Y e ,A nge w. Che m. Int. Ed. 2014,43,11478 .[9] F . S astre ,A . V. P uga ,L . C. L iu ,A . C orma ,H . G arcia ,J . Am. Chem. Soc. 2014,136,6798 .[10] J . A. H erron ,J . K im ,A . A. U padhye ,G . W. H uber ,C . T. M aravelias , E nergy Environ. Sci. 2015,8,126 .[11] K . K. Ghuman, T. E. Wood, L. Hoch, C. A. Mims, G. A. Ozin, C. Veer Singh, u npublished . [12] B rightness Sources Limitless, https://www.doczj.com/doc/e413472038.html,/ (accessed January 2015). [13] S olar Updraft Tower, https://www.doczj.com/doc/e413472038.html,/wiki/Solar _updraft _tower (accessed January 2015). [14] S olar Fuels and Arti? cial Photosynthesis , R SC, Advancing the Chemical Sciences, January 2012, https://www.doczj.com/doc/e413472038.html,/solar-fuel .[15] M . R omeroa ,A . S teinfeld ,E nergy Environ. Sci. 2012,5,9234 .[16] P . F urler ,J . S cheffe ,M . G orbar ,L . M oes ,U . V ogt ,A . S teinfeld ,
E nergy Fuel 2012,26,7051 .[17]
F uel Cells Bull. 2014,2014,8. [18] Q . S chiermeier ,N ature 2013,496,156 .[19]
G . C iamician ,S cience 1912,36,385 .[20] T homas A. Edison Edison nnovation Foundation, http://www.
https://www.doczj.com/doc/e413472038.html,/index.php/education/edison-quotes/ (accessed January 2015). [21] M . C alvin ,A cc. Chem. Res. 1978,11,369 .[22] G lobal CCS I nstitute, https://www.doczj.com/doc/e413472038.html,/publica-tions/accelerating-uptakeccs-industrial-use-captured-carbon-dioxide (accessed January 2015).