PERSPECTIVE pubs.acs.org/JPCL
New Directions for the Photocatalytic Reduction of CO2: Supramolecular, scCO2 or Biphasic Ionic Liquid-scCO2 Systems David C. Grills* and Etsuko Fujita Chemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York 11973-5000
ABSTRACT There is an urgent need for the discovery of carbon-neutral sources of energy to avoid the consequences of global warming caused by ever-increasing atmospheric CO2 levels. An attractive possibility is to use CO2 captured from industrial emissions as a feedstock for the production of useful fuels and precursors such as carbon monoxide and methanol. An active field of research to achieve this goal is the development of catalysts capable of harnessing solar energy for use in artificial photosynthetic processes for CO2 reduction. Transitionmetal complexes are excellent candidates, and it has already been shown that they can be used to reduce CO2 with high quantum efficiency. However, they generally suffer from poor visible light absorption, short catalyst lifetimes, and poor reaction rates. In this Perspective, the field of photocatalytic CO2 reduction is introduced, and recent developments that seek to improve the efficiency of such catalytic processes are highlighted, especially CO2 reduction with supramolecules and molecular systems in supercritical CO2 (scCO2) or biphasic ionic liquid-scCO2 mixtures.
R
esearch into artificial photosynthetic processes has intensified in recent years due to the twin problems of global warming and our diminishing stockpile of fossil fuels. The global demand for energy has increased dramatically since the Industrial Revolution, with the average energy consumption rate being ∼13.5 TW in 2001 and expected to double to ∼27 TW by 2050.1 The vast quantities of CO2 that are accumulating in the atmosphere as a result of this energy consumption, primarily via the burning of fossil fuels, will have a devastating impact on the global climate if steps are not immediately taken to reduce them.2,3 One option is to develop carbon capture and storage technology so that the majority of CO2 generated by industrial and energy-related sources can be captured and stored indefinitely. However, such technology is still in its infancy, and it is not yet known if the captured CO2 can be safely stored for many centuries without leakages. An alternative to storing the captured CO2 would be to make use of it as a C1 feedstock for the generation of useful fuels and precursors such as methanol, methane, and carbon monoxide in an essentially carbon-neutral cycle. Being the final product of combustion of carbon-based fuels and the most oxidized form of carbon, CO2 is by definition an extremely inert and thermodynamically stable molecule. The conversion of CO2 into higher-energy reduced forms (i.e., CO2 reduction) therefore requires the input of large amounts of energy, and for this to be an overall carbonneutral process, a renewable energy source will be required. The most promising source of energy is the sun since more energy from sunlight strikes the surface of the earth in 1 h
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than all of the energy currently consumed on the planet in 1 yr.1 In natural photosynthesis, upon absorption of sunlight, H2O is oxidized into its constituents, with the release of O2 as a byproduct. Thus, H2O acts as a sacrificial source of protons and electrons, which are used to produce the energy storage molecules, adenosine-50 -triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH). These ultimately reduce captured CO2 into higher-energy sugars via a complex light-independent reaction cycle. However, natural photosynthesis will not be able to keep up with our everincreasing demand for energy. Therefore, research into socalled artificial photosynthesis (AP) has intensified in recent years.4 AP involves the development of synthetic catalysts that can absorb solar energy and drive chemical transformations inspired by the more complex natural photosystems in order to create clean fuels, such as H2 and carbon-based compounds. Figure 1 shows a schematic diagram of the processes that occur in natural photosynthesis. Highlighted in red are four areas of AP research that draw inspiration from different components of these natural processes. Typically, only one or two of these areas are investigated at a time to reduce complexity, and in this Perspective, we will discuss light absorption and the reduction of CO2 to CO by transitionmetal complexes.
Received Date: July 24, 2010 Accepted Date: August 26, 2010 Published on Web Date: September 02, 2010
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DOI: 10.1021/jz1010237 |J. Phys. Chem. Lett. 2010, 1, 2709–2718
PERSPECTIVE pubs.acs.org/JPCL
Figure 1. Schematic diagram of a natural photosynthetic system, with four areas of artificial photosynthesis (AP) research highlighted in red and described in green text. Synthetic AP systems are designed to perform similar functions to the different components of natural photosystems. AP research typically focuses on one or two of these topics at a time to reduce complexity. However, the ultimate goal would be to combine each of these components into a single, functional AP system for the renewable generation of clean fuels and precursors such as CO or methanol from CO2, H2O, and sunlight.
catalysts that can drive such reactions is therefore a field of intensive research. The use of transition-metal complexes as homogeneous catalysts for these processes has received a lot of attention for several reasons. First, such complexes are synthetically flexible, so that their absorption properties can be tuned to capture visible light and their reduction potentials tuned to match the potential required for CO2 reduction. They also allow access to multiple redox states, which is essential when catalyzing multi-electron-transfer reactions. Finally, light absorption often generates charge-separated excited states, such as metal-to-ligand charge transfer (MLCT), that are sufficiently energetic and long-lived to couple with electrontransfer processes to reduce CO2 via low-energy pathways. Such pathways typically begin with an initial reduction of the catalyst by electron transfer from an electron donor, followed by coordination of CO2 to a vacant site at the metal center that either already existed or that was formed by the dissociation of one of the ligands following reduction. Sometimes metal-based catalysts are used in conjunction with photosensitizer molecules, which absorb light and transfer electrons to the catalyst following reductive quenching of their excited state. In other cases, the metal complexes behave as both the light absorber and catalyst. It should be noted that although the ultimate goal would be to couple CO2 reduction and water oxidation catalysts together (with water acting as a sacrificial source of electrons and protons), such a coupling of two types of catalysis is an extremely formidable challenge. It has therefore proved more practical to initially
Being the final product of combustion of carbon-based fuels and the most oxidized form of carbon, CO2 is by definition an extremely inert and thermodynamically stable molecule. The conversion of CO2 into higher-energy reduced forms (i.e., CO2 reduction) therefore requires the input of large amounts of energy. If we examine the thermodynamics of CO2 reduction, we find that the direct one-electron reduction of CO2 to CO2•- is a very energy intensive, unfavorable process (standard electrochemical potential, Eo0 = -1.90 V vs NHE in aqueous solution at pH 7).5 However, CO2 reduction via proton-assisted multielectron-transfer pathways is much more favorable. For example, the two-electron two-proton reduction of CO2 to CO and H2O occurs at a potential of -0.53 V versus NHE, and the six-electron six-proton reduction of CO2 to CH3OH and H2O occurs at a potential of -0.38 V.5 The development of new
r 2010 American Chemical Society
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DOI: 10.1021/jz1010237 |J. Phys. Chem. Lett. 2010, 1, 2709–2718
PERSPECTIVE pubs.acs.org/JPCL
Table 1. Some Examples of Mononuclear and Supramolecular Re-Based Photocatalytic Systems for the Reduction of CO2 to CO and Their Quantum Yields and Catalytic Activitiesa photosensitizer/catalyst
Φb (mol einstein-1)
donor
d
TON/TOFc
ref
ReCl(bpy)(CO)3
TEOA
0.14
23/11.5
21, 22
ReCl(bpy)(CO)3
TEA
n.r.
42/1.7e
23
Re(SCN)(bpy)(CO)3
TEOA
n.r.
26.4/13.2
24
[Re(4,40 -(MeO)2-bpy)(CO)3{P(OEt)3}]þ/ Re(bpy)(CO)3(CH3CN)]þ [Re(bpy)(CO)3{P(Ohex)3}]þ
TEOA
0.59
n.r.
11
TEA
n.r.
2.2/1.1f
25
[Re(bpy)(CO)3{P(OiPr)3}]þ
TEOA
n.r.
15.6/0.7e
26
[Re(dmb)(CO)3{P(OEt)3}]þ
TEOA
0.18
4.1/0.2
27
Ru(dmb)32þ/ReCl(dmb)(CO)3
BNAH
0.062
101/6.3
13
[(dmb)2Ru(L1)Re(CO)3Cl]2þ
BNAH
0.12
170/10.6
13
[(dmb)2Ru(L1)Re(CO)3{P(OEt)3}]3þ
BNAH
0.21
232/19.3
14
[Ru{(L1)Re(CO)3Cl}3]2þ
BNAH
0.093
240/15
13
[(dmb)2Ru(L2)Re(CO)3Cl]2þ [(dmb)2Ru(L3){Re(CO)3Cl}2]2þ
BNAH BNAH
0.13 (n = 2), 0.11 (n = 4, 6) n.r.
180/15 (n = 2), 120/10 (n = 4, 6) 190/11.8
17 15, 16
[{(dmb)2Ru}2(L3)Re(CO)3Cl]4þ
BNAH
n.r.
110/6.9
15, 16
a
In the majority of cases, a single complex is acting as both the photosensitizer and the catalyst for CO2 reduction. Abbreviations used: TEOA = triethanolamine, TEA = triethylamine, bpy = 2,20 -bipyridine, dmb = 4,40 -dimethyl-2,20 -bipyridine, BNAH = 1-benzyl-1,4-dihydronicotinamide, n.r. = not reported. For L1-L4, see Chart 1. b The quantum yield of product formation is defined as the formation rate (mol s-1) divided by the light intensity (einstein s-1), where 1 einstein = 6.022 1023 photons. c TON is the turnover number, defined as moles of CO formed divided by moles of catalyst used, and TOF is the turnover frequency, defined as the TON divided by the irradiation time in hours. d With 23 equiv of NEt4Cl added. e In DMF pressurized with CO2 gas (T = 26 °C, P = 1-2.5 MPa). f In liquid CO2 solution (T = 28 °C, P = 7.0 MPa).
are almost 100% selective in producing CO as the sole reduction product, and some examples are listed in Table 1. Quantum efficiencies for the production of CO have continued to improve, with the highest reported being 59% for a dual Re-based photosensitizer-catalyst system (where X = CH3CN and P(OEt)3).11 A common theme among all of the known homogeneous photochemical CO2 reduction catalysts is that they are inherently unstable, with complete catalyst deactivation generally occurring after only a relatively small number of turnover numbers (TON
