Photoredox Reactions and the Catalytic Cycle for Carbon Dioxide

Article pubs.acs.org/JPCC

Photoredox Reactions and the Catalytic Cycle for Carbon Dioxide Fixation and Methanogenesis on Metal Oxides Ilya A. Shkrob,*,† Timothy W. Marin,†,‡ Haiying He,§ and Peter Zapol†,§ †

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ‡ Chemistry Department, Benedictine University, 5700 College Road, Lisle, Illinois 60532, United States § Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: Photoirradiated metal oxide semiconductors are known to reduce carbon dioxide to methane. This multistep reaction is commonly represented as a sequence of proton-coupled two-electron reactions leading from carbon dioxide to formate to formaldehyde to methanol and to methane. We suggest that the actual reaction mechanism is more complex, as it involves two-carbon molecules and radicals in addition to these onecarbon species. The ″stepping stone″ of this mechanism for carbon dioxide fixation could be glyoxal, which is the product of recombination of two formyl radicals, or glycolaldehyde, which is its reduced form. We demonstrate the main steps of this reduction chain and suggest a catalytic cycle integrating these steps and the radical chemistry. In addition to methane, this cycle generates complex organic molecules, such as glycolaldehyde, acetaldehyde, and methylformate, which were observed in product analyses. This cycle can be regarded as one of the simplest realizations of multistep, photosynthetic fixation of atmospheric carbon in prebiotic nature.

1. INTRODUCTION The present study continues the theme addressed in some other recent publications from our laboratory,1−5 viz., the mechanism for catalytic reduction of CO2 on photo- and electroactive semiconducting metal oxides.6−11 Methane, which attracts the most attention due to its use as a fuel, is one of the products of this multistep reduction: other known volatile products include one-carbon (formate, formaldehyde, and methanol), two-carbon (methylformate and acetaldehyde), and three-carbon (acetone) molecules.6,11,12 In the following, hydrated TiO2 (anatase) serves as the model for all such oxide materials, as this material is particularly amenable to spectroscopic study. In refs 1−3, we argued that the first steps of this reduction involve (i) one-electron reduction of chemisorbed CO2 to CO2−•, which is doubly bound to Ti4+−O−Ti4+ centers at the surface through the two oxygens, (ii) a proton-coupled electron transfer to this Ti4+−OCO−•−Ti4+ center resulting in the formation of the doubly bound formate anion, Ti4+−OC(H)O−−Ti4+, and (iii) one-electron reduction of formic acid via O atom transfer to the resulting Ti3+ center resulting in the formation of the formyl radical, HC•O (here the bullet stands for the unpaired electron). Using eCB−• as the general notation for the conduction band (CB) or tail-band electron that is trapped by surface Ti4+ ions involved in the Ti−O bonds with the adsorbate (or the electron-accepting adsorbate itself), these reactions can be, respectively, written as eCB−• + CO2 → CO2−• © 2012 American Chemical Society

eCB−• + H+ + CO2−• → HCO2−

(2)

eCB−• + Ti4 + + HCO2 H → Ti4 +O−H + HC•O

(3)

The focus of the present study is to suggest how the 8-electron reduction of CO2 proceeds past reaction 3. As discussed elsewhere,2,5 it is not presently clear whether this reduction occurs as a sequence of one-electron reactions similar to reactions 1 to 3 or if it involves two-electron reactions that require concerted transfer of two electrons to a reactant on the TiO2 surface. The simplest reaction scheme is a sequence of four consecutive twoelectron proton-coupled reactions3,6 +2e−

+2e−

+2e−

+2H

+2H+ −H2O

+2H

CO2 ⎯⎯⎯⎯⎯⎯→ HCO2 H ⎯⎯⎯⎯⎯⎯⎯→ H2CO ⎯⎯⎯⎯⎯⎯→ CH3OH + + +2e−

⎯⎯⎯⎯⎯⎯⎯→ CH 4 +2H+ −H2O

(4)

While this way of visualizing the reaction is lucid and useful, it affords limited insight into the actual reaction mechanism. Furthermore, it tacitly assumes that two- and three-carbon products play no significant role in methanogenesis, which is not Received: January 4, 2012 Revised: February 16, 2012 Published: March 6, 2012

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dx.doi.org/10.1021/jp300122v | J. Phys. Chem. C 2012, 116, 9450−9460

The Journal of Physical Chemistry C

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Due to its π-conjugation, glyoxal has significant gas-phase adiabatic electron affinity (EA, estimated as 0.62 or 0.9 eV)17 as compared to monoaldehydes (e.g., acetaldehyde and formaldehyde have EA of a few millielectronvolts).18 While the electron affinity of monoaldehydes becomes slightly positive in solution, glyoxal is still a much more efficient electron acceptor than these monoaldehydes. Electrochemical reduction of glyoxal to trans-ethane-1,2semidione, C2H2O2−•,19 is facile, and so glyoxal can serve as a ″stepping stone″ to further reduction past reaction 3. In contrast, the CO2−• anions generated in reaction 1 cannot serve as such “stepping stones”: as shown in Section 2.5 below, the recombination of these radicals yields products which are difficult to reduce on TiO2. In ref 1, we found that photolysis of oxalate and oxalic acid on TiO2 yielded only oxidation products via a Kolbe reaction15

self-evident, given that such molecules are more easily reduced than one-carbon molecules. Can the latter molecules serve as the precursors of methane, e.g., as the progenitors of the methyl radical,2 as was recently suggested in ref 5? In a companion publication,1 we used electron paramagnetic resonance (EPR) spectroscopy to study the photoreactions of HCO2− and HCO2H on hydrated TiO2. The CO2−• and OC•OH (that is, protonated CO2−•) radicals arising from oneelectron oxidation of their parent molecules by valence band holes (hVB+•) h VB+• + HCO2− → H+ + CO2−•

(5)

and the formyl radical generated in reaction 3 were observed at low temperature. No methyl radicals were observed. Trifunac and co-workers13,14 and subsequently Shkrob et al.15 used EPR spectroscopy to study the photoreactions of methanol and methoxide on hydrated TiO2. No methyl radicals were observed under the normal illumination conditions. However, Trifunac and co-workers13 observed methyl radicals below 10 K when methoxide-dressed TiO2 was irradiated using high-fluence 248 nm laser pulses (>100 mJ/pulse). These conditions were so extreme that direct photoexcitation of methanol cannot be excluded. On the other hand, methyl radicals can be readily generated on TiO2 by photooxidation of acetate,15 and such radicals can be observed below 150 K; at higher temperature, the methyl radicals migrate and abstract H from the acetate, yielding carboxymethyl radicals. Thus, the failure of EPR spectroscopy to observe methyl radicals in methanol and formate solutions of aqueous TiO2 cannot be due to the unusually facile secondary radical chemistry (depleting the methyl radicals). In Sections 2.1 and 2.2, we revisit these EPR observations and demonstrate that methanol and formaldehyde do not yield radicals via one-electron reduction. This is not too surprising given that these molecules have no electron affinity and do not react even with the hydrated electron, which is a much stronger reducing agent than the electron on TiO2. The key assumption of reaction scheme 4 is envisioning the four proton-coupled two-electron (or two consecutive one-electron) transfers as H adatom reactions similar to those occurring on metal catalysts. Mechanistically, this picture is unlikely to lead to methane: in the aqueous bulk, the H atoms abstract hydrogen from formate, formaldehyde, and methanol, yielding the corresponding CO2−•, HC•O, and •CH2OH radicalsthat is, the same radicals that are produced by photooxidation of these molecules on TiO2. Such H atom reactions would close the catalytic cycle and direct the reduction toward H2 rather than CH4. This undesirable propensity makes one-carbon molecules in reaction scheme 4 unlikely progenitors of methane: it is much easier to remove a hydrogen atom than to add a hydrogen atom to these molecules. While this may not necessarily be correct regarding what occurs on the surface (which may reduce the barriers for H atom addition as opposed to H atom abstraction), an obstacle still presents itself: one-electron reduction of one-carbon molecules in reaction scheme 4 is energetically prohibitive. Therefore, it seems likely that the reduction is assisted by reactions joining two carbon atoms that yield molecules for which the reaction barriers for further reduction are significantly lowered. It is well-known16 that recombination of the formyl radicals generated in reaction 3 produces glyoxal (ethanedial) 2HC•O → C2H2O2

h VB+• + C2O4 2 − → CO2 + CO2−•

(7)

The involvement of glyoxal would solve another puzzle: in photoexcitation of TiO2 (in the absence of a sacrificial hole-scavenging reagent removing the trapped holes from the surface),11 for every electron, eCB−•, there is a valence band hole, hVB+•, that is trapped at the surface as the Ti4+−O• oxygen hole center or a bound hydroxyl radical.20 The reactions of these holes reverse scheme 4. In methanogenesis, such reactions are, in fact, necessary, as otherwise the photogenerated holes recombine with electrons on TiO2, inhibiting further reduction of the adsorbate: the products of CO2 reduction double as sacrificial hole scavengers. While such oxidation is clearly necessary, it is not clear how reduction can prevail over oxidation, as the latter tends to be more facile. This general consideration indicates that in the overall reaction scheme there must be a step involving a molecule which is more readily reduced than oxidized, and it is this step that allows the process to proceed to completion. If the oxidation reintroduces radicals whose reactions replenish this allimportant “stepping stone” molecule, the reduction can take advantage of this stepping stone and carry past reactions 1 to 3. We suggest that glyoxal and glycolaldehyde serve as these stepping stone molecules, and in Section 3.1, we suggest a catalytic cycle based on this assumption. Before we discuss this cycle in Section 3, much groundwork needs be laid, and this program is implemented in Section 2. In Sections 2.1 and 2.2, we demonstrate that methanol and formaldehyde can only be oxidized on TiO2; no radicals from their one-electron reduction are generated. Then, we turn to two-carbon molecules and in Section 2.3 demonstrate that both glyoxal and glycolaldehyde can be reduced on TiO2, with glycolaldehyde yielding the vinoxyl radical, the precursor of acetaldehyde. In Section 2.4, we show that acetaldehyde yields methyl radicals on TiO2; these methyl radicals can serve as the progenitors of methane. In Section 2.5, we demonstrate that neither glyoxylate nor glycolate (which are two other candidate two-carbon molecules) can serve as stepping stones in the reduction chain. The reaction scheme is further examined and elaborated upon in Section 3.1. In Section 3.2, we demonstrate that radicals generated on the TiO2 surface generally escape to the bulk or weakly interact with the oxide surface; the occurrence of such desorption is an important concern for the suggested reaction scheme, as the bound radicals tend to oxidize. Finally, in Section 4, we discuss some implications of our mechanistic study. We have omitted the experimental and computational section, as the methods we used are identical to ref 1, and we

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direct the reader to that study for more detail. To conserve space, some tables and figures have been placed in the Supporting Information. Such figures have designator ″S″ (e.g., Figure 1S).

2. RESULTS 2.1. Photoreactions of Methanol. Since the EPR study of Trifunac and co-workers,13,14 it has been widely accepted that photooxidation of methanol and other hydroxylated compounds (ROH) involves the alkyloxyl (Ti4+O−R) centers serving as hole traps; this trapping is followed by or concerted with deprotonation of the resulting O 2p hole centers, e.g. h VB+• + Ti4 +O−CH3 → Ti4 +O•CH3

(8a)

Ti4 +O•CH3 → Ti4 +O−C•H2 + H+

(8b)

(Here it is implied that the released proton is attached to the TiO2 surface, as the Ti24+OH+ or Ti4+OH2 centers, or water molecules, as hydronium ions). The key prediction of this mechanism is that the resulting radicals are bound to the oxide surface.15 However, as shown in Section 3.2, density functional theory (DFT) calculations (both ours and from other groups)21 and observations of the so-called ″current doubling″ on TiO222,23 suggest that such bound radicals may not be stable on TiO2, decaying through back electron transfer to the metal oxide Ti4 +O−C•H2 → eCB−• + Ti4 + + H2CO

Figure 1. (a) First-derivative EPR spectra obtained in aqueous solutions of TiO2 nanoparticles containing 5 wt % methanol-h3 with (ii) or without (i) 0.5 M KOH (see the legend in the plot). These samples were irradiated by 355 nm laser light at 77 K, and the EPR spectra were obtained at 50 K, using a microwave power of 2 mW and a modulation amplitude of 2 G (100 kHz modulation). The resonance lines of the surface and lattice Ti3+ centers are not shown. Trace (iii) is the EPR spectrum of the hydroxymethyl radical, •CH2OH, in neat methanol-h3 irradiated by 3 MeV electrons at 77 K. Panel (b): Like panel (a), for the three isotopomers of methanol, as indicated in the plot. The solid lines are the EPR spectra from pH 1.9 solutions, and the dashed lines are from the solutions containing 0.5 M KOH.

(9a)

If reaction 9a occurs, this implies that the reverse reaction (the reduction of formaldehyde) eCB−• + Ti4 + + H2CO → Ti4 +O−C•H2

(9b)

is energetically unfavorable. Thus, it is difficult to reconcile the formation of bound oxymethyl radicals and the reduction of the formaldehyde to methanol via one-electron reactions (to substantiate reaction scheme 4). However, to our knowledge, it has never been demonstrated that such bound radicals are indeed stable on TiO2. Figure 1a shows the EPR spectrum obtained from the frozen aqueous solution of anatase nanoparticles containing methanol and irradiated by 355 nm laser light at 77 K under acidic (trace (i)) and basic (trace (ii)) conditions. For comparison, we superimposed the EPR spectrum of the hydroxymethyl (•CH2OH) radical generated in 3 MeV β-radiolysis of frozen methanol at 77 K (trace (iii)). It is seen that traces (ii) and (iii) are almost identical, which strongly suggests that the radical observed on TiO2 is, actually, the •CH2OH radical rather than the Ti4+O−C•H2 radical or its protonated variant, Ti4+O(H)C•H2. In alkaline solution, the EPR spectrum is different, as the radical exhibits smaller hyperfine coupling constants (hfcc) on the two protons, which is consistent with the known properties of the •CH2O−Alk+ radical (Table 1S). Thus, the progenitor of the latter EPR spectrum could be either the postulated Ti4+O−C•H2 (that is, Ti4+−O−C•H2) radical or the physisorbed • CH2O−Alk+ radical, whereas the radical observed in acidic solution (actually, for pH < 8) is consistent with the •CH2OH radical. As the two radicals have rather different hfcc’s on their 13C nuclei (see Tables 1 and 1S and EPR simulations in Figure 1S), in Figure 1b we examined the EPR spectra obtained for 13CD3OD and 13CH3OH in these acidic and alkaline solutions (in D2O and H2O, respectively). These EPR spectra qualitatively correspond to simulations shown in Figure 1S, suggesting that the isotropic hfcc on 13C for the radical observed in acidic solutions is greater than

the hfcc for the radical observed in alkaline solutions. Figure 2 exhibits our simulations of the EPR spectra obtained for the TiO2/ 13 CD3OD/D2O solutions, and Table 1 gives the optimized hfcc parameters. It is seen that the hfcc for the radical observed in acidic solution is very close to the one observed for free •CH2OH radicals, whereas in alkaline solutions, this constant is lower than for the •CH2O− radical in aqueous solutions. These results suggest that the photooxidation results in release of the radical from the surface Ti4 +O•CH3 + H2O → Ti4 +OH2 + •CH 2OH

(10)

rather than reaction 8b. Trifunac and co-workers13 suggested that the bound alkoxyl groups can be reduced to alkyl radicals via the heteroatom transfer reaction1 eCB−• + Ti4 +O−R + H+ → Ti4 +O−H + •R

(11)

(in this case, for R = Me). We did not observe the methyl radical using 300 and 355 nm photoexcitation under acidic or basic conditions, at any concentration of methanol nor any sample temperature for all of the TiO2 matrices that we used. Our results suggest that reaction 11 does not occur. Figure 2S shows the analogous experiment for several ethanol isotopomers. These EPR spectra are fully accounted for by the contributions from the CH3C•HOH and •CH2CH2OH radicals 9452



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Table 1. Optimized g- and hfcc Tensor Parameters for 13 C Nuclei for Hydroxymethyl (Low pH) and Oxymethyl (High pH) Radicals on Aqueous TiO2 Nanoparticles (50 K) Compared to Experimental Data in the Literature and Gas-Phase DFT Calculations (B3LYP/6-31+G(d,p)) 13

radical •

CD2ODa CD2O−b • CH2OH •

• •

• •

CH2O− CH2OHf CH2O−Li+f CH2O−f

C

13

δg||

B||, G

aiso, G

27 32

36 26

44.4 39.7

42.4 32.9 47.4d 45.3e 37.7d 59.2g 37.5h 31.8h 14.2i

33

37c 46.5g 48.0h 41.4h 20.6i

2H2CO → HOCH2CHO

C

δg⊥

c

acetal form (formose reactions),24 which can be schematically written as (12a)

The same product, glycolaldehyde, can occur through recombination of the formyl and hydroxymethyl radicals HC•O + •CH2OH → HOCH2CHO

(12b)

While reaction 12a is interesting from a mechanistic standpoint, as it provides a molecular path to two-carbon molecules (Section 3.1), it complicates spectroscopic studies, as the aqueous formaldehyde is stabilized by methanol (“formalin”), which is photoreactive on TiO2 (Section 2.1). To avoid this complication, we generated formaldehyde via high-temperature acidic hydrolysis of its linear polymer (para-formaldehyde) and/or trimeric ortho-formaldehyde (Figure 3), rapidly added TiO2 to the mixture, adjusted the pH, and flash froze these solutions by immersion of the sample tube into liquid nitrogen. Figure 3a exhibits the EPR spectra from hydrolyzed paraformaldehyde under weakly acidic (pH 1.9), strongly acidic, and

a For hydroxymethyl radicals. bFor oxymethyl radicals. cIsotropic δgiso = (δg|| + 2δg⊥)/3. We assumed coaxial (axially symmetrical) g and A tensors (B⊥ − B||/2). The hfcc's are given in Gauss (1 G = 10−4 T); δg = (g − 2) × 104. dFrom ref 39. eFrom ref 40. fB3LYP/6-31G(d,p) calculation for gas-phase radicals. gNonplanar (optimized geometry) radical; HCOH dihedral angle is 25°. hCs symmetry planar radical. iC2v symmetry (optimized geometry).

Figure 2. Simulation of the EPR spectra for photoirradiated 13CD3OD on anatase from Figure 1b obtained under acidic and basic conditions, respectively (as indicated in the plot), using the parameters given in Table 1. The vertical lines indicate the resonance lines for the magnetic field oriented along the principal axes of (coaxial) g and A(13C) tensors.

Figure 3. EPR spectra obtained for photoirradiated solutions of 5 wt % formaldehyde on aqueous anatase. The formaldehyde was obtained by hydrolysis of (a) para- and (b) ortho-formaldehyde (see the insets for chemical structures). Trace (i) corresponds to the ″normal″ pH of 1.9; traces (ii) and (iii) were obtained in the presence of 2 wt % of 1 M HCl and KOH, respectively; and trace (iv) is the simulated EPR spectrum of the formyl radical (whose resonance lines are indicated by arrows). The resonance line from the oxygen hole centers on TiO2 contributes to these EPR spectra; there is no evidence for the hydroxymethyl radical or other radical products of formaldehyde reduction.

generated in the oxidation of the ethanol by TiO2; again, there is no indication that the ethyl radical is generated in this photoreaction. We conclude that methanol does not undergo one-electron reduction on TiO2; instead it is oxidized to a f ree hydroxymethyl radical. The last step of scheme 4 cannot be a sequence of two one-electron reactions, and the prevalent photoreaction of methanol is oxidation. Disproportionation of the released hydroxymethyl radical with other radicals in the system (see section 3.1) or reaction 9a yields formaldehyde, reversing the third step in scheme 4. 2.2. Photoreactions of Formaldehyde. Formaldehyde is chemically unstable in aqueous solutions, undergoing basecatalyzed aldol condensation to glycolaldehyde involving its

strongly basic conditions. In all of these systems, no organic radicals other than HC•O were observed. The identity of the latter radical is confirmed by 13C substitution (Figure 4 and Table 1S). While there are resonance lines at the center, the same lines are observed without formaldehyde, so these EPR signals are from trapped-hole 9453



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centers, which are known to be pH sensitive.20 The same observation applies to the formaldehyde generated from ortho-

Figure 4. Solid lines are the EPR spectra obtained in photoirradiated solutions of TiO2 nanoparticles containing 5 wt % of (i) 12CH2O or (ii) 13CH2O generated from para-formaldehyde. The arrows indicate the resonance lines from the isotopomers of the formyl radical, and the dashed lines are simulated EPR spectra. There is no evidence for other organic radicals present in this system. Trace (iii) is the spectrum of the H13C•O radical obtained by photoreduction of (CH3)2N13CHO on TiO2 (from ref 1).

Figure 5. (a) EPR spectra for photoirradiated aqueous anatase nanoparticles containing 1 M (i) glyoxal and (ii) glycolaldehyde at pH 1.9. Traces (i) were obtained at 2 mW, 50 K (solid lines), and 0.2 mW, 12 K (dashed line). The arrow indicates the low-field line from the formyl radical. In trace (ii), the dashed line is the EPR spectrum from hydroxymethyl as observed in the TiO2/MeOH system, replotted from Figure 1a. (b) EPR spectra from glyoxal on TiO2 obtained under the (i) acidic and (ii) basic conditions. These transformations correspond to the protonation equilibrium shown in the inset. See Figure 3S in the Supporting Information for simulations of these EPR spectra.

formaldehyde. There is no evidence for Ti4+O−C•H2 or •CH2OH radicals, which are the anticipated radical products of formaldehyde reduction, while the formyl radical is the product of oxidation h VB+• + H2CO → HC•O + H+

(13)

The methyl radical was also not observed in this photosystem. Like methanol (Section 2.1), formaldehyde is oxidized on photoexcited TiO2; there is no evidence for one-electron reduction under the relevant experimental conditions. Thus, the photoreactions of methanol and formaldehyde on TiO2 are biased toward oxidation rather than reduction. The reduction of these one-carbon molecules, if it occurs at all, likely proceeds via concerted two-electron reactions. 2.3. Photoreactions of Glyoxal and Glycolaldehyde. Given the negative results of Sections 2.1 and 2.2 and the general considerations in the Introduction, we turned to the two-carbon products. The most appealing candidates are the glyoxal generated in reaction 6 and glycolaldehyde generated via reactions 12a or 12b. Since glycolaldehyde is the product of two-electron reduction of glyoxal, both of these molecules are strong candidates for the elusive “stepping stone” (Section 1). Figure 5a exhibits the EPR spectra of glyoxal on photoilluminated TiO2 in weakly acidic solutions. In addition to a weak resonance line of the formyl radical, there is a doublet that can only originate from a >C•H radical. In an alkaline solution, this doublet collapses to a poorly resolved line (Figure 5b). As suggested by our simulations in Figure 3S (the reported hfcc’s for the corresponding radicals are given in Table 1S), the radical observed in acidic solutions is HOC•HCHO, and the radical observed in alkaline solutions is C2H2O−•. Thus, the main photoreaction is one-electron reduction, with oxidation as a side reaction eCB−• + C2H2OH + H+ → HOC•HCHO

(14)

h VB+• + C2H2O2 → OC•CHO + H+

(15)

OC•CHO → CO + HC•O

(16)

This unusual property of the glyoxal makes it a candidate species for overcoming the bottleneck mentioned in the Introduction, as this molecule is more readily reduced than oxidized. Furthermore, oxidation reaction 16 regenerates a formyl radical, which can recombine with another formyl radical (reaction 6) to reintroduce the glyoxal, completing the cycle. Turning to the reactions of glycolaldehyde, we first observe that one-electron reduction of this molecule, written as eCB−• + HOCH2CHO → HOCH2C•HO−

(17a)

produces the same radical anion as the one produced by H abstraction from ethylene glycol (HOCH2CH2OH) in strongly alkaline solutions.25,26 The well-known reaction of this radical anion is a 1,2-shift coupled to elimination of hydroxide25,26 HOCH2C•HO− → •CH2CHO + OH−

(17b)

Thus, the overall photoreaction can be written as eCB−• + HOCH2CHO + H+ → •CH2CHO + H2O (17c) •

yielding the vinoxyl radical, CH2CHO. Due to the energy gain accompanying dehydration, reaction 17c is facile even though the electron affinity of glycolaldehyde is low. The oxidation of glycolaldehyde can yield either the same HOC•HCHO radical that is generated by the reduction of glyoxal or HOCH2C•O 9454



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which is likely to decarbonylate to yield the hydroxymethyl radical

The broad EPR signal overlapping with the methyl radical appears to be from the vinoxyl radical that is generated via the alternative oxidation channel h VB+• + CH3CHO → •CH2CHO + H+

(19c)

There is no evidence for one-electron reduction (as is also the case for formaldehyde) in this acidic solution. However there could be photoreduction in alkaline solution (Figure 6), as the observed EPR spectrum resembles the EPR spectrum expected for CH3C•HO−, although the progenitor of this EPR spectrum can be a variant of the vinoxyl radical (as shown in Figure 4S). Since the acetaldehyde may undergo aldol condensation under such alkaline conditions, we did not pursue the matter further. 2.5. Photoreactions of Glyoxylate and Glycolate. We have suggested glyoxal and glycolaldehyde as possible candidates for our sought-after stepping stone; however, as both the HC•O and CO2−• radicals are present in the photosystem and the CO2−• radicals could be more abundant than the formyl radical, the joining of the two carbons may actually involve reaction 20 rather than reaction 6

Unfortunately, the EPR spectra of the vinoxyl and the hydroxymethyl radicals (which are both methylene radicals) are almost identical as both have negligible hfcc’s on their HO and HCO protons. Figure 5a shows the EPR spectrum obtained by photoirradiation of glycolaldehyde on TiO2. This EPR spectrum arises from a methylene radical, and the comparison of this EPR spectrum to the one replotted from Figure 1a emphasizes the similarity of the hfcc’s for this radical and •CH2OH on TiO2. While it is impossible to tell with certainty whether the observed radical is a variant of the hydroxymethyl radical formed in reaction 18b or the vinoxyl radical formed in reaction 17c, there are differences between the traces shown in Figures 5a that suggest the occurrence of reaction 17c. We conclude that our EPR results do not contradict the occurrence of reaction 17c, and this reaction looks plausible from the known radical chemistry. The important point is that the sequential reduction of glyoxal via glycolaldehyde leads to the vinoxyl radical, which is the precursor of acetaldehydeone of the known products of CO2 reduction on TiO2.11 2.4. Photoreactions of Acetaldehyde. Figure 6 (trace (i)) exhibits the EPR spectrum observed in 355 nm photolysis of

HC•O + CO2−• → HC(O)CO2−

(20)

Thus, it is prudent to examine the redox reactions of glyoxylate. Previous research suggested that carboxylic acids undergo a photo-Kolbe reaction with elimination of CO2,15 in this case h VB+• + HC(O)CO2− → HC•O + CO2

(21a)

However, the hydroxyl radical oxidizes glyoxylate by H abstraction rather than this reaction, so the second possibility is h VB+• + HC(O)CO2− → C•OCO2− + H+

(21b)

followed by decarboxylation of the unstable radical intermediate C•OCO2− → CO + CO2−•

(21c) •

In contrast, the reduction can either yield the HOC HCO2− radical (in analogy to reaction 14) or HCOC•O radical (in analogy to reaction 3). The latter radical would also decarboxylate, yielding the formyl radical Figure 6. EPR spectra from acetaldehyde on aqueous TiO 2 nanoparticles obtained under (i) acidic and (ii) basic conditions (0.5 M H+ or OH−). The quartet of lines from the methyl radical (dashed line) is visible in trace (i), being superimposed on the broader resonance lines of the •CH2CHO. In basic solutions, the αhydroxyethyl radical or its deprotonated form (see the legend in the plot) may contribute to the observed EPR spectrum. See Figure 4S in the Supporting Information for more simulations.

Thus, the formyl radical can be the product of either reduction or oxidation, while the CO2−• radical is the product of oxidation and the HOC•HCO2− radical the product of reduction. Figure 7 shows EPR spectra obtained in photolysis of glyoxylic acid and glyoxylate on TiO2. Glyoxylate yields a weak EPR signal from the HC•O radical (which was not observed at low pH, thereby excluding reaction 22b) and a strong EPR signal from CO2−• (which was significantly reduced at low pH; see Figure 7). The latter can be isolated by subtraction of the EPR spectrum obtained at low pH from the EPR spectrum observed at higher pH (Figure 7). It is clear that both radicals are the products of photooxidation. In addition to these two radicals, there is also a very saturable EPR signal that can be isolated by subtraction of the CO2−• (or OC•OH) trace from the composite EPR spectrum (Figure 7). This yields a doublet corresponding to a >C•H radical, whose EPR spectrum resembles the spectrum of HOC•HCHO from glyoxal (Section 2.3 and also Figure 8). This is clearly the HOC•HCO2H radical; i.e., both glyoxylate and glyoxylic acid are eff iciently reduced on TiO2 by proton-coupled electron attachment.

acetaldehyde on TiO2 (we emphasize that none of the examined molecules absorbs the 355 nm photons; the laser light was absorbed by the oxide nanoparticles only). The quartet of resonance lines from the methyl radical is the predominant feature of this EPR spectrum with a broad, poorly resolved line overlapping this quartet. The likely pathway to this methyl radical involves oxidation of acetaldehyde, yielding the unstable acetyl radical, which promptly decarbonylates27 h VB+• + CH3CHO → CH3C•O + H+

(19a)

CH3C•O → •CH3 + CO

(19b) 9455



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Figure 7. EPR spectra from (i) 0.35 M sodium glyoxylate and (ii) 0.7 M glyoxylic acid on aqueous TiO2 nanoparticles. The difference traces (iii) obtained by subtraction of trace (ii) from trace (i) reveal the presence of CO2−• (a narrow singlet line at 3375 G). Trace (iv) is a Gaussian fit to this EPR signal, and trace (v) is the difference of trace (ii) and trace (iv). This difference trace is compared to the EPR spectrum of the HOC•HCO2H radical from Figure 8a (trace (vi)). The distortions in the EPR spectra are due to the strong overlap with the low-field wing of the Ti3+ signal.

Further reduction of the HOC•HCO2− radical yields glycolic acid, HOCH2CO2−, which can also be generated in reaction 23 •

CH2OH + CO2−• → HOCH2CO2−

Figure 8. EPR spectra observed from 0.5 M glycolate on TiO2 at pH 1 (panel (a)) and pH 6 (panel (b)). In both cases, the EPR spectrum observed at 50 K slowly changes over time due to the occurrence of reaction 25. In panel (a), trace (i) shows the EPR spectrum obtained shortly after photolysis, while the solid lines indicate EPR spectra obtained 90 min later, at pH 1 (ii) and pH 10 (iii), respectively. The vertical marks indicate the centers. In panel (b), the EPR spectrum obtained shortly after the photolysis of a pH 6 solution (trace (i)) is compared to the spectrum of the hydroxymethyl radical from Figure 1 (trace (ii)). Forty minutes later, the latter contribution to the EPR spectrum (trace (iii)) is much reduced. Warming the sample to 130 K completes the reaction so that only the HOC•HCO2− radical is observed (trace (iv)). Superimposed on trace (iv) is trace (v), which is the difference of traces (i) and (ii), suggesting that trace (i) is a weighed sum of EPR signals from the HOC•HCO2H and •CH2OH radicals.

(23)

Glycolate can be readily oxidized back to HOC•HCO2− or • CH2OH radicals via h VB+• + HOCH2CO2− → •CH2OH + CO2

(24a)

→ HOC•HCO2− + H+ (24b)

Figures 8 and 5S show the EPR spectra obtained for glycolate at high (pH 1), medium (pH 6), and low (pH 10) acidity. In strongly acidic solutions (Figure 8a), the prevalent EPR signal is the doublet from the HOC•HCO2H radical: reaction 24b prevails. In neutral solutions, both •CH2OH and HOC•HCO2− radicals are observed at 50 K shortly after the photolysis (Figure 8b); however, over time the •CH2OH radical disappears, and only the HOC•HCO2− radical is observed (Figures 8b and 5S). This transformation is very rapid at 130 K (Figure 8b). Apparently, there is a reaction

yields the unstable RC•O radical, which promptly decarbonylates27 RC•O → R• + CO

so this reduction reintroduces smaller fragment radicals through which RCO2− is generated in an R• + CO2−• recombination. We conclude that for glycolate there is no reaction analogous to reaction 17c, which makes further reduction possible, and the cycle just loops back, as shown in Figure 6S. Thus, oxalate, glyoxylate, and glycolate cannot serve as stepping stones for further reduction.

•

CH2OH + HOCH2CO2− → CH3OH + HOC•HCO2− (25) •

−

−

(26b)

•

In alkaline solutions, both the CH2O and OC HCO2− radicals are initially observed (Figure 5S; the latter radical has a more positive g-factor and smaller hfcc on the α-proton than the protonated form, see Figure 8b), but over time the •CH2O− radical disappears in a reaction analogous to reaction 25. Except for these two radicals, no other species are observed. We point out that a reaction analogous to reaction 3 with a generic acid RCO2H (including the glycolic acid)

3. DISCUSSION 3.1. Overall Reaction Mechanism. To summarize Section 2, for TiO2 (under [weakly] acidic conditions): (i) neither methanol nor formaldehyde can be reduced via one-electron reactions, but both of these molecules are readily oxidized to the hydroxymethyl and formyl radicals, respectively; (ii) glyoxal, the product of HC•O + HC•O recombination, is more readily reduced to the HOC•HCHO radical than it is oxidized to the formyl radical;

eCB−• + Ti4 + + RCO2 H → Ti4 +O−H + RC•O (26a) 9456



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(iii) glycolaldehyde, the product of HC•O + •CH2OH recombination and/or aldol condensation of formaldehyde and methanol (and reduction of the glyoxal on TiO2), is either oxidized back to the hydroxymethyl radical or reduced to the vinoxyl radical; (iv) acetaldehyde is oxidized to the methyl radical; (v) neither HC•O + CO2−• nor CO2−• + CO2−• yields products that can be further reduced on the metal oxide (Figure 6S): oxalate can only be oxidized (Section 1), and glycoxylate, though it can be reduced to the HOC•HCO2− radical, has a reduced form, glycolate, that can only be oxidized back to •CH2OH and HOC•HCO2− radicals; it cannot be further reduced to glycoaldehyde (Section 2.5). Thus, the only two-carbon products generated in reactions of first-generation radicals (CO2−•, HC•O, and •CH2OH) that can carry the reduction chain further are glyoxal and glycolaldehyde (as the latter is reduced to the acetaldehyde). Another possibility is the reaction of two hydroxymethyl radicals, yielding ethylene glycol28 2•CH2OH → HOCH2CH2OH

(27)

In ref 29 we demonstrated that on TiO2 ethylene glycol can be photooxidized to the HOC•HCH2OH radical (reaction 28), which is known to dehydrate to the vinoxyl radical in protoncatalyzed reaction 29)25,26 h VB+• + HOCH2CH2OH → HOC•HCH2OH + H+ (28)

HOC•HCH2OH → •CH2CHO + H2O

(29)

Therefore, the vinoxyl radical can be generated through the reduction of glycolaldehyde (reaction 17c) or oxidation of ethylene glycol (reactions 28 and 29). No other reaction pathways apparently lead to the desired products. Thus, the methyl radical, the progenitor of methane, can be generated in the following way (Figure 9): (a) the CO2 is stepwise reduced to the formyl radical in reactions 1 to 3; (b) two formyl radicals recombine to yield glyoxal (reaction 6); (c) glyoxal is reduced to glycolaldehyde (reaction 14); (d) glycolaldehyde is reduced to acetaldehyde (reaction 17c); (e) acetaldehyde is oxidized to the methyl radical (reactions 19a and 19b); (f) the organic intermediates generated in these reactions (including formate, methanol, and formaldehyde) serve as sacrificial hole scavengers; (g) the hydroxymethyl radicals recombine, yielding ethylene glycol, which is oxidized to the vinoxyl radical (reactions 28 and 29). We stress that we have not demonstrated steps (c) and (d); rather we observed that glyoxal can be reduced to the HOC•HCHO radical, and glycolaldehyde can be reduced to the vinoxyl radical. To complete these transformations, one more H atom needs to be attached to these radicals: we have proved the elaborate scheme shown in Figure 9 only halfway, due to the limitations imposed by our method, that is, EPR spectroscopy, which is selective to paramagnetic species. Nevertheless, these transformations can be readily completed given that these radicals are involved in H-abstraction reactions involving H donors (such as methanol) R• + R′H → RH + R′•

Figure 9. ″Glyoxal cycle″. In this scheme, RH stand for the generic (molecular or radical) donor of H atoms in reactions 30 and 31.

and disproportionation involving suitable radicals, such as hydroxymethyl28 and formyl30 radicals, e.g. R• + •CH2OH → RH + H2CO

(31a)

R• + HC•O → RH + CO

(31b)

There is also possible proton-coupled reduction at the oxide surface eCB−• + R• + H+ → RH

(32)

Reaction 25 is a specific example of reaction 30 that was directly observed in our EPR experiments at low temperature (see Section 2.5 and Figure 5S). Reactions 30 and 31 indicate that one-carbon molecules and their radicals can serve as reservoirs of H atoms for reduction of two-carbon molecules whose oxidation, in turn, replenishes the stock of HC•O and •CH2OH radicals in the reaction mixture. Two important examples of the disproportionation are the selftermination reactions28,30 2HC•O → CO + H2CO

(33)

2•CH2OH → H2CO + CH3OH

(34)

which convert one-carbon radicals to oxidizable one-carbon molecules, allowing them to serve as H atom donors in reaction

(30) 9457



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30. The photoreactions shown in Figures 9 and 6S constantly reintroduce formyl and hydroxymethyl radicals, formaldehyde, and methanol, and it is this process that keeps the reduction going (via reactions 30 and 31). These one- and two-carbon molecules also serve as sacrificial hole acceptors, restocking the formyl radicals for glyoxal formation via reaction 6. The scheme in Figure 9 has two possible “shortcuts” that do not involve redox or radical chemistry. One is reaction 12a, yielding glycolaldehyde which is subsequently reduced in reaction 17c. The second possible shortcut is surface-catalyzed esterification CH3OH + HCO2 H → HCO2 CH3 + H2O

Table 2. Calculated Isotropic (aiso) and Anisotropic (Bνν) hfcc Parameters for 13C and 1Hα Nuclei in Optimized Geometries for Hydroxy- Or Oxymethyl Radicals in the Gas Phase, Aqueous Solution, and on an Anatase (101) Surfacea Using the B3LYP/6-31+G(2df,p) Method [Bxx, Byy, Bzz], G

aiso, G

C [−23.9,−23.2,47.1] 1 Hα [−12.6,−0.1,12.6] 1 Hα [−12.7,0.1,12.5] 13 C [−23.4,−22.8,46.3] 1 Hα [−12.4,0.1,12.3] 1 Hα [−12.5,0.2,12.3] 13 C [−10.9,−9.8,20.7] 2 1Hα [−7.0,2.0,5.0] 13 C [−16.1,−15.6,31.7] 2 1Hα [−8.9,0.8,8.4] 13 C [−25.7,−24.8,50.5] 1 Hα [−13.4,0.1,13.3] 1 Hα [−13.0,−0.4,13.4] 13 C [−9.5,−8.9,18.5] 1 Hα [−4.2,−0.3,4.4] 1 Hα [−5.0,0.0,5.0]

51.5 −15.3 −12.2 48.1 −14.8 −13.1 10.9 −12.1 34.2 −8.9 65.6 −14.4 −17.0 17.1 −8.6 −8.6

radical •

CH2OH gas phase

•

CH2OH in H2Ob,c

•

CH2O− gas phase

•

CH2O− in H2Oc,d

(35)

Methylformate is one of the known products of CO 2 reduction.12 In ref 1 we showed that the photooxidation of methylformate on TiO2 yields H loss radicals

Ti4+O(H)CH2• structure 1a

while the main photoreduction reaction is analogous to reaction 3

Ti4+O−CH2• structure 2a

eCB−• + Ti4 + + HCO2 CH3 → Ti4 +O−CH3 + HC•O (37)

13

a

See Figure 10. bThe experimental estimates for proton hfcc’s on Hα are −17.7 and −18.5 G (ref 40). cThe solvent was simulated using the polarizable continuum model (ref 41). dThe experimental estimates for proton hfcc’s on Hα are −14.3 G (ref 39).

In addition to reaction 37, methylformate can also react on TiO2 by dissociative electron attachment typical of other esters31 eCB−• + Ti4 + + HCO2 CH3 → Ti4 +O−CHO + •CH3

(structures 1 and 2 in Figure 10) and also several structures of • CH2OH radicals strongly interacting with undercoordinated

(38)

generating a methyl radical. While reaction 38 is a minor side reaction, it does generate the methyl radical through a (exclusively) reduction pathway, whereas the acetaldehyde pathway shown in Figure 9 requires a combination of the reduction and oxidation and, therefore, cannot occur in electrolytic reduction of CO2 on TiO2.32 Together, these reactions constitute a cycle for CO2 fixation that we suggest to call the ″glyoxal cycle″ to emphasize the unique role of glyoxal in the reduction chain on metal oxides. 3.2. Radicals on Metal Oxides. In the scheme in Figure 9, we suggest that the main mechanism for conversion of radical intermediates to molecules is via reactions 30 and 31 rather than reaction 32. Indeed, the very fact that organic radicals can be observed on the oxide surface by EPR argues against the occurrence of facile reactions 9a or 32 despite the fact that such radicals typically have lower ionization potentials and higher electron affinities than their parent molecules. In ref 1, we observed that CO2−• (OC•OH) radicals generated by the photooxidation of formate (formic acid) on TiO2 exhibited hfcc parameters indicating weak interaction with the oxide surface. The hfcc parameters for the formyl radical generated in reactions 3 and 5 were also indicative of weak matrix perturbation. Using the hfcc constants on carbon-13 in radicals generated in the TiO2/MeOH system (Section 2.1), we can determine whether these radicals are free (expelled to the bulk or weakly interacting with the oxide surface) or bound (strongly interacting with the oxide surface). To this end, we used a first-principles DFT method (see section 2 in ref 1 for more computational detail) to calculate EPR properties of radicals either in the gas phase, in aqueous solution, or on an anatase (101) surface (Table 2). For bound radicals, we consider Ti4+O(H)C•H2 and Ti4+O−C•H2 centers

Figure 10. Optimized structures for bound hydroxy- (1) and oxymethyl (2) radicals on the anatase (101) surface. See Table 2 for calculated EPR parameters.

Ti4+ atoms on the surface (Table 2S). The latter species can be immediately rejected, as the DFT calculations yield hfcc’s in the 1 Hα that strongly disagree with experimental estimates. The two structures shown in Figure 10 are also unsatisfactory. Structure 2 yields an hfcc on 13C that is 48% smaller than in the radical observed in alkaline TiO2 solutions (Figure 1 and Section 2.1) and 60% smaller than the radical in weakly acidic TiO2 solutions. Likewise, structure 1 gives an estimate for the hfcc on the 13C nucleus that is 55% greater than the experimental value (in acidic solutions). As seen from Table 2, (free) hydrated hydroxymethyl and oxymethyl radicals provide a much better match for the experimental estimates for aiso(13C) than either of these bound structures, and we believe that these DFT calculations and our results demonstrate that the radicals generated in the photooxidation of methanol are released f rom the surface even if the parent molecule is bound to the surface through 9458



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molecules and two-electron reactions. The photoredox, radical, and molecular reactions shown in Figures 9 and 6S comprise the complex “glyoxal cycle.” While the critical steps of this cycle have been demonstrated in this study, some of these steps remain tentative. In particular, we did not demonstrate how the radicals are reduced to molecules, though the radical reactions that we suggest as a means of such reduction are well-known. More work to prove or disprove this reaction mechanism using other techniques is needed. In addition to methanogenesis, the glyoxal cycle accounts for several known byproducts, such as methanol, formate, formaldehyde, acetaldehyde, and methylformate, for which (to our best knowledge) no specific reaction scheme has been suggested heretofore. We predict that this cycle also produces glyoxal and glycolaldehyde (and, possibly, glycoxylate and glycolate) as minor products (given their high reactivity). These products have not yet been observed, and the detection of these products could corroborate the scheme shown in Figure 9. While this glyoxal cycle is primitive as compared to the Calvin cycle through which biological nature fixes atmospheric carbon as glyceraldehyde-3-phosphate37 (with relatively low conversion efficiency, as the reaction intermediates are both generated and consumed), this glyoxal cycle yields complex organic molecules as byproducts of CO2 reduction, and it requires only a hydrated metal oxide surface (abundant on many planets) and UVA light as opposed to the sophisticated enzymatic machinery of a living cell. Apart from the obvious significance for production of fuels and carbon sequestration, the glyoxal cycle is one of the simplest realizations for abiogenic fixation of CO2 from the atmospheres of terrestrial planets that can operate over a wide range of conditions. Photoactive metal oxides (e.g., particulate iron(III) oxides that constitute most of the martian regolith) are ubiquitous on both Earth and Mars. Ages ago, the atmospheres of these planets were reducing and transparent to UVA radiation, and both planets had water.38 We argue that young Earth and Mars (the latter, both early and modern) could have been a vast ″solar panel factory″ abiotically fixing atmospheric CO2 and converting it to CH4 and complex organic molecules. These molecules look remarkably similar to the common microbial metabolites. Some of these molecules (e.g., glycolaldehyde) can undergo further aldol condensation24 producing 3- and 4atomic carbohydrates: the very building blocks of organic life. In other words, the glyoxal cycle could have been nature’s first draft of the photosynthetic cycle that after many millions of years of evolutionary tinkering became the Calvin cycle that gives us our daily bread.

its oxygen (reaction 10). Since the hydroxymethyl radical is released from the surface, further redox reactions involving this radical are suppressed until it is readsorbed on the surface. We suggest that this release is a general occurrence of photoreactions on metal oxides, as was previously suggested by Henderson.33 It is less clear, however, whether this release always occurs during photoreactions, as the bound radical may simply rapidly react further by oxidation (such as reaction 9a) or reduction (reaction 32) and in doing so may become invisible to EPR spectroscopy. Interestingly, the Ti4+−O−C•H2 radical shown in Figure 10 (structure 2) has low spin density on 13C chiefly because of the partial electron (and spin) transfer to the titanium ion; i.e., it has Ti3+···OCH2 character. Thus, it is possible that photooxidation can produce oxygen-bound radicals (reaction 8b) or free radicals (reaction 10), but the bound radical is unstable, decaying by reaction 9a, so that only a released radical is observed by matrix isolation EPR. Given the propensity of the hydroxylated radicals to oxidize back to aldehydes in reactions analogous to reaction 9a, we consider it unlikely that (nonconcerted) two-electron reduction (involving reaction 32 as the second step) plays a significant role in the catalytic cycle.

4. CONCLUDING REMARKS In this study, we have asked whether the reduction of CO2 to CH4 on the surface of semiconducting, photoactive metal oxides involves one-carbon intermediates and concerted two-electron reactions2,5,6,34,35 (scheme 4) or some other products that can be reduced by stepwise one-electron reactions.36 There is more than one consideration favoring the second alternative, as examination of the redox reactions of formate,1 formaldehyde, and methanol on TiO2 (Sections 2.1 and 2.2) showed no oneelectron reduction of these adsorbates. This prompted us to search for complementary mechanisms, and we realized that the ease of one-electron reduction considerably increases for twocarbon molecules as compared to the one-carbon molecules in reaction scheme 4. We recognized that glyoxal, which is the product of recombination of two formyl radicals (reaction 6), can serve as a strong electron acceptor, and we demonstrated that its reduction on TiO2 is more facile than its oxidation (Section 2.3). The products of recombination of CO2−• with CO2−•, HC•O, and •CH2OH do not yield molecules that can carry the reduction chain more than one step further without recycling the products (Section 2.5 and Figure 6S). The reduction of glycolaldehyde, which is the product of (i) glyoxal reduction, (ii) aldol condensation of formaldehyde, and (iii) recombination of HC•O and •CH2OH, is also facile, as the corresponding radical anion is unstable to dehydration and yields the vinoxyl radical. The latter can be reduced to acetaldehyde, which is photooxidized to the methyl radical, the precursor of the methane. Other pathways to the methyl radical could involve dissociative electron attachment to the methylformate that is generated through esterification of formic acid by methanol. We suggest that one-carbon molecules play only an auxiliary role in methanogenesis by serving as sacrificial hole scavengers and replenishing the pool of formyl and hydroxymethyl radicals whose reactions (i) lead to the formation of reducible twocarbon molecules and (ii) maintain the reduction chain through H abstraction and disproportionation reactions with the twocarbon radicals. Thus, we suggest that methanogenesis occurs through twocarbon molecules and involves mainly one-electron reactions in contradistinction to reaction scheme 4 that involves one-carbon

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ASSOCIATED CONTENT

S Supporting Information *

A file containing the list of reactions, Tables 1S and 2S, and Figures 1S to 6S with captions, including the experimental and simulated EPR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (630) 252-9516. E-mail [email protected]. Notes

The authors declare no competing financial interest. 9459



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ACKNOWLEDGMENTS This work is supported by Grant No. NNH08A65I from the Mars Fundamental Research Program of NASA (to I.A.S. and T.W.M.) and the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC0206CH11357, including the use of the Center for Nanoscale Materials. I.A.S. thanks N. Dimitrijevic, T. Rajh, D. Catling, and D. Tiede for many useful discussions.

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Photoredox Reactions and the Catalytic Cycle for Carbon Dioxide

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