Heteroatom-Transfer Coupled Photoreduction and Carbon Dioxide

Article pubs.acs.org/JPCC

Heteroatom-Transfer Coupled Photoreduction and Carbon Dioxide Fixation on Metal Oxides Ilya A. Shkrob,*,† Nada M. Dimitrijevic,*,†,‡ Timothy W. Marin,†,§ Haiying He,∥ and Peter Zapol†,∥ †

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 604393, United States ‡ Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 604393, United States § Chemistry Department, Benedictine University, 5700 College Road, Lisle, Illinois 605324, United States ∥ Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: Photoactive metal oxides, such as hydrated TiO2, are known to reduce carbon dioxide to methane, but the mechanism for this photoreaction is insufficiently understood. In particular, it is not known whether the reduction of crucial reaction intermediates, including the formate anion, involves one- or two-electron reactions. In this study, we demonstrate that formic acid and its derivatives can be reduced to the formyl radical via a concerted reaction in which the electron transfer is coupled to oxygen transfer to a Ti3+ center on the oxide surface. Several other examples of such heteroatomtransfer reactions are demonstrated, suggesting a general pattern. The implications of these reactions for photocatalytic methanogenesis, perchlorate diagenesis, and planetary chemistry on Mars are discussed.

1. INTRODUCTION The photochemistry on wide-gap metal oxide semiconductors, such as titanium(IV) and iron(III), attracts a lot of attention, as it could offer a path to affordable solar power, CO 2 sequestration, and fuel production.1 It is known1−9 that photoirradiated metal oxides can reduce (albeit, with a small quantum yield) carbon dioxide to methane as well as to methanol, acetone, methylformate, acetaldehyde, and formaldehyde.9 Despite many years of research, the mechanism for this multistage photoreaction remains conjectural, with many reaction schemes suggested,10−18 but few direct observations supporting such schemes are available. The chemical pathway to methane is thought to involve various one-carbon molecular intermediates, such as formate, formaldehyde, and methanol, for which one-electron transfer does not occur in aqueous solution even for the hydrated electron, which has >1 eV higher reducing power than TiO2 conduction band electrons.18 Thus, the occurrence of methanogenesis on oxides indicates either (i) the occurrence of concerted charge-transfer reactions involving two or more electrons from the oxide1,18,19 or (ii) a different mechanism for one-electron reduction from that in the bulk solvent, which includes trapped electron centers on the surface of metal oxides (such as Ti3+ centers on titania)18,20 and adsorbed surface species. While the former possibility has been extensively discussed in the literature, the latter mechanism had not yet been adequately addressed, and we seek to close the knowledge gap. © 2012 American Chemical Society

From a mechanistic standpoint, sequential one-electron reactions involving Ti3+ surface centers occur with reactant and intermediate molecules as electron acceptors (A) that are adsorbed on or in the vicinity of these centers. Ti 3 + + (A)ad → Ti4 + + A −•

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where the bullet indicates unpaired spin of the resulting radical. It is worth noting that the reactions presented throughout the text are surface reactions and that Ti3+ and Ti4+ in these reactions are ions present on the surface of anatase nanoparticles. In this study, which continues previous studies from our laboratory,17−19 we argue that in addition to reaction 1 there is another class of electron-transfer reactions on oxide surfaces. These reactions occur only for certain classes of molecular adsorbates, denoted as the RB adsorbates, where R is an organic group and B is a base that involves a heteroatom (such as O and N), for which the Ti3+ center becomes the acceptor, forming a bond to the heteroatom Ti 3 + + (RB)ad → Ti4 +B− + R•

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Received: January 4, 2012 Revised: February 14, 2012 Published: March 6, 2012 9461

dx.doi.org/10.1021/jp300123z | J. Phys. Chem. C 2012, 116, 9461−9471

The Journal of Physical Chemistry C

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The resulting Ti4+B− center can be protonated on a hydrated surface, either subsequently or in concert with reaction 2. Ti4 +B− + H+ → Ti4 + + HB

To emphasize the importance of Ti3+ centers and their role in heteroatom-type photocatalytic reactions on the surface of metal oxides, we studied the photoreduction of chlorate, perchlorate, formate, methylformate, and N,N′-dialkylformamide. The proposed mechanism of reaction 2 has broad implications for various processes from photocatalytic energy conversion to methanogenesis and perchlorate diagenesis in planetary chemistry on Mars.

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While, to the best of our knowledge, reactions of this type have not been examined on metal oxide surfaces, there is the wellknown precedent for reaction 2 involving Ti3+ ions in the aqueous bulk: the reduction of perchlorate.21 This reaction is thought to involve O− transfer from the ClO4− to the titanous ion with the formation of the titanyl ([TiO]2+) as its first step of the reaction Ti 3 +aq + ClO4−aq → Ti4 +O2 −aq + ClO3•

2. EXPERIMENTAL AND COMPUTATIONAL METHODS All of the reagents were obtained from Aldrich except for the (CD3)2NCHO that was obtained from C/D/N Isotopes, Inc. The chemicals were used as supplied. We remind the reader that the 12C nucleus (which has no spin, I = 0) does not couple to the unpaired electron, whereas the 13C nucleus (I = 1/2) strongly couples to the electron in carbon-centered radicals. Furthermore, the deuteron (I = 1) has a hyperfine coupling constant (hfcc) which is only 15.3% of the equivalent proton (I = 1/2), while nitrogen-15 (I = 1/2) has a hfcc which is 140% greater than nitrogen-14 (I = 1). Thus, not only EPR patterns for the natural-abundance isotopomers but also the characteristic changes in the EPR spectra for isotope-substituted radicals can be used to establish their structure. The radicals were observed using a 9.44 GHz Bruker ESP300E spectrometer, with the sample placed in a flow He cryostat (Oxford Instruments CF935). The magnetic field and the hfcc’s are given in units of Gauss (1 G = 10−4 T). If not stated otherwise, the first-derivative EPR spectra were obtained at 50 K using 5 G modulation at 100 kHz (for deuterated radicals, the modulation was decreased to 2 G). Typically, deaerated aqueous solutions of the molecule of interest were mixed with a pH 1.9 solution of anatase nanoparticles (∼5 nm diameter) synthesized as described in ref 19. These solutions were flash frozen and then irradiated at 77 K using 40 mJ, 6 ns pulses of 355 nm light from a Nd:YAG laser. We also carried out some experiments in which the solutions were illuminated using the water-filtered output of a 300 W Xe arc lamp (>200 nm). EPR spectroscopy was used not only to directly detect radicals upon illumination of TiO2 containing various adsorbates but also to distinguish “conduction-band” from surface-trapped electrons. It has been demonstrated previously18,20 that irradiation of TiO2 at 77 K results in localization of photogenerated electrons both in the interior, (Ti3+)latt, and at the surface of nanoparticles, (Ti3+)surf. The (Ti3+)latt electrons have energies close to the conduction band (shallow trapped electrons), while (Ti3+)surf ones can be in both energetically deep and shallow traps. The two species can be easily distinguished by EPR, as they exhibit characteristic g-tensors and line-shapes of their resonance lines (Figure 1S, Supporting Information). The calculations of the hyperfine coupling constants (hfcc's) and the structures for free radicals were carried out using a density functional theory (DFT) method with the B3LYP exchange-correlation functional28 as implemented in Gaussian 03.29 LANL2DZ for Ti and 6-31+G(2df,p) basis sets for all other atoms were employed. A Ti7O27H26 cluster model representing the anatase (101) surface was used in calculations of adsorbed species. Hydrogens were added to terminate dangling bonds on oxygen atoms of the cluster, and the resulting hydroxyls were kept fixed throughout partial geometry optimizations. An aqueous environment, when present, was described with the Polarizable Continuum Model.30 Other

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which is fully analogous to the postulated surface reaction 2. There have been reports in the literature indicating a dramatic slowing down of charge recombination and concurrent increase in yield of photooxidation when perchlorate was present in solutions of TiO2 nanoparticles.22,23 The contribution of reaction 2 in the overall photocatalytic transformation of carbon dioxide to methane on TiO2 could be significant because: (i) proposed reaction intermediates, such as formic acid, contain heteroatom oxygen which coordinates preferentially to Ti4+ ions upon adsorption, and (ii) trapped electrons on the surface of TiO2 exhibit a range of energetics (from 0.5 eV below the conduction band up to the edge of the conduction band). The energetics for heteroatom transfer reaction 2 can be more favorable than the charge-transfer reaction 1. We have chosen the photoreduction of formic acid on TiO2 to study the postulated reactions 2 and 3. The photochemical reduction of carbon dioxide is thought to yield formate (or, at low pH, formic acid) via sequential, protoncoupled, charge-transfer reactions 5 and 6 Ti 3 + + (CO2 )ad → Ti4 + + (CO2−•)ad

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Ti 3 + + (CO2−•)ad + H+ → Ti4 + + (HCO2−)ad

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which involve the radical anion of CO2 as a reaction intermediate.24−27 No such radical, however, was observed in the photolysis of CO2 on TiO2 by matrix isolation electron paramagnetic resonance (EPR) spectroscopy,18 suggesting that this radical anion is unstable on the TiO2 surface (presumably due to the occurrence of reaction 6). Since the same species should occur in photooxidation of formate and/or oxalate by the oxygen hole centers on the oxide Ti4 +−O• + (HCO2−)ad → Ti4 +OH + CO2−•

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Ti4 +−O• + (−O2 CCO2−)ad → Ti4 +O− + CO2−• + CO2

(8) −•

we used these two reactions to generate CO2 near the TiO2 surface. Our EPR results suggest that the CO2−• radical generated in reactions 7 and 8 is stable on the TiO2 surface. Had the CO2−• radical generated via reaction 5 been adsorbed/ stabilized on the surface in the same way as this radical is generated via reactions 7 and 8, it would also be observed by EPR spectroscopy (although with much lower yield). Thus, our study emphasizes the importance of the mode of surface binding for two-electron reduction of CO2 to formate and identifies the formyl radical as the next step in the reduction process. 9462



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details of the cluster model are the same as in our previous work.18,31 In the following, aiso denotes the isotropic hfcc, and Bνν denotes the three principal values of B, that is, the anisotropic part of the hfc tensor A. Simulations of the EPR spectra involved angular averaging of the fixed-tensor radicals using first-order perturbation theory. The same approach was used for fitting the experimental EPR spectra, with a genetic algorithm used for optimization of the model parameters. For convenience, the principal values of the g-tensor are reported as δgνν = (gνv − 2) × 104, where ν = x,y,z are the principal axes (which in all cases coincided with the principal axes for the leading spin-bearing atoms in the radicals). For some species, we needed to postulate distributions of the components of the g-tensors due to the matrix strain exerted on their antibonding orbitals. To this end, we assumed that (gνν − ge) = ( − ge)/(1 + Wνλ), where are the centroids of the distributions for the corresponding components ( stands for ensemble average); Wν is the width of the distribution; and λ is a random Gaussian variable with the mean = 0 and = 1. In this case, we optimized both the centroid positions and the widths. To calculate the reaction pathways of conversion of CO2− into HCO2− on the anatase (101) surface, we have used density functional theory with a Hubbard U correction for the on-site Coulomb repulsion (DFT + U).32 Periodic slab models were constructed with a 2 × 1 supercell for the anatase (101) surface along the [010] and [101̅] directions consisting of six TiO2 trilayers with four Ti atoms per layer. Atoms in the bottom trilayer were fixed to their bulk positions in the oxide. A vacuum layer of ∼1.1 nm was placed between slabs along the z direction. All the calculations were done using plane-wave basis sets and PAW-PBE33 potentials implemented in the VASP program.34 A Monkhorst−Pack grid of 2 × 2 × 1 was used to sample the first Brillouin zone. The total energy is converged to 10 μeV, and the force on each atom is converged to 0.3 eV/nm. A U value of 4.5 eV was employed for the Ti 3d states. This value of U gives the band gap of 3.14 eV for anatase, which is close to the experimental value of 3.2 eV.31 Transition states along the reaction pathways were located with the Climbing Image Nudged Elastic Band algorithm.35

Figure 1. (a) First-derivative EPR spectra (9.4 GHz, 100 kHz modulation) of an aqueous solution of anatase nanoparticles containing 0, 0.25, and 4 M sodium chlorate (see the legend in the plot) and irradiated by 355 nm laser light at 77 K. The EPR spectra were observed at 50 K using a microwave power of 2 mW. The EPR signals are mainly from the ClO2• radical. (b) Comparison of EPR spectra from photoirradiated sodium chlorate and nonirradiated sodium chlorite (which has a small equilibrium concentration of ClO2•). See Figure 2S (Supporting Information) for a simulation of this EPR spectrum. The arrows indicate the low-field component of lattice-trapped electrons in the nanoparticle core. Open circles indicate spectral regions where the signal from the oxygen hole centers overlaps with the resonance lines of the ClO2• radical.

simulated EPR spectra of this radical (at natural abundances of the spin = 3/2 35Cl and 37Cl nuclei) obtained using the magnetic parameters reported in the literature (Table 1S, Supporting Information). To generate this radical from ClO3−, it is necessary to lose one of the oxygens, and the only plausible reaction is the transfer of the oxide anion to a Ti3+ center at the surface such as in reaction 10, which is analogous to the postulated reaction 4 (occurring in the bulk), viz.

3. RESULTS 3.1. Photoreduction of Chlorate and Perchlorate. Figure 1a exhibits EPR spectra of frozen solutions of aqueous anatase nanoparticles irradiated by 355 nm light with and without the specified concentrations of sodium chlorate present in the reaction mixture. It is seen that, in addition to the resonance lines of oxygen hole centers (g > 2) and the sharp resonance lines from lattice-trapped electrons (cf. Figure 1S, Supporting Information), there is also another radical that occurs only in the chlorate solutions. This radical is not observed in photoirradiated aqueous solutions of sodium chlorate (without TiO2) or in the colloidal solutions of TiO2 containing chlorate before photoirradiation. In Figure 1b, we compared the EPR spectrum of this radical on TiO2 and the EPR spectrum of ClO2• observed in frozen acidified 5 M sodium chlorite (not irradiated), where this radical is formed via a dark reaction 5ClO2− + 4HCl → 5Cl− + 4ClO2• + 2H2O

Ti 3 + + (ClO3−)ad → Ti4 +O2 − + ClO2•

(10)

Spectroscopically, the expected feature of reaction 10 is the disappearance of the broad EPR signal of surface Ti3+ centers (Figure 1S, Supporting Information, and the lower trace in Figure 1a) reacting with the adsorbed chlorate. A closer inspection of Figure 1a does indicate that this EPR signal is absent, thereby confirming the suggested mechanism of reaction 10. These EPR spectra also suggest that the electrons trapped in the crystalline interior of the TiO2 nanoparticles do not react with the adsorbate; rather, reaction 10 occurs only with surface-trapped electrons. The same EPR signature of the ClO2• radical was observed when we photoilluminated TiO2/perchlorate solutions (Figure

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This comparison suggests that the species observed in Figure 1a is the ClO2• radical. This identification is confirmed by 9463



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absorbance experiments with room-temperature solutions; these authors did not suggest a specific reaction mechanism. 3.2. Photoreactions of Formate and Formic Acid. Photoirradiation of sodium formate in the presence of TiO2 nanoparticles yields a singlet resonance line as shown in Figure 3a. When the pH of the anatase solution is adjusted to less than

2). To observe these features, high concentrations of sodium or magnesium perchlorate were required (see the legend in Figure

Figure 2. Like Figure 1, for solutions containing the specified concentrations of sodium and magnesium perchlorate.

2), suggesting that surface absorption of this anion is poor. It is seen from the plot that the EPR signal from the oxygen hole centers and lattice-trapped electrons persists in all of these spectra, so the reaction cannot involve such centers; however, the broad resonance line of the surface Ti3+ centers disappears, and the resonance lines of the ClO2• radical are superimposed on the lines of the remaining spin centers in TiO2. The perchlorate is one of the most difficult anions to reduce (the gas-phase adiabatic electron detachment energy, ADE, for this anion is 5.25 eV), but the resulting ClO3• radical has a high ADE (4.25 eV) as opposed to ClO2• (2.15 eV).36 We suggest that the photogeneration of the ClO2• radical in the TiO2/ perchlorate system occurs via oxide-transfer reaction 4 coupled to the electron-transfer reaction 11 to the released ClO3• radical, which is a very strong electron acceptor Ti 3 + + (ClO3•)ad → Ti4 + + (ClO3−)ad

Figure 3. (a) Photoreactions of sodium formate and formic acid on photoirradiated anatase nanoparticles. Encircled is the Mz(1H) = +1/2 line of the formyl radical. (b) Appearance of the central resonance line for formic acid and sodium formate solutions in light (solid lines) and heavy (dotted lines) water solutions.

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Reactions 4 and 11 are followed by reaction 10, resulting in the overall three-electron reaction 12 3Ti 3 + + (ClO4−)ad → Ti4 + + 2Ti4 +O2 − + ClO2•

1.3−1.5 (pKa of formic acid is 3.75), the profile of this resonance line changes, and a weak side line (which is circled in Figure 3a) appears. As H/D isotope substitution in the formate has no effect on the central singlet resonance line, this suggests that the EPR line corresponds to the CO2−• radical generated via reaction 7. At pH > 2, the appearance of this singlet line changes very little when the light water in the solution is replaced with heavy water (Figure 3b, lower traces). However, at pH < 1, the line changes dramatically upon solvent replacement (Figure 3b), and the intermediate stages of this transformation can be observed when the content of D2O is gradually increased (Figure 5S(a), Supporting Information). The comparison of EPR spectra obtained at low and high pH suggests that the resonance lines shown in Figure 3b originate from different radical progenitors, as the shoulders in this line are observed only (i) at low pH and (ii) in light water. These observations indicate that the unpaired electron in the radical observed at low pH is coupled to a proton that ultimately originates from the solvent. This is consistent with protonation of the CO2−• radical at low pH, as this radical has a pKa value of ∼1.6.38 The simulated resonance lines for the CO2−•, OC•OH,

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Similar photoreactions for chlorate and perchlorate also occur on wet iron(III) oxide microparticles irradiated by 355 nm light: hematite, α-Fe2O3, and goethite, α-FeOOH (Figure 3S, Supporting Information). While the EPR signals of the ClO2• radical on these surfaces is weaker than on the anatase nanoparticles, the spectral signatures of this radical are unmistakable. Our observations rationalize the previously observed effect22,23,37 of perchlorate salts on the efficiency of electron−hole recombination on TiO2, where Na+ and Mg2+ ions in the double layer at the interface affected the energetics of charge recombination through a shift of the flat band potential.22 Our results suggest that the effect actually originates through the reactions of surface Ti3+ centers with perchlorate anions adsorbed on the TiO2 surface: this anion (which was previously presumed to be unreactive) is reduced on photoilluminated metal oxide surfaces. Slow perchlorate reduction by Ti3+ centers on TiO2 has been postulated by Safrany et al.37 to account for their pulse radiolysis−optical 9464



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and OC•OD radicals are presented in Figures 6S and 7S (Supporting Information), while Table 1 contains the optimized parameters for simulations. Table 1. Optimized A and g Tensor Parameters for the (Deprotonated and Protonated, trans-Conformer) Carbon Dioxide Radical Anion (pH > 2 and pH < 1, Respectively) Observed in Photooxidation of the Formate Anion (Formic Acid) on Aqueous Anatase Nanoparticlesa radical

structure

−•b

[δgxx, δgyy, δgzz]

CO2 CO2−•c CO2−•e OC•OHb

[−21,44.8, 19.4] [24.5, 24.5,22]d [−20,32,22] [−21.6, 52.5,16.4]

OC•OHc OC•OHg CO2−•h CO2−•i CO2−•j CO2−•k CO2−• + H+k CO2−• + OH−k CO2−• + H2Ok CO2−• + H+k OC•OHh

1 2

[24,24,16]d 2 − [28,−22,16] [−35,6,13] [−26,32,16] [−28,−26,−3]

3

[−12,1,−12]

4

[−17,11,−4]

5

[4,−22,0] −

OC•OHi OC•OHk

[−24,17,34] 6

[26,−14,8]

[Bxx, Byy, Bzz], G

aiso, G

C [−16.5,−13.1,29.5] C [−16.1,−16.1,32.1] 13 C [−17.7,−11.7,29.3] 13 C [−15.1,−10.7, 25.9] 1 H [∼0, 3.8, 8.7]f 13 C [−14.3,−14.3, 28.7] 1 H 13 C [−11.6,−9.9, 21.5] 13 C [−11.5,−10,21.5] 13 C [−13.6,−11.8,25.4] 13 C [−16.6,−13.4,30.1] 13 C [−15.8,−12.9,28.6] 1 H [−1.5,−1.3,1.9]f 13 C [−15,−12.4,27.3] 1 H [−2.4,−2.1,3.4]f 13 C [−16.2,−13.4,29.6] 1 H [0.6,0.9,12]f 13 C [−17,−13.8,30.8] 1 H [4.1,4.7,13.8]f 13 C [−15,−11,26] 1 H [−8.9,−5.4,8.0]f 13 C [−15.4,−12.3,27.7] 1 H [−8.4,−4.9,8.8]f 13 C [−20,−16.3,36.3] 1 H [−1.7,1.2,14.1]f

162.1 169 164.7 190.5 ∼4.5 192.4 9.0 114.8 104.9 127.8 191.4 176.1 −0.3 159.7 −0.4 174.4 4.5 190.6 7.5 190.4 −2.1 187.3 −1.5 233.6 4.5

13 13

Figure 4. EPR spectra of photoirradiated isotopomers of formic acid (D12CO2D and H13CO2H) in heavy and light water solutions of anatase nanoparticles at pH 1.3. The arrow points to the low-field line of the deuteroformyl radical. For comparison, the resonance lines (indicated with open circles) from 13CO2−• in the sodium formate-13C solution from Figure 5 are reproduced. It is seen that the resonance lines of the analogous radical in acidic solution (indicated with filled circles) correspond to a species with a greater hfcc on the carbonyl carbon-13 (see Figure 6).

a

The structures on the anatase (101) surface are indicated as shown in Figure 7. bIrradiated by pulsed laser 355 nm light and observed at 50 K. cIrradiated by continuous-wave 300 nm light and observed at 4 K. d Assumed axial g and A tensors. eFrom ref 39. fThe A tensor. gFrom ref 38. hB3LYP/6-31+G(d,p) calculation for a gas-phase radical. i B3LYP/6-31+G(2df,p) calculations in the gas phase. jB3LYP/631+G(2df,p) calculations in water (polarizable continuum model).30 k On TiO2 clusters (see Sections 2 and 3.3).

To further ascertain our radical attributions, we used carbon13 labeled formate and formic acid, as the isotropic hfcc on carbon-13 in 13CO2−• is large (Tables 1 and 1S), so the singlet line for 12CO2−• transforms into a widely spaced doublet of lines for 13CO2−•. This spectral transformation is apparent in Figures 4 and 5 (for low- and high-pH conditions, respectively). Figure 6 shows the best fit of these two EPR spectra, and Table 1 gives the corresponding optimized parameters. The high-pH species has a hfcc tensor that is very similar to the tensor obtained for the 13CO2−• radical in photoirradiated crystalline HCO2Na,39 which is also given for comparison in Table 1. In contrast, the low-pH species had a much greater isotropic hfcc than the typical 13CO2−• radicals in other matrices. This greater hfcc constant is also predicted by our DFT calculations for the lowest-energy trans-form (Figure 8S, Supporting Information) of the gas-phase and aqueous O13COH• radicals (Table 1). In

Figure 5. Comparison of the EPR spectra obtained from photoirradiated aqueous solutions of sodium formate and the corresponding 13 C isotopomer (see the legend). The arrows indicate the resonance lines from the lattice electron. Open circles indicate the resonance lines from the 12CO2−• and 13CO2−• radicals, respectively. The upper trace is obtained at a high microwave power of 200 mW to facilitate saturation of the spin transitions in the lattice Ti3+ center.

fact, the isotropic hfcc on 13C does not depend strongly on the OCOH dihedral angle, being systematically greater in this σradical as compared to the CO2−• radical, which is a π-radical. However, the proton coupling strongly depends on the degree of superconjugation in the bent radical, and our calculations indicate that only radicals with this angle between 120° and 180° are consistent with our EPR observations. Thus, we tentatively assign radicals observed at pH < 1 and pH > 2 as 9465



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The resulting dihydroxymethyl radical (pKa ∼ 9.5)40 can either abstract hydrogen from formic acid to yield the OC•OH radical and hydrated formaldehyde, H2C(OH)2 HC•(OH)2 + HCO2 H → H2C(OH)2 + OC•OH

(14)

or eliminate water to yield the formyl radical HC•(OH)2 → H2O + HC•O

In 12 M LiCl/H2O glass, reaction 15 did not occur: the characteristic 18 G doublet of the dihydroxymethyl radical (Table 1S, Supporting Information) generated in reaction 13 persisted to 170 K; at higher temperature, the radical is formed via reaction 1. There is no evidence for the presence of this HC•(OH)2 radical in our EPR spectra. Either low-temperature reaction 15 is catalyzed by the TiO2 surface or the formyl radical observed on TiO2 is not produced in a reaction analogous to reaction 13. Note that the fully hydrated electron (which is a stronger reducing agent than surface Ti3+ centers or conduction-band electrons) cannot reduce the formate or associated formic acid (although it can react with the hydronium ion in the dissociated acid).41 This suggests that the formyl radical is not formed in the surface analogue of reaction 13, but rather in reaction 2, that is

Figure 6. Simulation of the resonance lines of the 13CO2−• and transO13C•OH radicals obtained for formate-13C on aqueous anatase nanoparticles at pH > 2 and pH < 1, respectively. Solid lines are experimental EPR spectra (with the central components not shown), and dotted lines are simulated EPR spectra. The fit parameters are given in Table 1. The vertical lines indicate the resonances corresponding to the magnetic field aligned with the x, y, and z (principal) axes of the g-tensor.

Ti 3 + + (HCO2 H)ad → Ti4 +OH− + HC•O

free trans-OC•OH and CO2−•, respectively (see Section 3.3 for more discussion). Figure 9S (Supporting Information) exhibits the EPR spectra (at low and high pH) for formate-13C obtained using the prolonged photoirradiation with a 300 nm lamp and observed at 4 K; under these irradiation conditions, the EPR signals from the carbon dioxide radicals prevail over other defects, but there is much line broadening. In Figure 10S (Supporting Information) we simulated the EPR spectra in Figure 9S (Supporting Information) assuming axially symmetrical g- and A tensors, and the corresponding hfcc parameters are given in Table 1. The hfcc parameters are fairly close to the parameters for aqueous solutions irradiated at 355 nm and obtained at 50 K. Since both of the resonance lines are taken into account, the error in the isotropic hfcc for 13C is lower for the 300 nm trace. As mentioned already, the appearance of a new weak line in the EPR spectrum accompanies the presence of trans-OC•OH (pH < 1) but not CO2−• (pH > 2) radicals (Figure 3). By double integration, this line accounts for only 1% of the singlet line, indicating relatively low yield of the radical progenitor. The position of this line exactly corresponds to the position of the low-field line of the formyl radical, HC•O (Table 1S and Figure 5S(b), Supporting Information). The replacement of H by D in the formate results in disappearance of the HC•O doublet and appearance of another resonance line that corresponds to the exact position for the low-field component of the triplet of lines from DC•O. This suggests that these ″side″ components are from the free formyl radical. In our photosystem, the formyl radical can only be the product of reduction. One-electron reduction of HCO2H in γ-irradiated frozen neat formic acid and its solutions in 12 M LiCl/H2O has been studied by Sevilla and co-workers.40 This reaction is thought to occur through (concerted) dry (prehydrated) electron attachment and proton transfer HCO2 H + e−• + H+ → HC•(OH)2

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(16)

The formation of the Ti−O bond in the surface-bound hydroxide offsets the high energy cost of the one-electron reduction. Reaction 16 could be concerted with the protonation of the bound hydroxide (as this reaction occurs only at low pH); this protonation may provide the additional driving force for reduction. 3.3. Computational Study of CO2−• and OC•OH Species. Calculated hfcc’s for several structures in which the CO2−• radical was bound to the Ti4+ ion(s) on the anatase (101) surface through one or both of its oxygens and for one OC•OH structure are given in Table 1. These calculations were performed using the optimized cluster geometries illustrated in Figure 7. Structures 2 and 5 are representative of acidic conditions; structures 1 and 4 are representative of neutral conditions; and structure 3 is representative of alkaline conditions. According to our DFT calculations, surface binding of CO2−• to two Ti4+ ions through the oxygens (Figure 7, structure 1) is essential for facilitating reaction 6, as this binding primes the radical anion to become a proton acceptor at the carbon site (see below). It is also known from previous studies that the formate ions (and other carboxylate anions) are bound to the TiO2 surfaces in this fashion.42 Our calculations indicate that the bidentate Ti4+−OC•O−− 4+ Ti radical (structure 1 in Figure 7) has a significantly larger isotropic hfcc on the 13C nucleus (∼191 G) than the 162 G value measured for pH > 2 species (Section 3.2 and Table 1), as this bidentate radical has more σ-character to its singly occupied orbital than the free CO2−• radical. On the other hand, this calculated hfcc is close to the one observed for the pH < 1 species that we tentatively identified with the transOC•OH radical in Section 3.2. The main arguments for this identification in Section 3.2 were not only the greater isotropic hfcc for 13C but also the substantial proton coupling that disappeared when the light water was replaced by D2O. This proton coupling immediately excludes bidentate structure 1 in Figure 7 as the progenitor of pH < 1 species.

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hyperfine coupling on the proton involved in a H-bond with the CO2−• oxygen. Our DFT calculation indicates that the 1H and 13C hfcc’s for structure 2 are too low to account for the observed features of the pH < 1 species, but structure 5 has hfcc’s on 13C and 1H that are fairly similar to the pH < 1 species (Table 1). Regarding the surface-bound OC•OH as the possible species observed at pH < 1, we have included the Ti4+−OC•OH radical (structure 6 in Figure 7) results in Table 1. This structure exhibits too high of a hfcc on the 13C nucleus as compared to experiment. We conclude that the pH > 2 species cannot be the mono- or bidentate surface-bound CO2−• radical (that is, this species appears to be a free CO2−• radical in solution), while the pH < 1 species can be either a free trans-OC•OH radical or a monodentate Ti4+−OC•O−···+HO(Ti4+)2 center shown in Figure 7, structure 5. One reason that the structure 1 is not observed in EPR experiments could be its rapid further reduction by photogenerated electrons in the oxide. The mechanism for further reduction of bidentate CO2−• by one electron and one proton was examined using periodic DFT calculations. Results for the reaction barriers are graphically summarized in Figure 8 for

Figure 7. Optimized geometries (fragments) of bidentate (1) or monodentate (2−6) CO2−• and trans-OC•OH radicals on TiO2 clusters representing an anatase (101) surface with structures numbered as in Table 1. Atom colors: O (red), C (blue), H (gray, small), Ti (gray, large). Hydrogen bonds are indicated with a dotted line.

Considering other possible modes of surface binding, we examined monodentate Ti4+−OC•O− radicals, as the strongly coupled proton in the tentative “OC•OH radical” can potentially originate from physisorbed water (structure 4), a proton on a bridging oxygen (structures 2 and 5), or a hydroxide anion (structure 3) that is H-bound to the dangling oxygen (Figure 7). As seen from Table 1, the best candidate structure for the pH >2 species based on the observed hfcc values is the monodentate Ti4+−OC•O− radical in which the dangling oxygen forms a hydrogen bond with the proton in the Ti4+− OH− center (structure 3 in Figure 7); in other monodentate structures, the hfcc on the 13C nucleus is too high. However, it seems unlikely that such a structure could be present under mildly acidic conditions (pH ∼ 2). Structure 2 can potentially serve as the progenitor of the pH > 2 species. However, this possibility appears to be inconsistent with our EPR experiments, as the appearance of EPR spectra for the pH > 2 species does not change as the proticity changes from pH 2 to 10. Structure 4, despite larger hfcc on the 13C nucleus, is a contender which is likely to be present under acidic conditions. However, anisotropic hfcc’s in the proton bound to the dangling oxygen in structure 4 are substantial, and this coupling would be observed in our EPR experiments. So, this structure also appears to be inconsistent with our observations. We still need to find the bound radical that can serve as the progenitor at pH < 1. To this end, we considered clusters containing the Ti4+−OC•O− radicals bound to protons in two kinds of Ti4+−OH+−Ti4+ centers, as such centers are known to occur on the anatase surface under the acidic conditions (structures 2 and 5 in Figure 7). Both of these structures exhibit

Figure 8. Illustration of three reaction pathways: route I, from bidentate CO2− to bidentate HCO2−; route II, from monodentate CO2− to bidentate HCO2−; and route III, from monodentate CO2− to monodentate HCO2−. The monodentate and bidentate configurations are labeled as M and B, respectively, and TS denotes transition state. The change of electronic energy (eV) from one state to the other is also indicated in the figure along the reaction pathways. See Figure 11S in the Supporting Information for the geometries of the structures involved.

monodentate and bidentate CO2−•, and the corresponding structures are given in Figure 11S (Supporting Information). We have considered three reaction pathways, one starting from bidentate CO2−• and resulting in bidentate formate (pathway I in Figure 8) and two other pathways starting with monodentate CO2−• forming a hydrogen bond with a neighboring Ti4+− OH+−Ti4+ center and resulting in either bidentate formate (pathway II) or monodentate formate that retains a H-bond to the same surface proton (pathway III), as shown in Figure 11S(e) (Supporting Information). All of these reactions are exothermic, and the bidentate structures are lower in electronic energy than the corresponding monodentate structures (see Figure 8). However, pathway III has a prohibitively high reaction barrier and can be excluded from further consideration. Pathway II has a 0.33 eV barrier and requires that a neighboring bridging oxygen is deprotonated to form a bidentate product, which might be unfeasible under acidic conditions. The fact 9467



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that the reaction barrier (0.17 eV) is the lowest for the bidentate CO2−• (pathway I) that is formed in the photoreduction of CO2 on TiO2 could explain why a bidentate CO2−• species is not observed during this reduction, as this reaction intermediate is further reduced to bidentate formate. As the photooxidation of the formate yields f ree or monodentate CO2−•, this further reduction does not occur, and these radical species can be observed in low-temperature EPR experiments. 3.4. Photoreactions of Methylformate. A way of testing the suggested mechanism for reaction 16 is using a methylsubstituting group instead of an OH group, which is of particular interest from the standpoint of methanogenesis on TiO2, as methylformate (HCO2Me) is one of the identified minor products.9 The studies of the radiolytic reduction of esters of carboxylic acids in aqueous glasses revealed the general reactions of the following types43 RC(O)OR′ + eaq −• → RC(O−•)OR′

(17)

RC(O−•)OR′ → RCO2− + R′•

(18)

Figure 9. EPR spectra obtained for a photoirradiated aqueous solution of HCO2Me on anatase nanoparticles using the conditions indicated in the plot. The dotted line in the middle trace is the simulated EPR spectrum of the methyl radical (also indicated with arrows). The line arising from the formyl radical is observed in all three of these spectra. In the lower trace, the EPR signal from •CH2CO2− obtained via photooxidation of malonic acid on α-Fe2O346 is superimposed on the observed EPR spectrum for methylformate.

Reaction 18, therefore, can explain the formation of methyl radicals (that were directly observed by EPR)18 as these radicals can be generated by the dissociative electron attachment to methylformate. However, Sevilla et al.43 showed that reaction 18 did not occur for methylformate, while it occurred for ethylformate. For methylformate, the HC(O−•)OMe radical anion generated in reaction 17 was stable to 175 K; at higher temperature it abstracted hydrogen from another methylformate to yield the OC•OMe radical. The very weak resonance lines of HC•O superimposed on the resonance lines of these two radicals were observed, but their origin was not addressed. Given that the reduction chemistry of formic acid is shown to be different in aqueous glasses as compared to anatase surfaces, we tested whether the methylformate chemistry on TiO2 differs from homogeneous chemistry. To this end, we first examined what radicals are produced by ultraviolet light photolysis of neat HCO2Me in aqueous solutions. Methylformate cannot be fragmented by 355 nm light, but radicals can be generated in the samples photolyzed using 200−300 nm light (Figure 12S, Supporting Information). The three radicals making the largest contributions are the formyl, methyl, and a methylene radical which can be either • CH2OC(O)H, the product of H abstraction from the methyl group of methylformate (from the oxidation of the ester), or a hydroxymethyl radical resulting from the homolysis of the C− OMe bond and the subsequent conversion of the released methoxyl radical. The OC•OMe radical was not observed. On the other hand, in aqueous TiO2/HCO2Me excited by 355 nm light (Figure 9), the main contribution is from a radical that has a spectrum similar to the hydroxymethyl radical (obtained by photooxidation of methanol on TiO2)44,45 or the methylcarboxyl radical (obtained by photooxidation of acetate or photo-Kolbe reaction involving malonate).46 In addition to these EPR signals, possible (very weak) resonance lines of the methyl radical exist; the strong spectral overlap with the methylene radical precludes definite identification. There is also some structure observed in the central resonance line, suggesting the presence of the OC•OMe radical. The parsimonious interpretation of these EPR observations is that the photooxidation of methylformate involves deprotonation of the radical cation from the methyl and formyl groups, yielding the •CH2OC(O)H and OC•OMe radicals,

respectively. The possible contribution from the methyl radical can be rationalized via the occurrence of dissociative ″electron″ attachment reactions 17 and 18. However, the formation of the HC•O radical (whose yield is much greater than the yield of the formyl radical from the formic acid) suggests the occurrence of reaction 19 analogous to reaction 16 Ti 3 + + (HCO2 Me)ad → Ti4 +OMe− + HC•O

(19)

as (i) the homolysis would result in the parity of the EPR signals from the hydroxymethyl and formyl radicals (which was not observed experimentally) and (ii) such homolysis cannot be induced by 355 nm light, as the excitation energy is too low to dissociate the corresponding C−O bond. Thus, the photoreactions of methylformate on TiO2 are consistent with the occurrence of reaction 2. In Section 1S of the Supporting Information, we demonstrate that N,N′-dialkylformamide is reduced in reaction 20 Ti 3 + + (HCONR2)ad → Ti4 +NR2− + HC•O

(20)

which is fully analogous to reactions 16 and 19.

4. DISCUSSION In the previous section, we examined several examples of oneelectron photoreduction on TiO 2 that involve neutral molecules and anions which (in the solvent bulk) are very resistant to one-electron reduction in homogeneous systems, even when using very strong reducing agents, such as the hydrated electron (E0 = −1.9 V vs NHE). We have demonstrated that on the surface of TiO2 the one-electron reduction can proceed through heteroatom transfer to the surface Ti3+ centers. This bonding stabilizes the released anion, whose further stabilization may involve protonation at the surface. These reactions are surprisingly efficient despite the (nominally) unfavorable energetics. 9468



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CO2 reduction on TiO2. We suggest that the CO2−• radicals formed in one-electron reduction of surface-adsorbed CO2 are bound to two titanium ions

The heteroatom-transfer reaction of a general type (reaction 2) can explain how the multistep reduction of CO2 can proceed past the formic acid on the surface of TiO2 in the presence of only water as an electron donor. From a mechanistic standpoint, the conceptual difficulty is that the intermediate products of the multistep reduction of CO2 can be photooxidized, as the hole centers on TiO2 are very strong oxidizers, reacting with most organic molecules, whereas only some of these molecule can be reduced by the Ti3+ centers. In particular, no radical products of photoreduction have been observed for formaldehyde and methanol, the two tentative products of formate reduction.19,45 On the other hand, formaldehyde is readily oxidized to the formyl radical,45 and methanol is equally readily oxidized to the hydroxymethyl radical, •CH2OH.44−46 Thus, the reduction of these one-carbon molecules on TiO2 can only involve two-electron reactions. However, for HCO2H this is not the case due to the occurrence of reaction 16, resulting in the formation of the formyl radical. While reaction 16 rationalizes this vexing “bottleneck”, our observations pose another problem. Our EPR spectra indicate that photooxidation of formate on TiO2 yields the CO2−• radical anion (or its protonated form) which is stable toward further reduction at low temperature. Logically, the same radical should occur in one-electron reaction 5 of CO2 on TiO2, yet no CO 2 −• radical was observed by EPR in the corresponding photosystem.18 Thus, either the CO2−• formed in the CO2/TiO2 system is further transformed via facile reaction 6 or the CO2−• radical generated in reaction 5 is different from the CO2−• radical generated in reactions 7 and 8. As we noted in Section 3.2, the hfcc parameters of the CO2−• and trans-OC•OH radicals generated via photooxidation were similar to such parameters for free radicals. This observation suggests a simple explanation for the puzzle of reactiondependent CO2−• stability: the CO2−• radicals generated in reaction 7 (oxidation of formate) are stable because these radicals are either (i) expelled from the TiO2 surface or (ii) adsorbed on this surface in a different fashion than the CO2−• radicals generated in reaction 5 (reduction of carbon dioxide). Indeed, the latter reaction involves a surface-bound CO2 molecule yielding a surface-bound CO2−• radical (Section 3.3 and Figure 7, structures 1−5).18 Our computational studies in Section 3.3 exclude the bidentate Ti4+−OC•O−−Ti4+ radical as the progenitor of the pH > 2 and pH < 1 species observed in EPR spectra upon excitation of TiO2 in the presence of formate or formic acid, respectively. These species can be free transOC•OH and CO2−• radicals, respectively, that are weakly interacting with the TiO2 surface or, less likely, monodentate Ti4+−OC•O− centers at the surface (Section 3.3). The formyl radical also can be present on TiO2 as either a free (weakly bound) radical or an oxygen-bound Ti4+−OC•H radical. Isotropic hfcc constants for the 1H and 13C nuclei of this bound radical (Table 2S, Supporting Information) strongly differ from these constants for a free formyl radical (the 13C constant is 30 G too high, and the 1H constant is 35 G too low), whereas the experimental parameters are similar to those reported and calculated for the free radical. We conclude that the formyl radical generated in reactions 16,19, and 20 is always a f ree radical. This feature is fully anticipated from the suggested mechanism for these reactions, as the released formyl radical is free even if the parent molecule is oxygen-bound to the metal ion. On the strength of these observations and our DFT modeling, we refine the mechanism for the initial steps of

Ti4 + ··· OCO + Ti 3 + → Ti4 +−OC−•O−Ti4 + (21)

Such bidentate radicals can be further reduced by photoexcited conduction-band electrons eCB−• + Ti4 +−OC−•O−Ti4 + + H+ → Ti4 +−OC(H)O−−Ti4 +

(22)

CO2−•

For this reason, the bidentate radical intermediate cannot be observed by EPR. In contrast, f ree or monodentate CO2−• radicals can survive on the TiO2 surface at low temperature, as reaction 22 crucially involves binding of both oxygens with the surface, as illustrated in Figure 8.

5. CONCLUDING REMARKS 5.1. General Import for Photocatalysis. There has been a recent resurgence of interest to the kinetics and mechanisms for one-, two-, and three-electron reduction on metal oxides,37,47 but molecular detail for many of such (obviously, multistep) reactions is lacking. In this study, we examined the mechanism for one-electron reduction on photoactive metal oxides, such as TiO2. We demonstrated that this reduction essentially involves heteroatom transfer to the metal center, reaction 2. Gaining energy from the metal−heteroatom bond, such reactions can occur for substrates that are difficult to reduce in the water bulk, such as chlorine oxoanions and derivatives of formic acid. We argue that photocatalytic methanogenesis on metal oxides crucially involves such reactions on the path from carbon dioxide to methane. Furthermore, we suggest that the mode of surface binding of the substrate is also critically important, as the same species may or may not be reduced depending on the mode of binding. Our results suggest that the mode of chemisorption of the CO2−• radical (which is the reaction intermediate of one-electron reduction of CO2) fully determines the outcome: if this radical is doubly bound through its oxygens to the metal ions at the surface, this radical can be further reduced to formate; if it is singly bound or physisorbed by the oxide surface, this reduction is stalled. One-electron reduction or oxidation of organic molecules typically yields organic radicals that are much easier to oxidize or reduce than their parent molecules. Why is it possible to observe such radicals at all? There were previous suggestions48 that these radicals are stabilized because (unlike their parent molecules) they are not chemically bound to the oxide surface (or bound to the surface differently than the parent compounds). Our study supports this conjecture for the formyl radicals generated in reactions 16,19, and 20 and the CO2−• radicals generated in reactions 7 and 8. It appears that scission of the bond(s) between the metal ions and the organic adsorbate could be necessary to avoid f urther redox reactions involving radical intermediates, as the rate of the interfacial charge transfer dramatically depends on the mode of binding, whereas such reactions always compete with charge recombination eliminating the redox-active centers. 5.2. Import for Mars Chemistry. In addition to the obvious significance of these results for photosynthetic fuel production and mechanistic understanding of redox reactions on metal oxide surfaces, our study has a less obvious bearing on the planetary chemistry of Mars. 9469



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The surface of Mars is covered by dispersed iron(III) oxides (called the regolith) and also contains 1% TiO2.49 This mineral dust is exposed to ultraviolet light (200−400 nm) penetrating the thin martian atmosphere,50 which mainly consists of CO2; there is some moisture and hydrogen peroxide in this atmosphere and the soil.51 This planetwide photocatalyst can oxidize any extraneous or intrinsic organic material (Mars is bombarded by micrometeorites containing organic carbon).52,53 However, there were recent observations of methane in the atmosphere,54 and there is some (heavily disputed)55 evidence that the concentration of this methane is seasonably variable,56 suggesting that its production or depletion is controlled by some yet unknown surface (photo)chemistry.57 Photocatalytic methanogenesis on metal oxides was suggested as a possible mechanism for abiogenic, variable methane production,9 and this photoreaction may also produce two- and three-carbon byproducts closely resembling bacterial metabolites. As such photoactive surfaces can both reduce and oxidize, the balance depends on the volatility of the organic products as well as the ability of the surface to induce redox reactions. While possible photooxidative degradation of the tentative organic molecules on soil particles has long been recognized,52,53 photoreduction has never been systematically examined. Our study suggests that there could be an occurrence of complex organic carbon chemistry initiated by the reduction of atmospheric carbon dioxide on the surface of metal oxides. Mars could be a “photosynthetic” planet, both producing two to three carbon molecules (under certain conditions) and degrading organic molecules (under different conditions) on its sun-exposed soil. A particular example of such redox activity is the degradation of perchlorate, which is a major component of martian soil.58 The discovery of perchlorate on Mars was surprising; it was expected that chlorine is present as chloride. It was suggested that the perchlorate has an atmospheric origin: the reaction of volcanic HCl with ozone in the upper atmosphere yields oxychloride radicals, and the reaction of ClO3• with the hydroxyl radical yields perchloric acid, which precipitates from the atmosphere.59 The tacit assumption of such scenarios is the stability of perchlorate in the soil after it is generated elsewhere. Indeed, perchlorate has remarkable chemical, photo-, and radiation stability. Nevertheless, our study indicates that perchlorate is not photostable on light-exposed metal oxides, including the two iron(III) oxides that are most abundant on Mars (Section 3.1): there is three-electron reduction of perchlorate to volatile chlorine dioxide. As the O− ions removed from the perchlorate are permanently bound to metal ions on the surface of regolith particles, reoxidation of this radical to ClO4− in the soil is unlikely, even in the presence of oxidizers, such as the hydroxyl radical, hydrogen peroxide, and ozone. It appears, therefore, that light-exposed soil is constantly degrading the perchlorate anions, and some chemical process should continuously replenish them to permit their accumulation in the soil.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: I. A. Shkrob, [email protected]; N. M. Dimitrijevic, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Grant No. NNH08A65I from the Mars Fundamental Research Program of NASA (to IAS and TWM) and by 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.

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REFERENCES

(1) Roy, S. C.; Varghese, O. K.; Paulose, M.; Grimes, C. A.; Centi, G.; Perathoner, S. Catal. Today 2009, 148, 191−205. (2) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637−638. (3) Halmann, M.; Ulman, M.; Blajeni, B. A. Sol. Energy 1983, 31, 429−431. (4) Cook, R. L.; Macduff, R. C.; Sammells, A. F. J. Electrochem. Soc. 1988, 135, 3069−3070. (5) Anpo, M.; Chiba, K. J. Mol. Catal. 1992, 74, 207−212. (6) Sayama, K.; Arakawa, H. J. Phys. Chem. 1993, 97, 531−533. (7) Adachi, K.; Ohta, K.; Mijuma, T. Sol. Energy 1994, 53, 187−190. (8) Anpo, M.; Yamashita, H.; Ichihashi, Y.; Fujii, Y.; Honda, M. J. Phys. Chem. B 1997, 101, 2632−2636. (9) Bartoszek, M.; Wecks, M.; Jakobs, G.; Mohlmann, D. Planet. Space Sci. 2011, 59, 259−263. (10) Anpo, M.; Yamashita, H.; Ichihashi, Y.; Ehara, S. J. Electroanal. Chem. 1995, 396, 21−26. (11) Ku, Y.; Lee, W.-H.; Wang, W.-Y. J. Mol. Catal. A 2004, 212, 191−196. (12) Dey, G. R.; Belapurkar, A. D.; Kishore, K. J. Photochem. Photobiol. A 2004, 163, 503−508. (13) Tan, S. S.; Zou, L.; Eric, H. Catal. Today 2006, 115, 269−273. (14) Centi, G.; Perathoner, S.; Wine, G.; Gangeri, M. Green Chem. 2007, 9, 671−678. (15) Wu, J. C. S. Catal. Surv. Asia 2009, 13, 30−40. (16) Yui, T.; Kan, A.; Saitoh, C.; Koike, K.; Ibusuki, T.; Ishitani, O. ACS Appl. Mater. Interfaces 2011, 3, 2594−2600. (17) Chen, L.; Graham, M. E.; Li, G.; Gentner, D. R.; Dimitrijevic, N. M.; Gray, K. A. Thin Solid Films 2009, 517, 5641−5645. (18) Dimitrijevic, N. M.; Vijayan, B. K.; Poluektov, O. G.; Rajh, T.; Gray, K. A.; He, H.; Zapol, P. J. Am. Chem. Soc. 2011, 133, 3964− 3971. (19) Dimitrijevic, N. M.; Shkrob, I. A.; Gosztola, D. J.; Rajh, T. J. Phys. Chem. C 2012, 116, 878−885. (20) Rajh, T.; Poluektov, O. G.; Thurnauer, M. C. In Chemical Physics of Nanostructured Semiconductors; Kokorin, A. I., Ed.; NOVA Science Publ., Inc.: New York, 2003. Chiesa, M.; Giamello, E.; Che, M. Chem. Rev. 2010, 110, 1320. (21) Duke, F. R.; Quinney., P. R. J. Am. Chem. Soc. 1954, 76, 3800− 3803. Cope, V. W.; Miller, R. G.; Fraser, R. T. M. J. Chem. Soc. A 1967, 301−306. (22) Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Phys. Chem. C 2007, 111, 5259−5275 and references 68−74 therein. (23) Wang, C.; Huang, Z.; Lippincott, L.; Meng, X. J Hazard. Mater. 2010, 175, 159−164. (24) Perissinotti, L. L.; Brusa, M. A.; Crela, M. A. Langmuir 2001, 17, 8422−8427. (25) Hykaway, N.; Sears, W. M.; Morisaki, H.; Morrison, S. R. J. Phys. Chem. 1986, 90, 6663−6667.

ASSOCIATED CONTENT

S Supporting Information *

A file containing Section 1S on photoreactions of dimethylformamide on TiO2, the list of abbreviations and reactions, Tables 1S and 2S, and Figures 1S−13S with captions, including the experimental and simulated EPR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. 9470



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(26) Carraway, E. R.; Hoffman, A. J.; Hoffmann, M. R. Environ. Sci. Technol. 1994, 28, 786−793. Kesselman, J. M.; Weres, O.; Lewis, N. S.; Hoffmann, M. R. J. Phys. Chem. B 1997, 101, 2637−2643. (27) de Tacconi, N. R.; Wenren, H.; McChesney, D.; Rajeshwar, K. Langmuir 1998, 14, 2933−2935. Chawla, O. P.; Fessenden, R. W. J. Phys. Chem. 1975, 79, 2693−2700. (28) Becke, A. D. Phys. Rev. A 1988, 38, 3098. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford CT, 2004. (30) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (31) He, H.; Zapol, P.; Curtiss, L. A. J. Phys. Chem. C 2010, 114, 21474−21481. (32) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. Rev. B 1998, 57, 1505−1509. (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (34) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169−11186. (35) Henkelman, G.; Jonsson, H. J. Chem. Phys. 2000, 113, 9978− 9985. (36) Wang, X.-B.; Wang, L.-S. J. Chem. Phys. 2000, 113, 10928− 10933. (37) Safrany, A.; Gao, R.; Rabani, J. J. Phys. Chem. B 2000, 104, 5848−5853. (38) Anderson, N. H.; McMillan, M.; Norman, R. O. C. J. Chem. Soc. B 1970, 1075−1083. Edge, D. J.; Norman, R. O. C. J. Chem. Soc. B 1970, 1083−1086. (39) Dalal, N. S.; McDowell, C. A.; Park, J. M. J. Chem. Phys. 1975, 63, 1856−1862. Sharp, J. H.; Symons, M. C. R. J. Chem. Soc. A 1970, 3075−3080. Schlick, S.; Silver, B. L.; Luz, Z. J. Chem. Phys. 1971, 54, 867−871. (40) Sevilla, M. D.; Swarts, S.; Bearden, R.; Morehouse, K. M.; Vartanian, T. J. Phys. Chem. 1981, 85, 918−923. (41) Gordon, S.; Hart, E. J.; Matheson, M. S.; Rabani, J.; Thomas, J. K. Discuss. Faraday Soc. 1963, 36, 193−205. Schwarz, H. A. J. Phys. Chem. 1992, 96, 8937−8941. (42) Rotzinger, F. P.; Kesselman-Truttmann, J. M.; Hug, S. J.; Shklover, V.; Grätzel, M. J. Phys. Chem. B 2004, 108, 5004−5017. (43) Sevilla, M. D.; Morehouse, K. M.; Swarts, S. J. Phys. Chem. 1981, 85, 923−927. (44) Micic, O. I.; Zhang, Y.; Cromack., K. R.; Trifunac, A. D.; Thurnauer, M. C. J. Phys. Chem. 1993, 97, 7277−7283. Micic, O. I.; Zhang, Y.; Cromack., K. R.; Trifunac, A. D.; Thurnauer, M. C. J. Phys. Chem. 1993, 97, 13284−13288. (45) Shkrob, I. A.; Marin, T. W.; He, H.; Zapol, P. J. Phys. Chem. C 2012, DOI: 10.1021/jp300122v. (46) Shkrob, I. A.; Chemerisov, S. D. J. Phys. Chem. C 2009, 113, 17138−17150. (47) Mohamed, H. H.; Mendive, C. B.; Dillert, R.; Bahnemann, D. W. J. Phys. Chem. A 2011, 115, 2139−2147. Mohamed, H. H.; Dillert, R.; Bahnemann, D. W. J. Photochem. Photobiol. A: Chem. 2011, 217, 271−274. Mohamed, H. H.; Dillert, R.; Bahnemann, D. W. J. Phys. Chem. C 2011, 115, 12163−12172. (48) Henderson, M. A.; Deskins, N. A.; Zehr, R. T.; Dupuis, M. J. Catal. 2011, 279, 205−212. (49) Rieder, R.; Economou, T.; Wanke, H.; Turkevich, A.; Crisp, J.; Bruckner, J.; Drebius, G.; McSween, H. Y. Jr. Science 1997, 278, 1771−1774. (50) Ronto, G.; Berces, A.; Lammer, H.; Cockell, C. S.; MolinaCuberos, G. J.; Patel, M. R.; Selsis, F. Photochem. Photobiol. 2003, 77, 34. (51) Encrenaz, Th.; Bezard, B.; Greathouse, T.; Richter; Lacy, J.; Atreya, S. K.; Wong, A.; Lebonnois, S.; Lefevre, F.; Forget, F. Icarus 2004, 170, 424−429 and references therein.. (52) Shkrob, I. A.; Chemerisov, S. D.; Marin, T. W. Astrobiology 2010, 10, 425−436.

(53) Benner, S. A.; Devine, K. G.; Matveeva, L. N.; Powell, D. H. Proc. Natl. Acad. Sci. 2000, 97, 2425−2430. Shuerger, A. C.; Clark, B. C. Space Sci. Rev. 2008, 135, 233−243 and references therein. (54) Atreya, S. K.; Mahaffy, P. R.; Wong, A. S. Planet. Space Sci. 2007, 55, 358−369 and references therein. (55) Zahnle; Freedman, R. S.; Catling, D. C. Icarus 2011, 212, 493− 503. (56) Mumma, M. J.; Villanueva, G. L.; Novak, R. E.; Hewagama, T.; Bonev, B. P.; DiSanti, M. A.; Mandell, A. M.; Smith, M. D. Science 2009, 323, 1041−1045. Fonti, S., Marzo, G. Astron. Astrophys. 2010, 512, A51 (6 pages). (57) Lefèvre, F.; Forget, F. Nature (London) 2009, 460, 720−723. (58) Hecht, M. H.; Kounaves, S. P.; Quinn, R. C.; West, S. J.; Young, S. M. M.; Ming, D. W.; Catling, D. C.; Clark, B. C.; Boynton, W. V.; Hoffman, J.; DeFlores, L. P.; Gospodinova, K.; Kapit, J.; Smith, P. H. Science 2009, 325, 64−67. (59) Catling, D. C.; Claire, M. W.; Zahnle, K. J.; Quinn, R. C.; Clark, B. C.; Hecht, M. H.; Kounaves, S. J. Geophys. Res. 2010, 115, E00E11 (15 pages).

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Heteroatom-Transfer Coupled Photoreduction and Carbon Dioxide

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