Photooxidation Mechanism of Methanol on Rutile TiO2 Nanoparticles

Article pubs.acs.org/JPCC

Photooxidation Mechanism of Methanol on Rutile TiO2 Nanoparticles Dimitar A. Panayotov, Steven P. Burrows, and John R. Morris* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061-0212, United States S Supporting Information *

ABSTRACT: The use of nanoparticulate TiO2 as a photocatalyst for the conversion of organic molecules has grown tremendously in recent years; however, the roles of excited electrons, holes, and surface adsorbates in titania photochemistry remain poorly understood. In this work, detailed infrared measurements, which are sensitive to both vibrational and electronic transitions within the material, are used to uncover the mechanism of methanol oxidation on 4 nm rutile nanoparticles in both anaerobic and aerobic conditions. These experiments are performed in an ultrahigh vacuum cell where the coverage of methanol and exposure to oxygen are precisely controlled. Our measurements reveal that the primary pathway for initial methanol adsorption on TiO2 is dissociative, leading to the production of adsorbed methoxy groups. Upon exposure of the sample to ultraviolet photons, the results show that the electron−hole pairs (e−−h+) generated within TiO2 have significant lifetimes because the holes are efficiently trapped by the surface methoxy groups. The subsequent photochemistry induces a two-electron oxidative degradation process of the surface methoxy groups to formate. Formate production proceeds through the formation of a radical anion, the result of hole oxidation, followed by prompt electron injection by the radical anion into the TiO2. Furthermore, these studies show that the role of O2 in promoting methanol photodecomposition is to scavenge free electrons, which opens acceptor sites for the injection of new electrons during methoxy group oxidation. In this way, O2 increases the efficiency of methoxy oxidation by a factor of 5 relative to anaerobic conditions, yet does not affect the hole-mediated oxidation mechanism that leads to final formate production.

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+ characterized as hole traps (hVB → hT+ ), shallow electron traps − − − − (eCB → eST ), and deep electron traps (eCB → eDT ). Whereas the injection of electrons into the conduction band and the concomitant generation of holes in the valence band occur over femtoseconds,6,11,12 the trapping of holes and electrons into the gap states13−16 is a diffusion limited process that has been observed to occur within time scales ranging from femtoseconds14 to minutes.6,17 Conduction band electrons have been shown to become trapped at the particulate surface within 30 ps, and valence band holes have been observed to populate trap states up to 250 ns after excitation.18 At very low charge carrier concentrations, the intraparticle recombination of an e−−h+ pair typically tracks first-order kinetics. The lifetime of a single e−−h+ pair in a 12 nm-sized TiO2 particle may be as long as 30 ns.18 Unfortunately, for photochemical applications, only a fraction of photogenerated charge carriers reach the surface of nanoparticles to react with adsorbed electron acceptors and donors. This fraction of charge carriers defines the efficiency of the desired photochemical process.1,2,4−9,16,17,19 Titanium dioxide20,21 is a wide band gap semiconductor, commercially available as anatase and rutile polymorphs.5,22,23 Like other semiconductors, band gap characteristics of TiO2 are affected by the lattice structure of the material, which leads to different bulk band gaps for anatase and rutile.5,24 Further, the

INTRODUCTION TiO2 photochemistry is one of the most important and widely studied approaches for the catalytic conversion of organic compounds; however, scientists are only beginning to understand the roles of excited electrons, holes, and adsorbates in driving surface chemistry at TiO2 nanoparticles. Irradiation of the semiconductor metal oxide nanoparticles with ultraviolet light at energies exceeding their band gap generates mobile charge carriers within the particles.1,2 The photoinduced formation of electron−hole (e−−h+) pairs is the basis for numerous photochemical applications of TiO2, including solar energy conversion3 and heterogeneous reactions of environmental pollutants and other organic precursors.2,4−9 For example, TiO2 is currently utilized in very large-scale water purification applications10 and as a photocatalyst for selective oxidation reactions in organic chemistry.8 Despite the large number of promising applications for TiO2, surprisingly little is known about the mechanistic details of small molecule photochemistry on TiO2 nanoparticles. The work described below helps to bridge this gap by employing infrared spectroscopy to explore the important role of charge traps in the photodecomposition of methanol on the surface of rutile TiO2 nanoparticles. During photoillumination of titania, the majority of the charge carriers that populate the conduction band (CB) and the valence band (VB) of the semiconductor undergo direct − + nonradiative electron−hole pair recombination: eCB − hVB . Other charge carriers become trapped within band gap states, © 2012 American Chemical Society

Received: September 23, 2011 Revised: February 3, 2012 Published: February 8, 2012 6623

dx.doi.org/10.1021/jp209215c | J. Phys. Chem. C 2012, 116, 6623−6635

The Journal of Physical Chemistry C

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essential in preventing the charge recombination of electrons and holes within TiO2 nanoparticles.2,56,61,62 The dynamics of electron and hole migration and their role in surface chemistry has been recently investigated. For example, researchers have established that, in solution, oxidative reactions on TiO2 photocatalysts occur via photogenerated holes,54,55,63 as opposed to hydroxyl radicals, as suggested in other studies.64 In fact, a series of investigations into the photochemistry of nanocrystalline TiO2 films in liquids55,56 has identified the three most important reactive species as trapped holes, trapped electrons, and free electrons. Spectroscopically, trapped holes have been shown to exhibit an absorption band at 520 nm, a band at 770 nm was attributed to trapped electrons, and free electrons exhibited a very broad absorption band that spanned from the visible to the infrared. Trapped holes and electrons were found to be primarily localized at the particle surface, while free electrons most often reside in the bulk.56 Complementary studies53,59 have examined electron- and holecapture reactions under high vacuum (HV) conditions when a Pt/TiO2 photocatalyst was exposed to methanol vapor. In that work, a featureless absorbance in the infrared region of 4000− 1000 cm−1 was observed that depended inversely on wavenumber, according to the relationship: Δ(Absorbance) = Aṽ−p, where A is a constant, and p is the scattering constant. According to semiconductor theory,65 this featureless IR absorbance is due to free conduction band electrons. This infrared feature has now been observed in numerous studies of TiO2 materials.17,51,53,56,66−71 The photocatalytic oxidation of methanol is one of the most suitable model reactions to study for the development of a fundamental understanding of the mechanisms of photoassisted and photocatalytic processes. A number of previous studies into the photooxidation of methanol have been recently described in the review article cited in ref 2, which highlights the gaps in the current understanding of this reaction and reveals areas where several studies provide contradictory interpretations of the reaction dynamics. This body of work suggests that both direct69,72−76 (hole-mediated) and indirect72,73,75,77−79 (O2− mediated) oxidation processes may be involved. For example, in one set of studies, formate and formaldehyde are reported to be the primary intermediates of direct and indirect oxidation reactions, respectively,74,78 whereas other studies draw the opposite conclusion.73,75 The gas-phase photocatalytic oxidation of methanol has been studied on one of the most photoactive TiO2 materials, Degussa P25.69,75 These previous experiments have also indicated that, under anaerobic conditions, the photochemistry of methanol is initiated by holes trapped at methoxy groups.69,75 Under aerobic conditions, researchers suggested gas-phase oxygen plays a dual role: as an electron scavenger, due to its high electron affinity, and as a reactant that becomes incorporated into intermediates to form adsorbed formic acid and eventually yield CO2 and H2O.69,75 Despite this prior work, there has yet to be a study in which the dynamics of photogenerated e−−h+ pairs have been tracked while following the oxidation of a surface adsorbate, which is essential for the development of a complete understanding of this system. In this Article, we use transmission infrared spectroscopy to study the photochemistry of methanol on 4 nm rutile nanoparticles under both anaerobic and aerobic conditions in the well-controlled environment of an ultrahigh vacuum cell. The utility of infrared spectroscopy in the study of semiconductor surface chemistry stems from its ability to probe

fundamental absorption edge in nanosized TiO2 materials is often found to be blue-shifted, by ∼0.2 eV, for crystallite sizes below ∼10 nm, relative to their bulk analogues.5,25 Anatase nanoparticles are usually characterized by diameters of d < 50 nm, a band gap of ∼3.2 eV, and efficient UV light absorption at λ < 385 nm.5,22 Rutile nanoparticles are often larger in diameter (d > 200 nm), because they are usually produced by thermal transformation of anatase precursors. Rutile nanoparticles have a slightly smaller band gap than that of anatase at ∼3.02 eV and can be excited with longer wavelength radiation as high as ∼410 nm,5,22 which is advantageous for photochemical applications; however,5,22 anatase has been found to exhibit a 10-fold greater rate of hole trapping. The efficient charge-trapping rate of anatase accounts for many experimental results that indicate it is generally the more photoactive polymorph of titanium oxide.22,23 The charge-transport characteristics of nanoparticulate semiconductors fundamentally differ from those of bulk semiconductors. Most importantly, charge-diffusion coefficients for small nanoparticles are orders of magnitude smaller than those in bulk semiconductors due to the high concentration of defects that serve as charge trapping centers.6,26−28 The high ratio of surface area to volume for nanoparticulate materials and the presence of particle−particle grain boundaries give rise to a large number of interfacial energy trap states.6,12,25−28 In addition, surface ions have a reduced Madelung energy, because of their lower coordination number. In ionic materials, this effect lowers conduction-band level (cationic) and raises valence-band level (anionic) relative to the corresponding energy levels of the bulk bands.29 Each surface level may be further broadened into a distribution of surface-trap states for electron and hole charge carriers.29 Recent electron paramagnetic resonance (EPR) studies of trapped hole states in titania indicate that trapped holes are produced in the TiO2 lattice and exist as an O− species covalently bound to Ti atoms.17 The holes are most likely localized deep trap states that reside at the surface or in the subsurface region of the nanoparticles. In addition to hole-trap states, electrons can be efficiently trapped in the TiO2 lattice. Electronic trap states occur at Ti3+ sites, Ti4+ interstitials, Ti interstitial pairs, and other planar defects related to crystallographic shear planes.30,31 Evidence from a number of spectroscopic studies suggests that oxygen vacancies (Ovac) at bridging O atoms (Obr) (formally described as an Ovac lying between two Ti3+ sites)32,33 introduce electronic band gap states (BGS) about 1 eV below the conduction band minimum (CBM).2,31−40 While many studies suggest that Ovac are the source of BGS in TiO2,20,33,37,41,42 other work attributes BGS to titanium interstitial defects (Tiint).31,43−46 Whether the Ovac or the Tiint sites constitute the major contribution to the band gap states (and thus influence the photochemistry of titania) remains a topic of significant discussion in the literature.2,31,33,42−45,47−50 Transient absorption spectroscopy in the visible and mid-IR range has been employed to study the dynamics of photoinduced charge carriers.2,15,39,51−55 In addition, hole- and electron-scavenging reactions under pulse15,51−53,55 or steadystate30,56 UV-irradiation have been used to investigate the kinetics and mechanisms of charge recombination and trapping in titania. In the presence of a hole scavenger, holes can be rapidly removed, while long-lived electrons remain within the nanoparticles.2,5,56−60 Alternately, the abstraction of photogenerated electrons by molecular oxygen has been found to be 6624



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to 475 K, evacuated to 1 × 10−6 Torr, and cooled to room temperature. The methanol (97%) used for this work was obtained from Aldrich and further purified via five freeze−pump−thaw cycles and stored in glass bulbs. The oxygen used was obtained from Airgas Inc. and was of ultrahigh purity (99.999%). Dosing of either methanol or O2 was accomplished using a calibrated volume of the stainless steel gas line, and the gas pressure was measured by the capacitance manometer. The UV light source employed in this work was a highpressure 350 W Hg arc lamp (Oriel Corp.) equipped with a water filter to remove the IR radiation. The intensity of the UV radiation on the sample was 200 mW/cm2 in the energy range 3.0−5.0 eV. The methanol adsorption and photodecomposition experiments on the TiO2 sample were performed at 295 K. The clean (dehydroxylated and oxidized) TiO2 sample was exposed for 10 min to 0.1 Torr of methanol and then evacuated at 1 × 10−6 Torr for 20 min. This procedure was repeated until the desired coverage of methanol/methoxy species, Θa, was obtained. The coverage at which no further methanol uptake could be spectroscopically observed was assigned as saturation coverage, that is, Θa = 1.

discrete vibrational transitions of surface adsorbates, while simultaneously tracking electronic transitions within or close to the continuum of the conduction band. Together, these approaches have been used to clearly show that charge trapping readily occurs in the presence of hole or electron scavenging adsorbates and that the trapping processes are responsible for the decomposition chemistry of surface-bound organics.

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EXPERIMENTAL SECTION TiO2 Nanoparticles. Rutile TiO2 nanoparticles were prepared by laser vaporization of a TiO 2 tablet and condensation of nanoparticles in an O2 atmosphere (1 Torr) on a hemispherical collector.80 Figure S1 in the Supporting Information shows a transmission electron microscopy (TEM) image and particle size distribution of the ∼4 nm rutile nanoparticles employed in this work, as determined from TEM measurements. FTIR Spectroscopy. Rutile TiO2 nanoparticles were pressed as a circular spot (7 mm diameter) into a tungsten grid (80% optically transparent) with a hydraulic press at a pressure of 2.15 × 108 N m−2. The tungsten grid containing the TiO2 sample was held by Ni support clamps in a stainless steel high vacuum transmission IR cell described previously.81,82 The IR cell was equipped with two collinear KBr spectroscopic windows to transmit the IR beam and a third UV-transparent sapphire window in a perpendicular direction to the IR beam. Each window was sealed in 2 3/4 in. ConFlat flanges by differentially pumped Viton O-rings. The tungsten grid with the TiO2 sample was oriented at 45° with respect to both the IR and the UV beams. The sample cell was mounted on a computer-controlled translation system (Newport Corp.) capable of moving the cell to ±1 μm accuracy in the horizontal and vertical directions for precise alignment of the samples in the IR beam. The IR cell was connected to a stainless steel high vacuum system pumped down to 1 × 10−8 Torr background pressure by both an ion pump and a turbomolecular pump. An MKS capacitance manometer (Baratron, 0.001−1000 Torr) was used to monitor reactant gas pressure. A type K thermocouple welded to the top center of the tungsten grid was used to maintain the TiO2 sample temperature to within ±1 K via feedback control to the heater power supply. The sample temperature was regulated by ohmic heating of the grid and by filling a reservoir, which was in direct thermal contact with the sample holder, with liquid nitrogen (LN2).82 In addition, the LN2-filled reservoir served as a highly effective cryo-pump for condensable gases, such as H2O, which eliminated vapor from readsorbing on the TiO2 sample during the preparation procedure or during the UV photodecomposition of methanol. The IR spectra were acquired on a nitrogen gas purged Mattson Research Series I FTIR spectrometer equipped with an LN2 cooled MCT detector. The spectra were collected in the ratio mode with a resolution of 4 cm−1. Typically, 1000 scans were averaged for each spectrum for the steady state experiments, while 100 scans were averaged for the spectra recorded under dynamic UV-ON and UV-OFF regimes. TiO2 Activation: Methanol Adsorption and Photodecomposition Experiments. The oxide sample, typically weighing 6−7 mg/cm2, was activated in a vacuum at 673 K for 4 h and then treated with 20 Torr O2 for 60 min at the same temperature. This procedure removes the adsorbed water and traces of residual organic species. After evacuation at 673 K, oxygen was readmitted for 30 min; then the sample was cooled

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RESULTS AND DISCUSSION Infrared spectroscopy, which is sensitive to both electronic excitations within the titania and vibrations of surface adsorbates, has been used to help determine the mechanism of methanol photooxidation on rutile TiO2 nanoparticles. The following discussion progresses through a series of experiments that began with the initial uptake and dissociative adsorption of methanol in the absence of either UV radiation or background oxygen. This experiment was then repeated in the presence of UV radiation, which demonstrates the role of methoxy groups in serving as hole trapping sites. These hole trapping sites appear to serve as the primary oxidant in formate production. Finally, results for the simultaneous UV light and O2 exposure to methoxy-covered TiO2 reveal that electron extraction by oxygen is of critical importance to the overall chemistry. The role of O2 appears to be that of an electron scavenger, which accelerates oxidation by removing electrons that accumulate near the surface during photooxidation. In the absence of O2, accumulated surface charge blocks acceptor sites to prevent extensive oxidation of the surface adsorbates. Dissociative Adsorption of Methanol to Methoxy. In this work, we used infrared spectroscopy to measure the coverage of methanol and methoxy moieties on TiO2 under both conditions of UV irradiation and in the dark. Previous research has shown that methanol exposure to the surface of single crystal TiO2(110) leads primarily to nondissociative molecular adsorption.83−87 The small fraction of molecules that do dissociate appear to do so at defect sites where they produce methoxy groups and protons, which extract lattice oxygen atoms at 300 K. By increasing the number of defects on the surface through Ar+ bombardment, the coverage of methoxy groups was found to increase dramatically.83 Other experimental53,60,69,73,75,80,86,88−90 and theoretical studies predict79,91,92 that surface methoxy groups bond to coordinatively 4+ unsaturated Ti5c sites, and, under many conditions, the methoxy groups appear to be the dominant adsorbate in methanol-exposed titiania.30,53,54,60,69,73,75,80,83,88,89,93 Similar conclusions were drawn by the work of Bronkema et al., which reported that 90% of methanol adsorbed on high surface 6625



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Figure 1. Dissociative adsorption of methanol on clean TiO2 nanoparticles. (A) Transmission FTIR spectra of clean (a) and methanol covered (b,c) rutile TiO2 nanoparticles; and (B) difference spectra in the IR region of the CH-stretch modes for adsorbed methanol/methoxy species.

4+ 4+ Ti5c + CH3OH + O2br− ↔ Ti5c −O(CH3)H···O2br−

area anatase exists as surface-bound Ti−OCH3. The presence of adsorbed methoxy fragments following methanol exposure was also highlighted in work involving UV-photocatalytic chemistry of TiO2, which showed that methoxy groups can play a key role as the primary hole-scavenging trap, thereby prolonging the lifetime of conduction-band electrons.41,52,53,73 The FTIR spectra of clean and methanol covered TiO2, at two different coverages, Θb = 0.15 and Θc = 1, are presented in Figure 1A. The spectrum for the clean rutile sample (which was oxidized and highly dehydroxylated after the pretreatment procedure, see the Experimental Section) indicates the presence of trace amounts of free Ti−OH groups,80 as witnessed by the IR bands at 3720−3650 cm−1 (Figure 1A, spectrum a). The saturation of the rutile nanoparticles by methanol-derived surface species nearly eliminates the IR absorption bands at 3717, 3695, and 3672 cm−1 due to free Ti− OH groups. Both molecular, the bands at νas(CH3) = 2946 cm−1 and νs(CH3) = 2845 cm−1, and dissociative, the bands at νas(CH3) = 2923 cm−1 and νs(CH3) = 2824 cm−1, forms of adsorbed methanol are registered at 295 K (Figure 1A and B).60,73,75,80,89,94 In addition to the O−H and C−H stretching regions, the IR bands in the low-frequency region (Figure 1A) provide information about the bonding of methanol and methoxy groups to the surface. The band at 1154 cm−1 is attributed to the rocking mode ρ(CH3) of an adsorbed methyl group. The pairs of bands at 1439 cm−1 [δ(CH3)] and 1116 cm−1 [ν(OC)] and at 1459 cm−1 [δ(CH3)] and 1046 cm−1 [ν(OC)] indicate the presence of mono- and bridge-bonded methoxy groups, respectively.53,73,95 The small band at 1628 cm−1 is due to the δ(HOH) mode of water, a product of methanol-derived proton interaction with a surface hydroxyl group. The intense IR bands due to dissociatively adsorbed methanol reveal that methoxy groups are the primary surface adsorbate at both methanol coverages (Figure 1B), Θc = 1 and Θb = 0.15. The results for the room-temperature uptake of methanol on titania, shown in Figure 1, can be summarized by the following reactions: Ti4 +−OH + CH3OH ↔ Ti4 +−OH···O(H)CH3

(2)

4+ 4+ Ti5c + CH3OH + O2br− → Ti5c −O−CH3 + HO− br (3)

4+ 4+ 2Ti5c + CH3OH + O2br− → (Ti5c )2 −O−CH3 + HO− br (4)

Reactions 1 and 2 reflect the molecular adsorption of methanol on the surface of oxidized TiO2 through the formation of a hydrogen bond to free surface hydroxyl groups or with a bridging oxygen anion (Obr2−),79 respectively. Reactions 3 and 4 show the extraction of a proton from the methanol hydroxyl group to produce a methoxy species. The methoxy group appears to bond to a single 5-fold coordinated 4+ Ti5c cation (reaction 3) as a monodentate species or to two 4+ neighboring Ti5c sites (reaction 4) in a bridge-bonded configuration. In both cases, the proton depleted from methanol adds to a bridge oxygen anion to form another surface hydroxyl group HObr− . The results described here, along with previous reports,30,53,54,60,69,73,75,80,83,88−90,93,96−98 indicate that the methoxy adsorbates are very stable at room temperature. Studies of the thermal stability of methanol on TiO2 surfaces have shown that molecular desorption (the reverse of reactions 1 and 2), recombinative desorption of methanol from methoxy and hydroxyl constituents (the reverse of reactions 3 and 4), and decomposition of methoxy adsorbates, occur at elevated temperatures. Molecular and recombinative desorption of methanol from the defective rutile TiO2(110) surface occurs at ∼320−380 K.93,96 Parent methanol desorption at 350−400 K and recombinative desorption at 400−570 K have been observed for powder anatase TiO2.89,99 For anatase, conversion of methoxy groups into formaldehyde has been shown to occur over the temperature interval 350−500 K, dimethyl ether appears above 500 K, CH4 and CO appear above 575 K, and H2 appears above 600 K.89 Above 600 K, the remaining Ti−OCH3 groups have been shown previously to undergo combustion to form CO and H2O.89 Similar results have been reported for rutile80 and for Degussa P25 TiO2 particles.60,73 These previous

(1) 6626



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Figure 2. Difference spectra for TiO2 particles with an initial fractional coverage of methanol/methoxy groups of Θa = 0.4. (A) Upon exposure to 200 mW cm−2 UV light (3.0−5.0 eV) under vacuum; (B) upon removal of the UV excitation source under vacuum. The rise in the signal across the entire mid-IR range is due to effective hole trapping during UV irradiation under anaerobic conditions.

studies clearly show that TiO2 materials exhibit poor roomtemperature activity for the decomposition of methanol-derived species. However, when excited with supra-bandgap UV light at room temperature, TiO2 is known to behave as an active photodecomposition and photooxidation catalyst.4−7,60,69,72,75 Photodecomposition: Charge Trapping and Methoxy to Formate Chemistry. Following the initial dissociative adsorption of methanol, we began to explore the photochemistry of the surface adsorbates by recording infrared spectra in situ with UV-excitation of the particles. Figure 2A shows the response of the methanol/methoxy covered particles to UV excitation. Clearly, the most significant change to the spectra during irradiation is the large increase in absorbance across the entire spectrum, with a broad maximum at approximately 1800 cm−1.100,101 As observed in recent studies, this prominent IR absorption reflects the composite effect of two distinct optical processes: the direct infrared excitation of shallow-trapped (ST) electrons and the acoustic phononmediated infrared excitation of free conduction band (CB) electrons (see Figure S2 in the Supporting Information for the assignments of these two components). Although detection of excited electrons in n-type TiO2 has been accomplished previously, the rate of e−−h+ pair recombination usually precludes their observation at sample temperatures above 200 K.17 The data presented in Figure 2A indicate that excited electrons can have extremely long lifetimes, even at room temperature. That is, we find that efficient trapping of holes by the surface methoxy groups enables IR observation of the UVexcited electrons, the initial step in titania photochemistry. These findings are in agreement with recent theoretical studies of the electronic structure of methanol derived adsorbates on stoichiometric anatase(101)97 and rutile(110)98 TiO2 surfaces. Most importantly, it has been shown that the highest-occupied molecular orbital of specific methanol derived surface bound species exists just below the valence-band maximum in TiO2. These previous studies also show that the methoxy group produces a more favorable hole-trapping site than methanol.98

The efficient methoxy group trapping of holes and extended lifetime of excited electrons are clearly evidenced by the difference spectra, labeled b and c, in Figure 2A. As shown for spectra b and c, the population of both CB electrons (blue lines) and ST electrons (red lines) increases steadily with UV exposure. When the photogenerated holes are trapped by methanol/methoxy species, the complementary electrons that survive fast e−−h+ recombination become free CB electrons or are trapped (within picoseconds)6 at surface defects that might include morphological discontinuities, vacancies, interstitial or impurity atoms, etc.7,71 The shallow trap electrons populate states at energies 0.2−0.3 eV below the conduction band and become excited into the CB via an optical transition.25,102,103 When the excitation source is removed, the intensity of both absorbances returns slowly (within minutes) to the baseline (Figure 2B) as the electrons and holes eventually recombine. In addition to insight into the electronic nature of the particles, the spectra of Figure 2 highlight important information about UV light-induced conversion of adsorbed methanol/methoxy surface species. The spectrum labeled, a, in Figure 2A shows that no spectral changes, beyond those shown in Figure 1, occur for extended periods of time (>60 min) following initial exposure to methanol. However, conversion of surface methoxy species is initiated upon exposure to UV radiation (spectra b and c in Figure 2A). Upon irradiation, the photochemistry of surface adsorbates is revealed by the positive bands at 2862, 1568 cm−1 and the doublet at 1379 and 1360 cm −1 (Figure 2B) that are assigned to surface formates,94,104−106 most likely bound as a bidentate-bridge species.107−109 These spectral changes reflect a photochemical conversion of surface methoxy groups into surface-bound formates. The intensity of the IR bands for the methoxy groups at 2925, 2825, 1456, and 1436 cm−1 (the νa(CH3), νs(CH3), δa(CH3), and δs(CH3) modes, respectively) exhibits complex behavior during the UV-light ON/OFF stages, as seen in Figure 2A and 2B, respectively. Specifically, the spectral features associated with the methoxy groups appear to decrease substantially during UV irradiation of the sample, resulting in 6627



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Interestingly, the production of formate also appears to be anticorrelated with the electronic excitation of the particles. In fact, the rate of formate production is accelerated and reaches a maximum (point e in Figure 3) only after the UV photon source is extinguished. This kinetic information can be used to develop insight into the mechanistic details of the methoxy/ TiO2 photochemistry. Two-Electron Oxidation Mechanism. On the basis of the observations in Figures 2 and 3 and the above discussion, we propose the following mechanism for the photooxidation of methoxy to formate on the surface of rutile TiO2 nanoparticles. First, the continuous irradiation of TiO2 particles with UV light, having supra band gap photon energy, generates the charge + carriers: valence band holes (hVB ) and conduction band − electrons (eCB ) (reaction 5):

negative changes in absorptivity, while the background rises due to electron accumulation. However, the extent of the negative features in the difference spectra, especially in the CHstretching region, diminishes following removal of the excitation source and charge neutralization (spectrum f of Figure 2B). The nearly complete recovery of the IR modes associated with methoxy groups upon removal of the UV photon source suggests that these effects are due to the electric field changes within the particles, rather than significant chemical reactions or desorption. (Note that these measurements were performed under vacuum, where readsorption of background gases is minimal.) Such behavior has been observed in numerous previous studies and is well-known to be due to a trapped-carrier induced Stark effect.110 The Stark effect describes how the electric field, which develops within the particles as the result of electrons accumulated in a semiconductor, affects the electric dipole moment of an adsorbate.110−115 The very small frequency shift of methoxy group bands [1−3 cm−1; blue for the ν(CH3) and red for the δ(CH3) modes], together with the absence of peak broadening (see the inset in Figure 2A), imply that the electric field is homogeneous and its strength is relatively weak.111 The Stark effect, due to the electric field of excess CB and ST electrons, is more clearly highlighted in Figure 3, which shows the

hν> 3.02eV

+ TiO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ TiO2 (e− CB + h VB)

(5)

+ e− CB + h VB → annihilation + heat

(6)

− − − + + e− CB ↔ eST , eCB ↔ e DT , and h VB → hT

(7)

Although most charge carriers quickly recombine (reaction 6), a fraction of the electrons and holes become trapped at the surface or in the bulk, as depicted in reaction 7. In the presence of surface methoxy species, holes may also become trapped by the adsorbate (reaction 8): 4+ 4+ • Ti5c −O−CH3 + h+ VB → Ti5c −OC H3

(8)

The scavenging of holes by methoxy groups inhibits − + recombination between eCB and hVB , effectively increasing the population of free CB electrons as well as the population of available ST states in TiO2. The evidence for these reaction steps is provided in Figure 2A. That is, we observe efficient and stable charge separation, as revealed by the broad infrared absorption, which occurs due to trapping of photogenerated holes that prolongs the lifetime (and hence detection efficiency) of the free and trapped electrons. Following photogeneration of e−−h+ pairs and subsequent hole trapping by methoxy groups, the unstable methoxy radical 4+ likely decomposes via a radical anion intermediate, Ti5c − •− OC H2 (reaction 9), which readily transfers an electron into the CB of TiO2, as depicted in reaction 10:

Figure 3. Kinetics of electron accumulation (points a−d) and depletion (points d−f), as measured by the IR absorbance at 1800 cm−1, during the UV excitation and in the dark, according to the spectral changes of Figure 2. The changes in the integrated IR absorbance (CH-stretch) for methoxy groups (mainly due to a Stark effect-induced electric field) and the changes in the IR absorbance of formate groups (COO-stretch, accumulated as a product of methoxy group photooxidation) are also shown.

anticorrelation between the electronic excitation (blue squares) during UV illumination (beginning at point a) and the changes in the methoxy modes (green triangles). When the UV light is turned off at point d, the IR electronic signal decays due to e−− h+ recombination. This charge neutralization reduces the local electric field within the particles and the intensity of methoxy bands is largely restored (points d−f in Figure 3), except for the small reduction in final signal due to the conversion of methanol to formate. In addition to the changes in methoxy absorptivity due to the Stark effect, we observe conversion of a small fraction (approximately 3%) of the methoxy groups to formate species, the kinetics of which are also plotted in Figure 3 (red circles).

4+ 4+ Ti5c −OC•H3 → Ti5c −OC•−H2 + H+

(9)

4+ 4+ Ti5c −OC•−H2 → Ti5c −OCH2 + e− CB

(10)

H+ + O2br− → OH− br

(11)

Reactions 8−10 compose a two-electron oxidation process116,117 in which surface methoxy groups are converted to 4+ Ti5c −OCH2. Analogous photooxidation reactions of organics (including methanol, formate, and formaldehyde) have been observed to produce the so-called current-doubling effect on many metal oxide photoanodes (TiO2, ZnO, etc.).76,78,116,118,119 The absence of IR signal in the carbonyl region (1750−1650 4+ cm−1) of Figure 2 indicates that the Ti5c −OCH2 groups are either not readily converted into formaldehyde intermediates or, if formed, their surface concentration is low due to rapid desorption.60,75 6628



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Figure 4. Comparison of IR spectra observed (A) in the CH-stretch region for methoxy groups (before UV irradiation) and (B) in the COO-stretch region of formate groups (after UV irradiation) for two different initial methanol/methoxy coverages on TiO2 nanoparticles. (C) The kinetics of electron accumulation and depletion for the two different initial methanol/methoxy coverages. 4+ Our results suggest that, once formed, the Ti5c −OCH2 species are further oxidized to produce surface-bound formates, as revealed by the IR modes in Figure 2. Evidence for similar photooxidation pathways has been shown in other studies.74,76,78 The subsequent chemistry leading to formate production is summarized by the following reactions:

4+ 4+ • Ti5c −OCH2 + h+ VB → Ti5c −OC H2

(12)

4+ 4+ Ti5c −OC•H2 → Ti5c −OC•−H + H+

(13)

4+ 4+ Ti5c −OC•−H → Ti5c −OCH + e− CB

(14)

4+ 4+ Ti5c −OCH + O2br− → Ti5c −O−OCH + V − O

(15)

4+ 4+ 4+ Ti5c −O−OCH + Ti5c → (Ti5c −O)− 2 CH

(16)

were able to detect, by EPR, the formyl intermediate formed in methanol photodegradation on aqueous TiO2 colloids only at a very low temperature, 6 K. Furthermore, previous gas-phase studies indicate that, even on metal surfaces, these species are only detected as intermediates at temperatures below 200 K.106,107 As shown in spectrum f of Figure 2B, the fraction of methoxy groups that are converted into formate species after UVirradiation is low (∼3% as estimated by the depleted absorbance in the CH-stretching region). This result is in accord with other observations,60 which show that less than 5% of the initially adsorbed methanol can be photoconverted into products of methanol dehydrogenation on TiO2. Reaction 8 governs the dynamics of hole trapping during initial UV irradiation (points b,c in Figure 3), but the formate production channel, reactions 9−16, is accelerated only after the saturation of hole traps (points c and d in Figure 3). The overall rate of the sequential reactions 9−16 to produce surface formates may be slow because the chemistry depends on two key factors: (1) the number of available oxygen bridging sites for binding to the protons produced via reactions 9 and 13; and (2) the availability of surface sites for trapping the secondary electrons that are produced by reactions 10 and 14. Both of these types of sites (proton acceptor and electron traps), which are largely occupied during UV irradiation, appear to become available upon removal of the UV excitation source. In darkness, trapped electrons diffuse to recombine with trapped holes, and the electronic absorbance decreases (points d−f in Figure 3). This process produces empty electron traps that are required for the methoxy-to-formate conversion. As the e−−h+ recombination and the methoxy-to-formate conversion compete for holes, the rate of surface formate production passes through a maximum at point e and then slowly (points e and f in Figure 3) ceases. To summarize, we propose that the hole-induced conversion of methoxy into formate, according to reactions 9−16, is a process that depends on the availability of surface sites for trapping the secondary electrons and the number of available

The conversion of formyl intermediates into formate (reaction 15) requires interaction with surface oxygen ions (Obr2−) to create a surface oxygen vacancy (VO− ). Recently, we reported an energy barrier for the extraction of surface oxygen atoms by methoxy groups of 33 kJ mol−1 on the surface of ∼25 nm P25 TiO2 nanoparticles.120 Although this energy is much lower than the 52 kJ mol−1 energy barrier reported for the oxygen exchange reaction between an 18O-enriched TiO2(110) surface and C16O at the same temperature of ∼400 K, the highly defective nature of nanoparticles likely has a significant effect on the activity of available oxygen. For the very small, 4 nm rutile particles used in the current study, we find that the oxygen near the surface is sufficiently active that reaction 15 occurs at temperatures as low as 300 K. Although formaldehyde and formyl species are likely intermediates toward the stepwise dehydrogenation of methoxy to formate, the low surface concentration and short lifetime of these intermediates precluded direct spectroscopic observation (see Figure 2). The signatures for formyl species are expected to appear in the IR spectrum around 1850 cm−1.121 Micic et al. 6629



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Figure 5. Comparison of the kinetics for electron accumulation and depletion (closed symbols) along with surface formate production (open symbols) during methanol photooxidation for the two different initial methanol/methoxy coverages, as shown in Figure 4. Normalized IR absorbances are used for electron accumulation/depletion and the corresponding relative integrated absorbances for formate production.

sites that can bind the product, H+. Hence, the trapping of electrons is the key step in the mechanism of formate production. Such a mechanism would suggest that much more effective methoxy-to-formate-conversion would be achieved if the excess photogenerated electrons and the secondary electrons were efficiently removed from the electron trapping sites, a hypothesis that is explored further, below. Role of Surface Methoxy Coverage. Previous theoretical and experimental studies showed that Ti sites play the role of deep traps that capture excess electrons in the photoexcitation of TiO2.31,47,49 Interestingly, these Ti sites are also the location where methoxy groups reside following the dissociative adsorption of methanol. Therefore, we have explored the effect of methoxy group coverage on the efficiency of photogenerated e−−h+ charge separation and the subsequent conversion of methoxy to formate. The data presented in Figure 4 were obtained with rutile nanoparticles that were partially covered (Θa ≈ 0.15) or saturated (Θa′ ≈ 1) with methoxy adsorbates (Figure 4A). The IR signal from photogenerated electrons, which reflects the efficiency of charge separation, rises in direct proportion to the initial methoxy coverage, as shown in Figure 4C. That is, at the maximum, the IR signal for electronic excitation of the methoxy-saturated sample is 6 times greater than that obtained with the low-coverage sample, which reflects the ratio of the initial methanol coverages ∼6.7 (see Figure 4A). In addition, the inset of Figure 4C reveals that the initial kinetics of excess electron accumulation is much faster for the sample with the greatest coverage. In contrast to the near 1:1 rise in the population of excited electrons with methoxy coverage, the production of formate appears to be highly favored at low methoxy coverage, as highlighted in Figure 4B. The amount of adsorbed formate species obtained with the low-coverage sample is only a factor of 2 smaller than that produced with the saturated sample, although the methoxy coverages differ by a factor of 6.7. In the case of the methoxy-saturated sample, we estimate that only ∼6% of the initial methoxy coverage is photodepleted at point b′ in Figure 4C and very little formate production is observed throughout the first 30 min of UV light excitation of this

methoxy-saturated sample (see Figure 5A). The observation that high methoxy coverage actually limits the photooxidative production of formate provides support to the two-electron process proposed above. Specifically, the methoxy-to-formate conversion appears to require the availability of neighboring nonoccupied Ti sites to accommodate the secondary electrons generated by reactions 10 and 14. However, at high initial coverage, these sites are occupied by methoxy groups, and, as a result, the conversion of methoxy to formate is obstructed. In addition to the relative product formation rates, the IR spectra provide important insight into the rate of excited electron depletion. The e−−h+ recombination (reaction 6) annihilates the charge carriers by a reaction of the type A + B → 0 that is analogous to diffusion limited bimolecular recombination, but in a dispersive medium.122 As discussed in more detail in the Supporting Information (section 3), we find that the decay of IR electronic absorption observed at both methanol coverages can be described by quasi-first-order kinetics, that is, A = A0e−t/τ, according to the random walk model, which is shown by the insets of Figure 5. Methoxy Photodecomposition under Aerobic Conditions: O2 as an Electron Scavenger. The results shown in 4+ the previous section support the initial hypothesis that the Ti5c surface sites play a role in the fate of surface-bound methoxy species in two key ways. First, the Ti sites serve as the primary binding locations for methoxy groups upon the initial dissociative adsorption of the parent methanol (reactions 2− 4). The methoxy groups then participate as efficient hole traps during UV irradiation of the sample (reaction 8). Second, open Ti sites adjacent to adsorbed methoxy groups serve as trapping sites for the secondary electrons released by the conversion of methoxy groups to formate (reactions 10 and 14). To further test this hypothesis, we employed oxygen, a well-known electron scavenger,17,19,67,76 to remove electron density from the operative Ti sites. By employing oxygen to rapidly deplete the surface charge, one predicts that the equilibrium for reactions 10 and 14 would shift to the product side of the reaction, thereby enhancing the overall chemistry and ultimate formation of surface formates. 6630



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Figure 6. The role of O2 as an electron scavenger. Difference IR spectra, obtained during continuous UV irradiation of methanol/methoxy saturated TiO2 nanoparticles, demonstrating the depletion of excited electrons and the simultaneous growth of formate upon introduction of gas-phase oxygen (points b−d, panel A). Upon evacuation of oxygen (points d−f, panel B), the electronic absorbance is restored. Panel C summarizes the key changes in these spectra by showing the time evolution of the IR signal due to electronic excitation (blue symbols) and formate production (red symbols).

The data for the entire 175 min of UV exposure are summarized in Figure 6 C. In this figure, the IR intensity for the electronic population and the formate production are plotted as a function of time during three cycles: (1) in vacuum (points a and b), (2) upon introduction of O2 (points c and d), and (3) following evacuation of the O2 (points d−f). The surface coverage of methoxy-converted formate species rises almost linearly during the 60 min irradiation of the TiO2 sample in the presence of O2 (points c and d in Figure 6C), indicating a steady-state process according to reactions 9−16. Importantly, spectra c and d in Figure 6A do not show features due to surface species or gas-phase products that can be assigned to further oxidation of formate in the presence of O2 over the UVirradiated TiO2 sample. That is, the spectrum of the TiO2 sample obtained at point d (see Figure S2) after 60 min of UV illumination in the presence of gas-phase O2 shows only the presence of formate, which indicates that the O2•− formed during electron transfer does not react further with surface adsorbates over the duration of these measurements. Upon return of the background IR signal to baseline (see point d in Figure 6C), indicating near complete quenching of the UVexited electrons and maximum methoxy conversion, the system was rapidly evacuated (under continued UV exposure). Spectra d−f of Figure 6B and the corresponding rate data (points d−f) of Figure 6C show that, during evacuation, the broad IR baseline absorbance promptly returns, albeit at slightly reduced intensity. The slight reduction in the IR signal due to electronic excitations is attributed to the decrease in methoxy coverage, which reduces the number of hole traps, thereby increasing the rate of e−−h+ recombination. Photoinduced Methoxy to Formate Conversion in the Presence of Oxygen. While the previous section focused on the sequential introduction of UV radiation followed by oxygen exposure, Figure 7 shows the experimental data for UV irradiation of a methoxy-covered sample in the presence of oxygen, as would be used in a typical photocatalytic reactor. Initially, the exposure of TiO2 to gas-phase oxygen (prior to UV irradiation) did not produce new species (within the detection

The black trace in Figure 6 A (spectrum b) shows the infrared spectrum for a methoxy-saturated rutile TiO2 sample after 60 min of continual UV exposure in vacuum. As in Figure 2A, this spectrum shows the characteristic broad absorbance due to electronic excitations within the semiconductor. Following saturation of the electronic signal, the sample was exposed to 3 Torr of O2 for 60 min, while maintaining the constant UV exposure. Spectrum c in Figure 6 A, the first spectrum recorded immediately upon introduction of oxygen, shows rapid quenching of the broad electronic IR signal, as the spectral background returns to near baseline. The fast quenching is due to efficient interfacial transfer of the free CB and ST electrons to adsorbed O2, reactions 17 and 18: •− e− CB + O2 → O2

(17)

•− e− ST + O2 → O2

(18)

Ti 3 + + O2 → Ti4 + + O•− 2

(19)

An analogous description of O2-electron scavenging on titania has been previously observed by IR17,67,114,123,124 (reactions 17 and 18) and by EPR17,19 (reaction 19) for excited electrons in TiO2 nanoparticles. Specifically, deeply − trapped electrons (eDT ) detected as Ti3+ species by EPR were found to be readily transferred to O2 to produce two long-lived O2•− surface species associated with Ti surface sites.17 In support of the hypothesis for the role of open Ti sites in formate production, the sequence of spectra labeled c and d in Figure 6A shows a dramatic, 1 order of magnitude, increase in the IR intensity for peaks associated with the surface-bound formate in the presence of O2. For clarity, a direct comparison of the intensities of the peaks for the formate species, measured at point c (in a vacuum at 1 × 10−6 Torr) and d (in the presence of 3 Torr O2), is provided in the Supporting Information, Figure S3. In addition, this figure provides data from spectrum b′ of Figure 4B, which shows much less formate production under anaerobic conditions even after 220 min of reaction time. 6631



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The existence of isosbestic points, as highlighted in Figure 8A, together with the emergence of new formate modes in the lowwavenumber region of Figure 8B, indicate that methoxy groups transform directly into the formate product. As seen in Figure 8B, formates are the dominant species of methoxy photooxidation, and only trace amounts of surface carbonylcontaining species (1670 cm−1) and adsorbed molecular water (1635 cm−1) are observed. The kinetics of methoxy-toformate conversion during the 60 min UV irradiation of the rutile particles is presented in Figure 8C, which shows the corresponding normalized integrated absorbances from Figure 8A and B. As the surface concentration of methoxy groups decreases, the concentration of formates rises. The two kinetic curves of Figure 8C cross precisely at the point where conversion is 50% complete, which also suggests the direct conversion of methoxy into formate, with limited production of intermediates or other side reactions. Furthermore, we note that the kinetics of associated OHH‑bond group formation closely follows the kinetics of formate production (compare the blue and red lines in Figure 8C). The absence of negative bands in the IR region of OHfree vibrations indicates that the observed positive band at 3480 cm−1 is most likely due to newly formed OHH‑bond groups via reaction 11. Comparisons to Previously Proposed Mechanisms. One of the key observations regarding the mechanism of methanol photodegradation is the discovery that, under both anaerobic and aerobic conditions, the dominant decomposition pathway is the conversion of methoxy groups into formates that accumulate and persist on the surface of TiO2 during UV illumination. This result is highlighted by the comparison of spectrum b′ from Figure 4B to spectrum p from Figure 7, which is shown in Figure S4 of the Supporting Information. This comparison also shows that only trace amounts of adsorbed carbonyl species (1670 cm−1) and water (1635 cm−1) are observed in the presence of O2 (see the inset of Figure S4). These observations suggest that the same mechanism drives the UV-light induced transformation of methoxy to formate under both anaerobic and aerobic conditions. As suggested by

Figure 7. Methoxy to formate conversion in the presence of oxygen. Spectral developments during UV irradiation of methanol/methoxy saturated TiO2 nanoparticles in the presence of gas-phase oxygen.

limits of our spectrometer). However, upon initiation of UV irradiation, major spectral developments emerged. Figure 7 shows that several new peaks developed during UV irradiation, including negative bands at 2912, 2812, 1162, and 1077 cm−1, which reflect the consumption of methoxy groups in the photocatalytic production of surface formates. We estimate an efficiency of ∼15% for the methoxy-to-formate photoconversion, based on the integrated absorbance of depleted methoxy groups. The efficiency of methoxy photoconversion increases dramatically, by a factor of 5, in the presence of O2 as an electron scavenger. The newly produced formates are evidenced by positive bands at 2862, 1565, 1380, and 1359 cm−1. In addition, the positive broad band with a maximum at 3480 cm−1 is assigned to the production of newly formed OH groups that are associated with hydrogen bonds. Figure 8A and B more clearly highlights the changes in the spectral regions of CH-stretch and CH-deformation modes for this series of experiments, detailing the spectroscopic evidence for the conversion of methoxy groups into surface formates.

Figure 8. Scaled view of the spectra from Figure 7. Spectroscopic evidence for the conversion of methoxy groups into surface formate species in the presence of O2: (A) Changes in the IR absorbance of CH-stretch modes; (B) changes in the IR absorbance of COO-stretch modes; and (C) kinetics of methoxy-to-formate conversion. 6632



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neously characterizing the sequential development of products, will continue to advance the understanding of the roles of holes, excited electrons, and surface adsorbates in controlling photocatalytic process at the surface of titania.

reactions 9−16, the photodegradation follows hole-mediated oxidation of the surface organics via a two-electron process, involving formation of a radical through hole oxidation followed by prompt electron injection by the radical. The direct hole-mediated pathway involving an organic radical intermediate has been previously proposed for both gas/ surface69,75 and aqueous72,76,78 photocatalytic oxidation of methanol. However, the work shown here may be the first to provide direct spectroscopic evidence in support of the twoelectron photooxidation mechanism for methoxy to formate conversion in gas-surface reactions on TiO2-based materials. Under aerobic conditions, many researchers have shown indirect evidence that O2 participates as the primary oxidant in the conversion of surface organics.69,73,75 However, the results presented here clearly show that O2 plays a much less direct role in the overall photochemistry. Specifically, we find that the key role for O2 is as an electron scavenger. By extracting the photogenerated electrons (reaction 5) and the secondary electrons (reactions 10 and 14) from the TiO2 surface, O2 increases the lifetime of photogenerated holes and thereby increases the probability of secondary electron injection (radical oxidation) into the semiconductor. In this study, we observed only trace amounts of adsorbed carbonyl species and water that could be attributed to the incorporation of oxygen from the gas phase into the products. Apparently, the primary role of gas phase oxygen, in this system, is to extract electrons, but the resulting superoxide is not involved further in the surface chemistry. As indicated in refs 64 and 70, the photooxidation of methanol in the presence of oxygen is actually a very slow process. Previous studies suggest that the phenomenon of slow oxidation is “remote” in the sense that it occurs at regions of a TiO2 sample that are not directly exposed to light.2 Our results appear to be consistent with the hypothesis that a slow “remote” oxidation pathway plays a minor role in the overall chemistry to produce trace amounts of surface carbonyls and H2O.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Transmission electron microscopy (TEM) image and particle size distribution of as-prepared 4 nm rutile TiO2. (2) Charge separation in supra-bandgap UV-irradiated TiO2 rutile nanoparticles. (3) The long-lived electrons decay process. (4) The role of O2 as an electron scavenger. (5) References, and (6) complete ref 79. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the Army Research Office, W911NF-09-1-0150, and the Defense Threat Reduction Agency, W911NF-06-10111, is gratefully acknowledged. We would like to thank Professor Brian M. Tissue for many helpful discussions and generous use of his research group’s laser-evaporation nanoparticle synthesis apparatus.

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REFERENCES

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SUMMARY The data from this work show that methanol adsorbs predominantly via a dissociative pathway on the surface of 4 nm rutile nanoparticles, to produce surface methoxy and hydroxyl groups. These surface methoxy groups serve as effective hole trapping centers under irradiation of TiO2 with UV light. The hole trapping by methoxy groups provides effective charge separation that was tracked in these studies by the IR absorption of long-lived free and shallow trapped electrons. Trapped holes were found to induce a two-electron oxidative degradation process of surface methoxy groups to formates, involving formation of a radical anion, through hole oxidation, followed by prompt electron injection by the radical anion into the TiO2. We propose that, under both anaerobic and aerobic conditions, the major oxidation agent in this system is the photogenerated hole. The main role of adsorbed oxygen is to remove the photogenerated electrons by forming superoxide. As a consequence of electron extraction, the separation of charges is further improved, which, in turn, leads to a 5-fold increase in formate production. Overall, these results provide new insight into and help to establish a mechanism for the photooxidation of methanol on nanoparticulate titania. Hole-mediated reactions may also govern the photochemistry of other small organic compounds in this important catalytic system. Future work that probes electronic transitions within the semiconductor, while simulta6633



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