Final State Distributions of Methyl Photoproducts from the

Article pubs.acs.org/JPCC

Final State Distributions of Methyl Photoproducts from the Photooxidation of Acetone on TiO2(110) Daniel P. Wilson,† David Sporleder,†,§ and Michael G. White*,†,‡ †

Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States

‡

ABSTRACT: The UV photooxidation of acetone on a reduced TiO2(110) surface was investigated using a combination of photodesorption and thermal desorption measurements and pump−probe laser detection of gas-phase products. In agreement with earlier studies, acetone adsorbed on TiO2 does not undergo a UV photoreaction unless codosed with molecular oxygen. The only gas-phase photoproducts are methyl radicals originating from fragmentation of the active acetone surface species and photodesorbed molecular oxygen. Postirradiation TPD measurements show that acetate is the primary surface product remaining after photooxidation. The dependence of the methyl radical formation rate on oxygen and thermal pretreatment of the TiO2 surface is consistent with the formation of an acetone−oxygen (diolate) complex involving adsorbed acetone and oxygen adatoms. Pump-delayed-probe laser techniques were used to measure the velocity and translational energy distributions of methyl radicals resulting from fragmentation of the acetone diolate. The observed translational energy distributions are well described by empirical fits involving two components with average energies of 0.19 eV (“fast”) and 0.03 eV (“slow”). The latter are found to be insensitive to surface temperature or preannealing conditions, suggesting that the “fast” and “slow” components represent different final states of methyl radicals originating from fragmentation of a single photoactive species. The methyl kinetic energy distributions were also found to be independent of UV pump energy which is consistent with a substrate-induced process involving thermalized charge carriers, electrons or holes, which transfer to the acetone diolate to induce fragmentation. The results are discussed in terms of probable substrate-induced photoreaction mechanisms and analogous molecular photofragmentation processes.

I. INTRODUCTION Titania is finding increased application as a photooxidation catalyst where it is used to oxidize and remove organic pollutants in wastewater or air or to create self-cleaning surfaces.1,2 In general, the reaction pathways for photooxidation of organic molecules on titania are complex with many possible side branches and intermediates.3−5 This work is focused on photodecomposition of acetone on titania as it serves as a model for interactions of organic molecules containing carbonyl functional groups and is a common pollutant in indoor air. The majority of previous studies of acetone photooxidation have been performed on high surface area titania powders, with a focus on evaluating different catalyst or catalyst preparation methods or on the kinetics of the overall photooxidation to CO2.6−8 Mechanistic information can be inferred from spectroscopic studies on titania powders using EPR,9 FTIR,10 or NMR11 techniques which have identified multiple reaction intermediates including the peroxy species CH3(CO)CH2OO•, acetate, formate, acetaldehyde, and formaldehyde, and a few dimerized acetone species. These results suggest a relatively complex reaction mechanism involving many elementary and secondary reaction steps. Mechanistic studies performed on TiO2 single crystal surfaces under UHV conditions greatly simplify the reaction system by providing a well-defined substrate and better control © 2012 American Chemical Society

over the ketone and oxygen concentrations. The relevance of photooxidation investigations using UHV and TiO2 single crystals has been demonstrated by the identification of radical intermediates which also appear in powder studies.6 Henderson has studied the photooxidation reactions of a number of simple ketones (acetaldehyde,12 acetone,6 2-butanone,13 acetophenone,5 acetyl chloride,5 several chloroacetones,5 hexafluoroacetone14) on single crystal TiO2(110) surfaces using mass spectrometery to detect the gas-phase products as well as temperature-programmed desorption (TPD) to probe the surface products after UV irradiation. For these studies, a CW Hg arc lamp was used for the UV photoexcitation source. In general, it was found that the ketones chemisorbed on the TiO2(110) surface are themselves not photoactive but required coadsorbed O2 and thermal activation before photodecomposition was observed. The need for coadsorbed O2 requires the use of defective (reduced) surfaces created by vacuum annealing since O2 does not adsorb on fully oxidized TiO2(110) surfaces at temperatures above 100 K.4,15−17 The role of coadsorbed oxygen is the formation of a thermally activated ketone−oxygen complex (≥150 K) that was Received: April 24, 2012 Revised: June 19, 2012 Published: July 11, 2012 16541

dx.doi.org/10.1021/jp303947q | J. Phys. Chem. C 2012, 116, 16541−16552

The Journal of Physical Chemistry C

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postulated to be the photoactive species.18 In the case of acetone, TPD and HREELS studies of coadsorbed acetone and oxygen adlayers indicated that the photoactive species has an η2-acetone diolate structure.6,18 Interaction of the η2-acetone diolate species with charge carriers (electron−hole pairs) produced by UV photoexcitation of the TiO2 substrate results in α-carbon bond cleavage (breaking of a carbonyl carbon− carbon bond) yielding a surface bound acetate and ejection of a gas-phase methyl radical. The two step mechanism can by represented by the following reactions: η1‐acetone + oxygen* → η2‐acetone diolate

(1)

η2‐acetone diolate + hν → •CH3(g) + CH3COO(a)

(2)

(CMA; Physical Electronics) for Auger electron spectroscopy (AES), and a low-energy electron diffraction (LEED; Princeton Research) instrument. Photochemistry measurements were performed on the lower level which houses the time-of-flight mass spectrometer (TOF-MS) for species identification and velocity measurements. The sample holder is mounted on a liquid nitrogen cooled manipulator which rests on a xyztranslator with azimuthal rotation for precise alignment and movement between the two levels of the main chamber. An orientated, 10 × 10 × 2 mm rutile TiO2(110) crystal (CrysTec) was used in these experiments. Molybdenum (Mo) clips are used to clamp the crystal on top of the button heater which was held in a Mo ring attached to the manipulator by threaded Mo support rod. For improved heat transfer, Au foil was sandwiched between the button heater and the crystal. The temperature was measured with a type K thermocouple (chromel−alumel alloy) that was inserted into a small hole in the edge of the crystal and held in place with high-temperature ceramic cement (Omegabond 600). A crystal temperature of 100−900 K could be achieved with this sample mount. The TiO2 crystal surface was cleaned by successive cycles of low-energy sputtering with 500 eV Ne+ ions for 10 min, annealed to 850 K in vacuum for 30 min, and then annealed at 850 K in the presence of O2 gas (2 × 10−6 Torr). Defects states, primarily bridged-oxygen vacancies (Obr(V) and interstitial ions (Ti3+(i)), were introduced into the near surface region by annealing in vacuum at 850 K.23 A 10 min vacuum anneal at 850 K was also performed between consecutive TPD or photochemistry measurements in order to replenish the defect population since adsorbed O2 plus heat or UV radiation oxidizes the surface and fills the oxygen vacancies.15 The density of bridged oxygen vacancies (Obr(V)) on our crystal is estimated to be 6% based on D2O TPD measurements.23 To investigate the photodecomposition of acetone, the reduced TiO2(110) surface at 100 K was first exposed to a saturation coverage of molecular oxygen (80 langmuirs) using background dosing, followed by exposure to acetone via a directional doser. The directional doser consists of a 1 cm diameter stainless steel tube coupled to a small pinhole aperture that could be placed close to the surface for acetone adsorption. Photoexcitation of the adsorbate−oxide interface was induced by irradiating the sample surface at 45° with pulsed UV laser radiation generated by doubling the output of a Nd:YAGpumped dye laser (Spectra-Physics GCR-190; Sirah dye laser). Two Glan-Thompson polarizers (Lambrecht) in the UV beam path were used to establish p-polarized light relative to the TiO2 crystal face (along ⟨110⟩ direction) and allow for adjusting the UV pump energy. Calibrated fused-silica neutral density filters were also used to attenuate the UV laser energy to the desired level. Typical photon fluences used for photochemistry experiments were 1013−1014 photons/(cm2 s), which corresponds to an average fluence obtained by multiplying the per pulse value by the repetition rate (20 Hz). Gas-phase products ejected from the surface during UV excitation were detected by nonresonant, one-photon ionization using coherent VUV radiation and TOF-MS. For the acetone studies presented here, a VUV photon energy of 13.09 eV (94.74 nm) was used for ionization of desorbed products. The VUV radiation was produced by nonresonant third harmonic generation (THG) by focusing the doubled output of a Nd:YAG-pumped dye laser system (Spectra-Physics GCR230; Laser Analytical Systems, LDL-20505; Rhodamine 590 dye) at 284.2 nm into a pulsed-jet expansion of N2 gas. The

where oxygen* is an active oxygen species, e.g., O-adatoms or O2(ad). The methyl radical is ejected into the vacuum, and the remaining acetate species was identified by thermal desorption of ketene near 600 K which results from the thermal decomposition of acetate on reduced TiO2(110) surfaces.6,18 The photoreactions of other small ketones on TiO2(110) were also found to be consistent with the two-step mechanism, in which a gas-phase radical species is ejected from a thermally activated ketone diolate species by photofragmentation.5,13,19,20 In cases where α-cleavage could lead to two distinct radical products, e.g. methyl or ethyl radicals from 2-butanone, one radical loss channel is favored over the other.13 Recent DFT calculations suggest that the relative C−R bond energies and stabilities of the remaining carboxylate surface fragment determine which radical product is favored.20 In this work, we investigated the reaction dynamics of the photodecomposition of acetone on TiO2(110) surfaces using a two-color, pump−probe laser technique for initiating the reaction (pump) and detecting the gas-phase products (probe). For these studies, time-of-flight (TOF) mass spectra and postirradiation TPD were employed for identification of products formed in the primary photooxidation channels. Pump-delayed-probe studies were also used to determine final state velocity (or translational energy) distributions of the desorbing alkyl radical species. Translational energy distributions of gas-phase photoproducts reflect how the initial excitation energy is partitioned into product degrees of freedom.21−23 The latter are sensitive to the initial states of the photoactive molecule as well as any secondary reaction processes such as trapping-desorption or surface reactions. In addition, adlayers of O2 on TiO2 were prepared under different conditions to probe the identity and role of the oxygen species involved in thermal activation of the adsorbed ketone.

II. EXPERIMENT The surface photochemistry instrumentation used for these experiments has been described in detail elsewhere.23 Briefly, the apparatus consists of a surface science chamber with a base pressure of about 3 × 10−10 Torr and a smaller chamber which serves as a windowless VUV source containing a pulsed valve for gases used for harmonic generation of VUV light. The two chambers are connected via a doubly differentially pumped capillary light guide to allow passage of the VUV probe radiation while maintaining the UHV environment of the main chamber. The upper level of the main surface science chamber holds instrumentation for sample preparation and characterization including a quadrupole mass spectrometer (QMS; Hiden Analytical) for TPD studies, a cylindrical mirror analyzer 16542



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decomposes to ketene (parent ion, mass 42) above 500 K.6 The coadsorbed adlayers were prepared on a vacuum annealed TiO2(110) surface at 100 K by first adding a saturation dose of molecular oxygen (80 langmuirs) followed by ∼1.1 ML of acetone. The acetone coverage was estimated from comparisons with earlier TPD studies of Henderson.18 According to that work, the main desorption feature at 265 K corresponds to acetone binding at Ti4+ 5-fold coordinate sites (Ti4+(5f)) in the troughs of the (110) surface through the oxygen lone pair electrons in a η1 configuration. The peak near ∼170 K was attributed to compressive strain caused by filling all Ti4+(5f) sites and thereby appears at near-monolayer coverage. In this work, the low-temperature peak at 125 K does not saturate at higher exposures and is attributed to multilayer formation. The small shoulder and peak at ∼355 K is close to the desorption feature (∼375 K) observed by Henderson for acetone−oxygen adlayers which he attributed to the formation of the acetone diolate complex.6,18 The small peak at ∼400 K is observed in the absence of coadsorbed oxygen and is tentatively assigned to acetone binding at defects on the TiO2 surface (e.g., steps).32 At temperatures above 500 K, several TPD peaks (575, 665, and 725 K) are observed that only appear in the presence of coadsorbed oxygen. These are attributed to thermal reaction products between acetone and oxygen induced by heating of the surface during TPD measurements. A thermal reaction peak near 660 K was also observed by Henderson and attributed to ketene desorption from acetate decomposition.6 The dashed curve in Figure 1 shows the mass 42 TPD spectrum of an identically prepared acetone/O2/TiO2(110) surface after UV exposure at 335 nm for ∼4 min. Below 500 K, the TPD spectrum of mass 42 follows the temperature profile of the acetone parent ion (mass 58) but has decreased in intensity due to photoreaction, primarily near the main feature at 265 K and at the peaks corresponding to the photoactive diolate (∼355 K). More striking is the increased intensity of the peak near 600 K, which is associated with ketene desorption resulting from thermal decomposition of adsorbed acetate.6,18 The overall background intensity in the TPD spectrum above 500 K has also increased, but the relative intensities of the thermal reaction peaks at 665 and 724 K are unchanged. The appearance of the acetate decomposition peak at 600 K is clear evidence for methyl loss to photoreaction and supports the proposed diolate intermediate as the photoactive acetone− oxygen species. B. Final State Distributions of Gas-Phase Products. The TOF mass spectrum resulting from UV exposure of an acetone/O2/TiO2(110) adlayer at 100 K and a pump−probe laser delay of 28 μs is shown in Figure 2. The energy of the pump laser (3.7 eV, 335 nm) is well above the rutile TiO2 band gap energy (3.05 eV)33 to form excited e−/h+ pairs for interaction with surface adsorbates. Two narrow mass peaks are observed in the TOF spectrum at mass 15 and 32, which are not present when the pump laser is blocked. Mass peaks assigned to VUV ionization of background gases H2, H2O, and O2 are also labeled in Figure 2. (The split feature at mass 1 corresponds to H atom fragments resulting from dissociative ionization of H2 with H+ fragments directed toward and away from the detector.) Other small peaks above mass 25 are also part of the background and are present with and without the pump laser (no surface excitation). The narrow peaks at mass 15 and 32 are attributed to photofragmentation of acetone to form methyl radical (CH3) and photodesorption of molecular oxygen (O2), respectively.6,13 Note that the TOF peak for O2

VUV radiation was coupled to the ionization region of the TOF-MS by a doubly differentially pumped Pyrex capillary (35 cm long, 1 mm i.d.) which acts as a light guide and a differential pumping barrier between the VUV generation and surface science chambers. Photons at 13.09 eV can ionize most of the relevant gas-phase species expected for photoinduced reaction and/or desorption, i.e., acetone (9.71 eV),24 methyl radical (9.84 eV),25 oxygen (12.62 eV),26 formaldehyde (10.88 eV),27 and ketene (9.62 eV).28 Carbon monoxide and carbon dioxide cannot be detected using this setup due to their high ionization potentials.29,30 Methyl velocity distributions were derived from mass 15 ion intensity measurements taken as a function of the delay time between the excitation laser and ionization laser. The desorbed neutral CH3 radicals travel 28 mm before being intercepted by the probe ionization beam, and thus the time delay between the two lasers provides a direct measure of the velocity of the desorbing neutral. In order to improve signal-to-noise for pump−probe delay scans, at least three or more measurements were made for each delay range and averaged together. A second type of measurement (depletion curve) was performed at a fixed delay to measure the decrease of the methyl yield as a function of time due to acetone photoreaction on the surface (see Figure 3). Signal intensities for delay measurements were corrected for the decrease of the photoactive acetone species by using rates obtained from fits of the depletion curves. Pump− probe delay scans were converted to flux distributions by weighting by t−1 to correct for laser ionization being a density sensitive detection method.31 Plots of the product flux versus pump−probe laser delay are referred to as arrival time distributions. The neutral flight time and distance were used with the relevant Jacobian to transform the arrival time distributions to velocity and translational energy distributions.

III. RESULTS A. Thermal Desorption. Thermal desorption spectra of coadsorbed acetone and oxygen from a reduced TiO2(110) surface are displayed in Figure 1 before (solid) and after (dashed) exposure to UV laser light at 335 nm (3.7 eV). The mass 42 ion product is shown for both conditions as it has the same temperature profile as the acetone parent below 500 K (mass 42 cracking fragment) and can be used to follow the formation of the acetate photoproduct which thermally

Figure 1. Thermal desorption spectra for mass 42 from a surface prepared by codosing 80 langmuirs of O2 and ∼1.1 ML of acetone onto a reduced TiO2(110) surface held at 100 K. Solid line: thermal desorption of as-prepared surface; dashed line: thermal desorption spectrum following exposure to UV light at 335 nm for 300 s. 16543



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Figure 4. Arrival time distribution for methyl radicals (mass 15) desorbed from a reduced TiO2(110) surface pre-exposed to 80 langmuirs of O2 and ∼1.1 ML of acetone at 100 K. The UV pump and VUV probe photon energies were 3.7 eV (335 nm) and 13.1 eV (94.74 nm), respectively. The data have been converted to a flux distribution and corrected for depletion of the reactant coverage during UV exposure. Filled circles represent the corrected data, dashed lines are empirical fits to the “slow” and “fast” components, and the solid line is the sum of the two components.

Figure 2. Time-of-flight ion spectra resulting from VUV ionization of gas-phase species produced during UV irradiation of a reduced TiO2(110) surface pre-exposed to 80 langmuirs of O2 and ∼1.1 ML of acetone at 100 K. The UV pump and VUV probe photon energies were 3.7 eV (335 nm) and 13.1 eV (94.74 nm), respectively, and the pump−probe delay was set at 18 μs. The ion peaks marked H+, H2+, H2O+, and ∗ originate from background H2, H2O, and O2 in the vacuum chamber, respectively.

after 32 laser pulses which results in time steps of ∼1.6 s/point in our depletion curves. As observed in many previous studies of O2 photodesorption and molecular phootooxidation on TiO2(110) surfaces, the photoyield curves are not well fit by a single-exponential decay (solid line, Figure 3).23,34,35,32,36 In this work, the depletion curves were fit reasonably well by biexponential decays (dashed line, Figure 3), and these fitted decay rates were used to correct signal intensities from pump− probe delay scans from which velocity distributions were derived. Velocity distributions of methyl radicals produced in the photodecomposition of acetone were determined from measurements of the CH3 ion yield as a function of pump− probe laser delay. Figure 4 displays the CH3 flux from a surface as a function of laser delay time, i.e., the arrival time distribution, under identical conditions as the depletion curve presented in Figure 3. The arrival time distribution for the CH3 product exhibits a relatively sharp peak at 18 μs and a broader feature near ∼40 μs which tails off to longer time delays. Assuming that these two features correspond to two product distributions, the arrival time distribution was empirically fit to the sum of two asymmetric peaks shown as dashed lines in Figure 4, with the overall fit given by the solid line. The corresponding velocity and translational energy distributions are shown in Figures 5a and 5b, respectively, along with the empirical fits. The CH3 velocity distribution (Figure 5a) is reasonably well described by the two component fit, with a “slow” product distribution that peaks at ∼630 m/s and “fast” product distribution with a peak at ∼1500 m/s. The average translational energy (⟨Et⟩) of the entire distribution is 0.161 eV, whereas the “slow” and “fast” components have average energies of 0.036 and 0.183 eV, respectively. The derived translational energies have an estimated uncertainty of ±0.005 eV. The average energies that characterize the methyl product translational energy distributions are summarized in Table 1. Figure 6 shows the effects of changing the pump photon energy (3.24, 3.70, 4.38 eV) on the translational energies of the methyl fragment for identically prepared surfaces of coadsorbed acetone and oxygen on a reduced TiO2(110) surface at 100 K (no preheating). The lowest pump energy (3.24 eV) is slightly higher than the TiO2 rutile band gap (3.1 eV), and the highest energy (4.38 eV) is likely above the work function of a TiO2(110) surface with a saturated coverage of adsorbates.37−39

desorbing from the surface is narrower and arrives at a slightly earlier time than O2 from the background (indicated by asterisk). The narrow width and TOF arrival time shifts reflect the velocity and angular distributions of the photoproducts which are directed primarily along the detector axis compared to the randomly moving background gases with a 300 K Boltzmann velocity distribution. Photofragment peaks can also be identified by their TOF dependence on the pump−probe delay which determines the neutral flight time (velocity) of detected photofragments and thereby their initial velocity along the detector axis. Figure 3 shows a typical photoyield versus UV exposure time (depletion curve) for the methyl radical product (mass 15) for

Figure 3. Photodesorption signal for methyl radical (mass 15) as a function of UV (335 nm) exposure of a reduced TiO2(110) surface pre-exposed to 80 langmuirs of O2 and ∼1.1 ML of acetone at 100 K. The pump−probe laser delay was set at 17 μs. Time zero corresponds to the introduction of the UV light. Filled circles are data points; solid line and dashed lines are single-exponential and a biexponential fits to the data, respectively.

an acetone/O2/TiO2(110) surface at 100 K. These data were taken with a pump−probe delay of 17 μs which is near the maximum intensity of the methyl pump−probe delay scan (see Figure 4). The UV radiation was introduced at time zero, and at 380 s the UV light was blocked. The drop in signal after the UV laser is blocked shows that photoproducts persist at relatively long times. The signal is sampled every 50 ms (determined by the 20 Hz repetition rates of the lasers), but the data are binned 16544



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These intensity ratios should be seen as only approximate due to the sensitivity of the fits of the slow channel arrival time distributions at long laser delays where the methyl signals are very low (≥70 μs; see Figure 4). The accumulation of small signals at long delay times become compressed into only a few energy intervals when the data are transformed from arrival time to translational energy, and this can lead to significant variations in the slow channel intensity. As seen from Table 1, variations of the intensities of the slow channel do not have a significant effect on the average kinetic energies for the “fast” channels or the overall distributions which are essentially the same for all pump photon energies. C. Role of Adsorbed Oxygen. To gain more insight into the nature of the oxygen species responsible for the formation of the acetone diolate intermediate, we examined the photoyield of methyl radical for different surface preparations of the acetone/O2/TiO2(110) surface. For this series of experiments, the reduced TiO2 crystal was exposed to a saturation dose (80 langmuirs) of O2 at a temperature of 100 K, flash-annealed to 225 or 410 K, allowed to cool again to 100 K, and then dosed with ∼1 ML of acetone. For a reduced TiO2(110) surface prepared by sputtering and high-temperature vacuum annealing such as that done here, dosing at 100 K is expected to result primarily in molecularly adsorbed oxygen species at and near Obr(V) vacancy sites as well at near surface interstitials, Ti3+(i).15,36,40,41 Preannealing the oxygen adlayer to 225 or 410 K will lead to dissociation of molecules at Obr(V) sites with one oxygen atom filling the vacancy and the other becoming an O-adatom bound to a nearby Ti4+ 5-fold coordinate site.15,42 More recent studies also suggest that at saturation coverage other chemisorbed oxygen species, e.g., O4(a), can form by annealing or photoreaction which are stable against dissociation, photodesorption, and thermal desorption.36,43 Molecular oxygen that is active for photodesorption thermally desorbs at 410 K, so that an oxygen adlayer annealed to 410 K should contain mostly O-adatoms (and any strongly bound oxygen species that are not photoactive).43 Hence, pretreatment of the oxygen adlayer provides a means to change the nature of the oxygen species with which coadsorbed acetone interacts to form the acetone diolate. Photoyield curves for methyl radicals generated for different oxygen adlayer preannealing conditions are shown in Figure 7. The measurements were taken at a fixed pump−probe delay of 18 μs, which corresponds to the peak of the arrival time distribution for the methyl radical photoproduct (see Figure 4). The pump energy was 3.7 eV, and the beam flux was 6 × 1014 photons/(s cm2). The beam block that controls the UV pump beam from entering the chamber is opened at time zero and closed at about 320 s. The bottom curve corresponds to a surface that was not exposed to molecular oxygen prior to acetone dosing and shows essentially no methyl radical products. All the experiments in which TiO2 was first exposed to O2 exhibit a spike in the methyl yield upon introduction of the UV light followed by a slower decay at longer times. Rates of the fast and slow decays were obtained by fitting the data to a biexponential, and the integrated yields were obtained by extrapolating the biexponential fits to zero signal. The results of the fits are given in Table 2. The initial rates and total yields of methyl production may be somewhat underestimated for the 225 and 410 K data since the initial burst of methyl product may have exceeded the sampling rate of the TOF detector (50 ms).

Figure 5. Velocity (a) and translational energy (b) distributions derived from the methyl arrival time distribution in Figure 4. The dashed and solid lines correspond to the empirical fits of the “slow” and “fast” components of the arrival time distribution (Figure 4) and transformed to the corresponding methyl velocity and energy distributions.

Table 1. Methyl Radical Translational Energy Distributions from Coadsorbed Acetone and Oxygen on a Reduced TiO2(110) Surface at Different Photoexcitation Energies photoexcitation energy (eV)

product species

overall ⟨Et⟩ (eV)

fast ⟨Et⟩ (eV)

slow ⟨Et⟩ (eV)

fast to slow intensity ratio

3.70 3.25 4.38 3.70

CH3 CH3 CH3 CD3

0.161 0.161 0.158 0.127

0.183 0.197 0.197 0.151

0.036 0.030 0.031 0.021

4.9 3.8 3.2 4.2

Figure 6. Translational energy distribution for methyl radicals resulting from UV exposure of a reduced TiO2(110) surface preexposed to 80 langmuirs of O2 and ∼1.1 ML of acetone at 100 K for three different UV photon (pump) energies.

For the purposes of comparison, only the overall fits to the experimental data are shown in Figure 6. The average translational energies and the fast-to-slow intensity ratios taken from the empirical fits are shown in Table 1. It can be seen that the energy distributions are very similar for the different pump energies, with only the low-energy peaks exhibiting noticeable variations in intensities. Although no clear trend is seen in the slow channel average translation energies, the fast-to-slow intensity ratios derived from the fits show a decreasing trend with increasing photon energy (see Table 1). 16545



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excess thermal energy that they can overcome the barrier to form acetone diolate species at 100 K.37,41 The latter could give rise to the initial burst of methyl photoproducts, but the long time decay of the methyl photoyield (see Figure 7) suggests another mechanism for creating photoactive acetone species after this initial O-adatom concentration is consumed. Photodissociation of O2(ad) could be a continuing source of Oadatoms for diolate formation during UV-irradiation and could be responsible for the slow decay kinetics observed for surfaces held at 100 K or preannealed to 225 K. The near absence of a slow decay component in the methyl photoyield for the surface preannealed to 410 K is consistent with this explanation since this surface contains no molecular oxygen that can be photodissociated to O atoms. Direct evidence for O2(ad) photodissociation comes from a recent study by Petrik and Kimmel,43 who found that a substantial fraction of the O2(ad) adsorbed at Obr(V) sites undergo UV photodissociation (20−50%, depending on initial coverage). Photodissociation was attributed to dissociative electron attachment of O22− species at Obr(V) vacancies leading to O2− that fills the vacancy and an O− adatom bound at a fivecoordinate Ti4+ site. Hence, it is likely that UV-induced photodissociation of O2(ad) contributes to the formation of acetone diolate intermediate, especially at temperatures below ∼150 K where thermal dissociation is limited. Similar measurements of the integrated methyl photoyield versus preannealing temperature were performed by Henderson, but in those experiments a saturation dose of O2 was warmed to room temperature to preoxidize the surface, i.e., dissociate molecular oxygen and fill the Obr(V) vacancies, followed by dosing the acetone at 100 K.6 The codosed surface was then preheated to various temperatures and cooled back to 100 K at which temperature the methyl photoyield measurements were taken. In this way, Henderson started with the same O-species on the surface (mixture of O-adatoms and O2(a)) and explored the thermal activation of the acetone diolate complex. The integrated methyl photoyield was observed to increase by a factor of ∼5 for preheating the acetone−oxygen adlayer up to 250 K, above which it decreased due to the partial loss of acetone by thermal desorption. These measurements provided an estimate for the activation energy for the acetone complex formation (∼10 kJ/mol); however, they did not probe the nature of the oxygen species that are involved in complex formation.

Figure 7. Photoreaction yields of methyl radicals (mass 15) resulting from UV exposure (335 nm) from an acetone−oxygen adlayer on a reduced TiO2(110) surface prepared in different ways. Top three curves: a reduced TiO2(110) surface was dosed with 80 langmuirs of O2 at 100 K and then flashed to the temperature indicated to the right of each curve; the surface was then recooled to 100 K, dosed with ∼1.1 ML of acetone and irradiated with UV light. Bottom curve: photoreaction yield of methyl radicals resulting from UV exposure (335 nm) of a surface prepared by dosing only ∼1.1 ML of acetone without oxygen on a reduced TiO2(110) surface held at 100 K.

Table 2. Results of Biexponential Fits to Methyl Radical Photoyields versus Annealing Temperature of the O2 Adlayer

a

annealing temp (K)

fast decay rate (×102 s−1)

slow decay rate (×103 s−1)

fast to slow intensity ratio

integrated CH3 yielda

100 225 410

3.4 (6) 5.6 (8) 6.7 (4)

2.3 (1) 6.1 (1) 5.6 (1)

2.1 2.9 11.4

1.0 0.78 0.46

Normalized to integrated yield for 100 K data.

From the tabulated results, it is seen that the fast and slow decay rates increase significantly when the O2 adlayer is annealed from 100 to 225 K but are roughly the same for annealing at 225 and 410 K. The relative yields of the fast to slow components, however, show a dramatic increase when the oxygen adlayer is annealed to 410 K. These results suggest a transformation from an acetone species with slow photoreaction kinetics to those with a photoreaction rate ∼10 times higher. This transformation is induced by the reaction of acetone and surface oxygen species when the acetone is dosed onto the surface, which changes from weakly bound O2(a) at 100 K to mostly O-adatoms for preannealed surfaces at 410 K. If we associate the fast decay component to the prompt photoreaction of an acetone diolate complex, these results suggest that O-adatoms are the active species for complex formation. Hence, the thermal activation of an adlayer of oxygen and acetone codosed at low temperature is at least partially due to the need to thermally dissociate O2(a), with the resulting O atoms reacting with acetone to form the diolate complex. This is not the entire story, however, as the data in Figure 7 show that methyl photoproducts are produced even when the surface temperature is maintained at 100 K for both O2/ acetone dosing and UV irradiation. Indeed, the velocity measurements reported here (Figures 4−6) were taken for surfaces at 100 K that were not preannealed prior to UV exposure. It is possible that some of the O2(ad) at 100 K dissociates at Obr(V) sites to create O-adatoms with sufficient

IV. DISCUSSION A. Acetone Photodecomposition Products. The experiments reported here are for the most part in agreement with previous studies on the photodecomposition of acetone on single crystal TiO2(110) surfaces.6,13,18 Specifically, acetone photodecomposition on TiO2(110) surfaces does not precede without coadsorption of oxygen, and the photoyield of gasphase products is enhanced by preannealing an oxygen adlayer (see Figure 7). The latter are consistent with the formation of a diolate species which has a small thermal barrier for formation and represents the species with highest photoactivity. The structure of this intermediate was suggested to be η2-acetone diolate based on a O−C−O bond order of 1.5 as derived from vibrational spectra using electron energy loss spectroscopy.18 The adsorbed oxygen species involved in the formation of the intermediate are not known with complete certainty, but the results provided in Figure 7 suggest that O-adatoms are the most likely species, whereas O2(ad) species that are photo16546



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preparation conditions, e.g., acetone and/or oxygen doses and/or annealing temperatures, which would affect the relative amounts of the initial photoactive species and thereby affect the relative intensities of the two methyl velocity components. In fact, we find that the ratio of the two methyl velocity components is constant within experimental uncertainty for different initial conditions, e.g., with or without preannealing the coadsorbed acetone/oxygen adlayer. This strongly suggests that only one initial photoactive species is responsible for both the “fast” and “slow” components in the observed methyl velocity distributions. Photoproduct collisions with the surface prior to desorption have been invoked in previous studies to explain the appearance of “fast” and “slow” velocity components for methyl radicals released by UV photoexcitation of methyl iodide multilayers on TiO2(110).44 The “slow” methyl products appear at a CH3I coverage where some molecules are oriented perpendicular to the surface but with the methyl group pointed down. The methyl radicals released by photolysis from these molecules undergo inelastic collisions with the surface and/or other molecules before being able to escape to the vacuum. For the case of acetone diolate species, this mechanism would suggest a binding orientation in which one methyl group is pointed away from the surface (“fast” channel) and one pointed toward the surface (“slow” channel). This bonding configuration is inconsistent with that expected for an η2-bonded carboxylate or diolate species where the O atoms are bridge-bonded to Ti4+ cations in the troughs of the (110) surface and the molecular plane is perpendicular to the surface normal.5,18,45 Assuming this geometry is also applicable to the transition state prior to fragmentation, it seems likely that the methyl fragments would be ejected into the vacuum without collisions with the surface. Moreover, collisional energy loss would be expected to exhibit a surface temperature dependence since scattered fragments could be transiently trapped on the surface at lower temperatures. The translational energy distributions, however, are found to be unchanged over the temperature range studied in this work (95−200 K). Hence, energy loss by “fast” methyl fragments colliding with the surface is unlikely to be the origin of the “slow” channel. Velocity distributions have been observed for methyl fragments from the UV photodissociation of gas-phase acetone following excitation to singlet states (S1 or S2) which undergo internal conversion to a triplet state (T1) on which surface the molecule dissociates.46−49 For photolysis at 248 nm via S1, a fast methyl component (⟨Et⟩ ≈ 0.45 eV) is assigned to the loss of the first methyl group, while a second component (⟨Et⟩ ≈ 0.2 eV) results from fragmentation of the internally “hot” acetyl radical.46 For photodissociation of the acetone diolate species on TiO2(110), the observation of a stable acetate photoproduct in postirradiation TPD suggests that the acetate surface species is stable against further decomposition. It appears that the excess internal energy of the “hot” acetate fragment is insufficient to cross the barrier for dissociation into the second methyl fragment and gas-phase CO2. Although the VUV energies used here were too low to ionize CO2 (or CO), Henderson noted that no CO/CO2 were detected in his UV photoyield measurements using electron impact ionization detection.6 These considerations suggest that the secondary fragmentation process for the acetone diolate species is “quenched” by its interaction with the surface and/or energetically inaccessible so that the observed two component

desorbed by UV excitation and thermally desorbed above 400 K do not participate in acetone diolate formation.32 The UV photodecomposition of the acetone diolate species to yield methyl radicals is supported by our UV-pump, VUVprobe mass spectrometry measurements where mass 15 (CH3) and mass 32 (O2) are the only photoproducts originating at the surface (see Figure 2). The latter are readily distinguished from other detectible gas-phase species by their peak widths and dependence on pump−probe laser delay. The fact that the primary products from acetone ionization (mass 43 and 58) are absent in Figure 2 also indicates that direct photodesorption of acetone in the η1-bonded geometry or as a η2-diolate species does not occur during UV irradiation. As previously stated, postirradiation TPD measurements (Figure 1) are consistent with the formation of the corresponding surface-bound photoproduct, i.e., acetate, which thermally decomposes to produce ketene (mass 42).6,18,32 In addition, the acetone diolate may also thermally decompose to give acetate species on the surface which are responsible for additional ketene desorption peaks observed in the absence of UV irradiation (see Figure 1). Earlier investigations of acetone photodecomposition on TiO2 have demonstrated that annealing the acetone−oxygen adlayer to about 250 K prior to photolysis resulted in a significantly enhanced yield of methyl radical and suggested a relatively small activation barrier for formation of the acetone intermediate was overcome by annealing.6 In this work, we find that the initial rate (or cross section) of methyl radical ejection depends sensitively on preannealing conditions (see Figure 7), but the dynamics of photofragmentation as probed by the velocity distributions of the methyl fragments is unaffected by annealing conditions. Specifically, pump−probe delay scans of ejected methyl radicals were identical to within experimental error regardless of whether or not the oxygen/acetone adlayer was first flashed to 225 K or held at 100 K (data not shown). Variations in the rate for gas-phase product formation with annealing conditions reflect changes in the surface concentration of the acetone diolate species which is thermally activated. On the other hand, the photofragmentation dynamics depends only on the interaction of the acetone diolate with photoexcited charge carriers and the subsequent internal energy conversion process that leads to fragmentation. B. Acetone Photodecomposition: Methyl Fragment Velocity Distributions. In this work, pump-delayed-probe measurements were used to obtain velocity and energy distributions for methyl radical photoproducts (see Figures 4−6) resulting from the UV exposure of an acetone−oxygen adlayer on a reduced TiO2(110) surface at 100 K. The overall mean translational energy of the methyl products is 0.16 eV and is essentially independent of pump photoexcitation energy (see Table 1). The shoulder at longer arrival times suggests the presence of two velocity (energy) components, and the data were empirically fit to the sum of two asymmetric functions which gave a reasonable overall fit to the data. Averaged over the results for the three different pump excitation energies, the “fast” or “slow” methyl product distributions have mean translational energies of 0.191 ± 0.006 and 0.032 ± 0.005 eV, respectively (see Table 1). The observation of two velocity components in the methyl product distribution could be the result of multiple excitation channels (e.g., vibrationally excited products), postcollisional effects with the surface, or multiple initial states of the acetone diolate species. The latter should be sensitive to initial 16547



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components which are very similar to that of CH3 (see Figure 5b) from h6-acetone. The results from an empirical fit of the CD3 translational energy distribution at 335 nm is compared to that for CH3 in Table 1. It is seen that the overall translational energy of CD3 decreases by 21% compared to CH3 products, suggesting that less excess energy is channeled into translation for d6-acetone. More pertinent to vibrational excitation is the energy difference between the “slow” and “fast” channels, which for CD3 decreases to ∼0.13 eV as compared to ∼0.16 eV for CH3. The decrease in energy separation is qualitatively consistent with the isotope shift in the ν2 umbrella mode but is larger than expected for the 220 transition energy in CD3 (114 meV). This difference may be associated with the experimental accuracy of the “slow” distribution which originates from methyl products detected at long laser delay times with small signal levels. Incomplete integration of signal at long delay times would lead to a somewhat higher translational energy for the “slow” channel. Although the isotope measurements provide qualitative support for the production of vibrationally excited methyl products, a definite answer for CH3/CD3 vibrational excitation will require a more direct spectroscopic probe such as state-resolved (2 + 1) REMPI detection of methyl products.54,55 The latter can unambiguously establish the methyl final state vibrational distribution and possible coupling with translational energy. On the basis of the discussion above, we have established that the “fast” and “slow” components of the observed velocity/ energy distributions represent different final states of the methyl fragments resulting from cleavage of one of the C−CH3 bonds in the acetone diolate species. One possibility is that the methyl products are formed in different vibrational states, each of which corresponds to a different dissociation asymptote. Assuming the available energy of the transition state complex is fixed, a vibrationally excited methyl fragment would have less available energy for translation and appear as a “slow” fragment. More generally, the final state velocity/energy distributions reflect the dynamics of excited acetone diolate species which are strongly dependent on the internal energy distribution of the transition state, rates of internal energy transfer, and the topology of the potential energy surfaces leading to dissociation. From this perspective, the “fast” channel could be the result of prompt dissociation of an internally “hot” acetone diolate species, while the “slow” channel results from photoexcited molecules which have relaxed by internal conversion or energy transfer to the titania surface prior to fragmentation. C. Acetone Photodecomposition: Mechanism. Possible excitation mechanisms for the observed UV-induced photoreaction are direct optical excitation of adsorbed acetone or the acetone diolate species or substrate-mediated excitation of e−/ h+ pairs followed by charge/energy transfer. Laser-induced thermal reactions are not considered due to the large amount of energy required to cleave a C−C bond of acetone (about 350 kJ mol−1) and the low photon flux used in this work to initiate chemistry at the interface. Moreover, we recently demonstrated that O2 photodesorption from a reduced TiO2(110) surface using comparable UV pump laser fluences occurs via a onephoton process, whereas thermal heating would exhibit nonlinear dependence on laser fluence.23 The direct excitation and photofragmentation of adsorbed η1-acetone is unlikely due to the requirement of coadsorbed oxygen for the observation of methyl radical products. The need for coadsorbed oxygen implies a mechanism involving the

velocity distribution results from only one C−CH3 bond cleavage. The coupling of vibrational and translational degrees of freedom could also lead to multiple features in the velocity distributions, each of which would correspond to different vibrational final states of the methyl radical. Gas-phase photodissociation studies of d6-acetone show that the d3methyl radical leaves with considerable vibrational excitation, including the ν2 umbrella mode and the ν3 asymmetric stretch.47−49 At 248 nm (S1 excitation), 55% of the available energy appears as internal energy of the products (methyl and acetyl) while the remaining 45% is in center-of-mass translational energy.46 Desorbed methyl radicals from the UV photodissociation of CH3X (Cl, Br, I) on GaAs(110) and TiO2(110) also exhibited substantial vibrational excitation (only ν2 probed) which was similar to gas-phase CH3X photolysis.50−52 In this work, the energy difference ⟨Et(fast)⟩ − ⟨Et(fast)⟩ of the two methyl channels averaged for all three excitation energies is 0.16 ± 0.01 eV (see Table 1). This energy difference is the same, within experimental uncertainty, as the excitation energy of two quanta in the ν2 umbrella mode (220 transition; 2ω2 = 0.152 eV).53 Simulations of the velocity distributions with three Gaussian peaks separated by one quanta of ν2 resulted in an overall good fit to the data for methyl products produced at 335 nm (not shown). However, the relative intensities of the fitted peaks suggest an inverted vibrational population with the ν2 = 0:1:2 ratio being approximately 0.6:0.1:0.4. An inverted ν2 vibrational population in the methyl fragment seems unlikely and has not been observed in earlier acetone fragmentation studies from surfaces.49 To further investigate the possibility of vibrational excitation in the methyl product, we performed velocity distribution measurements on CD3 radicals resulting from the photoxidation of d6-acetone/O2/TiO2(110) prepared under identical conditions. Time-of-flight mass spectra obtained during UV exposure confirmed that the primary photoproduct was CD3 (mass 18) which could be distinguished from background H2O (mass 18) by its narrow velocity distribution and shift in arrival time with pump−probe delay. As seen in Figure 8, the CD3 photofragment translational energy distribution exhibits two

Figure 8. Translational energy distribution of d3-methyl radicals (mass 18) desorbed from a reduced TiO2(110) surface pre-exposed to 80 langmuirs of O2 and ∼1.1 ML of d6-acetone at 100 K. The UV pump and VUV probe photon energies were 3.7 eV (335 nm) and 13.1 eV (94.74 nm), respectively. The data were converted to a flux distribution and corrected for depletion of the reactant coverage during UV exposure. Filled circles represent the corrected data, dashed lines are empirical fits to the “slow” and “fast” components, and the solid line is the sum of the two components. 16548



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acetone diolate species given by reactions 1 and 2 or a mechanism in which adsorbed oxygen scavenges photoexcited electrons and thereby allows η1-acetone to interact with photoexcited carriers. In either scenario, direct photoexcitation and dissociation of η1-acetone is not responsible for the observed photoreaction. Direct excitation of the acetone diolate species is consistent with the need for coadsorbed oxygen, but the plausibility of such a mechanism is difficult to judge as there is no gas-phase analogue of the acetone diolate species with which to compare (dimethyldioxirane is closest in structure but is unstable and little is known of its photochemistry). Support for an indirect or substrate-induced photoexcitation mechanism is provided by Henderson, who showed that the methyl photoyield from an acetone/O2/TiO2(110) surface was negligible when UV light with energy above the TiO2 band gap was removed by a filter (

Final State Distributions of Methyl Photoproducts from the

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