Final State Distributions of the Radical Photoproducts from the UV

Apr 10, 2013 - The UV photooxidation of 2-butanone on TiO2(110) was studied using pump–probe laser methods and time-of-flight (TOF) mass spectrometr...
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Final State Distributions of the Radical Photoproducts from the UV Photooxidation of 2‑Butanone on TiO2(110) Daniel P. Wilson,† David P. 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

‡

S Supporting Information *

ABSTRACT: The UV photooxidation of 2-butanone on TiO2(110) was studied using pump−probe laser methods and time-of-flight (TOF) mass spectrometry to identify the gas-phase photoproducts and probe the dynamics of the photofragmentation process. A unique aspect of this work is the use of coherent VUV radiation for single-photon ionization detection of gas-phase products, which significantly reduces the amount of parent ion fragmentation as compared to electron impact used in previous studies. The pump−probe product mass spectra showed ions at mass 15 (CH3+) and mass 29 (C2H5+), which are associated with the primary α-carbon bond cleavage of the adsorbed butanone−oxygen complex, as well other C2Hx+ (x = 2−4) fragments, which could originate from ethyl radical secondary surface chemistry or dissociative ionization. Using two different VUV probe energies, it was possible to show that the fragment ions at mass 27 (C2H3+) and mass 28 (C2H3+) are not due to secondary reactions of ethyl radicals on the surface, but rather from dissociative ionization of the ethyl radical parent ion (mass 29). Another photoproduct at mass 26 (C2H2+) peak is also observed, but its pump−probe delay dependence indicates that it is not associated with nascent ethyl radicals. Pump-delayed-probe measurements were also used to obtain translational energy distributions for the methyl and ethyl radical products, both which can be empirically fit to “fast” and “slow” components. The ethyl radical energy distribution is dominated by the “slow” channel, whereas the methyl radical has a much larger contribution from “fast” fragments. The assignment of the C2Hx (x = 3, 4) fragments to ethyl (C2H5) dissociative ionization was also confirmed by showing that all three products have the same translational energy distributions. The origin of the “fast” and “slow” fragmentation channels for both methyl and ethyl ejection is discussed in terms of analogous neutral and ionic fragmentation processes in the gas phase. Finally, we consider the possible energetic and dynamical origins of the higher yield of ethyl radical products as compared to that for methyl radicals.

I. INTRODUCTION

photocatalysis using titania powders and product analyses under ambient conditions.6,10 Of particular interest is the photooxidation of simple ketones, for example, acetone and 2-butanone (methyl ethyl ketone), which are part of a larger class of oxygenated volatile organic compounds that are ubiquitous in urban atmospheres28−30 and serve as model systems for detailed mechanistic studies of photooxidation. For ketones on TiO2(110), Henderson showed that the primary fragmentation process involves cleavage of the α-carbon bonds resulting in the ejection of a radical product into the gas phase, for example, methyl radicals (CH3) from acetone.16,18−20,25,31 The yield of photoproducts was also found to be strongly dependent on the presence of coadsorbed oxygen, and warming of an oxygen/ketone adlayer strongly enhanced product formation. The latter observations were attributed to the formation of a ketone−oxygen complex, that is, η2-bonded ketone-diolate, which is the photoactive species, whereas the η1-bound ketone is not. In the case of 2-butanone,

The use of titania in heterogeneous photocatalysis has been a subject of sustained interest largely due to its effectiveness in “mineralizing” organic materials through UV induced photooxidation. Combined with its high stability and nontoxic nature, titania photocatalysts have found an ever expanding range of applications including air and water pollution remediation, selfcleaning coatings, and disinfecting surfaces.1−7 The strong oxidizing power of titania arises from its wide band gap, 3.0 eV (rutile) or 3.2 eV (anatase), and the position of the valence band maximum (VBM), such that UV excitation generates energetic holes (h+) at ∼7 eV below the vacuum level.8−10 The latter are capable of oxidizing most organic systems by sequential one-electron processes. Recent studies of UV photooxidation of simple organics on well-defined single crystal surfaces of TiO2(110) have provided considerable insight into the mechanism of molecular fragmentation, the role of adsorbed oxygen in promoting photoreaction, and the nature of the substrate-mediated charge transfer processes leading to molecular adsorbate excitation.11−27 In general, such information is difficult to derive from more conventional studies of © 2013 American Chemical Society

Received: February 21, 2013 Revised: April 9, 2013 Published: April 10, 2013 9290

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wherein α-cleavage could lead to methyl and/or ethyl radicals, the primary gas-phase photoproducts were identified as the ethyl radical (C2H5), and secondary reaction products, ethylene (C2H4) and ethane (C2H6).18 The latter assignments were based on known fragment ion cracking patterns resulting from electron impact ionization of neutral molecules, for example, ethyl radicals or ethane, in the quadrupole mass spectrometer (QMS). The small yield of mass 15 (methyl cation) was not assigned to direct ejection of methyl radicals, but to dissociative ionization of ethane and/or ethyl radical. More recent work in our laboratory using resonance enhanced multiphoton ionization (REMPI) has shown that direct ejection of the methyl radicals does occur, but the yield is small relative to the C2Hx products.22 In general, identifying neutral parent molecules from ion cracking patterns for room temperature samples can be problematic because the neutrals formed by photoreaction may have very different internal energies. The latter can significantly alter the dissociative ionization probabilities for specific fragments.32−34 In this work, we present new results for the photooxidation of 2-butanone on TiO2(110) using laser pump−probe techniques that provide information on the dynamics of photofragmentation and revisit the previous assignments of neutral photoproduct formation, for example, secondary reaction products. In particular, we use pump−probe laser methods combined with time-of-flight mass spectrometry to derive velocity and translational energy distributions for the gas-phase photoproducts generated by UV photooxidation of 2butanone, which is coadsorbed with molecular oxygen on a reduced TiO2(110) surface. Our previous pump−probe studies of methyl loss from acetaldehyde, acetone, 2-butanone, and acetophenone photooxidation on TiO2(110) have shown that energy distributions of the methyl fragments provide unique insight into the nature of the excited intermediate formed by charge transfer with the titania surface and the dynamics of the fragmentation process, for example, partitioning of excess energy and role of intramolecular relaxation.22,27 In the case of 2-butanone photooxidation, both methyl and ethyl radical photoproducts are accessible to pump−probe measurements, which provide a unique opportunity to compare the dynamics of different fragmentation channels of the same system. A key aspect of the present work is the use of VUV single-photon laser probes to detect photoproducts. By comparison to electron impact ionization used in conventional quadrupole mass spectrometers for photoproduct identification, the use of “soft” VUV ionization methods induces far less ion fragmentation, making it easier to identify the primary neutral species resulting from photooxidation.

programmed thermal desorption (TPD), a low-energy electron diffraction instrument (LEED; Princeton Research), and ion sputtering gun. Photochemistry measurements were performed on the lower level, which houses the time-of-flight mass spectrometer (TOF-MS) for species identification and velocity measurements, and a directed doser for depositing adsorbate molecules. A rotatable, xyz liquid nitrogen cooled manipulator was used to transport and align the sample between the two levels of the UHV chamber. Additional details concerning the apparatus can be found in previously published work.21,35 An oriented, 10 × 10 × 2 mm rutile TiO2(110) crystal (CrysTec) was used in these experiments. Molybdenum (Mo) clips were used to clamp the crystal on top of a commercial button heater (HeatWave), which was held in a Mo ring attached to the manipulator by a 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 that was inserted into a small hole in the edge of the crystal and held in place with hightemperature ceramic cement (Omegabond 600). A crystal temperature from ∼100 to ∼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 Ar+ 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.21 A 10 min vacuum anneal at 850 K was also performed between consecutive TPD or photochemistry measurements to restore Obr(V) sites that are healed by thermal or UV photodissociation of adsorbed O2.36 The density of bridged-oxygen vacancies on our crystal is estimated to be 6% based on D2O TPD measurements.21 The liquid samples of 2-butanone (Sigma-Aldrich) and d8butanone (CDN Isotopes) were degassed using multiple freeze−thaw cycles. Oxygen (Matheson, 99.8%) was used without further purification. Photoactive surfaces composed of an adlayer of coadsorbed O2 and butanone were prepared similarly to that in a previous study of ketone photooxidation on TiO2(110).22 Briefly, the vacuum annealed (reduced) TiO2(110) surface was cooled to 105 K and then given a saturation dose of O2 (80 L) followed by a ∼1 ML dose of butanone. Background exposure was used for O2 dosing, while the butanone was deposited using a stainless steel dosing tube (9.5 mm ID) placed ∼1 mm from the surface of the crystal. The photooxidation experiments were carried out at elevated temperatures (200 K) and with a background pressure of O2 (5 × 10−8 Torr), which served to remove multilayer butanone from the surface and enhance the yield of photoproducts.18,22 The latter has been attributed to the thermal reaction between adsorbed O2 and ketone to form a photoactive η2-bonded diolate species.25,37 Photooxidation experiments were carried out using a pump− probe technique for generation and detection of radicals. The pump (photoexcitation) laser was a Nd:YAG (Spectra-Physics GCR-190, 20 Hz, 532 nm) pumped dye setup using a combination of DCM and LDS 698 dyes to produce 670 nm photons, which were frequency doubled to 335 nm (3.7 eV). The UV pump beam passed through two Glan-Thompson polarizers (Lambrecht) and neutral density filters to adjust the laser fluence and provide p-polarized light onto the crystal.

II. EXPERIMENTAL SECTION The instrument used in this experiment consists of a two-tier ultrahigh vacuum (UHV) system with a base pressure of about 3 × 10−10 Torr, and a smaller chamber that serves as a windowless VUV source containing a pulsed gas valve used for harmonic generation of VUV light. The surface science and VUV vacuum chambers are connected by a doubly differentially pumped capillary light guide, which acts to collimate the VUV radiation and allow for differential pumping between the UHV sample chamber and the relatively high pressure VUV generation chamber under operating conditions (10−3−10−4 Torr). Sample preparation and characterization is carried out on the upper level, which includes a quadrupole mass spectrometer (QMS; Hiden Analytical) for temperature 9291

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Typical photon fluxes for this experiment were 2 × 1012 photons s−1 cm−2. Coherent VUV radiation (probe beam) was used for singlephoton ionization detection of the C2Hx photoproducts resulting from photooxidation of 2-butanone. Nonlinear conversion techniques were used to generate two different photon energies, 13.09 eV (94.74 nm) and 10.88 eV (114.42 nm), that could distinguish photoproducts by their ionization potentials and fragmentation patterns. For radiation at 13.09 eV, visible light (568.44 nm) from a second Nd:YAG pumped dye laser system (Spectra-Physics GCR-230; Laser Analytical Systems, LDL-20505) was frequency doubled (284.42 nm), and then focused into a pulsed-jet expansion of N2 gas where a small fraction is converted to VUV light by nonresonant third harmonic generation (THG).38 Radiation at 10.88 eV was produced by sum frequency mixing the visible (572.1 nm) and UV second harmonic (286.05 nm) output from the dye laser into a pulsed jet of Xe gas.39,40 After passing through the gas-jet, the VUV radiation is collected by a glass capillary (1.5 mm ID) and “guided” into the UHV chamber where it passes through the ionization region of the time-of-flight mass spectrometer (TOF-MS). Ions resulting from interaction of the neutral photoproducts with the VUV light are accelerated and focused into a time-of-flight ion mass spectrometer where they are separated by their mass. At the end of the flight tube, the ions are accelerated onto a dual microchannel plate (MCP) electron multiplier whose output is displayed on a phosphor screen and detected by a photomultiplier. Although single-photon VUV ionization could be used to detect the methyl radical fragments (see Figure 1), this detection scheme was not sensitive enough to obtain methyl radical final state distributions with acceptable signal-tobackground levels. In this case, second harmonic light (∼286.3 nm) from the probe dye-laser was used to ionize methyl radicals by (2 + 1) resonant multiphoton ionization (REMPI) via the strong Q-branch of the 2A2″(4pz) ← X2A2″

transition.41 The UV light was focused into the ionization region of the TOF-MS using a 30 cm lens. The (2 + 1) REMPI scheme resulted in significantly higher detection sensitivity (∼20×) for methyl radicals as compared to single-photon VUV ionization. Mass identification of the photoproducts uses their time-offlight taken at fixed time-delay between the pump and probe lasers. Differences in translational energy distributions are evident as changes in the photoproduct yields as the pump− probe delay is varied. Moreover, it is easy to distinguish which mass peaks correspond to surface photoproducts because their TOF changes with pump−probe delay, whereas background species are insensitive to pump−probe delay (see Figure 1). Photoproduct mass peaks typically have narrower TOF widths as well, which reflects their narrow velocity and angular distributions along the detector axis. Once we have identified a desorbing radical, we can take energy distributions by varying the pump−probe laser delay while collecting ion signal for a single mass. Final state energy distributions are obtained by measuring the intensity of a given mass peak as a function of pump−probe delay.21,22,27 These “arrival-time” distributions (see Figure 3) are readily converted to velocity distributions because the neutral free flight distance from the crystal surface to the ionization point is known (28 mm). To obtain better signal statistics, at least three arrival-time distributions were collected and averaged for each mass. Measurements of the photoyield versus pump laser exposure time (“depletion scans”) were also measured for each photoproduct to correct for the loss of surface reactant coverage during the collection of arrival time data. Depletion scans were obtained under identical conditions as the arrival time data (surface coverage, surface temperature, laser fluence), but at fixed pump−probe delay, typically corresponding to the peak signal in the arrival time distribution. Additional intensity corrections were necessary for the arrival time distributions for the C2Hx+ fragments due to the changes in background intensity as the pump−probe delay was scanned. The additional correction procedures are described in the Supporting Information. After intensity corrections for background and surface depletion, the arrival time distributions were converted to velocity and translational energy distributions using the known neutral flight distance and the appropriate Jacobian transformations.

III. RESULTS A. Identification of Gas-Phase Products. Time-of-flight mass spectra for gas-phase species produced during the UV photooxidation of an adlayer of coadsorbed oxygen and butanone are shown in Figure 1. These spectra were taken at a surface temperature of 200 K in a background of O2 gas at a pressure of 5 × 10−8 Torr. The UV pump energy was 3.70 eV (335 nm) and VUV radiation at 13.09 eV (94.7 nm) was used for the ionization probe. Mass spectra at two different pump− probe delay times are shown corresponding to the two maxima in the arrival-time distribution for the mass 29 fragment (ethyl radical) at ∼25 and ∼60 μs (see Figure 3). For comparison, the background spectrum taken under the same conditions but without the UV pump laser is also shown. The features marked with asterisks (*) in Figure 1 are artificially generated by incomplete gating of the multichannel plate detector (MCP) during the arrival time of O2+ ions generated from VUV ionization of background oxygen (used to replenish surface oxygen during photooxidation).27 The relatively high oxygen

Figure 1. TOF mass spectra of gas-phase species observed during UV photooxidation of 2-butanone on a TiO2(110) surface taken at a temperature of 200 K and in a background of O2 at 5 × 10−8 Torr. Bottom curve (black): background spectrum obtained with no pump laser. Middle curve (blue): TOF mass spectrum taken with a pump− probe laser delay of 60 μs. Top curve (red): TOF mass spectrum obtained with a pump−probe laser delay of 25 μs. The adsorbate adlayer was prepared as described in the Experimental Section. The photon energies of the pump and probe lasers were 3.7 and 13.09 eV (94.74 nm), respectively. 9292

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Table 1. Ionization Potentials for the Various Gas-Phase Molecular Species That Are Possible as Primary or Secondary Reaction Products from the Photooxidation of 2-Butanone on the TiO2(110) Surface (All Values in eV) methyl radicala CH3 9.84 a

ethyl radicalb C2H5 8.6

2-butanonec C4H7O

ethaned C2H6

9.52

11.5

ethylened C2H4 10.51

vinyl radicale C2H3 8.25

e

acetylened C2H2 11.40

From ref 53. bFrom ref 54. cFrom ref 44. dNIST Chemistry WebBook. eFrom ref 55.

background pressure (5 × 10−8 Torr) results in very large ion signals, and the MCP detector voltage is pulsed off during their arrival time to prevent charge saturation and MCP damage. The period during which the MCP is gated is observable as a zero baseline in the TOF spectra. Mass peaks corresponding to photoproducts from the surface are identified by their TOF dependence on the pump−probe delay, which effectively changes the velocity at which the fragments are detected. Changes in TOF with pump−probe delay are clearly evident for the mass peaks 26−29, and for mass 15, which only appears at short pump−probe delays. From Table 1, it is seen that the 13.09 eV probe light can ionize gas-phase methyl (CH3; mass 15) and ethyl (C2H5; mass 29) radicals expected from butanone fragmentation, as well as any neutral C2Hx (x = 2−4) products from possible secondary hydrogenation/dehydrogenation reactions of the nascent ethyl radicals at the surface.18 A number of small mass peaks including that for H2O+ (mass 18; IP = 12.62 eV) appear without the pump laser and are thereby attributed to VUV ionization of residual background gases in the vacuum chamber. The TOF positions of these background peaks do not shift with pump−probe delay. Mass peaks in the range 26−29 are only observed when the surface is irradiated with the UV pump light and are assigned to C2Hx+ fragments with x = 2−5, respectively. The assignment of these peaks to C2Hx+ fragments is confirmed by additional measurements using d8-butanone where the observed mass shifts are consistent with D-atom substitution to form C2Dx+ (x = 2−5) fragments (see Supporting Information, Figure S1). Mass 15 only appears at short pump−probe delays consistent with the arrival time distribution of methyl radical photoproducts determined in an earlier study.22 We also note that mass 26 (C2H2+) and a small feature near mass 30 are also enhanced at shorter pump−probe delays. The latter suggests that their velocity distributions are more similar to the methyl product rather than the other C2Hx+ (mass 27−29) fragments whose relative intensities are comparable at both 25 and 60 μs pump−probe delays (see Figure 1). From the relative intensities in the spectrum taken at 25 μs delay, it is also seen that the probability for methyl radical formation is significantly less than that for the sum of the C2Hx fragments, all of which presumably originate from ethyl radicals (C2H5) released by photooxidation of butanone. Peaks observed above mass 38 are also present without UV irradiation and thereby not associated with surface photoreactions. Mass spectra taken at other pump−probe delay times, that is, different final state kinetic energies, did not reveal any new mass peaks associated with surface photoreaction. Therefore, the only gas-phase fragments generated by the photooxidation of butanone on TiO2(110) are methyl radical (CH3; mass 15), ethyl radical (C2H5; mass 29), and C2Hx fragments with x = 2−4. As discussed below, the appearance of the weak feature near mass 30 at short pump−probe delays is attributed to an impurity. To explore the origin of the C2Hx+ mass peaks, mass spectra were also taken at a lower VUV photon energy, 10.88 eV, where it is possible to discriminate among possible products

Figure 2. TOF mass spectra of C2Hx+ species from UV photooxidation of 2-butanone on a TiO2(110) surface taken with two different VUV probe energies, 13.09 and 10.88 eV. Left panel: pump−probe laser delay of 25 μs. Right panel: pump−probe laser delay of 60 μs. The adsorbate adlayer was prepared as described in the Experimental Section. The photon energy of the pump laser was 3.7 eV.

with differing ionization potentials. Mass spectra for VUV energies of 13.09 and 10.88 eV are compared in Figure 2 at pump−probe delays of 25 μs and 60 μs. As shown in Table 1, a 10.88 eV photon can ionize all of the possible photoproducts except the stable molecules ethane (C2H6; mass 30) and acetylene (C2H2; mass 26). Mass 29 is a prominent peak in the mass spectra taken at both VUV photon energies, indicating that the ethyl radical is a primary photoproduct of butanone photooxidation. For masses 26−28, contributions from dissociative ionization of ethyl radical and direct ionization of ethylene, vinyl radical, and acetylene have to be considered. The fact that mass 28 is missing in the 10.88 eV spectrum, even though the photon energy is high enough to ionize ethylene, indicates that mass 28 in the 13.09 eV spectrum is from dissociative ionization of ethyl radical and not from ionization of neutral ethylene. The decrease in the mass 27 signal in the 10.88 eV is also likely to be a result of a smaller probability for dissociative ionization of ethyl radical, although a lower cross section for direct ionization of a vinyl radical photoproduct cannot be completely discounted. Overall, the observed trends in the intensities for masses 27−29 are consistent with previous studies of dissociative ionization of ethyl radical.42 Specifically, the C2H3+ (mass 27) and C2H4+ (mass 28) daughter fragments were found to have nearly the same cross section at a photon energy of 13.8 eV, while at 12.1 eV, the cross sections for these fragments decrease by factors of ∼2× and ∼30×, respectively.42 This would explain the loss of mass 28 peak and the decrease in the mass 27 peak in the 10.88 eV spectrum, and supports our claim that neutral vinyl radical (mass 27) is not a primary photoproduct. The loss of the mass 26 peak at the lower VUV photon energy (see Figure 2), especially evident at a pump−probe delay of 25 μs, is more difficult to explain because its disappearance could result from (1) the higher ionization potential of neutral acetylene (11.44 eV) or (2) a significant 9293

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30 in Figures 1 and 2 as due primarily to an impurity that is dosed onto the surface along with 2-butanone. Finally, Henderson also suspected that the small mass 15 signal was not due to methyl radical photoproducts, but a fragment ion of ethane or ethyl radical. Our data as a function of pump−probe delay in Figures 1 and 2 show that mass 15 is not associated with masses 27−29, but is more likely a result of methyl radicals formed by direct photofragmentation of the butanone−oxygen complex. This conclusion is strongly supported by the mass 15 velocity and translational energy distributions, which are very similar to neutral methyl products formed by photooxidation of similar molecules, acetaldehyde, acetone, and acetophenone, for which methyl radical is the primary gas-phase product.22,27 B. Final State Distributions. The arrival-time distribution for the ethyl radical fragment (mass 29) is shown in Figure 3.

decrease in the probability for dissociative ionization of ethyl radical to a mass 26 (C2H2+) fragment. A clue that mass 26 is not from dissociative ionization is that it exhibits a pump− probe delay dependence that is different from masses 27−29, all of which have been shown to originate with neutral ethyl radical. Ionic fragments that result from dissociative ionization of a single neutral parent will all exhibit the same initial velocity distribution, and hence the same pump−probe delay dependence. In addition, gas-phase photoionization studies of ethyl radical have not reported the formation of C2H2+ daughter fragments at similar photon energies.42 These considerations suggest that the mass 26 fragment is associated with a neutral photoproduct from the surface other than ethyl radical or it is a fragment ion of a higher mass photoproduct that we have not yet identified. The obvious candidate for a neutral photoproduct is acetylene, C2H2, which is consistent with our observations, but the process leading to its formation at the surface cannot be determined from this study. In a previous study of the UV photooxidation of 2-butanone on TiO2 (110), Henderson used electron impact mass spectrometry to identify the C2Hx gas-phase photoproducts.18 In particular, Henderson observed photodesorption yields for masses 27−30 and used the electron impact cracking patterns from tabulated mass spectra to assign mass 30, 29, and 28 to neutral photoproducts corresponding to ethane (C2H6; 28%), ethyl radical (C2H5; 15%), and ethylene (C2H4; 54%), respectively. No mention was made of a mass 26 product. The ethyl radical is due to direct photofragmentation of the butanone−oxygen complex (diolate), whereas ethane and ethylene were attributed to secondary reactions of nascent ethyl radicals with oxygen species on the TiO2 surface (adatoms, molecules, and hydroxyls). Henderson noted an increase in water desorption (estimated at ∼0.15 ML) in postUV TPD experiments and suggested that this increase results from a dehydrogenation reaction involving ethyl and a surface oxygen species. Our own water TPD measurements for 2butanone and O2/2-butanone adlayers before and after UV exposure did not show increases in water desorption following UV photoreaction (see Supporting Information, Figure S2). Furthermore, the data in Figures 1 and 2 taken with two different VUV probe energies, for which dissociative ionization is greatly reduced relative to electron impact ionization at 70 eV, are consistent with masses 27−29 originating from a common neutral parent molecule, that is, ethyl radical photoproducts, rather than three different neutral species (ethane, ethyl, and ethylene). More evidence to support this conclusion is provided in the next section using translational energy distributions. The mass 30 product, which Henderson attributed to ethane from secondary surface reactions,18 is a weak feature in our spectra seen only at shorter pump−probe delays (see Figures 1 and 2). We also found that the intensity of the mass 30 peak varied from day to day, whereas the relative intensities of the mass 26−29 peaks were unchanged for the same photooxidation conditions. We suspected an impurity, and this was confirmed by allowing the sample to sit in the stainless steel tubing of the dosing inlet system for about 3 days. The photoproduct TOF mass spectrum taken with this 2-butanone sample showed a very prominent mass 30 peak that was comparable in intensity to the mass 29 peak (13.09 eV probe energy). These data are presented in the Supporting Information (Figure S3). Hence, we regard signals near mass

Figure 3. Arrival-time distribution of ethyl radical photoproduct (mass 29) from UV photooxidation of 2-butanone on a TiO2(110) surface. ●: experimental data. Dotted curves: fitted distributions for the “slow” (red) and “fast” (blue) distributions. Full curve: sum of the “slow” and “fast” distributions (dark red). The dotted line (black) represents zero flux intensity. The adsorbate adlayer was prepared as described in the Experimental Section. The photon energies of the pump and probe lasers were 3.7 and 13.09 eV (94.74 nm), respectively.

These data were corrected for delay-dependent background described in the Supporting Information. Similar to that found for methyl radicals from 2-butanone photooxidation in a previous study,22 the arrival-time distribution of ethyl radicals can be reasonably fit to two components, designated as “fast” and “slow”. These are shown as dashed lines in Figure 3. The analytical forms of the “fast” and “slow” distributions are not significant (modified Boltzmann distributions) but represent the minimum number of simple curves that provide a good description of the data. The two components are more readily seen in velocity and translational energy distributions for the ethyl radical products shown in Figure 4. The latter were derived from the arrival-time distribution following depletion correction and the appropriate Jacobian transformations. The translational energies for the overall distribution, ⟨Et (total)⟩, and that for the “slow” and fast” channels, ⟨Et (slow)⟩ and ⟨Et (fast)⟩, are given in Table 2 along with those for the methyl radical loss channel determined in an earlier study.22 The overall average translational energy obtained from the experimental data is small, ⟨Et⟩ = 51 meV, which reflects the dominant contribution of the “slow” ethyl fragmentation channel, ⟨Et (slow)⟩ = 24 meV, relative to the “fast” channel, ⟨Et (fast)⟩ = 98 meV. Hence, the primary ethyl fragmentation process leads to photoproducts with low kinetic energies. In 9294

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Figure 4. Final state distributions of ethyl radical (mass 29) photoproducts from UV photooxidation of 2-butanone on TiO2(110) taken at a surface temperature of 200 K and in the presence of background O2 at 5 × 10−8 Torr. (a) Velocity distribution; (b) translational energy distribution. “●” are the experimental data, the dotted curves correspond to the fitted distributions for the “slow” (red) and “fast” (blue) distributions, and the full curve (dark red) is the sum of the “slow” and “fast” distributions. The dotted line (black) represents zero flux intensity. Other conditions are the same as in Figure 3.

Figure 5. Translational energy distributions of mass 27 (C2H3; top panel) and mass 28 (C2H4; bottom panel) photoproducts from UV photooxidation of 2-butanone on TiO2(110). The experimental data were taken at a surface temperature of 200 K and in the presence of background O2 at 5 × 10−8 Torr. “●” are the experimental data points, and the solid line (red) corresponds to the translational energy distribution for the ethyl radical (mass 29) scaled to give the best overlap with the experimental points. The dotted line (black) represents zero flux intensity. The adsorbate adlayer was prepared as described in the Experimental Section. The photon energies of the pump and probe lasers were 3.7 and 13.09 eV (94.74 nm), respectively.

Table 2. Average Translational Energies (Total, Fast, Slow) of Ethyl and Methyl Radical Translational Energy Distributions from UV Photooxidation of 2-Butanone on TiO2(110)a photoproduct methyl radical ethyl radical

⟨Et (total)⟩ (eV)

⟨Et (slow)⟩ (eV)

⟨Et (fast)⟩ (eV)

relative intensity fast/slow

0.195

0.026

0.221

6.3

0.051

0.024

0.098

0.3

dissociative ionization of ethyl radical, which is the main C2Hx species ejected from the surface as a result of 2-butanone photooxidation. This conclusion is also consistent with the aforementioned VUV photoionization study of ethyl radical where both C2H3+ and C2H4+ daughter fragments were observed with energy-dependent cross sections.42 Unfortunately, the signal-to-noise for mass 26 (C2H2+) was too poor using the VUV ionization probe to extract final state information. However, the data in Figures 1 and 2 show that mass 26 does not have the same pump−probe delay dependence as masses 27−29, and therefore would not be expected to exhibit the same velocity and translational energy distributions. Although we are unable to unambiguously establish the origin of the mass 26 peak, it does not appear to be associated with nascent ethyl radicals, which are ejected directly into the vacuum.

The average energies are taken from the empirical fits to the experimental arrival-time distributions. a

contrast, the previously measured translational energy distribution for methyl radicals from 2-butanone photooxidation is dominated by a “fast” channel with ⟨Et (fast)⟩ = 218 meV and an overall energy of ⟨Et⟩ = 190 meV. Translational energy distributions were also determined for masses 27 (C2H3+) and 28 (C2H4+) to aid in assigning the origin of these species, that is, parent ions or daughter fragments. The translational energy distributions for species corresponding to masses 27 and 28 are presented in Figure 5. Here, the “●” represent the data, and the solid lines are the translational energy distribution of the ethyl radical (mass 29) taken from Figure 4 and scaled for purposes of comparison. It can be seen that all three species have nearly identical translational energy distributions and thus are all likely to originate from a single neutral species, that is, the ethyl radical. This observation strongly supports the conclusion that masses 27 (C2H3+) and 28 (C2H4+) are daughter fragments from the

IV. DISCUSSION In Figure 6 we compare the translational energy distributions for the methyl and ethyl photoproducts from 2-butanone photooxidation. The energy curves shown in Figure 6 are the overall fits to the experimental data (see, for example, Figure 4b) and arbitrarily normalized to the highest flux value for ease of comparison. The methyl data were obtained using identical O2/butanone coverage (dosed at 100 K) and running conditions (200 K; 5 × 10−8 Torr).22 The low yield of methyl 9295

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secondary reaction products (e.g., ethylene and ethane) in our photoinduced mass spectra (see Figures 1 and 2) whose formation would have required collisions and possible trapping of the nascent ethyl radicals at the surface prior to desorption. Before discussing the possible origins of the “fast and “slow” fragmentation channels observed in the ethyl radical velocity and translational energy distributions, it is useful to briefly review the assignments of the analogous features in the energy distributions of the methyl radical fragments. In our earlier studies of methyl loss from the UV photooxidation of acetaldehyde, acetone, 2-butanone, and acetophenone, we attributed the “fast” and “slow” channels to fragmentation of an internally “hot” intermediate species formed by resonant electron transfer from the ketone−oxygen complex to a nearsurface hole (h+).22,27 The latter were created by photoexcitation of the TiO2(110) substrate above the band gap (hν ≥ 3.05 eV for rutile TiO2). The photoreaction can be written as

Figure 6. Comparison of translational energy distributions for methyl (solid) and ethyl (dashed) radicals produced by UV photooxidation of 2-butanone on TiO2(110). The dotted line (black) represents zero flux intensity. The individual curves were generated by analytical fits to the experimental data and arbitrarily scaled to each other at the peak of the “slow” channel. See text for details.

R(COO)(CH3)(ad) + h+ → [R(COO)(CH3)(ad)]* → R(COO)(ad)* + CH3(g)(slow, fast)

radicals (see Figure 1) required a more sensitive ionization probe, and VUV single-photon ionization was replaced by a (2 + 1) resonant multiphoton ionization scheme.22,41 From Figure 6, it is seen that the methyl translational energy distribution is clearly bimodal, but unlike ethyl radical, it is dominated by the “fast” channel whose energy extends out to ∼0.6 eV. As noted earlier, this methyl energy distribution is very similar to that observed for methyl photoproducts resulting from the photooxidation of the related molecules acetaldehyde, acetone, and acetophenone, indicating similar fragmentation dynamics.22 As a result of the dominant “fast” channel, the average energy of the methyl photoproducts (⟨Et (total)⟩ = 195 meV) is significantly higher than that for ethyl (⟨Et (total)⟩ = 51 meV; see Table 2). On the other hand, the “slow” components of both the methyl and the ethyl products have nearly identical average energies of ∼25 meV. The latter are close to the thermal energy of the surface at 200 K, that is, 2kT ≈ 35 meV, which could suggest that the “slow” channels for both methyl and ethyl products originate from thermal processes. A similar, nearthermal “slow” channel was observed in the translational energy distributions of O2 molecules photodesorbed by UV light from a reduced TiO2(110) surface.21 In that case, the average energy of the “slow” channel could be described by a Boltzmann distribution, which tracked the surface temperature. This “slow” channel was attributed to a trapping-desorption mechanism in which photoexcited O2 molecules become trapped on the surface and subsequently desorb thermally. By contrast, the methyl translational energy distributions from acetone photooxidation on TiO2(110) were found to be insensitive to surface temperatures between 100 and 200 K.22,27 As a result, the “slow” methyl products from acetone photooxidation are more likely associated with a distinct low energy fragmentation pathway and not trapping-desorption. Because of the obvious similarities between the methyl translational energy distributions from acetone and butanone, we assume that the fragmentation dynamics for methyl loss are also similar; that is, the “slow” and “fast” channels are distinct fragmentation pathways.22,27 Signal limitations prevented us from obtaining ethyl energy distributions at lower surface temperatures (e.g., 105 K); nonetheless, we favor a similar assignment of the “slow” and “fast” ethyl products to two distinct fragmentation channels. This conclusion is partially supported by the lack of

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We have written the initial adsorbate as the η2-diolate complex, which Henderson has proposed is the photoactive species formed by a thermal reaction between the adsorbed aldehyde (R = H) or ketone (R = CH3, CH3CH2, C6H5) and molecular oxygen.16,18,25,37 By charge balance, the excited asterisks on the intermediate and surface bound product can be formally interpreted as representing excited “cation” species, that is, [R(COO)(CH3)(ad)]+* and R(COO)(ad)+*, induced by resonant charger transfer to the photohole (h+) in the first step of reaction 1. These “cation” species will nonetheless be shortlived, and the surface bound fragment, R(COO)(ad)+*, is expected to be neutralized during relaxation to the ground state of the carboxylate, R(COO)(ad). To understand the fragmentation dynamics, however, it useful to consider both neutral and cation pathways because the analogous gas-phase photodissociation and dissociative ionization processes have been studied for many of the simple ketones, including 2butanone.43−45 By analogy with the UV (248 nm) photochemistry of gasphase acetone, the “fast” channel for the methyl fragment was assigned to dissociation over an exit barrier whose average energy and width are reflected in the “fast” channel energy distribution.46,47 The barrier is on the lowest energy triplet 3 (n,π*) surface, which is populated by intersystem crossing (ISC) from the photoexcited singlet 1(n,π*) S1 state. Some clue as to the dynamical origin of the “slow” channel comes from more recent studies of the secondary fragmentation of the internally hot acetyl radical to form CO and a second methyl fragment, that is, CH3CO* → CO + CH3.48 Specifically, stateresolved measurements show that CO products in low rotational states (J″ < 12) exhibit bimodal translational energy distributions with a “slow” component (⟨Et⟩ = 38 meV) that is very similar to what is observed for methyl fragments in our ketone photoionization studies. The origin of this low energy fragmentation channel was attributed to internal conversion (IC) from S1 to S0 followed by dissociation on S0 without a barrier, leading to very low translational energies for the subsequent CO and CH3 products. Following this mechanism, we ascribe the “slow” channel observed in the methyl translational energy distribution (see Figure 6) to a barrierless dissociation process following IC of the initially prepared 9296

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has a higher threshold energy, which would lead to less excess energy available for the neutral ethyl radical products and a concomitant lower average translational energy. Indeed, the increase in appearance potential for the formation of neutral ethyl radical (100−130 meV) is very close to the observed decrease (∼120 meV) in the average energy of the “fast” ethyl fragments relative to that for methyl fragments (see Table 2). The most striking difference between the ethyl and methyl fragment energy distributions in Figure 6 is that the former is dominated by the “slow” channel with a “fast” to “slow” intensity ratio ∼20× smaller than that for methyl fragments (see Table 2). Assuming that the overall mechanism yielding “slow” methyl and ethyl fragments is the same, the dominant “slow” channel for the ethyl fragments suggests that the ICfragmentation process is far more efficient for ethyl than methyl products. Additional insight into the nature of the “slow” channel in 2-butanone dissociation comes from recent ab initio theoretical calculations by Nádasci et al., who investigated the energetics and dynamics of fragmentation following photoexcitation to S1 using 308 and 248 nm light.45 An excitation wavelength of 335 nm was used in this work, so the dynamics at 308 nm are likely to be more relevant to our system. At 308 nm (∼0.3 eV excess energy), the calculations of Nádasci et al. suggest that the dominant mechanism involves a nonadiabatic transition from S1 to S0, followed by dissociation on the S0 surface with no barrier to form both ethyl or methyl products. Without an exit barrier, the ethyl and methyl fragments are likely to be formed at very low energies.48 This is analogous to the IC photodissociation pathway for acetone discussed above. Internal conversion from the electronically excited-state intermediate complex (η1-butanone or η2-butanone diolate in reaction 2) to internally excited levels of the ground state on the time scale of dissociation is certainly plausible and is analogous to DIET (desorption induced by electronic transitions) mechanisms previously used to describe molecular photodesorption from surfaces.52 Overall, this mechanism shows how “slow” fragment channels could result from IC of the excited intermediate; however, it still remains unclear why the low energy pathway should dominate for the ethyl fragments. It may be that the specific dynamics of the nonadiabatic S1→S0 conversion step and the higher density of states of the ethyl radical fragment combine to dramatically increase the rate of the IC-fragmentation process relative to the over-the-barrier dissociation in the excited state. Finally, the photooxidation of 2-butanone on TiO2(110) is one of a few systems where both products of α-bond cleavage have been observed, that is, methyl and ethyl radicals. As is evident from Figure 1 of this work, the ethyl radical fragment is the dominant loss channel. At the simplest level, we might expect the radical fragment with the lowest α-carbon bond energy to be favored. This argument is consistent with the known bond dissociation energies of gas-phase 2-butanone for which the ethyl group is slightly lower (3.61 ± 0.03 eV) than the methyl group (3.67 ± 0.04 eV) (values at 298 K).44 Recent density functional calculations by Henderson also examined the relative energies of the η2-butanone-diolate species bound on the TiO2(110) surface relative to the surface bound and gasphase species products (see reactions 1 and 2).25 The results show that the transformation from the η2-butanone-diolate species to the ethyl products ((COO)(CH3)(ad) + CH3CH2(g)) is less endothermic than the methyl products (CH3(COO)(ad) + CH3(g)) by 16 kJ/mol (0.17 eV). Although the overall energetics appear to favor ethyl products, they are only likely to

excited intermediate. This assignment is also consistent with the results of trajectory calculations for methyl loss from the acetone cation as a function of internal energy.49 Here, we identify the excited intermediate in reaction 1 as a short-lived cation formed by resonant electron transfer to the surface. In the calculations of Zhou et al., the low energy methyl fragments appeared at longer simulation times, consistent with a cation intermediate that undergoes IC before fragmentation.49 In the present example of 2-butanone, the “fast” to “slow” intensity ratio for methyl products is ∼6 (see Table 2), which suggests that the IC process is relatively inefficient as compared to the more direct process in which fragmentation to methyl products proceeds over a barrier in the excited (triplet) state. Returning to the ethyl products from 2-butanone, we can write a photoreaction analogous to that given in reaction 1, that is: (CH3CH 2)(COO)(CH3)(ad) + h+ → [(CH3CH 2)(COO)(CH3)(ad)]* → (COO)(CH3)(ad) * + CH3CH 2(g)(slow, fast)

(2)

By comparison to the methyl distributions, the appearance of “fast” and “slow” ethyl fragments suggests that the fragmentation mechanisms are similar; that is, the “fast” channel arises from dissociation over an exit barrier, whereas the “slow” channel originates from an excited intermediate that has undergone IC prior to fragmentation. The lower average energy of the “fast” ethyl fragments (98 meV) as compared to methyl (221 meV) indicates that the barrier height relative to the energy of the products is correspondingly smaller. This may reflect the slightly smaller dissociation energy of the α-carbon bond for the ethyl group (3.61 ± 0.03 eV) versus the methyl group (3.67 ± 0.04 eV).44 Another contributing factor could be the higher density of states of the ethyl group, which has 15 vibrational modes as compared to only 6 for methyl. A larger fraction of the ethyl vibrations are also low energy modes, which makes them more energetically accessible.50 A high density of states in low energy modes could act as a “sink” for excess energy in the ethyl fragments and thereby reduce the available energy for translation. Hence, the lower average energy of the “fast” ethyl fragments may suggest that the ethyl fragment has significant internal excitation. Our ethyl radical detection using one-photon ionization does not provide direct information on the ethyl internal state distribution, but an internally “hot” ethyl fragment could affect the dissociative ionization probabilities leading to the daughter fragments at mass 28 and 27 (see Figures 1 and 2). Alternatively, if we assume that the excited intermediate in reactions 1 and 2 is a cation species formed by resonant electron transfer to the surface photohole, then the relevant bond dissociation energies would correlate with the threshold energies for appearance of the methyl and ethyl radical products (and the corresponding “ionic” fragments on the surface). Threshold energies (or appearance potentials) for fragmentation of cations are typically measured by photoionization mass spectrometry or photoelectron−photoion coincidence (PEPICO) using tunable VUV radiation.51 For gas-phase 2-butanone, PEPICO studies determined the appearance potentials for the CH3CHCO+ + CH3 and CH3CO+ + CH3CH2 fragmentation channels at energies of 10.20 eV (10.35 eV) and 10.30 eV (10.48 eV) at 298 K (0 K), respectively.43,44 In this case, the formation of the ethyl radical 9297

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energies, we are able to show that the C2Hx (x = 4, 3) products at masses 27 and 28 are not due to secondary reactions of ethyl on the surface, but rather from dissociative ionization of the ethyl radical parent ion (mass 29). The origin of a C2H2 (mass 26) peak in the product mass spectrum is not known at this time, but must originate from some other species on the surface as its time delay dependence was very different from the ethyl product. Pump−probe measurements were used to obtain translational energy distributions for the ethyl radical, which were found to be bimodal with a dominant low energy component (⟨Et (slow)⟩ = 24 meV). The assignment of the daughter fragments (C2Hx; = 4, 3) to ethyl (C2H5) dissociative ionization was also confirmed by showing that all three products have the same translational energy distributions. The measured methyl translational energy distribution is also bimodal, but here the “fast” channel (⟨Et (fast)⟩ = 221 meV) is the dominant component while the “slow” channel makes only a small contribution (⟨Et (slow)⟩ = 24 meV). The energy distributions and relative yields of the methyl and ethyl fragments are discussed in terms of both neutral and ionic intermediates. For both methyl and ethyl fragments, we assign the “fast” channel to a prompt fragmentation of an excited intermediate that is formed via resonant electron transfer of the butanone−oxygen complex (diolate) to fill photoholes (h+) at the TiO2(110) surface. The average energy of the “fast” fragments is associated with the height of a barrier in the dissociation coordinate. The low energy fragments from the “slow” channel are thought to arise from an interval conversion process of the excited intermediate back to an internally excited ground state that then undergoes subsequent dissociation without a barrier. Finally, we note that the relative yield of the methyl to ethyl radical products is more likely a consequence of excited-state dynamics rather than relative bond energies or stabilities.

be a controlling factor for thermally induced reactions. Even here, one needs information concerning the relative transition state activation barriers to judge the efficacy of a given dissociation pathway. The photoinduced reactions 1 and 2, however, take place on excited-state potentials where small energetic differences may not play a significant role in controlling the dynamics of fragmentation. Returning to the alternative description of the excited intermediate as a short-lived cation formed by electron transfer to a surface photohole, we previously noted that the appearance threshold for ethyl fragments from the butanone cation is higher than that for methyl fragments by 100−130 meV.44 In this case, we might expect the methyl product to be favored, but again these small energetic differences may not influence the relative fragment yields because dissociative ionization is strongly dependent on the cation internal energy deposited by the ionization process. In particular, detailed “breakdown” curves have been measured for the dissociative ionization of gas-phase 2-butanone by PEPICO techniques and have shown that the methyl channel is strongly favored for small internal energies (0−0.5 eV), where the difference in appearance energies is important.44 At higher internal energies (≥0.65 eV), ethyl formation becomes the major fragmentation channel. Hence, the relative yields of the methyl to ethyl products will depend very strongly on the internal energy of the intermediate cation following electron transfer. Unlike the photoexcitation process where we have a sharp energy from which to estimate the internal excess energy, the “excitation energy” of the resonant electron transfer process to the photohole is not very well-defined, because the photoholes can have a range of energies and the HOMO levels of the adsorbate are likely to be substantially broadened by bonding interactions with the TiO2 substrate. Moreover, methyl translational energy distributions for acetone photooxidation on TiO2(110) do not show any dependence on photoexcitation energy (hν = 3.25 eV, 3.70, and 4.38 eV).27 This result suggests that the photoholes are probably thermalized at the top of the valence band, and hence it is not possible to change the internal energy of the excited complex. Assuming that the electron transfer process is instantaneous on the time scale of nuclear motion, we can nonetheless envisage a vertical transition from a neutral diolate complex to a cation species whose internal energy is determined by a Franck−Condon envelope. The latter will determine the internal energy of the intermediate, which in turn controls the IC and direct fragmentation rates and ultimately the relative methyl to ethyl fragment yields. Although it is not possible to extract such information from our data at present, comparisons with neutral and ionic fragmentation processes in gas-phase 2-butanone suggest that the internal energy is high enough (>0.1 eV) to overcome energetic differences in the initial or final product stabilities. These considerations suggest that excited-state dynamics are more important than static energetics in determining the ethyl to methyl product yields from photooxidation of 2-butanone on TiO2(110).

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ASSOCIATED CONTENT

* Supporting Information S

Additional product mass spectra and TPD results are presented that provide additional support for mass assignments related to ethyl radical formation, secondary reactions, and ionization detection. Also included are details of the procedure used to remove background signals from arrival-time-distributions for ethyl radical mass fragments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (631) 344-4345. Fax: (631) 344-5815. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experiments were carried out in the Chemistry Department at Brookhaven National Laboratory under Contract No. DE-AC02-98CH10086 with the U.S. Department of Energy (Division of Chemical Sciences).

V. SUMMARY The photooxidation of 2-butanone on TiO2(110) was studied under UHV conditions using a pump−probe TOF method to understand reaction dynamics upon exposure to UV light. Using VUV radiation to detect the gas-phase products via single-photon ionization, we observe both methyl (CH3) and C2Hx (x = 5, 4, 3) products, which appeared to be associated with the loss of ethyl radical (C2H5). Using two different VUV

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp401838r | J. Phys. Chem. C 2013, 117, 9290−9300