Reactions of Photodesorption Products with Ice Overlayers on the ...

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Impact of Solvent on Photocatalytic Mechanisms: Reactions of Photodesorption Products with Ice Overlayers on the TiO2(110) Surface Mingmin Shen and Michael A. Henderson* Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-87 Richland, Washington 99352, United States ABSTRACT: The effects of water and methanol ice overlayers on the photodecomposition of acetone on rutile TiO2(110) were evaluated in ultrahigh vacuum (UHV) using photon-stimulated desorption (PSD) and temperature-programmed desorption (TPD). In the absence of ice overlayers, acetone photodecomposed on TiO2(110) at 95 K by ejection of a methyl radical into the gas phase and formation of acetate on the surface. With ice overlayers, the methyl radicals are trapped at the interface between TiO2(110) and the ice. When water ice was present, the ejected methyl radicals reacted either with each other to form ethane or with other molecules in the ice (e.g., water or displaced acetone) to form methane (CH4), ethane (CH3CH3), and other products (e.g., methanol) with all of these products trapped in the ice. The new products were free to revisit the surface or to depart during desorption of the ice. Using isotopic labeling, we show that a significant portion (∼50%) of methane formed resulted from reactions of methyl radicals with water in the ice. Because the methane formation from reaction of methyl radical and water is highly endothermic, the ejected methyl radicals must be emitted hyperthermally with the reaction occurring during the initial collision of the radical with a neighboring water molecule. Formation of ethane (and other products) likely comes as a consequence of unfavorable methyl radical and water collisions in the ice. Similar results were obtained using methanol ice (instead of water) except that methane and ethane products slowly leaked through the methanol ice overlayers into vacuum at 95 K but not through the water ice overlayers. These results provide new insights into the product formation routes and solution-phase radical formation mechanisms that are important in heterogeneous photocatalysis.

1. INTRODUCTION Studies of radical species generated during heterogeneous photocatalysis are of extreme importance for understanding the mechanistic reaction pathways.16 The prevalence of radicals in heterogeneous photocatalysis should come as no surprise since the basis of photocatalytic reactivity involves single electron transfer events that drive surface redox chemistry. The photocatalyst surface can serve both as the origin of photogenerated radicals and as the host to subsequent reactions of these species. In some cases, radical species are believed to be ejected from the surface by a photoinitiated event (i.e., photodesorption) and to participate in reactions in the media above the photocatalyst (e.g., gas phase, solvent environment, or physisorbed layer surrounding the photocatalyst). The behavior of radical species at the interfaces between photocatalysts and other phases requires attention given the potential involvement of radicalsolvent side reactions in photocatalysis. Acetone represents a serious air pollutant for indoor environments, and photocatalytic decomposition of acetone (as well as other organic compounds) to less harmful compounds is a potential pathway to removal of these pollutants. TiO2 has attracted much attention in the literature because of its promising activity in UV-induced decomposition of organics. Heterogeneous photocatalytic oxidation of acetone on TiO2,710 as well as on other organic compounds,4,11,12 is a potential degradation method that r 2011 American Chemical Society

has been the subject of several investigations. Acetone photochemistry has been extensively studied on powder TiO2 (anatase and rutile) materials7,1320 and TiO2 single crystals in ultrahigh vacuum (UHV) systems.2126 UHV surface science has been instrumental in bringing new insights to the field of heterogeneous photocatalysis. For example, photodesorption products are easily detected in UHV offering new means for gaining insights to the mechanisms of photocatalytic reactions. However, the advantage of UHV in studying photocatalytic reactions on model single crystal surfaces, such as rutile TiO2(110), becomes a limitation when it comes to understanding the influence of solvent in photocatalysis. Numerous groups have shown that water plays an important role in photocatalytic reactions in high pressure and aqueous conditions. For example, Peral and Ollis reported that water vapor inhibited acetone oxidation on TiO2.7 Other studies13,17,2729 suggest that water is an important supplier of the raw materials for OH radical reactions. In this study, water and methanol ice overlayers on acetonecovered rutile TiO2(110) were used to model the solventsurface interface in exploration of the solvent effect in heterogeneous photocatalysis. The charge-transfer processes that generated Received: December 13, 2010 Revised: February 2, 2011 Published: March 10, 2011 5886

dx.doi.org/10.1021/jp111839j | J. Phys. Chem. C 2011, 115, 5886–5893

The Journal of Physical Chemistry C

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methyl radicals during photodecomposition of acetone on TiO2(110) under UHV conditions were not affected by the presence of water or methanol ice overlayers. That is, the charge-transfer reaction (likely, hole-mediated) that leads to photodecomposition of acetone diolate to acetate and to an ejected methyl radical did not appear to be significantly altered by the nearby presence of water or methanol. However, photodesorbed methyl radicals were trapped at the TiO2(110) interface by thick ice layers. These trapped radicals abstract hydrogen from different kinds of adsorbates at the iceTiO2 interface to form methane and to migrate together to form ethane. These reaction pathways were not observed on TiO2(110) in the absence of an ice overlayer.

2. EXPERIMENTAL SECTION The UHV chamber used in this study is described in detail elsewhere.30 The base pressure of experiments in this study was 4 1010 Torr. The rutile TiO2(110) crystal was obtained from Princeton Scientific with dimensions of 10 10 1.5 mm. Surface cleaning was accomplished through cycles of Arþ sputtering and annealing. Water temperature-programmed desorption (TPD) was used to characterize the quality of the surface and the concentration of oxygen vacancy sites.31,32 The crystal surface used in this study possessed an oxygen vacancy population of ∼0.05 ML, where 1 ML is defined as the surface-site density of fivecoordinated Ti4þ cations on the ideal TiO2(110) surface (5.2 1014 molecules/cm2). Routine daily cleaning was accomplished by annealing the crystal at 850 K for ∼1530 min in UHV. These annealing treatments did not significantly increase the oxygen vacancy population over the course of the experiments described here.25 TPD experiments were performed with a heating rate of 2 K/s. Oxygen was dosed by backfilling the chamber. The actual oxygen exposure to the crystal tended to be slightly greater than the cited values because of additional exposure occurring during pump-down. Dosing of other gases on the TiO2(110) surface was achieved using a triply differentially pumped molecular beam doser that delivered the reagent in an ∼5 mm spot centered on the crystal face. High-performance liquid chromatography (HPLC) grade acetone (Fisher Scientific, 99.7%), water (Aldrich), fully deuterated acetone (Cambridge Isotope Laboratories, 99.9%, note as D6-acetone hereafter), HPLC grade methanol (SigmaAldrich, 99.9%), and D3-methanol (CD3OH, Cambridge Isotope Laboratories, 99.9%) were further purified using liquid-nitrogen (LN2) freezepumpthaw cycles. For each experiment, the vacuum-annealed surface was exposed to 10 L O2 (1 L = 1 106 Torr s) at ∼95 K followed by flashing to 300 K (to dissociate adsorbed O2 and to remove any background water adsorbed during the O2 exposure) before recooling for gas exposures. Acetone (D6-acetone) was exposed to the oxidized TiO2(110) surface at 95 K through the molecular beam. The sample was then flashed to 225 K before recooling for water or methanol exposure. Acetone, water, and methanol coverages were determined through knowledge of the exposure needed to saturate the first layer.21,32,33 UV irradiation was accomplished at ∼95 K using a fiber optic light delivery system from a 100 W Hg arc lamp as described in detail elsewhere.22,26 For photon-stimulated desorption (PSD) measurements, the sample was oriented ∼45° relative to the entrance aperture of the quadrupole mass spectrometer (QMS). The only operational difference between this and previous work was that the distance between the crystal and the fiber optic tip was adjusted so that the entire crystal face was irradiated instead

Figure 1. Methyl radical (mass 15) photodesorption spectra from UV irradiation in UHV at 95 K of ∼0.5 ML acetone adsorbed on preoxidized TiO2(110) with various amounts of water ice overlayers. (See Experimental Section for details on the preoxidation procedure.) The inset shows methyl radical (mass 15) photodesorption yields as a function of water ice overlayer coverage.

of just the 5 mm diameter adsorbate spot on the crystal face. Irradiation of the entire TiO2(110) crystal face did not result in spurious desorption events of concern since only the 5 mm diameter adsorbate spot was responsible for methyl radical production. Under these conditions, only a 3 K temperature rise was registered at 95 K after 15 min UV irradiation. TPD was done with the crystal oriented normal to the apertured QMS.

3. RESULTS AND DISCUSSION 3.1. Acetone Photodecomposition with Water Ice Overlayers. The objective of this section is to examine the effect of

water ice overlayers on the yield of methyl radical photodesorption from acetone on TiO2(110). Figure 1 shows a series of methyl radical (CH3 3 , mass 15) photodesorption traces from UV irradiation at 95 K of ∼0.5 ML acetone adsorbed on preoxidized TiO2(110) with various amounts of water ice overlayers. The inset shows the integrated PSD yield at the different water coverages. The time designated 0 s corresponds to the start of UV exposure. Methyl radical PSD signal decreased dramatically when the water ice coverage increased from 0 to 10 ML, and no PSD signal could be detected when the water ice coverage was increased to 15 ML. As reported previously, water and acetone compete for adsorption sites (the five-coordinated Ti4þ cations) on the TiO2(110) surface with water generally displacing acetone from the first layer. However, water does not affect the acetone diolate species once it has formed.21,30 Assuming that the water ice overlayer should not affect the transmission of UV light to the crystal face (i.e., because water does not absorb UV light), similar amounts of electron/hole pairs should be generated in the TiO2(110) crystal with or without the ice, and similar rates of methyl radical production should ensue in both cases. The decreasing trend of methyl radical yield with increasing water ice overlayer shown in the inset of Figure 1 can be assumed to result from the inability of methyl radicals to escape the surface because of trapping by the water ice overlayer. Clearly, the gradually reduced PSD signal with water ice overlayers indicates 5887

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Figure 2. Mass 15 (a) and mass 16 (b) TPD spectra from ∼0.5 ML acetone adsorbed on preoxidized TiO2(110) and irradiated with UV light for 15 min at 95 K with various amounts of water ice overlayers.

that the trapping effect increased with water ice overlayer. However, some methyl radical species could still escape from the surface even after several water ice layers, which suggests that water, as deposited at 95 K, does not fully wet the surface with acetone and acetone diolate present. (The wetting of this surface by water dosed at 140 K was worse than at 95 K because water crystallized on the surface at this temperature.34) TPD after UV irradiation was used to explore for potential reaction products resulting from methyl radicals trapped in the ice. Figure 2a and b shows TPD results for mass 15 (left) and mass 16 (right) signals. Mass 15 depicts signal from any methylcontaining species (e.g., from QMS cracking of methane or unreacted acetone), whereas mass 16 signal resulted from either methane or O-containing species (e.g., from QMS cracking of water, acetone, or O2). There were two TPD peaks in these curves: one at low temperature, shifting from 125 to 155 K with increasing water ice coverage, and the other at high temperature, shifting from 150 to 160 K with increasing water ice coverage. On the basis of previous TPD results of coadsorbed water and acetone on TiO2(110), the higher temperature TPD peaks of mass 15 and 16 should be attributed to the cracking fragments of unreacted acetone ice23 and water ice, respectively, desorbing in coincident peaks. For low-temperature TPD peaks, the ratio of mass 15 and 16 at the same water ice coverage is ∼0.8 to 0.95, which matches the cracking pattern ratio of methane (CH4),35 indicating the low-temperature peaks were from methane desorption. In addition, the position and the full width at halfmaximum (fwhm) of the methane desorption peaks were dependent on the thickness of the water overlayer. For example, the desorption maxima shifted from 125 to 155 K, close to the ice desorption temperature, and sharpened with increasing ice overlayer thickness. This trend is similar to the variation of the TPD peak for O236 or CCl437 trapped under water ice as a function of water overlayer thickness. The abrupt sharpness of these desorption states is linked to the phase transition of the ice from a metastable amorphous solid water (ASW) structure to a crystalline ice (CI) structure.37 Therefore, the shifting and sharpening of the CH4 desorption peak with increasing water coverage was caused by thickening of the ice film and by the resulting ASW-toCI phase transition as the ice is thickened. Figure 3a and b shows TPD traces of masses 27 and 30. Similar desorption features as those in the mass 15 and 16 channels were

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Figure 3. Mass 27 (a) and mass 30 (b) TPD spectra from ∼0.5 ML acetone adsorbed on preoxidized TiO2(110) and irradiated with UV light for 15 min at 95 K with various amounts of water ice overlayers. Both Y axes are multiplied by 4 to match the intensity scale in Figure 2.

Figure 4. Methyl radical PSD yield (left, squares), methane TPD yields (right, triangles), and ethane TPD yields (right, diamonds) as a function of water ice overlayer coverage.

observed in mass 27 traces for the same set of experiments (left side of Figure 3). The high-temperature TPD peaks (at 150160, 188, and >250 K) are attributed to QMS cracking of acetone split into several states because of the influence of coadsorbed water.23 The mass 30 traces (right side of Figure 3), however, were only present in the low-temperature TPD peak and could not be assigned to QMS cracking of acetone. For the low-temperature TPD peaks, the mass 27, 29 (not shown), and 30 ratios at the same water coverages matched the QMS cracking pattern ratios expected for ethane (CH3CH3). A similar ASW-to-CI effect as seen with methane (see Figure 2) was also found for the ethane TPD peaks as a function of increasing water coverage. The data in Figures 2 and 3 therefore indicate that methane and ethane formed from reactions of trapped methyl radical and that these species were trapped at the iceTiO2(110) interface for sufficiently thick ice layers. Figure 4 shows the methyl radical PSD yield (squares), the methane TPD yields (triangles), and the ethane TPD yields (diamonds) as a function of water coverage. The CH3 PSD signal was collected over the initial 100 s of irradiation beyond which 5888



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to form CH4 can involve hydrogen abstraction by methyl radicals from other organics or from water ice, the latter shown in eq 2. To further identify which of these reactions dominate hydrogen abstraction leading to methane, D6-acetone was used to label generated methyl radicals. If photodesorbed CD3 radicals only react with water ice (eq 2), then CD3H (mass 19 in TPD) should be the main methane product as shown as eq 4. However, if photodesorbed CD3 radicals react mainly with other organic adsorbates to form methane (eq 3), then CD4 (mass 20 in TPD) should be the main methane product as shown in eq 5.

Figure 5. Mass 19 (a) and mass 20 (b) TPD spectra from D6-acetone adsorbed on preoxidized TiO2(110) and irradiated with UV light for 15 min at 95 K with various amounts of H2O ice overlayers.

the signal attenuated to background. The CH3 PSD yield (lefthand axis) decreased monotonically when the water ice coverage increased from 0 to 10 ML with no PSD signal detected for water ice layers above ∼15 ML. In contrast, the methane (CH4) and ethane (CH3CH3) TPD yields increased with increasing water coverages as plotted on the right-hand axis of Figure 4. Because methane and ethane were the main TPD products from reactions of trapped methyl radicals, the yields of these two species represent the yield of trapped methyl radicals. Because of different sample positioning for PSD and TPD as mentioned in the Experimental Section, no direct quantitative comparison can be made between the CH3 PSD yield and the TPD yield of methane and ethane. However, qualitatively, Figure 4 shows that these two yields inversely correlate as a function of water coverage. This implies that methyl radicals trapped by ice are fated to react and form methane or ethane. The crossing point, as a function of water coverage, between the CH3 PSD yield and the CH4 TPD yield was at ∼7 ML of water. If the ethane TPD yield (actually, doubled yield since two radicals are required) could be included, the crossing point would be at a slightly lower water coverage. The approximate water coverage required to effectively bury the diolate covered regions of the TiO2(110) surface is ∼10 ML. 3.2. Reactions of Trapped Methyl Radicals with Water Ice Overlayer. Possible reactions of trapped methyl radicals to form methane and ethane at the iceTiO2 interface include those shown in eqs 13 CH3 3 þ CH3 3 f CH3 —CH3

ð1Þ

CH3 3 þ H2 O f CH4 þ 3 OH

ð2Þ

CH3 3 þ other adsorbates f CH4 , CH3 —CH3

ð3Þ

As shown in the TPD results in Figures 2 and 3, methane and ethane were formed only when photodesorbed methyl radicals were trapped by water ice overlayers. The ethane product could conceivably be formed from trapped methyl radicals reacting with each other (eq 1) or with other organic adsorbates (eq 3). The organics present include unreacted acetone, unreacted acetone diolate, or possibly acetates generated by photodecomposition of other acetone diolates. In contrast, reaction pathways

CD3 3 þ H2 O f CD3 H þ 3 OH

ð4Þ

CD3 3 þ other adsorbates f CD4 , CD3 —CD3

ð5Þ

Figure 5 shows TPD traces for masses 19 (left) and 20 (right) from photolysis of D6-acetone with various overlayer coverages of H2O. As pointed out earlier, the low-temperature TPD peaks between 125 and 155 K, shifting with increasing water ice coverage, for mass 19 and mass 20 were attributable to CD3H and CD4 methane desorption, respectively. Because QMS cracking of CD3H does not have signal at mass 20, nor does CD4 at mass 19, the similar TPD yields in these two peaks as a function of water ice coverages suggest similar yields for the two pathways in eqs 4 and 5. These data provide evidence that methyl radicals trapped under water ice abstract hydrogen atoms with roughly equal probability from both organic adsorbates and water ice. With the current experiments, it is hard to clarify which organic adsorbates played a dominant role in CD4 formation. The observation of methane from the reaction of methyl radicals with water molecules in the ice is significant. As reported previously,3840 the reaction of methyl radical with water to yield methane and hydroxyl radical (eq 2) is endothermic with ΔH = 60.5 and 104.5 kJ/mol for gas-phase and liquid-phase water, respectively.41 Much work4246 has been done to study interactions between water and methyl radicals forming methyl radicalwater complexes, where the OH 3 3 3 C interaction involving a hydrogen-bonding interaction between the unpaired electron on the carbon atom and the hydrogen on water was found as the most stable complex. The interaction is not strong enough to result in homolytic OH bond cleavage. For hydrogen abstraction to occur between methyl radical and water according to eqs 2 and 4, the photodesorbed methyl radical must have sufficient energy to overcome the thermal barrier in the reaction. (This barrier in the gas phase has been estimated by Tantawy and Zipse38 to be ∼69.4 kJ/mol.) Observations of CD3H formation from the reaction of CD3 and H2O indicate that methyl radicals ejected during photodecomposition of acetone diolate on TiO2(110) must be hyperthermal. In other words, a significant fraction of the photoejected methyl radicals must possess sufficient kinetic energy to overcome the activation energy of eq 2 in order to form methane, most likely in the first collision with a surrounding water molecule. The fact that the amount of methane formed possessing the hydrogen isotope of water constitutes a significant percentage (∼50%) of the overall methane (compare right and left panels of Figure 5) indicates that the probability of eq 2 occurring, in this setting, is high. However, the fact that other methyl radical reactions also occur (e.g., eqs 1 and 3) indicates that not all of the ejected methyl radicals have sufficient energy to overcome the barrier associated with eq 2 and that the trajectories of some ejected methyl radicals are not favorable to eq 2. For example, a hot methyl could collide 5889



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Figure 6. TPD spectra from different types of acetone adsorbed on preoxidized TiO2(110) and irradiated with UV light for 15 min at 95 K with various amounts of water ice overlayers. (a) Mass 31, acetone with water ice; (b) and (c) masses 33 and 35, D6-acetone with H2O ice, respectively.

first with the O-end of a water molecule, not react, become thermalized to the ice temperature (which is too low to thermally promote eq 2), and then remain in the ice to follow some other reaction process. By extension, TPD data (Figure 5) that supports eq 2 also suggests that OH radicals should be formed from the reaction of methyl radicals with water ice. Detection of OH radicals with TPD is complicated by the many thermal reactions known to occur between coadsorbed water and oxygen on TiO2(110).4751 However, an indirect means of detecting OH radicals is in their reactions to form unique products. For example, CH3 or CD3 radical could react with an OH radical to form methanol (CH3OH) or D3-methanol (CD3OH), respectively, as shown in eqs 6 and 7 CH3 3 þ 3 OH f CH3 OH

ð6Þ

CD3 3 þ 3 OH f CD3 OH

ð7Þ

TPD data shown in Figure 6 provides evidence for the formation of methanol and D3-methanol. In the absence of UV irradiation, coadsorption of acetone and water ice overlayers on oxidized TiO2(110) did not result in methanol formation (data not shown). Also, no methanol was detected in TPD as the result of acetone photochemistry in the absence of water ice overlayers (lower trace of Figure 6a). However, for the same experiments depicted in Figures 2 and 3, TPD shows that methanol (mass 31 QMS cracking fragment corresponding to CH3Oþ) was produced as the water ice film thickness increased (Figure 6a). The methanol desorbed coincident with the water ice TPD feature (150160 K) suggesting it was formed in the ice and not on the surface. Analysis of the mass 30 (not shown) to mass 31 signals confirmed that these peaks were attributed to methanol. When D6-acetone and H2O ice overlayers were employed, complementary mass 33 and 35 TPD peaks were detected (Figure 6b and c) in the place mass 31 in the acetone and H2O case (Figure 6a). Perusal of the mass spectra for various isotopic combinations for methanol3840 indicates that these signals came from CD3OH. (The lower temperature mass 33 TPD peak in Figure 6b, particularly prominent for thick ice, resulted from C2D6 cracking, but the higher temperature feature was CD3OH.) These data

Figure 7. Deuterated methyl radical (mass 18) photodesorption spectra from UV irradiation in UHV at 95 K of D6-acetone adsorbed on preoxidized TiO2(110) with various amounts of methanol ice overlayers. The inset shows CD3 radical (mass 18) photodesorption yields as a function of CH3OH ice overlayer coverage.

Figure 8. Mass 19 (a) and mass 20 (b) TPD spectra from D6-acetone adsorbed on preoxidized TiO2(110) and irradiated with UV light for 15 min at 95 K with various amounts of CH3OH ice overlayers.

support the potential of OH radicals generated according to eq 2 participating in reactions with unreacted methyl radicals according to eq 7. The amount of methanol produced at 15 ML water ice coverage is about 0.013 ML. 3.3. D6-Acetone Photodecomposition with Methanol Ice Overlayers. Results in the preceding section revealed that photodesorbed methyl radicals were able to abstract hydrogen from both OH bonds (in water) and CH bonds (in organics). In this section, we explore how methyl radicals react with a solvent possessing both types of bonds. Methanol is widely used as a solvent and as a reagent. Figure 7 presents results exploring the effect of methanol overlayers on methyl radical PSD signals. Using D6-acetone, increasing methanol ice coverage decreased the CD3 radical PSD signal similarly to what was seen with H2O ice with no PSD signal detected for methanol ice coverages higher than ∼8 ML. The inset plot in Figure 7 shows the integrated CD3 radical PSD yield change with increasing 5890



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Figure 9. CD3 radical PSD yield (left, squares), TPD yields for CD3H (right, triangles), CD4 (right, inverse triangles), and C2D6 (right, empty diamonds) as a function of CH3OH ice overlayer coverage.

Figure 10. Mass 19 (a) and 20 (b) TPD results from the reaction of CD3 radicals with 16 ML of CH3OH (lower traces) and CD3OH (upper traces) ice.

methanol coverage attributable to the trapping of methyl radicals by methanol ice overlayers. The similarity between the coveragedependent decay of the CD3 PSD yield in the methanol and water cases suggests similar degrees of wetting of the two ice layers over the acetone diolate covered TiO2(110) surface. However, the postirradiation TPD data was different in the two cases. Figure 8 shows TPD traces for masses 19 (left) and 20 (right), which corresponded to CD3H and CD4, respectively. There are two main desorption peaks in the mass 19 TPD traces: one at ∼106 K which appeared for methanol coverages > 8.6 ML and another appearing at methanol coverages g 5 ML that increased and shifted from 141 to 144 K with increasing methanol coverage. On the basis of the cracking pattern and the TPD behavior of methanol ice, the higher mass 19 TPD peak was due to desorption of methanol ice. For the low-temperature TPD peak, the ratio of the mass 18 (not shown) to the mass 19 signal matched the QMS cracking pattern of D3-methane (CD3H). Surprisingly, the mass 20 TPD signals (Figure 8b) were weak or nonexistent (e.g., the mass 20 signal at ∼106 K for 16 ML of methanol was only ∼5% of the mass 19 signal at the same temperature). Also, the TPD behavior of the methane detected in the methanol ice was very different from that detected in the water ice case (Figure 2). For example, the leading edge temperature of CD3H desorption with methanol ice was very close to the substrate base temperature (95 K). Also, the CD3H peak appeared as a sharp state at various methanol ice coverages in comparison to the broader, weaker methane signal detected at comparatively lower water ice coverages. TPD signals for C2D6 (not shown) were also detected at ∼106 K for higher methanol coverages consistent with CD3—CD3 bond formation. Figure 9 shows the CD3 radical PSD yield (squares) along with the TPD yields for CD3H (triangles), CD4 (inverse triangles), and C2D6 (empty diamonds) as a function of methanol ice coverage. While the decay of the CD3 PSD signal with increasing methanol ice coverage in the 510 ML coverage regime was similar to that seen with water ice (Figure 4), the ability of methanol ice in the 510 ML coverage regime to result in the CD3 radical reaction products in TPD was different in the two cases. The yields of products resulting from trapped CD3 radicals, as reflected in the postirradiation TPD spectra of methane and ethane, did not correlate well with the CD3 PSD yield. For example, when the methanol ice coverage was 5 ML,

more than half of the ejected CD3 radicals should have been trapped by methanol ice, resulting in trapped methane and ethane, but no obvious methane or ethane TPD signals were detected at this coverage in TPD. Given that the presence of product at higher methanol ice coverages illustrated that reactions do occur between methyl radicals and methanol ice molecules, there are two possible explanations for this behavior in the 510 ML methanol ice coverage regime. First, methyl radicals were initially detained at 95100 K by methanol ice films between 5 and 10 ML, at least to the extent that they do not appear in the sharp PSD spike seen upon immediate UV exposure, but slowly leak through the ice during the short time between completion of the PSD experiment and commencement of the TPD experiment (usually a few minutes at most), or the radicals were trapped and the products were formed but the products leaked through the methanol ice prior to TPD. Given that the leading edge of methane and ethane desorption in postirradiation TPD was close to the base temperature for the methanol ice case (see Figure 8), we suspect the latter is the cause for the PSD and TPD traces in Figure 9 not mirroring each other for methanol ice as was seen for water ice. Our timedependent TPD results after UV light irradiation with the same amount of methanol ice coverage (12 ML) also showed decreasing methane and ethane TPD yields with increasing time separation (total time from starting UV irradiation to commencement of the TPD experiment), which confirmed the leaking of methane and ethane through methanol ice. Potential products resulting from reactions of methyl radicals trapped in methanol ice include ethane and methane produced in eqs 1 and 3, respectively, similarly to the case of water ice. However, for the direct reaction of methyl radical with ice molecules, eq 2 in the case of water can be replaced with eqs 8 and 9 in the case of D3-methanol CD3 3 þ CD3 O H f CD3 H þ CD3 O 3

ð8Þ

CD3 3 þ C D3 OH f CD4 þ 3 CD2 OH

ð9Þ

The ability to differentiate between the contributions of eqs 8 and 9, as well as that of reactions reflected by eq 3, was obtained by selectively labeling the hydrogen atoms in methanol with deuterium. Figure 10 presents mass 19 (CD3Hþ) and mass 20 (CD4þ) 5891


The Journal of Physical Chemistry C TPD traces obtained from reactions of CD3 radicals trapped by 16 ML’s of CH3OH versus CD3OH ice. In both sets of data, the lowtemperature peaks (at ∼106 K) were the result of CD3 radical reactions and the higher temperature peaks (at ∼145 K) were from QMS cracking contributions associated with the methanol ices. (Note: the abundance of D exposed to the QMS in these experiments resulted in ionizer reactions that provided small mass 19 and 20 signals during desorption of the methanol ice despite the fact that methanol typically would not register signal in the QMS at these masses. The signals typical for QMS cracking of methanol resulting from desorption of the 16 ML coverage were orders of magnitude greater than the weak mass 19 and 20 signals shown in Figure 10.) In the case of CH3OH (bottom traces), TPD shows that CD3 radicals abstracted mainly H atoms from CH3OH but not D atoms from unreacted D6-acetone or other CD3 or D3-acetate photoproducts as depicted in eq 3. Contributions from eq 3 in the methanol ice case were eq 9 > eq 8. Gray and Herod’s study54 also showed that the rate constant for eq 9 is slightly higher than that of eq 8 at a temperature range of 408523 K regardless of reaction probabilities, which is inconsistent with our observation here. Although the abstraction of deuterium from D6-acetone has a much higher reaction rate in liquid-phase study, no such reaction is observed in our methanol ice overlayer study, which is different from the methyl radicals reacting with organics when water ice was employed. Given that ΔH = 2.8 and 37.8 kJ/mol with gasphase methanol and all substances not deuterated for eqs 8 and 9, respectively, most of the ejected methyl radicals will perform either eq 8 or eq 9, that is, even some hot radicals could collide with the O-end of methanol molecules without reaction and become thermalized to ice temperature, so that they could then react again with methanol ice to abstract hydrogen.

4. CONCLUSION PSD and TPD data presented here demonstrate that both water and methanol ices can trap and react with photoejected methyl radicals. These trapped methyl radicals form methane and ethane as main products through different kinds of reactions in the two ices. In the presence of water ice, trapped methyl

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radicals can abstract hydrogen from both unreacted acetone (and other organic adsorbates present) and water ice to form methane, which is also trapped in water ice. In the presence of methanol ice, trapped methyl radicals mostly abstract hydrogen from methanol ice to form methane, which could slowly leak through the methanol ice at our substrate temperature of ∼95 K. Different reactions in water and methanol ice come from different reaction thermodynamics.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Work reported here was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, and was performed in the William R. Wiley Environmental Molecular Science Laboratory (EMSL), a Department of Energy user facility funded by the Office of Biological and Environmental Research. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by the Battelle Memorial Institute under contract DEAC06-76RLO1830. ’ REFERENCES (1) Serpone, N.; Khairutdinov, R. F. Stud. Surf. Sci. Catal. 1996, 103, 417. (2) Litter, M. I. Appl. Catal., B: Environ. 1999, 23, 89. (3) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol., C 2000, 1, 1. (4) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (5) Kamat, P. V. Chem. Rev. 1993, 93, 267. (6) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (7) Peral, J.; Ollis, D. F. J. Catal. 1992, 136, 554. (8) Lichtin, N. N.; Avudaithai, M.; Berman, E.; Dong, J. Res. Chem. Intermed. 1994, 20, 755. (9) Larson, S. A.; Widegren, J. A.; Falconer, J. L. J. Catal. 1995, 157, 611. (10) Alberici, R. M.; Jardim, W. E. Appl. Catal., B: Environ. 1997, 14, 55. (11) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (12) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (13) El-Maazawi, M.; Finken, A. N.; Nair, A. B.; Grassian, V. H. J. Catal. 2000, 191, 138. (14) Xu, W.; Raftery, D. J. Catal. 2001, 204, 110. (15) Xu, W.; Raftery, D.; Francisco, J. S. J. Phys. Chem. B 2003, 107, 4537. (16) Coronado, J. M.; Kataoka, S.; Tejedor-Tejedor, I.; Anderson, M. A. J. Catal. 2003, 219, 219. (17) Coronado, J. M.; Zorn, M. E.; Tejedor-Tejedor, I.; Anderson, M. A. Appl. Catal., B: Environ. 2003, 43, 329. (18) Vorontsov, A. V.; Stoyanova, I. V.; Kozlov, D. V.; Simagina, V. I.; Savinov, E. N. J. Catal. 2000, 189, 360. (19) Raillard, C.; Hequet, V.; Le Cloirec, P.; Legrand, J. J. Photochem. Photobiol., A: Chem. 2004, 163, 425. (20) Zorn, M. E.; Tompkins, D. T.; Zeltner, W. A.; Anderson, M. A. Appl. Catal., B: Environ. 1999, 23, 1. (21) Henderson, M. A. J. Phys. Chem. B 2004, 108, 18932. (22) Henderson, M. A. J. Phys. Chem. B 2005, 109, 12062. (23) Henderson, M. A. Langmuir 2005, 21, 3443. (24) Henderson, M. A. Langmuir 2005, 21, 3451. 5892



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