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May 5, 2015 - ABSTRACT: Angle-resolved photon-stimulated desorption. (PSD) combined with infrared reflection−absorption spec- troscopy and temperatu...
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Insights into Acetone Photochemistry on Rutile TiO2(110). 2. New Photodesorption Channel with CH3 Ejection along the Surface Normal Nikolay G. Petrik,* Michael A. Henderson, and Greg A. Kimmel* Physical Sciences Division, Pacific Northwest National Laboratory MSIN K8-88, P.O. Box 999, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Angle-resolved photon-stimulated desorption (PSD) combined with infrared reflection−absorption spectroscopy and temperature-programmed desorption reveal two distinct channels in the photochemistry of acetone on rutile TiO2(110). During UV irradiation of coadsorbed oxygen and acetone molecules, nonthermal methyl radicals (CH3) are ejected in two different directions: (i) normal to the surface and (ii) off-normal at ∼±66° to the surface normal in the [11̅0] azimuth (i.e., perpendicular to the O and Ti rows). The direction of the “off-normal” PSD component is consistent with the orientation of the C−CH3 bonds in the η2-acetone diolatea photoactive form of acetone chemisorption on the oxidized TiO2(110) surface proposed in earlier studies. The direction of the “normal” PSD component requires an orientation of a C−CH3 bond which is not consistent with the η2-acetone diolate structure. The “off-normal” PSD component dominates at lower acetone coverage ( 0° but are also shown for φdes < 0 for better visualization. The cartoons on the right show directions of the detector motion relative to the TiO2(110) crystal orientation (red, O ions; blue, Ti ions) with an adsorbed acetone diolate molecule.

and 0.60 ML, and for two different azimuthal orientations on the TiO2(110) surfacealong the [11̅0] and [001] azimuths (see illustrations in Figure 5). As shown in part 1,1 for θ(acetone) = 0.083 ML, the dominant trajectory of the desorbing CH3 radicals is peaked at φdes ∼ ±66° along the [11̅0] azimuth (Figure 5a, green traces). This component of the

Figure 4. Acetone decomposition (in ML, blue squares), acetate production (normalized, red triangles), and integrated CH3 PSD (normalized, black circles) at 30 K versus the initial acetone coverage. O2 and acetone were dosed using a standard procedure (see the text for details). QMS detector is set for angle-integrated measurement (out of line of sight, φdes = 94°). 12276

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Figure 6. Differential absorbance IRAS spectra in (a) p- and (b) s-polarized modes for 0.17 and 0.60 ML of hydrogenated (H6-) and deuterated (D6) acetone dosed on oxidized TiO2(110) surface without UV irradiation (red and light blue traces) and after UV irradiation (dark blue and green traces). The IR beam is parallel to the [11̅0] azimuth (s-vector is along the Ti and O rows). The samples were irradiated for 60 s with a photon fluence of ∼6 × 1017 photons/cm2 with energies >3 eV.

distribution, which we will refer to as the “off-normal” component, is consistent with the C−CH3 bond orientation of the η2-acetone diolate adsorbed on the Ti5c sites of TiO2(110). For θ(acetone) = 0.17 ML (Figure 5, blue traces), the “off-normal” peak is somewhat larger. However, more noteworthy is the development of a peak in the CH3 PSD distribution that desorbs normal to the surface, which we will call the “normal” component. For θ(acetone) = 0.60 ML, this normal component develops even more to become a significant reaction channel (Figure 5a, red traces).8 As θ(acetone) increases from 0.17 to 0.6 ML, the amplitude of the “offnormal” PSD component does not significantly change (Figure 5a, inset) and the peak of the distribution shifts to slightly smaller desorption angles, φdes. The observation that the offnormal component does not increase appreciably for θ(acetone) ≥ 0.17 ML is consistent with the IRAS data which indicate that the amount of acetone diolate also does not increase for θ(acetone) ≥ 0.17 ML (see Figure 6 and part 11). Measurements of the CH3 PSD along the [001] azimuth show only the “normal” component of the desorbing CH3 radicals for all acetone coverages (Figure 5b). Along this azimuth, the growth of the “normal” component in the CH3 PSD is clear. In Figure 5, the angular distributions for both components of the CH3 PSD are relatively narrow as compared to a thermally equilibrated, cosine distribution.9 This indicates that both components of the distribution are the result of nonthermal reactions and that the ejected CH3 have not had significant interactions with the surface after they were produced. As a result, the angular distributions provide information about the orientation of the bonds that were broken in the nonthermal reactions. A key point is that the “normal” PSD component of the CH3 PSD is inconsistent with the C−CH3 bond orientation of the η2-acetone diolate (see Figure 1 and illustrations in Figure 5). Therefore, these results show that there must be a second photochemical process that is associated with a precursor molecule that has a C−CH3 bond oriented normal to the surface of TiO2(110). Various forms of chemisorbed oxygen (molecular and atomic)10−25 can thermally react with acetone to form a

photoactive acetone−oxygen complex on TiO2(110). The different forms of chemisorbed oxygen may potentially be associated with different reaction pathways that lead to different trajectories for the photodesorbing CH3 radicals. To test this hypothesis, we changed the thermal regimes of the oxygen deposition to investigate the possible effects on the CH3 PSD angular distribution. Figure S6 in the Supporting Information shows the integrated CH3 PSD yields versus desorption angle for three different forms of the chemisorbed oxygen. In all three cases, both the “normal” and “off-normal” components of the CH3 PSD are observed, albeit with somewhat different relative intensities. Therefore, the CH3 PSD angular distribution is not sensitive to the particular form of the chemisorbed oxygen and the initial acetone coverage appears to be the main parameter affecting it. Figure 6 shows IRAS spectra before and after UV irradiation for acetone adsorbed on oxidized TiO2(110) for θ(acetone) = 0.17 and 0.6 ML. The O2 and acetone were dosed and annealed using the standard procedure described in the Experimental Procedure. The experiments were performed with both regular and deuterated acetone (H6-acetone and D6-acetone, respectively). Before UV irradiation, peaks for acetone diolate are observed in the p-polarized spectra at 1195, 1178, and 1013 cm−1 for H6-acetone (Figure 6a, red lines) and 1170 cm−1 for D6-acetone (Figure 6a, light blue lines). An acetone diolate peak is also observed at ∼1000 cm−1 in the s-polarized spectra for D6-acetone (Figure 6b, light blue lines). (See part 11 for a discussion of the IRAS spectra for acetone diolate.) These acetone diolate peaks all disappear after UV irradiation (Figure 6, dark blue and green lines), indicating that all the acetone diolate reacts. Note that the amplitudes for the acetone diolate peaks are similar for both acetone coverages. This result, which suggests that no additional acetone diolate is produced for the larger acetone coverage, is consistent with the observation that the off-normal component of the CH3 PSD distribution also does not appreciably increase over this coverage range (see Figure 5a). However, since the total amount of CH3 PSD is larger for θ(acetone) = 0.6 ML and the increase is primarily associated with the CH3 ejected normal to the surface, we 12277

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Figure 7. Schematic of a reaction channel for acetone adsorbed on oxidized TiO2(110) that leads to CH3 emission off-normal to the surface via the acetone diolate intermediate.2,3 Note that there are a number of chemisorbed oxygen forms that are active in the acetone photooxidation, such as O2−, O22−, Oa, etc. However, only Oa is shown.

IV. DISCUSSION The data presented here and in part 11 clearly indicate two distinct reaction pathways in the acetone photochemistry on oxidized TiO2(110). The first reaction pathway, which is evident in the “off-normal” component of the angle-resolved PSD, is dominant for θ(acetone) < ∼0.2 ML. The second reaction pathway, which is seen in the “normal” component of the angle-resolved PSD, grows with the acetone coverage and becomes significant for θ(acetone) > ∼0.2 ML. Angle-resolved PSD measurements, IRAS spectra, and TPD experiments with isotopically labeled oxygen, presented in part 1,1 show that the “off-normal,” low-coverage reaction pathway is the two-step photooxidation process, based on the η2-acetone diolate precursor, that was proposed previously by one of us (M.A.H.)2,3 and depicted schematically in Figure 7. The second reaction pathway grows with the acetone coverage in the whole studied range and it maximizes at the highest acetone coverage (Figure 5). This additional reaction channel has not been previously identified, and the normal emission of the CH3 during PSD is inconsistent with the structure and orientation of η2-acetone diolate. Other evidence for a second reaction channel is the observation that the total amount of the acetone lost due to photon-stimulated reactions exceeds the amount of acetone diolate that is formed during the adsorption process (Figure 4). Since acetate, which is the product of the diolate-mediated reaction channel, has a C−CH3 bond that is normal to the surface (Figure 1c), it might seem to be a promising candidate for the precursor for the second reaction channel. However, previous research has shown that acetate adsorbed on TiO2(110) is not photoactive.2 We have also tested for photoactivity in the acetate produced during the UV irradiation. In the experiment, oxygen and 0.17 ML of H6-acetone were dosed using the standard procedure and irradiated. Next, all the remaining H6-acetone was desorbed by ramping the temperature to 500 K, leaving just the photoproduced H3-acetate on the surface. Finally, 0.17 ML of D6-acetone was dosed, annealed at 200 K, and irradiated a second time. As shown in the Supporting Information, Figure S5, no CH3 PSD was detected during the second irradiation showing that the H3-acetate was not photoactive. Furthermore, any photoreactions that involved an acetate molecule for the CH3 PSD would contradict the product yield correlation shown in Figure 4. Previous research provides a plausible candidate for the second reaction pathway for the acetone photochemistry on TiO2(110). Studies of the thermal oxidation of acetone on oxygen-predosed Ag(110)27 and Ag(111)28 reveal two major

might hope to see evidence for a new precursor molecule in the IRAS spectra before UV irradiation at the larger acetone coverage. While some changes in the peaks associated with acetone were observed, we were not able to identify any new species from the IRAS spectra for θ(acetone) = 0.6 ML. After UV irradiation, new peaks appear in the IRAS spectra for θ(acetone) = 0.17 ML, as discussed in part 1.1 The new peaks in the p-polarized spectra at 1454 cm−1 (1434 cm−1) are attributed to the νs(OCO) mode of H3(D3)-acetate, which is left on the surface when a methyl radical is ejected from acetone diolate. For θ(acetone) = 0.17 ML, the small peak in the s-polarized spectra at ∼1510 cm−1 (1485 cm−1) should correspond to the νa(OCO) mode of H3(D3)-acetate. Since the CH3 PSD increases with increasing acetone coverage (Figure 4) and no other products are observed leaving the surface during UV irradiation, there must be more reaction products remaining on the surface after UV irradiation at higher coverages. For the p-polarized spectra after UV irradiation of H6-acetone, a peak at 1420 cm−1 increases when the acetone coverage is increased from 0.17 to 0.6 ML (Figure 6a, dark blue lines). In contrast, the peaks at 1454 cm−1 (1434 cm−1) assigned to H3(D3)-acetate shift to slightly lower frequencies, but the intensities remain about the same when the coverage of acetone increases. For the s-polarized spectra after UV irradiation, the small peaks observed for initial H6(D6)-acetone coverages of 0.17 ML at 1510 cm−1 (1485 cm−1) are replaced by much more intense peaks at 1534 cm−1 (1516 cm−1) for initial H6(D6)-acetone coverages of 0.6 ML. As noted in part 1,1 the ν(CO) mode of the molecular acetone appears as a doublet in the p-polarized spectrum on the oxidized TiO2(110) surface for θ(acetone) = 0.60 ML: 1705 (1700) cm−1 + 1723 (1715) cm−1 for H6(D6)-acetone (Figure 6a). After UV irradiation, the relative intensity of the blueshifted component of the doublet slightly increases. This peak splitting may be the result of intermolecular interaction in the adsorbed layer at higher coverages. This is also consistent with the growing ν(CO) peak in the s-polarized mode (Figure 6b), which may indicate tilting of some of the acetone molecules away from the preferential normal to the surface orientation. Alternatively, this could arise from new adsorption geometries for the acetone. For example, a doublet in the ν(CO) IR band of the molecular acetone on Pt(111) surface was associated with coexistence of two different molecular adsorption configurationsη 1 and η2having different orientations and binding energies.26 12278

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Figure 8. Schematic of a possible reaction pathway for acetone adsorbed on oxidized TiO2(110) that leads to CH3 emission normal to the surface via an acetone enolate intermediate. (a) → (b) UV irradiation leads to nonthermal reactions between acetone and adsorbed oxygen that produce enolate. (b) → (c) η2(C,O)-enolate is photoactive and ejects a CH3 radical normal to the surface leaving ketene. (c) → (d) Ketene converts to acetate, either during irradiation or during postirradiation temperature-programmed desorption. Note that there are a number of chemisorbed oxygen forms that are active in the acetone photooxidation, such as O2−, O22−, Oa, etc. However, only Oa is shown.

channel increases substantially, suggesting that acetone enolates are formed on this surface (Supporting Information, Figure S7b). The correlation that is observed for the entire range of acetone coverages between the acetone lost due to photoreactions, the CH3 PSD yield, and the acetate yield (Figure 4) is noteworthy. It indicates that the amount of the CH3 radicals ejected per acetone molecule is the same for both reaction channels and that the overall stoichiometry of both reaction channels is the same (i.e., both reactions leave behind C2H3O in some form). The loss of a methyl group from enolate due to a photon-stimulated reaction should leave ketene (OC CH2) adsorbed on the surface (see Figure 8c) plus an extra hydrogen in the form of a hydroxyl. However, ketene is known to react with OH groups to form acetate on some oxide surfaces, such as ZnO, where the reaction occurs at low temperature.51 Therefore, if this reaction occurs on TiO2(110), either during the UV irradiation or during the postirradiation TPD, then both reaction channels would lead to acetate (Figure 8c,d), explaining the correlation between the CH3 PSD and acetate yields seen in Figure 4.52 While the angular distribution of the CH3 PSD, the H/D isotope exchange experiment, and the correlation of the acetate yield with the CH3 PSD versus coverage provide evidence that a species such as enolate is involved in the second reaction channel, the IRAS results for larger acetone coverages (where the second channel is enhanced) are inconclusive. For θ(acetone) = 0.17 and 0.6 ML on oxidized TiO2(110) before UV irradiation, acetone and acetone diolate are the only species that are readily identifiable in the IRAS spectra (Figure 6). In IR studies of acetone adsorbed on metal and oxide surfaces, the most common peaks observed for acetone enolate at 1500− 1570 cm−1 are associated with ν(CO), or ν(CC), or ν(CCO) modes, and those at 1420−1450 cm−1 are associated with δa(CH3) mode.29−31,47,49,50 In our IRAS spectra, we do not have peaks in these ranges prior to UV irradiation (except for the initial acetone peaks). But since enolate is not expected to be formed by annealing at 200 K, its absence prior to UV irradiation is not surprising. For example, in the H/D exchange experiments described above, the increase in the H/D exchange signal (45 amu) occurs only for T > 300 K (see Supporting Information, Figure S7b). On the other hand, “enolization” of acetone may proceed nonthermally at lower temperature under UV irradiation. Chemisorbed oxygen species12,13,53 and the adsorbed acetone molecules6,54,55 are photoactive on the TiO2(110) surface reacting with the photogenerated electrons

reaction pathways: (i) nucleophilic attack of oxygen at the electron-deficient carbonyl carbon and (ii) C−H bond activation. The former path produces acetone diolate, while the latter path produces acetone enolate (CH2C(CH3) O−). Acetone enolate, the formation of which is related to the well-known ketone−enol transformation, CH3COCH3 ↔ CH2COHCH3, is a common product of acetone reactions in solutions and on the surface of various metals and oxides after adsorption of acetone.28−41 There are a number of possible enolate adsorption configurations on surfaces. In the η2(C,O) form, the enolate molecule is attached to the surface at two points, creating a bridgelike structure and leaving the CH3 group pointing up, which makes this species an attractive candidate for the “normal” component of the CH3 PSD. The η2(C,O)-enolate complexes were observed for acetone in coordination with palladium,42,43 or adsorbed on Ni(111),29 Pt(111),30,31 Si(001),44,45 Ge(001),46 CeO2(111),33 and ZnO.47 Figure 8a,b illustrates the possible η2(C,O)-enolate complex formation on TiO2(110). As we discuss below, it is likely that the formation of the enolate occurs during UV irradiation. In most cases, it takes higher temperatures to form enolate than it does to form acetone diolate.27,28,33,35,37,48 On TiO2, enolate was observed at elevated temperatures (>300 K) as an intermediate in the formation of mesityl oxide.49 Typically, “enolization” is initiated by a thermally activated proton transfer from a methyl group of an acetone molecule. On TiO2(110), the coordination of acetone to Lewis acidic Ti4+ sites would enhance the Brønsted acidity of hydrogen atoms in the methyl groups of acetone and would therefore make the base-catalyzed loss of a proton more favorable.49 The proton transfers to adjacent oxygen ions on the oxide surface (Figure 8b).48−50 In fact, efficient isotopic H/D exchange between coadsorbed acetone and water at elevated temperatures (∼400 K) during TPD on oxidized TiO2(110) was reported earlier,48 suggesting a role for enolate intermediates on this surface. We have also observed H/D exchange between normal and deuterated acetone on oxidized TiO2(110) (Supporting Information, Figure S7). In these experiments, 0.33 ML of H6- and D6-acetone was coadsorbed on TiO2(110) and annealed at 200 K. Desorption of acetone was then monitored during TPD by following masses 43, 45, and 46 amu, where the signal at 45 amu arises from H/D exchange. For experiments on reduced TiO2(110), only a small amount of H/D exchange is seen (Supporting Information, Figure S7a). For oxidized TiO2(110), the amount of H/D exchange seen in the 45 amu 12279

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acetone on oxidized, rutile TiO2(110) that are associated with normal and off-normal trajectories of the photoejected CH3 radicals. The “off-normal” component of the CH3 PSD, which was the focus of part 1,1 is related to the photoinduced dissociation of acetone diolate and is consistent with a previously proposed model for the acetone photochemistry. The “normal” component of the CH3 PSD is inconsistent with the structure of acetone diolate on TiO2(110) and instead indicates a precursor having a C−CH3 bond directed along the surface normal. Based on the chemistry of acetone on other surfaces, acetone enolate is a likely candidate for the photoactive species that leads to CH3 emission normal to the surface. In particular, η2(C,O)-enolate should have a CH3 group normal to the surface of TiO2(110) and H/D exchange that is observed between H6- and D6-acetone on oxidized TiO2(110) also indicates the formation of enolate. However, since IRAS failed to provide direct spectroscopic evidence for the enolate model, more work is needed to fully characterize this new photoreaction channel for acetone on TiO2(110).

and holes, which can potentially initiate the required proton transfer. The increasing decay time of the CH3 PSD signal at higher acetone coverage (Figure 2b) may be an indicator of such an additional photochemical reaction step (Figure 8a,b). Therefore, a structure similar to the η2(C,O)-enolate would account for a C−CH3 bond normal to the surface, providing the basis for the observed PSD data. It could also account for the previous high-resolution electron energy loss spectroscopic (HREELS) data without UV irradiation,4 where the observed 1425 cm−1 peak grew significantly after annealing at ∼300 K where thermal “enolization” of acetone begins. The product left on the surface after the “normal” PSD process is difficult to identify with available experimental data. In the TPD spectra of the irradiated samples at high acetone coverages, we only see ketene (besides the acetone) desorbing in a single temperature peak corresponding to decomposition of the surface acetate (Figure 2). On the other hand, some of this acetate may potentially be produced indirectly during thermal annealing in the TPD experiment (Figure 8c,d). The IRAS spectra obtained at low temperature after UV irradiation are also hard to interpret unambiguously. We do not see clear evidence for ketene or more acetate in the postirradiation IRAS spectra (Figure 6). When the initial acetone coverage was increased to 0.6 ML, the most notable change was in the postirradiation spectra for s-polarized light where comparatively large peaks appeared at 1534 and 1516 cm−1 for H6- and D6acetone, respectively (Figure 6b). The small isotopic shift indicates this peak is due to a skeletal mode. While their frequencies agree quite well with that expected for the νa(OCO) mode of acetate,29,34,56−58 there is no corresponding increase seen in the p-polarized spectra for the νs(OCO) mode of either H3- or D3-acetate. Another possibility is the ν(CO) mode of the enolate;27,29,32,34 however, no peaks corresponding to enolate are seen in the p-polarized spectra, even though the signal-to-noise ratio is typically better in p-polarized spectra (leading to better sensitivity). If enolate is the species responsible for the normal component of the CH3 PSD, then there should be very little, if any, left on the surface after UV irradiation (Figure 8). In fact, if the formation of enolate is the rate-limiting step in the reaction, then the concentration of enolate should be low throughout the experiment. In the postirradiation p-polarized spectra, the peak at ∼1420 cm−1 for the surface dosed with H6-acetone, which is missing in the corresponding deuterated spectra, is probably associated with a mode such as δa(CH3).27,28 However, many possible reaction channels involve species with methyl groups, so this mode is not that helpful in identifying the species. Despite the uncertainties associated with interpreting the IRAS spectra, we believe that the balance of the results presented here suggest that photon-stimulated reactions in the acetone adsorbed on oxidized TiO2(110) lead to the production of enolate. This enolate, presumably in an η2(C,O) configuration with its C−CH3 bond normal to the surface, is the photoactive species that gives rise to the normal component in the CH3 PSD distribution. The ejection of the CH3 group leaves some form of ketene adsorbed on the surface, which presumably converts to acetate nonthermally or during postirradiation TPD. Figure 8 shows a schematic of the proposed reaction pathway.

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ASSOCIATED CONTENT

* Supporting Information S

Figure S1 shows a schematic for IRAS geometry on TiO2(110). Figure S2 shows a semilog plot of the CH3 PSD yields versus time. Figure S3 shows acetone TPD spectra for different initial coverages on the oxidized surface of TiO2(110) after UV irradiation and without irradiation. Figure S4 shows acetone TPD spectra from the reduced and oxidized surface of TiO2(110). Figure S5 shows CH3 PSD yield (PSD-1 and PSD-2) versus time for the sequential irradiations with redosed D6-acetone. Figure S6 shows the integrated CH3 PSD yields versus desorption angle along the [110̅ ] azimuth on the TiO2(110) for various thermal treatments of the predosed oxygen. Figure S7 shows TPD spectra of 0.33 ML of D6acetone and 0.33 ML of H6-acetone codosed on the reduced and oxidized surface of TiO2(110). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02478.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 509-371-6151. *E-mail: [email protected]. Tel.: 509-371-6134. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. The work was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle.

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REFERENCES

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V. CONCLUSION Measurements of the angle-resolved PSD distribution of CH3 reveal two distinct reaction channels in the photochemistry of 12280

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DOI: 10.1021/acs.jpcc.5b02478 J. Phys. Chem. C 2015, 119, 12273−12282