Article pubs.acs.org/JPCC
Insights into Acetone Photochemistry on Rutile TiO2(110). 1. OffNormal CH3 Ejection from Acetone Diolate 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: Thermal- and photon-stimulated reactions of acetone coadsorbed with oxygen on rutile TiO2(110) surface are studied with infrared reflection−absorption spectroscopy (IRAS) combined with temperature-programmed desorption and angle-resolved photon stimulated desorption. IRAS results show that η2-acetone diolate ((CH3)2COO) is produced via thermally activated reactions between the chemisorbed oxygen and coadsorbed acetone. Formation of acetone diolate is also consistent with 18O/16O isotopic exchange experiments. During UV irradiation at 30 K, CH3 radicals are ejected from the acetone diolate with a distribution that is peaked at ∼±66° from the surface normal along the [110̅ ] azimuth (i.e., perpendicular to the rows of bridging oxygen and Ti5c ions). This distribution is also consistent with the orientation of the C−CH3 bonds in the η2-acetone diolate on TiO2(110). The acetone diolate peaks disappear from the IRAS spectra after UV irradiation, and new peaks are observed and associated with η2acetate. The data presented here demonstrate direct signatures of the previously-proposed two-step mechanism for acetone photooxidation on TiO2(110). when coadsorbed with oxygen.25,28 Henderson proposed a twostep reaction mechanism to explain the observed photochemistry.25 In the first step, a thermal reaction between acetone and coadsorbed oxygen produces an acetone−oxygen complex with an activation barrier of about 10 kJ/mol:
I. INTRODUCTION Acetone is a widely used solvent and chemical intermediate in the production of plastic, fibers, drugs, and other materials.1,2 The vast majority of acetone from all sources goes into the atmosphere, where it breaks down via three major routes: photolysis, reaction with OH radicals, and surface deposition.3 As a result acetone plays an important role in thermal and photoinduced reactions in the upper troposphere.3−5 Photodissociation of gas-phase acetone, which has been studied in detail, leads to C−C bond scission producing CH3 radicals via ionization (E > 10.5 eV) or excitation in the n → π* band (E ∼ 4.5 eV).6−10 Acetone on surfaces is important for the understanding of aerosol chemistry in the troposphere and stratosphere,11 for decomposition of acetone to less harmful compounds in polluted urban and indoor environments,12−14 for developing acetone sensors for medical diagnostics,15,16 and for many other applications. TiO2 is a broadly used material and one of the most-studied oxide photocatalysts.17−22 On single crystal, rutile TiO2(110), the thermal chemistry and photochemistry of the adsorbed acetone and other ketones has been investigated by one of us (M.A.H.) and co-workers23−29 and by White and coworkers.30−33 Acetone adsorbs molecularly in an η1 configuration at Ti5c sites on reduced TiO2(110) (see Figure 1a) and desorbs over a broad temperature range due to repulsive interactions among the adsorbates.23 Acetone is not photochemically active on reduced TiO2(110), but it is photoactive © 2015 American Chemical Society
η1‐acetone(ads) + O(ads) → [acetone−oxygen](ads)
(1)
In the second step, a photon-stimulated reaction ejects a methyl radical from the surface and converts the acetone− oxygen complex into acetate: [acetone−oxygen](ads) + UV(TiO2 ) → η2‐acetate(ads) + CH3(gas)
(2)
The photoactive acetone−oxygen precursor was investigated using the oxygen exchange between 18O2 and coadsorbed 16Oacetone,23 and the results pointed toward an η2-acetone diolate ((CH3)2COO) structure as a likely candidate for the photoactive precursor (see Figure 1b).28 Acetone diolate has previously been identified as one of the major products of acetone’s reactions with oxygen atoms adsorbed on various metal surfaces such as Ag(110), Ag(111), and Ni(111).34−36 While no peaks corresponding to acetone diolate were Received: March 13, 2015 Revised: May 4, 2015 Published: May 5, 2015 12262
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Figure 1. Schematic of the initial, intermediate, and final products of acetone thermal and photoinduced reactions on TiO2(110) surface (side and top views): (a) η1-acetone next to an oxygen adatom (Oa) on Ti5c site; (b) η2-acetone diolate on Ti5c site produced via thermally activated reaction between acetone and Oa; (c) η2-acetate on Ti5c site produced via photoinduced dissociation of the acetone diolate and ejection of the CH3 radical from the surface. The cartoons are consistent with the calculated structures.37 Note that there are a number of chemisorbed oxygen forms active in the acetone photooxidation (O2−, O22−, Oa, etc.). Only Oa is shown here for clarity.
eV (“slow”), representing different final states of the methyl fragments.32 Prompt dissociation of the photoactive precursor was responsible for the “fast” component and dissociation of the photoexcited molecules relaxed by internal conversion or energy transfer to the titania surface prior to fragmentation produced the “slow” component. The CH3 kinetic energy distributions were independent of UV pump energy, indicating a substrate-induced process involving reaction of photogenerated electrons or holes.32 It is important to note that the majority of the photoejected CH3 did not experience collisions with the surface or with neighboring acetone molecules.32 This observation is consistent with the proposed orientation of the C−CH3 bond in the η2-acetone diolate, which according to density functional theory (DFT) calculations are directed out of the surface with the C−C−C group in the (001) plane (Figure 1b).37,38 If the CH3 radical is ejected along this C−CH3 bond after photoexcitation, then this should be reflected in the angular distribution of the photoproducts. Photoejection of CH3 from the acetone diolate leaves acetate adsorbed on the surface (Figure 1c). Detection of the acetate was based on postirradiation temperature-programmed-desorption (TPD) measurements via desorption of ketene, produced by the thermal decomposition of acetate in the 600−700 K temperature range:23,25
observed with high-resolution electron energy loss spectroscopy (HREELS) on TiO2(110), a new vibrational feature at 1425 cm−1 was detected for acetone dosed on the oxygen-predosed surface, but assignment of this peak remains under discussion.23 The identities of all of the chemisorbed oxygen species (i.e., O2−, O22−, Oa, etc.) involved in the thermal reaction on TiO2(110) surface are not known in detail.23,25,28 However, oxygen adatoms on Ti5c sites are considered among the most reactive with acetone. Figure 1 shows a schematic of the proposed reaction on oxidized TiO2(110)28 in which acetone is initially adsorbed on a Ti5c site adjacent to an oxygen adatom, Oa (Figure 1a). A thermal reaction between the Oa and the carbonyl carbon atom weakens the CO bond, resulting in an O−C−O group of an acetone diolate oriented along the [001] axis (Figure 1b). Photon-stimulated desorption (PSD) of the methyl radicals has been used to study the photochemistry of acetone on oxidized TiO2(110). The ejection of methyl radicals was proved using isotopically labeled acetone25 and later confirmed with UV pump, vacuum ultraviolet (VUV) probe mass spectrometry31 and state-resolved, resonance-enhanced multiphoton ionization (REMPI) measurements.30 Recent pulsed pump− probe laser excitation measurements discovered two components in the translational energy distributions of the ejected CH3 radicals with average energies of 0.19 eV (“fast”) and 0.03 12263
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welded to the base plate. The base temperature for the samples was typically between 25 and 30 K. The TiO2 samples were prepared by sputtering with 2 keV Ne+ ions and then annealing for between 2 and 10 min in a vacuum at 950 K. Multiple ion sputtering/annealing cycles resulted in reduced TiO2 samples.21 On the ion sputtered and annealed surfaces, the concentration of vacancies in the bridging oxygen rows, θ(VO), was 0.04−0.05 monolayer (ML) based on the magnitude of the high temperature OH recombination peak during water temperature programmed desorption.54,55 The photodecomposition of acetone coadsorbed with chemisorbed oxygen was investigated. O 2 dosing was performed with ultra high purity gas (Air Liquid, 99.9995%) with fluxes of (∼1−3) × 1014 molecules/cm2·s, at normal incidence to the surface. Previous research has shown that the photoreactions are enhanced by first adsorbing oxygen and annealing the sample to ∼300 K and subsequently adsorbing acetone and annealing to ∼200 K.23,25,28 As described in section III, we have also found that this procedure enhances the photoreactions in our experimental setup. Unless otherwise noted, the “standard” procedure used to prepare the surface was as follows. The sputtered and annealed surfaces were exposed to a dose of O2 that exceeded the maximum amount that could chemisorb at the base temperature. (Previous research has shown that the maximum amount of oxygen that can be chemisorbed on reduced TiO2(110), θsat, is approximately twice the vacancy concentration, i.e. θsat ∼ 2θ(VO).56) The oxygen-dosed sample was then annealed for 60 s at 300 K. Finally, acetone was dosed at 80 K and the sample was annealed for 60 s at 200 K. HPLC grade acetone from Aldrich was used. The acetone coverage, θ(acetone), is given relative to the amount of the acetone that can be adsorbed on the Ti5c sites of TiO2(110),which is 5.2 × 1014 molecules/cm2.23 This coverage, which is defined as θ(acetone) = 1 ML, was determined from TPD spectra for several different exposures (Supporting Information, Figure S1), which is quite similar to the earlier data on acetone TPD from TiO2(110).23 The acetone coverage was measured using the most intense fragment, CH3C16O+, in the mass spectrum at 43 amu (or the corresponding masses for other acetone isotopomers). Photon irradiations were performed using a 100 W Hg lamp (Oriel No. 6281). The output of the lamp was focused onto a fiber optic cable that was used to introduce the light into the vacuum chamber. The infrared portion of the output was blocked using a water filter. The light was incident at 45° with respect to the surface normal. The distance of the fiber optic cable from the sample was chosen such that the illuminated spot on the sample was centered on, and somewhat larger than, the molecular beam spot on the sample. The entire UV portion of the lamp’s spectrum was used during photon irradiation. For the experiments reported here, all UV irradiations were performed at the base temperature (25−30 K). During photon irradiation, the increase in the sample temperature was 3 eV was ∼1 × 1016 photons/cm2·s. Angle-resolved PSD measurements were performed with a QMS mounted on a rotation stage and equipped with an integrating cup with a 5 mm aperture located ∼20 mm above the surface (see ref 53 for more details). The desorption angle, φdes, was set by the polar angle of the quadrupole mass spectrometer relative to the surface normal of the TiO2(110) crystal. Measurements in the line of sight to the front face of
[H3CCOO](ads) → H 2CCO(gas) + 0.5H 2O(gas) + 0.5Oa
(3)
To date, however, there has been no direct spectroscopic identification of the acetate formed by the loss of CH3 from the acetone diolate after UV irradiation. Recently, infrared reflection−absorption spectroscopy (IRAS) has emerged as an important tool to study molecules adsorbed on the surface of TiO2 single crystals.39−47 Using IRAS to investigate molecules adsorbed on a dielectric surface is a challenge because of the low sensitivity as compared to metal substrates. However, since the surface dipole selection rule does not apply on dielectrics, IRAS with both s- and ppolarized light can be used to provide valuable information on the molecular orientation.48−50 This paper is the first in a two-part series in which we use IRAS, angle-resolved PSD, and other methods to investigate the thermal- and photon-stimulated reactions of acetone coadsorbed with oxygen on rutile TiO2(110) and irradiated with UV photons with energies above the band gap. Together, the two papers show that the acetone photochemistry on TiO2(110) involves two distinct photochemical reaction pathways. In this paper, we focus on the reaction pathway that proceeds via the previously-proposed acetone diolate intermediate that was discussed above. This reaction pathway is most evident for initial coverages of acetone that are less than or approximately equal to the coverage of oxygen chemisorbed on TiO2(110) surface, but it occurs for all acetone coverages studied (i.e., 0− 0.6 monolayer). IRAS spectra show η2-acetone diolate forms via thermally activated reactions between chemisorbed oxygen and acetone. Angle-resolved measurements of the CH3 radicals that are ejected from the acetone diolate during UV irradiation show that they have a distribution that is peaked at ∼±66° from the normal [110] vector in the [110̅ ] azimuth (i.e., perpendicular to the rows of bridging oxygen (Ob) and Ti5c ions), consistent with the expected orientation of the C−CH3 bonds in η2acetone diolate on TiO2(110). After UV irradiation, the acetone diolate peaks disappear from the IRAS spectra and new peaks that are associated with η2-acetate are observed. The results provide direct signatures of the previously-proposed two-step mechanism of the acetone photooxidation on TiO2(110). In part 2,51 we report on a different and previously undetected photochemistry channel which becomes more important for larger initial coverages of acetone. This reaction channel manifests itself in the angular distribution of the photodesorbed CH3 as a second component that is peaked along the surface normal.
II. EXPERIMENTAL PROCEDURE The experiments were performed in two ultrahigh vacuum (UHV) systems that have been described previously.41,52,53 Both systems consisted of a closed-cycle helium cryostat, a molecular beamline for adsorbate deposition, a quadrupole mass spectrometer (QMS), a UV photon source, and surface diagnostic equipment. The typical base pressure was less than 1 × 10−10 Torr. One of the systems was equipped with a Fourier transform infrared spectrometer (Bruker Vertex 70); the other was equipped with a rotating QMS stage for angle-resolved photon stimulated desorption measurements. The 10 × 10 × 1 mm rutile TiO2(110) crystals (CrysTec GmbH) were mounted on a resistively heated tantalum base plate. For temperature monitoring and control, a K-type thermocouple was spot12264
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The Journal of Physical Chemistry C the crystal have two contributions: a direct, angle-resolved component and an approximately constant contribution from the angle-integrated component. Two different crystals mounted at 90° relative each other were used for the measurements of the angle-resolved PSD to enable the QMS rotation plane to be perpendicular to and parallel to the Ob rows (i.e., along the [11̅0] and [001] azimuths, respectively). However, some of the experiments reported here were conducted with the detector at a fixed position of φdes = 54° to the surface normal or out of line of sight to the front face of the TiO2(110) crystal at φdes = 94° in order to provide the angle-integrated desorption intensities. The infrared light was incident on the TiO2(110) single crystals at grazing incidence (∼84° with respect to the normal) and detected in the specular direction. Here, we use IRAS with s- and p-polarized light incident along the [11̅0] azimuth (Supporting Information, Figure S2). In this configuration, the s-polarized spectra are sensitive to vibrations that have a projection that is parallel to the surface and in the [001] azimuth. The p-polarized spectra reflect vibrations that either have a component normal to the surface or have a component parallel to the surface and in the [11̅0] azimuth. Note that for spolarized light on a dielectric substrate absorbance peaks are negative, while for p-polarized light the absorbance peaks can be positive or negative depending on a variety of factors including the orientation of the dipole moment of the corresponding vibrational mode.39−41,46,57 IRAS spectra were obtained using a resolution of 4 cm−1. Absorbances are defined as A(ν) ≡ log10[R0(ν)/R(ν)], where R(ν) and R0(ν) are the reflected signals from the TiO2(110) with and without adsorbed molecules. All IRAS spectra were measured at T ∼ 30 K. The total number of spectrometer scans varied from 20 000 to 60 000 depending on the size of the signal and the noise level.
Figure 2. CH3 PSD yields versus time for various sample conditions. Lower 3 traces: θ(O2) = 0, 0.5θsat, and θsat dosed at 30 K (black, red, and blue traces, respectively) and then annealed at 300 K; then 0.67 ML of acetone was dosed at 80 K and annealed at 200 K. Middle green trace: ∼30 L of O2 was dosed at 300 K, and then 0.67 ML of acetone was dosed at 80 K and annealed at 200 K. Upper purple trace: θ(O2) = θsat dosed at 30 K and then annealed at 100 K; then 0.67 ML of acetone was dosed at 80 K (no further annealing).
and/or O22− depending on coverage.52,56,59,60 The CH3 PSD is significantly reduced if the adsorbed O2 is only annealed to 100 K (Figure 2, purple line), indicating that the molecular anion forms are less reactive with acetone. This observation is consistent with previous results showing that a thermal reaction between the oxygen and acetone is needed to produce a photoactive acetone−oxygen complex.25,31 O2 adsorbs dissociatively at 300 K on TiO2(110), healing the vacancies and depositing oxygen adatoms Oa on the Ti5c sites.56,61−68 With oxygen adatoms, the CH3 PSD signal is significant (Figure 2, green line), indicating that acetone reacts with Oa to form the photoactive species. This signal is quite similar to the case where θ(O2) = 0.5θsat and Tann1 = 300 K (Figure 2, red line), where also Oa are produced as result of the O22− thermal dissociation in the vacancy.56 The largest yield for the CH3 PSD is achieved for θ(O2) = θsat (Figure 2, blue line, and Supporting Information, Figure S3), where the oxygen coverage is higher, but its chemical state is uncertain.52,56,64,67,69−76 During the high-temperature annealing of the saturation O2 dose, the O2− molecular anion converts into a form (or forms) that behave differently from Oa.56,68 These species (e.g., tetraoxygen, ozonide, or others) lead to higher PSD yields (compare blue trace in Figure 2 with the red and green), but the Oa would be equally or more efficient on a “per O atom” basis. However, with respect to the acetone photochemistry, we have found no appreciable differences besides the overall yieldfor surfaces with oxygen adatoms (O2 dosed at 300 K) or those prepared by annealing chemisorbed O2 (dosed at 30 K) to 300 K. In Figure 2, the initial acetone coverage was 0.67 ML. Qualitatively similar results are obtained for smaller acetone coverages and various oxygen dosing procedures (data not shown). See also part 2 in this series,51 which discusses the effect of the initial acetone coverage on the CH3 signals versus time for the case where a saturation coverage of O2 is dosed and annealed to 300 K. Figure 3 shows the integrated CH3 PSD versus annealing temperature. In Figure 3a, a saturation coverage of chemisorbed O2 was annealed at various temperatures, Tann1, for 60 s. Acetone was then adsorbed and annealed to Tann2 = 200 K and the CH3 PSD was measured at 30 K (Figure 3a, blue squares). For 100 K < Tann1 < 200 K, the CH3 PSD is approximately
III. RESULTS AND DISCUSSION The first step in the acetone photooxidation is a thermally activated reaction between acetone and chemisorbed oxygen. The acetone photooxidation yield increases with the O2 exposure,28 the O2 preannealing temperature,31 and the O2 + acetone second annealing temperature.25 Here we optimize these parameters for our system in order to maximize the yield of the photoactive complex. One of the goals is to overcome the small IRAS signals for submonolayer coverages of adsorbates on TiO2(110).39−41,43,46,58 The black, red, and blue lines in Figure 2 show the CH3 PSD signal versus time for oxygen coverages of 0, 0.5θsat, and θsat, respectively (see Experimental Procedure for the definition of θsat). In this experiment, the O2 was dosed at 30 K, and then the sample was annealed for 60 s at 300 K. Subsequently, 0.67 ML of acetone was dosed at 80 K and annealed for 60 s at 200 K. Finally, the CH3 PSD was measured at T = 30 K. As shown previously,25,28 no CH3 PSD was observed from the reduced surface without adsorbed oxygen (Figure 2, black line). For the oxidized surfaces, the CH3 PSD signal increases abruptly when the light is turned on and the maximum signal occurs at or near t = 0 s, and then decays in a complicated multiexponential fashion.31 The CH3 PSD signal increases with increasing coverage of adsorbed oxygen (Figure 2, red and blue traces), and the integrated CH3 PSD yield increases approximately linearly with the initial oxygen coverage up to θsat (Supporting Information, Figure S3, blue squares). Below 100 K, oxygen chemisorbs on reduced TiO2(110) as a molecular anion: O2− 12265
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following studies, and it is qualitatively consistent with previous reports.25,28,31 We can estimate the amount of acetone that reacts with the predosed oxygen by measuring the 16O/18O isotopic exchange between 16O-acetone and chemisorbed 18O2 on the TiO2(110) surface.23,36 For acetone diolate, the following reactions are expected to proceed during the second annealing and subsequent TPD:36 Tann
(CH3)2 C16O + 18O ⎯→ ⎯ (CH3)2 C16O18O
(4)
TPD
(CH3)2 C16O18O ⎯⎯⎯→ 0.5((CH3)2 C18O + 16O) + 0.5((CH3)2 C16O + 18O)
(5)
In this titration-like experiment, we dosed 18O2 (θ(O2) = θsat) and annealed at 300 K, and then various coverages of the 16 O-acetone were dosed at 80 K and annealed at 200 K to create acetone diolate according to reaction 4. (Note that several forms of chemisorbed oxygen (e.g., O2−, O22−, Oa, etc.) can participate in reaction 4 (see Figure 2). However, only an oxygen adatom is shown here for simplicity.) Acetone diolate is a weakly bound compound, easily giving the acetone molecule back during TPD.23 The reversion back to acetone and an oxygen adatom (reaction 5) results in a 50%:50% oxygen isotope scrambling. The 16O−18O isotopic exchange ratio was measured by monitoring masses 43 and 45 amu in TPD (Supporting Information, Figure S4). The fraction of C3H618O (45 amu) in the total acetone TPD yield is plotted in Figure 4
Figure 3. (a) Integrated CH3 PSD at 30 K versus O2 annealing temperature. O2 was dosed at 30 K (θ(O2) = θsat), then annealed at Tann1 = 100−450 K for 60 s, and then 0.67 ML of acetone was dosed at 80 K and annealed at Tann2 = 200 K (blue squares) or not annealed (black triangles), following with UV irradiation (PSD). (b) Integrated CH3 PSD versus acetone annealing temperature. O2 was dosed at 30 K (θ(O2) = θsat), then annealed at Tann1 = 300 K, and then 0.67 ML of acetone was dosed at 80 K and annealed at Tann2 = 80−500 K for 60 s (blue squares), following with UV irradiation (PSD) and TPD. During this annealing, acetone desorbs above 200 K, which is shown in red triangles.
constantprobably due to the acetone annealing step at 200 K. The CH3 PSD then gradually increases and goes through a broad maximum centered around Tann1 ∼ 350 K. At temperatures above 400 K, the CH3 PSD yield drops. For these temperatures, Ti interstitials diffuse to the surface and react with the oxygen to create TiOx islands,56,67,68 which apparently do not react with the acetone to form a photoactive complex. For a similar experiment where the acetone is not annealed to 200 K, the integrated CH3 PSD signal is much smaller, but it displays the same qualitative behavior versus the annealing temperature for the chemisorbed oxygen (Figure 3a, black triangles). Previous experiments investigating changes in the form of chemisorbed oxygen on TiO2(110) versus the annealing temperature56,72 have shown that the O2 electronand photon-stimulated-desorption yields versus annealing temperature are similar to the results in Figure 3a, suggesting that the current results are also due primarily to changes in the chemisorbed oxygen versus annealing temperature. The blue squares in Figure 3b show the integrated CH3 PSD signal versus the annealing temperature for the acetone (after first dosing with O2 and annealing to 300 K). The CH3 PSD increases approximately linearly until Tann2 = 200 K. For higher temperatures, acetone starts to desorb during the annealing step (Figure 3b, red triangles) and the CH3 PSD signal decreases. The results in Figure 3 show that a (nearly) optimal procedure for preparing the photoactive form of acetone on the surface is to chemisorb a saturation coverage of chemisorbed O2, θsat, at low temperature (e.g., ∼30 K), anneal the sample to ∼300 K, and then adsorb acetone at 80 K and perform a second annealing step at ∼200 K. This procedure is used in most of the
Figure 4. 16O−18O isotopic exchange ratio from 43 and 45 amu TPD between the 18O2 (θ(O2) = θsat, annealed at Tann1 = 300 K) and 16Oacetone (dosed at 80 K and annealed at Tann2 = 200 K) versus acetone coverage. Dashed lines show the expected ratio for the complete isotopic mixing versus acetone coverage for various coverages of 18O (blue lines).
versus the initial acetone coverage. This fraction is approximately 0.5 in the low acetone coverage range, but decreases for θ(acetone) > 0.1 ML. Dashed lines show the expected ratio for the complete isotopic mixing versus the acetone coverage for various amounts of chemisorbed 18O (blue lines). An 18O coverage of ∼0.1 ML provides the best fit. This indicates that the acetone diolate coverage is ∼0.1 ML and it is limited by the amount of reactive oxygen on the surface. For our samples with ∼0.05 ML vacancy concentration, ∼ 0.1 ML of oxygen molecules chemisorb at saturation.56 The formation of ∼0.1 ML of acetone diolate is also supported by the observation that 12266
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Figure 5. IRAS spectra in (a) p- and (b) s-polarized light for various coverages of acetone dosed on reduced and oxidized TiO2(110) surface (black and red traces, respectively). O2 and acetone were dosed using the standard procedure (see Experimental Procedure for details). Green traces show IRAS spectra for multilayer dose of acetone. The IR beam is parallel to the [11̅0] azimuth such that the electric field vector is along the Ti and Ob rows for s-polarized light).
acetone (green traces) an additional peak from the methyl rocking mode, ρ(CH3), at ∼1098 cm−1 is discernible in the spolarized spectrum, but not in the p-polarized. For acetone adsorbed on the oxidized TiO2(110), peaks for both acetone and acetone diolate are observed (Figure 5, red lines). In the p-polarized spectra, new peaks appear at ∼1195, ∼1178 (shoulder), and 1013 cm−1. These spectral features are quite close to spectra of η2-acetone diolate on Ag(110)36 with peaks at 980 and 1190 cm−1, and on the Ag(111)34 having peaks at 970, 1162, and 1188 cm−1. These peaks were assigned primarily to the ρ(CH3) mode coupled to νs(CCC) and/or νs(OCO) vibrations of acetone diolate,34,36 but the (uncoupled) νs(OCO) and νa(OCO) modes of the acetone diolate may also contribute.36 With s-polarized light, we do not see new peaks above the noise for the oxidized surface. However, since the symmetric vibration modes νs(CCC) and νs(OCO) for acetone diolate will be normal to the surface (Figure 1b), they should only be seen in the p-polarized spectra and not in the s-polarized spectra. Additionally, the sensitivity of our IRAS system decreases significantly at lower frequencies, making measurements below ∼1050 cm−1 very challenging, especially for s-polarized light and small adsorbate coverages. As mentioned in section II , modes that are parallel to the surface and directed along the [11̅0] azimuth can also be probed with p-polarized light (Supporting Information, Figure S2). On the other hand, the νa(OCO) mode in the η2-acetone diolate is directed along the [001] azimuth (Figure 1b) and it would not contribute to the p-polarized spectra with the chosen IR beam orientation. While acetone diolate has been proposed as the product of the thermal reaction between chemisorbed oxygen and acetone on TiO2(110), the results here provide conclusive spectroscopic evidence for acetone diolate in this system. The intensities of the acetone diolate peaks are approximately the same for all three acetone coverages, which is consistent with the results of the 16O−18O isotopic exchange experiment (Figure 4), limiting the acetone diolate coverage by ∼0.1 ML with increasing acetone coverage. These diolate peaks were not seen in the earlier HREELS studies of acetone dosed on the oxidized TiO2(110) surface.23 This may be due to interference
0.1 ML of acetone is lost from the acetone TPD spectra after UV irradiation (see Figure S5 in the Supporting Information). To gain more insight into the mechanism of the acetone photooxidation on TiO2(110), we used IRAS spectroscopy. The experimental details for the IRAS measurements and dosing the acetone are described in the Experimental Procedure. Figure 5 shows IRAS spectra taken with p- and spolarized light for several acetone coverages on reduced and oxidized TiO2(110) (black and red traces, respectively). The IR spectra were obtained at ∼30 K. For reference, green traces show IRAS spectra of a multilayer dose of acetone on the reduced TiO2(110) surface. As described in detail below, the IRAS results in Figure 5 show that acetone diolate is produced by thermal reactions between acetone and oxygen on TiO2(110). On the reduced surface, only peaks related to physisorbed acetone are observed (Figure 5a, black lines). For p-polarized light, the largest peak is from the carbonyl stretch of acetone, ν(CO), at 1693−1705 cm−1 (Figure 5a, black lines). This peak is blue-shifted and broadened as the acetone coverage increased. Asymmetric and symmetric deformations of the methyl groups, δa(CH3) and δs(CH3), are observed at 1422 cm−1, and as a doublet at 1372 and 1357 cm−1, respectively. For s-polarized light (Figure 5b, black lines), the signals are quite small relative to the noise level (note the difference in the absorbance scale between the p- and s-polarized spectra, Figure 5). The ν(CO) mode is barely identifiable for 0.17 ML of acetone, which is consistent with mostly a perpendicular orientation of the acetone on top of Ti5c sites as seen in DFT calculations.38 On the other hand, the larger ν(CO) peak in the s-polarized spectra for 0.33 and 0.6 ML indicates some inclination of the CO axis from the surface normal along the [001] azimuth, probably to reduce the intermolecular repulsive interactions at higher coverages. Additionally, the δs(CH3), ν(CC) at ∼1232 cm−1, and probably δa(CH3) modes are seen in s-polarized spectra at higher acetone coverages. These data are consistent with the HREELS spectra of acetone on TiO2(110),23 and also they are quite close to the IR spectra of the gas phase acetone.77,78 For multilayer coverages of 12267
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The Journal of Physical Chemistry C
relatively narrow as compared to a thermally equilibrated cosine distribution,53 indicating their nonthermal nature. The azimuth and polar angle of the “off-normal” component of the CH3 PSD is consistent with the orientation of the C−CH3 bonds expected for η2-acetone diolate adsorbed on TiO2(110) from the theoretical calculations37 (Figure 1b), where both oxygen atoms are attached to the Ti5c sites and the C−CH3 bonds are in the (001) plane (normal to the [001] azimuth). In addition to the off-normal peaks, there is also a small peak in the CH3 PSD distribution directed along the surface normal that is seen most clearly for the data along the [001] azimuth (Figure 6b). This peak, which indicates a second photochemical reaction pathway for acetone on TiO2(110), will be the primary subject of the second paper in this series.51 Figure 7 shows IRAS spectra of the nonirradiated (red and light blue lines) and UV-irradiated (dark blue and green lines) acetone dosed on oxidized TiO2(110) surface for two isotopomers: C3H6O (H6-acetone, red and dark blue lines) and C3D6O (D6-acetone, light blue and green lines). Oxygen and acetone, with θ(acetone) = 0.17 ML, were dosed using the standard procedure. The deuterated acetone diolate is associated with the peak at 1170 cm−1 in the p-polarized spectrum, which is red-shifted from the corresponding peak of the hydrogenated acetone diolate (Figure 7a). This is consistent with the acetone diolate isotopic shifts observed on the Ag(111) and Ag(110) surfaces.34,36 For the deuterated acetone diolate, the ρ(CD3) mode red-shifts relative to the ρ(CH3) by more than 100 cm−1 (which puts it out of the observable spectral range) and decouples from the skeletal νs(CCC) and νs(OCO) modes.34,36 On the other hand, the latter two modes now couple with δs(CD3),34 which is likely associated with the peak near 1000 cm−1 in our s-polarized spectra (Figure 7b). After UV irradiation, the acetone diolate peaks disappear from the spectra of the H6- and D6-acetone (Figure 7) showing that acetone diolate is the photoactive acetone−oxygen complex. While the acetone diolate peaks disappear after the UV irradiation, the molecular acetone ν(CO) peak intensity in p-polarized spectra remains practically unchanged (Figure 7a), indicating that photoactivity of the molecular acetone is low. New peaks are detected in the p-polarized spectra of the UVirradiated samples: at 1454 cm−1 with a shoulder at ∼1429 cm−1 for the H6-acetone, and a single peak at 1434 cm−1 for the D6-acetone (Figure 7a). For the H6-acetone, the new peaks overlap with the δa(CH3) peak of acetone in p-polarized spectra, but they are significantly more intense (there is no such overlap in the D6-acetone case). The new features are quite close to the νs(OCO) mode in the 1425−1454 cm−1 range for the H3-acetate (CH3COO−) adsorbed on TiO2,79−82 SnO2,83 Fe2O3,84 and Ni(111).35 This mode for the D3-acetate (CD3COO−) adsorbed on SnO2,83 Ni(111),35 and Pd(111)85 is reported at 1415 cm−1, which is red-shifted versus the H6acetone (consistent with our observation). The asymmetric νa(OCO) mode was reported in the 1513−1544 cm−1 range for the H3-acetate adsorbed on TiO2,79−82 SnO2,83 and Fe2O384 and at 1505 cm−1 for the D3-acetate adsorbed on SnO2.83 In our experiments, a small, relatively broad peak centered at ∼1485 cm−1 appears in the s-polarized spectrum of the UVirradiated D6-acetone (Figure 7b, green line), which is very close to the frequency of the νa(OCO) mode of D3-acetate. The corresponding feature in the s-polarized spectrum of the UV-irradiated H6-acetone is less clear, but a small peak centered at ∼1510 cm−1 (Figure 7b, dark blue line) may be the
with the multiple phonon loss peaks from the TiO2 substrate in this spectral range in HREELS. The production of acetone diolate from acetone on the oxidized TiO2(110) surface is accompanied by a decrease of the molecular acetone ν(CO) peak intensity as compared to the reduced surface (Figure 5a), as expected from reaction 1. The δa(CH3) and δs(CH3) peaks also decrease on the oxidized surface in the p-polarized spectra as seen for lower coverages. The ν(CO) peak shape experienced some changes for θ(acetone) = 0.60 ML: a shoulder near 1723 cm−1 appears in the p-polarized spectrum on the oxidized surface (Figure 5a), which may also be a result of intermolecular interactions. We will discuss the results for the higher acetone coverages in more detail in part 2,51 while here we will focus mainly on the low acetone coverages, where the acetone diolate precursor controls the photooxidation process. Valuable information on the structure and surface orientation of the photoactive acetone−oxygen complex can be obtained using angle-resolved PSD measurements.53,72 Time-integrated CH3 PSD yields for 0.083 ML of acetone adsorbed on oxidized TiO2(110) are shown in Figure 6 versus the desorption angle,
Figure 6. Integrated CH3 PSD yields versus desorption angle, φdes, relative to the surface normal along the [11̅0] azimuth (a) and [001] azimuth (b) on the TiO2(110) surface. O2 and 0.083 ML of acetone were dosed using a standard procedure (see the text for details). The data in (a) and (b) are taken for different TiO2(110) crystals with slightly different thermal histories and degrees of reduction. Data were obtained for φdes > 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 the adsorbed acetone diolate molecule.
φdes, for two different azimuthal orientations of the TiO2(110) surface: (a) along the [11̅0] azimuth and (b) along the [001] azimuth. Measurements along the [11̅0] azimuth show that the dominant trajectory of the desorbing CH3 radicals is off-normal and peaked at φdes ∼ ±66°. Measurements along the [001] azimuth show a small component desorbing along the surface normal. The angular distributions for both components are 12268
DOI: 10.1021/acs.jpcc.5b02477 J. Phys. Chem. C 2015, 119, 12262−12272
Article
The Journal of Physical Chemistry C
Figure 7. Differential absorbance IRAS spectra in (a) p- and (b) s-polarized modes for 0.17 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.
νa(OCO) of the H3-acetate. The fact that the νa(OCO) peak is observed for the electric field vector parallel to the Ti row indicates orientation of the acetate molecule on the surface along the [001] azimuth with both oxygen atoms on Ti5c sites. This observation is consistent with the orientation of the acetate on TiO2(110) deduced from electron-stimulated desorption ion angular distribution (ESDIAD) experiments69 and DFT calculations (see Figure 1c).37
product of the acetone diolate photodecomposition at low temperature (∼30 K). These results prove unambiguously the second step in the acetone photooxidation on the TiO2(110) surfacephotoinduced scission of the C−CH3 bond in the η2acetone diolate molecule leading to the CH3 radical ejection and leaving the η2-acetate on the surface (Figure 1b,c): η2‐acetone diolate(ads) + UV(TiO2 ) → η2‐acetate(ads) + CH3(gas)
IV. CONCLUSION The data presented here provide the first direct spectroscopic evidence for the main reaction steps in acetone photooxidation on TiO2(110) involving the acetone diolate intermediate. Our IRAS spectra show characteristic peaks of the η2-acetone diolate produced on TiO2(110) via thermal reaction of acetone with predosed oxygen. The loss of these features in the IRAS spectra after UV irradiation also indicates that the acetone diolate is the photoactive species. The ability of the η2-acetone diolate to reversibly exchange the oxygen atom was demonstrated experimentally and used to quantify the stoichiometry of the acetone thermal oxidation reaction yielding the acetone diolate (Figure 1 a,b): η1‐acetone(ads) + O(ads) → η2‐acetone diolate(ads)
(7)
The acetone diolate photodissociation is not direct but is initiated by the charge transfer from the photogenerated electronic excitations of TiO2 (e− and/or h+).25,28,31 The results presented here have focused primarily on the photochemistry occurring for acetone coverages of
