and High-Temperature Dehydration, Isotope Effects, and - American

Jul 4, 2007 - Oleksandr Bondarchuk,†,§ Yu Kwon Kim,† J. M. White,*,† Jooho Kim,‡,# Bruce D. Kay,*,‡ and. Zdenek Dohnalek*,‡. Pacific Nort...
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J. Phys. Chem. C 2007, 111, 11059-11067

11059

Surface Chemistry of 2-Propanol on TiO2(110): Low- and High-Temperature Dehydration, Isotope Effects, and Influence of Local Surface Structure Oleksandr Bondarchuk,†,§ Yu Kwon Kim,† J. M. White,*,† Jooho Kim,‡,# Bruce D. Kay,*,‡ and Zdenek Dohnalek*,‡ Pacific Northwest National Laboratory, Fundamental Sciences Directorate, Institute for Interfacial Catalysis, Richland, Washington 99352, and Center for Materials Chemistry, Texas Materials Institute, UniVersity of Texas, Austin, Texas 78712 ReceiVed: March 22, 2007; In Final Form: May 19, 2007

Dosed on rutile TiO2(110) at 100 K, the thermal chemistry of 2-propanol in three formssC3H7OH, C3D7OD, and C3H7ODswas characterized using temperature-programmed desorption. Only 2-propanol, propene, and water desorb with no evidence for acetone. The propene forms and desorbs by two paths, a heretofore unreported low-temperature path extending from 300 to 450 K and, concurring with prior work, a high-temperature path peaking between 570 and 580 K. Both paths exhibit isotope effects. The high-temperature path is interpreted in terms of decomposition of 2-propoxy species located on bridging oxygen atom rows. The low-temperature path is attributed to 2-propanol dehydration on undercoordinated Ti4+ ions of the Ti4+ rows. The low-temperature path characteristics vary with the long-range order and bridge-bonded oxygen atom vacancy concentration.

1. Introduction The (photo)oxidation of alcohols on rutile TiO2(110) represents a prototypical catalytic reaction often employed as a model for oxidation of organic contaminants.1-4 Additionally, alcohol decomposition is often used to identify catalytically active sites on metal oxide surfaces in single-crystal and powder form. At the surface of oxides that can be partially reduced by heating, point defects, i.e., oxygen atom vacancies, have been proposed as active centers for thermal and photon-driven catalysis.1-4 The TiO2(110) surface comprises rows of bridge-bonded oxygen anions (BBO’s) separated by rows of five-coordinate Ti cations, formally Ti4+. With the use of scanning tunneling methods, there have been a number of direct observations of bridge-bonded oxygen atom vacancies (BBOV’s) (ref 5 and references therein). Among small molecules that have been used to probe chemistry on TiO2, alcohols have proven interesting, particularly with regard to the roles played by BBOV’s. The thermal and photochemical surface science of 2-propanol, the alcohol of interest here, has been studied on two surfaces, (110) and (001), of rutile TiO2,1-4 and on powdered forms of rutile and anatase.6,7 On rutile TiO2(001), {011}-facetted, there is important early X-ray photoelectron spectroscopy (XPS) and temperatureprogrammed desorption (TPD) evidence that the O-H bond of RO-H (R ) CH3, CH3CH2, CH3CH2CH2, and (CH3)2CH) heterolytically dissociates to form an alkoxy group (RO-) bound to coordinatively unsaturated Ti4+ cation and a proton bound to a surface O2- anion.1 Once formed, these adsorbed species either react to reform and desorb the original alcohol or * Corresponding authors: E-mail: [email protected] (Z.D.); [email protected] (B.D.K.); [email protected] (J.M.W.). † University of Texas. ‡ Pacific Northwest National Laboratory. § Present address: Fritz-Haber Institute, Berlin, Germany. # Present address: LAM Research Corp., Fremont, CA 94538.

dehydrogenate/dehydrate to form products, e.g., alkenes, aldehydes, ketones, water, and, perhaps, hydrogen and carbon monoxide. On TiO2(110), the surface of interest here, there is recent directly relevant work on the surface chemistry of alcohols, including 2-propanol.4 Two reaction paths were identified. The lower temperature channel involves 2-propanol and water desorption with maximum rates at 345 K. The second channel involves 2-propanol desorption at 540 K followed by propene at 565 K and, in small amounts, water and hydrogen at 600 K. Some CO desorbs at 650 K. A radical mechanism was proposed to account for the high-temperature formation of propene. In this scheme, the C-O bond in the alkoxy breaks before the C-H bond. In related work using ethanol,2 two chemically distinct ethoxy, CH3CH2O(a), groups were proposedsone bound to coordinatively unsaturated Ti4+ cations that was responsible, during TPD, for the reformation and desorption of ethanol, CH3CH2OH, between 250 and 400 K, and a second form bound at a BBOV (filling that vacancy) that led to desorption of equal amounts of C2H4 and CH3CH2OH at much higher temperatures (650 K). When isotopically labeled CD3CH2OH was used, only CD2CH2 was formed evidencing a β-hydride elimination reaction involving cleavage of the β-C-D bond, not the R-C-H bond, in ethoxy, CD3CH2O. Finally, this interesting paper suggests that water desorbing below 450 K involves a reaction between two types of adsorbed OH, one located on a row of anions, while the other is bound to an accessible Ti4+. The formation of H2O during the dehydration of alcohols is important but difficult to quantify because of background water in typical vacuum systems, low-level impurities in the dosed alcohol, and facile isotope exchange reactions when D-labeled alcohols are used. In the context of reactivity on TiO2(110), the surface chemistry of dosed water has been widely studied.8-16 On TiO2(110), H2O dosed at T e 300 K, adsorbs nondissociatively, except at BBOV’s.8,9,14,15,17 In TPD, H2O multilayers start

10.1021/jp072298m CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007

11060 J. Phys. Chem. C, Vol. 111, No. 29, 2007 to desorb at 165 K. For the H2O in a direct contact with the surface, there is a peak at 175 K attributed to H2O adsorbed on BBO ions, a peak at 275 K peak attributed to H2O adsorbed on Ti4+, and a relatively small peak at 510 K attributed to recombination of surface hydroxyls formed previously by filling BBOV’s. In comparing results from different laboratories, differences in product yields and thermal profiles are often noted for reactions on TiO2(110). These variations, mostly undetectable by conventional low-energy electron diffraction (LEED) or Auger electron spectroscopy (AES), are attributed to subtle differences in surface order and stoichiometry. The point here is not to discuss the specific details but to emphasize that labto-lab reproducibility of reaction kinetics will likely improve if quantitative benchmarking comparisons with one or more test adsorbates, e.g., water, are made and benchmark preparation procedures involving scanning tunneling microscopy (STM) are used to minimize, or at least characterize, relevant structural variations, e.g., variations of BBOV concentrations and terracestep dimensions. In this study we investigated by TPD the surface chemistry of three isotopic forms of 2-propanolsC3H7OH (2-PrOH), C3D7OD (2-PrOD8), and C3H7OD (2-PrOD)son TiO2(110)-1 × 1 under UHV conditions. Only 2-propanol, propene, and water were desorbed in measurable amounts. We find, in addition to high-temperature propene desorption, ca. 570 K, evidence for heretofore undetected broad low-temperature propene evolution between 300 and 450 K. A local maximum is observed between 345 and 410 K depending on the coverage of 2-propanol and the history of the surface. Disrupting long-range order by light sputtering inhibits this low-temperature channel. There are discernible kinetic isotope effects for both the low- and hightemperature propene channels. In accord with prior literature, the high-temperature propene channel is discussed in terms of a surface reaction of individual 2-propoxy groups, positioned on BBO rows. The path for the high-temperature channel involves the cleavage of the C-O bond and one β-C-H(D) bond to form and desorb propene. The H(D) released by β-C-H(D) cleavage attaches to a neighboring BBO leading to recombinative water desorption at slightly higher temperatures. We speculate that the reaction path to the low-temperature propene formation involves a 2-propanol dehydration on undercoordinated Ti4+ rows. Our results for lightly Ne+-bombarded surfaces indicate that ordered 1-D Ti4+ rows are critical and that reducing their length lowers the lowtemperature propene yield. Detailed examination of lowtemperature 2-propanol and propene desorption profiles shows correlated subtle changes depending on the surface morphology and the BBOV concentration. 2. Experimental Section The experiments were conducted in a UHV molecular beamsurface scattering apparatus with base pressure less than 1 × 10-10 torr. The apparatus, typical sample mounting, sample temperature measurement, and thermocouple calibration procedure are described elsewhere.18 To exclude any concern of possible metal contamination, we have recently modified the sample holder by eliminating the front retaining ring noted in our prior TiO2 studies.18,19 The results obtained using a new sample mount were identical to these obtained from the sample mount having the front retaining ring. The 300 K effusive molecular beam directed normal to the TiO2(110) surface (10 × 10 × 1 mm3, single-side polish, Princeton Scientific) is important in minimizing exposure of surfaces other than the

Bondarchuk et al. TiO2(110) substrate. The beam diameter (7 mm) and position with respect to the center of the sample were determined visually by condensing a very thick layer (103 ML) of water at 50 K. The molecular beam was produced by expanding neat vapor through a 1 mm diameter circular aperture. Typically, a newly installed sample was cleaned by 15 sputter-anneal cyclesssputtering with Ne+ (60 s, 1500 eV, 10-5 A/cm2) at room temperature followed by ramping the temperature (5 K/s) to 900 K in vacuum. This removed impurities identified by AES (Ca, K, and C), and the substrate exhibited a sharp ordered (1 × 1) LEED pattern. After initial cleaning, a single sputter-anneal cycle each day was typically sufficient to give LEED and AES results indistinguishable from the above. After the initial cleaning, the substrate had a light-blue color indicative, as expected, of some reduction (Ti3+ centers) in the bulk of the crystal and BBOV’s at the surface.5 With use, the blue intensity grew, but only very slowly, indicating increasing bulk reduction. The BBOV concentration was quantified using TPD of H2O or D2O dosed at 50 K. For doses of slightly more than 1 ML, the water TPD spectrum exhibits peaks at 175, 275, and 510 K consistent with literature described in the Introduction.8,9,15 Moreover, the peak shapes of the 175 and 275 K peaks have very steep trailing edges with the desorption at 325 K being less than 5% of the peak intensity at 275 K. This reproduces important details of standards set by excellent work in the literature.8,9 The BBOV concentration (monitored in our work by the 510 K peak intensity for directly dosed water) was held at two valuess0.03 or 0.07 ML, where one ML is defined in terms of the areal density of five-coordinate Ti4+ cations in a perfect TiO2(110) surface (5.2 × 1014 cm-2). Prior to dosing, the 2-propanol sources were purified by repeated freeze-pump-thaw cycles. The molecular beam was produced with 1 torr of 2-propanol vapor behind the 1.0 mm diameter circular aperture. After dosing, TPD spectra (1.8 K s-1 ramp rate) were measured using a computer-controlled lineof-sight quadrupole mass spectrometer (UTI). Concurring with earlier work,3 the sticking probability for 2-propanol was close to unity and coverage-independent as measured using the beam reflection technique of King and Wells.20 Fragmentation pattern analysis was used to identify products desorbing in TPD. For isotopically labeled propanols (2-PrOH, 2-PrOD8, and 2-PrOD) these were determined by condensing a thick (10 ML) film of propanol at 50 K and raising the temperature to 147 K. During this process, some multilayer desorbs and the mass spectrum is used as the fragmentation pattern. Accounting for D-for-H substitution, these patterns compare favorably with a standard fragmentation pattern for C3H7OH.21 The following ions were used to track various desorbing species after the alcohol adsorption: PrOD8 (66, 50 amu), PrOD (60, 46 amu), PrOH (59, 45 amu), C3D6 (46, 42 amu), C3H6 (41, 39 amu), D2O (20, 18 amu), HDO (19 amu), and H2O (18, 17 amu). Relative adsorbate coverages in ML for water and 2-propanol are defined separately. For D2O, 1 ML is defined as the observed saturation intensity of the 275 K TPD peak for D2O dosed at 100 K. As determined in our previous study,18 this coverage corresponds to 5.2 × 1014 H2O/cm2 or 1 H2O per Ti4+ site. Similarly, 1 ML of 2-propanol is defined as the observed saturation intensity of the 315 K TPD peak for 2-propanol dosed at 100 K. In this case, the absolute coverage is estimated to be 2.6 × 1014 C3H7OH/cm2 or 0.5 C3H7OH per Ti4+ as further discussed below.

Surface Chemistry of 2-Propanol on TiO2(100)

Figure 1. Thermal desorption of 2-propanol from TiO2(110) following doses at 100 K of (CD3)2CDOD, panel A, and (CH3)2CHOD, panel C. The integrated intensity of 2-propanol desorbed as a function of the dose in monolayers (ML) is plotted in panels B and D. The inset in panel C shows (vertically scaled by 5×) the high-temperature desorption of 2-propanol from 2-PrOD8 (50 amu) and 2-PrOH (45 amu) for two doses (0.25 and 1 ML). The clean surfaces differ; in (A) the BBOV concentration was 7%, whereas that in (C) was 3%. The higher BBOV concentration of (A) is correlated with a larger ∆, the estimated amount of 1 ML that desorbs as a product other than 2-propanol, and with the appearance of the features II and III. From one sample to another, the peak temperatures vary by (5 K, a difference attributable to thermal contact with the heating plate (see the Experimental Section).

3. Results After dosing 2-propanol at 100 K, TPD spectra tracking desorption of the alcohol, Figure 1, show variations depending on the dose and on the detailed condition of the TiO2(110) substrate. For 2-PrOD8, using the 50 amu signal to monitor C3D7OD TPD, Figure 1A, from a substrate with 7% BBOV’s, desorption sets in as low as 140 K and continues to 650 K. After TPD to T g 700 K, carbon is not detectable in AES (not shown) and did not accumulate during cycles of dosing and TPD experiments. The overall spectrum can be divided into five regions (M2, M1, I, II, and III) as labeled in Figure 1A. Desorption in regions I, II, and III is ascribed to C3D7OD in direct contact with the substrate Ti4+ sites. The nature of the M1 region is unclear, but it most likely involves 2-propanol molecules also bound to the substrate. Since the desorption temperatures are below those for Ti4+-bound 2-propanol, we speculate that this state involves 2-propanol molecules bound to BBO ions. The desorption from M2 is ascribed to C3D7OD that is not in direct contact with TiO2. Desorption of species bound to the substrate is of most interest here and, assuming the above assignments are correct, occurs with coverage-dependent local maxima at T g 315 and g 410 K and a coverage-independent maximum at 560 K in regions I, II, and III, respectively. The peak in region II shifts to as low as 410 K and then appears to saturate and become a shoulder (discussed further below) on the peak used to identify region I. The desorption rate in region I is dose-dependent, and the peak shifts from 390 to 315 K, but no further, as the dose increases. For higher doses, the M1 region fills in followed by the emergence of the sharp unsaturable true multilayer peak (M2) obeying a zero-order desorption kinetics.

J. Phys. Chem. C, Vol. 111, No. 29, 2007 11061 For any dose exhibiting a peak at M2, there is broad desorption throughout the M1 region (170-250 K). Note that although this region may make a small contribution (gray area) to the curve marked with an arrow in Figure 1A, it is barely discernible and, as described in the Experimental Section, we define this coverage as 1 ML of C3D7OD. With the use of this scale, a plot of the integrated 50 amu intensities (Figure 1B) changes slope at 0.8 ML and the reaction products found in TPD (see below) approach saturation. A straight line (dashed) passing through the origin and parallel to the high-coverage data is the predicted 50 amu peak area dependence in the absence of surface reactions. The discrepancy (∆) above 0.8 ML coverage for this substrate is 0.3 ML of C3D7OD and is taken as a measure of the maximum amount of the alcohol undergoing reaction. It is instructive to convert the relative 1 ML of 2-propanol coverage to absolute coverage in terms of the number of Ti4+ sites. To accomplish this we cross-correlate exposures required to obtain 1 ML coverage of 2-propanol with that of water. For an effusive source, the ratio of 2-propanol and water fluxes at the same expansion-aperture backing pressure is simply inversely proportional to the square root of the ratio of their molecular weights. For 2-PrOD8 and 2-PrOD, we obtain fluxes that are 0.51 and 0.54 of that for H2O. Since the sticking coefficients are close to unity for both water and 2-propanol, the product of their relative saturation times (from TPD) and relative fluxes yields saturation coverage of 2-PrOD8 and 2-PrOD that are 0.57 and 0.60 of that for water. The estimated uncertainty of these values is approximately (10%. Since the saturation coverage of 1 ML of water on TiO2(110) corresponds to one H2O per each Ti4+ (5.2 × 1014 cm-2), we conclude that each 2-propanol molecule blocks two Ti4+ sites (2.6 × 1014 cm-2). In comparison to a surface with 7% BBOV’s, the TPD profiles for 2-propanol desorption from a substrate with 3% BBOV’s differ; regardless of the isotopic composition, the shoulder in region II is absent. This is illustrated in Figure 1C for 2-PrOD using the 46 amu signal (CH3CHOD+) to track 2-propanol desorption for four doses up to 1 ML. Figure 1D shows that ∆ ) 0.1 ML, which is significantly smaller than in Figure 1B. On this substrate, the same ∆ was measured for the two other 2-propanol isotopes (PrOD8, PrOH, not shown). In passing, there is no 46 amu intensity in region III when 2-PrOD is dosed because the 2-propanol desorbed in this region is fully hydrogenated, i.e., 2-PrOH not 2-PrOD (see the further discussion). Focusing on a 1 ML dose of 2-PrOD8, we searched for desorbing products other than C3D7OD and found evidence (Figure 2) for C3D6 (46 amu), D2O (20 amu), and HDO (19 amu), but no evidence for D2 or other products, in particular no acetone, (CD3)2CO (64 amu). The appearance of HDO+ (19 amu) is the result of H-for-D isotope exchange on the chamber walls and in the QMS ionizer. Detailed analysis of the region between 450 and 600 K shows that the C3D7OD desorption (50 amu) peaks at 560 K, 20 K below C3D6 (46 amu, 580 K) and 35 K below D2O (20 amu). Although both C3D7OD and propene (C3D6) give significant ion intensity at 46 amu, detailed analysis of Figure 2 shows that the (46 amu/50 amu) intensity ratio increases with temperature. Thus, at least one product in addition to C3D7OD desorbs; the evidence is consistent with propene. Underscoring this point for a submonolayer dose of 2-PrOD8, inset a in Figure 3A is an overlay of the unscaled (as-gathered) intensities of the 50 and 46 amu signals for a low (0.1 ML) C3D7OD dose. Fortuitously, both plots onset at the same temperature (ca. 350 K) and have equal intensities up to 400

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Bondarchuk et al.

Figure 2. TPD of monolayer (CD3)2CDOD dosed at 100 K on a substrate with 7% BBOV’s. The five signals track the following species: 50 amu, (CD3)2CDOD; 46 amu, (CD3)2CDOD and C3D6; 20 amu, D2O and (CD3)2CDOD; 19 amu, HDO; 64 amu, (CD3)2CO. The monotonically rising baseline of 19 amu is attributed to isotope exchange with the rising background water partial pressure as nonsample surface warms during TPD. The absence of structure in the 64 amu profile confirms that acetone desorption is negligible. Figure 4. Integrated propene desorption intensitiesstotal, hightemperature, and low-temperature regionssplotted against 2-propanol dose. Panel A, 2-PrOD8 dosed on the TiO2(110) with 7% BBOV’s; panel B, 2-PrOH, 2-PrOD, and 2-PrOD8 dosed on the TiO2(110) with 3% BBOV’s.

Figure 3. Propene desorption profiles for doses of (CD3)2CDOD, panel A, and (CH3)2CHOD, panel B. Propene intensities were determined by subtracting, from the C3H(D)5+ signal, the contributions from alcohol desorption. Fragmentation patterns, measured during multilayer desorption of the parent 2-propanol molecules, were used to determine the amounts to subtract. Unambiguously confirming desorption of species other than alcohol, inset a in panel A plots as-measured 50 and 46 amu profiles for a case that illustrates that these do not track for any temperature above 410 K. As in Figure 1C, panel B involves a surface with 3% BBOV’s, whereas for (A) there were 7% BBOV’s. The inset b in panel B is a vertically expanded version of the 300550 K regions for two lowest doses.

K. However, above 400 K the 46 amu intensity exceeds that of 50 amu and the ratio is temperature-dependent. Since the measured fragmentation pattern of C3D7OD, determined using multilayer TPD, gives I50/I46 ) 2.5, we conclude that the 46 amu intensity exceeds that attributable to C3D7OD throughout

the temperature range above 350 K and that C3D6 readily accounts for the excess. Assuming only C3D7OD and C3D6 contribute to the 46 amu signal and only C3D7OD contributes to the 50 amu signal, a scaled version of the latter, when subtracted from the raw 46 amu profile, provides the relative propene (C3D6) desorption rate. For four doses (e1 ML), Figure 3A shows the resulting propene desorption rate profiles for the 7% BBOV surface of Figure 1A. As shown by a relative propene yield plot (see Figure 4A), for all submonolayer coverages the two distinct local maxima (regions II and III) monotonically intensify as the dose increases. Upon saturation (1 ML), ∼50% of the total propene amount desorbs via the high-temperature BBOV-related channel. Since the total amount of reacted 2-propanol is 0.3 ML and the 1 ML saturation coverage is 2.6 × 1014 cm-2, we calculate that this amount corresponds to formation of 3.9 × 1013 C3D6/cm2 at high temperature. This value is in very good agreement with the BBOV concentration of 3.6 × 1013 BBOV/cm2 on this surface. Similar analysis for 2-PrOD dosed on the substrate with 3% BBOV’s, gives the propene, C3H6, desorption profiles of Figure 3B. To emphasize that region II exhibits intensity, albeit weak, for the lowest doses, the inset b shows a 5× vertical expansion of region II. As for the surface of Figure 3A, the hightemperature peak position does not vary with dose, but its position is 10 K lower than measured for propene derived from 2-PrOD8 (see Figure 5). Figure 4B shows integrated intensities for three adsorbates, 2-PrOH, 2-PrOD, and 2-PrOD8, gathered on the surface of Figure 3B. As already discussed with Figure 1C, the total propene yield is approximately the same for all isotopes and equal to 0.10 ( 0.01 ML. Upon saturation, the absolute amounts of propene for region III are calculated to be 2.0 × 1013 C3D6/cm2 for 2-PrOD and 2-PrOH and 1.6 × 1013 C3H6/cm2 for 2-PrOD8 in a good agreement with the BBOV concentration of 1.6 × 1013 BBOV/cm2.

Surface Chemistry of 2-Propanol on TiO2(100)

J. Phys. Chem. C, Vol. 111, No. 29, 2007 11063

Figure 5. For three doses of three forms of 2-propanol at 100 K, the variation of propene desorption with deuterium labeling. Of the three profiles, propene, C3D6, derived from (CD3)2CDOD, labeled 2-PrOD8 (dashed line), is most intense in the low-temperature region (T < 520 K) but is less intense and shifted 10 K higher in the high-temperature region (T > 520 K). The profiles for C3H6 derived from (CH3)2CHOD, labeled 2-PrOD (black line), and (CH3)2CHOH, labeled 2-PrOH (gray line), do not differ significantly.

There are interesting isotope effects (Figure 5, 3% BBOV’s) in the propene temperature profiles for propene derived from the three 2-propanol molecules used. Concurring with prior work,4 the high-temperature propene intensity following dose of 2-PrOD contains no D (not shown), i.e., only C3H6 desorbs indicating that the C-H bond cleavage is involved in the ratelimiting step leading to immediate propene desorption. Confirming that C-H bond breaking is involved in the rate-determining step, the high-temperature propene desorption kinetics (Figure 5) depends on the isotopic labeling of the dosed alcohol. For fully deuterated 2-PrOD8, the high-temperature D-labeled propene (C3D6) peak is 10 K higher for all doses and slightly less intense than the profiles for H-labeled propene (C3H6) desorption derived from 2-PrOH and 2-PrOD. We conclude that stretching the C-H(D) bond is key in forming the activated complex that relaxes to form propene and, presumably, hydroxyl. There is also an isotope effect in the low-temperature propene formation path (Figure 5); the formation rate of C3D6 is higher than C3H6. This is particularly evident in C3H6 derived from C3H7OH. Clearly, the fraction of the propene desorbed in the low-temperature channel is larger when C3D7OD is dosed. In addition, we conclude that fraction of the 2-propanol dosed that desorbs via the low-temperature propene route for C3D7OD is roughly twice that for C3H7OH. Clearly, this observation is surprising since C-H bond cleavage is easier than that of C-D as observed in the case of the high-temperature dehydration channel. Further experiments are required to address this issue. The low-temperature propene desorption channel is completely suppressed by Ne+ sputtering (Figure 6). Before sputtering, TPD of a PrOH dosed at 100 K (Figure 6a) gave the typical broadly distributed propene desorption beginning at 270 K with peaks at 375 and 570 K. After sputtering at 300 K (1.5 kV, fluence 4 × 1015 Ne+/cm2), the sharp 1 × 1 LEED pattern of a clean surface was diffuse but not destroyed. In subsequent TPD to 650 K of a dose of C3H7OH (Figure 6b), the lowtemperature propene channel is completely suppressed and there is significant intensification in the high-temperature region with a maximum at 510 K and a shoulder at 570 K. Upon cooling and redosing, TPD to 650 K gave much less propene, Figure

Figure 6. Effect of Ne+ sputtering of clean TiO2(110) (3% BBOV’s) on subsequent TPD of propene for 1 ML of (CH3)2CHOH dosed at 160 K: (a) before sputtering; (b) after sputtering, dosing at 100 K and TPD to 650 K; (c) dosing at 100 K after (b); (d) dosing at 100 K and TPD to 850 K after (c), thick dark curve. In (d) the dashed curve is an overlay of spectrum a. In the dark-gray regions the intensity of (a) > (d), and in the light-gray region (a) < (d).

6c, marginal evidence for propene desorption below 450 K, 5-fold reduced intensity at 510 K, and retention of strong intensity at 570 K. Repeating this cycle did not alter the profile measurably until the surface was annealed to 850 K (Figure 6d). Even the propene TPD from a 850 K annealed, 2-propanol dosed surface (Figure 6d) was still not superimposable on that found before Ne+ sputtering (Figure 6a). To fully recover the ordered TiO2(110) surface, extended annealing at 900 K was required. Figure 7 shows an overlay of the 45 and 46 amu TPD profiles following a dose of 1 ML of 2-PrOD at 100 K. In constructing this graph, the 45 amu signal was scaled to bring its leading edge into coincidence with the 46 amu profile. It is clear that the two profiles align to 325 K, but for higher temperatures, the scaled 45 amu profile is more intense (dark-gray region). The difference, shown separately in Figure 7, has two peaks. The 46 amu profile is uniquely diagnostic for 2-PrOD, whereas 45 has contributions from both 2-PrOD and 2-PrOH. The difference (dark-gray area of Figure 7) is interpreted as desorption of 2-PrOH from the surface dosed with 2-PrOD. Since 2-propanol dehydrates to form propene and water on TiO2(110), water formation and desorption is a central issue. After removing the contribution from alcohol fragmentation of dosed 2-PrOD8, there are four local water desorption maxima/ shoulders that vary with 2-propanol dose (Figure 8). Peak I, assigned to background and impurity D2O, shifts steadily to lower temperatures as the dose increases (40 K shift between lowest and highest doses). Region II appears as a shoulder and intensifies but does not shift perceptibly. This peak is attributed to water derived from alcohol chemistry involved in producing the low-temperature propene (see Figure 3 and discussion below). Peak III is attributed to water formed from hydroxyl

11064 J. Phys. Chem. C, Vol. 111, No. 29, 2007

Figure 7. Alcohol TPD following a 1 ML dose of (CH3)2CHOD at 100 K. The as-gathered 46 amu plot (light-gray area) and the 45 amu plot scaled to give superimposable leading edges clearly differ for T > 325 K; the scaled 45 amu signal exceeds the as-gathered 46 amu signal. The difference, indicated by the dark-gray area in the superposition, is plotted separately in the lower profile. The desorption of (CH3)2CHOH in the low-temperature region evidences C-H bond breaking at low temperatures and is consistent with low-temperature propene formation.

Bondarchuk et al.

Figure 9. Water desorption following a 0.7 ML dose of (CH3)2CHOD at 100 K. The contributions from fragmentation of (CH3)2CHOD have been subtracted. The 290 K peak is attributed to impurity in the dosed water, the 335 K contribution to low-temperature water involving C-H bond breaking in the dosed alcohol; the 520 K peak (dominated by D2O) to water formed from OD groups formed at low temperatures when the dosed alcohol deprotonates at BBOV’s to form 2-propoxy, (CH3)2CHO-, and OD groups, and the 580 K peak (dominated by H2O) to reaction of OH groups formed when C-H bonds break to form C3H6 from (CH3)2CHO- at high temperatures.

4. Discussion

Figure 8. Deuterated water TPD, 20 amu D2O+, for several (CD3)2CDOD doses at 100 K. The contribution from ion source fragmentation of (CD3)2CDOD has been subtracted. The peaks, I-IV, are assigned as follows: I, impurity in alcohol source; II, low-temperature water derived from reaction of (CD3)2CDOD that is coupled to the lowtemperature propene formation; III, D2O derived from OD species that were formed by deprotonation at BBOV’s of dosed (CD3)2CDOD; IV, D2O derived from OD formed at BBO’s when C-D bonds break at high temperatures to form C3D6.

groups located on BBO rows that formed at lower temperatures. Peak IV is attributed to water formed via reaction of hydroxyl groups formed on BBO rows in concert with the hightemperature propene evolution. Further insight regarding water desorption is gained by tracking isotopic water (18, 19, and 20 amu) in TPD after dosing 2-PrOD (Figure 9). As in Figure 8, there are four local maxima in all the profiles. The two new key points are (1) the 520 K peak (III) is dominated by D2O and (2) the 580 K peak (IV) is dominated by H2O.

Models for Low- and High-Temperature Propene Formation. From the above data, we construct the following picture of how 2-propanol interacts with TiO2(110) at 100 K during dosing and during subsequent TPD. As noted earlier, the protocol for preparing TiO2(110) surfaces is relevant if one is, as we are, interested in comparing data gathered from different substrates at different times in our own and other research laboratories. In our case, we have prepared a surface from which water desorbs with the particular profile described above and have focused on two surfaces, one with 3% BBOV’s, the other with 7% BBOV’s. On these surfaces, for any coverage of 2-propanol, even the lowest we used (0.02 ML), 2-propanol, propene, and water are found in TPD, and importantly, the desorption rates each maximize locally at two or three temperatures. This reflects multiple reaction paths to the products. It is not clear from the results whether or not there is dissociation during adsorption, but there is direct evidence from STM work22,23 that alcohols deprotonate at BBOV’s at 300 K with the alkoxy groups filling the BBOV’s and the proton attaching to a neighboring BBO, i.e., a hydroxyl. These species survive to high temperatures. In the proposed model, depicted in Figure 10, the hydroxyls form water that desorbs with a peak at 510 K, forming one BBOV. As shown in Figure 10, the hydrogen for this water originates from the hydroxyl hydrogen of the dosed alcohol. At completion, this reaction restores the BBOV concentration to half its initial value. As the temperature increases, the 2-propoxy groups thermally pass through a fiveatom (O-C-C-H-O) cyclic activated complex (center panel of Figure 10) that concertedly leads to propene and OH, the H coming from a methyl group of 2-propanol via β-hydride elimination. The resulting OH groups recombine (bottom panel of Figure 10) with the peak rate occurring when the transient OH concentration reaches its maximum instantaneous value, ca., 10 K above the maximum rate for propene formation. Between 300 and 450 K, additional dissociative chemistry is required to account for three observations: (1) low-temperature

Surface Chemistry of 2-Propanol on TiO2(100)

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Figure 10. Schematic of reactions of 2-propoxy and hydroxyl groups at high temperature (>500 K). The model involves species located on BBO rows. The red circles denote original positions of BBOV’s that are filled with oxygen in (CH3)2CHO- when alcohols deprotonate, and the yellow circles represent the accompanying hydrogens. At 520 K, the hydrogens are mobile along the BBO rows and, upon coming together, can react to form water that desorbs and a new BBOV (blue circle enclosed with dashed red line). At 570 K, 2-propoxy groups react by dehydration to form propene by C-H bond breaking. The resulting mobile proton concentration grows as propene forms and more water follows at 580 K carrying an oxygen atom from the location where the two protons, both derived from C-H bond breaking, interact.

Figure 11. Schematic of reaction path to form water and propene at low temperatures (300-450 K). Assuming (CH3)2CHOD is dosed, in (A), the proposed model involves dissociation of molecularly bound 2-propanol on Ti4+ rows. In (B), the 2-propoxy on Ti4+ rows undergoes dehydration reaction to produce H-labeled propene, a proton bound to a BBO, and an OD group bound to Ti4+. In (C), the OD groups on Ti4+ rows then react with the proton bound to a BBO to form water without creating a vacancy.

propene desorption, (2) low-temperature water desorption, and (3) when 2-PrOD is dosed, the desorption of 2-PrOH above 325 K (Figure 7). Comparison of Figures 3B, 9, and 7 indicates that these three profiles mimic each other. Observation 3 is particularly relevant because the hydrogen to form the excess 2-PrOH must come from C-H bonds in the dosed 2-PrOD. A plausible low-temperature reaction path model is depicted schematically in Figure 11. We assume that when the relatively low concentration of initial BBOV’s is filled, 2-propanol

chemisorbs on Ti4+ rows through a donor-acceptor (acid-base) process in which lone pairs on the oxygen of the alcohol interact with exposed Ti4+ cations increasing the coordination number from 5 to 6. In agreement with prior studies,2 we believe that some propanol molecules can dissociate on Ti4+ rows and form 2-propoxy bound to Ti4+ and hydrogen bound to a neighboring BBO (left-hand panel of Figure 11). Upon moderate heating (300-450 K), the Ti4+-bound 2-propoxy can recombine with the BBO-bound hydrogen as previously reported.2 Additionally

11066 J. Phys. Chem. C, Vol. 111, No. 29, 2007 as we demonstrate here, this 2-propoxy can also undergo further reaction to yield propene in this low-temperature range. Our experiments on lightly sputtered TiO2(110) (see Figure 6) reveal that this low-temperature dehydration channel requires extended rows of Ti4+ ions. Furthermore, an increased propene yield from PrOD8 as compared to that from PrOH and PrOD (see Figures 4 and 5) suggests that C-H(D) bond cleavage is involved in the rate-limiting step of propene formation. Despite these observations, the exact mechanism of this low-temperature dehydration channel is unclear and is currently the subject of our ongoing investigations. Another product (transient) generated during the alcohol dehydration is hydroxyl, labeled with the hydroxyl hydrogen of the dosed alcohol, and formed as the C-O bond breaks and binds to Ti4+. Once the propene and 2-PrOH leave the surface, the vacated local environment offers no steric impediment to water formation by recombination of the hydroxyl groups, one on Ti4+ and the other on a BBO row (right-hand panel of Figure 11); no BBOV forms in this process. The hydroxyl labeled with hydrogen from the methyl group can also participate in the recombinative 2-propanol desorption or undergo isotope exchange with the alcohol hydrogen thus accounting for the observation of excess 2-PrOH when 2-PrOD is dosed (Figure 7). The intrinsic mobility of the OH group on Ti4+ is not known, but STM data show that H atoms (protons) do migrate across the BBO rows when assisted by molecularly bound alcohol.22 In comparison to C3H6, regardless of dose, the peak reaction rate occurs at a higher temperature for C3D6 formation. The scheme depicted in Figure 11, middle panel, accounts for this difference in terms of the participation of the β-C-H(D) bond in the transition state leading from 2-propoxide to propene. In this framework, the activation enthalpy required to pass from the initial state to the transition state is higher for the D-labeled 2-propoxide. There is another important aspect: on identical surfaces, the integrated intensity of high-temperature C3H6 exceeds that of C3D6 by 10% (Figure 4B), and in a partial tradeoff, the high-temperature intensity of 2-PrOD8 is slightly higher than 2-PrOH (inset in Figure 1C). However, there must be an additional component since the increased high-temperature 2-PrOD8 is not sufficient to account for the lower C3D6. A clue is found in the low-temperature channels; the integrated C3D6 intensity exceeds the integrated C3H6 intensity. As shown in the integrated intensity plot of Figure 4B, the partitioning between the low- and high-temperature propene channels is quite different. The low-temperature channel favors 2-PrOD8 by an amount that compensates for the lower intensity in the hightemperature channel. The comparisons made here between surfaces with 3% and 7% BBOV concentrations show that the low-temperature propene desorption profiles set in at the same temperature but are broadened toward higher temperatures and more intense relative to the high-temperature channel for the 7% BBOV case. Furthermore, the 2-propanol desorption profiles are also broadened in the same way qualitatively. This indicates that BBOV’s have an impact extending beyond the deprotonation reaction. In particular, a larger fraction of the dosed 2-propanol is present at higher temperatures for the 7% BBOV surface. While surface characteristics, namely BBOV concentration and long-range order, have been identified here as playing important roles in the low-temperature dehydration, the importance of subsurface reduction, e.g., oxygen vacancies and titanium interstitials, remains an interesting and open question. We intuitively expect the alcohol surface chemistry to be influenced when these types

Bondarchuk et al. of defects are located within a few lattice spacings of the gassolid interface. In comparison with prior work, the key point is our detection and characterization, for the first time, of a low-temperature route to propene. The high-temperature reaction channel is a common feature of alcohol chemistry on titania.1,2,4,6 The reasons why we find the low-temperature channel, while earlier studies did not, remain speculative but may be related to one or more of the following: the ensemble average length of uninterrupted Ti4+ rows, the degree of subsurface reduction, and the surface BBOV concentration. However, BBOV concentration by itself is an unlikely source, since we find low-temperature propene for both 3% and 7% BBOV concentrations. It is possible that the primary reason stems from the difficulties in the isolation of the propene desorption from that of 2-propanol. As shown in Figure 2, 2-propanol desorption dominates from 250 to 400 K, and since the masses used to extract propene desorption have a significant contribution from 2-propanol desorption, only careful subtraction of high-quality TPD data can reveal this channel. In a forthcoming paper, we discuss the generality of this low-temperature reaction path for a series of primary, secondary, and tertiary alcohols.24 Regarding the high-temperature channel, our results are in general agreement with prior work that involved doses of 2-propanol at 300 K.4 On clean TiO2(110) with minimal concentrations of BBOV’s, these authors reported that 2-propanol and water desorbed with maximum rates at 345 K, additional 2-propanol desorbed at 540 K followed by propene at 565 K and, in small amounts, water and hydrogen at 600 K. There was no acetone desorption. Within experimental uncertainty, the peak temperatures and the distributions of products, except for H2, which we do not find, are in accord with our results. The high-temperature propene and water desorption was accounted for in terms of the reaction depicted in Figure 10, i.e., decomposition of 2-propoxide formed at lower temperatures and located on BBO rows. Differing from our scheme that relies on initial BBOV’s to deprotonate the alcohol, the authors’ model incorporates BBOV formation during TPD. Their model proposes low-temperature deprotonation of 2-propanol on Ti4+ rows to form 2-propoxy bound to the Ti4+ and protonation of a BBO to form hydroxyl. During TPD, the hydroxyls react at low temperatures to form and desorb water carrying a BBO leaving behind a BBOV. The latter is filled by 2-propoxy that migrates from a nearby Ti4+, and these 2-propoxy groups are the sole source of the high-temperature propene. The proposed formation path for a BBOV is open to question based on the previous studies8,14,15 that show that hydroxyl species located on BBO rows do react to form water, but only at higher temperatures, ca., 500 K, as we find in Figures 8 and 9. In a separate paper,25 we discuss routes to the formation of alkoxy groups on BBO rows that do not require an initial BBOV. 5. Summary The thermal chemistry of 2-propanol in three isotopically labeled formssC3H7OH, C3D7OD, and C3H7ODsdosed at 100 K on TiO2(110) surfaces with 3% and 7% BBOV’s involves only 2-propanol, propene, and water with no evidence for acetone. A low-temperature path to propene (300-450 K) is reported for the first time and described in terms of 2-propanol dehydration on undercoordinated Ti4+ ions of the Ti4+ rows. The low-temperature path characteristics vary with the longrange order and BBOV concentration. Destroying long-range order by Ne+ bombardment completely suppresses the lowtemperature path. It is restored by annealing at 850 K. A surface

Surface Chemistry of 2-Propanol on TiO2(100) with 7% BBOV’s exhibits more low-temperature propene than a surface with 3% BBOV’s. The high-temperature (peak between 565 and 575 K) path to propene concurs with prior work and is interpreted in terms of decomposition of 2-propoxy species located on bridging oxygen atom rows. Both paths exhibit isotope effects, the high-temperature path favoring hydrogenated 2-propoxy and the low-temperature path favoring deuterated 2-propoxy. Acknowledgment. This work was supported by the U.S. Department of Energy Office of Basic Energy Sciences, Chemical Sciences and Materials Sciences Divisions, and it was performed at the W.R. Wiley Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory. PNNL is operated for the U.S. DOE by Battelle under Contract No. DE-AC06-76RLO 1830. J.M.W. acknowledges support by the U.S. Department of Energy Office of Basic Energy Sciences, Chemical Sciences Division under Grant DE-FG02-03ER15480 to the University of Texas and by the Robert A. Welch Foundation and Center for Materials Chemistry at the University of Texas. References and Notes (1) (2) (3) (4)

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