Ambient-Pressure XPS Studies of Reactions of Alcohols on SrTiO3

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

Ambient-Pressure XPS Studies of Reactions of Alcohols on SrTiO3(100) Yafen Zhang, Aditya Savara, and David R. Mullins* Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201, United States ABSTRACT: Ambient-pressure X-ray photoelectron spectroscopy (APXPS) and temperature-programmed desorption (TPD) have been employed to elucidate the adsorption and reaction of simple alcohols on SrTiO3(100). TPD experiments indicate molecular desorption of alcohols with a small amount of aldehydes below 100 °C, whereas no gas-phase products are observed above this temperature. APXPS spectra at 0.1 Torr show that alcohols adsorb dissociatively onto SrTiO3(100) to form alkoxies. Surface methoxides appear to react with each other to produce acetate as a surface intermediate. These surface species are eventually oxidized to gas-phase CO, CO2, and H2O. Ethoxide is readily oxidized to acetate species that undergo further reactions to form gasphase products. CO2 is the only C-containing product during ethanol oxidation, whereas methanol can also produce the partial oxidation product, CO. When no O2 is present, alcohol oxidation yields gas-phase CO, CO2, or H2O and creates oxygen vacancies on the surface, resulting in the reduction of Ti4+. Without a source of oxygen replenishment, the availability of surface oxygen would be limited, and thus, the oxidation reactions could not progress indefinitely. At near-ambient pressures, the reactivity of the surface and the distribution of surface species and reaction products were changed by altering the alcohol/O2 ratio, consistent with an interpretation that surface vacancies are being created and that their concentration is altered when an oxygen source is present. The conversion from acetate to CO2 might be rate-limiting when sufficient O2 gas is present. als.15−17 Perovskites have the general formula ABO3 and are typically composed of alkaline, alkaline-earth, or lanthanide cations (A) and transition-metal cations (B), including Ti, Mn, Co, Fe, Ni, and Cr.17 By selectively choosing the constituents of ABO3, it is possible to alter the catalytic activity while maintaining nominally the same structure. SrTiO3(100) single crystals are employed in this work because of their availability and their extensive use as a model perovskite for the study of surface structure and chemical properties.18,19 SrTiO3 single crystals have the same basic building blocks, namely, TiO6 octahedra, as TiO2 (rutile).20 Based on studies of the photodecomposition of water under conditions of band-gap illumination on SrTiO3, scientists generally attribute the oxide reactivity to the presence of defect sites likely created by surface reduction.19,21 Thus far, few studies have focused on reactions of small organic molecules on SrTiO3 surfaces. Bowker and co-workers investigated the adsorptive and reactive behavior of the SrTiO3 powder surface toward methanol that was adsorbed at room temperature using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS).20 They observed that gas-phase CH3OH was desorbed from the surface as the main product, with H2, H2O, and CH4 being produced in much smaller quantities. In addition, comparison of the reactivity of methanol on SrTiO3 with that on TiO2 indicated a lower

1. INTRODUCTION Chemical reactions on oxide surfaces play an important role in a great number of applications, including the development of catalysts, the sensing and separation of gases, and the fabrication of semiconductor devices.1−5 The interfacial interaction of small molecules, such as methanol and ethanol, with metal oxides is often employed to probe physical properties and catalytic activities of the oxides.6,7 The catalytic activity of oxide surfaces can be characterized by three key aspects: (1) the coordination environment of surface atoms, (2) the acid−base and redox properties of the oxide surface, and (3) the oxidation state of the cations.8,9 The surface coordination environment can be tuned by changing the exposed crystal plane but can be susceptible to surface reconstruction.10 In addition, the acid−base or redox properties can also depend on the surface structure.8 Hence, these properties can be controlled by the choice of oxide, such as the adjustment of acid−base properties of the oxide surface through the selection of different transition metals.11 Further, the oxidation state can be altered by sample pretreatment. For example, the oxidation states of some easily reduced oxide surfaces were reported to be varied by either simple thermal treatment in a vacuum or chemical reduction with small molecules including H2 and methanol.12−14 A perovskite, such as that utilized in this study, offers great flexibility with respect to the tailoring and tuning of redox properties.11 Perovskites can show exceptional thermal stability, ionic conductivity, and electron mobility and have consequently emerged as an important new class of mixed-oxide materi© XXXX American Chemical Society

Received: June 28, 2017 Revised: September 26, 2017 Published: October 3, 2017 A

DOI: 10.1021/acs.jpcc.7b06319 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. TPD following the adsorption of (a) methanol and (b) ethanol at −83 °C from fully oxidized SrTiO3(100).

2. EXPERIMENTAL SECTION Experiments were performed in two separate stainless steel vacuum chambers, one based at the National Synchrotron Light Source II (NSLS II) at Brookhaven National Laboratory for ambient-pressure soft X-ray photoelectron spectroscopy (APXPS), and one used for performing temperatureprogrammed desorption/reaction (TPD/TPR) at Oak Ridge National Laboratory (ORNL). In each of the systems, the model SrTiO3(100) [STO(100)] catalyst was obtained from MTI Corporation and used as purchased without further ex situ processing. Ex situ reflection high-energy electron diffraction (RHEED) and XRD indicated that the surface and bulk, respectively, were well-ordered. Atomic force microscopy (AFM) indicated that the surface was relatively smooth, with terraces that were approximately 500 nm wide (data not shown). Methanol and ethanol were degassed by several freeze−pump−thaw cycles and dosed onto the surface through either an effusive gas doser24 (TPD) or by backfilling through a leak valve (APXPS). All APXPS spectra were recorded at Beamline 23-ID-2 (CSX-2) with a photon energy of 750 eV.25 The Ti 2p photoemission was used for binding-energy calibration based on the 458.4 eV (Ti 2p3/2) feature.20 The STO(100) surface was initially oxidized in the presence of 0.02 Torr of O2 at 500 °C to eliminate carbon contamination. The literature has shown that the surface is mostly terminated by TiO2 upon annealing in O2.26,27 Prior to the investigation of alcohol reactions on STO(100) at different surface temperatures, the surface was reoxidized at 500 °C in the presence of O2 until no carbon was detected and Ti was fully oxidized. For the catalysis of alcohols on the surface at various alcohol/O2 ratios, after the surface had been cleaned, it was cooled to the appropriate temperature, O2 was first introduced into the chamber, and then 0.1 Torr of alcohol was added. The TPD experiments were performed in a UHV chamber with a base pressure of 1 × 10−10 Torr at ORNL. The STO(100) surface was first cleaned by oxidation in ∼10−7 Torr of O2 at 527 °C for at least 30 min. The oxidized surface was then exposed to the equivalent of 7 L of methanol from the effusive tube doser at −83 °C, which is slightly higher than the multilayer desorption temperature for methanol.7 After dosing, the sample was positioned in front of a Hiden HAL/3F 301 mass spectrometer, with the sample face ca. 2 cm away from the

catalytic activity of SrTiO3, which they attributed to the structural and electronic influence from the presence of Sr on SrTiO3. However, according to their XPS results, some hydrocarbon contamination was present even after the surface had been heated in oxygen for 1 h at 773 K. The carbon contamination could have an effect on the distribution of reaction products observed by mass spectroscopy. Wang et al. also studied the adsorption and reaction of methanol on fully oxidized and reduced SrTiO3(100) surfaces.7 Their TPD and density-functional theory (DFT) calculation results suggested the nondissociative adsorption of methanol on the oxidized SrTiO3(100) surface because of weak interactions between methanol and the surface and, as a result, preferable desorption of methanol during heating of the surface. In contrast to the nonreactive behavior of the oxidized SrTiO3(100) surface, the reduced SrTiO3−x(100) surface exhibited a high catalytic activity toward methanol, with the production of gas-phase H2, H2O, CO, CH4, HCHO, and CO2. The authors attributed the enhanced reactivity of the reduced SrTiO3−x(100) surface to the presence of oxygen vacancies created by surface reduction. Similar results have been reported following methanol adsorption on the Ti-terminated rutile TiO2(110) surface.22,23 On a stoichiometric surface, methanol adsorbs primarily nondissociatively and desorbs near 300 K. The presence of oxygen vacancies leads to dissociative adsorption with methanol desorption at higher temperatures. At higher concentrations of O vacancies, a significant amount of methane was observed.22 These XPS and TPD studies on the catalytic activity of SrTiO3 single-crystal surfaces toward alcohols were performed under ultrahigh- or high-vacuum (UHV or HV) conditions. In this work, we focus on the reaction of two alcohols, methanol and ethanol, on the stoichiometric (fully oxidized) SrTiO3(100) surface under continuous exposure at an elevated pressure, which produces a higher collision frequency. We used ambient-pressure XPS (APXPS) to identify surface intermediates and gas-phase products in situ under steady-state reaction conditions and ultimately propose reaction mechanisms with relation to the creation of surface oxygen vacancies during the reaction. B


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Figure 2. TPR of (a) methanol and (b) ethanol with an equal amount of O2 exposed to fully oxidized SrTiO3(100) at a total pressure of ca. 10−7 Torr.

mass spectrometer aperture. During TPD, the temperature was ramped at 2 °C/s, and the sample was biased at −70 V to prevent electrons generated by the mass spectrometer ionizer from stimulating reactions at the surface. The following reaction products were monitored: H2 (m/z = 2), H2O (m/z = 18), CH2CH2 (m/z = 27), CO (m/z = 28), CH3CHO (m/ z = 29), HCHO (m/z = 30), CH3OH (m/z= 31), and CO2 (m/z = 44). Overlapping fragments, such as methanol at mass 30 or formaldehyde at mass 28, were subtracted to remove interferences. The relative amounts of the various products were obtained by multiplying the corresponding TPD traces by scale factors according to the method proposed by Ko et al.28

Conceivably greater reactivity would occur on reduced STO(100), as reported by Wang et al.7 Reduction could be induced by Ar+-ion sputtering or by repeated alcohol TPD cycles. The behavior of reduced STO(100) at low pressure was not examined in this work. Temperature-programed reaction (TPR) experiments were conducted by mixing alcohol and O2 in a stainless steel ballast with an alcohol/O2 ratio of 1:1 before directly dosing the mixture onto the surface through the effusive gas doser while monitoring products with the mass spectrometer. The effective pressure at the sample was estimated to be ca. 10−7 Torr.29 During TPR, the temperature was ramped at 1 °C/s from an initial sample temperature of 25 °C. As shown in Figure 2, TPR indicated that no reactions occurred between alcohols and O2 under these low-pressure conditions. TPRs of pure alcohols on STO(100), that is, with no O2 in the reactants, also showed no reaction (not shown here). We note that similar experiments on more highly active oxide surfaces such as CeO2(100) and LaMnO3(100) produced products with intensities that were 50% or more of the initial methanol intensity (unpublished data). 3.2. Reactions of Alcohols on SrTiO3(100) at Elevated Pressures. To complement the investigations of methanol and ethanol on STO(100) under UHV conditions, it is desirable to determine whether the observed surface chemistry changes at elevated pressures resulting from a so-called “pressure gap”. Studying such behavior is important for bridging the knowledge between “real-world” catalysis under practical conditions and measurements obtained from UHV surface science. Therefore, we also conducted measurements at near-ambient pressures using ambient-pressure X-ray photoelectron spectroscopy (APXPS). The intense X-rays utilized in these measurements raise a concern for possible X-ray- or secondary-electroninduced reactions. Although we cannot exclude these possibilities with 100% certainty, experiments conducted at different temperatures, gas pressures, and positions on the sample and with different X-ray exposure times suggest that Xray-induced reactions are not a significant concern. This is consistent with previous experiments conducted with a synchrotron light source on CeO2(111), where it was observed

3. RESULTS AND DISCUSSION 3.1. Reactions of Alcohols on SrTiO3(100) at Low Pressure. We first performed TPD of methanol adsorbed at −83 °C following an exposure equivalent to 7 L (Figure 1a) on the STO(100) surface. The TPD spectra in Figure 1a show that methanol mostly desorbs molecularly from the surface, with the formation of a small amount of decomposition products (e.g., HCHO and H2O) and with no desorption products above 100 °C, which is consistent with the observations reported by Wang et al.7 However, the same work found that methanol adsorbs mostly nondissociatively on a Ti-terminated SrTiO3(100) surface, with the oxygen atom bound to the Ti4+ site and three hydrogen atoms (one from OH plus two others from CH3) interacting with the surface through nonspecific electrostatic and dipolar attractions.7 Therefore, the low reactivity of methanol on STO(100) under UHV conditions could be attributed to the weak binding of methanol on STO(100). Similarly, we performed TPD experiments from STO(100) with 7 L of ethanol adsorbed on the surface at −83 °C, as shown in Figure 1b. We observed only the desorption of ethanol and the formation of a small amount of acetaldehyde and ethylene at surface temperatures less than 100 °C (Figure 1b), which suggests that either ethanol likely adsorbs molecularly on STO(100) under UHV conditions or the recombination of dissociated ethanol occurs rapidly below 100 °C. In addition, little to no products are produced above 100 °C, which we attribute to the weak interactions of ethanol with the oxidized surface. C


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Figure 3. C 1s, O 1s, and Ti 2p APXPS spectra of a SrTiO3(100) surface at a surface temperature of 250 °C before (black) and after (red) the addition of 0.1 Torr of methanol.

Figure 4. (a) C 1s and (b) O 1s spectra of the SrTiO3(100) surface in the presence of mixed gas with different methanol/O2 ratios at a surface temperature of 250 °C.

that aldehydes30,31 and ketones29 are susceptible to X-rayinduced effects whereas alcohols32 are relatively robust. 3.2.1. Methanol Reaction on SrTiO3(100). Figure 3 shows C 1s, O 1s, and Ti 2p APXPS spectra obtained from the STO(100) surface in the presence of 0.1 Torr of methanol at a surface temperature of 250 °C. For comparison, the XPS spectra of the clean surface correspond to the black lines in Figure 3. With an exposure of 0.1 Torr of methanol, the peak at 286.3 eV in the C 1s region is due to the formation of methoxy species on the SrTiO3(100) surface.6,33,34 The two other peaks at 285.3 and 288.7 eV (with a separation of 3.6 eV) in Figure 3 are attributed to the methyl carbon (CH3) and the carboxyl (COO−) in an acetate, respectively. The formation of acetate from methanol is surprising, but the C 1s assignments are consistent with those of previous studies. Liu et al. studied ethanol surface reactions on CeO2 and reported the formation of acetate species that had similar binding energies in the C 1s XPS spectrum.33 Moreover, in the work on reactivity and reaction intermediates for acetic acid adsorbed on CeO2(111) reported by Calaza et al., a pair of peaks at 285.7 and 289.1 eV in the C 1s XPS spectra were assigned to methyl and carboxyl carbons.35 The C 1s feature at 285.3 eV could be assigned to a CHx species resulting from methanol decomposition that is adsorbed directly to the surface.20 However, this would be equivalent to graphitic or so-called “adventitious” C, which has a lower binding energy of close to 284.6 eV.33 In addition, the peaks observed in Figure 3 have the same binding energies as

acetate formed from the oxidation of ethanol on STO(100) (see section 3.2.2). Finally, the intensities of the 285.3 and 288.7 eV peaks are virtually equivalent and remain equivalent under most conditions (see below), which suggests that they are contained in the same surface species. Therefore, the features in the C 1s XPS spectra of Figure 3 indicate that methanol adsorbs dissociatively on the surface at 250 °C to form methoxy, some of which appears to undergo surface oxidation and coupling to produce acetate species. The small peak at 287.6 eV is likely due to gas-phase methanol near the SrTiO3(100) (STO) surface or possibly an adsorbed formaldehyde species.36,37 In the O 1s region, the clean surface produces a single peak at 529.6 eV that is associated with the lattice oxygen from STO.20 When methanol is present, the broad shoulder at ∼532 eV is assigned to the O from methoxy/acetate species as well as hydroxyl groups (OH) that are produced upon the binding of H to lattice O of STO following the dissociative adsorption of methanol.6,7 Moreover, because Liu et al. reported a separation of ∼3 eV between oxygenates/hydroxyls and gasphase species in their APXPS study of the ethanol reaction on CeO2(111), the small peak at 534.4 eV is likely due to the presence of gas-phase methanol near the surface.33 Note that undissociated methanol adsorbed on the surface would have a similar binding energy near 534 eV.32,33,38 Because there is little intensity in this region, the O 1s spectrum supports the D


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Figure 5. APXPS spectra in the C 1s region of SrTiO3(100) exposed to (a) pure methanol and (b) a mixed gas with a methanol/O2 ratio of 1:1 at different surface temperatures.

phase CO and CO2 upon surface oxidation.33,37 When more O2 (0.1 Torr) was added to react with methanol on the STO(100) surface, the production of CO2(g) became more obvious, as is evident from the peak in the red curve at 536.2 eV. Meanwhile, the appearance of a peak at 534.8 eV in the O 1s spectrum (red), clearly differentiated from the methanol peak at 534.4 eV, indicates the formation of gas-phase water with increasing O2 content.39 To explore the effects of the surface temperature on the oxidation of methanol by STO(100) and on the selectivity of the reaction products, a comparison of the APXPS spectra in the C 1s region obtained at various surface temperatures was performed. Figure 5 shows the temperature dependence of Ccontaining surface species and gas-phase products when the STO(100) surface is exposed to methanol with and without O2. When there is no O2 present (Figure 5a), the most striking difference between the surface temperatures of 250 and 300 °C is that the amount of surface acetate species formed at 250 °C is at least twice the amount formed at 300 °C. The decrease in the intensity of the peaks assigned to surface acetate in the C 1s spectra at elevated surface temperature (300 °C) could be due to further oxidation of such species (which might be the source of the gas-phase CO, shown in Figure 5a.) Another possible path for consuming the surface acetate could be recombination with H to form acetic acid, although the formation of acetic acid was not evident in these experiments. Figure 5b displays the APXPS spectra in the C 1s region of STO(100) exposed to a gas mixture with a methanol/O2 ratio of 1:1 at three different surface temperatures. As discussed above, at 250 °C the reactions of methanol with O2 on the STO(100) surface result in the formation of surface acetate and gas-phase CO2. The reaction produced fewer surface species but more gas-phase CO2 when the surface temperature was increased to 300 °C, which is attributed to a more efficient conversion from acetate to CO2 as well as the enhanced possibility of desorption of surface species with elevated surface temperature. When the freshly cleaned surface was cooled to 100 °C and 0.1 Torr of O2 and 0.1 Torr methanol were introduced into the chamber, the spectrum (Figure 5b) obtained in the range of 284−290 eV could be fit by four peaks at 288.7, 287.7, 286.4, and 285.3 eV, which are assigned to a carboxylate group, gas-phase methanol, methoxy, and the methyl group of acetate, respectively. In addition, it should be

assignment of the C 1s feature at 286.3 eV to methoxy rather than methanol. The high-resolution Ti 2p XPS spectrum shows that the Ti is reduced by methanol exposure at 250 °C. When the sample is exposed to methanol, the shoulder at ∼456.5 eV in the Ti 2p3/2 peak is due to the presence of Ti3+ that results from reduction by methanol.20 The catalyst was then treated by annealing at 500 °C in an atmosphere containing 0.01 Torr of O2 until no carbon was detected in the C 1s spectrum and the Ti was fully reoxidized. Once the surface had been cleaned and cooled to 250 °C, the oxygen pressure was adjusted to either 0.02 or 0.1 Torr. Then, 0.1 Torr of methanol was added to produce a methanol-tooxygen ratio of 5:1 or 1:1. The corresponding APXPS results for the C 1s and O 1s regions are shown in Figure 4. As discussed previously, in the presence of methanol without O2, the most prevalent peaks shown in the C 1s region are those of methoxy and acetate species.33,35 The formation of acetate occurs as a consequence of the oxidation of surface-adsorbed methoxy. The small peak at 287.6 eV is attributed to gas-phase methanol.36 No gas-phase products were observed at higher binding energies in the APXPS spectra in the absence of O2. When methanol was mixed with a small amount of O2, the intensity of the methoxy peak decreased significantly, which suggests the consumption of the methoxy species in the presence of O2. The addition of O2 results in the formation of gas-phase CO and CO2, as evident by the emergence of two peaks at 291.4 and 292.6 eV, respectively.33,37 In addition, the formation of acetate is retained in the presence of 0.02 Torr of O2. In the case of a methanol/O2 ratio of 1:1, the methoxy is completely consumed, and CO2 is the only gas-phase product evident. Hence, by altering the ratio of methanol to O2, one can change the distribution of surface species and reaction products. The O 1s spectra at high binding energy are magnified to demonstrate the gas-phase species. The peak at 534.4 eV, which is assigned to gas-phase methanol, is evident only when O2 is absent.36 No other gas-phase product was observed in the absence of O2, which is in agreement with the observations in the C 1s spectra. As the ratio of methanol to O2 was changed from 1:0 to 5:1 (blue curve), the methanol peak disappeared, and two weak peaks (just above the noise level) emerged at 537.5 and 536.2 eV, which are due to the formation of gasE


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Figure 6. Ti 2p, Sr 3d, C 1s, and O 1s XPS spectra of a SrTiO3(100) surface at a surface temperature of 250 °C before (black) and after (red) the addition of 0.1 Torr of ethanol.

Figure 7. (a) C 1s and (b) O 1s spectra of the SrTiO3(100) surface in the presence of a mixed gas with different ethanol/O2 ratios at a surface temperature of 250 °C.

For the surface species evolution, Figure 6 also depicts the C 1s and O 1s APXPS spectra of the STO(100) surface during ethanol exposure at a surface temperature of 250 °C. In the C 1s spectrum, the methyl and alkoxy carbons in ethoxy are at 285.0 and 286.2 eV, respectively, and a small amount of acetate is evident at 285.3 and 288.8 eV.33 The small peak at 287.7 eV is assigned to the carbon bound to oxygen of gas-phase ethanol, and the methyl group is likely overlapped with the ethoxy peak at 286.2 eV. The binding energies of alkoxy, methyl, and carboxylate are similar to those observed for methoxy and acetate during the reaction of methanol on STO(100) (Figure 3). A small amount of gas-phase CO2 is also evident. The formation of CO2 was not observed in methanol reactions on STO(100) in the absence of O2, which might be indicative of a greater reactivity of ethanol on STO(100), relative to methanol. The formation of CO2 at this temperature was not observed in the TPD of ethanol in UHV (Figure 1b), which might simply be due to the low coverage of surface species remaining at 250 °C in the TPD experiment. In the O 1s region, the major peak can be clearly seen for lattice O of STO (529.6 eV), along with the emergence of a broad peak at 531.6 eV that represents a combination of surface hydroxyl and oxygenated hydrocarbons (e.g., ethoxy, acetate).7,33 As with methanol, the absence of a significant peak near 534 eV indicates the absence of undissociated ethanol on the surface.

noted that the peak intensity of the carboxylate group at 288.7 eV is significantly larger than that of the methyl group of surface acetate at 285.3 eV. One would expect equal intensities for this pair of peaks if they were assigned to the same species (surface acetate). Therefore, the peak at 288.7 eV in the C 1s XPS spectrum collected at 100 °C is likely due to the presence of formate as well as the contribution from the carboxylate group of the acetate. The intensities of the acetate peaks from the reaction of methanol at 100 °C are similar to, or even larger than, those at 250 and 300 °C, which could be due to a greater stability of surface acetate at lower temperatures. 3.2.2. Ethanol Reaction on SrTiO3(100). From the study of methanol reactions on STO(100) at 0.1 Torr with and without O2, we established that methanol adsorbs dissociatively on the surface at elevated temperatures and then appears to react to form surface acetate and gas-phase CO/CO2 upon further oxidation. Now, we turn to the role of the chain length of the alcohol and investigate the reaction of ethanol on STO(100). Following the same procedure as used for methanol, the surface was heated to 500 °C in the presence of O2 to clean carbon contamination and fully oxidize the Ti, and then 0.1 Torr of ethanol was subsequently introduced into the chamber once the surface had been cooled to 250 °C. The Ti was reduced by the ethanol as indicated by the shoulder at 456.3 eV in the Ti 2p3/2 region in Figure 6. F


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Figure 8. APXPS spectra in the C 1s region of SrTiO3(100) exposed to (a) pure ethanol and (b) a mixed gas with an ethanol/O2 ratio of 1:1 at various surface temperatures.

CO2, resulting from reactions of ethanol on the surface. The reaction of ethanol with an equal amount of O2 on STO(100) generates the greatest amount of gas-phase products compared to lower proportions of O2, which is consistent with observations in the C 1s APXPS spectra. Therefore, as for the reaction of methanol on the surface, one can tune the product distribution by altering the ratio of ethanol to O2. To gain insight into the effects of the surface temperature on the ethanol reaction, the surface was exposed to 0.1 Torr of ethanol, with or without an equal pressure of O2, at three different temperatures (100, 250, and 300 °C). As shown in Figure 8, in the absence of O2, surface ethoxy (284.9 and 286.1 eV) and acetate (285.3 and 288.8 eV) were evident at 100 °C. As the surface was heated to progressively more elevated temperatures, the acetate peaks were attenuated, and no other new features could be observed, which is probably due to the reaction of acetate to form various products such as CO, CO2, and ketene. However, the intensity of the ethoxy species was retained at 250 and 300 °C despite the diminishment of surface acetate. We attribute the persistence of the ethoxy species despite the decrease in intensity of the acetate peaks to the consumption of surface O, which inhibits the oxidation of ethoxy to acetate. Figure 8b presents the C 1s APXPS spectra of ethanol reacting with an equivalent amount of O2 on STO(100) at surface temperatures of 100, 250, and 300 °C. At 250 and 300 °C, the adsorbed ethoxy was entirely consumed to form surface acetate, and continuous production of the of gas-phase CO2 (a complete oxidization product) was observed. The lower intensities of the acetate peaks along with higher intensity of the CO2 peak suggest that the conversion from acetate to CO2 is more efficient at 300 °C than at 250 °C. However, at 100 °C, surface acetate and ethoxy species are both present, whereas only a small amount of CO2 is formed. We speculate that the effects of the surface temperature on the reactivity and selectivity of ethanol catalyzed on STO(100) are as follows: At 100 °C, the energy is high enough to readily cross the barrier for surface acetate formation but not sufficient to rapidly produce CO2. The latter process could be considered as a ratelimiting step and creates a bottleneck for further reactions. Therefore, we see the surface species dominated by ethoxy and acetate and observe only a small CO2 peak at 100 °C regardless of whether O2 is present, as shown in Figure 8. As the surface

Figure 7 shows the APXPS spectra in the C 1s and O 1s regions of STO(100) in the presence of ethanol mixed with various amounts of O2. In the previous discussion, we demonstrated that ethanol adsorbs dissociatively on the surface and forms surface ethoxy and acetate species and CO2 in the absence of O2, as evidenced by the features in Figure 6. However, when the surface was exposed to 0.1 Torr of ethanol plus 0.02 Torr of O2, we observed that the ethoxy peaks (284.9 and 286.1 eV) were attenuated and the peaks at 285.3 and 288.8 eV, assigned to surface acetate, increased.33 This indicates that the consumption of adsorbed ethoxy results in the formation of acetate species. In addition, the two peaks at 291.7 and 292.9 eV are attributed to gas-phase CO and CO2 production from further oxidation of surface oxygenates.33,36 The peak at 291.7 eV is barely discernible above the noise, and the gas-phase product is dominated by CO2. When the ratio of ethanol to O2 was increased to 1:1, the ethoxy peak vanished, and the reaction on STO(100) led to more surface-bound acetate and gas-phase CO2. It should be noted that the reaction of ethanol with O2 on STO(100) yields more acetate species than the reaction of methanol, and unlike methanol, ethanol undergoes a complete oxidation to form gas-phase CO2, suggesting a higher catalytic activity for ethanol conversion on STO(100). The enhanced reactivity of ethanol on STO(100) could be attributed to a lower reaction barrier to form acetate and gas-phase products, because the inductive effect introduced by the electron-donating alkyl groups of ethanol might stabilize the transition state for acetate formation and thereby lower the activation energy.40 The O 1s APXPS spectra are magnified in Figure 7b to demonstrate gas-phase species at high binding energies. When no O2 is present, a peak that is assigned to gas-phase ethanol is visible at 534.3 eV in the black spectrum. We could not see any gas-phase CO2 peak at ∼536.5 eV as we observed in the C 1s region. Because the O 1s spectrum was recorded after the C 1s spectrum, the CO2 might have been a transient product that disappeared after the O on the surface of the STO(100) was depleted. In the presence of 0.02 Torr of O2, the ethanol peak disappeared, and we could only observe two very broad peaks (barely above noise) appearing at ∼535 and ∼536.5 eV assigned to gas-phase water and CO2, respectively. When we set the ratio of ethanol to O2 at 1:1, two intense peaks appeared at 535.0 and 536.4 eV that are attributed to gas-phase H2O and G


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oxidation in the absence of O2, even at lower temperatures (100 and 250 °C), indicates that ethanol is more reactive than methanol on STO(100). For the reactions taking place in the absence of O2, acetate formation from methanol is more difficult than acetate formation from ethanol. Comparison of reactions 1 and 2 with reactions 5 and 6 indicates that more lattice oxygen is needed for methoxy to form acetate than for ethoxy to form acetate. As lattice O is depleted, acetate formation will be inhibited faster for methanol than for ethanol. We note, however, that surface O is likely never depleted completely because, even if the Ti4+ were fully reduced to Ti3+, the composition would be SrTiO2.5 and there would still be O available to form acetate. In the presence of O2 (e.g., alcohol/O2 = 1:1), oxygen vacancies created by surface reduction (reactions 3, 4, and 7) could be refilled by chemisorbed oxygen atoms. In this case, surface alkoxy species would continue to be oxidized to produce more acetate compared with the case of pure alcohol reactions. We did not observe an increase in the intensities of the acetate peaks when O2 was added to methanol (Figure 4a), but we did see an intensity increase for the acetate peaks when O2 was added to ethanol (Figure 7a). We attribute this difference in behavior for O2 plus methanol compared to O2 plus ethanol to a possibly higher reaction barrier in forming acetate from methoxy than in forming acetate from ethoxy, resulting in a lower rate of the acetate formation reaction for methanol (reaction 2). The difference in the reaction barriers likely results from an entropic difference affecting the prefactor. Acetate formation from methoxy requires coupling between two adjacent methoxy species, whereas acetate formation from ethoxy requires only a reaction between a single ethoxy and a fixed lattice O. In the presence of 0.02 Torr of O2, the consumption of the surface acetate produces gas-phase carbon oxides and water through further oxidation (reactions 3 and 7). With increasing amount of O2 (alcohol/O2 = 1:1) at 250 °C, the adsorbed alkoxy is completely oxidized to surface acetate and gas-phase CO2 and water (reaction 8), as evidenced in Figures 3 and 6.

temperature was increased, there is sufficient energy to convert acetate to CO2 and, as a result, the reaction is faster, allowing the ethoxy to form acetate completely, although the acetate is still not fully consumed. Further experiments and/or computational studies are needed to probe this hypothesis. 3.3. Possible Reaction Pathways for the Catalysis of Alcohols on SrTiO3(100). The APXPS data presented in the previous sections demonstrate that, at 0.1 Torr, methanol can readily adsorb onto the SrTiO3(100) surface and dissociate on acid−base pair sites, namely, Ti cations and O anions, respectively. This forms methoxy and surface hydroxyls, as shown in reaction 1, where the subscripts “a” and “l” represent “adsorbed” and “lattice”, respectively 2CH3OH + 2O(l) → 2CH3O(a) + 2OH(a)

(1)

CH3O(a) + CH3O(a) + 3O(l) → CH3COO(a) + 3OH(a)

(2)

CH3COO(a) + 3OH(a) → 3H 2O(g) + 2CO(g) + 3VO (3)

OH(a) + OH(a) → H 2O(g) + O(l) + VO

(4)

In the absence of O2, adsorbed methoxy species can interact with each other and form surface acetate at 250 °C, as shown in reaction 2. When the surface temperature is raised from 250 to 300 °C, the acetate species can then undergo further oxidation, leading to the production of gas-phase CO and water (reaction 3), where “g” refers to the gas phase. This is consistent with the observations in Figure 5a. Some of these processes result in the formation of oxygen vacancies, where VO represents an oxygen vacancy in reactions 3 and 4. The removal of O from the surface was indicated by the reduction of Ti4+ to Ti3+ in the Ti 2p APXPS spectrum (Figure 3). Once a portion of the STO(100) is reduced, the coordinatively unsaturated Ti cations and adjacent oxygen anions facilitate further adsorption and dissociation.7 As oxygen vacancies continue to be created by surface reactions, the methoxy species would become stabilized without further oxidation in the absence of available surface O atoms, which is consistent with the observations in the C 1s APXPS spectra. In the case of ethanol, similarly to what we observed in the methanol reaction on STO(100), the surface species is primarily ethoxy when the surface is exposed to pure ethanol, suggesting dissociative adsorption upon exposure (reaction 5). Additionally, we found that the adsorbed ethoxy is likely to be oxidized to form surface acetate, as shown in reaction 6. However, instead of observing gas-phase products in the C 1s XPS spectra only at a higher surface temperature (300 °C) as was the case for the methanol reaction, we could observe the formation of gas-phase CO2 at 100, 250, and 300 °C (see reaction 7 and Figure 7a). CH3CH 2OH + O(l) → CH3CH 2O(a) + OH(a)

(5)

CH3CH 2O(a) + 3O(l) → CH3COO(a) + 2OH(a)

(6)

CH3COO(a) + 3OH(a) + O2 (l) → 3H 2O(g) + 2CO2 (g) CH3O(a) + 3O(l) → HCOO(a) + 2OH(a)

(9)

As the surface temperature changes, the product distribution varies in the presence of O2, as indicated by Figures 5b and 8b. When the surface temperature is lowered to 100 °C, methanol produces formate in addition to methoxy and acetate (see reaction 9 and Figure 5b). According to the TPD studies of methanol and ethanol, we learned that the alkoxy is not stable on the surface, as shown by the desorption of the alcohols below 100 °C in Figure 1. Acetate is likely more stable than alkoxy, as indicated by the nondecreasing intensities of the acetate peaks at various alcohol/O2 ratios at the same surface temperature. However, the reactivity is not high enough to convert all of the surface alkoxy to acetate at 100 °C. Because we never observed any fully dehydrogenated surface species such as CO, CO2, or carbonate, the cleavages of CC and/or CH bonds are likely to be the rate-determining steps. Once these bonds are broken, CO or CO2 rapidly desorbs. When the surface temperature is raised to 300 °C, the reaction probability of acetate on STO(100) is enhanced, and more gas-phase CO2

CH3COO(a) + 3OH(a) + O(l) → 3H 2O(g) + CO(g) + CO2 (g) + 5VO

(8)

(7)

We did not observe the gas-phase CO proposed in reaction 7 in the APXPS spectra, possibly because of a low intensity that is below the noise level. The fact that ethanol undergoes complete H



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and H2O is formed. This is in agreement with the features observed in Figures 5b and 8b. Moreover, the difference in reactivity at low pressure versus elevated pressure suggests that transient surface coverage of the alcohols plays a role in the surface reaction. At low pressures, the surface is fully covered by alcohols only at low temperatures, as indicated by the desorption of the alcohol by room temperature (Figure 1). Under constant-exposure conditions during low-pressure TPR (Figure 2), the rate of desorption exceeds the rate of other reaction pathways. At elevated pressure, the chemical potential of the reactants is increased, and thus, the rate of adsorption could be equal to or even greater than the desorption rate, leading to an increased availability of surface alkoxy and enhanced reactivity on STO(100).

REFERENCES

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4. CONCLUSIONS We have investigated the adsorption and reaction of simple alcohols (i.e., methanol and ethanol) at near-ambient pressure (∼0.1 Torr). TPR experiments and APXPS measurements indicate that less reactivity can be detected by TPR at low pressure than under continuous exposure at near-ambient conditions. At elevated pressures, the alcohols dissociatively adsorb onto SrTiO3(100) to form alkoxy species and surface hydroxyls. Interestingly, the APXPS spectra show that methoxy likely undergoes coupling and produces acetate species as a surface intermediate in the absence of O2. In contrast, in the case of ethanol reactions, ethoxy is directly oxidized to form acetate, which undergoes further oxidation to gas-phase CO2 and H2O, suggesting a higher reactivity of ethanol than methanol on the surface. However, with increasing amount of O2, catalysis of both the methanol and ethanol reactions results in complete oxidation to CO2 and H2O. Therefore, the distribution of surface species and reaction products can be changed by altering the alcohol/O2 ratio. In addition, studies on the effects of surface temperature indicate an enhancement in reaction probability at elevated temperatures. However, the presence of O2 is the key factor for complete oxidation even at higher temperatures (250 and 300 °C) because the surface vacancies created by reactions with the alcohols need to be filled by O2 adsorption. The conversion from acetate to CO2 might be rate-limiting when sufficient O2 gas is present.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 865-574-2796. Fax: 865576-5235. ORCID

Aditya Savara: 0000-0002-1937-2571 David R. Mullins: 0000-0003-3495-7188 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, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. This research used Beamline 23-ID-2 of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract DE-SC0012704. I


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Ambient-Pressure XPS Studies of Reactions of Alcohols on SrTiO3

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