Mechanistic Study of Methanol Decomposition and Oxidation on Pt(111)

Article pubs.acs.org/JPCC

Mechanistic Study of Methanol Decomposition and Oxidation on Pt(111) Alexander V. Miller,† Vasily V. Kaichev,*,†,‡ Igor P. Prosvirin,† and Valerii I. Bukhtiyarov†,‡ †

Boreskov Institute of Catalysis, Lavrentieva ave. 5, Novosibirsk 630090, Russia Novosibirsk State University, Pirogova str. 2, Novosibirsk 630090, Russia

‡

S Supporting Information *

ABSTRACT: Decomposition and oxidation of methanol on Pt(111) have been examined between 300 and 650 K in the millibar pressure range using in situ ambient-pressure X-ray photoelectron spectroscopy (XPS) and temperature-programmed reaction spectroscopy (TPRS). It was found that even in the presence of oxygen, the methanol decomposition on platinum proceeds through two competitive routes: fast dehydrogenation to CO and slow decomposition via the C−O bond scission. The rate of the second route is significant in the millibar pressure range, which leads to a blocking of the platinum surface by carbon and to the prevention of further methanol conversion. As a result, without oxygen, the activity of Pt(111) converted to a turnover frequency is ∼0.3 s−1 at 650 K. The activity strongly increases with oxygen content, achieving 20 s−1 in an oxygen-rich mixture. The main products of methanol oxidation were CO, CO2, H2, and H2O. The CO selectivity as well as the H2 selectivity decrease with the increase in oxygen content. It means that the main reaction route is the methanol dehydrogenation to CO and hydrogen; however, in the presence of oxygen, CO oxidizes to CO2 and hydrogen oxidizes to water. At room temperature, the C1s spectra contain weak features of formate species. This finding points out that the “non-COinvolved” pathway of methanol oxidation realizes on platinum as well. However, the TPRS data indicate that at least under the oxygen-deficient conditions the methanol dehydrogenation pathway dominates. A detailed reaction mechanism of the decomposition and oxidation of methanol agreeing with XPS and TPRS data is discussed. Pt(110) single crystals.5−16 For example, Sexton,5 using lowenergy electron loss spectroscopy (LEELS) and thermal desorption spectroscopy (TDS), showed that on the clean Pt(111) surface methanol molecularly adsorbs at temperatures below 140 K, whereas it unselectively decomposes to CO and hydrogen at higher temperatures. According to the mechanism of methanol dehydrogenation on group VIII and Ib metals, as discussed by Davis and Berteau,17 the reaction proceeds via methoxy (CH3O), as a first intermediate, then by stepwise hydrogen abstraction via formaldehyde (CH2O), formyl (CHO), and CO:

1. INTRODUCTION Currently, the availability and low cost of methanol stimulate permanent searches for new fields for its application. In particular, methanol is considered as the most promising alternative energy source. For instance, methanol can be used both in classical internal combustion engines and in special fuel cells for electricity production.1,2 It is also known that spark and diesel engines operated on fuel with addition of hydrogen or synthesis gas generate much lower level of harmful exhausts at an even higher engine efficiency.3,4 The latter is of special interest due to the possibility of hydrogen and synthesis gas production from methanol right on board of the car. Previously, it was shown that both hydrogen and synthesis gas can be produced by the catalytic conversion of methanol on platinum. To design better catalysts for on-board catalytic converters, a deeper understanding of the methanol surface chemistry on platinum is required. Besides, methanol is a good model system for understanding specific aspects of some other processes because although it is a small molecule it contains three different chemical bonds (C−H, C−O, O−H) and thereby possesses many chemical characteristics of more complex organic molecules. Despite numerous investigations, there are several controversies surrounding the mechanism of methanol decomposition and oxidation on platinum. Most of experimental studies devoted to the mechanisms of these reactions were performed in ultra high vacuum (UHV) on Pt(111) and © 2013 American Chemical Society

CH3OHgas → CH3OHads → CH3Oads → CH 2Oads → CHOads → COads

(1)

The realization of this mechanism on platinum is confirmed by experimental and theoretical studies.7,8,11−14,18 In particular, the formation of methoxy species from methanol on the reconstructed (2 × 1)Pt(110) surface was detected by LEELS at 150−200 K.11 Akhter and White8 observed the formation of methoxy species on an oxygen-covered Pt(111) surface at 125− 150 K using secondary ion mass spectrometry (SIMS). Afterward, Liu et al.7 showed by means of infrared reflection Received: December 12, 2012 Revised: March 20, 2013 Published: March 22, 2013 8189

dx.doi.org/10.1021/jp3122177 | J. Phys. Chem. C 2013, 117, 8189−8197

The Journal of Physical Chemistry C

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using the nonmonochromatic Al Kα radiation in the fixed-pass energy mode. A double-side Pt(111) single crystal 1.5 mm thick and 8 mm in diameter was used as a catalyst. The crystal was mounted between two tungsten wires by which the sample could be resistively heated up to 1300 K. The sample temperature was measured with a chromel-alumel thermocouple spot-welded to the crystal edge. Before each experiment, both Pt(111) surfaces were cleaned by cycles of Ar+ etching at room temperature, heating up to 1200 K, and oxidating during the cooling from 1000 to 600 K in 2 × 10−7 mbar O2 for 2 min, and a final flashing up to 1250 K in UHV. After several such cycles, no contaminants were detected by XPS (including C and O), and the surface was characterized by a sharp (1 × 1) LEED pattern. High-purity methanol (>99.8%) was used in all experiments. High-vapor-pressure contaminants were removed from methanol by several freeze−pump−thaw cycles. Reaction pathways of methanol decomposition and oxidation were determined by the temperature-programmed reaction spectroscopy (TPRS). The TPRS experiments were performed by heating the Pt(111) single crystal in a reaction mixture flow. The heating rate was ∼0.25 K/s. The flows of methanol and oxygen were regulated separately with Horiba-Z500 flow-masscontrollers. The total pressure in the high-pressure gas cell was measured with a MKS type 121A baratron. The partial pressures of the products and the reactants at the gas cell outlet were measured with a PrismaPlus QMG-220 quadrupole mass-spectrometer equipped with a differential pump system. MS signals at m/z = 2, 18, 28, 29, 31, 32, and 44 were used for analysis of H2, H2O, CO, CH2O, CH3OH, O2, and CO2, respectively. Before the experiments, the mass spectrometer was calibrated with respect to methanol, oxygen, and the main reaction products: CO, CO2, H2, and H2O. For this, the gas cell was filled step-by-step with the gases at a fixed pressure, and the ion current at a selected m/z ratio was measured. In addition, the fragmentation pattern for each gas was obtained. In situ XPS was used for detailed analysis of the chemical composition of the surface and near-surface regions during the methanol conversion under steady-state conditions. In this case, the Pt(111) single crystal was placed into the gas cell and then heated in a stepwise manner in a reaction mixture flow. The C1s and Pt4f core-level spectra were obtained in situ at fixed temperatures. By deconvolution of the C1s spectra into separate peaks, the various intermediate species could be identified and their surface concentration as a function of temperature was determined. For all spectra, a Shirley-type background subtraction was applied. For fitting of all of the C1s peaks, a convolution of Gaussian and Lorentzian functions was used. For each spectrum, the shape of all components was the same, with the exception of the component corresponding to methanol in the gas phase. The shape of the latter component was fixed for all C1s spectra. To determine a platinum state, the Pt4f spectra were also curve-fitted using the FitXPS software. In this case, as model functions for the individual components, asymmetric Doniach−Sunjic line shape26 convoluted with Gaussian line shapes was used. To estimate the coverage of platinum surface by different carbon-contained species (θ), we used a simplified method described elsewhere.19,21,27 In short, the method is based on comparing the C1s spectrum intensity normalized to the Pt4f spectrum intensity with those obtained at a reference point. As the reference point, the relative intensity of the C1s spectra of the Pt(111) surface obtained in situ under 2 × 10−3 mbar CO

absorption spectroscopy (IRAS) and TDS that methanol dehydrogenated to produce methoxy species on a Pt(111)-(2 × 2)O surface in the temperature range from 130 to 170 K. At submonolayer coverages, the methoxy species dehydrogenated to yield formaldehyde at ∼180 K. There is also some experimental evidence that methanol can partially decompose through the C−O bond scission on some platinum surfaces to produce adsorbed CHx species (x = 0−3). For example, Levis et al.15 using SIMS showed that methanol adsorbed on Pt(111) can decompose through the C−O bond scission to produce adsorbed methyl species (CH3) at 180−220 K. At higher temperatures, the methyl species dehydrogenate to adsorbed carbon atoms (Cads). Wang and Masel16 using TDS and LEELS found that on the reconstructed (1 × 1)Pt(110) surface, adsorbed methanol can decompose through the C−O bond scission at temperatures above 140 K to produce water and a mixture of adsorbed CHx species. The CHx species then react to form Cads, Hads, methane, and some other higher hydrocarbons. In contrast, on the reconstructed (2 × 1)Pt(110) surface, no water, carbon, methane, or higher hydrocarbons were detected.12 At the same time, according to DFT calculations, the activation barrier for the C−O bond scission in methanol on Pt(111) is ∼210 kJ/mol, whereas the activation barriers for C−H and O−H bonds are only 65 and 78 kJ/mol, respectively.18 It means that the full dehydrogenation route of the methanol decomposition is more favorable than the route based on the C−O bond scission. It should be stressed that the question about the methanolic C−O bond scission is a matter of principle because the carbonaceous deposits formed, in this case, can block the platinum surface and thereby influence its catalytic activity in both the methanol decomposition and the methanol oxidation. Indeed, although the rate of C−O bond scission is low due to the high activation barrier, the carbonaceous deposits can accumulate on the catalyst surface, leading to its progressive poisoning. This effect can be negligible in UHV; however, at ambient pressure, it can lead to fast catalyst deactivation.19−23 Of course, the carbonaceous deposits show high reactivity toward oxygen and can be removed from the catalyst surface in the presence of oxygen.19 To better understand the mechanism of methanol decomposition and oxidation on Pt(111) and to examine the ideas discussed above, we have studied these reactions in the millibar pressure range using in situ X-ray photoelectron spectroscopy (XPS) combined with mass-spectrometry. XPS is one of the most useful tools to investigate both the surface composition and the nature of adsorbed species while mass spectrometry can provide the monitoring of products and reactants in the gas phase.24 Recently, the same approach has been used to study the decomposition and oxidation of methanol on Pd(111) in the millibar pressure range.19−23

2. EXPERIMENTAL SECTION The experiments were carried out using a VG ESCALAB HP electron spectrometer.23−25 The spectrometer was equipped with an X-ray source with a double Al/Mg anode, a hemispherical electron energy analyzer, LEED optics, and a VG AG-2 ion source. The special high-pressure gas cell incorporated into the analyzer chamber of the spectrometer as well as the differential pump systems of the X-ray sources and the electron energy analyzer were used to obtain in situ XPS spectra at pressures up to 0.1 mbar. All spectra were acquired 8190



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at 320 K was used. According to the published data,28−30 under these conditions, a saturated CO coverage near θ = 0.62 ML is formed. (1 ML is a monolayer, which equals the density of Pt atoms in the (111) plane; 1 ML = 1.5 × 1015 atom/cm2.) In fact, this approach can be easy to prove for XPS analysis of a semi-infinite substrate with a uniform overlayer of adsorbates under two assumptions. First, because the coverage of adsorbates in our case is sufficiently low (θ < 1 ML), it is possible to ignore the XPS signal attenuation in the adsorbate overlayer. Second, the XPS signal attenuation in the gas phase is not considered as well because the difference between the kinetic energies of C1s and Pt4f photoelectrons is negligible. For Al Kα radiation (hν = 1486.6 eV), these kinetic energies equal approximately 1200 and 1400 eV. The main formulas for the coverage calculation are presented in the Supporting Information.

on Pt(111). Indeed, Fuhrmann et al.,34 studying the adsorption of methane on Pt(111), showed that the CH3 adsorbed species are characterized by two peaks with the C1s binding energy of 282.45 and 282.85 eV, while the CH adsorbed species are characterized by two peaks with the C1s binding energy of 283.6 and 284.0 eV. Matsumoto et al.35 and Larciprete et al.36 observed one unresolved peak at 282.7 to 282.8 eV for methyl species adsorbed on Pt(111). At the same time, using XPS and STM, Starr et al.37 showed that small graphite clusters on Pt(111) are characterized by the C1s peak at 284.2 eV. Taking into account these data, it could be supposed that the interaction of methanol with the platinum surface results in the formation of adsorbed CHx species, which quickly dehydrogenate to carbon even at room temperature. The carbon concentration increases with a temperature increase. Another intense peak at 285.4 to 285.8 eV (Figure 1a) can be attributed to CO adsorbed on Pt(111), which is the product of methanol dehydrogenation. At 340 K, maximum of this component locates at 285.8 eV, whereas at higher temperatures it shifts to 285.4 to 285.5 eV. This reproducible shift is accompanied by a decrease in the peak intensity (Table 1). It is supposed that at low temperatures CO adsorbs on the platinum surface in both the bridge and on-top sites with the local coverage above 0.5 ML, whereas above 400 K, a part of the CO molecules desorbs mainly from the on-top sites.30 This identification is confirmed by XPS data represented elsewhere,32,33 where bridge-bonded CO on platinum was characterized by the C1s binding energy of 285.6 eV. According to high-resolution XPS studies,38−41 the C1s binding energy of CO adsorbed in the on-top sites is higher by 0.7 eV than the C1s binding energy of CO adsorbed in the bridge sites. Unfortunately, because of poor resolution of our XPS spectrometer it is not possible to resolve the C1s peaks of CO adsorbed in the on-top and bridge sites. The CO component in the C1s spectra at 285.4 to 285.9 eV (Table 1) corresponds to a superposition of the C1s peaks of CO adsorbed in the on-top and bridge sites. From this point of view, the observed shift of the CO components at 285.4− to 285.9 eV results in a relative occupation of the on-top and bridge sites by CO. This hypothesis is in good agreement with a model of CO adsorption on clean Pt(111). According to UHV studies,28,39,42,43 CO molecules adsorb on Pt(111) into the bridge and on-top sites to form well-ordering structures. At coverages up to 0.33 ML, the on-top sites are mainly populated in the (√3 × √3)R30° order structure. At coverages above 0.33 ML, the bridge sites start to be occupied. As a result, at 0.5 ML, the (√3 × √3)R30° structure uniaxially compresses into the c(2 × 4) structure, where half of CO molecules adsorb in the bridge sites. The further increasing of the CO coverage leads to the continuous transformation of the c(2 × 4) structure into a hexagonal close-packed layer with a saturation of 0.68 ML.28,30,39,44 As shown by Kinne et al.,39 for a coverage beyond θ = 0.5 ML, an increase in on-top species is observed, whereas the bridge site occupation decreases slightly. On the basis of this model, the shift of the CO component of the C1s spectrum can be interpreted in the following way. According to the XPS data (Table 1), the concentration of adsorbed CO at 340 K is corresponded to approximately θ = 0.5 ML. The temperature increase leads to the decrease in the surface CO coverage to θ = 0.35 ML. Taking into account the high concentration of carbon species even at 340 K (θ = 0.66 ML), it is possible to suppose that the local CO concentration

3. RESULTS 3.1. Methanol Decomposition. The first part of the study was devoted to the investigation of methanol decomposition on Pt(111) under steady-state conditions by ambient-pressure XPS. In these experiments, the C1s and Pt4f spectra of the Pt(111) surface were obtained in situ during stepwise heating from 340 to 500 K under 0.01 mbar CH3OH. The spectra are depicted in Figure 1. The results of curve-fitting analysis,

Figure 1. C1s (a) and Pt4f (b) core-level spectra obtained in situ during the methanol decomposition at different temperatures under 0.01 mbar CH3OH.

including binding energies, relative intensities recalculated to coverage, and full-widths at half maximum (fwhm) for all of the components are presented in Table 1. It can be seen that the C1s spectra consist of four components at 284.2−284.3, 285.4−285.8, 287.4, and 288.3−288.4 eV. The most intense peak at 284.2−284.3 eV corresponds to the CHx (x = 0−3) adsorbed species15,31−33 that are formed as a result of the methanolic C−O bond scission. Previously, Levis et al.15 observed CH3 species adsorbed on Pd(111) with the C1s binding energy of 284.2 eV resulting from activation of the C− O bond of methanol in the temperature range of 150−200 K and suggested that above 300 K these methyl species begin to decompose to Cads. In our previous work,19 it was shown that carbon species located in the surface and subsurface region of palladium were characterized by the C1s binding energy in the range of 283.8 to 284.1 eV, while for 2D and 3D carbon clusters the C1s binding energy is ∼284.4 eV. Therefore, the C1s component at 284.2 eV could be assigned to the carbon species 8191



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Table 1. Binding Energy (eV), fwhma (eV), and Related Intensity Recalculated to Coverages (θ, ML) of Different Components in the Pt4f7/2 and C1s Spectra Obtained in Situ during Decomposition and Oxidation of Methanol at Different Temperatures C1s reaction mixture

T (K)

Pt4f7/2

CHx,ads

COads

HCOOads

CH3OHgas

0.01 mbar CH3OH

340

71.05 (1.77)

288.57 (1.33)

71.14 (1.83)

450

71.11 (1.85)

500

71.08 (1.91)

340

71.06 (1.75)

400

71.13 (1.80)

450

71.08 (1.86)

500

71.05 (1.90)

340

71.10 (1.77)

400

71.09 (1.80)

450

71.05 (1.85)

500

71.05 (1.88)

340

71.13 (1.77)

400

71.10 (1.80)

450

71.05 (1.85)

500

71.11 (1.88)

285.79 (1.84) θ = 0.50 285.53 (1.72) θ = 0.36 285.44 (1.77) θ = 0.35 285.69 (1.85) θ = 0.35 285.90 (1.73) θ = 0.46 285.63 (1.77) θ = 0.28 285.61 (1.60) θ = 0.11 285.58 (1.76) θ = 0.12 285.85 (1.77) θ = 0.44 285.61 (1.78) θ = 0.27 285.56 (1.96) θ = 0.17 285.56 (1.75) θ = 0.13 285.83 (1.70) θ = 0.35 285.55 (1.98) θ = 0.34 285.51 (2.01) θ = 0.11 285.55 (1.60) θ = 0.13

287.40 (1.84) θ = 0.073

400

284.17 (1.84) θ = 0.66 284.17 (1.72) θ = 0.98 284.19 (1.77) θ = 1.3 284.28 (1.85) θ = 1.7 284.14 (1.73) θ = 0.57 284.16 (1.77) θ = 0.39 284.15 (1.60) θ = 0.18 284.11 (1.76) θ = 0.11 284.09 (1.77) θ = 0.57 284.18 (1.78) θ = 0.44 284.14 (1.96) θ = 0.16 284.14 (1.75) θ = 0.19 284.11 (1.70) θ = 0.55 284.16 (1.98) θ = 0.51 284.11 (2.01) θ = 0.14 284.06 (1.60) θ = 0.11

CH3OH/O2 2:1

CH3OH/O2 1:1

CH3OH/O2 1:2

a

288.25 (1.33) 288.43 (1.33) 288.36 (1.33) 287.58 (1.73) θ = 0.14

288.13 (1.33) 287.64 (1.33) 287.68 (1.33) 287.71 (1.33)

287.66 (1.77) θ = 0.14

288.02 (1.33) 287.67 (1.33) 287.56 (1.33) 287.80 (1.33)

287.76 (1.70) θ = 0.024

287.97 (1.33) 287.73 (1.33) 287.90 (1.33) 287.69 (1.33)

Values of fwhm set in parentheses.

However, the methoxy species on Pt(111) are unstable above 170 K and quickly dehydrogenate to adsorbed CO.5,8 Again, Liu et al.7 studying the methanol oxidation on Pt(110), showed that the methoxy species is partially dehydrogenated to yield adsorbed formaldehyde at ∼180 K, which is further oxidized to formate at ∼200 K. Correspondingly, the weak C1s peak at 287.4 eV (Figure 1a) was assigned to adsorbed formate species (HCOO).45,46 The formate species were detected only at 340

at 340 K is achieved at 0.68 ML and CO molecules adsorb on the platinum surface mainly in the on-top sites. At higher temperatures, a part of the on-top adsorbed CO molecules desorbs, which leads to the formation of surface structures like c(2 × 4). As a result, the shift of the C1s peak toward the lower binding energies is observed (Figure 1a). Certainly, a small number of CHxO adsorbed species, which are characterized by the same binding energy,15 could not be excluded as well. 8192



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C1s spectra obtained in situ at the oxygen-to-methanol ratio 1:2 during the stepwise heating of Pt(111) from 340 to 500 K. In this case, the partial pressures of O2 and CH3OH were 5.0 × 10−3 and 1.1 × 10−2 mbar, respectively. The experimental curves are well-described with the same four components at 284.1−284.2, 285.6−285.9, 287.6, and 287.6−288.1 eV (Table 1), corresponding to adsorbed C, CO, HCOO, and gas-phase methanol. A considerable amount of formate species was observed only at 340 K. The concentrations of all of the adsorbed species decrease with temperature. Similar spectra were observed for the other oxygen-to-methanol ratios. It should be noted that the position of the last C1s peak varies in a wide range depending on the gas phase composition and temperature. However, this peak has a small width (fwhm = 1.3 eV) that is typical of gas-phase species24 and can be unambiguously assigned to gas-phase methanol. The observed shift of the C1s peak can be stimulated by the formation of a volume charge due to the partial ionization of gas in the space between the sample and electron lens of the XPS spectrometer. It is important that under all experimental conditions platinum stayed in the metallic state. Even in the excess of gas-phase oxygen the Pt4f core-level spectra consist of two sharp peaks at 71.1 and 74.4 eV that correspond to the 4f7/2− 4f5/2 spin−orbit doublet (Figure 2b). The peaks have an asymmetric line shape with the parameter α = 0.2.47 No significant changes were observed with the temperature increase (Table 1). Because oxidized and reduced supported Pt-based catalysts showed the same activity in methanol oxidation and because methanol reduces platinum oxides,33 it can be speculated that methanol oxidation mainly occurs on platinum in the metallic state, even at atmospheric pressure. To test catalytic performance of Pt(111) in the methanol oxidation, we carried out TPRS experiments for different oxygen-to-methanol ratios. In this case, the Pt(111) single crystal was heated in the reaction mixture flow at a constant rate. Before heating, the partial pressure of methanol in the high-pressure gas cell was ∼0.01 mbar. The only products obtained under these conditions were CO, CO2, H2, and H2O. No formaldehyde was detected. The typical TPRS profiles are shown in Figure 3. The results of all experiments are summarized in Table 2. One can see that the methanol oxidation starts near 450 K. Below this temperature, according to the XPS data (Figure 2a), high concentrations of adsorbed CO and carbon species block the platinum surface. At higher temperatures, CO desorbs, which leads to the appearance of free sites for oxygen adsorption. This process initiates the further oxidation of CO and carbon species to CO2, which then quickly desorbs from the surface. As a result, the methanol conversion increases with temperature above 450 K and achieves 18% at 650 K in the oxygen-rich mixture (Table 2). In this temperature range, the yields of CO, CO2, and H2O usually increase monotonically, whereas the yield of H2 has a maximum; the position of this maximum depends on the oxygen-to-methanol ratio. The partial pressure of oxygen has a strong influence on the catalytic performance. The dependences of the methanol conversion rate and the CO selectivity versus the oxygen partial pressure are shown in Figure 4 for two temperatures: 550 and 650 K. In these experiments, the methanol partial pressure was kept at ∼0.01 mbar, while the oxygen partial pressure was varied. For both temperatures, the methanol conversion rate increases monotonically with the oxygen partial pressure. In contrast, the CO selectivity decreases as the oxygen

K (Table 1). The peak at 288.3 to 288.4 eV is attributed to gasphase methanol.19−24 It should be noted that during curvefitting analysis the amplitude of the component in the C1s spectra due to methanol in the gas phase at 340 and 400 K was fixed, which allowed us to identify the adsorbed formate species with high reliability. In the whole temperature range under study, platinum stayed in the metallic state. As seen in Figure 1b, the Pt4f core-level spectra can be well-approximated with two peaks at 71.1 and 74.4 eV that correspond to the 4f7/2−4f5/2 spin−orbit doublet (Table 1). All peaks have an asymmetric line shape with the parameter α = 0.2 that is typical of metallic platinum.47 It is a predictable result because supported platinum oxides are reduced under reaction conditions of methanol decomposition.33 Hence, the data clearly demonstrate that the methanol decomposition on platinum proceeds through two competitive routes: dehydrogenation to CO and decomposition via the C− O bond scission. Because the full dehydrogenation of methanol to CO on Pt(111) is observed even at 200 K in UHV,5−14 the rate of this process must be sufficiently high at room temperature in the millibar pressure range. In contrast, the rate of the second process (C−O bond scission) cannot be high due to the high activation barrier for the C−O bond scission in methanol on Pt(111).18 Nevertheless, this process can be significant at elevated pressure because the carbonaceous deposits can accumulate on the surface, block the platinum surface, and hence prevent the further methanol conversion. Indeed, the gradual deactivation of the platinum surface toward the adsorption of CO and C6H6 with increasing carbon coverage was observed and attributed to the nucleation and the growth of numerous inactive graphitic islands on Pt(111).48 As a result, in the TPRS spectra, slight signals of CO and H2 were observed after heating Pt(111) to 650 K in 0.01 mbar CH3OH (spectra are not shown here). The activity of Pt(111) when converted to a turnover frequency (TOF), which is the moles of methanol converted per mole of surface platinum atoms per second, was approximately 0.2 and 0.3 s−1 at 550 and 650 K, respectively. These values are in good agreement with data presented in refs 49−51. 3.2. Methanol Oxidation. The methanol oxidation on Pt(111) was studied at different molar oxygen-to-methanol ratios (2:1, 1:1, 1:2) in the same manner. Figure 2a shows the

Figure 2. C1s (a) and Pt4f (b) core-level spectra obtained in situ during the methanol oxidation at different temperatures; methanol pressure 0.011 mbar; CH3OH/O2 = 2. 8193



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4. DISCUSSION Concentrations of C, CO, and HCOO species adsorbed on Pt(111) versus temperature measured in situ during the decomposition and oxidation of methanol are shown in Figure 5. It can be seen that in pure methanol the concentration of

Figure 3. Typical TPRS profiles obtained during heating of Pt(111) in the reaction mixture flow (p(CH3OH) = 0.01 mbar, p(O2) = 0.0025 mbar). The MS signals from CH3OH, O2, CO, CO2, and H2O are vertically shifted by 6.5 × 10−3, 0.5 × 10−3, 0.5 × 10−3, 1.0 × 10−3, and 1.5 × 10−3, respectively; H2 signal is multiplied by 10. Figure 5. Surface concentrations of C, CO, and HCOO on Pt(111) versus temperature measured in situ: (a) in 0.01 mbar CH3OH and (b) in the mixture of methanol and oxygen; see details in the caption of Figure 2.

Table 2. Methanol Conversion (X) and Selectivities to CO (SCO) and H2 (SH2) at 550 and 650 K, Depending on the Molar Oxygen-to-Methanol Ratio (R) X (%)

SCO (%)

SH2 (%)

R

550 K

650 K

550 K

650 K

550 K

650 K

0.00 0.25 0.55 0.75 1.1 1.5 2.1

0.19 0.45 0.93 1.5 2.7 3.1 4.8

0.26 3.5 6.2 7.5 8.9 15 18

100 25 14 12 7.7 6.7 5.8

100 66 35 21 13 8.2 6.3

100 4.6 2.7 2.2 1.4 0.70 0.28

100 4.3 1.4 0.83 0.44 0.19 0.10

carbon increases with temperature to achieve ∼1.7 ML at 500 K (Figure 5a). The CO concentration is ∼0.5 ML and slightly decreases with temperature. The formate species are detected only at 340 K with the concentration below 0.1 ML. It means that the methanol decomposition on platinum proceeds mainly through two competitive pathways: full dehydrogenation to CO and decomposition of methanol via the C−O bond scission. In addition, methanol transforms to the formate species, which then decompose to CO2 and H at ∼300 K.7,52−56 CO is a product of the methanol dehydrogenation. According to DFT calculation,57,58 this pathway involves a scission of the O−H bond in methanol, followed by sequential hydrogen abstractions from the resulting methoxy, formaldehyde, and formyl intermediates (reaction 1). The ratelimiting step is the O−H bond scission of the adsorbed molecules of methanol, and the end product of the pathway, CO, is shown to be so strongly bonded that it could poison the platinum surface at low temperatures. As shown by Greeley and Mavrikakis,57 the intermediates in this pathway (CH3O, CH2O, and CHO) have very low activation barriers to hydrogen abstraction, which explains the extreme difficulty in detecting these adsorbed species in experimental studies. To the best of our knowledge, only adsorbed methoxy species and formaldehyde were observed by vibrational spectroscopy on Pt(111) and Pt(110).7,8,11 It should be noted that this finding initiated a question about the practical realization of the alternative pathway of the methanol dehydrogenation involving hydroxymethyl (CH2OH) as a first intermediate.18,49−51,59 Moreover, Watanabe et al.58 have recently shown that the reaction pathway starting from the C−H bond scission has a larger activation barrier and therefore is less kinetically favorable. This assumption is also confirmed by molecular beam technique studies.13 The carbon species is formed as a result of the decomposition of methanol via the C−O bond scission. Although this process is very slow, it is supposed to mainly determine the insufficient high activity of platinum in the

Figure 4. Methanol conversion rate (a) and CO selectivity (b) at 550 (open circles) and 650 K (filled circles).

partial pressure increases. The H2 selectivity also decreases with oxygen content (Table 2). Under the oxygen-deficient conditions at 650 K, the CO selectivity achieves 66% while the H2 selectivity is only ∼4%. It means that the main reaction route is the methanol dehydrogenation to CO and hydrogen. However, in the presence of oxygen, CO oxidizes to CO2 and hydrogen oxidizes to water. As a result, in the oxygen-rich mixture, the CO2 selectivity achieves 94%, whereas the detected H2 yield is very low. 8194



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addition, the CO selectivity at 650 K is higher than that at 550 K when the oxygen-to-methanol ratio is in the range of 0.25 to 2.1. In contrast, the H2 selectivity exhibits the reverse effect (Table 2). It means that an increase in the catalyst temperature significantly stimulates the CO desorption while hydrogen is rather oxidized at higher temperatures and then leaves the surface as water. To complete the description of the mechanism of methanol oxidation on platinum, the non-CO-involved route of methanol oxidation to CO2 and H27,52−56 should be discussed as well. Despite numerous investigations, a mechanism of the non-COinvolved pathway of methanol oxidation is not yet clear. According to an experimental study performed in UHV with IRAS and TDS,7 in the presence of oxygen, methanol on Pt(111) first dehydrogenates to produce methoxy species, which then dehydrogenates to formaldehyde at ∼180 K and then can further oxidize to formate at ∼200 K. The formate species decompose at ∼300 K, producing CO2 and hydrogen. This pathway may be described by the following scheme:7,67−70

methanol conversion in the absence of oxygen. Indeed, carbon species are able to accumulate on the surface, which leads to poisoning of platinum catalysts. Unfortunately, no detailed theoretical studies devoted to the mechanism of methanolic C− O bond scission on platinum were found in literature. By analogy to palladium,19,60,61 the decomposition of methanol via the C−O bond scission on platinum is more likely to take place in the intermediates with a single (σ-type) C−O bond, like CH3O. As shown by Yudanov et al.,61 CH2O and CHO intermediates with a π contribution to the C−O bond exhibit much higher activation barriers for the C−O bond scission. The highest activation barrier of the C−O bond scission was calculated for the final product of methanol dehydrogenation, CO, the species where this bond is strongest due to a notable πcontribution. Hence, it is supposed that the methanolic C−O bond scission on platinum occurs in the methoxy species, producing CH3. The adsorbed methyl species are unstable and quickly dehydrogenate to carbon, even at room temperature. (See the discussion above and Figure 1.) These carbon species can be found on the surface as isolated atoms or can form carbon chains and 2D/3D carbon clusters.37,62−64 Correspondingly, the decomposition of methanol via the C−O bond scission and the formation of carbonaceous deposits may be represented by the following scheme: CH3Oads → CH3,ads + Oads

(2)

CH3,ads → CH 2,ads → CHads → Cads

(3)

CH 2Oads + Oads → H 2COOads → HCOOads + Hads → CO2 + H 2

(4)

It should be noted that a small amount of formate was detected by XPS under the conditions of both the methanol decomposition and the methanol oxidation (Figures 1a and 2a). However, in the first case, due to the low concentration of adsorbed oxygen, the formation and decomposition of formate is hardly noticeable. In the presence of oxygen in the gas phase, the contribution of the non-CO-involved methanol oxidation pathway in the overall reaction can be significant. Indeed, a higher concentration of formate was detected in this case (Table 1). Unfortunately, under the used conditions, the low yield of hydrogen due to its fast oxidation to water, the contribution of the non-CO-involved methanol oxidation pathway cannot be understood. However, under the oxygendeficient condition at 650 K the CO selectivity achieves 66%, which points to the prevalence of the methanol dehydrogenation pathway.

In the presence of oxygen, the concentration of all adsorbed species decreases with the temperature (Figure 5b). For example, at room temperature, the carbon concentration is ∼0.6 ML, whereas at 500 K, it is below 0.2 ML. Obviously, in the presence of O2, the carbon species can oxidize to form CO2, which then proceeds to leave the surface. The steady-state concentration of CO achieves approximately 0.35−0.46 ML at room temperature and decreases to 0.12−0.13 ML at 500 K (Table 1). A considerable amount of formate species (θ ≈ 0.1 ML) was observed at only 340 K. At higher temperatures, the formate species are unstable, and the intensity of the HCOO signal sharply decreases. As seen in Figure 4a, the methanol conversion rate increases monotonically with the oxygen content. On one hand, it may be a result of removing adsorbed hydrogen, carbon, and CO that block the catalyst surface. On the other hand, the formation of different oxygen species in the surface and subsurface regions may increase the catalytic activity of platinum, as well. For example, it was found65 that there is a linear correlation between the concentration of a subsurface oxygen species dissolved in metallic copper and its catalytic activity in the selective oxidation of methanol to formaldehyde. Unfortunately, in this study, the O1s spectra could not be analyzed due to poor spectral resolution of the XPS spectrometer and the overlapping of the O1s and Pt4p3/2 spectra. However, these experimental problems can be easily overcome with the use of synchrotron facilities.24,66 From this point of view, the further in situ XPS study of the methanol oxidation over platinum using synchrotron radiation is necessary. Besides, an increase in the oxygen-to-methanol ratio leads to a decrease in the selectivities of CO and H2 (Table 2). For instance, a change in the molar oxygen-to-methanol ratio from 0.25 to 2.1 at 650 K leads to a decrease in the CO selectivity from 66 to 6.3% and from 4.3 to 0.1% in the H2 selectivity. In

5. CONCLUSIONS On the basis of the presented results and literature data, it can be concluded that even in the presence of oxygen, the methanol decomposition on platinum proceeds through two competitive routes: fast dehydrogenation to CO and slow decomposition of methanol via the C−O bond scission. The non-CO-involved pathway of methanol oxidation can also take place; however, at least under oxygen-deficient conditions, the methanol dehydrogenation pathway dominates. The methanol dehydrogenation proceeds via methoxy as a first intermediate, then by stepwise hydrogen abstraction via formaldehyde, formyl, and CO. The methanolic C−O bond scission occurs in the methoxy species with the formation of CH3. The adsorbed methyl species are unstable and quickly dehydrogenate to carbon even at room temperature. These carbon species can exist on the surface as isolated atoms or can form carbon chains and 2D/3D carbon clusters. Because of fast accumulation of these carbonaceous deposits, the activity of platinum in the methanol dehydrogenation is insufficient. These carbonaceous deposits have a high reactivity toward oxygen. As a result, in the presence of oxygen above 450 K, the catalyst surface restores its adsorption properties, which provides fast methanol dehydrogenation. The rate of methanol conversion increases with 8195



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oxygen content. However, the presence of oxygen leads to oxidation of CO to CO2 and of hydrogen to water.

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ASSOCIATED CONTENT

* Supporting Information S

Short description and the main formulas for calculation of the coverage of platinum surface by different carbon-containing species. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +7 383 3269 774. Fax: +7 383 330 83 65. E-mail: vvk@ catalysis.ru. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work was partially supported by RFBR (research project no. 12-03-31247) and Russian Academy of Sciences (the program of fundamental researches no. 24, project no. 70).

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REFERENCES

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