J. Phys. Chem. B 2001, 105, 1573-1577
1573
In Situ IRAS Observation of Catalytic Deep Oxidation of Methanol on Pt(111) under Ambient Pressure Conditions Makoto Endo, Taketoshi Matsumoto, Jun Kubota, Kazunari Domen, and Chiaki Hirose* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: September 13, 2000
The catalytic deep oxidation of methanol by oxygen on the Pt(111) surface under ambient pressure conditions has been studied by in situ infrared reflection absorption spectroscopy (IRAS). It was found that the Pt(111) surface was covered with formate in a flow of 0.16 kPa of methanol and >1.3 kPa of oxygen mixture at 350 K while it was covered with CO at the oxygen pressure below 1.3 kPa. CO2 was detected in the product gas and the reaction kinetics was examined. The relation between the production rate of CO2 and the coverage of adsorbates as a function of oxygen pressure indicated that the CO2 mainly originated from the formate which was produced from oxidation of methanol. Although CO produced by methanol decomposition seems to be one of precursors of CO2 formation, this reaction was found to be a minor pathway. The detailed mechanism of oxidation of methanol on Pt(111) was discussed.
Introduction Deep oxidation of molecules on platinum surfaces by oxygen is one of the simplest catalytic reactions and there is a great interest for understanding of the several catalytic reactions such as catalytic combustion, automotive-emission control, partial oxidation, fuel cells, etc.1-3 While the Pt(110) surface has activity for the production of formaldehyde via the partial oxidation of methanol,4-7 the Pt(111) surface promotes deep oxidation of methanol to CO2 and H2O. We have found a reaction pathway of CO2 production from methanol and oxygen on Pt(111) in ultrahigh vacuum (UHV); methanol is oxidized to surface formate prior to the formation of CO2.8,9 Formation of formate and production of CO2 were observed when methanol reacts with only molecularly adsorbed oxygen on Pt(111). The oxygen molecules on Pt(111) dissociate to hot atoms which may react with methanol.10-13 The production of CO2 in the temperature-programmed reaction (TPR) corresponded to the disappearance of formate peak in IRA spectra, indicating that the origin of CO2 was formate.8,9 There has been renewal of interest in the function of formate formation in a catalytic oxidation of methanol over Pt(111) under an ambient pressure conditions. Importance of study on catalytic reactions under ambient pressure conditions has been pointed out by many researchers because observation under atmospheric conditions is frequently different from that in UHV.14,15 Although many kinetic approaches from reaction rates have been carried out, in situ spectroscopic study under ambient pressure conditions has been delayed: only a few systems such as CO/H2,16 CO/O2,17 and C2H4/H218 have been examined. We have built a UHV apparatus which is equipped with high-pressure reaction cell connected to a gas flow reaction system and the surface under the flow of reactant gases at ambient pressures can be continuously monitored by IRAS.19 The absorption of infrared by reactant and product molecules in the gas above the sample surface is successfully canceled out by means of a polarization switching * Corresponding author. Fax: 81-45-924-5276.
system interfaced with Fourier transform infrared (FTIR) spectrometer. This system is applicable for observation of surface species in the deep oxidation of methanol under ambient pressure conditions without any interference by gaseous molecules. We have briefly reported in the previous short article8 that formate and CO were present on the Pt(111) surface under catalytic oxidation of methanol at ambient pressures. However, the function of formate and CO for the catalytic oxidation of methanol has not been clarified. In this paper, we compare the production rates of CO2 with the coverages of surface species and discuss the reaction kinetics in detail for understanding of the mechanism of methanol oxidation. Experimental Section Experiments were performed in an ultrahigh vacuum (UHV) system consisting of sample preparation and IRAS chambers as shown in Figure 1.8,9,19 The preparation and IRAS chambers were separately evacuated by an oil diffusion pump (DP) and a turbomolecular pump (TMP), respectively, to obtain an ultimate pressure of
