from first principles PP-I-1

OP-III-11Decomposition and Oxidation of Methanol on Pt and Pd:In situ XPS and Mass-Spectrometry StudyKaichev V.V., Miller A.V., Prosvirin I.P., Bukhtiyarov V.I.Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russiavvk@catalysis.ruDecomposition and oxidation of methanol on Pt(111) and Pd(111) single crystals wereexamined between 300 and 650 K in the mbar pressure range using in situ X-rayphotoelectron spectroscopy and temperature-programmed reaction spectroscopy. It was foundthat even in the oxygen presence, the methanol decomposition on both Pt and Pd proceedsthrough two competitive routes: fast dehydrogenation to CO and H 2 and slow decompositionof methanol via the C-O bond scission. In contrast to ultrahigh vacuum conditions, in thembar pressure range the rate of the second route is significant, which leads to a blocking ofthe catalyst surface by carbon and to a prevention of the further methanol conversion [1-2].These carbon deposits are very active toward adsorbed oxygen species and, as a result, in thepresence of oxygen, fast methanol oxidation occurs at temperature above 450 K [3]. CO, CO 2 ,H 2 , and H 2 O are detected as products in the gas phase. Increase in the oxygen content leads toincrease in methanol conversion as well as in CO 2 and H 2 O selectivities. For example, achange in the O 2 /MeOH molar ratio from 0 to 2 leads to an increase in methanol conversionrate (in turnover frequency, TOF) on Pt(111) at 650 K from 0.3 to 20 s -1 ; CO selectivitydecreases from 100 to 6 %. It means that the main route of methanol reforming is themethanol dehydrogenation, while in the presence of oxygen, CO is oxidized to CO 2 , and H 2 isoxidized to water. According to XPS in all used conditions, Pt and Pd are in the metallic state.Using “perfect” and “defect-rich” single-crystal surfaces, we found that the methanoloxidation is a structure-sensitive reaction on Pt. In our experiments, the “defect-rich” surfaceswere prepared by long Ar + -bombardment of the “perfect” single-crystal surfaces [1]. It wasfound that at 550 K, the methanol conversion rate on the “defect-rich” Pt(111) surface is a fewtimes higher than on the “perfect” Pt(111) surface. Moreover, the CO selectivity at lowoxygen content is also higher on the “defect-rich” Pt(111) surface. At 650 K, these effectsfade out due to annealing defects. Such effects were not observed on Pd. The mechanism ofdecomposition and oxidation of methanol on Pt and Pd is discussed.References:[1] V.V. Kaichev, M. Morkel, H. Unterhalt et al., Surf. Sci. 566-568 (2004) 1024.[2] M. Morkel, V.V. Kaichev, G. Rupprechter et al., J. Phys. Chem. B 108 (2004) 12955.[3] V.V. Kaichev, A.V. Miller, I.P. Prosvirin, V.I. Bukhtiyarov, Surf. Sci. 606 (2012) 420.54