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Abstract:
The decomposition of methanol on a Pd(111) surface has been investigated under steady reaction conditions and at temperatures T > 300 K by combined static secondary ion mass spectrometry (SSIMS) and X-ray photoelectron spectroscopy (XPS). Pulsed field, desorption mass spectrometry (PFDMS) has been applied in studies of the same reaction at 300 K on a Pd field emitter surface with (111) orientation of the terraces. The major reaction route on both types of surfaces involves O-H bond breaking in CH3OH with subsequent formation of a methoxy species. Further decomposition of this methoxy species to CO (and H2) proceeds via stepwise hydrogen abstraction as concluded from the observation of CH2O+ and CHO+ ions in the PFD mass spectra. The detection of PdCH+3 and PdO+ or PdOD+ in SSIMS suggests O-C bond scission in CH3OH as a side reaction leading to surface methyl- and/or surface oxygen/hydroxyl species. The C1s XP spectra taken at T ⩾ 373 K, i.e. at temperatures well above those necessary for recombination of CH3Oad and Had (with subsequent thermal desorption of methanol) give corroborating evidence for the simultaneous presence of adsorbed CO and CH3. The adsorbed methyl, which is present in quite small amounts, is thermally stable up to temperatures of at least 440 K.
