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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 CH₃O_ad and H_ad (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.