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Free keywords:
Alcohols; Carbon monoxide; Palladium; X-ray photoelectron spectroscopy; Vibrations of adsorbed molecules; Catalysis; Low index single crystal surfaces
Abstract:
Methanol adsorption/desorption and its time- and temperature-dependent decomposition on well-annealed and defect-rich (ion-bombarded) Pd(111) were examined by X-ray photoelectron spectroscopy (XPS) and polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS). Annealing CH3OH multilayers from 100 to 700 K mainly resulted in CH3OH desorption. Dehydrogenation to CO was a minor path and only trace amounts of carbon or carbonaceous species (CHx; x=0–3) were produced, i.e. C–O bond scission was very limited. By contrast, an exposure of 5 x 10_7 mbar CH3OH at 300 K produced CHx (~0.3 ML) on both surfaces but the rate of formation was not considerably enhanced by surface defects. On well-annealed Pd(111) isolated carbon atoms were identified by XPS in the early stages of carbon deposition, with carbon diffusion leading to the growth of carbon clusters in the later stages. Since carbon(aceous) species may either originate from C–O bond scission within methanol (or CHxO) or from a consecutive dissociation of the dehydrogenation product CO, analogous experiments were also carried out with CO. PM-IRAS spectra up to 170 mbar CO, acquired using a UHV-high-pressure cell, did not show any indications of CO dissociation, excluding CO as source of carbonaceous deposits.
