| Abstract: | Pulsed field desorption mass spectrometry (PFDMS) is used to investigate the early stages of ruthenium oxidation and the decomposition of methanol over ruthenium, rhodium, palladium and copper. Various oxides are identified during ruthenium oxidation at O₂ pressures below 10⁻³ Pa. RuO₂ is formed on the clean metal at 700 K but on a preoxidized surface, ionic species of RuO₃ and RuO₄ are detected. The presence of clusters ions -- Ru₂O₄⁺, Ru₂O₅⁺ and Ru₂O₆⁺ -- suggests nucleation of RuO₂ and RuO₃. The mobilities of the latter oxides are proved in a specially designed experiment. The catalytic decomposition of methanol over ruthenium, rhodium and palladium leads to carbon monoxide and hydrogen. Secondary reactions of carbon monoxide yield metal subcarbonyls. PFDMS allows the direct observation of the reaction intermediates, CH₃O, CH₂O and CHO. On ruthenium, at temperatures below 460 K, high intensities of CH₃⁺ (from CH₃O) are observed, indicating that the decomposition of adsorbed methoxy is inhibited by the final product, adsorbed carbon monoxide. At temperatures above 460 K, surface coverage of absorbed carbon monoxide is lowered by thermal desorption during the reaction interval. Methanol readily decomposes at empty sites and the intensities of the detected intermediates are small. Hence, the reaction is desorption rate limited below 460 K. The decomposition on rhodium shows a similar behavior as on ruthenium. Adsorbed carbon monoxide thermally desorbs at temperatures [greater than or equal to] 500 K. Below this temperature, the overall reaction is limited by the thermal desorption of CO[subscript ad]. A model is postulated for methanol decomposition based on stepwise H-abstraction. The slow step in the overall reaction is the decomposition of the methoxy. The influence of an electric field on the decomposition reaction is studied by applying a steady field during the reaction time between the pulses. At a field strength of about 4 V/nm, the intensities of CHO⁺, CH₂O⁺, and CH₃⁺ increase steeply. This increase is most pronounced for CH₂O⁺ and coincides with a decrease in CO⁺ and H⁺[subscript n] intensities. Thus, a steady electric field reduces the reaction rate and stabilizes the intermediate stages. Other field effects include a C-O bond breaking and a concerted C-H(D) bond cleavage in the methoxy. In the case of copper, mainly Cu⁺ ions are observed. The presence of an electric field leads to Cu(CH₃OH)[susbscript n]H⁺ complexes. |
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