Thermal Decomposition of CH₃OH Adsorbed on Pd{l 11}: A New Reaction Pathway Involving CH₃ Formation

The adsorption and subsequent decomposition of CH₃OH on Pdf 111} has been examined with X-ray photoelectron spectroscopy and secondary ion mass spectrometry (SIMS) over the temperature range 110–600 K. We find that the thermal decomposition reaction pathway after adsorption at 110 K is strongly dependent on the initial CH₃OH exposure. Below the saturation CH₃OH exposure of 1.5 langmuirs the primary decomposition products are observed to be CH_3ads, OCH_3ads, and H₂O_ads, This chemistry suggests that the methanolic C-O bond is activated at the metal surface. These products are observed to form at temperatures as low as 125 K. For exposures above 1.5 langmuirs this decomposition pathway is no longer observed, possibly due to a site blocking effect, and molecular desorption occurs with trace decomposition. Interestingly, we find no evidence that CH₃OH adsorbed at 110 K with initial exposures below 1.5 langmuirs decomposes to CO at any temperature below the thermal desorption temperature of CO at 490 K. Finally, the SIMS ion observed at m/e 15 (CH_3ads) and associated with the decomposition of CH₃OH is found to persist over the rather wide temperature range 125–400 K. The unusual stability of CH_3ads suggested by these findings may be related to the effectiveness of Pd-based catalysts to produce CH₃OH.