The adsorption and reaction of methanol on Pd(100). II. Thermal desorption and decomposition

Abstract

The reaction of methanol with a clean Pd(100) surface has been studied between 77 and 300 K using a variety of techniques. High-resolution electron energy loss spectroscopy, thermal desorption spectroscopy, UV photoemission work function measurements, and low energy electron diffraction reveal a complex reaction pattern. At 77 K, photoemission, vibrational loss spectra, thermal desorption data as well as the Δφ behavior indicate the initial formation of a methoxide species which has a maximum population around 1×1014/cm2. 80% of this methoxide species desorbs as methanol between 180 and 210 K via second-order kinetics with Edes ∼11.5-12 kcal/mol. Above ∼200 K, we find that 20% of all the initially adsorbed species have not desorbed but have converted to two other more stable species. These species have CH3O stochiometries as determined from thermal desorption product ratios, but cannot be further identified. The first of these is more stable and predominantly forms (95% of total remaining species). It decomposes at ∼300 K to produce CO+H on the surface and has UPS level intensities significantly different than those observed from the methoxide. The other species occur in very small concentrations and only for higher coverages of the adsorbate. It appears in small amounts, ∼5% of these remaining species as inferred from its decomposition to gaseous CO+H 2 at ∼530 K. A simple bond site model is proposed to account for the ability of the low temperature methoxide species to hydrogenate in the presence of chemisorbed hydrogen. These results are compared to those on Ni(100). The observation of this hydrogenatable form of methoxide on Pd as well as its large branching ratio into methanol provides a possible explanation for the selectivity of Pd in catalytically forming methanol from synthesis gas. © 1982 American Institute of Physics.