Steady state kinetics of the decomposition and oxidation of methanol on Pd(110)

Abstract

The decomposition of CH 3OH and the reaction of CH 3OH with oxygen on a Pd(110) surface have been studied in the pressure range 10 −710 −4 mbar at temperatures between 300 and 550 K. The reaction was investigated using Video-LEED, Δø measurements, thermal desorption spectroscopy (TDS) and mass spectrometry for analysis of the reaction products. TD spectra of adsorbed CH 3OH indicate complete decomposition into CO ad and H ad at 300 K. Higher CH 3OH exposures (> 500 L) result in the suppression of the H 2 TD states characteristic for pure hydrogen and in the formation of two distinct new adsorption states at 420 and 500 K which we attribute to subsurface hydrogen. The CH 3OH + O 2 reaction was investigated under steady flow conditions. Only CO 2 and H 2O were identified as oxidation products. Hysteresis effects which were observed upon cyclic variation of the temperature or of the partial pressures were of kinetic origin. The reaction rate exhibits a maximum at a P MeOH P O 2 ratio ≈ at temperatures between ~ 400 and 550 K. With increasing P MeOH P O 2 ratio the rate maximum is shifted towards higher temperature. The reaction is inhibited if either a too high CO coverage or a too high oxygen coverage forms under the applied reaction conditions. As has been verified by LEED measurements, the inhibitive effect in both cases can be associated with the formation of ordered adlayers: a (4 × 2) pattern in the case of CO and a c(2 × 4) structure in the case of oxygen.