STEADY-STATE KINETICS OF THE DECOMPOSITION AND OXIDATION OF METHANOL ON PD(110)

The decomposition of CH3OH and the reaction of CH3OH with oxygen on a Pd(110) surface have been studied in the pressure range 10(-7)-10(-4) mbar at temperatures between 300 and 550 K. The reaction was investigated using Video-LEED, DELTAphi measurements, thermal desorption spectroscopy (TDS) and mass spectrometry for analysis of the reaction products. TD spectra of adsorbed CH3OH indicate complete decomposition into CO(ad) and H(ad) at 300 K. Higher CH3OH 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 CH3OH + O2 reaction was investigated under steady flow conditions. Only CO2 and H2O 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(O2) ratio almost-equal-to 1:1 at temperatures between approximately 400 and 550 K. With increasing P(MeOH/p(O2) 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 x 2) pattern in the case of CO and a c(2 X 4) structure in the case of oxygen.