SURFACE KINETICS STUDIED BY USING ION-STIMULATED DESORPTION OF NEUTRAL MOLECULES - TIME-RESOLVED DECOMPOSITION OF CH3OH ADSORBED ON NI(110)

The time-resolved decomposition of methanol on Ni(110) has been studied by using a pulsed Ar+ beam at 2 keV followed by laser ionization at 193 nm of the desorbed (sputtered) neutral species and reflecting time-of-flight mass spectrometry of the resulting photoions. Methanol decomposition was monitored during temperature-programmed reaction (TPR) following molecular adsorption at 120 K for initial coverages of 0.09 and 0.29 monolayer (ML) with respect to the number of exposed nickel atoms, and at 170 K for 0.04, 0.15, and 0.29 ML. Cleavage of the O-H bond in molecularly adsorbed methanol and decomposition of methoxy to carbon monoxide and hydrogen were clearly resolvable even though the products for both of these reactions remained adsorbed on the Ni(110) surface. Reaction occurred exclusively via a methoxy intermediate that formed via O-H bond cleavage during TPR from 140 to 240 K. Methoxy decomposed to adsorbed carbon monoxide and hydrogen at 240-290 K. No evidence for reactions involving C-O bond cleavage in either molecularly adsorbed methanol or methoxy was seen. For an initial coverage of 0.29 ML, methanol desorption occurred together with decomposition and approximately 0.08 ML of methanol desorbed during TPR. Methoxy decomposition and O-H bond cleavage kinetics were monitored primarily using the decrease in the surface concentrations of methoxy and molecular methanol, respectively, with increasing temperature during TPR. The kinetics for both of these reactions appeared similar for the range of initial methanol coverages studied. Based on analysis of the kinetics using model calculations, details of the mechanism for both methoxy formation and decomposition on Ni(110) are discussed.