Stimulated Desorption of Neutrals as a Real-Time Probe of Chemistry on Surfaces: Reactions of Methyl Hydroxide and Carbon Monoxide on NICKEL(110)

The adsorption and decomposition of methanol and the adsorption of carbon monoxide on Ni(110) have been studied by stimulated desorption using a pulsed Ar ^+ beam at 2 keV, followed by laser ionization at 193 nm and reflecting time-of-flight mass spectrometry (TOF-MS) of the desorbed neutral species. Stimulated desorption is conducted under "static" or nondamaging conditions, and the sputtered neutrals are photoionized by a pulsed, focused ArF excimer laser beam (6.4 eV/photon). For low initial methanol coverages, methanol adsorption occurs dissociatively via a precursor-mediated adsorption mechanism to form adsorbed methoxy and atomic hydrogen for surface temperatures above 140-150 K, while at and above 0.29 ML some methanol adsorbs molecularly. The methanol decomposition intermediate, methoxy, decomposes to form adsorbed carbon monoxide and hydrogen between 240 and 270 K. The methoxy decomposition kinetics are independent of initial methanol coverage for initial methanol coverages between 0.04 and 0.29 ML. No evidence for C-O bond cleavage was observed. Kinetic modeling of methanol deprotonation and methoxy decomposition was conducted and the results are discussed. The activation barrier for methoxy decomposition was determined to be E^ddagger = 19.5 +/- 1.7 kcal/mol by the variation of heating rate method. The Ni signal was observed to vary monotonically with hydrogen coverage. The Ni signal was used as a crude measure of hydrogen coverage to determine that active methoxy decomposition sites may be blocked by hydrogen, thus explaining the observed coverage independent methoxy decomposition kinetics. Carbon monoxide adsorption on Ni(110) was also investigated. Signal intensities were monitored in real -time at a variety of constant surface temperatures between 196 and 429 K during exposure of the crystal to 3 times 10^{-8} Torr of carbon monoxide. Carbon monoxide adsorbs on Ni(110) via a precursor-mediated adsorption mechanism. Unusual adsorption kinetics, which have not been previously observed, were seen for surface temperatures between 350 and 400 K. Attempts to model the carbon monoxide adsorption kinetics were only partially successful.