THE DETERMINATION OF THE DECOMPOSITION MECHANISM AND KINETICS OF METHOXY ON CLEAN NI(100) AND THE EFFECTS OF COADSORBED HYDROGEN, CO, AND SULFUR BY TIME-RESOLVED FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

Time-resolved Fourier transform infrared spectroscopy (TR-FTIR) was used to simultaneously examine the mechanism and kinetics of methoxy decomposition on Ni(100) under varying conditions. The use of TR-FTIR to obtain kinetic parameters was first validated by comparison to temperature-programmed desorption using annealed CO adlayers. Assuming a first-order reaction with a constant pre-exponential factor of 5.2 x 10(12) s(-1), we find the activation energy for methoxy decomposition to CO and hydrogen to increase as the reaction proceeds according to 16.9 + 10 theta(CO) +/- 0.3 kcal/mol, where theta(CO) is the coverage of CO product in monolayers, for initial methoxy coverages between 0.18 and 0.056 ML. The addition of 0.10 +/- 0.02 ML of CO increases the energy for C-H bond activation to 19.2 +/- 0.4 kcal/mol. Coadsorbed sulfur was found to inhibit both methoxy formation and decomposition. The addition of 0.14 +/- 0.02 ML of sulfur raises the activation energy for methoxy decomposition by 1 +/- 0.4 kcal/mol, an effect similar in magnitude to coadsorbed CO. The preadsorption of 0.4 ML of hydrogen reduces the amount of methoxy formed from a methanol multilayer but does not raise the activation energy for methoxy decomposition. These results are compared to methoxy decomposition studies on Ni(111) and Ni(110).