DECOUPLING OF VIBRATIONAL-MODES AS A STRUCTURAL TOOL - COVERAGE INDUCED REORIENTATION OF METHOXIDE ON MO(110)

The structure and reactivity of methoxide adsorbed on Mo(110) was investigated using temperature programmed reaction, x-ray photoelectron, high resolution electron energy loss and infrared reflection absorption spectroscopies. Methanol decomposes through a methoxy surface intermediate on Mo(110), with dehydrogenation and carbon-oxygen bond scission occurring. at similar to 400 K. The structure of the methoxy moiety is dependent on coverage, by reference to data obtained using surface infrared spectroscopy in combination with selective isotopic labeling. We demonstrate that methoxy exhibits C-3v symmetry, i.e., the C-O bond is normal to the surface, at coverages below 0.17 ML. However, the C-O axis begins to tilt towards the surface at higher coverages, so that at saturation coverage (0.25 ML), two distinct methoxy species with C-s symmetry are observed with an average tilt angle of 25 degrees+/-15 degrees from the surface normal. In addition, we conclusively show that the intense features at similar to 2910 cm(-1) in the infrared spectrum of adsorbed methoxide are due to overtones of the methyl deformation modes, gaining intensity by Fermi resonance with the symmetric carbon-hydrogen stretching mode, in contrast to previous assignments to the in-plane asymmetric C-H stretch which, we demonstrate, occurs at above 2935 cm(-1). (C) 1995 American Institute of physics.