CH3 AND CH3I CHEMISTRY ON PT(111) - THE INFLUENCE OF CO

The surface chemistry of methyl iodide alone and coadsorbed with CO on Pt(111) was studied with high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), positive temperature programmed static secondary ion mass spectrometry (+TPSSIMS) and Auger electron spectroscopy (AES). Methyl iodide bonds to Pt(111) through lone pair electrons on the halide with a molecular symmetry less than C3v and is not significantly affected by coadsorbed CO. On the clean surface, CH3I decomposes to CH3 and I at about 250 K. Methane is formed at 290 K from the hydrogenation of CH3 groups, with hydrogen supplied from the decomposition of other CH3 groups. The competition between CH3 hydrogenation and dehydrogenation is coverage dependent. For a surface prepared by dosing sub-saturation amounts of CH3I and then saturating with CO, more thermal energy must be added to break the C-I bond. However, if CH3 is formed first, and then CO is added to saturation, CH4 formation occurs 40 K lower and with a higher yield than in the absence of CO. This enhanced methanation activity in the presence of saturation CO may be linked to destabilization of the Pt-CH3 and/or Pt-H bonds.