STUDY OF THE SURFACE-CHEMISTRY OF METHYL-IODIDE COADSORBED WITH HYDROGEN ON PT(111)

The surface chemistry of methyl iodide coadsorbed with hydrogen on Pt(111) was studied using thermal programmed desorption (TPD) spectroscopy. When methyl iodide is adsorbed on a surface previously exposed to high doses of hydrogen, heating of the sample yields methane almost exclusively. In the case of coadsorbed deuterium and normal methyl iodide, the methane product distribution shows maxima for CH3D and CD4 formation, a result typical of catalytic exchange reactions. That distribution results from competition between two different reaction pathways on the surface, both starting from chemisorbed methyl groups: single deuteriation takes place via a reductive elimination step that requires about 17 kcal/mol to be activated, while multiple exchange follows a series of surface transformations including methylene formation, rapid methylene-methylidyne equilibration, and methylene stepwise hydrogenation to methyl and then to methane. The observed energy barrier for this second process is about 6-8 kcal/mol, and is the result of a combined effect of at least two separate steps which contribute to the overall reaction rate. Additional experiments using deuterated methyl iodide were also performed to confirm some of the conclusions from the studies with D2 and CH3I.