Surface chemistry of CH3Br and methyl modified by copper deposition on Ru(001)

The chemistry of methyl bromide on Cu/Ru(001) has been studied utilizing wo rk function change (Delta phi) and temperature-programmed desorption (TPD) measurements. The remarkable modification in the methyl fragments dehydroge nation at the completion of a single copper layer and the significant diffe rence in reactivity of the Cu(2 ML)/Ru(001) or Cu(lll) surfaces are the foc us of this study. A decrease in work function at the completion of 1 ML CH3 Br of 2.15 +/- 0.02 eV and 1.33 +/- 0.05 eV was measured, respectively, for Ru(001) and Cu(2 ML)/Ru(001) held at 82 K. Methyl bromide does not dissoci ate upon adsorption on clean or the copper-covered surfaces, and it is boun d with the bromine down. Copper modifies the reactivity of the Ru substrate , gradually decreasing the dissociated fraction of CH3Br from 0.55 of the i nitial one monolayer on clean Ru(001) to 0.06 on Cu(2 ML)/Ru(001), probably because of defects in the copper layer. The methyl fragment dehydrogenatio n rate slows as the copper coverage increases. At a narrow copper coverage range between 0.8 and 0.95 ML, adsorbed hydrogen and methyl fragments coexi st on the surface in the temperature range 230-280 K. Sequential. decomposi tion channels of the parent molecules and the methyl fragment lead to a uni que enhancement of methane production rate, this on the account of further hydrocarbon dehydrogenation, as reflected in both Delta p and Delta phi TPD measurements. Methane is formed on top of copper terraces as a result of " spill-over" of both methyl and hydrogen atoms, similar to the chemistry ove r Cu(lll) and Cu(110) single-crystal surfaces. The dipole moment of adsorbe d methyl is reported here for the first time on metal surfaces, being 0.48 D on top of Cu(2 ML)/Ru(001).