Death from above: Exploring new reactions and establishing structure/reactivity relationships for gas-phase hydrogen radicals on metal surfaces.

Abstract

Examining the effect of surface structure and surface composition and exploring new reactions between gas-phase hydrogen radicals and adsorbates is the focus of this research. The interaction of the adsorbate with the surface plays an important role in the chemistry of gas-phase hydrogen radicals with adsorbed species. Surface structure kinetically controls the reactive pathways for cyclopropane hydrogenation by gas-phase hydrogen radicals. An additional propane formation pathway is observed for the hydrogenation of cyclopropane by gas-phase hydrogen radicals on the Ni(111) surface that is not observed on the Ni(100) surface. Therefore, surface structure must play an important role in the reactions of the impinging hydrogen radicals with adsorbates. The chemical composition of the surface also effects the chemistry of gas-phase' hydrogen radicals with adsorbates. Cyclopropane hydrogenation to form propane is 5 times faster on the Pt(111) relative to the Ni(111) surface. The stronger adsorption of cyclopropane on the Pt(111) surface (8.3 kcal/mol) compared to the Ni(111) surface (6.7 kcal/mol) correlates with the increased hydrogen radical addition rate. This reactivity trend parallels similar results obtained for thermally activated hydrogenation of cyclopropane on supported Pt and Ni catalysts at high pressures. The importance of surface structure and chemical composition is surprising since the energy for initiating hydrogen addition is supplied by the gas-phase hydrogen radical reactant. Gas-phase hydrogen radicals have been shown to be effective in desulfurization reactions. For temperatures as low as 120 K, hydrogen radicals remove adsorbed sulfur from the Ni(100) surface during exposure. Upon subsequent heating, the surface undergoes further desulfurization through hydrogenation of a sulfhydryl intermediate. For the first time, gas-phase hydrogen radicals have been shown to induce desulfurization through C-S bond activation. During hydrogen radical exposure, adsorbed methyl thiolate undergoes C-S bond scission at temperatures as low as 120 K forming methane by first order kinetic process in both adsorbate concentration and hydrogen radical pressure. Adsorbed methyl is also formed during reaction. Upon heating, the adsorbed methyl group is hydrogenated by surface hydrogen desorbing as methane at 230 K. Taken together this research demonstrates the interesting range of surface reactions which can be characterized using gas phase hydrogen radicals.