Desulfurization of the Ni(100) surface using gas-phase hydrogen radicals

Gas-phase hydrogen radicals cause desulfurization of the sulfided Ni(100) s urface even for temperatures as low as 120 K, resulting in H2S formation. I n contrast, no thermal desulfurization is observed in the presence of coads orbed hydrogen. During hydrogen radical exposure, sulfur is abstracted from the Ni(100) surface by a sequential Eley-Rideal mechanism. After hydrogen radical exposure, two additional H2S formation pathways involving coadsorbe d hydrogen are observed during subsequent heating. In the first pathway, HI S formation is observed at 150 K, involving a partially hydrogenated interm ediate formed during gas-phase atomic hydrogen exposure. The second pathway involves addition of desorbing subsurface hydrogen to adsorbed sulfur, lea ding to H2S formation at 190 K. Both the temperature and coverage dependenc e of the 150 K pathway support a sequential hydrogen addition mechanism wit h a sulfhydryl intermediate during temperature-programmed desorption (TPD) studies. Previous H2S decomposition studies on this surface show that the s ulfhydryl intermediate is not stable above similar to 190 K because of ther mal dehydrogenation. The temperature dependence of H2S formation and sulfur removal during exposure to the gas-phase hydrogen radical is also consiste nt with a sulfhydryl intermediate. Above 200 K, no desulfurization is obser ved during gas-phase hydrogen radical exposure. This thermal dehydrogenatio n of H2S also depends on the coverage of coadsorbed sulfur. Increasing sulf ur coverages inhibits dehydrogenation of both H2S and SH. With higher sulfu r coverages, H2S desorption is favored and substantial sulfur is removed du ring temperature-programmed reaction spectroscopy (TPRS) experiments after low-temperature hydrogen radical exposure. Taken together, the temperature- and coverage-dependent behavior indicates that sulfhydryl is an intermedia te for sulfur abstraction. Through control of gas-phase hydrogen radical ex posure, vacancies in sulfided nickel layers were generated. Hydrogen chemis orption studies were used to probe these sulfur vacancies. The new, low-tem perature hydrogen desorption peak at 230 K corresponds to hydrogen modified by coadsorbed sulfur.