Reactions of gas-phase atomic hydrogen and deuterium with chemically modified Ir(111) surfaces

The reactions of gas-phase atomic hydrogen (H(g)) and deuterium (D(g)) with the chemically modified Ir(111)-p(1 X 2)-O, and deuterium (D(a)) and hydro gen (H(a)) adatom precovered Ir(111) surfaces at 100 K have been studied us ing thermal desorption mass spectrometry. Although the Ir(111)-p(1 X 2)-O s urface at a temperature of 100 K is passivated with respect to adsorption o f gas-phase H-2 and D-2, the interaction of H(g) and D(g) with this surface at 100 K results in the subsequent desorption of water in thermal desorpti on spectra. These results suggest that while the dissociative chemisorption of molecular hydrogen on Ir(111) is precluded by the presence of the (1 X 2)-O oxygen overlayer, energetically "hot" H(g) reacts readily with this ox ygen-modified surface. Moreover, a strong kinetic isotope effect has been o bserved in the interaction of H(g) and D(g) with D(a) and H(a) precovered I r(111) surfaces at 100 K. We find that H(a) is more readily replaced by D(g ) (abstraction cross section of sigma(D(g)) = 4.7 +/- 0.4 X 10(-16) cm(2)) than is D(a) by H(g) (sigma(H(g)) = 2.6+/-0.2 x 10(-16) cm(2)). These calcu lated cross sections assume a unity probability for reaction of H(g) and D( g) with the Ir(111)-p(1 X 2)-O surface. This observed isotopic difference i n abstraction cross sections is consistent with the differences associated with the expected zero point energy of the transition state for the abstrac tion reaction and differences associated with the zero point energy between H and D adatoms. (C) 2000 American Vacuum Society. [S0734-2101(00)02804-9] .