The role of steps in the dynamics of hydrogen dissociation on Pt(533)

Gee, A. T., Hayden, B. E., Mormiche, C. and Nunney, T. S. (2000) The role of steps in the dynamics of hydrogen dissociation on Pt(533). The Journal of Chemical Physics, 112 (17), 7660-7668. (doi:10.1063/1.481360).

Abstract

The dissociative adsorption of H-2 and D-2 on Pt(533) (Pt{4(111)x(100)}) has been investigated using temperature programmed desorption and supersonic molecular beams. Associative desorption of D-2 from (100) step sites is observed at lowest exposures in TPD (assigned beta(3)) at 375 K. Saturation of this peak at Theta(H)=0.14 corresponds to the filling of half of the available four-fold sites at the (100) step edge. At higher coverages, additional desorption takes place from the (111) terraces in a broad peak below 300 K similar to that observed (assigned beta(1) and beta(2)) for the Pt(111) surface. The incident kinetic energy (E-i), surface temperature (T-s), coverage (Theta(D)), and incident angle (Phi) dependence of the dissociative sticking probability (S) was also measured. The initial dissociative sticking probability (S-0) first decreases with increasing kinetic energy over the range 0 < E-i(meV)< 150 (low energy component), and subsequently increases (high energy component). Comparison with D-2 dissociation on Pt(111), where (S-0) increases linearly with E-i, leads to the conclusion that it is the step sites that are responsible for the low energy component to dissociation on Pt(533). The high energy component is a result of a direct dissociation channel on (111) terraces of the Pt(533) surface. The probability of dissociation through the direct channel on the (111) terraces is found to be independent of T-s. The probability of dissociation through the low energy component associated with the (100) steps, over most of the range of E-i where it contributes, is also shown to be independent of T-s. Only at the very lowest value (6.6 meV) of E-i investigated does S-0 exhibit a (negative) temperature dependence. A (0.8-Theta(D))(2) dependence (where 0.8 is the measured saturation coverage) of S with Theta(D) is observed at E-i=180 meV where the direct channel dominates. However, the dependence of S on Theta(D) exhibits characteristics similar to those expected for precursor mediated dissociation at E-i=16 meV and E-i=6.6 meV where the low energy channel dominates. The angular dependence S-0(Phi) scattering in a plane perpendicular to the step direction is asymmetric about the Pt(533) surface normal at both E-i=6.6 meV and E-i=180 meV. At 180 meV S-0(Phi) can be understood by considering direct dissociation at the (111) terrace and (100) step plane. At 6.6 meV, S-0 tends to scale with total energy. The observed characteristics of the low energy channel is discussed in the light of models [specifically the role steps and defects, precursors (accommodated and dynamical), and steering] suggested to account for the low energy component for H-2/D-2 dissociation and exchange on metal surfaces presenting low activation barriers. At lowest energies (E-i=6.6 meV) dissociation through a conventional accommodated precursor takes place. In addition, more significant proportion of sticking in the range 0 < E-i(meV)< 150 takes place through an indirect channel involving an unaccommodated precursor dissociating at step sites, and is unlikely to be accounted for through a steering mechanism.

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