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

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. (C) 2000 American Institute of Physics. [S0021-9606(00)71416-4].