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

Abstract

The dissociative adsorption of H2 and D2 on Pt(533) (Pt{4(111)×(100)}) has been investigated using temperature programmed desorption and supersonic molecular beams. Associative desorption of D2 from (100) step sites is observed at lowest exposures in TPD (assigned β3) at 375 K. Saturation of this peak at ΘH=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 β1 and β2) for the Pt(111) surface. The incident kinetic energy (Ei), surface temperature (Ts), coverage (ΘD), and incident angle (Φ) dependence of the dissociative sticking probability (S) was also measured. The initial dissociative sticking probability (S0) first decreases with increasing kinetic energy over the range 0<Ei(meV)<150 (low energy component), and subsequently increases (high energy component). Comparison with D2 dissociation on Pt(111), where (S0) increases linearly with Ei, 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 Ts. The probability of dissociation through the low energy component associated with the (100) steps, over most of the range of Ei where it contributes, is also shown to be independent of Ts. Only at the very lowest value (6.6 meV) of Ei investigated does S0 exhibit a (negative) temperature dependence. A (0.8-ΘD)2 dependence (where 0.8 is the measured saturation coverage) of S with ΘD is observed at Ei=180 meV where the direct channel dominates. However, the dependence of S on ΘD exhibits characteristics similar to those expected for precursor mediated dissociation at Ei=16 meV and Ei=6.6 meV where the low energy channel dominates. The angular dependence S0(Φ) scattering in a plane perpendicular to the step direction is asymmetric about the Pt(533) surface normal at both Ei=6.6 meV and Ei=180 meV. At 180 meV S0(Φ) can be understood by considering direct dissociation at the (111) terrace and (100) step plane. At 6.6 meV, S0 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 H2/D2 dissociation and exchange on metal surfaces presenting low activation barriers. At lowest energies (Ei=6.6 meV) dissociation through a conventional accommodated precursor takes place. In addition, more significant proportion of sticking in the range 0<Ei(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.