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

The dissociative adsorption of H[sub 2] and D[sub 2] on Pt(533) (Pt{4(111)x(100)}) has been investigated using temperature programmed desorption and supersonic molecular beams. Associative desorption of D[sub 2] from (100) step sites is observed at lowest exposures in TPD (assigned β[sub 3]) at 375 K. Saturation of this peak at Θ[sub 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 β[sub 1] and β[sub 2]) for the Pt(111) surface. The incident kinetic energy (E[sub i]), surface temperature (T[sub s]), coverage (Θ[sub D]), and incident angle (Φ) dependence of the dissociative sticking probability (S) was also measured. The initial dissociative sticking probability (S[sub 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[sub 2] dissociation on Pt(111), where (S[sub 0]) increases linearly with E[sub 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[sub s]. The probability of dissociation through the low energy component associated with the (100) steps, over most of the range of E[sub i] where it contributes, is also shown to be independent of T[sub s]. Only at the very lowest value (6.6 meV) of E[sub i] investigated does S[sub 0] exhibit a (negative) temperature dependence. A (0.8-Θ[sub D])[sup 2] dependence (where 0.8 is the measured saturation coverage) of S with Θ[sub D] is observed at E[sub i]=18... [ABSTRACT FROM AUTHOR]