Modulated molecular beam study of the mechanism of the H2--D2 exchange reaction on Pt(111) and Pt(332) crystal surfaces

The interaction of hydrogen with platinum has been studied by the exchange reaction H2+D2=2HD on two crystal surfaces, a flat (111) and a stepped (332) (in step notation, (S)-[6(111) x (111)]). The adsorption of hydrogen appears to be an activated process on the Pt(111) surface, with a barrier height of approx.0.5--1.5 kcal/mole. On the Pt(332) surface, the adsorption of hydrogen requires no activation energy. The recombination of H and D atoms to form HD follows, on both surfaces, a parallel mechanism with one of the branches operative in the entire temperature range studied, 25--8000 C. This branch has an activation energy and pseudo-first-order preexponential of E1=13.0 +- 0.4 kcal/mole and A1= (8 +- 3) x 104sec-1 for the Pt(332) surface and E1=15.6 +- 0.5 kcal/mole and A1= (2.7 +- 1) x 105sec-1 for the Pt(111) surface. For temperatures above approx.3000 C, the second branch is observed, but the values of the preexponential factors and activation energies could not be uniquely determined for either of the two crystals. Below approx.3000 C, a third process appears in series with the first branch. The scatter in the data at low temperatures due to the small signal amplitude prevents accurate determination of the rate constants for this reaction step