Adsorption and reaction of gaseous H(D) atoms with D(H) adatoms on Pt(111) and Sn/Pt(111) surface alloys

The reaction of gas phase H(D) atoms with adsorbed D(H) atoms on Pt(111) and two different Sn/Pt(111) surface alloys was studied by temperature programmed desorption (TPD). The incident H(D) atoms were produced by thermal dissociation in a Pt tube source operated at 1300 K. The alloy surfaces were prepared in situ by vapor deposition of Sn onto a Pt(111) single crystal to form the (2 × 2) and (√3 × √3)R30°-Sn/Pt(111) surfaces, which have a well-defined structure and composition with relative Sn surface concentrations of 0.25 and 0.33, respectively. A kinetic barrier eliminates dissociative H₂(D₂) chemisorption on both of these surface alloys, but abstraction reactions of incident H(D) atoms with preadsorbed H or D adatoms occur at 110 K on Pt(111) and both Pt-Sn alloys. This is well below the temperatures for thermal recombination on these surfaces, indicating that the reactions proceed by a direct or pseudo-direct reaction mechanism. Values for the H→D abstraction cross-section, σ^R, on Pt(111) and the (2 × 2) and (√3 × √3)R30°-Sn/Pt(111) surface alloys were determined to be 0.21, 0.93, and 1.7 Ų, respectively. The corresponding D→H abstraction cross-sections for the two alloys were determined to be 0.8, and 1.5 Ų, respectively. The values of σ^R for both H→D and D→H reactions increase with Θ_Sn and indicate a significant structural sensitivity for H abstraction reactions. There is no significant kinetic isotope effect on either alloy surface, however there is evidence that incident H atoms are slightly more efficient in abstracting adsorbed D atoms than vice versa.