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) an d two different Sn/Pt(111) surface alloys was studied by temperature progra mmed desorption (TPD). The incident H(D) atoms were produced by thermal dis sociation in a Pt tube source operated at 1300 K. The alloy surfaces were p repared in situ by vapor deposition of Sn onto a Pt(111) single crystal to form the (2 x 2) and (root3 x root3)R30 degrees -Sn/Pt(111) surfaces, which have a well-defined structure and composition with relative Sn surface con centrations of 0.25 and 0.33, respectively. A kinetic barrier eliminates di ssociative H-2(D-2) chemisorption on both of these surface alloys, but abst raction reactions of incident H(D) atoms with preadsorbed H or D adatoms oc cur at 110 K on Pt(111) and both Pt-Sn alloys. This is well below the tempe ratures for thermal recombination on these surfaces, indicating that the re actions proceed by a direct or pseudodirect reaction mechanism. Values for the H --> D abstraction cross-section, sigma (R), on Pt(111) and the (2 x 2 ) and (root3 x root3)R30 degrees -Sn/Pt(111) surface alloys were determined to be 0.21, 0.93, and 1.7 Angstrom (2), respectively. The corresponding D --> H abstraction cross-sections for the two alloys were determined to be 0 .8, and 1.5 Angstrom (2), respectively. The values of sigma (R) for both H --> D and D --> H reactions increase with Theta (Sn) and indicate a signifi cant structural sensitivity for H abstraction reactions. There is no signif icant kinetic isotope effect on either alloy surface, however there is evid ence that incident H atoms are slightly more efficient in abstracting adsor bed D atoms than vice versa. (C) 2001 Elsevier Science B.V. All rights rese rved.