N-BUTANE HYDROGENOLYSIS ON PLANAR AND FACETED PT/W(111) SURFACES

The relationship between surface structure and reactivity is investigated by means of n-butane hydrogenolysis, a known structure sensitive reaction, for planar and faceted Pt/W(111) surfaces. The W(111) surface reconstructs to form pyramidal facets with [211] orientation upon vapor deposition of Pt (> 1.3 ML) and annealing above 750 K. The hydrogenolysis kinetics over the planar and the faceted surface are found to be quite different. The planar surface has a higher selectivity towards ethane formation and a higher reaction rate. The apparent activation energies are found to be 33 +/- 4 kJ/mol for the planar surface and 76 +/- 6 kJ/mol for a surface covered with approximately 20 nm facets. There appears to be a correlation between the concentration of fourfold coordination (C4) sites on the surface and the amount of ethane produced. The C4 concentration is altered by changing the facet size (annealing temperature). The results indicate the presence of a different intermediate on the C4 sites as evidenced by the differences in the apparent activation energy, the reaction rate and the overall selectivity.