Benzene Adsorption and Dehydrogenation on Pt/ZnO(0001)−O Model Catalysts

The dehydrogenation of benzene on Pt/ZnO(0001)−O model catalysts was studied with temperature-programmed desorption (TPD) and low-energy ion scattering spectroscopy (LEIS). Vapor-deposited Pt grows as two-dimensional (2D) islands on this oxygen-terminated ZnO(0001)−O surface up to ∼50% of a monolayer and as three-dimensional (3D) islands at higher coverage. The reactivity of these Pt islands is studied as a function of their thickness and coverage. Benzene desorbs molecularly at ∼250 K from Pt-free ZnO(0001)−O, unless the surface is preannealed in ultrahigh vacuum, in which case some of it dehydrogenates and populates the subsurface ZnO with H (or D when using perdeuterobenzene, c-C 6 D 6 ). Subsurface H (or D) reappears as H 2, HD, and D 2 gas above ∼700 K in TPD, isotopically scrambled with the bulk H (D) reservoir. The ZnO defect sites active in benzene decomposition are passivated by annealing in O 2 . On Pt islands, most of the adsorbed benzene dehydrogenates in TPD, giving a broad H 2 peak at ∼560 K with a shoulder extending to ∼750 K. This is attributed to the stepwise conversion of adsorbed benzene to graphitic C, as occurs at the same temperatures on bulk Pt surfaces. Some of the H abstracted by Pt diffuses into the ZnO bulk and reappears above 700 K as H 2 . These H 2 peaks all appear as D 2 when using perdeuterated benzene and scramble with the subsurface H reservoir to give simultaneous HD peaks. The LEIS signal also suggests that approximately one-third of the adsorbed c-C 6 D 6 on the Pt islands also desorbs molecularly at ∼300−480 K. Benzene surface reactions and desorption do not depend strongly on island size or thickness, but its dehydrogenation probability on these islands is higher than on Pt(111) and may depend on island dimensions.