Cyclohexane Dehydrogenation and H2 Adsorption on Pt Particles on ZnO(0001)−O

Abstract

The dehydrogenation of cyclohexane on Pt/ZnO(0001)-O model catalysts was studied with temperature programmed desorption (TPD), low-energy ion scattering spectroscopy, and X-ray photoelectron spectroscopy. Vapor-deposited Pt grows on this O-terminated ZnO face as 2-dimensional (2D) islands at low coverage, and later as 3D particles. The reactivity of these Pt nanoparticles toward H 2 and c-C 6 D 1 2 was studied versus Pt coverage, which controls their lateral dimension and then their thickness. Dissociatively adsorbed H 2 gives an H 2 TPD peak at ∼330 K from Pt particles, nearly independent of their size or thickness. The initial sticking probability for c-C 6 D 1 2 at 170 K is higher on Pt particles than on ZnO. Adsorbed c-C 6 D 1 2 on this ZnO surface desorbs intact at ∼220 K. When c-C 6 D 1 2 is adsorbed on Pt islands, some of it desorbs molecularly at ∼250 K, but most of it (>60%) dehydrogenates. The dehydrogenation pathway produces two main D 2 TPD regions: ∼300-400 K, peaking at ∼370 K, and 450-750 K, peaking at ∼580 K. This is similar to the dominant H 2 peaks seen from low-index faces of Pt crystals because of dehydrogenation of adsorbed c-C 6 H 1 2 to adsorbed benzene plus H (370 K) and further dehydrogenation of benzene to adsorbed C 2 H (∼560 K) and finally graphitic carbon (600-750 K). The ∼580 K peak is broadened considerably on particles compared to low-index faces. The probability for c-C 6 D 1 2 dehydrogenation is much higher on Pt particles than on Pt( 111), especially when the particles are 2D. The probability for loss of product D (or H) into the bulk of the ZnO is high at lower temperature, which decreases the apparent intensity ratio of the two main D 2 TPD peak regions (370 K: 580 K and above) relative to the stoichiometric 1:1 ratio seen for Pt( 111).