Reactivity of diatomics and of ethylene on zeolite-supported 13-atom platinum nanoclusters

Abstract

Monodisperse Pt clusters of 13 ± 2 atoms, supported on the zeolites NaY and KL and saturated with chemisorbed hydrogen, are investigated as well-defined model catalysts for reactions of CO, NO, O₂ and ethene. CO reacts within <10 min, leading to the formation of dinuclear Pt carbonyl molecular clusters. A similar behaviour of NO suggests an analogous reaction. In stark contrast, O₂ reveals a very sluggish reaction on a timescale of days although the reaction with chemisorbed hydrogen to form H₂O is thermodynamically still favoured. This is ascribed to the inability of O₂ to adsorb atop of Pt when all neighbouring sites are blocked by chemisorbed hydrogen. The hydrogenation reaction of ethene yields ethane as the only product. The turnover frequency at room temperature is somewhat lower than the one reported for the same reaction on Pt(111) single crystal surfaces or on Pt nanoparticles, but its activation energy is double that typically found in other systems. This means that the reaction which has been known to be structure-insensitive becomes structure-sensitive for catalyst clusters as small as 13 atoms. This fact is ascribed to a significantly larger binding energy of H on Pt as a consequence of the small cluster size and the influence of the support.