Understanding the reactivity of oxide-supported bimetallic clusters:: Reaction of NO with CO on TiO2(110)-supported Pt-Rh clusters

The reactions of CO, NO, and NO with CO have been studied on Pt, Rh, and bimetallic Pt-Rh clusters deposited on TiO2 (110). The following four cluster surfaces were investigated: 4 ML of Rh, 4 ML of Pt, 2 ML of Rh + 2 ML of Pt (Rh + Pt), and 2 ML of Pt + 2 ML of Rh (Pt + Rh). Scanning tunneling microscopy studies demonstrated that the surfaces exhibited similar cluster sizes and densities, and low-energy ion scattering experiments showed that the surfaces of the bimetallic clusters were Pt-rich (20-30% Rh) regardless of the order of metal deposition; therefore, both Pt and Rh atoms are capable of diffusing to the cluster surface at room temperature. Notably, heating the surface caused substantial encapsulation of the metal clusters by titania at 700 K and complete encapsulation at 800 K. In temperature programmed desorption experiments, the activities of the Pt and Rh clusters for CO and NO dissociation were found to be higher than those of the (111) surfaces of the corresponding single crystals. For both reactions, the activities of the Rh + Pt and Pt + Rh clusters were identical to each other and intermediate between that of pure Rh and pure Pt. For the reaction of NO with CO, the bimetallic clusters exhibited the greatest production of CO2 and the highest fraction of NO dissociation. On pure Rh clusters, CO2 production is inhibited by the preferential adsorption of NO over CO, whereas on the pure Pt clusters, CO adsorption is favored over NO. Only the Pt-Rh surfaces can provide sites for both NO dissociation and CO adsorption that are necessary for facilitating CO2 formation.