The morphology and catalytic activity of bimetallic nanoclusters supported on titanium dioxide (110)

Oxide-supported bimetallic nanoclusters may exhibit superior properties including increased catalytic activity when compared to the monometallic catalysts. However, the mechanisms for the increased activity are poorly understood. In this work, two model systems, the oxidation of CO with NO on TiO2 (110)-supported Pt-Rh bimetallic clusters and the oxidation of CO with O2 on TiO2(110)-supported Pt-Au bimetallic clusters were studied in ultrahigh vacuum to understand the influence of the support and elemental composition of the clusters on the catalytic activity. The supported nanoclusters were studied using scanning tunneling microscopy, x-ray photoelectron spectroscopy, low energy ion scattering, and temperature programmed desorption.;Pt-Rh nanoclusters supported on TiO2(110) were studied as a model system for the catalytic converter in automobiles. Sequential deposition of Pt and Rh provides a surface composed of bimetallic clusters with both Pt and Rh present in the surface layer, regardless of deposition order. Heating Rh and Pt clusters causes encapsulation by a reduced titania species at temperatures above 600K and 700K, respectively, with encapsulation complete by 800K. Upon exposure to a mixture of NO and CO, the Rh surface shows preferential adsorption of NO while the Pt surface shows preferential adsorption of CO. While the pure metal surfaces have an abundance of one reactant and a correspondingly low yield for N2 and CO2, the bimetallic surface has a NO:CO ratio much closer to stoichiometric and a high yield of N2 and CO2. Thus, surface Pt sites must be intermixed with surface Rh sites to obtain high activity for the oxidation of CO with NO.;TiO2(110)-supported Au clusters are active in the oxidation of CO with O2. The reaction is believed to occur at the interface between Au and TiO2, so by alloying Au with a metal that undergoes encapsulation, such as Pt, the Au-titania interface area may be increased. Sequential deposition of Au and Pt provides a surface composed of bimetallic clusters with Au enrichment at the surface, in agreement with thermodynamic predictions. Au exhibits low O2 adsorption and the bimetallic surfaces exhibit low activity for the oxidation of CO. However, on surfaces with high Au content Pt diffuses to the surface at 300K to facilitate CO adsorption.;The increased catalytic activity observed for the oxidation of CO by NO on Pt-Rh nanoclusters and for the oxidation of CO by O2 on Pt-Au nanoclusters can be explained by the elemental composition of the surface layer and its effect on the concentrations of the reactants preferentially adsorbed onto one of the two metals.;To facilitate the processing of scanning tunneling microscopy images, a program was written in C++ to filter an image to remove the substrate and calculate various statistics such as diameter, height, footprint area, surface area, volume, and nearest-neighbor distance for every cluster in the image as well as the total number, footprint area, surface area, and volume of all the clusters within the image.