Understanding the Growth and Chemical Activity of Co-Pt Bimetallic Clusters on TiO2(110): CO Adsorption and Methanol Reaction

Abstract

Pure and bimetallic Co-Pt clusters were grown on TiO2(110) and studied by scanning tunneling microscopy (STM). Despite the lower mobility of Co atoms on TiO2 compared to Pt, STM experiments demonstrate that exclusively bimetallic clusters can be deposited through either order of deposition, provided that deposition of the first metal results in a sufficient density of seed clusters to nucleate all of the metal atoms from the second deposition. Bimetallic clusters of varying compositions were prepared by depositing different Co:Pt ratios at room temperature with a fixed total coverage of 0.25 ML. The average cluster heights decrease with increasing Co fraction since the higher Co coverage results in a greater number of nucleation sites. After heating to 800 K, the pure Pt clusters exhibit the greatest increase in average cluster height; however, both Pt and Co clusters become encapsulated with titania upon annealing to 800 K, which is also the onset temperature for Co desorption. Low energy ion scattering (LEIS) experiments show that for all bimetallic compositions studied, both Co and Pt atoms reside at the cluster surfaces although the surface composition is always richer in Pt than the bulk. Co-Pt clusters with a 55% Co fraction have nearly identical surface compositions, regardless of the order of Co and Pt deposition, and this behavior suggests that diffusion of atoms within the clusters is facile at room temperature. Furthermore, 55% Co-45% Pt clusters exhibit identical activity in terms of CO desorption and methanol reaction for both Co on Pt and Pt on Co. Temperature programmed desorption (TPD) experiments with CO indicate that CO desorbs from both Pt and Co sites on surfaces with high Co fractions, and the desorption temperature of CO decreases with increasing Co fraction. In the reaction of methanol on the pure and bimetallic clusters, CO and H-2 are produced as the main gaseous products on all surfaces. On the bimetallic surfaces, the selectivity for methane formation is higher than on either pure Co or Pt, and the bimetallic clusters are less active for C H bond scission than pure Co or Pt. CO evolution from methanol reaction appears to be desorption-limited, and the surface CO prefers to adsorb at Pt sites over Co sites.