(324b) Support Effects On the Catalytic Properties of Pt-Au Bimetallic Nanoparticles: A Multi-Scale Simulation Study

Carbon nanotube-supported metal nanoparticles show promise as catalysts for a wide variety of reactions. The catalytic activity of these materials depends on their composition, structure, and the coordination numbers of exposed atoms. We have used all-atom molecular dynamics simulations to investigate the properties of bimetallic nanoparticles deposited on bundles of carbon nanotubes, focusing on nanoparticles composed of platinum and gold. The distribution of the atom types on the surface of the nanoparticle can be tuned by changing the composition of the nanoparticle and the geometry of the support. For example, it is possible to have isolated platinum atoms surrounded by gold, or chains or islands of platinum atoms. To obtain quantities that can be verified experimentally we further used ab initio density functional theory (DFT) calculations to study the adsorption of CO on small noble metal clusters. The clusters are composed of 13 noble metal atoms. The cluster compositions range from 100% Pt to 100% Au. Adsorption is only studied on the top atom site. This atom can be either Pt or Au, depending on the cluster considered. Results are analyzed in terms of CO adsorption energy, CO bond stretching frequency, and geometry of the CO+cluster system. Analysis of these data allows us to quantify how the local environment affects the adsorption of CO. It found that, as expected, the CO adsorption energy depends strongly on the adsorption site, with adsorption on Pt being >1 eV stronger than adsorption on Au. More importantly, the cluster composition affects the adsorption energy. When CO adsorbs on Pt, increasing Au content decreases the adsorption energy. In contrast, when CO is adsorbed on Au increasing Pt content increases the adsorption energy. Our results confirm that the relationship between C-O stretching frequency and CO adsorption energy is approximately linear, with higher adsorption energies leading to lower C-O stretching frequencies. We will also discuss the electronic properties of the clusters, calculating the charges of relevant atoms and the density of states of the CO-metal system. When extended to bimetallic nanoparticles, our multi-scale simulation studies will lead to the design of active and selective materials for heterogeneous catalysis.