Size-dependent catalytic and melting properties of platinum-palladium nanoparticles

Abstract

While nanocatalysis is a very active field, there have been very few studies in the size/shape-dependent catalytic properties of transition metals from a thermodynamical approach. Transition metal nanoparticles are very attractive due their high surface to volume ratio and their high surface energy. In particular, in this paper we focus on the Pt-Pd catalyst which is an important system in catalysis. The melting temperature, melting enthalpy, and catalytic activation energy were found to decrease with size. The face centered cubic crystal structure of platinum and palladium has been considered in the model. The shape stability has been discussed. The phase diagram of different polyhedral shapes has been plotted and the surface segregation has been considered. The model predicts a nanoparticle core rich in Pt surrounded by a layer enriched in Pd. The Pd segregation at the surface strongly modifies the catalytic activation energy compared to the non-segregated nanoparticle. The predictions were compared with the available experimental data in the literature. PACS: 65.80-g; 82.60.Qr; 64.75.Jk.

Figure 1

Size-dependent melting temperature of platinum versus the size for different shapes.

Figure 2

Size-dependent melting temperature of palladium versus the size for different shapes.

Figure 3

Phase diagram of the Pt-Pd system for different shapes. Different shapes at a size equal to 4 nm and at the bulk scale. The solid lines indicate the liquidus curves whereas the dashed lines indicate the solidus ones.

Figure 4

Phase diagram of the Pt-Pd system considering the surface segregation effect. Surface segregation effect at a size equal to 4 nm for a spherical nanoparticle.

Figure 5

Composition dependency of the catalytic activation energy for a spherical nanoparticle of Pt-Pd. Nanoparticle of Pt-Pd with a size equal to 4 nm.