Catalytic activation of Rh,Pd nanoclusters supported on Ceria

Abstract

Rh,Pd/CeO2 nanocatalysts exhibit excellent activity and stability for ethanol steam reforming (ESR) at 400°C. This thesis encompasses experimental and computational investigations of this nanocatalyst. The activation mechanism of Rh,Pd/CeO2 nanocatalysts under ethanol steam reforming conditions is unclear, though it has been previously suggested that oxidic forms of Rh and Pd initially present on the surface of CeO2-x are reduced to metallic form under the reducing environment of the ESR reaction. Here we report X-Ray Photoelectron Spectroscopy (XPS), X-Ray Absorption Spectroscopy (XAS) and High Resolution Transmission Electron Microscopy (HRTEM) studies on the catalytic activation of Rh,Pd/CeO2 nanocatalysts during ethanol steam reforming (ex-situ), as well as EXAFS and HRTEM studies of catalyst activation under reducing H2 (ex-situ) and in-situ EXAFS under CO atmospheres. The studies reveal the catalyst auto-activates under ESR conditions, with metallic Rh and Pd being the dominant noble metal species present after catalyst activation. By comparison, attempts to activate the catalyst using H2 proved ineffective. XAS and HRTEM showed the formation of the Rh2O3 clusters from RhCl3 after H2 reduction at 400°C. CO reduction showed different behaviour depending on the temperature. Bimetallic Rh-Pd cluster formation occurs within 80 minutes at 400°C during CO reduction. At lower temperatures, no transformation occurred. The metal/support interaction of Rh,Pd/CeO2 is investigated using DFT calculations for tetrahedral M10 clusters on the reduced and oxidised CeO2 support. A differing interaction with the supports is traced to the occupancy of the 4f states of ceria. The metal displays differing behaviour on the support, Pd10 does not show the donation to the support of Rh10. The bonding mechanism was seen to be different for both metal clusters where Pd10 shows a partial ionic mechanism while Rh10 shows only inter-metallic bonding. Rh4Pd6 is found to exhibit a bonding mechanism similar to Rh10. The interfacial Ce 4f display occupancy in the Ce-Rh interaction but remain empty for Ce-Pd interface. Both rhodium containing clusters show significant distortion as compared to their structure in the gas phase where Pd10 does not. The electronic and geometric properties of the interface are presented in delving the stability and activity of the bimetallic clusters.