Preparation and characterization of Co–Rh bimetallic model catalysts: From thin films to dispersed clusters

Abstract

Co–Rh bimetallic model catalysts were prepared as thin films on a Mo(1 0 0) substrate and as dispersed clusters on a SiO 2 support under ultrahigh vacuum conditions. The surface composition and electronic structure of the Co–Rh thin films on the Mo(1 0 0) substrate were characterized with low energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS). Co–Rh thin films form a stable alloy between 700 and 1000 K with substantial surface enrichment of Co. Annealing a 1:1 Co–Rh thin film at 700 K for 30 min yields an alloy with 79% (atomic concentration) of Co in the outmost surface layer. The surface composition and morphology of the Co–Rh dispersed clusters on the SiO 2 support were determined by LEIS, XPS and transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDX). The Co and Rh atoms mixed to form bimetallic clusters between 700 and 800 K with Co segregated to the cluster surface. EDX spectra provided direct proof for the formation of a Co–Rh alloy. TEM images showed that the Co–Rh clusters were highly dispersed on the SiO 2 support and the average particle size of the bimetallic clusters increased monotonically with the total Co fraction.