Genesis of surface and bulk phases in rhodium-copper catalysts

The temperature-programmed reduction of four copper-rich, Rh-Cu/Al2O3 bimet allic catalysts was studied using X-ray absorption near-edge spectroscopy ( XANES), infrared spectroscopy using CO as a probe molecule, and electron pa ramagnetic resonance (EPR). In the initial, fully oxidized state, copper an d rhodium do not form mixed oxide phases; that is, only copper aluminates a nd rhodium oxide are detected. The nature of the copper aluminate is contro lled by the Rh loading and pH of the preparation solution. Below 1.0 wt % o f rhodium charge, there is extensive formation of a superficial copper alum inate phase, which is coreduced with rhodium oxide to yield an alloyed phas e. The bimetallic particles formed have a heterogeneous composition with a Rh-rich core and a surface enriched in copper. Rh is electronically perturb ed in these binary particles in relation to a metallic reference and shows a net positive charge. The analysis of the genesis mechanism of the zeroval ent phases provides evidence of the critical role of the support and superf icial interactions between oxidized phases in alloy formation.