Abstract
The temperature-programmed reduction of four copper-rich, Rh−Cu/Al2O3 bimetallic catalysts was studied using X-ray absorption near-edge spectroscopy (XANES), infrared spectroscopy using CO as a probe molecule, and electron paramagnetic resonance (EPR). In the initial, fully oxidized state, copper and rhodium do not form mixed oxide phases; that is, only copper aluminates and rhodium oxide are detected. The nature of the copper aluminate is controlled by the Rh loading and pH of the preparation solution. Below 1.0 wt % of rhodium charge, there is extensive formation of a superficial copper aluminate phase, which is coreduced with rhodium oxide to yield an alloyed phase. The bimetallic particles formed have a heterogeneous composition with a Rh-rich core and a surface enriched in copper. Rh is electronically perturbed in these binary particles in relation to a metallic reference and shows a net positive charge. The analysis of the genesis mechanism of the zerovalent phases provides evidence of the critical ...
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