ADSORPTION OF HYDROGEN ON DISPERSED COPPER-RHODIUM BIMETALLIC CRYSTALLITES

Supported bimetallic Rh−Cu/A12O3 samples of different XCu [NCu/(NCu + NRh) atomic ratios] were prepared by impregnating a 3.8 wt % Rh/A12O3 sample with different amounts of Cu(NO3)2 solution and characterized with techniques of hydrogen chemisorption and 2H NMR spectroscopy. The irreversible hydrogen uptake of the bimetallic samples, measured from the chemisorption, increases upon impregnating a small amount of copper but decreases as the XCu becomes larger than 0.1. Deuterium atoms adsorbed on the bimetallic samples exhibit only a single 2H NMR peak. However, both the line width (Δυ) and the chemical shift (δ) of this peak vary significantly with the XCu ratio. Phenomena observed from chemisorption and NMR spectroscopy may be explained satisfactorily by the formation of two alloy phases, i.e., a rhodium-rich phase [(Rh), XCu ≈ 0.05] and a copper-rich phase, [(Cu), XCu ≈ 0.8], on the surface of bimetallic crystallites. Detected variations in the Δυ and the δ suggest a fast chemical exchange of the deuteri...