ADSORPTION OF HYDROGEN ON DISPERSED COPPER-RHODIUM BIMETALLIC CRYSTALLITES

Supported bimetallic Rh-Cu/Al2O3 samples of different X-Cu [N-Cu/(N-Cu + N-Rh) atomic ratios] were prepared by impregnating a 3.8 wt % Rh/Al 2O3 sample with different amounts of Cu(NO3)(2) solution and character ized with techniques of hydrogen chemisorption and H-2 NMR spectroscop y. The irreversible hydrogen uptake of the bimetallic samples, measure d from the chemisorption, increases upon impregnating a small amount o f copper but decreases as the X-Cu becomes larger than 0.1. Deuterium atoms adsorbed on the bimetallic samples exhibit only a single H-2 NMR peak. However, both the line width (Delta upsilon) and the chemical s hift (delta) of this peak vary significantly with the X-Ca, ratio. Phe nomena observed from chemisorption and NMR spectroscopy may be explain ed satisfactorily by the formation of two alloy phases, i.e., a rhodiu m-rich phase [(Rh), X-Cu approximate to 0.05] and a copper-rich phase, [(Cu), X-Cu approximate to 0.8], on the surface of bimetallic crystal lites. Detected variations in the Delta upsilon and the delta suggest a fast chemical exchange of the deuterium atoms adsorbed on the surfac es of these two alloy phases. The rate constant of this exchange proce ss is estimated from the variation of by to be k(ex) = (1.4 +/- 0.6) x 10(6) exp(-8.6 +/- 0.8 kJ mol(-1)/(RT)) s(-1). From the average, calc ulated from the delta of H-2 NMR and the X-Cu of the bulk composition estimated from ICP-MS, a surface enrichment of copper on supported Cu- Rh crystallites was indicated.