Supported bimetallic Rh−Cu/A12O3 samples of different X Cu [N Cu/(N Cu + N Rh) 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 X Cu 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 X Cu 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), X Cu ≈ 0.05] and a copper-rich phase, [(Cu), X Cu ≈ 0.8], on the surface of bimetallic crystallites. Detected variations in the Δυ and the δ suggest a fast chemical exchange of the deuterium atoms adsorbed on the surfaces of these two alloy phases. The rate constant of this exchange process is estimated from the variation of Δυ to be k ex = (1.4 ± 0.6) × 106 exp(−8.6 ± 0.8 kJ mol-1/(RT)) s-1. From the average X Cu calculated from the δ of 2H 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.

Pd/AI,O, samples of different palladium loadings were prepared by an incipient wetness technique. The palladium dispersion (0) of each sample was estimated from its isotherm of hydrogen uptake measured volumetrically at 298 K. The isosteric heat of hydrogen sorption (9, including absorption and adsorption) on palladium was determined from a series of uptake isotherms, measured at temperatures in the range 273-673 K, using the Clausius-Clapeyron equation. The value of q varied with the extent of the uptake and indicated three consecutive stages for sorption of hydrogen into palladium crystallites, i.e, chemisorption on the surface of the crystallites, penetration into sublayers of the crystallites to form the a-phase PdH and diffusion into bulk crystallites to form the /I-phase PdH. The dissipated heat of chemisorption (q,) decreased with increasing coverage (8) from 9c 2 22 kcal mol-' at low coverages (0 < 0.25), levelling-of momentarily at q, w 18 kcal mol-' before falling to 8 kcal mol-' at high coverages. The decrease in 9c from zero coverage to the point where it levels off reflects a distribution in the size of palladium crystallites on each sample. The subsequent decrease in qc is attributed to weak chemisorption.

Adsorption isosteres of hydrogen on a 5 wt.% Rh/AI,O, sample have been measured volumetrically in the temperature region between 273 and 523 K. The variation in the heat of adsorption (qad) with the hydrogen uptake ( N , ) was obtained from the isosteres using the Clausius-Clapeyron equation. A linear In P,, us. T ' Clausius-Clapeyron relationship was found at low hydrogen uptakes. From the slope of these linear relationships, a decrease in -qad from 100 kJ (mol H2)-' with an increasing uptake was noticed. This heat was attributed to the dissociative chemisorption of hydrogen. At high hydrogen uptakes, a curved Clausius-Clapeyron relationship was obtained. This curved relationship is attributed to the simultaneous presence of the two weakly adsorbed types, i.e. atomic and molecular adsorptions. The -qad of the atomic adsorption and the molecular adsorption in this weakly adsorbed region have a value of 42 kJ (mol H2)-' and 21 kJ (mol H2)-', respectively.