Characterization of Silver–Rhodium Bimetallic Nanocrystallites Dispersed on γ-Alumina

A monometallic 3.2 wt% Rh on Al2O3 is used to prepare a series of Ag–Rh/Al2O3 bimetallic samples with different silver molar fractions (XAg=0.00–0.91). Prepared bimetallic samples are characterized by a combination of the techniques of hydrogen chemisorption, X-ray diffraction (XRD), and 2H NMR. The irreversible hydrogen uptake of Ag–Rh/Al2O3 increases on an initial addition of Ag to Rh/Al2O3 (XAg<0.09) but decreases on further increase in XAg. The observed variation in uptake trend is explained in terms of the formation of two alloy phases, i.e., rhodium-rich phase (Rh) and silver-rich phase (Ag), on the surface of the dispersed bimetallic crystallites. Overall, the observed alloying behavior of Ag is similar to that of Cu when they are added onto Rh/Al2O3. However, experimental data from Au–Rh/Al2O3 catalysts indicated a nonalloying nature between Au and Rh. Deuterium species adsorbed on bimetallic Ag–Rh/Al2O3 samples showed a single 2H NMR peak, the chemical shift and linewidth of which changed with the molar fraction of silver. Observed variations in NMR peaks are attributed to a change in the surface composition of alloy crystallites as well as to a fast exchange of deuterium adsorbed on (Rh) and (Ag) phases.