On the structural characteristics of γ-alumina-supported Pd–Cu bimetallic catalysts Original Research Article

Bimetallic γ-alumina-supported Pd–Cu catalysts with different Pd:Cu atomic ratios (2:1, 1:1, 1:2) and different total metal loadings (Pd+Cu: 6.1, 1.3, 0.65 wt.% at constant Pd:Cu=2:1) were prepared by pore volume successive impregnation technique. The materials were characterized by means of X-ray powder diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, chemisorption of hydrogen, transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDX). TEM/EDX, H2 chemisorption, and EXAFS results gave strong evidence that particles of nearly spherical shape, dispersed on supporting γ-alumina crystallites, are bimetallic. Comparison with Pd/γ-alumina prepared under identical conditions clearly suggests that the addition of copper decreases the amount of palladium atoms that chemisorb hydrogen. The H2 chemisorption and EXAFS data indicated a dilution effect of Pd ensembles by copper atoms. It was found that both surface composition and bulk structure of the bimetallic particles varied with the Pd:Cu atomic ratio, while the size of particles did not change significantly. On the other hand, the Pd–Pd and Pd–Cu coordination values demonstrated that higher total metal loading at a constant Pd:Cu atomic ratio resulted in the formation of larger bimetallic particles. The highest H2 chemisorption ability and the smallest bimetallic nanoparticles with nonuniform distribution of Pd and Cu were observed for Pd–Cu catalyst (Pd:Cu=2:1) previously reported, that exhibited the highest selectivity in a liquid-phase nitrate reduction.