On the structural characteristics of γ-alumina-supported Pd-Cu bimetallic catalysts

Bimetallic gamma -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, H-2 chemisorption, and EXAFS results gave strong evidence that particles of nearly spherical shape, dispersed on supporting gamma -alumina crystallites, are bimetallic. Comparison with Pd/gamma -alumina prepared under identical conditions clearly suggests that the addition of copper decreases the amount of palladium atoms that chemisorb hydrogen. The H-2 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 H-2 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. (C) 2001 Elsevier Science B.V. All rights reserved.