Palladium/alumina catalysts: effect of the processing route on catalytic performance

Supported precious metal catalysts preparation usually includes two major steps, namely, the production of the ceramic support and its impregnation with the active species in the form of a salt solution, followed by a heat treatment. Alternatively, the active species can be incorporated simultaneously with the preparation of the support by combustion synthesis, which is a straightforward technique to produce homogeneous submicron crystalline oxide ceramic powders without the intermediate decomposition and/or calcination steps. In the present work, 0.5 and 1 wt.% of metallic palladium was incorporated through the impregnation technique in commercial Alcoa A-16 alumina and in alumina prepared by combustion synthesis. Moreover, alumina + Pd mixed powders, with the same nominal compositions, were prepared in a single step by combustion synthesis. All alumina-supported Pd catalysts are shown to be effective in ethanol oxidation reaction to acetic acid. The catalytic performance was investigated in terms of selectivity to acetic acid and total conversion of ethanol as a function of temperature (up to 200 degreesC). The effect of the addition of PdO on the sintering behavior and catalytic performance changes when the incorporation technique is changed. The results obtained suggest that, with impregnation, the metal remains outside the alumina grains and densification mostly occurs by diffusion along grain boundaries. On the other hand, during combustion synthesis the metal is highly dispersed and higher temperatures are required to activate diffusion through the bulk of the grains. (C) 2001 Elsevier Science B.V. All rights reserved.