LOW-LOADED PALLADIUM ON ALPHA-ALUMINA CATALYSTS - CHARACTERIZATION BYCHEMISORPTION, ELECTRON-MICROSCOPY, AND PHOTOELECTRON-SPECTROSCOPY

Combined X-ray photoelectron spectroscopy, transmission electron micro scopy (TEM), and chemisorption measurements of 0.09-0.30 wt% Pd/alpha- Al2O3 catalysts were performed. Pd(C5H7O2)(2) benzene solutions of var ying concentrations were used as precursors. It was found that the cor e level binding energies of Pd were totally independent of the particl e size in the 1.5-7.5 nm range, yet correlated with the metal loading. For the samples with Pd concentrations decreasing from 0.30 to 0.09%, a binding energy (BE) shift of about 1 eV toward higher BE has been o bserved. On the 0.09% Pd sample, a large interaction between metal and support is assumed, leading to the epitaxial growth of the particle. In the 0.30% Pd sample, 6-nm sized bulk-like Pd particles having only a little interaction with the support are obtained. Both types of part icles, interactive or noninteractive with the support, are formed in t he sample with an intermediate concentration (0.20% Pd) showing a clea r bimodal distribution by TEM analysis. A coherent explanation of the results emerges considering that two different phases are obtained whe n Pd(C5H7O2)2 is deposited on cu-alumina; the proportions of these two phases depends upon the concentration of the precursor solution. (C) 1995 Academic Press, Inc.