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

摘要

Combined X-ray photoelectron spectroscopy, transmission electron microscopy (TEM), and chemisorption measurements of 0.09-0.30 wt% Pd/alpha-Al2O3 catalysts were performed. Pd(C5H7O2)(2) benzene solutions of varying concentrations were used as precursors. It was found that the core level binding energies of Pd were totally independent of the particle 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 observed. 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 particles, interactive or noninteractive with the support, are formed in the sample with an intermediate concentration (0.20% Pd) showing a clear bimodal distribution by TEM analysis. A coherent explanation of the results emerges considering that two different phases are obtained when 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. [References: 34]