Nanostructural Features of Pd/C Catalysts Investigated by Physical Methods:  A Reference for Chemisorption Analysis

In this work we have determined, on a series of 0.5% Pd/C catalysts, the palladium particle sizes by the following physical techniques:  (i) X-ray diffraction (XRD) line broadening (LB) method, associated with the Rietveld method, (ii) small-angle X-ray scattering, and (iii) transmission electron microscopy. The catalysts, suitably aged at different temperatures (673, 773, 873, and 973 K), had significantly different metal dispersions. Since the XRD-LB technique is not able to measure directly very small metal particles or clusters (roughly ≤25 Å in size), because they give diffuse X-ray scattering spreading out into the background, we have tackled this problem by means of a suitably tailored Rietveld quantitative analysis. This analysis allowed determination of the Pd fraction “visible” in the Voigtian XRD peaks and its average crystallite size using the LB method. As to the nanoparticle size of the undetectable fraction, an average value of 20 Å was assumed, corresponding to the size of a cubooctahedral perfect cluster of the fourth order. Combining all these data, real effective Pd average particle sizes could be calculated and compared with the corresponding values found by CO chemisorption. It was found that a surface Pd/CO stoichiometry of 2 must be assumed, irrespective of Pd dispersion, to get correct values of the average Pd particle size.