Fine structure of supported and promoted catalysts as deduced from XPS intensity data

Abstract

AbstractFor model calculations it is assumed that metal spheroids are randomly distributed on spherical support particles of nearly closest‐packed arrangement. Both the metal and the support can be covered with layers or submonolayers of promoter(s) and carbon. The average diameters as well as n, the average number of metal particles on one support sphere, are estimated from total surface area and selective chemisorption measurements and preferably by electron microscopy too. In the computer programme two cases are distinguished depending on whether n < 4 or n > 4, since in the latter case the partial shadowing of coated metal particles by others should also be considered. Electron beams emitted by the metal, the support and their layers are calculated as a function of the ‘angle of latitude’ of the support spheres and the ‘angle of latitude and longitude’ of the metal spheroids. The computation variables are the thickness and sequence of different layers above the metal and the support, the extent of ‘immersion’ of metal particles into the support and/or its layers and the extent of diffusion of promotor cations into the support matrix. The calculated intensities are compared with the measured ones. By this method the probable fine structure of an alkali‐promoted Pd/SiO2 (n < 4) and a sodium‐promoted Ni/(Al2O3 + SiO2) catalyst (n > 4) has been determined.