Quantitative ESCA surface analysis applied to catalysts: Investigation of concentration gradients

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Abstract
ESCA intensity measurements for quantitative surface analysis can give more information than the average composition in the surface region, e.g. for substances which possess a concentration gradient vertical to the surface. This concerns almost all samples which were subject to solid‐state surface reactions, among them frequently also supported catalysts. Enrichment factors Γ (that is, the measured intensity ratio compared with the ratio corresponding to the stoichiometric composition of the sample) are introduced and calculated for an exponential concentration–depth profile by convolution with the exponential attenuation law for photoelectrons. The two characteristic parameters in a two‐component system (surface concentration x0 and profile parameter n) can be determined independently only by the measurement of two or more Γ values for one sample, varying either the escape angle or the escape depth of photoelectrons. We used the latter method and measured besides the Ni 2p/Si 2p intensity ratios also the Ni 2p/Ni 3p intensity ratios of a series of NiO/SiO2 catalysts. The application of this kind of depth profiling to high surface area substances (an enrichment of one components in the outer region is necessarily connected with a depletion of the same component in deeper regions) complicates the deduced equations and needs further model considerations. We determined the characteristic parameters of the Ni concentration gradient in these catalysts and from them calculated the most important first layer composition. In combination with the analysis of ESCA line positions and shapes and in agreement with the results of other measurements, we were able to set up a model of the catalyst surface structure.