Quantitative surface analysis of catalysts composition, dispersion and coverage

QUANTITATIVE SURFACE ANALYSIS OF CATALYSTS COMPOSITION, DISPERSION AND COVERAGE 

In most cases, XPS has been u.sed to acquire quantitative data concerning catalyst surfaces. Quantitative data of interest consist of elemental concentration ratios, dispersion of the catalytically active component, or promoter, in the case of supported catalysts, and the coverage of the support by the promoter. For a given loading and support surface area dispersion and coverage are not independent parameters. Dispersion itself is defined as the ratio of surface promoter atoms to total number of promoter atoms. It is correlated with the catalyst activity. 

There arc two principal ways in which surface concentration data may be obtained by the evaluation of XPS intensities. One relies on a first-principle description of photoelectron emission from a solid surface, the other on empirically determined elemental. sensitivity factors (cf. Chapter 4 and 5). Either approach can be used in its simplest form to estimate elemental surface concentration ratios from XPS intensity ratios for a catalyst surface, provided the sample is homogeneous and isotropic, i.e.. all elements are uniformly distributed in the surface layer sampled by XPS. However, a heterogeneous catalyst is in fact just that—heterogeneous it can be multipha.se and of complex structure. Operative words here are porosity, inner and outer surface, texture, segregation, etc. Nevertheless the simple procedures have been used extensively in catalyst characterization studies for straightforward interpretation of XPS in- 

Taking into account dispersion and coverage, for the important class of high-area supported catalysts, appropriate quantitative models have been derived which are based on the idea that such samples can be modeled as stacks of support material sheets each covered by promoter particles (cf Fig. 11 or Ref 63). The surface area of the support and the density of the support material define the corre.sponding sheet thickness d. For example, a typical porous silica is characterized by d = 2.6 nm. When considering this stratified layer model it turns out that the simple expression 

Standard Terminology Relating to Surface Analysis (E673 95c), Annual Book of ASTM Standards. Vol. 03.06, American Society for Te.sting and Materials. West Conshohocken. 1996. p. 845. 

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