Catalysts Auger maps

Figure 7. Secondary electron micrograph and carbon (KLL) map of untreated catalyst A. photomicrograph obtained at 8kV, O.lnA primary beam B. corresponding carbon (KLL) Auger map obtained in (peak-background)/ background mode, at 8kv, lOnA primary beam.

In a subsequent study, Ertl et al reported on the overall surface composition of a reduced catalyst which had a bulk composition similar to catalyst 1 in the present study. The surface composition of the two samples is, however, vastly different. In particular, the iron content is lower than the iron content of the present sample, in the unreduced state. The levels of potassium and aluminum are, on the other hand, much larger. It was pointed out that the surface composition as determined was consistent with earlier chemisorption experiments, and led to the conclusion that ca 40% of the accessible surface should be covered with potassium oxide. If the potassium and aluminum oxides were indeed present, as suggested, in the form of surface films, then the bulk of the catalyst would consist predominantly of metallic iron. If a sampling depth of 2-4 nm for XPS is assumed, then the analysis presented in Ref. 19 would represent a catalyst which is largely covered by a multilayer of promoter oxides. Consequently, a very much smaller fraction than 60% of the surface would consist of free iron. In accordance with the SEM and Auger map images the activation procedure of Ertl et led to a surface of elemental iron, which is covered almost entirely by a layer of promoter oxides that contain occasional crystals of embedded ternary oxides. 

Figure 3.1. Auger maps from an industrial ammonia synthesis catalyst after reduction, showing the lateral distribution of Fe, K, Al, and Ca at the surface.

Figure 14.30 Auger element distribution maps of Ni and S on the surface of a spent catalyst, obtained by rastering the electron beam across the surface of the catalyst.

The electron beam can scan a surface area systematically in what is called a raster scan. By monitoring the intensity of the Auger spectrum of a particular element during scanning, it is possible to map its distribution on the surface examined. An example of this technique is shown in Figure 14.30, which reveals the concentrations and location of sulfur and nickel on a spent catalyst... 

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