Heterogeneous structures in promoted chromia + alumina dehydrogenation catalysts

Abstract

The catalytic activity and various physical–chemical properties (crystallographic phase, specific surface area, electric conductivity, e.p.r.) of alkali metal doped Al₂O₃+ Cr₂O₃ catalysts (7.5 mol % Cr₂O₃) have been compared before and after dissolution of the superficial layers in water. The active promoters (i.e. K, Rb and Cs) extensively segregate from the bulk to the outer layers. A substantial amount of chromium is simultaneously segregated. Some of the segregated alkali metal ions are associated with segregated chromium, and form a chromate in the regenerated (i.e. oxidized) catalyst. The additional activity brought about by the promoters can be explained by the formation of incompletely coordinated Cr³⁺ upon reduction in isobutane in the first minutes of the catalytic run.