Temperature-programmed reduction of CoOAI2O3 catalysts

doi:10.1016/0021-9517(85)90149-6

Journal of Catalysis

Volume 93, Issue 1, May 1985, Pages 38-54

https://doi.org/10.1016/0021-9517(85)90149-6 Get rights and content

Abstract

It is shown that temperature-programmed reduction (TPR) is a sensitive technique for the characterization of Co- and CoAl-oxidic phases in $CoO Al_{2} O_{3}$ catalysts. Four different reduction regions can be present for $CoO AI_{2} O_{3}$ catalysts, which are assigned to four Co phases (I, II, III, and IV). Phase I (reduction at ca. 600 K in TPR at 10 K/min) consists of Co₃₄ crystallites. Phase II (reduction at ca. 750 K) consists of Co³⁺ ions, in crystallites of proposed stoichiometry Co₃AlO₆ or in well-dispersed surface species. Phase III (reduction at ca. 900 K) consists of surface Co²⁺ ions. Phase IV (reduction at ca. 1150 K) consists either of surface Co²⁺ ions (with more Al³⁺ ions in their surrounding than in phase III) or of subsurface Co²⁺ ions, occurring in diluted Co²⁺Al³⁺ spinel structures or in CoAl₂O₄. Al³⁺ ions influence the reducibility of Co ions strongly. This is explained by polarization of CoO bonds by Al³⁺ ions. Preparation conditions (calcination flow rate and calcination temperature) influence the structure of $CoO Al_{2} O_{3}$ , namely the Co valency, the extent of solid-state diffusion, and the dispersion. Solid-state diffusion of Co²⁺ and Al³⁺ ions occurs above ca. 800 K. The implications of this study for $CoO-MoO_{3} Al_{2} O_{3}$ hydrodesulfurization catalysts are discussed.

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¹: Present address: Koninklijke/Shell-Laboratorium Badhuisweg 3, 1031 CM Amsterdam, The Netherlands.

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