Selective catalytic reduction of NOx with C3H8 using Co-ZSM5 and Co-MOR as catalysts: A model to account for the irreversible deactivation promoted by H2O

• Deactivation of Co-zeolite deNOx catalysts promoted by H2O.
• The Co2+ active sites are lost during deactivation by migrations into the zeolite channels.
• The transformations of Co sites can be described as S1→S2→S3, with S2 as the active site.

A deactivation mechanism and a model are proposed for the H2O-induced deactivation of Co-ZSM5 and Co-MOR catalysts at high GHSV (150,000 and 90,000 h−1) during Selective Catalytic Reduction of NOx (SCR-NOx). The model is based on the analysis of the deactivation curves, as well as DR UV-Vis and H2-TPR, for the structural characterisation of the catalysts before and after long-term hydrothermal operation. We propose that Co2+ active sites are lost during deactivation by migration into the zeolite channels to isolated sites with low SCR-NOx activity and by the formation of cobalt oxide crystallites (CoOx). This process is promoted by the presence of H2O in the reaction medium. The deactivation curves can be modelled as resulting from a solid-state migration properly fitted using an equation for a packed bed reactor (PBR) coupled with the transformation of the cobalt sites in a reaction series S1→S2→S3, with S2 being the active sites for the SCR-NOx reduction reaction. The numerical solution of this model was found to be in good match with our experimental data, suggesting that the re-arrangements of Co in both zeolites by H2O promotion occurred under the same mechanism.

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