Transient CO Oxidation as a Versatile Technique to Investigate Cu2+ Titration, Speciation and Sites Hydrolysis on Cu–CHA Catalysts: The Cu Loading Effect

Abstract

Abstract The investigation of the ZCu 2+ (OH) − and Z 2 Cu 2+ ions modifications during NH 3 -SCR on Cu–CHA catalysts is a key aspect to clarify the still-debated low-T redox SCR mechanism. In previous works, the dry transient CO oxidation protocol has been employed to identify the generation of dinuclear Cu 2+ structures under conditions representative of the low-T SCR–RHC: NH 3 solvation promotes the inter-cage mobility and coupling of ZCu 2+ (OH) − , acting as the catalytic centers for the CO oxidation process, while Z 2 Cu 2+ results inactive. Herein the same protocol, with pre-stored NH 3 , has been applied to a set of Cu–CHA catalysts with variable Cu loading (0.7–2.4% w/w) but fixed Si/Al: an increasing Cu content produced a net positive effect on the CO 2 production, coherent with a growing ZCu 2+ (OH) − population, while a further enhancement was observed in the presence of H 2 O. The analysis of the integral CO 2 production enabled to predict the maximum CO conversion, corresponding to the titration of the whole ZCu 2+ (OH) − content for each catalyst under dry condition, verifying the initial mechanism. Conversely, in the presence of water, the analysis evaluated an asymptotic titration of the total catalyst Cu 2+ contents. This finding permits to generalize a recent study where combined TRMs, DFT and FTIR were used to probe the complete reversible Cu 2+ sites hydrolysis and pairing in the presence of H 2 O and NH 3 , thus activating the participation of Z 2 Cu 2+ species, too. These results also highlight the versatility and effectiveness of the CO oxidation protocol as a multi-purpose technique to study the Cu 2+ ions in Cu–CHA catalysts.

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