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

Abstract

The investigation of the ZCu2+(OH)− and Z2Cu2+ ions modifications during NH3-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 Cu2+ structures under conditions representative of the low-T SCR–RHC: NH3 solvation promotes the inter-cage mobility and coupling of ZCu2+(OH)−, acting as the catalytic centers for the CO oxidation process, while Z2Cu2+ results inactive. Herein the same protocol, with pre-stored NH3, 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 CO2 production, coherent with a growing ZCu2+(OH)− population, while a further enhancement was observed in the presence of H2O. The analysis of the integral CO2 production enabled to predict the maximum CO conversion, corresponding to the titration of the whole ZCu2+(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 Cu2+ contents. This finding permits to generalize a recent study where combined TRMs, DFT and FTIR were used to probe the complete reversible Cu2+ sites hydrolysis and pairing in the presence of H2O and NH3, thus activating the participation of Z2Cu2+ species, too. These results also highlight the versatility and effectiveness of the CO oxidation protocol as a multi-purpose technique to study the Cu2+ ions in Cu–CHA catalysts.