On the Redox Mechanism of Low-Temperature NH3-SCR over Cu-CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle

Cu-CHA is the state-of-the-art catalyst for the Selective Catalytic Reduction (SCR) of NOx in vehicle applications. Although extensively studied, diverse mechanistic proposals still stand in terms of the nature of active Cu-ions and reaction pathways in SCR working conditions. Herein we address the redox mechanism underlying Low-Temperature (LT) SCR on Cu-CHA by an integration of chemical-trapping techniques, transient-response methods, operando UV/Vis-NIR spectroscopy with modelling tools based on transient kinetic analysis and density functional theory calculations. We show that the rates of the Reduction Half-Cycle (RHC) of LT-SCR display a quadratic dependence on CuII, thus questioning mechanisms based on isolated CuII-ions. We propose, instead, a CuII-pair mediated LT-RHC pathway, in which NO oxidative activation to mobile nitrite-precursor intermediates accounts for CuII reduction. These results highlight the role of dinuclear Cu complexes not only in the oxidation part of LT-SCR, but also in the RHC reaction cascade

Highly Active and Stable CeO₂‑Promoted CuCl₂/Al₂O₃ Oxychlorination Catalysts Developed by Rational Design Using a Rate Diagram of the Catalytic Cycle

In this study, we have developed a method to predict the steady-state rate and Cu oxidation state during ethylene oxychlorination from a reaction rate diagram of the individual steps involved in the catalytic oxychlorination cycle. The steady state of the redox cycle is represented by a cross point of the reaction rates of the reduction and oxidation steps as a function of the Cu²⁺ in the rate diagram. Transient kinetics of elementary reactions and steady-state kinetics of the overall catalytic cycle were investigated in an operando study using combined mass and UV–vis-NIR spectrophotometry. The catalytic consequence of the promoters was then evaluated in terms of reduction and oxidation activity as well as number of active sites, site activity, and the catalyst oxidation state at steady state. Results revealed that the neat CuCl₂ catalysts operated at low Cu²⁺ at the steady-state conditions with stoichiometric feed composition, as a result of relatively low oxidation rate of Cu¹⁺. As a consequence of a high content of Cu¹⁺, ethylene conversion and selectivity are low, and the catalyst deactivates rapidly. By the promotion of the CuCl₂ catalyst by K, the reactor operates at a high Cu²⁺ concentration with much improved stability as a result of enhanced oxidation rate, but the catalyst has low activity due to significantly reduced reduction rate. Therefore, the rate diagram has been applied as a tool for a rational design of the CuCl₂-based oxychlorination catalysts, and Ce was proposed as the promoter due to its high promotion to the oxidation and low reactivity with Cu ions. It was found that the activity of the Ce-promoted catalysts increased 8 times compared to the neat CuCl₂ catalyst and moreover significantly improved the stability for the oxychlorination catalyst at steady state, due to the enhancement of both the rates of the reduction and oxidation. It is anticipated that the methodology developed here paves the way for a general method for catalyst design of heterogeneous catalysts where the catalyst undergoes oxidation state changes, in particular in redox reactions