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 pathwaysinSCR working conditions.Herein we address the redoxm echanism underlying Low-Temperature (LT) SCR on Cu-CHA by an integration of chemical-trapping techniques,transient-response methods,operando UV/Vis-NIR spectroscopyw ith modelling tools based on transient kinetic analysis and density functional theory calculations.W es how that the rates of the Reduction Half-Cycle (RHC) of LT-SCR displayaquadratic dependence on Cu II ,t hus questioning mechanisms based on isolated Cu II-ions.W ep ropose,i nstead, aCu II-pair mediated LT-RHC pathway,inwhichNOoxidative activation to mobile nitrite-precursor intermediates accounts for Cu II 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.
Ethylene oxychlorination is the key technology in vinyl chloride (VCM, the monomer of PVC, polyvinyl chloride) production to close the chlorine loop by consuming the HCl released from the former cracking step. Due to the high demand for PVC, this leads to ethylene oxychlorination being one of the most important processes in the industry. This Review covers an indepth analysis of the dynamic nature of active sites for the main and side reactions involved in ethylene oxychlorination, featuring the findings and viewpoints from the dynamic kinetics study and analysis under industrial operating conditions. A unified picture of the mechanism of the surface reactions, and the effect of supports and promoters, has been presented based on the decoupled redoxcycle experiments, which leads to a significantly better understanding of the mechanism and provides valuable guidelines for effective catalyst design. Operando techniques and kinetic tools of the rate-diagram, as well as their application to the study of the redox-cycle in ethylene oxychlorination and kinetic models on both the main product and byproduct, are also reviewed. Perspectives on challenges and new process development and future research focus for better study of the VCM production chemistry are also proposed.
We report a DFT and experimental study of the effects of the surface composition of a Ni/Ag alloy on methane activation and steam methane reforming compared to a pure Ni catalyst.
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