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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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.

Here we provide new mechanistic and kinetic insights into the functions of oxides on Ni catalysts in methane dry reforming combining kinetic studies with density functional theory (DFT) calculations. Hydrotalcite derived Ni catalysts with a small amount of oxide additive (CeO2, ZrO2, ZnO) as promoters are synthesized and characterized by different techniques, X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 physisorption, H2 chemisorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and thermogravimetric analysis combined with mass spectrometry (TGA-MS).Regarding H2/CO ratio, the CeO2-Ni shows the highest the values along all the temperatures. Moreover, the CeO2-Ni catalyst has the best stability among the four catalysts, while ZnO-Ni experiences the most severe deactivation. Kinetic studies in terms of reaction orders and activation energies are performed and compared to the DFT investigations, to assess the functions of oxide promoters. The CeO2-Ni catalyst shows the lowest apparent activation energy for CO2 activation, and it is also found that forward turnover rate is independent of CO2 partial pressure for all the samples. In DFT calculations, CO2 is more favorable to be activated on the support and the TOF obtained from G plot is in perfect agreement with our experiment value. In addition, it is also found that basicity and electron affinity of different oxide additives can be well correlated to the activation of CO2 and catalyst deactivation. In general, both the increased basicity of oxide and electron affinity of metal help to promote the CO2 activation and enhance the catalyst's stability. We propose that the CeO2-Ni catalyst has best performance for CO2 activation, thus leading to a higher surface oxygen concentration to oxidize the carbon on the catalysts, which prolongs the catalyst's life.

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By introducing Pt atoms into the surface of reduced hydrotalcite (HT)-derived nickel (Ni/HT) catalysts by redox reaction, we synthesized an enhanced active and stable Ni-based catalyst for methane dry reforming reaction. The bimetallic Pt−Ni catalysts can simultaneously enhance the catalyst activity, increase the H 2 /CO ratio by suppressing reverse water−gas shift reaction, and enhance the stability by increasing the resistance to the carbon deposition during the reaction. Kinetic study showed that 1.0Pt− 12Ni reduces the activation energy for CH 4 dissociation and enhances the catalytic activity of the catalyst and lowers the energy barrier for CO 2 activation and promotes the formation of surface O* by CO 2 adsorptive dissociation. It is beneficial to enhance the resistance to the carbon deposition and prolong its service life in the reaction process. In addition, density-functional theory calculations rationalized the higher coke resistance of Pt−Ni catalysts where CH is more favorable to be oxidized instead of cracking into surface carbon on the Pt−Ni surface, compared with Ni(111) and Pt(111). Even if a small amount of carbon deposited on the Pt−Ni surface, its oxidation process requires a lower activation barrier. Thus, it demonstrates that the bimetallic Pt−Ni catalyst has the best ability to resist carbon deposition compared with monometallic samples.