Studies On Supported Nickel Catalysts For Selective Hydrogenation Of Carbon Dioxide

CO₂ hydrogenation is a hot topic in catalytic research.Supported nickel catalysts have attracted more and more attentions,due to their high activity and low price.They are widely used for CO₂ hydrogenation to methane(i.e.,CO₂ methanation),but their catalytic performance requires a further improvement.A low temperature plasma was employed to control the structural properties of a ZrO₂ supported nickel catalyst,in order to enhance catalytic activity for CO₂ methanation.At present,studies for methanol synthesis from CO₂ hydrogenation over the nickel catalyst are few,and the catalytic activities are lower.To improve the utilization value of the nickel catalyst and broaden its applications in catalysis,we applied an unique support for methanol synthesis,and obtained higher CO₂ conversion and methanol selectivity.We used a dielectric barrier discharge plasma technology replacing traditional calcination decomposition to prepare a Ni/ZrO₂ catalyst.After hydrogen reduction,this catalyst exhibits improved low-temperature catalytic activity for CO₂ methanation.The low-temperature plasma can provide lots of high-energy electrons for rapid Ni O nucleation.Besides,lower bulk temperature and negatively charged sheathes around the Ni O nanoparticles suppress their agglomeration,leading to the formation of a highly dispersed Ni catalyst with smaller Ni particle sizes.We also found that this decomposition process can significantly enhance the Ni-ZrO₂ interaction which effectively promotes H₂ dissociation and hydrogen spillover.Hence,more active H*can be provided for the surface reduction of ZrO₂ to produce rich oxygen vacancies.In a word,this technology can not only control the Ni particle size and structural properties,but also effectively enhance the concentration and intensity of surface oxygen vacancies on the support,to further strengthen the synergistic effect between the Ni active sites and the surface basic sites.The reaction mechanism and kinetics of CO₂ methanation over the plasma-prepared Ni-based catalyst were studied.A series of operando DRIFTS analyses indicate that the sufficient active sites promote the formation of the intermediate carbon species and their rapid conversion,leading to higher low-temperature reaction activity.The principally exposed Ni(111)facets enhance the decomposition of formate and the stable adsorption of CO*.The reaction pathway is therefore changed from a formate-hydrogenation route to a CO*-hydrogenation route.The kinetic study proves that this catalyst possesses much higher pre-exponential factor,indicating more surface active sites provided for CO₂ conversion.To further study the CO*-hydrogenation mechanism for CO₂ methanation and the carbon resistance of the plasma-prepared Ni-based catalyst,we conducted a series of CO methanation tests at different H₂/CO ratios.Compared with the calcined catalyst,the plasma-prepared catalyst shows better resistance to carbon deposition,attributed to that its excellent structural properties enhance the hydrogenation rate of the active carbon formed from CO decomposition.The carbon deposit on the Ni particles is suppressed,resulting in a higher reaction activity and stability.Here we present an In₂O₃ supported nickel catalyst prepared via a wet chemical reduction method for the methanol synthesis.The Ni loading provides highly dispersed Ni species with a strong interaction with the In₂O₃ support.Such active Ni sites contribute significantly to H₂ activation and hydrogen spillover,thereby providing abundant H*atoms for oxygen vacancy creation on the In₂O₃ surface.An enhancement of the oxygen vacancies induced by Na BH₄ treatment was confirmed as well.As a result,an effective synergy between the active Ni sites and surface oxygen vacancies causes a superior catalytic performance of the Ni/In₂O_3-x catalyst,higher than most of reported In₂O₃-based catalysts.Ni catalyst generally acts as an excellent methanation catalyst,but the synergistic effect between the metallic Ni and the defective In₂O_3-x leads to a transition from the methanation to the methanol synthesis.