Investigation Of Ni-based Catalysts And The Reaction Mechanisms For Low-temperature CO₂ Methanation

Recently,CO₂ emissions were greatly increased owing to the massive burning of fossil fuels and led to environmental pollution.Thus,CO₂ conversion and utilization is one of the effective strategies,while CO₂ methanation is of great significance to the sustainable development of the world from energy and the environment.Now,Ni-based catalysts were widely used for CO₂ methanation and were considered one of the potentially active metals for industrialization due to their high activity and low cost.However,the Ni catalysts could not obtain high activity in low temperatures,while the hydrogenation mechanism is still unclear.To resolve the challenges and bottlenecks,contracting new catalytic systems and analysis of the structure-activity relationship will promote the technology’s further development.This work focused on the Ni-based catalyst for CO₂ methanation,regulating the physicochemical properties of the catalysts via different methods,and intends to understand the structure-activity relationship from the aspects of oxygen vacancies,metal-support interaction,hydrogenation routes,and deactivation mechanism.The main research contents of this paper are as follows:Part 1:In this chapter,a series of Ni-Ce catalysts fabricated via impregnation and electrospinning methods,and study the variation of CO₂ methanation performance from the dynamic analysis of intermediates and oxygen vacancies.The Ni NPs@Ce O₂NF catalyst prepared by the co-electrospinning method shows superior catalytic performance with CO₂ conversion of 50.6 and 82.3%at 250 and 300°C,respectively,as well as excellent stability of 60 h at a high temperature of 400°C.The achieved catalytic properties could be attributed to the confined environment and synergistic effect between Ni nanoparticles and Ce O₂ nanofibers.Additionally,in-situ Raman verified that nanofibers can generate more active oxygen vacancies and adsorb CO₂ well.In-situ DRIFTS analysis reveals that the monodentate and bridging bidentate formate were the key intermediates for CO₂ methanation.The catalytic performance of CO₂ methanation was critically related to oxygen vacancies,it is of great importance to understand the roles of oxygen vacancies.Part 2:In this chapter,an optimized hollow Ni/Ce O₂-Co₃O₄ catalyst is shown to be capable of CO₂ absorption and activation,the performance could be attributed to the strong synergistic effects among each component.In addition,the respective role was verified that the Co₃O₄ and Ce O₂contributed to the CO₂ adsorption and subsequently coupled with Ni for CO₂ activation.Furthermore,in-situ DRIFTS were used to demonstrate the hydrogenation mechanisms,and the promotion of CO₂absorption and CH₄ selectivity was confirmed in Ni/Ce O₂-Co₃O₄.The DFT results verified the synergistic effects of the established catalysts,which could also be a general approach for the designation of catalysts.CO₂ absorption and activation play key roles in the process of CO₂ methanation,this work provides new insights for the design of catalysts as the mechanism is still unclear in complex processes.Part 3:The catalysts will easily deactivate after long-term and high temperatures,while poisoning,carbon deposition,and coking are the causes of deactivation,but there are few studies to study the mechanism of deactivation.In this chapter,Ni catalysts with different Ce/Zr alloys as support were prepared by a modified sol-gel method and the structure-activity relationship was studied by series characterization.In-situ Raman was used to determine the existence of oxygen vacancies in different states,and there were two adsorption sites were recognized by CO₂-TPD analysis,and the preliminary analysis was the existence of oxygen vacancies with different strengths.It can be proved that different oxygen vacancies will lead the various strong adsorption sites by DFT results,which confirms the predicted results in the experiment.Moreover,CO was detected as the key factor for the stability from in-situ DRIFTS results,more CO generation will form carbon deposition and accelerate catalyst deactivation.At last,combining the stability and oxygen states verified that the inactive oxygen vacancies generate more CO and accelerate carbon deposition,while highly reactive oxygen vacancies produce less CO and improve stability.This work provides a reference for the understanding of oxygen vacancies.Part 4:In this work,the PT-Ni Fe/Ce O₂ catalyst was fabricated by plasma pretreatment,and the structure-activity relationships were demonstrated during the process.PT-Ni Fe/Ce O₂ catalyst showed considerable improvement compared to the calcined catalysts at low temperatures.The abundant hydroxyl functional group（OH^-）and high H₂ spillover capacity on the surface of the PT catalyst are the main promotion for the significant improvement of catalytic performance.In addition,TPSR was carried out to record the hydrogenation and CH₄ formation temperatures during the catalytic process,which revealed the promotion effect of the PT catalysts.Formate was considered to be the key intermediate in CO₂ methanation by in-situ DRIFTS.This work provides a reference for DBD plasma pretreatment and catalyst preparation,and also provides a research basis for the analysis of H₂ spillover in the reaction.H₂ spillover is a key process in the process of CO₂ methanation.It provides active hydrogen for hydrogenation reaction,which favors the reaction at a lower temperature.