Preparation Of Highly Efficient Ni/TiO₂ Catalyst In CO₂ Methanation And Low Temperature Carbon Deposition Via Plasma

CO₂ methanation is wildly used in life support system for the spacecraft and potentially functional for solving Greenhouse effect and energy storage problem.Ni-based catalyst has transcendent industrial prospect for its high CO₂ methanation activity and low price.Due to CO₂ methanation is a highly structure sensitive reaction,the morphology and structure of catalyst are directly relavant to the catalytic activity.Therefore,it is of great significant to synthesize benefitial structured catalyst for CO₂methanation.Recent years,dielectric barrier discharge?DBD?plasma attracts great attentions as a new catalyst synthesis technology for its characteristics like low environmental temperature and high energy.In this thesis,we use DBD plasma and water rinse process to synthesize Ni/TiO₂ catalyst,whose principal exposing facet is Ni?111?.The catalyst with Ni?111?as the principal exposing facet shows higher activity with higher methane selectivity.The maximum activity for CO₂ methanation is obtained at350°C with a CO₂ conversion of 73.2%and a CO₂ conversion rate of1.56mol g h at high GHSV of 60,000 h^-1.This rate is higher than those reported CO₂ conversion rates in the literature.According to the DRIFT analysis,CO₂methanation over catalyst with Ni?111?as the principal exposing facet follows the mechanism via CO intermediate,while the catalyst with multi-facets takes the pathway of the direct hydrogenation of formate,with which nickel is only functional for hydrogen dissociation.Ni/TiO₂ catalsyt with Ni?111?as the principal exposing facet inactivate quickly during high temperature.We take high temperature calcinations of the catalyst before reaction in argon atmosphere.It is found that the Ni metal on the catalyst got agglomeration.Meanwhile,the size of Ni metal turned bigger and the dispersion of it decreased.According to the HR-TEM and CO₂ methanation performance test,it is indicated that,after calcinations the activity of Ni/TiO₂ catalsyt with Ni?111?as the principal exposing facet become almost the same as conventional catalyst with multi Ni facets.In the HR-TEM images,Ni?111?exposing facet is jeopardized and developed into multi-facets.Therefore,it is clearly understandable that Ni?111?is not stable during high temperatureIn this thesis,we also studied the low temperature carbon deposition on DBD decomposed Ni/TiO₂ catalyst by CO CVD.It is indicated here that all deposited carbon within the range abounds in defects and edges.As the temperature decreases,the disorder tends to increase while the degree of graphitization becomes lower.The morphology of carbon also changes considerably,from nanotubes to flat plate,then to wrinkled silk graphene-alike films.According to the DSC and H₂-TPR analysis,the deposited carbon can be classified as C_?,C_?and C_?.With the decrease of CVD temperature,the carbon species transforms from C_?to C_?,ultimately to C_?and the reducibility of it increases accordingly.Monoatomic carbon species?C_??,which is the most reaction active,only exists in samples deposited in 350 and 300°C.This may well explain the superb low temperature activity for CO₂ or CO methanation over Ni?111?catalysts.