Catalysts’ Design And Their Catalytic Properties For Selective Reduction Of CO₂ To CO

To solve a series of environmental problems caused by the CO₂ emission,our government aimed to achieve carbon peaking in 2030 and carbon neutrality goals in 2060,respectively.Hence,the technologies for CO₂ reducing and utilization need to be urgently exploited.It is a mature and efficient method to transform CO₂ into CO,followed by CO transformation into high-valued chemicals through reactions like F-T synthesis.The CO₂ reduction to CO can be realized through dry reforming of methane（DRM）reaction,which converting two kind of greenhouse gases.Moreover,the transformation of these two molecules,which are difficult to be active,is also of important scientific significance.Besides that,the reduction of CO₂ to CO can be also realized with the help of H₂,which is the reverse water gas shift（RWGS）reaction.With the rapid development of hydrogen industry,the utilization of green hydrogen in RWGS reaction will achieve the“zero carbon emission process”,making this reaction an important way of CO₂ utilization.However,both DRM and RWGS reaction are endothermic reactions and rapid CO₂ conversion only occurred at high reaction temperature,but conventional supported catalysts usually tend to be deactivate due to metal sintering.Moreover,CO₂molecular is thermodynamic stable due to the linear structure and high bond energy,and thus it is difficult to be activated.Above all,promoting CO₂ activation,reducing reaction temperatures and developing highly stable catalysts are key scientific problems to be solved.In this thesis,an embedded Ni@S-1 catalyst with high-loading Ni nanoparticles in hollow silicalite-1 zeolites shells was designed by controlling the synthesis process,and the catalyst exhibited high activity and stability in DRM reaction.Due to the high capacity of activating CO₂ molecules over transition metal carbides,highly active and stable Fe₃C was developed via.in-situ reaction,and the influencing factors were explored simultaneously.Furthermore,β-Mo₂C nanorods with high surface area were prepared and used in RWGS reaction,which exhibited an even better performance than the Fe₃C under thermal-only condition.Moreover,β-Mo₂C nanorods were coupled with non-thermal plasma in RWGS to explore the synergy between nano-structrured catalysts and plasma.This thesis comprises several sections as following:（1）In attempting to solve the problem of nanoparticles aggregation and carbon deposition over Ni-based catalysts for DRM reaction,we developed a controlled‘dissolution-recrystallization’method of embedding high loading（20 wt%）and uniform（4-5 nm）Ni NPs into the shells of hollow silicalite-1 zeolites.According to the characterizations,the formation-Ni-O-Si-bond during the‘dissolution-recrystallization’process was essential,which linked the Ni O core and S-1 zeolites and kept the strong interactions between Ni O clusters and S-1upon calcination,and a composite structure with high density of uniform Ni NPs embedded into the shells of hollow S-1 zeolites was obtained after H₂ reduction.The sizes of Ni NPs maintained at ca.4-5 nm with increasing Ni loading from 3%to 20%,but the density of active sites have been augmented.Accordingly,the TOFs were well maintained in DRM reaction with Ni content variation.In DRM reaction,the mass specific reaction rate of 20%Ni@S-1 catalyst could reach as high as 20.0 mol _CH4·g_cat^-1·h^-1 at 800℃.Moreover,the embedded structure endowed an outstanding stability and CH₄ and CO₂ conversions were well-maintained in 150hours(800℃,100,000 m L·g^-1·h^-1,without any inert gas).（2）Fe-based catalysts show some advantages like good activity and high CO selectivity in RWGS reaction,and thus they have been well explored.However,the active phase still remain controversial due to the possible various phases in CO₂ hydrogenation reactions.Herein,uniform iron oxide nanoparticles were prepared by calcination of MIL-88A,and it transformed into Fe₃C during RWGS reaction,and Fe₃C was identified as the active and stable phase.The factors that influencing the transformation process were also well explored,including reaction temperatures,reduction degree of iron oxide,and feed gas.The results found that,high reaction temperatures promoted the phase transformation from iron oxide to Fe₃C,and the feed gas ratio of H₂/CO₂ showed a decisive effect on the in-situ formation of Fe₃C and an optimized ratio was2.Moreover,ex-situ synthesized Fe₃C via CO carbonization performed an inferior catalytic performance compared with the in-situ generated Fe₃C due to the unexpected but inescapable carbon deposition,which may cover active sites and restrain mass transfer.（3）To further improve the catalytic performance of carbide catalysts in RWGS reaction,β-Mo₂C nanorods with high BET surface area and abundant pores were prepared,and its BET surface area was 9 times of that overβ-Mo₂C nanoparticles.It showed higher CO₂ conversions but similar TOF values in comparison withβ-Mo₂C nanoparticles in thermal RWGS reaction,which was attributed to the abundant active sites.When non-thermal plasma was coupled withβ-Mo₂C nanorods,it performed an unprecedented intrinsic rate of 13,680μmol_CO2·g_cat^-1·s^-1without additional thermal input.Compared withβ-Mo₂C nanoparticles,β-Mo₂C nanorods showed higher TOFs and CO specific yields.β-Mo₂C nanorods performed stronger interplay with plasma.This may be because that,the unique pores promoted surface discharge and micro discharge,the discharge power was thus increased and“intermediate pathway”formed at the same time.