Study On Mechanism Of Carbon Dioxide Hydrogenation To Methanol Based On In₂O₃ Catalysts

Carbon dioxide（CO₂）,one of the greenhouse gases,is an abundant and inexpensive C1 resource on earth.The conversion of CO₂ into raw materials or fuels not only mitigates the greenhouse effect but also alleviates the energy shortage problem.In particular,coupling CO₂ with green hydrogen produced from renewable energy is a green and sustainable way for CO₂ hydrogenation to methanol,which achieves the green recycling of carbon.The development of efficient catalysts for CO₂ hydrogenation to methanol is an important topic with both academic significance and application value.However,CO₂ is kinetically and thermodynamically stable,leading to catalytic conversion of CO₂ is extremely challenging.In this thesis,CO₂ reduction approaches,the thermodynamic analysis of CO₂ hydrogenation to methanol reaction,the research progress of catalysts and the reaction mechanism were reviewed,focusing on In₂O₃-based catalyst and its reaction mechanism.Graphene oxide（GO）modified In₂O₃ catalyst,In₂O₃ hollow tube catalysts with different morphologies and sizes,carbon coated ZnOIn₂O₃ quantum dots（QDs）heterojunction catalyst and ZnO-In₂O₃ hollow tube solid solution catalyst with different ZnO locations were designed and prepared.The roles and status of GO or ZnO on In₂O₃ catalyst in CO₂ hydrogenation to methanol were investigated.The relationship between the microstructure of In₂O₃ and the reaction mechanism of methanol synthesis was revealed by combining different characterization methods and theoretical calculation.The main contents and conclusions were as follows:（1）Rod In₂O₃ composite catalysts modified with different GO contents were prepared by precipitation method via controlling the GO content.The effect of in situ formed c-In₂O₃（440）/hIn₂O₃（110）homojunction on the reaction activity of CO₂ hydrogenation to methanol was studied.It was proposed that the presence of GO significantly promoted the in-situ formation of hexagonal phase In₂O₃（h-In₂O₃）,decreased the In₂O₃ particle size,and inhibited the deep reduction of h-In₂O₃ to metal In.It also promoted the formation of homojunction between h-In₂O₃（110）and c-In₂O₃（440）,and maintained the number of the hexagonal phase and homojunction.The c-In₂O₃（440）/h-In₂O₃（110）homojunction strengthened the interaction between the two phases,motivated the reduction of surface In₂O₃ and facilitated the generation of oxygen vacancies,which was greatly beneficial to the formation of methanol.DFT manifested that formation energy of oxygen vacancies on c-In₂O₃（440）/h-In₂O₃（110）homojunction was lower than that of single h-In₂O₃ or c-In₂O₃,indicating that more oxygen vacancies were created easily at the interface of two phases.With the increase of GO content,the concentration of oxygen vacancy,the adsorption capacity of CO₂ and the space-time yield（STY）of methanol showed the trend of increasing and then decreasing,reaching the maximum at 8%GO.On the In₂O₃-8wt%GO catalyst,the CO₂ conversion was 10.4%,the methanol selectivity was 76%,and the methanol STY could reach up to 0.93 gMeOH h^-1g cat^-1 at 350 ℃.（2）In₂O₃ hollow tubes with different morphology and size using In-MOF as a sacrificial template were prepared by controlling the NaOAc concentrations.The results showed that the size of MIL-68（In）solid hexagonal prism decreased with the increase of the concentration of NaOAc.Too large or too small diameter of MIL-68（In）（NaOAc at 0.2 M or 1 M）led to the collapse of corresponding derivative In₂O₃ tubular structure and only haphazardly accumulated In₂O₃ nanoparticles were retained.However,the In₂O₃ hollow tubular-like structure could be obtained when NaOAc concentrations were in the range of 0.4 M to 0.65 M.The self-assembly MIL-68（In）-0.5 M-derived In₂O₃-0.5 M hollow tube was the most regular and perfect structure formed by the ordered arrangement of In₂O₃ nanoparticles.With the increase of NaOAc concentration,the oxygen vacancy concentration firstly elevated and then dwindled.The variation trend of the adsorption capacity of CO₂ and methanol STY was consistent with that of oxygen vacancy,reaching the maximum on In₂O₃-0.5 M.The methanol formation activity of perfect In₂O₃-0.5 M hollow tube was much higher than that of In₂O₃ catalysts prepared with other NaOAC concentration.In₂O₃ hollow tube nanostructures formed by orderly arrangement of nanoparticles could effectively inhibit the migration and aggregation of nanoparticles.The methanol STY was further improved because the space confinement effect improved the reaction efficiency of oxygen vacancy and the adsorption capacity of CO₂,and prolonged the residence time of the reactants.It achieved the CO₂ conversion of 14.0%,while methanol selectivity remained at 65%over In₂O₃-0.5 M catalyst.The methanol STY was 1.07 gMeOH h^-1g cat^-1.（3）Carbon coated In₂O₃ hollow tube catalysts embedded with ultra-low content ZnO QDs were synthesized using Zn-doped In-MOF as sacrificial template.The effect of ZnO QDs on the reaction activity of CO₂ hydrogenation to methanol over In₂O₃ catalyst was explored.The ZnO/ln2O3 QDs heterojunctions were formed at the interface between ZnO and In-2O3（222）.The ZnO/In₂O₃ heterojunctions,as a key structure to promote the CO₂ hydrogenation to methanol,not only enhanced the interaction between ZnO and In₂O₃ as well as CO₂ adsorption capacity,but also accelerated the electron transfer from In³⁺ to Zn²⁺.ZnO QDs boosted the dissociation and activation of H2.The carbon layer coated on In₂O₃ surface acted as a hydrogen spillover medium,and the dissociated H atoms were transferred to the CO₂ adsorption sites on the In₂O₃ surface through the carbon layer,promoting the reaction of H atoms with CO₂ more effectively.The conductivity of carbon enhanced the electron transfer from In³⁺ to Zn²⁺.The combination of the ZnO/In₂O₃ QDs heterojunctions and carbon layer greatly improved the methanol generation activity.The ZnO-In₂O₃-II catalyst exhibited the strongest adsorption capacity for CO₂ and H2,and the highest methanol yield.At 350℃,the CO₂ conversion of 8.9%with the methanol selectivity of 86%and the methanol STY of 0.98 gMeOH h-1 gcat-1 was obtained over ZnO-ln2O3-II catalyst.（4）ZnO supported at different locations of In₂O₃ hollow tube was used to designed and synthesized ZnO-In₂O₃ hollow tube solid solution catalyst.The effect of ZnO at three different locations inside,outside and on the wall of In₂O₃ hollow tube on the catalytic performance of CO₂ hydrogenation to methanol.All three catalysts were capable of forming solid solution.The presence of ZnO protected the structure of In₂O₃ hollow tubes from being destroyed in the process of CO₂ hydrogenation.For the three catalysts,the reaction process promoted the further in situ formation of ZnO-In₂O₃ solid solution,especially ZnO on the wall of In₂O₃ hollow tubes.After the formation of ZnO-In₂O₃ solid solution,the In and 0 binding ability of In₂O₃ was weakened,which increased the surface electron density of In₂O₃.ZnO not only transferred electrons to In₂O₃,but also facilitated electron delocalization in surface lattice oxygen.The surface electrons were transferred and concentrated in the surface oxygen vacancies to improve the electron density of the surface oxygen vacancies,i.e.,oxygen vacancy "quality" was enhanced,especially for the ZnO-In₂O₃-wall catalyst.The adsorption and activation of CO₂ and H2 was promoted over ZnO-In₂O₃-wall catalyst.The DFT calculation revealed that the formate formation energy barrer was significantly reduced and the carboxylate formation energy barrier was increased on the solid solution surface,indicating that the formation of the solid solution inhibited RWGS reaction and was more conducive to methanol formation.Experiments and DFT calculations confirmed that formate was intermediate in the formation of methanol.The methanol formation activity of ZnO-In₂O₃wall catalyst was the highest.At 350℃,the CO₂ conversion was 13.9%,the methanol selectivity was 68%,and the methanol STY reached 1.12 gMeOH h^-1g cat^-1.