Study Of CO₂ Hydrogenation To Methanol Over Pd/In₂O₃/SBA-15 Catalyst

In recent years,with the rapid increase of CO₂ emissions,the global warming caused by the greenhouse effect is becoming increasingly prominent,thus seriously threat the ecological balance on the earth.Therefore,the capture and effective use of CO₂ have become a hot research topic in the whole world.Synthesizing highly value-added industrial products such as methanol through the catalytic CO₂ hydrogenation is a very economical and effective way to reduce CO₂ emissions.In this work,the novel In₂O₃-based on SBA-15 mesoporous molecular sieve supported catalysts are designed and prepared,on which the catalytic hydrogenation performances of CO₂ to methanol are investigated systematically.Firstly,the In₂O₃/SBA-15 supported catalysts were designed and prepared and their preparation conditions were optimized.The effects of In₂O₃ loading amounts and preparation methods on the physicochemical properties and catalytic performance in the hydrogenation of CO₂ to methanol of catalysts were investigated.The experimental results showed that the catalytic performances in the CO₂ hydrogenation is directly related to the oxygen vacancies concentration on In₂O₃ surface and the oxygen vacancies concentration is too high or too low will not conducive to the catalytic hydrogenation process.Too low concentration of oxygen vacancies will lead to lack of catalytic activity sites,while too high concentration of oxygen vacancis will cause excessive reduction of surface In₂O₃ to metal In and lose catalytic activity.Therefore,the oxygen vacancies concentration should be kept within a suitable range.When the In₂O₃/SBA-15 catalyst sample was prepared by the citric acid method?CA?and with 10%In₂O₃ loading,the active components obtained good dispersion and suitable concentration of active oxygen vacancies,thus showing the best catalytic activity for CO₂ hydrogenation to methanol.The maximum CO₂ conversion,methanol selectivity and methanol STY are 14.17%,14.50%and 2.3×10^-3 mol·h^-1·g^-1_cat at the reaction temperature of 360?,respectively.In order to further improve the catalytic performances of In₂O₃/SBA-15 catalyst,the Pd/In₂O₃/SBA-15 catalysts were also designed and prepared.The effects of Pd doping on the physicochemical properties and catalytic performances in the hydrogenation of CO₂ to methanol of catalysts were investigated,and the stability test of Pd/In₂O₃/SBA-15 catalysts were also carried out.On the basis of the above works,the CO₂ hydrogenation reaction conditions including pressure and space velocity were optimized to further improve the catalytic performances in the methanol synthesis from CO₂ hydrogenation.The experimental results showed that the active components in all Pd/In₂O₃/SBA-15 catalyst samples with different Pd loadings were well dispersed on the SBA-15 support and no obvious agglomeration was observed.The Pd NPs not only have stronger ability to dissociate and adsorb hydrogen molecules,but also can facilitate the formation of more oxygen vacancies,thus the CO₂ conversion and methanol selectivity of the catalysts were greatly enhanced.The 10%Pd/In₂O₃/SBA-15catalyst sample exhibited excellent catalytic performance,and under the optimized reaction conditions of temperature of 260?,pressure of 5 MPa,gas hourly space velocity of 15000 cm³·h^-1·g^-1_cat,the CO₂ conversion,methanol selectivity and methanol STY can be up to 12.6%,83.9%and 1.1×10^-2 mol·h^-1·g^-1_cat,respectively.After the activity test was running for 12 h,the CO₂ conversion and methanol selectivity almost remained constant?decreased by only 1.5%and 0.4%,respectivly?,which indicated that the Pd/In₂O₃/SBA-15 catalyst had good stability under the hydrogenation reaction conditions.