Catalytic Carbon Dioxide Hydrogenation To Methanol On Copper-based And Indium Oxidebased Catalysts

The conversion of CO₂,a widespread carbon resource in nature,into high value-added products can not only reduce the emission of greenhouse gases,but also alleviate the over-dependence on fossil fuels,which is of great significance.Among several developed methods for the chemical utilization of CO₂,CO₂ hydrogenation to methanol using hydrogen produced from renewable energy,not only efficiently utilizes the excess effluent CO₂ from industrial gaseous waste,but also produces a clean and renewable chemical feedstock of methanol.This process achieves the green recycling of carbon,and it is an important research topic,the core of which is to develop highly efficient and stable catalysts for methanol formation.In this work,Cu-based and In₂O₃-based catalysts were prepared and applied to catalyze CO₂ hydrogenation.The relationship between the physicochemical properties and catalytic performance of the catalysts was studied,and possible reaction mechanisms were also discussed.The main content,results and conclusions are as follows:In₂O₃ catalysts with different crystal phases were synthesized via hydrothermal/solvothermal method,and by adjusting the types and compositions of solvent.The effect of crystal phases on the physicochemical properties and catalytic performance was studied.It is shown that the mixture of cubic and hexagonal In₂O₃ not only promotes oxygen vacancy formation,but also enhances medium-strength CO₂ desorption.Hence,the mixed cubic/hexagonal In₂O₃ catalyst exhibits more excellent catalytic performance.In addition,a single-active-site catalytic mechanism was proposed for CO₂hydrogenation to CH₃OH over In₂O₃ catalysts.Different Cu-In intermetallic catalysts were prepared by controlling reduction temperature.The reduction behavior of CuO-In₂O₃ and its effect on catalytic performance were investigated.The results indicate that the formation of Cu₁₁In₉ modulates Cu electronic structure and improves H₂adsorption strength.An interface exists between Cu₁₁In₉ and In₂O₃,illustrating that Cu₁₁In₉ interacts closely with In₂O₃ and thus affects CO₂ adsorption strength.It is also found that H₂ adsorption capacity is not a principal factor and that CH₃OH space-time yield is positively correlated with CO₂adsorption capacity.After 350 ^oC reduction,the obtained catalyst shows a medium H₂ adsorption,excellent CO₂ adsorption and numerous Cu₁₁In₉-In₂O₃ interfacial sites,leading to a high catalytic activity.Furthermore,a catalytic mechanism based on two active sites and their interface was proposed for CO₂ hydrogenation to CH₃OH over Cu-In intermetallic compounds.Cu-In intermetallic catalysts with different properties were also prepared by controlling metal compositions.The influence of metal composition on the Cu-In intermetallic formation and catalytic performance was studied.The results suggest that the phase of Cu-In intermetallic catalysts changes significantly with the Cu:In molar ratio,but only two Cu₁₁In₉ and Cu₇In₃ intermetallic compounds emerge.In addition,the synergistic effect between Cu-In intermetallic compound and In₂O₃ has an obvious effect on the catalytic activity.When the Cu:In molar ratio equals 1:2,a Cu₁₁In₉-In₂O₃intermetallic catalyst is formed,in which the contents of Cu and In₂O₃ are moderate.It is found that the most excellent synergistic effect between Cu₁₁In₉ and In₂O₃ exists in the catalyst,which leads to the highest metallic Cu dispersion,active surface area,surface oxygen vacancy concentration and CO₂ adsorption capacity,and thus to the best catalytic performance.On the basis of the above researches,a Cu-In intermetallic CuIn@SiO₂ catalyst with unique core-shell structure was designed and synthesized.The interaction between Cu and In,and the Cu₂In-In₂O₃interfacial sites were studied in detail.Moreover,the anti-sintering property of the core-shell structured Cu-In intermetallic catalyst was also investigated.After combination of Cu and In,Cu₂In intermetallic compound can be formed.It is found that the strong interaction between Cu and In,which improves the reducibility of the Cu-In bimetallic catalyst and enhances the formation of oxygen vacancies.CuIn@SiO₂ possesses high specific surface area,small metal particle size,high metal dispersion,and thus more interfacial sites can be obtained.All these lead to a high CH₃OH space-time yield.Furthermore,the core-shell structure design endows the metal active phases with superior anti-aggregation properties.Besides In₂O₃ and Cu-In intermetallic catalysts,two new Cu-based catalysts were also studied in this work.The ternary Cu-Ce-Zr mixed metal oxide catalysts were prepared by one step co-precipitation method.The influence of metal compositions on the physiochemical properties and catalytic performance of the catalysts was discussed in detail.The results show that the introduction of Zr⁴⁺leads to the formation of Ce³⁺which promotes the formation of more oxygen vacancies.With the increase of Zr⁴⁺concentration,more net electronic charges can be obtained,which causes more low-coordination oxygen atoms formation,and results in the production of more moderate basic sites.It is found that the ratio of CeO₂/ZrO₂ has a significant influence on the catalytic performance.When the ratio of CeO₂/ZrO₂ equals 1:1,the formed catalyst shows the most excellent catalytic activity.In addition,the catalytic mechanism based on two active sites and their interface is also suitable for Cu-Ce-Zr mixed metal oxide catalysts.TiO₂ nanotube-supported CuO-ZnO-CeO₂ catalysts were synthesized through a deposition-precipitation method.The effects of TNTs content on the structure,physicochemical properties and catalytic performance of the catalysts were investigated.The results indicate that the incorporation of TNTs support into CuO-ZnO-CeO₂ catalysts not only promotes CuO reducibility and improves the metallic Cu dispersion and active surface area,but also enhances CO₂ adsorption and increases the proportion of strong basic sites.Furthermore,the TiO₂ nanotube-supported CuO-ZnO-CeO₂ catalyst with 10 wt.%TNTs possesses the excellent reducibility,high metallic Cu surface area,superior CO₂adsorption and large proportion of strong basic sites,and thus gives the highest CH₃OH space-time yield.