Catalytic Performance And Structure-property Relationship Of Cu-based Catalysts For CO₂ Hydrogenation Reaction

In recent years,the gradual increase in carbon dioxide and other greenhouse gas emissions has caused global ecological problems.Therefore,reducing CO₂ emissions and increasing CO₂utilization are the keys to solving ecological problems.Hydrogenation is widely used in many industurial productions.Among them,CO₂ hydrogenation can generate many valuable chemical products,such as formic acid/fomate,methanol and so on.Methanol is an important chemical product and an ideal fuel substitute.Therefore,CO₂ hydrogenation to methanol has received extensive attention.Firstly,a series of Cu Zn catalysts were prepared by the traditional co-precipitation method and the forward/reverse deposition-precipitation method respectively,and the influence of the preparation method of the catalyst on its catalytic performance was explored.The results show that the ZnO@Cu catalyst prepared by the forward deposition-precipitation method has the smallest Cu species particle size,specific surface area and the strongest CO₂ adsorption capacity,can effectively adsorb and activate CO₂,and thus has the highest catalytic activity and methanol yield.The Cu@ZnO catalyst prepared by the reverse deposition-precipitation method has the strongest reducing ability,can promote the reduction of Cu O,provide sufficient metal Cu sites for methanol synthesis,and has the highest methanol selectivity.Secondly,a series of Cu@ZnO catalysts and Cu/Si O₂ catalysts were prepared by the reverse deposition-precipitation method and the traditional equal volume impregnation method.The formation mechanism of methanol and CO during CO₂ hydrogenation on Cu@ZnO catalyst was explored.In addition,the effect of reaction conditions on the catalytic performance of Cu@ZnO catalyst was investigated.The results show that,due to the weak adsorption capacity of Cu for CO₂ and the insufficient dissociation capacity of ZnO for H₂ at low temperature,Cu and ZnO alone cannot effectively convert CO₂.There is a synergistic catalytic effect between Cu and ZnO,and the Cu-ZnO interface sites are conducive to the formation of methanol.On the one hand,electrons are transferred from ZnO to Cu,which enhances the adsorption of CO₂and generates methanol through the*HCOO species;on the other hand,a large number of dissociated H atoms are generated on the Cu surface and diffuse to the Cu-ZnOH interface,which promotes the further hydrogenation of the adsorbed CO₂ molecules.In addition,the particle size of Cu species has a greater impact on the formation rate of CO.Smaller-sized Cu particles expose more low-coordination Cu sites and form more and stronger H-Cu bonds.There is an interaction between the H-Cu bond and the O atom in CO₂,which promotes the formation of*COOH.Because the C atom has a stronger affinity with the Cu site,it weakens the C-O bond of*COOH,making the C-O bond easier to break and generate more CO.The reaction conditions have a great influence on the performance of the Cu@ZnO catalyst:as the reaction temperature increases,the CO₂ conversion rate and methanol space-time yield increase significantly,and the methanol selectivity decreases;as the reaction space velocity increases,the CO₂ conversion rate decrease,methanol selectivity and space-time yield increase;the increase of H₂/CO₂ ratio can increase CO₂ conversion rate and methanol selectivity,and the space-time yield of methanol presents a volcanic trend.Finally,a series of Ga-Cu@ZnO catalysts with different Ga content were prepared by reverse deposition-precipitation method,and the effect of Ga promoter on the performance of the catalyst was studied.The results showed that the Ga promoter reduces the particle size of Cu O,increases the surface area of Cu species and the reduction ability,increases the number of active sites on the surface of the Cu@ZnO catalyst.In addition,the number of basic sites on the surface of the catalyst increases,the alkalinity increases,and the adsorption and activation of CO₂ are promoted.Among them,5Ga-Cu@ZnO catalyst has the highest CO₂ conversion rate and methanol yield.