Catalytic Performance And Structure-property Relationship Of CeO₂-supported Catalysts For CO₂ Hydrogenation Reaction

With the rapid development of renewable hydrogen,carbon dioxide utilization is considered to be an effective way to alleviate the rise of carbon dioxide level in the atmosphere.Among various options of CO₂ utilization,CO₂ hydrogenation to methanol（CO₂+3H₂→CH₃OH+H₂O）has attracted extensive attention all over the world with the so-called"methanol economy".Methanol is not only an important high-quality fuel,but also can be transformed into high value-added chemicals and low-carbon olefins,which has attracted wide attention.At the same time,the reaction can be regarded as a means of hydrogen and/or energy storage.Therefore,the preparation of highly active and selective catalysts for CO₂ hydrogenation to methanol has become the focus of academic research all over the world.In this paper,various catalysts were prepared by various methods with Ce O₂ as the major support.The effects of oxygen vacancy,crystal plane,additives,metal-support interaction and reaction conditions on CO₂ conversion and product selectivity were studied.The main research results are as follows:（1）By changing the way of introducing Ce,0.5%Ir/Ce O₂-R and 0.5%Ir+20%Ce/MCM-41 catalysts were prepared by equal volume impregnation method.The effects of oxygen vacancy and reaction path on the catalytic performance were studied.On the two catalysts,the CO₂ hydrogenation reaction is carried out through the synergy between Ce O₂activated CO₂ and Ir activated H₂,and the enhancement of CO₂ adsorption and activation is due to the H overflow on the Ir active site,which promotes the formation of oxygen vacancies on the support surface.During the reaction process,the chemical adsorption of CO₂ becomes stronger,which will cause CO₂ to occupy a large number of active sites and do not react,thus reducing the number of active sites that can activate CO₂.Therefore,the CO₂ conversion of the two catalysts gradually decreases with the progress of the reaction.In addition,Ce O₂ is over reduced due to hydrogen overflow,Ir species were gradually reduced to Ir⁰ and then agglomerated,which inhibited the adsorption and dissociation of H₂.Therefore,the methanol selectivity on the two catalysts gradually decreased with the progress of the reaction.The doping of Ir species inhibited the redox activity of 0.5%Ir+20%Ce/MCM-41 catalyst and promoted the redox activity of 0.5%Ir/Ce O₂-R catalyst.Therefore,the reaction path of methanol formation on the two catalysts was different.The 0.5%Ir/Ce O₂-R catalyst takes the RWGS（reverse water gas shift）path,As the reaction proceeds,the CO produced gradually increases,resulting in a slight decrease in methanol selectivity.However,0.5%Ir+20%Ce/MCM-41 catalyst adopts formate path.Due to the great changes in the properties of support and iridium during the reaction,the formate path tends to produce bidentate formate（b-HCOO*）intermediate products,which is easier to produce CO than the0.5%Ir/Ce O₂-R catalyst.In addition,the selectivity of the product also depends on the reaction temperature and reaction space velocity.（2）Ce O₂ nanorods（Ce O₂-R）and Ce O₂ nanotubes（Ce O₂-T）were prepared,and Ce O₂nanoparticles（Ce O₂-C）were purchased.Using three different morphologies of Ce O₂ as supports,0.1%Ir/Ce O₂ catalysts with different morphologies were prepared by chemical adsorption method.The effects of Ce O₂morphology on exposed crystal plane,oxygen vacancy concentration,metal support interaction and catalytic performance were systematically studied.The results show that the oxygen vacancy concentration on0.1%Ir/Ce O₂-T catalyst with nanotube structure is the highest,because it is not only easier to form oxygen vacancies at the interface of（110）and（111）crystal planes of Ce O₂-T,but also has strong reducibility and high surface oxygen activity.In addition,Ir and H₂ also contribute to the formation of oxygen vacancies.Ir can provide dissociated H atoms to promote the reduction of oxygen on the surface of Ce O₂.Among them,0.1%Ir/Ce O₂-T catalyst has the highest catalytic activity because it has more redox active oxygen vacancies for adsorbing and activating CO₂.In addition,Ir site improves the adsorption and dissociation of H₂.From the evaluation results,the selectivity of the product basically does not depend on the oxygen vacancy concentration.（3）The doping of Zr not only changes the crystal structure,phase composition,grain size and physical properties of Ce O₂ support,but also increases the concentration of oxygen vacancy.The role of Zr is mainly to increase the number of oxygen vacancies and enhance the adsorption of CO₂.Compared with the support,the oxygen vacancy concentration of gold loaded 1%Au/Ce O₂ and 1%Au/Zr_0.25Ce catalysts also increased,which further increased the strength of the basic site of the support.Among them,the medium strength basic site plays an important role as the active site of methanol synthesis in the process of CO₂ hydrogenation,which may be related to the stronger reducibility of supported Au catalyst.The strong interaction between Au and Zr_xCe support is conducive to the dissociation of H₂.The dissociated H atom can overflow onto the support to promote the formation of oxygen vacancy,which not only greatly promotes the adsorption and activation of CO₂,but also helps to disperse and stabilize Au species.In addition,the activity of 1%Au/Zr O₂ catalyst is the highest(X_CO2=23.3%),because the strength and number of basic sites on 1%Au/Zr O₂catalyst reach the maximum,and it has a large surface area.However,the addition of Zr changed the formation path of methanol,which was not conducive to the formation of CH₃OH.Formic acid in 1%Au/Zr_xCe catalyst is likely to be an intermediate in the formation of methanol.It can be seen from the conditional experiment that higher space velocity is conducive to desorption and rapid formation of methanol.