Catalytic Performance And Structure-Property Relationship Of CeO₂-Supported Catalysts For CO₂ Hydrogenation Reaction

researchers have found that converting carbon dioxide into chemicals is an effective way to slow the increase in atmospheric carbon dioxide concentrations with the rapid development of renewable energy.One of the most important conversion pathways is the hydrogenation of carbon dioxide to methanol（CO₂+3H₂→CH₃OH+H₂O）,known as the"methanol economy",which has attracted extensive attention from researchers around the world.Methanol is not only an important chemical fuel of high quality,but also can be converted into a variety of valuable chemicals and alkenes,which provides the possibility for the conversion of carbon dioxide.This reaction is also regarded as an important means of storing hydrogen energy.The preparation of highly active and highly methanol selective catalysts for the hydrogenation of carbon dioxide to methanol is the focus of academic research worldwide.In this study,CeO₂ was used as the carrier and catalysts with different structures were prepared by adding different auxiliaries and using different preparation methods.The influence of some factors on catalytic performance were researched in depth,such as oxygen vacancy number,oxygen vacancy with different properties,dispersion of precious metals on the carrier surface,reaction temperature.The main research contents of this paper are as follows:（1）The effects of La,Zr,Cr,Ca,Co,Cd,Zn,Bi and K on 2Pd/CeO₂ catalysts were investigated.The rod-like cerium dioxide was selected as the carrier because the oxygen vacancy on the surface of cerium dioxide nanorods is easy to adjust.Different auxiliaries and Pd were impregnated on cerium dioxide nanorods by co-impregnation method.The result showed that the number of oxygen vacancies on the surface of different catalysts had a linear relationship with the catalytic activity.A large number of experimental and characterization results showed that among these additives,Zr additive among these additives could be doped into the cerium lattice,promoting the formation of oxygen vacancy,and enhance the adsorption and activation of carbon dioxide of the catalyst.In addition,it can also promote the dispersion of Pd,leading to the catalyst was endowed with catalytic activity and methanol selectivity.At 240℃ and 2000 L g_cat^-1h^-1,the activity of 2Pd-1Zr/CeO₂ catalyst is 8.1%,and the selectivity of methanol is 37.2%,which is much higher than the 5.9%CO₂ conversion and 31.6%methanol selectivity of 2Pd/CeO₂catalyst.The results of catalytic activity again proved that the addition of Zr additive increased the concentration of oxygen vacancy on the catalyst surface and improved the catalytic activity,while the methanol selectivity was improved because oxygen vacancy promoted the formation of Pd with smaller particle size.The smaller the Pd particle size,the higher the methanol selectivity of the catalyst.（2）The 0.5Pd/CeO₂ and 0.5Pd/CePrO_x catalysts with different Pr doping amounts were prepared by traditional hydrothermal synthesis method and chemisorption method.A series of experiments and characterization were conducted to explore the effects of their physicochemical properties,reducibility,adsorption and activation of carbon dioxide and surface oxygen availability on their catalytic performance.At the same time,the effects of different reaction temperatures,different Pr loads and the concentration of oxygen vacancy on catalyst selectivity were investigated,and the reason for the large difference in catalyst selectivity was revealed.The investigated of the catalytic hydrogenation performance of 0.5Pd/CeO₂ and0.5Pd/CePrO_xcatalysts was measured at 200-300℃ and 3000-12000 L g_cat^-1h^-1.The result showed that the activity of 0.5Pd/Ce_0.8Pr_0.2O_x was 4.6%and the selectivity of methanol was72.6%at 240℃ and 3000 L g_cat^-1h^-1,which were higher than the 3.6%activity and 55%selectivity of methanol of 0.5Pd/CeO₂ catalyst.Finally,combined with the catalytic and characterization results,it was found that there were two types of oxygen vacancies with different properties on the surface of CeO₂,namely active oxygen vacancy and non-active oxygen vacancy with no activity.The increase of reactive oxygen vacancy can improve the catalytic activity by improving the adsorption and activation ability of the catalyst for CO₂.With the increase of non-reactive oxygen vacancy can enhance the adsorption of the catalyst for the reaction byproduct CO,thus improving the selectivity of methanol.The addition of Pr can make CeO₂ produce two oxygen vacancies at the same time,improve the catalytic activity of the catalyst and effectively prevent the formation of the main byproduct CO,finally improved the selectivity of methanol in the product.（3）The reason for the rapid deactivation of 0.1Ir/CeO₂ catalyst was explored.It was found that the catalyst deactivation was caused by the fact that Ir atoms on the catalyst surface were easily reduced to Ir⁰ at high temperature,which resulted in serious agglomeration.thus the catalyst lost the ability to activate H₂,leading to deactivation.Firstly,CeO₂ carrier was evenly dispersed on the surface of SiO₂ carrier to form an isolated CeO₂ nanoisland,and then the chemical adsorption Ir method of evenly dispersed CeO₂ carrier was used to prevent the surface Ir from sintering and causing catalyst inactivation.The experimental and characterization results show that the catalyst of 0.1Ir/8%CeO₂/SiO₂synthesized by us have very high stability,and the activity reached 4.6%at 240℃ and 6 L g_cat^-1h^-1,and the selectivity of methanol is close to 100%,which is not easy to deactivate.No Ir aggregation was observed after the reaction.On the surface of Ir/CeO₂ catalyst,large-scale Ir agglomeration occurred t high temperature,and other physical and chemical properties are basically unchanged.It was concluded that the cause of catalyst deactivation is the aggregation of Ir on the surface.The catalyst designed by us can well prevent agglomeration and obtain a catalyst with high stability,which provides a good strategy for us to solve the problem of catalyst inactivation caused by agglomeration in the future.