Cu-CeO₂ Based Catalysts For CO₂ Hydrogenation Reactions: Performance And Structure-activity Relationship

Fixation of cheap and abundant non-toxic CO₂ to high value-added chemicals is a promising powerful way to alleviate its detrimental effects on the environment and to mitigate the energy crisis at the same time.CH₃OH is not only used as a valuable fuel substitute and additive,but also a key feedstock for the synthesis of other high-value chemicals.Syngas?CO+H₂?obtained by reverse water gas shift?RWGS?reaction is an important chemical feedstock for the synthesis of a range of platform chemicals and synthetic fuels through existing processes such as methanol synthesis and Fischer-Tropsch process.Therefore,CO₂ hydrogenation to CO through RWGS reaction and CO₂ hydrogenation to methanol have been regarded as promising ways to utilize CO₂.Cu-CeO₂ based catalysts show good catalytic performance in both reactions.Moreover,the theoretical study of Cu-CeO_x based catalyst model showed that the RWGS reaction is the key step of CO₂hydrogenation to methanol.The study of the active site and CO₂ activation path of Cu-CeO₂based catalyst in the RWGS reaction is helpful for designing efficient Cu-CeO₂ based catalyst for CO₂ hydrogenation to methanol.In the third chapter,the physiochemical properties of CeO₂have been tuned through the morphological modification and then Cu was loaded to prepare Cu/CeO₂ catalysts for the RWGS reaction;then,in situ techniques were employed to investigate the active sites and mechanism of RWGS reaction under the real reaction conditions.The important role of oxygen vacancy on the surface of CeO₂ in CO₂ activation was revealed as well.Based on the results of the third chapter,the previous publication of the Cu/CeO_x/TiO₂?110?catalyst model,a typical Cu-CeOx model catalyst,and the research of Cu/TiO₂?001?nanosheet in our group,CeO₂ was introduced into the Cu/TiO₂?001?nanosheet catalyst to build a highly efficient Cu-CeO_x-TiO₂ catalyst for CO₂ hydrogenation to methanol in the fourth chapter.At the same time,the important promotion roles of CeO₂ in the Cu-CeO_x-TiO₂ catalyst were discussed in details.The main contents and conclusions of this thesis are as follows:?1?Self-assembled CeO₂with 3D hollow nanosphere,nanoparticle and nanocube morphologies were prepared and used to load Cu particles,the obtained samples denoted as Cu/CeO₂-hs,Cu/CeO₂-np and Cu/CeO₂-nc,respectively,which were then tested for the reverse water-gas shift?RWGS?reaction.Under the reaction conditions,V?H₂?:V?CO₂?=3:1and weight hourly space velocity?WHSV?=300,000 m L g^-1 h^-1,the Cu/CeO₂-hs sample exhibited the best catalytic performance among the as-prepared catalysts.In order to reveal the key factors affecting the catalytic performance,the physicochemical properties of the catalysts were analyzed by XRD,BET,SEM,TEM,H₂-TPR,quasi in-situ XPS and in situ UV-Raman techniques.The results point out that the dispersion of Cu is not the rate-determining factor while the amount of surface oxygen vacancies is highly correlated with the catalytic reaction rate.In situ UV-Raman and in situ DRIFTS experiments indicate the formation of bidentate carbonate and formate species on oxygen vacancy sites which is thought to be the key intermediates for the RWGS reaction.These findings unravel the crucial roles of surface oxygen vacancies on CO₂ adsorption and activation.?2?The anatase TiO₂ nanosheet with highly exposed?001?facets was prepared and used to load Cu and CeO₂ by deposition precipitation method.Under reaction conditions,V?H₂?:V?CO₂?:V?N₂?=69:23:8,weight hourly spaced velocity?WHSV?=3600 m L g^-1 h^-1 and pressure=3 MPa,Cu-CeO_x-TiO₂ catalyst with 5 wt%CeO₂ loading displayed the excellent catalytic performance and the maximum methanol yield reached 108.7 mg h^-1 g^-1 at 280?,.The physicochemical properties of the catalysts were studied by SEM,TEM,BET,XRD,H₂-TPR and quasi in situ XPS.The results indicate that the addition of CeO₂ not only promoted the dispersion of Cu species,but also increased the content of oxygen vacancies;both the dispersion of Cu and oxygen vacancies are the key factors affecting the performance of the catalysts.Combined with quasi in situ XPS,CO₂-TPD and in situ DRIFTS,it is revealed that the oxygen vacancies and Ce³⁺at the surface of Cu-CeO_x-TiO₂ participate in CO₂ activation through formation of bidentate carbonate and formate.At the same time,the synergistic transformation of oxygen vacancy disappearance?regeneration,Ce³⁺?Ce⁴⁺occurs under the reaction conditions.The synergistic effect of these defects and highly dispersed Cu⁰ can reduce the activation energy of carbon species,thus promoting the activation of CO₂ and directional conversion.