Study On Indium-Based Catalysts For Hydrogenation Of Carbon Dioxide

To tackle climate change,China proposes that the total emission of carbon dioxide（CO₂）will reach its peak by 2030 and achieve the carbon-neutral by 2060.Catalytic hydrogenation of CO₂ to methanol represents a promising pathway for carbon-neutral.Indium oxide（In₂O₃）catalyst with oxygen vacancies can activate CO₂ and H₂,and exhibits an excellent performance of CO₂ hydrogenation to methanol.It can also couple with zeolites to realize the highly selective conversion of CO₂ to hydrocarbons.However,In₂O₃ has low catalytic activity and is easy to sinter,which limit the catalytic performance of In₂O₃ for CO₂ hydrogenation.In this paper,different crystal phases of indium oxide are investigated,and the structure-activity relationship between the crystal phases and the catalytic performance of In₂O₃ is studied due to the importance of the structure of the In₂O₃ phases and the number of active sites to the performance of the catalysts.In order to increase the number of active sites of indium oxide,a series of transition-metal-supported mesoporous indium oxide catalysts were designed and prepared,which improved the activity of indium oxide for CO₂ hydrogenation to methanol and inhibited the sintering of indium oxide.Coupling them with zeolites can realize the highly selective hydrogenation of CO₂ to high value-added hydrocarbons.The primary conclusions are summarized as follows:Combining DFT simulations and in situ characterizations,we investigated the catalytic performance and process of phases transition of In₂O₃ with different crystal phases for CO₂ hydrogenation.The results showed that H₂ adsorption and formation of oxygen vacancies on the cubic indium oxide（c-In₂O₃）surface occurred more readily than on the hexagonal phase（h-In₂O₃）surface,and those oxygen vacancies have stronger adsorption of CO₂ than that of h-In₂O₃,which result in higher catalytic activities of c-In₂O₃ for reverse water gas shift reaction（0.1 MPa）and CO₂ hydrogenation to methanol（3.0 MPa）.At 450℃,the phase and activity of c-In₂O₃ remain constant,while the activity of h-In₂O₃ increases gradually with time on stream,which is caused by a phase transition from h-In₂O₃ to c-In₂O₃.The results of in situ XRD and controlled experiments suggested that the redox reaction induced phase transition,and this phase transition is much faster and deeper at higher temperature.The crystal phase transition occurs through two steps:H₂ reduces h-In₂O₃ to elemental indium and then CO₂ oxidizes elemental indium to c-In₂O₃.The performance of In₂O₃ for CO₂ hydrogenation to methanol can be effectively improved by increasing the number of active sites and supporting metal over c-In₂O₃.Ordered mesoporous indium oxide（meso-In₂O₃）with cubic phase,prepared by mesoporous silica（KIT-6）as a hard template,contains more active sites and exhibits a higher catalytic activity（2-3 times）than that of traditional In₂O₃（nano-In₂O₃）.MesoIn₂O₃ is more stable than nano-In₂O₃.Chemically reaction leads to the sintering for In₂O₃,and the reduction of In₂O₃ by H₂ to elemental indium is the main factor.MesoIn₂O₃ is more difficult to be reduced to elemental indium than nano-In₂O₃,which is an important reason for its excellent stability.Highly dispersed Ni particles supported on the meso-In₂O₃ favors H₂ activation and improves catalytic performance of meso-In₂O₃ for CO₂ hydrogenation to methanol.Coupling metal-supported meso-In₂O₃ with methanol-to-hydrocarbon catalysts（zeolites）can significantly improve the catalytic performance of In₂O₃&zeolites bifunctional catalysts with highly selective conversion of CO₂ to high value-added hydrocarbons.Ni/meso-In₂O₃&zeolites produce a large amount of alkanes,while Co,Pt/meso-In₂O₃&zeolites and In₂O₃/ZrO₂&zeolites can effectively improve the catalytic activity of CO₂ with high selectivity of lower olefins and C5+ hydrocarbons.Adjusting the structure and acidity of zeolites as well as the proximity of the two components can achieve high selectivity of products（lower olefin selectivity>65%,C5+ hydrocarbon selectivity>70%）,while the selectivity of by-product methane is less than 3%.