The Study Of Co/TiO₂-ZrO₂ Catalysts For Oxidative Dehydrogenation Of Ethane To Ethene With CO₂

As the main greenhouse gas,CO₂ emission reduction is urgent.The preparation of high-demand ethylene（C₂H₄）by using cheap and non-toxic CO₂ and ethane（C₂H₆）,abundant in natural gas and shale gas,as raw materials can not only alleviate the high carbon emissions in ethylene industry,but also further convert and utilize CO₂,contributing to the realization of the"emission peak"and"carbon neutrality".Co-based catalysts have been widely studied in the oxidative ethane dehydrogenation to ethylene with CO₂,direct dehydrogenation of light alkanes and CO₂ reduction reactions,which is a potential catalyst candidate for industrial applications.Researches have shown thatTiO₂-ZrO₂ composite oxides contribute to the adsorption and activation of ethane and CO₂ because of its acidic and basic properties.In this thesis,a series of Co/TiO₂-ZrO₂ catalysts were prepared for CO₂-ODHE reaction.The binding sites of Co were modified by regulating different Ti/Zr atomic ratios to study the effect of Co binding sites and valence on the reaction pathway.BecauseTiZrO₄ solid solution supported Co catalyst has the best catalytic performance,the loading amount of Co was adjusted to explore the regulatory effect of various Co species on the reaction pathways.1)Single metal oxide supportsTiO₂,ZrO₂ and a series of composite oxide supportsTiO₂-ZrO₂（Ti/Zr ratio ratio=10,5,1,0.2 and 0.1）were prepared by co-precipitation,and impregnated with 3 wt%Co to prepare the investigated catalysts named 3CT,3CZ and 3CTx Z1,respectively.The CO₂-ODHE reaction was evaluated at a space velocity（WHSV）of 9000h^-1 in the feed gas（C₂H₆:CO₂:N₂=1:2:17）.3CT1Z1 exhibited excellent co-activation capabilities of C₂H₆ and CO₂ during the whole process,the C₂H₆,CO₂ conversion and ethylene yield were33.77%,18.03%and 25.13%at 650℃,respectively.Specifically,T1Z1 only exhibited single crystal phase ofTiZrO₄,while other Tx Z1 support exhibited mixed crystal phases includingTiZrO₄ solid solution andTiO₂ or ZrO₂.The formation ofTiZrO₄ solid solution was accompanied with more oxygen vacancies,Ti³⁺and Zr³⁺.The surface interaction of Co-Ti ZrO₄was strong after loading Co,Co was doped intoTiZrO₄.Compared with 3CT and 3CZ,the3CTx Z1 catalyst had larger specific surface area,more oxygen vacancies and M³⁺（M=Ti or/and Zr）,which enabled the catalyst to effectively activate C₂H₆ and CO₂ while maintaining high C₂H₄ selectivity.In addition,pretreatment experiment by various temperature in 5%H₂/Ar of 3CT1Z1 found that low amount of Co⁰ promoted CO₂-ODHE,while high amount of Co⁰accelerated DRE and carbon deposition.2)On the basis of the above studies,in order to explore the influence of various Co species(isolated Co₃O₄ and Co²⁺-Ti ZrO₄)and Co valence evolution on the catalytic performance,Co/Ti ZrO₄ with different Co-loading（0.5,1,3,5 and 7.5 wt%）were prepared and their catalytic performance for CO₂-ODHE reaction was investigated.At low Co-loading（0.5 and 1 wt%）,most of the Co species exist in the structure of Co²⁺-Ti ZrO₄,and catalysts maintain good stability during the reaction process.After reaction,the proportion of Co³⁺decreased,while the amount of Co⁰ was very small,and Co²⁺was the main species.In this case,the catalyst has high C₂H₄ selectivity and relatively low conversion of C₂H₆ and CO₂.At high Co-loading（5 and 7.5wt%）,the aggregated Co₃O₄on the surface ofTiZrO₄ solid solution was easily reduced to generate a large amount of Co⁰ nanoparticles during the reaction,resulting in large amounts of side reactions,serious carbon deposition and obvious catalyst deactivation.When the Co loading was 3 wt%,the interactions between Co andTiZrO₄ solid solution was moderate,which not only promoted the conversion of C₂H₆ and CO₂ with a small amount of Co⁰,but also avoided the accumulation of Co⁰ and caused a large amount of coke.Through the above studies,the properties of Co-metal composite oxide（CoTiO₃）,isolated Co₃O₄ attached to the support surface and Co doped metal oxides(Co²⁺-Ti ZrO₄)in the reaction process was clarified.The important influence of the reduction degree of Co species on the reaction pathway was clear.The modulation effect of Co⁰ content on oxidative dehydrogenation,dry reforming and carbon deposition was verified from multiple aspects,which provided catalyst design ideas for achieving higher ethylene yield.