Study On The Structure-Function Relationship Of Reverse Water Gas Shift Reaction Catalyzed By Titanium Dioxide Supported Catalysts

In recent years,a large number of greenhouse gas CO2 emissions have a serious impact on the earth’s ecological environment.How to eliminate and use CO2 has become a common topic in the world.As one of the important sub-reaction in the CO2 hydrogenation,the reverse water gas shift(RWGS)reaction can effectively eliminate CO2.Meanwhile,it can also use CO2 as a cheap and green carbon sourse to generate CO,and further obtain other high-value carbon derived chemicals through Fischer-Tropsch synthesis to achieve the utilization of CO2.For the RWGS reaction,limited by its equilibrium,it is kinetically and thermodynamically favorable at high temperature.Non-noble metal supported catalysts for the RWGS reaction are commonly used to catalyze high-temperature reactions.They have the advantage of low price,abundant reserves,and high conversion.However,due to the high reaction temperature,the active metals are prone to agglomeration,which leads to the deactivation of the catalyst during the reaction process,greatly affecting the performance of the catalysts.Therefore,in this thesis,the classical support TiO2 was mainly used to load non-noble metal Ni or Co.Through the regulation of the support or the supported metal,the high activity of catalysts could be obtained while maintaining the high stability of the catalyst.Ex-situ and in-situ characterizations were used to explore the morphology,structure and structure-activity relationship of the catalysts.The specific research contents of this thesis are as follows.1.Hydroxylated TiO2 supported stabilized Ni cluster catalysts for the RWGS reactionNi-based catalysts have excellent catalytic performance in the RWGS reaction,however,Ni species are easily agglomerated and deactivated at high temperature,resulting in a decrease in the activity of the catalysts.In this work,we employed a hydrothermal method to reconstruct commercial TiO2 in an alkaline environment and construct-OH groups on its surface.10 wt.%Ni was deposited on it by a simple deposition-precipitation method,followed by air calcination.The results show that this catalyst had excellent catalytic activity,and the CO formation rate was 520.1 μmolCO·gcat-1·s-1 at 500℃,which was several times the rate reported in the literature.At the same time,it still maintained long-term stability for more than 300 h at a high temperature of 600℃ and a high space velocity of 400,000 mL·gcat-1·h-1.By means of HAADF-STEM,in-situ IR,quasi in-situ XPS and other characterization methods,we found that a large number of-OH species on the supports had a stabilizing effect on the active metals on the supports.Meanwhile,the catalysts can in situ generate Ni cluster around 1 nm in the RWGS reaction.The stable Ni clusters contributed to the catalytic performance of the catalysts.This work develops a method to prepare stable cluster Ni species via pre-hydroxylated support,which provides a new strategy to synthesize metal cluster species.2.Co/TiO2 catalysts catalyzed the RWGS reaction to achieve separation of active sites and coking sitesCobalt,as a common transition metal,is widely used in catalytic reactions.However,Co-based catalysts tend to grow carbon nanotubes in a carbon-containing atmosphere,which will cause the coverage of active sites,resulting in a decrease in catalytic activity and stability.In this work,we synthesized TiO2 by hydrothermal method,and successfully prepared supported catalysts with different Co loadings by deposition-precipitation method.By evaluating the catalytic performance of the catalysts for the RWGS reaction,it was found that the 15Co/TiO2-T-500 catalyst had more excellent catalytic performance.By means of XRD,Raman,TEM,XPS and other characterization methods,it was confirmed that the 15Co/TiO2-T-500 catalyst had a large number of positive Co species before the reaction,while the positive Co species still dominated the catalyst after the reaction.Meanwhile,the HRTEM images also showed that there were two forms of Co species on the catalyst:clusters and large particles.A large number of carbon nanotubes were formed near the large particles of Co,but it did not affect the CO2 conversion and stability of this catalyst,which indicated that the coking sites of the catalyst were not the active sites in this reaction,and the Co cluster played a major contribution to the activity.The separation of active sites and coking sites ensures the good catalytic performance of the catalyst,and also provides a new direction for the stability strategy of this type of catalyst.