Preparation Of Cobalt-based Catalysts And Catalytic Performance In Reactions Of Hydrogen Production

As a renewable and clean alternative to traditional fossil fuels,hydrogen is a promising choice.Therefore,the preparation of high-purity hydrogen and the design,development and utilization of high-efficiency hydrogen production catalysts have attracted much attention.The traditional industrial hydrogen production method is mainly through the burning of fossil fuels.In this process,the unavoidable production,CO,is not conducive to the subsequent reaction.The water-gas shift（CO+H2O=CO2+ H2,WGS）reaction is widely used to eliminate CO in the reaction gas,which is a necessary process for traditional hydrogen production.In addition to using the WGS reaction to remove CO in the reaction gas to obtain high-purity hydrogen,ammonia decomposition with high hydrogen storage density to produce high-purity hydrogen without CO is also widely used.However,considering that noble metal catalysts have limitations in large-scale and low-cost industrial applications,the catalytic performance requirements for reactions and ammonia decomposition reactions in actual industrial applications are more stringent.Thus,it is necessary to develop highly efficient,stablity and clear-structured non-noble metals catalysts.Thorough literature research,we found that the cobalt-based catalyst has certain catalytic activity in the WGS reaction and ammonia decomposition.However,due to the complex structure of the cobalt-based catalyst and the lack of certain in-situ characterization methods,such as the active site in the catalyst,the interaction between the components,the phase change of the catalyst itself,and the reaction mechanism still need to be confirmed.At the same time,for the non-precious metal cobalt-based catalyst,its activity needs to be further improved.Therefore,we have successfully prepared a cobalt-based catalyst with excellent catalytic performance by using the ultrasonic spray method,and added rare earth oxides CeO2 and Sm2O3 to further improve the activity and stability.In addition,a series of in-situ and ex-situ characterization methods were used to determine the phase change of the catalyst in each catalytic system and the interaction between the components,and to identify the active site of the catalyst in the reaction.The specific research contents are as follows:1.Hydrogen production by ammonia decomposition is an endothermic reaction that occurs at high temperatures.Cobalt-based catalysts require higher temperatures（usually above 600℃）to achieve complete conversion.Here,we synthesized various CoaSmbOx catalysts with different contents by ultrasonic spray method.The ammonia decomposition reaction activity is obviously improved after adding samarium element.At the same time,we found that the Co:Sm of the catalyst is 7:3 as the optimal configuration.It has the best catalytic activity for the decomposition of ammonia,and the conversion can be completed at 550℃ at a space velocity of 23,000 mL gcat-1 h-1.Besides,the catalyst has long-term stability due to the addition of Sm and exhibits excellent catalytic performance.We used TEM and SEM to characterize the morphology of the catalyst and found that CoaSmbOx has a fixed morphology and high dispersibility.Combining H2-TPR and in-situ XRD,we further found that there was a certain interaction between cobalt and samarium,which affected the reducibility of cobalt oxide.Using in-situ Raman characterization,it was found that Co0 was the active species of CoaSmbOx catalyst in ammonia decomposition.In addition,through the NH3TPD experiment,it is observed that the addition of Sm is beneficial to the adsorption of NH3.In the end,we successfully obtained a CoaSmbOx catalyst with high activity and good stability through the ultrasonic spray method;copper-cerium combined with a variety of characterization methods,explored the active species in the ammonia decomposition reaction process,and determined the structure-activity relationship of the catalyst.2.For the WGS reaction,since the reaction is difficult to proceed spontaneously,it is very important to find a suitable catalyst for the occurrence of the reaction.We synthesized the cobalt-based composite catalyst CoaCebOx by the ultrasonic spray method,combined with TEM,SEM,XRD,Raman and other characterizations to determine the morphology of the catalyst and the phases before and after the reaction.The catalyst structure is stable and the elements are highly dispersed.It exhibits extremely high activity in the WGS reaction.The most active catalyst is at 240℃ and the space velocity is 42 000 mL gcat-1 h-1.The conversion rate is over 90%.At the same time,the reaction rate of the catalyst at 300℃ is 312.5 μmol gcat-1 s-1,which is several times higher than many non-noble metal catalysts or even noble metal catalysts.Through the long-term stability test,the catalyst did not undergo significant deactivation during the continuous reaction at high space velocity for more than 100 hours,indicating that the catalyst has good anti-sintering ability.Then combined with in-situ and ex-situ characterizations such as H2-TPR,ex-situ/in-situ XPS,in-situ Raman,etc.,it was found that the presence of CeO2 in the CoaCebOx catalyst made it more difficult for Co3O4 to be reduced to elemental Co0.The active species is CoO.Combined with online mass spectrometry to detect the surface reaction of the catalyst,the TPSR experiment showed that the surface of the CoaCebOx catalyst could not directly dissociate water to generate H2,which ruled out the redox mechanism,and further combined with Ar-TPD and other means to verify that the catalyst follows a synergistic mechanism in the WGS reaction.