Preparation Of Supported Nickel And Palladium Based Catalysts And Their Applications In Hydrogenation

The design and preparation of high-performance catalysts are key to improve catalytic efficiency.Ni and Pd are two common active metals in supported metal catalysts.Ni-based catalyst with Ni as the active center has low price and excellent hydrogenation activity.However,Ni-based catalyst is prone to deactivation and instability,which is a major challenge for the application of Ni-based catalyst.Supported Pd-based catalyst has excellent hydrogenation performance,but Pd is a precious metal with high price,which poses a challenge for the industrial application of Pd-based catalyst.In addition,there are some common issues in exploring the hydrogenation mechanism of supported metal catalysts,such as what are the true active sites?What is the specific reaction mechanism?In response to the above issues,we took Ni-based and Pd-based catalysts as representatives of non-noble metal and noble metal supported catalysts,and designed high-performance Ni-based and Pd-based catalysts for different hydrogenation reactions and explored their hydrogenation mechanisms.In the first part of the paper,we prepared a Ni-based catalysts （Ni@Si）for the nitrobenzene hydrogenation.The Ni species in Ni@Si are stabilized by Si O_xligands,exhibiting excellent anti-sintering performance.The yield of aniline reaches 97.6%with a 100%nitrobenzene conversion,demonstrating excellent catalytic performance.In addition,in-depth exploration of catalytic mechanism was conducted,and physical characterization such as XPS confirmed the existence of mixed valence Ni species in the reduced Ni@Si catalyst.Kinetic experiments showed that the N=O group reduction is a decisive step in the entire reaction process.The exploration of the structure–performance relationship revealed the synergetic catalytic mechanism between Ni⁰and Ni²⁺species in the Ni@Si catalyst.Specifically,Ni⁰species activate and dissociate H₂to generate active hydrogen species,Ni²⁺species absorb and polarize the N=O group of nitrobenzene through the lone pair electrons on the O atom.Then,N=O group is reduced by active hydrogen species to generate nitosobenzene.This step is the decisive step of the whole reaction and plays a key role in nitrobenzene hydrogenation.Subsequently,the generated nitrosobenzene reacts with active hydrogen and is rapidly converted to aniline through a series of steps.This part of the work provides a reference for the design of efficient Ni-based catalyst.And it is of great significance to explore the catalytic mechanism of nitrobenzene hydrogenation and active metal with different valence states.In the second part of the paper,Pd-based catalyst for the acetophenone hydrogenation,reductive alkylation,and benzaldehyde hydrogenation was prepared,and oxygen vacancies were introduced in the support,greatly enhancing the catalytic activity of the catalyst.Firstly,the effects of different CeO₂supports,Pd precursors and the reduction temperature of catalyst were investigated.Commercial CeO₂and Pd（NO₃）₂·2H₂O is selected as the support and precursors respectively.And the optimal reduction temperature is 300°C.Subsequently,based on the reduction properties’differences between CeO₂and Al₂O₃supports,Pd/CeO₂and Pd/Al₂O₃catalysts with and without oxygen vacancy were obtained.These catalysts were used to catalyze three significant reactions,that is,acetophenone hydrogenation,reductive alkylation and benzaldehyde hydrogenation.Combining physical characterization such as ICP and CO pulse adsorption,as well as reaction data,the TOF values of each catalyst in the three reactions were calculated,and it was found that the intrinsic activity of Pd/CeO₂is much higher than Pd/Al₂O₃.XPS characterization confirmed the existence of a large number of oxygen vacancies in Pd/CeO₂.Based on literature,we believe that the high activity of Pd/CeO₂is attributed to the presence of a large number of oxygen vacancies in the catalyst,which not only enhance the adsorption of reactants but also promote the dissociation of H₂.This part of the work preliminarily explores the role of oxygen vacancy in catalytic hydrogenation,laying a foundation for further research on the role of oxygen vacancy in our laboratory.