Construction Of Hydrogen Storage Ortho Sites Of Alumina Supported Metal Catalysts For Hydrogenation Reation

Al₂O₃ has excellent stability,plasticity,and economy,making it the most commonly used support in industrial supported catalysts.The hydrogen spillover effect has been widely studied in hydrogenation catalysts,which can significantly improve the hydrogenation rate by utilizing the migration of active hydrogen on the support surface.However,as an irreducible support,the inherent properties of alumina determine that the migration of active hydrogen species on its surface is difficult,limiting the hydrogenation performance of alumina supported metal catalysts.Therefore,in the design process of efficient hydrogenation catalysts,it is necessary to develop effective strategies to expand the hydrogen migration and storage functions of irreducible oxide supports,in order to share the burdens of active metals and alleviate competitive adsorption.This paper aims to improve the hydrogenation performance of the catalyst by addressing the poor hydrogen migration ability of irreducible support.Alumina,the most widely used support,was chosen as the model to design a site adjacent to the active metal,which can transfer and store active hydrogen species,referred to as Hydrogen Storage Ortho Sites（HSOSs）.In this paper,it was studied that the HSOSs creation and controllable construction,and synergistic regulation of active site and HSOSs.The geometric distribution relationship and electronic interaction between active metals and HSOSs were investigated.The nature,characteristics,influencing factors of the formation process,and hydrogenation enhancement mechanism of HSOSs were revealed.As a result,we developed a catalyst performance enhancement strategy suitable for various hydrogenation reactions,providing guidance for the design of catalysts on irreducible supports represented by alumina.The specific content is as follows:（1）The control of O species properties in microsphere-shaped alumina was achieved by La elements bulk doping,and HSOSs were successfully created around Pd particles.Through element screening,it was found that rare earth elements had the best effect on support regulation.Systematic research was conducted with La element as the representative.Through bulk phase doping,La was atomically dispersed in the alumina crystal lattice,thereby maximizing the regulation of the local structure and O site properties of the support.The active metal Pd was loaded on the support surface containing HSOSs.It was found that Pd was dispersed in the form of nanoparticles and adjacent to a large number of O（-La）sites on the support surface,and exhibited electronic interactions.D₂ in-situ IR indicated that the HSOSs were the O sites connected to La in the support,capable of adsorbing active hydrogen species.DFT calculation results showed that the energy barrier of active hydrogen migrating from Pd particles to support was reduced by 0.49 e V,which promoted the migration of active hydrogen from Pd to support and stable storage.Thus,the catalytic activity of Pd/La-Al₂O₃ catalyst for hydrogenation of 2-ethylanthraquinone was doubled with a good reusability,and it showed higher activity in hydrogenation reactions of various functional groups（C=O,C=C,-NO₂,and benzene ring）,demonstrating good universality for unsaturated bond hydrogenation using H₂ as the hydrogen source.（2）The formation mechanism and characteristics of HSOSs were detailedly study by regulating the structure of lanthanum-containing supports,in order to achieve controllable construction and reveal structure-activity relationships.The structure of lanthanum-aluminum composite oxide supports was controlled by three preparation methods including coprecipitation,impregnation,and hydrothermal,with Al₂O₃,La₂O₃,and La Al O₃ as reference samples.It was found that La doping into Al₂O₃ crystal lattice to replace the tetrahedral Al position was the best way to construct HSOSs.The coordination structure of La and O in the support determined the maximum number of HSOSs.The in-situ hydrogenation and co-adsorption experiments with hydrogen isotope labeling were designed to study the characteristics of HSOSs in catalysts and the relationship between HSOSs relative quantity and catalytic performance.The results showed that HSOSs were synergistic sites of active metals,which cannot independently dissociate H₂ molecules and rely on the synergy of Pd particles to active hydrogen species.Compared with Pd/La-Al₂O₃-HT with uneven Pd dispersion,the amount of surface-OD_x species of Pd/La-Al₂O₃-CP catalyst with good Pd dispersion increased by 1.5 times when hydrogen adsorption was saturated,which indicated the degree of synergy between HSOSs and Pd particles restricted the effective number of HSOSs.It was found that the hydrogenation efficiency of the catalyst showed a good positive correlation with the effective number of HSOSs.（3）The synergistic control of Ru metal and HSOSs on millimeter-sized spherical alumina particles to inhibit the hydrogen poisoning phenomenon on the Ru particle surface and improve the performance of Ru catalyst for ammonia synthesis.The dispersion state of Ru on spherical alumina is influenced by multiple factors such as adsorption,diffusion,and surface tension,which greatly affect the effective synergy of HSOSs and catalytic performance.Spherical alumina for fixed bed reactors was prepared by improving the oil column forming method to incorporating La into alumina.Furthermore,the citric acid complexation impregnation method was created to balance the adsorption and diffusion rates of Ru precursors on the surface of alumina,achieving uniform Ru dispersion at both macroscopic and microscopic levels on the spherical alumina.The H⁺ionized of citric acid can weaken the interaction between alumina and Ru precursor,allowing Ru to diffuse into the interior of spherical alumina with water molecules,achieving macroscopic Ru dispersion at the millimeter scale.The IR spectra indicated that the carbonyl oxygen of citrate can coordinate with Ru³⁺,thereby avoiding the aggregation of Ru precursors during the impregnation process and achieving microscopic Ru dispersion at the nanoscale.The uniform dispersion of Ru enabled it to achieve maximum synergy with atomic level dispersed HSOSs.Consequently,the ammonia generation rate of Ru/La-Al₂O₃-CA catalyst at 475℃was 4.60 mmol NH₃ g^-1 h^-1,an order of magnitude higher than that of catalysts without HSOSs,and the intrinsic activity of Ru sites were significantly improved.Kinetic studies had shown that the construction of HSOSs caused the hydrogen reaction order over Ru catalysts to change from negative to positive,and the nitrogen reaction order to decrease from around 1 to about 0.5,indicating that hydrogen poisoning on the Ru surface was suppressed and competitive adsorption was alleviated.In addition,the electron-rich Ru site can further promote the dissociation and activation of nitrogen molecules.The two factors significantly reduced the activation energy of ammonia synthesis by more than 60%,and the activation temperature is reduced by 30℃,resulting in a substantial increase in activity.