| Catalysis is the core content of modern chemistry research.As an important branch of catalysis,heterogeneous catalysis is widely used in industrial production and academic research in the fields of energy,environment,and materials due to its excellent characteristics such as easy separation of products,easy regeneration,and environmental friendliness.In recent years,the rapid development of nanomaterials has not only gradually reduced the size of heterogeneous catalysts from the initial bulk to the nanoscale and even the sub-nanoscale,but also provided researchers with rich material modification methods,such as element doping and morphology regulation,to enhance the activity of the catalyst,the selectivity for target products,and the stability under long-term working conditions.Transition metal elements are widely used as catalytic materials because they have abundant d-orbital electrons and multiple spatial orientations of empty d-orbitals.Among them,platinum group noble metal elements represented by palladium(Pd)and 3d transition metals represented by cobalt(Co)have a wide range of applications in many important heterogeneous catalytic processes such as the synthesis of ammonia,Fischer-Tropsch synthesis,and electrochemical hydrogen/oxygen evolution,and have therefore received sustained attention from researchers.However,on the one hand,the actual surface structure of nanocatalysts is extremely rich and complex due to various factors such as morphology,size,valence state,and composition,and on the other hand,the active phase that truly catalyzes the reaction often forms during the activation or reaction process of the catalyst.The lack of complete understanding of the catalyst structure limits people’s understanding of the structure-performance relationship of catalysts.In addition,conventional research methods cannot provide information on catalysts in situ or under real working conditions,which has resulted in a relatively initial level of understanding of catalytic mechanisms.The lack of a precise description of the structure-performance relationship of catalysts,in turn,has led to a lack of theoretical guidance for the design and construction of new catalysts,making catalysis research time-consuming and laborintensive.Therefore,in this thesis,synchrotron X-ray absorption spectroscopy and other nuclear science and technology were used to conduct detailed research at the atomic-molecular level on the structure,catalytic mechanism,and structureperformance relationship of Pd and Co-based catalysts in several typical heterogeneous catalytic processes,including ammonia borane hydrolysis for hydrogen production,selective oxidation of benzyl alcohol,and CO2 hydrogenation to hydrocarbons.The main research content and achievements of this thesis are summarized as follows:1.Oxygen modified CoP2 supported palladium nanoparticles as highly efficient catalyst for hydrolysis of ammonia boraneAmmonia borane(AB)is regarded as a promising chemical hydrogen-storage material due to its high hydrogen content,non-toxicity,and long-term stability under ambient temperature.However,constructing advanced catalysts to further promote the hydrogen production still remains a challenge for the hydrolysis of AB.Herein,we report a novel oxygen modified CoP2(O-CoP2)material with dispersed palladium nanoparticles(Pd NPs)as a highly efficient and sustainable catalyst for AB hydrolysis.The modification of oxygen could optimize the catalytic synergy effect between CoP2 and Pd NPs,achieving enhanced catalytic activity with a TOF number of 532 min-1 and an Ea value of 16.79 kJ mol-1.Meanwhile,reaction kinetic experiments prove that the activation of water is the rate-determining step(RDS).The water activation mechanism is revealed by Quasi in-situ X-ray photoelectron spectroscopy(XPS)and in-situ X-ray absorption fine structure(XAFS)measurements.The activation of water leads to the production of-H and-OH groups,which are further adsorbed on the oxygen atoms in P-O bond and Pd atoms,respectively.In addition,density functional theory(DFT)calculations indicate that the introduced oxygen facilitates the adsorption and activation of water molecules.This novel modulation strategy successfully sheds new light on the development of advanced catalysts for hydrolysis of AB and beyond.2.Revealing the crystal facet effect of ceria in Pd/CeO2 catalysts toward the selective oxidation of benzyl alcoholCeria is an excellent catalyst or support for various oxidation reactions due to its abundant oxygen vacancies and strong metal-support interactions.In this study,ceria nanocrystals were hydrothermally prepared with different morphologies(nanorods,nanocubes,and nano-octahedra)and corresponding exposed crystal facets((110),(100)and(111)).The Pd/CeO2 catalysts were synthesized through a impregnation method by applying these ceria nanocrystals as supports.Selective oxidation of benzyl alcohol,involving multiple catalytic steps,was employed as a model reaction to study the crystal facet effect of ceria on the catalytic activity of various Pd/CeO2 catalysts.The Pd/CeO2 nanorod catalyst exhibits superior performance in each individual step,resulting in an improved catalytic activity.Specifically,the highest oxygen vacancy concentration and the optimum metal-support synergy of CeO2(110)crystal facet facilitate the dissociative adsorption of benzyl alcohol,α-H elimination and O2 activation.As a consequence,the Pd/CeO2-nanorod catalysts deliver a much higher benzyl alcohol conversion(74%)than Pd/CeO2-nano-octahedra(35%)and Pd/CeO2-nanocubes(28%).This work provides in-depth insight into the crystal facet-performance relationship of CeO2 in the selective oxidation of benzyl alcohol and sheds light on the catalyst support design strategy for other multistep reactions.3.The study of morphology effect of Co3O4 for catalytic hydrogenation of CO2 to hydrocarbons.In this study,we synthesized cubic-shaped Co3O4-C nanoparticles with exposed(100)crystal planes and octahedral-shaped Co3O4-O nanoparticles with exposed(111)crystal planes via solvothermal and hydrothermal methods,respectively.The synthesized nanoparticles were then used as catalysts for the CO2 hydrogenation reaction.Performance testing showed that Co3O4-C had higher activity and extremely high CH4 selectivity under all experimental reaction conditions.On the other hand,the main products of Co3O4-O catalysts were CO,CH4,and C2+hydrocarbons.Furthermore,we used temperature-programmed reduction(TPR),temperature-programmed desorption(TPD),quasi in-situ near-edge X-ray absorption fine structure(NEXAFS),and quasi in-situ X-ray photoelectron spectroscopy(XPS)to study the structureperformance relationship of Co3O4-C and Co3O4-O nanoparticles.The higher reducibility of Co3O4-C was favorable for the adsorption of CO2 molecules,which improved the catalytic activity.CO2 and H2 generated formate intermediates on the surface of Co3O4-C,while carboxyl intermediates were generated on the surface of Co3O4-O.The different reaction pathways were the reason for the different catalytic selectivity of Co3O4-C and Co3O4-O.This study not only broadened our understanding of the structure-performance relationship of Co3O4 nanoparticles in CO2 hydrogenation reactions but also provided guidance for the construction of more complex Co-based catalysts in the future. |
PDF Full Text Request