Structure-Performance Relationship Of Carbon Supported Noble Metal Catalysts In Liquid-Phase Catalytic Process

Nanocarbons are promising catalyst supports because of the high surface area,outstanding stability,and adjustable surface nucleation centers,which are in favor of providing the anchoring sites as well as regulating the chemical and electronic properties of active metal nanoparticles(NPs).The catalytic performance of nanocarbon supported noble metal NPs materials are highly dependent on the nature of functional species on the carbon surface,which can affect the particle size distribution(PSD),chemical state and sintering resistance of supported NPs.Herein,different nanocarbon materials supported platinum(Pt)or palladium(Pd)were used as model catalysts to explore the interactions between metal NPs and doping heteroatoms or functional groups on the nanocarbon surface in some probe reactions,e.g.,methanol electron-oxidation,selective hydrogenation of nitroarenes,and carbon-carbon coupling reaction.In addition,the structural evolution of the catalysts under liquid-phase reaction conditions was carefully studied by the identical location transmission electron microscopy(IL-TEM)method.By comparing and quantifying a series of TEM images,the structural evolutions of the catalysts were explored in detail,and the corresponding structure-function relationships were proposed correctly.The main results consist of the following parts:1.Enhanced stability of immobilized Pt NPs through nitrogen heteroatoms on doped carbon supports:The surface-modified carbon nanotubes(CNTs)prepared by nitric acid oxidation and ammonia post-treatment process were used as supports for Pt NPs.For the purpose of clarifying the critical role of CNTs with different surface properties as supports for Pt NPs,the detailed structural evolution of Pt NPs supported on oxidized as well as nitrogen-doped CNTs under thermal and electrochemical conditions were systemically characterized by in situ&IL-TEM methods.It was observed that both the oxygen functional groups and nitrogen-doped species are responsible for the fine dispersion of Pt NPs,which affords the catalysts with excellent pristine mass activity in methanol electro-oxidation reaction(MOR),while only the nitrogen species can effectively stabilize the Pt NPs under thermal and applied redox potential conditions through the strong metal-support interaction,thus contributing to the improved durability for MOR.Moreover,different degradation procedures occurred on the oxidized as well as nitrogen-doped CNTs supported Pt NPs catalysts were clearly observed by IL-TEM method and the deactivation mechanism of oxidized CNTs supported Pt catalyst was uncovered intuitively.2.Enhanced chemoselective hydrogenation through tuning the interaction between Pt NPs and carbon supports:High-level nitrogen-doped CNTs can be achieved by using nitrogen-containing precursor during the chemical vapor deposition(CVD)growth.The results indicated that the strong metal-support interaction along with the electron-donor capacity of nitrogen-sites on the high amount of nitrogen doping CNTs(H-NCNTs)was capable of stabilizing the Pt NPs and achieving related catalytic recyclability as well as approximately 100%selectivity towards a wide scope of substituent nitroarenes into their corresponding anilines.The good stability under reaction conditions was studied by IL-TEM method,which is the first time,applied to study and compare the catalysts at identical locations at any reaction time in the traditional liquid-phase catalytic process.The result intuitively proved that the introduction of the nitrogen species on the surface of CNTs played a significant role in stabilizing Pt NPs under the reaction conditions.Therefore,the case suggests a new route for the rational design of high-performance hydrogenation catalyst through tuning the electronic interaction between metal NPs and the surface structure of nanocarbon support.3.Exploring the structure evolution of heterogeneous Pd catalyst in Suzuki reaction by IL-TEM and designing of new generation catalyst:Highly dispersed Pd NPs supported on CNTs as a model catalyst before and after Suzuki-Miyaura coupling reactions were systemically characterized by modrnm electron microscopy.Through the hot filtration experiment,the catalytic active species,reaction mechanism and deactivation procedures of heterogeneous Pd catalyst were thoroughly uncovered.Leached single Pd site caused by the strong adsorption of reactants was responsible for the products and the strength for different substrates follows the order of iodobenzene>phenylboronic acid>bromobenzene.Meanwhile,the delicate structure evolution of Pd/oCNTs catalyst at different reaction stages was further studied by the IL-TEM)method and dissolution,deposition and growth,three typical sequential processes of supported Pd NPs along with the increase of the conversion during the reaction were intuitively demonstrated,which strongly implies a leached soluble Pd mediated quasi-homogenous reaction pathway.Additionally,the analysis results from the high-resolution TEM images obtained by IL-TEM investigation show that the low coordinate sites on the surface of Pd NPs,such as corners and edges would preferentially dissolve into reaction solution and the grown particle after reprecipitation process undergo a surface reconstruction in sequence.The performed work provides a valuable reference for further understanding of the mechanism of heterogeneous Pd catalyzed Suzuki coupling reaction and gives reasonable guidance for the development of the new generation of efficient and reusable catalysts for synthetic applications.