| Supported metal catalysts(SMCs)are a special type of heterogeneous catalysts that consist of nano-metal species(nanoparticles or clusters or single atoms)stabilized on solid supports as the effective catalytic sites.Taking the advantage of porous support,the stability of SMCs can be enhanced,the electronic and geometric features can also be adjusted,which makes SMCs promising candidates in both laboratory research and industrial application.It has become an important branch of heterogeneous catalysis.In recent years,the exploration of the size and composition of nanoparticles and metal-support interaction to provide the best surface properties for specific reactions has become more and more intriguing.Meanwhile,identification the key active sites in SMCs catalysts under catalytic conditions has always been a thematic issue in heterogeneous catalysis,because it is necessary to understand the catalytic reaction mechanism and establish effective structure-activity relationship.To this end,this thesis presents an in-depth study regarding SMCs through two types of catalytic reactions:hydrogenation of nitro compounds and hydrogenation of carbon dioxide.In these two catalytic reactions,several important parameters including the composition and the size of catalytic sites,the nanostructure of supports,the metal-support interaction,the evolution of catalytic sites under realistic conditions and the interaction between intermediates and catalysts,etc.have been well studied.This thesis consists of two parts.In the first part,we focus on the structure and hydrophilicity of Co-N-C catalysts,and the abundance of the CoNx active sites.For this,we carried out a series of studies on the preparation and synthesis of Co-N-C catalysts and their application on hydrogenation of nitro compounds(NCs).In Chapter 2,in order to achieve high activity and selectivity for the catalytic hydrogenation of NCs to amines,a facile solid-state nanocasting approach is developed for the controllable synthesis of Co,N co-doped ordered mesoporous carbon materials(denoted as CoNx-OMCs).Compared with the previous nanocasting synthesis of mesoporous catalysts,the current method requires no solvent and relies on melting and interfacial chemical interactions between silica and the precursors for loading and casting,and chemical coordination among the precursors for the formation and dispersion of the active sites.The resulting CoNx-OMCs possess high surface areas(～941 m2g-1),ordered mesopores(～4.0 nm),high N content(～6.8 wt.%)and abundant CoNx sites and fine metallic Co nanoparticles.With molecular H2 as the reducing agent,the optimized catalyst delivers very attractive catalytic activities(100%conversions),selectivities(close to 100%selectivities)and stability(no obvious performance decay after cycling)in the hydrogenation of a series of NCs carrying diverse groups in aqueous solutions under mild conditions.A comparative study clearly reveals that the CoNx sites,not the metallic Co nanoparticles,are the key active sites for the hydrogenation of NCs.The CoNx sites are found to preferentially adsorb nitro groups,thus activating them and promoting their reduction.A detailed study reveals that the high catalytic performance relies on the synergistic cooperation of the catalyst composition and structure,which are tunable by adjusting the synthetic conditions.In Chapter 3,to address the problem of low efficiency of hydrophobic carbonbase catalysts in aqueous solution,we propose the idea of amphiphilic mesoporous catalysts for selective hydrogenation of hydrophobic NCs in aqueous solution.The result amphiphilic catalyst Co@Co-N-C@SBA-15 with a sandwich-like structure is constructed by a one-step solvent-free melting coating method.The catalyst has an external hydrophilic silica support that facilitates catalyst dispersion in water.It has unique Co-N-C catalytic layers uniformly coated in the inner mesopore surfaces of the silica support,which enhance the selective adsorption and activation of hydrophobic NCs.It has a high surface area(～448.2 m2 g-1)and a uniform mesopore size(～7.0 nm)for fast mass transportation.It possesses ultrafine metallic Co nanoparticles uniformly anchored within the N-doped carbon(N-C)layers for easy magnetic separation.These features make the catalyst excellent for the selective hydrogenation of 4-nitrostyrene to form 4-aminostyrene compared with other control catalysts.The CoNx sites are the intrinsic active sites.They can selectively adsorb and activate the nitro groups other than the vinyl groups,leading to superior selectivity.Water as the solvent results in the best performance compared with typical organic solvents probably because of an enhanced water-mediated hydrogen spillover and transfer.In Chapter 4,the compositional heterogeneity of the two above catalysts which contains various Co species such as Co nanoparticles and single Co atoms,composes a key obstacle to unambiguously identifying the exact the active sites.In this work,we use freeze drying methods,to direct utilization of the well-controlled Co2+/histidine(His)coordination effect,the homogeneous distribution of silica nanospheres to isolate and anchor Co-His complex,and the adjustable binding with N-species related to temperature to sustain high content of atomically dispersed CoNx active sites.Compared with our previous work,this method effectively avoids embedding of Co nanoparticles,and no Co nanoparticles was observed after acid etching,thus a single atomic dispersed CoNx catalyst was obtained.In this chapter,the efficient catalytic effect of CoNx site was further demonstrated.The resultant CoNx-OMCs possess abundant CoNx sites with a high metal loading of 1.7 wt.%,where the highest catalytic activity is achieved in hydrogenation of NCs into amines.With molecular H2 as the reducing agent,the optimized catalyst also delivers excellent chemoselectivity and recyclability for hydrogenation of NCs with various reducible substituents in aqueous solutions under mild conditions.In the second part of the thesis,based on the modification effect of heteroatoms on the electronic structure of metal active sites,and the advantages of the colloidal synthesis methods,we designed a NixP catalyst supported on TiO2 and a single-site Co catalyst,their performance in the CO2 selective hydrogenation reaction and the structure-activity relationship have been studied in depth.In Chapter 5,although organic colloidal methods have being well studied in development of nano-catalysts with well-defined structure,the dynamic surface evolution of capping agent during the pretreatment and catalytic test is often neglected and still elusive.we report an in-situ elucidation of this dynamic process by multiple in situ techniques couple with theoretical simulations,which includes ambient pressure X-ray photoelectron spectroscopy,environment transmission electron microscopy and infrared spectroscopy.Our results show colloidal Ni nanoparticles with different capping ligands leads to different surface evolution under oxidative(air)or reductive gas(CO2+ H2)environments at elevated temperatures,thus result in different catalyst composition and different catalytic selectivity in hydrogenation of CO2.Amine-capped Ni exhibits conventional NiOx-Ni dynamics and provide metallic Ni surface for CO2 methanation.In contrast,phosphine-capped Ni undergoes an evident transition to NixP,which weaken CO binding and achieve a drastic product selectivity change from CH4 to CO(>90%).The study of this dynamic surface evolution inspires us to reconsider the traditional colloidal methods and allow more precisely control of the colloidal materials.In Chapter 6,a single atom Co catalyst supported on TiO2 was synthesized by colloidal methods,in which Co atoms were doped by partially replacing Ti atoms in TiO2 substrate and forming stable Co-O binding.Co species demonstrated a strong metal-support interaction(SMSI)with the TiO2 substrate.In the CO2 hydrogenation reaction,it was shown that the chemical state of Co species—induced by a SMSI—has a major impact on the reaction selectivity.The Co-O binding makes Co species still maintain partial oxidation under catalytic conditions,whereas Co nanoparticles post loaded on TiO2 supports which have much weaker metal-support interaction were totally reduced to metallic Co during CO2 hydrogenation reaction.Compared with metallic Co,partially oxidized Co weakens the adsorption with CO,inhibits its hydronation to CH4,and greatly improves the CO selectivity.It is demonstrated that modulation of the chemical state of metal species by a strong metal-support interaction is important for regulation of the observed selectivity on CO2 hydrogenation reaction. |
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