Rectifying Contact And Catalytic Performance Of Transition-Metal/N-Doped Carbon Heterojunctions

The design of highly efficient heterogeneous catalyst has a key role for a sustainable and green reaction process with low energy comsuption,fewer additives and side products.Recently cheap transition-metal/N-doped carbon hybrids with physicochemical property of large surface areas and highly thermal/chemical stability make them attractive for various catalysis application.The heterogeneous catalysts of reusable transition metal afford a relatively low reactivity in comparision with noble-metal catalysts,which seriously limited the development of transition-metal/N-doped carbon composite for the industrial application in the future.Vast quantities of reported evidence have been proved that transition-metal/N-doped carbon hybrid has advantages of programmable composition and structure.The construction of highly coupled and controlled metal/support composite structure provides an effective approach to break the restricts of charge state of specific metal surface via electron transfer effect of rectifying metal/support interface,which could enhance the ability of reactants or intermediate adsorbed with catalyst surface to promote the reactivity and chemical selectivity for a specific reaction.Hence,it is highly desirable and significant that relationship between controlled composite structure of rectifying transition-metal/N-doped carbon heterojunctions and catalytic activity was constructed using rational synthesis method,advanced characterization technology and simulation tool for the development of sustainable and green catalysis.In this dissertation,we boost the catalystic performance of rectifying transition-metal/N-doped carbon hybrids via constructing high-density integrated heterojunctions;tuning nitrogen dopent concentration in carbon support;optimizing metal content and decreasing active metal size.In addition,we use them in electrochemical and thermal catalysis application and investigate the work mechanism between the composite structure of metal/N-doped carbon and catalytical activity and selectivity.Detailed research content can bee seen as following:(1)Constructing high-density Co nanaoparticle encapsuled in carbon nanofibers to increase active sites for enhanced HER perforcance:In order to solve the problem that decreasing active sites and poor reactivity induced by supported transition-metal aggregation on N-doped carbon,the carbonization of metal-organic coordination complex nano-assembled on three-dimensional carbon paper could abtain high-density cobalt nanoparticle/carbon nanofibers(Co@CNF).The highly integrated structure of metal/carbon dyads and high concentration of well-dispersed Co nanoparticles can increase active sites and boost the ability of electron transfer from carbon support to substrates for significantly enhanced HER performance.At the same time,the typical embeded structure of metal nanoparticle encased in carbon shell can effectively avoid the oxidation and leaching of metal component to ensure the final catalytic activity and stability comparable with commercial Pt/C catalyst in electrocatalytic hydrogen evolution reaction.This research work reveals the relationship of embeded Co@CNF with high-concentration metal loading and electrochemical HER activity,which provides a new routine to engine and syntheze an efficient electrochecial catalyst.(2)Activating cobalt nanoparticles via the modified N-doped carbon support to enhance its oxidative power:In order to solve the issue that low reactivity of transition-metal/N-doped carbon hybrid for liquid organic oxidation,the Mott-Schottky-type catalyst of nitrogen-rich carbon enveloped cobalt nanoparticles(Co@NC)was sythesized via polycondensation of simple organic ligands and metal salts in the presence of carbon nitride powder.The structural characterization and catalytic activity result of catalyst have indicated that the carbon support modified by high-density of nitrogen dopant was used to tune electron density and reactivity of cobalt nanoparticles.The electron-dificient Co nanoparticle can significantly boost the dehydrogenation ability of the cobalt/carbon dyad.The Co@NC coupled excellent activity exhibited a high TOF value(8.12 mol_{methyl benzoate}/mol _Co h)for oxidation of benzyl alcohol and methanol to methyl benzoate under base-free condiction.Such efficient Co@NC catalyst can achieve diverse alkyl esters and even diesters in good yields.This research work reveals catalytic mechemism of rectifying Co/NC interface promoting liquid organic catalysis via electron transfer effect,which offers a novel activating strategy to enhance the activity of cheap transition-metal catalysts for liquid organic reactions.(3)The cooperative enhancement of accessible rectifying interface for bifunctional catalysis:In order to meet the requirement of transition-metal/N-doped carbon hybrid catalyzed bifunctional hydrogenation-dehydrogenation application,the nitrogen thermal reaction between Co species and nitrogen dopant was explored to establish an excellent tradeoff between the amount of exposed Co/NC interface and the most obvious interfacial synergetic effect via varying the composition of Co/NC hybrid.The bifunctional Co/NC catalyst exhibits good performance for both hydrogenation and dehydrogenation of heteroarene.The ultrahigh turn over frequency(TOF)value of 34mol_substrate/mol_metal h for hydrogenation of quinoline over the optimized Co/NC dyad outperforms the state-of-the-art heterogeneous catalysts.Both experimental and theoretical results indicate the key role of the optimized rectifying contact at the interface in simultaneously reinforcing the hydrogenation activity of electron-deficient Co nanoparticles and dehydrogenation activity of electron-rich nitrogen-doped carbons and ultimately achieving reversible dehydrogenation-hydrogenation reactions.This research work reveals the key importance of the sufficient exposure of the metal-carbon interfaces to boost different reaction activity simultaneously,which opens up the possibility to extend this hybrid in diverse catalytic process.(4)Constructing high-density single Ni atom/N-doped carbon hybrid to enhance metal utilization efficiency:In order to solve the issue of low active metal efficiency in catalytic conversion,using an efficient ligand-stabilized polycondensation approach prepares dense arrays of single Ni atoms dispersed at the N-doped carbon(Ni/NC)catalyst with a high concentration of single-site Ni(9.5 wt.%).The grouping effect of single Ni-N₄ sites in N-doped carbon support enhances electron density at each single Ni-N₄ site to accelerate hydrogen-atom transfer process on singe-Ni-site to for the coupling of benzyl alcohol and aniline into N-benzyl aniline with a turnover frequency(TOF)value of 7.0mol _{N-benzyl aniline}/mol_metal h,surpassing that of stable non-noble-metal-based catalysts in the reported literature by a factor of 2.This research work reveals the correlation between electronic structure of single Ni site and the hydrogen transfer process,which provides an efficient strategy to decrease the gap between the performance of heterogenesou and homogeneous catalysts.