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Metal/porous carbon composites for heterogeneous catalysis: old catalysts with improved performance promoted by N-doping Yueling Cao, Shanjun Mao, Mingming Li, Yiqing Chen, and Yong Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02335 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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ACS Catalysis

Metal/porous carbon composites for heterogeneous catalysis: old catalysts with improved performance promoted by N-doping Yueling Cao,‡ Shanjun Mao,‡ Mingming Li, Yiqing Chen, Yong Wang* Advanced Materials and Catalysis Group, Department of Chemistry, Zhejiang University, Hangzhou 310028, P. R. China Corresponding Author *

Corresponding author at: Advanced Materials and Catalysis Group, ZJU-NHC

United R&D Center, Department of Chemistry, Zhejiang University, Hangzhou 310028, P. R. China. E-mail address: [email protected]. ‡ These authors contributed equally to this work.

ABSTRACT

Developing novel and efficient catalysts is always an important theme for heterogeneous catalysis from fundamental and applied research points of view. In the past, carbon materials were served as supports for numerous heterogeneous catalysts because of their fascinating properties including high surface areas, tunable porosity, and functionality. Recently, the newly emerging N-doped carbon supported metal

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catalysts have arguably experienced great progress and brought most attention over the last decades in view of the fact that nitrogen doping can tailor the properties of carbon for various applications of interest. Compared with pristine carbon supported metal catalysts, these catalysts normally show superior catalytic performance in many heterogeneous catalytic reactions because of the introduced various metal-support interactions from N doping. In this perspective, we focus on the fabrication methods for N-doped carbon supported metal catalysts and the catalytic application of these fascinating

catalysts

in

several

industrially

relevant

reactions,

including

hydrogenation, dehydrogenation, oxidation and coupling. Notably, we try to elucidate the structure-activity correlations obtained from theoretical calculation, extensive characterization and observed catalytic performances, thereby providing guidance for the rational design of advanced catalysts for heterogeneous catalysis. KEYWORDS: N-doped carbon, metal/N-doped carbon, heterogeneous catalysis, hydrogenation, dehydrogenation, oxidation, coupling 1. Introduction Since the term “catalysis” was introduced as early as 1836 by Berzelius, catalysis has increasingly become of vital importance to the world’s economy.1, 2 It has been estimated that over 80% of all chemical processes in the chemical, food, pharmaceutical, automobile and petrochemical industries use catalysts.3 It is notable that among the numerous catalysts used, carbon supported metal catalysts have been viewed as one of the most important and common catalysts over past decades due to carbon-based support normally showing high specific surface area, large pore volume, good electricity and thermal conductivity, and low cost of manufacture.4-8 For


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example, the Pd/C catalysts are firstly commercialized carbon-supported metal catalysts and used industrially for the manufacturing of organic fine chemicals.4, 9-11 Later, in 1994, the Pd/C catalyst was found to be also active for Suzuki-Miyaura reaction, which is extraordinarily useful for carbon–carbon bond formation.12 Now, Pd/C catalyst has been extensively used for hydrogenation, hydrogenolysis and hydrodechlorination reactions, etc. Among various carbon-based catalysts, activated carbon supported metal catalysts are the most investigated and widely applied in chemical industry.13-16 Unfortunately, activated carbon suffers from limitations such as the microporous structure, which restricts the mass transfer of molecules in reactions, and the insufficient anchoring sites for the active phase, which leads to the non-uniform dispersion of metal NPs (nanoparticles) on the carbon surface and further easy aggregation and leach of these NPs. Therefore, highly active carbon materials with tunable porosity and surface chemistry are desperately required to obtain metal/carbon catalysts, which show great industrial prospect. In the past decades, much attention has been paid to the fabrication of heteroatom (such as B, N, F, P, S et. al) decorated porous carbon materials.17-28 The incorporation of heteroatoms in carbon materials considerably broadens their potential applications because of the favorably modified surface and bulk properties. Among various possible dopants, heteroatom nitrogen has been the most intensively studied option. In fact, since Rideal and Wright29 first reported that the presence of nitrogen atom in a carbon matrix has a promotional effect on the catalytic activity of carbons in the oxalic acid oxidation in 1926, the influence of nitrogen on the improvement of the catalytic performance of metal/N-doped carbon catalysts in heterogeneous catalytic reactions has attracted much attention.30, 31 Now, it is believed that the doped nitrogen


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can alter the acid-base properties of the support surface, increase the electronic interaction between metal and support, and accelerate the electron transfer in the catalytic system. To date, carbon-based nanomaterials, including porous carbons, N-doped carbons, and N-doped carbon supported metal/metal oxide composites have been widely applied in the energy and environmental areas, such as batteries (e.g., lithium ion),32-34 supercapacitors,35-37 as well as electrocatalysis (e.g., hydrogen/oxygen evolution reactions),38-42 which have already been reviewed in recent excellent papers.43-45 The application of metal/N-doped porous carbon composites in heterogeneous catalytic reactions has been a research hot spot in the last decade. In this perspective, we will mainly focus on the recent developments of N-doped carbon supported metal and/or metal oxide catalysts for heterogeneous catalytic applications, including the catalyst preparation techniques and their catalytic performance for reactions involved in hydrogenation, dehydrogenation, oxidation and coupling, during which, various N-doped carbon materials and the possible effect of heteroatom N are also highlighted. We hope that our perspective can be of assistance in sorting out the role of heteroatom nitrogen in carbon-based catalysts and catalytic reactions, and for developing advanced carbon-based catalyst with the desired structure and composition for boosting heterogeneous catalysis in sustainable chemistry. 2. N-doped carbon materials Normally, nitrogen doping means to chemically attach or incorporate nitrogen atoms into the backbone of the carbon materials. In fact, both “N-doped” and “N-functionalized” carbon materials are widely used to describe carbon materials with N atoms incorporated into the backbone and/or on the surface. However, doping is more often used to refer to the incorporation of N into the bulk while functionalization,


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in most cases, means the N dopants on the carbon surface. In general, N containing carbon materials synthesized by in situ synthesis approach, such as pyrolysis of various precursors containing both nitrogen and carbon, will possess N atoms both in the backbone and on the surface,46-56 while those obtained by post-treatment methods, such as pyrolysis of carbon material in ammonia atmosphere, will possess N atoms largely on the surface.57-63 Unfortunately, identification of the exact position of N atoms is still very difficult because of the limited characterization techniques and the presence of abundant defects in carbon materials.64 In general, the N-doped carbon materials can be obtained by modification of many types of well-studied carbon nanostructures including active carbon, carbon nanotubes, carbon nanofibers, and graphene, which are already commercialized and widely applied in environment and energy areas.43, 65-68 The methods of introducing N atoms into carbon materials have been described by our and others’ some reviews.69-73 Additionally, some N-doped carbon materials with entirely new structures such as hierarchical porous carbon, hollow carbon and onion-like carbon also have drawn much attention and can be achieved by post treatment and direct synthesis.65 There is little doubt that preparation of N-containing carbon materials with different features such as morphology is one of the key topics for the design of carbon materials with enhanced performance. Therefore, there is clearly a requirement to develop N-doped carbon with novel features to offer new directions for the applications of these new carbon-based catalysts. 2.1 Structure of doped N atoms Recently, N-doped carbon materials have received much attention because of the following three specific reasons.65 First of all, N and C have a similar atomic radius, thus the incorporation of N atoms into carbon matrix can prevent significant lattice


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mismatch. Secondly, N is right next to C on the periodic table, and thus only one electron will be tailored every time when one C is replaced with N in the carbon matrix. Lastly, they are abundant, accessible, and have a low health risk. N dopants in a given carbon matrix can be primarily divided into four types: graphitic nitrogen, pyridinic nitrogen, pyrrolic nitrogen and pyridinic N-oxide (as shown in Fig. 1a).74,

75

These different N species can be distinguished by

deconvolution of the N 1s XPS signals, the binding energies of about 398.3-399.8 eV, 400.1-400.5 eV, 401.0-401.4 eV, and 404.0-405.6 eV were assigned to pyridinic N, pyrrolic N, graphitic N, and different N-oxide species, respectively, as shown in Fig. 1b.76-78 Based on experimental and theoretical studies, the first three exhibit possible interactions with active components and/or reactants, and thus are more important for N-doped carbon supported catalysts and their application in heterogeneous catalysis.79-82 Graphitic N atoms are N atoms that substitute C atoms located in the carbon plane. For the graphitic N in carbon materials, four electrons are used to form σ and π bonds, and the fifth electron is in the higher-energy π* state, leading to the electron donor behavior of graphitic N.83, 84 Pyridinic N atoms are normally located at the edges or in vacancies of the graphitic carbon layers. In contrast with graphitic N, the pyridinic N uses two electrons to form σ-bonds with neighboring carbon, other two electrons to form a lone pair, and the remaining electron to occupy the N π-state, which results in a π-electron missing of pyridinic N, functioning as an electron acceptor.85,

86

Moreover, the lone pair of the pyridinic N also endows the N-doped

carbon materials with Lewis basicity. The pyrrolic N atom normally refers to the N atoms incorporated into the incomplete five-sided ring. Wu and coworker reported that the pyrrolic N is thermally unstable and could gradually transform into a graphitic nitrogen atom inside the graphitic carbon plane as the increase of heating


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temperature.87

Figure 1. (a) Common bonding configurations of N atoms in carbon matrices. (b) XPS spectra of typical binding energies of nitrogen atoms in various bonding configurations. Reproduced with permission from ref 78. Copyright 2014, American Chemical Society.

2.2 Effect of doped N on pristine carbon materials and metal/carbon composites Normally,

the

introduction

of

nitrogen

atoms

could

change

the

physicochemical properties of carbon materials such as the acid-base property, wettability, charge mobility and electronic structure. More specifically, the introduction of nitrogen can afford basic surface functionalities, which are capable of improving the interactions of carbon surface with acidic molecules though dipole-dipole, hydrogen bonding and/or covalent bonding.88 Stevenson et al. and Papp et al. reported that an aqueous suspension of N-CNTs has a pH of 8-9 and there is a correlation between the number of basic sites and pyridinic N content.89, 90 Moreover, the presence of nitrogen-containing groups can enhance their wetting properties, which is very important for the metal NPs deposition in liquid phase, for instance, by impregnation.91, 92 More importantly, in undoped carbon materials, only the surface carbon atoms exhibit weak charges because of the presence of hydrogen


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atoms on the surface. However, once introduced, the charge distribution of carbon atoms will be dramatically affected by the neighboring nitrogen dopants due to different electronegativity of nitrogen and carbon (3.04 and 2.55, respectively), which leads to the generation of vacancies or defects in the graphitic carbon structure.93-95 It should be noted that N-doped carbons normally show a lower oxidation resistance in that the incorporation of N atoms inevitably introduces defects to parent carbon materials. It is well known that the corrosion of carbon materials occurs more rapidly at defect sites.96 Furthermore, doped N atoms can alter the electronic structure of carbon materials at the N-doping sites, therefore causing localized charge accumulation.97 The changed charge distribution can be useful in the electron transfer reaction and promote the adsorption and/or activation of molecules, which endow N-doped carbon materials with a particular importance in heterogeneous catalysis.98 Once the metal is introduced on the surface of N-doped carbon, new influence resulting from N atoms emerges, which will tune the catalytic performance of metal/N-doped porous carbon composites. Firstly, nitrogen-containing groups can act as active sites for the nucleation and growth of metal NPs, which can stabilize the deposited metal NPs with small size and narrow distribution.80, 99 The stabilization of metal NPs on the surface of N-doped carbon materials is also supported by theoretical calculations.80, 100, 101 For Pt on N-doped carbon, the platinum atom binding energy on the different adsorption sites were calculated. Chen et al. found that the platinum atom bonding with the carbon atoms neighboring the graphitic nitrogen atoms was more energetically favorable than directly with these nitrogen atoms.80 Meanwhile, Jugroot and coworkers concluded that the platinum bonding with three pyridinic nitrogen atoms adjacent a carbon vacancy was the most energetically favorable.100 Similar


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ACS Catalysis

conclusions were reached for the Ni on N-CNTs system by Kim and coworkers, as shown in Fig.2.101 The binding energies of a single nickel atom adsorbed on the various adsorption sites decreased in the following order: Ni-NGraphitic-C (−0.34 eV) > Ni-C (−0.47 eV) > Ni-C-NGraphitic (−0.68 eV) > Ni-2NPyridinic (−1.62 eV) > Ni-3NPyridinic (−4.59 eV). Secondly, the chemical state of supported metal nanoparticles can be influenced significantly by N atoms through the electronic interaction between metal NPs and support.102,

103

For example, Ru on NHPC (hierarchical porous N-doped

carbon) is mainly in the reduction state, while the oxidation state will be dominant if it is supported on HPC (hierarchical porous carbon) in the as-prepared samples, implying that NHPC favors the formation of metallic Ru. In addition, the metal-nitrogen bonding may be formed, which could help secure the metal NPs on their original position thus inhibit their mobility.104 It should be noted that although the bonding between noble metals (such as Pd and Pt) with N was rarely directly detected, the metal-nitrogen bonding for non-noble transition metals like Co has been widely reported.105-107 All of the influences could be utilized to tune the physicochemical properties of supported metal NPs, thus improving their catalytic performance.


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Figure 2. Optimized structures of Ni bound to CNTs including (a) N-free, [(b)-(d)] graphitic N, (e) pyridinic N and (f) a vacancy-N complex (N: blue, Ni: red). Reproduced with permission from ref 101. Copyright 2007, AIP Publishing LLC. 3. Synthesis of metal/N-doped carbon composites The preparation of metal/N-doped carbon composites for various catalytic applications has been well developed over the past few decades. The dispersion of metals over suitable supports is one of the key issues for higher catalytic performance because of the fact that the higher dispersion normally means more surface atoms, and thus higher catalytic activity. On the other hand, it is well known that the catalyst performance is intimately connected to the choice of metal precursors, carbon supports and synthetic process.108,

109

Thus, it is very important to choose an

appropriate synthesis method to prepare metal/N-doped carbon composites based on the character of metal and/or metal oxide and N-doped carbon. Here, we will discuss the various strategies for preparing metal and/or metal oxide/doped carbon composites tested in catalytic reactions. The basic methods of synthesizing this kind of catalysts can be broadly categorized into the following three types: 1) traditional post-loading


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method that deposits the metal precursors on presynthesized N-doped carbon materials by traditional methods such as impregnation and deposition-precipitation; 2) simultaneous introduction of metal and nitrogen on presynthesized carbon supports and 3) in situ formation of metal and/or metal oxide supported on N-doped carbon materials. 3.1 Traditional post-loading method Generally speaking, most of N-doped carbon supported metal catalysts are prepared by post-loading method in the last decades. Specifically, N-doped carbon materials are firstly obtained through post-treatment process or direct synthesis from nitrogen containing precursors. Then, metal cation is loaded on the N-doped carbon by traditional post-loading methods such as impregnation, deposition-precipitation and sol-immobilization.110-112 Normally, the metal salts on the surface N-doped carbon are reduced further to the metallic state, and thus a final reduction step is always needed. Reducing agents such as H2, HCHO, N2H4·H2O, or NaBH4 are usually used.102, 113 3.1.1 Impregnation This method is one of the efficient and facile techniques for the synthesis of heterogeneous catalysts and can be readily scaled up for industrial production. Typically, the metal precursor, normally a salt (such as metal nitrates, acetates, chlorides, sulfates, carbonates, and organic metal complexes, such as metal acetylactonates), is first dissolved in the appropriate quantity of solvent. The porous N-doped carbon is then added to the metal salt solution with stirring. After the solvent being removed, the as-obtained solid is oven-dried, calcined and/or reduced before being tested as a catalyst. Two main impregnation methods are distinguished, namely, wet impregnation,


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whereby an excessive amount of solution is used, and pore volume impregnation, in which a certain amount of solution to just fill the pore volume of the support is used. The latter method is also known as incipient wetness impregnation or dry impregnation, because the impregnated material keeps a dry character at a macroscopic scale. Among various post-loading methods, impregnation is the most frequently used preparation method due to its simple execution and low waste streams. Almost all of carbon materials and soluble metal salts can be used as the supports and metal precursors, respectively. Moreover, with this method, the loading amount of the metal can be controlled by varying the concentration of the precursor solution. However, because of the difference in diffusion resistance between the external and internal surface, part of the metal precursors may be adsorbed on the outer surface of the support, leading to surface aggregation of some metal nanoparticles.109 Additionally, the chloride ions will remain during the process of solvent removal if metal chlorides are used as precursor, which will result in the agglomeration of metal NPs during calcining process.114 3.1.2 Deposition-Precipitation This method involves the dissolution of the metal precursor followed by a change of pH or temperature, or evaporation to achieve a complete precipitation of the metal compounds (often metal hydroxides), which is deposited on the surface of an existing N-doped carbon materials. After the precipitation-deposition step, the material is filtered, washed, dried and reduced. The most important step is the formation of small metal hydroxides in the solution phase, which should have done before the metal precursors adsorb onto the supports.115 Classically, deposition precipitation was developed to produce catalysts with


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metal loadings that exceed those obtained by impregnation which is limited by solubility. More importantly, all of the active components remain on the surface of the support and homogeneous particle size distribution can be obtained given the right conditions. However, it’s not easy to control the process of nucleation of the metal species, thus great care has to be taken to prevent local concentrations exceeding the critical supersaturation. Furthermore, an excess of reductant (such as H2, NaBH4, N2H4·H2O) is necessary to guarantee the complete formation of metal NPs, which has to be removed after reaction. 3.1.3 Sol-immobilization Sol immobilization is a widely used method to prepare supported metal (especially noble metal such as Au, Pd, Pt and Ru) catalysts.116 This preparation method often involves colloidal sols with a well-defined size distribution, in which the metal NPs are stabilized in solution.117 These nanoparticles are then typically immobilized on the surface of N-doped carbon. The deposition of pre-formed nanoparticles onto N-doped carbons is highly advantageous, as precise control of the particle size and structure of the pre-formed nanoparticles can be achieved. Moreover, the particle size of nanoparticles synthesized by the colloidal method is less affected by the surface chemistry and morphology of the N-doped carbon than other methods. Despite all of advantages mentioned above, this method still suffers from some limitations. Normally, the residual stabilizing agent (such as polymers, surfactants, etc) can block the active site of metal NPs through steric effects, while the removal of these stabilizing molecules is often problematic. The use of thermal and oxidative methods is not always highly reproducible since parameters such as the sample heating rate and catalyst loading can affect the specific structures of the supported metal nanoparticles.


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In conclusion, the traditional post-loading method is the firstly studied and the most widely used. The main advantages of this method are as follows: 1) the hierarchically porous structures of N-doped carbon materials can be tuned by using numerous synthesis approaches based on the need of a particular reaction;73 2) various physiochemical conditions and templating-effects could be used extensively to synthesize desired morphologies for supports; 3) adjustable surface functions of carbon materials such as hydrophobicity-hydrophilicity and acid-base properties can be achieved by introduction of other heteroatoms; 4) various conventional catalyst preparation methods can be chosen based on the properties of N-doped carbon materials. However, the greatest disadvantage of this method is the tedious preparation procedure. Typically, the N-doped carbon supported metal catalysts should go through three processes: the preparation of N-doped carbon, deposition of metal and reduction of metal precursor to the metallic state. 3.2 Simultaneous introduction of metal and nitrogen on presynthesized carbon supports As mentioned above, traditional post-loading method is a classical and effective method to synthesize N-doped carbon supported metal and/or metal oxide catalysts, but it normally needs time-consuming procedures. To obtain the metal/N-doped carbon composites more efficiently, a new strategy that simultaneous introduction of metal and nitrogen on presynthesized carbon was developed. Recently, Beller and coworkers prepared metal and/or metal oxide supported on N-doped carbon via pyrolysis of a mixture of defined non-volatile organometallic amine complexes and carbon supports at high temperature (as shown in Fig. 3).118 During the process of pyrolysis, the metal complexes are partially or completely decomposed, generating active metal and/or metal oxide NPs and N modified carbon. Since that, a series of


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metal and/or metal oxide supported on N-doped carbon catalysts using similar methods have been reported.118-120 Now it is believed that pyrolysis of a mixture of metal salt (nitrates, acetates and chlorides), N-containing precursors (melamine and dicyandiamide) and carbon support can obtain the metal and/or metal oxide supported on N-doped carbon catalysts. In some cases, it is worth noting that no extra reduction process is needed since metal(0) can be obtained through optimization of the pyrolysis step.121 In addition, the simultaneous introduction of metal and N species can reduce a number of steps required to modify carbon with nitrogen atoms and to deposit metal on support.

Figure 3. Schematic representation of cobalt oxide–N/C catalyst preparation. Reproduced with permission form ref 118. Copyright 2013, Nature. However, the catalysts obtained by this method normally contain nanoparticles with a wide size distribution. Except for small particles like 10-20 nm, some larger particles with 20-80 nm and even up to 800 nm also exist. This might be resulted from the highly chemical inertness of carbon material, leading to weak interaction between metal and support. Sometimes, acid leaching of these catalysts is necessary before conducting experiments.120 To sum up, the heterogeneity of such catalysts, not only complicates the identification of the real catalytic active sites, but also reduces the atomic efficiency, and even causes undesirable side-reactions.


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In order to obtain the catalysts with uniform NPs dispersion, recently Sung and coworkers improved Beller’s method and gained the FeP/C catalyst with uniform dispersed N-doped carbon-shell-coated FeP NPs through a single-step thermal treatment of polydopamine-coated iron oxide NPs (as shown in Fig. 4).122 During the pyrolysis process, polydopamine coating is converted to carbon shell, which could effectively prevent iron oxide NPs from aggregation. Compared with the catalyst obtained without the polydopamine coating step, obvious aggregation of NPs occurs during the heat treatment (Fig. 4e).

Figure 4. (a) Schematic representation of carbon-shell-coated FeP NP preparation. (b−e) TEM images of as-synthesized iron oxide NPs (b), carbon-shell-coated FeP NPs (c, d), and FeP NPs prepared without carbon shell (e). Reproduced with permission form ref 122. Copyright 2017, American Chemical Society. 3.3 In situ production of metal/N-doped carbon catalysts Although the above approaches show great success in the synthesis of metal/N-doped carbon composites with controlled composition and structure, a facile and efficient one-step synthesis is urgently desired. The most simple and direct


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approach is to mix the metal precursor, nitrogen precursor and carbon precursor together, followed by a pyrolysis process to afford metal/N-doped carbon composites. Recently, our group reported a novel NiOx@GCNTs (GCNTs: graphitic carbon nanotubes) catalyst through co-pyrolysis of a mixture of Ni(NO3)2·6H2O, D-glucosamine hydrochloride (GAH) and melamine.123 As illustrated in Scheme 1, the evolution of NiOx@GCNTs was achieved by the following process: (i) Low temperature condensation, during which melamine and GAH were converted into g-C3N4 (graphitic carbon nitride) and N-doped carbon, respectively. Meanwhile, metal salt underwent decomposition and in situ incorporation into the above hybrids. (ii) High-temperature annealing, during which g-C3N4 decomposed and GCNTs formed with the assistance of nickel NPs, thus generating the hybrids.

Scheme 1. Scheme for the synthesis of NiOx@GCNTs. Reproduced with permission from ref 123. Copyright 2015, Royal Society of Chemistry. Interestingly, we found that depending on the starting materials (such as carbon source and metal salt), heating temperature and heating steps employed, the nanostructures of the as-obtained catalysts can be modified, and significant


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differences in the catalytic performance are observed.123-126 For example, when the GAH and melamine are chosen as carbon and nitrogen sources, the morphology of the as-obtained catalyst can be tuned through changing the metal precursor. As shown in Fig.5, the Co-based catalyst with flake-like carbon can be obtained when cobalt acetate is used as metal source, while a wide range of carbon nanotubes will appear in the Co-based catalyst if the metal salt is changed to cobalt nitrate. In addition, the morphology and structure of the catalyst also can be influenced by carbon and nitrogen precursors. Severely aggregation phenomena were observed when GHA and melamine were chosen as carbon precursor, respectively.125 Only when both of them were used as carbon precursor, uniform Cobalt NPs could be obtained in Co-based catalyst. The last but not the least, a lasagna-like Fe-N co-doped carbon nanotubes and graphene framework (LMFC) can be achieved when oxalic acid is added to the mixture of Fe(NO3)·9H2O and melamine (as shown in Fig. 6).127 It’s believed that oxalic acid can function as chelating agent to stabilize Fe ions and structure-directing agent to achieve self-assembly. Meanwhile, Zhang and Wang et al. reported the Co–N–C catalyst with single-atom dispersion could be synthesized by using a support-sacrificed strategy (Fig. 7).128 Compared with the commonly used carbon supports, the utilization of Mg(OH)2 as sacrificial template exhibited great advantages in inhibiting the aggregation of cobalt. The synthesis of N-doped carbon supported non-noble metal (such as Fe, Co and Ni) catalysts by one-pot pyrolysis method has been widely reported, while the preparation of noble metal/N-doped carbon using this strategy was rarely observed. Recently, our group found that Rh/N-C catalyst with well-dispersed Rh NPs could be synthesized through one-pot pyrolysis of biowaste (chitin) and Rh salts.129


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Figure 5. Representative TEM image and the metal particle size distribution histogram of CoOx@GCNTs (a), Reproduced with permission from ref 124. Copyright 2015, American Chemical Society; and CoOx@CN (b), Reproduced with permission from ref 125. Copyright 2016, Royal Society of Chemistry.


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Figure 6. Scheme for the synthetic procedure (a); SEM images of LMFC at different planes and magnifications (horizontal: (b)–(d); vertical: (e)–(g)). Reproduced with permission from ref 127. Copyright 2017, Springer.

Figure 7.

SEM (A), HRTEM (B), and HAADF-STEM (C, D) images of

Co–N–C catalyst. The white dots in (C, D) are Co single atoms. Reproduced with permission from ref 128. Copyright 2016, Royal Society of Chemistry.

As mentioned above, this synthetic strategy is a time-saving method. More importantly, the morphology and structure of catalyst can be tuned through changing the synthesis process. However, it still suffers from some limitations. For example, part of the catalysts (especially for non-noble metals) prepared by in situ pyrolysis at high temperatures are composed of nanoparticles with very wide particle size distribution. Not only the metal NPs ranging from a few to tens of nanometers, both exposed and encapsulated, could be observed, but also highly dispersed single atoms, which are invisible with normal electron microscopy, might be contained in the


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catalysts. All of them make the identification of the real active sites a pending challenge. And even this method is unsuitable for synthesis of N-doped carbon supported noble metals due to most of noble metals intend to aggregate at this high temperature. Moreover, partial metal NPs can be completely embedded into the carbon matrix, which makes them deactive. In additional, metal-organic frameworks (MOFs) have gained particular attention in recent years as a novel approach for the synthesis of metal and/or metal oxide NPs supported on N-doped carbon due to porosity, three-dimensional (3D) structure, and diversity of metals and organic linkers.130 According to the literature, metal oxides or metals/metal oxides (Fe, Co, Ni, Cu, Zn and Al) supported on N-doped carbon can be obtained directly from the pyrolysis of corresponding MOFs (as shown in Fig. 8).50, 131-135

Still, it should be noted that the structures of MOF precursors and the pyrolysis

conditions have great influence on the microstructure and physicochemical properties of the as-obtained catalysts, which eventually affect their catalytic performance.130

Figure 8. Scheme for the synthesis a nitrogen doped graphite embedded Co catalyst from one-step pyrolysis of ZIF-67. Reproduced with permission from ref 50. Copyright 2015, American Chemical Society. Recently, Li and Wu et al. reported that ultrafine clusters or even isolated single atom catalysts can be prepared by converting atomically dispersed metal nodes in situ from MOFs.136-139 For example, they reported that uniform Ru3 clusters stabilized by


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nitrogen species (Ru3/CN) could be obtained by utilization the isolated metal precursor (Ru3(CO)12) being encapsulated in suitable molecular-scale cages of ZIF-8 (Fig.9a).138 Furthermore, they also found that isolated single ruthenium site supported on nitrogen-doped porous carbon (Ru SAs/N−C) can be obtained by utilization of strong coordination between Ru3+ and the free amine groups (−NH2) at the skeleton of a metal−organic framework (Fig. 9b).139 These efforts may initiate a new class of single-metal or single-molecule materials, which may exhibit great promising in heterogeneous catalysis.

Figure 9. Illustration of the Ru3/CN preparation process (a), Reproduced with permission from ref 138. Copyright 2017, American Chemical Society; and the proposed formation mechanisms for Ru SAs/N–C (b), Reproduced with permission from ref 139. Copyright 2017, American Chemical Society. .

Compared with conventional methods as mentioned above, using MOFs as sacrificing templates and precursors to fabricate various N-doped carbon-based nanomaterials has following advantages: 1) The most fascinating merit of this method


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is that various functions can be incorporated into the frameworks through carefully choosing the organic ligands and metal species; 2) Most MOF-derived catalysts have high specific surface area and tailorable porosity; 3) The special MOF structures that the periodic alternation of the metal ions with organic ligands can effectively prevent the aggregation of metal and/or metal oxide NPs during the pyrolysis;130 4) Last, but not the least, uniform dispersed metal clusters and even isolated single atoms anchored on N-doped carbon can be obtained if the MOFs precursors are built up strategically, which might significantly enhance the catalytic performance of metal/N-doped carbon catalysts.139 Despite great efforts dedicated to the investigation of MOF-derived N-doped carbon supported metal and/or metal oxide catalysts, researchers are still incapable of controlling the fine structures of these materials due to the limited knowledge of the precise mechanisms for the transformation process. Furthermore, all of them have been obtained only at the laboratory level with very low yields and using time-consuming procedures. 4. Applications In comparison to conventional undoped counterparts, there is definite superiority in the use of N-doped carbon supported metal and/or metal oxide catalysts in heterogeneous catalysis, which can be divided into two categories. Once introduced, N dopants generally provide basicity and wettability, which can enhance the catalytic performance of N-doped carbon supported metal and/or metal oxide catalysts in several aspects, as shown in Fig.10a. More importantly, N dopants can also influence the properties of the supported metal and/or metal oxide NPs, such as metal dispersion, metal-support interactions and metal NPs stability, which are beneficial for improving their catalytic performance (as shown in Fig.10b). Therefore, N-doped carbon


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supported metal catalysts bring new prospect for heterogeneous catalysis in the future. Here, four kinds of heterogeneous catalytic reactions including hydrogenation, dehydrogenation, oxidation and coupling will be reviewed to highlight the various beneficial effects of nitrogen doping.

Figure 10. The promotional effect of N doping on support (a) and catalyst (b). 4.1 The role of wettability in catalytic activity Given the fact that almost all heterogeneous catalytic reactions are present in a complex mixture, it is necessary to tune the wettability of the catalysts to improve dispersion stability, to limit side-reactions and/or to control how the reactant(s)


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chemisorb. Recently, our group found that the introduction of N atoms in carbon material could improve its hydrophilic property, which is favor of the dispersion of N-doped carbon supported metal catalyst in aqueous solution.47 As shown in Fig. 11a, all the [email protected] catalyst dispersed well in aqueous phase after the reaction, which could enhance the contact between the catalyst and the substrates (vanillin), therefore improving the catalytic activity dramatically (Table 1, entry 1 and 2). Additionally, we found that the incorporation of hydrophilic N-containing carbon (CN) with the pure MgO support can considerably improve its dispersion in aqueous solution.140 As shown in Fig. 12, the contact angle of CN with water is 78o and it can be dispersed homogeneously in water even after 3 h, indicating a good hydrophily of CN. More importantly, a better dispersion in water is achieved in comparison of pure MgO if the hydrophilic CN hybridizes with a traditional metal oxide such as MgO, the hybrid material (CN@MgO) (Fig. 11b), which is an important factor to improve its catalytic performance (Table 1, entry 3 and 4). Interestingly, this strategy of hybridizing N-doped carbon and traditional metal oxide also can be applied to prepare CN@ZnO (as shown in Fig. 11c), which accordingly results in better catalytic activity of PdZn/CN@ZnO (Table 1, entry 5 and 6).141 Further, this conclusion has been confirmed by our recent work.142 As shown in Fig. 13, there is a linear relationship between the zeta potential absolute values and TOFs. In other words, the TOF of various Ir-based catalysts increased along with increasing in the zeta potential absolute values. Jiang et al. also reported that complete vanillin conversion with a TOF of 100 h-1 over Pd/NPC-ZIF-8 catalyst could be obtained for the


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hydrodeoxygenation of vanillin in a water/oil mixed solution, while only 36% vanillin conversion with a TOF of 22 h-1 was observed over N-free carbon supported Pd catalyst (Table 1, entry 7 and 8).143

Figure 11. (a) The catalyst dispersion in biphasic interface, the upper was decalin, the bottom was water. Reproduced with permission from ref 47. Copyright 2012, American Chemical Society. (b) Water dispersions of MgO and CN@MgO after different times. Reproduced with permission from ref 140. Copyright 2014, Royal Society of Chemistry. (c) Water dispersions of ZnO (left) and CN@ZnO (right) after 10 min. Reproduced with permission from ref 141. Copyright 2017, Elsevier.


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Figure 12. Pictures of contact angle of CN with water (a) and its dispersion in water (b). Reproduced with permission from ref 140. Copyright 2014, Royal Society of Chemistry.

Figure 13. Relationship between the benzoic acid conversion on various catalysts (a) and zeta potentials of various supports (b). Reproduced with permission from ref 142. Copyright 2017, Royal Society of Chemistry.


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Table 1. Metal/N-doped carbon catalysts employed for various hydrogenation reactions. Entry

Catalyst

Application

Performance

1

Pd/CN0.132

Hydrodeoxygenation

Con. 100%, Sel. 100%

2

Pd/AC

of vanillin

Con. 98%, Sel. 74%

3

Pd/CN@MgO

Aldol

Con. > 99%, Sel. 82%

4

Pd/MgO

5

PdZn/CN@ZnO

Semihydrogenation

TOF 434 h-1

6

PdZn/ZnO

of alkynols

TOF 74 h

7

Pd/NPC-ZIF-8

Hydrodeoxygenation

TOF 100 h-1

Pd/AC

of vanillin

TOF 22 h

8

condensation–hydrog enation of furfural

Con. 86%, Sel. 42%

-1

Ref.

47

140

141

143

-1

Con.: Conversion, Sel.: Selectivity, TOF: Turnover Frequency. 4.2 The role of acid-base properties in catalytic activity 4.2.1 Hydrogenation As mentioned above, the basic properties can be introduced in the carbon material by N doping, leading to the enhanced interactions of the carbon surface with acidic molecules.69 In 2011, our group found that phenol can be adsorbed in nonplanar fashion on the surface of mpg-C3N4 (mesoporous graphitic carbon nitride) through the hydroxy group via weak O−H···N or O−H···π interactions (Scheme 2), which gives rise to the enhanced catalytic activity for the hydrogenation of phenol to cyclohexanone.144 Later, Xia et al. also found that there is a favorable interaction between benzoic acid (BA) and N-doped mesoporous carbon (MCN), which lead to higher adsorption capacity of BA over MCN (228 mg/g) that of AC (92 mg/g) and


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N-doped AC (131 mg/g), thus accelerating reaction rate on surface of Pd/MCN.145

Scheme

2.

Possible

reaction

mechanism

of

phenol

hydrogenation

to

cyclohexanone over Pd@mpg-C3N4. Reproduced with permission from ref 144. Copyright 2011, American Chemical Society. In addition, the basic sites brought by N dopants may play a crucial role in influencing the catalytic performance in some cases, for example the use of alcohols as hydrogen donors in transfer hydrogenation reactions. According to the literature, nitrogen functionalities on the surface of carbon material could function as Lewis-base sites,146 which can be utilized to adsorb the alcohol (hydrogen-donor) and thus enhance the formation of metal hydrides in transfer hydrogenation reactions. Inspired by that, Li et al. prepared nitrogen-doped carbon-supported cobalt (Co@C-N) catalysts by pyrolysis of a Co-MOF.147 As expected, Co@C–N exhibited high catalytic performance for the hydrogenation of a variety of unsaturated bonds with isopropanol without the assistant of base additives (Table 2, entry 4 and 5). Based on CO2-TPD result, it was found the strong basic sites existed in all as-prepared Co@C-N samples. It can be, therefore, speculated that the basic sites on Co@C–N might behave a similar role as that of base additives to adsorb isopropanol and promote the transfer of hydrogen to Co NPs, thus accelerating the transfer hydrogenation reactions. Later, Fu et al. obtained similar conclusion from the transfer hydrogenation of furfural (FF) to furfural alcohol (FFA) over Fe-L1/C-800 (prepared by using 1,10- phenanthroline as metal ligand) catalysts.148 It was believed that the Lewis-base sites and Lewis-acid sites were introduced by nitrogen doping and the


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loading of iron, respectively. The synergy effect between these acid and base site makes the hydrogen transfer reaction happen. Under the optimal reaction conditions, 83.0% FFA selectivity with a FF conversion of 91.6% could be obtained over N-doped carbon supported Fe catalyst, which is far higher than that of undoped carbon supported Fe catalyst (Table 2, entry 6-8). Above all, it can be concluded that doping carbon material with the electron-rich nitrogen atoms can significantly improve its basicity, and these basic sites are the real active centers for hydrogen transfer reaction through promoting the formation of metal hydrides from hydrogen donor.

Table 2. Metal/N-doped carbon catalysts employed for various hydrogenation reactions. Entry

Catalyst

1

Pd/MCN

2

Pd/NAC

3

Pd/AC

4

Co@NC-900

5

Co@C-900

6

Fe-L1/C-800

7

Fe/C-800

8

Fe-L1/C-800

Reaction conditions

Application

110 C, 2.5

Hydrogenation of

MPa H2

benzoic acid

o

Transfer 80 oC

hydrogenation of 2-methylbenzonitrile

120 oC 160 oC

Performance Con. 81.5% Con. 48.2%

145

Con. 7.2% Con. 45.5% Con. 99

80.4

0.4

C-EDA-2.3

2.3

98.6

82.8

0.7

C-EDA-4.1

4.1

>99

96.0

0

C-EDA-8.3

8.3

>99

93.4

0.8

C-NH3

3.8

97.8

84.1

7.8

C-DMA

3.2

>99

47.1

32.1

C-ACN

0.3

97.0

1.8

10.7

Support

Reaction conditions: reactant (0.5 mmol), catalyst (0.04 g), H2O (10 mL), O2 (1.0 MPa),

110

C,

o

12

h.

FDCA:

2,5-Furandicarboxylic

acid,

FFCA:

5-formyl-2-furancarboxylic acid, HTC: hydrothermal carbon. C-EDA, C-NH3, C-DMA and C-ACN mean the N-doped carbon prepared using ethylenediamine, NH3, N, N-dimethyaniline, and acetonitrile as nitrogen source, respectively.


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Figure 15. (a) CO2-TPD spectra of various supports and (b) Plot of relative intensity against the percentage of Pyridinic N (■), Pyrrolic N (●), and Graphitic N (▲). Reproduced with permission from ref 164. Copyright 2016, Elsevier.

Figure 16. CO2-TPD profiles and benzyl alcohol conversion of various Co/C-Nx materials. Reproduced with permission from ref 165. Copyright 2016, Royal Society of Chemistry. 4.2.4 Coupling Since cross-coupling catalysis was awarded the 2010 Nobel Prize in Chemistry, the formation of aryl–aryl bond, which

is ubiquitous in natural products,

commercial dyes, and organic conductors or semiconductors as well as the backbone


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of some ligands for asymmetric catalysis,

has become one of the most powerful

tools of modern organic synthesis.166-168 It is believed that incorporating nitrogen into carbon matrix leads to enhanced bonding ability with metal and basic property. In other words, the chemical environment of the nitrogen on the carbon surface is similar with that in the N-type ligands which are widely used as accelerators for the cross-coupling reactions. Thus, can the N-doped carbon behave as electron-rich ligands for the metal-catalyzed cross-couplings? Recently, our group presented that mesoporous N-doped carbon can act as active N-type heterogeneous promoters for copper-catalyzed cross-couplings.169 As shown in Fig. 17, good yields could be afforded for the Ullmann-type coupling of various aryl halides with phenols, amines, and thiophenols when CuI and N-doped carbon were used as the catalyst and the promoter, respectively. In addition, N-doped carbon supported CuO nanoparticles are prepared and found to be active in the C–O cross-coupling reactions. The strong interaction between the N-doped carbon and the copper NPs guarantees for the high stability, recyclability and almost no metal leaching at least for five runs. Meanwhile, Cabrele and coworkers also found that the nitrogen atoms in N-doped CNTs can not only offer strong bonding to the metallic copper, but also function as electron donors.170 After immobilization of copper NPs on the N-CNTs, the Cu/N-CNTs catalyst can be successfully applied for the A3-type coupling reaction under mild conditions (Fig.18).


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Figure 17. Mesoporous N-doped carbon for copper-catalyzed Ullmann-type C– O/–N/–S cross-coupling reactions. Reproduced with permission from ref 169. Copyright 2013, Royal Society of Chemistry.

Figure 18. A3-type coupling over Cu/N-CNTs catalysts. Reproduced with permission from ref 170. Copyright 2012, Elsevier. Later, Bazgir and corkers found that a highly nitrogen-doped graphene supported Pd NPs (Pd NPs-HNG) can also effectively catalyze the Suzuki–Miyaura and Ullmann-type coupling reactions (Scheme 3).171 It was believed that N-doped graphene could function as both the catalyst support and the electron source for Ullmann homocoupling. Subsequently, series of N-doped carbon supported Pd catalysts were developed and proven to be very active for the Suzuki-Miyaura cross-coupling reaction.172, 173


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Scheme 3. Ullmann homocoupling catalyzed by Pd/N-Graphene nanocomposite.

4.3 Improved metal dispersion and stability In terms of heterogeneous catalytic reactions, the preparation of supported catalysts with high metal dispersion is highly demanding. On the other hand, the support plays a crucial role both in dispersing and stabilizing these small metal NPs. Therefore, we focus discussion here on the role of N-doped carbon in preparing supported catalysts with higher metal dispersion. In 2008, Terrones et al. employed Pt NPs supported on N-CNTs for the selective hydrogenation of cinnamaldehyde.174 It’s found that the protonated N-sites of N-CNTs (CNTs–N+:H sites) could act as [PtCl6]2- nucleation centers for depositing platinum, which led to a high metal dispersion with an average size of 2.94 ± 0.53 nm for the Pt nanoparticles. Later, Chizari et al. also found that compared with pristine CNTs, the support after N doping possessed more surface nucleation sites.175 Therefore, Pd/N-CNTs (N/C: 3 at.%) catalyst had a smaller mean Pd particle size (3 nm) than that of Pd/CNTs catalyst (10 nm), which makes Pd NPs supported on the N-CNTs exhibiting higher C=C bond hydrogenation activity and selectivity. The difference of particle size distribution observed on different supports could be


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explained by the following facts: 1) N dopants on the N-CNTs surface could function as active sites for the adsorption of the palladium precursor and 2) these sites then would stabilize the active phase through the interaction of N species with metal, thus inhibiting the mobility of metal particles during heat-treating process. A similar conclusion has also been drawn in studies on N-doped carbon supported noble metal catalysts by our group.47, 104, 176 As shown in Fig. 19, for Ru/NHPC, highly-dispersed Ru NPs with an average size of 2.6 nm can be clearly seen, while slight aggregations of Ru NPs (3.3 nm) were observed in the Ru/HPC. The best dispersion (determined by H2–O2 titration) of Ru NPs was on the NHPC (58%), while only 42% was obtained on HPC.

Figure 19. HRTEM images and particle size distribution of (a) Ru/NHPC, (b) Ru/HPC. Reproduced with permission from ref 104. Copyright 2016, Royal Society of Chemistry. In addition to the enhanced metal dispersion brought by N doping, stabilization of deposited metal NPs can also be achieved by the introduction of defective C-N sites and N interstitials.141,

177, 178

Recently, our group found that compared with

carbon support, N-doped carbon could significantly improve the stability of Pd NPs for the chemoselective hydrogenation of chalcone.179 As shown in Fig.20, Pd/CN could be recycled at least 8 times with little loss of activity while the Pd/Cs showed a


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remarkable deactivation during the recycling. Meanwhile, we also found that N-doped carbon supported bimetallic RuPd catalyst exhibited better deactivation resistance and recyclability for the hydrogenation of benzoic acid in comparison with commercial Pd/AC catalyst (Fig. 21a).180 The similar phenomena were also observed by Su et al. They found that N-CNTs supported Pt catalyst showed good stability for the hydrogenation of nitrobenzene at both high and low conversion due to the stronger anchoring effect of N-CNTs support than that of N-free CNTs for Pt NPs (Fig. 21b).181

More importantly, they found more obvious particle detachment on

Pt/CNTs than that on Pt/N-CNTs after the reaction through the identical location transmission electron microscopy (IL-TEM) method (Fig. 22). Furthermore, the average particle size increases from 1.3 to 1.8 nm in Pt/CNTs, whereas a slight change in Pt/N-CNTs sample. Considering the fact that particle detachment and migration are a consequence of a weak interaction between Pt NPs and support, they thus concluded a stronger anchoring effect in N-CNTs than that in N-free CNTs for Pt NPs.

Figure 20. Recyclability of Pd/CN (a) and Pd/Cs (b). (35 °C, 2.5 h). Reproduced with permission from ref 179. Copyright 2015, Royal Society of Chemistry.


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Figure 21. (a) Comparison of the reusability of the RuPd/CN and commercial Pd/AC in the benzoic acid hydrogenation under the same reaction conditions. Reproduced with permission from ref 180. Copyright 2015, American Chemical Society. (b) Recycling test of Pt/N-CNTs for nitrobenzene hydrogenation to aniline. Reproduced with permission from ref 181. Copyright 2016, American Chemical Society.


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Figure 22. TEM images of Pt/CNTs (a) and Pt/N-CNTs (e) catalysts, the enlargement of selected area (outlined in white) before (b, f) and after the reaction for 30 min (c, g), and the corresponding PSD histograms (d, h) by quantifying a series of TEM images at identical locations. Reproduced with permission from ref 181. Copyright 2016, American Chemical Society.

4.4 Electronic interaction between metal and doped N atoms 4.4.1 Electron-transfer induced changes in electronic state of metal NPs In addition to size effect, N incorporation can also significantly influence the electronic state of metal NPs via the strong electronic interaction between metal and doped N atoms. Recently, our group found that the nitrogen contained in mpg-C3N4


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can result in highly dispersed Pd nanoparticles with high proportions of Pd0 on the surface of the catalyst (70%) (Fig. 23a), which makes the Pd@mpg-C3N4 showing high catalytic activity and selectivity for the hydrogenation of phenol to cyclohexanone.144 Almost complete conversion of phenol with higher than 99% selectivity could be obtained at 65 °C for 2 h under atmospheric pressure in aqueous solution. The similar phenomena were also observed in our later report that 71% percentage of Pd0 was obtained in Pd/CN0.132 sample (Fig.23b).47 Moreover, compared with undoped carbon (HPC), the Ru/NHPC possessed the greatest percentage of Ru0 (64.1%), larger than that of Ru/HPC (51.8%) and Ru/AC (46.6%) (Fig.23c), further confirming that the incorporation of nitrogen species is in favor of the reduction of Mn+ (M=Ru, Pd or Pt) to M0 because of the electron donating effects of nitrogen.104 In the case of hydrogenation of toluene, Ru/NHPC shows the highest turnover frequency (TOF) of 9160 h-1, which is almost two and nine times than that of Ru/HPC (4678 h-1) and Ru/AC (1041 h-1) (Table 4, entry 1, 2 and 3). To reveal the influence of M0 percentage on the catalytic activity, further studies were performed. After excluding impact of the particle size of metal NPs, the relationship between phenol conversion and the percentage of Pd0 was correlated. As shown in Fig.23d, when the percentage of Pd0 was high (>50%), the conversion of phenol was proportional to the percentage of Pd0.54 However, there was no obvious relationship between of them, if the percentage of Pd0 was low (70%) on the surface of the catalyst.60 On the other hand, according to the literature, the anionic metal plays a critical


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role in adsorbing O2 on the metal surface and activating molecular oxygen by donating an excess electron charge to the antibonding orbital, leading to the generation of anionic O2 (such as superoxo or peroxo oxygen).95 Considering these facts, it can be forecast that the introduction of N in carbon supported metal catalysts will considerably improve their catalytic performance in selective oxidation reactions. Recently, our group developed an efficient Pd NPs supported on nanoporous nitrogen-doped carbon catalyst for solvent-free aerobic oxidation of hydrocarbons and alcohols.184 An impressive turnover frequencies (up to 863 h-1 for hydrocarbon oxidation and up to 210,000 h-1 for alcohol oxidation), which are higher than most reported palladium catalysts under the same reaction conditions, were obtained (Table 5). The high catalytic performance could be explained by the following facts: (1) The N dopants could increase the electron density of Pd0, which can promote the activation of O-H bond and C-H; (2) The introduction of N atoms could increase structural defects of support, which can facilitate the adsorption of the substrates on the catalyst surface.


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Table 5. Oxidation of hydrocarbons and alcohols over 0.5%Pd@C-GluA-550 catalysts. Reproduced with permission from ref 184. Copyright 2013 Nature. Amount

Oxidant

Substrate

TOF T ( C) o

(~mL)

4

a

Product (Selectivity %)

(atm)

air (1)b

(h-1)c

120

863 (85)

4

air (1)

120

477 (80)

4

air (1)

120

188 (97)

4

air (1)

120

245 (94)

50

air (1)

120

50

air (1)

120

(63)

14802

13181 (95)

50

O2 (1)

160

212576 (99)

4

air (1)

120

86 (97)

The accurate amount of substrate was calculated by weight.

a

2-acetylpyridine (20 mg) as additive.

b

TOF=[reacted mol substrate]/[(total mol metal)/(reaction time)].

c


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The electron interaction between noble metal and doped N species has been widely reported. It is of great interest to know whether this interaction can be found in non-noble metals, especially those that can be easily oxidized in air. Inspired by these facts, our group investigated the effect of doped N species on the carbon supported Co catalysts.124 As shown in Fig. 25a, for the CoOx@G sample (glucose was used as carbon precursor), strong diffractions corresponding to Co3O4 were observed. However, once the N dopants were introduced into the catalysts such as CoOx@GAH (D-glucosamine hydrochloride was used as carbon and nitrogen precursor) and CoOx@M (melamine was used as carbon and nitrogen precursor), the peaks belonging to metallic Co were appeared while the peaks belonging to Co3O4 were almost disappeared. This may be resulted from the presence of electronic interaction between N dopants and cobalt, which promotes the reduction of cobalt oxide during catalyst preparation process. Similar conclusion that compared with NiO/HPC, the NiO NPs supported on NHPC had a lower initial reduction temperature because of the electronic interaction between Ni and N, was also obtained by our recent work.185 Additionally, this conclusion was further confirmed by Xia and coworkers.186 As shown in Fig. 25b, the decrease of the Co3O4 intensity and increase in metallic Co intensity was clearly observed when the nitrogen content on the CNT surface increased from 0 to 4.9 at. % (atomic percent), which are in line with XPS results (Fig. 25c). Additionally, based on XPS quantitative analysis, the relative surface content of N dopants remarkably decreased on both NCNTs-NH3 (synthesized by treating the OCNTs in NH3) and NCNTs-grown (synthesized by feeding a nitrogen source during CNTs growth process) after Co deposition (Fig. 25d and e), which can be ascribed to partial surface sites of N species covered by the anchored Co NPs. Among various Co-based catalysts, Co/NCNTs-growth showed the highest activity for the


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hydrogenation of nitrobenzene, which is believed due to the following reason. The surface N species have a promotional effect on the formation of metallic Co sites, which is favor to activate and dissociate both H2 and nitrobenzene.

Figure 25. (a) XRD patterns of various Co-based catalysts. Reproduced with permission from ref 124. Copyright 2015, American Chemical Society. (b) XRD patterns of the Co/CNT samples reduced in H2; (c) Co 2p spectra of the Co/CNT samples reduced in H2; N 1s spectra of NCNT-NH3 (d) and NCNT-grown (e) before and after Co loading. Reproduced with permission from ref 186. Copyright 2014, American Chemical Society.

4.4.2 Possible relationship between certain N species and metal NPs Considering the obvious proof of the interaction between metal NPs and surface


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N dopants on N-doped carbon materials, establishing a precise relationship between certain nitrogen species and the properties of the supported metal NPs is urgently required. Therefore, many researchers have done great work to try to answer this question. In 2015, a series of N-CNTs with different pyrrolic-N were synthesized and applied to examine the effect of pyrrolic nitrogen on the physicochemical properties and catalytic performance of Pd/N-CNTs catalysts by Nyamori and coworkers.187 XPS results revealed that there is a positive correlation between the amount of pyrrolic nitrogen and the percentage of Pd2+, which might be resulted from the interaction between Pd NPs and neighboring pyrrolic N atoms. To further confirm this conclusion, the FTIR spectra of N-CNTs-2 (using CNTs with 2 wt% oxygen as precursor) samples before and after Pd deposition were conducted. The peak assigned to C–N groups appeared at 1567 cm−1 for N-CNTs-2 while appeared at around 1552 cm−1 for Pd/N-CNTs-2, further confirming the interaction of pyrrolic nitrogen with Pd NPs. Contrary to above conclusion, Xia et al. found that there was a strong interaction between pyridinic N and Pd NPs, which could lower the reducibility of the Pd species, thus leading to the formation of ultrafine Pd clusters with a percentage of Pdδ+.188 For the benzoic acid hydrogenation, 9.2 times higher activity over Pd/N-AC catalyst than that of Pd/AC catalyst can be achieved (Table 4, entry 4 and 5). However, Su and coworkers concluded that the electronic interaction only occurs between graphitic nitrogen and Pt NPs.181 Compared with pristine CNTs, Pt NPs supported on N-CNTs tends to have more Pt0, indicating that the introduction of nitrogen on CNTs is in favor of forming Pt0 species. To investigate the interaction between the distinct N species and Pt NPs, a series of Pt/N-CNTs with various Pt loading amount from 0.5 to 3.0 wt% were prepared. Based on XPS analysis (Fig. 26), the binding energy of Pt 4f negatively shifted when gradually increasing Pt content. More importantly, graphitic


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N gradually shifted to higher binding energy while there was no change to the oxygen functional groups, implying the interaction of graphitic N and Pt. For the hydrogenation of nitroarenes, Pt/N-CNTs catalyst afforded 3-fold activity to that of Pt/CNTs catalyst and 4-fold to that of Pt/AC catalyst (Table 4, entry 6, 7 and 8). Additionally, our group found that both graphitic N and pyridinic N could interact with Rh NPs if the catalyst was prepared by one-pot pyrolysis of chitin and Rh precursor.129 The N atoms in chitin first serve as solid ligand to bond with metal ions, and then convert various kinds of N including graphitic N and pyridinic N during the pyrolysis process, leading to the interaction of Rh NPs with both graphitic N and pyridinic N. As shown in Fig.26c, the binding energy of graphitic N gradually shifted to the high-energy side with the increase of pyrolysis temperature, while that of pyridinic N showed the opposite trend, which is consistent with previous report that graphitic N and pyridinic N can function as electron donor and electron acceptor, respectively.

Figure 26. Pt 4f (a) and N 1s (b) XPS spectra of Pt/N-CNTs catalysts with different Pt loadings. Reproduced with permission from ref 181. Copyright 2016, American Chemical Society. (c) XPS spectra of N 1s in various samples pyrolyzed at different temperatures. Reproduced with permission from ref 129. Copyright 2017, American Chemical Society.


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Except for the promotional effect of the electronic interaction of metal NPs with doped N atoms in hydrogenation reaction, similar phenomena can also be observed in the dehydrogenation of formic acid. Bulushev found that the Pd/N-CM catalyst showed the highest catalytic activity and stability (30 h without activity loss) for vapor-phase formic acid decomposition (Fig. 27a).189 According to series of experimental and theoretical studies, it was confirmed that the real active sites for the formic acid decomposition reaction are the isolated Pd species bound to pyridinic N. Xu et al. have drawn a similar conclusion that the strong interaction between the Pd precursors and the N functionalities, making it difficult to reduce the Pd2+, which results in the Pd/N-C catalyst possessing smaller Pd0 proportion in comparison of Pd/C catalyst.190 However, the Pd/N-MSC-30-two-175 catalyst with smaller Pd0 proportion afforded the highest catalytic activity (TOF = 8414 h−1) at 333 K, which is far higher than that of the Pd/MSC-30 catalyst (TOF = 4109 h−1) (Fig. 27b).

Figure 27. (a) Temperature dependences of the formic acid conversion over catalyst supports and Pd catalysts prepared from palladium acetate in water (w) or acetone (ac) solutions. Reproduced with permission from ref 189. Copyright 2015, American Chemical Society. (b) Volume of the released gas (H2 + CO2) versus time for the dehydrogenation of formic acid-sodium formate at 60 °C over different catalysts. N-MSC-30-one: MSC-30 was thermally treated with urea at 300 °C, N-MSC-30-two-175: N-MSC-30-one was thermally treated at 175 °C Reproduced with permission from ref 190. Copyright 2017, American Chemical


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Society. (c) The dehydrogenation of formic acid at 45 °C over different catalysts. Reproduced with permission from ref 191. Copyright 2016, Elsevier. Contrary to above conclusion, Yoon and coworkers believe that electron-rich Pd of Pd/N-C resulted from nitrogen donating partial electrons to Pd NPs is the reason for enhanced catalytic activity as compared with Pd/C for the FA decomposition.191 They found that the Pd electron density of Pd/N-C slightly increased because of the electronic interaction between Pd NPs and N-doped carbon (Fig. 27c). More importantly, the Pd NPs with high electron density could facilitate the dissociation of the C-H bonds in H-COOH, which is the rate determining step of the reaction, thus enhancing catalytic activities of Pd/N-C for FA dehydrogenation. To elucidate the effect of the electronic structure of the supported Pd NPs on the catalytic activity for dehydrogenation of formic acid, Cao and coworkers presented an efficient and reliable process that can effectively modulate the surface electronic and acid-base properties of the Pd catalyst.192 In comparison to the existing catalysts, the engineered pyridinic-N-tuned Pd catalyst showed great improvements in terms of catalytic activity and durability toward FA dehydrogenation. Based on ATR-IR spectroscopy (Fig. 28a), nitrogen addition resulted in a lower wavenumber than that for the undoped counterpart, and the extent of the red shift was higher for samples with higher N content in the CNx materials, which confirmed that CNx supports have a strong electronic effect on Pd NPs, depending on the N content. Interestingly, the Pd/CN0.95 with the highest N content showed the lowest performance. Based on XPS analysis, it was found that pyridinic-N might be the main contributor to the electronic interaction with Pd NPs, and the surface pyridinic-N/Pd molar ratio was responsible for the catalytic activity (Fig. 28b). To verify this conclusion, they modified commercial composite Pd-based catalysts with N-containing model organic molecules.


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Modification with pyrrole decreased the performance, while pyridine and 3-aminopyridine improved the activity of both Pd catalysts. These results further confirmed that pyridinic-N beneficially modulates the electronic properties of the Pd catalyst surface, thereby promoting H2 generation.

Figure 28. a) ATR-IR of CO on Pd/CNx catalysts. b) Correlation between the surface pyridinic-N/Pd molar ratio and the initial TOF value of Pd/CNx catalysts. Reproduced with permission from ref 192. Copyright 2016, Wiley-VCH. Meanwhile, Wang et al. showed that Pt/N-CNTs catalysts could efficiently catalyze the selective oxidation of glycerol.86 It has been found that the improved catalytic activity was mainly resulted from the interaction Pt NPs and graphitic N sites.


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Based on XPS analysis (Fig. 29a), the binding energy (BE) of NG (graphitic N) positively shifted with the increase of Pt loading while the decreasing NP (pyridinic N) BE was only observed when the Pt loading changed from 0.98 to 3.87 wt.%. On the other hand, they found that if the Pt loading was 1.66 wt.%, the Pt:NG atomic ratio would be close to 0.9. Therefore, if the Pt loading was higher than this loading amount, excess Pt atoms would have to occupy other sites, leading to the mixed interaction between Pt and NG and NP. Based on above discussion, they concluded that the graphitic N atoms were preferentially interacted with Pt NPs, leading to the formation of smaller particle size of Pt NPs with an electron-enriched state, which showed higher intrinsic activity for the glycerol oxidation. To value the influence of Pt electronic properties on the catalytic activity, the TOF of catalysts was correlated with the corresponding BE of metallic Pt. Normally, the higher Pt4f7/2(0) BE had a lower activity in the glycerol oxidation (Fig.29b). It should be noted that the conclusion that the preferable deposition of Pt NPs over NG sites occurs at low Pt loadings is still controversial considering the following fact. The ratio of NG atom to Pt NPs is far larger than 1 at a Pt loading of 1.66 wt.%, although the atomic ratio of Pt to NG is close to 0.9. At the same time, Yuan and coworkers also found that compare with Pt/CNTs catalyst, Pt/N-CNTs catalyst demonstrated higher catalytic activity and selectivity for glycerol oxidation (Table 6).57 This can be explained by the following facts: 1) the Pt NPs showed higher ability to activate molecular oxygen due to the incorporated N donated part electron to Pt NPs, making them electron-rich; 2) the enhanced surface basicity resulted from the introduction of N accelerated the formation of alkoxide by abstracting proton from glycerol under base-free reaction conditions.


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Figure 29. (a) Dependences of binding energy of graphitic and pyridinic N on various Pt loadings, (b) Dependences of TOF in glycerol oxidation over various Pt-based catalysts on their Pt4f7/2(0) binding energies. Reproduced with permission from ref 86. Copyright 2015, Elsevier. (c) N 1s and (d) Co 2p XPS spectra of Co@NC-x, (e) The N/C mole ratio of Co@NC-x samples as determined by elemental analysis and the corresponding schematic structures (inset of e). (f) TOF values for methyl benzoate (MB) production via aerobic esterification of benzyl alcohol and methanol over Co@NC-x and the highest


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TOF values over a Co-based heterogeneous catalyst with (purple line) or without (orange line) the addition of base reported in the literature. Reproduced with permission from ref 193. Copyright 2017, American Chemical Society.

Table 6. Glycerol oxidation over various Pt-based catalysts. Reproduced with permission from ref 57. Copyright 2015, Royal Society of Chemistry. Catalyst

Selectivity (%)

Conversi

TOF

on (%)

GLYA

GLYDE

DHA

(h-1)

1% Pt/CNTs-353

51.5

43.2

26.4

23.2

248.3

1% Pt/N-CNTs(1.57)

49.0

48.3

19.4

25.8

249.2

1% Pt/N-CNTs(3.32)

58.5

47.6

19.5

24.6

312.7

1% Pt/N-CNTs(5.74)

76.1

55.6

19.0

20.3

459.2

1% Pt/N-CNTs(9.44)

14.5

40.7

29.3

20.4

152.4

1% Pt/N-CNTs(22.85)

7.9

32.5

33.5

20.6

74.3

1% Pt/C3N4(61.67)

2.5

20.9

55.1

24.0

-

Reaction conditions: glycerol aqueous solution (0.1 mol L−1) 10 mL, catalyst 0.039 g, 60 oC, 1 bar (O2: 10 mL/min), 4 h. GLYA: glyceric acid, GLYDE: glyceraldehyde, DHA: dihydroxyacetone. Contrary to the above conclusion that electron-rich metal NPs are beneficial to activate molecular oxygen, Li and coworkers concluded that the electron-poor Co NPs are responsible for attracting and activating O2 for the further removal of protons from the proposed Co-H intermediates.193 Based on XPS analysis (Fig. 29d), the Co 2p XPS gradually shifted toward higher energy, indicating the gradually depressed electron density of Co particles in Co@N-C samples with the introduction of more nitrogen dopants. On the other hand, nitrogen-rich carbon with a relatively higher flat band potential (or work function) than that of metallic Co materials, will accept


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electrons from Co nanoparticles until their Fermi level reaches equilibrium. Therefore, the authors concluded that electron-transfer might occur at the interface of metallic cobalt and N-doped carbon, which is the reason for improved catalytic activity of Co@N-C catalyst for the direct, base-free, aerobic oxidation of benzyl alcohols to methyl benzoate. It can be, based on the above discussion, concluded that it is still difficult to confirm which type of N species is responsible for the electronic interaction, and further the impacts to the reactivity. One of the main impediments is the controllable synthesis of the N-doped carbon with certain N species. Almost all the synthesized N-doped carbon materials so far possess a mixed N species. Additionally, the N species are often not uniformly but rather shape-selectively distributed on carbon materials. And one thing should be specially noticed is the possible interference to the judgement of the electronic interaction between metal and N species induced by the metal oxide (Mn+ species) in NPs have not received enough attention since Mn+ species may accelerate the reactions or the existence of oxygen can modify the electronic structures of metallic M0. All of these issues complicated the metal/N-doped carbon catalyst and thus impeded the full understanding of the interaction of N with metal. 5. Summary and Outlook The application of metal or metal oxide/N-doped carbon composites in heterogeneous catalysis has become a highly active and interesting area of research over the past few decades. In this perspective, we briefly introduced the synthesis of metal or metal oxide/N-doped carbon composites, and then discussed their application in hydrogenation, dehydrogenation, oxidation and coupling reactions in detail. According to the literature, this perspective summarized the following key statements: 1) The presence of N dopants on the surface of carbon material can improve their


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wettability, thus favoring the dispersion of the catalyst in polar solution, especially in aqueous solution. 2) The introduction of N atoms alters the acid-base properties of the support surface, which is beneficial for transfer hydrogenation, dehydrogenation and oxidation reactions. On the one hand, the presence of N species can function as basic sites to activate reactants. On the other hand, the incorporated N makes neighboring carbon atoms electron deficient due to nitrogen is more electronegative than carbon, thereby promoting oxygen adsorption on catalysts. 3) The incorporated N atoms can act as active sites for the deposition of metal NPs. Thus, the N-doped carbon supported metal catalysts normally show higher metal nanoparticle dispersion with smaller particle size and narrower size distribution compared with N-free carbon, which is mainly responsible for the enhanced activity. 4) The presence of N species in carbon materials favors the formation of metal species with a tunable electronic structure through the interaction between N atoms and metal NPs, which is an important factor affecting the catalytic activity, selectivity and stability of N-doped carbon supported catalysts. In spite of recent advances on the synthesis of metal or metal oxide/doped carbon composites and the understanding of the relationship between the nature of doped N and the catalytic properties of the corresponding catalysts, there are still some problems remained to be solved. First, it is still essential to develop simple, low cost and structurally adjustable methods for catalyst preparation, which are important for large-scale application. For example, N-doped carbon materials can be synthesized by exploring precursors such as biomass, which is low-cost, readily available and renewable. However, the biomass-derived carbon exhibits significantly different physicochemical properties


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when different biomass precursors are used. Fortunately, the “leaven method” developed by our group is very easy-to-handle with wide raw biomass materials,194 the relative catalysts base on the as made NHPC materials, for example Pd/NHPC, had already been used in the industry in Zhejiang NHC Company LTD. for the preparation of Vitamin H and Vitamin E, ETC. It should be noted that how to resolve the different catalytic performance of various biomass derived carbon supported catalysts is still the key target for future research although the reported “leaven method” could solve this problem in part. Additionally, different reactions may have different requirements for the catalysts. For example, some catalytic reactions occurring on the solid-fluid interface may be affected by the mass transfer effect, which could be avoided by using a three-dimensional (3D) interconnected hierarchical porous carbon material as the support (for example NHPC materials).104 Thus, in the future, carbon material with novel features, such as morphology and structure, and new catalyst preparation methods should be developed to meet the specific requirements of reactions. Finally, for the in-situ production of metal/N-doped carbon catalysts, researchers are still incapable of controlling the fine structures of these materials due to the limited knowledge of the precise mechanisms for the transformation process. Therefore, future work should pay more attention to a fundamental understanding of the formation mechanisms of the N-doped carbon and metal NPs and then find the key factors influencing the pyrolysis process. Second, it has been widely documented that there is an interaction between the nitrogen species and the metal NPs, which in turn could change the electronic structure of metal NPs. Unfortunately, the exact kind of nitrogen species interacting with metal NPs still remains unclear due to the diversity of nitrogen species. Furthermore, it should be noted that the different electronic state (electron-sufficiency


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or electron-deficiency) of metal NPs may be needed in different reactions. The solutions are expected to be found using more joint efforts of systematic experimental and theoretical studies. First, N-doped materials with only a single type of nitrogen, which is certainly beneficial to understand the relationship between the type of nitrogen species and the catalytic performance, should be synthesized with more precise methods. For example, Müllen and coworkers have synthesized the atomically precise nitrogen-doped graphene nanoribbons with single type nitrogen using bottom-up synthetic strategy by selective nitrogen substitution of the precursor monomers.195 On the other hand, the synthetic method of metal clusters with controllable atoms, which might be in favor of better understanding the real active sites in the metal catalysts, should be developed in the future. For example, Au clusters ranging from about a dozen to a few hundred atoms with precise size control can be synthesized with bulk solution synthetic methods.196 Recently, even ultrasmall metallic Au particles with few atoms have been successfully synthesized by Scheerder and coworkers.197 In addition, theoreitical calculation should be well utilized owing to the advantages for electronic structure analysis and the controlable models to be studied. However, the models are always too ideal to represent the real catalytic systems. And the conclusions sometimes could be quite questionable. Herein more efforts should be dedicated to building realistic models accessible to actual reaction systems

with

ab

initio

molecular

dynamic

simulations

or

quantum

mechanics/molecular mechanics approaches so that enviromental conditions such as pressure, temperature, solvent and etc, can be involved. To obtain an in-depth insight of the relationship between the nature of the nitrogen species and the catalytic properties of the corresponding catalysts, advanced characterization techniques for both the catalysts and reactions should be also


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developed. For example, extended X-ray absorption fine structure (EXAFS) can be used to characterize the coordination numbers and bond lengths of the catalysts, which could offer key information on the specific interaction between metal and N dopant. Advanced microscopy equipment such as spherical aberration-corrected and high-angle

annular

dark-field

scanning

transmission

electron

microscope

(HAADF-STEM) can be used to observe the ultrafine clusters or even isolated atoms, which can effectively avoid omitting possible active sites. Additionally, the development of in situ techniques and the introduction of operando studies are also very urgent to investigate the nature of active sites and the reaction mechanism over metal/N-doped carbon catalysts, excluding the effects of external environment on the characterization of catalyst. By combining in situ methodologies with characterization techniques such as infrared spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray absorption spectroscopy, Mössbauer spectroscopy, nuclear magnetic resonance, and high-resolution electron microscopy, the dynamic changes and atomic-scale insight of catalysts might be obtained beyond surface sciences. Finally, the investigation of reaction kinetics over these catalysts has always been underestimated. Reaction kinetics study is not only the basis for clarifying the mechanism of a reaction, but also a prerequisite for the industrial application in that it provides critical parameters for reactor design and optimation. ACKNOWLEDGEMENTS Financial support from the National Key R&D Program of China (2016YFA0202900), the National Natural Science Foundation of China (21622308, 91534114, 21376208), the Key Program Supported by the Natural Science Foundation of Zhejiang Province, China(LZ18B060002), the Fundamental Research Funds for the Central Universities, and the computing time supported by Special


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Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) are greatly appreciated. REFERENCES 1.

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ACS Catalysis

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