Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Molybdenum Carbide: Controlling the Geometric and Electronic Structure of Noble Metals for the Activation of O−H and C−H Bonds Yuchen Deng,§,# Yuzhen Ge,§,# Ming Xu,§ Qiaolin Yu,§ Dequan Xiao,◊ Siyu Yao,*,Δ and Ding Ma*,§ §
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Beijing National Laboratory for Molecular Engineering, College of Chemistry and Molecular Engineering and College of Engineering, BIC-ESAT, Peking University, Beijing 100871, P. R. China ◊ Center for Integrative Materials Discovery, Department of Chemistry and Chemical Engineering, University of New Haven, 300 Boston Post Road, West Haven, Connecticut 06516, United States Δ Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States CONSPECTUS: In the field of heterogeneous catalysis, transition metal carbides (TMCs) have attracted growing and extensive attention as a group of important catalytic materials for a variety of energy-related reactions. Due to the incorporation of carbon atoms at the interstitial sites, TMCs possess much higher density of states near the Fermi level, endowing the material with noble-metal-like electron configuration and catalytic behaviors. Crystal structure, site occupancies, surface termination, and metal/carbon defects in the bulk phase or at the surface are the structural factors that influence the behavior of the TMCs in catalytic reactions. In the early studies of heterogeneous catalytic applications of TMCs, the carbide itself was used individually as the catalytically active site, which exhibited unique catalytic performance comparable to precious metal catalysts toward hydrogenation, dehydrogenation, isomerization, and hydrodeoxygenation. To promote the catalytic performance, the doping of secondary transition metals into the carbide lattice to form bimetallic carbides was extensively studied. As a recent development, the utilization of TMCs as functionalized catalyst supports has achieved a series of significant breakthroughs in low-temperature catalytic applications, including the reforming of alcohols, water−gas shift reactions, and the hydrogenation of functional groups for chemical production and biomass conversion. Generally, the excellence of TMCs as supports is attributed to three factors: the modulation of geometric and electronic structures of the supported metal centers, the special reactivity of TMC supports that accelerates certain elementary step and influences the surface coverage of intermediates, and the special interfacial properties at the metal−carbide interface that enhance the synergistic effect. In this Account, we will review recent discoveries from our group and other researchers on the special catalytic properties of face-centered cubic MoC (α-MoC) as both a special catalyst and a functional support that enables highly efficient lowtemperature O−H bond activation for several important energy-related catalytic applications, including hydrogen evolution from aqueous phase methanol reforming, ultralow temperature water−gas shift reaction, and biomass conversion. In particular, α-MoC has been demonstrated to exhibit unprecedented strong interaction with the supported metals compared with other TMCs, which not only stabilizes the under-coordinated metal species (single atoms and layered clusters) under strong thermal perturbation and harsh reaction conditions but also tunes the charge density at the metal sites and modifies their catalytic behavior in C−H activation and CO chemisorption. We will discuss how to exploit the metal/α-MoC interaction and interfacial properties to construct CO-tolerant selective hydrogenation catalysts for nitroarene derivatives. Several examples of constructing bifunctional tandem catalytic systems using molybdenum carbides that enable hydrogen extraction and utilization in one-pot conversion of biomass substrates and Fischer−Tropsch synthesis are also highlighted.
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INTRODUCTION
strain of the metal lattice via the incorporation of C atoms. Most importantly, the electronic structure of TMCs is modified by the strong electronic interaction between C atoms and parent metals (i.e., the valence state and d-band center of parent metals are significantly modulated via the formation of TMCs). Owing to their unique structural and electronic properties, TMCs have been extensively studied as a potentially inexpensive alternative
Transition metal carbides (TMCs) are a class of typical compounds with carbon atoms incorporated into the interstitial sites of their parent metals, mainly involving group IV−VI metals.1 By virtue of the modification of C atoms into the lattice of metals, TMCs exhibit unique physical and chemical properties compared to parent metals, such as the appearance of extreme hardness, high melting point, and outstanding electric conductivity.2,3 In terms of chemical structure, the metal−metal distance of TMCs is increased due to the tensile © XXXX American Chemical Society
Received: April 11, 2019
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DOI: 10.1021/acs.accounts.9b00182 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
nities to build high-temperature, stable under-coordinated metal species for catalytic applications. In this Account, we will review the successful catalytic protocols using MoC as catalytic centers and supports in the energy related reactions from our groups and other researchers. We will present examples on how to exploit the strong metal− support interactions (SMSIs) between metal and α-MoC to fulfill specific catalytic tasks. Also, the strategy to construct bifunctional tandem catalysts using molybdenum carbide as the hydrogen extraction center for biomass conversion and Fischer−Tropsch synthesis from CO and H2O will be summarized.
to noble metals in heterogeneous catalysis. Since the pioneering work of Levy and Boudart in 1973 that described tungsten carbide exhibiting platinum-like catalysts in the isomerization of hydrocarbons,4 TMCs and dispersed TMCs on oxide supports have facilitated a series of important reactions as catalytically active components, particularly in the activation and selective transformation of C−H bonds of hydrocarbons (e.g., dehydrogenation,5 hydrogenation,6 and hydrogenolysis7). In these studies, it has been demonstrated that the crystal structure, site occupancy, metal/carbon ratio, surface termination, and defects are the factors that could significantly influence the reactivity of the TMC catalysts.8 Secondary promoters doped onto the surface or into the lattice (e.g., bimetallic carbides9) are also the effective methods to enhance the catalytic performance of TMCs in the syngas process,10 hydrodeoxygenation,11 and hydrogenation.12 Recently, significant research interest has been attracted by the idea of utilizing TMCs as functional supports for heterogeneous catalysts.3,13−15 Compared with traditional oxide supports, the TMC supports offer several functions to enhance catalytic performance. First, the TMC supports possess strong interaction with the loaded metal and exhibit tremendous influence on the geometric and electronic structure of the metal species.16 Owing to the strong mutual interaction between TMCs and the metals, the supported metal domains tend to exhibit under-coordinated (e.g., sub-nanometer clusters and single atoms) or tiled (e.g., raft-like particles and overlayers) geometries, which maximize the utilization efficiency of the loaded metals.17 Also, normally in an oxidative reaction environment, pure metal carbide tends to be oxidized and deactivated. For metal/metal carbide catalyst systems, if the reaction environment is an oxygen-containing environment, oxygen decoration on the carbide surface is inevitable (forming surface oxycarbides). However, if the turnover of the oxygencontaining species, such as OH or surface oxygen species, on the metal carbide (by CO or carbon-containing intermediates on metal) is sufficiently efficient, deep oxidation of the metal carbide and deactivation of the catalyst will not happen. In addition, the formation of metal−carbide bonding induces the charge redistribution at the interface, which reduces the electron density of the metal species, endowing the metal centers with special adsorption and catalytic behaviors in the reaction.16 Second, as an active component, the carbide support is capable of participating in certain steps in a tandem or cascade reaction or promoting the activation of special chemical bonds in the intermediates. Finally, the introduction of secondary metal centers on the surface of the original TMC generates novel interfacial sites with special electronic properties and coordination environment, which enhances the synergistic effect between the metals and the supports.18 As a class of most frequently studied TMCs, molybdenum carbides have been reported as excellent catalysts in hightemperature applications.19,20 In the past decades, the potential utilization of molybdenum carbide for low-temperature applications are extensively explored.21 In these attempts, the metastable, face-centered cubic α-MoC has gradually become a star material.22 Although not as active as the hexagonal β-Mo2C in the traditional C−H activation reaction, due to its less metallic properties, α-MoC exhibits unexpectedly high activity in the activation of O−H bonds at low temperatures.23 In additional, α-MoC has been discovered to possess much stronger interaction with noble metals such as Pt, Au, and others than the frequently studied β-Mo2C, providing potential opportu-
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A BRIEF REVIEW ON THE SYNTHETIC METHODS FOR MOLYBDENUM CARBIDES
The synthesis methods for molybdenum carbide originate from the metallurgical industry, in which the Mo precursors (metal powders, hydrides, or oxides) are mixed with appropriate amounts of carbon and then heated under an inert or a reductive atmosphere at high temperature, up to 1500 K. In these procedures, the products are the thermodynamically stable hexagonal phase, β-Mo2C, with relatively small surface area. In order to increase the surface area of the carbide materials and develop a universal synthesis method for fabricating loaded molybdenum carbides on different oxide supports, Boudart et al. developed the temperature programmed carburization method in which the molybdenum precursor (generally molybdenum oxides) is in the flow of the hydrocarbon and hydrogen mixture.24 With the gradually increased temperature, the hydrocarbon will decompose and the generated carbons will incorporate into the reduced Mo lattice to form the desired molybdenum carbide. The flow rate, the type and concentration of the carbon sources, and the heating programs are all tunable factors that will influence the physical and chemical properties of the molybdenum carbide. In order to obtain the meta-stable face-centered cubic structure α-MoC, a topological conversion procedure in which FCC structured Mo precursors are carburized without changing their crystal phase is required. The most frequently used precursors are the FCC structured molybdenum nitride (γMo2N) and the oxyhydride (MoOxHy), which can be obtained via ammonization or long-term H2 treatment.25 In the synthesis of metal/Mo carbide catalysts, two common strategies can be used. The first one is to introduce the metal precursors directly over presynthesized carbide materials using impregnation or other deposition methods. Further activation treatment to reduce the metal from its compounds and regenerate of the active molybdenum carbide surface are generally required before the reaction. Another common synthetic strategy is precipitating the metal precursor with the molybdenum compounds (e.g., ammonium molybdate) to form the metal−Mo oxide composite. The composite can be further carburized using a similar procedure to that of the bare Mo carbide materials. However, due to the autocatalytic behaviors of the loaded metals to the C and H sources after being reduced, the carburization procedures will be influenced and the final products will be varied with the different loaded metals. For instance, β-Mo2C formed when the Fe, Co, or Cu precursor was composited with MoO3, while supported Ni, Pd, or Pt18,21,26 facilitated the formation of α-MoC. B
DOI: 10.1021/acs.accounts.9b00182 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Accounts of Chemical Research
Figure 1. (A, E) High-resolution STEM Z-contrast images, (B, F) XPS characterizations, and (C, G) extended X-ray absorption fine structure fitting results of Pt/α-MoC with the Pt loadings of 0.2 and 2.0 wt %, respectively, (D) X-ray absorption near-edge structure of 0.2% Pt/α-MoC, and (H) EXAFS oscillation functions of 2% Pt/α-MoC. Reproduced with permission from refs 22 and 42. Copyright 2017 and 2019 Springer Nature.
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EXPANDING THE NEW HORIZON OF CATALYTIC APPLICATIONS OF α-MoC AT LOWER TEMPERATURES In traditional catalytic reactions tested over molybdenum carbide catalysts, such as hydrocarbon isomerization, dehydrogenation, and ammonia synthesis, β-Mo2C exhibited superior activity over α-MoC. As a result, understanding of the catalytic behaviors of α-MoC was quite limited from both experimental and theoretical studies. In the past few years, researchers started to revisit this material and have discovered special behaviors of this material as both individual catalyst and catalyst support for the reactions that prefer operating at low temperatures (normally below 300 °C), such as the alcoholysis of lignin,7 methanol reforming,22 and low temperature water−gas shift (WGS).23 The O−H bond activation and extremely strong interaction between metal and α-MoC have expanded the horizon of the understanding related to molybdenum carbide catalysts. Catalytic reforming reactions refer to the reactions between water and hydrocarbons such as CH4 or oxygenated hydrocarbons like methanol, ethanol, polyols, and biomass, in gasphase or aqueous-phase, aiming at producing hydrogen with the expense of converting low valence carbon to carbon oxides.27−29 Due to the low efficiency in direct H2 storage and distribution, the reforming of alcohols is of great importance for fulfilling an indirect H2 storage process, in which H2 will be stored and transported in the form of organic compounds and release only at the position in need via highly efficient catalytic reactions operating at ambient pressure and temperature. By the reforming of methanol and water (i.e., CH3OH + H2O → CO2 + 3H2), hydrogen with a high gravimetric density of 12 wt % can be released in situ for the following applications. The well investigated catalysts in industry for the reforming of alcohols were Cu-based catalysts such as Cu/ZnO/Al2O3 catalyst under an operating temperature of 513−533 K.28
However, the critical problem of the Cu/ZnO/Al2O3 catalyst was its serious deactivation under the flow of water and methanol, while other more stable catalysts using noble metals as active sites tend to catalyze the methanol decomposition reaction, which generates a significant amount of CO. Therefore, suitable catalytic systems are required to promote the application of methanol reforming. The use of β-Mo2C as an individual catalyst or a catalyst support for methanol reforming has been extensively investigated. The use of pure Mo2C was first reported by Széchenyi and Solymosi, where the conversion of methanol with high yield and selectivity of H2 was achieved at 623 K.30 But the catalyst showed significant deactivation after 10 h of reaction. Transition metals such as Pt, Fe, Co, Ni, and Cu supported on β-Mo2C were reported by Ma et al. for the MSR reaction.21,26 Compared with pure β-Mo2C, the metal-doped β-Mo2C showed higher methanol conversion and hydrogen production yield. Among these metal/β-Mo2C catalysts, Pt/β-Mo2C showed the highest catalytic activity and selectivity, with a methanol conversion of 100% even at a temperature as low as 473 K. Ni/β-Mo2C showed high long-term stability, and nearly 90% of its activity was preserved after 50 h of reaction with unchanged selectivity toward H2.31 In our studies,22 we discovered that the reactivity, the size of supported Pt domains, and the Pt−Mo interaction have positive correlations with the percentage of FCC α-MoC in the support (Figure 1). When pure α-MoC is used as the support, the mass specific activity of 2% Pt/α-MoC catalyst is over 10 times higher than that of the Pt/β-Mo2C with the same loading. In the catalyst, the Pt−Pt coordination number dropped to only 5, suggesting α-MoC exhibits much stronger interaction than βMo2C. Detailed characterizations (Figure 1) including highresolution STEM Z-contrast images and X-ray absorption fine structure proved the presence of a high percentage of atomically dispersed Pt1, which bonded with the surface Mo atoms at the range of Pt loading from 0.2 to 2 wt %. Besides controlling the C
DOI: 10.1021/acs.accounts.9b00182 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. Catalytic performance and mechanistic study of Pt/α-MoC for APRM: (A) Coordination numbers of Pt−Pt and Pt−Mo shells and their catalytic activity. (B) Eleven-cycle activity test. (C, D) DFT calculation of reaction path for APRM: (C) energy profiles for CH3OH dissociation into CO and H atoms on α-MoC(111), Pt(111), and Pt1/α-MoC(111) surfaces; (D) energy profiles for CO2 formation via the water−gas shift reaction on these three surfaces. Pt, Mo, C, O, and H atoms are shown in blue, cyan, gray, red, respectively; the C atom in CO is represented in black. Reproduced with permission from ref 22. Copyright 2017 Springer Nature.
CO2 and CO is inevitable. The further purification processes to separate hydrogen from the reforming gas for high purity H2 consume a large amount of energy. In the hydrogen economy, the H2 fuel cell requires the CO impurities in the gas feed to be less than 10 ppm.32 The WGS reaction (eq 1) is an important process for hydrogen generation and carbon monoxide removal in hydrogen production processes. Industrially, the WGS reaction was carried out in two stages: (i) high-temperature WGS (573− 623 K) using Fe-based catalysts (e.g., Fe2O3/Cr2O3) and (ii) low-temperature WGS (473−543 K) using Cu-based catalysts (e.g., Cu/ZnO/Al2O3 catalyst).
geometric structure of Pt1, modified electronic structure was also observed based on the XPS and XANES results. The electrons were partially transferred from Pt1 to the α-MoC support generating electron-deficient Pt1 atoms. Combining the distinct geometric and electronic structure of Pt1 and α-MoC, the usage efficiency of Pt metal is maximized and the Pt/α-MoC catalyst showed extraordinary performance for the aqueous-phase reforming of methanol (APRM). The average turnover frequency (TOF) could reach 18046 mol of H2 (mol of Pt)−1 h−1 at 463 K, which is 2 orders of magnitude higher than those of the traditional catalysts (Figure 2A). In an 11-cycle test, the 0.2% Pt/α-MoC catalyst achieved a total turnover number of more than 132000 for each platinum atom, generating about 1.68 mol hydrogen per gram catalyst, almost meeting the requirements for state-of-the-art PEMFC vehicle applications (Figure 2B). DFT calculations (Figure 2C,D) revealed that Pt/α-MoC was a bifunctional catalyst. With the α-MoC support dissociating O−H bonds of H2O and CH3OH at low temperature and atomically dispersed Pt scissoring C−H bonds of CH3OH, the intermediates of surface hydroxyls could be effectively reformed to generate CO2 and hydrogen at the interface of Pt and α-MoC. The excellent performance of Pt/α-MoC provided a new strategy for efficient low-temperature hydrogen production and storage. Currently, commercial hydrogen is mainly (∼95%) produced from coal gasification and steam reforming of hydrocarbons or alcohols. In this hydrogen production process, the generation of
CO + H 2O → CO2 + H 2
(1)
However, the industrial catalysts failed to meet the requirements for a mobile CO removal system (e.g., fuel cell vehicle), due to their high activation temperature and poor stability under repeated “start-up−cool-down” cycles. As a result, the state-ofthe-art catalysts in the laboratory for low-temperature WGS reactions were mainly gold or platinum supported on reducible oxides (CeOx, TiOx, etc.),33,34 which have higher stability and operate above 523 K. A recent development from FlytzaniStephanopoulos et al.35,36 demonstrated that alkali-promoted inert supports such as alumina, silica, or zeolites (Au−Na/ MCM41, Pt−Na/K−SiO2/Al2O3) have similar function to the reducible supports. As the WGS reaction is an equilibrium-limited reaction that favors low temperatures, reducing the operating temperature D
DOI: 10.1021/acs.accounts.9b00182 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 3. Structural characterizations of the 2% Au/α-MoC catalyst. (A, B) High resolution high-angle annular dark-field (HAADF)-STEM images with single atoms of Au marked in blue circles and layered Au structures highlighted in yellow. (C−E) STEM z-contrast image and EDS elemental maps of Au and Mo. (F−H) XRD patterns, XPS analysis, and extended X-ray absorption fine structure results. Reproduced with permission from ref 23. Copyright 2017 The American Association for the Advancement of Science.
The structural characterizations of the 2% Au/α-MoC catalyst demonstrate that the α-MoC support forms a strong interaction with the Au particles (Figure 3). Even after 2 h treatment at 863 K, the Au particles were still anchored with fine dispersion over the support. High-resolution STEM images and EDS element mappings revealed that two types of Au species existed on the surface of α-MoC: (1) small layered Au clusters with an average diameter of 1−2 nm and a thickness of 2−4 atoms (
