Chapter 1: Introduction to the Application of Nitrides, Carbides, Phosphides and Amorphous Boron Alloys in Catalysis
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Published:11 Jul 2018
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Special Collection: 2018 ebook collectionSeries: Catalysis Series
K. J. Smith, in Alternative Catalytic Materials: Carbides, Nitrides, Phosphides and Amorphous Boron Alloys, ed. J. S. J. Hargreaves, A. R. McFarlane, and S. Laassiri, The Royal Society of Chemistry, 2018, ch. 1, pp. 1-26.
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The incorporation of C, N, P and B into transition metals, passivates metal surface reactivity, suggesting the possibility of new materials for catalysis with tunable activity. In this introductory chapter, an overview of the application of these new materials in catalysis is provided. Most applications have focused on reactions that are known to be catalysed by metals, including hydrogenations, hydrotreating reactions (C–N, C–S and C–O bond cleavage by hydrogen), reforming and synthesis gas conversion. Hydrotreating applications (S and N removal) are limited by instability issues, although hydrodeoxygenation and biofuels upgrading applications show promise because of the unique reaction pathways that occur on carbides and phosphides. Although activities and selectivities for synthesis gas conversion are promising, the interstitial compounds produce significant amounts of CH4. The materials show promise in applications of hydrogen production and storage and in several electrocatalytic processes, including water splitting.
1.1 Introduction
Catalysts are used to enhance the rates of thermocatalytic, electrocatalytic and photocatalytic reactions.1 Catalysts function by interacting with reacting species to generate new reaction intermediates that only exist on the catalyst surface, thereby providing alternative, faster reaction pathways to the desired products. Key to the catalytic reaction is the breaking and forming of new bonds between the catalyst and the reacting molecules. Consequently, the efficacy of a particular catalyst is determined by its surface chemistry and new developments in catalysis science and technology are driven in part by the development of new materials with well controlled surface properties. Increasing attention has been given to the discovery of heterogeneous catalysts that have high thermal and chemical stability, using materials that are not strategically limited and are of low cost.2 Catalysts are used in a wide range of applications including motor vehicle emissions control, upgrading and refining of crude oils, and they are responsible for approximately 90% of the chemical processes currently operating world-wide in the chemical industry.1,3
Controlling the reactivity of a catalyst surface plays a key role in obtaining catalysts with high activity and selectivity. Sabatier's principle indicates that the interactions between reacting species and a catalyst surface must be of the appropriate strength—interactions that are too strong or too weak yield less active catalysts.1 Hence, in the classic paper of Levy and Boudart,4 C added to W to produce W2C, was shown to decrease the high reactivity of W toward O and thereby yield an effective catalyst for the H2+O2→H2O reaction at room temperature, a reaction typically catalysed by Pt. Similarly, the W2C was active for the isomerisation of 2,2-dimethylpropane to 2-methylbutane, a reaction also catalysed by Pt.4 This work spawned a large number of studies in which new materials with metallic character, formed by the incorporation of C, N, P and B into the lattices of early transition metals, have been investigated as potential catalysts for various reactions.5–7 These interstitial alloys adopt simple crystal structures with the carbides and nitrides forming face-centered cubic (fcc), hexagonal close packed (hcp) or simple hexagonal (hex) structures, as summarised in Table 1.1. Many metal phosphides are also known7 and their crystal structures as shown in Figure 1.1. The very rich chemistry of these interstitial alloys provides an opportunity for the development of new catalysts with a wide potential for application, especially with the synthesis of these materials as well dispersed nanoparticles.5
Crystal structure . | Compound . |
---|---|
Fcc | TiC, ZrC, HfC, VC, NbC, and TaC |
TiN, VN, NbN | |
γ-Mo2N, β-W2N, Re2N | |
Hcp | β-Mo2N, W2C, Re2C |
Hexagonal | WC, MoC, δ-WN |
Crystal structure . | Compound . |
---|---|
Fcc | TiC, ZrC, HfC, VC, NbC, and TaC |
TiN, VN, NbN | |
γ-Mo2N, β-W2N, Re2N | |
Hcp | β-Mo2N, W2C, Re2C |
Hexagonal | WC, MoC, δ-WN |
Crystal structures of metal-rich phosphides.9 Reprinted from Catal. Today, 143, S. T. Oyama, T. Gott, H. Zhao and Y. Lee, Transition metal phosphide hydroprocessing catalysts: A review, 94–107. Copyright (2009) with permission from Elsevier.
Crystal structures of metal-rich phosphides.9 Reprinted from Catal. Today, 143, S. T. Oyama, T. Gott, H. Zhao and Y. Lee, Transition metal phosphide hydroprocessing catalysts: A review, 94–107. Copyright (2009) with permission from Elsevier.
The unique catalytic behaviour of metals bound to C, N, P or S may be attributed to the changes in the electronic properties of the metal surface induced by the ligands and/or the geometry by which the metal and the ligands are arranged at the catalyst surface. Hence, the ligands passivate the metal surface reactivity and these effects have been quantified in some cases using molecular simulation.10 For example, Liu and Rodriguez11 have shown, by DFT calculation, that the CO adsorption and S adsorption energies on the (001) surfaces of MoN, MoC, MoP and Mo decrease with an increase in their d-band centre energy, as shown in Figure 1.2.
Calculated adsorption energy of CO (g) and S (l) as a function of the d-band centre for Mo in clean surfaces of Mo(001), γ-MoC(001), δ-MoN(001) and MoP(001). Here, the d-band centres are relative to the Fermi energy.11 P. Liu and J. A. Rodriguez, Catalysis Letters, 2003, 91, 247–252. With permission of Springer.
Calculated adsorption energy of CO (g) and S (l) as a function of the d-band centre for Mo in clean surfaces of Mo(001), γ-MoC(001), δ-MoN(001) and MoP(001). Here, the d-band centres are relative to the Fermi energy.11 P. Liu and J. A. Rodriguez, Catalysis Letters, 2003, 91, 247–252. With permission of Springer.
These observations suggest the possibility that metal surface reactivity can be controlled and tuned using interstitial atoms.12 Consequently, there exist several detailed reviews describing the incorporation of C,5,13 N,6,14 P7,9 and B15 into metals and the use of the resulting interstitial alloys for catalysis, with varying degrees of success. Since, in nearly all cases, the interstitial atoms are incorporated into metals, the application of these new materials is mostly focused on reactions catalysed by metals. In some cases, however, the catalysts may also have acidic or basic properties, broadening their potential application.16–18 In this chapter, the application of these materials is introduced with a view to demonstrating the wide scope of their application in heterogeneous catalysis. The focus is on their use as thermochemical catalysts and electrocatalysts, although they have also been used as catalyst supports. Selected examples are described to illustrate the application. Comprehensive reviews of these materials and their applications are presented in later chapters.
1.2 Hydrogenation Reactions
Hydrogenation reactions that use expensive metal catalysts are an essential part of chemical synthesis and product upgrading processes.3 Metal carbides,19–22 nitrides23–25 and especially borides26–28 are also effective hydrogenation catalysts for a wide range of applications and these materials have the potential to replace some metal catalysts.
The hydrogenation of aromatic compounds is possible on metal carbide, nitride, phosphide and boride catalysts. Mo2C has been shown to hydrogenate naphthalene to tetralin with high selectivity (>90%) at 340 °C and 4 MPa H2, although catalyst activity declined from 90% to 30% over a period of about 60 h.29 When supported on HY zeolite, the Mo2C also shows some ring-opening activity with about 10% selectivity to ring-opened products and 80% tetralin selectivity at 300 °C and 3 MPa H2.30 However, the Mo2C/HY catalyst was less active than a commercial Pd/HY catalyst operated at the same conditions. The bimetallic nitride Ni2Mo3N (unsupported) also has high catalytic activity for benzene hydrogenation25 and promotion with K reduces the deactivation of the catalyst in the presence of S.31 Amorphous RuB supported on ZrO2 hydrogenates benzene to cyclohexene with high yields (46% maximum at 433 K, 5 MPa and 55 min reaction time and liquid phase).32
The hydrogenation of toluene on a low surface area (12 m2 g−1) unsupported Mo2C has also been reported at 423–598 K and 2.76 MPa H2 pressure. Within the temperature range 473–523 K, the yield of methylcyclohexane was 100%, but at higher temperature, ethylcylopentane was also produced as a result of isomerisation reactions.33 The dehydrogenation of decalin has also been shown to occur on WC,34 while the dehydrogenation performance of a Ni–WC/AC catalyst was better than that of Ni/AC and WC/AC at 101 kPa and 400 °C. Among a series Ni–WC/AC catalysts, decalin dehydrogenation to naphthalene was highest on a 30 wt%Ni–20 wt%WC/AC catalyst. The Ni–WC/AC catalyst showed good stability for decalin dehydrogenation.34 The hydrogenation of olefins and dienes has also been reported on amorphous metal borides,35–37 nitrides23,38 and carbides.23
In the past decade, the transformation of residual biomass into fuels and chemicals has been the focus of much research. Hydrogenations of intermediate molecules is an important step in these processes and the Pt-group metals (PGMs) are often the catalyst of choice for the hydrogenation reactions. For example, levulinic acid (LA) has been identified as an important molecule for application in future biorefineries that can be produced from lignocellulosic wastes at low cost. LA can be hydrogenated to γ-valerolactone (GVL), which also has many potential applications. The heterogeneous catalytic hydrogenation of LA to GVL is therefore of interest and Ru supported on activated carbon is usually the catalyst of choice. Recently, Quiroz et al.39 reported that nanostructured β-Mo2C has a LA to GVL turnover frequency (TOF) of 2.3 min−1, measured at 30 bar H2 and 180 °C, the same order of magnitude as that reported for Ru/TiO2 (9.8 min−1)40 and a Ni–MoOx/C (TOF of 3.4 min−1), the latter performed under more severe conditions (50 bar H2 and 240 °C).41 The authors also reported that the nanostructured β-Mo2C was stable under continuous operation. Amorphous metal-boride alloy catalysts have also attracted attention for hydrogenation of bio-based feedstocks. The hydrogenation of furfural, crotonaldehyde and citral on CuB/SiO2 and Cr–CuB/SiO2 catalysts, has been carried out with 5 mL of substrate in 75 mL ethanol at 393 K for furfural and crotonaldehyde, and at 373 K for citral, under a constant pressure of 929 kPa.42 The CuB/SiO2 catalyst was more active than a Cu/SiO2 catalyst. Citral hydrogenation on a 5%NiB/SiO2 catalyst has a high yield (∼84%) at 30 °C.43 In the hydrogenation of fructose and fructose/glucose mixtures, bimetallic amorphous CoNiB catalysts operated at 343 K and 6 MPa were more active than NiB, and much more active than CoB and Raney Ni.44 In a separate study of glucose hydrogenation over an amorphous Ni–B/SiO2 catalyst at 373 K and 4.0 MPa, the catalyst had much higher activity than other Ni-based catalysts, including crystalline Ni–B/SiO2, Ni/SiO2 and a commercial Raney Ni catalyst.45,46 The hydrogenation of ethyl lactate derived from fermentation of renewable resources such as carbohydrates, to yield a green propanediol can also be achieved on a RuB/Sn-SBA-15 catalyst operated at 423 K and 5.5 MPa H2.47
The carbides, nitrides and borides have also been shown to have high activity and selectivity for the hydrogenation and reduction of nitro groups.20,48,49 The bimetallic nitride Fe3Mo3N promotes selective reduction of –NO2 in p-chloronitrobenzene to generate p-chloroaniline whereas Co3Mo3N favours C–Cl scission with the formation of nitrobenzene.25 Earlier work has also shown that the transition metal nitrides have activity for n-butane dehydrogenation, hydrogenolysis and isomerisation.50 Hydrogenation of p-chloronitrobenzene also occurs on La-doped NiMoB,51 NiFeB52 and NiCoB.53
Metal carbide catalysts have also been shown to be effective in the hydrogenation of dimethyl oxalate (DMO) to ethanol.54,55 This indirect synthesis route to ethanol, uses CO oxidative coupling to produce DMO. The authors report that Cu–Mo2C/SiO2 and Mo2C/SiO2 have very good stability and activity for the hydrogenation of DMO to ethanol (Figure 1.3) at low temperatures (473 K), compared to conventional Cu/SiO2, which although very active, degrades during hydrogenation because of agglomeration of the Cu particles.
The reaction pathway of the hydrogenation of DMO. Reproduced from ref. 54 with permission from The Royal Society of Chemistry.
The reaction pathway of the hydrogenation of DMO. Reproduced from ref. 54 with permission from The Royal Society of Chemistry.
1.3 Hydrotreating Reactions
Hydrotreating refers to a class of reactions in which heteroatoms (S, N, O) are removed from organic molecules by reaction with hydrogen to yield refined products suitable for use as fuels and chemicals.56 Conventional hydrotreating catalysts are based on supported MoS2 promoted with Co or Ni. These catalysts have been used commercially in the petroleum industry for decades, especially for hydrodesulfurisation (HDS) and hydrodenitrogenation (HDN) processes.3,56 With increased environmental legislation that has reduced allowable S and N levels in fuels, catalysts with higher HDN and HDS activity, especially for the more refractory heteroaromatics present in residue oils, has become the focus of research and in this regard, metal carbides,13 nitrides,13,14 and phosphides9,57 have been applied to both HDS and HDN. In the HDS of thiophene at 450 °C, both Co2B and Ni3B are partially sulfided,58 suggesting that the borides are unstable and for this reason there are few studies describing the catalytic activity of metal borides for HDN and HDS. With the growing interest in bio-oil upgrading in which O removal by hydrodeoxygenation (HDO) reactions is critical, the carbides, nitrides, phosphides and borides have also been assessed for HDO.6,59,60
Table 1.2 reports data from Sajkowski and Oyama61 who compared the hydrotreating activity of a commercial Ni–MoS/Al2O3 catalyst to that of a Mo2N and a Mo2C/Al2O3 catalyst. The catalysts were assessed using a residue oil and a gas oil, both derived from coal, so that the feedstocks had high S (810 and 116 ppm S, respectively) and N (4620 and 3580 ppm N, respectively) contents. The apparent first-order rate constants for desulfurisation, denitrogenation and aromatic saturation reactions, normalised for the number of active sites measured by suitable chemisorption experiments, show the Mo2C to be significantly more active than the commercial catalyst.
Estimated relative first-order rate constants per active site, adapted from Sajkowski and Oyama.61 Results measured at 633 K and 13.7 MPa with co-current upward flow of oil (4 cm3 h−l) and H2 (159 cm3 (STP) min−1).
. | Desulfurisation . | Denitrogenation . | Aromatic saturation . |
---|---|---|---|
Mo2N | 1.3 | 3.4 | 3.4 |
Mo2C/Al2O3 | 2.1 | 5 | 3.1 |
MoS2/Al2O3 | 0.67 | 0.98 | 0.41 |
Ni–Mo–S/Al2O3 | 1 | 1 | 1 |
. | Desulfurisation . | Denitrogenation . | Aromatic saturation . |
---|---|---|---|
Mo2N | 1.3 | 3.4 | 3.4 |
Mo2C/Al2O3 | 2.1 | 5 | 3.1 |
MoS2/Al2O3 | 0.67 | 0.98 | 0.41 |
Ni–Mo–S/Al2O3 | 1 | 1 | 1 |
However, in a comparison of NiMo sulfide, carbide and nitride catalysts for the HDS of thiophene and Maya crude oil, Villasana et al.62 reported that NiMoS was the most effective catalyst and this trend was the same for residue conversion.62 The HDN and HDS activities of bimetallic Ni–Mo carbide, nitride and sulfide catalysts have also been compared using light gas oil and heavy gas oil derived from Athabasca bitumen at 8.8 MPa in the temperature range 340–370 °C and 375–400 °C, respectively. The supported Ni–Mo sulfide catalyst was more active for HDN and HDS of light gas oil and heavy gas oil than the corresponding carbide and nitride catalysts, on a mass of catalyst basis.63–65 In another study, P-promoted Mo2C/Al2O3 was shown to be more active for deep desulfurisation of refractory compounds such as 4,6-dimethyldibenzothiophene than a commercial Ni–Mo sulfide.66 The metal phosphides are also effective for hydrotreating real feedstocks, as demonstrated by Oyama et al.57,67 who reported that Ni2P/SiO2 had better activity in hydroprocessing than a commercial Ni–Mo–S/Al2O3 catalyst when using a real gas oil feed at 593 K and 3.9 MPa and based on equal catalyst weights loaded in the reactor (HDS 85% vs. 80%). Subsequent studies also reported significant activity of bimetallic phosphides using realistic gas oil feedstocks.68–70 The gradual transformation of Ni–Mo carbide, nitride and phosphide phases into Ni–Mo sulfide phases was observed during the hydrotreating reactions and similar transformations have been reported in other studies.31,58,62 The impact of changes in composition and morphology on the long-term operation of these alternative catalysts under commercial conditions has not been reported in the literature, but these transformations likely impact the reported relative activity of the materials among the different studies.
Many other studies have demonstrated that the metal carbides, nitrides and phosphides are effective catalysts for HDS and HDN reactions using model reactants. Studies with model compounds allow reaction pathways to be clearly established. For example, in studies of the deep desulfurisation of refractory heteroaromatics on Mo2C,66,71 in which 4,6-dimethyldibenzothiophene (4,6 DMDBT) is used as model reactant, it was shown that the reaction proceeds by two parallel routes leading to 3,3′-dimethylbiphenyl (DMBP) by a direct desulfurisation (DDS) pathway or to 3-(3′-methylcyclohexyl)toluene (MCHT) through a hydrogenation (HYD) pathway, as summarised in Figure 1.4.72 On the Mo2C catalyst, the DDS route is more important (HYD/DDS=0.7) whereas over commercial Ni–Mo–S/Al2O3, operating under the same conditions of temperature (613 K) and pressure (4 MPa total pressure), the HDS of 4,6-DMDBT by the HYD pathway is favoured (80% for Ni–Mo–S/Al2O3).72 In another important study, a series of transition metal carbides (Mo2C, WC, NbC) and nitrides (TiN, VN, Mo2N) were tested in a three-phase trickle-bed reactor using a model liquid feed mixture containing 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt% aromatics (tetralin), and balance aliphatics (tetradecane).73 The Mo2C showed higher HDN activity than a commercial sulfided Ni–Mo/Al2O3 catalyst (on the basis of unit area of catalyst), while VN was found to exhibit excellent activity and selectivity for the HDO of benzofuran. The HDN activity of WC was found to be comparable to that of a commercial sulfided Ni–Mo/Al2O3 catalyst. A comparative study of the metal phosphides using dibenzothiophene as the model reactant for HDS and quinoline for HDN at 643 K and 3.1 MPa, showed the order of activity increased as follows: Fe2P<CoP<MoP<WP<Ni2P,57 on the basis of an equal number of active sites in the reactor. Few studies of HDN using metal nitrides other than Mo2N have been reported in the literature.14 Milad et al.74 reported that the activity per unit area of a series of unsupported metal nitrides for pyridine HDN was as follows: Co4N>Fe3N>Mo2N>W2N>NbN>CrN>VN. The activity of the metal nitrides decreased with the heat of formation of the metal nitrides.
Pathways for HDS of 4,6DMDBT.72 Reprinted from J. Mol. Catal. A: Chem., 184, P. Da Costa, C. Potvin, J.-M. Manoli, J.-L. Lemberton, G. Perot and J. Djega-Mariadassou, New catalysts for deep hydrotreatment of diesel fuel: Kinetics of 4,6-dimethyldibenzothiophene hydrodesulfurisation over alumina-supported molybdenum carbide, 323–333. Copyright (2002) with permission from Elsevier.
Pathways for HDS of 4,6DMDBT.72 Reprinted from J. Mol. Catal. A: Chem., 184, P. Da Costa, C. Potvin, J.-M. Manoli, J.-L. Lemberton, G. Perot and J. Djega-Mariadassou, New catalysts for deep hydrotreatment of diesel fuel: Kinetics of 4,6-dimethyldibenzothiophene hydrodesulfurisation over alumina-supported molybdenum carbide, 323–333. Copyright (2002) with permission from Elsevier.
Despite the promising activities of the metal carbides, nitrides and phosphides reported in the literature for the HDS and HDN reactions, these are well established, mature commercial technologies used to produce high quality fuels and it is difficult to displace the highly active Ni(Co)–Mo–S/Al2O3 catalysts that have been optimised for these processes over many years. However, with increased interest in the development of biorefinery technology for sustainable green fuels, and given that one major route to green fuels involves O-removal from intermediate bio-oils generated by pyrolysis processes,75 there has been a very significant effort to demonstrate the use of metal carbide,76–78 nitride79–81 and phosphide82,83 catalysts for HDO reactions.84 There are several examples in the literature where the carbides, nitrides and phosphides are shown to favour reaction pathways for O removal that are different to that which occurs on group 8 metal catalysts.76,85 For example, with phenol as a model reactant on metals such as Pt, Pd and Rh, hydrogenation reactions dominate, yielding cyclohexanol as product; whereas, on metals with higher O affinities such as Ru, Co and Ni, deoxygenation reactions occur more readily.86 On Mo2C and W2C, C–O bond cleavage is favoured with minimal hydrogenation occurring especially on oxygen-modified materials.76 Studies of the HDO of guaiacol (Figure 1.5) show that a key first step in the HDO over Ru, for example, is via catechol formation (by O–CH3 bond cleavage) followed by phenol formation (R–OH bond cleavage),85,87 whereas on Mo2C direct demethoxylation occurs to yield phenol.76 The same demethoxylation reactions occur on transition metal phosphide catalysts82 whereas both paths appear to occur on Mo2N.79
Reaction network for guaiacol conversion as reported by Chang et al.85 Reprinted with permission from ref. 85. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Although the unique characteristics of these materials for the HDO of model compounds is well documented, few studies of their application using real bio-oils are available. Recently, however, Guo et al.88 compared a series of metal phosphides for the HDO of a pyrolysis oil at 300 °C and 50 bar, using 40 g of pyrolysis oil and 2 g of catalyst reacted for 3 h (Figure 1.6). The authors reported that MoP/AC had HDO activity comparable to a Ru/C catalyst and the HDO activity of the metal phosphides were found to follow the order of NiP/AC>CoP/AC>MoP/AC.88 Figure 1.6 also shows that the oil fraction yield was significantly improved by addition of P and the MoP/AC catalyst had the highest oil fraction yield among the Mo-based catalysts.
Yield of oil fraction from the HDO experiments with different in-house prepared catalysts compared with the commercial Ru/C catalyst (300 °C, 50 bar H2, 3 h).88 © 2016 American Institute of Chemical Engineers.
Yield of oil fraction from the HDO experiments with different in-house prepared catalysts compared with the commercial Ru/C catalyst (300 °C, 50 bar H2, 3 h).88 © 2016 American Institute of Chemical Engineers.
Hydrotreating vegetable oils mainly composed of triglycerides is another important deoxygenation pathway to prepare high-grade diesel-like hydrocarbons from renewable biomass.89 Several studies have investigated metal carbides,90,91 nitrides92 and phosphides93,94 for the HDO of vegetable oils. Table 1.3 compares results from various vegetable oils using a Mo2C catalyst and Table 1.4 gives a comparison among Mo2N, WN and VN supported on γ-Al2O3 used in the hydrotreating of oleic acid.92 The Mo2C catalyst shows high yields of deoxygenated hydrocarbons for a range of vegetable oils, whereas Table 1.4 shows similar activity for the Mo2N catalyst, with the VN having significantly lower yield of hydrocarbon products. HDO of methyl palmitate (50% in decalin) over a 5–20 wt% Ni2P/MCM catalyst at 3 MPa H2, WHSV=3 h−1, and H2/oil=1000 at 290–350 °C has also been reported.93 At 290 °C, the conversion was 85% with C15 and C16 selectivities of 55% and 45%, respectively. The HDO of methyl laurate at 573–613 K and 3.0 MPa on Ni2P on various supports was reported by Shi et al.,95 showing that the conversion of methyl laurate and the selectivity to C11 and C12 hydrocarbons was highest on the Ni2P/SiO2 catalyst. The effect of the support mainly results from the support acidity and the metal-support interaction that limits reducibility.95 The high activity of the Ni2P for O-removal compared to other metal phosphides has also been observed for HDS and HDN reactions.
Vegetable oil . | Conversion (%) . | Yield (%) . |
---|---|---|
Maize oil | 100 | 95 |
Olive oil | 100 | 95 |
Rapeseed oil | 88 | 85 |
Soya bean oil | 94 | 92 |
Sunflower oil | 97 | 94 |
Vegetable oil . | Conversion (%) . | Yield (%) . |
---|---|---|
Maize oil | 100 | 95 |
Olive oil | 100 | 95 |
Rapeseed oil | 88 | 85 |
Soya bean oil | 94 | 92 |
Sunflower oil | 97 | 94 |
Note: The yield was defined as the mass of hydrocarbons produced (mainly in the range of C15–C18) divided by the mass theoretically produced.
Metal nitride catalysts supported on γ-Al2O3 for hydrotreating oleic acid at 380 °C, 7.15 MPa H2, 0.45 h−1 and 810 H2/L oleic acid. Adapted from ref. 92.
. | Mo2N . | WN . | VN . |
---|---|---|---|
Oleic acid conversion (%) | 99.9 | 97.1 | 97.0 |
O removal (%) | ∼100 | ∼100 | 71.8 |
Product yields (g/100 g oleic acid) | |||
Liquid organic products | 84.1 | 81.1 | 85.0 |
CO2 | 1.6 | 2.2 | 2.6 |
H2O | 9.7 | 4.2 | 2.4 |
. | Mo2N . | WN . | VN . |
---|---|---|---|
Oleic acid conversion (%) | 99.9 | 97.1 | 97.0 |
O removal (%) | ∼100 | ∼100 | 71.8 |
Product yields (g/100 g oleic acid) | |||
Liquid organic products | 84.1 | 81.1 | 85.0 |
CO2 | 1.6 | 2.2 | 2.6 |
H2O | 9.7 | 4.2 | 2.4 |
In a related study, the bulk metal-rich phosphide, Ni3P, prepared with high purity using a hydrothermal method followed by annealing at 773 K,96 has been shown to have high activity for selective glycerol hydrogenolysis at low temperature, with high selectivity to 1,2-propanediol.96
The direct catalytic conversion of raw woody biomass into phenols and diols over a carbon supported Ni-W2C catalyst has also been reported (Table 1.5).97 Using various sources of woody biomass, catalyst and 100 mL of water reacted at 235 °C and 6 MPa H2 for 4 h, the carbohydrate fraction in the woody biomass was converted to ethylene glycol and other diols. A synergistic effect in Ni–W2C/AC existed, not only in the conversion of the carbohydrate fraction, but also in lignin component degradation and the latter activity was comparable to noble metal catalysts.
The results of 4% Ni-30% W2C/AC catalysed hydrocracking of different sources of woody biomass at 235 °C and 6 MPa H2. Adapted from ref. 97.
Biomass source . | Phenol yield (%) . | Diol yield (%) . |
---|---|---|
Poplar | 32 | 75 |
Baswood | 37 | 71 |
Ashtree | 41 | 76 |
Beech | 26 | 58 |
Xylosma | 29 | 62 |
Bagasse | 23 | 60 |
Pine | 10 | 44 |
Yate | 11 | 31 |
Biomass source . | Phenol yield (%) . | Diol yield (%) . |
---|---|---|
Poplar | 32 | 75 |
Baswood | 37 | 71 |
Ashtree | 41 | 76 |
Beech | 26 | 58 |
Xylosma | 29 | 62 |
Bagasse | 23 | 60 |
Pine | 10 | 44 |
Yate | 11 | 31 |
In another potential application, the catalytic conversion of Kraft lignin by ethanolysis was reported over a α-MoC1−x catalyst supported on activated carbon (AC).98 The reaction was done in supercritical ethanol at 280 °C for 6 h in the absence of H2. Products included C6 monohydric alcohols, C8–C10 esters, C7–C10 monohydric phenols, C7–C10 benzylalcohols, and C8–C10 aromatic hydrocarbons. No oligomers or char were formed and both the solvent and catalyst affect the molecular yields and product composition. Wheat-straw Kraft lignin from black liquor can also be used as the feed.
In related work, the direct catalytic decomposition of alkaline lignin over WP catalysts in a hot compressed water–ethanol mixed solvent has also been reported (Table 1.6).99 The reactor was loaded with alkaline lignin (1.0 g), fresh catalyst (0.3 g) and a mixture of water and ethanol solvent (volume ratio 1 : 1, total 100 mL), pressurised to 2.0 MPa H2 and reacted at 553 K for 2 h. The products from the WP/Carbon catalyst consisted of 2-methoxy-phenol (guaiacol), 2-methoxy-4-methyl-phenol, 2-methoxy-4-ethyl-phenol, 2-methoxy-4-acetyl-phenol and 2-methoxy-4-propyl-phenol. The highest overall phenol yield was 67.0 mg g−1 lignin.99
Product yields from lignin conversion using WP catalysts at 553 K and 2 MPa H2. Adapted from ref. 99.
Catalyst . | Phenol selectivity . | Product yield (mg g−1) . | |
---|---|---|---|
Sulfur ethers . | Phenols . | ||
None | 72.6 | 7.4 | 19.6 |
WP | 66.6 | 22.4 | 44.6 |
WP/SiO2 | 54.1 | 26.0 | 30.7 |
WP/AC | 51.7 | 62.5 | 67.0 |
Ni-WP/AC | 73.1 | 18.0 | 48.8 |
Fe-WP/AC | 75.3 | 17.3 | 52.7 |
Catalyst . | Phenol selectivity . | Product yield (mg g−1) . | |
---|---|---|---|
Sulfur ethers . | Phenols . | ||
None | 72.6 | 7.4 | 19.6 |
WP | 66.6 | 22.4 | 44.6 |
WP/SiO2 | 54.1 | 26.0 | 30.7 |
WP/AC | 51.7 | 62.5 | 67.0 |
Ni-WP/AC | 73.1 | 18.0 | 48.8 |
Fe-WP/AC | 75.3 | 17.3 | 52.7 |
Carbide catalysts have also been used for the conversion of cellulose to polyols, especially to ethylene glycol (EG). The catalytic performance of tungsten carbides, molybdenum carbides and platinum on carbon supports reacted at 518 K and 6 MPa for 30 min is summarised in Table 1.7.100 Among all the catalysts, tungsten carbide supported on activated carbon, W2C/AC, showed a higher yield of EG than the Pt, Ni or Mo2C catalysts and the highest EG yield (61%) occurred with the Ni-promoted W2C.
Cellulose conversion and polyol yields over different catalysts at 518 K and 6 MPa for 30 min. Adapted from ref. 100.
Catalyst . | Conversion (%) . | EG yield (%) . |
---|---|---|
Pt/AC | 66 | 8.2 |
Ni/AC | 68 | 5.2 |
W2C/AC | 98 | 27.4 |
Mo2C/AC | 85 | 5.3 |
2%Ni-W2C/AC | 100 | 61.0 |
2%Ni-Mo2C/AC | 87 | 11.3 |
Catalyst . | Conversion (%) . | EG yield (%) . |
---|---|---|
Pt/AC | 66 | 8.2 |
Ni/AC | 68 | 5.2 |
W2C/AC | 98 | 27.4 |
Mo2C/AC | 85 | 5.3 |
2%Ni-W2C/AC | 100 | 61.0 |
2%Ni-Mo2C/AC | 87 | 11.3 |
1.4 Synthesis Gas Production
The metal carbides, nitrides and phosphides have been shown to be effective for a range of reactions involving molecules with one carbon atom, such as CO2, CO and CH4, although most applications have focused on the metal carbides. The production of synthesis gas from CH4 is possible by steam reforming, dry reforming, or partial oxidation yielding a synthesis gas with a H2/CO ratio close to 3, 1 or 2, respectively. Depending on the application, additional processing steps are required to adjust the H2/CO ratio to the desired value. Given the search for new technologies for the conversion and capture of CO2, there is a growing interest in the dry reforming of CH4 and Mo2C catalysts are active for this reaction.101 High activity of Mo2C on various oxide supports was reported by Brungs et al.,102 as summarised in Table 1.8. The relative stability of the catalysts is reported as Mo2C/Al2O3>Mo2C/ZrO2>Mo2C/SiO2>Mo2C/TiO2. Subsequent studies have investigated several other metal carbides as catalysts for dry reforming, as is summarised in Table 1.9.
Dry reforming of methane over supported Mo2C catalysts operated at 1220 K, 8 bar CH4/CO2=1 and GHSV=2600 h−1. Adapted from ref. 102.
Catalyst support . | Mo2C loading (wt%) . | Conversion (%) . | CO Yield (%) . | H2/CO . | |
---|---|---|---|---|---|
CH4 . | CO2 . | ||||
SiO2 | 18.3 | 91 | 86 | 89 | 0.95 |
γ-Al2O3 | 12.5 | 89 | 86 | 87 | 0.97 |
ZrO2 | 6.5 | 90 | 93 | 92 | 0.96 |
TiO2 | 10.1 | 31 | 27 | 29 | — |
Catalyst support . | Mo2C loading (wt%) . | Conversion (%) . | CO Yield (%) . | H2/CO . | |
---|---|---|---|---|---|
CH4 . | CO2 . | ||||
SiO2 | 18.3 | 91 | 86 | 89 | 0.95 |
γ-Al2O3 | 12.5 | 89 | 86 | 87 | 0.97 |
ZrO2 | 6.5 | 90 | 93 | 92 | 0.96 |
TiO2 | 10.1 | 31 | 27 | 29 | — |
Summary of carbide catalysts for dry reforming of CH4.
Catalyst . | Temperature (°C) . | Pressure (kPa) . | CH4/CO2 feed ratio . | CO2 conv. (%) . | CO yield (%) . | Ref. . |
---|---|---|---|---|---|---|
5%Ni/βSiC | 900 | 101 | 1 | 90 | — | 164 |
5%Mo2C/ZrO2 | 950 | 101 | 1 | 58 | — | 165 |
5%Mo2C–1%Bi/ZrO2 | 950 | 101 | 1 | 75 | — | 165 |
Co6W6C | 850 | 404 | 1 | 70 | 42 | 166 |
Ni–Mo2C/La2O3 | 800 | 110 | 1 | 78 | — | 167 |
20Co–Mo2C/ZrO2 | 850 | 101 | 1 | 97 | 87 | 168 |
Ni–WC | 800 | 101 | 1 | 90 | 80 | 169 |
Ni–Mo2C | 800 | 101 | 1 | 90 | 80 | 169 |
Catalyst . | Temperature (°C) . | Pressure (kPa) . | CH4/CO2 feed ratio . | CO2 conv. (%) . | CO yield (%) . | Ref. . |
---|---|---|---|---|---|---|
5%Ni/βSiC | 900 | 101 | 1 | 90 | — | 164 |
5%Mo2C/ZrO2 | 950 | 101 | 1 | 58 | — | 165 |
5%Mo2C–1%Bi/ZrO2 | 950 | 101 | 1 | 75 | — | 165 |
Co6W6C | 850 | 404 | 1 | 70 | 42 | 166 |
Ni–Mo2C/La2O3 | 800 | 110 | 1 | 78 | — | 167 |
20Co–Mo2C/ZrO2 | 850 | 101 | 1 | 97 | 87 | 168 |
Ni–WC | 800 | 101 | 1 | 90 | 80 | 169 |
Ni–Mo2C | 800 | 101 | 1 | 90 | 80 | 169 |
Combined reforming technologies including bi- and tri-reforming of CH4 have also attracted interest owing to the elimination of the need for a secondary process to adjust the H2/CO ratio to the required value.103–106 Small amounts of Mo2C (0.5–2 wt%) added to Ni/ZrO2 catalysts were reported to be beneficial for increasing the catalyst activity of the steam-CO2 bi-reforming of CH4 and the increased activity was ascribed to an increased Ni dispersion and a higher Mo(II) content of the catalyst.106 The best catalyst, with 0.5 wt% Mo2C–10 wt% Ni/ZrO2 also had higher stability in comparison with an unmodified 10 wt% Ni/ZrO2 catalyst.106 The ability of Ni/Mo2C to catalyse CH4 bi-reforming at 950 °C has also been demonstrated.107 A rapid loss in activity of the catalyst was noted after a certain reaction period, after which Mo2C oxidation to MoO2 occurred, but no evidence of coking, the usual limitation in dry reforming reactions, was observed.107 Tri-reforming in which the three reactions:
occur simultaneously to yield synthesis gas, has also been described using Ni–Mg catalysts supported on SiC.104,105
Several studies have focused on Mo2C for the water–gas shift (WGS) or the reverse water–gas shift (RWGS) reaction: CO+H2O⇌CO2+H2O. When steam reforming of CH4 is used for the industrial production of hydrogen from CH4 (CH4+2H2O→CO2+4H2), the product stream usually contains small amounts (1–5%) of CO as an impurity and the WGS, catalysed by Cu/ZnO/Al2O3 catalysts, is used to remove the CO and produce additional hydrogen. Mo2C has been shown to be a very effective support for Pt catalysts used to catalyse the WGS reaction.108,109 The synthesised catalyst results in a unique interaction between the Pt and the Mo2C raft like particles, and as shown in Figure 1.7, the rate of the WGS reaction for the Pt/Mo2C is significantly higher than that for several other Pt catalysts supported on metal oxides.
(a) Arrhenius plots of the WGS reaction rates for 2.7% Pt/Al2O3, 5% Pt/CeO2, 2% Pt/TiO2, and 4% Pt/Mo2C catalysts. (b) WGS rates at 240 °C for the Pt/Mo2C catalysts as a function of Pt loading including predicted rates from the surface site and perimeter site models.108,109 Reprinted with permission from N. M. Schweitzer, J. A. Schaidle, O. K. Ezekoye, X. Pan, S. Linic and L. T. Thompson, J. Am. Chem. Soc., 2011, 133, 2378–2381. Copyright (2011) American Chemical Society.
(a) Arrhenius plots of the WGS reaction rates for 2.7% Pt/Al2O3, 5% Pt/CeO2, 2% Pt/TiO2, and 4% Pt/Mo2C catalysts. (b) WGS rates at 240 °C for the Pt/Mo2C catalysts as a function of Pt loading including predicted rates from the surface site and perimeter site models.108,109 Reprinted with permission from N. M. Schweitzer, J. A. Schaidle, O. K. Ezekoye, X. Pan, S. Linic and L. T. Thompson, J. Am. Chem. Soc., 2011, 133, 2378–2381. Copyright (2011) American Chemical Society.
In a subsequent study,110 the metals Pt, Pd, Ni, Au, Ag and Cu supported on Mo2C were also shown to have high activity for the WGS reaction. At 120 °C using a feed gas of 7% CO, 22% H2O, 8.5% CO2, 37% H2, the WGS rate per unit area of catalyst for Pt, Au, Pd and Ni (1.5–2 wt%) supported on Mo2C was 4–8 times higher than that of the commercial Cu/ZnO/Al2O3 catalyst.110
1.5 Synthesis Gas Conversion
Synthesis gas conversion to fuels and chemicals provides a route to clean fuels that are typically S-free. In addition, depending on the catalyst and chosen process, the products from synthesis gas can be varied from heavy waxes to CH4 and may also include a range of oxygenated hydrocarbons, such as methanol and higher alcohols. The use of non-conventional metal carbides, nitrides, phosphides and the like have all been investigated for synthesis gas conversion.
The Fischer–Tropsch synthesis (FTS) converts synthesis gas into straight chain hydrocarbons by a surface polymerisation reaction with Fe111 and Co112 catalysts being used in industrial processes. Fe carbides, nitrides and carbo-nitrides were first studied in the 1950's,113 with the nitrided iron catalysts showing significantly higher amounts of oxygenated hydrocarbons than the iron carbide. The catalysts also underwent significant structural and chemical changes upon exposure to the synthesis gas.113 Other Fe-carbides synthesised by various methods are also known to be active for the FTS.114,115 During FTS on Fe catalysts, the presence of Fe carbides is well known, with several different Fe-carbides being identified, including ε-Fe2C, Fe7C3, χ-Fe5C2 and θ-Fe3C.116 Recently, the formation of θ-Fe3C, χ-Fe5C2, and ε-Fe2C was shown to depend on the thermodynamic stability of each phase at the gas phase composition and temperature of the reaction.116 Furthermore, it was shown that a significant part of the Fe carbide phases were amorphous. The χ-Fe5C2 was oxidised during FTS conditions, and the θ-Fe3C and amorphous carbide phases showed lower activity and selectivity than the other phases.
Other metal carbides, especially Mo2C, are also known to have significant activity and selectivity for CO hydrogenation to methane117,118 and other FTS products.119–121 For example, CO hydrogenation at atmospheric pressure, 570 K with a 3/1 : H2/CO ratio and SV of 10 000 h−1 on unsupported Mo2C, produced mostly C1–C5 paraffins while promotion of the Mo2C with K2CO3 yielded C2–C5 hydrocarbons with 80–100% olefins and reduced the methane selectivity.119 Several promoters (K, Na, Co, Ce, Ba) have been added to the Mo2C catalyst in an attempt to control product selectivity with moderate success.120–124 The use of Mo2C catalysts for CO hydrogenation at high pressure to yield alcohols has also been extensively investigated.10,125 According to Xiang et al.,126 Mo2C catalyst operated at T=573 K, P=8.0 MPa, GHSV=2000 h−1, H2/CO=1.0 yields mostly light hydrocarbons, whereas when the same catalyst is promoted with K2CO3, alcohols are also produced. At a K/Mo ratio of 0.2 the selectivity to alcohols (on a CO2-free basis) was 53% whereas the hydrocarbon selectivity was 47%.127 Adding Fe, Co or Ni to the K/Mo2C catalysts resulted in an increase in catalyst activity in the order Ni>Co>Fe; whereas, in terms of C2+OH selectivity (i.e. selectivity to alcohols above methanol) the increase was Ni>Co>Fe.128 Ni-Mo bimetallic carbides operated at H2/CO=2.0, T=513 K, P=7.0 MPa, GHSV=4000 h−1 also produce a product mix of alcohols and hydrocarbons that depends on the Ni content.129 As summarised in Table 1.10, the addition of the Ni significantly increases selectivity to alcohols, with the hydrocarbon fraction dominated by more than 67% CH4.129 In one study, the effect of support was investigated for a K promoted Mo2C catalyst, with CO conversion reaching a maximum with about 20 wt% Mo2C loaded onto an active carbon support.130 This study also demonstrated the need for the K promoter to be in contact with the Mo for effective promotion of the alcohols synthesis.130
Performance of CO hydrogenation over the Ni–Mo bimetallic carbide catalysts operated at H2/CO=2.0, T=513 K, P=7.0 MPa, GHSV=4000 h−1. Adapted from ref. 129.
Catalyst . | CO conv. (%) . | Selectivity . | |
---|---|---|---|
ROH . | CHx . | ||
MoC | 1.0 | 14.5 | 85.5 |
Ni0.17MoC | 35.0 | 46.5 | 54.0 |
Ni0.5MoC | 40.3 | 46.0 | 54.0 |
Ni1.0MoC | 41.1 | 34.6 | 65.4 |
Ni2.0MoC | 59.3 | 25.3 | 74.7 |
Catalyst . | CO conv. (%) . | Selectivity . | |
---|---|---|---|
ROH . | CHx . | ||
MoC | 1.0 | 14.5 | 85.5 |
Ni0.17MoC | 35.0 | 46.5 | 54.0 |
Ni0.5MoC | 40.3 | 46.0 | 54.0 |
Ni1.0MoC | 41.1 | 34.6 | 65.4 |
Ni2.0MoC | 59.3 | 25.3 | 74.7 |
Recent studies also report on metal nitrides, phosphides and borides as catalysts for synthesis gas conversion. Ultrafine catalysts of CuB and Me-CuB (Me=Cr, Th, Zr) have been investigated for methanol synthesis from CO/H2131 and CO2/H2.132 In one example, 20% Zr–CuB (approximately 77% Cu by mass) operated at 250 °C, 3.0 MPa and CO2/H2 of 1 : 3 in the liquid phase with hexadecane as solvent, produced CH3OH at a maximum selectivity of 55% and a CH3OH synthesis rate of 1.5 mol kgCu−1 h−1, equivalent to ∼37 g CH3OH kgcat−1 h−1.132 At 150 °C, 6 MPa H2/CO=2 : 1 the CH3OH synthesis rate was 46 mmol h−1 over the 20% Th-CuB catalyst (2 mmol Cu equivalent) in the presence of methylformate (80 mL) and potassium methoxide (3 g).131 Synthesis gas conversion to mixed higher alcohols on metal phosphides was first investigated by Zaman and Smith,133 in which synthesis gas conversion at 548 K, 8.3 MPa and a H2/CO=1 over a 10 wt% MoP/SiO2 catalyst yielded mostly hydrocarbons with 35% selectivity to methane.134 K addition to the MoP–SiO2 shifted selectivity to oxygenates, with the highest oxygenate space time yield of 147.2 g kgcat−1 h−1 obtained over a 5 wt% K–15 wt% MoP–SiO2 catalyst.133,134 The highest selectivity towards C2+ oxygenates (76.6 C atom%) and lowest selectivity towards CH4 (9.7 C atom%) occurred on a 5 wt% K–10 wt% MoP–SiO2 catalyst. The major oxygenates in the product were acetaldehyde, acetone and ethanol. Promotion of MoP by addition of Co and K has also been shown to improve selectivity to C2+ oxygenates.135 At 548 K, H2/CO=1.0, 5.0 MPa and GHSV=3600 h−1 the best catalyst (K1Co0.75MoP) had a CO conversion of 14.4%, a CH4 selectivity of 13% and C2+ oxygenate selectivity of 43%.135 The K promotion of Mo2N for synthesis gas conversion to oxygenates has also been reported recently.136 Catalysts with various K loadings (0.45–6.2 wt%) were operated at 275–325 °C, 7 MPa and 60 000 h−1. The highest total oxygenate selectivity (44% at 300 °C) was observed on a 1.3K-Mo2N catalyst, but the hydrocarbon selectivity on these catalysts remained high.
1.6 Ammonia and Hydrogen
The ammonia synthesis, N2+3H2→2NH3, is an important industrial process used in the manufacture of fertilisers that occurs at 450–500 °C and 30 MPa on Fe catalysts,3 while the reverse reaction is of current interest because of the potential for NH3 to be used as a vector for H2 storage, transportation and supply. Both reactions occur on interstitial metal nitrides and carbides. Ammonia synthesis rates, measured in a stoichiometric H2/N2 mixture at atmospheric pressure and 673 K were higher on β-Mo2C, MoOxCy and γ-Mo2N than on a Ru catalyst, but less than on a K2O-Fe catalyst.5 These promising results led to several studies focused on similar materials.6 In particular, the bimetallic nitrides Fe3Mo3N, Co3Mo3N and Ni2Mo3N have been shown to be highly active for the ammonia synthesis137,138 and a Cs promoted Co3Mo3N catalyst is reported to be more active for ammonia synthesis (15 mmol h−1 g−1) at 673 K under 3.1 MPa than a doubly promoted iron catalyst.139–141 In another recent study, ammonia synthesis under mild conditions (420–500 K, N2/H2=1 : 3, WHSV of 60 000 mL g−1 h−1 and 10 bar) has been demonstrated using LiH-transition metal nitride (V to Ni) composite catalysts, in which the LiH acts as strong reducing agent, which removes activated N atoms from the transition metal nitride, yielding LiNH2 which further reacts with H2 to yield NH3 and regenerate the LiH.142 A Ba-promoted Ru catalysts, supported on BN, has also been used for the ammonia synthesis, with high activities and catalyst stability reported at 360 and 400 °C and at pressures of 50 to 100 bar.143
There is significant interest in the production of hydrogen by means other than hydrocarbon reforming/partial oxidation reactions and the use of the interstitial carbides, nitrides and especially the borides as catalysts for these new processes has been proposed in several studies. H2 production that is free of COx is important in the operation of polymer electrolyte membrane (PEM) fuel cells, where the PGM catalysts are readily poisoned by trace amounts of COx. Ammonia decomposition is one approach to obtain COx-free H2 and several studies have focused on metal nitrides and carbides for this reaction. High ammonia decomposition activity of MoNx/γ-Al2O3 and NiMoNy/γ-Al2O3 catalysts at temperatures 600–750 °C and a GHSV of 1800–3600 h−1 was reported by Liang et al.144 who also identified the presence of both Mo2N and Ni3Mo3N phases in the active catalyst. Several other studies of the bimetallic nitrides have since been completed.145,146 In one case, the activity of a series of bimetallic nitrides was ranked in decreasing order for ammonia decomposition; Co3Mo3N>Ni3Mo3N>Fe3Mo3N>Mo2N.147 Zheng et al.148 report the use of a Mo2C catalyst with relatively high activity for ammonia decomposition, as compared to a Ru catalyst as shown in Figure 1.8.
Catalytic activity and stability of (circles) Mo carbide and (triangles) graphite-supported 2 wt% Ru catalyst for NH3 decomposition. Reaction conditions: 50 mg of the sample, NH3 space velocity 36 000 mL gcat−1 min−1, reaction temperature 600 °C (Mo) or 450 °C (Ru).148 Reprinted with permission from W. Zheng, T. P. Cotter, P. Kaghazchi, T. Jacob, B. Frank, K. Schlichte, W. Zhang, D. S. Su, F. Schueth and R. Schloegl, J. Am. Chem. Soc., 2013, 135, 3458–3464 Copyright (2013) American Chemical Society.
Catalytic activity and stability of (circles) Mo carbide and (triangles) graphite-supported 2 wt% Ru catalyst for NH3 decomposition. Reaction conditions: 50 mg of the sample, NH3 space velocity 36 000 mL gcat−1 min−1, reaction temperature 600 °C (Mo) or 450 °C (Ru).148 Reprinted with permission from W. Zheng, T. P. Cotter, P. Kaghazchi, T. Jacob, B. Frank, K. Schlichte, W. Zhang, D. S. Su, F. Schueth and R. Schloegl, J. Am. Chem. Soc., 2013, 135, 3458–3464 Copyright (2013) American Chemical Society.
Among the novel methods of hydrogen storage and supply, the hydride salts such as sodium borohydride (NaBH4, NH3BH3, LiBH4, etc.) are seen as safe hydrogen reservoirs that can readily produce COx-free hydrogen by hydrolysis or methanolysis at ambient temperature.15 Catalysts for the hydrolysis of NaBH4 have been reviewed recently.15 The most effective catalysts are based on CoB15,149–151 and for NaBH4 hydrolysis the Co–B is readily promoted by a second metal.152,153 Patel et al. reported that the activity of these materials for NaBH4 hydrolysis decreased in the order Co–Cr–B>Co–Mo–B>Co–W–B>Co–Cu–B>Co–Fe–B>Co–Ni–B>Co–B.15,154 NaBH4 methanolysis (NaBH4+4CH3OH→4H2+NaB(OCH3)4) also provides high yields of H2 and is catalysed by Ru/Al2O3, Co/TiO2, FeB, CuB and MoP. Recently, a significantly higher rate of H2 evolution per mass of catalyst during NaBH4 methanolysis was reported for a Ni2P/SiO2 catalyst, compared to Co and Ru supported on Al2O3.155 Recent efforts to demonstrate H2 storage and production on a practical scale include the use of a CuB honeycomb catalytic reactor for NaBH4 hydrolysis that produced 7.55 L min−1 gCo−1 at 70 °C.156 At 134 °C and 5 bar outlet pressure H2 production of up to 32 L min−1 gCo−1 was obtained.156
1.7 Electrocatalysis
The metal carbides have also been the focus of recent developments in electrocatalysis applied to various processes. Water splitting (H2O→H2+½ O2, ΔG=237.1 kJ mol−1, corresponding to a thermodynamic voltage requirement of 1.23 V)157 is seen as a promising approach to clean hydrogen. Electrochemical water-splitting includes a cathodic hydrogen evolution reaction (HER; 2H++2e−→H2) and an anodic oxygen evolution reaction (OER; H2O→½ O2+2H++2e−). Both the OER and HER reactions require substantial overpotentials and Pt-based metals have the best activity for the HER, and Ru/Ir-based materials are the benchmark catalysts for the OER. Supply and cost issues have meant that there is a major effort aimed at the development of cost-effective and efficient alternative catalytic materials for water splitting. For example, Yu et al.158 describe a new porous carbon-supported Ni/Mo2C (Ni/Mo2C-PC) composite catalyst derived from the thermal treatment of nickel molybdate nanorods coated with polydopamine, which catalyses the HER and OER. The catalyst affords small overpotentials of 179 mV for the HER and 368 mV for the OER at a current density of 10 mA cm−2. The home-made alkaline electrolyser, assembled with Ni/Mo2C-PC as a bifunctional catalyst, can enable a water-splitting current density of 10 mA cm−2 to be achieved at a low cell voltage of 1.66 V. In another study, the use of a WC catalyst co-doped with Co and N, for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), is reported.159 In other cases the metal carbide such as TiC has been used as a support for single atom metal catalysts, such as Pt on TiC used for the ORR. In a recent report, Co2B was shown to be an excellent catalyst for the OER and is simultaneously active for catalysing the HER.160,161 The catalyst achieves a current density of 10 mA cm−2 at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen-doped graphene.160,161
Metal carbides have also been used on other electrochemical processes, including Mo2C which is capable of catalysing CO2 conversion to CH4 at low potentials162 and a Fe–N–C catalyst, that includes graphitic carbon, graphene, iron carbides, FeN and Fe2N and that is suitable for SO2 electrooxidation.163
1.8 Conclusions
The metal carbides, nitrides, phosphides and borides have shown promise as new catalysts for a range of reactions normally catalysed by metals. The behaviour of these new materials is attributable to changes in the electronic properties of the metal surface induced by the ligands and by the metal–ligand surface geometry. The materials are active in catalytic hydrogenations, hydrotreating and synthesis gas production and conversion. In several cases, they provide unique reaction pathways and are more active or selective than conventional metal catalysts. Nevertheless, commercial application remains a challenge. The stability of the materials under the relatively severe reaction conditions encountered in hydrotreating and synthesis gas conversion, for example, is low, resulting in changes to the surface composition of the catalysts. Promising results have been reported in electrocatalytic applications such as water splitting and hydrogen production. The rich chemistry of these materials provides an opportunity for further discovery and development, with the potential to apply these materials to a range of unexplored catalytic reactions.