A Review of Preparation Strategies for α-MoC<sub>1–x</sub> Catalysts
A Review of Preparation Strategies for α-MoC1–x Catalysts
Transition metal carbides attract growing attention
Transition metal carbides are attracting growing attention as robust and affordable alternative heterogeneous catalysts to platinum group metals (pgms), for a host of contemporary and established hydrogenation, dehydrogenation and isomerisation reactions. In particular, the metastable α-MoC1–x phase has been shown to exhibit interesting catalytic properties for low-temperature processes reliant on O–H and C–H bond activation. While demonstrating exciting catalytic properties, a significant challenge exists in the application of metastable carbides, namely the challenging procedure for their preparation. In this review we will briefly discuss the properties and catalytic applications of α-MoC1–x, followed by a more detailed discussion on available synthesis methods and important parameters that influence carbide properties. Techniques are contrasted, with properties of phase, surface area, morphology and Mo:C being considered. Further, we briefly relate these observations to experimental and theoretical studies of α-MoC1–x in catalytic applications. Synthetic strategies discussed are: the original temperature programmed ammonolysis followed by carburisation, alternative oxycarbide or hydrogen bronze precursor phases, heat treatment of molybdate-amide compounds and other low-temperature synthetic routes. The importance of carbon removal and catalyst passivation in relation to surface and bulk properties are also discussed. Novel techniques that bypass the apparent bottleneck of ammonolysis are reported, however a clear understanding of intermediate phases is required to be able to fully apply these techniques. Pragmatically, the scaled application of these techniques requires the pre-pyrolysis wet chemistry to be simple and scalable. Further, there is a clear opportunity to correlate observed morphologies or phases and catalytic properties with findings from computational theoretical studies. Detailed characterisation throughout the synthetic process is essential and will undoubtedly provide fundamental insights that can be used for the controllable and scalable synthesis of metastable α-MoC1–x.
The pgms have been the cornerstone of many catalytic applications and will continue to be so as processes become more sustainable. Platinum is a highly reactive catalytic metal with applications in highly contemporary processes, such as low temperature water-gas shift (1, 2) proton exchange membrane fuel cells (3) and green hydrogen production through the hydrogen evolution reaction (4). Ruthenium and iridium oxides are state-of-the-art catalysts for the associated oxygen evolution reaction (5, 6). The pgms are also found to be highly active for aqueous phase hydrogenation and reforming reactions, of relevance for biomass platform chemical utilisation (7–12). Yet, it is often considered that pgm-rich catalysts cannot satisfy the requirements needed for the employment of these new technologies at scale, because of their relative scarcity, high cost and poor catalytic stability in certain key reactions. Carbon monoxide poisoning of platinum in fuel cell applications is an excellent example of catalyst poisoning and instability. Regarding scarcity and environmental cost, Nuss and Eckelman in a life cycle assessment showed that the global warming potential of rhodium, platinum, iridium and ruthenium are 35,100 kgCO2 kgM–1, 12,500 kgCO2 kgM–1, 8860 kgCO2 kgM–1 and 2110 kgCO2 kgM–1 (M = metal) respectively (13). Although the scale of use of these materials in relation to iron, steel and aluminium must be placed in context.
Strategies to mitigate against these issues can be broadly summarised as: (a) improving atom efficiency of pgms through control of atom cluster or nanoparticle sizes, i.e. improving performance per gram of pgm; (b) pgm-non-noble metal alloying to reduce pgm content and improve stability and poisoning resistance; and finally (c) complete replacement of pgms with other catalytic materials. Advancements in synthetic procedures and characterisation has seen a proliferation of work around controlled pgm clusters and single atom catalysts that incorporate the concept of a ‘single site catalyst’ (14–19). The term, coined by the late Sir John M. Thomas, refers to a single energetically equivalent active site providing maximum efficiency of catalytic material (20). However, control over metal nuclearity in an applied context is far from trivial, catalyst stability is challenging and proof of the existence or retention of ‘single-sites’ requires robust in situ or operando characterisation (21). Utilisation of non-noble metals (alone or in alloys) clearly reduces the environmental burden of catalysts. Straight replacement of pgms with first row transition metals can result in acceptable catalytic performance, but notably poorer than a pgm regarding activity and stability (22–24). Specifically, metals such as nickel and copper are more prone to oxidation and leaching into liquid reaction media. Alloying of these metals with pgms often removes these weaknesses and even on occasion produces superior catalytic performance, but perhaps fail in their main remit of sufficiently reducing reliance on pgms (25–28). It is worth noting that these strategies are not mutually exclusive, and combinations can be quite successful for specific applications (vide infra).
An alternative strategy is to diversify into abundant metal compounds. Transition metal carbides are a class of Group IV–VI metal compounds with carbon occupying interstitial sites of the relevant metal. In addition to being known for their hardness (29), the early transition metal carbides have been shown by Levy and Boudart to have ‘platinum-like’ catalytic behaviour (30). Since the seminal work by Levy and Boudart on tungsten carbide catalysed hydrogen oxidation and hydrocarbon isomerisation, metal carbides have been widely used as catalysts for a host of hydrogenation (31–34), dehydrogenation (35–38), hydrogenolysis (39–41), carbon monoxide or nitric oxide reduction (42), isomerisation (43, 44) and water-gas shift reactions (45–48). Molybdenum carbide phases, which are the focus of this review, have received significant attention for contemporary applications in green and sustainable processes. Interest in molybdenum carbide catalysis is driven by its catalytic properties, low cost, earth abundancy and resistance to poisoning from sulfur, nitrogen and carbon monoxide impurities present in both hydrocarbon and biomass feedstocks (49–52). Such properties make these catalysts appealing as robust alternatives to pgm catalysts. However, this generalisation of properties is overly simplistic, given the numerous possible structures of carbides, a vast array of potential applications that have subtly different requirements, and the fact that while similar, the properties of metal carbides and platinum are clearly not identical. The reader is directed to several excellent reviews which discuss these complexities in detail, such as discussing metal carbides properties in the context of other alternative catalytic systems (such as nitrides, phosphides and boron alloys (53)) and those that focus on specific catalytic applications or resistance to poisoning (49, 54, 55). A further recent observation is the exceptionally strong metal support interaction between platinum and some first row transition metals and molybdenum carbides, in particular the metastable α-MoC1–x phase. Several reports show that single-atom platinum can be supported on α-MoC1–x to improve catalytic properties (56–58).
The application of molybdenum carbides is more complicated than may be initially perceived, as demonstrated by the lack of its widespread adoption in industrial processes. One aspect to consider is the challenge in identifying and selecting a specific phase and morphology of molybdenum carbide, which is desirable for a particular process. In this context, theoretical studies of reaction mechanism with respect to termination and morphology of different carbide phases has provided great insight. Specifically, they provide understanding of the thermodynamic stability of specific surfaces with respect to synthesis, reaction conditions and Mo:C composition, then used this as a basis to elucidate a preferred structure to a specific catalytic application. A further challenge is a reliable, scalable and affordable preparation method that allows for the synthesis of such specific carbide phases and morphologies. This current review will provide a brief overview of molybdenum carbide phases and their general properties, then discuss the properties and synthetic routes of the metastable molybdenum carbide (α-MoC1–x) phase that has recently received significant interest. An overview of the potential synthetic strategies to produce α-MoC1–x is given in Scheme I.
1.1 Phases and Properties of Molybdenum Carbide
As alluded to above, all molybdenum carbide phases are generally characterised by outstanding mechanical properties and, like other transition metal carbides has (in parenthesis Mo2C values): high hardness (17 Gpa), tensile strength (530 GPa), high melting point (2520°C) and good thermal (15 W m–1 k–1) and electrical conductivity (57 μΩ cm) (59). The latter point on electrical conductivity being defined by the delocalisation of the molybdenum d -band due to increased Mo–Mo distances, which result in an increased density of states near the Fermi level (60). The novel catalytic properties of molybdenum carbide, which as we already have said, have been found to be comparable to the noble metals, can be explained by the surprising similarity of its electronic properties to platinum (61, 62) (Figure 1). Specifically, these metallic properties are due to the hybridisation of carbon sp and molybdenum d -orbitals, which allows the delocalisation of the d -band of the molybdenum, similar to the d -band of metallic platinum (61). It is this broad partially occupied d -band in MoCx that makes this material particularly successful as catalysts, in hydrogen evolution and purification, reforming reactions, hydrogenations and Fischer-Tropsch synthesis (53, 57, 61, 63).
Taking these general aspects and applications for which MoCx has grown in popularity into account, a more thorough analysis of the different phases is necessary. Phases will have differences according to geometric factors (Hagg’s rule regarding the ratio of the hard ball radii of nonmetal to metal (64)) and electronic factors (Engel and Brewer theory on s–p electron count (65)), which define their structure, stability and activity as illustrated by Oyama (60, 66). In addition, it is worth remembering that, as with any material, bulk and specific surface structures can have different features (67).
Molybdenum carbide forms a range of phases with 1:2 or 1:1 Mo:C stoichiometry (68, 69), although significant nonstoichiometry has been observed. In all cases they are considered interstitial compounds and changes in the content of carbon and a variation of the oxidation states of molybdenum are observed accordingly. Across this Mo:C composition range seven main phases are reported, as shown in Figure 2 and Table I. Some of them are thermodynamically stable at room temperature, others are only stable in a very small range of temperatures and compositions, while others are metastable. Importantly, the stability of the phases varies according to the stoichiometry. Hugosson et al. affirmed that a substoichiometric phase is not favourable for the γ and γ’ phase, while η and δ are present only with a certain amount of vacancies (δ-MoC is experimentally found only between MoC0.66–MoC0.75 (70–73)). Nonstoichiometry influences multiple properties, such as the bulk moduli and the electronic properties, with vacancies changing the density of states (DOS) due to the unscreened Mo–Mo (74, 75).
|Phase||Type||Range of temperature, stability||Structure||Lattice parameter, Å (ICSD)a||Space group (ICSD)a||C atom position||Stacking sequence|
|β-Mo2Cb hcp||α-Mo2C||ζ-Fe2N||Room temperature (76, 80, 81)||Orthorhombic (distorted) hcp|| a = 4.724
b = 6.004
c = 5.199
|ɛ-Mo2C||ɛ-Fe2N||Intermediate temperature (81, 92)||Hexagonal/trigonal hcp|| a = 5.190
b = 5.190
c = 4.724
|β-Mo2C||Nb2N/W2C||High temperaturesd (76, 80, 81, 93)||Hexagonal (filled) hcp|| a = 3.002
b = 3.002
c = 4.724
|η-MoC||η-Mo3C2||Metastable (84, 94, 95) Stable T > 1700°C (71)||Hexagonal (complex) hcp|| a = 3.010
b = 3.010
c = 14.610
|α-MoC1-xb fcc||δ-MoCc||NaCl||Metastable (84) Stable T > 1700°C (71)||Cubic fcc|| a = 4.270
b = 4.270
c = 4.270
|γ-MoC||WC type||Room temperature (84, 93, 96)||Hexagonal simple|| a = 2.903
b = 2.903
c = 2.828
|P-6m2 (187)||Trigonal prismatic||AAAA|
|γ’-MoC||TiAs/TiP||Always metastablee (84, 93, 95)||Hexagonal|| a = 2.932
b = 2.932
c = 10.97
|P63/mmc (194)||Trigonal prismatic/octahedral||AABB|
It is worth noting that terminologies are inconsistent across the literature, with the orthorhombic Mo2C being previously assigned as α (76, 77) or β (68, 69, 78). Experimentally and theoretically (79) it has been observed that orthorhombic Mo2C is the most thermodynamically stable phase and is extensively reported in numerous publications. Confusion arises as this phase is sometimes referred to as α-Mo2C to differentiate it from a high-temperature phase discussed in the relevant papers as β-Mo2C (80, 81). Also an intermediate temperature phase between these two, called ɛ, has also been observed, which has a hexagonal or trigonal structure (71, 81, 82). Due to the low scattering cross-section of carbon, laboratory source X-ray diffraction (XRD) patterns of these phases are quite similar and only recently advanced neutron scattering experiments have classified these structures conclusively (80–83). Consequently, materials focused research refers to the stable orthorhombic phase as α-Mo2C, while the catalytic community refers to this phase is as β-Mo2C as a way to easily differentiate it from the metastable face-centred cubic MoC1–x designated as α-MoC by Clougherty et al. (84). Further confusion is caused by this face-centred cubic MoC1–x being interchangeably referred to as α or δ. We will follow the commonly used notation of β-Mo2C (hcp) and α-MoC1–x (fcc).
The most commonly catalytically studied phase is the orthorhombic β-Mo2C. However, there has been increasing interest and research into the metastable MoC1–x phases, in particular the cubic α-MoC1–x, although the hexagonal η-MoC and γ-MoC phases have also been shown to be of interest in a limited number of studies (85–87). The stability and properties of these phases and the influence of Mo:C ratio (45, 88, 89) and nonstoichiometry in this context has been thoroughly investigated (71, 90, 91).
1.2 Surface Properties of the α and β Phases
Extending the earlier work by Chen and coworkers on β-Mo2C (61), Illas and coworkers investigated the projected density of states (PDOS) of MoCx phases (Figure 3) (97). The study highlighted the difference in the character of the fcc α-MoC1–x and hcp β-Mo2C phases, with the former having more covalent character due to the presence of localised states in the DOS, and the latter being more metallic with a broad region of states around the Fermi level. A combination of charge density difference (CDD) iso-surfaces and electron localisation function (ELF) plot analysis spatially confirmed the electron distribution being more localised on the carbon for the α-fcc phase, while being spread between the atoms in the β phase. Analysis of the low index planes, including non-polar and polar surfaces, of these phases showed significant differences work function with the β-Mo2C (011) being the most stable with a ~3.4 eV work function.
Since this initial work, significant effort has been expended on theoretically determining the properties and stability of specific surface terminations of a range molybdenum carbide phases. Surface free energies of different stoichiometric and nonstoichiometric surfaces (i.e. molybdenum, carbon, mixed terminations) have been determined at different temperatures and carbon chemical potential (μC) of a range of molybdenum carbide phases. Computed Wulff constructions then allow for determination of particle morphology at different conditions. Saidi showed that the prevalence of the (011) surface increases in β-Mo2C as the chemical potential is lowered to ca. –10 eV below which (021) and carbon terminated (100) begin to dominate (98). In comparable studies, Jin and coworkers (99) and recently Hao and coworkers (100) determined the surface stability and morphology of α-MoC1–x. Generally, molybdenum-rich (carbon deficient) surfaces decreased in stability with increasing μC, the inverse being observed for carbon-rich surfaces, and no change being observed for stoichiometric surfaces. The result was the domination of the (311) surface at ‘conventional’ synthesis conditions, while proposed syngas conditions result in the emergence of (100) and (211) surfaces.
In addition to corroborating the Wulff constructions suggested above (Figure 4(a)), Paolucci and coworkers theoretically considered the influence of particle size and predicted crystallisation pathways of all major molybdenum carbide phases (79). A clear observation was that small particle size is a predominant factor in the thermodynamic stability of the α-MoC1–x phase (Figure 4(b)). Further, the phase boundary between carbides and molybdenum metal changes with size and μC, which suggests that carbon vacancies may stabilise larger α-MoC1–x particles, as supported by work from Hugosson et al. and Zaoui et al. (72, 101). While these theoretical studies provide insight into the thermodynamic stability of certain structures, kinetic aspects must also be considered, particularly for the synthesis of the α-MoC1–x that frequently is formed from topotactic rearrangement of a suitable precursor phase (discussed in detail below). Finally, in a comprehensive theoretical study, Quesne and colleagues showed that significant surface reconstruction or rumpling was observed for low index planes, with higher relaxation being seen for less stable surfaces (102). The lowest energy surface was the (100) and the lowest work function (an indication of high reactivity during reductions) was the (110).
1.3 An Overview of the Catalytic Application of α-MoC1–x
In the following section we will briefly discuss the different catalytic properties of α-MoC1–x and β-Mo2C phases within the context of key catalytic applications (53, 103). Specifically, reforming (56, 104, 105) and water-gas shift reactions for sustainable hydrogen production (45, 46, 57, 88, 106), CO2 hydrogenation (31, 89) and the hydrogen evolution reaction (63, 107, 108). Given the focus of the article is α-MoC1–x synthesis, the discussion below is not exhaustive and the reader is directed to specific reviews regarding the catalytic applications highlighted above.
1.4 Thermal Catalytic Applications of α-MoC1–x
As Ma and coworkers have succinctly described in their recent review, the β-Mo2C phase is historically the most studied for high-temperature reactions, such as hydrocarbon isomerisations, dehydrogenations and ammonia syntheses (57, 109). The thermodynamic stability, superior C–H bond activation and strong dissociative hydrogen adsorption are key parameters for these reactions. On the other hand, α-MoC1–x has recently been shown to have superior catalytic performance at lower temperatures for reactions such as methanol reforming, water-gas shift and biomass utilisation (alcoholises of lignin (110), furfural and succinic acid hydrogenation (111, 112) and Kraft lignin decomposition (113)). The improved activity for these water-containing reactions is attributed to high O2 and O–H bond activation in the α-MoC1–x phase. A summary of reaction conditions and key findings from select studies regarding reforming and water-gas shift are given in Tables II and III respectively.
Several aspects of the α-MoC1–x, (70) phase and how it differs from β-Mo2C account for this improved O–H activation at low reaction temperatures. Firstly, it is important to note that α-MoC1–x is suitable only for processes at low temperatures (below 300°C), specifically because it is a metastable phase that at elevated temperatures is likely to transform into the thermodynamically stable β-Mo2C (57). The high surface area, compared to β-Mo2C, clearly increases the proportion of active sites available during a reaction. While clearly advantageous, this does not account for the improved reactivity in water activation. An intrinsic increase in catalytic activity of the α phase in these reactions is due to the fact that α-MoC1–x (0.25 < x < 0.35), with Mo:C ratio between 1.3–1.5, has a lower percentage of molybdenum, compared to β-Mo2C, which has a Mo:C ratio of 2 (70). According to Rodriguez et al. (88) this reduced molybdenum content is beneficial in limiting the reactivity of active oxygen species to produce oxycarbides (89, 119). A reduction of the metal:carbon ratio results in a lower reactivity of the surface because fewer metal atoms are present on the surface and with more positive charge. As noted by Palma et al. this will prevent catalyst deactivation from activated water species as it facilitates the desorption of OH (and O) species (120). The excellent low-temperature water-gas shift reactivity of this catalyst is explained by this reduced water activation vs. the β-Mo2C.
|Catalyst||Reaction||Reforming ratioa||TOF, h–1b||Conversion, %||Conditions||Selectivity CO, %||Hydrogen production, μmolH2 g–1cat s–1||GHSV/WHSV flow||Ref.|
20 bar of N2
|2% Pt/Al2O3||1:1||1077||<15|| 240°C
20 bar of N2
|ZnO-Pt/MoC||Methane steam reforming||1:3||1098||65.9|| 160°C
|not detected||29.7||1.2 ml h–1||(104)|
|not detected||28.8||1.2 ml h–1||(115)|
|β-Mo2C||1:1||–||100% at 400°C <20% at 250°C||150–400°C||10% at 400°C||–||9000 cm3 g–1 h–1||(105)|
|Fe1.6Mo98.4C hcp phase||1:1||–||100% at 350°C 70% at 250°C||150–400°C||<5%, peak 300°C||–||9000 cm3 g–1 h–1|
|Pt1.6Mo98.4C fcc phase||1:1||–||100% at 200°C||150–400°C||<5||22.1||9000 cm3 g–1 h–1|
|Catalyst||Reactant composition||CO conversion, %||Water-gas shift production rate, μmol g–1cat s–1||Apparent Ea, kJ mol–1||Ref.|
| 1.5% Pt/Mo2C
|120°C, 7% CO, 22% H2O, 37.5% H2, 8.5% CO2, 1 atm, flow rate of 75.4 cm3 min–1||<10|| 1.8
|Ni0.25Mo0.75C||180°C, 10.5% CO, 21% H2O, flow rate of 60 ml min–1||50||8||–||(48)|
| 3% Au/Mo2C
|130°C, 3% CO, 10% H2O, WHSV = 120,000 ml g–1cat h–1||<15|| 23.6
|2% Au/Mo2C||120°C and 200°C, 1 atm, GHSV = 180,000 h–1, 10.5% CO, 21% H2O, 20% N2||103||22||(46)|
|α-MoC1−x||120°C, 1 atm, GHSV = 180,000 h–1, 11% CO, 26% H2O, 26% H2, 7% CO2||2.05||64||(46)|
|Ir/α-MoC1−x||200°C, 1 atm, flow rate of 30 ml min–1, 18,000 ml g–1cat h–1, 2 vol% CO, 10 vol% H2O||20.5||43||(118)|
It has also been noted that α-MoC1–x has reduced hydrogen adsorption strengths, C–H and C–O bond activation in comparison to β-Mo2C. These changes in adsorption strengths are credited with the reasonable catalytic performance in the reverse water-gas shift reaction (CO2 reduction to carbon monoxide) (34, 121). Gao et al. noted the high carbon monoxide selectivity in α-MoC1–x and attributed it to its weak adsorption (121). Given the reverse water-gas shift reactivity and low methanation, CO2 hydrogenation to methanol has become an area of great interest regarding α-MoC1–x, as shown in Table IV.
|Catalyst||Reaction conditions||CO2 conversion, %|| Selectivity, %
|Bulk α-MoC1−x||300°C, 20 bar, WHSV = 40 h–1, H2:CO2 = 2.7||19.8||79.2||–||16.3||4.4||(34)|
|β-Mo2C||300°C, 20 bar, flow rate = 30 ml min–1, Ar:CO2:H2 = 10:15:75 (%)||24||28||4||45||23||(31)|
|Cu/ZnO/Al2O3||230°C, 3 MPa, H2:CO2 = 3:1||18.7||–||43||–||–||(123)|
|Mo2C||200°C, 40 bar, H2:CO2 = 3:1, liquid state||10||4.9||53||17||25.1||(32)|
|250°C, 30 bar, WHSV = 30,000 ml g–1cat h–1, CO2:H2:N2 = 20:60:20 (vol%)||<6||60–75||10–20||10–20||<2||(40)|
|α-MoC1−x nanowire||250–600°C, 1:4 molar ratio CO2:H2, WHSV = 36,000 ml g–1cat h–1||>60||99||–||<1||–||(121)|
|α-Mo2C||400°C, 0.1 MPa, CO2:H2:N2 = 1:1:3, GHSV = 3000 h–1||16||99||–||<1||–||(124)|
|β-Mo2C||300°C, He:CO2:H2 = 1:1.5:8, 20 bar GHSV = 3.8 h–1||11.5||82||2||14||2||(125)|
|MoxCy/SiO2||300°C, He:CO2:H2 = 1:1.5:8, 20 bar GHSV = 5.8 h–1||24.5||64||0||33||3|
|α-MoC1−x/TiO2||250°C, 30 bar, H2:CO2 = 3:1, 50 ml min–1||2.2||68||11||16||5||(126)|
The interplay between the reverse water-gas shift reaction, methanation and CO2 hydrogenation was studied by Rodriguez and Illas using model polycrystalline catalysts under ultrahigh vacuum conditions (33, 89). It is reported that with δ-MoC (α-MoC1–x) a reaction mechanism goes via a formate intermediate (COOH) with minimal C–O cleavage and results in reduced methane formation. It was observed that on β-Mo2C (001) surfaces methane selectivity was much higher due to facile CO2 (and carbon monoxide) activation and dissociation.
Theoretical and single studies have allowed for correlation of specific surface terminations with key elementary reaction steps in reforming and hydrogenation reactions. Taking Quesne’s observed stability of the (001) α-MoC1–x surface (102), Illas and coworkers investigated a range of different pathways for hydrogen adsorption on α-MoC1–x and other Group IV transition metal carbides. They showed that this surface is particularly favourable towards hydrogenation reactions with strong associated H* hydrogen adsorption energies vs. non-dissociative adsorption (i.e. a Kubas type interaction between hydrogen and surface). Moreover, α-MoC1–x is particularly suitable in this sense since it presents the highest adsorption energies among the transition metal carbides, although these values are still moderate and comparable to late transition metals (127). Ge and coworkers considered methane adsorption, dissociation and coupling on the (111) cubic α-MoC1–x and noted comparable properties to platinum (128).
Another useful property of α-MoC1–x is the strong interaction with (noble) metals which allows for the production of a stable supported catalyst with multiple functionalities in one material. In this context, the MoC phase and its O–H bond activation can be combined with noble metals with complementary functions (like C–H or C–C cleavages) to produce high reactivity (56, 57, 129, 130). Several papers have noted that this is specific to α-MoC1–x and that there is a significantly reduced interaction between metals, such as platinum or gold, and β-Mo2C, resulting in no significant improvement in activity for the latter carbide phase (45, 88) (Figure 5).
1.5 Hydrogen Evolution Reaction
As a critical component of water splitting the hydrogen evolution reaction (HER) remains a topic of great interest and requires significant improvement in catalytic performance, ideally with little or no pgm content. Reactivity can be correlated to the hydrogen adsorption free energy (taking into account the catalysts dependence of the rate constant) (131, 132). Molybdenum carbides have been extensively studied for the acid/alkaline hydrogen evolution reaction as a potential replacement to platinum (129, 133). Numerous molybdenum carbide phases have been explored, with many studies finding conflicting or unclear preference for one phase over another (Table V). However, through excellent synthetic control of molybdenum carbide phase (vide infra) Leonard and coworkers have provided compelling evidence for β-Mo2C being the most active (85). This observation that β-Mo2C is the best phase for electrochemical hydrogen production has been further verified (59, 134). Saidi and coworkers have, using density functional theory (DFT) approaches (vide supra), shown that the best HER activity is observed on the (011) surface followed by the C terminated (001). The much higher H* adsorption strength on β-Mo2C provides a clear explanation as to why the phase is superior for acidic based media. An open question remains regarding the potential of α-MoC1–x in alkaline media, where water dissociation becomes a key reaction step (135). Interestingly some reports, such as that by Wang and coworkers on HER in acidic media, have also proposed a synergistic effect through the formation of α-MoC1–x/β-Mo2C heterostructures (136). Therefore, significant interest in α-MoC1–x catalysts for this reaction remains.
|Catalyst|| Acid media (0.5 M H2SO4)
|| Alkaline media (1 M KOH)
||Loading, mg cm–2||Stabilityc||Ref.|
|η10, mVb||Tafel slope, mV dec–1||η10, mVb||Tafel slope, mV dec–1|
|Mo2C@C nanospheres||141||56||47||71||0.9|| 2000 cycles (acidic)
1000 cycles (alkali)
| 50 h
|α-MoC + β-Mo2C@Co||–||–||110||98||1.18||10 h||(140)|
|α-MoC1–x nanosheets||158||54||147||43.6||0.3||5000 cycles||(86)|
|α-MoC1–x + β-Mo2C + SiO2||155||48||–||–||0.213||5000 cycles and 30 h||(136)|
|14 (Pt)e||1000 cycles||(129)|
|α-MoC1−x/NCf||142||74||118||84||–|| 40 h in acid
60 h in alkali
In summary, a broad picture emerges in which the more metallic β-Mo2C is considered superior for C–H bond activation and strong dissociative hydrogen adsorption, but with moderate surface area values. The phase is also considered thermally stable for use in high-temperature applications. In contrast α-MoC1–x is considered to have notably better O–H bond activation, forming high hydroxyl group coverage in aqueous phase reactions or alkoxy intermediates in alcoholic media. In addition, α-MoC1–x has been applied as a non-benign support for both platinum and first row transition metals, with very strong metal support interactions being observed leading to formation of single-atom catalysts. These properties, coupled with higher surface areas, present an opportunity to develop novel and interesting catalyst technologies that operate at relatively low temperatures and require water tolerance or activation.
The scientific and technical challenges that arise from pursuing such metastable phase are twofold. Firstly, given the metastable nature of the phase, possible bulk phase changes cannot be discounted during catalytic reaction or handling under ambient conditions. An extension of this is the possibility, as highlighted by Hargreaves and Alexander (53, 144), that the presence of oxygen could influence the surface or subsurface structure of α-MoC1–x. Thus, the nature of the active catalyst may be different to that originally envisaged, complicating rational design. The second challenge is the successful synthesis of the α-MoC1–x without forming significant quantities of more thermodynamically favourable phases, such as β-Mo2C. The remainder of this review will focus on reported synthetic strategies, the associated challenges with such methodologies and their effectiveness.
2. Synthesis of α-MoC1–x
Historically, interstitial metal carbide synthesis techniques have been grouped (66, 145) as: (a) those associated with high temperature techniques; (b) temperature programmed techniques; and (c) thin film synthesis by plasma or chemical vapour deposition. Oyama provides, in his 1992 review, an excellent general overview of the first two techniques at the time (66). In summary, high-temperature methods, such as traditional carbonisation of oxides or metals with solid carbon, reaction of vapourised metals with hydrocarbons or solid combustion methods produce thermodynamically stable carbides of generally low surface area and are of limited scalability. Prior to the introduction of temperature programmed synthesis, the only procedure known for the synthesis of α-MoC1–x was quenching a mixture of Mo–C from around 2000°C (70).
Boudart and his group developed in 1973 the temperature programmed synthesis route to making tungsten and molybdenum carbides (30). In the direct temperature programmed reaction of molybdenum trioxide and various methane/hydrogen mixtures the β-Mo2C phase can be produced with surface areas between 50–100 m2 g–1 (146). Depending on the CH4:H2 ratio used a clean Mo2C was formed or a mixture of β-Mo2C and surface polymeric carbon, which could be removed by controlled treatment in hydrogen. The reaction proceeds through an initial reduction of MoO3 to MoO2, first via the formation of sub-MoO3 oxides, followed by carburisation to form Mo2C. Boudart and coworkers showed that control of ramp rate and the reduction potential of the gases is key to controlling the kinetics of oxygen removal, carbon migration into the structure and molybdenum species sintering (146). Synthesis of the α-MoC1–x phase was not observed by the group using this methodology, which can be rationalised by an aggressive carburisation process that facilitated the rearrangement of the molybdenum cations to form the thermodynamically favoured β-Mo2C phase. The method developed by Boudart and coworkers to produce α-MoC1–x via a modification of this temperature programmed synthesis remains today the most studied and adopted synthesis strategy, mainly for the very high purity of the carbides produced as well as for the high surface area.
2.1 Temperature-Programmed Synthesis of α-MoC1–x
The method developed by Volpe and Boudart is reported in a pair of seminal papers (147, 148) and involves the initial synthesis of γ-Mo2N, from the ca. 700°C heat treatment of MoO3 in pure ammonia, referred to as ammonolysis. Following this, the γ-Mo2N undergoes carburisation using CH4/H2 between 400–700°C to form highly pure α-MoC1–x. The importance of the intermediate γ-Mo2N for α-MoC1–x synthesis has been discussed in detail in the following papers (147, 149, 150). The overall process from MoO3 through the cubic γ-Mo2N intermediate to the fcc α-MoC1–x, is considered topotactic. The initial changes from orthorhombic MoO3 to the cubic γ-Mo2N structure involves only a ‘soft’ variation of the metal planes distance, and the final fcc cubic lattice of γ-Mo2N is retained in the formation of cubic α-MoC1–x (Figures 6 and 7). A lack of molybdenum rearrangement during this topotactic process is what is hypothesised to prevent the formation of the thermodynamically stable β phase (147, 149, 150). Interestingly the morphology of α-MoC1–x is dictated by that of the original precursor phase (pseudomorphism), i.e. the morphology of orthorhombic MoO3 is retained after synthesis (see microscopy in Figure 6). It has been observed that these platelet structures are secondary morphologies of very small (sub-10 nm) primary particles which provides α-MoC1–x with dramatically greater porosity and surface area than MoO3 (surface area of the oxide is often reported to be below 1 m2 g–1). These observations are corroborated by the much more recent theoretical studies by Paolucci and coworkers (vide supra) that highlight the thermodynamic stability of α-MoC1–x at low particle size (79). What is not clear is the morphology of these primary particles in these original temperature programmed reactions.
While the process is scientifically elegant and conceptually simple, it has several practical challenges that make synthesis of α-MoC1–x via this route non-trivial. Specifically, this concerns the ammonolysis step, which requires careful control of a number of parameters to facilitate the topotactic process, such as low heating rates and high space velocity of gases (151). The reader is directed to the review by Alexander and Hargreaves for a more detailed discussion on metal nitrides synthesis and characterisation (144). The challenging conditions of ammonolysis clearly present logistic issues, both at the laboratory scale (although this can be overcome with sufficient knowledge and investment) and in large-scale application. In addition to the more obvious issue around handling ammonia at elevated temperatures, control of flow dynamics and heat transfer issues are of concern in large-scale application. In the context of the synthesis of metastable carbides like α-MoC1–x, different strategies can be employed to overcome these issues. These vary from alternative routes to synthesise γ-Mo2N to avoiding nitride phases altogether.
Progress towards ammonolysis-free synthesis was made directly by Levi and Boudart, who found that adding transition metals like platinum to the starting molybdenum precursor was a successful way to produce α-MoC1–x (70). Lee and coworkers further hypothesised that specific transition metals, such as platinum, palladium and nickel, facilitate, via dissociative hydrogen adsorption and spillover, the formation of a hydrogen bronze HxMoO3 (70, 149). This phase lowered the reduction temperature of Mo(VI) from 600°C, seen for pure MoO3, to ca. 175°C and in the process produced a cubic oxycarbide MoOxCy phase, which provided the conditions for the topotactic transition to α-MoC1–x (152, 153). Despite the observation that pseudomorphism from MoO3 to α-MoC1–x was limited when using transition metals, the surface areas reported were comparable (200 m2 g–1) to those from the Boudart γ-Mo2N route (225 m2 g–1), making the process appear desirable for use in catalyst synthesis. Yet, it is possible that the presence of the transition metal impurities in the final product could be problematic, becoming sources of catalytic poisons or promoters of undesired catalytic side reactions. However, given that α-MoC1–x is being actively used as a non-benign support for these very transition metals, as seen in work from Ma and coworkers on aqueous phase reforming of methanol (56), such problems may appear unfounded in certain circumstances. Indeed, Sun et al. recently showed that metal loaded MoO3 transformed into metal/α-MoC1–x with higher activity for water-gas shift than an unloaded carburised MoO3 sample (154). Although the influence of the transition metal on activity cannot be deconvoluted from the molybdenum carbide phase produced, as the absence of the metal produced Mo2C as opposed to α-MoC1–x. Lastly, even if the transition metal is beneficial to catalytic performance, it is likely that most of said metal is ‘wasted’, from a catalytic perspective, in the bulk of the synthesised material and unusable for surface catalytic reactions (70, 149).
Later Bouchy et al. (152, 155–157) discovered that a prereduction of MoO3 in pure hydrogen at 350°C prior to carburisation could produce α-MoC1–x without requiring ammonolysis or transition metals. Through the same mechanism as shown with transition metals, hydrogen insertion into the oxide bulk to form a hydrogen bronze MoOxHy then facilitates the formation of the desired α-MoC1–x (155). However, it was observed that reduction of MoO3 under pure hydrogen resulted in a mixture of MoOxHy and MoO2, which subsequently on carburisation produced a mixture of α-MoC1–x and β-Mo2C. In addition, the procedure initially used pure methane as the carburisation gas, which resulted in significant carbon deposition and a relatively low surface area of 90 m2 g–1. Replacement of pure methane with 9:1 H2:CH4 mixture produced a carbon deposit free catalyst with a surface area of 179 m2 g–1. Unfortunately, no evidence for the MoC1–x:Mo2C ratio was provided for this higher surface area material and only a statement to the effect that Mo2C was a minor phase was provided. While the Mo2C was the minor phase it is unproven that this specific methodology can produce a pure single phase MoC1–x catalyst.
Several researches have studied the influence of using different n -CnH2n +2x carburising agents (152). The group of the late Malcolm Green reported that α-MoC1–x could be synthesised directly from MoO3 when the carbon chain is increased from C1 to C4 (158, 159). Through the use of extended X-ray absorption fine structure (EXAFS) and 13C nuclear magnetic resonance (NMR) they showed that ethane resulted in a mixed MoC1–x:Mo2C material while butane produced a poorly crystalline material comprised of only α-MoC1–x. Although intermediate phases were not reported by Green, it was shown in analogous experiments that an MoOxHyCz phase was formed during heat treatment of H2/C4H10 (157). A summary of the proposed topotactic synthesis via MoOxHyCz precursors and XRD evidence is shown in Figure 8.
Unfortunately, the reported surface area of α-MoC1–x produced via Green’s route was only 35 m2 g–1, although it should be noted that no attempt was made to identify the presence of carbon deposits or, if present, remove them. Following this work a host of different carbon sources (151), such as C2H2 (160), C2H6 (159, 161, 162), C3H8 (163), C4H10 (35), CO (150), C7H8 and C7H16 (164) have been used to produce different carbide phases. Bouchy and coworkers showed that this technique always produced MoO2 in addition to the oxycarbide (152, 156, 165). Perret and coworkers showed in TiO2/ZrO2 supported MoC1–x for succinic acid hydrogenation, that changing the carburisation temperature and the hydrocarbon percentage in the hydrocarbon/hydrogen mixture altered the carbon content in the α-MoC1–x, with a consequent influence on the catalytic activity (112).
2.2 Temperature-Programmed Synthesis of α-MoC1–x Using Carbon Supports and Amine Precursors
Temperature-programmed reactions to produce molybdenum carbides, including α-MoC1–x, are (as stated above) dependent on molybdenum precursor structure and the source of carbon used. Recently, as a natural progression of this statement, the technique has been successfully adapted to make use of carbon and nitrogen sources already present in the molybdenum precursor.
One concept is the addition of an excess of solid carbon phases which then facilitate carburisation in a range of atmospheres, including those excluding a carburisation gas or hydrogen. Through impregnation of a molybdenum precursor onto a carbon, a final catalyst of MoCx/C can be produced where the carbon acts like a conventional support structure. Several different support structures have been employed, such as activated carbons (166) graphene carbon nanofibres (41), carbon nanotubes (167, 168), nanowire (121), doped graphene oxide nanosheet (169), carbon nitride g-C3N4 (141) and glucose (133). Li and coworkers (110, 113) demonstrated that activated carbon impregnated with ammonium molybdate could be converted into either α-MoC1–x or Mo2C under methane/hydrogen, pure hydrogen or under nitrogen only (XRD evidence provided in Figure 9). A general trend on heating of ammonium molybdate → MoOx/MoO2 → α-MoC1–x → Mo2C was seen under all gas atmospheres, with methane/hydrogen producing pure α-MoC1–x at the lowest temperature. Interestingly, under nitrogen, mixed phases of α-MoC1–x and Mo2C were produced and found to be the most active catalysts for the dehydrogenation of lignin. It is interesting to note that no intermediate oxycarbide or hydrogen bronze phases are observed or discussed in the work, making it unclear as to how α-MoC1–x was formed.
Sun and coworkers performed an interesting analogous experiment where ammonium molybdate was impregnated onto C3N4. After heat treatment under an inert atmosphere γ-Mo2N was formed prior to α-MoC1–x (141). Under the conditions used by this group an analogous temperature programmed reaction using an activated carbon only produced β-Mo2C.
Another emerging strategy for bulk α-MoC1–x synthesis has been the utilisation of polymeric or composite molybdenum compounds as precursors during temperature programmed synthesis. Leonard and coworkers pioneered the controlled synthesis of a host of molybdenum carbide phases, including α-MoC1–x, through thermal decomposition of an amide-molybdenum oxide composite (85, 170). These composites were synthesised from the simple precipitation of an aqueous ammonium molybdate and amine solution by addition of hydrochloric acid to pH 3. The composite materials were then heat treated under argon up to 675°C to form α-MoC1–x with excellent reported crystallinity, but no reported surface area. However, their successful employment in HER suggests that the active surface areas must be reasonable. The choice of amine, amine:molybdate ratio and heat treatment temperature dictated the morphology and phase of carbide produced (α,β,η,γ). As far as we are aware this is the only reported synthetic method of controlling α-MoC1–x morphology and therefore has significant potential to experimentally explore surface specific reactivity as hypothesised from theoretical studies. Thermogravimetric analysis (TGA) of the composites indicated two main weight losses, the first being the decomposition of the amine and MoO2 formation and the second carburisation to form α-MoC1–x. Interestingly no evidence was sought to identify oxycarbide or γ-Mo2N and its requirement to produce the α-MoC1–x phase. A number of variations on the amine synthesis route have been reported using ammonium molybdate as precursor with different amino-containing compounds such as dopamine and aniline (121, 136, 171). Tang and Asefa et al. (86) proposed a similar synthesis method which is claimed to produce highly crystalline nitrogen-doped α-MoC1–x or η-MoC nanosheets by drying a solution in deionised water of melamine, cyanuric acid and ammonium and pyrolysing this under a nitrogen atmosphere at temperatures between 450–850°C. α-MoC1–x formed at lower temperature pyrolysis (550–650°C) while η-MoC formed at higher temperature (750°C or 850°C).
The use of alternative molybdenum precursors has also been reported, usually in combination with an amine but not exclusively. Ruddy and coworkers used a gel made from the evaporation of ethanol from a solution containing MoCl5 and 4-chloro-ortho -phenylenediamine, without any pH adjustment, which on heat treatment under nitrogen at 850°C produced α-MoC1–x (172). Inspired by the work of Hashimoto, Nakanishi and coworker who synthesised molybdenum carbonitride from a molybdate-polydiaminopyridine compound (173), Bayati et al. synthesised α-MoC1–x via an analogous polypyrole/phosphomolybdate/graphene oxide composite (169). The authors produced high surface area catalysts of 150–560 m2 g–1, comprised of a mixture of α-MoC1–x and graphitic carbon, after carburisation under an inert atmosphere at 900°C. Interestingly, a slight shift in the reflections of the observed fcc phase were taken to indicate residual molybdenum nitride, hypothesised to have been produced from the decomposition of the pyrrole. Bayati and coworkers noted, as with many other carbide preparation methods, that carbon deposits form during synthesis (169). Interestingly, they noted that these carbonaceous deposits encapsulated α-MoC1–x particles and considered this to be advantageous as a method of preventing α-MoC1–x sintering during preparation and use as an ammonia electro-oxidation catalyst. We provide a note of caution regarding such opportunistic C@α‐MoC1–x structures, as control over carbon layer thickness and porosity, which would be essential for practical catalytic use, requires controlling.
2.3 Temperature-Programmed Synthesis of α-MoC1–x With Non-Carbon Supports and Additives
The use of the heteropolymetalate, phosphomolybdate (H3Mo12PO40·12H2O), as a precursor to α-MoC1–x has also been reported in the elegant hard templating synthesis of a mesoporous hydrogen evolution catalyst (129). A summary of the preparation is shown in Figure 10. H3Mo12PO40.12H2O was impregnated into the mesoporous silica KIT-6 and then carburised under 2:1 CH4:H2 at 650°C to form α-MoC1–x, subsequently the KIT-6 was removed by hydrogen fluoride. Again a fine carbon overlay was formed and considered advantageous, even to the point where further etching of the α-MoC1–x produced an exciting ordered carbon framework (129, 174). Besides the elegant mesoporous structures, the work suggested that an intermediate molybdenum nitride phase was not required to access α-MoC1–x when using the phosphomolybdate precursor. Further, no pretreatment in hydrogen was required. It is apparent that use of comparable carburisation conditions with the conventional MoO3 precursor would exclusively produce the thermodynamically stable β-Mo2C phase. Unfortunately, neither publication performed a simple control synthesis of phosphomolybdate carburisation without the additional complexity of confined pore architectures.
As noted in the preceding paragraph, strategies for the formation of mesoporous or nanostructured α-MoC1–x make use of composite structures that include silica and α-MoC1–x. Work at the start of the millennium by Hamid, coworkers and Haldor Topsøe showed that α-MoC1–x could be formed within ZSM‐5 pores via Green’s carburisation with H2/C4H10, but not via a pure hydrogen treatment. These confined α-MoC1–x/ZSM-5 catalysts were shown to be superior catalysts, compared to β-Mo2C/ZSM-5, for methane to aromatic reactions (157). Ruddy and coworkers identified the challenge of incorporating molybdenum-amine precursors into mesoporous structures, which they overcame through the use of surface modified SBA-15 to produce 2 nm α-MoC1–x from an inert gas heat treatment. It was noted that the acid sites of SBA‐15 in close proximity to α-MoC1–x produced a highly active catalyst for acetic acid deoxygenation (172).
Specific influence on carburisation of the confinement of molybdenum precursors in mesoporous structures is limited. However, one interesting study is provided by Wang and coworkers, who used an amine synthesis route with varying weight percent silica nanospheres being present during polymerisation (during pH adjustment) (136). These spheres were suggested to form packed structures with the molybdenum-polydopamine within the voids (Figure 11). Using the same 800°C argon heat treatment the fraction of α-MoC1–x:Mo2C changed with the weight percent silica. The absence of silica resulted in α-MoC1–x being the only observable phase by XRD, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, while increasing the weight percent silica resulted in significant β-Mo2C content being observed (Figure 12). The authors hypothesise that this phenomenon was caused by confinement reducing the concentration of carbonaceous deposits during the heat treatment which then favours β-Mo2C formation. Despite the lack of evidence to support the hypothesis, the observation is interesting and warrants further study. The same group boarded the concept of additives influencing carbide phase formation by studying the doping of the molybdenum-polydopamine with boric acid and found that its inclusion had a modest effect on suppressing Mo2C formation (171).
Sun and coworkers report an alternative study, with the addition of zinc oxide to a ammonium molybdate and platinic chloride precursor (104). In this instance, traditional carburisation gases hydrogen and methane were employed and the addition of platinum during the carburisation will, as discussed above, influence carbide phase formation (115). It was observed that weight percent zinc up to ~30 wt% increased α-MoC1–x formation, while above this threshold XRD reflections of Mo2C began to dominate. In all instances the reported surface areas were low (maximum of 40 m2 g–1) compared to α-MoC1–x produced by conventional temperature programmed synthesis (>200 m2 g–1) and carbon deposits were not checked for.
Roy et al. (175) have proposed a synthesis of the α-MoC1–x by starting from Mg(MoO4) and Zn(MoO4), compounds prepared from the molybdic acid with MgO and ZnO. Avoiding the ammonolysis, a mixture of 20% carbon monoxide in hydrogen at 700°C was used in a so called ‘sacrificial support method’, in which the formation of magnesium/zinc oxide-phase blocks the formation of the MoO2 intermediate and consequently inhibits β-Mo2C formation. The residual zinc oxide and magnesium oxide were then leached from the catalyst using hydrochloric acid.
2.4 Alternative Preparation Routes to α-MoC1–x
Drawbacks of the Levi Boudart method of temperature-programmed reduction (TPR), based around challenges associated with scale-up of ammonolysis, carbon deposits from using amine polymer precursors and the issues of slow-ramping high-temperature procedures have led to a number of alternative synthesis methods being developed. These processes have attempted to eliminate, or minimise phase sensitivity to, high-temperature carburisation in the synthesis of ammonolysis α-MoC1–x.
Through an electrical wire-explosion technique, Kim et al. published a room temperature route to synthesise ultrafine α-MoC NPs (139). The process used molybdenum wire, which was electrically superheated, resulting in a proposed evaporative explosive process, the products of which were dispersed in oleic acid (chosen for its high carbon content). Nanoparticles of α-MoC were formed by this process alone, but according to transmission electron microscopy (TEM) and XPS, these sub-20 nm particles were embedded within a carbon shell. Heat treatment under a hydrogen environment at 600°C was then required to remove carbonyl functionality and reduce carbon content from 28 wt% to 17 wt%. However, the α-MoC1–x remained encapsulated within a graphitic shell. Despite this the catalysts were active for the HER. Platinum was then deposited on these MoC1–x@C core-shell catalysts to further improve their performance through the ethanol oxidation method, which utilises localised electrons, formed from ethanol oxidation to acetic acid, to reduce the platinic chloride to metal (176).
Separately, Cao et al. (177) and Du and coworkers (178) have grown α-MoC1–x with notable high surface area through an electrospinning technique followed by a pyrolysis of the MoC precursor. The former work by Cao et al. focused primarily on electrospinning as a method of producing a zinc-doped precursor that facilitated the formation of the α-MoC1–x phase during pyrolysis at 800°C. Analogous samples without zinc doping formed the Mo2C phase at the equivalent temperature, leading to the authors to hypothesise that zinc doping supresses the phase transition. The precursor was synthesised by passing a solution of zinc acetate, molybdenum acetylacetonate, polyvinylpyrrolidone in dimethylformamide through a needle with an applied potential of 15 kV. The pyrolysis process in hydrogen/argon at 800°C was proposed to cause zinc oxide reduction and sublimation, which resulted in exciting changes in final surface area depending on Zn:Mo molar ratio used, with a Zn:Mo of 8 producing the highest specific surface area of 418 m2 g–1. Lastly, the authors demonstrated that the synthesised α-MoC1–x was stable up to 1000°C that allowed for the preparation of remarkably high crystalline material. Du and coworkers utilised a similar electrospinning process but used a phosphomolybdic precursor and co-sprayed platinum acetylacetonate to synthesise single atom Pt/α-MoC1–x nanorods.
Malmstadt and coworkers (34) have recently proposed, in a highly cited paper, a continuous microfluidic (mF) synthesis of α-MoC1–x, which is particularly notable for not requiring any high-temperature (maximum temperature used <475°C) carburisation or pyrolysis. The reported method exploits the thermolytic decomposition of the molybdenum precursor Mo(CO)6, which has been reported previously to produce α-MoC1–x when heated under vacuum (179, 180) with partial pressures of carbon monoxide and hydrogen. The process was first adapted by Malmstadt and coworkers by thermolytic decomposition of Mo(CO)6 in a solution with oleylamine and 1-octadecene. The formation of a Mo(CO)4(OAm)2 species was reported to form at ca. 200°C followed by decomposition at >240°C into α-MoC1–x. This process was then further adapted into a continuous flow millifluidic reactor (Figure 13) consisting of the Mo(CO)6 solution being pumped through a borosilicate reactor, prepressurised with nitrogen and immersed in sand bath at 320°C. The product of the reaction was quenched in hexane before the MoC1–x was collected by centrifugation. The produced MoC1–x were characterised by molybdenum K-edge X-ray absorption fine structure (XAFS), XRD, XPS and X-ray pair distribution function (PDF, shown in Figure 14) to demonstrate that partially disordered nanostructured α-MoC1–x capped with oleylamine were formed. These particles were found to be stable under air without passivation, a hereto undiscussed point. The particles were then supported on activated carbon and shown to be highly active and selective for CO2 Fischer-Tropsch chemistry. The reported procedure represents, in our opinion, the greatest departure from the conventional process originally set out by Boudart, with α-MoC1–x being produced at significantly lower temperature and not requiring a temperature-controlled ramping process.
3. Passivation of α-MoC1–x
Passivation of as-synthesised α-MoC1–x (or any other transition metal carbide) is often required due to its highly reactive and pyrophoric nature. Without passivation there is significant surface instability in the presence of air, exacerbated by the high surface areas of α-MoC1–x (40). The consequence of uncontrolled exposure to air are, at a scientific level, a lack of understanding of the reactive surface of the catalyst and at an applied level a significant safety issue. Commonly, within the academic open literature, passivation procedures are given little discussion, or else outright ignored, in synthetic procedures. While this is not universally true, the result is often a lack of clear and precise protocols. Further, regarding this literature review on α-MoC1–x synthesis, the few dedicated passivation studies are related to the β-Mo2C phase, despite it still being an essential process for the application of α-MoC1–x.
Conventionally, passivation of pyrophoric materials is conducted at room temperature in a controlled atmosphere with low concentrations of oxygen (0.5–2% oxygen in helium or nitrogen) over relatively long reaction times (2–20 h) (70, 149, 181–183) The process can also be done by gradually increasing the oxygen partial pressure of the mixture 1–20% before exposing the sample to air (117). Alternatively, passivation can be done by using milder oxidants such as water/CO2 with and without oxygen at 400°C (181, 184), but while this can be less disruptive of the carbide’s subsurface structure, it has reduced effectiveness regarding long term stability on exposure to air (185).
Studies on passivation of β-Mo2C and WC phases show that it can strongly change the surface properties and the reactivity of the material (40, 181). Jentoft and coworkers showed that 0.1% and 1% oxygen passivation was also insufficient for long term storage, with reactivation with hydrogen showing significantly more oxygen content in the old sample than fresh (Figure 15). Demczyk et al. showed in the analogous case of nitrides an actual distinct subsurface phase of Mo2N3–xOx was formed during the passivation (144, 186). As noted by Hargreaves in his recent review, this observation raises the question as to how these subsurface phases influence activation and carburisation (144). Further, passivation is also a crucial consideration when wishing to deposit reactive metals on the MoCx surface. Simply, this is because the metal is no longer interacting with a MoC surface, but with an oxidised surface. This was highlighted clearly by the studies of Thompson et al. (47, 187, 188) with platinum on Mo2C, where platinum assumed different structural configurations on a passivated surface compared to an unpassivated one: namely forming agglomerates on the passivated Mo2C, while remaining finely dispersed on the unpassivated Mo2C. The water-gas shift activity in the two samples was notably different even though the surface areas were similar.
Passivation of catalysts frequently requires an accompanying reactivation step prior to further surface treatments, catalyst characterisation and employment in a catalytic reaction. However, the passivation-activation procedure is not innocuous and the reactivated surface is rarely the same as in the fresh catalyst (40). In the process of removing surface oxygen through the use of hydrogen, carbon atoms are lost from the near-surface (189, 190). The use of mixtures of methane/hydrogen appears to be milder, with reduced but still significant surface carbon loss (191). It is common to reactivate passivated catalysts by flowing hydrogen or alkene/hydrogen mixtures at 450–700°C for one or two hours prior to catalytic use (56, 182, 189, 192).
Potentially it is possible to avoid a direct passivation step, for example Gao et al. report a method to grow β-Mo2C on carbon nanotubes in a way that no passivation was needed afterwards (193). Another example is work by Malmstadt and coworkers where the synthesised α-MoC1–x was reportedly capped by organic compounds (34). Alternatively, some literature studies perform catalytic testing directly after synthesis and within the same reactor to avoid the need for passivation (167), although this is unlikely to be practically viable in an applied industrial reactor. In reality, such unpassivated catalysts will have significant surface reconstruction upon exposure to catalytic reactants (167). Although on a partially tangential topic to carbide synthesis, Román-Leshkov and coworkers have recently shown how reactive carbide surfaces are during hydrodeoxygenation reactions (194). Therefore, it is obvious and possibly not unexpected to think that a reactive carbide catalyst will have notably different surface and possibly subsurface structure as compared to an as-synthesised material.
4. Perspectives and Conclusion
Since Levy and Boudart’s seminal study of the ‘platinum-like’ catalytic behaviour of early transition metal carbides, there has been a significant scientific push to apply these compounds to contemporary catalysis. It has been noted that, while these compounds are generically platinum-like, they have their own unique properties and further that different transition metal carbides and carbide phases have their own specific catalytic behaviours and applications. Therefore, it has become clear that particular carbide structures and morphologies should be targeted using the correct synthetic procedures. This review focuses on the metastable fcc structured α-MoC1–x which has excellent OH bond activation properties for application in reforming, water-gas shift and biomass conversion technologies. In addition, the structure provides exceptionally strong metal support interactions with transition metals and can be used to produce highly dispersed metal supported catalysts. Table VI provides the collated synthetic procedures, parameters and structural properties of the resultant α-MoC1–x materials from this review.
|Method||Phase produced||Mo-precursor||Reaction conditions||Surface area, m2g–1||Particle size, nm||Morphology||Passivation||Ref.|
|Ammonolysis + carburisation (TPRe)||α-MoC1−x||α-MoO3||Ammonolysis: 700°C in NH3; flow rate: 70–130 ml min–1. Carburisation: gas: 20% CH4/H2; ramp: 5–100°C h–1 to 700°C; flow rate: 90–170 ml min–1||120–200||3–6a||Platelet-like shape||0.5–1% O2/He, RT||(70), (148), (149), (195)|
|Direct carburisation (TPRe) with metal||M/α-MoC1−x||M/α-MoO3||Carburisation: 20%–80% CH4/H2. Metals (M) = Ni, Pd, Pt giving α; Cu or Co giving β||<221||2.6a, 3.4b||Broken platelet-like particles||–||(70), (149)|
|Activation + Carburisation (TPRe)||α-MoC1−x traces of β-Mo2C||α-MoO3||Activation: H2 or H2/high-C alkane (157) at 350°C for >24 h. Carburisation: 10% CH4/H2, ramp: 3°C min–1 to 710°C, flow: <30 ml min–1||179||3.4b||Platelet-like shape, avg. pore size 3 nm||–||(155), (157)|
|Direct carburisation (TPRe)||α-MoC1−x||α-MoO3|| Carburisation (all at 1°C min–1, flow rate: 90–170 ml min–1)
(i) 20% CH4/H2, ramp to 750°C
(ii) 10% C2H6/H2, to 630°C
(iii) 5% C4H10/H2, to 550°C
| (i) 30.8
| (i) 5.2
| (i) Leaf-like
(iii) Platelet-like shape
|1% O2/He, RT||(158)|
|Carburisation: 10% C2H2/H2, ramp: 1°C min–1 to (i) 450°C; (ii) 500°C; (iii) 550°C; or (iv) 630°C for 4 h. Flow rate of 100 ml min–1||19–36 (size ∝1/temp)||Platelet-like shape||1% O2/Ar, RT||(160)|
|Activation + carburisation (TPRe) Mo-precursor with solid carbon containing source||α-MoC1−x/C, β-Mo2C/C, MoO2/C||(NH4)2MoO4 + activated carbon||(i) 350°C in H2, then carburisation in CH4/H2 gives: MoO2 + α-MoC at 500°C, α-MoC at 600–800°C; or (ii) H2 only gives: MoO2 + α-MoC at 500°C, α-MoC at 600°C, α-MoC + β-Mo2C at 625–650°C, β-Mo2C at 700–800°C or (iii) N2 gives: MoO2 at 500°C, α-MoC + β-Mo2C + MoO2 at 700°C, α-MoC + β-Mo2C >800°C||301–505||12–45c||Irregular particles||–||(113)|
|α-MoC1−x/C, β-Mo2C/C||(NH4)6Mo7O24·4HO + glucose||Hydrothermal treatment at 200°C for 10 h, freeze drying then annealing in 5% H2/Ar at 10°C min–1 to 800°C. β-Mo2C/C when glucose:(NH4)2MoO4 is 6:1. α-MoC1−x/C at 4:1||α 218; β 243||α 3.3b; β 3.2c||–||–||(133)|
|Pyrolysis of Mo-amine precursor||α-MoC1−x, β-Mo2C, η-MoC, γ-MoC||Amine-oxide hybrid precipitate: (NH4)6Mo7O24·4HO + amine||α-MoC1–x: with 4Cl-o PDA, 1,12-DDA, HMT or PDA in Ar at 100°C h–1 to 675–750°C for 12 h. η-MoC: with p PDA at 100°C h–1 to 1050°C, no dwell. γ-MoC: with 4Cl-o PDA to 850°C at 100°C h–1 for 24 h. β-Mo2C: with 4Cl-o PDA, 1,6-HDA, HTM to 850°C at 100°C h–1 for 12 hd||–||–||α-MoC1–x: rods, flakes, wires, layers and cubes. β-Mo2C: rods, spheres and flowers||55–60 days in air||(85), (170)|
|α-MoC1−x||Melamine + cyanuric acid + (NH4)6Mo7O24||Pyrolysis of the Mo-CN hybrid in N2 at 2°C min–1 to 550°C or 650°C for 3 h||153||–||Nanoporous 2D nanosheets||–||(86)|
|Graphene-@ α-MoC1−x||PMo12e + GO + pyrrole||Pyrolysis 900°C in N2 + etching in H2SO4||150||2–10c||Nanospheres 70–100 nm||–||(169)|
|Sacrificial support method||α-MoC1−x||(i) (NH4)6Mo7O24, (ii) (NH4)2Mg(MoO4)2||Carburisation: 20% CO/H2 at 15°C min–1 to 700°C for 3 h|| (i) 73
| (i) 5.8b
|–||1% O2/He, RT||(175)|
|Carbonyl decomposition||α-MoC1−x||Mo(CO)6|| (i) Sonochemical decomposition in hexadecane at 90°C + 20% CH4/H2 at 500°C for 48 h.
(ii) Flow reaction: 320°C in oleyamine and 1-octadecene. Particles stabilised in hexane
| (i) 2c
|Round isotropic crystallites||–||(34), (196)|
|Template assisted method||α-MoC1−x/C||Phosphomolybdic acid + KIT-6 (template)||Carburisation of the impregnated precursor on KIT-6 in a CH4:H2:Ar mixture (60.5:29.5:10 vol%) at a flow rate of 200 cm3 min–1, ramp rate of 10°C min–1 to 650°C for 1 h||25||–||Mesoporous: volume 0.07 cm3 g–1||–||(129), (174)|
|α-MoC1−x / SBA-15||MoCl5 + 4Cl-o PDA||Pyrolysis of the Mo-diamine gel in N2 at 100°C h–1 to 850°C for 4 h||10||4.2c||Quasi-spherical NPs||1% O2/N2||(172)|
Controllable synthesis of this phase is highly challenging on both a laboratory scale and on an applied industrial scale. Most carbide synthetic strategies revolve around a temperature programmed methodology, in which a molybdenum precursor (most often MoO3) is heated at a slow ramp rate with a high space velocity of hydrogen and an alkane. In the absence of specific synthetic strategies the process will result in the formation of the thermodynamically stable β-Mo2C. In order to successfully produce the metastable α-MoC1–x, the process is believed to have to undergo a topotactic reaction of carbon insertion into an equivalent cubic precursor phase. fcc γ-Mo2N, MoOxCy and MoOxHy have all been observed as such topotactic precursors. Certainly, in the case of γ-Mo2N a clear, measurable scientific correlation between this precursor phase and the final α-MoC1–x product has been reported. The process produces high surface area (225 m2 g–1) and high purity α-MoC1–x. However, it is frequently asserted that Boudart’s original ammonolysis of MoO3 prior to carburisation of the resultant γ-Mo2N is impractical. Handling of high-temperature pure ammonia gas streams is clearly challenging, but as demonstrated by numerous scientific groups, possible to achieve on the laboratory scale. Application on an industrial scale would however require significant control of process heat flows and would undoubtably be challenging.
Avoiding the use of ammonolysis to form α-MoC1–x, via hydrogen pretreatment, incorporation of hydrogen dissociating transition metals, use of higher alkanes (i.e. butane and higher) in carburisation and decomposition of amine containing precursors is well reported. A number of these synthetic protocols produce relatively high purity α-MoC1–x, although traces of β-Mo2C are frequently observed. Reported surface areas vary significantly, with to our knowledge only one higher (relative to Boudart’s method) reported value of >500 m2 g–1, several at comparable surface areas to the original 200 m2 g–1, some with dramatically lower surface areas and several papers not reporting surface area. The presence of additional graphitic carbon addressed in Boudart’s original publications is often not discussed in these papers. In some instances, evidence has been provided that certain cubic intermediates are produced during the temperature-programmed reaction of these ammonolysis free routes. Unfortunately, this is not always the case. This is particularly notable in more recently reported, non-conventional preparation routes, including using alternative molybdenum precursors and the formation of hybrid hierarchical materials, for example in the use of hard templating. Our perspective is that several control experiments require performing and that in situ characterisation studies of these synthesis routes would provide a solid scientific basis as to how they produce α-MoC1–x. Further, it is not clear if the use of more costly molybdenum precursors or processes makes these alternative routes more applicable on an applied scale than Boudart’s ammonolysis strategy. Each of these alternative routes will have specific advantages and disadvantages, for example the addition of <0.2 wt% platinum or first row transition metals might be viable or even beneficial in some catalytic applications. Indeed, several publications report excellent metal-support interaction between α-MoC1–x and platinum to make highly active catalysts. The incorporation of so called ‘single atom’ platinum onto α-MoC1–x presents an exciting topic area which bridges other contemporary areas of science that aim to maximise pgm efficiency.
DFT based theoretical studies have shown that there are interesting correlations between morphology, particle size and Mo:C ratio in regard to the α-MoC1–x phase. Validation of these findings through experiments requires further work. Specifically, the predicted morphologies are based entirely on the thermodynamic stability of specific surface terminations. Practically, access to the metastable phase requires topotactic rearrangement of a precursor phase (i.e. kinetic control) and so the morphology is dictated by that of the precursor. Most synthesis methods have used orthorhombic MoO3 which has large platelets with a high fraction of the (010) surface termination. Given that the (100) plane of Mo2N or α-MoC1–x is parallel to this, these products exhibit pseudomorphism. The validity of these very interesting theoretical studies can be interrogated by suitable characterisation of synthesised materials using modern advanced microscopy. In addition, very little research has been published on exploiting the interdependence of α-MoC1–x morphology on that of the precursor. It should be noted that amine-MoO3 composite synthesis appears to be one of the few synthetic strategies to date that can produce different α-MoC1–x morphologies.
Noticeable throughout a significant proportion of the literature is an absence of discussion around α-MoC1–x passivation. Given that the α-MoC1–x surface is highly reactive, passivation will create permanent surface and subsurface changes to catalysts, influencing performance and the ability to support additional metal nanoparticles. However, from a practical perspective the process is entirely necessary to ensure easy handling of this pyrophoric material. Fully exploring this parameter space, while less exciting than development of new preparation routes, is equally important. Systematic studies that consider correlations between passivation conditions (oxidant, its concentration and reaction temperature), specific carbide properties (surface area, surface terminations, Mo:C ratio and particle size), along with difference between different phases (α-MoC1–x vs. β-Mo2C), are required. Characterisation to elucidate these correlations could include the use of near-ambient XPS (NP-XPS) which would be highly beneficial for studying surface changes in situ.
There is a great deal of literature and understanding of transition metal carbide catalysts and an evolving understanding of specific phases, such as α-MoC1–x. However, controllable and practical synthesis methods still require further work. These include methodologies that make use of relatively low temperature and simple pyrolysis steps, which ideally will have greater tolerance to deviation in gas flow dynamics and heating ramp rate than current temperature-programmed techniques. In addition, it is important that any preceding wet chemical processing to make molybdenum precursors that facilitate ‘simple’ heat treatments are clearly understood and reproducible at scale. Ensuring a systematic understanding of the process and how it influences the final phase and surface properties of the catalysts is essential to scale up and application.
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Andrea De Zanet is currently a PhD student at the Chemistry department of Loughborough University, UK. Andrea has received the Bachelor’s and the Master’s degree in Material Science at the University of Padova. His research at Loughborough focuses on the synthesis of novel catalysts for the sustainable production of hydrogen, with a particular interest in molybdenum carbides for processes like aqueous-phase reforming and HER. His work is part of SLowCat, an interdepartmental project at Loughborough University, which is studying new low-dimensional catalysts, optimised for biomasses treatment and generation of chemicals and fuels.
Dr Simon A. Kondrat completed his Master’s in Chemistry from the University of Warwick, UK, in 2007, before completing his PhD under Professor Graham J. Hutchings at Cardiff University, UK in 2007. He is currently a lecturer at the University of Loughborough with an interest in heterogeneous catalysis for sustainable hydrogen production and operando spectroscopy using tender X-ray absorption spectroscopy and neutron scattering techniques. He has a particular interest in the inorganic chemistry of catalyst preparation in areas including metal carbides, perovskites and single atom catalysis.