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An overwhelming number of research reports on the catalytic activity of molybdenum carbides in the dry reforming of hydrocarbons refer to catalysts prepared using TPR method. The physicochemical properties and resulting catalytic activity of molybdenum carbide catalysts are influenced by the molybdenum-to-carbon ratio. Gao et al.
[79][55] reported a series of molybdenum carbide catalysts that differ in the weight content of Mo in order to use carbon nanotubes as a carbon source (Mo 0, 5, 10, 15, 30, 60, and 100 wt.%). Along with an increasing proportion of molybdenum in the catalyst, a decrease in the specific surface area, diameter, and pore volume was observed. A correlation was observed between the molybdenum content and catalytic activity in the dry methane reforming process. The highest activity was observed for the catalyst containing 30 wt.% of Mo. Another of the key structural parameters of the carbide for catalytic activity is the excess unbound carbon formed during the synthesis process. Roohi et al.
[216][56] found that the amount of excess carbon depends on the carburization temperature and the concentration of carbon-containing gas during the synthesis. Catalysis with lower contents of excess carbon exhibited an initial higher activity in the DRM reaction; however, during the long-term test, the molybdenum loading was a crucial factor.
Several articles have been published to investigate the effect of the crystal structure on catalytic activity in DRM
[78,217][57][58]. Liang et al.
[78][57] investigated the catalytic activity of β-Mo
2C and α-MoC
1-x phases in DRM. Both phases were characterized by a narrow size distribution of up to 5 nm. Better activity was observed for the -MoC
1-x phase. Oshikawa et al.
[217][58] observed the dependence of the η-Mo
3C
2 phase on the methane decomposition rate. They reported the key role of the η-Mo
3C
2 phase among other molybdenum carbide phases as an active species for methane reforming. During the DRM process, the molybdenum carbide may be partially oxidized to the form of an oxycarbide. Kurlov et al.
[218][59] reported that the oxycarbidic phase Mo
2C
xO
y exhibits high stability toward further oxidation to MoO
2, and the increase in β-Mo
2C/ Mo
2C
xO
y active sites correlates with higher efficiency in the DRM reaction.
5.4. Molybdenum Carbide Modified with Nickel Particles
3.4. Molybdenum Carbide Modified with Nickel Particles
Molybdenum carbide catalysts during DRM at atmospheric pressure may suffer from deactivation due to oxidation with carbon dioxide. The carbide structure is reconstructed with the carbon element from the dissociation of methane; however, oxidation with CO
2 is more favorable
[219][60]. The combination of molybdenum carbide with other metals: Ni
[222][61], Co
[223][62], and Fe
[24][63], allows controlled dissociation paths of CO
2 and CH
4, ensuring appropriate conditions for oxidation–recarburization cycles
[47,224][16][64]. The introduction of other metals into the carbide catalyst results in the generation of more moles of hydrogen, leading to a higher H
2/CO ratio in the outlet stream. Carbide and the introduced metal (Ni, Co) act as an active center for the dissociation of CO
2 and methane, respectively.
It is generally accepted that the catalytic activity of nickel catalysts is strictly connected with the size of the nickel particles: the smaller the Ni particles, the better the catalytic activity, resulting from the stronger active metal–support interactions, delayed sintering, and a lower rate of formation of carbon deposits
[3,5,14,225][3][5][65][66]. However, in the case of molybdenum carbide supported nickel catalysts, the ratio of Ni/Mo to the size of nickel particles plays a predominant role
[51,226][19][67]. The nickel-to-molybdenum ratio affects the morphology and catalytic activity of Mo
2C. Moreover, too high a dissociation of CH
4 promotes the formation of coke on the surface of the catalysts
[79,222][55][61]. Zhang et al.
[222][61] observed that with an increasing nickel content in nickel-modified Mo
2C supported on carbon nanotubes, the crystallite size of Mo
2C for Ni/Mo ratios = 0.5, 1, 1.5, and 2 was equal to 53, 38, 35, and 28 nm, respectively. Moreover, the increase in the Ni content resulted in an increase in the particle size. Catalytic activity increased with an increasing Ni/Mo ratio to the optimal value (1:1). After this value was exceeded, the activity decreased despite the higher content and smaller particle size of nickel. The DRM process is carried out mainly at temperatures above 800 °C. The performance of processes at lower temperatures results in lower methane and carbon dioxide conversions, as well as a lower H
2/CO ratio
[119,123][68][69]. However, Diao et al.
[227][70] recently reported the high catalytic activity of a Ni-Mo
2C/Al
2O
3 catalyst at 470 °C in a catalytic bed coupled with non-thermal plasma treatment. The molybdenum-nickel-alumina catalyst exhibited superior activity compared to Ni/Al
2O
3. The H
2/CO ratio was equal to 0.9, and the conversions of CH
4 and CO
2 were around 80% and 85%, respectively.
Both bare and nickel-modified molybdenum carbide catalysts are used, both supported and unsupported. Deposition on an inert substrate allows for dilution of the catalyst, thus eliminating channeling, and retarding heat transfer limitations and pressure drop across the catalytic bed
[194][34]. As a support, metal oxides: La
2O
3 [224][64], Al
2O
3 [12[71][72][67],
121,226], SiO
2 [10], ZrO
2 [116][73], MgO
[228][74], biochar
[77][75], carbon nanotubes
[222][61], zeolites
[26][76], and silicon carbide
[10], have been examined. Silva et al.
[10] investigated the effect of the support (SiO
2, Al
2O
3, and SiC) Ni-Mo
2C on catalytic activity and stability in the DRM reaction. The lowest DRM substrate conversions and H
2/CO ratios were observed for the silica support. As a reason for the low activity observed for the SiO
2-supported samples, there were weak interactions between Ni and SiO
2, leading to movement of Ni species at the surface of the catalysts, retarding the interface contact between Ni and Mo
2C responsible for the oxidation–recarburization cycle, Ni aggregates, and the formation of filamentous carbon.
5.5. MAX and MXenes for Dry Reforming of Hydrocarbons
3.5. MAX and MXenes for Dry Reforming of Hydrocarbons
A special family of transition metal carbides is constituted by multilayer metal carbides with a 2D nanosheet structure similar to that of graphene, belonging to the group of compounds called MXenes. The term MXenes denotes carbides and nitrides of transition metals, with the general formula Mn+1XnTx, where n = 1, 2, 3, or 4, M refers to the transition metal (M = Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Mo, Hf, Ta, and W [23,161][77][78]), and X refers to the p-block element (silicon, aluminum, gallium), while T describes the type of terminal groups (–O, –OH, –F, –Cl) in the amount of x per selected unit. They are obtained by selectively removing component A from the ternary MAX matrix. The MAX matrix consists of the elements of the transition metal M, a p group element (A), and carbon or nitrogen (X). MXene compounds are gaining importance due to their metal-like conductive properties, thermal and chemical stability, and the ability to manipulate properties through simple and effective modification of terminal groups [162,163][79][80]. Their unique properties allow for application in various branches of science: energy storage [163[80][81],164], electrocatalysis [165[82][83],166], photocatalysis [167[84][85],168], and heterogeneous catalysis [23,164,165,166,169][77][81][82][83][86].
Despite a broad examination in electrocatalysis, heterogeneous photocatalysis, and catalysis, to our best knowledge, to date, among titanium, molybdenum, and tungsten MXenes or MAX matrices, the catalytic activity in dry reforming of hydrocarbons has been reported only in a few articles. Ronda-Lloret et al. reported the catalytic activity of Co
3O
4 supported on Ti
2AlC in the dry reforming of butane
[230][87]. The levels of butane and carbon dioxide conversions for Co
3O
4/Ti
2AlC were 20% and 25% after 18 h of testing, respectively. The efficiency of butane conversion was higher compared to Co
3O
4/TiO
2; however, it was lower than that for Co
3O
4/Al
2O
3. Despite the lower activity, the Co
3O
4/Ti
2AlC catalyst exhibited higher stability and anticoking properties compared to the metal oxide-supported catalysts.
Kurlov et al.
[231][88] reported the catalytic activity of a 2D-Mo
2CO
x/SiO
2 catalyst in the dry reforming of methane. The catalyst was prepared by incipient wetness impregnation of multilayered m-Mo
2CT
x on a SiO
2 support in a colloidal alcoholic suspension, followed by reduction in hydrogen (20 vol.% H
2/N
2, 800 °C) and oxidation with CO
2. The authors found that the deposition on silica particles prevents the thermal sintering and oxidation of the Mo
2C and MoO
2 phases, respectively, while activation with CO
2 is crucial to protect the catalysis from complete oxidation. Furthermore, they found that long-term storage of 2D-Mo
2CO
x/SiO
2 leads to partial fragmentation of nanosheets and thus to deactivation of the catalyst.
Among others, the catalytic activity of V
2O
3−V
8C
7/m-V
2CT
x, obtained from the V
2AlC matrix, belonging to the MAX/MXene family, in the dry reforming of methane was reported
[226][67]. The V
2O
3−V
8C
7/m-V
2CT
x catalyst exhibited catalytic activity comparable to the nickel catalyst supported on ZSM-5 zeolite. After the catalytic process, the remaining V
2O
3−V
8C
7/m-V
2CT
x catalysts’ layered structure was slightly oxidized into an oxycarbide. The thermal stability at high temperatures, anti-oxidation properties under mild oxidants (CO
2), and ability to participate in oxidation–carburization cycles are crucial factors in terms of potential application in the dry reforming of hydrocarbons.