Constant CO(x)-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts.

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A series of bimetallic carbides of the form β-(Mo_1-xV_x)₂C (0 < x < 0.12) was synthesized by carbothermal reduction of corresponding h-Mo_1-xV_xO₃ precursors. The oxides were synthesized by precipitation, and the subsequent carbide phase development was monitored. The reduction mechanism is discussed on the basis of observed structural evolution and solid-state kinetic data. The reduction is observed to proceed via a complex mechanism involving the initial formation of defective Mo^IV oxide. Increasing the V content retards the onset of reduction and strongly influences the kinetics of carburization. The carbides exhibit a trend in the growth morphology with V concentration, from a particulate-agglomerate material to a packed, nanofibrous morphology. The high-aspect-ratio crystallites exhibit pseudomorphism, and in the case of the V-containing materials, some preferential crystal orientation of grains is observed. An increasing mesoporosity is associated with the fibrous morphology, as well as an exceptionally high surface area (80–110 m² /g). The synthesis was subsequently scaled up. By adapting the heating rate, gas flow, and pretreatment conditions, it was possible to produce carbide materials with comparable physical properties to those obtained from the small scale. As a result, it was possible to synthesize Mo₂C materials in multigram quantities (5–15 g) with BET surface areas ranging from 50 to 100 m² /g, among the highest values reported in the literature

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The effect of the gas-phase chemical potential on surface chemistry and reactivity of molybdenum carbide has been investigated in catalytic reactions of propane in oxidizing and reducing reactant mixtures by adding H2, O2, H2O, and CO2 to a C3H8/N2 feed. The balance between surface oxidation state, phase stability, carbon deposition, and the complex reaction network involving dehydrogenation reactions, hydrogenolysis, metathesis, water-gas shift reaction, hydrogenation, and steam reforming is discussed. Raman spectroscopy and a surface-sensitive study by means of in situ X-ray photoelectron spectroscopy evidence that the dynamic formation of surface carbon species under a reducing atmosphere strongly shifts the product spectrum to the C3-alkene at the expense of hydrogenolysis products. A similar response of selectivity, which is accompanied by a boost of activity, is observed by tuning the oxidation state of Mo in the presence of mild oxidants, such as H2O and CO2, in the feed as well as by V doping. The results obtained allow us to draw a picture of the active catalyst surface and to propose a structure-activity correlation as a map for catalyst optimization.