Experimental and Theoretical Investigation of Molybdenum Carbide and Nitride as Catalysts for Ammonia Decomposition

Abstract

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.

H2 Production via Ammonia Decomposition Using Non-Noble Metal Catalysts: A Review

“…The activity of MoN x was considerably improved after ball milling of the MoO 3 catalyst due to the increase in specific surface area from 1 to 13 m 2 g -1 [82]. Complementary experimental and theoretical studies by Zheng et al [83] demonstrated that the high rate of ammonia decomposition over molybdenum carbide and nitride can be attributed to the energetic sites comprising of twin boundaries, faults in stacking, steps and defect sites. Further development of the MoN x catalyst is needed to achieve a net cost effective system compared to the ruthenium-based catalysts, especially considering that the cost of molybdenum is half of that of ruthenium, the molybdenum-based catalyst have a very low surface area (bulk systems) compared to the highly dispersed ruthenium ones.…”

The bending machine: CO2activation and hydrogenation on δ-MoC(001) and β-Mo2C(001) surfaces

“…26 Furthermore, it has been predicted that d-MoC(001) can easily dissociate molecular oxygen 27 and DFT calculations showed the very high catalytic power of hexagonal a-Mo 2 C on ammonia dehydrogenation. 28 Also, hexagonal a-Mo 2 C(001) and orthorhombic b-Mo 2 C phases have been considered as catalysts for CO hydrogenation, [29][30][31] and very recently, b-Mo 2 C(001) has been proposed for CO dissociation.…”

Structure and site evolution of molybdenum carbide catalysts upon exposure to oxygen

“…Mo 2 C phase identification literature has discrepancies between surface science and experimental nomenclature. Experimentally, Mo 2 C is generally characterized by XRD as having a hexagonally closepacked structure (hcp b-Mo 2 C, JCPDS PDF # 00-035-0787) [23][24][25][26][27] or a metastable face-centered cubic structure (fcc a-Mo 2 C, JCPDS PDF # 00-015-0457) [21,25,27]. Surface science [28] and DFT [4,29] literature often refer to a-Mo 2 C as the orthorhombic unit cell indexed by Parthé and Sadagopan [30](JCPDS PDF # 01-072-1683).…”