Applied Catalysis A: General 408 (2011) 178–186 Contents lists available at SciVerse ScienceDirect Applied Catalysis A: General j ourna l ho me page: www.elsevier.com/locate/apcata Preparation and characterization of low-temperature nano-crystalline cubic molybdenum carbides and insights on their structures G. Vitale a,∗ , M.L. Frauwallner b , C.E. Scott b , P. Pereira-Almao b a Chemistry Department, University of Calgary, Calgary, Alberta, Canada T2N 1N4 b Schulich School of Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4 a r t i c l e i n f o Article history: Received 8 July 2011 Received in revised form 16 September 2011 Accepted 19 September 2011 Available online 24 September 2011 Keywords: -Mo2C Carbon vacancies Toluene hydrogenation XRD a b s t r a c t Direct conversion of precursors formed by drying mixtures of ammonium heptamolybdate (AHM) and sucrose solutions to nano-crystalline cubic molybdenum carbides by temperature-programmed reaction (TPR) with H 2 has been studied at low temperature (673 K). The precursors prepared with AHM and sucrose were able to produce nano-crystalline domains of cubic molybdenum carbide which seemed to show carbon vacancies affecting the intensity of the signals (2 0 0) and (2 2 0) of the X-ray powder diffrac- tion patterns. The number of carbon vacancies was affected by the time the precursor spent at 673 K; the longer the time at this temperature, the higher the carbon vacancies observed by reduction of the intensity of the (2 0 0) signal of the X-ray diffraction pattern. The nano-crystalline molybdenum carbides prepared showed activity for the hydrogenation of toluene at 473 K to selectively produce methyl-cyclohexane. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. 1. Introduction Transition-metal carbides have shown catalytic behavior simi- lar to the much more expensive noble metals [1–5] and they have attracted much attention as catalysts. Different transition-metal carbides have been tested in many of the reactions which nor- mally use noble metals. Bulk and supported carbides of group VI promoted or not, have been shown to be active in several reac- tions. Some interesting examples are: hydrogenation of aromatics [6–11], CO 2 hydrogenation [12–14], oxidation of hydrocarbons [15], water gas shift [16–19], alcohol synthesis [20,21], hydrazine decomposition [22,23], reforming [24–27] and hydrotreating reac- tions [28–35]. The commercial application of these materials as catalysts is restricted by the lack of complete control and under- standing of the relationship between the structure and its physical chemical properties, as well as a practical and cost-effective way of producing them and to their sensibility to oxidative/sulfidic envi- ronments [8,35]. Temperature programmed reaction (TPR) is the process typi- cally carried out to synthesize bulk metal carbides of group VI for their use as catalysts. Under the TPR route, a given amount of the oxide precursor (usually MoO 3 or WO 3 ) is heated while it is exposed to a mixture of hydrogen and a hydrocarbon, like CH 4 , C 2 H 6 , C 2 H 2 or n-C 4 H 10 [28,35–40]. During the thermal treatment, the oxide is ∗ Corresponding author. Tel.: +1 403 210 9587; fax: +1 403 210 3973. E-mail address: vitaleg@ucalgary.ca (G. Vitale). progressively reduced to lower valence oxide states further under- going carburization to yield the carbide. Typical preparations of molybdenum carbides under the TPR route require temperature step ramps to obtain the final product at high temperature, which normally is the stable and well known hexagonal molybdenum carbide phase. It is possible to produce other molybdenum carbide phases by modifying synthesis conditions or by using a different carbide pre- cursor. The face centered cubic (fcc) phase can be obtained under TPR route by first producing a cubic oxyhydrate (MoOxHy) from the MoO 3 , by applying first H 2 before the carburizing mixture [41], or by impregnating the MoO 3 with Pt, Pd or Ni which act as cubic structure stabilizing agents [42] before carburization. The prepara- tion of the cubic phase can also be accomplished with slurries of molybdenum hexacarbonyl by sonochemical methods [43] under lower temperatures but this has the commercial feasibility and cost-efficiency as drawbacks. The cubic phase can also be formed under plasma arc at very high temperature in which it is the sta- ble form [44,45] but cooling the produced materials seems to favor the transformation to the hexagonal phase. Preparation of precur- sors different from MoO 3 by solution route [46,47] with molybdic acid or ammonium heptamolybdate with organic compounds like ethylene glycol or sucrose in water also has been shown to produce the cubic molybdenum carbide at high temperature and with small crystal domain sizes which may be suitable for its use as a catalyst. Transition-metal carbides with the cubic rock-salt structure pre- pared in ways described above produce materials that belong to a family of non-stoichiometric or sub-stoichiometric compounds. 0926-860X/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.09.026