Molybdenum–Niobium Carbides: Synthesis, Characterization and Catalytic Activity in Toluene Hydrogenation
Ali Mehdad,1 Rolf E. Jentoft,1,2 and Friederike C. Jentoft1,2
1Chemical, Biological & Materials Engineering, University of Oklahoma
Norman, OK 73019-1004 USA
2present address: Department of Chemical Engineering, University of Massachusetts
686 North Pleasant Street, 159 Goessmann Laboratory, Amherst, MA 01003-9303, USA
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
Transition metal carbides are active catalysts for a variety of reactions, most notably hydrogenation, hydrogenolysis and hydrodesulfurization, –nitridation, and –oxygenation (HDS, HDN, HDO). However, in comparison to oxide catalysts, the number of well described families of carbides is small, although variation of the nonmetal (i.e., carbon, but also oxygen) content and the incorporation of another metal into the bulk or surface structure allows tuning the catalytic properties, including the stability. Mixed metal carbides have been advertised as superior to their respective monometallic carbides or traditional catalysts; such successes include molybdenum–tungsten carbides in reforming , supported molybdenum–niobium carbides in HDS and HDN [2,3], and molybdenum–nickel and molybdenum–cobalt carbides in HDN [4,5].
The goal of this work was to synthesize mixed metal carbides with balanced metallic (hydrogenation) and acid functions, and enhanced stability. To this end, molybdenum and niobium were chosen. Mo2C is known for its pronounced metallic behavior, in contrast to NbC. Molybdenum and niobium also differ in their affinity to oxygen and thus in their ability to generate acidic sites through oxycarbide formation.
Precursors for mixed metal carbides with different molybdenum/niobium ratios were prepared by hydrothermal and freeze-drying methods and converted to carbides in 20% CH4/H2 at final temperatures ranging from 650 to 950 °C. The carburization temperature was determined by the composition. XRD and EDS results showed that the mixed metal carbides are single phase solid solutions that follow Vegard's law. Depending on the metal ratio or precursor synthesis method, the carbides crystallize in the cubic NbC structure or the hexagonal Mo2C structure. Surface area and oxygen uptake during passivation were also determined by the nature of the precursors. Temperature-programmed reduction revealed that surface reduction and carbon vacancy formation is more facile for mixed metal carbides than for their monometallic counterparts.
Hydrogenation of toluene was used as the test reaction to investigate the reactivities of the samples. At 400 °C and 20 bar H2, the materials with Mo2C structure catalyzed hydrogenation and hydrogenolysis, whereas materials with NbC structure were less active and produced more acid-catalyzed products (ring contraction). NbC was inactive. The variations in product selectivity can be ascribed to the balance between sites with metallic and acidic properties. The acidic properties are likely to be associated with incomplete surface reduction and the presence of oxycarbide, which is favored by the highly oxophilic niobium. The catalytic activity of materials with NbC structure was generally more stable than that of materials with Mo2C structure.
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