(751f) Molybdenum–Niobium Carbides: Synthesis, Characterization and Catalytic Activity in Toluene Hydrogenation

Molybdenum?Niobium Carbides: Synthesis, Characterization and
Catalytic Activity in Toluene Hydrogenation

Ali Mehdad,¹ Rolf E. Jentoft,^1,2 and Friederike
C. Jentoft^1,2

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

mehdad@ou.edu,
rejentoft@umass.edu, fcjentoft@umass.edu

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 [1], 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.

References

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[2] V. Schwartz, S.T. Oyama, J.G. Chen, J. Phys. Chem. B 104 (2000) 8800-8806.

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512-518.

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