Synthesis, characterization, and reactivity of tungsten oxynitride
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Synthesis, characterization, and reactivity of tungsten oxynitride Toby E. Lucy, Todd P. St. Clair, and S. Ted Oyamaa) Environmental Catalysis and Materials Laboratory, Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0211 (Received 9 June 1997; accepted 28 October 1997)
High surface area tungsten oxynitride has been prepared by the temperature programmed reaction (TPR) of WO3 with NH3 . All samples were characterized by x-ray diffraction (XRD), nitrogen physisorption, CO chemisorption, and elemental analysis. Samples were prepared at different heating rates (b), and a Redhead analysis yielded an activation energy for nitridation of 109 kJ mol21 . A heating rate of 0.016 K s21 gave optimal synthesis conditions. Solid state intermediates were studied by interrupting the temperature program at various stages. No distinct suboxide phases were found using XRD. The nitridation step was determined to be a continuous transformation from oxide to oxynitride. Surface area, CO uptake, and nitrogen weight % were all found to increase as the reaction progressed. Reactivity experiments showed reasonable hydrodeoxygenation (HDO) and hydrodenitrogenation (HDN) activity, but little hydrogenation (HYD) or hydrodesulfurization (HDS) activity.
I. INTRODUCTION
Transition metal carbides and nitrides form a unique class of compounds that combine properties of both ceramics and metals. They are found over wide ranges of stoichiometry, and have very high melting points (.3300 K) and hardness values (.2000 kg mm22 ).1 In addition, their electrical and magnetic properties include Hall coefficients and electrical conductivities that fall in the range of metals.2 The industrial uses of these materials are numerous, and include coatings on cutting tools and diffusion barriers in semiconductor contacting technology.3 In carbides and nitrides, the nonmetal atoms occupy interstitial sites in the metal lattice, typically octahedral in face-centered cubic (fcc) and hexagonal close packed (hcp) structures, and trigonal prismatic sites in simple hexagonal structures. The formation of carbides and nitrides usually results in a crystal structure different from that of the parent metal. The particular type of crystal structure that is formed is governed by two factors: geometric and electronic.1 The geometric factor is based on an empirical rule by H¨agg which states that simple structures are formed when the hard ball radii of nonmetal to metal is less than 0.59.4 The electronic factor is based on the Engel–Brewer theory of metals which states that as s-p electron count is increased, the crystal structure will shift from bodycentered cubic (bcc) to hcp to fcc.5,6 For instance, Group 6 metals (W, Mo) are bcc, Group 7 metals (Tc, a)
Author to whom correspondence should be addressed. J. Mater. Res., Vol. 13, No. 8, Aug 1998
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Re) are hcp, and Group 10 metals (Pd, Pt) are fcc. In an analo
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