CCl 4 -Assisted Preparation of Highly Dispersed Molybdenum and Tungsten Nitrides
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ORIGINAL PAPER
CCl4-Assisted Preparation of Highly Dispersed Molybdenum and Tungsten Nitrides Pavel Afanasiev • Dorothe´e Laurenti
Published online: 15 August 2012 Ó Springer Science+Business Media, LLC 2012
Abstract Nanoscopic transition metal nitrides were prepared by means of a newly developed technique, using nanoparticles of scheelites (AMO4, A = Ca, Sr, Ba; M = Mo, W) as precursors. After treatment with CCl4– NH3 gas mixtures the precursor salts were transformed to mixtures of alkali earth chlorides (ACl2) and transition metal nitrides (M2N). The thermodynamic driving force of the process is the high free enthalpy of the simultaneous formation of carbon dioxide and an electropositive chloride, from the interaction between CCl4 and AMO4. As a result, the gas–solid reaction occurs at relatively low temperature (500–550 °C). After water extraction of soluble chlorides, finely dispersed solids Mo2NxCy and W2NxCy (x = 0.79–0.95, y = 0.03–0.15) were obtained, which demonstrated high specific surface areas and well developed mesoporosity. Their catalytic activity was compared in the thiophene hydrodesulfurization reaction, where the active phase is provided by MS2 sulfides formed in situ under the reaction conditions, as confirmed by temperatureprogrammed reduction. Due to their open porosity, the solids as obtained demonstrated high activity and increased stability as compared to the same nitrides prepared by means of other techniques. The catalytic activity of the newly prepared nitrides was evaluated in the guaiacol hydrodeoxygenation (HDO) reaction. Molybdenum nitride showed promising HDO performance and unusual selectivity, whereas the tungsten homolog demonstrated poor activity.
P. Afanasiev (&) D. Laurenti Institut de Recherches sur la Catalyse et l’Environnement de Lyon UMR5256, CNRS-Universite´ de Lyon 1, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France e-mail: [email protected]
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Keywords Nitride Molybdenum Tungsten Scheelites Carbon tetrachloride HDO
1 Introduction Transition metal nitrides are important inorganic materials with a widespread range of applications, from superconductivity to highly resistant alloys. High surface area nitride (and similar carbide) materials were first prepared by temperature-programmed reaction [1, 2]. Since then, transition metal nitrides have attracted a lot of interest from researchers in the catalysis field. It was speculated that due to the d-band narrowing, W and Mo nitrides possess electronic properties similar to noble metals [3]. Even if this analogy often falls short to explain catalytic behaviour of nitrides, it remains useful to account for hydrogen activation and enhanced hydrogenating properties of molybdenum and tungsten carbides and nitrides. Unsupported Mo2N and Mo2C with high specific surface areas were prepared [4–6] and studied in a number of catalytic reactions involving hydrogen activation, such as hydrodesulfurization and hydrodenitrogenation [7, 8], hydrogenation [9, 10], ammonia synthesis [11–13] and
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