Molybdenum Nitride Catalysts II. H 2 Temperature Programmed Reduction and NH 3 Temperature Programmed Desorption

Author： Colling C.W. Choi J.G. Thompson L.T.

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.160, Iss.1, 1996-01, pp. : 35-42

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Abstract

The surface chemistries of a series of Mo nitride catalysts with surface areas ranging up to 193 m 2 /g were characterized using thermal desorption spectroscopies. These materials were prepared by the temperature programmed reaction of MoO 3 with NH 3 . The passivated catalysts contained up to one monolayer of oxygen on or in the surface. This oxygen was removed as H 2 O via two pathways during temperature programmed reduction: (a) reaction with hydrogen from decomposed NH 3 and hydrogen residue left on the surface after synthesis at temperatures less than =~550 K and (b) reaction with gas phase H 2 at higher temperatures. Nitrogen and/or surface NH x species also reacted with gas phase H 2 producing NH 3 . The NH 3 adsorption capacities were different for each of the reduced Mo nitrides; however, other variations in the temperature programmed desorption spectra were qualitatively similar. At low coverages, all of the NH 3 decomposed and the products desorbed as H 2 at 500-700 K and N 2 above 600 K. Following saturation of the surface, most of the NH 3 desorbed molecularly at 300-500 K and the balance decomposed. The ratio of molecularly desorbed to decomposed NH 3 was approximately 3 : 1 for all the catalysts suggesting a connection between the desorption and decomposition of NH 3 . We have interpreted the results, in particular for the high surface area materials where the saturation coverage was low, in terms of localized NH 3 adsorption perhaps forming islands at high coverage. The pyridine HDN reaction rate increased linearly with the amount of NH 3 chemisorbed. The corresponding turnover frequency was 4.1 x 10 -4 s -1 at 633 K. Finally, the number of types of NH 3 and H 2 desorption sites was a function of the surface area. The low surface area, high activity Mo nitrides possessed low and high temperature desorption sites. Only the low temperature sites were observed for the high surface area, low activity materials. The rate limiting step for NH 3 desorption from the Mo nitrides was first order with desorption energies for the low and high temperature sites of 24 ± 4 and 32 ± 5 kcal/mol, respectively.