Molybdenum nitride catalysts .2. H-2 temperature programmed reduction and NH3 temperature programmed desorption

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 MoO3 with NH3. The passivated catalysts contained up to one monolayer of oxygen on or in the surface. This oxygen was removed as H2O via two pathways during temperature programmed reduction: (a) reaction with hydrogen from decomposed NH3 and hydrogen residue left on the surface after synthesis at temperatures less than similar to 550 K and (b) reaction with gas phase H-2 at higher temperatures. Nitrogen and/or surface NHx species also reacted with gas phase H-2 producing NH3. The NH3 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 NH3 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 NH3 desorbed molecularly at 300-500 K and the balance decomposed, The ratio of molecularly desorbed to decomposed NH3 was approximately 3:1 for all the catalysts suggesting a connection between the desorption and decomposition of NH3. We have interpreted the results, in particular for the high surface area materials where the saturation coverage was low, in terms of localized NH3 adsorption perhaps forming islands at high coverage. The pyridine HDN reaction rate increased linearly with the amount of NH3 chemisorbed. The corresponding turnover frequency was 4.1 x 10(-4) s(-1) at 633 K. Finally, the number of types of NH3 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 NH3 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. (C) 1996 Academic Press, Inc.