Temperature-programmed reduction and XRD studies of ammonia-treated molybdenum oxide and its activity for carbazole hydrodenitrogenation

The change in the structure and composition of molybdenum nitride catalysts with cooling in a stream of ammonia or helium gas after NH3 treating was d etermined using temperature-programmed reduction (TPR) and X-ray powder dif fraction analyses. The relationship between the molybdenum species and the catalytic activities of the molybdenum nitride catalysts for the hydrodenit rogenation (HDN) of carbazole was discussed. MoO2, gamma-Mo2N, and Mo metal were mainly formed during the temperature-programmed reaction of MoO3 with ammonia at 773, 973, and 1173 K, respectively. During the TPR experiment, a portion of the adsorbed NHx (x = 0-3) species caused further nitriding of the catalyst at higher temperatures. It was found that nitrogen desorption during TPR could be assigned to four types of nitrogen species: (1) NHx ad sorbed on MoO2, (2) NHx adsorbed on gamma-Mo2N, (3) N-2 during the transfor mation of gamma-Mo2N to beta-Mo2N0.78, and (4) N-2 during the reduction of beta-Mo2N0.78 to molybdenum metal. Purging the NH3-treated catalyst with he lium at 973 K not only removed the adsorbed NH2 and diffused nitrogen but a lso altered the structure of the molybdenum compounds, i.e., from gamma-Mo2 N to beta-Mo2N0.78 During the HDN of carbazole, gamma-Mo2N was the most act ive, followed by beta-Mo2N0.78 for CN hydrogenolysis, while molybdenum meta l had the highest activity for hydrogenation. NH3-treated MoO2 was much les s active for both C-N hydrogenolysis and hydrogenation during carbazole HDN . From these results, the C-N hydrogenolysis sites were most likely located on small nitrogen deficient particles and crystallites of the molybdenum n itrides. The hydrogenation sites were located on the surface grain boundary of molybdenum metals. (C) 1999 Academic Press.