Temperature-Programmed Reduction and XRD Studies of Ammonia-Treated Molybdenum Oxide and Its Activity for Carbazole Hydrodenitrogenation

Abstract

The change structure and composition of molybdenum nitride catalysts with cooling in a stream of ammonia or helium gas after NH3 treating was determined using temperature-programmed reduction (TPR) and X-ray powder diffraction analyses. The relationship between the molybdenum species and the catalytic activities of the molybdenum nitride catalysts for the hydrodenitrogenation (HDN) of carbazole was discussed. MoO2, γ-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 adsorbed on MoO2, (2) NHx adsorbed on γ-Mo2N, (3) N2 during the transformation of γ-Mo2N to β-Mo2N0.78, and (4) N2 during the reduction of β-Mo2N0.78 to molybdenum metal. Purging the NH3-treated catalyst with helium at 973 K not only removed the adsorbed NHx and diffused nitrogen but also altered the structure of the molybdenum compounds, i.e., from γ-Mo2N to β-Mo2N0.78. During the HDN of carbazole, γ-Mo2N was the most active, followed by β-Mo2N0.78 for C–N hydrogenolysis, while molybdenum metal had the highest activity for hydrogenation. NH3-treated MoO2 was much less 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 nitrides. The hydrogenation sites were located on the surface grain boundary of molybdenum metals.