Study of niobium oxynitride: Synthesis, characterization, and reactivity

Niobium oxynitride with a face-centered cubic structure was prepared by a temperature-programmed synthesis method, consisting of passing 20% (v/v) NH3/He over a Nb2O5 oxide precursor at a flow rate of 680 mu mol s(-1) (1000 cm(3) min(-1)) while the temperature was raised linearly at variable heating rates, 1-5 K min(-1) (0.016 67-0.083 33 K s(-1)), to a final temperature ranging from 1027 to 1173 K. The progress of the reaction was followed by mass spectroscopy of the gaseous products, and this revealed only a single stage in the transformation. In agreement with this, X-ray diffraction patterns of the product and intermediates of the reaction showed a direct transformation from the niobium pentoxide to niobium oxynitride during this step, as also evidenced by the gradual replacement of oxygen by nitrogen observed by elemental analysis. Heating rate variation experiments yielded an activation energy for the synthesis of 86 kJ mol(-1) by the Redhead equation. The specific surface area (S-g) increased with decreasing heating rate, and values ranging from 23 to 57 m(2) g(-1) were obtained. Scanning electron microscopy of intermediates in the synthesis showed an increasingly rougher surface morphology as S-g increased, but little change in particle shape or size, indicating that the transformation was pseudomorphous. The niobium oxynitride had moderate and stable hydrodenitrogenation activity over a period of 60 h.