Variations in the surface structure and composition of tungsten oxynitridecatalyst caused by exposure to air

A tungsten oxynitride (WN0.59O0.24) catalyst with a specific surface area o f 63.7 m(2) g(-1) has been prepared by the temperature-programmed reaction of WO3 with NH3 and characterized by elemental analysis, X-ray powder diffr action, N-2 sorption, and X-ray photoelectron spectroscopy (XPS) in order t o extensively investigate the textural variation of this material caused by exposure to ambient air. No noticeable changes in the bulk structure of tu ngsten oxynitride prepared here are caused by the sample storage in ambient conditions for a long period of time up to 80 days. However, its N-2 BET s urface area and total pore volume were found to severely decrease with incr easing the period of time of exposure to air. The XPS depth profile measure ments reveal that the amorphous tungsten trioxide (WO3) species is formed o n the surface of tungsten oxynitride upon exposure to air, which results in the significant modification of the surface structure and composition of t he oxynitride material. In contrast, no indications of other surface tungst en phases such as dioxidic, monooxidic, and oxynitridic species are detecte d. The overall results of this study strongly suggest that oxygen is mobile enough to diffuse into the surface or near-surface of the oxynitride latti ce even in ambient conditions, leading to the surface WO3 formation.