Catalytic activities of tungsten nitride for NO dissociation and reduction with hydrogen

Tungsten nitride (W2N) was synthesized from WO3 and NH3 by means of temperature-programmed nitridation, and its activities for NO dissociation as well as NO reduction with H2 were evaluated. In a 5% H2/1% NO/He stream at 400 °C, stable NO conversion to N2 of ca. 82% was observed within a test period of 40 h. When the H2/NO ratio was reduced to 3/1, the conversion was ca. 80% and the catalyst started to deactivate after 28 h of on-steam time. At a H2/NO ratio of 1/1, NO conversion to N2 was ca. 63% and such performance could last for only 23 h; even in the absence of H2, a conversion of ca. 52% lasted for 22 h. We applied the XRD and XPS techniques, respectively, to examine the structural and compositional changes of W2N during the reactions. The results indicated that the gradual loss in catalytic activity was due to the bulk oxidation of W2N into WO3. Combined the activity measurements with those of NO adsorption studies, we deduce that W2N is catalytically active for NO adsorption and dissociation, and a H2/NO ratio of 5/1 is required to avoid heavy accumulation of surface oxygen and deactivation of the catalyst.