Nanosized W2N was prepared by NH3-temperature-programmed reaction and characterized by XRD, transmission electron microscopy (TEM), BET, X-ray photoelectron spectroscopic (XPS) and temperature-programmed desorption (TPD) techniques. Transient-response experiments were applied in order to improve understanding of the mechanism of NO reduction with H-2 on W2N surface. Switches of He into 1% NO/0-7% H-2/He gas streams on W2N surface were performed at 500 degrees C. The 1% NO/He step-response experiment showed that NO dissociation in the absence of H-2 was not effective on W2N surface because the surface oxygen from dissociated NO was strongly bond to the catalyst surface, poisoning the NO dissociation sites and inhibiting further NO dissociation. In the 1% NO/1-7% H-2/He step-response experiments, N-2, NH3, H2O and N2O (little or none) were detected as reaction products. The formation of H2O on the catalyst surface was the most crucial step for NO dissociation to proceed and that it depended on the NO: H-2 ratio in the reaction mixture. Reasonable NO: H-2 ratio of 1:5 in feed gas was determined to keep the catalyst surface sites active for the catalytic dissociation of NO. Reaction pathways explaining the reduction of NO with H-2 were proposed. (C) 2008 Elsevier B. V. All rights reserved.