The active sites for direct decomposition of NO on supported palladium catalysts were investigated by a comparative study with single-crystal palladium surfaces. In the study of powder catalysts, the activity of Pd/Al2O3 was found to be significantly affected by the type of palladium precursor. Pd/Al2O3 catalysts prepared from palladium nitrate showed the highest activity, expressed in terms of turnover frequency (TOF), for NO decomposition to N-2. In studies using single-crystal model catalysts reported previously [I. Nakamura, T. Fujitani, and H. Hamada, Surf. Sci. 514 (2002) 409], the adsorption, dissociation, and desorption behavior of NO was closely related to the surface structures, the stepped surface palladium being active for dissociation of NO. NO adsorption by FT-IR spectroscopy confirmed the presence of the stepped palladium sites on Pd/Al2O3 catalysts, and the specific activity (TOFstep) for NO decomposition to N-2, calculated on the basis of number of step sites estimated from the amount of chemisorbed NO, was not changed for all the Pd/Al2O3 catalysts. These results suggest that direct decomposition of NO mainly proceeds on the step sites of palladium. This is in agreement with the results obtained for single-crystal model catalysts. The present study shows that the activity of powder catalyst can be well interpreted by its surface structure, indicating the usefulness of surface science techniques. (C) 2003 Elsevier Inc. All rights reserved.