Cobalt catalysts are prepared by impregnating zirconia and sulfated zirconia using an aqueous solution of cobalt(II) acetate. XRD results show that the catalysts with 5 wt% cobalt loading contain a small amount of Co3O4. Analysis of the FT-IR results on the adsorption of NO at room temperature reveals the formation of cobalt(II) mono- and dinitrosyls. It is shown that the nitrosyls formed on the sulfate-free catalyst with 5 wt% cobalt loading are unstable on prolonged contact with NO at room temperature due to the oxidation of adsorbed NO to NO2- (nitro) and NO3 species by cobalt(III) originating from the Co3O4 phase. This process does not occur in the case of the sulfated catalyst containing the same amount of cobalt, for which the existence of a Co3O4 phase is also detected. This experimental fact leads to the conclusion that the sulfate ions lower the reducibility of cobalt(III). Upon coadsorption of NO and O-2 at room temperature on the samples studied, various kinds of surface nitrates are observed differing in the modes of their coordination. In the case of CoOx/SO42--ZrO2 catalysts, part of the bidentate nitrates transform to NO2- (nitro) species after evacuation at 373 K. The nitro-nitrato species on the sulfated catalysts are characterized by a lower thermal stability than that of the nitrates on the CoOX/ZrO2 samples. (C) 2004 Elsevier Inc. All rights reserved.