The preparation of some new Pd and Pt catalysts dispersed on SnO2 prepared in different ways is reported. Supports differ in surface area and porosity. Samples are characterized by BET, TPR and powder XRD analysis. The hydrogenation of 100 ppm nitrate and nitrite solutions shows significant differences depending on the nature of the metal (Pd catalysts are much better than Pt) and the morphology of the support. The selectivity to nitrogen benefits from the use of low surface area, large pore supports. The use of CO2 in the gas feed is analyzed and the role of carbon dioxide as a buffer is demonstrated by comparison with acetate buffer. The results suggest the importance of intraparticle diffusion and local buffering in the control of the activity and selectivity of the reaction. (C) 2004 Elsevier B.V. All rights reserved.