Well-defined platinum nanocrystals (approximate to 52% cubic) with an average diameter of 12 nm were prepared by colloid method and then supported on alumina. The effect of amount of the exposed Pt (controlled either by metal loading or by the catalyst weight) on the catalytic performances for lean-burn deNO(x) reaction was taken firstly under investigation. An optimum peak NOx conversion of 56% was observed at 250 degrees C. Typically, the propylene conversion level was in the 85-100% range at peak NOx conversions. The catalytic data free of transport effects have been identified and then used to calculate kinetic parameters for NO and C3H6 conversions (turnover frequencies and activation energies). The average TOF values for NO and C3H6 conversions, determined in the 225-275 degrees C temperature domain, ranged between 0.04-0.27 and 0.05-0.50 s(-1), respectively. The average activation energies for NO and C3H6 conversions were 91 and 108 kJ mol(-1), respectively. The structure sensitivity of NO/O-2/C3H6 reaction as well as the morphological evolution of the well-structured platinum nanocrystals in reaction conditions have been clearly evidenced. The deNO(x) catalytic behavior was mainly related to the shape (facet effect) and to a lesser extent to the size (bulk effect) of the Pt nanoparticles. The N-2/N2O ratio was higher (approximate to 1: 1) for the catalyst statistically rich in Pt nanoparticles with low index facets, relatively free of defects, compared to the polycrystalline one (approximate to 1:2). High concentration of edges, corners, kinks, and surface defects of polycrystalline platinum particles was the main factor responsible for the overall catalytic activity for NOx conversion. Large (24 nm) as well as small (2.4 nm) polycrystalline Pt particles showed in great lines the same catalytic behavior for NO conversion. In light of experimental results, it is suggested that further improvement in the catalytic activity and selectivity for lean deNO(x) reaction (increase in the specific catalytic activity and N-2/N2O ratio) can be foreseen through an optimum morphological (size and facet) control of the supported Pt particles. (c) 2005 Elsevier B.V. All rights reserved.