In this paper, the integration of activity results for the selective catalytic reduction of NOx with C3H6 with a thorough characterization of Pt-beta catalysts with different platinum dispersion has been accomplished. The parent zeolite NH4-beta (Si/Al = 11.4) was ion-exchanged with a Pt(II) precursor and activated by calcination and reduction in H-2 before reaction. The evolution and nature of the Pt phase during the different preparation stages and after catalytic tests were investigated by CO chemisorption, TEM, XRD, and XPS. By using different heating rates during activation of the ion-exchanged material and upon the deNO(x) HC-SCR, the average particle size of platinum in the final catalyst was varied in the range of 2-25 nm. It has been clearly shown that deNO(x) HC-SCR conditions produce a decrease in the metal dispersion of the catalysts by sintering of Pt particles. The increase in average particle size has a positive effect on the activity of the catalysts. Thus, the larger the platinum particles, the higher the NOx conversion and the lower the operation temperature. The XPS results show that both Pt(0) and Pt(II) species are present in the calcined samples, after H-2 reduction and during reaction. Coke deposits, formed during reaction on the zeolite support, were studied by XPS, DRIFT, and TPO-TPD/MS. The structure sensitivity of the lean deNO(x) reaction toward the platinum phase has been confirmed by the direct correlation established between platinum particle size and TOR Based on previous results on a single crystal, it seems that the key steps are the NO dissociation on Pt(100) planes and Pt-O clean off which will be easily performed on large Pt particles. On the other hand, the independence of Pt(100)/Pt(I 11) ratios with particle size explains the similar N-2 and N2O selectivity values presented along all the samples. (C) 2003 Elsevier Inc. All rights reserved.