On the kinetic and structure sensitivity of lean reduction of NO with C3H6 over nanodispersed Pt crystals

Well-defined platinum nanocrystals (≈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 deNOx reaction was taken firstly under investigation. An optimum peak NOx conversion of ≈56% was observed at 250 °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 °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/O2/C3H6 reaction as well as the morphological evolution of the well-structured platinum nanocrystals in reaction conditions have been clearly evidenced. The deNOx 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 N2/N2O ratio was higher (≈1:1) for the catalyst statistically rich in Pt nanoparticles with low index facets, relatively free of defects, compared to the polycrystalline one (≈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 deNOx reaction (increase in the specific catalytic activity and N2/N2O ratio) can be foreseen through an optimum morphological (size and facet) control of the supported Pt particles.