The oxidation of NO to NO2 (NO+O2 = NO2) over a supported noble metal component is an important step involved in the NOx storage/reduction (NSR) process being developed for diesel engines. In fact, NO to NO2 oxidation is the first step that occurs in the cyclic NSR process. It is also a key step for soot removal.
The reaction kinetics of NO oxidation for two Pt/Al2O3 catalysts having different Pt particle sizes were investigated. The rate expression for the forward reaction on a fresh catalyst having a Pt cluster size of 2.4 nm was r = k[NO]1.09±0.07[O2]0.86±0.06[NO2]-0.85±0.06, where concentrations are in gas phase volume fraction, k = Exp(A-Ea/RT) is the rate constant ( in s-1) with an apparent activation energy (Ea) of 81.8 kJ mol-1±5 kJ mol-1 and A is 15.1±1.03. The sintered catalyst, with an average particle size of 7 nm, had similar Ea and apparent reaction orders with respect to NO and NO2, but the apparent O2 order decreased to 0.7, compared to the fresh catalyst. Thus, the product NO2 was found to inhibit the forward rate of NO oxidation reaction on both catalysts.
The turnover rate (TOR), defined as moles of NO reacted per second per mole of surface Pt, on the sintered catalyst was found to be ca. 4 times higher than the TOR on the fresh catalyst. The oxygen uptake by Pt on these catalysts was quantified using the CO titration method to determine the surface reactivity. After NO oxidation reaction reached steady state, both fresh and sintered catalysts showed oxygen uptake of about 1.5 times the number of Pt surface atoms (PtS). But when these catalysts were exposed to high concentration of NO2 at room temperature, the oxygen uptake increased to the equivalent of about two oxygen atoms per surface Pt and the NO oxidation TOR decreased by about 7 times with respect to the original steady-state level for both catalysts. Based on the XPS measurements, the observed catalyst deactivation was attributed to an increase in the bulk oxidation of the Pt particles towards PtO. The O/PtS values greater than one obtained in the CO titration suggest an increasing penetration of oxygen into the interior of the Pt particles and an approach to PtO stoichiometry. To explain the higher rates on catalysts with larger particle sizes, TEM measurements on the supported catalysts and rate measurements on a Pt foil were made. The TEM images of the two supported catalysts showed that the sintering procedure used to increase the Pt particle size on the fresh catalyst only broadened the particle size distribution, with less than 20% of the particles on the sintered catalyst having sizes larger than the average particle size seen on the fresh catalyst. Preliminary results indicate that the NO oxidation TOR on a Pt foil is about two orders of magnitude greater than the rate seen on the sintered supported Pt catalyst. The TEM and foil results suggest that only particles larger than about 10 nm are responsible for the observed NO oxidation activity. As a consequence, most Pt clusters on a supported catalyst with high dispersion are not active for NO oxidation.