In the work reported here, we have measured the kinetics (apparent activation energy and power rate law reaction orders) for NO oxidation on a series of Pt catalysts with varying average Pt particle sizes (ca. 2-9 nm) and support materials (γ-Al2O3, SBA-15, carbon, Nb2O5, TiO2). The rate of NO oxidation to NO2 for the catalysts studied was found to be close to first order with respect to both the reactants NO and O2, while it was close to negative first order with respect to the product NO2, over the concentration range studied. Thus, similar reaction kinetics, including the inhibition by the product NO2, was observed for all these catalysts, indicating that the apparent reaction orders are independent of Pt cluster sizes and support materials used in this study. A reaction mechanism that is consistent with the observed reaction orders has been proposed.

The turnover rate (TOR) of the reaction, defined as moles of NO reacted per second per mole of surface Pt, was found to be a strong function of both the Pt particle size and the catalyst support. The TOR increased about 50 times when Pt size increased from 2 to 9 nm on SBA-15 support; and about 20 times on alumina support (for the same size range). Based on the particle size distribution obtained by transmission electron microscopy (TEM) measurements, it is hypothesized that only the large Pt particles are responsible for the observed activity, with the smaller Pt particles deactivating under the oxidizing reaction conditions. From flow experiments, the observed catalyst deactivation was found to be Pt cluster size dependent; the 9 nm Pt/SBA-15 catalyst was found to deactivate to a lower extent, two fold, compared to the six fold decrease in the TOR of the 2 nm Pt/SBA-15 catalyst before they reached a steady rate. XPS measurements indicated that the catalyst deactivation was due to an increase in the oxidation of the Pt particles as the reaction progresses. This observation, combined with increased deactivation seen on smaller Pt particles, suggests that large Pt particles are more resistant to Pt oxidation (deactivation) than the smaller ones. High resolution TEM imaging done on the SBA-15 supported samples revealed the presence of crystalline large Pt particles with <111> faces predominant on the 9 nm Pt/SBA-15 catalyst, which was not the case with the other two SBA-15 supported samples having smaller Pt cluster sizes (2 and 4 nm). This result, along with the high TOR seen on the 9 nm Pt/SBA-15 sample, suggests that Pt(111) surfaces may be the active sites for NO oxidation reaction, capable of resisting oxidation and hence, deactivation, when compared to the more open surfaces. NO oxidation rate measurements on a Pt(111) single crystal indeed gave a TOR similar to that seen on the 9 nm Pt/SBA-15 catalyst (which has predominant (111) surface orientation), and may represent the limiting rate on Pt for NO oxidation reaction. With regards to the catalyst support, carbon and Nb2O5 were found to be better supports for this reaction, exhibiting 15 fold higher TOR and negligible deactivation, when compared to the Al2O3 or SBA-15 supports, even for the smaller average Pt particle size of about 2 nm. This suggests that even smaller Pt particles supported on carbon or Nb2O5 probably have (111) surfaces predominant, making them resistant to Pt oxidation under reaction conditions, thereby resulting in higher TOR when compared to Al2O3 or SBA-15 supports