Characterization and performance of Pt-USY in the SCR of NOx with hydrocarbons under lean-burn conditions

Pt-USY was used for the selective catalytic reduction of NOx with hydrocarbons in the presence of excess oxygen. The catalyst was prepared by an ion-exchange method and characterized by XRD, TEM, CO chemisorption, and Ar adsorption at 87 K. The platinum particle size distribution was found to be broad (2–20 nm), with no apparent sintering of the active phase during the HC-SCR process after 25 h time-on-stream. Generally, large metal clusters (>15 nm) are situated at the external surface of the zeolite, while the smaller ones are located in the pores of the support. Pt-USY shows an excellent activity in the deNOx reaction (molar NOx conversion 90% at 475 K) with propene as the reductant in 5 kPa O2, as well as stable operation during time-on-stream. Propane only yields a low NOx conversion compared to propene. The presence of high oxygen contents (5–10 kPa O2) slightly inhibits the reaction. No significant decrease in deNOx activity was observed at high space velocities (up to 100,000 h−1). The presence of SO2 and H2O in the feed stream did not significantly affect the deNOx activity. Pt-USY performs better under lean-burn conditions than other Pt-catalysts supported on e.g. ZSM-5, Al2O3, or SiO2. The selectivity to N2 was similar to the other Pt-based catalysts (∼30%), the other major product being N2O.