The beneficial effect of SO2 on platinum migration and NO oxidation over Pt containing monolith catalysts

In this work we investigated NO oxidation over Pt/Al2O3. The NO oxidation activity was slowly decreasing during the whole 36 h of experiment and we propose that this is due to formation of platinum oxides, which we detected in an earlier study using XPS. Further, it was not possible to remove these oxides at 250 degrees C, but some of them could be regenerated in Ar only at 400 degrees C. However, the NO oxidation activity was completely regained after the introduction of a reducing agent, in this case propene at 400 degrees C for 30 min. We also investigated the effect of long time exposure of SO2. The catalyst was exposed to 630 ppm NO+8% O-2 for 30 min, which was followed by 22 h of 630 ppm NO+8% O-2 + 30 ppm SO2. The NO oxidation activity decreased when introducing SO2. However, after about 3 h of SO2 the concentration of NO2 starts to increase from 30 ppm to 140 ppm after 22 h of SO2 exposure. The Pt dispersion before the long time SO2 exposure was 12%, but after this experiment only 3.5%. Thus, it is clear that SO2 exposure at this low temperature (250 degrees C) causes a large Pt migration and sintering. Earlier studies have shown that it is beneficial with large Pt particles for NO oxidation. This is due to either that the reaction is structure sensitive or that smaller particles more easily form platinum oxides, which are less active. This can explain the increased NO oxidation during the SO2 exposure. In addition, we conducted two temperature ramps with NO + O-2, where one was done prior to the SO2 experiment and one after. We observed a dramatical increase in NO oxidation after the long SO2 exposure experiment. For example the NO2 production increased from 27% to 92% at 200 degrees C. The long SO2 experiment and the two ramps were repeated on a corresponding catalyst, but the SO2 exposure was done at an even lower temperature (200 degrees C). We observed the same phenomenon also at this temperature, with a very large increase in NO2 production after the long time SO2 experiment. Thus, SO2 greatly enhances the Pt migration and sintering and large effects are observed already at 200 degrees C. (C) 2009 Elsevier B.V. All rights reserved