(141f) On the Activity and Selectivity of Ag/Al2O3 Catalyst During the Lean Reduction of NO by C3H8 Studied At High Space Velocities

Ag/Al₂O₃ is considered a promising catalyst for the direct selective lean reduction of NOx by hydrocarbons and oxygenates. It has been reported to have high activity, good selectivity to N₂, stability, durable activity in presence of water and SO₂, moderate operating temperature window and relatively low cost (1-4). Most of the studies have been made at low space velocities, however, and that difficults the analysis of intermediates and the effect of catalyst composition.

In this study we present results obtained at high space velocities (F/W =666-32,000 cm³.g^-1.min^-1) using a ~2 wt% Ag/Al₂O₃ catalyst prepared by impregnation. The feed gas mixture contains 200 ppm NO, 1750 ppm C₃H₈, 3% O₂ and 6% H₂O diluted in nitrogen as a balance gas. The reactions were carried out in a tubular quartz microreactor equipped with a mass flow controllers and PID temperature control. The gas analyses were done by GC (HP 6890 and Shimadzu GC-12A), on line with an FTIR (Bruker Tensor 27) equipped with 0.75 m path-length infrared gas cell heated at 120ºC to prevent condensation, and a chemiluminescence NO-NOx analyzer (Rosemount 951A).

We observed that the reduction of NO with C₃H₈ occurs in the 375-500°C temperature range in the absence of H₂. By adding a small amount of H2 to the feed the light-off temperature decreases nearly 200ºC. These results are in line with those reported in the literature (2), but operation at high F/W shows that our catalyst is significantly more active in the reduction of NOx. The conversion - W/F behavior, particularly at low temperature, suggests that the mechanism involves consecutive reactions. This issue is being studied and will be discussed further. Characterization of the catalysts using UVVis in situ allows us to understand the role of Ag nanoparticles and AgOx species present under reaction conditions and to dwelve into the reason for the high activity of this class of catalysts.

References

1. Midyadera, T., Appl. Catal. B 2 (1993) 199.
2. Zhu, T., Hao, J., Fu, L., Li, J., Liu, Z., React. Kinet. Catal. Lett. 84,1(2005) 61.
3. She, X., Flytzani-Stephanopoulos, M., Catalysis Today 127 (2007) 207.
4. DiMaggio, C., Fisher, G., Rahmoeller, K., Sellnau, M., SAE Paper 2009-01-0277