SCR of NO with hydrocarbons has been under development for more than two decades as a promising technology for NOx abatement. Ever since Ag/Al2O3 was reported as an active SCR catalyst [1], extensive work has been performed on elucidating the active structure, reaction mechanism, and the reported resistance of this catalyst to SO2/H2O [2,3].
In regard to the active sites on Ag-alumina for NO-SCR, literature reports [3,4] have pointed to the oxidized silver species as the catalytic sites for NOx-SCR and to nanoparticles of silver as the catalytic sites for HC-combustion. In recent work in our lab, a nitric acid leaching method was applied to remove the silver particles from Ag-alumina prepared by co-gelation [5] in order to examine the activity and stability of the resulting silver aluminate surface. Silver cations were found in the Ag-alumina catalysts by UV-VIS (DRS). NO-O2-TPD shows adsorption of NOx on both alumina sites and nano-silver particles. However, the kinetics of the SCR reaction are similar for the leached and parent catalyst and the same apparent activation energies were found. Temperature programmed surface reaction experiments showed that the reaction light-off temperature (~300oC), was higher than that of NOx desorption from the silver nanoparticles. Hence, we conclude that only the NOx species adsorbed on alumina participate in the SCR reaction. The SCR activity increases with the amount of oxidized alumina-bound silver. A reaction scheme involving alumina and Ag+-O-Al sites is proposed. Silver nanoparticles are spectator species for the CH4-SCR of NO.
On the other hand, silver particles are shown to be essential and even to promote the CH4-SCR of NO in high concentrations of SO2(0.1%) in the gas stream [5]. The role of silver nanoparticles in the presence of high amounts of SO2 and H2O, and with a different reductant, such as ethanol, was also investigated. A promotion of the reaction was observed as will be shown in this presentation.
References
[1] T. Miyadera, Appl. Catal. B 2 (1993) 199.
[2] T. Miyadera, Appl. Catal. B 13 (1997) 157.
[3] K.A. Bethke, H.H. Kung, J. Catal. 172 (1997) 93.
[4] A. Keshavaraja, X. She, M.Flytzani- Stephanopoulos, Appl. Catal. B 27 (2000) L1.
[5] X.She, M.Flytzani- Stephanopoulos, 19th North American Catalysis Meeting, Philadelphia, PA, 2005.
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