(143v) Selective Catalytic Reduction of No with Methane or Ethanol over Silver-Alumina Catalysts in the Presence of High Concentrations of So2 and H2o

SCR of NO with hydrocarbons has been under
development for more than two decades as a promising technology for NO_x
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 NO_x-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 NO_x 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 (~300^oC), was higher than that of NO_x
desorption from the silver nanoparticles. 
Hence, we conclude that only the NO_x 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.