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Cu-Zeolite SCR Catalyst Thermal Deactivation Studied with FTIR Spatial Resolution
Abstract:
The performance of a commercial Cu-zeolite SCR catalyst after
differing degrees of hydrothermal aging (aged for 72 hours at 500,
700 and 800°C with 10% moisture balanced with air) was studied by
spatially resolving different key reactions using gas-phase FTIR
measurements. Gases were sampled along a channel at different
positions and analyzed using FTIR, which overcomes the interference
of water and nitrogen on ammonia concentration detection
encountered in standard mass spectrometer-based spatial resolution
measurements. The NO:NO₂ concentration ratio was changed so that
the standard (NO:NO₂ = 1:0), fast (NO:NO₂ = 1:1) and NO₂ (NO:NO₂ =
0:1) SCR reactions could be investigated as a function of the
catalyst's hydrothermal aging extent. In addition, the effects
of hydrothermal aging on the activity of NH₃ and NO oxidation were
also investigated.
Hydrothermal aging had little effect on NO oxidation activity.
For standard and fast SCR processes, NOx conversion
attained its maximum at 300°C. Hydrothermal aging decreased
reaction rates for SCR and NH₃ overconsumption simultaneously.
However, selectivity to NH₃ overconsumption was high over the
highly aged catalyst at high temperatures. NO was oxidized to NO₂
at the outlet portion of the catalyst under standard SCR conditions
at high temperatures, suggesting a balance between NO oxidation and
SCR at limited NH₃ concentrations. NO conversion under standard SCR
conditions decreased more with hydrothermal aging compared to
NOx conversion under fast SCR conditions. Coincidently,
NH₃ overconsumption was more significant under standard SCR versus
fast SCR conditions.
Along a monolithic catalyst channel, the extent of reaction
changed with the NO:NO₂ composition. When both NO and NO₂ were
present in the inlet gas, the fast SCR reaction dominated. At
temperatures below 200°C, a small amount of reaction between NH₃
and NO₂ was also observed. Standard SCR became more significant
with NO₂ depletion along the catalyst. However, the fast SCR
reaction at high temperatures can be limited by the decomposition
of NO₂ at the front of the catalyst.
Hydrothermal aging decreased the extent of NO₂ decomposition
into NO at high temperatures, which helps maintain fast SCR
reaction conditions in the system. The effects of hydrothermal
aging on NH₃ oxidation was more complicated. Generally, aging
reduced NH₃ oxidation activity, but it did not change monotonically
with aging extent and neither did NOx produced via NH₃
oxidation. However, the highly aged catalyst did produce a high
proportion of NOx.