Cu-Zeolite SCR Catalyst Thermal Deactivation Studied with FTIR Spatial Resolution

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, NO_x 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 NO_x 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 NO_x produced via NH₃ oxidation. However, the highly aged catalyst did produce a high proportion of NO_x.