Hydrocarbon Deactivation of a Cu/Zeolite SCR Catalyst

Hydrocarbon Deactivation of a Cu/Zeolite SCR Catalyst

A Cu-exchanged zeolite catalyst was evaluated for selective catalytic reduction (SCR) performance before and after exposure to propylene at different temperatures, ranging from 150 to 500°C. A transient response method was used to characterize the standard, fast and NO₂ SCR reactions, as well as NH₃ and NO oxidation, in the presence and absence of propylene.

The results clearly show propylene inhibition of the standard and fast SCR reactions between 200 and 400°C. With the addition of propylene there is an immediate increase in outlet NO_X. According to the DRIFTS data obtained, propylene oxidation intermediates, for example acrolein-like species, rapidly form on the catalyst surface upon propylene introduction at these temperatures, explaining this immediate drop in performance. At 300°C there was a slower decrease in performance with prolonged exposure to propylene, after the initial rapid loss, which was not observed at 250°C. In the 300 to 400°C range, acrolein species were again observed, resulting in the rapid loss in performance observed, but coke formation was also observed, and was slow to build-up. The coke formation explains the slow decrease in performance noted. At 150°C, inhibition was not observed, nor was any propylene consumption via oxidation, suggesting that some extent of propylene oxidation is required for the onset of inhibition, further verifying the propylene oxidation intermediates causing inhibition.

In terms of NO oxidation, in all tests, the addition of propylene led to reduced NO₂ formed via NO₂ being used as an oxidant for propylene oxidation, and thus reduction of NO₂ to NO. NH₃ was found to inhibit propylene oxidation, due to the relatively stronger adsorption of NH₃ to the surface. For NO₂ SCR, the addition of propylene resulted in improved performance, through HC-SCR as well as reduction of NO₂ to NO, allowing some fast SCR reaction to occur.