Hydrocarbon poisoning of Cu-zeolite SCR catalysts

The effects of propylene (C₃H₆) and dodecane (n-C₁₂H₂₆) exposure on the NH₃-based selective catalytic reduction (SCR) performance of two Cu-exchanged zeolite catalysts were investigated. The first sample was a model Cu/beta zeolite sample and the second a state-of-the-art Cu/zeolite sample, with the zeolite material characterized by relatively small pores. Overall, the state-of-the-art sample performed better than the model sample, in terms of hydrocarbon inhibition (which was reduced) and N₂O formation (less formed). The state-of-the-art sample was completely unaffected by dodecane at temperatures lower than 300°C, and only slightly inhibited (less than 5% conversion loss), for standard SCR, by C₃H₆. There was no evidence of coke formation on this catalyst with C₃H₆ exposure. The model sample was more significantly affected by hydrocarbon exposure. With C₃H₆, inhibition is associated with its partial oxidation intermediates adsorbed on the catalyst surface. For example, at 150°C, no C₃H₆ oxidation was observed and no inhibition was noted. At 300°C, both the oxidation intermediates and coke formation led to deactivation. With C₁₂H₂₆, inhibition was observed over the whole temperature range. At 150°C NO_x conversion decreased from 50% to less than 20% due to the strong adsorption of C₁₂H ₂₆ blocking active sites either directly, or indirectly by blocking pores. At 300°C, due to both partial oxidation intermediates and strong hydrocarbon adsorption, NO_x conversion decreased from 90% to less than 30%. The differences in terms of resistance to HC poisoning, such as HC adsorption and coke formation, is related to the pore structure of the zeolite. The small pores in the state-of-the-art sample do not allow the diffusion of large hydrocarbon molecules into the pores, hindering adsorption onto active sites, as well as the formation of cokerelated molecules in the pores, and thus the active sites are preserved.