Active centers response to SO2 and H2O poisoning over Fe-W-Ni exchanged zeolite for high-temperature NH3-SCR: Experimental and DFT studies

Catalyst deactivation due to SO2 and H2O poisoning posed a grand challenge to the general application of selective catalytic reduction (SCR) of NOx with NH3 over metal-exchanged zeolites. In this work, we reported the activities of the synthesized Fe-Ni-W exchanged zeolite (0.5FNW-H) and pristine HZSM-5 were adversely affected by SO2 and/or H2O in the order of SO2 + H2O > H2O >> SO2 at high temperature (T > 500 degrees C). With the in-situ DRIFTs and in-situ UV-Vis analyses, the chelating bidentate HSO4-/SO42- was found to accumulate mainly on the framework Bronsted sites for HZSM-5, while on the Fe sites for the 0.5FNW-H, which caused the decline in the catalyst activities. Density functional theory study revealed that the dissociated hydroxyl from H2O facilitates the formation of sulfur complexes via bonding with the exchanged metal (ZFe-OH) and framework Bronsted sites (ZAl-OH) on the 0.5FNW-H and pristine HZSM-5, respectively. On the pristine HZSM-5, the dealumination of the tetrahedral coordinated Al (IV) atoms in zeolite lattice by H2O was identified by the Al-27 solid-state NMR, which caused the irreversible loss of SCR functionality of HZSM-5. On 0.5FNW-H however, a much higher activation barrier for Al extraction from its neighboring metal site was observed, indicating the exchanged metal sites suppressed the bonding of -OH and Al atom in the framework and thus inhibiting the irreversible deactivation of HZSM-5. The findings are expected to provide rational guidance for the development of effective zeolite catalysts with high stability by regulating exchanged metal sites and framework Al.

Automated summary

The deactivation of selective catalytic reduction (SCR) catalysts by SO2 and H2O was found to be more severe on pristine HZSM-5 compared to Fe-Ni-W exchanged zeolites, with the latter showing better stability at high temperatures. The study suggests that regulating exchanged metal sites and framework aluminum in zeolites could lead to the development of more stable and effective catalysts for SCR reactions.