Toward Excellent Na⁺ Poisoning Resistance of Fe₂O₃/HY: The SiO₂/Al₂O₃ Ratio of Zeolite Strongly Matters

Abstract

Fe2O3/HY zeolite has been recently identified as an efficient alkali-poisoning-resistant catalyst for NO x control by NH3-selective catalytic reduction. The HY zeolite is shown to be the “workhorse” building block for alkali trapping, improving the alkali resistance of catalyst. In this paper, we discovered the complex SiO2/Al2O3 ratio-dependent alkali-resistant behavior for this catalyst. At SiO2/Al2O3 ratios higher than 8, the protonated form of the HY zeolite in Fe2O3/HY introduces rather limited numbers of H+ centers as binding sites for alkali poisons. However, at SiO2/Al2O3 ratios lower than 8, the intercalative ion-exchange process for alkali trapping is limited by contracted pore entrances, probably because of structural distortions arising from excessive Al displacement as revealed by X-ray diffraction. Maximum resistance to alkali poisoning occurs at a SiO2/Al2O3 ratio of 8 in Fe2O3/HY. For this optimum catalyst, an adequate number of framework protons are present and large pore entrances of the HY framework are observed, facilitating the efficient cation diffusion and exchange processes for alkali trapping. With the aid of nuclear magnetic resonance spectroscopy, it is revealed that only protons at double 6-ring pores of HY zeolites undergo the exchange of protons for alkali poisons, whereas other framework proton sites behave as spectators for enhancing the overall alkali resistance.