Fe₂O₃/HY Catalyst: A Microporous Material with Zeolite-Type Framework Achieving Highly Improved Alkali Poisoning-Resistant Performance for Selective Reduction of NO_x with NH₃

Abstract

The commercially available V2O5/WO3–TiO2 is a well-known catalyst for selective catalytic reduction (SCR) of NO with NH3. When alkali ions are present in the exhaust (e.g., as impurities such as dust) of a reactor containing commercial V2O5/WO3–TiO2, alkali poisoning occurs, deactivating the catalyst. Consequently, there is substantial interest in the development of better-performing and more durable NH3–SCR catalysts with an improved resistance to alkali deactivation. For the present study, the protonated (H+) form of zeolite Y, HY, was used as a support and acted as buffer zone, leading to trapping (sticking) of foreign alkali poisons in the zeolite pore structure, preventing alkali poisoning of the Fe2O3/HY catalyst. Catalytic tests showed that the Fe2O3/HY retained 100% of its original catalytic reactivity for NH3–SCR reaction even after 1000 μmol Na+ g–1 poisoning. 1000 μmol Na+ g–1 treatment indicates a 26 000-h exposure under an alkaline dust-containing condition. In contrast, upon 1000 μmol Na+ g–1 treatment, severe alkali deactivation occurred for a commercial V2O5/WO3–TiO2. The catalyst activity of Fe2O3/HY remained unchanged because of the intercalation of Na+ in the internal HY zeolite pores that impedes the blocking of Na+ poison to the external active sites of Fe2O3. The findings in this work suggest that the zeolite HY may be revealed as an attractive building block for designing an alkali poisoning-resistant catalyst.

Improvement of Alkali Metal Resistance for NH3-SCR Catalyst Cu/SSZ-13: Tune the Crystal Size

“…For Cu/SSZ-13 with Si/Al of~25, Cu 2+ is mostly in the form of Z[Cu II OH] (Z represents the negatively charged framework site), rather than Z 2 Cu II , because the probability of having two Al atoms in a six-membered ring (6MR) at the same time is relatively small. 29 Si NMR results also show that Si (3Si, 1Al) species dominate the framework structure (Figure S4). However, an unexpected result is that the larger the crystal size, the fewer Cu species can be reduced.…”

“…The framework coordination structure of the catalyst was also characterized by NMR technology. Normalized 27 Al and 29 Si NMR spectra show that the coordination of Si and Al has little change before and after Na poisoning, independent of the crystal size of Cu/SSZ-13 (Figure S4). All samples present framework tetrahedral Al at 60 ppm.…”

“…The 29 Si and 27 Al NMR were carried out at a field strength of 14.1 T with a total scanning of 1200 and 300 times, relaxation delay of 10s and 2s, respectively. The tube diameter of 4 mm and 8 mm was chosen for testing 27 Al and 29 Si, respectively.…”

“…The following are available online at https://www.mdpi.com/article/ 10.3390/catal11080979/s1, Figure S1 27 Al NMR spectra and (c-d) 29 Si NMR spectra of CuSSZ-13 before and after Na poisoning; Figure S5. NH 3 -TPD profiles of Cu/SSZ-13 with different crystal sizes (a) before and (b) after Na poisoning; Figure S6.…”

“…However, the multiple Cu complex cannot fit in the CHA's steric pore size and the inactive CuO clusters are easily formed in the CHA zeolites [28]. Du et al compared the alkali resistance of conventional V 2 O 5 /WO 3 −TiO 2 catalysts and Fe 2 O 3 /HY zeolite catalysts and found that the HY zeolites played an alkalibuffer role to retain the catalytic activity [29]. Zha et al developed a hollandite Mn-Ti oxide promoted Cu-SAPO-34 (SAPO-34 is one framework type code of chabazite zeolite) catalysts (HMT@Cu-S).…”