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Iron-based molecular sieves show great promise for high-temperature NH3-SCR due to their intrinsic shape selectivity and thermal stability. However, excessive ammonia oxidation at high temperatures limits NOx conversion and long-term stability, and its kinetic transition remains poorly understood. A team led by Zhiqiang Sun, Hanzi Liu, and Xinlin Xie has developed a high-temperature Fe@ZSM-5 catalyst and established a coupled kinetic model to describe ammonia oxidation behavior at high temperatures. Their work is published in the journal Industrial Chemistry & Materials on December 2025.
The authors synthesized HZSM-5 zeolites via a hydrothermal method, followed by ion-exchange with Fe(acac)3 (Fe(C5H7O2)3). After stirring at 80 °C, washing, and drying at 110 °C for 12 h, the solids were calcined at 800 °C for 5 h to obtain Fe@ZSM-5. XRD and TEM confirmed the MFI structure, while HR-TEM revealed Fe2O3 nanoparticles anchored on the zeolite, mainly exposing the (110) and (104) planes. EDS mapping showed uniform distribution of Si, Al, and O, and AC-STEM images displayed bright spots of ~1.5 nm, indicating coexisting atomically dispersed Fe and nanoparticles. EELS confirmed the presence of Fe3+.
XPS indicated both Fe2+ and Fe3+ species at high Fe loadings, with O 1s spectra dominated by framework oxygen. NH3-TPD and H2-TPR showed that higher Fe loading increased surface acidity and enhanced Fe2O3-to-Fe3O4 reduction, while FeO-to-metal Fe reduction decreased, confirming Fe3+ as the predominant species. XANES and EXAFS analysis revealed that increasing Fe loading increased Fe-Fe coordination (~2.5 Å) and decreased Fe-O coordination (~1.8 Å), reflecting a transition to bulk Fe oxides.
In NH3-SCR, a Si/Al ratio of 27 gave optimal high-temperature NO conversion. NO conversion increased with temperature and Fe loading, reaching 95.1% at 400-700 °C, but decreased above 700 °C, more pronounced at higher Fe loadings, suggesting two kinetic regimes. NH3 oxidation increased with Fe content at high temperatures, contributing to NO conversion decline. Higher GHSV reduced NO conversion, correlating with decreased NH3 conversion. Stability tests at 700 °C for >50 h showed only a 2.5% decrease in NO conversion for 0.1Fe@ZSM-5.
Under 300 ppm SO2 and 8.3 vol% H2O at 700 °C, NO conversion for 0.1Fe@ZSM-5 decreased from 83.0% to 60.1% over 50 h but partially recovered to 71.5% after poison removal. HZSM-5 showed irreversible deactivation under similar conditions. Sulfur deposition, dealumination, loss of Lewis acid sites, and lattice oxygen explained the deactivation in Fe@ZSM-5.
To model high-temperature SCR, a dual-regime kinetic model was developed incorporating NH3 oxidation and NOx selectivity, successfully capturing the transition where NO formation surpasses N2 formation with increasing temperature. In-situ DRIFTS confirmed key intermediate evolution at high temperatures. Fe nanoparticle size influenced the mechanism: larger Fe particles increased Fe0 content, enhanced NH3 adsorption on Brønsted sites, and promoted NH3 over-oxidation to NO at high temperatures, consistent with SCR performance.
The research team includes Xinlin Xie, Jibin Yuan, Lei Liu, Hanzi Liu, and Zhiqiang Sun from the Central South University.
This research is funded by the National Natural Science Foundation of China and the Provincial Natural Science Foundation of Hunan.
Industrial Chemistry & Materials is a peer-reviewed interdisciplinary academic journal published by Royal Society of Chemistry (RSC) with APCs currently waived. ICM publishes significant innovative research and major technological breakthroughs in all aspects of industrial chemistry and materials, especially the important innovation of the low-carbon chemical industry, energy, and functional materials. Check out the latest ICM news on the blog.
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