Iron-based catalysts have garnered significant attention in environmental catalysis, particularly for their promising role in high-temperature selective catalytic reduction (SCR) of nitrogen oxides (NOx) using ammonia (NH3). A major challenge in this field, however, has been the undesirable over-oxidation of ammonia at elevated temperatures, which not only limits NOx conversion efficiency but also compromises catalyst stability over long-term operation. Recently, a breakthrough study led by Zhiqiang Sun and colleagues at Central South University, China, has brought new insight into the mechanistic intricacies and kinetic behaviors of high-temperature NH3-SCR by advancing a novel dual-pathway model and developing an innovative Fe@ZSM-5 catalyst. Their findings, published in the prestigious Industrial Chemistry & Materials journal in December 2025, offer a sophisticated understanding that could reshape future catalyst design for emission control technologies.
The research team synthesized their catalytic material using a meticulously controlled hydrothermal process to produce HZSM-5 zeolites, subsequently ion-exchanged with iron acetylacetonate (Fe(acac)3) to embed iron species within the zeolite framework. This was followed by a sequence of stirring, washing, drying, and high-temperature calcination at 800 °C, which yielded a robust Fe@ZSM-5 catalyst. Advanced characterization techniques, including X-ray diffraction (XRD) and transmission electron microscopy (TEM), confirmed the preservation of the MFI zeolite structure and revealed well-distributed Fe2O3 nanoparticles prominently exposing (110) and (104) crystal facets on the zeolite surface.
Delving deeper into the catalyst’s composition, energy-dispersive X-ray spectroscopy (EDS) mapping and aberration-corrected scanning transmission electron microscopy (AC-STEM) elucidated a dual presence of atomically dispersed iron atoms alongside ~1.5 nm iron oxide nanoparticles. Electron energy loss spectroscopy (EELS) further verified the dominance of Fe3+ oxidation states within the catalyst, implicating this as a key feature governing catalytic behavior. X-ray photoelectron spectroscopy (XPS) detected both Fe2+ and Fe3+ species, while the O 1s spectra indicated that framework oxygen substantially contributes to the catalyst’s surface chemistry, vital for SCR reactions.
Surface acidity and reducibility are critical parameters influencing catalytic performance. Through ammonia temperature-programmed desorption (NH3-TPD) and hydrogen temperature-programmed reduction (H2-TPR), the study demonstrated that increasing iron loading enhances surface acid sites and facilitates the reduction from Fe2O3 to Fe3O4 while suppressing reduction to metallic Fe phases. These findings coincide with the predominance of Fe3+ species under reaction conditions, maintaining an optimal balance between active site availability and structural stability. Complementary X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses revealed an increase in Fe–Fe coordination contacts as iron loading rose, indicating a gradual shift toward bulk iron oxide phases that correlate with catalytic activity trends.
The catalytic evaluation of Fe@ZSM-5 for NH3-SCR reactions unveiled a fascinating temperature-dependent dual kinetic regime. Optimal high-temperature NO conversion was observed with a Si/Al ratio of 27, reaching up to 95.1% NO conversion within the 400–700 °C temperature window. Above 700 °C, however, NO conversion declined markedly, especially with higher iron loadings, attributed to escalating ammonia oxidation competing pathways. This over-oxidation reduced the overall NOx reduction efficiency, confirming that precise control of operational parameters and catalyst composition is paramount for maximal efficacy.
Interestingly, gas hourly space velocity (GHSV) studies revealed an inverse relationship between feed flow rates and NO conversion, mirrored by a commensurate drop in NH3 conversion, suggesting diffusion limitations and kinetic constraints influencing reaction pathways at high throughput conditions. Durability tests at 700 °C extending beyond 50 hours showcased extraordinary catalyst stability, with the 0.1Fe@ZSM-5 variant only experiencing a minor 2.5% decrease in NO conversion, affirming its potential for practical long-term applications under harsh industrial environments.
The real-world pertinence of the catalyst was further tested under challenging conditions incorporating 300 ppm sulfur dioxide (SO2) and 8.3 vol% water vapor, simulating flue gas compositions. NO conversion initially dropped from 83.0% to 60.1% over 50 hours but intriguingly exhibited partial recovery to 71.5% once the poisoning agents were removed. This resilience contrasts sharply with sulfur-induced irrecoverable deactivation observed in parent HZSM-5 catalysts. The researchers pinpointed sulfur deposition, framework dealumination, loss of Lewis acid sites, and lattice oxygen consumption as the multiple intertwined factors responsible for the observed deactivation, providing insight for future catalyst improvements.
A pivotal advancement in this work lies in the development of a kinetic model that captures the dual-reactive pathways inherent in high-temperature NH3-SCR processes. By integrating NH3 oxidation dynamics alongside NOx selectivity transitions, the model successfully describes the experimental phenomenon where NO formation surpasses dinitrogen generation at elevated temperatures. In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) validated the emergence and role of NH2* intermediates central to the SCR mechanism, particularly under high-temperature conditions, corroborating the theoretical framework proposed.
Crucially, the size of iron nanoparticles exerts a significant influence on the reaction mechanism. Larger Fe particles enriched in metallic Fe0 species were found to enhance NH3 adsorption on Brønsted acid sites, catalyzing increased ammonia over-oxidation to NO at elevated temperatures. This size-dependent modulation creates a delicate balance governing catalytic performance, underscoring the importance of controlling iron dispersion and particle dimensions during catalyst synthesis to optimize SCR activity and minimize undesired side reactions.
The multidisciplinary team comprising Xinlin Xie, Jibin Yuan, Lei Liu, Hanzi Liu, and Zhiqiang Sun combined expertise in materials chemistry, surface science, and catalysis to deliver this comprehensive study. Their efforts were supported by the National Natural Science Foundation of China and the Provincial Natural Science Foundation of Hunan, highlighting the vital role of sustained funding in advancing frontier research tackling energy and environmental challenges.
This work not only advances fundamental understanding of iron-based catalysts under extreme reaction conditions but also propels industrial applications aimed at mitigating NOx emissions, a critical component of air pollution control strategies worldwide. By unraveling the complex kinetic interplay between NH3 oxidation and NO reduction pathways, and engineering tailored Fe@ZSM-5 catalysts with exceptional stability and activity, this research sets a new benchmark for the design of next-generation SCR catalysts capable of enduring rigorous operational demands while delivering superior environmental performance.
As the global community intensifies its commitment to reducing pollutant emissions and transitioning toward cleaner technologies, breakthroughs such as this illuminate pathways to more effective catalytic materials. Their implications span across automotive exhaust treatment, power generation, and chemical manufacturing sectors, contributing to more sustainable industrial practices and improved air quality.
Subject of Research: High-temperature selective catalytic reduction of NOx using ammonia over iron-modified ZSM-5 catalysts and kinetic modeling of competing reaction pathways.
Article Title: Dual kinetic effect from confined iron nanoparticles in zeolite modulates high-temperature catalytic NO reduction and NH3 oxidation.
News Publication Date: 15-Dec-2025.
Web References:
Industrial Chemistry & Materials Journal
DOI: 10.1039/D5IM00245A
Image Credits: Zhiqiang Sun, Central South University, China.
Keywords
Iron-based catalysts, high-temperature NH3-SCR, NOx reduction, Fe@ZSM-5, ammonia oxidation, catalytic mechanism, kinetic modeling, zeolite, iron nanoparticles, environmental catalysis, catalyst stability, selective catalytic reduction
Tags: advanced characterization techniquescatalyst stability and efficiencyemission control technologiesenvironmental catalysis researchFe@ZSM-5 catalyst developmenthigh-temperature selective catalytic reductionhydrothermal synthesis processiron-based catalystskinetic behaviors in catalysisNH3-SCR mechanismnitrogen oxides conversionzeolite framework embedding
