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

The oxidation of propylene, an essential chemical reaction in both environmental and industrial contexts, frequently demands catalysts that not only ensure high selectivity but also maintain efficiency under diverse operating conditions. Traditionally, the mechanistic pathways behind such catalytic processes have been regarded as relatively static under varying thermal regimes. However, this study challenges that notion by revealing that the dominant reaction mechanism in propylene oxidation can pivot dramatically based on temperature fluctuations when using Pt/CeO₂ catalysts.

Central to this research is the unique interplay between platinum nanoparticles and cerium oxide support materials. Platinum, well-known for its catalytic prowess in oxidation reactions, when coupled with ceria, known for its oxygen storage capacity and redox properties, forms an ensemble catalyst system. The synergy between these materials creates reactive sites that can adopt distinct catalytic behaviors depending on thermal energy input, which, as the study reveals, activates different reaction intermediates and pathways.

Using advanced spectroscopic techniques combined with kinetic analyses, the research team observed that at lower operational temperatures, the oxidation process proceeds via a Mars-van Krevelen (MvK) mechanism. This mechanism involves the direct participation of lattice oxygen from the ceria support, which temporarily vacancies are replenished by molecular oxygen from the gaseous phase. It is a pathway that has traditionally been credited for excellent catalytic activity and durability, owing to the dynamic involvement of the oxide support’s lattice oxygen.

However, as the temperature surpasses a critical threshold, the reaction mechanism exhibits a transition toward a Langmuir-Hinshelwood (LH)-type pathway. Here, adsorbed molecular oxygen and propylene species on adjacent platinum sites react without significant lattice oxygen involvement. This thermally induced switch was confirmed through in situ spectroscopy that captured distinct adsorbate species at elevated temperatures, revealing a mechanistic departure from the MvK pathway to an LH mechanism as thermal energy increases.

The implications of this mechano-thermal transition are profound for designing next-generation catalysts. By demonstrating that catalytic activity and selectivity can be dynamically manipulated by temperature, the study highlights an underexplored axis of catalyst optimization. This knowledge allows researchers and industries to envision catalysts specifically tailored for operating windows, enhancing efficiency and reducing unwanted byproducts such as greenhouse gases or unreacted propylene residues.

Moreover, the discovery underscores the importance of cerium oxide as more than a passive support material. Its active role in oxygen supply and redox dynamics enables a catalytic versatility that can be harnessed in designing highly selective oxidation catalysts for various hydrocarbon transformations beyond propylene, such as in automotive exhaust treatment or chemical manufacturing.

From a methodological perspective, the work brings together innovative experimental setups involving temporal analysis of products, temperature-programmed reaction studies, and operando spectroscopic measurements. Such an integrative approach allowed them to unambiguously track the evolution of catalytic phases and surface intermediates under conditions mimicking industrial catalytic reactors, ensuring that the findings bear direct relevance to real-world applications.

Furthermore, the study illuminates the subtle yet critical influence of catalyst ensemble structures, where the spatial proximity of metal nanoparticles and oxide supports governs not only reaction rates but also the predominance of reaction pathways. This insight propels forward the concept of engineered heterojunctions at the nanoscale as a frontier in catalytic science.

In a broader context, this temperature-responsive behavior holds potential in designing smart catalytic systems that adapt their function based on environmental or operational variables. Such systems could operate with enhanced flexibility, self-adapting to fluctuations in feedstock composition, temperature, or pressure, thereby achieving consistent performance without frequent manual intervention or catalyst replacement.

The strategic use of Pt/CeO₂ catalysts also points to a trend in exploiting the redox properties of rare-earth oxides integrated with precious metals to overcome conventional limitations. These composite catalysts not only improve reaction turnover frequencies but also exhibit improved resistance to common deactivation pathways such as coking or sintering under harsh conditions.

This discovery also raises intriguing questions regarding the nature of surface oxygen species on ceria and their interaction dynamics with transition metals under thermal stress. Understanding these relationships at atomic and subatomic levels could unlock further paradigms in catalysis, potentially leading to breakthroughs in clean energy production, selective oxidation processes, and green chemistry synthesis routes.

Notably, the team emphasizes the criticality of catalyst preparation methods, as subtle morphologies and particle distributions significantly affect the observed mechanistic transitions. This highlights the nuanced role of synthesis protocols, aging, and pretreatment in controlling catalyst performance and stability, factors essential for scaling up such materials for industrial deployment.

In conclusion, Li et al.’s recent work represents a seminal advancement in the catalytic oxidation of propylene, revealing the nuanced temperature-dependent switch of reaction mechanisms on Pt/CeO₂ ensembles. This revelation not only enriches fundamental catalysis knowledge but also delivers a clear pathway toward engineering smarter, more versatile catalysts for environmentally and economically vital chemical transformations. As the global demand for cleaner and more efficient catalytic processes escalates, such foundational insights will inevitably catalyze innovation in chemical industries worldwide.

Future explorations inspired by this research may delve deeper into the atomic-scale orchestration of oxygen vacancies, dynamic metal-support interactions, and the generalizability of temperature-induced mechanistic shifts to other catalytic systems. These could pave the way toward the rational design of adaptive catalysts capable of meeting the diverse and evolving needs of sustainable chemical manufacturing.

Subject of Research: Catalytic oxidation of propylene over Pt/CeO₂ catalysts and temperature-driven mechanistic transitions.

Article Title: Temperature-driven mechanistic transition in propylene oxidation over Pt/CeO₂ ensemble catalysts.

Article References:
Li, Z., Chen, X., Lv, Y. et al. Temperature-driven mechanistic transition in propylene oxidation over Pt/CeO₂ ensemble catalysts. Nat Commun 16, 9199 (2025). https://doi.org/10.1038/s41467-025-64243-y

Image Credits: AI Generated