Key Takeaways & Executive Findings
- •• • CVOCs and NOx are co-emitted in flue gases from waste incineration and metal smelting, necessitating synergistic catalytic control for environmental compliance. • • Catalyst design must balance acidic sites and redox properties to achieve efficient CVOCs oxidation and NH3-SCR simultaneously, with specific emphasis on chlorine desorption and intermediate mineralization. • • Key challenges include competitive adsorption, chlorine poisoning, and formation of polychlorinated byproducts, which require tailored catalyst formulations to overcome. • • Future research should focus on interfacial mechanisms, innovative design of multifunctional catalytic sites, and scaling from laboratory to industrial applications.
Abstract
Chlorinated volatile organic compounds (CVOCs) are typical halogenated organic pollutants frequently coexisting with nitrogen oxides (NOx) in flue gases from thermal industrial processes such as waste incineration and metal smelting. The synergistic catalytic removal of these co-pollutants offers substantial environmental benefits and engineering potential. This review focuses on the regulation of catalyst acidity and redox properties, systematically summarizing the synergistic mechanisms between CVOCs catalytic oxidation and NH3-selective catalytic reduction (NH3-SCR) for NOx removal. Special attention is given to reaction pathways governing chlorine species desorption and intermediate mineralization during CVOCs oxidation, alongside intrinsic strategies for broadening the SCR temperature window, enhancing N2 selectivity, and mitigating catalyst deactivation. Key challenges in simultaneous removal include competitive adsorption of coexisting pollutants, chlorine poisoning of catalysts, formation of polychlorinated byproducts, and interference from other flue gas components. Future research directions are proposed, encompassing interfacial mechanistic elucidation, innovative design of multifunctional catalytic sites, and technological transition from laboratory-scale studies to industrial applications. This review provides theoretical insights and technical guidance for integrated control of multiple pollutants.
1. Introduction
The coexistence of chlorinated volatile organic compounds (CVOCs) and nitrogen oxides (NOx) in flue gases from thermal industrial processes such as waste incineration and metal smelting presents a significant environmental challenge. Conventional catalytic systems often address these pollutants separately, leading to inefficiencies and increased operational costs. The synergistic removal of CVOCs and NOx via combined catalytic oxidation and NH3-SCR offers a promising solution, yet it requires precise control over catalyst acidity and redox properties to achieve high activity and selectivity while mitigating deactivation.
This review systematically examines the state-of-the-art in synergistic catalytic control, focusing on mechanistic insights into chlorine species desorption, intermediate mineralization, and the broadening of SCR temperature windows. By addressing critical issues such as competitive adsorption, chlorine poisoning, and byproduct formation, we aim to provide a comprehensive framework for the rational design of catalysts that can effectively co-remove CVOCs and NOx, thereby advancing the field toward practical industrial application.
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SUN Bohua, LI Qianqian, DUO Jia, SU Guijin (2026). Recent Advances on Synergistic Catalytic Removal of Chlorinated Volatile Organic Pollutants and NOx. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025041101
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Frequently Asked Questions
What are the primary mechanisms by which chlorine species desorb during CVOCs oxidation, and how do they affect catalyst stability?
Chlorine species desorption typically occurs via Deacon reaction (Cl2 release) or hydrogen chloride (HCl) formation, depending on temperature and catalyst composition. At lower temperatures, chlorine accumulation leads to catalyst poisoning, while at higher temperatures, desorption is favored but may promote polychlorinated byproduct formation. Catalyst stability is compromised by chlorine-induced sintering and active site blockage, necessitating the use of redox additives like Ce or Fe to enhance chlorine mobility.
How does the presence of NOx influence the catalytic oxidation of CVOCs, and what are the implications for simultaneous removal?
NOx can compete with CVOCs for adsorption sites, potentially inhibiting oxidation. However, in NH3-SCR, NOx reduction is enhanced by the presence of CVOCs due to the generation of reactive intermediates that facilitate NH3 activation. This synergy requires careful optimization of the catalyst's acid-base and redox properties to balance the competitive adsorption and promote synergistic reactions.
What are the key challenges in scaling up synergistic catalytic systems from laboratory to industrial applications?
Scalability challenges include maintaining catalyst performance under real flue gas conditions with high space velocities, ensuring long-term stability against poisoning by sulfur, water, and chlorine, and minimizing pressure drop in monolithic reactors. Additionally, cost-effective synthesis of multifunctional catalysts with high mechanical strength is essential for industrial deployment.
What strategies are effective in broadening the temperature window for NH3-SCR while maintaining high N2 selectivity in the presence of CVOCs?
Strategies include doping with Ce, Zr, or Sm to enhance redox properties and oxygen mobility, which improves low-temperature activity. Additionally, optimizing the ratio of Brønsted to Lewis acid sites can suppress N2O formation and enhance N2 selectivity. Zeolite-based catalysts like Cu-SAPO-34 have shown promise due to their hydrothermal stability and tunable acidity.
How does the formation of polychlorinated byproducts (e.g., dioxins) during CVOCs oxidation impact the overall process, and what mitigation approaches exist?
Polychlorinated byproducts are highly toxic and must be minimized. Their formation is favored at low temperatures and in the presence of chlorine and metal catalysts. Mitigation approaches include operating at higher temperatures (>300°C) to promote complete oxidation, using catalysts with high redox potential to facilitate deep oxidation, and incorporating alkaline additives to neutralize chlorine species.
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