Recent advances in simultaneous removal of NOx and VOCs over bifunctional catalysts via SCR and oxidation reaction

NOx and volatile organic compounds (VOCs) are two major pollutants commonly found in industrial flue gas emissions. They play a significant role as precursors in the formation of ozone and fine particulate matter (PM2.5). The simultaneous removal of NOx and VOCs is crucial in addressing ozone and PM2.5 pollution. In terms of investment costs and space requirements, the development of bifunctional catalysts for the simultaneous selective catalytic reduction (SCR) of NOx and catalytic oxidation of VOCs emerges as a viable technology that has garnered considerable attention. This review provides a summary of recent advances in catalysts for the simultaneous removal of NOx and VOCs. It discusses the reaction mechanisms and interactions involved in NH3-SCR and VOCs catalytic oxidation, the effects of catalyst acidity and redox properties. The insufficiency of bifunctional catalysts was pointed out, including issues related to catalytic activity, product selectivity, catalyst deactivation, and environmental concerns. Subsequently, potential solutions are presented to enhance catalyst performance, such as optimizing the redox properties and acidity, enhancing resistance to poisoning, substituting environment friendly metals and introducing hydrocarbon selective catalytic reduction (HC-SCR) reaction. Finally, some suggestions are given for future research directions in catalyst development are prospected.

Cu-VWT Catalysts for Synergistic Elimination of NOx and Volatile Organic Compounds from Coal-Fired Flue Gas

A dual-function catalyst, designated as Cu5-VWT, has been constructed for the synergistic removal of NO_x and volatile organic compounds under complex coal-fired flue gas conditions. The removal of toluene, propylene, dichloromethane, and naphthalene all exceeded 99% (350 °C), and the catalyst could effectively block the generation of polycyclic aromatic hydrocarbons. Mechanistic studies have shown that Cu sites on the Cu5-VWT catalyst facilitate catalytic oxidation, while V sites facilitate NO_x reduction. Thus, toluene oxidation and NO_x reduction can proceed simultaneously. The removal of total hydrocarbons and nonmethane total hydrocarbons from 1200 m³·h^-1 real coal-fired flue gas by a monolithic catalyst were determined as 92 and 96%, respectively, much higher than those of 54 and 72% over a commercial VWT catalyst, indicating great promise for industrial application.

Synergistic Catalytic Elimination of NOx and Chlorinated Organics: Cooperation of Acid Sites

The synergistic catalytic removal of NO_x and chlorinated volatile organic compounds under low temperatures is still a big challenge. Generally, degradation of chlorinated organics demands sufficient redox ability, which leads to low N₂ selectivity in the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR). Herein, mediating acid sites via introducing the CePO₄ component into MnO₂/TiO₂ NH₃-SCR catalysts was found to be an effective approach for promoting chlorobenzene degradation. The observation of in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) and Raman spectra reflected that the Lewis acid sites over CePO₄ promoted the nucleophilic substitution process of chlorobenzene over MnO₂ by weakening the bond between Cl and benzene ring. Meanwhile, MnO₂ provided adequate Brønsted acid sites and redox sites. Under the cooperation of Lewis and Brønsted acid sites, relying on the rational redox ability, chlorobenzene degradation was promoted with synergistically improved NH₃-SCR activity and selectivity. This work offers a distinct pathway for promoting the combination of chlorobenzene catalytic oxidation and NH₃-SCR, and is expected to provide a novel strategy for synergistic catalytic elimination of NO_x and chlorinated volatile organic compounds.

Improvement of Al2O3 on the multi-pollutant control performance of NOx and chlorobenzene in vanadia-based catalysts

We compared the influences of Al₂O₃ and SiO₂ on a traditional V₂O₅-MoO₃/TiO₂ for the simultaneous removal of NO_x and chlorobenzene (CB). The Al₂O₃ doping catalyst considerably broadens the active temperature window with higher NO_x reduction and CB oxidation efficiencies than the SiO₂ doping one and the V₂O₅-MoO₃/TiO₂. Furthermore, its resistance to SO₂ was preserved and the quantities of polychlorinated byproducts also decreased. The increase in activity at low temperatures could be due to the promotion of vanadia reducibility via interactions between V₂O₅ and Al₂O₃. Moreover, the high temperature activity could be due to the additional surface acidities provided by Al₂O₃, in which the Lewis acid sites played the predominant role in both NH₃ adsorptions and CB de-chlorination compared to the Brønsted acid sites. Finally, we proposed that Al₂O₃ is an effective addition for vanadia-based catalyst in NO_x and CB simultaneous removal from stationary sources.

Promotional effect of Sn additive on the chlorine resistance over SnMnOx/LDO catalysts for synergistic removal of NOx and o-DCB

Sn additive greatly improves the chlorine resistance of manganese-based catalysts by introducing more acid sites.

Simultaneous removal of NO and dichloromethane (CH2Cl2) over Nb-loaded cerium nanotubes catalyst

Herein, a series of niobium oxide supported cerium nanotubes (CeNTs) catalysts with different loading amount of Nb₂O₅ (0-10 wt.%) were prepared and used for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) in the presence of CH₂Cl₂. Commercial V₂O₅-WO₃-TiO₂ catalyst was also prepared for comparison. The physcial properties and chemical properties of the Nb₂O₅ loaded cerium nanotubes catalysts were investigated by X-ray diffractometer, Transmission electron microscope, Brunauer-Emmett-Teller specific surface area, H₂-temperature programmed reduction, NH₃-temperature programmed desorption and X-ray photoelectron spectroscopy. The experiment results showed that the loading amount of Nb₂O₅ had a significant effect on the catalytic performance of the catalysts. 10 wt.% Nb-CeNTs catalyst presented the best NH₃-SCR performance and degradation efficiency of CH₂Cl₂ among the prepared catalysts, due to its superior redox capability, abundant surface oxygen species and acid sites, the interaction between Nb and Ce, higher ratio of Nb⁴⁺/(Nb⁵⁺+ Nb⁴⁺) and Ce³⁺/(Ce³⁺ + Ce⁴⁺), as well as the special tubular structure of cerium nanotube. This study may provide a practical approach for the design and synthesis of SCR catalysts for the simultaneously removal NO_x and chlorinated volatile organic compounds (CVOCs) emitted from the stationary industrial sources.

Catalytic activity and stability of a Cr modified Co–Fe LDO catalyst in the simultaneous catalytic reduction of NOx and oxidation of o-DCB

In this paper, a Co–Fe LDO catalyst was prepared by combining K2Cr2O7 and Cr(NO3)3 to modify the LDH precursor.

Investigation of chlorine-poisoning mechanism of MnOx/TiO2 and MnOx-CeO2/TiO2 catalysts during o-DCBz catalytic decomposition: Experiment and first-principles calculation

The catalytic activity and stability of MnO_x/TiO₂ and MnO_x-CeO₂/TiO₂ catalysts for the oxidative degradation of 1,2-dichorobenzene (o-DCBz) at low temperatures (≤275 °C) were experimentally examined. The chlorine (Cl) poisoning mechanism of the catalysts was also clarified based on the catalyst characterization combined with theoretical calculations. Experimental results show that the MnO_x/TiO₂ catalyst is considerably deactivated during o-DCBz catalytic decomposition, mainly due to the chlorination of the catalytic active component. Ce addition and high temperature can effectively promote the resistance of MnO_x/TiO₂ catalyst to Cl poisoning. Density functional theory (DFT) calculations in the framework of first-principles reveal that Cl atom prefers to anchor on surface oxygen vacancy (OV) rather than on top site of Mn atom. The adsorption of Cl atom on surface OV hinders the dissociated adsorption of O₂ on surface OV and interrupts the regeneration of the surface reactive oxygen species. The adsorption of Cl atom on top site of Mn atom increases the formation energy of surface OV and damages the surface Lewis acid sites which act as the important adsorption sites for o-DCBz molecules. Ce addition causes Cl atom to adsorb preferentially onto the OV around Ce atom, which weakens the interaction between Cl atom and Mn atom. Consequently, the chlorination of the MnO_x species is prevented and the oxygen mobility of the catalyst is guaranteed to some extent.

Multipollutant Control (MPC) of Flue Gas from Stationary Sources Using SCR Technology: A Critical Review

The emission of gaseous pollutants from the combustion of fossil fuels is believed to be one of the most serious environmental challenges in the 21st century. Given the increasing demands of multipollutant control (MPC) via adsorption or catalysis technologies, such as NO_x, volatile organic compounds (VOCs), heavy metals (Hg etc.), and ammonia, and considering investment costs and site space, the use of existing equipment, especially the selective catalytic reduction (SCR) system to convert pollutants into harmless or readily adsorbed substances, is one of the most practical approaches. Consequently, many efforts have been directed at achieving the simultaneous elimination of multipollutants in a SCR convertor, and this method has been widely used to mitigate the stationary emission of NO_x. However, the development of active, selective, stable, and multifunctional catalysts/adsorbents suitable for large-scale commercialization remains challenging. Herein, we summarize recent works on the applications of SCR in MPC, describing the approaches of (i) SCR + VOCs oxidation, (ii) SCR + heavy metal control, and (iii) SCR + NH₃ reduction to reveal that the efficiency of simultaneous elimination depends on catalyst composition and flue gas parameters. Furthermore, the synergistic promotional/inhibitory effects between SCR and VOCs/ammonia/heavy metal oxidations are shown to be the key to the feasibility of the reactions.

The effect of additives and intermediates on vanadia-based catalyst for multi-pollutant control

The influence of additives, WO₃ and MoO₃, on the activity of V₂O₅/TiO₂ catalyst for NO_x and chlorobenzene (CB) removal (multi-pollutant control, MPC) was investigated.