Promoting C-Cl Bond Activation via a Preoccupied Anchoring Strategy on Vanadia-Based Catalysts for Multi-Pollutant Control of NOx and Chlorinated Aromatics

Regulating vanadia-based oxides has been widely utilized for fabricating effective difunctional catalysts for the simultaneous elimination of NOx and chlorobenzene (CB). However, the notorious accumulation of polychlorinated species and excessively strong NH3 adsorption on the catalysts lead to the deterioration of multipollutant control (MPC) activity. Herein, protonated sulfate (-HSO4) supported on vanadium-titanium catalysts via a preoccupied anchoring strategy are designed to prevent polychlorinated species and alleviate NH3 adsorption for the multipollutant control. The obtained catalysts with -HSO4 modification achieve an excellent NOx and CB conversion with turnover frequency values of ∼ 3.63 and 17.7 times higher than those of the pristine, respectively. The protonated sulfate promotes the formation of polymeric vanadyl with a higher chemical state and d-band center of V. The modulated catalysts not only substantially alleviate the competitive adsorption of multipollutant via the "V 3d-O 2p-S 3p" network, but also distinctly strengthen the Brønsted acid sites. Besides, the introduced proton donor of the -HSO4 connecting polymeric structure could markedly reduce the reaction barrier of breaking the C-Cl bond. This work paves an advanced way for low-loading vanadium SCR catalysts to achieve highly efficient NOx and CB oxidation at a low temperature.

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.

Progress of catalytic oxidation of typical chlorined volatile organic compounds (CVOCs): A review

Chlorinated volatile organic compounds (CVOCs) are still a part of the current atmospheric environmental problems that cannot be ignored, but unlike conventional VOCs, the presence of Cl causes various catalyst deactivations in the catalytic process. In this paper, we focus on six common CVOCs and discuss various behavioral mechanisms of the whole catalytic process from six aspects: catalyst selection, factors affecting the catalytic effect, changes in catalytic behavior in the presence of different gases, catalyst poisoning deactivation behavior, degradation products and degradation mechanisms to provide guidance for further development of low-temperature and efficient CVOCs catalysts.

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.

Impact of NOx and NH3 addition on toluene oxidation over MnOx-CeO2 catalyst

An increasing number of industries remove toluene from flue gas by the existing NH₃-selective catalytic reduction (NH₃-SCR) units. A thorough probe into the impact of NO_x and NH₃ addition on toluene oxidation is imperative but still lacks a unified understanding. In this work, NH₃-SCR reactants are found to inhibit the toluene oxidation process over the MnO_x-CeO₂ catalyst below 200 °C. The competitive adsorption between NH₃-SCR reactants and toluene, the NO₂ adsorption state, and carbon deposition are emphasized to play important roles in this deactivation. Within the NO₂ adsorption states, only the adsorbed NO₂ can enhance the toluene oxidation. The formed nitrate species (NO₃^-) on the surface is inactive. NO₂ adsorption is the weakest among the reactants with the smallest adsorption energy of -0.42 eV, restricting its promotion on toluene oxidation. NO and N₂O are both demonstrated to be inefficient to oxidize toluene. Meanwhile, MnO_x-CeO₂ catalyst suffers from serious acetonitrile and benzonitrile poisoning. The amount of nitrile species accounts for ~95% of total carbon deposition, while no simple substance carbon (C) can be generated from CO disproportionation. Special care should be considered towards the formation of environmentally hazardous benzamide in the off-gas from the simultaneous NO_x and toluene removal process.

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.

Key intermediates from simultaneous removal of NOx and chlorobenzene over a V2O5–WO3/TiO2 catalyst: a combined experimental and DFT study

Chlorobenzene suppresses the active cis-N2O22− formation and the dissociated Cl− combines with vanadium to form vanadium chloride which enhanced surface Brønsted acidity.

Selective catalytic reduction of NOx with NH3 over TiO2 supported metal sulfate catalysts prepared via a sol–gel protocol

Metal sulfate catalysts exhibited high SO₂ tolerance in the NH₃-SCR reaction. The NH₃-SCR reaction mechanism on metal sulfate catalysts should follow the Eley–Rideal mechanism.