Synergistic Elimination of NOx and Chlorinated Organics over VOx/TiO2 Catalysts: A Combined Experimental and DFT Study for Exploring Vanadate Domain Effect

Regulation Lattice Oxygen Mobility via Dual Single Atoms for Simultaneously Enhancing VOC Oxidation and NOx Reduction

Synergistic catalytic removal of multipollutants (e.g., volatile organic compound (VOC) oxidation and nitrogen oxide (NOx) reduction) is highly demanded due to the increasingly strict emission standards. The prevention of the key reactive intermediate species nitrite excessive oxidation over the supported noble-metal catalysts, rather than the traditional low-efficiency transition metal oxide catalysts, remains a great challenge. Herein, a sound strategy of Pd single atoms saturated with acidic transition element ligands is proposed. The coexistence of Pd and V dual single atoms strengthens the adsorption of reactants, while synergistic interaction between dual atoms and surface oxygen weakens activation of lattice oxygen, thus significantly reducing the overoxidation of nitrite. Meanwhile, the neutralization of the active Pd and inert V sites results in a rational decrease in the redox property of Pd and an obvious increase in that of V. The Pd1V1/CeO2 dual single-atom catalyst achieves 90% conversion of NOx and toluene at 238 and 230 °C and has a large temperature window (>150 °C) for NOx reduction. This research makes a breakthrough in the development of efficient supported noble-/transition-metal dual single-atom catalysts for VOC and NOx simultaneous purification.

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.

Changes of Nitrate Activity and Byproduct Distribution Characteristics for Synergistic NOx and Dioxin Abatement over V2O5/AC Catalyst

The simultaneous removal of NOx and dioxins has been considered an economical and effective technology of controlling multipollutant flue gas in the context of "carbon peaking and carbon neutrality". However, this technology has not yet been implemented in practical situations, because the interactive relationship between the selective catalytic reduction (SCR) reaction and dioxin catalytic oxidation lacks a deep understanding, especially on a carbon-based catalyst. In this research, the influence of NO and NH3 on the oxidation characteristics and byproducts distribution of dibenzofuran (DBF) was studied on V2O5/AC catalyst. Results indicated that NH3 has a stronger inhibition effect for DBF catalytic oxidation than NO due to obvious competitive adsorption between NH3 and DBF on the V2O5/AC catalyst. In addition, although both NO and NH3 inhibit the complete degradation of DBF, their effects on the byproduct distribution are not consistent. NO primarily affects the level of oxygen-containing byproducts, while NH3 primarily affects the level of alkane byproducts. Furthermore, the SCR reaction activity demonstrated a reduction when DBF was present. The occupation of V2O5 sites by DBF and its oxidizing intermediates has hindered the production of monodentate nitrate and the reactivity of bridged nitrate, resulting in a decrease in SCR activity via the L-H mechanism. This work aims to provide theoretical guidance for simultaneous removal of NOx and dioxins in industrial fumes.

Phosphotungstic Acid as a Dechlorination Agent Collaborates with CeO2 for Synergistic Catalytic Elimination of NOx and Chlorobenzene

The development of efficient technologies for the synergistic catalytic elimination of NOx and chlorinated volatile organic compounds (CVOCs) remains challenging. Chlorine species from CVOCs are prone to catalyst poisoning, which increases the degradation temperature of CVOCs and fails to balance the selective catalytic reduction of NOx with the NH3 (NH3-SCR) performance. Herein, synergistic catalytic elimination of NOx and chlorobenzene has been originally demonstrated by using phosphotungstic acid (HPW) as a dechlorination agent to collaborate with CeO2. The conversion of chlorobenzene was over 80% at 270 °C, and the NOx conversion and N2 selectivity reached over 95% at 270-420 °C. HPW not only allowed chlorine species to leave as inorganic chlorine but also enhanced the Bro̷nsted acidity of CeO2. The NH4+ produced in the NH3-SCR process can effectively promote the dechlorination of chlorobenzene at low temperatures. HPW remained structurally stable in the synergistic reaction, resulting in good water resistance and long-term stability. This work provides a cheaper and more environmentally friendly strategy to address chlorine poisoning in the synergistic reaction and offers new guidance for multipollutant control.

Uncovering the Dinuclear Mechanism of NO2-Involved NH3-SCR over Supported V2O5/TiO2 Catalysts

Commercial vanadium oxide catalysts exhibit high efficiency for the selective catalytic reduction (SCR) of NO with NH3, especially in the presence of NO2 (i.e., occurrence of fast NH3-SCR). The high-activity sites and their working principle for the fast NH3-SCR reaction, however, remain elusive. Here, by combining in situ spectroscopy, isotopic labeling experiments, and density functional theory (DFT) calculations, we demonstrate that polymeric vanadyl species act as the main active sites in the fast SCR reaction because the coupling effect of the polymeric structure alters the elementary reaction step and effectively avoids the high energy barrier of the rate-determining step over monomeric vanadyl species. This study unveils the high-activity dinuclear mechanism of the NO2-involved SCR reaction over vanadia-based catalysts and provides a fundamental basis for developing high-efficiency and low V2O5-loading SCR catalysts.

Structure-Directing Role of Support on Hg0 Oxidation over V2O5/TiO2 Catalyst Revealed for NOx and Hg0 Simultaneous Control in an SCR Reactor

The crystal structure of TiO₂ strongly influences the physiochemical properties of supported active sites and thus the catalytic performance of the as-synthesized catalyst. Herein, we synthesized TiO₂ with different crystal forms (R = rutile, A = anatase, and B = brookite), which were used as supports to prepare vanadium-based catalysts for Hg⁰ oxidation. The Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B was the best, followed by V₂O₅/TiO₂-A and V₂O₅/TiO₂-R. Further experimental and theoretical results indicate that gaseous Hg⁰ reacts with surface-active chlorine species produced by the adsorbed HCl and the reaction orders of Hg⁰ oxidation over V₂O₅/TiO₂ catalyst with respect to HCl and Hg⁰ concentration were approximately 0 and 1, respectively. The excellent Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B can be attributed to lower redox temperature, larger HCl adsorption capacity, and more oxygen vacancies. This work suggests that to achieve the best simultaneous removal of NO_x and Hg⁰ on state-of-the-art V₂O₅/TiO₂ catalyst, a combination of anatase and brookite TiO₂-supported vanadyl tandem catalysts is supposed to be employed in the SCR reactor, and the brookite-type catalyst should be on the downstream of the anatase-based catalyst due to the inhibition of NH₃ on Hg⁰ oxidation.

Insight into the Transformation Behaviors of Dioxins from Sintering Flue Gas in the Cyclic Thermal Regeneration by the V2O5/AC Catalyst-sorbent

Dioxins in the sintering flue gas are usually removed through integrated elimination technologies by carbonaceous catalysts. However, the regeneration of the used catalyst is poorly investigated, leading to the risk of leakage of dioxins. Herein, the influences of cyclic regenerations on the dioxin removal performance of a catalyst (V₂O₅/AC) were investigated systematically with dibenzofuran (DBF) as a model pollutant. It was demonstrated that the adsorption capacity and oxidation activity of catalysts significantly declined after several regeneration cycles due to the decreasing external specific surface area and V⁵⁺, respectively. Compared with 79.12% DBF directly emitted from a regenerator during N₂ regeneration, the emission of DBF was only 29.93% with the modification of the regeneration process through O₂ addition and temperature adjustment. The possible regenerated products were also analyzed to disclose the transformation behaviors of DBF. The regeneration mechanisms of DBF followed the transformation pathway of dibenzofuranol, benzofuran, anhydride species, and ultimately to CO₂ and H₂O. Moreover, the accumulated heavy aromatics on the surface could be decomposed by introducing O₂. This research provides a comprehensive understanding of dioxin transformation behavior and a theoretical basis for efficient control of dioxin removal in the whole integrated removal technologies.

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.