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.

Efficient synergistic catalysis of chlorinated aromatic hydrocarbons and NOx over novel low-temperature catalysts: Nano-TiO2 modification and interaction mechanism

For efficient and synergistic elimination of chlorinated aromatic hydrocarbons (e.g., dioxins and chlorobenzenes) and NO_x at low temperatures, a novel VO_x-CeO_x-WO_x/TiO₂ catalyst was systemically studied, involving the nano-TiO₂ modification and the interaction mechanism between 1,2-dichlorobenzen (1,2-DCB) catalytic oxidation (DCBCO) and NH₃-SCR. The VO_x-CeO_x-WO_x/TiO₂ performed excellent oxygen storage/release capacity (OSRC) and desirable 1,2-DCB conversion efficiency (95.1-97.4%) at 160-200 ℃ via M‒K and L‒H mechanism. The nano-TiO₂ modification slightly impaired the 1,2-DCB oxidation to 93.6-96.2% owing to the reduced surface area and Brønsted acidity, while it distinctly enhanced NO conversion and lowered the T₅₀ (from 162 to 112 ℃) and T₉₀ (from 232 to 205 ℃) by improving catalyst reducibility. Based on further synergistic catalysis evaluation and in-situ DRIFT analysis, NO enhanced the 1,2-DCB conversion and complete oxidation capacity of VO_x-CeO_x-WO_x/TiO₂ by promoting active oxygen (O₂^-, O^-, O^2-) generation and improving 1,2-DCB chemosorption and subsequent oxidation. In detail, the produced HCl and H₂O improved the catalyst acidity and promoted the formation of HONO and HNO₃. Moreover, their generation not only facilitated the chemisorption of NH₃ but also participated in the NH₃-SCR via L‒H mechanism. The ensuing problem was the competitive chemisorption among 1,2-DCB, NH₃, and their subsequent intermediates. As a result, NH₃ had distinct advantages in competing for acid sites and active oxygen species, especially at the higher temperature, resulting in the improved NO conversion with elevated reaction temperature but the reduced 1,2-DCB conversion. The results provided essential basics for developing new catalysts to synergistically control the emission of chloroaromatic organics and NO_x at low temperature.

Single-atom Ir1 supported on rutile TiO2 for excellent selective catalytic oxidation of ammonia

Gaseous ammonia (NH₃) in the atmosphere is potentially harmful to both human health and the environment. The selective catalytic oxidation of NH₃ (termed as NH₃-SCO) into N₂ and H₂O is a promising method for decreasing NH₃ emissions. A highly efficient catalyst is required for controlling NH₃ emissions by this method in practice. In this study, we prepared Ir/TiO₂ catalysts using different crystal structures of TiO₂ (rutile, P25 or anatase) as supports by a simple impregnation method and evaluated their performance in the NH₃-SCO. We found that the Ir/TiO₂-R (rutile) catalyst performed better than the Ir/TiO₂-P25 (mixed-phase) and Ir/TiO₂-A (anatase) catalyst. High-angle annular dark-field images of the aberration-corrected scanning transmission electron microscopy revealed that the Ir species were mainly atomically dispersed on the TiO₂ support in Ir/TiO₂-R with 1 wt% Ir loading, whereas the Ir species agglomerated to form clusters or nanoparticles in Ir/TiO₂-P25 and Ir/TiO₂-A. The combined results of X-ray absorption fine structure, H₂-temperature-programmed reduction, and in situ diffuse reflectance for infrared Fourier Transform spectroscopy studies suggested that atomically dispersed Ir species had stronger electronic metal-support interaction with rutile TiO₂, which resulted in easier to adsorb and activate O₂ at the interface and thus, better low-temperature activity of the Ir/TiO₂-R catalyst.

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

Vanadium-based catalysts have been extensively applied for the synergistic control of NO_x and chlorinated organics. However, how the vanadia species affect the reaction activity and products distribution, and what are the dominant reaction sites of these vanadia species are still unknown. Herein, we investigated the reaction characteristics of monomeric and polymeric vanadate domains for the catalytically synergistic elimination of NO_x and chlorobenzene (CB). Density functional theory (DFT) calculations and experimental investigations have been combined to clarify the effects of different vanadyl species on the synergistic reaction. It was noted that the main adsorption site of CB on the monomeric domain was V-OH bond, and that on the polymeric one was V═O bond. The monomeric vanadyl was favorable for converting the Lewis V═O into Brønsted V-OH, which provided sufficient H protons for HCl formation, whereas the polymeric species could effectively retain the V⁴⁺/V⁵⁺ redox cycle, and yielded superior activity in CB catalytic oxidation (CBCO) reaction. However, the abundant oxygen vacancies and the inclined accumulation of Cl by forming the V-Cl bands led to significant polychlorinated byproducts on the polymeric vanadyl catalysts. Our work gives the first insight into different vanadate domain effects on the synergistic reaction, and is expected to provide theoretical basis for efficient design of the vanadium-based catalysts toward multipollutants elimination.

Investigation of suitable precursors for manganese oxide catalysts in ethyl acetate oxidation

The control of ethyl acetate emissions from fermentation and extraction processes in the pharmaceutical industry is of great importance to the environment. We have developed three Mn₂O₃ catalysts by using different Mn precursors (MnCl₂, Mn(CH₃COO)₂, MnSO₄), named as Mn₂O₃-Cl, -Ac, -SO₄. The tested catalytic activity results showed a sequence with Mn precursors as: Mn₂O₃-Cl > Mn₂O₃-Ac > Mn₂O₃-SO₄. The Mn₂O₃-Cl catalyst reached a complete ethyl acetate conversion at 212℃ (75℃ lower than that of Mn₂O₃-SO₄), and this high activity 100% could be maintained high at 212℃ for at least 100 hr. The characterization data about the physical properties of catalysts did not show an obvious correlation between the structure and morphology of Mn₂O₃ catalysts and catalytic performance, neither was the surface area the determining factor for catalytic activity in the ethyl acetate oxidation. Here we firstly found there is a close linear relationship between the catalytic activity and the amount of lattice oxygen species in the ethyl acetate oxidation, indicating that lattice oxygen species were essential for excellent catalytic activity. Through H₂ temperature-programmed reduction (H₂-TPR) results, we found that the lowest initial reduction temperature over the Mn₂O₃-Cl had stronger oxygen mobility, thus more oxygen species participated in the oxidation reaction, resulting in the highest catalytic performance. With convenient preparation, high efficiency, and stability, Mn₂O₃ prepared with MnCl₂ will be a promising catalyst for removing ethyl acetate in practical application.

Selective catalytic reduction of NO over W–Zr-Ox/TiO2: performance study of hierarchical pore structure

A series of W–Zr-Ox/TiO2 catalysts with hierarchical pore structure were prepared and used for selective catalytic reduction of NO by NH3.

Significant promotion effect of the rutile phase on V2O5/TiO2 catalysts for NH3-SCR

High rutile content and low specific surface area of the vanadia-based catalysts contribute to an excellent NH₃-SCR activity.

Recent Progress on Improving Low-Temperature Activity of Vanadia-Based Catalysts for the Selective Catalytic Reduction of NOx with Ammonia

Selective catalytic reduction of NOx with NH3 (NH3-SCR) has been successfully applied to abate NOx from diesel engines and coal-fired industries on a large scale. Although V2O5-WO3(MoO3)/TiO2 catalysts have been utilized in commercial applications, novel vanadia-based catalysts have been recently developed to meet the increasing requirements for low-temperature catalytic activity. In this article, recent progress on the improvement of the low-temperature activity of vanadia-based catalysts is reviewed, including modification with metal oxides and nonmetal elements and the use of novel supports, different synthesis methods, metal vanadates and specific structures. Investigation of the NH3-SCR reaction mechanism, especially at low temperatures, is also emphasized. Finally, for low-temperature NH3-SCR, some suggestions are given regarding the opportunities and challenges of vanadia-based catalysts in future research.