Manganese supported on controlled dealumination Y-zeolite for ozone catalytic oxidation of low concentration toluene at low temperature

Oxygen vacancy-rich K-Mn3O4@CeO2 catalyst for efficient oxidation degradation of formaldehyde at near room temperature

Synthesis of catalysts with high catalytic degradation activity for formaldehyde (HCHO) at room temperature is highly desirable for indoor air quality control. Herein, a novel K-Mn3O4@CeO2 catalyst with excellent catalytic oxidation activity toward HCHO at near room temperature was reported. In particular, the K addition in K-Mn3O4@CeO2 considerably enhanced the oxidation activity, and importantly, 99.3 % conversion of 10 mL of a 40 mg/L HCHO solution at 30 °C for 14 h was achieved, with simultaneous strong cycling stability. Moreover, the addition of K species considerably influenced the chemical valence state of Mn from +4 (ε-MnO2) to +8/3 (Mn3O4) on the surface of CeO2, which obviously changed the tunnel structure and the number of oxygen vacancies. One part of K species is uniformly dispersed on K-Mn3O4@CeO2, and the other part exists in the tunnel structure of Mn3O4@CeO2, which is mainly used to balance the negative charge of the tunnel and prevent collapse of the structure, providing enough active sites for the catalytic oxidation of HCHO. We observed a phase transition from tunneled KMnO2 to Mn3O4 to tunneled MnO2 with the decreasing K+ content, in which K-Mn3O4@CeO2 exhibited higher HCHO oxidation activity. In addition, K-Mn3O4@CeO2 exhibited lower oxygen vacancy formation and HCHO adsorption energies in aqueous solution based on density functional theory calculations. This is because the K species provide more active oxygen species and richer oxygen vacancies on the surface of K-Mn3O4@CeO2, promote the mobility of lattice oxygen and the room-temperature reduction properties of oxygen species, and enhance the ability of the catalyst to replenish the consumed oxygen species. Finally, a possible HCHO catalytic oxidation pathway on the surface of K-Mn3O4@CeO2 catalyst is proposed.

Exceptional Ozone Decomposition over δ-MnO2/AC under an Entire Humidity Environment

Ozone (O3) pollution is highly detrimental to human health and the ecosystem due to it being ubiquitous in ambient air and industrial processes. Catalytic decomposition is the most efficient technology for O3 elimination, while the moisture-induced low stability represents the major challenge for its practical applications. Here, activated carbon (AC) supported δ-MnO2 (Mn/AC-A) was facilely synthesized via mild redox in an oxidizing atmosphere to obtain exceptional O3 decomposition capacity. The optimal 5Mn/AC-A achieved nearly 100% of O3 decomposition at a high space velocity (1200 L g-1 h-1) and remained extremely stable under entire humidity conditions. The functionalized AC provided well-designed protection sites to inhibit the accumulation of water on δ-MnO2. Density functional theory (DFT) calculations confirmed that the abundant oxygen vacancies and a low desorption energy of intermediate peroxide (O22-) can significantly boost O3 decomposition activity. Moreover, a kilo-scale 5Mn/AC-A with low cost (∼1.5 $/kg) was used for the O3 decomposition in practical applications, which could quickly decompose O3 pollution to a safety level below 100 μg m-3. This work offers a simple strategy for the development of moisture-resistant and inexpensive catalysts and greatly promotes the practical application of ambient O3 elimination.

Technological solutions for NOx, SOx, and VOC abatement: recent breakthroughs and future directions

NOx, SOx, and carbonaceous volatile organic compounds (VOCs) are extremely harmful to the environment, and their concentrations must be within the limits prescribed by the region-specific pollution control boards. Thus, NOx, SOx, and VOC abatement is essential to safeguard the environment. Considering the importance of NOx, SOx, and VOC abatement, the discussion on selective catalytic reduction, oxidation, redox methods, and adsorption using noble metal and non-noble metal-based catalytic approaches were elaborated. This article covers different thermal treatment techniques, category of materials as catalysts, and its structure-property insights along with the advanced oxidation processes and adsorption. The defect engineered catalysts with lattice oxygen vacancies, bi- and tri-metallic noble metal catalysts and non-noble metal catalysts, modified metal organic frameworks, mixed-metal oxide supports, and their mechanisms have been thoroughly reviewed. The main hurdles and potential achievements in developing novel simultaneous NOx, SOx, and VOC removal technologies are critically discussed to envisage the future directions. This review highlights the removal of NOx, SOx, and VOC through material selection, properties, and mechanisms to further improve the existing abatement methods in an efficient way.

Catalytic Ozonation of Polluter Benzene from -20 to >50 °C with High Conversion Efficiency and Selectivity on Mullite YMn2O5

Catalytic decomposition of aromatic polluters at room temperature represents a green route for air purification but is currently challenged by the difficulty of generating reactive oxygen species (ROS) on catalysts. Herein, we develop a mullite catalyst YMn₂O₅ (YMO) with dual active sites of Mn³⁺ and Mn⁴⁺ and use ozone to produce a highly reactive O* upon YMO. Such a strong oxidant species on YMO shows complete removal of benzene from -20 to >50 °C with a high CO_x selectivity (>90%) through the generated reactive species O* on the catalyst surface (60 000 mL g^-1 h^-1). Although the accumulation of water and intermediates gradually lowers the reaction rate after 8 h at 25 °C, a simple treatment by ozone purging or drying in the ambient environment regenerates the catalyst. Importantly, when the temperature increases to 50 °C, the catalytic performance remains 100% conversion without any degradation for 30 h. Experiments and theoretical calculations show that such a superior performance stems from the unique coordination environment, which ensures high generation of ROS and adsorption of aromatics. Mullite's catalytic ozonation degradation of total volatile organic compounds (TVOC) is applied in a home-developed air cleaner, resulting in high efficiency of benzene removal. This work provides insights into the design of catalysts to decompose highly stable organic polluters.

To promote catalytic ozonation of toluene by tuning Brönsted acid sites via introducing alkali metals into the OMS-2-SO42-/ZSM-5 catalyst

Although free hydroxyl radical (·OH) generated on OMS-2-based catalysts during the catalytic ozonation process have been shown as important reactive oxygen species (ROSs) for toluene degradation, improvement of surface ·OH formation ability remains challenging. Here, Na, K, Rb, and Cs-OMS-2-SO₄^2-/ZSM-5 catalysts were prepared, characterized and evaluated for catalytic ozonation of toluene. Both characterizations and DFT calculations showed that the appropriate alkali metal introduction made the catalyst possess with appropriate crystalline, reducibility, and acidity, which was favorable for catalytic ozonation of toluene. Characterizations also showed that alkali metal introduction resulted in water molecule adsorption on Brönsted acid sites of the catalysts, which made water molecule activation by ozone to form ·OH more easily. The introduction of K⁺ content of ∼ 5.9 wt% yielded K-OMS-2-SO₄^2-/ZSM-5 catalyst with the highest Brönsted acid sites and thus formed the most ·OH among the five prepared catalysts. As a result, the catalyst exhibited excellent toluene conversion and CO_x selectivity for catalytic ozonation of toluene at room temperature and ambient humidity. Furthermore, the catalytic activity of deactivated K-OMS-2-SO₄^2-/ZSM-5 catalyst was recovered after regeneration by a combination of water washing and heat treatment. Finally, a complete mechanism for toluene catalytic ozonation, catalyst deactivation, and regeneration was proposed.

Catalytic removal of 2-butanone with ozone over porous spent fluid catalytic cracking catalyst

To remove harmful volatile organic compounds (VOCs) including 2-butanone (methyl ethyl ketone, MEK) emitted from various industrial plants is very important for the clean air. Also, it is worthwhile to recycle porous spent fluid catalytic cracking (SFCC) catalysts from various petroleum refineries in terms of reducing industrial waste and the reuse of discharged resources. Therefore, Mn and Mn-Cu added SFCC (Mn/SFCC and Mn-Cu/SFCC) catalysts were prepared to compare their catalytic efficiencies together with the SFCC catalyst in the ozonation of 2-butanone. Since the SFCC-based catalysts have a structure similar to that of zeolite Y (Y), the Mn-loaded zeolite Y catalyst (Mn/Y) was also prepared to compare its activity for the removal of 2-butanone and ozone to that of the SFCC-based ones at room temperature. Among the five catalysts of this study (Y, Mn/Y, SFCC, Mn/SFCC, and Mn-Cu/SFCC), the Mn-Cu/SFCC and Mn/SFCC catalysts showed the better catalytic decomposition activity than the others. The increased distributions of the Mn³⁺ species and the O_vacancy sites in Mn/SFCC and Mn-Cu/SFCC catalysts which could supply more available active sites for the 2-butanone and ozone removal would enhance the catalytic activity of them.

Insights into the influence of water molecules on selective catalytic ozonation of gaseous ammonia into nitrogen on cryptomelane-type manganese oxide using in-situ DRIFTS

Catalytic ozonation is an environmentally friendly technology for the removal of gaseous NH₃ due to high NH₃ conversion and high N₂ selectivity at ambient temperature. However, the influence mechanism of ubiquitous water vapor on catalytic ozonation of NH₃ is unclear. In this study, cryptomelane-type manganese oxide (OMS-2) catalyst was prepared and tested for catalytic ozonation of NH₃ in different relative humidity. The results showed that water vapor significantly decreased the catalytic activity, which was due to the inhibition of water on NH₃ adsorption on Lewis acid sites and O₃ decomposition on oxygen vacancies, as well as the combination of water with active oxygen species (O₂^2- and O_atom). And the effect of water vapor on NH₃ conversion was more significant than O₃ decomposition because more Mn-OH were involved in the O₃ decomposition under humid conditions. Combining in-situ DRIFTS results with the performance of NH₃ oxidation, it is found that L-2 acid sites (the peak of NH₃ adsorption on Lewis acid sites at 1188 cm^-1) were the main active sites for adsorption and activation of NH₃ in the early stage of catalytic reaction; as the reaction progressed, L-2 acid sites were gradually occupied by water and more Brønsted acid sites participated in the catalytic reaction. This work deepened the understanding of the reaction process for selective catalytic ozonation of NH₃, and provided theoretical guidance for the design of efficient hydrophobic catalysts to eliminate gaseous NH₃ pollution.

Promoting the Catalytic Ozonation of Toluene by Introducing SO42- into the α-MnO2/ZSM-5 Catalyst to Tune Both Oxygen Vacancies and Acid Sites

Mn-based catalysts hold the promise of practical applications in catalytic ozonation of toluene at room temperature, yet improvement of toluene conversion and CO_x selectivity remains challenging. Here, an innovative α-MnO₂/ZSM-5 catalyst modified with SO₄^2- was successfully prepared, and both characterizations and density functional theory (DFT) calculations showed that SO₄^2- introduction facilitated the formation of oxygen vacancies, Lewis and Brönsted acid sites, and active oxygen species and enhanced the adsorption ability of toluene on α-MnO₂/ZSM-5. Characterizations also showed that SO₄^2- introduction made the catalyst possess larger specific surface area, superior reducibility, and stronger surface acidity. As a result, α-MnO₂/ZSM-5 with a S/Mn molar ratio of 0.019 exhibited the best toluene conversion and CO_x selectivity, 87 and 94%, respectively, after the reaction for 8 h at 30 °C under an initial concentration of 5 ppm toluene and 45 ppm ozone, relative humidity of 45%, and space velocity of 32,000 h^-1, far superior to those of non-noble catalysts reported to date under comparable reaction conditions. The synergistic role of increased oxygen vacancies and acid sites of α-MnO₂/ZSM-5 modified with SO₄^2- resulted in excellent toluene conversion and CO_x selectivity. The findings represented a critical step toward the rational design and synthesis of highly efficient catalysts for catalytic ozonation of toluene.

Tuning the Micro-coordination Environment of Al in Dealumination Y Zeolite to Enhance Electron Transfer at the Cu-Mn Oxides Interface for Highly Efficient Catalytic Ozonation of Toluene at Low Temperatures

The development of stable, highly active, and inexpensive catalysts for the ozone catalytic oxidation of volatile organic compounds (VOCs) is challenging but of great significance. Herein, the micro-coordination environment of Al in commercial Y zeolite was regulated by a specific dealumination method and then the dealuminated Y zeolite was used as the support of Cu-Mn oxides. The optimized catalyst Cu-Mn/DY exhibited excellent performance with around 95% of toluene removal at 30 °C. Besides, the catalyst delivered satisfactory stability in both high-humidity conditions and long-term reactions, which is attributed to more active oxygen vacancies and acidic sites, especially the strong Lewis acid sites newly formed in the catalyst. The decrease in the electron cloud density around aluminum species enhanced electron transfer at the interface between Cu-Mn oxides. Moreover, extra-framework octahedrally coordinated Al in the support promoted the electronic metal-support interaction (EMSI). Compared with single Mn catalysts, the incorporation of the Cu component changed the degradation pathway of toluene. Benzoic acid, as the intermediate of toluene oxidation, can directly ring-open on Cu-doped catalysts rather than being further oxidized to other byproducts, which increased the rate of the catalytic reaction. This work provides a new insight and theoretical guidance into the rational design of efficient catalysts for the catalytic ozonation of VOCs.

Study on the synergy effect of MnOx and support on catalytic ozonation of toluene

MnO_x has received widespread attention in low-temperature catalytic oxidation of VOCs, however, the synergy effect of MnO_x and support on the VOCs catalytic ozonation were rarely studied. In this study, five different MnO_x/X (X: MCM-41, 13X, ZSM-5, HY, USY) were synthesized and found their support greatly affect the catalytic oxidation activity. MnO_x/MCM-41 presents the largest specific surface area, pore volume and unique surface morphology, and thereby provides more sites for MnO_x loading and VOCs adsorption. Moreover, MnO_x/MCM-41 presents a high proportion of Mn³⁺, which helps to enhance the ion exchange capability, and thus promotes the regeneration of oxygen vacancies. Furthermore, a part of Mn was proved to be introduced into the MCM-41 lattice, which can promote the electron transfer between the active components and the support, and thereby effectively improve the surface electronic properties of the catalyst. The toluene catalytic experiments showed that MnO_x/MCM-41 exhibited the best catalytic activity, presenting complete degradation of O₃ and VOCs at room temperature. In addition, 5 wt%MnO_x/MCM-41 exhibited better catalytic activity than other loading, and its higher surface oxygen species endowed it with strong water resistance and stability. In-situ DRIFTs indicated that toluene was initially oxidized into benzyl alcohol during the adsorption process, and then decomposed to intermediate products (benzaldehyde, phenolate, etc.) during the catalytic ozonation process, and finally oxidized to carbon dioxide. In conclusion, the supply of loading sites and the improvement of interfacial electron transfer are the manifestations of the synergy between the support and MnO_x, leading to the promotion of the catalytic ozonation of VOCs.

Catalytic ozonation of VOCs at low temperature: A comprehensive review

VOCs abatement has attracted increasing interest because of the detrimental effects on both atmospheric environment and human beings of VOCs. The assistance of ozone has enabled efficient VOCs removal at low temperature. Thereby, catalytic ozonation is considered as one of the most feasible and effective methods for VOCs elimination. This work systematically reviews the emerging advances of catalytic ozonation of different VOCs (i.e., aromatic hydrocarbons, oxygenated VOCs, chlorinated VOCs, sulfur-containing VOCs, and saturated alkanes) over various functional catalysts. General reaction mechanism of catalytic ozonation including both Langmuir-Hinshelwood and Mars-van-Krevelen mechanisms was proposed depending on the reactive oxygen species involving the reactions. The influence of reaction conditions (water vapor and temperature) is fully discussed. This review also introduces the enhanced VOCs oxidation via catalytic ozonation in the ozone-generating systems including plasma and vacuum ultraviolet. Lastly, the existing challenges of VOCs catalytic ozonation are presented, and the perspective of this technology is envisioned.

Catalytic ozone oxidation toluene over supported manganese cobalt composite: influence of catalyst support

In this study, the manganese cobalt composite (Mn-Co)-loaded SiO₂, MgO, TiO₂, γ-Al₂O₃ and silicalite-1 were prepared by ultrasonic complexation method. The catalysts were characterized by XRD, BET, SEM, TEM, H₂-TPR and XPS, and the activity of catalytic oxidation of toluene was evaluated. It was found that Mn-Co loaded γ-Al₂O₃ (Mn₂CoO_x/γ-Al₂O₃) exhibited excellent catalytic activity. When the gas hour space velocity (GHSV) was 45,000 h^-1, the removal rate of toluene reached 91.2% within 5.5 h, and the selectivity of CO₂ was 71.10% at ambient temperature. The operation of Mn₂CoO_x/γ-Al₂O₃ at different temperatures was investigated, and the better toluene removal efficiency more than 80% after reacting 9h was obtained at 50 °C. The characterization results showed that better catalytic activity is related to smaller grain size, higher Mn³⁺/Mn⁴⁺ values and the relative content of active oxygen species (O_II + O_III). Increased amounts of low state species easily led to the imbalance of the catalyst surface charge and promoted the formation of more oxygen vacancies.

Investigation into the Phase-Activity Relationship of MnO2 Nanomaterials toward Ozone-Assisted Catalytic Oxidation of Toluene

Manganese dioxide (MnO₂ ), with naturally abundant crystal phases, is one of the most active candidates for toluene degradation. However, it remains ambiguous and controversial of the phase-activity relationship and the origin of the catalytic activity of these multiphase MnO₂ . In this study, six types of MnO₂ with crystal phases corresponding to α-, β-, γ-, ε-, λ-, and δ-MnO₂ are prepared, and their catalytic activity toward ozone-assisted catalytic oxidation of toluene at room temperature are studied, which follow the order of δ-MnO₂  > α-MnO₂  > ε-MnO₂  > γ-MnO₂  > λ-MnO₂  > β-MnO₂ . Further investigation of the specific oxygen species with the toluene oxidation activity indicates that high catalytic activity of MnO₂ is originated from the rich oxygen vacancy and the strong mobility of oxygen species. This work illustrates the important role of crystal phase in determining the oxygen vacancies' density and the mobility of oxygen species, thus influencing the catalytic activity of MnO₂ catalysts, which sheds light on strategies of rational design and synthesis of multiphase MnO₂ catalysts for volatile organic pollutants' (VOCs) degradation.