A novel γ-like MnO2 catalyst for ozone decomposition in high humidity conditions

Synergistic effects of copper and oxygen vacancies in enhancing the efficacy of partially crystalline CuMnxOy catalyst for ozone decomposition

To tackle the challenge of ground-level ozone pollution, this study proposed a potential catalytic design approach for ozone decomposition using Cu-Mn bimetallic oxide. This approach is grounded in an understanding of the intrinsic reactivity for catalyst and incorporates a novel potassium-driven low-temperature oxidation process for catalyst synthesis. The research highlights the creation of a highly reactive Cu-Mn oxide phase with extensive defect coverage, leading to significantly increased reaction rates. It also identifies the MnO2(100) facet as a crucial active phase, where oxygen vacancies simultaneously enhance O3 adsorption and decomposition, albeit with a concurrent risk of O2 poisoning due to the stabilization of adsorbed O2. Crucially, the incorporation of Cu offsets the effects of oxygen vacancies, influencing conversion rates and lessening O2 poisoning. The synergistic interplay between Cu and oxygen vacancies elevates the performance of the defect-rich Cu-Mn oxide catalyst. By combining computational and experimental methods, this study not only advances the understanding of the Cu-Mn oxide system for ozone decomposition but also contributes valuable insights into developing more efficient catalysts to mitigate ozone pollution.

Graphitized Carbon-Supported Co@Co3O4 for Ozone Decomposition over the Entire Humidity Range

Ground-level ozone (O3) pollution has emerged as a significant concern due to its detrimental effects on human health and the ecosystem. Catalytic removal of O3 has proven to be the most efficient and cost-effective method. However, its practical application faces substantial challenges, particularly in relation to its effectiveness across the entire humidity range. Herein, we proposed a novel strategy termed "dual active sites" by employing graphitized carbon-loaded core-shell cobalt catalysts (Co@Co3O4-C). Co@Co3O4-C was synthesized via the pyrolysis of a Co-organic ligand as the precursor. By utilizing this approach, we achieved a nearly constant 100% working efficiency of the Co@Co3O4-C catalyst for catalyzing O3 decomposition across the entire humidity range. Physicochemical characterization coupled with density functional theory calculations elucidates that the presence of encapsulated metallic Co nanoparticles enhances the reactivity of the cobalt oxide capping layer. Additionally, the interface carbon atom, strongly influenced by adjacent metallic Co nuclei, functions as a secondary active site for the decomposition of O3 decomposition. The utilization of dual active sites effectively mitigates the competitive adsorption of H2O molecules, thus isolating them for adsorption in the cobalt oxide capping layer. This optimized configuration allows for the decomposition of O3 without interference from moisture. Furthermore, O3 decomposition monolithic catalysts were synthesized using a material extrusion-based three-dimensional (3D) printing technology, which demonstrated a low pressure drop and exceptional mechanical strength. This work provides a "dual active site" strategy for the O3 decomposition reaction, realizing O3 catalytic decomposition over the entire humidity range.

Manganese-oxide-supported gold catalyst derived from metal-organic frameworks for trace PCl3 oxidation in an organic system

Polysilicon is widely used in the field of semiconductors and solar energy. Trichlorosilane feedstocks that are used to produce polysilicon in the mainstream production process contain PCl3 impurities that have adverse effects on the quality of the polysilicon. Traditional methods for dephosphorization cannot achieve the effect of complete removal, whereas oxidizing PCl3 to POCl3 in the presence of oxygen for removal via adsorption is a promising and appealing route for establishing a dephosphorization process; it has a high phosphorous removal rate due to the strong Lewis-base property of POCl3 in comparison with PCl3. In this work, we synthesized an active catalyst with an active interface between Au nanoparticles (NPs) and a manganese-oxide support (Mn3O4) by calcination of a corresponding composite, where Au NPs were embedded uniformly in a metal-organic framework (MOF). The catalyst shows a significantly active catalytic performance for trace PCl3 oxidation in an organic system that is an imitation of a trichlorosilane system, with a 99.13% yield of POCl3 in an 80 °C and 0.6 MPa reaction environment. The structure-performance-mechanism analysis shows that the possible reaction and catalytic mechanism is PCl3 oxidation by interface lattice oxygens, which bridge the Au NPs and the support, in a Mars van Krevelen (MvK) process; this process was promoted by the interaction between the Au NPs and Mn3O4 in terms of charge transfer and chemical potential changes. This work provides an effective way to dephosphorize trichlorosilane feedstocks in the polysilicon industry and gives guidance for constructing an efficient catalyst via the study of the structure and mechanism.

Highly efficient removal of ozone by amorphous manganese oxides synthesized with a simple hydrothermal method

Amorphous manganese oxides (MnOx) were synthesized by facile hydrothermal reactions between potassium permanganate and manganese acetate. Synthesis parameters, including hydrothermal time and temperature and molar ratio of precursors, significantly affected the ozone removal performance and structure property of MnOx. Amorphous MnOx-1.5, which was prepared at the Mn2+/Mn7+ molar ratio of 1.5 under hydrothermal conditions of 120°C and 2 hr, showed the highest ozone removal rate of 93% after 480 min at the room temperature, RH (relative humidity) = 80% and WHSV (weight hourly space velocity) = 600 L/(g·hr). The morphology, composition and structure of catalysts were investigated with X-ray diffractometer (XRD), Raman spectra, N2 physisorption, field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), O2 temperature-programmed desorption (O2-TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). It was confirmed that high catalytic activity of amorphous MnOx for ozone removal was mainly ascribed to its abundant oxygen vacancies, high oxygen mobility and large specific surface area.

Synthetic effect of supports in Cu-Mn-doped oxide catalysts for promoting ozone decomposition under humid environment

The escalating levels of surface ozone concentration pose detrimental effects on public health and the environment. Catalytic decomposition presents an optimal solution for surface ozone removal. Nevertheless, catalyst still encounters challenges such as poisoning and deactivation in the high humidity environment. The influence of support on catalytic ozone decomposition was examined at a gas hourly space velocity of 300 L·g-1·h-1 and 85% relative humidity under ambient temperature using Cu-Mn-doped oxide catalysts synthesized via a straightforward coprecipitation method. Notably, the Cu-Mn/SiO2 catalyst exhibited remarkable performance on ozone decomposition, achieving 98% ozone conversion and stability for 10 h. Further characterization analysis indicated that the catalyst's enhanced water resistance and activity could be attributed to factors such as an increased number of active sites, a large surface area, abundant active oxygen species, and a lower Mn oxidation state. The catalytic environment created by mixed oxides can offer a clearer understanding of their synergistic effects on catalytic ozone decomposition, providing significant insights into the development of water-resistant catalysts with superior performance.

Rapid Ozone Decomposition over Water-activated Monolithic MoO3 /Graphdiyne Nanowalls under High Humidity

Catalytic ozone (O3 ) decomposition at high relative humidity (RH) remains a great challenge due to the catalysts poison and deactivation under high humidity. Here, we firstly elaborate the role of water activation and the corresponding mechanism of the promoted O3 decomposition over the three-dimensional monolithic molybdenum oxide/graphdiyne (MoO3 /GDY) catalyst. The O3 decomposition over MoO3 /GDY reaches up to 100 % under high humid condition (75 % RH) at room temperature, which is 4.0 times as high as that of dry conditions, significantly surpasses other carbon-based MoO3 materials(≤7.1 %). The sp-hybridized carbon in GDY donates electrons to MoO3 along the C-O-Mo bond, facilitating water activation to form hydroxyl species. As a result, hydroxyl species dissociated from water act as new active sites, promoting the adsorption of O3 and the generation of new intermediate species (hydroxyl ⋅OH and superoxo ⋅O2 - ), which significantly lowers the energy barriers of O3 decomposition (0.57 eV lower than dry conditions).

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.

α-Fe2O3-supported Co3O4 nanoparticles to construct highly active interfacial oxygen vacancies for ozone decomposition

Ozone pollution has become one of the most concerned environmental issue. Developing low-cost and efficient catalysts is a promising alternative for ozone decomposition. This work presents a creative strategy that using α-Fe₂O₃-supported Co₃O₄ nanoparticles for constructing interfacial oxygen vacancies (Vo) to remove ozone. The efficiency of Co₃O₄/α-Fe₂O₃ was superior to that of pure α-Fe₂O₃ by nearly two times for 200-ppm ozone removal after 6-h reaction at 25 °C, which is ascribed to the highly active interfacial Vo. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy suggest that the Fe³⁺-Vo-Co²⁺ was formed when Co₃O₄ was loaded in α-Fe₂O₃. Furthermore, the density functional theory (DFT) calculations reveal the desorption and electron transfer ability of intermediate peroxide (O₂^2-) on Fe³⁺-Vo-Co²⁺ are higher than the Vo from other regions. In situ diffuse reflectance Fourier transform (DRIFT) spectroscopy also demonstrate the higher conversion rate of O₂^2- on Co₃O₄/α-Fe₂O₃. Base on the intermediates detected, we propose a recycle mechanism of interfacial Vo for ozone removal: O₂^2- is quickly converted to O₂^- and transformed into O₂ on interfacial Vo. Moreover, O₂-temperature-programmed desorption (TPD), H₂-temperature-programmed reduction (TPR), and electrochemical impedance spectroscopy (EIS) reveal that the oxygen mobility, reducibility, and conductivity of Co₃O₄/α-Fe₂O₃ are greatly superior to those of α-Fe₂O₃, which is contributed to the conversion of O₂^2-. Consequently, our proposed strategy effectively enhances the activity and stability of the bimetallic transition oxides for ozone decomposition.

Different degradation mechanisms of low-concentration ozone for MIL-100(Fe) and MIL-100(Mn) over wide humidity fluctuation

The synergistic control of ozone and fine particulate matter is a research hotspot in the current environmental fields. Among the ozone removal, wide humidity fluctuation and low concentration dynamic adsorption are two thorny problems. In this work, MIL-100(Fe) and MIL-100(Mn), synthesized by hydrothermal and solvothermal methods respectively, were selected to investigate the degradation of flowing ozone pollutants. The samples showed different ozone degradation mechanisms, namely photocatalytic degradation and normal temperature degradation. Notably, MIL-100(Fe) exhibited more outstanding photocatalytic activity than MIL-100(Mn), while the normal temperature catalytic efficiency of MIL-100(Mn) was much superior to MIL-100(Fe). For different humidity conditions, MIL-100(Fe) has the optimal photocatalytic performance at 10% humidity, which is 38%, while MIL-100(Mn) has basically no change in normal temperature catalytic degradation efficiency at different humidity levels of 10-90%. Furthermore, the degradation mechanism was proposed by in-situ DRIFTS and ESR, which was significantly correlated with oxygen vacancy and photogenerated electron efficiency. By the aid of Temperature Programmed Desorption (TPD), a large quantity of Lewis acid sites was detected in MIL-100(Mn), which was the critical factor that the selected materials could maintain excellent normal temperature degradation performance under high humidity. This work will expand the practical application of ozone removal and improve the degradation efficiency.

Evaluation of the Flexibility for Catalytic Ozonation of Dichloromethane over Urchin-Like CuMnOx in Flue Gas with Complicated Components

The evaluation of the poisoning effect of complex components in practical gas on DCM (dichloromethane) catalytic ozonation is of great significance for enhancing the technique's environmental flexibility. Herein, Ca, Pb, As, and NO/SO₂ were selected as a typical alkaline-earth metal, heavy metal, metalloid, and acid gas, respectively, to evaluate their interferences on catalytic behaviors and surface properties of an optimized urchin-like CuMn catalyst. Ca/Pb loading weakens the formation of oxygen vacancies, oxygen mobility, and acidity due to the fusion of Mn-Ca/Pb-O, leading to their inferior catalytic performance with poor CO₂ selectivity and mineralization rate. Noticeably, the presence of As induces excessively strong acidity, facilitating the inevitable formation of byproducts. Catalytic co-ozonation of NO/DCM is achieved with stoichiometric ozone addition. Unfortunately, SO₂ introduction brings irreversible deactivation due to strong competition adsorption and the loss of active sites. Unexpectedly, Ca loading protects active sites from an attack by SO₂. The formation of unstable sulfites and the released Mn-O structure offset the negative effect from SO₂. Overall, the catalytic ozonation of DCM exhibits a distinctive priority in the antipoisoning of metals with the maintenance of DCM conversion. The construction of more stable acid sites should be the future direction of catalyst design; otherwise, catalytic ozonation should be arranged together with post heavy metal capture and a deacidification system.

Catalytic ozonation of CH2Cl2 over hollow urchin-like MnO2 with regulation of active oxygen by catalyst modification and ozone promotion

This paper firstly reported efficient catalytic ozonation of CH₂Cl₂ (dichloromethane, DCM) at low temperature over hollow urchin-like MnO₂ with high chlorine resistance. Regulations on morphologies and Cu doping, as well as ozone promotion were conducted to optimize active oxygen of MnO₂ catalysts, contributing to excellent catalytic behaviors. Cu doping MnO₂ with hollow urchin-like morphology attained a stable 100% DCM conversion with O₃/DCM molar ratio of 10 at 120 °C. The ozone utilization rate, final products, and byproducts distribution were discussed. Abundant crystal defects, low-valance Mn/Cu, O_ads, and weak acidity, as well as better low temperature reducibility contributed to its superior performance. During DCM catalytic ozonation, DCM oxidation exhibited competitive effect on O₃ decomposition due to the occupation of intermediates (CH₂ClO₃·, O-CH₂Cl, and O-CH₂ -O) over active sites that should belong to O₃ originally. Nevertheless, O₃ decomposition exhibited synergistic effects on DCM oxidation with promotion on active oxygen. Density functional theory (DFT) calculations confirmed the positive effect on oxygen vacancy formation and O₃/DCM adsorption from Cu doping. The possible mechanism for DCM catalytic ozonation included four parts, including O₃/DCM adsorption, O₃ activation, DCM oxidation, and electron replenishment. This paper provides new insight for catalytic elimination of chlorinated alkanes at mild conditions.

Low-Temperature O3 Decomposition over Pd-TiO2 Hybrid Catalysts

In aircraft and spacecraft, outside air is not directly fed to the passenger because it contains ozone at elevated altitudes. The decomposition of low concentration ozone in the air was carried out at 25 °C by catalytic oxidation on Pd-based catalysts supported on a high surface area hybrid TiO2. The use of these hybrid catalysts has shown a beneficial effect, both on the catalytic activity and on the catalyst stability. Kinetic studies showed that the most promising catalytic phase (Pd/TiO2_100) was the one obtained from the TiO2 support containing the lowest content of citrate ligands and leading to small Pd particles (around 4 nm). The effect of catalyst synthesis on the decomposition of O3 gas (15 ppm) in a dry and humid (HR = 10%) stream in a closed environment such as aircraft or spacecraft was also investigated in this study and further elucidated by detailed characterizations. It was shown that the system could be used as an effective treatment for air coming from outside.

Relationship between Surface Hydroxyl Complexation and Equi-Acidity Point pH of MnO2 and Its Adsorption for Co2+ and Ni2

MnO₂ has shown great potential in the field of adsorption and has a good adsorption effect on heavy metal ions in aqueous solution, but there have been problems in the adsorption of heavy metal ions in high-concentration metal salt solutions. In this paper, different crystal forms of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂, δ₁-MnO₂, δ₂-MnO₂, and ε-MnO₂) were prepared and characterized by XRD, SEM, EDS, XPS, ZETA, and FT-IR. The reasons for the equi-acidity point pH change of MnO₂ and the complex mechanism of surface hydroxylation on metal ions were discussed. The results showed that the equi-acidity point pHs of different crystalline MnO₂ were different. The equi-acidity point pH decreased with the increase of reaction temperature and electrolyte concentration, but the reaction time had no effect on it. The equi-acidity point pHs of MnO₂ were essentially equal to the equilibrium pH values of adsorption and desorption between surface hydroxyl and metal ions on them. The change of equi-acidity points was mainly due to the complexation of surface hydroxyl, and the equi-acidity point pHs depended on the content of surface hydroxyl and the size of the complexation ability. According to the equi-acidity point pH characteristics of MnO₂, more hydroxyl groups could participate in the complexation reaction by repeatedly controlling the pH, so that MnO₂ could adsorb heavy metals Co²⁺ and Ni²⁺ in high-concentration MnSO₄ solution, and the adsorption rates of Co²⁺ and Ni²⁺ could reach 96.55 and 79.73%, respectively. The effects of MnO₂ dosage and Mn²⁺ concentration on the adsorption performance were further investigated, and the products after MnO₂ adsorption were analyzed by EDS and FT-IR. A new process for MnO₂ to adsorb heavy metals Co²⁺ and Ni²⁺ in high-concentration MnSO₄ solution was explored, which provided a reference for the deep purification of manganese sulfate solutions.