Detrimental role of residual surface acid ions on ozone decomposition over Ce-modified γ-MnO2 under humid conditions

Effect of Interlayer Anions on NiFe Layered Double Hydroxides for Catalytic Ozone Decomposition

NiFe layered double hydroxides (NiFe-LDH) exhibited an outstanding performance and promising application potential for removing ozone. However, the effect of interlayer anions on ozone removal remains ambiguous. Here, a series of NiFe-LDH with different interlayer anions (F-, Cl-, Br-, NO3-, CO32-, and SO42-) were prepared to investigate the effect of the interlayer anion on ozone removal for the first time. It was found that the interlayer anions are a key factor affecting the water resistance of the NiFe-LDH catalyst under moist conditions. NiFe-LDH-CO32- exhibited the best water resistance, which was much better than that of NiFe-LDH containing other interlayer anions. The in situ DIRFTS demonstrates that the carbonates in the interlayer of NiFe-LDH-CO32- will undergo coordination changes through the interaction with water molecules under moist conditions, exposing new metal sites. As a result, the newly exposed metal sites could activate water molecules into hydroxyl groups that act as active sites for catalyzing ozone decomposition. This work provides a new insight into the interlayer anions of LDH, which is important for the design and development of LDH catalysts with excellent ozone removal properties.

Enhanced water resistance mechanism in Ag-Hollandite for catalytic ozone decomposition

Catalytic ozone (O3) decomposition at ambient temperature is an efficient method to mitigate O3 pollution. However, practical application is hindered by the poor water resistance of catalysts. Herein, Ag-Hollandite (Ag-HMO) with varying Ag+ content was synthesized. Catalysts with more Ag+ exhibited improved efficiency and water-resistance, with the optimal one maintaining 98% O3 conversion at 70% relative humidity (RH) within 8 h. Physicochemical characterizations revealed that Ag+ had entered the tunnel of OMS-2, facilitating oxygen species removal. Notably, enhanced H2O desorption and the complete inhibition of chemisorbed water formation on Ag-HMO were the primary reasons for its high-efficiency O3 conversion across a wide humidity range. The underlying mechanism arises from the charge redistribution induced by the Ag-O interaction within the tunnel, which reduces acidity and modulates hydrophilicity. This study aims to contribute insights for designing catalysts with higher water-resistance.

Coordination-Controlled Catalytic Activity of Cobalt Oxides for Ozone Decomposition

Nowadays, it is still elusive and challenging to discover the active sites of cobalt (Co) cations in different coordination structures, though Co-based oxides show their great potency in catalytic ozone elimination for air cleaning. Herein, different Co-based oxides are controllably synthesized including hexagonal wurtzite CoO-W with Co²⁺ in tetrahedral coordination (Co_Td²⁺) and CoAl spinel with dominant Co_Td²⁺, cubic rock salt CoO-R with Co²⁺ in octahedral coordination (Co_Oh²⁺), MgCo spinel with dominant Co³⁺ in octahedral coordination (Co_Oh³⁺), and Co₃O₄ with mixed Co_Td²⁺ and Co_Oh³⁺. The valences are proved by X-ray photoelectron spectroscopy, and the coordinations are verified by X-ray absorption fine structure analysis. The ozone decomposition performances are Co_Oh³⁺ ∼ Co_Oh²⁺ ≫ Co_Td²⁺, and Co_Oh³⁺ and Co_Oh²⁺ show a lower apparent activation energy of ∼42-44 kJ/mol than Co_Td²⁺ (∼55 kJ/mol). In specific, MgCo shows the highest decomposition efficiency of 95% toward 100 ppm ozone at a high space velocity of 1,200,000 mL/gh, which still retains at 80% after a long-term running of 36 h at room temperature. The high activity is explained by the d-orbital splitting in the octahedral coordination, favoring the electron transfer in ozone decomposition reactions, which is also verified by the simulation. These results show the promising prospect of the coordination tuning of Co-based oxides for highly active ozone decomposition catalysts.

Heterojunctioned CuO/Cu2O catalyst for highly efficient ozone removal

In recent years, near surface ozone pollution, has attracted more and more attention, which necessitates the development of high efficient and low cost catalysts. In this work, CuO/Cu₂O heterojunctioned catalyst is fabricated by heating Cu₂O at high temperature, and is adopted as ozone decomposition catalyst. The results show that after Cu₂O is heated at 180°C conversion of ozone increases from 75.2% to 89.3% at mass space velocity 1,920,000 cm³/(g·hr) in dry air with 1000 ppmV ozone, which indicates that this heterojunction catalyst is one of the most efficient catalysts reported at present. Catalysts are characterized by electron paramagnetic resonance spectroscopy and ultraviolet photoelectron spectroscopy, which confirmed that the heterojunction promotes the electron transfer in the catalytic process and creates more defects and oxygen vacancies in the CuO/Cu₂O interfaces. This procedure of manufacturing heterostructures would also be applicable to other metal oxide catalysts, and it is expected to be more widely applied to the synthesis of high-efficiency heterostructured catalysts in the future.

Ozone catalytic oxidation of low-concentration formaldehyde over ternary Mn-Ce-Ni oxide catalysts modified with FeOx

Manganese oxide-based catalysts have attracted extensive attention due to their relatively low cost and remarkable performance for removing VOCs. In this research, we used the Pechini method to synthesize manganese-cerium-nickel ternary oxide catalysts (MCN) and evaluated the effectiveness of catalytic destruction of formaldehyde (HCHO) and ozone at room temperature. FeO_x prepared by the impregnation method was applied to modify the catalyst. After FeO_x treatment, the catalyst represented the best performance on both HCHO destruction and ozone decomposition under dry conditions and exhibited excellent water vapor resistance. The as-prepared catalysts were next characterized via H₂-temperature programmed reduction (H₂-TPR), temperature programmed desorption of O₂ (O₂-TPD), and X-ray photoelectron spectroscopy (XPS), and the results demonstrated that addition of FeO_x increased Mn³⁺ and Ce³⁺ concentrations, oxygen vacancies and surface lattice oxygen species, facilitated adsorption, and redox properties. Based on the results of in situ diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), possible mechanisms of ozone catalytic oxidation of HCHO were proposed. Overall, the ternary mixed-oxide catalyst developed in this study holds great promise for HCHO and ozone decomposition in the indoor environment.

Exploring an electric-aid ozone decomposition mode to enhance water resistance over manganese oxide monolith catalyst under high humidity

In this work, a facile, green, and effective reaction mode of electric-aid ozone decomposition (EAOD) was developed over a manganese-based monolith catalyst for eliminating ozone under high humidity. The catalyst was prepared by directly growing α-MnO₂ nanorods on Al honeycomb substrate (MnO₂/Al) via a simple hydrothermal process, and the EAOD mode was performed just by connecting the MnO₂/Al monolith catalyst with a DC power supply during ozone decomposition reaction. In the EAOD mode reaction, the MnO₂/Al catalyst exhibited a stable ozone conversion efficiency of over 82 % and excellent stability over 720 min under a relative humidity of 90%, well beyond the performance of catalyst in the conventional ozone decomposition reaction without the help of electric aid. Here, the water evaporation by the external electric field generated from the EAOD mode hinders the competitive adsorption of water vapor on the active sites of MnO₂/Al catalyst, consequently enhances its water resistance. Moreover, increasing input electric current of the DC power supply could further improve the catalytic activity and stability of the monolith catalyst for ozone decomposition in EAOD mode reaction.

Influence of water molecule on active sites of manganese oxide-based catalysts for ozone decomposition

Developing an efficient approach to decompose ground-level O₃ in humidity is crucial for preventing O₃ pollution in practical application scenes. In this study, MnO_x, CuO, and Cu/MnO_x were synthesized to investigate the influence of H₂O on the variation of active sites during O₃ decomposition. The structural characterizations of the as-synthetic catalysts were measured by N2 physisorption, XRD, SEM, O₂-TPD, H₂-TPR, TG, and FT-IR analyses. In dry conditions, the elimination rate of O₃ followed the sequence of MnO_x > Cu/MnO_x > CuO. The introduction of Cu to MnO_x enhanced the surface area and pore volume of Cu/MnO_x, accordingly diminishing the amounts of surface defects and the participation of sub-surface lattice oxygen for catalytic cycle, indicating that surface defects and oxygen vacancies (V_O) determined the catalytic activity for O₃ decomposition. In humid conditions, the elimination rate of O₃ changed to the sequence of Cu/MnO_x > MnO_x > CuO, with a variation rate compared to dry conditions of -62.9% for MnO_x, 14.2% for CuO, and 27.7% for Cu/MnO_x. The decrease of participant sub-surface lattice oxygen and the accumulation of intermediates in humidity diminished the decomposition of O₃ on MnO_x, while the active species such as superoxide radicals generating from the reaction of H₂O and Cu/MnO_x facilitated the participation of V_O and the desorption of O₂ from the occupied active sites, accelerating the catalytic cycle on Cu/MnO_x. This work developed a deeper understanding of the influence of H₂O on catalytic activity, promoting the performance of MnO_x-based catalysts for practical O₃ decomposition.

Layered Double Hydroxide Catalysts for Ozone Decomposition: The Synergic Role of M2+ and M3

In previous work, we successfully prepared a NiFe-layered double hydroxide (LDH) with superior activity and stability for catalytic ozone decomposition, which fundamentally avoids deactivation under high-humidity conditions. However, the role of the metal elements (M²⁺ and M³⁺) in LDH catalysts is not clear. Here, LDH materials containing different metals (NiFe, NiAl, NiMn, CoFe, and MgFe) were prepared by a simple co-precipitation method. It was found that the LDHs containing Ni²⁺ exhibited catalytic performance far superior to that of Co²⁺ and Mg²⁺ for ozone elimination, and NiFe-LDH had the best activity and stability among LDH materials prepared in this study. The NiFe-LDH can maintain 78% catalytic activity within 144 h at room temperature, even under a relative humidity of 65% and a space velocity of 840 L·g^-1·h^-1. Physicochemical characterizations demonstrated that chemical stability in an oxidizing atmosphere and the synergic role of M²⁺ and M³⁺ ions are crucial. The result of density functional theory calculation showed that the synergic role of Ni²⁺ and Fe³⁺ weakens the interaction between O and H in the O-H bond, which effectively lowers the reaction barrier of ozone decomposition compared with MgFe-LDH.

A novel Fe-Co double-atom catalyst with high low-temperature activity and strong water-resistant for O3 decomposition: A theoretical exploration

Developing catalysts with high activity, durability, and water resistance for ozone decomposition is crucial to regulate the pollution of ozone in the troposphere, especially in indoor air. To overcome the shortcomings of metal oxide catalysts with respect to their durability and water resistance, Fe-Co double-atom catalyst (DAC) is proposed as a novel catalyst for ozone decomposition. Here, through a systematic study using density functional theory (DFT) calculations and microkinetic modeling, the adsorption and catalytic decomposition of O₃ on Fe-Co DAC have been examined based on adsorption configuration, orbital hybridization, and electron transfer. Based on Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction mechanisms, the mechanisms of ozone decomposition on Fe-Co DAC were explored by analyzing reaction paths and energy variations. To confirm the water-resistant of Fe-Co DAC, competitive adsorption behavior between O₃ and dominant environmental gases was discussed through ab initio molecular dynamic (AIMD) simulation. The dominant reaction mechanism of ozone decomposition is L-H and the rate-determining step is the desorption of the first oxygen molecule from the surface of Fe-Co DAC which has an energy barrier of 0.78 eV. Due to this relatively low energy barrier and high turnover frequency (TOF), the optimal operation window of catalytic O₃ decomposition on Fe-Co DAC is <500 K suggesting that catalytic decomposition of O₃ on Fe-Co DAC can occur at room temperature. This theoretical study provides new insights for designing novel catalysts for ozone decomposition and fundamental guidance for subsequent experimental research.

High performance ozone decomposition over MnAl-based mixed oxide catalysts derived from layered double hydroxides

Mesoporous and dispersed MnAl-based mixed metal oxide catalysts (Mn_xAlO) were fabricated via the calcination of layered double hydroxide (LDH) precursors prepared by the coprecipitation method. Their physiochemical properties were characterized and their catalytic activities for ozone decomposition were evaluated. The results indicate that the prepared Mn_xAlO catalysts have excellent catalytic activity owing to their large specific surface area, abundant surface oxygen vacancies and lower average Mn oxidation states. The Mn/Al atomic ratio and calcination temperature are found to significantly affect the textural properties and catalytic activity for ozone decomposition. The Mn₂AlO-400 catalyst (Mn/Al = 2, calcined at 400 °C) exhibited 84.8% ozone conversion after 8 h reaction under an initial ozone concentration of 45 ± 2 ppm, 30 ± 1 °C, a relative humidity of 50% ± 3%, and a space velocity of 550 000 h⁻¹. The results also show that the catalytic activity of Mn₂AlO-400, which was deactivated owing to the accumulation of oxygen-related intermediates, was recovered by calcination at 400 °C under a N₂ atmosphere for 1 h. A possible reason for catalyst deactivation and regeneration is proposed. This work provides a facile method for fabricating Mn_xAlO catalysts with excellent characteristics to achieve better catalytic activity, which are promising candidates for practical ozone decomposition.

Mesoporous and highly dispersed MnAl-based mixed metal oxide catalysts (Mn_xAlO) were fabricated via the calcination of layered double hydroxides (LDHs), which presented excellent catalytic activity for ozone decomposition.

Engineering of Mn3O4@Ag microspheres assembled from nanosheets for superior O3 decomposition

A topochemical transformation route was designed for synthesis of Mn3O4 nanospheres using β-MnOOH as the precursor. Ag nanoparticles were doped via an in situ redox reaction to obtain Mn3O4@Ag-NF, which displayed an enhanced performance for O3 elimination.

Ce-promoted Mn/ZSM-5 catalysts for highly efficient decomposition of ozone

Manganese oxides supported by ZSM-5 zeolite (Mn/ZSM-5) as well as their further modified by Ce promoter were achieved by simple impregnation method for ozone catalytic decomposition. The yCe20Mn/ZSM-5-81 catalyst with 8% Ce loading showed the highest catalytic activity at relative humidity of 50% and a space velocity of 360 L/(g × hr), giving 93% conversion of 600 ppm O₃ after 5 hr. Moreover, this sample still maintained highly activity and stability in humid air with 50%-70% relative humidity. Series of physicochemical characterization including X-ray diffraction, temperature-programmed technology (NH₃-TPD and H₂-TPR), X-ray photoelectron spectroscopy and oxygen isotopic exchange were introduced to disclose the structure-performance relationship. The results indicated that moderate Si/Al ratio (81) of zeolite support was beneficial for ozone decomposition owing to the synergies of acidity and hydrophobicity. Furthermore, compared with 20Mn/ZSM-5-81, Ce doping could enhance the amount of low valance manganese (such as Mn²⁺ and Mn³⁺). Besides, the Ce³⁺/Ce⁴⁺ ratio of 8Ce20Mn/ZSM-5-81 sample was higher than that of 4Ce20Mn/ZSM-5-81. Additionally, the synergy between the MnO_x and CeO₂ could easily transfer electron via the redox cycle, thus resulting in an increased reducibility at low temperatures and high concentration of surface oxygen. This study provides important insights to the utilization of porous zeolite with high surface area to disperse active component of manganese for ozone decomposition.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

Recent advances in catalytic decomposition of ozone

Ozone (O₃), as a harmful air pollutant, has been of wide concern. Safe, efficient, and economical O₃ removal methods urgently need to be developed. Catalytic decomposition is the most promising method for O₃ removal, especially at room temperature or even subzero temperatures. Great efforts have been made to develop high-efficiency catalysts for O₃ decomposition that can operate at low temperatures, high space velocity and high humidity. First, this review describes the general reaction mechanism of O₃ decomposition on noble metal and transition metal oxide catalysts. Then, progress on the O₃ decomposition performance of various catalysts in the past 30 years is summarized in detail. The main focus is the O₃ decomposition performance of manganese oxides, which are divided into supported manganese oxides and non-supported manganese oxides. Methods to improve the activity, stability, and humidity resistance of manganese oxide catalysts for O₃ decomposition are also summarized. The deactivation mechanisms of manganese oxides under dry and humid conditions are discussed. The O₃ decomposition performance of monolithic catalysts is also summarized from the perspective of industrial applications. Finally, the future development directions and prospects of O₃ catalytic decomposition technology are put forward.

Unravelling the efficient catalytic performance of ozone decomposition over nitrogen-doped manganese oxide catalysts under high humidity

Nitrogen-doped Mn species, coated with a carbon layer of several nanometers in thickness, for enhanced water vapor resistance.