Ag-modified H-Beta, H-MCM-41 and SiO2: Influence of support, acidity and Ag content in ozone decomposition at ambient temperature

Enhancing ozone catalytic decomposition through acid treatment of α-MnO2 for improved activity and humidity resistance

Post-etching method using dilute acid solutions is an effective technology to modulate the surface compositions of metal-oxide catalysts. Here the α-MnO2 catalyst treated with 0.1 mol/L nitric acid exhibits higher ozone decomposition activity at high relative humidity than the counterpart treated with acetic acid. Besides the increases in surface area and lattice dislocation, the improved activity can be due to relatively higher Mn valence on the surface and newly-formed Brønsted acid sites adjacent to oxygen vacancies. The remnant nitro species deposited on the catalyst by nitric acid treatment is ideal hydrophobic groups at ambient conditions. The decomposition route is also proposed based on the DRIFTS and DFT calculations: ozone is facile to adsorb on the oxygen vacancy, and the protonic H of Brønsted acid sites bonds to the terminal oxygen of ozone to accelerate its cleavage to O2, reducing the reaction energy barrier of O2 desorption.

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.

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

MnO₂ catalysts have been widely studied for catalytic gaseous ozone decomposition. However, their poor moisture resistance often leads to undesirable catalytic effects in the presence of high humidity. In this study, a novel catalyst with γ-like MnO₂ was synthesized using the selective dissolution method on LaMnO₃ perovskites. The as-prepared catalyst exhibited quite stable ozone conversion of ~90% within 12 h under 75% relative humidity (400-800 ppm of ozone, 30 °C, 150 000 mL·g^-1·h^-1 of WHSV). In contrast, traditional γ-MnO₂ catalyst showed deficient resistance to H₂O and sensitivity to space velocity. Detailed characterizations showed that the larger number of oxygen vacancies induced by structure reconstruction of the γ-like MnO₂ and residual La³⁺ cations facilitated ozone decomposition in humid atmosphere. Finally, the reaction rate of ozone decomposition was proposed by a kinetic study, which further proved that the amount and hydrophilicity of oxygen vacancies are the determinants of the first-order reaction rate constant.

Terminal Hydroxyl Groups on Al2O3 Supports Influence the Valence State and Dispersity of Ag Nanoparticles: Implications for Ozone Decomposition

Ozone is a poisonous gas, so it is necessary to remove excessive ozone in the environment. Catalytic decomposition is an effective way to remove ozone at room temperature. In this work, 10%Ag/nano-Al₂O₃ and 10%Ag/AlOOH-900 catalysts were synthesized by the impregnation method. The 10%Ag/nano-Al₂O₃ catalyst showed 89% ozone conversion for 40 ppm O₃ for 6 h under a space velocity of 840 000 h^-1 and a relative humidity of 65%, which is superior to 10%Ag/AlOOH-900 (45% conversion). The characterization results showed Ag nanoparticles to be the active sites for ozone decomposition, which were more highly dispersed on nano-Al₂O₃ as a result of the greater density of terminal hydroxyl groups. The understanding of the dispersion and valence of silver species gained in this study will be beneficial to the design of more efficient supported silver catalysts for ozone decomposition in the future.

Tuning the Chemical State of Silver on Ag-Mn Catalysts to Enhance the Ozone Decomposition Performance

Ag-Mn catalysts with excellent water resistance and ozone decomposition activity were successfully synthesized by simple precipitation and impregnation methods. Under a relative humidity of 65% and space velocity of 840,000 h^-1, the 6%Ag/α-Mn₂O₃-I catalyst showed 99% conversion of 40 ppm O₃ after 6 h, which was far superior to the performance of the 6%AgMnO_x-C (49%), 6%Ag/MnCO₃-I (32%), and α-Mn₂O₃ (5%) catalysts. Physicochemical characterization indicated that the chemical state of Ag on the Ag-Mn catalysts determined the O₃ decomposition activity of the catalysts. The Ag species on the 6%Ag/α-Mn₂O₃-I catalyst were mainly metallic silver nanoparticles (Ag_n⁰), which exhibited much better ozone decomposition performance than the Ag_1.8Mn₈O₁₆ and oxidized silver clusters (Ag_n^δ+) existing on the 6%Ag/MnCO₃-I and 6%AgMnO_x-C catalysts. The 6%Ag/α-Mn₂O₃-I catalyst still had above 85% ozone conversion after 60 h under a relative humidity of 65% and space velocity of 840,000 h^-1. The slight deactivation of the catalyst was ascribed to the oxidation of Ag_n⁰, and its activity could be completely recovered by treatment at 350 °C under an N₂ atmosphere, which indicated that it is a promising catalyst for ozone decomposition. This research provides guidance for the subsequent development of Ag-Mn catalysts for ozone decomposition with high activity.

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.

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

In the study, the catalyst precursors of Ce-modified γ-MnO₂ were washed with deionized water until the pH value of the supernatant was 1, 2, 4 and 7, and the obtained catalysts were named accordingly. Under space velocity of 300,000 hr^-1, the ozone conversion over the pH = 7 catalyst under dry conditions and relative humidity of 65% over a period of 6 hr was 100% and 96%, respectively. However, the ozone decomposition activity of the pH = 2 and 4 catalysts distinctly decreased under relative humidity of 65% compared to that under dry conditions. Detailed physical and chemical characterization demonstrated that the residual sulfate ions on the pH = 2 and 4 catalysts decreased their hydrophobicity and then restrained humid ozone decomposition activity. The pH = 2 and 4 catalysts had inferior resistance to high space velocity under dry conditions, because the residual sulfate ion on their surface reduced their adsorption capacity for ozone molecules and increased their apparent activation energies, which was proved by temperature programmed desorption of O₂ and kinetic experiments. Long-term activity testing, X-ray photoelectron spectroscopy and density functional theory calculations revealed that there were two kinds of oxygen vacancies on the manganese dioxide catalysts, one of which more easily adsorbed oxygen species and then became deactivated. This study revealed the detrimental effect of surface acid ions on the activity of catalysts under humid and dry atmospheres, and provided guidance for the development of highly efficient catalysts for ozone decomposition.

Transition metals-incorporated zeolites as environmental catalysts for indoor air ozone decomposition

The present study aimed to prepare catalysts of Fe- and Cu-loaded zeolite via ion-exchange technique using dilute solutions of metal nitrate precursors followed by calcination at 600°C in the air for 4 h. Commercial zeolite ZSM-5 with specific surface area of 400 m²/g and diameter particle of 1.2-2 mm was used as a parent support. The prepared catalysts were characterized by Fourier transform infrared spectroscopy analysis. The IR absorbed bands of Cu-ZSM-5 and Fe-ZSM-5 revealed a shift in the frequency and a reduction in the intensity framework. This indicates that both catalysts have a significant change in the number of the zeolite structure bonds. The catalytic activity of the prepared materials compared to the parent zeolite was evaluated for the catalytic ozone decomposition. The ozone stream of the initial concentration (13 g/m³) with air flow rate (Q) of 0.18 m³/h was passed through a glass jacket column reactor filled with a fixed bed of 40 g zeolites. It was showed that the ozone removal efficiency by Cu-ZSM-5 and Fe-ZSM-5 was obviously higher than that found with the parent ZSM-5. In terms of O₃ removal efficiency, zeolite samples could be ranked as follows: Fe-ZSM-5 > Cu-ZSM-5> parent ZSM-5. The results revealed about 90% O₃ removal efficiency for Fe-ZSM-5 and 70% for Cu-ZSM-5 as compared to nearly 40% for the parent zeolite. Consequently, the incorporation of Fe and Cu metals onto the zeolite surface plays a key role for enhancing the gaseous ozone elimination.

Ozone decomposition on Ag/SiO2 and Ag/clinoptilolite catalysts at ambient temperature

Silver modified zeolite (Bulgarian natural clinoptilolite) and Ag/silica catalysts were synthesized by ion exchange and incipient wet impregnation method respectively and characterized by different techniques. DC arc-AES was used for Ag detection. XRD spectra show that Ag is loaded over the surface of the SiO(2) sample and that after the ion-exchange process the HEU type structure of clinoptilolite is retained. UV-VIS (specific reflection at 310 nm) and IR (band at 695 cm(-1)) spectroscopy analysis proved that silver is loaded as a T-atom into zeolite channels as Ag(+), instead of Na(+), Ca(2+), or K(+) ions, existing in the natural clinoptilolite form. The samples Ag/SiO(2) and Ag-clinoptilolite were tested as catalysts for decomposition of gas phase ozone. Very high catalytic activity (up to 99%) was observed and at the same time the catalysts remained active over time at room temperature.