Atomically Dispersed Y or La on Birnessite-Type MnO2 for the Catalytic Decomposition of Low-Concentration Toluene at Room Temperature

Synthesis of δ-MnO2 via ozonation routine for low temperature formaldehyde removal

Nowadays, it is still a challenge to prepared high efficiency and low cost formaldehyde (HCHO) removal catalysts in order to tackle the long-living indoor air pollution. Herein, δ-MnO2 is successfully synthesized by a facile ozonation strategy, where Mn2+ is oxidized by ozone (O3) bubble in an alkaline solution. It presents one of the best catalytic properties with a low 100% conversion temperature of 85°C for 50 ppm of HCHO under a GHSV of 48,000 mL/(g·hr). As a comparison, more than 6 times far longer oxidation time is needed if O3 is replaced by O2. Characterizations show that ozonation process generates a different intermediate of tetragonal β-HMnO2, which would favor the quick transformation into the final product δ-MnO2, as compared with the relatively more thermodynamically stable monoclinic γ-HMnO2 in the O2 process. Finally, HCHO is found to be decomposed into CO2 via formate, dioxymethylene and carbonate species as identified by room temperature in-situ diffuse reflectance infrared fourier transform spectroscopy. All these results show great potency of this facile ozonation routine for the highly active δ-MnO2 synthesis in order to remove the HCHO contamination.

Less Is More: Selective-Atom-Removal-Derived Defective MnOx Catalyst for Efficient Propane Oxidation

Defect manipulation in metal oxide is of great importance in boosting catalytic performance for propane oxidation. Herein, a selective atom removal strategy was developed to construct a defective manganese oxide catalyst, which involved the partial etching of a Mg dopant in MnOx. The resulting MgMnOx-H catalysts exhibited superior low-temperature catalytic activity (T50 = 185 °C, T90 = 226 °C) with a propane conversion rate of 0.29 μmol·gcat.-1·h-1 for the propane oxidation reaction, which is 4.8 times that of pristine MnOx. Meanwhile, a robust hydrothermal stability was guaranteed at 250 °C for 30 h of reaction time. The comprehensive experimental characterizations revealed that the catalytic performance improvement was closely related to the defective structures including the abundant (metal and oxygen) vacancies, distorted crystals, valence imbalance, etc., which prominently weakened the Mn-O bond and stimulated the mobility of surface lattice oxygen, leading to the elevation in the intrinsic oxidation activity. This work exemplifies the significance of defect engineering for the promotion of the oxidation ability of metal oxide, which will be valuable for the further development of efficient non-noble metal catalysts for propane oxidation.

Recent advances in catalytic oxidation of VOCs by two-dimensional ultra-thin nanomaterials

Catalytic oxidation, an end-of-pipe treatment technology for effectively purifying volatile organic compounds (VOCs), has received widespread attention. The crux of catalytic oxidation lies in the development of efficient catalysts, with their optimization necessitating a comprehensive analysis of the catalytic reaction mechanism. Two-dimensional (2D) ultra-thin nanomaterials offer significant advantages in exploring the catalytic oxidation mechanism of VOCs due to their unique structure and properties. This review classifies strategies for regulating catalytic properties and typical applications of 2D materials in VOCs catalytic oxidation, in addition to their characteristics and typical characterization techniques. Furthermore, the possible reaction mechanism of 2D Co-based and Mn-based oxides in the catalytic oxidation of VOCs is analyzed, with a special focus on the synergistic effect between oxygen and metal vacancies. The objective of this review is to provide valuable references for scholars in the field.

Interface engineering of Mn3O4/Co3O4 S-scheme heterojunctions to enhance the photothermal catalytic degradation of toluene

Transition metal oxides have high photothermal conversion capacity and excellent thermal catalytic activity, and their photothermal catalytic ability can be further improved by reasonably inducing the photoelectric effect of semiconductors. Herein, Mn₃O₄/Co₃O₄ composites with S-scheme heterojunctions were fabricated for photothermal catalytic degradation of toluene under ultraviolet-visible (UV-Vis) light irradiation. The distinct hetero-interface of Mn₃O₄/Co₃O₄ effectively increases the specific surface area and promotes the formation of oxygen vacancies, thus facilitating the generation of reactive oxygen species and migration of surface lattice oxygen. Theoretical calculations and photoelectrochemical characterization demonstrate the existence of a built-in electric field and energy band bending at the interface of Mn₃O₄/Co₃O₄, which optimizes the photogenerated carriers' transfer path and retains a higher redox potential. Under UV-Vis light irradiation, the rapid transfer of electrons between interfaces promotes the generation of more reactive radicals, and the Mn₃O₄/Co₃O₄ shows a substantial improvement in the removal efficiency of toluene (74.7%) compared to single metal oxides (53.3% and 47.5%). Moreover, the possible photothermal catalytic reaction pathways of toluene over Mn₃O₄/Co₃O₄ were also investigated by in situ DRIFTS. The present work offers valuable guidance toward the design and fabrication of efficient narrow-band semiconductor heterojunction photothermal catalysts and provides deeper insights into the mechanism of photothermal catalytic degradation of toluene.

Catalytic removal of toluene using MnO2-based catalysts: A review

Volatile organic compounds (VOCs) have serious hazard to human health and ecological environment. Due to its low cost and high activity, the catalytic oxidation technology considered to be the most effective method to remove VOCs. Toluene is one of the typical VOCs, hence its catalytic elimination is crucial for the regulation of VOCs. Manganese dioxide (MnO₂) has been extensively studied for its excellent redox performance and low-temperature operation conditions. In this review, we summarize the research progresses in the toluene catalytic oxidation of MnO₂-based catalysts, which contain single MnO₂, metal-doped MnO₂ and supported MnO₂ catalyst. In particular, we pay much attention on the relationship between the chemical properties and toluene oxidation performance over MnO₂ catalyst, as well as the catalytic reaction mechanisms. Moreover, the effects of different crystal forms and morphologies on the catalytic toluene reaction were discussed. And the perspective on MnO₂ catalysts for the catalytic oxidation of toluene has been proposed. We expect that the summary of these important findings can serve as an important reference for the catalytic treatment of VOCs.

Yttrium-preintercalated layered manganese oxide as a durable cathode for aqueous zinc-ion batteries

Rechargeable aqueous zinc-ion batteries (RAZIBs) are regarded as competitive alternatives for large-scale energy storage on account of cost-effectiveness and inherent safety. In particular, rechargeable Zn-MnO₂ batteries have drawn increasing attention due to high manufacturing readiness level. However, obtaining MnO₂ with high electrochemical activity and high cyclic stability toward Zn²⁺/H⁺ storage still remains challenging. Herein, we reveal that incorporating yttrium ions (Y³⁺) into layered MnO₂ can regulate the electronic structure of the MnO₂ cathode by narrowing its band gap (from 3.25 to 2.50 eV), thus boosting the electrochemical performance in RAZIBs. Taking advantage of this feature, the optimized Y-MnO₂ (YMO) sample exhibits greater capacity (212 vs. 152 mA h g^-1 at 0.5 A g^-1), better rate capability (94 vs. 61 mA h g^-1 at 8 A g^-1), reduced charge-transfer resistance (79 vs. 148 Ω), and promoted mass transfer kinetics (3.13 × 10^-11vs. 2.37 × 10^-11 cm² s^-1) in comparison with Y-free MnO₂ (MO). More importantly, compared to MO, YMO-0.1 exhibits enhanced energy storage capability by nearly 40% (309 vs. 222 W h kg^-1) and stable cycle performance (94 vs. 52 mA h g^-1 after 3000 cycles). In situ Raman microscopy further reveals that the presence of Y³⁺ endows MnO₂ with remarkable electrochemical reversibility during charge/discharge processes. This work highlights the importance of the Y³⁺ preintercalation strategy, which can be further developed to obtain better cathode materials for aqueous batteries.

Thermal-Driven Optimization of the Strong Metal-Support Interaction of a Platinum-Manganese Oxide Octahedral Molecular Sieve to Promote Toluene Oxidation: Effect of the Interface Pt2+-Ov-Mnδ. ACS APPLIED MATERIALS & INTERFACES 2022;14:56790-56800. [PMID: 36524882 DOI: 10.1021/acsami.2c16923] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Strong metal-support interactions (SMSIs) have a significant effect on the performance of supported noble-metal catalysts for volatile organic compound (VOC) elimination. Herein, the strength of the SMSI of Pt/OMS-2 between Pt and the OMS-2 support is regulated by simply changing calcination temperatures, and the catalyst calcined at 300 °C (Pt/OMS-2-300) performs the best in the catalytic combustion of toluene. Through systematic structural characterizations, it is revealed that much more Pt²⁺-O_v-Mn^δ+ species are formed in Pt/OMS-2-300, which can help facilitate the generation of more reactive oxygen species and promote lattice oxygen mobility. Moreover, the results of in situ DRIFTS experiments further confirm that abundant Pt²⁺-O_v-Mn^δ+ species at the Pt-MnO₂ interface on Pt/OMS-2-300 can better enhance the adsorption and activation of toluene, thus boosting the catalytic performance in toluene combustion. This newly developed strategy of thermal-driven regulation of the SMSI provides a novel perspective for constructing highly efficient catalysts for VOC emission control.

Review on Catalytic Oxidation of VOCs at Ambient Temperature

As an important air pollutant, volatile organic compounds (VOCs) pose a serious threat to the ecological environment and human health. To achieve energy saving, carbon reduction, and safe and efficient degradation of VOCs, ambient temperature catalytic oxidation has become a hot topic for researchers. Firstly, this review systematically summarizes recent progress on the catalytic oxidation of VOCs with different types. Secondly, based on nanoparticle catalysts, cluster catalysts, and single-atom catalysts, we discuss the influence of structural regulation, such as adjustment of size and configuration, metal doping, defect engineering, and acid/base modification, on the structure-activity relationship in the process of catalytic oxidation at ambient temperature. Then, the effects of process conditions, such as initial concentration, space velocity, oxidation atmosphere, and humidity adjustment on catalytic activity, are summarized. It is further found that nanoparticle catalysts are most commonly used in ambient temperature catalytic oxidation. Additionally, ambient temperature catalytic oxidation is mainly applied in the removal of easily degradable pollutants, and focuses on ambient temperature catalytic ozonation. The activity, selectivity, and stability of catalysts need to be improved. Finally, according to the existing problems and limitations in the application of ambient temperature catalytic oxidation technology, new prospects and challenges are proposed.

Yttrium-modified Co3O4 as efficient catalysts for toluene and propane combustion: Effect of yttrium content

A series of Y-modified cobalt oxides with various Y/(Co+Y) molar ratios (0.25 %, 0.5 %, 1 %, 3 % and 5 %) were prepared to study the effect of Y content on toluene and propane combustion. The characterization of the catalysts revealed that proper Y incorporation resulted in smaller crystallite sizes, larger specific surface areas, more oxygen vacancies and weaker Co-O bonds. As such, the Y-modified Co₃O₄ showed enhanced low-temperature reducibility, boosted oxygen mobility and better catalytic activity. However, excess Y (> 1 %) aggregates on the surface of Co₃O₄ and forms yttrium carbonate species, hindering the catalyst activity. A volcano-type relationship between the Y content and the catalytic activity was established. The optimal catalyst 1 % Y-Co (with Y/(Co+Y) molar ratio of 1 %) exhibited toluene oxidation rate of 24 nmol g^-1 s^-1 at 220 °C and propane oxidation rate of 69 nmol g^-1 s^-1 at 180 °C. Besides, 1 % Y-Co presented perfect cycling stability and long-term durability in propane oxidation. Regarding its low cost, high efficiency and good stability, 1 % Y-Co is a promising catalyst for the practical elimination of hydrocarbon emissions.

Advances in single-atom catalysts: Design, synthesis and environmental applications

Over the past few years, single-atom catalysts (SACs) on the horizon have driven rapid and extensive scientific advances in heterogeneous catalysis. Nevertheless, large-scale applications of SACs in the environment have been hindered by the problematic synthesis of catalysts, because the atomic-scale materials with high activation energy are easy to form nanoclusters and nanoparticles in the synthesis stage. The catalytic stability and catalytic activity of SACs in the treatment of complex environmental pollutants also need to be further researched. Herein, the review is built on a comprehensive discussion of the design and synthesis strategies of SACs. The shortcomings of traditional methods and the improvement from different angles like defect regulation are analyzed. Furthermore, the reaction mechanism of SACs in different reactions was summarized, and the environmental applications of SACs, such as wastewater treatment, carbon dioxide reduction, nitrogen reduction, hydrogen evolution, NO_x reduction and oxidation, volatile organic compounds removing and environmental monitoring are exemplified to deeply evaluate the prospects and challenges of SACs in the field of environmental protection.

Noble Metal Single-Atom Catalysts for the Catalytic Oxidation of Volatile Organic Compounds

Volatile organic compounds (VOCs) are detrimental to the environment and human health and must be eliminated before discharging. Oxidation by heterogeneous catalysts is one of the most promising approaches for the VOCs abatement. Precious metal catalysts are highly active for the catalytic oxidation of VOCs, but they are rare and their high price limits large-scale application. Supported metal single-atom catalysts (SACs) have a high atom efficiency and provide the possibility to circumvent such limitations. This Review summarizes recent advances in the use of metal SACs for the complete oxidation of VOCs, such as benzene, toluene, formaldehyde, and methanol, as well as aliphatic and Cl- and S-containing hydrocarbons. The structures of the metal SACs used and the reaction mechanisms of the VOC oxidation are discussed. The most widely used SACs are noble metals supported on oxides, especially on reducible oxides, such as Mn₂ O₃ and TiO₂ . The reactivity of most SACs is related to the activity of surface lattice oxygen of the oxides. Furthermore, several metal SACs show better reactivity and improved S and Cl resistance than the corresponding nanocatalysts, indicating that SACs have potential for application in the oxidation of VOCs. The deactivation and regeneration mechanisms of the metal SACs are also summarized. It is concluded that the application of metal SACs in catalytic oxidation of VOCs is still in its infancy. This Review aims to elucidate structure-performance relationships and to guide the design of highly efficient metal SACs for the catalytic oxidation of VOCs.

Catalytic oxidation of VOCs over 3D@2D Pd/CoMn2O4 nanosheets supported on hollow Al2O3 microspheres

Catalytic oxidation is a promising method for removing harmful volatile organic compounds (VOCs). Therefore, exploring high-efficiency catalysts for catalyzing VOCs is of great significance to the realization of an environment-friendly and sustainable society. Here, a series of 3D@2D constructed Al₂O₃@CoMn₂O₄ microspheres with a hollow hierarchical structure supporting Pd nanoparticles was successfully synthesized. The introduction of hollow Al₂O₃ for the in situ vertical growth of 2D CMO spinel materials constructs a well-defined core - shell hollow hierarchical structure, leading to larger specific surface area, more accessible active sites and promoted catalytic activity of support material. Additionally, theoretical calculations also indicate that the addition of Al₂O₃ as the support material strengthens the adsorption of toluene and oxygen on CoMn₂O₄, which promotes their activation. The dispersion of Pd further strengthens the low-temperature reducibility along with more active surface oxygen species and lower apparent activation energy. The optimum 1 wt% Pd/h-Al@4CMO catalyst possesses the lowest apparent activation energy for toluene of 77.4 kJ mol^-1, showing the relatively best catalytic activity for VOC oxidation, reaching 100% toluene, benzene, and ethyl acetate conversion at 165, 160, and 155 °C, respectively. Meanwhile, the 1 wt% Pd/h-Al@4CMO sample possesses excellent catalytic stability, outstanding selectivity, and good moisture tolerance, which is an effective candidate for eliminating VOCs contaminants.

Recycling electroplating sludge as a monolithic catalyst for effective catalytic purification of volatile organic compounds

Electroplating sludge had a high content of heavy metals and usually lacked high-value-added utilization. In this work, Cu-containing sludge was used to synthesize a spinel catalyst, which was applied in catalytic oxidization of toluene. As a result, the sludge-derived spinel removed 50% of toluene (1000 ppm, 9600 h^-1) at 280 °C. In comparison, a reagent-synthesized spinel with a similar component removed 50% of pollutant at 294 °C. The sludge-derived spinel also showed a stable performance for over 50 h at 370 °C. Even when the initial concentration was increased to 5000 ppm, or the gas hourly space velocity was increased to 40,000 h^-1, the temperature for 50% removal was only increased to 303 °C. According to characterizations, surface oxygens of the sludge-derived spinel were more active than those in the reagent-synthesized one. Besides, the former had more active surface oxygens (207.9 μmol/g) than the latter (183.1 μmol/g). Furthermore, the sludge-derived spinel was coated on a monolithic honeycomb, which were also effective in catalytic oxidization of toluene. The main results of this work were in favor of high-value-added utilization of hazardous solid waste and promoting its real industry application.

Zeolite-Based Materials for the Catalytic Oxidation of VOCs: A Mini Review

Catalysts for VOCs combustion have been widely studied and zeolite-based materials and have been structured to meet the need of particle use in this field. This review summarized several new trends in zeolite-based catalysts for VOCs catalytic oxidation. Intensive effort has been devoted to the optimization of composition and structure of catalysts, abatement of CVOCs, design of zeolite-based monolithic catalysts and adsorbent/catalyst bi-functional material. The suggestions for further work here presented are put forward based on the collation of recently published papers.