Synergistic effects in Mn-Co mixed oxide supported on cordierite honeycomb for catalytic deep oxidation of VOCs

Co-doped cryptomelane-type manganese oxide in situ grown on a nickel foam substrate for high humidity ozone decomposition

Monolithic catalysts with excellent O3 catalytic decomposition performance were prepared by in situ loading of Co-doped KMn8O16 on the surface of nickel foam. The triple-layer structure with Co-doped KMn8O16/Ni6MnO8/Ni foam was grown spontaneously on the surface of nickel foam by tuning the molar ratio of KMnO4 to Co(NO3)2·6H2O precursors. Importantly, the formed Ni6MnO8 structure between KMn8O16 and nickel foam during in situ synthesis process effectively protected nickel foam from further etching, which significantly enhanced the reaction stability of catalyst. The optimum amount of Co doping in KMn8O16 was available when the molar ratio of Mn to Co species in the precursor solution was 2:1. And the Mn2Co1 catalyst had abundant oxygen vacancies and excellent hydrophobicity, thus creating outstanding O3 decomposition activity. The O3 conversion under dry conditions and relative humidity of 65%, 90% over a period of 5 hr was 100%, 94% and 80% with the space velocity of 28,000 hr-1, respectively. The in situ constructed Co-doped KMn8O16/Ni foam catalyst showed the advantages of low price and gradual applicability of the preparation process, which provided an opportunity for the design of monolithic catalyst for O3 catalytic decomposition.

Engineering Oxygen Vacancies of Co-Mn-Ni-Fe-Al High-Entropy Spinel Oxides by Adjusting Co Content for Enhanced Catalytic Combustion of Propane

Transition metal-based oxides with similar oxidation activities for catalytic hydrocarbon combustion have attracted much attention. In this study, a new class of metal high-entropy oxides (CoxMnNiFeAl)3O4 (x = 1, 2, 3, 4, 5) with a porous structure was fabricated through a simple and inexpensive NaCl template-assisted sol-gel approach, which was employed for the catalytic oxidation of propane. The results indicated that the content of cobalt has a great impact on its activity, and the (Co4MnNiFeAl)3O4 catalyst exhibited the best catalytic activity. At the high space velocity of 60 000 mL·g-1·h-1, the optimized one with high-temperature treatment can still achieve 90% propane conversion at 309 °C, which is 68 and 178 °C lower than those of the (CoMnNiFeAl)3O4 catalyst and pure cobalt oxide, respectively. Meanwhile, it has the lowest apparent activation energy (46.6 KJ·mol-1) and the fastest reaction rate (26.976 × 10-6 mol·gcat-1·s-1 at 290 °C). The improved performance of the (Co4MnNiFeAl)3O4 catalyst could be attributed to the enhancement of low-temperature reducibility, the increased number of reactive surface oxygen species, and the cocktail effect of the high-entropy oxides. This work provides new insights into the preparation of efficient light alkane degradation catalysts and a realistic approach for the large-scale application of high-entropy oxides in the field of oxidation catalysts.

Radical-driven selective oxidation of benzyl alcohol on MnCoOx catalysts with no oxidant other than air in reactor

Mn reinforced Co3O4 catalysts (MnCoOx) were prepared by a facile solid phase mixed foaming method with an in-situ heating enhancement for the formation of spinel phase mixed oxide species, and studied in the selective oxidation of benzyl alcohol just the air in reactor as oxygen donor. It was found that the MnCoOx catalysts are composed of relatively minimal spinel MnCo2O4 mixed oxide and massive Co3O4 to form MnCo2O4-Co3O4 oxide pair. The micro-domains of MnCo2O4-Co3O4 oxide pair present two redox couples of Mn3+/Mn2+ and Co3+/Co2+ instead of the single one of Co3+/Co2+ in Co3O4, and then dramatically enhance the formation of superoxide radicals (•O2-) species from the O2 in air, which can efficiently initiate the conversion of benzyl alcohol to benzaldehyde in a Fenton-like processes. With no oxidant other than air in reactor, the interaction between MnCo2O4 and Co3O4 in MnCoOx catalysts leads to a benzyl alcohol conversion up to 98 % with a 100 % benzaldehyde selectivity at atmospheric pressure while single component Co3O4 can only present a benzyl alcohol conversion at 37 %. This embodiment of highly efficient heterogeneous selective oxidation just with air as oxidant provides a probability for developing a low-cost and super-facile radical-induced selective oxidation process for alcohols.

Supported nano-sized precious metal catalysts for oxidation of catalytic volatile organic compounds

Volatile organic compounds (VOCs) are common contaminants found as indoor as well as outdoor pollutants, which can induce acute or chronic health hazards to the human physiological system. The catalytic oxidation method is widely considered as one of the effective methods for removing VOCs, and the development of highly effective catalysts is highly urgent for booming this interesting field. This review focuses on the recent progress of VOC oxidation catalyzed by supported nano-sized precious metal catalysts, and discusses the effects of metal composition, supports, size, and morphology on the catalytic activity. In addition, the roles played by both nano-sized precious metals and supports in enhancing the performance of catalytic VOCs are also systematically discussed, which will guide the further development of more advanced VOC catalysts.

Promoted catalytic performance of Ag-Mn bimetal catalysts synthesized through reduction route

VOCs can exert great harm to both human and environment, and catalytic oxidation is believed to be an effective technique to eliminate these pollutants. In this paper, Ag-Mn bimetal catalysts with 10 wt.% of silver were synthesized using doping, impregnation, and reduction methods respectively, and then they were applied to the catalytic oxidation of benzene. Through series of characterizations it showed that the loading of silver using reduction method significantly resulted in improved physico-chemical properties of manganese oxides, such as larger surface area and pore volume, higher proportion of surface Mn3+ and Mn4+, stronger reducibility and more active of surface oxygen species, which were all beneficial to its catalytic activity. As a result, the Ag-Mn catalysts synthesized by reduction method showed a lower T90 value (equals to the temperature at which 90% of initial benzene was removed) of 203°C. Besides, both the used and fresh Ag-Mn catalysts synthesized by reduction method showed preferable stability in this research.

Study on the Catalytic Oxidation of Toluene Using CeO2@S-AZMB Prepared from Spent Zn-Mn Batteries

The recycling and utilization of waste alkaline zinc manganese batteries (S-AZMB) has always been a focus of attention in the fields of environment and energy. However, current research mostly focuses on the recycling of purified materials, while neglecting the direct reuse of waste batteries. Here, we propose a new concept of preparing thermal catalysts by combining unpurified S-AZMB with CeO2 by means of ball milling. A series of characterizations and experiments have confirmed that the combination with S-AZMB not only enhances the thermal catalytic activity of CeO2 but also significantly enhances the concentration of surface oxygen vacancies. In the toluene removal experiment, the temperature (T90) at 90% toluene conversions of CeO2@S-AZMB was 180 °C, lower than the 220 °C for CeO2. More noteworthy is that this S-AZMB-based thermal catalyst can maintain a good structure and thermal catalytic stability in cyclic catalysis.

Catalytic ozonation of methylethylketone over porous Mn-Cu/HZSM-5

The catalytic ozonation of methylethylketone (MEK) was performed at the room temperature (25 °C) using the synthesized Mn and Cu-loaded zeolite (ZSM-5, SiO₂/Al₂O₃ = 80) catalysts. The ZSM-5 zeolite was used as a porous support material due to the large surface area and high capacity for adsorption of volatile organic compounds. Since Mn and Cu-loaded zeolite catalysts were effective for the catalytic ozonation of VOCs such as MEK, according to the loaded concentration of Mn and Cu, there are four types of metal loaded ZSM5 catalysts synthesized [5 wt% Mn/ZSM-5, 5 wt% Cu/ZSM-5, 5 wt% Mn-1 wt% Cu/ZSM-5 (5Mn1CuZSM), and 5 wt% Cu-1 wt% Mn/ZSM-5]. The catalytic efficiency for the removal of MEK and ozonation using the different catalysts was also studied. Based on various experimental analysis processes, the characteristics of the synthesized catalysts were explored and the removal efficiencies of MEK and O₃ together with the CO_x concentration generated from the destruction of MEK and O₃ were explored. The results for the decomposition of MEK and O₃ at the room temperature indicated that the Mn dominant ZSM-5 catalysts showed better efficiency for the conversion of MEK and O₃. The 5 wt% Mn/ZSM-5 outweighed the rest of them for the removal of MEK while the 5Mn1CuZSM showed the best catalytic reactivity for the conversion of O₃ and the CO₂ selectivity. It was ascertained that during the reaction time of catalyst and reactants of 120 min the Mn dominantly deposited bimetallic catalyst, 5Mn1CuZSM, was determined as the most effective for the removal of MEK and O₃ due to the high capability of production of Mn³⁺ species and more available adsorbed oxygen sites compared to the other catalysts. Finally, the durability measurement for the 5Mn1CuZSM catalyst was performed together with the produced CO and CO₂ concentration for 420 min.

Component regulation in novel La-Co-O-C composite catalyst for boosted redox reactions and enhanced thermal stability in methane combustion

A novel La-Co-O-C (LC-C) composites were prepared via a facile co-hydrothermal route with oxides and glycerol and further optimized for methane catalytic activity and thermal stability via component regulation. It was demonstrated that Co₃O₄ phase was the main component in regulation. The combined results of X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of oxygen (O₂-TPD), temperature-programmed reduction of hydrogen (H₂-TPR), temperature-programmed desorption of ammonia/carbon dioxide (NH₃/CO₂-TPD) revealed that component regulation led to more oxygen vacancies and exposure of surface Co²⁺, lower surface basicity and optimized acidity, which were beneficial for adsorption of active oxygen species and activation of methane molecules, resulting in the excellent catalytic oxidation performance. Especially, the (3.5)LC-C (3.5 is Co-to-La molar ratio) showed the optimum activity and the T₅₀ and T₉₀ (the temperature at which the CH₄ conversion rate was 50% and 90%, respectively) were 318 and 367°C, respectively. Using theoretical calculations and in situ diffuse reflection infrared Fourier transform spectroscopy characterization, it was also found that the catalytic mechanism changes from the "Rideal-Eley" mechanism to the "Two-term" mechanism depending on the temperature windows in which the reaction takes place. Besides, the use of the "Flynn-Wall-Ozawa" model in thermoanalytical kinetics revealed that component regulation simultaneously optimized the decomposition activation energy, further expanding the application scope of carbon-containing composites.

Review of Emission Characteristics and Purification Methods of Volatile Organic Compounds (VOCs) in Cooking Oil Fume

Volatile organic compounds (VOCs) in cooking oil fumes need to be efficiently removed due to the significant damage they cause to the environment and human health. This review discusses the emission characteristics, which are influenced by different cooking temperatures, cooking oils, and cuisines. Then, various cooking oil fume purification methods are mainly classified into physical capture, chemical decomposition, and combination methods. VOCs removal rate, system operability, secondary pollution, application area, and cost are compared. The catalytic combustion method was found to have the advantages of high VOC removal efficiency, environmental protection, and low cost. Therefore, the last part of this review focuses on the research progress of the catalytic combustion method and summarizes its mechanisms and catalysts. The Marse-van Krevelen (MVK), Langmuir-Hinshelwood (L-H), and Eley-Rideal (E-R) mechanisms are analyzed. Noble metal and non-noble metal catalysts are commonly used. The former showed excellent activity at low temperatures due to its strong adsorption and electron transfer abilities, but the high price limits its application. The transition metals primarily comprise the latter, including single metal and composite metal catalysts. Compared to single metal catalysts, the interaction between metals in composite metal catalysts can further enhance the catalytic performance.

Non-thermal plasma synthesis of supported Cu-Mn-Ce mixed oxide catalyst towards highly improved catalytic performance for volatile organic compound oxidation

Compared with that of the transition metal mixed oxide pellet catalyst, the catalytic activity of the supported mixed oxide catalyst was significantly reduced, which was limited in practical industrial applications. In this work, supported Cu-Mn-Ce mixed oxide catalysts were prepared by non-thermal plasma. Catalyst characterization result demonstrated that plasma treatment could promote the proportion of oxygen vacancy and enhance the adsorptive strength of VOCs on the surface of catalyst. Meanwhile, plasma treatment process exerted a slight influence on the pore structure and morphological property of the catalyst. Consequently, CMC/SiO₂-P exhibited much higher catalytic activity than CMC/SiO₂ prepared by the incipient wetness impregnation method for the catalytic oxidation of toluene and n-hexane. Among the catalysts prepared, the 15%CMC/SiO₂-P catalyst even exhibited a high catalytic activity comparable to that of the supported noble metal catalyst for the oxidation of the inert hexane. The T₉₈ of toluene and n-hexane over 15%CMC/SiO₂-P was 260 and 280°C under the conditions of VOC concentration at 1000 ppm and WHSV at 20,000 mL·g^-1·h^-1, respectively. This work provided a novel method for the preparation of the supported transition metal mixed oxide catalyst.

Highly efficient removal of toluene over Cu-V oxides modified γ-Al2O3 in the presence of SO2

Developing efficient catalysts with good resistance to complex flue gas is essential for VOCs removal in coal-fired flue gas. In this study, by exploring the effect of transition metal oxide additive, metal loading and bimetallic synergy on toluene oxidation in coal-fired flue gas, 10Cu-3V/γ-Al₂O₃ is identified as the optimal catalyst. It achieves 90% of CO₂ generation at 350 ℃, which is decreased by ca. 46 ℃ compared with 13Cu/γ-Al₂O₃. And it also exhibits good resistance to H₂O and good stability. ICP-OES, N₂ adsorption-desorption isotherms, XRD, TEM, XPS, EPR and H₂-TPR analyses were applied to characterize the catalyst composition and physicochemical properties. Doping V into 13Cu/γ-Al₂O₃ not only leads to better dispersity of CuO and homogeneous elements distribution that benefits to produce more active centers, but also constitutes the redox cycle of V⁵⁺ + Cu⁺ ↔ V⁴⁺ + Cu²⁺ which induces more surface chemical oxygen (O_sur). Moreover, since SO₂ is the main inhibiting factor in toluene oxidation, the SO₂ poisoning mechanism was illustrated by XPS, TG and in situ DRIFT analyses in depth.

Effects of the Reaction Parameters and Light Hydrothermal Aging for Catalytic Combustion of Propane over Co-Mn-Ce Catalyst

A composite oxides’ Co-Mn-Ce catalyst was synthesized by a coprecipitation method, and the experiment was carried out to study the effects of reaction parameters and light hydrothermal aging on propane combustion over the Co-Mn-Ce catalyst. The influence of reaction temperature, propane concentration, oxygen concentration, water vapor, and hydrothermal aging was studied during the catalytic combustion of propane. The propane conversion significantly decreased by 10% when the propane concentration increased at 300°C and then further decreased from 80% to 40% as water vapor concentration increased from 0 to 10 vol.%. In addition, water vapor also prolonged the time required to reach equilibrium. After hydrothermal treatment, the catalyst obtained the lowest oxidation capacity of propane. Furthermore, the results of in situ DRIFTs and O2 temperature programmed desorption (O2-TPD) demonstrated that there were fewer oxygen species after hydrothermal aging, and carbonates were the main intermediates formed during the catalytic oxidation of propane.

Research progress of catalytic oxidation of volatile organic compounds over Mn-based catalysts – a review

With the rapid development of urbanization and industrialization, environmental pollution has become more severe. Volatile organic compounds (VOCs) could be originated from the following sources: domestic, mobile and industrial sources. As important air pollutants, VOCs could cause serious harm to the environment and human health. Therefore, removing VOCs has become a priority research direction of ecological issues. Among the many elimination methods, catalytic oxidation approaches are among the most effective and economical methods which can transform VOCs into CO2 and H2O. MnOx catalysts are among the most active catalysts, which can be further modified by different cations such as Cu2+, Co2+, Cr3+, Ni2+ and Ce4+ to form mixed oxides to improve the catalytic oxidation of VOCs activity. Moreover, MnOx can be loaded on the carrier, improving the redox and oxygen storage capacity and improving its stability and activity. This review explores the structure, preparation and oxidation state of Mn-based catalysts.