Catalytic oxidation of toluene on Mn–containing mixed oxides prepared in reverse microemulsions

Potential Strategy to Control the Organic Components of Condensable Particulate Matter: A Critical Review

More and more attention has been paid to condensable particulate matter (CPM) since its emissions have surpassed that of filterable particulate matter (FPM) with the large-scale application of ultralow-emission reform. CPM is a gaseous material in the flue stack but instantly turns into particles after leaving the stack. It is composed of inorganic and organic components. Organic components are an important part of CPM, and they are an irritant, teratogenic, and carcinogenic, which triggers photochemical smog, urban haze, and acid deposition. CPM organic components can aggravate air pollution and climate change; therefore, consideration should be given to them. Based on existing methods for removing atmospheric organic pollutants and combined with the characteristics of CPM organic components, we provide a critical overview from the aspects of (i) fundamental cognition of CPM, (ii) common methods to control CPM organic components, and (iii) catalytic oxidation of CPM organic components. As one of the most encouraging methods, catalytic oxidation is discussed in detail, especially in combination with selective catalytic reduction (SCR) technology, to meet the growing demands for multipollutant control (MPC). We believe that this review is inspiring for a fuller understanding and deeper exploration of promising approaches to control CPM organic components.

Catalytic Properties of the Spinel-Like CuxMn3−xO4 Copper Manganese Oxides—An Overview

Copper manganese oxide spinels and related (multiphase) materials with the formula CuxMn3−xO4 are the active catalysts in a wide variety of industrially important processes due to their great diversity in their phase relations, metal ion valence/site distribution, and chemical properties. In this review, we summarize the preparation methods and their effects on the composition, properties, and catalytic properties of various CuxMn3−xO4 catalysts with various Cu/Mn ratios. The main summarized catalytic reactions are the oxidation of carbon monoxide, nitrogen oxide, and hydrogen sulfide and the oxidative removal of organic solvents such as benzene, toluene, and xylene from the air. Some industrially important reactions (steam reforming of methanol or synthesis gas) and the manufacture of organic chemicals (methyl formate, propylene oxide, and benzyl alcohol) catalyzed by CuxMn3−xO4 spinels are also reviewed.

The Study of Thermal Stability of Mn-Zr-Ce, Mn-Ce and Mn-Zr Oxide Catalysts for CO Oxidation

MnO_x-CeO₂, MnO_x-ZrO₂, MnO_x-ZrO₂-CeO₂ oxides with the Mn/(Zr + Ce + Mn) molar ratio of 0.3 were synthesized by coprecipitation method followed by calcination in the temperature range of 400-800 °C and characterized by XRD, N₂ adsorption, TPR, TEM, and EPR. The catalytic activity was tested in the CO oxidation reaction. It was found that MnO_x-CeO₂, MnO_x-ZrO₂-CeO₂, MnO_x-ZrO₂ catalysts, calcined at 400-500 °C, 650-700 °C and 500-650 °C, respectively, show the highest catalytic activity in the reaction of CO oxidation. According to XRD and TEM results, thermal stability of catalysts is determined by the temperature of decomposition of the solid solution Mn_x(Ce,Zr)_1-xO₂. The TPR-H₂ and EPR methods showed that the high activity in CO oxidation correlates with the content of easily reduced fine MnO_x particles in the samples and the presence of paramagnetic defects in the form of oxygen vacancies. The maximum activity for each series of catalysts is associated with the start of solid solution decomposition. Formation of active phase shifts to the high-temperature region with the addition of zirconium to the MnO_x-CeO₂ catalyst.

The Synthesis of Cu–Mn–Al Mixed-Oxide Combustion Catalysts by Co-Precipitation in the Presence of Starch: A Comparison of NaOH with Organic Precipitants

Cu–Mn mixed oxides are well known as active combustion catalysts. The common method for their synthesis is based on co-precipitation, with NaOH as a precipitant, and is burdened with the possibility of introducing undesired Na contamination. This work describes the use of two organic bases, tetrabutylammonium hydroxide and choline hydroxide, as precipitating agents in a novel alkali-free route for Cu–Mn–Al catalyst synthesis. To obtain fine crystalline precursors, which are considered advantageous for the preparation of active catalysts, co-precipitation was carried out in the presence of starch gel. Reference materials prepared with NaOH in the absence of starch were also obtained. Mixed oxides were produced by calcination at 450 °C. The precursors contained MnCO3 doped with Cu and Al, and an admixture of amorphous phases. Those prepared in the presence of starch were less crystalline and retained biopolymer residues. The combustion of these residues during calcination enhanced the formation of larger amounts of the Cu1.5Mn1.5O4 spinel phase, with better crystallinity in comparison to catalysts prepared from conventionally synthesized precursors. Tests of toluene combustion demonstrated that the catalysts prepared with starch performed better than those obtained in starch-free syntheses, and that the mixed oxides obtained by the alkali-free route were more active than catalysts prepared with NaOH. Catalytic data are discussed in terms of property–performance relationships.

Recent Advances of Chlorinated Volatile Organic Compounds' Oxidation Catalyzed by Multiple Catalysts: Reasonable Adjustment of Acidity and Redox Properties

The severe hazard of chlorinated volatile organic compounds (CVOCs) to human health and the natural environment makes their abatement technology a key topic of global environmental research. Due to the existence of Cl, the byproducts of CVOCs in the catalytic combustion process are complex and toxic, and the possible generation of dioxin becomes a potential risk to the environment. Well-qualified CVOC catalysts should process favorable low-temperature catalytic oxidation ability, excellent selectivity, and good resistance to poisoning, which are governed by the reasonable adjustment of acidity and redox properties. This review overviews the application of different types of multicomponent catalysts, that is, supported noble metal catalysts, transition metal oxide/zeolite catalysts, composite transition metal oxide catalysts, and acid-modified catalysts, for CVOC degradation from the perspective of balance between acidity and redox properties. This review also highlights the synergistic degradation of CVOCs and NO_x from the perspective of acidity and redox properties. We expect this work to inspire and guide researchers from both the academic and industrial communities and help pave the way for breakthroughs in fundamental research and industrial applications in this field.

Approach to the Characterization of Monolithic Catalysts Based on La Perovskite-like Oxides and Their Application for VOC Oxidation under Simulated Indoor Environment Conditions

Catalysts are very important in controlling the pollutant emissions and are used for hundreds of chemical processes. Currently, noble metal-based catalysts are being replaced for other kinds of materials. In this study, three lanthanum-based perovskite-like oxides were synthesized (LaCo, LaCoMn, and LaMn) by the glycine-combustion method. The powder catalysts obtained were supported onto cordierite ceramic monoliths using an optimized washcoating methodology to obtain the subsequent monolithic catalysts (LaCo-S, LaCoMn-S, and LaMn-S). Sample characterization confirmed the formation of the perovskite-like phase in the powder materials as well as the presence of the perovskite phase after supporting it onto the monolithic structure. The XPS analysis showed a general decrease in lattice oxygen species for monolithic catalysts, mainly caused by the colloidal silica used as a binder agent during the washcoating process. Additionally, some variations in the oxidation state distribution for elements in Co-containing systems suggest a stronger interaction between cordierite and such catalysts. The catalytic activity results indicated that powder and monolithic catalysts were active for single-component VOC oxidation in the following order: 2-propanol > n-hexane ≅ mixture > toluene, and there was no evidence of loss of catalytic activity after supporting the catalysts. However, LaMn-S had a better catalytic performance for all VOC tested under dry conditions, achieving oxidation temperatures between 230–420 °C. The oxidation efficiency for the VOC mixture was strongly affected by the presence of moisture linking the oxidation efficiency at wet conditions to the VOC chemical nature. Additionally, for higher VOC concentrations, the catalyst efficiency decreased due to the limited number of active sites.

Recent advances in volatile organic compounds abatement by catalysis and catalytic hybrid processes: A critical review

Air pollution, particularly for toxic and harmful compounds to humans and the environment, has aroused increasing public concerns. Among air pollutants, volatile organic compounds (VOCs) are the main sources of air pollution. Many attempts have been made to control VOCs using catalysts, plasma, photolysis, and adsorption. Among them, oxidative catalysis by noble metals or transition metal oxides is considered one of the most feasible and effective methods to control VOCs. This paper reviews the experimental achievements on the abatement of VOCs using noble metals, transition metals and modified metal oxide catalysts. Although the catalytic degradation of VOCs appears to be feasible, there are unavoidable problems when only catalysis treatments are applied to the field. Therefore, catalysts including hybrid processes are developed to improve the removal efficiency of VOCs. This review addresses new hybrid treatments to remove VOCs using catalysts, including hybrid treatment combined with plasma, photolysis, and adsorption. The mechanism of the oxidation of VOCs by catalysts is explained by adsorption-desorption principles, such as the Langmuir-Hinshelwood, Eley-Rideal, and Mars-van-Krevelen mechanisms. A π-backbonding interaction between unsaturated compounds and transition metals is introduced to better understand the mechanism of VOC removals. Finally, several factors affecting the catalytic activities, such as support, component ratio, preparation method, metal loading, and deactivation factor, are discussed.

Catalytic removal of toluene over manganese oxide-based catalysts: a review

It is necessary to control the emissions of toluene, which is hazardous to both human health and the atmosphere environment and has been classified as a priority pollutant. Manganese oxide-based (Mn-based) catalysts have received increased attention due to their high catalytic performance, good physicochemical characteristic, availability in various crystal structures and morphologies, and being environmentally friendly and low cost. These catalysts can be classified into five categories, namely single manganese oxide, Mn-based composite oxides, Mn-based special oxides, supported Mn-based oxides, and Mn-based monoliths. This review focused on the recent progress on the five types of Mn-based catalysts for catalytic removal of toluene at low temperature and further systematically summarized the strategies improving catalysts, including improving synthetic methods, incorporating MnO_x with other metal oxides, depositing Mn-based oxides on proper supports, and tuning the supports. Moreover, the effect of coexisting components, the reaction kinetics, and the oxidation mechanisms toward the removal of toluene were also discussed. Finally, the future research direction of this field was presented.

Cordierite-supported metal oxide for non-methane hydrocarbon oxidation in cooking oil fumes

Cooking emission is an important reason for the air quality deterioration in the metropolitan area in China. Transition metal oxide and different loading of manganese oxide supported on cordierite were prepared by incipient wetness impregnation method and were used for non-methane hydrocarbon (NMHC) oxidation in cooking oil fumes (COFs). The effects of different calcination temperature and different Mn content were also studied. The SEM photographs and CO₂ temperature-programmed desorption revealed 5 wt% Mn/cordierite had the best pore structure and the largest number of the weak and moderate basic sites so it showed the best performance for NMHC oxidation. XRD analysis exhibited 5 wt% Mn/cordierite had the best dispersion of active phase and the active phase was MnO₂ when the calcination temperature was 400℃ which were good for the catalytic oxidation of NMHC.

Synthesis and characterization of some nanostructured composite oxides for low temperature catalytic combustion of dilute propane

Five nanosized perovskite and four ferrospinel powders were prepared by sol–gel self-combustion technique. The La_0.6Pb_0.2Mg_0.2MnO₃ perovskite was found to exhibit the best catalytic performance with respect to propane combustion at low temperatures.

Facile synthesis of Mn–Fe/CeO2 nanotubes by gradient electrospinning and their excellent catalytic performance for propane and methane oxidation

Nanotubes have been the focus of vital efforts in the catalysis community because of their unique properties.

Development of new transition metal oxide catalysts for the destruction of PCDD/Fs

Various transition metal oxide and vanadium-containing multi-metallic oxide catalysts were developed for the destruction of PCDD/Fs (polychlorinated dibenzo-p-dioxins and furans). A stable PCDD/Fs generating system was installed to support the catalytic destruction tests in this study. Nano-titania supported vanadium catalyst (VOx/TiO2) showed the highest activity, followed by CeOx, MnOx, WOx and finally MoOx. Multi-metallic oxide catalysts, prepared by doping WOx, MoOx, MnOx and CeOx into VOx/TiO2 catalysts, showed different activities on the decomposition of PCDD/Fs. The highest destruction efficiency of 92.5% was observed from the destruction test over VOxCeOx/TiO2 catalyst. However, the addition of WOx and MoOx even played a negative role in multi-metallic VOx/TiO2 catalysts. Characterizations of transition metal oxides and multi-metallic VOx/TiO2 catalysts were also investigated with XRD and TPR. After the catalysts were used, the conversion from high valent metals to low valence states was observed by XPS.

Recent Development of Catalysts for Removal of Volatile Organic Compounds in Flue Gas by Combustion: A Review

Volatile organic compounds (VOCs) emitted from anthropogenic sources pose direct and indirect hazards to both atmospheric environment and human health due to their contribution to the formation of photochemical smog and potential toxicity including carcinogenicity. Therefore, to abate VOCs emission, the catalytic oxidation process has been extensively studied in laboratories and widely applied in various industries. This report is mainly focused on the benzene, toluene, ethylbenzene, and xylene (BTEX) with additional discussion about chlorinated VOCs. This review covers the recent developments in catalytic combustion of VOCs over noble metal catalysts, nonnoble metal catalysts, perovskite catalysts, spinel catalysts, and dual functional adsorbent-catalysts. In addition, the effects of supports, coke formation, and water effects have also been discussed. To develop efficient and cost-effective catalysts for VOCs removal, further research in catalytic oxidation might need to be carried out to strengthen the understanding of catalytic mechanisms involved.

Rod-like CuMnOx transformed from mixed oxide particles by alkaline hydrothermal treatment as a novel catalyst for catalytic combustion of toluene

Rod-like copper manganese mixed oxides by alkaline hydrothermal treatment exhibit superior catalytic activity toward toluene combustion at 210 °C.

Promoter effect of Pd species on Mn oxide catalysts supported on rare-earth-iron mixed oxide

The loading of a small amount of Pd species dramatically promotes a catalytic performance of MnO_x species located on the hexagonal structure of YbFeO₃.

Reduction of mixed Mn–Zr oxides: in situ XPS and XRD studies

The reduction of the solid solutions Mn_xZr_1−xO_2−δ proceeds via two stages. During the first stage, the Mn cations incorporated into the solid solutions Mn_xZr_1−xO_2−δ undergo partial reduction. At the second stage, Mn cations segregate on the surface.

Low temperature catalytic oxidation of volatile organic compounds: a review

Volatile organic compounds (VOCs) are toxic and recognized as one of the major contributors to air pollution.

Total oxidation of isopropanol over manganese oxide modified ZSM-5 zeolite membrane catalysts

Manganese oxide modified ZSM-5 membrane catalysts were prepared for the catalytic combustion of a volatile organic compound (isopropanol) over a zeolite membrane reactor.

Catalytic Combustion of Low Concentration Methane over Catalysts Prepared from Co/Mg-Mn Layered Double Hydroxides

A series of Co/Mg-Mn mixed oxides were synthesized through thermal decomposition of layered double hydroxides (LDHs) precursors. The resulted catalysts were then subjected for catalytic combustion of methane. Experimental results revealed that the Co4.5Mg1.5Mn2LDO catalyst possessed the best performance with theT90=485°C. After being analyzed via XRD, BET-BJH, SEM, H2-TPR, and XPS techniques, it was observed that the addition of cobalt had significantly improved the redox ability of the catalysts whilst certain amount of magnesium was essential to guarantee the catalytic activity. The presence of Mg was helpful to enhance the oxygen mobility and, meanwhile, improved the dispersion of Co and Mn oxides, preventing the surface area loss after calcination.

Low-cost catalysts for the control of indoor CO and PM emissions from solid fuel combustion

Cu-Mn based mixed oxide type low-cost catalysts have been prepared in supported form using mesoporous Al(2)O(3), TiO(2) and ZrO(2) supports. These supports have been prepared by templating method using a natural biopolymer, chitosan. The synthesized catalysts have been characterized by XRD, BET-SA, SEM, O(2)-TPD and TG investigations. The catalytic activity for CO as well as PM oxidation was studied, in a view of their possible applications in the control of emissions from solid fuel combustion of rural cook-stoves. The trend observed for the catalytic activity of the synthesized catalysts for CO oxidation was ZrO(2)>TiO(2)>Al(2)O(3) while for PM oxidation it was observed to be TiO(2)>ZrO(2)>Al(2)O(3). The effect of CO(2), SO(2) and H(2)O on CO oxidation activity was also investigated, and despite partial deactivation, the catalysts show good CO oxidation activity. An effective regeneration treatment was attempted by heating the partially deactivated catalysts in presence of oxygen. Redox properties of TiO(2) and ZrO(2) and their structure appeared to be responsible for their promotional activity for CO and PM oxidation reactions. These unordered mesoporous materials could be useful for such reactions where mass transfer is more important than shape and size selectivity.

Chemical-physical properties of spinel CoMn2O4 nano-powders and catalytic activity in the 2-propanol and toluene combustion: Effect of the preparation method

Spinel-type CoMn(2)O(4)nano-powders are prepared using sol-gel auto combustion (SGC) and co-precipitation (CP) methods and their catalytic activities are evaluated in combustion of 2-propanol and toluene. The chemical-physical properties of the oxides are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N(2)-adsorption-desorption, temperature programmed reduction (TPR) and scanning electron microscopy (SEM). After calcination at 700°C, CoMn(2)O(4)-SGC shows higher amounts of the normal-type spinel phase and is more crystalline than CoMn(2)O(4)-CP. Higher calcination temperatures (850°C) do not affect very much the weight percentage of the normal-type spinel phase; although the crystal size slightly increased. The TPR analysis evidences a large number of Mn(3+) cations in CoMn(2)O(4)-SGC compared to CoMn(2)O(4)-CP. This difference, together with the higher surface area, could justify the higher activity of CoMn(2)O(4)-SGC in both the investigated reactions.