Efficient catalytic degradation of toluene at a readily prepared Mn-Cu catalyst: Catalytic performance and reaction pathway

Spinel-Based Catalysts That Enable Catalytic Oxidation of Volatile Organic Compounds

Volatile organic compounds (VOCs) have caused serious harm to human health and ecological environment, and have received much attention in recent years. Despite the successful applications of catalytic combustion of VOCs as the core technology of VOCs removal in industry, the development of efficient catalysts that can mineralize VOCs into nontoxic CO2 and H2O at low temperatures remains a great challenge. Recent studies show that spinel-based materials as efficient catalysts were extensively used in the catalytic oxidation VOCs field due to their synergistic effect, manifold compositions, and electron configurations. However, most of the pollutants are complex, consisting of multiple VOCs, water vapor, CO2, SO2 and other substances, which presents a significant challenge in constructing highly active and stable catalysts. To meet the future demand for efficient catalysts capable of removing various types of VOCs, it is urgent to rationally design and scientifically prepare spinel catalysts based on existing knowledge. This work reviews the research and development of various spinel catalysts with an emphasis on their catalytic performance in VOCs oxidation. The catalytic performance of spinel-based catalysts for different sorts of VOCs was summarized and compared. Moreover, the effects of the reaction conditions on the catalytic performance of spinel-based catalysts were examined to accommodate complicated operating conditions. Subsequently, the regulation of spinel oxides in structure and defect was coherently reviewed to guide the development and design of efficient catalysts. Especially, the research techniques for the reaction mechanism over spinel catalysts were displayed to better deepen the understanding of catalytic oxidation of VOCs. Finally, the current development and challenges were proposed and put forward for future research. This review provided a systematic understanding of the VOCs oxidation over spinel-based catalysts and offered guidance for the development of high-performance catalysts for VOCs elimination.

Defect-Rich Regulatory Activity Strategy: Disordered Structure for Enhanced Catalytic Interfacial Reaction of Chlorobenzene

In contrast to previous defect engineering methods, the preparation of amorphous materials can obtain abundant defect sites through a simple way, which is expected to effectively degrade Volatile Organic Compounds (VOCs) under milder conditions. However, in-depth and systematic studies in this area are still lacking. Novel types of amorphous CeMnx catalysts with abundant defects were prepared through simple hydrothermal synthesis and used for Cl-VOCs catalysis for the first time. Experimental characterizations and DFT calculations proved that Ce doping induced MnO2 lattice distortion, which led to the transformation of CeMnx into an amorphous structure and the formation of abundant defect sites. It was observed that CeMn0.16 was able to eliminate chlorobenzene (CB) at 200 °C, and the CO2 yields and the selectivity of inorganic chlorine was significantly higher than that of MnO2. The 18O isotope kinetic experiments revealed that the interfacial reaction process followed the MVK mechanism. The large number of oxygen vacancies accelerated the migration of lattice oxygen from the interior to the exterior, enhancing the ability to trap gas-phase oxygen. Mn4+ acted as the main active center to participate in CB catalysis, and the resulting reactive oxygen species (ROS) and Mn3+-[O2-]-Ce4+ further accelerated the entire oxidation cycle.

Unravelling the nature of the active species as well as the Mn doping effect over gamma-Al2O3 catalyst for eliminating AsH3 and PH3

To investigate the enhancing effect of Mn on the performance of simultaneous catalytic oxidation of AsH3 and PH3 by CuO-Al2O3 in a reducing atmosphere under micro-oxygen conditions, Cu-Mn modified γ-Al2O3 catalysts were prepared. The characteristics of the catalysts showed that Mn reduced the crystallinity of the active CuO component, increased the number of oxygen vacancies and acidic sites on the catalyst surface, enhanced the mobility of surface oxygen, and the interaction between copper and manganese promoted the redox cycling ability of the catalysts and improved their oxidation performance, which increased the conversion frequency (TOF) by 2.54 × 10-2 to 3.07 × 10-2 sec-1. On the other hand, the introduction of Mn reduced the production of phosphate and As2O3 on the catalyst surface by 30.96% and 44.9%, which reduced the coverage and inerting of the active sites by phosphate and As2O3, resulting in an 8 hr (6 hr) improvement in the stability of PH3 (AsH3) removal.

Synthesis of cordierite using municipal solid waste incineration fly ash as one additive for enhanced catalytic oxidation of volatile organic compounds

Municipal solid waste incineration (MSWI) fly ash is a hazardous waste, which needs various recycling in order to reach "net-zero waste". This work aimed to synthesize cordierite using MSWI fly ash as one additive and investigate influence of the additive on properties of the cordierite. As a result, the cordierite was successfully synthesized when the additive weight ratio was <15 % and the synthesis strategy was universally feasible for 14 kinds of different MSWI fly ashes. As a heat accumulator, the cordierite attained compressive strength of 42.1 MPa, water absorption of 26 %, bulk density of 1.87 g·cm-3, and open porosity of 47 %. After five cycles of thermal impact at 1200 °C, the strength was only decreased by 15 %. These properties were comparable to a commercial cordierite. As a catalyst carrier, after loading Mn and Cu species, the cordierite removed 100 % of toluene at 250 °C. In comparison, a commercial cordierite only got a removal of 34.4 %. The enhanced activity was attributed to co-existing spinel and bytownite as well as imbedded Zn and Cu in the MSWI fly ash-added cordierite. Therefore, this work devotes to hazardous recycling, green development, and cycled economy.

Acetone Efficient Degradation under Simulated Humid Conditions by Mn-O-Pt Interaction Taming-Triggered Water Dissociation Intensification

As a generally existing component in industrial streams, H2O usually inhibits the catalytic degradation efficiency of volatile organic compounds (VOCs) greatly. Here, we propose a novel strategy that accelerates the H2O dissociation and facilitates positive feedbacks during VOC oxidation by fabricating citric acid (CA)-assisted Pt(K)-Mn2O3/SiO2 (Pt-Mn/KS-xCA). Results reveal that the complexation of carboxyl groups of citric acid with Mn cations leads to the formation of small Mn2O3 (4.1 ± 0.2 nm) and further enhances the Mn-O-Pt interaction (strengthened by the Si-O-Mn interaction), which can transfer more electrons from Pt-Mn/KS-6CA to H2O, thus facilitating its breaking of covalent bonds. It subsequently produces abundant surface hydroxyl groups, improving the adsorption and activation abilities of acetone reactant and ethanol intermediate. Attributing to these, the acetone turnover frequency value of Pt-Mn/KS-6CA is 1.8 times higher than that of Pt-Mn/KS at 160 °C, and this multiple changes to 6.3 times in the presence of H2O. Remarkably, acetone conversion over Pt-Mn/KS-6CA increases by up to 14% in the presence of H2O; but it decreases by up to 26% for Pt-Mn/KS due to its weak dissociation ability and high adsorption capacity toward H2O. This work sheds new insights into the design of highly efficient catalytic materials for VOC degradation under humid conditions.

Self-assembly of random networks of zirconium-doped manganese oxide nanoribbons and poly(3,4-ethylenedioxythiophene) flakes at the water/chloroform interface

Owing to their magnificent chemical and physical properties, transition metal-based heterostructures are potential materials for applications ranging from point-of-care diagnostics to sustainable energy technologies. The cryptomelane-type octahedral molecular sieves (K-OMS-2) are extensively studied porous materials with a hollandite (2 × 2 tunnel of dimensions 4.6 × 4.6 Å2) structure susceptible to the isovalent substitution of metal cations at the framework of MnO6 octahedral chains. Here we report a facile in situ synthesis of framework-level zirconium (Zr)-doped K-OMS-2 nanoribbons in poly(3,4-ethylenedioxythiophene) (PEDOT) nanoflakes at a water/chloroform interface at ambient conditions. An oxidant system of KMnO4 and ZrOCl2·8H2O initiated the polymerisation at temperatures ranging from 5° to 50 °C. The lattice distortions arising from the framework-level substitution of Mn4+ by Zr4+ in the K-OMS-2 structure were evidenced with powder X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and N2 adsorption-desorption studies. Transmission electron microscopic and mapping images confirmed that PEDOT/Zr-K-OMS-2 comprises a highly crystalline random network of two-dimensional PEDOT flakes and Zr-doped K-OMS-2 nanoribbons. In this regard, the proposed interfacial strategy affirms an in situ method for the morphological tuning of heterostructures on polymer supports at low temperatures.

Coupling effect of reaction conditions on the catalytic activity of Cu-Mn composite oxide catalysts for toluene

Volatile organic compounds (VOCs) are one of the major components of air pollution. Catalytic combustion is a promising technology for the treatment of VOCs and at its center is the preparation of efficient and cheap catalysts. In this study, by loading copper (Cu) and manganese (Mn) on Santa Barbara Amorphous-15 (SBA-15) molecular sieve, the Cux-Mny/SBA-15 (x = 1, 2; y = 1, 2) composite metal oxide catalyst was prepared using the equal volume impregnation method. Their performance in the toluene catalytic combustion reaction was investigated by adjusting the molar ratio (x : y), and the loading of Cu and Mn. The results of the Brunner-Emmett-Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analyses show that the CuMnO spinel phase can be detected in the Cu-Mn composite metal oxide catalyst doped with a low concentration of Cu. The overall rod-like structure of the fibrous network structure provides a large specific surface area, and the particle crystallinity is low and the dispersion is good. Due to the synergistic effect of Cu and Mn, the greater the amount of Mn3+ and adsorbed oxygen species (Oads) that are available, and the higher the turnover frequency (TOF) value, the better and more superior catalytic performance and excellent stability is obtained, when compared with the single-component oxides used in toluene catalytic combustion. After a continuous catalytic reaction for 12 h, the toluene conversion rate remained above 95%. The coupling effect of the catalytic reaction temperature and concentration of oxygen on the catalytic combustion of toluene was also studied. At a low reaction temperature (<250 °C), the increase of the concentration of oxygen played a superior role in promoting the conversion of toluene. The kinetic analysis of the toluene catalytic combustion process showed that the catalytic combustion of toluene by Cu-Mn/SBA-15 followed both the Mars-Van Krevelen (MVK) and Langmuir-Hinshelwood (L-H) reaction mechanisms. With the increase of the Oads amount caused by the decrease of the Cu ratio, the proportion of the L-H reaction mechanism increases.

Investigation on removal of multi-component volatile organic compounds in a two-stage plasma catalytic oxidation system - Comparison of X (X=Cu, Fe, Ce and La) doped Mn2O3 catalysts

Mn₂O₃-X catalysts (X = Cu, Fe, Ce and La) were prepared based on γ-Al₂O₃ for the mixture degradation of muti-component volatile organic compounds (VOCs) composed of toluene, acetone, and ethyl acetate. The catalysts were characterized, and the density functional theory (DFT) simulation of ozone adsorption on Mn₂O₃-X were carried out to investigate the influence of adsorption energy on catalytic performance. The results showed that the removal efficiency (RE) of each VOC component was similarly improved by Mn₂O₃-X catalysts, and the greatest increase in VOCs' removal efficiency was obtained (7.8% for toluene, 86.2% for acetone, and 82.5% for ethyl acetate) at a special input energy (SIE) of 700 J L^-1 with Mn₂O₃-La catalyst. Characterization results demonstrated that Mn₂O₃-La catalyst had the highest content of low valence Mn elements and the greatest O_ads/O_latt ratio, as well as the lowest reduction temperature. Mn₂O₃-La catalyst also presented superior catalytic effect in improving carbon balance (CB) and CO₂ selectivity ( [Formula: see text] ). The CB and [Formula: see text] were increased by 47.7% and 12.61% respectively with Mn₂O₃-La at a SIE of 400 J L^-1 compared with that when only γ-Al₂O₃ was applied. The DFT simulation results of ozone adsorption on Mn₂O₃-X catalysts indicated that the adsorption energy of catalyst crystal was related to the catalytic performance of the catalyst. The Mn₂O₃-La/γ-Al₂O₃ catalyst, which had the highest absolute value of adsorption energy, presented the best performance in improving VOCs' RE.

Electronic properties and adsorption mechanism of Ru-doped copper clusters towards CH3OH molecule: A DFT investigation

In this study, we have investigated the stability and electronic properties of the Cu_nRu (n = 2-10) nanoclusters and their interaction with the CH₃OH molecule without and with the presence of O₂ molecule by using DFT calculations with TPSS/SDD/6-311g(d,p) level of theory. Based on the second energy difference (Δ²E), the results reveal that the Cu_nRu (n = 4, 6 and 8) clusters are relatively more stable than their neighboring clusters. The values obtained for the Fukui function (f^-) proves that the Ru atom in the Cu_nRu clusters is an excellent adsorption site for the molecules. The interaction of the Cu_nRu clusters with CH₃OH molecule exhibits that the Ru atom is the preferred adsorption site for the CH₃OH molecule, where the O atom of the CH₃OH molecule is strongly chemisorbed onto the Ru site of the clusters, forming a strong bond between the Ru and O atoms. The copper sites of the clusters were found less preferred for the adsorption of CH₃OH, and the complexes formed between both species are less stable than those obtained from the CH₃OH chemisorption over the Ru site of the clusters. The interaction of CH₃OH with the clusters was also evaluated in an oxidizing environment, and the results obtained reveal that the molecule is greatly chemisorbed over the ruthenium site with adsorption energies which vary from - 1.18 to - 2.05 eV. In the presence of the oxygen, the gap energy of the clusters was sharply changed after their interactions with the CH₃OH molecule, suggesting that these clusters can easily detect the above molecule with great sensitivity. Therefore, the presence of the oxygen not only does not prevent the adsorption process, but it considerably promotes the CH₃OH chemisorption onto the ruthenium site of the clusters and therefore significantly rises their sensitivity performance. In conclusion, the Cu_nRu clusters could be employed as effective nanosensors for the CH₃OH molecule detection.

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.

Revealing the mechanism of high water resistant and excellent active of CuMn oxide catalyst derived from Bimetal-Organic framework for acetone catalytic oxidation

Environmental H₂O is an influential factor in the low-temperature catalytic oxidation of volatile organic compounds (VOCs), and it significantly impacts the reaction process and mechanism. Here, a series of rod-like Cu-Mn oxides were synthesised by pyrolysing Cu/Mn-BTC for acetone oxidation. The results confirm that the formation of multiphase interfaces have more excellent catalytic performance compared to single-phase catalysis. This phenomenon can be attributed to the formation of multiphase interfaces, which resulted in the synthesized catalysts with more active oxygen species and defective sites. The CuMn₂O_x catalyst exhibited superior catalytic performance (T₉₀ = 150 °C), high water resistance and long-term stability. Furthermore, in situ diffuse reflectance infrared Fourier transform spectroscopy and thermal desorption-gas chromatography-mass spectrometry results indicated that the degradation pathway of acetone was as follows: acetone ((CH₃)₂CO*) → enolate complexes ((CH₂) = C(CH₃) O*) → acetaldehyde ((CH₃CHO*) → acetate (CH₃COO*) → formate (HCOO*) → CO₂ and H₂O. At a low-temperature, water vapour dissociated a large number of activated hydroxyl groups on the multiphase interface, which promoted the dissociation of enolate complexes and acetaldehyde species. This composite oxide is a promising catalyst for removing oxygenated VOCs at high humidity.

Accelerating the Low-Temperature Catalytic Oxidation of Acetone over Al-Substituted Mn-Al Oxides by Rate-Limiting Step Modulation

In order to enhance the catalytic activity and improve the stability of Mn-Al oxides in acetone oxidation, it is interesting to have found that modulating and accelerating the rate-limiting step by Al substitution rather than just mixing of Mn and Al is crucial for hydrocarbon efficient catalytic destruction. Here, a series of Mn-Al oxides with different Al substitution ratios were prepared by a scalable and facile hydrothermal-redox strategy. The reaction rate, selectivity, and stability of the representative α-MnO₂ catalyst in acetone oxidation can be remarkably promoted by simple replacing of the partial framework Mn with Al, which changes the rate-limiting step from acetic acid dissociation to ethanol decomposition accelerated by H₂O molecules. Among them, MnAl_0.5 displays the best catalytic performance with 90% of acetone converted at just 165 °C and a remarkable CO₂ yield. DFT results suggest that the p_y and p_x orbitals of the O element take part in the formation of the carbonyl group when the intermediate of removing H* from ethanol reacts with the hydroxyl group of H₂O. The d_xz orbital of Mn with p-electron of Al plays a vital role in the rate-limiting step. The work provides new insights into engineering catalysts for industrial VOC efficient and economical mineralization.

Promotional effects of calcination temperature and H2O on the catalytic activity of Al-substituted MnAlO catalysts for low-temperature acetone oxidation

In order to enhance the role of Al in the materials, Al-substituted MnAlO catalysts were synthesized via the hydrothermal-redox method at different calcination temperatures for acetone oxidation. There were Al-substituted α-MnO₂ and amorphous aluminum oxide existed with homogeneous dispersion of elements in the catalysts. The surface property, reaction rate, CO₂ yield and water resistance of MnAlO catalysts were greatly affected by calcination temperatures. MnAlO-450 catalyst exhibited the best catalytic performance (acetone conversion of 90% at 165 °C) with CO₂ yield higher than 99.7%, which was mainly related to the weaker Mn-O bond strength, lower temperature reducibility and abundant Lewis acid sites. The acetone conversion of MnAlO-450 increased by as much as 16% in the presence of 1 vol% H₂O compared to that in the absence of H₂O at T₅₀ (the temperature for 50% conversion of acetone). The acceleration consumption of ethanol as the main by-product by H₂O improved the catalytic performance. This work would shed light on the Al substitution based catalysts for OVOC oxidation with highly efficient and water resistance.

Low-Temperature Selective NO Reduction by CO over Copper-Manganese Oxide Spinels

Selective catalytic reduction of NO with CO (CO-SCR) has been suggested as an attractive and promising technology for removing NO and CO simultaneously from flue gas. Manganese-copper spinels are a promising CO−SCR material because of the high stability and redox properties of the spinel structure. Here, we synthesized CuxMn3-xO4 spinel by a citrate-based modified pechini method combining CuO and MnOx, controlling the molar Cu/Mn concentrations. All the samples were characterized by SEM, EDX, XRD, TEM, H2−TPR, XPS and nitrogen adsorption measurements. The Cu1.5Mn1.5O4 catalyst exhibits 100% NO conversion and 53.3% CO conversion at 200 °C. The CuxMn3-xO4 catalyst with Cu-O-Mn structure has a high content of high valence Mn, and the high mass transfer characteristics of the foam-like structure together promoted the reaction performance. The CO-SCR catalytic performance of Cu was related to the spinel structure with the high ratio of Mn4+/Mn, the synergistic effect between the two kinds of metal oxides and the multistage porous structure.

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.

Determination of trace compounds and artifacts in nitrogen background measurements by proton transfer reaction time-of-flight mass spectrometry under dry and humid conditions

A qualitative analysis was applied for the determination of trace compounds at the parts per trillion in volume (ppt_v ) level in the mass spectra of nitrogen of different qualities (5.0 and 6.0) under dry and humid conditions. This qualitative analysis enabled the classification and discovery of hundreds of new ions (e.g., [S_x ]H⁺ species) and artifacts such as parasitic ions and memory effects and their differentiation from real gas impurities. With this analysis, the humidity dependency of all kind of ions in the mass spectrum was determined. Apart from the inorganic artifacts previously discovered, many new organic ions were assigned as instrumental artifacts and new isobaric interferences could be elucidated. From 1140 peaks found in the mass range m/z 0-800, only 660 could be analyzed due to sufficient intensity, from which 463 corresponded to compounds. The number of peaks in nitrogen proton transfer reaction (PTR) spectra was similarly dominated by nonmetallic oxygenated organic compounds (23.5%) and hydrocarbons (24.1%) Regarding only gas impurities, hydrocarbons were the main compound class (50.2%). The highest contribution to the total ion signal for unfiltered nitrogen under dry and humid conditions was from nonmetallic oxygenated compounds. Under dry conditions, nitrogen-containing compounds exhibit the second highest contribution of 89% and 96% for nitrogen 5.0 and 6.0, respectively, whereas under humid conditions, hydrocarbons become the second dominant group with 69% and 86% for nitrogen 5.0 and 6.0, respectively. With the gathered information, a database can be built as a tool for the elucidation of instrumental and intrinsic gas matrix artifacts in PTR mass spectra and, especially in cases, where dilution with inert gases plays a significant role.

Effect of Cu/Co ratio in CuaCo1-aOx (a = 0.1, 0.2, 0.4, 0.6) flower structure on its surface properties and catalytic performance for toluene oxidation

Catalytic oxidation is considered a high-efficient method to minimize efficiently toluene emission. It is still a challenge to improve the catalytic performance for toluene oxidation by modifying the surface properties to enhance the oxidation ability of catalyst. Herein, a series of Cu_aCo_1-aO_x (a = 0.1, 0.2, 0.4, 0.6) catalysts were synthesized via solvothermal method and applied for toluene oxidation. The effects of the Cu/Co ratio on the texture structure, morphology, redox property and surface properties were investigated by various characterization technologies. The Cu_0.4Co_0.6O_x catalyst with dumbbell-shaped flower structure exhibited much lower temperature of 50% and 100% toluene conversion and far higher reaction rate (13.96 × 10^-2 μmol·g^-1·s^-1) at 220 °C than the Co based oxides in previous reports. It is found that the good activity can be attributed to the fact that the proper Cu/Co ratio can significantly improve the formation of more surface adsorbed oxygen and Co³⁺ species, leading to the much higher oxidation ability came from the strong interaction between Cu and Co oxides. It is suggested that toluene should be oxidized more rapidly to CO₂ and H₂O over the Cu_0.4Co_0.6O_x catalyst than Co₃O₄ based on the results of in situ DRIFTS.