Cryptomelane-type Manganese Oxide Research Articles

Evolution of Heterogeneous Tunnel Structures in Cryptomelane-Type Manganese Oxides and Their Geoinspired Implications.

Cryptomelane-type manganese oxides, α-MnO2 (KxMn8O16), play key roles in various fields such as geochemical processes, catalytic reactions, energy storage, and environmental sciences. The function of cryptomelane-type oxides can be affected by cation substitutions and the changes in tunnel structures. Research on natural cryptomelane minerals could provide geoinspiration for the design of new nanomaterials with cation substitutions, as well as a key to understanding the evolution of tunnel structures. In this study, natural cryptomelane minerals are characterized by the cosubstitution of iron and zinc. The localization of cosubstituted Fe and Zn in the tunnel framework has been revealed. Furthermore, the evolution of heterogeneous tunnel structures in cryptomelane has been demonstrated as a transition from large-size tunnels to small ones with high Mn(III) concentrations, indicating the significant role of Mn(III) in driving this transition. Lead (Pb2+) can be effectively trapped in the 2 × 2 tunnels. A mechanism for the attachment of cryptomelane crystals in different orientations has also been explored, showing that the migration of Mn atoms and the formation of (110) planes at specific sites contribute to lattice matching at the boundary. Our results provide geoinspired insights into controlled synthesis with Fe/Zn cosubstitution, a fundamental understanding of the evolution of tunnel structures, and functionalized applications of tunnel-based nanomaterials.

Nitrogen promoted electron transfer to create highly active Cu2+ and Mn3+ sites for efficient catalytic ozonation of Ammonia over copper and nitrogen co-doped Cryptomelane-Type manganese oxide at ambient temperature

Eliminating ammonia (NH3) from high-humidity exhaust gases by catalytic oxidation is a great challenge, especially at low temperatures. Herein, Cu and N co-doped cryptomelane-type manganese oxide (Cu-N-OMS-2) catalysts with high active Cu2+ and Mn3+ sites were synthesized and tested for catalytic ozonation of NH3 at 30 °C. The optimal Cu0.05-N2-OMS-2 catalyst exhibited 93.8 % NH3 conversion, 91.7 % O3 conversion, and 92.1 % N2 selectivity when the relative humidity (RH) was 40 %, whereas OMS-2 only demonstrated 39.3 %, 44.0 % and 83.1 %. Additionally, NH3 and O3 conversion at high RH could be recovered with just a basic water-washing, indicating that Cu0.05-N2-OMS-2 catalyst possessed outstanding renewable properties. The results of characterization revealed that N doping increased the amount of Cu fixed on the catalyst surface and promoted the electron transfer from Cu to Mn, resulting in an increase in electron-poor Cu2+ and electron-rich Mn3+ in Cu-N-OMS-2. In dry conditions, the Mn3+ sites provided O3 with electrons to form adsorbed oxygen species and •O2–, which were essential for oxidation of adsorbed NH3. The electron-poor Cu2+ sites combined with H2O and O3 to form Cu-OH and •HO3, which facilitated the generation of •OH and •O2–, thereby improving the stability under humid conditions. This study provides a new theoretical guidance for the efficient removal of NH3 in industrial exhaust gases.

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.

Improving the non-radical/radical transformation efficiency of halogenated polycyclic aromatic hydrocarbons by using metal ion-exchanged cryptomelane-type OMS-2 under moisture-unsaturated conditions

Halogenated polycyclic aromatic hydrocarbons (HPAHs) have recently remerged as major soil contaminants. However, limited studies have focused on the transformation and removal of HPAHs in soils. The transformation behaviors of HPAHs on cryptomelane-type manganese oxide (K(I)-OMS-2) and its metal ion-exchanged M-OMS-2 (M = Fe(III), Cu(II), and Zn(II)) under moisture-unsaturated conditions were systematically investigated. The transformation kinetics results showed that the transformation rate constants (k) of HPAHs were 0.077–0.829, 0.071–0.629, 0.068–0.530, and 0.064–0.259 d−1 for Fe(III)-OMS-2, Cu(II)-OMS-2, Zn(II)-OMS-2, and K(I)-OMS-2. The metal ion exchange treatment of K(I)-OMS-2 enhanced the transformation efficiency of HPAHs. The characterization (H2-TPR, CV, ESI, XPS, and EPR) and theoretical calculation (density of states and formation energy of oxygen vacancies) results indicated that the non-radical/radical transformation efficiency of HPAHs on M-OMS-2 was higher than that on K(I)-OMS-2. The transformation rate of HPAHs was positively correlated with the redox potential of metal ions and superoxide radical yield of M-OMS-2. Additionally, this study revealed that HPAHs with lower redox potential and more strongly bound to M-OMS-2 were more preferentially transformed. Finally, the experimental results of the addition of M-OMS-2 to the natural soil revealed that M-OMS-2 aids the transformation of HPAHs in soils. This study reveals the chemical transformation behavior of HPAHs in soils and suggests that M-OMS-2 can a promising agent for soil remediation.

Insights into the influence of water molecules on selective catalytic ozonation of gaseous ammonia into nitrogen on cryptomelane-type manganese oxide using in-situ DRIFTS

Catalytic ozonation is an environmentally friendly technology for the removal of gaseous NH3 due to high NH3 conversion and high N2 selectivity at ambient temperature. However, the influence mechanism of ubiquitous water vapor on catalytic ozonation of NH3 is unclear. In this study, cryptomelane-type manganese oxide (OMS-2) catalyst was prepared and tested for catalytic ozonation of NH3 in different relative humidity. The results showed that water vapor significantly decreased the catalytic activity, which was due to the inhibition of water on NH3 adsorption on Lewis acid sites and O3 decomposition on oxygen vacancies, as well as the combination of water with active oxygen species (O22− and Oatom). And the effect of water vapor on NH3 conversion was more significant than O3 decomposition because more Mn–OH were involved in the O3 decomposition under humid conditions. Combining in-situ DRIFTS results with the performance of NH3 oxidation, it is found that L-2 acid sites (the peak of NH3 adsorption on Lewis acid sites at 1188 cm−1) were the main active sites for adsorption and activation of NH3 in the early stage of catalytic reaction; as the reaction progressed, L-2 acid sites were gradually occupied by water and more Brønsted acid sites participated in the catalytic reaction. This work deepened the understanding of the reaction process for selective catalytic ozonation of NH3, and provided theoretical guidance for the design of efficient hydrophobic catalysts to eliminate gaseous NH3 pollution.

Vanadium doped OMS-2 catalysts for one-pot synthesis of imine from benzyl alcohol and aniline: Effects of vanadium content and precursor

A series of vanadium doped cryptomelane-type manganese oxide (V-OMS-2) catalysts were prepared by a simple, low-cost reflux method, and investigated for one-pot imine synthesis from oxidative coupling of benzyl alcohol and aniline with air. The physicochemical properties of the V-OMS-2 catalysts were characterized by various techniques including XRD, BET, SEM, TEM, XPS, H2-TPR and NH3-TPD. It was found that the surface area, Lewis acid sites, the amount of Mn3+ component and active surface oxygen species were much improved with vanadium doping. Consequently, the activity of V-OMS-2 catalyst for oxidative coupling of benzyl alcohol and aniline to imine was enhanced. The highest conversion and the imine yield were obtained over the 3 mol% V-OMS-2 catalyst, being ∼99% and 92%, respectively. Higher vanadium doping (≥ 6 mol%), however, hindered the preservation of OMS-2 crystal structure, leading to a drop in the catalytic performance. The high specific surface area was suggested to be the key contributor to the high catalytic activity of 3% V-OMS-2(1) catalyst. Among the vanadium precursors studied, the catalyst prepared with vanadium pentoxide exhibited a much higher catalytic activity, which can be attributed to its larger surface area, unique mesoporous structure, increased Lewis acid sites and more readily available surface oxygen species. In addition, the stability and recyclability of the catalyst were also studied, and the reaction mechanism was discussed.

Hierarchical structure and electronic effect promoted degradation of phenols over novel MnO2 nanoprisms via non-radical mechanism

In this work, a novel 3D hierarchical cryptomelane-type manganese oxide nanocomposite (APM-OMS-2) was prepared via pre-incorporation with ammonium phosphomolybdate (APM) as an additive for the first time. The APM dosage not only transferred the material morphology from 1D to 3D, but also affected the crystal phasze, textural properties and redox ability. APM-OMS-2 displayed a unique nanoprism morphology coated by APM as the outer-sphere and consisted of a hierarchical framework. The catalytic ability of APM-OMS-2 was examined by the degradation of phenolics via peroxymonosulfate (PMS) activation in various water matrix. The optimized catalytic system gave a 95 % degradation rate and a 83 % mineralization rate of 4-chlorophenol (20 mg/L) with 200 mg/L of APM-OMS-2 and 100 mg/L of PMS within 60 min. According the first-order kinetic study of 4-chlorophenol degradation, APM-OMS-2/PMS system gave a higher initial reaction rate constant at 0.2014 min−1 and a lower activation energy at 29.2 kJ/mol than OMS-2/PMS because its hierarchical structure and textural properties were beneficial to mass transfer, diffusion and reaction between catalyst and reactants. Characterizations further suggested that PMS activation was conducted by 1O2-mediated oxidation along with MnIV/MnIII redox cycles. Moreover, electrochemical analysis indicated that mediated electron transfer contributed to the catalytic performance via the outer-sphere electronic interaction of the core/shell nanocomposite. This study provides a very simple method for the synthesis of 3D cryptomelane-type MnO2 towards oxidative degradation of aqueous contaminants.

Comprehensive evaluation of cobalt incorporated cryptomelane-type manganese oxide molecular sieve as an efficient adsorbent for enhanced removal of europium from wastewater systems

Extraction of radionuclide contaminants from wastewater systems has recently drawn widespread attention, and then developing a novel and green extracting technology has also become an enormous challenge. Herein, a facile hydrothermal method was employed to fabricate cobalt-incorporated cryptomelane-type manganese oxide molecular sieve (Co-OMS-2) for extraction Eu(III) from wastewater under diverse experimental conditions. All kinds of characterized techniques, such as SEM, TEM, XRD, FTIR, BET, EDS and XPS had verified the qualified synthesis process and splendid structural features of the Co-OMS-2. The maximum adsorption capacity of Co-OMS-2 was 7.62 × 10−4 mol/g for Eu(III) at 298 K, which was superior than primarily traditional materials reported previous literatures. The high adsorption capacity of Eu(III) onto Co-OMS-2 was primarily attributed to high specific surface area and abundant surface functional groups, and the interactions were mainly contributed to strong surface complexation and electrostatic attraction. Under the condition of low pH, the outer-sphere surface complexation and cation exchange were primary mechanisms to Eu(III) adsorption onto Co-OMS-2 composites, while inner-sphere surface complexation was mainly assigned to Eu(III) adsorption onto Co-OMS-2 under the high pH sections. The Co-OMS-2 composite achieved equilibrium in a relatively short time, and this excellent performance was conducive to the treatment of Eu(III) under the extreme emergency conditions. In view of the extraordinary adsorption capacity and recycled reusability, the Co-OMS-2 composites can be as prospective adsorbents adopted for the extraction of Eu(III) in real wastewater management.

Anti-sulfur selective catalytic reduction of NOx on Sb-doped OMS-2

Sb doped cryptomelane-type manganese oxides (Sb-OMS-2) with Sb/Mn molar ratios of 0–0.4 were synthesized and investigated for low-temperature selective catalytic reduction of NO by CO (CO-SCR). The resulting catalyst was characterized by X-ray diffractometer (XRD), specific surface area analyzer (BET), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). As a result, the doping of Sb increased the specific surface area and oxygen vacancy concentration of the catalyst. At the same time, most Sb entered the lattice of MnO6 to form Sb–O bonds, and a small number of Sb ions replaced K+ in the tunnel. Sb and Mn synergistically catalyzed the reduction of NO. Compared with undoped OMS-2, the NO conversion rate of Sb0.2-OMS-2 increased from 60% at 150 ℃ to 83%, and from 80% at 300 ℃ to 95%. In the presence of SO2, the NO conversion rate of Sb0.2-OMS-2 at 300 ℃ was 88%, while that of OMS-2 was only 60%.

Manganese Oxide Minerals from the Xiangtan Manganese Deposit in South China and Their Application in Formaldehyde Removal

Because of the nano-scale tunnel constructed by the active Mn-O octahedron in cryptomelane, cryptomelane-type manganese oxides have high activity in the oxidation of several volatile organic compounds (VOCs). Natural cryptomelane, in the form of supergene oxide manganese ore, carpets much of South China. In the lower part of the Datangpo Formation of Nanhua System on the southeastern Yangtze Platform, cryptomelane is one of the major manganese oxides in black shale of the Xiangtan manganese deposit in this deposit. Formaldehyde is a dominant indoor pollutant among volatile organic compounds (VOCs), and applications of synthetic cryptomelane have been reported to eliminate it. To study the removal capacity of naturally outcropping cryptomelane, representative samples of manganese oxide (the primary mineral component of cryptomelane) from the Xiangtan Mn deposit were analyzed in this study. The chemical composition, crystal structure and micromorphology of the manganese oxide minerals were explored using ICP-AES, XRD, EPMA, SEM and HR-TEM techniques. Fine-grained and poorly crystalline, these minerals consist primarily of cryptomelane, along with minor amounts of pyrolusite, hollandite, lithiophorite, limonite and quartz. Natural cryptomelane is a monoclinic crystal, and its cell parameters are refined. The results of catalytic tests revealed that natural cryptomelane has obvious catalytic activity in the oxidation of formaldehyde in a static environment under room temperature. This study may provide a natural mineral material as an inexpensive and efficient catalyst for the purification of formaldehyde in industrial or indoor air treatment.

Promotional effect of cobalt doping on catalytic performance of cryptomelane-type manganese oxide in toluene oxidation

The cryptomelane-type manganese oxide (OMS-2)-supported Co (xCo/OMS-2; x = 5, 10, and 15 wt.%) catalysts were prepared via a pre-incorporation route. The as-prepared materials were used as catalysts for catalytic oxidation of toluene (2000 ppmV). Physical and chemical properties of the catalysts were measured using the X-ray diffraction (XRD), Fourier transform infrared spectroscopic (FT-IR), scanning electron microscopic (SEM), X-ray photoelectron spectroscopy (XPS), and hydrogen temperature-programmed reduction (H2-TPR) techniques. Among all of the catalysts, 10Co/OMS-2 performed the best, with the T90%, specific reaction rate at 245°C, and turnover frequency at 245°C (TOFCo) being 245°C, 1.23 × 10−3 moltoluene/(gcat·sec), and 11.58 × 10−3 sec−1 for toluene oxidation at a space velocity of 60,000 mL/(g·hr), respectively. The excellent catalytic performance of 10Co/OMS-2 were due to more oxygen vacancies, enhanced redox ability and oxygen mobility, and strong synergistic effect between Co species and OMS-2 support. Moreover, in the presence of poisoning gases CO2, SO2 or NH3, the activity of 10Co/OMS-2 decreased for the carbonate, sulfate and ammonia species covered the active sites and oxygen vacancies, respectively. After the activation treatment, the catalytic activity was partly recovered. The good low-temperature reducibility of 10Co/OMS-2 could also facilitate the redox process accompanied by the consecutive electron transfer between the adsorbed O2 and the cobalt or manganese ions. In the oxidation process of toluene, the benzoic and aldehydic intermediates were first generated, which were further oxidized to the benzoate intermediate that were eventually converted into H2O and CO2.

Low thermal oxidation of gaseous toluene over Cu/Ce single-doped and co-doped OMS-2 on different synthetic routes

In this study, cryptomelane-type manganese oxide (K-OMS-2) was used as a catalyst for the thermal oxidation of toluene. The catalyst was synthesized via the hydrothermal route and doped with Cu, Ce, and co-doped with the same metals by ex-situ and in-situ methods. The effects of dopant type, dopant concentration, and dopant impregnation method on the thermal catalytic activity of K-OMS-2 were examined. Initially, 1%, 2%, and 4% by mole Cu or Ce were incorporated by wet impregnation to the hydrothermally synthesized K-OMS-2 and in-situ direct hydrothermal method. The physicochemical properties were characterized by X-ray diffraction (XRD), N2 adsorption-desorption, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and thermal gravimetric analysis (TGA) techniques. Gaseous toluene was made to react with the doped catalysts in a packed bed reactor attached to gas chromatography equipment to measure the amount of unreacted toluene. Results showed K-OMS-2 doped with 4% by mole Cu and 1% by mole Ce gave excellent thermal catalytic efficiency results at 180 °C reaction temperature. Meanwhile, K-OMS-2 co-doped with Ce and Cu with a mole ratio of 0.25:3.75 per 100 moles of K-OMS-2 gave the best catalytic activity and complete oxidation at 180 °C reaction temperature. Comparing K-OMS-2 doped with the same metal ratios, in situ doped catalysts led to higher toluene removal percentages. Meanwhile, the 72-h stability test showed that the optimum co-doped catalysts are capable of entirely oxidizing toluene even after prolonged and continuous use.

Selective catalytic ozonation of gaseous ammonia to dinitrogen on cryptomelane-type manganese oxide catalysts: Role of oxygen vacancies and acid sites

Two kinds of cryptomelane-type manganese oxide (OMS-2-Ac and OMS-2-SO4) catalysts were prepared with MnAc2·4H2O and MnSO4 as precursors, respectively, and were tested for selective catalytic ozonation of gaseous ammonia (NH3). The results showed that two OMS-2 catalysts exhibited high NH3 conversion and N2 selectivity as well as complete decomposition of ozone. Combining the results of characterization and in-situ DRIFTS, it is revealed that the surface oxygen vacancies and Lewis acid sites of OMS-2 played key roles in excellent catalytic activity and N2 selectivity, while the contribution of Brønsted acid sites to NH3 conversion was much less than that of Lewis acid sites. In-situ DRIFTS studies suggested that catalytic ozonation of NH3 over OMS-2 catalysts followed the imide and i-SCR mechanisms. In addition, the Mn2+ precursor exhibited a certain effect on the catalytic activity of OMS-2. OMS-2-Ac catalysts presented higher NH3 conversion (89%) and N2 selectivity (91%) at 30 °C under dry condition; moreover, the influence of humidity on the conversion of NH3 and O3 over OMS-2-SO4 was greater than OMS-2-Ac at 30 °C, which were mainly due to more surface oxygen vacancies and better hydrophobicity of OMS-2-Ac. This work provided an improved understanding of the role of the oxygen vacancies and acid sites in selective catalytic ozonation of ammonia over cryptomelane-type manganese oxide.

The Effect of Support on the Catalytic Efficiency of MnO2/Activated Carbon for degradation of Methylene blue

Methylene blue (MB) is a blue cationic thiazine dye which is widely used in cotton fiber, wood and textile industries, but has an adverse effect to environment and living organism. It is urgent to find an alternative and effective solution for this recalcitrant compound. The Fenton process-based advanced oxidation process is considered as an effective method for the degradation of organic contaminants such as dyes. In this study, the composite α-MnO2/activated carbon (AC) was first synthesized by a facile, one-pot synthesis using sol-gel method and characterized by X-ray diffraction (XRD) and BET surface areas. The XRD results indicated that the tunnel cryptomelane-type manganese oxide (α-MnO2) was successfully synthesized as the minor phase in the activated carbon support. The composite α-MnO2/AC has higher specific surface area than the single α-MnO2. The catalytic studies indicated that the α-MnO2/AC has a much higher catalytic efficiency over single α-MnO2 for degradation of MB. The presence of the support and the increase in surface area of α-MnO2/AC could be responsible for its higher catalytic efficiency compared to single α-MnO2. In the optimum condition, the α-MnO2/AC was able to degrade 98.48 % and 99.2 % of MB within 10 and 120 minute of reaction time, respectively.

Enhanced Performance of the OMS-2-Supported CuOx Catalysts for Carbon Monoxide, Ethyl Acetate, and Toluene Oxidation

Different Cu contents (x wt%) were supported on the cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) (xCu/OMS-2; x = 1, 5, 15, and 20) via a pre-incorporation method. Physicochemical properties of the OMS-2 and xCu/OMS-2 samples were characterized by means of the XRD, FT-IR, SEM, TG/DTG, ICP-OES, XPS, O2-TPD, H2-TPR, and in situ DRIFTS techniques, and their catalytic activities were measured for the oxidation of CO, ethyl acetate, and toluene. The results show that the Cu species were homogeneously dispersed in the tunnel and framework structure of OMS-2. Among all of the samples, 15Cu/OMS-2 sample exhibited the best activities with the T50% of 65, 165, and 240 °C as well as the T90% of 85, 215, and 290 °C for CO, ethyl acetate and toluene oxidation, respectively, which was due to the existence of the Cu species and Mn3+/Mn4+ redox couples, rich oxygen vacancies, good oxygen mobility, low-temperature reducibility, and strong interaction between the Cu species and the OMS-2 support. The reaction mechanisms were also deduced by analyzing the in situ DRIFTS spectra of the 15Cu/OMS-2 sample. The excellent oxygen mobility associated with the electron transfer between Cu species and Mn3+/Mn4+ redox couples might be conducive to the continuous replenishment of active oxygen species and the constantly generated reactant intermediates, thereby increasing the reactant reaction rate.

Catalytic Elimination of Carbon Monoxide, Ethyl Acetate, and Toluene over the Ni/OMS-2 Catalysts

The Ni-loaded cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) catalysts (xNi/OMS-2: x = 1, 3, 5, and 10 wt%) were prepared by a pre-incorporation method. Physicochemical properties of the as-synthesized materials were characterized by means of various techniques, and their catalytic activities for CO, ethyl acetate, and toluene oxidation were evaluated.The loading of Ni played an important role in improving physicochemical propertiesof OMS-2. Among all of the samples, 5Ni/OMS-2 exhibited the best catalytic activity, with the T90 being 155 °C for CO oxidation at a space velocity (SV) of 60,000 mL/(g·h), 225 °C for ethyl acetate oxidation at an SV of 240,000 mL/(g·h), and 300 °C for toluene oxidation at an SV of 240,000 mL/(g·h), which was due to its high Mn3+ content and Oads concentration, good low-temperature reducibility and lattice oxygen mobility, and strong interaction between the Ni species and the OMS-2 support. In addition, catalytic mechanisms of the oxidation of three pollutants over 5Ni/OMS-2 were also studied. The oxidation of CO, ethyl acetate, and toluene over the catalysts took place first via the activated adsorption, then intermediates formation, and finally complete conversion of the formed intermediates to CO2 and H2O.

Insight Into the Role of Ceria on OMS-2 and OL Materials for Catalytic Degradation of Toluene

Catalytic oxidation is one of the most efficient approaches for industrial volatile organic compounds (VOCs) elimination. MnOx-based catalysts have attracted much attention due to their remarkable low-temperature catalytic activity. Here, octahedral layered birnessite-type manganese oxide (OL-1) with layer spacing of 7 Å and cryptomelane type manganese oxide (OMS-2) with tunneled pore diameter of 4.6 × 4.6 Å were synthesized by a reflux method. Following this, Ce-doped OL-1 and OMS-2 were further prepared by an impregnation method with target to improve catalytic performance in toluene oxidation. Results reveal that the OMS-2 material exhibits the best catalytic activity with 90% of toluene decomposed at 224 °C owing to the presence of a large quantity of active lattice oxygen species. Interestingly, the introduction of Ce leads to the formation of large amounts of acidic sites, which limit the oxidation process and enhance the yield of benzoic acid by-product. The findings in the present work are meaningful for deepening our understanding of the role of ceria on metal oxide catalysts and helping us to design effective catalysts for VOC destruction.

The Novel Synthesis of CaMnO3 Perovskite Type-Oxide and its Catalytic Activity for Degradation of Dye

The CaMnO3 perovskite type-oxide has been synthesized using a citrate sol-gel method using two different MnO2 precursors and calcium carbonate. The cryptomelane-type manganese oxide precursor produced well-crystalline CaMnO3 perovskite, whereas pyrolusite-type manganese oxide (commercial) generated mixtures of CaMnO3 perovskite and un-reacted MnO2 using the same CaCO3 as other precursor. The CaCO3/MnO2 mole ratio seems to play a crucial role to obtain high purity of the CaMnO3 perovskite. The CaMnO3 perovskite proves to have high catalytic activity for the degradation of methylene blue (MB) using hydrogen peroxide as an oxidant. This Fenton catalyst is able to degrade 98.5% of MB within 20 minutes of degradation times at pH 2.

Sol-gel synthesis and photocatalytic activity of biomimetic calcium manganese oxide catalysts

The water contaminations by dye residues are increasingly becoming serious concerns worldwide due to dye toxicity and persistent characteristics. Manganese oxide-based catalysts having similar structures with manganese compounds found in nature (biomimetic) have been considered as a very promising and effective photocatalyst for the degradation of organic pollutions in wastewater. This paper focuses on the synthesis of calcium manganese oxide catalysts from the octahedral layered birnessite-type manganese oxide and calcium carbonate via sol-gel method with citric acid as the complexing agent. The as-synthesized oxide was then tested as a photocatalyst for the degradation of methylene blue (MB) dye. The calcium manganese oxides prepared by the two different mole ratios of CaCO3/MnO2 resulted in the similar crystallinity and crystal phases but difference in the crystal sizes. The photocatalytic activities of the prepared calcium manganese oxides for the degradation of methylene blue are compared to that of the calcium manganese oxide prepared from tunnel cryptomelane-type manganese oxide. Both the calcium manganese oxides prepared from the different manganese oxide phases show the remarkable performance for the photocatalytic degradation of methylene blue after 10 minutes of reaction times. The birnessite-prepared calcium manganese oxide displays slightly higher photocatalytic performance than cryptomelane-prepared calcium manganese oxide.

Enhanced catalytic activity of OMS-2 for carcinogenic benzene elimination by tuning Sr2+ contents in the tunnels

Cryptomelane-type manganese oxides (OMS-2) have been intensively investigated for application in the catalytic oxidation of carcinogenic benzene, and doping metal ions in the OMS-2 tunnels are widely used for modifying its catalytic activity. In this study, we reported a novel strategy of enhancing catalytic activity of OMS-2 for carcinogenic benzene elimination by tuning Sr2+ concentration in the tunnels. The catalytic activity result revealed that an obvious decrease (△T50 = 27 °C and △T90 = 37 °C) in T50 and T90 (corresponding to benzene conversions at 50 % and 90 %, respectively; initial benzene concentration was 2000 mg m−3; contact time was 1.5 s) had been observed by increasing the Sr2+ concentration in the OMS-2 tunnels. The origin of Sr2+ doping effect on catalytic activity was theoretically and experimentally investigated by CO temperature-programmed reduction, 18O2 isotope labeling, and density functional theory calculations. The result confirmed that increasing Sr2+ concentration in the tunnels not only promoted the lattice oxygen activity, but also facilitated the generation of more oxygen vacancy defects, thus considerably improving the catalytic activity of OMS-2.