Cryptomelane-type Manganese Oxide Research Articles

The Effects of Mn2+ Precursors on the Structure and Ozone Decomposition Activity of Cryptomelane-Type Manganese Oxide (OMS-2) Catalysts

The effects of Mn2+ precursors on the structure and ozone decomposition activity of cryptomelane-type manganese oxide (OMS-2) catalysts were investigated under high-humidity conditions. The OMS-2 catalysts were synthesized using a hydrothermal approach. Characterization of OMS-2 was carried out using X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 physical adsorption, Raman spectroscopy, X-ray absorption fine structure (XAFS), H2 temperature-programmed reduction (H2-TPR), and inductively coupled plasma (ICP) spectroscopy. The OMS-2-Ac synthesized using MnAc2 as a Mn2+ precursor showed the best catalytic activity for ozone decomposition (∼80%) under RH = 90% and space velocity of 600000 h–1 and is a promising catalyst for purifying waste gases containing ozone under high-humidity conditions. Acetate groups could prevent the aggregation of manganese oxide particles, which may introduce more crystalline defects. On the basis of the characterization results, it is supposed that the greater sur...

Effect of oxygen species on the liquid product distribution in the oxidative carbonylation of phenol over Pd/M-OMS-2 catalysts

The role of oxygen species in the product distribution of oxidative carbonylation of phenol was investigated using metal doped cryptomelane-type manganese oxide octahedral molecular sieves M-OMS-2 (M = Pb, Cr, Ce, Zn) as catalysts supports. The oxygen species of Pd/M-OMS-2 catalysts were characterized by H2-TPR and X-ray photoelectron spectra techniques. The results show that the Pd/Pb-OMS-2 catalyst calcined at 300 °C exhibits lower onset reduction temperature and more active OH− groups compared with the other catalysts, so the best diphenyl carbonate (DPC) yield is obtained. When Pb-OMS-2 was used as a support, a vast majority of by-product was 4-bromophenol, and a small amount of 3-phenoxyphenol and biphenyldiol yielded. However, for the Pd/Cr-OMS-2, Pd/Ce-OMS-2 and Pd/Zn-OMS-2 catalysts, the decreased amount of 4-bromophenol was observed while 3-phenoxyphenol and biphenyldiol yielded remarkably. It is deduced that active OH− groups not only improve the DPC yield, but also promote the formation of 4-bromophenol and deep oxidation of phenol.

Modified manganese oxide octahedral molecular sieves M′-OMS-2 (M′ = Co,Ce,Cu) as catalysts in post plasma-catalysis for acetaldehyde degradation

Metal ions-modified cryptomelane-type manganese oxide octahedral molecular sieves M′-OMS-2 (M′=Co,Ce,Cu) were synthesized and characterized by XRD, BET, EDS, ICP, TEM, XPS, FTIR, H2-TPR in this work. The obtained materials were supported on Al2O3 pellets and investigated for acetaldehyde degradation in a post plasma-catalysis system. The results showed that the introduction of M′-OMS-2/Al2O3 catalysts improved acetaldehyde removal efficiency and inhibited ozone formation of plasma significantly, while the intermediates such as acetic acid, amine and nitromethane were suppressed. Co-OMS-2/Al2O3 exhibited the best catalytic activity to combine with plasma among all the prepared catalysts. The excellent activity is ascribed to the redox property, oxygen vacancies, oxygen mobility and surface area of catalyst. It can be inferred that, ozone formed in plasma was catalytically dissociated into active oxygen species by metal ions in M′-OMS-2 catalysts and contributed to acetaldehyde oxidation and intermediates removal.

A novel microwave absorption material of Ni doped cryptomelane type manganese oxides

Cryptomelane type manganese oxide α-MnO2 and Ni doped KMn8O16 nanostructures were synthesized by water-bathing methods at 80°C for 24h using NiSO4·H2O as the dopant sources. The structures, morphologies and physical properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results show that the products are Ni doped KMn8O16 nanorods after the introduction of NiSO4·H2O during the reaction process. The electromagnetic characteristics and microwave absorption properties of the materials were carried out with a vector network analyzer (VNA) and the transmission line (TML) theory. The dielectric loss and microwave absorption properties of the cryptomelane materials are improved after Ni doping. The thickness dependent reflection loss shows that the peak frequency and effective absorption bandwidth all decrease with the increasing material thickness. With the increase of Ni doping concentration, the peak frequency shifts to higher frequency bands and the effective absorption bandwidth increases. The electromagnetic performance of cryptomelane can be attributed to its unique tunnel structures and the improvement of Ni doping can be due to the enhanced electromagnetic polarization.

Vapor-Phase Oxidation of Benzyl Alcohol Using Manganese Oxide Octahedral Molecular Sieves (OMS-2)

Vapor-phase selective oxidation of benzyl alcohol has been accomplished using cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2) catalysts. A conversion of 92% and a selectivity to benzaldehyde of 99% were achieved using OMS-2. The role played by the oxidant in this system was probed by studying the reaction in the absence of oxidant. The natures of framework transformations occurring during the oxidation reaction were fully studied using temperature-programmed techniques, as well as in situ X-ray diffraction under different atmospheres.

Manganese oxide octahedral molecular sieve (OMS-2) as an effective catalyst for degradation of organic dyes in aqueous solutions in the presence of peroxymonosulfate

Cryptomelane type manganese oxide OMS-2 was prepared and tested in heterogeneous activation of peroxymonosulfate (PMS) for Acid Orange 7 (AO7) degradation in aqueous solutions. X-ray diffraction, scanning electron microscopy, N2 adsorption/desorption isotherms, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron spin resonance spectroscopy (ESR) were used to characterize the properties of the material. The effect of several parameters on its catalytic activity was also investigated. It was found that the catalyst was very active for AO7 and other organic dyes degradation, and presented a long-term stability during multiple runs. Results from XPS and ESR suggested that the highly catalytic efficiency possible involved the activation of PMS to sulfate radicals meditated by the redox pair of Mn(IV)/Mn(III) in OMS-2. Based on intermediate detections, the degradation pathway of AO7 in the OMS-2/PMS system was proposed.

Microwave-Assisted Hydrothermal Synthesis of α-MnO2: Lattice Expansion via Rapid Temperature Ramping and Framework Substitution

Cobalt-doped cryptomelane-type manganese oxide (K-OMS-2) microwires have been successfully synthesized and characterized. Their catalytic activity was tested in an oxidation reaction with benzyl alcohol as the substrate, and the cobalt-doped OMS-2 materials showed 100% selectivity toward benzyl aldehyde with a conversion of 55%. The cobalt-doped OMS-2 materials were also investigated as a desulfurization sorbent in a fixed bed reactor at 250 °C where high sulfur sorption capacities (49.4 g of sulfur/100 g of sorbent) were observed. Here, structure-controlled synthesis was performed using a facile one-step microwave-assisted hydrothermal method (MWHY) associated with a rapid temperature ramping (200 °C/min). The structural effects induced by the compositional control of transition metal dopants on the cryptomelane (space group I4/m) body centered tetragonal structure were identified with X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD and TEM results showed that the systematic v...

Efficient aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran on manganese oxide catalysts

A cryptomelane-type manganese oxide octahedral molecular sieve with a (2×2, 4.6Å×4.6Å) tunnel size (OMS-2) efficiently catalyzed aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) with a high yield of 97.2% at 383K and 0.5MPa O2 in N,N-dimethylformamide. OMS-2 was superior to other MnO2 catalysts with different morphologies, including OMS-1, OMS-6, and OMS-7 with various tunnel sizes, amorphous MnO2 and birnessite-type MnO2, apparently due to its (2×2) tunnel structure and consequently high reducibility and oxidizability. Kinetic and isotopic studies on OMS-2 showed near half-order dependence of the activities on HMF and O2 concentrations and marked kinetic isotope effects for deuterated HMF at its methylene group. These results, together with the similar initial rates under aerobic and anaerobic conditions, suggest that HMF oxidation to DFF on OMS-2 proceeds via a redox mechanism involving kinetically-relevant steps of C–H bond cleavage in adsorbed alcoholate intermediate, derived from quasi-equilibrated dissociation of HMF, using lattice oxygen and reoxidation of Mn3+ to Mn4+ by dissociative chemisorption of O2.

An investigation on the structure and catalytic activity of cryptomelane-type manganese oxide materials prepared by different synthesis routes

Cryptomelane is an octahedral molecular sieve with a 2 × 2 tunnel structure (OMS-2) and a stoichiometric formula of approximately KMn8O16. Cryptomelane OMS-2 is gaining increasing attention in a range of applications, including catalysis. However, the impacts of synthesis methods on catalytic performance are still not well understood. Cryptomelane OMS-2 was prepared in this study by three methods: (1) a sol–gel method, (2) a reflux method, and (3) a milling method. The physical and chemical characteristics of these materials were investigated, and the catalytic performances of these materials were compared for the gas phase oxidation of ethanol. A novel aspect of this study is that cryptomelane was prepared by the milling method using post-consumer alkaline battery waste, and this novel cryptomelane material was compared with others prepared from commercially-available precursors and published laboratory methods.

Manganese octahedral molecular sieve (OMS-2) catalysts for selective aerobic oxidation of thiols to disulfides

Selective aerobic oxidation of thiols to disulfides without any over oxidized products is studied using cryptomelane type manganese oxides (K-OMS-2) with a tunnel structure as catalysts. Using K-OMS-2 prepared by different synthetic procedures, complete conversion was obtained under air atmosphere without generating any overoxidized products. K-OMS-2 prepared by solvent free method (K-OMS-2SF) with the highest surface area (155±1m2/g) gave complete conversion, while materials prepared using hydrothermal method (K-OMS-2HY) with the lowest surface area (44±1m2/g) gave only 18% conversion at room temperature. Selective poisoning of the acid sites suggests that Lewis acid sites are the dominating active site during the reaction. Effects of surface area of the catalyst, solvent polarity, substrate effect, catalyst recyclability and temperature were studied. The catalyst could be recovered in the active form after the reaction without significant structural changes. The characterization of the catalyst using XRD, SEM, TGA, BET, TEM, and FT-IR are reported. The process developed is environmentally benign and is indeed heterogeneous.

Synthesis of P-doped mesoporous manganese oxide materials with three-dimensional structures for catalytic oxidation of VOCs

P-doped cryptomelane-type manganese oxide K-OMS-2 materials with three-dimensional structures were easily prepared using KMnO4 and MnPO4 as precursors via hydrothermal route. The scanning electron microscopy (SEM) and Energy dispersive X-ray spectra (EDS) results revealed that the morphology of P-doped K-OMS-2 transformed from hierarchical microspheres assembled from nanosheets and nanoneedles to nest-like structure with the increase of P/Mn molar ratio. Nitrogen sorption analysis showed that the isotherms of P-doped K-OMS-2 materials were typically type-IV with hysteresis loops, which meant that the samples had a mesoporous structure. The surface area and average pore diameter of the as-prepared K-OMS-2 materials were 138–153m2g−1 and 18–56nm respectively. In addition, the obtained materials showed good catalytic ability for the catalytic oxidation of a typical volatile organic compound (VOCs), o-xylene.

Substrate Control of Anisotropic Resistivity in Heteroepitaxial Nanostructured Arrays of Cryptomelane Manganese Oxide on Strontium Titanate

Resistivity and resistance measurements have been carried out for thin films of cryptomelane-type manganese oxide (OMS-2) grown onto (001), (110), and (111)STO single crystals substrates via pulsed laser deposition. While the symmetries of the (001) and (111)STO substrate surfaces give deposits consisting of multiple nanofiber arrays with isotropic in-plane resistivities, only a single nanofiber array is formed on (110)STO giving highly anisotropic electrical properties with very low resistivity values measured parallel to the fibers and similar to the lowest value ever reported.

Transition metal doped cryptomelane-type manganese oxide for low-temperature catalytic combustion of dimethyl ether

The catalysts of transition metals (Fe, Co, Ni, Cu, Cr) doped cryptomelane-type manganese oxide (M-OMS-2) were prepared and investigated for the catalytic combustion of dimethyl ether (DME). The catalysts were characterized by XRD, FT-IR, BET, TEM, ICP-AES, XPS and H2-TPR techniques. The effects of surface area, the average oxidation state, and the redox properties on catalytic activities were studied. The results showed that 100% CO2 selectivity can be realized over the doped OMS-2 catalysts. The M-OMS-2 catalysts displayed the nanorod morphology with a 13nm diameter and the length ranging from 50 to 220nm. The Cu doped OMS-2 catalyst exhibited the best activity, achieving the light-off temperature (the temperature acquired for 10% DME conversion, T10) and full-conversion temperature (the temperature acquired for 90% DME conversion, T90) of 171 and 180°C, respectively. DME catalytic combustion over M- OMS-2 catalysts was found to follow a Mars–van-Krevelen mechanism with the presence of redox between Mn4+/Mn3+ and Mm+/M(m−1)+ (M=Fe, Co, Ni, Cu, Cr).

Degradation of ciprofloxacin by cryptomelane-type manganese(III/IV) oxides

The objective of this study is to investigate and understand the oxidizing properties of a manganese oxide, specifically synthetic cryptomelane (KMn(8)O(16)) and its derivatives, in aqueous solution. Ciprofloxacin (CIP), a commonly used fluoroquinolone antibiotic, was used as the probe. Synthetic cryptomelane, known as octahedral molecular sieves (OMS-2), was synthesized, and its derivatives were prepared by adding transition metal oxides, V(2)O(5) or MoO(3), as dopants during synthesis. The solids were characterized by x-ray powder diffraction (XRD), SEM-energy-dispersive spectrometry (SEM-EDX), x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectra (FTIR), Raman spectra, and N(2)-Brunauer-Emmett-Teller method. Degradation of CIP by different doped OMS-2 was carried out. Process conditions were optimized using response surface methodology (RSM). XRD patterns indicated the crystal phase of regular and doped OMS-2 as the cryptomelane type. Presence of the dopants in doped cryptomelane was confirmed by SEM-EDX and XPS. FTIR and Raman results suggested that the dopants were substituted into the framework in place of manganese. SEM images, XRD analysis, and surface area analysis of doped OMS-2 indicated decreased particle size, decreased crystallinity, and increased surface area compared to regular OMS-2. Higher oxidizing reactivity of doped OMS-2 was also observed with increased CIP removal rates from aqueous solution. The enhancement of reactivity may be due to the increase of surface areas. Nine percent Mo/OMS-2, the most effective oxidant of all synthesized derivatives, was selected for optimization study. Favorable treatment conditions were obtained using RSM at pH 3 with molar ratio [9 % Mo/OMS-2]/[CIP] ≥ 50. Under such conditions, more than 90 % CIP can be removed in 30 min. The degradation kinetics was modeled by a modified first order rate with introduction of a retardation factor-α (R (2) > 0.98). Analysis of degradation products indicated that oxidation takes place mainly on the piperazine ring of CIP.

Effects of textural parameters and noble metal loading on the catalytic activity of cryptomelane-type manganese oxides for formaldehyde oxidation

Nanoporous manganese oxides were synthesized by a sol–gel method with maleic acid as an organic reducing agent. Noble metals (Pt and Ag) were then loaded by an impregnation method. The as-prepared materials are tunneled manganese oxides, cryptomelane (K-OMS-2). Further characterization was performed using X-ray diffraction (XRD), transmission electron microscopy (TEM) and N2 adsorption/desorption techniques. The structure and textural properties are heavily dependent on reaction conditions. Reaction solutions using low temperature produced well-crystallized nanorods, whereas solutions at high temperature yielded poorly-crystallized nanoparticles. Formaldehyde oxidation was carried out to evaluate the catalytic activity of these materials. The cryptomelane nanoparticles that possess higher specific surface area and more open pores showed better catalytic performance than the cryptomelane nanorods. Effects of Pt and Ag on the cryptomelane catalysts indicate that not only noble metal types, but also the existence form of metals on the catalyst surface greatly affects the catalytic activity. Pt/K-OMS-2 consisting of ca. 2nm Pt nanoparticles presents higher activity than Ag/K-OMS-2, but lower activity than the corresponding cryptomelane materials without metal loading, due to the cover of Pt on the active sites of manganese oxides.

Synthesis and Catalytic Activity of Magnetic Cryptomelane-Type Manganese Oxide Nanotubes

Magnetic cryptomelane-type manganese oxide (OMS-2) nanotubes were successfully prepared by grafting Fe3O4 nanoparticles onto the OMS-2 nanotubes. SEM and HRTEM images show that the prepared magnetic OMS-2 nanotubes exhibited diameters of 100 nm, lengths less than 3.0 μm, and the diameters of the Fe3O4 nanoparticles are less than 10 nm. The synthesized material exhibits excellent magnetic separation and catalytic properties for the degradation of methylene blue (MB) by a Fenton-like reaction.

Rapid synthesis of cryptomelane-type manganese oxide under ultrasonic process

Cryptomelane-type manganese oxide (OMS-2) was rapidly prepared under ultrasonic irradiation in short time. Characterization results using X-ray powder diffraction (XRD), surface area analyzer, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), oxygen temperature-programmed desorption (O 2-TPD) revealed that ultrasonic irradiation has tremendous effect on the surface area, morphology, surface defects, and redox properties of the OMS-2 materials. The OMS-2 prepared via ultrasonic irradiation shows nanoneedle morphology with smaller crystallite size, larger surface area (120.4 m 2/g), more surface defects, and higher oxygen mobility, thus it demonstrates excellent activity in the catalytic combustion of dimethyl ether with a start-off temperature of 160 °C and a complete combustion temperature of 172 °C.

Fabrication of magnetic cryptomelane-type manganese oxide nanowires for water treatment

Superparamagnetic Fe(3)O(4) nanoparticles were successfully grafted onto the manganese oxide OMS-2 nanowires, and the synthesized material exhibits excellent magnetic separation and adsorption properties.

Large-scale size-controlled synthesis of cryptomelane-type manganese oxide OMS-2 in lateral and longitudinal directions

A facile method was developed to synthesize size-tunable OMS-2 nanomaterials using a series of saturated fatty carboxylic acids as acid agents and regulators. The particle sizes, from 8.2 to 61.1 nm in width and 35.6 to 1376.1 nm in length, can be precisely controlled by decreasing alkyl chain lengths of carboxylic acids.

Synthesis, characterization and catalytic activities of vanadium–cryptomelane manganese oxides in low-temperature NO reduction with NH 3

Vanadium doped cryptomelane-type manganese oxides (V-OMS-2) with V/Mn molar ratios of 0–10% were synthesized and investigated for low-temperature selective catalytic reduction of NO by NH 3 (NH 3-SCR). The characterization results of XRD patterns and Raman spectra demonstrated that V 5+ isomorphously substituted for framework Mn 4+ of the V-OMS-2. The adsorption and desorption of NH 3 and H 2-TPR results revealed that both surface defect sites (Lewis acid) and redox abilities were efficiently controlled by amounts of vanadium doping, and more vanadium dopings resulted in more Lewis acid sites and weaker bulk redox abilities. The results of catalytic tests revealed that the 2%V-OMS-2 catalyst showed the highest catalytic activities among the V-OMS-2 catalysts. In fact, the desired doping amount of V 5+ can not only provide more Lewis acid sites for adsorption of NH 3 but also maintain excellent surface redox abilities for activation of NH 3, which resulted in higher catalytic activities in the low-temperature NH 3-SCR reactions.