Cryptomelane-type Manganese Research Articles

Effect of preparation method on the surface characteristics and activity of the Pd/OMS-2 catalysts for the oxidation of carbon monoxide, toluene, and ethyl acetate

The cryptomelane-type manganese oxide octahedral molecular sieve (OMS-2)-supported Pd (0.5wt% Pd/OMS-2-DP, 0.5wt% Pd/OMS-2-PI, and 0.5wt% Pd/OMS-2-EX) catalysts were prepared by the deposition-precipitation, pre-incorporation, and ion-exchanging strategies, respectively. It is shown that the preparation method exerted an important effect on the physicochemical property of the sample. Among the OMS-2-supported Pd catalysts, 0.5wt% Pd/OMS-2-DP possessed the highest surface (Mn2++Mn3+)/Mn4+ atomic ratio and the highest surface Pd loading and acid sites. The 0.5wt% Pd/OMS-2 catalysts outperformed the Pd-free counterpart, among which 0.5wt% Pd/OMS-2-DP presented the best catalytic activity (T50% and T90% were 25 and 55°C for CO oxidation, 240 and 285°C for toluene oxidation, and 160 and 200°C for ethyl acetate oxidation, respectively). We believe that the high Pd surface loading, high surface atomic ratio of (Mn2++Mn3+)/Mn4+, and good low-temperature reducibility, good oxygen mobility, and high acidity were responsible for the excellent performance of the 0.5wt% Pd/OMS-2-DP catalyst.

Transition metal doped cryptomelane-type manganese oxide catalysts for ozone decomposition

Transition metal (Cerium, Cobalt, and Iron) doped cryptomelane-type manganese oxide (M-OMS-2) catalysts have been successfully synthesized and characterized. The different metal-ion-doped K-OMS-2 catalysts showed distinct differences in their ozone decomposition activity. Cerium-doped OMS-2 materials showed ozone conversion of 90% under RH=90% and space velocity of 600,000h−1. X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) data suggested that the as-synthesized M-OMS-2 materials were all crystalline with no segregated metal oxide impurities. ICP-OES and XAFS results confirmed that Co3+ and Fe3+ replace Mn3+ in the cryptomelane structure and Ce4+mainly replaces the K+ in the tunnel and partially replaces the Mn4+ in the framework of the cryptomelane structure. Because of the differences in the substitution sites and the ionic radius of dopants, the morphologies of the catalysts were different. The Mn3+content and number of surface defects play a key role during the decomposition of ozone. Ce-OMS-2 is a promising catalyst for purifying waste gases containing ozone under high-humidity conditions.

Tremendous effect of oxygen vacancy defects on the oxidation of arsenite to arsenate on cryptomelane-type manganese oxide

Manganese oxide as a benign and low-cost oxidant has been widely investigated for the oxidation of As(III) to As(V), but its As(III) oxidation rate is easily hindered due to the thermodynamically favorable adsorption of the As(V) species produced and coexisting ions. Herein, we develop a novel strategy of tremendously increasing the As(III) oxidation activity via tuning the concentration of oxygen vacancy defects (OVD) in cryptomelane-type manganese oxides (OMS-2). The result of the batch experiment of the As(III) oxidation on OMS-2 shows that increasing the concentration of oxygen vacancy defects in OMS-2 remarkably increases the specific As(III) oxidation rate from 0.12 to 0.22μmolm−2min−1, and dramatically reduces the unfavorable effect of the As(V) adsorption on the As(III) oxidation activity. The origin of the tremendous effect of OVD in OMS-2 on the As(III) oxidation activity is experimentally and theoretically investigated by the adsorption of the As(III) and As(V) species on OMS-2, the point of zero charge (pHPZC), the evolution of the concentration of Mn2+ ions dissolved in the solution, and density functional theory (DFT) calculations. The results reveal that the presence of OVD in OMS-2 not only makes the adsorption of As(III) on OMS-2 more thermodynamically favorable, but also accelerates the charge transfer from As(III) to Mn atom, thus significantly improving the As(III) oxidation activity.

CO oxidation over gold supported on Cs, Li and Ti-doped cryptomelane materials

Cryptomelane-type manganese oxides were synthesized by redox reaction under acid and reflux conditions. Different metals (cesium, lithium and titanium) were incorporated into the tunnel structure by the ion-exchange technique. Gold was loaded onto these materials (1wt%) by a double impregnation method. The obtained catalysts were characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectrometry, scanning electron microscopy, X-ray diffraction and temperature-programmed reduction. The catalytic activity of these materials was evaluated in the oxidation of carbon monoxide. The incorporation of Cs, Li or Ti into cryptomelane was detrimental in terms of catalytic activity. Further addition of gold to cryptomelane doped materials significantly improved the catalytic performance, especially for Cs-K-OMS-2 and Li–K-OMS-2 (to a smaller extent). Addition of gold to the Ti containing material did not show a significant improvement. The observed trends are related to the effect of gold on samples reducibility and to the gold particle size. The lattice oxygen can also be considered accountable for the activity of the materials, since the most active cryptomelane catalysts are those with higher lattice oxygen donating ability for the oxidation of the CO molecule.

Oxidative coupling of anilines to azobenzenes using heterogeneous manganese oxide catalysts

Cryptomelane-type manganese oxide shows high efficiency for the NN formation reaction in the oxidative coupling of anilines to azobenzenes. The difference of the Hammett constants (Δσ) of two substituted groups determines the selectivity to unsymmetric azobenzenes in cross-coupling reactions, which are favored at Δσ< 0.32.

Conversion of inert cryptomelane-type manganese oxide into a highly efficient oxygen evolution catalyst via limited Ir doping

Inert cryptomelane oxide doping with Ir results in both higher OER performance and substantially reduced Ir consumption compared to IrO2.

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.

First-principles studies of carbon dioxide adsorption in cryptomelane/hollandite-type manganese dioxide

A critical challenge in developing engineered materials for low-energy-cost carbon capture and storage applications is to fundamentally understand adsorption, particularly when the materials exhibit hysteretic sorption behavior. We used first-principles calculations and combined with experiments to explore CO2 sorption in a nanoporous solid: cryptomelane-type manganese dioxide (OMS-2). Cations present in this material to not only stabilize its porous structure but also tailor CO2-pore interaction. The concentration, type and charge of cations have significant effects on CO2 adsorption, diffusion and hysteresis behavior. OMS-2 with lower-charge cation, e.g. K+ (Cryptomelane), exhibits less hysteresis than OMS-2 with higher-charge cation, e.g. Ba2+ (Hollandite).

Heterogeneously Catalyzed Aerobic Cross‐Dehydrogenative Coupling of Terminal Alkynes and Monohydrosilanes by Gold Supported on OMS‐2

The cross-dehydrogenative coupling of various terminal alkynes and monohydrosilanes efficiently proceeds in the presence of gold supported on a cryptomelane-type manganese oxide-based octahedral molecular sieve as catalyst and O2 as a terminal oxidant.