Hollandite-type Manganese Oxide Research Articles

Low-Temperature Benzene Abatement over Active Manganese Oxides with Abundant Catalytic Sites

Benzene is a typical volatile organic pollutant, and catalytic oxidation is promising for its abatement at low temperatures. Here, we prepared tunnel-structured hollandite-type manganese oxide (HMO) nanoparticles with abundant catalytically active sites (CASs). The HMO nanoparticles are more active than a traditional noble-metal catalyst, namely, Pt/Al2O3. The former catalyst can achieve 100% benzene conversion at 200 °C at a very high space velocity of 120 000 mL gcat–1 h–1. The results from relevant physicochemical characterizations and reaction kinetics studies demonstrate that the CASs are located at the tunnel openings of the HMO{001} surfaces. These CASs not only provide surface-active lattice oxygen species for benzene oxidation, but also efficiently activate O2 during the catalytic process. Moreover, HMO nanoparticles also have more CASs than HMO nanorods, thus leading to higher activity in benzene oxidation. This work may assist in the rational design of active transition-metal oxide catalysts fo...

Ambient hydrogenation of CO2 to methane with highly efficient and stable single-atom silver-manganese catalysts

This research focused on developing an efficient and stable single-atom silver-manganese catalysts (Ag-HMO) for converting the increasingly concerning CO2 to highly demanded methane (CH4) via ambient photocatalytic hydrogenation with renewable H2. The catalytic performances of Ag/P25, silver nanoparticles (Ag NPs), silver loading hollandite-type manganese oxide nanorods (Ag/HMO) and single-atom silver-manganese catalysts (Ag-HMO) were tested and compared under ambient conditions. It showed that the CH4 was the main product and the photocatalytic activity of single-atom catalysts reached the maximum value, 6.0 μmol h−1 g−1 catalyst, with the silver loading of 30%, which was >33.3% higher than that of sole silver nanoparticles (Ag NPs) and Ag/HMO. The activity of Ag-HMO was >71% higher than that of the benchmark Ag/P25. The enhancement of photocatalytic CO2 reduction activity was ascribed to the superior electrical conductivity, efficient Ag to HMO electron transfer, high adsorption of visible light and activation of adsorbed CO2 and H2. This work provided a promising efficient and cost-effective substitute for noble metal cocatalysts such as Pt, Au and Pd.

Electronic-Structure-Dependent Performance of Single-Site Potassium Catalysts for Formaldehyde Emission Control

Depending on the dispersion degree and electronic states, alkali metals such as potassium can function as either promoters or inhibitors in catalytic reactions, but the underlying reasons for that are still obscure. Herein, we fabricated two kinds of single-site potassium catalysts (K1/HMO and K1/HWO) by using hollandite-type manganese oxide (HMO) and hexagonal tungsten oxide (HWO) as supports. K1/HMO with the partially charged single K ions efficiently enhances the ability to activate both surface lattice oxygen and molecular oxygen, thereby leading to a higher catalytic activity for HCHO oxidation. In contrast, K1/HWO is less active than HWO due to the electron-lean K ions. This work provides a feasible explanation for the functions played by alkali metals in catalytic oxidation, and thus assists the design of efficient catalysts promoted by alkali metals.

Single-atom silver-manganese nanocatalysts based on atom-economy design for reaction temperature-controlled selective hydrogenation of bioresources-derivable diethyl oxalate to ethyl glycolate and acetaldehyde diethyl acetal

The research was designed to synthesize novel single-atom silver-manganese nanocatalysts with a simple method for producing highly demanded ethyl glycolate (EGly) and acetaldehyde diethyl acetal (ADA) by hydrogenating bioresources-derivable diethyl oxalate (DEO) under different temperatures. DEO conversions achieved with the catalyst are 80–100%. 100% EGly or 97% ADA selectivity was produced by simply tuning reaction temperatures. The single-atom silver-manganese nanocatalysts were formed through the entrance of Ag atoms into the lattice tunnels of hollandite-type manganese oxide (HMO) under H2 reduction at 200 °C. The excellent activities of the nanocatalysts are mainly ascribed to the activation of H2 and desirable chemical adsorption of DEO and ethanol. The EGly was formed through the first order hydrogenation of DEO, while the ADA was obtained via the hydrogenation, dehydration and reaction. With the help of high-efficiency nanocatalysts, bioresources are promising raw materials for producing various high-value chemicals in environmentally responsible manners.

Activating Inert Alkali-Metal Ions by Electron Transfer from Manganese Oxide for Formaldehyde Abatement.

Alkali-metal ions often act as promoters rather than active components due to their stable outermost electronic configurations and their inert properties in heterogeneous catalysis. Herein, inert alkali-metal ions, such as K+ and Rb+ , are activated by electron transfer from a Hollandite-type manganese oxide (HMO) support for HCHO oxidation. Results from synchrotron X-ray diffraction, absorption, and photoelectron spectroscopies demonstrate that the electronic density of states of single alkali-metal adatoms is much higher than that of K+ or Rb+ , because electrons transfer from manganese to the alkali-metal adatoms through bridging lattice oxygen atoms. Electron transfer originates from the interactions of alkali metal d-sp frontier orbitals with lattice oxygen sp3 orbitals occupied by lone-pair electrons. Reaction kinetics data of HCHO oxidation reveal that the high electronic density of states of single alkali-metal adatoms is favorable for the activation of molecular oxygen. Mn L3 -edge and O K-edge soft-X-ray absorption spectra demonstrate that lattice oxygen partially gains electrons from the Mn eg orbitals, which leads to the upshift in energy of lattice oxygen orbitals. Therefore, the facile activation of molecular oxygen by the electron-abundant alkali-metal adatoms and active lattice oxygen are responsible for the high catalytic activity in complete oxidation of HCHO. This work could assist the design of efficient and cheap catalysts by tuning the electronic states of active components.

Low-temperature NO-adsorption properties of manganese oxide octahedral molecular sieves with different potassium content

In this study, we synthesized hollandite-type manganese oxide octahedral molecular sieves (OMS) with different potassium content (KxMn8O16; K-OMS-2). Owing to crystal growth, the specific surface area of K-OMS-2 decreased with an increase in the K concentration for x>1.5. The K-OMS-2 samples with x<1.5 exhibited NO adsorption at low temperatures because of an oxidation reaction of NO. The amount of adsorbed NO depended on the oxidation activity of K-OMS-2 instead of the specific surface area (i.e., the number of NO adsorption sites). The amount of NO that was adsorbed at 323K was correlated to the increase in the amount of oxygen released from K-OMS-2. It is assumed that the thermally unstable oxygen sites acted as oxidation sites for NO at low temperatures and thus increased the amount of adsorbed NO.

Enhancing the Catalytic Activity of Hollandite Manganese Oxide by Supporting Sub-10 nm Ceria Particles

Sub-10 nm CeO2 particles were highly dispersed on Hollandite-type manganese oxide (HMO) to give a CeOx/HMO catalyst, which showed obviously higher activity than HMO and CeO2 alone in CO oxidation and selective catalytic reduction of NO by NH3. A combination of various characterization data with the catalytic tests revealed that the enhanced catalytic activity results from the strong interaction between CeO2 and HMO to activate the transfer of electron, the improved redox ability of surface oxygen due to an increased electron density, and the presence of more surface active oxygen species.

Oxygen Reduction Catalytic Activity of Hollandite-Type Manganese Oxides

Hollandite-type Mn oxides were synthesized by a co-precipitation (CP) and a hydrothermal (HT) method as the cathode catalysts for anion-exchange membrane fuel cells (AEMFCs), and their oxygen reduction reaction (ORR) activities and AEMFC single cell performances were evaluated. In this study, we prepared two kinds of hollandite oxides K0.14MnO2·0.12H2O(KMO-CP) and K0.12MnO2·0.06H2O(KMO-HT) and their partially Co-substituted ones K0.11(Mn0.88 Co0.12)O2・0.16H2O(KMC0.12-CP) and K0.11(Mn0.88Co0.12)O2· 0.08H2O(KMC0.12-HT), and examined the ORR activities with a rotating ring-disk electrode (RRDE) in 0.1 M KOH at 50°C. All the samples showed high onset potentials of ORR, ca. 0.9 V vs. reversible hydrogen electrode (RHE), and relatively high ORR currents at 0.75 V and high efficiencies for 4-electron reduction (Eff4) of around 90% were obtained for the HT samples owing to higher crystallinity and higher specific surface area than those of the CP ones. The AEMFC single cells prepared with KMO-HT and KMC0.12-HT cathode catalysts were operated stably and exhibited the maximum power densities of ca. 49 and 42 mW cm-2, respectively, which were comparable to that of the single cell using a conventional 50 wt% Ag/C catalyst.

Cr-hollandite: Breaking tradition with todorokite-type manganese oxides

The synthesis of a tunneled hollandite-type manganese oxide with interstitial and framework Cr3+ is described. This unique material is prepared from a layered buserite precursor under conditions previously believed to only yield todorokite-type manganese oxides with larger tunnels. The influence of Cr3+ in promoting the hollandite structure has been investigated by selectively placing the cation either in interstitial or framework sites. The use of framework Cr3+ in combination with other interstitial cations generates related hollandite and todorokite derivatives. Catalytic oxidation reactions with benzyl alcohol and carbon monoxide have also been examined.

Structural Modification and Electrochemical Characteristics of Co-substituted Hollandite-type Manganese Oxides using a Hydrothermal Method

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Catalytically Active Single‐Atom Sites Fabricated from Silver Particles

Silver chains fabricated from supported silver particles were embedded in Hollandite-type manganese oxide, thereby exposing single silver atoms as catalytically active sites (see picture). Excellent oxygen activation and high catalytic activity in the oxidation of formaldehyde was observed.

Hydrothermal Synthesis and Electrochemical Properties of Hollandite-type Manganese Oxides

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Synthesis and magnetic properties of single-crystalline Na2- x Mn8O16 nanorods

The synthesis of single-crystalline hollandite-type manganese oxides Na2-xMn8O16 nanorods by a simple molten salt method is reported for the first time. The nanorods were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and a superconducting quantum interference device magnetometer. The magnetic measurements indicated that the nanorods showed spin glass behavior and exchange bias effect at low temperatures. The low-temperature magnetic behaviors can be explained by the uncompensated spins on the surface of the nanorods.

Surface structure sensitivity of manganese oxides for low-temperature selective catalytic reduction of NO with NH3

Hollandite-type manganese oxides with K+ or H+ cations in the tunnels (K-Hol or H-Hol) were successfully synthesized and investigated in low-temperature selective catalytic reduction of NO by NH3. The results of the catalytic tests revealed that both K-Hol and H-Hol with almost same catalytic activities had much higher reaction rates than β-MnO2 under the same conditions, and even at a high gas hourly space velocity of 160,000h−1, K-Hol obtained more than 90% conversions of NO in a wide temperature window of 100–300°C. The high resolution transmission electron microscopy observations showed that both K-Hol and β-MnO2 were tetragonal prism-shaped nanorods with same exposed {110} planes, and the atoms in the {110} planes of K-Hol arranged to form semitunnel structures, while the {110} planes of β-MnO2 were relatively smooth surfaces. The temperature-programmed reduction by H2 and thermal gravimetric analyses indicated that active surface lattice oxygen atoms of K-Hol were around 1.6% with respect to total lattice oxygen atoms. The transition reactions of NH3 demonstrated that K-Hol with special semitunnel structured surface and active surface lattice oxygen showed much stronger ability to efficiently adsorb and activate NH3 molecules than β-MnO2. Hence, both efficient semitunnel structured external surfaces and high active surface lattice oxygen atoms predominantly accounted for the high catalytic activities.

Spin-glass-like behavior in hollandite Ba1+δMn8O16 nanoribbons synthesized by molten-salt method

Abstract Single-crystal hollandite-type manganese oxides Ba 1+ δ Mn 8 O 16 nanoribbons (width of 30–500 nm and up to several hundred micrometers long) are synthesized by a simple molten-salt method. At 43 K a rapid increase of magnetization is observed; the zero field-cooled (ZFC) and field-cooled (FC) magnetization curves diverge below this temperature with a sharp cusp around 26 K in the ZFC curve revealing spin-glass-like behavior in the Ba 1+ δ Mn 8 O 16 nanoribbons. Measurements of ac-susceptibility indicate the coexistence of a weak ferromagnetic phase with a spin-glass component in the nanoribbons, which is distinct from the canted antiferromagnetic state found in the counterpart bulk sample.

Significant enhancement of catalytic activities of manganese oxide octahedral molecular sieve by marginal amount of doping vanadium

Abstract Vanadium doped hollandite-type manganese oxide octahedral molecular sieve (OMS-2) was synthesized by a simple hydrothermal route. The results of catalytic tests showed that marginal amount of doping vanadium had greatly enhanced catalytic activities of OMS-2 not only in low-temperature complete oxidation of formaldehyde at 140 °C but also in high-temperature complete oxidation of methane at 450 °C. The results of XPS, HRTEM and H2-TPR measurements revealed that the enhancements of the catalytic activities primarily resulted from the increase of surface defect sites regardless of reaction temperatures and nature of reactants, rather than reducibility and average oxidation state of manganese.

An Electrochemical Sensor Based on Nanostructured Hollandite-type Manganese Oxide for Detection of Potassium Ions

The participation of cations in redox reactions of manganese oxides provides an opportunity for development of chemical sensors for non-electroactive ions. A sensor based on a nanostructured hollandite-type manganese oxide was investigated for voltammetric detection of potassium ions. The detection is based on the measurement of anodic current generated by oxidation of Mn(III) to Mn(IV) at the surface of the electrode and the subsequent extraction of the potassium ions into the hollandite structure. In this work, an amperometric procedure at an operating potential of 0.80 V (versus SCE) is exploited for amperometric monitoring. The current signals are linearly proportional to potassium ion concentration in the range 4.97 × 10−5 to 9.05 × 10−4 mol L−1, with a correlation coefficient of 0.9997.

Synthesis and Electrochemical Properties of Co-substituted Hollandite-type Manganese Oxides Using a Hydrothermal Method

not Available.

Single Crystal Growth of Birnessite- and Hollandite-Type Manganese Oxides by a Flux Method

By using a KNO3 flux, single crystals of birnessite- and hollandite- type potassium manganese oxides can be obtained. The formation reactions and products were investigated by XRD and Rietveld refinement, SEM observation, TG-DTA, and chemical analysis. KNO3 takes on two roles in the formation of potassium manganese oxides, performing as a reactant supplying K+ ions, and also as a solvent. Rod-shaped hollandite-type and hexagonal prismatic birnessite-type manganese oxide single crystals were obtained at low (500 and 600 °C) and high (700 °C) temperatures, respectively. The hollandite crystals have a tetrahedral symmetry with space group I4/m. The K+-form birnessite crystals have a hexagonal symmetry, and the presence of small unindexed peaks in the XRD pattern suggests the formation of superstructure. K+ ions were extracted from the interlayer of the birnessite by the treatment in a (NH4)2S2O8 aqueous solution, and the resulting K+-extracted birnessite has a hexagonal symmetry with space group P63/mmc.

Synthesis of a new hollandite-type manganese oxide with framework and interstitial Cr(III).

Hollandite with Cr(III) in both tunnel and framework sites has been prepared hydrothermally from layered manganese oxide precursors.