Robust Mesoporous Manganese Oxide Catalysts for Water Oxidation

Mesoporous Manganese Oxides with High-Valent Mn Species and Disordered Local Structures for Efficient Oxygen Electrocatalysis

Active and nonprecious-metal bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital components of clean energy conversion devices such as regenerative fuel cells and rechargeable metal-air batteries. Porous manganese oxides (MnO_x) are promising electrocatalyst candidates because of their high surface area and the abundance of Mn. MnO_x catalysts exhibit various oxidation states and crystal structures, which critically affect their electrocatalytic activity. These effects remain elusive mainly because the synthesis of oxidation-state-controlled porous MnO_x with similar structural properties is challenging. In this work, four different mesoporous manganese oxides (m-MnO_x) were synthesized and used as model catalysts to investigate the effects of local structures and Mn valence states on the activity toward oxygen electrocatalysis. The following activity trends were observed: m-Mn₂O₃ > m-MnO₂ > m-MnO > m-Mn₃O₄ for the ORR and m-MnO₂ > m-Mn₂O₃ > m-MnO ≈ m-Mn₃O₄ for the OER. These activity trends suggest that high-valent Mn species (Mn(III) and Mn(IV)) with disordered atomic arrangements induced by nanostructuring significantly influence electrocatalysis. In situ X-ray absorption spectroscopy was used to analyze the changes in the oxidation states under the ORR and OER conditions, which showed the surface phase transformation and generation of active species during electrocatalysis.

Caveat Emptor: Commercialized Manganese Oxide Nanoparticles Exhibit Unintended Properties

Nano-encapsulated manganese oxide (NEMO) particles are noteworthy contrast agents for magnetic resonance imaging (MRI) due to their bright, pH-switchable signal ("OFF" to "ON" at low pH), high metal loading, and targeting capability for increased specificity. For the first time, we performed a head-to-head comparison of NEMO particles from In-house and commercialized sources (US Nano vs Nanoshel) to assess their potential as bright T₁ MRI contrast agents. Manganese oxide nanocrystals (MnO, Mn₂O₃, and Mn₃O₄) were systematically evaluated for size, chemistry, release of manganese ions, and MRI signal pre- and post-encapsulation within poly(lactic-co-glycolic acid) (PLGA). Suprisingly, a majority of the commercialized formulations were not as advertised by displaying unintended sizes, morphologies, chemistry, dissolution profiles, and/or MRI signal that precludes in vivo use. US Nano's Mn₃O₄ and Mn₂O₃ nanocrystals contained impurities that impacted Mn ion release as well as micron-sized rodlike structures. Nanoshel's MnO and Mn₂O₃ nanoparticles had very large hydrodynamic sizes (>600 nm). In-house MnO and Nanoshel's Mn₃O₄ nanoparticles demonstrated the best characteristics with brighter T₁ MRI signals, small hydrodynamic sizes, and high encapsulation efficiencies. Our findings highlight that researchers must confirm the properties of purchased nanomaterials before utilizing them in desired applications, as their experimental success may be impacted.

Ni-Doped MnO2 Nanosheet Arrays for Efficient Urea Oxidation

Urea oxidation reaction (UOR), with a low thermodynamic potential, offers great promise for replacing anodic oxygen evolution reaction of electrolysis systems such as water splitting, carbon dioxide reduction, etc., thus reducing the overall energy consumption. To promote the sluggish kinetics of UOR, highly efficient electrocatalysts are required, and Ni-based materials have been widely investigated. However, most of these reported Ni-based catalysts suffer from large overpotentials, as they generally undergo self-oxidation to form NiOOH species at high potentials, which act as catalytically active sites for UOR. Herein, Ni-doped MnO₂ (Ni-MnO₂) nanosheet arrays were successfully prepared on nickel foam. The as-fabricated Ni-MnO₂ shows distinct UOR behavior with most of the previously reported Ni-based catalysts, as urea oxidation on Ni-MnO₂ proceeds before the formation of NiOOH. Notably, a low potential of 1.388 V vs reversible hydrogen electrode was required to achieve a high current density of 100 mA cm^-2 on Ni-MnO₂. It is suggested that both Ni doping and nanosheet array configuration are responsible for the high UOR activities on Ni-MnO₂. The introduction of Ni modifies the electronic structure of Mn atoms, and more Mn³⁺ species are generated in Ni-MnO₂, contributing to its outstanding UOR performance.

Rationally Designed Manganese-Based Metal-Organic Frameworks as Altruistic Metal Oxide Precursors for Noble Metal-Free Oxygen Reduction Reaction

The sluggish oxygen reduction reaction (ORR) at the cathode is challenging and hinders the growth of hydrogen fuel cells. Concerning kinetic values, platinum is the best known catalyst for ORR; however, its less abundance, high cost, and corrosive nature warrant the development of low-cost catalysts. We report the hydrothermal synthesis of two novel Mn-based metal-organic frameworks (MOFs), [Mn₂(DOT)(H₂O)₂]_n (Mn-SKU-1) and [Mn₂(DOT)₂(BPY)₂(THF)]_n (Mn-SKU-2) (DOT = 2,5-dihydroxyterephthalate; BPY = 4,4'-bipyridine). Mn-SKU-1 contains dimeric Mn(II) centers where the two corner-shared MnO₆ octahedra fuse to give rise to an infinite Mn₂O₁₀ cluster, whereas the two Mn(II) ions coordinate to DOT and BPY moieties to give rise to a pillared structure in Mn-SKU-2 and form a 3D → 3D homo-interpenetration MOF with a twofold interpenetrated net. The pyrolysis of as-synthesized Mn-MOFs at 600 °C under N₂ produced exclusively porous α-Mn₂O₃ composites (PSKU-1 and PSKU-2), with the BET surface area of 90.8 (for PSKU-1) and 179.3 m² g^-1 (for PSKU-2). These mesoporous MOF-derived α-Mn₂O₃ composites were modified as cathode materials for the electrocatalytic reduction of oxygen. The onset potential for the oxygen reduction reaction was found to be 0.90 V for PSKU-1 and 0.93 V for PSKU-2 versus RHE in 0.1 M KOH solution, with the current density of 4.8 and 6.0 mA cm^-2, respectively, at 1600 rpm. Based on the RDE/RRDE results, the electrocatalytic oxygen reduction occurs majorly via the four-electron process. The electrocatalyst PSKU-2 is cheap, easy to use, retains 90% of its activity after 10 h of continuous use, and offers higher recyclability than Pt/C. The onset potential maximum current density and kinetic values (J_k = 11.68 mA cm^-2 and Tafel slope = 85.0 mV dec^-1) obtained in this work are higher than the values reported for pure Mn₂O₃.

Recent Progress on Bimetallic-Based Spinels as Electrocatalysts for the Oxygen Evolution Reaction

Electrocatalytic water splitting is a promising and viable technology to produce clean, sustainable, and storable hydrogen as an energy carrier. However, to meet the ever-increasing global energy demand, it is imperative to develop high-performance non-precious metal-based electrocatalysts for the oxygen evolution reaction (OER), as the OER is considered the bottleneck for electrocatalytic water splitting. Spinels, in particular, are considered promising OER electrocatalysts due to their unique properties, precise structures, and compositions. Herein, the recent progress on the application of bimetallic-based spinels (AFe₂ O₄ , ACo₂ O₄ , and AMn₂ O₄ ; where A = Ni, Co, Cu, Mn, and Zn) as electrocatalysts for the OER is presented. The fundamental concepts of the OER are highlighted after which the family of spinels, their general formula, and classifications are introduced. This is followed by an overview of the various classifications of bimetallic-based spinels and their recent developments and applications as OER electrocatalysts, with special emphasis on enhancing strategies that have been formulated to improve the OER performance of these spinels. In conclusion, this review summarizes all studies mentioned therein and provides the challenges and future perspectives for bimetallic-based spinel OER electrocatalysts.

Inverse micelle fabrication of ordered mesoporous manganese oxide and degradation of tetracycline hydrochloride

Ordered mesoporous manganese oxides (MnO_x) were synthesized using the modified inverse micelle method. The crystal structure and surface morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The element content and changes in surface valence of catalysts were analyzed by X-ray photoelectron spectroscopy (XPS). The MnO_x were used to activate peroxymonosulfate (PMS) to degrade tetracycline hydrochloride (TCH). The catalytic activity of MnO_x was enhanced at a calcination temperature of 350 °C (MM-3). The degradation efficiency of TCH in MM-3/PMS system was 87.89% in 180 min. Appropriate dosages of catalyst and PMS improve the degradation efficiency of TCH. This system showed a wide range of pH application (3-9). In the presence of coexisting ions and humic acid, the degradation efficiency of TCH was still above 80%. The results of free radical capture experiment and electron paramagnetic resonance (EPR) test proved that the system activates PMS to produce three types of free radicals: SO₄^-, OH and ¹O₂. Therefore, MM-3 is a promising catalyst for the degradation of TCH in practical wastewater treatment.

Surprisingly Low Reactivity of Layered Manganese Oxide toward Water Oxidation in Fe/Ni-Free Electrolyte under Alkaline Conditions

So far, many studies on the oxygen-evolution reaction (OER) by Mn oxides have been focused on activity; however, the identification of the best performing active site and corresponding catalytic cycles is also of critical importance. Herein, the real intrinsic activity of layered Mn oxide toward OER in Fe/Ni-free KOH is studied for the first time. At pH ≈ 14, the onset of OER for layered Mn oxide in the presence of Fe/Ni-free KOH happens at 1.72 V (vs reversible hydrogen electrode (RHE)). In the presence of Fe ions, a 190 mV decrease in the overpotential of OER was recorded for layered Mn oxide as well as a significant decrease (from 172.8 to 49 mV/decade) in the Tafel slope. Furthermore, we find that both Ni and Fe ions increase OER remarkably in the presence of layered Mn oxide, but that pure layered Mn oxide is not an efficient catalyst for OER without Ni and Fe under alkaline conditions. Thus, pure layered Mn oxide and electrolytes are critical factors in finding the real intrinsic activity of layered Mn oxide for OER. Our results call into question the high efficiency of layered Mn oxides toward OER under alkaline conditions and also elucidate the significant role of Ni and Fe impurities in the electrolyte in the presence of layered Mn oxide toward OER under alkaline conditions. Overall, a computational model supports the conclusions from the experimental structural and electrochemical characterizations. In particular, substitutional doping with Fe decreases the thermodynamic OER overpotential up to 310 mV. Besides, the thermodynamic OER onset potential calculated for the Fe-free structures is higher than 1.7 V (vs RHE) and, thus, not in the stability range of Mn oxides.

Advanced Zinc-Air Batteries with Free-Standing Hierarchical Nanostructures of the Air Cathode for Portable Applications

It is today advanced that the development of a free-standing (binderless) air cathode via direct growth of nonprecious metal electrocatalysts onto the surface of the conductive collector would be a cutting-edge strategy to reduce the interfacial resistance, improve the mechanical stability, and reduce the final weight and the cost of manufacturing. Here, for Zn-air batteries (ZABs), we propose an innovative binderless noble-metal-free hierarchical nanostructured bifunctional air cathode in which high-density MnO_x nanorods (NRs) are directly grown on carbon nanotubes (CNTs) themselves synthesized on a microfibrous carbon paper (CP) substrate. All carbon/MnO_x air cathodes achieved specific capacities very close to the theoretical value of 820 mAh g_Zn^-1. A very stable voltage gap between the charge and discharge processes along hundred cycles was obtained, demonstrating the stability and good bifunctional electrocatalytic activities of these cathodes toward the oxygen reduction reaction/oxygen evolution reaction in a real ZAB device. As a proof-of-concept for handheld electronic applications, a ZAB assembled with CP/MnO_x NRs as the air electrode and a Zn plate anode operated a timer for 14 days successfully, whereas two ZAB-based CNTs/MnO_x cathodes connected in series powered a 2 V light-emitting diode (LED) bulb and a 3 V multimeter. The proposed strategy and results may pave the way for the rational design of hierarchical free-standing bifunctional electrocatalysts for ZABs, other metal-air batteries, and fuel cells.

A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na2WO4 catalytic active sites for CH4 heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO4 catalytic active sites resulting in surface WO5 sites with retarded kinetics for C-H scission. On the other hand, dimeric Mn2O5 surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH4 than the poisoned surface WO5 sites or the original WO4 sites. However, the surface reaction intermediates formed from CH4 activation over the Mn2O5 surface sites are more stable than those formed over the Na2WO4 surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO x , in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na2WO4 surface sites towards CH4 activation but can yield MnO x surface sites that activate CH4 faster than Na2WO4 surface sites, but unselectively.

Oxidative removal of sulfadiazine using synthetic and natural manganese dioxides

Studies on oxidation kinetics of sulfadiazine (SDZ) using δ-MnO₂ (birnessite) and natural MnO₂ are limited. Reaction order at different SDZ speciation was determined based on the effects of initial H⁺, MnO₂ and SDZ concentrations using initial rate method, which would be useful to determine the optimum pH and MnO₂ concentration. Birnessite and natural MnO₂ with different physico-chemical properties such as BET surface area, pH_PZC, d-spacing, and crystal size similarly showed good efficiencies in oxidizing neutral SDZ (pH 5) and anionic SDZ (pH 8). Activation energy (E_a) and thermodynamic parameters indicated the similar oxidation efficiencies in the temperature range of 10-40°C. The SO42- was produced from the SDZ oxidation coupled to the reduction of MnO₂ to Mn²⁺. The effect of co-solute ciprofloxacin (CIP) on the oxidation kinetics of SDZ was also studied. The rates of SDZ oxidation by both birnessite and natural MnO₂ were reduced by the presence of CIP due to competition in oxidation between SDZ and CIP. The SDZ was more rapidly oxidized than CIP in both single- and bi-solute systems, as indicated by the presence of CIP intermediate, whereas the intermediate of SDZ was not detected.

Ordered mesoporous metal oxides for electrochemical applications: correlation between structure, electrical properties and device performance

Ordered mesoporous metal oxides with a high specific surface area, tailored porosity and engineered interfaces are promising materials for electrochemical applications. In particular, the method of evaporation-induced self-assembly allows the formation of nanocrystalline films of controlled thickness on polar substrates. In general, mesoporous materials have the advantage of benefiting from a unique combination of structural, chemical and physical properties. This Perspective article addresses the structural characteristics and the electrical (charge-transport) properties of mesoporous metal oxides and how these affect their application in energy storage, catalysis and gas sensing.

Activity manifestation via architectural manipulation by cubic silica-derived Co3O4 electrocatalysts towards bifunctional oxygen electrode performance

A KIT-6-derived Co3O4 material demonstrates superior bifunctional activity due to its higher densities of Co3+ and Co2+ sites.

Heteroleptic manganese compounds as potential precursors for manganese based thin films and nanomaterials

Heteroleptic manganese compounds, [Mn(tmhd)(TMEDA)Cl]2 (1), [Mn(tmhd)(dmamp)]2 (2), Mn2(tmhd)2(edpa)2(μ-THF) (3), [Mn(dmampea)(NEt2)]2 (4), and Mn(dmampea)(iPr-MeAMD) (5), were synthesized and characterized. Compound 5 was a volatile liquid. Structural analysis revealed that 1-4 were dimers. Compounds 1 and 3, 2, and 4 had distorted octahedral, distorted trigonal-bipyramidal, and distorted tetrahedral geometries around the Mn centers, respectively. Based on thermogravimetric analysis, the residues of 2 and 3 were expected to be MnO and Mn3O4, respectively. According to thermogravimetric analysis, 4 showed a higher residual value, whereas 5 exhibited a lower value than those expected for manganese nitrides.

Control of the Porosity in Manganese Trimer-Based Metal-Organic Frameworks by Linker Functionalization

Manganese complexes have attracted significant interest in chemical industries and academic research for their application as catalysts owing to their ability to attain a variety of oxidation states. Generally, sterically bulky ligands are required to isolate molecular homogeneous catalysts in order to prevent decomposition. Herein, we capitalize on the catalytic properties of Mn and circumvent the instability of these complexes through incorporation of Mn-atoms into porous crystalline frameworks, such as metal-organic frameworks (MOFs). MOFs are able to enhance the stability of these catalysts while also providing accessibility to the Mn sites for enhanced reactivity. We solvothermally synthesized two trinuclear Mn-based MOFs, namely [Mn₃O(BDC)₃(H₂O)₃]_n (Mn-MIL-88, where H₂BDC = benzene-1,4-dicarboxylic acid) and [Mn₃O(BDC-Me₄)₃(H₂O)₃]_n (Mn-MIL-88-Me₄, where H₂BDC-Me₄ = 2,3,5,6-tetramethylterephthalic acid). Through comprehensive single-crystal X-ray diffraction, spectroscopic, and magnetic studies, we revealed that both MOFs are in a Mn(II/III) mixed-valence state instead of the commonly observed Mn(III) oxidation state. Furthermore, the use of a methylated linker (BDC-Me₄) allowed access to permanent porosity in Mn-MIL-88-Me₄, which is an analogue of the flexible MIL-88 family, yielding a catalyst for alcohol oxidation.

Vanadium-Substituted Tunnel Structured Silver Hollandite (Ag1.2VxMn8-xO16): Impact on Morphology and Electrochemistry

A series of tunnel structured V-substituted silver hollandite (Ag_1.2V_xMn_8-xO₁₆, x = 0-1.4) samples is prepared and characterized through a combination of synchrotron X-ray diffraction (XRD), synchrotron X-ray absorption spectroscopy (XAS), laboratory Raman spectroscopy, and electron microscopy measurements. The oxidation states of the individual transition metals are characterized using V and Mn K-edge XAS data indicating the vanadium centers exist as V⁵⁺, and the Mn oxidation state decreases with increased V substitution to balance the charge. Scanning transmission electron microscopy of reduced materials shows reduction-displacement of silver metal at high levels of lithiation. In lithium batteries, the V-substituted tunneled manganese oxide materials reveal previously unseen reversible nonaqueous Ag electrochemistry and exhibit up to 2.5× higher Li storage capacity relative to their unsubstituted counterparts. The highest capacity was observed for the Ag_1.2(V_0.8Mn_7.2)O₁₆·0.8H₂O material with an intermediate level of V substitution, likely due to a combination of the atomic composition, the morphology of the particle, and the homogeneous distribution of the active material within the electrode structure where factors over multiple length scales contribute to the electrochemistry.

Organic template-based ZnO embedded Mn3O4 nanoparticles: synthesis and evaluation of their electrochemical properties towards clean energy generation

To deal with fossil fuel depletion and the rise in global temperatures caused by fossil fuels, cheap and abundant materials are required, in order to fulfill energy demand by developing high-performance fuel cells and electrocatalysts. In this work, a natural organic agent has been used to synthesize nano-structured ZnO/Mn₃O₄ with high surface area and enhanced electrocatalytic performance. Upon pre-annealing treatment, mixed metal oxide precipitates are formed due to the complex formation between a metal oxide and organic extract. The thermally annealed mixed oxide ZnO/Mn₃O₄ was characterized by XRD diffractometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). Gas chromatography-mass spectrometry (GC-MS) identified methyldecylamine as a major stabilizing agent of the synthesized nanomaterial. Using a Tauc plot, the calculated band energy for the synthesized ZnO/Mn₃O₄ mixed metal oxide was 1.65 eV. Moreover, we have demonstrated the effects of incorporated organic compounds on the surface chemistry, morphology and electrochemical behavior of ZnO/Mn₃O₄. The phyto-functionalized ZnO/Mn₃O₄ was deposited on Ni-foam for electrocatalytic studies. The fabricated electrode revealed good performance with low over-potential and Tafel slope, suggesting it to be suitable as a potential catalyst for water splitting application, in particular for the oxygen evolution reaction (OER). The overall findings of the current study provide a cost-effective and efficient organic template for functionalization and sustainable fabrication of ZnO/Mn₃O₄ nanomaterial for application as an electrocatalyst.

To deal with fossil fuel depletion and the rise in global temperatures caused by fossil fuels, cheap and abundant materials are required, in order to fulfill energy demand by developing high-performance fuel cells and electrocatalysts.

Construction of Defect-Rich Ni-Fe-Doped K0.23 MnO2 Cubic Nanoflowers via Etching Prussian Blue Analogue for Efficient Overall Water Splitting

Designing elaborate nanostructures and engineering defects have been promising approaches to fabricate cost-efficient electrocatalysts toward overall water splitting. In this work, a controllable Prussian-blue-analogue-sacrificed strategy followed by an annealing process to harvest defect-rich Ni-Fe-doped K_0.23 MnO₂ cubic nanoflowers (Ni-Fe-K_0.23 MnO₂ CNFs-300) as highly active bifunctional catalysts for oxygen and hydrogen evolution reactions (OER and HER) is reported. Benefiting from many merits, including unique morphology, abundant defects, and doping effect, Ni-Fe-K_0.23 MnO₂ CNFs-300 shows the best electrocatalytic performances among currently reported Mn oxide-based electrocatalysts. This catalyst affords low overpotentials of 270 (320) mV at 10 (100) mA cm^-2 for OER with a small Tafel slope of 42.3 mV dec^-1 , while requiring overpotentials of 116 and 243 mV to attain 10 and 100 mA cm^-2 for HER respectively. Moreover, Ni-Fe-K_0.23 MnO₂ CNFs-300 applied to overall water splitting exhibits a low cell voltage of 1.62 V at 10 mA cm^-2 and excellent durability, even superior to the Pt/C||IrO₂ cell at large current density. Density functional theory calculations further confirm that doping Ni and Fe into the crystal lattice of δ-MnO₂ can not only reinforce the conductivity but also reduces the adsorption free-energy barriers on the active sites during OER and HER.

Facile inverse micelle fabrication of magnetic ordered mesoporous iron cerium bimetal oxides with excellent performance for arsenic removal from water

In this study, magnetic ordered mesoporous Fe/Ce bimetal oxides (OMICs) were successfully synthesized via the modified sol-gel-based inverse micelle method. The textural/structure properties, surface chemistry and adsorption behavior of OMICs could be easily adjusted by using the calcination temperature. The sintering of samples would decrease the surface area, while expand the pore and crystallite size, which resulted in the formation of highly ordered inner-connected structure. Compared with pure mesoporous iron oxides (MI) and mesoporous cerium oxides (MC), this ordered mesoporous iron-cerium bimetal oxides (OMIC-3, 450 °C) exhibited remarkable arsenic adsorption performance. The maximum adsorption capacities of As(III) and As(V) for OMIC-3 were 281.34 and 216.72 mg/g, respectively, and both As(III)/As(V) adsorption kinetics were well described by the pseudo-second order. The ionic strength and coexisting ions (except SiO₃^2- and PO₄^3-) did not affect arsenic removal, while humic acid (HA) significantly influenced on the arsenic removal even at a lower concentration. The adsorption mechanism study revealed that both the surface charge and surface M-OH groups of OMIC-3 were played the key roles in arsenic removal. The reusable property suggested that this magnetic OMIC-3 was a promising excellent adsorbent for decontamination of arsenic-polluted (especially As(III)-polluted) wastewater.

Interfacing Manganese Oxide and Cobalt in Porous Graphitic Carbon Polyhedrons Boosts Oxygen Electrocatalysis for Zn-Air Batteries

Rational design and synthesis of highly active and robust bifunctional non-noble electrocatalysts for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are urgently required for efficient rechargeable metal-air batteries. Herein, abundant MnO/Co heterointerfaces are engineered in porous graphitic carbon (MnO/Co/PGC) polyhedrons via a facile hydrothermal-calcination route with a bimetal-organic framework as the precursor. The in situ generated Co nanocrystals not only create well-defined heterointerfaces with high conductivity to overcome the poor OER activity but also promote the formation of robust graphitic carbon. Owing to the desired composition and formation of the heterostructures, the resulting MnO/Co/PGC exhibits superior activity and stability toward both OER and ORR, which makes it an efficient air cathode for the rechargeable Zn-air battery. Importantly, the homemade Zn-air battery is able to deliver excellent performance including a peak power density of 172 mW cm^-2 and a specific capacity of 872 mAh g^-1 , as well as excellent cycling stability (350 cycles), outperforming commercial mixed Pt/C||RuO₂ catalysts. This work highlights the synergy from heterointerfaces in oxygen electrocatalysis, thus providing a promising approach for advanced metal-air cathode materials.

Oxidative nucleation and growth of Janus-type MnOx-Ag and MnOx-AgI nanoparticles

Janus nanoparticles (NPs) containing two chemically distinct materials in one system are of great significance for catalysis in terms of harnessing catalytic synergies that do not exist in either component. We herein present a novel synthetic method of two Janus-type MnOx-Ag and MnOx-AgI NPs. The synthesis of Janus-type MnOx-AgI NPs is based on the oxidative nucleation and growth of Ag domains on MnO first and the subsequent iodization of Ag. A mild and non-disruptive iodization strategy is developed to yield Janus MnOx-AgI NPs, in which converting Ag to AgI domains with iodomethane (CH3I) is achieved through partial iodization. Simultaneously, Mn2+ species in the primary MnO octahedra are oxidized during the growth of Ag NPs, leading to the formation of amorphous p-type MnOx domains. Therefore, the as-resultant Janus-type MnOx-AgI NPs combining two semiconductors into an integrated nanostructure can be used as an efficient photocatalyst for visible-light-driven water oxidation. Janus MnOx-AgI NPs show an expected photocatalytic activity even in the absence of Ru(bpy)3Cl2 as an electron mediator. This intriguing synthesis may offer a new opportunity to develop asymmetric nanostructures of two semiconductors that will potentially be efficient photocatalysts for solar-driven water splitting.

Guided Assembly of Microporous/Mesoporous Manganese Phosphates by Bifunctional Organophosphonic Acid Etching and Templating

Manganese (Mn)-based compounds are important materials for both energy conversion and energy storage. Unfortunately, it has been a significant challenge to develop highly ordered microporous/mesoporous structures for them to provide more active sites for these applications. In order to do so using the soft-templating method, three conditions have to be met, namely, a strong interaction between the inorganic precursor and the organic templates; eliminating the formation of bulk manganese phosphate; and the preservation of the manganese phosphate framework without it collapsing upon template removal. Herein, a soft-templating approach is reported using an organophosphonic acid (n-hexylphosphonic acid) as both the etching and the templating agent, followed by high-vacuum-assisted annealing. This approach simultaneously satisfies the above conditions. Both microporous and mesoporous manganese phosphates with uniform pore sizes and well-defined pore structures are obtained. The utilization of the organophosphonic acid is shown to be the key in the transformation from bulk manganese oxide into a highly ordered microporous phosphate. A very high surface area of 304.1 m² g^-1 is obtained for the microporous manganese phosphate, which is the highest among the reported values for Mn-based compounds. The ultrafine micropores and high specific surface area of our manganese phosphate promote electrocatalytic activity for the oxygen evolution reaction.

Surface redox characters and synergetic catalytic properties of macroporous ceria-zirconia solid solutions

Macroporous CeO₂-ZrO₂ (CZ) solid solutions with gradually changing ceria content were prepared through the EISA method. Pore sizes of the samples are about 100 nm-1 μm and pore walls are 100 nm-1.5 μm. The surface and near surface reduction bands of Ce⁴⁺ below 600 °C were maximized for the Ce_0.5Zr_0.5O₂ sample (C5) according to the quantitative de-convolution to the acquired TPR curves. The area percentage of the O^2-2p⁶ → Ce³⁺3d⁹4f² electronic transition band on XPS spectra, which related to the concentration of the Ce³⁺, was found to be a function of the ceria content. The oxygen storage capacity showed a positive relationship with the chemical compositions. Redox reactions below 600 °C play a key role in determining the reduction performances of ceria based TWCs. Three-way catalytic performances of the Pd + Rh + Pt /C5 sample showed an ignition temperature for CO and NOx at about 240 °C, and finished before 300 °C. The ignition of C₃H₈ started at 270 °C while finished at differed samples. The maximum catalytic efficiencies of CO, NOx, and C₃H₈ on C5 sample were revealed to 100%, 98%, and 97%, respectively. The performances showed that porous CZ solid solutions are suitable for high performance catalytic applications.

Templated Growth of Crystalline Mesoporous Materials: From Soft/Hard Templates to Colloidal Templates

Mesoporous non-siliceous materials, in particular mesoporous transition metal oxides (m-TMOs), are of interest due to their fascinating electronic, redox, and magnetic properties for a wide range of applications in catalysis and energy storage. Control of the porosity (e.g., pore size, wall thickness, and surface area) and the crystalline degree (e.g., phase composition, crystallinity, and crystal grain size) of m-TMOs are critical for those applications. To crystallize TMOs, high temperature annealing is often needed to remove the amorphous defects and/or tune the compositions of different crystalline phases. This has brought many challenges to surfactant or block copolymer templates used in the process of evaporation-induced-self-assembly to prepare m-TMOs. In this review, we summarize the most recent achievements including the findings in our own laboratory on the use of organosilicate-containing colloids for the templated growth of mesoporous materials. We review a few key examples of preparing crystalline mesoporous oxides using different templating methods. The colloidal templating method by which mesoporous nanostructures can be stabilized up to 1,000°C is highlighted. The applications of m-TMOs and meso metal-oxide hybrids synthesized using organosilicate-containing colloidal templates in photocatalysis and high-temperature catalysis are also discussed.

Accelerating the water splitting kinetics of CoP microcubes anchored on a graphene electrocatalyst by Mn incorporation

CoP is considered as an efficient electrocatalyst for the hydrogen evolution reaction (HER) in acidic electrolytes but its performance in alkaline solutions is generally poor because of its slow reaction kinetics, which further limits its application in overall water splitting. Herein, we demonstrate a strategy to greatly accelerate its HER and OER kinetics in alkaline solutions through Mn incorporation. Ternary Mn x Co1-x P microcubes with a tunable Mn/Co ratio strongly anchored on rGO were synthesized using Prussian blue analogues as precursors. The synergy between the high activity of Mn x Co1-x P microcubes and the good conductivity of rGO leads to the superior performance of the hybrid toward water splitting in 1 M KOH. The optimized Mn0.6Co0.4P-rGO electrocatalyst shows high activity and stability towards both the HER and OER with low overpotentials of 54 and 250 mV at 10 mA cm-2, respectively. Furthermore, the water electrolyzer using Mn0.6Co0.4P-rGO as both the cathode and anode only requires a cell voltage as low as 1.55 V to reach a current density of 10 mA cm-2, making Mn0.6Co0.4P-rGO a competitive and cost-effective electrocatalyst for water splitting.

S, Adarakatti PS, Hughes JP, Rowley-Neale SJ, Banks CE, S. A. In situ addition of graphitic carbon into a NiCo2O4/CoO composite: enhanced catalysis toward the oxygen evolution reaction

A facile synthesis technique for the production of NiCo₂O₄/CoO and graphite composites that demonstrate efficient electrocatalysis towards the oxygen evolution reaction.

Stabilization of Cobalt-Polyoxometalate over Poly(ionic liquid) Composites for Efficient Electrocatalytic Water Oxidation

The key to unlock a renewable, clean, and energy-dense hydrogen fuel lies in designing an efficient oxygen evolving catalyst exhibiting high activity, stability, and cost-effectiveness. This report addresses an improved activity toward oxygen evolution by a composite of cobalt-polyoxometalate [Co₄(H₂O)₂(PW₉O₃₄)₂]^10- (CoPOM) and an ionic polymer, poly(vinyl butyl imidazolium) (PVIM), in highly alkaline media. PVIM provides a stable platform for CoPOM and acts as a conductive linker between CoPOM and the electrode surface, forming a concrete solid composite, which balances the multinegative charge of CoPOM synergistically. This improved stability and conductivity of CoPOM by PVIM in the PVIM-CoPOM composite performs remarkable electrocatalytic water oxidation with a very low overpotential of 0.20 V and a very high current density of 250 mA/cm² (at 1.75 V vs RHE) with a turnover frequency (TOF) of 52.8 s^-1 in 1 M NaOH.

Structural characteristic and ammonium and manganese catalytic activity of two types of filter media in groundwater treatment

Two types of filter media in groundwater treatment were conducted for a comparative study of surface structure and catalytic performance. Natural filter media was adopted from a conventional aeration-filtration groundwater treatment plant, and active filter media as a novel and promising filter media was also adopted. The physicochemical properties of these two kinds of filter media were characterized using numerous analytical techniques, such as X-Ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS) and Zeta potential. The catalytic activities of these filter media were evaluated for ammonium and manganese oxidation. XRD data showed that both active filter media and natural filter media belonged to birnessite family. A new manganese dioxide (MnO₂) phase (PDF#72-1982) was found in the structure of natural filter media. The SEM micrograph of natural filter media showed honeycomb structures and the active filter media presented plate structures and consisted of stacked particle. These natural filter media presented lower level of some trace elements such as calcium and magnesium, lower degree of crystallinity, lower Mn(III) content and lattice oxygen content than that of active filter media, which were associated with its poor ammonium and manganese catalytic activities. In addition, some γ-Fe₂O₃ and MnCO₃ were found in the coating which may hinder the ammonium and manganese catalytic oxidation. This study provides a thorough and comprehensive understanding about the most commonly used filter media in water treatment, which can provide a theoretical guide to practical applications.

Quasi-single-crystalline CoO hexagrams with abundant defects for highly efficient electrocatalytic water oxidation

Defects and structural long-range ordering have been recognized as two crucial characters for advanced electrocatalysts. However, these two features have rarely been achieved together. Herein, we report the preparation of single-crystalline CoO hexagrams and demonstrate their exceptional electrocatalytic properties for water oxidation. The quasi-single-crystalline CoO hexagrams, prepared at the critical phase transition point of β-Co(OH)2/Co(OH)F hexagrams, possess both abundant oxygen vacancies as defects and structural long-range ordering. The matching between the b-axis of Co(OH)F crystals and the a-axis of β-Co(OH)2 crystals is critical for the formation of the CoO hexagram single crystals. The quasi-single-crystalline CoO hexagrams with abundant defects are highly efficient for water oxidation by delivering a 10 mA cm-2 current density at a low overpotential of 269 mV in a 1 M KOH aqueous solution.

Solution growth of 3D MnO2 mesh comprising 1D nanofibres as a novel sensor for selective and sensitive detection of biomolecules

This work is the first report describing the solution grown 3D manganese oxide nanofibrous (MnO₂ NFs) mesh and its potential for the simultaneous detection of biomolecules such as ascorbic acid and uric acid. The mesh is synthesized by a facile, one-pot, and cost-effective hydrothermal approach without using any template or structure directing compound. The morphology consists of randomly placed nanofibres possessing a diameter in the range of 10-25 nm, and length of several micron; constituting a highly porous and flexible material. The electrochemical potential was examined by recording cyclic voltammetry signals towards ascorbic acid and uric acid. The special mesh morphology offers a large surface area to promote enhanced electrochemical activity, and also provided a macroporous network that supported efficient mass transport. Additionally, the strong electronic cloud and roughness of MnO₂ NFs mesh facilitated the fast oxidation of species at very low potential. The lower detection limit was found to be 1.33 µM (S/N = 3) and 1.03 µM (S/N = 3) for ascorbic acid and uric acid, respectively. The MnO₂ NFs mesh modified electrodes can robustly differentiate both of them by giving well separate signals (Δ = 500 mV) indicating capability of the material towards selective detection. The sensor has been successfully applied to human blood and urine samples and the recoveries were found statistically significant. These results demonstrate the practical feasibility of 3D mesh to develop sensors for the accurate diagnosis of clinically important molecules.

Supported Mn3O4 Nanosystems for Hydrogen Production through Ethanol Photoreforming

Photoreforming promoted by metal oxide nanophotocatalysts is an attractive route for clean and sustainable hydrogen generation. In the present work, we propose for the first time the use of supported Mn₃O₄ nanosystems, both pure and functionalized with Au nanoparticles (NPs), for hydrogen generation by photoreforming. The target oxide systems, prepared by chemical vapor deposition (CVD) and decorated with gold NPs by radio frequency (RF) sputtering, were subjected to a thorough chemico-physical characterization and utilized for a proof-of-concept H₂ generation in aqueous ethanolic solutions under simulated solar illumination. Pure Mn₃O₄ nanosystems yielded a constant hydrogen production rate of 10 mmol h^-1 m^-2 even for irradiation times up to 20 h. The introduction of Au NPs yielded a significant enhancement in photocatalytic activity, which decreased as a function of irradiation time. The main phenomena causing the Au-containing photocatalyst deactivation have been investigated by morphological and compositional analysis, providing important insights for the design of Mn₃O₄-based photocatalysts with improved performances.

Catalysis of the Oxygen Evolution Reaction by 4-10 nm Cobalt Nanoparticles

Electrolysis of water is key technology, not only for clean energy production, but to ensure a continued supply of hydrogen beyond fossil resources, essential to the manufacture of many chemical goods other than fuels. Cobalt nanomaterials have been widely identified as a promising candidate for the anode (oxygen evolution) reaction in this process, but much work has focused on applied materials or electrode design. Given the importance of oxidation state changes Co(III) → Co(IV) in the accepted reaction mechanism, in this work we look at size effects in small (4–10 nm) cobalt nanoparticles, where the ease of oxidation for lower cobalt oxidation states is known to change with particle size. To discriminate between geometric and chemical effects we have compared the catalysts in this study to others in the literature by turnover frequency (widely used in other areas of catalysis), in addition to the more commonly employed performance metric of the overpotential required to produce a current density of 10 mA cm⁻². Comparisons are drawn to key examples of using well defined nanomaterials (where the surface are of cobalt sites can be estimated). This has enabled an estimated intrinsic turnover rate of ~ 1 O₂ molecule per surface Co atom per second at an overpotential of 500 mV in the oxygen evolution reaction under typical alkaline reaction conditions (pH 14.0) to be identified.

In situ X-ray absorption spectroscopy of transition metal based water oxidation catalysts

X-ray absorption studies of the geometric and electronic structure of primarily heterogeneous Co, Ni, and Mn based water oxidation catalysts are reviewed. The X-ray absorption near edge and extended X-ray absorption fine structure studies of the metal K-edge, characterize the metal oxidation state, metal-oxygen bond distance, metal-metal distance, and degree of disorder of the catalysts. These properties guide the coordination environment of the transition metal oxide radical that localizes surface holes and is required to oxidize water. The catalysts are investigated both as-prepared, in their native state, and under reaction conditions, while transition metal oxide radicals are generated. The findings of many experiments are summarized in tables. The advantages of future X-ray experiments on water oxidation catalysts, which include the limited data available of the oxygen K-edge, metal L-edge, and resonant inelastic X-ray scattering, are discussed.

The laccase-like reactivity of manganese oxide nanomaterials for pollutant conversion: rate analysis and cyclic voltammetry

Nanostructured manganese oxides, e.g. MnO₂, have shown laccase-like catalytic activities, and are thus promising for pollutant oxidation in wastewater treatment. We have systematically compared the laccase-like reactivity of manganese oxide nanomaterials of different crystallinity, including α-, β-, γ-, δ-, and ɛ-MnO₂, and Mn₃O₄, with 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and 17β-estradiol (E2) as the probing substrates. The reaction rate behaviors were examined with regard to substrate oxidation and oxygen reduction to evaluate the laccase-like catalysis of the materials, among which γ-MnO₂ exhibits the best performance. Cyclic voltammetry (CV) was employed to assess the six MnO_x nanomaterials, and the results correlate well with their laccase-like catalytic activities. The findings help understand the mechanisms of and the factors controlling the laccase-like reactivity of different manganese oxides nanomaterials, and provide a basis for future design and application of MnO_x-based catalysts.