The Structures and Compositions Design of the Hollow Micro-Nano-Structured Metal Oxides for Environmental Catalysis

In recent decades, with the rapid development of the inorganic synthesis and the increasing discharge of pollutants in the process of industrialization, hollow-structured metal oxides (HSMOs) have taken on a striking role in the field of environmental catalysis. This is all due to their unique structural characteristics compared to solid nanoparticles, such as high loading capacity, superior pore permeability, high specific surface area, abundant inner void space, and low density. Although the HSMOs with different morphologies have been reviewed and prospected in the aspect of synthesis strategies and potential applications, there has been no systematic review focusing on the structures and compositions design of HSMOs in the field of environmental catalysis so far. Therefore, this review will mainly focus on the component dependence and controllable structure of HSMOs in the catalytic elimination of different environmental pollutants, including the automobile and stationary source emissions, volatile organic compounds, greenhouse gases, ozone-depleting substances, and other potential pollutants. Moreover, we comprehensively reviewed the applications of the catalysts with hollow structure that are mainly composed of metal oxides such as CeO2, MnOx, CuOx, Co3O4, ZrO2, ZnO, Al3O4, In2O3, NiO, and Fe3O4 in automobile and stationary source emission control, volatile organic compounds emission control, and the conversion of greenhouse gases and ozone-depleting substances. The structure-activity relationship is also briefly discussed. Finally, further challenges and development trends of HSMO catalysts in environmental catalysis are also prospected.

Controllable Anchoring of Graphitic Carbon Nitride on MnO2 Nanoarchitectures for Oxygen Evolution Electrocatalysis

The design and fabrication of eco-friendly and cost-effective (photo)electrocatalysts for the oxygen evolution reaction (OER) is a key research goal for a proper management of water splitting to address the global energy crisis. In this work, we focus on the preparation of supported MnO2/graphitic carbon nitride (g-CN) OER (photo)electrocatalysts by means of a novel preparation strategy. The proposed route consists of the plasma enhanced-chemical vapor deposition (PE-CVD) of MnO2 nanoarchitectures on porous Ni scaffolds, the anchoring of controllable g-CN amounts by an amenable electrophoretic deposition (EPD) process, and the ultimate thermal treatment in air. The inherent method versatility and flexibility afforded defective MnO2/g-CN nanoarchitectures, featuring a g-CN content and nano-organization tunable as a function of EPD duration and the used carbon nitride precursor. Such a modulation had a direct influence on OER functional performances, which, for the best composite system, corresponded to an overpotential of 430 mV at 10 mA/cm2, a Tafel slope of ≈70 mV/dec, and a turnover frequency of 6.52 × 10-3 s-1, accompanied by a very good time stability. The present outcomes, comparing favorably with previous results on analogous systems, were rationalized on the basis of the formation of type-II MnO2/g-CN heterojunctions, and yield valuable insights into this class of green (photo)electrocatalysts for end uses in solar-to-fuel conversion and water treatment.

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

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

Polymeric template assisted synthesis of monodisperse-porous manganese oxide microspheres: a new nanozyme with oxidase-like activity allowing biomolecule determination via bimodal sensing

New template assisted synthesis of monodisperse-porous MnO₂ microspheres and their usage as a nanozyme in the first bimodal sensing of ascorbic acid.

Multi-functional MnO2 nanomaterials for photo-activated applications by a plasma-assisted fabrication route

Supported MnO2-based nanomaterials were fabricated on fluorine-doped tin oxide substrates using plasma enhanced-chemical vapor deposition (PE-CVD) between 100 °C and 400 °C, starting from a fluorinated Mn(ii) diamine diketonate precursor. Growth experiments yielded β-MnO2 with a hierarchical morphology tuneable from dendritic structures to quasi-1D nanosystems as a function of growth temperature, whose variation also enabled a concomitant tailoring of the system fluorine content, and of the optical absorption and band gap. Preliminary photocatalytic tests were aimed at the investigation of photoinduced hydrophilic (PH) and solid phase photocatalytic (PC) performances of the present nanomaterials, as well as at the photodegradation of Plasmocorinth B azo-dye aqueous solutions. The obtained findings highlighted an attractive system photoactivity even under visible light, finely tailored by fluorine content, morphological organization and optical properties of the prepared nanostructures. The results indicate that the synthesized MnO2 nanosystems have potential applications as advanced smart materials for anti-fogging/self-cleaning end uses and water purification.

Controllable synthesis 3D hierarchical structured MnO2@NiCo2O4 and its morphology-dependent activity

The α-MnO₂@NiCo₂O₄ hierarchical structure with the nanowire- and nanosheet-like morphology was controlled synthesized under different solvents and alkalis. The α-MnO₂@NiCo₂O₄ heterogeneous structure exhibits a remarkably enhanced activity in the DME combustion reaction.

Hierarchical branched α-MnO2: one-step synthesis and catalytic activity

The hierarchical pine tree-like α-MnO₂are controlled synthesized at room temperature without any template. The α-MnO₂displays superior activity in the catalytic combustion of dimethyl ether.

Manganese Oxide Nanoarray-Based Monolithic Catalysts: Tunable Morphology and High Efficiency for CO Oxidation

A generic one-pot hydrothermal synthesis route has been successfully designed and utilized to in situ grow uniform manganese oxide nanorods and nanowires onto the cordierite honeycomb monolithic substrates, forming a series of nanoarray-based monolithic catalysts. During the synthesis process, three types of potassium salt oxidants have been used with different reduction potentials, i.e., K2Cr2O7, KClO3, and K2S2O8, denoted as HM-DCM, HM-PCR, and HM-PSF, respectively. The different reduction potentials of the manganese source (Mn(2+)) and oxidants induced the formation of manganese oxide nanoarrays with different morphology, surface area, and reactivity of carbon monoxide (CO) oxidation. K2Cr2O7 and KClO3 can induce sharp and long nanowires with slow growth rates due to their low reduction potentials. In comparison, the nanoarrays of HM-PSF presented shorter nanorods but displayed an efficient 90% CO oxidation conversion at 200 °C (T90) without noble-metal loading. Reducibility tests for the three monolithic catalysts by hydrogen temperature-programmed reduction revealed an activation energy order of HM-PSF > HM-DCM > HM-PCR for CO oxidation. The characterizations of oxygen temperature-programmed desorption and X-ray photoelectron spectroscopy indicated the abundant surface-adsorbed oxygen and lattice oxygen contributing to the superior reactivity of HM-PSF. The straightforward synthetic process showed a scalable, low-cost, and template-free method to fabricate manganese oxide nanoarray monolithic catalysts for exhaust treatment.

Enhancement of hydrocarbon production and catalyst stability during catalytic conversion of biomass pyrolysis-derived compounds over hierarchical HZSM-5

Alkali treated ZSM-5 with sheet-like mesopores showed higher yield of hydrocarbons in CFP of biomass, withstanding long-running over catalysis-regeneration cycles.

Template-free synthesis of complicated double-wall Cu2O hollow spheres with enhanced visible photocatalytic activities

In this study, complicated double-wall Cu₂O hollow spheres have been successfully prepared by a facile hydrothermal process in a ternary solvent system including water, ethanol and glycerol.

A facile one-pot hydrothermal synthesis of branched α-MnO2 nanorods for supercapacitor application

Branched α-MnO₂ nanorods are synthesized using a facile, green and effective one-pot hydrothermal approach without surfactants or templates. The branched α-MnO₂ nanorod electrode exhibited good supercapacitive properties due to its novel textural morphology and crystal structure.