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

Unveiling Enhanced PEC Water Oxidation: Morphology Tuning and Interfacial Phase Change in α-Fe2O3@K-OMS-2 Branched Core-Shell Nanoarrays

A simple fabrication method that involves two steps of hydrothermal reaction has been demonstrated for the growth of α-Fe2O3@K-OMS-2 branched core-shell nanoarrays. Different reactant concentrations in the shell-forming step led to different morphologies in the resultant composites, denoted as 0.25 OC, 0.5 OC, and 1.0 OC. Both 0.25 OC and 0.5 OC formed perfect branched core-shell structures, with 0.5 OC possessing longer branches, which were observed by SEM and TEM. The core K-OMS-2 and shell α-Fe2O3 were confirmed by grazing incidence X-ray diffraction (GIXRD), EDS mapping, and atomic alignment from high-resolution STEM images. Further investigation with high-resolution HAADF-STEM, EELS, and XPS indicated the existence of an ultrathin layer of Mn3O4 sandwiched at the interface. All composite materials offered greatly enhanced photocurrent density at 1.23 VRHE, compared to the pristine Fe2O3 photoanode (0.33 mA/cm2), and sample 0.5 OC showed the highest photocurrent density of 2.81 mA/cm2. Photoelectrochemical (PEC) performance was evaluated for the samples by conducting linear sweep voltammetry (LSV), applied bias photo-to-current efficiency (ABPE), electrochemical impedance spectroscopy (EIS), incident-photo-to-current efficiency (IPCE), transient photocurrent responses, and stability tests. The charge separation and transfer efficiencies, together with the electrochemically active surface area, were also investigated. The significant enhancement in sample 0.5 OC is ascribed to the synergetic effect brought by the longer branches in the core-shell structure, the conductive K-OMS-2 core, and the formation of the Mn3O4 thin layer formed between the core and shell.

Highly Active and Water-Resistant Cu-Doped OMS-2 Catalysts for CO Oxidation: The Importance of the OMS-2 Synthesis Method and Cu Doping

Porous cryptomelane-type Mn oxide (OMS-2) has an outstanding redox property, making it a highly desirable substitute for noble metal catalysts for CO oxidation, but its catalytic activity still needs to be improved, especially in the presence of water. Given the strong structure-performance correlation of OMS-2 for oxidation reactions, herein, OMS-2 is synthesized by solid state (OMS-2S), reflux (OMS-2R), and hydrothermal (OMS-2H) methods, aiming to improve its CO oxidation performance through manipulating synthesis parameters to tailor its particle size, morphology, and crystallinity. Characterization shows that OMS-2S has the highest CO oxidation activity in the absence of water due to its low crystallinity, high specific surface area, large oxygen vacancy content, and good redox property, but the presence of water can greatly reduce its CO oxidation activity. Doping Cu into an OMS-2 can not only improve its CO oxidation activity but also greatly improve its water tolerance. The Cu-doped OMS-2S catalyst with ∼4 wt % Cu can achieve a T90 of 49 °C (1% CO/10% O2/N2 and WHSV = 60,000 mL·g-1·h-1), ranking among the lowest reported T90 values for Mn oxide-based CO oxidation catalysts, and it can maintain nearly 100% CO conversion in the presence of 5 vol % water for over 50 h. In situ DRIFTs characterization indicates that the good water resistance of Cu-doped OMS-2S can be attributed to the significantly suppressed surface hydroxyl group generation because of Cu doping. This work demonstrates the importance of the synthesis method and Cu doping in determining the CO oxidation activity and water resistance of OMS-2 and will provide guidance for synthesizing highly active and water-resistant CO oxidation catalysts.

Novel Nanosized Spinel MnCoFeO4 for Low-Temperature Hydrocarbon Oxidation

The present paper reports on MnCoFeO₄ spinels with peculiar composition and their catalytic behavior in the reactions of complete oxidation of hydrocarbons. The samples were synthesized by solution combustion method with sucrose and citric acid as fuels. All samples were characterized by powder X-ray diffraction, N₂-physisorption, scanning electron microscopy, thermal analysis, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy. The catalytic properties of the spinels with Mn:Co:Fe = 1:1:1 composition were studied in reactions of complete oxidation of methane, propane, butane, and propane in the presence of water as model pollutants. Both prepared catalysts are nanosized materials. The slight difference in the compositions, structure, and morphology is due to the type of fuel used in the synthesis reaction. The spinel, prepared with sucrose, shows a higher specific surface area, pore volume, higher amount of small particles fraction, higher thermal stability, and as a result, more exposed active sites on the sample surface that lead to higher catalytic activity in the studied oxidation reactions. After the catalytic tests, both samples do not undergo any substantial phase and morphological changes; thus, they could be applied in low-temperature hydrocarbon oxidation reactions.

Biomimetic Tremelliform Ultrathin MnO2/CuO Nanosheets on Kaolinite Driving Superior Catalytic Oxidation: An Example of CO

Highly efficient three-dimensional (3D) kaolinite/MnO₂-CuO (KM@CuO-NO₃) catalysts were synthesized by a mild biomimetic strategy. Kaolinite flakes were uniformly wrapped by ultrathin tremelliform MnO₂ nanosheets with thicknesses of around 1.0-1.5 nm. Si-O and Al-O groups in kaolinite hosted MnO₂ nanosheets to generate a robust composite structure. The ultrathin MnO₂ lamellar structure exhibited excellent stability even after calcination above 350 °C. Kaolinite/MnO₂ exhibited abundant edges, sharp corners, and interconnected diffusion channels, which are superior to the common stacked structure. Open channels guaranteed fast transportation and migration of CO and O₂ during CO oxidation. The synthesized KM@CuO-NO₃ achieved a 90% CO conversion efficiency at a relatively low temperature (110 °C). Furthermore, the abundant oxygen vacancies on KM@CuO-NO₃ assisted the adsorption and activation of oxygen species and thus enhanced the oxygen mobility and reactivity in the catalytic process. The mechanism results suggest that CuO introduced to the catalyst not only acted as CO active sites but also weakened the Mn-O bond, subsequently improved the mobilities of the oxygen species, which was found to contribute to its high activity for CO oxidation. This study provides new conceptual insights into rationally regulating structural assembly between transition metal oxides and natural minerals for high-performance catalysis reactions.

Effect of MnO2 Polymorphs' Structure on Low-Temperature Catalytic Oxidation: Crystalline Controlled Oxygen Vacancy Formation

MnO₂ polymorphs (α-, β-, and ε-MnO₂) were synthesized, and their chemical/physical properties for CO oxidation were systematically studied using multiple techniques. Density functional theory (DFT) calculations and temperature-programmed experiments reveal that β-MnO₂ shows low energies for oxygen vacancy generation and excellent redox properties, exhibiting significant CO oxidation activity (T₉₀ = 75 °C) and stability even under a humid atmosphere. For the first time, we report that the specific reaction rate for β-MnO₂ (0.135 molecule_CO·nm^-2·s^-1 at 90 °C) is roughly approximately 4 and 17 times higher than that of ε-MnO₂ and α-MnO₂, respectively. The specific reaction rate order (β-MnO₂ > ε-MnO₂ > α-MnO₂) is not only in good agreement with reduction rates (CO-TPSR measurements) but also agrees with the DFT calculation. In combination with in situ spectra and intrinsic kinetic studies, the mechanisms of CO oxidation over various crystal structures of MnO₂ were proposed as well. We believe the new insights from this study will largely inspire the design of such a kind of catalyst.

Fabrication of MnCoOx composite oxides for catalytic CO oxidation via a solid-phase synthesis: The significant effect of manganese precursor

A series of Mn3Co16Ox composite oxides catalysts were fabricated via a solid-phase synthesis using different manganese precursors (namely as manganese acetate (A), nitrate (N), and sulfate (S)). It has been...

Tuning Metal-Support Interaction of Pt-CeO2 Catalysts for Enhanced Oxidation Reactivity

Metal-support interaction (MSI) has been widely recognized to be playing a pivotal role in regulating the catalytic activity of various reactions. In this work, the degree of MSI between Pt and CeO₂ support was finely tuned by adjusting the activation condition, and the obtained catalysts were tested for the oxidative abatement of CO and HCHO under ambient conditions. The characterization of catalysts shows that activation of strongly interacting Pt-CeO₂ at higher temperatures by H₂ leads to a weaker MSI with increased electron density of Pt, and this modification of local electronic properties is demonstrated to result in enhanced O₂ adsorption/activation to prevent the CO self-poisoning effect, while it abates the activity of CO adsorption/activation and oxidation of adsorbed CO. The Pt-CeO₂ catalyst with a moderate MSI, which is able to balance each step in the catalytic cycle over Pt and Pt-CeO₂ interface domains, displays the highest activity for CO/HCHO oxidation under ambient conditions.

Synergistic Effects of Ternary PdO-CeO2-OMS-2 Catalyst Afford High Catalytic Performance and Stability in the Reduction of NO with CO

We developed a robust ternary PdO-CeO₂-OMS-2 catalyst with excellent catalytic performance in the selective reduction of NO with CO using a strategy based on combining components that synergistically interact leading to an effective abatement of these toxic gases. The catalyst affords 100% selectivity to N₂ at the nearly full conversion of NO and CO at 250 °C, high stability in the presence of H₂O, and a remarkable SO₂ tolerance. To unravel the origin of the excellent catalytic performance, the structural and chemical properties of the PdO-CeO₂-OMS-2 nanocomposite were analyzed in the as-prepared and used state of the catalyst, employing a series of pertinent characterization methods and specific catalytic tests. The experimental as well as theoretical results, based on density-functional theory calculations suggest that CO and NO follow different reaction pathways, CO is preferentially adsorbed and oxidized at Pd sites (Pd^II and Pd⁰), while NO decomposes on the ceria surface. Lattice oxygen vacancies at the interfacial perimeter of PdO-CeO₂ and PdO-OMS-2, and the diffusion of oxygen and oxygen vacancies are proposed to play a critical role in this multicenter reaction system.

Elucidating the Nature of the Cu(I) Active Site in CuO/TiO2 for Excellent Low-Temperature CO Oxidation

Stabilized Cu⁺ species have been widely considered as catalytic active sites in composite copper catalysts for catalytic reactions with industrial importance. However, few examples comprehensively explicated the origin of stabilized Cu⁺ in a low-cost and widely investigated CuO/TiO₂ system. In this study, mass producible CuO/TiO₂ catalysts with interface-stabilized Cu⁺ were prepared, which showed excellent low-temperature CO oxidation activity. A thorough characterization and theoretical calculations proved that the strong charge-transfer effect and Ti-O-Cu hybridization in Ti-doped CuO(111) at the CuO/TiO₂ interface contributed to the formation and stabilization of Cu⁺ species. The CO molecule adsorbed on Cu⁺ and reacted directly with Ti doping-promoted active lattice oxygen via a Mars-van Krevelen mechanism, leading to the enhanced low-temperature activity.

Electro-activation of O2 on MnO2/graphite felt for efficient oxidation of water contaminants under room condition

The activation of oxygen (O₂) under room condition is highly desirable for oxidative removal of organic pollutants in water. Herein, we report a graphite felt (GF)-supported α-MnO₂ catalyst which is active for activating O₂ with assistance of an anodic electric field. The electro-assisted catalytic wet air oxidation (ECWAO) process on MnO₂/GF is able to rapidly degrade a variety of dyes, pharmaceutics and personal care products (PPCPs) under room condition. The congo red, basic fuchsin, neutral red and methylene blue are completely mineralized in 160 min, and the bisphenol A, triclosan and ciprofloxacin are mineralized by 89.9%, 81.5% and 65.4%, respectively, in 300 min. Mechanistic study indicates a surface-catalyzed non-free radical pathway for the oxidation of organic pollutants by O₂ in the ECWAO process. The oxygen vacancies on MnO₂ are identified as the catalytically active sites, at which oxygen atom is transferred from O₂ to organic molecule through chemisorbed oxygen species. The anodic electric field assists such an oxygen transfer pathway by activating the complex of chemisorbed oxygen species and organic molecule through electro-oxidation reaction. The ECWAO process on MnO₂/GF electrode exhibits a great potential for practical wastewater treatment under room condition.

Leaf-like Co-ZIF-L derivatives embedded on Co2AlO4/Ni foam from hydrotalcites as monolithic catalysts for toluene abatement

Herein, a series of distinctively monolithic catalysts were first synthesized by decorating leaf-like Co-ZIF-L derivatives on Co₂AlO₄ coral-like microspheres from CoAl layered double hydroxides (LDHs), which were coated on three-dimensional porous Ni foam. As a proof of concept application, toluene was chosen as a probe molecule to evaluate their catalytic performances over the as-synthesized catalysts. As a result, the L-12 sample derived from Co₂AlO₄@Co-Co LDHs displayed an excellent catalytic performance, cycling stability and long-term stability for toluene oxidation (T₉₉ = 272 °C, 33 °C lower than that of Co₂AlO₄ sample), where leaf-like Co-ZIF-L served as a sacrificial template to synthesize Co-Co LDHs. The improved catalytic performance was attributed to its distinctive structure, in which leaf-like Co-ZIF-L derivatives on Co₂AlO₄ resulted in its higher specific surface area, lower-temperature reducibility, rich surface oxygen vacancy and high valence Co³⁺ species. This work thus demonstrates a feasible strategy for the design and fabrication of hybrid LDHs/ZIFs-derived composite architectures, which is expected to construct other novel monolithic catalysts with hierarchical structures for other potential applications.

Macroporous Ni foam-supported Co3O4 nanobrush and nanomace hybrid arrays for high-efficiency CO oxidation

Herein, we reported the synthesis of well-defined Co₃O₄ nanoarrays (NAs) supported on a monolithic three-dimensional macroporous nickel (Ni) foam substrate for use in high-efficiency CO oxidation. The monolithic Co₃O₄ NAs catalysts were obtained through a generic hydrothermal synthesis route with subsequent calcination. By controlling the reaction time, solvent polarity and deposition agent, these Co₃O₄ NAs catalysts exhibited various novel morphologies (single or hybrid arrays), whose physicochemical properties were further characterized by using several analytical techniques. Based on the catalytic and characterization analyses, it was found that the Co₃O₄ NAs-6 catalyst with nanobrush and nanomace arrays displayed enhanced catalytic activity for CO oxidation, achieving an efficient 100% CO oxidation conversion at a gas hourly space velocity (GHSV) 10,000hr^-1 and 150°C with long-term stability. Compared with the other Co₃O₄ NAs catalysts, it had the highest abundance of surface-adsorbed oxygen species, excellent low-temperature reducibility and was rich in surface-active sites (Co³⁺/Co²⁺=1.26).

Vertically-aligned Co3O4 arrays on Ni foam as monolithic structured catalysts for CO oxidation: effects of morphological transformation

A generic hydrothermal synthesis route has been successfully designed and utilized to in situ grow highly ordered Co3O4 nanoarray (NA) precursors on Ni substrates, forming a series of Co3O4 nanoarray-based monolithic catalysts with subsequent calcination. The morphology evolution of Co3O4 nanostructures which depends upon the reaction time, with and without CTAB or NH4F is investigated in detail, which is used to further demonstrate the growth mechanism of Co3O4 nanoarrays with different morphologies. CO is chosen as a probe molecule to evaluate the catalytic performance over the synthesized Co-based oxide catalysts, and the effect of morphological transformation on the catalytic activity is further confirmed via using TEM, H2-TPR, XPS, Raman spectroscopy and in situ Raman spectroscopy. As a proof of concept application, core-shell Co3O4 NAs-8 presenting hierarchical nanosheets@nanoneedle arrays with a low density of nanoneedles exhibits the highest catalytic activity and long-term stability due to its low-temperature reducibility, the lattice distortion of the spinel structure and the abundance of surface-adsorbed oxygen (Oads). It is confirmed that CO oxidation on the surface of Co3O4 can proceed through the Langmuir-Hinshelwood mechanism via using in situ Raman spectroscopy. It is expected that the in situ synthesis of well-defined Co3O4 monolithic catalysts can be extended to the development of environmentally-friendly and highly active integral materials for practical industrial catalysis.

Synthesis, Characterization, and Thermal Properties of N-alkyl β-Diketiminate Manganese Complexes

A series of N, N'-dialkyl-β-diketiminato manganese(II) complexes was synthesized and characterized by single crystal X-ray diffraction, UV-vis and FTIR spectroscopy, and then assayed for volatility, thermal stability, and surface reactivity relevant to vapor-phase film growth processes. Bis( N, N'-dimethyl-4-amino-3-penten-2-imine) manganese(II), 1, and bis( N- N'-diisopropyl-4-amino-3-penten-2-imine) manganese(II), 2, specifically, emerge as the most promising candidates, balancing volatility (sublimation temperatures < 100 °C at 100 mTorr) with coordinative unsaturation and reactivity, as revealed by rapid release of ligand in the presence of a silica surface. Good correlation is observed between buried volume calculations and relative surface reactivity data, indicating that metal availability resulting from sterically open ligand alkyl substituents increases surface reactivity. The thermal stability, volatility, and reactivity exhibited by these compounds render them promising precursors for the growth of manganese oxide films via vapor-phase growth processes.

Low-temperature CO oxidation over integrated penthorum chinense-like MnCo2O4 arrays anchored on three-dimensional Ni foam with enhanced moisture resistance

Advanced integrated nanoarray (NA) catalysts have been designed by growing metal-doped Co₃O₄ arrays on nickel foam with robust adhesion.

Crystalline (Ni1-xCox)5TiO7 nanostructures grown in situ on a flexible metal substrate used towards efficient CO oxidation

Severe environmental contamination and the urgent demand for a clean atmosphere require an efficient and low-cost solution to address problems such as CO emission. In this study, we report the in situ integration of non-noble (Ni_1-xCo_x)₅TiO₇ nanostructures with different Co concentrations and tunable size on a flexible metal network support using a conventional PEO method; we further report their utilization for efficient CO oxidation. It was found that the Co/Ni ratios in the original electrolyte precursors directly result in the different size and morphology evolution. The (Ni_1-xCo_x)₅TiO₇ nanowire arrays with x = 0.16 exhibit the best performance towards CO catalytic oxidation along with a good catalytic stability. Further analysis using XPS indicates that the CO catalytic oxidation was mainly determined by the amount of defective oxygen, lattice oxygen and surface area. The single crystal nature, large surface area, excellent CO catalytic capability and strong substrate adhesion of the (Ni_1-xCo_x)₅TiO₇ nanostructures on a flexible metal substrate will open up more applications in CO oxidation ranging from processing autovehicle exhaust to chemical gas emissions in the industry.

Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review

Manganese oxide has been recognized as one of the most promising gaseous heterogeneous catalysts due to its low cost, environmental friendliness, and high catalytic oxidation performance. Mn-based oxides can be classified into four types: (1) single manganese oxide (MnOx), (2) supported manganese oxide (MnOx/support), (3) composite manganese oxides (MnOx-X), and (4) special crystalline manganese oxides (S-MnOx). These Mn-based oxides have been widely used as catalysts for the elimination of gaseous pollutants. This review aims to describe the environmental applications of these manganese oxides and provide perspectives. It gives detailed descriptions of environmental applications of the selective catalytic reduction of NOx with NH₃, the catalytic combustion of volatile organic compounds, Hg⁰ oxidation and adsorption, and soot oxidation, in addition to some other environmental applications. Furthermore, this review mainly focuses on the effects of structure, morphology, and modified elements and on the role of catalyst supports in gaseous heterogeneous catalytic reactions. Finally, future research directions for developing manganese oxide catalysts are proposed.

Solvent-Free Chemical Approach to Synthesize Various Morphological Co3O4 for CO Oxidation

Co₃O₄ nanomaterials with diverse morphologies were usually synthesized in liquid phase accompanied by the template or surfactant under harsh conditions, which further restricted their practical application. Herein, we reported an extremely simple and practical solid-state chemical method to synthesize Co₃O₄-octahedrons, -plates, and -rods. Among these, the shape control of Co₃O₄-octahedrons and Co₃O₄-plates involve variation of the amount of reactant, and the formation of Co₃O₄-rods with {110} facet can be achieved by replacing the reactant. The formation of the Co₃O₄ nanomaterials with different morphologies originated from the different microenvironments of reaction and the structure of reactants. The catalytic activity of Co₃O₄ samples for CO oxidation was evaluated in normal feed gas. The as-prepared Co₃O₄-rods exposed {110} facet exhibited superior catalytic activity for CO oxidation, which can be attributed to more oxygen defects on Co₃O₄-rods surface. Additionally, Co₃O₄-rods exhibited excellent durablility (without pretreatment) in normal feed gas, even in the presence of moisture, comparable or better than that reported in the literature. The practical and environmental friendly solvent-free strategy provided a new promising route for large-scale preparation of (metal) oxide with remarkable CO oxidation performance for practical application.

Nanoscale Cobalt-Manganese Oxide Catalyst Supported on Shape-Controlled Cerium Oxide: Effect of Nanointerface Configuration on Structural, Redox, and Catalytic Properties

Understanding the role of nanointerface structures in supported bimetallic nanoparticles is vital for the rational design of novel high-performance catalysts. This study reports the synthesis, characterization, and the catalytic application of Co-Mn oxide nanoparticles supported on CeO₂ nanocubes with the specific aim of investigating the effect of nanointerfaces in tuning structure-activity properties. High-resolution transmission electron microscopy analysis reveals the formation of different types of Co-Mn nanoalloys with a range of 6 ± 0.5 to 14 ± 0.5 nm on the surface of CeO₂ nanocubes, which are in the range of 15 ± 1.5 to 25 ± 1.5 nm. High concentration of Ce³⁺ species are found in Co-Mn/CeO₂ (23.34%) compared with that in Mn/CeO₂ (21.41%), Co/CeO₂ (15.63%), and CeO₂ (11.06%), as evidenced by X-ray photoelectron spectroscopy (XPS) analysis. Nanoscale electron energy loss spectroscopy analysis in combination with XPS studies shows the transformation of Co²⁺ to Co³⁺ and simultaneously Mn^4+/3+ to Mn²⁺. The Co-Mn/CeO₂ catalyst exhibits the best performance in solvent-free oxidation of benzylamine (89.7% benzylamine conversion) compared with the Co/CeO₂ (29.2% benzylamine conversion) and Mn/CeO₂ (82.6% benzylamine conversion) catalysts for 3 h at 120 °C using air as the oxidant. Irrespective of the catalysts employed, a high selectivity toward the dibenzylimine product (97-98%) was found compared with the benzonitrile product (2-3%). The interplay of redox chemistry of Mn and Co at the nanointerface sites between Co-Mn nanoparticles and CeO₂ nanocubes as well as the abundant structural defects in cerium oxide plays a key role in the efficiency of the Co-Mn/CeO₂ catalyst for the aerobic oxidation of benzylamine.

Scalable continuous flow synthesis of ZnO nanorod arrays in 3-D ceramic honeycomb substrates for low-temperature desulfurization

Hydrothermal based continuous flow synthesis demonstrates a highly efficient strategy of integrating nanostructure arrays onto 3-D channeled honeycomb substrates.