Wenqiao Song

Structure–Property Relationship of Bifunctional MnO2 Nanostructures: Highly Efficient, Ultra-Stable Electrochemical Water Oxidation and Oxygen Reduction Reaction Catalysts Identified in Alkaline Media

Manganese oxides of various structures (α-, β-, and δ-MnO2 and amorphous) were synthesized by facile methods. The electrocatalytic properties of these materials were systematically investigated for catalyzing both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline media. Extensive characterization was correlated with the activity study by investigating the crystal structures (XRD, HRTEM), morphologies (SEM), porosities (BET), surfaces (XPS, O2-TPD/MS), and electrochemical properties (Tafel analysis, Koutechy-Levich plots, and constant-current electrolysis). These combined results show that the electrocatalytic activities are strongly dependent on the crystallographic structures, and follow an order of α-MnO2 > AMO > β-MnO2 > δ-MnO2. Both OER studies and ORR studies reveal similar structure-determined activity trends in alkaline media. In the OER studies, α-MnO2 displays an overpotential of 490 mV compared to 380 mV shown by an Ir/C catalyst in reaching 10 mA cm(-2). Meanwhile, α-MnO2 also exhibits stability for 3 h when supplying a constant current density of 5 mA cm(-2). This was further improved by adding Ni(2+) dopants (ca. 8 h). The superior OER activity was attributed to several factors, including abundant di-μ-oxo bridges existing in α-MnO2 as the protonation sites, analogous to the OEC in PS-II of the natural water oxidation system; the mixed valencies (AOS = 3.7); and the lowest charge transfer resistances (91.8 Ω, η = 430 mV) as revealed from in situ electrochemical impedance spectroscopy (EIS). In the ORR studies, when reaching 3 mA cm(-2), α-MnO2 shows 760 mV close to 860 mV for the best ORR catalyst (20% Pt/C). The outstanding ORR activity was due to the strongest O2 adsorption capability of α-MnO2 suggested by temperature-programmed desorption. As a result, this discovery of the structure-related electrocatalytic activities could provide guidance in the further development of easily prepared, scalable, and low-cost catalysts based on metal oxides and their derivatives.

Photocatalytic Water Splitting—The Untamed Dream: A Review of Recent Advances

Photocatalytic water splitting using sunlight is a promising technology capable of providing high energy yield without pollutant byproducts. Herein, we review various aspects of this technology including chemical reactions, physiochemical conditions and photocatalyst types such as metal oxides, sulfides, nitrides, nanocomposites, and doped materials followed by recent advances in computational modeling of photoactive materials. As the best-known catalyst for photocatalytic hydrogen and oxygen evolution, TiO2 is discussed in a separate section, along with its challenges such as the wide band gap, large overpotential for hydrogen evolution, and rapid recombination of produced electron-hole pairs. Various approaches are addressed to overcome these shortcomings, such as doping with different elements, heterojunction catalysts, noble metal deposition, and surface modification. Development of a photocatalytic corrosion resistant, visible light absorbing, defect-tuned material with small particle size is the key to complete the sunlight to hydrogen cycle efficiently. Computational studies have opened new avenues to understand and predict the electronic density of states and band structure of advanced materials and could pave the way for the rational design of efficient photocatalysts for water splitting. Future directions are focused on developing innovative junction architectures, novel synthesis methods and optimizing the existing active materials to enhance charge transfer, visible light absorption, reducing the gas evolution overpotential and maintaining chemical and physical stability.

Monolithically integrated spinel M(x)Co(3-x)O(4) (M=Co, Ni, Zn) nanoarray catalysts: scalable synthesis and cation manipulation for tunable low-temperature CH(4) and CO oxidation.

A series of large scale Mx Co3-x O4 (M=Co, Ni, Zn) nanoarray catalysts have been cost-effectively integrated onto large commercial cordierite monolithic substrates to greatly enhance the catalyst utilization efficiency. The monolithically integrated spinel nanoarrays exhibit tunable catalytic performance (as revealed by spectroscopy characterization and parallel first-principles calculations) toward low-temperature CO and CH4 oxidation by selective cation occupancy and concentration, which lead to controlled adsorption-desorption behavior and surface defect population. This provides a feasible approach for scalable fabrication and rational manipulation of metal oxide nanoarray catalysts applicable at low temperatures for various catalytic reactions.

Ni- and Mn-Promoted Mesoporous Co3O4: A Stable Bifunctional Catalyst with Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction

Efficient bifunctional catalysts for electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable due to their wide applications in fuel cells and rechargeable metal air batteries. However, the development of nonprecious metal catalysts with comparable activities to noble metals is still challenging. Here we report a one-step wet-chemical synthesis of Ni-/Mn-promoted mesoporous cobalt oxides through an inverse micelle process. Various characterization techniques including powder X-ray diffraction (PXRD), N2 sorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) confirm the successful incorporation of Ni and Mn leading to the formation of Co-Ni(Mn)-O solid solutions with retained mesoporosity. Among these catalysts, cobalt oxide with 5% Ni doping demonstrates promising activities for both ORR and OER, with an overpotential of 399 mV for ORR (at -3 mA/cm(2)) and 381 mV (at 10 mA/cm(2)) for OER. Furthermore, it shows better durability than precious metals featuring little activity decay throughout 24 h continuous operation. Analyses of cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), Raman, and O2-temperature-programmed desorption (O2-TPD) reveal that redox activity of Co(3+) to Co(4+) is crucial for OER performance, while the population of surface oxygen vacancies and surface area determine ORR activities. The comprehensive investigation of the intrinsic active sites for ORR and OER by correlating different physicochemical properties to the electrochemical activities is believed to provide important insight toward the rational design of high-performance electrocatalysts for ORR and OER reactions.

Facile Synthesis of Co3O4@CNT with High Catalytic Activity for CO Oxidation under Moisture-Rich Conditions

The catalytic oxidation reaction of CO has recently attracted much attention because of its potential applications in the treatment of air pollutants. The development of inexpensive transition metal oxide catalysts that exhibit high catalytic activities for CO oxidation is in high demand. However, these metal oxide catalysts are susceptible to moisture, as they can be quickly deactivated in the presence of trace amounts of moisture. This article reports a facile synthesis of highly active Co3O4@CNT catalysts for CO oxidation under moisture-rich conditions. Our synthetic routes are based on the in situ growth of ultrafine Co3O4 nanoparticles (NPs) (∼2.5 nm) on pristine multiwalled CNTs in the presence of polymer surfactant. Using a 1% CO and 2% O2 balanced in N2 (normal) feed gas (3-10 ppm moisture), a 100% CO conversion with Co3O4@CNT catalysts was achieved at various temperatures ranging from 25 to 200 °C at a low O2 concentration. The modulation of surface hydrophobicity of CNT substrates, other than direct surface modification on the Co3O4 catalytic centers, is an efficient method to enhance the moisture resistance of metal oxide catalysts for CO oxidation. After introducing fluorinated alkyl chains on CNT surfaces, the superhydrophobic Co3O4@CNT exhibited outstanding activity and durability at 150 °C in the presence of moisture-saturated feed gas. These materials may ultimately present new opportunities to improve the moisture resistance of metal oxide catalysts for CO oxidation.

Tunable mesoporous manganese oxide for high performance oxygen reduction and evolution reactions

Understanding the origin of manganese oxide activity for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a key step towards rationally designing of highly active catalysts capable of competing with the widely used, state-of-art noble metal catalysts. Herein, we present a bifunctional, thermally stable cesium-promoted mesoporous manganese oxide (Cs-MnOx) tuned by simple heat treatment from an amorphous to a crystalline phase with controlled surface and bulk active Mn centers. The Cs-MnOx material exhibited the highest ORR activity (0.87 V vs. RHE at −3 mA cm−2) among all noble-metal-free manganese oxide catalysts reported to date with superior activity compared to state-of-the-art Pt/C catalyst. In addition, Cs-MnOx exhibited comparable OER performance with the highly active Ir/C and RuO2 catalysts. Extensive characterization and density functional theory (DFT) computations suggested that the stabilization of the surface and bulk enriched Mn3+ species, increase of relative basicity and maintaining active crystalline phase due to Cs incorporation, are the main decisive factors for the profound ORR and OER activities. Findings from our study provide general guidance for designing of cost effective and active metal oxide based electrocatalysts.

Crystalline Mesoporous K2–xMn8O16 and ε-MnO2 by Mild Transformations of Amorphous Mesoporous Manganese Oxides and Their Enhanced Redox Properties

Synthesis of crystalline mesoporous K(2-x)Mn8O16 (Meso-OMS-2), and ε-MnO2 (Meso-ε-MnO2) is reported. The synthesis is based on the transformation of amorphous mesoporous manganese oxide (Meso-Mn-A) under mild conditions: aqueous acidic solutions (0.5 M H(+) and 0.5 M K(+)), at low temperatures (70 °C), and short times (2 h). Meso-OMS-2 and Meso-ε-MnO2 maintain regular mesoporosity (4.8-5.6 nm) and high surface areas (as high as 277 m(2)/g). The synthesized mesoporous manganese oxides demonstrated enhanced redox (H2-TPR) and catalytic performances (CO oxidation) compared to nonporous analogues. The order of reducibility and enhanced catalytic performance of the samples is Commercial-Mn2O3 < nonporous-OMS-2 < Meso-Mn2O3 < Meso-OMS-2 < Meso-ε-MnO2 < Meso-Mn-A.

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.

Ligand‐Assisted Co‐Assembly Approach toward Mesoporous Hybrid Catalysts of Transition‐Metal Oxides and Noble Metals: Photochemical Water Splitting

A bottom-up synthetic approach was developed for the preparation of mesoporous transition-metal-oxide/noble-metal hybrid catalysts through ligand-assisted co-assembly of amphiphilic block-copolymer micelles and polymer-tethered noble-metal nanoparticles (NPs). The synthetic approach offers a general and straightforward method to precisely tune the sizes and loadings of noble-metal NPs in metal oxides. This system thus provides a solid platform to clearly understand the role of noble-metal NPs in photochemical water splitting. The presence of trace amounts of metal NPs (≈0.1 wt %) can enhance the photocatalytic activity for water splitting up to a factor of four. The findings can conceivably be applied to other semiconductors/noble-metal catalysts, which may stand out as a new methodology to build highly efficient solar energy conversion systems.

A facile synthesis of Fe3C@mesoporous carbon nitride nanospheres with superior electrocatalytic activity

We report a colloidal amphiphile-templating approach to preparing nanosized Fe3C encapsulated within mesoporous nitrogen-doped carbon nanospheres (Fe3C@mCN). The obtained Fe3C@mCN hybrids having a high surface area and ultrafine Fe3C nanocrystals exhibited superior activity and durability for oxygen reduction.