Insight into the Effect of Oxygen Vacancy Concentration on the Catalytic Performance of MnO2

Tuning of Oxygen Vacancies in Co3O4 Electrocatalyst for Effectiveness in Urea Oxidation and Water Splitting

The development of an excellent multifunctional electrocatalyst that is based on non-precious metal is critical for improving the electrochemical processes of the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the urea oxidation reaction (UOR) in alkaline media. This study demonstrates that incorporating Mo into Co3O4 facilitated the formation of rich oxygen vacancies (Vo), which promotes effective nitrate adsorption and activation in urea electrolysis. Subsequently, in situ/operando X-ray absorption spectroscopy is used to explore the active sites in Mo-Co3O4-3 under OER, indicating the oxygen vacancies are first filled with OH• in Mo-Co3O4; facilitated the pre-oxidation of low-valence Co, and promoted the reconstruction/deprotonation of intermediate Co-OOH•. Mo-Co3O4-3 electrocatalysts show impressive HER, OER, and UOR with low overpotentials of 141 mV, 220 mV, and 1.32 V, respectively, at 10 mA cm-2 in an alkaline medium. Furthermore, in situ/Operando Raman spectroscopy results reveal the importance of CoOOH active sites for enhanced electrochemical performance in Mo-Co3O4-3 compared to the pure Co3O4. The urea electrolyzer with Mo-Co3O4 electrocatalysts acts as an anode and the cathode delivers 1.42 V at 10 mA cm-2. A viable approach to creating effective UOR electrocatalysts involves synergistic engineering exploiting doping and oxygen vacancies.

Boost Electrocatalytic Activity of La0.6Sr0.4Co0.2Fe0.8O3-δ Air Electrode Prepared by High-Temperature Shock for Solid Oxide Electrochemical Cells

High-temperature shock (HTS) is an emerging material synthesis technology with advantages, such as rapid processing, low energy consumption, and high controllability. This technology can prepare ultrafine nanoparticles with uniform particle size distribution and introduce additional oxygen vacancies, offering significant potential for the preparation of key materials for solid oxide electrochemical cells (SOCs). In this study, the La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) air electrode was successfully prepared using HTS technology. Compared to the conventional muffle furnace calcination, the HTS-prepared LSCF exhibits a larger specific surface area and a higher oxygen vacancy concentration, and it demonstrates significant improvements in performance. The oxygen ion conducting SOC (O-SOC) with the HTS-LSCF air electrode achieved a peak power density (PPD) of 960 mW cm-2 and a current density of 0.38 A cm-2 (at 1.3 V) at 700 °C. Meanwhile, the proton conducting SOC (P-SOC) with HTS-LSCF air electrode reached a PPD value of 1.34 W cm-2 and a current density of 3.43 A cm-2 (at 1.3 V) at 700 °C. Additionally, the P-SOC with HTS-LSCF air electrode showed no significant degradation during over 200 h of long-term testing, reflecting the excellent stability of HTS-LSCF. This work provides a fast, efficient, and economical approach for synthesizing high-performance, high-stability SOC air electrode materials.

Electron Accumulation Induced by Electron Injection-Incomplete Discharge on NiFe LDH for Enhanced Oxygen Evolution Reaction

Optimizing the local electronic structure of electrocatalysts can effectively lower the energy barrier of electrochemical reactions, thus enhancing the electrocatalytic activity. However, the intrinsic contribution of the electronic effect is still experimentally unclear. In this work, the electron injection-incomplete discharge approach to achieve the electron accumulation (EA) degree on the nickel-iron layered double hydroxide (NiFe LDH) is proposed, to reveal the intrinsic contribution of EA toward oxygen evolution reaction (OER). Such NiFe LDH with EA effect results in only 262 mV overpotential to reach 50 mA cm-2, which is 51 mV-lower compared with pristine NiFe LDH (313 mV), and reduced Tafel slope of 54.8 mV dec-1 than NiFe LDH (107.5 mV dec-1). Spectroscopy characterizations combined with theoretical calculations confirm that the EA near concomitant Vo can induce a narrower energy gap and lower thermodynamic barrier to enhance OER performance. This study clarifies the mechanism of the EA effect on OER activity, providing a direct electronic structure modulation guideline for effective electrocatalyst design.

Advances in Defect Engineering of Metal Oxides for Photocatalytic CO2 Reduction

Photocatalytic CO2 reduction technology, capable of converting low-density solar energy into high-density chemical energy, stands as a promising approach to alleviate the energy crisis and achieve carbon neutrality. Semiconductor metal oxides, characterized by their abundant reserves, good stability, and easily tunable structures, have found extensive applications in the field of photocatalysis. However, the wide bandgap inherent in metal oxides contributes to their poor efficiency in photocatalytic CO2 reduction. Defect engineering presents an effective strategy to address these challenges. This paper reviews the research progress in defect engineering to enhance the photocatalytic CO2 reduction performance of metal oxides, summarizing defect classifications, preparation methods, and characterization techniques. The focus is on defect engineering, represented by vacancies and doping, for improving the performance of metal oxide photocatalysts. This includes advancements in expanding the photoresponse range, enhancing photogenerated charge separation, and promoting CO2 molecule activation. Finally, the paper provides a summary of the current issues and challenges faced by defect engineering, along with a prospective outlook on the future development of photocatalytic CO2 reduction technology.

Switching reactive oxygen species reactions derived from Mn-Pt anchored zeolite for selective catalytic ozonation

Rationally switching reactive oxygen species (ROS) reactions in advanced oxidation processes (AOPs) is urgently needed to improve the adaptability and efficiency for the engineering application. Herein we synthesized bimetallic Mn-Pt catalysts based on zeolite to realize the switching of ROS reactions in catalytic ozonation for sustainable degradation of organic pollutants from water. The ROS reactions switched from singlet oxygen (1O2, 71.01%) to radical-dominated (93.79%) pathway by simply introducing defects and changing Pt/Mn ratios. The oxygen vacancy induced by anchoring Mn-Pt species from zeolite external surface (MnPt/H-Beta) to internal framework (MnPt@Si-Beta) exposes more electron-rich Pt2+/Pt4+ redox sites, accelerating the decomposition of O3 to generate •OH via electron transfer and switching ROS reactions. The Mn site acted as a bridge plays a critical role in conducting electrons from organic pollutants to Pt sites, which solidly solves the electron loss of catalysts, facilitating the efficient degradation of pollutants. A 34.7-fold increase in phenol degradation compared with the non-catalytic ozonation and an excellent catalytic stability are achieved by MnPt@Si-Beta/O3. The 1O2-dominated ROS reaction originated from MnPt/H-Beta/O3 exhibits superior performances in anti-interference for Cl-, HCO3-, NO3-, and SO4-. This work establishes a novel strategy for switching ROS reactions to expand the targeted applications of O3 based AOPs for environmental remediation.

New mechanistic insight into catalytic decomposition of dioxins over MnOx-CeO2/TiO2 catalysts: A combined experimental and density functional theory study

Elucidation of the catalytic decomposition mechanism of dioxins is pivotal in developing highly efficient dioxin degradation catalysts. In order to accurately simulate the whole molecular structure of dioxins, two model compounds, o-dichlorobenzene (o-DCB) and furan, were employed to represent the chlorinated benzene ring and oxygenated central ring within a dioxin molecule, respectively. Experiments and Density Functional Theory (DFT) calculations were combined to investigate the adsorption as well as oxidation of o-DCB and furan over MnOx-CeO2/TiO2 catalyst (denoted as MnCe/Ti). The results indicate that competitive adsorption exists between furan and o-DCB. The former exhibits superior adsorption capacity on MnCe/Ti catalyst at 100 °C - 150 °C, for it can adsorb on both surface metal atom and surface oxygen vacancies (Ov) via its O-terminal; while the latter adsorbs primarily by anchoring its Cl atom to surface Ov. Regarding oxidation, furan can be completely oxidized at 150 °C - 300 °C with a high CO2 selectivity (above 80 %). However, o-DCB cannot be totally oxidized and the resulting intermediates cause the deactivation of catalyst. Interestingly, the pre-adsorption of furan on catalyst surface can facilitate the catalytic oxidation of o-DCB below 200 °C, possibly because the dissociated adsorption of furan may form additional reactive oxygen species on catalyst surface. Therefore, this work provides new insights into the catalytic decomposition mechanism of dioxins as well as the optimization strategies for developing dioxin-degradation catalysts with high efficiency at low temperature.

Insight into high-performance of La-Ce-MnOx oxides with different calcination temperatures for diesel soot combustion

A series of La-Ce-MnOx catalysts derived from the precipitation of acetate salt and ammonium carbonate were calcined at different temperatures and applied for the catalytic oxidation of soot from diesel exhausts. The structure states of the as-prepared catalysts and catalytic behaviour for soot oxidation were studied by many characterization techniques such as XRD, XANES, N2 adsorption-desorption, TPR, O2-TPD, XPS and TGA. XANES results display most Mn species are Mn4+ when the samples are calcined at 500 and 600°C. However, for MCLa-700 and MCLa-800 catalysts, the predominance valence is 3+ ions. The MCLa-500 and MCLa-600 catalysts possess higher concentrations of surface active oxygen evidenced by H2-TPR and soot-TPR. Therefore, the combustion rate of soot over MCLa-600 catalyst is remarkably increased with the T10, T50 and T90 at 318°C, 360°C and 388°C under NOx atmosphere respectively.

Harnessing Pseudo-Jahn-Teller Disordering of Monoclinic Birnessite for Excited Interfacial Polarization and Local Magnetic Domains

The symmetry in a polymorph is one of the most important elements for determining the inherent lattice nature. The MnO2 host tends to high-symmetry MnO6 octahedra as a result of the electronic structure t2g 3 eg 0 of Mn4+ ions, displaying an ordered structure accompanying with poor polarization loss and limiting its application toward high-performance microwave absorbers. Here, a pseudo-Jahn-Teller (PJT) distortion and PJT disordering design with abundant self-forming interfaces and local magnetic domains in the monoclinic birnessite-MnO2 host is first reported. The PJT distortion can give rise to asymmetric MnO6 octahedra, inducing the formation of interfaces and increased electron spin magnetic moment in the lattice. The resultant birnessite with PJT distortions and PJT disordering delivers an outstanding reflection loss value of -42.5 dB at an ultralow thickness of 1.7 mm, mainly derived from the excited interfacial polarization and magnetic loss. This work demonstrates an effective approach in regulating the lattice structure of birnessite for boosting microwave absorption performance.

Enhanced oxygen evolution performance of iron-nickel oxide catalyst through dual-defect engineering

Transition metal oxides have been extensively investigated for oxygen evolution reaction (OER). While the introduction of oxygen vacancies (V_o) was found to be an effective way to enhance the electrical conductivity and the OER electrocatalytic activity of transition metal oxides, the oxygen vacancies are easily damaged during the long-term catalytic process, resulting in rapid decay of the electrocatalytic activity. Herein, we proposed the strategy of dual-defect engineering to enhance the catalytic activity and stability of NiFe₂O₄ by filling the oxygen vacancies of NiFe₂O₄ with phosphorus atoms. The filled P atoms could form coordination with iron and nickel ions to compensate the coordination number and optimize the local electronic structure, which not only enhances the electrical conductivity but also improves the intrinsic activity of the electrocatalyst. Meanwhile, the filling of P atoms could stabilize the V_o and thus improving the cycling stability of the material. The theoretical calculation further demonstrates that the improvement in conductivity and intermediate binding by P refilling remarkably contributes to enhancing the OER activity of NiFe₂O₄-V_o-P. Benefiting from the synergistic effect of filled P atoms and V_o, the derived NiFe₂O₄-V_o-P exhibits fascinating activity with ultra-low OER overpotentials of 234 and 306 mV at 10 and 200 mA cm^-2, together with the good durability for 120 h at relatively high current density of 100 mA cm^-2. This work sheds light on the design of high-performance transition metal oxide catalysts through defect regulation in the future.

Mn anchored zeolite molecular nest for enhanced catalytic ozonation of cephalexin

The molecular nest structured catalysts have demonstrated better performance than the traditional supported catalysts. However, they have not been tried in antibiotics or other organic pollutants removal from water by advanced oxidation processes (AOPs). Here we synthesized Mn anchored zeolite molecular nest (Mn@ZN) for the catalytic ozonation of cephalexin (CLX), which is the widely used antibiotic and also a refractory pollutant in water. The ozonation catalyzed by Mn@ZN achieves 97% of CLX degradation in only 2 min and a reaction rate constant of 0.2454 L mg^-1·s^-1, which is 79.2 times higher than that of the non-catalytic ozonation. Even after ten cycles, the 0.46Mn@ZN/O₃ still achieves a CLX degradation efficiency higher than 88% in 2 min, presenting an excellent stability. Mn ions stabilized by the molecular nests facilitate Lewis acid sites and oxygen vacancies, providing active sites for O₃ sorption and decomposition into ·O^2- and ¹O₂ through electrons transfer for the radical reaction with CLX. DFT calculation indicates that both the oxygen vacancy formation energy and the O₃ adsorption energy of Mn@ZN are reduced by the Mn species introduction. This study finds a fascinating catalyst of Mn@ZN for the catalytic ozonation of antibiotics, and also a smart design strategy for zeolite confined metals catalysts for water treatment.

Catalytic Ozonation of Polluter Benzene from -20 to >50 °C with High Conversion Efficiency and Selectivity on Mullite YMn2O5

Catalytic decomposition of aromatic polluters at room temperature represents a green route for air purification but is currently challenged by the difficulty of generating reactive oxygen species (ROS) on catalysts. Herein, we develop a mullite catalyst YMn₂O₅ (YMO) with dual active sites of Mn³⁺ and Mn⁴⁺ and use ozone to produce a highly reactive O* upon YMO. Such a strong oxidant species on YMO shows complete removal of benzene from -20 to >50 °C with a high CO_x selectivity (>90%) through the generated reactive species O* on the catalyst surface (60 000 mL g^-1 h^-1). Although the accumulation of water and intermediates gradually lowers the reaction rate after 8 h at 25 °C, a simple treatment by ozone purging or drying in the ambient environment regenerates the catalyst. Importantly, when the temperature increases to 50 °C, the catalytic performance remains 100% conversion without any degradation for 30 h. Experiments and theoretical calculations show that such a superior performance stems from the unique coordination environment, which ensures high generation of ROS and adsorption of aromatics. Mullite's catalytic ozonation degradation of total volatile organic compounds (TVOC) is applied in a home-developed air cleaner, resulting in high efficiency of benzene removal. This work provides insights into the design of catalysts to decompose highly stable organic polluters.

Insights into the Electrochemical Behavior of Manganese Oxides as Catalysts for the Oxygen Reduction and Evolution Reactions: Monometallic Core-Shell Mn/Mn3 O4

Overcoming the sluggish electrode kinetics of both oxygen reduction and evolution reactions (ORR/OER) with non-precious metal electrocatalysts will accelerate the development of rechargeable metal-air batteries and regenerative fuel cells. The authors investigated the electrochemical behavior and ORR/OER catalytic activity of core-porous shell Mn/Mn₃ O₄ nanoparticles in comparison with other manganese dioxides (β- and γ-MnO₂ ), and benchmarked against Pt/C and Pt/C-IrO₂ . Under reversible operation in O₂ -saturated 5 M KOH at 22 °C, the early stage activity of core-shell Mn/Mn₃ O₄ shows two times higher ORR and OER current density compared to the other MnO₂ structures at 0.32 and 1.62 V versus RHE, respectively. It is revealed that Mn(III) oxidation to Mn(IV) is the primary cause of Mn/Mn₃ O₄ activity loss during ORR/OER potential cycling. To address it, an electrochemical activation method using Co(II) is proposed. By incorporating Co(II) into MnO_x , new active sites are introduced and the content of Mn(II) is increased, which can stabilize the Mn(III) sites through comproportionation with Mn(IV). The Co-incorporated Mn/Mn₃ O₄ has superior activity and durability. Furthermore, it also surpassed the activity of Pt/C-IrO₂ with similar durability. This study demonstrates that cost-effective ORR/OER catalysis is possible.

Investigation of Mg-doping effect on the activity and P tolerance of V2O5-MoO3 /TiO2 catalysts for NH3-SCR

V₂O₅-MoO₃/TiO₂ catalyst modified by Mg was studied to obtain higher NOx SCR activity and higher P, SO₂, and H₂O resistance than the V₂O₅-MoO₃/TiO₂. The results show that Mg modification can promote the denitration activity of V₂O₅-MoO₃/TiO₂ catalyst, and the maximum NOx removal efficiency of Mg₁-SCR catalyst was 97.5%; the optimum reaction temperature and flow rate were 350℃ and 1200 mL/min, respectively. Mg doping can broaden the reaction temperature window of the catalyst, and the denitration efficiency can reach more than 87% at 300℃. P₂O₅ solution was prepared as poisoning precursor and mixed with catalyst to simulate the process of catalyst P-poisoning. The step-wise study showed that Mg_0.5-SCR and Mg₁-SCR catalyst displays higher durable resistance to P, SO₂, and H₂O than original catalyst. The degree of denitrification efficiency reduction of Mg₁-SCR is 4% smaller than that of Mg₀-SCR after passing SO₂. Mg_0.5-SCR catalyst achieves 88% denitrification efficiency at 350 °C after simulating phosphorus poisoning. The catalysts have been characterized by X-ray diffraction, Brunner - Emmet - Teller, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscop, programmed temperature desorption, and programmed temperature reduction. The obtained results suggested that the Mg doping made the active components more dispersed on the surface of the supports, improved the thermal stability of the catalyst, promoted the transition of VOx from monomeric state to polymerized state, inhibited the interaction between P and V, and protected the acid site.

DRIFTS-MS Investigation of Low-Temperature CO Oxidation on Cu-Doped Manganese Oxide Prepared Using Nitrate Aerosol Decomposition

Cu-doped manganese oxide (Cu-Mn₂O₄) prepared using aerosol decomposition was used as a CO oxidation catalyst. Cu was successfully doped into Mn₂O₄ due to their nitrate precursors having closed thermal decomposition properties, which ensured the atomic ratio of Cu/(Cu + Mn) in Cu-Mn₂O₄ close to that in their nitrate precursors. The 0.5Cu-Mn₂O₄ catalyst of 0.48 Cu/(Cu + Mn) atomic ratio had the best CO oxidation performance, with T₅₀ and T₉₀ as low as 48 and 69 °C, respectively. The 0.5Cu-Mn₂O₄ catalyst also had (1) a hollow sphere morphology, where the sphere wall was composed of a large number of nanospheres (about 10 nm), (2) the largest specific surface area and defects on the interfacing of the nanospheres, and (3) the highest Mn³⁺, Cu⁺, and Oads ratios, which facilitated oxygen vacancy formation, CO adsorption, and CO oxidation, respectively, yielding a synergetic effect on CO oxidation. DRIFTS-MS analysis results showed that terminal-type oxygen (M=O) and bridge-type oxygen (M-O-M) on 0.5Cu-Mn₂O₄ were reactive at a low temperature, resulting in-good low-temperature CO oxidation performance. Water could adsorb on 0.5Cu-Mn₂O₄ and inhibited M=O and M-O-M reaction with CO. Water could not inhibit O₂ decomposition to M=O and M-O-M. The 0.5Cu-Mn₂O₄ catalyst had excellent water resistance at 150 °C, at which the influence of water (up to 5%) on CO oxidation could be completely eliminated.

Ultrafast short-range catalytic pathway modified peroxymonosulfate activation over CuO with surface oxygen defects for tetracycline hydrochloride degradation

The presence of antibiotics in water bodies seriously threatens the ecosystem and human health. Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS), an effective method to remove antibiotics, have a bottleneck problem that the low oxidant utilization is attributed to the hindered electron transfer between metal oxides and peroxides. Here, CuO with rich oxygen vacancies (OVs), MSCuO-300, was synthesized to efficiently degrade tetracycline hydrochloride (TTCH) (k = 0.095 min^-1). The dominant role of direct adsorption and activation of OVs and its regulated Cu-O, rather than surface hydroxyl adsorption, mediated a short-range catalytic pathway. The shortened catalytic pathway between active sites and PMS accelerated the charge transfer at the interface, which promoted PMS activation. Compared with Cu_xO-500 and Commercial CuO, the activation rate of PMS was increased by 11.97, and 12.64 times, respectively. OVs contributed to the production of ¹O₂ and O₂^•-, the main active species. In addition, MSCuO-300/PMS showed excellent adaptability to real water parameters, such as pH (3-11), anions, and continuous reactor maintained for 168 h. This study provides a successful case for the purification of antibiotic-containing wastewater in the design of efficient catalysts by oxygen defect strategies.

Catalytic removal of toluene using MnO2-based catalysts: A review

Volatile organic compounds (VOCs) have serious hazard to human health and ecological environment. Due to its low cost and high activity, the catalytic oxidation technology considered to be the most effective method to remove VOCs. Toluene is one of the typical VOCs, hence its catalytic elimination is crucial for the regulation of VOCs. Manganese dioxide (MnO₂) has been extensively studied for its excellent redox performance and low-temperature operation conditions. In this review, we summarize the research progresses in the toluene catalytic oxidation of MnO₂-based catalysts, which contain single MnO₂, metal-doped MnO₂ and supported MnO₂ catalyst. In particular, we pay much attention on the relationship between the chemical properties and toluene oxidation performance over MnO₂ catalyst, as well as the catalytic reaction mechanisms. Moreover, the effects of different crystal forms and morphologies on the catalytic toluene reaction were discussed. And the perspective on MnO₂ catalysts for the catalytic oxidation of toluene has been proposed. We expect that the summary of these important findings can serve as an important reference for the catalytic treatment of VOCs.

Ru-MnOx Interaction for Efficient Hydrodeoxygenation of Levulinic Acid and Its Derivatives

Metal-oxide interaction was widely observed in supported metal catalysts, playing a significant role in tuning the catalytic performance. Here, we reported that the interaction of Ru and MnO_x was able to facilitate the hydrodeoxygenation of levulinic acid (LA) to 2-butanol with a high turnover frequency (1.99 × 10⁶ h^-1), turnover number (4411), and yield (98.8%). Moreover, this catalyst was capable of removing the hydroxymethyl group of lactones and diol with high yields of products. The high activity of the Ru-MnO_x catalyst was due to the strong Ru-MnO_x interaction, which facilitated reduction of Ru oxide to Ru⁰ and Mn oxide to Mn²⁺. The increased fractions of Ru⁰ and Mn²⁺ provided metal and Lewis acid sites, respectively, and therefore facilitated LA hydrodeoxygenation. A linear correlation between the hydrodeoxygenation activity of the Ru-MnO_x catalyst and [Mn²⁺]ln([Ru⁰]) was observed.

Structure design mechanisms and inflammatory disease applications of nanozymes

Nanozymes are artificial enzymes with high catalytic activity, low cost, and good biocompatibility, and have received ever-increasing attention in recent years. Various inorganic and organic nanoparticles have been found to exhibit enzyme-like activities and are used as nanozymes for diverse biomedical applications ranging from tumor imaging and therapeutics to detection. However, their further clinical applications are hindered by the potential toxicity and long-term retention of nanomaterials in vivo. Clarifying the catalytic mechanism of nanozymes and identifying the key factors responsible for their behavior can guide the design of nanozyme structure, enlighten the ways to improve their enzyme-like activities, and minimize the dosage of nanozymes, leading to reduced toxicity to the human body for a real biomedical application prospect. In particular, inflammation occurring in numerous diseases is closely related to reactive oxygen species, and the active oxygen scavenging ability of nanozymes potentially exerts excellent therapeutic effects on inflammatory diseases. In this review, we systematically summarize the structure-activity relationship of nanozymes, including regulation strategies for size and morphology, surface structure, and composition. Based on the structure-activity mechanisms, a series of chemically designed nanozymes developed to target various inflammatory diseases are briefly summarized.

Mechanistic and thermodynamic insights into the SO2 oxidation on MnO2 catalysts: A combined theoretical and experimental study

Manganese oxide (especially manganese dioxide [MnO₂]) is an excellent catalytic material for SO₂ removal in flue gas desulfurization. In this study, the effect of crystalline structure of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂ and δ-MnO₂) on their activity for SO₂ oxidation was studied based on density functional theory with Hubbard U corrections (DFT + U). The calculated results showed that α-MnO₂ has mild energy barriers of 0.69 eV and 0.46 eV, and β-MnO₂ has poor redox performance on SO₂ molecules, which has the highest energy barrier of 2.17 eV and the largest oxygen formation energy of 1.74 eV, making it difficult for the oxygen atom to remove from the surface lattice to form reactive sites. Thermodynamic calculations showed that α-MnO₂ is suitable for SO₂ oxidation for its low energy barriers, reaction energy close to zero in the first half, and relatively high spontaneity in the whole reaction. Experimental tests showed that α-MnO₂ had the best catalytic oxidation effect, with the highest sulfur capacity (304.11 mg/g), but β-MnO₂ had poor catalytic oxidation performance, with a sulfur capacity of 41.59 mg/g. This work studies the catalytic performance and mechanism of SO₂ removal and proposes a strategy to improve the catalytic activity by phase structure.

Engineering Amorphous/Crystalline Structure of Manganese Oxide for Superior Oxygen Catalytic Performance in Rechargeable Zinc-Air Batteries

Although amorphous materials are popular in oxygen electrocatalysis, their performance requires further improvement to meet the need for rechargeable zinc-air batteries. In this work, an amorphous/crystalline layered manganese oxide (ACMO) was designed, and its unique amorphous/crystalline homogeneous structure activated its oxygen reduction activity with a positive half-wave potential of 0.81 V and oxygen evolution activity with a moderate overpotential of 407 mV at 10 mA cm^-2 . Moreover, the amorphous/crystalline structure endowed ACMO with excellent stability. While employed as the air-electrode material for rechargeable zinc-air batteries, ACMO overcame the poor cycling stability of manganese oxide and cycled stably for 1000 cycles (≈17 days) at 10 mA cm^-2 . Besides, it delivered a high power density of 159.7 mW cm^-2 and a narrow voltage gap of 0.66 V. This work gives an insight into designing oxide materials with amorphous/crystalline structure and feasible guidance for harmonizing electrochemical activity and stability.

A Novel Hybrid Point Defect of Oxygen Vacancy and Phosphorus Doping in TiO2 Anode for High-Performance Sodium Ion Capacitor

Although sodium ion capacitors (SICs) are considered as one of the most promising electrochemical energy storage devices (organic electrolyte batteries, aqueous batteries and supercapacitor, etc.) due to the combined merits of battery and capacitor, the slow reaction kinetics and low specific capacity of anode materials are the main challenges. Point defects including vacancies and heteroatoms doping have been widely used to improve the kinetics behavior and capacity of anode materials. However, the interaction between vacancies and heteroatoms doping have been seldomly investigated. In this study, a hybrid point defects (HPD) engineering has been proposed to synthesize TiO₂ with both oxygen vacancies (OVs) and P-dopants (TiO₂/C-HPD). In comparison with sole OVs or P-doping treatments, the synergistic effects of HPD on its electrical conductivity and sodium storage performance have been clarified through the density functional theory calculation and sodium storage characterization. As expected, the kinetics and electronic conductivity of TiO₂/C-HPD3 are significantly improved, resulting in excellent rate performance and outstanding cycle stability. Moreover, the SICs assembled from TiO₂/C-HPD3 anode and nitrogen-doped porous carbon cathode show outstanding power/energy density, ultra-long life with good capacity retention. This work provides a novel point defect engineering perspective for the development of high-performance SICs electrode materials.

Ozone Decomposition below Room Temperature Using Mn-based Mullite YMn2O5

A super-low-temperature ozone decomposition is realized without energy consumption on a ternary oxide catalyst mullite YMn₂O₅ for the first time. The YMn₂O₅ oxide catalyzed ozone decomposition from a low temperature of -40 °C with 29% conversion (reaction rate: 1534.2 μmol g^-1 h^-1) and quickly reached 100% (5459.5 μmol g^-1 h^-1) when warmed up to -5 °C. The superior low-temperature performance over YMn₂O₅ could surpass that of the reported ozone decomposition catalysts. The structure and element valence characterizations confirmed that YMn₂O₅ remained the same after 100 h of room-temperature reaction, indicating excellent durability of the catalyst. O₂-TPD (O₂-temperature-programmed desorption) showed that the active sites are the Mn³⁺ sites bonded with singly coordinated oxygen on the surface. Combined with in situ Raman measurements and density functional theory calculations, we found that the ozone decomposition reaction on YMn₂O₅ showed a barrier of only 0.29 eV, following the Eley-Rideal (E-R) mechanism with a rate-limiting step of intermediate O₂^2- desorption. The low barrier minimizes the accumulation of intermediate products and realizes the fast O₃ decomposition even at super-low temperatures. Fundamentally, the moderate Mn-O bonding strength in the low-symmetry ternary oxides is crucial to produce singly coordinated active species on the surface responsible for the efficient ozone degradation at low temperatures.

Effects of Mono-Vacancies of Oxygen and Manganese on the Properties of the MnO2/Graphene Heterostructure

The effects of the monovacancies of oxygen (VO) and manganese (VMn) on the structural and electronic properties of the 1T−MnO2/graphene heterostructure are investigated, within the framework of density functional theory (DFT). We found that the values of the formation energy for the heterostructure without and with vacancies of VO and VMn were −20.99 meVÅ2 , −32.11meVÅ2, and −20.81 meVÅ2, respectively. The negative values of the formation energy indicate that the three heterostructures are energetically stable and that they could be grown in the experiment (exothermic processes). Additionally, it was found that the presence of monovacancies of VO and VMn in the heterostructure induce: (a) a slight decrease in the interlayer separation distance in the 1T−MnO2/graphene heterostructure of ~0.13% and ~1.41%, respectively, and (b) a contraction of the (Mn−O) bond length of the neighboring atoms of the VO and VMn monovacancies of ~2.34% and ~6.83%, respectively. Calculations of the Bader charge for the heterostructure without and with VO and VMn monovacancies show that these monovacancies induce significant changes in the charge of the first-neighbor atoms of the VO and VMn vacancies, generating chemically active sites (locales) that could favor the adsorption of external atoms and molecules. From the analysis of the density of state and the structure of the bands, we found that the graphene conserves the Dirac cone in the heterostructure with or without vacancies, while the 1T−MnO2 monolayer in the heterostructures without and with VO monovacancies exhibits half-metallic and magnetic behavior. These properties mainly come from the hybridization of the 3d−Mn and 2p−O states. In both cases, the heterostructure possesses a magnetic moment of 3.00 μβ/Mn. From this behavior, it can be inferred the heterostructures with and without VO monovacancies could be used in spintronics.

Fabrication of Flexible Mesoporous Black Nb2 O5 Nanofiber Films for Visible-Light-Driven Photocatalytic CO2 Reduction into CH4

Achieving high selectivity and conversion efficiency simultaneously is a challenge for visible-light-driven photocatalytic CO₂ reduction into CH₄ . Here, a facile nanofiber synthesis method and a new defect control strategy at room-temperature are reported for the fabrication of flexible mesoporous black Nb₂ O₅ nanofiber catalysts that contain abundant oxygen-vacancies and unsaturated Nb dual-sites, which are efficient towards photocatalytic production of CH₄ . The oxygen-vacancy decreases the bandgap width of Nb₂ O₅ from 3.01-2.25 eV, which broadens the light-absorption range from ultraviolet to visible-light, and the dual sites in the mesopores can easily adsorb CO₂ , so that the intermediate product of CO* can be spontaneously changed into *CHO. The formation of a highly stable NbCHO* intermediate at the dual sites is proposed to be the key feature determining selectivity. The preliminary results show that without using sacrificial agents and photosensitizers, the nanofiber catalyst achieves 64.8% selectivity for CH₄ production with a high evolution rate of 19.5 µmol g^-1 h^-1 under visible-light. Furthermore, the flexible catalyst film can be directly used in devices, showing appealing and broadly commercial applications.

Mn-vacancy birnessite for photo-assisted elimination of formaldehyde at ambient condition

Visible light-assisted catalysis has recently attracted considerable attention because it is efficient, cost effective, and does not cause indoor air pollution. Several birnessite-type MnO₂ catalysts with different numbers of manganese vacancies (MVs) were synthesized in this study and used for photo-assisted catalytic oxidation of HCHO. Under visible light irradiation, MVs act as trapping centers to accelerate electrons transport and produce abundant reactive radicals to boost the activation of molecular oxygen, thereby improving the catalytic HCHO oxidation. The birnessite with the highest number of MVs exhibits remarkable oxidation activity with 80 ppm of HCHO (42% HCHO conversion was attained at ambient temperature) and a corresponding gas hourly space velocity (GHSV) of 60 L/(g·h) in a dynamic experiment. Moreover, it mineralizes 80 ppm of HCHO within 160 min in a static experiment, whereas it only takes 90 min under the same conditions with the visible light irradiation. The activity factor of birnessite with the highest MV content under visible light irradiation is 2.2 times that observed under dark conditions. Overall, this study elucidates the photothermal catalytic oxidation of HCHO, and concludes that the birnessite comprising MVs is a promising material for air purification applications.

Bimetal heterointerfaces towards enhanced electro-activation of O2 under room condition

Efficient catalysts for oxygen (O₂) activation under room condition are required for effective wet air oxidation (WAO) technology. Here, we report a novel manganese-cobalt-based composite (MnO-CoO@Co) fabricated on a graphite felt (GF) support for catalyzing the electro-activation of O₂ under room condition. Abundant Co-MnO and CoO-MnO heterointerfaces are formed in the composite. In comparison to the single-metal counterparts, i.e. CoO@Co/GF (16.99 wt% Co) and MnO/GF (26.83 wt% Mn), the bimetal MnO-CoO@Co/GF (5.29 wt% Co and 8.79 wt% Mn) displays an improved oxygen storage capacity and provides more active sites to accommodate surface adsorbed oxygen species. Notably, the strong synergy derived from bimetal heterointerfaces enhances the electron transfer and oxygen mobilization during the electro-activation of O₂, thereby significantly reducing the reaction barrier. MnO-CoO@Co/GF exhibits excellent efficiency and stability in electrocatalytic WAO (ECWAO) towards the removal of pharmaceuticals and personal care products (PPCPs) over a wide pH range from 4.0 to 10.0. A model pollutant sulfamethoxazole (SMX) acquires mineralization efficiency of 78.4 ± 2.1% and mineralization current efficiency of 157.89% at +1.0 V of electrode potential. The toxicity of PPCPs can be totally eliminated after the ECWAO treatment. This work highlights the synergy derived from bimetal heterointerfaces in O₂ electrocatalysis, and provides a promising approach for advanced WAO catalysts in PPCPs pollution control.

Realizing Superior Redox Kinetics of Hollow Bimetallic Sulfide Nanoarchitectures by Defect-Induced Manipulation toward Flexible Solid-State Supercapacitors

As a typical battery-type material, CuCo₂ S₄ is a promising candidate for supercapacitors due to the high theoretical specific capacity. However, its practical application is plagued by inherently sluggish ion diffusion kinetics and inferior electrical transport properties. Herein, sulfur vacancies are incorporated in CuCo₂ S₄ hollow nanoarchitectures (HNs) to accelerate redox reactivity. Experimental analyses and theoretical investigations uncover that the generated sulfur vacancies increase the active electron states, reduce the adsorption barriers of electrolyte ions, and enrich reactive redox species, thus achieving enhanced electrochemical performance. Consequently, the deficient CuCo₂ S₄ with optimized vacancy concentration presents a high specific capacity of 231 mAh g^-1 at 1 A g^-1 , a ≈1.78 times increase compared to that of pristine CuCo₂ S₄ , and exhibits a superior rate capability (73.8% capacity retention at 20 A g^-1 ). Furthermore, flexible solid-state asymmetric supercapacitor devices assembled with the deficient CuCo₂ S₄ HNs and VN nanosheets deliver a high energy density of 61.4 W h kg^-1 at 750 W kg^-1 . Under different bending states, the devices display exceptional mechanical flexibility with no obvious change in CV curves at 50 mV s^-1 . These findings provide insights for regulating electrode reactivity of battery-type materials through intentional nanoarchitectonics and vacancy engineering.

Boosting Oxygen Reduction Activity of Manganese Oxide Through Strain Effect Caused By Ion Insertion

Transition-metal oxides with a strain effect have attracted immense interest as cathode materials for fuel cells. However, owing to the introduction of heterostructures, substrates, or a large number of defects during the synthesis of strain-bearing catalysts, not only is the structure-activity relationship complicated but also their performance is mediocre. In this study, a mode of strain introduction is reported. Transition-metal ions with different electronegativities are intercalated into the cryptomelane-type manganese oxide octahedral molecular sieves (OMS-2) structure with K ions as the template, resulting in the octahedral structural distortion of MnO₆ and producing strains of different degrees. Experimental studies reveal that Ni-OMS-2 with a high compressive strain (4.12%) exhibits superior oxygen reduction performance with a half-wave potential (0.825 V vs RHE) greater than those of other reported manganese-based oxides. This result is related to the increase in the covalence of MnO₆ octahedral configuration and shifting down of the e_g band center caused by the higher compression strain. This research avoids the introduction of new chemical bonds in the main structure, weakens the effect of e_g electron filling number, and emphasizes the pure strain effect. This concept can be extended to other transition-metal-oxide catalysts.

Investigation of chlorine-poisoning mechanism of MnOx/TiO2 and MnOx-CeO2/TiO2 catalysts during o-DCBz catalytic decomposition: Experiment and first-principles calculation

The catalytic activity and stability of MnO_x/TiO₂ and MnO_x-CeO₂/TiO₂ catalysts for the oxidative degradation of 1,2-dichorobenzene (o-DCBz) at low temperatures (≤275 °C) were experimentally examined. The chlorine (Cl) poisoning mechanism of the catalysts was also clarified based on the catalyst characterization combined with theoretical calculations. Experimental results show that the MnO_x/TiO₂ catalyst is considerably deactivated during o-DCBz catalytic decomposition, mainly due to the chlorination of the catalytic active component. Ce addition and high temperature can effectively promote the resistance of MnO_x/TiO₂ catalyst to Cl poisoning. Density functional theory (DFT) calculations in the framework of first-principles reveal that Cl atom prefers to anchor on surface oxygen vacancy (OV) rather than on top site of Mn atom. The adsorption of Cl atom on surface OV hinders the dissociated adsorption of O₂ on surface OV and interrupts the regeneration of the surface reactive oxygen species. The adsorption of Cl atom on top site of Mn atom increases the formation energy of surface OV and damages the surface Lewis acid sites which act as the important adsorption sites for o-DCBz molecules. Ce addition causes Cl atom to adsorb preferentially onto the OV around Ce atom, which weakens the interaction between Cl atom and Mn atom. Consequently, the chlorination of the MnO_x species is prevented and the oxygen mobility of the catalyst is guaranteed to some extent.

Adjusting the Covalency of Metal-Oxygen Bonds in LaCoO3 by Sr and Fe Cation Codoping to Achieve Highly Efficient Electrocatalysts for Aprotic Lithium-Oxygen Batteries

Developing high-efficiency dual-functional catalysts to promote oxygen electrode reactions is critical for achieving high-performance aprotic lithium-oxygen (Li-O₂) batteries. Herein, Sr and Fe cation-codoped LaCoO₃ perovskite (La_0.8Sr_0.2Co_0.8Fe_0.2O_3-σ, LSCFO) porous nanoparticles are fabricated as promising electrocatalysts for Li-O₂ cells. The results demonstrate that the LSCFO-based Li-O₂ batteries exhibit an extremely low overpotential of 0.32 V, ultrahigh specific capacity of 26 833 mA h g^-1, and superior long-term cycling stability (200 cycles at 300 mA g^-1). These prominent performances can be partially attributed to the existence of abundant coordination unsaturated sites caused by oxygen vacancies in LSCFO. Most importantly, density functional theory (DFT) calculations reveal that codoping of Sr and Fe cations in LaCoO₃ results in the increased covalency of Co 3d-O 2p bonds and the transition of Co³⁺ from an ordinary low-spin state to an intermediate-spin state, eventually resulting in the transformation from nonconductor LCO to metallic LSCFO. In addition, based on the theoretical calculations, it is found that the inherent adsorption capability of LSCFO toward the LiO₂ intermediate is reduced due to the increased covalency of Co 3d-O 2p bonds, leading to the formation of large granule-like Li₂O₂, which can be effectively decomposed on the LSCFO surface during the charging process. Notably, this work demonstrates a unique insight into the design of advanced perovskite oxide catalysts via adjusting the covalency of transition-metal-oxygen bonds for high-performance metal-air batteries.

Significantly boosted oxygen electrocatalysis with cooperation between cobalt and iron porphyrins

Developing electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is of great importance. Herein, Co tetrakis(pentafluorophenyl)porphyrin (Co-P) and Fe chloride tetrakis(pentafluorophenyl)porphyrin (Fe-P) were loaded on carbon nanotubes (CNTs) for combining the electrocatalytic advantages of both Co-P and Fe-P. The resultant (Co-P)0.5(Fe-P)0.5@CNT composite displayed significantly boosted activity for the selective four-electron ORR with a half-wave potential of 0.80 V versus reversible hydrogen electrode (RHE) and for the OER with a potential of 1.65 V versus RHE to obtain 10 mA cm-2 current density in 0.1 M KOH. A Zn-air battery assembled from (Co-P)0.5(Fe-P)0.5@CNT exhibited a small charge-discharge voltage gap of 0.74 V at 2 mA cm-2, a high power density of 174.5 mW cm-2 and a good rechargeable stability (>120 cycles).

Plasma-assisted defect engineering of N-doped NiCo2O4 for efficient oxygen reduction

Defect control is a promising way to enhance the electrocatalysis performance of metal oxides. Oxygen vacancy enriched NiCo₂O₄ was successfully prepared using cold plasma. Oxygen as a plasma-forming gas introduces oxygen vacancies via electron etching. The concentration of oxygen vacancies can be controlled by different plasma-forming gas. CoO, which formed on the plasma samples, is beneficial for quick charge transfer and electrocatalytic performance. A high amount of nitrogen atoms of up to 10.1% was doped on NiCo₂O₄ because of the enriched oxygen vacancies and improved the stability of the oxygen defects and the conductivity of the catalyst. Electrocatalytic studies showed that the plasma-induced N-doped NiCo₂O₄ shows enhanced electrocatalytic performance for the oxygen reduction reaction (ORR). It shows a typical four-electron process that considerably improves the current density and onset potential. The HO₂^- % was as low as 0.59% and current density was 4.9 mA cm^-2 at 0.2 V (Vs. RHE) on the plasma-treated NiCo₂O₄. Calculations based on density functional theory reveal the mechanism for the promotion of the catalytic ORR activity via plasma treatment. This increases the electron density near the Fermi level, reducing the work function, and changing the position of the d-band center.

Aerobic oxidation of 5-hydroxymethylfurfural into 2,5-diformylfuran using manganese dioxide with different crystal structures: A comparative study

Aerobic oxidation of 5-Hydroxymethylfurfural (HMF) to 2,5-Diformylfuran (DFF) using O₂ gas represents a sustainable approach for valorization of lignocellulosic compounds. As manganese dioxide (MnO₂) is validated as a useful oxidation catalyst and many crystalline forms of MnO₂ exist, it is critical to explore how the crystalline structures of MnO₂ influence their physical/chemical properties, which, in turn, determine catalytic activities of MnO₂ crystals for HMF oxidation to DFF. In particular, six MnO₂ crystals, α-MnO₂, β-MnO₂, γ-MnO₂, δ-MnO₂, ε-MnO₂, and λ-MnO₂ are prepared and investigated for their catalytic activities for HMF oxidation to DFF. With different morphologies and crystalline structures, these MnO₂ crystals possess very distinct surficial chemistry, redox capabilities, and textural properties, making these MnO₂ exhibit different catalytic activities towards HMF conversion. Especially, β-MnO₂ can produce much higher DFF per surface area than other MnO₂ crystals. β-MnO₂ could achieve the highest C_HMF = 99% and Y_DFF = 97%, which are much higher than the reported values in literature, possibly because the surficial reactivity of β-MnO₂ appears to be highest in comparison to other MnO₂ crystals. Especially, β-MnO₂ could exhibit Y_DFF > 90% over 5 cycles of reusability test, and maintain its crystalline structure, revealing its advantageous feature for aerobic oxidation of HMF to DFF. Through this study, the relationship between morphology, surface chemistry, and catalytic activity of MnO₂ with different crystal forms is elucidated for providing scientific insights into design, application and development of MnO₂-based materials for aerobic oxidation of bio-derived molecules to value-added products.

Simultaneous removal of SO2 and NOx with OH from the catalytic decomposition of H2O2 over Fe-Mo mixed oxides

In this paper, the simultaneous removal of SO₂ and NO_x catalyzed by Fe-Mo mixed oxides at varying Mo/Fe atomic ratios was reported for the first time with the aim of reducing H₂O₂ consumption and elucidating the roles of Fe and Mo species in the catalytic process. Fe-Mo mixed oxides with varying Mo/Fe atomic ratios were synthesized and the catalytic performances were systematically studied. The catalyst with Mo/Fe atomic ratio of 2.0 exhibited the highest activity, with which removal efficiencies of 89.4 % for NO_x and 100 % for SO₂ can be attained at extremely low H₂O₂ dosage. Products analysis revealed that SO₂ was mainly removed via wet scrubber, while the adequate oxidation resulting from OH radicals was the prerequisite for NO_x removal. The redox pair of Fe²⁺/Fe³⁺ played a significant role in decomposing H₂O₂, while Mo species had double effect on catalytic activity. Higher Mo content resulted in abundant oxygen vacancies and stronger surface acidity, which favored OH formation. However, the excessive Mo content involved severe surface Mo enrichment and remarkably reduced the active sites of Fe species. The H₂O₂/Fe-Mo catalyst system showed excellent stability and had a promising prospect for simultaneously removing SO₂ and NO_x in coal-fired flue gas.