MnO2的晶相结构和表面性质对低温NH3-SCR反应的影响

采用水热法合成了两种具有相同形貌但是不同物相结构的MnO₂纳米棒, 分别为隧道状和层状结构, 考察其低温NH₃选择性催化还原NO_x (NH₃-SCR)的性能. 结果表明MnO₂纳米棒的比表面积不是影响活性的主要因素, 催化剂的晶相结构和表面性质对催化活性有很大影响, 隧道状α-MnO₂纳米棒的低温NH₃-SCR活性明显高于层状δ-MnO₂纳米棒. 结构分析和NH₃程序升温脱附(NH₃-TPD)实验表明, α-MnO₂纳米棒的暴露晶面(110)面存在大量的配位不饱和Mn离子, 形成较多的Lewis 酸性位点, 而且α-MnO₂较弱的Mn―O键和隧道结构都有利于NH₃的吸附; 而δ-MnO₂纳米棒的暴露晶面(001)面上的Mn离子已达到配位饱和, 所以其表面Lewis 酸性位点较少. X射线光电子能谱(XPS)和热重(TG)分析表明α-MnO₂纳米棒的表面更有利于NH₃和NO_x的活化. 具有有利于吸附NH₃和活化NH₃和NO_x的表面性质和晶型结构, 是α-MnO₂纳米棒活性高的主要原因.     

Effect of alkaline and alkaline–earth cations on the supercapacitor performance of MnO2 with various crystallographic structures

The electrochemical performances of the α-, γ-, and δ-MnO₂ with different crystallographic structures were systematically investigated in 0.5 mol/L Li₂SO₄, 0.5 mol/L Na₂SO₄, 1 mol/L Ca(NO₃)₂, and 1 mol/L?Mg(NO₃)₂ electrolytes. The results showed that the electrochemical performances of the manganese dioxides depended strongly on the crystallographic structures of MnO₂ as well as the cation in the electrolytes. Because the δ-MnO₂ consists with layers of structure and the interlayer separation is 7 Å, which is suitable for insertion/extraction of some alkaline and alkaline–earth cations, the δ-MnO₂ electrode showed the higher specific capacitance than that of α-MnO₂ and γ-MnO₂. We also found that the α-, γ-, and δ-MnO₂ electrodes in the Mg(NO₃)₂ electrolyte showed a higher specific capacitance, while all the α-, γ-, and δ-MnO₂ electrodes in the Li₂SO₄ electrolyte exhibited a better cycle life. The reason for the different behavior of Li⁺ and Mg²⁺ during the charge/discharge process can be ascribed to the charge effect of the cations in the electrolytes. The ex situ X-ray diffraction (XRD) and long-time cyclic voltammogram measurements were used to systematically study the energy storage mechanism of MnO₂-based electrodes. A progressive crystallinity loss of the materials is also observed upon potential cycling at the oxidized states. A reasonable charge/discharge mechanism is proposed in this work.     

铂纳米颗粒增强MnO2纳米棒对CO和挥发性有机化合物的氧化活性

通过水热法合成了纯相的α-MnO2和δ-MnO2纳米棒,并利用溶胶固定化工艺制备了负载铂纳米颗粒的Pt/MnO2材料.通过透射电镜(TEM),X射线粉末衍射(XRD),扫描电镜(SEM),X射线光电子能谱(XPS),N2吸附-脱附和H2程序升温还原(H2-TPR)技术研究了样品的微观结构和吸附活性位,探查了CO和挥发性有机化合物(VOCs)(苯和甲苯)在催化剂上的催化发光(CTL)性质.结果表明:铂颗粒在α-MnO2和δ-MnO2载体上以高分散状态存在,负载过程不会影响α-MnO2纳米棒的晶相结构,但会导致δ-MnO2纳米棒产生结构变化.经XPS证实不是Pt与其发生了反应.α-和δ-MnO2纳米棒对CO、苯和甲苯的催化氧化都具有很高的活性,δ-MnO2的活性略高于α-MnO2相.虽然N2吸附-脱附实验结果证实Pt负载会导致MnO2纳米棒比表面积的下降,但H2-TPR结果显示Pt和MnO2之间会产生强烈的相互作用,显著增强其催化活性,且Pt/δ-MnO2活性高于Pt/α-MnO2.催化氧化发光研究表明,这四种催化剂活性顺序是α-MnO2≤δ-MnO2相似文献   

Influence of ion size on the stability of chloroplumbates containing PbCl<Stack><Subscript>5</Subscript><Superscript>−</Superscript></Stack> or PbCl<Stack><Subscript>6</Subscript><Superscript>2−</Superscript></Stack>

The enthalpies of formation of PbCl₄, PbCl₅⁻ and PbCl₆²⁻, originating from quantum mechanics, have enabled the thermodynamic behaviour of these ions with respect to Cl-detachment to be assessed. The stability of salts containing PbCl₅⁻ and PbCl₆²⁻ as a function of the dimensions of these anions and complementary cations was studied using an approach combining the Kapustinskii-Yatsimirskii equation with basic thermochemical relationships. It was found that hexachloroplumbates of monovalent metal cations will not dissociate into metal chlorides and PbCl₄, provided the complementary cations are suitably large in size. Hexachloroplumbates of divalent metal cations have not yet been synthesised since no known metal cations attain the requisite large size. Such salts will not dissociate if the divalent metal cations are able to complex suitably large electron-donating ligands. The pentachloroplumbates of both monovalent and divalent metal cations are unstable, since no known metal cations have appropriately large ionic radii. The approach adopted appears to be useful for the examination of the thermal behaviour, stability and reactivity of chloroplumbates.     

A radiochemical study of the kinetics of exchange between manganese oxides and some cations in solution

The kinetics of the exchange between⁵⁶Mn-labelled manganese dioxide and cations in aqueous solution was studied by measuring the β⁻ activity acquired by the solution. The results of the exchange between a chemical γ MnO₂ and a divalent M²⁺ ion (M=Mn, Co, Cu or Zn) or a trivalent M³⁺ ion (M=Ga, Fe, In, Rh or Al) indicate a fast initial process followed by a diffusion—controlled exchange. It is assumed that M²⁺ ions exchange with Mn²⁺ ions and M³⁺ ions exchange with Mn³⁺ ions in MnO₂. The process depends on the radii of the host and substituent ions and on consideration of crystal field stabilisation energies. It seems that the γ MnO₂ studied contains more Mn³⁺ than Mn²⁺ ions. The possibility of the exchange between Mn ions and cations of a different charge cannot be ruled out. The exchange between Co²⁺ ions and MnO₂ was enhanced in presence of pyrophosphate, which stabilises Mn(III) as a complex. The fraction of Mn in different samples of MnO₂ exchanged with a given cation depends on the type and not on the surface area of the sample.     

Lattice vibration spectra. Part XCV. Infrared spectroscopic studies on the iron oxide hydroxides goethite (α), akaganéite (β), lepidocrocite (γ), and feroxyhite (δ)

Infrared spectra (IR, FIR, DRIFT, 90 and 295 K) and DSC measurements of the various polymorphs of iron oxide hydroxide, viz. goethite (α), akaganéite (β), lepidocrocite (γ), and feroxyhite (δ), and of deuterated specimens are reported. They are discussed with respect to the crystal structures proposed in the literature, the hydrogen bonds present, the energies of the OH stretching, OH bending (librational), and translational modes, and their thermal decomposition. From the two space groups proposed for β- and γ-FeO(OH), the groups I4/m and Cmc2₁, respectively, seem to be more reliable. The disorder of the OH⁻ ions of γ-FeO(OH) has not been confirmed in contrast to that of δ-FeO(OH). The intraionic O(H,D) distances of γ- and δ-FeO(OH) derived from neutron powder diffraction studies have to be doubted. The greater strength of the OHOH hydrogen bonds of lepidocrocite, for example, compared to that of the OHO hydrogen bonds of goethite despite the larger hydrogen bond acceptor capability of O²⁻ is due to the strong cooperativity of the hydrogen bonds of the γ-polymorph. The extremely different strength of the hydrogen bonds of isostructural α-AlO(OH) (v_OH = 2950 cm⁻¹, 295 K), α-MnO(OH) (v_OH = 2686 cm⁻¹), and α-FeO(OH) (v_OH = 3130 cm⁻¹) is caused by the different synergetic effect of the metal ions involved, especially that of Mn³⁺ due to its Jahn-Teller behaviour. The decomposition temperatures and heats of the various FeO(OH) modifications as well as the halfwidths of the DSC peaks evidence a much faster decomposition rate of akaganéite than those of the other polymorphs. This is obviously due to the Cl⁻ ion impurities present in this compound.     

Formation of β-distonic oxonium ions: Study of ROCH2CH2OR′ radical cations

Radical cations derived from the ethers ROCH₂CH₂OR′ (R, R′ = H, CH₃, C₂₅) were studied, since β-distonic oxonium ions are often prepared from ionized ethers of glycol. The first step in the fragmentation is a 1,5-transfer of an α-hydrogen to oxygen of a terminal alkoxy group leading to a δ-distonic oxonium ion. This step is thermo-neutral and reversible in the ROCH₂CH₂OH radical cations and exothermic and irreversible in the dialkyl ether radical cations. Depending on R and R,′ these δ-distonic oxonium ions fragment by three reactions: the loss of an alcohol or a water molecule, the formation of a β-distonic oxonium ion ˙CH₂CH₂O(H)⁺R and a 1,4-H migration between carbon atoms. Competition between these processes is discussed.     

Co-electrodeposition of MnO2/graphene oxide coating on carbon paper from phosphate buffer and the capacitive properties

MnO₂/graphene oxide sheet composite (MnO₂/GOS) has been co-electrodeposited on the thermally treated carbon paper (TTCP) in phosphate buffer solution containing GOS and KMnO₄. The resulted samples have been characterized by scanning and transmission electron microscopy, Raman, X-ray diffraction, and X-ray photoelectron energy spectroscopy. The results show that the synthesized MnO₂ may be δ-MnO₂ and the morphology of MnO₂/GOS is very different from that of MnO₂, indicating that the introduction of GOS in electrolyte can influence the morphology during the deposition. The capacitive properties of the samples are investigated by using cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The specific capacitance of MnO₂ for MnO₂/GOS can reach about 829 F g^?1 at discharged current density of 1.0 A g^?1 in 1 M Na₂SO₄ aqueous solution, which is larger than that of MnO₂ deposited on TTCP. The composite of MnO₂/GOS also exhibits excellent cyclic stability with a decrease of 18.5 % specific capacitance after 1,500 cycles.     

Twinning and structure of Eu0.6Sr0.4MnO3

The crystal structure of europium strontium manganese trioxide, Eu_0.6Sr_0.4MnO₃, has been refined using a multiply twinned single crystal containing six twin components. The MnO₆ octa­hedra show Jahn–Teller distortions with nearly fourfold symmetry, but the octa­hedral tilting scheme reduces the crystal symmetry to ortho­rhom­bic (space group Pbnm). The refinement of site occupancies and the analysis of difference Fourier maps show that the Eu³⁺ and Sr²⁺ cations occupy different crystallographic positions with eightfold and twelvefold coordination, respectively.     

Bismuth oxide coated amorphous manganese dioxide for electrochemical capacitors

With MnSO₄, NaOH and K₂S₂O₈ as the raw materials, the amorphous and δ-type manganese dioxide (MnO₂) is separately prepared by using different chemical precipitation-oxidation methods. The results of charge–discharge and electrochemical impedance spectroscopy (EIS) tests show that (i) the specific capacitance of the amorphous MnO₂ reaches to 301.2 F g⁻¹ at a current density of 200 mA g⁻¹ and its capacitance retention rate after 2000 cycles is 97%, which is obviously higher than 250.8 F g⁻¹ and 71% of the δ-type one, respectively; (ii) good electrochemical capacitance properties of the amorphous MnO₂ should be contributed to easy insertion/extraction of ions within the material; (iii) when 5 wt% Bi₂O₃ is coated on the amorphous MnO₂, its specific capacitance increases to 352.8 F g⁻¹ and the capacitance retention rate is 90% after 2000 cycles.     

Hybrid Superlattice-Triggered Selective Proton Grotthuss Intercalation in δ-MnO2 for High-Performance Zinc-Ion Battery

A great deal of attention has been paid on layered manganese dioxide (δ−MnO₂) as promising cathode candidate for aqueous zinc-ion battery (ZIB) due to the excellent theoretical capacity, high working voltage and Zn²⁺/H⁺ co-intercalation mechanism. However, caused by the insertion of Zn²⁺, the strong coulomb interaction and sluggish diffusion kinetics have resulted in significant structure deformation, insufficient cycle stability and limited rate capability. And it is still far from satisfactory to accurately modulate H⁺ intercalation for superior electrochemical kinetics. Herein, the terrace-shape δ−MnO₂ hybrid superlattice by polyvinylpyrrolidone (PVP) pre-intercalation (PVP−MnO₂) was proposed with the state-of-the-art ZIBs performance. Local atomic structure characterization and theoretical calculations have been pioneering in confirming the hybrid superlattice-triggered synergy of electron entropy stimulation and selective H⁺ Grotthuss intercalation. Accordingly, PVP−MnO₂ hybrid superlattice exhibits prominent specific capacity (317.2 mAh g⁻¹ at 0.125 A g⁻¹), significant rate performance (106.1 mAh g⁻¹ at 12.5 A g⁻¹), and remarkable cycle stability at high rate (≈100 % capacity retention after 20,000 cycles at 10 A g⁻¹). Therefore, rational design of interlayer configuration paves the pathways to the development of MnO₂ superlattice for advanced Zn−MnO₂ batteries.     

Hydroflux Synthesis and Characterization of the Non-Centrosymmetric Oxomanganate(V) KSrMnO4

Bluish-green crystals of the oxomanganate(V) KSrMnO₄ were synthesized under hydroflux conditions with KOH at about 200 °C starting from KMnO₄ and Sr(NO₃)₂. The product formation depends on the hydroxide concentration and the strontium-manganese ratio of the starting materials. By changing these parameters, also other reaction products, such as α-SrMnO₃ and Sr₅[MnO₄]₃OH, are obtained. KSrMnO₄ crystallizes in the non-centrosymmetric orthorhombic space group P2₁2₁2₁ with a = 743.26(7) pm, b = 575.72(6) pm and c = 987.2(1) pm. The crystal structure consists of isolated [MnO₄]^3– tetrahedra surrounded by Sr²⁺ and K⁺ cations. The group-subgroup relation to KBaMnO₄ and to the aristotype α-K₂SO₄ are discussed. UV/Vis spectra and magnetic data agree with the oxidation state +V of manganese. KSrMnO₄ is paramagnetic, but orders as an antiferromagnet below 4.6(1) K. Thermal analysis of KSrMnO₄ revealed the decomposition at about 950 °C to α-SrMnO₃.     

Boosting Water Oxidation through In Situ Electroconversion of Manganese Gallide: An Intermetallic Precursor Approach

For the first time, the manganese gallide (MnGa₄) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high‐performance and long‐term‐stable MnO_x‐based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnO_x minerals with distinct structures and induced defects: birnessite δ‐MnO₂, feitknechtite β‐MnOOH, and hausmannite α‐Mn₃O₄. The abundance and intrinsic stabilization of Mn^III/Mn^IV active sites in the three MnO_x phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa₄ precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious‐metal‐based catalysts, could be achieved at a current density of 10 mA cm^?2 with a durability of more than five days.     

Charge storage mechanism of MnO2 cathodes in Zn/MnO2 batteries using ionic liquid-based gel polymer electrolytes

Cathode reactions in Zn/MnO₂ batteries using aqueous electrolytes have been usually interpreted by the reduction of Mn^4 + to Mn^3 + while protons and/or cations penetrate inside the cathode. However, until now, the MnO₂ storage charge mechanism using a non-aqueous gel polymer electrolyte (GPE) has not been investigated. In this work, ionic liquid-based GPEs including BMIM Tf and ZnTf₂ have been employed in Zn/MnO₂ batteries. Different states of charge of MnO₂ cathodes used in Zn/IL-GPE/MnO₂ batteries have been analyzed by XPS and EDX techniques. XPS analysis showed that Mn^4 + is reduced during the discharge process at the same time as Zn^2 + cations are incorporated into the cathode. Besides, Zn^2 + cations insertion is accompanied by triflate anions.     

低温等离子体场内复合不同晶型氧化锰催化降解甲苯性能和机理

作为典型的挥发性有机化合物,甲苯通常来源于建筑涂料、交通运输和各种工业生产过程,是PM2.5、臭氧和光化学烟雾的重要前驱体,对环境和人类健康造成巨大影响.近年来,低温等离子体技术因具有在常温常压下就能通过高能电子、活性氧物种和羟基等活性粒子有效降解挥发性有机物的优点而受到广泛关注.然而,高能耗和大量副产物的产生是等离子体技术工业化应用的巨大障碍.当前最有效的策略之一是将等离体技术与催化技术结合,从而加快反应速率,提高产品的选择性和能源利用率.在所应用的催化剂中,MnO2因具有较好的O3分解效率而成为最有潜力的催化剂之一.但是MnO2具有不同的晶型结构、隧道结构和形貌,这些均会显著影响MnO2的催化活性.本文通过一步水热法制备了α-,β-,γ-和δ-MnO2四种MnO2催化剂,并将其用于等离子体催化降解甲苯研究,在此基础上系统考察了等离子体催化降解性能和MnO2不同晶型之间的关系.结果表明,当能量密度为160 J/L时,等离子体单独降解甲苯去除效率为32.5%.引入催化剂能够显著提高甲苯的降解效率,其中α-MnO2效果最显著,甲苯降解效率能够提升至78.1%,β-,γ-和δ-MnO2能够相应提升至47.4%,66.1%和50.0%.采用X射线衍射、拉曼光谱、扫面电子显微镜、透射电子显微镜、比表面积-孔结构分析、氢气程序升温还原和X射线光电子能谱等手段研究了催化剂的理化特性.结果表明,隧道结构、催化剂在等离子体中的稳定性、Mn–O键能和催化剂表面吸附氧均在等离子体催化降解甲苯中发挥了重要作用.在此基础上,通过GC-MS分析降解产生的气相副产物推断甲苯在等离子体和等离子体催化体系中的降解机理.在等离子体催化体系中,通过Mn4+,Mn3+和Mn2+价态的变化,等离子体产生的O3,O2*和其他活性自由基会被吸附到催化剂表面,随后与催化剂吸附的甲苯或中间副产物发生氧化还原反应,将甲苯氧化为CO2等小分子物质.此外,MnO2作为分解O3最有效的催化剂,可以吸附O3并将其分解为O?或者与H2O生成?OH参与到反应中,从而提高甲苯的降解效率.     

Factors Governing the Activity of α-MnO2 Catalysts in the Oxygen Evolution Reaction: Conductivity versus Exposed Surface Area of Cryptomelane

Cryptomelane (α-(K)MnO₂) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce⁴⁺ were compared. The decisive factors that explain this difference were identified in the catalysts’ microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H₂O₂ decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm⁻² at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions.     

Multifrequency ESR and ENDOR studies of ionomers

We describe methods for determining the local environment of cations and the process of ionic clustering in ionomers, using electron magnetic resonance spectroscopy. The distance between Cu²⁺ cations in perfluorinated membranes (Nafion) containing terminal sulfonic groups and swollen by water has been deduced from an analysis of ESR spectra at L (1.25 GHz), S (2.36 GHz) and X (9.36 GHz) bands, in membranes containing cupric ion concentrations in the range 1–30 percent of the total amount needed to fully neutralize the pendant acid groups. At higher cation concentrations ESR spectra indicate the presence of aggregated cations. The intercation distance determination is based on the simulation of spectra from isolated cations using distribution widths δg₁₁ and δA₁₁ and extraction of the residual width ΔH^R, which is due to dipolar interactions. No aggregation is detected in membranes swollen by less polar solvents such as methanol, dimethylformamide (DMF) and tetrahydrofuran (THF); these results are in contrast to SAXS experiments in membranes swollen by methanol, which exhibit the “ionic peak”. Cu²⁺-Cu²⁺ and Ti³⁺-Ti³⁺ dimers have been detected in Nafion swollen by water, methanol, DMF and THF, and have been characterized by an analysis of the spin-forbidden half-field Δm_s=2 transition, and by computer simulations. The intercation distance in the cupric dimers, deduced from the intensity ratio of the Δm_s=2 and Δm_s=1 dimer transitions, is 5.0±0.2 Å. A model for the dimer has been proposed, which explains the crosslinking of the polymer chains by the metal cations. ENDOR signals from ¹H, ²H and ¹⁹F nuclei have been detected in Nafion neutralized by Ti³⁺. The ENDOR results allow determination of the local environment of the paramagnetic cations, to a distance of ∼10 Å.     

Effects of sodium dodecyl sulfate on the electrochemical behavior of supercapacitor electrode MnO2

Supercapacitor electrode material MnO₂ was prepared by liquid co-precipitation with different concentration of anionic surfactant sodium dodecyl sulfate (SDS). As evidenced by X-ray diffraction, the obtained MnO₂ are all typical amorphous α-MnO₂ with poor crystallinity. Scanning electron microscopy reveals that the dispersity of MnO₂ initially get better and then worse with the increase of SDS, and the particle sizes first become smaller then larger as well. It is worthwhile noting that the morphology of MnO₂ tested by transmission electron microscopy undergoes a changeable process: fibrous, pine needle like, cotton like, round bubble like, flocculent, and nervous tissue like as SDS increases. Through cyclic voltammetry and galvanostatic charge/discharge tests, SDS addition amount 0.2 g (0.017 mol L^?1) is found to be the optimal effect value, and the as-prepared Mn-0.2 obtains the highest specific capacitance (C _sp) of 154.5 F g^?1 at a current density of 500 mA g^?1. Compared with the sample Mn-0 synthesized without SDS, the C _sp increases by about 50 % (±5 %), which can be attributed to its largest Brunauer–Emmett–Teller–specific surface area of 255.9 m² g^?1, best particle dispersity, and smallest particle size of approximately 50–80 nm. Meanwhile, the rate capability and cycle stability of Mn-0.2 also improves obviously, and the equivalent series resistance decreases a lot, only 0.120 Ω.     

MnO2 nanozyme induced the chromogenic reactions of ABTS and TMB to visual detection of Fe2+ and Pb2+ ions in water

In this study, we used a simple and rapid colourimetric reaction for visual sensing of Fe²⁺ and Pb²⁺ ions in water by employing nano-MnO₂ as a natural oxidase mimic to respectively catalyse ABTS and TMB in citrate-phosphate buffer solution (C-PBS) at 25°C and pH 3.8. It was found that nano-MnO₂ possessed highly oxidase-mimicking activity with the K_m values of 0.030 and 0.027 toward ABTS and TMB, respectively, indicating TMB had a stronger affinity on nano-MnO₂ than ABTS. Interestingly, the presence of 0.01 mmol·L^?1 Fe²⁺/Pb²⁺ ion was able to significantly down-regulate the activity of MnO₂ nanozyme in nano-MnO₂-mediated ABTS reaction processes (P < 0.01), which mainly due to the strong adsorption of metal ion toward nano-MnO₂ surface via the electrostatic attractions, thus leading to the passivation and inactivation of MnO₂ nanozyme catalytic activity. Thereinto, Fe²⁺ reacted with multivalent manganese by oxidation-reduction, while Pb²⁺ was specifically adsorbed onto the surface of MnO₂ nanozyme and formed complexes. Notably, only Fe²⁺ ion inhibited the activity of MnO₂ nanozyme-TMB with a detection limit as low as 1.0 μmol·L^?1. In MnO₂ nanozyme-ABTS sensing systems, Fe²⁺ and Pb²⁺ ions detection limit of 0.5 and 2.0 μmol·L^?1 were, respectively, achieved with a linear response range of 0–0.02 and 0–0.8 mmol·L^?1, implying the developed MnO₂ nanozyme-ABTS sensor was potentially applicable for the visual determination of Fe²⁺ and Pb²⁺ ions in water. In the real water samples, MnO₂ nanozyme-ABTS achieved high accuracy (relative errors: 3.4?10.5%) and recovery (96?110%) for respective detection of Fe²⁺ and Pb²⁺ ions. The simple and rapid MnO₂ nanozyme-ABTS sensing systems might provide a practical assay for visual detection of Fe²⁺ and Pb²⁺ ions in the environmental water samples.     

EQCM study of redox properties of PEDOT/MnO<Subscript>2</Subscript> composite films in aqueous electrolytes

Electrochemical behavior of poly-3,4-ethylenedioxythiophene composites with manganese dioxide (PEDOT/MnO₂) has been investigated by cyclic voltammetry and electrochemical quartz crystal microbalance at various component ratios and in different electrolyte solutions. The electrochemical formation of PEDOT film on the electrode surface and PEDOT/MnO₂ composite film during the electrochemical deposition of manganese dioxide into the polymer matrix was gravimetrically monitored. The mass of manganese dioxide deposited into PEDOT at different time of electrodeposition and apparent molar mass values of species involved into mass transfer during redox cycling of PEDOT/MnO₂ composites were evaluated. It was found that during the redox cycling of PEDOT/MnO₂ composite films with various MnO₂ content, the oppositely directed fluxes of counterions (anions and cations) occur, resulting in a change of the slope of linear parts of the Δf–E plots with changing the mass fraction of MnO₂ in the composite film.Rectangular shape of cyclic voltammograms of PEDOT/MnO₂ composites with different loadings of manganese dioxide was observed, which is characteristic of the pseudocapacitive behavior of the composite material. Specific capacity values of PEDOT/MnO₂ composites obtained from cyclic voltammograms were about 169 F g^?1. The specific capacity, related to the contribution of manganese dioxide component, was about 240 F g^?1.