Electrocatalysis and the chlorine evolution reaction—II. Comparison of anode materials

A number of electrodes which are catalysts for the chlorine evolution reaction have been investigated using steady state current-potential and impedance-potential measurements. Electrodes which show similar overvoltages for the chlorine evolution reaction in technical tests show large differences when subjected to this kinetic approach.     

Electrocatalysis and the oxygen evolution reaction

The oxygen evolution reaction has been investigated on a number of electrodes which are electrocatalysts for the chlorine evolution reaction, by making measurements in NaClO₄ solution. Steady state current—potential and impedance—potential measurements, obtained using automated equipment, have been used as the preferred experimental method. Analysis of the impedance has been undertaken by means of curve fitting, and the resulting parameter curves of the double-layer capacity, the charge transfer resistance, and the electrolyte ohmic resistance displayed as a function of potential. Some speculation is made about the interpretation of the parameter curves. The ohmic loss parameter, R_w, is used to correct the potentials to the true values. In comparison to some other gas evolution reactions, such as hydrogen and chlorine, R_w does not depend strongly on potential. The corrected log i—E curve is also considered. It is suggested that a more complete picture of the electrical performance of these electrodes can be obtained by using these electrochemical methods.     

Electrocatalysis in the anodic evolution of oxygen and chlorine

Requisites for electrode materials to be suitable for technological applications are outlined and discussed. Oxides with metallic or quasi-metallic conductivity meet these requirements best. Most of these electrodes are prepared by thermal procedures. It is shown that the temperature of preparation affects the catalytic activity through the surface area and the chemical composition (non-stoichiometry). The co-variation of these parameters is best followed in situ by voltammetric curves and point of zero charge measurements. Examples are given for pure RuO₂, IrO₂, Co₃O₄ and IrO₂ + RuO₂ mixtures. Kinetic and mechanistic details are discussed for O₂ evolution on RuO₂, IrO₂ and Co₃O₄ and for Cl₂ evolution on RuO₂ and Co₃O₄. Finally, the electrocatalytic properties of different oxides are correlated with the energy change involved in the lower → higher valency state transition. Experimental data for both O₂ and Cl₂ evolution can thus be organized into a “volcano” curve enabling predictive interpolations to be made.     

A consideration of the electrochemical mechanism in the chlorine evolution reaction

The catalytic activity for the anodic evolution of chlorine on the RuO₂, IrO₂ and graphite electrodes were discussed on the basis of the activation energy. It was deduced that the activation energy seems to be a linear function of the energy difference between the conduction band of the solid and the energy level of the adsorbed intermediate Cl_ads.     

Some oxide catalysts for the anodic evolution of chlorine: reaction mechanism and catalytic activity

The chlorine evolution reaction was investigated on several oxide electrodes. The active site on the electrode surface is concluded to be the metal site, based on the dependence of the anodic current on pH of the electrolyte. The reaction mechanism can be determined by the d-electron configuration in the oxide; ie the reaction proceeds by the Volmer-Heyrovsky mechanism on the electrode containing the transition metal cation with partially filledt_2g orbitals and empty e_g orbitals, whereas the Volmer-Tafel mechanism on the electrode containing the transition metal cation with just or half filled t_2g orbitals and partially filled e_g orbitals.     

Electrocatalysis of chlorine evolution on different materials and its influence on the performance of an electrochemical reactor for indirect oxidation of pollutants

Two different Ti/Pt–Ir materials (commercial and home made) and Ti/PdO + Co₃O₄ were investigated for their electrocatalytic properties versus Cl₂ evolution reaction. The materials were used in a batch electrochemical reactor to treat biologically recalcitrant di-azo compound. An electrochemically driven oxidation, mediated by a Cl₂/Cl⁻ couple, proved efficient for destruction of this complex organic molecule, causing cleavage of the conjugated double bonds and destruction of unsatured bonds. Both Ti/Pt–Ir materials performed well; lower kinetics obtained with the Ti/PdO + Co₃O₄ anode was caused by adsorption of the model compound, evidenced in preliminary voltammetric measurements. The dye oxidation reaction followed the second order kinetics with partial orders in the model compound and (time varying) chlorine concentrations equal to one. Specific energy consumption of 3.12 kWh m⁻³ proved the process more economic than the homogeneous phase oxidation.     

氧化石墨烯生长钨基氧化物酸环境电催化析氢

为了解决商用铂碳电极在电催化析氢反应（HER）领域生产成本高、催化剂使用周期短等问题,利用磷钨酸提供钨源,以单层氧化石墨烯（GO）作为载体,引入利于电子传输轨道的碳纳米管（CNTs）,通过一步水热法和空气中煅烧制备得到WO3-rGO-CNTs纳米异质结电催化剂。利用FT-IR、XRD、XPS、TG、SEM、EDAX、TEM和BET对WO3-rGO-CNTs的化学结构和物理形貌进行了表征。结果表明,在单层GO表面均匀生长WO3晶体,并引入CNTs后,纳米异质结WO3-rGO-CNTs在酸性电解质中表现出优异的HER催化活性。利用线性伏安法（LSV）和循环伏安法（CV）对WO3-rGO-CNTs进行HER测试,当电流密度为10 mA/cm2时,其过电势为218 mV;塔菲尔斜率为130.5 mV/dec。当过电势为-0.5 V vs. RHE时,其阻抗值为8.2 Ω。同时,WO3-rGO-CNTs纳米异质结可以在218 mV（电流密度为10 mA/cm2）过电势下,保持50 h的稳定性和耐久性;其双层电容值为1.2 mF/cm2。电化学数据表明,WO3与GO和CNTs间由于异质结构的存在,产生了协同效应：GO为WO3晶体提供了广阔的金属反应活性位点,而CNTs则提供了利于电子传输的活性轨道。     

The electrocatalytic activities of the Pd-doped MnO2 for the chlorine evolution reaction

The anodic evolution of chlorine on the massive β-MnO₂ doped with various amounts of Pd was investigated in acidic chloride solution. A second-order relation was found between the rate of the chlorine evolution reaction and the number of active Pd sites at the electrode surface. And it was confirmed that, in the relatively low doped electrodes, the active sites are more effectively dispersed on the surface with microscopic homogeneity. From the activation energies, it was also suggested that the reaction mechanism on the MnO₂ doped with relatively small amounts of Pd is somewhat different from that on the pure PdO electrode.     

A consideration of the activation energy for the chlorine evolution reaction on RuO2 and IrO2 electrodes

The electrocatalysis for chlorine evolution reaction on RuO₂ and IrO₂ electrodes was discussed on the basis of the activation energy. The activated complex with the pentagonal bipyramid-type structure was speculated to be formed in the transition state for the Heyrovsky reaction. The activation energy for this reaction was evaluated by using the difference of the crystal field stabilization energy between the initial and transition states. The good agreement was obtained between the calculated and found values.     

Kinetics of the reaction between chlorine and oleic acid in carbon tetrachloride solution

Kinetic data have been obtained for the reaction between chlorine and oleic acid in carbon tetrachloride solution by measuring the chlorine depletion and hydrogen chloride production as a function of residence time in a rod-like flow system. The data are well correlated by a kinetic model incorporating parallel, second-order addition and substitution reactions. Measured reaction rates are an order of magnitude less than those reported in a previous investigation which neglected substitution reactions.     

Kinetic investigation of the chlorine reduction reaction on electrochemically oxidised ruthenium

The rate and mechanism of the electroreduction of chlorine on electrooxidised ruthenium has been investigated with focus on the effect of solution pH. Current/potential curves for the reduction process in solutions with constant chloride concentration of 1.0 mol dm⁻³ and varying H⁺ concentration have been obtained with the use of the rotating disk electrode technique (RDE). It was found that the chlorine reduction rate is highly inhibited in solutions with high H⁺ concentrations and that it can be satisfactorily described by the Erenburg mechanism, previously suggested for the chlorine evolution on RuO₂ and RTO. The expression of the kinetic current as a function of chlorine and H⁺ concentration was obtained by solving the elementary rate equations of the kinetic mechanism. The kinetic constants obtained from the correlation of the kinetic current expression to the experimental data were used to simulate the dependence of the surface coverages and elementary reaction rates on overpotential.     

A graph theoretical approach to the elucidation of reaction mechanisms: Analysis of the chlorine electrode reaction

A methodology and algorithm are proposed in this work to conceive a limited set of dependent elementary reaction steps for a reaction mechanism by exploiting the reaction route (RR) graph of the mechanism. These steps are constrained by the conservation principles and must be subjected to thermodynamic feasibility studies. This method is expected to aid in the conception of new steps, which might otherwise be ignored, especially in the case of large mechanisms. In order to illustrate the method, it is applied to the chlorine electrode reaction mechanism. The result indicated that a reaction pathway consisting of one of the newly proposed steps is the dominant one. This example illustrated the potential of the proposed method to identify new steps to a mechanism and thereby help in mechanism elucidation. However, these new steps must be subjected to further critical study before they can be accepted.     

稀土元素对析氯电位的影响

介绍了在Ｒｕ、Ｔｉ中加入稀土元素Ｎｄ、Ｌａ、Ｅｕ对析氯电位的影响。结果表明该体系具有较低的析氯电位、极化率,但强化寿命不合要求。     

Electrocatalysis of oxygen evolution at stainless steel anodes by electrosynthesized cobalt hydroxide coatings

Controlled growth of cobalt hydroxide coatings (1–2 m thick) on stainless steel electrodes was achieved by electrogeneration of base by cathodic reduction of aqueous cobalt nitrate solutions. These coatings catalyse the oxygen evolution reaction in alkaline medium (1 M KOH) by as much as 100 mV, making cobalt hydroxide a candidate catalyst for these applications.     

Experimental activation energies for evolution of oxygen and chlorine on oxide electrodes

The chlorine and oxygen overpotential in dependence on the current density i (A cm^?2) and on the temperature in the range of 15–75°C was measured at γ-MnO₂ and β-PbO₂ electrodes in concentrated water solutions of sodium chloride and perchlorate. From the measured values the experimental activation energy in dependence on overpotential was calculated and, for the temperature of 25°C, the constants of Tafel's equation (a,b) (α, i_o respectively) were evaluated.     

Electrocatalysis of hydrogen evolution in the light of the brewer—engel theory for bonding in metals and intermetallic phases

A comprehensive survey of achievements in electrocatalysis for the hydrogen evolution reaction (h.e.r.) has been reviewed with the main emphasis on the composite d-metal catalysts. The basic concept of the Brewer—Engel valence-bond theory that relates the electronic state of highest multiplicity which corresponds to the electronic configuration of lowest energy level or the structure of atoms, and corresponding phase structure both in individual metallic and multicomponent intermetallic systems, is also given. On the basis of the Brewer—Engel model it has been pointed out that whenever metals of the left-half of the transition series that have empty or half filled vacant d-orbitals are alloyed with metals of the right-half of the transition series that have internally paired d-electrons not available for bonding in the pure metal, there arises well pronounced synergism in electrocatalysis for the h.e.r., which often exceeds individual effects of precious metals and each other (the synergism condition) and approaches the reversible behaviour within the wide range of current density. It has been inferred that the upmost electrocatalytic activity reach the composite d-metal catalysts of improved d-orbital overlap in intermetallic phases of highest symmetry and minimal entropy such as Cr₃Si(A₁₅) type like MoCo₃, WNi₃, MoNi₃, LaNi₅, that the Brewer—Engel theory predicts for the most stable systems.     

A laser interferometric study of chlorine evolution from concentrated NaCl solutions

A brief qualitative study was done on a laboratory-sized chlor-alkali cell by laser interferometry which made visible refractive index effects at the anode at low cds or alternatively up to the beginning of bubble formation. Change in the refractive index indicated the direction of convection prior to bubble formation, concentration profiles and diffusion layer thickness. Onset of bubble formation and bubble scavenging was clearly marked by a less steep refractive index profile over a greater diffusion layer thickness.     

氧化锰电催化析氧反应及其电极界面特性

析氧半反应是速控步骤。氧化锰（MnO_x）具有多价态,可形成多个子步骤降低活化能。二氧化钛（TiO₂）是抗氧化性半导体。考察两种氧化锰催化剂析氧性能,即滑动弧等离子体合成MnO_x（pM）和商业化（cM）,与商业化TiO₂ （cT）比较。通过理想极化电极系统（IPE）解析,表达电极界面特性,揭示三相界面与活性的关联。结果表明：对两种MnO_x 催化剂,pM比cM性能更优。碱性环境中,前者起始电位低180 mV,Tafel斜率近半。酸性中,均表现阶跃式极化,析氧性能相近。cM比cT催化剂析氧性能更优,起始电位低420 mV。溶液电阻R_s与活性变化趋势一致。对相同催化剂,考察I/C、担量、单双层结构,电容压降占比f_Cd与活性一致。对不同催化剂,考察cM, cT或pM, cM,f_Cd与活性不一致,这与多价态锰参与非法拉第过程有关。