Controlling Selectivity in the Chlorine Evolution Reaction over RuO2‐Based Catalysts

In the industrially important Chlor‐Alkali process, the chlorine evolution reaction (CER) over a ruthenium dioxide (RuO₂) catalyst competes with the oxygen evolution reaction (OER). This selectivity issue is elucidated on the microscopic level with the single‐crystalline model electrode RuO₂(110) by employing density functional theory (DFT) calculations in combination with the concept of volcano plots. We demonstrate that one monolayer of TiO₂(110) supported on RuO₂(110) enhances the selectivity towards the CER by several orders of magnitudes, while preserving the high activity for the CER. This win‐win situation is attributed to the different slopes of the volcano curves for the CER and OER.     

Single-atom rhodium anchored on S-doped black phosphorene as a promising bifunctional electrocatalyst for overall water splitting

Superior bifunctional electrocatalysts with ultra-high stability and excellent efficiency are crucial to boost the oxygen evolution reaction (OER) and the hydrogen evolution reduction (HER) in the overall water splitting (OWS) for the sustainable production of clean fuels. Herein, comprehensive density functional theory (DFT) computations were performed to explore the potential of several single transition metal (TM) atoms anchored on various S-doped black phosphorenes (TM/S_nx-BP) for bifunctional OWS electrocatalysis. The results revealed that these candidates display good stability, excellent electrical conductivity, and diverse spin moments. Furthermore, the Rh/S₁₂-BP catalyst was identified as an eligible bifunctional catalyst for OWS process due to the low overpotentials for OER (0.43 V) and HER (0.02 V), in which Rh and its adjacent P atoms were identified as the active sites. Based on the computed Gibbs free energies of OH*, O*, OOH* and H*, the corresponding volcano plots for OER and HER were established. Interestingly, the spin moments and the charge distribution of the active sites determine the catalytic trends of OER and HER. Our findings not only propose a promising bifunctional catalyst for OWS, but also widen the potential application of BP in electrocatalysis.     

Full Kinetics from First Principles of the Chlorine Evolution Reaction over a RuO2(110) Model Electrode

Current progress in modern electrocatalysis research is spurred by theory, frequently based on ab initio thermodynamics, where the stable reaction intermediates at the electrode surface are identified, while the actual energy barriers are ignored. This approach is popular in that a simple tool is available for searching for promising electrode materials. However, thermodynamics alone may be misleading to assess the catalytic activity of an electrochemical reaction as we exemplify with the chlorine evolution reaction (CER) over a RuO₂(110) model electrode. The full procedure is introduced, starting from the stable reaction intermediates, computing the energy barriers, and finally performing microkinetic simulations, all performed under the influence of the solvent and the electrode potential. Full kinetics from first‐principles allows the rate‐determining step in the CER to be identified and the experimentally observed change in the Tafel slope to be explained.     

镍锡析氢活性阴极的电化学制备及其在碱性溶液中的电催化机理

采用恒电流电沉积法在铜箔基底上获得镍锡合金镀层电极. 电子能谱(EDS)、X射线衍射(XRD)以及高分辨透射电镜(HRTEM)分析表明, 随着锡含量的增加, 镀层由镍晶胚与非晶镍锡构成的非晶态结构转变为Ni₃Sn₄与Ni₃Sn₂的混晶结构. 扫描电镜(SEM)分析发现, 非晶结构镍锡合金电极表面粒子分布均匀且粒径细小, Ni₃Sn₄与Ni₃Sn₂混晶结构的镍锡合金电极表面粗糙且断面呈分层自组装结构. 在25℃, 1 mol·L^-1 NaOH溶液中的稳态极化曲线表明非晶结构的镍锡合金电极具有良好的催化活性, 其析氢过电位仅为85 mV. 交流阻抗测试表明, 非晶以及混晶结构的镍锡合金在析氢电催化反应过程中由电化学吸附(Volmer)以及电化学脱附(Heyrovsky) 两个电荷转移过程控制, 且非晶结构电极相比于Ni₃Sn₄与Ni₃Sn₂混晶结构电极的高活性源于其活性氢具有更快的电化学吸附以及脱附速度.     

Optimal Geometrical Configuration of Cobalt Cations in Spinel Oxides to Promote Oxygen Evolution Reaction

MgCo₂O₄, CoCr₂O₄, and Co₂TiO₄ were selected, where only Co³⁺ in the center of octahedron (Oh), Co²⁺ in the center of tetrahedron (Td), and Co²⁺ in the center of Oh, can be active sites for the oxygen evolution reaction (OER). Co³⁺(Oh) sites are the best geometrical configuration for OER. Co²⁺(Oh) sites exhibit better activity than Co²⁺(Td). Calculations demonstrate the conversion of O* into OOH* is the rate‐determining step for Co³⁺(Oh) and Co²⁺(Td). For Co²⁺(Oh), it is thermodynamically favorable for the formation of OOH* but difficult for the desorption of O₂. Co³⁺(Oh) needs to increase the lowest Gibbs free energy over Co²⁺(Oh) and Co²⁺(Td), which contributes to the best activity. The coexistence of Co³⁺(Oh) and Co²⁺(Td) in Co₃O₄ can promote the formation of OOH* and decrease the free‐energy barrier. This work screens out the optimal geometrical configuration of cobalt cations for OER and gives a valuable principle to design efficient electrocatalysts.     

Understanding Synergistic Catalysis on Cu-Se Dual Atom Sites via Operando X-ray Absorption Spectroscopy in Oxygen Reduction Reaction

The construction and understanding of synergy in well-defined dual-atom active sites is an available avenue to promote multistep tandem catalytic reactions. Herein, we construct a dual-hetero-atom catalyst that comprises adjacent Cu-N₄ and Se-C₃ active sites for efficient oxygen reduction reaction (ORR) activity. Operando X-ray absorption spectroscopy coupled with theoretical calculations provide in-depth insights into this dual-atom synergy mechanism for ORR under realistic device operation conditions. The heteroatom Se modulator can efficiently polarize the charge distribution around symmetrical Cu-N₄ moieties, and serve as synergistic site to facilitate the second oxygen reduction step simultaneously, in which the key OOH*-(Cu₁-N₄) transforms to O*-(Se₁-C₂) intermediate on the dual-atom sites. Therefore, this designed catalyst achieves satisfied alkaline ORR activity with a half-wave potential of 0.905 V vs. RHE and a maximum power density of 206.5 mW cm⁻² in Zn-air battery.     

Effect of porosity on the kinetics of the two-route chlorine reaction: The chlorine evolution—ionization kinetics on iridium dioxide

The porosity effect on the kinetics of the chlorine reaction proceeding via two parallel paths is analyzed theoretically. An equation for the steady-state polarization curve is derived for these conditions. The chlorine evolution-ionization kinetics is studied by steady-state polarization measurements at a rotating disk electrode with an active iridium dioxide coating. A comparison of experimental and theoretical polarization curves shows the chlorine reaction to proceed via two parallel paths not only on DSA and RuO₂, but on IrO₂ as well     

Behavior of Porous Titanium and Catalytically Active Electrodes Based on It in Acidic Chloride Solutions

The behavior of porous titanium and electrodes based on it, which are activated with Pt, Au, RuO₂, Co₃O₄, and MnO₂, in 20-% LiCl solution (pH –0.4 to –0.5) is studied. On porous titanium in the potential ranges 0.1 < E< 0.5 and 0.5 < E< 1.1 V (NHE), the formation of titanium hydrides and passive oxide layers, respectively, is observed; the processes decay with time. In the ranges E< 0.1 and E> 1.1 V, the dissolved oxygen reduction and chlorine evolution, respectively, are observed on porous titanium at high overpotentials. On porous titanium activated with thin-layer Pt, Au, and RuO₂coatings, the functional Evs. pH dependence, which is typical for these electrocatalysts, breaks down due to the conjugate reactions of titanium oxidation. On porous titanium activated with Co₃O₄and MnO₂, at pH below unity, chlorine evolution is observed; its rate is limited by the chlorine mass transfer into the bulk solution. Under a gas-diffusion control, the chlorine evolution rate is determined by the diffusion of absorbed hydrogen chloride. The conditions of application of porous titanium as the support for catalytically active electrodes of electrochemical sensors in acidic chloride solutions are considered.     

Electrocatalytic activity of sol-gel-prepared RuO2/Ti anode in chlorine and oxygen evolution reactions

Electrocatalytic properties of RuO₂/Ti anode with different coating masses, which are prepared by the alkoxide sol-gel procedure, are investigated in chlorine and oxygen evolution reactions by polarization measurements and electrochemical impedance spectroscopy in H₂SO₄ and NaCl electrolytes. According to polarization measurements, the activity of anodes at overpotentials below 100 mV is independent of coating mass. However, impedance measurements above 100 mV reveal changes in the activity of anodes in chlorine evolution reaction for different coating masses. The diffusion limitations related to the evolved chlorine are registered in low-frequency domain at 1.10 V (SCE), diminishing with the increase in potential to the 1.15 V (SCE). The observed impedance behavior is discussed with respect to the activity model for activated titanium anodes in chlorine evolution reaction involving formation of gas channels within porous coating structure. Gas channels enhance the mass transfer rate similarly to the forced convection, which also increases the activity of anode. This is more pronounced for the anode of greater coating mass due to its more compact surface structure. The more compact structure appears to be beneficial for gas channels formation. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 10, pp. 1173–1179. The text was submitted by the authors in English.     

Coupling Magnetic Single‐Crystal Co2Mo3O8 with Ultrathin Nitrogen‐Rich Carbon Layer for Oxygen Evolution Reaction

Transition‐metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A₂Mo₃O₈ as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co₂Mo₃O₈@NC‐800 composed of highly crystallized Co₂Mo₃O₈ nanosheets and ultrathin N‐rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm^?2 and 422 mV@40 mA cm^?2, and a full water‐splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm^?2 in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t₂³e⁴), which improve the OER kinetics of rate‐determining step to form *OOH.     

Coupling Magnetic Single-Crystal Co2Mo3O8 with Ultrathin Nitrogen-Rich Carbon Layer for Oxygen Evolution Reaction

Transition-metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A₂Mo₃O₈ as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co₂Mo₃O₈@NC-800 composed of highly crystallized Co₂Mo₃O₈ nanosheets and ultrathin N-rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm⁻² and 422 mV@40 mA cm⁻², and a full water-splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm⁻² in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t₂³e⁴), which improve the OER kinetics of rate-determining step to form *OOH.     

Fast Modulation of d-Band Holes Quantity in the Early Reaction Stages for Boosting Acidic Oxygen Evolution

Synthesis of highly active and durable oxygen evolution reaction (OER) catalysts applied in acidic water electrolysis remains a grand challenge. Here, we construct a type of high-loading iridium single atom catalysts with tunable d-band holes character (h-HL−Ir SACs, ∼17.2 wt % Ir) realized in the early OER operation stages. The in situ X-ray absorption spectroscopy reveals that the quantity of the d-band holes of Ir active sites can be fast increased by 0.56 unit from the open circuit to a low working potential of 1.35 V. More remarkably, in situ synchrotron infrared and Raman spectroscopies demonstrate the quick accumulation of *OOH and *OH intermediates over holes-modulated Ir sites in the early reaction voltages, achieving a rapid OER kinetics. As a result, this well-designed h-HL−Ir SACs exhibits superior performance for acidic OER with overpotentials of 216 mV @10 mA cm⁻² and 259 mV @100 mA cm⁻², corresponding to a small Tafel slope of 43 mV dec⁻¹. The activity of catalyst shows no evident attenuation after 60 h operation in acidic environment. This work provides some useful hints for the design of superior acidic OER catalysts.     

Plasma-modified iron-doped Ni3S2 nanosheet arrays as efficient electrocatalysts for hydrogen evolution reaction

The development of efficient transition-metal catalysts for the hydrogen evolution reaction is significant to meeting global energy demands. In this study, to realize a high-performance electrocatalyst, we synthesize an Fe-doped Ni₃S₂ nanosheet material in situ on 3D structured nickel foam via the hydrothermal sulfide method, and then modify it by the dielectric barrier discharge plasma technique. Combining Fe atom doping and plasma modification increases the electrochemical surface area, provides an abundance of active sites, optimizes the electronic structure, and accelerates the reaction kinetics, thereby improving catalytic activity. As a result, the PA@Fe_1/4-Ni₃S₂/NF catalyst exhibits excellent hydrogen evolution reaction activity, only requires ultra-low overpotentials to achieve a current density of 10 mA cm^?2, and exhibits excellent durability. This study proposes a novel method for rationally designing non-noble metal electrocatalysts.     

Extent of exchange of36Cl with CAB in strong acid medium

Aqueous chloramine-B /C₆H₅SO₂NCl Na/ solution is known to contain species like RNCl^–, RNHCl, RNCl₂, RN⁺H₂Cl, HOCl and H²⁺OCl where R=C₆H₅SO₂. The exchange studies between³⁶Cl and CAB carried out in various media by ion-exchange method indicated that there is no exchange in solution at pH7. As the pH is decreased below 7, the extent of exchange increases reaching a maximum at pH 3. 3. The exchange decreases as the acidity is increased between pH 3.3 and 1N and again the exchange increases beyong 1N. The observed increase in exchange in strong acid medium is due to the evolution of chlorine.     

Effect of the basic surface sites of carbons on the degree of dispersion of platinum catalysts prepared by H<Subscript>2</Subscript>PtCl<Subscript>6</Subscript> adsorption

In the synthesis of Pt/C catalysts via H₂PtCl₆ adsorption onto a carbon support, NH₄Cl can be formed catalytically during the reduction of the precursor with H₂ at 250°C. This compound favors the sintering of metal particles. This effect is likely due to the weakening of metal-support bonding because of NH₄Cl adsorption on the Pt surface. The sources of nitrogen and chlorine atoms are basic surface sites of the support, which contain nitrogen atoms in their structure and adsorb Cl^? ions from the precursor solution. This effect is typical of active carbons, whose surface contains chemically bound nitrogen as amino groups, and weakens as the Pt/N atomic ratio in the supported catalyst precursors is increased.     

How does the extent of substitution of methane with chlorine influence the mechanism and kinetics of the reactions between chloromethanes and atomic chlorine

A theoretical investigation of the reaction mechanism and kinetics of the reaction between chloromethanes CH_4–xCl_x (x = 1–3) and chlorine atoms was performed. The height of the reaction barrier was found to decrease with the degree of substitution of chloromethanes with atomic chlorine. A direct dynamics method was employed to study the kinetic nature of these hydrogen-abstraction reactions. The sequence of calculated reaction rate coefficients is: k(CH₃Cl + Cl) < k(CH₂Cl₂ + Cl) < k(CHCl₃ + Cl).     

Amorphous Mo-doped NiS0.5Se0.5 Nanosheets@Crystalline NiS0.5Se0.5 Nanorods for High Current-density Electrocatalytic Water Splitting in Neutral Media

It is vitally important to develop highly active, robust and low-cost transition metal-based electrocatalysts for overall water splitting in neutral solution especially at large current density. In this work, amorphous Mo-doped NiS_0.5Se_0.5 nanosheets@crystalline NiS_0.5Se_0.5 nanorods (Am−Mo−NiS_0.5Se_0.5) was synthesized using a facil one-step strategy. In phosphate buffer saline solution, the Am−Mo−NiS_0.5Se_0.5 shows tiny overpotentials of 48 and 209 mV for hydrogen evolution reaction (HER), 238 and 514 mV for oxygen evolution reaction (OER) at 10 and 1000 mA cm⁻², respectively. Moreover, Am−Mo−NiS_0.5Se_0.5 delivers excellent stability for at least 300 h without obvious degradation. Theoretical calculations revealed that the Ni sites in the defect-rich amorphous structure of Am−Mo−NiS_0.5Se_0.5 owns higher electron state density and strengthened the binding energy of H₂O, which will optimize H adsorption/desorption energy barriers and reduce the adsorption energy of OER determining step.     

Efficient solar-to-chemical conversion with chlorine photoanode

The photoelectrochemical water splitting is an artificial photosynthetic approach that could provide a sustainable supply of clean energy, however, the sluggish kinetics of the oxygen evolution reaction (OER) has been a bottleneck to the solar-to-chemical conversion. Here we report an implementation of 8% efficient photoanode based on the photoelectrolysis of saturated NaCl solution. Replacing the OER with the chlorine evolution reaction (CER) has transformed both the thermodynamic basis and the kinetic process of the photoelectrolysis, more chemical energy can be produced with much less driving force. The RuO₂/TiO₂/n-Si photoanode exhibits a high rate of photoelectrochemical conversion (35 mA/cm² at equilibrium potential), which steadily and exclusively produces Cl₂ without detectable O₂.     

Formation and Decomposition of the Methylplatinum(IV) Complex in the Mechanically Activated K2PtCl4 Powder–MeI Vapor System

Mechanical treatment of the K₂PtCl₄ solid salt in a vibrating mill results in Pt–Cl bond heterolysis to form coordinatively unsaturated Pt(II) complexes. At room temperature, the freshly treated K₂PtCl₄ salt absorbs methyl bromide and evolves methyl chloride to the gas phase. The reaction mechanism involves the following sequence of steps: the oxidative addition of methyl iodide to Pt(II) with the intermediate formation of Pt(IV) methyl complexes and the decomposition of the latter due to intramolecular reductive elimination with methyl chloride formation. The first step of the reaction of MeI with the preactivated surface of the K₂PtCl₄ salt is assisted by active sites, which are regenerated in each act of the chemical transformation of MeI into MeCl involving in the chain substitution of halogen in methyl iodide. The coordinatively unsaturated surface platinum complexes can act as such active sites. Due to their effective positive charge, they can provide electrophilic assistance to nucleophilic substitution. Chain termination is probably due to the coordination of the complex with a coordination vacancy and an interstitial chloride ion to the inactive K₂PtCl₄ complex.     

Quantum chemical study of the reaction mechanism of ozone and methane with fluorine and chlorine atoms

Ab initio calculations of the potential energy surface for the F + O₃ and Cl + O₃ reactions have been performed using the G3 and G3MP2 methods, which optimize the geometry configuration of reactants, products, intermediates, and transition states. The results show that fluorine atoms react with ozone as violently as chlorine atoms. At the same time, we have studied the reaction mechanisms of F atoms and Cl atoms with methane. It is found that fluorine atoms prefer to react with methane and chlorine atoms with ozone when there is competition between ozone and methane. Therefore, we can reasonably explain why chlorine atoms play the main role of reactants depleting ozone, while the more active fluorine atoms deplete less ozone. © 2002 Wiley Periodicals, Inc.; DOI 10.1002/qua.10119