Diffusion of neutral and charged species under small-signal a.c. conditions

Several of the ways in which diffusion of an electroactive species may affect the small-signalresponse of an electrochemical system are examined, with particular attention to cases in which the electrode reaction produces or consumes a neutral species whose concentration at the electrode surface is determined by diffusion through the electrode. The conventional (time domain) rate and diffusion equations may be expressed in the frequency domain through the use of complex, frequency-dependent rate constants, whose form reflects the sequence of events in the overall reaction, including possible adsorption steps, and leads directly to equivalent-circuit representations of the pertinent parts of the system response. The complex rate constant formalism also allows the immediate generalization of existing exact treatments of unsupported systems to include such diffusion effects.     

Binary electrolyte small-signal frequency response

The frequency response is considered of a two-electrode linearized system containing a single positively charged species and a single negatively charged species. These species may have arbitrary valences and mobilities and may individually react at the electrodes. The results follow from a detailed solution of the equations of charge motion given earlier. Normalized response is exhibited for this unsupported, intrinsic-conduction situation for a wide range of mobility ratios, valence number ratios, and reaction rate ratios. Results are given in the form of specific formulas, impedance-plane plots, and the dependences on normalized frequency of series and parallel resistive and capacitative components of the normalized total impedance of the system.Impedance-plane plots exhibit from one to three connected arcs, depending on the specific situation. Approximate Warburg frequency response appears for the “interface” impedance over a certain frequency region when normalized reaction rate parameters differ, but it only shows up strongly in the total impedance when the mobility ratio departs appreciably from unity as well. Under such conditions, a plateau region, where the total parallel capacitance remains essentially independent of frequency over a wide frequency range, may appear at frequencies just above the Warburg region. The plateau capacitance is close to but not identical to the conventional double-layer capacitance present when both species of charge are completely blocked. In incomplete blocking cases, however, this double-layer capacitance only makes a significant appearance in the approximate equivalent circuit under slow reaction conditions; it is thus not present when one of the reaction rate constants is infinite.In general, the system can show ω^?m frequency response for the parallel capacitance over a wide frequency range with 0?m?2, and with the experimentally common regions where m≌0, 0.5, 1.5, and 2 especially likely. Particular attention is given to deviations from ideal Warburg behavior which led to a combined charge-transfer and heterogeneous chemical reaction resistance. Results are compared to those from conventional supported treatments and show both important similarities and differences. Finally, several new equivalent circuits are presented which are pertinent in various frequency ranges for the unsupported situation.     

Compact and diffuse double layer interaction in unsupported system small-signal response

Previous exact results for the small-signal impedance of an unsupported electrode/material/electrode system which include effects of the finite size of charge carriers are simplified and discussed. The material contains non-recombining charges of opposite sign with the positive one immobile and uniformly distributed. General boundary conditions which encompass the range from no electrode reaction to ohmic electrode behavior are employed. In the presence of an electrode reaction, the interaction of the compact and diffuse double layers leads to considerably more complexity in the equivalent circuit than might appear in simple treatments of the supported case, in which the diffuse double layer capacitance is neglected or the compact double layer and diffuse double layer capacitances are placed in series. Two different approximate equivalent circuits made up of frequency-independent elements are found which yield remarkable agreement with the exact results over the entire frequency range of interest. The first involves the ordinary approximate circuit (OAC) previously found in the absence of compact layer effects plus a series compact layer contribution involving a parallel resonant circuit with quality factor at resonance which may approach unity. Pseudo-inductance effects are found to be extremely significant in this representation. The second approximate equivalent circuit, simpler and almost as accurate as the first, has the same form as the original OAC but with its reaction element values altered by the presence of the compact layer. For non-Butler-Volmer electrode kinetics an upper limit is found for the experimentally determinable apparent reaction rate constant, a feature of practical importance for thin films or membranes. The response of thin films and membranes, including compact layer effects, can very readily be erroneously confused with pure bulk response, yielding entirely incorrect values for the geometrical capacitance and bulk resistance of the material.     

A systematic approach to the impedance of surface layers with mixed conductivity forming on electrodes

Electrode impedance can be evaluated on the basis of the electrode reaction kinetics in many systems, even for complicated electrode reactions. However, when a surface layer is present on the electrode surface, the theoretically well-established impedance model of the electrode reaction is often completed with phenomenological equivalent circuit elements in order to achieve the number of time constants as derived from the electrode impedance spectra measured. In these cases, the meaning of the phenomenological equivalent circuit elements are often unclear, though the presence of these elements is helpful to describe the system throughout the frequency domain used for the measurement. In the present work, an attempt will be shown to separate the effect of the electronic and ionic charge transfer in a surface layer and to identify the appropriate equivalent circuits. Examples are shown from the fields of lithium-ion batteries where a solid electrolyte interface as a surface layer is present at the negative electrode and the contribution of various charge carriers may be of importance.     

Sur la détermination générale de l'impédance de concentration (diffusion convective et réaction chimique) pour une électrode à disque tournant

A general relation is demonstrated between the concentration impedance in the presence of a homogeneous chemical reaction and the impedance due to convective diffusion. This relation is specified for the case where the diffusion coefficients of the substances involved in the reaction are equal. It can be used to treat experimental results if the impedance of convective diffusion can be obtained in an analytical form. Three approximation formulas for this relation are given, valid respectively for low, medium, and high values of the module of a dimensionless complex variable which is a function of the frequency and the rate constants. Examples are given of numerical applications of these formulas for predicting the value of the concentration impedance, given the frequency, the Schmidt number, the rate constants of the reaction, and the disc rotation rate.     

SYNTHESIS, CHARACTERIZATION AND CATALYTIC ACTIVITY OF N,N''''-BIS(3-ALLYL SALICYLIDENE)ETHYLENEDIAMINE COBALT(Ⅱ) SCHIFF BASE COMPLEX ANCHORED ON A NEW POLYMER SUPPORT

INTRODUCTIONAlthough homogeneous catalysts have higher selectivity and act under mild experimental conditions in variousreactions they have problems of separation and recovery from reaction products. Hence current research activitiesare aimed at developing catalysts of coordination compounds anchored on suitable supports[1,2]. This techniqueof immobilization of catalyst on an inert support increases the catalytic activity, selectivity, efficiency[3],operational flexibility and stability. …     

担载的Co2(CO)6(PBu3)2络合物的表面化学

红外光谱表明当Co_2(CO)_6(PBu_3)_2羰基络合物担载于不同的氧化物上后, 其羰基振动频带位置的波数顺序为: 络合物/SiO_2=络合物/TiO_2>络合物/Al_2O_3>络合物/ZrO_2。当这四种担载络合物在真空中加热脱羰基时, 以ZrO_2和Al_2O_3为载体者出现相当于桥式配位羰基的振动频带, 分别至300 ℃和200 ℃时羰基完全脱去。以TiO_2和SiO_2为载体者,基本没有出现桥羰基的频带, 而且在较低的温度下羰基即可脱去。紫外漫反射光谱表明担载络合物与担载前相比其表征Co-Co键和Co—Co配位键性质的特征带仍然存在, 脱羰基后出现相当于Co~(+2)或Co_3O_4的特征谱带。程序升温分解反应发现在H_2气氛中有表面加氢反应发生。     

SYNTHESIS, CHARACTERIZATION AND CATALYTIC ACTIVITY OF N,N''-BIS(3-ALLYL SALICYLIDENE)ETHYLENEDIAMINE COBALT(II) SCHIFF BASE COMPLEX ANCHORED ON A NEW POLYMER SUPPORT

A new chelating polymer support has been prepared by suspension copolymeriz a tion of synthesized N,N'-bis(3-allyl salicylidene)ethylenediamine monomer Schiff base (N,N'-BSEDA) with styrene (St) and divinylbenzene (DVB) using azobisisobutyronitrile (AIBN) as initiator in the presence of poly(vinyl alcohol). The content and complexation ability of monomer Schiff base (N,N'-BSEDA) for cobalt(II) ions in prepared crosslinked polymer beads have shown dependence on the amount of DVB used in reaction mixture. The amount of monomer Schiff base (N,N'-BSEDA) in crosslinked beads showed a substantial decreasing trend at high concentration of DVB in the reaction mixture (＞ 1.5 mol dm-3), hence the efficiency of complexation (EC%) and cobalt(II) ion loading (EL%) of polymer beads showed a decreasing trend. The structure of monomer Schiff base (N,N'-BSEDA) and its cobalt(II) complex on polymer support was elucidated by IR, UV and magnetic measurements. The catalytic activity of polymer bound cobalt(Ⅱ) Schiff base complex was evaluated by analyzing kinetic data of decomposition of hydrogen peroxide in the presence of either supported cobalt (II) complex or free cobalt(II) complex. The activation energy for the decomposition of hydrogen peroxide by polymer supported cobalt(II)complex was found to be low (33.37 kJ mol-l) in comparison with unsupported cobalt(II) complex (56.35 kJ mol-1). On the basis of experimental observations, reaction steps are proposed and a suitable rate expression derived.     

Thiophene Hydrogenation to Tetrahydrothiophene over Tungsten Sulfide Catalysts

Independent reactions of thiophene reduction to tetrahydrothiophene and thiophene hydrogenolysis to form hydrogen sulfide and C₄ hydrocarbons are shown to occur over supported tungsten sulfide catalysts and unsupported tungsten sulfide at an elevated temperature and a high pressure. The highest rate of tetrahydrothiophene formation over the supported catalysts is observed when alumina was used as a support, and the lowest reaction rate is found when silica gel was used as a support. Both catalysts are less active than unsupported tungsten disulfide. The rate of thiophene hydrogenation over tungsten disulfide increases with increasing thiophene concentration and hydrogen pressure and is inhibited by tetrahydrothiophene. The selectivity to tetrahydrothiophene is constant (70–90%) in the whole range up to high thiophene conversions. The high selectivity over tungsten sulfide catalysts is suggested to be due to the reaction pathway through thiophene protonation mediated with the surface SH groups and to the inhibition of hydrogenolysis.     

负载型钌基催化剂催化苯选择加氢合成环己烯

沉淀法制备的钌基催化剂催化苯选择加氢的工艺已成熟并工业化,新型高活性、高选择性的负载型催化剂因其独特的性质成为目前新的研究方向.本文重点讨论了负载型钌基催化剂制备过程中载体种类、载体修饰、活性组分和负载量等因素对催化剂活性、选择性等方面的影响,同时也介绍了反应温度、压力、转速和添加剂等因素对催化剂活性、选择性等方面的影响.     

连续电荷传递反应的阻抗谱与电极反应机制的关系

对于含有一个吸附中间物的不可逆连续电行传递反应，在较低的过电位下，电极过程的Faraday阻抗通常在复平面上显示一个低频感抗弧．在较高的过电位下，这个感抗弧将转化为容抗弧，这一转变预示着速控步骤的变化．相反，当反应速率一直被其中的一个步骤控制时，它的法拉第弛豫始终表现出容抗行为．     

量子电动学诱导电磁辐射研究纳米催化剂

Gold has been regarded as a poor heterogeneous catalyst because it is generally considered a nonreactive metal. But as nanocatalysts,gold and other metals somehow significantly enhance reactivity. It is generally thought chemical bonds of reactants are weakened by adsorption to nanocatalysts thereby allowing reactions to proceed more rapidly,but how this reaction proceeds to completion is not well understood. Here gold nanocatalysts are treated as unsupported nanoparticles (NPs) in a solution of reactant molecules from which extensions are made to gold NPs supported on titanium dioxide. Whether the NPs are supported or unsupported,enhanced catalytic reactivity depends on absorbed thermal kT (k is Boltzmann's constant and T is absolute temperature) energy accumulated from prior collisions of reactant molecules. The accumulated kT energy is treated as electromagnetic thereby allowing frequency up-conversion by quantum electrodynamics (QED) to the confinement frequency of the NP,typically beyond the vacuum ultraviolet (VUV). By this theory,the chemical reaction of reactant molecules having bonds weakened by adsorption is completed by QED induced VUV photolysis.     

SYNTHESIS OF A COMB GRAFT COPOLYMER VIA RING OPENING METATHESIS POLYMERIZATION CATALYZED BY SUPPORTED RHUTHENIUM CARBENE COMPLEX

Comb graft copolymers (CGC), a special kind of the hyper-branched polymer series, have unique and excellent properties [1,2]. Synthesizing CGC via ring opening metathesis polymerization (ROMP) of cycloolefinyl macromonomer catalyzed (initiated) by the molybdenum carbene complex (Schrock complex) is a newly-developed method [3,4]. Though showing excellent catalytic behavior the complex has the disadvantage of highly moisture and oxygen sensitive and is hence operation-inconvenient. We repor…     

SYNTHESIS OF A COMB GRAFT COPOLYMER VIA RING OPENING METATHESIS POLYMERIZATION CATALYZED BY SUPPORTED RHUTHENIUM CARBENE COMPLEX

5－Chloromethyl-2-norbornene was synthesized via Diels-alder reaction of cyclopentadiene and allyl chloride using a polymer supported boron trifluoride as the catalyst,and was then lithiated and used to initiate a livcing anionic polymerization of styrene to prepare a macromonome,5-polystryl-2-norbornene NB-PS,Comb graft copolymer PNB-g-PS was synthesized via ring opening metathesis polymerization of the macromonomer under the catalysis of ruthenium carbene complex RuCl2(PPh3)2(=CCHtBu)and its polymer supported correspondent.Experimetal results showed that the behavior of both the supported boron and supported ruthenium catalysts are superior to their unsupported counterparts.The possible promotion mechanism of the tailor-made supports is discussed.     

Investigation of ion-selective electrodes with neutral ionophores and ionic sites by EIS. I. Theory

Models of an ion selective electrode involving an ionophore and mobile sites in a membrane are proposed. The first model, called the phase boundary potential model, supposed thermodynamic equilibrium; it allows the concentrations of the various species to be calculated. Then, a kinetic model, which takes into account the ionic transfer at the membrane|solution interfaces, was derived. The impedance of the membrane was calculated. It shows that a membrane with nernstian behavior shows only one capacitive loop in the impedance diagram, which is related to the conductivity and dielectric properties of the material of the membrane. Non-nernstian behavior is related to slow ionic transfer at the membrane|solution interfaces or/and transport limitation of the species in the membrane. Finite rate constants of the ionic transfer lead to a capacitive loop in the middle frequency range, whereas finite rate transport leads to a diffusional impedance in the low frequency range.     

Relaxation spectra of the electrode impedance

A new analytical form is proposed enabling one to calculate the impedance of systems that contain no inductances. The method is based on breaking the overall impedance into a sum of isolated contours corresponding to a set of intrinsic frequencies of the system. Intrinsic frequencies, which exist in any real system, can be uniquely determined from the frequency characteristics. In this, they advantageously differ from speculative elements of equivalent circuits. For inductionless circuits, sets of intrinsic frequencies are replaced by intrinsic sets of reciprocal relaxation times. This allows one to construct a relaxation spectrum for the system, which describes quantitative contributions made by each relaxation contour to the overall impedance. As a result, one can estimate quantitatively conditions under which the system’s individual parameters may be observed, evaluate the resolving power of the experimental setup, and determine the full information capacity of an experiment (the maximum number of system’s parameters that can be determined).     

SYNTHESIS, CHARACTERIZATION AND CATALYTIC ACTIVITY OF N,N'-BIS(3-ALLYL SALICYLIDENE)ETHYLENEDIAMINE COBALT SCHIFF BASE COMPLEX ANCHORED ON A NEW POLYMER SUPPORT

 A new chelating polymer support has been prepared by suspension copolymerization of synthesized N,N'-bis(3-allyl salicylidene)ethylenediamine monomer Schiff base (N,N'-BSEDA) with styrene (St) and divinylbenzene (DVB) using azobisisobutyronitrile (AIBN) as initiator in the presence of poly(vinyl alcohol). The content and complexation ability of monomer Schiff base (N,N'-BSEDA) for cobalt(Ⅱ) ions in prepared crosslinked polymer beads have shown dependence on the amount of DVB used in reaction mixture. The amount of monomer Schiff base (N,N '-BSEDA) in crosslinked beads showed a substantial decreasing trend at high concentration of DVB in the reaction mixture (> 1.5 mol dm^-3), hence the efficiency of complexation (EC%) and cobaltⅡion loading (EL%) of polymer beads showed a decreasing trend. The structure of monomer Schiff base (N,N'-BSEDA) and its cobalt(Ⅱ)complex on polymer support was elucidated by IR, UV and magnetic measurements. The catalytic activity of polymer bound cobalt(Ⅱ)Schiff base complex was evaluated by analyzing kinetic data of decomposition of hydrogen peroxide in the presence of either supported cobalt(Ⅱ)complex or free cobalt(Ⅱ)complex. The activation energy for the decomposition of hydrogen peroxide by polymer supported cobalt complex was found to be low (33.37 kJ mol^-1) in comparison with unsupported cobalt(Ⅱ)complex (56.35 kJ mol^-1). On the basis of experimental observations, reaction steps are proposed and a suitable rate expression derived.     

负载型过氧磷钨杂多化合物上正己醇催化氧化成正己酸

由正己醇合成正己酸不仅可以提高附加值,还能生产己酸类香料以及多种医药产品．利用杂多过氧磷钨季铵盐催化剂,以对环境友好的H2O2 为氧化剂催化十八醇氧化成十八酸的研究已有报道［1,2],但合成负载型相转移催化剂并用于本课题的研究还未见报道．由于过氧磷钨十八烷基季铰盐催化剂（POWP）通常只有很小的比表面积,而SiO2不仅有大的比表面积,同时它所具有的表面羟基是杂多化合物的良好载体［2],POWP负载后的催化剂（POWPS）与未负载的POWP相比,将正己醇氧化为正已酸的收率提高了10多个百分点．     

In situ investigation of the ionisation of silica in aqueous ammonia by using a high frequency dielectric method

The reaction of silica gel and St?ber beads of silica with ammonia was studied in aqueous medium using a high frequency dielectric method. Measurements of the complex impedance of silica pulps in both static and dynamic modes were found to be a new rapid, sensitive and non-destructive way for the in situ characterisation of the surface silica ionisation process in aqueous ammonia. The influence of various parameters (field frequency, ionic strength, ammonia concentration) was discussed. The apparent equilibrium constant for the following reaction (SiOH) + NH(3)<-->(SiO(-)NH(4)(+)) was found to be strongly affected by the degree of dissociation of both silica samples, so that the complete neutralisation was never observed. The porosity of the St?ber silica towards the NH(3) species was demonstrated experimentally through the formation of (SiO(-)NH(4)(+)) entities in the interior of the silica beads. The reversibility of the neutralisation reaction was applied to successive on-column analysis for which the hydrolysis of ammoniated silica was found to be catalysed by the presence of acids.     

梳形接枝共聚物的合成(Ⅱ)——载体化钌卡宾络合物催化降冰片烯基聚苯乙烯的开环岐化聚合

由石油化工副产C₅馏份提取双环戊二烯(DCPD)、以聚合物负载三氟化硼为催化剂进行DCPD与烯丙基氯(AC)的Diels-Alder反应合成5-氯甲基-2-降冰片烯(NBCH₂Cl),经锂代反应后用以引发苯乙烯的活性阴离子聚合合成了降冰片烯(NB)基聚苯乙烯(PS)大分子环烯单体NB-PS,在聚合物负载钌卡宾络合物[RuCl₂(PPh₃)₂(=C=CHtBu)]催化(引发)作用下进行大分子单体NB-PS的开环歧化聚合(ROMP)合成了梳形接枝共聚物PNB-g-PS.实验结果表明所合成聚合物负载硼、钌络合物催化剂的性能均明显优于对应非负载体.讨论了上述催化剂的聚合物载体效应的机理及温度、溶剂等对活性阴离子聚合反应的影响.