Au_(core)@Pt_(shell)/GC电极上甲醇解离中间体CO氧化的SERS研究

本文采用共焦显微拉曼系统对不同介质中甲醇解离中间体CO在Aucore@Ptshell/GC电极上的氧化行为进行了研究。结果显示,不论在酸性、中性还是碱性介质中,甲醇均能在Aucore@Ptshell/GC电极上自发氧化解离出强吸附中间体CO;较低电位下,CO在酸性和中性介质中以线性吸附为主,碱性介质中则以桥式吸附为主。此研究表明,电极在中性及碱性介质中对甲醇解离中间体CO的电氧化比在酸性介质中有更好的催化活性,原位表面增强拉曼光谱技术有望拓展成为研究电催化反应的有效工具。     

A complex impedance study of Pt∣NaCl and C∣NaCl interfaces

The a.c. impedance of Pt∣NaCl∣Pt and C∣NaCl∣C cells was measured in the frequency range from 10^?3 to 10³ Hz. Two contributions to the interface impedance could be distinguished, hindered charge transfer and an adsorption/desorption or slow reaction contribution, the latter being the rate determining step in the frequency range studied. An approximately symmetric nonlinear dependence of both the capacitance and the conductance on overvoltage was found at the lowest frequencies, indicating that the crystal/electrode interface is in a state which can be characterized as close to equilibrium. It should be ensured by two electrochemical reactions of approximately equal rates. The concept of a temporarily reversible electrode is introduced. It is a result of the ability of ions to discharge at the interface and to produce a finite reservoir of adsorbed or absorbed reaction products. An external voltage increases or reduces this reservoir, and therefore only a transient electric current may be carried by a backward reaction.     

The influence of adsorption phenomena on the impedance spectroscopy of an electrolytic cell

In this work we investigate the influence of the adsorption of ions on the impedance spectroscopy of an electrolytic cell. We consider that the positive and negative ions present in a dielectric liquid are adsorbed in the electrode surfaces with different adsorption energies. This difference in adsorption energies causes an additional plateaux in the limit of the low-frequency range of the real part of the impedance Z . In the same frequency range, a second minimum in the imaginary part of Z is predicted. The theory is illustrated with measurements of the impedance of an electrolytic solution in the frequency range from 10^-2Hz to 1KHz. A comparison between the present model and others from the literature to describe the experimental results is also made.     

Stochastic model of adsorption of particles on macromolecules at an arbitrary filling

The theory of adsorption of particles on macromolecules is considered. The theory is based on the notion that the adsorption is a discrete Markovian process. In the context of this theory the average number and the variance of the number of ligands adsorbed on macromolecules are calculated exactly. The results are compared with the frequently used approximation where the number of adsorbed particles is smaller than that in the solution. It is shown that for small values of the equilibrium constant the exact and approximate values coincide practically.     

An “electrocapillary” study of the gold-perchloric acid solution interface

The bending experienced by a thin elastic electrode insulated on one side is used to obtain “electrocapillary” data on the gold-perchloric acid solution interface. The technique yields self-consistent and numerically reasonable values of the charge on the electrode and the cationic and anionic Gibbs' surfaces excesses. The results are consistent with a specific adsorption of perchlorate ions which is represented by an isotherm of Temkin type. Hydroxonium ions are not specifically adsorbed. The data are limited in number, the numerical value of Young's modulus for the system lacks accuracy : hence the uncertainty of the data is high.     

利用交流阻抗谱揭示Ir(111)电极在酸性溶液中的氢吸附动力学

本文利用阻抗谱研究Ir(111)电极在HClO₄和H₂SO₄中溶液中的氢吸附行为. 在HClO₄溶液中,随着施加电位从0.2 V降到0.1 V(vs RHE),Ir(111)电极上氢吸附速率从1.74×10^-8 mol·cm^-2·s^-1增大到 3.47×10^-7 mol·cm^-2·s^-1 . 与相同条件下Pt(111)电极上的氢吸附速率相比,Ir(111)上的氢吸附速率要小1∽2个数量级,这是由于Ir(111)电极与H₂O结合能力更强,因此位于水合氢键网络中的氢离子需要克服更高的能垒才能重新定向进而发生欠电位沉积. 在H₂SO₄溶液中,氢吸附电位负移了200 mV,吸附速率也下降了一个数量级,这是由于Ir(111)电极表面强吸附的硫酸根/硫酸氢根物种的阻碍作用. 结果表明,在电化学环境下,位于电极表面附近的水分子的取代和重新定向在很大程度上影响了氢吸附过程.     

不同pH酸性溶液中铂电极上氢析出和二氧化碳还原的电化学研究

本文利用循环伏安法和电化学原位红外光谱的联用,研究了Pt(111)和Pt膜电极在CO₂饱和的酸性溶液中氢析出和CO₂还原的竞争. 发现：(i)在pH>2的溶液中,主要反应是氢析出,界面pH值随着氢析出突然增加;(ii)通过红外光谱检测,CO_ad是CO₂还原过程中唯一的吸附中间体;(iii)CO_ad生成速率随着欠电位沉积氢(UPD-H)覆盖的增加而增大,并在氢析出的起始电位达到最大值;(iv)在氢析出时,CO_ad的减少与CO₂吸附和还原所必需的的中间产物(H_ad)有限的可用位点和停留时间相关.     

The special features of adsorption and the kinetics of decomposition of monosilane molecules on the epitaxial surface of silicon

Analytic equations relating the rate of the incorporation of silicon atoms into a growing crystal to the characteristic frequency of the pyrolysis of silane molecules on the surface of silicon were obtained over the temperature range corresponding to the epitaxial growth of silicon films. As distinct from the earlier works, it was assumed that adsorbed silicon atoms and monosilane molecules formed double bonds with the surface. The data of technological experiments for the most extensively used pyrolysis models obtained thus far were used to determine the region of the characteristic frequencies of the decomposition of hydride molecule radicals adsorbed on the surface of a silicon plate over the temperature range 450–700°C. The temperature dependence of the frequency of monosilane molecule decomposition was shown to be to a great extent determined by the form of the temperature dependence of the $ \tilde v_{SiH_2 }^0 $ \tilde v_{SiH_2 }^0 preexponential factor. It was also found that the characteristic frequency of the decomposition of silane molecules was sensitive to the stage of pyrolysis at which hydrogen atoms released from silane molecules were captured by the surface. Decomposition occurred at the highest rate if hydrogen molecules were adsorbed at the stage of the adsorption of monosilane. The lowest rate of decomposition was observed if hydrogen molecules were adsorbed at the stage of the decomposition of radicals already captured by the surface. The temperature dependence of the coefficient of adsorption of monosilane molecules was characterized by a negative activation energy of the process for almost all the most important system models over the temperature range of growth. At elevated growth temperatures, the adsorption of monosilane molecules by the surface of silicon proceeded via an intermediate state characterized by the difference of desorption and chemisorption energies on the order of 0.28 eV.     

Adsorption of firefly luciferase at interfaces studied by total internal reflection fluorescence spectroscopy

The adsorption of luciferase onto silica surfaces was studied by total internal reflection fluorescence (TIRF) spectroscopy. Two model surfaces were used: hydrophilic and hydrophobic silica. Luciferase adsorbed differently on these two surfaces. Initial kinetics of luciferase adsorption onto the hydrophilic surface showed that luciferase adsorbs over an adsorption energy barrier of 3 kT The quantum yield of luciferase fluorescence decreased at the hydrophilic silica surface, which indicated that the protein conformation was altered during adsorption. Luciferase adsorption onto the hydrophobic silica surface proceeded with a small adsorption energy barrier and the fluorescence efficiency of adsorbed protein remained unchanged after adsorption. The affinity of luciferase for luciferin was measured using quenching of luciferase fluorescence with luciferin. The binding constant of the adsorbed luciferase-luciferin complex at the hydrophilic silica surface was two orders of magnitude smaller than the respective binding constant in the solution. Adsorbed luciferase showed an absence of ATP-dependent visible luminescence, indicating that the adsorbed enzyme was not active at either of the two silica surfaces.     

Diffraction of sound by an elastic or impedance sphere located near an impedance or elastic boundary of a halfspace

An exact solution is obtained to the problem of sound diffraction by an elastic or impedance sphere located near an impedance or elastic boundary of a halfspace. The problem is solved using the Helmholtz integral equation in which the field of a point source in the halfspace with an elastic boundary is used as the Green function. The diffracted field is represented as a series expansion in spherical harmonics. The expansion coefficients are determined from a set of independent algebraic systems of equations. The matrix coefficients of these systems are determined as integrals of the products of the associated Legendre polynomials on the complex plane with respect to the real and complex angles of the sound incidence on the halfspace boundary. To decrease the number of such integrals, expansions using the Klebsh-Gordon coefficients are applied. As a result, algorithms for calculating the scattered field in the halfspace are obtained.     

Electrode reaction and microstructure of La0.6Sr0.4CoO3−δ thin films

Dense La_0.6Sr_0.4CoO_3−δ film electrodes were deposited by pulsed laser deposition (PLD) on Ce_0.9Gd_0.1O_1.95 electrolytes. The grain size of one film was 300–500 nm, and the other was 30–50 nm. DC polarization and AC impedance measurements were performed at 873 K–1073 K in O₂–Ar gas mixtures. From investigations of the electrochemical capacitances, the rate determining process for both electrodes were confirmed to be the surface reaction. The analyses in the electrochemical resistance revealed that the oxygen adsorption/desorption rate was faster on the electrode with smaller grain size. DC responses agreed with AC results, so the current density on the nano-grain electrode was larger by half an order than those of the sub-micron-grain electrode. Under a dilute oxygen atmosphere, the rate determining step transferred from a surface reaction to a gas phase diffusion.     

Capacitance dispersion in electrochemical impedance spectroscopy measurements of iodide adsorption on Au(1 1 1)

Electrochemical interfaces that display dispersive characteristics do not present the purely capacitive behaviour predicted by the theory of ideally polarised interfaces. For interfaces involving solid electrodes, capacitance dispersion phenomena in the double layer region are usually attributed to the structural characteristics of the electrode surface as well as to the interfacial region. This paper presents a study of the dispersive characteristics, in the double layer potential region, related to the iodide adsorption on an Au(1 1 1) electrode. The study was performed by using electrochemical impedance spectroscopy, and the corresponding spectra fitted with an equivalent circuit containing a constant phase element (CPE). The fitting results are compared with capacitance curves obtained by chronocoulometry, in order to analyse the relationship between the CPE and the interfacial capacitance. It was observed the occurrence of dispersive behaviour in the potential regions associated with phase transition processes in the adsorbed layer and to the potential induced reconstruction phenomena. On the other hand, in the potential regions where such phenomena do not occur, the interface presents almost pure capacitive behaviour. These observations provide evidence of the strong contribution of the solution properties to the capacitance dispersion.     

Evolution of imitation networks in Minority Game model

The Minority Game is adapted to study the “imitation dilemma”, i.e. the tradeoff between local benefit and global harm coming from imitation. The agents are placed on a substrate network and are allowed to imitate more successful neighbours. Imitation domains, which are oriented trees, are formed. We investigate size distribution of the domains and in-degree distribution within the trees. We use four types of substrate: one-dimensional chain; Erd?s-Rényi graph; Barabási-Albert scale-free graph; Barabási-Albert 'model A' graph. The behaviour of some features of the imitation network strongly depend on the information cost epsilon, which is the percentage of gain the imitators must pay to the imitated. Generally, the system tends to form a few domains of equal size. However, positive epsilon makes the system stay in a long-lasting metastable state with complex structure. The in-degree distribution is found to follow a power law in two cases of those studied: for Erd?s-Rényi substrate for any epsilon and for Barabási-Albert scale-free substrate for large enough epsilon. A brief comparison with empirical data is provided.     

STUDY OF HOMOGENEOUS ADSORPTION KINETICS OF CRYSTAL VIOLET

The adsorption kinetica of crystal violet (CV) on silver mirror and two different kinds of silver sols is studied. The adsorption of CV on silver is homogeneous. It is confirmed that the topography of the surface affects only adsorption rate constant, while it does not change heterogeneity parameter. The similar behavior of enhancement factor and adsorption rate constant of CV in the two silver sols is discussed. The adsorption energy of CV and the concentration of adsorbed sites on the surface are estimated by adsorption isotherm. It is found that the enhancement effect on the silver mirror is determined by the concentration of adsorbed sites on surface.     

Effect of longitudinal heat conduction on the catalytic ignition of carbon monoxide in a boundary layer

The catalytic ignition of dry carbon monoxide and air in a boundary layer flow over a palladium plate is studied in this paper. The heterogeneous reaction mechanism is modelled with the dissociative adsorption of the molecular oxygen and the non-dissociative adsorption of CO, together with a surface reaction of the Langmuir-Hinshelwood type and the desorption reaction of the adsorbed product, CO₂(s). The critical condition for catalytic ignition, represented by the ignition Damköhler number, has been deduced using high activation energy asymptotics of the desorption kinetics of the most efficiently adsorbed reactant, CO(s). Longitudinal heat conduction along the plate has been considered and its influence on the ignition temperature has been evaluated. This influence is rather weak, indicating that the flat plate boundary layer flow configuration is a robust device to determine the critical conditions for catalytic ignition.     

Reaction kinetics of the Pt, O2(g)|c-ZrO2 system: precursor-mediated adsorption

A micro kinetic model of the Pt, O₂(g)|c-zirconia electrode/electrolyte system was developed in state space form (model M3). The oxygen adsorption/desorption process was modeled as a precursor-mediated surface reaction. The surface diffusion of atomic oxygen and the electrochemical reduction of atomic oxygen near the three-phase boundary (tpb) were considered. It was shown that the simulated charge-transfer behavior of M3 is significantly different from models with ordinary Langmuir kinetics (model M2). The electrochemical rate constant was estimated from selected experimental data as k₁₀=(6.05±0.25)·10⁶ m³/(mol·s). From experimental results it was concluded that only one adsorbed oxygen species is relevant for the dynamic behavior. In porous Pt electrodes binary gas phase diffusion of oxygen in O₂/N₂ gas mixtures becomes relevant at oxygen partial pressures below 10⁻³ atm. The general procedure for state and parameter estimation can be well adopted for the investigation of further reaction mechanisms.     

Thermal desorption study of Oxygen adsorption on Pt, Ag & Au films deposited on YSZ

The Thermal Desorption or Temperature Programmed Desorption (TPD) technique has been used for the study of oxygen adsorption on Pt, Ag and Au catalyst films deposited on YSZ. The catalyst film was deposited on the one side of the YSZ specimen while on the other side gold counter and reference electrodes were deposited, constructing a three-electrode electrochemical cell similar to those used in Electrochemical Promotion studies. Oxygen adsorption has been carried out either by exposing the samples to gaseous oxygen (gas phase adsorption) or by the application of a constant current between the catalyst/working electrode and the counter electrode (electrochemical adsorption) or by mixed gas phase and electrochemical adsorption. Oxygen adsorption was carried out at temperatures between 200 and 480 °C. After exposure to gaseous oxygen, normal adsorbed atomic oxygen species have been observed on Pt and Ag surfaces while there was no detectable amount of adsorbed oxygen on Au. Electrochemical O²⁻ pumping to Pt, Ag and Au catalyst films creates strongly bonded “backspillover” anionic oxygen, along with the more weakly bonded atomic oxygen. Electrochemical O²⁻ pumping to Pt, Ag and Au catalyst films in presence of preadsorbed oxygen creates strongly bonded “backspillover” anionic oxygen, with a concomitant pronounced lowering of the T_p of the more weakly bound preadsorbed atomic oxygen. The two oxygen species co-exist on the surface. The activation energy for oxygen desorption or, equivalently, the binding strength of adsorbed oxygen was found to decrease linearly with increasing catalyst potential, for all three metal electrodes. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland Sept. 13–19, 1997     

Bringing statistical mechanics into chemistry: The early scientific work of Karl F. Herzfeld

The physicist Karl F. Herzfeld was instrumental in introducing the techniques of statistical mechanics to physical chemists in the years between 1920 and 1935. Herzfeld's work included an early derivation of the rate constant for a first order reaction. This was similar to and fifteen years earlier than Eyring's calculation of the rate constant. Herzfeld also carried out the first calculation of a relaxation equation for vibrational energy transfer. Herzfeld's success lay in his classical approach to physics, which, it is suggested, made his work accessible to chemists.     

The influence of weakly bound intermediate states on thermal desorption kinetics

Following a suggestion of Langmuir, models for adsorption kinetics have been proposed over the years which assume that a molecule approaching a surface is initially adsorbed into a mobile second layer in which migration could occur until an empty site is found. In this paper a model for desorption kinetics is developed which acknowledges the existence of this second layer, or intermediate state. The influence of this intermediate state on desorption kinetics is illustrated through computed desorption spectra, the major features being a broadening of the desorption peak and a shift towards higher peak temperatures. It is shown that under certain conditions lineshape analysis of desorption spectra made by ignoring the role of the intermediate state can lead to very large errors in the derived values for both desorption energies and rate constant pre-exponentials. Additionally, even for first order desorption a coverage dependence in the peak temperature is introduced.     

Effect of electrode microstructure on gas-phase diffusion in solid oxide fuel cells

The relation between electrode microstructure and gas diffusion has been investigated with different morphologies of Pt electrodes by using AC impedance techniques. The measurements were carried out at temperatures of 873–1273 K and oxygen partial pressure (P_O2) of 0.01–1 atm.Gas-phase diffusion was observed only for high-performance electrodes at the high-temperature (1073–1273 K) and low-oxygen-partial-pressure regions (<0.1 atm P_O2). Considering the physical and electrochemical characteristics of impedance arcs, it was found that the arc at the frequency of below 1 Hz was related to gas conversion resistance, while the arc at the frequency of around 10 Hz represented pore diffusion resistance through the current-collecting part. For a thick electrode with a low porosity, however, gas diffusion resistance through pores of an electrode was observed at a frequency of around 100 Hz.From the results of a comparison of electrode performances with different electrode microstructures, electrochemical reaction sites (ERS) are supposed to be located at the peripheral line of Pt and YSZ as well as the Pt/YSZ interfaces where reaction gas can easily diffuse.