Oscillatory reactions on single crystal surfaces

Heterogeneous catalytic reactions exhibit under certain conditions kinetic oscillations which have been investigated both with polycrystalline materials and with single crystal surfaces as catalysts. The present paper reviews single-crystal experiments conducted under isothermal, low pressure conditions (p < 10^-3 mbar). Two different reaction systems have been investigated: catalytic CO oxidation on various Pt and Pd orientations and catalytic NO reduction on Pt(100) using CO, H₂, or NH₃ as the reducing agent. The different reaction systems exhibit a wide variety of interesting phenomena which are well-known in nonlinear dynamics, for example, such as spatiotemporal pattern formation, the existence of Turing structures and the appearance of deterministic chaos, and chemical turbulence. The mechanistic steps leading to the observed phenomena have been investigated and appropriate mathematical models have been formulated and analyzed using bifurcation theory. The driving force for the rate oscillations has been shown to result from structural changes of the substrate in the case of catalytic CO oxidation on Pt surfaces, subsurface oxygen formation in the case of catalytic CO oxidation on Pd surfaces, and in the chemical reaction network described by a vacancy model in the case of the NO reduction reactions.     

Stochastic resonance in surface catalytic oxidation of carbon monoxide induced by colored noise

Noise is generally considered as a disadvanta-geous factor, which would smear weak signals, there-fore, people always try to reduce its influence. How-ever, recent studies show that, for a nonlinear system, under certain circumstances, noise can enhance sys-tem抯 response to a signal. At specific noise intensity, the response of a system to a weak signal may reach its maximum, which is called 搒tochastic resonance?(SR). The concept of SR was originally put forward by Benzi and his collaborat…     

Canard explosion and internal signal stochastic bi-resonance in the CO oxidation on platinum surface

Canard explosion means a dramatic change fromsmall amplitude quasi-harmonic oscillation to largeamplitude relaxation oscillation, accompanied by anexponential increase of period, of a limit cycle withina very narrow interval of a control parameter. Thisphenomenon was first found in the Van Der Pol equa-tions[1], and later has been found also in chemical[2]and biological[3] systems. Generally speaking, it is theresult of multi-time scales in the system, and can bedealt with singular perturbati…     

Vibrational properties of CO at the Pt(111)-solution interface: the anomalous Stark-tuning slope

Vibrational properties of CO have been studied on Pt(111) in acid and alkaline electrolytes by synchronous measurements of CO oxidation current (0.5 mV/s) and IRAS spectra (one spectrum for every 1 mV). We found that in acid solutions the frequency-tuning rate (dnu(CO)/dE) as well as the potential-dependent bandwidth (dDeltanu1/2/dE) deviates from expected linear relationships. This unusual potential-dependent behavior is interpreted in terms of compression/dissipation of CO islands during the CO oxidation, engendered by competitive adsorption between inactive anions from a supporting electrolyte and the reactive OH species.     

Periodic and random perturbation of catalytic oxidation of CO

When a controllable input is modulated by noise and signal, the response of a nonlinear system may exhibit a synchronized effect, which is referred to as stochastic resonance(SR). With the help of noise, the detection of weak signal may become possible and its signal-to-noise ratio can be increased. A model to describe catalytic oxidation of CO on single crystal was adopted, and its stability was studied by linear analysis method. Through computer simulation, the responses under periodic and random perturbation were analyzed. Stochastic resonance behavior was found in a narrow bistable region, or near the oscillatory region. The results shows more characteristics than those of 1-dimensional system does.     

Low-Temperature Direct Electrochemical Methanol Reforming Enabled by CO-Immune Mo-Based Hydrogen Evolution Catalysts

Hydrogen storage in the form of intermediate artificial fuels such as methanol is important for future chemical and energy applications, and the electrochemical regeneration of hydrogen from methanol is thermodynamically favorable compared to direct water splitting. However, CO produced from methanol oxidation can adsorb to H₂-evolution catalysts and drastically reduce activity. In this study, we explore the origins of CO immunity in Mo-containing H₂-evolution catalysts. Unlike conventional catalysts such as Pt or Ni, Mo-based catalysts display remarkable immunity to CO poisoning. The origin of this behavior in NiMo appears to arise from the apparent inability of CO to bind Mo under electrocatalytic conditions, with mechanistic consequences for the H₂-evolution reaction (HER) in these systems. This specific property of Mo-based HER catalysts makes them ideal in environments where poisons might be present.     

铜电极阳极溶解过程恒电位电流振荡的动力学模型

研究了铜电极在酸性氯化钠溶液中的恒电位电流振荡行为,分析了电极过程中的非线性步骤及电化学耦合因素,提出了一个可能的电极过程动力学模型,并借助线性稳定性分析及分支分析得到了参数坐标空间中的动力学行为区域图。在此基础上,将极化曲线视为稳定非平衡定态区态函（电流）与外控参数（电位）的关系,同时将恒电位电流振荡模拟为稳定极限环振荡,分别计算出了极化曲线与时间－电流振荡曲线,其结果与实验数据相符,表明该类电     

Effects of internal noise for rate oscillations during CO oxidation on platinum surfaces

We have studied the influence of internal noise on the reaction rate oscillation during carbon-monoxide oxidation on single crystal platinum surfaces using chemical Langevin equations. Considering that the surface is divided into small well-mixed cells, we have focused on the dynamic behavior inside a single cell. Internal noise can induce rate oscillations and the performance of the stochastic rate oscillations shows double maxima with the variation of the internal noise intensity, demonstrating the occurrence of internal noise coherent biresonance. The relationship between such a phenomenon with the deterministic bifurcation features of the system is also discussed.     

Particle size dependent adsorption and reaction kinetics on reduced and partially oxidized Pd nanoparticles

Combining scanning tunneling microscopy (STM), IR reflection absorption spectroscopy (IRAS) and molecular beam (MB) techniques, we have investigated particle size effects on a Pd/Fe(3)O(4) model catalyst. We focus on the particle size dependence of (i) CO adsorption, (ii) oxygen adsorption and (iii) Pd nanoparticle oxidation/reduction. The model system, which is based on Pd nanoparticles supported on an ordered Fe(3)O(4) film on Pt(111), is characterized in detail with respect to particle morphology, nucleation, growth and coalescence behavior of the Pd particles. Morphological changes upon stabilization by thermal treatment in oxygen atmosphere are also considered. The size of the Pd particles can be varied roughly between 1 and 100 nm. The growth and morphology of the Pd particles on the Fe(3)O(4)/Pt(111) film were characterized by STM and IRAS of adsorbed CO as a probe molecule. It was found that very small Pd particles on Fe(3)O(4) show a strongly modified adsorption behavior, characterized by atypically weak CO adsorption and a characteristic CO stretching frequency around 2130 cm(-1). This modification is attributed to a strong interaction with the support. Additionally, the kinetics of CO adsorption was studied by sticking coefficient experiments as a function of particle size. For small particles it is shown that the CO adsorption rate is significantly enhanced by the capture zone effect. The absolute size of the capture zone was quantified on the basis of the STM and sticking coefficient data. Finally, oxygen adsorption was studied by means of MB CO titration experiments. Pure chemisorption of oxygen is observed at 400 K, whereas at 500 K partial oxidation of the particles occurs. The oxidation behavior reveals strong kinetic hindrances to oxidation for larger particles, whereas facile oxidation and reduction are observed for smaller particles. For the latter, estimates point to the formation of oxide layers which, on average, are thicker than the surface oxides on corresponding single crystal surfaces.     

单原子催化剂M1/FeOx(M=Pt,Fe)的理论研究:CO氧化反应

负载型纳米贵金属催化剂是用于多相催化反应的重要的催化剂之一,也是各国催化科学与技术研发的重点,其工业应用也越来越广泛.理论和实验的研究结果均表明,当载体表面的金属粒子尺寸减小至亚纳米级乃至更小的低配位、不饱和的原子团簇时,它们常常成为诱发催化反应的活性中心,呈现更高的催化活性和选择性.将负载的金属尺寸由纳米量级减小至分散的金属团簇甚至单原子而使每个原子成为反应的活性位点已成为研究的重点.最近,由张涛等首次合成的单原子催化剂(SAC)Pt1/FeOx引起了国内外催化及表面科学工作者的极大关注.单原子催化剂作为连接均相催化剂和多相催化剂的桥梁,不仅具有非均相催化剂的稳定、易于与反应体系分离、易表征等优点,而且具有均相催化剂活性中心结构均一、活性中心原子利用率百分之百等优点.一方面,单原子催化剂给多相催化领域注入了新的活力,另一方面也更有利于运用量子与计算化学的研究方法建立与实验相匹配的理论模型并从原子水平上进一步理解多相催化反应的微观作用机理.实验和理论的研究结果表明,其它单原子催化剂如Ir1/FeOx,Au1/FeOx和Ni1/FeOx催化CO氧化反应表现出不同的活性.然而,底物FeOx中的Fe同样是第VIII族中的3d过渡金属,却在低温下对CO氧化反应没有催化活性.我们围绕这一问题,重点研究了底物FeOx在负载单原子Pt1前后催化CO氧化的反应机理和活性,解释了单原子催化剂Pt1/FeOx相比于底物FeOx为何具有如此高的催化活性的原因.我们采用Vienna Ab-initio Simulation Package(VASP)从头算模拟软件和密度泛函理论(DFT)的广义梯度近似(GGA)进行了理论计算.其中,选择PBE泛函描述体系的交换关联相互作用,用投影缀加波(PAW)赝势基组方法描述体系中的电子和离子实之间的相互作用,对Fe原子采用了DFT+U方法进行d电子强相关校正,并使用Dimer计算方法搜寻反应过渡态.研究结果表明,底物FeOx中氧空位的再生伴随第二个CO2分子从催化剂表面脱附的过程需要较高的活化势垒(1.09 eV),这一过程是整个CO氧化反应的决速步.与此相比较,Pt1/FeOx催化剂中,由于Pt原子代替了表面Fe原子,导致电子结构及性质的显著变化,有利于CO的活化、氧化和CO2的脱附.我们从电子能量态密度(DOS)和Bader电荷分析及模型分子团簇的轨道相互作用的角度进一步分析了两种催化剂存在差异的本质;揭示了单原子催化剂Pt1/FeOx中Pt1和底物FeOx之间的相互作用的机理及催化剂表面Pt单原子在催化反应过程中的关键作用.     

Electro-oxidation of formic acid catalyzed by FePt nanoparticles

The electrocatalytic oxidation of formic acid at a gold electrode functionalized with FePt nanoparticles was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a mixed solution of 0.1 M HCOOH and 0.1 M HClO4. The FePt bimetallic nanoparticles, with a mean diameter of 3 nm, were prepared by a chemical reduction method. The Au/FePt nanostructured electrode was prepared firstly by the deposition of FePt nanoparticles onto a clean Au electrode surface, followed by ultraviolet ozone treatment to remove the organic coating. In CV measurements, two well-defined anodic peaks were observed at +0.20 and +0.51 V (vs. a Ag/AgCl quasi-reference). The anodic peak at +0.20 V was attributed to the oxidation of HCOOH to CO2 on surface unblocked by CO, whereas the peak at +0.51 V was ascribed to the oxidation of surface-adsorbed CO (an intermediate product of HCOOH oxidation) and further oxidation of bulk HCOOH. From the onset potential and current density of the electro-oxidation of HCOOH, FePt nanoparticles exhibit excellent electrocatalytic activities as compared to Pt and other metal alloys. EIS measurements were carried out to further examine the reaction kinetics involved in the HCOOH electro-oxidation. The EIS responses were found to be strongly dependent on electrode potentials. At potentials more positive than -0.25 V (vs. Ag/AgCl), pseudo-inductive behavior was typically observed. At potentials between +0.3 and +0.5 V, the impedance response was found to reverse from the first quadrant to the second quadrant; such negative Faradaic impedance was indicative of the presence of an inductive component due to the oxidation of surface-adsorbed CO. The impedance responses returned to normal behavior at more positive potentials (+0.6 to +0.9 V). The mechanistic variation was attributed to the formation of different intermediates (CO or oxygen containing species) on the electrode surface in different potential regions. Two equivalent circuits were proposed to model these impedance behaviors.     

Parametric analysis of kinetic models, 8. Two-center catalytic oscillators

The results of parametric analysis for a kinetic model of CO oxidation reaction on two types of active centers of Pt are given. The local bifurcation curves are constructed for every type of center. The regions with self-oscillations are singled out in a plane (p_CO, T). A sum of rate oscillations on every center can have a quasi-chaotic character.     

Relevance of electronic effects on the yield of CO2 from methanol oxidation

The yields of products of methanol oxidation (HCHO, HCOOH, and CO(2)) were studied for carbon-supported PtRu nanoparticles having different amounts of alloyed and oxide phases. It is demonstrated that the increase in the Pt 5d-band vacancy enhances the production of CO(2), which is not directly related with the catalytic activity for CO oxidation. Results prove the relevant role of oxides and, at the same time, shed some new light on mechanistic aspects of methanol oxidation on PtRu nanocatalysts. It is also demonstrated that extrapolating from the behavior of smooth surfaces to nanoparticle systems is not always valid.     

非线性化学反应动力学*

本文从实验和理论的角度综述非线性化学反应动力学。介绍了若干非线性化学反应动力学现象如时钟反应、反馈、混合振荡、化学混沌及反应扩散图案等; 对时空序列、功率谱、Lyapunov 指数、吸引子及其维数、Poincare 截面及分叉理论等常用于分析非线性动力学特征的手段作了介绍; 探讨了对非线性化学反应的实验装置、测试方法、机理和模型研究的方法; 简要回顾了非线性化学反应动力学的发展历史, 并展望了其发展趋势。     

Kinetic model of CO oxidation on heterophase surface analyzed regarding COads spillover, I. Homotopic method. Effect of temperature and CO pressure

The homotopic method has been used to analyze the kinetic models of CO oxidation on two surface patches conjugated by CO_ads spillover. On each patch reaction proceeds via a three-stage mechanism but with different constants. The stability of steady-states solution has been studied. CO_ads spillover from one patch to another changes substantially the bifurcation picture of steady states and produces islands.     

Mechanistic study of electrocatalytic oxidation of formic acid at platinum in acidic solution by time-resolved surface-enhanced infrared absorption spectroscopy

Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltammetry or chronoamperometry has been utilized to examine kinetic and mechanistic aspects of the electrocatalytic oxidation of formic acid on a polycrystalline Pt surface at the molecular scale. Formate is adsorbed on the electrode in a bridge configuration in parallel to the adsorption of linear and bridge CO produced by dehydration of formic acid. A solution-exchange experiment using isotope-labeled formic acids (H(12)COOH and H(13)COOH) reveals that formic acid is oxidized to CO(2) via adsorbed formate and the decomposition (oxidation) of formate to CO(2) is the rate-determining step of the reaction. The adsorption/oxidation of CO and the oxidation/reduction of the electrode surface strongly affect the formic acid oxidation by blocking active sites for formate adsorption and also by retarding the decomposition of adsorbed formate. The interplay of the involved processes also affects the kinetics and complicates the cyclic voltammograms of formic acid oxidation. The complex voltammetric behavior is comprehensively explained at the molecular scale by taking all these effects into account.     

Rh和Pt超薄层电极上的CO吸附行为(英文)

利用沉积在粗糙金电极上的过渡金属超薄层电极技术 ,我们获得了氢及一氧化碳在Rh和Pt表面上吸附的拉曼信号 ,并对两者之间的相互作用进行了分析 ..我们还进行了二氧化碳在这两种金属表面的还原行为的初步研究 ,以及对不同方式获得的一氧化碳吸附拉曼信号的特点进行了分析 .     

Rupture of thin films with resonant substrate patterning

We study the stability and rupture of thin liquid films on patterned substrates. It is shown that striped patterning on a length scale comparable to that of the spinodal instability leads to a resonance effect and an imperfect bifurcation of equilibrium film shapes. Weakly nonlinear analysis gives predictions for film shapes, stability, growth rates, and rupture times, which are confirmed by numerical solution of the thin-film equation. Film behavior is qualitatively different in the resonant patterning regime, but with sufficiently large domains rupture occurs on a spinodal length scale regardless of patterning. Instabilities transverse to the patterning are examined and shown to behave similarly as disturbances to films on uniform substrates. We explain some previously reported effects in terms of the imperfect bifurcation.     

Pt-CNTs 修饰玻碳电极(Pt-CNTs/GC)电氧化活性的研究

采用透射电镜(TEM)和选区电子衍射(SAED)技术, 分别表征了Pt-CNTs/GC电极的表面形貌和所负载铂纳米原子簇的结构. 以CO和CH3OH为探针分子, 用循环伏安和计时电流等常规电化学方法检测了CO和CH3OH在Pt-CNTs/GC电极上的氧化行为. 研究结果表明, CO在Pt-CNTs/GC电极上有3个氧化电流峰(Ⅰ, Ⅱ, Ⅲ), 其中峰Ⅰ为CO桥式吸附的氧化峰, 而峰Ⅱ和Ⅲ则分别为CO线形吸附在碳纳米管负载的不同粒径的Pt纳米原子簇以及Pt原子薄膜上所分裂的氧化峰; CH3OH在Pt-CNTs/GC电极上也能自发解离吸附强吸附中间体CO; Pt-CNTs/GC电极对CH3OH的氧化峰电流不总是随CNTs上载铂量的增加而增大, 表明在制备直接甲醇燃料电池阳极时, 应选择合适的载铂量.