Theoretical study on the effects of internal noise for rate oscillations during CO oxidation on platinum(110) surfaces

Very recently, the effects of internal molecular noise in mesoscopic chemical reaction systems have gained growing attention. Using a mesoscopic stochastic model, the effect of internal noise for rate oscillation during CO oxidation on Pt(110) surface is studied analytically. In a parameter region outside but close to the supercritical Hopf bifurcation, a stochastic normal form is obtained from the chemical Langevin equation. By stochastic averaging procedure, the system is simplified and solvable. Noise-induced oscillation and internal noise coherent resonance (which is related to an optimal system size), observed from simulations, are well reproduced by the theory. The theoretical analysis helps to clearly figure out when and how the internal noise affects the system's oscillating dynamics.     

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.     

Co3O4纳米晶表面晶格氧催化氧化CO的理论研究

近年来,纳米科学技术的迅速发展给催化领域,特别是多相催化带来了新的机遇和挑战.科学家们开始着眼于在纳米尺度上对催化剂结构和催化性能进行表征、控制和设计.Co3O4作为一种重要的半导体金属氧化物材料,由于其优异的氧化还原性质,在锂电池、气体传感器以及多相催化领域得到了十分广泛的应用.最近,研究者发现Co3O4纳米晶在催化CO低温氧化和CH4活化等一系列重要反应中表现出显著的反应活性和晶面效应,表明有效设计和合成特定的高活性、高选择性的纳米晶面,对催化领域的发展将具有十分重要的意义.因而,从原子层面对纳米晶所表现出的这种高活性和晶面效应进行深入解释,将为高效催化剂设计提供重要指导.低温CO氧化作为一种重要的催化反应在燃料电池、空气净化与汽车尾气处理中具有重要的应用价值.本文采用密度泛函理论对Co3O4纳米晶催化CO氧化反应的机理、晶面效应以及结构敏感性进行了理论研究.首先,研究了CO在Co3O4(001)和(011)表面Co,CoOo和Co-Ot三种不同位点的吸附扩散行为,发现CO在Co位点表现出较强的吸附行为,但这种吸附构型需要克服很高的能垒(～1 eV)才能转变到Co-O离子对位点,在低温下这种转变将不可能发生,因此我们推断CO在Co位点的吸附对Co3O4催化CO氧化的晶面效应没有显著影响.接着,对CO在Co-O离子对位点抽提晶格氧生成CO2的反应机理进行了研究.我们发现,(011)表面Co-Ot位点可以较强地吸附CO(吸附能-1.15 eV),并十分容易夺取晶格氧离子(能垒0.26 eV),具有很低的势能面,因而其CO氧化活性明显大于(001)面.为了更清楚地理解这种晶面效应和结构敏感效应的本质,我们提出将CO2形成步的过渡态在反应路径上的能级作为反应活性指标.这种活性指标兼顾考虑了CO在Co-O氧位点的吸附覆盖度和CO2形成步的反应能垒,可以近似理解为反应的表观活化能.据此我们得出,Co3O4不同表面不同品格位点催化CO氧化的反应活性顺序为:(011)-Co-Ot＞＞(001)-Co-Oo＞(011)-Co-Oo＞(001)-Co-Ot.由于CO吸附和CO2形成步都涉及到表面被还原的过程,我们因此发现CO催化氧化活性的高低与表面晶格氧位点的可还原性具有正相关性.这种表面不同位点的还原性可以直接通过对空穴形成能的计算获得,降低表面氧空穴的生成能将有利于提高CO氧化的活性.催化设计的终极目标是在对催化活性位点的本质及反应机理深入认识的基础上在原子层面上对催化剂进行可控设计,从而实现催化剂材料的高效、经济的利用.本文研究表明离子对活性位点是Co3O4纳米晶催化CO氧化反应的活性位点,其中阳离子负责对CO的吸附,阴离子则负责CO2的形成过程,这种协同作用实现了Co3O4纳米晶的高反应活性.我们相信,寻找有效的方法在催化剂表面增加离子对位点活性中心的数目是一种实现高性能催化剂设计的途径.     

Simulation of the effect of surface-oxide formation on bistability in CO oxidation on Pt-group metals

The kinetics of CO oxidation on Pt-group metals are known to often exhibit bistability. During the low-reactive regime observed at relatively high CO pressure, the surface is primarily covered by CO and the reaction rate is controlled by O2 dissociation. During the high-reactive regime at relatively low CO pressure, in contrast, the surface is mainly covered by oxygen and the reaction rate is proportional to CO pressure. In the latter case, the adsorbed oxygen may be in the chemisorbed state and/or may form surface oxide. The experiments indicate that the formation of surface oxide often occurs via the island growth and accordingly should be described in terms of the theory of first-order phase transitions. Here, the author proposes a generic lattice-gas model satisfying this requirement and allowing one to execute the corresponding Monte Carlo simulations. Systematically varying the model parameters determining the oxide stability, he classifies the likely scenarios of the bistable reaction kinetics complicated by oxide formation.     

Structure sensitivity in the oxidation of CO on Ir surfaces

We report results on the catalytic oxidation of carbon monoxide (CO) over clean Ir surfaces that are prepared reversibly from the same crystal in situ with different surface morphologies, from planar to nanometer-scale facets of specific crystal orientations and various sizes. Our temperature-programmed desorption (TPD) data show that both planar Ir(210) and faceted Ir(210) are very active for CO oxidation to form CO2. Preadsorbed oxygen promotes the oxidation of CO, whereas high coverages of preadsorbed CO poison the reaction by blocking the surface sites for oxygen adsorption. At low coverages of preadsorbed oxygen (< or = 0.3 ML of O), the temperature Ti for the onset of CO2 desorption decreases with increasing CO coverage. At high coverages of preadsorbed oxygen (> 0.5 ML of O), T(i) is < 330 K and is independent of CO coverage. Moreover, we find clear evidence for structure sensitivity in CO oxidation over clean planar Ir(210) versus that over clean faceted Ir(210): the CO2 desorption rate is sensitive to the surface morphological differences. However, no evidence has been found for size effects in CO oxidation over faceted Ir(210) for average facet size ranging from 5 to 14 nm. Energetically favorable binding sites for O/Ir(210) are characterized using density functional theory (DFT) calculations.     

Promoter effect of BaO on CO oxidation on PdO surfaces

The effect of bulk BaO promoter on CO oxidation activity of palladium oxide phase was studied by density functional calculations. A series of BaO(100) supported Pd(x)O(y) thin layer models were constructed, and energy profiles for CO oxidation on the films were calculated and compared with corresponding profiles for the most stable PdO bulk surfaces PdO(100) and PdO(101). The most stable of the thin films typically exhibit the same PdO(100) and PdO(101) surface planes; the PdO(100) dominates already with double layer thickness. The supporting promoter improves the CO oxidation activity of the Pd(x)O(y) phase via a direct electronic effect and introduced structural strain and corrugation. Changes in CO adsorption strength are reflected in oxidation energy barriers, and the promoting effect of even 0.3 eV can be seen locally. Easier oxygen vacancy formation may partially facilitate the reaction.     

Active surfaces for CO oxidation on palladium in the hyperactive state

Hyperactivity was previously observed for CO oxidation over palladium, rhodium, and platinum surfaces under oxygen-rich conditions, characterized by reaction rates 2-3 orders higher than those observed under stoichiometric reaction conditions [Chen et al. Surf. Sci. 2007, 601, 5326]. In the present study, the formation of large amounts of CO(2) and the depletion of CO at the hyperactive state on both Pd(100) and polycrystalline Pd foil were evidenced by the infrared intensities of the gas phase CO(2) and CO, respectively. The active surfaces at the hyperactive state for palladium were characterized using infrared reflection absorption spectroscopy (IRAS, 450-4000 cm(-1)) under the realistic catalytic reaction condition. Palladium oxide on a Pd(100) surface was reduced eventually by CO at 450 K, and also under CO oxidation conditions at 450 K. In situ IRAS combined with isotopic (18)O(2) revealed that the active surfaces for CO oxidation on Pd(100) and Pd foil are not a palladium oxide at the hyperactive state and under oxygen-rich reaction conditions. The results demonstrate that a chemisorbed oxygen-rich surface of Pd is the active surface corresponding to the hyperactivity for CO oxidation on Pd. In the hyperactive region, the CO(2) formation rate is limited by the mass transfer of CO to the surface.     

纳米金催化一氧化碳氧化反应的理论研究

近年来,纳米金催化剂独特的催化性质,特别是其优异的低温催化氧化活性,引起了人们极大的研究热情.除低温选择氧化外,在精细化学品合成、大气污染物消除、氢能的转换和利用等领域也开发出了一系列有广泛应用前景的金催化反应.此外,体相金的化学惰性和纳米金的超高活性之间差异的“鸿沟”也引起了理论工作者浓厚兴趣,试图从原理上理解体相金和纳米金活性差异的根源. CO催化氧化是最具有代表性的研究金催化活性的化学反应,本文主要综述了近十多年来金催化 CO氧化反应理论计算方面的研究工作.一般认为, CO在纳米金表面的吸附是 CO氧化反应的初始步骤.密度泛函理论研究表明, CO在金表面的吸附强度主要与被吸附金原子的配位数有关：金配位数越低, CO的吸附能越强,部分研究结果表明两者之间存在近似的线性关系.我们研究发现, CO吸附强度也与被吸附金周围配位金原子的相对位置有关,其中位于正下方的配位金原子加强 CO吸附,而位于侧位的配位金原子则弱化 CO吸附,这显然削弱了 CO吸附与金配位数线性关系的可靠性.理论研究表明,在纯金表
　　面上 O2吸附强度一般很弱,只有在一些特殊结构的金团簇上才有较强的吸附,但在 Au/TiO2界面及 CeO2表面上 O2吸附较强.金表面原子氧的吸附和金的表面结构有关.我们发现,原子氧倾向于在金的表面形成一种线性的 O–Au–O结构以增加其稳定性.当金表面的氧覆盖度增大时,会形成一种金氧化物薄膜结构,其结构依赖于氧的化学势和金的表面结构.纳米金催化 CO氧化反应机理可能因体系、载体等的差异而不同.大部分理论计算结果表明,在纯金表面上 O2很难直接解离形成原子氧,因此反应机理可能是吸附的 CO先与 O2反应形成了一种 CO–O2中间体,然后解离形成 CO2.在 Au/TiO2和 Au/CeO2催化剂上 CO催化氧化机理争议很大,均有计算结果支持 LH机理和 M–vK机理.另外,根据实验上观察到了负载型纳米金能直接活化分子氧的结果,理论上也提出了分子氧先解离为原子氧再与 CO反应的氧解离机理.针对如何解离分子氧问题,人们分别提出了低配位金模型、正方形金结构模型、Ti5c模型及 Au/Ti5c模型等.我们也提出了一种独特的双直线 O–Au–O模型来理解 Au/TiO2或 Au/CeO2界面解离活化分子氧.理论计算结果表明,低配位的金,金和载体之间的电荷转移,以及金所表现出的强相对论效应对于纳米金的活性影响很大.需要特别指出的是,金的强相对论效应有助于理解金表面的 CO吸附与金配位的关系、金表面原子氧的吸附特性、金氧化物薄膜的结构和分子氧的活化等过程.我们认为,金的强相对论作用导致了体相金的化学惰性以及纳米金的活性,因此相对论效应的深入研究将有助于理解金催化 CO氧化反应机理,从而有助于深层次理解纳米金催化活性来源.     

Theoretical study of CO and NO chemisorption on RhCu(111) surfaces

A systematic density functional theory study using periodic models is presented concerning the chemisorption of CO and NO on various sites of RhCu(111) surfaces. The properties of the adsorbed molecules on various mono- and bimetallic sites of these alloy surfaces have been obtained and compared to those corresponding to the pure Rh(111) and Cu(111) surfaces. It is shown that that the interaction of small probe molecules such as CO or NO on RhCu alloys is essentially dominated by the atomic nature of the surface active site with little influence of the rest of the metallic system. Moreover, it is suggested that it is possible to control the adsorption site of these molecules by appropriate choice of the surface composition.     

Theoretical evidence of PtSn alloy efficiency for CO oxidation

The efficiency of PtSn alloy surfaces toward CO oxidation is demonstrated from first-principles theory. Oxidation kinetics based on atomistic density-functional theory calculations shows that the Pt3Sn surface alloy exhibits a promising catalytic activity for fuel cells. At room temperature, the corresponding rate outstrips the activity of Pt(111) by several orders of magnitude. According to the oxidation pathways, the activation barriers are actually lower on Pt3Sn(111) and Pt3Sn/Pt(111) surfaces than on Pt(111). A generalization of Hammer's model is proposed to elucidate the key role of tin on the lowering of the barriers. Among the energy contributions, a correlation is evidenced between the decrease of the barrier and the strengthening of the attractive interaction energy between CO and O moieties. The presence of tin modifies also the symmetry of the transition states which are composed of a CO adsorbate on a Pt near-top position and an atomic O adsorption on an asymmetric mixed PtSn bridge site. Along the reaction pathways, a CO2 chemisorbed surface intermediate is obtained on all the surfaces. These results are supported by a thorough vibrational analysis including the coupling with the surface phonons which reveals the existence of a stretching frequency between the metal substrate and the CO2 molecule.     

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…     

CO oxidation on Ir(111) surfaces under large non-Gaussian noise

In this article we consider the CO oxidation on Ir(111) surfaces under large external noise with large autocorrelation imposed on the composition of the feed gas, both in experiments and in theory. We report new experimental results that show how the fluctuations force the reaction rate to jump between two well defined states. The statistics of the reaction rate depend on those of the external noise, and neither of them have a Gaussian distribution, and thus they cannot be modeled by white or colored noise. A continuous-time discrete-state Markov process is proposed as a suitable model for the observed phenomena. The model captures the main features of the observed fluctuations and can be modified to accommodate other surface reactions and other systems under non-Gaussian external noise.     

Theoretical analysis of the telegraph-like fluctuations in bridged electrochemical contacts

Steady-state current-voltage characteristic of an electrochemical bridged contact and the telegraph noise power are calculated in terms of the model of two redox states. It is shown that the overvoltage dependence of the tunnel current at a constant bias voltage is S-shaped, which was observed in works on the electron tunneling in similar systems. The overvoltage dependence of the contact conductance has a maximum near the equilibrium potential of the bridge-molecule redox group. The overvoltage dependence of the noise power at zero frequency has a maximum whose position is determined by the bias voltage.     

Laser-induced nanoscale superhydrophobic structures on metal surfaces

The combination of a dual-scale (nano and micro) roughness with an inherent low-surface energy coating material is an essential factor for the development of superhydrophobic surfaces. Ultrashort pulse laser (USPL) machining/structuring is a promising technique for obtaining the dual-scale roughness. Sheets of stainless steel (AISI 304 L SS) and Ti-6Al-4V alloys were laser-machined with ultraviolet laser pulses of 6.7 ps, with different numbers of pulses per irradiated area. The surface energy of the laser-machined samples was reduced via application of a layer of perfluorinated octyltrichlorosilane (FOTS). The influence of the number of pulses per irradiated area on the geometry of the nanostructure and the wetting properties of the laser-machined structures has been studied. The results show that with an increasing number of pulses per irradiated area, the nanoscale structures tend to become predominantly microscale. The top surface of the microscale structures is seen covered with nanoscale protrusions that are most pronounced in Ti-6Al-4V. The laser-machined Ti-6Al-4V surface attained superhydrophobicity, and the improvement in the contact angle was >27% when compared to that of a nontextured surface.     

Catalytic spatiotemporal thermal patterns during CO oxidation on cylindrical surfaces: experiments and simulations

Dynamics of spatiotemporal thermal patterns during the catalytic CO oxidation over Pd supported on a glass-fiber catalytic cloth rolled into a tube of 20 mm diameter and 80 mm length has been studied in a continuous flow reactor by IR thermography. A specially designed aluminum mirror built in the reactor provided image of the entire surface of the horizontally held catalytic tube. With flow in the main axial direction and through the tube surface, we observed periodic motions of a pulse, which was born downstream and propagated upstream. The temperature pulse motion was accompanied by conversion oscillations of CO2. With flow in the main axial direction, parallel to the surface, we observed a stationary hot zone after an oscillatory transient. These patterns can be simulated with a plug-flow-reactor-like heterogeneous reactor model that incorporates previously determined kinetic and transport parameters.     

Pt-表层限域的表面FeOx结构用于低温CO氧化反应

通过简单的浸渍-还原并随后在酸性溶液中处理制备了Pt-Cu双金属催化剂. 利用电感耦合等离子体发射光谱、X射线衍射和X射线光电子能谱对不同处理条件下的Pt-Cu纳米粒子的结构和组成进行了表征. 研究发现, Pt-Cu催化剂在高温H₂中还原形成了PtCu₃合金结构, 酸洗处理后形成了包含Pt-骨架(Pt-skeleton)的表面结构和PtCu₃合金核的纳米粒子. 而Pt-骨架结构又可通过在H₂中退火转变为规整的Pt-表层(Pt-skin)结构. Pt-表层表面修饰Fe氧化物后在CO选择氧化反应中表现出较好的催化性能. 通过此方法制备的三金属Pt-Cu-Fe催化剂可达到与Pt-Fe相近的高活性, 并且催化剂中Pt的用量大大降低.     

Theoretical analysis of the internal rotation in aminoborane and borylphosphine

Using a recently proposed orbital deletion procedure and the block-localized wavefunction method, the rotational barriers in H₂BNH₂ and H₂BPH₂ are analyzed in terms of conjugation, hyperconjugation, steric effect and pyramidalization. With the zero-point energy corrections, the π-binding strengths in the planar H₂BNH₂ and H₂BPH₂ are both around 20 kcal/mol at the HF level using the 6-311+G** basis set. With the deactivation of the π atomic orbitals on the boron atom and the evolution from a planar structure to a 90°-twisted structure, the steric repulsion between the B‐H and the N‐H or P‐H is relieved and moreover, the negative hyperconjugation from the lone electron pair or pairs on the nitrogen or phosphorus atoms to the antibonding orbital ^χ* _B H ₂ of the BH₂ group stabilizes the twisted structure by 7.4(8.8) or 4.0(5.0) kcal/mol at the HF/6-31G*(6-311+G**) level. However, the repulsive interaction between the lone pair(s) and the two BH σ bonds is so prominent that the overall steric effect contributes 20.3(22.9) and 19.3(19.8) kcal/mol to the rotational barriers in H₂BNH₂ and H₂BPH₂ with the 6-31G*(6-311+G**) basis set. The present techniques and analyses may also give some clues to justify the parameterization in the empirical molecular mechanics methods. Received: 17 April 1998 / Accepted: 17 September 1998 / Published online: 1 February 1999     

A DFT study of the CO adsorption and oxidation at ZnO surfaces and its implication for CO detection

Recently,ZnO-based gas sensors have been successfully fabricated and widely studied for their excellent sensitivity and selectivity,especially in CO detection.However,detailed explorations of their mechanisms are rather limited.Herein,aiming at clarifying the sensing mechanism,we carried out density functional theory(DFT) calculations to track down the CO adsorption and oxidation on the ZnO(101 0) and(1120) surfaces.The calculated results show that the lattice O of ZnO(1010) is more reactive than that of ZnO(1120) for CO oxidation.From the calculated energetics and structures,the main reaction product on both surfaces can be determined to be CO_2 rather than carbonate.Moreover,the surface conductivity changes during the adsorption and reaction processes of CO were also studied.For both ZnO(1010) and(1120),the conductivity would increase upon CO adsorption and decrease following CO oxidation,in consistence with the reported experimental results.This work can help understand the origins of ZnO-based sensors' performances and the development of novel gas sensors with higher sensitivity and selectivity.     

Template-assisted patterning of nanoscale self-assembled monolayer arrays on surfaces

We report a general, simple, and inexpensive approach to pattern features of self-assembled monolayers (SAMs) on silicon and gold surfaces using porous anodic alumina films as templates. The SAM patterns, with feature sizes down to 30 nm and densities higher than 10(10)/cm(2), can be prepared over large areas (>5 cm(2)). The feature dimensions can be tuned by controlling the alumina template structure. These SAM patterns have been successfully used as resists for fabricating gold and silicon nanoparticle arrays on substrates by wet-chemical etching. In addition, we show that arrays of gold features can be patterned with 10-nm gaps between the dots.     

Theoretical analysis of growth of ZnO nanorods on the amorphous surfaces

Semiconductor nanorod arrays on a substrate have a preferential alignment orientation that minimizes the excessive free energy of the system. In the case of wet chemically synthesized zinc oxide (ZnO) nanorod on the amorphous surfaces, the thermodynamic driving force determines the orientation to be normal to the surface. Among the various kinds of amorphous surfaces, the spherical seed layer composed of ZnO precursors gives isotropic radially aligned arrays. For other surfaces, such as wrinkled and planar ZnO precursor thin film, nanorod arrays are aligned to be perpendicular to the tangential line of the surface. The maximum value of the aspect ratio of the nanorod is determined by the thermodynamic relationship. The number density of nanorods per unit precursor particles decreases with increasing contact angle of the seed particles.