In-situ infrared study of the adsorption and oxidation of oxalic acid at single-crystal and thin-film gold electrodes: a combined external reflection infrared and ATR-SEIRAS approach

The adsorption and oxidation of oxalic acid at gold electrodes were studied by in-situ infrared spectroscopy. External reflection experiments carried out with gold single-crystal electrodes were combined with internal reflection (ATR-SEIRAS) experiments with gold thin-film electrodes. These gold thin films, with a typical thickness of ca. 35 nm, were deposited on silicon substrates by argon sputtering. As previously reported for evaporated gold films, the voltammetric curves obtained in sulfuric acid solutions after electrochemical annealing show typical features related to the presence of wide bidimensional (111) domains with long-range order. The in-situ infrared data collected for solutions of pH 1 confirmed the potential-dependent adsorption of either oxalate (Au(100)) or a mixture of bioxalate and oxalate (Au(111), Au(110), and gold thin films) anions in a bidentate configuration. The better signal-to-noise ratio associated with the SEIRA effect in the case of the gold thin-film electrodes allows the observation of the carbonyl band for adsorbed bioxalate that was not detected in the external reflection experiments. Besides, additional bands are observed between 2000 and 3000 cm(-)(1) that can be tentatively related to the formation of hydrogen bonds between neighboring bioxalate anions. The intensities of these bands decrease with increasing solution pH values, disappearing for pH 3 solutions in which adsorbed oxalate anions are the predominant species. The analysis of the intensities of the nu(s)(O-C-O) and nu(C-OH) + delta(C-O-H) bands for adsorbed oxalate and bioxalate, respectively, suggests that the pK(a) for the surface equilibrium between these species is significantly lower than that for the solution equilibrium.     

Hydrogen electrocatalysis on overlayers of rhodium over gold and palladium substrates--more active than platinum?

We have investigated the stability and catalytic activity of epitaxial overlayers of rhodium on Au(111) and Pd(111). Both surfaces show a strong affinity for hydrogen. We have calculated the energy of adsorption both for a strongly and a more weakly adsorbed species; the latter is the intermediate in the hydrogen evolution reaction. Both the energy of activation for hydrogen adsorption (Volmer reaction) and hydrogen recombination (Tafel reaction) are very low, suggesting that these overlayers are excellent catalysts.     

Imaging the Bonds of Dehalogenated Benzene Radicals on Cu(111) and Au(111)

Dissociative adsorption of doubly substituted benzene molecules leads to formation of benzyne radicals. In this study, co‐adsorbed hydrogen molecules are used in scanning tunneling hydrogen microscopy to enhance the contrast of the meta‐ and the para‐isomers of these radicals on Cu(111) and Au(111). Up to three hydrogen molecules are attached to one radical. One hydrogen molecule reveals the orientation of the carbon ring and its adsorption site, allowing discrimination between the two radicals. Two hydrogen molecules reflect the bond picture of the carbon skeleton and reveals that adsorption on Cu(111) distorts the meta‐ isomer differently from its gas‐phase distortion. Three hydrogen molecules allow us to determine the bond picture of a minor species.     

A First‐Principle Calculation of Sulfur Oxidation on Metallic Ni(111) and Pt(111), and Bimetallic Ni@Pt(111) and Pt@Ni(111) Surfaces

Sulfur, a pollutant known to poison fuel‐cell electrodes, generally comes from S‐containing species such as hydrogen sulfide (H₂S). The S‐containing species become adsorbed on a metal electrode and leave atomic S strongly bound to the metal surface. This surface sulfur is completely removed typically by oxidation with O₂ into gaseous SO₂. According to our DFT calculations, the oxidation of sulfur at 0.25 ML surface sulfur coverage on pure Pt(111) and Ni(111) metal surfaces is exothermic. The barriers to the formation of SO₂ are 0.41 and 1.07 eV, respectively. Various metals combined to form bimetallic surfaces are reported to tune the catalytic capabilities toward some reactions. Our results show that it is more difficult to remove surface sulfur from a Ni@Pt(111) surface with reaction barrier 1.86 eV for SO₂ formation than from a Pt@Ni(111) surface (0.13 eV). This result is in good agreement with the statement that bimetallic surfaces could demonstrate more or less activity than to pure metal surfaces by comparing electronic and structural effects. Furthermore, by calculating the reaction free energies we found that the sulfur oxidation reaction on the Pt@Ni(111) surface exhibits the best spontaneity of SO₂ desorption at either room temperature or high temperatures.     

Electrochemical and FTIRS characterisation of NO adlayers on cyanide-modified Pt(111) electrodes: the mechanism of nitric oxide electroreduction on Pt

We report here a study, using cyclic voltammetry and FTIRS, of NO irreversibly adsorbed on a cyanide-modified Pt(111) electrode. NO adlayers were formed by immersion of the cyanide-modified Pt(111) electrode in an acidic solution of KNO(2). The behaviour of NO adsorbed on the cyanide-modified electrode is very similar to that of NO on the clean Pt(111) surface, suggesting that adsorbed cyanide (saturation coverage theta(CN) = 0.5) behaves simply as a third body, blocking some of the surface sites but leaving the free Pt sites unaffected. Comparison of the voltammetric profile for NO electroreduction on Pt(111) and on cyanide-modified Pt(111) electrodes has allowed us: (i) to confirm that the reduction of three-fold hollow NO and atop NO on Pt(111) electrodes occurs in two distinct reduction peaks, as previously proposed by Rosca et al. (Langmuir, 2005, 21, 1448); (ii) to suggest that the reduction of irreversibly adsorbed NO layers on Pt electrodes can proceed through two possible paths, one involving an EE mechanism in which the rate-determining step (rds) is an Eley-Rideal reaction, with a direct proton transfer from the solution to adsorbed NO, and the other involving an EC mechanism in which the rds is a Langmuir-Hinshelwood reaction of adsorbed NO with adsorbed H. The availability of adsorbed hydrogen determines which path is followed by the reaction; (iii) to identify the smallest atomic ensemble for the reduction of NO on Pt as being composed of two adjacent Pt atoms.     

Ordered molecular assemblies of substituted bis(phthalocyaninato) rare earth complexes on Au(111): in situ scanning tunneling microscopy and electrochemical studies

Substituted bis(phthalocyaninato) rare earth complexes ML2 (M = Y and Ce; L = [Pc(OC8H17)8]2, where Pc = phthalocyaninato) were adsorbed onto single crystalline Au(111) electrodes from benzene saturated with either YL2 or CeL2 complex at room temperature. In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to examine the structures and the redox reactions of these admolecules on Au(111) electrodes in 0.1 mol dm(-3) HClO4. The CVs obtained with YL2- and CeL2-coated Au(111) electrodes respectively contained two and three pairs of redox peaks between 0 and 1.0 V (versus reversible hydrogen electrode). STM molecular resolution revealed that YL2 and CeL2 admolecules were imaged as spherical protrusions separated by 2.3 nm, which suggests that they were oriented with their molecular planes parallel to the unreconstructed Au(111)-(1 x 1). Both molecules when adsorbing from approximately micromolar benzene dosing solutions produced mainly ordered arrays characterized as (8 x 5 radical3)rect (theta = 0.0125). The redox reactions occurring between 0.2 and 1.0 V caused no change in the adlayer, but they were desorbed or oxidized at the negative and positive potential limits. The processes of adsorption and desorption at the negative potentials were reversible to the modulation of potential. Electrochemical impedance spectroscopy (EIS) and CV measurements showed that YL2 and CeL2 adlayers could block the adsorption of perchlorate anions and mediating electron transfer at the Au(111) electrode, leading to the enhancement of charge transfer for the ferro/ferricyanide redox couple.     

An investigation of the oxidation of formaldehyde on noble metal electrodes in alkaline solutions by electrochemically modulated infrared spectroscopy (emirs)

In-situ spectroscopy was used to detect adsorbed species formed during the oxidation of formaldehyde at polycrystalline Au, Pt, Rh and Ir electrodes in alkaline solution. At the gold electrode, the spectra showed the oxidation of the gem-diol form of formaldehyde and the formation of an adsorbed formate species. At platinum, rhodium and iridium, adsorbed CO was observed in the hydrogen adsorption region where the oxidation of formaldehyde is strongly inhibited on these metals. Alternative routes for producing the poison are discussed.     

Cyclotrimerization‐Induced Chiral Supramolecular Structures of 4‐Ethynyltriphenylamine on Au(111) Surface

Cyclotrimerization‐induced chiral supramolecular structures of 4‐ethynyltriphenylamine (ETPA) have been synthesized on the Au(111) surface through alkyne‐based reactions. Whereas the ETPA molecules adsorbed on the Au(111) surface remain inert and form a close‐packed self‐assembled structure at room temperature, the combination of scanning tunneling microscopy observations and theoretical calculations unambiguously reveal that the ETPA molecules cyclotrimerize to form new trimer‐like species—1,3,5‐tris[4‐(diphenylamino)phenyl]benzene (TPAPB)—after annealing at 323 K. Further annealing drives these cyclotrimerized TPAPB molecules to form chiral hexagonal supramolecular structures with an extraordinary self‐healing ability.     

A Periodic Density Functional Study on Adsorption Properties of Methoxy on Au（111） Surface

1INTRODUCTION Methoxy(CH3O)has been identified as the first intermediate in the decomposition of methanol on extensive list of clean transition metal surfaces,such as Ni(100)[1],Cu(100)[2,3],Cu(111)[4],Ag(111)[5],Au(110)[6],Pd(111)[7]and Ru(0001)[8].The electronic structure of the metal is a determining factor in OH bond scission.In fact,group IB clean surfaces have shown very low activity towards this reaction,al-though there are reports on low amounts of methoxy formed on clean Cu(…     

Electrocatalytic properties of Au(111)-Pd quasi-single-crystal film electrodes as probed by ATR-SEIRAS

Electrochemical and electrocatalytic properties of thin films Au(111-25 nm), which are quasi-single-crystal electrodes 25 nm thick made of gold with the (111) preferential orientation, and same electrodes modified with a monolayer (ML) of palladium are studied in 0.1 M solutions of HClO₄ and H₂SO₄ employing voltammetric techniques and surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS). Spectroscopic experiments demonstrate strong adsorption of electrolyte species (H₂O, OH_ads, anions) on the Pd surface. The weak and reversible adsorption of CO on Au(111-25 nm) does not change the interfacial-water structure. Adsorption of CO on the Pd-modified film results in an irreversibly adsorbed CO adlayer stabilized by co-adsorbed isolated water species. Various electrooxidation mechanisms are discussed. Electrochemical and spectroscopic investigations on the adsorption and electrooxidation of HCOOH on bare and 1 ML Pd-Au(111-25 nm) electrodes reveal that electrooxidation proceeds in both cases via a direct or dehydrogenation pathway. This mechanism involves the formation of formate as intermediate, which is detected by in situ ATR-SEIRAS. The reactivity on Pd-modified surfaces is higher than on bare gold. The specifically adsorbed anions (sulfate/bisulfate) and the oxide formation on the substrate surface lower the reactivity for CO and HCOOH on both surfaces. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1312–1329. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Transient hydroxyl formation from water on oxygen-covered Au(111)

We present evidence for the formation of transient hydroxyls from the reaction of water with atomic oxygen on Au(111) and investigate the effect of adsorbed oxygen on the hydrogen bonding of water. Water is evolved in peaks at 175 and 195 K in temperature programed reaction experiments following adsorption of water on oxygen-covered Au(111). The peak at 175 K is ascribed to sublimation of multilayers of water, whereas the peak at 195 K is associated with oxygen-stabilized water or a water-hydroxyl surface complex. Infrared reflection absorption spectra are consistent with the presence of molecular water over the entire range of coverages studied, indicating that isolated stable hydroxyls are not formed. Isotopic exchange of adsorbed (16)O with H(2)(18)O following adsorption and subsequent temperature programed reaction, however, indicates that transient OH species are formed. The extent of oxygen exchange was considerable--up to 70%. The degree of oxygen exchange depends on the initial coverage of oxygen, the surface temperature when preparing oxygen adatoms, and the H(2)(18)O coverage. The hydroxyls are short-lived, forming and disproportionating multiple times before water desorption during temperature programed reaction. It was also found that chemisorbed oxygen is critical in the formation of hydroxyls and stabilizing water, whereas gold oxide does not contribute to these effects. These results identify transient hydroxyls as species that could play a critical role in oxidative chemical reactions on gold, especially in ambient water vapor. The crystallinity of adsorbed water also depended on the degree of surface ordering and chemical modification based on scanning tunneling microscopy and infrared spectra. These results demonstrate that oxidation of interfaces has a major impact on their interaction with water.     

贵金属原子在CeO2(111)表面吸附的密度泛函理论研究

利用密度泛函理论系统研究了贵金属原子(Au、Pd、Pt和Rh)在CeO₂(111)表面的吸附行为。结果表明,Au吸附在氧顶位最稳定,Pd、Pt倾向吸附于氧桥位,而Rh在洞位最稳定。当金属原子吸附在氧顶位时,吸附强度依次为Pt > Rh > Pd > Au。Pd、Pt与Rh吸附后在Ce 4f、O 2p电子峰间出现掺杂峰;Au未出现掺杂电子峰,其d电子峰与表面O 2p峰在-4～-1 eV重叠。态密度分析表明,Au吸附在氧顶位、Pd与Pt吸附在桥位、Rh吸附在洞位时,金属与CeO₂(111)表面氧原子作用较强,这与Bader电荷分析结果相一致。     

Electrochemistry of self-assembled monolayers of the blue copper protein Pseudomonas aeruginosa azurin on Au(111)

We report the self-assembly and electrochemical behaviour of the blue copper protein Pseudomonas aeruginosa azurin on Au(111) electrodes in aqueous acetate buffer (pH=4.6). The formation of monolayers of this protein is substantiated by electrochemical measurements. Capacitance results indicate qualitatively that the protein is strongly adsorbed at sub-μM concentrations in a broad potential range (about 700 mV). This is further supported by the attenuation of a characteristic cyclic voltammetric peak of Au(111) in acetate solution with increasing azurin concentration. Reductive desorption is clearly disclosed in NaOH solution (pH=13), strongly suggesting that azurin is adsorbed via its disulphide group. An anodic peak and a cathodic peak associated with the copper centre of azurin are finally observed in the differential pulse voltammograms. These peaks are, interestingly, indicative of long-range electrochemical electron transfer such as paralleled by intramolecular electron transfer between the disulphide anion radical and the copper atom in homogeneous solution, and anticipated by theoretical frames. Together with reported in situ scanning tunnelling microscopy (STM) results they constitute the first case for electrochemistry of self-assembled monolayers of azurin, even redox proteins. This integrated investigation provides a new approach to both structure and function of adsorbed redox metalloproteins at the molecular level.     

In situ STM imaging and direct electrochemistry of Pyrococcus furiosus ferredoxin assembled on thiolate-modified Au111 surfaces

We have addressed here electron transfer (ET) of Pyrococcus furiosus ferredoxin (PfFd, 7.5 kDa) in both homogeneous solution using edge plane graphite (EPG) electrodes and in the adsorbed state by electrochemistry on surface-modified single-crystal Au111 electrodes, This has been supported by surface microscopic structures of PfFd monolayers, as revealed by scanning tunneling microscopy under potential control (in situ STM). Direct ET between PfFd in phosphate buffer solution, pH 7.9, and EPG electrodes is observed in the presence of promoters. Neomycin gives rise to a pair of redox peaks with a formal potential of ca -430 mV (vs SCE), corresponding to [3Fe-4S]1+/0. The presence of an additional promoter, which can be propionic acid, alanine, or cysteine, induces a second pair of redox peaks at approximately -900 mV (vs SCE) arising from [3Fe-4S]0/1-. A robust neomycin-PfFd complex was detected by mass spectrometry. The results clearly favor an ET mechanism in which the promoting effect of small organic molecules is through formation of promoter-protein complexes. The interaction of PfFd with small organic molecules in homogeneous solution offers clues to confine the protein on the electrode surface modified by the same functional group monolayer and to address diffusionless direct electrochemistry, as well as surface microstructures of the protein monolayer. PfFd molecules were found to assemble on either mercaptopropionic acid (MPA) or cysteine-modified Au111 surfaces in stable monolayers or submonolayers. Highly ordered (2 radical 3 x 5)R30 degrees cluster structures with six MPA molecules in each cluster were found by in situ STM. Individual PfFd molecules on the MPA layer are well resolved by in situ STM. Under Ar protection reversible cyclic voltammograms were obtained on PfFd-MPA/Au111 and PfFd-cysteine/Au111 electrodes with redox potentials of -220 and -201 mV (vs SCE), respectively, corresponding to the [Fe3S4]1+/0 couple. These values are shifted positively by 200 mV relative to homogeneous solution due to interactions between the promoting layers and the protein molecules. Possible mechanisms for such interactions and their ET patterns are discussed.     

Interfacial electrostatics of self-assembled monolayers of alkane thiolates on Au(111): work function modification and molecular level alignments

We have isolated at T < 150 K a weakly adsorbed dimethyl disulfide (DMDS) layer on Au(111) and studied how the vibrational states, S core hole level shifts, valence band photoemission, and work function measurements evolve upon transforming this system into chemisorbed methylthiolate (MT) self-assembled monolayers (SAM) by heating above 200 K. By combining these observations with detailed theoretical electronic structure simulations, at the density functional level, we have been able to obtain a detailed picture of the electronic interactions at the interface between Au and adsorbed thiolates and disulfides. All of our measurements may be interpreted with a simple model where MT is bound to the Au surface with negligible charge transfer. Interfacial dipoles arising from Pauli repulsion between molecule and metal surface electrons are present for the weakly adsorbed DMDS layer but not for the chemisorbed species. Instead, for the chemisorbed species, interfacial dipoles are exclusively controlled by the molecular dipole, its interaction with the dipoles on neighboring molecules, and its orientation to the surface. The ramifications of these results for alignment of molecular levels and interfacial properties of this class of materials are discussed.     

Exploring three-dimensional nanosystems with Raman spectroscopy: methylene blue adsorbed on thiol and sulfur monolayers on gold

Resonant Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tunnel microscopy and electrochemical techniques, have been used to obtain information about the amount and spatial distribution of methylene blue (MB) molecules immobilized on sulfur and four ultrathin molecular alkanethiolate films self-assembled on Au(111) and rough Au electrodes. The intensity of the Raman signals allow one to estimate the amount of immobilized MB at different organic films, whereas the decrease in the SERS intensity as a function of distance for the rough Au electrodes is used to locate the average position of the MB species with respect to the Au substrate. We found that significant amounts of cationic MB species are able to diffuse into methyl-terminated thiols, but they are stopped at the outer plane of the self-assembled monolayer (SAM) by negatively charged carboxylate groups. The relative shift of C-N stretching Raman modes indicates that the binding of MB to S is different from that found for MB on thiols. Most of the molecules immobilized on methyl- and carboxylate-terminated thiols are electrochemically inactive, suggesting that strong coupling between the Au electrode and the MB molecules is needed for charge transfer. Our results are consistent with a small population of electrochemically active MB species very close to the Au surface that reach this position driven by their lipophilic (hydrophobic) character through defects at SAMs.     

Enhanced catalytic activity of Fe phthalocyanines linked to Au(111) via conjugated self-assembled monolayers of aromatic thiols for O2 reduction

We have obtained and characterized self assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) and 1-(4-mercaptophenyl)-2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDPP) functionalized with iron phthalocyanine (FePc) adsorbed on gold (111) electrodes. The catalytic activity of these SAMs/FePc was examined for the reduction of O₂ in aqueous media (pH = 4) and compared with that of bare gold and of gold coated directly with FePc molecules. Scanning tunneling microscopy (STM) studies confirm the functionalization of the 4-ATP by FePc. The electrocatalytic studies carried out with Au/FePc, Au/4-ATP/FePc and Au/MDPP/FePc electrodes show that the O₂ reduction takes place by a 4-electron transfer to give water in contrast to a 2-electron-transfer process observed on the bare gold. The activity of the electrodes increases as follows: Au < Au/FePc < Au/4-ATP/FePc < Au/MDPP/FePc.     

Dynamics of CO at the solid/liquid interface studied by modeling and simulation of CO electro-oxidation on Pt and PtRu electrodes

We consider theoretical models for CO monolayer oxidation on stepped Pt single-crystal electrodes and Ru-modified Pt(111) electrodes. For both systems, our aim is to assess the importance of CO surface diffusion in reproducing the experimental chronoamperometry or voltammetry. By comparing the simulations with the experimental chronoamperometric transients for CO oxidation on a series of stepped Pt surfaces, it was concluded that mixing of CO on the Pt(111) terrace is good, implying rapid diffusion (N. P. Lebedeva, M. T. M. Koper, J. M. Feliu and R. A. van Santen, J. Phys. Chem. B, submitted). We discuss here a more detailed model in which the CO adsorbed on steps is converted into CO adsorbed on terraces as the oxygen-containing species occupy the steps (as observed experimentally on stepped Pt in UHV), followed by a subsequent oxidation of the latter, to reproduce the observed chronoamperometry on stepped surfaces with a higher step density. On Ru-modified Pt(111), the experimentally observed splitting of the CO stripping voltammetry into two stripping peaks, may suggest a slow diffusion of CO on Pt(111). This apparent contradiction with the conclusions of the experiments on stepped surfaces, is resolved by assuming a weaker CO binding to a Pt atom which has Ru neighbors than to "bulk" Pt(111), in agreement with recent quantum-chemical calculations. This makes the effective diffusion from the uncovered Pt(111) surface to the perimeter of the Ru islands, which are considered to be the active sites in CO oxidation electrocatalysis on PtRu surfaces, very slow. Different models for the reaction are considered, and discussed in terms of their ability to explain experimental observations.     

On the performance of Au(111) for ethylene epoxidation: a density functional study

This work presents a periodic density functional study of the epoxidation mechanism of ethylene on Au(111). It is found that, once atomic oxygen is adsorbed on the surface, partial oxidation to ethylene oxide becomes possible. Calculated transition state theory rate constants for the elementary steps involved in the reaction predict that the selectivity of Au(111) toward epoxide is of approximately 40% in good agreement with recent experimental findings for styrene epoxidation on Au(111).     

Polymyxin-coated Au and carbon nanotube electrodes for stable [NiFe]-hydrogenase film voltammetry

We report on the use of polymyxin (PM), a cyclic cationic lipodecapeptide, as an electrode modifier for studying protein film voltammetry (PFV) on Au and single-walled carbon nanotube (SWNT) electrodes. Pretreating the electrodes with PM allows for the subsequent immobilization of an active submonolayer of [NiFe]-hydrogenase from Allochromatium vinosum ( Av H2ase). Probed by cyclic voltammetry (CV), the adsorbed enzyme exhibits characteristic electrocatalytic behavior that is stable for several hours under continuous potential cycling. An unexpected feature of the immobilization procedure is that the presence of chloride ions is a prerequisite for obtaining electrocatalytic activity. Atomic force microscopy (AFM) relates the observed catalytic activity to enzymatic adsorption at the PM/Au(111) surface, and a combination of concentration-dependent CV and AFM is used to investigate the interaction between the enzyme and the PM layer.