Two types of vertically adsorbed 3-pyridinecarboxylate on Au(1 1 1), Au(1 0 0) and Au(1 1 0) electrodes

3-Pyridinecarboxylic acid (3-PC) adsorbed on low index face electrodes in NaF solution were investigated using in situ Fourier transform-infrared (FTIR) spectroscopy and differential capacity measurements. Two types of vertically adsorbed 3-PC were confirmed on the electrodes, while flat-lying adsorbed species were not observed at the potentials measured. At negative potentials, 3-PC molecules adsorb vertically on the electrode via the lone pair electrons of the pyridine ring nitrogen atom, and at positive potentials, 3-PC adsorbs vertically via the two oxygen atoms of the carboxyl group.     

Adsorption behavior of terephthalic acid on Au(1 0 0), Au(1 1 1) and Au(1 1 0) electrodes in neutral solution

Adsorption of terephthalic acid on Au(1 0 0), Au(1 1 1) and Au(1 1 0) electrodes in neutral solution has been investigated using in situ Fourier transform infrared (FT-IR) spectroscopy, differential capacity measurements and scanning tunneling microscopy (STM). At negative potentials, the terephthalate anions in solution adsorb in a flat orientation onto the electrode via the π electrons of the aromatic ring. At positive potentials, the terephthalate anions adsorb in a vertical orientation via the oxygen atoms of one of the carboxyl groups as a form of dianion. At more positive potentials, the anions adsorb in a vertical orientation as a form of hydrogen terephthalate. For the three electrodes examined, the overtone and/or combination bands, due to vertically oriented hydrogen terephthalate, were observed at 2642 and 2517 cm⁻¹, respectively. For the Au(1 1 1) electrode, STM observations indicated a flat orientation in the form of terephthalic acid.     

IR and UV-visible spectroscopic studies of the adsorption of 3-fluorobenzoic acid on Au(1 1 1), Au(1 0 0) and Au(1 1 0) electrodes in neutral solution

3-Fluorobenzoic acid adsorbed on Au(1 1 1), Au(1 0 0) and Au(1 1 0) electrodes has been investigated using in situ IR reflection spectroscopy, UV-visible reflectivity measurements, and differential capacity measurements. As determined by IR reflection spectroscopy, the vertically adsorbed 3-fluorobenzoate is the predominant species at positive potentials, and the flat-lying type is adsorbed at negative potentials. In the UV-visible reflectivity measurements, the reflectivity changes for the vertically adsorbed and flat-lying 3-fluorobenzoates on the Au(1 0 0) electrode were observed at different frequency regions. No UV-visible reflectivity signal was observed for the Au(1 1 0) electrode, and it was proved that the UV-visible reflectivity change has a strong single crystal surface dependence. For the vertical configuration, a large contribution between the 5d orbital of Au and the 2p orbital of oxygen in 3-fluorobenzoate was found for the cluster model by DFT calculation.     

Electrochemical reduction of nitrate on Pt(S)[n(1 1 1) × (1 1 1)] electrodes in perchloric acid solution

The electrochemical reduction of nitrate ion was studied by cyclic voltammetry on Pt(1 1 1) and [n(1 1 1) × (1 1 1)] stepped Pt surfaces, where n (=14, 10, 7, 6, 5, 4, 3, 2) is the number of terrace atoms, in 0.1 M HClO₄ + 10 mM KNO₃. The electrocatalytic nitrate reduction was found to hardly proceed on Pt(1 1 1) in the hydrogen adsorption region, while the electrocatalytic activity was improved with the increase in the step density. Inactivation was observed in the presence of adsorbed hydrogen or nitrate-derived reduced adsorbate, i.e. adsorbed NO, on (1 1 1) step sites. It was, therefore, concluded that the electrocatalytically active NO₃⁻ species does not adsorb on the (1 1 1) terraces but on the (1 1 1) monoatomic steps. The nitrate reduction current increased with the step density in a non-linear relationship. The overall current density at 0.21 V (RHE) corresponding to the peak potential of the main electrocatalytic nitrate reduction wave which was maximum at n = 2, abruptly increased with short terraces, i.e. n < 5, where the current wave of adsorbed hydrogen on the Pt stepped surface with comparatively narrow (1 1 1) terraces, denoted as H_nt, also appeared unmodified for n < 5 on voltammograms recorded in 0.1 M HClO₄ in the absence of nitrate.     

Dependence of electrocatalytic activity on film thickness for the hydrogen evolution reaction of Pd overlayers on Au(1 1 1)

The hydrogen evolution reaction has been studied for ultrathin Pd overlayers of various thickness on Au(1 1 1) in 0.1 M H₂SO₄. A clear correlation of the electrocatalytic activity as expressed by the exchange current density and the binding energy of adsorbed hydrogen has been found. While hydrogen is bound strongest on the second Pd monolayer (ML), the respective catalytic activity is poorest for all the surface structures under study. The exchange current density increases in the order 2 ML Pd < 1 ML Pd < bulk Pd (more than 2 ML). The electronic ligand effect, a geometric effect due to pseudomorphic growth and the surface defect density belong to the most crucial parameters in relations between structure of the electrode surface and its electrocatalytic activity. The experimental results are supported by an excellent agreement with theoretical predictions.     

Electrosorbed carbon monoxide monolayers on Pt(1 1 1)

We review structures of high-density CO monolayers on Pt(1 1 1) surfaces in CO-saturated electrolytes or in gaseous CO at near atmospheric pressure, using surface X-ray scattering (SXS) and scanning tunneling microscopy (STM). In electrolytes, we confirmed the well-known (2 × 2)-3CO and (√19 × √19)-13CO structures and were able to study the transition between them. For gas-phase studies, we were able to stabilize extremely well-ordered CO monolayers by emersion transfer from an electrochemical cell. We found that the hexagonal close-packed (2 × 2)-3CO structure is the equilibrium phase at room temperature in ∼1 atm CO gas pressure. This commensurate (C) phase transforms continuously to an incommensurate (IC) phase at elevated temperature (a second-order phase transition). We also confirm that the (√19 × √19)-13CO structure is stable at lower CO partial pressure. This C phase transforms discontinuously to an IC phase (a first-order phase transition). A tentative phase diagram and a brief review of structure details of the (2 × 2)-3CO and (√19 × √19)-13CO phases will be presented.     

Adenine adsorption on Au(1 1 1) and Au(1 0 0) electrodes: Characterisation, surface reconstruction effects and thermodynamic study

Adsorption of adenine on Au(1 1 1) and Au(1 0 0) electrodes is studied by cyclic voltammetry, impedance and chronoamperometric measurements in 0.1 M and 0.01 M KClO₄ and in 0.5 M NaF solutions. The experiments performed with flame-annealed electrodes at different contact potentials, scan potential limits and scan rates, suggest different adsorption behaviour on the unreconstructed and reconstructed surface domains. This is confirmed by comparing the results obtained with electrochemically annealed unreconstructed and with flame-annealed reconstructed surfaces. In both cases the initial electrode surface state is characterised by the E_pzc values. The adsorption on reconstructed surfaces takes place at more positive potentials than on the unreconstructed surfaces and induces the lifting of the reconstruction.The thermodynamic analysis is performed on the chronoamperometric data for adenine desorption on well characterised unreconstructed Au(1 1 1) surfaces. To this end a new methodology of the chronoamperometric experiments is introduced. Quantitative thermodynamic adsorption parameters such as surface tension, Gibbs surface excess, Gibbs energy of adsorption, potential versus Gibbs excess slope and electrosorption valency are determined. Weak chemisorption of adenine is inferred with a molecular orientation independent on the coverage and on the electrode potential. It is proposed that adsorbed adenine molecules adopt a tilted orientation at the surface to facilitate the coordination to the gold atoms.     

Kinetic study of nitrate reduction on Pt(1 1 0) electrode in perchloric acid solution

The kinetics of electrocatalytic reduction of nitrate on Pt(1 1 0) in perchloric acid was studied with cyclic voltammetry at a very low sweep rate of 1 mV s⁻¹, where pseudo-steady state condition was assumed to be achieved at each electrode potential. Stationary current-potential curves in perchloric acid in the absence of nitrate showed two peaks at 0.13 V and 0.23 V (RHE) in the so-called adsorbed hydrogen region. The nitrate reduction proceeded in the potential region of the latter peak in the pH range studied. The reaction orders with respect to NO₃⁻ and H⁺ were observed to be close to 0 and 1, respectively. The former value means that the adsorbed NO₃⁻ at a saturated coverage is one of the reactants in the rate-determining step (rds). The latter value means that hydrogen species is also a reactant above or on the rds. The Tafel slope of nitrate reduction was −66 mV per decade, which is taken to be approximately −59 mV per decade, indicating that the rds is a pure chemical reaction following electron transfer. We discuss two possible reaction schemes including bimolecular and monomolecular reactions in the rds to explain the kinetics and suggest that the reactants in the rds are adsorbed hydrogen and adsorbed NO₃⁻ with the assistance of the results in our recent report for nitrate reduction on Pt(S)[n(1 1 1) × (1 1 1)] electrodes: the nitrate reduction mechanism can be classified within the framework of the Langmuir-Hinshelwood mechanism.     

CO monolayer oxidation on stepped Pt(S) [(n − 1)(1 0 0) × (1 1 0)] surfaces

The electrochemical oxidation of CO has been studied on Pt(S)[(n − 1)(1 0 0) × (1 1 0)] electrodes to investigate the effect of the step density in the reaction. This series shows two different trends for long (n ≥ 7) and short terraces. For long terraces, the voltammetric peak shifts towards higher potential as the step density increases, unlike the behaviour observed for other stepped surfaces, which exhibit the opposite behaviour in agreement with the Smoluchowski effect. For short terraces, the “normal” behaviour is observed, that is, as the step density increases the peak shifts towards lower potentials. Chronoamperometric measurements were used to determine rate constants and Tafel slopes using the mean field Langmuir-Hinselwood kinetics. Rate constants follow the same trends as the peak potentials in voltammetry. A Tafel slope of 75 ± 4 mV has been obtained for the surfaces with long terraces whereas a value of the surfaces with short terraces showed a value of 100-120 mV is obtained. This change of slopes is interpreted as a change in the electrochemical behaviour of the species involved in the mechanism, probably, a change in the adsorption isotherm of adsorbed OH. Pt(5 1 0) electrode exhibits an intermediate behaviour between those of long and short terraces with two different peaks that can be associated with both behaviours previously described.     

Electroreduction of oxygen on gold-supported nanostructured palladium films in acid solutions

The electrochemical reduction of oxygen on thin Pd films with a nominal thickness of 0.25-10 nm on polycrystalline Au substrate (Pd/Au) was studied. The Pd films were prepared by electron beam evaporation and oxygen reduction was studied in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions using the rotating disk electrode (RDE) method. The surface morphology of Pd overlayers was examined by scanning tunnelling microscopy (STM). O₂ reduction predominantly proceeds through 4e⁻ pathway on all Pd/Au electrodes. The specific activity (SA) of oxygen reduction was lower in H₂SO₄ solution and decreased slightly with decreasing the Pd film thickness. In HClO₄, the SA was higher and not significantly dependent on the film thickness. The Tafel slope values close to −60 mV at low current densities and −120 mV at high current densities were found for all electrodes.     

The oxygen reduction reaction mechanism on Pt(1 1 1) from density functional theory calculations

We study the oxygen reduction reaction (ORR) mechanism on a Pt(1 1 1) surface using density functional theory calculations. We find that at low overpotentials the surface is covered with a half dissociated water layer. We estimate the barrier for proton transfer to this surface and the barrier for proton transport parallel to the surface within the half dissociated water network. We find both barriers to be small. The only potentially dependent step is the proton transfer from water to the half dissociated water layer. We find that ORR proceeds via four direct e⁻ reductions without significant peroxide formation. We show that the oxygen-oxygen bond breaking is dependent on the local surface environment. The minimum energy pathway is constructed and we confirm that OH removal from the surface determines the overpotential.     

Oxidation of formaldehyde and ethanol on Au(1 1 1) and Au(1 1 1) modified by spontaneously deposited Ru in sulfuric acid solution

The oxidation of formaldehyde and ethanol on both pure Au(1 1 1) and Au(1 1 1) modified by approximately 0.3 monolayer (ML) of spontaneously deposited Ru was studied by cyclic voltammetry (CV) in 0.5 M H₂SO₄ solution containing either 0.25 M formaldehyde or 0.35 M ethanol. In situ scanning tunneling microscopy (STM) and CV were employed to characterize the Au(1 1 1) and Ru/Au(1 1 1) surfaces. The oxidation of HCHO on Ru/Au(1 1 1) commences at 0.1 V more negative potential than on pure Au(1 1 1). From 0.25 to 0.55 V vs. (Ag/AgCl), the reaction occurs with increasing current, showing a peak at a potential of 0.43 V. It is assumed that the increasing anodic activity of the Ru/Au(1 1 1) surface is associated with the oxidation of some reaction intermediates, facilitated by the presence of Ru in its metallic state. On the other hand, the oxidation of ethanol on Ru/Au(1 1 1) commences at 0.1 V more positive potential than on pure Au(1 1 1), and proceeds in the potential region from 0.2 to 0.5 V with significantly smaller currents, showing a peak at 0.43 V. This inhibiting effect is explained by the deactivation of the most active Au(1 1 1) step sites by high coverage with Ru islands. The appearance of a small peak at 0.43 V is most likely associated to the oxidation of some intermediates during ethanol oxidation at the Ru/Au step sites formed on the Au(1 1 1) terraces by the presence of a small coverage with Ru islands.     

First principles mechanistic study of borohydride oxidation over the Pt(1 1 1) surface

The mechanism of borohydride oxidation and the competing hydrolysis reaction are examined over Pt(1 1 1) using density functional theory (DFT) methods. Adsorption of BH₄⁻ over Au(1 1 1) and Pt(1 1 1) is examined. Adsorption over Pt(1 1 1) is dissociative and extremely exothermic at potentials of interest, leading to a high surface coverage of H^* for which gaseous hydrogen evolution is competitive with oxidation. Elementary surface reactions oxidizing B-containing intermediates are favorable over Pt(1 1 1) at −0.85 V (SHE), consistent with experimental voltammetry results in the literature. The energetics of the initial adsorption step dictate the activity limitation of gold anodes and the selectivity limitation of platinum electrodes. This adsorption energy can be rapidly calculated with DFT methods, enabling screening of pure metals, alloys, poisons, and promoters to optimize borohydride oxidation catalyst design.     

On the electrocatalysis of nanostructures: Monolayers of a foreign atom (Pd) on different substrates M(1 1 1)

We have applied the electrocatalysis theory based on the Santos-Koper-Schmickler Hamiltonian to investigate the reactivity of nanostructures. A good agreement with experimental results has been obtained for monolayers of Pd on different substrates. The strain induced by the mismatch in the lattice constants of substrate and adsorbate is extensively discussed. We show that it is not the only effect for changing the electrocatalytic properties; also the specific chemical interaction between the monolayer and the substrate plays an important role. The catalytic properties of the nanostructures can be different for oxidation and reduction reactions.     

Effects of concentration and temperature on the formation process of decanethiol self-assembled monolayer on Au(1 1 1) followed by electrochemical reductive desorption

The formation process of self-assembled monolayer (SAM) of decanethiol (C10SH) on Au(1 1 1) single crystal electrode has been investigated for wide range of C10SH concentration (0.1-500 μM) and temperature (253-298 K). The amount and quality of C10SH SAM were determined from area and position, respectively, of reductive desorption peak of the SAM modified Au(1 1 1) electrode measured in 0.5 M KOH aqueous solution. The kinetic analysis indicates that SAM formation proceeded by two steps, diffusion-limited physisorption followed by chemisorption.     

Nanostructured Pt-M/C (M = Fe and Co) catalysts prepared by a microemulsion method for oxygen reduction in proton exchange membrane fuel cells

Nanostructured Pt-M/C (M = Fe and Co) catalysts have been synthesized by a microemulsion method and a high-temperature route. They have been characterized by cyclic voltammetry in 1 M H₂SO₄ and for oxygen reduction in proton exchange membrane fuel cells (PEMFC). The Pt-M alloy catalysts synthesized by the microemulsion method show higher electrochemical active surface area than those prepared by the high-temperature route, and some of them exhibit improved catalytic activity towards oxygen reduction compared to pure Pt. Among the various alloy catalysts investigated, the Pt-Co/C catalyst prepared by the microemulsion method shows the best performance with the maximum catalytic activity and minimum polarization loss. Mild heat treatment of the catalysts prepared by the microemulsion method at moderate temperatures (200 °C) in reducing atmosphere is found to improve the catalytic activity due to a cleaning of the surface and an increase in the electrochemical surface area.     

Self-assembly of 2,6-naphthalenedicarboxylic acid and 4,4′-biphenyldicarboxylic acid on highly oriented pyrolytic graphite and Au(1 1 1) surfaces

Self-assemblies of 2,6-naphthalenedicarboxylic acid (NDC) and 4,4′-biphenyldicarboxylic acid (BDC) molecules on the highly oriented pyrolytic graphite (HOPG) and reconstructed Au(1 1 1) surfaces has been, respectively, studied by using scanning tunneling microscopy (STM). The NDC molecules form an incommensurate structure, while BDC molecules form a commensurate structure on the HOPG surface. The aromatic rings take the same orientation with that of the underlying HOPG substrate to gain optimum adsorbate-substrate interactions. On the Au(1 1 1) surface, both molecules form incommensurate structures. The orientations of aromatic ring locate about 20° with respect to the gold rows, the 〈1 1 0〉 direction. On the HOPG surface, the whole carboxyl groups are invisible, while the carbon atoms of the carboxyl groups may appear bright on the gold surface.     

Comparative STM studies on island equilibrium shapes, shape fluctuations and island coalescence on Au(0 0 1) electrodes in chloric and sulfuric acid solutions

We present scanning tunneling microscopy data on the dynamics of monatomic high islands on gold electrodes in different electrolytes. From a statistical analysis of the island fluctuations around the equilibrium shape and from the investigation of the local perimeter curvature of the latter we determine the step line tension and the kink energy of gold islands on an Au(0 0 1) surface as a function of the electrode potential. We find that in chloric acid the step line tension is smaller than for Au(0 0 1) in sulfuric acid, however, the kink energy is much higher which leaves us with the puzzling result that island fluctuations are large, however, island edges are comparatively straight. Both energies show merely a weak dependence on the electrode potential in contrast to our previous findings in sulfuric acid. Furthermore, by analyzing the relaxation time of island coalescence events we determine the dominant mass transport on the Au(0 0 1) electrode to be terrace diffusion where the time limiting step of the migration process is the creation of transport species at the island edge rather than the hopping of species on the surface. This is in apparent contradiction to previous results from island decay studies where a diffusion limited terrace diffusion was found. All results can be explained if one assumes that the relevant mass transport species on Au(0 0 1) in chloride containing electrolytes are larger (linear) structure units rather than single adatoms. Possible candidates for these units are Au-Cl complexes.     

Overpotential deposition of copper on an iodine-modified Au(1 1 1) electrode

Cyclic voltammetry, chronoamperometry and in situ electrochemical scanning tunneling microscopy were used to study the kinetics of nucleation and crystal growth during the initial stages of copper overpotential deposition (OPD) on a previously iodine-modified Au(1 1 1) electrode, from an aqueous solution 10⁻³ M CuSO₄ in 0.05 M H₂SO₄. The starting potential during step experiments was chosen in the region where the gold electrode was completely free of the copper deposit. The recorded current transients for copper deposition onto the iodine-modified Au(1 1 1) electrode surface appear to be very complex, with the unusual presence of two or more current maxima. A new method was used for quantitative evaluation of current transients that involves the transition UPD-OPD, developed by our group [M. Palomar-Pardavé, I. González, N. Batina, J. Phys. Chem. B 104 (2000) 3545], was used for the quantitative interpretation. Our results show that, within a single current transient, copper adsorption and two types of nucleation process: two-dimensional (2D) and three-dimensional (3D) limited by lattice incorporation of copper adatoms and diffusion of Cu(II) ion, respectively, take place simultaneously. STM images revealed the enhanced growth of 3D copper on edge of I-Au(1 1 1) during the early stages of deposition. Moreover, our results strongly suggest that the iodine adlayer is constantly present, even after the striping Cu that was overpotential deposited.