Carbon monoxide oxidation on Au(1 1 1) surface decorated by spontaneously deposited Pt

Platinum is deposited spontaneously on Au(1 1 1) surface from 1 mM H₂PtCl₆ + 1 M HClO₄ solution using multiple deposition procedure. X-ray photoelectron spectroscopy (XPS) analysis has shown that after immersion into the Pt containing solution and rinsing with water, Pt(OH)₂ resides on the Au(1 1 1) substrate. Consecutive depositions as well as in situ scanning tunneling microscopy (STM) and electrochemical measurements are performed on previously electrochemically reduced Pt/Au(1 1 1) surfaces. Only homogeneous distribution of thus deposited Pt islands is observed by in situ STM. With subsequent depositions, the width of deposited Pt islands increases, but stays lower than 10 nm, while a significant increase of Pt islands height is observed, leading to moderate increase of the coverage. Cyclic voltammetry (CV) profiles of obtained Pt/Au(1 1 1) surfaces, and CO stripping curves are recorded in 0.5 M H₂SO₄ solution. CO oxidation takes place only at higher potentials shifting negatively with increasing coverage. This is discussed with respect to Pt islands width and height distributions and to the influence of the Au(1 1 1) substrate surface.     

Surface X-ray scattering of high index plane of platinum containing kink atoms in solid-liquid interface: Pt(3 1 0) = 3(1 0 0)-(1 1 0)

Surface structure of Pt(3 1 0) = 3(1 0 0)-(1 1 0), which contains kink atoms in the step, has been determined with the use of in situ surface X-ray scattering (SXS) in the double layer region (0.50 V(RHE)) in 0.1 M HClO₄. Clean Pt(3 1 0) surface has pseudo (1 × 1) structure on which lateral displacements of 2-9% and 0.3-1% are found along a and b directions, respectively, whereas the surfaces of Pt(1 1 0) = 2(1 1 1)-(1 1 1) and Pt(3 1 1) = 2(1 0 0)-(1 1 1) are reconstructed to (1 × 2) according to previous reports. Interlayer spacing between the first and the second layers d₁₂ is contracted about 5% compared with the bulk spacing, whereas those between underlying layers are expanded down to fourth layer. Fully adsorbed CO has no effect on the surface structure of Pt(3 1 0). This result differs from that on Pt(1 1 1), where d₁₂ is expanded after CO adsorption.     

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.     

Electrochemical characterization of kinked Pt(7 5 1) surface in acidic media

Kinked Pt(7 5 1) surface was prepared and its electrochemical behaviors under different pretreatment conditions in acidic media were investigated systematically by using cyclic voltammetry. The results demonstrated that the upper limit of potential scanning and cooling atmospheres after the Pt(7 5 1) having been flame-annealed significantly influence the voltammetric behavior of Pt(7 5 1) electrode. The electric charge of hydrogen adsorption-desorption slightly increases with increasing the upper limit of potential scanning. Different cooling atmospheres give rise impacts to the surface structure of Pt(7 5 1) electrode, but hardly change the amount of hydrogen adsorption-desorption sites on the electrode. In addition, the so-called third oxidation peak appears near −0.08 V in H₂SO₄ media and −0.05 V in HClO₄ solution because of the presence of (1 1 0) terrace sites on this surface, and a plausible mechanism for the formation of this current peak is discussed. The results are of importance in understanding the electroadsorption properties of the kinked Pt(7 5 1) surface, as well as in further exploration of this kinked electrode in electrocatalysis.     

Crystalline alloys produced by mercury electrodeposition on Pt(1 1 1) electrode at room temperature

In situ scanning tunneling microscopy (STM) and reflection high energy electron diffraction (RHEED) were used to characterize mercury film electrodeposited onto a Pt(1 1 1) electrode at room temperature. Depending on the amount of Hg deposit, two different growth modes were observed. At low Hg coverage, crystalline (0 0 0 1)Hg adlayer accompanied by 30°-rotated (1 1 1)-Pt patches was found on Pt(1 1 1). Deposition of multilayer Hg resulted in layered PtHg₂ and PtHg₄ amalgams, which grew epitaxially by aligning their (2 0 1) and planes, respectively, parallel to the Pt(1 1 1) substrate. The preference of these epitaxial relationships for the electrochemically formed Pt-Hg intermetallic compounds on Pt(1 1 1) could result from minimization of the surface energy.     

Electrodeposition of Ag and Pd on a reconstructed Au(1 1 1) electrode surface studied by in situ scanning tunneling microscopy

Electrochemical deposition of Ag and potential-induced structural change of the deposited Ag layer on a reconstructed surface of Au(1 1 1) electrode were followed by in situ scanning tunneling microscope (STM). A uniform Ag monolayer was formed on a reconstructed Au(1 1 1) surface in a 50-mM H₂SO₄ solution at +0.3 V (vs. Ag/AgCl) after adding a solution containing Ag₂SO₄ so that the concentration of Ag⁺ in the STM cell became ca. 2 μM. No characteristic height corrugation such as the Au reconstruction was observed on the surface, indicating that the lifting of the substrate Au reconstruction occurred by Ag deposition. The formed Ag monolayer was converted to a net-like shaped Ag nano-pattern of biatomic height when the potential was stepped from +0.3 to −0.2 V in the solution containing 2 μM Ag⁺. This result indicates that the substrate Au(1 1 1)-(1 × 1) surface was converted to the reconstructed surface even in the presence of Ag adlayer. Quite different structure was observed for Pd deposition on a reconstructed surface of Au(1 1 1) electrode at +0.3 V and the origin for this difference between Ag and Pd deposition is discussed.     

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.     

XPS study of irreversibly adsorbed arsenic on a Pt(1 1 1) electrode

The chemical composition of an irreversibly adsorbed layer of arsenic on Pt(1 1 1) in sulfuric acid solution has been studied by X-ray photoelectron spectroscopy (XPS). From the chemical shift of the As 3d level, a change in the valence state from As(0) to As(III) with positive-going electrode potential is deduced, the total amount of As on the surface remaining constant. The As coverage derived from XPS is around 0.33 ML, which is in agreement with the charge under the current peak in the cyclic voltammogram. From the coadsorption of anions, accompanying the valence transition of As, As(III) is assumed to exist as As(OH)₃ on the surface.     

Electroreduction of nitrate ions on Pt(1 1 1) electrodes modified by copper adatoms

Kinetics and mechanism of nitrate ion reduction on Pt(1 1 1) and Cu-modified Pt(1 1 1) electrodes have been studied by means of cyclic voltammetry, potentiostatic current transient technique and in situ FTIRS in solutions of perchloric and sulphuric acids to elucidate the role of the background anion. Modification of platinum surface with copper adatoms or small amount of 3D-Cu crystallites was performed using potential cycling between 0.05 and 0.3 V in solutions with low concentration of copper ions, this allowed us to vary coverage θ_Cu smoothly. Following desorption of copper during the potential sweep from 0.3 to 1.0 V allowed us to estimate actual coverage of Pt surface with Cu adatoms. Another manner of the modification was also applied: copper was electrochemically deposited at several constant potentials in solutions containing 10⁻⁵ or 10⁻⁴ M Cu²⁺ and 5 mM NaNO₃ with registration of current transients of copper deposition and nitrate reduction.It has been found that nitrate reduction at the Pt(1 1 1) surface modified by copper adatoms in sulphuric acid solutions is hindered as compared to pure platinum due to induced sulphate adsorption at E < 0.3 V. Sulphate blocks the adsorption sites on the platinum surface and/or islands of epitaxial Cu(1 × 1) monolayer thus hindering the adsorption of nitrate anions and their reduction. The extent of inhibition weakly depends on the copper adatom coverage. Deposition of a small amount of bulk copper does not affect noticeably the rate of nitrate reduction.Nitrate reduction on copper-modified Pt(1 1 1) electrodes in perchloric acid solutions occurs much faster as compared to pure platinum. The steady-state currents are higher by 4 and 2 orders of magnitude at the potentials of 0.12 and 0.3 V, respectively. The catalytic effect of copper adatoms is largely caused by the facilitation of nitrate adsorption on the platinum surface near Cu_ad and/or on the islands of the Cu(1 × 1) monolayer (induced nitrate adsorption).Hydrogen adatoms block the adsorption sites on platinum for NO₃⁻ anion adsorption and inhibit reactions of nitrate reduction even at moderate surface coverage.The products of nitrate reduction in sulphuric and perchloric acids are essentially the same (NO and ammonia) irrespective of the presence or absence of Cu on the platinum surface.     

In situ EC-STM studies of n-Si(1 1 1):H in 40% NH4F solution at pH 10

In situ electrochemical-scanning tunneling microcopy (EC-STM) was employed to investigate the etching dynamics of the moderately doped n-Si(1 1 1) electrode during cyclic voltammetric perturbation and at the seven different potentials including the open circuit potential (OCP) in 40% NH₄F solution at pH 10, which was prepared from 40% NH₄F and concentrated NH₄OH solution. The etching rate was significant at OCP and showed an exponential dependence on the potential applied to the silicon substrate electrode. Although some triangular pits were generated at the Si(1 1 1) surface, at the potentials more negative than OCP the site dependence in the removal of surface silicon atoms prevailed and led to the atomically flat Si(1 1 1):H surfaces with sharply defined steps of the step height 3.1 Å, where the interatomic distance of 3.8 Å was observed with a three-fold symmetry. At the potentials sufficiently more positive than OCP, macroporous hole was formed to limit further in situ EC-STM study. The results were compared with in situ EC-STM studies of the etching reaction of n-Si(1 1 1):H in the aqueous solution of dilute ammonium fluoride at pH 5, 40% NH₄F at pH 8, and 1 M NaOH reported in the literature.     

Potential of zero free charge of Pd overlayers on Pt(1 1 1)

Differential capacitance measurements of Pd overlayers on a Pt(1 1 1) electrode in dilute aqueous NaF solutions have been performed as a function of film thickness in order to determine the potential of zero free charge (pzfc). The pzfc of the first, pseudomorphic Pd monolayer on Pt(1 1 1) is −0.21 V versus SCE. By increasing the amount of deposited Pd, a clear shift of the pzfc to more positive values is observed. After deposition of an equivalent of 10 monolayers, the value approaches that of a massive Pd(1 1 1) electrode (−0.12 V versus SCE). The pzfc's for the various Pd coverages are correlated with surface structure information, derived from STM images (R. Hoyer, L.A. Kibler, D.M. Kolb, Electrochim. Acta 49 (2003) 63). Variations in the pzfc are discussed in the context of an electronic modification by the underlying substrate and are compared with corresponding data for Pd overlayers on Au(1 1 1).     

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.     

Adsorption of 2-X-benzoic acids (X = fluoro, chloro, and bromo) on Au(1 0 0) electrode in acidic media studied by IRAS

Adsorption behaviors of 2-fluoro-, 2-chloro-, and 2-bromobenzoic acids on Au(1 0 0) electrode in 0.1 M HClO₄ have been investigated by using in situ reflection adsorption IR spectroscopy and differential capacity measurements. It is found for the 2-substitued benzoic acids that the flat lying adsorbed species is present at negative potentials, and the vertically adsorbed benzoate with both oxygen atoms oriented toward the metal surface is present at positive potentials. A new adsorbed state due to the vertical orientation was observed around 0.75 V only in 2-fluorobenzoic acid solution, as well as the case of benzoic acid. This indicates that the new band is due to the formation of an ordered adsorption layer of the vertical 2-fluorobenzoate, and the formation of the arrangement adsorption is dependent on the size of the adsorbed molecule.     

Adsorption of phosphate species on poly-oriented Pt and Pt(1 1 1) electrodes over a wide range of pH

The adsorption of phosphate anions from phosphate solutions at poly-oriented and single-crystal platinum electrodes, primarily Pt(1 1 1), was studied over a wide range of pH by cyclic voltammetry. The features observed at the poly-oriented Pt electrode in phosphate solution may be related to the different crystalline facets, the (1 1 1) orientation presenting the most significant behavior in terms of phosphate adsorption. On the reversible hydrogen electrode (RHE) scale, the phosphate adsorption strength decreases with increasing alkalinity of the solution. Qualitatively, three different pH regions can be distinguished. At pH < 6 only a broad reversible peak is observed, corresponding to the adsorption of H₂PO₄⁻ and further deprotonation to adsorbed HPO₄⁻. For 6 < pH < 11 a butterfly feature followed by one or two anodic peaks (depending on scan rate) is observed, ascribed to the adsorption of HPO₄⁻ followed by its subsequent deprotonation to adsorbed PO₄³⁻. The splitting into two or three voltammetric features, and the irreversibility of the two features at more positive potential, is ascribed to the deprotonation reaction leading to a surface species (i.e. phosphate) which needs to change its surface coordination. At pH > 11 a reversible pre-wave and a sharp spike are observed, ascribed to the co-adsorption of phosphate and hydroxide.     

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.     

In situ radiotracer and voltammetric study of the formation of surface adlayers in the course of Cr(VI) reduction on polycrystalline and (1 1 1) oriented platinum

This paper is focused on the in situ radiotracer and voltammetric studies of the induced HSO₄⁻/SO₄²⁻ adsorption at Pt(poly) and Pt(1 1 1) surfaces in 0.1 mol dm⁻³ HClO₄ solution in the course of Cr(VI) electroreduction. Besides this, the sorption behavior of HSO₄⁻/SO₄²⁻ ions on bare Pt(poly) and Pt(1 1 1) electrodes is compared and discussed. From the experimental results it can be stated that: (i) although the extent of bisulfate/sulfate adsorption is strongly dependent upon the crystallographic orientation of Pt surfaces, the maximum coverage on the Pt(1 1 1) does not exceed 0.2 monolayer; (ii) the Cr(VI) electroreduction on both poly- and (1 1 1) oriented platinum proceeds via a ce (chemical-electron-transfer) mechanism to yield Pt surfaces covered with intermediate surface adlayers containing Cr(VI) particles (and reduced Cr-containing adspecies) and ‘strongly bonded’ HSO₄⁻/SO₄²⁻ ions; (iii) while the coverage of platinum surfaces by the intermediate complexes formed in the course of Cr(VI) electroreduction at E > 0.20 V is basically independent of the crystallographic orientation of the Pt electrode, the onset for rapid Cr(VI) reduction is highly affected by the nature and crystallographic orientation of the electrode.     

Heteroepitaxial growth and characterization of 3C-SiC films on on-axis Si (1 1 0) substrates by LPCVD

Cubic SiC (3C-SiC) film has been deposited on Si (1 1 0) substrate by the low pressure chemical vapor deposition (LPCVD) with gas sources of SiH₄, C₃H₈ and carrier gas of H₂. The 3C-SiC crystalline film can be confirmed through the observations using reflection high-energy electron diffraction (RHEED) images. The X-ray diffraction (XRD) pattern and the rocking curve indicate that the (1 1 1) plane of SiC film is parallel to the surface of the Si (1 1 0) substrate and the film is of high crystallinity. The results of the field emission scanning electron microscope (FESEM) images show that the film has smooth surface morphology. Transmitted electron diffraction (TED) pattern and high resolution transmission electron microscope (HRTEM) image further confirm the high quality of the film.     

In situ STM study of underpotential deposition of bismuth on Au(1 1 0) in perchloric acid solution

The underpotential deposition (UPD) of Bi on Au(1 1 0) was investigated in HClO₄ solution using in situ scanning tunneling microscopy. The UPD of Bi occurred in three steps. A structure, in which Bi atoms formed dimers, was found for the first UPD adlayer. A (1 × 1) image was obtained by STM at the second UPD peak. For the third UPD peak, Bi atoms formed an incommensurate adlayer, and stripes of Bi were observed on terraces. After the third UPD, a structural reconstruction caused by adsorbed Bi was observed.     

Electrochemical characterization of irreversibly adsorbed germanium on platinum stepped surfaces vicinal to Pt(1 0 0)

The electrochemical behavior of germanium irreversibly adsorbed at stepped surfaces vicinal to the Pt(1 0 0) pole is reported. The process taking part on the (1 0 0) terraces is evaluated from charge density measurements and calibration lines versus the terrace dimension are plotted. On the series Pt(2n − 1,1,1) having (1 1 1) monoatomic steps, the charge involved in the redox process undergone by the irreversibly adsorbed germanium is able to account for (n − 0.5) terrace atoms, thus suggesting some steric difficulties in the growth of the adlayer on the (1 0 0) terraces. Conversely, no steric problems are apparent in the series Pt(n,1,0) in which more open (1 0 0) steps are present on the (1 0 0) terraces. In this latter case the charge density under the germanium redox peaks is proportional to the number of terrace atoms. Some comparison is made with other stepped surfaces to understand the behavior and stability of germanium irreversibly adsorbed on the different platinum surface sites.