Surface Structure Of Pt Research Articles

Atomic Scale Surface Structure of Pt/Cu(111) Surface Alloys

Pt–Cu bimetallic alloys are a key component in many heterogeneous catalysts that have the potential to be used in a range of industrially important reactions. Given the catalytic differences between Pt and Cu, the surface composition and geometry of Pt–Cu alloys can have a large influence on their chemistry. Extensive characterization of bulk Pt–Cu alloys has been performed; however, only a few studies have addressed surface and subsurface alloying of Pt with Cu, and none have examined the atomic scale surface structure of Pt–Cu. In this study, scanning tunneling microscopy was used to determine the local structure of surface alloys formed by physical vapor deposition of Pt onto Cu(111) over a range of alloying temperatures (315–550 K). Our results indicated that Pt and Cu were capable of intermixing at 315 K and forming multiple metastable states. Increasing the temperature of the Cu surface during the deposition of Pt altered the surface geometry and further enhanced the dispersion of Pt. The results ar...

Structure and Chemical State of the Pt(557) Surface during Hydrogen Oxidation Reaction Studied by in Situ Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy

The surface structure of Pt(557) during the catalytic oxidation of hydrogen was studied with in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. At 298 K, the surface Pt oxide formed after exposing Pt(557) to approximately 1 Torr of O2 can be readily removed by H2, at H2 partial pressures below 50 mTorr. Water is detected as the product in the gas phase, which also coadsorbs with hydroxyl groups on the Pt(557) surface.

An influence of pretreatment conditions on surface structure and reactivity of Pt(100) towards CO oxidation reaction

We present a combined electrochemical and in situ STM study of the surface structure of Pt(100) single crystal electrodes in dependence on the cooling atmosphere after flame annealing. The following cooling conditions were applied: Ar/H2 and Ar/CO mixtures (reductive atmosphere), argon (inert gas) and air (oxidative atmosphere). Surface characterization by in-situ STM allows deriving direct correlations between surface structure and macroscopic electrochemical behavior of the respective platinum electrodes. We investigated the influence of defect type and density as well as long range surface order on the kinetics of the CO electro-oxidation reaction. The defect-rich Pt(100) electrodes as cooled in air or Ar, and followed by immersion in the hydrogen adsorption region display higher activities as compared to the rather smooth Pt(100)-(1 × 1) electrode cooled in an Ar/H2-atmosphere.

Effect of the Surface Structure of Pt(100) and Pt(110) on the Oxidation of Carbon Monoxide in Alkaline Solution: an FTIR and Electrochemical Study

The electrochemical oxidation and infrared spectral features for carbon monoxide adsorbed on Pt(100) and Pt(110) in 0.1 M NaOH were studied as a function of the annealing/cooling treatment with the objective to establish the influence of the platinum surface site geometry and the role of surface order in the electro-oxidation of CO on these two surfaces. Two common cooling methods (Ar and H2/Ar atmospheres) were employed for both surface electrodes. Additionally, CO cooling and I2 vapour cooling methods were used for Pt(100) and Pt(110), respectively. Multiple CO vibration bands were observed on both electrode surfaces, their distribution and potential dependence strongly depending on the surface treatment.

Promotion by hydrous ruthenium oxide of platinum for methanol electro-oxidation

Pt–(RuO x H y ) m electrocatalysts ( m being the atomic Ru/Pt ratio) supported on multi-walled carbon nanotubes, in which amorphous hydrous ruthenium oxide (RuO x H y ) is the exclusive Ru-containing species, were prepared and comprehensively characterized by X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, thermogravimetric analysis and transmission electron microscopy techniques. Cyclic voltammetry (CV) and chronoamperometry studies of CO stripping and methanol electro-oxidation indicated that the CO tolerance and catalytic activity of Pt improved remarkably by the co-presence of RuO x H y . Repeated CV pretreatments in 0.5 M H 2SO 4 up to potentials higher than 0.46 V (vs. SCE) induced significant dissolution of RuO x H y , which changed the RuO x H y content, Pt–RuO x H y proximity and surface structure of Pt, and consequently altered the electrocatalytic activity of Pt in the final electrode. However, RuO x H y dissolution was not identified when the pretreatment potentials was set no higher than 0.46 V. Discussion on the promotional function of RuO x H y was made based on the peculiarity of RuO x H y as a mixed electron/proton conductor.

Effect of surface composition of Pt-Au alloy cathode catalyst on the performance of direct methanol fuel cells

A pure Pt cathode catalyst in direct methanol fuel cells is not only favored for oxygen reduction but also for the unwanted oxidation of methanol that permeates from the anode. Based on the idea that alloying another metal can alter the surface structure of Pt and hence reduce the active sites for methanol adsorption, in this work we prepare carbon supported Pt-Au nanoparticles with different surface compositions with the dimethylformamide co-reduction method by adjusting the pH value from 14 to 12. The electrochemical characterizations indicate that the alloyed Pt-Au catalyst prepared under pH = 13 exhibits lower catalytic activity to methanol oxidation but retains the oxygen-reduction activity similar to that of the Pt/C. The cell performance tests show that the PtAu/C can almost double the peak power density of the cell with the Pt/C cathode, although the Pt loading in the PtAu/C cathode is only half of that in the Pt/C cathode.

Superstructures and Order−Disorder Transition of Sulfate Adlayers on Pt(111) in Sulfuric Acid Solution

The surface structure of Pt(111) in a 0.1 M H2SO4 electrolyte was investigated in the potential range of sulfate adsorption with electrochemical scanning tunneling microscopy (STM) and cyclic voltammetry. Two ordered anion structures were observed coexisting in the potential range between 0.49 and 0.79 V (vs RHE): the well-known (radical3xradical7)R19.1 degrees superstructure with an anion coverage of 0.20 monolayer and a new, high-density (3x1) superstructure with a coverage of 0.33 monolayer. Both superstructures undergo a reversible order-disorder transition at 0.8 V. Simultaneous imaging of the adsorbed ions and of topographic details of the Pt substrate lattice allows us to study the local adsorption geometry of the sulfate. In the (radical3xradical7)R19.1 degrees, structure the sulfate ions are adsorbed close to depressions in the STM image of the Pt substrate which may be identified with face-centered cubic (fcc) hollow sites. In addition to the sulfate ions, a coadsorbed species, possibly water molecules, is observed in the unit cell of the (radical3xradical7)R19.1 degrees superstructure. Preliminary potentiodynamic STM data indicate that the transformation of the ordered sulfate adlayer into a disordered structure at 0.8 V is not directly related to adsorption/desorption features in the voltammogram commonly attributed to the adsorption/desorption of OH, and that the sulfate adlayer remains on the surface for potentials well above the adsorption potentials of OH.

Effect of Specific Adsorption of Anions and Surface Structure of Pt(111) Electrode on Kinetics of Dissociative Adsorption of Ethylene Glycol

The effect of specific adsorption of anions and the surface structure of a Pt (111) electrode on the kinetics of dissociative adsorption of ethylene glycol (EG) was studied using cyclic voltammetry and a programmed potential step technique. Quantitative results demonstrated that the specific adsorption of anions remarkably influenced the dissociative adsorption of EG. Both the initial reaction rate (vsubscript i)) of the EG dissociative adsorption and the saturated coverage of dissociative adsorbates, measured in perchloric acid media (without specific adsorption), are significantly larger than the corresponding values acquired in sulfuric acid media (with specific adsorption of SO(superscript 2- subscript 4)/HSO(superscript - subscript 4). We illustrate that the variation of the average reaction rate (average)v of EG dissociative adsorption on Pt(111) in perchloric acid media with an electrode potential yields a volcano-like distribution with a maximum value near 0.22 V (vs SCE). Furthermore, different surface structures of the Pt(111) electrode that were obtained by different treatments also significantly affect this surface process.

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 HClO 4. 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 d 12 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 d 12 is expanded after CO adsorption.

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 H 2SO 4 media and −0.05 V in HClO 4 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.

Kinetics of dissociative adsorption of ethylene glycol on Pt(1 0 0) electrode surface in sulfuric acid solutions

The dissociative adsorption of ethylene glycol (EG) on Pt(1 0 0) electrode surface cooled in air after flame annealing was investigated by using programmed potential step technique and in situ FTIR spectroscopy. The stable adsorbates derived from EG dissociative adsorption on Pt(1 0 0) were determined by in situ FTIR spectroscopy as linear- and bridge-bonded CO. The quantitative results demonstrated that the average rate of dissociative adsorption of EG on Pt(1 0 0) surface varies with electrode potential, yielding a volcano-type distribution with a maximum value located near 0.10 V versus SCE. From the variation of the quantity of CO adsorbates generated in EG dissociative adsorption with the adsorption time t ad, the initial rate ( ν i) of this surface reaction was evaluated quantitatively. The maximum value of ν i has been determined to be 2.64 × 10 −11 mol cm −2 s −1 in a solution containing 2 × 10 −3 mol L −1 EG. The influence of the surface structure of Pt(1 0 0) electrode obtained by different pretreatment as well as of the specific adsorption of (bi)sulfate anions on the kinetics of EG dissociative adsorption has been also investigated and discussed. In comparison with a Pt(1 0 0) surface cooled in air atmosphere after flame treatment, the Pt(1 0 0) surface cooled in an Ar–H 2 stream or subjected to a treatment of fast potential cycling decreased significantly the initial rate ν i of EG dissociative adsorption. Similar effect was also observed for the specific adsorption of (bi)sulfate anions. However, the maximum attainable coverage ( θ DA S ) of adsorbates derived from EG dissociative adsorption is not affected either by the surface structure of Pt(1 0 0) or by (bi)sulfate anions adsorption.

Surface processes and kinetics of CO2 reduction on Pt(100) electrodes of different surface structure in sulfuric acid solutions

The reduction of CO2 on a Pt(100) electrode in CO2 saturated 0.5 M H2SO4 solutions was studied by in situ FTIR reflection spectroscopy and a programmed potential step technique. Different surface structures of Pt(100) electrode were prepared by different treatments including fast potential cycling (200 V s−1) for a known time. The Pt(100) surface was characterized by a parameter γ that designates the relative amplitude of the current peak of hydrogen adsorption on (100) sites distributed on the one-dimensional surface domains to that on the two-dimensional surface domains. The in situ FTIR spectroscopic results demonstrated that the reduction of CO2 on the Pt(100) dominated by two-dimensional surface domains produced only bridge-bonded CO (COB) species, which give rise to IR absorption near 1840 cm−1. However both bridge- and linear-bonded CO (COL, yielding IR absorption at around 2010 cm−1) species are found for CO2 reduction on the Pt(100) dominated by one–dimensional surface domains. The small intensity of the COL and COB bands indicates that coverage by reduced CO2 species (r-CO2, or COL and COB species) is low. The cyclic voltammetric (CV) studies confirmed quantitatively the in situ FTIRS results, and revealed that the r-CO2 species adsorb preferentially on (100) sites distributed on the two-dimensional surface domains. The initial rate of CO2 reduction υi, i.e., the rate of CO2 reduction on a clean Pt(100) surface, has been determined quantitatively from studies using a programmed potential step technique. It has been demonstrated that the maximum values of υi (υim) measured on Pt(100) electrodes with different surface structures all appeared at − 0.19 V. From analysis of the relationship between υim and γ we have determined that the υim of CO2 reduction on (100) sites distributed on the two-dimensional surface domains is 0.53 × 10−11 mol cm−2 s−1 and that on (100) sites distributed on the one-dimensional surface domains is approximately 2.66 × 10−11 mol cm−2 s−1. Based on in situ FTIRS and electrochemical studies a migration process of the r-CO2 from the one-dimensional surface domains to the two-dimensional surface domains has been proposed to be involved in CO2 reduction. The present study has thrown new light on the electrocatalytic activity of different surface structures of a Pt(100) electrode and the surface processes and kinetics of CO2 reduction.

SFG-surface vibrational spectroscopy studies of structure sensitivity and insensitivity in catalytic reactions: cyclohexene dehydrogenation and ethylene hydrogenation on Pt (1 1 1) and Pt (1 0 0) crystal surfaces

The effect of the surface structure of Pt (1 1 1) and Pt (1 0 0) has been investigated for cyclohexene hydrogenation and dehydrogenation, and ethylene hydrogenation by using sum frequency generation. Cyclohexene dehydrogenation is a structure sensitive reaction, and the rate was found to proceed more rapidly on the Pt (1 0 0) crystal surface than on the Pt (1 1 1) crystal surface. On Pt (1 0 0), the major reaction intermediate during cyclohexene dehydrogenation was 1,3-cyclohexadiene, whereas on Pt (1 1 1), both 1,3- and 1,4-cyclohexadiene were present. Both 1,3- and 1,4-cyclohexadiene can dehydrogenate to form benzene, although the reaction proceeds more rapidly through the 1,3-cyclohexadiene intermediate. Because of this, the structure sensitivity of cyclohexene dehydrogenation is explained by noting that there are both a fast and slow reaction pathway for Pt (1 1 1), whereas there is only a fast reaction pathway on Pt (1 0 0). Ethylene hydrogenation is a structure insensitive reaction. Both ethylidyne and di-σ-bonded ethylene are present in both Pt (1 1 1) and Pt (1 0 0) under reaction conditions, although the ratio of the concentrations of the two species are different. The rate of the reaction was found to be 11±1 and 12±1 molecules per site per second for Pt (1 1 1) and Pt (1 0 0), respectively. Since the reaction rate is essentially the same on the two surfaces, while the concentration of ethylidyne and di-σ-bonded ethylene are different, these species must not be the active species which turnover under catalytic ethylene hydrogenation. The most likely species which turnover are π-bonded ethylene and ethyl, and their concentrations are near the detection limit of SFG.

Scanning tunneling microscopy and electrochemical study of the surface structure of Pt(10,10,9) and Pt(11,10,10) electrodes prepared under different cooling conditions

The effect of three surface electrochemical preparation techniques for single crystal surfaces on two platinum stepped surfaces [Pt(10,10,9) and Pt(11,10,10)] has been investigated by scanning tunneling microscopy and cyclic voltammetry. The preparation techniques consisted of a thermal treatment followed by a cooling step in (a) a hydrogen+argon atmosphere, (b) air (c) iodine vapor+air. For Pt(10,10,9) and Pt(11,10,10) surfaces, the hydrogen+argon treatment provides surfaces that have a very narrow distribution of terrace widths around the nominal value and monatomic steps. On the other hand, facetted surfaces with terrace widths and steps four to five times their nominal values are obtained when the cooling is in the presence of iodine vapor. The air treatment generates surfaces in which the terrace width is not as uniform as in the H2+Ar case and some kink sites are created. The changes in the surface topography can be followed in the voltammetric profile of the surfaces recorded in a sulfuric acid solution.

Electrochemical Epitaxial Growth of a Pt(111) Phase on an Au(111) Electrode

The electrochemical deposition of platinum on an Au(111) single-crystal electrode in acidic solutions containing H2PtCl6 was studied using an electrochemical scanning tunneling microscope (STM) and electrochemical quartz crystal microbalance (EQCM). The STM investigation showed an ordered adlayer of PtCl62- on the electrode surface during the electrochemical deposition of platinum and a Pt(111)-(1×1) structure on the electrode surface after the electrode was rinsed with a Pt complex-free solution. Formations of the Pt(111) bulk phase and surface structure of Pt(111)-(1×1) were confirmed by X-ray diffraction (XRD) and the underpotential deposition of copper and hydrogen, respectively.

Hydrogen-adsorption-induced phase transitions on Pt(100)-hex and the surface structure of Pt(100)-(1 x 1)H.

While adsorption of 100 L (1 L = 10(-6) Torr s) H-2 molecules at room temperature does not cause any phase transitions from a clean reconstructed Pt(100)-hex surface, a 10-L dose of dissociated H-2 can induce a phase transition from the Pt(100)-hex to an atomic-hydrogen-stabilized Pt(100)-(1 X 1)structure as observed using low-energy electron diffraction (LEED), This Pt(100)-(1 X 1) surface was subsequently cooled down using liquid nitrogen to a temperature around 80 K and LEED I-V (intensity vs electron energy) curves were measured at normal incidence. Dynamical tenser LEED calculations were performed using different surface structure models including neglecting hydrogen, ordered (1 X 1) hydrogen at atop, bridge, and fourfold hollow sites. It has been found that within the converging range of atomic displacements, the minimum overall Pendry R factors (R(p)) for the above models are 0.28, 0.35, 0.31, and 0.28, respectively. For the first model, Pt(100) interlayer relaxations down to the fifth layer were calculated. The fourfold hollow site model which also gave an overall small R(p) showed scattered R(p) for different beams and it is likely that the surface contained disordered hydrogen.

Note on the hydrogen adsorption—desorption voltammogram on Pt(111) in sulfuric acid solution

The remarkable shapes of hydrogen adsorption-desorption voltammograms of basal planes of platinum single crystal electrodes have been noted since the first reports by Clavilier’s group [1,2]. Special attention has been paid to the sharp peaks observed at 0.45 V vs. RHE on Pt(ll1) in 0.5 M sulfuric acid solution. In order to understand the structure of the interface between Pt(ll1) and sulfuric acid solutions, many attempts have been made by applying different instrumental techniques, such as Auger electron spectroscopy, low energy electron diffraction [3], infrared (IR) absorption spectroscopy [4,5], second harmonic generation 161, Uv/Vis reflectance [7], scanning tunneling microscopy (STM) [S] and radioactive labeling [9]. Despite these efforts, we do not have a complete and clear picture of the behavior of the hydrogen atom, water, and anions of sulfuric acid and of the surface structure of Pt(ll1). For details of the electrochemistry at the platinum single crystal surfaces a well documented article [lo] should be referred to. In this note we show some findings on the hydrogen adsorption-desorption voltammograms and on the admittance plots at Pt(ll1) in sulfuric acid solution.

Influence of catalyst parameters and operational variables on the photocatalytic cleavage of water

The photocatalytic cleavage of water over Pt-RuO 2/TiO 2 catalysts under illumination in the near UV (250–400 nm) is investigated in a batch and in a semicontinuous flow photoreactor cell. The effects of operational variables and parameters related to chemical and physical characteristics of the catalyst on the rate of hydrogen production are investigated. It is shown that the rate of the process is insensitive to the details of surface structure of Pt; the rate exhibits a weak maximum at a Pt loading of 0.5 wt% and is independent of Pt loading when this exceeds 1 wt%. The rate of H 2 production is strongly dependent on the pH of the slurry, increasing significantly at pH levels higher than 10. The rate was also observed to be a linear function of the intensity of illumination in the near UV.

I n s i t u, real-time monitoring of electrode surfaces by scanning tunneling microscopy. III. Surface structure of Pt and Pd electrodes

Pt and Pd electrodes were monitored by STM in situ in real time. In the case of Cu deposition on Pd, small domains of Cu were first formed and they gradually merged to form terraces. The surface finally became relatively smooth. Surface structure change of Pt electrode during oxide formation/reduction was also investigated. Growth of parallel ridge and domelike structures was observed.

Electrochemical behaviour of irreversibly adsorbed bismuth on Pt (100) with different degrees of crystalline surface order

Pt (100) oriented electrodes yield a variety of voltammogram profiles in the region of hydrogen adsorption-desorption in aqueous sulphuric acid which have been shown to correspond to different degress of bidimensional long range order. Two main profiles, S I and S II are reached, depending on the conditions of pretreatment of the (100) surface in the presence of either oxygen or hydrogen, respectively, interacting with the surfaces. From the S I or S II states, others with various degrees of surface perturbation, S n III, can be achieved after n voltammetric cycles involving full oxygen adsorption-desorption. A general view of the different ways of interchanging the various surface structures, S I, S II or S n III, is given. The aim of the present work is to investigate the behaviour of bismuth adsorbed irreversibly on the various surface structures of Pt (100). It is shown that surface defects deliberately created on the surface like randomly distributed steps are the preferential place for the dissolution of the adsorbed bismuth. The blocking of hydrogen adsorption sites as a function of the amount of electric charge exchanged at oxidation in the surface redox reaction of bismuth gives a linear relation in the whole range of bismuth coverage. Its slope indicates that one bismuth atom would block two hydrogen adsorption sites, independently of the type of structure, S I or S II. The recovery of these two structures after bismuth electrochemical desorption was completed, has been established.