Cyclic voltammograms for H on Pt(111) and Pt(100) from first principles

Cyclic voltammetry is a fundamental experimental method for characterizing electrochemical surfaces. Despite its wide use, a way to quantitatively and directly relate cyclic voltammetry to ab initio calculations has been lacking. We derive the cyclic voltammogram for H on Pt(111) and Pt(100), based solely on density functional theory calculations and standard molecular tables. By relating the gas phase adsorption energy to the electrochemical electrode potential, we provide a direct link between surface science and electrochemistry.     

杂质对吸附系统O/RhxPt1-x衬底、合金(110)表面偏析的影响

构造了考虑吸附与偏析相互作用情况下无序二元合金Rhx Pt1-x(110)吸附氧表面的原子集团模型,其中O的覆盖度为0.5;构造了考虑杂质Ni,Cu,W对合金可能产生影响的吸附表面原子集团模型,杂质的掺入采用替位式.应用recursion方法计算了合金表面的环境敏感镶嵌能和电子结构.环境敏感镶嵌能计算表明杂质Ni,Cu和W均使O吸附RhxPt1-x(110)合金表面偏析情况发生逆转,Ni对Rh-Pt合金偏析的影响最大,其次是Cu,W对合金偏析的影响最小;电子结构计算表明杂质Ni,Ca W存在于合金表面时,使Rh与O的共价相互作用减弱,使表面偏析发生逆转,Pt再次偏析于表面.     

Thermally activated crystallization of Nb2O5 grown on Pt electrode

The influence of the local crystallographic orientation of the polycrystalline bottom platinum electrode on the crystallization of niobium pentoxide thin films during their rapid thermal annealing was investigated by X-ray diffraction, X-ray reflectivity and transmission electron microscopy. The Nb₂O₅ thin films under study were reactively sputtered in a mixed O₂/Ar atmosphere and subsequently subjected to the annealing in argon atmosphere at temperatures ranging from 500 ^°C to 700 ^°C. The X-ray diffraction confirmed a transition from the amorphous niobium oxide to the crystalline orthorhombic Nb₂O₅ for temperatures between 500 ^°C and 600 ^°C. The X-ray reflectivity measurements showed that the crystallization process was accompanied by a continuous increase of the electron density in Nb₂O₅ and by a rapid increase of the surface roughness at 700 ^°C. It was further observed by transmission electron microscopy that Nb₂O₅ crystallizes selectively and that the crystalline domains of Nb₂O₅ possess a strong orientation relationship to the platinum from the bottom electrode. The orientation relationship $(\bar{1} 1 1)_{\mathrm{Pt}}\,{\parallel}\, (\bar{1} \bar{6}0)_{\mathrm{Nb}_{2}\mathrm{O}_{5}}$ was identified as the most beneficial one for crystallization of Nb₂O₅.     

Reaction kinetics of the Pt, O2(g)|c-ZrO2 system: precursor-mediated adsorption

A micro kinetic model of the Pt, O₂(g)|c-zirconia electrode/electrolyte system was developed in state space form (model M3). The oxygen adsorption/desorption process was modeled as a precursor-mediated surface reaction. The surface diffusion of atomic oxygen and the electrochemical reduction of atomic oxygen near the three-phase boundary (tpb) were considered. It was shown that the simulated charge-transfer behavior of M3 is significantly different from models with ordinary Langmuir kinetics (model M2). The electrochemical rate constant was estimated from selected experimental data as k₁₀=(6.05±0.25)·10⁶ m³/(mol·s). From experimental results it was concluded that only one adsorbed oxygen species is relevant for the dynamic behavior. In porous Pt electrodes binary gas phase diffusion of oxygen in O₂/N₂ gas mixtures becomes relevant at oxygen partial pressures below 10⁻³ atm. The general procedure for state and parameter estimation can be well adopted for the investigation of further reaction mechanisms.     

Behaviour of a dense In2O3/ YSZ electrode in oxygen containing atmospheres

A study of electrical and electrochemical properties of a dense In₂O₃ electrode in contact with a single crystal YSZ electrolyte was carried out by d.c. and a.c. methods. As a result, it was found that dense In₂O₃ electrodes have high electrical conductivity but very low electrochemical activity. In a vicinity of the equilibrium potential and under the anodic polarisation, the rate of Faraday reaction at the In₂O₃ electrodes was as low as to consider the electrode a blocking one. The blocking properties of the In₂O₃ electrodes were used to measure the hole conductivity of the YSZ electrolyte in the temperature range between 795 to 1163 K and oxygen partial pressure from 1 to 10⁵ Pa. A comparison with the literature data confirmed that the dense In₂O₃ electrode blockes the ionic transfer through the YSZ. A set of experiments indicated that the oxygen exchange between the indium oxide surface presented to the oxygen containing gaseous phase and this phase is very poor. A route of the electrode process at O₂, In₂O₃ / YSZ electrode was proposed a limiting stage of which is the discharge of the oxygen ions to the atomic oxygen adsorbed on the electrode surface: $$O_0 ^x \left( {In_2 O_3 } \right)_s = V_0 ^{ \bullet \bullet } \left( {In_2 O_3 } \right)_s + O_{ad} \left( {In_2 O_3 } \right)_s + 2e'\left( {In_2 O_3 } \right).$$ The polarisation resistance decreases when platinum or the praseodymium oxide is deposited on the surface of the In₂O₃ electrodes. The cathodic polarisation also increases the electrochemical activity of the electrodes. Both the establishment of the steady state of the electrode under polarisation and the recovery of the equilibrium state by the electrode are very long processes, which are probably related to the diffusion mechanism by which the stoichiometry of the indium oxide is changed.     

Polarisation conductivity of the O2,Pt/Zr(Y)O2 and H2-H2O, Pt/Zr(Y)O2 electrodes

Pt electrodes with defined contact geometries were studied by using impedance spectroscopy. The specific polarisation conductivity per unit length of the three-phase boundary was determined. It is found to be 1 × 10⁻⁴ S·cm⁻¹ at 977 °C in an atmosphere of “pure” hydrogen with an oxygen partial pressure of 10⁻²⁰ atm at 1000 °C. Investigations carried out in an atmosphere of pure oxygen revealed a pronounced dependence of the polarisation conductivity on the electrode history. The polarisation conductivity was found to be in a range of 2 × 10⁻⁴ to 6.5 × 10⁻⁴ S·cm⁻¹ at a temperature of 977 °C. It was possible to estimate the area of the electrolyte surface which takes part in the electrode reaction. The real exchange current density was determined.     

Electrochemical promotion of the NO reduction by C2H4 on Pt/YSZ and by CO on Pd/YSZ

The effect of electrochemical promotion was investigated for the catalytic reduction of nitric oxide with ethylene and carbon monoxide on polycrystalline Pt and Pd, respectively, deposited on yttria-stabilized zirconia (YSZ). It was found in both cases that applying negative potentials and thus lowering the catalyst work function results in a pronounced increase in the catalytic rate and in the selectivity to nitrogen. A 7-fold increase was observed for the NO+C₂H₄ reaction on Pt while a 2-fold increase was obtained for the NO+CO reaction on Pd. The induced changes in catalytic rates were found to be 7 to 50 times higher than the rates of ion transfer from the catalyst surface. In both reactions, the observed electrophilic behavior can be attributed to the strengthening of the chemisorptive NO bond and concomitant enhanced dissociation of NO as the catalyst potential and work function is decreased. Forced periodic oscillations of the applied current was investigated and resulted in a enhanced production of CO₂, but an intermediate selectivity towards N₂, as compared to constant current application. The effect of the cycling waveform, frequency and amplitude was studied and provided evidence that the synergy observed during the cycling experiment results from a favorable transient coverage of adsorbed species on the catalyst surface as the catalyst potential oscillates from negative to positive values. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

The effects of hidden impurities on the reactivity of a Pt(111) crystal

The interaction of water with a Pt(111) surface contaminated with Si has been examined by thermal desorption spectroscopy (TDS), work function (Δφ) and sticking coefficient measurements. Water is found to dissociate on the contaminated surface when all traces of oxide have been removed. When the silicon is oxidized, no water dissociation is observed and desorption order apparently changes from one to zero. A model is discussed which explains observed sticking coefficient changes and thermal desorption spectra in terms of island size.     

Improved catalytic activity of Pt/rGO counter electrode in In2O3-based DSSC

A platinum/reduced graphene oxide (Pt/rGO) nanocomposite acting as a counter electrode (CE) was fabricated using a chemical bath deposition method for In₂O₃-based dye-sensitized solar cell (DSSC) via sol-gel technique. The report analyzes the morphological and electrochemical impedance spectroscopy of the annealing Pt/rGO films at 350, 400, and 450 °C. Micrograph images obtained from field emission scanning electron microscopy demonstrated the annealed films are highly porous. The energy-dispersive X-ray results show that the carbon atoms were homogeneously distributed on the film annealed at 400 °C. A good photovoltaic performance was exhibited with high photocurrent density of 8.1 mA cm⁻² and power conversion efficiency (η) of 1.68 % at the Pt/rGO CE annealed at 400 °C. The employed electrochemical impedance spectroscopy analysis quantifies that the Pt/rGO films annealed at 400 °C provide more efficient charge transfer with the lowest effective recombination rate and high electron life time, hence improving the performance of Pt/rGO CE.

     

H2O on Pt(111): structure and stability of the first wetting layer

We study the structure and stability of the first water layer on Pt(111) by variable-temperature scanning tunneling microscopy. We find that a high Pt step edge density considerably increases the long-range order of the equilibrium √37 × √37R25.3°- and √39 × √39R16.1°-superstructures, presumably due to the capability of step edges to trap residual adsorbates from the surface. Passivating the step edges with CO or preparing a flat metal surface leads to the formation of disordered structures, which still show the same structural elements as the ordered ones. Coadsorption of Xe and CO proves that the water layer covers the metal surface completely. Moreover, we determine the two-dimensional crystal structure of Xe on top of the chemisorbed water layer which exhibits an Xe-Xe distance close to the one in bulk Xe and a rotation angle of 90° between the close-packed directions of Xe and the close-packed directions of the underlying water layer. CO is shown to replace H(2)O on the Pt(111) surface as has been deduced previously. In addition, we demonstrate that tunneling of electrons into the antibonding state or from the bonding state of H(2)O leads to dissociation of the molecules and a corresponding reordering of the adlayer into a √3 × √3R30°-structure. Finally, a so far not understood restructuring of the adlayer by an increased tunneling current has been observed.     

Interaction of H2O,CO, and H2 with Pt(111) and Pt(111)+K surfaces

The potential energy and surface dipole were calculated as a function of the geometry of the coadsorbed systems using the cluster method and theoretical oscillation frequencies and work function changes were compared with experiment. It was found that the K fills unoccupied Pt 5d states and reduces the local polarizability of the metal. The H₂O molecule binds to the K atom, such that the H atoms point towards the surface inducing an increase in the work function. For the CO molecule a charge transfer (KCO) through the surface stabilizes the bond and induces a change of adsorption place (on-topbridge). The K increases the tendency to H₂ dissociation because of the local decrease of the work function. Zero-point energy effects add important dynamical features to the electronic H₂- surface interaction. Three examples for the Pt(111)-H₂ system are presented: (i) A virtual attractive potential well produced by the softening of the H-H bond near the surface, (ii) a virtual potential barrier for dissociation due to the hindering of molecular rotations at the surface, and (iii) a change in the apparent surface temperature in H₂ desorption processes.     

Characterization of oxide impurities on Pt and their effect on the adsorption of CO and H2

The interactions between oxide support materials and Pt have been studied by incorporating silica, alumina, titania, and niobia into the surface of a clean Pt foil. Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of CO and H₂ were used for surface characterization. For all of these oxides, TPD indicated no change in the adsorption properties of CO and H₂. Peak temperatures were unaffected by the presence of oxide impurities. For silica and alumina, AES results indicated that suboxides could be formed after oxidation at 400 and 800 K respectively. Al₂O₃ and SiO₂ were formed at higher temperatures. Relatively large quantities of these oxides were required to substantially decrease the saturation coverages of CO and H₂, indicating that these oxides probably form clusters on the metal surface. For titania and niobia, AES indicated that these oxides dissolved into the Pt above 1300 K, but segregated back to the surface below 500 K. These segregated layers cover the Pt evenly and both oxides completely suppress H₂ and CO adsorption at an oxygen coverage of 1 × 10¹⁵/cm². These results are used to discuss the possible reasons for differences in the catalytic properties of Pt on these four oxide supports.     

Effect of impurities on the surface morphology of Fe(111)

The surface composition and morphology of Fe(111) have been examined through a combined analysis that includes low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The preferential segregation of sulfur has been clearly identified by AES upon annealing. The STM images exhibit numerous triangular pits of various sizes, and the LEED patterns show diffused n × 1 spots. The triangular pits reveal a Sierpinski gasket fractal. For sulfur-free Fe(111), nitrogen segregates to the surface upon annealing, forming a 4√3 × 4√3 superstructure that is identified by LEED patterns and STM images. The STM images show nanoscale triangular clusters regularly aligned in a hexagonal 4√3 × 4√3 configuration. Ultra-thin chromium film deposited on a nitrogen-segregated Fe(111) surface with post-annealing induces further nitrogen segregation, resulting in the formation of triangular pyramid-shaped CrN nanoclusters.     

Interaction of O2 with Pt(100): I. Equilibrium measurements

Two newly discovered phases on the Pt(100) surface produced by the adsorption of oxygen have been investigated using Rutherford baekscattering (RBS), nuclear microanalysis (NMA), work function changes (Δφ) and LEED. One phase is associated with the oxygensaturated surface (0.63 ± 0.03 monolayers0.81 × 10¹⁵ O atoms cm^?2), where a very complex LEED pattern is observed; the other is observed at an average coverage of 0.44 ± 0.05 monolayers and gives rise to a (3 × 1) LEED pattern (when observed at room temperature). For both surfaces, RBS measurements indicate large (? 0.025 nm) Pt atom displacements. Also discussed is a new method for preparing the “clean” (1 × 1)-Pt(100) surface without the need for NO adsorption/decomposition.     

The desorption of CO from small Pt particles on Al2O3

The adsorption of CO on small Pt particles supported on alumina was studied using temperature programmed desorption (TPD). Samples were prepared by vapor deposition of Pt onto a flat substrate in ultra high vacuum. Metal coverages were reproducibly obtained using a film thickness monitor which was calibrated with Auger electron spectroscopy (AES). AES results indicated that Pt grew in a layer-by-layer manner on alumina at both 90 and 300 K and that these metal films aggregated into particles when heated above 650 K in vacuum. The average particle size could be estimated from the amount of CO desorbing in TPD and from the metal coverage and could be varied from 1.1 nm up to a continuous film. For the smallest particles, CO desorbed in a single state at 510 K. For larger particles, a second desorption state at 400 K was also observed. Since the desorption of CO occurs at similar temperatures on single crystals of Pt, these results indicate that the adsorption properties for CO on small particles of Pt on alumina are very similar to those for CO on bulk Pt. The change in the relative populations of the two desorption states with increasing particle size is interpreted as evidence for the formation of (111)-type facets on the larger particles.     

Ultrathin Y2O3(111) films on Pt(111) substrates

The growth of ultrathin films of Y₂O₃(111) on Pt(111) has been studied using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), and low energy electron diffraction (LEED). The films were grown by physical vapor deposition of yttrium in a 10^? 6 Torr oxygen atmosphere. Continuous Y₂O₃(111) films were obtained by post-growth annealing at 700 °C. LEED and STM indicate an ordered film with a bulk-truncated Y₂O₃(111)–1 × 1 structure exposed. Furthermore, despite the lattices of the substrate and the oxide film being incommensurate, the two lattices exhibit a strict in-plane orientation relationship with the [11?0] directions of the two cubic lattices aligning parallel to each other. XPS measurements suggest hydroxyls to be easily formed at the Y₂O₃ surface at room temperature even under ultra high vacuum conditions. The hydrogen desorbs from the yttria surface above ~ 200 °C.     

The adsorption of CO,O2, and H2 on Pt: II. Ultraviolet photoelectron spectroscopy studies

Ultraviolet photoelectron spectroscopy (UPS) has been used to study the chemisorption of CO, O₂, and H₂ on platinum. Three single crystal surfaces ((111), 6(111) × (100), and 6(111) × (111)) and two polycrystalline surfaces were studied. These studies yielded three important results. First, the most dominant change in the Pt valence band upon gas adsorption was a decrease in the height of the peak immediately below the Fermi level. This decrease was nearly identical for all three gases studied. Second, CO adsorption resulted in the formation of a resonance state ～8 eV below the Fermi level which was attributed to CO molecular orbitals. In contrast, no dominant resonance states were observed for adsorbed O or H. The lack of an O resonance state on platinum is in contrast to the results observed for O adsorbed on Fe and Ni and suggests important differences between the OPt chemisorption bond and the OFe and ONi chemisorption bonds. Finally, adsorption of CO at steps or defects led to a decrease in work function while its adsorption on terraces led to an increase in work function. For H, adsorption at steps led to an increase in work function while adsorption on terraces led to a decrease in work function. The adsorption of O led to an increase in work function on all of the surfaces studied.     

The adsorption of CO,O2, and H2 on Pt: I. Thermal desorption spectroscopy studies

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O₂, and h₂ on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O₂ adsorption at steps or defects than on terraces.