Borate adsorption at Pt(111) in acidic medium

The adsorption/desorption process of borate was studied at Pt(111) in acidic solution by cyclic voltammetry. A so-called butterfly wave in the cyclic voltammogram of Pt(111) in HClO₄ shifted to negative direction upon the addition of boric acid with the disappearance of its sharp spikes. The shift in potential was found to be −57 mV with a tenfold increase of boric acid concentration. This illustrates that this anomalous wave is due to borate adsorption/desorption by a one-electron transfer process. The borate adsorption/desorption wave was observed to shift by −63 mV/pH. At pH>3, the anomalous wave splits forming two separate waves, depending on the pH and the scan rate. The appearance of two waves is assigned to the change in the adsorption mode of borate or the participation of OH in the adsorption process.     

Reduction of NO adlayers on Pt(110) and Pt(111) in acidic media: evidence for adsorption site-specific reduction

We present a combined in situ Fourier transform infrared reflection-absorption spectroscopy and voltammetric study of the reduction of saturated and subsaturated NO adlayers on Pt(111) and Pt(110) single-crystal surfaces in acidic media. The stripping voltammetry experiments and the associated evolution of infrared spectra indicate that different features (peaks) observed in the voltammetric profile for the electrochemical reduction of NO adlayers on the surfaces considered are related to the reduction of NO(ads) at different adsorption sites and not to different (consecutive) processes. More specifically, reduction of high- and intermediate-coverage (ca. 0.5-1 monolayers (ML)) NO adlayers on Pt(110) is accompanied by site switching from atop to bridge position, in agreement with the ultra-high-vacuum data. On Pt(111) linearly bonded (atop) NO and face-centered cubic 3-fold-hollow NO species coexist at high coverages (0.25-0.5 ML) and can be reduced consecutively and independently. On Pt(111) and Pt(110) electrodes, linearly bonded NO species are more reactive than multifold-bonded NO species. Both spectroscopic and voltammetric data indicate that ammonia is the main product of NO(ads) reduction on the two surfaces examined.     

The kinetic study of specific adsorption of phosphate species on Pt(111) in acidic solutions

The study of the adsorption/desorption mechanism of phosphate anions at Pt(111) in acidic solution of pH 4.3 and 0.8 was performed by the potential step method in order to reveal the kinetics of anion adsorption. The current-time curve due to phosphate adsorption/desorption showed various decay features, being dependent on the potential region. The rate of current decay depended on pH, being faster in a lower pH solution. Specific adsorption processes were analyzed by the Langmuir and Elovich adsorption equations and also in terms of a two-dimensional nucleation-growth mechanism in different adsorption/desorption regions. In the case of adsorption in 0.3M phosphate buffer solution of pH 4.3, random adsorption without interaction following the Langmuir adsorption, takes place at low coverage, while random adsorption with repulsive force was observed at high coverage. In the desorption process, random desorption with repulsive force takes place at high coverage, and the repulsive force disappears where random adsorption without interaction takes place at medium coverage. When the surface coverage becomes further lower, the desorption mechanism changes dramatically into a two-dimensional nucleation-growth type, suggesting that an ordered adsorbate structure is formed after a rapid discharge process of anion adsorption.     

Adsorption of formaldehyde on Pt(111) and Pt(100) electrodes: cyclic voltammetry and scanning tunneling microscopy

The adsorption of formaldehyde (HCHO) on Pt(111) and Pt(100) electrodes was examined by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in 0.1 M HClO(4). The extent of HCHO adsorption at both Pt electrodes was evaluated by comparing the CVs, particularly for the hydrogen adsorption and desorption between 0.05 and 0.4 V, obtained in 0.1 M HClO(4) with and without HCHO. The adsorption of HCHO on these Pt electrodes was significant only when [HCHO] >/= 10 mM. Adsorbed organic intermediate species acted as poisons, blocking Pt surfaces and causing delays in the oxidation of HCHO. Compared to Pt(111), Pt(100) was more prone to poisoning, as indicated by a 200 mV positive shift of the onset of HCHO oxidation. However, Pt(100) exhibited an activity 3 times higher than that of Pt(111), as indicated by the difference in peak current density of HCHO oxidation. Molecular resolution STM revealed highly ordered structures of Pt(111)-( radical7 x radical7)R19.1 degrees and Pt(100)-( radical2 x radical2) in the potential region between 0.1 and 0.3 V. Voltammetric measurements further showed that the organic poisons produced by HCHO adsorption behaved differently from the intentionally dosed CO admolecules, which supports the assumption for the formation of HCO or COH adspecies, rather than CO, as the poison. On both Pt electrodes, HCHO oxidation commenced preferentially at step sites at the onset potential of this reaction, but it occurred uniformly at the peak potentials.     

First principles study of sulfuric acid anion adsorption on a Pt(111) electrode

A first principles theory combined with a continuum electrolyte theory is applied to adsorption of sulfuric acid anions on Pt(111) in 0.1 M H(2)SO(4) solution. The theoretical free energy diagram indicates that sulfuric acid anions adsorb as bisulfate in the potential range of 0.41 < U ≤ 0.48 V (RHE) and as sulfate in 0.48 V (RHE) < U. This diagram also indicates that sulfate inhibits formations of surface oxide and hydroxide. Charge analysis shows that the total charge transferred for the formation of the full coverage sulfate adlayer is 90 μC cm(-2), and that the electrosorption valency value is -0.45 to -0.95 in 0.41 < U ≤ 0.48 V (RHE) and -1.75 to -1.85 in U > 0.48 V (RHE) in good agreement with experiments reported in the literature. Vibration analysis indicates that the vibration frequencies observed experimentally at 1250 and 950 cm(-1) can be assigned, respectively, to the S-O (uncoordinated) and symmetric S-O stretching modes for sulfate, and that the higher frequency mode has a larger potential-dependence (58 cm(-1) V(-1)) than the lower one.     

Electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) surfaces

The electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) has been studied using voltammetry, chronoamperometry, and in situ infrared spectroscopy. The oxidative adsorption of ammonia results in the formation of NH(x) (x = 0-2) adsorbates. On Pt(111), ammonia oxidation occurs in the double-layer region and results in the formation of NH and, possibly, N adsorbates. The experimental current transients show a hyperbolic decay (t(-1)), which indicates strong lateral (repulsive) interactions between the (reacting) species. On Pt(100), the NH(2) adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH(2) fragments on Pt(111) and Pt(100), respectively, is in an interesting agreement with recent theoretical predictions. The Pt(111) surface shows extremely low activity in ammonia oxidation to dinitrogen, thus indicating that neither NH nor N (strongly) adsorbed species are active in dinitrogen production. Neither nitrous oxide nor nitric oxide is the product of ammonia oxidation on Pt(111) at potentials up to 0.9 V, as deduced from the in situ infrared spectroscopy measurements. The Pt(100) surface is highly active in dinitrogen production. This process is characterized by a Tafel slope of 30 mV decade(-1), which is explained by a rate-determining dimerization of NH(2) fragments followed by a fast decay of the resulting surface-bound hydrazine to dinitrogen. Therefore, the high activity of the Pt(100) surface for ammonia oxidation to dinitrogen is likely to be related to its ability to stabilize the NH(2) adsorbate.     

The temperature dependence of hydrogen and anion adsorption at a Pt( 100) electrode in aqueous H2SO4 solution

Research on the underpotential deposition of H (upd H) and anion adsorption on Pt( 100) in 0.5 M aqueous H₂SO₄ solution by cyclic voltammetry (CV) indicates that the overall adsorption/desorption charge density is affected strongly by variation of the temperature T, and that it decreases by about 1/3 when T is raised from 293 to 328 K. The sharp peak at 0.375 V vs. RHE assigned to the anion adsorption decreases its potential, current density and charge density. The CV feature assigned to the upd H, a wide shoulder overlapping the sharp peak, also decreases with T augmentation, but the decline of its charge density is less pronounced. The results indicate that the H_upd and anion surface coverages, θ_HAN and θ_AN respectively are strongly temperature-dependent. This behavior may be assigned to lateral repulsive interactions.     

Kinetics of dissociative adsorption of formic acid on Pt(100), Pt(610), Pt(210) and Pt(110) single-crystal electrodes in perchloric acid solutions

The kinetics of dissociative adsorption of HCOOH on Pt(100), Pt(610), Pt(210) and Pt(110) single-crystal electrodes has been investigated. The oxidation of dissociative adsorbate (DA) derived from the dissociative adsorption of HCOOH was used as a probe reaction together with the potential-step technique of short time windows. The quantity Q_ad of DA produced at a given potential E_ad and in a defined time window t_ad of adsorption has been determined systematically. At fixed t_ad peaked curves of Q_ad versus E_ad in the potential range between −0.25 and 0.25 V/SCE have been obtained on all four electrodes. The maximum rate of dissociative adsorption of HCOOH decreases in the order Pt(110) Pt(210) Pt(610) Pt(100).     

Determination of O and OH adsorption sites and coverage in situ on Pt electrodes from Pt L(2,3) X-ray absorption spectroscopy

The adsorption of atomic oxygen and hydroxide on a platinum electrode in 0.1 M HClO4 or H2SO4 electrolytes was studied in situ with Pt L(2,3) X-ray absorption spectroscopy (EXAFS and XANES). The Pt L3 edge absorption data, mu, were collected at room temperature in transmission mode on beamline X-11A at the National Synchrotron Light Source using a custom built cell. The Pt electrode was formed of highly dispersed 1.5-3 nm particles supported on carbon. A novel difference procedure (delta mu = mu(O[H]/Pt) - mu(Pt)) utilizing the L3 XANES spectra at different applied voltages was used to isolate the effects of O[H] (O or OH) adsorption in the XANES spectra. The Deltamu results are compared with results obtained from real-space full-multiple scattering calculations utilizing the FEFF8 code on model clusters. The experimental results, when compared with theoretical calculations, allow the adsorption site to be identified. At low coverages OH adsorbs primarily in 1-fold coordinated atop sites. As the coverage increases, O binds in the bridge/fcc sites, and at still higher coverages above 1.05 V RHE, O adsorbs into a higher coordinated n-fold or subsurface site, which is thought to be the result of Pt-O site exchange and oxide formation. These results are similar to those found in the gas phase. Direct specific adsorption of bisulfate anions in H2SO4 is spectroscopically seen in both the EXAFS and XANES data and is seen to impede oxygen adsorption consistent with previous reports.     

Adsorption isotherm of CO on Pt(111) electrodes.

We have determined, for the first time, the equilibrium CO coverage of Pt(111) electrodes at room temperature in 0.1 M H(2)SO(4) as a function of the CO partial pressure using CO-stripping cyclic voltammetry. Fourier-transform infrared (FT-IR) spectroscopy was used to confirm qualitatively the coverage values obtained.     

Spectroscopic investigation of the adsorption and oxidation of thiourea on polycrystalline Au and Au(111) in acidic media

In the present paper, a systematic electrochemical investigation on thiourea (TU) electrooxidation was developed on polycrystalline and (111) single-crystal gold electrodes in 0.1 M perchloric acid. The combination of cyclic voltammetry with in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry techniques have allowed the nature of the species formed during the electroadsorption and electrooxidation of TU to be established. FTIRS experiments were performed in D2O to clean up the region of the H2O bending around 1600 cm(-1). It was concluded that TU adsorbs tilted on the surface in the 0.05-0.40 VRHE potential range. A dual-path reaction mechanism was evidenced in the oxidation process. The first pathway takes place from adsorbed TU at E > 0.40 VRHE and implies the formation of [Au(I)-(TU)2]+, which is oxidized to NH2CN and S0 at E > 0.80 VRHE. In a following oxidation step at E > 1.20 V, N2, CO2, and HSO4-/SO4(2-) were produced. The second parallel reaction occurs from TU in solution at E > 0.50 VRHE to form (TU)2(2+). All these species were characterized from the spectroscopic experiments. Similar results were obtained for both surfaces.     

n-alkanes on Pt(111) and on C(0001)Pt(111): chain length dependence of kinetic desorption parameters

We have measured the desorption of seven small n-alkanes (C(N)H(2N+2), N=1-4,6,8,10) from the Pt(111) and C(0001) surfaces by temperature programed desorption. We compare these results to our recent study of the desorption kinetics of these molecules on MgO(100) [J. Chem. Phys. 122, 164708 (2005)]. There we showed an increase in the desorption preexponential factor by several orders of magnitude with increasing n-alkane chain length and a linear desorption energy scaling with a small y-intercept value. We suggest that the significant increase in desorption prefactor with chain length is not particular to the MgO(100) surface, but is a general effect for desorption of the small n-alkanes. This argument is supported by statistical mechanical arguments for the increase in the entropy gain of the molecules upon desorption. In this work, we demonstrate that this hypothesis holds true on both a metal surface and a graphite surface. We observe an increase in prefactor by five orders of magnitude over the range of n-alkane chain lengths studied here. On each surface, the desorption energies of the n-alkanes are found to increase linearly with the molecule chain length and have a small y-intercept value. Prior results of other groups have yielded a linear desorption energy scaling with chain length that has unphysically large y-intercept values. We demonstrate that by allowing the prefactor to increase according to our model, a reanalysis of their data resolves this y-intercept problem to some degree.     

Metallization of cyanide-modified Pt(111) electrodes with copper

Journal of Solid State Electrochemistry - The reduction of Cu2+ ions irreversibly attached to the surface of a cyanide-modified Pt(111) electrode via non-covalent or weakly covalent interactions...     

Surface characterization of Pt electrodes using underpotential deposition of H and Cu: Part III. Surface improvement of the flame-annealed Pt(100) and Pt(111) electrodes via potential cycling

In Parts I and II of this series it was shown that the Pt(100) and Pt(111) surfaces pretreated by flame-annealing and quenching in aqueous electrolyte contain a high density of defects such as vacancies, Pt adatoms and clusters of Pt adatoms. In this paper we show that potential cycling including scans into the oxygen region in sulfuric or perchloric acid removes most of these sites and that a limited number of cycles yield hydrogen adsorption-desorption profiles (cyclic voltammograms) that compare favorably with those published by authors who established the structure using electron diffraction techniques. Some loss of longer-range surface order as a result of the potential cycling is indicated by an increase in the width at half-height of the monolayer copper stripping peaks. The possibility of surface improvement in the absence of surface oxidation and reduction is explored by potential cycling in hydrochloric acid.     

Electrochemical induction of changes in the distribution of the hydrogen adsorption states on Pt (100) and Pt (111) surfaces in contact with sulphuric acid solution

Electrochemical adsorption-desorption of hydrogen has been shown to be a structure-sensitive reaction. The influence of the crystallographic surface structure on this process is typically represented by the voltammogram of such a surface in contact with 0.5 M H₂SO₄ solution.For a given crystallographic plane, the voltammetric profile shows several peaks which are ascribed to various H adsorption states. Defined polarization programmes, applied to such electrodes, changed in a controlled way the population of each state with the possibility of inducing new ones.These transformations in the shape of the Pt (100) voltammogram in the hydrogen adsorption region suggest that the surface undergoes reconstruction due to hydrogen adsorption above 0.3 of a monolayer. The combined effect of low oxygen coverage with high hydrogen coverage induces a new type of voltammogram. At least four types of voltammogram have been observed for this orientation, each related to a particular surface structure which remains unknown in the present state of knowledge. This behaviour is in agreement with the known behaviour of this surface in contact with the gas phase at low pressure whose surface structure depends on the presence or not of an adsorbate such as hydrogen and on thermal treatment. This work gives preliminary evidence for the existence of possibilities of passing reversibly, by an electrochemical route, from one surface structure to another.For the Pt (111) surface, only one ordered state and a disturbed state with a different degree of disorder are observed. There is no known electrochemical possibility of passing reversibly from one state to the other; the transformation always occurs from the ordered to the disordered state.Comparison of these various voltammograms with that of a polyoriented model surface represented by a sherical Pt single crystal suggest that for both orientations the most strongly bonded hydrogen states are directly connected with the presence of two-dimensional long-range ordered domains on these low index surfaces.     

Heat of adsorption of naphthalene on Pt(111) measured by adsorption calorimetry

The heat of adsorption of naphthalene on Pt(111) at 300 K was measured with single-crystal adsorption calorimetry. The heat of adsorption on the ideal, defect-free surface is estimated to be (300 - 34 - 199(2)) kJ/mol. From this, a C-Pt bond energy for aromatic hydrocarbons on Pt(111) of approximately 30 kJ/mol is estimated, consistent with earlier results for benzene on Pt(111). There is higher heat of adsorption at very low coverage, attributed to step sites where the adsorption heat is >/=330 kJ/mol. Saturation coverage, = 1 ML, corresponds to 1.55 x 10(14) molecules/cm(2). Sticking probability measurements of naphthalene on Pt(111) give a high initial value of 1.0 and a Kisliuk-type coverage dependence that implies precursor-mediated sticking. The ratio of the hopping rate to the desorption rate of this precursor is approximately 51. Naphthalene adsorbs transiently on top of chemisorbed naphthalene molecules with a heat of adsorption of 83-87 kJ/mol.     

New understanding of the nature of OH adsorption on Pt(1 1 1) electrodes

The adsorption of hydroxyl on Pt(1 1 1) single crystal electrodes from aqueous acidic solutions is carefully reinvestigated. The effect of small additions (10⁻⁸–10⁻⁵ M) of chloride and bisulphate anions on the OH adsorption region in perchloric acid solution has been studied. Two regions can be differentiated in the voltammetric profile, that behave differently after the addition of the foreign anion. The initial broad adsorption process is unaffected until the highest concentration is attained. However, the sharper peak at higher potentials is affected even at the lower anion concentration. Since mass transport limitations allow to discard the anion adsorption as the main process giving this peak, we propose that the two processes are due to the dissociative adsorption of two different kinds of water, that are affected by the anion in a different way. From this idea, a new model, based on the Frumkin adsorption isotherm, is proposed, which gives an excellent fit of the experimental results.