In situ FTIRS and EQCM studies of glycine adsorption and oxidation on Au(1 1 1) electrode in alkaline solutions

Dissociative adsorption and oxidation of glycine on Au(1 1 1) single crystal electrodes in alkaline solutions were studied in the present paper using cyclic voltammetry (CV), in situ FTIR spectroscopy (FTIRS) and electrochemical quartz crystal microbalance (EQCM). In situ FTIRS results demonstrated that adsorbates derived from glycine dissociative adsorption are adsorbed cyanide anions (CN_ad⁻). The CN_ad⁻ species are stable on Au(1 1 1) surface in the potential region from −0.8 to 0.0 V, and can be oxidized when electrode potential is increased above 0.1 V. The oxidation of CN_ad⁻ releases surface active sites for further glycine oxidation. The products of CN_ad⁻ oxidation were determined by in situ FTIRS as cyanate (OCN⁻), aurous cyanide (AuCN) and aurous di-cyanide (Au(CN)₂⁻). The formation of Au(CN)₂⁻ may initiate a dissolution of Au(1 1 1) surface atoms, which has been confirmed by a loss of surface mass determined in EQCM studies. It has revealed also that at high electrode potential region glycine may be split on Au(1 1 1) surface to form AuCH₂NH₂ and AuCOO⁻ adsorbates. Further oxidation of these species yielded CO₂ and -NH₂, and the AuCH₂NH₂ may be also combined with surface Au oxide to form methylamine. The CO₂ species produced in glycine oxidation are all retained in alkaline solutions to generate carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) species that were clearly determined by in situ FTIRS studies.     

Dimethoxymethane electrooxidation on low index planes of platinum single crystal in acid media

Conventional electrochemical methods have been applied to study the oxidation of a possible alternative fuel for a direct oxidation fuel cell. The electrooxidation of dimethoxymethane (DMM) was investigated on the three low index planes, (1 0 0), (1 1 0) and (1 1 1) of platinum single crystals and compared with its oxidation on a platinum polycrystalline electrode. Among platinum electrodes, electroreactivity of DMM observed is Pt(1 1 1) > Pt(1 0 0) > Pt(1 1 0) ∼ Pt poly. Hydrogen adsorption is limited by the presence of DMM, except for Pt(1 1 1) plane. In situ IR experiments show the presence of bands of CO_ads with all electrodes except Pt(1 1 1). This work shows that the mechanism of DMM electrooxidation is structure sensitive. A path takes place on Pt(1 0 0) and Pt(1 1 0) which is favourable to the formation of CO_ads. Another path proceeds on Pt(1 1 1), where CO_ads is not present and reaction does not occur at low potential. Results indicate that peak intensities are higher in perchloric acid than in sulphuric acid. So DMM adsorption is dependent on the specific adsorption of the anions. In situ IR reflectance spectroscopy identified some intermediates and reaction products of DMM adsorption and electrooxidation on Pt electrodes: CO_L (linearly bonded) and CO_B (bridge bonded), adsorbed CHO and CH₃O species, methanol and CO₂. The electrochemical and spectroelectrochemical experiments suggest a complex mechanism of DMM electrooxidation.     

Electrooxidation of ethanol at polycrystalline and platinum stepped single crystals: A study by differential electrochemical mass spectrometry

The electrooxidation of adsorbed and bulk solution of 10⁻² M ethanol and D₆-ethanol at polycrystalline platinum, smooth, roughened and Ru modified Pt(3 3 2), Pt(3 3 1) and Pt(1 1 1) electrodes was studied by on-line differential electrochemical mass spectroscopy (DEMS) using a dual thin layer flow through cell.On polycrystalline Pt, the main (or even single) product is acetaldehyde; due to the flow through conditions the amount of acetaldehyde further oxidized to acetic acid is negligible. At stepped single crystals with (1 1 1) terraces (Pt(s)[n(1 1 1) × (1 1 1)], acetic acid is produced at a lower potential than acetaldehyde. This demonstrates that in addition to the reaction path involving C-C bond splitting leading to CO₂ (via adsorbed CO and CH_x) and the reaction path leading to acetaldehyde there is a third, direct reaction path leading to the formation of acetic acid.Step decoration by Ru does not lead to an increased reactivity. This is different from the strong cocatalytic effect of Ru at step sites on the oxidation of CO. Furthermore, Ru does not influence the relative amount of acetaldehyde formed.     

The stability of adsorbed quinoline and cinchonine on poly- and monocrystalline platinum surfaces

The adsorption of quinoline and cinchonine on Pt (1 1 1), Pt (3 3 2) and polycrystalline Pt electrode has been studied by differential electrochemical mass spectrometry (DEMS). From the surface-coverage data and from the potential dependence of both the faradaic oxidation current and the rate of CO₂ formation during the oxidation of quinoline and cinchonine on Pt (1 1 1), we conclude that both molecules are bound through the π-system to the electrode surface. The only anodic desorption product found was CO₂. Surface concentrations for both molecules were found to be around 0.1-0.2 nmol cm⁻². It was also found that quinoline completely desorbs from the Pt (1 1 1) electrode around 0 V, provided that the electrolyte in the thin-layer cell is exchanged for fresh electrolyte; in contrast, desorption from Pt (3 3 2) and polycrystalline Pt is incomplete. Cinchonine does desorb in part from polycrystalline Pt, but not notably from Pt (1 1 1) due to an additional binding interaction of the exocyclic vinyl group linked to the quinuclidine moiety. No decomposition products, e.g. alkanes, were detected during such cathodic potential sweeps.Further experiments revealed that coadsorption of CO on polycrystalline Pt notably reduces the amount of carbon dioxide formed during subsequent anodic potential sweeps for pre-adsorbed quinoline and cinchonine, pointing to a partial displacement of the modifiers. In contrast, ethene is coadsorbed without displacing the original adsorbate and can still undergo hydrogenation when a negative potential is applied. Benzene is also coadsorbed to some extent, but its hydrogenation, which usually occurs on an unmodified surface, is largely diminished.     

Understanding CO-stripping mechanism from NiUPD/Pt(1 1 0) in view of the measured nickel formal partial charge number upon underpotential deposition on platinum surfaces in sulphate media

We recently showed nickel-underpotential deposition (Ni-UPD) occurs on polycrystalline or single crystal platinum electrodes in acidic media. Whereas the decoupling of the nickel and hydrogen adsorption/desorption peaks is difficult for low pH, these processes can be better separated for higher pH values, typically pH > 3. However, even for platinum single crystals, high pH solutions do not enable to sufficiently separate nickel from hydrogen phenomena. As a result, electrochemistry alone cannot yield important information about Ni-UPD, such as the formal partial charge number (valency of electrosorption) and the role of the sulphate or hydrogen sulphate anions.So, we decided to couple cyclic voltammetry to electrochemical quartz crystal microbalance (EQCM). EQCM measurements enable to decorrelate the simultaneous hydrogen and nickel adsorption/desorption peaks, which we could not attempt solely with electrochemistry. The coupling between gravimetric and electrochemical measurements allows us to detect the contribution of the anions and thus to isolate that of nickel: nickel coverage can then be determined. Nearly 4/5 Ni_UPD monolayer (θ_Ni ≈ 0.8) over platinum is reached at nickel equilibrium potential for high pH solutions (5.5). The QCM and electrochemistry coupling further allows the determination of nickel formal partial charge number: ι_Ni,EQCM = 1.3 ± 0.13. Direct electrochemistry measurements (Swathirajan and Bruckenstein method) yield: ι_Ni,Pt(poly) = 1.5 ± 0.17. These two values are close, which validates the electrochemical method for the nickel/platinum system. In consequence, we used Swathirajan and Bruckenstein method for Pt(1 1 0)-(1 × 2) crystal and found: ι_{Ni,Pt(1 1 0)} ≈ 1.4 ± 0.1. Whatever the system (Ni_UPD/Pt(poly) or Ni_UPD/Pt(1 1 0)-(1 × 2)) or the experimental technique, nickel formal partial charge number is lower than nickel cation charge: ι_Ni < z_Ni = 2. In consequence, upon underpotential deposition on platinum surfaces, nickel cations discharge and then undergo additional charge exchange processes, such as anion (or water) adsorption, resulting in apparent partial nickel cation discharge. Moreover, Ni_UPD/Pt(1 1 0) surface displays high activity towards CO_ad oxidation reaction. We explain such positive effect by the possible existence of a bifunctional mechanism in which oxygenated-species-covered Ni_UPD adatoms provide the oxygen atom to CO_ad?Pt species, enabling its facile oxidation.     

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.     

Effect of the structure of Pt-Ru/C particles on COad monolayer vibrational properties and electrooxidation kinetics

In this paper, we combined FTIR spectroscopy and CO_ad stripping voltammetry to investigate CO_ad adsorption and electrooxidation on Pt-Ru/C nanoparticles. The Pt:Ru elemental composition and the metal loading were determined by ICP-AES. The X-ray diffraction patterns of the Pt-Ru/C indicated formation of a Pt-Ru (fcc) alloy. HREM images revealed an increase in the fraction of agglomerated Pt-Ru/C particles with increasing the metal loading and showed that agglomerated Pt-Ru/C nanoparticles present structural defects such as twins or grain boundaries. In addition, isolated Pt-Ru/C nanoparticles have similar mean particle size (ca. 2.5 nm) and particle size distributions whatever the metal loading. Therefore, we could determine precisely the effect of particle agglomeration on the CO_ad vibrational properties and electrooxidation kinetics. FTIR measurements revealed a main CO_ad stretching band at ca. , which we ascribed to a-top CO_ad on Pt domains electronically modified by the presence of Ru. As the metal loading increased, the position of this band was blue shifted by ca. 5 cm⁻¹ and a shoulder around 2005 cm⁻¹ developed, which was ascribed to a-top CO_ad on Ru domains. The reason for this was suggested to be the increasing size of Ru domains on agglomerated Pt-Ru/C particles, which lifts dipole-dipole coupling and allows two vibrational features to be observed (CO_ad/Ru, CO_ad/Pt). This is evidence that FTIR spectroscopy can be used to probe small chemical fluctuations of the Pt-Ru/C surface. Finally, we comment on the CO_ad electrooxidation kinetics. We observed that CO_ad was converted more easily into CO₂ as the metal loading, i.e. the fraction of agglomerated Pt-Ru/C nanoparticles, increased.     

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.     

Ethanol electrooxidation on novel carbon supported Pt/SnOx/C catalysts with varied Pt:Sn ratio

Novel carbon supported Pt/SnO_x/C catalysts with Pt:Sn atomic ratios of 5:5, 6:4, 7:3 and 8:2 were prepared by a modified polyol method and characterized with respect to their structural properties (X-ray diffraction (XRD) and transmission electron microscopy (TEM)), chemical composition (XPS), their electrochemical properties (base voltammetry, CO_ad stripping) and their electrocatalytic activity and selectivity for ethanol oxidation (ethanol oxidation reaction (EOR)). The data show that the Pt/SnO_x/C catalysts are composed of Pt and tin oxide nanoparticles with an average Pt particle diameter of about 2 nm. The steady-state activity of the Pt/SnO_x/C catalysts towards the EOR decreases with tin content at room temperature, but increases at 80 °C. On all Pt/SnO_x/C catalysts, acetic acid and acetaldehyde represent dominant products, CO₂ formation contributes 1-3% for both potentiostatic and potentiodynamic reaction conditions. With increasing potential, the acetaldehyde yield decreases and the acetic acid yield increases. The apparent activation energies of the EOR increase with tin content (19-29 kJ mol⁻¹), but are lower than on Pt/C (32 kJ mol⁻¹). The somewhat better performance of the Pt/SnO_x/C catalysts compared to alloyed PtSn_x/C catalysts is attributed to the presence of both sufficiently large Pt ensembles for ethanol dehydrogenation and C-C bond splitting and of tin oxide for OH generation. Fuel cell measurements performed for comparison largely confirm the results obtained in model studies.     

In situ FTIR spectroscopic studies of electrooxidation of ethanol on Pd electrode in alkaline media

Electrooxidation of ethanol on a polycrystalline Pd disk electrode in alkaline media was studied by in situ Fourier transform infrared (FTIR) reflection spectroscopy. The emphasis was put on the quantitative determination of intermediates and products involved in the oxidation. It has revealed that most of ethanol was incompletely oxidized to acetate. The selectivity for ethanol oxidation to CO₂ (existing as CO₃²⁻ in alkaline media) was determined as low as 2.5% in the potential region where Pd electrode exhibited considerable electrocatalytic activity (−0.60 to 0.0 V vs. SCE). Nevertheless, the ability of Pd for breaking C-C bond in ethanol is still slightly better than that of Pt under the same conditions. Besides, a very weak band of adsorbed intermediate, bridge-bonded CO (CO_B) was identified on the Pd electrode for the first time, suggesting that CO₂ and CO₃²⁻ species may also be generated through CO pathway (i.e., indirect pathway).     

Electrocatalytic oxidation of hydrazine on platinum electrodes in alkaline solutions

The mechanism and structure sensitivity of the electrocatalytic oxidation of hydrazine on platinum in alkaline solutions were investigated using cyclic voltammetry, steady-state current measurements, and on-line electrochemical mass spectrometry. The voltammetry of hydrazine oxidation on platinum in alkaline media is characterized by a single diffusion-controlled wave. The on-line electrochemical mass spectrometry measurements of hydrazine oxidation on Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0) surfaces indicated the formation of molecular nitrogen. No oxygen-containing nitrogen compounds were detected under the given experimental conditions. A comparative analysis of voltammetric data for hydrazine oxidation in alkaline solutions on the three surfaces at low overpotentials points to a structure sensitivity of the reaction. The electrocatalytic activity of basal planes increases in the order Pt(1 1 0) > Pt(1 0 0) > Pt(1 1 1), as deduced from the onset of the oxidation wave. The structure of the electrocatalyst surface affects the mechanism of the reaction, although without affecting the selectivity. At low overpotentials the hydrazine oxidation on the Pt(1 1 0) and Pt(1 1 1) surfaces is limited by the rate of electrochemical steps, whereas on Pt(1 0 0) a chemical step that probably involves N₂H₂ adsorbed intermediate is the rate-determining step. Platinum is more active in hydrazine oxidation in alkaline solution than in acidic solutions. In contrast to hydrazine oxidation on platinum in acidic media, the stabilization of chemisorbed hydrazine does not occur to a significant extent in alkaline media.     

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.     

CO tolerance and catalytic activity of Pt/Sn/SnO2 nanowires loaded on a carbon paper

Electrochemical activities and structural features of Pt/Sn catalysts supported by hydrogen-reduced SnO₂ nanowires (SnO₂NW) are studied, using cyclic voltammetry, CO stripping voltammetry, scanning electron microscopy, and X-ray diffraction analysis. The SnO₂NW supports have been grown on a carbon paper which is commercially available for gas diffusion purposes. Partial reduction of SnO₂NW raises the CO tolerance of the Pt/Sn catalyst considerably. The zero-valence tin plays a significant role in lowering the oxidation potential of CO_ads. For a carbon paper electrode loaded with 0.1 mg cm⁻² Pt and 0.4 mg cm⁻² SnO₂NW, a conversion of 54% SnO₂NW into Sn metal (0.17 mg cm⁻²) initiates the CO_ads oxidation reaction at 0.08 V (vs. Ag/AgCl), shifts the peak position by 0.21 V, and maximizes the CO tolerance. Further reduction damages the support structure, reduces the surface area, and deteriorates the catalytic activity. The presence of Sn metal enhances the activities of both methanol and ethanol oxidation, with a more pronounced effect on the oxidation current of ethanol whose optimal value is analogous to those of PtSn/C catalysts reported in literature. In comparison with a commercial PtRu/C catalyst, the optimal Pt/Sn/SnO₂NW/CP exhibits a somewhat inferior activity toward methanol, and a superior activity toward ethanol oxidation.     

Study of the kinetics and the influence of Biirr on formic acid oxidation at Pt2Ru3/C

Electrochemical oxidation of HCOOH in H₂SO₄ and HClO₄ solutions was examined on thin film Pt₂Ru₃/C electrode. XRD pattern revealed that Pt₂Ru₃ alloy consisted of the solid solution of Ru in Pt and the small amount of Ru or solid solution of Pt in Ru. According to STM images, Pt₂Ru₃ particles size was between 2 and 6 nm. It was established that electrochemical oxidation of HCOOH commenced at −0.1 V versus SCE at Pt sites in the catalyst. Kinetic parameters indicated that dehydrogenation path was predominant. Dehydration occurs in parallel, but without significant poisoning by CO_ad owing to oxidative removal by OH species on Ru atoms. The coverage of Pt₂Ru₃ surface by CO preadsorbed from the solution was found to be 24% lower when the surface was modified by irreversibly adsorbed Bi. Modification by Bi also shifted the onset potential for HCOOH oxidation for about 50 mV towards more negative values and consequently, increased the reaction rate for a factor of two. It was proposed that Ru acts through bifunctional mechanism, i.e. OH species adsorbed on Ru oxidizes CO_ad from Pt sites, while Bi hinders the adsorption of CO on Pt sites via electronic and/or ensemble effects.     

Electrooxidation of acetaldehyde on carbon-supported Pt, PtRu and Pt3Sn and unsupported PtRu0.2 catalysts: A quantitative DEMS study

The oxidation of acetaldehyde on carbon supported Pt/Vulcan, PtRu/Vulcan and Pt₃Sn/Vulcan nanoparticle catalysts and, for comparison, on polycrystalline Pt and on an unsupported PtRu_0.2 catalyst, was investigated under continuous reaction and continuous electrolyte flow conditions, employing electrochemical and quantitative differential electrochemical mass spectroscopy (DEMS) measurements. Product distribution and the effects of reaction potential and reactant concentration were investigated by potentiodynamic and potentiostatic measurements. Reaction transients, following both the Faradaic current as well as the CO₂ related mass spectrometric intensity, revealed a very small current efficiency for CO₂ formation of a few percent for 0.1 m acetaldehyde bulk oxidation under steady-state conditions on all three catalysts, the dominant oxidation product being acetic acid. Pt alloy catalysts showed a higher activity than Pt/Vulcan at lower potential (0.51 V), but do not lead to a better selectivity for complete oxidation to CO₂. C–C bond breaking is rate limiting for complete oxidation at potentials with significant oxidation rates for all three catalysts. The data agree with a parallel pathway reaction mechanism, with formation and subsequent oxidation of CO_ad and CH _{x, ad} species in the one pathway and partial oxidation to acetic acid in the other pathway, with the latter pathway being, by far, dominant under present reaction conditions.     

Adsorption behavior of acetonitrile on platinum and gold electrodes of various structures in solution of 0.5 M H2SO4

The variation of electrode nature and surface structure (the use of stepped single crystal faces with controlled width of (1 1 1) terraces and monoatomic steps of (1 0 0) or (1 1 0) orientation) allows to determine peculiarities of co-adsorption of acetonitrile molecules, hydrogen adatoms and (bi)sulfate anions. It has been shown that first of all acetonitrile blocks adsorption sites at the steps. Anion adsorption at terraces of stepped platinum surfaces in 0.5 M H₂SO₄ solution with additions of acetonitrile depends on terrace width and the step orientation. This demonstrates the important role of structural factors in competitive adsorption processes. The decrease in adsorption of hydrogen and anions on narrow terraces is substantially due to the influence of acetonitrile molecules placed at the steps or nearby sites. At E < 1.0 V, electrochemical conversion of acetonitrile has not been detected at single crystal Pt surfaces. However, acetonitrile oxidation might proceed on polycrystalline platinum followed by product desorption. On Au(1 1 1) surface acetonitrile adsorption is considerably weaker than that on platinum electrodes.     

CO adsorption kinetics and adlayer build-up studied by combined ATR-FTIR spectroscopy and on-line DEMS under continuous flow conditions

Using a novel combined spectro-electrochemical DEMS/ATR-FTIRS technique, the CO adsorption kinetics on a Pt film electrode were studied, performing transient CO adsorption experiments at different constant potentials (0.06-0.6 V). CO adsorption rate and CO_ad coverage were determined continuously from the CO consumption measured by on-line differential electrochemical mass spectrometry (DEMS). Simultaneously measured FTIR spectra, recorded in situ in an attenuated total reflection (ATR) configuration, allow a direct correlation of the IR band intensity and frequency with CO_ad surface coverage at different constant potentials. The data show that (i) the CO adsorption kinetics are independent of the adsorption potential up to 0.5 V, (ii) a significant potential dependence of the ratio between CO_L and CO_M for the same coverage, (iii) in the regime of very high CO_ad coverages there is no proportional relation between CO_ad coverage and CO_L,M intensity, and (iv) a distinct tendency for CO_ad island formation at E_ads < 0.2 V and > 0.4 V, most likely due to coadsorption of H-upd at the lower potentials and (bi-)sulfate at higher potentials. Finally, at 0.6 V, CO_ad oxidation follows a Langmuir-Hinshelwood mechanism with the highest CO₂ formation rate at a relative CO_ad coverage of ∼0.4.     

Hydrogen oxidation kinetics on model Pd/C electrodes: Electrochemical impedance spectroscopy and rotating disk electrode study

This work reports on the kinetics of the hydrogen oxidation reaction (HOR) on model Pd nanoparticles supported on a low surface area carbon substrate. Two Pd/C samples, with the average particle size 2.6 and 4.0 nm were used. The structure of the catalysts was characterized with the ex situ (electron microscopy) and in situ (electrochemical) methods. We utilized the electrochemical impedance spectroscopy (EIS) and the rotating disk electrode (RDE) voltammetry to study the kinetics of the HOR on Pd/C. The relevance of these techniques for elucidating the kinetics and the mechanism of the HOR on Pd/C was explored. The experimental results suggest that the catalytic activity of Pd in the HOR is more than 2 orders of magnitude lower than that of Pt, and does not depend on the particle size in the range from 2.6 to 4.0 nm. Computational modeling of the experimental steady-state (RDE) and non-steady-state (EIS) data shows that the reaction kinetics can be adequately described within Heyrovsky-Volmer mechanism, with the rate constants υ_0H = (8.8 ± 1.5) × 10⁻¹⁰ mol cm⁻² s⁻¹ and υ_0V = (1.0 ± 0.3) × 10⁻⁸ mol cm⁻² s⁻¹. The model suggests that underpotentially deposited hydrogen H_UPD is unlikely to be the active intermediate H_ad of the HOR. It is concluded that the surface coverage of H_ad deviates from that of H_UPD with increasing overpotential, and the lateral interactions within H_ad adlayer are weak.     

A new amperometric glucose biosensor based on platinum nanoparticles/polymerized ionic liquid-carbon nanotubes nanocomposites

A new amperometric glucose biosensor has been developed based on platinum (Pt) nanoparticles/polymerized ionic liquid-carbon nanotubes (CNTs) nanocomposites (PtNPs/PIL-CNTs). The CNTs was functionalized with polymerized ionic liquid (PIL) through directly polymerization of the ionic liquid, 1-vinyl-3-ethylimidazolium tetrafluoroborate ([VEIM]BF₄), on carbon nanotubes and then used as the support for the highly dispersed Pt nanoparticles. The electrochemical performance of the PtNPs/PIL-CNTs modified glassy carbon (PtNPs/PIL-CNTs/GC) electrode has been investigated by typical electrochemical methods. The PtNPs/PIL-CNTs/GC electrode shows high electrocatalytic activity towards the oxidation of hydrogen peroxide. Taking glucose oxidase (GOD) as the model, the resulting amperometric glucose biosensor shows good analytical characteristics, such as a high sensitivity (28.28 μA mM⁻¹ cm⁻²), wide linear range (up to 12 mM) and low detection limit (10 μM).     

Studies of the morphology, composition and reactivity of osmium nanodeposits on platinum single crystal electrodes

In the present work, the morphology and composition of osmium deposits formed in submonolayer amount on Pt(1 0 0) and Pt(1 1 1) single crystal surfaces either by spontaneous deposition or by electrolysis at 50 mV in different deposition times (t_dep), were studied. The cyclic voltammetric curves for Pt(1 0 0)/Os and Pt(1 1 1)/Os were recorded in the potential range where the oxidation of deposited osmium species occurs, which suggests that the chemical composition of the deposits is a mixture of metallic Os and OsO₂. The ratio of each species deposited depends directly on the deposition potential, as well as the surface structure characteristics. The composition of the modified platinum single crystal surfaces has also been estimated by X-ray photoelectron spectroscopy (XPS), whose results agree with those obtained by CV, about the amount of osmium oxide on Pt(1 1 1) to be higher than on Pt(1 0 0). This feature of the Pt(1 1 1)/Os becomes its surface more active for ethanol oxidation at lower potentials than that observed for Pt(1 0 0)/Os, although at high potentials the high presence of OsO₂ is prejudicial to the catalytic activity of the electrode. The Pt-Os surfaces were also explored by use of the scanning tunneling microscopy (STM) technique in order to image the features of osmium deposits on platinum, after a short and a long deposition time. It was found, by using spontaneous deposition procedure, that the dimensions of osmium deposits islands do not grow with the increase of degree of coverage of osmium on the surface, as well as they keep the monoatomic thickness.