Carbon monoxide adsorption on palladium electrodes modified with silver adatoms

Adsorption of carbon monoxide on palladium deposits and smooth polycrystalline palladium is studied in conditions of preliminary accumulation of various amounts of adsorbed silver (?_Ag) on them. It is discovered that carbon monoxide undergoes adsorption on palladium as it does on platinum, i.e. at the sites free of adsorbed silver, virtually without forcing out preliminarily adsorbed silver. At small and medium values of ?_Ag in the region of electrodesorption of a mixed layer on palladium, as opposed to Pt/Pt, only two peaks are observed in an anodic voltammetric curve. The nature of the processes that are responsible for the presence of two peaks pertaining to the oxidation of a mixed adsorption layer is discussed.     

Carbon monoxide adsorption on Pt/Pt electrodes modified with silver adatoms

Adsorption of carbon monoxide on the Pt/Pt surface in the presence of preliminarily accumulated different amounts of UPD silver (_Ag) is studied by using voltammetry, electrode washings, and analytical techniques. It is found that carbon monoxide is adsorbed only on the sites unoccupied by Ag_ad and rather than displacing the latter atoms to the noticeable extent. The fraction of carbon monoxide chemisorbed in the linear form increases with _Ag. The shape of anodic voltammetric curves and the number of peaks in the region where the mixed layer CO_ads + Ag_ad is desorbed strongly depend on the CO_ads : Ag_ad ratio; at small and medium _Ag, three and more peaks are observed. Electroanalytic detection of silver passed to solution in various potential ranges in the course of the mixed-layer oxidation allowed us to identify the processes corresponding to different peaks.Translated from Elektrokhimiya, Vol. 41, No. 2, 2005, pp. 241–246.Original Russian Text Copyright © 2005 by Maksimov, Podlovchenko.This revised version was published online in April 2005 with corrections to the article note and article title and cover date.     

Kinetically Limited CO Adsorption: Spill-Over as a Highly Effective Adsorption Pathway on Bimetallic Surfaces

We demonstrate that the (local) adsorbed carbon monoxide, CO_ad, coverage on the Pt-free areas of bimetallic Pt/Ru(0001) surfaces (a Ru(0001) substrate partly covered by Pt monolayer islands) can be increased to ∼0.80 monolayers (ML), well above the established saturation CO_ad coverage of 0.68 ML, even under ultrahigh vacuum conditions by using spill-over of CO adsorbed on the Pt islands to the Ru areas as an highly effective adsorption channel. The apparent CO_ad saturation coverage of 0.68 ML on pure Ru(0001) is identified as due to kinetic limitations, hindering further uptake from the gas phase, rather than being caused by thermodynamic reasons. This spill-over mechanism is proposed to be a general phenomenon for adsorption on bimetallic surfaces.     

Abnormally fast mobility of CO at electrochemical interfaces

A very fast rate of CO transport at the Pt/electrolyte interface is reported. The mass transport was monitored by following the change of the electrode capacitance caused by H_ad displacement during carbon monoxide adsorption at potentials in the hydrogen adsorption region. A diffusion parameter was calculated assuming the validity of Fick’s law. No dependence was found on the surface structure.     

The influence of hydrogen peroxide on carbon monoxide electrooxidation at Pt/C and Pt:Ru/C electrodes

Polymer electrolyte fuel cells constitute one of the most important efficiency energy converters for non-centralised uses. However, the use of fuels arising from reformate processes significantly lowers the current efficiency because of anodic catalytic poison coming from adsorbed carbon monoxide (CO_ad). In this work, the influence of the addition of hydrogen peroxide in the flow current is studied, considering the adsorption and electrochemical oxidation of carbon monoxide on carbon-supported Pt (20% Pt/Vulcan) and Pt:Ru (1:1, 20% Pt:Ru/Vulcan) catalysts in 2 M sulphuric acid. The investigation was conducted applying cyclic voltammetry and on-line differential electrochemical mass spectrometry. A series of experiments has been performed to investigate the influence of the temperature as well as the time of contact and the concentration of hydrogen peroxide. Oxidation of CO_ad to carbon dioxide occurs at lower potentials in the presence of hydrogen peroxide. Moreover, it is possible to remove ca. 70% of CO_ad on Pt/C electrodes. On the other hand, for PtRu/C electrodes, similar charge values to those of Pt/C electrodes were obtained for the CO stripping, but the process occurs at more negative potentials. In this case, the effect of partial desorption for CO_ad by interaction with hydrogen peroxide is added to the bifunctional mechanism usually considered for this alloy. This paper is dedicated to Prof. Francisco Nart, in memoriam.     

Catalytic effects of metal submonolayers formed by faradaic adsorption on the anodic oxidation of ascorbic acid at a platinum electrode

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO₄ were studied by linear sweep voltammetry. The anodic peak due to a two-electron oxidation of ascorbic acid shifts to the negative potential side on the addition of Bi³⁺. This indicates the accelerating effect of Bi³⁺ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb²⁺, Hg²⁺, Tl⁺, Ag⁺ and Sb³⁺, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as M_ad) still remain on the electrode surface until the electrode potential goes up to and beyond the peak potential of the oxidation of ascorbic acid. On the other hand, metal ions forming no adsorbed layer on Pt, such as Co²⁺, Zn²⁺, Fe³⁺ and Ni²⁺, exhibit no catalytic effect. These facts suggest that the presence of a M_ad on Pt is essential for the promotion of the anodic oxidation of ascorbic acid. However, there is a difference in the catalytic action among the M_ad, for example, Cu_ad, Cd_ad, In_ad, Sn_ad and Mo_ad display no catalytic action.The catalytic activity depends on the degree of surface coverage by the M_ad. The maximal effect of the M_ad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the M_ad plays its major role in the removal of the adsorbed ascorbic acid occupying active sites on the electrode surface, and provides effective sites for the activation of adjacent water molecules. Furthermore, from the ¹³C NMR spectra for the oxidation products, the adsorbed water on the M_ad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.     

Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions

Kinetics of the oxygen reduction reaction (orr) and the hydrogen evolution–oxidation reactions (her/hor) were studied on the Pt(111) and Pt(100) surfaces in 0.05 M H₂SO₄ containing Cl⁻. The orr is strongly inhibited on the (100) surface modified by adsorbed Cl⁻ (Cl_ad), and it occurs as a 3.5e⁻ reduction via solution phase peroxide formation. In the hydrogen adsorption (H_upd) potential region, the orr is even more inhibited, and corresponds only to a 2 e⁻ reduction at the negative potential limit where the electrode is covered by one monolayer of H_upd and some (unknown) amount of Cl_ad. On the Pt(111)---Cl_ad surface, the orr is inhibited relatively little (in addition to that caused by strong bisulfate anion adsorption on this surface), and the reaction pathway is the same as in Cl⁻ free solution. The kinetics of the hor on Pt(111) are the same in pure solution and in a solution containing Cl⁻, since Cl_ad does not affect platinum sites required for the breaking of the H---H bond. A relatively large inhibition of the hor is observed on the (100) surface, implying that strongly adsorbed Cl_ad is present on the surface even near 0 V.     

Specific features of the formation of co-adsorption layers on metal surfaces with the participation of simple molecules: NH<Subscript>3</Subscript> + CO on Pt(111)

The co-adsorption of ammonia and carbon monoxide on the Pt(111) surface was studied at temperatures <300 K using high-resolution electron energy loss spectroscopy (HREELS). The state of ammonia and carbon monoxide molecules in the co-adsorption layer was established to differ significantly from their state in individual adsorption layers. The adsorption of CO on a clean surface occurs with the primary filling of single-bound terminal sites, whereas the bridging sites are filled preferably by CO molecules in the presence of NH_3,ads. The symmetry axis of ammonia molecules adsorbed on the clean surface is parallel to the normal to the surface, whereas in the co-adsorption layers the interaction with CO_ads molecules results in the deviation of the symmetry axis toward the surface. Presumably, the observed changes in the state of adsorbed molecules are due to the donor-acceptor interaction inducing the electron density transfer from ammonia molecules across the metal surface to CO molecules.     

522—Catalytic oxydation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of glucose

The catalytic oxidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HClO₄ by linear sweep voltammetry. The adsorbed metals (denoted as M_ad, such as Bi_ad and Pb_ad) formed on Pt in the potential region more positive than the reversible potential of an M⁼⁺/M^o couple, lead to a marked increase in the anodic c?urrent of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M_ad. The strongly adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pt electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_ad were discussed on the basis that M_ad plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.     

Insights into the enhanced tolerance to carbon monoxide on model tungsten trioxide-decorated polycrystalline platinum electrode

Polycrystalline platinum decorated by WO₃ nanoparticles (WO₃/Pt_pc) is used as a model electrode to gain insights into the enhanced tolerance to carbon monoxide (CO) observed on such composite materials. Bifunctional-type reactions involving WO₃ and Pt active sites are observed, such as hydrogen spill-over or the electrooxidation of CO molecules adsorbed on Pt sites neighboring the WO₃ nanoparticles. The resulting CO_ad-free Pt sites are active for the hydrogen oxidation reaction (HOR), thereby enhancing the HOR activity for WO₃/Pt_pc electrode relatively to bare Pt_pc in 300 ppm CO/H₂ saturated HClO₄ electrolyte. However, this bifunctional effect occurs exclusively for CO molecules weakly adsorbed on Pt, i.e. only for a small fraction of the CO_ad fully covering the Pt surface.     

DEMS study on the nature of acetaldehyde adsorbates at Pt and Pd by isotopic labelling

The formation of acetaldehyde adsorbates on Pt and Pd has been studied applying cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). The adspecies were isolated on the metal surface at selected adsorption potentials (E _ad) applying a flow cell procedure under potential control, and the anodic stripping were performed for each E _ad. For Pt, two different contributions were established during oxidation: one at E < 0.80 V and the second in the range 0.80–1.50 V in the Pt oxide region. For Pd, the voltammetric profile resembles that for the oxidation of adsorbed CO. DEMS experiments have shown that CO₂ was the sole electro-oxidation product in both cases. The oxidation of each C atom in acetaldehyde adsorbates has been distinguished using the isotopic-labelled aldehyde in DEMS experiments at selected E _ad. It was observed that, on Pt, acetaldehyde molecules loose part of the CH₃ groups during adsorption at E _ad < 0.40 V, whereas the CHO groups are easily oxidized at E _ad > 0.40 V. Therefore, both C₁ and C₂ species are present on the surface, and their yields depend on E _ad. On the contrary, on Pd, most of the CH₃ groups are lost during adsorption at all E _ad, and the main adsorbed species seems to be CO_ad. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

Adsorption of Carbon Monoxide on Platinized Platinum Electrode with Preadsorbed Iodine and Iodide Anions

Adsorption of carbon monoxide in the presence of adlayers formed upon exposure of Pt/Pt to I^– anions and I₂ (0.5 M ₂SO₄ as the supporting electrolyte) is studied using the method of electrode washing. Transients of current and potentiodynamic curves show that the displacement of iodine adatoms from the Pt/Pt surface by CO is virtually complete when CO is adsorbed in the range of hydrogen adsorption potentials (E 0.35 V (RHE)) and incomplete at higher potentials. It is concluded that the bond formed by iodine adatoms with the surface strengthens with an increase in the potential. Possible reasons for the striking difference in the behavior of adatomic monolayers formed from KI and I₂ solutions is discussed. The surface charge of Pt/Pt is observed to drastically change as a result of the displacement of iodine adatoms by carbon monoxide.     

A DFT Investigation of Surface‐Enhanced Raman Scattering of Adenine and 2′‐Deoxyadenosine 5′‐Monophosphate on Ag20 Nanoclusters

We present a detailed analysis of the surface‐enhanced Raman scattering (SERS) of adenine and 2′‐deoxyadenosine 5′‐monophosphate (dAMP) adsorbed on an Ag₂₀ cluster by using density functional theory. Calculated Raman spectra show that spectral features of all complexes depend greatly on adsorption sites of adenine and dAMP. The complexes consisting of adenine adsorbed on the Ag₂₀ cluster through N3 reproduce the measured SERS spectra in silver colloids, and thus demonstrated that adenine interacts with the silver surface via N3. We also investigate the SERS spectrum of adenine at the junction between two Ag₂₀ clusters and demonstrate that adenine can bind to the clusters through N3 and the external amino group, while dAMP can be adsorbed on the cluster in an end‐on orientation with the ribose and phosphate groups near to or away from the silver cluster. In contrast to the adenine–Ag₂₀ complexes, the dAMP–Ag₂₀ complexes produce new and strong bands in the low‐ or high‐wavenumber region of the Raman spectra, due to vibrations of the ribose and phosphate groups. Furthermore, the spectrum of dAMP bound to the Ag₂₀ cluster via N7 approaches the experimental SERS spectra on silver colloids.     

Effect of silver ions on codischarge of copper and silver at a platinum electrode at the copper underpotentials

Codischarge of copper and silver ions at potentials more positive than the equilibrium potential of the Cu⁺²/Cu system is studied by the voltammetry technique. At the Ag⁺ concentrationc below 10^-7 M, silver ions make no noticeable impact on the formation of a Cu_ad monolayer but decelerate that of supermonolayers of Cu at almost equilibrium potentials. Atc ≥ 4 × 10^-5 M, voltammograms indicate the concurrent adsorption of Cu_ad and Ag_ad and the nucleation and deposition of phase Ag     

Application of Statistical Lattice Models to the Analysis of Oscillatory and Autowave Processes in the Reaction of Carbon Monoxide Oxidation over Platinum and Palladium Surfaces

Statistical lattice models which imitate oscillatory and wave dynamics in the adsorbed layer during of carbon monoxide oxidation over Pt(100) and Pd(110) single crystals differing in the mechanism of autooscillation formation are compared. In the case of platinum, oscillations are due to phase transitions of the catalyst surface structure and surface reconstruction under the action of the reaction medium. In the case of palladium, the driving force of oscillations is phase transitions in the adsorbed layers on the catalyst surface, namely, the reversible formation of subsurface oxygen in the course of the reaction, which modifies the adsorption and catalytic properties of the surface. It is shown that, according to the proposed models, a change in the coverages (CO_ads O_ads) in the autooscillation regimes occurs via the formation of a surface wave whose front is characterized by the high concentration of catalytically active sites that provide the maximal rate of CO₂ molecule formation. Under certain conditions, the formation of various spatiotemporal structures is observed in simulation experiments.     

Surface spectators and their role in relationships between activity and selectivity of the oxygen reduction reaction in acid environments

We use the rotating ring disk (RRDE) method to study activity–selectivity relationships for the oxygen reduction reaction (ORR) on Pt(111) modified by various surface coverages of adsorbed CN_ad (Θ_CNad). The results demonstrate that small variations in Θ_CNad have dramatic effect on the ORR activity and peroxide production, resulting in “volcano-like” dependence with an optimal surface coverage of Θ_CNad = 0.3 ML. These relationships can be simply explained by balancing electronic and ensemble effects of co-adsorbed CN_ad and adsorbed spectator species from the supporting electrolytes, without the need for intermediate adsorption energy arguments. Although this study has focused on the Pt(111)–CN_ad/H₂SO₄ interface, the results and insight gained here are invaluable for controlling another dimension in the properties of electrochemical interfaces.     

Über die Reduktion von Silbersulfid mit atomarem Wasserstoff

Zusammenfassung Die Kinetik der Reduktion von Ag₂S durch atomaren Wasserstoff zwischen 250 und 350°C wird untersucht. In der Silbersulfidprobe, die eine Elektrode der galvanischen Festkörperkette Pt/Ag/AgJ/Ag₂S/Pt bildet, können während der Reduktion definierte Schwefelaktivitäten eingestellt werden. Die Reduktion verläuft nach 1. Ordnung bezüglich der Wasserstoffatomkonzentration und ist von der Schwefelaktivität im Ag₂S unabhängig. Zur Deutung des Ablaufs der Reaktion wird angenommen, daß ein adsorbiertes Wasserstoffatom und ein Wasserstoffatom aus der Gasphase mit Schwefel an der Silbersulfidoberfläche reagieren.

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The rate of reaction of Ag₂S with atomic hydrogen has been measured at temperatures between 250° and 350°C. Definite sulphur activities in the Ag₂S during the reduction were obtained using the galvanic solid state cell Pt/Ag/AgJ/Ag₂S/Pt. It was found that the reduction is of first order with respect to the atomic hydrogen concentration and does not depend on the activity of the sulphur in silver sulphide. A mechanism for the reaction is proposed in such a way that the hydrogen atom from the gaseous phase interacts with an adsorbed hydrogen atom and sulphur on the silver sulphide surface.

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Chemical Adsorption onto an ITO Substrate of Single‐Wall Carbon Nanotube Functionalized by Carboxylic Groups as an Efficient Support for Electrocatalyst

A single‐wall carbon nanotube functionalized by carboxylic groups (SWNT‐CA) was found to be adsorbed on an indium tin oxide (ITO) electrode by chemical interaction between carboxylic groups and the ITO surface. The adsorption experiments indicated that the narrow pH conditions (around pH 3.0) exist for its adsorption which is restricted by preparation of stable fluid dispersion (favorable at higher pH) and by the chemical interaction (favorable at lower pH). Atomic force microscopic (AFM) measurements suggest that fragmented SWNT‐CA are adsorbed, primarily lying on the surface. Electrochemical impedance analysis indicated that an electrochemical double layer capacitance of the SWNT‐CA/ITO electrode is considerably higher than that for the ITO electrode, suggesting that the interfacial area between the electrode surface and the electrolyte solution is enlarged by the SWNT‐CA layer. Pt particles were deposited as a catalyst on the bare ITO and SWNT‐CA‐coated ITO (SWNT‐CA/ITO) electrodes to give respective Pt‐modified electrodes (denoted as a Pt/ITO electrode and a Pt/SWNT‐CA/ITO electrode, respectively). The cathodic current for the Pt/SWNT‐CA/ITO electrode was 1.7 times higher than that for the Pt/ITO electrode at 0.0 V, showing that the Pt/SWNT‐CA/ITO electrode works more efficiently for O₂ reduction at 0.0 V due to the SWNT‐CA layer. The enhancement by the SWNT‐CA layer is also effective for electrocatalytic proton reduction. It could be ascribable to the enlarged interfacial area between the electrode surface and the electrolyte solution.     

CO and methanol adsorption on (2 × 1)Pt(110) and ion‐eroded Pt(111) model catalysts

Methanol adsorption on ion‐sputtered Pt(111) surface exhibiting high concentration of vacancy islands and on (2 × 1)Pt(110) single crystal were investigated by means of photoelectron spectroscopy (PES) and thermal desorption spectroscopy. The measurements showed that methanol adsorbed at low temperature on sputtered Pt(111) and on (2 × 1)Pt(110) surfaces decomposed upon heating. The PES data of methanol adsorption were compared to the data of CO adsorbed on the same Pt single crystal surfaces. In the case of the sputtered Pt(111) surface, the dehydrogenation of H_xCO intermediates is followed by the CO bond breakage. On the (2 × 1)Pt(110) surface, carbon monoxide, as product of methanol decomposition, desorbed molecularly without appearance of any traces of atomic carbon. By comparing both platinum surfaces we conclude that methanol decomposition occurs at higher temperature on sputtered Pt(111) than on (2 × 1)Pt(110). Copyright © 2010 John Wiley & Sons, Ltd.     

Molecular simulation of interactions between silver clusters and an α-quartz surface

The adsorption of silver clusters Ag _n (n = 1–7) on different sites of an α-SiO₂ (0001) surface is studied using density functional theory calculations in the general gradient approximation. The most stable configurations of Ag _n (n = 1–7) adsorbed on the α-SiO₂ surface are determined, and the correlations between cluster size and adsorption energy are found. It is established that the optimized geometry parameters of Ag _n (n = 1–7) and the values of their adsorption energy are in good agreement with the results from earlier investigations and confirm the validity of the chosen theoretical model.