Positive shift of the potential of zero total charge of stepped Pt(1 1 1) electrodes decorated by irreversibly adsorbed bismuth

The value of the potential of zero total charge (pztc) of stepped Pt(1 1 1) electrodes, whose step sites have been blocked by irreversibly adsorbed bismuth, has been determined by means of the CO displacement experiment. It has been observed that the pztc of the decorated surfaces shift positively with respect to that of the substrate stepped surface electrode. In this way the electrode total charge at constant potential diminishes by effect of the adatom adsorption. The oxidation of adsorbed CO takes place at higher potentials on the decorated surfaces, pointing out a direct effect of the pztc shift on their reactivity as electrocatalyzers.     

A spectroscopic proof of a surface equilibrium between on top and bridge bonded CO at Pt(1 1 0) in acid solution

According to most of works in the literature, adsorbed carbon monoxide at Pt(1 1 0) electrodes in acid media presents only linear bonded (CO_L) so-called, atop geometry. In the present work, the formation of bridge bonded carbon monoxide (CO_B) is shown via in situ infrared FT spectra, measured on a Pt(1 1 0) electrode covered with 25% CO, in HClO₄ solutions. For the first time, the inter conversion between atop and bridge bonded CO at potentials in the hydrogen adsorption region is reported in acid solution. Band intensity and band center frequency indicate dipole–dipole coupling effects in spite of the low CO total coverage.     

Preferential oxidation of carbon monoxide adsorbed on Pd submonolayer films deposited on Pt(1 0 0)

The electrochemical behavior of palladium modified Pt(1 0 0) in perchloric acid solution has been studied by means of cyclic voltammetry, CO bulk oxidation and in-situ Fourier transform infrared (FTIR) spectroscopy. FTIR spectra on Pd submonolayers reveal that at low potentials (about 0.4 V) Pd-bounded CO is oxidized preferentially, whereas at the same time the Pt–CO oxidation rate is rather slow.     

Exploring the interfacial neutral pH region of Pt(111) electrodes

The interfacial properties of Pt(111) single crystal electrodes have been investigated in the pH range 3 < pH < 5 in order to obtain information about the acidity of electrosorbed water. Proper experimental conditions are defined to avoid local pH changes while maintaining the absence of specifically adsorbed anions and preserving the cleanliness of the solution. For this purpose, buffer solutions resulting from mixtures of NaF and HClO₄ are used. Total charge curves are obtained at different pHs from the integration of the voltammetric currents in combination with CO charge displacement experiments. Analysis of the composition of the interphase as a function of the pH provides information for the understanding of the notion of interfacial pH.     

Oxygen reduction reaction activities of Ni/Pt(111) model catalysts fabricated by molecular beam epitaxy

Oxygen reduction reaction (ORR) activities were evaluated for clean Pt(111) and Ni/Pt(111) model catalysts fabricated by molecular beam epitaxy. Exposure of clean Pt(111) to 1.0 L CO at 303 K produced linear-bonded and bridge-bonded CO-Pt IR bands at 2093 and 1858 cm^? 1. In contrast, 0.3-nm-thick Ni deposited on Pt(111) at 573 K (573 K-Ni_0.3 nm/Pt(111)) produced broad IR bands for adsorbed CO at around 2070 cm^? 1; the separation of reflection high-energy electron diffraction (RHEED) streaks is slightly wider for 573 K-Ni_0.3 nm/Pt(111) than for the clean Pt(111). For 823 K-Ni_0.3 nm/Pt(111), the separation of the RHEED streaks is the same as that for the Pt(111), and a single sharp IR band due to adsorbed CO is located at 2082 cm^? 1. The results suggest that for the 823 K-Ni_0.3 nm/Pt(111), a Pt-enriched outermost surface (Pt-skin) was formed through surface segregation of the substrate Pt atoms. ORR activities for the 573 K- and 823 K-Ni_0.3 nm/Pt(111) as determined from linear sweep voltammetry curves were five times and eight times higher than that for clean Pt(111), respectively, demonstrating that Pt-skin generation is crucial for developing highly active electrode catalysts for fuel cells.     

Platinum-macrocycle co-catalyst for electro-oxidation of formic acid

In this work, a new promoter, tetrasulfophthalocyanine (FeTSPc), one kind of environmental friendly material, was found to be very effective in both inhibiting self-poisoning and improving the intrinsic catalysis activity, consequently enhancing the electro-oxidation current during the electro-oxidation of formic acid. The cyclic voltammograms test showed that the formic acid oxidation peak current density has been increased about 10 times compared with that of the Pt electrode without FeTSPc. The electrochemical double potential step chronoamperometry measurements revealed that the apparent activity energy decreases from 20.64 kJ mol⁻¹ to 17.38 kJ mol⁻¹ after Pt electrode promoted by FeTSPc. The promoting effect of FeTSPc may be owed to the specific structure and abundant electrons of FeTSPc resulting in both the steric hindrance of the formation of poisoning species (CO) and intrinsic kinetic enhancement. In the single cell test, the performance of DFAFC increased from 80 mW cm⁻² mg⁻¹ (Pt) to 130 mW cm⁻² mg⁻¹ after the anode electrode adsorbed FeTSPc.     

CO preoxidation on Ru-modified Pt(111)

Electrochemical scanning tunneling microscopy was used to study the structural evolution of adsorbed CO during preoxidation on Pt(111) modified with spontaneously deposited Ru. During the preoxidation process, a phase transition was observed from (2 × 2)-3CO-α to (√19 × √19)R23.4°-13CO via the transient structures (2 × 2)-3CO-β and (1 × 1)-CO. A comparison of these structural changes with those that occur on unmodified Pt(111) revealed that the presence of Ru resulted in higher populations of transient structures at lower potentials and a cathodic shift in the potential at which preoxidation is complete. These observations are discussed in terms of increased mobility of adsorbed CO in the presence of Ru.     

A topographic view of the Pt(1 1 1) surface at the electrochemical interface in the presence of carbon monoxide

In this communication we present topographic images of the Pt(1 1 1) surface in CO saturated 0.1 M HClO₄, obtained by scanning tunneling microscopy.The topography presents two different structures, depending on the CO adsorption potential (E_ad = 0.15 V or E_ad = 0.5 V vs RHE). For adsorption at 0.15 V the system presents a heterogeneous appearance, which totally covers the surface and impedes the observation of steps on the substrate surface. When CO is adsorbed at 0.5 V large clusters forming chains along the steps are observed. These aggregates can be, tentatively, correlated with the H-bonded water structure suggested earlier on the basis of FTIR spectroscopy. The clusters have inhibitory effects on CO oxidation.     

Direct electrochemistry of hemoglobin and its electrocatalytic effect based on its direct immobilization on carbon ionic liquid electrode

Direct electrochemistry of hemoglobin (Hb) has been achieved by its direct immobilization on carbon ionic liquid electrode (CILE). CILE was immersed in a solution containing Hb and ionic liquid, octylpyridinium chloride ([OcPy][Cl]), to directly immobilize Hb on CILE. Cyclic voltammetry of modified electrode exhibited quasi-reversible peaks corresponding to Hb. The oxidation and reduction peak potentials of immobilized Hb in acetate buffer solution, pH 5.0 and at a scan rate of 0.1 V s⁻¹ were obtained at about –150 mV and –290 mV, respectively. The average surface coverage of the electroactive Hb adsorbed on the electrode surface was calculated as 8.4 × 10⁻¹¹ mol cm⁻². Hb retained its bioactivity on modified electrode and showed excellent electrocatalytic activity towards oxygen, hydrogen peroxide and nitrite. Hydrogen peroxide can be determined in the range of 1.0 × 10⁻⁴–5.0 × 10⁻³ M.     

Voltammetric analysis of the co-adsorption of cyanide and carbon monoxide on a Pt(1 1 1) surface

Carbon monoxide has been adsorbed at controlled potential on a Pt(1 1 1) electrode, which was pre-covered with cyanide up to saturation. From the results obtained, it has been deduced that CO occupies platinum sites between CN molecules with no distortion of the pre-adsorbed cyanide layer. The anodic stripping of CO does not induce modifications of the CN adlayer either. These observations allow the estimation of θ_CO rather accurately by means of the coulometric method.     

Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. −0.44 V (vs. Ag|AgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed.     

Influence of surface oxygen groups on V(II) oxidation reaction kinetics

The role of surface oxygen groups on the kinetics of the V(II) oxidation reaction was studied on modified glassy carbon (GC) electrodes by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The reaction was found to be sensitive to the presence of oxygen groups on the electrode surface. Higher O/C ratios determined by X-ray photoelectron spectroscopy (XPS) corresponded to higher reactivities and lower charge transfer resistances measured in a 1 M V(II) electrolyte. The stability of an oxidised GC surface was also investigated in a 1 M V(II) electrolyte by potential holding and cycling experiments. It was found that after holding and cycling to successively more negative potentials up to − 0.8 V/RHE, the electrode surface lost its initial reactivity.     

Surface poisoning during electrocatalytic monosaccharide oxidation reactions at gold electrodes in alkaline medium

In the present study, the surface poisoning of electrocatalytic monosaccharide oxidation reactions at gold electrodes were investigated. In the cyclic voltammetric studies, the electrocatalytic oxidation of aldohexose and aldopentose type monosaccharides, aminosugars, acetyl-glucosamine and glucronamide were observed at gold plate electrodes in alkaline medium. However, in controlled-potential electrolytic studies ranging −0.3 to −0.2 V in reaction solutions, current flows during electrolyses decreased quickly with time, except when glucosamine was used as a substrate.Results from surface enhanced infrared adsorption (SEIRA) spectroscopic measurements at an evaporated gold electrode for the electrocatalytic oxidation of glucose in 0.1 mol dm⁻³ NaOH at −0.3 V and Gaussian simulated spectra indicated that the gluconic acid as a 2-electron oxidation product and/or its analogs adsorbed onto the gold surface. Electrochemical quartz crystal microbalance (EQCM) measurement results, along with surface adsorption results from surface poisoning at the gold electrode during electrolytic reactions, suggested that gluconic acid and/or its analogs adsorbed vertically onto electrode surfaces in a full monolayer packing-like conformation. In the case of the electro oxidation of glucosamine in 0.1 mol dm⁻³ NaOH at −0.2 V, the obtained SEIRA spectra and EQCM results, clearly indicated that the glucosaminic acid as a 2-oxidation glucosamine product did not strongly bind onto the gold electrode surface.     

Unusual adsorption state of carbon monoxide on single-crystalline gold electrodes in alkaline media

We report an unusual and not previously reported adsorption state of CO on a Au(1 1 1) electrode and a hexagonally reconstructed Au(1 0 0) surface. Chemisorbed CO exhibits a reversible adsorption feature close to the onset of the oxidation of CO in solution. We suggest that the voltammetric feature is associated with the co-adsorption (and desorption) of OH^- from the alkaline solution at low potential.     

Kinetics of copper UPD on stepped platinum single crystals in the presence of acetonitrile

The kinetics of copper underpotential deposition on stepped Pt(h k l) electrodes with controlled width of (1 1 1) terraces in acidic solutions of copper sulfate with 0–200 mM of acetonitrile (AcN) has been studied by means of cyclic voltammetry. In the presence of AcN Cu UPD process is hindered both at (1 0 0) steps and (1 1 1) terraces of Pt(17 15 15) and Pt(7 5 5) faces due to blocking of the electrode surface with organic molecules, strongly adsorbed at the steps and nearby ones. The decoration of (1 1 0) steps with copper adatoms is slightly accelerated for Pt(7 7 5) electrode in the solution with 0.04 mM AcN. Increase in AcN concentration leads to inhibition of the UPD process. The difference in behavior of the stepped platinum electrodes is controlled by competitive adsorption of AcN, (bi)sulfate and Cu atoms at the step sites. AcN adsorption at (1 0 0) steps is stronger as compared with (1 1 0) ones.     

Adsorption of hydrogen on Pt(1 1 1) and Pt(1 0 0) surfaces and its role in the HOR

Hydrogen adsorption isotherms, evaluated by combination of cyclic voltammetry and chronoamperometry, are reported on Pt(1 1 1) and Pt(1 0 0) surfaces in 0.1 M HClO₄. We found that at E > 0.05 V Pt(1 1 1) and Pt(1 0 0) are only partially covered by the adsorbed hydrogen (H_ad). On both surfaces, a full monolayer of the adsorbed hydrogen is completed at −0.1 V, i.e. the adsorption of atomic hydrogen is observed in the hydrogen evolution potential region. We also found, that the activity of the hydrogen oxidation reaction is mirrored by the shape of the hydrogen adsorption isotherms, implying that H_ad is in fact a spectator in the HOR.     

Correlations between hydrogen electrosorption properties and composition of Pd-noble metal alloys

Hydrogen adsorption on and absorption into Pd alloys with other noble metals was studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry. Correlations were found between the potentials of adsorbed/absorbed hydrogen oxidation peaks and surface/bulk compositions of Pd–Rh alloys. The potential of the α–β-phase transition depends linearly on Pd bulk content in Pd–Au, Pd–Rh, Pd–Pt and Pd–Pt–Rh alloys. The obtained relationships can be utilized for the determination of the composition of homogeneous Pd-noble alloys from hydrogen electrosorption experiments.     

O2 surface concentration change and its implication on oxygen reduction mechanism and kinetics at platinum in acidic media

O₂ concentration near Pt surface during oxygen reduction reaction (ORR) in 0.1 M HClO₄ has been monitored by rotating ring-disk electrodes system. At 0.8 V < E < 1.0 V (vs. RHE), O₂ concentration near Pt surface increases with potential accompanying with the decrease of ORR current at the disk electrode; O₂ concentration in the negative-going scan is larger than that at the same potential in the positive-going scan, while ORR current shows the opposite trend at ω > 400 rpm. At E > 0.8 V accumulation of O_ad|OH_ad at Pt disk electrode with ORR time is evident, revealing that O_ad|OH_ad formation rate is faster than that for the removal of OH_ad to H₂O under such conditions. At relatively lower rotation speed and faster scan rate, the cathodic current during ORR in the negative-going scan can be larger than that in the positive-going scan with a current peak at ca. 0.8 V, which is attributed to the superimposition of ORR current increase due to change of O₂ concentration near the surface and the additional reduction of O_ad|OH_ad formed from decomposed O₂ at higher potentials.     

Ensemble size estimation in formic acid oxidation on Bi-modified Pt(111)

The physical dimensions of ensemble structures formed by adsorbed Bi on Pt(111) were estimated and correlations were established between ensemble size and oxidation activity. Measurements were made by examining electrochemical scanning tunneling microscopy (EC-STM) images of Bi irreversibly adsorbed on Pt(111). Percentage coverages of Bi, CO, poison, and ensemble were determined by both EC-STM and cyclic voltammetry. As the fractional Bi coverage increased, from 0.03 to 0.21, the ensemble size decreased, from approximately 9 to 1 nm. Poison formation was inhibited at ensemble sizes less than 2 nm and the turnover frequency of formic acid was enhanced by a factor of 100–200. Bi is hypothesized to not only physically produce the ensembles, but may also chemically alter their electronic properties.     

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.