Electrochemical modification of steel by platinum nanoparticles

The aim of this study was to synthesize low-concentration catalysts with a highly developed surface based on nanodispersed platinum deposited onto the steel surface electrochemically modified with the use of an ionic liquid. Sintered fibers of austenitic steel without pretreatment (steel 1) and etched with hydrochloric acid to remove surface oxides (steel 2) were used as substrates. 1-Butyl-4-methylimidazolium acetate [BMIM][Ac] ionic liquid was used. Variation of the current intensity and anodic treatment time leads to the formation of different structures at the steel surface. For steel 2, optimal conditions leading to self-organization, namely, formation of hexagonal structures, have been selected. It has been demonstrated that formation of nanostructures at the steel surface can occur without the participation of fluoride ions. Low-concentration (Pt/steel 2) catalysts with a uniform distribution of platinum nanoparticles over the surface were prepared via galvanostatic deposition from an aqueous solution of H₂PtCl₆.     

AFM characterization of dendrimer-stabilized platinum nanoparticles

This work describes the use of atomic force microscopy (AFM) to measure the size of dendrimer-stabilized Pt nanoparticles (Pt DNs) deposited from aqueous solutions onto mica surfaces. Despite considerable previous work in this area, we do not fully understand the mechanisms by which PAMAM dendrimers template the formation of Pt DNs. In particular, Pt DN sizes measured by high-resolution transmission electron microscopy (HRTEM) are reported to be larger than expected if one assumes that each PAMAM molecule templates one spherical Pt nanoparticle. AFM provides a vertical height measurement that complements the lateral dimension measurement from HRTEM. We show that AFM height measurements can distinguish between "empty" PAMAM and Pt DNs. If the complexation of Pt precursor with PAMAM is prematurely terminated, AFM images and feature height distributions show evidence of arrested precipitation of Pt colloids. In contrast, sufficient Pt-PAMAM complexation time leads to AFM images and height distributions that have relatively narrow, normal distributions with mean values that increase with the nominal Pt:PAMAM ratio. The surface density of features in AFM images suggest that these Pt DNs reside on the mica surface as two-dimensional surface aggregates. These observations are consistent with an intradendrimer templating mechanism for Pt DNs. However, we cannot determine if the mechanism obeys a fixed loading law because we do not have definitive information about Pt DN shape. A second peak in the Pt DN height distribution appears when the Pt loading exceeds about 66% of PAMAM's theoretical capacity for Pt. Excluding these secondary particles, the dependence of mean feature height on the Pt:PAMAM ratio follows a power-law relationship. Also considering the magnitudes of the measured mean height values, the data suggest that Pt DNs exist as ramified, noncompact aggregates of Pt atoms interspersed within the PAMAM framework.     

Electrochemical DNA biosensors based on platinum nanoparticles combined carbon nanotubes

Platinum nanoparticles were used in combination with multi-walled carbon nanotubes (MWCNTs) for fabricating sensitivity-enhanced electrochemical DNA biosensor. Multi-walled carbon nanotubes and platinum nanoparticles were dispersed in Nafion, which were used to fabricate the modification of the glassy carbon electrode (GCE) surface. Oligonucleotides with amino groups at the 5′ end were covalently linked onto carboxylic groups of MWCNTs on the electrode. The hybridization events were monitored by differential pulse voltammetry (DPV) measurement of the intercalated daunomycin. Due to the ability of carbon nanotubes to promote electron-transfer reactions, the high catalytic activities of platinum nanoparticles for chemical reactions, the sensitivity of presented electrochemical DNA biosensors was remarkably improved. The detection limit of the method for target DNA was 1.0 × 10⁻¹¹ mol l⁻¹.     

Electrochemical characterization of Prussian Blue nanoparticles

Mixing of FeCl₃ solution with the excess of K₄Fe(CN)₆ solution results in well-dispersed Prussian Blue (PB) nanoparticles that are stable over at least one month. Polyaniline was deposited onto the PB-nanoparticle-modified electrode to provide its stability. Promising results of the enhanced detection of H₂O₂ with these PB nanoparticles are described. The text was submitted by the authors in English.     

Electrochemical reactivity and stability of platinum nanoparticles in imidazolium-based ionic liquids

Journal of Solid State Electrochemistry - The electrocatalytic activity of synthesized quasi-spherical Pt nanoparticles (NPs) has been studied, taking as a model the COads electrooxidation reaction...     

Electrochemical behaviour of aldehyde-N-arylhydrazones at platinum electrode and their characterization

A series of chemically and pharmaceutically interesting 2,5-disubstituted-1,3,4-oxadiazoles were efficiently synthesized from the anodic oxidation of aldehyde-N-arylhydrazones at platinum anode by using NaClO₄ as supporting electrolyte in MeOH solution under controlled potential electrolysis at room temperature. The products were characterized by spectroscopic methods and a mechanism was deduced from voltammetry studies. The salient features of this protocol are mild reaction conditions, short reaction time, accelerated rate, high yield and simple work-up procedure.     

Kinetic modeling of CO(ad) monolayer oxidation on carbon-supported platinum nanoparticles

We present a theoretical study of CO(ad) electrooxidation on Pt nanoparticles. Effects of size and surface texture of nanoparticles on the interplay of relevant kinetic processes are investigated. Thereby, strong impacts of particle size on electrocatalytic activities, observed in experiments, are rationalized. Our theoretical approach employs the active site concept to account for the heterogeneous surface of nanoparticles. It, moreover, incorporates finite rates of surface mobility of adsorbed CO. As demonstrated, the model generalizes established mean field or nucleation and growth models. We find very good agreement of our model with chronoamperometric current transients at various particle sizes and electrode potentials (Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem., in press, doi:10.1016/j.jelechem.2006.02.024). The full interplay of on-site reactivity at active sites and low surface mobility of CO(ad) unfolds on the smallest nanoparticles ( approximately 2 nm). In this case, the solution of the model requires kinetic Monte Carlo simulations specifically developed for this problem. For larger nanoparticles (>4 nm) the surface mobility of CO(ad) is high compared to the reaction rate constants, and the kinetic equations can be solved in the limiting case of infinite surface mobility. The analysis provides an insight into the prevailing reaction mechanisms and allows for the estimation of relevant kinetic parameters.     

Electrochemical behavior of CO2 reduction on palladium nanoparticles: Dependence of adsorbed CO on electrode potential

The electrochemical reduction of CO₂ is strongly influenced by both the applied potential and the surface adsorption status of the catalyst. In this work a gas diffusion electrode (GDE) coated with Pd nanoparticles/carbon black (Pd/XC72) was used to study the electrochemical reduction of CO₂. Cyclic voltammetric (CV) analysis of Pd/XC72 between 1.5 V and − 0.6 V (vs. RHE) shows the formation of intermediates and the blocking of hydrogen absorption on the Pd nanoparticles (NPs) under a CO₂ atmosphere. The relationships between the Faradaic efficiency/current density and the applied potential reveal that the onset potential of CO formation is around − 0.4 V. Moreover, the presence of adsorbed CO was confirmed through CV analysis of Pd/XC72 under CO₂ and CO/He atmospheres. This demonstrates that H atoms and CO intermediates co-adsorb on the surface of the Pd NPs at an applied potential of around − 0.4 V. When the applied potential is more negative than − 0.6 V, adsorption of CO intermediates on the surface of the Pd NPs becomes dominant.     

Electrochemical and optical characterization of triarylamine functionalized gold nanoparticles

This paper describes the synthesis, structural analysis, and investigations of the optical and electrochemical properties of some gold nanoparticles (AuNPs) which consist of a triarylamine ligand shell attached to small gold cores (Au-Tara). The triarylamine chromophores were attached to small 4-bromobenzenethiol covered gold nanoparticles (ca. 2 nm in diameter) by Sonogashira reaction. This procedure yields triarylamine redox centers attached via π-conjugated bridging units of different length to the gold core. The AuNPs were analyzed with (1)H NMR spectroscopy, diffusion ordered NMR spectroscopy (DOSY), thermogravimetric analysis (TGA), and scanning transmission electron microscopy (STEM). Cyclic voltammetry (CV) technique was used to determine the composition of the redox active particles via the Randles-Sevcik equation. The optical and electrochemical properties of the Au-Tara nanoparticles and of their corresponding unbound ligands (Ref) were investigated with UV/vis/NIR absorption spectroscopy, Osteryoung square wave voltammetry (OSWV), and spectroelectrochemistry (SEC). These data show that the assembling of triarylamines in the vicinity of a gold nanoparticle can change the optical and electrochemical properties of the triarylamine redox chromophores depending on the kind and length of the bridging unit. This is due to gold core-chromophore and chromophore-chromophore interactions.     

Electrochemical and surface characterization of 4-aminothiophenol adsorption at polycrystalline platinum electrodes

The formation of a self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) on polycrystalline platinum electrodes has been characterized by surface analysis and electrochemistry techniques. The 4-ATP monolayer was characterized by cyclic voltammetry (CV), linear sweep voltammetry, Raman spectroscopy, reflection-absorption infrared (RAIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). CV was used to study the dependence of the adsorption time and 4-ATP solution concentration on the relative degree of coverage of 4-ATP monolayers on polycrystalline Pt electrodes. The adsorption time range probed was 24-72 h. The optimal concentration of 4-ATP needed to obtain the highest surface at the lowest adsorption time was 10 mM. RAIR and Raman spectroscopy for 4-ATP-modified platinum electrodes showed the characteristic adsorption bands for 4-ATP, such as nuNH, nuCH(arom), and nuCS(arom), indicating the adsorption on the platinum surface. The XPS spectra for the modified Pt surface presented the binding energy peaks of sulfur and nitrogen. High energy resolution XPS studies, RAIR, and Raman spectrum for platinum electrodes modified with 4-ATP indicate that the molecules are sulfur-bonded to the platinum surface. The formation of a S-Pt bond suggests that ATP adsorption leads to an amino-terminated electrode surface. The thickness of the monolayer was evaluated via angle-resolved XPS (AR-XPS) analyses, giving a value of 8 A. As evidence of the terminal amino group on the electrode surface, the chemical derivatization of the 4-ATP SAM was done with 16-Br hexadecanoic acid. This surface reaction was followed by RAIR spectroscopy.     

Catalytic oxidation of CO on platinum

Using concepts recently developed in thermal decompositions of solids and reduction of bulk oxides by gases and (re)analysis of experimental literature data, a novel mechanism for the catalytic oxidation of CO by PtO₂ is proposed. Instead of the conventional Mars–van Krevelen scheme, the reactions proposed are: PtO₂(s) + 2CO ? Pt(g) + 2CO₂ and Pt(g) + O₂ ? PtO₂(g) → PtO₂(s). The first reaction determines the kinetics of CO oxidation and the second determines the kinetics of restoration of the PtO₂ layer. Thermochemical consideration of the kinetic features of this model, based on Langmuir’s quasi-equilibrium equations for evaporation of simple substances, allowed calculation of the reaction enthalpy and the absolute rate of CO oxidation. These results are in good agreement with experimental data. The proposed mechanism explains the origin of the surface-retexturing effect, the limited loss of Pt metal from the catalyst, the mechanism of PtO₂ regeneration by oxygen, the strong effect of CO₂ in reducing the CO oxidation rate and the three-fold variation of the Arrhenius E parameter with temperature.     

Electrochemical deposition of platinum nanoparticles on carbon: a study by standard and anomalous X-ray diffraction.

This paper is devoted to an alternative method to characterize platinum nanoparticles: X-ray powder diffraction with synchrotron radiation in classical and anomalous dispersion modes. We could straightforwardly determine the mean diameter and the surface concentration of carbon-supported platinum nanoparticles, even down to diameters of 2-3 nm and catalyst amounts of 0.03 mgcm(-2). We could study early stages of the formation of electrochemically prepared platinum nanoparticles from [PtCl4(2-) species preadsorbed on carbon inside a carbon-Nafion layer, to obtain a fuel-cell electrode. Our X-ray diffraction (XRD) results demonstrate that, provided the superficial concentration is not too high, new and smaller particles appear for each current pulse, since there is not any strong nucleation limitation for the high overvoltages obtained. Hydrogen evolution becomes the main electrochemical phenomenon on particles of sufficient size and it explains the noteworthy size limitation. Better yields of Pt metal are obtained for smaller current densities and longer times: the rate-determining step is then not electrochemical, but chemical or related to superficial diffusion.     

Highly active platinum nanoparticles on graphene nanosheets with a significant improvement in stability and CO tolerance

Graphene nanosheets (GNS) supporting Pt nanoparticles (PNs) are prepared using perfluorosulfonic acid (PFSA) as a functionalization and anchoring agent. Transmission electron microscope (TEM) results indicate that the prepared Pt NPs are uniformly deposited on GNS with a narrow particle size ranging from 1 to 4 nm in diameter. A high catalytic activity of this novel catalyst is observed by both cyclic voltammetry and oxygen reduction reaction (ORR) measurements due to the increasing of proton (H(+)) transmission channels. Significantly, this novel PFSA-functionalized Pt/GNS (PFSA-Pt/GNS) catalyst reveals a better CO oxidation and lower loss rate of electrochemical active area in comparison with that of the plain Pt/GNS and conventional Pt/C catalysts, indicating our PFSA-Pt/GNS catalysts hold much higher stability and CO tolerance by virtue of introduction of PFSA.     

Hydrogen peroxide reduction on single platinum nanoparticles

Understanding oxygen reduction, key to much of electrochemical energy transformation technology, crucially requires exploration of the role of hydrogen peroxide as a possible intermediate especially on catalysts such as Pt which can bring about the 4e reduction of O₂ to water. We reveal that at the single nanoparticle scale the direct platinum catalysed reduction of hydrogen peroxide is found – even at high overpotentials – not to be controlled by the rate mass-transport of the reagents to the interface but by a surface limited process. Further under alkaline (pH 12.3) and near mass-transport free conditions, the single nanoparticle hydrogen peroxide reduction rate goes through a maximum at potentials comparable to the surface deposition of hydrogen (H_upd) with the highest reaction rate occurring when the surface is partially covered in hydrogen.

At the single platinum nanoparticle scale the hydrogen peroxide reduction reaction is a surface limited process.     

Alumina-supported vanadium nanoparticles: structural characterization and CO adsorption properties

Alumina-supported vanadium particles were prepared under ultrahigh vacuum (UHV) conditions and characterized with respect to their structural and CO adsorption properties. As supporting oxide, we used a thin, well-ordered alumina film grown on NiAl(110). This allows the application of scanning tunneling microscopy (STM), infrared reflection-absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS) without charging effects. Vanadium evaporation under UHV conditions leads to the growth of nanometer-sized particles which strongly interact with the alumina support. At very low vanadium coverages, these particles are partially incorporated into the alumina film and get oxidized through the contact to alumina. Low-temperature CO adsorption in this coverage regime permits the preparation of isolated vanadium carbonyls, of which we have identified mono-, di-, and tricarbonyls of the V(CO)(y)() type. A charge-frequency relationship was set up which allows one to quantify the extent of charge transfer from vanadium to alumina. It turns out that this charge transfer depends on the V nucleation site.     

Electrochemical polymerization of acetylene on a surface of platinum

Polyacetylene (PA) deposited on a platinum surface is synthesized by electrochemical polymerization of acetylene in a cell with platinum strip as cathode, nickel strip as anode, and nickel bromide in acetonitrile as electrolyte. The electrolytic solution is presaturated with acetylene. The PA so produced has a granular morphology and high surface area (79 m²/g), and is insoluble. Polymerization at lower temperature gives higher content in cis units. It has the same chemical structure as that produced using the Shirakawa method as examined by elemental analysis, infrared spectroscopy, x-ray diffraction, and differential scanning calorimetry.     

Coadsorbed H and CO interaction on platinum

The behavior of hydrogen near a platinum-surface-adsorbed carbon monoxide molecule is described using a potential energy term constructed from density functional theory. A clear nonattractive interaction of hydrogen with CO is confirmed, most notably with oxygen, which retains its strong H-repulsive traits in the Pt-bound CO case. Inhibiting effects of CO greater than what is expected from simple adsorption site exclusion are discussed with regard to adsorption/desorption and mobility on platinum, as well as possibilities of COH and HCO formation.