Platinum surface additive based nanostructured CuO films for ethanol sensing

In the present work, Platinum (Pt)/Copper (II) oxide (CuO) thin film based ethanol sensors were fabricated by sputtering of Pt in varying concentrations over pre-sputtered nanostructured CuO films. The responses of these sensors as a function of Pt concentrations were studied using operating temperature modulation (200-450 °C) and ethanol concentration modulation (100-2500 ppm). During these modulations, it was found that the sensing response was maximum at operating temperature near 400 °C for all the samples irrespective of the Pt concentration dispersed over them. Moreover, the sensing behavior improves for lower Pt concentration (Pt/CuO-60s) and deteriorates for higher Pt concentration (Pt/CuO-120s). In comparison with bare CuO sample, the sensitivity of Pt/CuO-60s increased up to 22% in the linear range and 33% for maximum ethanol concentration. Hence, the well dispersed optimum Pt additive concentration improves the overall sensing behavior including sensitivity, linear working range and response as well as recovery time.

Growth mechanism of graphene on platinum: Surface catalysis and carbon segregation

A model of the graphene growth mechanism of chemical vapor deposition on platinum is proposed and verified by experiments. Surface catalysis and carbon segregation occur, respectively, at high and low temperatures in the process, representing the so-called balance and segregation regimes. Catalysis leads to self-limiting formation of large area monolayer graphene, whereas segregation results in multilayers, which evidently "grow from below." By controlling kinetic factors, dominantly monolayer graphene whose high quality has been confirmed by quantum Hall measurement can be deposited on platinum with hydrogen-rich environment, quench cooling, tiny but continuous methane flow and about 1000°C growth temperature. © 2014 AIP Publishing LLC.

Platinum surface modification with cerium species and the effect against the methanol anodic reaction

Spontaneous deposition and electrochemical deposition by potential perturbation programs were used to place cerium-containing species on platinum surfaces in acid solution. Cyclic voltammetric profiles of cerium-modified platinum surfaces obtained after potentiostatic or potentiodynamic procedures (applied in the true hydrogen evolution region) differ from those recorded after spontaneous methods. However, the catalytic effects are nearly the same on these cerium-modified platinum surfaces for methanol electrooxidation, i.e. lower onset potential values for the anodic reaction. Besides, a different electrocatalytic effect was observed at large positive potentials on methanol oxidation due to the cerium oxide capability of oxygen storage. This effect is observed on platinum modified by a drastic potentiostatic procedure (by applying -2.0 V) in cerium(IV) acid solution. (C) 2008 Elsevier B.V. All rights reserved.

The generator coordinate Hartree-Fock method as strategy for building Gaussian basis sets to ab initio study of the process of adsorption of sulfur on platinum (200) surface

The scheme named generator coordinate Hartree-Fock method (GCHF) is used to build (22s14p) and (33s22p16d9f) gaussian basis sets to S ((3)P) and Pt ((3)D) atoms, respectively. Theses basis sets are contracted to [13s10p] and [19s13p9d5f] through of Dunning's segmented contraction scheme and are enriched with d and g polarization functions, [13s10p1d] and [19s13p9d5flg]. Finally, the [19s13p9d5f1g] basis Set to Pt ((3)D) was supplemented with s and d diffuse functions, [20s13p10d5flg], and used in combination with [13s10p1d] to study the effects of adsorption of S ((3)D) atom on a pt ((3)D) atom belonged to infinite Pt (200) surface. Atom-atom overlap population, bond order, and infrared spectrum of [pt(_)S](2 -) were calculated properties and were carried out at Hartree-Fock-Roothaan level. The results indicate that the process of adsorption of S ((3)P) on pt ((3)D) in the infinite Pt (200) surface is mainly caused by a strong contribution of sigma between the 3p(z) orbital of S ((3)P) and the 6s orbital of pt ((3)D). (c) 2004 Elsevier B.V. All rights reserved.

THE REVERSIBLE ELECTRON-TRANSFER REACTION OF MYOGLOBIN AT BRILLIANT CRESYL BLUE MODIFIED PLATINUM GAUZE ELECTRODE

The electrochemical behavior of myoglobin at a Brilliant Cresyl Blue (BCB) modified platinum gauze electrode and spiral pt wire in the BCB solution in optically transparent thin layer cell base been investigated by using cyclic potential-absorbance method and double potential step chronoabsorptometry. The results reveal a reversible electron transfer resection of myoglobin. Exhaustive reductive and oxidative electrolyses are achieved at the modified platinum surface in 20 and 100s respectively. The formal h...

A combined transient and computational study of the dissociation of N(2)O on platinum catalysts

The energetics of the low-temperature adsorption and decomposition of nitrous oxide, N(2)O, on flat and stepped platinum surfaces were calculated using density-functional theory (DFT). The results show that the preferred adsorption site for N(2)O is an atop site, bound upright via the terminal nitrogen. The molecule is only weakly chemisorbed to the platinum surface. The decomposition barriers on flat (I 11) surfaces and stepped (211) surfaces are similar. While the barrier for N(2)O dissociation is relatively small, the surface rapidly becomes poisoned by adsorbed oxygen. These findings are supported by experimental results of pulsed N(2)O decomposition with 5% Pt/SiO(2) and bismuth-modified Pt/C catalysts. At low temperature, decomposition occurs but self-poisoning by O((ads)) prevents further decomposition. At higher temperatures some desorption Of O(2) is observed, allowing continued catalytic activity. The study with bismuth-modified Pt/C catalysts showed that, although the activation barriers calculated for both terraces and steps were similar, the actual rate was different for the two surfaces. Steps were found experimentally to be more active than terraces and this is attributed to differences in the preexponential term. (C) 2004 Elsevier Inc. All rights reserved.

The role of low coverage sodium surface species on electrochemical promotion in a Pt/YSZ system

The effect of sodium-modification on the catalyst and electrocatalytic properties of a platinum catalyst supported on a YSZ solid electrolyte was studied. Increasing the sodium coverage on the catalyst surface appears to block some of the three-phase boundary (tpb) sites and reduces the rate of the charge transfer reaction. The promotion of the platinum surface reaction (ethylene oxidation) seems to a first approximation to be a function of the rate of oxygen supply or removal to or from the surface irrespective of whether this is contaminated by sodium or not (samples with sodium contamination require a higher overpotential to achieve the same current density as a clean sample because of poisoning in the tpb). At high negative polarisations (oxygen removed from the surface) the sodium contaminated samples show a significant increase in rate, possibly due to the decomposition of e.g. sodium hydroxides and carbonates. © 2012 Elsevier B.V.

Electrocatalysis of hydrogen evolution reaction on Pt electrode surface-modified by S-2 chemisorption

Kinetic studies of hydrogen evolution reaction (HER) at the surface of Pt in alkaline conditions, reported in this paper, show that electrocatalytic activity is enhanced after adsorption of S-2 ions. EIS and steady-state polarization curve data pointed to an undoubted improvement in performance with the Pt-S cathode that was attributed to higher adsorbed hydrogen coverage. Experimental findings suggested an increase in the electronic density of the modified surface sites that may strengthen the interaction between H2O and the adsorption site and, consequently, accelerates the Volmer step. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Reaction intermediates of ethanol electro-oxidation on platinum investigated by SFG spectroscopy

Although electrochemical oxidation of simple organic molecules on metal catalysts is the basic ingredient of fuel cells, which have great technological potential as a renewable source of electrical energy, the detailed reaction mechanisms are in most cases not completely understood. Here, we investigate the ethanol-platinum interface in acidic aqueous solution using infrared-visible sum frequency generation (SFG) spectroscopy and theoretical calculations of vibrational spectra in order to identify the intermediates present during the electro-oxidation of ethanol. The complex vibrational spectrum in the fingerprint region imply on the coexistence of several adsorbates. Based on spectra in ultra-high-vacuum (UHV) and electrochemical environment from the literature and our density functional theory (DFT) calculations of vibrational spectra, new adsorbed intermediates, never before observed with conventional infrared (IR) spectroscopy, are proposed here: g2-acetaldehyde, g2-acetyl, ethylidyne, monodentate acetate, methoxy, tertiary methanol derivative, COH residue, g2-formaldehyde, mono and bidentate formate, CH3 and CH2 residues. In addition, we present new evidences for an ethoxy intermediate, a secondary ethanol derivative and an acetyl species, and we confirm the presence of previously observed adsorbates: a tertiary ethanol derivative, bidentate acetate, and COad. These results indicate that the platinum surface is much more reactive, and the reaction mechanism for ethanol electro-oxidation is considerably more complex than previously considered. This might be also true for many other molecule-catalyst systems.

Electrochemical and In Situ FTIR Study of o-Cresol on Platinum Electrode in Acid Medium

The electrochemical behaviour of o-cresol in acidic medium on platinum electrode has been studied by cyclic voltammetry and in situ Fourier transform infrared spectroscopy. The o-cresol suffers hydrolysis during oxidation giving rise to the formation of methyl-p-benzoquinone. In situ FTIR spectroscopic studies also reveal the presence of CO2, formed as a consequence of the rupture of the aromatic ring. Moreover, the oxidation of o-cresol in acidic medium produces a polymeric film on the platinum surface that precludes further oxidation of o-cresol. The reduction of o-cresol at potentials below 0 V produces in the first step the partial reduction of the aromatic ring and when the potential goes to values below 0 V, methyl-cyclohexanone.

Using Surface Spectroscopy to Help Interpret STM Images of Chemisorbed Molecules: Application to the Study of the Hydrogenation of Ketones and a-Ketoesters on Pt

Depuis que la haute énantiopureté est nécessaire dans l’industrie pharmaceutique, les études visant à découvrir les mécanismes pour l’hydrogénation énantiosélective de cétones ou céto-esters sur les surfaces, et à rechercher de nouveaux et plus performants catalyseurs asymétriques, sont d’une grande importance. La microscopie à effet tunnel (STM), la spectroscopie infrarouge de réflexion-absorption, la spectroscopie de désorption à température programmée et la spectrométrie de photoélectrons induits par rayons X sont des méthodes performantes facilitant la compréhension des mécanismes de réaction. En plus de nous permettre de comprendre les mécanismes réactionnels, les études peuvent fournir des informations sur la dynamique des réactions en catalyse hétérogène ainsi que sur le développement de la théorie de la fonctionnelle de la densité (DFT) afin de calculer des interactions faibles dans les processus de surface. D’autres parts, les calculs DFT fournissent une aide essentielle à l’interprétation des données de STM et spectroscopie de surface. Dans cette thèse, certains cétones et céto-esters sur la surface de platine sont étudiées par les techniques sophistiquées mentionnées ci-dessus. Mes études démontrent que la combinaison de l’utilisation de la spectroscopie de routine, des nanotechnologies et de nombreux calculs élaborés, est une méthode efficace pour étudier les réactions à la surface car ces techniques explorent les différents aspects de la surface ainsi que s’entraident mutuellement lors de certaines interprétations.

Superactivation of metal electrode surfaces and its relevance to COads oxidation at fuel cell anodes

The inhibiting effect of COads on platinum-based anodes is a major problem in the development of ambient temperature, polyelectrolyte membrane-type fuel cells. One of the unusual features of the response for the oxidative removal of the species in question is that the response observed for this reaction in the positive sweep is highly dependent on the CO admission potential, for example, when the COads is formed in the Hads region it undergoes oxidation at unusually low potentials. Such behaviour is attributed here to hydrogen activation of the platinum surface, with the result that oxide mediators (and COads oxidation) occurs at an earlier stage of the positive sweep. It is also demonstrated, for both platinum and gold in acid solution, that dramatic premonolayer oxidation responses may be observed following suitable preactivation of the electrode surfaces. It is suggested that the defect state of a solid electrode surface is an important variable whose investigation may yield improved fuel cell anode performance.

Theoretical and computational approaches on heterogeneous nucleation

Nucleation is the first step of a first order phase transition. A new phase is always sprung up in nucleation phenomena. The two main categories of nucleation are homogeneous nucleation, where the new phase is formed in a uniform substance, and heterogeneous nucleation, when nucleation occurs on a pre-existing surface. In this thesis the main attention is paid on heterogeneous nucleation. This thesis wields the nucleation phenomena from two theoretical perspectives: the classical nucleation theory and the statistical mechanical approach. The formulation of the classical nucleation theory relies on equilibrium thermodynamics and use of macroscopically determined quantities to describe the properties of small nuclei, sometimes consisting of just a few molecules. The statistical mechanical approach is based on interactions between single molecules, and does not bear the same assumptions as the classical theory. This work gathers up the present theoretical knowledge of heterogeneous nucleation and utilizes it in computational model studies. A new exact molecular approach on heterogeneous nucleation was introduced and tested by Monte Carlo simulations. The results obtained from the molecular simulations were interpreted by means of the concepts of the classical nucleation theory. Numerical calculations were carried out for a variety of substances nucleating on different substances. The classical theory of heterogeneous nucleation was employed in calculations of one-component nucleation of water on newsprint paper, Teflon and cellulose film, and binary nucleation of water-n-propanol and water-sulphuric acid mixtures on silver nanoparticles. The results were compared with experimental results. The molecular simulation studies involved homogeneous nucleation of argon and heterogeneous nucleation of argon on a planar platinum surface. It was found out that the use of a microscopical contact angle as a fitting parameter in calculations based on the classical theory of heterogeneous nucleation leads to a fair agreement between the theoretical predictions and experimental results. In the presented cases the microscopical angle was found to be always smaller than the contact angle obtained from macroscopical measurements. Furthermore, molecular Monte Carlo simulations revealed that the concept of the geometrical contact parameter in heterogeneous nucleation calculations can work surprisingly well even for very small clusters.

An XPS study on oxidation states of Pt and its alloys with Co and Cr and its relevance to electroreduction of oxygen

Cathodic reduction of oxygen in fuel cells is known to be enhanced on platinum alloys in relation to the platinum metal. The higher performance of the platinum alloys is as a result of the improved oxygen-reduction kinetics on the alloys but there is hardly any increase in the electrode platinum-surface-areas for the platinum alloys as compared to the platinum metal, and thus the higher performance is solely due to the enhanced electrocatalytic activity of the alloys as compared to the platinum metal. The present X-ray photoelectron spectroscopic (XPS) study on carbon-supported Pt, Pt–Co and Pt–Co–Cr electrocatalysts suggests the presence of a relatively lower Pt-oxide content on the alloys. The X-ray powder diffraction patterns for these electrocatalysts show that while the carbon-supported platinum electrocatalyst has a face-centered cubic (fcc) phase, carbon-supported Pt–Co and Pt–Co–Cr electrocatalysts exhibit a face-centered tetragonal (fct) phase. But, Pt electrocatalyst has a lower particle-size and, hence, a higher dispersion. Previous studies have shown higher activities on the Pt-alloys than on Pt, and have attributed it to changes in the electronic and structural characteristics of Pt. These changes can be correlated with the lower oxidation-state of Pt sites, as found in this study.

Effect of pretreatment of black carbon on electrocatalytic activity of Pt/C catalyst for methanol oxidation

Direct methanol fuel cell (DMFC) has attracted wide attention due to its many advantages. However, its practical application is limited by the low electrocatalytic activity of the anodic Pt/C catalyst usually used for the methanol oxidation. In this paper, in order to increase the electrocatalytic performance of the Pt/C catalyst for the methanol oxidation, the black carbon, usually used as the supporter, was pretreated with CO2, air, HNO3 or H2O2. The cyclic voltarnmetric results indicated that the current densities of the anodic peak of methanol oxidation at the Pt/C catalysts with the black carbon pretreated with CO2,air, HN03, H202 and untreated black carbon were 39, 33, 32, 20 and 18 mA center dot cm(-2), respectively, illustrating that among the above five kinds of the Pt/C catalysts, the Pt/C catalyst with the black carbon pretreated with CO2 shows the best electrocatalytic activity and stability for the methanol oxidation. Its main reason is that the CO2 pretreatment could reduce the content of the oxygen-containing groups on the surface of the black carbon and increase the content of graphite in the black carbon, leading to the low resistance of the black carbon and the increase in the dispersion extent of the Pt particles in the Pt/C catalyst.