Preparation and characterization of ordered intermetallic platinum phases for electrocatalytic applications

Ordered intermetallic phases of Pt with several transition metals have been prepared and their electrocatalytic properties studied. In light of these tests it is proposed that these catalysts could be used as electrodes in fuel cells, as they combine an excellent capacity to adsorb organic fuels at the Pt sites with low susceptibility to being poisoned by intermediates and reaction products at the transition-metal sites. An experimental procedure used to obtain the four intermetallic phases Pt-M (M = Mn, Pb, Sb and Sn) is described. The phases thus produced were characterized by X-ray diffraction, scanning electron microscopy with surface analysis by energy-dispersive X-ray spectrometry, scanning tunneling microscopy and X-ray photoelectron spectroscopy. The data thus obtained support the conclusion that the method described here is highly effective for the preparation of Pt-M phases featuring a range of structural and electronic modifications that will allow a useful relation to be established between their physicochemical properties and predicted electrocatalytic activity. (C) 2007 Elsevier Ltd. All rights reserved.

On the apparent lack of preferential site occupancy and electrooxidation of CO adsorbed at low coverage onto stepped platinum surfaces

We report time evolution studies of low coverage CO adsorption (surface hydrogen site blocking < 40%) and oxidative stripping on stepped Pt(776) and Pt(554) surfaces. It was observed that there is no preferential site occupancy for CO adsorption on step or terrace. It is proposed that CO adsorption onto these surfaces is a random process, and after CO adsorption there is no appreciable shift from CO-(111) to CO-(110) sites. This implies that after adsorption, CO molecules either have a very long residence time, or that the diffusion coefficient is much lower than previously thought. After CO electrooxidation the sites released included both terrace (111) and step (110) orientations. For surface hydrogen site blocking > 40%, the lateral interactions might play a role in the preferential CO site occupancy. (C) 2011 Elsevier B.V. All rights reserved.

Direct Electrochemical Determination of Glyphosate at Copper Phthalocyanine/Multiwalled Carbon Nanotube Film Electrodes

A copper phthalocyanine/multiwalled carbon nanotube film-modified glassy carbon electrode has been used for the determination of the herbicide glyphosate (Gly) at -50 mV vs. SCE by electrochemical oxidation using differential pulse voltamtnetry (DPV). Cyclic voltammetry and electrochemical impedance spectroscopy showed that Gly is adsorbed on the metallic centre of the copper phthalocyanine molecule, with formation of Gly-copper ion complexes. An analytical method was developed using DPV in pH 7.4 phosphate buffer solution, without any pretreatment steps: Gly was determined in the concentration range of 0.83-9.90 mu mol L(-1), with detection limit 12.2 nmol L(-1) (2.02 mu g L(-1))

Ethanol oxidation reaction on PtCeO(2)/C electrocatalysts prepared by the polymeric precursor method

This paper presents a study of the electrocatalysis of ethanol oxidation reactions in an acidic medium on Pt-CeO(2)/C (20 wt.% of Pt-CeO(2) on carbon XC-72R), prepared in different mass ratios by the polymeric precursor method. The mass ratios between Pt and CeO(2) (3:1, 2:1, 1:1, 1:2, 1:3) were confirmed by Energy Dispersive X-ray Analysis (EDAX). X-ray diffraction (XRD) structural characterization data shows that the Pt-CeO(2)/C catalysts are composed of nanosized polycrystalline non-alloyed deposits, from which reflections corresponding to the fcc (Pt) and fluorite (CeO(2)) structures were clearly observed. The mean crystallite sizes calculated from XRD data revealed that, independent of the mass ratio, a value close to 3 nm was obtained for the CeO(2) particles. For Pt, the mean crystallite sizes were dependent on the ratio of this metal in the catalysts. Low platinum ratios resulted in small crystallites. and high Pt proportions resulted in larger crystallites. The size distributions of the catalysts particles, determined by XRD, were confirmed by Transmission Electron Microscope (TEM) imaging. Cyclic voltammetry and chronoamperometic experiments were used to evaluate the electrocatalytic performance of the different materials. In all cases, except Pt-CeO(2)/C 1:1, the Pt-Ceo(2)/C catalysts exhibited improved performance when compared with Pt/C. The best result was obtained for the Pt-CeO(2)/C 1:3 catalyst, which gave better results than the Pt-Ru/C (Etek) catalyst. (C) 2009 Elsevier B.V. All rights reserved.

Nanogravimetric study of the complex voltammetric response in the electro-oxidation of methanol on platinum

Instead of a time-invariant voltammetric profile, many electrochemical systems display a cycle-dependent current-potential response. This phenomenon has been referred to as complex voltammetric response and it has been observed during the electro-oxidation of several molecules such as methanol, ethanol, propanol and hydrogen. There are currently two explanations for the surface mechanism underlying this behavior. In one scenario, the complex voltammogram would result from the specific kinetic pathway taken during the forward sweep. In the other explanation, the phenomenon is discussed in terms of the interplay among the surface roughening and subsequent relaxation, and the ohmic drop coupled to a negative differential resistance. We report in this paper a nanogravimetric investigation of the complex voltammetric response in the electro-oxidation of methanol on platinum electrode in both acidic and alkaline media. Different periodic patterns composed of intercalated small and large hysteresis cycles were observed as a function of the applied voltage and the series resistance between the working electrode and the potentiostat. Independently, nanogravimetric results indicated no detectable difference in the delta-frequency versus voltage profile between small and large hysteresis cycles. These findings were interpreted as experimental evidence of the secondary, if any, role played by the very electrochemical reaction on the emergence of complex voltammetric response. (C) 2009 Elsevier Ltd. All rights reserved.

Numerical simulation of magnetic interactions in polycrystalline YFeO3

The magnetic behavior of polycrystalline yttrium orthoferrite was studied from the experimental and theoretical points of view. Magnetization measurements up to 170 kOe were carried out on a single-phase YFeO3 sample synthesized from heterobimetallic alkoxides. The complex interplay between weak-ferromagnetic and antiferromagnetic interactions, observed in the experimental M(H) curves, was successfully simulated by locally minimizing the magnetic energy of two interacting Fe sublattices. The resulting values of exchange field (H-E = 5590 kOe), anisotropy field (H-A = 0.5 kOe) and Dzyaloshinsky-Moriya antisymmetric field (H-D = 149 kOe) are in good agreement with previous reports on this system. (C) 2007 Elsevier B.V. All rights reserved.

Phytase immobilization on modified electrodes for amperometric biosensing

Phytase (myo-inositol hexaphosphate phosphohydrolase) and phytic acid (myo-inositol hexaphosphate) play an important environmental role, in addition to being a health issue in food industry. Phytic acid is antinutritional due to its ability to chelate metal ions and may also react with proteins decreasing their bioavailability. In this work, we produced biosensors with phytase immobilized in Layer-by-Layer (LbL) films, which could detect phytic acid with a detection limit of 0.19 mmol L-1, which is sufficient to detect phytic acid in seeds of grains and vegetables. The biosensosrs consisted of LbL films containing up to eight bilayers of phytase alternated with poly(allylamine) hydrochloride (PAH) deposited onto an indium-tin oxide (ITO) substrate modified with Prussian Blue. Amperometric detection was conducted in an acetate buffer solution (at pH 5.5) at room temperature, with the biosensor response attributed to the formation of phosphate ions. In subsidiary experiments with the currents measured at 0.0 V (vs. SCE), we demonstrated the absence of effects from some interferents, pointing to a good selectivity of the biosensor. (c) 2007 Elsevier B.V. All rights reserved.

Silsesquioxane as a New Building Block Material for Modified Electrodes Fabrication and Application as Neurotransmitters Sensors

Nanostructured films comprising a 3-n-propylpyridiniunn silsesquioxane polymer (designated as SiPy(+)Cl(-)) and copper (II) tetrasulfophthalocyanine (CuTsPc) were produced using the Layer-by-Layer technique (LbL). To our knowledge this is the first report on the use of silsesquioxane derivative polymers as building blocks for nanostructured thin films fabrication. Deposition of the multilayers were monitored by UV-Vis spectroscopy revealing the linear increment in the absorbance of the Q-band from CuTsPc at 617 nm with the number of SiPy(+)Cl(-)/CuTsPc or CuTsPc/SiPy(+)Cl(-) bilayers. FTIR analyses showed that specific interactions between SiPy+Cl- and CuTsPc occurred between SO(3)(-) groups of tetrasulfophthalocyanine and the pyridinium groups of the polycation. Morphological studies were carried out using the AFM technique, which showed that the roughness and thickness of the films increase with the number of bilayers. The films displayed electroactivity and were employed to detection of dopamine (DA) and ascorbic acid (AA) using cyclic voltammetry, at concentrations ranging from 1.96 x 10(-4) to 1.31 x 10(-3) molL(-1). The number and the sequence of bilayers deposition influenced the electrochemical response in presence of DA and AA. Using differential pulse technique, films comprising SiPy(+)/CuTsPc were able to distinguish between DA and ascorbic acid (AA), with a potential difference of approximately with 500 mV, in the concentration range of 9.0 x 10(-5) to 2.0 x 10(-4) molL(-1), in pH 3.0.

Absorption and emission properties of LBL PPV films deposited on ITO electrodes

In this work we studied the properties of absorption and emission line shape of layer-by-layer (LBL) poly(p-phenylene vinylene) (PPV) on indium-tin oxide (ITO) electrode. To minimize the PPV thermal conversion effects during the polymer processing, we used a less aggressive leaving group in the precursor polymer; minimizing electrode degradation. LBL ITO/PPV films showed the same absorption and emission line shape compared with LBL PPV films deposited on non-metallic substrates (glass). With this analysis we indirectly observe the decrease in the ITO degradation. Atomic force microscopy (AFM) technique was used to analyze quantitatively the microscopic morphology of the film surface. Results indicated that the substrate topology is not affected, to a large extent, by the use of dodecylbenzensulfonate (DBS) ion. (C) 2008 Elsevier B.V. All rights reserved.

Disposable Graphite Foil Based Electrodes and Their Application in Pharmaceutical Analysis

This paper explores a new source of graphite for working electrodes, which presents advantages such as low electrical resistance, good flexibility, favorable mechanical performance, versatility to design electrodes in almost any size and very low cost. The new electrodes were investigated in batch electrochemical cells as associated with flow injection analysis systems. Cyclic voltammetry, stripping voltammetry, and amperometry associated with flow injection analysis techniques were applied for the determination of ascorbic acid, zinc and paracetamol in pharmaceutical formulations, respectively. Well-established analytical methods were applied for comparison purposes. The results herein demonstrate the potential of graphite foils as working electrodes in different electroanalytical methods, offering the possibility of producing disposable sensors for routine applications.

Amperometric detection of benzoyl peroxide in pharmaceutical preparations using carbon paste electrodes with peroxidases naturally immobilized on coconut fibers

This paper describes the applications of anew carbon paste electrode containing fibers of coconut (Cocus nucifera L) fruit, which are very rich in peroxidase enzymes naturally immobilized on its structure. The new sensor was applied for the amperometric quantification of benzoyl peroxide in facial creams and dermatological shampoos. The amperometric measurements were performed in 0.1 mol L(-1) phosphate buffer (pH 5.2), at 0.0 V (versus Ag/AgCl). On these conditions, benzoyl peroxide was rapidly determined in the 5.0-55 mu mol L(-1), with a detection limit of 2.5 mu mol L(-1) (s/n = 3), response time of 4.1 s (90% of the steady state) and sensitivity limit of 0.33 A mol L(-1) cm(-2). The amperometric results are in good agreement with those obtained by spectrophotometric technique, used as a standard method. (C) 2009 Elsevier B.V. All rights reserved.

Identification of species formed after pyridine adsorption on iron, cobalt, nickel and silver electrodes by SERS and theoretical calculations

The adsorption of pyridine (py) on Fe, Co, Ni and Ag electrodes was studied using surface-enhanced Raman scattering (SERS) to gain insight into the nature of the adsorbed species. The wavenumber values and relative intensities of the SERS bands were compared to the normal Raman spectrum of the chemically prepared transition metal complexes. Raman spectra of model clusters M(4)(py) (four metal atoms bonded to one py moiety) and M(4)(alpha-pyridil) where M = Ag, Fe, Co or Ni were calculated by density functional theory (DFT) and used to interpret the experimental SERS results. The similarity of the calculated M(4)(py) spectra with the experimental SERS spectra confirm the molecular adsorption of py on the surface of the metallic electrodes. All these results exclude the formation of adsorbed alpha-pyridil species, as suggested previously. Copyright (C) 2009 John Wiley & Sons, Ltd.

Nickel hydroxide electrodes as amperometric detectors for carbohydrates in flow injection analysis and liquid chromatography

The present paper describes the utilization of nickel hydroxide modified electrodes toward the catalytic oxidation of carbohydrates (glucose, fructose, lactose and sucrose) and their utilization as electrochemical sensor. The modified electrodes were employed as a detector in flow injection analysis for individual carbohydrate detection, and to an ionic column chromatography system for multi-analyte samples aiming a prior separation step. Kinetic studies were performed on a rotating disk electrode (RDE) in order to determine both the heterogeneous rate constant and number of electrons transferred for each carbohydrate. Many advantages were found for the proposed system including fast and easy handling of the electrode modification, low cost procedure, a wide range of linearity (0.5-50 ppm), low detection limits (ppb level) and high sensitivities. (C) 2009 Elsevier B.V. All rights reserved.

Flow injection amperometric determination of hydrogen peroxide in household commercial products with a ruthenium oxide hexacyanoferrate modified electrode

Hydrogen peroxide was determined in oral antiseptic and bleach samples using a flow-injection system with amperometric detection. A glassy carbon electrode modified by electrochemical deposition of ruthenium oxide hexacyanoferrate was used as working electrode and a homemade Ag/AgCl (saturated KCl) electrode and a platinum wire were used as reference and counter electrodes, respectively. The electrocatalytic reduction process allowed the determination of hydrogen peroxide at 0.0 V. A linear relationship between the cathodic peak current and concentration of hydrogen peroxide was obtained in the range 10-5000 mu mol L(-1) with detection and quantification limits of 1.7 (S/N = 3) and 5.9 (S/N = 10) mu mol L(-1), respectively. The repeatability of the method was evaluated using a 500 mu mol L(-1) hydrogen peroxide solution, the value obtained being 1.6% (n = 14). A sampling rate of 112 samples h(-1) was achieved at optimised conditions. The method was employed for the quantification of hydrogen peroxide in two commercial samples and the results were in agreement with those obtained by using a recommended procedure.

Copper hexacyanoferrate nanoparticles modified electrodes: A versatile tool for biosensors

Copper hexacyanoferrate nanoparticles of about 30 nm in size have been prepared by the sonochemical irradiation of a mixture of aqueous potassium ferricyanide and copper chloride solutions. The nanoparticles were immobilized onto fluorine doped tin oxide (FTO) electrodes by using the electrostatic deposition layer-by-layer technique (LbL), obtaining electroactive films with electrocatalytic properties towards H2O2 reduction, providing higher currents than those observed for electrodeposited bulk material, even in electrolytes containing NH4+, Na+ and K+. The nanoparticles assembly was used as mediator in a glucose biosensor by immobilizing glucose oxidase enzyme by both, cross-linking and LbL. techniques. Sensitivities obtained were dependent on the immobilization method ranging from 1.23 mu A mmol(-1) L cm(-2) for crosslinking to 0.47 mu A mmol(-1) L cm(-2) for LbL; these values being of the same order than those obtained with electrodes where the amount of enzyme used is much higher. Moreover, the linear concentration range where the biosensors can operate was 10 times higher for electrodes prepared with the LbL immobilization method than with the conventional crosslinking one. (C) 2008 Elsevier B.V. All rights reserved.