Oscillatory electro-oxidation of glycerol on platinum

This paper describes the experimental study of the oscillatory electro-oxidation of glycerol on platinum under galvanostatic control. The system was investigated in both acidic and alkaline media and for different glycerol concentrations. In acidic supporting electrolyte, the oscillatory behavior is rather simple and the main features such as period, amplitude and waveform are barely affected by the glycerol concentration. A more complex picture including the presence of different temporal patterns and strong dependence with glycerol concentration emerges in alkaline medium. In both media, the composite oscillations visit two clearly discernible potential windows, and potential oscillations in alkaline media are observed only when the working electrode is kept stationary. The mechanistic implications of our findings are discussed in connection with available data obtained under close-to-equilibrium conditions. (C) 2012 Elsevier Ltd. All rights reserved.

Performance and selectivity of PtxSn/C electro-catalysts for ethanol oxidation prepared by reduction with different formic acid concentrations

Carbon supported Pt-Sn catalysts were prepared by reduction of Pt and Sn precursors with formic acid and characterized in terms of structure, morphology and surface properties. The electrocatalytic activity for ethanol oxidation was studied in a direct ethanol fuel cell (DEFC) at 70 degrees C and 90 degrees C. Electrochemical and physico-chemical data indicated that a proper balance of Pt and Sn species in the near surface region was necessary to maximize the reaction rate. The best atomic surface composition, in terms of electrochemical performance, was Pt:Sn 65:35 corresponding to a bulk composition 75:25 namely Pt3Sn1/C. The reaction products of ethanol electro-oxidation in single cell and their distribution as a function of the nature of catalyst were determined. Essentially, acetaldehyde and acetic acid were detected as the main reaction products; whereas, a lower content of CO2 was formed. The selectivity toward acetic acid vs. acetaldehyde increased with the increase of the Sn content and decreased by decreasing the concentration of the reducing agent used in the catalyst preparation. According to the recent literature, these results have been interpreted on the basis of ethanol adsorption characteristics and ligand effects occurring for Sn-rich electrocatalysts. (C) 2012 Elsevier Ltd. All rights reserved.

The influence of the Pt crystalline surface orientation on the glycerol electro-oxidation in acidic media

We investigated the electrochemical oxidation of glycerol on low-index Pt single crystals in acidic media (H2SO4 and HClO4) by cyclic voltammetry and Fourier Transform Infrared (FTIR) spectroscopy and we verified that this is a surface sensitive reaction. Pt(100) and Pt(110) surface structures favor the breaking of the C-C-C bond at low potentials (say 0.05 V), as seen by the formation of CO, one of the adsorbed residues of the glycerol dissociation, which poisons these surfaces even at high potentials. Pt(111) surface structure does not favor the C-C-C bond breaking at potentials as low as 0.05 V. However, Pt(111) is less poisoned by residues of glycerol dissociation and, for this reason, it is more active for glycerol oxidation than Pt(100) and Pt(110) at low potentials. Carbonyl containing compounds and CO2 were detected as reaction products of the glycerol oxidation on all investigated single-crystal Pt surfaces. The ratio between CO2 and carbonyl containing compounds is clearly much higher for Pt(100) and Pt(110) than for Pt(111). (C) 2012 Elsevier Ltd. All rights reserved.

Low content cerium oxide nanoparticles on carbon for hydrogen peroxide electrosynthesis

A comparative study using different proportions of CeO2/C (4%, 9% and 13% CeO2) was performed to produce H2O2, a reagent used in the oxidation of organic pollutants and in electro-Fenton reactions for the production of the hydroxyl radical (OH center dot), a strong oxidant agent used in the electrochemical treatment of aqueous wastewater. The CeO2/C materials were prepared by a modified polymeric precursor method (PPM). X-ray diffraction analysis of the CeO2/C prepared by the PPM identified two phases. CeO2 and CeO2. The average size of the crystallites in these materials was close to 7 nm. The kinetics of the oxygen reduction reaction (ORR) were evaluated by the rotating ring-disk electrode technique. The results showed that the 4% CeO2/C prepared by the PPM was the best composite for the production of H2O2 in a 1 mol L-1 NaOH electrolyte solution. For this material, the number of electrons transferred and the H2O2 percentage efficiency were 3.1 and 44%, respectively. The ring-current of the 4% CeO2/C was higher than that of Vulcan carbon, the reference material for H2O2 production, which produced 41% H2O2 and transferred 3.1 electrons per molecule of oxygen. The overpotential for this reaction on the ceria-based catalyst was substantially lower (approximately 200 mV), demonstrating the higher catalytic performance of this material. Gas diffusion electrodes (GDE) containing the catalyst were used to evaluate the real amount of H2O2 produced during exhaustive electrolysis. The 4% CeO2/C GDE produced 871 mg L-1 of H2O2, whereas the Vulcan carbon GDE produced a maximum amount of only 407 mg L-1. Thus, the 4% CeO2/C electrocatalyst prepared by the PPM is a promising material for H2O2 electrogeneration in alkaline media. (C) 2011 Elsevier B.V. All rights reserved.

Structural and textural studies of NiO impregnation in mesoporous ZrO2-CeO2 for catalysis applications.

The synthesis of zirconia-based ordered mesoporous structures for catalytic applications is a research area under development. These systems are also potential candidates as anodes in intermediate temperature solid oxide fuel cells (it-SOFC) due to an enhancement on their surface area [1-4]. The structural features of mesoporous zirconia-ceria materials in combination with oxygen storage/release capacity (OSC) are crucial for various catalytic reactions. The direct use of hydrocarbons as fuel for the SOFC (instead of pure H2), without the necessity of reforming and purification reactors can improve global efficiency of these systems [4]. The X-ray diffraction data showed that ZrO2-x%CeO2 samples with x>50 are formed by a larger fraction of the cubic phase (spatial group Fm3m), while for x<50 the major crystalline structure is the tetragonal phase (spatial group P42/nmc). The crystallite size of the cubic phase increases with increase in ceria content. The tetragonal crystallite size decreases when ceria content increases. After impregnation, the Rietveld analysis showed a NiO content around 60wt.% for all samples. The lattice parameters for the ZrO2 tetragonal phase are lower for higher ZrO2 contents, while for all samples the cubic NiO and CeO2 parameters do not present changes. The calculated densities are higher for higher ceria content, as expected. The crystallite size of NiO are similar (~20nm) for all samples and 55nm for the NiO standard. Nitrogen adsorption experiments revealed a broader particle size distribution for higher CeO2 content. The superficial area values were around 35m2/g for all samples, the average pore diameter and pore volumes were higher when increasing ceria content. After NiO impregnation the particle size distribution was the same for all samples, with two pore sizes, the first around 3nm and a broader peak around 10nm. The superficial area increased to approximately 45m2/g for all samples, and the pore volume was also higher after impregnation and increased when ceria content increased. These results point up that the impregnation of NiO improves the textural characteristics of the pristine material. The complementary TEM/EDS images present a homogeneous coating of NiO particles over the ZrO2-x%CeO2 support, showing that these samples are excellent for catalysis applications. [1] D. Y. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D. Stucky, Science 279, 548-552 (1998). [2] C. Yu, Y. Yu, D. Zhao, Chem. Comm. 575-576 (2000). [3] A. Trovarelli, M. Boaro, E. Rocchini, C. de Leitenburg, G. Dolcetti, J. Alloys Compd. 323-324 (2001) 584-591. [4] S. Larrondo, M. A. Vidal, B. Irigoyen, A. F. Craievich, D. G. Lamas, I. O. Fábregas, et al. Catal. Today 107–108 (2005) 53-59.