Claude Poleunis

Activation and characterization of Fe-based catalysts for the reduction of oxygen in polymer electrolyte fuel cells

Fe-based catalysts for the reduction of oxygen in polymer electrolyte fuel cells (PEFCs) have been prepared from a precursor containing 10 wt% Fe as-Fe(OH)(2) adsorbed on carbon black (Fe(OH)(2)/C). Activation of the precursor was performed in two steps: (i) H-2 reduction at 600 degrees C; (ii) pyrolysis in acetonitrile (AN) vapor at various temperatures (400, 600, 800, 1000 degrees C). The electrocatalytic properties of all catalysts were tested in rotating disk electrode (RDE) experiments and in single H-2/O-2 gas diffusion electrode (GDE) assemblies. Catalysts for O-2 reduction were obtained for Fe(OH)(2)/C pyrolyzed at 600 degrees C and higher. Stable currents in the fuel cell assembly were observed for the catalysts prepared at 800 degrees C and higher. Leaching the excess iron from the catalysts by exposure to an H2SO4 solution increased their catalytic activities. Exposure of the acid leached catalyst prepared at 1000 degrees C to Cl(2 )at 650 degrees C removed additional quantities of excess iron and increased the catalyst activity even further. The catalyst obtained after these treatments retained an iron content of 3.3 wt%. The survival of the catalytic activity in that material even after the Cl-2 treatment suggests that either Fe is present in the active site in a high oxidation state unleachable under the form of FeCl3, or that Fe is not a constituent of the active site. In this case, its role would be limited to catalyzing the formation of the carbon and nitrogen based active site. The XRD, XPS, ToF-SIMS and TEM analyses performed on the catalysts prepared during this study were inconclusive in resolving this issue as they all were dominated by the presence of inactive iron particles and AN pyrolysis products

Kinetics of conditioning layer formation on stainless steel immersed in seawater

Adhesion of microorganisms to surfaces in marine environments leads to biofouling. The deleterious effects of biofilm growth in the marine environment are numerous and include energy losses due to increased fluid frictional resistance or to increased heat transfer resistance, the risk of corrosion induced by microorganisms, loss of optical properties, and quality control and safety problems. Antifouling agents are generally used to protect surfaces from such a biofilm. These agents are toxic and can be persistent, causing harmful environmental and ecological effects. Moreover, the use of biocides and regular cleaning considerably increase the maintenance costs of marine industries. An improved knowledge of bio‐film adhesion mechanisms is needed for the development of an alternative approach to the currently used antifouling agents. The aim of this study is to characterise the chemical composition of the molecules first interacting with stainless steel during the period immediately following immersion in natural seawater and to elucidate the kinetics of the adsorbtion process. Proteins are shown to adhere very rapidly, closely followed by carbohydrates. The distribution on the surface of organic molecules is also examined. The ad‐sorbate on the surface is not a continuous film but a heterogeneous deposit, whose average thickness varies widely. The cleaning procedures used affect the adsorption kinetics. In particular, cleaning with hexane results in slower adsorption of nitrogen‐containing species than does cleaning in acetone.

Sizing Removal and Functionalization of the Carbon-fiber Surface Studied By Combined Tof Sims and Xps

Time-of-flight secondary ion mass spectrometry (TOF SIMS) and X-ray photoelectron spectroscopy (XPS) have been jointly used to study a two-step surface processing of AS4 carbon fiber: extraction of sizing in CH2Cl2 and functionalization with trimellitic anhydride. The combined information on molecular specificity obtained with TOF SIMS and quantification obtained with XPS allows us to follow qualitatively and quantitatively the changes in functional groups on the carbon surface. The results show that the sizing on AS4 contains at least four different compounds. These compounds can be extracted in CH2Cl2 and the elimination is almost complete for silicone. The functionalization of AS4 with trimellitic anhydride has been realized. The reaction takes place between the amine groups on the carbon fiber and the two types of functional groups in trimellitic anhydride.

Synthesis of Polystyrene Thin Films by Means of an Atmospheric-Pressure Plasma Torch and a Dielectric Barrier Discharge

In this paper, the deposition and characterization of plasma-polymerized polystyrene (pp-PS) using PECVD under atmospheric pressure on a variety of substrates was investigated. An atmospheric RF plasma torch and an HF dielectric-barrier-discharge (DBD) system were used to deposit thin pp-PS coatings on PTFE, HDPE, stainless steel, glass, and silicon wafer. The styrene vapor was carried by Ar or He. The pp-PS films were characterized by Fourier transform infrared spectroscopy (FTIR) (infrared reflection absorption spectroscopy), X-ray photoelectron spectroscopy (XPS), water contact angle (WCA), static secondary ion mass spectroscopy (SSIMS), and optical microscopy, and the plasma phase was studied by optical-emission spectroscopy. The major features that characterize PS are present in the FTIR, SSIMS, and XPS spectra of our films, although some differences are observed between pp-PS and their conventionally polymerized counterparts: oxygenation, branching, degree of cross-linking, and unsaturation. According to the WCA and XPS results, the films deposited by the RF plasma torch (placed in a Plexiglass chamber) are more oxygenated than those deposited by DBD, which is operated under a much more controlled atmosphere. A comparison of the chemical structure of the deposited coatings (branching, cross-linking) as a function of the nature of the carrier gas was established by FTIR: pp-PS synthesized in the presence of Ar (for both processes) exhibit more branching and a higher degree of cross-linking than pp-PS synthesized with He as the main plasma gas. The optical microscopy points out a diversity of structures that depend on the nature of the substrate and the plasma parameters.

Organic secondary ion mass spectrometry: Signal enhancement by water vapor injection

The enhancement of the static secondary ion mass spectrometry (SIMS) signals resulting from the injection, closely to the sample surface, of H2O vapor at relatively high-pressure, was investigated for a set of organic materials. While the ion signals are generally improved with increasing H2O pressure upon 12 keV Ga+ bombardment, a specific enhancement of the protonated ion intensity is clearly demonstrated in each case. For instance, the presence of H2O vapor induces an enhancement by one order of magnitude of the [M+ H]+ static SIMS intensity for the antioxidant Irgafos 168 and a ∼1.5-fold increase for polymers such as poly(vinyl pyrrolidone).

Antifouling properties of poly(methyl methacrylate) films grafted with poly(ethylene glycol) monoacrylate immersed in seawater.

Biofouling of all structures immersed in seawater constitutes an important problem, and many strategies are currently being developed to tackle it. In this context, our previous work shows that poly(ethylene glycol) monoacrylate (PEGA) macromonomer grafted on preoxidized poly(methyl methacrylate) (PMMAox) films exhibits an excellent repellency against the bovine serum albumin used as a model protein. This study aims to evaluate the following: (1) the prevention of a marine extract material adsorption by the modified surfaces and (2) the antifouling property of the PEGA-g-PMMAox substrates when immersed in natural seawater during two seasons (season 1: end of April-beginning of May 2007, and season 2: end of October-beginning of November 2007). The antifouling performances of the PEGA-g-PMMAox films are investigated for different PEG chain lengths and macromonomer concentrations into the PEGA-based coatings. These two parameters are followed as a function of the immersion time, which evolves up to 14 days. The influence of the PEGA layer on marine compounds (proteins and phospholipids) adsorption is evidenced by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). It was found that the antifouling efficiency of the PEGA-grafted surfaces increases with both PEGA concentration and PEG chain length.

Time-of-flight SIMS study of heterogeneous catalysts based on praseodymium and molybdenum oxides

Time-of-flight secondary ion mass spectrometry (TOF SIMS) analyses have been performed on MoO3–Pr6O11 mixtures before and after their use in the selective oxidation of isobutene to methacrolein at 673 K. The three pure molybdate phases (Pr2MoO6, Pr2Mo3O12, Pr6MoO12) obtained independently by the citrate route have also been examined to determine their respective SIMS fragmentation patterns. Although the Pr2MoO6 and Pr2Mo3O12 phases were found to give the same molecular fragments, these phases could be distinguished by the relative intensity ratios of some particular species. Identification of Pr6MoO12 was easier owing to the presence of characteristic high-mass mixed Pr–Mo fragments. A linear relationship was observed when plotting the experimental intensity ratios of several molecular fragments against the Pr/Mo bulk composition. The same behaviour was also noticed for a series of Pr6O11–MoO3 mixtures. Moreover, the examination of the 3-D crystal structure of the pure molybdate phases indicates that the fragmentation patterns of these phases were directly related to their structure. As far as the used catalysts were concerned, the SIMS results showed that: (i) the experimental intensity ratios of given fragments were much smaller in the used catalysts than in the fresh ones; (ii) fragments characteristic of the Pr6MoO12 phase were absent; and (iii) differences in the relative SIMS intensities of certain fragments showed the dominant presence of the Pr2Mo3O12 phase.

Antioxidant segregation and crystallisation at polyester surfaces studied by ToF-SIMS

Thin amorphous layers of poly(ethylene terephthalate-ethylene isophthalate) (PETI) containing variable concentration of an antioxidant additive (Irgafos 168) were deposited by spin casting from solution onto silicon wafers. ToF-secondary ion mass spectrometry (SIMS) results showed that a very low bulk percentages (similar to1 wt.%) of additive led to an almost uniform covering of the polymer surface, indicating a strong surface segregation (blooming effect). Multivariate statistical methods (PCA) allowed us to quantify the additive surface concentration in the lower concentration range (less than or equal to0.3 wt.%). ToF-SIMS imaging confirmed the uniform surface covering and, for higher concentration (5.5 additive wt.%), showed the formation of additive crystals on the top of the additive covered PETI surface

ToF-SIMS chemical mapping study of protein adsorption onto stainless steel surfaces immersed in saline aqueous solutions

It is now well established that protein adsorption constitutes the first step of the biofouling process. As sea. water contains several salts (mainly, NaCl, MgCl2 and CaCID and their influence on the protein adsorption is not yet clear, the aim of this contribution was to bring new insight on their role. For this purpose, different aqueous solutions containing salts mixtures have been prepared while keeping the total salt concentration the same as in natural seawater (36 g/l). A model protein, the bovine serum albumin (BSA) was added to these solutions. Stainless steel surfaces were immersed into these different solutions for 24 h then rinsed and freeze-dried. The time-of-flight-secondary ion mass spectrometry (ToF-SIMS) molecular images show that all the salts dissolved in water produce micrometer particles at the sample surfaces. These particles are mainly consist of sodium, magnesium and calcium. Because there are few space correlations with any counter ions (Cl, SO4-, etc.), these particles are not pure salt residues. Moreover, these particles were found to be the preferred adsorption sites for the proteins. Indeed, the images show that the BSA surrounds these metallic spots to form annular shape patterns (diameter 20 mm)

ToF-SIMS and XPS study of sulphur on carbon black surface

A very broad range of carbon blacks (CB) from different origins and different manufacturing processes (furnace CB, plasma CB, acetylene CB, plasma-treated CB, etc.) were analysed by surface-sensitive techniques: time-of-flight secondary ion mass spectrometry (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS), The data were compared with conventional bulk properties of industrial CB: specific surface area and sulphur titration. Different types of sulphur functional groups were identified at CB surfaces by ToF-SIMS (HS-, SOx-, HSOx-, SCN-, etc.). A linear relationship between the ToF-SIMS intensity of the sulphur-containing ions and the total sulphur bulk quantity was observed. The total intensity of ToF-SIMS spectra was found to be dependent on the specific surface area of the CB samples. The total quantity of sulphur measured by XPS, when detected, was in good agreement with the total sulphur bulk quantity. However, the ToF-SIMS sensitivity for sulphur functional groups was much higher than the XPS sensitivity, Copyright (C) 2000 John Wiley & Sons, Ltd.