F. Gaillard

Catalytic combustion of methane over copper- and manganese-substituted barium hexaaluminates

Copper- and manganese-substituted barium hexaaluminates were prepared by sol–gel method from metal alkoxides. The preparation conditions strongly influence the textural properties of the solids obtained. Manganese and copper occupy different crystallographic sites in the hexaaluminate structure: Mn3+ ions are located in octahedral sites, while Cu2+ enters only tetrahedral positions. The Cu sites are intrinsically more active than Mn sites for methane combustion, but the Cu-based catalysts are penalized by lower surface areas and by the lower limit of copper incorporation in the hexaaluminate matrix: manganese substitution for aluminium is possible up to ≈3 Mn per unit cell, while copper substitution is limited to about 1.3 Cu per unit cell. The catalytic activity of the Mn-substituted barium hexaaluminates increases with Mn substitution, the optimum composition being obtained when about 3 Mn ions per unit cell are incorporated, as regards the activity and the resistance to ageing at 1200°C in the presence of steam. This is related not only to the Mn content, but also to the higher Mn3+/Mn2+ ratio when the amount of manganese introduced increases, as shown by TPR and Auger parameter measurements. The catalytic activity can be correlated with the fraction of reducible manganese species.

Measuring Water Diffusion in Polymer Films on the Substrate by Internal Reflection Fourier Transform Infrared Spectroscopy

Among the various analysis modes which can be used in FTIR spectroscopy, the internal reflection mode enables us to gain near-surface information on solids or liquids. The interaction between the evanescent field created upon internal reflection of the infrared beam and a sample can be used to monitor the uptake of water molecules by a polymeric film. In this technique, a polymer film of sufficient thickness is applied to a substrate and a special cell is used to study the water diffusion in the polymer films. Spectra were taken automatically at specified time intervals without disturbing either the specimen or the instrument. Results for polystyrene and poly(methyl methac- rylate) films applied to a ZnS substrate are presented to demonstrate the method. The diffusion coefficients of water in these polymers are calculated using the sorption kinet- ics approach, and the diffusion process in each type of polymer is discussed. The effects of the molecular weight and defects in the films on water transport in the polymers are illustrated. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2465-2473, 1997

A Spectroscopic Technique for Studies of Water Transport Along the Interface and Hydrolytic Stability of Polymer/Substrate Systems

Abstract Information on water transport along the polymer/substrate interface is valuable for understanding the mechanisms and the controlling factors affecting the water-induced adhesion loss of polymer-coated metals, adhesive-bonded joints, and polymer/fiber composites subjected to aqueous environments. This paper presents data to demonstrate the capability of a technique, which combines a vertical cell with Fourier transform infrared spectroscopy in the multiple internal reflection mode, for studying water transport along the polymer/substrate interface and interfacial hydrolytic stability of polymeric composites and systems exposed to water and high relative humidities. The technique can distinguish water transport through the film from that along the interface; the latter transport is predominant for polymer/untreated substrate systems. Spectroscopic analyses of fractured surfaces of poor and well-bonded polymer/substrate systems after water exposure indicate that the technique is capable of discerni...

Dynamic adsorption on activated carbons of SO2 traces in air

Abstract Sulphur dioxide is an atmospheric pollutant which, among numerous others, has to be eliminated by habitacle filters. Breakthrough curves of low concentration SO2 streams through beds of activated carbons have been obtained. Two carbons were studied, an activated PAN fiber (CF) and a granulated activated carbon (CN) under SO2 concentrations lower than 100 ppm. Carbon CN used ‘as received’ is able to trap SO2 in air at concentrations as low as 2.5 ppm. At this concentration, the adsorption of SO2 is essentially irreversible. The fraction of reversibly adsorbed SO2 rapidly increases when SO2 content in air increases from 2.5 to 100 ppm. As expected, the amounts of SO2 adsorbed per gram of carbon are much smaller than in the case of high SO2 contents in air (>1000 ppm). The presence of water in carbon micropores enhances both reversible and irreversible adsorption of SO2. The reversibly adsorbed part is physisorbed while the irreversibly adsorbed part results in oxidation of SO2 at the carbon surface. This oxidation was evidenced by TPD from carbon samples after adsorption. The mechanism of SO2 adsorption is discussed in relation to the mechanisms proposed in literature for high SO2 contents (>1000 ppm).

The relationship of the catalytic activity and the open-circuit potential of Pt interfaced with YSZ

This study deals with the relationship between open-circuit potential and catalytic activity of the system Pt/YSZ. Temperature-programmed desorption (TPD) of oxygen was carried out in order to investigate the link between the oxygen coverage and the potential. Then, catalytic activity in parallel with the potential value was measured between 200 °C and 500 °C for NO oxidation, C3H8 combustion, C3H6 combustion and the selective catalytic reduction (SCR) of NO by C3H6 in the presence of oxygen. It was found that potential measurement can give precious information on the competitive adsorption between oxygen and other reactants. Finally, it seems to be also a good way to anticipate the NEMCA behavior for reactions, which involve oxygen.

Zinc Coated Steel/Epoxy Adhesive Systems: Investigation of the Interfacial Zone by FTIR Spectroscopy

Abstract The present study takes advantage of the ability of Fourier Transform Infrared Spectroscopy (FTIR) for the analysis of ultrathin organic films on metals. FTIR in the reflection mode (IRRAS) is used in order to study the interaction of ultrathin films of dicyandiamide (hardener of most one-pack epoxy resins) with various substrates, model ones such as gold or zinc and industrial ones such as steel and zinc-coated steels. Pure zinc surfaces and, to a lesser extent, zinc-coated steels are shown to react with dicyandiamide after heating at 180°C, as evidenced by the frequency shift of the absorption band (at about 2200 cm−1) characteristic for nitrile groups. As real systems consist of thick layers of a fully formulated adhesive cured onto a metallic substrate, the direct investigation of such a buried interphase is no longer possible by FTIR and by most of the known spectroscopies. Some mechanically tested specimens are then analysed, after failure, by FTIR microspectrometry. The spectra obtained, c...

Electrochemical promotion of NO reduction by propene on Pt/YSZ

The reduction of NO by C3H6 in the presence of oxygen, is of great environmental importance. Platinum-based catalysts are very active but not selective towards N2 production and mainly convert NO into N2O, which participates to the greenhouse effect. Moreover, their operating temperature window is quite narrow. Electrochemical promotion was used to improve platinum catalytic behaviour. Platinum was deposited on YSZ (Y2O3 — stabilised ZrO2), an O2-conductor. It was found that a negative current increased the rate of NO reduction and CO2 formation. This rate enhancement was non-Faradaic with an apparent Faradaic efficiency (Λ) close to 180 indicating the manifestation of a NEMCA effect. However, the current application had no effect on the N2 selectivity

Catalytic combustion of methane : Surface characterization of manganese-substituted barium hexa-aluminate catalysts

In order to achieve a good activity with respect to methane combustion, a good route would be via catalysts incorporating transition metals. For devices operating at high temperatures, such as gas turbines, substituted hexa-aluminates present the advantage of good thermal stability when compared to impregnated Al2O3. For the purpose of this study, manganese-substituted hexa-aluminates were prepared either by the sol–gel method or by carbonate precipitation. Surfaces were analysed by soft x-ray emission, namely low-energy electron-induced x-ray spectroscopy (LEEIXS), and by photoelectron spectroscopy, namely x-ray photoelectron spectroscopy. The results on samples incorporating increasing amounts of manganese show the existence of an upper limit to the number of atoms substituted at the catalyst surface. This limit is about 2.7 manganese ions per unit cell, for samples prepared either by the sol–gel or by the carbonate precipitation technique and then calcinated at 1200°C. The interpretation of the Auger parameter variations shows that the relative amount of Mn in oxidation state III increases when the surface amount of Mn increases, as well as with the Mn substitution level. This fact can be related to an increased activity for methane conversion.

Electrochemical promotion of environmental catalysis

Electrochemical catalysts were used for environmental applications, such as the clean production of energy from propane and propene combustion, the elimination of VOCs like propene and the NOx abatement All the selected electrochemical catalysts were composed of a Pt film deposited on YSZ, NASICON or CGO. It was found that all these chemical reactions can be electropromoted. Moreover, the reaction rates can be in-situ tuned by applying a polarisation. Furthermore, the selectivity of Pt-based electrochemical catalysts can be modified in order to avoid the formation of pollutant. Finally, EPOC can improve the lifetime of a catalyst by inhibiting its poisoning by carbonaceous species.

FT-IR and Temperature-programmed Desorption (TPD) Study of the Adsorption of Probe Molecules Used to Model Epoxy Resin Adhesion to Chromium, Iron and Stainless Steel Substrates

Abstract In this study, we investigated the reactivity of chromium, iron, and surface-treated 304L stainless steels (SS) toward molecules representing model epoxy resins. These molecules were ammonia (a basic probe molecule also representative of the hardener amine group), 1,2-epoxybutane (for epoxy groups) and 2(methylamino)ethanol (for the β-amino alcohol resulting from the reaction of epoxy with amine). These molecules were analyzed in the adsorbed state by either FT-IR or temperature-programmed desorption (TPD). Surface analysis showed that the top surface of the treated 304L samples only contains chromium and iron as metallic elements. The chromium/iron ratio can be varied within a wide range according to the SS surface treatment used. When increasing the SS surface chromium enrichment, we show a simultaneous increase of (i) both density and strength of surface acid sites, (ii) the amount of β-amino alcohol adsorbed. In addition, there is a marked improvement of the epoxy resin/304L bond strength when the SS surface is more chomium-enriched.