Jacques Lemaitre

Temperature-programmed Characterization of Synthetic Calcium-deficient Phosphate Apatites

Abstract Several calcium-deficient hydroxyapatites (DAp) samples were synthesized by aqueous precipitation under controlled conditions, and compared with a commercial hydroxyapatite (HAp) and an equimolar mixture of HAp and dicalcium phosphate (DCP). The samples were analysed using temperature-programmed dehydration (TPR). The results were compared with X-ray diffraction (XRD), and IR spectroscopy data. They show that a variety of calcium phosphate precipitates with Ca/P close to 1.50, ranging from DAp to mere mixture of HAp and DCP, can be obtained according to the synthesis conditions. Typical DAp moieties have two characteristic TPR bands: a wide band in the range 300–680 °C, and a sharper one in the range 700–900 ° C. The first band can be assigned to the slow conversion of DAp into a transitory apatite structure partially substituted by hydrogenophosphate and pyrophosphate groups; at higher temperatures, the pyrophosphate groups and the remaining hydrogenophosphate ones will react with the hydroxyl groups of the apatite structure, thereby producing a TPR peak at 700–900 °C. β-Tricalcium phosphate is formed as a result of the latter thermal transformation. Samples containing residual DCP, even in small amounts (⪢3%), exhibit a sharp TPR peak in the range 400–600 °C, which can be ascribed to the conversion of DCP into β-calcium pyrophosphate. The high sensitivity of TPR to the presence of DCP allows true DAp to be discriminated from mixtures of calcium phosphates exhibiting the same Ca/P atomic ratio.

Synthesis and Thermal-behavior of Well-crystallized Calcium-deficient Phosphate Apatite

Large crystals (0.5*0.5*10 mu m) of calcium-deficient hydroxyapatites (DAps) have been synthesized by aqueous precipitation under controlled conditions of pH and temperature. The samples have been characterized using chemical analysis, temperature-programmed dehydration (TPR), X-ray diffraction, IR spectroscopy, and transmission electron microscopy. They show that very well-crystallized DAps with Ca/P close to 1.50 can be obtained under well-selected synthesis conditions. The specimens presented typical TPR patterns, consisting of a diffuse evolution of water in the range 300-700 degrees C and a sharper one in the range 700-900 degrees C. The maximum temperatures of the TPR bands were found to be markedly higher for the DAp specimen having the largest crystal size.

Control of the catalytic activity of tungsten carbides. I: Preparation of highly dispersed tungsten carbides

Abstract A study of the preparation of carbides using hydrous oxide precursors is presented. Two different precursors (WO3 · H2O and hexagonal WO3 · xH2O), obtained by homogeneous precipitation from ammonium tungstate solutions, have been converted into carbides after preliminary dehydration and reduction steps. The carburization step was performed either in pure CO or in COue5f8CO2 mixture ( CO CO 2 ratio = 10:1 ). Carburization in pure CO always resulted in the deposition of free carbon on the surface of the carbides, as was shown by XPS analysis. The use of a CO CO 2 mixture made it possible to minimize the coverage of the carbide particles by free carbon, probably as a result of a slackening of the CO dismutation reaction. Hence, the amount of free carbon in excess over the quantity necessary to convert W2C to WC was maintained at the lowest possible level. The so obtained carbides proved to be extremely well dispersed (30 m2/g). They appeared to consist of particles which were pseudomorphic with the starting oxides. A passivation treatment, consisting of contacting the samples with diluted oxygen at 21 °C, was found necessary in order to prevent their spontaneous ignition in air.

Lysozyme on apatites: A model of protein adsorption controlled by electrostatic interactions

Abstract Hen egg-white lysozyme (isoelectric point near pH 11) has been adsorbed at 20°C on synthetic hydroxyapatites and fluorapatite, and on a commercial hydroxyapatite used as chromatography support. The influences of pH (5.5–8), ionic strength, addition of calcium and phosphate ions to the solution have been examined and related to the electrophoretic mobility of the adsorbent. Adsorption was rapid and reversible with respect to dilution. The maximum amount adsorbed was in the range 0–55 nmol M −2 depending on the adsorbent and experimental conditions, the highest value being typical of a loosely packed monolayer. Adsorption occurred only when the surface carried an electric charge opposite to that of the protein. The dependence of adsorption on the solution composition demonstrated that it was driven by electrostatic interactions. Electrophoretic mobility measurements in the presence of lysozyme confirmed that the latter ensures surface charge neutralization. Accordingly, a correlation has been found between the maximum amount adsorbed and the apparent zeta potential of the adsorbent at zero coverage. The variation of the affinity constant deduced from adsorption isotherms indicated that the potential acting on the molecules adsorbed at low coverage is close to the apparent zeta potential of the adsorbent. A formulation of the adsorption isotherm derived from the Langmuir equation has been worked out in order to account for the progressive decrease of the interaction potential with increasing coverage

Control of the catalytic activity of tungsten carbides: II. Physicochemical characterizations of tungsten carbides

Abstract The bulk and surface characterization of tungsten carbide samples, using X-ray diffraction, XPS and chemisorption techniques are described. The aim is to establish links between the preparation factors of tungsten carbides and their catalytic properties. XPS observations have shown that tungsten carbide samples were superficially oxidized as a result of the passivation treatment to which they were submitted in order to prevent them from igniting spontaneously upon exposure to air. According to XPS data, the surface oxide was completely removed by reduction in H 2 at 300 °C. No detectable variation of the lattice constants of the carbide samples could be evidenced by X-ray diffraction, which could be ascribed either to the nature of their oxide precursor or to their final composition ( W 2 C WC ratio). On the other hand, variations of the surface C W ratio (as estimated by XPS) versus the conversion to WC have been evidenced and tentatively related to the mechanism of WC formation. The results of H 2 ue5f8O 2 titration experiments have confirmed the analogy reported previously between the chemisorptive properties of tungsten carbides and noble metal catalysts; they have been interpreted in terms of the surface accessible to gaseous molecules such as H 2 and O 2 (referred to below as “pseudometallic surface”). The extent of the pseudometallic surface has been found to be negligible for carbides prepared in pure CO, and it decreases for increasing conversions to WC of samples prepared in COue5f8CO 2 mixtures. These observations are correlated with the accumulation of free carbon on the surface of the samples during the progress of the carburization reaction. The pseudometallic surface of tungsten carbides can therefore be interpreted as the fraction of their surface not covered by an impervious layer of pregraphitic carbon.

Control of the catalytic activity of tungsten carbides. III: Activity for ethylene hydrogenation and cyclohexane dehydrogenation

Abstract The catalytic activity of tungsten carbide samples for ethylene hydrogenation and cyclohexane dehydrogenation is described and discussed in the light of their physicochemical characteristics (phase composition, presence of free carbon or superficial oxygen) and correlated with the parameters of their preparation. A lack of activity presented by samples prepared in pure CO can be ascribed to their surface being extensively covered by pregraphitic carbon. The activity of samples prepared in COue5f8CO 2 mixtures has been found to depend on their conversion to WC, and to increase markedly after being heated at 300 °C in H 2 ; this treatment was necessary to eliminate the oxygen fixed on the samples as a result of a previous passivation treatment. A kinetic model based on a previously proposed mechanism on the W 2 C-WC transformation has enabled the variation of the catalytic activity versus the conversion to WC to be simulated. According to the model, the active sites of the carbide catalysts can be poisoned by the atomic carbon deposited on their surface as a result of the dismutation of CO. At this stage, the samples can still be reactivated on being heated in H 2 at 500 °C. Higher levels of contamination of free carbon can result in the progressive and irreversible formation of a pregraphitic layer which will prevent the gaseous reactants from being adsorbed on the surface of the carbide samples, thereby depressing their catalytic activity.

Influence of the lithium content on the state, dispersion and reducibility of nickel oxide supported on γ-alumina

The influence of lithium on the dispersion, active phase-carrier interaction and reducibility of nickel oxide supported on alumina carriers has been investigated using X-ray diffraction analysis (X.r.d.), ultraviolet diffuse reflectance spectroscopy (d.r.s.), X-ray photoelectron spectroscopy (X.p.s.) and temperature programmed reduction (t.p.r.).On Li-free alumina, nickel is present in the form of a NiAl2O4 compound and dispersed NiO. Increasing the Li content leads to a decrease in the formation of NiAl2O4, due, presumably, to the concurrent formation of Li-containing spinel phases, and a concurrent increase in the reducibility of the nickel-containing phase.

Determination of the Zeta-potential of Macroscopic Specimens Using Microelectrophoresis

Abstract The rectangular cell of a commercial microelectrophoresis apparatus has been modified in such a way that its main walls can be fitted with specimen plates. The measurement of the velocity profile of a probe colloid between the plates allows the electroosmotic mobility of the liquid, and thereby the zeta potential of the specimen plates, to be determined. A convenient procedure has also been developed for correcting mobility measurements for the influence of side walls, in either conventional or modified microelectrophoresis cells. The method has been applied to a variety of specimen surfaces. It has shown that the surfaces of polystyrene flasks for animal cell culture are negatively charged at pH around 6. It has also demonstrated that glass plates treated by aqueous solutions of aluminum ions or culture flasks exposed to chitosan underwent a reversal of their surface charge. The classical theory of the electroosmotic flow in microelectrophoresis cells has been worked out in order to allow asymmetrical velocity profiles to be analyzed. The extended theory has made it possible to derive the zera potentials of two different specimen plates fitted in the modified microelectrophoresis cell from a single experimental velocity profile. It has also allowed anonalous velocity profiles, which can sometimes occur as a result of surface contamination of the walls or sedimentation of the probe particles, to be investigated.

Effect of sodium on the CoMo/γ-Al2O3 system. Part 1.—Influence of sodium content on the state of dispersion and on the nature of the cobalt supported on γ-Al2O3

The effect of sodium content on the dispersion of cobalt on the surface of γ-Al2O3 as well as on the type of cobalt species formed on it have been studied using diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, X-ray powder analysis, electron microscopy and thermo-programmed reduction; two series containing, respectively, 2.1 and 8.8 % Co have been investigated.The former series showed a marked increase in the dispersion of cobalt on the surface of the support, as revealed by X.p.s., when the sodium content increased from 2.3 to 5.7 %. In this composition range, a considerable transformation of Co3O4 into CoAl2O4, as well as a shift of the distribution of cobalt species from the homogeneous to heterogeneous, have been observed.In contrast, virtually no change was observed for the 8.8 % Co series throughout the sodium composition range.

Preparation and Properties of Colloidal Spherical-particles of Molybdenum and Cobalt Sulfides

Abstract Sols of molybdenum and cobalt sulfides consisting of ultrafine spherical particles with a narrow size distribution were obtained by homogeneous precipitation from solutions of ammonium orthomolybdate or cobalt(II) acetate, together with thioacetamide, hydrazine, acetic acid, and ammonia. The sols were formed upon aging the mother solutions at 50°C for about 20 hr. The use of hydrazine for accelerating the hydrolysis of thioacetamide to generate hydrogen sulfide in weakly acidic solutions was found to be indispensable to the preparation of uniform spherical particles of both molybdenum and cobalt sulfides. Particle size of the sols could be varied from 0.09 to 0.70 μm and 0.08 to 0.42 μm for molybdenum sulfide and cobalt sulfide, respectively, by selecting the pH of the mother solution in the range 4–7. The elemental and XPS analysis of freeze-dried samples showed that oxygen was contained in both sulfide particles. Reproducible results of zeta potential measurements on the sols could be obtained by using a washing solution which contains hydrazine acetate (1 × 10−3 M), acetic acid (2 × 10−3 M), and ammonium sulfide (1 × 10−3 M). The isoelectric points of the sols were then found at pH 1.9 and 3.1 for molybdenum sulfide and cobalt sulfide, respectively.