Existence de quatre types de sites reactionnels dans l'oxydation du graphite

We have studied the temperature programmed decomposition of graphite-oxygen surface complexes using a gas flow apparatus under atmospheric pressure with infrared CO and CO₂ detection. We have shown that the oxygen is chemisorbed on four types of sites forming surface complexes which on decomposition give mainly CO and some CO₂. The first two types of sites, A and B, are formed by ‘labile’ carbon atoms created during the degassing carried out prior to each experiment. These two types of sites disappear without reconstitution upon desorption of the complexes. The other two, C and D, are formed by edge carbon atoms, normally linked to other atoms in the graphite lattice. The C type sites form by oxidation only at temperatures below about 950°C and give rise to hexagonal pits with sides oriented in the (101?0) crystallographic direction (‘arm-chair’ configuration). The D type sites form at temperatures above about 950°C and give rise to hexagonal pits oriented in the (11?20) crystallographic direction (‘zig-zag’ configuration). Water inhibits the formation of oxygen surface complexes on the C sites and it may be considered that it is essentially on these sites that the water is chemisorbed.     

Application de la thermoluminescence a l'etude de l'hydratation du silicate bicalcique

Hydraulic properties differences between β and γ dicalcium silicate varieties are well known. In this work, a water vapour chemisorption on the surface of both solid phases is shown by thermoluminescence and by quadrupole mass spectrometry.The similar behaviour of β and γ dicalcium silicate varieties in those experiments means that the fundamental difference between their hydraulic reactivities is not due to this hydration step.     

Etude de la formation d'un complexe de surface graphite-eau relation avec la reaction d'oxydation

Using a gas flow apparatus under atmospheric pressure with infrared CO and CO₂ detection and an apparatus operating under low pressure with a mass spectrometer for analysis, we have found the following: The chemisorption of water becomes appreciable above 200°C. The water chemisorbed on graphite previously degassed at 1000°C forms a surface complex which on raising the temperature decomposes with simultaneous evolution of CO and H₂, thus showing that hydrogen enters into the composition of the complex. The programmed thermal decomposition of the complex shows two peaks. The first peak corresponds to the sites which disappear with the degassing of the CO and H₂; these sites are formed by the ‘labile’ carbon atoms created by the prior degassing. The second peak corresponds to the sites which are constantly renewed after degassing the CO molecules, and which are therefore formed by graphite atoms normally linked to the graphite lattice. The reaction rate of the oxidation of graphite by water shows two transient rates before attaining a stationary value. Correlations between the CO thermo-desorption and these reaction rates are found which indicate that the formation of a surface complex is a necessary step in graphite-H₂O reaction.     

Stereochimie de l'electroreduction de bromocyclopropanes—III. Diastereoselectivite de la reduction electrochimique de gem-dibromocyclopropanes

Cleavage of a single carbon bromine bond occurs in the two electrons reduction of 2,2 disubstituted 1,1 dibromo cyclopropanes.Analysis of the diastereoisomeric mixture of monobromo compounds obtained at the term of electrolysis at a mercury cathode in a hydroethanolic media allows to determine which of the two bromine atoms is preferentially cleaved.

When R² = Ph, R¹ = H or CH₃ only a low stereoselectivity is generally observed. Nevertheless, yields up to 84% of the cis isomer are achieved when R¹ = H.Best results are obtained when R² is a carboxy group (CO₂H, CO₂Me or CO₂Et) especially when the reduction is carried out in the presence of quaternary ammonium cations. The cis isomer is always preferentially formed; yields up to 92% when R² = CO₂H and R¹ = Ph, and up to 99% when R² = CO₂Et and R¹ = CH₃ can be achieved.Influence of electrolyses conditions (working potential, nature of the supporting electrolyte) on the stereochemical course of the reduction is investigated.     

Chimisorption de l'hydrogene par les composes d'insertion graphite-potassium

At low temperature, graphite-potassium intercalation compounds of stage higher than one behave as molecular sieves as regards hydrogen [1,2]. At ambient and higher temperature, however, these compounds including the first stage one, chemically fix hydrogen by a completely different process. This reaction, which has been the object of previous papers [3,4], has been further studied so as to remove some of the previous uncertainties and imprecisions.The brown compound KC₈ after fixing hydrogen becomes blue, and saturates for an $\text{H}\text{K}$ ratio of $\text{2}\text{3}$. The reaction is reversible but does present noticeable hysteresis. Radiocrystallographic examination of the hydrogenated product shows that it involves a pure ternary phase of formula $\text{KC}{}_8\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$. The indexing of the 00l reflections indicates a second stage compound: each intercalated layer is composed of two metallic planes in the presence of hydrogen (Fig. 1). This stage change can be interpreted by the pleating of the graphite layers [5]. Neutrocrystallographic studies confirm the preceding results and allow (Table 2) to conclude that the hydrogen forms a layer between the two potassium planes (Fig. 2). The belonging to the second stage of the phase $\text{KC}{}_8\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ is further confirmed by the fact that the free graphite spacings are susceptible of fixing new alkaline metal layers (K, Rb or Cs) to form new compounds referred to as “bi-insertion compounds” [6].Attempts at hydrogen physisorption on the product during the course of chemical hydrogenation (Fig. 3), and the radiocrystallographic and kinetic measurements, clarify the finer points of the reaction. The hydrogen starts to be chemisorbed into the lattice of the compound KC₈ without any stage change (0A), then there appears progressively a second stage phase, unsaturated with hydrogen (AB). Saturation is only attained when the first stage phase has disappeared (BC).Low temperature hydrogen physisorption tests have been carried out on the second stage compound KC₂₄ during chemical hydrogenation. Figure 4 shows a linear decrease in the physisorption capacity, when the hydrogenation ratio increases. By extrapolation it is seen that this capacity should become zero for a chemisorption ratio $\text{H}\text{K}$ in the neighbourhood of $\text{2}\text{3}$.The results suggest the following hydrogenation model. Under the action of the gas, the intercalated metal forms double, hydrogenated layers, identical to those in the ternary $\text{KC}{}_8\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$. There also appear three types of graphitic spacings: free intervals, those occupied by lacunar single layers and those occupied by hydrogenated double layers. These spacings are distributed in such a manner that there appears a binary phase KC_12n coexisting with a ternary phase $\text{KC}{}_{\mathrm{4N}}\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ of equal or neighbouring stages n and N. Stoichiometry imposes a relationship between n and N. Figure 5 represents the variation of N as a function of n for various values of the hydrogenation ratio. At saturation the system would once again become single phase: a ternary, $\text{KC}{}_{24}\text{H}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ of sixth stage.In Table 3 are given the strongest 00l reflections and the corresponding interplanar distances, characteristic of the compound KC_12n and $\text{KC}{}_{\mathrm{4N}}\text{H}\text{2}\text{3}$. The experimental data taken from the X-ray diagrams, obtained for three KC₂₄H_y, compounds are compared to the values predicted by our model in Table 4. It is seen that the positions of the observed peaks are in good agreement with the 00l reflections of the model, confirming the interpretation we propose.In conclusion, hydrogenation of the potassium layers of KC₈ or of KC₂₄ always leads to the formation of dense and hydrogenated metallic double layers of formula $\text{KH}\text{2}\text{3}$. This process thus creates ternaries of higher stage than the starting binary.     

Etude de l'effet retardateur du zinc sur l'hydratation de la pate de ciment Portland

The retarding effect of zinc on the hydration of C₃S and C₃A, the two principal Portland cement components, has been investigated by X - ray diffraction. The results show that the C₃S retardation is more important than that of C₃A. This retardation is due to the precipitation of an amorphous layer of zinc hydroxide around the anhydrous grains. The effect of this coating depends on its permeability. The hydration reaction starts again through the transformation of the zinc hydroxide into the crystalline calcium zinc hydroxide Ca Zn₂ (OH)₆, 2H₂ O.     

Transport cationique dans l'oxyde de magnesium monocristallin

The method of coupled coulometric-linear expansion measurements is used to determine the cationic transport number t_c in MgO single crystal. Temperature ranges between 1100°C and 1400°C, and oxygen partial pressure in the limits 1–10^?10 atm t_c is always smaller than 0,5 and decreases with increasing temperature. A comparison of t_c with the total ionic transport number (emf method) shows a preferential cationic mobility.     

Reduction of CO<Subscript>2</Subscript> to CO via reverse water-gas shift reaction over CeO<Subscript>2</Subscript> catalyst

CeO₂ catalysts with different structure were prepared by hard-template (Ce-HT), complex (Ce-CA), and precipitation methods (Ce-PC), and their performance in CO₂ reverse water gas shift (RWGS) reaction was investigated. The catalysts were characterized using XRD, TEM, BET, H₂-TPR, and in-situ XPS. The results indicated that the structure of CeO₂ catalysts was significantly affected by the preparation method. The porous structure and large specific surface area enhanced the catalytic activity of the studied CeO₂ catalysts. Oxygen vacancies as active sites were formed in the CeO₂ catalysts by H₂ reduction at 400 °C. The Ce-HT, Ce-CA, and Ce-PC catalysts have a 100% CO selectivity and CO₂ conversion at 580 °C was 15.9%, 9.3%, and 12.7%, respectively. The highest CO₂ RWGS reaction catalytic activity for the Ce-HT catalyst was related to the porous structure, large specific surface area (144.9 m²?g^?1) and formed abundant oxygen vacancies.     

Decomposition de l'acetylene,de l'ethylene et du benzene sur le carbone aux tres hautes temperatures et sous de basses pressions

At high temperatures (1000–2000°C) and low pressures (10^?5?10^?2 Torr) ethylene, acetylene and benzene decompose helerogeneously on pyrolytic carbon giving mainly hydrogen and deposited carbon, with collision yields of the order of 10^?4. The kinetics of these carbon deposition reactions show some striking similarities with carbon removal reactions by oxygen or oxygenated compounds.The true reaction order of these decomposition reactions is one above 1400°C, but becomes smaller at lower temperatures. This behaviour, common in gas-solid reactions, is generally interpreted as an inhibition due to chemisorption of some intermediate or reaction product. Evidence is also obtained that decomposition of the hydrocarbon molecules only occurs on peculiar sites of the carbon surface, i.e. the decomposition is not a purely thermal process, but involves a specific chemical interaction with the surface.Moreover, the behaviour of the pyrocarbon surface in carbon deposition reactions is similar to that observed in gasification reactions, i.e. the reactivity of the surface accommodates itself to the temperature and pressure conditions, as revealed by the observation of “transitory” and “stationary rates”. Transitory rates show that the surface deactivates with increasing temperatures (Figs. 4 and 5) [from which a maximum in the stationary rate results (Figs. 1–3)] and decreasing pressures (Figs. 7 and 8). The interpretation assumes that reaction sites are continuously created as an effect of carbon atoms deposition, but also deactivated by a thermal healing process.A main difference between carbon deposition reactions from hydrocarbons and carbon gasification reactions concerns the temperature range where reactivity is temperature dependent: in carbon deposition reactions, deactivation of the pyrocarbon surface is still effective up to much higher temperatures (Fig. 12).     

L'insertion dans le graphite des amalgames de potassium et de rubidium

Potassium and rubidium amalgams are able to intercalate into graphite and produce ternary compounds of two types. With amalgams rich in alkali metal, binary compounds with small amounts of mercury in the intercalated layers are obtained: MC₈ (Hg). With amalgams of composition around MHg, one can prepare ternary multilayered compounds named mercurographitides: in these compounds, the intercalated layers consist of two alkali metal planes with a sheet of mercury in between.The data for the first stage mercurographitides MHgC₄ are given in Table 2. The identity period along the c axis was obtained from X-rays data (Fig. 1, Table 1). The hk0 reflexions show the octal epitaxy of the metal layers between the carbon planes (Fig. 2, Table 3).The intercalation of the amalgam occurs in two steps: first a quasiselective intercalation of the alkali metal (Fig. 3) and then a simultaneous intercalation of mercury and alkali metal (Fig. 4).The first step gives the compound MC₈ (Hg) and the second MHgC₄. The second stage mercurographitides MHgC₈ are characterized by their identity period along the c axis (Fig. 5, Table 4) and are given in Table 5. A mechanism of addition of mercury to the MC₈ phases is proposed in Fig. 6.Finally, in an effort to prepare mercurographitides of stages higher than 2, the identity period along the c axis for the third stage compound was found, although this compound was not obtained in a pure state.     

Etude par thermoluminescence de l'hydration de l'aliminate tricalcique

Thermoluminescence shows that the hydration of tricalcium aluminate is deeply influenced by structural defects of the solid. Hydration reaction excites centers of the solid detected by thermoluminescence; this energy storage is related to trapped electrons. Hydration of tricalcium aluminate in the presence of gypsum is not influenced by traps but thermoluminescence allows to corroborate the well-known mechanism of this reaction.     

Cinetique de l'adsorption du phenol en couche fluidisee de charbon actif

Phenol adsorption from dilute aqueous solutions was studied using fluidized bed of active carbon. Isotherms adsorption at 14 and 18°C were determined and Langmuir type equation is proposed. For three different particles sizes, d_s = 0,129 cm, d_s = 0,093 cm and d_s = 0,071 cm, and various flow rate the controlling diffusional mechanisms were checked, i.e. external and/or internal diffusion.     

Solubilite de l'eau dans le melange gazeux (N2 + 3 H2) comprime—teneurs a la saturation—methode de calcul

The solubility of water in the synthesis gas (N₂ + 3H₂) has been measured between ?20°C and +15°C, at pressures varying from 100 to 400 atm. These results are coherent with those formerly published concerning higher temperatures.An easy method for calculating solubility is explained. It gives an accurate account of measures relating to the systems containing water or ammonia and a nonpolar component. For instance, the author presents the results obtained for the mixtures (H₂O + N₂ + 3H₂) and the mixtures (NH₃ + N₂ + 3H₂).     

L'electrolyse de l'alumine dans la cryolithe fondue. Etude du mecanisme de decharge cathodique a l'aide de l'electrode a disque tournant

The use of a rotating disk platinum electrode, on which aluminium has been previously deposited, has allowed us to determine the mechanism of the cathodic reaction during the electrolysis of cryolite—alumina melts. This mechanism requires three stages within the Nernst's diffusion layer. The first of these is the dissociation of the ion AlF^3?₆, at δ′ from the electrode, forming the ions Al³⁺ and F^?. The second one is the discharge of ions Al³⁺ diffusing towards the cathode and the third one, at δ″ from the electrode, is the reaction of the ions F^? with the ions AlF^?₄ to give again the ions AlF^3?₆. The ions Na⁺, which transport most of the current serve to maintain electro-neutrality.     

Vapour liquid equilibrium calculations for dilute aqueous solutions of CO2, H2S,NH3 and NaOH to 300°C

Henry's law constants for aqueous CO₂, H₂S and NH₃ up to 300°C have been recalculated from literature vapour pressure, enthalpy and heat capacity data. The high vapour pressure of water above 150°C causes significant solute-water interactions in the gas phase, which were calculated using the Peng-Robinson cubic equation of state. The results were combined with selected ionization constant data to derive a vapour-liquid equilibrium model for dilute solutions. The model reproduces experimental data for binary systems at solute molalities of up to 0.5 m at 100°C, 1.0 m above 250°C and ionic strengths below about 0.1 m.     

Solid-Gas Reactions,Part III. Interaction of Solid Dibenzoylethylenes with Bromine and Iodine Vapours

The cis and trans isomers of solid 1,2-dibenzoylethylene, 1,2-di-p-chlorobenzoylethylene and 1,2-di-p-methyl-benzoylethylene yield, on exposure to bromine vapour at room temperature, the trans-adducts in quantitative yield except in the case of cis-1,2-di-p-methyl-benzoylethylene where isomerisation takes place prior to or during the bromination process yielding the same dibromide as the trans-isomer. The cis-trans isomerisation of the three solid cis-isomers by iodine vapour has been analysed by determining the rate of the reaction from the change in IR transmittance. The isomerisation follows a first-order mechanism. A plot of ΔH⁺₊ against ΔS⁺₊ is linear.