Xavier Duval

Two-dimensional phase transitions as displayed by adsorption isotherms on graphite and other lamellar solids

Abstract An introductory review is given to the study of two-dimensional phase transitions, as most easily observed on lamellar solids. Two examples have been chosen, namely the xenon-graphite and krypton-graphite couples, which have been the subject of the most extensive and detailed studies. These illustrate the development of research in the field, providing significant and typical results leading to fundamental questions on the nature of two-dimensional phases and their transitions.

Stepwise isotherms and phase transitions in physisorbed films

Abstract The present paper outlines the evolution during the last thirty years of research in the field of physisorption of gases on solids with a homogeneous surface. It relates this evolution as lived by some researchers of a physical chemistry laboratory whose objective was not originally to study physisorption as such, but to use it as a method to determine the specific surface area of solids participating in gas reactions. Consequently, the aim is not to give a complete review even of only those results obtained from adsorption isotherms, but simply to recall the way which led to the discovery of several of the most typical adsorption phenomena, i.e.: “gas-liquid-solid” and “commensurate-incommensurate” 2D transitions, 2D polymorphism, wetting transitions and specific behaviour of mixed films.

Comparative study of the first adsorbed layer of xenon and krypton on boron nitride and graphite

Abstract The thermodynamic properties of the first adsorbed layer of xenon and krypton on crystalline boron nitride have been studied between 95 and 125°K and between 77 and 93°K, respectively. They are compared to those previously determined for the first layer of the same gases adsorbed on graphite. With both xenon and krypton the final state of the layer is probably the same as on graphite: a self-determined two-dimensional (2D) solid state having approximately the same density and structure as a (111) plane of the 3D crystal which forms at saturation. As on graphite, the xenon layer also undergoes two successive first-order transitions above a definite temperature: a 2D gas → 2D liquid transition and a 2D liquid → 2D solid transition. But with krypton, only one phase transition has been observed whereas three occur on graphite: two first-order transitions (2D gas → 2D liquid and 2D liquid → 2D localized solid) and a localized → delocalized transition which is of higher order. The only transition observed with krypton on boron nitride is first order, but the nature of this transition has not yet been determined.

Étude comparée de la première couche d'adsorption de méthane sur nitrure de bore et graphite

Abstract The adsorption of methane on crystalline boron nitride has been studied between 77 and 90 K by the volumetric method, especially in the first monolayer domain. It is shown that the thermodynamical properties of the layer are similar to those previously obtained on graphite. It can be inferred from the results that below 77 K, the layer undergoes two first-order phase transitions: a 2D gas-2D liquid transition and a 2D liquid-2D solid transition. In its solid state the localization of the layer is certainly the same on both substrates.

Critical and triple point temperatures of the first adsorbed layer of nitric oxide on graphite, boron nitride and lamellar halides

Abstract The two-dimensional critical temperature T c (2D) of the first layer of nitric oxide adsorbed on homogeneous surfaces of graphite, boron nitride, cadmium halides (CdI 2 , CdBr 2 , CdCl 2 ) and magnesium bromide has been determined as accurately as possible. A correlation curve between T c (2D) and the adsorbent crystalline parameter is compared to the correlation curve of this parameter with the two-dimensional triple point temperature T t (2D) of the same layer on the above substrates and on those previously investigated by Enault and Larher. It appears from this comparison that T c (2D) is less sensitive than T t (2D) to the nature of the substrate: the difference between the extreme values of T c (2D) is 25 K, whereas it is larger than 70 K for T t (2D). Moreover the lowest values of T c (2D) correspond to the substrates for which the lowest values of T t (2D) have also been observed, namely graphite and boron nitride, both standing out by the smoothness of the adsorption potential of their cleavage faces. Furthermore we are led to reconsider the nature of the less dense phase (α phase) which hitherto has been inferred to be a 2D solid essentially composed of dimers lying flat on the surface.

Etude de la reaction du gaz carbonique avec le carbone aux temperatures elevees et sous de basses pressions

Abstract In order to complete and to determine more precisely previous data a new investigation on the kinetics of the C + CO 2 reaction has been carried out over a broader range of temperature (800–2000°C) and pressure (between 10 −4 and 10 −1 Torr) with an improved apparatus and using more varied carbon samples. All types of carbons exhibit the same peculiarities in kinetics related to changes in the intrinsic reactivity of the surface. The general features of the kinetics are more similar than hitherto believed to those of the C + O 2 reaction. Indeed, the two reactions differ only in their absolute rates. This difference is discussed on the basis of the mechanism previously proposed by Duval.

Décomposition de l'acétylène, de l'éthylène et du benzène sur le carbone aux très hautes températures et sous de basses pressions

Abstract 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).

Adsorption de krypton sur membranes et fibres de carbone

Abstract Surface properties of carbon membranes heattreated at 1000 and 2800°C (made from graphitic oxide) and carbon fibres made from acrylic (AG 12 – 2500°C) and rayon fibres (WYB 2500–3000°C) are studied by physical adsorption. The surface properties reflect the degree of organization in the bulk material: surface homogeneity of graphitized membranes (2800°C) compares favorably with that of the best graphitic surfaces yet known; the surface of the non-graphitized rayon fibres are markedly more heterogeneous than the ones of also non-graphitized but better oriented acrylic fibres.

Etude de la reaction du carbone avec le sulfure d'hydrogene H2S a hautes temperatures et sous basses pressions

Reaction of carbon with hydrogen sulfide at high temperature (1000–2000°C) and low pressure (10−4–10−2 Torr) exhibits the following features: —carbon disulfide CS2 is the only carbonaceous reaction product. There is no indication that CS2 would originate from a secondary reaction of the unstable carbon monosulfide CS. Some decomposition of H2S into its elements is also observed (Figs. 1–3). —as already observed in other high temperature carbon gasification reactions, the intrinsic reactivity of the sample surface is temperature and pressure dependent. Consequently, transitory or stationary rates are observed, depending respectively upon a changing or a stationary surface state of the carbon sample (Fig. 4). The changes in the surface state are more marked for amorphous than for graphitized samples (Fig. 5). —below 1700°C, the true reaction order is smaller than one, as a consequence of the high stability of the carbon-sulfur surface complexes. —for ungraphitized samples, the carbon surface loses slowly and irreversibly its ability to change with changes in pressure and temperature. All these features were previously observed in the reaction of carbon with sulfur vapor: consequently hydrogen sulfide appears to behave simply as a gaseous sulfur carrier. The kinetic behaviour is in agreement with former interpretations assuming presence of peculiar reactive sites, which originate from chemical attack of the solid but disappear due to a surface thermal heating process. Confirming also previous assumptions, sulfur chemisorption strongly affects the reaction kinetics, as shown by the influence of H2S traces on the kinetics of the C-O2 reaction (Fig. 10): —at lower temperatures (< 1300°C) there is an important inhibiting effect on the CO production: H2S is adsorbed strongly on the reactive sites which become inaccessible to O2 molecules. —in the intermediate temperature range an enhancing effect is observed which is attributed to a hindrance of thermal healing (caused by chemisorption still present). Finally, the kinetic features as a whole are tentatively summarized in a comprehensive diagram where the consistency of experimental results appears clearly.

Methane adsorption on dichloromethane preplated graphite: 2D phase transitions on dual surfaces

Adsorbing increasing amounts of methane (CH 4 ) at 80 K on graphite preplated with one monolayer of dichloromethane (CH 2 Cl 2 ) results in a succession of transitions: partial displacement of the preadsorbed film and replacement by a CH 4 monolayer; incorporation of CH 4 in the remaining CH 2 Cl 2 patches so as to form a 2D solution: alternate layer-by-layer condensation on the resulting dual surface. When CH 2 Cl 2 preplating is in the submonolayer range, CH 4 monolayer condensation occurs first on the bare part of the surface. When the graphite surface is saturated with CH 2 Cl 2 the displacement is suppressed.