Joaquín Cortés

Surface methylation of aerosil and its influence on the adsorption of water

Abstract Reversible adsorption isotherms at different temperatures have been used to study the system of water adsorbed on Aerosil and on Aerosil whose surface has been partially methylated with trimethylchlorosilane. The change in the shape of the isotherm upon surface methylation from type II to type III, according to Brunauers classification, renders difficult the calculation of the capacity of the monolayer; however, in this case it can be obtained by Kaganers method. The variation in the mobility of the monolayer on surface methylation can be shown by comparing the experimental and theoretical integral entropies, while the molecules adsorbed in the multilayer region always have a high degree of freedom.

Effect of the coordination of the superficial site in the monomer–dimer reaction on a disordered substrate

Abstract Using Monte Carlo simulation experiments, a study is made of the effect of the superficial coordination number in a square lattice of sites for the monomer–dimer surface reaction (Ziff, Gulari and Barshad model) in the case of disordered substrates showing geometric heterogeneity of the sites, such as the percolation clusters. An analysis is made of the change in character of the phase transitions and in the size of the reactive window in the phase diagram, and the results were also compared with mean field theoretical calculations for disordered systems.

Mean-field hierarchical equations for some A+BC catalytic reaction models

A mean-field study of the (A+BC→AC+12B2) system is developed from hierarchical equations, considering mechanisms that include dissociation, reaction with finite rates, desorption, and diffusion of the adsorbed species. The phase diagrams are compared to Monte Carlo simulations.

On the pair approximation method in a nonreactive catalytic system

The nonreactive catalytic system made up of the reduction of NO by CO on a square lattice of sites is studied by means of dynamic equations in the pair approximation, showing some characteristics and limitations of the model in systems with a nontrivial behavior.

Calculation of the gas–solid energy distribution from the surface potential energy of adsorption

The calculation of the energy distribution of a solid’s surface for a cleaved perfect crystal from the surface potential energy of adsorption is proposed. The calculation is made for argon adsorbed on the 100 face of potassium chloride, and for the physisorption of a monoatomic molecule on the 100 face of a simple cubic crystal. The same method is also used to simulate some simple cases of other forms of surface heterogeneity such as defects and impurities.

The monomer–trimer and dimer–trimer superficial reactions on geometrically disordered substrates

A Monte Carlo study is made of monomer–linear trimer, monomer–bent trimer and dimer–linear trimer surface reactions on substrates made of percolation clusters having varying degrees of surface disorder and on a statistical fractal, the incipient percolation cluster. The phase diagrams show that the branching of the disordered substrate alters the width of the reactive zone, the production value and the character of the irreversible phase transitions. At the ends of the diagrams a linear decrease in the jamming fractions with increasing substrate branching is seen, together with a discontinuous transition between the jamming states and the neighboring absorbent zones.

Gas-solid adsorption isotherms: The art and science of determining monolayer capacity

Abstract Some aspects related to the isotherm for the adsorption of a gas on the surface of a solid are reviewed. Two main aspects should be pointed out: 1) a conceptual analysis of the shape of the adsorption isotherm shows that it is a direct source of information on the system being studied (at the same time, some analogies with isotherms belonging to other physicochemical systems are suggested); and 2) a critical review of the large number of theoretical models of the isotherm found in the literature is made, stressing an order based on the principle of maximum likelihood.

The application of temperature programmed desorption in the study of the surface of aerosil

Abstract Temperature programmed desorption (TPD) is applied to the study of the surface of Aerosil. The resultant chromatograms differentiate between molecular water adsorbed on the surface and various silanol groups that exist on it. It is found that molecular water is restored even though the solid is subjected to temperatures at which its total removal would be expected. The TPD method also allows the heterogeneity of the surface silanol groups to be shown, as well as the influence of physisorbed water on the dehydration of the Aerosil surface.

The Hill-de Boer equation in the adsorption of water on quartz

Abstract Within the limitations imposed by the number of experimental points, it has been possible to establish that the system of water adsorbed on pure and methylated quartz has adsorption isotherms in which the quantity adsorbed increases with increasing temperature when plotted in terms of relative pressure. This behavior can be adequately described by the Hill-de Boer equation, even though the interpretation of the data by means of parameter K 1 of the equation suggests that the water shows only dispersion interactions with the silica surface. This rather anomalous behavior of the system is in agreement with the previously published values of parameter S of the Frenkel-Halsey-Hill equation.

Geometric heterogeneity of the lattice and its effect on the kinetics phase transitions of surface reactions

Abstract Two novel phenomena are discussed in this paper. The first one refers to the effect of the catalyst’s surface heterogeneity on the smoothing of the first-order transition observed in the ( A + B 2 ) reaction (ZGB model). The second effect corresponds to obtaining information on the surface heterogeneity from the shape of the transition curve. Two types of heterogeneity were considered: the structure obtained by the random blocking of reactive sites, and the existence of a distribution in independent strips or terraces on the catalyst’s surface.