G. Zgrablich

Thermal desorption of interacting molecules from heterogeneous surfaces; Application to co desorption from MgO

The behaviour of thermal desorption of interacting particles from heterogeneous surfaces is discussed by means of a Monte Carlo simulation method. It is shown how desorption spectra are affected by site energy heterogeneity, site structure and adsorbate-adsorbate interactions. The results obtained and their analysis can be used as a guide to the interpretation of experimental spectra and this is done for CO desorption from MgO surfaces.

Lattice-gas study of the kinetics of catalytic conversion of NO–CO mixtures on rhodium surfaces

The kinetics of the catalytic reduction of NO by CO on Rh(111) was simulated by using a lattice-gas model and a Monte Carlo algorithm. These simulations were designed to incorporate some new experimental results, which reveal that the formation of a N–NO intermediate is necessary for molecular nitrogen production. The steady-state phase diagram for the overall NO reduction reaction was studied in terms of several parameters representing different reaction schemes. It was found that, under the assumptions made in the model, an Eley–Rideal mechanism which includes a NO(gas)+N(ads)→N2(gas)+O(ads) step is absolutely necessary to be able to sustain a steady-state catalytic regime.

Mechanistic study of surface processes on adsorbents: I. Statistical description of adsorptive surfaces

Abstract A dual description based on a network of “sites” and “bonds” is developed for the characterization of the adsorptive energy of a heterogeneous surface. This description is more complete than previous ones based on only one of those elements. The joint site-bond energy distribution is determined through a correlation function in such a way that the maximum degree of randomness is attained in the network. The degree of randomness is limited by the “Construction Principle”: according to this, the adsorptive energy at a site must be deeper than that of any bond connected to that site. This correlation function contains valuable information about the topology of the energy surface, which plays an important role in adsorption equilibrium and dynamics.

Dimer physisorption on heterogeneous substrates

Abstract Physisorption of dimers on heterogeneous surfaces is studied by combining theoretical modelling, Monte Carlo simulation and experimental results. Monte Carlo simulation of dimer physisorption is carried out for substrates modelled in such a way that random, patchwise and intermediate adsorption site topographies can be generated. Simulated adsorption isotherms are used to test a model for adsorption of polyatomic molecules on heterogeneous surfaces, recently developed by Nitta et al.. Experimental adsorption isotherms for O2 and N2 adsorbed on zeolites 5A and 10X, as well as adsorption heats, are used to test the reliability of the simulation model. A simple theoretical form for the adsorption isotherm, based on the Fermi-Dirac approach, is finally proposed. Close agreement between simulated, theoretical and experimental results supports the validity of the proposed equation to describe interactions of diatomic gases with zeolites.

Lattice-gas model for calculating thermal desorption spectra: Comparison between analytical and Monte Carlo results

Abstract Thermal desorption spectra calculated using analytical approximations (the quasi-chemical and the mean-field approximation) and Monte Carlo simulations, are compared. The results are in good agreement when no order-disorder transitions occur in the overlayer during thermal desorption. If lateral interactions are such that the system crosses through ordered phases, various differences arise in the spectra which are discussed in terms of the adsorbate configurations involved.

The diffusion-controlled annihilation reaction in random adsorptive fields

The kinetics of the A+A to 0 reaction on a correlated heterogeneous one-dimensional chain is studied. As a novel result it is found that the temporal behaviour of the density of A particles depends on the energetic topography of the surface when A particles are initially located preferentially on the more energetic sites, as happens in the case of thermodynamical equilibrium for the absorbate.

Theoretical analysis of the coverage dependence of enantioselective chemisorption on a chirally templated surface

The effect of the formation of ensembles of chiral templates on the enantioselectivity of model heterogeneous catalysts is studied theoretically in the framework of a cooperative sequential adsorption model. Analytical solutions are presented for random adsorption onto a chirally templated surface which indicate that the surface exhibits a maximum enantioselectivity of ∼2.5, in agreement with results of enantioselective chemisorption experiments carried out in ultrahigh vacuum. It is suggested that the high enantioselectivity (≳90%) encountered in commercial catalysts could be due to correlated adsorption of the template molecules, and that these effects can be modeled using Monte Carlo calculations.

Scaling behavior in adsorption on bivariate surfaces and the determination of energetic topography

Adsorption of particles with repulsive nearest-neighbor interactions is studied through Monte Carlo simulation on bivariate surfaces characterized by patches of weak and strong adsorbing sites of size l. Patches can be either arranged in a deterministic chessboard structure or in a random way. Quantities are identified which scale obeying power laws as a function of the scale length l. Consequences of this finding are discussed for the determination of the energetic topography of the surface from adsorption measurements.

Pore-level modelling of wetting on correlated porous media

The displacement of a non-wetting fluid by a wetting one in a porous medium is influenced, among other things, by two competing mechanisms: the flow of the wetting phase along crevices, giving the possibility of snapping off in throats; and its advance through the centres of the pore space under various pore- and throat-filling conditions, leading to a cooperative filling. The percolation process associated with these two mechanisms on porous networks in two and three dimensions is well understood; it is a classic bond percolation problem competing with an invasion percolation one. We present a three-dimensional pore-level model that describes these effects on a site and bond network representing pores and throats, respectively. The network elements may have various degrees of correlation among their sizes. Site and bond-size distributions may be any kind of function representing a real pore space. In this work, we are able to predict various kinds of patterns that arise when the two aforementioned mechanisms compete and to study the effect of the correlations strength on the onset of each pattern, revealing the strong influence of the topology of the network in determining which process will dominate. Buoyancy forces are not taken into account in the present work.

The effect of energetic topography on the structure of the adsorbate

The distributions of potential energy for particles adsorbed on heterogeneous surfaces with different energetic topographies have been obtained by Monte Carlo simulation. The surfaces themselves were constructed using Monte Carlo simulations. The energy distributions are analyzed to give information about the roles of heterogeneity and lateral interactions in the adsorption process. In addition, these distributions are shown to be closely related to the conventional local adsorption isotherm.