Monte Carlo based methods applied to heterogeneous catalysis and gas separation

Abstract

The research work presented in this thesis is divided in two main topics: gas separation and heterogeneous catalysis. Although the systems studied in one part and another are quite different, they share two fundamental features: both topics have a special industrial interest and they have been studied through stochastic Monte Carlo based methods. The present work on gas separation aims to assess the performance of several faujasite structures, a well-known family of zeolites, in CO2 capture processes. Concretely, ten faujasite structures with different Al content have been evaluated in the separation of post-combustion CO2 mixtures via simulation of swing adsorption processes. Through GCMC simulations performed on a wide range of pressures and temperatures, the pure and mixture adsorption isotherms and isobars for the different structures are obtained. This data is used to calculate several performance criteria such as purity, working capacity, selectivity and energy required per ton of CO2 captured. The results show that high Al content structures are suitable for operating under a TSA unit, while intermediate and low Al content structures show better performance in PSA and VSA units, respectively. On the other hand, the research work on chemical reactivity focus on the study of the water-gas shift reaction (WGSR) on copper surfaces both from a thermodynamic and from a kinetic point of view. First, a kMC study is performed on the flat Cu(111) surface. The lattice model is quite simple, consisting on an hexagonal periodic grid of points. All sites are considered equivalent, and only repulsive lateral interactions between neighboring CO adsorbed species are included. However, even with this simple model, the kMC results agree quite well to the available experimental data, and demonstrate that the dominant reaction mechanism is the COOH mediated associative mechanism. The effect of van der Waals interactions is then studied, by performing DFT calculations of the WGSR on Cu(321) using the Grimme D2 correction accounting for the dispersion forces. The results obtained are compared with previous DFT results

published in the literature where no van der Waals corrections were included. The comparison shows big differences on the adsorption energies of some gas species, as well as important differences in some energy barriers, hence demonstrating the importance of including dispersion terms in order to obtain a meaningful description of the energetics of the WGSR. New kMC simulations are then performed for the WGSR on Cu(321) surface to study the effect of step sites on the WGSR activity. The recently developed graph- theoretical kMC framework is used, coupled with cluster expansion Hamiltonians to account for the lateral interactions between neighboring species. The simulation results show that the activity is much lower on the stepped Cu(321) surface. Analysis of the kMC simulations suggests that the reason in the poisoning of step sites by CO species, as well as the presence of low energy barriers for some key steps on the reverse direction (e.g. water dissociation and COOH formation). Finally, the thesis ends with a brief tutorial on kMC simulations where several issues are discussed, like the importance of including diffusion processes of the effect of lateral interactions.

El treball de recerca presentat en aquesta tesi es divideix en dos temes principals: separació de gasos i catàlisi heterogènia. Tot i que els sistemes estudiats en ambdues parts són molt diferents, comparteixen dues característiques fonamentals: ambdós temes tenen un elevat interès industrial i s'han estudiat mitjançant mètodes estocàstics de Monte Carlo. El treball corresponent a la separació de gasos pretén avaluar el rendiment de diverses estructures de faujasites, una coneguda família de zeolites, en processos de captura de CO2. Concretament, s'han avaluat deu estructures de faujasites amb diferent contingut d’alumini en la separació de mescles post-combustió. Mitjançant simulacions GCMC realitzades en una àmplia gamma de pressions i temperatures, s’han obtingut les isotermes i isòbares d'adsorció pures i de mescla per les diferents estructures. Aquesta informació s'ha emprat per calcular diversos criteris de rendiment com ara la puresa, la capacitat de treball, la selectivitat i l'energia requerida per tona de CO2 capturat. D'altra banda, els treballs de recerca sobre reactivitat química es centren en l'estudi de la reacció water-gas shift (WGSR) sobre superfícies de coure tant des d'un punt de vista termodinàmic com cinètic. En aquest context, s’ha estudiat l’efecte de les superfícies esglaonades i de les forces de van der Waals mitjançant càlculs d’estructura electrònica i simulacions amb el mètode de Monte Carlo cinètic (kMC) en la superfície plana Cu(111) i la superfície esglaonada Cu(321). Els resultats mostren que les superfícies esglaonades no sempre són més actives que les planes, i que la introducció de les interaccions de van der Waals és crucial per a obtenir una descripció correcta dels diferents processos que ocorren en superfície.