Toward Realistic DFT Description of Complex Systems: Ethylene Epoxidation on Ag-Cu Alloys and RPA Correlation in van der Waals Molecules

In this thesis we have studied two different aspects of Density Functional Theory (DFT): (i) the application of DFT with the generalized gradient approximation (GGA) functional for exchange-correlation energy in modeling an heterogeneous catalysis problem, and (ii) the development of a new self-consistent field (scf) strategy to solve the Kohn-Sham (KS) equations that allows to improve the accuracy of DFT method with exact exchange (EXX) and RPA correlation energy functionals in the description of weak chemical interactions. Ethylene epoxidation, one of the largest-scale catalytic processes in the chemical industry, were studied in Chapter 2 of this thesis. The formation of the desired product ethylene oxide (EO) in this reaction is promoted by a Ag-Cu alloy catalyst. In this study, the oxidation of ethylene is considered to occur on the Ag-Cu structures formed by thin copper-oxide layers on an Ag slab. These structures have been determined by theoretical and experimental works to be the favorable structures on the surfaces of Ag-Cu alloys in the high pressure and temperature conditions relevant to experiment. According to the calculations for reaction pathways, we found that the structures of Ag-Cu alloys are selective towards the formation of the EO final product, rather than the undesired product acetaldehyde (Ac) which is readily converted to carbon dioxide. The selectivity of Ag-Cu alloys is found to be higher than pure Ag, in agreement with experimental results. To do this, we carried out a study of the stability of the surface structures in thermodynamic equilibrium conditions ( at T = 600 K and pO2 = 1 atm), and we have shown that the higher selectivities relate to the formation of copper-oxide layers on the Ag slab. Moreover, our theoretical results show that the high selectivity of a copperoxide layer is maintained even when the thickness of the oxide is increased to two layers. In particular, we have found that a very high selectivity could be obtained by structure containing 1.25 ML of Cu and 0.25 ML of sub-surface oxygen. Another important result is the finding of a selectivity indicator that allows to determine the selectivity of the pure metals and alloy catalysts even with the thin oxide structures in ethylene epoxidation reaction. In further works, this indicator could be applied to predict the selectivity of other Ag-based alloys such as Ag-Pd, Ag-Pt, etc. These alloys were found experimentally to be selective catalysts towards the formation of EO. In spite of the great success of DFT when employing the well-known approximations such as LDA or GGA exchange-correlation functionals, the standard DFT approaches exhibit several serious shortcomings, and one of them is the poor or even wrong evaluation of long-range dispersion interactions (i.e., van der Waals interactions). Calculations with the EXX/RPA-correlation energy within the adiabatic connection uctuation-dissipation theorem (ACFDT) formalism have shown as a promising approach that can give the correct description not only of weak bonds but also of systems with covalent bonds. In Chapter 3, we developed the complete scf procedure that enables the optimization of KS systems whose total energy is computed with the EXX/RPA-correlation energy functionals. The implementation has been applied to the study of some simple molecules. In future work, EXX/RPA calculations could be applied to heterogeneous catalysis systems, where the role of van der Waals interactions is still largely unknown. Moreover, improvement of the accuracy of EXX/RPA calculations is also needed. According to ACFDT, one can go beyond the RPA formalism by taking into account higher-level approximations of the exchange-correlation kernel in the Dyson equation such as the time-dependent EXX kernel.