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