Catalytic and surface oxidation processes on transition metal surfaces

Abstract

Transition metals are technologically important catalytic materials. The transition metal catalysts are used for example in petroleum and fertilizer industry. In the car industry the catalytic materials are used in the catalytic converters. Because of the industrial importance the catalytic metals have been widely studied throughout the past decades. Nonetheless, the oxidation mechanisms of small molecules and the effect of alloying to catalytic properties of metals are not fully understood.

In this thesis the catalytic effect of a palladium-silver alloy on the oxidation of a carbon monoxide molecule is studied with a computational approach. With ab initio methods we have been able to investigate the underlying microscopic processes of the oxidation. We have studied how the Pd atoms introduced to an Ag surface change the catalytic properties of the material. It has been seen that the Pd atoms enhance the dissociation of the O2 molecules, strengthen the binding of the CO molecules and lower the reaction barrier of the oxidation process.

In addition to the oxidation of the carbon monoxide, the oxidation of copper surfaces has been studied. Copper is widely used for example in construction, water pipes and other large scale facilities. On the other hand, it is used in a microscopic level in electronics as s conductor and the copper oxide as an insulator. In all of these examples the oxidation of copper plays a crucial role and it also possesses a great commercial value. In some applications the oxidation is an unwanted phenomenon, whereas in some other applications the properties of grown oxide are exploited. The oxidation of copper surfaces is also scientifically interesting since the oxygen is known to induce a reconstruction of the surface. The reconstruction structure has intensively been studied, but still the initial steps of the oxidation and reconstruction are not completely explained.

In this thesis, the reconstruction and the growth of the oxide layer on Cu(100) are studied with a multi-scale modeling involving both the atomistic electronic structure calculations and the larger scale statistical simulations. With the combined modeling we have been able to reproduce and further explain the structures observed in the experiments. We have shown that the dissociation and diffusion of oxygen is more rapid on the clean Cu(100) surface than on the reconstructed parts of the surface. The Cu adatoms form rapidly stable (100) islands and O adatoms form separate c(2 × 2) domains.