• Moreira, Raphaell

Abstract

Nanoporous gold (np-Au) has emerged as a very promising catalyst for a variety of catalytic processes. In recent years' significant efforts have been made to elucidate the role of structural aspects as well as admixtures of a second metal, oftentimes silver, which remains in the nanoporous material produced by corrosion of an appropriate alloy such as AuAg. Investigations on well-defined single crystalline model systems provide evidence that the reactivity of np-Au can be rationalized by the properties of low index Au surfaces. However, various open questions still remain such as the role of steps or the importance of water for the understanding of the catalytic properties. In order to investigate reaction kinetics of catalytic reaction on well-defined single crystal surfaces, an ultra-high vacuum molecular beam chamber was setup. The oxidation reactions were performed under isothermal conditions using pulsed molecular beam techniques combined with mass spectrometric measurements to monitor the kinetics of products in the gas-phase. In addition, the apparatus allows for an IR spectroscopic characterization of the surface under reactions conditions to elucidate the nature of surface species during the reaction. As Au surfaces do not dissociate molecular oxygen under UHV conditions the experiment utilizes an effusive beam of atomic oxygen as created by a thermal cracker. In this thesis CO oxidation was studied on the Au(332) surface. Initially, IRAS studies were performed to obtain a detailed assignment of IR vibration modes of 13CO on the pristine and O pre-covered Au(332) surface. In agreement with the literature results, IRAS studies showed a 13CO band on pristine Au(332) with a maxima at about 2075 cm-1 at low coverages, which shifts to lower wavenumber as the coverage rises. The O pre- covered Au(332) surface shows initially two distinguishable vibrational bands which suggest that different adsorption sites were created after exposure the surface to O atoms and additionally the results show that CO is more strongly bound on the oxygen pre-covered surface than on the pristine Au(332). Transient kinetics of CO oxidation reaction on Au(332) provide clear evidence for a much more complex reaction scenario than anticipated by simple oxidation of CO by oxygen atoms. Moreover, the results showed the beneficial effect of water being known to be in fast exchange with adsorbed oxygen, which hence can compete with CO oxidation. The transient kinetic shows clear evidence for fast and slow processes as a function of water pressure. At high water partial pressure, the system adopts steady state kinetics, while it does deactivate at lower pressures due to the formation of Au-O phases being less reactive towards CO oxidation. Due to the quantitative nature of the molecular beam experiments it is possible to show that the system exhibits a significant transient oxygen concentration on the surface while being in steady state. The latter oxygen does, however, react with CO in case the oxygen atom supply is switched off. The ability of water to utilize oxygen present in Au-O phases for CO oxidation being otherwise not reactive under the chosen experimental conditions can be directly correlated to observation for np-Au. For these systems water was found to be beneficial not only for the steady state reactivity of the system, but perhaps even more interesting for a reactivation of a deactivated catalyst, which can be explained by the utilization of otherwise unreactive oxygen species poisoning the surface.

Topics

• impedance spectroscopy
• surface
• single crystal
• silver
• corrosion
• phase
• experiment
• Oxygen
• reactive
• laser emission spectroscopy
• gold