Abstract:
This study involves the examination of chemical reactions and the adsorption behavior of ammonia, carbon dioxide, acetic acid, acetaldehyde, acetylene, propyne and bromoacetic acid on TiO2(001) single crystal surface under ultra high vacuum (UHV) conditions. The surfaces of TiO2(001) in its stoichiometric and sub-stoichiometric forms were characterized with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) while the reaction chemistry was followed via Temperature Programmed Desorption (TPD), dark conditions and steady state flow reactions under ultra-violet light. On the stoichiometric surface and in the absence of UV light ammonia, carbon dioxide, acetylene and propyne were found to be molecularly adsorbed and desorbed intact at low temperature upon heating. Simulation of TPD results were conducted using ASTEK program in order to extract kinetic information such as activation energy, pre-factor and to see the into the coverage effect on the stability of the adsorbates. For most of the studied molecules increasing the surface coverage has resulted in decreasing both the activation barrier and pre-factor. Substituting H by Br in the CH3 group of acetic acid has resulted in a complex reaction chemistry that triggered both carbon-carbon coupling and dehydration reactions. The sub-stoichiometric surface that was created by Ar+-bombardment shows a rich chemistry for the organic molecules. Among the most important observation from this study was the coupling of CO2 to ethylene on O-defected surface.
In the presence of UV light, acetic acid gave ethane, methane and CO2, the photo-Kolbe products. This reaction was followed in steady state conditions on the {011} and {1l4}-faceted surfaces with the former shown to have a higher quantum yield. Cluster model computational methods using mostly semi-empirical PM3(d), but some using Hartree-Fock and DFT(B3LYP) were also employed to compliment (and compare) the experimental.