| dc.description.abstract | The work in this thesis was carried out with the overall goal to investigate surface dynamics of model catalyst surfaces. Single crystals were utilized as model systems, namely Co(11-20) and Pd3Au(100). The Co(11-20) surface, being known from previous surface science investigations to reconstruct at room temperature upon CO adsorption, was chosen to investigate the effect of FTS relevant adsorbates on the surface reconstruction. The bimetallic system Pd3Au(100) was chosen to investigate the role of the alloying element on the surface chemistry during CO oxidation, with the long term goal of elucidating segregation effects.
The surfaces were studied with a combination of techniques, both experimental and theoretical. The characterization methods applied for Co(11-20) were scanning tunneling microscopy, low energy electron diffraction and high resolution photoelectron spectroscopy at synchrotron facilities. The Pd3Au(100) surface was investigated with a combination of near ambient photoelectron spectroscopy and quadropole mass spectrometry. The Co model surfaces investigated by density functional theory were derived from experimental results. This combination of methods allowed for a more complete understanding of the processes than would be possible with the techniques applied separately to the surfaces.
The Co(11-20) surface is known to undergo a (3×1) surface reconstruction upon exposure to CO. The results pertaining to the restructuring of the Co(11-20) surface interpreted together suggest that obstruction of this process causes changes in the stability and amount of CO adsorbed. Depositon of submonolayer amounts of K resulted in significant changes to the restructuring and amount of CO adsorbed on the surface. Relatively large differences in desorption temperatures were measured for low temperature and room temperature CO exposure on Co(11-20), with room temperature exposure having the highest desorption temperature. Calculations inferred that CO adsorbed with highest stability on the reconstructed model surfaces. Therefore, the higher desorption temperature for initial room temperature exposure was attributed to the CO-induced reconstruction. The combined results showed that small amounts of K present on the surface could result in a higher activation barrier for the restructuring process, and that this could contribute to the drop in activity observed for Co-based FTS catalysts with small amounts of alkali metals.
The deposition of submonolayer amounts of K to Co(11-20) was also found to influence the adsorption and decomposition of ethylene. The presence of K induced a new adsorption state for ethylene at 100 K as measured by photoelectron spectroscopy. Heating this ethylene covered surface to above 640 K showed significant differences in the C 1s core levels as compared to the the clean surface, indicating differences in decomposition. The (2×5) carbon overlayer, known to form on clean Co(11-20) is also formed with small amounts of K present, as verified by scanning tunneling microscopy and photoelectron measurements. However, the results suggest that K increases the resistance towards the formation of polymeric carbon as heating the overlayer to 640 K did not produce a shift in the C 1s core level towards higher binding energy, contrary to heating the overlayer on the clean surface. Hence the presence of K has a significant effect on both the adsorption and decomposition of ethylene, and the formation of carbon species at higher temperatures.
The investigation of the Pd3Au(100) surface under CO oxidation with near ambient photoelectron spectroscopy and quadropole mass spectrometry revealed differences in the behavior of Ag and Au as alloying elements. The Pd-alloy with Au behaved comparatively to pure Pd, with the active surface towards CO2 production being a (√5×√5)R27◦ surface oxide, and displaying normal hysteresis. This is in contrast to Pd75Ag25(100) which is covered by atomic oxygen under CO2 production, and displays reversed hysteresis. | nb_NO |
