Heterogeneous catalytic reactions studied with transient gas supply and event-averaged ambient pressure x-ray photoelectron spectroscopy

Sammanfattning: CO oxidation over Pd(100) is studied with ambient pressure x-ray photoelectron spectroscopy (APXPS). Cyclic gas composition pulsing of the CO:O2 ratio, combined with a new method using software based event-averaging by image recognition over many pulses, is used to improve the signal-to-noise ratio substantially. Hereby, it becomes possible to follow the dynamics of Pd(100) restructuring between an oxidized surface and CO covered surface in situ while the reaction is running and with sub-second time resolution. Even though the experiments suffered from CO2 production at a platinum based heater filament the time-resolved measurements made it possible to follow the surface phase transitions and in particular distinguish when increased CO2 production is caused by changes on the Pd(100) surface. By time evolution analysis of components from both gas phase and surface, it is demonstrated that the surface turns active before it becomes oxidized. Finally, the work includes one of the first statistical analysis of multi-pulse experiments. This analysis reveals large fluctuations of the time the Pd(100) surface is CO-covered between individual experiments. However, the detailed analysis of many experiments performed at different temperatures demonstrates a clear temperature dependence of the time the Pd(100) surface is CO-covered.