Near-ambient pressure velocity map imaging

Sammanfattning: Catalytic reactions on solid surfaces have been studied under Ultra-high vacuum (UHV) conditions for decades. These studies provide crucial information for catalytic reactions, such as surface structures, adsorption sites, and reaction mechanisms. However, industrial catalysis operates under high gas pressure to increase the reaction rate, so the knowledge we learn from the previous UHV studies may not be able to directly transfer to the industry. This difference is referred to as the “pressure gap”, and it represents the difficulties that scientists encounter when attempting to investigate and comprehend catalytic reactions at high pressure. To address this issue, in situ/operando techniques and instruments have been developed to study reactions at pressures closer to real-world applications.The present thesis aims to showcase the new instrument, Near-Ambient Pressure Velocity Map Imaging (NAP-VMI), and its applications to molecular spectroscopy and surface science at near-ambient pressures. This instrument features a velocity map imaging (VMI) setup with redesigned ion optics and uses differential pumping to achieve a working pressure of 10−3 mabr. It allows time-resolved measurements at microsecond time scales using the pump-probe technique with a pulsed molecular beam and a pulsed laser. The performance is validated using N2O photodissociation and N2 surface scattering. CO oxidation on Pd(110) and Pd(100) surfaces is studied at elevated oxygen pressure (1×10−5 mbar) where the surfaces reconstruct.The results show the suppression of CO2 production in oxygen rich environments for both surfaces. The difference in kinetics and dynamics behavior between the two surfaces also suggests that surface structures and adsorption sites are important in the reaction mechanisms. These findings highlight the importance of surface structure in catalytic reactions and pave the way for more effective catalysts to be designed by tailoring surface properties and reaction conditions.