CO adsorption on Ni(100) and Pt(111) studied by infrared-visible sum frequency generation spectroscopy: design and application of an SFG-compatible UHV-high-pressure reaction cell

Infrared-visible sum frequency generation (SFG) surface vibrational spectroscopy was applied to monitor CO stretching vibrations on Ni(100) and Pt(111) in the range from submonolayer coverages up to 200 mbar. Since SFG can operate in a pressure range from ultrahigh vacuum (UHV) to ambient conditions, it is particularly suited for in situ studies of adsorbates at elevated pressure or during a catalytic reaction. At high coverages, a compressed overlayer was formed on Ni(100) at 100 K that can be modeled by a coincidence structure. On Pt(111), terminally bonded (on-top) CO was the only species observed at 230 K, independent of gas pressure. At low pressure the SFG spectra were complemented by LEED, AES and TPD. The experiments were carried out in an SFG-compatible elevated pressure reactor that is attached to a UHV surface analysis chamber. After preparation and characterization in UHV, model catalysts can be transferred in vacuo into the reaction cell. The reactor is separated from the UHV chamber by an arrangement of differentially pumped spring-loaded teflon seals and can be pressurized to 1 bar without degrading the vacuum in the UHV analysis system.