Infrared spectroscopy of O2 and N2 adsorption on the clean and hydrogen covered Pt(111) surface

Doktorsavhandling, 2005

This thesis is devoted to the investigation of O2 and N2 adsorption on the clean and the hydrogen covered Pt(111) surface. Using infrared reflection absorption spectroscopy (IRAS) both physisorbed and chemisorbed molecular adsorption states have been identified and characterized via their characteristic internal vibration spectra. On Pt(111) the IRAS measurements reveal a physisorbed O2 state at temperatures below 40 K while O2 chemisorption in peroxo- and superoxo-like states are observed in the investigated temperature range 30-90 K. The temperature dependence of the O2 chemisorption probability and the thermal conversion of physisorbed 2 into primarily the superoxo state show that the physisorbed state acts as a precursor to the O2 chemisorption. On the hydrogen saturated Pt(111)(1x1)H surface no O2 chemisorption occurs and the physisorbed molecules do not act as precursors to chemisorption but simply desorb at temperatures above 45 K. When the Pt(111) surface is partially covered with p(1x1)H islands, O2 chemisorption in the peroxo and superoxo state occur on the bare Pt(111) regions. However, the thermal activation of the O2 precursor is altered presumably due to loss of trapped precursor molecules by thermal desorption from the chemically inert p(1x1)H islands. N2 adsorption on Pt(111) at 30 K involve, apart from a weak defect related signal, two N2 states, one chemisorbed and one physisorbed. Almost identical uptake behavior in these two states indicates that they are intrinsic constituents of the growing (3x3) ordered nitrogen islands, and that there is at least one chemisorbed molecule in the (3x3) unit cell. The hydrogen saturated Pt(111)(1x1)H surface is inert with respect to N2 chemisorption but below 45 K N2 physisorbs and at 30 K a commensurate (3x3) ordered N2 adlayer forms. The infrared absorbance and the polarization induced dynamic dipole moments related to the fundamental internal vibration transition of N2 and O2 physisorbed on the chemically inert Pt(111)(1x1)H surface have been analyzed together with the induced static dipole moments derived from work function data. A good estimate of the induced dynamic as well as static dipole moments can in general be obtained from a van der Waals model.