POLARIZATION MODULATION INFRARED REFLECTION-ABSORPTION SPECTROSCOPY OF CO ADSORPTION ON CO(0001) UNDER A HIGH-PRESSURE REGIME

The adsorption of CO on Co(0001) has been investigated in situ by pola rization modulation infrared reflection absorption spectroscopy (PM-RA IRS), which has been applied for the first lime in a study of a model system for a heterogeneous catalyst. The CO/Co(0001) system was studie d in the pressure range from 10(-10) to 600 mbar at temperatures betwe en 300 and 550 K, showing the in situ potential of PM-RAIRS and the si gnificant scope of this method for catalysis research. Linearly and br idge-bonded CO species could be distinguished on well-annealed surface s. High-pressure RAIRS experiments done at room temperature were in ag reement with previous low-energy electron diffraction (LEED) investiga tions in ultrahigh vacuum (UHV) at 100 K,(3,4) indicating a transition in the CO layer from a (root 3 x root 3)R30 degrees to a (2 root 3 x 2 root 3)R30 degrees structure with increasing CO coverage. By compari son of well-annealed and Ar-sputtered (defective) surfaces, we could i dentify, at a high frequency of around 2080 cm(-1), a CO species attac hed to defect sites. It is shown that annealing at 450-490 K at 100 mb ar of CO pressure leads to the creation of defects at the cobalt surfa ce. The defects influence the structure of the CO overlayer, The natur e of this ''defect''-bound CO is discussed. Postreaction X-ray photoel ectron spectroscopy (XPS) showed the development of surface carbide up on annealing in CO, which is in good agreement with the vanishing of t he RAIRS signal of adsorbed CO at temperatures above 520 K.