Infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy and temperature-programmed desorption study on the absorption and decomposition of Fe(CO)5 over silver surfaces

Abstract

The adsorption and decomposition of Fe(CO)₅ over Ag surfaces have been studied using infrared reflection adsorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The thermal desorption spectrum (TDS) showed that Fe(CO)₅ is partly chemisorbed at temperatures below 120 K on the Ag surface cleaned by Ar-ion sputtering. The active sites for chemisorption may be defect sites produced by the sputtering. XPS revealed that the chemisorbed species involve a small amount of dissociated species even at 95 K and undergo decarbonylation upon heating the substrate to ca. 180 K to form an intermediate species, Fe(CO)₄, which is stable at temperatures up to 280 K and then decomposed completely above 330 K, depositing Fe metal on the surface. An intense C—O stretching band at 2062 cm^–1 and a weak band at 2116 cm^–1 in the IRA spectra of adsorbed Fe(CO)₅ decrease and eventually disappear upon elevating the temperature to 230 K, and a new band grows at 2052 cm^–1 upon heating the substrate from 230 to 280 K. These IRAS results were discussed in terms of a surface selection rule for IRAS. The geometry of adsorbed Fe(CO)₅ was concluded to be square pyramidal with the square base toward the surface, and the Fe(CO)₄ moiety trigonal pyramidal. The deposited Fe metal from Fe(CO)₄ was found to dissolve to some extent into the bulk Ag at temperatures below 400 K.