ADSORPTION AND DECOMPOSITION OF FORMIC-ACID ON MGO(001) SURFACE AS INVESTIGATED BY TEMPERATURE-PROGRAMMED DESORPTION AND SUM-FREQUENCY GENERATION SPECTROSCOPY - RECURRENCE INDUCED DEFECT SITES

The change of MgO(001) surface during the adsorption and thermal desor ption of formic acid was investigated by temperature programmed desorp tion (TPD) and infrared-visible sum-frequency generation SFG spectrosc opy. The TPD results revealed that the feature of desorption signals c hanged as the adsorption/thermal desorption process was repeated, desp ite no significant change of the low energy electron diffraction (LEED ) pattern. The surface formate, produced by dissociative adsorption of formic acid, decomposed at 560 K and 700 K, giving H2O+CO and H-2+CO2 as primary and minor products, respectively, but the broad desorption peak of H2O started to appear at 300 K. The 560 K peak appeared from an early stage, while the 700 K peak gained intensity as the experimen t was repeated and the density of defects increased. The adsorption si te for the 700 K peak was ascribed to the catalytic reaction of magnes ia powder. The SFG spectra gave three resonance peaks of CH stretching bands at 2850 cm(-1), 2870 cm(-1), and 2920 cm(-1) with a different d ependence on the dose time, history of surface, and polarization of in cident visible beam. The former two bands were assigned to the formate s with bridged configuration, and the third band to the formate with u nidentate configuration. The polarization dependence suggested that th e CH bonds of bridged formates stood perpendicular to the surface, and that of the unidentate formate tilted approximately 30 degrees from t he surface normal. A vibration nonresonant background signal was prese nt on the SFG signal, showing characteristic dependence on the polariz ation of incident light beams, as well as the coverage and history of the surface, and was interpreted as originating from the defect sites on the surface. (C) 1997 American Institute of Physics.