Adsorption and Reactions of Formic Acid on (2×2)-NiO(111)/Ni(111) Surface. 1. TPD and IRAS Studies under Ultrahigh Vacuum Conditions

Abstract

The adsorption and decomposition of formic acid on (2×2)-NiO(111) surface have been studied using temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS) under ultrahigh vacuum (UHV) condition. Formic acid dissociated to surface formate at 163 K for low exposure and molecular adsorption occurred at higher exposure of formic acid. Absorption bands observed at 2858, 1570, 1360, and 778 cm-1 on the IRA spectra were assigned to the C−H stretching, O−C−O asymmetric stretching, O−C−O symmetric stretching, and O−C−O deformation modes of adsorbed formate, respectively. Cumulative consideration of vibrational frequencies, assignment of the bands, and the selection rule of IRAS revealed that the formate is in bidentate configuration but is tilted sideways to the surface. The adsorbed formate decomposed through two reaction pathways on raising the temperature; dehydrogenation producing H2 and CO2 occurred at 340, 390, and 520 K and dehydration producing CO occurred at 415 and 520 K. The temperature-dependent IRA spectra reflected the thermal desorption behavior of adsorbed formate.