Unstable intermediates are indispensable ingredients of chemical reactions, but their identification at solid surfaces has been hampered by the lack of techniques to detect short-lived and fractionally generated species. We succeeded in identifying the intermediate in a thermal decomposition reaction by utilizing the picosecond temperature jump induced by the irradiation of laser pulses and the observation by sum-frequency generation spectroscopy. We verified that the decomposition of formate on the NiO(111) surface is preceded by the transformation of stable bidentate formate to unstable monodentate formate, which is the reactive intermediate, and two species are found to be in equilibrium before decomposition.

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.

Product gas analysis using quadrupole mass spectrometer (QMS) and infrared reflection absorption spectroscopy (IRAS) have been performed to investigate the catalytic decomposition of formic acid on NiO(111) film grown on Ni(111) surface under steady-state conditions and the results have been compared with the previous results obtained in vacuum condition. The rates of the catalytic reactions as measured by product gas analysis at various pressures of formic acid and also at different substrate temperatures revealed two reaction paths for the decomposition of surface formate under equilibrium with gas-phase formic acid. The dehydrogenation producing H 2 and CO 2 with the activation energy of 22 ( 2 kJ/mol was preferred even at the low temperature of 373 K over the dehydration producing CO and H 2 O which occurred at a higher temperature of above 423 K with the activation energy of 16 ( 2 kJ/mol, where the activation energy refers to the reaction of formic acid in gas phase to the product substances. The apparent reaction order was 0.5 with respect to the pressure of formic acid for both reactions. IRA spectroscopy using both normal and deuterated formic acid revealed the presence of two kinds of formate species: bidentate formate aligned normal to the surface and monodentate formate. The result is in contrast with our previous finding of only the tilted-bidentate formate species on the surface under vacuum. Pressure-dependent features of the IRA spectra suggested that the monodentate formate is the intermediate for the catalytic decomposition reactions. The combined use of gas analysis and IRAS results enabled us to derive the values of 58 ( 3 and 49 ( 3 kJ/mol as the activation energy for the dehydrogenation and dehydration reactions, respectively, of the monodentate formate on the surface.

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