Adsorption and Reactions of Formic Acid on (2×2)-NiO(111)/Ni(111) Surface. 2. IRAS Study under Catalytic Steady-State Conditions

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 H2 and CO2 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 H2O 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.