Adsorption and decomposition of formic acid (DCOOD) on NiO(111) and Ni(111) surfaces probed by SFG

Adsorption and decomposition of formic acid (DCOOD) on NiO(111)/Ni(111) and Ni(111) surfaces under ultrahigh vacuum (UHV) conditions and the steady flow of DCOOD gas were investigated by sum-frequency generation (SFG) spectroscopy. Formates adsorbed via two carbonyl oxygen atoms were produced by the adsorption and the species on the oxide surface had two orientations in UHV; the species which decomposed at around 300 K was tilted from the surface normal and that decomposing at similar to 400 K was vertical to the surface. We induced the decomposition on the oxide surface by irradiation with picosecond 1064 nm pulses which caused the surface temperature to jump by similar to 300 K with similar to 100 ps decay time. The transient temperature jump transformed reversibly the stable formates to the monodentate formate adsorbed via one oxygen atom and the decomposition occurred at 100 K above the onset temperature 275 K of the transformation. The transformation was ascribed to the shift of chemical equilibrium due to the laser induced temperature jump. The energy difference between the two species and the activation energy for the decomposition of monodentate formate were derived.