ADSORPTION AND REACTION OF FORMIC-ACID ON A (2 X-2) NIO(111) NI(111) SURFACE - 3 - IRAS STUDIES ON THE CHARACTERIZATION OF REACTION SITES USING CO AND THE BEHAVIOR OF SURFACE HYDROXYL SPECIES/

The decomposition of formic acid on (2 x 2) NiO(111)/Ni(111) was studi ed by infrared reflection absorption spectroscopy (IRAS). Formic acid molecules adsorbed dissociatively on the surface to form formate which decomposed to H-2, CO2, H2O, and CO at 340-415 and 520 K. The vacant sites after the decomposition of formate were probed by the adsorption of CO at 100 K. On clean NiO(111) at 100 K, IRAS peaks of two types o f adsorbed CO were observed at 2146 and 2079 cm(-1), which were assign ed to the CO on fully-oxidized Ni cation sites and less-oxidized Ni ca tion sites, respectively. The CO peaks were not observed on the format e precovered surface. The IRAS peak of adsorbed CO at 100 K on the ful ly-oxidized Ni cation sites appeared when the surface was heated to de compose formate around 340-415 K. After decomposition at 520 K, the in tensities of both peaks of CO were recovered to the initial intensitie s. It was thus concluded that the sites of decompositions at 340-415 a nd 520 K are the fully-oxidized Ni cation sites and less-oxidized Ni c ation sites, respectively. The behavior of hydroxyl species produced b y dissociative adsorption of formic acid was next investigated using d euterated formic acid (DCOOD). The hydroxyl species (-OD) formed by th e dissociative adsorption of formic acid (DCOOD) was observed at 2536 cm(-1) below 343 K, while the isolated OD groups appeared at 2713 cm(- 1) and around 500 K. The mechanism of decomposition of formate at each temperature is discussed.