Observation of the Decomposition of Formic Acid on NiO(111) by IRAS

Abstract

The adsorption and decomposition of formic acid on a (2×2)-NiO(111) surface were studied by TPD and IRAS under ultrahigh vacuum conditions. It was found by TPD that formic acid decomposed to hydrogen and carbon dioxide at 340-390 and 520 K and to carbon monoxide at 415 and 520 K. An IRA spectra showed that the formic acid adsorbs dissociatively to form formate in a tilted-bidentate configuration. It is known that adsorbed CO on NiO(111) gives two IRAS peaks, of which the peak at the higher wavenumber is assigned to the CO on fully-oxidized Ni cation sites and that at lower fre-quency to the CO on less-oxidized sites. The less-oxidized sites are considered to be located at oxygen vacancy, boundaries of the NiO crystals, and/or steps. We examined the IRA spectra of adsorbed CO on the formate-covered NiO(111) at 100 K after heating to various temperatures to characterize the reaction sites of formate giving each TPD peak. When the formate-covered NiO(111) was heated to 340-415 K, the higher frequency band of adsorbed CO appeared, indicating that the decomposition of formate at 340-415 K takes place on the fully-oxidized sites. On the other hand, the intensity of the CO band at lower frequency increased above 473 K, and the decomposition of formate at 520 K was considered to arise from the less-oxidized sites.