The adsorption and decomposition of formic acid on Ni(111): The identification of formic anhydride by vibrational spectroscopy

Abstract

The adsorption and decomposition of formic acid (HCOOH) on a Ni(111) surface has been examined using thermal desorption spectroscopy (TDS), electron energy-loss spectroscopy (EELS), and reflection–absorption infrared spectroscopy (RAIRS). At 90 K several adsorption states above monolayer coverage are detected. Hydrogen bonding of HCOOH is indicated by strongly red-shifted OH stretching modes. The first layer, possibly consisting of adsorbed HCOOH dimers, is dehydrated above 250 K and forms a stable surface intermediate. Further heating leads to the known autocatalytic decomposition of the surfaces species and produces extremely sharp TDS peaks of CO2 and H2 . Above 360 K only CO is left on the surface. After forming the stable surface intermediate using DCOOH, modes at 776, 1267, 1329, 2181, and 2199 cm−1 are observed by RAIRS. The latter two modes, which are not resolved by EELS, are interpreted as the symmetric and antisymmetric CD stretching modes of the surface intermediate. Consequently, whereas most vibrational studies on other surfaces favor a formate ion (HCOO) as stable surface intermediate, we propose a formic anhydride on the Ni(111) surface. The modes observed at 1323 and 1267 cm−1 are interpreted as symmetric and asymmetric CO stretching modes. The existence of a formic anhydride intermediate is finally confirmed by an experiment using a mixture of HCOOH and DCOOH: The RAIRS spectra show the presence of all three different kinds of intermediates which are (HCO)2 O, (DCO)2 O, and HCO–O–DCO.