ADSORPTION AND DECOMPOSITION OF FORMIC-ACID ON NI(110)

The adsorption of formic acid on Ni{110} has been extensively studied over a wide temperature range using RAIRS. At 300 K, the decomposition of the acid produces adsorbed CO and formate, the ratio of which is dependent on the exposure. The IR spectra show initially the presence of CO alone. Subsequently, however, coadsorbed CO and formate do exist on the surface. With prolonged exposure the coadsorbed CO is completely removed, leaving only formate on the surface. These observations are correlated with two competitive mechanisms for the decomposition of the surface formate: C-H and C-O bond cleavage to produce CO2 and adsorbed CO, respectively. For low exposures there is a high probability for C-O bond cleavage, and with increasing exposure this changes to favour C-H bond cleavage. The change in selectivity with exposure is attributed to accumulation of very small quantities of carbon from the initial decomposition sequence, which stabilise the formate and destabilise the adsorbed CO. With preadsorbed carbon and oxygen IR spectra reveal significant stabilisation of the surface formate, with the change in the decomposition pathway. Adsorption at 120 K shows the presence of both CO and formate regardless of exposure. At this temperature coadsorbed CO only accounts for a small fraction of the overlayer and is produced from the dissociation of the acid at defect sites.