The chemistry of formic acid on oxygen modified Ru(001) surfaces

The effect of oxygen pre-coverage on the chemistry of formic acid on modified Ru(001) surfaces was analysed by reflection-absorption infrared spectroscopy (RAIRS). The O/Ru(001) surfaces (theta(O) between 0.25 and 0.6 ML) were exposed to a very low dose (0.05 L) of formic acid at 90 K, and subsequently annealed. A low oxygen pre-coverage (theta(O) = 0.25 ML) promotes the production of the intermediate formate by O-H bond cleavage, at 90 K. On the contrary, high oxygen pre-coverages (theta(O) >= 0.5 ML) inhibit the deprotonation reaction, leading to some physical adsorption of formic acid. At 90 K, monodentate formate was observed on all the O/Ru(001) surfaces, co-adsorbed with bidentate (C-2V) formate or with formic acid, for low or high oxygen pre-coverage, respectively. A thermal activation is required for converting the monodentate into the more stable bidentate formate. This process occurs irreversibly at 100 K (for theta(O) = 0.25 ML) or 110 K (for theta(O) >= 0.5 ML). The stability of bidentate formate is maximized for an oxygen coverage of 0.25 ML, as its initial decomposition temperature increases from 120 K (on the clean surface) to 130 K, dropping again to 120 K for higher oxygen coverages. Since carbon monoxide was detected as a reaction product both on clean and modified surfaces, there is no evidence for a mechanism involving exclusively C-H bond breaking (leading only to CO2) on the oxygen modified surfaces. (c) 2005 Elsevier B.V. All rights reserved.