Reactions and reaction intermediates on iron surfaces. IV. Thermodynamics of surface reactions

The reactions of methanol, formaldehyde, and formic acid on copper, silver, iron, and tungsten surfaces have been examined in terms of reaction mechanisms and thermodynamic driving forces. Thermochemical techniques were used to estimate the heats of formation of adsorbed surface intermediates. The enthalpies of reaction were then compared to determine the thermodynamic feasibility of the reactions. It was found that common surface intermediates were found for the reactions on all surfaces. Methanol reacted via a methoxy surface intermediate and formic acid reacted via a formate surface intermediate. Formaldehyde was found to undergo nucleophilic addition forming either methoxy or formate. The observed reaction paths could be accounted for in terms of their thermodynamic driving forces, including the effect of preadsorbed oxygen on the adsorption of methanol on copper and silver. The thermochemical model could also quantitatively account for the "volcano plot" correlation between catalytic activity and the heats of formation of metal formates for formic acid decomposition on transition metal catalysts.