Insight into the role of adsorbed formate in the oxidation of formic acid from pH-dependent experiments with Pt single-crystal electrodes

Abstract

The concentration dependence of the activity for the oxidation of formic acid at pH 1.2, 2.4, and 3.9 has been studied on Pt(111), Pt(100), and stepped Pt[n(111) x (110)] electrodes. The results clearly demonstrate that, in this pH range, formate, and not formic acid, is the active species. To analyse the data, a kinetic model in which adsorbed monodentate formate is the active intermediate has been proposed. On Pt(111), a steady increase in the reaction rate with increasing pH and a reaction order of 1 for the bulk formate concentration was obtained, in agreement with the proposed model. On the other hand, the competition between adsorbed hydrogen and formate for the adsorption sites on Pt(100) cancels both the concentration and the pH dependence of the reaction rate. On this electrode, the reaction rate for the dehydrogenation of formic acid to COad was also studied and found to go through a maximum both at pH 1.2 and at pH 3.9, although the rate of dehydrogenation is slower and the maximum is broader at pH 3.9. The slower rate at the higher pH is consistent with the well-known fact that dehydrogenation of HCOOH is an acid-catalysed reaction. On Pt[n(111) x (110)] electrodes the behaviour observed is similar to that of Pt(111), with a reaction order of 1 for formate. The only significant difference with respect to the Pt(111) surface is the formation of COad due to the presence of (110) steps.