Mechanism of formic acid oxidation on Bi modified Pt(111): Implication from the concentration effect of formic acid and different coverages of Bi
Zhen Wei, Anni Yu, José M. Gisbert-González, Jun Cai, Yanxia Chen, Juan M. Feliú, Enrique Herrero
Abstract
The concentration dependence of the activity for the formic acid oxidation at pH 1.2 and 4 has been studied on Pt(111) with different coverages of bismuth. The results clearly show: (1) The FAOR on bismuth-modified Pt(111) has no relation with HCOOb adsorbed on platinum but depends on the bismuth coverage. (2) The Tafel slope is ca. 90 - 110 mV in lower potential and increases to nearly the vertical asymptote when the potential increases. (3) Free Pt sites are required for FAOR on bismuth-modified Pt(111). (4) Solution formate, rather than formic acid, is the active species. Combined with previous reports, a kinetic model in which monodentate formate, adsorbed on a Pt-Bi ensemble, is the active intermediate and solution formate is the active species has been proposed. The model is able to reproduce the experimental behavior and agrees with the reaction order measured experimentally. For low potentials, the reaction order for formate is 1, whereas a reaction order of 0.5 is obtained for high potentials.