Formic Acid Oxidation on Platinum- a Simple Mechanistic Study

The oxidation of small organic acids on noble metal surfaces under electrocatalytic conditions is important in industrial processes and of scientific interest, but the basic reaction mechanisms continue to be a matter of debate.  Formic acid oxidation on platinum is one of the simplest of these reactions, yet even this model system remains poorly understood.  Historically, proposed mechanisms for the oxidation of formic acid involve the acid molecule as a reactant, but recent studies suggest that the formate anion is the key reactant instead. Ab initio studies of this reaction do not address formate as a possible reactant, due to the difficulty of calculating a charged species near a solvated surface under potential control.  Using the recently-developed Joint Density Functional Theory (JDFT) framework for electrochemistry, we perform ab initio calculations of this reaction on a Pt(111) surface to explore this reaction and help resolve the debate.  When a formate anion approaches the platinum surface with H pointing towards the surface, it reacts to form CO₂ and adsorbed H with no barrier on a clean Pt surface at relevant voltages.  This mechanism explains the elusiveness of an active surface intermediate and leads to a reaction rate proportional to formate concentration and number of available platinum sites.  Additionally, high coverages of adsorbates lead to large reaction barriers, and consequently, we expect availability of pristine metal sites to limit the experimentally observed reaction rate.