Adatom Coverage Dependences and Specificity Effects on the Rate of Formic Acid Electro‐oxidation at Polycrystalline Platinum

Abstract

The catalysis and inhibition of formic acid electro-oxidation that may accompany metal-submonolayer formation on polycrystalline platinum was studied quantitatively using the rotating ring-disk electrode (RRDE) technique. Under controlled flux conditions one can attain limiting current behavior for the oxidation of formic acid and thereby determine the upper limit for the optimum adatom coverage for lead and bismuth. These values are found to be 30-35% of the real area of the platinum electrode. The electro-oxidation experiments in this paper are correlated with the literature dealing with polycrystalline and single-crystal studies of hydrogen adsorption and formic acid oxidation at platinum to explain differences of the behavior of various underpotential deposited (UPD) species. The poisoning of the electro-oxidation process of formic acid requires the presence of adsorbed hydrogen or unhindered access to the first adsorbed intermediate C. OOH that is produced. During the initial stages of UPD, submonolayers of silver or copper do not deposit significantly on Pt(100), the plane that is responsible for most ofthe electro-oxidation offormic acid. Therefore, UPD silver or copper cannot inhibit poisoning of the formic acid oxidation process by decreasing hydrogen adsorption, or restricting access to C OOH via a third body mechanism. Bismuth, lead, and other UPD species with radii larger than platinum do deposit significantly on Pt(100) at low UPD coverages, and can therefore slow the poisoning of the formic acid process on this plane. Research on the oxidation of formic acid on nobIe metals and its electrocatalysis has been going on for more than a century (1-3). Interest in this process has gained considerable momentum in the past two decades due to the process's significance in the fuel cell technology and because the simplicity of the formic acid molecule provides a model for the electro-oxidation of other organic fuels. A formic acid oxidation mechanism that describes the principal experimental results involves, as the initial stage, an oxidative electrosorption step (4, 5) resulting in the weakly ad-sorbed carboxyl adradial, COOH, and H +. The car-boxyl adradial is bonded to ~ne electrode site and may then either be electro-oxidized directly to CO2 and H + or react with other adsorbed species on the platinum surface and transform into organic poisons. These poisons inhibit further oxidation of formic acid by blocking those reaction sites required in the initial stage of the oxidation. The similarity between adsorption products produced from formic acid and methanol oxidation under potentiodynamic (4) and potentiostatic (6, 7) control, and that the initial interaction of CH3OH with an activated electrode results in the loss of the three hydrogens on the carbon atom (8), led Capon and Parsons to deduce that the chemical composition of the major poisoning species in formic acid oxidation is COH, which is strongly adsorbed at three adjacent electrode sites. A minor poisoning species, with the structure C(OH)e occupying two adjacent sites, has also been suggested as being involved in the inhibition process (4). A detailed review of the work in acidic media prior to 1973 (9) and the interpretation of the cyclic voltammograms on noble metals (10) are available. Significant catalysis of the oxidation of formic acid is caused by the presence of submonolayer amounts of some foreign metal atoms, e.g., Hg (11), T1, Cd, Bi, and Pb (12). These submonolayers form by the underpo-tential deposition (UPD) on platinum and other noble metals. Also, other strongly adsorbed species, such as SH-, CH~CN, etc., catalyze the electro-oxidation of formic acid (11). UPD of Cu and Ag on platinum inhibit this reaction (12). Two theories are currently used to explain the catalytic effect of UPD monolayers: (1) UPD prevents hydrogen adsorption on platinum and thereby interferes with COH formation (12, 13); and (ii) the UPD species f't~nctions as a so-called * Electrochemical Society Active Member, Key words: surface coverage, form~:c acid oxidation, electro-eataly~l~ underpotential deposition, platinum active sites, third body e~ect. "third body" which diminishes the probability of an encounter between two adsorbed species that results in the formatio ~ of poisons (11, 14). A similar process is believed to decrease the hydrogen evolution rate along with UPD of copper (15). The poisoning of the hydrogen evolution process by species such as R-CN, S, and SH-is also explained through this effect. Thus, the word "catalysis" is used in the context of formic acid electro-oxidation in the sense of preventing a decrease in the initially rapid rate, rather than increasing the initial rate at an activated electrode. In this work, the rotating ring-disk electrode (RRDE) technique was applied to study the effect of varying UPD submonolayer coverages of lead and bismuth on the catalysis of formic acid electro-oxidation at platinum electrodes in acidic media. Concentrations of formic acid in the range of 10-6 to 10-~M were used. Under these conditions, mass-transfer limited currents could be achieved that not only allowed the study of the catalytic effects in a simple manner, but also made possible the control of the rate of electrode poisoning during the establishment of the UPD adatom equilibrium coverage isotherm. The electrocatalysis by UPD lead and bismuth and the inhibition effects of UPD copper and silver are explained in terms of platinum site preference on Pt(100) and Pt(lll) of the UPD species and the formic acid oxidation reactions .