Study of the Formic Acid Electrooxidation on Rhodium on Steady State Using a Flow Cell: Potential Dependence of the CO Coverage

Abstract

The HCOOH electrooxidation reaction (FAO) was experimentally studied on rhodium electrode using a flow cell in a 0.5-M HCOOH + 0.5-M H2SO4 solution. It was obtained the steady-state current-potential curve by chronoamperometry, which demonstrated that the FAO is verified in the potential region 0.45 < E/V < 0.70. The amount of adsorbed CO on the electrode surface as a function of potential was quantified by stripping voltammetry after changing to a 0.50-M H2SO4 solution at closed electric circuit. It was established that the spontaneous dissociative adsorption of HCOOH on rhodium produces irreversibly adsorbed CO only for potential values E ≤ 0.40 V, and therefore, the FAO takes place in the absence of COad. These results can explain the current-potential profiles obtained when the FAO is studied by cyclic voltammetry at different values of the potential scan limits, where the behaviour is strongly influenced by the inhibition of reaction sites due to the adsorbed CO. A kinetic mechanism for the FAO on rhodium is proposed on the basis of the results obtained, which are also in agreement with recently published spectroscopic measurements.