Mechanistic Insight into Formic Acid/Formate Oxidation at the Au(111) Electrode: Implications from the pH Effect and H/D Kinetic Isotope Effect

Formate adsorption and formic acid oxidation reaction (FAOR) on Au(111) in solutions with 0 < pH < 14 with either HCOOH/HCOO– or DCOOH/DCOO– as precursors have been systematically investigated by cyclic voltammetry. The results are analyzed with the help of computer simulation and careful comparison with similar data obtained at Pt(111). We found that (i) the pH at the electrode/electrolyte interface (pHs) drops to ca. 6 during FAOR in solutions with 6.5 < pH < 10, and this leads to apparently higher FAOR currents; (ii) after correcting the reaction-induced pHs shift, the peak current (j p) for FAOR at Au(111) displays an intrinsic volcano-shaped pHs dependence; j p first increases with pHs up to 5 and remains a maximum plateau at 5 < pHs < 6, and then it decreases with a further increase of pHs; (iii) the H/D kinetic isotope effect factors for HCOOH/DCOOH are ca. 5 in all solutions with 0 < pH < 10.5; (iv) a mechanism with HCOO– ⇌ COOad λ– + H+ + (2 – λ)­e– as the rate-determining step with proper consideration of the electric double layer (EDL) effect can simulate well all of the data for FAOR on Au(111); and (v) the values of the electrostatic factor β used for describing the EDL effect, which fit well the data for FAOR on both Pt(111) and Au(111), are nearly the same. This can be rationalized by the fact that the EDL effect comes from the electrostatic interaction between the charge adsorbate and electrode, which is just the function of the charge of the adsorbate and the parameters of EDL, such as the capacitance of the inner and outer Helmholtz layers. The different pH dependences and H/D kinetic isotope effects for FAOR at Au(111) and Pt(111), as well as the reasons why other previously proposed mechanisms for FAOR at Au(111) are unlikely, are also briefly discussed.