Electro-oxidation of l-ascorbic acid on platinum in acid solutions: an in-situ FTIRRAS study

Studies of in-situ Fourier transform infrared reflection-absorption spectroscopy (FTIRRAS) showed that l-ascorbic acid (AA) undergoes a spontaneous dissociative or destructive adsorption at a Pt electrode in acid solutions at the hydrogen adsorption-desorption region with linearly bonded carbon monoxide (CO)_L as the adsorbate. This dissociative adsorption process most likely takes place via an interaction between the side-chain of AA and the Pt surface. The overall electro-oxidation of AA at Pt in acid solutions may involve a contribution from its ethylene glycol (EG)-like side-chain portion as well as its lactone ring portion. This process may consist of three major stages: (1) dissociative adsorption via the EG-like side-chain to form (CO)_L on Pt at the hydrogen adsorption-desorption region; (2) direct oxidation via the lactone ring to form dehydro-l-ascorbic acid (DHA) and its hydrated derivatives in the potential region from the onset of oxidation to the current peak; (3) the EG-like side-chain and (CO)_L undergo further oxidation to form CO₂ as the final product when the potential is driven to a more positive region. This study indicates that AA undergoes a spontaneous dissociative adsorption at the Pt electrode in acid solution prior to its direct oxidation. This dissociative adsorption probably takes place via an interaction between the side-chain of the AA molecule and the Pt surface, with (CO)_L as the adsorbate. The overall electro-oxidation of AA at Pt in acid solutions may consist of three major stages: (1) dissociative adsorption; (2) direct oxidation via the lactone ring to form DHA and its hydrated derivatives in the potential-region from the onset of oxidation to the current peak; (3) further oxidation via side-chain and (CO)_L to form CO₂ as the final product when the potential is driven to a more positive region.