An EQCM study of adsorption and oxidation of glycine at a Pt electrode in acid and alkaline solutions

Adsorption and oxidation of glycine at a Pt electrode were studied by using electrochemical quartz crystal microbalance(EQCM) in both acid and alkaline solutions. The results demonstrate that the behavior of adsorption and oxidation of glycine depends strongly on the acidity of solutions. The adsorption of glycine is weak in acid solution. However, glycine can be decomposed in alkaline solution to produce adsorbates that are stable on Pt surface below 0.0 V(vs. SCE). It has been revealed that the activity of glycine oxidation is higher in alkaline solutions than that in acid solutions. The surface mass change(Δm) and the average molar mass(M/n) of all adsorbates on Pt electrode surface per electron transferred in the above processes have been determined quantitatively through EQCM. An increase of 58.6 ng/cm 2 in Δm was detected in acid solutions for the process of hydrogen desorption from -0.25 to 0.11 V(vs. SCE) at Pt electrode, which was ascribed to the replacement of adsorbed hydrogen by the adsorption of water molecules or/and glycine anions. The EQCM frequency change in the oxygen region(between 0.35 and 1^20 V) was mainly caused by the adsorption of oxygen, the oxidation of adsorbed species and the oxidation of glycine, which resulted in an increase of 43.4 ng/cm 2 in the surface total mass change. This value is smaller than that(61.2 ng/cm 2) acquired in a sulfuric acid solution free of glycine in the same potential region. In the case of alkaline solutions, the surface mass change of Pt electrode is related directly to the dissociative adsorption of glycine. When the upper limit of potential scan was set at 0.60 V, an increase in surface mass of 17.6 ng/cm 2 in the hydrogen region and an increase in surface mass of 43.0 ng/cm 2 in the oxygen region at Pt electrode have been determined.