State of the pre-adsorbed sulfur on a rough platinum electrode in voltammetric conditions: Microgravimetric measurements with electrochemical quartz crystal microbalance

The removal of the chemisorbed sulfur from the surface of the rough Pt electrode was investigated by combined coulometric and gravimetric measurements using the cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCMB) methods. Pt surface was covered with elemental sulfur in 1 M solutions of Na2S and tiourea (TU) at open circuit potential, for a period between 60 s and 300 s. CV was performed in 0.1 M HClO4, 0.1 M NaOH and 1 M NaOH solutions, while EQCM in 1 M NaOH solution, in the air, at 296 K. The balance of the oxidation reaction was calculated with the anodic and cathodic charges involved in CV and the mass variation of the rough Pt electrode through the frequency shift of the piezoelectric sensor. Discrepancies in the mass balance calculated by coulometry and microgravimetry were ascribed to the complexity of the removal mechanism. By each cycle, S is removed from the surface as SO3 in two stages: a fraction in the anodic scan, simultaneous with the oxidation process (faradaic process) and a remaining fraction in the cathodic scan, by electro-desorption (non-faradaic process). The whole process is biased by the potential of the electrode (the electrical field at the interface). The balance can be thoroughly equilibrated by invoking a “hidden” process embedded in the cathodic scan, accountable for the non-faradaic removal of additional mass of sulfur. This process could be caused by the adsorbate–adsorbate (S*/SO3* or S*/O*) interaction and is not detectable in CV measurements. Consequently, the S coverage calculated in CV by anodic and cathodic charge balance yields lower values than the real chemisorbed amount (the symbol * represents the adsorbed state of the element).