An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes

In this work we have studied the electrochemical behaviour of a bismuth surface compound formed spontaneously without applying external potential when platinum is put in contact with a solution of a Bi(III) salt. When studied in a sulphuric acid solution, the surface compound undergoes a redox reaction where oxidized and reduced species are both adsorbed at the platinum surface. The redox reaction depends on the crystalline surface structure of the platinum substrate. The behaviour of the adsorbate at the platinum (111), (100), (110) and (332) faces as well as at the surface of a spherical platinum single crystal has been investigated. The redox reaction has been found to be highly reversible at the Pt (111) electrode and has been studied as a function of pH and sweep rate at this orientation. This reaction appears as a good probe for detection of (111) surface domains on the platinum surface, as shown by the results obtained with Pt (332), otherwise noted in the terrace-step notation as 5 (111) × (110). The results show also that steps may be blocked selectively by the bismuth adsorbate while terrace sites remain free for hydrogen adsorption. The stability of the bismuth surface compound at the various surface orientations has been investigated. The voltammetric behaviour of the surface compound at Pt (100) has been compared to the voltammetric UPD of bismuth. The comparison allows the assignment of peaks in the voltammogram of the UPD of bismuth at this orientation to a change in the valency of the surface bismuth compound without change in the coverage because the adsorbate is already present at the surface. A similar conclusion is valid for other orientations. The spontaneous deposition has been assumed to be due to an incipient underpotential deposition of bismuth at potentials higher than that of incipient Pt-OH formation. Three surface compounds have been identified at the surface from the reversible behaviour of the adsorbate at Pt (111): Biad at low potential, (BiOH)ad as an intermediate in the redox process and (BiO)ad or (Bi(OH)2)ad, the stable species, at higher potential.