REVERSIBLE REDOX CHEMISTRY, HYDRODESULFURIZATION, AND ANODIC-OXIDATION OF THIOPHENOLS CHEMISORBED AT SMOOTH POLYCRYSTALLINE IRIDIUM ELECTRODES

The reversible redox chemistry, electrocatalytic hydrodesulfurization, and anodic oxidation of thiophenols chemisorbed at smooth polycrystalline iridium electrodes have been investigated. The experimental measurements were based upon thin-layer electrochemical methods. 2,5-Dihydroxy-thiophenol (DHT), 2,5-dihydroxy-4-methylbenzylmercaptan (DHMBM), and pentafluorothiophenol (PFT) were used as the model compounds. The principal findings of this study are as follows: (i) The surface coverages of DHT, DHMBM, and PFT are consistently lower on Ir than on Pt or Au. (ii) Whereas surface-chelated chemisorption occurs for DHT on Ir which explains its lower coverage and electrochemical inactivity, no such mode of binding is evident for DHMBM and PFT: Pentafluorobenzene itself is inert towards Ir and a sharp quinone/diphenol redox is displayed by DHMBM which indicates that its diphenolic moiety is pendant. The decreased surface packing densities suggest that the strength of the metal-thiolate interaction is weaker on Ir than on Pt, especially at higher coverages, (iii) All three thiophenols undergo hydrodesulfurization (HDS), the extent of which increases in the order: DHMBM (15%) < DHT (55%) < PFT (100%). The same trend has been observed at Pt electrodes, (iv) Although the diphenolic group in DHT is directly bonded to the Ir surface, there is only minimal ring hydrogenation accompanying the HDS reaction, (v) The anodic oxidation of the subject thiophenols at Ir is similar to that at Pt. Specially, PFT is totally resistant to anodic oxidation even at potentials beyond the oxygen evolution reaction. © 1990.