Electrochemistry of mixed-metal interfaces: Chemisorption and electrochemical reactivity of thiophenols at Au 90Pt 10 electrodes

Abstract

The surface electrochemical properties of thiophenolic compounds chemisorbed at a smooth polycrystalline Au 90Pt 10 alloy electrode have been studied in comparison with those at the pure monometal electrodes. 2,5-Dihydroxythiophenol (DHT) and pentafluorothiophenol (PFT) were used as model compounds since these are strongly chemisorbed on Au and Pt, and their electrochemical reactivities [reversible redox activity, hydrodesulfurization (HDS), and anodic oxidation] in the chemisorbed state depend strongly upon the composition of the electrode surface. Experimental measurements were based on thin-layer electrochemical methods. The major findings are as follows: (i) The packing density and the mode of binding (through the -S group) of DHT and PFT at Au 90Pt 10 are the same as at the pure Pt and Au electrodes, (ii) Substrate-mediated DHT-DHT interactions which occur on Pt but not on Au are promoted, albeit to a small extent, at the alloy interface, (iii) The extent of HDS is greater at Au 90Pt 10 than at Au but much lower than at Pt. (iv) HDS of DHT at Au 90Pt 10 unlike that at Pt, is not accompanied by hydrogenation and/or hydrogenolysis of the pendant aromatic ring; that is, HDS at the alloy surface is selective towards simple cleavage of the C-S bond. (v) Irreversible oxidation of DHT chemisorbed at Au 90Pt 10 yields two distinct anodic peaks; the first appears to be associated with oxidation involving the Pt surface sites, whereas the second is suggestive of oxidation involving Au surface atoms, (vi) Chemisorbed PFT, which on pure Pt is inert towards anodic oxidation, undergoes irreversible oxidation at Au 90Pt 10 similarly but not identically to that at pure Au. (vii) Based upon the separate Pt-oxide and Au-oxide reduction peaks, the surface composition of the Au 90Pt 10 alloy is the same as that in the bulk. (viii) On the unoxidized alloy surface, the distribution of the Au and Pt sites is predominantly homogeneous, (ix) Phase separation occurs after the alloy surface is oxided; reduction of the surface oxide appears to restore the homogeneity of the bimetallic interface, (x) Under the conditions of the reactions studied, no surface roughening or segregation occurred at the alloy electrode. The present results reflect the complex interplay between the relative surface activities and catalytic reactivities of the Au and Pt sites at the Au 90Pt 10 interface.