Surface electrochemistry of CO on Pt(111): anion effects

In situ studies of CO adsorption by surface X-ray scattering and Fourier transform infrared (FTIR) spectroscopy techniques are used to create the link between the macroscopic kinetic rates of CO oxidation and the microscopic level of understanding the structure/site occupancy of CO on Pt(1 1 1). A remarkable difference in activity is observed between alkaline and acid solutions. In alkaline solution the oxidation of CO occurs at low overpotential (<0.2 V) by the surface reaction between the adsorbed CO and OH, the latter forming selectively in the hydrogen underpotential potential region at defect sites. In acid solution these sites are blocked by specific adsorption of anions, and consequently in a solution containing Br- the ignition potential is shifted positively by 0.6 V. Anions of supporting electrolytes also have dramatic effects on both the potential range of stability and the domain size of the p(2,x 2)-3CO structure which is formed at 0.05 V. The stability/domain size of this structure increases from KOH (approximate to:30 Angstrom between 0.05 < E < 0.3 V), to HClO4 (approximate to 140Angstrom between 0.05 < E < 0.6 V) to HClO4 + Br- (approximate to 350 Angstrom between 0.05 < E < 0.8 V). The larger the ordered domains of the p(2 x 2)-COad structure, the less active the surface is towards CO oxidation. (C) 2001 Elsevier Science B.V. All rights reserved.