Surface Defects as Ingredients That Can Improve or Inhibit the Pathways for CO Oxidation at Low Overpotentials Using Pt(111)-Type Catalysts

All scientific reports regarding the electro-oxidation reaction of CO on Pt electrodes, without exception, show that this reaction is favored on Pt surfaces rich in defects. In fact, ordinary or simple CO preoxidation is a typical process of surface-defected catalysts obeying the conditions of a full COads layer formed at suitably low potentials, such as those of the hydrogen under-potential deposition region. Among the Pt(111)-type surfaces, nondefected Pt(111) has the lowest catalytic activity for CO electro-oxidation. As a novelty, this paper reports that an unusual CO preoxidation appears on a nondefected Pt(111) surface, depending on the COads layer preparation, that is, for a COads layer prepared by cooling the flame-annealed Pt(111) electrode in a CO atmosphere. The magnitude of this unusual CO preoxidation decreases as the Pt surface becomes rich in steps (defects), and using stepped surfaces, this unusual stage of CO electro-oxidation at low overpotentials is revealed to be connected with a long-range order on the (111) plane. Interestingly, both ordinary and unusual CO preoxidations take place on the (111) terrace domains, but only ordinary CO preoxidation is favored on surfaces rich in defects. We propose a mechanism of indirect participation of defects in catalysis, according to which, at low overpotentials, defects on the surface act as ingredients that can improve or inhibit the pathways for the CO electro-oxidation reaction but do not serve as the most active sites themselves. Therefore, at least on extended platinum surfaces, there is no general condition whereby surface defects always favor the CO electro-oxidation reaction. From experiments at temperatures ranging from 283 to 313 K, the activation energies for the unusual CO preoxidation and the main CO oxidation processes were about 108 and 117 kJ mol(-1), respectively. Furthermore, it was found that the increase of the CO stripping temperature caused a proportionally greater reduction of charge density of the CO electro-oxidation processes at low overpotentials, that is, the unusual CO preoxidation, in comparison with the charge density of the oxidation of CO at the main peak (at higher overpotentials).