FORMATION AND DISSIPATION OF ADLAYER DOMAINS DURING ELECTROOXIDATION OF CARBON-MONOXIDE ON ORDERED MONOCRYSTALLINE AND DISORDERED PLATINUM AND RHODIUM SURFACES AS PROBED BY FTIR SPECTROSCOPY

The stability of adlayer domains ("islands") of carbon monoxide formed during electrooxidation on ordered and disordered monocrystalline platinum and rhodium electrodes in aqueous 0.1M HClO4 has been characterized by means of real-time surface FTIR spectroscopy. The presence of such domains is discerned in part from the retention of high-frequency C-O stretching (nu-CO) bands for both terminal and twofold bridging CO, characteristic of close-packed adlayer structures, during electrooxidative CO removal down to low average coverages, theta. Further evidence for the presence of stable adlayer domains on ordered Pt(111) and Pt(100) was obtained by sequential dosing and partial electrooxidation of (CO)-C-13 followed by (CO)-C-12 dosing. The dipole-dipole coupling properties of such adlayers indicate partial segregation of the isotopic forms. Subsequent rapid electrooxidation of these mixed isotope layers yields initially preferential formation of (CO2)-C-12, pointing to the presence of reactive domain regions on the ordered surfaces. While the adlayer domains are stable for extended periods (ca. 1 h) on most ordered monocrystalline surfaces, they undergo relaxation on Pt(100) as discerned from decreases in the nu-CO frequency with time at a fixed-theta as well as with decreasing coverage. The dissipation of adlayer structures on disordered monocrystalline Pt and Rh was also observed during slow potentiostatic oxidation from theta-induced nu-CO frequency decreases, which become progressively greater for longer oxidation timescales. Similar findings were also obtained for CO on both mechanically polished and annealed polycrystalline platinum; domain dissipation is especially facile on the former surface. Such electrooxidation-induced CO domain dissipation is attributed to the nucleation of the reaction front at surface defect sites. The stability of adlayer domains during electrooxidation on most ordered surfaces, and for short reaction timescales on disordered surfaces, is ascribed to a lower density of nucleation sites operative under these conditions.