STUDY BY UV VISIBLE POTENTIAL-MODULATED REFLECTANCE SPECTROSCOPY AND CYCLIC VOLTAMMETRY OF THE ELECTROADSORPTION AND ELECTROOXIDATION OF CO ON AU IN ALKALINE MEDIA

The stationary curve of the electro-oxidation of CO on Au in 1 M NaOH shows a wide plateau between 0.4 and 1.3 V vs. RHE, whose height and width are almost unaffected by changes in the potential at which CO is admitted to the cell. The current at the plateau was only 40% of the calculated diffusion-limited current. Chemisorption of CO on Au in 1 M NaOH takes place at potentials lower than 0.3 V vs. RHE, this chemisorbed CO being oxidized at potentials greater than 0.8 V. As already reported (J.A. Caram and C. Gutierrez, J. Electroanal. Chem., 306 (1991) 301), potential-modulated reflectance (PMR) spectra of this chemisorption system showed a maximum at 330 nm and a bipolar band centred at 650 nm. The energy of the maximum increased from 3.45 to 3.95 eV when the potential was increased from -0.25 to +0.57 V. This maximum was attributed, following our previous work (J.A. Caram and C. Gutierrez, J. Electroanal. Chem., 291 (1990) 289), to CO located in a hollow surface site, i.e. bonded to three surface Au atoms. Since the transition energy increased linearly with increasing positive potential, we believe that the transition corresponds to the excitation of an electron from a filled state in the metal to an empty state in the CO molecule. As for the bipolar band in the visible range, the energy of its centre decreased from 2.0 to 1.75 eV with increasing potential. We also attribute this bipolar band to a charge-transfer transition, but from a filled CO level to an empty level in the metal.