Electrooxidation of CO and H2/CO Mixtures on Pt(111) in Acid Solutions

Electrocatalysis of CO oxidation and the interfacial structure of the CO adlayer (COad) on the Pt(111) surface in 0.5 M H2SO4 were examined by using the rotating disk electrode method in combination with in situ surface X-ray diffraction measurements. The results presented here elucidate the roles played by two different forms of COad: one which is oxidized at lower overpotentials, in the so-called preoxidation region, we characterize as a weakly adsorbed state (COad.w), and a strongly adsorbed state (COad,s) which is oxidized at higher overpotentials. The COad,w state forms at saturation coverage by adsorption at E < 0.15 V and assumes a compressed p(2 × 2) structure containing 3 CO molecules in the unit cell (0.75 CO/Pt). We propose that oxidative removal of COad,w is accompanied by simultaneous relaxation of the CO adlayer, and that the remaining COad (≈0.6 CO/Pt) assumes a new bonding state which we identify as COad,s. The COad,s state is present in a structure lacking long-range order. Despite the reduced coverage by COad, H2 electrooxidation is still completely poisoned at potentials below 0.6 V. The electrooxidation of CO in solution is proposed to occur via the COad,w state at 0.6−0.8 V, and via the COad,s at higher potentials.