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

Abstract

Electrocatalysis of CO oxidation and the interfacial structure of the CO adlayer (CO ad ) on the Pt(111) surface in 0.5 M H 2 SO 4 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 CO ad: one which is oxidized at lower overpotentials, in the so-called preoxidation region, we characterize as a weakly adsorbed state (CO ad.w ), and a strongly adsorbed state (CO ad,s ) which is oxidized at higher overpotentials. The CO ad,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 CO ad,w is accompanied by simultaneous relaxation of the CO adlayer, and that the remaining CO ad (≈0.6 CO/Pt) assumes a new bonding state which we identify as CO ad,s . The CO ad,s state is present in a structure lacking long-range order. Despite the reduced coverage by CO ad, H 2 electrooxidation is still completely poisoned at potentials below 0.6 V. The electrooxidation of CO in solution is proposed to occur via the CO ad,w state at 0.6−0.8 V, and via the CO ad,s at higher potentials.