A NEW HIGH-DENSITY CO OXYGEN COADSORBATE LAYER ON PT(111) AND ITS ROLE IN CO OXIDATION/

A high density CO/oxygen coadsorbate layer with maximal composition 2C O + O can be prepared by exposing the O(2 x 2)/Pt(111) layer to large CO doses (up to 10(5) L). It has been studied by HREELS, TPD and LEED. At 120 K all CO molecules are adsorbed in linear form, irrespective o f the coverage, showing another case of noncorrelation of adsorption s ite and internal CO stretch frequency. Heating to 260 K, i.e. in the r ange of incipient CO oxidation, leads to partial conversion of CO into two higher coordinated states with v(C-O) at 1750 and 1850 cm(-1). Wh ile only one well known CO oxidation channel between 330 and 360 K is observed for CO coverages below Theta(CO)=0.25 ML, a new CO oxidation channel at 290-300 K opens up for higher Theta(CO).