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 2CO + 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 of the coverage, showing another case of noncorrelation of adsorption site and internal CO stretch frequency. Heating to 260 K, i.e. in the range 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). While 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).