CO adsorbs upside-down on small PtmAun clusters

The DFT results predicted that CO upside-down adsorption can take place on the PtmAun clusters with m, n = 0, 4 but not on the bulk surfaces such as Pt(1 1 1), Pt0.25Au0.75(1 1 1), Au(1 1 1). As the circled data in the picture have indicated, one may detect the CO upside-down adsorption experimentally by using combined FTIR and TPD techniques.