NMR EVIDENCE OF TEMPERATURE-DEPENDENT STRUCTURAL-CHANGES OF THE CO ADLAYER ON SUPPORTED PD CLUSTERS

C-13 NMR studies of (CO)-C-13 adsorbed on supported Pd clusters are reported. The C-13 NMR spectrum consists of two lines, each line reflecting a particular bonding site (bridge or on-top) of the CO molecules. The relative CO population of the two sites varies with temperature and cluster size, corresponding to differing arrangements (ordered arrays) of the CO's on the surface. In going from a low-temperature arrangement to the high-temperature one, a massive rearrangement of bonding sites must occur. The structure of the low-temperature arrangement depends on cluster size: on small clusters the CO is bonded mainly to on-top sites, but on large clusters is bridge bonded. In the high-temperature arrangement, the CO is bridge bonded.