An NMR study of diffusion of CO on metal surfaces

The author reports the study of diffusion of carbon monoxide on supported palladium and platinum clusters by Nuclear Magnetic Resonance. As part of these studies, a novel NMR technique for studying diffusion (called an S-shape sequence) has been implemented. He finds that the CO adlayer on Pd may be present in one of two phases, a low and a high temperature phase. The transition temperature from one phase to the other depends on cluster size and has been expressed by a phase diagram. In the low temperature phase CO is bridge bonded to large clusters though on small clusters the bonding appears to be linear. In this phase, the packing of the CO adlayer on large clusters is equivalent to the one of CO on Pd (111) surfaces whereas on small clusters the packing is less dense. The CO diffusion rate in this phase has been measured by the Sshape method. For the high temperature phase CO is much more mobile than for the low temperature phase as is evidenced by motional narrowing of the NMR line. At a given temperature, the diffusion rate in this phase is faster on larger clusters. Using the S-shape method to study diffusion of CO on Pt, the author has found that the barrier for diffusion is larger for small clusters (11 kcal/mol) than for large ones (6 kcal/mol) . The value for the large clusters agrees with results in the literature for single crystal surfaces. Reducing the CO coverage increases the diffusion rate, as would be the case if diffusion went by a CO vacancy mechanism. These experiments suggest that below the motional narrowing regime the CO molecules that are bridge bonded are diffusing while the linearly bonded remain fixed. Thus there are two interpenetrating lattices, one of mobile CO's the other of fixed CO's.U of I OnlyThesi