Anomalous diamagnetic susceptibility in 13-atom platinum nanocluster superatoms.

We are used to being able to predict diamagnetic susceptibilities χD to a good approximation in atomic increments since there is normally little dependence on the chemical environment. Surprisingly, we find from SQUID magnetization measurements that the χD per Pt atom of zeolite-supported Pt13 nanoclusters exceeds that of Pt(2+) ions by a factor of 37-50. The observation verifies an earlier theoretical prediction. The phenomenon can be understood nearly quantitatively on the basis of a simple expression for diamagnetic susceptibility and the superatom nature of the 13-atom near-spherical cluster. The two main contributions come from ring currents in the delocalized hydride shell and from cluster molecular orbitals hosting the Pt 5d and Pt 6s electrons.

Internal concentration gradients of guest molecules in nanoporous host materials: measurement and microscopic analysis.

Evolution of internal concentration profiles of methanol in 2-D pore structure of ferrierite crystal was measured in the pressure range of 0 to 80 mbar with the help of the recently developed interference microscopy technique. The measured profiles showed that both a surface barrier and internal diffusion controlled the kinetics of adsorption/desorption. Furthermore, they indicated that in the main part of the crystal, the z-directional 10-ring channels were not accessible to methanol and that the transport of methanol mainly occurred via 8-ring y-directional channels. The roof-like part of the crystal was almost instantaneously filled/emptied during adsorption/desorption, indicating accessible 10-ring channels in this section. The measured profiles were analyzed microscopically with the direct application of Fick's second law, and the transport diffusivity of methanol in ferrierite was determined as a function of adsorbed phase concentration. The transport diffusivity varied by more than 2 orders of magnitude over the investigated pressure range. Transport diffusivities, calculated from measured profiles from small and large pressure step changes, were all found to be consistent. Simulated concentration profiles obtained from the solution of Fick's second law with the calculated functional dependence of diffusivities on concentration compared very well with the measured concentration profiles, indicating validity and consistency of the measured data and the calculated diffusivities. The results indicate the importance of measuring the evolution of concentration profiles as this information is vital in determining (1) the direction of internal transport, (2) the presence of internal structural defects, and (3) surface/internal transport barriers. Such detailed information is available neither from common macroscopic methods since, they measure changes in macroscopic properties and use model assumptions to predict the concentration profiles inside, nor from microscopic methods, since they only provide information on average displacement of diffusing molecules.