Unique Dynamics of Water Confined on the Surface of Metal Oxide Clusters.

Water confined on metal oxide surface plays significant roles in heterogeneous catalysis. Heteropolyacid, a 1.2 nm-metal oxide cluster with well-defined structure, is applied as a model to understand the dynamics of water on its surface. The surface water strongly associates with heteropolyacid cluster and form the so-called 'pseudoliquid phase' where catalytic reactions are conducted. Broadband dielectric spectroscopy and differential scanning calorimetry have been applied to probe the dynamics of water in this pseudoliquid phase. A supercooling phase transition of water below its normal melting temperature and a dipolar glassy relaxation behaviour due to the hindered dynamics of water have been observed. The rich dynamic behavior on the surface of such well-defined metal clusters provide new perspectives to understand the properties of surface water and their relation to catalytic performance of heteropolyacid.