Experimental and theoretical studies of small clusters of metal atoms are aimed at revealing how properties change in the ultrafinely divided state. Thermodynamic, electronic, and structural features can be size-dependent and may be involved in determining the activity of small-particle catalysts. Correlations between these properties give indications of the features important in catalysis.

Pre-exponential factors for surface reactions have been estimated by use of transition state and hard sphere reaction models. We find that calculated values can be used to determine rate limiting steps in various surface reactions. In the case of H 2-D 2 exchange, adsorption is the• rate limiting step at high temperature while bond breakage becomes rate limiting at lm'l temperature. Typical. pseudo-first order pre-exponential factors for various rate limiting steps are: adsorption 10 2-10 4 sec~1 , diffusion 10 7-1011 sec-1 , surface react~on 1o 10-1o 13 sec-1 , desorption 1o 13-1o 16 sec-1 • The calculated values are compared to'numerous examples of surface reactions compiled from the literature. In addition, conditions heeded to apply these roodels to catalyzed reactions are discussed.

We present the results of calculations of the {[FeCl6-n(CN)n]m-×(VNa)5-m}5- impurity complex incorporated in the NaCl crystal lattice. A number of characteristic defect configurations are calculated quantum mechanically using an embedded cluster method, and compared with the results of Mott-Littleton (ML) calculations. We investigate the electron affinity and the orientation of CN- ligands. The dependence of vibrational frequencies of the CN stretching mode on the ligand orientation, charge of the impurity and the relative position of the ligands and the charge compensating vacancies are calculated. Analysis of these results demonstrates the ability of ML calculations to predict the relative stability of different configurations of this particular family of defects. Comparison of the ML and embedded cluster calculations reveals the contributions to the defect energy and stability.

AgBr microcrystals that have been treated with reducing agents to form small silver clusters on their surface have been examined with low-temperature optical and optically detected magnetic resonance methods. These investigations have indicated that these treatments produce two closely related Ag clusters that are spectroscopically active. One of the centers manifests itself by low-temperature emission bands at 550 and 640 nm and an absorption band at 442 nm. The other center, which absorbs at 430 nm, acts as a surface hole trap that completely changes the nature of the AgBr donor acceptor recombination. These centers are only observable on crystals with {111} faces. Spectroscopic and theoretical considerations suggest that these spectroscopically prominent centers are silver dimers in two different surface configurations.