Size effects in catalysis by metal nuclei

A method has been developed for determining the minimum size for a nucleus of metal atoms to achieve catalytic properties. Submonolayer vacuum deposits of the metal in question are used for this purpose. The deposits consist of a distribution of atomic clusters of various sizes, depending upon the coverage. To be useful, the size distributions in the deposits must be known, and these are determined by special techniques using electron microscopy. The nuclei are then employed as catalysts for reduction–oxidation chemical reactions analogous to photographic physical‐development processes. Minimum size is determined by comparison of data from counts of particles of the reaction product with the size–frequency calibration of the deposits. The significant changes in catalytic properties are found to occur in the size range of a few atoms. Attempts to relate these changes to the physical properties of the nuclei involve such approaches as XPS determination of the density of occupied states, spectroscopic investigation of the electronic transitions, and molecular orbital calculations of all of these properties.