Heterogeneous catalysis by metals

Catalysis by metals will be reviewed in this paper because of the progress in this sub-field of heterogeneous catalysis since 1965. Preparation of supported metals has become reproducible under controlled conditions, and reaction rates can now be measured in the absence of limitations by mass and heat transfer. With correct rates obtainable on reproducible catalysts, it is increasingly common to report rates not as an ill-defined ‘activity’ but as turnover rates, i.e., molecules reacting per second per surface metal atom titrated by standardized methods of selective chemisorption. Turnover rates for a number of reactions on several metals have been reproduced by different investigators using different preparations. Agreement has been found between turnover rates on catalysts with 100% of metal exposed and on large single crystals. Hence, for reactions that are called structure-insensitive, no bulk metal (subsurface atoms) is needed to obtain the surface behavior of bulk samples. It is also clear that metalsupport interactions can be totally absent. For other reactions, i.e. those which are structure-sensitive, certain sites consisting of ensembles of surface atoms exhibit enhanced turnover rates. Identifying and counting these metallic sites are challenging tasks that have been tackled successfully only in the case of the synthesis of ammonia on iron.