(77e) Challenges with Model Catalysts

Anchoring of molecular catalysts to solid supports has long offered the prospect of creating heterogeneous catalysts with uniform sites. We have explored over many years the physical, chemical and electronic properties of polyoxometalates – a class of discrete molecular metal oxide catalysts – that are well known for acid and oxidation catalysis. Studies in which these species were dispersed on surfaces of graphite, graphene, nitrogen-containing carbons, and conventional catalyst supports such as silica, have generally shown that they behave as discrete molecules with respect to electronic and chemical properties. We have also shown that spectroscopic techniques can provide useful correlation and prediction of catalyst redox properties. However, our recent studies of catalytic reactions suggest that there are cooperative effects in surface arrays of these molecules, leading to turnover rates that depend on surface concentrations of active sites. These challenge notions about single site catalysts and about appropriate ways to describe their activity.