(261c) Thermodynamic Activity Coefficients Describe the Catalytic Activity of Supported Polyoxometalates

Anchoring of molecular catalysts to solid supports has long offered the prospect of creating heterogeneous catalysts with uniform sites. Polyoxometalates – a class of discrete molecular metal oxide catalysts well known for acid and oxidation catalysis – have been widely explored in both supported and unsupported form, for both gas and liquid phase reactions. Spectroscopic studies in which these species were dispersed on support surfaces have generally shown that they behave as discrete molecules with respect to electronic and chemical properties. However, our studies of catalytic reactions with cation-exchanged phosphomolybdates (PMo₁₂O₄₀)^3- supported at submonolayer coverages on silica have demonstrated turnover rates for methanol dehydration and oxidation that depend on the extent of exchange of protons for other cations These results can be explained quantitatively in terms of the non-ideal thermodynamics of concentrated acid solutions, described by Harned’s rule, even though the reactions nominally involve gas-solid contact catalysis. This implies that the surface species are solvated under reaction conditions, and may have implications for the performance of other catalysts operating at modest temperatures in the presence of water.