Electron exchange processes in olefin oxidation

Abstract

A new experimental technique is described for measurement of the relative Fermi energy of various catalysts used for partial oxidation of olefins and alcohols. The technique is based on a measurement of the conductance of a semiconducting support (TiO 2). For a number of catalyst compositions of industrial importance (in terms of selectivity and activity) the bulk Fermi energy is found to have a common value. This recurring value is near the electrochemical potential of electrons on adsorbed oxygen. Catalyst systems studied were the bismuth/molybdenum, copper oxide, vanadium oxide and iron/molybdenum systems. An attempt is made to separate the macroscopic requirements (where the Fermi energy is considered a macroscopic parameter) from the microscopic ones (where local bonding orbitals and acid centers may be considered microscopic parameters). We conclude that for reactions for which the rate-limiting step depends on electron transfer to the catalyst that the Fermi energy should be near or just above the electron exchange level for oxygen. Also the bulk Fermi energy should be stable against small variations in reactant gas compositions. Impurity band pinning of the Fermi energy may account for the excellent stability of bismuth molybdate.