Fifty years of my romance with vanadium oxide catalysts

The fifties of XXth century were times when solid state physics and chemistry scored great successes both experimental and theoretical – transistors were discovered and the theory of electronic structure of solids made rapid progress. It was believed that this theory will permit the understanding of the mechanism of chemical reactions at metal and oxide surfaces and electronic theory of catalysis was developed. Vanadium oxides were a good example to be studied, because extremely rich chemistry of vanadium oxides and vanadium oxide based catalysts results from a number of different interrelated electronic and structural factors. These compounds have partially filled d-orbitals which are responsible for a wide variety of electronic, magnetic and catalytic properties. The phase diagram shows that vanadium atoms exist in different formal oxidation states, which vary from two to five. The easy conversion between oxides of different stoichiometry and formation of oxygen vacancies enables the oxide to function as catalyst in selective oxidation. Exposed to gas phase of sufficient redox potential reduction and reconstruction of the surface into V6O13 phase may take place. Due to the anisotropy of crystallites different crystal planes have different adsorption properties and different catalytic active sites are present. Reducibility depends on the degree of dispersion. When deposited on other oxide supports their wetting is observed and surface migration from one support onto another one may take place depending on the surface free energy ratio. On exposure to water vapour, the vanadium monolayer transforms into polyanions. Depending on the type of support vanadium oxide shows various catalytic properties. The quantum-chemical calculations show that nonbonding d-orbitals of vanadium ions have the LUMO character and act as Lewis acid sites, whereas the lone electron pairs of bridging oxygen ions have the HOMO character and behave as Lewis basic sites. Cleavage along (1 0 0) planes leaves coordinatively unsaturated vanadium and oxygen ions which develop Brønsted acid–base interactions with reacting molecules and cause their heterolytic chemisorption. On the basis of the results collected in the last 50 years a book on physical chemistry of vanadium–oxygen system could be written.