Structure and reactivity of transition-metal oxide monolayers

Abstract

Transition-metal oxides supported on and chemically interacting with the surface of SiO₂, Al₂O₃, TiO₂etc. form discrete monolayers of species unlike those found at the surface of the unsupported oxides, being in effect two-dimensional compounds. Areas of their application include alkene metathesis and polymerisation, selective reduction of NO by NH₃, and selective oxidation of aromatics. They are readily detected by Raman spectroscopy, FTIR and EXAFS/XANES, and thermal methods and XPS assist their characterisation. They can exhibit strong acidic character, mainly of the Brønsted type; whereas isopropyl alcohol decomposition on MoO₃TiO₂ gives chiefly dehydration, the main product over V₂O₅/TiO₂ is acetone. The unique ability of the latter system to catalyse the selective oxidation of o-xylene to phthalic anhydride is attributed to the ability of the surface species to undergo oxidative addition at VO groups and to effect the elimination of H atoms in adsorbed radicals as water. When the amount of supported oxide exceeds the monolayer capacity, two further forms of the oxide are detectable, both differing in their reactivity from that of the bulk oxide.