Many useful compounds are produced by the reactions of hydrocarbons with oxygen, a conversion that is catalyzed by metal oxides.  The process operates via the transfer of "lattice" oxygen to the hydrocarbon substrate, a step that temporarily generates a vacancy (or defect).  In a subsequent step, the missing oxygen is replenished by O. Such catalysts rely on the ability of the metal oxide to form phases that are not stoichiometric.  An analogous sequence of events describes other kinds of atom-transfer reactions including hydrogenation and hydrodesulfurization catalysed by solid catalysts.  These considerations also highlight the fact that stoichiometry is determined by the interior of crystals: the surfaces of crystals often do not follow the stoichiometry of the bulk.  The complex structures on surfaces are described by the term "surface reconstruction".