Metal oxides excel at catalyzing gas phase reactions by photo-activation, as well as thermal activation of the catalyst. Oxidation of hydrocarbons, alcohols, carbon monoxide, and ammonia occurs when stimulated with light of greater energy than the band gap of the metal oxide. Homophasic and heterophasic light-induced oxygen isotope exchange has also been observed over TiO and ZnO. Homophasic isotope exchange is the production of  from  and . Heterophasic isotope exchange is the chemisorption of an oxygen isotope to the lattice of the metal oxide (lat), and replacement of one of the oxygens in the gas phase with the lattice oxygen as shown in the following reaction.