In heterogeneous catalysis the catalyst is in a different phase from the reactants. Heterogeneous photocatalysis is a discipline which includes a large variety of reactions: mild or total oxidations, dehydrogenation, hydrogen transfer, O–O and deuterium-alkane isotopic exchange, metal deposition, water detoxification, and gaseous pollutant removal.
Most heterogeneous photocatalysts are transition metal oxides and semiconductors. Unlike metals, which have a continuum of electronic states, semiconductors possess a void energy region where no energy levels are available to promote recombination of an electron and hole produced by photoactivation in the solid. The difference in energy between the filled valence band and the empty conduction band in the MO diagram of a semiconductor is the band gap. When the semiconductor absorbs a photon with energy equal to or greater than the material's band gap, an electron excites from the valence band to the conduction band, generating a electron hole in the valence band. This electron-hole pair is an exciton. The excited electron and hole can recombine and release the energy gained from the excitation of the electron as heat. Such exciton recombination is undesirable and higher levels cost efficiency. Efforts to develop functional photocatalysts often emphasize extending exciton lifetime, improving electron-hole separation using diverse approaches that may rely on structural features such as phase hetero-junctions (e.g. anatase-rutile interfaces), noble-metal nanoparticles, silicon nanowires and substitutional cation doping. The ultimate goal of photocatalyst design is to facilitate reactions of the excited electrons with oxidants to produce reduced products, and/or reactions of the generated holes with reductants to produce oxidized products. Due to the generation of positive holes (h) and excited electrons (e), oxidation-reduction reactions take place at the surface of semiconductors irradiated with light.
In one mechanism of the oxidative reaction, holes react with the moisture present on the surface and produce a hydroxyl radical. The reaction starts by photo-induced exciton generation in the metal oxide (MO) surface by photon (hv) absorption:
:MO + hν  →  MO (h + e)
Oxidative reactions due to photocatalytic effect:
:h + HO → H + •OH         	
:2 h + 2 HO → 2 H + HO   	
:HO→ 2 •OH
Reductive reactions due to photocatalytic effect:
:e + O → •O		
:•O + HO• + H → HO + O
:HO → 2 •OH
Ultimately, both reactions generate hydroxyl radicals. These radicals are oxidative in nature and nonselective with a redox potential of E = +3.06 V. This is significantly greater than many common organic compounds, which typically are not greater than E = +2.00 V. This results in the non-selective oxidative behavior of these radicals.
Titanium dioxide|, a wide band-gap semiconductor, is a common choice for heterogeneous catalysis. Inertness to chemical environment and long-term photostability has made  an important material in many practical applications. Investigation of TiO in the rutile (bandgap 3.0 eV) and anatase (bandgap 3.2 eV) phases is common. The absorption of photons with energy equal to or greater than the band gap of the semiconductor initiates photocatalytic reactions. This produces electron-hole (e /h) pairs:
Where the electron is in the conduction band and the hole is in the valence band. The irradiated  particle can behave as an electron donor or acceptor for molecules in contact with the semiconductor. It can participate in redox reactions with adsorbed species, as the valence band hole is strongly oxidizing while the conduction band electron is strongly reducing.