Photocatalysis: fundamentals and applications in JEP 2011

Abstract

As part of catalysis—and more precisely of heterogeneous catalysis—heterogeneous photocatalysis is an area of chemistry impacting many reactions as varied as total or mild oxidation reactions, dehydrogenation reactions, metal deposition, hydrogen transfers, etc. These different reactions are mainly facing applications in the field of water and air purification treatments—targeting both chemical pollutants and biological ones, self-decontaminating or self-cleaning products, organic fine chemistry as well as energy-related areas with hydrogen production from water. Relative to the history of catalysis processes, coined by Berzelius at the beginning of nineteenth century, photocatalysis remains a recent discipline, as marked by J.M. Herrmann in its introductory paper of this Special Issue. Basically, photocatalysis differentiates from conventional catalysis by the activation of the catalytic solid, which occurs through photon absorption rather than by thermal activation. This photonic activation thus requires the use of a semiconductor material as catalyst, provided that the radiation wavelengths are greater than its band gap, which corresponds to the energy gap between both conduction and valence bands of the semiconductor. Activating a semiconductor leads to the promotion of an electron from the valence to the conduction band, with the simultaneous creation of a photogenerated hole within the valence band. Further, the transfer of photogenerated charge carriers to the photocatalyst surface allows redox reactions to occur with adsorbed reactants, coming from gas or liquid phase depending on the application. At the catalyst surface, the redox reactions are separated into reduction and oxidative steps, involving on one hand, conduction band electrons and adsorbed electron acceptors such as, e.g., oxygen molecules (eCB +A→A), and on the other hand, valence band holes and adsorbed electron donors such as, e.g., organic molecules or more generally the targeted pollutant (hVB + D→D). Indirect oxidation reactions also occur through the formation of highly oxidative hydroxyl radicals generated by the oxidation of water by holes. Thus, the photocatalysis discipline exists through the ability of a material, in this case a semiconductor (usually TiO2), to simultaneously interact with light and reactants, through both aband adsorption phenomena, respectively. Nowadays, the “Advanced Oxidation Process” (AOP) nature of photocatalysis remains the most exploited, facing applications in the field of water and air purification treatments and self-decontaminating/self-cleaning products. On one hand, the implementation of photocatalysis for environmentally friendly cleaning water or air fits into a sustainable development approach, driven by the search for alternative S. Lacombe (*) Institute for Analytical Sciences and Physico-Chemistry for Environment and Materials (IPREM), UMR 5254 CNRS, University of Pau and Pays de l’Adour, Helioparc : 2 avenue of President ANGOT, 64053 Pau Cedex 09, France e-mail: sylvie.lacombe@univ-pau.fr