Photocatalysis is a phenomenon by which light can lower the activation energy of a chemical reaction.

In a semiconductor, the energy levels available for the electrons are divided into a valence band (VB), which is completely filled with electrons, and an empty conduction band (CB). The bands are separated by a forbidden energy gap (“Energy gap”: Eg). When a photon with hv energy, which is higher than the Eg value of any given semiconductor, is absorbed by the latter, the “promotion" of an electron from the valence band to the conduction occurs. This leads to the formation of two charge carriers, namely the electron in the conduction band, e^ˉ_CB, and an empty positive electron within the valence band, which is known as a "hole" and generally indicated with h⁺_VB. Thus, a charge carrier is generated. Electrons in excited state within the conduction band and holes within the valence band may recombine and dissipate the initial energy in the form of heat. They could also be trapped in metastable surface states or react with the accepting electrons and with donating electrons and absorbed on the semiconductor surface.

Through this mechanism, excited states are generated within the material. These can start chain processes such as redox reactions and molecular transformations in molecules with which the material comes into contact. h⁺_vb is a strong oxidant which can either oxidize a compound directly or react with donating electrons such as water or hydroxide ions to form hydroxyl radicals and reacts with organic contaminant compounds. The oxygen molecule is a good acceptor of electrons, as it can form a radical anionic superoxide O₂^• after capturing the electron

Titanium dioxide (TiO₂) is a well-known semiconductor material which is particularly interesting for its photocatalytic properties. Amongst different crystalline forms of titanium dioxide (or the amorphous), anatase and rutile are those which have a photocatalytic activity. Both have a high refractive index, n, respectively equivalent to 2,52 (anatase) and 2,72 (rutile), which determines a high level of light "trapping" in the material structure, and thus a higher probability of photon absorption.

Furthermore, the photocatalytic degradation processes may be accelerated by hydrogen peroxide. During the reaction, the peroxide can produce hydroxyl radicals by reacting with O₂^•- or by direct photolysis. In addition, it can act as a conductive band to accept electrons such as oxygen and form hydroxyl radicals.

Main equations that govern a photocatalysis process are:

TiO₂ + hv →  TiO₂(h⁺_VB+e^-_CB)

h⁺_vb + e^-_CB →  heat

TiO₂(e^-_CB) + O₂ →  TiO₂ + O₂^•-

O₂^•- + H⁺ →  HO₂^•

HO₂^• + O₂^•- + H⁺ →  H₂O₂ + O₂

2HO₂^• →  H₂O₂ + O₂

HO₂^• + H⁺ + TiO₂(e^-_CB)  →  TiO₂ + H₂O₂

H₂O₂ + O₂^•- →  OH^• + OH^- + O₂

H₂O₂ + TiO₂(e^-_CB)  →  TiO₂ + HO^• + OH^-

H₂O + TiO₂(h⁺_VB)  →  TiO₂ + HO^• + H⁺

OH^- + TiO₂(h⁺_VB)  →  TiO₂ + HO^•

H₂O₂ + hv →  2HO^•

contaminant + HO^• →    degradated contaminant
contaminant + TiO₂ (h^-_VB)  →  oxidated contaminant
contaminant + TiO₂ (e-_CB) →  reduced contaminant

Nowadays, the major problems which have been detected in the use of nano anatase in the advanced oxidation photocatalytic process are the following:

• the recombination of the e^ˉ_CB/h⁺_VB pairs, a phenomenon which influences and limits the treatment yield;
• the difficult, if not impossible, separation of the titanium dioxide nanoparticles after the treatment, as a result of the nanometric dimension and the enhanced hydrophilia.

The literature describes attempts to avoid the recombination of the electron-hole pairs, based on the doping of the titanium dioxide with noble metals  (Ag, Pt, Au, etc.) and ignoble metals (eg. Fe³⁺); the combination of TiO₂ with CdS or non-metallic elements, such as fluoride, nitrogen, sulphur and other compounds.

All these attempts failed, mainly for environmental problems, for the low efficiency of the modified catalyst and for the cost of the process. Up to now, these problems have been the major limiting factor to the industrial application of this technology.

With the use of RECAM^® RE.44 it is now possible to solve both those critical points.

Graphenes (GRF) contained in RECAM^® have a very interesting structure and excellent electronic properties. Actually, they are semiconductors.
SA Envitech has developed a specific type of RECAM^® named RE.44 charged with anatase.

Thus, the electrons which are free after the titanium dioxide activation are easily transported to the GRF cells. Then, the possibility for the e^-_CB/h⁺_VB to recombine is strongly reduced, which increases the process yield.

The photocatalytic process reactions which regulate the process are:

TiO₂(e^-_CB) + GRFs → TiO₂ + GRFs(e^-)

GRFs(e^-) + O₂ → GRFs + O₂^•

GRFs(e^-) + H⁺ + O₂^•- →  HO₂^•-

HO₂^•- + H⁺ →  H₂O₂

O₂^•- + H₂O  →  HO₂^- + OH^•

contaminant + OH^• →  degraded contaminant

In the Table below are reported the main parameters affecting the photocatalysis process with RECAM^® RE.44 and titanium anatase.

Table - Main parameters that affect the photocatalysis process with titanium dioxide and RECAM^® RE.44.