Photocatalysis, among all the various advanced oxidation processes is the one that has found wider use: it’s a chemical reaction that mimics trees photosynthesis to absorb and transform pollutants into non-harmful elements.
If we try to dig into details of this revolutionary discovery, we find that its operations follows just exactly what happens in nature. Photocatalysis imitates, as we said, the well-known photosynthesis in transforming harmful substances for mankind. The chemical process that is at the base is in fact an oxidation that starts thanks to the combined action of light (solar or artificial) and air.
Photocatalysis is increasingly used as a method to clean up air and water, and the reasons why this process is becoming increasingly popular are the following:
- It significantly degrades recalcitrant pollutants present in the air and water
- The pollutant is mineralized to CO2 and H2O and not transferred into another phase
- It’s cheap and does not require regeneration
- It’s not selective, can eliminate lots of pollutants of different nature
- It takes advantage of UV light and water (in the case of air, humidity)
How it works
To better understand how the photocatalytic process works we must define some concepts:
- Oxidation reaction: it’s a reaction in which a chemical species, atom or ion loses electrons and its oxidation number increases.
Fe > Fe2++2e- (NB e- is the symbol of the electron with negative charge)
- Reduction reaction: it’s a reaction where a chemical species, atom or ion acquires electrons. Its oxidation number decreases.
2H++2e- > H2.
It’s obvious that if an element in a chemical reaction oxides losing electrons it must exists another element that reduces, acquiring electrons. Therefore, the oxidation and reduction reactions must occur simultaneously: in this case we talk about oxidation-reduction, or redox reaction. We define the species which oxidized as reducing agent over the other species and instead agent the chemical species reduced.
- Catalyst: it’s a substance capable of increasing the speed of a chemical reaction. It has the ability to lower the activation energy and allow a greater number of molecules to react with each other. It actively participates in the chemical reaction, and in the end of this reaction remains unchanged.
- Photochemistry: it’s a branch of chemistry that studies the chemical reactions generated by the interaction of an electromagnetic radiation (visible light/UV) with matter.
- Photocatalyst: it’s a substance capable of promoting a chemical reaction by absorbing an electromagnetic radiation. The photocatalyst decreases the activation energy of a given reaction and accelerates the speed of that same reaction. Semiconductors are substances capable of performing this kind of processes.
This is what happens in photocatalysis. If there were no catalytically active substances, oxidation of most of the hydrocarbons would proceed rather slowly.
Several studies have led to the creation of materials able to “eat” the organic and inorganic air pollutants, precisely exploiting the photocatalysis process. The best catalysts used in photocatalysis to oxidize harmful substances until complete mineralization are, as we said, solid semiconductors. Among the semiconductors titanium dioxide (TiO2) is one of the most widely used photocatalyst for the preparation of a variety of products.
In a heterogeneous photocatalytic system, semiconductor (photocatalyst) particles, in close contact with a mean of liquid or gaseous reaction, are excited by sunlight and this excitation promotes a series of chain reactions such as Redox reactions and molecular transformation.
Semiconductors (metal oxides or sulfides such as ZnO, TiO2 and ZnS) have a particular electronic structure, characterized by a full valence band (VB) and an empty conduction band (CB).
Thanks to their structure they can behave as sensitizers for Redox photo-induced processes. The energy difference that exists between the lowest level of the valence band energy and the highest energy level of the conduction band is called “Energy Gap” (Eg). This energy difference (Eg) is nothing more than the required minimum light energy in order to make a material a good conductor. If a semiconductor such as TiO2 is irradiated with photons of higher energy compared to its Eg, an electron of its valence band is able to overcome the energy gap and therefore is promoted to the conduction band.
TiO2 + hv > h+ + e‐
Vacancies of semiconductor metal oxides have the characteristic of having a strong oxidizing power and thus be able to oxidize a donor who loses an electron.
The electrons which instead are promoted to the conduction band can reduce the electron acceptor molecules due to their strong reducing power. In this way the vacant h+ can react with the absorbed water on their surface forming highly reactive hydroxyl radical (•OH). Vacants and hydroxyl radicals are both strongly oxidizing and, as such, can be used to oxidize the majority of organic contaminants.
H2O+h+ > •OH + H+
Air’s oxygen, however, acts as an electron acceptor by reacting with the electrons of the conduction band and thereby forming super-oxide ion
O2 + e- > •O2-
Super-oxide ions are highly reactive particles able to oxidize organic materials.
The electrons and vacants in TiO2 do not recombine immediately, instead they originate chain-photocatalytic reactions. Indeed, opposed to metals, semiconductors has a lack of a continuous of interbanda energy state to assist the recombination of elettron-hole pairs. That ensures to the excited state a sufficiently long life to allow photo-excited electron (e-) and the hole (h+) to interact with the chemical species adsorbed on the semiconductor surface. At the end the organic materials decompose into carbon oxide and water, similarly to what happens in photosynthesis where carbon dioxide and water, thanks to solar energy, are converted into glucose and oxygen. In conclusion, photocatalysis is therefore a catalytic method applied to photochemical reactions, conducted with the aid of a catalyst which exerts its action when irradiated with a radiation of appropriate wavelength.
Therefore in heterogeneous photocatalysis it is necessary:
a semiconductor / H2O / UV radiations.
Thanks to the photocatalytic process the decomposition of organic and inorganic substances (equivalent to all the fine particles – PM10), microbes, nitrogen oxides, aromatic polycondensates, sulfur dioxide, carbon monoxide, formaldehyde, acetaldehyde, methanol, ethanol, ethylbenzene, mexilene, monoxide and nitrogen dioxide can take place. As long as in nature nothing is created and nothing is destroyed, even the photocatalytic reaction produces residues resulting from its oxidizing action. Generally, the compounds resulting from the processing of pollutants are: minerals and limestone products in small quantities (parts per billion), invisible and harmless.
Pollutants and toxic substances in fact are transformed, through the photocatalytic process, in sodium nitrate (NaNO3), sodium carbonate (Ca(NO3)2) and limestone (CaCO3), harmless and measured in ppb (parts per billion). We can therefore deduce that the residuals of the photocatalysis can be considered absolutely negligible. The result is a significant reduction in toxic pollutants from cars, factories, home heating and other sources.
Among the photocatalysts, TiO2 is the one who found more use so far:
- It’s a crystalline white powder
- Particularly effective if irradiated by UV rays
- Very efficient in photocatalytic processes
- It’s used as a colorant (E171) for different products (e.g. foodstuffs, toothpastes, paints), and it is therefore available in large quantities.
Titanium dioxide can exist in amorphous form or in three different crystalline forms:
- Rutile, tetragonal shape. It is the form used industrially. It may present a black color, reddish brown in larger crystals, or yellow in finer crystals
- Symmetry rhombic bipyramidal brookite. In nature it’s present in the form of very small flattened tubular crystals ranging in color from pink to brown
- Anatase in distorted tetragonal form. It’s the most stable polymorph at low values of pressure and temperature. It’s the more active form catalytically speaking. The anatase Is in the form of small isolated crystals having a color ranging from blue to yellow (they all are octahedral structures (Ti06) where Ti has coordination number 6).
There are several materials that show photocatalytical properties (Ti02, ZnO, Sn02, CdS, etc.), however not all are quite efficient and stable over time to be used for this purpose. This is because the ability to transfer the charge of a semiconductor is governed by the position of its band and by the potential of oxidation-reduction.
The oxidation-reduction potential is a measure of the tendency of a chemical species to acquire electrons, that is, to be reduced.
Between two species that interact with each other:
- the species with lower potential E oxidizes (yields electrons)
- the species with higher potential E reduces (acquires electrons)
The red-ox potential of the hole in the valence band must be sufficiently positive to allow the acceptor function.
The red-ox potential of e- in the conduction band must be sufficiently negative to allow the donor function.
Other photocatalysts (like GaP, GaAs, CdSe, CdS or Fe2O3) appear to be less stable in air and to degrade more easily:
- ZnO, despite having a band gap width that lends itself well to promote the photocatalytic degradation processes of organic compounds in aqueous solution, forms a passivating layer of Zn(OH)2 on its surface which seriously compromises the characteristics
- SnO2 has an excessively high band gap
Titanium dioxide is without doubt the photocatalyst that, for many reasons, find more uses from a commercial point of view; its limit is the fact that it is activated only under UV rays illumination, and this is a problem in case of a low percentage solar energy. For this reason, to overcome this limitation, our company has used a sensible catalyst to visible light also.
Now that we’ve understood the operations of photocatalysis, we can list its advantages.
The photocatalytic process really brings the occurrence of three realities:
These properties are simply the result of oxidation of substances that encounter a photocatalytic surface. If they are pollutants (nitrogen dioxide, sulfur dioxide, carbon monoxide, fine particulate matter) we can speak of antipollution reaction, if they are fouling substances (carbon blacks, dyes) we can talk about self-cleaning reaction, if they are bacteria, molds, fungi and microorganisms, we can speak of antibacterial reaction.
Nowadays there is a long series of products using the concept of photocatalysis to improve the environment where we live, and at the same time to be compatible with the needs and style of the modern world. The photocatalytic process is used in various engineering applications, which is why many companies in the construction and air/water treatment industry are investing in this green technology.
Without doubt the photocatalytic process, in the next few years, will play an increasingly important role in sustainable treatment processes.
There are many products that could potentially be included in the daily use and that would give significant benefits to improve the air we breathe (e.g.: tiles with photocatalytic surface, photocatalytic cement and paints, water purification plants).
It’s in this optics of taking advantage of a simple and natural process, but at the same time effective and widely consolidated such as photocatalysis, that comes the latest invention of our company: PAL (Purification Air Lamp).
PAL is a LED lamp that uses the photocatalytic process to purify indoor environments: we’re talking about an effective product for indoor pollution.