Photocatalysis an ally against pollution

Definition of photocatalysis

The Diccionario de la Real Academia de la Lengua defines the term photocatalysis as “increase in the speed of a chemical reaction by the effect of light or other forms of radiant energy”.

We can explain photocatalysis as a chemical reaction involving a source of light or radiation and a catalyst that accelerates the reaction. These reactions can be either oxidation of organic compounds or reduction of inorganic ions and other organic compounds.

To make it easier to understand, the example of plant photosynthesis is often used.

In photosynthesis, plants use the energy of sunlight to transform carbon dioxide from the environment into nutrients and oxygen, which they expel to the outside world. For this reason, plants and trees are said to be the green lungs of the Earth.

This same transformation process is produced by photocatalysis. The difference is that this mechanism involves a catalyst that increases the speed of the reaction.

Generally, titanium dioxide is used as a catalyst because of its low cost and because it is very easy to obtain.

Who discovered photocatalysis?

The discovery of this chemical process is relatively recent and its usefulness in fighting contamination and as a disinfectant is still being investigated.

It was in 1967 that Fujishima and Honda discovered the photolysis of water or the phenomenon of photocatalytic decomposition of water. Known as the Honda-Fujishima effect.

This discovery laid the groundwork for further research into the ability of catalysts such as titanium dioxide to decontaminate and sanitize.

What is involved in the photocatalysis process?

This chemical process involves sunlight which is transformed into chemical energy by the presence of a catalyst. Both oxidation and reduction reactions occur during this process.

Below we see the role of each of the parts involved in the photocatalytic process.

A photocatalyst

It is a semiconductor material that acts by accelerating the rate of chemical oxidation reactions.

The most widely used is titanium dioxide (TiO2) because of its low cost and its ability to mineralize contaminants and not generate other harmful substances.

An oxidizer

Oxygen is used so that the photocatalyst can oxidize and perform its function within the chemical reaction.

An element that supplies electrons

This chemical element is needed to obtain the electrons necessary for the chemical reaction and to allow the oxidation of the pollutants.

Radiation or sunlight

A light source, either natural or artificial, is necessary for the photocatalyst to accelerate the rate of the reaction.

What are the two types of catalysis that occupy the photocatalytic reactor?

There are two types of catalysis that a photocatalytic reactor can generate. These are: homogeneous photocatalysis and heterogeneous photocatalysis.

What is homogeneous photocatalysis?

In homogeneous photocatalysis, reactants and photocatalysts exist in the same phase.

What is heterogeneous photocatalysis?

Heterogeneous photocatalysis has the catalyst in a different phase from the reactants. This type of photocatalysis is a discipline that includes a wide variety of reactions.

What is photocatalysis used for?

As we have said, the discovery of photocatalysis is very recent, so its possible uses and applications are still being investigated.

The White Paper on Photocatalysis, prepared by the Iberian Photocatalysis Association, lists some of its main applications and benefits.

Photocatalysis for air decontamination

This chemical process is capable of removing from the environment a large number of pollutants such as: NOx, SOx, volatile organic compounds (VOCs), CO, methyl mercaptan, formaldehyde, chlorinated organic compounds, polys aromatic compounds…

But not only can they remove contaminants, they can also be used to disinfect the environment.

The use of the photocatalyst titanium dioxide allows the mineralization of pollutants. In this way, the air or rain in the case of outdoor environments makes these minerals disappear.

Moreover, as pointed out in the White Paper, it not only allows the decontamination of indoor and outdoor air. Photocatalysis also achieves “the elimination of odors, elimination of ethylene formed in fruit and vegetable stores, soil remediation by desorption, pollen reduction, etc.”.

It is very useful, therefore, to combat bad odors in enclosed spaces such as smoking rooms, parking, kitchens, restaurants, bars, garbage rooms in buildings. In addition, photocatalytic materials have the ability to “reduce the capacity for mold and algae formation, and also as a bactericide, in common indoor spaces, such as health centers, offices or the home itself”.

This publication highlights that one of “the characteristics of photocatalysis by means of TiO2 and ultraviolet light is its unspecificity, which means that it can eliminate a wide range of atmospheric pollutants, particularly organic ones. This is due to the oxidizing capacity of the radicals formed during the process which, in principle, are capable of attacking any organic compound, although in
practice some are more resistant to this attack than others”.

For water treatment

Photocatalysis is a way to purify and detoxify water. Therefore, it can be very useful for making water drinkable and obtaining a greater benefit from the planet’s water reserves.

To obtain materials with great benefits

These include photocatalysis applied to pavements, facades, roofs, fabrics (for residential awnings and protection of facade rehabilitation works), photocatalytic islands, and self-cleaning, self-sterilizing, biocidal, anti-fogging or anti-reflecting materials.

The Iberian Photocatalysis Association points out that “indoor applications are based on the decomposition capacity of photocatalytic technology on organic matter. This effect is what prevents the adherence of dirt (organic) on vertical surfaces in interiors and exteriors, so that the aesthetics of the photocatalytic material applied can be maintained over time, compared to a conventional material, thus reducing conservation and maintenance costs (delaying cleaning campaigns over time – normally costly due to the use of hanging scaffolding – or repainting)”.

Therefore, the self-cleaning effect of photocatalytic surfaces considerably reduces the budget and time required for cleaning and maintenance.

Healthcare applications

As indicated in the White Paper on Photocatalysis, “there is the possibility of incorporating a photocatalyst material in the plastic materials used in the manufacture of catheters and other sanitary utensils, so that not only a self-cleaning effect can be achieved in these products, but also a self-disinfecting effect, since the oxidizing effect of the photocatalyst is capable of inactivating bacteria and viruses, as has been demonstrated in various scientific studies”.

Energy source

Another of the avenues opened up by photocatalysis research is the possibility of obtaining sustainable fuels from renewable raw materials (CO2, water, biomass) and solar energy. The Iberian Association points out in its White Paper that “photocatalysis offers a way to develop artificial photosynthesis”.

What are photocatalytic materials?

Research has led to the development of a wide variety of photocatalytic materials that include the photocatalyst, titanium dioxide, in their composition.

These materials are already applied in a wide variety of fields related to construction and industry.

Among the materials with the capacity to perform photocatalysis reactions we find:

• Cement-based photocatalytic materials: the photocatalyst is part of the mixture, not a coating. This makes it possible to develop walls, walls, facades with the capacity to eliminate contamination and disinfect surfaces.
• Pavements that include the photocatalyst.
• Asphalt sheets used for waterproofing roofs and decks.
• Ceramic materials with titanium dioxide bath, which also increases their durability.
• Photocatalytic textile used in sun protection structures.
• Photocatalytic furniture, both indoor and outdoor, which also repels dust and dirt.
• Paints with photocatalytic treatment to cover all types of interior and exterior surfaces.

At Albergrass we are committed to environmental care and sustainability. In future articles of our blog we will continue talking about the benefits of photocatalysis.