We knew Titanium dioxide was photoactive. It is a photocathalyst, meaning it can build up an electric charge which can then be used by molecules in chemical reactions. Alternative the electrons can be picked up by oxygen forming O3, which is very reactive and will break down bacteria, toxic gasses, NO etc. In short, Titanium Oxide can rinse the air of pollutants found in a.o. car exhaust.
A while back we found that TiO2, which is the chemical name for Titanimum Dioxide, can be found in paint pigments you can buy in the art store, it’s non toxic, and also used in toothpaste to make it look more white. We wondered if we could make a paint that would have the air cleansing properties. We figured it would have to be water based and bring out the pigment in such a way that it would be in contact with the air. We bought Arabic Gom, (made out of camel bones?) but it turned into a covering yellowish layer over the pigment.
We hade seen several examples of ceramic tiles with TiO2 but those would be heavy and cumbersome and explensive and only usable for specific interiors. Then it struck us : we could mix it with gypsum. No idea if this would produce a working substance. Let’s google the idea (We usually (not always) come up with ideas that others have tried or investigated already).
We found a study of exactly what we are after : “Design of a Novel Photocatalytic Gypsum Plaster with the Indoor AirPurification Property” by Yu ad Brouwers out of Eindhoven University. Perfect! They set up an experiment to see what happened to NO (a car combustion pollutant) if they ran it by plaster with varying percentages of Ti2O. Their TiO2 was of one specific crystal type (there are three) called Anatase, the other two are Rutile and Brookite, this will become important later..
Four mixes and a reference mix where tested with roughly 2% and 4% TiO added to the mix (two with accelerant and two without). The mix used was anatase TiO2. NO, nitrogen oxide, a pollutant in car exhaust, was fed over a sheet of the prepared gypsum. The goal being conversion of NO into harmless HNO3.
A measurable effect can be seen to the NOx concentration (see image above). The NO however also gets turned into NO2, which is still harmfull. The experimenters measured significant amounts of NO turned into NOx or HNO3, even if some NO passed unchanged, probably due lack of contact with the gypsum.
The paper concludes that :
“The experimental results indicate that the photocatalytic oxidation is an effective indoor air purification technology. All photocatalytic oxidation experiments are carried out at ambient conditions under visible light, which shows its convenience for the indoor air purification.”
The pigment we have seems to be pure Anatase, according to the code it has with an A at the end (C 101101 A). The Iso specs say “Type A : Anatase type”. so this means the results achieved in the Yu/Brouwers study can be replicated by mixing our pigment into gypsum.
Worried which type of TiO2 we needed, Anatase or Rutile (Brookite is not sold as pigment) we looked for information about the photocatalytic effectiveness of both types of TiO2, wondering if it mattered. We found a poster (presentation of research) that gave us a precise answer.
The above graph shows that having a mix of Rutile and Anatase TiO2 has the strongest photocatalytic effect. Adding Rutile to pure Anatase pigment can increase the effectiveness by about 30%. A 50/50 mix is commercially available, it is not as cheap as the pigment though.
Our 40% TiO2 gypsum causes a noticable freshness. It would be nice to have a real measurement of the effect..
We think we could make gypsum panels which when exposed to the sun will reduce NO concentrations and remove other pollutants. We think we could add UV light to panels that are used inside (UV activates the TiO2). We think a specific surface pattern could increase the effectiveness and a TiO2 coating, so a last covering layer instead of massive TiO2 gypsum, will save money. We wonder if we could make a TiO2 active white wallpaint for indoor use (our initial goal). Lastly we wonder if adding other pigments (dye sensitization) could strengthen the effect as white paint reflects most energy. Many dies will turn light into an electric charge, with could add to the charge generated by TiO2. Lastly we think cities could increase the air quality (also in terms of germs) by painting walls with TiO2 gypsum where people will allow it.
Considering about 3 million people die each year due to air pollution and it’s the third cause of death (in Holland), there’s a case to be made for TiO2 plastered walls in places with lots of (IC) traffic. Time for an experiment?