I gave quite a quick answer in the chat but I can explain fully!
I use titanium dioxide, which is a solid and a white powder. It is truly a catalyst in the reaction and specifically a heterogeneous catalyst which means it is in a different state to the reactants. When ultra violet lights hits titanium dioxide the surface becomes excited and this initiates a series of reactions which creates a “hydroxyl radical”. The radical is the key – it’s highly reactive and this is the what destroys the surrounding pollutants .
Titanium dioxide is great because it it’s safer than other process used to make radicals, such as ones that use ozone or hydrogen peroxide, and it can be reused unlike the others too so it’s also cheaper. The problem is it’s less efficient, my job is to make it better 🙂