Its been known for a while now that Titanium dioxide is a photo-catalysts - it can attract smog compounds like NOx and other Volatile Organic Compounds from cars and industry and use sunlight to break them down.
There are restrictions - there must be a high surface area ... often by coating a mesoporous surface (think of Pumice stone) and other compounds (like soot) can accumulate and inhibit its abilities. On the other hand, bacteria and biofilms cannot adhere to this surface...one in part to how TiO2 attracts water as well under UV and breaks it into OH and H compounds that kill them off easily. When night falls, the un-energized TiO2 molecules release the H20, carrying off bacteria and other contaminants. And if that's not enough for you, we can use TiO2's properties to cool houses down very easily. If water is slowly applied to the surface, we can cool the building and everything inside through evaporation - just like the Human Body does with sweating. (There's a wonderful paper from Hong Kong called "Photocatalytic construction and building materials: From fundamentals to applications" ... I can't link the .pdf, but it might be out there on the internet somewhere.)
So, I see a few things coming out of this. First, highways and roads can have median barriers replaced with something resembling a 'sponge', printed with a Dshape. TiO2 would either be part of the material, or applied later. I suspect that we could get higher surface areas that with precast, and thus passively clean air better.
Another one uses TiO2s properties to cool housing efficiently and thus saving money on AC. I'm really amazed by this possibility....mostly because it provides another reason for using 3D printing. Small holes for water would be part of the structure ... placed strategically to cool the whole building. The massive savings on energy costs would be a big part of selling large-scale 3D printing as a better bet in construction.
Anyways....i've been trying to figure out how to 3D model an ordinary Concrete median with Voronoi diagrams to get my idea across.....Meshlab keeps on crashing..woot! Autocad 2011 for Mac is still giving me trouble.....Having a mac is not the best for 3D design sometimes...