[the other nigel (OP)]

I have heard that Titanium Dioxide can act as a photocatalyst and, when mixed with cement, it will convert airborne pollutants (such as carbon dioxide etc) into a benign substance that can be safely washed away with water.  Is there a simple way of demonstrating whether this is true or not? 
 
And if this is true, then what does that mean for toothpaste, sunblockers, etc all of which seem to do the opposite (ie they block UV light) because of the Titanium Dioxide?
 
i.e. the first situation sets up a chemical reaction whereas the second situation prevents a chemical raction - or am I completely missing the point!!!!!!!!!!!!?

[adianadiadi]

Nice question.
No doubt, the first case is to catalyse the reaction. It shows a different path to the reaction and thus by decreasing the activation energy. More at MOD EDIT: LINK REMOVED

But the second one is for blocking the uv radiation. It absorbs the harmful uv radiation which otherwise sets chemical reaction. So indirectly, it stops the unwanted reaction. Here it is not showing any different path to the reaction.

MOD: Sorry, no advertising

[diogenesNY]

While I cannot add  much to the photocatalyst portion of the question, perhaps this will go partway to enlightening the issue of the nature of Titanium Dioxide. 

Titanium dioxide is _very_ white.  It is used as the main pigment in the artist's paint (acrylic) called 'Titanium White' which is the most opaque and photo-stable of all of the artist's white paints.  In fact Titanium White is simply a mixture of acrylic medium (which is clear) and very finely ground titanium dioxide.  It has extraordinary staying power, resistance to degradation by UV radiation and opaque covering power.  (Zinc white is a transparent white pigment). It can be thinned an amazing amount and still maintain its covering power.  It is used as a whitening pigment in many other applications as well including industrial paints and coatings, and is very effective at shielding UV radiation and that is why it is used as a sun blocker in a number of varied applications.

Hope this was of some small help.

diogenesNY

[Bored chemist]

it looks lik a conflict but it's not.
Whitness depends on the abillity to reflct whit (visible light). The other effects depend on UV.
TiO2 absorbs UV light, dependng on things lke the particle size, crystal form and what else is around it, the energy it picks up from the UV can be transfered to another molecule, in which case you get photocatalysis or dissipated as heat, in which case you get staightforward absorpbtion (as in a sunblock).

[the other nigel (OP)]

Thanks guys

But the plot thickens!!  It seems to do so many different things.  Is there a simple way of demonstrating how it cleans pollutants from the air - I'm absolutely "hooked" on this now.

Thanks
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[Make it Lady]

You have to have quite big crystals to maximise the surface area to make it be an efficient photocatalyst. I've heard it is also used to clean nitrous oxides from polluted streets. Will look for more info on this as I too luff things that do loads of good stuff.

[Bored chemist]

You might want to look again at this "You have to have quite big crystals to maximise the surface area ".
When you cut a big crystal in half yoiu make some more surface area- the area of the newly cut surface.

[the other nigel (OP)]

I've now seen that it has been used as a coating on glass to keep the glass clean ie the new St Pancras Station roof in London. This again seems a "different" process - althogh nearer in substance! So that therefore is cleaning the surface pollutants.

The next step is how to demonstrate that its "pulling" air borne pollutants out of the air!! That's what I'm really struggling with.  How does that work??
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