Photocatalysis: Buy my snake oil September 30, 2009Posted by calvinus in Energy, Green Chemistry, Photocatalysis.
Tags: Energy, Photocatalysis
Was at a one-day meeting the other day. There were couple of companies trying to sell themselves and their products. Photocatalysis was mentioned a couple of times in despatches.
Photocatalysis, for those that do not know, is a way of using light (usually sunlight) to drive a “useful” chemical reaction. In the case below, this could be splitting water into oxygen and hydrogen, but it might also involve turning organic chemicals into water and carbon dioxide.
I should say that it is telling that I see this figure appear quite often but I do not know who was the originator of the image.
Normally the photocatalyst is a semiconductor and the vast, vast majority of scientific articles published use nanoparticles of titanium dioxide to do this. The popularity of titanium dioxide is partly historical (Fujishima and Honda discovered the phenomenon using titanium dioxide) but mostly due to titanium dioxide’s efficiency at converting light that has enough energy to a chemical product.
And therein lies an important drawback: the light must have enough energy to overcome barriers in the semiconductor. For titanium dioxide, this means the light has to be particularly high in energy – it can only use ultraviolet light. Given that this is only around 4% of natural sunlight, this is a problem if photocatalysis is to be a useful process. This is especially the case for indoor applications where room lights tend to have negligible ultraviolet light.
There are, it would appear, companies out there that claim to be able to do everything through photocatalysis. They can:
- destroy nasty pollutants in water
- make glass, concrete, roads, buildings that clean themselves
- kill bacteria such as “E. coli, MRSA, swine flu, bird flu”
- destoy spores and turn them into carbon dioxide (!)
Some of the claims are fanciful, to say the least, but there are some basis in these claims. Destruction of organic pollutants has been known for a while and substantially researched. Self-cleaning glasses are known (see St Pancras station for an example of their use). Anti-bacterial propoerties are again reported in the scientific literature although the jury is out – I don’t believe anyone fully understands the possible mechanism and I have certainly NEVER seen any reports of photocatalysis being used to combat the bacteria du jour, swine flu or bird flu. You might, if you were being charitable, sugest that these claims may have been the result of someone not fully understanding the background science.
However, what triggers my abject cynicism is that one company went as far to say that their photocatalysts worked in both outside applications AND indoors. When asked how the performance compared, indoor and out, I was very surprised to hear them openly admit that they hadn’t actually tested it. So how could they make these claims? “Er…”
This narks me. This narks me a lot. Not only does it border on fraudulent, it does give the whole area an air of “buy my snake oil”. For those who are trying to do good, honest work in this area, it pollutes the pool of potential investors and cheapens what could be a useful area of renewables research.
Of course, preying on people’s fears (“Kills MRSA and Swine Flu dead”) is one thing, but would they use the same bombast if they were a pharmaceutical company and had people’s lives hanging on their results?
 Fujishima A, & Honda K (1972). Electrochemical photolysis of water at a semiconductor electrode. Nature, 238 (5358), 37-8 PMID: 12635268