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Photocatalysis: Buy my snake oil September 30, 2009

Posted by calvinus in Energy, Green Chemistry, Photocatalysis.
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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[1]) 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.

Photocatalysis Book

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.

Maybe.

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?

ResearchBlogging.org [1] Fujishima A, & Honda K (1972). Electrochemical photolysis of water at a semiconductor electrode. Nature, 238 (5358), 37-8 PMID: 12635268

Comments»

1. Graham Tyrie - April 7, 2010

Swine flu and bird flu are caused by viruses not bacteria. Given what we do know about the mechanism of photocatalysis (PC) on breaking hydrogen bonds, PC probably deactivates viruses. Researching this is more difficult and we have not yet done this work.

Our company, Albagaia, has done research applying photocatalysis (PC) on both organic pollutants and bacteria. Both successfully destroyed either much better than UVC, or at least as well, using UVA/B (tanning lamps). We are currently developing devices for a range of applications.

There is still an energy issue, but as the comparison above shows if the application warrants the use of UVC, PC will not require more energy.

There is in fact a huge literature on PC research and along list of target molecules.

2. calvinus - April 13, 2010

You are quite correct. Swine and bird flu are caused by viruses, not bacteria, but this was precisely my point. These were claims made by one of the companies at the meeting (and I forget who it was, but it wasn’t Albagaia). In my humble opinion, there is a little too much over-hyping of the ability of photocatalysis to destroy whatever happens to be pricking the public conciousness at that time. Too much hyperbole backed up by too much fluff and nonsense.
I am typing this in the middle of the Semiconductor Photocatalysis 3 conference in Glasgow and we are still seeing claims of wonder treatments backed up by utterly slack science. Again, no names mentioned. In terms of anti-bacterial photocatalysis, there is an elephant in the room. What is the real effect of local temperature gradients upon irradiating the catalyst? What is the difference between the kill rate for gram-positive and gram-negative bacteria? How does the solution composition effect the function of the catalyst? Why are people still using light-sensitive dyes as a test “organic”?
I appreciate that rigorous testing may, in some cases, be difficult to realize, but for the time being, overselling a technology is not going to help people who do actually have something worth looking at. I’d be interested in hearing your experiences of taking photocatalysis to market.

3. Antibacterial Tiles that Clean and Sanitize Themselves | ARCHITERIALS - November 5, 2010

[…] Buy my Snake Oil” Post Tenebras Lux.  09/30/09.  Accessed 11/05/10.  URL. […]


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