Paper: Efficient and Clean Photoreduction of CO2 to CO by Enzyme-Modified TiO2 Nanoparticles Using Visible Light (Woolerton et al., J. Am. Chem. Soc. (2010))
Regardless of how much of all the global cooling warming climate change is hype, human error or falsification… it’s possible to measure current levels of CO2 fairly easily and accurately. And even though the concentration is still quite low (100s of parts per million), there’s still a lot of it around. From a “green” perspective, it’d be nice to find some way to get rid of it – or at least turn it into something that won’t destroy the earth. From an economic perspective, it’s a waste product and also a vast untapped resource. One way to tap it would be to convert it into more useful chemicals – like methanol, which can be burned, or carbon monoxide, which is a delightfully versatile starting point for all kinds of things.
Problem: CO2 is pretty stable, and converting it into anything else (like CO) requires a net addition of energy. (We get energy out of the reaction 2 CO + O2 → 2 CO2, therefore we have to put energy IN to run that in reverse.) So most attempts to “do something useful” with all that CO2 have been doomed from the start.
Now there’s a report of a nice way to use sunlight to supply the energy. If you shine sunlight on nanoparticles of titanium dioxide (common as dirt), some of the energy gets absorbed. A lot more gets absorbed if you add in a little bit of the right ruthenium compound, which grabs the photons and transfers their energy (as electrons) into the titanium dioxide particle. That’s not useful all by itself; the electrons can’t get from the titanium dioxide into the CO2 in the right way.
What’s new is that they added an enzyme taken from a bacteria, that converts CO2 into CO: CODH I. The enzyme needs a source of electrons; if the enzyme is close to the TiO2 surface, it can pick up electrons that way. So: Add sunlight, the ruthenium grabs photons and feeds them to TiO2 as electrons, the enzyme grabs the electrons and stuffs them into CO2, producing CO.
Not a lot of CO output, yet; they report an output of just 250 µmol per gram of TiO2 particles, per hour. (That’s 7 mg CO produced; it would have to run nonstop for 2 weeks to fill up a 2 L soda bottle with CO.) They suggest that the electron transfer from the particles to the enzyme is the problem; they just mixed things loosely in solution, and not much of the enzyme is going to be close enough to pick up electrons the right way. There’s ways to lock the enzyme down on the surface, so this could probably be accelerated significantly.
They even did this directly with sunlight, showing that it really is “solar powered” conversion of a waste product back into very useful starting material.

