The Ergosphere
Thursday, February 24, 2005

One small step for carbon

Over at Peak Oil Optimist, Rob waxes optimistic about a catalyst for the electrolytic reduction of CO2.  He specifically claims "Using a platinum-based catalyst, the team was able to convert raw CO2 to various hydrocarbon chains (benzenes and simple sugars)."
Maybe there's a flaw in the PowerPoint translation system of OOImpress and it leaves something out, but I have been over both slideshows and I don't see it.  The presentation mentions sugars, but I could not find anything to indicate that such complex molecules had been produced.  As far as I can tell, the accomplishment of the researchers is to have reduced CO2 to oxylate ion (C2O42-); the significance of oxylate is that it has covalently bonded carbons.  I am not a chemist and I'm in no position to say if this is a major step toward synthesis of less-oxidized molecules.  The cell-voltage requirements of ~1.7 V is more significant, as 1.7 electron-volts is in the energy range of visible photons; the relevance for artificial photosynthesis is obvious.
As far as I'm concerned it would be just as good to produce carbon monoxide (CO) and carbonate ion (CO32-); carbon monoxide is both a fuel and a base for synthesis, and carbonate ion can be immobilized relatively easily.  (Perhaps too easily, if it can't be kept from clogging equipment like it clogs my tea kettle.)
But that's just one way to convert CO2 into fuel.  Other pathways are better-understood, and if you have a source of pure CO2 and a source of hydrogen it appears that the doorways are already open.  Consider the synthesis of methanol.  Methanol synthesis plants are well-understood and inexpensive, to the point that methanol synthesis is being combined with IGCC electric plants.  (I thought I had several links regarding this subject, but I seem to have mislaid them.)  Once you have methanol, you can synthesize gasoline with what appears to be relative ease.  Other possibilities include synthesis of methane via the Sabatier reaction (proposed for Mars in-situ propellant production) and heavier hydrocarbons and olefins (ditto).
None of this gets you anywhere unless you have a method of capturing and purifying carbon dioxide, and a source of energy (either in the form of electricity or hydrogen) to power the process.  The source of the energy is all-important.  The prime mover in all cases comes back to one of the usual suspects, and adds further difficulties into the bargain.  It appears to this observer that chemosynthesis is one of the smaller difficulties here. 
EP -- suggest you look at the slide labeled "Results of CO2 reduction" in the Ana Marcus slide. If I read this correctly, this is a gas chromatagraph readout of the contents of the "oily liquid" they got. The "variable organic group" on the top graph has various benzene rings attached to whatisits on the right; on the left in the middle, many such.

I dunno, maybe I'm getting my panties in a bunch for no reason, but somehow this strikes me as really, really interesting.
If you look at slide 14 of the Ana Ciric presentation, you'll see that the "variable organic group" (ethyl iodate, if I have the nomenclature correct) is introduced externally and not generated as part of the reduction process.  The ethyl iodate is reacted with the potassium oxylate created by electrolytic reduction; that's where the "variable organic group" comes from.

The presentation does mention on slide 4 that CO2 reduction is the first step in photosynthesis, so maybe this is a bit more significant than I thought at first.  On the other hand, photolytic production of hydrogen would be just as significant, and research there has already produced H2 (though not with a complete cycle).  Both types of effort deserve more R&D money, IMHO.
Isn't the problem with hydrogen, though, the big "where does it come from" issue? From my understanding, commercial hydrogen comes from one of three (?) sources: natural gas (which we are running out of), electrolysis of water, and/or steam reforming. Electrolysis is about 30% efficient, if memory serves, and requires energy from some other source; the more steps required, therefore, the worse off you are because of inefficiencies down the chain. This is why I'm not too excited about the "hydrogen economy"; the problems associated with getting there from here are awfully large.
WRT CO2 reduction, yeah, that's why I was interested in this in the first place. But the other part of it is that hydrogen is harder to store and has other issues as well. The infrastructure for dealing with carbon as an energy carrier is already with us; something like this could be next big thing.
Electrolysis is considerably more efficient than 30%; a quick search shows figures of 65%, and I seem to recall much bigger numbers for the high-temperature processes.

Hydrogen is hard to store, but (as I mentioned in the post, existing chemistry is sufficient to produce storable fuels with nothing more than a stream of H2 and a stream of CO2.  Getting the hydrogen and CO2 is the problem; hydrogen will reduce CO2 quite easily without new catalysts.  If we can find a way to power CO2 reduction with photolysis, I view that as an alternate method of energy capture rather than a better method of chemosynthesis.

The electric process is interesting, but absent a combination of a source of CO2 and an electric power supply (e.g. excess power production from a wind farm) it doesn't look to be terribly useful by itself.
EP -- I spoke with Dr. Granger a couple days ago. I was more or less correct in that they were making benzoates and other long-chain carbon molecules with the polymer catalyst. One thing he did mention is that the problem he's had with this work is that it's terribly dependent on the input gas stream, so they're working on settling it down so they can get repeatable outputs.

I've asked for his permission to post a summary of our conversation on my blog, which should hopefully be up in a week or so. He also suggested I should read the papers of Clifford Kubiak at UCSD.
I looked up "benzoic acid"* and found that it has a benzene (phenyl) group with a -COOH radical. -COOH isn't terribly reduced; you really need to replace oxygen with hydrogen to get useful fuels. Unless the phenyl group was generated as part of the catalysis instead of being part of the "variable organic group", I stand by my my previous appraisal: it's a beginning, but not a world-changing discovery.

* Do you know how many pages in the CRC are devoted to benzoic acid and its derivatives? Nineteen and a half by my count.
The purpose of the iodoethane was to make the oxylate volitile... so it could be analyzed by GC/MS

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