As some had speculated, the Nobel in chemistry did take a turn toward physical chemistry this year, for the first time in some while. Gerhard Ertl has won for his work on reactions that take place on solid surfaces, an extremely important (and extremely difficult) field of research.
It’s hard because chemists and physicists have an easier time of it with bulk phases – all solid, liquid, or gas. When you start mixing them, or start trying to understand what happens where they meet, things get tricky. The border between two phases is very different from what’s on either side of it. The key zone is only a few atoms thick, and the interesting stuff there happens extremely quickly.
But some of the most important chemical reactions in the world take place down there. Take the Haber-Bosch process for producing ammonia – “Right,” I’m sure some readers of today’s newspaper are saying, “you take the Haber-Bosch process, whatever it is, and get it out of here.” But by making ammonia from nitrogen in the air, it led to (among other things) the invention of man-made fertilizers. That reaction has kept billions of people from starving to death, and kept huge swaths of wilderness from being turned into farmland. (Read up on Norman Borlaug if you haven’t already for more on this).
You can Haber-Bosch yourself some ammonia simply enough – just take iron powder, mix it with some drain cleaner (potassium hydroxide) and stir that up with some alumina and finely ground sand (silica). Heat it up to several hundred degrees and blow nitrogen and hydrogen across it; ammonia gas comes whiffing out the other end. Now, bacteria do this at room temperature in water, down around the roots of bean plants, but bacteria can do a lot of things we can’t do. For human civilization, this is a major achievement, because nitrogen does not want to do this reaction at all.
The industrial process was discovered in its earliest form nearly one hundred years ago, and was the subject of a Nobel all its own. But no one knew how it worked, which is a good example of how difficult surface interface work can be. You can see what has to happen eventually: the triple bond between two nitrogen atoms has to be broken and replaced by three bonds to hydrogen, whose own H-H bond is also broken. But that nitrogen triple is one of the strongest bonds in all of chemistry, so how is it breaking? Do the nitrogen molecules soak into the iron somehow, and if so, what does “soak in” mean on an atomic level, anyway? Do they sit on the surface, instead – and if they do, what keeps them there? Is that triple bond still in force when that happens, or has it started to break? If so, what on earth is strong enough on the surface of iron powder to do that? Where’s the hydrogen during all this, and how does its single bond get broken? What happens first, and why do you need the hydroxide and the other stuff? And so on.
Ertl and others had long studied hydrogen’s behavior on metal surfaces, while helping to figure out how catalytic hydrogenation works. (That was a reaction accurately described to me as an undergraduate in 1981 as “witchcraft”, and Ertl is one of the people who have helped to exorcise it). So they’d seen how hydrogen got broken into individual atoms and spread between iron atoms on the surface – the surprise for him and his co-workers was that nitrogen turned out to do the same thing, breaking that fearsome triple bond in the process. The biggest step in the whole mechanism happened very early. By running the reaction forward and in reverse (turning ammonia back into nitrogen and hydrogen, an otherwise perverse act for the most part), they were able to work out all the individual steps and the energies involved. Along the way, they figured out what the potassium hydroxide was doing in there, too (donating some key electrons to the iron atoms).
Observing this and other surface processes has pushed the limits of several spectroscopic techniques, such as Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), various forms of photoelectron spectroscopy, and others. Ertl's work has been notable for using a wide variety of methods, since there's no one tool that can give you the answers to questions like these.
He and his associates have studied many other surface reactions, such as the sorts of things that go on in the catalytic converters in exhaust systems. Metal-surface reactions like this are crucial to industrial civilization, and their importance is, if anything, growing. If we're ever going to get fuel cells to work economically, use hydrogen as an energy medium, or do a better job cleaning up industrial wastes, we're going to be using such things. And keeping them in the category of witchcraft won't cut it. It never does. Congratulations to Gerhard Ertl!