Through the Looking Glass: A Glimpse at Nitrogen Fixation Using Iron Catalysts
“This is one of the most challenging transformations in chemistry, owing in large part to the stability of the N2 molecule.” I’m talking with Nik Thompson, a graduate student in the lab of Professor Jonas Peters at Caltech and the lead author on a recent paper in Journal of the American Chemical Society. Nik’s paper is all about nitrogen fixation, or taking nitrogen gas (“N2”), which is 78% of the air we breath, and turning it into useful compounds like fertilizer and proteins. The main problem, however, is that nitrogen gas really likes to be nitrogen gas, so it’s difficult to chemically convince this stable, unusable molecule to become something useful.
One useful thing we want to make out of nitrogen gas is ammonia, an important molecule that’s a key ingredient for fertilizers used all over the world. However, nitrogen gas is so stable that it’s almost impossible to make ammonia out of it, at least with the techniques most chemists use on a daily basis. Happily, in 1909 Fritz Haber and his assistant Robert Le Rossingol made a breakthrough: nitrogen gas CAN be turned into ammonia using chemical methods, but you need a little bit of a special ingredient, called a catalyst, for this to work.
A catalyst is basically a little molecular machine that helps make things work faster and more efficiently, and nitrogen fixation is impossible without one. Haber and Rossingol found that iron metal is a good catalyst for making ammonia out of nitrogen gas; still, even this molecular machine is only able to work under high temperatures and pressures. How high? If you wanted to experience the conditions yourself, you would need to balance a bulldozer on your head while being heated to 700 ºF! These conditions are so extreme that it took the work of a brilliant chemical engineer, Carl Bosch, to figure out how to actually use this to make more than a few drops of ammonia at a time. The final design, called the Haber-Bosch process, has become a key part of life in the 20th and 21st centuries.
Today we make more than 275 billion pounds of ammonia per year using the Haber-Bosch Process, but there is still a big push to find a better way to make ammonia out of nitrogen gas. Nik puts this in perspective, “The Haber-Bosch process is exquisitely engineered, yet there are major drawbacks, the most obvious ones being the huge amount of energy and natural gas needed to drive it. In all, about 2% of global energy, and more than half of all hydrogen produced from the steam reformation of natural gas go to this single process.”
It’s tempting to say that we can’t do better, but biochemists have known for decades that nature is able to do this same nitrogen fixation reaction at room temperature under atmospheric pressure. The catalyst nature uses to do this is an enzyme called nitrogenase, a complicated collection of atoms stuck together just right, that’s made by bacteria in the roots of legumes like beans and alfalfa. Just how nitrogenase is able to catalyze this difficult reaction under such comparatively mild conditions remains largely a mystery, but the big benefit of having all these helper atoms stuck together has given chemists clues about how we might be able to make it better.
A team led by Professor Jonas Peters at Caltech decided to implement this suggestion by taking iron (Fe), a key element in both Haber-Bosch and nitrogenase, and covering it in a blanket of other atoms called a ligand. Prior work in the group demonstrated that the resulting molecule not only makes ammonia, but it makes ammonia at low temperatures. As Nik explains, “The Fe-based N2 fixation catalysts developed in our group operate at very low temperatures [–78 ºC, the temperature of dry ice], and are quite fast even at those temperatures.” However, these catalysts are only able to make a little bit of ammonia—or “turn over”—before breaking; further, sometimes the catalysts end up making the wrong product, hydrogen, instead of ammonia. To try to get a handle on why this is happening, Nik set out to take a closer look at the reactions.
Nik’s investigations started with figuring out how one of the catalyst even makes ammonia in the first place, something that might shed some light on how to make this catalyst better. Since the reaction is still fast at –78 ºC, Nik had to work in supercooled solvents (as low as –180 ºC!) and use a combination of X-ray absorption and Mössbauer spectroscopies to look at how the catalyst works. Together, these techniques showed them something surprising: their catalyst makes an iron-nitrogen triple bond, a “nitride,” on the way to making ammonia. Many researchers believed that these catalysts wouldn’t make nitrides because it would mean that the iron needs to gain and lose too many electrons (changing by up to 6!) for each ammonia molecule it makes. As Nik summarizes, “such a mechanism has been considered difficult for iron, as opposed to a metal like molybdenum, which supports a larger range of oxidation states.”
“Now that we know the system has a propensity to form this type of intermediate--a nitride--we can begin to ask why,” Nik continues. The key piece of data for this comes from the Mössbauer spectroscopy, a technique that uses gamma rays to catch a glimpse of how many electrons the iron atom has. “In our work, we have shown that our Fe catalyst can support up to six formal oxidation states,” Nik starts, “but we actually think that the Fe center really only cycles through perhaps two physical oxidation states, and this six electron couple is facilitated by 'non-innocence' of the framework supporting the Fe center.” In essence, Nik’s work shows that the ligand functions as a kind of shock absorber for the iron, stopping it from gaining or loosing too many electrons on the way to making ammonia.
While this essential role of the ligand is an important discovery on its own, Nik is excited about the potential it has to make iron catalysts even better at making ammonia. “I believe this behavior will be key in the design of novel late-metal N2 fixation catalysts.”