May 2006

Nitrogen Fix

Richard Schrock describes why finding an elusive catalyst could have a surprising impact on energy consumption.

By Richard Schrock

Molecular nitrogen (dinitrogen, N=N) makes up about 78 percent of the atmosphere. It is the most unreactive diatomic species known. Interestingly, however, nitrogen is required for all life; it is used to build proteins and DNA. Therefore, dinitrogen must be turned into a molecule that can be assimilated readily by plants. That molecule is ammonia, NH₃.

Prior to World War I, the iron-catalyzed Haber-Bosch process for ammonia synthesis at high temperatures (350 to 550 °C) and pressures (150 to 350 atmospheres) from dinitrogen and dihydrogen (H₂) was discovered. It is perhaps the most important industrial process ever developed and responsible for a dramatic increase in the population of the earth during the 20th century, because it supplies a reliable source of nitrogen for fertilizers. But because the Haber-Bosch process requires high temperatures and pressures, it consumes tremendous amounts of energy; it is estimated that as much as 1 percent of the world's total energy consumption is devoted to the process.

Nature also reduces dinitrogen using metalloenzymes in bacteria and blue-green algae, but at only one atmosphere of pressure and mild temperatures. The metalloenzymes, called nitrogenases, contain iron and usually molybdenum. Ever since their discovery more than 40 years ago, chemists have speculated about how reduction of dinitrogen occurs and whether an "artificial" nitrogenase could be developed that would lead to a more energy-efficient process than Haber-Bosch. Perhaps a thousand man-years and billions of dollars have been spent studying how nitrogenases work and trying to make artificial ones.

In 2003, my group showed that it is possible to make ammonia catalytically from dinitrogen, protons, and electrons. This is accomplished at a single molybdenum metal center. In the presence of protons and electrons in a non-aqueous medium, dinitrogen is reduced to ammonia with an efficiency in electrons of around 65 percent; the remaining electrons are used to make dihydrogen, which is in this context a wasteful and undesirable product. Our catalyst is not great, but it is a start.

Nature has developed a highly optimized version of the nitrogen reduction process over a period of a few billion years. Ours is an "artificial" nitrogenase that is barely catalytic. We are trying to identify the key problem or problems that prevent it from working well. Perhaps then we can improve its efficiency.

Can we design catalysts that will be as efficient as natural nitrogenases? Possibly. Will the Haber-Bosch process ever be replaced by catalysts that do not operate at high pressures and temperatures? Unknown. Only time, money, and ingenuity will reveal the answer.

Richard R. Schrock, the Frederick G. Keyes Professor of Chemistry at MIT, won the 2005 Nobel Prize in chemistry.