Bettering Haber-Bosch

Cambridge researchers think they’ve figured out a way to improve the efficiency of ammonia synthesis process, research which could have wide-reaching implications in the agricultural and energy sector.

Ammonia is currently produced from natural gas via the Haber (or Haber-Bosch) process, but researchers from the University of Cambridge think they’ve devised a more efficient process using ultra high vacuum (uhv) conditions.

“The Haber-Bosch process was developed in the early twentieth century but little has changed since that time,” said Dr Steve Jenkins from the department of chemistry, “Clearly, given the massive scale of worldwide ammonia production, even a tiny improvement in the efficiency of the ammonia synthesis process can have massive implications, not only for the economics of fertiliser production, but also for global energy demand.”

“The surface science approach uses single-crystal iron samples of high purity, and typically involves experiments carried out under uhv conditions (i.e. pressures less than one millionth of a millionth of an atmosphere,” Jenkins continued, “We have conducted experiments that combine some of the attractive features of single-crystal uhv surface science with those of higher pressure techniques.”

Researchers exposed an iron sample to nitrogen to build up coverage of nitrogen atoms on the surface – one atom per two top-layer iron atoms at the surface. Under uhv conditions, they quantified nitrogen coverage using Auger Electron spectroscopy (AES) before exposing the sample to 0.6mbar H₂ gas for several minutes – a much lower pressure than used in industrial processes which allows the reaction to proceed rapidly enough for researchers to take meaningful measurements over a timescale of minutes.

Researchers work episodically, exposing the sample to cycles of AES and uhv and measuring the drop in surface nitrogen and so the corresponding production of ammonia as a function of time and temperature.

“Our results suggest, that under certain conditions – namely when the ammonia pressure is kept low – the hydrogenation steps (from N to NH to NH₂ to NH₃) may actually be the most important,” said Professor Sir David King.