Biological Feedback from Monica Hughes

By David Bradley

Monica Hughes holds a chair in Plant Molecular Genetics at the University of Newcastle upon Tyne in the North East of England. Her research focuses on the genetic modification of cassava with a view to reducing the toxin content of this staple food of millions of people the world over. Her proudest achievement was attaining a personal chair at Newcastle but she would not reveal her most embarrassing moment.

Greener catalysts

A group of efficient catalysts has been developed by Danish researchers that might replace the well-worn iron catalysts used to synthesise ammonia without the need for expensive noble metal replacements.
  Claus Jacobsen of the Haldor Topsøe Research Laboratories, Lyngby, Denmark points out that about one percent of world energy usage is expended in the production of ammonia, which, he says, means even small improvements in efficiency could have a significant impact on fossil fuel consumption and consequently emissions.

  There have been several attempts to improve on the Haber-Bosch process beloved of school textbooks and the iron-based catalyst discovered by Mittasch. Industry still prefers the century-old multi-promoted iron catalyst method first described by Haber in which osmium and ruthenium catalysts lead to increased ammonia synthesis activities in the original process. Although a ruthenium promoted version that uses a graphite carrier has been introduced commercially by some plants. Avoiding the use of noble metals would, however, cut costs.


  Jacobsen and his team has now found several new, active and stable catalysts including Fe3Mo3N, Co3Mo3N and Ni2Mo3N which produce ammonia under 'industrially relevant conditions'.
  Catalytic activities were measured at 400 Celsius and 100 bar pressure. The inlet gas contained 4.5% ammonia in 3:1 dihydrogen-dinitrogen. He says that adding a small amount of caesium to Co3Mo3N results in a catalyst with higher activity than the commercial multi-promoted iron catalyst. For comparison the activity of a commercial multi-promoted iron catalyst, KM1, is about 750 ml ammonia per hour per gram under similar conditions but at 410 Celsius Jacobsen's caesium-promoted Co3Mo3N has the same activity. He and his team are working on improving the efficacy still further but he points out that at 50 bar and 400°C the ternary nitride catalyst is more than twice as active as the traditional iron-based synthesis catalyst.

The research is reported in more detail in Chem Commun (2000, 1057).  

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Cleaner biogas

British chemists have developed a low-temperature catalyst that allows 'biogas' to be burnt without producing toxic emissions.
  Barry Southward and Robert Burch (pictured) of The Queen's University of Belfast, Northern Ireland, point out that using renewable energy sources, such as biomass, could both limit carbon dioxide emissions and extend fossil fuel reserves. Problems arise, however, in converting biomass to a usable gaseous form because nitrogen content is converted into significant quantities of ammonia (some 600-4000 ppm, says Southward), which on burning releases nitrogen oxide (NOx) pollutants.
  Conventional fuel-lean catalytic combustion techniques have so far failed to surmount this ammonia obstacle. In contrast, Southward and Burch had previously demonstrated that it was possible to convert the ammonia component of biogas to nitrogen gas at close to 100% selectivity but only at elevated temperatures above 600 Celsius. They have now developed a new catalyst that can operate at the much lower temperature of 200 Celsius, 1%Pt/20%CuO/Al2O3 (PtCu). They tested the efficacy of this material on various simulated biogas streams containing 1000 ppm ammonia, and two levels of oxygen, employed to establish net fuel-lean or fuel-rich conditions.

  Using this new catalyst they obtained high nitrogen yields (>90%) during the catalytically mediated combustion of their simulated biogas. Additionally, by cyclically switching from fuel-lean to fuel-rich conditions for fifteen seconds and back to fuel-lean for 45 seconds they could obtain 100% nitrogen. The team suggests this cyclical approach to treating biogas might be used to effectively 'clean' the gas of ammonia. This they see as an example of making catalysts work harder by perturbing the reaction conditions to get better results.
  However, a further important element in the success of this method arises from the choice of the PtCu catalyst as this material is unique in that it is not a single catalyst material but, explains Southward, has 'two very specific but different active sites.' The Pt portion provides the first site which activates the ammonia to produce NOx, which is the rate-limiting step in N2 formation via the internal Selective Catalytic Reduction (iSCR) i.e. NH3 + 5/4O2 -> (3/2H2O) + NO + NH3 +1/4O2 -> N2 + 3/2H2O. The second - CuO- site adsorbs ammonia to produce an 'NHx' species which then reacts with the NOx formed on the Pt to give innocuous dinitrogen gas.

The team reports its findings in Chem. Commun. 2000, 1115.