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.
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.
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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. |
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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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British chemists have developed a low-temperature catalyst that allows
'biogas' to be burnt without producing toxic emissions. |
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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.