PROCESS CHEMISTRY
of the development of his process from 1908 onwards. In the 1960s, Atsumu Ozaki of Tokyo Institute of Technology and his PhD student Ken- ichi Aika discovered that ruthenium is an efficient catalyst for ammonia synthesis. After initial problems, an improved version of the ruthenium catalyst is now used commercially in several production facilities. Ruthenium catalysis is also one of the foundations of Japan’s continuing search for more efficient methods. One approach is to apply super
promoters to provide electrons that destabilise nitrogen by weakening the triple bond and making the molecule more reactive for ammonia synthesis. This was first reported in 2012 by the group of Hideo Hosono at the Tokyo Institute of Technology, Japan,3
in combination with 'electrides' – a new class of ionic materials where electrons serve as the anions. Electrides typically contain ‘caged’ electrons that have a very low work function, meaning that very little energy is needed to liberate them from the solid state. Other research groups have
achieved the injection of electrons into the N-N-triple bond using basic metal oxides. For instance, the group of Masaya Matsuoka at Osaka Prefecture University, Japan, also reported promising results with the ruthenium-loaded alkaline earth titanate BaTiO3
in 2013.4 In a recent review study, Hosono
and colleagues conclude that 'the combination of ruthenium catalysis and the electride C12A7:e- as electron donor operates with a reaction mechanism different from conventional ruthenium catalysts'. In this new mechanism, dissociative adsorption of the nitrogen molecule is no longer the rate limiting step of the reaction. The method operates at atmospheric pressure and temperatures between 250 and 400°C, and hydrogen poisoning of ruthenium catalysts is no longer a problem.5 Hosono’s group also developed
a more efficient catalyst ruthenium- loaded Ca(NH2
)2
researchers derived from their work with electride catalysts.6
exhibits the highest activity and excellent long-term stability,’ says Hosono, who sees the future of his methods in distributed, small scale
based on an idea the ‘This catalyst
who used ruthenium catalysts
applications of ammonia synthesis. ‘We are aiming at on-site ammonia synthesis. This is like a PC while the Haber Bosch method is a mainframe computer,’ Hosono explains. Meanwhile, the group of
Katsutoshi Nagaoka at the University of Oita, Japan, recently claimed a new record synthesis rate with a nanostructured ruthenium catalyst
In the Haber Process to produce ammonia the hydrogen (red) and nitrogen (blue) are first cleaned (lower left), mixed, and then compressed (centre left, 200 atmospheres). The mixture cycles through the reaction tower (right) and a catalyst (iron, on hori- zontal trays) and heat (450°C) form am- monia. A cooling loop (water, pale blue) at the base of the reaction tower condenses the ammonia into a liquid (orange) that is piped off at lower right
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SCIENCE PHOTO LIBRARY
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