Progress in Ammonia Combustion Catalysts

On February 14 the Journal of Physical Chemistry published a paper entitled “Local Structures and Catalytic Ammonia Combustion Properties of Copper Oxides and Silver Supported on Aluminum Oxides.”  The paper, by Satoshi Hinokuma of Kumamoto University in Kumamoto, Japan and four co-authors, reports on a catalyst system that is well adapted for use in ammonia energy applications.

The research builds on work by scientists in other parts of the world focused on managing fugitive ammonia emissions.  A 2013 paper by Magdalena Jablonska and two co-authors published in the journal Chemik notes that “the problem of NH3 emission into atmosphere is becoming an increasingly important issue due to the numerous processes in which ammonia is used as reactant or produced as byproduct.”  The paper cites production of urea and other nitrogen fertilizers, gasification of coal and biomass, and the use of urea in selective catalytic reduction (SCR) processes designed to control emission of nitrogen oxides (NOx).

In Japan, ammonia is building momentum as an important element in a hydrogen-oriented energy economy.  Hinokuma’s article starts with the premise that ammonia is a “renewable and carbon-free energy source” and quickly cites the work of Hideaki Kobayashi and his team at Tohoku University to develop a gas turbine powered by ammonia (recently covered here in Ammonia Energy News).  For ammonia to reach its potential as an energy carrier, the article says, “new NH3 combustion systems [are] required,” with particular promise seen for catalytic combustion.

As indicated by the Jablonska article, the previous work in this area was focused primarily on oxidizing low-concentration ammonia emissions.  Applying catalytic combustion in this context makes sense because the oxidation reaction can be sustained at the relatively low temperatures (c. 400 degrees C) that would occur when only a small amount of a combustible species is present in a flue stream.  A key feature of this approach, according to the Hinokuma article, is that “NOx emission is greatly diminished by the low operating temperatures.”

This sets up the development target that Hinokuma and his team set themselves: a catalyst system that could withstand the elevated temperatures found in devices whose purpose is to extract energy from ammonia (c. 900 degrees C) — working both efficiently and durably, and without generating the levels of NOx characteristic of high-temperature combustion.

The approach Hinokuma and his colleagues developed is a variant of a “binary system” that involves both silver (Ag) and oxides of copper (CuOx) supported on alumina (Al2O3).  Other researchers had established the system’s effectiveness at low temperatures.  Working with a variety of tools that reveal material microstructures, for example, “high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM),” the Kumamoto University team developed a process that employs thermal pretreatment, sequential impregnation of the alumina material with the catalytic species, and subsequent thermal aging to prepare, apply, and condition the catalytic system.

The team’s investigation showed that the key to the system’s success is the “highly dispersed and closely proximate CuOx and Ag nanoparticles” that are distributed throughout the supporting alumina.  The paper concludes, “The present study demonstrates that binary CuOx−Ag catalysts supported on Al2O3 suppress N2O/NOx production from the catalytic combustion of NH3 as a carbon-free energy source.”

The paper does not identify specific applications for the catalyst system.  However, in other reporting this week, a story has been posted on about two Japanese industrial demonstrations involving ammonia in dual-fuel electricity generation installations.  In both cases the second fuel has a fossil origin.  Progress toward full sustainability – in other words more ammonia and less fossil fuel — may very well be contingent upon the availability of catalyst systems like the one developed by Hinokuma and his colleagues that can support high-temperature ammonia combustion without excessive NOx production.

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