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A carbon-free energy economy using hydrogen as a clean energy source has been a dream of many scientists since the hydrogen economy was first proposed in the 1970s. The reaction of hydrogen with oxygen produces only energy and water, making it the ideal clean energy source.
However, hydrogen-based technologies have failed to reach mainstream popularity, possibly due to the low energy density of hydrogen and the difficulties associated with handling gasses. The use of hydrogen carrier compounds such as methanol, formic acid, and ammonia have been proposed to overcome these problems.
Ammonia is a particularly promising hydrogen carrier due to its relatively low cost, high energy density, and ease if liquefaction. Until now the use of ammonia as a hydrogen carrier has been limited by the absence of an efficient process for decomposing ammonia to hydrogen and nitrogen.
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Due to the highly endothermic nature of ammonia decomposition, the production of hydrogen from ammonia typically requires temperatures of 400 °C combined with a catalyst such as RuO2/La2O3.
A group of researchers from Japan has now discovered a new method of producing hydrogen from ammonia at room temperature, which may represent a step forward for the hydrogen economy.
Figure 1. Schematic diagram of the catalytic cycle for the oxidative decomposition of ammonia.
A recent article in Science Advances describes the new method. A pre-treated RuO2/γ-Al2O3 catalyst was exposed to ammonia. The ammonia then adsorbed to the catalyst surface, an exothermic process leading to the production of heat.
As a result, the temperature of the catalytic bed increased and eventually exceeded the autoignition temperature of ammonia, resulting in oxidative decomposition of ammonia and the production of hydrogen (Figure 1). The lead author, Dr. Nagaoka, described the method as utilizing “a simple fundamental physicochemical process, namely adsorption, to operate a reaction with minimal energy input.”
The performance of the pretreated RuO2/γ-Al2O3 catalyst was compared with a RuO2/La2O3 catalyst pretreated in the same way. While the RuO2/γ-Al2O3 catalyst was able to initiate the oxidative decomposition of ammonia under a variety of conditions, the RuO2/La2O3 catalyst was unable to produce any hydrogen without the use of an external heat source.
Microcalorimetry combined with a volumetric gas adsorption analyzer was used to reveal the differences between the two catalysts. The catalyst samples were pretreated in helium at 300 °C, then cooled to 50 °C and evacuated. Next, 11 μmol of ammonia was gradually added to the sample cell, and the differential heat was detected using heat flux transducers.
The heat evolved by the adsorption of ammonia was much greater for the RuO2/γ-Al2O3 catalyst than the RuO2/La2O3 catalyst (Figure 2). Microcalorimtery was able to successfully demonstrate the self-heating ability of the RuO2/γ-Al2O3 catalyst upon addition of ammonia, and the results suggest that when the RuO2/γ-Al2O3 catalyst was exposed to ammonia, more heat was evolved due to an increase in both chemisorption onto the catalyst surface, and physisorption of further layers of ammonia (Figure 3).
Figure 2. Total heat evolution and amount of ammonia absorbed as a function of the adsorption equilibrium pressure.
Figure 3. Schematic diagram of ammonia adsorption on RuO2/γ-Al2O3.
The RuO2/γ-Al2O3 catalyst was found to be stable, with no significant reduction in hydrogen production after 100 hours under reaction conditions. Furthermore, cycle tests revealed that terminating the reaction, cooling, and re-initiating the reaction resulted in no losses of activity.
Although more research is required to realize the utilization of this method in hydrogen energy applications such as fuel cells, engines, and turbines, Dr. Nagaoka commented that the team “expect this to contribute to the development of efficient, carbon-free energy production and thus to global solutions for energy and climate crises."
Nagaoka K., Eboshi T., Takeishi Y., Tasaki R., Honda K., Imamura K., Sato K., Carbon-free H2 production from ammonia triggered at room temperature with an acidic RuO2 /γ-Al2O3 catalyst. Science Advances, 3(4), 2017, e1602747.
Oita University. "Discovery of a facile process for hydrogen production using ammonia as a carrier." ScienceDaily. ScienceDaily, 29 April 2017.