Researchers at Oita University in Japan have developed an innovative process for the production of hydrogen from ammonia without the need for an external heat source to initiate or maintain the reaction. An open access paper on their work is published in the journal Science Advances.
Liquid ammonia (NH3) has been considered as a carrier (storage medium) for hydrogen that could alleviate the challenges of transporting, handling and storing hydrogen for commercial applications. However, the adoption of ammonia as a H2 carrier, especially for household and transportable devices, has been limited due to the lack of an efficient process for producing H2 and nitrogen by the oxidative decomposition of ammonia.
Conventional production of hydrogen from ammonia by catalytic decomposition is challenging because the endothermic nature of ammonia decomposition requires that the catalyst be continuously heated by an external heat source during the reaction. Typically, the Oita team noted, a high temperature is needed for ammonia decomposition; for example, equilibrium calculations show that a temperature of 400°C is required to convert 99.1% of ammonia to its decomposition products at 0.1 MPa. Heating the catalyst from room temperature to the required reaction temperature using an external heat source takes time and energy.
The research team, led by Dr. Katsutoshi Nagaoka and Dr. Katsutoshi Sato, set out to develop a process that could be initiated rapidly, and that could produce H2 at a high rate without the need for external heat.
They found that exposing ammonia and O2 to a pretreated catalyst consisting of RuO2 nanoparticles supported on γ-Al2O3 at room temperature (~25°C) triggers the exothermic oxidative decomposition of ammonia, producing hydrogen is produced at a high rate.
|Schematic of the catalytic cycle developed for oxidative decomposition of ammonia. Nagaoka et al. Click to enlarge.|
Before use, the RuO2/γ-Al2O3 catalyst is treated under an inert gas (helium) at 300°C to remove H20 and CO2 adsorbed on the catalyst, resulting in the formation of ammonia adsorption sites. Upon addition of the mixed ammonia and O2 to the catalyst at room temperature, ammonia is adsorbed onto the catalyst, thereby generating substantial heat. This heat rapidly increases the catalyst bed temperature to the catalytic autoignition temperature of ammonia combustion, and the oxidative decomposition of ammonia begins.
Because the temperature of the catalyst bed during the reaction is higher than 300°C, the adsorbed ammonia is desorbed in situ (self-regeneration of NH3 adsorption sites). If the catalyst is cooled without exposure to ammonia, then the ammonia adsorption sites remain unoccupied.
To reboot the process, the oxidative decomposition of ammonia can be repeatedly triggered from room temperature without heat treatment in an inert gas. This completes a catalytic cycle that requires no external energy source, the team said.
Our discovery utilizes a simple fundamental physicochemical process, namely adsorption, to operate a reaction with minimal energy input. We expect this to contribute to the development of efficient, carbon-free energy production and thus to global solutions for energy and climate crises.—Dr. Nagaoka
Katsutoshi Nagaoka, Takaaki Eboshi, Yuma Takeishi, Ryo Tasaki, Kyoko Honda, Kazuya Imamura, and Katsutoshi Sato (2017) “Carbon-free H2 production from ammonia triggered at room temperature with an acidic RuO2/γ-Al2O3 catalyst” Science Advances Vol. 3, no. 4, e1602747 doi: 10.1126/sciadv.1602747