Simultaneous Production of Ammonia and Hydrogen By P/L Reaction Utilizing Efficient Atomic Nitrogen Generation By Dielectric Beads-Filled Packed-Bed Dielectric Barrier Discharge
Authors: Yoshida Souma , Murakami Naoya , Takatsuji Yoshiyuki , Haruyama Tetsuya 
Published: December 2024

Ammonia is a compound that has a wide range of uses, such as fertilizers, chemical raw materials, denitrification reducing agents, carbon-free fuels, and energy carriers, and is in high demand. All of its industrial production of ammonia relies on the Haber-Bosch process. Although the Haber-Bosch process is an excellent reaction process, it requires hydrogen (H2) as a raw material, so there is a desire to develop new synthetic methods that are more flexible. Various ammonia synthesis methods have been studied, but they still require H2 as a raw material. Currently, H2 is mostly produced by decomposing fossil fuels such as natural gas and coal. Not only will this involve emitting carbon dioxide during the H2 production, but also it will prevent us from moving away from fossil fuels.

Our research group has succeeded in developing the Plasma/Liquid (P/L) reaction, a method for synthesizing ammonia that doesn’t require H2, for the first time in the world.

The P/L reaction is a reaction that proceeds at room temperature and pressure without a catalyst. The P/L reaction involves two steps: (1) activation of nitrogen (discharge or electromagnetic wave irradiation), and (2) self-reduction reaction due to hydrogen abstraction reaction from water molecules by activated (dissociated and excited) nitrogen. In this study, we demonstrated that dissociated nitrogen (atomic nitrogen; Natom) promotes the ammonia synthesis reaction more efficiently and selectively than excited nitrogen molecules in P/L reaction.

The activated nitrogen species can be roughly divided into three states: Natom, excited nitrogen molecules (N2*), and nitrogen molecular ions (N2
+). Of these, N2
* and N2
+ are also called metastable nitrogen. On the other hand, Natom is highly reactive, so the P/L reaction will proceed quickly, abstracting hydrogen atoms from water molecules.

To verify ammonia synthesis in the P/L reaction by Natom, we performed quantitative analysis of Natom. Focusing on the fact that Natom has an absorption maximum in vacuum ultraviolet light, we analyzed the relationship between discharge conditions and quantitative results of generated Natom by vacuum ultra violet (VUV) spectroscopy. It has become clear that Natom can be generated with high selectivity by using a barrier discharge device that we designed and constructed, in which the discharge space is filled with titania beads. By directly connecting this discharger to the reactor that performs the P/L reaction, we constructed a connected reactor that eliminates oxygen contamination. In the P/L reaction using this reactor, ammonia was synthesized with high selectivity in the water phase, and the production of nitrate ions, which are typical by-products of nitrogen fixation reactions, was suppressed.

When a P/L reaction proceeds in which ammonia is produced with high selectivity, it is necessary to consider how the oxygen in the water molecule behaves. In our previous research, using electron spin resonance (ESR) demonstrated that activated nitrogen species abstracts hydrogen atoms from water molecules by forming hydroxyl radicals (·OH) after the P/L reaction. In this case, we also show that nitrates are formed by the reaction of ·OH with activated nitrogen species. However, when we performed ESR analysis on the P/L reaction which is dominated by Natom, we found that ·OH wasn’t present. This suggests that Natom abstracts two hydrogen atoms from the water molecule, leaving only the oxygen atom in the water phase. Therefore, it is necessary to clarify the behavior of this oxygen atoms. Therefore, we collected gas after the P/L reaction and performed gas analysis. As a result, it was revealed that NO and H2 were generated in the gas phase.

Based on the above results, we considered that the following reactions proceed in the P/L reaction in which Natom production selectivity was improved by PbDBD.

Natom + H2O → NH2 + O

2Natom + H2O → 2NH + O

Natom + O → NO

Natom + O → NO + H2

What is noteworthy is that H2 gas can be generated in parallel with ammonia synthesis through the P/L reaction.

The above results indicate that in the P/L reaction using PbDBD, ammonia can be synthesized with 100 % selectivity in the water phase, and H2 and NO gas can be generated in addition to ammonia in the gas phase. As mentioned earlier, ammonia has a wide range of uses and is in high demand. NO gas can be reacted with water to synthesize nitric acid. H2 can of course be used as energy. We successfully demonstrate that three useful compounds can be synthesized at once from nitrogen and water through the P/L reaction.