(433a) Carbon-Free H2 Production from NH3 Triggered at Ambient Temperature with Oxide Supported Ru Catalysts

1. Introduction

Hydrogen produced from renewable energy has received a lot of attentions as a clean energy and development of a hydrogen storage and transportation system using hydrogen carrier has been greatly demanded. Among different kinds of hydrogen carrier, NH₃ is regarded as one of the promising candidates, due to high energy density, high hydrogen capacity, and ease of liquification at room temperature. Furthermore, a carbon-free hydrogen storage and transportation system could be realized by using NH₃ as hydrogen carrier. In this system, hydrogen produced from NH₃ is used in engines, fuel cells, and turbines. However, use of NH₃ as a hydrogen carrier, peculiarly for transportable devices and household, has been limited due to lack of an efficient process for producing hydrogen from NH₃.

To solve this issue, it is necessary to develop a process that can be initiated rapidly, produces hydrogen from NH₃ at a high rate, and does not need external heat input. NH₃ oxidative decomposition (Eq. 1) is a combination of NH₃ combustion (Eq. 2) and NH₃ decomposition (Eq. 3) and thus the heat generated by NH₃ combustion is utilized for NH₃ decomposition, which results in high H₂ production rate. Moreover, after ignition of NH₃ oxidative decomposition at catalytic auto-ignition temperature, the catalyst is automatically heated to reaction temperature owing to exothermic nature of the reaction and then the reaction proceeds spontaneously. Therefore, challenge is how to heat the catalyst from ambient temperature to catalytic auto-ignition temperature of NH₃ oxidative decomposition. In this research, we have discovered that the heat generated by self-heating of the catalyst heats the catalyst bed rapidly. Our results demonstrated that two kinds of heats are usable for self-heating, i.e. heat produced by NH₃ adsorption on the catalyst¹ and oxidation of the reduced catalyst. With this process, hydrogen is produced by only supplying NH₃ and O₂ at ambient temperature to pre-treated oxide-supported Ru catalysts.

NH₃ (g) + 0.25O₂ (g) → H₂ (g) + 0.5N₂ (g) + 0.5H₂O (g) ΔH = –75 kJ mol^–1 (Eq. 1)

NH₃ (g) + 0.75O₂ (g) → 0.5N₂ (g) + 1.5H₂O (g) ΔH = −317 kJ mol^–1 (Eq. 2)

NH₃ (g) + 0.25O₂ (g) → H₂ (g) + 0.5N₂ (g) + 0.5H₂O (g) ΔH = –75 kJ mol^–1 (Eq. 3)

We also succeeded in construction of cyclic process where NH₃ oxidative decomposition is triggered repeatedly without pre-treatment from 2^nd to 5^th cycle.

2. Methods

RuO₂/γ-Al₂O₃ and RuO₂/Ce_0.5Zr_0.5O₂ were prepared by wet impregnation method.

After the pre-treatment shown in the section of results and discussion, acitivty tests were carried out under quasi-adiabatic conditions. An NH₃/O₂/He (NH₃/O₂/He ratio, 150:37.5:20.8 ml min^−1;GHSV, 62.5 Lh^−1g^−1) gas mixture was then fed at room temperature to the catalyst. We set the gas composition by assuming the Eq. 1. The composition of the exit gas was monitored with a quadrupole mass spectrometer and analyzed with a thermal conductivity detector after 30 min.

3. Results and discussion

At first, theRuO₂/γ-Al₂O₃ catalyst was treated in He at 300°C to remove adsorbate and create NH₃ adsorption site and then the catalyst was cooled to ambient temperature. After supplying the NH₃/O₂/He mixture, H₂ production rate increased suddenly to more than 30 Lh^−1g^−1 within 0.5 min and catalyst bed temperature rose to 522 ^oC at the same time¹. After the reaction for 30 min, O₂ was consumed completely: NH₃ conversion was 93% and H₂ yield was near the maximum value, 67 %. These results demonstrate that NH₃ oxidative decomposition was triggered in a very short time without any external heat input and proceeds spontaneously. After 35 min, the reaction was terminated by substitution of He for the NH₃/O₂/He mixture, and the catalyst was cooled to ambient temperature. Then, O₂ was supplied over the catalyst to oxidize the Ru metal formed during the reaction, and an NH₃/O₂/He mixture was fed to the catalyst. This purge-feed sequence was repeated for three more times. For all cycles, the NH₃ oxidative decomposition was repeatedly triggered at ambient temperature, and high H₂ yields as well as high NH₃ and O₂ conversions were maintained for all cycles. After the second cycle, ammonia was desorbed in situ (regeneration of NH₃ adsorption sites) during the reaction.

Next, RuO₂/Ce_0.5Zr_0.5O₂ catalyst was treated in H₂ at ambient temperature to reduce the catalyst. After supplying the NH₃/O₂/He mixture, catalyst bed temperature increased dramatically and H₂ production starts. After the reaction for 30 min, O₂ as well as NH₃ were consumed and H₂ yield reached 67 %. These results demonstrate that NH₃ oxidative decomposition was triggered without any external heat input. After 35 min, the reaction was terminated by stopping flow of O₂ and He in NH₃/O₂/He mixture, and the catalyst was cooled to ambient temperature. Then, an NH₃/O₂/He mixture was fed to the catalyst. This purge-feed sequence was repeated for three more times. For all cycles, the NH₃ oxidative decomposition of ammonia was repeatedly triggered at ambient temperature. After the second cycle, the catalyst was reduced to Ru/Ce_0.5Zr_0.5O_2-xin situ by H₂ produced during the reaction.

4. Conclusions

Our results revealed that self-heating of the catalyst bed from ambient temperature is used as a trigger for the NH₃ oxidative decomposition. To heat the catalyst to the catalytic auto-ignition temperature, we successfully used the heat generated by NH₃ adsorption onto RuO₂/γ-Al₂O₃ or oxidation of reduced Ru/Ce_0.5Zr_0.5O₂. This study demonstrates the concept of self-heating of catalysts, which is a novel strategy for the cold-start process for H₂ production from NH₃.

Reference

1. Nagaoka, T. Eboshi, Y. Takeishi, R. Tasaki, K. Honda, K. Imamura, K. Sato Sci. Adv. 2017; 3: e1602747.