Ammonia is an important chemical for synthesized fertilizers traditionally and for a potential energy vector increasingly. Industrial ammonia synthesis through the Haber–Bosch process is energy-intensive and requires advanced materials to catalyze ammonia synthesis under mild conditions, whereas the utilization of ammonia as a hydrogen carrier via ammonia decomposition is facing a similar situation as well. The developed second-generation Ru-based catalysts performs superior activities over commercial Fe- or Ni-based catalysts; however, it remains challenging to construct effective Ru catalysts to achieve the usage of Ru metal affordably as well as further understanding the nature of Ru catalysis. This Perspective summarizes the recent contributions in engineering Ru-based catalysts via various strategies and related catalysis in ammonia synthesis and decomposition, and it discusses the similarities and differences of Ru catalysis in these reactions. Finally, an overview of this area and opportunities for further investigation are also provided. 

Challenges and Opportunities of Ru-Based Catalysts toward the Synthesis and Utilization of Ammonia

The emerging H2 economy faces storage and transport challenges, and the use of ammonia (NH3) as a COx-free source of H2 via NH3 decomposition has recently attracted attention. Noble Ru-based catalysts are considered the best choice for highly efficient NH3 decomposition; however, their high cost and limited availability are disadvantages in large-scale applications. Otherwise, among non-noble metal-based catalysts, Ni-based catalysts are the most active, and Ni is considered a good alternative candidate material for NH3 decomposition because of its low cost. At present, some challenges remain in efforts to improve the efficiency of both Ru- and Ni-based systems. This review covers recent developments regarding these catalysts and can serve as a comprehensive work for evaluating effective long-term strategies.

A review on the recent developments of ruthenium and nickel catalysts for COx-free H2 generation by ammonia decomposition

On-site ammonia decomposition has been considered as a potential candidate to alleviate the challenges of hydrogen storage and transportation by utilizing NH 3 (with a high hydrogen content of 17.6 wt%) as hydrogen carrier, along with the flourish of renewable energy. Although the decomposition of NH 3 into H 2 is thermodynamically favorable at above 400 °C, the reaction kinetics remains sluggish due to the high activation energy for N–H bond cleavage and N 2 desorption. This motivates the design and construction of functional catalysts with high-efficiency and low-cost. To date, a variety of metal-based catalysts have been investigated for NH 3 decomposition, among which Ru often shows the highest activity due to its optimal metal–nitrogen binding energy. Efforts are being devoted to the further improvement of catalytic performance through tuning the morphology, electronic structure, defect/doping and metal–support interaction. Meanwhile, advanced techniques are employed to disclose the structure–performance relationship of catalyst. Herein, this review identifies the fundamental principles of catalytic NH 3 decomposition, addresses the advances on current catalyst design, summarizes the strategies to enhancing efficiency, and provides recommendations for further material design. A comprehensive consideration of recent development of noble metal and transition metal catalysts are provided, as well as the promoters and supports. Moreover, the advantage of using bimetallic materials, which offers a synergy to modulate the electronic structure and improve the intrinsic activity, is emphasized. This review may serve as an informative work to inspire the future development of ammonia decomposition catalyst for practical application. • Ammonia is a promising carrier for long-distance hydrogen transportation. • The key of “H 2 –NH 3 –H 2 ” transportation is the design of ammonia decomposition catalyst. • A comprehensive outlook of novel catalysts for ammonia decomposition is provided. • The in-depth mechanism and its principal origin of catalyst is highlighted. 

Ammonia as hydrogen carrier: Advances in ammonia decomposition catalysts for promising hydrogen production

Applications of rare earth oxides in catalytic ammonia synthesis and decomposition

Development of efficient catalysts for ammonia decomposition is crucial for hydrogen production from ammonia. Herein, Ru nanoparticles (NPs) highly dispersed on Pr2O3 (Ru/Pr2O3) were prepared and investigated as catalyst for ammonia decomposition reaction. Under relatively high weight hourly space velocity (WHSV = 30,000 mL gcat−1 h−1), the Ru/Pr2O3 achieves a H2 production rate of 20.9 mmol gcat−1 min−1 at 450 °C, which is superior to Ru/Pr6O11 and many other typical oxides supported Ru catalysts. Detailed characterization results demonstrate that Pr2O3 has high density of basic sites, abundant oxygen vacancies, and strong interaction with Ru, which can modulate electronic properties of Ru NPs and enhance the intrinsic activity of the catalyst, making the catalyst highly active for ammonia decomposition. Moreover, the Ru/Pr2O3 catalyst also exhibits excellent stability and can maintain its high activity for over 100 h. The use of Pr2O3 as support offers opportunity to improve the catalytic performance of Ru-based catalysts for ammonia decomposition. 
The high density of basic sites, abundant oxygen vacancies of Pr2O3, and the strong metal–support interaction effectively promote the electron donation to Ru nanoparticles, enabling the superior catalytic performance of Ru/Pr2O3 for ammonia decomposition.

Ru Nanoparticles on Pr2O3 as an Efficient Catalyst for Hydrogen Production from Ammonia Decomposition

Cl-free Ru/CeO2 spherical catalysts were prepared by incipient wetness impregnation and subsequent washing processes using RuCl3·3H2O and a commercial spherical CeO2 support. Cs addition improved c...

Development of a Cs-Ru/CeO2 spherical catalyst prepared by impregnation and washing processes for low-temperature decomposition of NH3: Characterization and kinetic analysis results

Koji Kimura

Papers

Development of a Cs-Ru/CeO2 spherical catalyst prepared by impregnation and washing processes for low-temperature decomposition of NH3: Characterization and kinetic analysis results