Highly Dispersed Ru/Al2O3 Catalysts for Enhanced Ammonia Decomposition: A Simple Atmosphere-Induced Approach

Hydrogen energy is a renewable fuel that holds great potential as a clean energy source in various fields. However, the transportation and storage of hydrogen face limitations due to its low volume density. Ammonia, with its high hydrogen content (17.7 wt%) and energy density (3000 Wh/kg), has emerged as a promising hydrogen storage material for green energy cycles. Additionally, ammonia is already widely produced in industries. However, the lack of an efficient catalyst for ammonia decomposition has hindered the industrial application of hydrogen from ammonia.

Extensive research has been conducted on catalysts to enhance the decomposition of ammonia for hydrogen production, and ruthenium (Ru) has been recognized as the most efficient catalyst for this process. The efficiency of NH3 decomposition on Ru catalysts is known to be structure-sensitive, dependent on factors such as Ru particle size, morphology, and support properties. Recently, Ru nanoparticles supported on reducible metal oxides like CeO2, ZrO2, or Pr2O3 have shown good catalytic activity. Ru/CeO2 catalysts, in particular, have exhibited slightly higher catalytic activity due to the strong metal-support interaction (SMSI) and electron modification of Ru active sites by ceria. However, Ru/CeO2 catalysts tend to rapidly deactivate over time, likely due to the gradual decrease in exposed Ru sites. Aluminum oxide, a non-reducible and widely used catalyst support, offers high surface area, structural stability, and commercial availability. However, under high reduction conditions, such as during ammonia decomposition, highly dispersed nanoparticles supported by oxides are prone to sintering. In Ru-based catalytic systems, the size and morphology of active Ru particles and the support greatly affect the catalyst's performance. By changing the crystal phase of the support and the calcination temperature of the catalyst, the metal dispersion and morphology can be controlled. Ru catalysts with different particle sizes exhibit varying ammonia decomposition activities, with the maximum turnover frequency (TOF) occurring at particle sizes of 2.2 nm or 7 nm.

The process of ammonia decomposition involves a stepwise dehydrogenation process, with the rate-limiting step being the desorption of recombined N2 on the Ru catalyst during the reaction. The 'electron-donating effect' within the catalyst enhances electron transfer and accelerates the rate-limiting step. In Ru/ZrO2 catalysts, the electron-donating effect plays a crucial role in ammonia decomposition activity. The Ru/LaxCe1-xOy catalyst, which exhibits relatively small Ru particle size, strong interaction, and appropriate acid-base properties, effectively enhances the desorption of H2 and N2 during ammonia decomposition. Basic promoters such as K and Cs can provide electrons to the surface sites, promoting the desorption of recombined N2 and thus facilitating the decomposition of ammonia. However, ammonia decomposition is an endothermic reaction that typically requires high-temperature reaction conditions (>450 oC) on noble catalysts, especially when using metal catalysts supported on irreducible alumina supports, which have weak interactions between the active metal and support, resulting in catalyst deactivation. Recent studies have found that utilizing the abundant hydroxyl groups on alumina can control the dispersion of nanoparticles. The hydroxyl group content and density play a key role in the redispersion of metal particles on the oxide surface, influencing reactivity. Additionally, the interaction between the metal and support can be controlled by changing the reaction atmosphere, as the pretreatment time and temperature promote the migration and dispersion of Ru nanoparticles, leading to the formation of small-sized active metals and the exposure of more active sites. The strength of the metal-support coupling, metal morphology, surface atomic configuration, and SMSI mode of the metal-support interaction are closely related to the catalyst's preparation process and conditions.

In this study, we developed highly dispersed Ru/Al2O3 catalysts using a simple atmosphere-induced method, which exhibited high ammonia decomposition activity and long-term stability. By employing different pretreatment atmospheres and temperatures, we obtained Ru/Al2O3 catalysts with different particle sizes. In the reducing atmosphere (Ru/Al2O3-R), the Ru particle size remained highly dispersed, measuring less than 2 nm. To validate the universality of our synthesis strategy, we also synthesized Ru/S (S=SiO2 and TiO2) catalysts. Under a reduction condition of 500 oC, the Ru nanoparticles supported on different supports maintained high dispersion. Additionally, we used alkali metal K as an electron donor to enhance the electron density of Ru, thereby improving the ammonia decomposition activity. Consequently, we successfully prepared highly dispersed small-scale Ru catalysts by the atmosphere-induced method, which exhibited high ammonia decomposition activity and excellent stability.