Graphene aerogels decorated with metal (ru and ni) nanoparticles as high-performance catalysts for cox-free production of hydrogen from ammonia decomposition

Hydrogen is one of the most promising alternative energy sources to reduce the consumption of fossil fuels and their negative consequences. However, storage and transportation of hydrogen require harsh conditions, which can be overcome by storing hydrogen chemically in ammonia thanks to the advantages it offers, such as high hydrogen content, well-established infrastructure, and more importantly carbon-free elemental composition. Ruthenium is the most active metal for ammonia decomposition, which suffers from scarcity and high cost. Therefore, efficient use of this precious metal is crucial in design of the catalysts. Furthermore, nickel is the second most active metal for ammonia decomposition, and it is widely utilized owing to its broader availability and much lower cost than ruthenium. However, it requires a basic support with high surface area to reach high rates for ammonia decomposition. In this thesis study, to overcome these challenges, graphene aerogel (GA), a promising support material with high specific surface area, highly porous three-dimensional structure, high electrical conductivity, and abundant oxygen-containing functional groups, was utilized as a support to disperse both Ru and Ni nanoparticles to be able to reach high catalytic activity in COx-free hydrogen production from ammonia. Firstly, a family of Ru catalyst was prepared by dispersion of Ru on GA at high loadings to achieve a high performance in ammonia decomposition. Catalytic performance measurements on ammonia decomposition showed that the GA-supported catalyst with a Ru loading of 13.6 wt% provides an ammonia conversion of 71.5% at a space-velocity of 30,000 mL NH3 gcat-1 h-1 and at 450 °C, corresponding to a hydrogen production rate of 21.9 mmol H2 gcat-1 min-1. The addition of K increased the ammonia conversion to a record high value of 97.6% under identical conditions, reaching a hydrogen generation rate of 30.0 mmol H2 gcat-1 min-1, demonstrated to be stable for at least 80 h. A comparison of the turnover frequencies of catalysts indicated that this increase in performance upon the addition of K originated from an increase in the number of the active Ru sites and the corresponding electron density available for reaction. Secondly, three different Ni precursors with different pH of impregnation solutions were used to load Ni nanoparticles on GA as catalysts for ammonia decomposition to utilize the correct precursor based on the surface characteristics. Characterization and catalytic performance measurements on ammonia decomposition showed that the best dispersion and homogeneity, as well as catalytic performance were achieved with Ni (II) acetylacetonate (Ni(acac)2) precursor. An average Ni nanoparticle size of 13.6 ± 4.3 nm was obtained on the GA-supported Ni catalyst prepared by using Ni(acac)2 precursor and an impregnation solution with a pH of 10.2. This catalyst with a Ni loading of 11.1 wt% provided an ammonia conversion of 70.2% at a space velocity of 30,000 mL NH3 gcat-1 h-1 and 600 °C corresponding to a hydrogen production rate of 21.5 mmol H2 gcat-1 min-1. Data illustrated that the Ni precursor determines the pH of the impregnation solution, and the pH is one of the major parameters affecting the average Ni nanoparticle size and homogeneity of nanoparticles as well as the catalytic activity. Also, controlling the surface characteristics and interaction between support material and corresponding metal enhance the catalytic performance of ammonia decomposition. As a result, since impregnation solution prepared via Ni(acac)2 has the highest pH as ammonia decomposition is more favorable with basic catalysts, it provided better catalytic properties than impregnation solution prepared via Ni(NO3)2.6H2O.