Influence of the catalyst precursor for cobalt on activated carbon applied in ammonia decomposition

Abstract

Ammonia is a key compound for storing and transporting green hydrogen. However, the efficient release of stored hydrogen through thermocatalytic ammonia decomposition is achievable only at temperatures around 600 °C, particularly with non-noble metal-based catalysts, which prove to be both ecologically and economically more feasible. In this study, electrically conductive activated carbon was selected as the catalyst support, chosen specifically for its suitability in achieving more energy-efficient direct reactor heating through the ohmic resistance of the catalysts. Cobalt salts were wet impregnated on activated carbon investigating the influence of different precursors (cobalt nitrate and cobalt acetate) and pyrolysis temperatures (400 °C and 600 °C) under N2 flow on the cobalt particle size and the incorporation of cobalt into the carbon matrix. TEM imaging and CO-chemisorption revealed well dispersed cobalt particles with sizes below 10 nm for the catalysts synthesized from the cobalt nitrate precursor. On the other hand, cobalt acetate led to about nine times larger Co agglomerates, which were partially detached from the carbon matrix. Moreover, this substantial difference in the Co particle size results in a significantly higher ammonia conversion for cobalt nitrate-based catalysts, achieving 94 % of ammonia conversion at 600 °C. Furthermore, the long-term stability test of the cobalt nitrate-based catalyst resulted in a slight deactivation of only 2 % ammonia conversion at 500 °C.