Alumina-supported bimetallic catalysts with ruthenium and CoNi for enhanced ammonia decomposition

Abstract

Ammonia, which comprises 17.6 wt% hydrogen, has potential as a hydrogen storage medium. Catalysts made from cost-effective transition metals such as cobalt (Co) and nickel (Ni), supported on Al₂O₃, are favored as substitutes for ruthenium in the process of ammonia decomposition for hydrogen generation at the point of use. A series of catalysts with the composition x%CoNi/Al₂O₃ (where x = 5, 10, 20, 30, and 50 wt%; and Co : Ni = 1 : 1) were synthesized using the dry mixing technique, and their activity was evaluated against analogous catalysts containing a trace amount of Ru (0.5%Ru/x%CoNi/Al₂O₃). The catalysts were analyzed through SEM, EDS, mapping, TEM, BET, and TPR techniques. However, they show highly agglomerated spherical particles, a trend seen from Ni/Al₂O₃ to CoNi/Al₂O₃, potentially affecting catalyst performance. The 5%CoNi bimetallic catalyst supported on Al₂O₃ (5%CoNi/Al₂O₃) demonstrated superior reactivity for ammonia decomposition (95%) at 674 °C, in contrast to its monometallic counterparts, Co/Al₂O₃ (691 °C) and Ni/Al₂O₃ (677 °C), at a gas hourly space velocity (GHSV) of 20 400 mL g_cat⁻¹ h⁻¹ based on total feed. Increasing the bimetallic content to 50% on Al₂O₃ sustained 95% ammonia decomposition at 500 °C, with a minor enhancement (498 °C) noted for 0.5%Ru–50%CoNi/Al₂O₃. The improved low-temperature ammonia cracking efficiency of the CoNi bimetallic system results from the synergistic interaction between Co and Ni, which promotes effective engagement with NH₂⁻ species and establishes a balanced basic character of the bimetal on the Al₂O₃ support. The established reactivity trend is Co < Ni < CoNi ≈ 0.5%Ru/CoNi on Al₂O₃ under identical reaction conditions. Both 50%CoNi/Al₂O₃ and 0.5%Ru/50%CoNi/Al₂O₃ maintained their decomposition activity for 65 hours without any deterioration.