Turning properties of Ni/Al2O3 catalyst to improve catalytic ammonia decomposition for green hydrogen production: pH does matter!

In this study, the influence of solution pH (6.0–12.5) of the cation–anion double hydrolysis (CADH) method in the formation of Ni/Al₂O₃ catalyst and its catalytic performance for green hydrogen production NH₃ decomposition was studied for the first time. The physicochemical properties of the prepared catalysts were systematically characterized by various analysis techniques. The results indicated that pH conditions significantly influenced both the structural properties and catalytic activity of the derived Ni/Al₂O₃ catalysts. The better catalytic activity for NH₃ decomposition over catalysts prepared under high pH conditions (pH ≥ 10.0) is mainly due to the appropriate synergic effect of the interaction between Ni and Al₂O₃ support, large active Ni surface area, and suitable porosity, particle size, and basicity of the catalyst. The correlation analysis and density functional theory (DFT) calculations confirm that the percentage of surface metallic Ni plays a crucial role in controlling the catalytic activity of Ni/Al₂O₃ catalysts. The 40Ni/Al₂O₃ catalyst (Ni = 40 wt%) could achieve over 94.5 % NH₃ conversion under the harsh reaction conditions of 600 °C and NH₃ WHSV of 54000 mL/g_cat./h, and that stably maintained for 200 h without any obvious deactivation. Overall, our work not only highlights the critical role of pH conditions in the CADH solution for cost-effective Ni/Al₂O₃ catalyst preparation but also proposes a promising strategy for developing a highly active, stable, and Ru-free catalyst for practical hydrogen production via NH₃ decomposition.