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Hydrous hydrazine decomposition is one of many key promising reactions for clean hydrogen generation, particularly with potential applications in sustainable energy systems. However, the reaction's selectivity towards hydrogen production versus undesired byproducts, such as ammonia and nitrogen oxides, remains a critical challenge. Metal nanoparticles such as Nickel has been found to be inactive for the catalytic decomposition of hydrous hydrazine but are active with about 33% in the gas phase catalytic decomposition process at elevated temperatures of 323K. Experimental works by Wang and co-workers have reported that at a reaction temperature of about 343 K and the addition of sodium hydroxide, NaOH (0.50–1.00 M), unsupported Ni NPs showed a hydrogen selectivity of 100%1. Experimental studies have shown that the hydroxide, OH, could suppress the formation of NH3 in the hydrazine solution during decomposition, while the rate-determining deprotonation step (N2H4 /N2H3 + H) of hydrazine decomposition could also be accelerated by the increase of OH. This study investigates the mechanistic pathways governing hydrogen selectivity during hydrous hydrazine decomposition using a combination of density functional theory (DFT) calculations and experimental validation. We explore the role of catalytic surfaces, reaction intermediates, and kinetic barriers in influencing the selectivity.