Magnetically separable Co@Co–Al–O catalysts for NaBH4 hydrolysis: Water activation impact on activity and stability

The kinetic isotope effect (k_H/k_D, KIE) during replacement of H₂O with D₂O is a valuable tool for evaluating water activation in NaBH₄ hydrolysis. However, its correlation with catalyst activity and stability remains unclear. In this study, two magnetically separable catalysts of different activity with a Co⁰ core and a “Co–Al–O″ shell were synthesized via galvanic replacement. Compared to the non-catalytic process, water activation on the catalyst surface occurs more efficiently, resulting in reduced k_H/k_D. The active Co-O_max catalyst contains a higher amount of oxidized phases (Co₃O₄, CoO, Co(OH)₂, Co–Al layered double hydroxide), which reduce to the active component in the reaction medium. Their deficiency in Co-O_min causes a sharp activity decline and a drastic increase in k_H/k_D (>5) upon reuse, indicating a shift toward the non-catalytic pathway. In contrast, highly active Co-O_max demonstrates an efficient catalytic water activation supported by a stable k_H/k_D of 2.34 ± 0.05 over five cycles. The observed monotonic decrease in H₂ generation rate for this catalyst is probably attributable to the hydride activation step.