Facile synthesis of hexagonal α-Co(OH)2 nanosheets and their superior activity in the selective reduction of nitro compounds

Abstract

Novel hexagonal α-cobalt hydroxide nanosheets are synthesized through a 2-methylimidazole-induced hydrolysis strategy with cetyltrimethylammonium bromide (CTAB) as a surfactant. The weak alkaline environment provides favorable conditions for the formation of metastable α-Co(OH)₂, while the same raw material will produce β-Co(OH)₂ when a strong alkali solution is used. CTAB plays a vital role not only in hexagonal oriented growth, but also in the formation of the hydrotalcite-like structure of α-Co(OH)₂ with high crystallinity. The crystallinity of both α- and β-Co(OH)₂ is very poor without CTAB as a surfactant. The Co in this Co(OH)_2−x layer presents most of the Co^II and a small part of the Co^III, and the interlayer nitrate anion balances the positive charge of the host layer. The redox function produced by the Co^II and Co^III of α-Co(OH)₂ together with the large layer spacing jointly promotes the electron and mass transfer. The use of hydrazine hydrate for transfer hydrogenation involves the transport of protons and electrons produced by decomposition, and the rapid transport is bound to be conducive to the reduction process. Nitro compounds with varieties of functional groups can be smoothly reduced to the corresponding amines with high selectivity, when α-Co(OH)₂ was used as a catalyst under mild conditions.