Catalytic activity of two-dimensional cobalt-ZIF catalyst for NaBH4 hydrolysis

In this work, cobalt-ZIF catalysts were synthesized by water-based synthesis methods. Various hydrogen bond acceptor (tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB), tetrapropylammonium bromide (TPAB), tetrabutylammonium bromide (TBAB) and trimethylphenylammonium bromide (TMPAB)) were co-added during cobalt-ZIF catalyst synthesis procedures to promote the deprotonation of 2-methyl imidazole and the deprotonated 2-methyl imidazole was then further coordinated with cobalt ion to form a ZIF framework. Co-addition of TMPAB during synthesis resulted in 3D imperfect rhombic dodecahedrals with high surface area (432.3 m2/g), while, the rest of used hydrogen bond acceptors (TMAB, TEAB, TPAB and TBAB) provided 2D ZIF-L structure with relatively small surface area and pore volume. All synthesized cobalt-ZIF with different structures and morphologies were used as a catalyst for H2 generation by NaBH4 hydrolysis reaction. It was found that all 2D cobalt-ZIF structures exhibited higher hydrogen production rate than that of the 3D imperfect rhombic dodecahedral structure. This can be explained by the cushion-shaped cavity of a two-dimensional framework exhibiting higher flexibility which can promote more diffusion of reactants than the tetrahedral structure of a 3D-framework. X-ray absorption spectroscopy was used to investigate the electronic state of cobalt and to monitor the changing of cobalt probe atom during hydrolysis reaction. The electronic state of cobalt for all catalysts was Co2+ with the same tetrahedral symmetry. The lowering of pre-edge peaks of cobalt K edge XANES spectra during hydrolysis reaction indicated to distortion of the symmetry which is related to the reduction of the catalytic performance.