Cobalt Phosphonates as Precatalysts for Water Oxidation: Role of Pore Size in Catalysis

We report a simple approach for the synthesis of cobalt phosphonate (CoOP) nanocages with two distinct types of pore diameters by utilizing a novel tetra-constituent assembly of CoCl₂ ⋅6 H₂ O, nitrilotris(methylene)triphosphonic acid (NMPA), F127 surfactant, and polyvinyl alcohol (PVA, co-surfactant). Transmission electron microscopy images revealed the formation of large nanocages in spheres (pore diameter: 20-60 nm) and the existence of narrow micro/mesopores (pore diameter: 1.5-5 nm) on their walls. Brunauer-Emmett-Teller adsorption/desorption experiments led to the observation of dual porosity and indicated that the contribution of micro/mesopores increased gradually with increasing concentration of PVA during synthesis from CoOP-0 to CoOP-15 (where the number gives the wt % of PVA used in CoOP synthesis). These materials acted as precatalysts for heterogeneous water oxidation at pH 13.9 (1 m KOH) and electrochemical studies revealed that the reactivity improved remarkably with increasing contribution of narrow micro/mesopores. Among these catalysts, the best catalyst (CoOP-15) exhibited an overpotential of 380 mV and turnover frequency of 1.6×10^-2  s^-1 . The improvement of reactivity was due to significant enhancement of electrochemically accessible surface area with increasing contribution of narrow micro/mesopores, which facilitated contact between the catalyst and water molecules by improving mass transport inside the nanomaterials. Hence, this study suggests narrow micro/mesopores are beneficial towards enhancement of water oxidation catalysis.