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 CoCl(2)6H(2)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: 20-60nm) and the existence of narrow micro/mesopores (pore diameter: 1.5-5nm) 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 pH13.9 (1m 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 380mV and turnover frequency of 1.6x10(-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.