Matrix dependent spatial distributions of in situ formed rhodium nanostructures in ion-exchange membranes

Abstract

Rhodium nanomaterials (Rh NPs) are important heterogeneous catalysts in many organic reactions. Therefore, Rh NPs were synthesized in the commercially available cation-exchange membranes (Nafion-117 and Selemion-CMV) and anion-exchange membrane (Selemion-AMV) using sodium borohydride or formamide as the reductants. These membranes have different physical and chemical architectures. The Rh NPs were prepared by loading Rh3+ cations in the cation-exchange membranes and RhCl63− anions in the anion-exchange membrane followed by the reduction with either BH4− ions at room temperature or formamide at 65 °C. The size and spatial distributions of the Rh NPs across the thickness of the membrane were studied by the high resolution transmission electron microscopy of the thin-sliced samples. It was observed that the fractal aggregates of Rh NPs were formed by reduction with BH4− ions in all three membrane hosts. However, larger nanostructures such as rods and cubes were formed during formamide reduction of Rh3+-loaded Nafion-117. Contrary to this, uniformly distributed smaller spherical Rh NPs were observed in the Selemion-CMV by the formamide reduction. The formation and distribution of different shaped Rh nanostructures were attributed to three processes, (i) atomic diffusion-driven Ostwald ripening to form the particles, (ii) migration of the particles leading to collision and coalescence, and (iii) reshaping of the particle’s assemblies. These processes were highly dependent on the supply of in situ reduced precursor atoms and local physical conditions of the membrane matrix. The redox catalytic activities of Rh NPs in Selemion-AMV and Nafion-117 were studied using the model redox reactions involving BH4− ions reduction of p-nitrophenol and methylene blue, respectively. This study suggested that the Rh NPs were accessible to the reactants in the membrane matrix and exhibited a reasonably good catalytic efficacy of Rh NPs in the model catalyzed redox reactions.