Novel porous rhodium catalysts

Nanomaterials containing an array of nano-sized pores have large surface area-to-volume ratios. They have been actively researched and developed due to their potential as catalytic and adsorption materials, offering sites for novel chemical reactions. Various forms of porous materials, such as metal organic frameworks (or porous coordination polymers), mesoporous silica and zeolites have been previously reported. In particular, nanoporous materials with metal frameworks have great potential for totally new applications.

Rhodium is an industrially important chemical element, as it has been used as a catalyst in automobile exhaust gas purification due to its high nitric oxide (NO) reduction activity compared to other metals. Because rhodium is relatively scarce and expensive, many research projects focus on maximizing its catalytic activity.

In this research project, we formed spherical micelles (molecular assemblies) of uniform size by adjusting the concentrations of polymers with both hydrophobic and hydrophilic properties (amphiphilic block polymers) in a dilute solution. We then allowed rhodium ions to undergo chemical reductions in the presence of the spherical micelles -- which functioned as templates -- under precisely controlled conditions. As a result, we succeeded in forming nanoporous rhodium particles with pore sizes corresponding to the sizes of the micelle templates used.

The resulting mesoporous rhodium exhibited NO reduction activity superior to that of commercially available rhodium catalysts, and high electrocatalytic activity for methanol oxidation reactions. Current catalyst design concepts have not adopted the technique of increasing catalytic activity by forming nanopores in metals. However, this study demonstrated that this technique may be applicable in various fields related to catalytic and adsorption materials.