Porous Polyurea Supported Pd Catalyst: Easy Preparation, Full Characterization, and High Activity and Reusability in Reduction of Hexavalent Chromium in Aqueous System

Developing efficient supports for transition-metal nanoparticles (M-NPs) to prevent their agglomeration and to facilitate their handling and recycled use has been a constant objective. Herein, porous polyurea (PPU) is prepared by simple precipitation polymerization of toluene diisocyanate through its reaction with water in water–acetone, followed by Pd­(OAc)₂ immobilization on a PPU surface to get Pd­(OAc)₂@PPU and by Pd­(OAc)₂ reduction to get a hybrid composite Pd@PPU with Pd-NPs on a PPU surface. Multiple characterizations revealed that the porous structure of PPU was retained in Pd­(OAc)₂@PPU and Pd@PPU. Pd NPs were well dispersed on the Pd@PPU surface with a mean diameter of 7.18 nm. Pd@PPU was used as catalyst for reduction of toxic hexavalent chromium Cr­(VI) to benign trivalent chromium with formic acid as the reducer. A higher catalytic performance was detected compared to that of reported heterogeneous catalysts under similar conditions. A slight decrease in catalytic activity was detected in recycled use of the catalyst, which is ascribed to CO poisoning, a byproduct in formic acid decomposition. Full regeneration was obtained by heating the catalyst under N₂ atmosphere for 2 h at 180 °C. This work provides therefore a novel approach for the preparation of a Pd@PPU catalyst of high performance for Cr­(VI) reduction, featured by low cost, facile synthesis, easy handling, and reusability.