Porous polyurea microspheres with Pd immobilized on surface and their catalytic activity in 4-nitrophenol reduction and organic dyes degradation

Abstract

Transition-metal nanoparticles (NPs) have been widely used as high performance catalyst in organic reactions, and it is well recognized that these metal species are highly toxic, their handling is hazardous, and featured also by high-costing. Their incorporation in or immobilization on a support is one of the most opted strategies. We report here a facile two-step protocol to obtain composite microspheres (Pd@PPM2) of porous polyurea microspheres (PPM) and Pd-NPs: highly uniform PPM were prepared first by interfacial polymerization of toluene diisocyanate in water through a microfluid device, followed by Pd immobilization on their surface. Pd@PPM2 was characterized by NMR, XPS, and temperature-programmed reduction. The results indicated that Pd was immobilized on PPM surface through a coordination interaction with urea ligand. A parallel version of the composite microspheres, Pd@PPM1, was also prepared through one single step, by dissolving Pd(OAc)2 in the monomer in PPM preparation. The catalytic performance of the two composite microspheres, Pd@PPM1 and Pd@PPM2, was evaluated in the reduction of 4-nitrophenol (4-NP) and in degradation of several dyes, using NaBH4 as the reducer in both, and the results compared. In all the tested processes, Pd@PPM2 out-performed Pd@PPM1, thanks to their higher specific surface area of immobilized Pd-NPs and a larger number of active sites on the surface. The catalysts were shown also to have sustainable reusability in dyes degradation. In contrast, the catalytic activity was obviously deteriorated in its reuse in 4-NP reduction, owing to the formation of a complex between Pd-NPs and the reduction product of 4-NP, which resulted in the Pd active sites being covered by the complex, leading to the decrease in their catalytic performance. Their good reusability in dyes degradation confirmed that the decrease in catalytic activity was limited only to the processes where 4-aminophenol and similar amine containing compounds were involved. The high uniformity, easy recovery, high catalytic activity and sustainable reusability make Pd@PPM2 a great catalyst with good potential for applications in different fields.