Pd–Rh Nanocrystals with Tunable Morphologies and Compositions as Efficient Catalysts toward Suzuki Cross-Coupling Reactions

Abstract

Delicate elaboration of the nanostructures of multimetal catalytic materials with well-defined shapes and compositions to reveal their potential use as heterogeneous nanocatalysts for organic synthetic reactions with the combined merits of heterogeneous and homogeneous catalysis is both scientifically and technologically important, but this type of investigation has remained rarely pursued. In this work, we demonstrated a facile hydrothermal approach toward the one-pot shape-selective syntheses of Pd–Rh nanocrystals with tunable compositions and morphologies, including hollow nanocubes (NCs), nanoicosahedrons (NIs), and nanotruncated octahedrons (NTOs), using poly(vinylpyrrolidone) as both reductant and capping agent and halide anions (Br–/I–) as shape control agents. The formation of Pd–Rh hollow NCs was induced by an iodine adsorbate-induced reconstruction mechanism with KI, whereas the formation of Pd–Rh NIs and NTOs were realized by controlling the selective nucleation of twinned seeds or single crystal seeds and their relative growth rates along different facets (e.g., (111) and (100) facets) through finely adjusting the Pd/Rh ratio and the amount of KBr added in the absence of KI. Due to the great significance of Pd-catalyzed organic reactions, the catalytic performances of Pd–Rh nanocrystals for Suzuki cross-coupling reactions with different reactants were evaluated. The measured turnover frequencies (TOFs) suggested that Pd–Rh hollow NCs held considerably enhanced catalytic activities (at least 3 times) than other Pd-based solid nanocrystals including Pd–Rh NIs, Pd–Rh NTOs, Pd–Rh NCs, Pd NCs, and commercial Pd/C, with iodobenzene as the reactant. In addition, even for more inert reactants such as bromobenzene or 4-bromotoluene, the catalytic activities of Pd–Rh hollow NCs were still impressive (showing similar TOFs to those of other shapes for reactions with iodobenzene as the reactant), indicating the promising application of Pd-based nanocatalysts for other powerful Pd-catalyzed organic synthesis reactions. Meanwhile, Pd-based solid NCs, enclosed with (100) facets only, showed better catalytic performance than NIs as well as NTOs, which had a larger fraction of (111) facets other than (100) ones, further suggesting that morphology differences were vitally significant to tune the catalytic performances of bimetallic nanocatalysts.