A synthetic strategy for carbon nanospheres impregnated with highly monodispersed metal nanoparticles

Abstract

N-doped mesoporous carbon nanospheres (N-MCN@M) impregnated with uniformly dispersed noble-metal (Au, Pt, Rh, Ru, Ag, Pd and Ir) nanoparticles are rationally designed and synthesized for hydrogenation reactions. This facile and generally applicable synthetic strategy ensured confinement of the noble-metal nanoparticles within different carbon morphologies, including mesoporous spheres, hollow particles and core–shell particles. High loading of the noble-metal nanoparticles from 8 to 44% was accomplished by tuning the initial concentration of metal salts. Even at very high loadings (>40 wt%), a homogeneous dispersion of uniform metal nanoparticles throughout the carbon nanostructures was achieved. The proposed synthesis is also well suited for the fabrication of carbon spheres loaded with bimetallic nanoparticles (AuPt, AuRh and PtRh). Examination of these metal-loaded carbon particles as catalysts for the hydrogenation of benzaldehyde gave 100% selectivity toward carbonyl group at room and higher reaction temperatures. The outstanding performance of Au nanoparticles gave an unprecedented turn over frequency 2–4 times greater than those of Pt nanoparticles with the same size, loading and support. A strategy that packs more metallic nanocatalysts into carbon support materials than usual can improve catalytic hydrogenation reactions. Nanoparticles made from gold, platinum and other noble metals are finding favour as catalysts that speed up the addition of hydrogen atoms to organic molecules because of their large, active surface areas. However, to avoid agglomeration, these metals need to be ‘loaded’, or spatially separated, on solid supports. By reacting porous carbon nanospheres with metal salt precursors, Jian Liu from Curtin University in Australia and co-workers have doubled typical catalyst loading rates. Thermal treatments transformed the metals into uniformly sized nanoparticles embedded throughout the carbon support. When impregnated with gold nanoparticles, the catalyst hydrogenated benzaldehydes with 100% selectivity at carbonyl positions and had turnover frequencies two to four times higher than with platinum metals. Herein, we elaborated a facile and generally applicable synthetic strategy to ensure confinement of uniformly dispersed noble-metal nanoparticles (Au, Pt, Rh, Ru, Ag, Pd and Ir) within various carbon morphologies with controlled loadings from 8 to 44%. The metal nanoparticles were small (~2 nm), but importantly were evenly distributed throughout the carbon support even at the highest loading, allowing for a significantly higher surface area for rapid and even selective catalysis.