Synthesis of highly dispersed Pd nanoparticles supported on multi-walled carbon nanotubes and their excellent catalytic performance for oxidation of benzyl alcohol

Abstract

Narrow sized and highly homogeneous dispersed Pd nanoparticles have been synthesized on nitric acid-functionalized multi-walled carbon nanotubes (CNTs) without a capping agent. The TEM images show that the extremely small Pd nanoparticles with an average size of about 1.5 nm were homogeneously dispersed on the surface of the CNTs. The characterization results indicate that the pretreatment with nitric acid not only improved the dispersion of Pd, but also enhanced the strong interaction between the Pd nanoparticles and the CNTs, thereby preventing their agglomeration and leaching in the liquid phase. On pretreatment with HNO₃, it is possible to generate more acidic groups on the surface of CNTs without a significant change in textural properties. The catalytic performance of the aforementioned material was investigated for selective oxidation of benzyl alcohol. Pd/CNTs exhibits high activity (~98% conversion) and selectivity (~90%) to benzaldehyde with excellent reusability. The high activity of the catalyst was attributed to the small size, high dispersion of Pd nanoparticles and higher accessibility of reactants. A careful analysis of the kinetic data suggests that there are different sites for the disproportionation and oxidation reactions. The excellent reusability of Pd/CNT catalysts makes this material a promising candidate for selective benzyl oxidation. Further, the results of the present study show that it is possible to synthesize uniformly dispersed Pd nanoparticles on various carbon supports without a capping agent.