Anatomy of high recyclability of graphene oxide based palladium nanocomposites in the Sonogashira reaction: On the nature of the catalyst deactivation

Although graphpne based palladium (Pd) nanocomposites are known as efficient catalysts in the Sonogashira C-C cross-coupling reaction, it has been reported that the catalysts are easily deactivated after several uses. To evaluate the nature of the deactivation, we have prepared Pd nanoparticles (NPs) dispersed on reduced graphene oxide (Pd/RGO) and graphene oxide (Pd/GO), and characterized their morphological and electronic structures in the recycling of the Sonogashira reactions using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and transmission electron microscopy (TEM). Here, we demonstrate that Pd/GO revealed higher recyclability in the Sonogashira reaction when compared with Pd/RGO. The origin of the remarkable recyclability of Pd/GO is the presence of oxygen functionalities on the surface of GO, which can provide nucleation sites for the detached Pd adatoms during the Sonogashira reaction. As a result, Pd/GO can continuously promote the dispersion of smaller Pd NPs and keep providing highly reactive catalytic sites for the consecutive Sonogashira reactions. We also found strong evidence that oxidation states of Pd in the catalysts are not closely associated with catalytic efficiency and recyclability in the Sonogashira reaction.