Palladium/graphene oxide nanocomposites with carbon nanotubes and/or magnetite for the reduction of nitrophenolic compounds

Abstract

Graphene oxide (GO) was synthesised via the oxidation of graphite and was characterised using ATR FTIR, PXRD, SEM, TEM and TGA. These techniques confirmed the presence of characteristic oxygen-containing functional groups and the resulting increase in interlayer spacing in the nanostructure. GO is used as the support to form nanocomposites composed of combinations of the following: iron oxide nanoparticles (Fe₃O₄), carbon nanotubes (CNT) and palladium nanoparticles (Pd). The four final nanocomposites formed are: Pd/GO, Pd/Fe₃O₄/GO, Pd/CNT/GO, and Pd/CNT/Fe₃O₄/GO. Key intermediates were analysed using ATR FTIR for the confirmation of the modification. Additionally, all composites and their precursors underwent electron microscopic analysis to visually assess composite morphologies and the size distribution of deposited nanoparticles. The Fe₃O₄ and Pd nanoparticles were indistinguishable from each other in their spherical shape and particle diameters, which were no bigger than 32 nm. From the TGA, incorporation of Fe₃O₄, CNT and finally Pd into the nanocomposites increased total thermal stability in terms of mass percentage lost over the temperature programme. GO showed significant decomposition, with all nanocomposites remaining relatively stable up to 120 °C. ICP OES results showed total Pd content by mass percentage for each final composite, varied from 7.9% to 9.1% mass Pd/collective mass. XPS confirmed the expected elemental compositions of composites according to their structures and the Pd⁰ : Pd^II ratios are obtained. The nanocomposites were tested for the catalytic reduction of nitrophenols. Pd/CNT/Fe₃O₄/GO gave the highest TOF′ for the reduction of 4-NP and 2-NP. For the reduction of 3-NP, Pd/GO showed the highest TOF′. Nitrophenol's pK_a and catalyst TOF′ correlated in a direct proportional relationship for Pd/GO and Pd/Fe₃O₄/GO. It was found that Pd⁰ surpassed Pd^II in catalytic activity. Reduction of Pd^II to Pd⁰ took place during the first catalytic cycle.