Pd/[C2NH2mim][Br] Thin Film Versus Pd/[C8mim][Cl] or Pd/[C8mim][BF4]: Catalytic Applications in Electrooxidation of Methanol, p-Nitrophenol Reduction and C–C Coupling Reaction

Abstract

In this study, different ionic liquids including 1-aminoethyl-3-methyl-imidazolium bromide [C2NH2mim][Br] (1), 1-methyl-3-octylimidazolium chloride [C8mim][Cl] (2) and 1-methyl-3-octylimidazolium tetrafluoroborate [C8mim][BF4] (3) were applied to stabilize Pd nanoparticles (NPs) at toluene/water interface as thin films. Field emission-scanning electron microscopy (FE-SEM) images showed puckered chains of ionic liquid (1) around the Pd NPs as flower nanostructures. Transmission electron microscopy (TEM) image of Pd/1 showed clearly core–shell nanostructures. Furthermore, applications of Pd/1, Pd/2 and Pd/3 were investigated in the Suzuki–Miyaura C–C coupling reaction in the presence and absence of ultrasonic waves, hydrogenation catalysis of p-nitrophenol reduction and methanol oxidation reaction. Interestingly, Jf (forwarding current density due to methanol oxidation) was observed only for Pd/1. We believe that interactions of –NH2 and imidazolium groups of ionic liquid 1 with Pd particles are very important in producing of puckered shells around the Pd NPs. Injection of electron density from –NH2 group and Br− of ionic liquid 1 to Pd content tends to Pd be softer than other ionic liquids (2 or 3). This effect weakens the strength of Pd–O and facilitates the intermolecular reductive elimination between Pd–O and Pd–C≡O in rate-determining step of methanol oxidation to produce CO2 product. However, electron releasing group accelerates the increasement in negative charge density of metal accelerates intramolecular or intermolecular reduction elimination. Stabilization of Pd nanoparticles from organometallic precursor in the presence of various ionic liquids is investigated. The obtained Pd/ionic liquid thin films were applied in catalytic reactions such as Suzuki–Miyaura C–C coupling reaction, p-nitrophenol reduction and methanol oxidation process for fuel cells. In spite of Pd nanoparticle thin films used for methanol oxidation reaction that exhibit no considerable methanol oxidation, these catalysts exhibit good electrocatalytic activity.