Preparation of Palladium Catalysts Supported on Carbon Nanotubes by an Electrostatic Adsorption Method

Pd nanoparticles are excellent catalysts in C C coupling, hydrogenation, oxidation, and some other reactions. 10] Generally, Pd nanoparticles are immobilized on supports to hinder their aggregation during a reaction and also to ease separation and recovery from the reaction solution. To date, many materials have been used to support Pd catalysts, such as metal oxides, polymers, and carbon materials. Among these supports, carbon materials are attractive because of their ability to be tailored to meet specific needs. In addition, the carbonaceous supports can be burned off, which allows the facile recovery of precious metals. Since the report of Iijima in 1991, there has been a growing interest in the application of carbon nanotubes (CNTs) as supports in many catalytic reactions. As a result of their special physical and chemical properties, CNTs are considered to be potentially useful supports in heterogeneous catalysis. Metal particle size is one of the most crucial parameters for supported catalysts. For structure-insensitive reactions, the main target is to prepare highly dispersed catalysts. As far as the preparation of Pd catalysts is concerned, the incipient wetness impregnation (IW) method is used commonly, but the particles are usually large and disperse nonuniformly, especially in the fabrication of catalysts with a high Pd loading. The strong electrostatic adsorption (SEA) method is a simple approach to synthesize supported metal catalysts, which is based on an electrostatic mechanism in which the oxygen surface functional groups (SFGs) on the support can be protonated and deprotonated to be positively or negatively charged as a function of pH relative to the point of zero charge (PZC; the pH value at which the electrical charge density on the support surface is zero). Pt nanoparticles supported on silica and carbon materials with high dispersion have been synthesized by the SEA method. Silica-supported Pd catalysts have also been synthesized by the SEA method, in which the Pd nanoparticles have a higher dispersion than that prepared by the traditional IW method. However, there is no report on the synthesis of CNTs-supported Pd catalysts using the SEA method. In the SEA method, the PZC of the support is one of the most important factors as it can assist in the choice of an appropriate metal precursor to be used for the SEA method. The PZC value of CNTs depends on the chemical and electronic properties of the functional groups on its surface, which can be tuned by different functionalization methods. Herein, we report the preparation of CNTs-supported Pd catalysts by a facile SEA method. The CNTs supports functionalized with various kinds of surface oxygen groups (SFGs) were obtained by oxidation of the raw CNTs with concentrated acids (HNO3 and HNO3/H2SO4 with a molar ratio of 3:2) at different temperatures and characterized by TEM, N2 adsorption, zeta potential measurement, and temperature-programmed desorption (TPD). The effect of the SFGs on the PZC value of the CNTs and the adsorption of Pd cationic complexes were investigated systematically. The Suzuki reaction was used as a probe reaction to test the Pd/CNTs catalysts prepared by different methods. Pd nanoparticles supported on carbon nanotubes (CNTs) with an average Pd size of around 1 nm were synthesized successfully by a facile strong electrostatic adsorption (SEA) method. CNTs functionalized with various surface oxygen functional groups (SFGs) were obtained by treating the raw CNTs with concentrated HNO3 or HNO3/H2SO4. The SFGs could decrease the point of zero charge (PZC) of the CNTs and promoted the adsorption of Pd cationic complexes on the CNTs. In particular, the intrinsic and in situ formed (hydrolyzed from carboxylic anhydride groups) carboxylic acid groups of the CNTs during the adsorption process mainly influenced the adsorption of Pd cationic complexes. The SEA method was superior to the traditional incipient wetness impregnation method for the synthesis of highly dispersed Pd catalysts, which exhibited a high specific activity for the Suzuki coupling reaction.