Reduction Of Hexavalent Chromium Over Palladium Nanocatalysts Supported On Carbon Materials

With the rapid development of the industry, chromium pollutants containing Cr(Ⅵ) ions have been focused due to their high mobility and high toxicity. Cr(Ⅲ) ion is less toxic and can form Cr(OH)3precipitate that can be removed easily. Conversion of Cr(Ⅵ) to Cr(Ⅲ) by chemical reduction method is proved to be an effective way to remove poisonous chromium. Formic acid with strong reducing property is good for reducing Cr(Ⅵ) to Cr(Ⅲ) in the presence of Pd, which only provides two products as CO2and H2. Although the colloidal Pd nanocatalysts show high catalytic activity in the reduction of Cr(Ⅵ) to Cr(Ⅲ), studies on the supported Pd nanocatalysts are more valuable in order to recycling the noble metal catalysts.In this dissertation, three kinds of carbon composites (expanded graphite EG, SBA-15/EG, and Fe3O4@C) are prepared as supports. The supported palladium nanocatalysts are synthesized through wet-impregnation with the aid of polymer. The synthesized palladium nanocatalysts are investigated in the reduction of Cr(Ⅵ) using formic acid as reductant. Effects of reaction temperature, pH, amounts of catalysts and recycle times are discussed on the catalytic reduction of Cr(Ⅵ) to Cr(Ⅲ). Details are shown below:(1) Three kinds of carbon materials are synthesized including expanded graphite(EG), SBA-15/EG and Fe3O4@C. The carbon composites are porous and stable, which benefit the immobilizing and dispersing of Pd nanoparticles on them. Among these carbon materials, Fe3O4@C has a biggest specific surface area and easily adsorbs more Pd particles.(2) Comparing with the colloidal Pd nanoparticles, the supported Pd nanocatalysts exhibit high catalytic activity and thermal stability. In the range of reaction temperature25℃-65℃, catalytic activities of the three supported Pd nanocatalysts rise with the increase of the temperature, while catalytic activities of the colloidal Pd nanocatalysts decrease at the high reaction temperature. The optimum pH of reaction is4.0and3.0for EG-Pd and Fe3O4@C-Pd catalysts, respectively, however, the catalytic activity of SBA-15/EG(1:1)-Pd gradually increases with the decrease of pH. With the amount of catalysts increasing, the reduction rate increases correspondingly and then keeps constant, which means the reaction is diffusion controlled when too much catalysts are used.(3) The recycling stabilities of the three supported Pd nanocatalysts in the reduction of Cr(Ⅵ) are different. The EG-Pd catalysts exhibit stable catalytic performances even after seven recycles. The SBA-15/EG(1:1)-Pd catalysts keep stable catalytic activities for five recycles and its activity decreases at the six recycling reaction. The recycling performance of Fe3O4@C-Pd is not good. The catalytic activity decreases gradually in the several recycling reactions, especially in the fourth times.