The Preparation And Application Of Pd/C Catalyst With Ultrafine Grain

As a kind of noble metal, value of palladium can be improved greatly and the metallurgy industry chain can be prolonged if the palladium compounds are further transforming to palladium-carbon catalyst (Pd/C catalyst). Pd/C catalysts are widely applied in the industries of petrochemical, pharmaceutical, dye and new energy. One case among which is the hydrofining of purified terephthalic acid (PTA).To avoid disadvantages of uneven distribution and low dispersity of Pd grains in the Pd/C catalysts by the “immersion-precipitation” method, the conventional preparation methods for Pd/C catalysts used in hydrofining of PTA, this work proposed a superfine grain sol-gel method to prepare the Pd/C catalyst. This method is composed of three steps as pretreatment of carrier (coconut shell activated carbon), preparation of superfine grain and absorption of activated carbon. The effects of the main parameters in each step on the microstructure and activity of the catalysts were investigated by means of SEM, TEM, XRD, EDS, HPLC and chemisorptions. And the relationship between the dispertiy of Pd grain on the carrier and the catalytic performance were analyzed. The optimal processing conditions for preparation of Pd/C catalysts were obtained and the results may play some role in the research and development of superfine grain Pd/C catalysts with high activity.The results show that HNO3, HCl and H2O2have significant influences on pore structures of Pd/C catalyst.A suitable dosage of these reagents can enlarge the pore on surface of carrier, while corrosion products appear when the concentration is too high, which bring negative effects on catalytic performance. HNO3treatment has better effects than HCl, and then followed by H2O2. The proper concentration of HNO3is0.8-1.6mol/L. In the process of preparation of superfine grain, with the increase in reductant dosage and palladium concentration, catalytic activity decreases firstly and then increases. With the increase in temperature and pH value, catalytic activity decreases. In the absorption of activated carbon, the catalytic activity increases firstly and then decreases with the increase in pH value and the activity produces a less reduce with the increase in temperature. The optimal conditions of superfine grain sol-gel method for preparation of Pd/C catalysts are that HNO3concentration of1.2mol/L (in pretreatment process), reductant(NaHB4) dosage of three times(based on the theoretical quantity), reducing temperature of10℃, reducing pH value of-0.5, palladium concentration of0.006mol/L, additive concentration X of0.67g/L, Y of0.67mL/L and Z of0.67g/L, adsorbing pH value of0.5-1, adsorbing temperature of10℃. The Pd grains with cubic crystals are well dispersed in the aqueous colloidal solution and the average size is5-6nm. By loading Pd nanocrystalline on activated carbon Pd/C catalysts were prepared, and the catalytic activity reached at99.93%. Sizes of Pd particle on the AC carrier are nearly the same with that in colloid and the Pd dispersity of catalyst reaches12.63%by chemisorptions method.The additives are believed to the main factor of the colloid stability in this method. The roles of additives are toform a layer of “coatings” on the surface of palladium grain. The coating formed by ternary surfactant mixture is assemble in the solid-liquid interfaces(interfaces between palladium grains and aqueous solution), which makes the palladium grains highly dispersed in the sol system, and prevents the grains from aggregation and growth.