还原温度对溶胶法制备Pd/C催化剂性能与微观结构的影响

INFLUENCE OF REDUCTION TEMPERATURE ON CATALYTIC ACTIVITY AND MICROSTRUCTURE OF Pd/C CATALYSTS PREPARED BY COLLOIDAL METHOD

采用钯溶胶法制备对苯二甲酸加氢精制用 Pd/C 催化剂，考察了还原温度对 Pd/C 催化剂活性的影响。通过 SEM、TEM、能谱、物理吸附、化学吸附等手段对 Pd/C 催化剂结构进行表征。结果表明：随着还原温度的升高，催化剂活性和分散度快速下降，当还原温度为10℃时，对羧基苯甲醛转化率接近100％，同时分散度也达到最高，所得催化剂 Pd 晶粒细小均匀，平均粒径为6.45 nm；30℃下制备的催化剂载体表面存在明显的团聚现象，Pd 晶粒平均粒径为15～20 nm，30℃下制备催化剂表面 Pd 含量约为10℃时的5倍；随着温度升高，聚氧乙烯基与水之间的氢键断裂，其增溶能力大幅下降，同时胶体中的 Pd 颗粒碰撞几率增加，造成胶体钯中 Pd 颗粒长大聚集，催化活性降低。

A series of Pd/C catalysts for hydropurification of terephthalic acid were prepared using sol-gel process. The effect of reduction temperature on the microstructure and activity of the catalysts were investigated by means of SEM,TEM,EDS,physisorption,chemisorption and HPLC. The results show that the activity and dispersion of Pd sharply decreases with the increase of reduction temperature.The conversion ratio of 4-CBA is close to 100% at reduction temperature of 10 ℃ and the dispersion of Pd also reaches the highest. The Pd particles at this condition are fine and equally distributed with aver-age size of about 6.45 nm. However,a lot of larger particles with size of 15-20 nm gather at the sur-face at reduction temperature of 30 ℃. The amount of Pd on the catalyst surface reduced at 30 ℃ is about five times the amounts of Pd on the surface reduced at 10 ℃. Higher temperature generally causes the breaking of hydrogen bond between polyoxyethylene group and water,leading to a great reduction of solubilization capacity,which further results in the growth of Pd grain and the decrease of activity of Pd/C catalysts because of increased collision probability of Pd colloids.