还原制备时的pH值对Pd/C催化剂性能的影响

采用液相还原法,以甲酸为还原剂制备了负载量为5%的Pd/C催化剂。考察了还原过程中p H值对Pd/C上硝基苯液相催化加氢性能的影响,并用XRD、SEM、TEM等手段对催化剂的结构进行了表征。结果表明,制备过程中随着p H的增大,催化剂活性先增大后减小;适宜的p H值能加快Pd还原反应的速率,有利于粒子快速成核,降低生成大粒径Pd粒子的几率,减少团聚,使Pd粒子均匀分散在活性炭载体上,获得较高的催化活性。p H=9时制得的催化剂上Pd晶粒分布均匀,平均粒径为3.63 nm;在50℃、H2压力0.8 MPa、搅拌速率100 r/min,反应45 min的条件下,这一催化剂对硝基苯加氢液相催化为苯胺的转化频率(TOF)达到8758 h-1。

Effect of pH Values in Reduction Preparative Process on the Performance of Pd/C Catalysts

Pd/C catalysts with a loading of 5%Pd content were prepared by liquid-phase reduction method using formic acid as a reducing agent, and characterized by means of XRD, SEM and TEM techniques. The effect of pH values in the reduction preparative process on the performance of catalysts for the liquid phase hydrogenation of nitrobenzene was investigated. The results show that, the catalytic activity firstly increases and then decreases with the augment of pH value during preparation. Suitable pH value can accelerate the rate of Pd reduction reaction and promote rapid nucleation of Pd particles, which will reduce the probability of generating large particles and aggregation phenomenon, producing uniform Pd particles required for higher activity. Pd particles with a mean diameter of 3.63 nm were found to be distributed uniformly on activated carbon carrier when reduction was carried out at pH 9. Catalyzed by the catalyst such prepared, the turnover frequency (TOF) reached 8758 h-1 in the reaction of liquid phase hydrogenation of nitrobenzene to aniline at 50℃for 45 min, when H2 partial pressure was 0.8 MPa with stirring at 100 r/min.