硝酸水热处理活性炭对其负载的Ba-Ru-K氨合成催化剂性能的影响

采用N2物理吸附、Boehm滴定、He-TPD-MS、CO化学吸附和透射电镜等手段考察了硝酸水热处理对活性炭(AC)及其负载的Ru基催化剂的孔结构、表面含氧基团、Ru分散度的影响,并评价了Ba-Ru-K/AC催化剂氨合成反应性能.结果表明,经硝酸水热处理后,AC表面含氧基团明显增多,但其孔结构变化不大.随着水热处理硝酸浓度的增加,AC表面含氧基团的数量增加,而相应催化剂的Ru分散度有所降低,Ru粒子尺寸增大.当硝酸浓度为2.0mol/L时,Ru分散度较高,且粒子尺寸(2.0nm)适宜,分散均匀,因此催化剂活性较高.在10MPa和10000h1条件下,400和375oC时,出口氨浓度分别达到17.80%和11.10%,较4.6mol/L硝酸回流处理AC负载的Ru基催化剂分别提高了16.8%和21.3%.水热处理AC的适宜条件为硝酸浓度2.0mol/L,150oC处理4h,填充度为70%.因此,通过调节水热处理时所用硝酸浓度可有效调控AC表面含氧基团的数量及其负载Ru的粒子尺寸.

Effect of Hydrothermal Treatment of Activated Carbon by Nitric Acid on Activity of Ba-Ru-K/AC Catalyst for Ammonia Synthesis

Activated carbon (AC) was functionalized by HNO3 hydrothermal treatment at 180 oC. Ruthenium catalyst samples were prepared through incipient wetness impregnation with aqueous solution of RuCl3·3H2O precursor. The influence of HNO3 concentration on the support structure, surface oxygen functional groups (SOFGs), Ru dispersion, and the catalytic activity of Ru/AC for ammonia synthesis was investigated by N2 physisorption, He temperature-programmed desorption, CO chemisorption, and transmission electron microscopy. The results indicated that the amount of SOFGs, Ru particle size, and activity depend strongly on the HNO3 concentration used in the hydrothermal treatment. The porous texture of carbon support almost keeps unchanged with low HNO3 concentrations (≤ 2.0 mol/L) treatment, while it decreases significantly at high acid concentrations (2.4 mol/L). However, the amount of SOFGs increases with the concentration of nitric acid as well as the Ru particle size. Ammonia synthesis activity of the Ba-Ru-K/AC catalyst samples, whose support was treated with HNO3 (2.0 mol/L), is as high as 17.80% at 400 oC, 10 MPa, and 10000 h.1, which is 16.8% higher than that of the catalyst obtained via washing with HNO3 (4.6 mol/L). The optimal conditions of hydrothermal treatment are HNO3 (2.0 mol/L) at 150 oC for 4 h with filling degree 70%. Therefore, the amount of SOFGs and Ru particle size are tunable by varying the HNO3 concentration during the hydrothermal treatment.