Research Of Pretreatment Of Activated Carbon And Ru-Based Catalyst For Ammonia Synthesis

Activated carbon (AC) supported ruthenium catalyst for ammonia synthesis has higher catalytic activity at low temperature and low pressure. The cost of Ru-based catalyst is too high due to the precious of Ru metal. The cost can be lowered by reducing the loading of ruthenium or the amount of catalysts needed. Therefore, to explore the catalysts with high performance at low loading is necessary. In present work, the preparation of Ru catalysts was optimized by detailed study on the pretreatment of activated carbon support and preparation method. The effect of functionalization methods on the support structure and surface oxygen functional groups (SOFGs), Ru dispersion and the catalytic activity of Ru-based catalyst for ammonia synthesis were investigated by N2physisorption, He-temperature-programmed desorption (He-TPD), CO chemisorption and transmission electron microscopy (TEM). The main results areas the followings:First, a two-step wet oxidation method (HNO3+H2O2), which HNO3 and H2O2treatments were sequential not simultaneous, was investigated for the pretreatment of AC. Compared to pretreatment of nitric acid, the two-step wet oxidation significantly improve the amount of surface oxygen functional groups without destroy the physical structure of AC. The total amount of acidic groups is1.55mmol/g. The dispersion of Ru increases with increasing of the amount of SOFGs on AC and catalytic activity of Ba-Ru-K/AC catalyst for ammonia synthesis was greatly improved. The optimized HNO3concentration is6.1mol/L; temperature is90℃; the optimized H2O2concentration is30wt%; temperature is60℃; time is5h; and the ratio of liquid-solid is10ml/g.Secondly, a HNO3hydrothermal treatment method was studied for the pretreatment of AC. The texture of AC support almost keeps unchanged while the amount of SOFGs increases with the increasing of the concentration of nitric acid. The optimal concentration of HNO3is2.0mol/L; the hydrothermal temperature is150℃; time is4h; filling degree of70%. The catalytic activity of the Ba-Ru-K/AC catalyst for ammonia synthesis is greatly improved when supported on the modified activated carbon under the above optimal conditions and, treated by3%O2/97%N2mixture gas. However, if the modified activated carbon was further treated by H2O2, the catalytic activity of Ba-Ru-K/AC catalyst is reduced.Finally, an ultrasound-assisted precipitation method for the preparation of Ba-Ru-K/AC catalyst is investigated. Compared to the NH3H2O precipitation, the catalytic activity of supported Ba-Ru-K/AC catalyst prepared by ultrasound-assisted NH3H2O precipitation is improved. The optimal ultrasound time is15min. The catalytic activity of Ba-Ru-K/AC catalyst for ammonia synthesis prepared by the above optimized condition is improved from19.19%and13.72%to19.88%and15.61%, respectively, at400and375℃, under the reaction conditions of10MPa,10000h"1.