不同MgO担体对Ba-Ru/MgO氨合成催化剂物化性质和反应性能的影响

采用不同沉淀剂制备了MgO材料,以其为载体制备了Ba-Ru/MgO氨合成催化剂,考察了沉淀剂种类和BaO助剂对其氨合成性能的影响.通过X射线衍射(XRD)、N2物理吸附、X射线荧光光谱(XRF)、透射电镜(TEM)、H2程序升温还原(H2-TPR)、CO2程序升温脱附(CO2-TPD)、H2程序升温脱附(H2-TPD)和N2程序升温脱附(N2-TPD)表征手段,对不同沉淀剂影响Ba-Ru/MgO催化剂氨合成性能的原因进行了探索.结果表明:采用(NH4)2CO3作沉淀剂制备的Ba-Ru/MgO催化剂表面Ru物种易于在低温下还原,催化剂表面在低温区具有较多数量的弱碱性吸附位,在450℃、5.0 MPa和5 000 h-1条件下,由(NH4)2CO3做沉淀剂制备的Ba-Ru/MgO催化剂活性最高,出口氨浓度为3.74%.BaO助剂的加入大大减少了Ba-Ru/MgO催化剂表面吸附氢的数量,增大表面脱附氮的数量,从而易于N2解离吸附,提高氨合成反应速率.

Effect of Different Supports on Physical and Chemical Properties and Catalytic Activity over Ba-Ru/MgO Ammonia Synthesis Catalysts

MgO supports were prepared by different precipitants and further examined as the supports for barium-promoted ruthenium catalyst for ammonia synthesis.The reason resulting in their difference of catalytic activity were explored by X-ray diffraction,N2 adsorption-desorption,X-ray fluorescence spectroscopy,transmission electron microscopy,H2 temperature-programmed reduction,CO2 temperature-programmed desorption,H2 temperature-programmed desorption and N2 temperature-programmed desorption.It was found that MgO prepared by(NH4)2CO3 precipitant facilitated the reduction of ruthenium oxide for Ba-Ru/MgO catalyst,and the weak basic sites of Ba-Ru/MgO((NH4)2CO3 as precipitant) appeared at lower temperature and showed higher weak basic site densities compared with the others.At 450 ℃、 5.0 MPa and 5 000 h-1,catalytic activity of Ba-Ru/MgO catalyst employing(NH4)2CO3 as the precipitant was higher than that of the catalyst prepared by other precipitants under the same reaction conditions,the ammonia concentration in the effluent reached 3.74%.The addition of Ba promoter significantly decreased the amount of hydrogen chemisorption,increased the number of dissociatively adsorbed N* on the Ru surface and thus accelerated the reaction rate of ammonia synthesis.