Modification Of Doped Ba Nano-Magnesia Support Towards Enhancing Catalytic Activity Of Its Supported Ru Catalysts For Ammonia Synthesis

A series of Ba-doped nano-magnesia supports（Ba-MgO） for Ru catalysts,having higher surface area and steady characteristic,were firstly prepared by chemical precipitation-strong electrical adsorption（SEA） method with the aid of ultrasonic wave,and further modified of the support by coprecipitating rare earth（La,Sm,Ce） and adding organic molecule.Ru catalysts were then prepared by impregnation method using the modified Ba-MgO as the supports and 2wt%Ru₃（CO）₁₂ as the precursor.The effect of supported Ru catalysts with the modified technology on its surface performance,matter phase and texture was investigated by FE-SEM,HR-TEM,X-ray diffraction,N₂ physical-adsorption,etc.And the activity of ruthenium catalysts for ammonia synthesis was measured at 10000 h^-1,10.0 MPa and 375℃ ～450℃.The main research contents and results are as follows:1.There serial of Ba-MgO supports and its support Ru catalyst were prepared by ultrasonic-precipitation-strong electrical adsorption（SEA）, coating precipitation（CP） and direct impregnation（DI） method respectively.It was found that the Ru/Ba-MgO catalyst prepared by SEA method exhibit higher activity than the Ba-promoted Ru/MgO in which the Ba promoter was introduced by DI and conventional impregnation（CI） techniques,and CP was falling in between.The results showed that Ba-doped in support prepared by SEA method exist as the form of BaCO₃ which was uniform,single layer and strong adsorb,and that could be divided into two differently stable barium carbonate phases, low-temperature barium carbonate（LT-BaCO₃） and high-temperature barium carbonate（HT-BaCO₃）.This method would increase the promoted effect of the Ba and the dispersion of Ru particles and improve the surface performance too.So the activity of catalysts prepared by SEA method was the highest.2.The Ba was existed as the form of BaCO₃ in the support which was prepared by the method of strong electrical adsorption,thus the Ru/ （La-）Ba-MgO catalysts was reduced due to the BaO acting as the most activate form.The effect of reduce condition on catalytic performance of ruthenium catalysts was studied by heating-holding-heating reduce program,and the reduction mechanism of ruthenium catalysts at hydrogen atmosphere was discussed.The results show that the thermal stability of BaCO₃ and the agglomeration of Ru particles were the main factors for the catalytic activity of the ruthenium catalysts.It was also found that coupling reaction had occurred at the hydrogen atmosphere, and furthermore Ru could accelerate the coupling reaction,thus promoting the reduction of the catalyst and enhancing its catalytic activity.3.A series of RE-doped modified supports and its Ru catalysts（Ru/ RE-Ba-MgO,RE=La,Sm and Ce） were further prepared by ultrasonic-coprecipitation-SEA method.The results showed that the RE was introduced by co-precipitation and the Ba was brought in by the strong electronic adsorption,the introducing of rare earth not only changed the chemical,textural properties of nano-Ba-MgO supports but also controlled the doping amount and formation state of barium by change the charged type and amount of the particles.That is to say,the amount of LT-BaCO₃ was remarkably changed after introduced RE,and the Ru-catalysts showed different catalytic activities at low temperature. When La:Ba:Ru=5:0.8:1,the Ru/La-Ba-MgO catalyst exhibited the highest catalytic activity for the ammonia synthesis with a reaction rate of 65.49 mmol/（g·h）(400℃,10 MPa,10000 h^-1),upgrading 66.3% compared with the catalytic activity of Ru/Ba-MgO.4.A series of organic molecule modified supports were further prepared by ultrasonic-precipitation-strong adsorption method.The results showed that the supports modified by citric acid and potassium oleate support Ru catalysts exhibit the highest catalytic activity for the ammonia synthesis.It showed that the adding of citric acid not only changed the chemical,textural properties of nano-Ba-MgO supports but also the adsorbing form of barium.Ba was not only introduced as the form of physical adsorption,but also the chemical adsorption.So the doping amount of barium in the supports decreased firstly and increased secongdly with the enhancement of citric acid amount.When C_C6H8O6=2.0mmol/l,the modified Ru/Ba-MgO catalyst exhibited the highest catalytic activity for the ammonia synthesis with a reaction rate of 63.32 mmol/（g·h）(400℃, 10 MPa,10000 h^-1),upgrading 58%compared with that of unmodified Ru/Ba-MgO.