Research Of Ammonia Synthesis Over Ruthenium Catalyst Supported On Composite Oxides

In this work,we reported that MgAl-layered double oxide(LDO),which is the calcined product of MgAl-hydrotalcite(LDH),as the carrier of ruthenium based ammonia synthesis catalyst,and these samples were characterized by XRD,N2 physical adsorption and CO2-TPD to study the effect of preparation method and calcination temperature on the structure and catalytic performance of Ru/MgAl-LDO.Besides,in order to further improve the ammonia synthesis activities of Ru/MgAl-LDO,we modified supports with higher alkalinity and electronic conductivity,and the characterizations of H2-TPR,XPS,TEM revealed the reasons for this improvement results systematically.Preparation condition has a very big impact on the performance of Ru/MgAl-LDO ammonia catalyst.The results show that MgAl-LDO prepared by co-precipitation at nMg2+/nAl3+=3 after being calcined at 500? is more beneficial to ammonia synthesis.It is attributed to the larger surface,more alkaline,smaller grain size and better pore distribution on the supports.The addition of rare earth elements can effectively improve the carrier's basic and enhance the interaction between support and active metal.In this work,a series of MgAl-LDO with Y,La,Ce were successfully synthesized by modified co-precipitation to improve the activity of Ru/MgAl-LDO catalyst.The study shows that the number and strength of basic site both enhance when MgAl-LDO was doped with different rare earth elements,specially in the case of Y and La doping.Ru/Y-LDO is showing higher activity compared to Ru/Ce-LDO and Ru/La-LDO.Because yttrium could be introduced to the hydrotalcite layered structure in a larger amount,which resulted from smaller ionic radius of Y3+.The addition of Y not only increases alkalinity and surface area of support,but also induces the strong metal-support interaction which is responsible for more active surface Ru metal.In addition,this interaction is also effective to prevent the Ru particles sinter to improve the distribution of ruthenium on the carrier.However,too much incorporation of Y not show any considerable higher activity as expected.It is because Y does not fully enter into the shelves with the increasing of Y content,so the improvement of alkalinity and surface area of support would be less obvious.It is shown that the NH3 concentration of Ru/Y-LDO in effluent is higher when nMg2+/(nAl3+++nY3+)=3,nY3+/(nAl3++nY3+)=3%.In this chapter,the influence of Zr content doping on Ru/MgAl-LDO performence was investigated.nMg2+/(nAl3++nZr4+)=3,nZr4+/(nAl3++nZr4+)=10%,the addition of Zr is effective to increase the alkalinity,reduce the particle size of the Ru particles,improve the dispersion of Ru,exhibited the highest activity for ammonia synthesis than others.BaZrO3 has been proven to be an excellent electron-conducting carrier for ammonia synthesis,but its specific surface area is too small to have a broader application.The MgAl-LDO possess high specific surface,we can have a material which both possess large specific surface area and good electronic conductivity when we Combining MgAl-LDO with BaZrO3.And the results shows that when the mass fractions both are 50%,the catalyst which is supported by composite oxide shows best activity for ammonia synthesis.