Deactivation Mechanism Of Carbon-supported Ruthenium Catalyst For Ammonia Synthesis

Ruthenium catalyst is the next generation catalyst for ammonia synthesis following iron catalyst.The commercial Ru catalyst was composed of three major components,including support,Ru metal and promoter.These components may have a strong influence on the catalytic activities and the stabilities of Ru catalyst.This thesis was devoted to study the effect of the properties of carbon support,and the existing forms of K or Ba promoter on the catalytic activities and the stabilities of carbon-supported Ru catalyst.Then the deactivation mechanism of carbon-supported ruthenium catalyst was discussed.Some results were obtained as follow:(1)The presence of iron significantly increased the graphitization degree of carbon during the heat treatment process of carbon in argon gas.The area of the heated carbon would not decrease if potassium was added into carbon.The coexistence of iron and potassium not only increased the graphitization degree of the heated carbon,but also enhanced the area of the as-obtained sample.The increase of the treatment temperature also greatly increased the graphitization degree of carbon.However,the increased of the graphitization degree of carbon support had a limited impacted on the ammonia synthesis activities and the stabilities of Ru catalyst.These resulted indicated that the differences in the graphitization degree of carbon was not the only factor contributing to the stabilities of Ru catalyst.(2)The changes in K precursors affected the ammonia synthesis activities and the stabilities of K-promoted Ru catalyst.the sample with KNO3showed the highest heat resistance.The presence of K promoter increased the number of hydroxyl groups,and potassium hydrogen carbonate can be found in the reduced catalysts obtained from KOH or K2CO3.Theseresults led to the increase of methane formed at low temperature,which did not significantly affect the catalytic activities of K-promoted Ru catalysts.However,during the long time run,potassium hydrogen carbonate would be reduced to form carbon oxides.Furthermore,the high temperature treatment at above 500 ? may destroy the structures of carbon support,and then more CO and CO2 could form.The production of CO or CO2 was accompanied by the consumption of hydrogen atoms,thus a less amount of hydrogen atoms could be used to synthesis ammonia,and then the ammonia synthesis rates and the stabilities decreased significantly.(3)The existing forms of Ba promoter for Ru catalyst were changed by using the different treatment gas,including hydrogen,water vapor,CO and CO2.The addition of barium promoter significantly decreased the amount of hydroxyl groups.More methane was produced for the sample treated with CO,but this catalyst showed the highest thermal stability.The production of water and methane was unavoidable for all Ru catalyst during the ammonia synthesis reaction.The high temperature treatment or the long time run ammonia synthesis has a benefit on the removal of hydroxyl groups,then the amount of water and methane decreased greatly.Thus the activities and the stabilities of Ru catalyst both greatly increased because more atomic hydrogen can be used to synthesis ammonia.