Structure Control And Properties Of Ruthenium Catalysts Supported On Activated Carbon

Ruthenium catalyst has the high ammonia synthesis activity at low temperature and low pressure reaction conditions,which makes it a new generation of ammonia synthesis catalyst.In order to further improve the cost-effectiveness and stability of ruthenium catalyst,a series of carbon-supported ruthenium catalysts were prepared in this paper.The ammonia synthesis activity and stability of the catalyst were tested at a certain temperature and pressure.The effects of HNO3 preoxidation and ammonia reduction on carbon support and catalyst performance were investigated by using N2 adsorption,CO chemical adsorption,H2-TPR,H2-TPD,N2-TPD,MS,gas chromatography,TEM and other means of characterization.The effects of active metal Ru content,K promoter content and the addition of rare earth metal La promoter on the activity and stability of the catalyst were studied.The main conclusions are as follows:Activated carbon is preoxidized by HNO3 and then treated in the ammonia atmosphere under high temperature,which can optimize the physical texture of carrier carbon.The carbon carrier was treated at 1100 ? to prepare activated carbon with a large surface area,pore volume,and higher mesopore ratio.Then the Ru/AC ammonia synthesis catalyst was prepared.The difference of nitric acid concentration and ammonia treatment temperature has a great influence on the performance of the catalyst.Among these catalysts,the ruthenium catalyst prepared under 700-900 ?treatment conditions has better activity and stability.In the ammonia synthesis reaction,the increase of ruthenium content significantly improved the ammonia synthesis activity of catalysts,but methanation reaction of the carrier carbon was evidently aggravated,and then the structure of the catalyst was destroyed.For catalyst in which weight ratio of ruthenium to carbon is 10 wt.%,its structure is damaged quite serious,and its specific surface area dropped to 86 m2/g,the decrease rate is 56.8%.Similarly,in the series of RulOBalOKx/AC catalyst,the ammonia synthesis activity of the potassium-containing ruthenium catalyst has been remarkably improved because of the electronic effect of the potassium promoter.However,the presence of potassium also has a significant effect on the methanation reaction and the physical structure of catalysts.For the Ru10Ba10K12/AC catalyst,the content of K promoter is higher,so that the methanation reaction of the carbon carrier was more serious during the ammonia synthesis reaction.After 300 h heat resistance,the specific surface area and pore volume of the catalyst decreased,and the specific surface area decreased to 152 m2/g,the decrease rate was 54.20%.Eventually,the stability of the catalyst deteriorated and the catalytic performance decreased.On the other hand,the increase of active metal ruthenium content and the electronic effect of potassium promoter promoted the reduction of active metal and promoter,which enhanced the N2 and H2 adsorption of Ru/AC catalyst,and improved the efficiency of ammonia synthesis reaction.Therefore,in this paper the effect of content of ruthenium and potassium on the activity and stability of the catalyst was investigated.It was suggest that the stability of Ru/AC ammonia synthesis catalyst was better and the cost performance of catalyst was higher when the content of ruthenium and potassium is 8 wt.%.The addition of a small amount of La promoter can inhibit the adsorption of H2 on the catalyst and enhance the adsorption capacity of N2,thus the ammonia synthesis activity of ruthenium catalyst is improved.At the same time,it also enhances the anti-methanation capacity of the catalyst and improves the thermal stability of the ruthenium catalyst.However,the addition of a larger number of La promoter has many limitations on the further improvement of the catalyst performance,so the optimum weight ratio of La promoter to carbon content is 0.5 wt.%.Moreover,the further addition of the potassium promoter would enhance the interaction between the active metal Ru and the promoter.Consequently,the ammonia synthesis activity,thermal stability and conversion efficiency of the catalyst have been improved remarkably.