| As an excellent hydrogen-energy carrier without carbon,ammonia has many significant advantages,such as high hydrogen content（17.7 wt.%）,high energy density and its property of COx-free as well.It has been widely concerned and focused on research in the field of hydrogen energy storage and transportation.However,there are some serious kinetic obstacles to rhe progress of ammonia decomposition,which usually requires high temperature（≥700℃）to achieve high conversion of ammonia decomposition.Consequently,it is urgent to develop ammonia decomposition catalysts with lower temperature and higher efficiency.In this paper,different catalysts of Ru/ZrO2-p and Ru/K-Al2O3 were prepared by solid phase grinding method（m）,chemical precipitation method（p）and impregnation method（i）step by step.In the meantime,their catalytic properties were regulated in several ways such as adjusting the preparation parameters,adding promoters and controlling the contents of ruthenium.Multifarious characterization methods including XRD,BET,TEM,CO2-TPD,H2-TPR,XPS and others were selected to analyze the particle size and dispersion of Ruthenium nanoparticles（Ru NPs）,the surface properties of the supports and the interaction between the carrier and the active component Ru were analyzed as well.The activity and stability of catalysts in ammonia decomposition were measured by a fixed bed gas chromatograph.The main conclusions of this paper are as follows:（1）catalysts of Ru/ZrO2-m and Ru/ZrO2-p prepared by different preparation methods have no obvious difference in their microstructures.The average particle size of Ru NPs is 1.2-2.1nm,and the dispersion is 50-78%.However,the catalytic performance test results showed that at 450℃ with the WHSV of 3.0×104 mL·gcat-1·h-1,the ammonia conversion rate of Ru/ZrO2-p over setted-condition is 39.5%,which is significantly higher than that of Ru/ZrO2-m（22.5%）,indicating that the catalyst prepared by precipitation method can promote the activation and dissociation of ammonia molecules more effectively,and show higher ammonia decomposition activity and intrinsic activity.（2）Catalysts with different Ru NPs loads of 1.5 and 3.0 wt.%Ru/ZrO2-p were compared,and the results showed that the ammonia conversion of 3.0 wt.%Ru/ZrO2-p at 450℃ was 39.5%and 78.2%respectively.The apparent activation energy of these two catalysts was similar,but the turnover frequency of hydrogen production（TOFH2）at 400℃ was very different.Higher Ru loading capacity could obtain higher intrinsic activity,and the enhanced metal interaction between the carrier and the promoter component would make it obtain higher activity and stability over long periods of time（120 h）.（3）Impregnation method was used to prepare Ru/K-Al2O3 catalysts doped with various K additives.The characterization results of CO2-TPD showed that adding K additives changed the surface properties of the support Al2O3 to produce some super alkaline sites.Meanwhile,with the increase of K contents（the weight ratio of K-Al2O3 are 0.06,0.11,0.17 and 0.23）,the super alkaline sites on the surface of supports was increased gradually too,the peak value of the super alkaline sites increased as well,and the number of super basic sites increased with some weak basic sites changing to strong basic sites.（4）Compared with Ru/Al2O3 catalyst without K promoter,the ammonia conversion rate of Ru/0.23K-Al2O3 catalyst with a mass ratio of K/Al2O3 was increased to 82.4%at 450℃ and 3.0×104 mL·gcat-1·h-1,which was increased by 1.54 times.The interaction between the carrier and the active metal Ru can be enhanced by adding K additive to generate super alkaline sites on the carrier surface,which can effectively regulate the electronic structure of Ru,enhance its intrinsic activity,and show high stability.（5）Whether Ru/ZrO2-m,Ru/ZrO2-p,or Ru/K-Al2O3 catalysts,the conversion rate of ammonia is closely related to the temperature and the WHSV.In one hand,for the catalyst of Ru/ZrO2-p,the ammonia conversion rate decreases from 100%to 0,when the temperature decreases from 500℃ to 300℃ gradually,and the ammonia conversion rate decreases obviously from 100%to 23%indeed when the airspeed increases from 0 to 8.0×104 mL·gcat-1·h-1.In the other hand,for the catalyst of Ru/0.23K-Al2O3,when the temperature decreases from 550℃ to 300℃ gradually,the ammonia conversion rate decreases from 100%to 0,and when the airspeed increases from 0 to 3.0×104 mL·gcat-1·h-1,the conversion rate of ammonia decreases from 100%to 81%. |
