Synthesis Of Alkaline Earth Niobates And Their Applications To Ruthenium-Based Ammonia Synthesis Catalysis

Ammonia is the main source of nitrogen fertilizer,and it also plays an important role in the industries of chemical industry,metallurgy,medicine and explosives.Ammonia is an important commodity chemical manufactured globally at a scale of 1500 million metric tons per year in 2017.Compared with the iron-based catalyst,the ruthenium-based ammonia synthesis catalysts show high activity at low temperature and low pressure,therefore,we focused on empoldered ruthenium-based ammonia synthesis catalysts.The ruthenium-based ammonia synthesis catalysts was support-dependent and structure-sensitive,it means the size and morphology of Ru and the characteristics of support were related to the ammonia synthesis rate.Constantly,the characteristics of support have effect on the size and morphology of ruthnium.Niobates have the characteristics of stablision,alkalinity,large specific surface area and controllable morphologies,which can be an ideal candidate of the support for the ruthenium based ammonia synthesis catalyst.In this paper,a mild hydrothermal synthesis method was used to synthesize a niobate with larger surface area by changing the reaction conditions.And we construct a ruthenium based catalyst which use niobate as supports and Ba or Cs as promoter to realize superior activity and stablibity for ammonia synthesis.The main research contents and conclusions are shown as follow:1.The synthesis of alkaline erath niobte.First,a precursor of niobium(Nb2Os·nH2O)was synthesized from NbCl5 via a hydrothermal method.Second,the resultant Nb2O5·nH2O and Sr(OH)2·8H2O or Ba(OH)2·8H2O with different molar ratio were mixed to get niobat.e The results show that when Nb:Sr is greater than 1 and Nb:Ba is greater than 5:4 the barium niobate and strontium niobate can be obtained repectively.The strontium niobate and barium niobate are all flaky structures.The results of NH3-TPD show that they are all basic carriers,and barium niobate is stronger than strontium niobate.2.We choosed Sr2Nb2O7(Sr/Nb = 2.0)as the support for 2 wt%Ru catalysts.Addition of Ba-or Cs-promoter increase the ammonia synthesis rate and the optimum activity temperature will decrease accordingly.The corresponding optimal activation temperature was 773 and 673 K for Ba-and Cs-promoted catalysts,respectively and the optimal molar ratio of Ba or Cs promoter to Ru is 4 or 8,respectively.The ammonia synthesis of Ba-Ru/Sr2Nb2O7 and Cs-Ru/Sr2Nb2O7 at 673K were 2317μmol g-1cat h-1 and 4986 μmol g-1cat h-1 at the best condition.After the stability test of 72h,XRD and BET results both showed that Ba-Ru/Sr2Nb2O7 and Cs-Ru/Sr2Nb2O7 catalysts had good stability.3.We choosed BasNb4O15(Ba/Nb = 2.0)as the support for 4 wt%Ru catalysts.Addition of Ba-or Cs-promoter increase the ammonia synthesis rate and the optimum activity temperature will decrease accordingly.The corresponding optimal activation temperature was 773 and 673 K for Ba-and Cs-promoted catalysts,respectively and the optimal molar ratio of Ba or Cs promoter to Ru is 1 or 2,respectively.It is worth noting that the ammonia synthesis rate of Ba/Cs-Ru/BN system at 723K was similar to that of Ba/Cs-Ru/SN system at 723K.The ammonia synthesis of Ba-Ru/Ba5Nb4O15 and Cs-Ru/BasNb4O15 at 673K were 3720 μmol g-1cat h-1 and 4900 pmol g-1cat h-1 at the best condition.After the stability test of 72h,XRD and BET results both showed that Ba-Ru/BasNb4O15 and Cs-Ru/BasNb4O15 catalysts had good stability.Ba5Nb4O15 as support can reach a high ammonia synthesis rate at 623K which was similar to the Sr2Nb2O7as a support at 673K.