Promoter effect of Sm2O3 on Ru/Al2O3 in ammonia synthesis

Ammonia synthesis (N-2 + 3H(2) = 80 kPa) and the isotopic equilibration reaction (IER) of dinitrogen (N-14(2) + N-15(2) = 20 kPa) were studied on promoted and unpromoted Ru/Al2O3 catalysts around 588 K. Sm2O3 promoter was compared with CsOH promoter or the nonpromoted state. On the Ru-Sm2O3/Al2O3 catalyst, the ammonia synthesis rate was higher than the IER rate, whereas on the Ru/Al2O3 and Ru-CsOH/Al2O3 it was lower than the IER rate, For Ru/Al2O3 and Ru-CsOH/Al2O3 catalysts the IER rate was retarded by coexisting hydrogen, These results were explained with the Langmuir-type competitive adsorption model, Detailed kinetic parameters were obtained for Ru/Al2O3, where hydrogen adsorption (Q(H) = 9.9 kcal mol(-1)) was stronger than nitrogen adsorption (Q(N) = 4.2 kcal mol(-1)) with an activation energy of dissociative adsorption of N-2 14.3 kcal mol(-1). On the other hand, Sm2O3 released hydrogen poisoning against nitrogen adsorption. Hydrogen retardation was not observed in the IER on Ru-Sm2O3/Al2O3. The promotion by Sm2O3 was mainly due to hydrogen release, which might be related with the morphological modification of Ru by nascent Sm2O3, We also found that the Ru-Sm2O3/Al2O3 catalyst was active even under the high pressure ammonia synthesis condition due to the lesser hydrogen inhibition. (C) 1996 Academic Press, Inc.