Structure sensitivity of supported ruthenium catalysts for ammonia synthesis

The catalytic ammonia synthesis activities of four supported ruthenium cata lysts are reported. It is seen that Ru/MgAl2O4 is more active than two simi lar Ru/C catalysts, which are significantly more active than Ru/Si3N4. The activity differences cannot be satisfactorily explained solely by the diffe rences in dispersion. Recent results from single crystal studies and DFT ca lculations have shown that ammonia synthesis over ruthenium catalysts is a very structure sensitive reaction, more so than on iron catalysts. It is su ggested that special Bs-type sites are primarily responsible for the cataly tic activity of the present supported Ru catalysts. It is shown how the num ber of such Bs-type sites depends on the Ru crystal size for a given crysta l morphology. We have found that the activity of the Ru/MgAl2O4 catalyst in creases significantly during the initial part of a test run. This activity increase is paralleled by the disappearance of crystals smaller than ca. 1. 0nm due to sintering and a resulting formation of larger crystals. We concl ude that there exists a lower limit to the desired crystal size of Ru in su pported ammonia synthesis catalysts. This is in agreement with a low number of Bs-type sites expected for such crystal sizes. Furthermore, we suggest that the support plays a decisive role in controlling the morphology of the Ru crystals and the resulting change in abundance of Bs-type sites is the main cause for the significant activity variations observed for Ru catalyst s with different supports. Finally, the support may also influence the elec tronic and catalytic properties of neighboring Bs-type sites. (C) 2000 Else vier Science B.V. All rights reserved.