Influence of the oxide supports on ammonia synthesis activity of unpromoted ruthenium catalysts

Abstract

Supported ruthenium (Ru) catalysts with Ru loading of 3.5% by weight have been prepared by the incipient impregnation method using MgO, <em>γ</em>-Al₂O₃ and ZrO₂ as supports. After complete reduction, irreversible hydrogen chemisorption has been measured on these catalysts at 25°C and as atmospheric pressure. Ammonia synthesis activities of these catalysts have been evaluated in a microflow reactor at 400°C under atmospheric pressure. It is found that Ru dispersion and metal area change in the order Ru/<em>γ</em> - Al₂O₃ > Ru/ZrO₂ > Ru/MgO<em>. </em>The ammonia synthesis activity per gram of the catalyst and the turnover frequency change in the decreasing order: Ru/MgO > Ru/<em>γ</em> -Al₂O₃≥ Ru/ZrO₂. The activity of Ru/MgO is found to be five-times higher than the activities of the other two catalysts. The rate of ammonia synthesis is observed to increase with time on Ru/MgO while it remains steady on Ru/<em>γ</em> -Al₂O₃ and Ru/ZrO₂ catalysts. The exceptional behaviour of Ru/MgO is explained in terms of pronounced electron donation by MgO to ruthenium and in terms of possible changes in the crystallographic planes of ruthenium under the influence of nitrogen and ammonia under synthesis conditions.