Ammonia synthesis over ruthenium catalysts supported on high surface area MgO and MgO–Al 2O 3 systems

Abstract

Ruthenium catalysts (5–15 wt.%) deposited on the high surface area magnesia ( S BET=94 m 2/g), magnesium–aluminum spinel (96 m 2/g) and magnesia–alumina mixture (Mg:Al=1:2 molar ratio, 65 m 2/g) were characterized by the chemisorption techniques and tested in NH 3 synthesis. The chemisorption experiments (O 2, CO) have shown that ruthenium is highly dispersed (FE=60–95%), even if the Ru loading is high (∼15 wt.%). The catalytic properties (TOF) of small ruthenium particles supported on Mg–Al spinel (the most effective unpromoted systems) proved to be only slightly, by 40%, more advantageous in NH 3 synthesis at 400 °C and 63 bar than the properties of particles supported on MgO–Al 2O 3 mixture (the least active system), TOF over the former being roughly independent of the Ru crystallite size (1.0–1.5 nm). In contrast, the activities of the Ba-doped samples differed significantly, i.e. the TOF values (63 or 90 bar, 400 °C) over optimally promoted Ba–Ru/MgO were by four to eight times higher than those over Ba–Ru/Mg–Al spinel and by 30 times higher than over Ba–Ru/MgO–Al 2O 3 mixture. The effect of the support on the Ba promotion is discussed. Under comparable conditions (400 °C, 63 bar, 8.5% NH 3 and 400 °C, 90 bar, 11% NH 3), the Ba–Ru/MgO catalyst (10 wt.% Ru) was by several times more active than multipromoted fused iron, the comparison being based on the volumetric reaction rates.