Ammonia synthesis over ruthenium catalysts supported on high surface area MgO and MgO–Al2O3 systems Original Research Article

Ruthenium catalysts (5–15 wt.%) deposited on the high surface area magnesia (SBET=94 m2/g), magnesium–aluminum spinel (96 m2/g) and magnesia–alumina mixture (Mg:Al=1:2 molar ratio, 65 m2/g) were characterized by the chemisorption techniques and tested in NH3 synthesis. The chemisorption experiments (O2, 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 NH3 synthesis at 400 °C and 63 bar than the properties of particles supported on MgO–Al2O3 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–Al2O3 mixture. The effect of the support on the Ba promotion is discussed. Under comparable conditions (400 °C, 63 bar, 8.5% NH3 and 400 °C, 90 bar, 11% NH3), 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.