Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and power-law kinetics

Supported Ru catalysts for NH, synthesis were prepared from Ru-3(CO)(12) and high-purity MgO and Al2O3. In addition to aqueous impregnation with alkali nitrates, two non-aqueous methods based on alkali carbonates were used to achieve alkali promotion resulting in long-term and high-temperature stable catalysts. For the reliable determination of the Ru particle size, the combined application of H-2 chemisorption, TEM and XRD was found to be necessary. The power-law rate expressions were derived at atmospheric pressure and at 20 bar which were shown to be efficient tools to investigate the degree of interaction of the alkali promoter with the Ru metal particles. The following sequence with respect to the turnover frequency (TOF) of NH, formation was found: Cs2CO3-Ru/MgO > CsNO3-Ru/MgO > Ru/MgO > Ru/K-Al2O3 > Ru/Al2O3. The Cs-promoted Ru/MgO catalysts turned out to be more active than a multiply-promoted Fe catalyst at atmospheric pressure with an initial TOF of about 10(-2) s(-1) for the non-aqueously prepared Cs2CO3-Ru/MgO catalyst at 588 K. The strong inhibition by H-2 was found to require a lower molar H-2:N-2 ratio in the feed gas than 3:1 in order to achieve a high effluent NH, mole fraction. The optimum ratio for Cs2CO3-Ru/MgO at 50 bar was determined to be about 3:2, resulting in an effluent NH, mole fraction which was just a few percent lower than that of a multiply-promoted Fe catalyst operated at 107 bar and at roughly the same temperature and space velocity. Thus, alkali-promoted Ru catalysts are an alternative to the conventionally used Fe catalysts for NH3 synthesis also at high pressure.