Preparation and characterization of chlorine-free ruthenium catalysts and the promoter effect in ammonia synthesis: 3. A magnesia-supported ruthenium catalyst

Abstract

The support and promoter effects of Ru in ammonia synthesis were studied systematically using Ru carbonyl, Ru 3(CO) 12, as a precursor of the Ru catalyst. MgO was found to be the most effective support among several pure oxides. This high activity is thought to be due to the high Ru dispersion, the Cl-free system, and the basicity of the support. Modifying MgO by adding other oxides was also tried. Na 2OMgO and Al 2O 3MgO supports were found to be as effective as the pure MgO. Fourteen kinds of metal nitrates, including alkali metals, alkali earth metals, and lanthanides, were tested as promoters of Ru/MgO prepared from Ru 3(CO) 12. Alkali metal nitrates (Cs +, Rb +, and K +), especially CsNO 3, were most effective. The synthesis rate was greatest when the Cs/Ru ratio was as low as 1.0, where the activity was 20 times as high as that without CsNO 3 at 588 K and under 80 kPa of N 2 + 3H 2. CsNO 3 was considered to be disproportionated to CsOH during the hydrogen treatment. The alkali promoter proved to act not only as a Cl scavenger in the case of a Cl-containing Ru catalyst but also as an electronic surface modifier to Ru. Hydrogen chemisorption studies disclosed that alkali covered mostly the Ru surface but had a weak interaction with the MgO surface. The importance of surface morphology in the combination of Ru and promoter was noted. RuCsOH/MgO (5 wt%; Cs/Ru = 1) was even more active than 4.7 wt% RuK/AC and was comparable to the Raney RuCsNO 3 catalyst, which had previously been reported to be the most active catalyst at 573 K and under 80 kPa of N 2 + 3H 2.