Carbon-supported ruthenium catalyst for the synthesis of ammonia. The effect of the carbon support and barium promoter on the performance

The dispersion and activity of carbon-based ruthenium catalysts for ammonia synthesis were examined. Barium was used as a promoter. Parallel XRD and e xtensive chemisorption (H-2, O-2 and additionally CO) studies of a series o f unpromoted ruthenium/carbon materials showed that a more developed textur e (porosity and surface area) of the carbon support results in a significan t increase in the ruthenium dispersion. It was found that the presence of u ltra small mesopores (diameters<3 nm) in the support results in the formati on of ultra fine ruthenium particles. Barium influences the adsorption of t he three reactant gases: (i) chemisorption of oxygen increases slightly (15 -20%); (ii) hydrogen chemisorption increases significantly (up to 100%); (i ii) carbon monoxide chemisorption drops to a negligible value. Barium is su pposed to decorate the ruthenium particles and to retard the migration of c arbon-hydrogen species onto the metal surface. The promoted ruthenium catal ysts exhibit extremely high activities in ammonia synthesis. Under the expe rimental conditions (p=6.0 MPa, T=673 K, x(NH3) = 8% H-2:N-2=3:1) the react ion rate referred to the number of Ru surface atoms (TOF) was by an order o f magnitude higher than that of a commercial fused iron catalyst. The TOF v alues are about constant for large Ru crystallites (diameters>3 nm) but the y decrease, very likely, for smaller ones. (C) 1999 Elsevier Science B.V. A ll rights reserved.