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

Abstract

The dispersion and activity of carbon-based ruthenium catalysts for ammonia synthesis were examined. Barium was used as a promoter. Parallel XRD and extensive chemisorption (H 2, O 2 and additionally CO) studies of a series of unpromoted ruthenium/carbon materials showed that a more developed texture (porosity and surface area) of the carbon support results in a significant increase in the ruthenium dispersion. It was found that the presence of ultra small mesopores (diameters<3 nm) in the support results in the formation of ultra fine ruthenium particles. Barium influences the adsorption of the three reactant gases: (i) chemisorption of oxygen increases slightly (15–20%); (ii) hydrogen chemisorption increases significantly (up to 100%); (iii) carbon monoxide chemisorption drops to a negligible value. Barium is supposed to decorate the ruthenium particles and to retard the migration of carbon–hydrogen species onto the metal surface. The promoted ruthenium catalysts exhibit extremely high activities in ammonia synthesis. Under the experimental conditions ( p=6.0 MPa, T=673 K, x NH 3 =8%, H 2:N 2=3:1) the reaction rate referred to the number of Ru surface atoms (TOF) was by an order of magnitude higher than that of a commercial fused iron catalyst. The TOF values are about constant for large Ru crystallites (diameters>3 nm) but they decrease, very likely, for smaller ones.