Structure sensitivity of ammonia synthesis over promoted ruthenium catalysts supported on graphitised carbon

A series of carbon-based ruthenium catalysts differing in Ru loading (1-32 wt%) was characterised (XRD, TEM, O-2 and CO chemisorption) and, after promotion with Ba or Cs or both, was studied in NH3 synthesis. Partly graphitised carbon with a high surface area (S-BET = 1310 m(2)/g) was used as a support for catalyst preparation. Ruthenium chloride and barium nitrate and/or caesium nitrate were precursors of the active phase and promoters, respectively. The chemisorption experiments have shown that the mean size of ruthenium particles (d) increases monotonically with Ru loading, from about 1 nm for 1 wt% Ru to about 4 nm for 32 wt% Ru. The NH3 synthesis studies have revealed that the reaction rates (400 degrees C, 63 bar, 8.5% NH3 or 400 degrees C, 90 bar, 11.5% NH3), expressed in terms of TOF, increase versus particle size, regardless of the promoter type. Extrapolation to lower sizes indicates that crystallites smaller than 0.7-0.8 nm might be totally inactive. The co-promoted catalysts (Ba + Cs) were found to be more active than the singly doped systems over the whole range of the particle diameter. The trends in TOF versus d have been attributed to the promoter/promoters location, on the faces of the Ru crystallites, or, alternatively, to changes in crystallite morphology-larger particles (3-4 nm) may expose more B-5 sites than the smaller ones (1-2 nm). The effect of co-promotion has been ascribed to different modes of the promoters' action: whereas the main role of Cs is to lower the barrier for N-2 dissociation (electronic effect), barium acts predominantly as a structural promoter-the reconstructed surfaces in Ba-Ru/C are more resistant to poisoning by hydrogen when operating, thus making more sites available for N2 adsorption. (c) 2004 Elsevier Inc. All rights reserved.