Ammonia synthesis over barium-promoted cobalt catalysts supported on graphitised carbon

作者：Rarog Pilecka Wioletta; Miskiewicz Elzbieta; Kepinski Leszek; Kaszkur Zbigniew; Kielar Katarzyna; Kowalczyk Zbigniew^*

来源：Journal of Catalysis, 2007, 249(1): 24-33.

DOI：10.1016/j.jcat.2007.03.023

• ammonia synthesis
• cobalt catalysts
• barium promoter
• graphitic carbon support

A group of the cobalt catalysts supported on carbon and doped with barium was prepared, characterised (by TEM, XRD, and H-2 chemisorption), and tested in ammonia synthesis under industrially relevant conditions (9.0 MPa, 400-470 degrees C, H-2:N-2 = 3: 1). Three partly graphitised carbons (A, B, and Q, with different BET surface areas (S-A = 122 m(2)/g, S-B = 840 m(2)/g, and S-C = 1280 M-2/g) were used as supports. Cobalt nitrate 14 (index N) or cobalt acetate (index Ac) and barium nitrate were used as precursors of the active phase and a promoter precursor, respectively. TEM studies of the unpromoted Co/carbon materials treated differently (calcined in air, also reduced in hydrogen or reduced in H2 and recalcined in air), as well as studies of the post-NH3 synthesis catalysts promoted with barium, have shown that the specimens are heterogeneous. Hydrogen chemisorption studies of the promoted catalysts revealed that the Co dispersion in an operating system depends on the carbon texture (i.e., the higher the carbon surface area, the lower the average Co crystallite size), metal loading (i.e., a higher Co content results in a lower dispersion), and preparation procedure (i.e., reduction of the calcined Co/carbon precursor in H2 before impregnation with barium nitrate is disadvantageous for the dispersion of the resultant catalyst, especially when highly loaded materials are prepared). Cobalt salt (nitrate vs acetate) does not influence the size of Co particles in Ba-Co/carbon. Kinetic studies of ammonia synthesis demonstrated that the surface-based reaction rates (TOF) over the Ba-promoted catalysts prepared from reduced Co/carbon precursors are lower than the rates corresponding to the catalysts derived from calcined/re-calcined precursors. The TOF values for the systems based on calcined materials (N series) proved to be independent of the average Co particle size (over the range of 5-20 nm). The apparent energy of activation for Ba-Co/carbon (92 kJ/mol) is significantly higher than that for fused iron (57 kJ/mol for KM 1), but lower than that for promoted ruthenium on carbon (112 kJ/mol). Measurements of the reaction rate versus NH3 content in the gas mixture showed in turn that cobalt is kinetically more "sensitive" to ammonia than ruthenium but is considerably less sensitive than iron. At high conversion, corresponding to 11 % NH3, the weight-based reaction rate over the most active Ba-Co/carbon catalyst (27% Co) is 7 times higher than that for iron (KM 1).

• 出版日期2007-7-1