Catalysts for ammonia synthesis

#The subject of the present invention are catalysts for ammonia synthesis, characterised in particular by a new, advantageous kind of support. Furthermore, the invention relates to innovative methods concerning both the preparation of the support and of the catalyst, and the pre- and post-treatment of the same.

#Since one century ammonia is produced industrially by catalytic reduction of nitrogen with hydrogen (Haber-Bosch process), the most employed catalyst being iron, with addition of promoters, such as potassium oxide, alumina and other non reducible oxides. Aiming at the best compromise between thermodynamic and kinetic factors, it has been found particularly advantageous to work at 120–220 bar pressure and 380–520° C. temperature (see e.g. J. R. Jennings (Editor), Catalytic Ammonia Synthesis, Fundamentals and Practice, Plenum Press, New York, 1991, and A. Nielsen, Ammonia Catalysts and Manufacture, Springer Verlag, Heidelberg, 1995).

#In order to operate at a pressure lower than those previously indicated (which allows noticeable advantages from the points of view of plant engineering, economy and security) several other catalytic materials have been considered and ruthenium has been found particularly promising. However, the high cost of this metal entrains a high surface area support, so to allow a higher metal dispersion, i.e. the use of a reduced amount of metal. To this end several supports have been investigated, such as: SiO2: see Lopez et al., React. Kinet. Catal. Lett., 41 (1990) 217; Al2O3: Y. Kadowaki et al., J. Catal., 161 (1996) 178; and S. Murata et al., J. Catal., 136 (1992) 118; Zeolites: C. T. Fishel et al., J. Catal., 163 (1996) 148; and J. Wellenbüscher et al., Stud. Surf. Sci. Catal., Vol. 84, part B, Elsevier 1994, 941; MgO: O. Hinrichsen et al., Chem. Eng. Sci., 51 (1996) 1983; Carbon-covered alumina: K. S. Rama Rao et al., Appl. Catal., 62 (1990) L19; and S. K. Mashtan et al., J. Molec. Catal., 67 (1991) L1; MgAlO4 spinel: B. Fastrup, Catal. Lett., 48 (1997) 111; Lanthanides oxides: Y. Niwa et al., Chem. Lett., 1996, 3; and, particularly, Graphitised carbon: Z. Kowalczyk et al, Appl. Catal., A: General, 138 (1996) 83.

#A sample of commercial graphite, labelled as HSAG 300, produced by Timcal S.A., Bodio, Switzerland, in form of fine powder, with BET specific surface area over 290 m2/g, whose X-ray diffraction pattern does not show any relevant bands due to amorphous carbon, has been impregnated with a solution of potassium ruthenate, containing, in the minimal amount of water, needed for the impregnation, the exact amount of ruthenium, needed for obtaining the desired loading of metal in the final catalyst. Water has been then removed in rotating evaporator in vacuo at 70° C. and the solid so obtained has been dried overnight in oven at 80° C. The ruthenate has been then reduced to metallic Ru by treating in flowing hydrogen, in tubular oven, at 320° C. e then cooled down to room temperature in flowing nitrogen.

#After cooling, the solid has been washed repeatedly with distilled water, to remove the residual potassium, till the pH of the washing solution attained the neutral value. After another drying in oven at 80° C. for 4 hours, the promoters (Ba, Cs and K) have been added by impregnation, using aqueous solutions containing, in the minimal amount of water needed for impregnation, the exact amount of BaNO3, and of CsOH+KOH, in the order. After each impregnation the excess water has been removed in rotating evaporator and in vacuo, at 35–40° C.

#The Ru loading in the finished catalyst was 8.9 weight % and the atomic ratios of promoters and Ru were Ba/Ru=0.6, Cs/Ru=1 and K/Ru=3.5, respectively.