4. Carbon-supported promoted Ru catalyst for ammonia synthesis
5. A series of alkali- and/or alkali-earth-promoted, carbon-supported Ru catalysts have been prepared by impregnation from aqueous solutions of the precursors. The supports have been pretreated by heating in inert atmosphere at various temperatures, followed by partial oxidation in air at 425℃ and then by hydrogen treatment at 900℃. The catalyst samples, diluted 1/22 with quartz powder, have been studied in ammonia synthesis by means of a bench-scale, downflow, continuous, tubular reactor, under standard reaction conditions (430℃, 100 bar total pressure, H2/N2 = 1.5/1 feeding ratio). Caesium and Barium proved to be much more effective promoters than Potassium as promoters and the optimal temperature range for support pretreatment was found to be about 1900℃. The ammonia productivity, on a catalyst volume basis, of our best Ru catalyst was about twice higher than that of the most widely used Fe-based commercial catalysts. ?1999 Elsevier Science B.V. All rights reserved.
6. During the last decades a continuous trend can be recognized towards lower pressure in ammonia synthesis process, which clearly requires more active catalysts. As the intrinsic activity of the presently used Fe catalyst was already optimized a long time ago, this requirement was firstly accomplished by decreasing catalyst particle size, through the introduction of radial reactors [1]. This was really a breakthrough in ammonia technology, but still not sufficient. So the use of Fe-based alloy catalysts was also tried (mainly Fe–Co) [2], but there remain still many doubts about their advantages. Fe catalysts preparation by magnetite precipitation, instead of melting, was tried too [3], but, in spite of the good activity, industrial production by this procedure appeared very cumbersome. So the problem seems unlikely to be resolved by the development of a modified Fe catalyst.
7. The most important results of the present investigation may be summarised as follows: (i) a chlorine-free procedure, easily amenable to scale up, has been set up for the preparation of a stable carbon-supported Ru-based catalyst, much more active for the synthesis of ammonia than the usual Fe-based one; (ii) the much higher promoting effect of Ba and Cs, with respect to K, has been confirmed; (iii) the beneficial effect of heat pretreatment of the carbon has been outlined in terms of both activity and stability of catalyst and (iv) the heat pretreatment to ca. 1900℃ allows to obtain a very active catalyst, in the absence of any further oxidation–reduction treatment.
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