Ammonia synthesis over the Ba-promoted ruthenium catalysts supported on boron nitride
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Catalysis Letters Vol. 100, Nos. 1–2, March 2005 ( 2005) DOI: 10.1007/s10562-004-3089-6
Ammonia synthesis over the Ba-promoted ruthenium catalysts supported on boron nitride Dariusz Szmigiel,a Wioletta Raro´g-Pilecka,a El_zbieta Mis´ kiewicz,a Ewa Maciejewska,a Zbigniew Kaszkur,b Janusz W. Sobczak,b and Zbigniew Kowalczyka,* a
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-662 Warsaw, Poland b Institute of Physical Chemistry PAS, Kasprzaka 44/52, 01-224 Warsaw, Poland
Received 11 August 2004; accepted 21 October 2004
Barium promoted ruthenium catalysts deposited on the boron nitride supports were characterised (XRD, O2 and CO chemisorption) and tested in NH3 synthesis. Prior to use, the raw BN materials marked as BNS (Starck, 96 m2/g) and HCV (Advanced Ceramics Corporation Cleveland USA, 40 m2/g) were heated in an ammonia stream at 700–800 C for 120 h. As a result, the oxygen content was reduced from 7.0 at% (BNS) to 3.5 at% (BNSNH3) and from 3.8 to 2.7 at% (HCVNH3), as evidenced by XPS. The kinetic studies of NH3 synthesis (63 or 90 bar; H2:N2 = 3:1) revealed that the catalysts based on the modified supports were more active, respectively, than those derived from starting nitrides, the difference being especially pronounced in the case of BNS and BNSNH3. Studies of the catalysts activation have shown, in turn, that the stabilisation in a H2:N2 = 3:1 mixture at 1 bar is very slow, i.e. the reaction rate increases slowly versus time on stream even at a high temperature of 550 – 600 C. Stabilisation is faster and the NH3 synthesis rates are higher when the activation is performed with an ammonia rich mixture (10% NH3 in H2:N2 = 3:1) flowing under high pressure of 90 bar. It is suggested that boron oxide (an impurity) acts as a deactivating agent for Ba–Ru/BN and that the reaction between NH3 and B2O3 (B2O3 + 2NH3 = 2BN + 3H2O) is responsible for the activity increase. A poisoning mechanism of B2O3 is discussed. KEY WORDS: ammonia synthesis; ruthenium catalyst; boron nitride supports; support modification with ammonia.
1. Introduction Although the ammonia synthesis from hydrogen and nitrogen has been commercialised almost 100 years ago, it still remains a very important but also a high energy consuming industrial process [1]. The catalytic NH3 synthesis is very attractive as a model reaction in the fundamental studies, too [2]. For the above reasons, the researchers from industrial centres and academia are still active in the field of NH3 synthesis and they try to work out a completely new catalyst [1,3–10] or to improve the formula and properties of the conventional iron one [11–15]. Among several new catalytic systems investigated in the last 30 years, only ruthenium supported on high surface area graphite (HSAG) was implemented to the industrial practice so far. A combination of both conventional iron (first catalytic bed) and modern Ru/C catalyst (last three beds) in a so-called Kellog Brown & Root Advanced Ammonia Process (KBRAAP) was shown to be very advantageous [16,17], i.e. the pr
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