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 Kowalczyk a, * 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, O 2 and CO chemisorption) and tested in NH 3 synthesis. Prior to use, the raw BN materials marked as BNS (Starck, 96 m 2 /g) and HCV (Advanced Ceramics Corporation Cleveland USA, 40 m 2 /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% (BNS NH3 ) and from 3.8 to 2.7 at% (HCV NH3 ), as evidenced by XPS. The kinetic studies of NH 3 synthesis (63 or 90 bar; H 2 :N 2 = 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 BNS NH3 . Studies of the catalysts activation have shown, in turn, that the stabilisation in a H 2 :N 2 = 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 NH 3 synthesis rates are higher when the activation is performed with an ammonia rich mixture (10% NH 3 in H 2 :N 2 = 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 NH 3 and B 2 O 3 (B 2 O 3 + 2NH 3 = 2BN + 3H 2 O) is responsible for the activity increase. A poisoning mechanism of B 2 O 3 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 NH 3 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 NH 3 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 sup- ported on high surface area graphite (HSAG) was implemented to the industrial practice so far. A combi- nation 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 pressure in ammonia loops could be significantly reduced (to 90 bar), thus resulting in a lower energy consumption. The KBRAAP catalyst has been operat- ing successfully in several retrofitted as well as totally new plants, some of them being of high capacity (about 2000 ton/day) [16]. The commercialisation of the Ru/HSAG catalyst has resulted in intensive studies of ruthenium catalysts deposited on various supports such as magnesia [18–30], magnesium–aluminum spinel [31–34], zeolites [35–38] or carbon [39–55], Recently, Jacobsen [56] and Hansen et al. [57] from H. Topsoe have revealed that ruthenium supported on boron nitride and promoted with barium is the most efficient catalytic system for NH 3 synthesis that has ever been found. Our paper is also devoted to the Ba–Ru/BN catalysts or, more precisely, to their catalytic properties in NH 3 synthesis. To get a closer insight into the role of the BN substrate, both the effect of the support modification with ammonia at high temperature and that of the activation procedure were examined. The kinetic measurements of NH 3 synthesis were supplemented with the characterisation studies of the BN supports (XRD, XPS) and catalysts (XRD, oxygen and carbon monoxide chemisorption). 2. Experimental 2.1. The BN supports Two commercial boron nitride powders of 190 m 2 /g (marked as BNS) and of 40 m 2 /g (marked as HCV) BET surface areas were received from H.C. Starck GmbH and Advanced Ceramics Corporation Cleveland USA, *To whom correspondence should be addressed. E-mail: zbyko@ch.pw.edu.pl Catalysis Letters Vol. 100, Nos. 1–2, March 2005 (Ó 2005) 79 DOI: 10.1007/s10562-004-3089-6 1011-372X/05/0300–0079/0 Ó 2005 Springer Science+Business Media, Inc.