Ammonia synthesis over the Ba-promoted ruthenium catalysts supported on boron nitride

Abstract

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.

Letter to Editor

“…Hansen et al observed that the Ru crystals exhibited a hexagonal morphology in situ TEM studied of BN supported ruthenium catalysts [11]. Therefore, the reported results of Szmigiel et al [1] are more justifiably explained by the likely phase transition of boron nitride. When the support materials were modified by heating in ammonia, the disorder BN can be converted to highly ordered BN, as indicated by the sharpening of the (002) reflection in XRD diffraction pattern, thus the activity for ammonia synthesis was higher.…”

“…In the recent article by Szmigiel and co-workers [1], the authors reported results in the ammonia synthesis using the barium-promoted ruthenium catalysts supported on boron nitride. They attributed the observed enhancement in the ammonia synthesis rates of the catalysts with modifying the supports at high temperature to the decrease of boron oxide, which might act as a deactivation agent.…”

Response to Letter to the Editor

“…1 that all the BN materials exhibit a turbostratic structure. This was also stressed in [10]. It is seen as well that the structure of both starting nitrides (BNS and HCV) is fully resistant to overheating in an ammonia stream at 700-800°C for 120 h. In contrast to what the authors of the letter claim, there is no reason to suppose that ''disordered BN can be converted to highly ordered BN'' when heating in NH 3 .…”

“…To evaluate the alternative proposed by the authors of the letter (the proposal is addressed exclusively to the Ba-Ru/BN catalysts), a reference should be given to the XRD data characterizing the BN supports used in our studies [10] (see Fig. 1).…”

“…To facilitate the discussion and to avoid misunderstanding, the form of Fig. 1 should precisely be the same as that presented in [10] and this has been done. It is clearly seen in Fig.…”