Wustite as a new precursor of industrial ammonia synthesis catalysts
Introduction
The traditional ammonia synthesis catalyst, developed in the first years of 20th Century by BASF researchers in Germany [1], is prepared from magnetite, promoted with small amounts of unreducible oxides, typically of Al, K and Ca. Magnetite is then reduced to metallic iron by synthesis gas in the reactor itself. The industrial catalyst presently used is not basically different from that developed about 100 years ago. It is widely accepted that magnetite is indispensable as precursor, because it allows an easy cationic substitution of Al3+ for Fe3+, thus giving homogeneous distribution of Al in the solid [2]. This gives, after reduction of magnetite to metallic iron, Fe particles either partly covered by well-dispersed Al oxide isles [3] or including paracrystalline Fe aluminate species [4], [5], [6], both of which prevent Fe from sintering, thus increasing the catalyst life. It is also thought that the Fe surface produced from magnetite by reduction with synthesis gas contains the largest amount of sites having the geometrical configuration most convenient for the dissociative chemisorption of dinitrogen, thus giving high catalytic activity. In fact, it has been experimentally found that, when the Fe2+/Fe3+ ratio of the catalyst is different from 0.5 (stoichiometric magnetite), either higher or lower, the activity decreases, thus giving support to the above-mentioned opinions [3]. On the whole, this catalyst was considered well consolidated and no special improvement was still expected, so that a completely different Ru/C catalyst was recently developed for low-pressure ammonia synthesis [7], [8], [9].
In 1996, some Chinese researchers reported [10] and patented [11] a new high-activity ammonia synthesis catalyst based on wustite as precursor in place of magnetite, the oxidic promoters being substantially the same. It was shown that the reaction rate of the new catalyst is 30–90% higher than that of the traditional one and that such an activity difference increases when temperature decreases, thus rendering this catalyst well suited to low-pressure synthesis. Moreover, as wustite can be reduced to Fe more easily than magnetite, the new catalyst allows to shorten appreciably the duration of plant start-up. Resistance to sintering and mechanical strength are reported to be the same of the traditional catalyst, while resistance to CO impurities is higher. It was also reported [10] that the new catalyst is being used successfully in Chinese ammonia plants since 1991. The ammonia productivity was claimed to be increased by 20%, but further advantages are to be expected when the new catalyst is used in big modern plants.
Very recently, some Polish researchers reported [12] that wustite-based ammonia synthesis catalysts are slightly less active than those based on magnetite and much less resistant to thermal deactivation. The high industrial impact of the ammonia synthesis catalyst and also the underlying scientific implications led us to face the problem of such contradictory data. To this purpose the Chinese wustite-based A301 catalyst and a widely used magnetite-based industrial catalyst have been extensively characterized for both physical and catalytic properties, aiming at reaching definite conclusions about the relative performance of the two catalyst types. It is clear that, should wustite be a more effective precursor than magnetite, a substantial revision of the present knowledge of ammonia synthesis catalysts should be worth.
Section snippets
Catalysts
Two commercial catalysts, based on wustite and on magnetite, respectively, were chosen for this investigation. The wustite-based catalyst (A301) is manufactured by Shangyou Catalyst Co. Ltd., while the magnetite-based one is a widely used European product. Catalyst A301 is reported to contain the same promoters as the traditional ones [13], so that the main difference between the two catalysts is in the precursor. This has been confirmed by our analyses (Table 1).
Physical characterizations
The bulk density was measured
Bulk density
The following values have been obtained for the bulk densities of the two catalysts (particle size 1.5–3 mm):A301 wustite catalyst (g/cm3) 3.25 Magnetite catalyst (g/cm3) 2.85
As the whole available volume of industrial reactors is usually filled with the catalyst, the activity per volume (for instance, moles of ammonia produced per hour per cubic meter of catalyst) must be considered for industrial purposes. Therefore, an activity increase of 14% for catalyst A301 is simply due to the higher bulk
Discussion
The first announcement of wustite as a better precursor than magnetite for ammonia synthesis catalysts was given in 1996 in an international journal by the Catalyst Group of the Zhejiang University of Technology in Hangzhou [10]. It is clear that such a discovery has a strong impact on the consolidated scientific knowledge of ammonia catalysts and moreover opens the way to interesting industrial application. It was therefore surprising that after 6 years no papers had appeared from other groups
Conclusions
The higher activity of ammonia synthesis catalysts from wustite, with respect to those obtained from magnetite, has been definitively established. The presence of wustite as the only crystalline phase in the catalyst precursor is indispensable for high activity. The activity gap between the two catalysts increases at lower temperatures. The higher activity, referred to bed volume, of the wustite-based catalyst is due in part to its higher density, but mostly to the higher efficiency of Fe
References (27)
- et al.
J. Catal.
(1972) - et al.
J. Catal.
(1972) - et al.
J. Catal.
(1978) - et al.
Appl. Catal. A
(1999) - et al.
Appl. Catal. A
(2001) - et al.
Appl. Catal. A
(1996) - et al.
Appl. Catal. A
(2002) - et al.
Appl. Catal. A
(2001) - et al.
Appl. Catal. A
(1997) - et al.
Appl. Catal. A
(1998)
Appl. Catal.
Cited by (0)
- 1
Deceased.