Structure Sensitivity Of Supported Ruthenium Catalysts For Ammonia Synthesis

Journal of Molecular Catalysis A: Chemical 163 (2000) 19–26

Structure sensitivity of supported ruthenium catalysts for
ammonia synthesis
Claus J.H. Jacobsen a,∗ , Søren Dahl a , Poul L. Hansen a , Eric Törnqvist a , Lone Jensen a ,
Henrik Topsøe a , Dorthe V. Prip b , Pernille B. Møenshaug b , Ib Chorkendorff c
a Haldor Topsøe Research Laboratories, Nymøllevej 55, DK-2800 Lyngby, DenmarkInstitute for Applied Chemistry, Technical University of Denmark, Building 307, DK-2800 Lyngby, Denmark
Interdisciplinary Research Center for Catalysis (ICAT), Technical University of Denmark, Building 307, DK-2800 Lyngby, Denmark
b

c

Received 27 March 2000; accepted 15 May 2000

Abstract
The catalytic ammonia synthesis activities of four supported ruthenium catalysts are reported. Itis seen that Ru/MgAl2 O4
is more active than two similar Ru/C catalysts, which are significantly more active than Ru/Si3 N4 . The activity differences
cannot be satisfactorily explained solely by the differences in dispersion. Recent results from single crystal studies and DFT
calculations have shown that ammonia synthesis over ruthenium catalysts is a very structure sensitive reaction, more sothan on iron catalysts. It is suggested that special B5 -type sites are primarily responsible for the catalytic activity of the
present supported Ru catalysts. It is shown how the number of such B5 -type sites depends on the Ru crystal size for a given
crystal morphology. We have found that the activity of the Ru/MgAl2 O4 catalyst increases significantly during the initial
part of a test run.This activity increase is paralleled by the disappearance of crystals smaller than ca. 1.0 nm due to sintering
and a resulting formation of larger crystals. We conclude that there exists a lower limit to the desired crystal size of Ru in
supported ammonia synthesis catalysts. This is in agreement with a low number of B5 -type sites expected for such crystal
sizes. Furthermore, we suggest that thesupport plays a decisive role in controlling the morphology of the Ru crystals and
the resulting change in abundance of B5 -type sites is the main cause for the significant activity variations observed for Ru
catalysts with different supports. Finally, the support may also influence the electronic and catalytic properties of neighboring
B5 -type sites. © 2000 Elsevier Science B.V. All rightsreserved.
Keywords: Ruthenium; Ammonia synthesis; Structure sensitivity; B5 sites

1. Introduction
The catalytic ammonia synthesis technology has
played a central role in the development of the chemical industry during the 20th century. This industrial
importance has been paralleled by a significant scientific interest in understanding and improving the
∗ Corresponding author. Tel.:+45-45-27-22-02;
fax: +45-45-27-29-99.
E-mail address: chj@topsoe.dk (C.J.H. Jacobsen).

ammonia synthesis catalyst. Often new techniques,
methods, and theories of catalysis have initially been
developed and applied in connection with studies of
this system. Similarly, new discoveries in the field of
ammonia synthesis have been extended to other areas
of catalysis. The combined influence of refinedcharacterization techniques, improved kinetic analyses,
and new possibilities in theoretical modeling has led
to a detailed insight into the fundamentals of ammonia synthesis catalysts. Several recent reviews give a
comprehensive account of the current understanding

1381-1169/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 1 - 1 1 6 9 ( 0 0 ) 0 0 3 9 6 - 420

C.J.H. Jacobsen et al. / Journal of Molecular Catalysis A: Chemical 163 (2000) 19–26

of catalytic ammonia synthesis [1–4]. It is an interesting paradox that despite these efforts the industrial
ammonia synthesis catalyst is still very similar to that
developed by Mittasch and co-workers.
Ammonia synthesis over iron-based catalysts is the
standard textbook example of a structure...