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Catalytic Synthesis of AmmoniaЧA УNever-Ending StoryФ.

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Ammonia Synthesis
Ammonia Synthesis
Catalytic Synthesis of Ammonia—A “Never-Ending
Story”?
Robert Schlgl*
Keywords:
ammonia · heterogeneous catalysis · iron ·
ruthenium · surface chemistry
N
itrogen atoms are essential for the function of biological molecules
and thus are and important component of fertilizers and medicaments.
Bonds to nitrogen also find nonbiological uses in dyes, explosives, and
resins. The synthesis of all these materials requires ammonia as an
activated nitrogen building block. This situation is true for natural
processes and the chemical industry. Knowledge of the various techniques for the preparation of ammonia is thus of fundamental importance for chemistry. The Haber–Bosch synthesis was the first
heterogeneous catalytic system employed in the chemical industry and
is still in use today. Understanding the mechanism and the translation
of the knowledge into technical perfection has become a fundamental
criterion for scientific development in catalysis research.
Recently a paper bearing the title: “All quiet at the
nitrogen front” was published.[1] The author laments the low
visibility of research in nitrogen fixation in general, and in
organometallic biochemical nitrogen fixation in particular,
despite significant advances in this subject. However, it would
not be convincing to argue that a higher research effort in
ammonia synthesis (which would certainly be desirable)
would be justified by the necessity for developing totally
different routes with higher degrees of efficiency than the
modern Haber–Bosch processes, particularly when the following thermodynamic efficiency numbers are considered.
Liquid ammonia at atmospheric pressure is needed as starting
material, made from air, liquid water (T = 300 K), and natural
gas. The resulting ammonia theoretically contains 6.8 Gcal t 1
as chemical energy if the process is energetically isolated.
Industrial processes achieve total degrees of efficiency of up
to 70 % for the processes described above (approximately
4.81 Gcal t 1 NH3), whereas the biggest losses in efficiency are
made in the methane reforming process (1.18 Gcal t 1). Only
a slight loss is made during the actual ammonia synthesis
(0.37 Gcal t 1).[2] These efficiencies are achieved with com-
[*] Prof. Dr. R. Schlgl
Fritz-Haber-Institut der MPG
Department of Inorganic Chemistry
Faradayweg 4—6, 14159 Berlin (Germany)
Fax: (+ 49) 30-84-13-44-01
E-mail: acsek@fhi-berlin.mpg.de
2004
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
mercially available iron catalysts[3, 4]
and with modern ruthenium-based
catalysts. Any further improvement in
the catalysts would help to increase the
thermodynamic efficiency and reduce
the specific price of this basic compound through a reduction in the
pressure during synthesis. However, any new proposals of
this kind[5] have to meet the high standards of the “conventional” routes. It should be further kept in mind that the main
synthesis step has been optimized already, and that the main
obstacles arise from synthesis of the precursors. Figure 1
shows a modern ammonia plant. All the large aggregates in
the foreground are needed to generate hydrogen from
methane and purify it, the actual rector is in comparison very
small and can not be seen. The picture clearly illustrates
where the development potential lies and where the optimization has reached a high maturity.
As well as being of high industrial relevance the catalytic
synthesis of ammonia is also a key reaction for creating new
life and a prototypical model reaction[6] that helps in gaining a
fundamental understanding of catalysis in general and therefore of considerable scientific and cultural importance. It is
mainly this reason that drives the research in ammonia
synthesis forward, especially since evidence for a knowledgebased improvement of a catalyst would have a strong
signaling effect on other fields of catalysis research.
For the still industrially important iron catalyst, a good
picture of its structure and its activity as an accelerator for the
dissociation of dinitrogen, which is the kinetically determining step, can be drawn. Results from pioneering experiments
on iron crystals[7] at lower pressures, were quantitatively
corroborated by high-pressure experiments.[8] This apparent
lack of a pressure gap allowed the theoretical description of
the nitrogen activation and therefore the quantitative extrap-
DOI: 10.1002/anie.200301553
Angew. Chem. Int. Ed. 2003, 42, 2004 – 2008
Angewandte
Chemie
R. Schl$gl
Figure 1. Overview of a modern ammonia synthesis plant. The rubbish
skip in the foreground indicates the dimensions. The large structure to
the left is the methane steam reformer unit. The generated hydrogen is
purified in all the other aggregates seen at the front (source BASF AG).
olation of the kinetics from ultra-high vacuum (UHV)
conditions to industrial conditions.[9–11] Furthermore, studies
on single crystals[8] showed that the lack of a pressure gap only
occurred at low nitrogen coverages, whereas at high nitrogen
coverages a considerable deviation from the theoretical
adsorption behavior is caused by the formation of a subnitride
surface phase.
Under industrial conditions the nitrogen coverage is low
(hence the theoretical description applies), but in the bulk
phase the catalyst is nitrided, which causes a distorted crystal
structure. Through suitable pretreatments this structure has
also been included in model experiments, however, it was not
further characterized. After the required activation[12]—a
process now well understood[13, 14]—the industrial catalyst
shows a distorted iron structure, that has been demonstrated
to be essential for its catalytic function.[15] This distortion
manifests itself as metastable plates in the (111) orientation,[16] which are formed by the topotactic reduction of the
magnetite precursor at extremely mild conditions,[12, 17] but
also by stress states in the regularly orientated (100) regions[18]
of the ammonia iron.[*] These stress states participate in the
structure-sensitive activation of nitrogen, which is particularly
efficient on the (111) faces,[19, 20] but also on different faces and
on strain-induced[21, 22] step defects.[23] These recent theoretical[25] and experimental[13, 26–29] results require that the discussion about the role of the promoters (especially the
potassium), which is normally considered as closed,[24] is
reopened.
The modern industrial iron catalyst is a nanostructured
metastable substance, which is formed during the surprisingly
complex synthesis of the oxide precursor.[12, 18] This differs
significantly from pure iron especially in the mesoscopic
area[30] despite the similarities in the local structure. Its
metastability is the reason for its sensitivity towards thermal
overstress during activation and oxidation of the activated
material. Alternative preparation routes for the nanostructured system are possible,[26, 31–33] and confirm the complex
structure of this seemingly very simple ammonia iron.
A quantitative analysis has shown[34] that only 5 % of the
geometric iron surface participates in the activation of
nitrogen on an industrial catalyst. The remaining adsorption
sites are filled from these centers by surface diffusion. Thus,
the duality of plates and blocks has a functional equivalent as
nitrogen activation and storage regions, which coincides
quantitatively with theoretical and experimental findings.
Ruthenium and molybdenum compounds stand out in the
group the non-iron catalysts. A comprehensive report about
the development and background of the empirically motivated search for suitable catalysts is given elsewhere.[35]
20 years ago, in a review, the catalytic efficiency of the
elements for the synthesis and decomposition of ammonia
was correlated with the chemisorption energy of nitrogen. An
inverted parabolic function (volcano curve) was obtained, in
which iron, ruthenium, and osmium mark the top of the
volcano.[36] The idea of optimizing the performance of the
elements on the slope of the curve though the formation of
alloys of the elements which react either too slowly or too
vigorously with nitrogen, was proposed in the same paper.
The formation of metal subnitrides and their effectiveness as
inhibitors for the decomposition of ammonia was described
about 30 years ago.[37] Thus, the routes for development were
set out a long time ago. Not surprisingly, an efficient nonruthenium catalyst on the basis of cobalt–molybdenum nitride
was recently presented,[38–42] which can be regarded as the
current climax of a systematically but empirically justified
development.[43, 44]
Ruthenium-based catalysts have a much a longer history
of development and are routinely in use.[45] As a consequence
these systems have formed the basis of much fundamental
research. The established single-crystal approach[7] did not
lead to immediate success because carefully prepared samples
are extremely inactive in the dissociative adsorption of
nitrogen.[46–48] The discrepancy with kinetic studies, which
showed the high efficiency of ruthenium particles for the
activation of nitrogen,[49] was resolved by experiments with
single crystals containing step defects. It was shown that these
types of defect, which are normally minimized during
[*] Ammonia iron is a term used to describe iron that, through the action
of ammonia, has become structurally strained and as a result,
catalytically active.
Angew. Chem. Int. Ed. 2003, 42, 2004 – 2008
www.angewandte.org
Robert Schlgl was born in Munich in 1954
and completed his chemistry studies at the
Universitt Munchen in 1982. Working in
Cambridge (Prof. Sir John Thomas) and
Basel (Prof. Hans-Joachim G/ntherodt)
drew him to the areas between chemistry
and solid-state physics. Following some time
in industry he completed his Habilitation in
Berlin (Prof. Gerhard Ertl) in 1989 and
then became Professor of Inorganic
Chemistry in Frankfurt. In 1993 he joined
the Fritz Haber Institute of the Max Planck
Society where he is studying the basis for
heterogeneous catalytic processes and the
development of in situ spectroscopic
methods.
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2005
Angewandte
Chemie
Ammonia Synthesis
conventional single-crystal preparation, are essential for the
catalytic effectiveness of ruthenium in nitrogen activation.[50–53] This experiment demonstrates impressively how
important the mesoscopic structure, which reaches beyond
the confines of the local structure of the surface, is for surface
science. In conjunction with the experiments a theoretical
description about the dissociation process was presented as
well as an elegant explanation for the importance of the
steps.[23, 52, 54]
In analogy to the iron system the suitable nanostructure of
ruthenium is induced through nitrogen being dissolved in the
bulk. An experiment on a Ru (0001) single crystal was carried
out, in which a stream of nitrogen was applied at 1600 K under
UHV conditions. The structural characterization (Figure 2)
shows that the combined effects of heat and nitrogen
exposure lead to the transformation of the single-crystal
surface from its preferred orientation (still visible on the
reverse side of the sample) into a complex but ordered new
structure.
The SEM image shows the decay of the single crystal into
domains with a system of grain boundaries. On this nanostructured surface (Figure 3) the chemisorption of dinitrogen
could be verified by valence band and core-level photoelectron spectroscopy (XPS). The spectroscopic signatures
seen in Figure 3 confirm the presence of end-on-bound
nitrogen molecules as the majority of surface species under
the chosen in situ conditions. The weak interaction of the N2
molecule with the ruthenium metal is shown by the small
distortion of the metal 3d band. (Figure 3, region I, curve A)
in comparison to the adsorption of the strongly interacting
CO molecules (curve B). The double structure in the corelevel spectrum of the nitrogen (Figure 3) is evidence for the
adsorption geometry, and energetically corresponds very well
to the data obtained from the iron system. These experiments
show the merit of a strategy in which model systems with
increasing structural complexity (starting from planar then
stepped single crystals up to surfaces restructured in situ) are
used towards a characterization of the nanoparticles of
industrially relevant ruthenium catalysts.
The synthesis of highly efficient, stable, and reproducible
supported systems, which in light of the high cost of
ruthenium is essential for any possible application, has
recently attained a high level through the continuation of
previous studies on carbon and basic oxides as supports.[55–66]
These systems are now classified as a new generation of
ammonia-synthesis catalysts.[45] In parallel, a functional characterization of carbon- and basic-oxide-supported materials
was performed with temperature-programmed methods[49]
and led to a quantitative micro kinetic model of the ammonia
synthesis over ruthenium particles. It is remarkable that the
system that has shown the highest efficiency in previous
studies was found to be rather inefficient during a highthroughput-screening project.[67] The results of the investigation based on theoretical knowledge with thorough techniques (see above) turned out to be superior to the too quick
high-throughput-screening testing.[68]
The convergence of knowledge about model catalysis and
real catalysis is directly observable for the ruthenium system
since high-resolution electron microscopy can deliver images
2006
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 2. Powder X-ray diffraction characterization of a Ru (0001) single crystal of 0.8 mm thickness after being heated at 1600 K for 4 h.
Whilst the front side of the crystal was exposed to a nitrogen flow of
10 7 mbar (A), the reverse side was heated (B). Both images show the
characteristic lines for hexagonal Ru metal (ICDD 6–663) but with substantially different texture. The reverse side is strongly oriented in
[0001] direction, as expected (dominant reflexes of the (001) series)
whilst the front side shows strong intensities of other reflex groups,
which do not correspond to the series of intensity for statistical powder orientation. The scanning electron microscopic survey image
(15 keV) and detail image with 15-times higher magnification shows
the polycrystalline morphology with grains, grain boundaries, and a
surface layer with very finely structured parts. Energy-dispersive X-ray
fluorescence (EDX) shows traces of nitrogen in the metal.
of activated ruthenium particles and their barium oxide
promoters.[69] These images show facetted particles of 2–5 nm
in diameter, which are partly covered by disordered fragments, which are interpreted as being barium oxide particles.
Parts of these fragments show the same orientation as the
stepped structures in the model experiments.[50]
Experiments carried out by Ertl[7] and Somorjai [19] were
designed to enhance the understanding of the ammonia
synthesis and allow a quantitative theoretical description of
www.angewandte.org
Angew. Chem. Int. Ed. 2003, 42, 2004 – 2008
Angewandte
Chemie
R. Schl$gl
Figure 3. Above: Adsorption experiment of dinitrogen on the pretreated Ru crystal (see Figure 2) after not more than 4 Langmuir N2 exposure (40 s, 10 7 mbar) at 120 K. In sequence A the growth of a molecular species with orbital energies of 8.0 eV and 11.9 eV below the Fermi
level could be observed. This species could not be CO, which adsorbs
very well as impurity. The effect of CO is illustrated in spectrum B,
which was taken for comparison (orbital energies 8.7 eV and 12.2 eV)
after exposition of 1 Langmuir CO on a pure sample. In region I the intensity of the metal d band is decreasing, whilst in region II the emission of the molecular N2 3s and 2s states is increasing. This data corresponds very well to earlier measurements on iron and other metals.[75] Below: Core-level photoelectron spectrum of the N1s level of
molecular adsorbed nitrogen. The measurement was taken with Mgk
excitation and with a sample temperature of 300 K and a background
pressure of 10 6 mbar N2. The positions of the lines with 405.6 eV and
400.5 eV correspond very well to the observations on iron, but the distribution of intensity between the high energetic main signal and the
lower energetic satellites does not. These bands are too intense and
thereby show that a further species must be present. This species
could not be atomic nitrogen, which has a single XPS line at 397.0 eV
in complete agreement to measurements on iron.[76] The inset shows
an atomic force microscopy (AFM) image of the microstructure of the
sample surface in air. The height of the about 100 nm wide steps is
about 2 nm. The multiple step edges might contain the active centers.
this reaction. Two predictions for the future development of
this subject resulted from this work.
First of all a quantitative description of the volcano curve
for the catalytic efficiency of the elements in the ammonia
synthesis could be given, which was based on fundamental
knowledge of the reaction path and the theory of the
transition state. This description allowed a specific prediction
of the catalytic efficiency of alloy systems[70, 71] with extreme
properties on the volcano curve. The studies on the Co-Mo-N
system,[39, 70] which were carried out almost simultaneously but
with totally different motivation, led to similar results. They
confirm in a remarkable way, how theory and chemical
intuition are becoming equally useful with respect to the
Angew. Chem. Int. Ed. 2003, 42, 2004 – 2008
choice of research topics. Disregarding the “old empirical
knowledge”, one could speak of a remarkable example of
“designing” a catalytic system,[72] based on pure theoretical
knowledge about the process of the target reaction. Despite
these significant advances, this good catalytic behavior is only
achieved at relative small synthesis rates; also there are
significant stability problems. Much more research effort is
needed here.
A different challenge thrown down by theory for the
experimentalist lies in the prediction of a biomimetic
ammonia-synthesis path at room temperature and atmospheric pressure. This should be achieved preferably by
electrochemical processes with an efficiency similar to the
synthesis strategies that are available today.[73]
The answer to the question raised at the beginning should
therefore be, that there is no other practically relevant
reaction that leads to such a close interconnection between
theory, model catalysis, and experiment as the high-pressure
synthesis of ammonia by Haber and Bosch. This statement is
not only valid for the iron system but also for the ruthenium
system and, in foreseeable future, for the Co-Mo-N alloy
system. The time frame for the acquirement of knowledge is
truly remarkable, it took decades for the case of the iron
system but just ten years for the ruthenium system, and it will
take only a few years for the case of the alloy system. This is
despite the catalyst getting more complex both chemically
and structurally. Analysis of the reasons leading to this story
of success reveals the factors that stand out to be, the
persuasiveness of the pioneering experiments that persists
even 30 years later, the enormous maturation of experimental
and theoretical methods, and the increased precision of the
world-wide research efforts. Here, the Danish national network of industrial and academic research, which has existed
for more than 10 years, plays an important part.
The never-ending story has not ended yet. In addition to
questions about the elementary steps of the reaction and the
importance of the real structure and subnitrides for the
catalyst efficiency, as well as the wide-open question about
new catalyst materials, there are also new challenges for new
applications. Ammonia would make a convenient storage
molecule for hydrogen for operation in fuel cells, because it is
free of carbon and so the hydrogen would be less likely to
poison the fuel cell, neither would any harmful contribution
to the ecobalance be produced. Synthesis and decomposition
of ammonia out of and to the elements are well controlled. If
a practical and safe transfer of the industrial logistic[74] into an
end-consumer field were to be obtained, then a powerful
alternative for the generation of hydrogen could be at our
disposal.
Received: March 1, 2002 [M1553]
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