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appliedsurface science
ELSEVIER

Applied Surface Science l 21 / 122 (1997) 488-491

Molecular sensing techniques for the characterization and designof new ammonia catalysts
K. Aika *, J. Kubota, Y. Kadowaki, Y. Niwa, Y. Izumi
Department of Em, ironmental Chemisto, and Engineering, To~'o Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226,
Japan

Received 5 November 1996; accepted 24 February 1997

Abstract
The N - N stretching frequency of adsorbed dinitrogen on the actual (supported or promoted) Ru catalysts has been shown
to be extremely sensitive to the electronic state of the active surface which is usually hard to detect even by XPS. The
wavenumber was also a good index of the N 2 activation and ammonia synthesis on these Ru catalysts. Atomically adsorbedhydrogen and its effect o n N 2 activation was also observed by FTIR. The addition of alkali promotes N 2 activation on Ru
although it causes strong hydrogen poisoning. These various techniques have been discussed with regard to the developmentof the 'second generation ammonia catalyst'. © 1997 Elsevier Science B.V.
Keywords: Ammonia synthesis; Nz activation; Ruthenium catalyst; FTIR of adsorbed dinitrogen; FTIR of adsorbed hydrogen; Alkali effect

1. Introduction
The authors have devoted long years to the studyof ruthenium catalysts for ammonia synthesis [1]. A
well developed Ru catalyst has been shown to be
more active than iron catalysts [2]. In 1992, the firstcommercial plant containing a ruthenium catalyst toproduce ammonia was established in Canada. Manyfactors determine the rate of ammonia synthesis on
the ruthenium catalyst, however, Ru had yet to bestudied in detail compared with the iron catalyst [1].
For the ruthenium catalyst, the activity and themechanisms are very much influenced by the support
and promoter [2]. Thus, the kinetic features differ

Corresponding author. Fax: +81-45-9245441; e-mail: kenaika@chemenv.titech.ac.jp.

depending on the different Ru catalysts having different supports and promoters [3].
The mechanism behind a m m o m a synthesis on Ru
has been proposed as follows [1,3]:
N2+2*(=N2(a)+*)--+2N(a),(1)H 2 + 2. = 2H(a),(2)N ( a ) + 3 H ( a ) = NH 3 + 4 , .

(3)The dissociative adsorption of N 2 (through theshort-lived Nz(a)) determines the rate of the synthesis (Eq. (1)), while strongly adsorbed hydrogen (Eq.
(2)) blocks the vacant site ( * ) and retards Eq. (1)under the equilibrium of H2 and the NH 3 produced.
When the retardation index is described as - z forthe hydrogen atom and - 2 y for the nitrogen atom,0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
Pll S0169-4332(97)00343-7

K. Aika et al./ Applied Surface Science 121 / 122 (1997) 488-491

the rate of ammonia synthesis can be described asfollows [3]:R = kPNz(PNHPu ,1)5 ":bPp 3 v - z p--NH;"-2v"-N2-~:2y( P n ,_)

,z

(4)

(5)

FFIR techniques have been employed to characterizethese mechanisms at the molecular level. Since therate-determining step is the activation of dinitrogen(Eq. (1)), the nature of the precursor N2(a) relates tothe activity. Also, when the rate is controlled byH(a), it may cause the adsorption properties of N a.
Recently we have successfully observed both N2(a)and H(a) [4-6].
In this paper, we would like to discuss how
molecular sensing techniques used for characterization (especially the FTIR spectra of adsorbed
molecules) are important in understanding the natureof the Ru catalyst for ammonia synthesis.
A comprehensive understanding will lead to thedesign of a better catalyst for ammonia synthesis.
2wt%Ru/A1203, 2wt%Ru-Cs+/A1203 (Cs/Ru = 8mol/mol), 2wt%Ru/MgO,and 2 w t % R u C s + / M g O (Cs/Ru = 0.1 and 1 mol/mol)were used
as model catalysts in this work.

2. Experimental
y-Al203 (184 m2/g) and MgO (90 m2/g) werecalcinated in air at 773 K for 5 h and then impregnated w i t h Ru3(CO)~2 in the stirred tetrahydrofuran
solution at room temperature for 4 h. The solvent
was evaporated at room temperature. Then, the samples were heated at 673 K under vacuum (ca. 10 6Torr; 1 Tort = 133 Pa) for 2 h.
Cesium promoted samples were obtained by impregnating the prepared supported Ru catalysts with
the CsNO 3 aqueous solution at ambient temperaturefor 4 h and drying at 373 K in air.
The dried samples were pressed to infrared disks(20 mm diameter, 20-50 mg), and then mounted in
the center of the infrared cell with NaCI windows.
The samples were heated under vacuum at the desired pretreatment temperatures and then treated withcirculated He (200 Torr) with a liquid nitrogen trapfor 12 h. The cesium nitrate in the samples wasdecomposed under H 2 atmosphere. The samples were489

then evacuated at the same temperature for 1-2 h toremove the hydrogen. After pretreatment, the sample
temperature was quickly controlled before measurement.
The spectra were taken by an FTIR spectrometer(Japan Spectroscopic, Model FT/IR5300) which wasequipped with a triglycine sulphate (TGS) detector.
The spectrometer was placed on a movable table toposition the infrared beam to the center of the sample.
Commercial grade N 2, H a, and He were purifiedthrough a reduced active copper catalyst (473 K)
connected to a liquid nitrogen trap. One gram of thecatalyst was used to measure the rate of ammonia
synthesis in the flow system at 588 K under atmospheric pressure of N 2 + 3 H 2. The catalysts werereduced with hydrogen at 623 K before use forammonia synthesis.

3. Results and discussion

3.1. Rate of ammonia synthesis and wm'enumber ofadsorbed dinitrogen on 2 wt% Ru catalysts
The rate of ammonia synthesis on the catalysts
was measured at 588 K under N 2 + 3H 2 = 101 kPa
and shown in Fig. 1. The activities of the Ru catalysts range in the order of the basicity of theirsupports or promoters with supports as follows:
AI20 3 < MgO < Cs+/A1203(Cs/Ru = 8mol/mol) < C s + / M g O (Cs/Ru = 0.1 mol/mol) <C s + / M g O (Cs/Ru = 1.0 mol/mol).
On the other hand, FTIR measurements of Nz(a)were carried out and it was found that when 50 Torrof N 2 was introduced onto the five Ru catalysts,
sharp peaks attributed to the adsorbed dinitrogen
could be observed. The wavenumbers are shown inFig. 1. The peaks for R u / M g O and R u - C s + / M g O(Cs/Ru = 0.1 mol/mol) had shoulders at higher
frequencies, which are also shown in the figure (twopoints are shown for each sample). The wavenumbets of the N - N stretching frequencies are all lower
than that of the free molecule (2331 cm-~). Thismeans that the N 2 molecule is activated ( N - N
stretching is loosened) on the Ru metal surface. The
extent of the looseness (wavenumber shift from thatof the free molecule) ranges in the order of basicity

K. Aika et a l . / Applied SurJ~tce Science 121 / 122 (1997) 488-491

490

Observed frequencies (cm ~) ofdinitrogen
adsorbed on 2 wt% Ru catalysts
2200

2100

2000

II

fI/

¢n1900~!*"~"

Table 1Kinetic expression of NH 3 synthesis under 101 kPa total pressure
over 0.5 g of 3 wt% Ru catalyst at 588 KCatalyst• ~ / ~ Ru)~/CsOH

1000--

Ru/MgO
Ru-Cs+/MgO"6Cs/Ru

Reaction order in(tool/tool)

N,H~NH01.00.90
0.97

-0.33
-0.78

0.34
-0.28

400--

3.2. tlydrogen inhibition against ammonia synthesison the Ru su~aceZ100/.........

Eo=~ R ~sOHL]

AI~D,40--//@÷
I,.oIz,.¢ZsZ_ ~I AIzOa
Fig. I. Rate of ammonia synthesis on 5 Ru catalysts underN 2 + 3 H , = 101 kPa and at 588 K and the observed frequency ofadsorbed dinitrogen on the same catalysts at 298 K.

(or the electron donation property) of the support (orthe promoter-support combination).
The rate-determining step of ammonia synthesis is
the dissociation of dinitrogen on the ruthenium surface (the same as on the iron surface). If the firstinteraction between N 2 and Ru atom is the end-on
type adsorbed state, Ru metals with a high electron
concentration can donate more electrons back to theanti-bonding state of dinitrogen to weaken the N - Nbond. Such a Ru surface thus must be more active
for ammonia synthesis. Adsorbed dinitrogen should
not be observed by FTIR at a reaction temperature of588 K because of the low concentration, it must be
the short lived intermediate. Thus, the FTIR spectraof adsorbed dinitrogen were shown to be successful
to tell the degree of electron-donation from the support a n d / o r the promoter and can be a measure of
the catalyst activity. Here the effects of hydrogenand ammonia were covered because no kinetic measurement was done, which will be shown in the next
section.

The reaction order with respect to ammonia wasobtained by changing the total flow rate. The N 2 (orH~) order was obtained by keeping the H 2 (or N 2)
pressure constant and by compensating NH 3 ordercontributions. The results are shown in Table 1.
Contrary to the iron catalyst, the hydrogen orders arenegative (see Eqs. (4) and (5)). The typical orders foriron catalysts are 1.0 for nitrogen, 1.5 for hydrogen,and - 0 . 7 5 for ammonia ( y = 0.375 and z = 0). Cs +promoted R u / M g O is more active, but has highnegative value of hydrogen order. If we analyze thisresult in the manner of Eqs. (4) and (5), the nitrogen
retardation indexes y are 0.17 and 0.14 and thehydrogen retardation indexes z are 0.84 and 1.20 for
R u / M g O and R u - C s + / M g O , respectively. Thus,
the nitrogen retardation order is iron > R u / M g O >R u - C s + / M g O and the hydrogen retardation order isiron << R u / M g O < R u - C s + M g O .
Industrially, ammonia must be produced underhigh pressure because of the equilibrium constraint.
The negative order in hydrogen means that the reaction rate is not high enough at high pressure. Hydrogen seems to be more strongly adsorbed on R u C s - / M g O than on R u / M g O . The rate-determining
N 2 activation process is inhibited by the strong
adsorption of hydrogen.
67 kPa of H 2 was introduced at 293 K on to theTable 2
VI'IR wavenumber of adsorbed hydrogen on Ru catalysts at 293 K
Catalyst

2wt%Rh/MgO
2wt%Ru-Cs+/MgO

Cs+/RuWavenumber (cm(tool/tool)

on-topthree-holdi)

02.0

1717s
1781s

1120m, 933m
1233m, 940m

K. Aika et al. /Applied Surface Science 121 / 122 (1997) 488-491
4035305~, 25~- 20I5-\105, ",---,--,,,--7--

1'0 20 30 40 50 60 7'0 80 90 I00
0HI%
Fig. 2. Amount of chemisorbed dinitrogen as a function ofpreadsorbed hydrogen atoms on R u / M g O (solid line) and on
R u - C s + / M g O (dotted line). Both are identified by ~q?IR andestimated quantitatively by GC. N 2 was introduced at 183 K andevacuated at the same temperature for 5 min.

Ru catalysts. The observed wavenumbers are shownand identified as hydrogen atom as in Table 2. Since
the wavenumber of R u - H on R u - C s + / M g O ishigher than that on R u / M g O , hydrogen must be
strongly adsorbed on the former. This result is inaccordance with the above kinetic results.
Adsorbed N 2 observable by FTIR was found to be
decreased by introducing hydrogen at 183 K. The
estimated chemisorbed amount of N 2 is shown as afunction of the amount of adsorbed hydrogen in Fig.
2.
It is seen that Cs + promoted R u / M g O inhibits
N 2 adsorption more strongly. These FTIR measurement was not done under the reaction temperature,
but the results reflect the performance under thereaction.

3.3. Possibilities and problems of the molecularsensing characterization for new ammonia catalyst
designModem surface science techniques such as EELS,XPS, TPD, LEED devoted much on the understanding the catalyst surface especially through the single

491

crystal work. Of course these are important, butsometimes it differs much from the real catalyst
which is supported and promoted.
The real catalyst can be easily studied by FTIR,EXAFS, and XPS. FTIR seems to be more powerfulthan the others. The electronic state of Ru surface
was once studied by XPS, however, the extents ofBE shift were not remarkable [7]. N - N frequencies
by FTIR instead showed a remarkable difference inseveral catalysts here. The N2(a) and H(a) interaction
was also observable by FTIR because it can beobserved among the gaseous reactants. XPS maygive such information but only for the strongly adsorbed species. EXAFS is only effective for a model
catalyst with a small size metal cluster.
The relation between a reaction and the active
site-nature is called 'the mechanism'. A study torelate the active site-nature and chemical nature of
the catalyst may be called 'the surface characterization'. A detailed study of 'the mechanism' only
judges the effectiveness of 'the surface characterization'. The ammonia catalyst is now being improved.
A meaningful surface characterization should help
the design of new catalyst. One of the conclusions tohelp the design is that alkali promotes N: activation
but suffers from strong hydrogen inhibition.

References[1] A. Ozaki, K. Aika, in: J.R. Anderson, M. Boudart (Eds.),
Catalysis - - Science and Technology, vol. 3, Springer-Verlag,
Berlin, 1981, p. 139.
[2] K. Aika, T. Takano, S. Murata, J. Catal. 136 (1992) 126.
[3] K. Aika, M. Kumasaka, T. Oma, O. Kato, H. Matsuda, N.
Watanabe, K. Yamazaki, A. Ozaki, T. Onishi, Appl. Catal. 28(1986) 57.
[4] J. Kubota, K. Aika, J. Phys. Chem. 98 (1994) 11293.
[5] J. Kubota, K. Aika, J. Chem. Soc. Chem. Commun. (1992)661.
[6] Y. Izumi, M. Hoshikawa, K. Aika, Bull. Chem. Soc. Jpn. 67(1994) 3191.
[7] K. Aika, A. Ohya, A. Ozaki, Y. Inoue, I. Yasumori, J. Catal.
92 (1985) 305.