Selective catalytic reduction of nitric oxide by ammonia over egg-shell MnOx/NaY composite catalysts

Article abstract of DOI:10.1006/jcat.2001.3468

A novel composite catalyst system for the SCR of NO_x by NH₃ was described operating at < 470 K in the presence of water with NO conversions of 80-100% at space velocities of 30,000-50,000/hr. The applied precipitation technique for preparation of a composite material with intended egg-shell arrangement of amorphous MnO_x around zeolite NaY microcrystals led to catalysts with high performance in the SCR of NO_x by NH₃. The catalyst tolerated water contents of up to 30 vol % in the feed. Full conversion of NO_x was achievable at 325-450 K up to GHSV values of ～ 50,000 cc/g_cat-hr in the presence of 5-10 vol % water. The most relevant feature of the composite MnO_x/NaY catalyst was its high activity at temperatures where commercial V₂O₅/WO₃/TiO₂ catalysts were only marginally active. These properties would fulfill the requirements of a low-temperature NO_x reduction catalyst for mobile diesel engines if an ammonia supply was implemented "on board", e.g., by urea decomposition.

Full text of DOI:10.1006/jcat.2001.3468

Journal of Catalysis 206, 98–113 (2002)
doi:10.1006/jcat.2001.3468, available online at http://www.idealibrary.com on
Selective Catalytic Reduction of Nitric Oxide by Ammonia
over Egg-Shell MnOx/NaY Composite Catalysts
∗
∗
∗
M. Richter,∗,1 A. Trunschke, U. Bentrup, K.-W. Brzezinka,† E. Schreier,
∗
∗
∗
M. Schneider, M.-M. Pohl, and R. Fricke
∗
Institute for Applied Chemistry Berlin-Adlershof, Richard-Willsta¨tter-Str. 12, D-12489 Berlin, Germany; and †Federal Institute
for Materials Research and Testing (BAM), Richard-Willsta¨tter-Str. 11, D-12489 Berlin, Germany
Received July 31, 2001; revised November 5, 2001; accepted November 5, 2001; published online January 14, 2002
commonly used vanadia-based catalysts operate at tem-
peratures between 575 and 725 K with high activity and
selectivity. Over the last decade, the potential for the ap-
plication of ammonia SCR to exhaust gas purification of
diesel-driven vehicles has been investigated (4, 5). For mo-
bile diesel engines the low NOx emission limits of further
legislative regulations can only be obeyed if the oxidation
catalysts presently in use are supplemented by a NOx reduc-
tionfunction. Onestrategyistoincludetherobustammonia
SCR technology into the exhaust gas aftertreatment of mo-
bile diesel engines. Because the handling of toxic ammonia
is problematic, the production of ammonia on board is pre-
ferred. This can be accomplished by thermal decomposition
of urea. From several companies solutions are offered for
heavy-duty engines (5, 6). However, for application of the
urea SCR technology in diesel passenger cars, the volume of
the catalyst system has to be minimized and the main SCR
catalyst has to be active at exhaust gas temperatures of 425–
525 K. This requires alternative catalysts with high intrinsic
activity. TheV2O5/TiO2 catalystcommonlyusedforstation-
ary SCR systems has insufficient performance at low tem-
peratures. Much work has been focused on zeolite-based
materials. While the application for lean-burn otto engines
failed due to the limited hydrothermal stability of zeolites
at the high exhaust gas temperatures of 700–1000 K nor-
mally encountered, the lower diesel exhaust gas tempera-
tures do not exclude the application of zeolite-based cata-
lysts. It is well-known that the SCR of NOx by ammonia
over nonmodified protonic zeolites or their ammonium
forms is possible up to temperatures of 600 K (8–16). Inter-
action of ammonia with protonic zeolites leads to formation
of ammonium ions fixed to the zeolite framework. It could
be shown (17) that the reaction of ammonium ions with
NOx is stoichiometrically possible according to the equa-
tion
A novel composite catalyst system for the selective catalytic re-
duction (SCR) of NOx by NH3 is described operating at temper-
atures lower than 470 K in the presence of water with NO con-
versions of 80–100% at space velocities of 30,000–50,000 h−1. The
catalyst is prepared by egg-shell precipitation of MnO2 on the exter-
nal surface of zeolite NaY. Structural and thermal stability of pre-
cipitated MnO2 as well as of the MnO2/NaY composite catalyst were
characterized by N2 adsorption, X-ray diffraction, laser Raman
spectroscopy, temperature-programmed reduction, and electron
microscopy. MnO2 precipitated on zeolite NaY (15 wt% loading) re-
tained its amorphous state up to calcination temperatures of 775 K.
The zeolite component remained structurally intact. Calcination
at higher temperatures destroyed the zeolite structure and trans-
formed MnO2 into Mn3O4. DRIFT spectroscopic investigations re-
== – – ==
vealed the presence of symmetric O N O N O species formally
corresponding to N2O3 on the composite catalyst after contact with
NO. Catalytic measurements under integral flow conditions showed
that the catalyst performance is associated with a close coupling of
nitrite formation and its drain off from equilibria with NO/NO2
and nitrate by ammonia. Several results are in line with the “diazo-
tation” mechanism, including NH3 protonation to NH+4 , whereas
prevailing Lewis acid sites should enable NH3 activation via amide
species, thus leading to a parallel “amide/nitrosamide” SCR reac-
tion route. The activity-temperature profile fulfills the requirements
of a low-temperature NOx reduction catalyst for mobile diesel en-
gines if an ammonia supply is implemented “on board,” e.g., by
c
ꢀ 2002 Elsevier Science (USA)
urea decomposition.
Key Words: SCR of NOx by NH3; egg-shell composite MnOx/NaY
catalyst; low-temperature activity; laser Raman spectroscopy;
DRIFT spectroscopy; temperature-programmed reduction.
INTRODUCTION
The selective catalytic reduction (SCR) of NO/NO2
(NOx ) by NH3 as reductant is the state-of-the-art technol-
ogyfortheabatementofnitrogenoxidesfromexhaustgases
of stationary plants and combustion engines (1–3). The
4 Z−NH+4 + 4 NO + O2 → 4 N2 + 6 H2O + 4 Z−H+, [1]
1 To whom correspondence should be addressed. Fax: +49 30 6392 4350.
E-mail: mr@aca-berlin.de.
where Z− denotes the anionic zeolite framework. If no NH3
98
0021-9517/02 $35.00
c
ꢀ 2002 Elsevier Science (USA)
All rights reserved.

NOx SCR BY NH3 OVER MnOx /NaY COMPOSITE CATALYSTS
EXPERIMENTAL
99
is fed, the zeolite is converted into the protonic form and
NOx is reduced selectively to N2. This transient reaction
proceeds in dry gas atmosphere already at temperatures
slightly above ambient. But the reaction rates are slow
and complete NOx conversion requires high residence
times, i.e., low space velocities which are not appropriate
for mobile diesel engines. Moreover, most studies of the
NOx SCR process by ammonia over zeolites have been
performed with dry model exhaust streams. A reasonable
performance was reported (9) for the conversion of ca.
480 ppm NO and 4 vol% O2 with NH3 preadsorbed on ze-
olite mordenite (nSi/nAl = 5). NOx conversions amounted
to 35% at 400 K and to 100% at 475 K at a space velocity
of ca. 30,000 h−1 (9).
Nonmodified zeolites, however, are poor catalysts for
the NOx reduction in the presence of moisture, which is
inevitably present in exhaust gas streams of combustion
engines. Necessarily, the zeolites have to be promoted by
components which accelerate one or more reaction steps,
preferentially by the initial oxidation of NO to NO2. Mod-
ification of the zeolite by ion exchange processes removes
the protons and hence the Brønsted acidity of the zeolite
matrix. This annihilates the beneficial influence of the
Brønsted acid sites for storage and activation of NH3 and
decreases the low-temperature activity.
Sample Preparation
The preparation of the composite catalyst is described
in Ref. (18). The key step is the precipitation of MnO2
according to Eq. [2] in aqueous solution in the presence
of suspended zeolite powder.
3 Mn(CH3COO)2 + 2 KMnO4 + 2 H2O
→ 5 MnO2↓ + 4 CH3COOH + 2 CH3COOK [2]
Commercial zeolite NaY (nSi/nAl atomic ratio = 2.3)
(Aldrich) was suspended in distilled water at room temper-
ature and MnO2 precipitated in the presence of the zeolite
by simultaneous admixture of Mn(II) acetate solution and
KMnO4 in stoichiometric concentrations to yield a MnO2
loading of 15 wt%. From geometrical considerations, the
amount of manganese oxide was chosen sufficiently high to
guarantee a theoretical MnO2 shell thickness around the
zeolite microcrystals of about 1 µm. The suspension was
heated to 353 K and kept at this temperature for an addi-
tional 30 min. As follows from Eq. [2] the residual solution
contains diluted acetic acid and, hence, the pH value was
found to be near 4. Nevertheless, it has been ascertained
27
by Al magic-angle spinning (MAS) NMR (18) that no
The approach we proposed (18, 19) was to combine the
properties of the zeolitic component with the redox prop-
erties of a suitable manganese oxide without pore plug-
ging of the zeolite in order to keep the interior available
for deNOx reaction steps. This has been achieved by pre-
cipitating an amorphous, mesoporous/macroporous MnOx
phase (1.5 < x ≤ 2.0) on the external surface of zeolite
microcrystals (18). It has been discussed in Ref. (17) that
the ammonium form of the zeolite enables a cyclic opera-
tion with zeolite-fixed ammonium ions as reductant for
gaseous NOx . For high NOx conversion, the close neigh-
borhood of manganese oxide and zeolitic ammonium ions
is indispensable (19). However, a deterioration of the cata-
lyst performance was observed at longer time on stream
in wet feed. A slight dealumination of the zeolite could be
identified to be one reason for the capacity loss in NH3
storage (18).
To improve the catalyst stability the sodium form of
zeolite Y was chosen as catalyst component because of its
higher hydrothermal stability. This results in a loss of the
ability to fix gaseous ammonia in the form of ammonium
ions and requires a permanent supply of ammonia re-
ductant. In view of the application in exhaust gas
cleaning of diesel internal combustion engines, thermal
decomposition of urea is one alternative to providing am-
monia on demand.
acid leaching of tetrahedrally coordinated aluminum out
of the zeolite lattice has occurred. After filtration and dry-
ing the solid was calcined at 773 K for 2 h in air (standard
procedure). The calcination temperatures are indicated as
subscripts (in K) of the sample designation chosen in accor-
dance with that given in Ref. (18): 15 MnNaY775 denotes
the composite catalyst containing 15 wt% MnO2 deposited
on zeolite NaY, calcined at 775 K. Precipitated manganese
(IV) oxide samples are abbreviated as Mn, with postcalci-
nation temperatures given as subscripts. Textural data are
summarized in Table 1. Reference crystalline manganese
oxides were purchased from AlfaꢀR , Germany.
Sample Characterization
Surface areas and pore volumes were determined
by nitrogen adsorption on the ASAP 2010M facility
(Micromeritics). Calculation of pore sizes followed the
TABLE 1
Textural Data of Samples
Surface area Micropore volume Mesopore volume
Sample
Mn425
Mn775
15 MnNaY775
NaY775
(m2 g−1
)
(cm3 g−1
)
(cm3 g−1
)
170
27
664
928
0.05
0.02
0.28
0.44
0.14a
0.17b
0.11
—
The present work aims at a more detailed understand-
ing of the relevant factors determining the performance of
the composite MnOx /NaY catalyst in relation to its specific
structure.
a Average pore diameter, 3.3 nm.
b Cumulative volume of pores between 1 and 300 nm diameter.

100
RICHTER ET AL.
method of Barrett-Joyner-Halender (20) according to im- Catalytic Measurements
plemented software routines.
Catalytic measurements were performed in a flow system
X-ray diffraction (XRD) powder diffraction measure-
ments were carried out with a STOE powder diffraction sys-
tem using Cu Kα radiation. Reference spectra were taken
from the ASTM data base.
equipped with an integral flow reactor (internal diameter
10 mm) and online sampling system. Standard feed compo-
sitions comprised 1000 ppm NO, 1000 ppm NH3, 10 vol%
O2, and 7 vol% H2O at a space velocity of 48,000 cm3
Laser Raman spectroscopic measurements were per-
formed on a DILOR XY-Raman spectrometer equipped
with a nitrogen-cooled charge-coupled devise multichan-
nel detector and a BH2-Olympus microscope (microprobe
technique, objective 50×) providing a laser beam focus of
ca. 2 µm. The power transferred to the sample surface was
0.11–1.06 mW µm−2. The spectra were taken within the
range 100–1375 cm−1 and accumulation times of 100–600 s
using the 514.5-nm Ar+ line for excitation. The accu-
racy of band positions was checked using monocrystalline
Si(100) that provides the Raman active ꢀ25 mode of Si at
520 cm−1 (21).
DiffusereflectanceFouriertransform(DRIFT)measure-
ments were accomplished with a diffuse reflectance attach-
ment coupled to a modified reaction chamber (Harrick).
The spectra were collected on a spectrometer FTS-60A
(Bio-Rad) at a resolution of 2 cm−1. Some 256 scans have
been accumulated. Before measurements, all samples were
treated in situ with He at 425 K for 1 h and subsequently
cooled to ambient temperature. Flow rates amounted to
20 cm−3 min−1 with the exception of catalyst 15 MnNaY,
where 10 cm−3 min−1 was used. The concentration of NO
was 5 vol%. For recording NO adsorption, the spectra of
the samples pretreated at the same temperature were used
for reference.
−1
g
h
−1. The apparent density of the composite catalysts
cat
amounted to ca. 0.5 g cm−3, so that the true gaseous hourly
space velocity (GHSV) amounts to 24,000 h−1
.
Analysis of the product flow was conducted using gas
chromatography (HP 6890 series) on two parallel capil-
lary columns linked to thermal conductivity detectors. A
HP-PLOT MoleSieve column served for separation of N2,
O2, and CO, and a Poraplot Q column for separation of
NO, N2O, H2O, NH3, and CO2. Simultaneously, the exit
stream composition was continuously analyzed by a mul-
tigas sensor (Multor 610, Maihak GmbH, Hamburg,
Germany) equipped with nondispersive infrared channels
for NO, CO, and SO2 registration and a channel for elec-
trochemical O2 detection as well as with a NO2/NO con-
verter reducing NO2 to NO over a molybdenum catalyst.
The difference between overall NOx and NO concentration
corresponds to the NO2 percentage.
Activity is expressed as the conversion of NOx to N2
at fixed experimental conditions. The N2 selectivity of the
reaction is defined as S (%) = 100 [N2])/([N2]) + [N2O]).
Formal reaction orders were determined from the linear
fit of lg r vs lg c diagrams, where reaction rates r are de-
rived from the variation of NO conversion with W/FNO
(W, catalyst weight; FNO, NO feed rate) by numerical
differentiation.
The Fourier transform infrared (FTIR) characterization
of acidity by pyridine adsorption was carried out on a
Bruker IFS 66 spectrometer (2 cm−1 resolution, 100 scans).
The probe molecule pyridine (20 mbar) was adsorbed at
room temperature until saturation on the sample powder
pressed to self-supporting disks which were pretreated at
673 K under vacuum for 30 min. Results are presented as
difference spectra obtained by subtraction of the spectrum
of the pretreated sample at room temperature. Integrated
absorbances were related to unit surface areas of the
samples.
Temperature-programmed reduction (TPR) was achie-
ved applying the characterization system AMI-1 (Altamira
Instruments) equipped with a thermal conductivity detec-
tor. For reduction, a feed of 5 vol% H2 in Ar was used at a
heating rate of 5 K min−1. The overall flow rate amounted
to 25 cm3 min−1. The consumption of H2 was determined
from the integrated peak areas by calibration.
RESULTS
Structural Characterization of MnOx
and Composite Catalyst
XRD analysis. XRD diagrams of Mn425, Mn775, Mn875
,
and Mn975 are shown in Figs. 1a–1d. Sample Mn425 is com-
pletely amorphous whereas Mn775 reveals diffraction lines
attributable to α-MnO2 (Fig. 1b). The intensity of the ob-
served diffraction lines is low, and the percentage of crys-
tallinity for the calcined sample is estimated to be ca. 15%.
Samples Mn875 and Mn975 show additional diffraction lines
attributable to γ -Mn2O3 (bixbyite-C). The intensity of the
Mn2O3 lines increases with higher calcination temperature
(cf. Figs. 1c and 1d).
Several modifications of MnO2 and Mn2O3 phases are
known (22) but all have in common the fact that the ther-
mal conversion of MnO2 phases proceeds via intermediate
Mn2O3 to Mn3O4.
Electron microscopic images of samples were taken
by scanning (SEM) as well as transmission-electron mi-
croscopy (TEM), applying an Amray 2000B microscope
and a Philips CM20 at 200 kV (LaB6), respectively. For
TEM images, the samples were prepared by depositing the
materials without any pretreatment on Lacey carbon films.
The XRD diagram of the composite 15 MnNaY875 ma-
terial shows exclusively reflections of the zeolite (Fig. 2A).
A contribution from crystalline manganese oxide phases
could not be resolved. Calcination of the composite catalyst

NOx SCR BY NH3 OVER MnOx /NaY COMPOSITE CATALYSTS
101
FIG. 1. XRD diagrams of precipitated samples Mn425 (a), Mn775 (b), Mn875 (c), and Mn975 (d). Reflections of α-MnO2 and of γ -Mn2O3 (bixbyite-C)
are marked by o and x, respectively.
at 975 K, however, leads to destruction of the zeolite crys- by in situ decomposition of MnO2 due to the high laser
tallinity to a large extent and to conversion of the amor- power applied (50 mW).
phous MnOx phase into Mn3O4 (hausmannite), at least
Bands at 507 and 579 cm−1 prove the existence of MnO2.
partially (Fig. 2B). This is different from the behavior of The band at 632 cm−1 is observed for all samples. A phase
precipitated MnO2 (cf. Fig. 1), where a mixture of Mn2O3 transformation in situ due to local heating is rather unlikely,
and MnO2 is formed after calcination at 975 K. The fact that because a laser power of only 1 mW has been used for
no Mn3O4 is observed for sample Mn975 is explained by a spectrum acquisition. Hence, it has to be assumed that a
higher thermal stability of the γ -Mn2O3 phase formed, i.e., distorted Mn2O3 phase not detectable by XRD is already
a higher crystallinity, whereas the γ -Mn2O3 phase of the present in sample Mn425. Bands within the 182–187 cm−1
composite catalyst is obviously in a more distorted state range for samples Mn775 and Mn975 point to the existence
and more easily convertible to Mn3O4.
of γ -Mn2O3. A Raman line at 194–199 cm−1 is indicative
for α-Mn2O3 (cf. Fig. 3B,b).
Laser Raman spectroscopy. Laser Raman spectra of
samples Mn425, Mn775, and Mn975 are shown in Fig. 3A. Ref-
erence spectra of crystalline manganese oxides are given in
Fig. 3B. The spectra were taken with low laser power (1 mW
at maximum) to minimize thermal transformations of man-
ganese oxides during spectrum accumulation.
The laser Raman spectra of the precipitated MnOx
samples reveal bands at 186, 271, 378, 474, 507, 579, and
632 cm−1, with different intensities according to the
calcination temperature. A comparison with reference
spectra of crystalline manganese oxides (Fig. 3B) shows
that the structure of unsupported precipitated MnOx
does not correspond to either crystalline β-MnO2 (py-
rolusite) (Fig. 3B,a), α-Mn2O3 (Fig. 3B,b) or Mn3O4
(Fig. 3B,c).
The spectra of Mn775 and Mn975 contain contributions
from MnO2 and Mn2O3. Considering XRD results, phase
structures correspond to α-MnO2 and γ -Mn2O3, respec-
tively. No additional contributions from Mn3O4 were
detected.
Laser Raman spectra of zeolite NaY and the composite
material15MnNaY775, thelattertakenfromsampleregions
differing in color, are shown in Fig. 4. Zeolite NaY has an
intense white color under the microscope whereas sample
15 MnNaY775 contains crystallites with grey–white to dark-
brown color. The optical appearance of the sample under
the microscope proves the deposition of manganese oxide
on the zeolite microcrystals. But inhomogeneities of the
coverage are evident. This is reflected by the modified laser
Raman spectra according to the region they were taken
from. The nonmodified NaY has its most intense Raman
line at 501 cm−1. This line is attributed to the symmetrical
deformation (A1 symmetry) of the O atoms along the half
angle of the T -O-T bridges (T = Si, Al) of the zeolite
Amorphous MnO2 is reported (23) to exhibit bands at
ca. 510 and 580 cm−1. A band observed at 633 cm−1 was as-
signed to γ -Mn2O3 with a distorted hausmannite structure
by Bernard et al. (24), formed during spectrum acquisition

102
RICHTER ET AL.
FIG. 2. XRD diagrams of samples 15 MnNaY875 (A) and 15 MnNaY975 (B). Residual zeolite reflections in (B) are marked by asterisks.
framework. Further Raman lines with lower intensity were Instead, a band at 639 cm−1 dominates the spectrum, with
found at 305, 359, 821, 1053, and 1113 cm−1. The Raman a shoulder at 577 cm−1. This corresponds to a mixture of
spectra of the composite include contributions of the zeo- MnO2 and γ -Mn2O3 phases with varying proportions ac-
lite and the supported manganese oxide according to the cording to the local Mn loading on the zeolite microcrystals.
local loading. The spectrum shown in Fig. 4b, taken from
grey–white surface regions is dominated by the typical zeo-
Temperature-programmed reduction. Reduction pro-
lite lines but additionally reveals two broad lines at 574 and files for samples Mn425, Mn575, Mn775, and 15 MnNaY775
639 cm−1. The manganese oxide layer is sufficiently thin to are shown in Fig. 5, together with those of reference sam-
allow the acquisition of the zeolite spectrum in the grey– ples. Reduction of samples Mn425 and Mn575 by H2 starts at
white and light-brown colored sample regions, which is not 375 K and proceeds almost identically in several superim-
the case in the dark-brown colored regions. Raman lines of posedprocesses. Formally, theprofilesofsamplesMn425 and
the zeolite are hardly recognizable in the spectrum shown Mn575 can be deconvoluted into four single processes (not
in Fig. 4d, taken from dark regions of the sample surface. shown). A clear modification of the TPR profile occurred

NOx SCR BY NH3 OVER MnOx /NaY COMPOSITE CATALYSTS
103
FIG. 4. Laser Raman spectra of zeolite NaY (a) and of sample
15 MnNaY775 for different surface regions from (b) grey–white to (c) light-
brown and (d) dark-brown appearance under the microscope.
occurred during calcination up to 775 K. However, the over-
all H2 consumption during TPR does not correspond to the
hypothetical value of 0.5 mol of O2 per mol of MnO2. These
low reduction degrees confirm the nonstoichiometric com-
position of the precipitated oxide. Stoichiometric reduc-
tion to MnO is fulfilled for crystalline commercial β-MnO2,
α-Mn2O3, and Mn3O4 with sufficient accuracy.
The TPR profile of sample 15 MnNaY775 (Fig. 5d) agrees
well with earlier measurements (18). It reveals a reduci-
bility at relatively low temperatures comparable to the
FIG. 3. Laser Raman spectroscopic characterization of (A) samples
Mn425 (a), Mn775 (b), and Mn975 (c) and (B) reference crystalline man-
ganese oxides β-MnO2 (a), α-Mn2O3 (b), and Mn3O4 (c). Laser power,
1 mW; 600-s accumulation time; Ar+ line excitation at 514.5 nm.
for sample Mn775. The onset of reduction is shifted to higher
temperatures (around 525 K). The reduction of sample
Mn775 by H2 proceeds in one sharp peak centered at 580 K.
The profiles of all three samples are different from those
observed for crystalline β-MnO2, α-Mn2O3, and Mn3O4
reference oxides (Figs. 5e–5g).
Reduction of precipitated MnOx samples starts at lower
temperatures compared with the crystalline oxide β-MnO2.
The reduction profile of crystalline β-MnO2 revealed two
overlapping peaks centered at 620 and ca. 670 K, which is in
goodagreementwithresultsreportedbyKapteijnetal. (25).
The two-peak pattern is interpreted as stepwise reduction
of MnO2 to MnO via intermediate Mn2O3 or Mn3O4.
Formal reduction degrees calculated as a ratio between
experimentally determined H2 consumption and the H2
consumption that is theoretically possible assuming MnO
as the end product are summarized in Table 2. Nearly
FIG. 5. H2 consumption TPR profiles of samples Mn425 (a), Mn625 (b),
Mn775 (c), and 15 MnNaY775 (d) as well as of crystalline β-MnO2 (e),
identical overall H2 consumptions were found for Mn425
,
Mn575, and Mn775. Obviously, no thermal loss of oxygen α-Mn2O3 (f), and Mn3O4 (g). Profiles are vertically shifted for clarity.

104
RICHTER ET AL.
TABLE 2
Reduction Degrees of Manganese Oxides and Sample
15 MnNaY775 According to TPR
H2 consumption
(mmol g−1), calc.a
H2 consumption
Reduction
degree (%)
Samples
(mmol g−1), exp.b
Mn425
Mn575
Mn775
15 MnNaY775
β-MnO2
α-Mn2O3
Mn3O4
11.50
11.50
11.50
1.75
11.50
6.33
9.97
10.03
9.74
1.63
11.48
6.42
86.7
87.2
84.7
93.1
99.8
101.4
101.4
4.37
4.43
a Calculated for reduction to MnO for all samples.
b According to peak areas of the TPR profiles.
FIG. 6. FTIR spectroscopic acidity characterization of zeolite NaY
(dotted line) and of sample 15 MnNaY775 (solid line) by pyridine as probe
molecule at room temperature (samples evacuated at 673 K for 30 min).
precipitated oxides Mn425 and Mn575. This first reduction
process between 425 and 600 K indicates a superimposi-
tion of at least two processes, resembling the behavior
of the precipitated amorphous oxide. A second reduction
peak not found for the unsupported oxides emerges be-
tween 600 and 700 K. The location on the temperature axis
coincides with the second H2 consumption peak observed
for reduction of β-MnO2 (Fig. 5e). But as long as the con-
secutive reduction process of the MnOx phase mixture is
not clarified, assignments of single reduction peaks to phase
transformations are not justified.
The overall H2 consumption of sample 15 MnNaY775 is
higher than for the unsupported oxide (cf. Table 2) but, nev-
ertheless, does not correspond to a stoichiometric reduc-
tion of MnO2 to MnO. This confirms the nonstoichiometric
characterofthemanganeseoxide. ButtheMnOx phasesup-
ported on the zeolite support has higher thermal stability.
In comparison, calcination of sample Mn775 initiated ther-
mal decomposition and crystallization of γ -Mn2O3, thus
leading to lower H2 consumption during TPR.
acid sites per unit of surface area is higher for the compos-
itecatalystthanforzeoliteNaY. ThehigherdensityofLewis
acid sites found for the composite material implies that the
MnOx phase possesses its own Lewis acidity. The maximum
at 475 K observed for NaY cannot be interpreted so far.
Electron microscopy. Electron microscopic images
(SEM) taken from sample Mn775 and from the composite
catalyst 15 MnNaY775 showed that the precipitated MnOx
forms agglomerates consisting of small primary particles of
1 µm size or less whereas the micrograph of the catalyst is
dominated by zeolite crystals of cubic shape and a size of
2–3 µm. The loading of the zeolite crystals by MnOx is not
resolved. A TEM image of the composite catalyst showed
that the MnOx particles are arranged around the zeolite
crystals but do not form a coherent shell. Rather, patches of
Acidity characterization with pyridine as probe molecule.
Gaseous pyridine interacts with both Brønsted and Lewis
acid sites of solid materials. A differentiation is readily pos-
sible by IR spectroscopy. Pyridine adsorbed at Brønsted
sites of a solid material forms PyH+ ions with characteristic
vibration bands at about 1540 and 1640 cm−1, but if coordi-
natively bound to Lewis sites, bands at 1440–1460, 1488, and
1580–1630 cm−1 appear. FTIR spectra of zeolite NaY and
the composite catalyst 15 MnNaY775 after pyridine adsorp-
tion at room temperature and outgassing at 673 K under
vacuum for 30 min are displayed in Fig. 6. Exclusively, the
spectra reveal vibration bands of pyridine coordinated to
Lewis acid sites. The dependence of the integral intensi-
ties of the Lewis band at 1442 cm−1 normalized to the sur-
face area is shown in Fig. 7. Thermal desorption of pyridine
occursattemperatureshigherthan475K. Beneaththistem-
perature, pyridine is stably coordinated to Lewis acid sites.
FIG. 7. Integrated IR absorbances of pyridine fixed to Lewis acid sites
Within the low-temperature region, the density of Lewis of (ꢀ) 15 MnNaY775 and of (ꢁ) NaY vs sample temperature.

NOx SCR BY NH3 OVER MnOx /NaY COMPOSITE CATALYSTS
105
deposited manganese oxide are recognizable. Other parts
of the manganese oxide are loosely accumulated near the
zeolite surface. This explains the variations in laser Raman
spectra depending on the position of the microscope above
the catalyst surface.
Interaction of NO with MnOx and the Composite Catalyst
DRIFT measurements were applied for characterizing
surface species during interaction of NO with MnOx and
the composite catalyst. The DRIFT spectrum (not shown)
of Mn425 annealed in the IR cell at 425 K under a flow of He
for 1 h shows the presence of molecularly adsorbed water
and adsorbed CO2, as well as residual carbonate species.
Obviously, the mild annealing at 425 K is not sufficient for
removing these species from the surface completely. Mn–O
vibrations that are located within the range 700–400 cm−1
(26, 27) cannot be seen due to the poor reflection of the
dark-colored sample at wavenumbers below 1000 cm−1
.
Immediately after addition of NO to Mn425 at 298 K, one
absorption band at 1208 cm−1 developed whose intensity
does not change with a longer time of interaction (Fig. 8A).
Further bands emerge at 1400 and 1326 cm−1. The inten-
sity of the band at 1326 cm−1 increases with time of NO
interaction and the band position is shifted to 1315 cm−1
.
Simultaneously, the band at 1400 cm−1 is transformed into
a peak at 1419 with a shoulder at 1395 cm−1. These bands
indicate the presence of nitrite species (surface nitrito and
nitro complexes) (27). If nitrates are coordinated to man-
ganese ions in the chelating bidentate form (cf. Table 3)
bands around 1290 and 1555 cm−1 should be present in
the IR spectrum (28). Bridging bidentate nitrato species
would cause a high-frequency absorption at a wavenumber
higher than 1600 cm−1 (27). A weak absorption is seen in
this region. Additionally, the weak band at 1038 cm−1 which
emerges after 5 min of NO interaction is attributable to the
ν1 vibration of those nitrates. The band at 1208 cm−1 is diffi-
cult to assign. In Ref. (28) a band at 1230 cm−1 is attributed
to bridging bidentate nitrito complexes. Bulk monodentate
nitrito complexes are characterized by vibration bands in
the spectral regions 1206–1065 and 1470–1375 cm−1 (26).
Bridging nitro–nitrito complexes possess bands at 1260–
1180 and 1520–1390 cm−1 (27). Therefore, it is reasonable
to assume that bridging nitro–nitrito species are formed af-
ter interaction of NO with Mn425 at room temperature. The
bands at 1315 and 1419 cm−1 are due to nitro complexes,
whose formation proceeds slowly (29). The formation may
cause a reduction according to Eq. [3],
2 Mnn+ + NO + H2O → 2 Mn(n−1)+ + NO−2 + 2 H+, [3]
where n may be 4 or 3 according to the valence state of in-
teracting manganese ions. Three-valent manganese ions are
likely present, as follows from the nonstoichiometric com-
position of the MnO2 phase. Evidence for the reduction
FIG. 8. Interaction of 5 vol% NO/He with sample Mn425 at 298 K.
DRIFT spectra at progresive times of interaction. (a) Immediately after
contact, and after (b) 5, (c) 10, (d) 15, and (e) 20 min. (A) Spectral region
2000–800 cm−1; (B) hydroxyl region.

106
RICHTER ET AL.
TABLE 3
Species Formed after Interaction of NOx with Metal Cations (M)
and Characteristic IR Vibration Frequencies
Main vibrational
Species
Structural sketch
frequencies
Reference
26, 30
Chelating bidentate
nitrato
1500–1565
1260–1300
Bridging bidentate
nitrato
1600–1650
1170–1225
26
26, 27
27
Bridging bidentate
nitrito
νas (NO2) 1266–1314
νs (NO2) 1176–1203
–
Monodentate
nitrito
ν (N O) 1065–1206
==
ν (N O) 1375–1470
–
Bridging nitro
nitrito
ν (N O) 1180–1260
27
==
ν (N O) 1390–1520
Nitro
νas (NO2) 1375–1650
νs (NO2) 1250–1350
26, 29
FIG. 9. Interaction of 5 vol% NO/He with sample Mn425. DRIFT
spectra at (a) 298, (b) 373, and (c) 423 K and (d) after cooling to 298 K
under He flow. Time of contact, 20 min.
process (Eq. [3]) is provided by spectra shown in Fig. 8B,
where the protonation of the surface leads to the emergence
of OH groups linked to Mn (bands at 3680 and 3620 cm−1).
A temperature increase leads to the following modifica-
tions of the spectrum (Fig. 9). The intensity of the band at
1208 cm−1 decreases and the intensity of bands attributed to
nitro species increases. After cooling to room temperature,
the spectrum is characterized by absorption of nitro com-
plexes at 1450 and 1320 cm−1, bridging bidentate nitrato
complexes at 1038 cm−1 (weak), OH groups at 3680 and
3620 cm−1, and adsorbed water (superimposed to nitrate)
at 1635 cm−1. The observed species are different from those
reported in the literature for Mn2O3 and MnOx /Al2O3 (28)
or Mn–ZSM-5 (31).
Figure 10 compares the DRIFT spectra of NaY775, Mn425
,
and 15 MnNaY425 after admission of NO at 298 K in the
1500–2200 cm−1 region. Nitrosyls of Mnn+ cations that
produce bands in the 1750–1966 cm−1 region (26) were
not formed. Different from NaY775 and Mn425, sample
15 MnNaY425 developed two weak bands at 1685 and
1664 cm−1 at 298 K. These bands may be attributed to the
==
in-phase and out-of-phase motions of linked N O stretch
motions, which could indicate the presence of symmet-
== – – ==
ric O N O N O species (30) formally corresponding to
N2O3. The spectral region below 1400 cm−1 is not suitable
for discussion because of the dominating strong absorption
of the catalyst itself under the NO/He flow.
FIG. 10. DRIFT spectra of sample NaY775 (a) and sample Mn425
(b) after 5 min of interaction with 5 vol% NO/He at 298 K and of sample
15 MnNaY425 after 1 (c), 5 (d), and 20 min (e).

NOx SCR BY NH3 OVER MnOx /NaY COMPOSITE CATALYSTS
107
but the catalyst performance is not relevant for real wet
exhaust. With wet exhaust gas compositions (7 vol% H2O)
the composite standard catalyst 15 MnNaY775 achieves an
average NOx conversion of 70% at 423 K over at least 70 h
time-on-stream (TOS) and, hence, is more efficient than the
other cited Mn-based catalysts.
Influence of feed composition. Three feed variants were
considered at a reaction temperature of 423 K: 1000 ppm
NO2, 500 ppm NO/500 ppm NO2, and 1000 ppm NO/
10 vol% O2, each completed by addition of 1000 ppm NH3
and 7 vol% H2O as well as He for dilution. Results are
shown in Fig. 12.
The highest NOx conversion to N2 of ca. 80% is observed
with a feed containing equimolar amounts of NO and NO2,
and the lowest NOx conversion to N2 (ca. 50%) with a feed
containing exclusively NO2. Regarding formation of N2,
results for the NO/O2 feed are comparable with those for
the NO/NO2 feed. These findings confirm that a mixture of
NO/NO2 is beneficial to the reaction. Formally, an equimo-
lar mixture of NO/NO2 corresponds to N2O3, the source of
NO−2 anions in contact with moisture. Nitrite is viewed as
theintermediatespeciesleadingtoN2 inthereactioncourse
(see later). Obviously, the good performance of the com-
posite catalyst is related to the ability to produce NO2 in
sufficient concentration by oxidation of NO, thus providing
a NO/NO2 mixture. Therefore, comparable NOx conver-
sions to N2 are achieved in comparison with the NO/NO2
premixed feed. With all feed compositions, N2O was found
in comparable concentrations of ca. 100 ppm. N2O origi-
nates from decomposition of ammonium nitrate, which is
formed if intermediate nitrite is converted to nitrate and
residual ammonia is present (conversion levels less than
100%).
FIG. 11. SCR of NOx by NH3 over Mn775 (A) and over 15 MnNaY775
(B) vs reaction temperatures. NO conversion to N2 (ꢁ) and overall con-
version (dotted line). Selectivity of N2 formation (ꢀ). Reaction condi-
tions: 1000 ppm NO, 1000 ppm NH3, 10 vol% O2, 7 vol% H2O. SV =
48,000 cm3 gc−a1t h−1
.
Catalytic Properties
Oxidation of NO by gaseous O2. Conversion of NO
Steady state SCR of NOx . Results of measurements for to NO2 by wet and dry air over the composite catalyst is
sample Mn775 and the composite catalyst 15 MnNaY775 per- shown in Fig. 13 in dependence on the reaction temper-
formed with standard−f1eed composition at a space velocity ature, together with the observed level of homogeneous
(SV) of 48,000 cm3 gcat h−1 within the temperature range NO oxidation proceeding in the connection lines and in the
400–500 K are shown in Figs. 11A and 11B, respectively. free reactor volume. The homogeneous oxidation amounts
The overall NOx conversion over Mn775 increases from to about 10%, whereas conversions from 60 to 80% are
30% at 413 K to 100% at 485 K. However, the N2 selec- observed with dry feed in the presence of the catalyst.
tivity of the NOx conversion is poor. Nearly 50% of NOx The dependence of NO conversion on temperature pro-
is converted to N2O at 483 K. This finding is in line with ceeds through a minimum between 350 and 400 K with dry
results reported by Kapteijn et al. (25), who also found a feed.
high percentage of side reaction to N2O for the SCR of
This was earlier reported for pure zeolites by Halasz et al.
NOx over pure manganese oxides. At the same experimen- (39) and attributed to a superimposition of thermodynamic
tal conditions, the composite catalyst 15 MnNaY775 reveals and kinetic contributions. At low temperature, the adsorp-
both higher activity for the SCR of NOx and higher selecti- tion equilibrium of NO is decisive for the catalytic transfor-
vity to N2 (Fig. 11B) compared with sample Mn775
.
mation to NO2. This adsorption equilibrium is shifted to the
Table 4 summarizes relevant reaction data for 15 MnNaY775 gas phase if temperatures exceed 375–400 K, and the oxi-
in comparison with literature data for Mn-based SCR cata- dation of NO to NO2 becomes kinetically controlled. At
lyst systems. Most published data have been obtained with higher temperatures (>600 K) the NO/NO2 equilibrium
dry feed. This allows high conversions at low temperatures limits the maximum conversion of NO.

108
RICHTER ET AL.
TABLE 4
Stationary Conversion of NOx by NH3 over Mn-Based Catalysts
Feed composition
Reaction conditions
NO
(ppm)
NH3
(ppm)
O2
(%)
H2O
(%)
T
(K)
XNO
(%)
SN
(%)
SV
TOS
(h)
x
2
Catalyst
(cm3 g−1 h−1
)
Reference
25
b
Pure bulk
Mn oxidea
MnO2
500
550
2
0
383
398
423
473
60
70
92
90
90
?
15
82
75
38
c
c
Mn/γ –Al2O3 (2 wt% Mn)
Mn/γ –Al2O3 (2 wt% Mn)
500
450
550
500
2
2
1–5
383–698
?
?
25,000
31,000
10–15
32
0
423
10
40
38
n.d.
65
100
150
33, 34
Mn/γ –Al2O3 (2 wt% Mn)
45
550
500
500
500
550
2
5
0
0
0
423
37
55
≈100
37,500
91,400
24,000
35
36
37
Mn2O3–WO3/γ -Al2O3
(10.8/6.8/82.4 wt%)
10
398
423
25
45
100
3
d
MnOx /Al2O3
2
383
423
443
473
25
63
80
97
≈100
90
80
70
Mn/carbonized
silica–alumina
(2.5 wt% Mn,
impregnated)
800
800
3
0
7
413
453
493
533
573
94
98
96
85
67
91
79
71
65
44
12,000
48,000
38
15 MnNaY775
1000
1000
10
423
74
74
77
71
69
92
91
92
1
10
20
40
70
This work
92
15 MnNaY775
15 MnNaY775
1000
1000
1000
1000
10
5
7
7
443
443
88
82
93
93
48,000
48,000
22
32
This work
This work
82
94
75–80
a Further investigated oxides with lower activities but partly higher selectivities: Mn5O8, Mn2O3, Mn3O4.
b Only the feed rate was given: 50 cm3 min−1
.
c No conversion data given, but relative rate constants k/kref, related to reference values of the rate constant kref without water addition.
Addition of 2 vol% H2O leads to a reversible decrease from 1 to 0.3.
d Impregnation of commercial Al2O3 (55 m2 g−1) with HMnO4 solution and calcination at 423, 573, 873, and 1273 K for 5 h. Values are given for the
sample calcined at 573 K. n.d., Not determined.
Oxidation of NH3. The catalytic oxidation of NH3 to containing NO a priori. This “internal” SCR is reported
N2 is described to proceed on catalysts according to
as the reaction route of NH3 oxidation to N2 by Amblard
et al. (40). The N2O formation might occur for the same rea-
son as discussed before, indicating thermal decomposition
of ammonium nitrate. Kapteijn et al. (25) considered the
reaction
4 NH3 + 3 O2 → 2 N2 + 6 H2O
[4]
but, particularly at higher temperatures, oxidation to NO
takes place:
2 NH3 + 2 O2 → N2O + 3 H2O
[6]
4 NH3 + 5 O2 → 2 NO + 6 H2O.
[5]
Intermediate NO in the presence of O2 and NH3 corre- as the source for N2O formation, dominating the oxidation
sponds to a gas composition containing all components for of NH3 over MnOx at higher temperatures.
the SCR of NOx by NH3. Therefore, first traces of NO could
As follows from the reaction routes of NH3 oxidation, the
effectively be converted to N2 in the same way as with a feed N balance of the NOx reduction might be disguised if side

NOx SCR BY NH3 OVER MnOx /NaY COMPOSITE CATALYSTS
109
FIG. 14. NH3 oxidation over catalyst 15 MnNaY775 vs reaction tem-
perature. Conditions: 1000 ppm NH3, 5 vol% O2, 7 vol% H2O. SV =
48,000 cm3 gc−a1t −1. (ꢃ) NH3, (ꢀ) N2O, (ꢁ) N2, (ꢂ) NO.
h
FIG. 12. Influence of feed composition on product distribution
for the SCR of NOx by NH3 over 15 MnNaY775 at 425 K. SV =
48,000 cm3 gc−a1t h−1
.
selective way according to Eq. [7]:
4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O.
[7]
reactions [4]–[6] accompany the NOx conversion to rele-
vant degrees. For this reason, the activity of the composite
catalyst has been checked for its oxidation properties with
a feed of 1000 ppm NH3, 5 vol% O2, and 7 vol% H2O at
standard space velocity.
Results are shown in Fig. 14. Oxidation of NH3 is ob-
served at temperatures higher than 450 K, leading to for-
mation of N2 and N2O. The concentration of both pro-
ducts proceeds through a maximum located at about 600 K.
Consumption of NH3 is as high as 60% at this temperature.
NO is detectable in the gas phase at higher temperatures.
For both cases, conversion data of NO versus the NO con-
centration at a reaction temperature of 425 K are shown
in Fig. 15A. A feed containing 1000 ppm of each reac-
tant yields 86% of NO conversion. With 2000 ppm NO but
nNO/nNH equal to 2, the conversion decreases considerably
3
due to the nonstoichiometric low NH3 concentration. On
the other hand, at 500 ppm NO and a ratio of nNO/nNH of
3
0.5, the conversion of NO reaches 95%. If the NH3 supply is
correspondingly changed with the NO concentration fixing
the nNO/nNH ratio at 1, there is practically no influence on
3
Influence of reactant concentrations. The NO concen-
tration has been varied by either keeping the NH3 con-
centration constant, i.e., higher NO concentration causes a
feed composition increasingly lean of NH3, or by maintain-
the conversion of NO between 500 and 2000 ppm.
The influence of oxygen on the SCR of NOx at reaction
temperatures of 425, 445, and 455 K is shown in Fig. 15B.
The influence persists over the investigated concentration
range but is lower from 5 to 10 vol%. The reaction is pro-
moted by enhanced O2 concentration. At the highest O2
content, NOx conversion amounts to 86, 95, and 100% at
425, 445, and 455 K, respectively, with N2 selectivities of
90% or higher.
ing the nNO/nNH molar ratio at 1. This corresponds to the
3
required reaction stoichiometry if the conversion runs in a
For a reaction temperature of 425 K the influence of H2O
concentration has been investigated. The result shown in
Fig. 15C provides evidence that the NO conversion to N2
decreases from 90% at 6 vol% H2O to ca. 60% at a water
content of 30 vol%. Full conversion is achieved with dry
feed.
Formal reaction orders. Formal reaction orders were
determined from a logarithmic representation of NO con-
sumption rates versus component concentrations. Figure 16
displays the result for variation of the NO concentration
between 500 and 2000 ppm.
The linearization performed is valid for a simple power
rate model
FIG. 13. Oxidation of NO to NO2 over catalyst 15 MnNaY775 in dry
feed (ꢀ) and wet feed (ꢂ). Feed composition: 1000 ppm NO, 10 vol%
−1
O2 7 vol% H2O. SV = 8000 cm3
g
h
−1. (Dotted line) Homogeneous
.
cat
−rNO = dX/d(W/FNO) = kapp. cNn O
[8]
oxidation at a feed flow rate of 2 cm3 s−1

110
RICHTER ET AL.
FIG. 16. Rate of NO consumption vs NO concentration for catalyst
15 MnNaY775 in logarithmic coordinates. Reaction temperature, 425 K.
Conditions: 500–2000 ppm NO, 1000 ppm NH3, 10 vol% O2, 7 vol% H2O.
SV = 48,000 cm3 gc−a1t h−1
.
other authors. Overwhelmingly, orders of or near +1 are re-
ported (8, 9, 14) independent of catalyst type and proposed
reaction mechanism. A reaction order of 2 implies that two
NO molecules are involved in the rate-limiting step. This is
reconcilable with a mechanism where after fast initial oxi-
dation of NO to NO2 both molecules form an intermediate
(formally N2O3) that reacts with NH3. This conclusion is
in line with the observed beneficial influence of a mixed
NO/NO2 feed on the SCR process.
The integral reaction order of oxygen is 0.3. No mean-
ingful results could be obtained, however, for variation of
H2O and NH3 concentrations. NH3 in excess leads to bal-
ance discrepancies, pointing to formation of ammonium ni-
trate. H2O variation in such large excess over NO and NH3
has an overall deactivating effect on the NOx conversion
but its interference with the reaction kinetics is obviously
not straightforward. Acquired reaction orders with stan-
dard deviations and correlation coefficients of the linear fit
are summarized in Table 5.
FIG. 15. SCR of NOx by NH3 over 15 MnNaY775. Conversion of NO
to N2 in dependence on (A) NO concentration at nNO/nNH3 = 1 (ꢃ) and
at nNH3 = constant (1000 ppm) (ꢁ), T = 425 K; (B) O2 concentration at
425 (ꢀ), 445 (ꢃ), and 455 K; and (C) H2O concentration, T = 425 K. SV =
DISCUSSION
It was shown in this article that zeolite NaY in conjunc-
tion with a precipitated nonstoichiometric MnO2 phase
48,000 cm3 gc−a1t h−1
.
if gas-phase concentrations of all other components are
kept constant. In Eq. [8], X denotes the NO conversion, W
the catalyst weight, and FNO the feed rate of NO. The ap-
parent rate constant kapp. comprises the true rate constant
and the numerical contribution from fixed concentrations
of other reaction partners; n is the integral reaction order
of NO.
TABLE 5
Formal Reaction Orders of Components for the SCR of NOx
over 15 MnNaY775 at 425 K
Component
n
Linear fit
SDa
Range
NO
O2
1.8 0.10
0.3 0.02
0.9955
0.9935
0.06
0.02
500–2000 ppm
1.25–10 vol%
It turned out that the reaction order of NO is approx-
imately 2. This reaction order is higher than reported by
a Standard deviation.

NOx SCR BY NH3 OVER MnOx /NaY COMPOSITE CATALYSTS
111
represents a promising catalyst for the low-temperature Nitrous acid can disproportionate into nitric acid and nitric
SCR of NOx by NH3. Deposition of 15 wt% MnO2 by oxide according to Eq. [12].
precipitation reaction [2] on the external surface of zeolite
3 HNO2 ↔ HNO3 + 2 NO + H2O
[12]
NaY microcrystals leads to patches of amorphous MnOx ,
as recognizable from TEM images, but does not completely
cover the surface of the zeolite microcrystals. In the pres-
ence of excess gas-phase oxygen and water vapor, oxidation
of NO to NO2 proceeds to a considerable extent at reac-
tion temperatures above 370 K. Without gas-phase oxygen,
the participation of MnOx bulk oxygen in NO oxidation to
NO2 could be proven to occur (18, 41). The redox activity
of the MnOx component provides a mixed NO/NO2 feed,
formally equivalent to N2O3. However, N2O3 is known as
a very labile oxide decomposing easily to NO/NO2 in the
gas phase (Eq. [9]).
The same components are formed upon interaction of NO2
with moisture (Eqs. [13] and [14]).
2 NO2 + H2O ↔ HNO2 + HNO3
3 NO2 + H2O ↔ 2 HNO3 + NO
[13]
[14]
Conversions described by Eqs. [9]–[14] are equilibrium
reactions. Therefore, several componentsmaybe presentsi-
multaneously in the gas phase during steady state catalysis.
But the formation of nitrite (nitrous acid) and the transfor-
mation of nitrite to nitrate (nitric acid) are viewed as the key
steps of the reaction. Reaction progress demands the effi-
cient removal of nitrite before its reverse decomposition
into NO/NO2 (Eq. [9]) or its further transformation into
nitrate (Eq. [12]). Practically, HNO2 has to be drained off
from the set of equilibrium reactions [11]–[13] by reaction
with NH3. It is reasonable to conclude from Eq. [14] that ex-
cess NO2 enhances the nitrate (nitric acid) formation. Pre-
vailing nitrate formation should diminish the conversion of
NO to N2. This is confirmed by results of feed variation
presented in Fig. 12. A feed of NO2 yielded a conversion
of ca. 50% into N2, which is far less than that observed
for an equimolar NO/NO2 mixture or for NO/O2 over the
composite catalyst. The comparable results achieved for
NO/NO2 and NO/O2 feed compositions indicate the good
oxidation capability of the manganese oxide for the cata-
lytic conversion of NO to NO2. Rate limitations should
have kinetic reasons because, thermodynamically, NO2 is
favored at low temperatures and complete conversion of
NO to NO2 is achievable.
N2O3 ↔ NO + NO2
[9]
Sultana et al. (42) showed recently that zeolite NaY is able
to stabilize N2O3 inside the super cage by displacing water
molecules adsorbed at Na+ cations. The authors empha-
sized that this behavior of zeolite NaY is unique. Identifica-
tion relies on the presence of IR absorption bands at 1270,
1590, and 1905 cm−1. Szanyi and Paffett (43) attributed IR
bands at 1587 and 1875 cm−1 observed for gas-phase spectra
over zeolite H-ZSM-5 to N2O3. Similar assignments were
given by other authors (e.g., Ref. (44)). DRIFT spectro-
scopic results obtained in the present work point to the
existence of adsorbed N2O3 in a symmetric configuration.
The configuration is thought as a specific coordination of
two NO molecules to surface oxygen of the catalyst rather
than to the existence of molecular N2O3, because NO was
used in an inert He stream so that oxidation of NO by gas-
phase oxygen could not take place. Most of literature re-
sults attributed IR bands to an asymmetric N2O3 configu-
ration. The asymmetric N2O3 molecule is described as a NO
The high reaction order of NO near 2 confirms a reac-
tion stoichiometry requiring formally two NO (NO/NO2)
molecules for the reaction to proceed. This is the prerequi-
site for nitrite formation and supports the conclusion that
the diazotation mechanism
== –
molecule weakly linked to a NO2 molecule (O N NO2).
Consequently, the stretching vibrations of asymmetric
N2O3 are similar to the modes of NO and NO2, which
complicates the identification. In the presence of water, the
equilibrium
NH+4 + NO2− → N2 + 2 H2O,
[15]
2 NO+ + H2O ↔ N2O3 + 2 H+
[10]
which has been proven to be valid for zeolite structures
in their ammonium form (17), proceeds even on the alkali
form of the zeolites. It could be shown by in situ FTIR ex-
periments (41) that during interaction of NOx with the sur-
face of the composite catalyst 15 MnNaY775, the characteri-
stic vibration bands of zeolitic Brønsted acid sites emerge.
These Brønsted acid sites are formed during the interaction
of zeolite NaY with NOx in the presence of water, indicating
an in situ exchange of Na+ ions by H+ at Al-O−-Si units of
the zeolite framework. Thus, formation of NH+4 ions from
NH3 can take place during reaction. DRIFT spectroscopic
measurements revealed the in situ formation of hydrox-
is suggested to exist (41, 44). Nitrosonium ions NO+ and
nitrate anions NO−3 resulted from disproportionation of
N2O4. N2O4 was observed after room temperature inter-
action of NO/O2 with the catalyst (41). Moreover, both
in situ FTIR investigations discussed in Ref. (41) and in situ
DRIFT measurements of the present work confirmed the
formation of nitrite and nitrate surface species. In the pre-
sence of moisture, interaction of N2O3 with water leads to
nitrous acid (Eq. [11]).
N2O3 + H2O ↔ 2 HNO2
[11] yls located at manganese ions. The role of these hydroxyl

112
RICHTER ET AL.
groups and how far they interact with ammonia cannot be mercial V2O5/WO3/TiO2 catalysts are only marginally
stated at present. It is unlikely that the concentration and active (46).
acid strength of detected Mn–OH groups are sufficiently
These properties would fulfill the requirements of a low-
high to ensure NH3 activation as NH+ ions, as is reported temperature NOx reduction catalyst for mobile diesel en-
for V–OH groups of the V/TiO2 SC4R catalyst (45). The gines if an ammonia supply is implemented “on board,” e.g.,
predominance of Lewis acid sites on the MnOx /NaY com- by urea decomposition.
posite implies an activation of ammonia by dissociation,
forming amide species with concomitant hydrogen trans-
fer to adjacent oxygen atoms, as reported for other oxides
(45). Thisisdenotedthe“amide/nitrosamide”SCRreaction
route, proposed for NOx conversion by NH3 over 2 wt%
Mn/Al2O3 catalysts (35). But as follows from data given in
Table 4, these and other Mn-based catalyst formulations do
not achieve comparable activities to the 15 MnNaY cata-
lyst with wet feed. Impregnation of nonzeolitic supports
with Mn salt solutions and calcination of the catalyst
lead to the emergence of crystalline Mn oxides which
are not as active and selective as the amorphous MnOx
bulk phase on the microporous zeolite. Whereas the
MnOx phase precipitated on zeolite NaY retains its amor-
phous state with mainly tetravalent manganese after cata-
lyst calcination at 775 K, MnOx precipitated in the ab-
sence of the zeolite is mostly converted to crystalline
α-MnO2 and γ -Mn2O3 after calcination at 775 K. Cor-
respondingly, the catalytic behavior of sample Mn775 is
characterized by moderate NOx conversion activity and
a high N2O by-product formation (cf. Fig. 11A), as is
characteristic for crystalline manganese oxides (25) and
also for manganese oxides supported on Al2O3 at higher
loading (36).
ACKNOWLEDGMENTS
The authors are indebted to U. Wolf and R. Eckelt for technical assis-
tance. The work was supported by the Federal Ministry for Education and
Research of the FRG and the Senate of Berlin (project 03C3005). M.R.
and R.F. wish to thank the Verband der Chemischen Industrie (VCI) for
financial support.
REFERENCES
1. Janssen, F. J., in “Handbook of Heterogeneous Catalysis” (G. Ertl,
H. Kno¨zinger, and J. Weitkamp, Eds.), Vol. 4, p. 1633. VCH,
Weinheim, 1997.
2. Parvulescu, V. I., Grange, P., and Delmon, B., Catal. Today 46, 233
(1998).
3. Forzatti, P., Catal. Today 62, 51 (2000).
4. Held, W., Ko¨nig, A., Richter, T., and Puppe, L., Technical Paper Series
900496. Society of Automotive Engineers, Warrendale, 1990.
5. Amon, B., Fischer, S., Hofman, L., and Zu¨rbig, J., in “Preprints of the
Fifth International Congress on Catalysis and Automotive Pollution
Control,” Brussels, April 12–14, 2000, Vol. 1, 215. Universite Libre de
Bruxelles, Brussels, 2000.
6. Koebel, M., Elesener, M., and Kleemann, M., Catal. Today 59, 335
(2000).
7. Jacob, E., EP 0487886 B1, MAN Technologie AG, Munich (1994).
8. Stevenson, S. A., Vartuli, J. C., and Brooks, C. F., J. Catal. 190, 228
(2000).
Due to its favorable pore structure, zeolite NaY is able to
9. Eng, J., and Bartholomew, C. H., J. Catal. 171, 14 (1997).
stabilize and to store NOx in the form of nitrate, nitrite, and 10. Roberge, D., Raj, A., Kaliaguine, S., On, D. T., Iwamoto, S., andInui, T.,
Appl. Catal. B 10, L237 (1996).
11. Andersson, L. A. H., Brandin, J. G. M., and Odenbrand, C. U. I., Catal.
Today 4, 173 (1989).
12. Odenbrand, C. U. I., Andersson, L. A. H., Brandin, J. G. M., and
N2O3-like intermediates. The performance of the compo-
site catalyst 15 MnNaY775 is related to the close neighbor-
hood of the catalyst components coupling nitrite formation
and its drain off from equilibria with NO/NO2 and nitrate
Ja¨ras, S., Catal. Today 4, 155 (1989).
by activated ammonia. Redox properties of MnOx promote 13. Brandin, J. G. M., Andersson, L. A. H., and Odenbrand, C. U. I., Catal.
Today 4, 187 (1989).
14. Choi, E.-Y., Nam, I.-S., and Kim, Y. G., J. Catal. 161, 597 (1966).
15. Medros, F. G., Eldridge, J. W., and Kittrell, J. R., Ind. Eng. Chem. Res.
oxidation of NO to NO2, and the ion exchange properties of
the zeolite, besides its Lewis acidity, allows reaction path-
ways of NOx reduction by both the diazotation and the
amide/nitrosamide mechanism.
28, 1171 (1989).
16. Kieger, S., Delahay, G., Coq, B., and Neveu, B., J. Catal. 183, 267
(1999).
17. Richter, M., Eckelt, R., Parlitz, B., and Fricke, R., Appl. Catal. B 15,
CONCLUSIONS
129 (1998).
18. Richter, M., Berndt, H., Eckelt, R., Schneider, M., and Fricke, R.,
Catal. Today 54, 531 (1999).
19. Richter, M., Kosslick, H., and Fricke, R., Stud. Surf. Sci. Catal. 130,
1517 (2000).
20. Barrett, E. P., Joyner, L. S., and Halenda, P. P., J. Am. Chem. Soc. 73,
373 (1951).
The applied precipitation technique for preparation of a
composite material with intended egg-shell arrangement of
amorphous MnOx around zeolite NaY microcrystals leads
to catalysts with high performance in the SCR of NOx by
NH3. The catalyst tolerates water contents of up to 30 vol% 21. Veprek, S., Sarott, F.-A., and Iqbal, Z., Phys. Rev. B 36, 3344 (1987).
22. R. J. Meyer, Ed. “Gmelins Handbuch der Anorganischen Chemie,”
Vol. 56, Part C1, p. 126, Verlag Chemie, Weinheim 1973.
23. Buciuman, F., Patcas, F., Cracium, R., and Zahn, D. R. T., Phys. Chem.
Chem. Phys. 1, 185 (1999).
24. Bernard, M.-C., Hugot-Le Goff, A., Thi, B. V., and Cordoba de
in the feed. Full conversion of NOx is achievable at 325–
450 K up to GHSV values of ca. 50,000 cm3 gc−a1t h−1 in the
presence of 5–10 vol% water.
The most relevant feature of the composite MnOx /NaY
catalyst is its high activity at temperatures where com-
Terresi, S., J. Electrochem. Soc. 140, 3065 (1993).

NOx SCR BY NH3 OVER MnOx /NaY COMPOSITE CATALYSTS
113
25. Kapteijn, F., Singoredjo, L., and Andreini, A., Appl. Catal. B 3, 173
(1994).
26. Hadjiivanov, K., Catal. Rev.–Sci. Eng. 42, 71 (2000).
27. Nakamoto, K., “IR and Raman Spectra of Inorganic and Organic
Compounds,” 3rd ed. Wiley, New York, 1978.
28. Kapteijn, F., Singoredjo, L., Dekker, N. J. J., and Moulijn, J. A., Ind.
Eng. Chem. Res. 32, 445 (1993).
29. Hadjiivanov, K., Saussey, J., Freysz, J.-L., and Lavalley, J.-C., Catal.
Lett. 52, 103 (1998).
36. Kapteijn, F., Singoredjo, L., van Driel, M., Andreini, A., Moulijn,
J. A., Ramis, G., and Busca, G., J. Catal. 150, 105 (1994).
37. Kijlstra, W. S., Poels, E. K., Bliek, A., Weckhuysen, B. M., and
Schoonheydt, R. A., J. Phys. Chem. B 101, 309 (1997).
38. Grzybek, T., Klinik, J., Krzyzanowski, A., Papp, H., and Zyla, M.,
Catal. Lett. 63, 107 (1999).
39. Halasz, I., Brenner, A., and Ng, K. Y. S., Catal. Lett. 34, 151 (1995).
40. Amblard, M., Burch, R., and Southward, B. W. L., Catal. Lett. 68, 105
(2000).
30. Varetti, W. L., and Pimentel, G. C., J. Phys. Chem. 55, 3813 (1971).
31. Aylor, A. W., Lobree, L. J., Reimer, J. A., and Bell, A. T., J. Catal. 170,
390 (1997).
32. Kijlstra, W. S., Daamen, J. C. M. L., van de Graaf, J. M., van der Linden,
B., Poels, E. K., and Bliek, A., Appl. Catal. B 7, 337 (1996).
41. Bentrup, U., Bru¨ckner, A., Richter, M., and Fricke, R., Appl. Catal. B
32, 229 (2001).
42. Sultana, A., Loenders, R., Monticelli, O., Kirschhock, C., Jacobs,
P. A., and Martens, J. A., Angew. Chem. 112, 112
(2000).
33. Kijlstra, W. S., Brands, D. S., Poels, E. K., and Bliek, A., J. Catal. 171, 43. Szanyi, J., and Paffett, M. T., J. Catal. 164, 232 (1996).
208 (1997).
44. Raj, A., Le, T. H. N., Kaliaguine, S., and Auroux, A., Appl. Catal. B
34. Kijlstra, W. S., Brands, D. S., Smit, H. I., Poels, E. K., and Bliek, A.,
J. Catal. 171, 219 (1997).
15, 259 (1998).
45. Busca, G., Lietti, L., Ramis, G., and Berti, F., Appl. Catal. B 18, 1
35. Kijlstra, W. S., Brands, D. S., Poels, E. K., and Bliek, A., Catal. Today
50, 133 (1999).
(1998).
46. Richter, M., et al., unpublished results.