In situ IR spectroscopic and XPS study of surface complexes and their transformations during ammonia oxidation to nitrous oxide over an Mn-Bi-O/α-Al2O3 catalyst

Article abstract of DOI:10.1007/s10975-005-0109-5

Surface complexes resulting from the interaction between ammonia and a manganese-bismuth oxide catalyst were studied by IR spectroscopy and XPS. At the first stage, ammonia reacts with the catalyst to form the surface complexes [NH] and [NH₂] via abstraction of hydrogen atoms even at room temperature. Bringing the catalyst into contact with flowing air at room temperature or with helium under heating results in further hydrogen abstraction and simultaneous formation of [N] from [NH₂] and [NH]. The nitrogen atoms are localized on both reduced (Mn²⁺) and oxidized (Mn ^δ+, 2 < δ < 3) sites. Atomic nitrogen is highly mobile and reacts readily with the weakly bound oxygen of the oxidized (Mn ^δ+-N) active site. The nitrogen atoms localized on oxidized sites play the key role in N₂O formation. Nitrous oxide is readily formed through the interaction between two Mn^δ+-N species. N₂ molecules result from the recombination of nitrogen atoms localized on reduced (Mn²⁺-N) sites.

Full text of DOI:10.1007/s10975-005-0109-5

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Kinetics and Catalysis, Vol. 46, No. 4, 2005, pp. 555–564. Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 590–600.
Original Russian Text Copyright © 2005 by Slavinskaya, Chesalov, Boronin, Polukhina, Noskov.
In situ IR Spectroscopic and XPS Study of Surface Complexes
and Their Transformations during Ammonia Oxidation
to Nitrous Oxide over an Mn–Bi–O/a-Al2O3 Catalyst
E. M. Slavinskaya, Yu. A. Chesalov, A. I. Boronin, I. A. Polukhina, and A. S. Noskov
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received September 10, 2003; in final form December 21, 2004
Abstract—Surface complexes resulting from the interaction between ammonia and a manganese–bismuth
oxide catalyst were studied by IR spectroscopy and XPS. At the first stage, ammonia reacts with the catalyst to
form the surface complexes [NH] and [NH2] via abstraction of hydrogen atoms even at room temperature.
Bringing the catalyst into contact with flowing air at room temperature or with helium under heating results in
further hydrogen abstraction and simultaneous formation of [N] from [NH2] and [NH]. The nitrogen atoms are
localized on both reduced (Mn2+) and oxidized (Mnδ+, 2 < δ < 3) sites. Atomic nitrogen is highly mobile and
reacts readily with the weakly bound oxygen of the oxidized (Mnδ+–N) active site. The nitrogen atoms localized
on oxidized sites play the key role in N2O formation. Nitrous oxide is readily formed through the interaction
between two Mnδ+–N species. N2 molecules result from the recombination of nitrogen atoms localized on
reduced (Mn2+–N) sites.
The mechanism of low-temperature ammonia oxi- deeper insight into the nature of catalysts and into the
dation has been the subject of numerous publications. mechanism of catalysis.
Highly selective catalysts producing nitrogen have
Study of ammonia oxidation over metals [7–13]
mainly been studied since this process has been suc-
showed that [N] and [NO] form on the surface and their
cessfully used to reduce ammonia emission. The major-
recombination or desorption results in N2, N2O, and
ity of works have been devoted to platinum catalysts.
NO. [N] species can both result from successive hydro-
Three main schemes for the reaction mechanism were
gen abstraction yielding amide–imide species [NH2–
suggested in earlier studies. In Andrussov’s mechanism
NH] and form from [HNO] species produced by the
[1], it is assumed that nitroxyl species [HNO] form and
interaction between NH3 and O2 adsorbed molecularly
then react to yield N2, N2O, and NO. According to
at the early stages.
Bodenstein’s hypothesis [2], the first step is the forma-
Study of the surface complexes forming upon NH3
adsorption on metal oxides showed that ammonia suc-
cessively loses a number of hydrogen atoms, depending
on the acidity of the oxide [14]. The formation of [NH2]
and its dimer, hydrazine, eventually yielding N2 was
observed by IR spectroscopy [15–18]. The species
[NH] and [HNO], yielding N2O and NO, were also
found [15, 16]. It was shown that ammonia is oxidized
via the selective reduction of NO (key intermediate)
with ammonia (SCR mechanism) [19]. The activation
of ammonia, that is, its oxidation to [NO], is the rate-
determining step.
tion of hydroxylamine [NH2OH], which then trans-
forms to [HNO]. Raschig [3] and Zavadzki [4] pro-
posed an imide mechanism in which the first step is
assumed to be the formation of an imide [NH]. It was
deduced from secondary ion mass spectrometry data
that such intermediates as [HNO], [NH2OH], [HNO2],
and [N2O] do not appear during ammonia oxidation at
partial pressures of about 10–4 Torr and that [NO] is the
only intermediate and dinitrogen is formed via the reac-
tion between [NO] and ammonia [5]. Autoion mass
spectrometry data [6] showed that ammonia interacts
with the surface covered with adsorbed atomic oxygen
to form species of the imide ([NH]) type, which trans-
form to nitroxyls [HNO]. Nitrogen results from the
interaction between [NH] and [HNO], whereas nitrous
oxide is formed by the recombination of two nitroxyls.
Intermediate surface complexes forming during
ammonia oxidation were studied in [20]. The authors
simultaneously measured the rate of surface species
transformation by in situ IR spectroscopy and the rate
The schemes proposed in the above-cited works of formation of reaction products. [NH2], [NH], and
were not strongly supported by experimental data, and [NO] species were detected. The interaction of [NO]
this fact explains their multiplicity. A new impetus to with ammonia was shown to occur more rapidly than
the study of the ammonia oxidation mechanism was NO desorption. It was suggested that N2O forms by the
given by the development of modern techniques for interaction between [NH] and [NO] and N2 forms by
surface characterization, which provided a much the interaction between [NH2] and [NO].
0023-1584/05/4604-0555 © 2005 MAIK “Nauka /Interperiodica”

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
556
SLAVINSKAYA et al.
Selective ammonia oxidation has recently been pro- bismuth nitrate Bi(NO3)3 · 5H2O (Aldrich, No. 38.307-4)
posed as an efficient method for the production of were used as starting chemicals.
nitrous oxide. Nitrous oxide attracts attention due to the
fact that it is widely used as a mild oxidizing agent for
Methods
benzene and other organic compounds [21]. To produce
N2O on a commercial scale, a supported Mn–Bi oxide
catalyst affording N2O with 90% selectivity was devel-
oped [22]. We studied the structure and surface compo-
sition of this catalyst as well as kinetics of the reaction
NH3 + O2 [23, 24]. According to XRD data, the catalyst
Mn–Bi–O/α-Al2O3 contains manganese and bismuth
oxides and a Mn–Bi–O mixed phase [23]. Transmission
electron microscopy showed that structurally disor-
dered layers of the manganese–bismuth oxide phase are
localized on the surface of α-Al2O3 and replicate the
shape of its particles, and the microcrystals of manga-
nese oxide, 10 nm in size, are bound to the Mn–Bi–O
phase. The high defectiveness of this phase is believed
to be responsible for the presence of highly mobile
active oxygen in the catalyst [24]. This inference is con-
firmed by data obtained by the pulse method for the
reduction of the catalyst with ammonia and for NH3
oxidation. In the course of the reduction process,
approximately 4 oxygen monolayers are removed from
the catalyst, and this amount is half the total oxygen of
the active component. Replacing 16é2 with 18é2 in the
gas phase demonstrated that the products of the cata-
lytic reaction contain unlabeled oxygen. It became
clear from the consumption of 16é2 and 18é2 in the
course of the reaction that 1/3 of the oxygen in the cat-
alyst is removed with the reaction products. This con-
firms that subsurface oxygen, that is, the lattice oxygen
of the Mn–Bi–O phase is involved in the reaction and is
highly mobile. The above data give evidence that
ammonia oxidation involves lattice oxygen via an alter-
nating reduction and reoxidation of the catalyst surface.
In situ IR spectroscopy. In our in situ IR spectro-
scopic studies, we used a BOMEM MB-102 Fourier
transform spectrophotometer equipped with a flow-
through high-temperature reactor cell with CaF2 optical
windows. A 50-mg catalyst pellet 1 × 3 cm in size was
placed in a 1.5-cm3 cell. Before each experiment, the
pellet was conditioned in flowing air at 523 K for 1 h in
the cell. Next, it was cooled to a preset temperature and
the air flow was replaced with an ammonia flow. While
100% ammonia was passed, the spectra of surface com-
pounds overlapped and intense absorption in the fre-
quency range 1800–1300 cm–1 due to gaseous NH3 was
observed. To monitor the evolution of the IR spectra of
surface compounds, the experiment was conducted as
follows. Ammonia was passed through the cell for
2 min, and then the flow was stopped and the cell was
sealed. After several minutes, over which the concen-
tration of gaseous ammonia decreased below the deter-
mination limit, a spectrum was recorded and the proce-
dure was repeated. The concentration of surface com-
plexes no longer changed after the third ammonia
admission. The observed spectra were the superposi-
tion of the spectra of the catalyst and adsorbed com-
pounds. The data acquisition time was 1.5 min
(30 scans). To separate out the spectrum of the surface
compounds, the background spectrum of the catalyst,
recorded before ammonia admission into the reactor
cell, was subtracted from the observed spectrum.
XPS. To study the surface compounds formed upon
the treatment of the catalyst with ammonia and to deter-
mine their surface concentrations and chemical states,
In this work, we studied, by in situ IR spectroscopy, X-ray photoelectron spectroscopy was used. XPS spec-
XPS, and by the temperature-programmed surface tra were recorded on a VG ESCALAB spectrometer
reaction (TPSR) method, the surface complexes that (AlKα radiation, hν = 1486.6 eV). The spectrometer
result from the interaction between ammonia and the was calibrated against the Au4f7/2 (Eb = 84.0 eV) and
catalyst and their transformation into reaction products Cu2p3/2 (Eb = 932.7 eV) lines [25]. The catalyst, rubbed
during ammonia oxidation to nitrous oxide in order to into a fine-meshed nickel gauze, was fixed in a holder.
elucidate the reaction mechanism.
To take into account the charging effect arising from
photoemission, both the internal standard (the Al2p line
in the spectrum of the α-Al2O3 support, Eb = 73.8 eV)
and the C1s line (Eb = 284.8 eV) from carbonaceous
surface impurities [25, 26] were used. The relative con-
centrations of the elements in the catalyst were derived
from the observed integral intensities of XPS lines
(Iï, IAl) using the formula
EXPERIMENTAL
Catalyst Preparation
The catalyst Mn–Bi–O/α-Al2O3 was prepared by
double impregnation of the support. At the first stage, a
weighed portion of the support was impregnated to
incipient wetness with a certain volume of a mixed
solution containing appropriate proportions of Mn(II)
IX/(ASF)X
----------------------------
nï/nAl =
,
IAl/(ASF)Al
and Bi(III) nitrates, and the resulting material was dried where nï is the concentration of the element X (at. %)
at 353–403 K [23]. The product was again impregnated and (ASF)i are the atomic sensitivity factors of the ele-
with an appropriate volume of the mixed solution, dried ments. The following ASF values were used [26]: Al2p,
at 353–403 K and calcined at 833 K for 2 h. Manganese 0,193; N1s, 0.477; O1s, 0.711; Mn2p, 2.420, and Bi4f,
nitrate Mn(NO3)2 · 6H2O (Aldrich, No. 28.864-0) and 7.632. The lineAl2p from the support was chosen as the
KINETICS AND CATALYSIS Vol. 46 No. 4 2005

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
In situ IR SPECTROSCOPIC AND XPS STUDY OF SURFACE COMPLEXES
557
1560
NH2
1545
Absorbance, arb. units
NH
1411
Absorbance, arb. units
1600
NH3
1610
NH+4
1475
0.08
0.10
NH+4
1674
1362
1386
0.08
0.06
0.06
0.04
0.02
0
1547
3
1
2
1610
1410
0.04
1655
2
1
0.02
1460
0
1800
1700
1600
1500
1400
1300
Frequency, cm–1
1800
1700
1600
1500
1400
1300
Frequency, cm–1
Fig. 1. IR spectra of surface compounds resulting from the
Fig. 2. IR spectra of surface compounds resulting from the
interaction between NH and the catalyst Mn–Bi–O/α-
interaction between NH and the catalyst Mn–Bi–O/α-
3
3
Al O at 300 K after (1) one and (2) two ammonia admis-
Al O at 300 K after ammonia admission: (1) oxidized cat-
2
3
2 3
sions. Spectrum 3 was recorded after N H admission at
alyst and (2) catalyst reduced with NH at 673 K. The
2
4
3
300 K.
deconvolution of these spectra is shown.
reference, and the other lines of the photoelectron spec- tion bands indicates the formation of several types of
trum were normalized to its intensity.
surface compounds under the above conditions. The
absorption bands at 1655 and 1460 cm–1 are due to the
symmetrical and asymmetrical bending vibrations of
For detailed processing of the spectra, we used the
original program package, which allows the deconvolu-
tion of a complex spectrum into individual components
each having a mixed, Gaussian–Lorentzian shape with
a variable fraction of the Gaussian.
The catalyst Mn–Bi–O/α-Al2O3 calcined at 833 K
was preliminarily conditioned in helium at 473 K for 1 h.
Next, it was treated with ammonia at room temperature
for 1 h and purged with air for 30 min. XPS spectra
were recorded between 300 and 769 K.
TPSR technique.A catalyst sample was loaded into
the reactor, conditioned in flowing helium at 723 K, and
pumped at room temperature. After ammonia adsorp-
tion at room temperature and removal of reversibly
adsorbed ammonia with flowing helium, a temperature-
programmed surface reaction was carried out by heat-
ing the reactor from room temperature to 923 K at a rate
of 15 K/min. Substances that were desorbed into the
gas phase during TPSR were analyzed with a quadru-
pole mass spectrometer.
NH+4 ion, respectively. The weak band at 1610 cm–1 is
likely due to the symmetrical bending vibration of the
molecularly adsorbed ammonia. The band due to the
asymmetrical bending vibration of this surface com-
plex likely occurs at a frequency below 1300 cm–1 and,
therefore, cannot be observed in the IR spectrum. The
bands at 1547 and 1410 cm–1 are likely to be assignable
to the products of ammonia oxidation. During adsorp-
tion, the ratio between the intensities of these bands
remains nearly unchanged. These bands can be consid-
ered to be induced by the same surface complex. How-
ever, in a study of ammonia adsorption on MgFe2O4
[27], bands near 1550 and 1410 cm–1 were assigned to
two different surface compounds, namely, coordinated
[NH2] and [NH], respectively. To verify the assignment
of these bands, ammonia adsorption was studied on the
catalyst Mn–Bi–O/α-Al2O3 preliminarily reduced at
673 K in an NH3 atmosphere. As can be seen in Fig. 2,
this pair of bands is also observed in the IR spectra of
the reduced sample. However, the ratio of the integral
intensities of the bands at 1547 and 1410 cm–1 in the
spectrum of the oxidized sample is ~1.5 times higher.
Taking into account this fact, we suggest that the bands
at 1547 and 1410 cm–1 are due in fact to different sur-
face complexes, namely, the [NH2] and [NH] species,
RESULTS
In situ IR Spectroscopic Studies
Figure 1 presents the spectra of surface compounds
resulting from the interaction between NH3 and the cat-
alyst surface at room temperature (300 K) after a single
(spectrum 1) and two (spectrum 2) admissions of
ammonia to the IR cuvette. Absorption bands at 1655 respectively. Another admission of ammonia resulted in
an increase in the concentration of the above surface
(shoulder), 1610, 1547, 1460, and 1410 cm–1 are
observed in the spectra. This large number of absorp- compounds. Their surface concentrations decreased
KINETICS AND CATALYSIS Vol. 46 No. 4 2005

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
558
Absorbance, arb. units
SLAVINSKAYA et al.
Absorbance, arb. units
0.12
0.20
0.15
0.10
0.05
0
0.10
0.08
0.06
0.04
0.02
0
1
2
3
4
1
2
5
3
4
–0.05
–0.02
1900 1800 1700 1600 1500 1400 1300
Frequency, cm–1
1900 1800 1700 1600 1500 1400 1300 1200
Frequency, cm–1
Fig. 4. IR spectra of surface compounds resulting from the
Fig. 3. IR spectra of surface compounds resulting from the
interaction between NH and the catalyst Mn–Bi–O/α-
interaction between NH and the catalyst Mn–Bi–O/α-
3
3
Al O recorded while purging the sample with air at 300 K:
Al O recorded as the temperature rose during purging of
2
3
2
3
(1) initial spectrum and (2–4) spectra recorded after the
sample is purged for (2) 1, (3) 2, and (4) 5 min.
the sample with helium. T = (1) 300, (2) 333, (3) 363,
(4) 393, and (5) 413 K.
Examination of the wide-range photoelectron spectra
showed that, after ammonia adsorption and purging
with air, an intense N1s line appeared, which was virtu-
ally absent before the ammonia treatment. To estimate
the nitrogen concentration and elucidate the chemical
state of nitrogen, the highly resolved spectrum in the
N1s region was analyzed. Figure 5 presents a series of
N1s spectra obtained directly in the chamber of the
spectrometer analyzer during stepwise heating of the
sample. Spectrum 1 corresponds to the initial state of
the catalyst before the ammonia treatment. A broad,
low-intensity line near 398 eV is observed, which is
most probably due to the strongly bound species of
residual nitrogen that are localized on the lattice defects
of the active component after catalyst calcination at
833 K. After ammonia adsorption followed by purging
the sample with air, a broad asymmetrical line at
Eb(N1s) ≈ 399 eV appeared. When the sample was
heated above 400 K, the N1s line changed its shape and
its intensity decreased (Fig. 5, curves 2–9). The inten-
sity of the nitrogen line observed at the rather high tem-
perature of ~800 K (Fig. 5, curve 9) is higher than the
background intensity, indicating that part of the nitro-
gen stays on the surface under the above conditions.
with an increase in adsorption temperature from 300 to
353 K and were below the sensitivity limit at 393 K.
Purging the catalyst with helium at room tempera-
ture changed the concentration of surface compounds
only slightly. The increase in temperature during purg-
ing resulted in a gradual decrease in the intensity of the
absorption bands due to the surface compounds. These
bands disappeared completely at 393 K (Fig. 3). Purg-
ing with air for 5 min was sufficient to decrease the
intensities of the absorption bands due to the surface
compounds below the sensitivity limit of the IR tech-
nique (Fig. 4). During purging with air, the concentra-
tion of surface compounds decreases but the intensity
ratio of the absorption bands of [NH2] and [NH]
remains nearly unchanged. Note that the decrease in the
concentration of surface compounds in this experiment
was not accompanied by the appearance of reaction
products or NH3 in the gas phase. Hence, the decrease
in the concentration of the above surface compounds is
due not to their desorption to the gas phase but to their
further transformation into surface compounds that do
not give rise to any absorption bands in the IR region
available for measurements (4000–1300 cm–1).
Figure 1 shows the spectrum recorded after the
interaction of N2H4 with the catalyst surface at room
temperature (spectrum 3). The absorption bands at
1600 and 1560 cm–1 are due to the symmetrical vibra-
tions, whereas the absorption bands at 1386 and
1362 cm–1 are due to the asymmetrical vibrations of the
hydrazine NH2 group [28, 29].
The N/Mn surface concentration ratio calculated
from the intensity ratio of the N1s and Mn2p lines is
~0.5. While the sample was heated from room temper-
ature to 800 K, the relative nitrogen concentration
decreased by a factor of 3.5.
As mentioned above, the N1s line has a complicated
shape, suggesting that nitrogen is in different states. To
identify these states, we used a stepwise procedure to
deconvolve the spectra. At the first stage, the spectra
XPS Study of the Mn–Bi–O/α-Al2O3 Catalyst
To identify the surface compounds formed upon were normalized to the background of the spectrum of
ammonia adsorption, the catalyst was studied by XPS. the starting sample (Fig. 5, curve 1). Then the spectrum
KINETICS AND CATALYSIS Vol. 46 No. 4 2005