molecules

Article
Nitrous Oxide Adsorption and Decomposition on Zeolites and
Zeolite-like Materials
Leonid M. Kustov 1,2,3, *, Sergey F. Dunaev 1,2,3 and Alexander L. Kustov 1,2,3

1 Chemistry Department, Moscow State University, 1 Leninskie Gory, Bldg. 3, 119991 Moscow, Russia;
dunaev@general.chem.msu.ru (S.F.D.); kyst@list.ru (A.L.K.)
2 N.D. Zelinsky Institute of Organic Chemistry RAS, 47 Leninsky Prosp., 119991 Moscow, Russia
3 Institute of Ecotechnologies and Engineering, National University of Science and Technology MISiS,
4 Leninsky Prosp., 119049 Moscow, Russia
* Correspondence: lmkustov@mail.ru or lmk@ioc.ac.ru

Abstract: Decomposition of N2 O on modified zeolites, crystalline titanosilicalites, and related amor-

phous systems is studied by the catalytic and spectroscopic methods. Zinc-containing HZSM-5
zeolites and titanosilicalites with moderate Ti/Si ratios are shown to exhibit a better catalytic per-
formance in N2 O decomposition as compared with conventionally used Cu/HZSM-5 zeolites and
amorphous Cu-containing catalysts. Dehydroxylation of the HZSM-5 zeolite by calcination at 1120 K
results in an enhancement of the N2 O conversion. The mechanism of the reaction and the role of
coordinatively unsaturated cations and Lewis acid sites in N2 O decomposition are discussed on the
basis of the spectroscopic data.

Keywords: HZSM-5 zeolite; N2 O decomposition; titanosilicalites; Lewis acid sites; diffuse-reflectance

IR spectroscopy






Citation: Kustov, L.M.; Dunaev, S.F.; 1. Introduction
Kustov, A.L. Nitrous Oxide The problem of N2 O decomposition remains to gradually attract attention in view of
Adsorption and Decomposition on the development of green technologies. This problem is related to the synthesis of adipic
Zeolites and Zeolite-like Materials. acid, which yields N2 O as a side product, as well as NOx abatement in exhaust gases of
Molecules 2022, 27, 398. https://
power plants or waste anesthetic gas purification. Furthermore, the reaction of N2 O decom-
doi.org/10.3390/molecules27020398
position was shown to be the initial and key stage in the processes of selective oxidation
Academic Editor: Giuseppe Cirillo of aromatic compounds with N2 O under mild conditions using zeolites as catalysts [1–6].
It was shown that coordinatively unsaturated cations (iron species and framework Lewis
Received: 18 December 2021
acid sites) are responsible for the catalytic activity of dehydroxylated HZSM-5 zeolites both
Accepted: 6 January 2022
in N2 O decomposition and in the reactions of oxidation of various aromatic substrates
Published: 8 January 2022
using N2 O [3–5]. In the art, oxide systems are known as N2 O decomposition catalysts, with
Publisher’s Note: MDPI stays neutral amorphous copper oxide, for instance, Cu-Me/Al2 O3 , as well as cobalt oxide systems [7],
with regard to jurisdictional claims in magnesium cobaltite Mgx Co1−x Co2 O4 [8], CoOx -CeO2 [9] or Co-Ce spinel [10] as quite
published maps and institutional affil- active, although most of the Co-based catalysts, except for the Co-Ce spinel, demonstrate
iations. high conversion only at high temperatures (870–1070 K). Ceria-zirconia behaves nearly
similar to Co-oxide materials [11].
Other supported catalysts, such as rhodium on lanthanum silicate Rh/La10 Si6−x Fex O27−δ
or Pt/ZrO2 providing a 100% N2 O conversion at temperatures as high as 870 K [12,13] or
Copyright: © 2022 by the authors.
Pt, Ir, and Pd supported on Al2 O3 [14] have been also studied. However, the use of noble
Licensee MDPI, Basel, Switzerland.
This article is an open access article
metals seems to be an expensive way to N2 O abatement. Furthermore, carbon nanotubes
distributed under the terms and
were predicted by DFT calculations to catalyze this reaction [15].
conditions of the Creative Commons Among the most efficient catalysts used for N2 O decomposition, high-silica zeolites
Attribution (CC BY) license (https:// modified with iron [5,16], rhodium [17], copper [18], ruthenium [19], and mixed Co-In [20]
creativecommons.org/licenses/by/ ions were shown to demonstrate the best performance. The reported catalysts provide a
4.0/). complete conversion of nitrous oxide to nitrogen and oxygen at 620 K. Ru(0) nanoclusters

Molecules 2022, 27, 398. https://doi.org/10.3390/molecules27020398 https://www.mdpi.com/journal/molecules

Molecules 2022, 27, 398 2 of 10

prepared by the reduction of Ru(III) ions, as well as osmium(III) species were found to
be less active compared with ruthenium ions. The systems containing Fe, Cu, Co, and
Ru metal ions exhibited a much better catalytic performance in N2 O decomposition as
compared with other modified and non-modified zeolites [21], as well as other amorphous
oxide systems [22,23]. The main disadvantages revealed, for example, by the Cu-catalysts
for N2 O decomposition are their low thermal stability (they irreversibly lose the activity
after overheating to T > 870 K) and poor tolerance to admixtures of H2 O, CO, CO2 , and
hydrocarbons, which are present in real gas mixtures and act as poisons. The behavior
of catalysts definitely depends on the presence of water vapor in the feed, as well as
other residual components (NO, O2 , NO2 ) that may interfere with the N2 O decomposition
process [24]. However, in the mixture, we will limit our scope with the model conditions,
without the introduction of other potentially important ingredients.
The aim of this work was to find new zeolite and zeolite-like catalysts that are active
in N2 O decomposition and to study the nature of active sites and plausible reaction mecha-
nisms, with an emphasis on the role of coordinatively unsaturated cations. Three groups of
catalysts were chosen for the investigation:
− Dehydroxylated HZSM-5 zeolites and ZSM-5 zeolites modified with zinc oxide, which
have been studied earlier from the point of view of the nature and strength of Lewis
acid sites [25,26], i.e., the systems containing strong coordinatively unsaturated cations
(Lewis acid sites);
− Crystalline Ti-silicalites that are widely used as efficient catalysts for the selective
oxidation of phenol into diphenols by H2 O2 in the liquid phase [27];
− Amorphous catalysts, based on the Ti/SiO2 system, which differ in the Ti/Si ratio and
in the preparation method.
For comparative purposes, the well-known Cu-ZSM-5-type catalysts for N2 O decom-
position, as well as amorphous Cu-containing oxide systems were also studied.

2. Results and Discussion

To evaluate the relative strength of coordinatively unsaturated cations in the modified
zeolites under study and to rank the samples according to the relative concentration of
strong electron-acceptor centers, IR spectra of molecular hydrogen, as a probe for low-
coordinated cations [28], were measured.
Figure 1 shows the IR spectra of H2 adsorbed at 77 K on three representative samples
containing rather strong coordinatively unsaturated cations: (1) Dehydroxylated HZSM-5
zeolite, (2) Cu/HZSM-5, and (3) Zn/HZSM-5. The absorption bands in the region of
4100–4120 cm−1 correspond to weakly bonded H2 complexes with bridging Si(OH)Al
and terminal SiOH groups, respectively [28], whereas the bands below 4100 cm−1 are
shown [28] to belong to complexes of molecular hydrogen with coordinatively unsaturated
cations (or Lewis acid sites) that exhibit electron-accepting properties. The stronger the
interaction in the complex, i.e., the stronger the electron-acceptor center, the larger the shift
of the corresponding band of adsorbed H2 toward lower frequencies measured relative to
the frequency of the H-H stretching vibration in the gas phase (νH -H = 4163 cm−1 ) [28]. As
seen from the spectra shown in Figure 1, the strongest coordinatively unsaturated cations
are present in the Zn/ZSM-5 zeolite (νH-H = 3955, 4010 and 4070 cm−1 , for these bands
∆νH -H = 208, 153, and 93 cm−1 , respectively), whereas the weakest centers among the three
catalysts under consideration are revealed in the dehydroxylated HZSM-5 zeolite (νH -H
= 4010 and 4035 cm−1 , ∆νH -H = 153 and 128 cm−1 , respectively). The Cu/ZSM-5 zeolite,
which is the well-known active catalyst for N2 O decomposition, manifests an intermediate
strength of the electron-acceptor centers (νH -H = 3970 and 4060 cm−1 , ∆νH -H = 193 and
103 cm−1 , respectively). Of note, the concentration of the strongest electron-acceptor centers
is the highest for the Zn/HZSM-5 zeolite. Moreover, it is noteworthy that a further increase
in the loading of copper in the Cu/HZSM-5 zeolite from 1 to 3 wt%, as well as an increase in
the loading of zinc in the Zn/HZSM-5 zeolite over 5 wt%, have no appreciable effect on the
Molecules 2022, 27, x FOR PEER REVIEW 3 of 11

Molecules 2022, 27, 398 loading of zinc in the Zn/HZSM-5 zeolite over 5 wt%, have no appreciable effect on the3 of 10
spectral pattern, i.e., the concentration of strong Lewis acid sites. Furthermore, this in-
crease does not improve the catalytic performance of the Cu-zeolite and Zn-zeolite cata-
lysts.spectral pattern, i.e., the concentration of strong Lewis acid sites. Furthermore, this increase
does not improve the catalytic performance of the Cu-zeolite and Zn-zeolite catalysts.

Figure 1. IR spectra of H2 adsorbed at 77 K on: (1) Dehydroxylated HZSM-5 zeolite; (2) Cu/HZSM-5;
Figure(3)1.Zn/HZSM-5.
IR spectra of H2 adsorbed at 77 K on: (1) Dehydroxylated HZSM-5 zeolite; (2) Cu/HZSM-
5; (3) Zn/HZSM-5.
The pre-adsorption of a small amount of N2 O at 300 K on the zeolite samples,
The pre-adsorption
which precedes the of a small amount
adsorption of N2Oin
of H2 results at the
300 disappearance
K on the zeoliteofsamples, which
the low-frequency
precedes the adsorption
absorption of H2 results
bands attributed to thein the disappearance
H2 complexes withoflow-coordinated
the low-frequency absorp-How-
cations.
tion ever,
bandsthis
attributed
has no to the H2 complexes
considerable influence withonlow-coordinated
the intensity of cations. However, this
the high-frequency bands
has no considerable −
influence 1 on the intensity of the high-frequency
(νH -H = 4100–4120 cm ) assigned to the complexes with OH groups. This experiment bands (ν H - H = 4100–
4120shows
cm−1) assigned to the complexes
that adsorption of N2 O occurs with on OHthe groups. This experiment
low-coordinated metalshows
cationsthatthatad-are re-
sorption of N2O
sponsible foroccurs on the low-coordinated
the appearance of the corresponding metal cations that are
absorption responsible
bands in the IRfor the of
spectra
adsorbed
appearance of hydrogen.
the corresponding absorption bands in the IR spectra of adsorbed hydro-
gen. The adsorption of N2 O on the zeolite samples results in the appearance of the ab-
sorption bands at −1 (Figure 2). The frequency of gaseous N O is 2224 cm−1 .
The adsorption of2285–2230
N2O on thecm zeolite samples results in the appearance of 2the absorp-
−1 , which is close to the gas-phase value (physi-
tion bands at 2285–2230 cm (Figure 2). cm
Herein, we observe one band
−1 at 2230 The frequency of gaseous N2O is 2224 cm−1.
callyweadsorbed −1 due to complexes with zinc species
Herein, observeN 2 O)band
one and aatshifted
2230 cm band
−1 at 2285
, which is cm
close to the gas-phase value (physi-
cally(electron-acceptor
adsorbed N2O) and centers).
a shifted The largest
band at 2285 shiftcmof the
−1 dueNto2 Ocomplexes
band withwith respect
zinc to the corre-
species
sponding bandcenters).
(electron-acceptor positionThe for N 2 O molecules
largest shift of the in the gasband
N 2O phase is observed
with respect to forthe Zn/HZSM-5
corre-
zeolites (ν =position
2285 cm −1 , ∆ν = 50 cm−1 ), which indicates the strongest polarization and
sponding band for N 2O molecules in the gas phase is observed for Zn/HZSM-5
activation
zeolites (ν = 2285 of the
cm−1N 2O =
, Δν molecule
50 cm−1),by the electron-acceptor
which indicates the strongest sites of the Zn/HZSM-5
polarization and acti-zeolite.
vation of the N2O molecule by the electron-acceptor sites of the Zn/HZSM-5 zeolite. Ac- the
According to our previous spectroscopic data and quantum-chemical calculation [1,2],
N2 Otomolecule
cording our previousis preferably adsorbed
spectroscopic data onandthe quantum-chemical
Lewis acid center (for instance, [1,2],
calculation on trigonally
the
N2O coordinated
molecule is aluminum
preferably ions)
adsorbedby a two-point
on the Lewis mechanism,
acid center which
(for also involves
instance, a neighboring
on trigonally
oxygen atom
coordinated aluminumof theions)
surface
by acluster.
two-point In this case, adsorption
mechanism, which alsoof Ninvolves
2 O is accompanied
a neighbor- by a
ing oxygen atom of the surface cluster. In this case, adsorption of N2O is accompanieddecrease
considerable change of the geometry of the molecule, in particular, by a substantial by
of the NNO
a considerable angle of
change (from
the 180 to 140◦of
geometry ) and by a stronginpolarization
the molecule, particular, by of the N-O bond,de-
a substantial which
favors the further decomposition of the N O molecule with
crease of the NNO angle (from 180 to 140°) and2by a strong polarization of the N-O bond, the evolution of N 2 into the
gas phase and chemisorption of atomic oxygen [1,2]. Evidently,
which favors the further decomposition of the N2O molecule with the evolution of 2N2 into the extent of N O polariza-
tionphase
the gas and activation, and thus,ofthe
and chemisorption rate of
atomic decomposition
oxygen are governed
[1,2]. Evidently, the extent by of
theNstrength
2O po-
of
coordinatively unsaturated cations. Correspondingly, heating of the Zn/HZSM-5 zeolite
larization and activation, and thus, the rate of decomposition are governed by the strength
with pre-adsorbed N2 O at 520 K for 1 h directly in the IR cell (under static conditions)
of coordinatively unsaturated cations. Correspondingly, heating of the Zn/HZSM-5 zeo-
leads to the complete decomposition of N2 O, and the bands at 2285–2240 cm−1 vanish
lite with pre-adsorbed N2O at 520 K for 1 h directly in the IR cell (under static conditions)
from the spectrum, whereas the corresponding band at 2355 cm−1 appears due to the
leads to the complete decomposition of N2O, and the bands at 2285–2240 cm−1 vanish from
molecular nitrogen formed upon the N2 O decomposition. For comparison, heating of the
the spectrum, whereas the corresponding band at 2355 cm−1 appears due to the molecular
dehydroxylated HZSM-5 zeolite with pre-adsorbed N2 O at 520 K for 1 h results only in a
partial decomposition of N2 O, in accordance with a weaker strength of the low-coordinated
cations (Lewis acid sites).
 nitrogen formed upon the N2O decomposition. For comparison, heating of the dehydrox-
ylated HZSM-5 zeolite with pre-adsorbed N2O at 520 K for 1 h results only in a partial
decomposition of N2O, in accordance with a weaker strength of the low-coordinated cati-
Molecules 2022, 27, 398 ons (Lewis acid sites). 4 of 10

Figure 2. (1) IR spectra of N2 O adsorbed at 300 K on Zn/HZSM-5; (2) IR spectra upon heating the
Figure 2. (1) IR spectra
Zn/HZSM-5 ofat
zeolite N520
2O adsorbed at 300pre-adsorbed
K for 1 h with K on Zn/HZSM-5; (2) K).
N2 O (300 IR spectra upon heating the
Zn/HZSM-5 zeolite at 520 K for 1 h with pre-adsorbed N2O (300 K).
To reveal subtle distinctions in the properties of the modified zeolites related to N2 O
To reveal subtle we
decomposition, distinctions
tested theinsamples
the properties of the
in the flow modified
catalytic unitzeolites
at 620–900related to Nreaction
K. The 2O

decomposition,
conditions and we tested the samples
the conversion in the
degrees forflow
the catalytic unit at 620–900
N2 O decomposition on theK.modified
The reaction
HZSM-5
conditions
zeolitesandandthesome
conversion degrees for
Cu-containing the N2O decomposition
amorphous catalysts usedon fortheNO modified HZSM- are
x decomposition
5 zeolites and some
summarized inCu-containing amorphous
Table 1. In agreement with catalysts used for NOdata,
the spectroscopic x decomposition
the dehydroxylatedare
HZSM-5in
summarized zeolite
Tableexhibits a poor conversion
1. In agreement even at enhanced
with the spectroscopic data,temperatures
the dehydroxylated (720 K), and
HZSM-5the Zn-containing
zeolite exhibitszeolites
a poor reveal the best
conversion even performance.
at enhancedThese catalysts(720
temperatures are active
K), and at low
temperatures zeolites
the Zn-containing as 620 Kreveal
(the conversion of 85%), while
the best performance. Thesethe known are
catalysts Cu/HZSM-5
active at low system
exhibits aasconsiderable
temperatures inferior performance
620 K (the conversion of 85%), while (the the
conversion does not exceed
known Cu/HZSM-5 20%)
system ex-under
the same conditions. Of note, both samples of the amorphous
hibits a considerable inferior performance (the conversion does not exceed 20%) under the Cu-containing catalysts
sameshow a poorOf
conditions. performance as compared
note, both samples with the Zn-
of the amorphous and Cu-zeolites.
Cu-containing The show
catalysts presencea of
poor low-coordinated
performance as comparedmetal ionswith (zinctheorZn-copper) should clearly
and Cu-zeolites. be considered
The presence as the pre-
of low-coor-
requisite
dinated metal forionsthe efficient
(zinc N2 O decomposition.
or copper) should clearly be Therefore,
consideredtheasspectroscopic
the pre-requisite and catalytic
for
data indicate
the efficient that strong electron-acceptor
N2O decomposition. (low-coordinated)
Therefore, the spectroscopic metal ions,
and catalytic datawhich
indicateshould
that strong electron-acceptor (low-coordinated) metal ions, which should actually be con-metal
actually be considered as Lewis acid-base pair sites containing a low-coordinated
ionas
sidered and an oxygen
Lewis anion
acid-base pairofsites
the framework,
containing aare presumably themetal
low-coordinated activeioncenters
and an responsible
oxy-
for the N
gen anion of the 2 O decomposition on the modified zeolites. With the
framework, are presumably the active centers responsible for the Nanalogy from our previous
2O

decomposition on the modified zeolites. With the analogy from our previous studies [1,2],
studies and taking into account the results of quantum-chemical calculations and we
takingmay
intopropose
accountthe thefollowing
results ofmechanism
quantum-chemicalof N2 O decomposition
calculations [1,2],on westrongmaycoordinatively
propose
the following mechanism of N2O decomposition on strong coordinatively unsaturated5 and
Molecules 2022, 27, x FOR PEER unsaturated
REVIEW metal ions, which involves strong perturbation of the N 2 O molecule of 11
metal ions, which involves strong perturbation of the N2O molecule and further formation the
further formation of the chemisorbed oxygen atom. Here, the latter is consumed for
of therecombination
chemisorbed or scavenged
oxygen atom.by the second
Here, the latterN2 Ois molecule,
consumedyielding N2 and O2 (Scheme
for the recombination or 1):
scavenged by the second N2O molecule, yielding N2 and O2 (Scheme 1):

Scheme 1. Mechanism of N2 O decomposition on Lewis acid-base pair sites.

Scheme 1. Mechanism of N2O decomposition on Lewis acid-base pair sites.

Table 1. N2O decomposition (45 cm3 N2O/min).

Reaction TOF,
Catalyst N2O Conversion, %
Temperature, K mmol N2O g−1 s−1
Conventionally calcined
720 12 0.020
HZSM-5 (770 K)
Dehydroxylated HZSM-
720 55 0.046
5 (970 K)
3% Zn/HZSM-5 620 85 0.071
Molecules 2022, 27, 398 5 of 10

Table 1. N2 O decomposition (45 cm3 N2 O/min).

Reaction TOF,
Catalyst N2 O Conversion, %
Temperature, K mmol N2 O g−1 s−1
Conventionally calcined
720 12 0.020
HZSM-5 (770 K)
Dehydroxylated
720 55 0.046
HZSM-5 (970 K)
3% Zn/HZSM-5 620 85 0.071
645 90 0.075
660 100 0.084
5% Zn/HZSM-5 620 85 0.071
645 100 0.084
3% Cu/HZSM-5 620 20 0.017
645 55 0.046
660 85 0.071
670 100 0.084
Co/Cr/Cu/Al2 O3 645 45 0.038
670 80 0.067

The second group of catalysts studied in the reaction of N2 O decomposition comprised

crystalline Ti-silicalites with different Si/Ti ratios and amorphous TiO2 -SiO2 systems. The
Ti-silicalites have been chosen for the investigation in the title reaction, since they exhibit
unique catalytic properties in the reactions of selective oxidation of phenol into diphenols
using H2 O2 as an oxidizing agent [27,29]. The active centers responsible for these properties
of the Ti-silicalites are should be titanyl groups Ti=O or isolated tetrahedral Ti+4 ions [29].
Accordingly, the reaction is assumed to involve Ti-OOH fragments in the coordination
sphere of the isolated Ti+4 ions. Taking into account the fact that the N2 O molecule contains
labile oxygen, similar to the H2 O2 molecule, and with due regard to the similarity of
the reaction mechanisms for the selective oxidation with N2 O and H2 O2 , which include
activation and decomposition of the molecule of the oxidizing agent, we may assume that
the catalysts which are active in the reactions involving H2 O2 , i.e., Ti-silicalites, will be
active in the reaction of N2 O decomposition.
Table 2 presents the results of catalytic testing of various Ti systems in the reaction of
N2 O decomposition. The N2 O conversion for the crystalline Ti-silicalites of the TS-1 type
(four samples) passes through a maximum at Si/Ti = 32. Of note, the performance of the
crystalline Ti-silicalite with the Si/Ti ratio equal to 32 is higher than the known Cu/HZSM-
5 catalyst, especially at low temperatures (620–645 K). The dome-shaped dependence of
the N2 O decomposition rate for the Ti-silicalites versus the Si/Ti ratio may be accounted
for in the following way. Evidently, a decrease in the performance with the increasing
Si/Ti ratio from 32 to 38 results from a diminution of the concentration of active isolated
Ti+4 ions in the framework. A decrease in the conversion degree when the Ti content in
the samples increases (the Si/Ti ratio decreases to 20–15), may be equally explained by a
decrease in the concentration of the active isolated Ti+4 species as a result of the growth
of the concentration of the pair Ti+4 centers, which are likely inactive (or less active, as
compared with the isolated Ti+4 species). The concentration of the isolated Ti+4 species
may also decrease due to the formation of octahedral Ti+4 centers, in particular, extra-
framework octahedral species, for instance, in the form of anatase, which is inactive in the
N2 O decomposition, at least in the temperature range studied. The latter hypothesis is
consistent with the data presented by Bellussi et al. [30], who showed that the probability
of the formation of anatase (or in general, octahedral Ti+4 ions) during the synthesis of
Ti-silicalites of the TS-1 type drastically increases, when the Si/Ti ratio approaches 20 and
lower values.
 Molecules 2022, 27, 398 6 of 10

Table 2. Results of catalytic testing of various Ti-systems in the reaction of N2 O decomposition.

N2 O Flow Conversion TOF,

Catalyst Si/Ti T, K
Rate, cm3 /min of N2 O, % mmol N2 O g−1 s−1
TS-1 (I) 14.4 770 20 15 0.006
870 20 0.008
TS-1 (II) 22.9 770 20 10 0.004
870 50 0.019
TS-1 (III) 32.0 620 45 55 0.046
645 80 0.067
670 85 0.071
TS-1 (IV) 37.9 620 45 45 0.038
645 60 0.050
670 80 0.067
TiO2 -SiO2 17.0 720 45 10 0.008
820 50 0.042

For a comparison with the crystalline Ti-silicalites, we also tested the performance
of amorphous TiO2 -SiO2 samples. Here, it is known [31] that the amorphous Ti/SiO2 is
completely inactive in the reaction of selective oxidation of phenol with aqueous solutions
of H2 O2 . Moreover, these samples exhibited a very low conversion in the reaction of
N2 O decomposition. Furthermore, they revealed some N2 O conversion only at high
temperatures of 720–770 K, while the crystalline Ti-silicalites with a close Si/Ti ratio were
very active at 620 K.
To ascertain the coordination state of titanium ions in the crystalline and amorphous
Ti-systems, we used the XPS method. Figure 3 depicts the representative XP spectra of
two crystalline samples and one amorphous catalyst. The spectra contain a characteristic
line of Ti 3p3 /2 in the range of the binding energies of 460.0–458.7 eV. For the Ti-silicalite
with a moderate Si/Ti ratio (32), a sharp peak at 460.0 eV is observed, which is ascribed to
tetrahedrally coordinated Ti+4 ions, whereas for Ti-silicalites with lower Si/Ti ratios (22.9
Molecules 2022, 27, x FOR PEER REVIEW
and 14.4), a superposition of the peak at 460.0 eV with the second line with7the maximum
of 11
at 458.7 eV is revealed as a result of the presence of octahedrally coordinated Ti+4 ions. A
similar spectral pattern is observed for the amorphous TiO2 -SiO2 sample.

Figure 3. Representative XP spectra: (1) TS-1(I) (Si/Ti = 14.4); (2) TS-1 (II) (Si/Ti = 22.9); (3) TS-1(III)
Figure 3. (Si/Ti
Representative
= 32). XP spectra: (1) TS-1(I) (Si/Ti = 14.4); (2) TS-1 (II) (Si/Ti = 22.9); (3) TS-1(III)
(Si/Ti = 32).

With an analogy regarding the chemistry of processes based on H2O2, one may con-
sider two plausible reaction mechanisms for N2O decomposition on the Ti-silicalites
(Scheme 2):
Molecules 2022, 27, 398 7 of 10
Figure 3. Representative XP spectra: (1) TS-1(I) (Si/Ti = 14.4); (2) TS-1 (II) (Si/Ti = 22.9); (3) TS-1(III)
(Si/Ti = 32).

With
Withanananalogy
analogyregarding
regarding thethe
chemistry
chemistryof processes based
of processes on Hon
based 2O2H, one
2 O2 ,may
one con-
may
sider two two
consider plausible reaction
plausible mechanisms
reaction mechanisms forfor
N2NO2 O
decomposition
decompositionononthe
theTi-silicalites
Ti-silicalites
(Scheme
(Scheme2):
2):

Scheme 2. Mechanism of N2 O decomposition on Ti-centers.

Scheme 2. Mechanism
The of N2Oisdecomposition
first mechanism similar to theonone
Ti-centers.
proposed for the modified zeolites contain-
ing coordinatively unsaturated cations, such as Zn/HZSM-5, except for the fact that Ti+4
ionsThe firstinmechanism
are not the trigonalisbutsimilar
in thetotetragonal
the one proposed for the
coordination. modified
However, zeolites
taking intocontain-
account
ing
thecoordinatively
fact that (1) forunsaturated
Ti ions thecations,
+4 such ascoordination
characteristic Zn/HZSM-5,numbers except for arethe fact 6,
4 and that
andTi(2)
+4

ions
tetragonal +4
are notTiin the ionstrigonal but in the of
in the framework tetragonal coordination.
Ti-silicalites are capableHowever, takingadditional
of coordinating into ac-
count the fact
adsorbate that (1) for
molecules, weTimay+4 ions the characteristic coordination numbers are 4 and 6, and
consider the first mechanism as one of the possible ways of
(2)
N2tetragonal Ti+4 ionsThe
O transformation. in the framework ofXPS
aforementioned Ti-silicalites
data lendaresomecapable
supportof coordinating
for this mechanismaddi-
tional adsorbate molecules, we may consider the first mechanism
of N2 O decomposition. However, of note, the presence of five-coordinated titanium ions as one of the possible
ways
cannot ofbe N2excluded,
O transformation.
since the XPS The pattern
aforementioned
representsXPS data lend some
a superposition of at support
least two,for this
maybe
mechanism
three lines. of The N2five-coordinated
O decomposition.titanium However, ionsofare
note,
alsothe presence of five-coordinated
coordinatively unsaturated and
titanium
therefore, ions
cancannot be excluded,
take part since the
in the reaction. TheXPS pattern
second represents
mechanism a superposition
involves of at
a cyclic peroxo
complex,
least which also
two, maybe threeseems
lines.quite probable in viewtitanium
The five-coordinated of the dataions obtained
are alsofor the so-called
coordinatively
“reactive silica”
unsaturated and[32]. In anycan
therefore, case, discrimination
take betweenThe
part in the reaction. these two mechanisms
second mechanism shouldinvolves be
done in the future research, probably, with the help of labelled isotopes
a cyclic peroxo complex, which also seems quite probable in view of the data obtained for of oxygen.
Figure 4“reactive
the so-called displays the IR spectra
silica” [32]. Inofany twocase,
samples of the crystalline
discrimination between Ti-silicalites
these twomeasured
mecha-
after N
nisms 2 O adsorption
should be done at inroom temperature
the future research, and after heating
probably, with the
the sample
help ofwith pre-adsorbed
labelled isotopes
Noxygen.
of 2 O at 520 K directly in the IR cell under static conditions. Unlike Zn/HZSM-5 zeolites,
the adsorption
Figure 4 displaysof N2 Othe
doesIR not result
spectra of in
twoa considerable
samples of the polarization
crystalline and perturbation
Ti-silicalites meas- of
the molecule, and the band position for adsorbed N O (∆ν - = 2235–2225 cm −1 ) is very
ured after N2O adsorption at room temperature and2 after H heating
H the sample with pre-
close to the
adsorbed N2 Ophysically adsorbed
at 520 K directly in N 2 O.
the IRNevertheless,
cell under staticheating of the samples
conditions. at 570 K for
Unlike Zn/HZSM-5
1 h results in the complete (the sample with Si/A = 32) or considerable (the sample with
Si/Al = 14.4) disappearance of the N2 O absorption bands. Simultaneously, the bands of N2
at 2360–2340 cm−1 are formed, thereby indicating the decomposition of N2 O. These data
agree fairly well with the catalytic data presented in Table 2.
 K for 1 h results in the complete (the sample with Si/A = 32) or considerable (the sample
with Si/Al = 14.4) disappearance of the N2O absorption bands. Simultaneously, the bands
of N2 at 2360–2340 cm−1 are formed, thereby indicating the decomposition of N 2O. These
data agree fairly well with the catalytic data presented in Table 2.
Molecules 2022, 27, 398 8 of 10

Figure 4. IR spectra of N2 O adsorbed on TS-1 (Si/Ti = 14.4) (a) and TS-1 (Si/Ti = 32) (b) at 300 K (1)
Figure 4. IR spectra of N2O adsorbed on TS-1 (Si/Ti = 14.4) (a) and TS-1 (Si/Ti = 32) (b) at 300 K (1)
and upon heating the samples at 570 K for 1 h with pre-adsorbed N O (300 K) (2).
and upon heating the samples at 570 K for 1 h with pre-adsorbed N22O (300 K) (2).
3. Materials and Methods
3. Materials and Methods
The dehydroxylated HZSM-5 zeolite was prepared by calcination of the HZSM-5
The(Si/Al
zeolite dehydroxylated
= 20) in aHZSM-5 vacuum zeolite
at 1120was prepared
K for by calcinationcatalysts
2 h. Zn/HZSM-5 of the HZSM-5 ze-
with ZnO
olite (Si/Al = 20) in a vacuum at 1120 K for 2 h. Zn/HZSM-5
loadings of 1–5 wt% were synthesized by wet impregnation of the HZSM-5 zeolite with catalysts with ZnO loadings
of
a 11–5
M wt%
aqueousweresolution
synthesized by wet3 )impregnation
of Zn(NO of the HZSM-5 zeolite with a 1 M aque-
2 , with further drying at 390 K in air and successive
ous solution of Zn(NO ) , with further drying
calcination in air at 820 K for 4 h and 920 K for 4 h. Four
3 2 at 390 K insamples
air and ofsuccessive
crystallinecalcination in
Ti-silicalites
air at 820 K for 4 h and 920 K for 4 h. Four samples of crystalline
of the TS-1 type with Si/Ti ratios of 14.4, 22.9, 32.0, and 37.9 were prepared according toTi-silicalites of the TS-1
type with Si/Ti
the known ratios of
procedure [30].14.4,
The22.9, 32.0, and 37.9
Cu/HZSM-5 were
zeolite prepared
with 3 wt% Cu,according
which to the known
corresponded
procedure
to the maximum [30]. The Cu/HZSM-5
conversion zeolite
on the zeolitewithin3Nwt%
2 O Cu, which
decomposition, corresponded
was to
prepared theby
max-
wet
imum conversion
impregnation onHZSM-5
of the the zeolite in Nsimilar
zeolite 2O decomposition,
to the Zn/HZSM-5was prepared
samples.byThe
wetcrystallinity
impregna-
tion
of theofzeolites
the HZSM-5 zeolite similar
and Ti-silicalites under tostudy,
the Zn/HZSM-5
monitored by samples.
XRD, was Theclose
crystallinity
to 95–100%.of the
zeolites and Ti-silicalites under study, monitored by XRD, was
Samples of amorphous Ti/SiO2 catalysts were prepared by the method of chemical close to 95–100%.
vapor Samples
depositionof amorphous
(CVD) using Ti/SiO catalysts
TiCl42 and were of
the sample prepared
silica gelbywith
the successive
method ofhydrolysis
chemical
vapor deposition (CVD)
or by coprecipitation using
of TiO TiClSiO
2 and 4 and the sample
2 . The resulting ofTi/SiO
silica gel with successive
2 catalysts hydrolysis
were characterized
byby
or TiO 2 loadings of 0.5–80
coprecipitation of TiO wt%.
2 and The
SiOCo-Cr-Cu/Al
2. The resulting 2 O3Ti/SiO
catalyst for N2 Owere
2 catalysts decomposition
characterizedwas
prepared by the co-precipitation of equimolar amounts of Co, Cr, and Cu from their nitrate
precursors, in the presence of γ-Al2 O3 (surface area, 170 m2 /g) with further calcination at
770 K for 2 h.
In this paper, the structure of all the studied zeolite samples, including the starting
HZSM-5, dehydroxylated HZSM-5, Zn/HZSM-5, Cu/HZSM-5, and TS-1 samples with any
Si/Ti ratio, present the same MFI type, as determined by XRD.
Prior to the catalytic tests, all of the samples were activated at 770 K for 4 h in an air
flow. The catalytic reaction of N2 O decomposition was studied in a flow setup at 620–900 K
and an N2 O + He (1:1) flow rate of 20–60 mL/min. The sample loading was 0.2 g. The
catalyst (0.5–1 mm particle size) was diluted with quartz (1:1). The reaction products and
Molecules 2022, 27, 398 9 of 10

unreacted N2 O were analyzed by gas chromatography (a Krystalux chromatograph) using a

1-m Porapak Q column. The only products of N2 O decomposition were N2 and O2 . Diffuse-
reflectance IR spectra were measured in the range of 4000–8000 and 2000–4000 cm−1 with
a Beckman Acta-M-VII; and Perkin-Elmer 580 B spectrophotometer, respectively, according
to the reported procedures [28]. Molecular hydrogen adsorbed at 77 K and a pressure of
30 Torr were used as a probe for coordinatively unsaturated cations [28,29]. Nitrous oxide
was adsorbed on the samples at 300 K and a pressure of 10–30 Torr.
XP spectra were measured with a XSAM-800 spectrometer using the MgKa excitation.
The C1S line at 285.0 eV was used as a reference.

4. Conclusions
In conclusion, the obtained catalytic and spectroscopic data allow the arrangement
of the systems under study in the following sequence, and according to their perfor-
mance in N2 O decomposition: Zn/HZSM-5 > TS-1 (III) > Cu/HZSM-5 > TS-1 (IV) > (Co,
Fe)/Cr/Cu/Al2 O3 > HZSM-5 > TS-1 (I), TS-1 (II), TiO2 -SiO2 . In addition, from these data,
two new catalytic compositions, i.e., ZnO/HZSM-5 and Ti-silicalite, with a moderate Si/Ti
ratio, exhibit a better performance in the reaction of N2 O decomposition, as compared with
the conventional Cu-containing zeolite and oxide catalysts. The key role played by coordi-
natively unsaturated Zn, Cu or Ti ions, as non-framework (Zn, Cu) or framework ions in
the N2 O decomposition has been revealed. Even the dehydroxylated HZSM-5 zeolite, con-
taining rather strong Lewis acid sites (but still weaker than those in Zn/HZSM-5 catalysts),
is more active in the reaction of N2 O decomposition compared with the conventionally
calcined HZSM-5 zeolite, which contains predominantly Bronsted acid sites. Furthermore,
a considerably high N2 O conversion reaching 85% is observed for the most active catalysts
(Zn/HZSM-5) under rather mild reaction conditions (T = 620 K).

Author Contributions: Conceptualization, L.M.K. and S.F.D.; methodology, A.L.K.; investigation,

A.L.K.; writing—original draft preparation, L.M.K.; writing—review and editing, L.M.K. All authors
have read and agreed to the published version of the manuscript.
Funding: This work was supported by the Institute of Organic Chemistry of the Russian Academy of
Sciences (Program of supporting scientific schools).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the modified ZSM-5 zeolites and titanosilicates are available from
the authors.

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