Cu, Fe and Mn oxides intercalated SiO2 pillared magadiite and ilerite catalysts for NO decomposition

Article abstract of DOI:10.1016/j.apcata.2021.118100

Synthesized magadiite and ilerite samples were pillared with SiO₂ and then intercalated with Cu, Fe and Mn oxides to utilize for direct NO decomposition between 400 and 600 °C. Cu-SiO₂-pil-ile and Cu-SiO₂-pil-mag catalysts exhibited high NO decomposition activity compared to Fe and Mn oxide intercalated catalysts. Remarkably, Cu-SiO₂-pil-ile offered 90 % NO conversion and 83 % N₂ selectivity at 600 °C. Elemental analysis, XRD, FESEM, DR UV-vis, Raman spectroscopy, N₂-adsorption, H₂-TPR, O₂-TPD and XPS were utilized to study physicochemical characteristics of the materials. The results from XRD and N₂ adsorption demonstrated that the samples possessed different pore structures from SiO₂-pillared silicates, due to different nature of metal oxides. The Cu-SiO₂-pil-ile and Cu-SiO₂-pil-mag samples possess a smaller number of Lewis and Br?nsted acid sites compared with Fe and Mn oxide intercalated samples. Presence of Cu²⁺/Cu⁺ and Fe³⁺/Fe²⁺, and synergism between redox centers are major reason for superior performance in NO decomposition. Therefore, the impact of redox properties and NO adsorption on the surface of the catalyst are significant.

Full text of DOI:10.1016/j.apcata.2021.118100

Applied Catalysis A, General 616 (2021) 118100
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Cu, Fe and Mn oxides intercalated SiO2 pillared magadiite and ilerite
catalysts for NO decomposition
,
,
,b,
Katabathini Narasimharaoa *, Islam Hamdy Abd El Maksoda b, Mohamed Mokhtara
*
a Surface Chemistry and Catalytic Studies (SCCS) Group, Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah, 21589,
Saudi Arabia
b Physical Chemistry Department, National Research Centre, Dokki, Cairo, 12622, Egypt
A R T I C L E I N F O
A B S T R A C T
Keywords:
Synthesized magadiite and ilerite samples were pillared with SiO2 and then intercalated with Cu, Fe and Mn
oxides to utilize for direct NO decomposition between 400 and 600 ◦C. Cu-SiO2-pil-ile and Cu-SiO2-pil-mag
catalysts exhibited high NO decomposition activity compared to Fe and Mn oxide intercalated catalysts.
Remarkably, Cu-SiO2-pil-ile offered 90 % NO conversion and 83 % N2 selectivity at 600 ◦C. Elemental analysis,
XRD, FESEM, DR UV-vis, Raman spectroscopy, N2-adsorption, H2-TPR, O2-TPD and XPS were utilized to study
physicochemical characteristics of the materials. The results from XRD and N2 adsorption demonstrated that the
samples possessed different pore structures from SiO2-pillared silicates, due to different nature of metal oxides.
The Cu-SiO2-pil-ile and Cu-SiO2-pil-mag samples possess a smaller number of Lewis and Brønsted acid sites
compared with Fe and Mn oxide intercalated samples. Presence of Cu2+/Cu+ and Fe3+/Fe2+, and synergism
between redox centers are major reason for superior performance in NO decomposition. Therefore, the impact of
redox properties and NO adsorption on the surface of the catalyst are significant.
SiO2 pillared silicates
Magadiite
Ilerite
Cu, Fe, Mn oxides
NO decomposition
1. Introduction
including noble metals [7], metal oxides [8] and ion exchanged zeolites
[9]. Noble metals such as Rh and Pd were proved as an active species in
Some of the emission gases such as nitrogen monoxide (NO), nitro-
gen dioxide (NO2) sulfur dioxide (SO2) and carbon monoxide (CO) are
considered as indirect greenhouse gases [1], while carbon dioxide
(CO2), nitrous oxide (N2O), and methane (CH4) are designated as the
direct greenhouse gases [2]. The direct NO decomposition is the best
suitable approach for NO removal due to its simplicity [3]. The direct
NO decomposition is thermodynamically favorable at temperatures
below 1700 ◦C, however the rate of the reaction is low [4]. The catalytic
decomposition of NO is well studied due to the fact that it is recognized
as the suitable process for NO elimination from exhaust streams [5]. The
two major issues those need to be addressed for development of NO
decomposition catalyst are catalyst activity and durability [4,5]. The
design and development of an effective heterogeneous catalyst for direct
NO decomposition would eliminate the utilization of different types of
reducing agents. This process could greatly help to simplify the process
of elimination of NO and decreases the cost of NO control in different
industrial sectors [6].
three-way catalytic converters; these catalytic systems exhibited supe-
rior performance for the NO decomposition [1]. However, the catalysts
containing noble metals are expensive and unstable at high temperature
as well as the catalysts which contained noble metals deactivate quickly
in the presence of oxygen [10]. Transition metal oxides (e.g. Co, Cu, Fe
and Mn oxides) based catalysts were also exhibited better catalytic ac-
tivities for NO decomposition due to their superior redox behavior [11].
Utilization of the transition metal oxides is highly explored due to their
high abundance in nature and inexpensive. It was also observed that the
transition metal ion exchanged mesoporous zeolites, primarily
Cu-exchanged ZSM-5 materials were good catalysts for NO decomposi-
tion [12]. However, several tests with the transition metals contained
zeolite catalysts had demonstrated that zeolites inefficiently catalyze NO
decomposition [13]; the catalysts were oxidized when the reaction
progresses and finally the NO decomposition was inhibited. Moreover,
the zeolite materials are suffering from a major limitation that they are
not hydrothermally stable [14].
A variety of catalysts were utilized for the decomposition of NO gas,
Interestingly, in the layered silicates, the distance between silica
* Corresponding authors at: Surface Chemistry and Catalytic Studies (SCCS) Group, Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O.
Box 80203, Jeddah, 21589, Saudi Arabia.
E-mail addresses: nkatabathini@kau.edu.sa (N. Katabathini), mmoustafa@kau.edu.sa (M. Mokhtar).
https://doi.org/10.1016/j.apcata.2021.118100
Received 18 November 2020; Received in revised form 7 March 2021; Accepted 10 March 2021
Available online 15 March 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
layers can be adjusted by swelling and pillaring, thereby enabling the
large molecules to diffuse to reach active centers located in between the
layers; this unique property provides layered silicates a significant
advantage over the microporous zeolites [15]. The synthesis of pillared
layered silicates expanded considerably due to the scope of their utili-
zation as heterogeneous catalysts for different chemical reactions [16].
It was observed that layered materials pillared with inorganic oxides
possessed meso/macro pore structure, high thermal stability and high
surface area as similar as meso porous zeolites [17]. Synthetic layered
autoclave without aging. The synthesis temperature was set as 100 ◦C
and the crystallization was carried out for 21 days under static
conditions.
2.2. Post-synthetic modifications
2.2.1. Swelling of silicates with hexadecyltrimethylammonium bromide
(CTAB)
In the first step of post-modification, the layered silicates (magadiite
and ilerite) were swollen with quaternary ammonium surfactant, CTAB
(C19H42BrN) at 40 ◦C under high pH conditions (pH~14). Typically,
9.0 g of aqueous clay slurry (20 wt. % solid) was added to 35.0 g of an
aqueous solution of 29 wt. % of CTAB and 11.0 g of an aqueous solution
of 40 wt. % tetrapropyl ammonium hydroxide (TPAOH). The mixture
was then stirred for 24 h at 40 ◦C.
silicates such as magadiite (Na2Si14O29.xH2O) and ilerite (Na2Si8O17
.
xH2O) are considered to be useful hosts for many pillared materials [18].
These materials possessed similar characteristics as natural clay mate-
rials and the interlayer spaces could expanded simply by intercalation of
guest molecules in the layers. Several years ago, Landis et al. [19] syn-
thesized high surface area contained SiO2-pillared magadiite (around
530 m2 gꢀ 1) by reacting the tetraethyl orthosilicate with octylamine.
Later, Kosuge and Tsunashima [20] reported the preparation of very
high surface area (1000 m2 gꢀ 1) containing silica-pillared ilerite by
intercalation of octylamine and tetraethyl orthosilicate into the layered
ilerite. In the past, Schwieger et al. synthesized Pt ion exchanged ilerite
silicate using Pt(NH3)24+ as Pt precursor and observed that Pt loaded
ilerite materials exhibited bifunctional character [21,22]. Schwieger
et al. also synthesized Pt loaded magadiite materials and observed that
the structure of the silica layers was not affected even after 22.2 wt% Pt
loading [23]. Kim et al. [24] prepared and characterized metal (Ti, Fe,
Zr) oxide pillared layered ilerite samples. Introduction of transition
metals such as Cu, Fe and Mn could generate redox active sites, which
enhance the activity of layered silicate materials for redox type reactions
[25]. Ahn et al. [26] synthesized Cu, Zn - pillared ilerite materials and
used as catalysts for direct synthesis of dimethyl ether from synthesis
gas. Ozawa et al. [27] synthesized well dispersed Ag and ZnO magadiite
nanocomposites with size between 3ꢀ 5 nm. However, synthesis and
application of manganese oxide-pillared layered silicates were not re-
ported well in the literature [28].
2.2.2. Pillaring with SiO2
The materials after swelling procedure were recovered by repetitive
cycles of centrifugation and washed with deionized water, then dried at
75 ◦C for 12 h. Typically, 1.0 g of swollen silicate was mixed with 5.0 g
of tetraethyl orthosilicate and stirred for at 80 ◦C for 24 h. Finally, the
impregnated product was filtered and dried at 30 ◦C for 12 h to remove
the excess silica from the solid surface. Then, a known amount (1.0 g) of
dried impregnated solid was hydrolyzed with 10 g of water for at 40 ◦C
for 5 h. Then, the hydrolyzed product was filtrated and washed with
deionized water. Finally, the filtered cake was dried at 75 ◦C for 12 h
and calcined at 600 ◦C for 5 h under air flow (120 mL minꢀ 1) in a muffle
furnace with a heating rate of 1.0 ◦C minꢀ 1
.
2.3. Intercalation of pillared silicates with Cu, Fe and Mn oxides
To synthesize the catalytically active pillared layered silicates,
transition metal (Cu, Fe and Mn, 15 wt.%) oxides were intercalated with
pillared layered silicates. Typically, calculated amount of metal oxide
precursor [Cu(NO3)2⋅3H2O or Fe(NO3)3⋅9H2O or Mn(NO3)2.4H2O] was
dissolved in deionized water and the pillared layered silicate containing
the same volume was added under constant stirring. Then, the excess
water was removed in vacuum oven and the dried powders were ther-
mally treated at 600 ◦C for 4 h in the air under static conditions to obtain
the final catalysts.
Ding et al. [29] reviewed the catalytic activities of different pillared
silicates for the selective catalytic reduction (SCR) of NO with NH3. It
was observed that the pillared layered silicate catalysts were stable in
presence of SO2 and H2O and exhibited higher activity compared to
conventional vanadium-titanium oxide catalysts. Although, the
NH3-SCR and direct NOx decomposition processes follow different re-
action mechanisms, the transition metal oxide contained pillared
layered silicates could be better catalysts for direct NO decomposition
[30]. In the present work, magadiite and ilerite were first pillared with
SiO2 and then intercalated with Cu, Fe and Mn oxides. The synthesized
materials were utilized for direct NO decomposition and also analyzed
by different characterization techniques to study their physico-chemical
properties. The role of structural and electronic properties of the ma-
terials with NO decomposition activity was established.
2.4. Characterization of materials
The chemical composition (atomic ratios of elements) of the samples
was measured by using Perkin-Elmer ICP-AES instrument. The XRD
patterns of the samples were recorded using Bruker D8 Advance X-ray
diffractometer using Ni filtered Cu Kα radiation (λ = 1.5405 Å). The FE-
SEM analysis of the samples was carried out using JEOL Model JSM-
6390LV microscope. The N2 physisorption measurements were carried
out using Quantachrome Autosorb ASiQ instrument at liquid nitrogen
temperature. The DR UV-vis analysis of the samples was performing
using Thermo-Scientific evolution spectrophotometer in the wavelength
range of 220ꢀ 1000 nm. The Raman spectra of the synthesized samples
were obtained using Bruker Equinox 55 FT-IR spectrometer. The XPS
spectra of samples were obtained using a SPECS GmbH X-ray photo-
electron spectrometer. The H2-TPR and O2-TPD analyses of the synthe-
sized samples were carried out using Quantachrome CHEMBET-3000
instrument [32]. DRIFT spectra of pyridine adsorbed samples were ob-
tained to determine the acidic properties of the synthesized samples. The
detailed experimental procedures are provided in the electronic sup-
plementary information (ESI).
2. Experimental
2.1. Synthesis of magadiite and ilerite materials
The synthesis procedure of Na-magadiite is described as reported in
the literature [23]. The synthesis solution was prepared by mixing the Si
and Na precursors containing known amount of water (a weak alkaline
and SiO2-rich ternary system with SiO2:Na2O:H2O molar ratio of
5:1:75). Afterwards, Na2CO3 was added in small portions, until a dense
gel was formed (pH~12.5). A hydrothermal method at 140 ◦C for 48 h
was used to crystallize the Na-magadiite sample. Then the obtained solid
sample was filtered and washed and dried at 110 ◦C for 12 h. The
preparation procedure to synthesize sodium form of ilerite is similar
¨
same as that described by Karger at al [31]. Na-ilerite was synthesized
2.5. Catalytic NO decomposition
using colloidal silica (Ludox AS-40), sodium hydroxide pellets and
deionized water as starting materials for the preparation of the synthesis
gel. The obtained synthesis gel was directly transferred into the
Catalytic direct NO decomposition reactions over synthesized cata-
lysts were performed using a quartz micro reactor (id of 80 mm) at
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N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
atmospheric pressure. Calculated amount (0.1 g) of sieved catalyst
particles loaded into the quartz reactor. The reactor consisted of a quartz
presence of magadiite phase in the sample. The 2θ positions for char-
acteristic reflections for Na-ilerite structure are clearly observed at 8.0◦,
18.6◦, 21.8◦, 25.6◦, 29.2◦ in the X-ray pattern for as-synthesized Na-i-
lerite sample and these findings are in excellent agreement with cited
literature [31]. No additional reflections for any other phases were
detected in the XRD patterns of Na-magadiite and Na-ilerite samples
indicating the effective synthesis of the layered silicates.
frit support and the catalyst particles (grain size of 60ꢀ 80 μm, volume of
the catalyst bed is 0.5 cm3) was placed on the frit, and covered with a
layer of quartz wool and the catalyst was thermally treated at 600 ◦C in
flow of helium gas before start of the reaction. The catalytic performance
was evaluated using 0.1 vol.% NO balanced by helium gas (total flow of
the gas is 200 mL minꢀ 1) at temperature range of 400ꢀ 600 ◦C. The NO
concentration was continuously monitored by a chemiluminescent NO/
NOx analyzer (Thermo Electron Corporation, model 10). A Shimadzu GC
model 14A was used to analyze the concentrations of N2 and N2O in the
product stream. The analytical system had two capillary columns: a
Poropak-Q column used to analyze N2O and a molecular sieve SA used
to analyze N2, O2, and NO. The NO conversion, N2 yield and rate of N2
formation were evaluated with every 50 ◦C increment. The reaction was
carried out at each temperature for at least 30 min. to allow reaction to
reach steady state. The NO conversion and N2 selectivity were calculated
using the following equations.
Presence of (00 L) reflection corresponding to the basal spacing,
d(001) after swelling and pillaring of layered silicates with CTAB and SiO2
can be clearly observed (electronic supplementary information, ESI).
The basal spacing corresponding to the d(001) reflection was calculated
for both the samples and it was observed that the SiO2 pillared magadiite
and ilerite samples possessed basal spacing of 1.54 nm and 1.11 nm
respectively and these values are in accordance with the previously re-
ported results [34]. Fig. 1 (A) & (B) shows the XRD patterns for the Cu,
Fe and Mn oxides intercalated SiO2-pillared magadiite (A), ilerite (B)
samples. These samples show dissimilar XRD patterns from
Na-magadiite and Na-ilerite samples due to SiO2 pillaring and interca-
lation of metal oxide between the layers of silicates. The XRD patterns of
the samples are clearly exhibiting the presence of broad reflection in the
range of 2θ = 15ꢀ 33◦, which could be attributed to the amorphous SiO2
as it forms as a pillar between the interlayers of silicates. The pillared
structures are found to be created by the intercalation of SiO2 between
the silicate layers for each calcined product [35]. It was reported that
transition metal oxides could form due to the decomposition of metal
hydroxide, which formed by metal ions reacting with the ꢀ OH groups
intercalated between the layers [36]. There is another clear possibility
for the reaction between the SiO2 and metal precursors to form amor-
phous metal silicates.
NO conversion (%) = {[NO]inlet - [NO]outlet / [NO]inlet} x (100)
(1)
N2 selectivity (%) = {[NO]inlet-[NO]outlet-[NO2]outlet-[N2O]outlet/[NO]inlet
-
[NO]outlet}x(100)
(2)
[NO]inlet = concentration of NO in the inlet, [NO]outlet = concentration
of NO in the outlet, [N2O]outlet = concentration of N2O in the outlet,
[NO2]outlet = concentration of NO2 in the outlet
3. Results and discussion
The powder X-ray diffraction patterns for as-synthesized Na-mag-
adiite and Na-ilerite samples are shown in Fig. S1 (ESI). The Na-
magadiite sample displayed characteristic reflections of well-defined
magadiite at 2θ = 5.7◦, 11.3◦, 17.11◦ corresponding to the (001),
(002), and (003) crystalline planes, respectively as per JCPDS 42–1350
[33]. The reflections found between 23◦ and 30◦ are revealing the
The XRD patterns of Cu oxide intercalated magadiite and ilerite
samples exhibited sharp reflections at 35.4◦, 38.4◦ and 48.9◦ due to
(110), (002) and (111) planes of the monoclinic phase of CuO (crystal-
lite size of 20 nm) [JCPDS#45ꢀ 0937]. Lim et al. [37] observed the
appearance of reflections due to copper hydroxide phase in case of Cu
oxide intercalated magadiite sample. The Fe intercalated magadiite and
Fig. 1. Powder XRD patterns of Cu, Fe and Mn oxides intercalated SiO2-pillared (A) magadiite (B) ilerite samples.
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N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
ilerite samples also showed the reflections due to presence of pure
on the external surface of the SiO2 pillared ilerite. Interestingly, the XRD
patterns of Mn oxide intercalated magadiite and ilerite samples exhibits
low intense reflections at 32.95◦ and 38.23◦ corresponding to (222) and
α
-Fe2O3 phase [JCPDS#24ꢀ 0072] with rhombohedral structure. The
presence of highly intense reflections due to CuO and
an evidence for the formation of aggregated CuO and
α
-Fe2O3 phases is
-Fe2O3 particles
α
(440) planes of α-Mn2O3 [JCPDS 41–1442]. Presence of low intense
Fig. 2. FE-SEM images of Cu, Fe and Mn oxides intercalated SiO2-pillared magadiite and ilerite samples.
4

N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
reflections could be due to the fact that the formed
α
-Mn2O3 was in a
existence of the magadiite phase, were detected. This sample consisted
of agglomerates of rosette-shaped crystals with amorphous material
present on their surface, which correlates very well with the XRD results.
The DR UV-vis absorption spectra of synthesized samples are shown
in Fig. 3. It is clear from the figure that all SiO2 pillared magadiite and
ilerite samples displayed absorption band centered at 247 nm. Chen
et al. [41] reported that the layered silicate sample exhibits a large ab-
sorption band in the range of 200ꢀ 300 nm with a shoulder around
250 nm attributed to silica wavelength region. As anticipated, the metal
oxide intercalated SiO2 pillared magadiite and ilerite samples exhibited
additional UV-vis absorption bands. The Cu-SiO2-pillar-mag and
Cu-SiO2-pillar-ile samples showed a broad band in between 400 nm and
850 nm, which could be attributed to d-d transition of Cu2+ species [42].
The appearance of pronounced asymmetric peak shape suggests that the
formed copper oxide have different size distribution broad peak which
could be attributed to the presences of copper oxide nanoparticles [43].
The DR UV-vis spectra for Fe-SiO2-pillar-mag and Fe-SiO2-pillar-ile
samples shows absorption peaks due to presence of different Fe (III)
species. It is known that O-Fe charge transfer transitions generally ap-
pears at 240ꢀ 300 nm and peaks in the region of 400ꢀ 600 nm could be
ascribed to the d-d electron pair transition in isolated Fe(III) species. The
peaks appears beyond 500 nm could be attributed to large iron oxide
particles [44]. The Fe-SiO2-pillar-mag and Fe-SiO2-pillar-ile samples
show broad absorption peak at the range of 400ꢀ 600 nm. It can be
observed that the peak maximum for magadiite is at wavelength of
460 nm. The center of absorbance peak for Fe-SiO2-pillar-ile sample was
at the wavelength around 520 nm. It was previously reported that the
absorbance peak shifts towards a higher wavelength with increase in the
particle size [45]. The peak identified around 900 nm in both the sam-
small size, and any bulk aggregation was not occurred. In this study, Cu,
Fe and Mn oxides intercalated SiO2 pillared magadiite and ilerite sam-
ples were calcined at 550 ◦C and the samples still exhibited the ordered
basal spacing, proving that the expanded galleries did not collapsed after
removal of CTAB. This observation revealing that the synthesis pro-
cedure used in this study are suitable for the formation of strong metal
oxide pillars. It is well known that the 2θ values of d
reflections
(001)
reveal the extent of layer expansion due to metal oxide pillaring, and the
expansion levels are different with type of pillared oxide. The XRD
analysis results are also indicating that Cu, Fe and Mn oxides interca-
lated SiO2 pillared magadiite and ilerite samples shows increase in the
basal spacing. The values are higher (1ꢀ 2 nm) compared to SiO2 pil-
lared magadiite and ilerite samples reported in the literature [35]. This
behavior is probably due to the presence of large hydrated Cu, Fe and
Mn oxides, confirming the intercalation of the metal oxides between the
silicate layers.
Fig. 2 shows FE-SEM images for all the synthesized samples including
Cu, Fe and Mn oxides intercalated SiO2 pillared ilerite and magadiite
samples. It was previously reported that natural magadiite sample pos-
sesses thin sheet-like morphology, however, synthetic magadiite sam-
ples normally exists in rosette-like shaped crystals [38]. Contrary to
synthesized magadiite morphology, the synthetic ilerite crystals shows
plate-like morphology [39]. It is clear from the FE-SEM images that
Na-magadiite and Na-ilerite samples exhibited morphology as previ-
ously reported. It is clear that thin rectangular plate shaped Na-ilerite
crystallites were preserved even after swelling with CTAB, SiO2 pillar-
ing and calcination at 600 ◦C. The crystallites of the Cu, Fe and Mn
intercalated SiO2-pillared ilerite samples exhibited
a
similar
morphology as as the Na-ilerite shape. Metal oxide modified SiO2 pil-
lared ilerite samples consisted of relatively large agglomerates of
irregular and rectangular plate-shape crystals typical for the ilerite
phase.
ples is due to the quantum size effect of α-Fe2O3 nanoparticle structure
[46] and the size of the iron oxide particles were more in case of ilerite
compared to magadiite. The DR UV-vis absorption spectra of
Mn-SiO2-pillar-mag and Mn-SiO2-pillar-ile samples exhibited broad ab-
sorption peaks at 390 and 400 nm, respectively. It was previously re-
ported that layer structured manganese oxide materials exhibit UV-vis
absorption peak centered on 380 nm [47]. The broad band at 400 nm
with a tail extending to 900 nm in both the samples, this is possibly due
to the presence of isolated Mn oxide clusters on the external surface of
the layered silicates. A similar observation was previously reported in
case of Mn exchanged zeolites [48].
The typical ilerite thin plate shape structure was still preserved after
metal oxide intercalation as shown in SEM images. The SiO2 pillared
magadiite sample exhibited small separated rosette-shaped islands with
quite large interparticle spaces. The diameter for “flower” and “petals”
were reduced to about 0.5–1.0 μm. The observed suppression in particle
size and creation of interparticle spaces could enhance the surface area
and the mesoporous nature of pillared samples. The subsequent
impregnation of Cu and Fe oxides over SiO2 pillared magadiite led to
change of morphology to sheet like crystals, with sharp edges fairly
defined [40]. Interestingly, the Mn modified SiO2 pillared magadiite
sample showed a similar morphology as the SiO2 pillared magadiite
sample. Only small amounts of rosettes-shaped crystals, confirming
The XPS measurements were used to evaluate the elemental
composition and oxidation states of metals presented in the synthesized
catalysts. The deconvoluted XP spectra of the synthesized samples are
shown in Fig. 4. It is clear from the figure that Si2p XPS peaks could be
deconvoluted into two different peaks at binding energies (BE) of
Fig. 3. DR UV-vis spectra of Cu, Fe and Mn oxides intercalated SiO2-pillared (A) magadiite and (B) ilerite samples.
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N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
Fig. 4. The deconvoluted XPS spectra of Cu, Fe and Mn oxides intercalated SiO2-pillared ilerite and magadiite samples.
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N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
103.3 eV and 102.2 eV in all the ilerite samples. It was previously re-
ported that the Si2p peak at 103.3 eV could be attributed to Si in the
tetrahedral sites of layer silicate [49] and this peak shifts to lower BE
value, when Si atoms are bonded with another metal atoms [50].
Therefore, the minor Si2p peak appeared at 102.2 eV could be assigned
to the Si-O-M (M = Cu, Fe and Mn) species presented in the metal
intercalated SiO2 pillared ilerite samples. In contrary, the magadiite
samples exhibited a single peak at BE of 103.3 eV corresponding to the
Si in tetrahedral sites of layered silicate as bulk magadiite sample
(Fig. S2) and absence of Si2p peak at 102.2 eV in these samples reveal
that the surface Si species are not in direct contact with metal oxide
species in case magadiite samples.
Table 1
Elemental analysis of the synthesized samples.
Elemental composition (wt. %)
Catalyst
Bulk
Na
Surface
Si
O
Metal
Na
Si
O
Metal
Na-mag
4.2
1.1
0.2
0.1
0.1
6.7
1.3
0.1
0.1
0.1
43.5
57.4
54.6
54.0
54.1
32.8
48.2
46.3
46.0
45.8
52.2
41.4
33.0
33.5
33.4
60.3
50.4
41.5
41.3
41.1
–
3.9
0.6
–
43.3
41.3
35.4
34.9
34.8
32.7
48.0
42.4
43.5
44.1
52.5
58.1
54.1
53.8
54.3
60.2
50.3
46.2
46.0
45.5
–
–
SiO2-pil-mag
Mn-SiO2-pil-mag
Cu-SiO2-pil-mag
Fe-SiO2-pil-mag
Na-ile
–
12.2
12.4
12.3
–
10.4
11.3
10.8
–
–
–
6.5
0.7
–
SiO2-pil-ile
–
–
Mn-SiO2-pil-ile
Cu-SiO2-pil-ile
Fe-SiO2-pil-ile
12.1
12.6
12.9
11.4
10.5
10.4
The both Cu oxide intercalated SiO2 pillared samples exhibited two
Cu 2p3/2 peaks at 933.1 eV and 935.3 eV. Huang et al. [51] assigned the
peak at 933.1 eV for the Cu2+ of CuO and the other peak at 935.3 eV for
the copper phyllosilicate species. This observation reveal that Cu is
chemically bonded with Si-O groups in both ilerite and magadiite
samples to form an amorphous copper phyllosilicate phase on the sur-
face. It is clear from the intensity of the peaks that the CuO contribution
is more compared to the copper phyllosilicate reveal presence more CuO
particles on the surface of the layered silicates. The Fe oxide intercalated
SiO2 pillared ilerite sample exhibited two different Fe 2p3/2 peaks; a
major peak at BE of 711.1 eV and a minor peak at 709.8 eV. Grosvenor
–
–
starting parent material after intercalation process, suggest that the
layered structures are preserved. A minor difference in the metal (Cu, Fe
and Mn) concentration in bulk (high) and surface (low) composition is
observed; this is due to the presence of metal oxide particles inside the
layers of ilerite and magadiite.
To verify the existence and chemical environment of the metal oxide
species on the catalyst surface, Raman spectral analysis (Fig. S3) for bulk
magadiite, ilerite and Cu, Fe and Mn oxides intercalated to SiO2 pillared
magadiite and ilerite samples was performed. A strong band was
observed at 465 cmꢀ 1 in the Raman spectra of magadiite sample; it was
previously reported that the Raman bands in the range of
– –
et al. [52] reported that fitting of Fe 2p3/2 peak for α-Fe2O3 phase yields
peaks at 709.4 eV and 711.1 eV for Fe2+ and Fe3+ species, respectively.
Therefore, presence of
α
-Fe2O3 particles (majority Fe3+) on the surface
of ilerite is evidenced from XPS analysis. On other hand, the Fe oxide
intercalated SiO2 pillared magadiite sample exhibited two equally
intense peaks at 710.6 eV and 712.3 eV due to Fe2+ and Fe3+ species,
respectively. The shift in the BE towards higher levels could be attrib-
400ꢀ 700 cmꢀ 1 could assigned to stretching vibration of Si
O Si
modes of six-membered rings of silicon-oxygen tetrahedral units [57].
On other hand, two well separated strong bands at 450 and 480 cmꢀ 1
were appeared in the Raman spectrum of bulk ilerite sample. These
bands are also attributed to the symmetric stretching vibrational modes
of Si-O-Si linkages characteristic of two distinct types of six-membered
rings [58]. It is clear that distinct Raman bands were observed in the
Raman spectra of ilerite and magadiite samples; therefore, the structure
of ilerite is quite different from the magadiite structure.
uted to the decrease in the amount of Fe3+ species in the
α-Fe2O3 par-
ticles. These observations are in agreement with the XRD results that this
sample exhibited low intense XRD reflections due to α-Fe2O3 phase.
The deconvoluted Mn2p peaks for the Mn oxide intercalated SiO2
pillared samples also presented in the Fig. 4. As shown in the figure,
Mn2p spectra of the samples were broadened by multiplet splitting,
which is due to the coupling between the excited unpaired 2p core-
electron and unpaired 3d valence electrons. The deconvoluted Mn2p
spectra also includes the shake-up satellite peaks. It was previously re-
ported that Mn 2p3/2 spectrum of Mn cations exhibits four major
multiplet peaks separated by approximately 1.0 eV [53]. The XPS
spectrum of Mn oxide intercalated SiO2 pillared ilerite sample also
showed four Mn 2p3/2 peaks at 640.5 eV, 641.9 eV, 643.2 eV and
644.8 eV, with a minor satellite peak. Similarly, Mn oxide intercalated
SiO2 pillared magadiite sample showed four peaks at 640.4 eV,
641.6 eV, 642.7 eV and 643.8 eV along with a major satellite peak. The
average BE of the peaks is 642.1 eV and 642.6 eV for ilerite and mag-
adiite samples and this observation indicating that the both samples
possessed Mn3+ species [54]. In case of Cu, Fe and Mn oxides interca-
lated SiO2 pillared ilerite samples, the O1s spectra were deconvoluted
into two components at 530.3 eV and 532.7 eV. The lower BE peak
could be attributed to metal oxide species and higher BE peak could be
assigned to the Oꢀ2 species of the silicate layers [55]. The O1s spectra of
Cu, Fe and Mn oxides intercalated SiO2 pillared magadiite samples also
exhibited two peaks at 530.8 eV and 532.8 eV corresponding to oxygen
species in metal oxide and silicate layers. Slight increase of BE in the
magadiite samples is due to the small size particles presented in these
samples.
The Raman spectra of Cu oxide intercalated SiO2 pillared magadiite
and ilerite samples exhibited two additional bands at 290 and 340, cmꢀ 1
correspond to CuO phase [59]. It is interesting to observe another
additional Raman band at 1120 cmꢀ 1, which is not observed in bulk
CuO phase, but found in CuO-SiO2 catalysts due to presence of Cu-O-Si
interactive species [60]. The Raman spectra of Fe oxide intercalated
SiO2 pillared magadiite and ilerite samples also showed three additional
Raman bands at 330, 495 and 1160 cmꢀ 1. These bands are attributed to
the framework of Fe-O-Si species. [61]. In other hand, The Raman
spectra of Mn oxide intercalated SiO2 pillared magadiite and ilerite
samples exhibited only one new broad Raman band in the range of
640ꢀ 650 cm-1 was due to a Mn-O mode [62]. These observations clearly
indicating that presence of M-O-Si be identified in case Cu and Fe oxide
intercalated SiO2 pillared magadiite and ilerite samples, but the inter-
active species were not observed in case of Mn oxide intercalated SiO2
pillared magadiite and ilerite samples.
Fig. S4 shows the nitrogen adsorption-desorption isotherms and pore
size distribution patterns (in the inset) for Cu, Fe and Mn oxides inter-
calated SiO2 pillared ilerite and magadiite samples. For the sake of
comparison, the patterns for bulk SiO2 pillared samples were also
included in the figure. The N2 adsorption-desorption isotherms of the
SiO2-pillared magadiite and ilerite samples are analogous to a type IV
isotherm, distinctive for the mesoporous materials, according to the
Brunauer, Deming, Deming and Teller (BDDT) classification [63]. Also,
for the ilerite and magadiite samples, the adsorption-desorption iso-
therms clearly confirm the lack of micropores and the presence of meso-
and macropores. As the synthesized ilerite and magadiite normally do
not contain large pores, the observed large pores could be due to voids
occurred between the crystals of the agglomerates. The hysteresis loop is
shown in the isotherms for the SiO2 pillared materials is of Type H3 as
The bulk and surface elemental composition of the synthesized
samples were obtained using ICP-AES and XPS techniques. The obtained
results are tabulated in Table 1. The bulk composition of the sodium salts
of ilerite (Na2Si8O17.10H2O) and magadiite (Na2Si14O29.83H2O) sam-
ples were quite matching with the idealized formulas proposed in the
literature [56]. The bulk chemical composition data obtained for the
SiO2 pillared and metals intercalated pillared ilerite and magadiite
samples indicate that all these materials retain the structure of the
7

N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
per the IUPAC classification [64], which is characteristic for the
slit-shaped pores due to plate-like particles with voids among the par-
ticles [65]. The isotherms of metal intercalated SiO2 pillared samples
exhibited diminutive hysteresis loops and no steps at high relative
pressures were observed. This is an indication that the metal oxide
intercalated SiO2 pillared silicates have few external surfaces between
the particles. The pore size distribution patterns for all synthesized
materials were obtained using NLDFT method (Fig. S4). A typical
unimodal pore size distribution curves, in the mesoporous range (above
2 nm), were detected for the SiO2 pillared samples. The Cu, Fe and Mn
oxides intercalated SiO2 pillared ilerite and magadiite exhibit slightly
different pore size distribution compared to parent materials due to the
incorporation of large amount of metal oxides resulted a broad range of
pore size distribution, demonstrating the development of irregular pore
structure. The pore volume and pore width were decreased with inter-
calation of metal oxides.
The BET surface area observed for Na-ilerite and Na-magadiite were
26 and 21 m2 gꢀ 1 respectively, however after SiO2 pillaring the surface
area was increased drastically to 582 and 478 m2 gꢀ 1, which is due to
the pillar formation between the silicates layers, similar observations
were previously reported by the other researchers [36]. As shown in the
Table 2, intercalation of metals in the layered structure altered the
porosity of the materials. The metal oxide intercalated SiO2 pillared
materials showed decrease in the BET-surface area. The BET-surface
area of Fe oxide intercalated samples is lower compared to Cu and Mn
oxides intercalated samples. The formation of highly crystalline Fe and
Cu oxide inside and outside of the silicate layers, as evidenced by XRD
analysis, could be reason for lower surface area in the Fe and Cu oxides
intercalated SiO2 pillared materials. The decrease in porosity values
reveal the metal oxide particles might be located near to edges layers
resulting in blockage of interlayers [66].
The H2-temperature programed reduction (H2-TPR) was used for the
characterization of layered silicates comprising reducible metal oxides.
The reduction of isolated metal oxides, in the layered silicate structure,
usually proceeded at lower temperature than that required for the
reduction of bulk metal oxides or spinel structures formed due to
calcination at higher temperatures [67]. The reduction profile of syn-
thetic SiO2 pillared magadiite (Fig. 5) displayed no reduction peaks,
indicating that the cations presented (Na+ and Si4+) could not be
reduced under the experimental conditions selected (25ꢀ 800 ◦C). On
the other hand, SiO2 pillared ilerite sample showed low intense noise
peaks at around 300 ◦C and 450 ◦C, which are not belong to any
reducible species, but probably due to complete thermal decomposition
of residual CTAB remained in the mesopores of ilerite sample. The Mn
oxide intercalated SiO2 pillared magadiite and ilerite samples showed
two different reduction peaks at low temperature region (250ꢀ 450 ◦C)
and one peak at high temperature region (above 650 ◦C) with H2 con-
Fig. 5. H2-TPR patterns of Cu, Fe and Mn oxides intercalated SiO2-pillared
magadiite and ilerite samples.
ilerite samples exhibited three different peaks. It was previously re-
ported in the literature that Fe based catalysts could be reduced via three
different steps [69]. The first one corresponds to reduction of Fe2O3 to
Fe3O4 and FeO occurring between 250 ◦C and 450 ◦C; subsequently the
second one is due to reduction of Fe3O4 and FeO to Fe above 500 ◦C; and
third one could be attributed to Fe metal creation from the meta-stable
FeO phase. It is clear that the both the samples are following similar
trend as reported in the literature. The reduction peaks for Fe oxide
intercalated ilerite sample are appeared at higher temperature
compared to Fe intercalated magadiite sample; this is possibly due to
strong interaction between the Si-O and Fe3+ species in ilerite than in
the magadiite, as in later sample presence of isolated bulk Fe2O3 parti-
cles were observed. The Cu intercalated magadiite and ilerite samples
showed two reduction peaks at temperature range of 250ꢀ 350 ◦C. The
TPR peak appear in this range could be attributed to the reduction of
Cu2+ species [70]. The first reduction peak appeared in the synthesized
samples can be assigned to the reduction of isolated CuO species on the
surface of silicate layer and the second reduction peak can be attributed
to the reduction of CuO interacted with Si-O moieties. The position and
intensity of reduction peaks clearly reveal that the fraction of interacted
CuO species is high in ilerite compared to magadiite. It is clear that the
area of second peak is larger than that of the first peak in case of
Cu-SiO2-pil-ile sample indicating that the relative content of Cu-O-Si
species are more in this sample. This is mainly because of ilerite sam-
ple able to interact with Cu-O species more effective than magadiite
sample and prevented the aggregation of CuO species. The H2 con-
sumption values for all the synthesized catalysts are tabulated in
Table S1, and it is clear that Cu oxide intercalated magadiite and ilerite
sumption of 1360 μmol/g and 1618 μmol/g, respectively. These reduc-
tion peaks could be associated with the reduction of Mn species via
MnO2 → Mn3O4 → MnO [68].
The reduction profiles of Fe intercalated SiO2 pillared magadiite and
Table 2
BET surface area, pore volume and pore width of the synthesized samples.
Catalyst
Surface area
Pore volume (cc
Pore width
(nm)
(m2 gꢀ 1
)
gꢀ 1
)
SiO2-pil-mag
Mn-SiO2-pil-
mag
478
392
0.426
0.389
2.8
2.7
Cu-SiO2-pil-
mag
364
0.321
2.5
Fe-SiO2-pil-mag
SiO2-pil-ile
Mn-SiO2-pil-ile
Cu-SiO2-pil-ile
Fe-SiO2-pil-ile
308
582
488
469
364
0.285
0.521
0.482
0.418
0.382
2.2
3.2
3.0
2.7
2.5
8

N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
catalysts are easily reducible compared to other catalysts and exhibited
high H2 consumption values. It is anticipated that Cu oxide intercalated
magadiite and ilerite catalysts could exhibit superior activity in direct
catalytic NO decomposition process.
intercalated with SiO2 pillared ilerite samples can desorb slightly more
lattice oxygen than the magadiite samples because their smaller particle
size favors the migration of lattice oxygen. The O2-TPD spectra of the
synthesized samples suggests that Cu and Fe oxides intercalated samples
possessed a significant concentration of oxygen species on the catalyst
surface at lower temperatures (between of 350ꢀ 450 ◦C). Meanwhile, in
case of Mn oxide intercalated samples, the oxygen desorption started at
high temperature (above 400 ◦C). Moreover, the intensity and area
under the sample desorption peak clearly show that Cu oxide interca-
lated samples’ oxygen adsorption capacity is higher compared to Fe and
Mn oxides intercalated samples (Table S1).
The O2-TPD patterns of Cu, Fe and Mn oxides intercalated SiO2 pil-
lared magadiite and ilerite samples are presented in Fig. 6. It was re-
ported that the oxygen desorption could occur in three stages; The first
one at lower temperatures (< 300 ◦C) could be due to the surface O2, O2ꢀ
and Oꢀ species. The second and third stage desorption peaks appear
relatively at higher temperatures (<500 ◦C), which could be attributed
to the release of lattice oxygen of transports from bulk of metal oxide to
surface at high temperatures [71]. The both Mn intercalated catalysts
exhibited major desorption peaks at high temperatures (greater than
500 ◦C), which is an indication that they have more of lattice oxygen
species. The slight difference in the peak maximum in the magadiite and
ilerite could be due to the difference in the particle size, which could
retard the release of the bulk lattice oxygen.
For direct NO decomposition, the catalytic efficiency of synthesized
Cu, Fe and Mn oxides intercalated SiO2 pillared magadiite and ilerite
catalysts were tested. Initially, a blank test was performed without any
catalyst and a negligible NO conversion was observed at 400ꢀ 600 ◦C
temperature range under the tested reaction conditions. It is well known
that NO usually decomposes into N2 and O2 in the direct NO decom-
position process [3], but there is a strong possibility of forming un-
wanted N2O and NO2 molecules as side products during the reaction.
Different potential catalytic reactions are described below [72].
It is clear from the figure that the Cu oxide intercalated samples
exhibited a sharp major O2 desorption peak around 400 ◦C, and two
broad desorption peaks at higher temperatures. The major desorption
peak at low temperature is indicating the high coverage of molecularly
adsorbed O2 or even the diffusion of O2 molecules within the silicate
layers. Presence of two high temperature O2 desorption peaks suggesting
that two reduction mechanisms are responsible for the evolution of O2,
as we observed two high-temperature reduction processes in the two
samples (H2-TPR analysis). It is interesting to note that the Fe interca-
lated SiO2 pillared ilerite sample’s O2-TPD patterns showed two major
desorption peaks at low temperatures at 155 ◦C and 350 ◦C, and a minor
broad peak at 550 ◦C with a shoulder at 670 ◦C. The Fe oxide interca-
lated SiO2 pillared magadiite sample showed very similar pattern, but
the peaks were slightly moved to higher temperatures. Metal oxides
2NO → N2 + O2
(3)
(4)
(5)
4NO → 2N2O + O2
NO + Olattice → NO2
The influence of reaction temperature in the direct NO decomposi-
tion performance over Cu, Fe and Mn oxide intercalated SiO2 pillared
magadiite and ilerite catalysts were studied. It was observed that parent
SiO2 pillared magadiite and ilerite samples exhibited low catalytic ac-
tivity in the temperature range of 400ꢀ 600 ◦C. However, the
Fig. 6. O2-TPD patterns of Cu, Fe and Mn oxides intercalated SiO2 pillared magadiite and ilerite samples.
9

N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
intercalation of transition metal (Cu Fe and Mn) oxide resulted in a
significant improvement in catalytic activity for direct NO decomposi-
tion [Fig. 7 (A)]. A similar observation was reported for CeO2 supported
catalysts that addition of first row transition metals to the CeO2 support
enhanced the direct NO decomposition activity [73]. As shown in the
Fig. 7 (A) & (B), with the increase of reaction temperature from 400 ◦C
to 600 ◦C, both the NO conversion and N2 selectivity were increased in
all transition metal oxide intercalated catalysts.
selectivity, however these catalysts offered less N2 selectivity compared
to Cu oxide intercalated samples. Interestingly, in case of Fe and Mn
oxides intercalated samples; on increase of reaction temperature from
400 to 600 ◦C resulted in enhancement in selectivity to N2 from 48 % to
72 % in case Fe-ilerite sample and 40%–65% for Mn-ilerite sample.
Intercalation of Cu led to high improvement in N2 selectivity compared
to Fe and Mn oxides intercalation at low reaction temperature. The Mn
oxide intercalated samples offered high selectivity to NO2, probably due
to the fact that the mobility of lattice oxygen and the ability of this
catalyst to provide the oxygen species to NO molecule to form NO2 is
high.
Among the synthesized samples, the Cu oxide intercalated magadiite
and ilerite samples exhibited superior catalytic activity. Intercalation of
Fe and Mn oxides to the SiO2 pillared layered silicates had limited in-
fluence on the direct NO decomposition activity at all reaction tem-
peratures. The Cu-SiO2-pil-ile sample showed NO conversion of 34.4 %
at 400 ◦C and with increase of reaction temperature to 500 ◦C resulted
increase of NO conversion to 73.8 %. It is widely accepted that devel-
opment of effective catalyst, which could offer high N2 selectivity and
NO conversion is desirable [2]. Interestingly, the synthesized catalysts in
this study not yielded any N2O as a side product under the investigated
reaction conditions; the N2 and NO2 are the two products detected. It is
clear from the stoichiometric reactions that NO2 is the product which
form due to the oxidation of NO, while N2 is the product due to the direct
decomposition of NO.
The Arrhenius plots for NO decomposition for the synthesized cata-
lysts in the temperature range of 400ꢀ 600 ◦C are shown in Fig. 8. The
apparent activation energy and reaction rate for NO decomposition over
most active catalyst; Cu-SiO2 pillar-lie sample is calculated as
23.2 kcal molꢀ 1 and 2.6
μ
mol N2 s-1 g-1 respectively at 500 ◦C. It was
reported that the most active Cu-ZSM-5 catalyst exhibited an activation
energy of 22 kcal molꢀ 1 and reaction rate of 12
μ
mol N2 s-1 g-1 [74]. For
comparison purpose, bulk CuO, α-Fe2O3, Mn2O3 and SiO2 supported
CuO,
α-Fe2O3 and Mn2O3 samples were tested for the direct NO
decomposition. The surface area normalized rates for these catalysts are
tabulated in Table S2. It is clear that these samples exhibited very low
NO conversion at 400 and 450 ◦C. The NO conversion rates were
increased with increasing the reaction temperature to 500–600 ◦C. The
investigated bulk Cu, Fe, Mn oxides and SiO2 supported Cu, Fe and Mn
oxide samples exhibited lower NO conversions and N2 selectivity
compared to synthesized Cu, Fe and Mn oxide intercalated SiO2-ma-
gadiite and ilerite samples. These observations indicate that the higher
direct NO decomposition of metal oxide intercalated layered silicate
The N2 and NO2 selectivites over the Cu, Fe and Mn oxides interca-
lated SiO2 pillared magadiite and ilerite samples at different reaction
temperatures are shown in Fig. 7 (B) & (C). The intercalation of Cu oxide
led to significant improvement in the N2 selectivity, as both magadiite
and ilerite samples exhibited around 90 % selectivity toward N2 at
400ꢀ 500 ◦C and 85 % selectivity to N2 at 600 ◦C. The intercalation of Fe
and Mn oxides to the magadiite and ilerite also improved the N2
Fig. 7. Influence of reaction temperature on (A) NO conversion (B) N2 selectivity (C) NO2 selectivity over synthesized catalysts.
10

N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
Fig. 8. The Arrhenius plots for NO decomposition for the synthesized catalysts.
Fig. 9. The time on stream analysis of Cu-SiO2-pil-ile catalyst.
samples is not due to aggregated bulk metal oxide on the catalyst
surface.
undecomposed NO with lattice oxygen of the catalyst and this catalytic
oxidation appeared be high in case of Mn and Fe oxide intercalated SiO2
pillared layered silicate samples, a similar observation was reported in
case of Cu-Mn oxide catalyst [76]. High NO conversion rates observed in
case of Cu, Fe and Mn oxide intercalated layered silicate samples higher
reaction temperature could be attributed to high mobility of lattice ox-
ygen, as O2-TPD patterns of the samples exhibited major desorption
peak of lattice oxygen at high temperatures (500–700 ◦C). Overall, the
formation of NO2 observed with Cu-SiO2-pil-ile is influenced by
lowering the residence time at reaction temperature smaller than 600 ◦C
compared to Fe and Mn oxide intercalated samples.
In industrial catalyst development, the catalytic activity at high
GHSV is a crucial aspect, therefore the most active catalyst, Cu-SiO2-pil-
ile sample was chosen to study the influence of GHSV on NO decom-
position performance and the obtained results are shown in Table 3.
With low GHSV, NO decomposition conversions appeared a maximum;
gradual increase of GHSV, NO conversion steadily decreased. With the
further increase of GHSV to 18,000 hꢀ 1, the selectivity to NO2 has not
changed significantly, whereas the selectivity of N2 steadily increased
with the increase of GHSV. This observation reveals that NO conversion
to N2 and NO2 can be accomplished through separate rate-limiting steps.
It is important to study the mass transport and diffusion limitations
for any catalyzed reaction. The external and internal diffusional limi-
tations for the synthesized catalysts in using reported procedures [75]
and relevant information is included in the electronic supplementary
information. In order to determine the influence of pore diffusion on
It was previously reported that the low activity of the catalysts in NO
decomposition process could be due to catalyst inability to desorb ox-
ygen formed from NO dissociation [77]. In case of some catalysts, the
formed oxygen due to NO dissociation is strongly bounded to surface of
catalyst, negatively influencing the active centers and hindering the
progression of NO dissociation. The characterization data from XRD,
XPS, H2-TPR and O2-TPD techniques for Cu, Fe, and Mn intercalated
layered silicate samples could be helpful to understand the structure and
NO adsorption properties of the investigated samples in this study.
Several decades ago, Iwamoto et al. [78] reported that oxygen species
adsorbed on Cu2+ ions in the aluminosilicates can easily desorb at low
temperatures (less than 300 ◦C), which is the contributing factor for
superior activity of the Cu contained aluminosilicates catalysts for NO
decomposition. The same research group indicated that large quantity of
Cu ions in aluminosilicates could be advantageous to enhance the NO
decomposition activity. Later on, Yahiro et al. [79] indicated that Cu+
ions in the aluminosilicates generated at high reaction temperatures
regarded as active centers and NO decomposition progresses through the
reaction rate the effectiveness factor ‘
that the internal effectiveness factor (
spherical catalyst pellet. It was observed that ‘
η
’ was determined. It is well known
) for a first-order reaction in a
’ has value is between
η
η
0.95 and 1 for all the investigated catalysts in the present study, there-
fore there is no internal diffusion resistance is taking place. The influ-
ence of external diffusion resistance over the reaction rate is determined
by using general diffusion laws, the value of the mass transfer coefficient
of reactants and fluid dynamic parameters. It is observed that the
calculated kc values are greater than k for all the catalysts, therefore the
NO decomposition proceeds in the kinetic regime and external diffusion
is not influencing the NO decomposition rate.
The time on stream analysis (durability of catalyst) is also a signifi-
cant aspect in catalyst development, and Fig. 9 demonstrates the results
of time on stream analysis for Cu-SiO2-pil-ile sample. It can be seen that
the NO conversion and N2 selectivity were slightly reduced (NO con-
version from 89.8% to 85% and N2 selectivity from 86 % to 82 %) after
continuous operation of 48 h. The slight decrease of activity could be
due to sintering of active metal oxide species. The observed formation of
NO2 in the process could be due to the reaction between the
creation of NOδꢀ or (NO)2 species on Cu+species. Groothaert et al.
δꢀ
[80] reported that presence of O-bridged Cu planar complexes showed
an influence on NO decomposition activity of Cu based catalysts, while
Hayes et al. [81] observed that CuO particles are the active catalytic
species. Da Costa et al. [82] provided an evidence for the participation of
redox cycle of Cu-O-Cu species in NO decomposition activity. The au-
thors also indicated that mobile oxygen atoms in the Cu-O-Cu species are
required for rapid redox cycles but not the isolated Cu2+ species for NO
decomposition. Therefore, it is important to understand the nature of the
Cu species in Cu-SiO2-pil-ile and Cu-SiO2-pil-mag responsible for the
superior catalytic NO decomposition performance.
Table 3
Influence of GHSV on NO conversion, selectivities to N2 and NO2 over Cu oxide
intercalated SiO2 pillared ilerite sample.
GHSV (hꢀ 1
)
NO conversion (%)
Selectivity to N2 (%)
Selectivity to NO2 (%)
In the synthesized Cu oxide intercalated layered silicates, majority of
Cu is instituted in the form of CuO particles as observed from XRD
analysis. It was also observed that the average particle size of CuO was
observed as 20 nm, which is too big to move inside the pores of ilerite or
magadiite. It is our interpretation that Cu2+ ions in CuO particles are at
3000
75.2
73.8
61.5
42.9
35.2
88.2
90.0
92.1
93.3
94.1
11.0
9.0
8.2
7.6
7.1
6000
12,000
18,000
24,000
11

N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
the interface with SiO2 pillars of layered silicate surface, as the XPS
results clearly indicated that Cu is chemically bonded with Si-O groups
in both ilerite and magadiite samples to form an amorphous copper
phyllosilicate on the surface. Gong et al. [83] reported that copper
phyllosilicates produce stable Cu+ species on its surface, which are
responsible for hydrogenation reaction.
measurements demonstrated that Cu, Fe and Mn oxide-SiO2 pillared
silicate samples possessed different pore structures from SiO2-pillared
silicates, due to the different nature of metal oxides. The Cu-SiO2-pil-ile
and Cu-SiO2-pil-mag samples possesses a smaller number of Lewis and
Brønsted acid sites compared with Fe and Mn oxide intercalated sam-
ples. The XPS analysis of the samples reveals that intercalation of both
Cu and Fe resulted in the presence of Cu2+/Cu+ and Fe3+/Fe2+, and the
synergism between the redox centers is the major reason for superior
performance in NO decomposition. Conversely, Mn remains in single
oxidation states (Mn3+) in the Mn intercalated catalysts. The H2-TPR
and O2-TPD results indicated that the Cu-SiO2-pil-ile and Cu-SiO2-pil-
mag catalysts possessed easily reducible Cu species and mobility of the
oxygen atoms is also high, which are directly related to the superior NO
decomposition activity. Thus, the activity of Cu-SiO2-pil-ile and Cu-
SiO2-pil-mag samples could be accredited to both existence of Cu2+ and
Cu+ species and also to oxygen desorption from Cu species at moderate
temperatures.
The H2-TPR and O2-TPD results indicated that the Cu-SiO2-pil-ile and
Cu-SiO2-pil-mag catalysts possessed easily reducible Cu species and
mobility of the oxygen atoms is also high, which are directly related to
the superior NO decomposition activity. Thus, the activity of Cu-SiO2-
pil-ile and Cu-SiO2-pil-mag samples could be ascribed to both existence
of Cu2+ and Cu+ species and also to oxygen desorption from Cu species
at moderate temperatures. In addition, it is well known that NO is an
acidic molecule and the NO adsorption greatly depends on the surface
acid/base character of the catalyst. The DRIFT spectra of the pyridine
adsorbed catalysts are shown in the Fig. 10 and the Cu-SiO2-pil-ile and
Cu-SiO2-pil-mag samples possesses less number of Lewis and Brønsted
acid sites compared with Fe and Mn oxide intercalated samples. This
factor is also affecting the reactivity of NO with surface of Cu-SiO2-pil-ile
and Cu-SiO2-pil-mag catalysts. The current best catalysts for NO
decomposition were tabulated in Table S3 (ESI) with the details on the
tested reaction temperatures, space velocities, NO conversions and N2
selectivities. It is clear from the literature review that the multi metal
Intellectual property
We confirm that we have given due consideration to the protection of
intellectual property associated with this work and that there are no
impediments to publication, including the timing of publication, with
respect to intellectual property. In so doing we confirm that we have
followed the regulations of our institutions concerning intellectual
property.
containing perovskite catalysts such as Ba3Y3Sc0.6Cu0.4O9, Ba3Y3.6
-
Cu0.4O9 (700 ◦C) and zeolite-based catalysts such as Fe/Mel-10@ZSM-5
(350 ◦C) exhibited high NO decomposition activity compared to the
most active catalyst [Cu-SiO2-pil-ile] in the present study. However, Cu-
SiO2-pil-ile catalyst offered 90 % NO conversion and 83 % N2 selectivity
at 600 ◦C with very high GHSV value (12,000 hꢀ 1) compared to the
catalysts reported in the literature. Therefore, it is meaningful to state
that the Cu-SiO2-pi-ile catalyst possessed remarkable NO decomposition
activity.
Research ethics
We further confirm that any aspect of the work covered in this
manuscript that has involved human patients has been conducted with
the ethical approval of all relevant bodies and that such approvals are
acknowledged within the manuscript.
4. Conclusions
Authorship
In this work, layered silicates (magadiite and ilerite) were pillared
with SiO2 and then intercalated with Cu, Fe and Mn oxides to utilize the
synthesized materials as catalysts for direct NO decomposition in the
temperature region of 400ꢀ 600 ◦C. The catalytic activity results indi-
cated that Cu intercalated SiO2 pillared magadiite and ilerite samples
are efficient catalysts among the synthesized samples (90 % NO con-
version and 83 % selectivity to N2 at 600 ◦C). Elemental analysis, XRD,
FE-SEM, DR UV-vis, Raman spectroscopy, N2-adsorption, H2-TPR, O2-
TPD and XPS measurements were used to study the physico-chemical
properties of the synthesized metal oxide intercalated layered silicate
materials. The results from XRD and N2 adsorption-desorption
1 Substantial contributions to the conception or design of the work; or
the acquisition, analysis, or interpretation of data for the work; AND
2 Drafting the work or revising it critically for important intellectual
content; AND
3 Final approval of the version to be published; AND
4 Agreement to be accountable for all aspects of the work in ensuring
that questions related to the accuracy or integrity of any part of the
work are appropriately investigated and resolved.
All those designated as authors should meet all four criteria for
Fig. 10. DRIFT spectra of pyridine adsorbed SiO2 pillared ilerite and magadiite samples.
12

N. Katabathini et al.
Applied Catalysis A, General 616 (2021) 118100
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Katabathini Narasimharao: Conceived and designed the research
work, collected the data, wrote the paper. Islam Hamdy Abd El Mak-
sod: Collected the data, wrote the paper. Mohamed Mokhtar:
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Declaration of Competing Interest
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[30] A. Swięs, A. Kowalczyk, M. Rutkowska, U. Díaz, A.E. Palomares, L. Chmielarz,
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This project was funded by the Deanship of Scientific Research (DSR)
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The authors, therefore, acknowledge with thanks DSR for technical and
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