High Zr-loaded SBA-15 cobalt catalyst for efficient NOx reduction in lean-burn exhaust

Article abstract of DOI:10.1016/j.apcata.2015.10.004

A high Zr-loaded SBA-15 mesoporous material with high surface area, well-ordered structure, homogeneous Zr dispersion and high surface acidity has been prepared by a novel direct-post synthesis technique (DP). While maximum Zr loadings of 5.1 wt% and 16.7 wt% have been achieved in direct and post synthesis methods, respectively, the support synthesized by the DP method (ZrSBADP2) has yielded a Zr loading of 32.1 wt%. Also, a high acidity of 136 μmol g^-1, favorable for catalysis reactions, has been reached for ZrSBADP2. The effect of cobalt concentration on the catalyst has also been examined on the NO to N₂ conversion via selective reduction (C₃H₈-SCR) with propane. It has been demonstrated that the catalyst with 2 wt% cobalt has shown the highest N₂ yield (70%) at 400°C in the presence of 6 vol% O₂. The catalyst performance has significantly improved in the presence of water vapor where the N₂ yield increased to 54% and 70% at 300°C and 350°C, respectively, compared with 18% and 34% in the absence of water vapor. Also, at high temperatures (≥400°C), the presence of water vapor has not resulted in any decrease in the catalyst performance. The long term stability test has proved that the catalyst structure is retained in extreme prolonged reaction conditions, making it a suitable catalyst for real applications.

Full text of DOI:10.1016/j.apcata.2015.10.004

Accepted Manuscript
Title: High Zr-loaded SBA-15 cobalt catalyst for efficient
NOx reduction in lean-burn exhaust
Author: Aghil Sabbaghi Frank L.Y. Lam Xijun Hu
PII:
S0926-860X(15)30181-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.10.004
APCATA 15580
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
18-5-2015
18-8-2015
4-10-2015
Please cite this article as: Aghil Sabbaghi, Frank L.Y.Lam, Xijun Hu, High Zr-loaded
SBA-15 cobalt catalyst for efficient NOx reduction in lean-burn exhaust, Applied
Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2015.10.004
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High Zr-loaded SBA-15 cobalt catalyst for efficient NOx
reduction in lean-burn exhaust
Aghil Sabbaghi, Frank L Y Lam and Xijun Hu*
Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong
*Corresponding author. Email: kexhu@ust.hk, Tel: +852 23587134, Fax: +852 23580054
Graphical abstract
Highlights
 High Zr-loaded SBA-15with a direct-post synthesis method was achieved.
 Non-noble metal-based catalyst was developed by doping cobalt on ZrSBADP2.
 The highest C3H8-SCR activity was achieved over Co2/ZrSBADP2.
 A high N2 yield (70 %) was obtained in the presence of water vapor at 350ꢀ.

The catalyst structure was stable in the extreme prolonged reaction conditions.

Abstract
A high Zr-loaded SBA-15 mesoporous material with high surface area, well-ordered structure,
homogeneous Zr dispersion and high surface acidity has been prepared by a novel direct-post
synthesis technique (DP). While maximum Zr loadings of 5.1 wt% and 16.7 wt% have been
achieved in direct and post synthesis methods, respectively, the support synthesized by the DP
method (ZrSBADP2) has yielded a Zr loading of 32.1 wt%. Also, a high acidity of
136 μmol. gꢁꢂ, favorable for catalysis reactions, has been reached for ZrSBADP2. The effect of
cobalt concentration on the catalyst has also been examined on the NO to N2 conversion via
selective reduction (C3H8-SCR) with propane. It has been demonstrated that the catalyst with 2
wt% cobalt has shown the highest N2 yield (70%) at 400ꢀ in the presence of 6 vol% O2. The
catalyst performance has significantly improved in the presence of water vapor where the N2
yield increased to 54% and 70% at 300ꢀ and 350ꢀ, respectively, compared with 18% and 34%
in the absence of water vapor. Also, at high temperatures (ꢃ 400ꢀ), the presence of water vapor
has not resulted in any decrease in the catalyst performance. The long term stability test has
proved that the catalyst structure is retained in extreme prolonged reaction conditions, making it
a suitable catalyst for real applications.
Keywords
Cobalt-based catalysts, NOx reduction, Zr-SBA-15, C3H8-SCR, Direct-Post synthesis

1 Introduction
Growing environmental concerns on NOx emission as a major source for air pollution have
driven industrial and governmental sectors to seek for an efficient method to reduce NOx
emission from mobile and stationary sources.
Selective catalytic reduction (SCR) of NOx has proved as an effective strategy to convert this
poisonous emission into environmental-benign gaseous components. Among many SCR
techniques, NOx reduction using hydrocarbon as the reducing agent (HC-SCR) in the lean-burn
conditions has been considered as a simple and efficient technique in comparison to other SCR
methods, such as NH3-SCR and Urea-SCR. The catalytic system in HC-SCR requires certain
properties, such as hydrothermal stability and high surface acidity. Moreover, specific
characteristics of the support used for catalyst preparation, such as high surface area and good
interaction with active components, will greatly assist the catalyst to exhibit higher activity in the
catalytic reactions [1–4].
So far, various types of catalysts including zeolites, metal oxides, transition metal-supported
compounds and noble metal-based catalysts, have been developed for HC-SCR [4–6]. Amongst,
zeolite-supported cobalt catalyst has exhibited promising results in NO reduction with
propane[7–9] that is the main component of unburned hydrocarbons in lean-burn gas, diesel and
natural gas-fueled appliances exhausts [10–13]. Shichi et al. [7] reported that Co-MFI zeolite
catalyst had 30% and 80% NO conversion at 350 and 400ꢀ, respectively. In another report, 30%
and 40% of NO conversion was observed over CoZSM-5 catalyst in the presence of propane at
the same temperature range [8]. However, zeolite-based catalysts suffer from instability in
hydrothermal conditions leading to catalysts deactivation [14]. Therefore, other catalytic systems
such as zirconium oxide-based catalysts have been attracted more attention for HC-SCR due to

high acidity, hydrothermal stability and water tolerance [15,16]. Nonetheless, their low surface
area and weak interaction with active components [5,6] keep them far from being a proper
catalyst for industrial applications. Hence, many researches have been devoted to overcome these
disadvantages and improve catalytic performance. For instance, a combination of zirconium with
a mesoporous silica results in increased catalyst surface area, enhanced interaction with active
components and formation of some Lewis and Brönsted acid sites which is desirable for a
catalyst [17–21].
Two distinct methods, i.e. direct and post synthesis methods, can be used to synthesize Zr-
incorporated SBA-15 [22]. In the direct synthesis method (D), Zr is introduced to the SBA-15
structure during the hydrothermal synthesis of SBA-15 [23], while in the post synthesis method
(P), incorporation of the metal is carried out through the chemical grafting on the synthesized
SBA-15 as a second step [24]. Although high amounts of Zr is favored to achieve increased
acidity and to enhance the interaction between the support and the active components, both
techniques have limitations in Zr loading content, while retaining the intact structure of the SBA-
15. High Zr loading brings about structural collapsing in the case of the former and pore
blockage and surface area reduction in the case of the latter.
The objective of this paper is to develop a high Zr-loaded SBA-15 support material by using a
combination of direct and post synthesis methods (DP). The application of the cobalt-doped
support, synthesized by this innovative approach, was evaluated in NOx reduction under lean
conditions and compared with those prepared by the conventional methods.

2 Experimental
2.1 Support and Catalyst preparation
Zr-SBA-15 synthesis by direct synthesis method (D): The established method reported by Gracia
et al. [25] with some modification was applied for Zr-SBA-15 preparation by direct synthesis
method. Amphiphilic triblock copolymer P123 (EO20PO70EO20) as the structure directing agent
was dissolved in hydrochloric acid solution at 35ꢀ to produce a clear micelle solution. Then,
appropriate amount of zirconium precursor (ZrONO3.xH2O) was added to the solution prior to
tetraethyl orthosilicate addition. The mixture was kept under stirring at 35ꢀ for 24 h. The
resulting gel with a molar composition of 0.0171 P123: 1 Si: 0.05-0.2 Zr: 0.3 HCl: 206.35 H2O
was then transferred into a Teflon-lined stainless steel autoclave for hydrothermal treatment at
100ꢀ for 24 h under autogenous pressure. The final product, ZrSBADx (x represents the mole
⁄
ratio of Si Zr), was filtrated, washed several times with deionized water, dried at 60ꢀ overnight
and calcined at 600ꢀ for 8 h with a heating rate of 1ꢀ. minꢁꢂ.
Zr-SBA-15 synthesis by post synthesis method (P): The method reported by Gutiérrez et al. [24]
was applied with some modification for the preparation of the post-synthesized Zr-SBA-15.
SBA-15 was prepared according to the method described elsewhere [26]. Then, appropriate
amount of SBA-15, zirconium precursor (Zirconium (IV) propoxide [Zr(n-PrO)4] (70 wt %
solution in 1-propanol)) and absolute ethanol, as solvent for chemical grafting, with a molar
composition of 1 Si: 0.22 – 0.88 Zr: 13.7 solvent, were mixed overnight at room temperature
under nitrogen gas. Excess Zr(n-PrO)4 was eliminated by filtering and washed with three
sequential aliquots of absolute ethanol. The obtained material, ZrSBAPx (x represents the mole
⁄
ratio of Si Zr), was then dried overnight at room temperature under nitrogen gas, and finally
calcined at 550ꢀ for 6 h with a heating rate 1ꢀ. minꢁꢂ.

Zr-SBA-15 synthesis by combined direct-post synthesis method (DP): ZrSBAD10 was first
⁄
prepared using the above-explained procedure with Si Zr of 10. Then, appropriate amount of
prepared ZrSBAD10, zirconium precursor and absolute ethanol with a weight composition of 1
ZrSBAD10: 1.74-6.96 Zr(n-PrO)4:10.5 absolute ethanol, were mixed overnight under nitrogen
gas. The slurry was filtered and washed with three sequential aliquots of absolute ethanol to
eliminate excess Zr(n-PrO)4. The obtained material, ZrSBADPx (x represents the weight ratio of
⁄
Si Zr), was dried overnight at room temperature under nitrogen gas prior to calcination at 550˚C
for 6 h with a heating rate 1ꢀ. minꢁꢂ.
Catalyst preparation: ZrSBAD10, ZrSBAP2 and ZrSBADP2 and SBA-15 were used as support
for Co-based catalyst preparation via the incipient wetness impregnation technique [27]. A
required volume of aqueous solution of cobalt acetate tetrahydrate (Co (CH3COO)2·4H2O) was
impregnated into the support to obtain a desired content of Co on the support structure (1, 2, 4
and 10 wt %). The impregnated supports were dried overnight at 70ꢀ and calcined at 550ꢀ for
5 h in the open air with a heating rate of 1ꢀ. minꢁꢂ. These catalysts were denoted by the support
preparation method and the cobalt content. For instance, Co4/ZrSBADP2 indicates that the
⁄
support was prepared by DP technique with a weight ratio of Si Zr equal to 2 and the catalyst
contains 4wt% cobalt (Table 1). Moreover, a 2%Ag/Al2O3 catalyst as a benchmark was prepared
with the same impregnation and heat treatment procedures. A requisite volume of an aqueous
solution containing the required amount of AgNO3 was added dropwise onto -Al2O3.
2.2 Characterization
Prepared supports and catalysts were characterized by means of various methods including
Powder X-ray diffraction technique (XRD), nitrogen adsorption and desorption, X-ray

photoelectron spectroscopy (XPS), X-ray fluorescence spectroscopy (XRF), Fourier transform
infrared spectroscopy (FTIR).
PANalytical X-ray diffractometer (model X'pert Pro) and powder X-ray diffraction system
(model PW1830 (Philips), 2KW, Cu anode, graphite monochromator) were used to determine
ꢄ
ꢆ
the crystallinity of the materials. The scanning range 2ꢅ was adjusted in the range of 0.3-3˚
and 10-90˚ for low and high angle spectra, respectively.
N2 adsorption-desorption were conducted by a Coulter SA3100 surface area analyzer at liquid
nitrogen temperature. The samples were outgassed at 250ꢀ for 120 min prior to the analysis.
The specific surface areas of the supports and catalysts were determined in the linear region of
the Brunauer–Emmett–Teller (BET) equation (relative pressure between 0.05 and 0.22). The
total pore volume was calculated at relative pressure of 0.98.
XPS experiment was carried out by a Kratos Axis Ultra DLD multi-technique XPS instrument
under ultra-high vacuum to analyze the surface chemistry of the materials. A monochromatic Al
Kα X-ray source (excitation energy, hυ = 1486.6 eV) at a voltage of 10 kV and a current of 15
mA was used as the energy source. The low and high resolution spectra were acquired at 70 and
20 eV pass energy, respectively. C(1s) at a binding energy of 284.6 eV was used as reference for
the spectra correction.
Elemental composition of the prepared supports and catalysts was determined by XRF using a
JEOL JSX-3201Z XRF. The X-ray source for the experiments was obtained when the voltage
and current of the tube was adjusted at 30 kV and 0-1 mA, respectively.
FTIR was used to identify the functional moieties of prepared supports. Infrared spectrum of
samples was collected by KBr pellet technique in the wave number range of 4000-400 ꢇꢈꢁꢂ at
room temperature with a FTS 6000 FTIR spectrometer.

The surface acidity was measured by a pulse chromatographic technique approaching conditions
of gas-chromatography linearity. The experiment was carried out in gas-phase at 300ꢀ. The
adsorption of pyridine (PY) and 2,6-dimethylpyridine (DMPY) as probe molecules (basic
compounds) in a dynamic mode was conducted to measure the total acidity (sum of Brönsted and
Lewis acid sites) and Brönsted acid sites, respectively [28]. Typically, a very small amount of
probes molecules were injected into a modified gas chromatograph through a microreactor
containing 50 mg of samples. The probe molecules were adsorbed by the sample until the
saturation level from where the peaks of the compounds were detected by GC. Subsequently, the
amounts of adsorbed PY and DMPY were measured.
2.3 Catalytic activity tests
The HC-SCR reaction was carried out in a fixed bed stainless steel reactor and fed with a
continuous feed gas consisted of nitric oxide (500ppm), propane (500-2000ppm), oxygen (3-10
vol%), water vapor (0-7 vol%) and helium as balance. The gas flow rate was regulated at
100ml. minꢁꢂ (GHSV equal to 12000hꢁꢂ). Typically, 0.5 g of catalyst powder was loaded
between two glass wool layers into the reactor. The loaded reactor was placed in a Carbolite tube
furnace in order to provide the desired temperature (controlled by an additional thermometer) for
the reaction. Effluent gases were analyzed by a gas chromatograph (HP 6890 series) equipped
with Porapak Q, Hayesep D columns and a TCD detector to determine the concentration of N2,
CO2 and C3H8 at steady state condition. The effects of various parameters including reaction
temperature, catalyst composition and reactant concentration were evaluated. N2 yield was
considered as the NOx reduction performance of the catalyst and was calculated as follows:

ꢖꢉꢔꢗꢘꢐꢌꢙꢚꢋꢏꢙ
ꢖꢉꢊꢗꢒꢍ
ꢄ ꢆ
ꢉꢊ ꢋꢌꢍꢎꢏꢐꢑꢒꢌꢍ ꢓꢌ ꢉꢔ % ꢕ
(Eq1)
where [N2]out and [NO]in represent the moles of N2 in the outlet and NO in the feed stream,
respectively.
3 Results and discussion
Production of a support with 2-D hexagonal structure as well as homogeneous dispersion and
deposition of Zr was aimed in the preparation of a high Zr-loaded SBA-15. These two
characteristics are essential for the material to be used as support for catalyst preparation
applicable in HC-SCR reaction.
3.1 Catalysts and supports characterization
3.1.1 Elemental analysis
In order to find out the effect of synthesis method on Zr content of the prepared materials, the
metal contents (Si and Zr) of the prepared supports was measured and tabulated in Table 1.
The elemental analysis results indicate that the Zr content in the direct synthesized supports
⁄
(ZrSBADx) is inversely proportional to Si Zr ratio in the initial synthesis gel. Although the
⁄
highest Zr loading is achieved at a Si Zr ratio of 5 (ZrSBAD5), the hexagonal structure of SBA-
15 was completely distorted according to the XRD pattern (section 3.1.2). Therefore, the highest
⁄
content of Zr (5.1 wt%) was obtained when Si Zr was regulated at 10 while the well-ordered
mesoporous structure of the SBA-15 remained intact. These results are in good agreement with
literature [5].
The loading of Zr in the post synthesized support (ZrSBAPx) was considerably increased
compared to the ZrSBADx while retaining the intact structure of the SBA-15. In ZrSBAP2, 16.7
⁄
wt % of Zr loading was achieved when Si Zr ratio was adjusted to a value of 2.24, where the 2-

D hexagonal structure of SBA-15 was almost remained intact. It was also observed that the SBA-
15 structure could accommodate the heteroatoms up to a saturation level and decreasing the
ꢛꢜ/ꢝꢞ ratio from 4.54 to 1.13 did not result in any increase in the Zr content. The highest amount
of loaded Zr into the SBA-15 structure in this work (16.7 wt%) is lower than that obtained by
Gutiérrez et al. (23.4 wt%) [29] which might be due to difference in solvent used (dry
ethanol, 99.999%). A part of the alkoxide (zirconium precursor) can take part in hydrolysis as a
side reaction in the presence of the water content of the absolute ethanol solvent leading to the
formation of zirconium hydroxide precipitate [30]. However, from the practical point of view,
using dry ethanol in industry scale is not applicable due to its high cost.
According to Table 1, the introduction of higher amounts of Zr into the SBA-15 structure has
been facilitated by using the DP technique. Zr content in the SBA-15 structure significantly
increased and reached 32.1 wt% (ZrSBADP2) which is 6 and 2 times higher than those of the
direct (ZrSBAD10) and post synthesized (ZrSBAP2) supports, respectively. Moreover, the
hexagonal structures of the prepared supports remained unchanged compared with that of the
pure SBA-15 with no considerable collapses (Section 3.1.2). The considerably high Zr loading
can be attributed to an enhanced interaction between the acid sites on the ZrSBAD10 and
zirconium precursor (Zr(n-PrO)4) used in the post-synthesis method. In addition, according to
Degirmenci et al.[31] , the concentration of the silanol groups, (SiO)3Si — OH, is significantly
increased upon direct synthesis of support. This high content of silanol groups might lead to an
enhanced interaction between the zirconium precursor and the support through chemical grafting,
subsequently resulting in more Zr contribution in the ZrSBADPx structure through either
substitution in the framework or deposition on the surface.

3.1.2 X-ray diffraction pattern
The low and high angle diffraction patterns of the SBA-15, supports and catalyst, depicted in
Figure 1a - b demonstrate that the intensity of the diffraction peaks related to DP-synthesized
material with the highest Zr content slightly decreased, however well-ordered mesoporous
crystal structure of the SBA-15 almost remained intact. This can be ascribed to the structural
stability of the supports and catalyst at high calcination temperature essential for NOx reduction
catalysts [32,33]. In addition, although a high Zr-loaded material (as high as 30 wt%) could be
⁄
synthesized through the direct method using a Si Zr ratio of 5 (ZrSBAD5), the well-ordered
mesoporous structure was completely distorted, as testified by the XRD pattern.
High-angel X-ray diffraction pattern of the Zr-loaded materials (Figure 1b) did not exhibit
diffraction peaks at 2θ = 30˚, 35˚, 50˚, and 60˚ related to the tetragonal ZrO2 crystalline phase on
the supports [34]. This indicates that despite the significantly high Zr loading made by the DP
method, Zr was dispersed homogeneously throughout the SBA-15 structure without any
agglomeration.
3.1.3 Textural properties
The textural properties of the developed supports, SBA-15 as a reference and the prepared
catalysts with various content of the cobalt are tabulated in Table 2. The surface area and pore
volume are inversely proportional to the contribution of Zr into the SBA-15 framework. SBA-15
with no Zr content showed the highest specific surface area (905 mꢟ. gꢁꢂ) and total pore volume
(1.2 cmꢠ. gꢁꢂ), while these two parameters decrease to 548 mꢟ. gꢁꢂ and 0.7 cmꢠ. gꢁꢂ for
ZrSBADP2 with the highest Zr loading. A slight 2-D hexagonal structural modification as a
result of Zr incorporation accounts for a reduction in the surface area and pore volume [5,24,25],
which is confirmed by XRD results as well (Figure 1). A slight decrease in BET surface area and
pore volume was observed after cobalt loading. The decreasing trend was directly proportional to

the cobalt content. A 29% reduction of the surface area was observed when the cobalt content
increased from 1 wt% to 10 wt%. This trend demonstrates that the structural modification was
more pronounced in the presence of a higher content of the cobalt species. Despite this reduction,
the surface area of the catalysts was still higher than those of the zeolites and noble metal-based
catalysts [15,35,36]. The large surface area of the prepared catalysts, resulting in more active
sites for the chemisorption of the reactants and thus higher reaction rate, can be considered as
one of the main advantages of the DP synthesis technique [37].
Nitrogen adsorption/desorption isotherms, shown in Figure S1, imply that all the samples
exhibited the typical type IV isotherm with H1-type hysteresis loop characteristic of the
mesoporous materials with a highly uniform size distribution according to the IUPAC
classification [38].
3.1.4 XPS analysis
The chemical state of Si, Zr and O atoms of the prepared supports was investigated by XPS. The
XPS spectra of the Si-2p3/2 (Figure 2) revealed that two distinct peaks for all the prepared
supports were distinguished at a binding energy (BE) of approximately 103 and 105 eV. By
comparing the spectra of the supports and SBA-15, a new Si environment was observed after the
modification of SBA-15 with Zr irrespective to preparation method. A peak with a binding
energy centered at about 104.5 eV emerged after the SBA-15 modification which is
characteristic of the Si — O — Zr, while the bond at a BE of 103.6 eV is ascribed to
Si — O — Si [39]. This suggests that Zr atoms were successfully introduced into the SBA-15
framework by all the techniques used.
XPS spectra of O1s (Figure 3a-c) show three distinct peaks associated with Si — O (at ca. 533
eV), Si — O — Zr (at ca. 532.5 eV) and Zr— O (at ca. 531 eV) for the various supports. In

addition, an obvious difference can be detected between the intensity of Zr— O in different
supports. The intensity of Zr — O is directly proportional to the Zr content of the supports and in
the order of ZrSBADP2 > ZrSBAP2 > ZrSBAD10. In addition, no peak at 528-529 eV related to
Zr — O of pure ZrO2 was observed during XPS experiment [5,25,40]. As shown in Figure 4, no
crystalline phase of ZrO2 was distinguished in the XPS spectra of Zr3d for different Zr-modified
SBA-15. The Zr 3d5/2 spectra of ZrSBADP2, ZrSBAP2 and ZrSBAD10 centered at 183, 183.2
and 183.4 eV, respectively, are far from 182.1 eV related to Zr4+ in ZrO2 cluster [41].
The deconvoluted XPS spectra of the catalysts are shown in Figure 5a-d and Table 3. For
Co1/ZrSBADP2 and Co2/ ZrSBADP2, the core level binding energies of Co2p3/2 were centered
at 782.3 and 782.4 eV, respectively. Two satellites corresponding to two spin-orbital peaks also
emerged at 787 eV and 786.8 eV, respectively. These BE values associated with satellite shake-
up features indicate that the oxidation state of Co in these two catalysts is 2+ [42]. In addition,
the core level BE values of Co2p1/2 for both samples was determined at 798.1 eV which is close
to that of Co2+ ions. Also, the satellite for Co2p1/2 was observed at 803.9 and 803.7 eV for
Co1/ZrSBADP2 and Co2/ ZrSBADP2, respectively. These results further suggest that the
majority of the cobalt atoms are present as Co2+ clusters in tetrahedral coordination in
Co1/ZrSBADP2 and Co2/ZrSBADP2 catalysts [43,44]. Co4/ZrSBADP2 showed two BE values
for each of Co2p3/2 and Co2p1/2 at 782 and 783.7 eV, and 797.2 and 798.5 eV, respectively. In
addition, two satellite related to the two spin-orbital peaks were observed 787.1 and 803.8 eV,
respectively. These results further verify that cobalt atoms are present in oxidation state of (II) in
Co4/ZrSBADP2. The existence of the second peak for Co2p3/2 (centered at 783.7 eV) and
Co2p1/2 (centered at 798.5 eV) implies that some of the cobalt (II) species exist in octahedral
coordination and/or cobalt oxide in Co4/ZrSBADP2 [45]. Similar trend can be observed for

Co10/ZrSBADP2. However, the intensities of the second peaks of Co2p3/2 and Co2p1/2 were
higher than those of Co4/ZrSBADP2 (Table 3).
3.1.5 FTIR analysis
The incorporation of Zr into the SBA-15 framework was studied by FTIR spectroscopy (Figure
S2). All the samples exhibited a band around 3440 cmꢁꢂ and at 1634 cmꢁꢂ assigned to O — H
stretching vibration mode of the silanols groups bonded with the hydrogen and Zr — OH
stretching vibration, respectively[46]. The intensity of the broad vibrational bands centered at
3440 cmꢁꢂ and 1634 cmꢁꢂ are directly proportional to the concentration of the hydroxyl and
adsorbed water molecules which are affected by Zr content [47]. ZrSBADP2 with highest Zr
content exhibited the highest intensity of O — H vibrational bands.
A broad strong band between 1200 and 810 cmꢁꢂ was observed for all the samples. This band is
comprised of several bands; the bands at 1200 cmꢁꢂand 1080 cmꢁꢂ are related to asymmetric
stretching vibration of SiO4 units, the band at around 970 cmꢁꢂ is assigned to Si— O— Zr
symmetric stretching, the band at 880 cmꢁꢂ is assigned to Si— Oδ- (non-bridging oxygen atoms),
and the band at 803 cmꢁꢂ is ascribed to symmetric stretching of Si— O [46,48]. The presence of
the band at 970 cmꢁꢂ in spectrum of Zr-loaded SBA-15 (related to Si— O — Zr) suggests that
Zr is effectively introduced into the SBA-15 framework [16]. Since no IR band in the range of
910 and 970 cmꢁꢂ was observed for the SBA-15, the presence of the band at 970 cmꢁꢂ for the
Zr-modified supports can be considered as an obvious evidence for the isomorphous contribution
of Zr ions [49]. Furthermore, the intensity of the band at 970 cmꢁꢂ is directly proportional to the
Zr loading where the largest intensity of the band was observed for ZrSBADP2. This
phenomenon might be related to greater substitution and deposition of Zr species either in the
framework or on the surface of the SBA-15 structure during the DP modification process.

The broad adsorption band between 600 cmꢁꢂand 400 cmꢁꢂ centered at 460 cmꢁꢂ , is assigned
to Zr — O— Si and/or Si — O— Si vibration [16,46,50]. Since, the same band was observed for
SBA-15 as well, the contribution of Si — O — Si band cannot be excluded.
FTIR analysis of catalyst with highest cobalt content (Co10/ZrSBADP2) was performed for
comparison with the support (ZrSBADP2). The result showed that the spectrum pattern of the
catalyst was comparable to the support with negligible difference (Figure S2).
3.1.6 Surface acidity analysis
As given in Table 2, the supports and catalysts exhibited both Brönsted and Lewis acid sites. The
quantities and simultaneous presence of these two types of acid sites play a critical role in HC-
SCR reactions [1]. It has been demonstrated that the quantities of the Brönsted and Lewis acid
sites are affected by the material preparation method. The Brönsted and Lewis acid sites of the
ZrSBAD10, ZrSBAP2 and ZrSBADP2 were measured to be 92 and 29μmol. gꢁꢂ, 96 and
32 μmol. gꢁꢂ, and 99 and 37 μmol. gꢁꢂ, respectively. This trend is in line with the trend of Zr
content in the supports, where ZrSBAD10 with the lowest content of Zr exhibited the lowest
acidity, and ZrSBADP2 with the highest Zr content showed the highest acidity. The relationship
between the Zr content and acidity is in good agreement with literature where a direct
proportional relationship between acidity and Zr content of mesoporous silica were reported
[5,51,52]. The acidity of the catalysts did not change significantly after Co loading into the
support. Lewis and Brönsted acidity decreased slightly from 37 to 34 μmol. gꢁꢂ and from 97 to
91 μmol. gꢁꢂ, respectively, when Co content increases from 1wt% to 10 wt%. Since the
concentration and strength of Brönsted and Lewis acid sites play an important role in NOx
reduction with hydrocarbons [35,53–55], ZrSBADP2-supported cobalt catalysts possess a high
potential to be applied as active catalysts in C3H8-SCR reaction.

3.2 Catalytic performance
3.2.1 Effect of cobalt content
NO conversion to N2 (%) over the catalysts with various cobalt contents at a temperature range
of 250-500˚C is shown in Figure 6. Three distinct temperature ranges was considered, namely
250-300ꢀ, 350-400ꢀ and Tꢃ 450ꢀ. At 300ꢀ, the catalysts with higher content of cobalt,
Co10/ZrSBADP2 and Co4/ZrSBADP2, exhibited 25 % and 21 % N2 yield, respectively, which
were higher than those of Co2/ZrSBADP2 (18 %) and Co1/ZrSBADP2 (5%). At 400ꢀ, the
catalyst with 2 wt% cobalt exhibited 70 % N2 yield which was 50% and 20% higher than those
of Co10/ZrSBADP2 and Co4/ZrSBADP2. Also, the cobalt content of 1 wt% was proved to be
sufficient to show higher NO conversion (67 %) compared to the Co10/ZrSBADP2 and
Co4/ZrSBADP2 at the same temperature. In the high temperature region, the activity of the
catalysts were inversely proportional to the cobalt content. Co1/ZrSBADP2 showed 45 % N2
yield at 500ꢀ while Co10/ZrSBADP2 had only 12 % N2 yield. Therefore, it is demonstrated that
the activity of the catalysts was strongly affected by the cobalt content. Since ZrSBADP2
exhibited neither NO reduction through the entire temperature range, nor CO2 formation at
T ꢡ 350 ꢀ, it is concluded that cobalt species play a crucial role in reduction of NO through
propane activation that results in the formation of intermediate components, namely, oxygenated
hydrocarbons (CxHyOz). In addition, no N2O formation was detected as the by-product of the
reaction during the NO reduction over the catalysts at different temperatures. In order to
elucidate the effect of cobalt content on the catalyst performance, two general competitive
reactions were considered in the SCR of NO with propane:
ꢔꢉꢊ ꢢ ꢣꢤꢥꢦ ꢢ ꢧꢊꢔ → ꢉꢔ ꢢ ꢧꢥꢔꢊ ꢢ ꢤꢣꢊꢔ
ꢣꢤꢥꢦ ꢢ ꢨꢊꢔ → ꢧꢥꢔꢊ ꢢ ꢤꢣꢊꢔ
(R 1)
(R 2)

It should be noted that reactions (R1) and (R2) are not intended as a SCR mechanism over the
catalysts. In addition, the oxidation of NO to NO2 might also occur during the NO reduction in
the presence of the oxygen [44].
The proceeding of reactions (R1) and (R2) is strongly affected by the nature of the cobalt species
on the catalysts, which is, in turn, influenced by the amount of cobalt content [13,56,57]. High
concentration of cobalt in Co10/ZrSBADP2 results in the formation of cobalt species which has
weak interactions with the support and can be easily reduced at low temperature. Moreover, the
production of cobalt oxides as a form of nano-sized Co3O4, active in NO reduction at low
temperature, might occur in the high-loaded cobalt catalyst which was already validated by XPS
analysis and has been reported by Martinez et al. [58]. The oxidation of NO to NO2, as one the
main steps in HC-SCR, is improved in the presence of these cobalt oxide species which in turn
facilitate the activation of hydrocarbons for NOx reduction at low temperatures. However, at
high temperatures, reaction (R2) is favored to a greater extent by Co3O4 than reaction (R1) [59]
which leads to a significant deactivation of Co10/ZrSBADP2.
Co4/ZrSBADP2 was less active than Co10/ZrSBADP2 at low temperature however it showed a
better activity at high temperature. This behavior might be due to the presence of various cobalt
species on the catalyst. A mixture of Co2+ ions and cobalt oxide is present on the two catalysts.
However, from the XPS results (Table 3), the intensity of the cobalt oxide species in
Co10/ ZrSBADP2 was much higher than Co4/ZrSBADP2. Thus, Co4/ZrSBADP2 was less
active than Co10/ZrSBADP2 at low temperatures whereas it showed a better activity at higher
temperature. Furthermore, it is well known that the formation of NOx-ad species over the
catalyst is one the main important steps in HC-SCR reaction [60,61]. The types of the formed
NOx-ad species are affected by the metal loading. Highly stable nitrate species including

monodentate and bidentate nitrates can be produced over the high metal-loaded catalysts [62].
Considering that the dissociation of these stable species occurs at high temperatures, direct
oxidation of propane (R2) is also accelerated at such high temperatures by high-loaded metal
catalyst. Thus, the overall NO reduction decreases over Co10/ZrSBADP2 and Co4/ZrSBADP2
at high temperatures.
The cobalt oxide species (active at ꢩ ꢡ 300 ꢀꢆ are not present in the catalyst with low cobalt
content. Therefore, Co2/ZrSBADP2 possesses merely Co2+ ions in tetrahedral coordination and
cobalt silicate which favors the reaction (R1) at ꢩ ꢪ 300 ꢀ. Moreover, further decrease in the
cobalt content, from 2 wt% to 1 wt%, did not lead to any significant change in the catalytic
activity of Co2/ZrSBADP2 and Co1/ZrSBADP2 at ꢩ ꢃ 400 ꢀ. However, a slight change in the
performance at ꢩ ꢫ 400 ꢀ can be assigned to the difference in the dispersion of the cobalt
species on the support and/or interaction with the support. The intensity of the shake-up satellite
related to Co2p3/2 and 2p1/2 of the Co2/ZrSBADP2 were higher than that of Co1/ZrSBADP2
(Table 3). This phenomenon indicates that the interaction of the cobalt species in
Co1/ZrSBADP2 with the support is stronger than that of Co2/ZrSBADP2 [43] which results in a
slight decrease in the reducibility of the cobalt species at low temperatures [58] and therefore,
lower activity of the Co1/ZrSBADP2 at ꢩ ꢫ 400 ꢀ.
Since Co2/ZrSBADP2 exhibited better NO reduction compared to the other catalysts, a 2 wt %
cobalt loading was considered as an optimum loading ratio for further study.
3.2.2 Effect of Zr content
In order to evaluate the effect of Zr content on the catalyst performance, the supports,
ZrSBAD10, ZrSBAP2,ZrSBADP2, and SBA-15 were loaded with 2 wt % cobalt and their NOx
conversion efficiency was determined. As depicted in Figure 7, the catalyst performance was

directly proportional to the Zr content. Co2/ZrSBADP2 with the highest Zr concentration
exhibited the highest N2 yield at all temperatures, while Co2/SBA and Co2/ZrSBAD10 did not
show any considerable N2 yield. This phenomenon can be attributed to the difference in the
acidity of the supports by the Zr content, where the amount of Lewis and Brönsted acidity is
ZrSBADP2 > ZrSBAP2 > ZrSBAD10. The conversion of NO to N2 occurs via the formation of
nitrates adsorbed (NOx-ad) species through several sequential reactions on the Lewis acid sites
[35]. Subsequently, the NOx-ad species are reduced to N2 by the oxygenated hydrocarbons
(CxHyOz) [60,61]. Thus, it can be inferred that an increase in the Lewis acid density (increase in
the Zr content) will elevate the amount of the NOx ad-species, which, in turn, will increase the
overall conversion of NO to N2. Moreover, it has been reported that HC-SCR is also promoted
by Brönsted acidity of the catalysts [63–67]. Orlyk [64] showed that the catalytic efficiency of
the oxide systems is directly proportional to the concentration of Brönsted acid sites of the
catalysts surface. This can be explained by the fact that the formation of oxygenated
hydrocarbons (CxHyOz), as intermediate components in HC-SCR, and hydrocarbon activation are
promoted by Brönsted acid centers [63,67]. Through the novel DP technique, higher contents of
Lewis and Brönsted acid sites can be achieved which can highly enhance the NO to N2
conversion.
3.2.3 Effect of propane concentration
The performance of the optimum catalyst was evaluated at two concentrations of propane (500
and 1250 ppm) while other parameters were maintained constant (500 ppm of NO, 6 vol % of O2
in helium as balance with the total flow rate of 100 ml. minꢁꢂ). According to Figure 8a, different
trends at two temperature regimes ꢄT ꢡ 350˚C and T ꢃ 400 ˚Cꢆ were observed during NO
reduction. At T ꢡ 350ꢀ, increasing the propane concentration from 500 ppm to 1250 ppm did
not lead to any enhancement in the NO to N2 conversion. This result is in line with the amount of

C3H8 conversion to CO2 at T ꢡ 350 ˚C (Table 4). C3H8 conversion to CO2 was about 2 % and
15% at 300 and 350ꢀ, respectively, with respect to that at 500ꢀ in the presence of 1250 ppm of
propane. Nonetheless, the conversion increased to 7% and 37% in the presence of 500 ppm of
propane at the same temperatures. This phenomenon might be explained by the fact that in the
presence of high concentration of propane, some of the active centers of the catalyst are blocked
by the higher fraction of unreacted hydrocarbon species, because the temperature is not high
enough to proceed the reaction towards N2 and CO2 formation in order to recover the active
centers.
At 400, 450 and 500ꢀ, N2 yield increased significantly from 70% to 100 %, 60% to 98% and
36% to 68%, respectively, with increasing the propane concentration. At high temperatures,
higher concentration of reducing agent increases the formation of oxygenated hydrocarbon
intermediate species (CxHyOz) which results in a strong enhancement in the catalytic activity
towards N2 formation [36,68,69]. Moreover, at higher reducing agent concentration, the metal
sites on the catalyst are reduced more efficiently which results in higher NOx reduction [36].
3.2.4 Effect of oxygen concentration
The efficiency of Co2/ZrSBADP2 was evaluated under various concentrations of oxygen (Figure
8b). In the presence of 3 vol % of oxygen, a 23 % and 65 % N2 yield was achieved at 350 and
400ꢀ, respectively. By increasing the oxygen concentration from 3 vol % to 6 vol %, NO
conversion to N2 was favored at ꢩ ꢡ 400ꢀ and reached 54% and 70% at 350ꢀ and 400ꢀ,
respectively. These results are consistent with the previous studies in which the oxidation of NO
to NO2 has been shown as one of the main steps in HC-SCR reaction in the presence of oxygen
[70,71]. In addition, the formation of partial oxidized hydrocarbon species (CxHyOz) proceeds via
the interaction with oxygen atoms and/or nitrogen containing intermediate species which leads to

the reduction of nitrogen oxide species to N2. This phenomenon can be more pronounced in the
presence of oxygen, [71–73]. On the other hand, the activity of the catalyst was slightly
suppressed when the concentration of O2 increased from 6 vol % to 10 vol % over the entire
temperature range. The maximum NO conversion to N2 decreased to 60% in the presence of 10
vol % of oxygen at 400ꢀ. This observation is in accordance with the results reported by Lou et
al. [74] and Kotsifa et al. [75]. They have attributed this reduction to the increase in the oxidation
rate of C3H8 to CO2 (R 2) at higher oxygen concentration [76], as validated in Table 4. It was
shown that the increase in oxygen concentration from 6 vol % to 10 vol % resulted in 13 %, 21
% and 16 % increase in the quantity of CO2 formation at 300, 350 and 400ꢀ, respectively.
Moreover, it is well-recognized that the ratio of hydrocarbon to oxygen surface coverages
(ꢅꢬꢭ/ꢅꢮ) plays an important role in HC-SCR reaction rate [77]. In the presence of high
concentration of propane and the absence of oxygen, hydrocarbonaceous species are deposited
on the catalyst active sites which diminish the NO reduction. Nonetheless, deposited
hydrocarbonaceous species are removed with the adsorbed oxygen (O-ad) as a result of the
introduction of O2 in the feed gas. Hence, the active sites are recovered and become available for
further NO reduction. However, excessive increase in oxygen concentration leads to active site
poisoning with O-ad due to insufficient amount of propane to recover the active sites [75,77]. It
is interesting to note that the catalyst could restore its activity when the oxygen concentration
was returned to 6 vol % of the inlet concentration. These results confirm the high tolerance of
Co2/ZrSBADP2 under severe condition, such as car exhaust [14].
3.2.5 Effect of water vapor
Figure 8c shows the performance of the catalyst in the presence of 7 vol% water vapor, as a
typical value for a lean-burn engine exhaust [36]. The activity of the catalyst was remarkably

enhanced by the presence of water vapor in the range of 300-350ꢀ, where the N2 yield increased
from 18 % to 34 % and from 54 % to 70 % at 300 and 350ꢀ, respectively. This can be explained
by the fact that, in the absence of water vapor, the active surface sites of catalysts might be
covered by the carbonaceous species resulting in the suppression of the catalyst performance.
Presence of water vapor has a depression effect on the accumulation of carbonaceous species on
the catalyst surface, and thus leads to an increase in the catalytic activity [78]. This rationale can
be validated by comparing the CO2 formation in the presence and absence of the water vapor
(Table 4) as well as propane conversion (Figure 8c). In the presence of water vapor, the propane
conversion was higher than that in the dry feeding at all examined temperatures. The results
indicate that the presence of water vapor causes a significant increase in the formation of CO2 in
the range of 300-350ꢀ. The formation of CO2 in the presence of water vapor was about 2 and
2.5 times more than those in the absence of the steam at 300 and 350ꢀ, respectively. These
findings are in good agreement with previous reports [7,79]. Furthermore, the formation of
organic nitroso or nitro species (R — NO, R — NO2) via gas phase or surface reaction has been
evidenced as intermediate species in HC-SCR system [7,80,81]. Nitroso or nitro species are
adsorbed as NCO and CN species through several reaction steps. Ultimately, NCO and CN
species can be converted to N2, CO2 and H2O through the following reaction pathways; (i)
oxidation by NO2 gas in which the reaction rate might increase in the presence of water vapor ,
(ii) conversion to ammonia through hydrolysis by water vapor and then selective reduction of
NO by the formed NH3 [7,82]. Thus, the presence of water vapor enhances the NO reduction to
N2 irrespective to the reaction pathways.
Figure 8c also shows that the activity of the catalyst was not affected by the water vapor at
temperatures higher than 400ꢀ. The extent of CO2 formation was not affected by the steam

at ꢩ ꢃ 400 ˚C (Table 4) which is accordance with the previous report [78]. This indicates that the
presence of water vapor did not change the reaction pathway or the availability of the propane
for NO reduction at high temperatures [83]. Furthermore, the competition between water ad-
species and reactants/intermediate species for the active sites are weakened at high temperatures,
because the abundance of water ad-species is considerably decreased and therefore the NO
reduction will be independent of the water vapor presence [15].
In contrast to the most developed catalysts which are deactivated by H2O [82,84,85], the high
tolerance of Co2/ZrSBADP2 towards H2O renders it suitable for practical NOx reduction
applications in lean burn HC-SCR.
3.2.6 Long-term activity
The long-term stability of the prepared catalyst in the real exhaust condition is a crucial concern
for practical applications. The activity of Co2/ZrSBADP2 was evaluated for longer reaction time
(60 h) at 400ꢀ in the presence and absence of water vapor (Figure 9).No significant decrease in
N2 yield was observed during the prolonged reaction. The catalyst could retain a 70 % NO
conversion to N2 for 60 h regardless of the presence or absence of water vapor. This indicates the
hydrothermal stability of the catalyst structure and the cobalt active components on the support
under reaction conditions.
The efficiency of the developed catalyst was more pronounced when it was compared with
2%Ag/Al2O3 as a benchmark catalyst. 2%Ag/Al2O3 did not show any NOx reduction at
T ꢡ 350ꢀ. Moreover, NO conversion to N2 of 8%, 20% and 28% was achieved at 400, 450 and
500ꢀ, respectively (Figure 6). These results are in a good agreement with the literature in which
Ag/Al2O3 has been used as a catalyst in HC-SCR [14,68,86,87]. Shimizu et al.[86]reported that
10% of NO was converted to N2 by 2%Ag/Al2O3 catalyst in the presence of 6000 ppm of

propane at 400ꢀ. Similar result was observed by Hernández-Terán et al. [87]. This implies that
Co/ZrSBADP2 is more active than the benchmark catalyst, which is beneficial for the practical
application in NOx reduction and depletion of unburned HC emission (Figure S3).
4 Conclusion
Highly loaded Zr-SBA-15 mesoporous materials were prepared by a novel DP technique. Zr
content of the prepared material (32.1 wt%) was considerably higher than those prepared through
the conventional methods. Characterization of the catalysts and supports showed an ordered
mesoporous structure with high specific surface area (548mꢟ. gꢁꢂ), homogeneous dispersion of
Zr and high acidity. A cobalt-based catalyst was prepared by loading various amounts of the
cobalt into the developed support. A 70 % N2 yield was achieved over the catalyst with a cobalt
content of 2 wt% in the absence of water vapor at 400ꢀ. Presence of water vapor not only did
not suppress the NOx conversion efficiency, but also significantly increased the N2 yield at
relatively low temperatures. The evaluation of the long-term stability of the catalyst at high
temperatures showed that the catalyst was able to retain its high efficiency for a prolonged period
of 60 h without any deactivation.
The promising activity of Co2/ZrSBADP2 under various extreme conditions, i.e. long-term
stability and presence of water vapor, confirms the potential applicability of the catalyst for NOx
conversion in real lean-burn gas and natural-fueled engines. Meanwhile, it is cost effective since
no noble metal was used.

Acknowledgment
We acknowledge the financial support from the Environment and Conservation Fund of Hong
Kong under the grant No. ECWW15EG02 and the Hong Kong University of Science and
Technology under the grant No. RPC10EG24.
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