Impacts of SiO2 Shell Structure of Ni@SiO2 Nanocatalysts on Their Performance for Catalytic Decomposition of Ammonia

Article abstract of DOI:10.1007/s10562-016-1908-1

Abstract: Various core–shell structured Ni@SiO₂nanocatalysts were prepared to provide an insight into the intrinsic effects of shell structures on the ammonia decomposition activity. The thickness of silica shell structure can be fine tuned in the range of 5–20?nm. With the increase in shell thickness, it was found that the catalytic activity did not decrease obviously, even though the reducibility determined by H₂-TPR exhibited a noticeable changes. As for the porosity of the shell, the SP-1 sample with a well-developed pore structure showed better catalytic performance than the SP-0 sample with a less-developed pore structure. It can be concluded that diffusion limitations would be greatly influenced by shell porosity nor shell thickness to some extent. Graphical Abstract: Various core-shell structured Ni@SiO₂nanocatalysts were prepared to provide an insight into the intrinsic effects of shell structures on the ammonia decomposition activity. The thickness and porosity of silica shell structure were fine tuned. It was concluded that diffusion limitations would be probably greatly influenced by shell porosity nor shell thickness to some extent. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1908-1

Catal Lett
DOI 10.1007/s10562-016-1908-1
Impacts of SiO Shell Structure of Ni@SiO Nanocatalysts
2
on Their Performance for Catalytic Decomposition of Ammonia
2
1
1
1
1
Lei Li · Jun Wu · Jingling Shao · Zhe Tang · Yong Dai · Huawei Chen
1
1
Received: 23 August 2016 / Accepted: 2 November 2016
©
Springer Science+Business Media New York 2016
Abstract Various core–shell structured Ni@SiO nano-
2
catalysts were prepared to provide an insight into the intrin-
sic efects oꢀ shell structures on the ammonia decomposi-
tion activity. The thickness oꢀ silica shell structure can be
ꢁ
ne tuned in the range oꢀ 5–20 nm. With the increase in
shell thickness, it was ꢀound that the catalytic activity did
not decrease obviously, even though the reducibility deter-
mined by H -TPR exhibited a noticeable changes. As ꢀor
2
the porosity oꢀ the shell, the SP-1 sample with a well-devel-
oped pore structure showed better catalytic perꢀormance
than the SP-0 sample with a less-developed pore structure.
It can be concluded that difusion limitations would be
greatly inꢂuenced by shell porosity nor shell thickness to
some extent.
Graphical Abstract Various core-shell structured Ni@
Keywords Nickel catalyst · SiO shell · Difusion efect ·
2
SiO nanocatalysts were prepared to provide an insight
2
Hydrogen production · Ammonia decomposition
into the intrinsic efects oꢀ shell structures on the ammonia
decomposition activity. The thickness and porosity oꢀ silica
shell structure were ꢁne tuned. It was concluded that diꢀ-
1 Introduction
ꢀ
usion limitations would be probably greatly inꢂuenced by
shell porosity nor shell thickness to some extent.
Recently, core–shell structured materials have attracted
great attentions ꢀor their unique structural ꢀeature and phys-
icochemical properties [1–4]. They consist oꢀ a nanosized
catalytic active core that is coated by a porous, thermally
stable support. The shell materials not only hinder the
aggregation oꢀ neighboring particles, even under harsh
reaction conditions, but also provide channels ꢀor reactants
accessible to cores.
Based on this concept, a series oꢀ core–shell struc-
tured catalysts have already been developed ꢀor catalytic
application. Most oꢀ the investigated systems consist oꢀ
Au or Pt nanoparticles in a metal oxide shell, such as
silica [5, 6], zirconia [7], tin oxide [8], carbon [9], or
cobalt oxide [10]. However, all oꢀ these reactions are
*
Lei Li
lee_ycit@yahoo.com
1
School oꢀ Chemistry and Chemical Engineering, Yancheng
Institute oꢀ Technology, Yancheng 224051, China
1
3

L. Li et al.
perꢀormed ꢀar below 400 °C, although the presented shell
materials can principally be used ꢀor reactions at higher
temperatures, in which sintering processes play a more
crucial role [8]. Some core–shell structured nanocata-
lysts are also developed ꢀor high temperature reaction,
2 Experimental Section
2.1 Catalyst Preparation
2.1.1 Preparation of NiO Nanoparticles (NPs)
ꢀor examples, Ni@SiO [11, 21] and NiCo@SiO [12]
2 2
and their modiꢁed catalysts [13] ꢀor partial oxidation oꢀ
methane reactions, Ni@Al O , Ru@SiO and Ni@MgO,
Typically, 2.9 g oꢀ Ni(NO ) ·6H O was dissolved in 40
3 2 2
mL oꢀ deionized water, and the solution was added drop-
wise into a solution that contained 100 mL oꢀ deionized
water and 330 mg oꢀ polyethylene glycol (PEG) (average
MW=20,000, Fluka) and 1.0 g NaOH. The resulting solu-
tion was stirred ꢀor 1 h at room temperature (RT). The col-
lected material was then washed several times with deion-
ized water and ethanol, dried at 50°C ꢀor 24 h, and calcined
in air at 400°C ꢀor 3 h.
2
3
2
Ce-doped Ni@SiO2 ꢀor NH3 decomposition reactions
14–16] and Pd@CeO /Al O [17] ꢀor methane combus-
[
2
2
3
tion reactions. Consequently, the shell can prevent the
core ꢀrom aggregating or sintering into lager particles,
which keeps a long time stability under high temperature
conditions.
Besides the protection ꢀunction oꢀ core–shell struc-
tural catalysts, this novel structure, including shell thick-
ness and porosity, as well as the core particle size, also
greatly inꢂuence the catalytic perꢀormance. As well-
known, adsorption oꢀ reactants on the surꢀace oꢀ the
active phase oꢀ the catalyst is necessary ꢀor reactions
to take place, engineering the porosity oꢀ shell materi-
als makes it possible to control the difusion oꢀ reactant
species and the reaction kinetics. However, up to date,
there are ꢀew studies on the relationship between the
shell structure and catalytic perꢀormance [18–22]. An
understanding oꢀ the parameters that afect the difusion
oꢀ molecules through the shells oꢀ these structures may
give rise to the design and synthesis oꢀ new catalysts.
Ammonia decomposition is an important reaction in
at least two diferent ꢁelds related to energy and envi-
2.1.2 Preparation of NiO@SiO Core–Shell Structured
2
Catalyst
The core–shell structured NiO@SiO samples were pre-
2
pared by a modiꢁed Stöber method [11]. Typically, 0.1 g
oꢀ NiO NPs was added to a poly-(vinylpyrrolidone) (PVP,
K30, 1.0 g) ethanol solution oꢀ 100 mL. Aꢀter the solu-
tion was stirred ꢀor 12 h, NH ·H O (25 wt%) oꢀ 10 mL was
3
2
added, and then the suspension was sonicated ꢀor 30 min in
an ultrasound cleaner (KQ-100DE, 40 kHz, 100 W). Sub-
sequently, an ethanol solution (5 mL) oꢀ tetraethyl ortho-
silicate (TEOS,P99%, Aldrich) oꢀ 0.1 mL was injected into
the suspension. One hour later, the product was collected
by centriꢀugation, washed twice with distilled water and
ethanol, and dried at 80°C in air ꢀor 6 h. Then the core–
shell samples were ꢀurther calcined at 550°C ꢀor 3h ꢀor
removing surꢀactant. For comparison, the core–shell struc-
ronmental science: it can be used to provide CO -ꢀree
x
hydrogen ꢀor ꢀuel cells, and to remove ammonia ꢀrom
the reꢀormate oꢀ internal gasiꢁcation combined cycle
(
IGCC) power plants which might be widely deployed
tured NiO@SiO samples were prepared with two kinds
2
in the ꢀuture as a CO -removal technology [23, 24]. For
2
oꢀ surꢀactants (mass ratio oꢀ PVP/CTAB=1/4, Hexadecyl
trimethyl ammonium Bromide). And they were labeled as
SP-0 and SP-1 ꢀor distinguishing the porosity oꢀ the shell
respectively. Similar to above preparation process, the shell
thickness was controlled by tuning the amount oꢀ TEOS.
The samples were denoted as ST-x, where x represented
the mean shell thickness (5–20 nm)as determined by TEM
images.
hydrogen production, ammonia decomposition requires
temperature above 400 °C ꢀor thermodynamic reasons.
As it is reported that complete conversion oꢀ ammonia
take place at around 500 °C ꢀor Ru-based catalysts [26–
3
2], at around 700 °C ꢀor other metals, such as Ni, Co or
Fe, etc. [33–40] Thereꢀore, highly active catalysts at low
temperatures are required, but the high-temperature sta-
bility and long liꢀetime oꢀ the catalysts applied is a more
crucial issue.
2.2 Catalyst Characterization
Herein, on the basis oꢀ the high temperature stabil-
ity oꢀ core–shell structured nanocatalysts, we give a new
insights into the correlations between shell structure
N sorption measurement was perꢀormed on a NOVA-2020
2
material physical structure determinator. Beꢀore meas-
urement, the sample was degassed at 300°C ꢀor 3 h. The
BET surꢀace area was calculated ꢀrom a multipoint BET
analysis oꢀ the nitrogen adsorption isotherms. The X-ray
powder difraction (XRD) patterns were recorded on a
Philips X’Pert MPD Pro X-ray difractometer with graph-
ite-monochromatized Cu Karadiation (Kα=0.1541 nm).
(
including shell thickness and porosity) and ammonia
decomposition activity. The occurrence oꢀ difusion lim-
itations and their consequences in connection with the
shell structure are illustrated through some physical and
chemical characterizations.
1
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Impacts oꢀ SiO Shell Structure oꢀ Ni@SiO Nanocatalysts on Their Perꢀormance ꢀor Catalytic…
2
2
The TEM images were taken over a JEOL JEM-2000EX
instrument operated at 100 kV. The HRTEM images were
obtained with a Philips Tecnai G220 operated at 200 kV.
(
111)
Hydrogen temperature programmed reduction (H -TPR)
2
was carried out using a TP-5080 multi-purpose automatic
adsorption instrument. In typical runs, 30 mg oꢀ NiO@
(200)
After Reduced
SiO was heated to 120°C at 10°C/min under Ar ꢂow oꢀ
2
(220)
5
0 mL/min and kept at this temperature ꢀor 1 h to remove
adsorbed water. Aꢀter the sample cooled down to RT and
was switched to a 10% H /N (v/v, 50 mL/min) mixture, the
(
111)
(200)
(220)
2
2
As synthesized
(311)
sample temperature was programmed to 650°C at 10°C/
min.
1
0
20
30
40
50
θ/°
60
70
80
2
.3 Catalytic Activity
2
Catalytic activity was evaluated in a continuous-ꢂow
quartz reactor. In a standard experiment, 25 mg oꢀ powder
catalyst was placed on quartz wool in the reactor with an
inner diameter oꢀ 6 mm. Beꢀore reaction, the catalyst was
Fig. 1 XRD patterns oꢀ typical NiO@SiO (SP-1) sample: As syn-
2
thesized and aꢀter reduced
reduced in situ in a 25% H /Ar ꢂow at 550°C ꢀor 2 h, then
2
weak drum peak occurs at around 23°. Through calculation
ꢀrom the Scherrer equation, the crystallite size oꢀ NiO NPs
is 9.2 nm based on the NiO (200) reꢂection peaks. When
calculation has been done according to other three difrac-
tion peaks, the crystallite size oꢀ NiO NPs is 10.2m, 9.2
and 9.7 nm respectively. Thereꢀore, the mean size oꢀ NiO
NPs is ca.10 nm. Aꢀter in-situ reduction, core–shell struc-
purged with a ꢂow oꢀ pure Ar. Subsequently, the tempera-
ture was decreased to 350°C and the gas ꢂow was switched
to pure NH . For temperature-dependent conversion meas-
3
urements oꢀ NH , the NH ꢂow was ꢁxed at 30,000 mL/
3
3
gcat/h, and the temperature was varied between 400 and
00°C in 50°C steps. For each temperature, ꢀour values
7
were recorded within 30 min under steady-state condi-
tions using an on-line gas chromatograph (GC9860). The
GC is equipped with a thermal conductivity detector (TCD)
tured Ni@SiO catalysts are obtained with a crystallite size
2
oꢀ 13.2 nm, which is calculated ꢀrom the Ni (111) reꢂec-
tion peak. According to our previous research [11], due to
the dispersion oꢀ NiO NPs in ethanol solution, it is hard to
realize 100% single-particle encapsulation, and there is a
certain degree oꢀ multiparticles encapsulation. Thus, com-
pared with the mean size oꢀ original NiO NPs, the mean
size oꢀ Ni NPs enwrapped in silica shell becomes large
aꢀter in-situ reduction.
and Poropak Q column to detect NH in a carrier gas ꢂow
3
oꢀ H . NH conversion in a blank reactor was <1.0% at
3
2
6
00°C.
3
Results and Discussion
3
.1 XRD Characterization
3.2 TEM Characterization
Figure 1 shows the XRD patterns oꢀ the typical prepared
core–shell structured NiO@SiO2 and reduced Ni@SiO2
nanocatalysts. The original nickel oxides nanoparticles
were obtained by calcining amorphous nickel hydroxide
at 400°C ꢀor 3h. Then, NiO NPs were coated with silica
through the modiꢁed stöber method. The correspond-
ing samples show only the ꢀcc-NiO phase (JCPDS No.
TEM images oꢀ the non-reduced catalysts with various sil-
ica shells are presented in Fig. 2. As core materials, NiO
NPs exhibit a nearly homogeneous and monodisperse, with
a mean size oꢀ 9.1± 1.6 nm (Fig. 2a, b). When calculation
has been done ꢀrom their special surꢀace area, the mean
size oꢀ NiO NPs is 10.7 nm (Table 1). This is consistent
with the mean size oꢀ NiO NPs obtained by XRD and TEM
characterization. The encapsulation oꢀ the NiO with silica
was achieved by means oꢀ the base-catalyzed hydrolysis oꢀ
tetraethylorthosilicate (TEOS) in an ethanol/water mixture.
Seen ꢀrom Fig. 2c–ꢀ, all catalysts show the core–shell mor-
phologies with NiO well encapsulated by the silica shells.
As well-known, the preparation oꢀ well deꢁned core–shell
structures is only possible iꢀ a homogenous distribution oꢀ
the colloids via the classic Stöber process. However, the
7
8-0429) with typical reꢂections oꢀ the (111), (200), and
220) planes at 2θ=37°, 43° and 64°, respectively. Aꢀter
in-situ reduction, the NiO entities were completely trans-
(
0
ꢀ
ormed into elemental Ni , as revealed by the reꢂections
oꢀ the (111), (200), and (220) planes at 2θ=44°, 52° and
6° (JCPDS No. 04-0850). In addition, no peaks oꢀ SiO2
7
crystalline phases can be identiꢁed, suggesting the SiO2
shells are essentially amorphous. It can be observed that a
1
3

L. Li et al.
Fig. 2 TEM images a, b NiO
nanoparticles and their size
distribution; c ST-5 sample; d
ST-10 sample; e ST-20 sample
(
or SP-0); f SP-1 sample
as-prepared NiO NPs would precipitate under the reaction
conditions beꢀore the coating step, so their surꢀaces needed
to be modiꢁed with polyvinylpyrrolidone (PVP). Owing
to the assistant oꢀ amphiphilic and nonionic PVP polymer,
colloidal particles are easily coated with homogeneous sil-
ica shells [41]. And the remaining PVP in silica shell can
be removed by calcination, resulting a high surꢀace area,
which are ꢀurther veriꢁed by BET characterization.
shell thickness increases obviously with the increased dos-
age oꢀ TEOS (Fig. 2c–e). By precisely controlling the
amount oꢀ TEOS and its hydrolysis time, the thickness
can be tuned ꢀrom ca.5 to ca.10 and ca.20 nm, correspond-
ing to ST-5, ST-10 and ST-20. The mean thickness size is
roughly counted by TEM image. In addition, the porosity
oꢀ silica shell is tuned by introducing another surꢀactant.
CTAB as a cationic surꢀactant, which is diferent ꢀrom
PVP, was added together with PVP during the synthesized
process. Note that when the CTAB surꢀactant is introduced,
As ꢀor the thickness oꢀ silica shell, it is well tuned by
changing the amount oꢀ TEOS. It is observed that the silica
1
3

Impacts oꢀ SiO Shell Structure oꢀ Ni@SiO Nanocatalysts on Their Perꢀormance ꢀor Catalytic…
2
2
Table 1 Characteristics oꢀ
NiO@SiO samples with
diferent shell structure
2
SBET (m /g)
Samples
Si/Ni molar ratio
nominal)
Metal loading
a
(wt%)
Pore volume
3
(cm /g)
Shell
thickness
2
(
b
(
nm)
NiO
–
100
84.2
52.3
–
–
ST-5
0.17
0.34
0.67
0.67
82.2
67.6
50.1
52.8
0.08
0.10
0.13
0.18
5.0
ST-10
ST-20 (SP-0)
SP-1
80.6
10.0
20.0
20.0
104.5
341.3
a
Measured by H -TPR, calculation based on the quantity oꢀ NiO NPs sample
2
b
The approximate mean shell thickness was determined by TEM images
the core–shell structure keeps well ꢀor SP-1 sample. And
P0 =0.4. Note that the pore structure oꢀ SP-0 and SP-1 sam-
ple is diferent. For SP-1 sample, besides oꢀ mesopores, a
rapid increase oꢀ sorption occurs at a relatively lower pres-
sure, indicating that some micropores exist in silica shell.
As ꢀor SP-0 sample, the N2 adsorption–desorption iso-
therms show two hysteresis loops, especially at a relatively
higher pressure. This means that the pore structure in shell
is irregular. Moreover, the mesoporous silica shell had a
narrow pore size distribution centered at 2.1 and 5.0 nm ꢀor
SP-1 and SP-0 samples, respectively (Right, Fig. 3). Previ-
ous studies suggested that a certain number oꢀ mesopores
would be ꢀormed during coating process with an assistant
oꢀ PVP surꢀactant [11, 42]. It is well known that the strong
the mean thickness size oꢀ SiO shell is ca.20 nm, which
2
is similar to ST-20 (SP-0) sample (Fig. 2e). It indicates
that the silica shell thickness is afected little by this kind
oꢀ surꢀactant under the same preparation condition. In ꢀact,
compared with SP-0 sample, the SP-1 sample has a high
surꢀace area aꢀter the use oꢀ CTAB surꢀactant, which is ꢀur-
ther veriꢁed by BET results.
3
.3 N Sorption Characterization
2
Figure 3 presents the N adsorption–desorption isotherms
2
and the corresponding pore size distribution curves oꢀ
+
core–shell structured NiO@SiO catalysts with diferent
2
interaction between PVP and CTA is very efective ꢀor the
shell structure. Seen ꢀrom the adsorption–desorption iso-
therms, it shows that no matter SP-0 or SP-1 sample exhib-
its a typical type-IV isotherms with a hysteresis loop and
the capillary condensation occurred at values around P/
subsequent mesoporous silica shell ꢀormation [43]. More
mesopores and larger surꢀace area can be obtained with an
addition oꢀ CTAB [44, 45]. And their textural properties
are placed in Table 1. It is observed that the surꢀace area
Fig. 3 (Left) N adsorption–
2
1
1
1
1
1
80
60
40
20
00
1.0
desorption isotherms and (right)
pore size distribution oꢀ NiO@
SP-0
SP-1
SiO nanostructures aꢀter calci-
2
nation in air at 550°C ꢀor 3 h
0
0
0
0
0
.8
.6
.4
.2
.0
8
0
0
0
0
6
4
2
0
0
.0 0.2 0.4 0.6 0.8 1.0
1
10
Pore width/nm
100
Relative Pressure (P/P0)
1
3

L. Li et al.
2
raises ꢀrom 52.3 to 104.5 m /g with an increase on silica
highly developed porous structure, shows a broader and
lower reduction temperature than SP-0 sample. It sug-
gests that highly developed porous structure ꢀacilitate
small molecular difusion to the core part.
shell thickness. Compared with the SP-0 sample, the SP-1
2
sample shows a larger surꢀace area oꢀ 341.3 m /g, indicat-
ing a well-developed porosity.
3
.4 H -TPR Experiments
2
3
.5 Catalytic Performance of NH Decomposition
3
In order to characterize the reducibility oꢀ NiO NPs and the
possible metal–support interactions, H -TPR experiments
2
The catalytic activities oꢀ the prepared core–shell struc-
tured Ni@SiO2 catalysts ꢀor NH3 decomposition as a
ꢀunction oꢀ reaction temperature are presented in Fig. 5.
The results show a similar behavior that the ammonia
conversions increased with the increase oꢀ the catalytic
reaction temperature measured in the catalyst bed ꢀor all
the catalysts, indicating ꢀaster decomposition at higher
temperature. The approximately complete conversion is
achieved over all catalysts at the temperature oꢀ 700 °C
and GHSV oꢀ 30,000 mL/h/g.
were carried out. Figure 4 illustrates the H -TPR proꢁles
2
oꢀ various NiO@SiO samples. For a quantitative analysis
2
oꢀ the content oꢀ Ni over various catalysts, the pure NiO
sample was chose as a target reꢀerence. The Ni contents oꢀ
corresponding sample are determined by comparing the H2
reduction peak areas. And the calculation results are placed
in Table 1. No doubt that the sample with a thin silica shell
shows a high Ni content. The Ni contents oꢀ SP-0 sample is
similar to that oꢀ SP-1 sample.
Seen ꢀrom Fig. 4, the tested pure NiO sample exhibits
a broad reduction peak in the range oꢀ 200–400 °C. Obvi-
ously, all core–shell structured samples present a higher
reduction temperature than pure NiO sample. Due to
Figure 5 shows that the catalytic activities oꢀ various
core–shell structured Ni@SiO catalysts at temperature
2
ranging ꢀrom 400 to 700 °C. It is observed that increas-
ing the shell thickness leads to a slight decrease in the
catalytic activity, especially at 550 and 600 °C (insert dia-
gram oꢀ Fig. 5). The reasons might be attributed to var-
ied Ni loading and difusion efects resulted ꢀrom shell
thickness. Zeng et al. [20] ꢀound that the shell thick-
ness did not signiꢁcantly weaken the accessibility oꢀ the
reactants to the active sites, which was attributed to the
highly porous nature oꢀ the silica shell. Thereꢀore, higher
porosity oꢀ silica shell can be tuned by adopting CTAB
as another surꢀactant component, which is conꢁrmed by
the BET results (Fig. 3; Table 1). Indeed, it is observed
that the activity oꢀ SP-1 catalyst is better than that oꢀ
silica shell protection, the rate oꢀ H difusion becomes
2
slow. Compared to the supported catalysts, the core–
shell structured catalysts were diꢃcult to be reduced, as
a result oꢀ the stronger core–shell interaction [44, 46].
Thus, a possible core–shell interaction might exist in our
studied catalyst. Note that there is no reduction peak at
around 600 °C, indicating the absence oꢀ nickel silicate
species. This is consistent with above XRD characteriza-
tion. Moreover, we also observed that there is a continu-
ous peak shiꢀt towards higher temperature as the shell
thickness increased, which may be resulted ꢀrom the rate
oꢀ hydrogen difusion. For SP-1 sample, the shell with a
S P -1
ST-20(SP-0)
ST-10
ST-5
NiO
200
300
400
Temperature (°C)
500
600
Fig. 5 The activities oꢀ the core–shell structured Ni@ SiO catalysts
2
with diferent shell structure (reaction conditions: T=400–700°C;
GHSV=30,000 mL/h/g; mcat = 25 mg)
Fig. 4 H -TPR proꢁles ꢀor various samples
2
1
3

Impacts oꢀ SiO Shell Structure oꢀ Ni@SiO Nanocatalysts on Their Perꢀormance ꢀor Catalytic…
2
2
SP-0 catalyst, though they have a similar Ni loading. Pre-
vious studies also pointed out that the higher porosity oꢀ
silica shell indeed enhance catalytic perꢀormance [14, 20,
suggesting that a lower utilization oꢀ Ni active phase and
un-notable difusion limitations. Owing to the internal diꢀ-
ꢀusion efects, the conversion and H production oꢀ SP-1
2
2
2], indicating that difusion efects played an important
sample are higher than that oꢀ SP-0 sample. For compari-
son, Table 3 summarizes the catalytic perꢀormance oꢀ typi-
role during reaction process. Zhang et al. [22] described
a “surꢀace-protected etching” approach that conveniently
converts dense coatings into porous shells so that chemi-
cal species can reach the core material to participate in
reactions. The results indicated that the reaction rate
could be controlled by varying shell porosity. However,
direct characterization oꢀ difusion processes in these
system is diꢃcult, and only a ꢀew examples have been
reported oꢀ such studies to date, mostly in gas phase [18,
cal Ni@SiO catalyst (SP-1) and other typical Ni-based
2
catalysts reported previously ꢀor ammonia decomposition.
Compared with other supported Ni catalysts reported in the
literature under the same conditions, our prepared catalyst
(SP-1) shows good catalytic perꢀormance. In terms oꢀ SP-1
catalyst, the stability in ammonia decomposition was tested
at 600°C ꢀor a period oꢀ 50 h. As shown in Fig. 6, ammonia
conversion is almost constant, indicating a very stable cata-
lytic perꢀormance at a high temperature. As mentioned in
the introduction section, lots oꢀ core–shell structured nano-
catalysts reported in the literature have a high-temperature
stability.
2
1] but also in liquids [19]. FTIR spectroscopy had been
used to monitor the transport oꢀ CO to the cores oꢀ yolk-
shell nanostructures [21]. It was ꢀound that the mecha-
nism oꢀ difusion through the oxide shells was dominated
by transport at the grain boundaries rather than at the
pores.
In addition, the apparent activation energy oꢀ ammo-
nia decomposition reaction (E ) were obtained ꢀrom the
a
The typical data oꢀ ammonia conversion and H pro-
2
Arrhenius relationship between the rate constant (k) and
the temperature (T), which can be described by the equa-
duction are shown in Table 2. It is observed that the
NH3 decomposition activity decreases with an increase
tion: ln(k)=−E /RT+constant, and the results are listed in
a
oꢀ shell thickness. However, the H production over per
2
Table 2. It can be seen that the value oꢀ E is in the range
a
gram oꢀ Ni increases with an increase oꢀ shell thickness,
ꢀrom 77 to 82 kJ/mol ꢀor diferent catalysts. This result is
Table 2 The catalytic activities
oꢀ various catalysts
a
Conversion H production
a
a
a,b
TOF
(mmolNH3/h/
a,c
Samples
H production
2
E
(kJ/mol)
2
(
%)
(mmol/min/gcat
)
(mmol/min/gNi)
mol )
Ni
ST-5
72.0
69.2
66.3
78.9
24.2
23.1
22.2
26.4
27.6
31.0
36.2
43.1
62.0
69.7
81.4
96.9
82.3
82.1
79.6
77.2
ST-10
ST-20 (SP-0)
SP-1
a
The data were measured at 600°C under pure NH ꢂow oꢀ 12.5 mL/min
3
b
3
TOF units as reported ꢀor the literature data ꢀor a ꢂow oꢀ 3 dm /h [15, 52]; iꢀ calculated ꢀor NH (data in
3
text), TOF=Vα / n, where V is the molar ꢂow rate oꢀ NH , α is the conversion degree and n is the moles oꢀ
3
Me NPs (MeNP
c
)
The apparent activation energy was measured under pure NH ꢂow at 400–500°C
3
Table 3 Comparison oꢀ
the catalytic perꢀormance
Samples
Temperature
°C)
Conversion (%)
H production rate
2
Reꢀerences
(
(mmol/min/gcat)
oꢀ Ni@SiO with several
2
typical Ni-based ammonia
decomposition catalysts at
GHSV=30 000 mL/h/gcat
1
0% Ni/SiO2
600
600
600
500
600
650
700
600
600
600
36.4
79.5
71.6
–
11.4
24.9
24.0
2.9
[48]
6
5% Ni/SiO –Al O
3
[48]
2
2
Ni/MCM-41 (TIE)
% Ni/CNTs
Ni/Al O
[49]
5
[26]
–
32.6
36.4
32.8
29.09
25.6
26.4
[33]
2
3
Ni/CNFs
5% Ni/MRM
Ce10–NiO–SiO -350
90.5
97.9
86.9
–
[50]
1
[51]
[47]
2
7
1%Nano-Ni@SiO2
[15]
Ni@SiO (SP-1)
2
78.9
This study
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comparable to the reported values oꢀ core–shell structured
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lowest activation energy among all samples. It indicates
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a
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In summary, core–shell structured Ni@SiO nanocatalysts
2
(
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with well tuned structure (shell thickness/porosity) were
prepared, characterized and applied to catalytic ammonia
decomposition. With increasing shell thickness, the NH3
decomposition activity decreased slightly. While a distinct
diference in activity was observed by tuning the porosity
oꢀ silica shell. No matter the NH conversion or H produc-
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