Tailoring catalytic properties of Pd/Co3O4 catalysts via structure engineering for methane oxidation

Article abstract of DOI:10.1166/jnn.2018.14294

The catalytic behavior of Co₃O₄ catalysts loaded by Pd for methane oxidation can be tailored by distinct spatial architectures and surface structures of such catalysts. Pd nanoparticles nested in Co₃O₄ with hexagonal-like microflakes exhibited superior catalytic activity, that is, T10 = 250°C and T90 = 325°C are correlated to 10% and 90% conversion of methane. Further Pd/Co₃O₄ microflakes catalyst can almost restore to its initial value in the absence of water when water vapor was cut off. This excellent catalysis should be attributed to its exposed more open surface, more active oxygen species and stronger redox properties.

Full text of DOI:10.1166/jnn.2018.14294

Article
Journal of
Nanoscience and Nanotechnology
Vol. 18, 2673–2679, 2018
Copyright © 2018 American Scientific Publishers
All rights reserved
Printed in the United States of America
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Tailoring Catalytic Properties of Pd/Co O Catalysts via
3
4
Structure Engineering for Methane Oxidation
∗
Lufei Chen and Yan Zhu
Department of Chemistry, School of Science, Shanghai University, Shanghai 200444, China;
Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China
The catalytic behavior of Co O catalysts loaded by Pd for methane oxidation can be tailored by
3
4
distinct spatial architectures and surface structures of such catalysts. Pd nanoparticles nested in
ꢀ
Co O with hexagonal-like microflakes exhibited superior catalytic activity, that is, T = 250 C and
3
4
10
ꢀ
T90 = 325 C are correlated to 10% and 90% conversion of methane. Further Pd/Co O microflakes
3
4
catalyst can almost restore to its initial value in the absence of water when water vapor was cut off.
This excellent catalysis should be attributed to its exposed more open surface, more active oxygen
species and stronger redox properties.
Keywords: Methane Oxidation, Crystal Planes, Co O , Doped Active Sites.
3
4
1
. INTRODUCTION
IP: 185.46.86.122 On: Mon c, o0u 2l dA bp er g2 r0e 1a 8t ly1 5a f: f3e 8c :t 1e d5 by the properties of the catalyst
Copyright: American S ec mi e pn l toi fyi ce d P. uC bu l ri sr eh net r ps opular catalysts for the catalytic com-
Methane, a primary constituent of natural gas, has attracted
Delivered by Ingenta
6ꢀ7
8–10
widespread attention in the energy sector as a fuel by
a direct burning with oxygen or a catalytic combustion
to obtain heat energy. Methane is recognized as a potent
greenhouse gas and it warms the earth 25 times as much
bustion of methane are noble metals, metal oxides,
1
1–13
14
perovskite
and hexaaluminate. Among these, sup-
ported Pd has been extensively studied due to the excellent
activity and thermo stability for methane initial combus-
tion. However, Pd-based catalysts are not stable at high
temperature, and meanwhile water produced might react
with active Pd or PdO to form inactive palladium hydrox-
1
–3
as the same mass of CO2.
Methane is a very stable
and symmetrical molecule that does not possess any dipo-
lar moment or functionality to allow for directing chemi-
1
2ꢀ15ꢀ16
1
ide further leading to the poison of Pd.
To improve
cal reactions, resulting in a low chemical reactivity. Most
the catalytic performance of Pd-based catalysts, usually
Pd confined in oxides enhances the intimate interaction of
noble metal and oxides. Recently, Co O is a promising
of methane conversions are thermodynamically unfavor-
able at low temperature and high temperature is basi-
cally required to break the C–H bond. The removal of
methane from the environment is challenging, as the high-
temperature combustion without catalysts or homogeneous
3
4
catalyst and exhibits high activity for methane oxidation,
which is possibly owing to the structures and electronic
properties of spinel type cobalt oxides and multifold sur-
face lattice oxygen anions with diversified Lewis basicity
combustion leads to the emission of toxic nitrogen oxides
and CO.4ꢀ5 Therefore, it is important to limit these emis-
8
ꢀ17ꢀ18
as well as the lower bonding energy of Co–O bonds.
sions into the atmosphere as low as possible and it is nec-
essary to oxidize methane into carbon dioxide and water in
order to suppress their release into the atmosphere. From
the chemocatalytic point of view, the catalytic oxidation
of methane can lead to the desirable product at low tem-
perature. Many research efforts are underway in academia
and industry to remove methane from the environment,
especially such as the removal of methane from transport
emissions.1 The actual reactivity of methane with oxygen
With the aim of understanding the catalytic behaviour
of Co O loaded by Pd on methane oxidation, herein,
3
4
four distinct spatial structures of Co O were designed
3
4
and they showed different active surfaces and composi-
tion of the catalytic active sites, resulting in the variant
catalytic performance for methane oxidation. In particu-
lar, Pd/Co O microflakes catalyst gave rise to the high-
3
4
est activity, where methane could be initially light-off at
ꢀ5
ꢀ ꢀ
50 C and completely oxidized at 350 C with a space
velocity of 30000 mL · g · h . Such architecture is also
2
−1
−1
∗
Author to whom correspondence should be addressed.
crucial for the structural stability and sintering resistance
J. Nanosci. Nanotechnol. 2018, Vol. 18, No. 4
1533-4880/2018/18/2673/007
doi:10.1166/jnn.2018.14294
2673

Tailoring Catalytic Properties of Pd/Co O Catalysts via Structure Engineering for Methane Oxidation
Chen and Zhu
3
4
of active sites. Even in case of wet methane, Pd/Co O
2.2. Combustion of Methane Catalytic Testing
3
4
microflakes showed excellent catalysis and almost restored
to its initial value in the absence of water when water was
cut off. It indicates that this catalyst would be very inter-
esting for this application.
The catalytic performance for combustion of methane was
evaluated at atmospheric pressure in a fixed-bed quartz
tubular reactor. All the catalysts used were sieved to
80–100 mesh and 100 mg catalyst were placed in the reac-
tor. The reactant feeds were CH , O and N (1% CH ,
4
2
2
4
1
0% O and 89% N ) and the gas hour space velocity
2 2
2
. MATERIAL AND METHODS
.1. Synthesis of Catalysts
.1.1. Synthesis of Co O Microflakes
−1
−1
(
GHSV) was fixed in 30000 mL · g · h . The combus-
2
2
2
tion of methane reaction temperature was controlled from
3
4
ꢀ ꢀ
00 C to 500 C. Water vapor from the exit gas stream
2
g Co(CH COO) · 4H O and 0.15 g polyvinyl pyrroli-
3
2
2
was separated by the quartz tube full with calcium chloride
anhydrous. The reaction products (CO, CO ) were ana-
lyzed by an online gas chromatograph (GC) equipped with
a thermal conductivity detector.
done (PVP) was added into a mixed solution of 60 mL
ethylene glycol and 6 mL distilled water, followed by vig-
orous stirring for 3 h, and then the obtained solution was
2
transferred into 100 mL autoclave, sealed and heated at
ꢀ
1
80 C for 24 h. The system was then allowed to cool to
2
.3. Characterization
room temperature naturally. The products were centrifuged
and washed with distilled water and ethanol several times,
and the solid obtained was dried at 70 C overnight and
X-ray diffraction (XRD) patterns were recorded on a
Japanese Rigaku UltimaIV powder diffractometer using
Cu Kꢂ radiation with a condition of wavelength of
ꢀ
ꢀ
calcined in air at 300 C for 3 h.
1
.54056 Å, 40 kV and 40 mA. The morphology and size
of catalysts were characterized using scanning electron
microscopy (ZEISS, SEM, 2014, German). The BET spe-
cific surface area was measured by nitrogen adsorption,
using Quanta chrome Autosorb-iQ2-MP analyzer, which
was performed after degassing the sample at 200 C for
overnight under vacuum. Pore volume and pore size dis-
2
0
3
.1.2. Synthesis of Co O Nanoballs
3 4
.5824 g Co(NO ꢁ was added into a mixed solution of
3
2
5 mL ammonia solution and 40 mL distilled water, and
then the obtained solution was transferred into 100 mL
autoclave after stirred 1 h, heated at 180 C for 6 h. The
following steps are as the synthesis of Co O microflakes.
ꢀ
ꢀ
IP: 185.46.86.122 On: Mon, 02 Apr 2018 15:38:15
3
4
tribution were determined by the Barrett-Joyner-Halenda
Copyright: American S c( Bi eJ nH t i) fi cm Pe tuh bo ldi s. hMe re st hane temperature program reductions
.1.3. Synthesis of Co O Nanoparticles
Delivered by( CI nHg e- nT tPa R) were carried out on a chemical adsorption ana-
2
3
4
4
5
mmol Co(CH COO) · 4H O was dissolved in 15 ml
lyzer (Autochem II 2920, Micromeritics). 50 mg catalyst
was pretreated in air flow at 300 C for 2 h and after
3
2
2
ꢀ
of ethylene glycol, and the mixture was gradually heated
to 80 C under vigorous stirring. Dissolved in 50 mL of
1
added the above solution. Then, the reaction vessel poured
into a continuous flow of Ar and the suspension was main-
tained at 80 C for 1 h under vigorous stirring, and then
transferred into 100 mL autoclave, heated 150 C for 3 h.
The resulting product was centrifuged and washed with
distilled water several times, and the solid obtained was
ꢀ
ꢀ
−1
cooled to 60 C, a flow (50 mL ·min ) of 10 vol.% CH
4
ꢀ
.06 g anhydrous sodium carbonate aqueous was slowly
and 90 vol.% He mixture was introduced and kept at 60 C
for 1 h, finally the sample was heated from 60 to 800 C
ꢀ
ꢀ
−1
at a ramp of 10 C · min in the CH –He mixture flow.
4
ꢀ
X-ray photoelectron spectroscopy (XPS) (Thermo Scien-
tific K-Alpha) using Al Kꢂ (hꢃ = 1486ꢄ6 eV) as the exci-
tation source. Correction of the charge effect was made
with the C (1s) peak at 284.8 eV. The Pd contents were
measured by the inductively coupled plasma atomic emis-
sion spectroscopic (ICP-AES) technique on a PerkinElmer
Optima 8000 spectrometer.
ꢀ
ꢀ
dried at 60 C overnight under vacuum and calcined at
3
ꢀ
00 C for 3 h.
2
0
.1.4. Synthesis of Co O Nanocubes
3
4
2
.4. Analytic Methods
.04 mol Co(NO ꢁ · 6H O and 0.01 mol NaOH was dis-
3
2
2
The methane conversion (XCH4 ) and TOF based on Pd
mass were calculated according to the following Eq.:
solved into 40 mL distilled water, followed by vigorous
stirring for 1 h and then transferred into 100 mL auto-
clave, heated at 180 C for 5 h. The resulting product was
centrifuged and washed with distilled water several times,
dried at 60 C overnight and calcined at 400 C for 3 h.
The Pd/Co O catalysts were synthesized by incipient
wetness impregnation with a water solution of palladium
nitrate. A certain amount of Pd(NO ꢁ water solution was
dropped into Co O crystals and then dried at 60
overnight under vacuum and calcined at 300 C for 3 h.
ꢀ
ꢅCH /CH ꢁ
4
4
XCH4 = 1 −
ꢅCH /N ꢁ
ꢀ
ꢀ
4
0
20
N
3
4
CH conversion
4
TOFPd = 1 −
ꢅmasscatalysts ×Pd wt%ꢁ/MPd
Where CH , N are corresponding the CH and N of the
3
2
ꢀ
C
3
4
4
20
4
2
0
ꢀ
steady mixture flow before reaction.
2674
J. Nanosci. Nanotechnol. 18, 2673–2679, 2018

Chen and Zhu
Tailoring Catalytic Properties of Pd/Co O Catalysts via Structure Engineering for Methane Oxidation
3 4
3
. RESULTS AND DISCUSSION
spacing of 0.283 nm were observed in Figure 1(c3). The
inset in Figure 1(d3) shows the FFT image from Co
The four representative morphologies of Pd/Co O cata-
3
O
4
3
4
lysts are shown in Figure 1. Pd/Co O microflakes look
like hexagon with 5 ꢆm edgelength (Fig. 1(a1)). We can
with [001] direction. Drawn from the lattice fringes and
FFT image, it can be seen that the predominantly exposed
3
4
see the (101) lattice fringe with interplanar spacing of
plane is (001) in cubic Co
3 4
O . The Figure 1(d3) shows
0
0
.264 nm on the PdO. The interplanar spacings of 0.244,
.285 and 0.488 nm on the HRTEM image analyzed from
that the lattice structure of Pd/Co O nanoparticles is dis-
3
4
cerned with spacings of 0.211 and 0.234 nm, close to the
lattice spacings measured for (400) and (222) planes. For
four samples, the mainly exposed plane of the PdO is of
(101) orientation, which is reflected by (101) crystal plane
with corresponding lattice spacing of 0.267 nm.
the circle of Figure 1(a2) have lattice fringe directions
attributed to (311), (220) and (111), respectively, seen from
Figure 1(a3). Combined with the fast Fourier transform
(
FFT) analysis in inset of Figure 1(a3), the stacking direc-
tion of Co O in microflakes is the [112]. For ball- like
As shown in Figure 2, all of the calcined samples show
diffraction peaks at the 2ꢇ of 19.0, 31.3, 36.8, 38.5, 44.8,
55.7, 59.4, 65.2, and 77.3 , which are typically indexed
3
4
Pd/Co O , the main exposed planes are (111), indicated
3
4
ꢀ
by the lattice spacing and electronic diffraction, which
are one plane corresponding to the (220) with the lattice
spacing of 0.285 nm (Fig. 1(b3) and inset). In term of
nanocubic Pd/Co O , a set of crystal planes with a lattice
as the (111), (220), (311), (222), (400), (422), (511),
(440), and (533) diffractions. The diffraction peaks of the
fours as-synthesized Pd/Co O samples match well with
3
4
3
4
IP: 185.46.86.122 On: Mon, 02 Apr 2018 15:38:15
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Figure 1. SEM, TEM and HRTEM images of Pd/Co O catalysts: microflakes (a1–a3), nanoballs (b1–b3), nanoparticles (c1–c3), nanocubes (d1–d3).
3
4
Insets in a3, b3, c3 and d3 are the corresponding FFT patterns.
J. Nanosci. Nanotechnol. 18, 2673–2679, 2018
2675

Tailoring Catalytic Properties of Pd/Co O Catalysts via Structure Engineering for Methane Oxidation
Chen and Zhu
3
4
catalytic activity for methane oxidation at low temper-
ꢀ
ature. Methane can be light-off at 250 C and com-
ꢀ
pletely oxidized at 350 C at a space velocity (SV) of
−
1
−1
3
0000 mL·g ·h with 1% CH and 10% O in N , as
4
2
2
shown in Figure 3(a). Pd/Co O nanoballs showed similar
3
4
T10 and T to microflakes catalyst, while T90 of the former
50
ꢀ
ꢀ
(
350 C) is slightly higher than that of the latter (325 C).
The cubic Pd/Co O catalyst gave the lowest activity, and
3
4
ꢀ
at 500 C, the conversion of methane is only 50%. Accord-
ing to the values of T10 and T90 listed in Table I, the cat-
alytic activity follows the order of Pd/Co O microflakes >
3
4
balls > nanoparticles > cubes. Almost linear evolution is
ꢀ
obtained between ln(rate) and 1/T at 225–280 C except
ꢀ
3
00–270 C for cubic catalyst (Fig. 3(b)). The reaction
rate of methane oxidation over Pd/Co O microflakes is
3
4
the highest among the four catalysts. The apparent acti-
vation energy for Pd/Co O microflakes is approximate
Figure 2. The XRD patterns of the (A) Pd/Co O microflakes,
3
4
(
B) Pd/Co O4 nanoballs, (C) Pd/Co O4 nanoparticles, (D) Pd/Co O
3
4
3
3
3
4
−1
nanocubes, respectively.
87.6 kJ · mol , and lower than those of other three cata-
lysts shown in Table I.
Pd/Co O microflakes catalyst exhibited a resistance
the cubic spinel Co O (JCPCD Card No. 43–1003, space
3
4
3
4
toward sintering of active sites at high temperature. When
group: Fd3m). Pd phases were not detected by XRD due
to their tiny content (∼1 wt% Pd) and well distribution.
Catalytic performance toward methane oxidation was
used to probe the effect of exterior structures on
ꢀ
the temperature was kept at 330 C, the conversion of
methane gradually dropped from 94% to 84% within
50 hours (Fig. 3(c)). When the temperature increased to
ꢀ
the catalytic behaviour of Pd/Co O4 catalysts. The
500 C, the 100% conversion of methane over Pd/Co O
3
3
4
Pd/Co O microflakes catalyst gave rise to an excellent
can be obtained during the reaction time was up to
3
4
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Figure 3. (a) Methane conversion as a function of reaction temperature on Pd/Co O nanoparticles, Pd/Co O nanoballs, Pd/Co O microflakes and
3
4
3
4
3
4
Pd/Co O nanocube; (b) Corresponding kinetic rate data for methane combustion; (c) Evolution of the catalytic performance of Pd/Co O microflakes
3
4
3
4
−1
−1
.
with respect to the reaction time. (d) The effect of 5 vol.% water vapour on the catalytic performance of Pd/Co O microflakes. SV = 30000 mL·g ·h
3
4
Feed: 1% CH , 10% O , and 89% N .
4
2
2
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Chen and Zhu
Tailoring Catalytic Properties of Pd/Co O Catalysts via Structure Engineering for Methane Oxidation
3 4
Table I. Pd contents of the samples calculated from ICP-AES, BET specific surface area (S ) from N absorption isotherms, adsorbed oxygen/lattice
BET
2
oxygen molar ratio (Oads/Olatt), Pd2 /Pd0 molar ratio, apparent activation energy (E ), and the temperatures for 10%, 50%, and 90% conversion of
+
a
methane (T , T and T ).
1
0
50
90
Pd content
(wt%)
S
Oads/Olatt
molar ratio
Co2 /Co
+ 3+
molar ratio
Pd /Pd0
2+
BET
−1
2
−1
ꢀ
ꢀ
ꢀ
Catalysts
Pd/Co O microflake
(m ·g
)
molar ratio
E (kJ·mol
)
T10 ( C)
T50 ( C)
T90 ( C)
a
1.13
1.04
1.03
1.12
19ꢄ1
5ꢄ2
81ꢄ9
5ꢄ4
0.41
0.33
0.32
0.31
0.70
0.65
0.62
0.60
40.70
52.19
31.66
62.42
87ꢄ63
96ꢄ53
118ꢄ39
117ꢄ06
250
257
275
340
300
294
340
500
325
350
400
3
4
Pd/Co O nanoball
3
4
Pd/Co O nanoparticle
3
4
Pd/Co O nanocube
>500
3
4
5
0 hours. We also investigated the effect of water vapour
have main effect, leading to distinct redox properties and
different surface active species.
The redox ability of the catalysts is responsible for the
catalytic activity of these catalysts. Figure 5(a) shows the
on the reactivity of the Pd/Co O microflakes catalyst
3
4
ꢀ
(
Fig. 3(d)). When the temperature was kept at 330 C,
the conversion of methane gradually dropped from 95% to
7% within 6 hours in the presence of 5 vol.% H O (g).
5
methane temperature program reduction (CH -TPR) pro-
2
4
Then the water was cut off, the conversion of methane can
files of catalysts obtained by monitoring the formation of
CO , CO, H and H O, and the consumption of methane.
restore to above 90%. When the temperature increased to
2
2
2
ꢀ
ꢀ
For Pd/Co O microflakes, the first band at 295 C is asso-
3 4
3
50 C and water vapour was introduced again, the con-
version of methane decreased from 100% to 86% within
ciated with the reaction process of the Co O with CH
from the reduction of Co to Co . The second band
3 4 4
ꢀ
3+
2+
6
hours. As the temperature decreased to 330 C again,
ꢀ
the conversion of methane dropped by 39% (from 89%
at 495 C is assigned to the reduction reaction of Co O
3
4
ꢀ
2+
0
to 50%). And when the temperature increased to 350 C,
with CH from Co to Co . The consumption peak at
583 C corresponds to the formation of CO , H O, CO,
and H , which is attributed to the cracking of CH .
4
ꢀ
the conversion of methane dropped from 100% to 77%.
When water vapour was cut off, the conversion of methane
can again restore to its initial value in the absence of water.
The excellent performance of the Pd/Co O microflakes
2
2
1
7ꢀ19
2
4
3
+
2+
For nanoballs, the reduction band of Co to Co shifted
ꢀ
2+
0
to 320 C, and the one of Co to Co moved toward
3
4
ꢀ
ꢀ
catalyst for methane oxidation prompted us to explore
522 C, respectively. The third band appeared at 782 C.
For nanoparticles and nanocubes, the three reduction bands
IP: 185.46.86.122 On: Mon, 02 Apr 2018 15:38:15
the structure-dependent catalytic properties. Firstly the N
2
Copyright: American Scientific Publishers
adsorption isotherms and pore size distribution of the
shifted toward higher temperatures. Taking into consid-
Delivered by Ingenta
3
+ 2+
four catalysts are shown in Figure 4, and the porous
characteristics are summarized in Table I. The sur-
face area of Pd/Co O catalysts descend as the follow-
eration the reduction temperatures of Co to Co and
2
+
0
Co to Co , it is clearly demonstrated that, the reduc-
tion temperature of Pd/Co O microflakes is lower than
3
4
3
4
ing sequence: nanoparticles > microflakes > nanocubes >
nanoballs, which is not well agreement with the catalytic
activity. It indicates that the higher catalytic activity of the
microflakes cannot be ascribed to the surface area. The
structural and electronic factors of the microflakes must
that of other three Pd/Co O catalyst. The stronger redox
3 4
properties of Pd/Co O microflakes can cause more oxy-
3
4
gen vacancies on the surface as compared to other three
catalysts, since it easier generates oxygen vacancies for
microflakes catalyst that is more susceptibly reducible.
Figure 4. (a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of the catalysts: (A) Pd/Co O microflakes, (B) Pd/Co O
4
3
4
3
nanoballs, (C) Pd/Co O nanoparticles, (D) Pd/Co O nanocubes.
3
4
3
4
J. Nanosci. Nanotechnol. 18, 2673–2679, 2018
2677

Tailoring Catalytic Properties of Pd/Co O Catalysts via Structure Engineering for Methane Oxidation
Chen and Zhu
3
4
Figure 5. The methane temperature program reduction spectra of (a) Pd/Co O microflakes, (b) Pd/Co O nanoballs, (c) Pd/Co O nanoparticles and
3
4
3
4
3
4
(
d) Pd/Co O nanocubes.
3 4
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Figure 6. (a) Co 2p, (b) O 1s, and (c) Pd 3d XPS profiles of Pd/Co O microflakes (A), Pd/Co O nanoballs (B), Pd/Co O nanoparticles (C), and
3
4
3
4
3
4
Pd/Co O nanocubes (D), respectively.
3
4
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J. Nanosci. Nanotechnol. 18, 2673–2679, 2018

Chen and Zhu
Tailoring Catalytic Properties of Pd/Co O Catalysts via Structure Engineering for Methane Oxidation
3 4
The surface active species on the surface of catalysts
are also not ignored to elucidate the shape-specific effects.
X-ray Photoelectron Spectroscopy (XPS) were carried out
to detect the surface composition and metal valence state.
Seen from the Co 2p XPS profiles in Figure 6(a), the
the catalytic reaction via structure engineering might aid
in improving the activation and conversion of methane.
Acknowledgments: We are grateful for financial sup-
port by National Natural Science Foundation of China.
2
+
3+
presence of Co and Co is in Co 2p spectra of all sam-
ples, in which 779.63 eV belongs to the electron binding
energy (BE) of Co , and 781.38 eV is assigned to the
References and Notes
3
+
1
2
3
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2
+ 20
2+
BE of Co . Co is considered to be more active than
Co
(2002).
3
+15ꢀ16
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2
+
tive amount of Co species on the surface of catalysts, as
shown in Table I. Therefore microflakes catalyst exhibited
the better ability for the activation of methane. As for O
4. T. V. Choudhary, S. Banerjee, and V. R. Choudhary, Applied Catal-
ysis A: General 234, 1 (2002).
1
s XPS in Figure 6(b), an asymmetrical peak is obtained
5
. J. Jansson, A. E. C. Palmqvist, E. Fridell, M. Skoglundh, L. Öster-
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istry C 118, 33 (2014).
and decomposed into four components: O (529.78 eV)
I
and OII (531.1 eV) are ascribable to the surface lattice
6
2
−
oxygen (O , O ) and adsorbed oxygen (O , e.g., O
latt
ads
2−
−
21
or O ). The OIII (532.2 eV) and OIV (533.3 eV) could
7. Y. N. Li, Y. B. Xie, and C. J. Liu, Catal. Lett. 125, 1 (2008).
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ence and Technology 3, 12 (2013).
arise from carbonates, hydroxyl groups or adsorbed water
molecule.2
2ꢀ23
The ratios of the surface-adsorbed oxygen
9. C. Chen, Y. H. Yeh, M. Cargnello, C. B. Murray, P. Fornasiero, and
R. J. Gorte, ACS Catalysis 4, 11 (2014).
species to lattice oxygen is 0.41 for the microflakes, 0.33
for the nanoballs, 0.32 for the nanoparticles, and 0.31 for
the nanocubes, respectively, as shown in Table I. As well
known, more surface adsorbed oxygen species (peroxide
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2+
4+ 27ꢀ28
to the Pd species (Pd and Pd ).
The molar ratios
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+
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2
+
0
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. CONCLUSIONS
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2
2
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Received: 25 October 2016. Accepted: 5 January 2017.
J. Nanosci. Nanotechnol. 18, 2673–2679, 2018
2679