Efficient selective oxidation of propylene by dioxygen on mesoporous-silica-nanoparticle-supported nanosized copper

Article abstract of DOI:10.1016/j.jcat.2018.07.014

A direct synthetic route was developed to prepare mesoporous-silica-nanoparticle-supported copper catalysts featuring ordered mesostructures with extensive intraparticle voids, a high degree of silica condensation, and a high dispersion of copper species. After hydrogen reduction, the catalysts containing nanosized metallic copper showed superior and stable catalytic activity for the selective oxidation of propylene by dioxygen, with high conversion of both reactants and high yield and formation rate of acrolein. The best catalyst outperformed the state-of-the-art silica-supported copper catalysts, representing one of the most active acrolein-production catalysts. Results of in situ X-ray absorption spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy, and other characterizations suggested that the metallic copper transformed and mainly stayed in the 1+ oxidation state under reaction conditions and that factors including the small size of copper and the presence of silanol groups on the silica support were crucial for the catalytic performance of the catalysts.

Full text of DOI:10.1016/j.jcat.2018.07.014

Journal of Catalysis 365 (2018) 411–419
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Efficient selective oxidation of propylene by dioxygen on
mesoporous-silica-nanoparticle-supported nanosized copper
Nien-Chu Lai , Ming-Chieh Tsai , Chun-Hsia Liu , Ching-Shiun Chen c,d, Chia-Min Yang a,e,
a
a
b
⇑
a
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
Center for General Education, Chang Gung University, Taoyuan City 33302, Taiwan
Department of Pathology, Chang Gung Memorial Hospital, Taoyuan City 33302, Taiwan
b
c
d
e
Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A direct synthetic route was developed to prepare mesoporous-silica-nanoparticle-supported copper
catalysts featuring ordered mesostructures with extensive intraparticle voids, a high degree of silica
condensation, and a high dispersion of copper species. After hydrogen reduction, the catalysts containing
nanosized metallic copper showed superior and stable catalytic activity for the selective oxidation of
propylene by dioxygen, with high conversion of both reactants and high yield and formation rate of
acrolein. The best catalyst outperformed the state-of-the-art silica-supported copper catalysts, represent-
ing one of the most active acrolein-production catalysts. Results of in situ X-ray absorption spectroscopy,
in situ diffuse reflectance infrared Fourier transform spectroscopy, and other characterizations suggested
that the metallic copper transformed and mainly stayed in the 1+ oxidation state under reaction
conditions and that factors including the small size of copper and the presence of silanol groups on
the silica support were crucial for the catalytic performance of the catalysts.
Received 22 April 2018
Revised 25 June 2018
Accepted 4 July 2018
Keywords:
Nanosized copper
Mesoporous silica nanoparticles
Metal dispersion
Selective propylene oxidation
In situ measurement
Ó 2018 Elsevier Inc. All rights reserved.
1
. Introduction
Supported nanocatalysts containing highly dispersed metals
Obviously, there is a strong need for new strategies to prepare
high-surface-area silica-supported nanosized copper catalysts.
Among the reactions catalyzed by copper, the selective
oxidation of propylene with dioxygen (O ) to produce acrolein is
generally exhibit high activity in heterogeneous catalysis, mainly
owing to their high surface area with an abundance of low-
coordination surface sites [1–8]. The preparation of metal nanocat-
alysts on high-surface-area supports remains challenging because
nanosized metals tend to agglomerate or sinter during catalyst
preparation and/or under catalytic reactions [1,3,4,6,8]. This is
especially true for the preparation of copper catalysts, which have
been widely applied in important industrial processes [9,10] and
are promising for selective oxidation [11–21], hydrogenation
2
of great interest [11,15,17,20,36,37] because it is a green reaction
that produces the high-value-added acrolein for the manufacture
of acrylic acid, methionine, and other important chemicals
[11,38–40]. Supported copper catalysts have been shown to be
active for this reaction, with the product selectivity mainly
associated with the oxidation state of copper and the reaction
temperature [1,11,17,19,20,41]. Supported CuO
copper-incorporated zeolites [41,42] are reported to produce acro-
lein, with CO and ethanal as main byproducts, and higher reaction
temperatures result in higher propylene conversion accompanied
by decreased acrolein selectivity. Making small Cu (or CuO ) parti-
x
[11,15] and
[
22–25], photocatalysis [26–29], and other reactions. Methods
2
including ammonia-induced deposition precipitation [6,30], fatty-
acid-assisted assembly [31,32], solid state grinding [33,34], vac-
uum thermal preparation [15], and co-gelation [35] have been
reported to deposit copper particles on high-surface-area silicas,
but the dispersion of copper remains low for these materials.
x
cles on high-surface-area supports has been suggested to be a way
of achieving high propylene conversion and high acrolein selectiv-
ity simultaneously at relatively low temperature [1,6]. Recently,
we reported a method of preparing highly dispersed copper
nanoparticles on mesoporous SBA-15 silica [15]. The SBA-15-
supported copper catalysts exhibited high conversion of propylene
and high selectivity for acrolein and outperformed other silica-
supported copper catalysts [43]. The study indicates that the small
⇑
Corresponding author at: Department of Chemistry, National Tsing Hua
University, Hsinchu 30013, Taiwan.
E-mail address: cmyang@mx.nthu.edu.tw (C.-M. Yang).
https://doi.org/10.1016/j.jcat.2018.07.014
0
021-9517/Ó 2018 Elsevier Inc. All rights reserved.

4
12
N.-C. Lai et al. / Journal of Catalysis 365 (2018) 411–419
size of copper is the most crucial factor in high catalytic activity for
the reaction. In addition to the size effect, the surface properties of
the support may also affect the catalytic performance of the sup-
ported catalysts. For example, Fukuoka et al. found a promotional
effect of the surface silanol groups in the preferential oxidation
of carbon monoxide catalyzed by platinum nanoparticles sup-
ported on mesoporous FSM-16 silica [44]. Nevertheless, for sup-
ported copper catalysts, the possible influence of the surface
properties of the support on their catalytic performance has sel-
dom been discovered and studied.
(ICP-MS) data were obtained using a Perkin–Elmer SCIEX-ELAN
5000 device. Powder X-ray diffraction (PXRD) patterns were
obtained on a Mac Science 18MPX diffractometer using CuK
a radi-
ation. N physisorption isotherms were measured at 77 K using a
2
Quantachrome Autosorb-1-MP instrument. The desorption
branches were analyzed by the density functional theory (DFT)
method to evaluate pore sizes, and the adsorption branches in
the relative pressure range 0.05–0.30 were used to calculate sur-
face areas by the Brunauer–Emmett–Teller (BET) method. Pore vol-
umes were evaluated at a relative pressure of 0.95. Solid-state 29Si
MAS NMR spectra were measured on a Bruker AVANCE III spec-
trometer using a 4-mm MAS probe. SEM images were obtained
with a field emission JEOL JSM-7000F microscope operating at
10 kV and equipped with an energy dispersion X-ray (EDX) spec-
trometer. The samples were coated with Pt before measurements
were performed. TEM images were taken using a JEOL JEM-2010
microscope operated at 200 kV and equipped with an EDX spec-
trometer. Diffuse reflectance UV–visible absorption spectra were
recorded on a JASCO V-650 spectrophotometer equipped with a
diffuse reflectance accessory.
Here we report a direct synthesis of MCM-41-type mesoporous
silica nanoparticle (MSN)-supported nanosized copper catalysts
and catalytic studies on the selective oxidation of propylene with
2
O . The synthesis was based on the ‘‘pH-jump” synthesis of meso-
porous MCM-41 silica [45], with the ammonia complex of Cu(II) as
the metal precursor. The addition of ethyl acetate (EA) caused a
drastic drop in solution pH and simultaneously initiated the coop-
erative self-assembly of ordered mesophases and the condensation
of silicates, as well as the deposition of copper oxide species. The
copper oxide species in the resulting materials (abbreviated as
CuMSN) were subsequently transformed to nanosized metallic
copper by hydrogen reduction. We found that the reduced CuMSN
exhibited superior catalytic activity and stability for the selective
2.3. Hydrogen temperature-programmed reduction
2
oxidation of propylene with O to produce acrolein, and that the
2
Hydrogen temperature-programmed reduction (H TPR) was
silanol groups on the silica surface of CuMSN facilitated
the adsorption of propylene, thereby enhancing the conversion of
the reactant. The acrolein yield for the best CuMSN was almost four
times the value for the state-of-the-art SBA-15-supported copper
catalyst under relatively mild reaction conditions [15]. Techniques
including hydrogen temperature-programmed reduction, in situ
X-ray absorption spectroscopy, and in situ diffuse reflectance infra-
red Fourier transform spectroscopy were employed to identify the
promotional effect in the CuMSN-catalyzed oxidation of propylene
performed at atmospheric pressure in a conventional flow system.
A sample (40 mg) was placed in a tubular reactor and heated at a
ꢀ1
rate of 10 °C min in a mixed gas stream (10% H
2
2
and 90% N ) with
ꢀ1
a flow rate of 30 mL min . The K- and P-values for the measure-
ments were 55–140 s and 5–20 K, respectively [46,47]. A cold trap
that contained a gel formed by the addition of liquid nitrogen to
acetone in a Thermos flask was used to prevent water from enter-
ing the thermal conductivity detector.
2
by O .
2.4. Estimation of surface area and dispersion of metallic copper
2
. Experimental
The surface area and the dispersion of metallic copper in the
reduced CuMSN catalysts were estimated by a method combining
2.1. Materials preparation
H2 TPR and N O oxidation [15,30]. The freshly prepared catalyst
2
2
was first analyzed by H TPR to calculate the total number of cop-
The preparation of CuMSN was based on the ‘‘pH-jump” synthe-
per atoms in that catalyst. After H2 TPR, the reduced catalyst was
sis of pure silica MSNs [45]. The solution containing cetyltrimethy-
lammonium bromide (1.14 g, Acros), sodium metasilicate (2.02 g,
J.T. Baker), and water (675 mL) was mixed well with a calculated
amount of the stock solution of the ammonia complex of Cu(II)
cooled to 80 °C under N , and the atmosphere was then switched
2
to 10% N O/N for 30 s to oxidize the surface atoms of the metallic
2
2
copper nanoparticles to Cu (I) according to the reaction 2Cu(s)
+
N O ? Cu O + N2(g). The thus-produced Cu O monolayer was
2
(g)
2
(s)
2
(
prepared by adding ammonia solution (50 mL, 3.0 M) into an
aqueous solution (50 mL, 0.1 M) of copper nitrate (Showa)). EA
9.76 mL, Alfa) was then quickly added to the mixture, which
quantified by H2 TPR and the amount of H2 consumed was used
to calculate the surface area of copper by assuming an average sur-
1
9
ꢀ2
(
face density for the metal of 1.4 ꢁ 10 atoms m [15,30]. The dis-
persion of copper was calculated from the number of surface
copper atoms and the total copper content of the catalyst. The data
were also used to estimate the average particle size of copper
nanoparticles in the reduced catalyst by assuming a spherical par-
ticle shape. The equations to calculate surface area (SCu), dispersion
(DCu), and average particle size (dCu) of copper nanoparticles are
was vigorously stirred for 30 s, then kept static at 35 °C for 24 h,
and finally aged at 90 °C for 24 h. The surfactant molecules in the
as-synthesized samples were removed by calcination at 540 °C or
by repeated solvent extraction (with an ethanol solution of ammo-
nium nitrate (0.025 M) at 50 °C for 15 min for three times). The cal-
cined and solvent-extracted samples with varied copper-to-silicon
(
Cu/Si) ratios of x% are denoted as xCuMSN-C and xCuMSN-E,
À
Á
2
19
respectively.
S
Cu in m =gCu ¼ ð2 ꢁ Y ꢁ NavÞ=ðX ꢁ MCu ꢁ 1:47 ꢁ 10
ꢂ 1353 ꢁ Y=X;
Þ
A reference sample with Cu/Si of 3% was prepared by impregna-
tion of the calcined pure silica material (i.e., 0CuMSN-C) with an
aqueous solution of copper nitrate followed by calcination at
D
Cu ðin %Þ ¼ ð2 ꢁ Y=XÞ ꢁ 100%;
5
40 °C. The reference sample is denoted as 3CuMSN-IC.
dCu ðin nmÞ ¼ 6=ðS ꢁ qCuÞ;
2.2. Materials characterization
where X is the H
in the catalyst, Y is the H
2
2
consumption for complete reduction of the copper
consumption for the reduction of surface
Cu(I) species (formed by N O oxidation), Nav is Avogadro’s constant
(6.02 ꢁ 10 mol ), MCu is the relative atomic mass of copper
Dynamic light scattering (DLS) measurements were conducted
on a particle size analyzer (Brookhaven Instruments Corporation,
Holtsville, NY). Inductively coupled plasma–mass spectroscopy
2
2
3
ꢀ1

N.-C. Lai et al. / Journal of Catalysis 365 (2018) 411–419
413
(
63.546 g molꢀ1), and
q
Cu is the density of metallic copper (8.92 g
3. Results and discussion
ꢀ
3
cm ).
The silica-supported copper materials were synthesized by
applying the pH-jump method reported for MCM-41-type MSNs,
with the ammonia complex of Cu(II) as a metal precursor [45].
The ammonia complex was stable under the alkaline conditions
and the synthesis solution remained clear and showed no signal
in DLS analysis before EA was added. After EA addition, the drastic
drop of solution pH caused by the hydrolysis of EA destabilized the
copper complex, triggered the polycondensation of silicate and
copper species, and in the meantime initiated the cooperative
assembly of an ordered organic–inorganic mesophase. All the cop-
per species were precipitated out in the solid products, as con-
firmed by ICP-MS analysis of the filtrates of the synthesis
solutions and EDX analysis of the solid products. The materials
were composed of rather uniform nanoparticles with sizes of
2
.5. In situ X-ray absorption spectroscopy measurements
Cu K-edge X-ray absorption spectroscopy (XAS) data were col-
lected on the beamlines 17C at the National Synchrotron Radiation
Research Center (NSRRC, Taiwan) with a storage ring energy of 1.5
GeV and an electron beam current of 360 mA in a top-up injection
mode. The powdered catalyst was packed into a holder before
being mounted in an in situ cell for transmission measurements.
The X-ray absorption near-edge structure (XANES) and the
extended X-ray absorption fine structure (EXAFS) were recorded
after exposure to varied conditions (to mimic those for catalyst
pretreatment and catalytic reactions) and cooling to room temper-
ature in He to monitor the change of oxidation state and the coor-
dination of copper species [15]. Multiple scans were averaged to
improve the signal-to-noise ratio.
1
00–200 nm (cf. Fig. S1 in the Supplementary Material for a typical
SEM image of 3CuMSN). Since the solution pH eventually became
nearly neutral, the degree of silica condensation for the samples
4
was high, as evidenced by the relatively high intensity of the Q
2.6. Catalytic studies
n
29
line (Q : (Si(OSi)
n
(OH)4ꢀn, n = 2, 3, 4) in the solid-state Si MAS
NMR spectra of the solvent-extracted samples (cf. Fig. S2 for the
A fixed-bed flow reactor was used for the studies. Before the
spectra of 3CuMSN-E and the calcined sample 3CuMSN-C). The fact
reaction, the catalyst (50 mg) was pretreated on line under a pure
flow at 350 °C for 1 h and was then cooled to a designated reac-
tion temperature (typically 180, 220, or 260 °C) under He. The
reaction gas, consisting of 5% propylene and 5% O in He with a
flow rate of 25 mL min (corresponding to a space velocity of
3
2
that the Q and Q lines for 3CuMSN-E were more intense than
those for 3CuMSN-C was associated with the presence of more sila-
nol groups in the solvent-extracted sample. Although the total
H
2
2
2
3
intensity ratios of Q and Q for 3CuMSN-C (28%) and for
CuMSN-E (32%) did not differ very much, the difference is signif-
icant, especially for highly condensed silica materials such as
CuMSN-E and 3CuMSN-C. It was noted that the process of solvent
ꢀ1
3
3
ꢀ1 ꢀ1
3
0,000 cm h
gcat), was preheated before passing over the cata-
lyst. The gas leaving the reactor was analyzed every 45 min by
an Agilent 7890A gas chromatography system equipped with a
Molsieve 5A and a Porabond Q column, each with a thermal con-
ductivity detector and a flame ionization detector. The carbon bal-
ance was close to 100 ± 3%. The parameters of propylene
conversion, acrolein selectivity, acrolein yield (YAC), acrolein for-
mation rate (RAC), and turnover frequency (TOF) are defined as
follows:
3
extraction did not result in leaching of copper from the samples,
and the Cu/Si ratios for the calcined and solvent-extracted samples
were nearly identical to the corresponding nominal values of the
molar ratios of sodium metasilicate and the ammonia complex of
Cu(II) in the synthesis solutions (as confirmed by ICP-MS). The cop-
per in the extracted and calcined CuMSN samples formed oxide-
like species, as suggested by the Cu K-edge XANES measurements.
Fig. S3 shows the XANES spectra and their first derivatives for
Propylene conversion = moles of (oxygenates + CO
in feed,
Acrolein selectivity = moles of acrolein/moles of (oxygenates +
CO /3),
2
/3)/moles of
3
CuMSN-E, 3CuMSN-C, and reference materials. Obviously the
3 6
C H
spectra of the two samples resembled that of CuO with a shift of
absorption edge toward higher energy. The observed energy shift
may be correlated with the chemical environment of the copper
species in the samples. The sample 3CuMSN-E was further sub-
jected to a leaching test by stirring in an aqueous solution of
hydrogen chloride (0.15 N) at 50 °C for 2 h, and about 67% of the
copper survived the test. This suggested that most copper species
were strongly bound to the silica framework, some of which might
be resident and isolated in the silica framework, to exhibit such
stability against acid leaching.
2
Y
R
AC = propylene conversion ꢁ acrolein selectivity,
AC = (YAC ꢁ flow rate ꢁ propylene concentration)/(molar vol-
ume of a gas at NTP ꢁ weight of copper),
TOF = (propylene conversion ꢁ flow rate ꢁ propylene concen-
tration)/(molar volume of a gas at NTP ꢁ number of surface
copper atoms).
The moles of surface copper atoms were derived from the sur-
face area of copper and an average surface density for the metal
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jcat.2018.07.014.
As expected, similar textural properties of the calcined and
1
9
ꢀ2
of 1.46 ꢁ 10 atoms m [15,30].
solvent-extracted samples were observed (cf. Fig.
1
and
2
.7. In situ diffuse reflectance infrared Fourier transform spectroscopy
Table S1). Small-angle PXRD patterns of xCuMSN-C indicated that
the calcined samples with Cu/Si ratio up to 9% exhibited a well-
ordered 2D-hexagonal mesostructure (cf. Fig. 1A), and further
increase of the Cu/Si ratio resulted in materials with a lower degree
of structural order. The absence of reflections in the wide-angle
PXRD regime suggested that the oxide species of copper dispersed
well in the materials (cf. Fig. S4). The xCuMSN-C samples showed
measurements
The study was performed using a Thermo-Nicolet 6700 spec-
trometer equipped with a mercury cadmium telluride (MCT)
detector. The instrument was operated at a 1 cm
and 256 scans were collected. A sample was mounted in a diffuse
reflectance infrared Fourier transform spectroscopy (DRIFTS) cell
ꢀ1
resolution,
type IV N
tive pressure (P/P
3.7-nm-wide channel-type mesopores. The H2-type hysteresis
loops at P/P = 0.50–0.95 may be associated with the presence of
extensive intraparticle voids, and a similar phenomenon was
2
physisorption isotherms (cf. Fig. 1B) with steps at rela-
(
Harrick) equipped with ZnSe windows and a heating cartridge.
0
) at 0.25–0.35, indicating the presence of ꢃ3.4–
The cell was heated to a designated temperature under a controlled
atmosphere and was then purged with an He stream for 1 h to
remove residual gases before measurements.
0

4
14
N.-C. Lai et al. / Journal of Catalysis 365 (2018) 411–419
of the hysteresis loop and the slope of the adsorption/desorption
curves at P/P = 0.50–0.95. No discernable domains composed
0
solely of copper oxides were observed, even for 9CuMSN-C. For
comparison, a reference sample, 3CuMSN-IC (with Cu/Si = 3%),
was prepared by impregnation. PXRD and N
ses indicated that 3CuMSN-IC contained CuO particles (JCPDS file
8–1548) that clogged the mesopores of the sample (cf. Fig. S5
2
physisorption analy-
4
and Table S1). In line with the data, some CuO nanoparticles near
the pore openings could be observed by TEM (cf. Fig. 2D). Diffuse
reflectance UV–visible spectra were further recorded to probe the
coordination state of the copper species. As shown in Fig. S6,
3
CuMSN-E and 3CuMSN-C showed a strong absorption peak at
ca. 225 nm and a shoulder absorption between 275 and 350 nm,
which could be assigned to the charge transfer transition between
2
+
n
isolated Cu with framework oxygen and [CuAOACu] cluster
species, respectively [48]. On the other hand, 3CuMSN-IC exhibited
noticeable broad absorption at about 400–800 nm, attributed to
2
+
the d–d transition of Cu in the octahedral ligand environment,
indicating the presence of CuO particles [48]. In summary, the
results showed the simplicity and uniqueness of the pH-jump
method of preparing copper-containing mesoporous silica materi-
als with highly dispersed metal species. The method may be
applied to prepare the same type of materials containing other
metals such as Ti, Cr, Mn, Co, Ni, and Zn using ammonia or suitable
amine-type ligands. Detailed studies are in progress and will be
reported elsewhere.
Fig. 1. Small-angle PXRD patterns (A) and N
2
physisorption isotherms (B) of
selected CuMSN samples. The isotherms are shifted (from bottom to top) by 0, 200,
ꢀ1
5
00, 800, and 1000 cm3
g
STP, respectively.
observed for MSNs synthesized by the pH-jump method [45]. The
samples exhibited high surface area and large pore volume, and the
values decreased with increasing Cu/Si ratio, accompanied by a
small increase in pore wall thickness (cf. Table S1). The results sug-
gested that the mesopores in xCuMSN-C were fully open without
being clogged by the copper species.
2 2 2
H TPR and combined N O oxidation/H TPR analyses were then
conducted on the samples with Cu/Si = 3% to gain information
about the dispersion of the metallic copper after hydrogen reduc-
tion. As shown in Fig. 3 (solid lines), 3CuMSN-C and 3CuMSN-E
Fig. 2 shows TEM images of selected samples. For the pure-silica
0
CuMSN-C, identical to the MCM-41 MSNs previously reported
exhibited intensive and relatively sharp but slightly tailing H
2
[
45], the presence of extensive voids that may be beneficial for
TPR peaks at 235 and 255 °C, respectively, representing the reduc-
tion of small and finely dispersed copper oxide as well as isolated
copper species [15,48–50]. On the other hand, 3CuMSN-IC started
to take up hydrogen at ꢃ110 °C and showed broad overlapping
peaks at 226 and 260 °C, suggesting the presence of both relatively
mass transport in catalytic processes was observed in each particle.
When the ammonia complex of Cu(II) was introduced into the syn-
thesis solution, the voids became slightly wider and more intercon-
nected, while the 2D-hexagonal mesostructure was preserved (cf.
Fig. 2B and 2C). This was also reflected in the changes of the shape
Fig. 2. TEM images of 0CuMSN-C (A), 3CuMSN-C (B), 9CuMSN-C (C), and 3CuMSN-IC (D).

N.-C. Lai et al. / Journal of Catalysis 365 (2018) 411–419
415
B
Fig. 4. (A) Cu K-edge XANES spectra of H
2
-reduced 3CuMSN-E, 3CuMSNB-C, and
3
CuMSN-IC. The spectrum of Cu foil is also shown for comparison. (B) FT profiles of
3
Cu K-edge k -weighted EXAFS data (–) and the fitted results (o) of the three reduced
samples.
sintering [51] of the nonuniform metallic copper particles during
the first H TPR to form relatively large and nonspherical particles
2
Fig. 3. H
lines are measured profiles of hydrogen consumption for the first and second TPR
after N O oxidation, respectively.
2
TPR profiles of 3CuMSN-C, 3CuMSN-E, and 3CuMSN-IC. Solid and dashed
in the mesopores of host silica. In summary, the results of UV–vis-
ible spectroscopy and TPR and XAS measurements indicated that
the one-pot pH-jump synthesis resulted in materials with highly
2
2
+
dispersed CuO clusters and isolated Cu species that could be
reduced to form very small metallic copper nanoparticles.
nonuniform CuO and isolated copper species in this sample [48].
After the first H TPR, the samples were subjected to N O oxidation
of the surface copper atoms, followed by H TPR. For the second H
The H
IC were applied as catalysts for the oxidation of propylene by O
at 180–260 °C. Fig. 5 compares the changes in the conversion of
propylene and the selectivities of acrolein and CO with time on
stream at 220 °C. Decreases in propylene conversion and CO selec-
2
-reduced samples 3CuMSN-E, 3CuMSN-C, and 3CuMSN-
2
2
2
2
2
TPR (cf. the dashed lines in Fig. 3), 3CuMSN-E and 3CuMSN-C
exhibited major hydrogen uptake at ꢃ165 °C, while a small and
broad peak at ꢃ150 °C was observed for 3CuMSN-IC. The amounts
of hydrogen uptake in the temperature range of 100–350 °C were
used to calculate the surface area of the metallic nanoparticles,
from which the dispersion and particle size of copper were esti-
mated. As shown in Table S2, the metal exhibited high dispersion
2
2
tivity and an increase in acrolein selectivity were observed during
the first 90 min of reaction for all the catalysts. Steady-state activ-
ities were reached within ꢃ2 h and remained nearly unchanged
even after further reaction for 12 h.
(
>50%) for both 3CuMSN-C and 3CuMSN-E, and the estimated par-
The high stability of the copper catalysts may be associated
with the confinement effect of mesopores and relatively strong
interaction between the nanosized copper and silica support. The
initial decrease in propylene conversion for 3CuMSN-E and
3CuMSN-C (14–18%) was much smaller than that (ꢃ50%) for the
impregnated sample 3CuMSN-IC. Table 1 summarizes the steady-
ticle sizes were ꢃ1.6 and 2.0 nm, respectively. The copper disper-
sion for 3CuMSN-IC was relatively low (<10%), suggesting a
larger particle size of the metal in this sample.
2
In addition to H TPR, Cu K-edge XAS measurements were fur-
ther conducted for particle size estimation. Each sample was
reduced by hydrogen at 350 °C in an in situ cell prior to the mea-
surements, and complete reduction of Cu was confirmed by XANES
state values of propylene conversion, O
selectivity, acrolein yield, acrolein formation rate, and turnover fre-
2
quency (TOF). The catalysts mainly produced acrolein and CO ;
2
conversion, products
(
cf. Fig. 4A). The data of EXAFS were further analyzed, and the Four-
3
ier transform (FT) profiles of the k -weighted EXAFS data and the
curve-fitting results are shown in Fig. 4B and Table S2. The FT pro-
files of the three samples mainly consist of a first-shell peak corre-
sponding to the nearest-neighbor CuACu distance of 2.50–2.53 Å,
which was reasonable for nanosized Cu particles [15]. The derived
CuACu coordination numbers (CNs) for 3CuMSN-C and 3CuMSN-E
were significantly smaller than that for 3CuMSN-IC, and the esti-
mated average sizes of Cu particles in the two one-pot synthesized
samples were 0.9 and 1.3 nm, respectively, in comparison with
that of 1.9 nm for 3CuMSN-IC. For the impregnated sample, the
inconsistency between the particle sizes estimated from TPR and
XAS (ꢃ10 and ꢃ2.0 nm, respectively) might be due to possible
ethanal and trace amounts of propylene oxide, propanal, acetone,
ethylene, and methane were also detected. It was noted that while
acrolein was the main product for these catalysts, some reported
silica-supported copper catalysts mainly produced propylene
oxide at temperatures below 250 °C. This may be attributed to
the distinct structural and chemical properties of the copper spe-
cies in the catalysts prepared by different routes, since these prop-
erties may affect the detailed mechanisms of the catalytic
2
reactions. The conversion of propylene and O increased, accompa-
nied by the rise of acrolein yield (despite a slight drop in acrolein
selectivity) as the reaction temperature was elevated from 180 to
260 °C.

4
16
N.-C. Lai et al. / Journal of Catalysis 365 (2018) 411–419
ꢀ
1
ꢀ1
ꢀ1
2
36 mmolacrolein
ꢀ1
g
Cu
ꢀ1
h
and TOF of 60.0 h (cf. 196 mmolacrolein
ꢀ
1
g
Cu
h
and 21.7 h for F-1-V [15]). To provide a better comparison
of the catalysts’ selectivity, catalytic tests with varied catalyst
ꢀ1
weights (1.5–60 mg) and flow rates (3–25 mL min ) at 220 °C
were performed to make a plot of acrolein selectivity versus propy-
lene conversion (cf. Fig. S7), from which (combined with the results
2
of H TPR) the initial TOF (per surface copper atom) was calculated
(
cf. Table S3). The initial TOF for 3CuMSN-E was found to be around
twice the value for 3CuMSN-C, which was significantly higher than
that for3CuMSN-IC. A comparison of catalytic activity and reaction
conditions for selected copper-based catalysts in the selective oxi-
dation of propylene to acrolein is shown in Table S4.
The sample 3CuMSN-E was further analyzed to unveil factors
contributing to its superior catalytic performance. Since the pro-
duct selectivity of copper catalysts for the selective oxidation of
propylene with O is known to be strongly associated with the oxi-
2
dation state of the metal [1,11,17,19,20,41], in situ XANES mea-
surements were employed to follow the change in oxidation
2
state of the copper species in the H -reduced 3CuMSN-E after
exposure to the reaction mixture at different temperatures. All
the spectra were acquired after the sample was exposed to differ-
ent conditions for 2 h to ensure that the steady state was reached.
As shown in Fig. 6, the first maximum of the first derivative shifted
from 8978.8 to 8979.9 eV right after the H
exposed to the O -containing mixture at room temperature, indi-
cating that the exposure resulted in immediate oxidation of a sig-
2
-reduced sample was
2
1
+
nificant fraction of surface metallic copper to Cu in 3CuMSN-E. A
similar change was observed for the SBA-15-supported copper
sample prepared by the same impregnation procedure as
3
CuMSN-IC [15], but the fraction of copper being oxidized to
Fig. 5. Changes in propylene conversion (closed circles) and product selectivities
open symbols) with on-stream time at 220 °C for the H -reduced 3CuMSN-E (A),
CuMSN-C (B), and 3CuMSN-IC (C).
1+
Cu was larger for 3CuMSN-E (mainly on the surfaces of metallic
nanoparticles), as evidenced by the more pronounced shift of the
first maximum of the first derivative. The Cu cations that were
in direct contact to the silica support may form CuAOASi bonds.
Copper remained predominantly in the 1+ oxidation state after
the sample was further heated to 260 °C under the same atmo-
sphere. The results suggested that Cu should be the active form
in all the samples studied for the selective oxidation of propylene
to produce acrolein as a major product, and the correlation of the
oxidation state of copper and the product selectivity is consistent
with previous studies on other types of copper catalysts [18,52,53].
On the other hand, the reduced 3CuMSN-E exhibited similar
(slightly lower) copper dispersion yet much higher propylene con-
version than the reduced 3CuMSN-C did. This pointed to the pres-
ence of some other factors that also contributed to the catalytic
performance of 3CuMSN-E. Since the major difference between
3CuMSN-E and 3CuMSN-C was the number of silanol groups on
(
3
2
1
+
Obviously, the catalytic activity of 3CuMSN-E and 3CuMSN-C
was much better than that of 3CuMSN-IC, which may be attributed
mainly to the small size and high dispersion of the metallic copper
in the one-pot-synthesized catalysts. The acrolein yield for
1
+
3
CuMSN-C at 260 °C was about an order of magnitude higher than
that for 3CuMSN-IC. Moreover, 3CuMSN-E exhibited much higher
catalytic activity than 3CuMSN-C in terms of the conversion of
propylene and O and the yield and formation rate of acrolein and
2
TOF. As compared with the state-of-art SBA-15-supported copper
catalyst (denoted as F-1-V with 1 wt.% copper loading in Ref. [15])
catalyzing the same reaction at 260 °C [15], 3CuMSN-E gave almost
four times higher acrolein yield of 12.1% (cf. 3.2% for
F-1-V) and a significantly higher acrolein formation rate of
Table 1
Steady-state catalytic performance of selected H
2
-reduced catalysts.a
Selectivity %
Catalyst Conversion %
T
Y
AC
R
AC
TOF
E
a
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
°
C
O
2
C
3
H
6
Acrolein
CO
2
Ethanal
Others
%
mmolacrolein
g
Cu
h
h
KJ mol
74
3
3
3
3
CuMSN-C
CuMSN-E
CuMSN-IC
CuMSN-EM
180
2
2
1.6
9.4
33.9
0.7
3.9
13.9
58
51
50
31
42
41
8
6
4
3
1
5
0.4
2.0
6.9
8
39
136
1.4
7.7
27.3
20
60
180
3.4
26.0
64.5
1.7
9.8
25.1
56
51
48
37
45
42
6
3
3
1
1
7
1.0
5.0
12.1
19
98
236
4.1
23.4
60.0
67
74
57
2
2
20
60
180
0.2
0.7
3.2
0.1
0.4
1.6
59
54
46
29
35
43
8
8
6
4
3
5
0.1
0.2
0.7
1
4
14
1.0
5.0
19.0
2
2
20
60
180
1.1
3.2
7.8
0.5
1.9
5.2
63
69
70
26
22
17
8
7
6
3
2
7
0.3
1.3
3.6
6
25
69
0.2
0.7
1.9
2
2
20
60
a
a
YAC: acrolein yield; RAC: acrolein formation rate; TOF: turnover frequency; E : activation energy.

N.-C. Lai et al. / Journal of Catalysis 365 (2018) 411–419
417
Fig. 6. Cu K-edge XANES spectra (A) and first-derivative XANES spectra (B) of the
as-prepared 3-CuMSN-E (a) and the sample after reduction under H at 350 °C (b),
cooling down to 25 °C and exposure to the reaction mixture of propylene (5%), O
2
2
(
5%), and He (90%) (c), and finally heating to 260 °C under the same atmosphere (d).
2
The spectra of reference samples of CuO, Cu O, and Cu foil are also shown. The
dashed lines in (B) indicate the positions of the first maxima for Cu foil (8978.8 eV),
Cu O (8979.9 eV), and CuO (8983.5 eV).
2
the silica support (cf. Fig. S2), it was speculated that the surface
silanol groups might become involved in the overall catalytic pro-
cess. To examine this, 3CuMSN-E was reacted with hexamethyldis-
ilazane (HMDS, Showa) to passivate the surface silanol groups via
silylation. The reaction was confirmed by solid-state Si MAS
NMR, and the spectrum of the resulting sample (designated as
Fig. 7. In situ DRIFTS results of 3-CuMSN-E (A) and 3-CuMSN-C (B) after H
2
reduction (a), cooling down to 25 °C and exposure to the reaction mixture of
propylene (5%), O2 (5%), and He (90%) at 25 °C for 45 min followed by He purge (b),
and finally heating to 260 °C and again exposure to the reaction mixture for 45 min
followed by He purge (c).
2
9
3
CuMSN-EM) contained a new line attributed to the trimethylsilyl
group (M line), weaker Q and Q3 lines, and a stronger Q line as
compared with the spectrum of 3CuMSN-E (cf. Fig. S8). The sample
was reduced by the same process and the dispersion and particle
size of the metallic copper in the reduced 3CuMSN-EM were nearly
2
4
ꢀ1
containing new peaks at 3100, 2954, 1442, 1477, and 912 cm
,
which could be assigned to the vibrational modes of propylene
55,56]. This indicated that the surface silanols in 3CuMSN-E, prob-
[
1+
ably those adjacent to the Cu species immediately formed upon
exposure to the reaction mixture, jointly and greatly promoted
the adsorption of propylene and thereby facilitated the overall cat-
alytic reaction. The silanol groups at the periphery of copper
nanoparticles might also become involved in the oxidation of cop-
per during catalytic reaction.
It was noted that peaks roughly at the same positions were
observed for both samples after exposure to the reaction mixture
at 260 °C followed by a He purge. The peaks might be attributed
to the unreacted adsorbed propylene or some intermediates of
the catalytic reaction on the catalysts. When all the results were
combined, it could be concluded that the superior catalytic activity
identical to those in the reduced 3CuMSN-E, as evidenced by H
TPR and oxidation/H TPR (cf. Fig. S9). The reduced
CuMSN-EM was applied to catalyze the reaction of propylene
and O under identical conditions, and the steady-state catalytic
performance (reached within 2 h) is also shown in Table 1. Com-
pared with the values for 3CuMSN-E, the conversion of propylene
2
N
2
O
2
3
2
2
and O for 3CuMSN-EM was much smaller, yet the acrolein selec-
tivity was higher and relatively insensitive to reaction tempera-
ture. The high acrolein selectivity seems to correlate well with
the low apparent activation energy for the passivated catalyst (cf.
Table 1), which implies that the relative energy of the transition
state for the reaction pathway leading to the formation of acrolein
for 3CuMSN-EM might be lower than that for 3CuMSN-E.
2
of 3CuMSN-E for the selective oxidation of propylene with O to
acrolein was mainly attributable to the small size of the metallic
The fact that much lower propylene conversion was observed
for the surface-passivated 3CuMSN-EM suggested a possible role
of the surface silanols in the adsorption of the reactant. In situ
DRIFTS measurements on 3CuMSN-E and 3CuMSN-C were further
conducted, and the results are shown in Fig. 7. A peak at 3740
copper nanoparticles, which resulted in the oxidation of a large
fraction of Cu0 to Cu1+ upon exposure to the reaction mixture,
and to the presence of surface silanols facilitating the adsorption
of propylene on the copper catalyst and thereby enhancing the
overall reaction rate.
ꢀ1
cm attributed to the isolated silanol groups and a band centered
ꢀ1
at 3400 cm associated with the hydrogen-bonded silanol nests
54] were observed for both the H -reduced samples. It was noted
that while the signal of isolated silanol groups is stronger for
CuMSN-C, the band attributed to the silanol nests is more intense
[
2
4. Conclusions
3
A direct synthesis of mesoporous-silica-nanoparticle-supported
nanosized copper catalysts has been reported. The catalysts feature
ordered mesostructures with extensive voids, high surface area
and large pore volume, high degree of silica condensation, and high
for 3CuMSN-E. After being exposed to the reaction gas mixture at
room temperature, followed by an He purge to remove unadsorbed
molecules, interestingly, only 3CuMSN-E exhibited a spectrum

4
18
N.-C. Lai et al. / Journal of Catalysis 365 (2018) 411–419
dispersion of copper species, which are strongly bound to the silica
framework. The ionic copper species transform to highly dispersed
and nanosized metallic copper after hydrogen reduction, and the
reduced catalysts exhibit superior activity for the selective oxida-
tion of propylene with dioxygen. The conversion of both reactants
and the yield and formation rate of acrolein are high and stable.
The excellent catalytic performance may be attributed mainly to
the small size of metallic copper in the one-pot synthesized cata-
lysts, which transforms to Cu1 under reaction conditions, and
the surface silanol groups contributed to propylene adsorption
and facilitated the overall reaction.
impact of catalyst preparation variables on material structure and catalytic
performance, Catal. Sci. Technol. 3 (2013) 2944–2957.
18] D. Duzenli, E. Seker, S. Senkan, I. Onal, Epoxidation of propene by high-
throughput screening method over combinatorially prepared Cu catalysts
supported on high and low surface area silica, Catal. Lett. 142 (2012) 1234–
[
[
1
243.
19] C.L. Bracey, A.F. Carley, J.K. Edwards, P.R. Ellis, G.J. Hutchings, Understanding
the effect of thermal treatments on the structure of CuAu/SiO catalysts and
2
their performance in propene oxidation, Catal. Sci. Technol. 1 (2011) 76–85.
20] H. Tüysüz, J. Galilea, F. Schüth, Highly diluted copper in a silica matrix as active
catalyst for propylene oxidation to acrolein, Catal. Lett. 131 (2009) 49–53.
21] W. Su, S. Wang, P. Ying, Z. Feng, C. Li, A molecular insight into propylene
[
[
+
2 2
epoxidation on Cu/SiO catalysts using O as oxidant, J. Catal. 268 (2009) 165–
174.
[
[
[
[
22] Q. Hu, G. Fan, L. Yang, F. Li, Aluminum-doped zirconia-supported copper
nanocatalysts: surface synergistic catalytic effects in the gas-phase
hydrogenation of esters, ChemCatChem 6 (2014) 3501–3510.
23] A. Dhakshinamoorthy, S. Navalon, D. Sempere, M. Alvaro, H. Garcia, Reduction
of alkenes catalyzed by copper nanoparticles supported on diamond
nanoparticles, Chem. Commun. 49 (2013) 2359–2361.
24] A. Ungureanu, B. Dragoi, A. Chirieac, S. Royer, D. Duprez, E. Dumitriu, Synthesis
of highly thermostable copper-nickel nanoparticles confined in the channels of
ordered mesoporous SBA-15 silica, J. Mater. Chem. 21 (2011) 12529–12541.
25] P. Munnik, M. Wolters, A. Gabrielsson, S.D. Pollington, G. Headdock, J.H. Bitter,
P.E. de Jongh, K.P. de Jong, Copper nitrate redispersion to arrive at highly active
silica-supported copper catalysts, J. Phys. Chem. C 115 (2011) 14698–14706.
[26] G. Yin, M. Nishikawa, Y. Nosaka, N. Srinivasan, D. Atarashi, E. Sakai, M.
Miyauchi, Photocatalytic carbon dioxide reduction by copper oxide
nanocluster-grafted niobate nanosheets, ACS Nano 9 (2015) 2111–2119.
Acknowledgments
The authors acknowledge Dr. Jyh-Fu Lee for help with the XAS
measurements. The authors gratefully acknowledge the financial
support of the Ministry of Science and Technology, Taiwan, under
Contracts MOST 106-2113-M-007-025-MY3 and MOST 107-3017-
F-007-002. This study was also supported by the Frontier Research
Center on Fundamental and Applied Sciences of Matter from The
Featured Areas Research Center Program within the framework
of the Higher Education Sprout Project by the Ministry of Education
[
27] S. Paria, O. Reiser, Copper in photocatalysis, ChemCatChem 6 (2014) 2477–
483.
(
MOE) in Taiwan.
2
[
28] L. De Rogatis, M. Cargnello, V. Gombac, B. Lorenzut, T. Montini, P. Fornasiero,
Embedded phases: a way to active and stable catalysts, ChemSusChem 3
(
2010) 24–42.
29] H.W. Slamet, E. Nasution, S. Purnama, J. Kosela, Gunlazuardi, photocatalytic
reduction of CO on copper-doped titania catalysts prepared by improved-
References
[
[
[
2
[
[
[
1] M.B. Gawande, A. Goswami, F.-X. Felpin, T. Asefa, X. Huang, R. Silva, X. Zou, R.
Zboril, R.S. Varma, Cu and Cu-based nanoparticles: synthesis and applications
in catalysis, Chem. Rev. 116 (2016) 3722–3811.
2] M. Behrens, Coprecipitation: an excellent tool for the synthesis of supported
metal catalysts - from the understanding of the well known recipes to new
materials, Catal. Today 246 (2015) 46–54.
3] M. Yang, S. Li, Y. Wang, J.A. Herron, Y. Xu, L.F. Allard, S. Lee, J. Huang, M.
Mavrikakis, M. Flytzani-Stephanopoulos, Catalytically active Au-O(OH)x-
species stabilized by alkali ions on zeolites and mesoporous oxides, Science
impregnation method, Catal. Commun. 6 (2005) 313–319.
30] Z.-Q. Wang, Z.-N. Xu, S.-Y. Peng, M.-J. Zhang, G. Lu, Q.-S. Chen, Y. Chen, G.-C.
Guo, High-performance and long-lived Cu/SiO
hydrogenation, ACS Catal. 5 (2015) 4255–4259.
31] L. Mo, E.T. Saw, Y. Du, A. Borgna, M.L. Ang, Y. Kathiraser, Z. Li, W. Thitsartarn,
M. Lin, S. Kawi, Highly dispersed supported metal catalysts prepared via in-situ
self-assembled core-shell precursor route, Int. J. Hydrogen Energy 40 (2015)
2
nanocatalyst for CO
2
1
3388–13398.
[
32] L. Mo, S. Kawi, An in situ self-assembled core–shell precursor route to prepare
3
46 (2014) 1498–1501.
ultrasmall copper nanoparticles on silica catalysts, J. Mater. Chem. A 2 (2014)
7
33] P. Gaudin, S. Dorge, H. Nouali, M. Vierling, E. Fiani, M. Moliere, J.-F. Brilhac, J.
Patarin, CuO/SBA-15 materials synthesized by solid state grinding: influence of
CuO dispersion and multicycle operation on DeSO
B 181 (2016) 379–388.
34] Y. Yin, W.-J. Jiang, X.-Q. Liu, Y.-H. Li, L.-B. Sun, Dispersion of copper species in a
confined space and their application in thiophene capture, J. Mater. Chem. 22
[
[
4] F. Schüth, M. Hesse, K.K. Unger, Precipitation and Coprecipitation, in:
Handbook of Heterogeneous Catalysis, Wiley-VCH, 2008.
5] S. Sato, R. Takahashi, T. Sodesawa, K.-I. Yuma, Y. Obata, Distinction between
837–7844.
[
surface and bulk oxidation of Cu through N
2000) 195–199.
2
O decomposition, J. Catal. 196
X
performances, Appl. Catal.
(
[
6] R. van den Berg, C.F. Elkjaer, C.J. Gommes, I. Chorkendorff, J. Sehested, P.E. de
Jongh, K.P. de Jong, S. Helveg, Revealing the formation of copper nanoparticles
from a homogeneous solid precursor by electron microscopy, J. Am. Chem. Soc.
[
[
(
2012) 18514–18521.
1
38 (2016) 3433–3442.
35] T. Yousefi Amiri, J. Moghaddas, Cogeled copper-silica aerogel as a catalyst in
hydrogen production from methanol steam reforming, Int. J. Hydrogen Energy
[
[
[
7] D. Wang, D. Astruc, The recent development of efficient earth-abundant
transition-metal nanocatalysts, Chem. Soc. Rev. 46 (2017) 816–854.
8] P. Liu, R. Qin, G. Fu, N. Zheng, Surface coordination chemistry of metal
nanomaterials, J. Am. Chem. Soc. 139 (2017) 2122–2131.
9] H. Shimizu, I. Nagasaki, K. Matsumura, N. Sayo, T. Saito, Developments in
asymmetric hydrogenation from an industrial perspective, Acc. Chem. Res. 40
4
0 (2015) 1472–1480.
[
36] C.R. Adams, T.J. Jennings, Investigation of the mechanism of catalytic oxidation
of propylene to acrolein and acrylonitrile, J. Catal. 2 (1963) 63–68.
37] A. Marimuthu, J. Zhang, S. Linic, Tuning selectivity in propylene epoxidation by
plasmon mediated photo-switching of Cu oxidation state, Science 339 (2013)
[
(
2007) 1385–1393.
1
590–1593.
[
10] D. Ma, Q. Cai, Copper/amino acid catalyzed cross-couplings of aryl and vinyl
halides with nucleophiles, Acc. Chem. Res. 41 (2008) 1450–1460.
[
[
[
38] Z. Zhai, A.B. Getsoian, A.T. Bell, The kinetics of selective oxidation of propene
on bismuth vanadium molybdenum oxide catalysts, J. Catal. 308 (2013) 25–36.
39] J. Huang, M. Haruta, Gas-phase propene epoxidation over coinage metal
catalysts, Res. Chem. Intermed. 38 (2012) 1–24.
40] T.A. Nijhuis, M. Makkee, J.A. Moulijn, B.M. Weckhuysen, The production of
propene oxide: catalytic processes and recent developments, Ind. Eng. Chem.
Res. 45 (2006) 3447–3459.
[
11] W. Song, D.M. Perez Ferrandez, L. van Haandel, P. Liu, T.A. Nijhuis, E.J.M.
Hensen, Selective propylene oxidation to acrolein by gold dispersed on
2 4
MgCuCr O spinel, ACS Catal. 5 (2015) 1100–1111.
[
12] G. Li, P. Vassilev, M. Sanchez-Sanchez, J.A. Lercher, E.J.M. Hensen, E.A. Pidko,
Stability and reactivity of copper oxo-clusters in ZSM-5 zeolite for selective
methane oxidation to methanol, J. Catal. 338 (2016) 305–312.
[
41] J.S. Yu, L. Kevan, Catalytic partial oxidation of propylene to acrolein over
[
13] R.V. Goncalves, R. Wojcieszak, H. Wender, C. Sato, B. Dias, L.L.R. Vono, D.
Eberhardt, S.R. Teixeira, L.M. Rossi, Easy access to metallic copper
nanoparticles with high activity and stability for CO oxidation, ACS Appl.
Mater. Interf. 7 (2015) 7987–7994.
2+
2+
+
+
+
copper(II)-exchanged M-X and M-Y zeolites where M = Mg , Ca , Li , Na , K ,
and H : evidence for separate pathways for partial and complete oxidation, J.
Phys. Chem. 95 (1991) 3262–3271.
+
[
[
42] F. Schüth, W. Schmidt, Microporous and mesoporous materials, Adv. Mater. 14
[
14] S.S. Acharyya, S. Ghosh, S. Adak, D. Tripathi, R. Bal, Fabrication of CuCr
spinel nanoparticles: potential catalyst for the selective oxidation of
cycloalkanes via activation of Csp3–H bond, Catal. Commun. 59 (2015) 145–
50.
2 4
O
(
2002) 629–638.
a
43] K.L. Boyesen, T. Kristiansen, K. Mathisen, Dynamic redox properties of
vanadium and copper in microporous supports during the selective
oxidation of propene, Catal. Today 254 (2015) 21–28.
44] A. Fukuoka, J.-I. Kimura, T. Oshio, Y. Sakamoto, M. Ichikawa, Preferential
oxidation of carbon monoxide catalyzed by platinum nanoparticles in
mesoporous silica, J. Am. Chem. Soc. 129 (2007) 10120–10125.
45] N.-C. Lai, C.-J. Lin, W.-C. Huang, C.-M. Yang, ‘‘pH-jump” synthesis of 2D-
rectangular c2mm mesoporous silica materials with helical morphology and
extensive void defects, Micropor. Mesopor. Mater. 190 (2014) 67–73.
46] D.A.M. Monti, A. Baiker, Temperature-programmed reduction. Parametric
sensitivity and estimation of kinetic parameters, J. Catal. 83 (1983) 323–335.
1
[
15] C.-H. Liu, N.-C. Lai, J.-F. Lee, C.-S. Chen, C.-M. Yang, SBA-15-supported highly
dispersed copper catalysts: vacuum-thermal preparation and catalytic studies
in propylene partial oxidation to acrolein, J. Catal. 316 (2014) 231–239.
16] S.D. Senanayake, D. Stacchiola, J.A. Rodriguez, Unique properties of ceria
nanoparticles supported on metals: novel inverse ceria/copper catalysts for CO
oxidation and the water-gas shift reaction, Acc. Chem. Res. 46 (2013) 1702–
[
[
[
[
1
711.
17] S. Belin, C.L. Bracey, V. Briois, P.R. Ellis, G.J. Hutchings, T.I. Hyde, G. Sankar,
CuAu/SiO catalysts for the selective oxidation of propene to acrolein: the
[
2

N.-C. Lai et al. / Journal of Catalysis 365 (2018) 411–419
419
[
[
[
47] P. Malet, A. Caballero, The selection of experimental conditions in
temperature-programmed reduction experiments, J. Chem. Soc. Faraday
Trans. 1 (84) (1988) 2369–2375.
48] G. Zhang, J. Long, X. Wang, Z. Zhang, W. Dai, P. Liu, Z. Li, L. Wu, X. Fu, Catalytic
role of Cu sites of Cu/MCM-41 in phenol hydroxylation, Langmuir 26 (2010)
[52] J.B. Reitz, E.I. Solomon, Propylene oxidation on copper oxide surfaces:
electronic and geometric contributions to reactivity and selectivity, J. Am.
Chem. Soc. 120 (1998) 11467–11478.
[53] H. Chu, L. Yang, Q. Zhang, Y. Wang, Copper-catalyzed propylene epoxidation by
+
molecular oxygen: superior catalytic performances of halogen-free K -
1
362–1371.
x
modified CuO /SBA-15, J. Catal. 241 (2006) 225–228.
49] B. Dragoi, A. Ungureanu, A. Chirieac, C. Ciotonea, C. Rudolf, S. Royer, E.
Dumitriu, Structural and catalytic properties of mono- and bimetallic nickel–
copper nanoparticles derived from MgNi(Cu)Al-LDHs under reductive
conditions, Appl. Catal. A 504 (2015) 92–102.
[54] A. Chang, H.-M. Hsiao, T.-H. Chen, M.-W. Chu, C.-M. Yang, Hierarchical
silicalite-1 octahedra comprising highly-branched orthogonally-stacked
nanoplates as efficient catalysts for vapor-phase Beckmann rearrangement,
Chem. Commun. 52 (2016) 11939–11942.
[
50] C. Tang, J. Sun, X. Yao, Y. Cao, L. Liu, C. Ge, F. Gao, L. Dong, Efficient fabrication
[55] P. Concepcion, P. Botella, J.M.L. Nieto, Catalytic and FT-IR study on the reaction
pathway for oxidation of propane and propylene on V- or Mo–V-based
catalysts, Appl. Catal. A 278 (2004) 45–56.
2
of active CuO-CeO /SBA-15 catalysts for preferential oxidation of CO by solid
state impregnation, Appl. Catal. B 146 (2014) 201–212.
[
51] G. Agostini, R. Pellegrini, G. Leofanti, L. Bertinetti, S. Bertarione, E. Groppo, A.
Zecchina, C. Lamberti, Determination of the particle size, available surface
area, and nature of exposed sites for silica–alumina-supported Pd
[56] C. Zhao, I.E. Wachs, Selective oxidation of propylene to acrolein over supported
V
2
O
5
/Nb
2 5
O catalysts: an in situ Raman, IR, TPSR and kinetic study, Catal.
Today 118 (2006) 332–343.
nanoparticles:
0485–10492.
a multitechnical approach, J. Phys. Chem. C 113 (2009)
1