What is the effect of Sn and Mo oxides on gold catalysts for selective oxidation of benzyl alcohol?

Article abstract of DOI:10.1039/C8NJ05642K

A foam-like mesoporous silica (MCF)-supported gold (Au) nanoparticle (NP) catalyst was prepared by a facile one-pot synthesis strategy. SnO_x and MoO_x were introduced as promoters by a wet impregnation method to synthesize promoted ca

Full text of DOI:10.1039/C8NJ05642K

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What is the Effect of Sn and Mo oxides on Gold Catalyst for
Selective Oxidation of Benzyl Alcohol?
a
b
b
b
a
b
Jing Xu, Yue Wang, Yunxiang Cao, Zhengke He, Lianming Zhao, Ubong Jerome Etim, Peng
Received 00th January 20xx,
Accepted 00th January 20xx
b
b
b
Bai, Zifeng Yan, Pingping Wu *
DOI: 10.1039/x0xx00000x
A foam-liked mesoporous silica (MCF) supported gold (Au) nanoparticle (NP) catalyst was prepared by a facile one-pot
www.rsc.org/
synthesis strategy. SnO
catalysts Au-SnO /MCF and Au-MoO
shows that introduction of a low amount of SnO
of SnO (1.0 wt%) has lower catalytic activity than the unpromoted Au NP catalyst. Both characterization results and density
functional theory (DFT) calculation suggest that the most important effect of SnO species on gold catalyst is the strong
x
and MoO
x
were introduced as promoters by wet impregnation method to synthesize promoted
/MCF, respectively. The evaluation of prepared catalysts in benzyl alcohol oxidation
(0.2 wt.%) lead to a higher catalytic activity, while a relatively high amount
x
x
x
x
x
interaction between them which leads to the electrons transfer and lattice distortion of Au NPs, providing more low-
coordinated Au active sites. This structural modification enhanced the chemisorption of reactants and lowers the energy
barrier for the dissociation of O
alcohol oxidation. However, a weaker interaction was detected between MoO
no lattice modification was observed, thus no improvement of catalytic activity for benzene alcohol oxidation occurred on
catalyst Au-MoO /MCF.
2
molecules on the surface of gold catalyst, improving the catalytic performance for benzyl
x
and Au NPs on catalyst Au-MoO /MCF and
x
x
were effective catalysts for the aerobic oxidation of benzyl alcohol.
17
Introduction
Furthermore, bimetallic gold (Au) and palladium (Pd) catalysts have
been widely used in alcohol oxidation and the combination of Au and
The selective catalytic oxidation of benzyl alcohols is of paramount
interest in industry, due to the huge demand for benzaldehydes and
benzoic acids as important intermediates in pharmaceuticals,
fragrances, and flavoring additives.1 Homogeneous acid catalysts
Pd as alloy NPs show enhanced catalytic activity for the oxidation of
18-23
benzyl alcohol.
However, the selectivity of the catalyst to
-5
benzaldehyde is low, with a significant amount of by-product toluene.
In order to improve the selectivity of benzaldehyde on gold catalyst,
transition metal oxides are used as supports or auxiliary components.
On the other hand, the introduction of transition metals or their
oxides into gold NP catalysts can change the surface electronic
properties of gold catalysts and effectively improve the stability and
(
such as heteropoly tungstates and heteropoly molybdate) are
generally used to synthesize benzaldehydes and benzoic acids in
6
, 7
industry, which is not convenient for large-scale production and
usually causes serious pollution. So it is urgent to find an economical
and environmentally friendly heterogeneous catalyst to replace
homogeneous ones.
24-26
27
activity of gold NP catalysts.
Della et al.
reported that the
Heterogeneous catalysis is promising in industrial processes due to
its simple operation flow and easy separation of products from
introduction of Cu effectively dispersed Au NPs and improved the
stability of the NP catalyst. Silica-supported Au-Cu alloy catalyst
exhibited good activity for oxidation of benzyl alcohol in air, affording
8
catalysts. In 1987, Haruta et al. found that the supported gold
catalyst showed high catalytic activity for CO oxidation at -70 C and
was stable in a moistened gas atmosphere. Subsequent studies
further confirm that gold catalysts have excellent catalytic activity in
partial oxidation of organic compounds.
27, 28
29
benzaldehyde with high selectivity and high yield.
Yi et al.
9
synthesized an Au-Ni bimetallic catalyst with excellent catalytic
activity for the oxidation of benzyl alcohol in gas phase, and the
improved selectivity to benzaldehyde (over 99%). The superior
activities of these Au-Cu and Au-Ni catalysts were attributed to the
10-16
Yoskamtorn reported
that the supported Au25 clusters prepared by the sol-gel method
presence of transition metal oxides (CuO
x
and NiO
x
) on the transition
25, 29
metal oxides-Au interfaces.
Therefore, transition metals or their
oxides are a good choice to promote gold catalysts for selective
benzyl alcohol oxidation, however, the mechanistic effect of these
transition metal oxides on Au NP catalyst was seldom studied.
In this work, a foam-liked mesoporous silica (MCF) supported Au NP
catalyst was prepared by a one-pot synthesis strategy. And
a.State Key Laboratory of Heavy Oil Processing, School of Materials Science and
Engineering, Institute of Advanced Materials, China University of Petroleum (East
China), Qingdao, Shandong 266580, China.
State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, College
of Chemical Engineering, China University of Petroleum (East China), Qingdao
b.
2
66580, China
subsequently, transition metal oxides, tin oxide (SnO
x
) and
Tel: 86-532-86984615. E-mail: wupp@upc.edu.cn (Wu P).
molybdenum oxide (MoO ) were introduced as an auxiliary
x
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components by wet impregnation method to synthesize catalysts Au- Au-MoO
x
/MCF samples have a typical type IV isotherm with a H-2
View Article Online
DOI: 10.1039/C8NJ05642K
SnO /MCF and Au-MoO /MCF, respectively. The effects of transition hysteresis loop, indicating a typical ink-bottle mesoporous structure
x
x
3
0
metal oxides on the structure and catalytic performance of Au NPs for all catalysts, similar to pristine MCF. As shown in Fig. 1, all
have been systematically explored and theoretically validated. This samples have similar pore size distribution in the range of 3.44.8
study suggests a theoretical basis for the preparation of highly nm. The introduction of transition oxides has no significant effect on
efficient transition metal oxide promoted Au NP catalyst.
the mesoporous structure of the catalysts, mirrored by the N
2
absorption/desorption isotherms and the pore size distribution
curves. After loading transition metal oxides, the specific surface
areas of the catalyst are slightly lower than those of pure Au/MCF
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Results and Discussion
Physicochemical properties
2
-1
2 -
catalyst, but still as large as 726.7793.6 m g and 596.1659.1 m g
1
x x
for Au-SnO /MCF and Au-MoO /MCF catalysts, respectively. In
addition, compared to Sn, the Mo component with higher oxidation
state occupies larger volume of catalyst pores, resulting in a lower
The N
2
absorption-desorption isotherms and pore size distribution
/MCF and Au-MoO /MCF are shown in Fig.
and the results are summarized in Table 1. Both Au-SnO /MCF and
curves of catalysts Au-SnO
1
x
x
x
x
specific surface areas of Au-MoO /MCF.
2 x
Fig. 1 N adsorption-desorption isotherms (left) and pore size distribution curves (right) of (a) and (b) Au-SnO /MCF and (c) and (d) Au-
MoO
The morphologies of the prepared catalysts were characterized by 6.6±0.45 and 5.1±0.45 nm for 0.5%Au-0.2%SnO
TEM (see Fig. 2). As shown in Fig.2, the catalyst support shows a 0.2%MoO /MCF, respectively, indicating that the introduction of
x
/MCF.
x
/MCF and 0.5%Au-
x
distinct foamed silica structure for all prepared samples, indicating transition metal oxide results in the agglomeration of Au NPs to some
that the introduction of metal ions in the preparation process does extent. The bulk and surface Sn(Mo)/Au content was characterized
not have a significant effect on the morphology of the catalyst by inductively coupled plasma optical emission spectroscopy (ICP-
support. It is also seen that the metal NPs uniformly distributed on OES) and X-ray photoelectron spectroscopy (XPS), respectively, as
the surface and in the pores of the support. For 0.5%Au/MCF, the shown in Table 1. The surface Sn/Au ratio was always higher than that
average particle diameter is 1.9±0.45 nm with a narrow particle size in bulk because the transition metal was introduced by impregnation
distribution of 1-4 nm. After loading transition metal oxides, the method to one-pot synthesized 0.5%Au /MCF. On catalysts 0.5%Au-
dispersion of the NPs is lowered, and the particle size increased to 0.2%SnO
x
/MCF, the difference of surface and bulk Sn/Au ratio was
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small, however, this difference is growing up with the increase of
transition metal loading, indicating that the surface Au atoms were
covered by transition metal oxide.
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View Article Online
DOI: 10.1039/C8NJ05642K
Table 1 Textural properties of catalysts.
Au
Pore
Mean pore
size
Atomic ratio
BET surface area
d
(111)
Catalysts
volume
2
-1
ICP
XPS
(
m g )
0
1
2
3
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0
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(nm)
3
-1
(
cm g )
(nm)
Sn(Mo)/Au
Sn(Mo)/Au
0
.5%Au/SiO
2
793.2
726.7
737.3
729.6
723.6
659.1
601.6
596.1
621.8
0.95
0.90
0.90
0.87
0.87
0.86
0.78
0.78
0.75
5.5
4.9
4.8
4.8
4.8
5.1
5.1
5.2
4.8
0.2351
0.2359
0.2360
0.2359
0.2358
--
--
--
0.89
4.32
10.25
21.10
--
0.5%Au-0.2%SnO
.5%Au-0.5%SnO
0.5%Au-0.8%SnO
.5%Au-1.0%SnO
x
x
x
x
/SiO
/SiO
/SiO
/SiO
2
2
2
2
0.69
1.72
2.77
3.44
0.86
2.14
3.42
4.28
0
0
0
0
0
.5%Au-0.2%MoO
.5%Au-0.5%MoO
.5%Au-0.8%MoO
x
x
x
x
/SiO
/SiO
/SiO
/SiO
2
2
2
2
0.2351
0.2350
0.2351
--
--
0.5%Au-1.0%MoO
--
Wide-angle XRD patterns of Au/MCF, Au-SnO
MoO /MCF catalysts shown in Fig. 3 confirmed the TEM results.
Sample 0.5%Au/MCF shows no obvious diffraction peaks at 2θ =
8.25°, 44.46°, 64.69°, and 77.72°, which corresponds to the
diffractions of the (111), (200), (220), (311) planes of cubic Au
PDF#65-8601), respectively. This indicates that the gold NPs are
evenly distributed on the MCF surface. After introducing SnO to
Au/MCF, four diffraction peaks were observed on catalysts Au-
SnO /MCF, but the diffraction peak positions of Au-SnO /MCF is
x
/MCF, and Au-
x
3
(
x
x
x
slightly shifted toward low angles, compared to the 0.5%Au/MCF
catalyst. Therefore, according to the Bragg equation, the crystal
inter-planar spacing of the cubic Au(111) is increased in Au-
/MCF, due to the interaction between SnO and Au NPs. The
x x
lattice spacing of the (111) plane is calculated to be 0.2351, 0.2359,
SnO
0
.2360, 0.2359, and 0.2358 nm for 0.5%Au/MCF, 0.5%Au-
.2%SnO /MCF, 0.5%Au-0.5%SnO /MCF, 0.5%Au-0.8%SnO /MCF,
/MCF, respectively, indicating a distortion of
the lattice spacing with the SnO introduced (as shown in Table 1). In
addition, all the calculated lattice spacing of Au-SnO /MCF are larger
than that of Au/MCF, further confirming that the addition of SnO
caused an increase in the inter-planar spacing of gold. Compared to
.5%Au/MCF, the intensity of the diffraction peaks for Au-SnO /MCF
0
x
x
x
and 0.5%Au-1.0%SnO
x
x
x
x
0
x
is increased, indicating that the introduction of tin oxide increases
the size of the gold NPs. This is well consistent with the TEM results
(
Fig. 2). After introducing molybdenum oxide to Au/MCF, the
intensity of the four diffraction peaks for cubic gold phase slightly
enhanced on catalyst Au-MoO /MCF but not as high as those on
catalysts Au-SnO /MCF. Moreover, compared to 0.5%Au/MCF, no
obvious change in XRD spectra of Au-MoO /MCF was observed,
x
x
Fig. 2 TEM images and particles size distribution histograms of (a)
x
0
0
.5%Au /MCF, (b) 0.5%Au-0.2%SnO
.2%MoO /MCF.
x
/MCF, and (c) 0.5%Au-
suggesting that the introduction of molybdenum oxide has no
x
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obvious effect on the structure of gold NPs. In addition, catalysts Au- transition metal species, probably due to the low loading of
SnO /MCF and Au-MoO /MCF have no obvious diffraction peaks of transition metal species and/or their uniform dispersion.
x x
View Article Online
DOI: 10.1039/C8NJ05642K
(
a)
(b)
(
111)
(
200)
(
220)
(311)
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.5%Au-1.0%SnOX
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.5%Au-1.0%MoOX
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.5%Au-0.8%SnOX
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.5%Au-0.5%SnOX
0.5%Au-0.2%SnOX
.5%Au
0.5%Au-0.5%MoOX
0.5%Au-0.2%MoOX
0.5%Au
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2
0
30
40
50
60
70
80
20
30
40
50
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2
Theta (degree)
2 Theta (degree)
Fig. 3 XRD spectra of catalysts with different ratios, (a) Au-SnO
x
x
/MCF and (b) Au-MoO /MCF catalysts.
0
0
(b)
Au
(a)
Au
525 nm
0
.5 Au-0.8 MoO /MCF
0
.5 Au-1.0 SnO /MCF
x
x
0
.5 Au-0.5 MoO /MCF
0
0
0
.5 Au-0.8 SnO /MCF
x
x
0.5 Au-1.0 MoO /MCF
.5 Au-0.5 SnO /MCF
x
x
.5 Au-0.2 SnO /MCF
x
5
25 nm
0.5 Au-0.2 MoO /MCF
x
0
.5 Au/MCF
0
.5 Au/MCF
3
50
400
450
500
550
600
3
50
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 4 UV-vis spectra of (a) Au-SnO
Fig. 4 shows the UV-vis spectra of the catalysts. The 0.5%Au/MCF The surface chemical properties of Au/MCF, Au-SnO
sample exhibits two broad peaks at 420 nm and 520 nm, MoO /MCF catalysts were revealed by XPS analysis and shown in Fig.
x x
/MCF and (b) Au-MoO /MCF catalysts.
x
/MCF, and Au-
x
31, 32
0
respectively. According to literature,
corresponds to the partially charged Au
the peak at 420 nm 5 and ESI Fig. S1. As shown in Fig. 5(a), the typical Au species with
clusters, while the peak at binding energies (BE) of 83.9 eV (Au4f7/2) and 87.6 eV (Au4f5/2) were
n
5
20 nm is attributed to the surface plasmon resonance (SPR) of Au0 observed on catalyst 0.5%Au/MCF.32-34 When the transition metal
NPs. When transition metal oxide is added, the peak at 520 nm is oxides are introduced, a positive shift of Au 4f peaks for catalysts Au-
enhanced on catalysts Au-SnO /MCF with the increase of SnO SnO /MCF was observed, indicating the interaction of Au with Sn
content, while on catalysts Au-MoO /MCF, nearly no enhancement species. In the Sn 3d region as shown in Fig. 5b, signals positioned at
was detected until 1% of MoO added. This enhancement is 487.4 and 495.7eV were observed on 0.5%SnO /MCF which were
attributed to the enhanced SPR of Au NPs with gold NPs growing up, ascribed to 3d5/2 and 3d3/2 photoelectrons of tetrahedrally
suggesting the strengthened interaction between transition metal coordinated SnO /MCF
species,26 while on catalyst 0.5%Au-0.5%SnO
oxide and Au NPs. In addition, compared to 0.5%Au/MCF, the peak at the binding energy of 3d5/2 and 3d3/2 electrons shifted to higher value
20 nm shows a blue shift for Au-SnO /MCF, whereas its position is of 487.8 and 496.4 eV respectively, confirming the interaction
nearly unchanged for Au-MoO /MCF, indicating tin oxide has stronger between Au and Sn, leading to the formation of SnO species. As
interaction with Au NPs than molybdenum oxide. These results are shown in Fig. S1, a slight positive shift of Au 4f binding energy on
consistent with the XRD and TEM data. On the other hand, as the catalyst 0.5%Au-0.5%MoO /MCF was observed, indicating the weak
transition metal oxides (SnO and MoO ) content increases in the interaction between Au and Mo species. However, the Mo 3d signal
catalyst, the absorption peak at 420 nm weakens or reduces, due to was undetectable even with a MoO loading of 2% on pure
the aggregation and growth of Au clusters as well as active surface MoO /MCF sample, which is probably due to the low response of Mo
coverage by SnO /MoO
x
x
x
x
x
x
0
2
x
5
x
x
x
x
x
x
x
n
x
x
x
.
species.
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.5%Au/MCF
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.5SnO /MCF
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.5%Au-0.5%SnO /MCF
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.5%Au-0.2%SnO /MCF
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485
Binding Energy(eV)
Binding Energy (eV)
Fig. 5 Au(4f) and Sn (3d) XPS spectra of 0.5%Au/MCF, 0.5%Au-0.2%SnO
x x x
/MCF, 0.5%Au-0.5%SnO /MCF and 0.5%SnO /MCF.
The elemental distribution of Au and Sn species has a profound effect The results of energy dispersive line scan (inset of Fig.6) also reveals
on their catalytic performance and can be probed by STEM.
33, 34
Fig. that the Sn species mainly dispersed on the surface of Au NPs,
6
shows the representative STEM-HAADF images and the especially on catalyst 0.5%Au-1.0%SnO
x
/MCF with higher SnO
19
x
corresponding colour-coded EDS spectral maps of catalysts 0.5%Au- loading, which shows a core-shell structure.
.2%SnO /MCF and 0.5%Au-1.0%SnO /MCF. The aggregation of Au
NPs was observed on both catalysts. EDS mapping shows that Sn
purple) elements are well distributed on the catalyst surface, and
0
x
x
0
.5% Au-0.2% SnO /MCF
x
(
R-OH
0.5% Au/MCF
the energy dispersive line scan across NPs indicated the dispersion of
Sn species aligned with Au, indicating the interaction between them.
H
-CH2-
3
500
3000
2500
-21000
1500
Wavenumber (cm )
Fig.7 FTIR spectra of benzyl alcohol adsorbed on catalysts 0.5%Au-
.2%SnO /MCF and 0.5%Au /MCF.
0
x
In order to understand the states of benzyl alcohol-adsorbed catalyst,
the infrared spectra of benzyl alcohol adsorbed 0.5%Au/MCF and
x
/MCF were measured and the results are shown in
Fig. 7. The infrared spectrum of the benzyl alcohol molecules can be
0
.5%Au-0.2%SnO
35
clearly confirmed from Fig. 7. The FTIR characterization indicated
Fig. 6 Representative STEM-HADDF images of (a) 0.5%Au- that benzyl alcohol can be chemisorbed on gold catalyst. From Fig. 7,
0
.2%SnOx/MCF and (b) 0.5%Au-1.0%SnOx/MCF (inset is the energy it was also observed that the peak intensity on catalyst 0.5%Au-
dispersive line scan across NPs using the Au-M and Sn L X-rays) and
corresponding color-coded EDS spectral maps of Au (green), Sn
0
.2%SnO
indicating the benzyl alcohol adsorption capacity on catalyst 0.5%Au-
.2%SnO /MCF is higher than that on catalyst 0.5%Au /MCF.
x
/MCF are much higher than those on catalyst 0.5%Au/MCF,
(
purple).
0
x
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ARTICLE
In summary，high specific surface area and pore volume was conversion to 9.1% on 0.5%Au-1%SnO
maintained after introducing transition metal oxides to Au/MCF benzaldehyde decreases to 51.1%, wherDeOasI: 10th.1e039b/eCn8zNoJi0c56a4c2iKd
catalyst. XPS data revealed that electron transfer occurred between selectivity increases to 40.3%. This indicates that a low amount of tin
transition metals oxide and Au, furthermore, gold lattice distortion oxide introduced results in a high catalytic activity, which is probably
Journal Name
/MCF, and the selectivity of
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
x
View Article Online
was detected on SnO
results, indicating the presence of strong interaction between SnO
and Au NPs, which also led to the aggregation of gold NPs to some SnO
x
promoted catalyst (Au-SnO
x
/MCF) from XRD attributed to the strong interaction between SnO
leading to the lattice distortion of Au NPs, but a higher amount of
may cover the Au active sites and leads to low catalytic activity.
x
and gold NPs
x
x
extent. EDS mapping results revealed the consistent dispersion of Au Meanwhile, with the increase of tin oxide content, the deep oxidation
and Sn and the Sn species mainly distributed on the surface of Au NPs. reaction is enhanced, resulting a higher benzoic acid selectivity on
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
.5% Au-1.0%SnO
0.2%SnO /MCF (15.8%). This is because the presence of large amount
of SnO on catalyst surface may act as an oxygen transfer bridge to
x
/MCF (40.3%) than that on 0.5%Au-
Catalytic performance
x
x
x
The catalytic performance of the Au/MCF, Au-SnO /MCF, and Au-
MoO /MCF catalysts was evaluated in selective oxidation of benzyl
x
alcohol with molecular oxygen. The conversion, selectivity and
product distribution are summarized in Table 2. The pure Au catalyst
enhance the deep oxidation of adsorbed benzaldehyde. It is worth
noting that the addition of transition metal oxide as promoter
switched off the toluene formation. Hutchings and co-workers
applied Pt as promoter to Au-Pd bimetallic catalyst to terminate the
toluene formation from disproportionation reactions. They
attributed this to the electronic or strain modification of Pt to the Au-
Pd NPs. In this work, the electronic effect between Au and transition
metal oxide (especially SnO ) was certified by XPS analysis and the
x
strain effect was confirmed by XRD analysis which would be further
verified by DFT calculation.
3
6
(
0.5%Au/MCF) exhibits a benzyl alcohol conversion of 15.4% with the
primary products of 50% benzaldehyde, 31.5% benzoic acid, 6.8%
toluene, and 8% benzyl benzoate. After introducing SnO to the
Au/MCF catalyst, the benzyl alcohol conversion dramatically
increased to 25.7% on catalyst 0.5%Au-0.2%SnO /MCF. The
x
x
corresponding selectivity to benzaldehyde increases to 75.9%, while
the benzoic acid selectivity decreases to 15.8%. However, further
increase in tin content leads to the decrease of benzyl alcohol
Table 2 Catalytic performance of the catalysts in the oxidation of benzyl alcohol.
Selectivity（%）
Samples
Conversion（%）
Benzyl
Toluene
Benzaldehyde
Benzoic acid
others
benzoate
Blank
2.1
15.4
25.7
20.5
13.6
9.1
15.4
6.8
0
45.6
50
27.1
31.5
15.8
30.7
37.4
40.3
6.0
6.8
8
5.1
3.7
2
0
.5%Au/MCF
0.5%Au-0.2%SnO
x
x
x
/MCF
/MCF
/MCF
75.9
60.9
54.7
51.1
78.3
57.6
55.0
42.4
6.3
7.1
7.3
8.1
6.7
2.7
0
0
0
.5%Au-0.5%SnO
.5%Au-0.8%SnO
0
1.3
0.6
0.5
9.0
6.3
0.0
0
0
0
.5%Au-1%SnO
.5%Au-0.2%MoO
0.5%Au-0.5%MoO
.5%Au-0.8%MoO
.5%Au-1%MoO
x
/MCF
0
0
x
x
x
/MCF
/MCF
/MCF
13.0
9.9
0
0
33.4
45
0
5.6
0
0
x
/MCF
2.6
0
57.6
0
o
Reaction conditions: 50 mg of catalysts mixed with 10.8g of benzyl alcohol react at 110 C for 1h under an O
2
pressure of 8 bar.
As the content of molybdenum oxide in the catalyst increases, the catalyst Au-SnOx/MCF was observed on catalyst Au-MoO
x
/MCF; that
conversion of benzyl alcohol decreases gradually. The conversion of is, the benzaldehyde selectivity decreased with the increase of
benzyl alcohol on catalyst 0.5%Au-1.0%MoO /MCF is only 2.58%, due molybdenum oxide content, which is also explained as the enhanced
to large amount of active sites covered by molybdenum oxide, deep oxidation reactions on transition metal oxides.
preventing the adsorption and activation of benzyl alcohol and O on
x
2
gold NPs, thus leading to low catalytic activity. Another possible DFT calculation
reason may be that compared to tin oxide, the weaker interaction
To further explore the effect of transition metal oxides on gold
catalysts for benzyl alcohol oxidation, DFT calculations were carried
between molybdenum oxide and gold NPs (confirmed by the XRD and
XPS results) hardly changes the structure of the gold surface, and
thus no obvious improvement of catalytic activity was observed on
out. XPS results (as shown in Figure 5) and previously reported
37, 38
literature results
suggested that when the transition metal
and SnO tend to form high-valence oxides,
x
catalyst Au-MoO /MCF. Similar benzaldehyde selectivity trend as
contents are low, MoO
x
x
6
| NJC., 2018, 00, 1-3
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Journal Name
SnO and MoO
ARTICLE
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
3
, due to the sufficient oxidation in the air. Therefore, Table 3 Adsorption energies, Eads (in eV) of O
2
and O as well as O-O
View Article Online
DOI: 10.1039/C8NJ05642K
considering the representativeness and operability of modeling, SnO
and MoO are selected as the models of our theoretical study. As
shown in Fig. 8 and Fig. S2, when SnO is adsorbed on Au(111), SnO
2
2
bond length dO-O (in nm) of adsorbed O on catalyst surfaces.
Catalysts
d
O-O
E
ads(O
2
)
E
ads(O)a
3
2
2
has a stronger interaction with Au(111) (Eads = -2.48 eV) and induces
obvious deformation of surface structure for Au(111), improving the
activity of the Au(111) surface. This is because the deformation of Au
NPs would lead to the presence of more low-coordinated Au sites as
Au(111)
0.1274
0.1356
0.1261
0.17
-0.18
0.21
0.04
-0.46
0.08
SnO
2
/Au(111)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
MoO
3
/Au(111)
active sites for benzyl alcohol oxidation. However, for MoO
the adsorption energy of MoO on Au(111) is relative smaller (Eads
1.90 eV) and the surface structure of Au(111) has almost no change
Fig. 8 and Fig. S2), suggesting the weaker influence of MoO on the
3
/Au(111),
3
=
a
2
Energies are relative to the total energy of 1/2 gaseous O molecule
-
(
and the clean catalyst.
3
structure of Au(111). In addition, more electron transfer between the
Au(111) surface and SnO (0.52 e) than MoO (0.30 e) further
and Au(111).
2
The energy profiles for the O dissociation into 2O on the Au(111),
SnO /Au(111), and MoO /Au(111) surfaces are shown in Fig. 8. The
2 3
optimized geometries for the initial states, transition states, and final
states involved in the reaction are presented in Fig. S4. As shown in
2
3
confirms the stronger interaction of SnO
2
Fig. 8, the dissociation of O
by 0.25 eV, and the reaction is impeded by a high energy barrier of
1.72 eV. When SnO is introduced to the Au(111) surface, the O
2
into 2O on pure Au(111) is endothermic
Au(111)
2
2
dissociation barrier is reduced to 1.29 eV and the reaction is only
slightly endothermic by 0.04 eV, further confirming that tin oxide can
SnO
2
/Au(111)
enhance the reactivity of O
catalytic performance for benzyl alcohol oxidation. However,
compared to pure Au(111), MoO /Au(111) has a higher O
2
on the Au surface and thus improve the
3
2
dissociation barrier of 1.74 eV and stronger endothermicity of 0.29
eV. Therefore, introduction of molybdenum oxide reduces the
catalytic activity of gold towards O and thus shows a lower catalytic
2
MoO
3
/Au(111)
O (ads)
2 O(ads)
2
Fig. 8 Energy profiles for O
SnO /Au(111), and MoO /Au(111) surfaces, with reaction energies
ΔE, energy barriers E , and the gas phase O chemical potential
shown for reference (in eV).
2
dissociation on the Au(111),
activity in benzyl alcohol oxidation.
2
3
a
2
Conclusions
Previous studies suggested that the rate-determining step in benzyl
alcohol oxidation by molecular oxygen is the activation of O
25, 27-29
2
In this study, Au-SnO
synthesized with transition metal oxides (SnO
promoters. When a small amount of SnO was introduced, a strong
interaction between SnO and Au NPs was observed, which leads to
x
/MCF and Au-MoO
x
/MCF catalysts were
on Au NP catalyst. Therefore, the adsorption and dissociation of O
were calculated on the Au(111), SnO /Au(111), and MoO /Au(111)
surfaces in this work. On the pure Au(111) surface, an oxygen
molecule preferably adsorbs on the Au -bridge site with the O-O
bond length of 0.1274 nm, while an oxygen atom prefers to adsorb at
Au -fcc site (see Fig. S3). However, Au(111) is inert for oxygen
adsorption as mirrored by the positive adsorption energies of 0.17 eV
for molecular O and 0.04 eV for atomic O (relative to 1/2 O in gas
2
x
and MoO ) as
x
2
3
x
x
2
the deformation of the surface structure of Au NPs, confirmed by the
characterization results as well as DFT calculations. The distortion of
Au NPs results in the formation of more low-coordinated Au active
sites and thus the catalytic performance for benzyl alcohol oxidation
3
2
2
is improved. While introducing a high content of SnO
x
into Au-
phase). This situation is in accordance with previous experimental
SnO /MCF catalyst, the surface of the Au NPs are covered by SnO
x
x
39
and theoretical studies. After introduction of SnO
2
on Au(111), both
−Au
interface (see Fig. S3). The adsorption energy of molecular O is
strengthened to -0.18 eV, and the O-O bond length is stretched to
.1356 nm. In addition, the adsorption energy of atomic O is also
strengthened to -0.46 eV, relative to 1/2 O in gas phase. The
enhanced adsorption of oxygen suggests SnO /Au(111) has a higher
catalytic activity toward O activation compared to Au(111). On
MoO /Au(111), the adsorption energies of molecular O and atomic
O are calculated to be 0.21 and 0.08 eV, respectively, and the O-O
bond length for adsorbed O is 0.1261 nm, suggesting the lower
activity of MoO /Au(111) toward O than pure Au(111). This may be
caused by the more electrostatic repulsion of MoO group with
oxygen on MoO /Au(111).
species, which prevents the adsorption of reactants, resulting in a
decrease of catalytic performance. On the other hand, because of the
x
and Au NPs and partial coverage
of active sites, Au-MoO /MCF catalysts exhibited a low catalytic
activity for benzyl alcohol oxidation. The characterization results
revealed the presence of electronic and strain modification of
transition metal oxide to Au NPs, while DFT calculation visualized
these results. Therefore, this work provides an effective route for
promoters screening through combing the experimental results with
DFT calculation, which will shed light on the design of transition
metal/transition metal oxide promoted noble metal catalysts with
improved catalytic performance.
2 2
the molecular O and atomic O prefer to adsorb at the SnO
2
weaker interaction between MoO
x
0
2
2
2
3
2
2
3
2
3
3
Experimental
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Journal Name
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Materials
Barrett-Joyner-Halenda (BJH) equation was used to calculate the
View Article Online
DOI: 10.1039/C8NJ05642K
pore size distribution from the desorption branch of isotherms. In
HAuCl
Chemical), ammonium molybdate ((NH
Chemical), 1,3,5- trimethylbenzene (C
triblock co-polymer PEO20PPO70PEO20 (P123, Sigma-Aldrich),
mercaptopropyltrimethoxysilane (MPTMS, 97%, Aladdin),
tetraethoxysilane (C Si, 98%, Sinopharm Chemical), benzyl
4
·3H
2
O (Aladdin), stannous chloride (SnCl
Mo
12, Sinopharm Chemical),
2
·2H
2
O, Sinopharm
0
addition, the pore volume of catalysts was obtained at P/P of 0.989.
The surface properties of the samples were analyzed using X-ray
photoelectron spectroscopy (XPS) and the solid ultraviolet-visible
4
)
H
6
O24·4H O, Sinopharm
7 2
9
(
UV-vis) absorption spectrophotometer. XPS data were obtained on
a Thermo-VG Scientific K-Alpha spectrometer equipped with an Al
anode (Al Ka = 1486.6 eV) and the binding energies were determined
utilizing the C1s spectrum as reference at 284.5 eV. The UV-vis
absorption spectra were acquired on a HitachiU-4100 UV-vis-NIR
8 20 4
H O
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
alcohol (99.99%, Sinopharm Chemical), absolute ethanol (99.98%,
Sinopharm Chemical), and hydrochloric acid (37%, Sinopharm
Chemical) were used as received without further purification.
4
scanning spectrophotometer by using BaSO as an internal reference.
Scanning transmission electron microscopy (STEM) high angle
annular dark field (HAADF) images and X-ray energy dispersive
spectra (XEDS) of the metallic particles were obtained using a JEOL
JEM-2100F with an accelerating voltage of 200kV. The Fourier
infrared diffuse reflection spectra of benzyl alcohol adsorption were
recorded on a NEXUS FTIR (Thermo Fisher Scientific, USA) using
degassed pure catalyst as blank. Prior to the measurement, the
samples were degassed in a vacuum at 573 K for 2 h to ensure the
removal of moisture. The degassed samples were soaked in benzyl
alcohol for 24h and dried in vacuum oven for 8 h. And then the
samples were degassed in a vacuum at 573 K for 3 h to remove the
physically adsorbed benzyl alcohol. The FT-IR spectra were finally
Catalysts synthesis
Synthesis of Au/MCF
The synthesis procedure of Au/MCF catalysts was similar to that
reported in our previous work. In a typical synthesis, P123 (4 g) was
first dissolved into a mixture of 37% HCl (5 mL) and deionized water
12
(
65 mL) at room temperature. Then, 1,3,5-trimethylbenzene (3g) was
introduced to the clear solution as a pore-foaming agent and stirred
o
at 38 C for 2 h, followed by dropwise adding of MPTMS (0.57 g) and
tetraethoxysilane (9.2 g) to the mixture. After that, HAuCl
4 2
·3H O
(
3.55 mL, 0.02 mol/L) was added into the synthesis system under
o
stirring, and the mixture was further crystallized at 100 C for 24 h in
an autoclave. Finally, the as-synthesized sample was washed with
deionized water and ethanol, sequentially, dried in a vacuum oven at
−1
recorded from 4000 to 400 cm using the average record of 64 times
scanning.
o
o
8
0 C for 12 h, and then calcined in air at 550 C for 6 h. The resulting
sample was designated as 0.5%Au/MCF, where 0.5% is the weight
percentage of Au.
Calculation models and method
All density functional theory (DFT) calculations were carried out using
the Perdew–Burke–Ernzerhof (PBE) functional with the generalized
x x
Synthesis of Au–SnO /MCF and Au–MoO /MCF
40
3
gradient approximation (GGA) method implemented in the DMol
The 0.5%Au/MCF sample (2 g) was added to a certain amount of
stannous chloride solution (0.1 mol/L) and stirred to a gelatinous
mixture. The gelatinous mixture was dried in a vacuum oven at 100
41, 42
code
of Material Studio. The metal ion cores were treated with
density functional semicore pseudopotential (DSPP), while the
valence electrons were calculated using the double numerical plus
polarization (DNP) basis set. All calculations were performed using
o
o
C for 12 h, and then calcined in air at 300 C for 4 h to obtain nAu–
mSnO /MCF, where n and m represent the weight percentage of Au
and SnO , respectively. The nAu–mMoO /MCF catalysts were
prepared by the same method as that for nAu-mSnO /MCF except
x
-5
spin-polarization with the convergence criteria of 1×10 Ha for
x
x
-3
-1
-3
energy change, 2×10 Ha Å for max force, and 5×10 Å for
displacement.
x
that stannous chloride solution (0.1 mol/L) was replaced by
ammonium molybdate solution (0.1 mol/L).
The Au(111) surface was built using a periodic four-layer slab with a
p(4 × 4) unit cell. Periodic images of the slab were separated by a 15
Å vacuum gap, and a 5×5×1 k points was sampled in the reciprocal
space. In the calculation process, the atoms in the two upmost layers
Characterization
The crystalline structure of the samples was characterized by as well as adsorbates were fully relaxed, while those in the two
measuring X-ray diffraction (XRD) patterns on an X’Pert PRO MPD bottom layers were fixed to the bulk positions. The SnO /Au(111)
instrument with a Cu Kα radiation (λ = 0.15418 nm) at 35 kV and 40 and MoO /Au(111) models were built via SnO and MoO adsorbed
mA. Transmission Electron Microscope (TEM) was performed to at the lowest energy position of the Au(111) surface, respectively.
examine the morphologies of the samples using JEOL JEM 2010 The adsorption energy (Eads) was calculated by Eads = Eadsorbate/substrate
microscope operated at 200 kV. The particle size distribution − (Eadsorbate + Esubstrate), where Eadsorbate/substrate is the total energy of the
obtained by counting 150-200 particles and the average particle size substrate with an adsorbed molecule, and Eadsorbate and Esubstrate are
of samples were analyzed from TEM data. Metal loadings in the the energies of the free adsorbate and the clean substrate,
catalysts were determined by inductively coupled plasma optical respectively.
2
3
2
3
emission spectroscopy (ICP-OES) using a VISTA-MPX Varian system.
The textural properties of the samples were analyzed from nitrogen Catalytic Tests
adsorption–desorption isotherms measured at 77 K by a surface area
The selective oxidation of benzyl alcohol with O
performed in a 100 mL autoclave with polytetrafluoroethylene lining
Model: SLM100, Beijing Easychem Science and Technology
2
as oxidant was
and porosity analyzer (Micromeritics, TriStar 3000). The Brunauer-
Emmett-Teller (BET) equation was used to calculate the specific
surface area in the relative pressure P/P of 0.05-0.25, while the
0
(
Development Company, China). In the catalytic test, benzyl alcohol
8
| NJC., 2018, 00, 1-3
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Please do not adjust margins

Page 9 of 11
Pl eNa es we dJ oo u nr no at l ao df jCu hs et mm i as tr rgy ins
Journal Name
10.8 g) and solid catalyst (50 mg) were added into the reactor. Then, 18.
ARTICLE
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1
1
1
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2
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(
J. H. Carter, S. Althahban, E. Nowicka, S. J. VFierweaAkrtlieclye,ODn.linJe.
Morgan, P. M. Shah, S. GolunskDi,OCI:.10J..10K3ie9l/yC8aNnJd05G64.2JK.
the autoclave was sealed, purged with O
heated to 110 C. The reaction was carried out for 1 h under the O
2
for three times, and then
o
Hutchings, ACS Catal., 2016, 6, 6623.
2
1
9.
0.
T. A. G. Silva, E. Teixeira-Neto, N. Lopez and L. M. Rossi, Sci.
Rep., 2014, 4, 5766.
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pressure of 0.8 Mpa. After reaction, the reactor was cooled in an ice
bath to 30 C and the pressure was released. The reaction products
o
2
were separated by centrifuge and analyzed using
a gas
324, 59.
chromatogram (Agilent 6870) equipped with an FID detector and a
DB-1 column (30*0.32*0.25).
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Conflicts of interest
There are no conflicts to declare.
23.
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This work was supported by National Natural Science Foundation of
China (51601223, 21805307), the Fundamental Research Funds for
the Central Universities (17CX05018, 17CX02056), PetroChina
Innovation Foundation (2018D-5007-0504), State Key Laboratory of
Heavy Oil Processing（SKLZZ-2017008) and Qingdao Postdoctoral
Applied Research Project (BY20170215).
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