A nitrogen-doped PtSn nanocatalyst supported on hollow silica spheres for acetic acid hydrogenation

Article abstract of DOI:10.1039/c8cc03649g

A novel PtSn/NHSS bimetallic catalyst with N-doped hollow silica spheres as the support was synthesized for the gas-phase hydrogenation of acetic acid to ethanol. Its specific activity was 30% higher than that of PtSn/SiO₂ due to the enhanced surface exposure of Pt active sites, induced by the strong interaction between Pt and N.

Full text of DOI:10.1039/c8cc03649g

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Commun., 2018, DOI: 10.1039/C8CC03649G.
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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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DOI: 10.1039/C8CC03649G
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Nitrogen-doped PtSn Nanocatalyst Supported on
Hollow Silica Sphere for Acetic Acid Hydrogenation
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,
a
a
b
Jiahua Zhou, Yujun Zhao, * Jian Zhang, Yue Wang, Oliver Y. Gutiérrez, Shengnian
c
d
d,
a
a
b,e
Wang, Zhengxiang Li, Peng Jin, * Shengping Wang, Xinbin Ma, Johannes A. Lercher
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel PtSn/NHSS bimetallic catalyst with N-doped hollow noble-based catalysts such as Pt-Sn, Pt-Fe, Ru-Fe, Pd-Re have been
silica sphere as support was synthesized for the gas-phase reported to be effective for AcOH hydrogenation.[10-13] Among these
hydrogenation of acetic acid to ethanol. Its specific activity catalysts, Pt-Sn catalysts exhibited higher catalytic activity and
2
exhibited 30% higher than that of PtSn/SiO due to the selectivity for the hydrogenation of AcOH to EtOH. For this dual-
enhanced surface exposure of Pt active sites, induced by the sites catalysis system, the role of Pt site is considered to provide
[14]
strong interaction between Pt and N.
mobile and activated hydrogen atoms,
adsorption of AcOH to acyl mainly depends on SnO
could provide Lewis acid sites. The reaction rate of AcOH
and the dissociative
x
species which
Highly dispersed noble metal nanoparticle (NP) catalysts are widely hydrogenation could be determined by the formation of acyl
used in heterogeneous reactions because of their excellent catalytic species or reaction between H atoms and acyl species, depending
[1, 2]
performance.
the utilization efficiency of precious metals by maximizing their exposed Pt active sites. Recently, we have developed a modified
dispersion because a higher dispersion increases the exposure of two-step sol-gel method (MTSG, Sn/SiO support was prepared first
Numerous researchers are focused on improving on the ratio of the amount of Lewis acid sites to the amount of
[
15]
2
surface sites, their intrinsic reactivity due to under-coordination, by sol-gel (SG) process and then the Pt NPs were loaded by strong
and quantum effects, which results in superior catalytic electrostatic adsorption method (SEA)) for the preparation of
[3-7]
[16]
performance of the involved noble metal NPs.
2
PtSn/SiO catalysts. An excellent activity was achieved in AcOH
However, owing to their thermodynamic instability, hydrogenation. However, further improving the utilization
nanoparticles easily aggregate when enough energy is available to efficiency of Pt is still a challenge for this catalyst.
overcome diffusion barriers, resulting in the deactivation of the
heterogeneous catalyst. The strong metal-support interactions on carbon surface, which presented higher activity in the
Nitrogen has been reported as a dopant to stabilize Pt catalysts
[8]
[17]
(
SMSI) could stabilize well-dispersed metal NPs and prohibit their decomposition of formic acid. The stabilization effect of N on Pt
aggregation during thermal pre-treatment and under harsh reaction catalyst led to the formation of electron-deficient, sub-nano Pt
[
9]
conditions.
As a model reaction for the conversion of organic acids to the without N doping. However, no research focused on the N doped
corresponding alcohols, the hydrogenation of acetic acid (AcOH) to PtSn/SiO catalyst as well as the role of N on PtSn catalyst in acetic
ethanol (EtOH) has attracted much attention. Supported bimetallic acid hydrogenation.
Herein, Sn supported on N-doped hollow silica spheres (Sn/NHSS)
was facilely synthesized by a novel micelle and emulsion dual
clusters with one order of magnitude higher activity than catalysts
[18]
2
a.
Key Laboratory for Green Chemical Technology of Ministry of Education.
Collaborative Innovation Center of Chemical Science and Engineering, School of
Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.
Prof. Y. Zhao, E-mail: yujunzhao@tju.edu.cn
Institute for Integrated Catalysis, Pacific Northwest National Laboratory,
Richland, WA99352, US.
Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA71272, US.
School of Materials Science and Engineering, Hebei University of technology,
Tianjin, 300130, China.
[19-22]
templating method (one-pot method).
The detailed
preparation process of this one-pot method can be found in the ESI.
A novel PtSn/NHSS catalyst with 0.5 wt.% Pt and 1 wt.% Sn was
prepared and tested for the gas-phase hydrogenation of AcOH to
EtOH, achieving excellent catalytic performance with 86.6%
conversion of AcOH and 90.0% selectivity of EtOH under a WHSV of
b.
c.
d.
-1
Prof. P. Jin, E-mail: china.peng.jin@gmail.com
Department of Chemistry and Catalysis Research Center, TU München, Garching
5748, Germany.
2
1 h (Table S1). Compared with the PtSn/SiO catalyst prepared by
e.
2
the MTSG method, and PtSn/N-SiO post-treated by (3-Aminopropyl)
8
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
triethoxysilane (APTES), the introduction of N using the one-pot
method enhances the SMSI between Pt and N-doped Sn/HSS, and
thus remarkably increasing the Pt dispersion. To the best of our
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knowledge, PtSn/NHSS catalyst is used for the first time in a gas-
phase hydrogenation of AcOH to EtOH. The great improvement of
this strategy on Pt specific activity gives a rational guideline on the
preparation of industrial catalyst for hydrogenation of carboxylic
acids.
View Article Online
DOI: 10.1039/C8CC03649G
The morphology and structure of the PtSn/NHSS catalyst was
characterized by SEM and TEM. The SEM image (Fig. 1 (a)) shows
that N-doped spherical silica nanoparticles with different particle
sizes were successfully synthesized by the O/W microemulsions
method. Numerous cracks on the surface of the silica spheres
indicate high permeability of the silica shell, which provide
mesoscale pathways for reactants/products, similar to what was
[22]
reported by Yamashita et al. Fig. 1 (b) shows that 0.5Pt1Sn/NHSS
has a hollow silica structure with a shell thickness of approximately
1
8 nm. Besides, STEM and energy dispersive spectroscopy (EDS)
mapping images further reveal the spatial distribution of Pt, Sn and
N. As shown in Fig. 1 (c), Pt and Sn are dispersed evenly on the
hollow silica sphere, indicating that Pt could be well anchored on
Fig. 1 (a) SEM, (b) TEM, (c) STEM image and the corresponding EDS
mapping images of 0.5Pt1Sn/NHSS catalyst, (d) Schematic depiction
of 0.5Pt1Sn/NHSS.
[18]
the Sn/NHSS support.
Additionally, N is detected to be well
catalysts (Fig.
dispersed on PtSn/NHSS (Fig. 1 (c)) and PtSn/N-SiO
2
S4). The well dispersed N is believed to play an important role in the
surface character of Pt catalysts. Based on the above analysis, a
structural model for 0.5Pt1Sn/NHSS was proposed as shown in Fig.
(5236 µmol gPt-1). It suggests
much lower than that of 0.5Pt1Sn/SiO
2
that CO adsorption on Pt is inhibited as the result of the
modification effect by N, indicating the presence of strong
interaction between Pt and the N-modified support. The low
CO/Metalexposed ratio could result from the weakened backdonation
1
x
(d), where Pt cluster and SnO species are dispersed evenly on the
hollow silica sphere, which might ensure the close contact between
the Pt clusters and SnOx species, thus enhancing the synergy
between Pt and Lewis acid sites. TEM images of the reduced
catalysts are displayed in Fig. 2 and the particles of Pt in the
samples are measured and listed in Table 1. The 0.5Pt1Sn/NHSS
catalyst shows the smallest particle size of Pt (1.6 nm) and largest Pt
∗
of Pt d-electrons to π antibonding orbital of CO due to the strong
dispersion (70%), while the Pt dispersion of 0.5Pt1Sn/SiO
2
and
0
.5Pt1Sn/N-SiO are only 50% and 46%, respectively. The results
2
suggest that the post-treatment by N precursor could not promote
Pt dispersed well. Conversely, Sn/NHSS support synthesized by the
one-pot method could enhance the interaction between Pt and
support, resulting in the higher dispersion of Pt. These observations
confirm that the presence of N as electron and structure dopant
[18]
helps anchoring Pt clusters and improving their dispersion.
CO is preferentially adsorbed on Pt instead of Sn.[17, 23, 24] It has
been reported that the irreversible adsorption of CO is weakened
and even can become undetectable due to the ionic/electron-
deficient state of Pt induced by N dopant.[4, 25, 26] This provides a
direct method to characterize the effect of N on Pt. As listed in Fig. 2 TEM images of (a) 0.5Pt1Sn/SiO
2 2
, (b) 0.5Pt1Sn/N-SiO ; HAADF-
STEM micrograph of (c) 0.5Pt1Sn/NHSS after reduction at 300 ˚C by
2
Table 1, the N-doped catalysts (0.5Pt1Sn/N-SiO and 0.5Pt1Sn/NHSS)
-1
2
H . Below are the particle size distributions of the Pt nanoparticles.
show similar CO/Metalexposed ratio of about 2900 µmol gPt , which is
Table 1 Textural and physicochemical characteristics of catalysts.
D c Pt dispersion d
e
Loading a
Content b
SBET
D pore
V pore
CO/Metalexposed
Catalysts
Pt
Sn
0.47 1.27
0.59 1.21
N
-
(
m g )
407
2
-1
(nm)
3.8
(cm ·g )
0.33
3
-1
(nm)
2.27
2.47
(%)
50
(µmol gPt-1)
0
.5Pt1Sn/SiO
2
5236
2986
2839
0
.5Pt1Sn/N-SiO
.5Pt1Sn/NHSS
2
3.0
3.0
404
3.7
0.24
46
0
0.48 1.33
322
8.1
0.67
1.63
70
a
b
c
d
e
Determined by ICP-OES. Nominal content. Particle size determined by TEM. Determined by TEM. Determined by CO-chemisorption.
2
| J. Name., 2012, 00, 1-3
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interactions between Pt and N.[17] It is reasonable that the
electronic state of the metal atoms was drastically changed because
of the strong interaction with the N-doped support.
View Article Online
DOI: 10.1039/C8CC03649G
Density functional theory (DFT) calculations were then carried
out to verify the electronic effects of N doping on Pt and compare
the interactions between Pt and different supports. Based on the
2
stable adsorption configurations for both Pt/SiO and Pt/NHSS (Fig.
S6), the plotted total density of state (TDOS) and partial density of
state (PDOS) (Fig. S7) showed that the N atoms and Pt atoms both
contributed to the density of state at the energy of 0 eV, indicating
substantial Pt-N orbital overlaps around the Fermi level.[27]
Consistent with the DOS analysis, NHSS exhibited larger adsorption
4 2
energies (Eads=-3.22 eV) in adsorbing Pt cluster than SiO (Eads=-1.26
eV), and the bond lengths of Pt-N (davg=2.03 Å) was shorter than Pt-
O (davg=2.56 Å) (Table S2). These results revealed that Pt clusters on
the reduced catalyst can be adsorbed much stronger by NHSS via N
sites than by SiO
Pt dispersion of 0.5Pt1Sn/NHSS than 0.5Pt1Sn/N-SiO
deduced that the current one-pot method for the preparation of
the N-doped PtSn catalyst might prompt the homogeneous Fig. 3 Schematic structural representation of (a) 0.5Pt1Sn/SiO
2
via O sites. When further considering the higher
2
, it can be
2
, (b)
distribution of N species on the support, ensuring the intimate 0.5Pt1Sn/N-SiO
2
, (c) 0.5Pt1Sn/NHSS catalysts.
and 0.5Pt1Sn/N-SiO , 0.5Pt1Sn/NHSS exhibits
the highest conversion of AcOH (67.1%) and EtOH selectivity
90.7%). In addition, the apparent activation barriers of the three
contact between Pt and N species during the reduction process.
with 0.5Pt1Sn/SiO
2
2
Thus, the contribution of N species on Pt dispersion is enhanced
due to their strong interaction. The charge density difference was
further calculated to verify the electronic effects of N doping on Pt
(
catalysts are almost the same (51 kJ/mol, Fig. 4 (b)), indicating the N
dopant does not affect the intrinsic catalytic activity of Pt. Thus, the
large amount of exposed Pt sites for 0.5Pt1Sn/NHSS catalyst could
provide affluent activated H atoms, which accelerates the reaction
(
Fig. S8). It was found that more electron-deficient Pt species are
formed in Pt/NHSS due to the electronic donation from Pt to N
induced by the strong interaction between them. The difficulty in
the reduction of the PtSn/NHSS catalyst can also well approve the
suggested stronger interaction (Fig. S9).
Pt was loaded by strong electrostatic adsorption (SEA) method[28,
2
9]
in this work. During the loading process, the silica surface was
negatively charged by controlling the pH value of the impregnation
solution to deprotonate the hydroxyl groups at the silica surface.
2
+
Thus, the metal ammine complexes [Pt(NH
absorbed onto the amorphous silica. Since the given pH value
10.60) was much higher than the PZC values[29] of the three
supports (Fig. S10), the ability of these supports in adsorbing
3
)
4
]
could be easily
(
2+
[
3 4
Pt(NH ) ] should be identical according to the nature of the SEA
method. Based on above results and discussions, hypothetical
structural models of these catalysts were proposed as shown in Fig.
3
. For 0.5Pt1Sn/N-SiO
2
catalyst, the post-treatment by N precursor
on the support
positively charged. However, the small
(
APTES) might result in the formation of -NH
2
[30]
surface, making Sn/SiO
2
amount of APTES was used during the catalysts preparation, so that
the PZC values can be hardly affected, which led to little influence
2+
on electrostatic adsorption of [Pt(NH
Thus, the reduced 0.5Pt1Sn/N-SiO presented a similar Pt dispersion
with 0.5Pt1Sn/SiO catalyst as the result of the nature of the post-
3 4 2
) ] on the Sn/N-SiO support.
2
2
treatment method. On the contrary, for Sn/NHSS support
synthesized by one-pot method, N species are highly dispersed in
Fig. 4 (a) Catalytic performance. Reaction conditions: P=2.6 MPa,
the SiO
2
framework. The strong interaction between Pt and the
-1
T=270 ˚C, H
2
/AcOH=20, WHSV=2 h ; (b) TOF plot for the AcOH
support caused by N could help anchoring Pt species, increasing Pt
dispersion on the NHSS support.
2
hydrogenation over the catalysts, R >0.99. (c) Specific activity and
TOF of the catalysts. Reaction conditions: P=2.6 MPa, T=270 ˚C,
The catalytic performance of these catalysts was examined in
-1
H
2
/AcOH=20, WHSV=2 h . (d) Stability of 0.5Pt1Sn/NHSS, Reaction
-1
AcOH hydrogenation under a WHSV of 2 h (Fig. 4 (a)). Compared
-1
conditions: P=2.6 MPa, T=270 ˚C, H
2
/AcOH=20, WHSV=1 h .
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rate between H and acyl species, promoting the conversion of AcOH [3] L. Li, A. Larsen, N. Romero, V. Morozov, C. Glinsvad, F. AbViieldw-PAertdicelersOennli,nJe.
DOI: 10.1039/C8CC03649G
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[
Conflicts of interest
2
There are no conflicts to declare.
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4
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