A well fabricated PtSn/SiO2 catalyst with enhanced synergy between Pt and Sn for acetic acid hydrogenation to ethanol

Article abstract of DOI:10.1039/c6ra09199g

This work reported a well-fabricated PtSn/SiO₂ catalyst prepared by a modified two-step sol-gel (MTSG) method. The adoption of strong electrostatic adsorption (SEA) and sol-gel (SG) method could give both a significantly high Pt dispersion and a large amount of Lewis acid sites. The homogenous distribution of Sn species could not only provide more opportunity for the Pt precursor to disperse on the support by forming Pt-(O-Sn)_y^2-y analogous species, but also enhance the synergy between Pt and Sn species. Consequently, an excellent activity was achieved in the hydrogenation of acetic acid (AcOH) with a conversion of 100% and ethanol (EtOH) selectivity of 93%. Investigations on the effect of Sn/Pt molar ratio showed that the dispersion of Pt decreases obviously with the increasing Sn/Pt ratio due to the geometric or electronic effects caused by SnO_x species. A balancing effect between Pt active sites and Lewis acid sites was found to be responsible for the superior catalytic performance in AcOH hydrogenation. Moreover, a parallel reaction path model was proposed for the hydrogenation of AcOH over PtSn/SiO₂ catalyst, in which ethanol and ethyl acetate (AcOEt) are formed competitively through the adsorbed ethoxy intermediate.

Full text of DOI:10.1039/c6ra09199g

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A well fabricated PtSn/SiO catalyst with enhanced
2
synergy between Pt and Sn for acetic acid
Cite this: RSC Adv., 2016, 6, 51005
hydrogenation to ethanol†
Guozhen Xu, Jian Zhang,* Shengping Wang, Yujun Zhao* and Xinbin Ma
2
This work reported a well-fabricated PtSn/SiO catalyst prepared by a modified two-step sol–gel (MTSG)
method. The adoption of strong electrostatic adsorption (SEA) and sol–gel (SG) method could give both
a significantly high Pt dispersion and a large amount of Lewis acid sites. The homogenous distribution of
Sn species could not only provide more opportunity for the Pt precursor to disperse on the support by
2ꢀy
forming Pt–(O–Sn^)
y
analogous species, but also enhance the synergy between Pt and Sn species.
Consequently, an excellent activity was achieved in the hydrogenation of acetic acid (AcOH) with
a conversion of 100% and ethanol (EtOH) selectivity of 93%. Investigations on the effect of Sn/Pt molar
ratio showed that the dispersion of Pt decreases obviously with the increasing Sn/Pt ratio due to the
x
geometric or electronic effects caused by SnO species. A balancing effect between Pt active sites and
Received 10th April 2016
Accepted 16th May 2016
Lewis acid sites was found to be responsible for the superior catalytic performance in AcOH
hydrogenation. Moreover, a parallel reaction path model was proposed for the hydrogenation of AcOH
DOI: 10.1039/c6ra09199g
over PtSn/SiO
2
catalyst, in which ethanol and ethyl acetate (AcOEt) are formed competitively through
www.rsc.org/advances
the adsorbed ethoxy intermediate.
4
monomers for polymer production. Among the numerous
carboxylic acid hydrogenation processes, hydrogenation of
1
. Introduction
Increasing concerns about global climate change and depletion AcOH into ethanol is of great interest. AcOH is the simplest
of fossil fuel resources have promoted the development of carboxylic acid so it is used as a good model to investigate C–C
producing industrial chemicals and fuels from natural bond and C–O or C]O bond activation. Meanwhile, ethanol is
resources. Carboxylic acids are a family of important platform an attractive product that has been widely used as a fuel addi-
5,6
chemicals that can be produced by biological and chemical tive, solvent and antiseptic.
pretreatments of biomass. In recent years, the catalytic reduc-
AcOH hydrogenation has been studied since the 1990's. It
tion of carboxylic acid has attracted a lot of attention since it was found that the direct catalytic hydrogenation of AcOH
7
offers a simple and direct route to synthesize aldehydes, alco- requires the presence of both metal and metal oxide phases.
hols and esters.
1–3
Hydrogenation of mono- and multi- Further researches showed that the adsorption of AcOH on the
functional carboxylic acids can make alcohols and polyols, oxide phases plays a major role in determining the hydro-
respectively, which can be used as fuels, solvents and conversion kinetics and the role of metal is to provide mobile,
activated hydrogen atoms. However, the mechanism of AcOH
hydrogenation is still on debate. Two types of mechanism
Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative
involving Mars–Van Krevelen mechanism and Langmuir–Hin-
Innovation Centre of Chemical Science and Engineering, School of Chemical
shelwood mechanism have been proposed. The difference
Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail:
zhangjian1985@tju.edu.cn; yujunzhao@tju.edu.cn
Electronic supplementary information (ESI) available: Fig. S1 shows the N
between the two mechanisms is the denition of active site for
AcOH activation. The former attributes the active sites to the
surface vacancies, while in the later it is assigned to the Lewis
†
2
adsorption–desorption isotherms and pore size distribution of reduced
catalysts. Fig. S2 displays the HAADF-STEM and EDX elemental-mapping
1,2,8,9
acid sites/metal cations on the catalyst surface.
images of the reduced PtSn1.6/SiO
2
-MTSG catalyst. Fig. S3 shows the TEM
Recently, bimetallic supported noble-based catalysts such as
images of reduced PtSnx/SiO -MTSG catalysts. Fig. S4 represents the
2
Pt–Sn, Pt–Fe, Ru–Fe, Pd–Re etc., are reported to be effective for
deconvolution of the t about FTIR spectra of chemisorbed pyridine on reduced
7,9–12
catalysts. Fig. S5 displays the hydrogenation performance of AcOH over AcOH hydrogenation.
Among these catalysts, PtSn sup-
PtSn1.6/SiO
tted results of H
quantity of Lewis acid sites of reduced catalysts. Table S3 illustrates the
2
-MTSG catalyst as a function of WHSV(AcOH). Table S1 shows the
ported bimetallic catalyst acquires a lot of attention since it
exhibits good activation for the hydrogenation of AcOH to
ethanol. Nevertheless, most of the PtSn catalysts are generally
prepared by impregnation (IM) method nowadays, which is

2
-TPR experiments of catalysts. Table S2 displays the integral
hydrogenation performance of AcOH over PtSn1.6/SiO
DOI: 10.1039/c6ra09199g
2
-MTSG catalysts. See
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quite limited to control the structural parameters. Conse- Sinopharm Chemical Reagent Co., Ltd) precursor with variable
quently, the hydrogenation process still suffers from a variety of Sn content from 0 to 5 wt%. The molar ratio of TEOS : C H -
2
5
issues such as lower ethanol selectivity, harsh reaction condi- OH : H O was 1 : 3.85 : 10.20. TEOS was mixed with ethanol and
2
12,13
tions and high catalyst costs.
an alcoholic solution of SnCl $2H O. Aer that, deionized water
2 2
PtSn bimetallic catalysts have been widely used in many was added dropwise into the solution with vigorously stirring at
catalytic processes. Various ways of preparing PtSn bimetallic room temperature. The pH of the solution was kept to be ca. 4
catalyst have been developed to tailor the electronic and during the water dropping process. Aer that, the clear sol was
1
4–16
geometric structures for different applications.
Up to now, allowed to gel in air for 48 h. Subsequently, the obtained gel was
ꢁ
supported PtSn bimetallic catalysts could be prepared by co- aged for 24 h at 60 C. The resulting solids were ltered, washed
reduction method, co-precipitation method, sol–gel (SG) with ethanol until negative chloride test, and then dried in oven
method, hydrothermal approach, carbonyl route, and at 105 C. The obtained xerogel was then calcined in air at 500
ꢁ
13,17,18
ꢁ
a controlled surface reaction method.
reported to be one of the promising technologies to prepare IM method with controlled concentration of SnCl
PtSn catalysts with well-dened physical properties. It increased wt/wt Sn loading), and the acquired substrate was denoted as
the homogeneity and purity of the solid, with better micro- Sn/SiO -IM.
structural control of the support, well-dened pore size distri- Supported Pt catalysts were obtained by depositing the metal
bution, and higher thermal stability of the catalysts. Two types precursor onto the support surface by SEA method. Platinum(II)
of sol–gel methods have been reported in preparing PtSn/Al O tetra-ammine chloride ([Pt(NH ) ]Cl , 99%, Sinopharm Chem-
SG method has been
C for 4 h. For comparison, Sn precursor was also uploaded by
2
solution (1%
2
2
3
3 4
2
19
catalyst for n-heptane reforming. One is the one-step sol–gel ical Reagent Co., Ltd) was used as the metal precursor. The SiO2
OSG) method, which starts from both the support and metal or Sn/SiO -SG support was rstly suspended in NH OH solution
(
2
4
precursors and then go all way to the nal catalysts in one (aq., pH ¼ 10.6) under stirring for 0.5 h. The solution of
batch. The other is two-step sol–gel (TSG) method, where plat- [Pt(NH ]Cl was then added with controlled concentration to
3
)
4
2
inum is uploaded by IM method aer tin is introduced into obtain 1% wt/wt Pt loading on the support. The suspension was
alumina support by SG method. Both of them could give cata- kept under stirring for 10 h and then was separated by ltration
lysts with higher activity and stability.
under vacuum. The solid was rinsed with water, and then was
ꢁ
On another hand, high dispersion of metal nanoparticles is dried at 105 C for 5 h. These catalysts were named as PtSnx/
thought to be crucial in the design of supported noble metal SiO -MTSG and x represents for the theoretical Sn/Pt molar
2
catalysts, in order to achieve high performance and minimize ratio. For comparison, other two types of PtSn/SiO catalysts
2
the cost. It was reported that Pt-based catalysts prepared by were also prepared. One was obtained by loading Pt onto Sn/
strong electrostatic adsorption (SEA) method exhibited higher SiO
dispersion of Pt than those prepared by IM method. By MIM. The other was prepared by impregnation of [Pt(NH ]Cl
2
-IM by SEA method, which was denoted as PtSn1.6/SiO
2
-
20
3
)
4
2
controlling the pH value of the impregnation solution, the onto Sn/SiO
hydroxyl groups that bonding on oxide surface become
protonated or deprotonated, thus become positively charged
2 2
-SG, and was denoted as PtSn1.6/SiO -TSG.
2
.2. Catalyst testing
below a characteristic pH value or negatively charged above the
characteristic pH value. Therefore, the metal complex could The hydrogenation performances of the catalysts were carried
deposit onto the support surface by SEA method, resulting in out in a stainless steel tubular xed bed reactor. Catalysts (0.8 g,
21–23
a relative higher metal dispersion.
ca. 2 mL, 40–60 mesh) were placed in the center of the reactor,
Herein, we use a modied two-step sol–gel (MTSG) method and both sides of the catalyst bed were packed with quarts
for the synthesis of highly dispersed and homogeneously powder (20–40 mesh). The catalysts were rstly reduced in situ
ꢀ1
ꢁ
distributed PtSn catalyst, in which Sn/SiO was rstly synthe- in owing hydrogen (99.999%, 100 mL min ) at 300 C for 4 h.
2
sized by SG method and then Pt was uploaded by SEA method Aer that, AcOH (99.5%) was pumped into the reactor system
sequentially. The effects of the synthesis methods on the using a constant-ow pump and vaporized through a preheater
ꢁ
surface structure and catalytic properties of AcOH hydrogena- maintained at 200 C. The hydrogenation products in liquid-
tion were studied. It shows that this MTSG strategy could phase were analyzed by BEIFEN 3420A gas chromatograph
enhance both the catalytic activity and selectivity for the equipped with an HP-INNOWAX (30 m ꢂ 0.25 mm ꢂ 0.25 mm)
hydrogenation of AcOH into ethanol, due to the enhanced capillary column and a ame ionization detector (FID). Giving
synergy between Pt and Sn species. Moreover, the inuence of that neglected C
2 6 4 2
H , CH , CO and CO were detected in the
Sn/Pt molar ratio was investigated and a reaction model for the effluent gas stream, AcOH conversion was determined from
formation of ethanol and AcOEt was proposed.
a carbon balance based on the analysis of compounds detected
in the liquid phase, and product selectivity was based on the
carbon-containing products, which excluded the amount of
water formed. Only the selectivity of ethanol and AcOEt were
presented here because other hydrogenation products like
2. Experimental
2.1. Catalyst preparation
Sn/SiO (denoted as Sn/SiO -SG) and SiO in this study were aldehyde could be neglected in the selected reaction conditions.
2
2
2
prepared by SG method with Si(OC
pharm Chemical Reagent Co., Ltd) and SnCl
2
H
5
)
4
(TEOS, 99.5%, Sino-
The turn over frequency (TOF) of the reaction was measured
ꢁ
ꢀ1
2
$2H
2
O (98%, under the conditions (270 C, 2.6 MPa, H
2
/HOAc (mol mol ) ¼
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20) where AcOH conversion was 60–70%. The TOFPt value was the intensity of the spectrum was enlarged by the same multiple
based on exposed Pt atoms, indicating the moles of AcOH as the disk weight used to be enlarged to 1 g.
converted per hour by per mole of exposed Pt atoms at the
catalyst surface.
Fourier transform infrared (FTIR) spectra of pyridine
adsorption were also collected on a Thermo Scientic Nicolet
ꢀ
1
6700 (32 scans, 4 cm ). The pretreatment for the sample was
same as that for CO adsorption. Aer the pretreatment, the disk
2.3. Catalyst characterization
ꢁ
was re-reduced in 10 vol% H
2
–Ar at 300 C for 0.5 h and then
Textual properties of the catalysts were measured using was evacuated under reduced pressure at the same temperature.
ꢁ
a Micromeritics Tristar 3000 analyzer by nitrogen adsorption at Aer cooling to 50 C, the samples were saturated with pyridine
ꢁ
the boiling temperature of liquid nitrogen. The specic surface vapor for 0.5 h and then evacuated at 150 C for 0.5 h, respec-
areas were calculated from the isotherms using the BET tively. Spectra were recorded at the evacuation temperature in
ꢀ
1
method. Platinum and tin contents were determined by ICP- the range of 4000–650 cm . The amount of the Lewis acid sites
OES (VISTA-MPX, Varian) and were expressed in metal weight in samples was calculated by peak deconvolution of character-
ꢀ
1
24
percent. The size distribution of metal particles was determined istic band at ca. 1450 cm
.
by transmission electron microscopy (TEM) on a FEI Tecnai G2 X-ray photoelectron spectrometer (XPS) was carried out on
F20 transmission electron microscope at 200 kV. The sample a Thermo ESCALAB 250Xi system, using a monochromatic Al-
powder was dispersed in ethanol by sonication, and then drops Ka (1486.6 eV) source operating at 10.8 mA ꢂ 15 kV and a pass
ꢀ5
of the suspension were applied onto a copper grid-supported energy of 50 eV under an operating pressure of 7.1 ꢂ 10 /7.1 ꢂ
ꢀ7
transparent carbon foil and dried in air.
10 Pa. The energy resolution is 1 eV. Catalysts were mounted
The in situ DRIFTs experiments were carried out on on a special-designed holders. Aer reduction in situ in the
a Thermo Scientic Nicolet 6700 spectrometer (32 scans, 4 rapid sampling chamber and pre-treatment chamber, samples
ꢀ
1
cm ), equipped with a Harrick Scientic DRIFT cell and were transferred into analysis chamber by vacuumed gradually.
a mercury–cadmium–telluride (MCT) detector cooled by liquid Possible deviations caused by electric charge on the samples
ꢁ
N
2
. All the samples were pretreated at 100 C under Ar (30 mL were corrected through taking the Si 2p line as an internal
ꢀ
1
min ) for 1 h, and then FTIR spectra was collected using KBr as standard at 103.4 eV. Peak tting was achieved using XPSPeak
background spectrum.
41 soware.
2
Temperature-programmed reduction (H -TPR) tests were
carried out on a Micromeritics AutoChem 2920 TPD/TPR
3
. Results and discussion
apparatus using ca. 0.15 g of catalyst. The catalyst was prior
ꢁ
3.1. PtSn/SiO
3.1.1. Catalytic testing. Table 2 shows the hydrogenation
performance of monometallic Pt/SiO -MTSG and Sn/SiO -SG
A thermal conductivity detector (TCD) was employed to deter- catalysts as well as bimetallic PtSn catalysts prepared by
mine the amount of hydrogen consumed. different methods. All the bimetallic PtSn catalysts exhibit
The metal dispersion of the catalysts was determined remarkable higher catalytic performance than the mono-
2
synthesized by MTSG method
treated at 200 C for 2 h in owing He. A ow rate of 30 mL
ꢀ
1
min of 10 vol% H /Ar was used for the reduction with the
2
ꢁ
ꢁ
ꢁ
ꢀ1
temperature range from 50 C to 900 C at a rate of 10 C min
.
2
2
12
according to literature by pulsed hydrogen titration of oxygen metallic catalysts, indicating that the hydrogenation of AcOH is
12,13
pre-covered sites using a Micromeritics 2920 TPD/TPR appa- catalyzed by the synergy between Pt and Sn species.
It also
ratus. Prior to the analysis, catalysts (0.15 g) were reduced in shows that the catalyst prepared by MTSG method has the
ꢁ

owing 10 vol% H /Ar at 300 C for 2 h, followed by purging highest catalytic performance. Conversion of AcOH over
2
with He to remove any physical adsorbed hydrogen. Aer PtSn1.6/SiO
cooling to 50 C in owing He, the catalysts were exposed to 10 of ethanol is 91.8%. PtSn1.6/SiO
2
-MTSG could reach up to 98.3% and the selectivity
-MIM presents a lower AcOH
ꢁ
2
2
vol% O /He for 0.5 h followed by purging with Ar. Titration with conversion and a similar ethanol selectivity. However, for
pulses of 10 vol% H
uptake occurred.
2
/Ar was then employed until no further H
2
PtSn1.6/SiO
2
-TSG catalyst, both AcOH conversion and ethanol
selectivity decrease dramatically. As mentioned above, SG
The in situ FTIR of chemisorbed CO on reduced catalysts method could give uniform distribution of tin throughout
ꢀ
1
19
were collected on a Nicolet 6700 (32 scans, 4 cm ) spectrometer silicon matrix. Thus, the homogeneous distribution of Sn
using a stainless steel cell connected to a gas dosing and evac- could be one of the important reasons for the excellent catalytic
uation system. The sample powders were prior reduced at 300 activity of catalyst prepared by MTSG method. Another possible
ꢁ
2
C in H for 4 h and then cooled to room temperature. Aer reason may be the higher dispersion of Pt, resulting from the
that, the powder was pressed into a self-supporting disk (ca. 20 SEA method instead of commonly used IM method. It is note-
mg) under the pressure of 8 MPa and then was introduced into worthy that the deteriorative selectivity of ethanol for the
ꢁ
the IR cell. The disk was re-reduced in 10 vol% H
2
/Ar at 300 C PtSn1.6/SiO
2
-TSG could be ascribed to the less Pt active sites,
ꢁ
for 0.5 h and then cooled to 30 C under a high-purity He ow induced by the IM method. Considering the adsorption of AcOH
for collecting the background spectrum. Aerward, the sample is the rate control step, it could be concluded that the hydro-
was exposed to the ow of CO at 30 C for 0.5 h. FTIR spectra genation activity of PtSn catalyst could be determined by the
ꢁ
9
were collected aer purging He for 0.5 h and referenced to the amount of surface active Sn species. While the selectivity of
background spectrum of reduced catalysts. For normalization, ethanol has close relationship with the amount of surface
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metallic Pt species. However, further experiments and charac-
terizations are necessary to reveal the explanation for the effects
of these preparation methods.
3
3
.1.2. Catalyst characterization
.1.2.1. TEM. Fig. 1 shows TEM images of the reduced Pt/
-MTSG and PtSn/SiO catalysts prepared by different
SiO
2
2
methods. All the catalysts prepared by SEA method exhibit
small particles homogeneously distributing on the silica
support, similar to the report that monolayer adsorption of Pt
complexes by SEA method can lead to small ra-like metal
2
2,23
particles.
.4–1.8 nm. However, the catalyst prepared by impregnation of
Pt(NH ]Cl (PtSn1.6/SiO -TSG) gives much larger particles
with a wide particle size distribution. According to the litera-
The average diameter of metal particles is about
1
[
3
)
4
2
2
2
+
ture, the ionic interaction between [Pt(NH
hydroxyl group of SiO could be responsible for the high
dispersion of Pt over SiO
similar surface properties of SnO (x < 2) and SiO , it could be Fig. 2 FTIR spectra of hydroxyl groups of SiO and Pt/SiO -MTSG.
3
)
4
]
and the surface
2
22,23
2
surface.
Thus, considering the
x
2
2
2
2
+
speculated that the interaction between [Pt(NH ) ] and the
3
4
surface hydroxyl provided by oxide tin could also exist. There-
2
ꢀy
ꢀ
1
ꢀ1
fore, Pt–(O–Sn^)
catalyst surface.
y
5,26
analogous species could be formed on the
Consequently, the interaction between Pt
silanol groups (3510 cm
and 3743 cm ), suggesting
2
consumption of silanol groups in the strong electrostatic
adsorption process. It was speculated that Pt–(O–Si^)
2
y
ꢀy
and Sn species could be enhanced when Pt is uploaded by SEA
method. However, such interaction should be weakened when analogous species might be formed by the interaction between
Pt is uploaded by IM method because of the lack of such [Pt(NH ) ] and SiOH groups.
2
+
28,29
4
3
interaction between Pt and Sn species.
3.1.2.3. HAADF-STEM. Fig. S2† shows the HAADF-STEM
3
.1.2.2. FTIR. The FTIR spectra of SiO and Pt/SiO -MTSG
2
2
images of reduced PtSn1.6/SiO -MTSG catalyst. Small discrete
2
are shown in Fig. 2. FTIR spectra in OH stretching region
suggests that both SiO and Pt/SiO -MTSG contained isolated
external silanol groups (3743 cm ), free internal silanol groups
particles are observed on the surface of the support, further
demonstrating the homogenous distribution of Pt and Sn
species (Fig. S2A and B†). The distribution of Pt and Sn species
was determined by EDX elemental-mapping analysis (shown in
2
2
ꢀ1
ꢀ
1
(
3732 cm ), and delocalized hydrogen-bonded groups (3510
ꢀ
1
27
cm ) of lattice defects. Meanwhile, compared to the FTIR Fig. S2C and D†). It shows that most of the nanoparticles
spectra of SiO
2
, Pt/SiO
2
-MTSG shows a signicant decrease in contain both Pt and Sn elements, indicating that MTSG method
could provide more possibility for the close contact between Pt
30
and Sn atoms.
.1.2.4. Temperature-programmed reduction (H
2
-TPR proles of the monometallic Pt/SiO -MTSG and Sn/SiO -
3
2
-TPR). The
H
2
2
SG catalysts along with PtSn/SiO
in Fig. 3. Two reduction peaks centered at around 206 C and
2
bimetallic catalysts are shown
ꢁ
Fig. 1 TEM images of the reduced catalysts. (A) Pt/SiO
2
-MTSG, (B)
-TSG.
PtSn1.6/SiO -MTSG, (C) PtSn1.6/SiO -MIM, (D) PtSn1.6/SiO
2
2
2
2 2 2
Fig. 3 H -TPR profiles of Sn/SiO -SG, Pt and PtSn/SiO catalysts.
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2
Table 1 Physical properties of supported PtSn/SiO catalysts
a
Loading (wt%)
2
ꢀ1
3
ꢀ1
b
c
Catalyst
Pt
Sn
S
BET (m g
)
Dpore (nm)
V
pore (cm g
)
D (nm)
Pt dispersion (%)
Sn/SiO
Pt/SiO
2
-SG
-MTSG
—
0.87
—
975
451
492
538
582
395
449
483
2.91
2.98
3.86
3.22
2.81
4.01
3.30
3.31
0.40
0.19
0.49
0.35
0.18
0.38
0.30
0.33
—
—
2
0.89
0.87
0.90
0.86
0.89
0.89
0.88
1.42
1.46
1.84
4.16
1.43
1.70
1.98
70.1
55.6
47.9
30.2
61.9
34.4
17.6
PtSn1.6/SiO
PtSn1.6/SiO
PtSn1.6/SiO
PtSn1.0/SiO
PtSn3.3/SiO
PtSn8.2/SiO
2
2
2
2
2
2
-MTSG
-MIM
0.89
0.81
0.89
0.62
1.64
4.29
-TSG
-MTSG
-MTSG
-MTSG
a
b
c
Metal loading determined by ICP-OES. Particle size determined by TEM images. Pt dispersion determined by H
2
–O
2
titration method.
ꢁ
3
66 C are observed over Pt/SiO
2
-MTSG catalyst. According to the strong interaction between Pt and Sn exists in the MTSG
the literature, the peak in the lower temperature is attributed to preparation process. Based on the TPR results, it could be
the reduction of Pt having a weak interaction with the support. proposed that Pt species in PtSn1.6/SiO -MTSG catalyst have the
2
The other at higher temperature corresponds to the reduction closest interaction with Sn species.
2
y
ꢀy
31
of Pt–(O–Si^)
species. In the case of Sn/SiO
2
-SG catalyst,
3.1.2.5.
2 2
H –O titration. Table 1 shows the dispersion of Pt
the reduction prole is characterized by a broad peak with particles in reduced Pt and PtSn catalysts. The dispersion of Pt
ꢁ
a maximal temperature (T
M
) at 503 C, and the reduction peak in monometallic Pt/SiO
2
-MTSG catalyst is 70.1%. For PtSn
ꢁ
ꢁ
begins at 300 C and ends at 700 C, which is due to the bimetallic catalysts, the dispersion of Pt particles decrease
1
4
reduction of SnO
2
species.
because of the geometric or electronic effect induced by Sn
32
H -TPR proles of the PtSn/SiO catalysts display features species. The dispersion of Pt in PtSn1.6/SiO -MTSG is slightly
2
2
2
related to the preparation methods. Comparing with the Pt/ higher than that of PtSn1.6/SiO -MIM catalyst, which could
2
SiO -MTSG catalyst, PtSn1.6/SiO -MTSG catalyst presents be attributed to the different reduction temperature of Pt–
2
2
ꢁ
2ꢀy
2ꢀy
a overlapped reduction peak at 282 C, which could be attrib- (O–Sn^)
uted to the co-reduction of Pt species with weak interaction with dispersion of Pt particles in PtSn1.6/SiO
the support and Pt–(O–Sn^)
reduction peak at 430 C could be ascribed to the reduction of consistent with the TEM images.
y
and Pt–(O–Si^)
y
species. Nevertheless, the
-TSG is 30.2%, result-
analogous species. A tiny ing from the aggregation of Pt particles. These results are
2
2
ꢀy
y
ꢁ
14
SnO
2
.
For the PtSn1.6/SiO
2
-MIM catalyst, two reduction peaks
3.1.2.6. Pyridine-FTIR. It was claimed that metal–acid
ꢁ
ꢁ
are centered at 272 C and 360 C, the former could be assigned bifunctional sites are indispensable for the catalytic hydroge-
33,34
to the co-reduction of Pt and Sn species and the later could be nation of carboxylic acid.
Therefore, the infrared experi-
species. Considering the ments of pyridine adsorption were carried out to detect the
nonuniform distribution of Sn over PtSn1.6/SiO -MIM, the surface acidity of PtSn catalysts. The FTIR spectra of pyridine
species is reasonable. The catalyst adsorbed on SiO support, monometallic catalysts Sn/SiO -SG
-TSG) in which Pt precursor is uploaded by IM and Pt/SiO -MTSG are shown in Fig. 4A. Two bands are observed
2
ꢀy
the reduction of Pt–(O–Si^)y
2
2
y
ꢀy
presence of Pt–(O–Si^)
PtSn1.6/SiO
method exhibits three peaks, approximately at 91 C, 193 C and at 1445 cm and 1596 cm over SiO
2
2
(
2
2
ꢀ1
ꢁ
ꢁ
ꢀ1
2
, which are related to the
20 C. The peaks at 91 C and 193 C could be associated to the adsorbed pyridine molecules bonded on the surface hydroxyl
ꢁ
ꢁ
ꢁ
4
35
reduction of Pt precursor with no and weak interaction with the groups of silica through weak hydrogen bonds. In the case of
support respectively. The peak at 420 C is attributed to the Pt/SiO -MTSG catalyst, the FTIR spectrum is similar to that of
ꢁ
2
reduction of SnO . The reduction temperature of PtSn1.6/SiO - SiO , suggesting that the presence of Pt would not modify the
2
2
2
TSG is lower than that of PtSn1.6/SiO
2
-MTSG, indicating that acid properties of SiO
2 2
. However, FTIR spectrum of Sn/SiO -SG
ꢀ
1
ꢀ1
ꢀ1
shows new bands at 1452 cm , 1490 cm , 1545 cm , 1612
ꢀ
1
ꢀ1
cm and 1640 cm , indicating that Sn/SiO
2
-SG possesses both
a
Table 2 Hydrogenation of AcOH over supported PtSn/SiO
2
catalysts
36
Lewis and Brønsted acid sites. According to literature reports,
2
+
4+
Lewis acid sites are provided by Sn or unsaturated Sn
cations, while Brønsted acid sites are related to the surface
AcOEt hydroxyl groups bonded on Sn . FTIR spectra over PtSn/SiO2
Selectivity (%)
EtOH
Conv.
(%)
4+
34
Catalyst
catalysts are also shown in Fig. 4A. It is observed that the acidity
of all the PtSn catalysts decreases comparing with the Sn/SiO -
2
SG substrate, which could be due to the increasing reduction
degree of oxide tin caused by H-spillover. Moreover, the amount
Pt/SiO
Sn/SiO
PtSn1.6/SiO
PtSn1.6/SiO
PtSn1.6/SiO
2
-MTSG
-SG
11.3
0.4
98.3
78.4
52.4
76.6
15.5
91.8
90.0
73.8
21.4
84.5
7.6
9.2
25.0
2
2
2
2
-MTSG
-MIM
-TSG
of Lewis acid sites decreases in the order of PtSn1.6/SiO
2
-MIM <
a
ꢁ
PtSn1.6/SiO -MTSG < PtSn1.6/SiO -TSG. Such phenomenon
Reaction conditions: 270 C, 2.6 MPa, H
2
/AcOH (mol ratio) ¼ 20,
2
2
ꢀ1
WHSV(AcOH) ¼ 2.0 h
.
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Fig. 4 FTIR spectra of chemisorbed pyridine on reduced catalysts.
could also be attributed to the difference in dispersion of Pt and
Fig. 5 XPS spectra of the reduced catalysts. (A) Pt 4f spectra, (B) Sn 3d
spectra.
distribution of Sn species as well as the interaction between reduction degree of Sn species, it is clear that the interaction
both species. For PtSn1.6/SiO
-TSG catalyst, the Pt particle size between Pt and Sn species could be strengthened by SEA method
is relatively larger than the other two catalysts. Therefore, the and the distribution of Sn could be improved by SG method,
reduction of Sn species caused by H-spillover is limited, which is consistent with the results of H -TPR and pyridine-FTIR.
retaining the maximum Lewis acid sites. While the reason for
Based on these characterization results, it could be
the lowest Lewis acid sites over PtSn1.6/SiO
-MIM could be its concluded that the MTSG method was an effective approach to
nonuniform distribution of Sn, though it presents similar Pt fabricate the PtSn/SiO catalyst with high dispersion of Pt
particle sizes as PtSn1.6/SiO -MTSG.
particles. Besides, this method presented a larger amount of
2
2
2
2
2
8,9,38–40
3
.1.2.7. XPS. The surface chemical states and composition of Lewis acid sites, which is crucial for AcOH adsorption.
PtSn catalysts were analyzed by XPS experiments aer in situ Moreover, the homogenous distribution of Sn species in the
reduction. As shown in Fig. 5, the Pt 4f spectra are described by catalyst prepared by MTSG method can give abundant surface
two features. The dominant core-level feature is located at 71.3 eV, hydroxyl, providing more opportunity for Pt precursor to
0
2ꢀy
which is assigned to metallic Pt (Pt ). The other relatively broad disperse on the support by forming Pt–(O–Sn^)
analogous
species. Herein, the synergy between Pt and Sn species was
According to the literature, the Sn core-level spectra in Sn/SiO -SG enhanced over PtSn1.6/SiO -MTSG, so it exhibited excellent
is described by two oxide states (Sn and Sn ), located at 486.6 catalytic performance. Nevertheless, PtSn catalysts prepared by
y
35,37
feature is located at 72.7 eV, which could be attributed to PtO.
2
2
2
+
4+
37
eV and 487.9 eV. However, with the addition of Pt, a new feature other methods either have low Pt dispersion or lack enough
2+
assigned to metallic Sn shows up and the percentage of Sn
Lewis acid sites, thus resulting in poor catalytic activity and
increases, suggesting the increasing reduction degree of oxide tin lower ethanol selectivity.
caused by H-spillover. Moreover, the percentage of metallic Sn,
2
+
4+
Sn and Sn (obtained by peak deconvolution) in the compared
PtSn catalysts is totally different. In PtSn/SiO -MIM catalyst, the
reduction degree of Sn is the highest. While in the case of PtSn/
3
.2. Effect of Sn/Pt molar ratio
2
The inuence of Sn/Pt molar ratio on the catalytic performance
4+
SiO -TSG, majority of the Sn species are still in the Sn state
2
over the PtSnx/SiO -MTSG (x ¼ 1.0, 1.6, 3.3, 8.2) catalysts were
2
because of its poor dispersion of Pt. According to the different
investigated. As shown in Fig. 6, both AcOH conversion and
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Fig. 7 FTIR spectra of chemisorbed CO on reduced catalysts.
Fig. 6 Effect of Sn/Pt molar ratio on the catalytic performance.
ꢁ
ꢀ1
2
Reaction conditions: 270 C, 2.6 MPa, H /AcOH (mol mol ) ¼ 20,
ꢀ1
WHSV(AcOH) ¼ 1 h
.
catalysts exhibit obvious Lewis acid sites evidenced by the
ꢀ
1
ꢀ1 34
bands at 1452 cm and 1609 cm
. The catalysts with lower
ꢀ1
ethanol selectivity display a volcano trend with the increase of Sn/Pt ratio (below 1.6) show new bands at 1446 cm and 1596
Sn/Pt ratio, and the optimal Sn/Pt ratio is found to be 1.6. When cm , which could be attributed to the adsorbed pyridine
Sn/Pt ratio increases from 1.0 to 1.6, the conversion of AcOH molecules bonded on the surface hydroxyl groups of silica.
ꢀ
1
35
increases from 74.2% to 99.7% with the selectivity of ethanol However, both bands disappear with the increasing Sn/Pt ratio
remaining at ca. 90%. The increase of AcOH conversion could (above 3.3), on account of the coverage of surface hydroxyl
be caused by the increase of active sites for AcOH adsorption. groups by Sn species. It is clear that the intensity of the band at
ꢀ1
However, the conversion of AcOH and ethanol selectivity 1452 cm increases with the increasing Sn/Pt ratio, indicating
decreases signicantly when Sn/Pt ratio is further increased the increasing surface acidity of PtSn catalysts. In order to
from 1.6 to 8.2. In order to understand the effect of Sn/Pt ratio correlate catalytic activity with the Lewis acid sites, the relative
24
on the physicochemical properties of the catalysts, PtSn bime- amount of Lewis acid sites in the PtSn catalysts are calculated.
tallic catalysts with different Sn/Pt ratio were also examined by As listed in Table S2,† the relative amount of Lewis acid sites
ꢀ
1
ꢀ1
TEM, CO-FTIR, pyridine-FTIR and H
2
–O
2
titration.
increase from 5.1 mmol g to 56.4 mmol g with the Sn/Pt ratio,
Fig. S3† shows the TEM images and particle size distribu- resulting from the increment of surface SnO species.
x
tions of bimetallic PtSnx/SiO -MTSG catalysts with various Sn/Pt
It was proposed that the reaction rate of acetic acid hydro-
ratio. It shows that the average particle sizes increase with the genation could be determined by the formation of acyl species
increase of Sn/Pt ratio. According to the H –O titration results, or the reaction between H atoms and acyl species, depending on
the dispersion of Pt particles decreases from 61.9% to 17.6% the ratio of the amount of Lewis acid sites versus the amount of
2
2
2
7–9
with the increasing Sn/Pt ratio (listed in Table 1). It may be exposed Pt active sites (denoted as L/Pt). However, none of
resulted from the geometric or electronic effect between Pt and these literatures distinguished the balance problem between
41
Sn species. The decrease of Pt dispersion suggests that the Lewis acid sites and the exposed Pt active sites. Thus, in order to
hydrogen adsorption ability of PtSn catalysts decreases with the elucidate this balancing effect, the TOF based on exposed Pt
42
increasing Sn/Pt ratio.
atoms was correlated with the amount of Lewis acid sites. As
CO-FTIR proles of PtSnx/SiO -MTSG are shown in Fig. 7.
2
The adsorbed CO on the Pt/SiO
a sharp peak at 2067 cm with a shoulder peak at 2043 cm
2
-MTSG catalyst clearly presents
ꢀ
1
ꢀ1
,
which corresponds to CO linearly bonded onto surface Pt atoms
CO , adsorbed atop of Pt atoms). The broad peak in the range
of 1800–1900 cm is attributed to the bridge-bound CO (CO
(
L
ꢀ1
B
,
32,43,44
adsorbed on Pt–Pt pair sites).
The linear CO peak intensity
decreases notably and the bridge-bound CO peak disappears
simultaneously with the increasing Sn/Pt ratio. Such decrease
could be due to the electronic effect caused by Sn species or the
migration of Sn species onto Pt particle surfaces, suggesting the
close interaction between Pt and Sn species exists in the
32,45
prepared PtSn catalysts.
results detected by H –O
This agrees well with the dispersion
2
titration method (listed in Table 1).
2
2
Pyridine-FTIR spectra of the bimetallic PtSnx/SiO -MTSG
catalysts with different Sn/Pt ratio are shown in Fig. 4B. All the
Fig. 8 Effect of L/Pt molar ratio on the catalytic activity. Reaction
ꢁ
ꢀ1
2
conditions: 270 C, 2.6 MPa, H /AcOH (mol mol ) ¼ 20.
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shown in Fig. 8, the TOFPt increases with the increasing molar
ratio of L/Pt in region I (from 0.18 to 0.54). Molar ratio of L/Pt is
very low in this region, thus there is enough exposed Pt to
provide dissociative hydrogen. So this increasing trend could
originate from the increasing adsorption sites of AcOH. Hence,
the determining step is the formation of acetyl. On the other
hand, the catalytic activity barely changes with the further
increment of L/Pt in region II (from 0.54 to 7.10). Such trend
could be deduced that the Lewis acid sites in region II was so
abundant that all the exposed Pt on catalyst surface could be
involved in the reaction. Owing to the relative insufficient
dissociated hydrogen, the hydrogenation is therefore controlled
by the reaction between H and acyl species. Thus, conversion of
AcOH decreases when the Sn/Pt ratio increases from 1.6 to 8.2.
The decrease of ethanol selectivity could be attributed to either
the decreasing of Pt dispersion or the increasing surface acidity
of catalysts. Explanation for this speculation will be discussed
in the next part.
Scheme 1 Schematic model of AcOH hydrogenation to ethanol and
AcOEt over the PtSn1.6/SiO -MTSG catalyst.
2
2
parameters such as temperature, pressure and H /AcOH molar
ratio (listed in Table S3†). According to the variation of the
catalytic performance, parallel reactions may exist in AcOH
hydrogenation system.
Hence, a schematic model regarding the hydrogenation of
AcOH over PtSn1.6/SiO -MTSG catalyst is proposed (Scheme 1),
2
in which the formation of ethanol and AcOEt are proposed to be
parallel reactions. According to literatures, AcOH is rstly
dissociated into adsorbed acetyl on Lewis acid sites. Then the
acetyl intermediates could combine with H atoms activated by
metallic Pt to form adsorbed acetaldehyde. The adsorbed acet-
3.3. Reaction model
aldehyde species then can undergo hydrogenation to form
9
As reported in literature, activated AcOH intermediate (acti- ethoxy groups. In the parallel reactions model, ethoxy groups
x
vated by SnO species) can be hydrogenated to produce ethanol can not only react with activated H atoms but also those
in the presence of surface-bonded H atoms activated by Pt active adsorbed acetyl, which produced ethanol and AcOEt respec-
9
sites. However, the reaction path for the formation of AcOEt is tively. Affluent activated H atoms could favor the formation of
still under ambiguous, which is essential to further understand ethanol, while the limited amount of activated H atoms may
the nature of the AcOH hydrogenation system. Based on the lead to the formation of AcOEt. Therefore, the competitive effect
analysis on the inuence of kinetics parameters, some prelim- of two reactions could lead to the changes of products selec-
inary results were achieved.
tivity. Moreover, the hydrogenation of AcOEt to ethanol could
The inuence of residence time on the conversion of AcOH occur by deep-hydrogenation when more H atoms were
as well as the selectivity of ethanol and AcOEt was evaluated by provided. Herein, one can infer that the synergy effect among Pt
varying the space velocity of AcOH. As shown in Fig. 9, AcOH and SnO species is responsible for the hydrogenation of AcOH
x
conversion increases with the increase of residence time and into ethanol. The parallel reaction mechanism for AcOH
achieve ca. 100% aer the residence time exceeds 130 s. When hydrogenation proposed by this study may provide valuable
the residence time is not long enough, the selectivity of ethanol help in catalyst design and process development.
remains constant. However, when the residence time is further
The long-term catalytic behavior results are also shown in
increased, for example above 130 s, the selectivity of ethanol Fig. 9. It is demonstrated that the well fabricated PtSn1.6/SiO2-
shows an increasing trend, indicating that partial of AcOEt MTSG catalyst remains its excellent activity and ethanol selec-
could be conversed to ethanol through deep-hydrogenation. tivity in the hydrogenation of AcOH into ethanol. It shows
Similar results were obtained by the study on other kinetic a prospective future in the massive synthesis of ethanol by
AcOH hydrogenation from an industrial application viewpoint.
4. Conclusions
2
In summary, a well fabricated PtSn/SiO catalyst with high
dispersion of Pt particles and large amount of Lewis acid sites
was prepared by MTSG method. The synergy between Pt and Sn
species was remarkably enhanced compared with those
prepared by IM method, consequently resulting in excellent
activity and ethanol selectivity. According to the investigations
on the inuence of Sn/Pt ratio, the amount of Lewis acid sites
increased accompanied with a decline of Pt dispersion with the
increasing Sn/Pt ratio. Excess high Sn/Pt ratio could inevitably
lead to severe coverage of Pt particle surface by SnO species.
x
The TOFPt increased with the increasing molar ratio of L/Pt and
then remained nearly unchanged. Therefore, a balancing effect
between exposed Pt and Lewis acid sites should be responsible
2
Fig. 9 Hydrogenation performance of AcOH over PtSn1.6/SiO -
MTSG catalyst as a function of residence time. Reaction conditions:
ꢁ
ꢀ1
2
70 C, 2.6 MPa, H
2
/AcOH (mol mol ) ¼ 20.
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for the superior catalytic performance of PtSn/SiO
2
-MTSG
L ´o pez, M. C. Barrera, J. Escobar, J. Navarrete and
catalyst. Meanwhile, a parallel reaction paths model was
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competitively through the adsorbed ethoxy, derived from the 23 J. T. Miller, M. Schreier, A. J. Kropf and J. R. Regalbuto, J.
dissociated adsorption of AcOH.
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2
2
4 C. A. Emeis, J. Catal., 1993, 141, 347–354.
5 D. F. Cox, G. B. Hound and H. A. Laitinen, Langmuir, 1985,
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Nature Science Foundation of China (21276186, 21325626,
9
1434127, U1510203) and the Tianjin Natural Science Founda-
tion (13JCZDJC33000). We also thank professor Shishan Sheng
Dalian Institute of Chemical Physics, Chinese Academy of
(
Sciences) for his constructive advances in the XPS results.
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