Production of ethanol by gas phase hydrogenation of acetic acid over carbon nanotube-supported Pt-Sn nanoparticles

Article abstract of DOI:10.1016/j.cattod.2013.05.002

Supported bimetallic Pt-Sn catalysts with different carriers were prepared through a one-step reduction method for the gas phase hydrogenation of acetic acid to ethanol. The structure of the catalysts was characterized by X-ray diffraction, transmission electron microscopy and energy-dispersive X-ray spectroscopy. Among these catalysts, the carbon nanotube-supported bimetallic Pt-Sn catalyst (Pt-Sn/CNT) showed the best performance, exhibiting over 97% conversion and 92% selectivity to ethanol under relatively mild conditions. The addition of Sn to Pt catalyst inhibited the CC bond cleavage activity and enhanced the selectivity of the catalyst to ethanol, due to the formation and well-dispersion of PtSn alloy on the CNT surfaces. The catalytic performance depended on the ratio of Sn and Pt and the particle size of PtSn alloy. The one-step reduction method was conductive to preparing catalyst with smaller PtSn alloy particles and higher performance, which was shown that the optimized Pt-Sn/CNT with average PtSn alloy particle size at 3.0 nm can retain its high catalytic performance for over 300 h on stream.

Full text of DOI:10.1016/j.cattod.2013.05.002

Catalysis Today 215 (2013) 260–266
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Production of ethanol by gas phase hydrogenation of acetic acid over
carbon nanotube-supported Pt–Sn nanoparticles
∗
∗
Subin Zhang, Xinping Duan, Linmin Ye, Haiqiang Lin, Zhaoxiong Xie , Youzhu Yuan
State Key Laboratory for Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters,
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Supported bimetallic Pt–Sn catalysts with different carriers were prepared through a one-step reduction
method for the gas phase hydrogenation of acetic acid to ethanol. The structure of the catalysts was
characterized by X-ray diffraction, transmission electron microscopy and energy-dispersive X-ray spec-
troscopy. Among these catalysts, the carbon nanotube-supported bimetallic Pt–Sn catalyst (Pt–Sn/CNT)
showed the best performance, exhibiting over 97% conversion and 92% selectivity to ethanol under rel-
atively mild conditions. The addition of Sn to Pt catalyst inhibited the C C bond cleavage activity and
enhanced the selectivity of the catalyst to ethanol, due to the formation and well-dispersion of PtSn alloy
on the CNT surfaces. The catalytic performance depended on the ratio of Sn and Pt and the particle size of
PtSn alloy. The one-step reduction method was conductive to preparing catalyst with smaller PtSn alloy
particles and higher performance, which was shown that the optimized Pt–Sn/CNT with average PtSn
alloy particle size at 3.0 nm can retain its high catalytic performance for over 300 h on stream.
© 2013 Elsevier B.V. All rights reserved.
Received 30 October 2012
Received in revised form 1 May 2013
Accepted 9 May 2013
Available online 1 July 2013
Keywords:
Ethanol
Acetic acid
Platinum
Tin
Alloy
Hydrogenation
1
. Introduction
Ethanol (EtOH) is an important fundamental chemical. It can also
On the other hand, the technology of methanol carbonylation
to acetic acid (AcOH) has been well developed [15,16] and the
global AcOH had an overcapacity of more than 40% in 2010 [17].
AcOH can also be synthesized from syngas with supported Rh
catalysts [18,19]. Thus, one of the alternative processes for the syn-
thesis of EtOH via syngas is through the selective hydrogenation of
be used as a gasoline additive to increase octane number and reduce
COx, NOx, and SOx emissions [1]. Conventionally, EtOH is produced
from feedstocks, such as oil and edible grains [2–5]. However, with
the soaring prices of those resources, the development of new pro-
cesses for EtOH production is highly demanded. The conversion
of renewable cellulosic materials to EtOH is promising; however,
considerable R&D efforts must be made before commercial-scale
production can be achieved [6]. The direct approach of syngas-to-
EtOH has drawn a great deal of attention because of the availability
of syngas from coal, biomass, and natural gas (including conven-
tional natural gas and unconventional gas such as shale gas, coal
bed gas and tight gas) [7–11]. Nonetheless, the process has unsolved
drawbacks, including low conversion and poor selectivity of EtOH
because of the slow initial formation kinetics of the C−C bond
and the fast chain growth of the C2 intermediates [7,12]. EtOH
can be obtained through the hydrogenation of dimethyl oxalate or
the combined carbonylation of dimethyl ether and hydrogenation
of methyl acetate [13,14]; however, disadvantages such as lower
selectivity or overall atomic efficiency exist.
AcOH (CH COOH + 2 H → CH CH OH + H O, ꢀH
= −77 kJ/mol)
3
2
3
2
2
298
(Scheme 1) [20–22]. This process is an integrated technology con-
sisting of syngas to methanol, coupling with CO to form AcOH, and
subsequent hydrogenation to produce EtOH. The overall atomic
efficiency from syngas to EtOH using the process is as high as 71.9%,
similar to that obtained via the direct approach of syngas-to-EtOH.
A number of noble metal-based catalysts are capable of catalyzing
the conversion of different acids to corresponding alcohols [23,24].
Several groups have also reported that AcOH hydrogenation to
EtOH can be readily accomplished over Pt-, Ru-, Pd-, and Re-based
catalysts based on the density functional theory calculations or
experiments [25–29]. A number of patents indicated that bimetallic
catalysts containing Pt are effective for the hydrogenation of acetic
acid to EtOH [30,31]. However, most of these catalysts suffer from
lower EtOH productivity or harsh reaction conditions. Therefore,
more detailed studies on the correlation between catalyst structure
and performance are still necessary.
Bimetallic catalysts often exhibit novel properties because of the
bifunctional mechanism or electronic effects [32–34]. The addition
of a second metal also restrains the aggregation of monometals
[35,36]. Meanwhile, the supermechanical, thermal, and electrical
∗ Corresponding author. Tel.: +86 592 2181659; fax: +86 592 2183047.
E-mail addresses: zxxie@xmu.edu.cn (Z. Xie), yzyuan@xmu.edu.cn (Y. Yuan).
0
920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.05.002

S. Zhang et al. / Catalysis Today 215 (2013) 260–266
261
as x%Pt–y%Sn/CNT, where x and y represent the weight percentage.
The Pt–Sn catalysts with other supports, such as ZrO , TiO , SiO ,
2
2
2
and SiC, were prepared using the same procedure.
For comparison, the 1%Pt–2%Sn/CNT-IM catalyst was prepared
via the co-impregnation method. Up to 1.0 g CNTs were dissolved in
−
1
the mixture of the aqueous solution of H PtCl (3.7 mg mL ) and
2
6
−
1
SnCl (4.0 mg mL ) under stirring. After completely dried at 333 K,
2
the solid was calcined at 623 K for 4 h in air, and then reduced in a
5
%H –95%N2 at 623 K for 4 h.
2
2.2. Catalyst characterization
N2 sorption isotherm was measured at 77 K using a Micro-
meritics TriStar II 3020 porosimetry analyzer. The samples
were degassed at 573 K for 3 h prior to the measurements.
The specific surface area was calculated according to the
Brunauer–Emmett–Teller (BET) method.
Scheme 1. Comparison of direct and indirect approaches for EtOH synthesis from
syngas.
Powder X-ray diffraction (XRD) patterns were taken using a
Philips PANAlytical X’pert Pro diffractometer equipped with a
graphite monochrometer and Cu K␣ radiation (40 kV and 30 mA).
The diffraction lines were identified by matching them with refer-
ence patterns in the JCPDS database.
Transmission electron microscopy (TEM) images and energy-
dispersive X-ray spectroscopy (EDS) were taken using a Tecnai F30
electron microscope operated at an acceleration voltage of 300 kV.
The powder was dispersed in ethanol by ultrasonication for 10 min,
and then drops of suspensions were deposited on a copper grid
coated with carbon. The statistical mean diameter of the nanopar-
ticles was measured by counting at least 300 particles for each
catalyst.
X-ray photoelectron spectroscopy (XPS) was performed using
a Quantum 2000 Scanning ESCA Microprobe instrument (Physical
Electronics). The samples were reduced and sealed immediately
under Ar and then they were transferred to a UHV chamber
to avoid contacting with air as much as possible. The raw data
were corrected for substrate charging with the BE of C 1s peak
(284.6 eV). Peak deconvolution and fitting were performed using
the peak-fitting software with the spin–orbit splitting and the rel-
ative intensities of the spin–orbit components fixed. No trace of
boron or sodium was detected on the reduced catalyst samples
using XPS.
properties of carbon nanotubes (CNTs) and their one-dimensional
characteristic have led to increased interests in the study of CNT-
based devices and composites, including heterogeneous catalyst
supports and promoters [37–42].
It is well-known that Pt–Sn bimetallic catalysts can be used
in many reactions, such as hydrogenation [43–45], dehydrogena-
tion [46,47], and oxidation reactions [48,49]. In most cases, Sn is
added to Pt catalysts to enhance the selectivity. For example, Sn
species can interact with the oxygen atom in the carbonyl group of
an unsaturated aldehyde and hinder the activity of Pt toward the
hydrogenation of the C C bond. As a result, Pt–Sn catalysts show
enhanced selectivity to unsaturated alcohol [50]. The current study
reports that the Pt–Sn bimetallic catalyst supported on CNTs can
exhibit remarkable performance for the hydrogenation of AcOH to
EtOH under mild conditions. The catalytic behavior and the struc-
tural feature of the catalyst are addressed with a combination of
kinetic and characteristic studies.
2
. Experimental
2.1. Catalyst preparation
Supported bimetallic Pt–Sn catalysts were prepared via a one-
step reduction method. CNTs (10–20 nm in diameter) with a purity
of 95% were purchased from Shenzhen Nanotechnologies Port
Co., Ltd. Hexachloroplatinic acid (H PtCl ·6H O), tin(II) chloride
The metal dispersions and active surface area of the catalyst
sample were determined by O2–H2 titration on a Mircometrics
ASAP 2020 instrument. The reduced sample was evacuated at 313 K
for 30 min, followed titrated with O2 to form Pts–O and Sns–O
species on the surface. After that, the H2 adsorption was taken at
313 K to measure the amount of Pts–O species, according to litera-
ture [51], Sns–O species could not titrated with H2. So the dispersion
of metal Pt was calculated by assuming H/Pts = 3 stoichiometry due
to the following reactions:
2
6
2
dihydrate (SnCl ·5H O), ethylene glycol (EG), sodium borohydride
2
2
(
NaBH ), and sodium dodecyl sulfate (SDS) were purchased from
4
Sinopharm Chemical Reagent Co., Ltd. First, the pristine CNTs were
purified and functionalized in concentrated HNO (68 wt%) at 353 K
3
for 16 h under refluxing conditions to remove amorphous carbon
and the remaining catalyst residues. The treated CNTs were filtered,
extensively washed with deionized water until the pH of the rins-
ing water became neutral, and then dried at 373 K overnight. During
the treatment, abundant surface functional groups, such as COOH,
Pts + 1/2O → Pts–O
(1)
(2)
2
Pts–O + 3/2H → Pts–H + H O
2
2
ꢀ
C O, OH and so on, were created on the surfaces of CNTs, which
facilitated the uniform deposition of metal precursors. In a typical
synthesis, initially purified CNTs were dispersed in EG by ultrasonic
2.3. Catalytic testing
−
1
treatment (160 W and 40 Hz) for 30 min. The H PtCl (3.7 mg mL
and SnCl2 (4.0 mg mL ) solutions were uniformly mixed. Subse-
quently, the mixture and a SDS solution (0.6 mg mL ) were mixed
with the CNTs under agitation for 1 h. Here, EG acted as a dispersing
agent to efficiently separate CNTs, whereas SDS acted as a stabilizer
to drive the high dispersion of small nanoparticles on the surface of
)
2
6
−1
The hydrogenation evaluation of the catalysts was carried out
in a stainless steel tubular fixed bed reactor equipped with a
computer-controlled auto-sampling system. About 200 mg cata-
lyst was placed in the center of the reactor, and both sides of
the catalyst bed were packed with quartz powders. Pure H2 was
fed into the reactor and the system pressure was held at 2.0 MPa.
AcOH was pumped into the reactor with varying weight liquid
hourly space velocities (WLHSV(AcOH)) using a Series III digital HPLC
pump (Scientific Systems, Inc.). The outlet stream line from the
−
1
CNTs. Afterward, excess NaBH solution was added dropwise to the
4
mixture. After stirring for 5 h, the mixture was filtered, rinsed with
deionized water, and then dried. Subsequently, the solid sample
was calcined at 623 K for 4 h in air. The obtained solid were labeled

2
62
S. Zhang et al. / Catalysis Today 215 (2013) 260–266
reactor was heated at about 473 K to avoid condensation of reac-
tion products. After depressured to ambient pressure, the outlet
stream was auto-sampled to a gas chromatograph (GC) equipped
with a flame ionization detector and KB-Wax capillary column
(
30 m × 0.32 mm × 0.33 ␮m) to quantify EtOH, acetaldehyde (Ace-
Al), ethyl acetate (EtOAc), acetone, CH , and AcOH. The outlet
4
stream was then cooled to below 278 K. The uncondensed gas was
auto-sampled to another GC equipped with a thermal conductiv-
ity detector and dual columns of Gaskuropack 54 (3 m) and active
carbon (3 m) to quantify CH , CO , CO, C H . AcOH conversion and
4
2
2
6
product selectivities were calculated using the calibrated area nor-
malization method.
The initial turn over frequency (TOF) of the reaction was mea-
sured under the conditions where the AcOH conversion was lower
than 20%. The TOF value was based on the Pt dispersion, indicating
the moles of AcOH converted per hour by per mole of metal at the
−1
−1 −1
, for short h ).
Fig. 1. Comparison of the catalytic performance of 1%Pt–2%Sn/CNT and
%Pt–2%Sn/CNT-IM under the reaction conditions of 623 K, 2.0 MPa,
WLHSV(AcOH) = 0.6 h , and H2/AcOH = 80.
catalyst surface (mol-AcOH mol-Ptsurf
h
1
−
1
3
. Results and discussion
(
SBET) of the catalyst, but the AcOH conversion may be related to the
3
.1. Effect of support
Pt surface area. The results on this issue are presented hereinafter
in connection to the TOF of the catalysts.
Effect of support for the catalytic activity of AcOH hydrogenation
to EtOH was investigated on several supported Pt–Sn bimetallic cat-
alysts with different carriers. Table 1 shows that 1%Pt–5%Sn/CNT
yielded an AcOH conversion of 49% and 83% selectivity to EtOH.
By contrast, the catalysts with other supports exhibited a much
lower selectivity to EtOH, although some of them showed higher
3.2. Effect of catalyst preparation method
The catalytic performance of the 1%Pt–2%Sn/CNT catalyst was
compared with that prepared via the conventional impregna-
AcOH conversion under the conditions of 623 K, 2.0 MPa, H /AcOH
tion method (1%Pt–2%Sn/CNT-IM) under the following condi-
2
−1
−1
molar ratio of 40, and 3.0 h
WLHSV(AcOH). The XRD patterns
tions: 623 K, 2.0 MPa, H /AcOH molar ratio of 80, and 0.6 h
2
of 1%Pt–5%Sn on different supports indicated that the diffraction
peaks of possible Pt, Sn or PtSn alloy were hardly observed in all
samples (Fig. S1). Further, the TEM images in Fig. S2 showed that
the metal particles were smaller than 5 nm in average size and
well dispersed on supports for all samples. However, the property
of support has a major function on the catalyst performance for
the hydrogenation of AcOH. Pt–Sn catalyst on supports like TiO2
and ZrO2 gave acetone as the main product, probably due to the
strong chemical interactions between acetic acid and these oxides
WLHSV(AcOH). The results are shown in Fig. 1. It is easier to conclude
that AcOH conversion and EtOH selectivity over 1%Pt–2%Sn/CNT
are higher than those over 1%Pt–2%Sn/CNT-IM. We noticed that
the activity of metal particles within the channels or dispersed
on the outside surfaces of CNTs may have much different proper-
ties in catalysis [55]. We checked that the most of PtSn particles
in 1%Pt–2%Sn/CNT were formed on the external surface of the
nanotubes but some of them were positioned within the internal
channels. Then we tried to prepare the catalysts with PtSn nanopar-
ticles dispersed on the inside or outside surfaces of CNTs in different
ratios. The experiments for the sample preparation were tough to
reproduce. However, the catalytic performance was producible if
the PtSn alloy nanoparticles were smaller and highly dispersed on
the inside or outside surfaces of CNTs, always showing high perfor-
mance for the hydrogenation of AcOH. Nevertheless, we could not
exclude the confinement effect of the PtSn nanoparticles at present.
The structure of the Pt–Sn catalysts was then examined by
XRD and TEM. The diffraction peaks marked as “#” correspond to
the contribution of CNTs (Fig. 2). The diffraction peaks of possible
PtSn alloy were observed in 1%Pt–2%Sn/CNT-IM sample, but were
negligible in 1%Pt–2%Sn/CNT. The results suggest that the bimetal-
lic nanoparticles in 1%Pt–2%Sn/CNT-IM were larger than those in
1%Pt–2%Sn/CNT. Further, the TEM images of the as-reduced cata-
lysts in Fig. 3 show that the metal particles in 1%Pt–2%Sn/CNT-IM
[
52]. Pt–Sn catalyst on a nearly neutral support SiO showed active
2
for the partial hydrogenation of acetic acid, giving higher selectiv-
ity to aldehyde. Previous works also reported that Pt–Sn/SiO2 was
used for the partial hydrogenation of furfural [53] and Pt–Fe/SiO2
catalyzed the hydrogenation of acetic acid to acetaldehyde and
ethyl acetate [26]. SiC and CNTs have high mechanical strength
properties and are chemical inertness, making the Pt–Sn/SiC and
Pt–Sn/CNT catalysts show an improved selectivity to EtOH. In addi-
tion, CNTs are useful to concentrate the reactants in an area close
to the active metal [54]. The better thermal conductivity of CNTs
may also be beneficial for the exothermic catalytic reduction of
AcOH [42]. The effects endow the Pt–Sn/CNT with higher catalytic
performance in terms of conversion and selectivity.
The results in Table 1 also indicated that there is no direct rela-
tionship between the performance and BJH specific surface area
Table 1
a
Hydrogenation of acetic acid over several supported 1%Pt–5%Sn catalysts .
SBET, m2
g
−1
SPt, m2
g
−1
Conv., %
Selec., %
Catalyst
EtOH
Ace-Al
EtOAc
Acetone
CHx + COx
1%Pt–5%Sn/ZrO2
1%Pt–5%Sn/TiO2
1%Pt–5%Sn/SiO2
1%Pt–5%Sn/SiC
1%Pt–5%Sn/CNT
11
44
235
29
5.2
8.4
2.3
4.9
6.2
39.7
95.8
16.5
17.0
49.1
8.7
18.9
11.1
41.5
82.5
8.5
3.6
28.2
18.0
9.0
25.4
7.1
52.2
37.4
7.4
57.1
62.1
6.9
1.6
0.4
0.3
8.3
1.6
1.5
0.7
180
a
−1
Reaction conditions: 623 K, 2.0 MPa, WLHSV(AcOH) = 3 h , H2/AcOH = 40; CHx = CH4 + C2H6, COx = CO + CO2.

S. Zhang et al. / Catalysis Today 215 (2013) 260–266
263
The EDS in Fig. 4b revealed that the Pt–Sn alloy nanoparticle was
consisted of Pt and Sn with an atomic ratio of approximately 1:1.
The above results strongly suggest that the PtSn alloy was formed
on the CNT surfaces. The 1%Pt–2%Sn/CNT with smaller size of PtSn
alloy afforded higher performance of AcOH hydrogenation to EtOH.
#
PtSn (PDF 01-089-2055)
1%Pt 2%Sn/CNT-IM
1%Pt 2%Sn/CNT
CNT
a
b
#
3.3. Effect of Pt to Sn atomic ratio
A series of supported Pt–Sn bimetallic catalysts with fixed Pt
#
loading at 1 wt% and varying Sn loadings at 0% to 5 wt% were pre-
pared. The TEM images of the 1 wt% Pt monometallic and 1%Pt–x%Sn
bimetallic catalysts supported on CNTs and the corresponding
metal particle size distributions are represented in Fig. S3. The
metal particles for all samples were well dispersed on the CNTs.
However, the sizes of the nanoparticles for bimetallic catalysts
were smaller than monometallic Pt. The presence of an appropriate
amount of Sn can improve the dispersion of Pt nanoparticles and
prevent the Pt nanoparticles from aggregation.
Fig. 5 displays the AcOH conversion and EtOH selectivity as a
function of Sn loading in 1%Pt–x%Sn/CNT. The addition of Sn into
Pt/CNT significantly enhanced EtOH selectivity. With the increase
of Sn content from 0 wt% to 5 wt%, AcOH conversion displayed a
volcano trend, where the optimal Sn loading was around 1.2 wt%.
Table 2 shows the COx + CHx selectivity of AcOH hydrogenation,
indicating that the monometallic Pt catalyst must have a high activ-
ity of C C bond cleavage to form equal and much higher amounts
of CO and CH4 compared with bimetallic Pt–Sn/CNT.
#
#
b
a
20
30
40
50
60
0
70
80
90
2
/
Fig. 2. XRD patterns of (a) 1%Pt–2%Sn/CNT-IM and (b) 1%Pt–2%Sn/CNT.
were aggregated, whereas those in 1%Pt–2%Sn/CNT were smaller
and uniformly dispersed on the CNT surfaces. Fig. 4 shows the HR-
TEM image and EDS of 1%Pt–2%Sn/CNT. The EDS measurements
were taken for the selected Pt–Sn nanoparticles. Well-resolved lat-
tice fringes were clearly observed, and two types of lattice fringes
with interplanar spacings of 0.30 nm and 0.22 nm appeared, which
are ascribed to the (1 0 1) and (1 1 0) planes, respectively. The angle
Scheme 2 illustrates the Pt–Sn/CNT with different Sn contents
and their catalytic behaviors in AcOH hydrogenation. Monometallic
Pt nanoparticle is active for C C bond cleavage. The PtSn alloy phase
is formed when Sn is added to the Pt catalyst. With the increase of Sn
content, Pt was almost completely alloyed with Sn and the catalyst
◦
between the planes was 47 (Fig. 4a). The results were approximate
to the theoretical value of PtSn alloy nanocrystal (PDF 01-89-2055).
Fig. 3. TEM images and the metal nanoparticle distributions of (a) 1%Pt–2%Sn/CNT-IM and (b) 1%Pt–2%Sn/CNT.

2
64
S. Zhang et al. / Catalysis Today 215 (2013) 260–266
Fig. 4. HR-TEM image (a) and EDS (b) of 1%Pt–2%Sn/CNT.
Scheme 2. Schematic for the preparation of Pt–Sn/CNT catalysts with different Sn contents and their catalytic behaviors in AcOH hydrogenation.
carboxyl group because of the formation of PtSn alloy. However,
excess Sn loading can partially block the PtSn alloy active sites and
100
EtOH selectivity
AcOH conversion
suppress the catalytic activity.
80
60
40
20
0
In order to investigate the presence of oxidized Sn in as-reduced
1
%Pt–3%Sn/CNT catalyst where excess Sn was loaded, the XPS mea-
surement was carried out and the results are displayed in Fig. S4.
The Sn 3d5/2 spectra showed two peaks. The first one with a binding
energy of 485.5 eV can be attributed to zero-valent Sn, the second
peak with a binding energy of 487.1 eV can be assigned to oxidized
species Sn(II) and/or Sn(IV). It should be indicated that the distinc-
tion between Sn(II) and Sn(IV) species is not possible from the XPS
measurements, since the two species have a slight difference in the
binding energy [56].
There is a strong correlation between Pt dispersion (or Pt surface
area) and hydrogenation performance of the catalyst. The TOF over
the samples at 603 K increased markedly with the incorporation of
small amount of Sn to Pt/CNT catalyst (Table 3). The highest TOF
was obtained with the Pt–Sn/CNT catalyst containing 1.0 wt % Pt
and 1.2 wt% Sn. Further increase of Sn loading caused a drop in
TOF value. The results are in good accord with AcOH conversion as
shown in Fig. 5.
0
1
2
3
4
5
Sn content / wt%
Fig. 5. Dependence of AcOH conversion and EtOH selectivity over 1%Pt–x%Sn/CNT
as a function of Sn content under the reaction conditions of 623 K, 2.0 MPa,
WLHSV(AcOH) = 3 h , and H2/AcOH = 40.
−1
thus formed showed excellent activity for AcOH to EtOH by car-
boxyl reduction. However, excess Sn may present in oxidized form
and cover the surface of PtSn alloy. In other words, when adequate
Sn is added to the Pt catalyst, the resultant decreases the rate of
3
.4. Effect of reaction parameter
Fig.
6
shows the catalytic performance of optimized
1
%Pt–1.2%Sn/CNT under different reaction parameters, including
C
C bond cleavage [29], but increases the reduction ability toward
WLHSV, H /AcOH molar ratio, pressure, and temperature. Clearly,
2
EtOH selectivity increased with the increase of AcOH WLHSV from
−
1
−1
−1
Table 2
0.6 h
to 1.5 h , and then decreased when AcOH WLHSV was
a
Comparison of selectivities to COx and CHx over 1%Pt/CNT and 1%Pt–1.2%Sn/CNT.
higher than 3.6 h . AcOH conversion gradually decreased with the
increase in WLHSV(AcOH). AcOH conversion and EtOH selectivity
Catalyst
(COx + CHx) Selectivity, %
˙, %
decreased and increased with the increase of H /AcOH molar
2
CO
CH4
CO2
C2H6
ratio from 20 to 100, respectively. Both AcOH conversion and
EtOH selectivity slightly increased along with hydrogen pressure.
However, AcOH conversion dramatically increased when the
reaction temperature increased from 523 K to 623 K and reached a
1
1
%Pt/CNT
%Pt–1.2%Sn/CNT
33.1
1.3
33.2
1.3
0.7
0.3
0.8
0.4
68.7
3.3
a
Reaction conditions are the same as those in Table 1.

S. Zhang et al. / Catalysis Today 215 (2013) 260–266
265
Table 3
Results of O2–H2 titration and TOF over several supported Pt and Pt–Sn catalysts.
SPt, m2
g
−1
−1
TOF at 603 K, h
Sample
Metal dispersion, %
By TEMa
By O2–H2 titration
1
1
1
1
%Pt/CNT
25.1
28.3
43.5
37.7
16.2
18.3
30.8
20.5
1.1
2.4
10.7
8.6
416.9
979.8
3242.1
2265.8
%Pt–0.5%Sn/CNT
%Pt–1.2%Sn/CNT
%Pt–2%Sn/CNT
a
All the Pt particles were regarded as the spheres. Thus, the Pt dispersion, D, was calculated as D = (6␯m/␣md) × 100%, according to literature [57], where, D is the Pt
dispersion; ␯m is the volume of a Pt atom, ˛m = (M/ꢁNo), M is the molecular weight of Pt, ꢁ is the density of Pt, No is Avogadro constant; ˛m is the area of a Pt atom on the
surface, ˛m = 1/(1.25 × 1019); and d is the Pt particle diameter determined by TEM. Therefore, D = 1.13/d.
1
00
a
100
90
80
70
60
50
b
9
8
7
6
5
0
0
0
0
0
AcOH conv.
EtOH selec.
AcOH conv.
EtOH selec.
1
2
13
4
20
40
60
80100
LHSV(AcOH) / h
H /AcOH molar ratio
2
1
00
100
c
d
8
6
4
0
0
0
9
8
7
6
5
0
0
0
0
0
AcOH conv.
EtOH selec.
AcOH conv.
EtOH selec.
20
0
1
2
3
4
500
550
600
650
Pressure / MPa
Temperature / K
Fig. 6. Hydrogenation of AcOH over 1%Pt–1.2%Sn/CNT catalyst as a function of (a) AcOH WLHSV, (b) H2/AcOH molar, (c) reaction pressure, (d) reaction temperature.
−
1
Reaction conditions: 623 K, 2.0 MPa, H2/AcOH = 80, WLHSV(AcOH) = 1.5 h
.
maximal plateau of 98.9% at temperatures higher than 623 K. EtOH
selectivity obviously dropped at 653 K, probably due to the C
100
C
bond cleavage at high temperature. Therefore, the vapor-phase
hydrogenation of AcOH is sensitive to reaction parameters of
80
AcOH conversion
EtOH selectivity
reaction temperature, H /AcOH molar ratio, hydrogen pressure,
2
and AcOH WLHSV.
60
40
20
0
3
.5. Catalyst stability
The long-term stability and activity of the catalysts are impor-
tant for AcOH hydrogenation to EtOH from both academic
and industrial viewpoints. The long-term catalytic behavior of
optimized 1%Pt–1.2%Sn/CNT is depicted in Fig. 7. The results
demonstrated that 1%Pt–1.2%Sn/CNT retained its excellent activity
for over 300 h at optimized reaction conditions of 623 K, 2.0 MPa,
0
20
40
60
240 260 280 300
−1
H /AcOH molar ratio of 80, and 1.5 h WLHSV(AcOH). The XRD pat-
Time on stream / h
2
terns of as-reduced 1%Pt–1.2%Sn/CNT and those after the reaction
test for 300 h showed that only the diffraction peaks of the CNTs
were observed on both catalysts, indicating that the PtSn alloy
nanoparticles were not significantly aggregated after the catalytic
Fig. 7. Catalytic performance of 1%Pt–1.2%Sn/CNT as
time under the reaction conditions of 623 K, 2.0 MPa, WLHSV(AcOH) = 1.5 h , and
H2/AcOH = 80.
a function of reaction
−
1

2
66
S. Zhang et al. / Catalysis Today 215 (2013) 260–266
test for 300 h (Fig. S5). Consequently, the extraordinary activity and
stability of the 1%Pt–1.2%Sn/CNT under the given operating condi-
tions suggest that the catalyst is feasible and promising for EtOH
production via the selective hydrogenation of AcOH.
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