Insight into the structure evolution and the associated catalytic behavior of highly dispersed Pt and PtSn catalysts supported on La2O2CO3 nanorods

Article abstract of DOI:10.1039/c7ra10084a

The current work introduces highly dispersed Pt and PtSn catalysts supported on La₂O₂CO₃ nanorods prepared via ultrasonic impregnation, which are used as probe catalysts for the liquid-phase crotonaldehyde hydrogenation. T

Full text of DOI:10.1039/c7ra10084a

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Insight into the structure evolution and the
associated catalytic behavior of highly dispersed Pt
and PtSn catalysts supported on La O CO
Cite this: RSC Adv., 2017, 7, 48649
2
2
3
nanorods†
ab
a
Huanling Song,* Lingjun Chou, *ac Jun Zhao,a
a
Fengjun Hou, Huahua Zhao,
a
a
Jian Yang and Liang Yan
2 2 3
The current work introduces highly dispersed Pt and PtSn catalysts supported on La O CO nanorods
prepared via ultrasonic impregnation, which are used as probe catalysts for the liquid-phase
crotonaldehyde hydrogenation. The physicochemical properties of the catalysts are assessed by means of
2
various techniques, including XRD, TEM, XPS, H -TPD, in situ CO-DRIFT and X-ray adsorption fine structure
(XAS). A close combination of catalyst surface experiments and the reactive performances reveals that the
distinct reactive performance of the Pt and PtSn catalysts is tentatively attributed to the composition-
dependent architecture of Pt–lanthanum interfaces and bimetallic particles while excluding the particle size
effect. Catalytic activity tests demonstrate that incorporation of Sn into Pt catalyst brings great significance
to the selective hydrogenation of carbonyl groups as it results into the structure evolution of bimetallic
particles. An optimization of Sn loading and reaction conditions achieves a 5-fold and 7-fold improvement
in the selectivity and yield to crotyl alcohol over the parent Pt catalyst. Lastly, it is found from the catalyst
reusability study that metal particles of PtSn catalysts suffers easily from particle migration and growth
compared to the Pt catalyst, most likely resulting from a weaker metal–support interaction.
Received 10th September 2017
Accepted 9th October 2017
DOI: 10.1039/c7ra10084a
rsc.li/rsc-advances
desired product is one of the main problems for CRAL hydro-
genation over the Pt catalysts.
1
. Introduction
Unsaturated alcohols are widely employed as important inter-
mediates for the production of fragrances and pharmaceuticals, Sn) are reported to perform high selectivity towards unsaturated
Heteroatoms of Pt and a transition metal (M ¼ Fe, Co, Ge or
9
–14
and can be attained by the chemoselective hydrogenation of alcohols and produce changes in the reactivity.
The promo-
1
unsaturated aldehydes. However, this selective hydrogenation tion effect of M on the catalytic behavior of Pt catalyst can be
process is challenging since the hydrogenation of the carbonyl generally interpreted in terms of geometric (dilution of Pt sites
group is thermodynamically unfavorable over the olenic by PtM alloys or MO
x
species) or electronic (the electron density
2
bond. To circumvent this issue, signicant efforts have been of Pt) aspects. A plot of reaction selectivity to cinnamyl alcohol
devoted to the development of effective catalysts with superior from cinnamaldehyde hydrogenation versus the rst row of
carbonyl selectivity. For another, crotonaldehyde (CRAL) transition metals as dopers of Pt gave a typical volcano shape
hydrogenation has been also studied as a model reaction to curve, which is proposed to be derived from the difference in d-
probe the catalyst surface and active sites of heterogeneous band center position from the Fermi level when the Pt surface is
3–8
15
catalysts. Among them, platinum is one of the most exploited electronically modied by the transition metal. In addition,
metals for its ability to easily capture and dissociate hydrogen. the composition variation is reported to induce changes in the
Regardless of the high activity, the limited selectivity to the particle size and structure of bimetallic nanoparticles (NPs).
Rong et al. reported the structure evolution of PtSn NPs from the
SnO2 -patched PtSn alloy to the SnO -patched Pt cluster as
the Sn/Pt ratio increases, accompanied by an increase in the
ꢀx
2ꢀx
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. E-mail:
ljchou@licp.cas.cn; songhl@licp.cas.cn; Fax: +86 931 4968 129; Tel: +86 931 4968
particle size, improving the desired selectivity for CRAL hydro-
16
genation. In this sense, various preparative methods, e.g.
0
66
b
atomic layer deposition (ALD) and surface organometallic
University of Chinese Academy of Sciences, Beijing 100049, PR China
c
Suzhou Research Institute of LICP, Chinese Academy of Sciences, Suzhou 215123, PR chemistry (SOMC), have been adopted to precisely control the
17,18
China
architecture of bimetallic NPs.
However, it is still difficult to
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra10084a
obtain homogeneous composition and structure of bimetallic
1
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NPs in consideration of their sensitivity to the synthetic Brunauer–Emmett–Teller (BET) method in the relative pressure
parameters.
Besides, support plays a crucial role in the chemical compo-
range of 0.05–0.30.
All catalysts were fabricated via an ultrasonic impregnation
sition and structure of active metal, and therefore inuences the procedure. The Pt/LOC was obtained from the mixture of H -
2
ꢀ
3
ꢀ1
catalytic behavior of its supported catalyst. In this case, devel- PtCl
6
$6H
2
O ethanol solution (1.13 ꢂ 10
gPt mL , 9 mL) and
oping of Pt or PtM crystals on an active support is one of the most the support (1.0 g) with ultrasonic treatment for 30 min. This
common strategies adopted to improve the unsaturated alcohol mixture was kept standing for 12 h, and then evaporated at
ꢁ
yield. Novel carbon materials (CNTs, CNFs, OMCs, RGOs) and 60 C to remove the excessive solvent. Obtained powder was air-
ꢁ
ꢁ
2 2 x
reducible oxides (TiO , CeO , FeO ), are usually employed as dried at 120 C overnight, calcinated at 400 C for 2 h, and
ꢁ
supports and demonstrated to enhance the selectivity towards thereaer reduced at 600 C for 1 h under a owing gas mixture
8
,13,19–23
the carbonyl hydrogenation.
at the metal–support interface, involving either reduced cations
or oxygen vacancy sites, are reported to activate the carbonyl prepared by the impregnation of the air-dried Pt/LOC (1.0 g)
The new catalytic sites located of 20% H /N (v/v). The nominal loading of Pt was 1.00 wt%.
2 2
The PtSn catalysts with different Sn/Pt molar ratios were
22,23
bond and increase the desired selectivity.
2 2
with SnCl $2H O (0.0080 g, 0.0120 g or 0.0180 g) following an
Since studies on its coke inhibition and impressive activity identical process. As determined by inductively coupled plasma
for oxidative coupling of methane (OCM) are reported, optical emission spectrometry (ICP-OES) analyses (Table 1), Pt
lanthanum oxy-carbonate (La
2
O
2
CO
3
) receives considerable content of the prepared catalysts is in the range of 0.99–
24,25
attention.
specic morphology are scarcely studied in the hydrogenation corresponding to the Sn/Pt molar ratio of 0.69, 0.94, and 1.33;
process, although they are widely employed as important hence, the PtSn catalysts are denoted as PtSn0.69/LOC, PtSn0.94
ingredients of heterogeneous catalysts in oxygen reduction LOC and PtSn1.33/LOC, respectively. In addition, the PtSn cata-
reaction, steam reforming of glycerol and biofuel production lyst supported on the La CO powders (PtSn0.94/LOC-NP) and
because of their unique hydrotalcite-like structure and abun- the PtSn catalyst prepared by conventional impregnation
As far as we know, La O CO materials with 1.11 wt%; Sn content is 0.46 wt%, 0.57 wt%, and 0.87 wt%,
2
2
3
/
2
O
2
3
26–29
dant active oxygen species.
tial deposit of Pt on the {101} facets of La
denoted as Pt/LOC) is accounted for its superior reactive
performance for CRAL hydrogenation to the Pt randomly
In our previous work, preferen- method without ultrasonic treatment (PtSn0.94/LOC-CI) are
2
O
2
CO nanorods fabricated for comparison.
3
(
2.2 Catalyst characterization
30
dispersed on the particle-shaped counterpart. Herein we carry
out a systematic study on the physicochemical properties and
the associated catalytic behavior of PtSn catalysts. An ultrasonic
impregnation procedure is used to synthesize well-dispersed Pt
and PtSn NPs on the support. The chemical composition and
structure of the catalysts are elaborately analyzed by a series of
techniques. Catalytic activity tests for CRAL hydrogenation are
performed as a probe reaction to discern the structure-activity
correlation of PtSn catalysts with varying Sn/Pt molar ratios.
The amount of Pt and Sn was determined by ICP-OES on an
Agilent 725-ES.
X-ray diffractograms (XRD) was performed on X'Pert Pro
Multipurpose diffractometer (PANalytical, Inc.) with Cu Ka
ꢁ
radiation (0.15406 nm) at room temperature from 10 to 80 .
Measurements were conducted using a voltage of 40 kV, current
setting of 20 mA, step size of 0.021, and count time of 4 s.
Sample morphology and dispersion was observed on the
TECNAI G2 F20 high-resolution transmission electron micros-
copy under a working voltage of 200 kV. Sample preparation
involved ultrasonic dispersion of the sample in ethanol and
deposition of the mixture on a carbon-coated copper grid. The
particle size distributions (PSD) were estimated by measuring at
2. Experimental
2
.1 Catalysts preparation
La CO
the lanthanum hydroxide nanostructures at 500 C for 3 h.
La O CO powders and nanorods has specic surface area of
2
O
2
3
powders and nanorods was obtained by annealing least 300 particles from several TEM micrographs for each
ꢁ
30
sample.
Temperature programmed desorption of hydrogen (H -TPD)
2
2
3
ꢀ1
2
2
2
ꢀ1
1
03 m g and 45 m g , which is calculated based on the was taken on a ChemBET Pulsar TPR/TPD analyzer (Quantachrome
Table 1 Physicochemical properties of the samples measured by ICP-OES and H
2
-TPD
b
Ptꢀ1
)
Amount of desorbed hydrogen (mmol g
a
a
a
Catalyst
wt% Pt
wt% Sn
Sn/Pt
Peak 1
Peak 2
Peak 3
Total
Pt/LOC
1.11
1.09
0.99
1.08
—
—
1.46(355)
1.30(350)
1.12(341)
0.74(344)
0.58(404)
0.64(406)
0.63(402)
0.63(413)
1.33(456)
1.19(449)
0.92(437)
0.51(456)
3.37
3.14
2.67
1.87
PtSn0.69/LOC
PtSn0.94/LOC
PtSn1.33/LOC
0.46
0.57
0.87
0.69
0.94
1.33
a
b
2
From the ICP-OES analyses. From the H -TPD measurement.
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Instruments U.S.) equipped with a TCD detector. 50 mg of reduced products were identied and analyzed on Agilent 7890A-5975C
catalyst was loaded in a U-tube quartz reactor and pretreated at equipped with an FID detector. The catalytic activity and
ꢁ
4
00 C for 1 h with Ar ushing to remove the impurities. The TPD selectivity of the products was determined by using the internal
ꢁ
experiment was carried out by heating the furnace to 900 C at normalization method with n-hexanol as internal standard.
ꢁ
ꢀ1
ꢀ1
a ramp of 20 C min in owing Ar (40 mL min ) aer hydrogen- CRAL conversion (Conv.CRAL) was dened as the adductive
ꢁ
adsorption at 30 C for 60 min.
percentages of CRAL converted into different products. The
X-ray photoelectron spectroscopy (XPS) was conducted on selectivity to a given product (i) was calculated as the ratio
a Thermo Scientic ESCALAB250xi spectrometer to determine between the moles of product i and the total moles of products.
the surface atomic composition and chemical states of the
The catalyst reusability study was conducted under the
ꢁ
catalysts. All binding energies were referenced to C 1s hydro- optimization reaction conditions (T ¼ 160 C, p ¼ 2.0 MPa, t ¼
carbon peak at 284.8 eV. The spectra of each element were 60 min). Monometallic Pt/LOC and bimetallic PtSn0 /LOC
.94
deconvoluted with a curve tting routine in XPSPEAK41 so- were selected as the catalysts for reutilization test. The spent
ware. Curve tting of the Pt 4f, Sn 3d and O 1s spectra for the catalysts were regenerated from the mixture of hydrogenation
catalysts was performed aer subtracting the background and system by rinsing with absolute ethanol, and drying overnight
ꢁ
ꢁ

tting with the least squares best tting routine, assuming an at 120 C before activation in air at 300 C for 1 h.
80/20 Gaussian/Lorentzian product function.
The CO-DRIFT spectra of the samples were recorded on the
3
. Results
BRUKER V70 Fourier transform infrared spectrometer equip-
ped with an in situ infrared reaction device. Measurements were 3.1 Catalysts characterization
ꢀ
1
conducted using a spectral resolution of 4 cm , step size of
0 kHz, and accumulation of 64 scans. Briey, self-supporting
3.1.1 XRD patterns. XRD technique was adopted to study
2
the crystalline phase of the catalysts, and the patterns are pre-
sented in Fig. 1. The diffraction signals of the Sn/LOC are in
accordance with typical patterns of hexagonal La O CO (JCPDS
2 2 3
No. 25-0424). By contrast, Pt-involved catalysts show intensied
diffraction signals, which can be indexed to a mixture of
hexagonal La O CO (JCPDS No. 25-0424) and hexagonal La O
pellet was prepared from the reduced sample and placed
directly in the IR quartz cell equipped with KBr windows. The
ꢁ
reduced sample was pretreated at 300 C for 90 min in owing
H
2
followed by 30 min ushing in Ar. Aer pretreatment, the
ꢁ
sample was cooling down to 40 C. And then a gas mixture of
2
2
3
2 3
0
4
.5% CO/Ar was admitted into the cell and le to equilibrate for
0 min. And then the CO-adsorption spectra of the sample were
continuously recorded with 40 min ushing in Ar ow.
(JCPDS No. 02-0688). Given that La O CO has a hydrotalcite-
2 2 3
n+
2ꢀ
like structure consisting of (La
2
O
2
)
layers and CO
CO surfaces may
induce the phase transformation of lanthanum upon thermal
3
ions,
incorporation of a noble metal onto the La
2
O
2
3
The Pt LIII-edge X-ray absorption spectroscopy (XAS) was
measured at the 4W1B station of the Beijing Synchrotron
Radiation Facility (BSRF). Experiments were performed at
ambient temperature in transmission mode using a Si (111)
solid state detector for selection of energy. The data reduction
and process was performed using the ATHENA and ARTEMIS
25,32
treatment.
It is plausible to accept that the presence of La
2 3
O
is related to the phase transformation of La
Pt atoms at the Pt–lanthanum interfaces upon reduction, sug-
gesting a strong metal–support interaction. It is noted that the
2
O
2
CO catalyzing by
3
31
soware. Each EXAFS function (c) was attained by subtracting
the post-edge background from the overall absorption and then
3
normalized relating to the edge jump step. Then, k -weighted
EXAFS oscillation in the k-space for Pt LIII-edge, were Fourier
transformed (FT) to the r-space to isolate the EXAFS contribu-
tions from different coordination shells. The Pt–Pt scattering
path was used in the tting of Pt foil; the Pt–O and Pt–Pt scat-
tering paths were used in the tting of the Pt/LOC; the Pt–O, Pt–
Pt and Pt–Sn scattering paths were used in the tting of the
PtSn
x
/LOC samples. The structural parameters, such as the
coordination number CN, the inter-atomic distance R, the
2
Debye–Waller factor s , and the edge-energy shi DE
allowed to vary during the tting process.
0
, were
2.3 Catalytic test
The catalytic behavior for liquid-phase CRAL hydrogenation was
operated in a 100 mL Parr reactor. 100 mg of catalyst, 1 mL of
CRAL and 19 mL of ethanol was charged into the autoclave. The
autoclave was purged three times and then pressurized with H2.
The reaction was heated to the desired temperature for reaction
Fig. 1 Standard patterns of hexagonal La
and cubic Pt crystalline. XRD patterns of the Pt/LOC, Sn/LOC and
/LOC series catalysts.
2 2 3 2 3
O CO , hexagonal La O
with vigorous stirring. The compositions of reactants and PtSn
x
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ꢁ
2 2 3
phase transformation of La O CO is much promoted over observed even aer reduction at 600 C. Fig. 2f shows the
PtSn0.69/LOC, as indicated by the increased phase composition histogram of particle sizes measured from the TEM images,
of La O , assuming an increase in the number of individual Pt indicating that Pt NPs in the reduced Pt/LOC have an average
2
3
atoms possibly segregated from lanthanum surface. However, particle size of 1.4 nm. As clearly seen from Fig. 2c–e, the tin
further increasing tin loading causes an obvious drop in overall addition does not practically affect the size distribution of metal
peak intensities as a result of the passivation effect of tin on the particles compared to the Pt/LOC. By contrast, Fig. S1† shows an
catalyst surface. In addition, no reection associated with obvious increase of metal particle size over the PtSn0.94/LOC-CI
metals or alloys is detected, presumably due to the small crys- (4.2 nm) and PtSn0.94/LOC-NP (3.3 nm). It is evident that the
talline size and good dispersion of metals, which is conrmed employment of lanthanum nanorods and ultrasonic impreg-
by the following TEM images.
.1.2 TEM images. As shown in Fig. 2a, LOC displays highly dispersed metal particles at a conned size range, which
diameters of 15–20 nm and lengths of 200–400 nm. The particle makes it feasible to probe the composition-dependent proper-
size and dispersion of metals on the catalysts estimated by TEM ties of the PtSn /LOC series catalysts while excluding the
images are displayed in Fig. 2b–e, which clearly shows that the particle size effect.
rod-shaped morphology of lanthanum support is well main- 3.1.3 XPS spectra. XPS measurement was conducted to
nation process plays a decisive role in controlling over the
3
x
tained aer the decoration of metals. A typical TEM micrograph analyze the surface composition and chemical states of the
of the Pt/LOC (Fig. 2b) demonstrates that uniform Pt NPs are catalyst surface; the results are presented in Fig. 3 and Table 2.
highly dispersed on the catalyst surface and no agglomeration is The Pt 4f7/2 and Pt 4f5/2 lines for the Pt/LOC exhibit a distinct
Fig. 2 Typical TEM images of (a) LOC, (b) Pt/LOC, (c) PtSn0.69/LOC, (d) PtSn0.94/LOC and (e) PtSn1.33/LOC, (f) histograms showing the size
distributions of metal particles over the catalysts.
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Fig. 3 XPS spectra of (a) Pt 4f, (b) Sn 3d, (c) La 3d for the samples. The curve fitting of the Pt 4f and Sn 3d spectra for the samples are performed
using the XPSPEAK41 software according to the uniform constraints on the fitting parameters after subtracting the background.
Table 2 Chemical composition and valences of the catalysts determined by XPS measurement
2
+
0
0
2+
4+ a
b
c
d
Sample
Species
Binding energy (eV)
Pt /Pt or Sn /(Sn + Sn
)
Pt/La
Sn/Pt
0.00
1.60
O/La
3.19
2.60
0
Pt/LOC
Pt 4f7/2
Pt 4f7/2
Sn 3d5/2
Pt
71.7
73.3
71.6
0.26
0.25
0.17
0.020
0.014
2
0
2
+
Pt
Pt
Pt
PtSn0.69/LOC
+
73.3
0
Sn
Sn
Sn
Pt
Pt
Sn
Sn
Sn
485.4
485.9
486.8
71.5
2
+
+
4
0
PtSn0.94/LOC
PtSn1.33/LOC
Pt 4f7/2
0.05
0.07
0.016
0.015
1.95
2.67
2.59
2.50
2+
73.2
0
2
4
Sn 3d5/2
485.1
485.9
486.7
71.5
+
+
0
Pt 4f7/2
Pt
0.06
0.04
2+
Pt
73.2
0
2
4
Sn 3d5/2
Sn
Sn
Sn
485.2
485.9
486.8
+
+
a
2+
0
2+
0
0
2+
4+
0
Pt /Pt : the atomic ratio of oxidized platinum (Pt ) to metallic platinum (Pt ); Sn /(Sn + Sn ): the atomic ratio of metallic tin (Sn ) to oxidized
2
+
4+
b
c
d
tin (Sn + Sn ). The molar ratio of Pt to La. The molar ratio of Sn to Pt. The molar ratio of O to La.
asymmetric shape inclined toward high binding energies and expected, with higher Sn/Pt molar ratio, the Sn 3d5/2 signals are
the peak area ratio deviates from the theoretical value of 1.33, evidently intensied. The Sn/Pt atomic ratio for the PtSn cata-
33
suggesting the multiple chemical states of Pt. The peaks of lysts assessed by XPS analyses is 1.60, 1.95 and 2.67, much
reduced Pt/LOC at 71.7 eV and 73.3 eV are assigned to the Pt 4f higher than that from ICP results. This demonstrates a signi-
7
/2
0
2+
electrons of Pt and Pt species, respectively. The oxidized plat- cant surface enrichment of tin presumably because the
inum is related to Pt catalyst involving the chlorinated precur- impregnation order leads to the partially covered Pt NPs by tin.
sors, which helps platinum remain unreduced in the nal Besides, the atomic ratio of Sn /(Sn + Sn ) is higher for
0
2+
4+
19,34
2+
0
catalysts.
As shown in Table 2, the Pt /Pt atomic ratio of the PtSn0.69/LOC (0.17) than that for PtSn0.94/LOC (0.07) and
Pt/LOC and PtSn0.69/LOC is 0.26 and 0.25, much higher than PtSn1.33/LOC (0.04), suggesting that most of tin is hardly
0
that of the PtSn0.94/LOC (0.05) and PtSn1.33/LOC (0.06), sug- reduced to metallic Sn and exists as SnO
gesting that the reducibility of Pt is greatly improved most likely loading.
x
entities at high Sn
resulting from the crippling Pt–lanthanum interfaces with
increasing Sn loading.
The La 3d spectra for Pt-involved catalysts, as shown in
Fig. 3c, contain peak at 834.2 eV and its accompanying satellite
As shown in Fig. 3b, the Sn 3d5/2 peaks can be deconvoluted peaks that can be attributed to the electrons of La 3d5/2 for
0
2+
4+
into three features assigned to Sn , Sn and Sn species. It is La
found that large quantities of tin exist as oxidative status sponds to the electrons of La 3d5/2 for La
according to the curve-tting results of the Sn 3d5/2 spectra. As verify the phase evolution of lanthanum catalyzed by Pt atoms at
2
O
3
. The peak at higher value (835.5 eV) for Sn/LOC corre-
2
O
2
CO . These results
3
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the Pt–lanthanum interfaces, which is consistent with the changes the exposure degree of surface Pt sites available for
previously discussed XRD patterns. It is noteworthy that the O/ hydrogen chemisorption. At low Sn content, the dilution effect
La ratio decreases from 3.19 to 2.50 as Sn/Pt value increases. of Pt sites by tin prevails, favoring the exposure of Pt sites; while
This suggests that a proportion of oxygen species on the catalyst at high Sn content, the blocking effect of tin predominates and
surface or subsurface is consumed, producing more oxygen decreases the number of exposed Pt sites. Besides, incorpora-
vacancies by the promotion effect of tin. According to above tion of tin lowers the overall peak intensity, in particular for the
results, it can be inferred that the tin addition induces an mid-temperature peak, conrming the strong passivation of
interaction between both metals, which affects the redox catalyst surface by tin.
property of Pt and Sn, generating the Pt–SnO
same time, more surface oxygen vacancies are produced due to behavior of carbonyl groups on the catalyst surface, in situ CO-
the formation of Pt–SnO species. DRIFT spectroscopic investigation was conducted. As shown in
.1.4 H -TPD measurements. To assess the H -metal Fig. 5, the spectra of fresh catalysts present three sets of peaks
x
entities; at the
3.1.5 CO-FTIR spectra. In order to probe the adsorption
x
3
2
2
ꢀ
1
ꢀ1
bonding sites, H
the temperature range of 100–900 C was performed. As shown 2100 cm , which are assigned to gas phase CO
in Fig. 4, the TCD signals for bare LOC contains two peaks lanthanum and linear-CO adsorption on low-coordinated Pt
2
desorption vs. temperature for the catalysts in located at 2300–2400 cm , 2150–2200 cm
and 2000–
ꢁ
ꢀ1
2
, CO(ad) on
ꢁ
ꢁ
35,36
centered at 565 C and 800 C, respectively. The H
2
-TPD proles sites, respectively.
It is evident that addition of Sn into the
for Pt-containing catalysts show an analogous shape, which can Pt/LOC leads to a signicant increase of the linear-CO signals
be divided into three regions according to the desorption on PtSn0.69/LOC compared to the Pt/LOC, suggesting a dilution
ꢁ
temperatures. The low-temperature region of 100–500
C
effect of Pt sites by tin. In comparison, the linear-CO signals for
corresponds to hydrogen desorbed from the metal sites; the PtSn0.94/LOC and PtSn1.33/LOC are drastically decreased, most
ꢁ
mid-temperature one (500–700 C) is related to hydrogen des- likely resulting from a decreasing number of surface Pt sites due
orbed from the catalyst surface/subsurface, most likely due to to the passivation of catalyst surface by tin. These changes share
lanthanum polarization by interaction with vicinal Pt atoms; similarities with the variations of XRD patterns and H
while the high-temperature one above 700 C originates from proles. In addition to the changes of peak intensity, it is
2
-TPD
ꢁ
the reductive decomposition of bulk lanthanum.
observed that addition of Sn leads to a systematic red shi of
ꢀ
1
In the case of the Pt/LOC, three overlapped desorption peaks the linear adsorbed CO signals. It is located at 2079 cm for Pt/
ꢁ
ꢀ1
ꢀ1
centered at 355, 404 and 455 C, are assigned to weak, medium LOC, while 2074 cm for PtSn0.69/LOC, 2072 cm for PtSn0.94
/
ꢀ1
and strong bonding sites with hydrogen, respectively. Table 1 LOC and 2069 cm for PtSn1.33/LOC. This indicates more back-
summarizes the H desorption capacity (in mmol gPt ) of the donation of electrons from Pt to the 2p anti-bonding orbital of
2
ꢀ
1
prepared catalysts calculated on the categorized temperature CO, suggesting an increased interaction between Pt and CO
ꢁ
below 500 C. The hydrogen desorption capacity for the molecules.
samples is followed in a decreasing order of PtSn0.69/LOC
It is proposed that the binding strength of CO on the Pt sites
ꢀ1
ꢀ1
(
(
3.14 mmol gPt ) > PtSn0.94/LOC (2.67 mmol gPt ) > Pt/LOC can somewhat reect the adsorption of carbonyl group and
2.30 mmol gPt ) > PtSn1.33/LOC (1.87 mmol gPt ). It is known desorption of the product.
ꢀ
1
ꢀ1
37,38
The linear-CO adsorption on
that Pt is the effective sites for hydrogen adsorption and fresh Pt/LOC seems to be completely reversible as this band
dissociation, whereas tin oxides has a relatively low capacity for disappears when ushing with Ar for 24 min. By contrast, the
hydrogen activation, and therefore the hydrogen desorption linear-CO adsorption on the PtSn0.69/LOC can be easily removed
behaviors can mostly be ascribed to the noble metal Pt. The when ushing with Ar for 8 min; the linear-CO adsorption on
calculated hydrogen desorption capacity reveals that tin deposit the PtSn0.94/LOC seems to be mostly desorbed on Ar ushing for
40 min; however, high tin concentration in PtSn1.33/LOC does
not result into noticeable changes of the linear adsorbed CO
signals even aer Ar ushing for 40 min. These results further
manifest that the interaction between CO and Pt is greatly
enhanced at high tin loadings. This enhanced binding force, on
one hand, results into a chemisorption mode favoring the
activation and hydrogenation of carbonyl groups on the metal
Pt. On the other hand, it inhibits the product desorption and
the recovery of active sites. In summary, the CO-DRIFT studies
reveal that the coordination environment and exposure degree
of Pt atoms in the catalysts is greatly changed upon tin addition;
further, metal particles consist of Pt and Sn atoms leads to the
composition-dependent variations in the interaction between
CO and Pt. These two factors work together to determine the CO
chemisorption behavior on Pt centers.
3.1.6 XAS spectra. To further study the electronic modi-
cation and the local environment of Pt, X-ray absorption spec-
troscopy (XAS) experiments were carried out on the reduced
Fig. 4
2
H -TPD profiles of the samples.
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Fig. 5 DRIFT spectra of CO adsorbed on the prepared (a) Pt/LOC, (b) PtSn0.69/LOC, (c) PtSn0.94/LOC and (d) PtSn1.33/LOC samples. The spectra
ꢁ
were continuously recorded on the samples (50 mg) at 40 C during Ar flushing for 40 min after introducing 0.5 Ar/CO for 40 min.
catalysts. X-ray adsorption near-edge spectroscopy (XANES) can CN of Pt–O bond decreases to around 1.0, suggesting that some
provide crucial information on the electronic environments of Pt atoms segregated from lanthanum surface are greatly reduced
0
2+
the central absorbing atom. Fig. 6a presents the normalized Pt aer the decoration of tin, in consistence with higher Pt /Pt
LIII edge XANES spectra of Pt foil and the catalysts. The white ratios for tin-promoted samples from XPS results.
line at 11 570.3 eV ascribes to the 2p3/2 to 5d transition in Pt foil
and is very sensitive to the average oxidation state of Pt. The 2.72 A, with an average CN of 5.9. By contrast, the CN of Pt–Pt
For Pt/LOC, there is a Pt–Pt contribution at a distance of
˚
white line intensities of the Pt-involved samples are higher than bond for tin-promoted samples is continuously decreased to
˚
that of Pt foil, verifying the existence of positively charged Pt 4.2. The Pt–Pt bond of PtSn0.94/LOC (2.62 A) and PtSn1.33/LOC
39,40
˚
˚
atoms that consists with the XPS results.
In the case of Pt- (2.65 A) shows shorter distance than that of Pt/LOC (2.72 A) and
˚
involved samples (the inset of Fig. 6a), XANES shis to higher PtSn0.69/LOC (2.75 A), suggesting the contraction of the bond
energy by 0.5 eV, and the region between 11 578 eV and distances owing to the formation of small Pt clusters at high Sn
1
1 600 eV shows a different ne structure, which is indicative of loading. In addition, the best t for the third shell of PtSn0.69/
˚
a different geometry around the Pt atoms depending on the Sn/ LOC is a Pt–Sn contribution at a distance of 2.66 A, with an
Pt ratios.
average CN of 1.1. The Pt–Sn contribution is greatly decreased to
3
Fig. 6b shows the Fourier transforms (FTs) of k -weighted 0.3 for PtSn0.94/LOC and 0.2 for PtSn1.33/LOC, implying that
extended X-ray absorption ne structure (EXAFS) oscillations at a phase segregation of PtSn alloys occurs at high Sn loading.
the Pt LIII-edge. The main peaks of FTs for the catalysts located in These changes give important clues on the structure evolution
˚
the range of 2–3 A can be assigned to the Pt–O, Pt–Pt or Pt–Sn of bimetallic NPs from PtSn alloys to small Pt clusters with
41
contributions. The structural parameters can be attained by increasing Sn/Pt ratios.
curve-tting the EXAFS data and summarized in Table 3. There is
It is found that the total CN of absorbing platinum atoms
˚
Pt–O bond at a distance of 2.02 A for the Pt/LOC; its coordination (SNPt-i) for Pt/LOC is 7.4, much higher than that for PtSn0.69
/
number (CN) is 1.5. The contribution of Pt–O shell gives further LOC (7.2), PtSn0.94/LOC (6.5), and PtSn1.33/LOC (5.4). The
evidence on the presence of oxidized Pt coordinated by oxygen decreased SNPt-i values for tin-dopped samples lead to
atoms of lanthanum. In the case of bimetallic PtSn catalysts, the a hypothesis that a number of Pt atoms are neither directly
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3
Fig. 6 (a) Normalized XANES spectra and (b) Fourier transforms (FTs) of Pt LIII-edge k -weighted EXAFS oscillations for Pt foil, Pt/LOC and PtSn
LOC catalysts.
x
/
Table 3 Structural parameters derived from curve fitting results of EXAFS oscillations at Pt LIII-edge
a
b
c
2
ꢀ3
2 d
e
˚
R (A)
˚
Sample
Shell
CN
DE
0
(eV)
s ꢂ 10
(A )
R factor
SNPt-i
Pt foil
Pt/LOC
Pt–Pt
Pt–O
Pt–Pt
Pt–O
Pt–Sn
Pt–Pt
Pt–O
Pt–Sn
Pt–Pt
Pt–O
Pt–Sn
Pt–Pt
12.0
1.5
5.9
1.1
1.1
5.0
1.0
0.3
5.3
1.0
0.2
4.2
2.76
2.02
2.72
2.04
2.66
2.75
1.96
2.57
2.62
2.03
2.62
2.66
8.0
10.9
8.6
15.4
ꢀ5.2
7.1
2.6
14.3
2.2
12.0
10.1
1.8
4.57
3.47
9.36
2.80
11.46
10.0
2.00
2.32
9.00
5.00
5.07
9.00
0.001
0.008
12.0
7.4
PtSn0.69/LOC
PtSn0.94/LOC
PtSn1.33/LOC
0.004
0.008
0.006
7.2
6.5
5.4
a
b
c
d
2
e
N: the average coordination number. R: the interatomic distance. DE
0
: the inner potential shi. s : the Debye–Waller factor. SNPt-i: the
ꢀ1
overall coordination number. The data range used for data tting in k-space (Dk) and R-space (DR) are 2.0–10.7 A˚ and 1.2–3.5 A, respectively.
˚
interacted with the Sn species nor with the support, presumably desired unsaturated alcohol (crotyl alcohol, CROL), saturated
instead by forming the Pt–SnO
As revealed by the XPS results, more SnO
x
entities through Pt–O–Sn bond. aldehyde (butanal, BUAL), and saturated alcohol (butanol,
entities with oxygen BUOL). The main hydrogenation product distributions over the
x
vacancy sites are produced at high tin loadings, which could be prepared catalysts are exhibited in Fig. 8. From the analysis of
the probable reason for lower SNPt-i values for the PtSn cata- catalytic behavior, it is found that tin exerts great inuence on
lysts. Analogous results are previously discussed by Taniya on the hydrogenation activity and carbonyl selectivity. The overall
38
ꢀ1 ꢀ1
the tin-modied SiO -coated Pt catalyst. In summary, the reactivity (g ) of 0.34 mmol g
s
over the Pt/LOC, corre-
2
i
Pt
EXAFS analyses throw light on the structure evolution of sponds to a CRAL conversion and CROL selectively of 66.2% and
bimetallic PtSn NPs with increasing Sn content. At low Sn 14.1%, respectively. The reactivity of PtSn0.69/LOC is improved
ꢀ
1
ꢀ1
concentration, the structure of PtSn0.69/LOC is PtSn alloys to 0.59 mmol gPt
patched by SnO entities; whereas at high Sn concentration, the CRAL conversion (92.8%) and CROL selectivity (64.9%). It is
structure of PtSn0.94/LOC and PtSn1.33/LOC is Pt clusters noteworthy that an increased yield of alcohol products (CROL
patched by SnO entities. The composition-dependent structure and BUOL), in contrast to the decreased saturated aldehyde
evolution of bimetallic NPs has also been observed on unsup- (BUAL) selectivity, clearly demonstrates the additional activity
s , accompanied by a steep increase in
x
x
41,42
ported PtSn NPs and PtNi NPs.
As shown below, these Pt to C]O hydrogenation for bimetallic systems. When the Sn/Pt
atoms diluted by PtSn alloy or SnO species exhibit distinct ratio increases to 0.94, it reaches a maximum value of CROL
x
reactive behaviors for CRAL hydrogenation.
selectivity (70.7%) at the expense of a slight decrease in the
ꢀ1
ꢀ1
reactivity (0.51 mmol gPt
s ) and CRAL conversion (91.7%).
Further increasing Sn content leads to a sharp drop in reactivity
and CROL selectivity for PtSn1.33/LOC. The CRAL conversions
over the catalysts follows a decreasing order of PtSn0.69/LOC >
PtSn0.94/LOC > Pt/LOC > PtSn1.33/LOC. These results coincide
3
.2 Reactive performance
.2.1 Catalytic behavior in CRAL hydrogenation reaction.
As shown in Fig. 7, CRAL is mainly hydrogenated to produce the
3
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Fig. 7 The reaction diagram of CRAL hydrogenation.
Fig. 8 (a) Liquid phase hydrogenation results over the PtSn
x
/LOC catalysts with different Sn/Pt molar ratios. Reaction conditions: 1 mL CRAL
ꢁ
catalyzed by 100 mg of the catalyst in 19 mL ethanol, T ¼ 160 C, p ¼ 2.0 MPa, t ¼ 60 min. Liquid phase hydrogenation results of CRAL over the
2
PtSn0.94/LOC as a function of: (b) temperature, (c) H pressure and (d) time.
with the desorbed hydrogen/CO capacity determined by H
2
-TPD a conversion of 79.6% for PtSn0.94/LOC-NP, much lower than
and CO-DRIFT analyses, which most likely result from the that obtained on the PtSn0.94/LOC, highlighting the superiority
composition-dependent variations in the exposure degree of of La
active Pt sites and the interaction between carbonyl groups and work.
Pt centers. By contrast, as shown in Table S1,† the selectivity to
2
O
2
CO
3
nanorods and sonochemical approach in this
Reaction parameters, e.g. hydrogen pressure, temperature
crotyl alcohol (62.0%) at a conversion of 81.2% is obtained on and time, can affect the kinetics of competitive adsorption and
PtSn0.94/LOC-CI, and the value is decreased to 54.3% at hydrogenation of the reactant. Reaction parameters are tested
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in order to evaluate their inuence on the activity and selectivity
for liquid-phase CRAL hydrogenation. Reaction parameters cycled catalysts still exists as a mixture of Pt and Pt , showing
have been optimized by correlating reaction rates with product a higher Pt /Pt value than the fresh ones presumably resulting
distributions over the PtSn0.94/LOC. As can be seen from Fig. 8b, from calcinations in air to regenerate the catalysts. No metallic
high hydrogenation rate is attainable with increasing reaction tin is detected for PtSn0.94/LOC-C and tin exists as ionic states
As is clear from XPS analyses in Fig. S5 and Table S2,† Pt in the
0
2+
2
+
0
2
+
4+
temperature, mainly from the formation of alcohol products (Sn or Sn ), suggesting a complete phase segregation of PtSn
coupled with a decreased yield of saturated aldehyde. In alloys. In addition, the Sn/Pt value of PtSn0.94/LOC-C is much
a period reaction of 60 min, the CROL selectivity presents lower than the fresh one, implying that a small amount of tin
a “parabola” curve: it ascends to 71.5% as the temperature rises species leaches out during the reaction process. According to the
ꢁ
to 160 C, and then descends slightly at higher temperature previously discussed XRD and XPS results, the decreased phase
possibly due to the distortion of the catalyst structure. When composition of La O for fresh PtSn0.94/LOC compared to the Pt/
2
3
ꢁ
reaction temperature is 160 C, increasing hydrogen pressure LOC clearly illustrates that the tin addition much weakens the Pt–
and reaction time displays an analogous promotion effect on lanthanum interfaces, resulting into more Pt atoms segregated
the reactivity, while the CROL selectivity retains at the initial from lanthanum surfaces to interact with the tin species.
level (around 70%) with additional formation of BUOL, result- Therefore, it is plausible that the heat-treatment in air can easily
ing from the deep hydrogenation of BUAL. Noticeably, an induce the phase segregation of PtSn alloys and further result
x
optimization of Sn content and reaction conditions gives rise to into the loss of SnO layers, assuming that a weak interaction
43
a 1.5-fold and 5-fold improvement in reactivity and CROL between lanthanum and tin oxides. Moreover, ICP-OES results
selectivity over the Pt/LOC. The extremely high activity and in Table S2† clearly indicate a decrease of Sn content (0.46 wt%)
CROL selectivity over the PtSn0.94/LOC clearly demonstrate that and Sn/Pt molar ratio (0.73) in PtSn0.94/LOC-C compared to the
the coexistence of Pt and the partially reduced SnO
essential to achieve high efficiency in carbonyl hydrogenation. leaching of Sn during the cycle experiment. Above results lead us
.2.2 Recyclability of the catalysts. Recyclability is one of to a conclusion that the phase segregation of PtSn alloys and the
the most vital parameters for the evaluation of heterogeneous loss of SnO patches in the vicinity of Pt centers during the cycle
x
species is fresh PtSn0.94/LOC, which provides further evidence on the
3
x
catalysts. It is known that the liquid-phase hydrogenation experiments, may be the primary reason responsible for the
process produces not only the main products, but also the by- catalyst deactivation of the PtSn0.94/LOC.
products from condensation reaction. In our test, trace
amount of acetals and hemiacetals (total amount below 5%) is
4. Discussion
detected, usually covering on or even poisoning the catalytic
ꢁ
active sites. Therefore, calcination at 300 C in air for 1 h is
According to the reactive performance over the resultant cata-
lysts, bimetallic PtSn systems are highly active and selective for
CRAL hydrogenation to the desired CROL compared to the
parent Pt/LOC. It seems that the enhanced catalytic behaviors of
tin-modied catalysts most likely result from the interaction
between Pt and the tin species, which affects the Pt–support
interfaces and the bimetallic structure. Therefore, it is worth-
while to study the interaction between both metals and
lanthanum surfaces on the basis of detailed characterization
data.
adopted as a treatment to regenerate the used catalysts for
characterizations or cycle experiment. Fig. S2† shows the
hydrogenation results of CRAL on the selected Pt/LOC and
PtSn0.94/LOC during 5 runs. It is found that no signicant
decrease in reactivity occurs on the Pt/LOC and the CROL
selectivity increases from 13.8% to 24.6%. By contrast, the
reactivity over the PtSn0.94/LOC vibrates around 90% during the

rst 3 cycles, aer which an evident deactivation is observed but
the CROL selectivity still remains above 60%.
To explore the reason for catalyst deactivation, character-
izations including XRD patterns, HAADF-STEM images and XPS
spectra on the cycled catalysts (denoted as Pt/LOC-C and
4.1 Pt–lanthanum interfaces
PtSn0.94/LOC-C) are elaborately performed. As shown in The electronic and structural properties of the Pt/LOC are esti-
Fig. S3,† the diffraction patterns of the cycled catalysts can be mated to probe the metal–support interaction. XRD patterns in
indexed to hexagonal La
2 2 3
O CO (JCPDS No. 25-0424). HAADF- Fig. 1 suggest that Pt atoms onto the lanthanum surfaces
STEM images in Fig. S4† reveal that the cycled catalysts catalyze the phase transformation of La O CO to La O , which
2
2
3
2 3
consist of highly dispersed metal NPs with narrow size distri- is supported by the variations of La 3d spectra in Fig. 3d. As
0
butions, which can explain why no diffraction signals related to shown in Fig. 3a, the split of Pt 4f spectra into metallic Pt and
2
+
metals are detected. The almost unchanged Pt particle size of oxidized Pt clearly demonstrates that two kinds of platinum
Pt/LOC-C relative to the Pt/LOC, provide direct evidence that the species coexist in the catalyst. On account of the high-
2
+
working catalyst is rather stable probably due to the strong temperature reduction, Pt species can be ascribed to Pt
anchoring sites on the lanthanum surfaces, which inhibits the atoms that are strongly interacted with lanthanum through the
migration and growth of Pt atoms even aer ve reduction– Pt–O–La bond. The Pt–O contribution to the Pt L -edge EXAFS
III
oxidation cycles. While for PtSn0.94/LOC-C, an obvious increase oscillation provide further evidence on the existence of O-
in metal particle size is observed, showing an average size of coordinated Pt atoms at the Pt–lanthanum interfaces. On one
2.2 nm, higher than that for PtSn0.94/LOC (1.3 nm).
hand, the strong metal–support interaction (SMSI) plays
a decisive role in retaining the high platinum dispersion even
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ꢁ
aer reduction at 600 C (see Fig. 2), further inhibiting the Pt/LOC (1.5). In addition to the decreased O/La ratio from XPS
metal sintering during the cycle experiment (see Fig. S4†). On analyses, it is suggested that surface oxygenic species are
the other hand, this SMSI effect usually decreases the exposure partially consumed at high Sn loading. One plausible reason is
degree of Pt sites, since it produces Pt atoms partially embedded that surface oxygen vacancy sites are generated, presumably due
x
by oxides patches. Analogous phenomenon has been discussed to the reduction of Pt–SnO sites through electron capture and
3
5,44
41,45
in other studies on the Pt doped ceria surfaces.
As introduced into the Pt/LOC, the tin species can be
transfer upon hydrogen spillover.
From these results we can conclude that bimetallic samples
2
+
0
randomly dispersed on the support surface or located in the are composed of O-coordinated Pt atoms and Pt clusters,
vicinity of Pt atoms. TEM images of tin-promoted samples in which are diluted by the PtSn alloys and SnO species. Although
x
Fig. 2 show that no obvious changes in the metal particle size most of tin cannot be reduced to metallic form and incorpo-
were observed upon the tin addition, hence we can discuss the rated into an alloy, it affects the geometric structure of platinum
composition-dependent properties of the PtSn catalysts while and helps to stabilize the Pt atoms. Furthermore, the
2
+
0
excluding the particle size effect. A decreasing Pt /Pt values composition-dependent changes in bimetallic structure are
and the reducing CN values of Pt–O contribution for tin- proposed to account for the distinct catalytic behavior.
modied catalysts reveal that the reducibility of platinum is
greatly improved possibly owing to the crippling Pt–lanthanum
interfaces aer the decoration of tin, which is validated by an
increase in metal particle size of tin-dopped catalysts during
4
.3 The correlations between structure evolution and
catalytic behavior
cycle experiment. As indicated by XRD analyses in Fig. 1, The above results reect an interaction between Pt and Sn,
introducing a limited quantity of tin results into an increase in which is related to the formed PtSn alloys and Pt–SnO species.
x
2 3
the phase composition of La O , while an opposite trend is Such an interaction exerts a great inuence on the reaction
observed when Sn/Pt ratio $ 0.94. These results suggest two route. The product distribution demonstrates that the reaction
different modication effects of tin on the Pt–lanthanum proceeds through preferential hydrogenation of C]O bond for
interfaces depending on the Sn/Pt ratios. At low Sn content, the bimetallic system. In particular, over the PtSn0.94/LOC, the
dilution effect of Pt by tin predominates and produces indi- selectivity and yield to CROL achieves a 5-fold and 7-fold
vidual Pt atoms segregated from lanthanum surfaces; whereas improvement over the Pt/LOC. Additionally, PtSn0.94/LOC
at high Sn content, more Pt atoms are interacted with the tin retains its selectivity at the initial level (around 70%), irre-
species and the passivation effect of tin on the catalyst surface spective of the reaction time and hydrogen pressure.
prevails.
It seems reasonable to accept that the enhanced carbonyl
selectivity of bimetallic system is originated from the intrinsic
nature of Pt, as our tests show that tin oxides are inactive for
CRAL hydrogenation and participate only in the isolation of Pt
4.2 Bimetallic particles
The interaction between Pt and the tin species is estimated by atoms. It is proposed that the chemisorption behaviors of H
XPS, CO-DRIFT and XAS techniques. The Pt–Sn contributions to and CO on metal sites originate from the interacting surfaces of
2
41
the Pt LIII-edge oscillation conrm the dilution of Pt atoms by Pt and Sn at the atomic level. The H
2
-TPD and CO-DRIFT
PtSn alloys or SnO species. XPS characterization manifests that measurements are employed to extract the correlation
x
the number of the O-coordinated Pt atoms is greatly decreased between the bimetallic structure and the catalytic behavior. It is
0
accompanied by a limited quantity of metallic Sn when Sn/Pt found that incorporating a small amount of tin (Sn/Pt ratio
ratio $ 0.94, suggesting the generation of Pt–SnO entities at equals to 0.69) substantially increases the number of active Pt
x
high Sn loading.
sites for hydrogen, presumably due to the prevailing effect of
˚
In the case of small particles (particle size below 50 A), the site isolation generated by tin. Meanwhile, this dilution effect
composition-dependent structural changes are reected in favors the reversible adsorption of linear Pt–CO, which is
varying CNs of the scattering paths from central atom.
9,18
The accounted for the improvement in hydrogenation rate for
PtSn0.69/LOC compared to the Pt/LOC. However, when Sn/Pt
Pt–O, Pt–Pt and Pt–Sn contributions in the curve tting of Pt LIII
-
edge provide further information on the bimetallic structure. ratio $ 0.94, excessive tin results into the passivation effect of
The average CN of the Pt–Sn shell for PtSn0.69/LOC is 1.1, and the catalyst surface and greatly reduce the exposure of Pt sites
this value is decreased to 0.3 for PtSn0.94/LOC and 0.2 for for H and CO adsorption. Besides, the interaction between CO
2
PtSn1.33/LOC, most likely resulting from the phase segregation and Pt is greatly enhanced with increasing Sn loadings, as evi-
41
of PtSn alloys. Meanwhile, the shorter distances of Pt–Pt bond denced by the red-shi of linear Pt–CO band and the desorption
for PtSn0.94/LOC and PtSn1.33/LOC than that for Pt/LOC and behavior from CO-DRIFTS spectra. This increased interaction
PtSn0.69/LOC, suggests a contraction of the interatomic distance plays a decisive role in the competitive adsorption and hydro-
due to the formation of small Pt clusters possibly segregated genation of carbonyl groups, whereas lowers the desorption rate
from PtSn alloys. These EXAFS tting results give a clear illus- of the products and hinders the recovery of active sites.
tration on the possible structure evolution of bimetallic NPs Consequently, the hydrogenation rate of CRAL is sharply
from SnO -patched PtSn alloys to SnO -patched Pt clusters with decreased at high Sn loadings.
x
x
increasing Sn/Pt ratios. Besides, the CN value of Pt–O contri-
It is known that the hydrogenation product of unsaturated
bution to the PtSn1.33/LOC is decreased to 1.0 with respect to the aldehyde is strongly dependent on the adsorption mode of
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37,46
reactant molecule on the catalyst surface.
According to the
Conflicts of interest
reactive performance over the prepared catalysts, two possible
adsorption models of CRAL over the prepared Pt and PtSn There are no conicts to declare.
catalysts are proposed. On the Pt/LOC, the support effect is the
primary factor for the selective hydrogenation. The carbonyl
Acknowledgements
2
+
oxygen is suggested to interact with the positively charged Pt ,
0
and the Pt site is the adsorption center of C]C bond. In this Sincere acknowledge is made to the nancial support from the
model, the hydrogenation of C]C and C]O bonds is National Science Foundation of China (21401204), Innovation
competitive, which dominates the selectivities to BUAL and Promotion Association CAS (2017460), the Western Light
CROL, respectively. While for tin-promoted catalysts, additional Program of Chinese Academy of Sciences (2015), and Science
Pt–SnOx sites are generated onto the catalyst surface. The and Technology Project of Suzhou City (SYG201627). The X-ray
carbonyl group could chemisorb on the Pt–SnOx entities absorption spectroscopy is granted by 4W1B station of Beijing
0
through the carbonyl carbon with the Pt site and carbonyl Synchrotron Radiation Facility, Institute of High Energy
n+
oxygen with electropositive Sn species to form a di-sCO Physics, Chinese Academy of Sciences. The staff members of
adsorption mode, which is proposed to facilitate the polariza- 4W1B are acknowledged for their support in measurements and
21,38
tion and activation of the C]O bond.
Evidently, the activa- data reduction.
tion of the C]O bond on the Pt–SnO entity is much easier than
x
0
2+
that on the Pt –Pt couple, which accounts for the additional
activity attained on the tin-modied catalysts compared to the
Pt/LOC. In addition, oxygen vacancies related to the generation
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