Tuning the catalytic performance for the semi-hydrogenation of alkynols by selectively poisoning the active sites of Pd catalysts

Article abstract of DOI:10.1039/c9gc01356c

Semi-hydrogenation of alkynols to alkenols with Pd-based catalysts is of great significance in fine chemical industries. Industrial Lindlar catalysts, employing Pb to modify the Pd nanoparticles for higher selectivity toward alkenols, however, generally suffer from both a severe activity decrease and environment pollution caused by using heavy metal Pb and additives. Therefore, how to overcome the selectivity-activity paradox remains a great challenge in industry. Here, we report a controllable strategy for the synthesis of semi-hydrogenation catalysts, which successfully improves the catalytic performance through selectively poisoning the edge and corner sites of Pd nanoparticles. When the integrity of the crystal face is reserved, both higher activity (～1340 h^-1) and selectivity (～95% at 99% conversion) are achieved in the semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY) in ethanol, an industrially important intermediate product for the synthesis of vitamin E, without adding any toxic additives. What's more, the yield could exceed 98% at 99% conversion under no solvent and organic adsorbate conditions, which had never been achieved before. This work provides a different perspective to design and develop high performance catalysts for semi-hydrogenation of alkenols or even substituted alkynes.

Full text of DOI:10.1039/c9gc01356c

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DOI: 10.1039/C9GC01356C
Tuning the Catalytic Performance for the
Semi-hydrogenation of Alkynols by
Selectively Poisoning the Active Sites of Pd
Catalysts
Shanjun Mao,‡a Bowen Zhao,‡a Zhe Wang,a Yutong Gong,a Guofeng Lü,b Xiao Ma,b
Lili Yu,b and Yong Wang*a
aAdvanced Materials and Catalysis Group, Institute of Catalysis, Department of
Chemistry, Zhejiang University, Tianmushan Road, Hangzhou, 310028 (China).
bZhejiang NHU Company Ltd., Xinchang County, Zhejiang Province.
Corresponding Author
* Corresponding author at: Advanced Materials and Catalysis Group, Institute of
Catalysis, Zhejiang University, Tianmushan Road, Hangzhou, 310028 (China).
E-mail address: chemwy@zju.edu.cn.
‡ These authors contributed equally to this work.
Abstract
Semi-hydrogenation of alkynols to alkenols with Pd-based catalysts, is of great
significance in fine chemical industries. Industrial Lindlar catalysts, employing Pb to
modify the Pd nanoparticles for higher selectivity toward alkenols, however, generally
suffer from both severely activity decrease and environment pollution caused by using

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heavy metal Pb and additives. Therefore, how to overcome the selectivity-activity
paradox remains a great challenge in industry. Here, we reported a controllable
strategy for semi-hydrogenation catalysts synthesis, which successfully improves the
catalytic performance through selectively poisoning the edge and corner sites of Pd
nanoparticles. When integrity of the crystal face is reserved, both higher activity
(~1340 h-1) and selectivity (~95% at 99% conversion) are achieved in the
semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY) in ethanol, an industrial
important intermediate product for synthesis of Vitamin E, without adding any toxic
additives. What’s more, the yield could exceed 98% at 99% conversion under no
solvent and organic adsorbate conditions, which had never been achieved before. This
work provides a different perspective to design and develop high performance
catalysts for semi-hydrogenation of alkenols or even substituted alkynes.
Keywords
semi-hydrogenation, selectively poison, Lindlar catalyst, Vitamin E,
2-methyl-3-butyn-2-ol
1. Introduction
The semi-hydrogenation of alkynols to alkenols is critical for many important
manufacturing processes of bulk and fine chemicals including vitamins, drugs,
fragrances and synthetic material such as corrosion inhibitor and artificial fiber etc.1, 2
Among them, the synthesis of Vitamin E involves various alkenol such as isophytol,
linalool and 2-methyl-3-buten-2-ol (MBE). Generally, these alkenols are achieved via
semi-hydrogenation of alkynols.3-6 Pd-based catalysts are frequently used in

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semi-hydrogenation reaction of alkynes in industry. However, they usually suffer
from limited selectivity, especially at high conversions.7 The general proposal is to
modify catalysts with another metal or reversible organic adsorbates such as pyridine
and quinoline to block or isolate the surface active sites at a certain degree.8 One of
the most famous cases is Lindlar catalysts,9 in which Pd/CaCO3 is modified with Pb
and quinoline. However, the addition of Pb leads to serious heavy metal pollution and
activity loss.10 Therefore, developing lead-free catalysts with higher performance for
selective hydrogenation is highly desired. Based on this idea, much work has been
focused on the isolation of the active sites by forming metal compounds or alloys,11-13
single-atom catalysts14-16 and so on17-19
.
The semi-hydrogenation of alkynols is structure-sensitive.20, 21 The activity and
selectivity are strongly influenced by both the size and shape of Pd nanoparticles
(NPs), since the ratio of different types of active sites (e.g., facet, corner and edge) on
the surface varies.22 In 2011, Xia22 et al. synthesized uniform Pd nanocrystals of
different shapes with different ratios of low-coordinated sites (edge and corner atoms)
through a solution-phase method with poly(vinyl pyrrolidone) (PVP) serving as a
stabilizer, and then tested them in the hydrogenation of MBY (Scheme 1). In view of
the mechanism, the direct route with MBY being hydrogenated to MBA straightly
occurred on the conditions that the adsorption of MBE was stronger than that of
MBY. In this way, adsorbed MBE intermediate would not be replaced by MBY on the
catalytic surface, and then was further hydrogenated to 2-methyl-butan-2-ol (MBA).
Otherwise, MBE would desorb into the solvent as the semi-hydrogenation product

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along the sequential
Scheme 1. Reaction scheme of 2-methyl-3-butyn-2-ol (MBY) hydrogenation.
route. The results from Xia demonstrate that the reaction mainly proceeded along the
sequential route on Pd catalysts and that the facet sites are the main active sites for the
transformation of alkynols to alkenols in semi-hydrogenation reaction, while the
corner and edge sites are responsible for over-hydrogenation.23-25 However, the
influence of PVP on catalytic performance cannot be ruled out. In our group, a
PdZn/CN@ZnO catalyst was recently utilized for the semi-hydrogenation of MBY.26
The selectivity to MBE was significantly improved over PdZn/CN@ZnO catalyst
compared with that over the monometallic Pd/CN catalyst. Density functional theory
(DFT) calculation results demonstrate that it is the decrease of corner and edge Pd
atoms caused by Zn doping, rather than planar poisoning, that was responsible for the
enhanced selectivity. However, it is hard to control the degree of Zn doping since
PdZn alloy are easily formed. This unselective doping would dramatically reduce the
catalytic activity due to the excessive poisoning of the surface active-sites (e.g. TOF =
434 h-1 and Sel./% = 92% for PdZn/CN@ZnO with 1 wt% Pd loading). One would be

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easily trapped into a mindset that the isolation of the Pd sites is pre-reqDuOisI:i1t0e.10t3o9/C9GC01356C
prevent the polymerization of alkynes during the catalyst preparation since organic
adsorbates are always used.27, 28 However, our previous investigation suggests that the
OH group in alkynols/alkenols actually bind to the Pd sites upon adsorption.26 This
reactant induced self-poisoning or self-isolation effect may suppress the
polymerization, and then the isolation of the Pd sites through Pb poisoning or organic
absorbates may no longer be neccessay.
In this work, a controllable strategy is adopted to fabricate catalysts in which
only corner and edge Pd atoms can be blocked. The synthesized catalysts were proved
to be highly efficient for the selective hydrogenation of MBY to MBE in ethanol
(TOF = 1340 h-1 and Sel./% = 95% with 0.3 wt% Pd loading). What’s more, to the
best of our knowledge, higher than 98% yield at 99% conversion were firstly achieved
under no solvent and organic adsorbate conditions, while commercial Lindlar
catalysts only gave ~1/4 activity and 93% selectivity under the same conditions. The
enormous improved performance of this catalyst originates from the appropriate
poisoning degree of Pd NPs, which can be precisely controlled by reduction
temperatures. Besides, the catalysts could be reused at least ten times without
significant loss of activity and selectivity. We believe this work would shed light on
another insight to design and synthesize efficient catalysts for industrial
semi-hydrogenation of alkenols or even substituted alkynes.
2. Experimental
2.1 Materials

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Indium oxide (50 nm, Macklin), In(NO3)3, NaBH4 (Sinopharm Chemical
Reagent Co. Ltd), SiO2 (Alfa Aesar), PdPb/CaCO3 (5 wt% Pd, Lindlar catalyst) and
2-methyl-3-butyn-2-ol (98%) were used as received from Aladdin Chemistry Co., Ltd.
Palladium(II) nitrate dehydrate was purchased from Energy Chemical.
Dehydrolinalool and dehydronerolidol were purchased from NHU Co., Ltd. All
chemicals were used as purchased.
2.2 Catalyst synthesis
A series of Pd catalysts were prepared by wetness impregnation method. The
prepared catalysts were named as Pd-In/In2O3-x (x stands for reduction temperature).
1.00 g In2O3 support was impregnated with a certain amount of palladium nitrate
aqueous solution to achieve 0.3 wt% Pd loading. After magnetic stirring at 70 ºC for
12 h, the precursor was dried at 70 °C in an oven overnight, followed by reduction at
various temperatures from 200 to 350 ºC in H2 for 1 h. The solid was ground before
use.
The prepared catalysts with no coating upon Pd particles, denoted as Pd/In2O3, were
prepared by an ultrasound reduction method, which was tested as contrast sample. In
detail, 1.00 g In2O3 support was dispersed in 30 mL deionized water in a beaker by
shaking it with the assistance of ultrasound, and 300 uL Pd(NO3)2 aqueous solution
(0.01 g mL-1) was added into this suspension with ultrasound for 30 min. Then, 10 mL
newly prepared NaBH4 solution (1 mg mL-1) was added into this system quickly, and
the mixture was shaken for another 30 min. Next, the solution was centrifuged to get
the gray residue. The residue was washed several times with deionized water. After

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drying in vacuum oven at 40 ºC overnight, the catalyst was obtained. Pd/SiO2DOcaI: t1a0.l1y03s9t/C9GC01356C
with 0.3 wt.% Pd loading was also prepared by wetness impregnation method.
PdIn/Al2O3 catalyst with 0.3 wt.% Pd and 0.3 wt.% In was prepared by
co-impregnation method with reduction temperature of 400 oC.
2.3 Characterizations
High-resolution transmission electron microscopy (HRTEM) was performed on a
Field Emission Transmission Electron Microscopy with an acceleration voltage of
200 kV. The samples to be tested were ultrasonically dispersed in ethanol. A drop of
the solution was deposited onto a carbon-coated copper grid, followed by evaporation
for TEM, HRTEM characterization and elemental mapping.
The IR spectra of CO adsorbed on solid samples were performed on a Bruker
Vector 22. All spectra were obtained at room temperature (25 ºC). Before the tests,
the as-synthesized catalysts (150 mg) were purged for 1 h with He at preset
temperatures to remove the vapor and impurities. After cooled to room temperature,
the background spectrum was recorded and a feed gas (5 vol% CO in He) was
introduced into the sample cell with flow rate at 30 mL/min for 1h. Then, He was
introduced with flow rate at 30 mL/min for 30 min to remove the physically absorbed
CO, and the IR spectra were collected.
After water removal under Ar atmosphere at 120 ºC, H2 temperature
programmed reduction (TPR) was conducted in a quartz reactor connected to a
thermal conductivity detector (TCD) under an atmosphere of 10% H2/Ar from room
temperature to 500 ºC at a heating rate 10 ºC/min. Temperature programmed
desorptions of CO (CO-TPD) and H2-TPD were conducted with a TCD under pure
CO and Ar respectively with H2 in situ reduction pretreatment at a heating rate 10
ºC/min from room temperature for 2 h before calcining at 250 oC for 3 h.

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The X-ray photoelectron spectra (XPS) were obtained on an ESCALAB MARK
II spherical analyzer with an aluminum anode (1486.6 eV) X-ray source, and the
binding energy was calibrated by C1s peak (284.5 eV). Powder X-ray diffraction
(PXRD) was performed using a wide-angle X-ray diffractometer (D/tex-Ultima TV)
with Cu Kα raiation (λ = 1.54 Å) (tube operating at 40 kV and 40 mA). The Pd
content in the prepared catalysts was analyzed by inductively coupled plasma-atomic
emission spectroscopy (ICP-AES, Perkin-Elmer 3300 DV). Scanning electron
microscopy (SEM) study was performed on Hitachi SU8010 microscopy. The textual
properties were collected on Micromeritics ASAP 2020 HD88 instrument. The
specific surface area was calculated by the Brunauer–Emmett-Teller (BET) method.
Prior to analysis, the samples were degassed in vacuum at 573 K for 12 h.
2.4 Catalytic tests
In a typical reaction, 1.25 mmol MBY, 5 mL ethanol, and 20 mg
freshly-prepared catalysts were added into a 50 mL 3-necked round bottom flask and
thoroughly mixed. A balloon filled with H2 was connected to the flask, and the flask
was purged with H2 for three times before reaction. Then, the reaction was maintained
at 30 ºC under vigorous stirring. To track the reaction progress, the reaction mixture
was collected with a syringe at a certain time interval. After sampling and
centrifugation, the product distribution of the solution was analyzed by gas
chromatography-flame ionization detector (GC-FID), while the products were
identified by gas chromatography-mass spectrometer (GC-MS). By utilizing
isopropanol as the internal standard, the mass ratios of MBE, MBY, and MBA were
eventually calculated.

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The conversion, selectivity, and mass balance of the reaction can be calculated as
following equations. These formulae imply that no components other than MBY,
MBE and MBA were obtained, which was confirmed by the carbon balance of 100 ±
2% for all reactions.
Conv.%= 1-m(MBY),
Sel.%= m(MBE)-m(MBY),
Mass balance= m(MBY)+m(MBE)+m(MBA)
where m(MBY), m(MBE), and m(MBA) are the mass ratios of MBY, MBE, and
MBA in the reaction system.
2.5 TOF calculations
The calculation method of TOF values in this article is reported in our earlier work.26
To be specifically, it could be denoted as follows:
푛푀퐵푌
푛푃푑
Conv.
TOF =
×
,
푇
Where nMBY is the moles of MBY, nPd is the moles of Pd in the catalyst, Conv. and T
are conversion and reaction time of MBY respectively. Conv./T is the value of the
slope taken from the T-Conv. curves of MBY in the kinetic regime.
2.6 Catalyst reusing
In order to examine the stability of the catalysts, they were recycled in repeated
runs. Parallel experiments were conducted during the recycle test. One 50 ml
3-necked round bottom flask, with a balloon filled with 1 Bar H2, contained 120 uL
MBY, 5 mL ethanol and 20 mg Pd-In/In2O3-250. Another, with a balloon filled with 1
Bar H2, contained 30 mg Pd-In/In2O3-250 with proportional amplified substrate and

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solvent and was used as catalyst supplier for the first flask. The catalytic
hydrogenation was carried out at 30 °C for 1 h separately. After each run, the
recovered catalyst was washed several times with ethanol and dried overnight at
40 °C under vacuum and stored in the vacuum desiccator for no more than 1 day
before the next run. The mass loss of the catalyst in the first flask was supplied from
the second flask before the next run. Subsequent reusability tests were carried out
following the same procedures as stated above.
3. Results and discussion
The Pd-In/In2O3-x catalysts with 0.3 wt% Pd loading were prepared by wetness
impregnation method. X-ray diffraction (XRD) patterns of all these catalysts exhibit
diffraction peaks indexed to only In2O3 (Figure 1A). No Pd diffraction peaks were
detected because of the low metal content. The representative images and
corresponding size distribution of Pd NPs are presented in Figure S1. It clearly shows
that the Pd NPs sizes in all samples are similar. No obvious particle sintering is
observed. The average particle size is about 3.9 nm, which is just in the optimum size
range (3−5 nm) of Pd nanoparticles for high performance of semi-hydrogenation of
MBY.22 Besides, the hemispherical shape of the Pd NPs indicates the existence of
interaction between Pd and In2O3. High resolution transmission electron microscopy
(HRTEM) images show that the Pd NPs on the catalysts are covered by a thin layer
except for

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Figure 1. The compositions and Structures of the catalysts. (A) XRD pattern for
Pd/In2O3, Pd-In/In2O3-200, Pd-In/In2O3-250 and Pd-In/ In2O3-300. (B) H2 TPR files of
Pd-In2O3 and In2O3 without reduction.
reduction temperature. One can observe that the carrier gradually migrated onto the
Pd-In/In2O3-200. The degree of coverage is different and closely depends on the
surfaces of Pd NPs (Figure 2A-D) under H2 atmosphere. In addition, the lattice
spacing of the thin cover layer is 0.29 nm which is consistent with the (222) plane of
In2O3, as revealed

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Figure 2. The interaction between Pd NPs and In2O3 under different reduction
conditions. HRTEM images of (A) Pd-In/In2O3-200, (B) Pd-In/In2O3-250, (C)
Pd-In/In2O3-300, (D) Pd-In/In2O3-350. (E) Pd 3d XPS spectra of the catalysts for (A),
(B) and (C). (F) An enlarged view of Pd-In/In2O3 -250.
in Figure 2D. Element mapping analysis also confirms the composition of the cover
layer (Figure S2). The above results fully prove the migration process of the carriers
to the surface of Pd NPs.
The interaction between Pd NPs and In2O3 was then studied using
H2-temperature programmed reduction (H2-TPR) and the results are shown in Figure

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1B. For In2O3, the reduction starts at around 170 °C. Two broad peaks centered at 250
°C and 410 °C are assigned to the reduction of surface In2O3.29 For sample
impregnated with Pd precursor, H2-TPR shows one peak centered at about 100 °C,
which can be attributed to the reduction of Pd2+.30 Two reduction peaks located at 240
ºC and 400 ºC can be attributed to the reduction of surface In2O3. And another small
peak centered at about 140 °C, which is not present in the sample In2O3, can be
attributed to the reduction of a small part of surface In2O3 by the spillover hydrogen
from Pd NPs.31
To further verify the electronic interaction between Pd and In2O3, X-ray
photoelectron spectroscopy (XPS) investigation was performed for samples
Pd-In/In2O3-200, Pd-In/In2O3-250 and Pd-In/In2O3-300 (Figure 2E). Two peaks at the
binding energy of 335.4 and 340.8 eV are observed for Pd-In/In2O3-200, in
accordance with the values for metallic Pd 3d5/2 and 3d3/2.32 For samples
Pd-In/In2O3-250 and Pd-In/In2O3-300, the peaks of Pd 3d5/2 and 3d3/2 gradually shift
to lower binding energies with the increase of reduction temperature. Meanwhile, the
layer over the Pd NPs becomes thicker (Figure 2A-D). The negative shift of Pd 3d
may indicate the electron transfer from reduced In to Pd. However, no obvious
difference for the binding energies of In 3d5/2 and 3d3/2 are observed for the three
samples (Figure S3) due to the existence of massive bulk In2O3. According to
previous researches, in the strong metal-support interaction (SMSI) state which
occurred under reductive condition, the Fermi level of the partially reduced metal
oxide support becomes higher than that of the metal NPs.33 Therefore, we surmise that

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the electron might transfer from the support to the metal NPs in order to attain the
Fermi-level equilibration at the interfaces. In our case, when pretreated with H2, the
Fermi level of species containing In which located on the top surface of Pd NPs is
higher than that of Pd NPs, which would facilitate the electron transfer from the
support to Pd NPs.34
To demonstrate the effect of poisoning degree on tuning the catalytic
performance, MBY semi-hydrogenation reaction was used as a probe reaction, as
shown in Figure 3A. The possible mass-transfer limitations has been minimized as
seen in Figure S4 and the involved discussion. When the reduction temperature is
below 200 ºC, the reaction activity for MBY hydrogenation gerenarally remains
unchanged, but the selectivity toward MBE rises. As reduction temperature further
enhances, the activity for MBY hydrogenation decreased significantly, while the
selectivity toward MBE continues to rise. Until the reduction temperature is beyond
250 ºC, the selectivity toward MBE begins to decline. Combined with Figure 1, the
above results illustrate that an appropriate poisoning extent is responsible for the
excellent selectivity. By comparing the selectivity of these catalysts, we found that
Pd-In/In2O3-250 was the optimal (Figure 3A). The yield of MBE could reach above
95% at 99% conversion, without adding any toxic additives such as heavy metal Pb or
organic ligand quinoline. Nearly twice higher activity (TOF = 1340 h-1) with identical
selectivity are achieved compared with our

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Figure 3. The catalytic performance of different catalysts. (A) Comparison of the
catalytic performance among different catalysts. Reaction conditions: 20 mg catalysts,
120 uL substrate, 5 mL ethanol, 30 ºC, 1 atm H2, stirring rate of 1000 rpm. (B) The
proposed catalyst model. And infrared spectroscopy of CO adsorption (C) and
CO-TPD spectra (D) on various Pd samples.
industrial catalyst PdZn/CN@ZnO.26
As a control experiment, the Pd/In2O3 sample reduced by NaBH4 without
decoration on the surface of Pd NPs, which can be clearly proved by TEM images
(Figure S5 and S6), was tested. As shown in Figure 3A and S7A, the selectivity of
Pd-In/In2O3-250 catalyst is much better than that of Pd/In2O3 during the whole
reaction process. Since the particle size of Pd/In2O3 is close to that of Pd-In/In2O3-250

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samples (Figure S8), the different performance cannot be attributed to the particle size
simply. Note that different reducing temperature may change the morphology of the
Pd NPs, which may also cause the different catalytic performance. Therefore, we also
choose SiO2, always seen as an inert oxide, as the carrier to probe the possibility of
the above affection. According to previous researches, the surface of SiO2 could not
be activated even when loaded with noble metals at nearly 400 ºC upon H2
atmosphere.1, 35 A series of Pd/SiO2 (0.3 wt% Pd loading) catalysts reduced at various
temperatures were tested (Figure S7B). Results showed that in contrast to Pd-In/In2O3,
the selectivity of Pd/SiO2 catalysts improved little with the change of reduction
temperature. So the proper surface decoration or poisoning of Pd NPs is the key for
Pd-In/In2O3-250 to achieve higher MBE selectivity while maintaining a remarkable
activity.
Different degree of poisoning on the surface of NPs would result in the structural
changes of active sites, further influence the catalytic performance.36, 37 So in-situ
diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as the
probe molecule was carried out to explore the relationship between the structure of
active sites and catalytic performance. As shown in Figure 3C, the peaks at 2000-2050
cm−1 could be attributed to the adsorption of CO linearly bound to (111)/(111) and
(111)/(100) particle edges.38-41 The bands at appropriately 2079 cm-1 could be
assigned to the adsorption of linear CO bound to corner atoms of Pd NPs.41 The peaks
appearing below 2000 cm-1 are generally attributed to the CO species bound to the
facet sites of the Pd NPs. Specifically, the bands at 1950 cm-1, 1900 cm-1 and 1850

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cm-1 can be assigned to compressed-bridge bonded CO, isolated-bridge bonded CO
and threefold-hollow bonded CO on facets of Pd NPs, respectively.7, 39
.
Compared with other samples, the intense peaks only appear on Pd/In2O3 at
appropriately 2051 cm-1 could be assigned to the adsorption of linear CO bound to the
edge atoms of Pd NPs, which indicates the existence of a large number of edge sites
on the Pd NPs. However, only the peaks for bridge-bonded CO adsorption are
observed on Pd-In/In2O3-200 sample. The disappearance of the signals for
linear-bonded CO adsorption under He purging conditions suggests that the edge and
corner sites of Pd NPs are blocked. In theory, it is thermodynamically feasible for
these low-coordinated atoms to preferentially bond with carriers which migrate onto
the surface of Pd NPs.42-44 In contrast to Pd/In2O3, the selectivity of Pd-In/In2O3-200
has been enhanced. According to our density functional theory (DFT) results (Table
S1), the major active sites for the semi-hydrogenation from MBY to MBE are facet
sites, while the edge and corner sites are responsible for the over-hydrogenation to
product MBA.26 Therefore, as the reduction temperature gradually rises, the improved
selectivity of Pd-In/In2O3-200 mainly ascribes to the selective poisoning of edge and
corner sites by carrier migration. CO-temperature programmed desorption (CO-TPD)
also corroborates the above conclusion. As shown in Figure 3D, the peak attributed to
facet sites (50 ~ 100 ºC) remained, while the peak corresponding to edge and corner
sites (150 ~ 200 ºC)45 disappeared when raising the reduction temperature. The
preservation of high activity further confirms that the facet sites are responsible for
semi-hydrogenation of alkynols. It can be concluded that there is no necessary to

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poison facet sites. Further raising the reduction temperature to 250 ºC, the signal
intensity for bridge-adsorbed CO adsorption (Pd-In/In2O3-250) is attenuated,
accompanied with the appearance of a stretching peak at 2070 cm-1, which is
attributed to the linear-bonded CO adsorption at facet sites.41 It implies that partial
facet sites have been alloyed with In, resulting in the break of the Pd-Pd bond and the
poisoning of the facet sites. The additional peak at 140 °C (Figure 1B) and the binding
energy shift of Pd 3d for Pd-In/In2O3-250 (Figure 2E) also corroborate the above
deduction. The sharp decline in activity is then well expected as shown in Figure 3A.
The continual rise of selectivity toward MBE beyond 250 °C may be caused by the
continual blocking of the remained small fraction of edge and corner sites (Figure 2F),
which is beyond the detectability of instrument. This detectable limitation factor can
be confirmed in the case of Pd-In/In2O3-300. After reduced at 300 ºC, the signal of
bridge-bonded CO adsorption on the facet sites disappears, which means the further
poison of facet sites. In fact, no valid signals were detected at all on Pd-In/In2O3-300.
However, this catalyst still shows activity, which means the partial exposure of facet
active sites. The selectivity decline for catalysts reduced beyond 250 °C means
poisoning the facet sites by breaking the Pd-Pd bond is detrimental to the
semi-hydrogenation of alkynols. Therefore, it is feasible to achieve both intriguing
catalytic activity and selectivity for semi-hydrogenation of alkynols by selectively
blocking corner and edge Pd sites in the synthesized catalysts as illustrated in Figure
2B.
One would suspect that may be it is the progressive inhibition of Pd-hydride

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formation with increased reduction temperatures that actually dominated improved
selectivity since several work has disclosed the impact of Pd-hydride to the catalytic
46
performance of semi-hydrogenation of alkynes.7,
In particular, bimetallic
PdIn/Al2O3 catalyst, with conventional Lindlar’s concept, has been successful in the
semi-hydrogenation of Acetylene, where Pd-hydride was entirely suppressed.47 Then
H2-TPD was conducted for the involved catalysts reduced at different temperatures.
As shown in Figure S9, a progressive inhibition of hydride formation was indeed
observed as the reduction temperature increased in that the released amount of H2
decreased monotonously along with the increase of the reduction temperature. And
the desorption temperature of H2 increased gradually as well. However, the selectivity
for the semi-hydrogenation of MBY went through a volcano, rather than a linear
decline. Therefore, the impact of Pd-hydride was not thought to be a dominant role to
the selectivity of MBY semi-hydrogenation. To further confirm this, supported PdIn
alloy (PdIn/Al2O3), which has been proved to completely inhibit the formation of
Pd-hydride,47 was used for the semi-hydrogenation of MBY. The results in Figure 3A,
yet, showed that this catalyst was totally inactive on these conditions, which also rules
out the possible contribution to the improved selectivity by surface facet Pd-In
intermixing on the above Pd-In/In2O3 catalysts, i.e., PdIn is not the active phase for
MBY semi-hydrogenation under this condition. In addition, the formation of
Pd-hydride, together with the CO-IR results in Figure 3C clearly showed the early
stage for selectively covering the edge/corner sites in order when increasing the
o
reduction temperature (< 200 C), and the late stage for surface Pd-In intermixing at

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facet sites when the reduction temperature went beyond 200 oC.
Note that solvent would also influence the selectivity and no solvent conditions
are actually used in industry.3, 48 The catalytic performance of Pd-In/In2O3-250 in pure
MBY were evaluated to make a comparison with the dominated commercial Lindlar
catalyst PdPb/CaCO3. As shown in Figure 4, the reaction rate on Pd-In/In2O3-250
(TOF =44208 h-1) is 5.5 times higher than that on PdPb/CaCO3 (TOF =8034 h-1).
What’s more, the selectivity of MBE were well maintained in the whole process,
achieveing unexpected more than 98% yield at 99% conversion for Pd-In/In2O3-250.
However, the selectivity decreased sharply at the late conversion stage for
PdPb/CaCO3 (~93% at 99% conversion). The superior catalytic performance for
Pd-In/In2O3-250 clearly demonstrates the validity of the proposed catalyst design
strategy.
In addition, other industrially relevant alkynols, such as dehydrolinalool (D1)
and
Figure 4. The comparison of the catalytic performance on Pd-In/In2O3-250 and
PdPb/CaCO3 for the semi-hydrogenation of MBY. (A) The time-conversion profile;

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(B) A comparison of selectivity vs. conversion. Reaction conditions: 2.77×D1O0I:-210.a10n3d9/C9GC01356C
4.16×10-2 wt.% of Pd for Pd-In/In2O3-250 and PdPb/CaCO3 respectively, 5 mL
substrate, 80 ºC, 1 Bar H2, stirring rate of 1000 rpm. The amount of catalyst loaded is
referred to previous work to ensure reasonable evaluation conditions.48
dehydronerolidol (D2), which play an important role in the synthesis of Vitamin E,
were also tested in ethanol (Table 1 and Figure S10). Their semi-hydrogenation
products were named linalool (L) and nerolidol (N), respectively. As expected, for
semi-hydrogenation of D1, the selectivity to L increases from 89.6% to 95% at a
conversion of over 99%. And for semi-hydrogenation of D2, the selectivity to N
increases from 84% to 89% at a conversion of over 94%. Above results indicate the
general scope of this catalyst in semi-hydrogenation of alkynols.
The recycling experiment of Pd-In/In2O3-250 at both high and middle
conversions
Table 1. Result of hydrogenation reaction on dehydrolinalool and
dehydronerolidol by Pd/In2O3 and Pd-In/In2O3-250.
Substrate
D1
Catalysts
Pd/In2O3
Conv./%
99
Sel./%
89

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Pd-In/In2O3-250
Pd/In2O3
99
94
94
95
84
89
D2
Pd-In/In2O3-250
Reaction condition: 20 mg catalysts, 2 mmol substrate, 5 mL ethanol, 30 ºC, 1 atm H2,
stirring rate of 1000 rpm.
for MBY semi-hydrogenation was conducted due to the importance of reusability for
catalyst in industrial application (Figure 5A and S11). It was observed that MBE
selectivity stay unchanged even after ten recycle runs. Slightly activity decline was
observed after seven recycle runs. According to the ICP results, there is no leaching of
Pd (Table S2). XRD patterns for the recycled catalyst after ten runs also show no
obvious changes (Figure 5B). Considering the sticking effect in Figure S4B, we

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Figure 5. The reusability evaluation. The reusability (A) and XRD results after
recycling (B) of Pd-In/In2O3-250. (C) Evolution of reactant and product
concentrations during hydrogenation over Pd-In/In2O3-250. Reaction condition: 20
mg catalysts, 120 uL substrate, 5 mL ethanol, 30ºC, 1atm H2, stirring rate of 1000
rpm.
suspected the activity loss would be caused by the decreased agglomerative sizes of
the catalysts during the long-term stirring in the recycle process. Besides, the stability
49
is much better than the newly-developed Pd/ZnO/SMF catalyst.25,
The good
stability of the catalyst may be attributed to the protection of the cover layer outside
the Pd NPs from SMSI. And the time-conversion curve in Figure 5C suggested a
typical sequential hydrogenation mechanism.

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4. Conclusions
Through the selective poisoning of the edge and corner sites of the Pd NPs
responsible for over-hydrogenation, both much better selectivity and activity of the
Pd- based catalysts were achieved. The comparable selectivity and much better
activity on barely Pd-In/In2O3-250 catalysts, compared with that on PdZn/CN@ZnO
or other Pd-based Lindlar catalysts with additives, suggests that the integrality of the
facet sites is necessary when the self-poisoning or self-isolation effects induced by
reactants are considered. This work offers a distinctive insight to solve the
selectivity-activity paradox on Pd-based Lindlar catalysts for the semi-hydrogenation
of alkynols or even other substituted alkynes.
Acknowledgement
This work has been funded by the National Natural Science Foundation of China
(21872121, 21622308), the Key Program Supported by the Natural Science
Foundation of Zhejiang Province, China (LZ18B060002), and the Fundamental
Research Funds for the Central Universities (2017XZZX002-16) are greatly
appreciated.
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139x78mm (150 x 150 DPI)