PdZn/α-Al2O3 catalyst for liquid-phase alkyne hydrogenation: effect of the solid-state alloy transformation into intermetallics

Article abstract of DOI:10.1016/j.mencom.2018.03.014

The complex characterization of a PdZn /Al₂O₃ catalyst by XPS, XRD, TEM and H₂-TPD techniques revealed the route of solid-state alloy → intermetallics transformation. The influence of this transformation on the performance of PdZn /Al₂O₃ in the liquid-phase hydrogenation of diphenylacetylene was estimated.

Full text of DOI:10.1016/j.mencom.2018.03.014

Mendeleev
Communications
Mendeleev Commun., 2018, 28, 152–154
PdZn/a-Al2O3 catalyst for liquid-phase alkyne hydrogenation:
effect of the solid-state alloy transformation into intermetallics
Igor S. Mashkovsky,*a Pavel V. Markov,a Galina O. Bragina,a Galina N. Baeva,a Aleksandr V. Rassolov,a
Andrey V. Bukhtiyarov,b Igor P. Prosvirin,b Valery I. Bukhtiyarovb and Aleksandr Yu. Stakheeva
a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. Fax: +7 499 135 5328; e-mail: im@ioc.ac.ru
b G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences,
630090 Novosibirsk, Russian Federation
DOI: 10.1016/j.mencom.2018.03.014
The complex characterization of a PdZn/Al2O3 catalyst by
XPS, XRD, TEM and H2-TPD techniques revealed the
route of solid-state alloy ® intermetallics transformation.
Zn
150°C
400°C
The influence of this transformation on the performance of
PdZn/Al2O3 in the liquid-phase hydrogenation of diphenyl-
acetylene was estimated.
Pd
PdZn alloy
PdZn IMCs
Nowadays, mono- and bimetallic clusters with controlled catalytic
properties are extensively used as catalysts for a number of
chemical processes due to the possibility of tuning their activity,
selectivity and stability under reaction conditions.1–6 Intermetallic
compounds (IMCs) are of growing interest as highly selective
heterogeneous catalysts.7–9 In contrast to substituted alloys, IMCs
are stable against segregation having ordered crystal structure
different from the structure of the constituting metals. The
intermetallic PdZn system is a promising catalytic composition,
which is successfully used in industry for selective alkyne
hydrogenation, methanol steam reforming, biomass conversion,
etc.10 Several reviews have been focused on the general properties
of IMCs and on the properties of PdZn compositions in parti-
cular.6,7,11 It is important that the PdZn system can form either
substituted solid-state alloy or IMCs.12 However, the effect of
PdZn solid-state alloy–PdZn IMCs transformation on the catalytic
performance was not studied in details. Here, to trace this effect,
the PdZn/Al2O3 catalyst was reduced stepwise at 150, 200, 250,
300, 350 and 400°C; after each reduction stage, a sample of the
catalyst was taken for characterization and catalytic study. Details
of catalyst preparation and characterization by XPS, XRD, TEM
and H2-TPD techniques can be found in Online Supplementary
Materials.
observe a minor shift of Pd 3d5/2 toward a lower binding energy
by 0.3–0.4 eV. This shift can be tentatively ascribed to the forma-
tion of PdZn intermetallics (see XRD data below). Alternatively,
it may be caused by Zn evaporation and partial depletion of the
PdZn particle surface in zinc17 since the Zn0/Pd0 ratio decreases
from ~1.0 to ~0.5 after reduction at 400°C.
Figure 2 shows XRD patterns for the PdZn/a-Al2O3 supported
catalysts reduced at different temperatures. Profiles of Pd/a-Al2O3
and unreduced PdZn/a-Al2O3 are also displayed for comparison.
For the reference monometallic Pd/Al2O3 catalyst, two XRD
reflexes were observed at ~40.0° and ~46.7°, corresponding to
Pd(111) and Pd(200), respectively.18 The XRD pattern of the
unreduced PdZn catalyst displays a group of signals at 31.0–35.0°.
A wide and asymmetrical peak at ~34.0° can be attributed to
PdO,19 and low-intensity peaks at ~31.8°, ~34.3° and ~36.3° are
assigned to (100), (002) and (101) of ZnO with a polycrystalline
wurtzite structure, respectively.20–22 No typical Zn0 signals at
38.9°, 43.1° or 54.2° were observed.14,23
337.2
335.8
1022.3
Zn 2p
Pd 3d
335.5
1020.5
The XPS study of the formation of Pd-Zn bimetallic particles
was carried out by monitoring the Pd3d and Zn2p regions
(Figure 1) after each step of catalyst reduction. Before reduc-
tion, Pd 3d5/2 and Zn 2p3/2 peaks were observed at 337.2 and
1022.3 eV for PdO and ZnO, respectively.13,14 After reduction
at 200–250°C, the Pd 3d5/2 peak shifted to 335.8 eV indicating
Pd2+ ® Pd0 reduction and characteristic of Pd0 in PdZn alloy.7,15,16
In the Zn 2p region, an additional signal appeared at 1020.5 eV
characteristic of Zn0. This observation suggests that the reduction
of Zn is facilitated in the presence of Pd via hydrogen activated
on Pd. Note that the Zn 2p3/2 binding energy is lower than that
of metallic Zn (~1021.5 eV) denoting the formation of PdZn
alloy.12 XPS analysis also demonstrates that the Zn0/Pd0 ratio
is about 1.0, and this value is achieved after reduction at 200°C.
After reduction at 300 and 400°C, Zn0 2p3/2 predominates
indicating further reduction of Zn. Simultaneously, we can
parent
parent
200 °C
250 °C
200 °C
250 °C
300 °C
400 °C
300 °C
400 °C
335.3
335
Binding energy/eV
345
340
1028
1024
Binding energy/eV
1020
1016
Figure 1 Pd 3d and Zn 2p XPS spectra of parent PdZn/a-Al2O3 catalysts
and those reduced (H2, 250 mbar, 1 h) at different temperatures.
© 2018 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 152 –

Mendeleev Commun., 2018, 28, 152–154
– PdZn IMC
evidence for the formation of PdZn IMCs.9,14 The asymmetry of
– Pd
– PdO
– ZnO
this reflex indicates some irregularity of IMCs structure.10,15,27
Since our XPS and XRD data suggest the formation of PdZn
alloy after reduction at 150–200°C, we additionally verify its
formation by an H2-TPD method. A specific characteristic of Pd
is its ability to absorb a considerable amount of H2 to form a-
and/or b-PdHx hydride phases.28,29 The hydrogen of this hydride
structure is capable to migrate to Pd surface under reaction
conditions enhancing the complete alkyne hydrogenation. How-
ever, upon the formation of a bimetallic PdM (M = Cu, Ag, In,
Pb, etc.) alloy, the solubility of hydrogen significantly decreases;
this makes it possible to avoid or minimize the PdH phase
formation.30,31 Thus, the higher selectivity of bimetallic or
intermetallic hydrogenation catalysts can be attributed to the
inhibition of palladium hydride phase formation.8,32
An H2-TPD study was performed to compare Pd hydride
formation ability in PdZn/Al2O3 and Pd/Al2O3 (Figure S1, Online
Supplementary Materials). The H2-TPD profile of Pd/Al2O3
displays a desorption peak at 75–90°C with a maximum at 83°C
characteristic of PdHx decomposition.26,28,33 In contrast, no signal
was observed in the TPD pattern of the PdZn/Al2O3 catalyst
reduced at 150 and 400°C. This observation provides evidence of
PdZn solid-state alloy formation even after reduction at 150°C,
in agreement with XPS and XRD data.
Thus, XPS, XRD and H2-TPD results clearly indicate the
formation of solid-state PdZn alloy after reduction at 150–300°C
and its transformation to IMCs as the reduction temperature
increases to 350–400°C. To reveal the effect of this transforma-
tion on the catalytic performance in alkyne hydrogenation, the syn-
thesized catalysts were studied in the liquid-phase hydrogena-
tion of diphenylacetylene. Figure 4 shows the kinetic profiles of
hydrogen uptake for Pd/Al2O3 and PdZn/Al2O3 samples reduced
at different temperatures. The PdZn/Al2O3 catalyst reduced at
200°C exhibited a distinctly different kinetic behavior, as compared
to that of the parent Pd sample. The kinetic profile of PdZn/Al2O3
catalyst exhibits pronounced downward bending after the uptake
of 1 equiv. of H2. A comparison of TOF1 (alkyne to alkene
hydrogenation) and TOF2 (alkene to alkane hydrogenation) values
shows that TOF1 for PdZn/Al2O3 decreases by a factor of 4,
while TOF2 decreases by a factor of ~17 in comparison with that
of the monometallic Pd/Al2O3 (see Table S1 in Online Supple-
mentary Materials). As a result, the TOF1/TOF2 ratio (so-called
kinetic selectivity) increases by a factor of 4: from 9.61 for
Pd /Al2O3 to ~38 for PdZn/Al2O3. Taking into account H2-TPD
data, we can suggest that the lower TOF2 value for PdZn/Al2O3
catalysts originates from the disappearance of a PdHx hydride
×1
×1
1
2
×2.5
×2.5
×2.5
×2.5
×2.5
×1
3
4
5
6
7
8
30
32
34
36
38
40
42
44
46
48
50
2q/deg
Figure 2 Comparison of XRD patterns for (1) Pd/a-Al2O3 and (2) PdZn/
a-Al2O3 catalysts, parent and (3)–(8) reduced at (3) 150, (4) 200, (5) 250,
(6) 300, (7) 350 and (8) 400°C.
After PdZn/a-Al2O3 reduction at 150°C, the intensities of
PdO and ZnO signals diminished and a wide reflex appeared
at ~39.0–41.5° Note that the position of its maximum does
coincide with the position of Pd(111) ~ 40.0° in Pd/Al2O3 and
is shifted toward higher 2q with a maximum at 40.6°, which
suggests the formation of PdZn substituted alloy even at a tem-
perature of 150°C. Low intensity of a signal, its asymmetry
and the absence of the Pd(200) reflex at 46.7° indicate that the
structure of the alloyed particles is largely disordered and the
formed particles are small. These results are consistent with
published data24 and TEM data (Figure 3). The following H2
treatment at 200 and 250°C results in a gradual shift of the
peak at 40.6° toward higher angles indicating the enrichment
of PdZn solid solution in Zn.25 Simultaneously, the intensity of
ZnO reflex notably decreases.
The transformation of PdZn solid-state alloy to the inter-
metallic structure begins as the temperature is raised above
300°C. A wide Bragg peak appears at ~41.4° ascribed to PdZn
intermetallic compound.9,22,26 However, the most part of PdZn
particles is still as a solid-state alloy, as evidenced by the
broadening and low intensity of a peak at ~41.4°. With a further
increase in H2 reduction temperature to 400°C, the intensity of this
signal grows, and it becomes more symmetrical and intense.
Moreover, an additional reflex appears at 44.0° giving distinct
parent
150°C
200°C
250°C
200 nm
300°C
200 nm
350°C
200 nm
400°C
200 nm
200 nm
200 nm
200 nm
Figure 3 TEM images of parent PdZn/a-Al2O3 catalysts and the catalysts reduced at 150–400°C.
– 153 –

Mendeleev Commun., 2018, 28, 152–154
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1.0
0.8
0.6
0.4
0.2
0.0
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t/min
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Pd/Al2O3 and PdZn/Al2O3 catalysts in the liquid-phase hydrogenation of
diphenylacetylene.
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We are grateful to the Department of Structural Studies of the
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy
of Sciences for the characterization of the catalysts by electron
microscopy.
This study was supported by the Russian Foundation for Basic
Research (project no. 16-29-10788).
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Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2018.03.014.
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