Supported Intermetallic PdZn Nanoparticles as Bifunctional Catalysts for the Direct Synthesis of Dimethyl Ether from CO-Rich Synthesis Gas

Article abstract of DOI:10.1002/anie.201906256

The single-step syngas-to-dimethyl ether (STD) process entails economic and technical advantages over the current industrial two-step process. Pd/ZnO-based catalysts have recently emerged as interesting alternatives to currently used Cu/ZnO/Al₂O₃ catalysts, but the nature of the active site(s), the reaction mechanism, and the role of Pd and ZnO in the solid catalyst are not well established. Now, Zn-stabilized Pd colloids with a size of 2 nm served as the key building blocks for the methanol active component in bifunctional Pd/ZnO-γ-Al₂O₃ catalysts. The catalysts were characterized by combining high-pressure operando X-ray absorption spectroscopy and DFT calculations. The enhanced stability, longevity, and high dimethyl ether selectivity observed makes Pd/ZnO-γ-Al₂O₃ an effective alternative system for the STD process compared to Cu/ZnO/γ-Al₂O₃.

Full text of DOI:10.1002/anie.201906256

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition Chemie
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Accepted Article
Title: Supported intermetallic PdZn nanoparticles as bifunctional
catalysts for the direct synthesis of dimethyl ether from CO-rich
synthesis gas
Authors: Manuel Gentzen, Dmitry E. Doronkin, Thomas L. Sheppard,
Anna Zimina, Haisheng Li, Jelena Jelic, Felix Studt, Jan-Dierk
Grunwaldt, Jörg Sauer, and Silke Behrens
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906256
Angew. Chem. 10.1002/ange.201906256
Link to VoR: http://dx.doi.org/10.1002/anie.201906256
http://dx.doi.org/10.1002/ange.201906256

Angewandte Chemie International Edition
10.1002/anie.201906256
COMMUNICATION
Supported Intermetallic PdZn Nanoparticles as Bifunctional
Catalysts for the Direct Synthesis of Dimethyl Ether from
CO-rich Synthesis Gas
[
a]
[a,b]
[a,b]
[a,b]
Manuel Gentzen, Dmitry E. Doronkin,
Thomas L. Sheppard,
Anna Zimina,
[
a]
[a]
[a,b]
[a,b]
[a]
Haisheng Li, Jelena Jelic, Felix Studt,
Jan-Dierk Grunwaldt,
Jörg Sauer, and Silke
[
a]
Behrens*
Abstract: The single-step syngas-to-dimethyl ether (STD) process
entails economic and technical advantages over the current
industrial two-step process. Pd/ZnO-based catalysts have recently
Conventional Cu/ZnO-based methanol catalysts have been one
focus of study, however these systems can be prone to
deactivation which drives the search for new catalysts.
[
4]
emerged as interesting alternatives to currently used Cu/ZnO/Al
2
O
3
Recently, Pd-based intermetallic compounds have emerged as
interesting alternatives providing improved methanol selectivity
and thermal stability. Pd/ZnO systems in particular have been
catalysts, but the nature of the active site(s), the reaction mechanism
and the role of Pd and ZnO in the solid catalyst are not well
established. Here Zn-stabilized Pd colloids with a size of 2 nm
served as the key building blocks for the methanol active component
[
5]
successfully employed in methanol steam reforming (MSR)
[
6]
and the water-gas-shift (WGS) reaction, and have recently
proven to be a suitable methanol synthesis component for the
in bifunctional Pd/ZnO--Al
2
O
3
catalysts. The catalysts were
[
3b,7]
characterized by combining high-pressure operando X-ray
absorption spectroscopy and density functional theory calculations.
The enhanced stability, longevity and high dimethyl ether selectivity
STD process in combination with a solid acid.
The activity of
these catalysts was ascribed to the formation of intermetallic
[
3b,6,8]
PdZn during the reductive catalyst pretreatment.
observed makes Pd/ZnO--Al
the STD process compared to Cu/ZnO/-Al
2
O
3
an effective alternative system for
While Pd/ZnO-based catalysts have been investigated
experimentally and theoretically, e.g. for MSR and WGS, the
exact nature of the active site(s) and the reaction mechanism for
bifunctional Pd/ZnO based STD catalysts is not well defined,
2 3
O .
Dimethyl ether (DME) is industrially produced via dehydration of
methanol derived from synthesis gas (CO/H ) with a global
production of 5 Mtons/year. This two-step process typically
[3b,7,9]
and depends strongly on the synthesis route employed.
2
Here we focus on the colloidal synthesis of highly uniform
Pd/Zn-based nanoparticles (NPs). This method allows for evenly
distributed metal content, sizes and shapes, which are ideal for
mechanistic investigation using complementary spectroscopy,
[
1]
employs Cu/ZnO/Al
solid acids (e.g. -Al
2
O
3
catalysts for methanol synthesis and
3
or zeolites) for methanol dehydration.
2
O
The development of an alternative single-step STD process is
highly desirable as it entails economic and technical
advantages, e.g. cost saving by process simplification and
higher syngas conversions. This necessitates the design of
suitable bifunctional catalysts with high methanol synthesis and
dehydration activity. Recent efforts have involved preparing
[10]
and computational modeling. We show how Pd/Zn catalyzes
the hydrogenation of CO to DME by employing high-pressure
operando X-ray absorption spectroscopy (XAS), in conjunction
with density functional theory (DFT) calculations.
Uniform Pd/Zn-based NPs were synthesized by reacting
2 2
Pd(acac) and Et Zn, yielding particles with the Pd core in close
[
1]
[
2]
physical mixtures of the two components
as well as
which couple the two sequential
reactions on active sites in close proximity to each other.
contact to the Zn-containing, organic shell without the need for
[
3]
bifunctional catalysts
0
additional ligands or surfactants (see SI). Since Pd promotes
the formation of methane over methanol, it is critical to avoid the
0
0
2+
formation of Pd via reactive Pd -Zn interfaces by ensuring the
sufficient supply of Zn atoms during catalyst preparation and
activation. This synthesis concept is versatile and provides
various NP building blocks for the methanol active
[
a]
Dr. M. Gentzen, Dr. D. E. Doronkin, Dr. T. L. Sheppard, Dr. A.
Zimina, Dr. H. Li, Dr. J. Jelic, Prof. F. Studt, Prof. J.-D. Grunwaldt,
Prof. J. Sauer, Dr. S. Behrens
Institute of Catalysis Research and Technology
Karlsruhe Institute of Technology (KIT)
[
10a-c]
component.
TEM images revealed highly uniform Pd/Zn
NPs of 2.0 ± 0.4 nm diameter (Fig. 1a). The molar Pd:Zn ratio of
the NP powder was 1:2.9, in agreement with the
Herrmann-von-Helmholtz-Platz 1, 76344 Eggenstein-
Leopoldshafen, Germany
E-mail: silke.behrens@kit.edu
Dr. D. E. Doronkin, Dr. T. L. Sheppard, Dr. A. Zimina, Prof. Felix
Studt, Prof. J.-D. Grunwaldt
Institute for Chemical Technology and Polymer Chemistry
Karlsruhe Institute of Technology (KIT)
Engesserstr. 20, 76131 Karlsruhe, Germany
Pd(acac)
-Al and calcination, two bifunctional STD catalysts with Pd
loadings of 8.4 wt.-% (Pd/Zn (8)--Al ) and 14.3 wt.-% (Pd/Zn
14)--Al ) were obtained (see SI (Table S1) for elemental
composition, specific surface area, number of acid sites).
2 2
:Et Zn ratio used during synthesis. After deposition on
[b]

2 3
O
2 3
O
(
2 3
O
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201906256
COMMUNICATION
1.4
1.2
1.0
0.8
0.6
a
a
Pd/Zn (8)--Al O3
2
Pd/Zn (14)--Al O3
2
Cu/Zn (12)--Al O3
2
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0.4
250
260
270
280
290
300
Temperature (°C)
70
0
1
2
3
4
5
NP diameter (nm)
b
60
50
40
equilibrium conversion
PdO ZnO PdZn
b
calcined
Pd/Zn (8)--Al
2
O
3
30
20
Pd/Zn (14)--Al
2
O
3
Cu/Zn (12)--Al
O
2 3
Pd/Zn (8)--Al O3
2
10
Pd/Zn (14)--Al O3
2
PdZn
GGW
spent
0
Pd/Zn (8)--Al
Pd/Zn (14)--Al
-Al
2
O
3
250
260
270
280
290
300
Temperature (°C)
2
O
3
c

2
O
3
80
60
40
20
40
60
80
SDME
2
 / °
Figure 1. a) TEM image of the Pd/Zn NPs (Inset: Particle size distribution). b)
XRD pattern of the calcined and spent Pd/Zn-based STD catalysts (Inset:
PdZn structure (P4/mmm; Pd blue, Zn red).
SCO2
XCO
20
SMeOH
0
X-ray diffraction (XRD) of the calcined STD catalysts showed
reflections of PdO, Pd, ZnO and -Al O (Fig. 1b) and no
2 3
50
100
150
200
250
TOS (h)
reflections from mixed oxides or alloyed species. Following
catalytic tests, the spent catalysts revealed reflections at 41.2°
Figure 2. a) Reaction rates and b) CO conversions for STD catalysts derived
from Pd/Zn and Cu/Zn NPs (50 bar, H :CO = 1, 70 vol.-% inert gases). c)
Conversion and selectivity with TOS (250 °C) (Pd/Zn (8)--Al ).
2
0
and 44.1° (2) characteristic of the L1 PdZn phase, respectively
2
O
3
0
and no Pd reflections (Fig. 1b). Topotactic alloy formation
involves considerable reduction of ZnO in close contact to a
2
noble metal capable of dissociative H adsorption (i.e., Pd).
PdZn alloys showed good selectivity to DME while maintaining
Since NP synthesis afforded a molar Pd/Zn ratio of 1:3, sufficient
Zn was available for complete formation of an intermetallic PdZn
high activity (SDME 65 % at 250 °C - 270 °C, SDME 62 % at 300
°C). CH
4
formation was negligible between 250 °C and 280 °C
2+
phase. Residual Zn species remained as ZnO in the spent
STD catalyst. According to the Scherrer equation, particle sizes
were 8-11 nm for PdO and 16 nm for PdZn in the calcined and
spent catalysts, respectively. TEM and SEM images of the
calcined and spent catalysts revealed Pd/Zn-based NPs well
(
SCH4 < 1 %) and increased only slightly to 2.4 % at 300 °C. In
contrast, Pd
selectivity.
2
10c]
Ga NPs showed a lower DME and a higher CH
4
[
The ratio of methanol to dehydration catalyst is
[
11]
also known to influence catalytic performance.
the reaction rate normalized to amount of Pd (in mol) was
superior for Pd/Zn (8)--Al compared to Pd/Zn (14)--Al
illustrating the influence of well-balanced catalyst functionalities.
Pd/Zn (14)--Al revealed SMeOH of 9 % (250°C) indicating
incomplete methanol dehydration, probably due to the lower -
Al content (SMeOH 3 % for Pd/Zn (8)--Al ) (Tables S1, S2;
Fig. S2). Therefore, less methanol was withdrawn from the
equilibrium, and lower reaction rates compared to Pd/Zn (8)--
Al
Interestingly,
distributed over the Al
O
2 3
dehydration catalyst (Fig. S1)
O
2 3
2 3
O
facilitating close proximity between the consecutive catalytic
functions of methanol synthesis and dehydration.
The catalytic properties of the bifunctional PdZn catalysts
in the STD reaction were determined in a fixed-bed reactor
2 3
O
O
2 3
2 3
O
(
50 bar, 250 °C - 300 °C, 70 vol.-% inert gas (Ar/N
using syngas with a H :CO composition of 1:1, simulating that
derived from renewable biomass feedstock. For Pd/Zn (8)--
Al , CO conversion increased with temperature until
2
)) (Table S2)
2
2 3
O were observed. Compared to the Cu/Zn-based reference
O
2 3
a
catalyst prepared via a similar synthetic approach, Pd/Zn
maximum at 290 °C (XCO 37 %) (Fig. 2b). At higher
temperatures, thermodynamic equilibrium constraints become
more important, resulting in decreased CO conversion of 34 %
at 300 °C.
catalysts showed enhanced CO turnover rates and less CH
4
[10a]
formation (Fig. 2a; Fig. S2).
Most notably, the Pd-based
catalysts showed excellent stability over time on stream (TOS)
Fig. 2c), with CO conversion and DME selectivity remaining
(
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Angewandte Chemie International Edition
10.1002/anie.201906256
COMMUNICATION
a
b
Pd K edge
0 °C
45 °C
6 °C
108 °C
67 °C
Zn K edge
constant after 270 h. After testing the catalyst at temperatures
up to 300 °C, XCO was reproducible at 250 °C. In contrast, the
2
6
Cu/Zn-based
deactivation (Fig. S3).
reference
catalyst
revealed
progressive
1
[
10a]
Compared to Cu, both higher thermal
226 °C
Pd
2
4
1
1
2
2
0 °C
8 °C
00 °C
52 °C
00 °C
50 °C
stability of the intermetallic PdZn compound and higher
hydrogenation activity of Pd may prevent particles sintering and
deposition of carbonaceous species under these conditions,
respectively.
2
4390
24420
ZnO
9680
Energy (eV)
2
4330
24360
24390
24420
9660
9700
Formation of the active catalyst phase under reducing H
atmosphere was monitored by in situ XAS at pressures up to
0 bar. XAS spectra (Fig. 3a,b) recorded at room temperature
approx. 3 min after switching to reducing H flow showed
2
Energy (eV)
c
Pd K edge
d
Zn K edge
Pd/Zn (8)--Al
2
O
3
reacted
2 3
Pd/Zn (8)--Al O reduced
2
Pd/Zn (8)--Al O3 reduced
Pd/Zn (8)--Al O3 calcined
2
2
Pd-Pd (metal)
Pd/Zn (8)--Al
2
2
O
3
calcined
Zn-Zn (Metal)
Zn-O
complete reduction of PdO in the calcined catalyst (Fig. 3c),
while ZnO remained oxidized (Fig. 3b, d) (reference spectra, Fig.
Zn
Pd-O
Pd-Pd (oxide)
Zn-Zn (oxide)
Pd
S3). The reduction of PdO
been previously reported for Pd-based catalysts.
absorption near edge structure (XANES) spectrum (20 °C,
% H /He) showed approx. 0.8 eV shift of the rising edge to
x 2
NPs with H at low temperatures has
[
12]
The X-ray
ZnO
PdO
0
1
2
3
4
5
0
1
2
3
4
5
5
2
Radial distance (Å)
Radial distance R (Å)
higher energies and shifts of all maxima to lower energies
compared to the Pd reference spectrum (Fig. 3a). Since no
0
Figure 3. XANES spectra measured during in situ TPR experiments at a) Pd
K- and b) Zn K-edges (RT to 250 °C; 5 % H /He; heating rate 1 °C/min) (Pd/Zn
8)--Al ). FT k²-weighted EXAFS spectra (not corrected for the phase shift)
measured during in situ XAS experiments at c) Pd K-and d) Zn K-edges after
calcination, reduction at 250 °C.
2
reduction of Zn occurred at 20 °C, the observed shifts of the
XANES features of Pd NPs could be attributed only to formation
(
2 3
O
[
13]
of PdH
x
hydrides.
While extended X-ray absorption fine
structure (EXAFS) is not sensitive to light atoms like hydrogen,
Pd K-edge XANES is prone to changes in the structure of
unoccupied electronic states and is thus affected by the
to Pd-Pd CN ratio remained 2:1, in good agreement with the
intermetallic bulk structure (Fig. 1). Hence, the intermetallic
PdZn phase remained stable during the STD process. A similar
formation of PdH
hydrogen and palladium.
x
due to mixing of unoccupied states of
[
14a]
The observed shifts of the XANES
[
16]
features and the difference spectrum compared to metallic Pd
decrease in CN was reported by Tao et al.
for a pure Pd
[
14]
(Fig. S4c) are consistent with the formation of PdH
x
NPs.
catalyst under CO and attributed to the formation of small
clusters on the catalyst surface. The CN decrease observed
herein suggests similarly an increase in dispersion or an
increase in surface area. DFT calculations were performed on
(adsorbate-induced) Pd segregation to investigate the surface
structure under reaction conditions. They strongly indicate that
the stability of the PdZn intermetallic compound prevents
segregation of Pd, even in the presence of CO and OH (Table
S4; Fig. S7).
They disappeared at 45 °C confirming decomposition of PdH
x
to
0
Pd species (inset in Fig. 3a; Fig. S4d). The Pd K-edge position
continuously shifted to lower energies during heating, probably
caused by alloying of Pd with Zn via spillover of hydrogen from
[
13]
PdH
x
NPs to the neighbouring ZnO species.
No significant
changes in the Pd K spectra occurred above 180 °C (Fig. S5a).
Linear Combination Analysis (LCA, Fig. S5b) of the Zn K-edge
0
0
XANES spectra showed that a Pd :Zn ratio of 1:1 was reached
at 180 °C. The EXAFS region in the Pd K-edge spectra exhibited
pronounced changes until 250 °C, indicating alloying with Zn
DFT calculations furthermore confirmed prominent reaction
paths for a high activity of PdZn alloys in CO hydrogenation to
(
Fig. 3a). Gradual reduction of Zn was also observed during
0
methanol, similar as for Cu. For this purpose, a PdZn L1 slab
[
15]
activation up to 250 °C (Fig. 3b), where 31 % (±10 %) of ZnO
was reduced to Zn . Following activation, the atomic Zn to Pd
ratio was 1.3:1, suggesting the formation of alloyed PdZn NPs
with an excess of Zn species, probably on the surface of PdZn,
favored by lower surface energy of Zn. This is further supported
by the Fourier-transformed (FT) EXAFS (Fig. 3c, d). At the Pd K-
edge, EXAFS revealed bond distances of 2.58 ± 0.03 Å (Pd-Zn)
and 2.76 ± 0.03 Å (Pd-Pd) for the activated STD catalyst
model with (111) termination was used to calculate adsorption
energies and transition states of the CO hydrogenation reaction
(Fig. 4a, b). Calculations of PdZn(111) (red line) are compared
to earlier calculations of CO hydrogenation on Cu(111) (black
0
0
0
0
[
17]
line) and Cu(211) (blue line) surfaces.
Although these
[
18]
calculations were performed with a different functional (RPBE)
[
19]
than that used here (BEEF-vdW),
we note that calculations
comparing RPBE and BEEF-vdW for CO hydrogenation on
[
20]
indicating the formation of the PdZn phase during H
2
activation
Cu(211) revealed almost identical results.
As shown in Fig.
[
8]
(Figure S6, Table S3).
4a, the strength with which intermediates and transition states
are bound to the PdZn surface are strikingly similar to those
calculated for Cu(211), which is believed to be the active site in
The STD process was then studied at high pressure using
operando XAS and reaction products monitored by on-line mass
spectrometry (Fig. S4f). Spectra at Pd K edge were measured
after the formation of methanol and DME had stabilized. The
coordination numbers (CN, Table S3, Figure S6) decreased
from 7.1 (Pd-Zn) and 3.6 (Pd-Pd) in the H
5
[
17,21]
CO hydrogenation.
In an attempt to understand why PdZn
surfaces have common properties to that observed for Cu, we
investigated the d-projected density of states (DOS) and the
corresponding d-band center often used as a descriptor of
2
-activated catalyst to
.5 (Pd-Zn) and 2.6 (Pd-Pd) during DME synthesis. The Pd-Zn
[
22]
surface reactivity. The d-projected DOS of PdZn is compared
with the DOS of Cu(111) and Pd(111) in Fig. 4c. The d-states of
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Angewandte Chemie International Edition
10.1002/anie.201906256
COMMUNICATION
Acknowledgements
M.G. acknowledges financial support of the Helmholtz Research
School “Energy-Related Catalysis”. H.L., J.J. and F.S. gratefully
acknowledge support by the state of Baden-Württemberg
through bwHPC (bwunicluster and JUSTUS, RV bw16G001 and
bwl17D011). We thank the CAT-ACT beamline at the
synchrotron radiation source at KIT for beamtime and Dr. T.
Prüßmann for support during measurements. The STN and SCI
programs are acknowledged for financial support of the high
pressure XAFS infrastructure. We thank D. Neumann-Walter
and Dr. T. Otto for support with BET measurements and Dr. Ch.
Kübel, Institute of Nanotechnology, and the Karlsruhe Nano
Micro Facility (KNMF, KIT) for HRTEM.
Keywords: Single-step dimethyl ether synthesis • Pd/Zn
nanoparticles • bifunctional catalysts • operando XAS • density
functional theory calculations
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Figure 4. a) Gibbs-free energy diagram for CO hydrogenation on Cu(111),
Cu(211) and PdZn(111)-L1
p(H )=40 bar, p(CO)= 10 bar and p(CH
and Cu(211) are taken from ref. [16], b) PdZn(111)-L1
grey, Zn purple, O red, C brown), c) DOS calculated for Cu(111), Pd(111) and
PdZn(111)-L1 . Calculated d band centers are marked with dark red line.
0
. Gibbs free energies are calculated for T=500 K,
OH)= 1 bar. Values for the Cu(111)
surface model (Pd light
[7]
2
3
0
[
[
[
8]
9]
115, 8457-8465.
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Pd(111) are not fully filled, resulting in a d-band center of -1.73
[
23]
eV, similar to earlier reports. For PdZn, however, the d-band
is now completely filled and shifted downwards resulting in a d-
band center of -2.32 eV. This is again strikingly similar to the -
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[
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even closer; -2.35 eV, Fig. S8). The local density of states of Pd
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Angewandte Chemie International Edition
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Dr Manuel Gentzen, Dr Dmitry E.
Doronkin, Dr Thomas L. Sheppard, Dr
Anna Zimina, Dr Haisheng Li, Dr Jelena
Jelic, Prof Felix Studt, Prof Jan-Dierk
Grunwaldt, Prof Jörg Sauer, Dr Silke
Behrens*
Zinc-stabilized palladium colloids with a size of about 2 nm are the key building
units for bifunctional syngas-to-dimethyl ether catalysts with enhanced stability,
longevity and high dimethyl ether selectivity. The catalysts were characterized by
combining high-pressure operando X-ray absorption spectroscopy and density
functional theory calculations.
Page No. – Page No.
Supported intermetallic PdZn
nanoparticles as bifunctional
catalysts for the direct synthesis of
dimethyl ether from CO-rich
synthesis gas
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