Visualizing Formation of Intermetallic PdZn in a Palladium/Zinc Oxide Catalyst: Interfacial Fertilization by PdHx

Article abstract of DOI:10.1002/anie.201812292

Controllable synthesis of well-defined supported intermetallic catalysts is desirable because of their unique properties in physical chemistry. To accurately pinpoint the evolution of such materials at an atomic-scale, especially clarification of the initial state under a particular chemical environment, will facilitate rational design and optimal synthesis of such catalysts. The dynamic formation of a ZnO-supported PdZn catalyst is presented, whereby detailed analyses of in situ transmission electron microscopy, electron energy-loss spectroscopy, and in situ X-ray diffraction are combined to form a nanoscale understanding of PdZn phase transitions under realistic catalytic conditions. Remarkably, introduction of atoms (H and Zn in sequence) into the Pd matrix was initially observed. The resultant PdH_x is an intermediate phase in the intermetallic formation process. The evolution of PdH_x in the PdZn catalyst initializes at the PdH_x/ZnO interfaces, and proceeds along the PdH_x ?111? direction.

Full text of DOI:10.1002/anie.201812292

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201812292
German Edition: DOI: 10.1002/ange.201812292
Intermetallic Nanoparticles
Visualizing Formation of Intermetallic PdZn in a Palladium/Zinc Oxide
Catalyst: Interfacial Fertilization by PdHx
Yiming Niu+, Xi Liu+, Yongzhao Wang, Song Zhou, Zhengang Lv, Liyun Zhang, Wen Shi,
Yongwang Li, Wei Zhang,* Dang Sheng Su,* and Bingsen Zhang*
Abstract: Controllable synthesis of well-defined supported
intermetallic catalysts is desirable because of their unique
properties in physical chemistry. To accurately pinpoint the
evolution of such materials at an atomic-scale, especially
clarification of the initial state under a particular chemical
environment, will facilitate rational design and optimal syn-
thesis of such catalysts. The dynamic formation of a ZnO-
supported PdZn catalyst is presented, whereby detailed
analyses of in situ transmission electron microscopy, electron
energy-loss spectroscopy, and in situ X-ray diffraction are
combined to form a nanoscale understanding of PdZn phase
transitions under realistic catalytic conditions. Remarkably,
introduction of atoms (H and Zn in sequence) into the Pd
matrix was initially observed. The resultant PdHx is an
intermediate phase in the intermetallic formation process.
The evolution of PdHx in the PdZn catalyst initializes at the
PdHx/ZnO interfaces, and proceeds along the PdHx h111i
direction.
a lack of convincing evidence about the key elementary steps
associated with the intermetallic process; thus, mechanistic
clarification and rational design of catalysts is hindered.
Firstly, PdZn formation was investigated under ultrahigh-
vacuum conditions, based on some model systems, in which
the interaction between deposited Zn layers and the single-
crystal Pd surface was studied at elevated temperatures in the
absence of a gaseous atmosphere.[8–10] According to the
(sub)surface structural evolution of PdZn, determined by
surface-sensitive techniques, it was determined that the
deposited Zn layer starts to diffuse into Pd (111) at about
350 K, followed by formation of a 1:1 PdZn multilayer
intermetallic at about 500 K because of further diffusion of
Zn into the Pd bulk. Similar processes also took place on a Pd
(110) surface, but at higher temperatures.
However, considering the very different chemical envi-
ronment and material gap between the model and real
systems, a study on technologically applied PdZn supported
catalyst under working conditions is necessary to gain
insightful understanding regarding the real intermetallic
process. Based on in situ X-ray absorption spectroscopy
(XAS) investigations, it was reported that the intermetallic
process was initiated from the surface of Pd nanoparticles
(NPs) and continued inwards during MSR.[11] But details
about the microstructural evolution of the PdZn intermetallic
at nano or atomic scales remains challenging. Furthermore,
the role of H2 in formation of the bimetallic alloy has not been
rationalized. Visualization of the formation of the active
species with high spatial resolutions will contribute to
fundamental studies as well as practical applications. To
date, rapid development of transmission electron microscopy
(TEM) and complementary techniques, such as in situ gas-
I
ntermetallic materials with well-defined structures and
tunable compositions afford unique catalytic properties, and
are thus highly desirable for heterogeneous catalysis.[1] For
instance, L10-type PdZn with lattice parameters of a = b =
4.0915 ꢀ and c = 3.3426 ꢀ in space group P4/mmm, is
regarded as a promising catalyst for methanol steam reform-
ing (MSR), the water–gas shift reaction (WGS), alkane
dehydrogenation, and selective hydrogenation reactions.[2–7]
The excellent catalytic performance of intermetallic PdZn is
derived from reduction of the ZnO-supported Pd, which give
rise to unique electronic properties and structural stability.
However, despite enormous efforts to acquire supported
PdZn intermetallic catalysts in a controllable fashion, there is
[*] Y. Niu,[+] Y. Wang, Dr. L. Zhang, W. Shi, Prof. D. S. Su, Prof. B. Zhang
Shenyang National Laboratory for Materials Science, Institute of
Metal Research, Chinese Academy of Sciences
Shenyang 110016 (China)
S. Zhou
School of Chemistry and Chemical Engineering
University of Chinese Academy of Sciences
Beijing, 100049 (China)
E-mail: dangsheng@fhi-berlin.mpg.de
bszhang@imr.ac.cn
Y. Niu,[+] Y. Wang, W. Shi
School of Materials Science and Engineering
University of Science and Technology of China
Shenyang 110016 (China)
Prof. W. Zhang
School of Materials Science & Engineering, Electron Microscopy
Center, and Key Laboratory of Automobile Materials MOE
Jilin University
Changchun 130012 (China)
E-mail: weizhang@jlu.edu.cn
Dr. X. Liu,[+] S. Zhou, Dr. Z. Lv, Prof. Y. Li
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry
Chinese Academy of Sciences
Taiyuan 030001 (China)
and
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201812292.
SynCat@Beijing, Synfuels China Technology Co., Ltd.
Beijing 101407 (China)
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heating and electron energy-loss spectroscopy (EELS),
facilitate direct detection of solid–gas interactions at nano-
scales, and some promising in situ TEM results regarding
Fischer–Tropsch synthesis, the low-temperature WGS reac-
tion, and other catalytic process, have been reported.[12–21]
To the best of our knowledge, herein we present the first
atomic-scale visualization of a complete phase transition from
Pd/PdHx to PdZn on a ZnO support under H2 atmosphere,
which has been systematically analyzed by integrating in situ
TEM, EELS, and X-ray diffraction (XRD) data. Significantly,
the interfacial PdHx species were generated quickly under an
H2 atmosphere and at elevated temperature these species
were identified as key intermediates in the following trans-
formation. Subsequently, a consecutive phase transition
started at the PdHx/ZnO interface at higher temperature,
then proceeded along the PdHx h111i direction until the
whole NP was converted to the PdZn structure. The acetylene
partial hydrogenation reaction was selected as a fingerprint
reaction to examine the catalytic activity of the formed PdZn
in comparison to monometallic Pd catalysts.
Figure 1. a) In situ XRD patterns of a Pd/ZnO sample under H2
atmosphere during heating treatment across a temperature range.
b) Selected in situ XRD patterns related with the evolution of PdHx
species at elevated temperatures ranging from 508C to 1308C. c) Four
kinds of distinct Pd-based species during the phase-transition process.
d) Selected in situ XRD patterns related with the phase-transition
process from PdHx to PdZn above 2608C.
ZnO nanorods with an approximate 15.6 nm diameter
prepared by a surfactant-assisted alcohol thermal method[22]
were chosen as supports to uniformly anchor Pd NPs with
controllable size, while serving as a potential Zn source. The
surfactant on ZnO nanorods was removed by calcination at
4008C in air for 2 h. After calcination, the ZnO nanorods
(Supporting Information, Figure S1) remained as a wurtzite
phase (Figure S2). Subsequently, Pd was decorated on the
calcined ZnO support through a wet impregnation method. A
subsequent calcination process in static air at 4008C was
performed to produce oxidized Pd NPs with a uniform size
distribution. XRD results (red curve in Figure S3) confirm
that Pd NPs have been completely oxidized into PdO NPs
(JCPDS 85-0624). PdO NPs are uniformly dispersed upon
well-preserved ZnO nanorods (Figure S4a) and the size
distribution of PdO NPs ranges from 2.0 to 8.0 nm with an
average size of approximately 4.1 nm (Figure S4a inset).
Temperature-programmed reduction mass spectrometry
(TPR-MS) was conducted to determine the phase-transition
temperatures and the variation of gas-phase composition
during the PdO/ZnO reduction process. The mass signals at
m/e = 2 and 18 representing H2 and H2O were detected and
normalized as a function of time (Figure S5a), respectively.
With increasing temperature, a sharp negative peak associ-
ated with the H2 signal appeared at about 908C, concomitant
with a corresponding positive peak associated with the H2O
signal at 1008C. At higher temperature, another broad
positive peak corresponding to the H2O signal emerged at
2608C and ended at 4008C. Based on additional ex situ XRD
results (Figure S3), we conclude that metallic Pd and PdZn
were formed consecutively in the two processes, respectively.
In situ XRD experiments were performed to gain insights
into the evolution of the crystalline structure under 1 bar of
H2 pressure and at elevated temperature. As shown in
Figure 1a, Pd transformed into b-PdH NPs under an H2
atmosphere at 508C. As the temperature increased, the
b-PdH phase disappeared gradually while the a-PdH
remained and finally transformed into a PdZn phase at
3408C. The crystal structure of ZnO was well-preserved
during the process. The hydrogen content in PdH was
determined from peak positions through Vegardꢁs law.[23] It
was observed that there were two PdH peaks at 508C,
indicating that there were two phase structures of PdH NPs
assigned to PdH0.9 (45.38) and PdH0.6 (46.18). When the
temperature was increased, the peak of PdH0.9 gradually
weakened and disappeared at 1308C, but the peak associated
with PdH0.6 shifted to PdH0.1 (46.78) and remained until 2608C
(Figure 1b). At about 2708C, the PdH0.1 phase started to
transform into PdZn and the process completed at 3408C
(Figure 1d). The in situ XRD experiments explicitly revealed
the structural evolution and identified the PdHx phase acting
as an intermediate phase for the formation of intermetallic
PdZn (Figure 1c).
To record the structural evolution and the hydrogen
distribution in Pd NPs during the intermetallic process at
nanoscales, in situ TEM experiments were carried out using
a commercial gas-heating holder with a homemade gas supply
system (Supporting Information) under various conditions.
According to the aforementioned in situ XRD experiments
and previous investigations, the PdH structure exhibits
a larger lattice parameter (a = b = c = 4.035 ꢀ) compared to
Pd (3.891 ꢀ). Thus, Pd NPs kept in H2/He (10 vol%) and pure
He atmospheres at 1008C were studied using in situ TEM.
High-resolution (HR)TEM images of the Pd/ZnO sample
under these two atmospheres (Figures 2a,b) were compared
in terms of lattice constants. Therein, Pd (111) was chosen as
a typical plane for studying lattice-constant differences. The
average d-spacing of the NP was measured as 2.24 ꢀ under
a He atmosphere (Figure 2c), which was indexed as the Pd
(111) plane. After introducing a H2 atmosphere to replace
pure He, the average d-spacing of the identical NP at the same
temperature expanded to 2.30 ꢀ (Figure 2c). The induced
variation is attributed to the incorporation of H into the
Pd NPs. Considering that the atomic ratio of palladium
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reciprocal spacings of PdHx (111) planes (red circle in
Figure S7) for NPs under H2 atmosphere verify expansion
of the lattice parameter. These results confirm that the PdHx
NPs were formed under a H2/He (10 vol%) atmosphere at
1008C and are consistent with the in situ XRD results.
Previous studies have demonstrated that interfacial met-
allic atoms exhibit different properties compared with those
away from the interface.[24–26] In this case, two kinds of Pd
hydride species (for example, PdH0.9 and PdH0.6) appeared
during in situ XRD experiments. Considering the uniform
size distribution of Pd NPs, it is reasonable to assume that H
may display an inhomogeneous distribution in Pd NPs
supported on ZnO and the interfacial Pd may have a stronger
capability for binding with H. To verify the hypothesis, the
formed hydride was analyzed using EELS to obtain spectro-
scopic evidence (Figure 3; Figure S10). Previous investiga-
tions have demonstrated that Pd exhibits a red shift from
approximately 7 to 5 eV in its plasmon resonance after
hydrogen absorption.[27–29] Thus, we used the peak at about
5 eV as the fingerprint assigned to PdHx formation in the
following EELS measurements, whereas quantitatively iden-
tifying the hydrogen content in Pd NPs under H2 atmosphere
is still challenging in a real system. EEL spectra of a hydride
particle were acquired at 508C under He atmosphere, which
had formed at 1008C under H2 atmosphere. The EELS signal
from 2–50 eV was recorded across the Pd NP to the ZnO
support, as shown in Figure 3a. No PdHx signals were
observed in the area (curves a and b in Figure 3b) away
from the interface, between Pd NP and ZnO. On approaching
Figure 2. HRTEM images of the same Pd NP on ZnO in a) He and
b) H2/He (10 vol%), which has been turned into PdHx; both images
were taken from the [011] zone axis. c) Corresponding lattice distance
measurements of the (111) plane based on the HRTEM images shown
in (a) and (b), respectively. d) Superimposed FFTs corresponding to
Pd and PdHx in (a) and (b); the reciprocal spacings of Pd(11-1) (green
circles) and PdHx(11-1) planes (red circles). e) Atomic diagrams
describing the interplanar spacing of (111) planes in Pd (top) and
PdH (bottom); Pd (dark green balls) and H (white balls) atoms.
Working conditions: 700 mbar, flow rate 1.5 mLminꢀ1, 1008C, dose
rate 900 eꢀꢀ2 sꢀ1
.
hydride is elusive because of its close relevance to H2
pressure, the measured value is slightly less than that of
PdH (111) (2.33 ꢀ, JCPDS 65-0557). The generated palla-
dium hydride is denoted as PdHx (0 < x < 1). Such exper-
imental results agree with the theoretical interplanar spacing
of Pd and PdH (111) planes described by atomic diagrams
(Figure 2e). The superimposed local fast Fourier transforms
(FFTs) correlate with the HRTEM images (Figure 2d), and
the larger reciprocal spacings of Pd (11-1) planes (green circle
in Figure 2d) indicates a smaller d-spacing, and vice versa.
Selected area electron diffraction (SAED) was also con-
ducted for further confirmation (Figure S6), and HRTEM
images and superimposed FFTs (Figure S7) were included to
obtain an accurate comparison of the variations in the lattice
parameter. To get a broader picture of the structural
evolution, 32 sets of FFT spots were examined from the Pd
(111) plane of different Pd NPs under different atmospheres
(Figures S8 and S9). Similar to Figure 2d, the smaller
Figure 3. a) High-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) image of the PdHx/ZnO sample at
508C under He atmosphere after in situ reduction. b) EEL spectra of
different regions on the PdHx/ZnO sample as marked in (a).
c) HRTEM image of the PdZn/ZnO sample at 3008C under an H2
atmosphere; the Figure inset is the corresponding FFT. d) Zn and Pd
L-edge EEL spectra of the area of interest in Figure S10b.
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the interface area, the PdHx peak emerged at approximately
5 eV and gradually enhanced (Figure 3b; Figure S10a). For
the ZnO supports, no peaks were found in the EELS results
(curves f and g in Figure 3b). Combined with the aforemen-
tioned analysis, it is concluded that Pd atoms in the interface
area with ZnO have a stronger capability for binding with
hydrogen to form the PdHx structure. The PdZn structure was
acquired after 3008C reduction and corroborated by HRTEM
and EELS (Figures 3c,d).
The phase transition from PdHx to PdZn was investigated
using HRTEM. According to the aforementioned XRD
results, a Pd NP in an a-PdH state at elevated temperature
was chosen as the starting material. When the temperature
increased to 3008C, the PdZn phase appeared at the NP/ZnO
support interface, as indicated by the lattice d-spacing
ascribed to the PdZn composite and a lattice mismatch that
emerged at the PdHx/PdZn interface. Co-existence of PdHx
(11-1) (red lines) and PdZn (1-1-1) (green lines) planes was
evident in the HRTEM images (Figures 4a,b), which was also
demonstrated in the corresponding FFTs (Figures 4c,d). The
corresponding inverse FFTs (IFFTs), correlated with the
spots in Figures 4c,d, were conducted (Figures 4e,f) to obtain
the distribution of PdHx and PdZn phases at 20 and
40 minutes, respectively. For the phase transition after
20 minutes, the PdZn structure (green dots) occupied a rela-
tively small area at the edge of the PdHx structure (red dots).
For phase transition after 40 minutes, the green area invaded
the red area, and the PdHx structure was transformed into the
PdZn structure. The expanding green area corresponds to the
gradual growth of the PdZn phase at the expense of the PdHx
phase. The shared lattice plane (white lines in the HRTEM
images, Figures 4e,f) of PdZn and PdHx phases were ascribed
to PdHx (11-1), indicating that the growth direction of PdZn
was parallel to PdHx h111i, as demonstrated by the atomic
model in Figure S12. This is consistent with previous studies,
which indicate that Zn atoms diffuse more readily into Pd for
the formation of the PdZn structure on the Pd (111) plane
than on the Pd (110) plane.[9,10,30] Therefore, the PdHx
structure was initially generated under H2 atmosphere and
acted as an intermediate during the phase transition. Zn
atoms that are reduced from ZnO by interstitial H in PdHx
preferentially diffuse into PdHx NPs along the PdHx h111i
direction to form the PdZn phase as H atoms are removed in
the form of water. The process is illustrated in Figure 5.
Although the electron dose was controlled to relatively
low levels (below 900 eꢀꢀ2 sꢀ1) for in situ HRTEM inves-
tigation, electron-beam irradiation was nevertheless inevita-
ble and likely accounts for the few amorphous areas that
emerged on the Pd NP surface after long periods of observa-
tion by TEM, similar to previous ex situ TEM investiga-
tions.[31] Thus, an ex situ TEM image of Pd/ZnO was acquired
at the initialized state and at the end of this process
(Figure S15). The Pd and PdZn NPs are homogenously
dispersed in ZnO nanorods (Figures S15a,b). Particle size
distribution (PSD) histograms (Figures S15a,b insets) confirm
the uniform size distribution of supported Pd and PdZn NPs.
The microstructural features of Pd/ZnO and PdZn/ZnO
catalysts exhibited an evolution from face-centered cubic
(fcc) Pd to PdZn NPs with a tetragonal structure, according to
Figure 4. HRTEM images of an identical Pd/ZnO sample at 3008C
maintained under H2 atmosphere for a) 20 and b) 40 minutes. c,d) The
corresponding local FFTs are displayed; crystal planes assigned to
PdHx (red circles), the crystal plane indexed to PdZn (green circles).
e,f) Superimposed IFFTs formed using spots in the corresponding FFT
images shown in (c) and (d); PdHx (red), PdZn (green). White lines
indicate the PdHx/PdZn interface, which is confirmed to be PdHx
(11-1) planes. White arrows show the growth direction of PdZn, which
is perpendicular to the PdHx (11-1) planes.
HRTEM images (Figures S15c,d), in good agreement with the
in situ TEM (Figures 2–4) and XRD (Figure 1; Figure S3)
analysis. Thus, the possibility of phase transition induced by
electron-beam irradiation is excluded. The catalytic perfor-
mance of the PdZn structure was also tested and compared
with mono Pd catalysts (Pd/Al2O3; Figure S16) for the partial
hydrogenation of acetylene. The PdZn catalyst exhibited
superior selectivity and stability compared to Pd/Al2O3
catalyst (Figure S17), which confirmed the efficacy of the
preparative method as a basis for the in situ TEM study.
In summary, our work demonstrates the direct visual-
ization of the phase transition from Pd to PdZn on a ZnO
nanorod support at atomic scale under real chemical environ-
ments. The in situ XRD results provide structural information
for the transition from Pd to PdHx and PdZn, in sequence,
under H2 atmosphere. Combined with H2-TPR and in situ
TEM studies, H atoms diffuse into the Pd NP at a relatively
low temperature, leading to the formation of PdHx phases.
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We gratefully acknowledge financial support provided by the
National Natural Science Foundation of China (91545119,
21761132025, 21773269, 21673273, 21872163) and Youth
Innovation Promotion Association CAS (2015152).
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Keywords: EELS · heterogeneous catalysis · in situ TEM · PdHx ·
PdZn intermetallics
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Manuscript received: October 25, 2018
Revised manuscript received: December 5, 2018
Accepted manuscript online: January 16, 2019
Version of record online: && &&, &&&&
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Communications
Intermetallic Nanoparticles
A palladium catalyst supported on ZnO is
monitored in situ and in chemical envi-
ronments relevant to catalysis. Structural
evolution of PdZn intermetallic metal
nanoparticles (NPs), visualized at an
atomic scale, reveals the evolution of
a PdHx intermediary species at the inter-
face between PdHx and the ZnO support.
Y. Niu, X. Liu, Y. Wang, S. Zhou, Z. Lv,
L. Zhang, W. Shi, Y. Li, W. Zhang,*
D. S. Su,* B. Zhang*
&&&& — &&&&
Visualizing Formation of Intermetallic
PdZn in a Palladium/Zinc Oxide Catalyst:
Interfacial Fertilization by PdHx
Angew. Chem. Int. Ed. 2019, 58, 1 – 7
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