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Adsorption of ethylene on palladium, a key step in various catalytic reactions, may result in a variety of surface-adsorbed species and formation of palladium carbides, especially under industrially relevant pressures and temperatures. Therefore, the application of both surface and bulk sensitive techniques under reaction conditions is important for a comprehensive understanding of ethylene interaction with Pd-catalyst. In this work, we apply in situ X-ray absorption spectroscopy, X-ray diffraction and infrared spectroscopy to follow the evolution of the bulk and surface structure of an industrial catalysts consisting of 2.6 nm supported palladium nanoparticles upon exposure to ethylene under atmospheric pressure at 50 °C. Experimental results were complemented by ab initio simulations of atomic structure, X-ray absorption spectra and vibrational spectra. The adsorbed ethylene was shown to dehydrogenate to C2H3, C2H2 and C2H species, and to finally decompose to palladium carbide. Thus, this study reveals the evolution pathway of ethylene on industrial Pd-catalyst under atmospheric pressure at moderate temperatures, and provides a conceptual framework for the experimental and theoretical investigation of palladium-based systems, in which both surface and bulk structures exhibit a dynamic nature under reaction conditions.
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nanomaterials
Article
Dehydrogenation of Ethylene on Supported
Palladium Nanoparticles: A Double View from Metal
and Hydrocarbon Sides
Oleg A. Usoltsev 1, Anna Yu. Pnevskaya 1, Elizaveta G. Kamyshova 1,
Andrei A. Tereshchenko 1, Alina A. Skorynina 1, Wei Zhang 2, Tao Yao 2,
Aram L. Bugaev 1, * and Alexander V. Soldatov 1
1The Smart Materials Research Institute, Southern Federal University, 178/24 Sladkova,
344090 Rostov-on-Don, Russia; oleg-usol@yandex.ru (O.A.U.); annpnevskaya@yandex.ru (A.Y.P.);
kamyshova.liza@gmail.com (E.G.K.); tereshch1@gmail.com (A.A.T.); alinaskorynina@gmail.com (A.A.S.);
soldatov@sfedu.ru (A.V.S.)
2National Synchrotron Radiation Laboratory, University of Science and Technology of China,
Hefei 230029, China; zwei2319@mail.ustc.edu.cn (W.Z.); yaot@ustc.edu.cn (T.Y.)
*Correspondence: abugaev@sfedu.ru; Tel.: +7-863-305-1996
Received: 8 July 2020; Accepted: 18 August 2020; Published: 21 August 2020


Abstract:
Adsorption of ethylene on palladium, a key step in various catalytic reactions, may result
in a variety of surface-adsorbed species and formation of palladium carbides, especially under
industrially relevant pressures and temperatures. Therefore, the application of both surface and bulk
sensitive techniques under reaction conditions is important for a comprehensive understanding of
ethylene interaction with Pd-catalyst. In this work, we apply in situ X-ray absorption spectroscopy,
X-ray diraction and infrared spectroscopy to follow the evolution of the bulk and surface structure
of an industrial catalysts consisting of 2.6 nm supported palladium nanoparticles upon exposure
to ethylene under atmospheric pressure at 50
C. Experimental results were complemented by ab
initio simulations of atomic structure, X-ray absorption spectra and vibrational spectra. The adsorbed
ethylene was shown to dehydrogenate to C
2
H
3
, C
2
H
2
and C
2
H species, and to finally decompose
to palladium carbide. Thus, this study reveals the evolution pathway of ethylene on industrial
Pd-catalyst under atmospheric pressure at moderate temperatures, and provides a conceptual
framework for the experimental and theoretical investigation of palladium-based systems, in which
both surface and bulk structures exhibit a dynamic nature under reaction conditions.
Keywords: palladium nanoparticles; ethylene dehydrogenation; XANES; EXAFS; DRIFTS; DFT
1. Introduction
Palladium catalysts are being extensively used for hydrogenation of unsaturated hydrocarbons
(e.g., selective hydrogenation of acetylene traces in ethylene-rich mixtures) [
1
,
2
]; a large number of
studies was aimed to get structural insights into the catalytic processes [
3
10
]. An important step in
such reactions is the adsorption of hydrocarbon molecules on the palladium surface, since dierent
adsorption geometries can lead to dierent reaction products [1].
In contrast to the adsorption of ethylene and other hydrocarbons over platinum extensively
examined by the group of Zaera [
11
13
] and numerous other groups [
8
,
9
,
12
,
14
21
], less attention has
been paid to ethylene on palladium [
8
,
9
,
15
,
17
,
20
24
]. In most of these studies, infrared spectroscopy is
considered as a main source of information due to sensitivity to surface adsorbed species. Density
functional theory (DFT) was also widely applied to obtain the structure of intermediate species; however,
only a few studies exploited DFT to go beyond the qualitative analysis of infrared spectra towards
Nanomaterials 2020,10, 1643; doi:10.3390/nano10091643 www.mdpi.com/journal/nanomaterials
Nanomaterials 2020,10, 1643 2 of 13
their theoretical modeling [
18
,
25
28
]. For both Pd and Pt surfaces,
π
- and di-
σ
-bonded ethylene was
reported, with further transformation to ethylidyne which was considered as an important intermediate
in ethylene hydrogenation reaction [
22
]. The possibility of ethylidyne layers formation over the Pd (111)
surface at high pressure was also shown [
24
]. In the recent works of Bowker [
29
31
], it was highlighted
that under industrially relevant pressures on real catalysts, the processes are dierent from that on
ideal surfaces and high-vacuum conditions, which are typical for surface science studies. In addition,
palladium can easily form carbide phase in the presence of hydrocarbons [
29
,
31
43
], which aects its
catalytic properties. For this reason, the application of bulk-sensitive structural methods in addition to
surface-sensitive ones is important to understand the structural evolution of the metal.
In a number of previous works, we have successfully applied in situ and operando X-ray absorption
fine structure (XAFS) spectroscopy to follow the evolution of the catalyst under hydrogenation reaction
conditions [
32
,
33
,
36
,
38
,
44
46
]. The extended XAFS (EXAFS) spectra provide direct information on the
local atomic structure (interatomic distances and coordination numbers) around the element of interest
(averaged over all of its possible locations). The X-ray absorption near-edge structure (XANES) is
formed by the excitation of the core-level electron to the unoccupied states, being, therefore, sensitive
to the electronic structure of material. This was eciently used to discriminate palladium hydrides
and carbides [32,36,38,47,48], which is usually a problem for hard X-ray-based techniques.
Here, we report the combined experimental and theoretical study of the industrial catalysts
consisting of 2.6 nm supported palladium nanoparticles (NPs) probed by in situ EXAFS, XANES,
X-ray diraction (XRD) and diuse reflectance infrared Fourier transform spectroscopy (DRIFTS),
complemented by DFT calculations screening over a wide range of possible C
n
H
m
adsorption
intermediates. The use of complementary techniques allowed us to obtain a complete picture of the
process, describing the evolution of both the structure of the palladium particles and ethylene molecules
adsorbed on its surfaces. Beyond the standard fingerprint assignment of the spectroscopic features,
XANES and infrared spectra were theoretically calculated based on DFT-relaxed atomic structure.
This strategy allowed us to successfully follow the adsorption and dehydrogenation of ethylene on the
surface of nanoparticles, with its further decomposition and formation of palladium carbide.
2. Materials and Methods
2.1. Materials
Commercial Pd/C and Pd/Al
2
O
3
catalysts with average size of palladium particles was provided
by Chimet S.p.A. The detailed characterization the catalysts and the support was performed
elsewhere [4951]
. The metal loading was 5 wt.% for both samples and the average particle size was
2.6 nm with a small standard deviation of 0.4 nm. The choice of carbon support for synchrotron studies
was explained by the possibility of XRD data collection for Pd phase, since in case of alumina support,
the Pd reflections are overshadowed by those of alumina [
52
]. However, the carbon supported sample
produced very poor DRIFTS signal; therefore, alumina support was chosen for infrared studies.
2.2. In Situ XAFS and XRD Measurements and Analysis
In situ Pd K-edge XAFS and XRD measurements were performed simultaneously for the Pd/C
sample at BM31 beamline [
53
] of ESRF (Grenoble, France). The sample was loaded inside 2 mm
quartz glass capillaries and connected to a remotely controlled gas line. The gas blower located under
the sample was used to control the temperature. Prior to exposure to hydrocarbons, the samples
were activated in 20 mL/min flow of 20% H
2
/He at 125
C for 30 min and then purged by He flow.
This procedure was to remove palladium oxides formed after continuous exposure to air and ensure
the initial metallic state of palladium. Then, the sample was cooled down to 50
C and exposed to pure
ethylene flow (20 mL/min).
XAFS spectra were collected in transmission geometry using ionization chambers. The energy
was selected by Si (111) double-crystal monochromator operated in continuous scanning mode and
Nanomaterials 2020,10, 1643 3 of 13
detuned to 80% of the maximum intensity to reduce the contribution of higher harmonics. For energy
calibration, palladium foil was measured simultaneously with the sample using a 3rd ionization
chamber. XRD patterns were collected in Debye–Scherrer geometry using a Dexcela CMOS 2D detector.
The photon wavelength of 0.51105 Å was selected using a channel-cut Si (111) monochromator. The total
time needed for one XAFS and XRD measurement was about 10 min.
XAFS data processing (energy calibration, normalization, background removal) and first-shell
Fourier-analysis of EXAFS spectra were performed in Demeter software with standard parameters.
The fit was performed independently for each spectrum using first-shell interatomic distance (R
Pd-Pd
),
Debye–Waller factor (
σPd-Pd
), coordination number (N
Pd-Pd
), and zero energy shift (
E
0
) as fitting
variables. Theoretical analysis of the XANES spectra was performed in PyFitIt code [
54
], which included
principle component analysis (PCA) and fitting the experimental spectra by theoretical ones calculated
within the finite dierence method implemented in FDMNES code [
55
,
56
]. All calculations were
performed with relativistic corrections and the radius of the computational sphere of 5.2 Å.
2.3. In Situ DRIFTS Measurements
In situ DRIFTS measurements for Pd/Al
2
O
3
were performed on Vertex 70 spectrometer (Bruker,
Billerica, MA, USA) equipped with a highly sensitive liquid mercury telluride detector. The choice
of alumina support was determined by the better DRIFTS signal comparing to carbon support.
The Praying Mantis low temperature reaction chamber (Harrick Scientific Products Inc., New York, NY,
USA) was installed for DRIFTS measurements. Measurements were performed in range 5000–400 cm
1
with a 1 cm
1
resolution, 40 scans, and automatically transformed into absorption units using the
Kubelka–Munk function. The self-written python code was then used to normalize the spectra by area
and subtract the spectrum of activated state of the sample. The powdered sample (12.8 mg) was loaded
into a cell enabling to control the temperature and the gas flow. An external gas system equipped with
mass flow controllers (EL-FLOW, Bronkhorst High-Tech B.V., Ruurlo, Netherlands) was used to set the
gas flows of Ar, H
2
, and C
2
H
4
through the cell. The gas mixture flowed either through the cell with the
sample, or through a by-pass. Switching was carried out using 3-way valves.
Similar to the procedure described in Section 2.2, the sample was activated for 30 min at 125
C
in 50 mL/min 5% H
2
/Ar and then cooled down to 50
C (using liquid nitrogen as a refrigerant) and
purged with inert gas (50 mL/min of Ar) to exclude the possible hydride phase. The lower hydrogen
content compared to that used in Section 2.2, does not aect the finally obtained structure of metallic
palladium. The higher values of gas flow were used due to the dierent geometry of the in situ reaction
chamber compared to the capillary and bigger mass of the sample. The spectrum collected under such
conditions was used as a background for the following spectra. The flow was then switched to 10%
C
2
H
4
in Ar and the spectra were collected continuously with the time-step of ca. 1 min. After the
saturation in the spectra was observed, the sample was purged first by pure Ar and then exposed to
5% H2/Ar and purged again with Ar.
2.4. DFT Calculations
Atomic structures of the adsorbed C
n
H
m
molecules on palladium (111) and (100) surfaces and
their vibrational spectra were obtained using VASP 5.3 code [
57
,
58
] with PBE exchange-correlation
potential [
59
]. Five layer Pd surfaces in (111) and (100) geometries were constructed with initial Pd–Pd
distances a=2.75 Å (that corresponded to the equilibrium unit cell parameter of the bulk structure)
and then optimized within conjugated gradient algorithm. The surface approximation in periodic
condition of VASP code was obtained by adding the vacuum above the top layer resulting the total
height of the unit cell of 20 Å. The cut-oenergy for the plane-wave basis set of 500 eV was used.
The Monkhorst–Pack method was used to generate k-points 5
×
5
×
1 and 9
×
9
×
1 grids for (111)
and (100) surfaces, respectively. The convergence criteria were set to 10
6
eV for self-consistent field
calculations and 10
5
eV for geometry relaxation. The infrared spectra of the optimized structures
with adsorbed hydrocarbon molecules were simulated keeping all palladium atoms fixed, which was
Nanomaterials 2020,10, 1643 4 of 13
checked not to aect the spectra for several selected cases. Calculation for an isolated ethylene molecule
was performed in square 10
×
10
×
10 Å box with a single k-point. The above model does not consider
the lower-coordinated palladium sites, which should be present on the surface of 2.6 nm NPs. However,
this model was able to reproduce the experimentally observed features, which prevented us from its
further complication.
3. Results
3.1. Evolution of the Bulk Structure of the NPs
Evolution of EXAFS and XANES spectra collected during exposure of the sample to ethylene
at 50
C is shown in Figure 1(parts a and b, respectively). The process was characterized by a slow
and gradual shift of the first-shell peak in the Fourier transformed EXAFS (FT-EXAFS) data due to
the expansion of palladium lattice. After almost 4 h of exposure to ethylene, Pd–Pd distances were
increased by ca. 0.7% (in the fresh sample R
Pd-Pd
=2.736
±
0.003 is close to previously reported values
for metallic Pd NPs [
43
,
50
,
52
,
60
]). In addition, the intensity of the first shell peak decreases due to
the increasing Debye–Waller factor (see Figure S1 and Table S1 in the Supplementary Information).
Both observations are indicative of palladium carbide formation [
34
,
36
,
38
,
40
]. It should be noted that
the coordination numbers are stable along the whole series of collected spectra indicating the stability
of Pd particles (see Figure S1 and Table S1 in the Supplementary Information). The average value of
NPd-Pd =10 is consistent with 2.6 nm particle size.
Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 14
to affect the spectra for several selected cases. Calculation for an isolated ethylene molecule was
performed in square 10 × 10 × 10 Å box with a single k-point. The above model does not consider the
lower-coordinated palladium sites, which should be present on the surface of 2.6 nm NPs. However,
this model was able to reproduce the experimentally observed features, which prevented us from its
further complication.
3. Results
3.1. Evolution of the Bulk Structure of the NPs
Evolution of EXAFS and XANES spectra collected during exposure of the sample to ethylene at
50 °C is shown in Figure 1 (parts a and b, respectively). The process was characterized by a slow and
gradual shift of the first-shell peak in the Fourier transformed EXAFS (FT-EXAFS) data due to the
expansion of palladium lattice. After almost 4 h of exposure to ethylene, PdPd distances were
increased by ca. 0.7% (in the fresh sample RPd-Pd = 2.736 ± 0.003 is close to previously reported values
for metallic Pd NPs [43,50,52,60]). In addition, the intensity of the first shell peak decreases due to the
increasing Debye–Waller factor (see Figure S1 and Table S1 in the Supplementary Information). Both
observations are indicative of palladium carbide formation [34,36,38,40]. It should be noted that the
coordination numbers are stable along the whole series of collected spectra indicating the stability of
Pd particles (see Figure S1 and Table S1 in the Supplementary Information). The average value of NPd-Pd
= 10 is consistent with 2.6 nm particle size.
The changes in XANES are characteristic for palladium carbides [32] (see Section 3.3 for the
interpretation), which supports the idea that the expansion of Pd lattice was induced by incorporation
of carbon atoms into the octahedral interstitial sites. Results of linear combination fitting (LCF) of
XANES data performed using the spectra of the metallic state and the sample after continuous
exposure to ethylene and PdPd interatomic distance obtained from EXAFS (Figure 2) demonstrate
that the evolution of palladium structure proceeds in three steps: (i) “induction” period during the
first ca. 20 min, (ii) fast evolution during the following 1 h, and (iii) the subsequent slower evolution.
Figure 1. (a) Experimental FT-EXAFS spectra (phase uncorrected) and (b) difference XANES spectra
(obtained by subtracting a spectrum of metallic Pd NPs) collected during exposure to ethylene at 50
°C (from blue to red) with the time step of ca. 10 min.
Figure 1.
(
a
) Experimental FT-EXAFS spectra (phase uncorrected) and (
b
) dierence XANES spectra
(obtained by subtracting a spectrum of metallic Pd NPs) collected during exposure to ethylene at 50
C
(from blue to red) with the time step of ca. 10 min.
The changes in XANES are characteristic for palladium carbides [
32
] (see Section 3.3 for the
interpretation), which supports the idea that the expansion of Pd lattice was induced by incorporation
of carbon atoms into the octahedral interstitial sites. Results of linear combination fitting (LCF) of
XANES data performed using the spectra of the metallic state and the sample after continuous exposure
to ethylene and Pd–Pd interatomic distance obtained from EXAFS (Figure 2) demonstrate that the
evolution of palladium structure proceeds in three steps: (i) “induction” period during the first ca.
20 min, (ii) fast evolution during the following 1 h, and (iii) the subsequent slower evolution.
Nanomaterials 2020,10, 1643 5 of 13
Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 14
Figure 2. The results of LCF of XANES (blue squares, left ordinate axis) and first shell PdPd
interatomic distance obtained from EXAFS (red circles, right ordinate axis) for the experimental XAFS
data collected during exposure to ethylene at 50 °C. The error bar corresponds to the maximal
uncertainty in the fitted values of RPd-Pd.
The experimental spectral changes in XANES spectra reported in Figure 1b have two different
origins. The changes in the higher energy region, starting from the peak at 24,380 eV are mainly
influenced by the peak shift according to the Natoli rule due to the lattice expansion. In contrast, the
region close to the edge position (see features A and B in Figure S2) is reflective of the new
antibonding state forming due to the mixing of PdC orbitals. Since the observed lattice expansion is
quite small (0.7%) compared to previously reported palladium carbides at higher temperatures
[36,38], the expected carbon concentration in the bulk PdCx is x 0.03 [61]. Therefore, the major
contribution to the reshaping of near-edge structure is explained by surface adsorbed hydrocarbons.
Figure S2 demonstrates that the experimental features can be reproduced by the model of di-σ-
adsorbed-ethylene. Due to the fact that XANES signal is averaged over all atoms in the NPs, its
sensitivity to the surface adsorbed species is significantly lower comparing to DRIFTS. Therefore, the
discrimination of different surface species would be too ambiguous. However, the fact of PdC
bonding is unambiguously proved. In addition, the analysis of Pd oxidation state performed by
fitting the XANES spectra by those of bulk Pd metal and PdO references (see Table S1) reveals a small
increasing trend from a Pd2+ fraction of 0.14 to 0.20, which is close to the experimental uncertainty of
0.04. The observed trend can also be explained by the fact that characteristic features of carbidic
palladium in the region of the first XANES peak [32,34,39,60] are visually similar to those obtained
by the addition of a PdO spectrum. The corresponding Pd2+ fraction for the fresh sample after
activation in hydrogen is 0.16.
The averaged cell parameter obtained from XRD data was consistent with the RPd-Pd values from
EXAFS. However, the evolution of XRD patterns shown in Figure S3 evidences that there is a stepwise
change of the cell parameter due to the phase transition to α-carbide. This fact can be best appreciated
on higher hkl reflections (e.g., 202 and 113, highlighted in Figure S2): the position of the metallic Pd
peaks remains constant, while additional peaks appear at lower 2θ values.
3.2. Detection of Surface Species by In Situ DRIFTS
Upon exposure to ethylene, a number of bands are observed in DRIFTS spectra (Figure 3) with
different positions with respect to gas-phase ethylene, which evidences its adsorption on palladium.
In particular, in the high-frequency region corresponding to C-H stretching, the bands around 2900
cm1 (shifted to the lower frequencies with respect to the gas phase ethylene) appear in the beginning
of exposure and slowly decrease during the first 8 min (red arrows in Figure 3). This can be explained
by the fact that ethylene initially adsorbed in di-σ configuration is then dehydrogenate to other
Figure 2.
The results of LCF of XANES (blue squares, left ordinate axis) and first shell Pd–Pd interatomic
distance obtained from EXAFS (red circles, right ordinate axis) for the experimental XAFS data collected
during exposure to ethylene at 50
C. The error bar corresponds to the maximal uncertainty in the fitted
values of RPd-Pd.
The experimental spectral changes in XANES spectra reported in Figure 1b have two dierent
origins. The changes in the higher energy region, starting from the peak at 24,380 eV are mainly
influenced by the peak shift according to the Natoli rule due to the lattice expansion. In contrast,
the region close to the edge position (see features A and B in Figure S2) is reflective of the new
antibonding state forming due to the mixing of Pd–C orbitals. Since the observed lattice expansion is
quite small (0.7%) compared to previously reported palladium carbides at higher temperatures [
36
,
38
],
the expected carbon concentration in the bulk PdC
x
is x
0.03 [
61
]. Therefore, the major contribution
to the reshaping of near-edge structure is explained by surface adsorbed hydrocarbons. Figure S2
demonstrates that the experimental features can be reproduced by the model of di-
σ
-adsorbed-ethylene.
Due to the fact that XANES signal is averaged over all atoms in the NPs, its sensitivity to the surface
adsorbed species is significantly lower comparing to DRIFTS. Therefore, the discrimination of dierent
surface species would be too ambiguous. However, the fact of Pd–C bonding is unambiguously proved.
In addition, the analysis of Pd oxidation state performed by fitting the XANES spectra by those of
bulk Pd metal and PdO references (see Table S1) reveals a small increasing trend from a Pd
2+
fraction
of 0.14 to 0.20, which is close to the experimental uncertainty of 0.04. The observed trend can also
be explained by the fact that characteristic features of carbidic palladium in the region of the first
XANES peak [
32
,
34
,
39
,
60
] are visually similar to those obtained by the addition of a PdO spectrum.
The corresponding Pd2+fraction for the fresh sample after activation in hydrogen is 0.16.
The averaged cell parameter obtained from XRD data was consistent with the R
Pd-Pd
values from
EXAFS. However, the evolution of XRD patterns shown in Figure S3 evidences that there is a stepwise
change of the cell parameter due to the phase transition to
α
-carbide. This fact can be best appreciated
on higher hkl reflections (e.g., 202 and 113, highlighted in Figure S2): the position of the metallic Pd
peaks remains constant, while additional peaks appear at lower 2θvalues.
3.2. Detection of Surface Species by In Situ DRIFTS
Upon exposure to ethylene, a number of bands are observed in DRIFTS spectra (Figure 3) with
dierent positions with respect to gas-phase ethylene, which evidences its adsorption on palladium.
In particular, in the high-frequency region corresponding to C-H stretching, the bands around 2900 cm
1
(shifted to the lower frequencies with respect to the gas phase ethylene) appear in the beginning of
exposure and slowly decrease during the first 8 min (red arrows in Figure 3). This can be explained
by the fact that ethylene initially adsorbed in di-
σ
configuration is then dehydrogenate to other
Nanomaterials 2020,10, 1643 6 of 13
intermediates (vide infra). The other bands in this region are also present but are significantly
broadened which complicates their assignment. This may be due to the higher disorder in the stricture
of 2.6 nm NPs in comparison with well-defined surfaces, and the presence of multiple species with
overlapping C-H stretching modes. Therefore, the assignment below was made considering the
1000–1700 cm
1
region. Moreover, a weak band is observed at 2165 (see Figure S4 in the Supplementary
Information), attributed to CC triple bond stretching.
The decreasing trend is also observed for two bands at 1325 and 1235 cm
1
with a parallel
growth of the bands at 1340 and 1273 cm
1
(blue arrows in Figure 3). These features are not related
to di-
σ
-adsorbed ethylene but can be explained by dierent frequencies (and intensities) which are
expected for similar molecules on the surfaces with dierent interatomic distances (see Figure S7).
Therefore, such behavior is related to the transition of palladium into its carbide phase with increased
cell parameters as observed by EXAFS and XRD in Section 3.1.
The two growing peaks near 1600 cm
1
are characteristic for C =C double bond stretching. The one
at a lower frequency appears with some delay with the respect to the higher one, and continues growing
after ethylene was switched o(see Figure 4). The C =C bond can be explained by the formation
of vinyl and vinylidene [
16
,
62
64
], the former also explains the peak at 1340 cm
1
discussed above
related to C-H scissoring (see also Section 3.3). This peak was also reported for ethylidyne [
12
,
14
,
16
]
but absence of C–C stretching near 1100 cm1allows us to exclude it.
The most intense peak observed at 1417 cm
1
(green arrow in Figure 3) is shifted to the lower
frequencies with respect to C-H scissoring peak of gas phase ethylene. This band may be related to
π
-
or di-
σ
-adsorbed ethylene [
16
,
62
,
63
,
65
,
66
], ethylidene [
16
,
24
,
62
,
67
], vinylidene [
16
] or methyl group
(CH3) [19].
Most of the formed species remain stable not only upon purging with argon, but also after
flowing hydrogen which is supposed to hydrogenate the surface adsorbed hydrocarbons (Figure 4).
The addition of hydrogen leads to a formation of a distinct peak at around 1450 cm
1
, which should
be related to the partial hydrogenation of one of the adsorbed species. To highlight the eect of
ethylene adsorption, the spectrum of fresh activated catalyst was subtracted from all the reported data.
This procedure allowed excluding the vibrational frequencies related to the support itself and possible
hydroxyl groups on alumina [6870].
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 14
intermediates (vide infra). The other bands in this region are also present but are significantly
broadened which complicates their assignment. This may be due to the higher disorder in the
stricture of 2.6 nm NPs in comparison with well-defined surfaces, and the presence of multiple
species with overlapping C-H stretching modes. Therefore, the assignment below was made
considering the 1000–1700 cm1 region. Moreover, a weak band is observed at 2165 (see Figure S4 in
the Supplementary Information), aributed to CC triple bond stretching.
The decreasing trend is also observed for two bands at 1325 and 1235 cm1 with a parallel growth
of the bands at 1340 and 1273 cm1 (blue arrows in Figure 3). These features are not related to di-σ-
adsorbed ethylene but can be explained by different frequencies (and intensities) which are expected
for similar molecules on the surfaces with different interatomic distances (see Figure S7). Therefore,
such behavior is related to the transition of palladium into its carbide phase with increased cell
parameters as observed by EXAFS and XRD in Section 3.1.
The two growing peaks near 1600 cm1 are characteristic for C = C double bond stretching. The
one at a lower frequency appears with some delay with the respect to the higher one, and continues
growing after ethylene was switched off (see Figure 4). The C = C bond can be explained by the
formation of vinyl and vinylidene [16,62–64], the former also explains the peak at 1340 cm1 discussed
above related to C-H scissoring (see also Section 3.3). This peak was also reported for ethylidyne
[12,14,16] but absence of CC stretching near 1100 cm1 allows us to exclude it.
The most intense peak observed at 1417 cm1 (green arrow in Figure 3) is shifted to the lower
frequencies with respect to C-H scissoring peak of gas phase ethylene. This band may be related to
π- or di-σ-adsorbed ethylene [16,62,63,65,66], ethylidene [16,24,62,67], vinylidene [16] or methyl
group (CH3) [19].
Most of the formed species remain stable not only upon purging with argon, but also after
flowing hydrogen which is supposed to hydrogenate the surface adsorbed hydrocarbons (Figure 4).
The addition of hydrogen leads to a formation of a distinct peak at around 1450 cm1, which should
be related to the partial hydrogenation of one of the adsorbed species. To highlight the effect of
ethylene adsorption, the spectrum of fresh activated catalyst was subtracted from all the reported
data. This procedure allowed excluding the vibrational frequencies related to the support itself and
possible hydroxyl groups on alumina [68–70].
Figure 3. Evolution of experimental DRIFTS data (after normalization and subtraction of spectrum of
the activated state) for Pd catalyst during exposure to ethylene at 50 °C (from blue to red) with the
acquisition time of ca. 1 min. Grey line corresponds to the reference spectrum of gas-phase ethylene
[71]. The scale bars demonstrate that the left part is enlarged with respect to the right one.
Figure 3.
Evolution of experimental DRIFTS data (after normalization and subtraction of spectrum of
the activated state) for Pd catalyst during exposure to ethylene at 50
C (from blue to red) with the
acquisition time of ca. 1 min. Grey line corresponds to the reference spectrum of gas-phase ethylene [
71
].
The scale bars demonstrate that the left part is enlarged with respect to the right one.
Nanomaterials 2020,10, 1643 7 of 13
Figure 4.
Background subtracted experimental DRIFTS data (after normalization and subtraction of
spectrum of the activated state) for Pd catalyst after exposure to C
2
H
4
. The subsequent spectra were
collected in argon (red), in hydrogen (blue) and again in argon (green). Black line corresponds to the
reference spectrum of gas-phase ethylene [
71
]. For clarity, the spectra are shifted in vertical direction.
The scale bars demonstrate that the left part is enlarged with respect to the right one.
3.3. DFT Relaxation of CnHmon Pd Surfaces and Their Vibrational Spectra
To correlate the experimentally observed features in XANES and DRIFTS data with the structure
of the adsorbed species, we have considered a wide range of possible C
n
H
m
molecules on the surface
of palladium. For XANES calculation, the bulk PdC structure, with carbon atoms occupying octahedral
interstitial sites of fcc lattice, was also considered. The DFT-relaxed atomic structures on Pd (111) are
shown in Figure 5. For Pd (100), similar results were obtained, except for structures (d) and (k), due to
the fact that there is no three-fold hollow site, and vinylidene (vide infra). The first two structures,
(a) and (b), correspond to
π
- and di-
σ
-adsorbed ethylene, respectively. The two following structures
are ethylidene (c) and ethylidyne (d), which were observed experimentally for dierent noble metal
surfaces after interaction with ethylene [
13
,
21
]. The next three structures are on top adsorbed ethyl (e),
µ
-vinyl (f), and ethynyl (g). Vinylidene (h) was initially placed in bridge configuration orthogonal
to the surface (
µ
-vinylidene). This configuration was preserved after relaxation for the surface of
(100), but for the surface of (111), the
µ3
-
η2
-vinylidene was formed after relaxation which correlates
with previous reports [
21
]. Finally, the C
1
-species (methyl (i), methylene (j) and methine (k) groups)
were also considered to account for the possible decomposition of ethylene. In addition,
π
- and
di-
σ
-adsorbed acetylene (C
2
H
2
) molecules were also relaxed, but are not shown in the Figure 5since
their formation after ethylene adsorption is the least expected.
The theoretical vibrational spectrum of an isolated ethylene molecule was in good agreement in
both positions and intensities with the experimental data for gas-phase ethylene (see Section S5 in
the Supplementary Information), which is an important prerequisite for the further comparison of
theoretical and experimental spectra of unknown species. Below, the comparison of theoretical and
experimental spectra is made based on both the absolute positions and the relative shifts with respect
to gas-phase ethylene. The figures are reported in Section S2 of the Supplementary Information.
Nanomaterials 2020,10, 1643 8 of 13
Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 14
Figure 5. Visualization of the DFT-relaxed structures of different C
n
H
m
species on Pd (111) surface: π-
(a) and di-σ-adsorbed (b) ethylene, ethylidene (c), ethylidyne (d), ethyl (e), μ-vinyl (f), ethynyl (g).
vinylidene (h), methyl (i),
methylene (j) and methine (k). For better visualization, only the top surface
layer of palladium is shown.
The shift of the C-H stretching bands of di-σ-adsorbed-ethylene (b) towards lower frequencies
(Figure S6) supports the assignment of the decreasing bands near 2900 cm
1
made in Section 3.2. The
C=C=C C stretching frequency of μ-vinyl is 60 cm
1
higher than in μ-vinylidene (Figures S7 and S8),
which explains the two bands at 1627 and 1524 cm
1
in the experimental data (although the absolute
values of theoretical frequencies are lower). The calculated frequency of CC bond of ethinyl (g) was
also underestimated by almost 200 cm
1
, which may be attributed to the higher interatomic distances
(2.77–2.82 Å) compared to experimental EXAFS results (2.76 Å). In particular, in the presence of
hydrogen, when the Pd-hydride is expected with increased PdPd distances, the experimental CC
band also shifts by more than 100 cm
1
to the lower frequencies (Figure S4b). The above structures
demonstrate progressive dehydrogenation from C
2
H
4
to C
2
H
1
(Figure 6). As already mentioned, it is
difficult to confirm or discard the presence of C
1
-species. The increase in the shoulder around 1450
cm
1
might be explained by hydrogenation of some part of the adsorbed species to ethyl (Figure S9).
Figure 6. Ethylene dehydrogenation pathway based on XRD, EXAFS, XANES and DRIFTS data.
4. Discussion
The synergetic coupling of EXAFS, XANES, XRD and DRIFTS data allows following the
evolution of the bulk structure of palladium NPs and the speciation of C
n
H
m
molecules at their
surfaces in situ. Immediately after exposure to ethylene, the adsorption of gas phase ethylene on the
Pd surface occurs, which does not induce any changes in the bulk structure of the NPs. The adsorbed
ethylene is then partially converted to vinyl as of the stable intermediates. This process is
accompanied with lattice expansion monitored by EXAFS, which means that the decomposition of
ethylene to atomic carbon takes place. In this case, XANES acts as a bridge between EXAFS and
DRIFTS, being sensitive to both lattice expansion and PdC bonding. With no evidence of palladium
hydride formation, XANES supports the hypothesis that the observed lattice expansion is indeed due
Figure 5.
Visualization of the DFT-relaxed structures of dierent C
n
H
m
species on Pd (111) surface:
π
- (
a
) and di-
σ
-adsorbed (
b
) ethylene, ethylidene (
c
), ethylidyne (
d
), ethyl (
e
),
µ
-vinyl (
f
), ethynyl (
g
).
vinylidene (
h
), methyl (
i
), methylene (
j
) and methine (
k
). For better visualization, only the top surface
layer of palladium is shown.
The shift of the C-H stretching bands of di-
σ
-adsorbed-ethylene (b) towards lower frequencies
(Figure S6) supports the assignment of the decreasing bands near 2900 cm
1
made in Section 3.2.
The C=C=C C stretching frequency of
µ
-vinyl is 60 cm
1
higher than in
µ
-vinylidene (Figures S7
and S8), which explains the two bands at 1627 and 1524 cm
1
in the experimental data (although the
absolute values of theoretical frequencies are lower). The calculated frequency of C
C bond of ethinyl
(g) was also underestimated by almost 200 cm
1
, which may be attributed to the higher interatomic
distances (2.77–2.82 Å) compared to experimental EXAFS results (2.76 Å). In particular, in the presence
of hydrogen, when the Pd-hydride is expected with increased Pd–Pd distances, the experimental C
C
band also shifts by more than 100 cm
1
to the lower frequencies (Figure S4b). The above structures
demonstrate progressive dehydrogenation from C
2
H
4
to C
2
H
1
(Figure 6). As already mentioned, it is
dicult to confirm or discard the presence of C
1
-species. The increase in the shoulder around 1450 cm
1
might be explained by hydrogenation of some part of the adsorbed species to ethyl (Figure S9).
Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 14
Figure 5. Visualization of the DFT-relaxed structures of different C
n
H
m
species on Pd (111) surface: π-
(a) and di-σ-adsorbed (b) ethylene, ethylidene (c), ethylidyne (d), ethyl (e), μ-vinyl (f), ethynyl (g).
vinylidene (h), methyl (i),
methylene (j) and methine (k). For better visualization, only the top surface
layer of palladium is shown.
The shift of the C-H stretching bands of di-σ-adsorbed-ethylene (b) towards lower frequencies
(Figure S6) supports the assignment of the decreasing bands near 2900 cm
1
made in Section 3.2. The
C=C=C C stretching frequency of μ-vinyl is 60 cm
1
higher than in μ-vinylidene (Figures S7 and S8),
which explains the two bands at 1627 and 1524 cm
1
in the experimental data (although the absolute
values of theoretical frequencies are lower). The calculated frequency of CC bond of ethinyl (g) was
also underestimated by almost 200 cm
1
, which may be attributed to the higher interatomic distances
(2.77–2.82 Å) compared to experimental EXAFS results (2.76 Å). In particular, in the presence of
hydrogen, when the Pd-hydride is expected with increased PdPd distances, the experimental CC
band also shifts by more than 100 cm
1
to the lower frequencies (Figure S4b). The above structures
demonstrate progressive dehydrogenation from C
2
H
4
to C
2
H
1
(Figure 6). As already mentioned, it is
difficult to confirm or discard the presence of C
1
-species. The increase in the shoulder around 1450
cm
1
might be explained by hydrogenation of some part of the adsorbed species to ethyl (Figure S9).
Figure 6. Ethylene dehydrogenation pathway based on XRD, EXAFS, XANES and DRIFTS data.
4. Discussion
The synergetic coupling of EXAFS, XANES, XRD and DRIFTS data allows following the
evolution of the bulk structure of palladium NPs and the speciation of C
n
H
m
molecules at their
surfaces in situ. Immediately after exposure to ethylene, the adsorption of gas phase ethylene on the
Pd surface occurs, which does not induce any changes in the bulk structure of the NPs. The adsorbed
ethylene is then partially converted to vinyl as of the stable intermediates. This process is
accompanied with lattice expansion monitored by EXAFS, which means that the decomposition of
ethylene to atomic carbon takes place. In this case, XANES acts as a bridge between EXAFS and
DRIFTS, being sensitive to both lattice expansion and PdC bonding. With no evidence of palladium
hydride formation, XANES supports the hypothesis that the observed lattice expansion is indeed due
Figure 6. Ethylene dehydrogenation pathway based on XRD, EXAFS, XANES and DRIFTS data.
4. Discussion
The synergetic coupling of EXAFS, XANES, XRD and DRIFTS data allows following the evolution
of the bulk structure of palladium NPs and the speciation of C
n
H
m
molecules at their surfaces in
situ. Immediately after exposure to ethylene, the adsorption of gas phase ethylene on the Pd surface
occurs, which does not induce any changes in the bulk structure of the NPs. The adsorbed ethylene
is then partially converted to vinyl as of the stable intermediates. This process is accompanied with
lattice expansion monitored by EXAFS, which means that the decomposition of ethylene to atomic
carbon takes place. In this case, XANES acts as a bridge between EXAFS and DRIFTS, being sensitive
to both lattice expansion and Pd–C bonding. With no evidence of palladium hydride formation,
Nanomaterials 2020,10, 1643 9 of 13
XANES supports the hypothesis that the observed lattice expansion is indeed due to the incorporation
of carbon atoms to the bulk of NPs. As noted in Section 3.3, the region close to 24,350 eV mainly
reflects formation of Pd–C bond. This explains the fact that the changes in this region occur almost
immediately—since the main part of such bonds corresponds to the surface coverage of palladium by
hydrocarbon molecules. In contrast, the changes in the higher energy region of XANES spectra are
more gradual due to slow lattice expansion. Based on the correlation of DRIFTS peaks and the order of
their appearance and the lattice expansion of the Pd lattice the dehydrogenation path ethylene
vinyl
vinylidene
ethinyl
palladium carbide can be suggested (Figure 6). In addition, it should be
noted that the obtained hydrocarbons were stable under hydrogen flow. Considering the activity of
palladium in hydrogenation reaction even at low temperatures [
38
], this result indicate that the revealed
intermediates are the ones that are responsible for ethylene hydrogenation in catalytic reactions.
The observed decomposition of ethylene with the formation of carbide correlates with a number
of previous reports [
29
31
,
33
,
36
40
]. The lattice expansion at 50
C is, however, significantly reduced,
in terms of both the kinetics and saturated values compared to the studies performed at higher
temperatures [
40
]. The collection of high-quality synchrotron data was of utmost importance for
highlighting these small changes. It should be noted that no evidence of ethylidyne formation was found,
which was commonly observed during ethylene conversion over platinum [
17
,
18
,
21
,
22
]. A similar
result was observed for the Pd (100) surface, at which ethylene dehydrogenates to vinyl without
the formation of ethylidyne [
63
]. However, the main reason for the dierence is the experimental
conditions (atmospheric pressure and 50
C temperature), under which the dehydrogenation and
subsequent decomposition of ethylene to Pd carbide is the dominant reaction pathway, which is in
correlation with a recent study by Jones et al. [
29
]. It should be noted that although having similar
size distribution, the NPs proved by surface sensitive DRIFTS and bulk sensitive XANES/EXAFS/XRD
techniques had two dierent types of supports—carbon and alumina, respectively—due to the features
of the experimental techniques employed. We believe that the evolution of the catalyst and the
substrate is similar with respect to the used techniques; however, one should keep in mind the role of
the metal-support interaction [
69
,
72
], which may aect, in particular, the electronic state of palladium
in these two cases, and lead to slightly dierent ethylene adsorption and dehydrogenation.
5. Conclusions
In conclusion, we have revealed the dehydrogenation of ethylene via vinyl, vinylidene and
ethinyl to palladium carbide as the dominant reaction on the surface of 2.6 nm palladium NPs
under atmospheric pressure and moderate temperature (50
C). The combination of in situ XAFS,
XRD and DRIFTS data provided not only complementary information, but also facilitated the mutual
interpretation of the data from dierent techniques. This synergetic coupling, supported by theoretical
simulations, was demonstrated to be an ecient approach for the in situ investigation of surface and
bulk structures of palladium-based catalysts under reaction conditions.
Supplementary Materials:
The following are available online at http://www.mdpi.com/2079-4991/10/9/1643/s1,
Table S1: Pd–Pd distances, Debye-Waller factors and coordination numbers obtained from EXAFS and the fraction
of Pd
2+
obtained by fitting the XANES spectra, Figure S1: Evolution of Debye-Waller factor and coordination
number from EXAFS, Figure S2: (a) Experimental dierence XANES spectrum obtained by subtraction the
spectrum of metallic NPs and simulated ones. Part (b) shows the same experimental spectrum, but compared
with simulations for the atom inside the bulk metallic Pd and bulk Pd carbide with increased cell parameter,
Figure S3: XRD patterns collected during exposure to ethylene at 50
C with the time step of ca. 10 min. Figure S4:
(a) Background subtracted experimental DRIFTS data for Pd catalyst after exposure to C
2
H
4
. The subsequent
spectra were collected in argon, in hydrogen and again in argon. Part (b) shows the time evolution of DRIFTS
during hydrogen treatment, Figure S5: Experimental and theoretical spectra of ethylene (gas phase), Figure S6:
Theoretical spectra of an isolated ethylene molecule and di-
σ
-adsorbed-ethylene on Pd (111), Figure S7: Theoretical
spectra of an isolated ethylene molecule and
µ
-vinyl on Pd (111), Figure S8: Theoretical spectra of an isolated
ethylene molecule and
µ
-vinylidene on Pd (100), Figure S9: Theoretical spectra of an isolated ethylene molecule
and ethyl on Pd (111).
Nanomaterials 2020,10, 1643 10 of 13
Author Contributions:
Conceptualization, T.Y., A.L.B. and A.V.S.; Data curation, O.A.U., A.Y.P., E.G.K. and A.A.S.;
Formal analysis, O.A.U., E.G.K., A.A.S., W.Z. and T.Y.; Funding acquisition, T.Y. and A.V.S.; Investigation, O.A.U.,
A.Y.P., E.G.K., A.A.T., A.A.S., W.Z. and A.L.B.; Methodology, A.L.B.; Project administration, A.V.S.; Supervision,
A.L.B.; Writing—Original draft, O.A.U., E.G.K. and A.L.B. All authors have read and agreed to the published
version of the manuscript.
Funding:
This research was funded by the State assignment
0852-2020-0019 to the Southern Federal University
(2020). W.Z. and T.Y. received the funding from the Youth Innovation Promotion Association CAS (CX2310007007
and CX2310000091).
Acknowledgments:
Research was financially supported by the Ministry of Science and Higher Education of the
Russian Federation (State assignment
0852-2020-0019 to the Southern Federal University, 2020). We thank
Riccardo Pellegrini from Chimet S.p.A. for providing the samples for investigation. We acknowledge ESRF for
providing the beamtime and especially Vladimir Dmitriev, Wouter van Beek and Dragos Stoian for their help
during the experiment at BM31. W.Z. and T.Y. acknowledge the Youth Innovation Promotion Association CAS
(CX2310007007 and CX2310000091).
Conflicts of Interest:
The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
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2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
... Clearly, in situ FTIR during reaction would be preferable [120][121][122][123], but this is difficult due to the low coverage and flat orientation of π-bonded species around 200 °C. More extended studies would be beneficial, but the results of the current catalytic investigations already provided some insight into TCE hydrodechlorination, i.e., how to control selectivity towards ethylene or ethane by shape-controlled synthesis. ...
... Clearly, in situ FTIR during reaction would be preferable [120][121][122][123], but this is difficult due to the low coverage and flat orientation of π-bonded species around 200 • C. More extended studies would be beneficial, but the results of the current catalytic investigations already provided some insight into TCE hydrodechlorination, i.e., how to control selectivity towards ethylene or ethane by shape-controlled synthesis. ...
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