Citation: Nejadsalim, A.; Bashiri, N.;
Godini, H.R.; Oliveira, R.L.; Tufail
Shah, A.; Bekheet, M.F.; Thomas, A.;
Schomäcker, R.; Gurlo, A.; Görke, O.
Core-Sheath Pt-CeO2/Mesoporous
SiO2Electrospun Nanofibers as
Catalysts for the Reverse Water Gas
Shift Reaction. Nanomaterials 2023,13,
485. https://doi.org/10.3390/
nano13030485
Academic Editor: Jose M. Palomo
Received: 30 December 2022
Revised: 13 January 2023
Accepted: 23 January 2023
Published: 25 January 2023
Copyright: © 2023 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 (https://
creativecommons.org/licenses/by/
4.0/).
nanomaterials
Article
Core-Sheath Pt-CeO2/Mesoporous SiO2Electrospun Nanofibers
as Catalysts for the Reverse Water Gas Shift Reaction
Aidin Nejadsalim 1,† , Najmeh Bashiri 2,3,†, Hamid Reza Godini 4, Rafael L. Oliveira 5, Asma Tufail Shah 1,6 ,
Maged F. Bekheet 1, Arne Thomas 2, Reinhard Schomäcker 3, Aleksander Gurlo 1and Oliver Görke 1,*
1Chair of Advanced Ceramic Materials, Institute of Material Science and Technology, Faculty III Process
Sciences, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany
2Functional Materials, Institute of Chemistry, Faculty II Mathematics and Natural Sciences, Technische
Universität Berlin, Hardenbergstr. 40, 10623 Berlin, Germany
3Chemical Engineering/Multiphase Reaction Technology, Institute of Chemistry, Faculty II Mathematics and
Natural Sciences, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany
4Inorganic Membranes and Membrane Reactors, Department of Chemical Engineering and Chemistry,
Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
5Low Temperature and Structure Research Institute of the Polish Academy of Science, Okólna 2,
50-422 Wroclaw, Poland
6
Interdisciplinary Research Centre in Biomedical Materials, COMSATS University Islamabad Lahore Campus,
Defence Road, Off-Raiwand Road, Lahore 54000, Pakistan
*Correspondence: oliver[email protected]
† These authors contributed equally to this work.
Abstract:
One-dimensional (1D) core-sheath nanofibers, platinum (Pt)-loaded ceria (CeO
2
) sheath on
mesoporous silica (SiO
2
) core were fabricated, characterized, and used as catalysts for the reverse
water gas shift reaction (RWGS). CeO
2
nanofibers (NFs) were first prepared by electrospinning (ES),
and then Pt nanoparticles were loaded on the CeO
2
NFs using two different deposition methods:
wet impregnation and solvothermal. A mesoporous SiO
2
sheath layer was then deposited by sol-gel
process. The phase composition, structural, and morphological properties of synthesized materials
were investigated by scanning electron microscope (SEM), scanning transmission electron microscopy
(STEM), X-ray diffraction (XRD), nitrogen adsorption/desorption method, X-ray photoelectron
spectroscopy (XPS), inductively coupled plasma—optical emission spectrometry (ICP-OES) analysis,
and CO
2
temperature programmed desorption (CO
2
-TPD). The results of these characterization
techniques revealed that the core-sheath NFs with a core diameter between 100 and 300 nm and a
sheath thickness of about 40–100 nm with a Pt loading of around 0.5 wt.% were successfully obtained.
The impregnated catalyst, Pt-CeO
2
NF@mesoporous SiO
2
, showed the best catalytic performance
with a CO
2
conversion of 8.9% at 350
◦
C, as compared to the sample prepared by the Solvothermal
method. More than 99% selectivity of CO was achieved for all core-sheath NF-catalysts.
Keywords:
electrospinning; nanofibers; core-sheath; tandem catalyst; reverse water gas shift reaction
1. Introduction
Depending on the desired structural properties, one-dimensional (1D) structures
could be synthesized using various methods such as electrospinning (ES), hot-filament
metal-oxide vapor deposition, sacrificial-template method, etc., [
1
–
4
]. For nanofibers
(NFs) as a promising 1D structure, achieving a high surface-to-volume ratio is always
desirable. Among these synthesis techniques, ES is a facile method to produce several
NFs materials. Moreover, ES is a straightforward and cost-effective technique to create 1D
fiber structures on the scale of nanometers to several micrometers and with various shapes,
including solid, hollow, core-sheath, and hierarchical structures [
5
–
7
]. Such flexibility
and potentials are owed to the controllable parameters of the ES process. In a typical ES
process, a polymeric viscous-enough solution loaded in a needle (spinneret) is exposed to
Nanomaterials 2023,13, 485. https://doi.org/10.3390/nano13030485 https://www.mdpi.com/journal/nanomaterials
Nanomaterials 2023,13, 485 2 of 20
an electric field provided by a high-power supply. Then, the solution is drawn to make
a jet that finally results in NFs being deposited on a collector after drying the solvent
out. Electrospun NFs have been utilized in a broad range of applications, such as gas
sensing [
8
], filtration membranes [
9
,
10
], biomedicine [
11
–
13
], and catalysis [
14
–
16
]. ES as
an engineering technique has the potential to produce NFs on a large scale that is a relative
advantage of this technique compared to the batch chemical synthesis methods, which
produce limited amounts of the material [
17
,
18
]. Moreover, NFs prepared by ES technique
can be further modified to enhance physical and chemical properties in order to achieve
the desired features [
9
]. These can be a motivation to bridge the lab-scale production of NF
to large-scale production through ES, which certainly reduces the cost of synthesis. It is
practical to use the ES technique for chemistry and material fabrication to reduce the price.
In this work, systematic ES NFs with the core-sheath structure were designed and
used for catalytic CO
2
conversion, a process with significant environmental and economic
implications. CO
2
can be converted to value-added products through a tandem system,
including reversed water-gas shift (RWGS) and Fischer–Tropsch (FT) reactions [
19
]. De-
signing novel multi-metallic catalysts with a controllable morphological structure will
play an important role in developing a complex tandem system. Nanoparticles (NPs) as
multi-metallic catalysts have been widely studied in the past few years in tandem systems.
However, NF core-sheaths have been investigated in much lesser extent. In this regard, de-
signing NF core-sheaths as a base formulation, here for RWGS, can be useful for developing
multi-steps reactions, as an example, CO2hydrogenation.
Various metal oxides, such as In
2
O
3
, SrTiO
3
, CeO
2
, TiO
2
, and Al
2
O
3
have been uti-
lized as catalysts in RWGS [
20
–
24
]. Among these, CeO
2
is the promising catalytic mate-
rial, which is redox-active and shows oxygen storage capacity due to abundant oxygen
vacancies [
19
,
25
–
27
]. These features and the strong metal-support interactions make CeO
2
not only interesting as a catalyst for CO generation, but also for several other applica-
tions [
19
,
28
–
30
]. CeO
2
is widely used as an efficient support for noble metals such as Pt.
Pt-supported on CeO
2
has received a lot of attention due to the exceptionally strong metal-
support interaction [
31
,
32
]. CeO
2
provides a better Pt dispersion than other metal oxides
and Pt dispersion remains intact and stable even at high temperatures [
33
,
34
]. It is revealed
that the synergistic effect between Pt and CeO
2
could improve the catalytic properties in
CO
2
hydrogenation and RWGS [
35
–
37
]. However, these NPs suffer from sintering and
aggregation during the catalytic process, especially in harsh conditions, which causes a loss
of catalytic performance [
38
–
43
]. Embedding NPs into well-designed materials, such as
core-sheath structures, can effectively minimize sintering and thus enhance the catalytic
performance. Sang Hoon Joo et al. developed core-shell NPs of Pt@mSiO
2
in which Pt was
surrounded by a mesoporous SiO
2
layer to prohibit Pt from agglomeration and improve
thermal stability [
42
]. In another study conducted by Ji Su et al., Pt was deposited on CeO
2
NPs and covered by a mesoporous SiO
2
shell, and used for the conversion of ethylene
to propanal via tandem hydroformylation [
44
]. Jones et al. studied the effect of the ceria
morphology employed (nanocubes and nanorods) as supports for an iron-based catalyst
for CO
2
conversion to hydrocarbons and found that ceria nanocubes provided a high
olefin-to-paraffin ratio, while a higher selectivity toward hydrocarbons was achieved using
ceria nanorods [
45
]. Tan et al. illustrated that CeO
2
nanotube-supported Cu-Ni shows
a higher catalytic performance for CO
2
hydrogenation to methanol compared to CeO
2
nanoparticle-supported Cu-Ni owing to the existence of abundant oxygen vacancies and
exposed (100) and (110) facets [
46
]. Tang et al. fabricated Pt-CeO
2
NFs using ES technique
and investigated the catalytic properties toward the water–gas shift reaction obtaining a CO
conversion of 98% [
47
]. However, Pt particles were entrapped within CeO
2
in electrospun
Pt-CeO
2
NFs, reducing the accessibility of the Pt active sites because of the pre-mixing of
Pt and CeO
2
solutions. To overcome this issue, Lu et al. developed CeO
2
NFs with a hierar-
chical porous structure, then dispersed Pt NPs on the CeO
2
surface using a photochemical
method and obtained a uniform Pt distribution on the porous CeO
2
[
48
]. However, the high
porosity could decrease the mechanical stability of the NFs, which makes the formation of a
Nanomaterials 2023,13, 485 3 of 20
homogenous sheath layer challenging. Here, to obtain a uniform sheath layer around CeO
2
,
nonporous smooth CeO
2
NFs were fabricated using the ES technique. To the best of our
knowledge, electrospun NF core-sheath structures have not been investigated for RWGS so
far. Moreover, two different methods were used to deposit Pt on CeO
2
NFs, including wet
impregnation and solvothermal. Deposition of pre-synthesized Pt NPs on CeO
2
NFs using
the solvothermal method have not been reported before.
In this study, we designed a systematic multi-step synthesis method for developing
the core-sheath structure of Pt-CeO
2
NFs@mSiO
2
. Non-porous smooth CeO
2
NFs were
first produced by ES technique, then two different approaches i.e., wet impregnation and
solvothermal, were applied and compared for deposition of Pt NPs, and at the last step, a
mesoporous SiO
2
was homogeneously formed around the core using the sol-gel process.
The obtained catalysts were tested in the RWGS. A comprehensive characterization of the
materials was further carried out to investigate the Pt distribution on NFs, core-sheath
morphology, and the chemical properties of metals.
2. Materials and Methods
2.1. Materials
Polyvinylpyrrolidone (PVP, M = 1,300,000 gmol
−1
and 29,000 gmol
−1
), ethanol (ab-
solute, 99.5%,), and N,N-dimethylformamide (DMF, 99.5%) were purchased from Sigma-
Aldrich. Cerium nitrate hexahydrate (Ce(NO
3
)
3·
6H
2
O, >98.5%) was supplied from Merck.
Commercial Cerium(IV) oxide (labeled as Com-CeO
2
) was purchased from Sigma-Aldrich.
Tetradecyltrimethylammonium bromide (TTAB, 98%), and tetraethyl orthosilicate (TEOS,
98%) were obtained from Sigma-Aldrich. Cetyltrimethylammonium bromide (CTAB, 99%)
and ethylene glycol (EG, 99.5%) were provided by Carl Roth. Ammonia solution 25% was
purchased from Chemsolute. Tetraammineplatinum (II) nitrate ([Pt(NH
3
)
4
](NO
3
)
2
) and
potassium tetrachloroplatinate(II) (K
2
PtCl
4
) were obtained from Merck. Deionized (DI)
water was used in all experiments, and all materials were used without further purification.
2.2. Fabrication of CeO2NFs
2.2.1. Preparation of Spinnable Solution
In a typical synthesis, 1.4 mmol of Ce(NO
3
)
3·
6H
2
O and 1.4 mmol of PVP
(M
w
= 1,300,000 gmol
−1
) were separately dissolved in 2 and 3 mL of DMF, respectively. The
polymer solution was stirred at 50
◦
C to facilitate PVP dissolution. After 2 h, Ce(NO
3
)
3·
6H
2
O
solution was added dropwise into the polymeric solution under vigorous stirring. A yel-
lowish spinnable solution was obtained after stirring overnight and was used for electro-
spinning.
2.2.2. Axial Electrospinning of CeO2
Electrospun CeO
2
NFs were prepared using an electrospinning apparatus, Yflow
®
2.2.D-300. The axial electrospinning equipment consists of a high-voltage supply, a peri-
staltic pump, a syringe with a 21-gauge silver-coated needle, and a ground collector. The
spinnable solution was loaded into the syringe and pumped to the needle at a constant
flow rate. NFs were obtained by adjusting the operating parameters with a voltage of 15 kV,
a needle-to-collector distance of 15 cm, and a flow rate of 0.5 mL/h. The as-spun NFs were
peeled off from the collector and dried at 80
◦
C in an oven for 24 h. Then the dried NFs
were calcined at 600
◦
C for 2 h with a heating rate of 1
◦
C/min in air to remove PVP and
obtain CeO
2
NFs. The commercial CeO
2
powder was used for comparison and was labeled
as CeO2-Com.
2.3. Synthesis of CeO2NF@SiO2Core-Sheath Structure
2.3.1. Preparation of Electrospun CeO2NFs for Sol-Gel Synthesis
In a typical synthesis, 20 mg of electrospun CeO
2
NFs mat were ultrasonicated for
10 min to separate the nonwoven NFs to access all NFs surfaces in all dimensions. The
Nanomaterials 2023,13, 485 4 of 20
separated powder-like CeO
2
NFs were further used in sol-gel synthesis to obtain the
core-sheath structure.
2.3.2. Sol-Gel Synthesis of CeO2NF@SiO2(CeSi)
A sol-gel method was implemented to synthesize the core-sheath structure, according
to the previous work [
49
]. About 20 mg of ultrasonicated CeO
2
NFs were dispersed in
45 mL of DI water. Then, a solution of 0.62 mmol of CTAB in 30 mL ethanol was added to
the CeO
2
NFs aqueous solution. A total of 0.2 mL of ammonia solution was added dropwise
to the above solution. Subsequently, 100
µ
L of diluted TEOS in ethanol (1 vol%) was slowly
added into the solution, followed by stirring at room temperature for 6 h. A centrifugation
was performed at 4000 rpm for 5 min to collect the as-synthesized core-sheaths. The solid
was calcined at 360 ◦C for 2 h to obtain CeO2NF@SiO2(labeled as CeSi).
2.4. Fabrication of Pt-CeO2NF@SiO2
2.4.1. Wet Impregnation of Pt on Electrospun CeO2NF
Pt NPs were loaded on the ultrasonicated CeO
2
NFs using the incipient wetness
impregnation technique. Theoretical loading of Pt (10 wt.% and 7 wt.%) on CeO
2
NFs was
done as follows: 0.0058 mmol (2.24 mg) of [Pt(NH
3
)
4
](NO
3
)
2
was dissolved in 500
µ
L of
DI water. Then this solution was added dropwise into 15 mg of CeO
2
NFs. Subsequently,
wetted CeO
2
NFs were dried using a rotary evaporator at 45
◦
C for 45 min at 176 mbar. In
order to obtain Pt-impregnated CeO
2
(labeled as IM-PtCe), dried [Pt(NH
3
)
4
](NO
3
)
2
/CeO
2
NFs were kept at 150 ◦C overnight to stabilize Pt on CeO2NFs.
2.4.2. Preparation of Pre-Synthesized Pt NPs
Preparation of Pt NPs was carried out as follows. Briefly, 21.8 mg PVP
(M
w
= 29,000 gmol
−1
) and 36.8 mg TTAB were dissolved in 10 mL EG and transferred to
an argon-protected three-necked flask equipped with a condenser in Argon protection. A
total of 21.9 mmol (9.1 mg) K
2
PtCl
4
was dissolved in 4 mL EG using sonication for 20 min.
Then, the Pt solution was injected into the flask and stirred for 15 min with a stirring rate
of 250 rpm at room temperature. Afterwards, the reaction temperature was increased
to 175
◦
C, kept for 30 min, and then cooled to room temperature naturally. The Pt NPs
dispersed in EG were used for the solvothermal method.
2.4.3. Loading of Pre-Synthesized Pt on CeO2NF by Solvothermal Method
To prepare the solution, a certain amount of CeO
2
NFs was ultrasonically dispersed in
20 mL of ethanol. Then, the pre-synthesized Pt NPs dispersed in EG solution were added
dropwise into the CeO
2
NF/ethanol suspension to prepare a mixture solution of Pt/CeO
2
NF/ethanol. The mixture was transferred into a Teflon-lined stainless steel container and
placed in an oven at 140
◦
C for 6 h. Afterwards, the obtained solution was centrifuged
at 6000 rpm for 15 min to separate the Pt-loaded CeO
2
NFs and then, dried overnight at
150 ◦C. This sample was labeled as ST-PtCe and was used for further synthesis.
2.4.4. Sol-Gel Synthesis of Pt-CeO2@SiO2
The same sol-gel procedure (Section 2.3.2) was used to prepare the core-sheath of
Pt-CeO
2
@SiO
2
. In a typical synthesis, 20 mg of Pt-CeO
2
NFs were dispersed in 45 mL of
DI water. Then as a separate solution, 225 mg of CTAB was dissolved in 30 mL of ethanol.
The CTAB/Ethanol was subsequently added to Pt-CeO
2
suspension and stirred for some
minutes. About 0.2 mL of ammonia solution was added to the above solution to control
the pH between 9 and 11. Subsequently, 100
µ
L of TEOS (diluted with ethanol) was slowly
added to this solution under stirring. The core-sheath of Pt-CeO
2
@SiO
2
was then separated
by centrifuging at 4000 rpm for 5 min and dried overnight at 80
◦
C. Finally, the solid
products were kept at 360
◦
C for 2 h to remove the CTAB template and to get solid oxides.
Figure 1a–g schematically shows the fabrication and synthesis procedures performed in
this study. All used samples described in the above sections are shown in Table 1.
Nanomaterials 2023,13, 485 5 of 20
Nanomaterials 2023, 13, 485 5 of 20
by centrifuging at 4000 rpm for 5 min and dried overnight at 80 °C. Finally, the solid prod-
ucts were kept at 360 °C for 2 h to remove the CTAB template and to get solid oxides.
Figure 1a–g schematically shows the fabrication and synthesis procedures performed in
this study. All used samples described in the above sections are shown in Table 1.
Figure 1. Schematic of the fabrication procedure for core-sheath NFs: (a) Preparation of spinnable
solution of Ce(NO3)3·6H2O/PVP/DMF, (b) electrospinning of Ce(NO3)3·6H2O/PVP NFs, (c) calcina-
tion of Ce(NO3)3·6H2O/PVP NFs to remove PVP and obtain CeO2 NFs, (d) ultrasonication of CeO2
NFs, (e) Pt nanoparticles were deposited on CeO2 NFs by (e-1) wet impregnation using
[Pt(NH3)4](NO3)2, and (e-2) solvothermal using pre-synthesized Pt from K2PtCl4 following by a heat
treatment for all deposited NFs samples, (f) core-sheath synthesis using sol-gel method of (f-1) IM-
PtCe@TEOS/CTAB, and (f-2) ST-PtCe@TEOS/CTAB, and (g) calcination of core-sheath NFs to obtain
IM-PtCeSi and ST-PtCeSi catalysts.
Table 1. Prepared samples with or without Pt loadings by electrospinning (ES), wet and solvother-
mal impregnation and sol-gel methods. The table is visually represented in Figure 1.
Sample Material/Precursor
Preparation
Method(s)
Heat Treatment/
Calcination Tempera-
ture (°C)
Desired Structure
Com-CeO2 Commercial CeO2 - - CeO2 powder
Ce(NO3)3·6H2O/PVP
NF Ce(NO3)3·6H2O/PVP ES No Composite poly-
mer/metal nitrate fibers
CeO2 NF CeO2 Calcination/
Ultrasonication 600 CeO2 fibers
IM-PtCe [Pt(NH3)4](NO3)2-CeO2 Wet-impregnation 150 Pt-CeO2 fibers
ST-PtCe K2PtCl4-CeO2 Solvothermal deposi-
tion 150 Pt-CeO2 fibers
Figure 1.
Schematic of the fabrication procedure for core-sheath NFs: (
a
) Preparation of spinnable
solution of Ce(NO
3
)
3·
6H
2
O/PVP/DMF, (
b
) electrospinning of Ce(NO
3
)
3·
6H
2
O/PVP NFs, (
c
) cal-
cination of Ce(NO
3
)
3·
6H
2
O/PVP NFs to remove PVP and obtain CeO
2
NFs, (
d
) ultrasonication
of CeO
2
NFs, (
e
) Pt nanoparticles were deposited on CeO
2
NFs by (
e-1
) wet impregnation using
[Pt(NH
3
)
4
](NO
3
)
2
, and (
e-2
) solvothermal using pre-synthesized Pt from K
2
PtCl
4
following by a
heat treatment for all deposited NFs samples, (
f
) core-sheath synthesis using sol-gel method of (
f-1
)
IM-PtCe@TEOS/CTAB, and (
f-2
) ST-PtCe@TEOS/CTAB, and (
g
) calcination of core-sheath NFs to
obtain IM-PtCeSi and ST-PtCeSi catalysts.
Table 1.
Prepared samples with or without Pt loadings by electrospinning (ES), wet and solvothermal
impregnation and sol-gel methods. The table is visually represented in Figure 1.
Sample Material/Precursor Preparation Method(s)
Heat Treatment/
Calcination Temperature
(◦C)
Desired Structure
Com-CeO2Commercial CeO2- - CeO2powder
Ce(NO3)3·6H2O/PVP NF Ce(NO3)3·6H2O/PVP ES No Composite polymer/metal
nitrate fibers
CeO2NF CeO2Calcination/
Ultrasonication 600 CeO2fibers
IM-PtCe [Pt(NH3)4](NO3)2-CeO2Wet-impregnation 150 Pt-CeO2fibers
ST-PtCe K2PtCl4-CeO2Solvothermal deposition 150 Pt-CeO2fibers
IM-PtCeSi aIM-PtCeO2-SiO2Sol-gel method 360 Core-sheath fibers
(IM-Pt-CeO2NF@SiO2)
ST-PtCeSi aST-PtCeO2-SiO2Sol-gel method 360 Core-sheath fibers
(ST-Pt-CeO2NF@SiO2)
aPt loading wt.% in core-sheath NFs is found to be 0.5% based on ICP-OES measurement.
Nanomaterials 2023,13, 485 6 of 20
2.5. Characterization
The crystalline phase composition of CeO
2
NFs and core-sheath NFs after synthesis
and calcination was assessed by X-ray diffraction (XRD, Bruker D8 Advance, Germany)
in the reflection mode using Co K
α
radiation (
λ
= 1.789 Å) in the 2
θ
range of 10–90
◦
with
the step size and time of 0.019
◦
and 192 s, respectively. The indexing of crystalline phases
was performed based on powder diffraction data distributed from the International Centre
for Diffraction Data (ICDD
®
) [
50
,
51
]. Rietveld refinement was implemented using FullProf
Suit software [
52
]. The refinement of all samples was performed by the profile function
7. The resolution of the instrument was provided from the structure refinement of LaB
6
as standard. The parameters corresponding to the refinement consisted of the scale factor,
zero-point of the detector, background parameters, lattice parameters, isotropic atomic
displacement parameters (Biso), asymmetric parameters, and the fractions of side phases.
The identification of functional groups was implemented by Fourier-transform in-
frared spectroscopy (FTIR) in Vertex 70 (Bruker, Germany) in the wavenumber range of
400–4000 cm
−1
. The surface composition of the materials, as well as the chemical state of
the corresponding elements, was analyzed by X-ray photoelectron spectroscopy (XPS). The
XPS measurement was implemented with a source gun type of Al K
α
, a spot size of 400
µ
m,
an energy step size of 0.1 eV, and energy steps of 601 (Thermo Fischer Scientific, Waltham,
MA, USA). The fitting of curves was performed using Origin 2018, and the deconvolution
of the curves was performed by adjusting a shared full width at half maximum (FWHM)
in a Gaussian function. All XPS spectra were corrected based on C1s binding energy of
284.8 eV.
The specific surface area, pore size, and pore volume of NFs were investigated using
N
2
adsorption–desorption at a cryogenic temperature of 77K by QuadraSorb SI device
(Quantachrome Instruments, Boynton Beach, FL, USA). The NFs were outgassed for 12 h at
the temperature of 150
◦
C. Brunauer–Emmett–Teller (BET) theory was employed to assess
the surface area of NFs. The QuadraWin software (Quantachrome Instruments, USA) was
used to explore the BET data.
The microstructure of NFs and core-sheath products were investigated by scanning
electron microscopy (SEM, LEOGEMINI 1530, Zeiss, Jena, Germany). The elemental
analysis was performed using energy-dispersive X-ray spectroscopy (EDS). The samples
were prepared by scattering a layer of carbon to inhibit the charging during characterization.
Morphology and fine microstructure of core-sheath structures were investigated by
transmission electron microscopy (TEM), using a 200 kV LaB6 TECNAI from FEI com-
pany, operated at 200 kV and a high-resolution scanning electron microscopy (STEM),
using a 300 kV cold FEG and probe-corrected JEM-ARM300F2 from JEOL Ltd., Freising,
Germany, operated at 300 kV. Samples were prepared by dispersing a certain amount of
electrospun and synthesized solids in ethanol using ultrasonication. Mapping analysis
was implemented to evaluate the distribution of the elements in core-sheath NFs. The
microscope was operated at 300 kV, equipped with a dual SDD EDX System (JEOL Ltd.)
with a detection area of 2
×
158 mm
2
and an energy resolution of 134 eV. STEM Images
were acquired with a camera length of 8 cm, which corresponds to a HAADF detection
angle of 68–280 mrad.
The amount of Pt loading on CeO
2
NFs was determined using inductively coupled
plasma measurement by a Horiba Scientific ICP Ultima2 (Horiba, Kyoto, Japan).
2.6. Catalytic Activity Test
Reverse water gas shift (RWGS, Equation (1)) is one of the common reactions in
the industry in which CO
2
reacts with hydrogen (H
2
) to produce carbon monoxide
(CO) and water (H
2
O). Due to its endothermic nature, the RWGS is favored at high
temperatures [
19
,
53
]. Low-temperature CO
2
and CO methanation (Equations (2) and (3),
respectively) are side reactions of RWGS.
CO2+H2↔CO +H2O∆H◦=42.1 kJ·mol−1(1)
Nanomaterials 2023,13, 485 7 of 20
CO2+4H2↔CH4+2H2O∆H◦=−165 kJ·mol−1(2)
CO +3H2↔CH4+H2O∆H◦=−206.1 kJ·mol−1(3)
In this study, RWGS was carried out in a stainless-steel fixed-bed tubular column
reactor (inner diameter = 4 mm and length = 70 cm). A 50 mg amount of catalyst was first
diluted in 450 mg of SiC with the mesh sieve of 100–200
µ
m, then loaded into the reactor.
The bottom of the column was packed with a layer of pure SiC (400–500
µ
m) and quartz
wool on which the diluted catalyst was placed. The bed temperature of the reactor was
measured by an installed thermocouple inside the center of the column. Before operating
the catalytic activity test, the catalyst was in situ reduced at 350
◦
C for 2 h in the flow
rates of H
2
(40 mL/min) and N
2
(30 mL/min) at atmospheric pressure. Next, the reactor
was cooled down to the reaction temperature, and pressure was increased to 6.2 bar. The
catalyst was tested under the gas flows of H
2
(30 mL/min), N
2
(15 mL/min), and CO
2
(10 mL/min) with the molar ratio of H
2
:N
2
:CO
2
(3:1.5:1) and a gas hour space velocity
(GHSV) of 66,000 mL gcat
−1
h
−1
. The operating pressure was set to 6.2 bar in all reaction
temperatures. The catalyst performance was measured at three different temperatures of
250, 300, and 350
◦
C. The reaction was set by heating the reactor to the desired temperature
at a rate of 10
◦
C/min. The concentrations of gas products were analyzed online by a gas
chromatography instrument (Schimatzu 7890A) equipped with a thermal conductivity
detector (TCD) and a flame ionized detector (FID). CO and CH
4
were the main products of
the process. The CO
2
conversion (
XCO2
) and selectivity of CO (
SCO
) and CH
4
(
SCH4
) were
calculated through the following Equations (4)–(6):
XCO2=CO2(in)−CO2(out)
CO2(in)
×100% (4)
SCO =CO(out)
CO(out)+CH4(out)
×100% (5)
SCH4=CH4(out)
CO(out)+CH4(out)
×100 (6)
where (in) and (out) are denoted for the mole of reactant and effluent corresponding gases
respectively.
3. Results and Discussion
CeO
2
NFs were fabricated using ES technique, followed by a calcination step at 600
◦
C.
The NFs were further used for synthesizing the core-sheath structure of CeO
2
@SiO
2
via
a sol-gel route. Figure 2shows XRD patterns of all samples, including electrospun CeO
2
NFs, CeSi core-sheath, ST-PtCeSi, and IM-PtCeSi NFs. The measured pattern for CeO
2
NFs
can be attributed to a pure CeO
2
crystal structure based on JCPDS 34-0394 representing
the fluorite cubic structure. The reflections at 28.5
◦
, 33.14
◦
, 47.5
◦
, 56.5
◦
, 59
◦
, 69.5
◦
, 77
◦
,
79
◦
, and 88.5
◦
are associated with (111), (200), (220), (331), (222), (400), (331), (420), and
(422) crystal planes of CeO
2
, respectively [
48
]. After synthesizing a silica layer around the
electrospun CeO
2
NFs, the intensity of XRD reflections is significantly reduced, and the
diffuse scattering at 20–25
◦
increased without the appearance of a new XRD reflection,
suggesting the amorphous structure of the SiO
2
overlayer. Similar changes were also
observed in the XRD patterns of IM-PtCeSi and ST-PtCeSi samples loaded with Pt and
coated with silica layers. Additionally, very tiny XRD reflections are observed at 39.8
◦
,
46.2
◦
, and 67.5
◦
for both catalyst samples of IM-PtCeSi and ST-PtCeSi, which are attributed
to Pt (111), (200), and (220) planes of metallic Pt, respectively.
Nanomaterials 2023,13, 485 8 of 20
Nanomaterials 2023, 13, 485 9 of 20
identified, with a core diameter of about 340 nm and a sheath thickness of about 70 nm.
Although well-designed core-sheath structures have been achieved, a few extra spherical
SiO2 particles can be seen in some regions. These particles might have formed first sepa-
rately during sol-gel synthesis and then attached on the silica sheath. To verify the crys-
tallinity of structure, SAED analyses were performed for both core and sheath layers (Fig-
ure 3e,f). The SAED images confirm the presence of a polycrystalline CeO2 material with
a fluorite cubic structure in the core (coded by 1) of CeO2 NFs and an amorphous silica
sheath (coded by 2).
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
CeO
2
#034-0394
♦
♦
ST-PtCeSi
IM-PtCeSi
CeSi
Intensity (a.u.)
2Theta (deg.)
(111)
(200)
(220) (311)
(222) (400)
CeO
2
-NF
(331)(420) (422)
Pt
Pt
Figure 2. XRD patterns of CeO2-NFs, CeSi, IM-PtCeSi, and ST-PtCeSi samples.
Figure 3. SEM images of (a) as-spun Ce(NO3)3·6H2O/PVP NFs, (b) CeO2 NFs after calcination at 600
°C, Insert shows the size distribution plots of fibers diameter, (c) CeO2 NF after 10 min ultrasoni-
cation, (d) TEM image of CeSi, (e,f) SAED of CeSi, core (1) and sheath (2).
As mentioned before, the ultrasonicated electrospun CeO2 NFs have been used to sup-
port Pt nanoparticles by two different methods: wet impregnation and solvothermal depo-
sition. In the impregnation method, Pt was directly deposited on CeO2 NFs, while PVP and
TTAB were used as capping agent and template for synthesis of Pt NPs and its deposition
on CeO2. After the deposition of Pt, a silica sheath was grown around the Pt-CeO2 NFs to
obtain the core-sheath structure. STEM was used to investigate the Pt distribution and core-
Figure 2. XRD patterns of CeO2-NFs, CeSi, IM-PtCeSi, and ST-PtCeSi samples.
To gain further insight about lattice parameters, crystallite size, microstrain, and
preferred orientation in the samples, Rietveld refinement was performed on the diffraction
patterns. The preferred orientation was determined with the March–Dollase model [
54
],
as implemented in the FULLPROF program. Figure S1 shows the observed, calculated,
and difference profile for the final cycle of the structure refinement. The results of Rietveld
refinement reveal that the lattice parameter of CeO
2
does not change significantly with
synthesizing a silica layer around the electrospun CeO
2
NFs or Pt loading, suggesting that
the silica and Pt are mainly added to the surface of the CeO
2
NFs without altering the
lattice CeO
2
phase. In contrast, the crystallite size of CeO
2
increases from 14.9 nm in CeO
2
NFs to 21.3 nm and 36.5 nm in IM-PtCeSi and ST-PtCeSi, respectively, with the addition
of silica and Pt. The March-Dollase (MD) parameter ralong the <110> directions was
found to be higher than unity in all samples, with increasing from 1.1 CeO
2
NFs to 1.3 and
1.4 for IM-PtCeSi and ST-PtCeSi samples, respectively. This MD parameter rdefines the
crystallites’ habit distribution and is unity for an ideal random-orientation (i.e., no preferred
orientation), greater than one for needle-habit crystals and less than one for platy crystals
pack along the diffraction vector. Thus, the CeO
2
crystals in the NFs are grown as needle-
habit along the <110> directions in all samples. However, the values of MD parameter
rindicate the percentage of excess crystallites with preferential orientation in comparison
with randomly oriented crystallites in the NFs, which means that ST-PtCeSi sample has
crystallites with the highest total preferential orientation along the <110>, followed by the
IM-PtCeSi sample. Moreover, the weight fraction of nanocrystalline Pt metals (~4.4 nm)
was found to be 2.5 (0.2) wt.% and 19.2 (0.5) wt.% in IM-PtCeSi and ST-PtCeSi samples,
respectively. The higher amounts of Pt detected in the samples from XRD compared to the
experimental value (7 wt.%) can be explained by the amount of amorphous silica excluded
in the Reitveld refinement analysis. These results suggest that the Solvothermal deposition
of Pt on the CeO
2
NFs enhances the growth of CeO
2
crystallites along <110> direction and
the formation of a high amount of Pt nanocrystallites (4.4 nm). In contrast, although wet-
impregnation of Pt increases the preferred orientation of CeO
2
along the <110> direction, a
slight increase in the crystallite size of CeO
2
and a low amount of metallic Pt are observed.
Since the same amount of Pt loading was used for both impregnation methods, the low
weight fraction of Pt in the IM-PtCeSi suggests the presence of a high amount of metallic Pt
with a very small crystallites size to be detected by XRD (<2 nm).
The FTIR spectra of CeSi from 4000 to 430 cm
−1
are illustrated in Figure S2. All
absorption bands corresponding to Si-O and Ce-O groups can be seen in the FTIR spectra.
Nanomaterials 2023,13, 485 9 of 20
The band at about 440 cm
−1
corresponds to the Ce-O vibration. The two absorption bands
at 1063 cm
−1
and about 810 cm
−1
can be ascribed as symmetric and asymmetric Si-O-Si
bonding groups, respectively [
55
]. The small band at 1650 cm
−1
can be related to O-H
stretching bond. Likewise, a small absorption band can be observed at 3750 cm
−1,
which is
attributed to the OH vibrations of free silanol groups [56,57].
SEM images ofthe electrospun NFs are showninFigure3. As-spunPVP/Ce(NO
3
)
3·
6H
2
O
NFs are illustrated in Figure 3a. The average diameter of NFs before calcination is about
200–250 nm, while the diameters have been reduced to 90–100 nm after heat treatment of the
NFs at 600
◦
C due to the removal of PVP along with other organic moieties and oxidation of
Ce(NO
3
)
3·
6H
2
O into CeO
2
, as can be seen in Figure 3b. This shows that the fibrous shape
of CeO
2
NFs remains intact while removing PVP from the structure. Figure 3b shows that
the surface of produced NFs is smooth with a relatively uniform average diameter of 143 nm.
Prior to introducing the SiO
2
sheath, the mat of CeO
2
NFs was ultrasonicated for better
accessibility of the entire CeO
2
surface. Figure 3c shows that after ultrasonication, the average
length of the NFs decreased to 500–1000
µ
m. TEM image of the CeSi core-sheath NFs confirms
the successful formation of core-sheath structure (Figure 3d). A clear interface between CeO
2
NFs as core and SiO
2
as sheath can be identified, with a core diameter of about 340 nm and a
sheath thickness of about 70 nm. Although well-designed core-sheath structures have been
achieved, a few extra spherical SiO
2
particles can be seen in some regions. These particles
might have formed first separately during sol-gel synthesis and then attached on the silica
sheath. To verify the crystallinity of structure, SAED analyses were performed for both core
and sheath layers (Figure 3e,f). The SAED images confirm the presence of a polycrystalline
CeO
2
material with a fluorite cubic structure in the core (coded by 1) of CeO
2
NFs and an
amorphous silica sheath (coded by 2).
Nanomaterials 2023, 13, 485 9 of 20
identified, with a core diameter of about 340 nm and a sheath thickness of about 70 nm.
Although well-designed core-sheath structures have been achieved, a few extra spherical
SiO2 particles can be seen in some regions. These particles might have formed first sepa-
rately during sol-gel synthesis and then attached on the silica sheath. To verify the crys-
tallinity of structure, SAED analyses were performed for both core and sheath layers (Fig-
ure 3e,f). The SAED images confirm the presence of a polycrystalline CeO2 material with
a fluorite cubic structure in the core (coded by 1) of CeO2 NFs and an amorphous silica
sheath (coded by 2).
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
CeO
2
#034-0394
♦
♦
ST-PtCeSi
IM-PtCeSi
CeSi
Intensity (a.u.)
2Theta (deg.)
(111)
(200)
(220) (311)
(222) (400)
CeO
2
-NF
(331)(420) (422)
Pt
Pt
Figure 2. XRD patterns of CeO2-NFs, CeSi, IM-PtCeSi, and ST-PtCeSi samples.
Figure 3. SEM images of (a) as-spun Ce(NO3)3·6H2O/PVP NFs, (b) CeO2 NFs after calcination at 600
°C, Insert shows the size distribution plots of fibers diameter, (c) CeO2 NF after 10 min ultrasoni-
cation, (d) TEM image of CeSi, (e,f) SAED of CeSi, core (1) and sheath (2).
As mentioned before, the ultrasonicated electrospun CeO2 NFs have been used to sup-
port Pt nanoparticles by two different methods: wet impregnation and solvothermal depo-
sition. In the impregnation method, Pt was directly deposited on CeO2 NFs, while PVP and
TTAB were used as capping agent and template for synthesis of Pt NPs and its deposition
on CeO2. After the deposition of Pt, a silica sheath was grown around the Pt-CeO2 NFs to
obtain the core-sheath structure. STEM was used to investigate the Pt distribution and core-
Figure 3.
SEM images of (
a
) as-spun Ce(NO
3
)
3·
6H
2
O/PVP NFs, (
b
) CeO
2
NFs after calcination
at 600
◦
C, Insert shows the size distribution plots of fibers diameter, (
c
) CeO
2
NF after 10 min
ultrasonication, (d) TEM image of CeSi, (e,f) SAED of CeSi, core (1) and sheath (2).
As mentioned before, the ultrasonicated electrospun CeO
2
NFs have been used to
support Pt nanoparticles by two different methods: wet impregnation and solvothermal
deposition. In the impregnation method, Pt was directly deposited on CeO
2
NFs, while
PVP and TTAB were used as capping agent and template for synthesis of Pt NPs and
its deposition on CeO
2
. After the deposition of Pt, a silica sheath was grown around
the Pt-CeO
2
NFs to obtain the core-sheath structure. STEM was used to investigate the
Pt distribution and core-sheath morphology. Distinguishable interfaces between core
and sheath can be observed for IM-PtCeSi with the theoretical 10 wt.% of Pt loading on
CeO
2
NF. The core diameter is about 100 nm, and the sheath thickness is around 40 nm
(Figure S3). Elemental mapping of Ce, Si, O, and Pt was also performed and is shown in
Figure S3. As can be seen, Pt presents in some areas of the sheath layer. To improve the
Nanomaterials 2023,13, 485 10 of 20
Pt stabilization on the CeO
2
core, a heat treatment at 150
◦
C for 24 h was carried out after
wet impregnation. Furthermore, the theoretical loading of Pt on CeO
2
NF was decreased
from 10 wt.% to 7 wt.% for IM-PtCeSi. As shown in Figure 4, the successful formation of
the core-sheath structure and the well distribution and stabilization of Pt on CeO
2
were
obtained. It can be concluded that no migration of Pt into the sheath has taken place. The
elemental mapping analysis of IM-PtCeSi confirms as well that Pt is well distributed on
CeO
2
. The diameter of core-sheath NF and the sheath thickness were about 160 nm and
55 nm, respectively. Solvothermal deposition of pre-synthesized NPs on metal oxides has
been utilized to increase the interaction between metal and support [
58
]. So, in another
approach, solvothermal deposition of Pt on CeO
2
NFs was adopted to further enhance the
distribution and stabilization of Pt on the substrate. Figure S4 illustrates STEM images of
ST-PtCeSi sample which also shows the migration of Pt into the SiO
2
sheath. To improve the
Pt stabilization and prevent its detachment from the surface of CeO
2
NFs, a heat treatment
at 150
◦
C for 24 h was applied after the solvothermal method,
Figures 5and S5.
A well-
defined core-sheath structure is obtained with a core diameter of about 110 nm and a sheath
thickness of about 42 nm.
The particle size of Pt in ST-PtCeSi was measured to be 3.8 nm, consistence with crys-
tallite size determined from XRD analysis (4.4 nm). The particle size of the pre-synthesized
Pt NPs used in the solvothermal method is larger than that used in the wet impregna-
tion method, Figure S5, and it agrees with XRD results. Considering Figure 4, it can be
concluded that the Pt particle size in IM-PtCeSi is less than 3.8 nm.
Nanomaterials 2023, 13, 485 10 of 20
As mentioned before, the ultrasonicated electrospun CeO2 NFs have been used to sup-
port Pt nanoparticles by two different methods: wet impregnation and solvothermal depo-
sition. In the impregnation method, Pt was directly deposited on CeO2 NFs, while PVP and
TTAB were used as capping agent and template for synthesis of Pt NPs and its deposition
on CeO2. After the deposition of Pt, a silica sheath was grown around the Pt-CeO2 NFs to
obtain the core-sheath structure. STEM was used to investigate the Pt distribution and core-
sheath morphology. Distinguishable interfaces between core and sheath can be observed for
IM-PtCeSi with the theoretical 10 wt.% of Pt loading on CeO2 NF. The core diameter is about
100 nm, and the sheath thickness is around 40 nm (Figure S3). Elemental mapping of Ce, Si,
O, and Pt was also performed and is shown in Figure S3. As can be seen, Pt presents in some
areas of the sheath layer. To improve the Pt stabilization on the CeO2 core, a heat treatment
at 150 °C for 24 h was carried out after wet impregnation. Furthermore, the theoretical load-
ing of Pt on CeO2 NF was decreased from 10 wt.% to 7 wt.% for IM-PtCeSi. As shown in
Figure 4, the successful formation of the core-sheath structure and the well distribution and
stabilization of Pt on CeO2 were obtained. It can be concluded that no migration of Pt into
the sheath has taken place. The elemental mapping analysis of IM-PtCeSi confirms as well
that Pt is well distributed on CeO2. The diameter of core-sheath NF and the sheath thickness
were about 160 nm and 55 nm, respectively. Solvothermal deposition of pre-synthesized
NPs on metal oxides has been utilized to increase the interaction between metal and support
[58]. So, in another approach, solvothermal deposition of Pt on CeO2 NFs was adopted to
further enhance the distribution and stabilization of Pt on the substrate. Figure S4 illustrates
STEM images of ST-PtCeSi sample which also shows the migration of Pt into the SiO2 sheath.
To improve the Pt stabilization and prevent its detachment from the surface of CeO2 NFs, a
heat treatment at 150 °C for 24 h was applied after the solvothermal method, Figures 5 and
S5. A well-defined core-sheath structure is obtained with a core diameter of about 110 nm
and a sheath thickness of about 42 nm.
Figure 4. STEM of IM-PtCeSi in different imaging: (a) high-angle annular dark field mode, (b) an-
nular bright-field mode, (c) bright-field mode, (d) secondary electron mode, (e) EDS elemental map-
ping of Ce, (f) EDS elemental mapping of Pt, and (g) EDS elemental mapping of Si.
Figure 4.
STEM of IM-PtCeSi in different imaging: (
a
) high-angle annular dark field mode, (
b
) annular
bright-field mode, (
c
) bright-field mode, (
d
) secondary electron mode, (
e
) EDS elemental mapping of
Ce, (f) EDS elemental mapping of Pt, and (g) EDS elemental mapping of Si.
Nanomaterials 2023,13, 485 11 of 20
Nanomaterials 2023, 13, 485 11 of 20
Figure 5. STEM of ST-PtCeSi in different imaging: (a) high-angle annular dark field mode, (b) an-
nular bright-field mode, (c) bright-field mode, (d) secondary electron mode, (e) EDS elemental map-
ping of Ce, (f) EDS elemental mapping of Pt, and (g) EDS elemental mapping of Si.
The particle size of Pt in ST-PtCeSi was measured to be 3.8 nm, consistence with crys-
tallite size determined from XRD analysis (4.4 nm). The particle size of the pre-synthesized
Pt NPs used in the solvothermal method is larger than that used in the wet impregnation
method, Figure S5, and it agrees with XRD results. Considering Figure 4, it can be con-
cluded that the Pt particle size in IM-PtCeSi is less than 3.8 nm.
Figure 6 shows the nitrogen adsorption–desorption isotherms for electrospun CeO2
NFs with a specific surface area of 18 m2·g−1. A type IV BET isotherm is obtained for CeSi
(Figure 6a), from which a surface area of 476.5 m2·g−1 can be calculated. While a type IV of
BET isotherm appeared for the core-sheath sample of CeSi with a hysteresis loop, Figure
6b. The significant increase in surface area can be attributed to the presence of the porous
SiO2 layer. The inset graph in Figure 6b shows the Barret–Joyner—Halenda (BJH) pore
size distribution curve for CeSi, which gives a pore diameter of about 2.8 nm for the silica
layer.
For a better understanding of the chemical states of samples, the XPS analysis was
performed and compared with CeO2-Com. Figure 7 and Figure S6 show the XPS spectra
of Ce 3d and O 1s [59–64], respectively, for CeO2-Com, CeO2 NF, IM-PtCe, SN-PtCe, IM-
PtCeSi, and ST-PtCeSi. Multiplets of u and v corresponding to the spin-orbital splitting of
Ce 3d3/2 and 3d5/2 are observed in Figure 7. The spin-orbit splitting of Ce 3d is reported to
be about 18.4 eV [65]. Typically, the Ce 3d spectrum displays five doublet pairs. The dou-
blet pairs of (u0-v0), (u′-v′) ascribes to Ce(III), whereas (u-v), (u″-v″), and (u‴-v‴) are as-
cribed to Ce(IV) [66,67]. The doublets with corresponding oxidation states are indicated
in the XPS spectrum for each sample. In the case of CeO2 NFs, in addition to the Ce4+ peaks,
the peaks with the binding energies (BEs) of about 903 and 885 eV indicate the presence
of Ce3+ species in the electrospun CeO2 NFs. Both oxidation states are also observed in
CeO2-Com.
Figure 5.
STEM of ST-PtCeSi in different imaging: (
a
) high-angle annular dark field mode, (
b
) annular
bright-field mode, (
c
) bright-field mode, (
d
) secondary electron mode, (
e
) EDS elemental mapping of
Ce, (f) EDS elemental mapping of Pt, and (g) EDS elemental mapping of Si.
Figure 6shows the nitrogen adsorption–desorption isotherms for electrospun CeO
2
NFs with a specific surface area of 18 m
2·
g
−1
. A type IV BET isotherm is obtained for CeSi
(Figure 6a), from which a surface area of 476.5 m
2·
g
−1
can be calculated. While a type IV of
BET isotherm appeared for the core-sheath sample of CeSi with a hysteresis loop, Figure 6b.
The significant increase in surface area can be attributed to the presence of the porous SiO
2
layer. The inset graph in Figure 6b shows the Barret–Joyner—Halenda (BJH) pore size
distribution curve for CeSi, which gives a pore diameter of about 2.8 nm for the silica layer.
Nanomaterials 2023, 13, 485 12 of 20
0.0 0.2 0.4 0.6 0.8 1.0
0
5
10
15
20
25
(a)
CeO
2
-NF
Quantity adsorbed (cm
3
/g STP)
Relative pressure (P/P
0
)
Adsorption
Desorption
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
250
300
350
(
b
)
Quantity adsorbed (cm
3
/g STP)
Relative pressure (P/P
0
)
CeSi
0 2 4 6 8 10 12 14 16 18
0.0
0.2
0.4
0.6
0.8
1.0
1.2
dV/dD (cm
3
nm
−1
g
−1
)
Pore diameter (nm)
Adsorption
Desorption
Figure 6. Nitrogen adsorption–desorption isotherm of (a) electrospun CeO2 NFs, and (b) core-sheath
of CeSi, insert graph shows pore diameter of CeSi.
The fractional amount of Ce4+ and Ce3+ was calculated using Equations (S1)–(S3) [66].
As shown in Figure S7, the commercial CeO2 powder consists of 24.4% of Ce3+ and 75.6%
of Ce4+, while a Ce3+ concentration of 13.6% was obtained for electrospun CeO2-NFs. Dur-
ing wet impregnation of Pt on the CeO2-NFs, the amount of Ce3+ species increases to 20.4%,
indicating that more oxygen vacancies have been generated. Similarly, the fraction of Ce3+
species in the ST-PtCe sample rises to 17.5%. The higher oxygen vacancies observed in
IM-PtCe might be related to the more uniform distribution of Pt on the surface of CeO2
NFs compared to ST-PtCe (see Figures 4 and 5). Since XPS is a surface-sensitive technique,
a core-level etching XPS of the core-sheath NFs was performed to obtain precise infor-
mation for Ce species. As can be seen in the spectra of IM-PtCeSi and ST-PtCeSi (Figure
7), the intensity of the peaks at 881.8 eV, 900.3 eV, 916.5 eV related to Ce4+ significantly
decreases, whereas for Ce3+ increases, indicating that the oxidation state of Ce in the vicin-
ity of SiO2 sheath was changed to non-stoichiometric (CeO2−X) [68–70]. The observed re-
duction of CeO2 on the surface can be likely attributed to the presence of free OH− groups
in the synthesis medium during the SiO2 sheath formation. As discussed in Rietveld re-
finement analysis (Figure S1), the lattice parameter of CeO2 was not significantly changed
in all samples meaning that the reduction of Ce4+ to Ce3+ did not occur in the bulk of the
materials and considering to the XPS analysis, it can be concluded that Ce3+ species was
merely attributed to the surface.
The Pt 4f region (Figure 8) is deconvoluted into two spin-orbit split doublets of 4f5/2
and 4f7/2. All peaks in IM-PtCe are attributed to the oxidized form of Pt with the charac-
teristic peaks at 75.95 eV (Pt/
) and 72.6 eV (Pt/
) [71-76]. The oxidation of Pt in IM-PtCe
can be due to the heat treatment before growing the silica sheath. For the core-sheath sam-
ple of IM-PtCeSi, an etching-XPS measurement was performed to access the Pt deposited
on the CeO2 core. The spectra looks similar and comparable to IM-PtCe. The Pt 4f spectra
for ST-PtCe and ST-PtCeSi are shown in Figure 8b. In ST-PtCe, Pt exposes two doublets at
the BEs of 70.96 and 74.31 eV related to the metallic form of Pt (Pt/
and Pt/
), and BEs
of 73.38 eV and 76.73 eV associated with the oxidized forms of Pt. A tiny doublet corre-
sponding to Br3d present in ST-PtCe spectra at the BEs of 67.74 eV and 68.78 eV [77], can
be attributed to bromide ions in TTAB used for the preparation of pre-synthesized Pt and
left on CeO2 NFs surface. After introducing the SiO2 sheath and heat treatment at 360 °C,
the peaks at the BEs of 71.25 eV, 74.6 eV were observed in ST-PtCeSi, which can be as-
signed to the metallic form of Pt [78,79]. The larger particle size in ST-PtCe and ST-PtCeSi
Figure 6.
Nitrogen adsorption–desorption isotherm of (
a
) electrospun CeO
2
NFs, and (
b
) core-sheath
of CeSi, insert graph shows pore diameter of CeSi.
Nanomaterials 2023,13, 485 12 of 20
For a better understanding of the chemical states of samples, the XPS analysis was
performed and compared with CeO
2
-Com. Figures 7and S6 show the XPS spectra of Ce
3d and O 1s [59–64], respectively, for CeO2-Com, CeO2NF, IM-PtCe, SN-PtCe, IM-PtCeSi,
and ST-PtCeSi. Multiplets of u and v corresponding to the spin-orbital splitting of Ce 3d
3/2
and 3d
5/2
are observed in Figure 7. The spin-orbit splitting of Ce 3d is reported to be about
18.4 eV [
65
]. Typically, the Ce 3d spectrum displays five doublet pairs. The doublet pairs
of (u
0
-v
0
), (u
0
-v
0
) ascribes to Ce(III), whereas (u-v), (u”-v”), and (u”’-v”’) are ascribed to
Ce(IV) [
66
,
67
]. The doublets with corresponding oxidation states are indicated in the XPS
spectrum for each sample. In the case of CeO
2
NFs, in addition to the Ce
4+
peaks, the
peaks with the binding energies (BEs) of about 903 and 885 eV indicate the presence of Ce
3+
species in the electrospun CeO
2
NFs. Both oxidation states are also observed in CeO
2
-Com.
Nanomaterials 2023, 13, 485 13 of 20
causes a peak shift to lower BE, as also has been reported in the literature [80]. The peak
positions, oxidation states, and corresponding integrated areas for Ce and Pt are summa-
rized in Tables S1 and S2.
930 920 910 900 890 880 870
u'
u
0
v' v
0
Ce3+3d5/2
v
Ce3+3d5/2
v''
v'
u
Ce3+3d3/2
u'
u''
ST-PtCeSi
IM-PtCeSi
ST-PtCe
IM-PtCe
CeO
2
-NF
CeO
2
-Com
Raw data
Fitting curve
Fitting curve
Cumulative
Ce 3d
v'''
Intensity (a.u.)
Ce3+3d3/2
Ce3+3d3/2
Ce4+3d5/2
Ce4+3d5/2
Ce4+3d3/2
Ce4+3d5/2
Ce4+3d3/2
Ce4+3d3/2
Binding Energy (eV)
u'''
Ce3+3d5/2
Figure 7. Ce 3d X-ray photoelectronic spectra for different samples.
To investigate the basicity of the materials, CO2-TPD was performed for IM-PtCeSi
and ST-PtCeSi and presented in Figure S8. The high-temperature desorption peaks ob-
served at 550 and 650 °C for ST-PtCeSi and IM-PtCeSi, respectively, reveal relatively
strong basicity in both catalysts. The higher desorption temperature for IM-PtCeSi con-
firms its stronger basicity compared to ST-PtCeSi at high temperature. Moreover, the de-
sorption peak for IM-PtCeSi is much more intense than ST-PtCeSi, revealing a higher
amount of adsorption centers for CO2 molecules.
Figure 7. Ce 3d X-ray photoelectronic spectra for different samples.
The fractional amount of Ce
4+
and Ce
3+
was calculated using Equations (S1)–(S3) [
66
].
As shown in Figure S7, the commercial CeO
2
powder consists of 24.4% of Ce
3+
and 75.6% of
Ce
4+
, while a Ce
3+
concentration of 13.6% was obtained for electrospun CeO
2
-NFs. During
wet impregnation of Pt on the CeO
2
-NFs, the amount of Ce
3+
species increases to 20.4%,
indicating that more oxygen vacancies have been generated. Similarly, the fraction of Ce
3+
species in the ST-PtCe sample rises to 17.5%. The higher oxygen vacancies observed in
IM-PtCe might be related to the more uniform distribution of Pt on the surface of CeO
2
NFs
Nanomaterials 2023,13, 485 13 of 20
compared to ST-PtCe (see Figures 4and 5). Since XPS is a surface-sensitive technique, a core-
level etching XPS of the core-sheath NFs was performed to obtain precise information for
Ce species. As can be seen in the spectra of IM-PtCeSi and ST-PtCeSi (Figure 7), the intensity
of the peaks at 881.8 eV, 900.3 eV, 916.5 eV related to Ce
4+
significantly decreases, whereas
for Ce
3+
increases, indicating that the oxidation state of Ce in the vicinity of SiO
2
sheath
was changed to non-stoichiometric (CeO
2−X
) [
68
–
70
]. The observed reduction of CeO
2
on
the surface can be likely attributed to the presence of free OH
−
groups in the synthesis
medium during the SiO
2
sheath formation. As discussed in Rietveld refinement analysis
(Figure S1), the lattice parameter of CeO
2
was not significantly changed in all samples
meaning that the reduction of Ce
4+
to Ce
3+
did not occur in the bulk of the materials and
considering to the XPS analysis, it can be concluded that Ce
3+
species was merely attributed
to the surface.
The Pt 4f region (Figure 8) is deconvoluted into two spin-orbit split doublets of
4f
5/2
and 4f
7/2
. All peaks in IM-PtCe are attributed to the oxidized form of Pt with the
characteristic peaks at 75.95 eV (
Pt2+
5/2
) and 72.6 eV (
Pt2+
7/2
) [
71
–
76
]. The oxidation of Pt in
IM-PtCe can be due to the heat treatment before growing the silica sheath. For the core-
sheath sample of IM-PtCeSi, an etching-XPS measurement was performed to access the Pt
deposited on the CeO
2
core. The spectra looks similar and comparable to IM-PtCe. The Pt
4f spectra for ST-PtCe and ST-PtCeSi are shown in Figure 8b. In ST-PtCe, Pt exposes two
doublets at the BEs of 70.96 and 74.31 eV related to the metallic form of Pt (
Pt0
7/2 and Pt0
5/2
),
and BEs of 73.38 eV and 76.73 eV associated with the oxidized forms of Pt. A tiny doublet
corresponding to Br3d present in ST-PtCe spectra at the BEs of 67.74 eV and 68.78 eV [
77
],
can be attributed to bromide ions in TTAB used for the preparation of pre-synthesized
Pt and left on CeO
2
NFs surface. After introducing the SiO
2
sheath and heat treatment
at 360
◦
C, the peaks at the BEs of 71.25 eV, 74.6 eV were observed in ST-PtCeSi, which
can be assigned to the metallic form of Pt [
78
,
79
]. The larger particle size in ST-PtCe and
ST-PtCeSi causes a peak shift to lower BE, as also has been reported in the literature [
80
].
The peak positions, oxidation states, and corresponding integrated areas for Ce and Pt are
summarized in Tables S1 and S2.
Nanomaterials 2023, 13, 485 14 of 20
86 84 82 80 78 76 74 72 70 68 66
(a)
Intensity (a.u.)
Binding Energy (eV)
Raw data
Fitting curve
Fitting curve
Cumulative curve
Baseline
Pt 4f
IM-PtCe
IM-PtCeSi
76.15 eV
72.8 eV
75.95 eV
72.6 eV
86 84 82 80 78 76 74 72 70 68 66
(b)
ST-PtCe
Intensity (a.u.)
Binding Energy (eV)
ST-PtCeSi
Pt 4f
Br 3d
Raw data
Fitting curve
Fitting curve
Cumulative curve
Baseline
73.38 eV
76.73 eV
74.6 eV
71.25 eV
74.31 eV
70.96 eV
Figure 8. Pt 4f X-ray photoelectronic spectra of (a) IM-PtCe and IM-PtCeSi, and (b) ST-PtCe, and ST-
PtCeSi.
Both core-sheath catalysts, IM-PtCeSi and ST-PtCeSi, were tested for the RWGS reac-
tion at different temperatures, 250, 300, and 350 °C. Figure 9 shows the temperature profile
and CO2 conversion during the reaction. The selectivity of CO and CH4 for both catalysts is
illustrated in Figure S9. As RWGS is an endothermic reaction, the CO2 conversion increases
with increasing temperature [81-83]. The highest CO2 conversion of 9% and 6.7% was ob-
tained for IM-PtCeSi and ST-PtCeSi, respectively, at 350 °C. Figure S10 shows the equilib-
rium conversion of CO2 between 200 and 600 °C. The experimental CO2 conversions ob-
tained in this study, are less than the equilibrium one, for example, at 350 °C a CO2 conver-
sion of 30% can be achieved at the equilibrium conditions. The CO2 hydrogenation mecha-
nism has been reported previously [35], in which CO2 can be converted through the redox
mechanism and directly reacts with the oxygen vacancies in CeO2, producing CO. In this
case, Pt mainly contributes to generate oxygen vacancies by activating H2, and then the ox-
ygen vacancies migrate to CeO2. XPS spectra of core-sheath samples (Figure 7) confirmed
the existence of oxygen vacancies in the corresponding interface of ceria core and SiO2
sheath. Moreover, IM-PtCe possesses higher Ce3+ species (Figure S7), so more oxygen va-
cancies are generated in this sample, therefore it can be expected that the catalytic perfor-
mance of IM-PtCeSi becomes higher than ST-PtCeSi. In addition, due to the fact that Pt can
spill over hydrogen to the neighboring CeO2 [84], smaller particle size and higher distribu-
tion of Pt can lead to more hydrogen placing in its coordination to CeO2, which might en-
hance the catalytic performance. The smaller Pt particle size and its high distribution present
in IM-PtCeSi sample compared to ST-PtCeSi can provide a higher surface area for Pt parti-
cles on CeO2 NF that improves corresponding catalytic efficiency. With regard to CO2-TPD
results, IM-PtCeSi shows higher CO2 uptake compared to St-PtCeSi (Figure S8), thus CO2
has more opportunity to be reduced to CO in IM-PtCeSi catalyst leading to its higher cata-
lytic performance. Moreover, the presence of a smaller crystallite size of CeO2, grown along
the <110> direction, in IM-PtCeSi catalyst compared to ST-PtCeSi could be another reason
for higher catalytic performance. Tan et al. reported that the exposed (110) facets and the
existence of abundant oxygen vacancies in CeO2 nanotube-supported Cu-Ni contribute to
its higher catalytic performance for CO2 hydrogenation [46]. Table 2 summarizes the perfor-
mance of different tested catalysts for RWGS with various catalysts and the Pt loadings% of
Figure 8.
Pt 4f X-ray photoelectronic spectra of (
a
) IM-PtCe and IM-PtCeSi, and (
b
) ST-PtCe, and
ST-PtCeSi.
Nanomaterials 2023,13, 485 14 of 20
To investigate the basicity of the materials, CO
2
-TPD was performed for IM-PtCeSi and
ST-PtCeSi and presented in Figure S8. The high-temperature desorption peaks observed at
550 and 650 ◦C for ST-PtCeSi and IM-PtCeSi, respectively, reveal relatively strong basicity
in both catalysts. The higher desorption temperature for IM-PtCeSi confirms its stronger
basicity compared to ST-PtCeSi at high temperature. Moreover, the desorption peak for
IM-PtCeSi is much more intense than ST-PtCeSi, revealing a higher amount of adsorption
centers for CO2molecules.
Both core-sheath catalysts, IM-PtCeSi and ST-PtCeSi, were tested for the RWGS reac-
tion at different temperatures, 250, 300, and 350
◦
C. Figure 9shows the temperature profile
and CO
2
conversion during the reaction. The selectivity of CO and CH
4
for both catalysts
is illustrated in Figure S9. As RWGS is an endothermic reaction, the CO
2
conversion in-
creases with increasing temperature [
81
–
83
]. The highest CO
2
conversion of 9% and 6.7%
was obtained for IM-PtCeSi and ST-PtCeSi, respectively, at 350
◦
C. Figure S10 shows the
equilibrium conversion of CO
2
between 200 and 600
◦
C. The experimental CO
2
conversions
obtained in this study, are less than the equilibrium one, for example, at 350
◦
C a CO
2
conversion of 30% can be achieved at the equilibrium conditions. The CO
2
hydrogenation
mechanism has been reported previously [
35
], in which CO
2
can be converted through
the redox mechanism and directly reacts with the oxygen vacancies in CeO
2
, producing
CO. In this case, Pt mainly contributes to generate oxygen vacancies by activating H
2
, and
then the oxygen vacancies migrate to CeO
2
. XPS spectra of core-sheath samples (Figure 7)
confirmed the existence of oxygen vacancies in the corresponding interface of ceria core
and SiO
2
sheath. Moreover, IM-PtCe possesses higher Ce
3+
species (Figure S7), so more
oxygen vacancies are generated in this sample, therefore it can be expected that the cat-
alytic performance of IM-PtCeSi becomes higher than ST-PtCeSi. In addition, due to the
fact that Pt can spill over hydrogen to the neighboring CeO
2
[
84
], smaller particle size
and higher distribution of Pt can lead to more hydrogen placing in its coordination to
CeO
2
, which might enhance the catalytic performance. The smaller Pt particle size and
its high distribution present in IM-PtCeSi sample compared to ST-PtCeSi can provide a
higher surface area for Pt particles on CeO
2
NF that improves corresponding catalytic
efficiency. With regard to CO
2
-TPD results, IM-PtCeSi shows higher CO
2
uptake compared
to St-PtCeSi (Figure S8), thus CO
2
has more opportunity to be reduced to CO in IM-PtCeSi
catalyst leading to its higher catalytic performance. Moreover, the presence of a smaller
crystallite size of CeO2, grown along the <110> direction, in IM-PtCeSi catalyst compared
to ST-PtCeSi could be another reason for higher catalytic performance. Tan et al. reported
that the exposed (110) facets and the existence of abundant oxygen vacancies in CeO
2
nanotube-supported Cu-Ni contribute to its higher catalytic performance for CO
2
hydro-
genation [
46
]. Table 2summarizes the performance of different tested catalysts for RWGS
with various catalysts and the Pt loadings% of 0.025 to 2%wt, showing that increasing the
loading amount of Pt enhances the CO
2
conversion. Considering Table 2, the obtained
CO
2
conversions in our study are comparable to the catalysts with the same Pt loading
tested at a similar range of temperatures [
23
,
24
,
36
,
37
,
85
–
88
]. Besides the catalyst type,
several parameters corresponding to reactor configuration (reactor type, reactor dimension,
etc.,) and operation conditions (temperature, pressure, GHSV, feed ratio) can also affect
the catalytic performance; therefore, a precise comparison study is challenging in RWGS.
A conversion of 6.7% has been reported by Chen et al. for Pt-CeO
2
nanoparticles with
the Pt loading of 1 wt.% at 300
◦
C [
36
], whereas in this study, core-sheath NFs showed
less conversion with 0.5 wt % Pt loading in a higher GHSV, at the same temperature. In
another study performed by Zhao et al. [
37
], 8.5% CO
2
conversion with more than 98%
CO selectivity had been reported approximately at similar testing conditions, while the
core-sheath catalysts tested in our study showed nearly the same conversion, 9%, and a
higher CO selectivity of 99%. Considering this comparison, it can be assumed that the
access of the reactants to the core is not completely prohibited by silica sheath and Pt
active sites are accessible. Pt-CeO
2
@SiO
2
-Co spherical NPs have been studied for CO
2
hydrogenation tandem systems to olefins. A CO
2
conversion of 25% and more than 98%
Nanomaterials 2023,13, 485 15 of 20
selectivity could be achieved at 350
◦
C over Pt-CeO
2
@SiO
2
nanoparticles for RWGS reaction
with the loading of 4.4% Pt [
49
]. Whereases in this study, the core-sheath NFs showed
conversions of 9% and 6.8% with IM-PtCe and ST-PtCeSi, respectively, with around nine
times lower Pt loading. Although the obtained conversion in this study is not very high
considering the thermodynamic conversion, the synthesis approach can be optimized in
future to improve the catalytic performance. The proposed catalysts can be potentially
used in a tandem system as a basic structure by introducing a second active metal on the
sheath layer. It should be noticed that in a tandem system such as CO
2
hydrogenation, the
obtained conversion of RWGS should be optimized considering the second active site to
reach the best ratio of subsequent reactions. Based on the systematic synthesis approach
developed in this article, the proposed method can be used to synthesize different catalysts
for a variety of tandem application.
Nanomaterials 2023, 13, 485 15 of 20
0.025 to 2%wt, showing that increasing the loading amount of Pt enhances the CO2 conver-
sion. Considering Table 2, the obtained CO2 conversions in our study are comparable to the
catalysts with the same Pt loading tested at a similar range of temperatures [23,24,36,37,85–
88]. Besides the catalyst type, several parameters corresponding to reactor configuration (re-
actor type, reactor dimension, etc.,) and operation conditions (temperature, pressure,
GHSV, feed ratio) can also affect the catalytic performance; therefore, a precise comparison
study is challenging in RWGS. A conversion of 6.7% has been reported by Chen et al. for Pt-
CeO2 nanoparticles with the Pt loading of 1wt.% at 300 °C [36], whereas in this study, core-
sheath NFs showed less conversion with 0.5 wt % Pt loading in a higher GHSV, at the same
temperature. In another study performed by Zhao et al. [37], 8.5% CO2 conversion with more
than 98% CO selectivity had been reported approximately at similar testing conditions,
while the core-sheath catalysts tested in our study showed nearly the same conversion, 9%,
and a higher CO selectivity of 99%. Considering this comparison, it can be assumed that the
access of the reactants to the core is not completely prohibited by silica sheath and Pt active
sites are accessible. Pt-CeO2@SiO2-Co spherical NPs have been studied for CO2 hydrogena-
tion tandem systems to olefins. A CO2 conversion of 25% and more than 98% selectivity
could be achieved at 350 °C over Pt-CeO2@SiO2 nanoparticles for RWGS reaction with the
loading of 4.4% Pt [49]. Whereases in this study, the core-sheath NFs showed conversions
of 9% and 6.8% with IM-PtCe and ST-PtCeSi, respectively, with around nine times lower Pt
loading. Although the obtained conversion in this study is not very high considering the
thermodynamic conversion, the synthesis approach can be optimized in future to improve
the catalytic performance. The proposed catalysts can be potentially used in a tandem sys-
tem as a basic structure by introducing a second active metal on the sheath layer. It should
be noticed that in a tandem system such as CO2 hydrogenation, the obtained conversion of
RWGS should be optimized considering the second active site to reach the best ratio of sub-
sequent reactions. Based on the systematic synthesis approach developed in this article, the
proposed method can be used to synthesize different catalysts for a variety of tandem ap-
plication.
0 50 100 150 200 250 300 350
0
2
4
6
8
10
12
14
240 260 280 300 320 340 360
0
1
2
3
4
5
6
7
8
9
10
CO
2
Conversion (%)
Temperature ( °C)
IM-PtCeSi
SN-PtCeSi
CO
2
Conversion (%)
IM-PtCeSi
ST-PtCeSi
Temperature profile
Ramping rate = 10 °C/min
Time on stream (min)
240
250
260
270
280
290
300
310
320
330
340
350
360
Temperature (°C)
Figure 9. Catalytic testing of IM-PtCeSi and ST-PtCeSi, CO2 conversion, and temperature profile vs.
time on stream for RWGS. Inset figure: CO2 conversion (in average values) vs. temperature.
Figure 9. Catalytic testing of IM-PtCeSi and ST-PtCeSi, CO2conversion, and temperature profile vs.
time on stream for RWGS. Inset figure: CO2conversion (in average values) vs. temperature.
Table 2.
A perspective of different catalysts performance for RWGS based on Pt loading (%wt.). All
reactions were performed in a fixed-bed reactor.
Catalyst Structure
Pt Loading Temperature GHSV XCO2SCO Ref.
%wt ◦C mL gcat−1h−1% %
Pt-CeO2NF@SiO2Core-sheath NFs 0.5 350 66,000 9 ≈99 This work
Pt-Al2O3Nanoparticles 0.0125–0.25 300 80,000 9 >99 [23]
Pt-TiO2Nanoparticles 0.025 and 2 250 80,000 <2 100 [85]
Pt-CeO2Nanorods 0.3 350 72,000 8.5 >98 [37]
Pt-TiO2Nanoparticles 0.5 400 6000 15 ≈98 [24]
Pt-CeO2Nanoparticles 1 300 30,000 6.7 NA [36]
Pt/20%CeO2-TiO2Nanoparticles 1 300 12,000 6.5 NA [86]
TiO2-supported Pt Nanoparticles 1 300 12,000 20 NA [87]
Pt-CeO2NA 2 290 600,000 20 NA [88]
In order to investigate the morphology of the tested catalysts, a TEM analysis was
performed after reaction, Figure S11. As seen, the morphology of catalysts is not changed,
Nanomaterials 2023,13, 485 16 of 20
and a clear interface between the core and sheath can be observed in both catalysts. This in-
dicates that the core-sheath structures remained intact under the catalytic testing conditions.
However, Pt aggregation has been observed for ST-PtCeSi, Figure S11d.
4. Conclusions
The electrospinning technique has the potential to produce NFs on a large scale,
compared to the chemical syntheses in which the product might be yielded on limited
scales. The NFs morphology can also be tuned using this technique so that potentially a
bulk texture of nonwoven NFs can be directly utilized for further surface modifications
and tested in a reactor for catalysis. The electrospun NFs can provide a higher surface
area which might improve the catalytic performance. In this work, core-sheath NFs of
Pt-CeO
2
NF@mSiO
2
were successfully fabricated using the electrospinning technique and
further sol-gel synthesis. The sheath layer of SiO
2
was grown directly on electrospun NFs
without using a capping agent. Pt was deposited on CeO
2
NFs by two different methods,
including wet-impregnation and solvothermal deposition of pre-synthesized Pt. Structural
and morphological studies revealed that Pt was more homogeneously dispersed on CeO
2
NF in IM-PtCeSi compared to ST-PtCeSi. The results of Rietveld refinement of XRD data
revealed that the Pt impregnation led to the formation of small weight fractions of metallic
Pt nanoparticles (
≤
4.4 nm) without inducing a significant change in the lattice parameter
of CeO
2
, indicating that the Pt is mainly added to the surface of the CeO
2
NFs without
altering the lattice CeO
2
phase. Although the CeO
2
crystals in the NFs were grown as
needle-habit along the <110> directions in all samples, Pt impregnation increased the
preferred orientation of CeO
2
along this direction. The XPS results regarding Ce 3d showed
that the oxygen vacancies were increased by reducing Ce
4+
species to Ce
3+
in IM-PtCe after
wet impregnation of Pt on CeO
2
NF. A uniform and intact porous SiO
2
sheath layer were
obtained in both catalyst samples. Both catalysts were then tested for the RWGS reaction.
IM-PtCeSi showed better performance compared to ST-PtCeSi, with a CO
2
conversion of
8.9% and a CO selectivity of 98.9% (at P = 6.2 bar, 350
◦
C, GHSV = 66,000 mLgcat
−1
h
−1
). It is
also demonstrated that by increasing the operating temperature, the catalytic performance
is enhanced in all catalysts. Such a porous silica layer can be potentially used as a second
substrate and interlayer to design tandem bifunctional catalysts as well.
Supplementary Materials:
The following supporting information can be downloaded at: https://
www.mdpi.com/article/10.3390/nano13030485/s1. Figure S1: XRD Rietveld refinement of (a) CeO
2
-
NF with the lattice parameter of 5.411(15) Å, (b) IM-PtCeSi with the lattice parameter of 5.410(30)
Å and 3.922(85) Å for CeO
2
and Pt, respectively, and Pt weight fraction of 2.5(0.2) wt.% and (c)
ST-PtCeSi with the lattice parameter of 5.410(14) Å and 3.923(47) Å for CeO
2
and Pt, respectively, and
Pt weight fraction of 19.2(0.5) wt.%. The lattice parameter of CeO
2
was kept intact after introducing
Pt, meaning the reduction of CeO
2
occurred only on the surface, not in bulk. Figure S2: FTIR spectra
of CeSi. Figure S3: STEM and EDS elemental mapping of IM-PtCeSi with 10% loading of CeO
2
with
Pt in bright field (BF) mode and mapping of elements, red color is Pt, green is Si and Blue is Ce.
Figure S4: STEM and EDS elemental mapping of ST-PtCeSi with 10% loading of CeO
2
with Pt. Figure
S5: STEM of ST-PtCeSi after optimization of Pt with the Pt particle size distribution in ST-PtCeSi
histogram plot (the average particle size is 3.8 nm); Equations (S1)–(S3): The fractions of Ce
4+
and
Ce
3+
. Figure S6: XPS for O 1s for all samples. Figure S7: Ce
3+
species are present in the samples of
CeO
2
-Com, CeO
2
-NF, IM-PtCe, and ST-PtCe which all are without the SiO
2
sheath. Table S1: Peak
positions, Ce oxidation state, and integrated area of all samples showed in Figure 7, compared to the
reference peak positions and corresponded oxidation states. Table S2: Pt peaks with corresponded
oxidation states occurred in Figure 8. Figure S8: CO
2
-TPD profile of IM-PtCeSi and ST-PtCeSi core-
sheath NF catalysts. Figure S9: CO selectivity and CH
4
selectivity at different reaction temperatures
for two tested catalysts. Figure S10: Equilibrium conversion of CO
2
at different temperature with
different H
2
:CO
2
compositions. Figure S11: TEM images of tested catalysts (a) and (b) IM-PtCeSi,
(c) and (d) ST-PtCeSi. References [59–64,66,70] are cited in the Supplementary Materials.
Nanomaterials 2023,13, 485 17 of 20
Author Contributions:
A.N.: conceptualization, methodology, investigation, visualization, formal
analysis, data curation, validation, project administration, writing—original draft. N.B.: concep-
tualization, methodology, investigation, validation, writing—original draft, writing—review and
editing. H.R.G.: supervision, conceptualization, project administration, methodology, writing—
review and editing. R.L.O.: methodology, writing—review and editing. A.T.S.: writing—review and
editing. M.F.B.: formal analysis, writing—review and editing. A.T.: supervision, resources, writing—
review and editing. R.S.: supervision, resources. A.G.: supervision, resources, funding acquisition,
writing—review and editing. O.G.: conceptualization, funding acquisition, supervision, project
administration, resources, writing—review and editing. All authors have read and agreed to the
published version of the manuscript.
Funding:
This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Re-
search Foundation) under Germany’s Excellence Strategy—EXC 2008–390540038—UniSysCat.
Data Availability Statement: Data are contained within the article or Supplementary Information.
Acknowledgments:
Aidin Nejadsalim is grateful for the financial support of the Berlin Graduate
School of Natural Sciences and Engineering and Unifying Systems in Catalysis (UniSysCat) (founded
by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)) for a PhD fellowship
(2018–2021). The authors also acknowledge Stephanie Reich and Svitlana E. Trotsenko for all their
support in using the Electrospinning equipment at the Department of Physics, Freie Universität Berlin,
as well as Sören Selve and Jan R. J. Simke for providing TEM and STEM analysis at ZELMI (JEM-
ARM300F2 founded by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)-
GZ: INST 131/789-1 FUGG)., Christina Eichenauer for BET measurement, Maria Unterweger and
Hüseyin Küçükkeçeci for their efforts to perform XPS measurements, Harald Link for making
ICP-OES measurements, and Maik A. Rudolph for implementing CO
2
-TPD analysis at Technische
Universität Berlin. We acknowledge support by the German Research Foundation and the Open
Access Publication Fund of TU Berlin.
Conflicts of Interest: The authors declare no conflict of interest.
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