Spectroscopic and kinetic insights into the methane reforming over Ce-pyrochlores

Article abstract of DOI:10.1016/j.mcat.2020.110964

High thermal stability and oxygen conductivity of pyrochlores makes them a potential catalyst for the dry reforming of methane (DRM). In this study, noble metal (Pt, Ru) substituted Ce₂Zr₂O₇ and Ce₂Ti₂O₇ pyrochlores have been synthesized by modified Pechini method. The effect of variation of dopant concentration and position was examined to achieve strong metal-support interaction. Ce₂Zr_1.96Pt_0.04O_7-δ exhibited the best catalytic activity towards DRM and hence, been studied for partial oxidation (POM) and autothermal reforming of methane (ATR). Syngas ratio (H₂/CO) was tuned by varying the O₂ concentration in the feed. Insights obtained from both ex-situ and in situ characterization were exploited to discern the reaction mechanism. The surface intermediates were examined by DRIFTS on Ce₂Zr_1.96Pt_0.04O_7-δ for DRM, POM and ATR. Besides adsorbed CH₄ and CO on Pt, the presence of carbonate and formate species were also identified. Thus, a dual-site mechanism via carbonate and formate formation was proposed for DRM over Ce₂Zr_1.96Pt_0.04O_7-δ.

Full text of DOI:10.1016/j.mcat.2020.110964

Molecular Catalysis 492 (2020) 110964
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Spectroscopic and kinetic insights into the methane reforming over Ce-
pyrochlores
Disha Jain*, Shreya Saha
Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
DRIFTS
Methane reforming
Kinetics
Pyrochlores
Surface chemistry
High thermal stability and oxygen conductivity of pyrochlores makes them a potential catalyst for the dry
reforming of methane (DRM). In this study, noble metal (Pt, Ru) substituted Ce Zr O and Ce Ti O pyrochlores
2
2
7
2
2 7
have been synthesized by modified Pechini method. The effect of variation of dopant concentration and position
was examined to achieve strong metal-support interaction. Ce 7-δ exhibited the best catalytic ac-
tivity towards DRM and hence, been studied for partial oxidation (POM) and autothermal reforming of methane
ATR). Syngas ratio (H /CO) was tuned by varying the O concentration in the feed. Insights obtained from both
ex-situ and in situ characterization were exploited to discern the reaction mechanism. The surface intermediates
were examined by DRIFTS on Ce 7-δ for DRM, POM and ATR. Besides adsorbed CH and CO on Pt,
the presence of carbonate and formate species were also identified. Thus, a dual-site mechanism via carbonate
and formate formation was proposed for DRM over Ce
2
Zr1.96Pt0.04O
(
2
2
2
Zr1.96Pt0.04
O
4
2
7-δ
Zr1.96Pt0.04O .
1
. Introduction
deposition on active metal. These difficulties can be overcome by the
incorporation of active metal into the lattice of oxide. Substitution of
active metal in the oxide lattice alters the reaction mechanism by in-
fluencing the activation of the reactant species and by modifying the
stability of the reaction intermediates [12]. Metal substitution elim-
inates the problem of metal sintering and also reduces carbon deposi-
tion by tailoring the reducibility of the support.
CH
4
and CO
2
are the major greenhouse gases, with CH
4
having
higher potency towards global warming than CO
2
[1]. Among the
various routes exploited for transforming methane into value-added
products, syngas production is the most feasible process [2]. Moreover,
many natural gas reserves and biogas obtained from anaerobic de-
gradation of organic matter contain considerable amount of CO
Carbon dioxide or dry reforming of methane (DRM) can utilize the
natural gas with large CO content, thereby avoiding the costly gas
separation processes [4]. The syngas ratio (H /CO) attained via DRM is
2
[3,4].
4
Activation of CH is a critical step in the dry reforming of methane
due to high dissociation energy of CeH bond (439 kJ/mol) [8]. In
supported metal catalyst, CeH bond splits by the addition of Pt atom
(or any active metal) into CeH bond with back donation of electron
2
2
nearly unity, which is suitable for Fischer-Tropsch synthesis of long
chain hydrocarbons and oxygenated chemicals [3,5,6]. The major
problems encountered in reforming reaction are methane activation,
coke deposition due to methane decomposition and Boudouard reac-
tion, thermal deactivation of support and side reactions like reverse
water-gas shift reaction [7,8].
Numerous permutations of active metal and support have been in-
vestigated in the literature to enhance metal-support interaction [7,9].
For instance, to reduce the deactivation due to sintering, the dispersion
of active metal over the support can be increased that leads to enhanced
metal-support interaction. The basic support suppresses coke deposition
3
from Pt to antibonding state of CeH, resulting in (H C–Pt—H) transi-
tion state [13]. However, the mechanism of activation of CeH is dif-
ferent for Pt-substituted oxide due to the electronic modifications in-
duced by metal doping. New energy levels are created due to the
hybridization of d-states of dopant and parent atom. The delocalized d-
states of transition metal hybridize with O 2p creating energy levels
that are responsible for the formation of various impurities in the oxide
[14]. Additionally, for aliovalent dopant, oxide vacancies are formed
that results in creation of defect energy levels between conduction band
and valence band [15]. Schwarz et al. suggests that the high charge
density at the terminal oxygen plays a key role in CeH bond cleavage
by improving the adsorption of CO
2
, which is mildly acidic [10,11].
by the transfer of hydrogen atom from CH
4
to oxide [16]. Further, as an
Nonetheless, the supported catalysts suffer from the problem of coke
effect of doping on the hydrogen atom transfer, there is an increase in
⁎
Corresponding author.
E-mail address: disha@iisc.ac.in (D. Jain).
https://doi.org/10.1016/j.mcat.2020.110964
Received 3 December 2019; Received in revised form 12 April 2020; Accepted 13 April 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
Scheme 1. Reaction steps involved in modified Pechini method.
charge density on the bridging oxygen of MgO after doping with Ga
2
O
3
easily occur in these structures, while retaining the cubic symmetry
[30].
[
17]. Therefore, the integration of metal in the oxide lattice can result
in formation of new active sites besides forming stable catalyst towards
methane reforming.
In this work, we have studied dry reforming of methane over noble
metal (NM: Pt, Ru) substituted Ce
synthesized by modified Pechini method. The catalytic activity of
Ce 7-δ towards partial oxidation of methane and auto-
2 2 7 2 2 7
Zr O and Ce Ti O pyrochlores
Pyrochlores are ternary oxides of type A
high temperature and have high oxygen conductivity. For a stable
pyrochlore structure, the radius ratio (r /r ) should lie between 1.46
2 2 7
B O , which are stable at
2
Zr1.96Pt0.04O
A
B
thermal methane reforming has been investigated. The synthesized
samples were characterized by X-ray diffraction (XRD), X-ray photo-
electron spectroscopy (XPS) and transmission electron microscopy
(TEM). Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) was performed to determine the abundant surface inter-
mediates formed during the reaction. Subsequently, a comprehensive
reaction mechanism was proposed over Ce Zr1.96Pt0.04O7-δ and a de-
2
tailed kinetic model was derived based on the insights obtained from
the above studies.
and 1.78. These oxides have vacancies at A and O sites that enhances
the migration of oxygen ion in the lattice [8]. These properties make
pyrochlore a potential catalyst for dry reforming of methane. However,
pyrochlore supports have low surface area that might limit the dis-
persion of active metal leading to reduction in the catalytic activity and
coke deposition [18]. Consequently, the substitution of active metal in
the pyrochlore lattice will yield a stable and active catalyst. Pakhare
2 2 7
et al. have studied the kinetics of the Rh-substitution on La Zr O
pyrochlore towards dry reforming of methane and proposed a dual-site
mechanism [19] La-pyrochlores are extensively studied in literature for
dry reforming of methane with noble metal substitution at B site
2. Experimental
[
20,21]. However, there are limited studies on Ce-pyrochlores for me-
thane reforming. CeO has been studied for DRM due to its oxygen
2
2.1. Materials
storage capacity, which is a beneficial attribute in mitigating coke de-
position during methane reforming. Owing to its low thermal stability,
Two series of cerium based pyrochlores were synthesized viz.
the mixed oxides of Ce1-xZr
investigated, however, the problem of formation of carbon filaments
and NiO still remains [22–25]. Mizuki et al. reported Ni/Ce Zr for
steam reforming of methane, but, the kinetics over Ce Zr pyro-
chlores remains unexplored [26]. The change in oxygen concentration
of Ce Zr 7+x compounds by detailed XRD analysis has been discussed
x
O
2
, Ce1-xPr
x
O
2
supported Ni catalysts were
Ce
these pyrochlores were cerium (III) nitrate hexahydrate (Ce
(NO .6H O, S.D. fine Chemicals, India), zirconium (IV) oxynitrate
(ZrO(NO , Sigma Aldrich, U.S.A.) and titanium isopropoxide (Ti(OCH
(CH , Sigma Aldrich, U.S.A.). Ruthenium chloride (RuCl , TCI
chemicals, Japan) and tetraammine platinum nitrate (Pt(NH (NO
2 2 7 2 2 7
Ti O and Ce Zr O . The metal precursors used for the synthesis of
2
2
O
x
3
)
3
2
2
2
O
x
3 2
)
3
)
2
)
4
3
2
2
O
3
)
4
3 2
) ,
in several studies [27–29] The comprehensive structural studies re-
Alfa Aesar, U.S.A.) were used as dopant precursors, while, citric acid
(Merck, India) and ethylene glycol (S.D. fine Chemicals, India) were
used as complexing agent and polymerizing agent, respectively.
ported by Achary et al. highlights the excellent oxygen storage capacity
3
+
4+
2 2
of Ce Zr O7+x [27]. The facile conversion between Ce and Ce can
2

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
2
2
.2. Methods
was packed inside the quartz tube (I.D. - 4 mm) and reduced in 5% H
Ar gas flow of 30 mL/min. H uptake was measured using thermal
conductivity detector (TCD) as a function of temperature, increased at
2
/
2
.2.1. Synthesis of Ce-pyrochlores
Noble metal (NM- Pt, Ru) doped Ce-pyrochlores i.e.
constant ramp rate of 10 °C/min. The quantity of H
calibrated with CuO.
2
consumed was
Ce
prepared by modified Pechini method [21]. For the synthesis of
Ce 7-δ, 2 mol of Ce(NO .6H O, 1.96 mol of ZrO(NO
and 0.04 mol of Pt(NH (NO were dissolved in deionized water. An
2 2 2
Zr1.96Pt0.04O7-δ, Ce Zr1.96Ru0.04O7-δ, and Ce Ti1.96Pt0.04O7-δ were
2
Zr1.96Pt0.04
O
3
)
3
2
3
)
2
2.5. CO temperature programmed desorption
3
)
4
3 2
)
aqueous solution of citric acid (CA) was added to the above solution,
maintaining the molar ratio of citric acid and total metal ion as 1.2:1.
The temperature of the solution mixture was raised to 70 °C, with
continuous stirring to ensure complete chelation of metal ions with
citric acid. Now, ethylene glycol (EG) was added to the solution with
EG:CA as 1:1. The solution was maintained at 70 °C until a viscous
transparent gel was obtained. The temperature of the solution was then
increased to enhance the degree of esterification reaction by removing
water. During this stage, violent bubbling and frothing of gel was ob-
Temperature programmed desorption was performed in the pre-
sence of CO over Ce 7-δ, to determine the exposed Pt atom
on the surface of the catalyst. The sample (20 mg) was filled in the
quartz tube (I.D. - 4 mm) between two ceramic wool plugs. In first step,
the sample was degassed in He flow at 200 °C to remove the adsorbed
2
Zr1.96Pt0.04O
2
water and CO from the surface and was then cooled to room tem-
perature. Pulse injection of CO was done and CO was allowed to adsorb
over the sample under He flow of 25 mL/min. The desorption of CO was
studied by increasing the temperature from 30 to 650 °C at constant rate
of 10 °C/min and quantity of CO was recorded by Thermal conductivity
detector (TCD).
x
served with the emission of brown fumes of NO formed as a result of
the decomposition of metal nitrates. An amorphous solid product thus
obtained was calcined at 750 °C for 3 h to achieve a crystalline pyro-
chlore oxide. Ethanol was used as solvent for synthesizing
2.6. Catalytic activity
Ce
various reaction steps involved in the synthesis of Ce
are depicted in Scheme 1. 4 at% Pt impregnated over Ce
synthesized by impregnation method, where the slurry of synthesized
Ce Zr was prepared in deionized water and stoichiometric quantity
of Pt(NH (NO . Pt ions were reduced with the help of hydrazine
hydrate over Ce Zr . The aging of this mixture was done for an hour
and thereafter, the mixture was washed with ethanol several times to
obtain pH = 7. The sample was dried and then reduced in H flow at
2
Ti1.96Pt0.04
O
7-δ, as titanium isopropoxide decomposes in water. The
Zr pyrochlores
Zr was
2
2
O
7
The catalytic studies i.e. dry reforming of methane (DRM), partial
oxidation of methane (POM) and autothermal reforming (ATR) were
conducted in the packed bed reactor. 100 mg of catalyst was packed in
the quartz tube between the ceramic wool plugs in the form of granules,
which are in the size range of 150−300 μm. The catalyst was diluted
with an inert material i.e. silica gel so as to maintain the bed length as
1 cm. The reactor was placed inside the furnace and the thermocouple
was positioned in the center of the catalyst bed. A PID controller was
used for maintaining the temperature of the catalyst bed within ± 1 °C.
The product gases were analyzed at various temperatures under iso-
2
2 7
O
2
2 7
O
3
)
4
3 2
)
2
2 7
O
2
2 2 7
700 °C for 2 h and was represented as 4% Pt/ Ce Zr O .
2.3. Characterization
thermal conditions. The feed gas mixture for DRM consisted of 3% CH
4
(
99 %, Chemix gases, Bangalore), 3 % CO
2
(99 %, Chemix gases,
(99 %, Noble gases, Bangalore), however, few
with
– 1, to study coke deposition. POM reaction was performed
with the reactant gas ratio i.e. CH :O as 1:0.5 with 3% CH . ATR was
performed at three different O (99 %, Noble gases, Bangalore) con-
The structural characterization of the synthesized compounds was
done using XRD, XPS and TEM. XRD pattern was acquired at 25 °C with
Bangalore) and rest N
2
reaction were performed at high concentration- 20 % CH
CH :CO
4
Rigaku X-ray diffractometer employed with Cu-Kα radiation
4
2
ͦ
(
λ = 1.5418 A) and Ni filter. XRD pattern was collected in the 2θ range
4
2
4
from 10° to 80° with steps of 0.02°. XRD was performed to do phase
analysis in the synthesized samples. X-ray photoelectron spectra (XPS)
were recorded on AXIS ULTRA instrument with Al-Kα (1486.6 eV) as
source and pass energy of 20 eV. The samples were etched with Ar+ for
2
centrations i.e. CH
determine the effect of O
flow rate was maintained at 80 mL/min for all the reaction studies. The
4
:CO
2
:O
2
was varied as 1:1:0.1, 1:1:0.2 and 1:1:0.5 to
concentration on syngas ratio. The total gas
2
−1
30 s and 4 keV to remove the adsorbed oxygen and hydroxyl species.
GHSV of 38,120 h
was calculated based on the total bed volume
3
XPS was performed to determine the elemental states of the samples
before and after the reaction. The charging correction for all the sam-
ples has been done using adventitious carbon (i.e. CeC) with binding
energy of 284.8 eV. The particle size and crystalline phases of the ma-
terial were determined by TEM on Technai F-30, operating at 200 kV.
The solution of the material dispersed in ethanol was drop casted on the
carbon coated Cu grids (400 mesh size). The surface area measurement
was performed on Belsorb surface area analyzer (Smart instruments)
using BET nitrogen sorption method at 77 K. The synthesized samples
were regenerated at 150 °C for 3 h to remove any adsorbed water and
(0.125 cm ) and was maintained constant for all the experiments. The
product gases were analyzed using gas chromatograph (Mayura
Analytical Ltd., India), which was fitted with flame ionization detector
(FID), TCD and methanator. FID was used for analyzing CO, CH
CO , whereas, H was detected in TCD. The methanator (positioned
before FID) was equipped with Ru catalyst which was maintained at
350 °C and used for converting CO and CO to CH , due to weak signals
of CO and CO in FID. A moisture trap was placed at the exit of the
4
and
2
2
2
4
2
reactor to condense water, if formed during the reaction, as water
cannot be allowed to enter the gas chromatograph.
2
CO . Coke deposition on the spent catalyst was determined by ther-
mogravimetric analysis (TGA) by NETZSCH STA 409. A crucible con-
taining 6–8 mg of sample was heated in a furnace from 30 to 800 °C in
nitrogen atmosphere. The bulk composition of noble metals was de-
termined by inductively plasma coupled mass spectroscopy (ICP-MS,
Thermo X series). For ICP analysis, 20 mg of sample was dissolved in
aqua regia and then after dissolution, aqua regia was evaporated. The
2.7. In situ FTIR (DRIFTS) studies
In situ FTIR (DRIFTS) studies were performed on Frontier, Perkin
Elmer, which was fitted with DTGS detector. The experimental set up
consisted of an in situ DRIFTS cell with ZnSe windows (Harrick, Model
# HVC-DWM-3), which is placed inside Praying Mantis (Model # HVC-
DRP-4). This high temperature reaction chamber was fitted with a
heating cartridge and was coupled with a chiller for maintaining tem-
perature. A PID controller (Harrick, Model # ATC-024-4) was employed
for maintaining the temperature of the cell within ± 1 °C. The catalyst
(∼50 mg) was filled in the DRIFTS cell and the reactant gases were
passed over the catalyst. The composition of the reactant gas for DRM,
digested samples were diluted with 2% HNO
3
for analysis.
2
.4. H temperature programmed reduction
2
2
Reduction studies by H were performed to investigate the re-
ducibility of the noble metal substituted pyrochlores. 10 mg of sample
3

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
2 7-δ
Fig. 1. (a) XRD pattern of the noble metal substituted pyrochlores; (b) Rietveld refined XRD pattern of Ce Zr1.96Pt0.04O .
POM and ATR reaction was 3 % CH
4
, 3 % CO
2
, rest N
2
, 3 % CH
4
, 1.5 %
due to Ru substitution. However, for Pt substituted compounds there is
slight increase in the surface area compared to the pristine oxide. This
can be attributed to the higher number of exposed Pt compared to Ru.
O
2
, rest N and 3 % CH , 3 % CO , 1.5 % O
2
4
2
2
, rest N
2
, respectively. The
composition was maintained constant for a particular reaction studies.
The spectra were acquired at four different temperatures i.e. 28, 350,
4
50 and 600 °C. The background was recorded at each temperature
before starting the analysis under N flow. All the spectra were recorded
with 32 scans and the resolution of 4 cm . The chamber was purged
with N while the temperature was increased. DRIFTS investigations
were performed for Ce
3
.1.2. X-ray photoelectron spectroscopy
Core level spectra of Ce 3d in Ce
, and Ce 7-δ are shown in Fig. 2a–c, respectively. The no-
2
2 2 7-
Zr1.96Pt0.04O7-δ, Ce Zr1.96Ru0.04O
−1
δ
2
Ti1.96Pt0.04O
2
tations for the spectral components of Ce 3d are adopted from Bur-
roughs et al. [32]. In the final state, the charge transfer occurs between
Ce 4f levels and O 2p valence band. This results in 3d 4f O 2p (v ) and
3d 4f O 2p5 (v') configurations for Ce , while 3d 4f O 2p (v) and
2
7-δ
Zr1.96Pt0.04O .
9
2
4
0
9
1
5
3+
9
2
4
3
. Results and discussion
9 1 4+
3
d 4f O 2p (v”) configurations are observed for Ce with a pure final
state 3d 4f O 2p6 (v”'). A strong hybridization between these config-
urations gives rise to bonding and antibonding states leading to the
formation of double peak spectra. Multiplet coupling does not result in
formation of extra peaks in Ce 3d, however, broadening in spectral
width is observed as a result of multiplet effect [32]. The binding en-
9
0
3.1. Structural characterization
3.1.1. X-ray diffraction
Fig. 1a shows the XRD pattern of all the synthesized samples. It is
evident from the XRD patterns that Ce 7-δ and
Ce 7-δ crystallized in the pyrochlore structure. However,
the pyrochlore phases of Ce 7-δ were accompanied by the
presence of some phase of TiO . The shift in 2θ values of some reflec-
tions in the synthesized samples may be attributed to the doping of NM
Pt. Ru) in the oxide lattice. Owing to the presence of intrinsic oxide
vacancy in Ce Zr , it can accept oxygen atom to annihilate oxide
vacancies forming Ce Zr . Arai et al. observed the existence of
Ce Zr 7.5, which is a metastable phase, as a result of adsorption of
2
Zr1.96Pt0.04O
Zr1.96Ru0.04
O
ergy and FWHM of the component peaks of Ce 3d are listed in Table 1.
2
The ratio of Ce3+:Ce was calculated for Ce
4+
Zr1.96Pt0.04O7-δ and it was
2
Ti1.96Pt0.04
O
2
found to be 0.45:0.55 before the reaction and 0.46:54 after the reaction.
Thus, no significant change in the elemental states was seen after the
reaction, which is indicative of the regeneration of the support sites. For
2
(
3+
4+
2
O
7
Ce
2
Zr1.96Ru0.04
O
7-δ
,
the ratio of Ce :Ce
was changed from
2
3
+
O
8
0.51:0.48 to 0.45:0.55 after the reaction, indicating the oxidation Ce
2
2
to Ce4 due to annihilation of oxide vacancies by CO
+
. However, there
7-δ, from the composition
2
2
O
2
is rise in Ce3 state for Ce
+
Ti1.96Pt0.04
O
ambient oxygen at room temperature [31]. However, due to the
dominant scattering from heavy metals like Ce and Zr, the small change
in stoichiometry of oxygen cannot be observed in XRD [27].
2
0.54:0.45 to 0.58:0.42 after reaction. Due to existence of metastable
states of these compounds, different composition of Ce3 and Ce
+
4+
XPS (see Section 3.1.2) was employed to acquire the elemental
states were observed for the synthesized samples [27,31].
3
+
4+
composition of Ce. The ratio of Ce :Ce
was obtained from the de-
2 2 7-δ
XP spectra of Zr 3d in Ce Zr1.96Pt0.04O7-δ and Ce Zr1.96Ru0.04O
convolution of Ce 3d XP spectra and was found to be 0.45:0.55. This
suggests the possibility of existence of synthesized samples in
recorded both before and after reaction are shown in Fig. 3a and b. The
energy separation between Zr 3d5/2 and 3d3/2, due to spin-orbit split-
ting is 2.4 eV. The Gaussian broadening of the spin-orbit component
and satellite peak is maintained as 1.25 eV and 3.5 eV, respectively
[33]. Both the materials display the presence of Zr4+ with 3d5/2 peak at
Ce
Ce
2
Zr
Zr
2
O
O
7.5 phase. As a result of rearrangement, Fd-3 m symmetry in
is changed to F-43 m in Ce Zr 7.5 [27,31]. Hence, the XRD
2
2
7
2
2
O
pattern calculated by Sasaki et al. (from ICSD) was used to refine the
experimental XRD pattern of Ce 7-δ [29]. XRD pattern of
Ce 7-δ was refined by GSAS and EXPGUI and the fitted
Zr1.96Pt0.04
O
182.2 eV. Ce
ever, the satellite feature was absent in the spectra of Ce
. Morant et al. explained that the occurrence of satellite peaks might be
2
Zr1.96Ru0.04O7-δ manifest satellite peak at 186 eV, how-
2
Zr1.96Pt0.04
O
2 7-
Zr1.96Pt0.04O
2
pattern is shown in Fig. 1b. Manual background subtraction was per-
formed with 18 terms and pseudo-voigt function was used for fitting
profile. The goodness of the fit was determined by the residuals Rp and
Rwp with values of 4.1 % and 5.6 %, respectively. The decrease in
lattice parameter to 10.63713(8) Å indicates the substitution of Pt in
the lattice. The absence of some reflections in the experimental data
from calculated profile can be attributed to the synthesis of oxide by
different methods and pretreatment.
δ
ascribed to the metal-to-ligand charge transfer due to the presence of
unoccupied 4d orbitals [33]. Zr 3d spectra displays slight shift in the
binding energy to the higher value after reaction as a consequence of
small change in particle size after reaction [34]. There was no change in
the elemental composition of Zr even after the reaction for both the
materials. As discussed by Arai et al., the presence of oxide vacancy
near Zr ion does not affect its state and the charge is compensated by
the change in oxidation state of Ce ion [31].
The specific surface area of Ce
2
Zr
2 7 2 7-δ
O , Ce Zr1.96Ru0.04O ,
Ce
2
Zr1.96Pt0.04
O
7-δ and Ce
2
Ti1.96Pt0.04
O
7-δ were found to be 35, 38, 42
Fig. 3c shows the XP spectra of Ti 2p in Ce
2
Ti1.96Pt0.04O7-δ collected
2
and 40 ( ± 2) m /g. There is no marked difference in the surface area
both before and after the reaction. In TiO , Ti 2p3/2 occurs at 458.5 eV,
2
4

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
Fig. 2. XP spectra of Ce3d in (a) Ce
2
Zr1.96Pt0.04
FWHM
2 2 7-δ
O7-δ, (b) Ce Zr1.96Ru0.04O7-δ, (c) Ce Ti1.96Pt0.04O .
Table 1
the synthesized sample was 0.23:0.35:0.42, which changed to
0.41:0.48:0.11 after the reaction. The increase in metallic Pt after the
reaction can be attributed to the presence of reducing atmosphere of
Binding energy and FWHM of Ce 3d [35].
3
d
5/2
Binding Energy
3d3/2
Binding Energy
CH
4
and H
Fig. 4b shows the comparison of Ru 2p3/2 spectra of
7-δ before and after reaction. The binding energy of Ru
2
.
v
o
880.2 ± 0.4
884.8 ± 0.4
881.9 ± 0.5
887.6 ± 0.8
897.9 ± 0.4
u
u'
U
u"
u”'
o
899.1 ± 0.2
903.3 ± 0.3
900.5 ± 0.4
906.2 ± 0.6
916.2 ± 0.6
2.0
3.0
2.0
3.2
3.2
v'
v
v"
v”'
Ce
2
Zr1.96Ru0.04
3/2 for Ru°, Ru
The satellite peaks is also noted for Ru
O
4+
3+
2p
and Ru
is 459.2, 462.7 and 465, respectively.
4+
3+
and Ru . Morgan has per-
formed detailed XPS characterization of Ru compounds and values of
binding energy and FWHM is adopted from his work [41]. Ru 2p1/2
however, in this study, Ti 2p3/2 was observed at 458.2 eV and the spin-
orbit split was 5.6 eV [36]. The shift in the binding energy was observed
due to development of lattice strain induced as a result of doping at Ti
site [37]. As evident from the XP spectra, Ti 2p1/2 is broader and shorter
than Ti 2p3/2 than expected. This can be attributed to the decay of core
hole by Coster-Kronig process, which results in the Lorentzian broad-
ening of the core electron line [38]. XP spectra indicate the presence of
only Ti4 in the synthesized sample. However, no change was observed
component is not shown here, due to the obscure nature of the spectra.
The ratio of Ru°:Ru3+:Ru
4+
before the reaction was observed to be
0.15:0.53:0.32, while after the reaction the composition changes to
0.35:0.40:0.25. There was, however, an increase in the concentration of
3
+
metallic Ru after the reaction that shows the involvement of Ru and
4+
Ru species. The uncertainty in Ru 3p (in Ce
2
Zr1.96Ru0.04
O
7-δ) and Pt
4f (in Ce
spectra.
2
Ti1.96Pt0.04O7-δ) is high due to the obscure nature of the
+
in the electronic state of Ti after the reaction as CH
4
does not adsorb on
the Ti site [39].
3
.1.3. Transmission electron microscopy
Fig. 4a depicts the XP spectra of Pt 4f in Ce Zr1.96Pt0.04O7-δ acquired
before and after reaction. Owing to spin-orbit splitting, the Pt 4f spectra
split into 4f7/2 and 4f5/2 component with a separation of 3.1 eV. The
2
TEM was performed for Ce
2
Zr1.96Pt0.04
O
7-δ, Ce
7-δ. Fig. 5a, e and i represents the bright field TEM
δ, and
7-δ, respectively. Fig. 5m depicts the particle size dis-
2
Zr1.96Ru0.04O7-δ, and
Ce
images
Ce
2
Ti1.96Pt0.04
O
2
+
4+
of
2
Ce Zr1.96Pt0.04O
2
Ce Zr1.96Ru0.04O7-δ,
binding energies of Pt 4f7/2 of Pt°, Pt and Pt states are 71, 72.4 and
4.4 eV, respectively. The linear background correction was performed
7-
2
Ti1.96Pt0.04
O
7
2
tribution of Ce Zr1.96Pt0.04O7- δ and the average particle size of the
and FWHM of 2 eV was maintained for all the component peaks [40].
The deconvoluted Pt 4f spectrum before reaction reveals the presence of
synthesized samples was found to be ∼ 10 nm. HRTEM micrographs
2
+
4+
demonstrate that the samples were highly crystalline in nature. Fig. 5b
shows the HRTEM micrograph of Ce
lysis depicted in Fig. 5b inset and c revealed the presence of (222),
Pt and Pt in the ratio of 0.87:0.13. However, Pt 4f spectrum of the
spent catalyst showed the formation of Pt° state in conjunction with
2
Zr1.96Pt0.04O7-δ and its FFT ana-
2
+
4+
2+
4+
Pt
.43:0.42:0.15. Fig. 4c shows the XP spectra of Pt 4f in
Ce 7-δ. It is evident from the spectra that Pt was present in
metallic form in the synthesized material. The ratio of Pt°:Pt :Pt in
and Pt
state and the ratio of Pt°:Pt :Pt
was found to be
(
400) and (444) planes of the pyrochlore. The shift in the d-spacing
0
2+
might be due to the doping of Pt
The electron diffraction pattern (Fig. 5d) confirmed the presence of
various planes i.e. (222), (400), (440) and (444), which corroborates
2 2 7
ion in the lattice of Ce Zr O .
2
Ti1.96Pt0.04O
2
+
4+
5

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
2 2 2 7-δ
Fig. 3. XP spectra (a) Zr 3d in Ce Zr1.96Pt0.04O7-δ, (b) Zr 3d in Ce Zr1.96Ru0.04O7-δ, (c) Ti 2p in Ce Ti1.96Pt0.04O .
Fig. 4. XP spectra of (a) Pt 4f Ce
2
Zr1.96Pt0.04
O
2 2 7-δ
7-δ, (b) Ru 3p in Ce Zr1.96Ru0.04O7-δ, (c) Pt 4f in Ce Ti1.96Pt0.04O .
6

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
Fig. 5. TEM micrograph of Ce
2
Zr1.96Pt0.04
O
7-δ: (a) Bright field images of (b) HRTEM micrograph, inset denotes the FFT pattern of the yellow marked region, (c)
Inverse FFT pattern, (d) SAED pattern; TEM micrograph of Ce
yellow marked region, (g) Inverse FFT pattern, (h) SAED pattern; TEM micrograph of Ce
2
Zr1.96Ru0.04
O
7-δ: (e) Bright field images of (f) HRTEM micrograph, inset denotes the FFT pattern of the
7-δ: (i) Bright field images of (j) HRTEM micrograph, inset
2
Ti1.96Pt0.04O
2
denotes the FFT pattern of the yellow marked region, (k) Inverse FFT pattern, (l) SAED pattern; (m) Particle size distribution of Ce Zr1.96Pt0.04O7-δ. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web version of this article.)
with the XRD result (Fig. 1a) [29]. However, no planes of CeO
TiO were observed in Ce 7-δ, indicating the presence of
pure pyrochlore phase. HRTEM micrograph of Ce 7-δ is
shown in Fig. 5f and the inset represents the FFT pattern of selected
yellow area. The inverse FFT pattern (Fig. 5g) of the spots shows the
presence of pyrochlore planes i.e. (311) and (400). Fig. 5h depicts the
2
and
pyrochlore along with (301) and (520) plane of Ce
pattern of Ce 7-δ (Fig. 5l) depicts the presence of (200)
in conjunction with (311), (440) and (622) planes of
2 2 8
Zr O . The SAED
2
2
Zr1.96Pt0.04
O
2
Ti1.96Pt0.04O
2
Zr1.96Ru0.04
O
plane of CeO
pyrochlores.
2
SAED pattern of Ce
400), (311), (622) and (440) planes of pyrochlore. Fig. 5j shows the
HRTEM micrograph Ce 7-δ and the FFT pattern of the
highlighted yellow region is depicted in the inset. The inverse FFT
pattern (Fig. 5k) of the spots exhibited the presence of (311) plane of
2
Zr1.96Ru0.04
O
7-δ, representing the presence of
3.1.4. Inductively plasma coupled mass spectroscopy (ICP-MS)
(
The noble metal composition in pyrochlores was investigated using
2
Ti1.96Pt0.04
O
ICP-MS. The amount of Pt in Ce
was found to be 3.17 and 3.03 at %, respectively, and Ru content in
Ce 7-δ was 3.77 at %.
2 2 7-δ
Zr1.96Pt0.04O7-δ and Ce Ti1.96Pt0.04O
2
Zr1.96Ru0.04O
7

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
3.3. CO temperature programmed desorption (CO-TPD)
Substitution of noble metal in the lattice reduces the metal exposed
at the surface of the catalyst and thus the amount of Pt at the surface of
Ce 7-δ was determined by CO TPD measurements. It is as-
2
Zr1.96Pt0.04O
sumed that each mol Pt adsorbs one mol of CO molecule while calcu-
lating metal dispersion. The complexity in estimating the amount of CO
adsorbed on Pt increases in substituted oxides. CO uptake by lattice
oxygen increases in substituted oxides in comparison to the pristine
oxide due to the weakening of metal oxygen bond, thus yielding higher
2
CO desorption (shown in H -TPR studies). Fig. S3 (Supplementary in-
formation) shows that the desorption initiates immediately as the
temperature is increased and this signal at low temperatures (< 100 °C)
is attributed to the molecular desorption of CO from the Pt site. The
features observed at higher temperature are assigned to the desorption
of CO
of Pt exposed on the surface was calculated using the following relation
45],
2
as a result of oxidation of CO by lattice oxygen [44]. The amount
[
VmAw 104
W% Sf
Dispersion=
Fig. 6. TPR profile of the synthesized samples. The inset depicts the oxygen
storage capacity of the samples.
where, V denotes the moles of CO per gram of sample, A is the atomic
m w
weight of metal, W% is the weight of metal in the sample and Sf is the
stoichiometric factor indicating the molecules of CO per metal atom.
2 2
3.2. H temperature programmed reduction (H TPR)
2 7-δ
The amount of exposed Pt atom on the surface of Ce Zr1.96Pt0.04O
was found to be 77 %. Turnover frequency was determined using the
following formula
The effect of noble metal substitution in pyrochlore lattice was
studied by H TPR (Fig. 6). Substitution of metal in the oxide can lead to
weakening of the metal oxygen bond in metal oxide and also H acti-
2
2
vation at lower temperature can be achieved by the presence of noble
metal. The metal atom at B site (dopant site) is coordinated by six
oxygen atom, however, the coordination of the surface atom can vary
mol
CH4 conversion rate
/s
(
g
)
cat
TOF (s 1) =
−
mol
Amount of CO desorbed from Pt
(
g
)
between 1−5. It is evident from Fig. 6 that the reduction of Ce
2
Zr
2
O
7
cat
occurs between 350−500 °C, similar observation was made by Baidya
et al. [42]. TPR studies performed by Gaur et al. on 1% Ru substituted
La Zr O lattice, showed only one reduction peak at 280 °C, which was
2 2 7
3
.4. Catalytic studies
ascribed to the reduction of surface Ru species [21] Haynes et al. in-
vestigated the effect of Sr and Ru substitution in La and Zr site, re-
spectively, and they observed two peaks at 182 and 555 °C. The ap-
pearance of peak at low temperature (182 °C) was attributed to the
change in the chemical environment near Ru in the pyrochlore lattice
3
.4.1. Dry reforming of methane
DRM was studied over Ce
7-δ and the ratio of CH
2
Zr1.96Pt0.04
O
7-δ, Ce
2
Zr1.96Ru0.04O7-δ and
Ce
2
Ti1.96Pt0.04
O
4
:CO
2
was maintained as 1:1 for
all the samples (Fig. 7). Among all the samples, Ce
showed highest conversion of CH and CO at all temperatures.
Ce 7-δ showed increase in conversion of CH and CO from
2 7-δ
50 to 700 °C, however, Ce 7-δ and Ce Ti1.96Pt0.04O
2 7-δ
Zr1.96Pt0.04O
4
2
[
43]. In this study, the incorporation of Pt and Ru in the Ce
2 2 7
Zr O led to
2
Zr1.96Pt0.04
O
4
2
increase in reducibility of the support. Reduction for Ce
Zr1.96Pt0.04O
2 7-δ
3
2
Zr1.96Ru0.04
O
began at temperature as low as 60 °C, which was assigned to the re-
duction of ionic Pt. The appearance of multiple reduction features in the
exhibited lower conversions up to 650 and 700 °C, respectively. The
reason for low catalytic activity between 400−650 °C of
Ce
accumulation of active carbon or CH
tion. With further increase in temperature the desorption of these
species will result in the availability of these sites for adsorption and
TPR profile of Ce
strong metal support interaction. A couple of intense peaks were ob-
served for Ce 7-δ at 96 and 105 °C. Similar feature were
observed for Ru/Al by Haynes et al. and were ascribed to the re-
duction of RuO species to metallic Ru [43]. Ce 7-δ showed
similar reduction profile as Ce 7-δ and Ce Zr1.96Ru0.04
which indicates the formation of similar sites in Ce-pyrochlores. The
comparison of TPR profiles of Ce Zr and Ce Ti is included in Fig.
S1 in Supplementary information. Although, the reduction of Ce Ti
occurs at higher temperature as compared to Ce Zr , the H uptake
was higher in Ce Ti uptake or oxygen
storage capacity Ce
Ce 7-δ and Ce 7-δ were found to be 83, 271,
2 2
Zr1.96Pt0.04O7-δ and Ce Zr1.96Ru0.04O7-δ indicates the
2
Zr1.96Ru0.04
O7-δ and Ce
2
Ti1.96Pt0.04
O
7-δ might be attributed to the
x
species formed by CH dissocia-
4
2
Zr1.96Ru0.04
O
2 3
O
x
2
Ti1.96Pt0.04O
activation of oxidant (CO
2
) leading to increase in conversion at higher
7-δ and Ce 7-δ showed 7.1 %
conversion at 600 °C, respectively, whereas
conversion. Nearly, 97.6 %
conversion was obtained at 650 and 800 °C for
7-δ and Ce 7-δ, respectively. However, only
conversion was observed for Ce 7-δ at 850 °C.
The low catalytic activity of Ce 7-δ can be due to the low
oxygen storage capacity compared to noble metal substituted Ce Zr
Furthermore, the presence of metallic Pt on the surface of
2
Zr1.96Pt0.04
O
2
7-δ
O ,
temperature. Ce
and 14.6
Ce
and 96.4 % CH
Ce
5 % CH
2
Ti1.96Pt0.04
O
2
Zr1.96Ru0.04O
%
CH
4
2
2
O
7
2
2 7
O
2
Zr1.96Pt0.04O7-δ exhibited 90.8 % CH
4
2
2 7
O
4
2
2
O
7
2
2
Zr1.96Pt0.04
O
2
Zr1.96Ru0.04O
2
2
O
(OSC)
7
(135 μmol O/g). The H
2
8
4
2
Ti1.96Pt0.04O
of
Ce
2
Zr
2
O
7
,
2 7-δ
Zr1.96Pt0.04O ,
2
Ti1.96Pt0.04O
2
Zr1.96Ru0.04
O
2
Ti1.96Pt0.04O
2
2 7
O .
3
37 and 166 μmol O/g, respectively. These values indicate that oxygen
storage capacity of noble metal substituted catalyst is greatly enhanced
by doping. However, the substitution of Pt in Ce Ti does not result
in significant enhancement in OSC. H uptake of these samples was not
as high as the CeO -ZrO catalyst reported in literature. Nonetheless,
Ce
2
Ti1.96Pt0.04
O
7-δ can cause sintering of Pt at higher temperature. For
7-δ and Ce 7-δ, the CH conversion was
conversions for temperatures below 600 °C, in-
dicating the occurrence of reverse water gas shift reaction (RWGS).
Ce 7-δ showed similar conversion of CH and CO , re-
presenting very low rate of RWGS. Noble metal substituted Ce Zr
compounds exhibited higher catalytic activity that Pt substituted
Ce Ti , demonstrating strong metal-support interaction between
2
2 7
O
Ce
2
Zr1.96Pt0.04
O
2
Zr1.96Ru0.04
O
4
2
higher than the CO
2
2
2
the reduction starts at lower temperatures in these catalysts indicating
the ease of removal of lattice oxygen [22,25].
2
Ti1.96Pt0.04
O
4
2
2
2 7
O
2
2 7
O
8

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
2 2 2 4 2
Fig. 7. Variation of fractional conversion with temperature of Ce Zr1.96Pt0.04O7-δ, Ce Zr1.96Ru0.04O7-δ and Ce Ti1.96Pt0.04O7-δ (a) CH , (b) CO ; Reaction conditions-
−
1
CH
4
:CO
2
- 3:3, GHSV =38,120 h
.
noble metal and Ce
exhibited highest catalytic activity towards DRM and hence, further
studies were performed on it. The catalytic studies were also performed
2
Zr
2
O
7
. Amongst all the samples, Ce
2
Zr1.96Pt0.04
O
7-δ
that Ce
temperature (< 600 °C), whereas the conversions increases with rise in
temperature for Ce 7-δ. Further increasing the Pt loading to
6 at% had no significant effect on conversions. T99% of CH for
7-δ were found to be 700 °C and
2
Zr1.98Pt0.02O7-δ showed decrease in catalytic activity at lower
2
Zr1.96Pt0.04O
at 20 % CH
comparison of conversion and reaction rates of CH
in Supplementary information (Fig. S4). The catalytic activity of
Ce 7-δ was compared with 4% Pt/ Ce Zr towards DRM
and it was observed that Pt-substituted Ce Zr exhibited significantly
higher catalytic activity compared to Pt impregnated over Ce Zr
Fig. S5, Supplementary information).
Fig. 11a shows the concentration profiles of various species at the
reactor outlet during DRM reaction over Ce 7-δ. The rate of
formation of H was higher than CO up to 550 °C as a consequence of
4
retaining the reactant gas (CH
4
:CO
2
) ratio as 1 and the
4
4
and CO is included
2
Ce
2
Zr1.96Pt0.04
O7-δ and Ce
2
Zr1.94Pt0.06
O
800 °C, respectively, whereas Ce
2
Zr1.98Pt0.02O7-δ showed a maximum of
2
Zr1.96Pt0.04
O
2
2
O
7
92 % conversion at 800 °C. The increase in activity was expected as the
Pt doping percentage was increased. Besides, increasing the number of
active sites (Pt), the increase in dopant concentration leads to the for-
2
2 7
O
2
2 7
O
(
mation of defects like oxide vacancies, which is favorable for CO
tivation. However, on further increasing concentration of Pt to 6%,
there was no increase in the conversion of CH and CO . This can be
2
ac-
2
Zr1.96Pt0.04
O
4
2
2
explained by (a) an incomplete substitution of the Pt atom in the oxide
lattice resulting in deactivation due to C-deposition, or (b) the possi-
bility of formation of more complex defects upon increasing the dopant
concentration, which reduces the number of free vacancy concentration
[46].
methane decomposition and Boudouard reaction. Above 550 °C, the
increase in CO might be due to the occurrence of RWGS or oxidation of
carbon deposited on the catalyst surface. The water formed as a result
of RWGS can also oxidize the carbon deposited on catalyst surface [19].
A small increase in concentration of CO
2
and low concentration of CO
The effect of doping of Pt on Ce (A) and Zr (B) site on the catalytic
activity towards DRM was also investigated (Fig. 9). Substitution of Pt
at the B site (Zr4+) exhibited higher DRM activity than that of Pt
substituted at the A site. This can be illustrated by the formation of
anionic defects and the difference in the ionic radii of the dopant.
Firstly, the doping of Pt2+ at Ce site leads to the creation of lesser
was observed at temperature below 350 °C, which could be due to
RWGS. However, the effect of these reactions disappeared after 650 °C,
indicating the selectivity of the catalyst towards DRM. Syngas ratio
(
H
2
/CO) plays a vital role in determining the process employed for its
conversion into value added products. Fig. 11f shows the syngas ratio
for Ce 7-δ, calculated at different temperatures for different
reforming routes. The H /CO ratio was determined at various tem-
peratures for Ce 7-δ and it was observed that the ratio was
constant at 0.84 after 650 °C.
Fig. 8 displays the influence of Pt loading on catalytic activity of
Zr2-xPt 7-δ (x = 0.02, 0.04, 0.06) towards DRM. It was observed
2+
2
Zr1.96Pt0.04
O
number of oxide vacancies as compared to doping of Pt at Zr site to
2
maintain charge neutrality. Secondly, the difference in the ionic radii of
3+
2+
Zr1.96Pt0.04
O
the Ce
Zr
(1.28 Å) and Pt
(0.94 Å) is higher than the difference in
(0.86 Å) and Pt , making substitution of Pt arduous at Ce site.
2
4
+
2+
Butler et al. suggested that the ionic radii of the dopant strongly in-
fluence the binding energy of the associated defects, which can be
Ce
2
x
O
Fig. 8. Effect of Pt doping % in Ce
2
Zr1.96Pt0.04
O
4 2 4 2
7-δ on fractional conversion with temperature (a) CH , (b) CO ; Reaction conditions- CH :CO - 3:3, GHSV
−
1
=38,120 h
.
9

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
2 4 2 4 2
Fig. 9. Effect of Pt doping position in Ce Zr1.96Pt0.04O7-δ on fractional conversion with temperature (a) CH , (b) CO ; Reaction conditions- CH :CO - 3:3, GHSV
−
1
=38,120 h
.
minimized if the ionic radii of dopant is close to that of parent ion [47].
The reduction studies by H over these materials indicate the doping of
Pt at Ce did not have increase in OSC of the material (Fig. S2, in Sup-
plementary information). Therefore, Ce Zr pyrochlores with Pt
determined by Arrhenius plot using the rates calculated from Fig. 10a
and b. The apparent activation energies for Ce 7-δ as shown
in Fig. 10c were found to be 146 ± 5 kJ/mol for CH and 112 ± 4 kJ/
2 7-δ
mol for CO . At 700 °C, the yield of H and CO for Ce Zr1.96Pt0.04O
2
2
Zr1.96Pt0.04O
4
2
2
O
7
2
2
substitution on the Zr site exhibited increased catalytic activity towards
DRM.
was found to be 77.7 % and 94.5 %, respectively. Table 2 exhibits the
comparison of rates, TOF and apparent activation energy over different
catalysts reported in the literature with respect to the samples reported
in this study.
Fig. 10a and b shows the variation in conversion of CH
with W/FCH4 and W/FCO at different temperature for Ce
4
and CO
2
2
Zr1.96Pt0.04
O
7-
2
δ
. These studies were performed to find the kinetic regime in the dif-
ferential reactor. The reaction rate at various temperatures was de-
termined by the slope of the plot (initial linear region) and was calcu-
3.4.2. Partial oxidation and autothermal reforming of methane
Ce Zr1.96Pt0.04O7-δ was studied for partial oxidation of methane
2
XCH
W
4
lated using
F
r
CH = FCH4
, where W is the weight of the catalyst and
. The apparent activation energy was
with CH :O2 as 1:0.5. Fig. 11b depicts the concentration of various
4
4
CH is the molar flow rate of CH
4
species obtained at the exit of the reactor. CO formation was observed
4
2
2 4 2
Fig. 10. Variation of fractional conversion with W/F for Ce Zr1.96Pt0.04O7-δ (a) CH , (b) CO , (c) Arrhenius plot.
10

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
2 4 2 2 4 2 2
Fig. 11. Variation of gas concentration with temperature on Ce Zr1.96Pt0.04O7-δ for (a) DRM; (b) POM; ATR (c) CH :CO :O – 1:1:1:0.1, (d) CH :CO :O – 1:1:1:0.2,
4 2 2 2
(e) CH :CO :O – 1:1:1:0.5; (f) Variation of H /CO for various reforming reaction.
between 400−600 °C due to RWGS and total oxidation of methane.
However, as the temperature was raised, the rate of formation of CO
CO
side reaction like total oxidation of methane and RWGS also increased.
Fig. 11f shows the H /CO ratio at different temperatures for DRM, POM
and ATR (with different O concentration). H /CO ratio for DRM above
650 °C was 0.84, which then improved to 0.94 with the introduction of
0.6 % of O in the feed. It is thus apparent from this study that by
varying the reactant concentration in ATR, H /CO ratio can be tuned,
which is in agreement with the previous studies [52,53].
2
increased between 350−600 °C. This indicates that the extent of
increased. The H
the present study the H
above 650 °C (Fig. 11f). Autothermal reforming of methane was per-
formed over Ce 7-δ at three different composition of O
with CH :CO :O as 1:1:0.1, 1:1:0.2 and 1:1:0.5. It is evident from
Fig. 11c–e that with increasing concentration of O , the formation of
2
/CO ratio is expected to be close to 2 for POM and in
2
2
/CO ratio for POM was found to be ∼1.60
2
2
2
Zr1.96Pt0.04
O
2
2
4
2
2
2
2
11

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
Table 2
Comparison of catalytic results with the literature.
a
a
TOF (s−1
Catalyst
E
(kJ/mol)
E
(kJ/mol)
Rate (μmol/g/s)
)
Carbon (wt%)
Nature of carbon
Ref
CH4
CO2
5
2
3
wt% Ni/Ce0.8Pr0.2
O
2
4.35 (550)b,a
–
–
–
–
–
130 (700)
43.3 (800)
48.4 (500)
5.0 (550)
–
–
–
15.7 (700)
4.2 (850)
–
0.63 (800)
2.5 (500)
29.9 (25)c,b
5 (10)
negligible
4.1
3.8 (100)
–
Filamentous
Graphitic
–
Filamentous
filamentous, amorphous
–
–
[23]
[48]
[48]
[25]
[5]
[49]
[50]
[51]
c
wt% Ru sub -La
2
Zr
2
O
7
60
152
69
–
111
68
105
48
–
–
–
.78 wt% Pt sub-La
2
Zr
2
O
7
Ni/CeO
NiCe/SBA-16
.4 wt% Rh/Al
2
(44)/ZrO
2
(56)
0
O
2 3
LaNiO
LaNiO
Ce
3
77
–
3
/CeSiO
2
0.76 (24)
3.2 (24)
–
d
2
Zr1.96Pt0.04
O
7-δ
146
112
amorphous, graphitic
This work
a
b
c
Temperature in °C in parentheses.
Time on stream in h in parentheses.
Substituted.
d
Values are reported for feed- 20 % CH
4
2
/20 % CO .
inverse FFT pattern shown in Fig. 13c reveals the presence of (333) and
−
(
620) planes of pyrochlore along with (31
2
) plane of Ce
2
Zr
2
O
8
. Ad-
ditionally, the deposition of (111) plane of graphitic carbon (denoted in
white) was also observed. The electron diffraction pattern (Fig. 13e)
recorded from region B reveals high crystallinity. Different planes of
Ce
Ce
2
Zr
Zr
2
O
O
7
- (311), (440), (622) were observed along with (400) plane of
.
2
2
8
3.6. In situ FTIR (DRIFTS) studies on Ce
2
Zr1.96Pt0.04
O
7-δ
DRIFTS study was performed on Ce
2
Zr1.96Pt0.04O7-δ for DRM, POM
and ATR at different temperatures i.e. 30, 350, 450 and 600 °C. The
shown spectra were recorded once the peak intensities attained stable
−
1
values (∼30 min.). The bands at 3728 and 3687 cm
correspond to
eOH group bonded to the surface, whose intensity decreased with in-
creasing temperature. At temperature above 350 °C, the presence of
eOH species can be attributed to the dissociation of H
2
O formed via
RWGS. All the spectra (Figs. 14 and 15) exhibited the presence of
−
1
Fig. 12. Time on stream CH
on Ce
4
and CO
2
conversion for dry reforming of methane
gaseous CH
4
at 3016 (ν
3
) and 1305 cm (ν
4 2
) and gaseous CO at 2360
−1
−1
2
Zr1.96Pt0.04
O
7-δ
.
and 2344 cm . It is to be noted that the bands at 2917 cm (ν
1
) and
) were absent at room temperature as they are infrared
inactive due to symmetrical structure of CH [54]. Fig. 15a shows the
formation of PteCO band at 2058 and 2036 cm
−1
1533 cm
(ν
2
3.5. Stability studies
4
−1
and gaseous CO at
at temperature as low as 350 °C for DRM. The
shift in frequency of PteCO band can be attributed to the co-adsorption
of CO and CH on Pt. This can be clearly seen in Fig. S6 d in Supple-
mentary information, where the IR spectra were recorded in the pre-
sence of CH and CO at different temperatures. The intensity of gaseous
−1
2172 and 2112 cm
The time on stream studies was performed for 24 h on
2
Zr1.96Pt0.04O7-δ to investigate the stability of the catalyst. Fig. 12
Ce
depicts the excellent stability of the catalyst with no sign of deactiva-
tion for the continuous cycle. To study the amount of coke deposited
4
over the spent catalyst TGA was performed under N
evaluation was found to be 1.5 wt% for 3% CH with CH
tained as 1. The stability studies were also performed at 20 %. CH
concentration (CH :CO – 1) and the catalyst showed high stability for
2
flow, which upon
4
CO band increased with temperature indicating the low extent of
Boudouard reaction on the catalyst surface. However, the gaseous CO
band for POM (Fig. 15b) and ATR (Fig. 15c) was apparent only at
higher temperature. It is evident from the spectra that no bands were
4
4
:CO main-
2
4
4
2
period of 24 h with 3.2 wt% of deposited carbon. Table 2 depicts the
comparison of coke deposition over different catalysts after time on
stream studies.
TEM micrographs were recorded subsequent the stability studies of
2
Ce Zr1.96Pt0.04O7-δ, to gain insight in the effect of reaction condition on
the catalyst structure and the nature of carbon deposited. Fig. 13a
displays the bright field TEM image of the spent catalyst, which exhibits
no significant change in the particle size after the reaction. Ad-
ditionally, the presence of amorphous layer of carbon over the catalyst
surface (marked with pink arrow) was noted and was marked as region
A in Fig. 13a. The electron diffraction pattern of region A is depicted in
Fig. 13d. Due to the amorphous layer of carbon, only few Laue rings
−1
observed at 2163 and 2169 cm , which corresponds to CO adsorption
on Ce3+ and Ce
presence of adsorbed CO
4+
[55]. Fig. 15b (POM) and 15c (ATR) shows the
band even at higher temperature due to the
oxidation of CO in the presence of oxygen.
A fluctuating band was observed in the region 2893−3152 cm
with a sharp characteristic mode of gaseous CH
2
−1
−1
4
at 3017 cm
3
(ν ) in
all reforming processes (Fig. 14). However, the small peaks at 2975 and
−1
2875 cm
appeared at higher temperatures, which were assigned to
adsorbed on Pt. These bands arise due to red shift in gaseous CH
CH
4
4
−1
band at 3017 and 2917 cm , as a consequence of CH
4
adsorption on
−
1
Pt. The vibrational mode at 2917 cm (ν
to T symmetry of the CH . However, the strong interaction of CH
molecule with the catalyst surface leads to lower symmetry structure
1
) is forbidden in infrared due
were seen that corresponds to (222) and (400) planes of Ce
2 2 7
Zr O
d
4
4
(
shown in yellow) and (400) and (531) planes of Ce Zr ((shown in
2
2 8
O
−1
like C3ν, which can be seen with the red shift in ν
54,56]. The oscillating band was noticed on the either side of the ν
all the spectra. Munera et al. observed a similar kind of spectra on
1
band at 2875 cm
orange). Fig. 13b exhibits the HRTEM micrograph of the spent catalyst
and inset denotes the FFT pattern of the marked yellow region. The
[
3
in
12

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
Fig.
Ce
13. TEM
micrographs
of
2
Zr1.96Pt0.04
O
7-δ recorded after sta-
bility studies (a) Bright field TEM
image showing amorphous carbon
(shown by pink arrow), (b) HRTEM
micrograph, inset represents FFT pat-
tern yellow marked region, (c) Inverse
FFT pattern, (d) and (e) SAED pattern
from Region A and B, respectively. (For
interpretation of the references to
colour in this figure legend, the reader
is referred to the web version of this
article.)
which are attributed to carbonate, formyl and formate species (Fig. 16).
Heyl et al. [60] reported that the band due to C]O stretching of formyl
−
1
species was observed at 1780 cm
on Rh catalysts, whereas, Chafik
−1
et al. [61] observed the same band at 1635 cm . In the present case,
there were no bands in these regions indicating the absence of formyl
species. This confirms that the CH
surface of Ce 7-δ. To distinguish between carbonate and
formate bands, the spectra were recorded in the presence of CH
CO
appears between 1510−1550 cm
x
species were not formed over the
2
Zr1.96Pt0.04O
4
and
2
alone (Fig. S6a–c in Supplementary information). Formate species
−
1
asym
−1
sym
(ν
), 1310−1375 cm
and 1088 cm , whereas carbonate vibrational bands were observed at
(ν
)
OCO
OCO
−
1
−1
1
550, 1464 and 1088 cm
[59,61,62]. It is thus apparent that (Fig.
S6a–c) that not only carbonate species decomposes as the temperature
is increased, but also different types of formate species were formed.
2 7-δ
4. Kinetic and mechanism development on Ce Zr1.96Pt0.04O
Noble metal are highly active for dry reforming of methane and
their activity declines in the following order Ru > Rh > Ni ∼
Ir > Pt > Pd, while the coke formation decreases in the order of
Ni > > Pt > Ru [63,64]. Platinum group metals are responsible for
the activation of C-H bond in methane and also enhance the spillover of
hydrogen and oxygen species between metal support [3]. However, in
the present study, Pt-substituted oxide exhibited better catalytic ac-
tivity than the Ru catalysts. This result prompts that the support plays a
crucial role in not just controlling coke deposition but also in modifying
the stability of various surface intermediates [12,65]. Previous studies
on DRM revealed that the CeH bond activation is the only rate con-
trolling step over supported metal [66]. Although, there are few studies,
2
Fig. 14. CeH stretching band at different temperature on Ce Zr1.96Pt0.04O7-δ for
4 2 2
(a) DRM, (b) POM, (c) ATR, CH :CO :O – 1:1:1:0.5.
La
lating nature of the spectra in this region are: (a) at low temperatures,
these oscillations are seen due to the interaction of CH with the hy-
droxyl group on the surface. The interaction of CH with hydroxyl
species induces dipole moment in CH , which causes the ν mode
symmetric stretch) to become weakly active [58], (b) at high tem-
perature, the occurrence of these bands is probably due to the inter-
action of CH with the surface species like formates, carbonates or
adsorbed hydrogen, (c) there is also possibility of occurrence of CeH
2 3 2 4
O in the presence of CO and CH [57] The reasons for the oscil-
4
4
4
1
where, the adsorption of CO
67]. Vasiliades et al. demonstrated with the help of SSTIKA studies
that the oxidation of carbon occurs by lattice oxygen over Ni/
and that the carbonate type species are inactive for the
reaction [22]. However, studies on La support showed the formation
2
is also considered as kinetically-relevant
(
[
4
2 δ
Ce0.38Zr0.62O -
−
1
2 3
O
vibration of formates in the range of 2950−2800 cm
above mentioned interactions are weak in nature, when the system is
purged with N , no bands were seen in this range. This also suggests
that CH bands were absent.
In the region 1700−1000 cm−1, multiple bands were observed
[59]. As the
of oxycarbonates and gasification of carbon by carbonate formation was
found to be rate determining step [68,69]. Pakhare et al. proposed a
dual-site mechanism for Rh-substituted La Zr O pyrochlores, with CH
2 2 7 4
activation being the rate determining step. However, the participation
of formates and carbonate species were not considered in the
2
x
13

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
2 7-δ
Fig. 15. DRIFT spectra recorded at different temperature on Ce Zr1.96Pt0.04O
−
1
in the region 2450–1800 cm
:1:1:0.5.
4 2 2
for (a) DRM, (b) POM, (c) ATR, CH :CO :O –
1
mechanism [19].
There are many studies on the mechanistic and kinetic study of DRM
where Eley-Rideal and Langmuir-Hinshelwood mechanisms have been
proposed. However, to the best of our knowledge, there is no study
where the formation of both carbonate and formate is taken into ac-
count while deriving the rate expression. The kinetic studies done by
Tsipouriari and Verykios over Ni/La O , CH dissociation and C-gasi-
2 3 4
fication via surface carbonates were proposed to be the rate de-
termining step [68]. In the present work, DRIFTS study showed the
2 7-δ
Fig. 16. DRIFT spectra recorded at different temperature on Ce Zr1.96Pt0.04O
−
1
in the region 1750–1000 cm
:1:1:0.5.
4 2 2
for (a) DRM, (b) POM, (c) ATR, CH :CO :O –
1
presence of carbonates and formates on the catalyst surface in addition
to adsorbed CH , CO and CO. However, no signs of CH and CH
formyl) species were observed. TEM studies showed the presence of
carbon on the surface of spent catalyst. Hence, the oxidation of the
4
2
x
x
O
(
14

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
Fig. 17. Variation of rate of CH
4
over Ce
2
Zr1.96Pt0.04
O7-δ with (a) partial pressure of CH
4
at constant pCO2 (b) partial pressure of CO
2
at constant pCH4
.
Table 3
Rate parameters obtained after fitting of Ce
1
r
1
k
2
1
P
CH
1
1
CO
⎛
⎜
⎞
⎟
⎛
⎜
⎞
⎟
=
+
+
2
Zr1.96Pt0.04
7-δ
O .
k
K
k
P
P
7
4
4
7
CH
4
2
(3)
⎝
⎠
⎝
⎠
Parameters Ce
2
Zr1.96Pt0.04
O
7-δ
Eqs. (2) and (3) indicate that the linear dependence between 1/r and
1/PCH at constant
the inverse of experimental rate vs 1/PCH at (constant
k
2
2.49 ± 0.05) × 105 exp
−17817 ± 65
P
CO and 1/r and 1/PCO2 at constant
P
P
CH4. Therefore,
(
(
(
4
2
(
)
T
CO ) and 1/PCO
4
2
2
K
4
7
−1269 ± 17
2.2 ± 0.6) exp(
)
T
(constant
lues of k
two linear plots, using the dependence from Eqs. (2) and (3). Then, Eq.
1) was fitted with the experimental data using the values of parameters
P
CH4) were plotted and found to be linear. The regressed va-
, K and K were obtained from the slope and intercept of the
K
4
−16129 ± 70
8.46 ± 0.8) × 10 exp
2
4
7
(
)
T
(
obtained by the linear regression. Fig. 17 shows the experimental data
with the rate Eq. (1). Table 3 enlists the value of rate parameters ob-
tained after fitting.
deposited carbon by lattice oxygen or by carbonates is an important
step in the mechanism. High apparent activation energy of CH than
CO (see Section 3.4.1), suggests that CH dissociation can be rate de-
termining step over Ce 7-δ [48]. Thus, based on the above
discussion, the following mechanism has been proposed on
4
2
4
2
Zr1.96Pt0.04O
5. Conclusions
Ce
2
7-δ
Zr1.96Pt0.04O :
Detailed structural characterization (XRD, XPS, TEM) was per-
formed for the synthesized samples. XRD exhibited the presence of pure
K1
CH4 + Pt⟷CH4 − Pt
(R-1)
pyrochlore phase for NM substituted Ce
Pt substituted Ce Ti pyrochlores showed the formation of other
oxide phases. XPS analysis on Ce 7-δ showed the presence
2 2 7
Zr O pyrochlores. However,
k
2
CH
4
− Pt⟶C − Pt + 2H
2
(RDS)
(R-2)
(R-3)
(R-4)
(R-5)
(R-6)
2
2 7
O
2
Zr1.96Pt0.04O
K
3
3+
4+
of Ce
and Ce
Zr
in the XP spectra of Ce 3d, which suggests the for-
7+x compound. The appearance of Pt in the XP
H
2
+ 2Pt⟷2H − Pt
0
mation of Ce
2
2
O
K4
spectra of Pt 4f recorded after the reaction was due to the presence of
reducing atmosphere. TEM exhibited the narrow size distribution of
particle size in all the synthesized samples and no significant increase in
particle size was observed in the spent catalysts, indicating high
CO2 + CZO⟷CZ − CO3
K5
CO2 + CZ⟷CO2 − CZ
K6
thermal stability. Amongst the synthesized materials, Ce
was found to be highly active for DRM and the time on stream studies
demonstrates high stability and coke resistance. Low coke deposition is
indicative of the ease of availability of lattice oxygen in Ce Zr 7+x, as
observed in H2 reduction studies. Syngas ratio was enhanced by the
addition of O in the feed (ATR) indicating the suppression of RWGS. At
higher temperature, DRIFTS studies on Ce 7-δ displayed the
2 7-
Zr1.96Pt0.04O
CO2 − CZ⟷CO + CZO
δ
k7
CO3 − CZ + C − Pt⟶2CO + Pt + CZO
(R-7)
(R-8)
2
2
O
K8
CO2 − CZ + H − Pt⟷HCO2 − CZ + Pt
2
K9
2
Zr1.96Pt0.04O
HCO
2
− CZ⟷CO + HO − CZ
(R-9)
development of CO band as a result of decomposition of formates and
carbonates, which was true for all the three reforming routes. Based on
DRIFTS and kinetic studies, the reaction mechanism was proposed to
occur on Ce Zr1.96Pt0.04O7-δ via formate formation indicating the in-
2
volvement of support during the reaction.
k10
C − Pt + CZO⟶CO − Pt + CZ
(R-10)
(R-11)
K11
CO − Pt⟷CO + Pt
The comprehensive derivation of mechanism based on above ele-
mentary step has been included in Supplementary information. The
CRediT authorship contribution statement
2
final rate law over Ce Zr1.96Pt0.04O7-δ was found to be
Disha Jain: Conceptualization, Methodology, Data curation,
Visualization, Investigation, Writing - original draft, Writing - review &
editing. Shreya Saha: Investigation.
4 2
2 7 4 CH CO
k k K P P
r =
k
7
K
4
P
CH
4
P
CO + k
2
(1 + K
4
PCO2)
(1)
(2)
2
Eq. (1) can be rewritten as
Declaration of Competing Interest
1
1
1 + K
4
P
P
CO
CO
1
CH
⎛
⎜
2
⎞
⎟
=
+
r
k
2
K
4
k
7
P
⎝
2
⎠
4
The authors declare that they have no known competing financial
15

D. Jain and S. Saha
Molecular Catalysis 492 (2020) 110964
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
reactivity of active and inactive carbon formed during drm over ni/ce0.38zr0.62o
2-δ
studied by transient isotopic techniques, Catal. Today 299 (2018) 201–211.
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Appendix A. Supplementary data
4
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