Ce promoted lanthana-zirconia supported Ni catalyst system: A ternary redox system for hydrogen production

Article abstract of DOI:10.1016/j.mcat.2021.111498

Hydrogen production from dry reforming of methane (DRM) over chief Ni-based catalyst is cutting edge research area due to environmental consciousness about reducing global warming gases (CH₄ and CO₂) and greener route of synthesis. Herein, ceria promoted lanthanum-zirconia supported Ni catalyst system (5NixCe/LaZr; x = 0, 1, 1.5, 2, 2.5, 3, 5 wt.%) is prepared by the wet impregnation method. It is further characterized using X-ray diffraction, infrared spectroscopy, ultraviolet spectroscopy, temperature-programmed and cyclic temperature-programmed experiments. 2.5 wt.% ceria promoted lanthana-zirconia supported Ni catalyst (5Ni2.5Ce/LaZr) has thermally stabilized the support, wide Ni-LaZr interface for CH₄ decomposition, wide basic surfaces (La₂O₃-ZrO₂) for CO₂ interaction, CO₂ coordinated La⁺³ sites (as La₂O₂CO₃ species), low band gap and oxygen vacancy carrying mobile oxygen (which is replenished by CO₂). Altogether, it produces more than 87 % hydrogen yield. This thorough study and fine correlation will help design industrially suitable catalyst for hydrogen production through DRM.

Full text of DOI:10.1016/j.mcat.2021.111498

Molecular Catalysis 504 (2021) 111498
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Ce promoted lanthana-zirconia supported Ni catalyst system: A ternary
redox system for hydrogen production
Jyoti Khatria, Ahmed S. Al-Fateshb *, Anis H. Fakeehab, Ahmed A. Ibrahimb,
,
,
Ahmed E. Abasaeedb, Samsudeen O. Kasimb, Ahmed I. Osmanc, Rutu Patela, Rawesh Kumara *
a Department of Chemistry, Sankalchand Patel University, Visnagar, Gujarat, 384315, India
b Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh, 11421, Saudi Arabia
c School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast, BT9 5AG, Northern Ireland, UK
A R T I C L E I N F O
A B S T R A C T
Keywords:
Hydrogen production from dry reforming of methane (DRM) over chief Ni-based catalyst is cutting edge research
area due to environmental consciousness about reducing global warming gases (CH4 and CO2) and greener route
of synthesis. Herein, ceria promoted lanthanum-zirconia supported Ni catalyst system (5NixCe/LaZr; x = 0, 1,
1.5, 2, 2.5, 3, 5 wt.%) is prepared by the wet impregnation method. It is further characterized using X-ray
diffraction, infrared spectroscopy, ultraviolet spectroscopy, temperature-programmed and cyclic temperature-
programmed experiments. 2.5 wt.% ceria promoted lanthana-zirconia supported Ni catalyst (5Ni2.5Ce/LaZr)
has thermally stabilized the support, wide Ni-LaZr interface for CH4 decomposition, wide basic surfaces (La2O3-
ZrO2) for CO2 interaction, CO2 coordinated La+3 sites (as La2O2CO3 species), low band gap and oxygen vacancy
carrying mobile oxygen (which is replenished by CO2). Altogether, it produces more than 87 % hydrogen yield.
This thorough study and fine correlation will help design industrially suitable catalyst for hydrogen production
through DRM.
Ceria promotion
DRM
La2O3-ZrO2 supported Ni
H2-production
Temperature programmed cyclic experiments
1. Introduction
route for hydrogen production.
Ni supported on SiO2, Al2O3 and ZrO2 were found promising for H2
Hydrogen is a clean and green source of energy. It is produced from
biomass pyrolysis, biomass gasification [1], ethanol [2], glycerol [3],
glucose [4], starch and catechol [5] through either stream or thermal
reforming. Albeit, hydrogen production through clean sources as
methane over a heterogeneous catalyst is highly demanded due to
environmental concern, atom economy and easy separation of H2 from
the product mixture. Thermal reforming of methane [6–8], steam
reforming of methane [9] and dry (CO2) reforming of methane are some
popular choice for hydrogen production. Hydrogen production from dry
reforming of methane (DRM) belongs to cutting edge research which
opens the hope of the efficient conversion of two global warmings gas
CO2 and CH4 together. Among heterogeneous catalysts, Pt [10], Ru [11],
Rh [12] and Ni-based catalyst system was found efficient in hydrogen
production. In recent year, cheap Ni-based catalyst system has drawn
most of the attention in this field. However, under highly endothermic
DRM reaction, Ni undergoes serious aggregation and loss its catalytic
property. So, the Ni-supported system is indeed a successful catalytic
production through DRM. All three supports can sustain the high-
temperature reaction condition for DRM. The possible activation of
CO2 at Ni/Zr interface [13] as well as inducing Ni dispersion through the
anchoring effect of Zr4+ species [14] makes ZrO2 support specific than
SiO2 and Al2O3. Apart from that, the redox behaviour of ZrO2 makes it
superior than that of irreducible SiO2 and Al2O3 support. Redox
behaviour enables the support to release oxygen for possible carbon
deposit oxidation [12,15–17]. Further addition of modifier/promoter in
zirconia supported Ni catalyst system was found to enhance the catalytic
activity. A brief literature survey of promotor/modifier utilized over
zirconia supported Ni catalysts is shown in Table 1.
Research activity for preparing Ni dispersed catalyst over binary
metal/non-metal oxide is also noticed in tuning acid-basic property or
thermal stability [33,34]. La2O3 also has potent redox chemistry with
La2O3/La2O2CO3 set [35–37]. It will be interesting to observe the effect
of the strong synergy between La2O3-ZrO2 if used to support the Ni
dispersed system. Presence of La2O3 may also influence CO2 adsorption
* Corresponding authors.
E-mail addresses: aalfatesh@ksu.edu.sa (A.S. Al-Fatesh), kr.rawesh@gmail.com (R. Kumar).
https://doi.org/10.1016/j.mcat.2021.111498
Received 25 November 2020; Received in revised form 4 February 2021; Accepted 16 February 2021
Available online 3 March 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

J. Khatri et al.
Molecular Catalysis 504 (2021) 111498
Seelze, Germany, Ce (NO3)3.6H2O was purchased Williams Ltd, Essex,
England, respectively. The support, La2O3+ZrO2, was obtained as a gift
from Daiichi Kigenso Kagaku Kogyo Co., Ltd. Osaka – Japan.
Table 1
A brief literature survey of promotor/modifier over ZrO2 supported Ni Catalysts.
Modifier/Promoter
Support
References
Period 1
Period 2
–
–
–
–
2.2. Catalyst preparation
–
–
–
–
–
Na
Mg
Mg
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
ZrO2
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[21]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[27]
–
The catalyst was synthesized via wet impregnation method as
described in our earlier work [66]. Weighted amount of Ni (NO3)2.6H2O
equivalent to 5.0 wt. % loading of nickel was dissolved in an aqueous
medium. Weighted amount of Ce (NO3)3.6H2O corresponding to 1.0,
1.5, 2.0, 2.5, 3.0, 5.0 wt. % loading of cerium was added to the nickel
nitrate aqueous solution. Further, the support 9 wt%La2O3+91wt%ZrO2
was added to the aqueous mixture under stirring at 80 ◦C. The slurry was
dried at 120 ℃ overnight in an oven and subsequently calcined in air at
600 ℃ for 3 h. Zirconia supported Ni catalyst, 1 % ceria promoted zir-
conia supported Ni catalyst, lanthana-zirconia supported Ni catalyst and
1–3 % ceria promoted lanthana-zirconia supported Ni catalyst are
abbreviated as 5Ni/Zr, 5Ni1Ce/Zr, 5Ni/LaZr, 5NixCe/LaZr (where; x =
1, 1.5, 2, 2.5, 3, 5).
Period 3
Al
Al
K
–
Mg
Ca
—
Fe
Period-4
—
Co
Mg
—
Co, Al
Y
—
Period-5
Period-6
Mg
—
La
—
—
Ce
Mg
—
Sm
2.3. Catalyst characterization
(due to its basic property) [38] and Ni-support interaction [39,40] &
NiO size optimization [41,42] whereas it may inhibit H2 consumption
through RWGS reaction [43,44]. Tou et al.; have prepared nickel
impregnated La2O3-ZrO2 via a sol-gel method for dry reforming of coke
oven gas and observed great coke resistance property [44].
Catalysts are characterized by X-ray diffraction (XRD), Infra-red
spectroscopy, Ultraviolet-visible spectroscopy (UV–vis), CH4-Tempera-
ture programmed surface reaction (CH4-TPSR), H2-temperature-pro-
grammed reduction (H2-TPR), CO2-Temperature programmed
desorption (CO2-TPD), temperature-programmed hydrogenation (TPH)
and O2-Temperature programmed oxidation (O2-TPO). The details
specification of the instrument and characterization procedure are given
in supporting information (S1).
Further addition of a small amount of CeO2 in the catalyst system
enhances lattice oxygen mobility, which is beneficial in eliminating
carbon [45]. The synergistic role of La-Ce was also claimed as guaran-
teed catalyst tolerance against carbon deposition as well as lattice oxy-
gen replenishment for the oxidation reaction [46]. Overall, we can
expect as good carbon resistant and high H2 yield with ceria promoted
La2O3-ZrO2 supported Ni catalytic system.
2.4. Catalyst activity test
It is generally accepted that H2 production (yield) through DRM is
greatly driven by dissociation of CH4 over Ni-supported surface followed
by subsequent gasification of carbon deposit by CO2 for cleaning the
surface for the next set of CH4 decomposition reaction (CH4 + CO2 → CO
+ H2). Later gasification of carbon deposit by H2 due to spillover effect
of hydrogen on the surface (C + 2H2 → CH4) [47] as well as reverse
water gas shift (RWGS) reaction (CO2 + H2 → CO + H2O) affects the H2
yield seriously. However, gasification of carbon deposit by water (as a
product of RWGS) has a positive contribution in H2 yield.
The dry reforming of methane experiments was carried over 0.1 g of
Ce promoted 5%Ni/La2O3+ZrO2 catalyst at 700 ℃ under 1 atm pressure
in stainless steel vertical fixed tubular reactor (PID Eng. & Tech Micro
Activity Reference, 9.1 mm i.d. and 30 cm long). The catalysts were
positioned by the help of a ball of glass wool during activity tests. The
temperature of the reactor was monitored by an axially positioned
thermocouple (K-type stainless sheathed) at the center of the catalyst
bed. Before the catalytic tests, reductive pretreatment of catalyst sam-
ples was carried out under the flow of hydrogen (20 mL/min) for 60 min
at 600 ℃. The mixture of gases feed CH4/CO2/N2 in the respective
volumes as 6:6:2, and the volume flow rate was 70 mL.minꢀ 1 and 42,000
Herein, we have prepared a lanthanum-zirconia supported Ni cata-
lyst system and is further promoted by 1–5 % ceria. This catalytic system
is employed for hydrogen production through DRM. It is characterized
via X-ray diffraction, Infrared spectroscopy, Ultraviolet spectroscopy,
temperature-programmed reduction (TPR), temperature-programmed
desorption (TPD), temperature-programmed surface reaction (TPSR),
mL(h.gcat
)
ꢀ 1 gas hourly space velocity was passed through the reactor.
The product gas stream was analyzed by GC (GC-2014 Shimadzu) unit
equipped with a thermal conductivity detector and two columns, Por-
apak Q and Molecular Sieve 5A. H2 yield % and H2/CO molar ratio are
determined by the following expressions:
temperature-programmed
hydrogenation
(TPH),
temperature-
programmed oxidation (TPO) and cyclic temperature-programmed ex-
periments, i.e. CH4-TPSR, H2TPR-CO2TPD-H2TPR cycle, CO2TPD-
O2TPO cycle and TPH-O2TPO cycle. A fine correlation is established
between characterization results and catalytic activity. This study will
be helpful to design industrially robust catalyst for hydrogen production
through dry reforming of methane. The objective of the research is to
explore the surface catalytic excellence of Ce promoted Lanthana-
Zirconia supported Ni catalyst system for hydrogen production
through DRM by X-ray diffraction, Infrared & Ultra-violet spectroscopy,
different temperature-programmed experiments & cyclic temperature-
programmed experiments.
Mole of H2 in product
H2 Yield% =
× 100
2 x m ol of CH4
in
H2
CO
Moles of H2 produced
Moles of CO produced
=
3. Result and discussion
3.1. Catalytic activity results
The catalytic activity results in terms of H2 yield is shown in Fig. 1.
Zirconia supported Ni catalysts are efficient to produce hydrogen with a
43 % yield. After adding the ceria promoter (5Ni1Ce/Zr), a rise in H2
yield (47 %) was noticed. However, a marked increase of H2 yield about
80 % was only noticed if lanthana-zirconia support was used in place of
only zirconia support. 5Ni/LaZr catalyst showed 80 % H2 yield. Further
2. Experimental
2.1. Materials
Ni (NO3)2.6H2O (98 %) was purchased from Riedel-De Haen AG,
2

J. Khatri et al.
Molecular Catalysis 504 (2021) 111498
composition that has both high and stable performance toward H2 yield.
5Ni2.5Ce/LaZr catalyst retained 86–87 % H2 yield up to more than 420
min. 2.5 % ceria loading was found to be the optimum loading in term of
catalytic activity. Further increases in ceria loading caused a drop in H2
yield; however, the activity of the catalysts remains stable. 5Ni3Ce/LaZr
and 5Ni5Ce/LaZr showed 81–82 % and 80–81 % H2 yield up to more
than 420 min of reaction, respectively. H2/CO ratio of all ceria promoted
lanthana-zirconia supported Ni catalyst system are found close to 1 (Fig
S2). Thermogravimetric analysis (TGA) analysis shows decreased car-
bon deposits over the catalyst surface with increasing ceria loading
(Fig. S3).
3.2. Characterization results
The XRD profiles of spent catalysts of 5NixCe/Zr (x = 0, 1) are shown
in (Fig. 2 (A)). Spent zirconia supported Ni (Spent 5Ni/Zr) catalyst
shows monoclinic zirconium oxide phase (space group: P21/C, JCPDS
no. 00-007-0343), crystalline carbon phases (space group: P63mc,
JCPDS no. 01-075-1621) and cubic nickel oxide phase (space group:
Fm3m, JCPDS no. 01-073-1519, 2θ = 37.32◦ at 111 plane, 2θ = 43.57◦
at 200 plane; 2θ = 62.92◦ at 220 plane). It indicates that zirconia sup-
port is prone to crystalline carbon deposition during the DRM reaction.
Ceria promoted catalyst system is known to be coke resistance due to
prompt mobile oxygen availability for coke oxidation. Ceria promoted
Fig. 1. Catalytic performance of 5NiyCe/Zr and 5NixCe/LaZr (x = 0, 1, 1.5, 2,
2.5, 3, 5 and y = 0, 1) in different catalytic system for dry reforming of
methane (DRM).
promotional effect of ceria was also noticed up to 2.5 % Ceria addition.
5Ni1Ce/LaZr, 5Ni1.5 Ce/LaZr and 5Ni2Ce/LaZr showed 82 %, 85 % and
91 % H2 yield, respectively. Our target was to find out a catalytic system
Fig. 2. XRD profile of fresh and spent 5NixCe/Zr and 5NixCe/LaZr (x = 0, 1) catalyst used for dry reforming of methane (DRM).
3

J. Khatri et al.
Molecular Catalysis 504 (2021) 111498
spent catalyst (spent 5Ni1Ce/ZrO2) had minimum carbon phases, but it
had prominent defective zirconia oxides phase as ZrO1.96 (monoclinic
Zirconium oxide, space group: P21/C, JCPDS no. 01-081-1319) and
Zr0.93O2 (cubic Zirconium oxide, space group: Fm3m, JCPDS no. 01-
081-1551). That means ceria promotional addition caused rapid wash
away of crystalline carbon (by oxidation of deposit).) Overall, defective
zirconia and phase transformation of zirconia phase from monoclinic to
cubic zirconia at high-temperature condition limits the high catalytic
performance of this catalytic system. Overall, zirconia was not a good
support, and some phase stabilizer is curiously needed for higher cata-
lytic performance.
for stretching vibration of Zr-O [50–52] whereas IR spectra of La2O3
shows a characteristic absorption band at 3,608 cmꢀ 1 for stretching
vibration of OH and 640 cmꢀ 1 for bending vibration of OH group at
surface of La2O3 and peak at 3,450 cmꢀ 1 is due to La(O)OH species
(Fig. 3(A)).
absorption bands at 1,464, 1,370, 1,084 and 859 cmꢀ 1 are attributed
to La2O2CO3 species. Among which absorption bands at 1,464 and 859
cmꢀ 1 are due to CO23ꢀ symmetric stretching, vibration belongs to the
pure form of typical ionic anhydrous lanthanum carbonate (La2(CO3)3)
[52,53]. The absorption band at 1370 cmꢀ 1 is due to “oxycarbonate
strongly coordinated with La3+” (La2O3.CO2). The absorption band due
to the symmetric stretch vibration of CO23- is infrared inactive for free ion
case, but in the lattice, it became active and gave an FTIR band at 1084
cmꢀ 1 due to change of D3h symmetry of free ion to lower symmetry C2v
or Cs [54]. This peak was selected to estimate the proportional amount
of La2O2CO3 species because it gave only one band in the hexagonal
polymorph form of La2O2CO3 [55].
The XRD profile of 5NixCe/LaZr (x = 0, 1) are shown in (Fig. 2(B)–
(F)). After mixing lanthanum in zirconia support, no such defective
phases and phase transformations were noticed (Fig. 2(B)). It indicates
that lanthana stabilized the zirconia phase and now lanthana-zirconia
became a good catalytic support for a high-temperature DRM reaction.
However, in all spent catalyst system; hexagonal lanthanum oxide phase
(Space group: P-3m1, JCPDS no. 01-083-1345; 2θ = 25.51◦ 100 plane,
2θ = 29.37◦ at 011 plane) was noticed. It indicates that the La2O3 phase
crystallized during the high-temperature reaction. Spent 5Ni/LaZr
catalyst also showed intense NiO peak at 37.32◦ than that without lan-
thana catalyst. It also an indication of poor NiO dispersion after the
reaction. To resolve this issue, a small addition of ceria was found
beneficial. After promotional addition of ceria in this catalyst system,
spent 5Ni1Ce/LaZr showed tetragonal cerium zirconia oxide peaks
(Space group: P4212, JCPDS no. 00-026-0359; 2θ = 29.46◦ at 002 plane,
2θ = 34.31◦ at 102 planes; 2θ = 49.34◦ at 212 plane) and diffused NiO
crystallite peaks. It indicates that the presence of ceria or ceria-zirconia
mixed oxide enhances the NiO dispersion and control the size of NiO
during the high-temperature DRM reaction.
The FTIR signal of La-Zr samples shows the disappearance of the
1
–
absorption band of 748 cmꢀ 1 for ZrO, 3608 and 640 cmꢀ
for OH
group vibration at the surface of La2O3. It indicates that the bonding
between atoms was greatly modified and isolated identity ZrO2 and
La2O3 were lost. The absorption band at 3450 cmꢀ 1 is due to La(O)OH
species and was either disappeared or merged with hydroxyl peaks of
zirconia/Lanthana-zirconia interface. As ZrO2 was present in major
quantity, so peaks due to vibration of OH were preserved at 3425 and
1625 cmꢀ 1. Absorption bands at 1084 and 1370 cmꢀ 1 for “oxycarbonate
strongly coordinated with La3+” in La2O2CO3 species were also found in
La-Zr samples whereas peak due to anhydrous lanthanum carbonate
(La2(CO3)3) were disappeared. Small new absorption bands also
appeared below 800 cmꢀ 1 due to La-O-La and La-O-Zr [56]. If IR spectra
was taken after CO2-TPD of 4Ni1CeLaZr sample, the absorption band of
physically adsorbed CO2 (2349 cmꢀ 1) had increased, but absorption
band due to surface hydroxyl (3425 and 1625 cmꢀ 1) and La2O2CO3
species (1084 and 1370 cmꢀ 1) also decreased. It indicates that at
high-temperature reaction condition these species were present over
catalyst surface.
The XRD profiles lanthana-zirconia sample had zirconia oxide pha-
ses, but not the lanthanum oxide phase. It indicates that lanthanum
oxide is well dispersed. After NiO addition, the cubic NiO diffraction
peaks appeared at 2θ = 37.32◦ and 43.57◦ (Fig. 2(C)). Moreover, after
the reaction; zirconium oxide diffraction peak intensity was suppressed
predominantly, lanthanum oxide (01-083-1345) diffraction peak
appeared, and NiO peaks about (111) intensified. After addition of ceria
to “lanthana-zirconia supported Ni system”, a marginal rise of zirconia
oxide peaks was noticed, but no significant changes were noticed in the
mean crystallite size of NiO (Fig. 2 (E) and (F)). After the reaction, the
lanthanum oxide (01-083-1345) peak appeared whereas zirconium
oxide as well as NiO peaks, were suppressed. It indicates that ceria does
not influence NiO dispersion during catalyst preparation, but during the
high-temperature DRM reaction, its presence is essential for high
dispersion of NiO.
The FTIR signal of “ceria promoted Lanthana-Zirconia supported Ni
catalyst” shows the disappearance of 1084 cmꢀ 1 absorption bands of
La2O2CO3 species concerning only “Lanthana-Zirconia supported Ni
catalyst” Fig. 3(B). However, as ceria loading increased from 1 % to 2 %,
it appeared again. It is remarkable that as ceria loading increased ab-
sorption bands of OH vibration and peaks due to La2O2CO3, they were
increasing. It indicates that ceria addition influences the surface hy-
droxyl as well as La2O2CO3 population.
Pure ZrO2 shows the UV band at 226 and 280 nm due to charge
transfer from the 2p energy state of O (valence band) to 4d (x2-y2, z2)
energy state of Zr (conduction band) (Fig. 4(A)). It is simply represented
as charge transition O2ꢀ to Zr+4 [57,58]. Because Zr has
d◦ configuration, so no characteristic peak for d-d transition was found.
It shows a band gap of 5.14 eV (Fig. 4(B)). After the addition of 9%
The Infra-red spectroscopy (IR) absorption bands about 3,425 and
1,625 cmꢀ 1 were found in all samples which are attributed to stretching
and bending vibration of OH, respectively Fig. 3. Physically adsorbed
CO2 gas showed an IR absorption band at 2,349 cmꢀ 1 [48,49]. The IR
spectra of ZrO2 shows absorption bands at 748 and 500 cmꢀ 1 which are
Fig. 3. IR spectra of the different catalysts used for dry reforming of methane (DRM).
4

J. Khatri et al.
Molecular Catalysis 504 (2021) 111498
Fig. 4. Ultraviolet-visible spectroscopy (UV–vis) and band gap of the different catalyst samples.
Lanthana in Zirconia, the UV band at 226 nm was demolished
completely. It indicates perturbance of charge transfer from O2ꢀ to M+n
system. The band gap of the lanthana-zirconia system was found at 5.26
eV, which is 0.12 eV above the band gap of pure ZrO2 (5.14 eV). It in-
dicates that lanthana addition caused a rise of band gap, which prohibits
the charge transfer from O2ꢀ to M+n in the new support. Thus, we can
say that electronically the support lanthana-zirconia is stable than that
of only zirconia.
5Ni/LaZr catalyst showed UV band at about 280 nm for the transition
from O2ꢀ to octahedral Ni+2 (O2ꢀ → Ni2+) in NiO lattice (Fig. 4(A)) [59,
60]. The bands about 377 and 410 nm were associated with the d-d
3
3
transition from A2g(F) state to T1g(P) state of Ni+2 in an octahedral
environment, whereas band about 718 nm shows the d-d transition from
3
3A2g(F) state to T1g(F) state of the Ni+2 octahedral environment [58,
61–63]. This transition is more electronically favourable because the
band gap of the 5Ni/LaZr catalyst system is found to reduce to 3.46 eV
with respect to lanthana-zirconia system (band gap 5.14 eV) (Fig. 4(B)).
After the addition of 1% ceria promoter, the band at 280 nm was
intensified. It indicates that charge transfer band from O2ꢀ to Ce+3/Ce+4
merged with charge transfer band of O2ꢀ → Ni2+ [64] As the ceria
proportion was increasing, the band at 280 nm was intensified (Fig. 4
(C)). The band of d-d transition about 410 nm for 3A2g(F) state to 3T1g(P)
state of Ni+2 (octahedral) was found to be demolished as well as ceria
loading was increasing; however, the d-d transition band from 3A2g(F)
Fig. 5. CH4-TPSR profile of catalyst used in DRM reaction.
For mesoporous lanthana-zirconia support (LaZr), a single prominent
consumption peak at about 870 ◦C was observed due to thermal
decomposition of CH4 and promotional role of the ZrO2 redox for
oxidizing carbon species (which was formed after CH4 thermal decom-
position) [65]. At 5Ni/LaZr catalyst, a low intense low-temperature CH4
consumption peak (~400 ◦C) and an intermediate temperature broad,
large peak (450–800 ◦C) were also found. The lower temperature peak
was due to CH4 decomposition at Ni, whereas intermediate temperature
peak was for CH4 decomposition at the intimate contact between Ni and
lanthana-zirconia (LaZr) interface. Promotional addition of ceria on
lanthana-zirconia supported Ni catalyst did not show a significant
3
state to T1g(F) state of Ni+2 (octahedra) did not be affected by ceria
loading as a promoter. In all ceria loading, the band gaps of catalyst
systems remained below 3.40 eV.
To study the conditions and sites of CH4 decomposition, CH4-tem-
perature programmed surface reaction (CH4-TPSR) experiment over
LaZr, 5Ni/LaZr and 5Ni2.5Ce/LaZr were carried out as shown in Fig. 5.
5

J. Khatri et al.
Molecular Catalysis 504 (2021) 111498
change in the CH4-TSPR pattern. Overall, it indicated that the majority
of CH4 decompositions seemed to happen at Ni-LaZr interface.
whereas higher temperature, a high-intensity sharp peak is related to
moderately oxidizable β-carbon species/less inert carbon species) [73,
74]. CO2-TPD of fresh and spent 5Ni2.5CeLaZrO2 catalyst showed that
basic sites density of all types are demolished after the reaction (Fig. S4).
However, if O2-TPO is carried out after CO2-TPD, it shows more intense
peaks than simple O2-TPO peaks. It indicates that CO2 may react with
carbon deposit and forms oxygenated species [75]. Oxygenated species
are more oxidizable and give higher intensity peaks during O2-TPO
experiment [68]. So, if O2-TPO is carried out after TPH, most intense
peaks about 400 ◦C and 550 ◦C (than simple O2-TPO peaks) are shown.
In TPH, it was discussed that H2 stream hydrogenates weakly bond
carbon intermediates/species (CHx) that desorbed as CH4. H2 stream
may also hydrogenates strongly bond carbon intermediate/species that
did not desorb during TPH but can be oxidized by oxygen during O2-TPO
experiment.
To understand the reducing-oxidizing surface behaviour of 5Ni/LaZr
and 5Ni2.5Ce/LaZr, we have carried out “H2TPR-CO2TPD-H2TPR
cycle”. H2-TPR of 5Ni/LaZr catalyst sample shows a remarkable peak in
the low-temperature range (250ꢀ 310 ◦C) and a prominent intense peak
at high-temperature range (450–550 ℃) (Fig. 6 (A)). The lower peak
was attributed to reduction peak of “NiO weakly interacted with the
support” whereas the high-temperature peak signifies the reduction
peak of “NiO strongly interacted with the support”. H2-TPR of 5Ni2.5Ce/
LaZr catalyst sample showed prominent growth in lower temperature
peaks than the high-temperature peak. It indicates that ceria addition
promotes the cultivation of easier reducible NiO species (Fig. 6 (B)).
CO2-TPD is carried out in continuation with H2-TPR over 5Ni/LaZr
and 5Ni2.5Ce/LaZr catalyst system. In our previous publication, the
CO2-TPD of 5Ni2.5Ce/LaZr, Samsudeen et al. [66] have reported the
presence of a low concentration of weak basic sites (surface hydroxyl) at
266 ℃, intense intermediate strength basic sites (surface oxygen anion)
about 400 ℃ and broad but diffuse strong basic sites (bulk oxygen
anion/oxygen vacancy) about 650 ℃. As per the expectation here, when
CO2-TPD is carried out after H2-TPR; both intermediate and
high-temperature peaks disappeared. It indicates that during the
reduction under H2 stream, oxygen anion responsible for “intermediate
and strong basic sites” are removed. It will be interesting to observe that
may this loss be overcome by CO2. Possibly during the CO2-TPD, these
sites may replenish oxygen from CO2. So, in continuation of CO2-TPD;
again, H2-TPR was carried out on the same catalyst. It gives interesting
and informative results. Both peaks in low-temperature ranges, as well
as high temperature, are suppressed, and an intense peak in the inter-
mediate temperature 391 ◦C region appeared. Alfatish et al. claimed that
4. Discussion
CH4 decomposition over supported Nickel is well known [76]
whereas ZrO2 system is quite basic to interact with CO2 for a possible
DRM reaction [77,78]. However, XRD profiles showed that “zirconia
supported Ni catalyst” system is prone for crystalline carbon deposition
during reaction which encounters inferior catalytic performance. It re-
sults in inferior catalytic H2 yield (43 %), which slows down to 37 %
after 460-minute time on stream. Ceria promotional addition to “zir-
conia supported Ni catalyst” causes rapid wash away of carbon (by
oxidation of deposit) and so the catalytic activity increased to 47 % H2
yield (against 43 % at zirconia supported Ni catalyst). Constant deposit
wash from the catalytic active sites also causes constant yield up to 460
min time on stream. However, defective zirconia phases and fractional
phase transformation of monoclinic zirconia to cubic zirconia at high
temperature reaction conditions limits catalytic performance.
– –
the peak was due to the reduction of CeO2-ZrO2 and Ni OC e solid
solutions [33]. The “H2TPR-CO2TPD-H2TPR cycle” shows that 5Ni/LaZr
and 5Ni2.5Ce/LaZr catalyst surface are reducible in H2 stream, the
reduced surface was re-oxidized in CO2 stream, and then again, the
oxidized surface is reducible in H2 stream.
When support is made up of “9%lanthana-91% zirconia”, infrared
results show the bonding between La2O3and ZrO2 species. The band gap
of support lanthana-zirconia was increased by 0.06 eV than pure zir-
conia (5.08 eV). XRD profile did not show La2O3 related phase, but it
showed phase for ZrO2. It indicates that lanthana addition stabilizes the
zirconia phase. The CO2-TPD profile of zirconia (prepared by Silva et al.
[79]) and lanthana-zirconia (prepared by Alfathesh et al. [66] are re-
ported very similar in earlier works. After the dispersion of Ni over
thermally stable lanthana-zirconia (5Ni/LaZr), defective zirconia phase
was not found after the reaction. IR spectra of 5Ni/LaZr also show the
presence of La2O2CO3 species. The UV band shows a reduction of band
gap to 3.12 eV (concerning 5.14 eV of lanthana-zirconia), favourable d-d
transition and presence of Ni+2 in an octahedral environment. CH4-TPSR
shows CH4 dissociation over Ni-support (LaZr) interface. Overall, the
increasing CO2 interaction with ZrO2 as well as La2O3, prominent CH4
decomposition at Ni-LaZr interface, excellent thermal stability, low
band gap over “lanthana-zirconia supported Ni” catalyst pivots the path
of DRM and ensures high H2 yield up to 80 %. H2 yield decreased to 75 %
To understand the reactive nature of carbon deposit in reducing at-
mosphere, Temperature Programmed Hydrogenation (TPH) was
carried out by 10%H2 in Ar over spent 5Ni2.5CeLaZr, and the corre-
sponding profile is shown in (Fig. 7(A)). A sharp peak and shoulder were
found in the temperature range of 100ꢀ 300 ◦C, whereas a broad but less
intense peak was found in the region of 375–600 ◦C. The peak temper-
ature position is related to the chemical equilibrium between CHx spe-
cies (x = 3, 2, 1) and their activity towards hydrogen [67,68]. Overall,
these peaks are claimed due to hydrogenation of CHx species into CH4 by
hydrogen.
O2-Temperature Programmed Oxidation (O2-TPO) was carried out
by 10%O2 in Ar over spent 5Ni2.5CeLaZr, and the corresponding profile
is shown in (Fig. 7(B)). It shows two oxidizing peaks at 400 and 550 ◦C.
Low temperature, the low-intensity peak is related to easily oxidizable
amorphous
α-carbon (intermediate of methane decomposition),
Fig. 6. (A) H2TPR-CO2TPD-H2TPR cycle of 5Ni/LaZr (B) H2TPR-CO2TPD-H2TPR cycle 5Ni/2.5CeLaZr catalyst that were used in the DRM reaction.
6

J. Khatri et al.
Molecular Catalysis 504 (2021) 111498
Fig. 7. (A) TPH profile of spent catalyst 5Ni2.5Ce/LaZr (B) Different O2-TPO profile of spent catalyst 5Ni2.5Ce/LaZr.
in 460 min in time on stream test (TOS). However, the XRD profile of the
spent catalyst shows the presence of high-intensity NiO. It indicates that
NiO crystallites assemble during the reaction, and at the end of the re-
action, NiO is poorly dispersed.
(Reaction 5). Ceria promotional addition generates an oxygen vacancy
in the catalytic system by reduction of Ce+4 to Ce+3. It becomes new
adsorption as well as decomposition sites for CO2. Very soon, oxygen
vacancies are replenished by these reactive oxygen species from the CO2
decomposition (Reaction 6). These reactive oxygen species are mobile
and vital oxidizing species for carbon species oxidation at Ni (Reaction
7). Delay in the interaction between C-Nin and mobile oxygen/-
interacted CO2 results in oligomer/polymer formation, leading to cata-
lytic deactivation (Reaction 8).
XRD profile indicates that on the promotional addition of ceria in
lanthana-zirconia supported Ni catalyst (5Ni1Ce/LaZr), a ceria-zirconia
mixed oxide is formed as well as NiO dispersion is maintained during the
reaction. It indicates that the ceria controls the NiO dispersion during
the reaction by increasing NiO-support interaction. As the loading of
ceria increases, IR peak due to La2O2CO3 gets intensified, the UV band
for charge transfer transition from O2ꢀ to Ce+3/Ce+4 get intensified, and
the d-d transition band for 3A2g (F) to 3T1g (F) in Ni+2 (octahedral) was
demolished. It indicates that the presence of ceria, prominently influ-
ence the electronic transition over the catalyst surface where the band
gap remained below 3.38 eV. At particular, ceria loading 2.5 %, Alfatish
et al. has shown increased CO2 adsorption at intermediate/strong
strength basic sites or oxygen anion [66]. “H2TPR-CO2TPD-H2TPR
cycle” showed that CO2 causes increased NiO-support interaction as well
as oxygen replenishment form CO2 to the catalytic surface to increase
oxygen mobility for the oxidation of C-Nin species (which was formed
after CH4 decomposition at Ni surface). Overall, it can be said that CO2
interaction with entire system (ZrO2, La2O3, CeO2), CH4 decomposition
over stabilized/dispersed Ni-LaZr interface, subsequent interaction of
C-Nin species with mobile oxygen (replenished by CO2)/Interacted CO2
to the surface, excellent thermal stability, low band gap over
5Ni2.5Ce/LaZr catalyst governs highest H2 yield ~87 % which remains
constant throughout the 460 min in TOS. The TPH experiment shows
that even ceria promoted catalyst may have some carbon deposits;
however, it is reducible and removable in the early temperature range
CH4 + Nin → C-Nin + 2H2
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
ZrO2 + CO2 → ZrO2——CO2
La2O3+ CO2 → La2O3——CO2
La2O3 + CO2 → La2O2CO3
C-Nin + CO2 (adsorbed) → 2CO + Nin
Vo + CO2 → CO + O; where Vo is oxygen vacancy
C-Nin + O → CO + Nin
m C-Nin → C..-C..-C..
5. Conclusion
High yield of hydrogen solely depends on the decomposition of CH4
largely at Ni-LaZr interface into H2 and C-Nin species as well as wash
away of carbon deposit through oxidation by CO2. Zirconia supported Ni
catalyst is potent in H2 production, but graphitic carbon rendered it for
inferior 43 % H2 yield. Ceria promotion also washes the carbon deposit
by the mobile oxygen and amends progress in activity, but the defective
support limits the H2 yield to 47 %. The addition of 9% lanthana into
zirconia support causes stabilization of zirconia phase. lanthana-
zirconia supported Ni catalyst has thermally stabilized support, Ni-
LaZr interface for CH4 decomposition, wide basic surfaces CO2 interac-
tion, CO2 coordinated La+3 sites (as La2O2CO3 species) and low band
gap. Altogether, it governs 80 % H2 yield. Promotional addition of ceria
controls the NiO dispersion during the reaction and adds mobile oxygen
in the system, which is replenished back by oxygen from CO2. 5Ni2.5Ce/
LaZr shows marked 87 % H2 yield, which remained constant up to 460
min. More than 2.5 % ceria loading shows comparably lower H2 yield
(~80 %) due to consumption of the H2 in a reverse water-gas shift
(RWGS) reaction (CO2 + H2 → CO + H2O).
◦
◦
(100ꢀ 300 C) or below the reaction temperature 650 C. As the ceria
loading increases more than 2.5 %, comparable lower, but constant H2
yield ~80 % is noticed. Ni/CeO2 system is also a well-known catalyst for
reverse water gas shift reaction (RWGS) [80]. So, possibly comparably
lower H2 yield is due to consumption of H2 in a reverse water gas shift
(RWGS) reaction (CO2 + H2 → CO + H2O) which is highly favoured at
the high reaction temperature. “TPH followed by O2-TPO” and “CO2TPD
followed by O2-TPO” shows that H2 and CO2 environment may also
induce oxygenated and hydrogenated carbon deposit at catalyst surface
which may deactivate the catalyst if not removed from the catalyst
surface.
Based on all characterization results, the reaction mechanism of H2
production through DRM can be summarized in two sections. The first is
the decomposition of CH4 largely at Ni-LaZr interface into H2 and C-Nin
species (Reaction 1) [16]. The second is washing away of carbon de-
posits over Ni metal through the oxidation by CO2. The second step
should be emphasized more deeply. CO2 may interact with basic ZrO2 or
La2O3-ZrO2 surfaces, and it may be potentially adsorbed on La2O3 in the
form of La2O2CO3 (Reaction 2–4). At high reaction temperature, this
interacted/potentially adsorbed CO2 tends to be mobile, reaches Ni-LaZr
interface, attacks carbon species (C-Nin) and forms two CO molecules
Disclaimer
The views and opinions expressed in this paper do not necessarily
reflect those of the European Commission or the Special EU Programmes
Body (SEUPB).
7

J. Khatri et al.
Molecular Catalysis 504 (2021) 111498
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Jyoti Khatri: Writing - original draft, Conceptulization, Investiga-
tion. Ahmed S. Al-Fatesh: Supervision, Project administration, Vali-
dation, Writing - review & editing, Funding acquisition. Anis H.
Fakeeha: Project administration, Funding acquisition, Supervision,
Editing. Ahmed A. Ibrahim: Methodology, Formal analysis. Ahmed E.
Abasaeed: Methodology, Formal analysis. Samsudeen O. Kasim:
Funding acquisition, Data curation, Formal analysis. Ahmed I. Osman:
Supervision, Editing. Rutu Patel: Software Formal, analysis. Rawesh
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Declaration of Competing Interest
[18] F. Somodi, Z. Schay, Na-promoted Ni/ZrO2 dry reforming catalyst with high
efficiency: details of Na2O–ZrO2–Ni interaction controlling activity and coke
formation, Catal. Sci. Technol. 7 (2017) 5386–5401.
There are no known conflicts of interest associated with this publi-
cation and this work is supported by the Deanship of Scientific Research
project # No. RGP—119.
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Acknowledgements
The KSU authors would like to sincerely appreciate the Deanship of
Scientific Research at the King Saud University for its funding for this
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