Low- empe a u e pa ial oxida ion o me hane o e
PdeNi bime allic ca alys s suppo ed on CeO
2
Shi a Fazlikesh eli, Xa ie Vend ell
**
, Jo di Llo ca
*
Ins i u e o Ene gy Technologies, Depa men o Chemical Enginee ing and Ba celona Resea ch Cen e in Mul iscale
Science and Enginee ing, Uni e si a Poli 
ecnica de Ca alunya, EEBE, Edua d Ma is any 16, 08019, Ba celona, Spain
highligh s g aphical abs ac
Bime allic PdeNi/CeO
2
p epa ed by
ball milling show excellen pe -
o mance o POM.
Maximum me hane con e sion
and syngas wi h 0.12 w % Pd and
1.38 w % Ni.
Supe io s abili y o PdeNi/CeO
2
ball milled a 50 Hz o 20 min.
Ball milling yields an imp essi e
dispe sion o me al species on
CeO
2
su ace.
a icle in o
A icle his o y:
Recei ed 1 Feb ua y 2022
Recei ed in e ised o m
26 June 2022
Accep ed 3 July 2022
A ailable online 13 Augus 2022
Keywo ds:
Me hane pa ial oxida ion
Bime allic ca alys s
Ce ia ca alys s
Palladium
Nickel
Mechanochemis y
abs ac
Monome allic Pd and Ni and bime allic PdeNi ca alys s suppo ed on CeO
2
a e p epa ed ia
mechanochemical and con en ional incipien we ness imp egna ion me hods and es ed
o he p oduc ion o syngas by he pa ial oxida ion o me hane. Compa ed wi h mono-
me allic Ni/CeO
2
and Pd/CeO
2
, bime allic PdeNi/CeO
2
ca alys s show conside able highe
me hane con e sion and syngas yield. Addi ionally, he bime allic ca alys s p epa ed by
ball milling p oduce syngas a lowe empe a u e. Di e en p epa a ion pa ame e s, such
as me al loading, Pd/Ni a io, milling ene gy, milling ime and o de o inco po a ion o he
me als a e examined. The bes pe o mance is ob ained wi h a bime allic ca alys p epa ed
a 50 Hz o 20 min wi h only 0.12 w % Pd and 1.38 w % Ni. S abili y es s demons a e
supe io s abili y o bime allic PdeNi/CeO
2
ca alys s p epa ed by a mechanochemical
app oach. F om he cha ac e iza ion esul s, his is explained in e ms o an imp essi e
dispe sion o me al species wi h a s ong in e ac ion wi h he su ace o CeO
2
.
©2022 The Au ho (s). Published by Else ie L d on behal o Hyd ogen Ene gy Publica ions
LLC. This is an open access a icle unde he CC BY-NC-ND license (h p://
c ea i ecommons.o g/licenses/by-nc-nd/4.0/).
*Co esponding au ho .
** Co esponding au ho .
E-mail add esses: xa ie . end ell. illa [email p o ec ed] (X. Vend ell), [email p o ec ed] (J. Llo ca).
A ailable online a www.sciencedi ec .com
ScienceDi ec
jou nal homepage: www.else ie .com/loca e/he
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035
h ps://doi.o g/10.1016/j.ijhydene.2022.07.020
0360-3199/©2022 The Au ho (s). Published by Else ie L d on behal o Hyd ogen Ene gy Publica ions LLC. This is an open access a icle unde he CC BY-NC-
ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/).
In oduc ion
Me hane is a majo componen o na u al gas (>90%) and i s
con e sion in o syngas (a aluable eeds ock composed by a
mix u e o gases domina ed by he p esence o ca bon mon-
oxide CO and hyd ogen) has a ac ed inc easing a en ion in
he las ew decades o he p oduc ion o nume ous chemical
p oduc s and uels [1e3]. Depending on he applica ion, he
ca aly ic syngas p oduc ion om me hane can be ealized
h ough h ee gene al ou es: (1) s eam e o ming o me hane
(SRM) (Eq. (1)), (2) d y e o ming o me hane (DRM) (Eq. (2)), and
(3) pa ial oxida ion o me hane (POM) (Eq. (3))[4e6].
CH
4
þH
2
O⟶CO þ3H
2
DH
298K
¼206 kJ mol
1
(Eq. 1)
CH
4
þCO
2
⟶2H
2
þ2CO DH
298K
¼247 kJ mol
1
(Eq. 2)
CH
4
þ1/2O
2
/CO þ2H
2
DH
298K
¼36 kJ mol
1
(Eq. 3)
In con as o SRM and DRM me hods, POM is a mildly
exo he mic eac ion. Fo his eason, POM is seen as an
ene gy-sa ing p ocess, and is conside ed he mos economic
echnology o syngas p oduc ion [7,8]. Mo eo e , he mola
a io o H
2
/CO in he POM eac ion is close o 2, which is
sui able o he p oduc ion o me hanol and highe hyd o-
ca bons h ough he Fische -T opsch p ocess [9,10]. Besides,
POM can handle a la ge olume o he eed gas wi h only a
small amoun o ca alys [11,12].
Usually, he POM p ocess in o syngas occu s in wo s eps
[4,13]. In he i s s ep, CH
4
is combus ed by O
2
o p oduce CO
2
and H
2
O (Eq. (4)), and hen he emaining un eac ed CH
4
is
e o med wi h wa e (Eq. (5)) and CO
2
(Eq. (6)) o p oduce CO
and H
2
[13,14]. These eac ions a e always accompanied by he
wa e gas-shi equilib ium (Eq. (7))[13].
CH
4
þ2O
2
%CO
2
þ2H
2
ODH
298
¼803 kJ mol
1
(Eq. 4)
CH
4
þH
2
O%CO þ3H
2
DH
298
¼þ206 kJ mol
1
(Eq. 5)
CH
4
þCO
2
%2CO þ2H
2
DH
298
¼þ247 kJ mol
1
(Eq. 6)
CO
2
þH
2
%CO þH
2
ODH
298
¼41 kJ mol
1
(Eq. 7)
A e nea ly 100 yea s o in es iga ion on he ca aly ic
pa ial oxida ion o me hane in o syngas, nume ous ca alys
o mula ions ha e been es ed. They can be di ided basically
in o h ee main ypes: (1) nickel, cobal , o i on ca alys s, (2)
ansi ion me al ca bide ca alys s, and (3) noble me al ca a-
lys s [15,16]. Cu en ly, he indus ial ca alys s used o POM
a e p ima ily nickel and noble me al suppo ed ca alys s [1].
Nickel-based ca alys s ha e been widely in es iga ed due o
hei low cos ; howe e , hey a e easily deac i a ed owing o
ca bon deposi ion and me al loss a empe a u es highe han
700 C[4,5,17]. To imp o e he mal s abili y and ac i i y and
dec ease ca bon deposi ion, o he componen s like noble
me als (Pd, P , Rh, Ru, e c.) a e added o Ni ca alys s. These
noble me als a e mo e ac i e, less sensi i e o coke o ma ion
and ha e highe capaci y o oxidize hyd oca bons, bu a e
expensi e. The e o e, bime allic ca alys s wi h Ni and noble
me als ha e been p oposed o p omo e me hane con e sion
[15,17]. Palladium is one o he mos e ec i e noble me als
used o me hane con e sion p ocesses [18e20]. The suppo
plays also an impo an ole on he POM p ocess [21,22]. Mos
ca alys s show high ac i i y when me als a e dispe sed on a
educible oxide suppo [4]. Ce ium oxide (CeO
2
) is one o he
mos signi ican educible oxide suppo s used in indus ial
ca alysis as i educes ca bon deposi ion and has a ema kable
oxygen s o age/exchange capaci y due o acile oxygen a-
cancy o ma ion and change o oxida ion s a es be ween Ce
3þ
and Ce
4þ
[19,23].
The main objec i e o he p esen in es iga ion is o
de elop an e ec i e bime allic NiePd/CeO
2
ca aly ic sys em
o POM o p oduce syngas a low empe a u e. To ha end, a
se ies o monome allic Ni, Pd, and PdeNi bime allic ca alys s
suppo ed on CeO
2
we e p epa ed, cha ac e ized and es ed.
Two di e en me hods we e used o p epa e he ca alys s, d y
ball milling (BM) and con en ional incipien we ness imp eg-
na ion (IWI). The BM echnique is a g een echnology wi h
eno mous po en ial o he p epa a ion o ca alys s; i has an
easy ope a ion and i is as , cos -e ec i e and en i onmen-
ally iendly [24e26].
Ma e ials and me hods
P epa a ion o CeO
2
Ce ium ni a e hexahyd a e (Ce(NO
3
)
3
$6H
2
O, 99.5%) and
palladium (II) ni a e (Pd(NO
3
)
2
, 93%) we e pu chased om
Al a Aesa . Nickel (II) ni a e hexahyd a e (Ni(NO
3
)
2
$6H
2
O,
98%) was ob ained om Fishe chemical. Ammonia solu ion
(NH
3
, 28%) was ob ained om Scha lab. All eagen s we e
used wi hou u he pu i ica ion. Ce ium dioxide (CeO
2
) was
ob ained by adding d opwise NH
3
o an aqueous solu ion o
Ce(NO
3
)
3
$6H
2
O un il he pH eached a alue be ween 9 and 10
and a yellowish p ecipi a e was ob ained. A e ha , he
p ecipi a ed was il e ed and washed ho oughly wi h deion-
ized wa e . The p ecipi a e was d ied o e nigh a 90 C and
calcined a 650 C o 4 h (5 C min
1
).
P epa a ion o ca alys s by mechanochemical me hod
Monome allic Pd(x)/CeO
2
/BM (x ¼0.5, and 1 w %) and Ni(y)/
CeO
2
/BM (y ¼0.5 w %) ca alys s we e p epa ed ia ball mill
(labeled as BM). Ni(NO
3
)
2
$6H
2
O and Pd(NO
3
)
2
we e used as Ni
and Pd p ecu so s, espec i ely. The desi ed amoun s o me al
ni a es we e mixed di ec ly wi h CeO
2
in a zi conium oxide
essel using a F i sch Pul e ise e 23 mini-mill appa a us and
one zi conium oxide ball o 15 mm diame e (ball o powde
a io, BPR ¼10.2). Two di e en ou es we e used o p epa e
he bime allic Pd(x)-Ni(y)/CeO
2
/BM ca alys s (x ¼0.06e1 w %,
y¼0.5 o 1.44 w %; x þy¼1.5) by he BM me hod: (1) co-BM
and (2) sequen ial-BM. In he co-BM me hod, he bime allic
PdeNi/CeO
2
/BM ca alys s we e p epa ed in one-s ep by mill-
ing oge he CeO
2
and he me al p ecu so s. In he sequen ial-
BM me hod, i s ly, one o he me al p ecu so s was mixed
wi h ce ium oxide by BM, and in he nex s ep he o he p e-
cu so was added and subjec ed again o BM. These samples
we e labeled as PdeCeO
2
/BM/Ni/BM o NieCeO
2
/BM/Pd/BM
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 12025
depending i ce ia was i s milled wi h he Pd p ecu so o
wi h he Ni p ecu so , espec i ely. The e ec o ball mill
equency (15, 30 and 50 Hz) and milling ime (5e40 min) was
also in es iga ed. All he esh ca alys s we e used wi hou
any u he ea men .
P epa a ion o ca alys s by incipien we ness imp egna ion
Monome allic Pd, Ni, and bime allic PdeNi ca alys s we e also
p epa ed by con en ional incipien we ness imp egna ion o
compa ison (labeled as IWI). Fo he p epa a ion o mono-
me allic Ni(y)/CeO
2
/IWI (y ¼0.5 w %) and Pd(x)/CeO
2
/IWI
(x ¼0.5, and 1 w %), Ni(NO
3
)
2
$6H
2
O and Pd(NO
3
)
2
aqueous
solu ions we e used as p ecu so s. Samples we e d ied a 90 C
and calcined a 650 C o 4 h (5 Cmin
1
). Two di e en ou es
we e used o p epa e he bime allic Pd(x)-Ni(y)/CeO
2
/IWI ca -
alys s (x ¼0.06e1 w %, y ¼0.5 o 1.44 w %; x þy¼1.5) by he
IWI me hod: (1) co-IWI and (2) sequen ial-IWI. In he co-IWI
me hod, he PdeNi/CeO
2
/IWI ca alys s we e p epa ed in
one-s ep, while in sequen ial-IWI wo subsequen imp egna-
ions we e ca ied ou , wi h a calcina ion s ep a 650 C o 2 h
be ween each imp egna ion. These samples we e labeled as
NieCeO
2
/IWI/Pd/IWI o PdeCeO
2
/IWI/Ni/IWI depending i he
i s imp egna ion was ca ied ou wi h he Ni o he Pd p e-
cu so , espec i ely. No u he ea men s we e pe o med
on he calcined samples be o e he ca aly ic es .
Ca aly ic es s
The ca aly ic pe o mance o he ca alys s o he POM p ocess
was e alua ed in a con inuous- low ixed-bed qua z ubula
eac o . Reac ions we e ca ied ou a a mosphe ic p essu e
be ween 300 and 550 C using a mix u e o CH
4
:ai :N
2
¼4:11:85
(mola , CH
4
/O
2
¼1.73) and F/W ¼60 L h
1
g
1
(gas hou ly space
eloci y GHSV ¼12 10
3
h
1
). Typically, 0.1 g o ca alys was
mixed wi h silicon ca bide o ob ain a ixed bed olume o
0.5 cm
3
. The eac o was hea ed in an elec ical u nace
(Ca boli e CTF) and he empe a u e in he cen e o he
ca alys bed was measu ed wi h a ype K he mocouple; no
axial he mal g adien s we e measu ed. CH
4
con e sion
ðXCH4Þand syngas selec i i ies ðSH2;SCOÞwe e e alua ed om
300 C o 550 C in s eps o 50 C. A each empe a u e, a dwell
ime o 45 min was applied, hus ensu ing s eady-s a e con-
di ions. The eac ion p oduc s we e analyzed online e e y
4 min wi h a gas ch oma og aph (Va ian CP-4900) equipped
wi h Molecula Sie e o 5 
A, Plo U and S abilwax columns o
a comple e analysis o p oduc s. The me hane con e sion
ðXCH4Þ(Eq. (8)), selec i i y o hyd ogen ðSH2Þ(Eq. (9)), selec i i y
o ca bon monoxide (SCO) (Eq. (10)), and yield o syngas ðYsgÞ
(Eq. (11)) we e calcula ed acco ding o he ollowing equa ions:
xCH4ð%Þ¼FinCH4Fou CH4
FinCH4
100 (Eq. 8)
whe e FinCH4and Fou CH4a e he inle and ou le mola low o
me hane, espec i ely
SH2ð%Þ¼ moles o H2p oduced
moles o ðH2þCO þCO2Þp oduced 100 (Eq. 9)
SCOð%Þ¼ moles o CO p oduced
moles o ðH2þCO þCO2Þp oduced 100 (Eq. 10)
Ysgð%Þ¼xCH4ðSH2þSCOÞ
100 (Eq. 11)
Ca alys cha ac e iza ion
Cha ac e iza ion o he su ace was accomplished by X- ay
pho oelec on spec oscopy (XPS) wi h a SPECS sys em
equipped wi h a XR50 sou ce ope a ing a 250 W and a Phoibos
150 MCD-9 de ec o . The pass ene gy o he high- esolu ion
spec a was se a 0.1 eV. Binding ene gy (BE) alues we e
e e ed o he Ce
4þ
3d
5/2
peak a 916.9 eV. The mic os uc u e
o he ca alys s was in es iga ed wi h high esolu ion ans-
mission elec on mic oscopy (HRTEM) and ene gy-dispe si e
X- ay analysis (EDX) using a FEI TECNAI F20 ins umen
ope a ed a 200 kV. Raman spec oscopy was pe o med using
a Renishaw inViaQon o con ocal Raman mic oscope equip-
ped wi h a 532.1 ±0.3 nm lase wi h a nominal 100 mW ou pu
powe di ec ed h ough a specially adap ed Leica DM2700 M
mic oscope (x50 magni ica ion). Spec a we e acqui ed in wo
anges, 50-800 cm
1
and 1000-2000 cm
1
, wi h an exposu e
ime o 0.5 s, 1% o maximum lase powe and 18 epe i ions.
Tempe a u e p og ammed educ ion (TPR-H
2
) was pe o med
wi h a Chems a appa a us wi h TCD de ec o . Samples we e
i s ea ed a 450 C in A (50 mL min
1
,10C min
1
), and
hen TPR was ca ied ou om 30 o 850 C (10 C min
1
) unde
10% H
2
and kep a his empe a u e o 30 min.
Resul s and discussion
Cha ac e iza ion o esh ca alys s
Raman spec oscopy
The Raman spec a eco ded o he bime allic PdeNi/CeO
2
ca alys s p epa ed by BM and IWI me hods be o e he ca aly ic
es s a e shown in Fig. 1. In all cases, he Raman we e domi-
na ed by he symme ical s e ching mode o he CeO
2
la ice
s uc u e (F
2g
mode) a ~464 cm
1
[27]. Addi ionally, o he
bime allic PdeNi/CeO
2
ca alys p epa ed by BM, Raman
spec a indica ed a weak band a ~1050 cm
1
, which co e-
sponds o he symme ic n
1
s e ching mode o he ni a e
anion [28,29]. The NO
3band was absen in he bime allic
ca alys s p epa ed by IWI because ni a e esidues dis-
appea ed ollowing he calcina ion ea men pe o med a
650 C. Also, o he bime allic PdeNi/CeO
2
ca alys p epa ed
by IWI, a b oad band om ~530 o 620 cm
1
is obse ed, which
can be asc ibed o NiO species [29], al hough a de ec -induced
ib a ional mode o ce ia la ice de ec s in he ce ia s uc u e,
such as oxygen acancies, canno be disca ded (~595 cm
1
,D
band) [30].
T ansmission elec on mic oscopy (HRTEM)
Wi h he aim o unde s anding he mic os uc u e o he
ca alys s, he PdeNi dispe sion and he possible in e ac ions
be ween me al and suppo , high esolu ion ansmission
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e1203512026
elec on mic oscopy (HRTEM) and ene gy-dispe si e X- ay
analysis (EDX) we e used (Fig. 2). A ep esen a i e image o he
monome allic Pd/CeO
2
ca alys p epa ed by BM is shown in
Fig. 2a. The sample is e y homogeneous and con ains sub-
nanome ic Pd en i ies (some o hem indica ed wi h a ows
in he image), which a e well dispe sed o e he CeO
2
suppo .
A la ge Pd pa icle is highligh ed wi h a dashed squa e. The
Fou ie T ans o m (FT) image o his a ea is shown in he inse
and shows spo s a 1.9 
A om CeO
2
(220) planes and a 2.2 
A
om Pd (111) planes. The EDX spec um eco ded in he same
a ea con i ms he na u e o he pa icle and shows he
simul aneous occu ence o Ce, Pd and O ( he Cu signal o ig-
ina es om he TEM g id). Fig. 2b co esponds o he bime allic
PdeNi/CeO
2
ca alys s p epa ed by IWI me hod. I is di icul o
dis inguish any Pd, Ni o PdeNi pa icle in TEM analysis,
which indica es an excellen dispe sion o Ni and Pd on he
CeO
2
suppo . The EDX spec um eco ded om he a ea
enclosed by he squa e shows only weak Pd signals; no signals
o Ni a e obse ed, p obably due o he low loading o Ni (0.5 w
%). Fig. 2c shows he mic os uc u e o he bime allic PdeNi/
CeO
2
ca alys s p epa ed by he BM me hod. The mechano-
chemical p epa a ion o PdeNi/CeO
2
c ea es an amo phous
laye a ound he ce ia c ys alli es (ma ked be ween a ows).
This shell exhibi s an a e age hickness o abou 2e4 nm. The
EDX spec um eco ded om he a ea enclosed by he squa e
conclusi ely iden i ied bo h Pd and Ni in his amo phous shell.
The amo phous shell is simila o ha ecen ly epo ed in Pd/
CeO
2
ca alys s p epa ed by BM [31] and co esponds o an
unp eceden ed a chi ec u e.
X- ay pho oelec on spec oscopy (XPS)
The su ace composi ion o he bime allic PdeNi/CeO
2
ca a-
lys s p epa ed by ball milling was analyzed by X- ay
Fig. 1 eRaman spec a o bime allic ca alys s wi h 1 w % Pd
and 0.5 w % Ni suppo ed on CeO
2
p epa ed by BM a 50 Hz
o 20 min (black), and IWI ( ed). (Fo in e p e a ion o he
e e ences o colo /colou in his igu e legend, he eade is
e e ed o he Web e sion o his a icle.)
Fig. 2 eHRTEM images o (a) monome allic Pd (1)/CeO
2
ca alys p epa ed by BM a 50 Hz o 20 min; (b) bime allic
Pd (1)-Ni(0.5)/CeO
2
ca alys p epa ed by IWI; (c) bime allic
Pd (1)-Ni(0.5)/CeO
2
ca alys p epa ed by BM a 50 Hz o
20 min.
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 12027
pho oelec on spec oscopy (XPS). Fig. 3a shows he Ce 3d
spec um. The peaks ma ked as (882.7 eV),
2
(889 eV),
3
(899.8 eV), u (901.1 eV), u
2
(907.1eV) and u
3
(916.9 eV) co e-
spond o he p esence o Ce
4þ
species, and hose labeled as
0
(881.6 eV),
1
(885 eV), u
0
(898.4 eV), and u
1
(903 eV) co espond
o Ce
3þ
species [32,33]. Abou 40% o ce ium a he su ace
appea s as Ce
3þ
species. Fig. 3b shows he Pd 3d spec um. A
single double co esponding o he 3d
5/2
and 3d
3/2
spli ing is
p esen , wi h a binding ene gy o 3d
5/2
a 335.3 eV, which
co esponds well o educed Pd. The su ace Pd/Ce and Ni/Ce
a omic a ios a e 0.09 and 0.04, espec i ely, which poin s o
an excellen dispe sion o he wo me als o e he ce ia
suppo .
Tempe a u e p og ammed educ ion (H
2
-TPR)
The H
2
-TPR p o iles eco ded o he in es iga ed samples a e
shown in Fig. 4. All he samples show a high- empe a u e
educ ion peak loca ed abo e 700 C, which co esponds o
he bulk educ ion o CeO
2
, as epo ed elsewhe e [34]. How-
e e , i is wo h men ioning ha he high- empe a u e peak is
shi ed o lowe empe a u es when samples a e p epa ed by
he BM me hod wi h espec o analogous samples p epa ed
by IWI. The Ni/CeO
2
/IWI ca alys shows a b oad H
2
con-
sump ion a ~450 C, which co esponds o he educ ion o
NiO s ongly in e ac ing wi h he CeO
2
suppo [35,36], ac-
co ding o an enhanced me al-suppo in e ac ion ollowing
calcina ion [37,38]. In con as , he H
2
consump ion on he Ni/
CeO
2
ca alys p epa ed by BM appea s a much lowe em-
pe a u e (~185 C) and wi h highe in ensi y, which can be
assigned o he educ ion o well-dispe sed NiO species [39]. I
is in e es ing o no e ha he ball mill me hod no ably im-
p o es he me al educibili y a lowe empe a u e. The
monome allic ca alys Pd/CeO
2
p epa ed by BM me hod a
b oad educ ion peak cen e ed a ~285 C, which can be
assigned o he educ ion o PdO species ancho ed on CeO
2
[40,41].
The bime allic PdeNi/CeO
2
ca alys s p epa ed by bo h IWI
and BM me hods show simila H
2
-TPR p o iles wi h a b oad H
2
consump ion a abou ~250 C, due o PdOeNiO educ ion.
Compa ed o Pd/CeO
2
monome allic ca alys s, in he bime-
allic PdeNi/CeO
2
ca alys s he educ ion appea s a a lowe
empe a u e, indica ing ha he addi ion o Ni acili a es he
educ ion o PdO. In o he wo ds, he e is a s ong syne gy
be ween bo h me als, which is in acco dance o he HRTEM
esul s (Fig. 2c).
Fig. 3 eCe 3d (a) and Pd 3d (b) X- ay pho oelec on spec a o he bime allic Pd (1)-Ni(0.5)/CeO
2
/BM ca alys (50 Hz o 20 min).
Fig. 4 eH
2
-TPR p o iles co esponding o Pd (1)/CeO
2
/BM
(black), Ni(0.5)/CeO
2
/BM (g een), Ni(0.5)/CeO
2
/IWI (o ange),
Pd (1)-Ni(0.5)/CeO
2
/IWI (blue) and Pd (1)-Ni(0.5)/CeO
2
/BM
( ed). Ball mill condi ions we e 50 Hz o 20 min. (Fo
in e p e a ion o he e e ences o colo /colou in his igu e
legend, he eade is e e ed o he Web e sion o his
a icle.)
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e1203512028

Ca aly ic es s
The empe a u e-dependen ca aly ic pe o mance o he
di e en ca alys s p epa ed ollowed simila pa e ns. As a
ep esen a i e example, Fig. 5 shows he mola low a es o
eac an s (me hane and oxygen) and p oduc s (syngas, wa e
and ca bon dioxide) om 300 o 550 C o bime allic PdeNi/CeO
2
ca alys s p epa ed by BM (Fig. 5a) and IWI (Fig. 5b) me hods.
Acco ding o he modynamics, he con e sion o me hane in-
c eases p og essi ely wi h empe a u e. Howe e , signi ican
di e ences a e obse ed be ween BM and IWI ca alys s. Fo he
bime allic ca alys p epa ed by BM, all he oxygen is consumed
a 450 C and om his empe a u e he consump ion o
me hane uns pa allel o he consump ion o wa e and he
p oduc ion o syngas is obse ed. The e o e, his pe o mance
below 450 C obeys o he equa ion: CH
4
þ2O
2
/CO
2
þ2H
2
O.
A highe empe a u es, when all he oxygen is consumed, he
eac ion ha akes place is he me hane s eam e o ming e-
ac ion: CH
4
þH
2
O%CO þ3H
2
, whe e he con e sion o
me hane is accompanied by he consump ion o H
2
Oand he
p oduc ion o syngas inc eases d as ically. Simul aneously, he
d y e o ming o me hane likely occu s: CH
4
þCO
2
%2CO þ
2H
2
. This mechanisms co esponds o he well-known “com-
bus ion and e o ming eac ion (CRR)”mechanism ou lined in
he in oduc ion sec ion [16,42], al hough a de ailed kine ic
analysis ou o he modynamic con ol would be necessa y o
discuss he eac ion mechanism in mo e de ail [43,44]. Simila
end dis ibu ion and appea ance o p oduc s a e ob ained
when he bime allic ca alys was p epa ed by he con en ional
IWI me hod, con i ming ha a simila eac ion mechanism
akes place independen ly o he p epa a ion me hod. Howe -
e , all he eac ions occu a highe empe a u es, indica ing
lowe ca aly ic ac i i y.
Conside ing he wo consecu i e s eps in ol ed in he POM
eac ion and gi en ha below 450 C he p oduc ion o syngas
is no signi ican , only he pe o mance o he ca alys s a
empe a u es abo e 450 C will be conside ed u he . Table 1
compiles he ca aly ic esul s o all p epa ed samples in e ms
o me hane con e sion ðxCH4Þ,H
2
selec i i y ðSH2Þ, CO selec-
i i y ðSCOÞand yield o syngas ðYsgÞ. Ba e CeO
2
esul s a e also
included as a blank es , as well as chemical equilib ium
alues.
The CeO
2
suppo in he absence o Pd and Ni is o ally
inac i e o POM unde he eac ion condi ions es ed. Mo e-
o e , he ca aly ic ac i i y esul s o he ca alys s wi h 0.5 w
% Ni suppo ed on CeO
2
p epa ed by ei he IWI o BM me hods
showed low me hane con e sion and he o ma ion o syngas
was no obse ed a all. On he o he hand, i is clea ha he
ca aly ic ac i i y o Pd/CeO
2
is much g ea e han Ni/CeO
2
,as
he me hane con e sion alues o Pd/CeO
2
ca alys s a 550 C
doubled hose ob ained o e Ni/CeO
2
ca alys s and he syngas
yield was 42e48%. The milling pa ame e s had negligible e -
ec on he ca aly ic pe o mance o he monome allic ca a-
lys s, as me hane con e sion a 550 C oscilla ed be ween 51
and 55% and syngas yield om 42 o 47%.
F om Table 1, i can be clea ly seen ha he coope a i e
e ec be ween Pd and Ni in he bime allic ca alys s is
ema kable, wi h a d ama ic inc ease in bo h he me hane
con e sion and he syngas yield compa ed o monome allic
ca alys s. A 550 C, bime allic PdeNi/CeO
2
ca alys s show
me hane con e sion le els om 62 o 75%, selec i i y owa ds
H
2
o 64e66%, selec i i y owa ds CO o 19e24%, and syngas
yield be ween 52 and 67%. These esul s demons a e ha he
syne gy be ween Pd and Ni suppo ed on CeO
2
yields ca alys s
wi h imp o ed ac i i y and selec i i y in he POM wi h espec
o monome allic Ni/CeO
2
and Pd/CeO
2
.
The p epa a ion me hod (BM s. IWI), he o de o inco -
po a ion o he me als (co-BM/co-imp egna ion s. sequen ial
BM/IWI) play an impo an ole on he ca aly ic pe o mance
o he bime allic PdeNi/CeO
2
ca alys s. Fig. 6a shows he
me hane con e sion a di e en empe a u es o he bime-
allic Pd (1)-Ni(0.5)/CeO
2
ca alys s p epa ed by co-
imp egna ion and sequen ial-imp egna ion. In e es ingly, a
low empe a u e (450 C) he me hane con e sion o he ca -
alys s p epa ed by sequen ial imp egna ion is much highe
han ha o he ca alys p epa ed by co-imp egna ion,
ega dless o he o de o addi ion used o Pd and Ni. In
con as , a high empe a u es (500 and 550 C), he me hane
con e sion a ained by he ca alys p epa ed by co-
imp egna ion is highe han hose o he ca alys s p epa ed
Fig. 5 eMola low a es eco ded o bime allic ca alys s Pd (1)-Ni(0.5)/CeO
2
p epa ed by BM a 50 Hz o 20 min (a) and IWI (b)
me hods. GHSV ¼12 £10
3
h
¡1
, F/W ¼60 L h
¡1
g
¡1
.
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 12029
by sequen ial imp egna ion. No ema kable di e ences we e
obse ed be ween bime allic ca alys s p epa ed by co-BM o
sequen ial-BM (Fig. 6b). In all cases, he me hane con e sion
was simila a each empe a u e. In any case, he addi ion o
Pd i s gi es be e ca aly ic esul s han when Ni is i s
inco po a ed, which poin s o a be e ca aly ic pe o mance
when Pd in e ac s s ongly wi h CeO
2
, as opposed o Ni.
On he o he hand, he e ec o milling condi ions ( e-
quency and ime) on he ca aly ic ac i i y o bime allic PdeNi/
CeO
2
/BM ca alys a di e en empe a u es is shown in Fig. 6c
and d. The ene gy applied du ing he ball milling in e ms o
equency o ib a ion has a s ong e ec on ca aly ic ac i i y;
he highe he ene gy he be e he ca aly ic ac i i y o he
ca alys (Fig. 6c). The e o e, i is clea ha high-ene gy milling
has a bene icial in luence on he numbe and quali y o he
ac i e si es [45,46]. This wo k adds an addi ional e idence ha
ball milling is able o c ea e unp eceden ed a chi ec u es ha
can exhibi ou s anding ca aly ic ac i i y. The ball milling
ime is ano he impo an ac o ha can a ec ca aly ic
pe o mance. Fig. 6d shows he e ec o he milling ime ( om
5 o 40 min) on he me hane con e sion alues. A good
comp omise be ween me hane con e sion and milling ime is
ound o he ca alys p epa ed using 20 min o milling ime.
Acco dingly, a ball milling equency o 50 Hz o 20 min ap-
pea s as he op imal syn hesis condi ions.
An impo an aspec ega ding bime allic ca alys s is he
ela i e amoun o he wo me als in ol ed. In ou case, i
should be also conside ed ha he cos o Pd is much highe
han ha o Ni, which ob iously has a di ec impac on he
inal cos o he ca alys . Fig. 7 shows he me hane con e sion
and syngas yield a 450 C and 550 C exhibi ed by PdeNi/
CeO
2
/BM and PdeNi/CeO
2
/IWI ca alys s con aining di e en
amoun s o Pd and Ni bu keeping he o al me al loading a
1.5 w % (Pd w .% om 0.06 o 1% and Ni w .% om 0.5 o
1.44%). Two aspec s me i pa icula a en ion. Fi s , i is e y
in e es ing o obse e ha a 450 C bo h he me hane con-
e sion and syngas yield a e much highe o all he ca alys s
p epa ed by BM wi h espec o hei espec i e coun e pa s
p epa ed by he IWI me hod. This clea ly demons a es ha
e y eac i e and speci ic ac i e si es a e c ea ed by he BM
me hod, which a e highly ac i e o he POM p ocess a low
empe a u e. A 550 C, he ca aly ic pe o mance o PdeNi/
CeO
2
/BM and PdeNi/CeO
2
/IWI ca alys s a e simila . Second,
he e ec o he Pd amoun on he ca aly ic pe o mance is
lowe han expec ed and depends also on he p epa a ion
me hod. Fo he PdeNi/CeO
2
/IWI ca alys s, a end is obse ed
Table 1 eMe hane con e sion, hyd ogen and ca bon monoxide selec i i y alues and syngas yields ob ained o e ba e
CeO
2
, Ni/CeO
2
, Pd/CeO
2
and PdeNi/CeO
2
bime allic ca alys s p epa ed by incipien we ness imp egna ion (IWI) and d y ball
milling (BM) using di e en syn hesis pa ame e s and me al loading alues. Reac ion condi ions: CH
4
:ai :N
2
¼4:11:85, F/
W¼60 L h
¡1
g
¡1
, GHSV ¼12 £10
3
h
¡1
.
Sample Hz min w .%Pd w .%Ni x450C
CH4x550C
CH4S550C
H2S550C
CO Y550C
syngas
Equilib ium - - - - 51.3 77.9 67.2 26.3 70.9
CeO
2
/BM 50 10 0 0 0 0 0 0 0
Ni/CeO
2
/BM 50 10 0 0.5 3.2 22 0 0 0
Ni/CeO
2
/IWI - - 0 0.5 3 18.6 0 0 0
Pd/CeO
2
/BM 50 10 0.5 0 32.6 52.7 63.3 17.7 42.7
Pd/CeO
2
/IWI - - 0.5 0 39.7 51 63.8 23 44.5
Pd/CeO
2
/BM 50 10 1 0 30.5 53.2 64.8 19.3 44.8
Pd/CeO
2
/IWI - - 1 0 34.5 57.4 64.5 19.4 48.2
Pd/CeO
2
/BM 50 20 1 0 32.7 55.6 65.2 20.2 47.6
Pd/CeO
2
/BM 15 10 1 0 35.2 54.9 63.5 18 44.7
Pd/CeO
2
/BM 30 10 1 0 31.4 51 64 18.2 41.9
PdeNi/CeO
2
/BM 15 10 1 0.5 36.4 62.4 64.2 19.2 52.1
PdeNi/CeO
2
/BM 30 10 1 0.5 39.9 67.1 64.9 20.5 57.4
PdeNi/CeO
2
/BM 50 10 1 0.5 44 71.8 65.5 21.7 62.7
PdeCeO
2
/BM/Ni/BM 50 10 1 0.5 42.5 71.6 65.5 22 62.6
NieCeO
2
/BM/Pd/BM 50 10 1 0.5 41.6 68 64.9 20.5 58.1
PdeNi/CeO
2
/BM 50 5 1 0.5 37.5 67.2 65.5 20.7 57.9
PdeNi/CeO
2
/BM 50 15 1 0.5 38 66.3 65.5 21.2 57.5
PdeNi/CeO
2
/BM 50 20 1 0.5 42.1 72.4 66 22 63.7
PdeNi/CeO
2
/BM 50 40 1 0.5 41.4 71.5 65.8 21.7 62.6
PdeNi/CeO
2
/BM 50 20 0.75 0.75 37.9 67.4 65.4 21.7 58.7
PdeNi/CeO
2
/BM 50 20 0.5 1 35.5 68.2 65.3 21.7 59.4
PdeNi/CeO
2
/BM 50 20 0.25 1.25 37.7 70.6 65.9 23.8 63.4
PdeNi/CeO
2
/BM 50 20 0.12 1.38 40.8 69.5 65.9 23.6 62.2
PdeNi/CeO
2
/BM 50 20 0.06 1.44 36.6 69.5 66 23.1 62
NieCeO
2
/IWI/Pd/IWI - - 1 0.5 38.5 51.5 64.6 19.5 54
PdeCeO
2
/IWI/Ni/IWI - - 1 0.5 42.2 71.7 66 21.8 63
PdeNi/CeO
2
/IWI - - 1 0.5 12.6 75.8 66.1 22.8 67.4
PdeNi/CeO
2
/IWI - - 0.75 0.75 11.6 74.8 66.2 22.5 66.3
PdeNi/CeO
2
/IWI - - 0.5 1 7.7 74.4 66.3 22.8 66.3
PdeNi/CeO
2
/IWI - - 0.25 1.25 6.9 70 66 21.8 63
PdeNi/CeO
2
/IWI - - 0.12 1.38 2.8 67.7 65.8 23 60.2
PdeNi/CeO
2
/IWI - - 0.06 1.44 3.3 65.4 66 22.9 62.6
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e1203512030
be ween me hane con e sion and Pd con en : he highe he
Pd amoun he highe he con e sion o me hane. This is no
he case o he PdeNi/CeO
2
/BM ca alys s, whe e bo h he
me hane con e sion and syngas yield a e a he unsensi i e
o he ela i e amoun s o Pd and Ni in he ange s udied. I
should be highligh ed ha he Ni/CeO
2
/BM sample showed
Fig. 6 eCon e sion o me hane a 450, 500 and 550 C o e PdeNi/CeO
2
ca alys s p epa ed by IWI (a), PdeNi/CeO
2
ca alys s
p epa ed by BM a 50 Hz o 10 min (b), PdeNi/CeO
2
/BM p epa ed a di e en milling equency o 10 min (c), and PdeNi/
CeO
2
/BM p epa ed a 50 Hz o di e en milling ime (d). GHSV ¼12 £10
3
h
¡1
, F/W ¼60 L h
¡1
g
¡1
.
Fig. 7 eCon e sion o me hane (a) and yield o syngas (b) o he bime allic Pd(x)-Ni(y)/CeO
2
ca alys s con aining di e en
amoun s o Pd and Ni p epa ed by BM (black) and IWI ( ed) me hods a 450 C and 550 C. All ca alys s con ain a o al me al
loading o 1.5 w %. GHSV ¼12 £10
3
h
¡1
, F/W ¼60 L h
¡1
g
¡1
. (Fo in e p e a ion o he e e ences o colo /colou in his
igu e legend, he eade is e e ed o he Web e sion o his a icle.)
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 12031
low me hane con e sion (3.2% a 450 C and 22% a 550 C,
Table 1) and no syngas p oduc ion, whe eas wi h only 0.06 w
% Pd he me hane con e sion inc eased up o 36.6 and 69.5%
a 450 and 550 C, espec i ely, and syngas yield was 62%.
Ce ainly, he bime allic PdeNi sys em appea s as a pa icu-
la ly appealing me al combina ion o POM. I has ecen ly
been shown by using DFT calcula ions ha he subs i u ion o
Ce by Pd ions in he su ace o he ca alys s o igina es highly
ac i e and s able species based on Pd
x
Ce
1-x
O
d
which a e highly
e ec i e in he apid CeH bond ac i a ion [47,48]. F om he
HRTEM images, we canno conclude i Pd subs i u es Ce in he
ce ia s uc u e bu , in any case, we ha e e idence ha bo h Pd
and Ni a e highly dispe sed a he subnanome ic le el. A low
empe a u e (450 C), whe e he bime allic PdeNi ca alys s
p epa ed by BM exhibi a pa icula high ac i i y o POM as
ou lined abo e, he bes pe o mance (bo h me hane
con e sion and syngas yield) was obse ed o PdeNi/CeO
2
/
BM wi h 0.12 w % Pd and 1.38 w % Ni.
Cha ac e iza ion o he ca alys s a e eac ion
The Raman spec a eco ded on he samples PdeNi/CeO
2
/BM,
and PdeNi/CeO
2
/IWI a e he ca aly ic es discussed abo e
a e shown in Fig. 8. In all cases, he posi ion o he F
2g
band o
CeO
2
did no shi a e he ca aly ic es wi h espec o he
alues eco ded be o e eac ion (Fig. 1), and he esidual ni-
a e signals in he sample PdeNi/CeO
2
/BM disappea ed a e
eac ion due o decomposi ion. In addi ion, he wo cha ac-
e is ic g aphi e D and G bands a abou ~1345 and ~1595 cm
1
[49,50] we e obse ed in he Raman spec a o he ca alys s
a e eac ion, being hei ela i e con ibu ion in he spec a
conside ably mo e in ense in he case o he bime allic sample
p epa ed by imp egna ion. The e o e, less coke deposi ion
occu ed on he bime allic sample p epa ed by ball milling.
Ca alys PdeNi/CeO
2
/BM was s udied by HRTEM and EDX
(Fig. 9). In e es ingly, a la ge numbe o oids a e iden i ied in
he ce ia c ys alli es, which we e no isible in he sample
be o e eac ion (Fig. 2c). These a e en a i ely asc ibed o
clus e s o oxygen acancies, p obably in con ac wi h he
me als, c ea ed unde he educing en i onmen c ea ed by
syngas du ing POM. On he o he hand, e en i he amo phous
shell is no longe obse ed a e he ca aly ic es , Pd and o Ni
pa icles escape de ec ion in HRTEM bu a e p esen in EDX
analyses, indica ing ha he high dispe sion o he me als is
main ained du ing he ca aly ic eac ion. Also, and in acco -
dance wi h he Raman esul s, coke is iden i ied by i s cha -
ac e is ic spacing a 3.8 
A, which co esponds o he (0001)
in e plana spacing o poo ly o de ed g aphi e. Howe e , he
amoun o deposi ed coke is a he low.
S abili y es s
In o de o p o e he long- e m pe o mance o he ca alys s,
s abili y es s o mo e han 100 h a 550 C we e ca ied ou .
Since bime allic ca alys s showed good me hane con e sion
and selec i i y o syngas (Table 1), PdeNi/CeO
2
ca alys s wi h
1 w % Pd and 0.5 Ni loading p epa ed by bo h IWI and BM
Fig. 8 eRaman spec a o bime allic Pd (1)-Ni(0.5)/CeO
2
ca alys s p epa ed by BM a 50 Hz o 20 min (black), and
IWI ( ed) a e eac ion (CH
4
:ai :N
2
¼4:11:85, F/
W¼60 L h
¡1
g
¡1
, GHSV ¼12 £10
3
h
¡1
). (Fo in e p e a ion
o he e e ences o colo /colou in his igu e legend, he
eade is e e ed o he Web e sion o his a icle.)
Fig. 9 eHRTEM images o he bime allic Pd (1)-Ni(0.5)/CeO
2
/BM ca alys (50 Hz o 20 min) a e he ca aly ic es .
in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e1203512032