In luence o Di e en Palladium P ecu so s on he
P ope ies o Solu ion-Combus ion-Syn hesized Palladium/
Ce ia Ca alys s o Me hane Combus ion
Sa a Colussi,*
[a]
A up Gayen,
[b]
Ma a Boa o,
[a]
Jo di Llo ca,
[c]
and Alessand o T o a elli
[a]
A se ies o Pd/CeO
2
ca alys s was p epa ed by solu ion com-
bus ion syn hesis (SCS) using di e en Pd p ecu so s. The pow-
de s we e cha ac e ized by complemen a y echniques such as
BET su ace a ea measu emen s, X-Ray di ac ion analysis, X-
Ray pho oelec on spec oscopy, empe a u e-p og ammed e-
duc ion, empe a u e-p og ammed oxida ion, and high- esolu-
ion TEM. The esul s ob ained e idenced he o ma ion o
a Pd-Ce solid solu ion on all SCS samples. This solid solu ion is
in he o m o an o de ed supe cell s uc u e only in he SCS
ca alys p epa ed om palladium ni a e. This was co ela ed
o he hea o eac ion be ween Pd(NO
3
)
2
and he uel. The
samples we e es ed o me hane ca aly ic combus ion, and
he eac ion a es on all SCS samples we e app oxima ely
wice ha o he imp egna ed coun e pa , i espec i e o he
p ecu so . This was a ibu ed o he p esence o he Pd-O-Ce
solid solu ion, which ga e ise o s ong Pd–ce ia in e ac ions.
In oduc ion
Solu ion combus ion syn hesis (SCS) is an a ac i e me hod
o he p epa a ion o ex emely dispe sed ca alys nanopow-
de s in a single, cos -e ec i e s ep.
[1]
This echnique is based
on he di ec eac ion o ca alys p ecu so s (oxidize s) wi h
a uel ( educing agen ). All eac an s a e dissol ed in wa e ,
and he solu ion is hea ed in a u nace in which he combus-
ion akes place wi h he o ma ion o a solid p oduc . The
gene al scheme o he SCS o a me al oxide using oxalyl dihy-
d azide (ODH, C
2
H
6
N
4
O
2
) as a uel is shown in Equa ion (1):
[2]
nanopa icles compa ed o con en ional me hods. The e a e
se e al examples ha demons a e how, wi h his echnique, i
is possible o p epa e s uc u ed ca alys s (i.e. , monoli hs) by
loading he ca alys powde di ec ly on o he su ace o he ce-
amic o me al suppo .
[3]
Mo eo e , some pa en s desc ibe he
possibili y o he indus ial scale-up o his syn hesis ap-
p oach.
[4]
As a esul o he in iguing cha ac e is ics o he
powde s and he simple p epa a ion me hod, many e o s
ha e been de o ed o s udy he pe o mances o SCS ma e ials
o di e en applica ions, which ange om h ee-way ca alys s
o au omo i e exhaus s a e ea men [3a, 5] o composi e ma e-
M
ð
NO
3
Þ2
ð
aq
Þ
þ
C
2
H
6
N
4
O
2
ð
aq
Þ
!
MO
s
Þ
þ
2
CO
þ
3N
þ3H O
ð 2ðgÞ
2ðgÞ
2
ð
g
Þ
ð
1
Þ
ials o ion ba e ies,
[6]
ce amic powde s o solid oxide uel
cells,
[7]
me al-based ca alys s o e o ming eac ions,
[8]
lumines-
cen ma e ials,
[9]
bioma e ials o medicine use,
[10]
and ca alys s
The powde s ob ained by his me hod show in e es ing
p ope ies, such as small pa icle size and ine dispe sion, be-
cause o he e olu ion o gases du ing eac ion ha can de-
s oy la ge agglome a es and lead o he o ma ion o smalle
o oxida ion eac ions.
[11]
Wi hin he las opic, we in es iga ed
he pe o mances o Pd/CeO
2
ca alys s o me hane combus-
ion p epa ed by SCS and compa ed hei ca aly ic ac i i y
wi h ha o he co esponding imp egna ed samples.
[12]
We
obse ed a signi ican inc ease in me hane combus ion a es
[a]
D . S. Colussi, P o . M. Boa o, P o . A. T o a elli
Depa men o Chemis y, Physics and En i onmen
Uni e si y o Udine
Via del Co oni icio 108, IT-33100 Udine (I aly)
E-mail : sa a.coluss[email p o ec ed]
[b]
D . A. Gayen
Depa men o Chemis y
Jada pu Uni e si y
Kolka a 700032 (India)
[c]
P o . J. Llo ca
Insi u de Tecniques Ene ge iques
and Cen e o Resea ch in Nanoenginee ing
Uni e si a Poli ecnica de Ca alunya, 08028 Ba celona (Spain)
on SCS samples wi h espec o he imp egna ed ones. SCS
samples p esen ed a peculia Pd-O-Ce supe cell s uc u e in
which al e na e Ce a oms we e subs i u ed by Pd a oms,
which led o he o ma ion o o de ed a ays o oxygen acan-
cies oge he wi h highly unde coo dina ed O a oms. DFT cal-
cula ions pe o med by Maye nick and Janik
[13]
p o ed heo e i-
cally ha his supe s uc u e is ex emely ac i e o me hane
combus ion, in ag eemen wi h ou expe imen al esul s. O he
au ho s indica ed ha he inco po a ion o me al ions in o an
oxide la ice, which implies he o ma ion o oxide ion acan-
cies, is esponsible o he highe ca aly ic ac i i y in combus-
ion eac ions ha ypically in ol e oxygen and uel dissocia-
ion on he ca alys su ace.
[11b]
SCS samples p esen o he cha ac e is ics ha could be in-
ol ed in he highe ac i i ies eco ded o hese ca alys s. Fo
Table 1. Pd p ecu so sal s, Pd loading, su ace a eas, and la ice pa ame e s o he ca alys s conside ed in his
wo k.
Sample
Pd p ecu so
Pd loading
Su ace a ea
R
wp
R
F2
c
2
Cell pa ame e
Composi ion
[b]
[w %][a]
[m
2
g
-
1
]
[%]
IWI
Pd(NO
3
)
2
1.74
12.2
6.8
4.7
2.41
5.4110 (1)
–
SCS2-N
Pd(NO
3
)
2
1.85
6.3
7.7
4.8
1.69
5.4103 (1)
Ce
0.97
Pd
0.03
O
2
-
d
SCS2-Cl
PdCl
2
1.84
5.7
6.4
4.9
1.16
5.4108 (1)
Ce
0.99
Pd
0.01
O
2
-
d
SCS2-Ac
Pd(O
2
CCH
3
)
2
1.41
8.9
6.2
4.5
1.23
5.4105 (1)
Ce
0.98
Pd
0.02
O
2
-
d
CeO
2
SCS
–
–
16.2
6.9
7.1
3.10
5.4113 (1)
–
[a] As measu ed by induc i ely coupled plasma a omic emission spec oscopy (ICP-AES). [b] Calcula ed acco d-
ing o Vega d’s law and assuming a Ce
4+
ionic adius o 0.97 Ç and a Pd
2+
ionic adius o 0.86 Ç.
R
wp
=
weigh ed
2
=
s uc u e R ac o . c
2
=
(
R
/
R
)
2
, whe e
R
is he expec ed R ac o
example Specchia e al. epo ed he p esence o dispe sed
Pd
2+
ions oge he wi h Pd and PdO on Pd-ce ia-zi conia sam-
ples p epa ed by SCS and a ibu ed he high ac i i y obse ed
o a syne gis ic e ec be ween he di e en Pd species.
[11a]
We
ha e epo ed a simila si ua ion o he ca aly ic combus ion
o p opane and dime hyl e he on Pd/CeO
2
ca alys s, in which
he coexis ence o highly ac i e Pd
II
cen e s wi h Pd and PdO
species was conside ed o be esponsible o he highe eac-
ion a es o SCS samples wi h espec o imp egna ed ones.
[14]
To ob ain he desi ed p ope ies and maximize he po en ial
o SCS ca alys s, one should be able o inely une he syn he-
sis p ocedu e. F om a p ac ical poin o iew, SCS is a e y
simple echnique and he heo e ical backg ound is clea ; ne -
e heless, he in luence o indi idual pa ame e s (i.e., ole o
uel, p ecu so s, combus ion empe a u e, e c.) on he syn he-
sized ma e ials a e complex o s udy because o he high exo-
he mici y o he eac ion and i s as e olu ion. Recen ly, some
s udies ha e ocused on he ole o he uel- o-oxidize a io
and on eac ion he modynamics modeling,
[10b, 15]
bu many as-
pec s s ill need o be in es iga ed in dep h. In his wo k, we
y o add ess his issue by cha ac e izing SCS Pd/CeO
2
samples
p epa ed om di e en Pd p ecu so s o e alua e he e ec o
his pa ame e on he o ma ion o he supe cell s uc u e.
Mo eo e , we s udy he ca aly ic beha io o he ma e ials o
he combus ion o me hane by eac ion a e measu emen s
pe o med in a ecycle eac o , wi h he aim o gi e mo e in-
sigh s in o hei peculia ca aly ic ac i i y.
Resul s and Discussion
The Pd p ecu so , Pd loading, and su ace a ea o he samples
conside ed in his wo k a e epo ed in Table 1. The measu ed
Pd amoun is close o he nominal loading o 2 w %, and he
loading on SCS2-Ac is sligh ly lowe han ha o he o he
samples. In he ollowing, all he quan i a i e esul s will be
calcula ed on a pe g am o Pd basis. In gene al all ca alys s
p esen low su ace a eas, i espec i e o he p epa a ion
me hod and he Pd p ecu so . The addi ion o Pd sal du ing
SCS, e en i in e y low amoun , dec eases he su ace a ea o
he ca alys s signi ican ly, as e idenced by compa ing Pd/CeO
2
wi h he CeO2 suppo p epa ed by he solu ion combus ion o
ce ic ammonium ni a e wi h ODH.
Figu e 1. XRD p o iles o he SCS samples. Inse : Pd me al peak (2 q
=
40.28).
XRD p o iles o he SCS samples a e shown in Figu e 1. The
XRD pa e ns o he ce ia suppo s a e almos iden ical o all
samples, whe eas a di e ence can be obse ed o he Pd
me al peaks (2 q
=
40.28). The p esence o Pd me al on SCS2-N
can be seen clea ly in he inse o Figu e 1, and he Pd me al
peak has a lowe in ensi y o SCS2-Cl and SCS2-Ac. No peaks
om palladium oxide (2 q
=
33.98) can be dis inguished on any
o he SCS samples. The mean c ys al size o ce ia calcula ed
by he Sche e equa ion is be ween 30 and 45 nm o all ca a-
lys s. On he sample p epa ed by incipien we ness imp egna-
ion (IWI ; Figu e S1) no Pd me al is de ec ed, bu small PdO
peaks a 2 q
=
33.9 and 42.38 a e obse ed. The calcula ed c ys-
al size o ce ia in he IWI ca alys is 16 nm.
Rie eld analysis o he SCS ca alys s indica es ha he s uc-
u al ea u es a e compa ible wi h he p esence o a solid solu-
ion in which Pd pa ially en e s he ce ia la ice (Table 1). The
amoun o Pd in solid solu ion, calcula ed acco ding o Ve-
ga d’s law, a ies sligh ly wi h he Pd p ecu so . The o ma ion
o a Ce1-xPdxO2-d solid solu ion on 1 a % Pd/CeO2 samples p e-
pa ed by SCS s a ing om a PdCl
2
p ecu so was epo ed by
P iolka and co-wo ke s.
[16]
Acco ding o hei wo k, his solid
solu ion comp ises Pd
2+
ions s abilized in Ce
4+
si es. Fo he
IWI sample, he a ia ion in he la ice pa ame e is oo small
o assume he inclusion o Pd in o he ce ia la ice.
The X- ay pho oelec on spec oscopy (XPS) esul s o he
as-p epa ed and spen ca alys s a e summa ized in Table 2 (see
Figu e S7 o he Suppo ing In o ma ion o XPS spec a). By
using XPS, i has been possible
o iden i y di e en Pd species
ha ha e been asc ibed o Pd0
me al (binding ene gy (BE)
=
335.6 eV), Pd
II
species (BE
=
337.6 eV), and nons oichiome ic
palladium oxide wi h an in e -
media e oxida ion s a e (BE
=
336.5 eV).
[17]
I can be obse ed
ha , in ag eemen wi h he XPS
spec a o esh samples, Pd
me al is de ec ed only on he
SCS samples, whe eas IWI shows
p o ile R ac o .
R
F
wp exp
exp
he p esence o oxidized Pd spe-
Table 2. XPS analysis o all samples.
Sample
a omic a io
Pd
0
PdO
x
Pd
II
Pd/Ce
[%]
[%]
[%]
IWI
0.078
0
22.6
77.4
SCS2-N
0.053
10.6
81.1
8.3
SCS2-Cl
0.107
20.9
79.1
0
SCS2-Ac
0.046
24.7
66.5
8.8
IWI
[a]
0.063
–
13.7
86.3
SCS2-N
[a]
0.067
–
19.6
80.4
SCS2-Cl
[a]
0.067
–
28.5
71.5
SCS2-Ac
[a]
0.058
–
17.0
82.9
[a] Used ca alys s, ha is, samples a e one hea ing/cooling cycle du ing
me hane combus ion.
cies only. Nons oichiome ic PdO
x
species a e p esen in a con-
side ably lowe amoun compa ed o ha on he SCS ca alys s,
which indica es ha bulk PdO is he p edominan phase on
he imp egna ed sample. Howe e , he ca alys s p epa ed by
SCS comp ise mainly PdO
x
, and he amoun o Pd me al de-
c eases in he o de SCS2-Ac
>
SCS2-Cl
>
SCS2-N. PdO is equally
p esen on SCS2-N and SCS2-Ac and absen on SCS2-Cl. The
amoun o nons oichiome ic Pd oxide shows an in e se end
wi h espec o he Pd me al con en (SCS2-N
>
SCS2-Cl
>
SCS2-Ac). The use o di e en Pd p ecu so s has an in luence
on he dis ibu ion o Pd species in he esh samples, and
om XPS esul s i is clea ha SCS leads o he o ma ion o
less oxidized ca alys s.
A di e en pic u e is e ealed i we look a he spec a o
he used ca alys s, collec ed a e one hea ing/cooling cycle o
me hane combus ion. The Pd me al has disappea ed, and oxi-
dized PdII species domina e he su ace composi ion. This is
e iden pa icula ly o SCS ca alys s, bu i is ue also o IWI,
which emains sligh ly iche in PdO wi h espec o SCS sam-
ples.
High- esolu ion ansmission elec on mic oscopy (HRTEM)
images o he SCS samples a e shown in Figu es 2–4. In gene -
al, all ca alys s show well-de ined ce ia c ys alli es wi h pa icle
sizes be ween 20 and 50 nm, in good ag eemen wi h XRD
da a. The sample p epa ed om palladium ni a e (SCS2-N) e-
eals he p esence o he supe cell s uc u e iden i ied in ou
p e ious wo k.
[12]
A cha ac e is ic image o his ca alys is
shown in Figu e 2. In he Fou ie ans o m (FT) image, c ys al-
log aphic spacings a 2.71 and 3.13 Ç a e eco ded, which co -
espond o he (2 0 0) and (111) c ys allog aphic planes o
CeO
2
. In he o he pa o he c ys al (Figu e 2, igh ), la ice
inges a 5.7 Ç co espond o he o bidden {1 0 0} di ac ion,
which becomes isible in he o de ed (2 x 1)(11 0) supe s uc-
u e in which one Pd a om subs i u es one Ce a om e e y wo
in he c ys al s uc u e.
[12]
HRTEM images o SCS2-Cl and SCS-Ac a e shown in Figu es 3
and 4, espec i ely. Bo h hese samples show no e idence o
he o ma ion o he o de ed supe cell s uc u e. Fo SCS2-Cl,
only CeO
2
c ys alli es a e iden i ied and no (2 2 2) la ice inges
ha a e a ibu ed o he Pd-O-Ce supe s uc u e a e obse ed
in he HRTEM image. The analysis o SCS2-Ac e eals ha his
sample is cons i u ed by well-de ined CeO
2
c ys alli es. A high-
Figu e 2. HRTEM images o SCS2-N ( op) and hei associa ed FT pa e ns
(bo om).
Figu e 3. HRTEM images o SCS2-Cl. a and b co espond o di e en pa icles
o he sample.
magni ica ion HRTEM image ha shows he p esence o he
(111) c ys allog aphic planes o CeO
2
is shown in Figu e 4 b. In
addi ion o CeO
2
c ys alli es, elec on-dense pa icles a e also
iden i ied in he image shown in Figu e 4 a. These a e oo
small (
ð
1 nm in diame e ) o an accu a e la ice inge analy-
sis, bu hei elec on-dense na u e sugges s ha hey co e-
spond o me allic Pd. The esul s o XPS (Table 2) and XRD, in
which a small peak a 2 q
=
40.28 is obse ed (inse o Fig-
u e 1 a), suppo his iden i ica ion. F om he HRTEM analysis, i
Figu e 4. HRTEM images o SCS2-Ac. a and b co espond o di e en pa i-
cles o he sample.
is possible o conclude ha he o de ed supe cell s uc u e de-
sc ibed in ou p e ious wo k
[12]
is p esen only on SCS2-N,
which sugges s he ole o he Pd p ecu so in he o ma ion
o his pa icula s uc u e. Fo he o he samples, he subs i u-
ion o Pd
2+
ions in o he ce ia la ice, as in e ed om XRD, is
no obse ed by HRTEM possibly because i is no o de ed
and/o i is limi ed o a e y hin laye . This supe cell s uc u e
can be ob ained only by he dissolu ion o Pd
2+
species wi h
he o de ing o he oxygen acancies; he p esence o a Pd-
Ce-O solid solu ion does no necessa ily imply he o ma ion o
a supe s uc u e.
The HRTEM image o IWI does no e eal any pa icula ea-
u es o his sample. Spo s a 2.1 Ç in he FT image co espond
o (11 0) planes o PdO, which indica es ha Pd is p esen in
he o m o PdO, in ag eemen wi h XPS and XRD indings. The
palladium oxide pa icle size on IWI is be ween 5 and 10 nm
(Figu e S2).
We in es iga ed he spen SCS2-N ca alys , ha is, he
sample a e one me hane combus ion cycle, by HRTEM. A la -
ice inge image eco ded a a magni ica ion o 10
6
is shown
in Figu e 5. The p esence o a supe cell s uc u e is e iden be-
cause he e a e spo s no pe mi ed in CeO
2
. This supe cell
s uc u e is again gene a ed by he pa ial subs i u ion o Ce4+
Figu e 5. HRTEM image o spen SCS2-N.
by Pd2+ in CeO2 wi h he concomi an c ea ion o o de ed
oxygen acancies.
To in es iga e he ole o he Pd p ecu so on he p ope ies
o he ca alys s p epa ed by SCS, we s udied he e ec o he
di e en sal s on he en halpy o eac ion D
H
eac
o each SCS
sample, and he esul s a e p esen ed in Table 3. The calcula-
Table 3. D
H
eac
calcula ed o he syn hesis o each SCS sample.
Sample
Pd p ecu so
DH eac [kcal mol-1]
Pd
sal
+CAN+ODH Pd
sal
+ODH
SCS2-N
Pd(NO
3
)
2
-
748
-
291
SCS2-Cl
PdCl
2
-
737 4
SCS2-Ac
Pd(O
2
CCH
3
)
2
-
740
-
85
ion o D
H
eac
was pe o med o bo h he global combus ion
eac ion [acco ding o Equa ions (2)–(4) in he Expe imen al
Sec ion] and he eac ion o he Pd sal wi h ODH alone. This
la e si ua ion can occu i some Pd sal is no dissol ed com-
ple ely o in he immedia e su oundings o Pd sal molecules
ha eac di ec ly wi h he uel. I can be obse ed ha he e-
ac ion pe o med wi h he ni a e p ecu so has a highe
D
H
eac
(absolu e alue), and he di e ence is mo e e iden i
we exclude he con ibu ion o ce ic ammonium ni a e. The
highe exo he mici y obse ed o he palladium ni a e p e-
cu so migh play a ole in he o ma ion o he o de ed supe -
cell s uc u e obse ed only o he SCS2-N sample. In gene al,
he he modynamic p ope ies o a solid solu ion change ac-
co ding o he mu ual posi ion o he a oms,
[18]
so i is eason-
able o assume ha di e en en halpies o eac ion can esul
in he di e en o de ing o Pd, O, and Ce a oms.
Tempe a u e-p og ammed educ ion (TPR) and empe a u e-
p og ammed oxida ion (TPO) we e pe o med o in es iga e
he edox beha io o he ca alys s. The TPR p o iles o he IWI
and SCS ca alys s a e shown in Figu e 6a and b, espec i ely.
Gene ally, all samples show h ee hyd ogen-up ake egions :
a low- empe a u e (LT) egion be ween 233 and 423 K,
a medium- empe a u e (MT) egion be ween 573 and 873 K,
and a high- empe a u e (HT) egion abo e 923 K. F om a quali-
a i e poin o iew, he TPR p o iles o SCS and IWI ca alys s
a e clea ly di e en in he LT egion. In e es ingly, IWI p esen s
a single, sha p peak o hyd ogen up ake, whe eas he SCS
samples show a complex p o ile ha includes a combina ion
o hyd ogen-up ake and - elease peaks in a wide empe a u e
ange. The di e ence be ween SCS ca alys s and IWI sugges s
ha on he imp egna ed sample Pd is p esen in a mo e ho-
mogeneous o m han ha on he SCS ones ; his e ec migh
be ela ed o di e en pa icle size dis ibu ion and/o o he
p esence o di e en PdO
x
species, acco ding o ha de ec ed
by XPS analysis.
The peaks in he LT egion can be a ibu ed o he educ-
ion o Pd- ela ed species as epo ed widely in he li e a-
u e ;
[19]
whe eas he peaks in he MT and HT egions can be a -
ibu ed o he educ ion o su ace and bulk ce ia species, e-
spec i ely.
[20]
On he SCS samples, H
2
up ake s a s
ð
50 K
lowe han ha on IWI, which indica es ha he SCS ca alys s
Figu e 6. TPR p o iles o a) IWI and b) SCS samples.
a e educed mo e easily han IWI. This e idence ma ches well
wi h he XPS esul s, which indica e ha IWI is he mos oxi-
dized ca alys . MT peaks a e mo e p onounced and co e
a b oade empe a u e ange o SCS2-Ac and SCS2-Cl, whe eas
o SCS2-N hey a e simila o hose o IWI. In he HT egion, all
he samples show a simila end o hyd ogen up ake.
SCS2-N and SCS2-Ac show a simila beha io , wi h he p es-
ence o wo hyd ogen-consump ion peaks ollowed by a hyd o-
gen- elease peak caused by he decomposi ion o b- ype palla-
dium hyd ides.
[19]
In he case o SCS2-Cl, he peak caused by
hyd ogen elease appea s in he middle o he wo hyd ogen-
up ake ea u es and i s e olu ion is clea ly s opped by he ap-
pea ance o he second peak. The gene al iden i ica ion o
e e y peak o he LT egion is no s aigh o wa d. Acco ding
o Luo and Zheng,
[21]
he spli ing o he hyd ogen-consump-
ion peak a low empe a u e o e a PdO/Ce
0.5
Z
0.5
O
2
ca alys is
ela ed o he condi ions o Pd oxida ion and can be caused
by he nonuni o m size dis ibu ion o PdO and/o o he inclu-
sion o some PdO in o he suppo a e edox cycles. In his
case, he inclusion o PdO in he ce ia la ice is obse ed o all
SCS samples ; mo eo e , he pa icula cha ac e is ics o SCS
( as e olu ion, exo he mici y, di e en local empe a u es
du ing combus ion) migh gi e ise o nonuni o m Pd pa icle
size and di e en Pd oxida ion s a es. F om XPS analysis, i is
obse ed ha he p edominan phase on hese ca alys s is
a nons oichiome ic palladium oxide ha is likely o eac wi h
hyd ogen in a mo e complex way han bulk PdO, which p e-
domina es on IWI. I is also known ha noble me als de elop
s ong in e ac ions on ce ia unde educing condi ions ha
can esul , o example, in he deco a ion o Pd pa icles by
CeO
2
.
[22]
Unde he condi ions o SCS, s ongly educing en i-
onmen s migh be es ablished locally du ing eac ion leading
o he o ma ion o such deco a ed pa icles and/o o o he
ypes o Pd-Ce alloys
[23]
beside he o de ed supe cell s uc u e
obse ed on SCS2-N.
F om he quan i a i e analysis
epo ed in Table 4, i appea s
ha he e is an amoun o hy-
d ogen spillo e on o he sup-
po o SCS2-Ac as he heo e i-
cal pe cen age o PdO educ ion
exceeds 100 %. The same is ue
o IWI because he eco ded hy-
d ogen consump ion is
ð
60 %
highe han he s oichiome ic
amoun calcula ed o PdO e-
duc ion. Fo SCS2-N and SCS2-
Cl, a deg ee o educ ion close
(SCS2-Cl) o sligh ly abo e uni y
(SCS2-N) likely includes a educ-
ion o he ce ia suppo as
hese samples p esen a ce ain
amoun o me allic Pd (seen
om XRD, XPS, and HRTEM e-
sul s). The spillo e o H
2
on o
a educible suppo in he p es-
ence o a noble me al is a well-
known phenomenon.
[24]
Table 4. Quan i a i e analysis o TPR expe imen s (H
2
up ake/ elease cal-
cula ed up o 423 K).
Sample
H
2
up ake
H
2
elease
PdO ed
PdH dec
[mL]
[mL]
[%]
[a]
[%]
[b]
IWI
0.39
–
164
–
SCS2-N
0.28
0.06
107
22
SCS2-Cl
0.23
–
90
–
SCS2-Ac
0.27
0.02
142
10
[a] % o PdO educed du ing TPR, calcula ed on he basis o he ac ual Pd
loading. [b] % o PdH decomposed du ing TPR, calcula ed on he basis o
he ac ual Pd loading.
The hyd ogen consump ion in he MT egion is e y small
o IWI and SCS2-N and app oxima ely h ee o ou imes
highe o SCS2-Cl and SCS2-Ac, espec i ely. This hyd ogen
up ake is smalle han ha obse ed on pu e CeO
2
p epa ed
by SCS (Suppo ing In o ma ion). This is consis en wi h he
spillo e obse ed in he LT egion: in he p esence o he
noble me al, ce ia ends o be educed a a lowe empe a u e.
The di e ences be ween he samples p epa ed om di e en
p ecu so s a e di icul o explain in he MT egion, bu hese
migh be ela ed o he di e en e olu ion o ca bon-con ain-
ing species ha a e s ill p esen on he ca alys su ace.
TPO expe imen s add some o he de ails o he edox p op-
e ies o he ca alys s. The oxygen- elease and -up ake p o iles
o SCS samples du ing he hi d TPO cycle a e shown in
Figu e 7. The hi d cycle is chosen as ep esen a i e as no u -
he modi ica ion o TPO p o iles can be de ec ed om his
cycle onwa ds; mo eo e , his cycle is somehow ep esen a i e
o he s a e o he ca alys s du ing he combus ion cycles.
F om he cooling pa o he cycles, i can be obse ed ha
he empe a u e and he dynamics o Pd eoxida ion a e simi-

Table 5. Quan i a i e analysis o TPO expe imen s
.
Sample
Oxygen elease
Oxygen up ake
PdO decomposi ion
Pd
oxida ion
[x 10-3 mmol gPd-1]
[x 10-3 mmolgPd-1]
[%]
[%]
IWI
3.76
3.79
80
81
SCS2-N
3.34
3.28
71
69
SCS2-Cl
3.42
3.40
73
72
SCS2-Ac
4.06
4.29
86
91
Table 6. Reac ion a es o all samples calcula ed a 573 K.
Sample
Reac ion a es
mmol gPd-1 s-1
x102 mmol m-2 s-1
IWI
10.0
1.4
SCS2-N
23.6
6.9
SCS2-Cl
23.4
7.6
SCS2-Ac
25.2
4.0
2PdIW_CeSCS
9.7
1.3
2Pd/Al
2
O
3
23.9
0.5
Figu e 7. TPO p o iles o he hi d cycle o all SCS samples (solid line : hea -
ing; do ed line: cooling).
la o all ca alys s, exhibi ing a single peak wi h a minimum a
ð
920 K. A di e en pic u e appea s i we look a he oxygen-
elease p o ile (hea ing amp): in his case a clea di e ence is
obse ed be ween SCS2-N and SCS2-Ac on one side and SCS2-
Cl on he o he . PdO decomposi ion akes place mos ly below
1073 K on SCS2-Cl, whe eas a signi ican pa o PdO is decom-
posed abo e 1073 K o SCS2-N and SCS2-Ac. Fo alumina-sup-
po ed Pd, he ac ion o PdO ha decomposes a a highe
empe a u e has been a ibu ed en a i ely o su ace PdO, o
PdO ha in e ac s s ongly wi h he suppo ,[17] and o he de-
composi ion o a co e o PdO co e ed by a shell o Pd
me al.[25] On hese Pd-ce ia sys ems, we can hypo hesize a simi-
la scena io in which he occu ence o sepa a e s eps du ing
PdO decomposi ion migh co espond o di e en PdO species
o o PdO in di e en en i onmen s (bound mo e s ongly o
he suppo , co e ed by Pd me al e c.). In gene al, hese s eps
a e mo e e iden on he SCS samples han on he IWI sample
(Figu e S3) in ag eemen wi h he esul s om TPR and wi h
XPS analysis, which shows he p edominance o PdOx on he
SCS ca alys s.
Quan i a i e analysis pe o med du ing TPO indica es clea ly
ha a highe amoun o Pd unde goes oxida ion-decomposi-
ion on SCS2-Ac (Table 5). The amoun o Pd cycling is in he
o de SCS2-Ac > IWI > SCS2-Cl
SCS2-N.
The esul s o eac ion a e measu emen s du ing me hane
combus ion pe o med in he ecycle eac o a e epo ed in
Table 6. The use o he ecycle eac o ensu es he di e en ial
condi ions necessa y o ob ain consis en a e alues. Mo e-
o e , he measu emen o he a e du ing he second ca aly ic
cycle a oids he p oblems ela ed o sin e ing and edispe sion
o Pd pa icles du ing he eac ion
[26]
and ensu es s able ca a-
ly ic beha io because he hi d cycle is almos iden ical o he
second one. The eac ion a es measu ed o SCS samples
based on noble me al loading a e mo e han wice he a es
measu ed on IWI. The di e ence is e en highe i we compa e
he esul s based on he su ace a ea as he su ace a ea o he
SCS samples is much lowe han ha o IWI. This e ec is al-
eady known o SCS2-N o he combus ion o me hane,
[12]
p opane, and dime hyl e he .
[14]
To e alua e whe he his e ec
could be ela ed o a pa icula s uc u e o he ce ia suppo
made by SCS ha can in e ac di e en ly wi h Pd, a sample
ha consis ed o Pd imp egna ed on CeO
2
made by SCS has
been p epa ed and es ed. This ca alys , 2PdIW_CeSCS,
showed a eac ion a e e y close o ha o IWI, which hus ex-
cludes an e ec o he SCS ce ia suppo in he enhancemen
o he ca aly ic pe o mance. In addi ion, we measu ed he e-
ac ion a e on a 2 w % Pd/Al
2
O
3
ca alys p epa ed by incipien
we ness imp egna ion wi h a simila p ocedu e o ha epo -
ed o he IWI sample. The esul s indica e ha , al hough he
su ace a ea is much highe han ha o he SCS samples
(93.5 m
2
g
-
1
), he eac ion a e a 573 K is simila . As widely
known om he li e a u e,
[27]
he alumina-based ca alys shows
a much highe deac i a ion du ing he cooling b anch o he
ligh -o cu e (Figu e S7) wi h espec o ce ia-based ma e ials,
which makes he ce ia-based ma e ials e y in e es ing om
he poin o iew o p ac ical applica ions.
The alues o he eac ion a es measu ed on SCS samples
indica e ha he e is no signi ican e ec o he di e en Pd
p ecu so s on ca aly ic ac i i y, al hough i is clea ha he syn-
hesis ou e plays a majo ole. XPS analysis o esh samples
e idences a highe deg ee o oxida ion o IWI han he SCS
ca alys s; e en so his di e ence almos disappea s a e one
me hane combus ion cycle as e ealed by XPS o he spen
ca alys s. These e idences end o exclude ha di -
e en Pd oxida ion s a es a e esponsible o he
highe eac ion a es eco ded on all SCS samples.
Howe e , he p esence o he o de ed Pd-O-Ce su-
pe s uc u e on SCS2-N a e eac ion de ec ed by
HRTEM suppo s he ole o his s uc u e, and ha
o i s nono de ed coun e pa , in he highe me hane
combus ion ac i i y. On he basis o hese esul s, i
is easonable o conclude ha he p esence o
highly dispe sed Pd
2+
ions in he ce ia la ice, o -
de ed o no , is key o achie e supe io me hane combus ion
ac i i y. This explana ion is in ag eemen wi h he wo k o
Ca gnello e al. in which a e y high ac i i y o me hane com-
bus ion on Pd/ce ia ca alys was eco ded, which was ela ed
o he p esence o unde coo dina ed Pd a oms in close in e ac-
ion wi h CeO
2
.
[28]
P iolka and co-wo ke s a ibu ed he highe
0:03PdðNO3Þ2þ0:97CANþ2:358ODH
!
0:03PdOþ0:97CeO2
þ
4:716 CO
2 þ
10:954 H
2
O
þ
8:626 N
2
0:03 PdCl2
þ0:97 CANþ2:328 ODH ! 0:03 PdOþ0:97 CeO2
þ
4:641 CO
2 þ
10:864 H
2
O
þ
8:536 N
2 þ
0:03 Cl
2 þ
0:135 C
ð
2
Þ
ð
3
Þ
ac i i y obse ed o CO oxida ion and NO educ ion o he
o ma ion o a Pd-O-Ce solid solu ion.
[16]
Mo eo e , he heo e -
ical wo k pe o med by Maye nick and Janik indica es ha he
p esence o subs i u ional Pd ions in he ce ia la ice lowe s
no only he ene gy ba ie o he o ma ion o oxygen acan-
cies bu also he eac ion ene gy o me hane adso p ion, ha
is, he i s s ep o ca aly ic combus ion.
[13]
Conclusions
We cha ac e ized di e en Pd-ce ia ca alys s p epa ed by solu-
ion combus ion syn hesis (SCS) s a ing om di e en Pd p e-
cu so s and es ed o me hane ca aly ic combus ion. Cha ac-
e iza ion esul s indica e he p esence on all SCS samples o
a Pd-Ce solid solu ion, which appea s as an o de ed supe cell
s uc u e on SCS2-N. The occu ence o his o de ed s uc u e
has been ela ed en a i ely o he en halpy o eac ion DH
eac
be ween palladium ni a e and he uel (oxalyl dihyd azide)
du ing SCS. The alue o DH
eac
o Pd(NO
3
)
2
is much mo e
exo he mic han ha o Pd(O
2
CCH
3
)
2
and PdCl
2
(sligh ly endo-
he mic). The esul s ob ained in his s udy explain he highe
ca aly ic ac i i y o me hane combus ion eco ded on all SCS
samples in ela ion o hei peculia s uc u e : on one side his
pa icula Pd–ce ia in e ac ion is e iden om s uc u al and
mo phological cha ac e iza ion (X- ay pho oelec on spec os-
copy, XRD, high- esolu ion TEM), on he o he i is unde lined
by he edox beha io o hese samples ( empe a u e-p o-
g ammed educ ion and oxida ion), which is clea ly di e en
and mo e complex wi h espec o ha o he imp egna ed
coun e pa , ega dless o he Pd p ecu so . I seems hen plau-
sible o asc ibe he supe io ac i i y o me hane combus ion
eco ded on SCS samples o he subs i u ion o Pd
2+
ions in o
he ce ia la ice, which is andom o SCS2-Cl and SCS2-Ac and
o de ed o SCS2-N. Ou esul s indica e ha hese ca aly ic
sys ems show p omise in he ield o me hane aba emen o
he con ol o g eenhouse gas emissions in ligh o hei com-
pa ison wi h s a e-o - he-a Pd-alumina ca alys s.
Expe imen al Sec ion
A se ies o 2 w % Pd/CeO
2
ca alys s was p epa ed by SCS. In his
echnique Pd and Ce p ecu so sal s a e mixed in a sui able
amoun o wa e o which a educe , o uel, is added. In his wo k,
(NH
4
)
2
Ce(NO
3
)
6
(ce ic ammonium ni a e, CAN) was used as he
CeO
2
p ecu so , and Pd(NO
3
)
2
, PdCl
2
, and Pd(O
2
CCH
3
)
2
we e used as
Pd p ecu so s. The uel was ODH (C
2
H
6
N
4
O
2
). Fuel and p ecu so s in
a s oichiome ic amoun we e dissol ed in wa e and placed in
a p ehea ed u nace (T
ð
623 K) o he combus ion o ake place.
A e comple e combus ion, he samples we e emo ed om he
u nace and cooled o RT. The o e all heo e ical combus ion eac-
ions a e shown in Equa ions (2)–(4):
0:03 Pd
ð
O
2
CCH
3 Þ2 þ
0:97 CAN
þ
2:328 ODH
!
0:03 PdO
þ
0:97 CeO
2
þ
4:656 CO
2 þ
10:954 H
2
O
þ
8:536 N
2 þ
0:12 C
ð
4
Þ
The amoun o uel o each eac ion was calcula ed o balance he
ne oxidizing alence o he sal s in ol ed by conside ing only ni-
a e compounds as he oxidize .
[2]
The IWI ca alys ha consis ed
o 2 w % Pd on comme cial CeO
2
(G ace Da ison) was p epa ed by
adi ional incipien we ness imp egna ion o compa ison wi h
SCS samples om a solu ion o 10 w % Pd(NO
3
)
2
(Ald ich,
99.999 %). The imp egna ed sample was d ied o e nigh a 393 K
and hen calcined a 1173 K in a u nace wi h ci cula ing ai o 4 h.
Cha ac e iza ion me hods a e epo ed in he Suppo ing In o ma-
ion. Ca aly ic es s and eac ion a e measu emen s we e pe -
o med in a ecycle eac o
[14]
wi h a o al low a e o 40 mL min
-
1
and a ecycle a io o 35. The eed gas composi ion was 0.5 % CH
4
and 2% O
2
in He. The sample (100 mg, p essed and sie ed) was
loaded in o a qua z mic o eac o (i.d. 4 mm, leng h 400 mm) on
a qua z wool bed, and he empe a u e was moni o ed by using
a he mocouple posi ioned in he ca alys bed. The e luen gases
we e moni o ed by using an Omnis a Quad upole Mass Spec om-
e e and a Va ian CP 4900 mic o gas ch oma og aph equipped
wi h molecula sie e and Po aplo Q columns. Fo eac ion a e
measu emen s, a i s hea ing/cooling cycle was pe o med up o
1173 K, du ing which he ligh -o cu es o me hane combus ion
we e eco ded (Suppo ing In o ma ion), hen he empe a u e was
inc eased o 573 K a a hea ing a e o 10 K min
-
1
. The a e was cal-
cula ed using he con e sion alue measu ed a e 2 h o holding
ime a 573 K; du ing his pe iod no signi ican a ia ion in con e -
sion alues was obse ed.
Acknowledgemen s
We hank egione FVG o suppo . J.L. is a Se a Hffln e Fellow
and is g a e ul o he ICREA Academia p og am and MINECO
g an ENE2012-36368.
Keywo ds: ce ium · he e ogeneous ca alysis · hyd oca bons ·
palladium · educ ion
[1] a) S. T. A una, A. S. Mukasyan,
Cu . Opin. Solid S a e Ma e . Sci.
2008,
12
,
44 –50; b) S. L. Gonz llez-Co ƒs, F. E. Imbe ,
Appl. Ca al. A
2013,
452
,
117– 131.
[2] K. Su esh, K. C. Pa il in
Pe spec i es in Solid S a e Chemis y
(Ed.: K. J.
Rao), Na osa Publishing House, New Delhi, 1995, pp. 376 –388.
[3] a) S. Sha ma, M. S. Hegde, R. N. Das, M. Pandey,
Appl. Ca al. A
2008,
337
,
130 –137; b) A. Vi a, G. C is iano, C. I aliano, S. Specchia, F. Cipi i, V.
Specchia,
In . J. Hyd ogen Ene gy
2014,
39
, 18592– 18603.
[4] a) P. Dinka, A. Mukasyan, WO2007019332A1, 2007; b) Q. Weng, D. Sun,
S. Lin, J. Chen, S. Xie, R. Huang, L. Zheng, CN101503188 (A), 2009.
[5] P. Be a, K. C. Pa il, M. S. Hegde,
Phys. Chem. Chem. Phys.
2000,
2
, 3715 –
3719.
[6] a) B. T. Zhao, X. Yu, R. Cai, R. Ran, H. T. Wang, Z. P. Shao,
J. Ma e . Chem.
2012,
22
, 2900 –2907; b) X. Li, Q. Xiao, B. Liu, H. C. Lin, J. B. Zhao,
J.
Powe Sou ces
2015,
273
, 128 –135.
[7] a) E. Thomas, D. H. Lee, M. Y. Yoon, S. H. Eh man, H. J. Hwang,
Ce am.
In .
2011,
37
, 2269 –2274; b) C. Y. Zhu, A. Nobu a, Y. W. Ju, T. Ishiha a, T.
Akiyama,
In . J. Hyd ogen Ene gy
2013,
38
, 13419 –13426.
[8] a) A. Kuma , A. S. Mukasyan, E. E. Wol ,
Appl. Ca al. A
2011,
401
, 20– 28;
b) A. Vi a, G. C is iano, C. I aliano, L. Pino, S. Specchia,
Appl. Ca al. B
2015,
162
, 551 –563.
[9] a) G. Ramak ishna, H. Nagabhushana, D. V. Suni ha, S. C. P ashan ha,
S. C. Sha ma, B. M. Nagabhushana,
Spec ochim. Ac a Pa A
2014,
127
,
177 –184 ; b) Y. S. Vidya, K. S. Anan ha aju, H. Nagabhushana, S. C.
Sha ma, H. P. Nagaswa upa, S. C. P ashan ha, C. Shi akuma a, Dani hku-
ma ,
Spec ochim. Ac a Pa A
2015,
135
, 241 –251.
[10] a) S. K. Ghosh, A. P akash, S. Da a, S. K. Roy, D. Basu,
Bull. Ma e . Sci.
2010,
33
, 7 –16; b) M. A. Aghayan, M. A. Rod iguez,
Ma e . Sci. Eng. C
2012,
32
, 2464 –2468.
[11] a) S. Specchia, E. Finocchio, G. Busca, P. Palmisano, V. Specchia,
J. Ca al.
2009,
263
, 134 –145 ; b) V. M. Shinde, G. Mad as,
Ca al. Today
2012,
198
,
270 –279 ; c) R. Mis i, M. Rahaman, J. Llo ca, K. R. P iolka , S. Colussi,
B. C. Ray, A. Gayen,
J. Mol. Ca al. A
2014,
390
, 187 –197; d) M. Piume i,
D. Fino, N. Russo,
Appl. Ca al. B
2015,
163
, 277 –287.
[12] S. Colussi, A. Gayen, M. F. Camellone, M. Boa o, J. Llo ca, S. Fab is, A.
T o a elli,
Angew. Chem. In . Ed.
2009,
48
, 8481 –8484 ;
Angew. Chem.
2009,
121
, 8633 –8636.
[13] a) A. D. Maye nick, M. J. Janik,
J. Phys. Chem. C
2008,
112
, 14955 –14964;
b) A. D. Maye nick, M. J. Janik,
J. Ca al.
2011,
278
, 16– 25.
[14] S. Colussi, A. Gayen, J. Llo ca, C. de Lei enbu g, G. Dolce i, A. T o a elli,
Ind. Eng. Chem. Res.
2012,
51
, 7510 –7517.
[15] a) H. Nasi i, E. B. Mo lagh, J. V. Khaki, S. M. Zeba jad,
Ma e . Res. Bull.
2012,
47
, 3676 –3680 ; b) A. B. Salunkhe, V. M. Kho , M. R. Phada a e,
S. H. Pawa ,
J. Alloys Compd.
2012,
514
, 91– 96 ; c) S. Haja pou , K. Ghei-
sa i, A. H. Raou ,
J. Magn. Magn. Ma e .
2013,
329
, 165 –169 ; d) R. Ianos¸,
R. Babu a, R. Lazau,
Ce am. In .
2014,
40
, 12207– 12211.
[16] K. R. P iolka , P. Be a, P. R. Sa ode, M. S. Hegde, S. Emu a, R. Kumashi o,
N. P. Lalla,
Chem. Ma e .
2002,
14
, 2120 –2128.
[17] S. Colussi, A. T o a elli, E. Vesselli, A. Ba aldi, G. Comelli, G. G oppi, J.
Llo ca,
Appl. Ca al. A
2010,
390
, 1 –10.
[18] a) Y. R. Wu, W. Y. Hu, S. C. Han,
J. Alloys Compd.
2006,
420
, 83– 93;
b) V. L. Vinog ad, M. H. F. Slui e , B. Winkle ,
Phys. Re . B
2009,
79
,
104201.
[19] a) G. Chen, W.-T. Chou, C. T. Yeh,
Appl. Ca al.
1983,
8
, 389 –397; b) T. C.
Chang, J. J. Chen, C. T. Yeh,
J. Ca al.
1985,
96
, 51– 57; c) H. Lieske, J.
Vol e ,
J. Phys. Chem.
1985,
89
, 1841 –1842.
[20] A. T o a elli,
Ca al. Re .
1996,
38
, 439 –520.
[21] M. F. Luo, X. M. Zheng,
Appl. Ca al. A
1999,
189
, 15– 21.
[22] S. Be nal, J. J. Cal ino, M. A. Cauqui, J. M. Ga ica, C. La ese, J. A. P. Omil,
J. M. Pin ado,
Ca al. Today
1999,
50
, 175 –206.
[23] L. Kepin´ ski, M. Wolcy z, J. Okal,
J. Chem. Soc. Fa aday T ans.
1995,
91
,
507 –515.
[24] W. C. Conne , J. L. Falcone ,
Chem. Re .
1995,
95
, 759 –788.
[25] X. Y. Chen, J. W. Schwank, G. B. Fishe , Y. S. Cheng, M. Jagne , R. W.
McCabe, M. B. Ka z, G. W. G aham, X. Q. Pan,
Appl. Ca al. A
2014,
475
,
420 –426.
[26] O. Demoulin, G. Rupp ech e , I. Seunie , B. Le Cle , M. Na ez, P. Ruiz,
J.
Phys. Chem. B
2005,
109
, 20454– 20462.
[27] a) R. J. Fa au o, M. C. Hobson, T. Kennelly, E. M. Wa e man,
Appl. Ca al.
A
1992,
81
, 227 –237; b) J. G. McCa y,
Ca al. Today
1995,
26
, 283 –293.
[28] M. Ca gnello, J. J. D. Jaen, J. C. H. Ga ido, K. Bakhmu sky, T. Mon ini,
J. J. C. Gamez, R. J. Go e, P. Fo nasie o,
Science
2012,
337
, 713 –717.