Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
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E ec o in e ac ion deg ee be ween Mn and Ce o MnO
X
-CeO
2
o mula ion
on NO educ ion and o-DCB oxida ion pe o med simul aneously
J.A. Ma ín-Ma ín, M.P. Gonz´
alez-Ma cos, A. A anzabal
*
, J.R. Gonz´
alez-Velasco
G oup o Chemical Technologies o En i onmen al Sus ainabili y, Depa men o Chemical Enginee ing, Facul y o Science and Technology, The Uni e si y o he Basque
Coun y, UPV/EHU, P.O. Box 644, E-48080 Bilbao, Spain
ARTICLE INFO
Edi o : Fumi ake Takahashi
Keywo ds:
MnO
X
-CeO
2
P epa a ion me hod
NO educ ion
O-DCB oxida ion
Mn and Ce in e ac ion
ABSTRACT
MnO
X
-CeO
2
ca alys s we e p epa ed by di e en me hods in o de o assess he in luence o Mn and Ce in e -
ac ion on he ca aly ic ac i i y o NO educ ion and 1,2-dichlo obencene (o-DCB) oxida ion ca ied ou simul-
aneously. Changes on ca aly ic p ope ies we e e alua ed by XRD, Raman, N
2
-physiso p ion, H
2
-TPR and NH
3
-
TPD. The samples p epa ed by co-p ecipi a ion and sol-gel exhibi ed he bes ca aly ic ac i i y o e he whole
empe a u e ange. These p epa a ion me hods p o ide high in e ac ion be ween Mn and Ce a su ace and bulk,
which was e idenced by he o ma ion o mixed oxide phase. Acco ding o cha ac e isa ion, he enhancemen o
ca aly ic ac i i y wi h Mn and Ce in e ac ion is associa ed o he p omo ion o s uc u al de ec s, which imp o e
edox and acid p ope ies. Oxida i e capabili y is also imp o ed as a consequence o he oxygen acancies
gene a ed by he p esence o hose s uc u al de ec s. On he o he hand, he p omo ion o Mn and Ce in e ac ion
a ou s he p oduc ion o N
2
O in NO educ ion a a lowe empe a u e, and also leads o o-DCB o al oxida ion by
dec easing CO p oduc ion.
1. In oduc ion
Cu en was e managemen egula ions es ablish a hie a chy in
which any way o eco e y is encou aged be o e disposal [1]. In his
con ex , municipal solid was e incine a ion is p omo ed o e land illing
because i allows he possibili y o eco e ing he ene gy eleased in he
combus ion o was e o p oduce hea and/o elec ici y. Howe e ,
incine a ion p ocesses gene a e a wide a ie y o pollu an s among
which NO
X
and PCDD/Fs a e some o he mos ha m ul o he en i on-
men and li ing beings.
In incine a ion plan s, NO
X
a e emo ed om he combus ion
exhaus gases by selec i e educ ion wi h ammonia p ecu so com-
pounds, such as u ea, ammonium hyd oxide, e c. The e a e wo p o-
cesses: he non-ca aly ic educ ion, ha occu s by adding he educing
agen in o he u nace; and he ca aly ic educ ion, ha equi es a spe-
ci ic ca aly ic uni o his pu pose. Selec i e ca aly ic educ ion (SCR)
ea u es highe e iciency and educing agen sa ings [2,3], hence i is
p omo ed in he ligh o he expec ed igh ening o emission limi s in he
u u e. On he o he hand, PCDD/Fs a e usually emo ed by adso p ion
p ocesses which allow o mee he emission limi s, bu ha e he disad-
an age o no des oying he pollu an (which is only mo ed om he
gas phase o he adso ben ) and gene a e a was e ha needs o be ea ed
[4,5].
The con en ional ca alys o SCR in s a iona y sou ces is based on V
oxide as he ac i e phase and p omo ed wi h W and/o Mo oxide [2].
This ca aly ic o mula ion has also been epo ed o be ac i e in he
emo al, by ca aly ic oxida ion, o PCDD/Fs bo h a ull and pilo scale
[6–8]. This abili y o he SCR ca alys b ings he possibili y o aba ing
simul aneously wo di e en pollu an s, such as NO
X
and PCDD/Fs
(and/o chlo ina ed o ganic compounds) in a single s age. In ac , his
p ocess has al eady been conside ed in he Bes A ailable Technique
Re e ence Documen o Was e Incine a ion by he Join Resea ch
Cen e, Eu opean Commission’s in-house science se ice [9].
Cu en ly, in o de o pe o m he SCR a lowe empe a u es, some
al e na i e o mula ions o he con en ional SCR ca alys a e unde
in es iga ion. These al e na i e ca aly ic o mula ions mainly s udy Mn
and Fe oxides as ac i e componen s [10–13] and hei combina ion wi h
o he ansi ion me als, such as Cu [14], Ce [15–17], Co [18,19], e c.,
whose in e ac ions wi h he ac i e me al can imp o e he ca aly ic
pe o mance. Mo eo e , exchanged zeoli es wi h Fe and Cu a e also
conside ed, because o hei good ca aly ic pe o mance in he high
empe a u e ange [10,20]. Among hem, MnO
X
-based ca alys s a e
* Co esponding au ho .
E-mail add ess: [email p o ec ed] (A. A anzabal).
Con en s lis s a ailable a ScienceDi ec
Jou nal o En i onmen al Chemical Enginee ing
jou nal homepage: www.else ie .com/loca e/jece
h ps://doi.o g/10.1016/j.jece.2023.110200
Recei ed 22 Decembe 2022; Recei ed in e ised o m 27 Ap il 2023; Accep ed 24 May 2023
Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
2
ex ensi ely s udied due o hei high ca aly ic ac i i y o low empe -
a u e SCR, especially hose doped wi h Ce. Ce-doping imp o es oxygen
s o age and mobili y along he ca aly ic s uc u e a bo h su ace and
bulk, which a o s edox and acid p ope ies. Hence, Mn and Ce com-
posi e oxides a e epo ed o each ull NO con e sion be ween 100 and
200ºC [15,17,21].
Mn and Ce composi e oxides a e also epo ed o be highly ac i e o
oxida ion eac ions, which means ha NO
X
and PCDD/Fs can also be
aba ed simul aneously. Howe e , he majo i y o wo ks in he li e a u e
ocus on he s udy o ca aly ic oxida ion ac i i y wi h model compounds
o PCDD/Fs and in he absence o NO
X
. In his ega d, Liu and co-
wo ke s epo ed o al con e sion in o-DCB oxida ion wi h MnO
X
as
he ac i e phase suppo ed o e se e al ma e ials [22]. Ca aly ic ac i i y
o Mn-based ca alys s is imp o ed by doping wi h one o se e al an-
si ion me als, such as Cu, Ti and Co [23–25]. None heless, Mn doped
wi h Ce is he mos e e enced o mula ion and i has been ex ensi ely
s udied in he ca aly ic oxida ion o di e en VOCs [26,27], such as
hexane, oluene, e c.; and chlo ina ed o ganic compounds, such as
chlo ina ed benzene and ichlo oe hylene [28,29]. Mo eo e , Ma -
ín-Ma ín and co-wo ke s [30] s udied he easibili y o MnO
X
-CeO
2
o mula ion in he simul aneous emo al o NO and o-DCB and
co obo a ed ha ca alys s wi h high Mn con en s allow high con e sion
o bo h pollu an s in he same empe a u e ange, be ween 200 and 250
ºC.
The e o e, Mn and Ce composi e oxides a e conside ed o be ac i e in
bo h low empe a u e SCR and ca aly ic oxida ion eac ions. None he-
less, he in e ac ion be ween ac i e me als is well known o play a key
ole in ca aly ic o mula ions based on composi e oxides because i a -
ec s hei s uc u al and mo phological p ope ies, which de ine
educibili y and acidi y. In his sense, he p epa a ion me hod is e-
po ed o be one o he a iables mos s ongly a ec ing he in e ac ion
be ween ac i e me als: i can p omo e su ace and/o bulk in e ac ion,
and e en he o ma ion o a mixed oxide phase by he inco po a ion o
one o he ac i e me als o he s uc u e o he o he . Acco ding o he
li e a u e, se e al p epa a ion me hods ha e been s udied o he syn-
hesis o Mn and Ce composi e oxides, such as imp egna ion [31], edox
p ecipi a ion [21], sol-gel [16]. None heless, he e is no ag eemen on
which is he mos app op ia e o pe o m SCR and ca aly ic oxida ion
eac ions.
The main goal o his wo k is o e alua e se e al p epa a ion
me hods o MnO
X
-CeO
2
ca aly ic o mula ion in o de o assess he
in luence o Mn and Ce in e ac ion on he ca aly ic ac i i y o NO
educ ion and o-DCB oxida ion ca ied ou simul aneously.
2. Expe imen al
2.1. Ca alys p epa a ion
Mn and Ce composi e oxides we e syn he ized by se e al p epa a ion
me hods, which we e selec ed because hey p o ide di e en deg ees o
in e ac ion be ween Mn and Ce in he esul ing samples, as well as hey
a e simple, ep oducible and easy o scale. Pu e ce ium oxide and pu e
manganese oxide we e also p epa ed.
Pu e oxides we e p epa ed by p ecipi a ion and using Mn(NO
3
)
2
⋅4
H
2
O o Ce(NO
3
)
3
⋅6 H
2
O as p ecu so s. In his p ocedu e, he p ope
amoun o he p ecu so compounds was sol ed in dis illed wa e a
oom empe a u e, ob aining p ecu so solu ions o Mn o Ce a ound
0.55 and 0.35 M, espec i ely. Subsequen ly, he p ecipi a ions o me al
ca ions we e ca ied ou by d opwise addi ion o a 1.3 M solu ion o
ammonium ca bama e (H
2
NCOONH
4
), un il a pH alue o 9 was
eached. The esul ing suspensions we e aged o 2 h and, subsequen ly,
il e ed and washed wi h dis illed wa e .
Fo he p epa a ion o Mn and Ce composi e oxides, ou p epa a ion
me hods we e used: mechanical mixing, imp egna ion, co-p ecipi a ion
and sol-gel. Mn and Ce con en s we e selec ed wi h he in en ion o
ob ain a simila concen a ion o bo h me als a he ca aly ic su ace. In
his way, an equimola concen a ion o Mn and Ce was selec ed o all
bime allic samples, excep o ha p epa ed by imp egna ion in which a
2% Mn (w %) was used ( he one co esponding o he heo e ical hal
monolaye co e age o he CeO
2
su ace). Nex , he expe imen al p o-
cedu es o each p epa a ion me hod a e de ailed:
2.1.1. Mechanical mixing
This me hod consis ed o he physical mixing o pu e manganese and
ce ium oxides a e being calcina ed and sie ed.
2.1.2. Imp egna ion
The p epa a ion was ca ied ou in a o a y e apo a o . Fi s ly, he
p ope amoun o suppo (CeO
2
, esul ing om di ec calcina ion o Ce
(NO
3
)
3
⋅6 H
2
O a 550 ºC o 3 h) was p e- ea ed in acuum condi ions a
40 ºC o 1 h. Secondly, a solu ion wi h he p ope amoun o Mn p e-
cu so was pou ed o he suppo . The esul ing slu y was homoge-
neously s i ed o 1 h, hen, he empe a u e was inc eased up o 40 ºC
o a ou he comple e e apo a ion o he sol en .
2.1.3. Co-p ecipi a ion
The p ocedu e is simila o ha desc ibed abo e o pu e oxides, he
only di e ence being ha he p ecu so solu ion, o which he p ecipi-
a ing agen is added, con ains bo h Mn and Ce p ecu so s.
2.1.4. Sol-gel
In his me hod, he app op ia e amoun o Mn and Ce p ecu so s was
sol ed in dis illed wa e a oom empe a u e. Ci ic acid, o a mola
concen a ion equal o 0.3(Mn+Ce), was also added as chela ing agen .
Subsequen ly, he solu ion empe a u e was p og essi ely inc eased o
80 ºC unde con inuous s i ing, o e apo a e he sol en and lead o he
o ma ion o he gel.
Finally, he esul ing solid om each p epa a ion me hod was d ied
o e nigh a 110 ºC and calcined a 500 ºC o 3 h wi h a hea ing amp o
1 ºC/min in s a ic ai . Then, he ca alys was sie ed o 0.3–0.5 mm.
2.2. Ca alys cha ac e iza ion
The di e en c ys al phases o he ca alys s we e iden i ied by X- ay
di ac ion (XRD). The analysis was ca ied ou on a Philips PW 1710 X-
ay di ac ome e wi h Cu K
α
adia ion (λ =1.5406 Å) and Ni il e .
Each measu emen was anged be ween 20 and 60º (2θ) wi h s ep size o
0.026º and coun ing ime o 528 s. Iden i ica ion o he c ys al phases
was made by compa ing wi h JCPDS da abase ca ds.
Raman spec oscopy analysis was pe o med in a Renishaw Sys em
1000 Raman spec ome e wi h a 706 nm solid-s a e lase used as
exci a ion line, dosing a powe on he sample o 1 mW. Each measu e-
men was pe o med a oom empe a u e wi h 20 s pe scan and 5
accumula ed scans.
N
2
adso p ion-deso p ion iso he ms a –196 ºC we e used o e alua e
ex u al p ope ies o he samples. The measu emen s we e ca ied ou
on a Mic ome i ics TRISTAR II 3020. Be o e each analysis, he samples
we e p e- ea ed a 350 ºC o 4 h wi h a low o N
2
. Speci ic su ace a ea
was calcula ed acco ding o BET p ocedu e, using he da a ob ained
om he adso p ion iso he m b anch in he ela i e p essu e be ween
0.03 and 0.3. On he o he hand, BJH me hod was used o es ima e po e
a e age size and dis ibu ion, using he da a om he deso p ion
iso he m b anch.
Redox p ope ies o he ca alys s we e assessed by empe a u e
p og ammed educ ion wi h H
2
(H
2
-TPR). A Mic ome i ics Au oChem
2920 ins umen was used o ca y ou hese expe imen s. Fi s ly, he
samples (15–20 mg) we e p e- ea ed a 500 ºC o 45 min wi h 50 cm
3
/
min o 5% O
2
/He mix u e and, hen, cooled down o 100 ºC in helium.
A e ha , he samples we e hea ed om 100 o 900 ºC a 10 ºC/min
unde 50 cm
3
/min o 5% H
2
/A . H
2
consump ion in he expe imen was
eco ded wi h a TDC. In o de o a oid in e e ences be ween wa e
p oduced and he TCD measu emen , wa e was apped in a cold ap.
J.A. Ma ín-Ma ín e al.
Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
3
Time in eg a ion o TCD signal allowed o calcula e he o al H
2
consumed.
Tempe a u e p og ammed deso p ion o ammonia (NH
3
-TPD) was
used o s udy he acidi y o he ca alys s. A Mic ome i ics Au oChem
2920 ins umen was used o ca y ou he NH
3
-TPD expe imen s. All
samples (15–20 mg) we e p e- ea ed a 500 ºC o 45 min wi h 50 cm
3
/
min o 5% O
2
/He mix u e and, hen, cooled down o 50 ºC in helium.
The adso p ion s ep o ammonia was pe o med by eeding 130 cm
3
/min
o 1% NH
3
/He mix u e gas a 50 ºC du ing 60 min. Ammonia weakly
adso bed on he su ace o he samples was emo ed wi h helium (130
cm
3
/min o 60 min). Finally, ammonia deso p ion was pe o med by
hea ing he sample om 50 o 500 ºC (10 ºC/min) and using helium as a
ca ie . A TCD was used o eco d he NH
3
deso bed, and he o al
acidi y was calcula ed by ime-in eg a ion o TCD signal.
2.3. Reac ion se -up and ca aly ic es s
The expe imen al eac ion se -up allows o p epa e a gas eeding
s eam wi h simila componen s and concen a ions o hose ound a
he inle o a SCR uni wi h ail-end con igu a ion a an incine a ion
plan . The lows o gaseous and liquid eagen s a e egula ed by mass
low con olle s (B onkho s ® High-Tech F-201CV and B onkho s ®
High-Tech
μ
-Flow L01-AAA-99–0–20S, espec i ely). The comple e
e apo a ion o liquid componen s in he sys em is ensu ed by a
con olled-e apo a o -mixed (B onkho s ® High-Tech W-102A-111-K),
which a o s a homogenous mix u e wi h he gas s eam as well. All
pipes in he expe imen al se -up a e hea ed wi h elec ical esis ances o
a oid gas adso p ion and condensa ion.
Fo he ca aly ic ac i i y es s, he eeding s eam was composed by
NO (300 ppm), NH
3
(300 ppm), O
2
(10%), o-DCB (100 ppm) and A o
balance. I is impo an o ema k ha o-DCB was used as model com-
pound PCDD/Fs because o he high oxici y and ope a ional p oblems
ha in ol es wo king wi h PCDD/Fs a lab le el. Mo eo e , al hough
CO and CO
2
a e componen s o he gas eeding s eam a ull scale, hey
we e no conside ed in his wo k because we ha e al eady checked hei
p esences do no a ec he eac ions unde s udy and makes i di icul
o analyze he selec i i y o he oxida ion eac ion.
On he o he hand, he ca aly ic bed is loca ed inside a ubula U-
shape qua z eac o , which is in u n placed inside a con ec i e low
o en. The ca aly ic bed is composed o 1.5 g o ca alys wi h 0.3–0.5 mm
size dilu ed in ine qua z un il illing a bed olume o 3 cm
−3
. Be o e
each expe imen , he ca aly ic bed was p e- ea ed a 200 ºC wi h a low
o 2 L
N
/min o pu e A o 2 h in o de o clean he ca aly ic su ace. The
ca aly ic ac i i y has been e alua ed by ligh -o expe imen s wi h a
eeding s eam o 2 L
N
/min (GHSV 40,000 h
−1
) a a p essu e o 1.5 a m
and inc easing eac ion empe a u e om 100 o 450 ºC wi h a hea ing
amp o 1.5 ºC/min.
Fo he analysis o eac ion eagen s and p oduc s, se e al analyze s
we e used: an ABB Limas 21 o con inuously measu e he concen a ion
o NO, NO
2
and NH
3
; an ABB U as 26 In a ed analyze o con inuously
measu e he concen a ion o N
2
O, CO and CO
2
; and, inally, a gas
ch oma og aph (Agilen Technologies 7890 A) coupled o a mass se-
lec i e de ec o (Agilen Technologies 5975 C) o quan i y o-DCB con-
cen a ion and he chlo ina ed o ganic by-p oduc s o o-DCB oxida ion.
NH
3
concen a ion could no be quan i ied as a consequence o o-DCB
in e e ence in ABB Limas 21.
NO and o-DCB con e sion was calcula ed om Eqs. 1 and 2,
espec i ely. Selec i i y o CO
2
and CO was calcula ed h ough Eqs. 3
and 4, espec i ely.
XNO =CNO,in −CNO,ou
CNO,in
⋅100 (1)
XoDCB =Co−DCB,in −Co−DCB,ou
Co−DCB,in
⋅100 2)
SCO2=Cco2,ou
6⋅Co−DCB,in −Co−DCB,ou
⋅100 (3)
SCO =Cco,ou
6⋅(Co−DCB,in −Co−DCB,ou )⋅100 (4)
3. Resul s and discussion
3.1. Ca alys cha ac e iza ion
The in luence o p epa a ion me hod on ca aly ic s uc u al p ope -
ies has been i s ly s udied by XRD. Fig. 1 shows he di ac ion pa e ns
o he composi e oxides p epa ed by he s udied p epa a ion me hods,
and wo addi ional ones co esponding o pu e ce ium and manganese
oxides. Pu e ce ium oxide exhibi s di ac ion peaks associa ed o luo-
i e cubic s uc u e, whe eas he di ac ion peaks o pu e manganese
oxide a e associa ed o
α
-Mn
2
O
3
c ys al phase.
The sample p epa ed by mechanical mixing shows di ac ion peaks
o bo h luo i e and
α
-Mn
2
O
3
, as a consequence o he physical mixing o
hese pu e oxides. Howe e , he di ac ion pa e ns o he samples
p epa ed by imp egna ion, co-p ecipi a ion and sol-gel indica e he
majo i y p esence o luo i e phase, which is cha ac e is ic o pu e
ce ium oxide. The absence o manganese oxide di ac ion peaks in he
sample p epa ed by imp egna ion is associa ed o i s low Mn con en . In
he case o he samples p epa ed by co-p ecipi a ion and sol-gel, his ac
is associa ed o Mn inco po a ion o luo i e s uc u e o ming a mixed
oxide phase o solid solu ion [30,32,33], as deno es he b oaden o
luo i e di ac ion peaks and he sligh shi o highe B agg angles o
he s onges peak o his c ys al phase wi h espec o pu e ce ium
oxide, clea ly no iceable in he inse g aph o Fig. 1. None heless, he
sample p epa ed by co-p ecipi a ion also shows some e y weak peaks
a ound 33 and 38º, which sugges s ha a e y small pa o Mn has
seg ega ed as Mn
2
O
3
.
In o de o e alua e he s uc u al dis o ion caused by he o ma ion
o he mixed oxide phase, he la ice pa ame e o luo i e phase was
es ima ed and is epo ed in Table 1. The alues o ca alys s p epa ed by
co-p ecipi a ion and sol-gel a e smalle han ha o pu e ce ium oxide.
This esul is associa ed o he con ac ion o luo i e s uc u e as a
consequence o he smalle ionic adius o Mn ions wi h espec o Ce
ions [28], hence i co obo a es he o ma ion o a mixed oxide phase.
Fig. 1. XRD pa e ns o MnO
X
-CeO
2
samples p epa ed by di e en p epa a ion
me hods. Inse igu e: zoom o 2ϴ egion be ween 26 and 32º.
J.A. Ma ín-Ma ín e al.
Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
4
On he con a y, he la ice pa ame e o he ca alys s p epa ed by
mechanical mixing and imp egna ion is close o ha o pu e ce ium
oxide, which means ha he amoun o Mn inco po a ed o CeO
2
s uc u e is e y low o p ac ically null.
Mo eo e , Table 1 epo s he c ys al size o luo i e phase, which
was es ima ed by applying Sche e equa ion o he mos in ense peak o
his c ys al phase, ha is, he one loca ed a 28.6º. The samples p epa ed
by mechanical mixing and imp egna ion exhibi a c ys al size simila o
ha o pu e ce ium oxide, whe eas he samples p epa ed by co-
p ecipi a ion and sol-gel show a lowe alue. This esul is in acco -
dance wi h he end in la ice pa ame e explained abo e, and is
associa ed o s uc u al de ec s gene a ed by Mn inco po a ion o he
luo i e s uc u e, which has a nega i e e ec on he g ow h o luo i e
c ys al domains o ce ium oxide.
S uc u al p ope ies o he ca alys s we e also cha ac e ized by
Raman spec oscopy. The spec a o he samples p epa ed by di e en
me hods and he wo pu e oxides a e shown in Fig. 2. Raman spec um
o pu e ce ium oxide has an in ense peak a 460 cm
−1
ela ed o F
2 g
mode o CeO
2
[34]. On he o he hand, pu e manganese oxide exhibi s
se e al peaks a 693, 640 and 306 cm
−1
co esponding o
ν
7
,
ν
6
and
ν
2
ib a ional modes o
α
-Mn
2
O
3
(in ag eemen wi h XRD, whe e his
manganese oxide phase was he only one iden i ied) [35].
The F
2 g
band is common in all Raman spec a o he ca alys s p e-
pa ed by di e en me hods, al hough, in e es ingly, i s loca ion sligh ly
changes depending on he samples (Table 1). The posi ion in he sample
p epa ed by mechanical mixing is he same as ha o pu e ce ium oxide,
whe eas i is shi ed o lowe wa enumbe s in all he o he bime allic
samples, al hough his shi becomes la ge in hose p epa ed by co-
p ecipi a ion and sol-gel. F
2 g
band co esponds o he a oms
belonging o he s uc u e a ound Ce
4+
and hei symme ical s e ching
ib a ion [36]. Thus, he displacemen jus discussed abo e e idences an
al e a ion in he CeO
2
la ice s uc u e as a esul o he o ma ion o
mixed oxide phase, which in u n is in ag eemen wi h he esul s ob-
ained om XRD.
Mo eo e , Raman spec a o he samples p epa ed by co-
p ecipi a ion and sol-gel exhibi a b oad band in he 700–600 cm
−1
wa enumbe egion. This band is p obably composed by wo con ibu-
ions: one associa ed o manganese oxide (i has been p e iously
obse ed in he spec um o pu e manganese oxide), and o he associ-
a ed o s uc u al de ec s o luo i e phase, i.e., oxygen acancies, which
a e epo ed in he li e a u e a ound 600 cm
−1
[36]. Focusing on oxygen
acancies, hey appea as a compensa ion o he nega i e cha ge p o-
duced by he inco po a ion o a la ice s uc u e o a doping ca ion wi h
di e en na u e and oxida ion s a e (in his speci ic case, he Mn
n+
inco po a ion o CeO
2
la ice s uc u e) [32]. The e o e, he appea ance
o oxygen acancies is ano he e idence o he o ma ion o he mixed
oxide phase.
Since oxygen acancies a e linked o s uc u al de ec s, he size o
luo i e c ys al domains will be a ec ed, as i was p e iously obse ed
by XRD. This ac could be also co obo a ed ollowing he FWHM o he
F
2 g
peaks, which is epo ed in Table 1. The samples p epa ed by me-
chanical mixing and imp egna ion ha e a simila FWHM as pu e ce ium
oxide, whe eas he FWHM alue o hose samples p epa ed by co-
p ecipi a ion and sol-gel is much la ge . This esul e idences a
dec ease in he size o luo i e c ys al domain, bu only in he samples
p epa ed by he me hods ha p omo e a mixed oxide phase.
Tex u al p ope ies o he samples we e s udied by N
2
-physiso p ion.
Fig. 3A shows he ob ained adso p ion/deso p ion iso he ms. Acco ding
o IUPAC classi ica ion, all samples exhibi ype IV iso he ms wi h H3
hys e esis loop in he ela i e p essu e be ween 0.4 and 1, which a e
cha ac e is ics o mesopo ous ma e ials.
Calcula ed speci ic su ace a eas and po e olumes a e epo ed in
Table 1. The sample p epa ed by co-p ecipi a ion exhibi s he highes
speci ic su ace a ea and po e olume, whe eas he emaining bime allic
samples show alues anged be ween pu e ce ium oxide and manganese
oxide. These esul s a e e y in e es ing because hey show no iceable
di e ences a he mo phological le el be ween hose ca alys s p epa ed
by co-p ecipi a ion and sol-gel, despi e hei simila s uc u al p ope -
ies ( o ma ion o a mixed oxide phase as a consequence o he Mn
inco po a ion in o CeO
2
s uc u e). The e o e, his ac co obo a es he
key ole o p epa a ion me hod on ex u al p ope ies.
Fig. 3B shows he po e dis ibu ion o he s udied ca alys s. All ca -
alys s exhibi a po e dis ibu ion cha ac e is ic o mesopo ous solids, in
acco dance wi h he ype o iso he m. None heless, p epa a ion me hods
a ec po e size dis ibu ion o he samples. The ca alys p epa ed by
mechanical mixing p esen s a po e size dis ibu ion which is a combi-
na ion o hose o he pu e oxides, whe eas ha p epa ed by imp eg-
na ion exhibi s po es wi h size a ound 10 nm. The ca alys s p epa ed by
co-p ecipi a ion and sol-gel show a po e dis ibu ion composed by wo
po e sizes: one a ound 4 nm, which is common o bo h ca alys s; and
he o he one a ound 25 nm (co-p ecipi a ion) o 10 nm (sol-gel). These
di e ences in po e sizes could be ela ed o he di e en mo phology
p o ided by each p epa a ion me hod.
Rega ding po e dis ibu ion, i is impo an o no e he e a e some
disc epancies in he li e a u e abou he p esence o po es wi h size
a ound 4 nm in mesopo ous ma e ials when he hys e esis loop is
Table 1
S uc u al and ex u al p ope ies o MnO
X
-CeO
2
samples p epa ed by di e en me hods.
Sample Fluo i e la ice pa ame e (nm) Fluo i e c ys al size (nm) F
2 g
posi ion (cm
−1
) FWHM o F
2 g
(cm
−1
) S
BET
(m
2
/g) V
p
(cm
3
/g)
CeO
2
5.4135 9 462 19 80 0.12
Sol-gel 5.3854 3 457 56 63 0.16
Imp egna ion 5.4161 8 460 23 63 0.17
Co-p ecipi a ion 5.4035 4 456 41 88 0.21
Mechanical mixing 5.4117 10 462 20 67 0.14
MnO
X
– – – – 34 0.19
Fig. 2. Raman spec a o MnO
X
-CeO
2
samples p epa ed by di e en me hods.
J.A. Ma ín-Ma ín e al.
Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
5
ab up ly closed in he P/P
0
ange a ound 0.45 [37], as occu s in some o
he samples he e s udied. This phenomenon is usually ela ed o ensile
s eng h e ec . In his sense, he inse g aph in Fig. 3 A shows a zoom o
he iso he ms in he P/P
0
egion be ween 0.3 and 0.6 o he ca alys s
p epa ed by di e en p epa a ion me hods. Compa ing he inse g aph
and Fig. 3B, i can be seen ha he mo e ab up ly he iso he m is closed,
he mo e no iceable is he peak associa ed wi h po es o 4 nm. The e-
o e, hese esul s sugges ha po es o 4 nm a e no ep esen a i e o
ex u al p ope ies o he samples s udied.
H
2
-TPR was used o in es iga e edox p ope ies. The educ ion
p o iles o pu e oxides, showed in Fig. 4, a e qui e di e en : pu e ce ium
oxide exhibi s wo b oad educ ion peaks a 430 and 830 ºC ela ed o
su ace and bulk ce ium oxide educ ion [17], espec i ely; whe eas he
educ ion p o ile o pu e manganese oxide is composed by wo peaks a
290 and 420 ºC, which a e associa ed o he educ ion o Mn
2
O
3
o
Mn
3
O
4
and Mn
3
O
4
o MnO, espec i ely [38].
The educ ion p o iles o bime allic samples p epa ed by di e en
me hods exhibi wo s ong educ ion peaks below 500 ºC, mainly
associa ed o he educ ion o Mn and, o a lesse ex en , o su ace Ce
educ ion, al hough he la e con ibu ion is indis inguishable. In he
case o he sample p epa ed by imp egna ion, H
2
up ake in his em-
pe a u e ange is e y low as a consequence o i s low Mn con en (only
su ace). On he o he hand, a high empe a u e, all he samples show
he educ ion peaks o bulk ce ium educ ion. In e es ingly, Mn educ-
ion o bime allic samples p epa ed by co-p ecipi a ion and sol-gel s a s
a lowe empe a u e wi h espec o pu e oxides and he bime allic
sample p epa ed by mechanical mixing. This esul sugges s ha co-
p ecipi a ion and sol-gel me hods lead o an imp o emen in he
educibili y o he samples due o mixed oxide phase p omo ion. Mixed
oxide phase has been co obo a ed by Raman o gene a e oxygen a-
cancies, which imp o es he mobili y o oxygen in he la ice s uc u e
and, consequen ly, acili a es he educ ion o he sample.
The imp o emen o edox p ope ies as a esul o he p omo ion o
mixed oxide phase is no only e idenced by Mn educ ion a lowe
empe a u es, bu also by he shape o educ ion p o iles. Thus, he
ca alys s p epa ed by co-p ecipi a ion and sol-gel show a la ge deg ee o
o e lap in he educ ion peaks associa ed o Mn educ ion, no obse ed
in he sample p epa ed by mechanical mixing no in he pu e Mn oxide.
Mo eo e , he samples p epa ed by co-p ecipi a ion and sol-gel show a
small H
2
up ake a ound 100 ºC, which is associa ed in he li e a u e o
he educ ion o su ace Mn embedded in CeO
2
s uc u e [39,40].
On he o he hand, Table 2 shows he a e age oxida ion s a e o Mn,
which was es ima ed om he o al H
2
consump ion wi h some as-
sump ions, such as conside ing ha he H
2
consump ion o ce ium a low
empe a u e is negligible (compa ed o ha o Mn) and MnO is he inal
educ ion s a e o Mn. The oxida ion s a e o pu e manganese oxide is
e y close o 3, which is in acco dance wi h XRD esul s, whe e Mn
2
O
3
was he only c ys al phase iden i ied. The bime allic samples p epa ed
by di e en me hods show oxida ion s a es o Mn be ween 3 and 4.
In e es ingly, he oxida ion s a e o Mn is signi ican ly highe and close
o 4 in he ca alys s p epa ed by co-p ecipi a ion and sol-gel, whe eas i
is close o 3 o he sample p epa ed by mechanical mixing. This esul
sugges s ha he p omo ion o mixed oxide phase a ou s he p esence
o Mn in highe oxida ion s a e.
Since he es ima ion o Mn oxida ion s a e in bime allic samples has
Fig. 3. A) N
2
physiso p ion iso he ms. Inse igu e: de ailed iew o he iso he ms in he P/P
0
ange 0.3–0.6. B) Po e size dis ibu ion o MnO
X
-CeO
2
samples
p epa ed by di e en me hods.
Fig. 4. H
2
-TPR p o iles o MnO
X
-CeO
2
samples p epa ed by di e en me hods.
J.A. Ma ín-Ma ín e al.
Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
6
e idenced he p esence o Mn
4+
species, he assignmen o he educ ion
peaks used o he pu e manganese oxide would no be app op ia e o
bime allic samples, because Mn
4+
species would no be conside ed. In
his sense, se e al wo ks in he li e a u e ag ee ha Mn
4+
educ ion is
indis inguishable om ha o Mn
3+
, so H
2
up ake in he s ong educ-
ion peak loca ed a lowe empe a u e is associa ed o he educ ion o
Mn
4+
/Mn
3+
o Mn
3
O
4
, and H
2
up ake in s ong peak placed a highe
empe a u e o he educ ion o Mn
3
O
4
o MnO [28,41,42].
Acid p ope ies we e e alua ed by NH
3
-TPD, and he NH
3
deso p ion
p o iles o he samples a e shown in Fig. 5. Pu e oxides exhibi wo
impo an NH
3
deso p ion zones: a common one a ound 100 ºC associ-
a ed o NH
3
adso bed o e weak acid si es, and ano he one be ween 150
and 250 ºC o pu e ce ium oxide and a 180 ºC o pu e manganese
oxide, associa ed o NH
3
adso bed o e s ong acid si es [43]. Mo eo e ,
a highe empe a u es, a small NH
3
deso p ion is also obse ed be ween
300 and 400 ºC in bo h pu e oxides, which sugges s he p esence o a
small amoun o e en s onge acid si es. A simila esul was obse ed
by Yao and co-wo ke s [44] a e s udying he NH
3
adso p ion-deso p ion o e pu e CeO
2
. Rega ding bime allic samples
p epa ed by di e en me hods, all o hem exhibi he NH
3
deso p ion
peak a ound 100 ºC associa ed o weak acid si es. Howe e , he
deso p ion peak associa ed o s ong acidi y is ound o be shi ed o-
wa ds highe empe a u es in he ca alys s p epa ed by imp egna ion
and co-p ecipi a ion. This esul e idences ha he p esence o su ace
Mn and Ce a ou s s onge acid si es. In he case o he sample p epa ed
by sol-gel, he peak ela ed o s ong acidi y is placed a simila em-
pe a u e han pu e ce ium oxide, al hough bo h Mn and Ce a e p esen
in he ca aly ic su ace o his sample. This esul could be ela ed o
sol-gel led o he highes dis o ion o luo i e phase a e Mn inco po-
a ion, which caused a dec ease in he c ys allini y o he sample.
Table 2 summa izes o al acidi y o he samples pe uni a ea. The
eason o e alua ing he acidi y pe uni a ea is o sup ess he con i-
bu ion o ca aly ic su ace a ea o acidi y, hus analysing exclusi ely he
e ec o p epa a ion me hod. Mn is he me al ha con ibu es mos o
he o al acidi y, since pu e manganese oxide is he mos acidic sample.
Among bime allic samples, hose p epa ed by co-p ecipi a ion and sol-
gel a e he mos acidic ones. This esul e idences he o ma ion o
mixed oxide phase p omo es o al acidi y compa ed o he o he bime-
allic samples.
Table 2 also epo s he con ibu ion o p epa a ion me hods o weak
and s ong acidi y. Weak acidi y was conside ed o be ha om NH
3
deso bed below 170 ºC, while NH
3
deso p ion abo e ha empe a u e
was ela ed o s ong acidi y. In e es ingly, he con ibu ion o s ong
acidi y o weak acidi y is highe o bime allic samples (especially o
hose p epa ed by imp egna ion, co-p ecipi a ion and sol-gel) han o
pu e oxides. This ac deno es a p omo ion o he s ong acidi y as a
consequence o he p esence o su ace Mn and Ce, which is in acco -
dance wi h he no able con ibu ion o s ong acidi y in he TPD p o iles
o he samples p epa ed by co-p ecipi a ion and imp egna ion.
The e o e, cha ac e iza ion esul s discussed abo e ha e co obo-
a ed ha he p epa a ion me hods s udied in his wo k a ec he ca -
aly ic p ope ies di e en ly. This e ec is associa ed o he deg ee o
in e ac ion be ween Mn and Ce p o ided by each p epa a ion me hod.
Co-p ecipi a ion and sol-gel me hods a e he ones ha a ou ed he
o ma ion o he mixed oxide phase, h ough Mn inco po a ion o CeO
2
s uc u e. The ac ha Mn and Ce belong o he same s uc u e, bo h a
bulk and su ace le el, leads o a high in e ac ion be ween bo h me als.
The absence o mixed oxide phase in he ca alys s p epa ed by me-
chanical mixing and imp egna ion e idences he low in e ac ion deg ee
be ween Mn and Ce a bo h samples. Howe e , he esul s om acidi y
analysis, ha showed a simila p omo ion o s ong acidi y in he sam-
ples p epa ed by imp egna ion and co-p ecipi a ion, sugges ha he
sample p epa ed by imp egna ion exhibi s a simila in e ac ion deg ee
be ween he ac i e me als a he su ace le el han ha p esen in he
samples p epa ed by co-p ecipi a ion and sol-gel.
The impo ance o p o iding a high in e ac ion be ween Mn and Ce is
ha i p oduces he s uc u al de ec s ha p omo e he o ma ion o
oxygen acancies, which imp o e he educibili y and a ou he p es-
ence o di e en Mn species wi h high oxida ion s a es (Mn
4+
and
Mn
3+
). Acidi y is also enhanced by p epa a ion me hods ha p omo e a
high in e ac ion be ween Mn and Ce a he su ace le el. This p ope y is
also no o iously a ec ed by he mo phology, especially by su ace a ea.
In his sense, p epa a ion me hods ha e been also co obo a ed o a ec
ex u al p ope ies, e en when hey p omo e simila s uc u al
p ope ies.
In he ollowing sec ion, he e ec o he deg ee o in e ac ion be-
ween ac i e me als (p o ided by he di e en p epa a ion me hods) on
ca aly ic pe o mance on he eac ions unde s udy will be add essed.
Table 2
Resul s om H
2
-TPR and NH
3
-TPD analysis.
Samples H
2
consump ion (mmol H
2
/g) Mn oxida ion s a e Acidi y
To al (µmol NH
3
/m
2
) Weak (µmol NH
3
/m
2
) S ong (µmol NH
3
/m
2
)
CeO
2
1.3 – 2.6 1.2 1.4
Sol-gel 4.2 3.8 3.1 1.2 1.9
Imp egna ion 1.4 2.8
a
2.6 1.0 1.6
Co-p ecipi a ion 3.1 3.5 3.5 1.2 2.3
Mechanical mixing 3.6 3.3 1.2 0.5 0.7
MnO
X
6.6 3.1 5.0 2.4 2.6
a
The es ima ion o Mn oxida ion s a e o he sample p epa ed by Imp egna ion has a high con ibu ion o e o associa ed o he low amoun o Mn loaded.
Fig. 5. NH
3
-TPD p o iles o MnO
X
-CeO
2
samples p epa ed by
di e en me hods.
J.A. Ma ín-Ma ín e al.
Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
7
3.2. Ca aly ic pe o mance
A e discussing he mos impo an poin s o cha ac e iza ion, his
sec ion aims o s udy he e ec o p epa a ion me hod on he ca aly ic
ac i i y in he simul aneous NO educ ion and o-DCB oxida ion. Fo his
pu pose, NO and o-DCB con e sion as well as by-p oduc s o ma ion will
be s udied.
Fig. 6 shows he NO and o-DCB con e sion p o iles o he samples
p epa ed by di e en me hods oge he wi h hose co esponding o
pu e manganese and ce ium oxides. Acco ding o Fig. 6A, pu e ce ium
oxide has a negligible NO con e sion below 200 ºC, al hough an inc ease
in he empe a u e leads o alues abo e 80% a ound 350 ºC, empe -
a u e abo e which NO con e sion d ops. On he o he hand, pu e
manganese oxide is e y ac i e a low empe a u e, wi h NO con e sion
abo e 80% below 175 ºC. Howe e , u he empe a u e inc ease causes
a small d op in NO con e sion and, a e an in e media e peak, a mo e
accen ua ed d op can be obse ed a highe empe a u e.
Fig. 6B shows he NO con e sion p o iles o bime allic samples.
Simila ly o pu e manganese oxide sample, con e sion p o iles o some
bime allic ca alys s show an oscilla o y end (below 250 ºC), which
sugges s he ac i a ion, as a unc ion o empe a u e, o a se o mul iple
eac ions ha p omo e he consump ion and o ma ion o NO. The
ca alys p epa ed by mechanical mixing has a di e en con e sion
p o ile o ha calcula ed as he weigh ed sum o pu e ce ium and
manganese oxide (showed in Fig. 6 A), al hough i s s uc u al and
ex u al p ope ies we e enclosed be ween hose obse ed o he pu e
oxides. Acco ding o his compa ison, highe NO con e sion is ob ained
wi h he sample p epa ed by mechanical mixing below 275 ºC. Thus,
his esul deno es ha he ac o mixing bo h pu e oxides, wi hou he
need o a p epa a ion me hod ha p omo es a close con ac be ween
ac i e me als, a ec s o he ca aly ic ac i i y.
The sample p epa ed by imp egna ion exhibi s a simila con e sion
p o ile o ha o he sample p epa ed by mechanical mixing, al hough
wi h lowe con e sion below 300 ºC and highe abo e ha empe a u e.
On he o he hand, he ca alys s p epa ed by co-p ecipi a ion and sol-gel
allow o al NO con e sion below 275 ºC. Thus, he ca alys s p epa ed by
co-p ecipi a ion and sol-gel a e he mos ac i e in NO educ ion, espe-
cially in he low and medium empe a u e ange. These p epa a ion
me hods we e he only ones ha p omo ed he o ma ion o a mixed
oxide phase, so hei excellen ac i i y is associa ed o he posi i e
con ibu ion o mixed oxide phase o he ca aly ic p ope ies.
In he high empe a u e ange, app oxima ely abo e 275 ºC, NO
con e sion s ongly d ops in all bime allic samples simila ly o pu e
oxides. Then, his nega i e beha iou is no associa ed o he ype o
p epa a ion me hod, bu i is in insic o he eac ion. In ac , a simila
d op in NO con e sion a high empe a u e is also epo ed o MnO
X
based ca alys s, ei he suppo ed o doped [12,45], and o he ca aly ic
o mula ions such as VO
X
/TiO
2
[46], Fe-ZSM5 [47] and P -Al
2
O
3
[48].
This beha iou is associa ed o he p e alence o side eac ions. Below,
he analysis o he by-p oduc s p oduced in NO educ ion will p o ide
mo e in o ma ion abou his ac .
None heless, al hough he NO con e sion d op a high empe a u e is
cha ac e is ic o NO educ ion, i seems o be a ec ed by p epa a ion
me hod, because in he ca alys s p epa ed by mechanical mixing, co-
p ecipi a ion and sol-gel, he d op s a s a ound 270 ºC, whe eas in
he ca alys p epa ed by imp egna ion, i occu s a ound 330 ºC.
Rega ding o-DCB oxida ion, all he samples exhibi an S-shape con-
e sion p o ile wi h wo s eps in which o-DCB con e sion inc eases wi h
empe a u e. Be ween hese wo s eps, o-DCB con e sion decays. This
d op in o-DCB con e sion could be caused by a deac i a ion o he ac i e
si es in ol ed in he eac ion a low empe a u e. Howe e , he u he
inc ease in he eac ion empe a u e a ou s a new inc ease in o-DCB
con e sion, which could be associa ed o an ac i a ion o he ac i e si es
p e iously deac i a ed o o he pa icipa ion o o he ac i e si es no
ac i e a low empe a u e.
As o Fig. 6C, pu e ce ium oxide does no allow o ob ain o-DCB
con e sions abo e 70% in he expe imen al condi ions, whe eas o al
con e sion is eached wi h pu e manganese oxide abo e 350 ºC. In he
case o bime allic samples, Fig. 6D, he ca alys p epa ed by mechanical
mixing exhibi a be e o-DCB con e sion, especially a medium and
Fig. 6. NO con e sion ob ained wi h A) pu e oxides and and B) bime allic ca alys s p epa ed by di e en me hods. o-DCB con e sion ob ained wi h C) pu e oxides
and D) bime allic ca alys s p epa ed by di e en me hods.
J.A. Ma ín-Ma ín e al.
Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
8
high empe a u e (abo e 275 ºC), han ha calcula ed as he weigh ed
sum o pu e oxides (showed in Fig. 6C), which is in ag eemen wi h ha
obse ed o NO educ ion. To al con e sion is ob ained wi h he sample
p epa ed by mechanical mixing a 385 ºC. The ca alys p epa ed by
imp egna ion shows he wo s ca aly ic ac i i y. I s con e sion p o ile is
shi ed owa ds highe empe a u e and con e sions abo e 80% a e no
ob ained in he s udied condi ions.
On he o he hand, he ca alys s p epa ed by co-p ecipi a ion and sol-
gel a e he bime allic samples ha allow o ob ain o al o-DCB con e -
sion a he lowes empe a u e, 350 ºC. This esul e idences hei highe
oxida i e capabili y, which is ela ed o he ac ha hese me hods
p omo ed he o ma ion o a mixed oxide phase ha a ou s he oxygen
mobili y. This cha ac e is ic is key o pe o m oxida ion ype- eac ions
a lowe empe a u e. Mo eo e , hese samples lead o a no able in-
c ease in o-DCB con e sion in he in e media e empe a u e ange,
coinciding wi h he i s s ep. Thus, o-DCB con e sion abo e 80% a e
ob ained be ween 200 and 260 and 220–240 ºC o co-p ecipi a ed and
sol-gel ca alys s. This di e ence in he ca aly ic ac i i y could be asso-
cia ed o he di e en ex u al p ope ies.
The e o e, he esul s discussed abo e ha e shown ha he samples
p epa ed by co-p ecipi a ion and sol-gel exhibi he bes ca aly ic pe -
o mance in bo h NO educ ion and o-DCB oxida ion. The be e ca a-
ly ic ac i i y is app eciable o e he whole empe a u e ange, al hough
i is mo e no iceable a low and in e media e empe a u es.
The imp o emen in he ca aly ic ac i i y o he samples p epa ed by
co-p ecipi a ion and sol-gel is associa ed o he high in e ac ion deg ee
be ween Mn and Ce, which is achie ed h ough he o ma ion o a mixed
oxide phase. The enhancemen o eac ion a e and eagen adso p ion
h ough he in e ac ion o wo ac i e me als was ea ly p oposed by
Solymosi and Kiss [49] and la e suppo ed by o he au ho s on Mn and
Ce-based ca alys s [15,50]. This posi i e e ec o in e ac ion be ween
ac i e me als o e he ca aly ic ac i i y is in ag eemen wi h he
conclusion ob ained in he cha ac e iza ion sec ion, whe e impo an
ca aly ic p ope ies, such as educibili y, acidi y and mo phology we e
enhanced by he p omo ion o a mixed oxide phase. Thus, he
in e ac ion be ween Mn and Ce is pos ula ed as a key ac o o he
imp o emen o ca aly ic pe o mances.
Nex , an analysis o he by-p oduc p oduc ion in each eac ion will
be add essed. Rega ding NO educ ion, N
2
O and NO
2
a e he main by-
p oduc s iden i ied. As can be seen in Fig. 7A, pu e ce ium oxide
ha dly p oduces N
2
O, so he p oduc ion o his by-p oduc is mainly
associa ed o he p esence o Mn species. In ac , a no able amoun o
N
2
O is p oduced wi h pu e manganese oxide o e he whole empe a-
u e ange. I s p oduc ion p o ile is maximum be ween 200 and 300 ºC,
empe a u e om which he p oduc ion o N
2
O dec eases.
In bime allic ca alys s, Figu e 7B, N
2
O is gene a ed o e he whole
empe a u e ange. The p oduc ion p o iles a e qui e di e en o hose
obse ed o pu e oxides because hey a e composed by wo peaks,
which a e placed a lowe empe a u es in he samples p epa ed by co-
p ecipi a ion and sol-gel. Acco ding o li e a u e [51,52], NH
3
oxida ion
is one o he SCR side eac ions in ol ed in N
2
O p oduc ion, so he shi
o N
2
O p o ile o lowe empe a u e could be ela ed o he highe
oxida i e capabili y o he co-p ecipi a ed and sol-gel samples, which
would p omo e he NH
3
oxida ion a lowe empe a u e. In ac , in
simila way, an imp o emen in he ca aly ic ac i i y o o-DCB oxida ion
has been p e iously ob ained wi h hese ca alys s.
As o NO
2
p oduc ion, Figs. 7C and 7D show he p oduc ion o his
by-p oduc akes place a empe a u es abo e 300 ºC and i s concen-
a ion inc eases p og essi ely wi h empe a u e. Bime allic samples
p epa ed by mechanical mixing and co-p ecipi a ion exhibi he highes
NO
2
p oduc ion. Cha ac e iza ion esul s showed he p esence o a
mixed oxide phase in he sample p epa ed by co-p ecipi a ion, which
was no o med in he samples p epa ed by mechanical mixing. The ac
ha bo h bime allic samples ha e a simila NO
2
p oduc ion, despi e
hei no able di e ences a he s uc u al le el sugges s ha he p esence
o mixed oxide phase does no play a key ole in he p oduc ion o NO
2
.
Acco ding o he p e ious esul s discussed in he analysis o ca a-
ly ic ac i i y, a s ong d op in NO con e sion was obse ed s a ing
a ound 275–300 ºC (Fig. 6). This beha iou was ela ed o he appea -
ance o side eac ions, which would consume he NH
3
needed o SCR.
Fig. 7. N
2
O gene a ion wi h A) pu e oxides and B) bime allic ca alys s p epa ed by di e en me hods. NO
2
gene a ion wi h C) pu e oxides and D) bime allic ca alys s
p epa ed by di e en me hods.
J.A. Ma ín-Ma ín e al.
Jou nal o En i onmen al Chemical Enginee ing 11 (2023) 110200
9
This p oposal is in ag eemen wi h he esul s ob ained in he analysis o
he by-p oduc s gene a ed in NO educ ion, since an impo an p o-
duc ion o N
2
O and NO
2
was ound a ound 300 ºC.
On he o he hand, he main by-p oduc s in o-DCB oxida ion a e CO
and chlo ina ed o ganic compounds ( ichlo obenzene, e achlo o-
me hane and e achlo oe hylene). Fig. 8 shows he selec i i y owa ds
bo h compounds. I is impo an o no e ha selec i i y o hese by-
p oduc s is shown abo e 200 ºC, due o he low le el o o-DCB con e -
sion below his empe a u e.
Selec i i y owa ds CO, Fig. 8A and B, inc eases g adually o pu e
ce ium oxide wi h empe a u e, al hough alues abo e 20% a e no
exceeded. In he case o pu e manganese oxide, CO selec i i y inc eases
un il eaching alues a ound 25% be ween 250 and 350 ºC, empe a u e
om which i s ongly dec eases. Bime allic ca alys s exhibi simila
p o iles o ha o pu e manganese oxide, al hough wi h lowe alues,
especially he sample p epa ed by imp egna ion and also hose p epa ed
by co-p ecipi a ion and sol-gel. The lowe alues o CO selec i i y o
samples p epa ed by co-p ecipi a ion and sol-gel could be associa ed o
hei highe oxida i e capabili y (as a consequence o he high oxygen
mobili y esul ing om he p esence o a solid solu ion), which a ou s a
dec ease in he selec i i y owa ds hose p oduc s o pa ial oxida ion.
As o selec i i y owa ds chlo ina ed o ganic compounds, Fig. 8C
shows a di e en end o ha obse ed in he p oduc ion o CO. In his
case, he p oduc ion o chlo ina ed o ganic compounds is highe and
occu s o e a wide empe a u e ange wi h pu e ce ium oxide han wi h
pu e manganese oxide. Among bime allic samples (Fig. 8D), he one
p epa ed by imp egna ion exhibi s he highes p oduc ion o chlo ina ed
o ganic compounds, i s selec i i y p o ile is simila o ha o pu e
ce ium oxide. The emaining bime allic samples show a selec i i y
p o ile e y simila o ha o pu e manganese oxide, so, simila ly o he
p oduc ion o NO
2
in NO educ ion, he p esence o absence o a mixed
oxide phase does no seem o a ec o a g ea ex en he p oduc ion o
chlo ina ed o ganic compounds.
The e o e, he p omo ion o a high in e ac ion deg ee be ween Mn
and Ce as a esul o he o ma ion o a mixed oxide phase, which has
p e iously conside ed o play an impo an ole in he ca aly ic pe o -
mance, does no ha e a signi ican impac on he amoun o N
2
O and
NO
2
p oduced. None heless, he in e ac ion be ween ac i e me als does
a o he p oduc ion o N
2
O a a lowe empe a u e, due o he
enhancemen o he oxida i e capabili y. The la e also p omo es a
lowe p oduc ion o CO in o-DCB oxida ion, bu i does no signi ican ly
a ec he p oduc ion o chlo ina ed o ganic compound.
4. Conclusions
Se e al MnO
X
-CeO
2
ca alys s we e p epa ed by di e en me hods in
o de o s udy he ole hey play on he Mn and Ce in e ac ion and, also,
o analyse hei e ec on he simul aneous NO educ ion and o-DCB
oxida ion. Co-p ecipi a ion and sol-gel me hods a e he ones ha p o-
mo e he highes in e ac ion deg ee be ween Mn and Ce a bo h su ace
and bulk, which is a ou ed by he o ma ion o a mixed oxide phase in
hese samples. The in e ac ion deg ee in he sample p epa ed by me-
chanical mixing is e y low, whe eas in he imp egna ed sample i is
only a a su ace le el.
The p omo ion o Mn and Ce in e ac ion is impo an o he ca aly ic
ac i i y, since hose ca alys s wi h he p esence o mixed oxide phase a e
he mos ac i e ones o e he whole empe a u e ange, bu especially a
low and medium empe a u e, whe e hey allow o each o al NO
con e sion and o-DCB con e sions abo e 80%. This beha iou lies on
mixed oxide phase p omo es oxygen acancies, which imp o e oxygen
mobili y and s o age. Mo eo e , he in e ac ion be ween ac i e me als
imp o es edox p ope ies and acidi y, especially s ong acidi y.
Mo phology o he sample is also a ec ed by p epa a ion me hods, e en
when hey lead o simila s uc u al p ope ies.
Rega ding selec i i y, N
2
O and NO
2
we e ound as he main by-
p oduc s o NO educ ion. A highe in e ac ion be ween ac i e me als
a ou s he p oduc ion o N
2
O a lowe empe a u e, as a consequence o
he p omo ion o oxida i e capabili y. This ac also imp o es o al o-
DCB oxida ion by dec easing CO p oduc ion, al hough selec i i y o-
wa ds chlo ina ed o ganic compounds is no a ec ed.
Fig. 8. CO selec i i y ob ained wi h A) pu e oxides and B) bime allic ca alys s p epa ed by di e en me hods. Chlo ina ed o ganic compounds ob ained wi h C) pu e
oxides and D) bime allic ca alys s p epa ed by di e en me hods.
J.A. Ma ín-Ma ín e al.
