1
Jon A. On ubia, B. Pe eda-Ayo, U. De-La-To e, Juan R. González-Velasco*
Depa amen o de Ingenie ía Química, Facul ad de Ciencia y Tecnología, Uni e sidad del País
Vasco, UPV/EHU, Campus de Leioa, P. O. Box 644, ES-48080 Bilbao, Bizkaia, Spain
KEYWORDS: pe o ski e, LaCoO3, LaMnO3, S -doping, su ace oxygen acancies, NO
oxida ion, P - ee ca alys
*Co esponding au ho : [email p o ec ed]
This is he accep ed manusc ip o he a icle ha appea ed in inal o m in Applied Ca alysis B: En i onmen al
213 :198–210 (2017), which has been published in inal o m a h ps://doi.o g/10.1016/j.apca b.2017.04.068. © 2017
Else ie unde CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/)
Key ac o s in S -doped LaBO3 (B = Co o Mn) pe o ski es o NO
oxida ion in e icien diesel exhaus pu i ica ion
2
ABSTRACT
Pe o ski es ha e a ac ed a en ion in ecen yea s as an economic al e na i e o noble me als
in oxida ion p ocesses. Syn hesis condi ions o LaCoO3 and LaMnO3 pe o ski es ha e been
s udied a ying ci a e o ni a e mola a io in he s a ing solu ion, pH and calcina ion
p o ocol, wi h he aim o ob aining high pu i y pe o ski es, absence o impu i ies, and wi h
enhanced ex u al p ope ies. Once syn hesis condi ions we e es ablished, s on ium was
inco po a ed in he pe o ski e la ice by subs i u ing lan hanum wi h di e en doping le els,
i.e. La0.9S 0.1BO3, La0.8S 0.2BO3, La0.7S 0.3BO3, La0.6S 0.4BO3 and La0.5S 0.5BO3 wi h B=Co o
Mn. The p epa ed solids we e cha ac e ized in e ms o c ys alline phase iden i ica ion
(XRD), speci ic su ace a ea (N2 adso p ion-deso p ion a -196 ˚C), educibili y and oxida ion
s a e o ansi ion me al ions (H2-TPR), quan i ica ion o adso bed oxygen species (O2-TPD)
and su ace elemen al composi ion (XPS). Cha ge imbalance associa ed o s on ium (S 2+)
inco po a ion in he pe o ski e la ice in subs i u ion o lan hanum (La3+) was p e e en ially
balanced by Mn4+ p omo ion in La1-xS xMnO3 pe o ski es, whe eas o ma ion o oxygen
acancies seems o be he mechanism o cha ge compensa ion in La1-xS xCoO3 pe o ski es,
whe e Co ions emained as Co3+ ions. S on ium doped pe o ski es u he imp o ed NO
con e sion compa ed o he non-subs i u ed o mula ions. The bes NO oxida ion
pe o mance was ob ained wi h La0.7S 0.3CoO3 and La0.9S 0.1MnO3 samples, achie ing
maximum NO con e sion o 83 and 63% a 300 and 325 C, espec i ely. Highe oxida ion
capaci y o La0.7S 0.3CoO3 sample was associa ed o he highe oxygen mobili y and exchange
capaci y be ween oxygen in he la ice and gas phase oxygen. I is wo h no ing ha p epa ed
pe o ski es p esen ed a highe NO oxida ion capaci y han pla inum-based NSR model
ca alys s, con i ming pe o ski es as an economic al e na i e o ca alyze NO oxida ion
eac ions in au omo i e ca alysis.
3
1. INTRODUCTION
Diesel engines p esen highe uel e iciency han s oichiome ic gasoline engines and
he eby emi less CO2 o he a mosphe e. In con as , he clean-up o diesel exhaus gases is
e en mo e challenging han gasoline engines exhaus gases, due o he na u e o he emission,
including soo , and also due o he ne oxidizing en i onmen ha limi s NOx educ ion [1] In
o de o mee s ingen emission s anda ds, diesel engines implemen ed ca aly ic p ocesses
such as Diesel Oxida ion Ca alys (DOC) [2] o Diesel Pa icula e Fil e (DPF) [3] In he
DOC, unbu ned hyd oca bons as well as ca bon monoxide a e comple ely oxidized o CO2
and H2O while NO is con e ed in o NO2. A e wa ds, soo is apped in DPF and
con inuously egene a ed by oxida ion wi h NO2 p oduced in he DOC. Howe e , his
s a egy ails in mee ing EURO VI s anda ds ega ding NOx emissions, and consequen ly
addi ional ca aly ic s a egies a e manda o y o be implemen ed, such as NOx S o age and
Reduc ion (NSR) [4] o Selec i e Ca aly ic Reduc ion (SCR) [5] In he NSR echnology, NO-
o-NO2 oxida ion is conside ed a p ima y s ep o p omo e NOx s o age ia ni i es o ni a es
[6, 7] o med o e an alkali o alkali-ea h ma e ial, because NO2 adso p ion occu s much
as e han NO [4] In he SCR echnology NO- o-NO2 oxida ion is also o c i ical impo ance
[8] being well known ha he as SCR eac ion ac i a es when NO/NO2=1/1 (mola a io),
achie ing high NOx con e sion e en a lowe empe a u e. Thus, he signi icance o NO- o-
NO2 oxida ion eac ion in au omo i e exhaus pu i ica ion is ele an and me i s u he
esea ch on no el ca alys o mula ions.
Noble me al based ca alys s ha e been commonly employed in o de o p omo e NO
oxida ion in au omo i e ca alysis. Howe e , he use o noble me al (especially P ) esul s in
high cos and poo he mal s abili y unde highly oxida i e condi ions [9] . In ha sense,
pe o ski es ha e a ac ed a en ion in ecen yea s as po en ial, low-cos al e na i e o noble
me al in oxida ion p ocesses [10] In he pe o ski e la ice (wi h he gene al o mula ABO3),
4
he B ca ion coo dina es wi h oxygen in oc ahed al s uc u e, and A ca ion loca es in he
cen e o he dodecahed al s uc u e. Pe o ski es ac i i y o oxida ion eac ions seems o be
ela ed wi h some speci ic s uc u al p ope ies, such as change o oxida ion s a e o B ca ion,
ac i e oxygen mobili y and ion acancy de ec [11] Du ing oxida ion eac ions, la ice
oxygen is hough o be ca aly ically ac i e and i s consump ion and egene a ion is ela i e o
cycling he oxida ion s a e o neighbo ing ansi ion me al ions (B+3 B+4) and/o (B+2
B+3) [12] In ha sense, i has been epo ed ha he ca aly ic ac i i y o pe o ski es is a
in luenced by he ype o B ca ion and i s oxida ion s a e cycling easibili y [13]
LaMnO3 and LaCoO3 pe o ski es ha e been p oposed as ac i e pe o ski es o NO
oxida ion eac ions [11, 14] . In pa icula , LaMnO3 pe o ski e in which Mn3+/Mn4+ mixed
oxida ion s a e is usually obse ed e en o nominal s oichiome y LaMnO3+ (wi h Mn4+
con en o 2 pe o mula uni ), opposi e o mos o he pe o ski es. Howe e , i has been
p oposed ha elec oneu ali y o he la ice is accomplished by gene a ing ca ion acancies,
ins ead o oxygen o e -s oichiome y, as pe o ski e la ice canno accommoda e in e s i ial
oxygen ions [15, 16] . On he o he hand, s oichiome ic LaCoO3 pe o ski e usually p esen s
some oxygen de iciency in he la ice, ep esen ed by LaCoO3-, and he cha ge balance is
accomplished by he p esence o some cobal as Co2+. This Mn3+/Mn4+ and Co2+/Co3+
e e sible oxida ion s a e is hough o be a key ac o o oxida ion eac ions [17-20]
The oxida ion s a e o B ca ion can be modula ed by a ying p epa a ion s eps du ing he
pe o ski e syn hesis, such as calcina ion empe a u e [21] , non-s oichiome y o ca ions A o
B [12, 14] o subs i u ing La+3 by lowe oxida ion s a e ca ions, such as Ca2+, Ba2+ o S 2+, o
e en highe oxida ion s a e ca ion such as Ce4+. S on ium-doped LaMnO3 and LaCoO3
pe o ski es ha e been used o a wide ange o applica ions [10, 22-24] . The in oduc ion o
lowe oxida ion s a e S 2+ in subs i u ion o La3+ in LaMnO3 and LaCoO3 la ice gene a es a
ne cha ge imbalance ha may be compensa ed by al e a ion o he oxida ion s a e o a
5
ac ion o ansi ion me al, e.g. Mn4+ o Co4+. Al e na i ely, he oxida ion s a e o ansi ion
me al could be main ained unal e ed (Mn3+ o Co3+), bu ins ead oxygen acancies could be
gene a ed in he la ice o a ain he cha ge balance. E en a mixed si ua ion showing al e ed
oxida ion s a e o ansi ion me al along wi h oxygen acancies in he la ice could be
expec ed.
Oxide-based ca alys s ha e ypically shown much lowe ac i i y han P -based ca alys s
unde he kine ics-con olled empe a u e egime. Howe e , Kim e al [25] p epa ed
La0.9S 0.1CoO3 and La0.9S 0.1MnO3 pe o ski es and epo ed NO oxida ion ac i i y simila o
o highe han hose o P -based ca alys s unde ealis ic au omo i e condi ions. Li el al. [26]
p epa ed a se ies o La1-xS 0.xCoO3 (x=0.1, 0.2, 0.3, 0.4 and 0.5), and hey ound, a e he
NOx s o age es s, he La0.7S 0.3CoO3 pe o ski e had he bes NO- o-NO2 pe o mance and he
la ges NOx s o age capaci y (NSC) a 300 ºC. These au ho s also sugges ed possible NOx
s o age ou es on La1-xS xCoO3. To ou knowledge, a simila s udy o s on ium-doped Mn-
based pe o ski es has no been epo ed and esul s compa ed wi h hose o Co-based
pe o ski es.
Mo e ecen ly, Dong e al. [22] p epa ed a se ies o La0.7S 0.3MnO3 pe o ski e- ype
ca alys s by he sol-gel me hod using ci ic acid as he complexan . They concluded ha
a ia ion o syn hesis condi ions, namely he cale ac i e eloci y, he calcina ion empe a u e
and he pH o he p ecu so solu ions, g ea ly a ec he mo phology o he pe o ski e ca alys
and, consequen ly, he NO- o-NO2 ac i i y and NOx s o age capaci y o La0.7S 0.3MnO3
pe o ski e- ype ca alys s.
In his wo k, we look deeply in o he iden i ica ion o he main changes in physico-
chemical p ope ies induced by he modi ica ion o di e en pa ame e s (i.e. syn hesis
condi ions o La pa ial subs i u ion by s on ium). Consequen ly, ele an in o ma ion on he
key ac o s o he NO- o-NO2 oxida ion is ex ac ed, along wi h he main di e ences de i ed
6
om he chemical na u e o B ca ion (Co o Mn) in he based-pe o ski e ca alys , o gain
unde s anding on key ac o s o design noble-me al- ee pe o ski e-based ca alys s
compe i i e o subs i u e ma e ial o he con en ional P -con aining NSR ca alys s. We
p epa e, cha ac e ize and es NO oxida ion ac i i y o s oichiome ic LaMnO3 and LaCoO3
as well as s on ium doped La1-xS xMnO3 and La1-xS xCoO3 pe o ski es (x=0.1, 0.2, 0.3, 0.4
and 0.5). A e op imizing condi ions du ing he sol-gel syn hesis, including ci a e o ni a e
a io, pH o he gel and and calcina ion p o ocol o ge pu e pe o ski es wi h no phase
seg ega ion and enhanced ex u al p ope ies, special a en ion is ocused on he co ela ion o
NO- o-NO2 ac i i y wi h he physico-chemical p ope ies o he p epa ed s on ium-doped
pe o ski e-based ca alys s.
2. EXPERIMENTAL
2.1. Pe o ski e ca alys p epa a ion. All pe o ski es we e p epa ed he by ci ic acid sol-
gel me hod [27] . App op ia e amoun s o La(NO3)3·6H2O (Fluka), Co(NO3)2·6H2O (Sigma
Ald ich), Mn(NO3)2·4H2O (Me ck) we e dissol ed in dis illa ed wa e unde igo ous
s i ing. Then ci ic acid (C6H8O7·H2O, CA) was added as a complexing agen and o ganic
loading o he combus ion p ocess wi h a ci a e o ni a e (CA/N) mola a ios o 0.7, 1.1 o
1.5. The pH alue was adjus ed o 3, 4.5, 6, 7, 8 o 9 by ammonia (25% as NH3, Pan eac).
A e sol en e apo a ion a 80 °C, he gel was u he d ied a 120 °C o e nigh and hen
calcined in 5% O2/He (60 ml min-1) o s a ic ai a desi ed empe a u e (600 °C, 700 °C, 800
°C o 900 °C) o 4h. Table 1 esumes he nomencla u e and syn hesis condi ions o all
ca alys . The gene al nomencla u e se o each sample was as ollows: LaCoO3 (LCO) o
LaMnO3 (LMO) wi h he speci ic syn hesis condi ions in b acke s (ci a e o ni a e mola
a io/pH/calcina ion empe a u e), as o example LCO (1.1/7/700). In o de o de e mine i
he syn hesis me hod was epe i i e his sample was p epa ed wice (wi h * in Table 1).
7
In o de o syn hesize s on ium doped La1-xS xCoO3 and LaxS 1-xMnO3 pe o ski es,
adequa e amoun o S (NO3)2 (Sigma Ald ich) was added o he ini ial solu ion ollowing he
same p ocedu e as s a ed be o e. The ollowing samples wi h inc easing La subs i u ion
deg ee by S we e syn he ized: La0.9S 0.1BO3, La0.8S 0.2BO3, La0.7S 0.3BO3, La0.6S 0.4BO3,
La0.5S 0.5BO3, wi h B=Mn o Co.
2.2. Ca alys cha ac e iza ion. X- ay di ac ion (XRD) pa e ns we e ob ained on a
Philips PW1710 di ac ome e . The samples we e inely g ound and we e subjec ed o Cu Kα
adia ion in a con inuous scan mode om 5° o 70° 2θ wi h 0.02° pe second sampling
in e al. PANaly ical X`pe HighSco e speci ic so wa e was used o da a ea men .
Tex u al p ope ies o he samples we e de e mined by N2 adso p ion-deso p ion a - 196
°C using Mic ome i ics T iS a equipmen .
The he mog a ime ic (TG) analysis was conduc ed wi h Se a am Se sys E olu ion TG
ins umen coupled o he mass spec ome e P ei e Vacuum DUO 2.5, using 10 mg o
sample. The expe imen was conduc ed in 5% O2/He mix u e o ai (100 mL min-1) om
oom empe a u e o 950 °C a a hea ing a e o 2 °C min-1.
The educibili y o he samples was in es iga ed by empe a u e p og ammed educ ion
(H2-TPR) using Mic ome i ics Au oChem II equipmen . The qua z ube eac o was loaded
wi h 0.1 g o sample and p e ea ed wi h 30 mL min-1 o 5% O2/He mix u e a 600 C o 30
min, and hen cooled down o 50 C. A e wa ds, samples we e hea ed om oom
empe a u e o 900 C wi h 10 C min-1 hea ing a e in a 5% H2/A gas mix u e wi h 30 mL
min-1 low a e. Wa e gene a ed du ing sample educ ion was emo ed using a cold ap
be o e gas analysis by TCD.
The s eng h and concen a ion o di e en oxygen species was in es iga ed by Oxygen
empe a u e p og ammed deso p ion (O2-TPD) using Mic ome i ics Au oChem II equipmen .
The sample (0.1 g) was in oduced in a qua z ube eac o and p e ea ed wi h 50 mL min-1
8
o 5% O2/He mix u e a 600 °C o 5 min and hen cooled down o 40 C. A e wa ds,
samples we e hea ed om oom empe a u e o 900 ˚C wi h a hea ing a e o 10 C min-1 in a
50 mL min-1 o Helium gas low.
X- ay pho oelec onic spec oscopy (XPS) cha ac e iza ion was ca ied ou in a SPECS
elec on spec ome e wi h a Phoibos 150 1D-DLD ene gy analyze using Al-Kα (1486.7 eV)
adia ion sou ce. To ob ain he XPS spec a, he p essu e o he analysis chambe was
main ained a 5 × 10−10 mba . The binding ene gy (BE) scale was adjus ed by se ing he C1s
ansi ion a 284.6 eV.
2.4. NO- o-NO2 oxida ion expe imen s. Oxida ion es s we e ca ied ou in a e ical
s ainless s eel eac o illed wi h 0.5 g ams o 0.3-0.5 mm pelle ized ca alys , and placed
inside a 3-zone ube u nace. The eed composi ion was 650 ppm o NO, 6% O2 and A as
balance gas. Gases we e eed ia mass low con olle s and he o al low a e was se a 634
mL min-1, which co esponded o a space eloci y (GHSV) o 123,500 h-1. Tempe a u e was
measu ed by a he mocouple inside he ca alys bed.
The ou le gas composi ion was con inuously measu ed using a MKS Mul iGas 2030 FT-IR
analyze o quan i a i e analysis o NO and NO2 (no addi ional ni ogen compounds a e
de ec ed). The con e sion o NO- o-NO2 was calcula ed a e s eady s a e a e en empe a u e
was eached, which usually needed a ound 20 minu es a e empe a u e s abiliza ion,
acco ding o Equa ion (1):
2
in ou
NO NO
NO- o-NO in
NO
(%) 100
FF
XF
(1)
3. RESULTS ANS DISCUSSION
9
3.1. LaCoO3 pe o ski es. The e a e se e al pa ame e s a ec ing pe o ski es physico-
chemical p ope ies du ing he syn hesis, such as ci a e o ni a e mola a io in he s a ing
solu ion, pH alue o he gel and calcina ion p o ocol [22, 28, 29]
3.1.1. In luence o ci a e o ni a e a io. The in luence o ci a e o ni a e a io (CA/N)
was s udied by p epa ing LaCoO3 pe o ski e wi h a ci a e o ni a e mola a io o 0.7, 1.1
and 1.5 ( i s sec ion in Table 1), i.e. de ec , nea s oichiome ic and excess o ci a e o he
s oichiome ic eac ion wi h ni a es, espec i ely. The he mal decomposi ion o he p epa ed
samples was cha ac e ized by he mog a ime ic analysis, as shown in Figu e 1. P io o he
analysis, he ob ained gels we e d ied o e nigh in an o en a 120 C and hus, only a weak
weigh loss (<5%) was obse ed below 150 C, which was asc ibed o emo al o esidual
adso bed and hyd a ed H2O. F om 150 C and onwa ds, wo main weigh losses we e
iden i ied cen e ed a ound 200 C and 300-350 C (DTGA cu e), asc ibed o he eac ion
be ween ci a es and ni a es.
FIGURE 1
In o de o gain unde s anding on he decomposi ion eac ions leading o he inal
o ma ion o he pe o ski e, TGA-MS expe imen s in 5% O2/He we e ca ied ou wi h ba e
p ecu so s, i.e. cobal ni a e, lan hanum ni a e and ci ic acid (no shown). Excluding he
weigh loss a ibu ed o dehyd a ion p ocesses, i was obse ed ha ci ic acid s a ed o
decompose a low empe a u e wi h a maximum weigh loss de ec ed a 170 C and a
p og essi e decomposi ion un il 400 C. CO2 was he main gas p oduc along wi h aces o
highe molecula weigh in e media es. On he o he hand, cobal ni a e decomposi ion
s a ed a 180 C and was comple ed a 280 C, whe eas lan hanum ni a e was mo e s able
and i s decomposi ion only s a ed a e empe a u e was aised abo e 400 C, concluding he
16
In o de o de e mine he in luence o s on ium doping, La1-xS xBO3 pe o ski es (B=Mn o
Co) we e syn hesized wi h La subs i u ions x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, using S (NO3)2 as
p ecu so , and he op imum syn hesis p ocedu e de e mined in he p e ious sec ion.
3.3.1. X- ay di ac ion (XRD). Figu e 6 shows he XRD pa e ns o he p epa ed
La1-xS xCoO3 (Figu e 6a) and La1-xS xMnO3 (Figu e 6b) pe o ski es. Pu e pe o ski e
di ac ion pa e ns we e ob ained o low S subs i u ion deg ees, in he absence o
impu i ies. Howe e , phase seg ega ion in he o m o Co3O4, La(OH)3 and S CO3 s a o be
de ec able by XRD o La0.6S 0.4CoO3 pe o ski e, and become e en mo e signi ican o
La0.5S 0.5CoO3. On he o he hand, in he case o Mn-based pe o ski es, he p esence o
impu i ies in he o m o S CO3 was only e iden o he highes S subs i u ed sample, i.e.
La0.5Mn0.5CoO3. Thus, i can be concluded ha LaMnO3 pe o ski es allow highe S
accommoda ion in he la ice wi hou phase seg ega ion in compa ison o LaCoO3
pe o ski es.
FIGURE 6
I espec i e o B ca ion, Mn o Co, inc easing S subs i u ion deg ee leads o a widening o
he di ac ion peaks (Figu e S3) due o smalle c ys al size o he solids, which has been
quan i ied by Sche e ´s equa ion (Table 3). In ac , he la ges c ys al size, 32 and 27 nm, is
de ec ed o he non-subs i u ed LaCoO3 and LaMnO3 samples, espec i ely, and he smalles ,
15 and 14 nm, o he highes subs i u ion deg ee La0.5S 0.5CoO3 and La0.5S 0.5MnO3,
espec i ely.
Fu he mo e, he in ensi y dec ease and o he di ac ion peaks wi h he displacemen
owa ds highe di ac ion angles by S doping, sugges s ha La+3 ca ions we e success ully
subs i u ed by Co+3 o by Mn+3/Mn+4 in he pe o ski e la ice s uc u e. In addi ion, g adual
changes in he cha ac e is ic pa e n a e obse ed om hombohed al LaCoO3 (PDF numbe :
17
048-0123) and La0.88MnO2.91 (PDF numbe : 089-0679) o hombohed al dis o ed
La0.5S 0.5CoO2.91 (PDF numbe : 048-0122) and ombohed al La0.5S 0.5MnO3 (PDF numbe :
048-0122), espec i ely [10, 33-35]
3.3.2. Tex u al p ope ies. The in luence o S con en on ex u al p ope ies was s udied by
N2 adso p ion-deso p ion a low empe a u e (Table 3). The e olu ion o he speci ic su ace
a ea (SSA) wi h espec o he subs i u ion deg ee e eals a maximum o La0.8S 0.2CoO3 and
La0.6S 0.4MnO3, samples. The SSA is enhanced by 32% o La0.8S 0.2CoO3 (21.4 m2 g-1) wi h
espec o he non-subs i u ed sample (16.2 m2 g-1) whe eas he SSA enhancemen was
no ably highe (70%) o La0.6S 0.4MnO3 (47.9 m2 g-1) in compa ison o he non-subs i u ed
sample (28.2 m2 g-1). The SSA de elopmen o S subs i u ed samples can be di ec ly linked
o he lowe c ys al size o he samples, as i was e ealed by XRD (Table 3). Al hough
c ys al size was u he dec eased o highe S subs i u ion, SSA was penalized, which was
a ibu ed o he p esence o phase seg ega ions ha may block he access o he po es [26]
No e ha his SSA penaliza ion esul ed e iden o S subs i u ions highe han 0.3 and 0.4
o LaCoO3 and LaMnO3 pe o ski es, espec i ely, which again is a clea e idence o he
highe abili y o LaMnO3 o accommoda e S in he la ice wi h no phase seg ega ion, and
hus allowing highe p omo ion e ec on ex u al p ope ies due o S inco po a ion.
TABLE 3
3.3.3. Reducibili y (H2-TPR). I has been ound ha he ca aly ic ac i i ies o lan hanum
pe o ski es we e mainly de e mined by he B si e elemen p ope ies [20] . In his sense, he
edox p ope ies o Mn and Co-based pe o ski es we e in es iga ed by H2-TPR expe imen s.
Figu e 7 illus a es he H2-TPR o La1-xS xCoO3 samples, whe e wo main H2 consump ion
egions can be dis inguished, i.e. below and abo e 500 C. Hyd ogen consump ion below 500
C can be decon olu ed in o h ee di e en con ibu ions cen e ed a ound 200, 325 and 375
18
C, which a e assigned o he educ ion o quimiso bed oxygen in he ca alys su ace,
supe icial educ ion o Co3+ o Co2+ and bulk educ ion o Co3+ o Co2+, espec i ely,
main aining he pe o ski e s uc u e,
1-x x 3 2 1-x x 2.5 2
La S CoO +1 2H La S CoO +1 2H O
(2)
On he o he hand, hyd ogen consump ion abo e 500 C can be assigned o he inal
educ ion o Co2+ o Co0 which is again accomplished in wo di e en consump ion peaks
a ibu ed o supe icial and bulk educ ion, esul ing in he des uc ion o he pe o ski e
s uc u e,
0
1-x x 2.5 2 2 3 2
(1-x)
La S CoO + H La O + xS O + Co H O
2
(3)
The a o emen ioned educ ion peaks can be clea ly obse ed o pe o ski e samples wi h
low S subs i u ion up o La0.7S 0.3CoO3. In con as , educ ion peak assigna ion is no
s aigh o wa d o highe S subs i u ion deg ee samples. In hose cases, an addi ional
educ ion peak can be obse ed a highe empe a u e (800 C) which is no ela ed wi h
hyd ogen consump ion bu ins ead is asc ibed o su ace decomposi ion o esidual
ca bona es in he o m o CO2 which al e s he TCD signal, as de ec ed by MS [26, 36] . As a
gene al end, i can be obse ed ha inc easing S con en esul s in he displacemen o
educ ion peaks o lowe empe a u e, ela ed wi h inc easing BET su ace a ea (SSA) which
p omo es sample educibili y.
FIGURE 7
Focusing on low S subs i u ed samples, he o al hyd ogen consump ion ela ed o he o al
amoun o cobal in he sample (Table 4) can be used o es ima e he mean oxida ion s a e o
cobal . Pe o ski e educ ion s oichiome y (equa ions 2 and 3) e eals ha 1.5 mol o H2 is
needed o educe 1 mol o Co3+ o Co0. Calcula ed H2/Co a ios a e in all cases close o 1.5,
19
his con i ming an oxida ion s a e o Co3+ in he ini ial pe o ski e and disca d he p esence o
Co4+. Consequen ly, i can be deduced ha gene a ion o oxygen acancies is he only
mechanism a ailable o accomplish cha ge compensa ion in LaCoO3 pe o ski es when
s on ium is doped in o he s uc u e. In ac , inc easing H2 consump ion due o educ ion o
chemiso bed oxygen in su ace acancies was obse ed wi h inc easing S doping (Table 4),
eaching his con ibu ion abou 4 imes o La0.5S 0.5CoO3 wi h espec o non-subs i u ed
LaCoO3 pe o ski e. Fu he mo e, i was expe imen ally e i ied ha he amoun o hyd ogen
consumed in he educ ion o Co3+Co2+ was hal o ha consumed o he educ ion o
Co2+Co0, which alida es he peak decon olu ion assignmen s. Co3+ ela i e con en wi h
espec o o al cobal in he sample was calcula ed om decon olu ed peaks a eas ela ion,
i.e. di iding he H2 consump ion ela ed o Co3+Co2+ educ ion by hal he H2 consump ion
ela ed o Co2+Co0 educ ion. I can be obse ed ha cobal exis s only in he 3+ oxida ion
s a e in he pe o ski e la ice o low S subs i u ed samples. On he con a y, o high S
subs i u ed samples lowe Co3+/Co a ios we e obse ed. This ac can be explained by he
p esence o a signi ican amoun o cobal as Co3O4, as obse ed by XRD, wi h a mixed
+2/+3 oxida ion s a e, which con ibu es o educe H2 consump ion and dec eases he
obse ed Co3+/Co a io.
TABLE 4
Figu e 8 shows TPR p o iles o Mn pe o ski es, all cu es exhibi ing simila shapes wi h
wo main educ ion peaks, below and abo e 550 C. H2 consump ion below 550 C was
decon olu ed in o h ee con ibu ions loca ed a 150, 300 and 400 C, which a e asc ibed o
he educ ion o nons oichiome ic excess oxygen accommoda ed wi hin he la ice, educ ion
o Mn4+ o Mn3+ and educ ion o Mn+3 o Mn+2 loca ed in a coo dina ion unsa u a ed
mic oen i onmen [22, 37, 38] , espec i ely. Theo e ically, same con en s o S 2+ and Mn4+
20
a e expec ed in he pe o ski e s uc u e in o de o accomplish cha ge balance. Fu he mo e,
i has been epo ed ha e en s oichiome ic LaMnO3 pe o ski es can show mixed
Mn3+/Mn4+ oxida ion s a e balanced by excess oxygen in he la ice. Thus, educ ion sequence
o La1-xS xMnO3 pe o ski es can be w i en as:
3+ 4+ 3+ 4+
1-x x 1-x x+δ/2 3 2 1-x x 1-x x 3 2
La S Mn Mn O + δH La S Mn Mn O + δH O
(4)
3+ 4+ 3
1-x x 1-x x 3 2 1-x x 3- 2 2
11
La S Mn Mn O + xH La S Mn O + xH O
22
x
(5)
3
1-x x 3- 2 2 2 3 2
1 1 1
La S Mn O + H 1-x La O +MnO+xS O+ H O
2 2 2
x
(6)
whe e x ep esen s he S mola ac ion and he excess o oxygen accommoda ed wi hin
he la ice. On he o he hand, H2 consump ion abo e 550 C was a ibu ed o bulk educ ion
o Mn3+ o Mn2+ and des uc ion o he pe o ski e. As p e iously obse ed o LaCoO3
pe o ski es, an addi ional peak was obse ed a highe empe a u e (800 C) o high S
subs i u ed samples due o S CO3 decomposi ion.
FIGURE 8
To al H2 consump ion ela ed o Mn con en in he sample (H2/Mn) is indica i e o an
a e age Mn oxida ion s a e. 1 mol o H2 is needed o educe 1 mol o Mn4+ o Mn2+ whe eas
0.5 moles o H2 is needed o educe 1 mol o Mn3+ o Mn2+. Calcula ed H2/Mn a ios esul ed
in be ween 0.5 and 1 o all samples, e ealing a hyb id Mn4+/Mn3+ oxida ion s a e (Table 5).
Obse ing nume ical alues o decon olu ed and in eg a ed signals, i can be obse ed ha
H2 consump ion ela ed o educ ion o oxygen excess is main ained ai ly cons an wi h S
doping. The obse ed inc easing amoun o H2 consump ion is a ibu ed o educ ion o Mn4+
o Mn3+, e ealing ha S doping p omo es he p esence o Mn4+ in he pe o ski e s uc u e o
compensa e he cha ge imbalance p oduced by S . A simila end obse ed o H2
consump ion is assigned o supe icial educ ion o Mn3+ o Mn2+, which co ela es
21
adequa ely wi h inc easing su ace a ea o he samples. Reduc ion o bulk Mn3+ o Mn2+ is
complemen a y o su ace Mn3+ o Mn2+ educ ion. Thus, H2 consump ion ela ed o bulk
educ ion o Mn3+ o Mn2+ dec ease wi h S doping. Mn4+ con en de e mined om in eg a ed
H2 consump ion o decon olu ed signals show, as a gene al end, an inc easing con en o
Mn4+ wi h inc easing S doping, ob aining almos 50% o manganese as Mn4+ o high S
doped pe o ski es.
TABLE 5
As a gene al end i can be obse ed ha inc easing S con en esul s in a displacemen o
educ ion peaks o lowe empe a u e, ela ed wi h an inc easing BET su ace a ea (SSA)
which p omo es sample educibili y, as i was obse ed o LaCoO3 pe o ski es.
3.3.3. Concen a ion and s eng h o he oxygen species (O2-TPD). Pa ial subs i u ion o
La+3 by S +2 can modi y Co o Mn oxida ion s a e as p e iously obse ed by H2-TPR, bu
oxygen acancies can be also p omo ed in o de o accomplished cha ge compensa ion. In
his sense, empe a u e p og ammed deso p ion o oxygen (O2-TPD) is a eliable echnique o
analyze he amoun and he s eng h o adso bed oxygen species. Figu es 9a and 10a show he
e olu ion o TCD signal ob ained du ing O2-TPD expe imen s o La1-xS xCoO3 and
La1-xS xMnO3 samples, espec i ely. The p o iles clea ly show wo dis inc egions whe e
oxygen is deso bed, ela ed o di e en O2 species: he so-called α-oxygen (< 550 ˚C ) and β-
oxygen (> 550 ⁰C) [39] . α-Oxygen comp ises weakly adso bed species a he su ace and
la ice oxygen gene a ed om he disloca ions o g ain on ie s [27] . Only he oxygen
eleased om acancies loca ed e y nea o o on he su ace is likely o deso b a his
empe a u e. On he o he hand, β-oxygen accoun s o oxygen being eleased om inne
laye s o he la ice, which could cause he educ ion o B si e ca ion [27]
22
Figu e 9a shows O2-TPD p o iles o La1-xS xCoO3 samples and Figu e 10a he e olu ion o
di e en oxygen species (Oα, Oβ and OTOTAL) wi h subs i u ion deg ee. As al eady obse ed
by H2-TPR s on ium doping does no induce any change in he cobal oxida ion s a e, and
hus, he posi i e cha ge de ec due o he subs i u ion o La+3 by S +2 should be balanced by
he o ma ion o oxygen acancies [33, 40-44] . As can be obse ed, subs i u ion deg ee up o
30%, i.e. La0.7S 0.3CoO3, nea ly iples he amoun o Oα species wi h espec o he non-
subs i u ed sample, which could be also ela ed o he p omo ion o speci ic su ace a ea [45]
On he con a y, subs i u ion deg ees highe han 0.4 penalize he amoun o Oα species due o
o ma ion o su ace impu i ies which educe su ace a ea. Wi h espec o Oβ, La subs i u ion
by S enhanced he o ma ion o hese species in he whole ange. Howe e , we e i ied by
MS ha TCD signal a ia ions o empe a u es highe han 600 C we e no only due o O2
deso p ion bu also due o CO2 o ma ion by su ace ca bona es decomposi ion. This explains
he ema kable inc ease in he TCD signal a empe a u es highe han 600 C o pe o ski e
samples wi h high subs i u ion deg ee, as hose samples p esen ed high amoun o su ace
impu i ies, as e ealed by XRD.
FIGURE 9
La1-xS xMnO3 samples (Figu e 9b) show he same deso p ion egions iden i ied o Mn
based samples a ibu ed o Oα and Oβ. Howe e , s on ium doping does no esul in a
signi ican change o Oα species in La1-xS xMnO3 samples, as opposi e o La1-xS xCoO3
(Figu e10b). Thus, cha ge compensa ion in La1-xS xMnO3 samples seems o be accomplished
p e e ably by modi ica ion o Mn oxida ion s a e (as obse ed by H2-TPR) a he han
o ma ion o oxygen acancies. In he high empe a u e egion, he oxygen deso p ion peak
si ua ed a ound 600-700 C is assigned o he elease o la ice oxygen which esul s in he
educ ion o Mn4+ ions o Mn3+. This peak shi s o lowe empe a u e and ends o inc ease as
23
S subs i u ion deg ee is inc eased, which sugges s a highe acili y o oxygen o di use
along he la ice and a highe Mn4+ con en [33] . I is wo h o no e ha an addi ional peak
appea s a ound 800 C o high S subs i u ed samples which is assigned o he
decomposi ion o su ace impu i ies in he o m o ca bona es, mainly S CO3. Highe con en
o impu i ies changes he O2 deso p ion p o ile o La0.5S 0.5MnO3.
FIGURE 10
3.3.5. Su ace elemen al composi ion and oxida ion s a es (XPS). X-Ray pho oelec on
spec oscopy (XPS) is a su ace-sensi i e echnique and p o ides in o ma ion o he su ace
elemen al composi ion and he oxida ion s a e o each compound, which could be modi ied
by S doping [44, 46-48] . Su ace composi ion and binding ene gies o La 3d5/2, Co 2p3/2, S
3d5/2 and O 1s co e le els we e eco ded by XPS o LaCoO3 pe o ski es (Figu e S4). La
3d5/2 ansi ion was decon olu ed in wo di e en con ibu ions loca ed a 832.9 and 834.6 eV
wi h hei co esponding shake up sa elli es. These con ibu ions we e assigned o La
accommoda ed wi hin he la ice and seg ega ed lan hanum as an oxide (La2O3), espec i ely.
Simila beha io was obse ed o S 3d5/2 ansi ion, i ing he obse ed spec a wi h wo
con ibu ions loca ed a 131.6 and 133.5 eV wi h hei co esponding shake up sa elli es, and
again assigned o con ibu ions due o la ice and seg ega ed s on ium phases, mainly as
S CO3, as obse ed by XRD. O 1s ansi ion shows a b oad emission line be ween 526 eV
and 536 eV, which can be decon olu ed in o h ee dis inc peaks a ound 528.8, 531.2 and
534.0 eV. Lowe BE alue signal co esponds o la ice oxygen bonded o ca ions in he
c ys al s uc u e (O2-la ). In e media e binding ene gy con ibu ion (Oads) is asc ibed o O-C o
O=C bonds, as o example in ca bona es, oxygen bonded o ca ions as seg ega ions (La2O3
o Co3O4) o weakly bonded O2 (associa ed wi h su aces de ec s, i.e. su ace acancies). The
signal a highes binding ene gy alues is due o wa e and hyd oxyl g oups on he su ace
24
[33, 49-52] . Co 2p3/2 ansi ion shows a s ong asymme ic peak a ound 779.5 eV wi h a
weak sa elli e peak a 790 eV. The o me signal could be esol ed in o wo componen s
a ibu able o Co+3 (779.5 eV) and Co+2 (780.7) in a hyb id oxida ion s a e en i onmen ,
espec i ely. Besides, an addi ional con ibu ion a 782.1 eV (wi h a shake-up sa elli e a
784.5) is needed in o de o i expe imen al da a, which is assigned o Co2+ in a CoO
seg ega ed phase [37] .
Table 6 p esen s S /Co and La/Co a omic su ace a ios along wi h a ios o la ice oxygen
and seg ega ed s on ium wi h espec o o al oxygen and s on ium con en , espec i ely. As
can be obse ed he e is a su ace en ichmen o S and La i espec i e o s on ium doping
deg ee, i.e. su ace a omic a io o s on ium and La wi h espec o cobal is no ably highe
han he heo e ical alue shown in b acke s, which is in line wi h lan hanum and s on ium
seg ega es obse ed by XRD. On he o he hand, i can be obse ed ha seg ega ed s on ium
ac ion inc eases wi h inc easing s on ium doping deg ee, e ealing a limi ed
accommoda ion capaci y o S in he La1-xS xCoO3 la ice. Wi h espec o O2, dec easing
con ibu ion o la ice oxygen is de ec ed in he O 1s ansi ion as S doping inc eases,
balanced wi h an inc ease con ibu ion o oxygen adspecies o oxygen bonded o ca ions as
seg ega ions.
TABLE 6
Simila p ocedu e was ollowed wi h S doped LaMnO3 pe o ski es, eco ding La 3d5/2,
Mn 2p3/2, S 3d5/2 and O 1s ansi ions by XPS (Figu e S5). Fo hese samples, i was possible
o decon olu e La 3d5/2 ansi ion in a unique componen si ua ed a 834.0 eV along wi h he
shake-up sa elli e, assigned o lan hanum being pa o he pe o ski e la ice. In con as , as
al eady obse ed o LaCoO3 pe o ski es, wo di e en componen s we e needed o i
expe imen al S 3d5/2 ansi ion, assigned o la ice and seg ega ed s on ium. Asymme ical
25
signal a 641.6 eV was eco ded o Mn 2p3/2 ansi ion wi h a weak sa elli e. Quan i ica ion
o Mn oxida ion s a e is a he di icul by XPS, due o small di e ences in he binding
ene gy o Mn+4 and Mn+3 ions. Binding ene gy o Mn 2p3/2 ansi ion appea s nea o he
heo e ical alue a ibu ed o Mn+4 and Mn+3, which is indica i e o a mix oxida ion s a e
[10, 33, 49, 53, 54] . O 1s ansi ion o La1-xS xMnO3 samples can be decon olu ed
ollowing he same p ocedu e as ha ollowed o LaCoO3.
Table 6 p esen s S /Mn and La/Mn a omic su ace a ios along wi h a ios o la ice oxygen
and seg ega ed s on ium wi h espec o o al oxygen and s on ium con en , espec i ely.
Su ace en ichmen o S and La can also be obse ed o La1-xS xMnO3. In con as ,
seg ega ed s on ium e sus o al s on ium con en shows a cons an alue i espec i e o
s on ium doping deg ee, which e eals a la ge accommoda ion capaci y o S in he la ice
o La1-xS xMnO3 samples wi h espec o La1-xS xCoO3. No e also ha o he same S doping
deg ee seg ega ed s on ium is much highe in La1-xS xCoO3 han La1-xS xMnO3.
3.3.6. NO- o-NO2 oxida ion o S -doped pe o ski es. NO- o-NO2 oxida ion capaci y o Co
and Mn pe o ski es wi h di e en S con en s a e plo ed in Figu e 11, including he
e olu ion o he equilib ium con e sion wi h empe a u e o he chosen eac ion condi ions
(b oken lines). The oxida ion capaci y o a pla inum model ca alys (1.5% P -
5% CeO2/Al2O3, is also included in bo h cases o compa ison pu pose. S on ium doped
LaCoO3 samples u he imp o e NO con e sion in compa ison o non-subs i u ed sample,
especially a in e media e empe a u es. Among p epa ed samples, La0.7S 0.3CoO3 pe o ski e
p esen ed he bes NO- o-NO2 pe o mance (Figu e 11a), wi h a maximum NO con e sion o
83% a 300 C (71% achie ed wi h LCO and 30%, and only 30% a 420 ºC wi h P -based
con en ional ca alys ) . We sugges ha op imum ac i i y o La0.7S 0.3CoO3 pe o ski e is he
esul o bes balance be ween α oxygen species (O2-TPD) and BET su ace a ea. I has been
obse ed ha s on ium doping p omo es he o ma ion o α oxygen species bu does no
induce any change in he oxida ion s a e o Co, emaining as Co3+ i espec i e o S doping.
Thus, i can be deduced ha highe α oxygen species implies highe oxygen mobi li y and
highe exchange capaci y be ween la ice and eed s eam oxygen, which inc eases he
amoun o ac i e oxygen o NO oxida ion [55] Besides, among S doped pe o ski es,
32
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36
TABLES AND FIGURES CAPTIONS
TABLE 1. Nomencla u e, syn hesis condi ions and ex u al p ope ies o he p epa ed
LaCoO3 and LaMnO3 pe o ski es.
TABLE 2. NO- o-NO2 oxida ion con e sions and eac ion a es o LaMnO3 and LaCoO3
pe o ski es in a di e en ial eac o .
TABLE 3. Tex u al p ope ies o La1-x S xCoO3 and La1-x S xMnO3 pe o ski es, wi h x om 0
o 0.5.
TABLE 4. Decon olu ed hyd ogen consump ion ela ed o di e en educ ion s eps o
La1-xS xCoO3 pe o ski es, wi h x om 0 o 0.5.
TABLE 5. Decon olu ed hyd ogen consump ion ela ed o di e en educ ion s eps o La1-x
S xMnO3 pe o ski es, wi h x om 0 o 0.5.
TABLE 6. Su ace a omic a ios o di e en compounds in La1-xS xCoO3 and La1-xS xMnO3
pe o ski es wi h x om 0 o 0.5.
TABLE 7. NO- o-NO2 oxida ion ac i i y o di e en pe o ski e o mula ions epo ed in he
li e a u e, unde di e en eac ion condi ions, in compa ison o La1-xS xMnO3 and La1-
xS xCoO3 p epa ed in his wo k.
Figu e 1. TGA and DTGA cu es o LaCoO3 gel p ecu so p epa ed wi h di e en ci ic
acid o ni a e mola a ios (CA/N): 0.7, 1.1 and 1.5.
Figu e 2. XRD di ac og ams o LaCoO3 pe o ski es syn hesized wi h di e en ci ic acid
o ni a e mola a ios (CA/N): 0.7, 1.1 and 1.5.
37
Figu e 3. NO- o-NO2 oxida ion capaci y o LaCoO3 pe o ski es p epa ed wi h di e en
syn hesis condi ions: a) ci ic acid o ni a e mola a ios (CA/N = 0.7, 1.1 and 1.5), b) pH
alues o he gel (pH = 3, 6, 7, 8 and 9) and c) calcina ion empe a u e (Temp. = 600, 700,
800, 800 °C).
Figu e 4. NO- o-NO2 oxida ion capaci y o LaCoO3 and LaMnO3 pe o ski es p epa ed by
op imal syn hesis condi ions.
Figu e 5. Linea iza ion o A henius equa ion in o de o ob ain ac i a ion ene gies o NO- o-
NO2 eac ion o e LaCoO3 and LaMnO3 pe o ski es.
Figu e 6. XRD di ac og ams o a) La1-x S xCoO3 and b) La1-x S xMnO3 wi h x anging om
0 o 0.5 (▲ S CO3, ■ La(OH)3 and ○ Co3O4).
Figu e 7. H2-TPR p o iles o La1-x S xCoO3 samples wi h x anging om 0 o 0.5.
Figu e 8. H2-TPR p o iles o La1-x S xMnO3 samples wi h x anging om 0 o 0.5.
Figu e 9. O2-TPD p o iles o a) La1-x S xCoO3 and b) La1-x S xMnO3, wi h x anging om 0
o 0.5 pe o ski es.
Figu e 10. E olu ion o , and o al deso bed oxygen species wi h subs i u ion deg ee o :
a) La1-x S xCoO3 and b) La1-x S xMnO3 pe o ski es.
Figu e 11. NO- o-NO2 oxida ion capaci y o a) La1-xS xCoO3 and b) La1-xS xMnO3
pe o ski es wi h x anging om 0 o 0.5, oge he wi h model P based ca alys .
38
TABLE 1
Nomencla u e, syn hesis condi ions and ex u al p ope ies o he p epa ed LaCoO3
and LaMnO3 pe o ski es.
Pe o ski e
Nomencla u e
CA/N
pH
T. Calc., C
SSA, m2 g-1
dc, nm
LaCoO3
LCO (0.7/7/700)
0.7
7
700
5.1
51
LCO (1.1/7/700)
1.1
7
700
10.6
40
LCO (1.5/7/700)
1.5
7
700
10.1
67
LCO (1.1/3/700)
1.1
3
700
11.3
57
LCO (1.1/4.5/700)
1.1
4.5
700
9.7
45
LCO (1.1/6/700)
1.1
6
700
9.7
41
LCO (1.1/8/700)
1.1
8
700
12.6
38
LCO (1.1/9/700)
1.1
9
700
13.2 (15.2*)
46 (34*)
LCO (1.1/7/600)
1.1
7
600
14.6
24
LCO (1.1/7/700)(+)
1.1
7
700
11.0 (11.6*)
41 (39*)
LCO (1.1/7/800)
1.1
7
800
7.0
164.
LCO (1.1/7/900)
1.1
7
900
4.3
212
LaMnO3
LMO (1.1/4.5/700)
1.1
4.5
700
21.6
42
LMO (1.1/6/600)
1.1
6
600
31.3
31
LMO (1.1/6/700)
1.1
6
700
26.0
34
LMO (1.1/7/700)
1.1
7
700
23.0
45
LMO (1.1/8/700)
1.1
8
700
13.9
51
(*) Calcina ion in 5% O2/He low.
39
TABLE 2
NO- o-NO2 oxida ion con e sions and eac ion a es o LaMnO3 and LaCoO3 pe o ski es in a di e en ial eac o .
LaCoO3 (LCO 1.1/8/700*)
LaMnO3 (LMO 1.1/6/600*)
Tempe a u e, °C
XNO- o-NO2, %
(- A), µmol min-1 m-2
Tempe a u e, °C
XNO- o-NO2, %
(- A), µmol min-1 m-2
140
2.64
0.260
140
5.74
0.281
164
3.72
0.367
163
5.85
0.286
188
7.56
0.746
189
8.10
0.396
211
14.57
1.437
218
12.71
0.621
236
25.65
2.531
238
19.01
0.929
40
TABLE 3
Tex u al p ope ies o La1-x S xCoO3 and La1-x S xMnO3 pe o ski es, wi h x om 0 o 0.5.
Sample
Su ace a ea, m2 g-1
Vp, cm3 g-1
dc, nm
LaCoO3(*)
16.2
0.09
32
La0.9S 0.1CoO3(*)
17.2
0.09
27
La0.8S 0.2CoO3(*)
21.4
0.11
18
La0.7S 0.3CoO3(*)
20.3
0.12
18
La0.6S 0.4CoO3(*)
18.8
0.11
16
La0.5S 0.5CoO3(*)
15.7
0.09
15
LaMnO3(**)
28.2
0.14
27
La0.9S 0.1MnO3(**)
33.1
0.17
21
La0.8S 0.2MnO3(**)
35.8
0.18
20
La0.7S 0.3MnO3(**)
39.1
0.17
15
La0.6S 0.4MnO3(**)
47.9
0.19
15
La0.5S 0.5MnO3(**)
39.6
0.16
14
(*) All samples syn hesized unde same condi ions ha LCO (1.1/8/700)
(**) All samples syn hesized unde same condi ions ha LMO (1.1/6/600)
41
TABLE 4
Decon olu ed hyd ogen consump ion ela ed o di e en educ ion s eps o La1-xS xCoO3 pe o ski es, wi h x om 0 o 0.5.
Sample
H2/Co
O2 chem (a), µmol H2 g-1
Co+3Co+2 (b), µmol H2 g-1
Co+2Co0 (c), µmol H2 g-1
Co+3/Co (d)
LaCoO3
1.53
145
2028
3990
0.98
La0.9S 0.1CoO3
1.55
222
2144
4130
0.96
La0.8S 0.2CoO3
1.53
282
2139
4357
1.02
La0.7S 0.3CoO3
1.49
315
2158
4237
0.98
La0.6S 0.4CoO3
1.38
542
2137
3245
0.76
La0.5S 0.5CoO3
1.34
590
1796
2086
0.58
(a) In eg a ion o peak cen e ed a ound 225 C.
(b) Sum o in eg a ed peaks cen e ed a ound 325 and 375 C.
(c) Sum o in eg a ed peaks cen e ed a ound 500, 575 and 650 C.
(d) Es ima ion o ela i e Co3+ con en based on decon olu ed peak a eas ela ion, Co3+/Co= (Co+2Co0)/(Co+3Co+2/2).
Figu e 2.
Figu e 3.
Figu e 4.
Figu e 5.
Figu e 6.
Figu e 7.
Figu e 8.
Figu e 9.
Figu e 10.
Figu e 11.
