ma e ials
A icle
Compa a i e S udy o he E iciency o Di e en Noble Me als
Suppo ed on Hyd oxyapa i e in he Ca aly ic Lean Me hane
Oxida ion unde Realis ic Condi ions
Zouhai Boukha *, Bea iz de Ri as, Juan R. González-Velasco, JoséI. Gu ié ez-O iz and Rubén López-Fonseca
Ci a ion: Boukha, Z.; de Ri as, B.;
González-Velasco, J.R.;
Gu ié ez-O iz, J.I.; López-Fonseca,
R. Compa a i e S udy o he
E iciency o Di e en Noble Me als
Suppo ed on Hyd oxyapa i e in he
Ca aly ic Lean Me hane Oxida ion
unde Realis ic Condi ions. Ma e ials
2021,14, 3612. h ps://doi.o g/
10.3390/ma14133612
Academic Edi o : Lucjan Chmiela z
Recei ed: 25 May 2021
Accep ed: 23 June 2021
Published: 28 June 2021
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2021 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
Chemical Technologies o En i onmen al Sus ainabili y G oup, Chemical Enginee ing Depa men , Facul y o
Science and Technology, Uni e si y o The Basque Coun y UPV/EHU, 48940 Leioa, Bizkaia, Spain;
[email p o ec ed] (B.d.R.); [email p o ec ed] (J.R.G.-V.);
[email p o ec ed] (J.I.G.-O.); [email p o ec ed] (R.L.-F.)
*Co espondence: zouhai [email p o ec ed]
Abs ac :
The combus ion o lean me hane was s udied o e palladium, hodium, pla inum, and
u henium ca alys s suppo ed on hyd oxyapa i e (HAP). The samples we e p epa ed by we ness im-
p egna ion and ho oughly cha ac e ized by BET, XRD, UV-Vis-NIR spec oscopy, H
2
-TPR, OSC, CO
chemiso p ion, and TEM echniques. I was ound ha he Pd/HAP and Rh/HAP ca alys s exhibi ed
a highe ac i i y compa ed wi h P /HAP and Ru/HAP samples. Thus, he deg ee o oxida ion o he
suppo ed me al unde he eac ion mix u e no ably in luenced i s ca aly ic pe o mance. Al hough
Pd and Rh ca alys s could be easily e-oxidized, he e-oxida ion o P and Ru samples appea ed o
be a slow p ocess, esul ing in small amoun s o me al oxide ac i e si es. Feeding wa e and CO
2
was
ound o ha e a nega i e e ec , which was mo e p onounced in he p esence o wa e , on he ac i i y
o Pd and Rh ca alys s. Howe e , he inhibi ing e ec o CO
2
and H
2
O dec eased by inc easing he
eac ion empe a u e.
Keywo ds: noble me als; hyd oxyapa i e; lean me hane oxida ion; OSC; wa e and CO2e ec s
1. In oduc ion
The implemen a ion o s ic en i onmen al egula ions imposes he use o mo e
e icien and cleane uels. In his sense, he u iliza ion o lean bu n na u al gas (NG)
engines is one o he mos a ac i e s a egies compa ed o hose based on gasoline o
diesel [
1
–
6
]. In addi ion o i s high calo i ic alue, he combus ion o his uel esul s in
ad an ageous CO
2
emissions and emi s lowe le els o oxic CO and NO
x
. Howe e ,
he elease o unbu n me hane p esen s a signi ican p oblem ha con ibu es o he
g eenhouse e ec . Though i is less abundan in he a mosphe e, me hane is signi ican ly
mo e ac i e in apping adia ion han CO2.
As a esul , i is essen ial ha ac i e and du able ca alys s a e de eloped o CH
4
emission aba emen . I is known ha noble me als exhibi a high ac i i y and good
esis ance o coke o ma ion [
1
,
4
–
7
]. Among he noble me als, Pd-based ca alys s suppo ed
on alumina a e clea ly he mos in es iga ed sys ems. Ne e heless, ew ca alys s ha e
demons a ed a easonable ac i i y and long- e m s abili y unde ealis ic eac ion mix u es,
in he p esence o la ge amoun s o H
2
O (10–15%) and CO
2
(10–15%), and ace le els
o sul u compounds (SO
2
o H
2
S) p esen in he exhaus gas o NG engines [
4
]. To
add ess hese majo disad an ages, cu en esea ch is ocused on he use o al e na i e
suppo s ha p esen sui able in e ac ions wi h he ac i e phase [
1
,
2
,
6
–
9
]. Simila ly, a ious
p omo e s and bime allic o mula ions ha e been examined o imp o e he ac i i y and
he esis ance o he ca alys s [10–13].
Nume ous epo s ha e claimed ha he oxida ion s a e o he ca alys plays a key ole
in con olling i s pe o mance o me hane oxida ion. The e is b oad consensus ha he
ac i e si es a e gene ally composed o he me al oxide [
1
,
4
,
7
,
13
]. Howe e , he occu ence
Ma e ials 2021,14, 3612. h ps://doi.o g/10.3390/ma14133612 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2021,14, 3612 2 o 15
o oxygen acan si es in he p oximi y o he ac i e phase o a he me al–suppo in e ace
also seems o be ex emely impo an . This con e s o he ca alys a signi ican abili y o
unde go edox p ocess and oxygen mobili y, in acco dance wi h he Ma s–Van K e elen
mechanism. Cullis e al. [
13
] demons a ed ha he na u e o he used suppo in luenced
he abili y o noble me als o adso b oxygen, and he e was a co ela ion be ween he
oxygen adso p ion capaci y o he suppo ed p ecious me al and i s ca aly ic ac i i y o
me hane oxida ion.
Due o hei dis inc p ope ies, HAP ma e ials ha e a ac ed conside able a en ion
as ca alys suppo s o nume ous applica ions [
14
–
24
]. P e ious epo s no ed ha he
ad an age o he use o HAP as a suppo lies in i s capaci y o e ain i s s uc u al p ope ies
when i unde goes signi ican changes in composi ion [
7
,
24
]. The p omising esul s a ising
om he use o HAP we e ound o be connec ed o i s high he mal s abili y, acid-base
p ope ies, and he ac ha i could p o ide bene icial syne gis ic e ec s wi h a a ie y o
me allic ac i e phases [14–24].
In he p esen s udy, we examined he sui abili y o HAP as a suppo o a se ies o
noble me al ca alys s (Pd, Rh, P and Ru) o he me hane oxida ion eac ion. The ca alys s
we e ex ensi ely cha ac e ized by BET, XRD, TEM, H
2
-TPR, UV-Vis-NIR spec oscopy, CO
chemiso p ion, and OSC echniques o co ela e hei ac i i y wi h he s uc u al ea u es.
To he bes o ou knowledge, no simila compa a i e s udy has been conduc ed on he
ac i i y o he se ies o noble me al ca alys s suppo ed on HAP.
2. Ma e ials and Me hods
2.1. P epa a ion o he Ca alys s
The HAP suppo was syn hesized using he co-p ecipi a ion me hod, adding d op
wise a calcium ni a e solu ion o a solu ion o (NH
4
)
2
HPO
4
in a basic medium (pH = 10).
Unde s i ing, he mix u e was hea ed o 80
◦
C and hen main ained o 16 h o accele a e
i s c ys alliza ion p ocess. A e il a ion, he eco e ed solid was washed wi h dis illed
wa e , d ied o e nigh a 120 ◦C, and hen calcined in s a ic ai a 500 ◦C o 4 h.
The noble me al ca alys s (Pd/HAP, Rh/HAP, P /HAP, and Ru/HAP), wi h a me al
loading o 0.5 w .%, we e p epa ed by incipien we ness imp egna ion o he suppo wi h
aqueous solu ions o he espec i e p ecu so sal s; namely Pd(NH
3
)
4
Cl
2·
H
2
O, RhCl
3·
3H
2
O,
[P (NH
3
)
4
](NO
3
)
2
, and Ru(NO)(NO
3
)
3
, espec i ely. The imp egna ed samples we e d ied
o e nigh a 120 ◦C and hen calcined a 500 ◦C o 4 h.
2.2. Cha ac e iza ion Techniques
The ex u al p ope ies o he p epa ed ca alys s we e in es iga ed by N
2
physiso p-
ion expe imen s a
−
196
◦
C on a Mic ome i ics (TRISTAR II 3020) appa a us (Mic omi i ics
Ins umen Co p, No c oss, GA, USA). P io o analysis, he samples we e pu ged wi h N
2
low a 300
◦
C o 8 h. The s uc u al p ope ies we e in es iga ed by XRD. The analyses
we e pe o med on a X’PERT-MPD X- ay ins umen (Mal e n Panaly ical L d., Roys on,
UK). The coo dina ion and he oxida ion s a e o he me allic species we e s udied by di use
e lec ance UV-Vis-NIR spec oscopy wi h a Ca y 5000 appa a us (Agilen Technologies,
San a Cla a, CA, USA).
Tempe a u e-p og ammed educ ion wi h hyd ogen (H
2
-TPR) expe imen s we e
ca ied ou on a Mic ome i ics Au oChem 2920 appa a us (Mic omi i ics Ins umen Co p.
No c oss, GA, USA). The samples we e p e- ea ed in a low o 5% O
2
/He a 500
◦
C o
30 min and hen cooled o
−
30
◦
C in He. Finally, hey we e educed in 5% H
2
/A gas
low (50 cm
3
min
−1
) by inc easing he empe a u e om
−
30 o 750
◦
C wi h a amp o
10 ◦C min−1.
The size and dispe sion o he me allic pa icles o he educed samples we e in es-
iga ed by ansmission elec on mic oscopy (TEM). The obse a ions we e pe o med
on a TECNAI G2 20 TWIN mic oscope (FEI Company, Hillsbo o, OR, USA) ope a ed a
200 kV and equipped wi h a LaB6 ilamen . The me allic dispe sion was also es ima ed
by CO chemiso p ion a 40
◦
C using he same ins umen used o H
2
-TPR s udies. The
Ma e ials 2021,14, 3612 3 o 15
samples we e p e- educed by lowing 5% H
2
/A (50 cm
3
min
−1
) a 400
◦
C o 1 h. Then,
hey we e cooled unde He low o 40
◦
C and submi ed o a se ies o CO pulses (using
5% CO/He and a loop olume o 0.5 cm
3
) un il hei sa u a ion. The me allic dispe sion,
de ined as he exposed ac i e me al ac ion, was de e mined on he assump ion o a uni y
adso p ion s oichiome y.
The oxygen s o age capaci y (OSC) o he syn hesized samples was s udied by oxygen
olume ic chemiso p ion in he empe a u e ange o 350–500
◦
C on a Mic ome i ics
Au oChem 2920 ins umen (Mic omi i ics Ins umen Co p. No c oss, GA, USA). The
samples (100 mg) we e p e- educed wi h a low o 5% H
2
/A (50 cm
3
min
−1
) a 500
◦
C
o 1 h and, hen, e acua ed in He o 1 h. A e hei cooling o he analysis empe a u e,
hey we e submi ed o wen y O
2
pulses (5% O
2
/He, loop olume: 0.5 cm
3
), injec ed in
He ca ie (50 cm3min−1).
2.3. Ca aly ic Pe o mance Tes ing
The ca aly ic uns we e ca ied ou in a ixed-bed eac o ope a ing a a mosphe ic
p essu e. The ca alys s (200 mg, 160–250
µ
m) dilu ed wi h qua z pa icles we e p e-
ea ed unde a 5% O
2
/He low (100 cm
3
min
−1
) a 500
◦
C o 1 h. The s anda d eac ion
mix u e was composed o 1%CH
4
and 20%O
2
balanced wi h He (F
o
= 100 cm
3
min
−1
)
co esponding o a weigh hou ly space eloci y (WHSV) o 300 cm
3
CH
4
h
−1
g
−1
. The
eac ion empe a u e was inc eased om 200 o 500
◦
C wi h a amp o 1
◦
C min
−1
. The
he mocouple was posi ioned a he inle o he ca alys bed. Addi ional expe imen s we e
pe o med o s udy he in luence o he addi ion o wa e (10%) and CO
2
(10%). The wa e
apo was ed using a GILSON 307 pump (Gilson Inc., Middle on, OR, USA). The analysis
sys em consis ed o a gas ch oma og aph (Agilen Technologies 490 Mic o GC) (Agilen
Technologies, San a Cla a, CA, USA) equipped wi h a TCD de ec o .
3. Resul s
3.1. Cha ac e iza ion o he Ca alys s
The N
2
adso p ion/deso p ion iso he ms co esponding o he ba e suppo and he
ou noble me al-modi ied samples, espec i ely, a e p esen ed in Supplemen a y Ma e ials
Figu e S1. All samples exhibi simila shapes o bo h he iso he ms and hys e esis loops.
The adso p ion b anches a e analogous o hose o ype II whe eas he deso p ion iso he ms
p esen a hys e esis loop o H3, ypically ound o e agg ega es p esen ing sli -shaped
po es. Da a om Table 1 e eal ha he modi ica ion o he HAP suppo wi h he noble
me als does no p o oke signi ican changes in i s ex u al p ope ies. The measu ed
speci ic su ace a eas we e ound o ange be ween 48 and 53 m
2
g
−1
(55 m
2
g
−1
o he
ba e suppo ).
Table 1. Cha ac e iza ion da a o he noble me al ca alys s.
Sample BET H2-TPR
SBET, m2g−1Vp(a), cm3g−1dp(b), nm mmolH2 g−1(H2/M) (c) (H2/M) (d)
HAP 55 0.40 27.8 – – –
Pd/HAP 53 0.35 26.5 3.8 10−20.8 1
Rh/HAP 51 0.40 29.7 6.2 10−21.3 1.5
P /HAP 51 0.37 28.7 1.1 10−20.4 2
Ru/HAP 48 0.36 29.1 8.7 10−21.8 2
(a)
Po e olume, as de e mined by applica ion o he BJH me hod.
(b)
Mean po e diame e , as de e mined by applica ion o he BJH me hod.
(c)
Ac ual H
2
/M mola a io as de e mined om he in eg a ion o he educ ion peaks a T < 250
◦
C.
(d)
Theo e ical H
2
/M mola a io
equi ed o he educ ion o s oichiome ic PdO, Rh2O3, P O2, and RuO2.
The XRD pa e n o he HAP ba e suppo (Figu e 1) shows ha he posi ions o all
di ac ion peaks a e iden ical o hose expec ed o he hyd oxyapa i e s uc u e (JCPDS
01-082-2956), which c ys allizes in an hexagonal sys em and belongs o he P6
3
/m space
g oup. The addi ion o noble me als (Pd, Rh, P , and Ru) o he HAP and he successi e
Ma e ials 2021,14, 3612 4 o 15
he mal ea men a 500
◦
C does no signi ican ly a ec he s uc u e o he suppo and i s
la ice pa ame e s (Supplemen a y Ma e ials Table S1). Mo eo e , no addi ional di ac ion
peaks due o he deposi ed me als a e obse ed, which can be explained by hei small
amoun s (0.5 w .%) and/o hei high dispe sion.
Ma e ials 2021, 14, x FOR PEER REVIEW 4 o 15
The XRD pa e n o he HAP ba e suppo (Figu e 1) shows ha he posi ions o all
di ac ion peaks a e iden ical o hose expec ed o he hyd oxyapa i e s uc u e (JCPDS
01-082-2956), which c ys allizes in an hexagonal sys em and belongs o he P63/m space
g oup. The addi ion o noble me als (Pd, Rh, P , and Ru) o he HAP and he successi e
he mal ea men a 500 °C does no signi ican ly a ec he s uc u e o he suppo and
i s la ice pa ame e s (Supplemen a y Ma e ials Table S1). Mo eo e , no addi ional di -
ac ion peaks due o he deposi ed me als a e obse ed, which can be explained by hei
small amoun s (0.5 w .%) and/o hei high dispe sion.
Figu e 1. XRD pa e ns o he noble me al ca alys s.
The op ical p ope ies o he suppo ed me allic species we e in es iga ed using
UV-Vis-NIR spec oscopy. Figu e 2 shows he co esponding abso p ion spec a. The
spec um o he HAP suppo exhibi s a s ong UV abso p ion peaked a 200 nm and a
shoulde nea 280 nm, assigned o O2− → Ca2+ cha ge ans e s. I espec i e o he na u e
o he added me al phase, hese UV bands appea o in ensi y ma kedly, he eby indi-
ca ing he con ibu ion o O2− → Mx+ cha ge ans e s. Mo eo e , new bands appea in he
isible domain due o he d-d ansi ions o he ca ionic o ms o he imp egna ed me als.
Hence, he Pd/HAP sample exhibi s a band cen e ed a 420 nm, assigned o d-d (ν1 + ν2)
ansi ions o Pd2+ ions in a e ahed al coo dina ion [1,7]. The spec um o he Rh/HAP
sample con ains a s ong abso p ion peaked a 340 nm accompanied by a shoulde nea
440 nm. Acco ding o ou p e ious s udy, he obse ed shape and posi ions indica e he
main occu ence o Rh2O3 species [18]. The spec um o he P /HAP sample is cha ac e -
ized by he p esence o a much less in ense isible band loca ed a 430 nm, hus sug-
ges ing a deposi ion o ela i ely smalle amoun s o he P ca ionic species. The posi ion
o his ea u e is e y close o ha obse ed on he spec um o a P O2 phase, as epo ed
by Lie z e al. [25]. The deposi ion o Ru on he HAP suppo induces he occu ence o a
b oad abso p ion band ex ended om he isible o he NIR egion and peaked a 850
nm. Xiao e al. [26] obse ed a simila ea u e, which was assigned o hyd ous RuO2 na-
nopa icles (RuO2·xH2O).
Figu e 1. XRD pa e ns o he noble me al ca alys s.
The op ical p ope ies o he suppo ed me allic species we e in es iga ed using
UV-Vis-NIR spec oscopy. Figu e 2shows he co esponding abso p ion spec a. The
spec um o he HAP suppo exhibi s a s ong UV abso p ion peaked a 200 nm and
a shoulde nea 280 nm, assigned o O
2−→
Ca
2+
cha ge ans e s. I espec i e o he
na u e o he added me al phase, hese UV bands appea o in ensi y ma kedly, he eby
indica ing he con ibu ion o O
2−→
M
x+
cha ge ans e s. Mo eo e , new bands appea
in he isible domain due o he d-d ansi ions o he ca ionic o ms o he imp egna ed
me als. Hence, he Pd/HAP sample exhibi s a band cen e ed a 420 nm, assigned o
d-d (
ν1
+
ν2
) ansi ions o Pd
2+
ions in a e ahed al coo dina ion [
1
,
7
]. The spec um
o he Rh/HAP sample con ains a s ong abso p ion peaked a 340 nm accompanied
by a shoulde nea 440 nm. Acco ding o ou p e ious s udy, he obse ed shape and
posi ions indica e he main occu ence o Rh
2
O
3
species [
18
]. The spec um o he P /HAP
sample is cha ac e ized by he p esence o a much less in ense isible band loca ed a
430 nm, hus sugges ing a deposi ion o ela i ely smalle amoun s o he P ca ionic
species. The posi ion o his ea u e is e y close o ha obse ed on he spec um o a P O
2
phase, as epo ed by
Lie z e al. [25]
. The deposi ion o Ru on he HAP suppo induces
he occu ence o a b oad abso p ion band ex ended om he isible o he NIR egion
and peaked a 850 nm.
Xiao e al. [26]
obse ed a simila ea u e, which was assigned o
hyd ous RuO2nanopa icles (RuO2·xH2O).
Ma e ials 2021,14, 3612 5 o 15
Ma e ials 2021, 14, x FOR PEER REVIEW 5 o 15
Figu e 2. UV-Vis-NIR spec a o he noble me al ca alys s.
The educibili y o he p epa ed ca alys s was in es iga ed by H2-TPR. The co e-
sponding p o iles and hei quan i ica ion da a a e included in Figu e 3 and Table 1, e-
spec i ely. The Pd/HAP ca alys exhibi s a ypical diag am o he educ ion o suppo ed
PdO species [7]. The i s up ake, which peaked a 26 °C, is asc ibed o he PdO phase
whe eas he second, loca ed a 58 °C (nega i e peak), is due o he decomposi ion o pal-
ladium hyd ide (Pd-H). The p o ile o he Rh/HAP ca alys p esen s a b oad educ ion
peak cen e ed a 103 °C. This posi ion is compa able o ha ound in a p e ious s udy on
Rh/Al2O3 and Rh/CeO2 sys ems [27]. In good ag eemen wi h he UV-Vis-NIR da a, he
amoun s o consumed H2 sugges ha he oxidized Rh species is mainly composed o he
Rh2O3 phase (Table 1). The H2-TPR p o ile o he P /HAP ca alys is cha ac e ized by he
p esence o a much less in ense educ ion peak loca ed a 98 °C. Mo eo e , he es ima ed
H2/P mola a io does no exceed 0.4 (Table 1). This esul could be explained by he
p esence o addi ional species, p obably exhibi ing a s ong in e ac ion wi h he suppo
ha equi e a high educ ion empe a u e [22]. Ins ead, acco ding o he UV-Vis-NIR
da a, hese esul s indica e he deposi ion o ela i ely small amoun s o oxidized P spe-
cies. The H2-TPR p o ile o he Ru/HAP sample displays wo educ ion peaks loca ed a
97 and 130 °C. These ea u es could be assigned o he educ ion o wo dis inc u he-
nium oxide species. Gene ally, he low empe a u e peak is a ibu ed o RuO2 phase,
whe eas he peak a ela i ely highe empe a u es can be associa ed wi h Ru species
exhibi ing a s onge in e ac ion wi h he suppo [28]. The quan i ica ion o H2 con-
sump ion e eals ha he calcula ed H2/Ru mola a io is close o ha co esponding o a
s oichiome ic RuO2 phase (Table 1). I is wo h no ing ha he educ ion empe a u es o
he Ru species ound on ou Ru/HAP sample a e signi ican ly lowe han hose obse ed
Figu e 2. UV-Vis-NIR spec a o he noble me al ca alys s.
The educibili y o he p epa ed ca alys s was in es iga ed by H
2
-TPR. The co e-
sponding p o iles and hei quan i ica ion da a a e included in Figu e 3and Table 1,
espec i ely. The Pd/HAP ca alys exhibi s a ypical diag am o he educ ion o suppo ed
PdO species [
7
]. The i s up ake, which peaked a 26
◦
C, is asc ibed o he PdO phase
whe eas he second, loca ed a 58
◦
C (nega i e peak), is due o he decomposi ion o pal-
ladium hyd ide (Pd-H). The p o ile o he Rh/HAP ca alys p esen s a b oad educ ion
peak cen e ed a 103
◦
C. This posi ion is compa able o ha ound in a p e ious s udy on
Rh/Al
2
O
3
and Rh/CeO
2
sys ems [
27
]. In good ag eemen wi h he UV-Vis-NIR da a, he
amoun s o consumed H
2
sugges ha he oxidized Rh species is mainly composed o he
Rh
2
O
3
phase (Table 1). The H
2
-TPR p o ile o he P /HAP ca alys is cha ac e ized by he
p esence o a much less in ense educ ion peak loca ed a 98
◦
C. Mo eo e , he es ima ed
H
2
/P mola a io does no exceed 0.4 (Table 1). This esul could be explained by he
p esence o addi ional species, p obably exhibi ing a s ong in e ac ion wi h he suppo
ha equi e a high educ ion empe a u e [
22
]. Ins ead, acco ding o he UV-Vis-NIR da a,
hese esul s indica e he deposi ion o ela i ely small amoun s o oxidized P species.
The H
2
-TPR p o ile o he Ru/HAP sample displays wo educ ion peaks loca ed a 97
and 130
◦
C. These ea u es could be assigned o he educ ion o wo dis inc u henium
oxide species. Gene ally, he low empe a u e peak is a ibu ed o RuO
2
phase, whe eas
he peak a ela i ely highe empe a u es can be associa ed wi h Ru species exhibi ing a
s onge in e ac ion wi h he suppo [
28
]. The quan i ica ion o H
2
consump ion e eals
ha he calcula ed H
2
/Ru mola a io is close o ha co esponding o a s oichiome ic
RuO
2
phase (Table 1). I is wo h no ing ha he educ ion empe a u es o he Ru species
ound on ou Ru/HAP sample a e signi ican ly lowe han hose obse ed on o he ca alys
suppo s [
28
,
29
]. Lanza e al. [
29
] a ibu ed his e ec o he occu ence o easily educible
Ma e ials 2021,14, 3612 6 o 15
species (RuO
2-y
(OH)
y
), esul ing om he in e ac ion o he RuO
2
species wi h su ace
hyd oxyl g oups.
Ma e ials 2021, 14, x FOR PEER REVIEW 6 o 15
on o he ca alys suppo s [28,29]. Lanza e al. [29] a ibu ed his e ec o he occu ence
o easily educible species (RuO2-y(OH)y), esul ing om he in e ac ion o he RuO2 spe-
cies wi h su ace hyd oxyl g oups.
Figu e 3. H2-TPR diag ams o he noble me al ca alys s.
Figu e 4 includes TEM images o he ca alys s educed a 400 °C. In all cases, qua-
si-sphe ical me allic pa icles can be obse ed. Table 2 summa izes he a e age pa icle
size and he dispe sion o suppo ed samples. In e es ingly, he es ima ed a e age size
(dM) is ound o depend on he na u e o he ac i e me al ollowing his o de : dRh (1 nm) <
dP (2 nm) < dPd (4.6 nm) < dRu (8.3 nm). Al hough he dispe sion da a es ima ed by CO
chemiso p ions (Table 2) di e om hose gi en by TEM, he gene al endency is ound
o be main ained, i espec i e o he echnique used.
Figu e 3. H2-TPR diag ams o he noble me al ca alys s.
Figu e 4includes TEM images o he ca alys s educed a 400
◦
C. In all cases, quasi-
sphe ical me allic pa icles can be obse ed. Table 2summa izes he a e age pa i-
cle size and he dispe sion o suppo ed samples. In e es ingly, he es ima ed a e age
size (d
M
) is ound o depend on he na u e o he ac i e me al ollowing his o de :
dRh (1 nm) < dP (2 nm) < dPd (4.6 nm) < dRu (8.3 nm).
Al hough he dispe sion da a es-
ima ed by CO chemiso p ions (Table 2) di e om hose gi en by TEM, he gene al
endency is ound o be main ained, i espec i e o he echnique used.
Ma e ials 2021,14, 3612 7 o 15
Ma e ials 2021, 14, x FOR PEER REVIEW 7 o 15
Figu e 4. TEM mic og aphs o he educed noble me al ca alys s.
Table 2. CO chemiso p ion, TEM, and ca aly ic ac i i y da a o he noble me al ca alys s.
Ca alys
CO Chem
TEM
Me hane Oxida ion Reac ion
(b)
D
Chem
, %
D
TEM
, %
d
M
, nm
T
10
T
50
T
100
Ea, kJ/mol
Pd/HAP
29 (22)
(a)
18.6
4.6
300
375
475
77
Rh/HAP
96 (81)
(a)
73
1
300
390
500
76
P /HAP
47
41.2
2
430
–
–
105
Ru/HAP
4.7
8.2
8.3
500
–
–
95
(a) Da a co esponding o he ca alys s submi ed o h ee cycles o he eac ion (Figu e 9). (b) Da a
ex ac ed om Figu e 6.
The esul s co esponding o he OSC expe imen s, in he empe a u e ange be-
ween 350 and 500 °C, a e displayed in Figu e 5. These s udies p o ide aluable in o -
ma ion abou he abili y o he educed ca alys s o be e-oxidized. I should be no ed ha
he samples we e p e ea ed unde a educing a mosphe e (5% H2/A ) a 500 °C o 1 h.
As expec ed, o e all analyzed samples he OSC ac i i y inc eases wi h he empe a u e.
Pd/HAP
Rh/HAP
P /HAP
Ru/HAP
Figu e 4. TEM mic og aphs o he educed noble me al ca alys s.
Table 2. CO chemiso p ion, TEM, and ca aly ic ac i i y da a o he noble me al ca alys s.
Ca alys CO Chem TEM Me hane Oxida ion Reac ion (b)
DChem, % DTEM, % dM, nm T10 T50 T100 Ea, kJ/mol
Pd/HAP 29 (22) (a) 18.6 4.6 300 375 475 77
Rh/HAP 96 (81) (a) 73 1 300 390 500 76
P /HAP 47 41.2 2 430 – – 105
Ru/HAP 4.7 8.2 8.3 500 – – 95
(a) Da a co esponding o he ca alys s submi ed o h ee cycles o he eac ion (Figu e 9). (b) Da a ex ac ed om Figu e 6.
The esul s co esponding o he OSC expe imen s, in he empe a u e ange be ween
350 and 500
◦
C, a e displayed in Figu e 5. These s udies p o ide aluable in o ma ion abou
he abili y o he educed ca alys s o be e-oxidized. I should be no ed ha he samples
we e p e ea ed unde a educing a mosphe e (5% H
2
/A ) a 500
◦
C o 1 h. As expec ed,
o e all analyzed samples he OSC ac i i y inc eases wi h he empe a u e. Mo eo e , a
compa ison o he OSC alues o he di e en ca alys s e eals ha hey ollow his gene al
Ma e ials 2021,14, 3612 8 o 15
end: Rh/HAP > Pd/HAP > P /HAP > Ru/HAP. In e es ingly, a 500
◦
C, he oxygen
up akes measu ed on he Pd and Rh ca alys s (23.1 and 36.1
µ
mol
O2
g
−1
, espec i ely)
a e e y simila o he heo e ical alues equi ed o he o ma ion o s oichiome ic PdO
and Rh
2
O
3
phases, espec i ely. Howe e , ega dless o he analysis empe a u e, he P
and Ru ca alys s show e y low OSC. Despi e inc easing he empe a u e o 500
◦
C, he
amoun s o s o ed oxygen do no exceed 6.1 and 3.1
µ
mol
O2
g
−1
, espec i ely, he eby
sugges ing ha hei comple e oxida ion is a e y slow p ocess which p obably equi es
highe empe a u es. Ins ead, his low OSC ac i i y can be ela ed o he oxygen s o age
si es loca ed a he nea su ace only.
Ma e ials 2021, 14, x FOR PEER REVIEW 8 o 15
Mo eo e , a compa ison o he OSC alues o he di e en ca alys s e eals ha hey
ollow his gene al end: Rh/HAP > Pd/HAP > P /HAP > Ru/HAP. In e es ingly, a 500
°C, he oxygen up akes measu ed on he Pd and Rh ca alys s (23.1 and 36.1 μmolO2 g−1,
espec i ely) a e e y simila o he heo e ical alues equi ed o he o ma ion o s oi-
chiome ic PdO and Rh2O3 phases, espec i ely. Howe e , ega dless o he analysis
empe a u e, he P and Ru ca alys s show e y low OSC. Despi e inc easing he em-
pe a u e o 500 °C, he amoun s o s o ed oxygen do no exceed 6.1 and 3.1 μmolO2 g−1,
espec i ely, he eby sugges ing ha hei comple e oxida ion is a e y slow p ocess
which p obably equi es highe empe a u es. Ins ead, his low OSC ac i i y can be e-
la ed o he oxygen s o age si es loca ed a he nea su ace only.
Figu e 5. OSC s udies on he noble me al ca alys s.
3.2. Ca aly ic Ac i i y
Figu e 6 displays he ligh -o cu es o he p epa ed ca alys s in he me hane oxi-
da ion eac ion. No e ha ca bon dioxide was he main ca bonaceous p oduc de ec ed
and only negligible amoun s o ca bon monoxide could be obse ed. As expec ed, he
HAP ba e suppo shows a low ac i i y, which does no each 4% a 500 °C. O e he
Pd/HAP and Rh/HAP samples, me hane con e sion s a s a 225 °C and inc eases slowly
wi h he empe a u e o each 30% a 350 °C. A highe empe a u es, he Pd/HAP ca a-
lys becomes mo e e icien han he Rh/HAP sample. Ne e heless, o e he la e he
o al oxida ion o me hane can be eached when aising he eac ion empe a u e o 500
°C. Howe e , he P /HAP and Ru/HAP samples exhibi a poo e pe o mance, because
me hane oxida ion is only isible abo e 350 °C and he con e sion le els a 500 °C do no
exceed 35% and 15%, espec i ely. Thus, he ca alys e iciency ollows his gene al end:
Pd/HAP > Rh/HAP > P /HAP > Ru/HAP. This is in good ag eemen wi h he li e a u e
Figu e 5. OSC s udies on he noble me al ca alys s.
3.2. Ca aly ic Ac i i y
Figu e 6displays he ligh -o cu es o he p epa ed ca alys s in he me hane oxida ion
eac ion. No e ha ca bon dioxide was he main ca bonaceous p oduc de ec ed and only
negligible amoun s o ca bon monoxide could be obse ed. As expec ed, he HAP ba e
suppo shows a low ac i i y, which does no each 4% a 500
◦
C. O e he Pd/HAP and
Rh/HAP samples, me hane con e sion s a s a 225
◦
C and inc eases slowly wi h he
empe a u e o each 30% a 350
◦
C. A highe empe a u es, he Pd/HAP ca alys becomes
mo e e icien han he Rh/HAP sample. Ne e heless, o e he la e he o al oxida ion o
me hane can be eached when aising he eac ion empe a u e o 500
◦
C. Howe e , he
P /HAP and Ru/HAP samples exhibi a poo e pe o mance, because me hane oxida ion
is only isible abo e 350
◦
C and he con e sion le els a 500
◦
C do no exceed 35% and 15%,
espec i ely. Thus, he ca alys e iciency ollows his gene al end: Pd/HAP > Rh/HAP >
P /HAP > Ru/HAP. This is in good ag eemen wi h he li e a u e da a, in which palladium
ca alys s a e gene ally ega ded as he mos p omising candida es o comple e me hane
oxida ion [
30
–
33
]. Fo ins ance, a simila end was epo ed by
Oh e al. [30]
in hei
Ma e ials 2021,14, 3612 9 o 15
s udy on he ac i i y o
γ
-Al
2
O
3
-suppo ed ca alys s unde bo h oxidizing and educing
condi ions (Pd > Rh > P ). On he o he hand, when subjec ed o a second ligh -o
expe imen , he Pd/HAP and Rh/HAP ca alys s show a mode a e deac i a ion p ocess
(Figu e S2). The ea e , hey main ain a s able pe o mance.
Ma e ials 2021, 14, x FOR PEER REVIEW 9 o 15
da a, in which palladium ca alys s a e gene ally ega ded as he mos p omising candi-
da es o comple e me hane oxida ion [30–33]. Fo ins ance, a simila end was epo ed
by Oh e al. [30] in hei s udy on he ac i i y o γ-Al2O3-suppo ed ca alys s unde bo h
oxidizing and educing condi ions (Pd > Rh > P ). On he o he hand, when subjec ed o a
second ligh -o expe imen , he Pd/HAP and Rh/HAP ca alys s show a mode a e deac-
i a ion p ocess (Figu e S2). The ea e , hey main ain a s able pe o mance.
Figu e 6. Compa ison o he pe o mance o he p epa ed ca alys s in he me hane oxida ion. The
eac ion mix u e was composed o 1% CH4, 20% O2, and 79% He.
Because he e is no co ela ion be ween he pe o mance and he me allic dispe sion,
i can be concluded ha he dependence o me hane oxida ion on he me allic pa icle
size, sp ead on he HAP suppo , is seconda y. Ins ead, he supe io i y o Pd/HAP and
Rh/HAP samples can be explained by hei g ea e OSC ac i i y. This high endency o be
e-oxidized p obably gene a es mo e ac i e me al oxides. P e ious epo s demons a ed
ha he ac i i y o noble me als in me hane oxida ion p oceed h ough he Ma s–Van
K e elen mechanism, in which hei abili y o unde go edox p ocesses unde he eac-
ion mix u e is a de e mining ac o [13,31,32]. Acco dingly, he Ru/HAP and P /HAP
ca alys s showed a low ac i i y, because a la ge ac ion o he suppo ed me al p obably
emains in educed o ms a e being exposed o he eac ion mix u e, e en in he p es-
ence o a high concen a ion o oxygen (20%).
The appa en ac i a ion ene gy (Ea) alues we e es ima ed assuming a i s o de
eac ion. Thus, a linea co ela ion can be obse ed be ween ln[-ln(1-XCH4)] and 1/T (Fig-
u e 7). As expec ed, he mos ac i e ca alys s (Pd/HAP and Rh/HAP) show he lowes Ea
alues (76–77 kJ mol−1) compa ed wi h hose es ima ed o e P /HAP and Ru/HAP ca a-
lys s (105 and 95 kJ mol−1, espec i ely) (Table 2). No e ha ou es ima ed alues a e
compa able o hose epo ed o e noble me als suppo ed on di e en ma e ials [31–35].
Figu e 6.
Compa ison o he pe o mance o he p epa ed ca alys s in he me hane oxida ion. The
eac ion mix u e was composed o 1% CH4, 20% O2, and 79% He.
Because he e is no co ela ion be ween he pe o mance and he me allic dispe sion,
i can be concluded ha he dependence o me hane oxida ion on he me allic pa icle
size, sp ead on he HAP suppo , is seconda y. Ins ead, he supe io i y o Pd/HAP and
Rh/HAP samples can be explained by hei g ea e OSC ac i i y. This high endency o be
e-oxidized p obably gene a es mo e ac i e me al oxides. P e ious epo s demons a ed
ha he ac i i y o noble me als in me hane oxida ion p oceed h ough he Ma s–Van
K e elen mechanism, in which hei abili y o unde go edox p ocesses unde he eac ion
mix u e is a de e mining ac o [
13
,
31
,
32
]. Acco dingly, he Ru/HAP and P /HAP ca alys s
showed a low ac i i y, because a la ge ac ion o he suppo ed me al p obably emains in
educed o ms a e being exposed o he eac ion mix u e, e en in he p esence o a high
concen a ion o oxygen (20%).
The appa en ac i a ion ene gy (E
a
) alues we e es ima ed assuming a i s o de
eac ion. Thus, a linea co ela ion can be obse ed be ween ln[-ln(1-X
CH4
)] and 1/T
(Figu e 7). As expec ed, he mos ac i e ca alys s (Pd/HAP and Rh/HAP) show he lowes
E
a
alues (76–77 kJ mol
−1
) compa ed wi h hose es ima ed o e P /HAP and Ru/HAP
ca alys s (105 and 95 kJ mol
−1
, espec i ely) (Table 2). No e ha ou es ima ed alues a e
compa able o hose epo ed o e noble me als suppo ed on di e en ma e ials [31–35].
