Stability of a NiAl2O4 Derived Catalyst in the Ethanol Steam Reforming in Reaction-Regeneration Cycles: Effect of Reduction Temperature

Ci a ion: Iglesias-Vázquez, S.;
Valecillos, J.; Remi o, A.; Bilbao, J.;
Gayubo, A.G. S abili y o a NiAl2O4
De i ed Ca alys in he E hanol
S eam Re o ming in
Reac ion-Regene a ion Cycles: E ec
o Reduc ion Tempe a u e. Ca alys s
2022,12, 550. h ps://doi.o g/
10.3390/ca al12050550
Academic Edi o : Binlin Dou
Recei ed: 26 Ap il 2022
Accep ed: 16 May 2022
Published: 17 May 2022
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ca alys s
A icle
S abili y o a NiAl2O4De i ed Ca alys in he E hanol S eam
Re o ming in Reac ion-Regene a ion Cycles: E ec o
Reduc ion Tempe a u e
Se gio Iglesias-Vázquez , JoséValecillos * , Ainge u Remi o , Ja ie Bilbao and Ana Guadalupe Gayubo
Depa men o Chemical Enginee ing, Uni e si y o he Basque Coun y (UPV/EHU), P.O. Box 644,
48080 Bilbao, Spain
; [email p o ec ed] (S.I.-V.); ainge u. [email p o ec ed] (A.R.); ja ie [email p o ec ed] (J.B.);
[email p o ec ed] (A.G.G.)
*Co espondence: [email p o ec ed]; Tel.: +34-946-01-53-41
Abs ac :
The ca alys egene a ion is s ill a challenge o make he e hanol s eam e o ming (ESR)
p ocess easible o sus ainable H
2
p oduc ion. NiAl
2
O
4
spinel de i ed ca alys s a e highly ac i e
and selec i e o ESR, bu hey equi e a oiding i e e sible deac i a ion o ensu e hei egene a ion.
Thei s abili y depends on he ca alys s uc u e, and he ein we epo di e en Ni/Al
2
O
3
-NiAl
2
O
4
ca alys s ob ained upon educ ion o a NiAl
2
O
4
spinel a 700, 750, o 850
◦
C. The ca alys s we e es ed
in ESR eac ion- egene a ion cycles, wi h eac ion a 600
◦
C and egene a ion by coke combus ion a
850
◦
C ollowed by educ ion a he co esponding empe a u e. The esh, spen , and egene a ed
ca alys s we e cha ac e ized using X- ay di ac ion, N
2
physiso p ion, empe a u e p og ammed
educ ion and oxida ion, and scanning elec on mic oscopy. The i e e sible deac i a ion is due o
Ni ola iliza ion and ca alys pa icle agmen a ion. These phenomena a e p omp ed by a high
ilamen ous ca bon deposi ion a o ed by he Al
2
O
3
con en in he ca alys . The educ ion in he
700–750
◦
C ange is op imum o con olling he Al
2
O
3
con en , inc easing he NiAl
2
O
4
/Al
2
O
3
a io
in he esul ing ca alys . These ca alys s show a pe iod o pa ial e e sible deac i a ion by coke wi h
a change in he H
2
o ma ion mechanism eaching a pseudo-s able s a e wi h a H
2
yield o 40% and a
ep oducible pe o mance in successi e eac ion- egene a ion cycles.
Keywo ds:
hyd ogen; e hanol s eam e o ming (ESR); Ni ca alys ; NiAl
2
O
4
spinel; ca alys deac i a ion;
coke; ca alys egene a ion; educ ion empe a u e
1. In oduc ion
Hyd ogen is a p omising ene gy ec o [
1
] and aw ma e ial o he syn hesis o
commodi ies [
2
], complying wi h sus ainabili y p ecep s when i is ob ained om enew-
able aw ma e ials. H
2
p oduc ion om e hanol ia s eam e o ming (ESR) is a sui able
al e na i e since e hanol can be ob ained sus ainably om biomass (bio-e hanol) by e men-
a ion/hyd olysis wi h no need o sepa a e he wa e and is easy o handle wi h ela i e
low isks in compa ison wi h o he eeds ock o he H
2
p oduc ion [
3
,
4
]. Mo eo e , he
ESR undamen als a e simila o hose o CH
4
s eam e o ming (MSR) which is he basis
o he la ges echnology o H
2
p oduc ion om na u al gas ex ended wo ldwide [
5
],
ep esen ing an ad an age o making an easie adap a ion and ansi ion o sus ainable
H2p oduc ion om e hanol.
The ESR p ocess in ol es se e al eac ions in a complex se ies-pa allel scheme, in
which e hanol e o ming (Equa ion (1)) and wa e gas shi (Equa ion (2)) a e he main eac-
ions gi ing he global s eam e o ming eac ion ep esen ed by Equa ion (3) [
3
,
6
–
8
]. How-
e e , e hanol may unde go dehyd ogena ion (Equa ion (4)), dehyd a ion (Equa ion (5)), and
decomposi ion (Equa ion (6)) yielding up mo e H
2
o in e media es (e hylene and ace alde-
hyde). Ace aldehyde and e hylene may unde go s eam e o ming (
Equa ions (7) and (8)
,
espec i ely) and decomposi ion (Equa ions (9) and (10), espec i ely). CH
4
may be o med
Ca alys s 2022,12, 550. h ps://doi.o g/10.3390/ca al12050550 h ps://www.mdpi.com/jou nal/ca alys s
Ca alys s 2022,12, 550 2 o 16
om some decomposi ion eac ions (Equa ions (6) and (9)) o me hana ion (Equa ion (11)),
and simul aneously may unde go s eam e o ming ( e e se o Equa ion (11)) and de-
composi ion (Equa ion (12)). CO may unde go disp opo iona ion (Boudoua d eac ion,
Equa ion (13)). The ca bon o med om decomposi ion (Equa ions (10) and (12)) o CO
disp opo iona ion may unde go gasi ica ion (Equa ion (14)).
C2H6O+H2O→2CO + 4H2(1)
CO + H2OCO2+ H2(2)
C2H6O + 3H2O→2CO2+ 6H2(3)
C2H6O→C2H4O+H2(4)
C2H6OC2H4+ H2O (5)
C2H6O→CH4+CO+H2(6)
C2H4O+H2O→2CO + 3H2(7)
C2H4+ 2H2O→2CO + 3H2(8)
C2H4O→CH4+ CO (9)
C2H4→2C + H2(10)
CO + 3H2CH4+ H2O (11)
CH4→C + 2H2(12)
2CO C + CO2(13)
C+H2O→CO + H2(14)
The mos sui able ca alys s in e ms o ac i i y and cos s o he ESR p ocess a e based
on Co o Ni suppo ed on esilien oxides, being Ni/Al
2
O
3
ca alys s he wides s udied [
7
].
Ni is highly ac i e o b eaking C-C bonds and abso bing and dissocia ing wa e [
9
,
10
]. On
he o he hand, Al
2
O
3
, o en combined wi h La
2
O
3
o CeO
2
, is an app op ia e suppo o
a high Ni dispe sion, which imp o es he ca aly ic pe o mance and p o ides good me-
chanical s eng h o he ca alys pa icles o ope a ion in mos ypes o eac o s. Howe e ,
Ni ca alys s a e p one o su e deac i a ion due mainly o ca bon (coke) o ma ion and
deposi ion ha blocks he access o ac i e si es, and sin e ing o ac i e si es ha dec eases
he su ace a ea o amoun o ac i e si es [
11
,
12
]. The deac i a ion by ca bon deposi ion
migh be e e sible since ca bon deposi s a e elimina ed by combus ion and he ca alys
migh eco e i s ac i i y. This ca alys egene a ion capaci y is a key ac o o make he
ESR p ocesses easible o la ge-scale ope a ions.
The ca bon elimina ion by combus ion is a c i ical s ep o a success ul ca alys e-
gene a ion since he uncon olled combus ion may sin e Ni si es due o he la ge amoun
o hea eleased [
13
], causing he i e e sible deac i a ion o he ca alys . Thus, many
ca alys o mula ions ha e been p oposed o he end o dec easing he ca bon o ma ion,
inc easing he ca alys li e ime and making possible i s egene a ion. Acco dingly, Mon-
e o e al. [
14
] demons a ed he ep oducible pe o mance o a Ni/
α
Al
2
O
3
-La
2
O
3
ca alys
in ESR eac ion- egene a ion cycles ( eac ion a 500 and 700
◦
C) upon an equilib a ion
ea men consis ing o a eac ion- egene a ion cycle wi h a eac ion empe a u e o 700
◦
C.
Campos e al. [
15
] showed ha a 1.0%Rh10%Ni/15%La
2
O
3
10%CeO
2
alumina suppo ed
ca alys can inc ease he H
2
selec i i y wi h a s able ope a ion and egene a ion capaci y
o wo ESR eac ion- egene a ion cycles a 500
◦
C. Con e as e al. [
16
] demons a ed ha
a ious Ni, Co and Ni-Co ca alys s suppo ed on hyd o alci e (Mg and Al oxides) s abilized
wi h W oxides ha e a good egene a ion capaci y in a ious ESR eac ion- egene a ion cy-
cles a 600
◦
C. Boudadi e al. [
17
] ound a good ac i i y eco e y o a ious Ni ca alys s wi h
modi ied suppo s, including Ni/La-Al
2
O
3
, Ni/La/TiO
2
-Al
2
O
3
, and Ni/La-clay, in ESR
Ca alys s 2022,12, 550 3 o 16
eac ion- egene a ion cycles a 500
◦
C. Di Michele e al. [
18
] p o ed Ni/MgAl
2
O
4
is a s able
ca alys o he ESR eac ion wi h low ca bon o ma ion leading o an easy egene a ion.
A ema kable and ep oducible Ni ca alys easily p epa ed om he educ ion o
NiAl
2
O
4
spinel is qui e ac i e o he s eam e o ming o bio-oil [
19
,
20
] and e hanol [
21
].
Acco dingly, he NiAl
2
O
4
spinel p ecu so is ob ained by co-p ecipi a ion o Ni and Al
sou ces ollowed by calcina ion, and hen he syn hesized spinel is comple ely educed in
H
2
o ob ain a Ni/Al
2
O
3
ca alys wi h high Ni dispe sion. This ca alys has been p o en
o ha e a mode a e ca bon deposi ion in he bio-oil s eam e o ming (BSR) and eco e s
i s ac i i y when i is used in eac ion- egene a ion cycles [
22
]. The egene a ion o his
ca alys consis s o wo s eps: (i) coke elimina ion by combus ion a 850
◦
C in ai , which also
econs uc s he NiAl
2
O
4
spinel s uc u e; and (ii) educ ion o he spinel a 850
◦
C in H
2
esul ing in he highly dispe sed Ni/Al
2
O
3
ca alys . This pe o mance in he s abili y and
egene a ion capaci y makes i a p omising al e na i e o e o he s o he s eam e o ming
o oxygena es, since he o mula ion is simple and ep oducible. Howe e , he use o his
ca alys in he ESR leads o a high ca bon o ma ion due o he p esence o acidic Al
2
O
3
ha a o s he e hanol dehyd a ion eac ion yielding e hylene (Equa ion (5)), and a o ing
he subsequen eac ions o oligome iza ion, a oma iza ion and condensa ion in o ca bon
s uc u es (coke) (Equa ion (10)) [
21
]. This high ca bon o ma ion and na u e may a ec he
egene a ion capaci y o his ca alys due o he possible uncon ollable combus ion and
o he issues ha cause an i e e sible deac i a ion.
In his wo k, we p opose o a enua e he deac i a ion and achie e he egene a ion
o he ca alys de i ed om NiAl
2
O
4
spinel, by dec easing he Al
2
O
3
p esence o con ol
he ca bon o ma ion. The simple and ep oducible s a egy o his end is dec easing he
educ ion empe a u e o he spinel, which leads o an incomple e educ ion o he Ni
species esul ing in a Ni/Al
2
O
3
-NiAl
2
O
3
ca alys wi h lowe Al
2
O
3
con en han in he
Ni/Al
2
O
3
ca alys p epa ed om he spinel educ ion a 850
◦
C. This s a egy has been
e ec i e o he ca alys equilib a ion in he glyce ol aqueous-phase e o ming [
23
], and i
is s udied in his wo k o he ESR in eac ion- egene a ion cycles.
2. Resul s
2.1. F esh Ca alys P ope ies
Figu e 1a shows he empe a u e p og ammed educ ion (TPR) p o ile o he NiAl
2
O
4
syn he ized. The TPR p o ile e idences ha he Ni educ ion akes place be ween 500 and
900
◦
C wi h a maximum a 800
◦
C, cha ac e is ic o he educ ion o Ni species in he spinel
s uc u e. Likewise, he absence o educ ion peaks a lowe empe a u es indica es ha
no ee nickel oxides a e signi ican ly p esen . This is e i ied by he XRD pa e n (plo ed
in Figu e 1b), e idencing he absence o NiO c ys alline phases and he only p esence o
NiAl
2
O
4
spinel. Thus, he educ ion o all Ni species esul s in Ni c ys als suppo ed on
Al
2
O
3
, which is possible a educ ion empe a u es abo e 850
◦
C o 4h[
19
]. Likewise, he
Ni con en calcula ed om he TPR p o ile is 34.7 w % in NiAl
2
O
4
o 38.7 w % in Ni/Al
2
O
3
,
being e y close o he s oichiome ic alues o 33.2 and 36.5 w %, espec i ely.
The ca alys s ha e been p epa ed by educing he NiAl
2
O
4
spinel p ecu so a 700, 750
and 850
◦
C, and hey a e named acco ding o hei educ ion empe a u e, as R-700, R-750
and R-850, espec i ely. These ca alys s ha e been cha ac e ized by se e al echniques
o de e mine hei mo e ele an p ope ies. Figu e 1b shows he X- ay di ac ion (XRD)
pa e ns o he ca alys s and he NiAl
2
O
4
p ecu so o de e mine he c ys alline phases
p esen in each ma e ial. The NiAl
2
O
4
shows di ac ion peaks a 2
θ
= 37.2, 45.3, 59.9 and
65.7
◦
co esponding o a ypical spinel cubic s uc u e (PDF 01-071-0965) [
21
,
23
]. Upon
educ ion a 850
◦
C, he NiAl
2
O
4
is almos comple ely con e ed in o educed Ni c ys als
(2
θ
= 44.6, 51.9 and 76.5
◦
) (PDF 04-010-6148) and Al
2
O
3
(2
θ
= 37.5, 45.9 and 66.9
◦
) (PDF
04-005-4662), being a Ni/Al
2
O
3
ca alys (R-850). In con as , he ca alys s ob ained upon
educ ion a 700
◦
C (R-700) o 750
◦
C (R-750) show peaks o he NiAl
2
O
4
, educed Ni
c ys als and Al
2
O
3
, indica ing ha a pa ial Ni educ ion akes place since he educ ion
empe a u e is insu icien o a comple e educ ion acco ding o he TPR da a (Figu e 1a).
Ca alys s 2022,12, 550 4 o 16
Thus, hese ca alys s a e composed o educed Ni c ys als suppo ed on NiAl
2
O
4
and Al
2
O
3
(Ni/Al
2
O
3
-NiAl
2
O
4
ca alys s). The a e age Ni c ys al size was de e mined om he XRD
da a using he Sche e equa ion (wi h di ac ion peak a 51.9
◦
), and he esul s (lis ed in
Table 1) show ha he Ni c ys al size inc eases wi h inc easing educ ion empe a u es.
Likewise, he con en o educed Ni c ys als was es ima ed om he TPR da a acco ding
o he educ ion empe a u e employed, and he esul s (lis ed in Table 1) con i m ha
his con en inc eases wi h inc easing educ ion empe a u es, as expec ed. The e o e, he
dec ease in he educ ion empe a u e leads o a lowe con en o educed Ni c ys als bu i
also sho ens he ex en o Ni sin e ing, which esul s in lowe a e age Ni c ys al sizes.
Ca alys s2022,12,5504o 16


(XRD)pa e nso  heca alys sand heNiAl2O4p ecu so  ode e mine hec ys alline
phasesp esen ineachma e ial.TheNiAl2O4showsdi ac ionpeaksa 2θ=37.2,45.3,
59.9and65.7°co esponding oa ypicalspinelcubics uc u e(PDF01‐071‐0965)[21,23].
Upon educ iona 850°C, heNiAl2O4isalmos comple elycon e edin o educedNi
c ys als(2θ=44.6,51.9and76.5°)(PDF04‐010‐6148)andAl2O3(2θ=37.5,45.9and66.9°)
(PDF04‐005‐4662),beingaNi/Al2O3ca alys (R‐850).Incon as , heca alys sob ained
upon educ iona 700°C(R‐700)o 750°C(R‐750)showpeakso  heNiAl2O4, educedNi
c ys alsandAl2O3,indica ing ha apa ialNi educ ion akesplacesince he educ ion
empe a u eisinsu icien  o acomple e educ ionacco ding o heTPRda a(Figu e1a).
Thus, heseca alys sa ecomposedo  educedNic ys alssuppo edonNiAl2O4and
Al2O3(Ni/Al2O3‐NiAl2O4ca alys s).Thea e ageNic ys alsizewasde e mined om he
XRDda ausing heSche e equa ion(wi hdi ac ionpeaka 51.9°),and he esul s
(lis edinTable1)show ha  heNic ys alsizeinc easeswi hinc easing educ ion em‐
pe a u es.Likewise, hecon en o  educedNic ys alswases ima ed om heTPRda a
acco ding o he educ ion empe a u eemployed,and he esul s(lis edinTable1)con‐
i m ha  hiscon en inc easeswi hinc easing educ ion empe a u es,asexpec ed.
The e o e, hedec easein he educ ion empe a u eleads oalowe con en o  educed
Nic ys alsbu i alsosho ens heex en o Nisin e ing,which esul sinlowe a e age
Nic ys alsizes.
300 400 500 600 700 800 900 1000
0
2
4
6
8
10
(a)
Ni educed (mg/(g min))
Tempe a u e (ºC)
30 40 50 60 70 80
NiAl
2
O
4
R-700
R-750
R-850
21
13
3
3
2
In ensi y (a.u.)
Di ac ion angle (º)
12
1
1. NiAl
2
O
4
2. Al
2
O
3
3. Ni
(b)

Figu e1.Cha ac e iza iono  heNiAl2O4p ecu so andca alys sob aineda di e en  educ ion
empe a u es.(a)TPRp o ileo  heNiAl2O4;(b)XRDpa e nso  heNiAl2O4andca alys s.
Table1.Mainp ope ieso  heNiAl2O4spinelandde i edca alys sa di e en  educ ion empe ‐
a u es.
SampleNiCon en 
(w %)
ReducedNi1
(w %)
SBET
(m2g−1)
Vpo e
(cm3g−1)
Dpo e
(nm)
NiC ys al
Size
(nm)
Acidi y
(mmolg−1)
NiAl2O4spinel34.7**‐ 78.30.1778.50‐ ‐
R‐700ca alys 38.7*11.876.70.19710.59.100.026
R‐750ca alys 38.7*17.774.00.20510.912.00.029
R‐850ca alys 38.7*38.770.30.21212.617.00.044
SBET,speci icsu acea eacalcula edusing heB unaue ‐Emme ‐Telle (BET) heo y;Vpo e, o al ol‐
umeo po es;Dpo e,a e agepo ediame e .*Amoun o  educedNiin heNi/Al2O3o Ni/Al2O3‐
NiAl2O4ca alys .**Amoun o Niin heNiAl2O4spinel.1Es ima ed om heTPRda a.
The ex u alp ope ieslis edinTable1alsoe idence hee ec o  he ansi iono 
heNiAl2O4spinel oNi/Al2O3‐NiAl2O4ca alys s.Acco dingly, hespeci icsu acea ea
(SBET)dec easeswi hinc easing educ ion empe a u es,whe eas he o alpo e olume
(Vpo e)anda e agepo ediame e (Dpo e)inc ease.Theca alys acidi ywasde e minedby
Figu e 1.
Cha ac e iza ion o he NiAl
2
O
4
p ecu so and ca alys s ob ained a di e en educ ion
empe a u es. (a) TPR p o ile o he NiAl2O4; (b) XRD pa e ns o he NiAl2O4and ca alys s.
Table 1.
Main p ope ies o he NiAl
2
O
4
spinel and de i ed ca alys s a di e en educ ion empe a u es.
Sample Ni Con en
(w %)
Reduced Ni 1
(w %)
SBET
(m2g−1)
Vpo e
(cm3g−1)
Dpo e
(nm)
Ni C ys al Size
(nm)
Acidi y
(mmol g−1)
NiAl2O4spinel 34.7 ** - 78.3 0.177 8.50 - -
R-700 ca alys 38.7 * 11.8 76.7 0.197 10.5 9.10 0.026
R-750 ca alys 38.7 * 17.7 74.0 0.205 10.9 12.0 0.029
R-850 ca alys 38.7 * 38.7 70.3 0.212 12.6 17.0 0.044
S
BET
, speci ic su ace a ea calcula ed using he B unaue -Emme -Telle (BET) heo y; V
po e
, o al olume o po es;
D
po e
, a e age po e diame e . * Amoun o educed Ni in he Ni/Al
2
O
3
o Ni/Al
2
O
3
-NiAl
2
O
4
ca alys . ** Amoun
o Ni in he NiAl2O4spinel. 1Es ima ed om he TPR da a.
The ex u al p ope ies lis ed in Table 1also e idence he e ec o he ansi ion o
he NiAl
2
O
4
spinel o Ni/Al
2
O
3
-NiAl
2
O
4
ca alys s. Acco dingly, he speci ic su ace a ea
(S
BET
) dec eases wi h inc easing educ ion empe a u es, whe eas he o al po e olume
(V
po e
) and a e age po e diame e (D
po e
) inc ease. The ca alys acidi y was de e mined by
means o NH
3
adso p ion, and he esul s a e lis ed in Table 1. As seen, he acidi y inc eases
wi h inc easing educ ion empe a u es om 0.026 mmol g
−1
a 700
◦
C o 0.044 mmol g
−1
a 850
◦
C, which should be ela ed o he inc easing Al
2
O
3
con en , wi h he p esence o
acidic γ-Al2O3phases [21].
In summa y, he educ ion a 850
◦
C is su icien o educe all o he Ni species in
he NiAl
2
O
4
spinel and ob ain a Ni/Al
2
O
3
ca alys (R-850 ca alys ), whe eas he educ-
ion a 700 and 750
◦
C pa ially educes he Ni species in he NiAl
2
O
4
spinel leading o
ob ain Ni/Al
2
O
3
-NiAl
2
O
4
ca alys s (R-700 and R-750 ca alys s). The e o e, he con en s o
educed Ni species (ac i e o he ESR eac ion) and acidic Al
2
O
3
dec ease as he educ ion
empe a u e dec eases. In consequence, o he ESR eac ion- egene a ion cyclic es s, he
space ime is adjus ed by using highe ca alys amoun s when he educ ion empe a u e
Ca alys s 2022,12, 550 5 o 16
is lowe , o p o iding simila amoun o educed Ni species in he ca aly ic bed wi h he
pu pose o ob aining compa able alues o e hanol con e sion and H2yield.
2.2. Ca alys Pe o mance in ESR Reac ion-Regene a ion Cycles
The pe o mance o he ca alys s in eac ion- egene a ion cycles was s udied quan i y-
ing he ime on s eam (TOS) e olu ion o he e hanol con e sion and p oduc yields o he
ESR using he R-850 (Figu e 2), R-750 (Figu e 3) and R-700 (Figu e 4) ca alys s. The main
p oduc s obse ed a e H
2
, CO, CO
2
, CH
4
, and C
2
H
4
and ca bon deposi ed on he ca alys .
The ca bon yield was es ima ed om C balance aking in o accoun he ca bonaceous
componen s in he eed and e luen s eams.
Ca alys s2022,12,5506o 16


Incon as , o  heR‐700andR‐750ca alys s, heini ial alueso H2,CO,CO2,and
CH4yieldsand hei TOSe olu ionsin hesecondand hi d eac ionsa equi ecompa a‐
blewi h hoseo  he i s  eac ion o eachca alys .Howe e , heTOSe olu iono  he
C2H4yieldno ablydi e s,wi haninc easing endin he aluesandappa en p olonged
pe iodo  hemaximum aluein hesuccessi ecycles,whichmakes hee hanolcon e ‐
sionalsoinc easecompa edwi h he i s  eac ion o eachca alys .
I shouldbeno ed ha  heH2yieldisalmos cons an upon4hons eam,whe eas
heyieldso o he p oduc schangeasaconsequenceo apa ialca alys deac i a ion o 
heex en o  a ious eac ions(Equa ions(1)–(14)).Thisobse a ionisconsis en wi h he
H2 o ma ionmechanismonca alys sde i ed omaNiAl2O4spinelp oposedinap e i‐
ouswo k[21].Acco dingly, hepa ialca alys deac i a ionsequen iallya ec s heH2
andca bon o ma ion ome hylene(Equa ion(10)) ha explains heinc easein heC2H4
yieldo e TOS,and hee hanoldehyd a ion(Equa ion(5)) ha explains hemaximum
obse edin heC2H4yieldin heR‐700andR‐750ca alys s.The e o ming eac ionsca a‐
lyzedbyNisi eskeepcons an  heH2yield,and hesesi eskeep hei accessibili y o 
hese eac ionssince heyalsoca alyze heca bongasi ica ion[24].
0123456
0
0.2
0.4
0.6
0.8
1.0
Con e sion o yield
Time on s eam (h)
Con e sion
H
2
CO
2
CO
2
CH
4
C
2
H
4
C
2
H
4
O
C
i s eac ion
egene a ion
egene a ion
01234
second eac ion
Time on s eam (h)
01234
hi d eac ion
Time on s eam (h)

Figu e2.TOSe olu iono  hee hanolcon e sionandp oduc syieldwi hR‐850ca alys inESR
eac ion‐ egene a ioncycles.
0123456
0
0.2
0.4
0.6
0.8
1.0
Con e sion
H2
CO2
CO
CH4
C2H4
C2H4O
C
Con e sion o yield
Time on s eam (h)
i s eac ion
egene a ion
second eac ion
egene a ion
hi d eac ion
01234
Time on s eam (h)
01234
Time on s eam (h)

Figu e3.TOSe olu iono  hee hanolcon e sionandp oduc syieldwi hR‐750ca alys inESR
eac ion‐ egene a ioncycles.
Figu e 2.
TOS e olu ion o he e hanol con e sion and p oduc s yield wi h R-850 ca alys in ESR
eac ion- egene a ion cycles.
Ca alys s2022,12,5506o 16


Incon as , o  heR‐700andR‐750ca alys s, heini ial alueso H2,CO,CO2,and
CH4yieldsand hei TOSe olu ionsin hesecondand hi d eac ionsa equi ecompa a‐
blewi h hoseo  he i s  eac ion o eachca alys .Howe e , heTOSe olu iono  he
C2H4yieldno ablydi e s,wi haninc easing endin he aluesandappa en p olonged
pe iodo  hemaximum aluein hesuccessi ecycles,whichmakes hee hanolcon e ‐
sionalsoinc easecompa edwi h he i s  eac ion o eachca alys .
I shouldbeno ed ha  heH2yieldisalmos cons an upon4hons eam,whe eas
heyieldso o he p oduc schangeasaconsequenceo apa ialca alys deac i a ion o 
heex en o  a ious eac ions(Equa ions(1)–(14)).Thisobse a ionisconsis en wi h he
H2 o ma ionmechanismonca alys sde i ed omaNiAl2O4spinelp oposedinap e i‐
ouswo k[21].Acco dingly, hepa ialca alys deac i a ionsequen iallya ec s heH2
andca bon o ma ion ome hylene(Equa ion(10)) ha explains heinc easein heC2H4
yieldo e TOS,and hee hanoldehyd a ion(Equa ion(5)) ha explains hemaximum
obse edin heC2H4yieldin heR‐700andR‐750ca alys s.The e o ming eac ionsca a‐
lyzedbyNisi eskeepcons an  heH2yield,and hesesi eskeep hei accessibili y o 
hese eac ionssince heyalsoca alyze heca bongasi ica ion[24].
0123456
0
0.2
0.4
0.6
0.8
1.0
Con e sion o yield
Time on s eam (h)
Con e sion
H
2
CO
2
CO
2
CH
4
C
2
H
4
C
2
H
4
O
C
i s eac ion
egene a ion
egene a ion
01234
second eac ion
Time on s eam (h)
01234
hi d eac ion
Time on s eam (h)

Figu e2.TOSe olu iono  hee hanolcon e sionandp oduc syieldwi hR‐850ca alys inESR
eac ion‐ egene a ioncycles.
0123456
0
0.2
0.4
0.6
0.8
1.0
Con e sion
H2
CO2
CO
CH4
C2H4
C2H4O
C
Con e sion o yield
Time on s eam (h)
i s eac ion
egene a ion
second eac ion
egene a ion
hi d eac ion
01234
Time on s eam (h)
01234
Time on s eam (h)

Figu e3.TOSe olu iono  hee hanolcon e sionandp oduc syieldwi hR‐750ca alys inESR
eac ion‐ egene a ioncycles.
Figu e 3.
TOS e olu ion o he e hanol con e sion and p oduc s yield wi h R-750 ca alys in ESR
eac ion- egene a ion cycles.

Ca alys s 2022,12, 550 6 o 16
Ca alys s2022,12,5507o 16


0123456
0
0.2
0.4
0.6
0.8
1.0
Con e sion
H2
CO2
CO
Con e sion o yield
Time on s eam (h)
CH4
C2H4
C2H4O
C
i s eac ion
egene a ion
second eac ion
egene a ion
hi d eac ion
01234
Time on s eam (h)
01234
Time on s eam (h)

Figu e4.TOSe olu iono  hee hanolcon e sionandp oduc syieldwi hR‐700ca alys inESR
eac ion‐ egene a ioncycles.
2.3.Ca alys Cha ac e iza iona e Reac ion‐Regene a ionCycles
Theca alys swe echa ac e izeda e  he hi d eac ionusingse e al echniques
wi h hepu poseo e alua ing heca bondeposi ionand he eco e yo  heca alys 
p ope ies.
2.3.1.Ca bonDeposi ion
Thecon en andcombus ioncha ac e is icso deposi edca bonwe ede e minedby
subjec ingsampleso  hespen ca alys s o empe a u ep og ammedoxida ion(TPO).
Figu e5shows heTPOp o ileso  he h eespen ca alys sa e  he hi d eac ion,in
whichauniquecombus ionpeakisobse ed o eachca alys .Thedeposi edca bons a s
obu nabo e400°Candmos o  heca bonspeciesbu na 525°C o  heR‐850ca alys 
anda 545°C o  heR‐700andR‐750ca alys s.Thus, heca boncon en (es ima ed om
hea eao eachTPOp o ile)isno o iouslyhighe  o  heR‐850ca alys (2.37g(gca a‐
lys )−1) han o  heR‐700(0.12g(gca alys )−1)o R‐750(0.15g(gca alys )−1)ca alys s.The
alueso  hea e ageca bon o ma ion a e( C)[25,26],(alsoindica edinFigu e5)a e
signi ican lylowe  o R‐700andR‐750ca alys s han o R‐850ca alys .Likewise, hehigh
combus ion empe a u ea  hemaximumcombus ion a e(maximumpeakposi ionin
heTPOp o ile)indica es ha  heca bondeposi edon he h eeca alys sishighlys uc‐
u edandcondensedwi halowH/C a io,expec edlycomposedo ca bon ilamen s
[21,27].Theshi  ohighe combus ion empe a u es o  heca bon o medon heR‐700
andR‐750ca alys sindica es ha  heca bons uc u esa emo e e ac o y han hose
o medon heR‐850ca alys [28].
300 400 500 600 700
0
0.02
0.04
0.06
0.08
C
C
= 0.12 g/g
C
= 0.0032 g/(g g h)
C
C
= 0.15 g/g
C
= 0.0040 g/(g g h)
R-700
R-750
R-850
dTG (g/(min g))
Tempe a u e (ºC)
C
C
= 2.37 g/g
C
= 0.063 g/(g g h)

Figu e5.TPOp o ileo  he h eespen ca alys sa e  he hi dESR eac ion.CCis heca boncon en 
e e ed o heca alys massand Cis heca bon o ma ion a e e e ed o heca alys mass,Cmass
ed,and imeons eam.
Figu e 4.
TOS e olu ion o he e hanol con e sion and p oduc s yield wi h R-700 ca alys in ESR
eac ion- egene a ion cycles.
Compa ing he pe o mance o he ca alys s in he i s eac ion, he ini ial e hanol
con e sion and H
2
, CO, CO
2
, CH
4
and C
2
H
4
yields a e simila . The TOS e olu ion o
he H
2
, CO, CO
2
and CH
4
yields is also qui e simila o he h ee ca alys s, showing a
signi ican dec ease in he i s 2 h on s eam and mo e s able beha io a e wa ds, which
indica es a pa ial ca alys deac i a ion o he eac ions o ming hese p oduc s. The
e hanol con e sion dec eases as e and eaches lowe alues in he pseudo-s a iona y
s a e as he educ ion empe a u e dec eases (in his ca alys o de : R700 > R750 > R850).
Abo e 4 h on s eam, he h ee ca alys s each a pseudo-s able s a e wi h cons an p oduc
yields, in pa icula ha o H
2
. The da a e idence ha he deac i a ion a e o he H
2
o ma ion eac ions is simila o he h ee ca alys s. Howe e , he TOS e olu ion o he
C
2
H
4
and ca bon yields is di e en among he h ee ca alys s. Fo he R-850 ca alys ,
he C
2
H
4
and ca bon yields inc ease wi h TOS and keep a s able beha io up o 4 h on
s eam and a e wa ds he C
2
H
4
yield sligh ly inc eases coinciding wi h he dec ease in
he ca bon yield. In con as , o he R-700 and R-750 ca alys s, he C
2
H
4
yield inc eases,
eaches a maximum alue (much lowe han he alues obse ed o he R-850 ca alys )
and dec eases a e wa ds, whe eas he ca bon yield is negligible. The slowe dec ease and
highe alues in he e hanol con e sion wi h inc easing educ ion empe a u es is due
o he highe p esence o acidic Al
2
O
3
(Table 1) ha ca alyzes he e hanol dehyd a ion
yielding mo e C2H4[21].
Upon he i s eac ion, he co esponding spen ca alys was subjec ed o he egene -
a ion p ocedu e desc ibed in Sec ion 4(combus ion wi h ai a 850
◦
C o coke emo al and
econs uc ion o he spinel, ollowed by spinel educ ion wi h H
2
-N
2
s eam a di e en
empe a u es o 700, 750 o 850
◦
C o he R-700, R-750 and R-850 ca alys s, espec i ely).
The whole p ocedu e is epea ed o a hi d eac ion and he esul s a e also shown in
Figu es 2–4. Fo he R-850 ca alys , he TOS e olu ion o he p oduc yields comple ely
changes om he i s o he second and hi d eac ions, wi h he p oduc dis ibu ion o
he second and hi d eac ions being almos iden ical ( ep oducible beha io upon he
egene a ions). Acco dingly, he ini ial H
2
, CO, CO
2
and CH
4
yields a e almos compa able
wi h hose o he esh ca alys , bu hey apidly dec ease wi h TOS eaching much lowe
alues in he second and hi d eac ions in compa ison wi h he i s eac ion. Likewise,
he C
2
H
4
and ca bon yields no ably inc ease in he second and hi d eac ions, making he
e hanol con e sion also inc ease in compa ison wi h he i s use o he ca alys .
In con as , o he R-700 and R-750 ca alys s, he ini ial alues o H
2
, CO, CO
2
, and
CH
4
yields and hei TOS e olu ions in he second and hi d eac ions a e qui e compa able
wi h hose o he i s eac ion o each ca alys . Howe e , he TOS e olu ion o he C
2
H
4
Ca alys s 2022,12, 550 7 o 16
yield no ably di e s, wi h an inc easing end in he alues and appa en p olonged pe iod
o he maximum alue in he successi e cycles, which makes he e hanol con e sion also
inc ease compa ed wi h he i s eac ion o each ca alys .
I should be no ed ha he H
2
yield is almos cons an upon 4 h on s eam, whe eas he
yields o o he p oduc s change as a consequence o a pa ial ca alys deac i a ion o he
ex en o a ious eac ions (Equa ions (1)–(14)). This obse a ion is consis en wi h he H
2
o ma ion mechanism on ca alys s de i ed om a NiAl
2
O
4
spinel p oposed in a p e ious
wo k [
21
]. Acco dingly, he pa ial ca alys deac i a ion sequen ially a ec s he H
2
and
ca bon o ma ion om e hylene (Equa ion (10)) ha explains he inc ease in he C
2
H
4
yield
o e TOS, and he e hanol dehyd a ion (Equa ion (5)) ha explains he maximum obse ed
in he C
2
H
4
yield in he R-700 and R-750 ca alys s. The e o ming eac ions ca alyzed by Ni
si es keep cons an he H
2
yield, and hese si es keep hei accessibili y o hese eac ions
since hey also ca alyze he ca bon gasi ica ion [24].
2.3. Ca alys Cha ac e iza ion a e Reac ion-Regene a ion Cycles
The ca alys s we e cha ac e ized a e he hi d eac ion using se e al echniques wi h
he pu pose o e alua ing he ca bon deposi ion and he eco e y o he ca alys p ope ies.
2.3.1. Ca bon Deposi ion
The con en and combus ion cha ac e is ics o deposi ed ca bon we e de e mined by
subjec ing samples o he spen ca alys s o empe a u e p og ammed oxida ion (TPO).
Figu e 5shows he TPO p o iles o he h ee spen ca alys s a e he hi d eac ion, in which
a unique combus ion peak is obse ed o each ca alys . The deposi ed ca bon s a s o
bu n abo e 400
◦
C and mos o he ca bon species bu n a 525
◦
C o he R-850 ca alys and
a 545
◦
C o he R-700 and R-750 ca alys s. Thus, he ca bon con en (es ima ed om he
a ea o each TPO p o ile) is no o iously highe o he R-850 ca alys (2.37 g (g ca alys )
−1
)
han o he R-700 (0.12 g (g ca alys )
−1
) o R-750 (0.15 g (g ca alys )
−1
) ca alys s. The
alues o he a e age ca bon o ma ion a e (
C
) [
25
,
26
], (also indica ed in Figu e 5) a e
signi ican ly lowe o R-700 and R-750 ca alys s han o R-850 ca alys . Likewise, he high
combus ion empe a u e a he maximum combus ion a e (maximum peak posi ion in he
TPO p o ile) indica es ha he ca bon deposi ed on he h ee ca alys s is highly s uc u ed
and condensed wi h a low H/C a io, expec edly composed o ca bon ilamen s [
21
,
27
].
The shi o highe combus ion empe a u es o he ca bon o med on he R-700 and R-750
ca alys s indica es ha he ca bon s uc u es a e mo e e ac o y han hose o med on he
R-850 ca alys [28].
Ca alys s2022,12,5507o 16


0123456
0
0.2
0.4
0.6
0.8
1.0
Con e sion
H2
CO2
CO
Con e sion o yield
Time on s eam (h)
CH4
C2H4
C2H4O
C
i s eac ion
egene a ion
second eac ion
egene a ion
hi d eac ion
01234
Time on s eam (h)
01234
Time on s eam (h)

Figu e4.TOSe olu iono  hee hanolcon e sionandp oduc syieldwi hR‐700ca alys inESR
eac ion‐ egene a ioncycles.
2.3.Ca alys Cha ac e iza iona e Reac ion‐Regene a ionCycles
Theca alys swe echa ac e izeda e  he hi d eac ionusingse e al echniques
wi h hepu poseo e alua ing heca bondeposi ionand he eco e yo  heca alys 
p ope ies.
2.3.1.Ca bonDeposi ion
Thecon en andcombus ioncha ac e is icso deposi edca bonwe ede e minedby
subjec ingsampleso  hespen ca alys s o empe a u ep og ammedoxida ion(TPO).
Figu e5shows heTPOp o ileso  he h eespen ca alys sa e  he hi d eac ion,in
whichauniquecombus ionpeakisobse ed o eachca alys .Thedeposi edca bons a s
obu nabo e400°Candmos o  heca bonspeciesbu na 525°C o  heR‐850ca alys 
anda 545°C o  heR‐700andR‐750ca alys s.Thus, heca boncon en (es ima ed om
hea eao eachTPOp o ile)isno o iouslyhighe  o  heR‐850ca alys (2.37g(gca a‐
lys )−1) han o  heR‐700(0.12g(gca alys )−1)o R‐750(0.15g(gca alys )−1)ca alys s.The
alueso  hea e ageca bon o ma ion a e( C)[25,26],(alsoindica edinFigu e5)a e
signi ican lylowe  o R‐700andR‐750ca alys s han o R‐850ca alys .Likewise, hehigh
combus ion empe a u ea  hemaximumcombus ion a e(maximumpeakposi ionin
heTPOp o ile)indica es ha  heca bondeposi edon he h eeca alys sishighlys uc‐
u edandcondensedwi halowH/C a io,expec edlycomposedo ca bon ilamen s
[21,27].Theshi  ohighe combus ion empe a u es o  heca bon o medon heR‐700
andR‐750ca alys sindica es ha  heca bons uc u esa emo e e ac o y han hose
o medon heR‐850ca alys [28].
300 400 500 600 700
0
0.02
0.04
0.06
0.08
C
C
= 0.12 g/g
C
= 0.0032 g/(g g h)
C
C
= 0.15 g/g
C
= 0.0040 g/(g g h)
R-700
R-750
R-850
dTG (g/(min g))
Tempe a u e (ºC)
C
C
= 2.37 g/g
C
= 0.063 g/(g g h)

Figu e5.TPOp o ileo  he h eespen ca alys sa e  he hi dESR eac ion.CCis heca boncon en 
e e ed o heca alys massand Cis heca bon o ma ion a e e e ed o heca alys mass,Cmass
ed,and imeons eam.
Figu e 5.
TPO p o ile o he h ee spen ca alys s a e he hi d ESR eac ion. C
C
is he ca bon con en
e e ed o he ca alys mass and
C
is he ca bon o ma ion a e e e ed o he ca alys mass, C mass
ed, and ime on s eam.
Ca alys s 2022,12, 550 8 o 16
To in es iga e he ex en o ca bon deposi ion and he ca bon mo phology, samples o
he spen ca alys we e analyzed by means o scanning elec on mic oscopy (SEM) using
a backsca e ed elec on (BSE) o seconda y elec on (SE) de ec o s, wi h he equipmen
desc ibed in Sec ion 4. Figu e 6shows he BSE-SEM images o he spen ca alys s a e he
hi d eac ion, which p o ide insigh s in o he ex en o ca bon deposi ion on he ca alys
pa icles acco ding o he b igh ness in ensi y le els. All o he spen ca alys pa icles
show a s ong ca bon deposi ion on he ex e nal su ace based on he almos homogeneous
da k colo o all samples. Rema kably, he size and ex u al appea ance o some R-850
ca alys pa icles (Figu e 6a) no ably changed in compa ison wi h hose o he R-700 and
R-750 ca alys s (Figu e 6c,d). Some R-850 ca alys pa icles a e la ge han he esh ca alys
(0.15–0.25 mm) and wi h a highly ough and meso- and mac o-po ous ex u e cha ac e is ic
o ca bon s uc u es. Likewise, some b igh small agmen s can be seen on he ex e nal
su ace (magni ied in Figu e 6b), which is indica i e o he p esence o ca alys agmen s
(Ni and Al would gi e his b igh ness in ensi y) o e he ca bon deposi ed. The pa icle
sizes o he spen R-750 (Figu e 6c) and R-700 (Figu e 6d) ca alys s did no signi ican ly
change by he ca bon deposi ion.
Ca alys s2022,12,5508o 16


Toin es iga e heex en o ca bondeposi ionand heca bonmo phology,samples
o  hespen ca alys we eanalyzedbymeanso scanningelec onmic oscopy(SEM)us‐
ingabacksca e edelec on(BSE)o seconda yelec on(SE)de ec o s,wi h heequip‐
men desc ibedinSec ion4.Figu e6shows heBSE‐SEMimageso  hespen ca alys s
a e  he hi d eac ion,whichp o ideinsigh sin o heex en o ca bondeposi ionon he
ca alys pa iclesacco ding o heb igh nessin ensi yle els.Allo  hespen ca alys pa ‐
iclesshowas ongca bondeposi ionon heex e nalsu acebasedon healmos homo‐
geneousda kcolo o allsamples.Rema kably, hesizeand ex u alappea anceo some
R‐850ca alys pa icles(Figu e6a)no ablychangedincompa isonwi h hoseo  heR‐700
andR‐750ca alys s(Figu e6c,d).SomeR‐850ca alys pa iclesa ela ge  han he esh
ca alys (0.15–0.25mm)andwi hahighly oughandmeso‐andmac o‐po ous ex u e
cha ac e is ico ca bons uc u es.Likewise,someb igh small agmen scanbeseenon
heex e nalsu ace(magni iedinFigu e6b),whichisindica i eo  hep esenceo ca alys 
agmen s(NiandAlwouldgi e hisb igh nessin ensi y)o e  heca bondeposi ed.The
pa iclesizeso  hespen R‐750(Figu e6c)andR‐700(Figu e6d)ca alys sdidno signi ‐
ican lychangeby heca bondeposi ion.

Figu e6.BSE‐SEMimageso  heca alys pa iclesa e  he hi dESR eac ion:(a)R‐850ca alys 
(×50);(b)R‐850ca alys (×100);(c)R‐750ca alys (×50);(d)R‐700ca alys (×50).
Figu e7shows heSE‐SEMimagesp o idingmo ede ailso  heca bonmo phology
bymagni yingsomea easo  ep esen a i epa icleso eachsample.Thepho ose idence
ha  he o medanddeposi edca bonismos lycomposedo ca bon ilamen sas ypically
expec ed o  heESR[21,27–29],beingpa icula lyabundan inallo  hespen R‐850ca ‐
alys pa icles(Figu e7a,b).Anappa en ca alys  agmen iscap u edinFigu e7b,sug‐
ges ing hese agmen sa eon he ipo ca bon ilamen s.On heo he hand,in hepa ‐
icleso  hespen R‐700andR‐750ca alys s, wodi e en su acemo phologiescanbe
dis inguished,onesimila  o ha o  heR‐850ca alys wi habundan ca bon ilamen s
(Figu e7c,e)andano he wi hanincipien p esenceo ca bon ilamen sandappa en 
amo phousmasso ca bon(Figu e7d, ).
Figu e 6.
BSE-SEM images o he ca alys pa icles a e he hi d ESR eac ion: (
a
) R-850 ca alys
(×50); (b) R-850 ca alys (×100); (c) R-750 ca alys (×50); (d) R-700 ca alys (×50).
Figu e 7shows he SE-SEM images p o iding mo e de ails o he ca bon mo phology
by magni ying some a eas o ep esen a i e pa icles o each sample. The pho os e idence
ha he o med and deposi ed ca bon is mos ly composed o ca bon ilamen s as ypically
expec ed o he ESR [
21
,
27
–
29
], being pa icula ly abundan in all o he spen R-850
ca alys pa icles (Figu e 7a,b). An appa en ca alys agmen is cap u ed in Figu e 7b,
sugges ing hese agmen s a e on he ip o ca bon ilamen s. On he o he hand, in he
pa icles o he spen R-700 and R-750 ca alys s, wo di e en su ace mo phologies can
be dis inguished, one simila o ha o he R-850 ca alys wi h abundan ca bon ilamen s
(Figu e 7c,e) and ano he wi h an incipien p esence o ca bon ilamen s and appa en
amo phous mass o ca bon (Figu e 7d, ).
Ca alys s 2022,12, 550 9 o 16
Ca alys s2022,12,5509o 16



Figu e7.SE‐SEMimageso  heca alys pa iclesa e  he hi dESR eac ion:(a,b)R‐850ca alys ;
(c,d)R‐750ca alys ;(e, )R‐700ca alys .
2.3.2.Regene a edCa alys P ope ies
To e i y hespinel econs uc ionabili y om hespen ca alys sbycombus ion/cal‐
cina iona 850°C,sampleso  he econs uc edNiAl
2
O
4
spinel om heusedR‐700,R‐
750andR‐850ca alys swe echa ac e izedbyusingTPRandXRD(Figu e8).TheTPR
p o iles(Figu e8a)a ecompa edwi h ha o  heNiAl
2
O
4
p ecu so .In he econs uc ed
NiAl
2
O
4
spinels,pa icula ly om heusedR‐850ca alys , heNi educ ion akesplacein
wo empe a u e anges,400–600and600–950°C,indica ing hep esenceo  woNispe‐
cies.TheXRDpa e ns(Figu e8b) e i y hep esenceo  woc ys allinephases,NiAl
2
O
4

spinel(PDF01‐071‐0965)andNiO(PDF01‐080‐5508),being hespinels uc u ep edom‐
inan inallo  hesamplesand hep esenceo NiOmo eno o iousin hespinel econ‐
s uc ed om hespen R‐850ca alys .Thus, heNi educ iona 400–600°Cmaybeasso‐
cia ed o he educ iono NiO,whe eas heNi educ ionin heNiAl
2
O
4
spinel akesplace
a 600–950°Cwi hmaximaa a ound810–820°C.In e es ingly, he educ ionpeakand
XRDpeaksassocia ed o heNiAl
2
O
4
spinel end obena owe  o  heallo  he econ‐
s uc edNiAl
2
O
4
spinels han o  hespinelp ecu so ,whichisindica i eo mo ehomo‐
geneousNiAl
2
O
4
s uc u es.Likewise, heTPRp o ilese idences ha  he o alamoun o 
educedNi(lis edinTable2)islowe  o  he econs uc edNiAl
2
O
4
spinels han o  he
spinelp ecu so .The e o e, hedec easein heNicon en wi h espec  o he eshca alys 
indica esapa ialNilossa e  heESR eac ioncycles.Theselossesa e3.90,9.50,and
14.5% o  heR‐700,R‐750,andR‐850ca alys s, espec i ely.P esumably, heNilossis
Figu e 7.
SE-SEM images o he ca alys pa icles a e he hi d ESR eac ion: (
a
,
b
) R-850 ca alys ;
(c,d) R-750 ca alys ; (e, ) R-700 ca alys .
2.3.2. Regene a ed Ca alys P ope ies
To e i y he spinel econs uc ion abili y om he spen ca alys s by combus ion/calcina ion
a 850
◦
C, samples o he econs uc ed NiAl
2
O
4
spinel om he used R-700, R-750 and
R-850 ca alys s we e cha ac e ized by using TPR and XRD (Figu e 8). The TPR p o iles
(Figu e 8a) a e compa ed wi h ha o he NiAl
2
O
4
p ecu so . In he econs uc ed NiAl
2
O
4
spinels, pa icula ly om he used R-850 ca alys , he Ni educ ion akes place in wo
empe a u e anges, 400–600 and 600–950
◦
C, indica ing he p esence o wo Ni species.
The XRD pa e ns (Figu e 8b) e i y he p esence o wo c ys alline phases, NiAl
2
O
4
spinel
(PDF 01-071-0965) and NiO (PDF 01-080-5508), being he spinel s uc u e p edominan
in all o he samples and he p esence o NiO mo e no o ious in he spinel econs uc ed
om he spen R-850 ca alys . Thus, he Ni educ ion a 400–600
◦
C may be associa ed
o he educ ion o NiO, whe eas he Ni educ ion in he NiAl
2
O
4
spinel akes place a
600–950
◦
C wi h maxima a a ound 810–820
◦
C. In e es ingly, he educ ion peak and XRD
peaks associa ed o he NiAl
2
O
4
spinel end o be na owe o he all o he econs uc ed
NiAl
2
O
4
spinels han o he spinel p ecu so , which is indica i e o mo e homogeneous
NiAl
2
O
4
s uc u es. Likewise, he TPR p o iles e idences ha he o al amoun o educed
Ni (lis ed in Table 2) is lowe o he econs uc ed NiAl
2
O
4
spinels han o he spinel
p ecu so . The e o e, he dec ease in he Ni con en wi h espec o he esh ca alys
indica es a pa ial Ni loss a e he ESR eac ion cycles. These losses a e 3.90, 9.50, and 14.5%
o he R-700, R-750, and R-850 ca alys s, espec i ely. P esumably, he Ni loss is due o he
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