Synthesis, characterisation and performance evaluation of spinel-derived Ni/Al2O3 catalysts for various methane reforming reactions

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Zouhai Boukha, C is ina Jiménez-González, 6
Bea iz de Ri as, Juan Ramón González-Velasco, 7
Jose Ignacio Gu ié ez-O iz and Rubén López-Fonseca* 8
Chemical Technologies o En i onmen al Sus ainabili y G oup, 9
Depa men o Chemical Enginee ing, Facul y o Science and Technology, 10
Uni e si y o The Basque Coun y UPV/EHU, 11
P.O. Box 644, E-48080 Bilbao, Spain. 12
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*Co esponding au ho :16
Phone: +34-94-6015985 17
Fax: +34-94-6015963 18
E-mail add ess: [email p o ec ed]19
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SYNTHESIS, CHARACTERIZATION AND PERFORMANCE
EVALUATION OF SPINEL-DERIVED Ni/Al2O3 CATALYSTS FOR
VARIOUS METHANE REFORMING REACTIONS
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 158/159 :
190-201 (2014), which has been published in inal o m a h ps://doi.o g/10.1016/j.apca b.2014.04.014. © 2014 Else ie unde
CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/)
2
Abs ac 21
The ca aly ic pe o mance o bulk nickel alumina e ca alys s syn hesised by co-22
p ecipi a ion (NiAl2O4-CP) and co-dissolu ion (NiAl2O4-D) was examined o a ious 23
me hane e o ming eac ions, namely pa ial oxida ion, s eam e o ming and oxida i e 24
s eam e o ming. The calcined and educed spinels we e ho oughly cha ac e ised by a 25
wide numbe o analy ical echniques including WDXRF, N2 physiso p ion, XRD, UV–26
isible–NIR DRS, XPS, TEM, H2-TPR and NH3-TPD. The cha ac e isa ion esul s o 27
he calcined samples a 850 ºC showed ha nickel alumina e phase was mainly ob ained 28
on he NiAl2O4-CP nea -su ace oge he wi h a small NiO excess. By con as , a la ge 29
amoun o NiO phase was o med on NiAl2O4-D and deposi ed on he spinel su ace. 30
The educ ion a 850 ºC o he wo samples p oduced monodispe sed Ni pa icles 31
(10.6 nm) in he NiAl2O4-CP while la ge me allic nickel (17.7 nm) was deposi ed on 32
he NiAl2O4-D. In he h ee in es iga ed e o ming eac ions he NiAl2O4-CP ca alys 33
p o ed o be highly ac i e, s able and esis an owa d ca bon deposi ion. Mo eo e , he 34
pe o mances o he NiAl2O4-CP ca alys appea ed o be gene ally compa able wi h ha 35
o a comme cial 1%Rh/Al2O3 ca alys . The di e ence in ca aly ic beha iou be ween 36
he wo nickel alumina es was ela ed o he Ni dispe sion, i s pa icle size and i s 37
capaci y o minimize he acid cha ac e o he alumina by co e ing a la ge pa o i s 38
su ace. 39
40
41
42
43
Keywo ds: Nickel alumina e, Co-p ecipi a ion, Co-dissolu ion, Ni C ys alli e size, 44
Me hane e o ming 45
46
3
1. In oduc ion 47
Me hane s eam e o ming (SRM) is he mos used echnology o p oducing syn hesis 48
gas (CO and H2) om na u al gas [1-5]. Mo eo e , his echnology is a p e e able 49
choice o p oducing clean and la ge hyd ogen yield. Howe e , because o he high 50
cos o he endo he mic cha ac e o SRM, he exo he mic me hane pa ial oxida ion 51
(POM) appea s as an al e na i e as i o e s some in e es ing ad an ages especially 52
when he pos e io use o he syngas s eam equi es a suiTable H2/CO a io [6-8]. 53
Cu en ly indus ial acili ies a e equipped wi h sys ems ha o e he possibili y o 54
using POM and/o SRM echnologies. The combina ion o he POM and SRM s a egies 55
called oxida i e s eam e o ming (OSRM) has he ad an age o con olling he p ocess 56
hea and he dis ibu ion o he p oduc s by adjus ing he O/C mola a io [9-10]. 57
Ni-based ca alys s a e widely used o me hane e o ming eac ions due o hei high 58
ac i i y and hei compe i i e cos s compa ed o noble me als [11-13]. Alumina 59
suppo ed Ni sys ems ha e pa icula ly ecei ed conside able a en ion because o hei 60
p ac ical applica ions o a a ie y o e o ming eac ions [14-18]. Ne e heless, despi e 61
exhibi ing ad an ageous ca aly ic p ope ies, alumina lacks an adequa e he mal s abili y 62
which p o okes he sin e ing o suppo ed me als. Fu he mo e, he use o alumina as 63
suppo ma e ial has he d awback o apid deac i a ion o he ca alys due o ca bon 64
deposi ion on i s ac i e acid su ace [19-20]. Fo his eason, he in e es o ecen 65
in es iga ions has been ocused on he p epa a ion o o mula ions leading o a high 66
dispe sion o Ni species which can co e alumina su ace, hus minimising he nega i e 67
e ec o i s acid cha ac e . In his sense, he e ec o he addi ion o some alkali and 68
alkali ea h me als, used as chemical p omo e s, on he ac i i y and s abili y o 69
Ni/alumina ca alys s has been ex ensi ely in es iga ed. Typically hese addi i es help o 70
4
a oid ca bon deposi ion and enhance hei s abili y bu a he expense o a educ ion o 71
hei ac i i y due o he blocking o he mo e eac i e Ni si es [21,22]. 72
Wi hin he same objec i e ( he need o imp o e s abili y o Ni-based ca alys s) nickel 73
alumina e syn hesis is one o he ou es p oposed in he li e a u e o he design o a 74
p ecu so capable o p oduce, a e i s educ ion, me allic Ni p esen ing s ong 75
in e ac ion wi h alumina [14,15,23-25]. Fo so long, Ross e al. [25] had e idenced he 76
pa icipa ion o he educed su ace nickel alumina e si es in he s eam e o ming o 77
me hane o e Ni/alumina ca alys s. Since hen, a numbe o s udies dealing wi h he 78
p epa a ion o nickel alumina e as p ecu so o ac i e Ni me allic pa icles o me hane 79
e o ming eac ions a e a ailable in he li e a u e. A g ea majo i y o hese wo ks ha e 80
been de o ed o he in es iga ion o he in luence o p epa a ion me hods on he 81
s abili y and he ac i i y o he inal o med Ni s uc u es in e o ming eac ions 82
[14,15,24,26,27]. As s a ed in a ious e e ences, i is howe e complica ed o 83
syn hesise pu e o s oichiome ic nickel alumina e by means o con en ional me hods o 84
syn hesis. This is because o he o ma ion o NiO as an excess e en when hea ing a 85
e y high empe a u es (>1300 ºC) [28-30]. The e o e, many wo ks ha e examined he 86
impac o his pa icula i y on he ca alys s pe o mance [15,16,24,27,31]. 87
In ou p e ious s udy, we compa ed wo se ies o Ni/Al2O3 ca alys s wi h simila me al 88
con en p epa ed by co-imp egna ion, using solu ions con aining a mix u e o nickel 89
ace a e and aluminium ni a e o ob ain a s oichiome ic NiAl2O4 suppo ed on alumina, 90
and simple imp egna ion o nickel on alumina [15]. We demons a ed ha , in me hane 91
s eam e o ming, he co-imp egna ed ca alys was mo e ac i e and we a ibu ed his 92
beha iou o i s highe Ni–suppo in e ac ion. We also no iced ha ob aining he ac i e 93
me allic Ni, wi h small pa icle size, was con olled by he NiO/NiAl2O4 a io ini ially 94
p esen in he calcined samples. Achou i e al. [16] also s udied he in luence o he 95
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p epa a ion me hod on he ca aly ic ac i i y in diesel s eam e o ming o wo alumina-96
suppo ed nickel alumina e ca alys s p epa ed by we -imp egna ion and co-p ecipi a ion. 97
They concluded ha he imp egna ed ca alys exhibi ed a highe ca aly ic pe o mance 98
and s abili y o e ime han he co-p ecipi a ed coun e pa . This di e en beha iou 99
was explained by he highe ca bon deposi ion endency shown by he co-p ecipi a ed 100
sample. 101
In his pape he in luence o he physico-chemical cha ac e is ics o wo bulk nickel 102
alumina e ca alys s, p epa ed by co-p ecipi a ion and co-dissolu ion me hods, on hei 103
ca aly ic beha iou in POM, SRM and OSRM eac ions is in es iga ed. To each his 104
a ge , an ex ensi e cha ac e isa ion has been ca ied ou by a wide numbe on analy ical 105
echniques including BET measu emen s, XRD, XPS, UV– isible–NIR DRS, TEM and 106
H2-TPR. Likewise, TPD o NH3, a well-known p obe molecule o acid si es, has been 107
used o cha ac e ise he su ace chemis y o he in es iga ed samples. A special 108
a en ion has been paid o he de e mina ion o he di e en nickel species o med a e 109
high- empe a u e calcina ion and subsequen high- empe a u e educ ion. In o de o 110
examine and compa e hei ca aly ic e iciency he wo p epa ed ca alys s we e 111
e alua ed in pa ial oxida ion, s eam e o ming and oxida i e s eam e o ming o 112
me hane. In e es ing conclusions a e d awn om he co ela ion be ween he 113
dis ibu ion o Ni ac i e species o he wo ca alys s and hei pe o mance in hese h ee 114
e o ming eac ions. 115
116
2. Expe imen al 117
2.1. Ca alys s p epa a ion 118
Bulk nickel alumina e samples we e syn hesised by co-p ecipi a ion (CP) and co-119
dissolu ion (D) me hods. The sample named NiAl2O4-D was syn hesised using wo 120

6
aqueous solu ions o Ni(CH3-COO)2·4H2O and Al(NO3)3·9H2O. Then hey we e mixed 121
(leading o a mix u e wi h he desi ed 1:2 Ni/Al mola a io), and e apo a ed on a ho 122
pla e (150 ºC). In he case o NiAl2O4-CP, aqueous ammonia was added o he mixed 123
aqueous solu ions o adjus he inal sui able pH (=8). Fo compa a i e pu poses NiO 124
was also p epa ed by simple calcina ion in ai o Ni(CH3-COO)2·4H2O. All he samples 125
we e d ied a 110 ºC o e nigh and hen calcined a 850 ºC in s a ic ai o 4 h a a 126
hea ing a e o 10 ºC min-1. Finally, pelle s o bulk (nickel alumina e o nickel oxide) 127
samples we e p epa ed by a p ocess o comp essing he powde s in o lakes in a 128
hyd aulic p ess (Specac), c ushing and sie ing (0.3-0.5 mm). 129
130
2.2. Ca alys cha ac e isa ion 131
The ca alys s we e cha ac e ised by N2 physiso p ion a -196 ºC, wa eleng h dispe si e 132
X- ay luo escence (WDXRF), X- ay di ac ion (XRD), ul a iole - isible-nea 133
in a ed di use e lec ance spec oscopy (DRS), X- ay pho oelec on spec oscopy 134
(XPS), empe a u e p og ammed educ ion wi h hyd ogen (H2-TPR) and empe a u e 135
p og ammed deso p ion o NH3 (NH3-TPD). The expe imen al de ails o each analy ical 136
echnique a e desc ibed elsewhe e [14,15]. Addi ionally, he mo phology and pa icle 137
size dis ibu ion o he nickel pa icles was examined by ansmission elec on 138
mic oscopy (TEM). P io o analysis, he samples we e dispe sed in absolu e e hanol 139
ul asonically o 30 min, and 10 cm3 o each sample we e hen placed on lexible ilm 140
(Pa a ilm® M). Glow-discha ged ca bon-coa ed coppe g ids we e in e ed on o he 141
d ople s o each sample. A e incuba ion o 1 min a oom empe a u e, he g ids we e 142
manually blo ed wi h il e pape ai -d ied. Digi ally eco ded 2D images o each 143
solu ion we e aken a oom empe a u e a a nominal magni ica ion o 80000 on a 144
Jeol JEM-1230 ansmission elec on mic oscope, wi h a LaB6 ilamen as he sou ce o 145
7
elec ons and ope a ed a 100 kV. Digi al images we e eco ded on an O ius SC1000 146
cooled slow-scan CCD came a, 4008×2672 pixels (GATAN), ob aining a inal pixel 147
size o 0.85 Å pixel-1. The pa icle size dis ibu ion was ob ained om he measu emen 148
o a leas 300 pa icles using ImageJ so wa e, and he a e age diame e was calcula ed 149
by dM =di·ni/ni, whe e ni is he numbe o he pa icles o diame e di. 150
On he o he hand, he amoun o ca bonaceous deposi s on he used ca alys was 151
de e mined by dynamic he mog a ime y using a Se a am Se sys E olu ion appa a us 152
unde a mosphe ic p essu e coupled o a P ei e P isma mass spec ome e (TPO-MS). 153
The mass loss and he sample empe a u e we e con inuously eco ded by a 154
compu e ised da a acquisi ion sys em. P e iously, he samples (20 mg) we e d ied om 155
oom empe a u e o 150 ºC. Then, he empe a u e was inc eased om 150 o 850 ºC a 156
a cons an hea ing a e o 5 ºC min-1. The oxidan s eam was 5%O2/He (50 cm3 min-1) 157
lowing downwa ds on o he cylind ical sample holde . 158
159
2.3. Ca aly ic es s 160
The h ee me hane e o ming eac ions we e s udied in a bench-scale ixed-bed eac o 161
ope a ed a a mosphe ic p essu e. The eac o was made o s ainless s eel wi h an 162
in e nal diame e o 9 mm and a heigh o 305 mm. P io o he eac ion he ca alys 163
(0.125 g) was dilu ed wi h ine qua z (0.875 g, 1-1.25 mm). The ca alys bed was 164
main ained in he eac o on a qua z wool plug. The empe a u e was measu ed by a 165
he mocouple placed be ween he pa icles o he ca alys . Be o e he eac ion he 166
NiAl2O4-D and NiAl2O4-CP ca alys s we e educed in si u wi h a mix u e o 5%H2/N2 167
a 850 °C o 2 h. Likewise, a 1%Rh/Al2O3 (Al a Aesa , 132 m2 g−1) comme cial 168
ca alys was educed a 700 ºC and i s ac i i y was used o compa a i e pu poses in he 169
h ee me hane e o ming eac ions. 170
8
Th ee di e en eed gas mix u es balanced wi h N2 (38400 cm3 CH4 g−1 h−1) we e used 171
in each e o ming eac ion as ollows: 172
i) 10% CH4 and 5% O2 in POM eac ion. 173
ii) 10% CH4 and 30% H2O in SRM eac ion. 174
iii) 10% CH4, 30% H2O and 5% O2 in OSRM eac ion. 175
The uns we e sequen ially ca ied ou by inc easing and dec easing he eac ion 176
empe a u e (450 ºC-550 ºC-650 ºC-550 ºC-450 ºC) wi h an accumula ed ime online o 177
abou 63 h. Ca aly ic ac i i y, p oduc yields and s abili y we e eco ded du ing 12.5 h 178
a each eac ion empe a u e. Feed and e luen s eams we e analysed online by a 179
Mic oGC (Agilen 3000) equipped wi h a TCD de ec o . Two columns, Molecula Sie e 180
5A and Plo U, we e used in a se ies/bypass a angemen o he comple e sepa a ion o 181
H2, N2, O2, CH4, CO and CO2. A cold ap a he ou le o he eac o was used o 182
condense ou any wa e om he p oduc gas s eam. On basis o he mola low a he 183
inle and ou le o he eac o , con e sion and p oduc yields we e calcula ed, acco ding 184
o he ollowing equa ions: 185
     
 
2
4
4
,% 100


ou ou
in
F CO F CO
X CH F CH
(1) 186
 
 
2
2
4
()2

ou
in
FH
YH F CH
(2) 187
 
 
4
()ou
in
F CO
Y CO F CH
(3) 188
 
 
2
2
4
()ou
in
F CO
Y CO F CH
(4) 189
The he modynamic da a we e calcula ed ia he HSC Chemis y so wa e package by 190
he GIBBS p og am using he so-called Gibbs Ene gy Minimiza ion Me hod. Fo hese 191
calcula ions, only en halpy, en opy and hea capaci y da a o all p e ailing compounds 192
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we e needed. The so wa e calcula ed he amoun s o p oduc s a equilib ium unde 193
iso he mal and isoba ic condi ions. The subs ances o be aken in o accoun in he 194
calcula ions, he amoun o eac an s, he po en ially sTable phases as well as he 195
empe a u e o aw species we e speci ied as inpu . In addi ion o solid ca bon, he 196
ollowing subs ances in he gas phase we e conside ed: CH4, O2, N2, CO, CO2, H2 and 197
H2O. Calcula ions we e pe o med in he 450-650 ºC empe a u e ange a a mosphe ic 198
p essu e. Hence, he GIBBS p og am ound he mos sTable phase combina ion and 199
de e mined he phase composi ion whe e he Gibbs ene gy o he sys em eached i s 200
minimum a cons an p essu e and empe a u e. 201
202
3. Resul s and discussions 203
3.1 Cha ac e isa ion o he samples 204
3.1.1. N2-physo p ion (BET measu emen s) 205
N2 adso p ion a -196 ºC on NiAl2O4-D and NiAl2O4-CP showed ha he iso he ms (no 206
shown) we e cha ac e is ic o mesopo ous solids o ype IV acco ding o he IUPAC 207
classi ica ion. In he case o NiAl2O4-D he ni ogen deso p ion ga e ise o a hys e esis 208
loop, H2 ype, which was cha ac e is ic o diso de ed po ous ma e ials. Table 1 lis s he 209
da a ob ained om he analysis o he ex u al p ope ies o he calcined and educed 210
samples. The p epa ed NiAl2O4-CP spinel (76 m2 g-1) had a highe su ace a ea han 211
NiAl2O4-D (55 m2 g-1). This di e ence p obably esul ed om he p epa a ion me hod 212
and he co esponding p opo ion o NiO o med in each sample, which will be 213
discussed in he XRD, H2-TPR and XPS sec ions. Fig. 1 shows he po e sizes 214
dis ibu ion o he NiAl2O4-D and NiAl2O4-CP ca alys s. The po e size dis ibu ion 215
ace o he NiAl2O4-D sample exhibi ed one maximum a 7 nm in he low mesopo ous 216
ange (<10 nm) whe eas he NiAl2O4-CP samples exhibi ed a peak cen ed a 20 nm. 217
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Fig. 5 368
The di e ence in Ni/Al a omic a io de e mined by XPS be ween NiAl2O4-CP and 369
NiAl2O4-D also suppo ed ou conclusion abou he s uc u al composi ion o nea -370
su ace. Acco ding o Table 2, he Ni/Al a omic a io o NiAl2O4-CP (0.6) was smalle 371
han ha de e mined o NiAl2O4-D (1), sugges ing ha he su ace o he co-372
p ecipi a ed sample was compa a i ely de icien in nickel. Thus, in iew o he 373
s oichiome y o he pu e spinel (Ni/Al=0.5), i could be concluded ha nickel 374
alumina e phase was mainly ob ained on he NiAl2O4-CP nea -su ace oge he wi h a 375
small NiO excess. A la ge amoun o NiO phase was howe e o med on NiAl2O4-D. 376
3.1.6. T ansmission elec on mic oscopy (TEM) 377
NiAl2O4-CP and NiAl2O4-D samples, educed a 850 °C, we e in es iga ed by 378
ansmission elec on mic oscopy as well. Fig. 6(a) and 6(b) show he TEM 379
mic og aphs and size dis ibu ion diag ams (ob ained om he measu emen o abou 380
300 pa icles). The mic og aphs o bo h samples showed no signi ican di e ences in 381
mo phology. In bo h cases sphe ical pa icles o nickel we e obse ed. Ne e heless, i 382
was isualised ha nickel was homogeneously dispe sed in NiAl2O4-CP while i was 383
much mo e he e ogeneous on he NiAl2O4-D ca alys . The pa icle size dis ibu ion in 384
he co-p ecipi a ed sample p esen ed a unimodal shape wi h an a e age size o 10.6 nm 385
(dispe sion o 9.5%). By con as a b oad dis ibu ion anging om 10 o 50 nm was 386
ob ained o he NiAl2O4-D ca alys wi h a co esponding a e age size o 17.7 nm 387
(dispe sion o 4%). By co ela ing he Ni pa icle size dis ibu ions wi h he esul s o 388
H2-TPR, XRD, XPS and UV- isible-NIR DRS i could be easonably assumed ha he 389
smalle mono-dispe sed pa icles p esen on he NiAl2O4-CP co esponded o pa icles 390
esul ed om he educ ion o nickel alumina e phase whe eas he ange o la ge 391
pa icles deposi ed on he NiAl2O4-D was o igina ed by he educ ion o he ee NiO. 392

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As s a ed p e iously, he NiAl2O4-D ca alys con ained wo Ni species as a mix u e o 393
ee NiO and nickel alumina e phase while NiAl2O4-CP was composed mainly by nickel 394
alumina e oge he wi h a ela i ely small ac ion o highly dispe sed NiO (as p o ed 395
by H2-TPR). Fu he mo e, we hough ha he di e ence be ween NiAl2O4-D and 396
NiAl2O4-CP p obably esul ed om he ex u al p ope ies o each ca alys as well. The 397
NiAl2O4-D a e age po e size was smalle (7.5 nm) compa ed o i s Ni pa icles a e age 398
size, sugges ing ha nickel was mainly deposi ed on he ex e nal su ace. Howe e , he 399
NiAl2O4-CP a e age po e size was a ound 15 nm and his migh limi he g ow h o he 400
Ni c ys alli es du ing he educ ion. 401
Fig. 6 402
3.1.7. Tempe a u e p og ammed deso p ion o NH3 (NH3-TPD) 403
The su ace acidi y o he wo ca alys s was cha ac e ised by means o empe a u e 404
p og ammed deso p ion, using ammonia as p obe molecule, ollowed by dynamic 405
he mog a ime y and coupled o a mass spec ome e [15]. The o e all acidi y o he 406
samples (calcined and educed spinel ca alys s) was quan i ied om he ne weigh gain 407
du ing he adso p ion s ep a 100 ºC ollowed by he emo al o physically bound 408
ammonia om he su ace wi h lowing helium. 409
The decon olu ion o he NH3-TPD he mog ams o he calcined NiAl2O4-D and 410
NiAl2O4-CP samples exhibi ed wo bands co esponding o wo ypes o acid si es 411
(Fig. 7). The i s one consis ed o a peak loca ed a low empe a u es (<220 ºC) 412
accompanied by a second much mo e in ense ea u e a ela i ely high empe a u es 413
(250-370 ºC), he la e being p obably due o a ac ion o s ong acid si es p esen a 414
he su ace o he ca alys s. I mus be poin ed ou ha , in he case o he wo educed 415
samples, he MS analysis o he exi s eam e ealed he p esence o ace amoun s o 416
N2 and H2 ela ed o he decomposi ion o he p obe molecule on he educed si es. 417
18
The ob ained quan i a i e esul s we e summa ised in Table 1. I was ound ha he 418
su ace densi y o NH3 adso bed on he calcined NiAl2O4-CP, 3.82 mol NH3 m-2, was 419
smalle han ha de e mined o he calcined NiAl2O4-D, 4.95 mol NH3 m-2. The 420
di e ence could be ela ed o he s uc u al composi ion o each ca alys . As 421
demons a ed by XPS analysis he NiAl2O4-D ca alys con ained alumina phase in i s 422
nea -su ace which could imp o e i s su ace acid p ope ies. I should be poin ed ou 423
ha he H2 educ ion a 850 ºC o he wo ca alys s appea ed o inc ease bo h he amoun 424
and he he mal s abili y o he adso bed NH3 by shi ing he deso p ion peaks owa ds 425
highe empe a u es. The acid su ace densi y o educed NiAl2O4-CP was es ima ed o 426
each 4.38 mol NH3 m-2 in compa ison wi h 5.7 mol NH3 m-2 o NiAl2O4-D 427
(Table 1). These esul s migh be ela ed o he o ma ion o alumina ins ead o nickel 428
alumina e s uc u e in he wo educed samples, as p o ed by XRD analysis. In addi ion, 429
he p esence o homogeneous and highly dispe sed nickel in he educed NiAl2O4-CP 430
ca alys , as con i med by TEM analysis, could explain he occu ence o a small ac ion 431
o su ace acid si es in compa ison wi h NiAl2O4-D. By con as , he low Ni dispe sion 432
(4%) on he la e led o a la ge unco e ed acid suppo su ace. 433
Fig. 7 434
435
3.2. Ca aly ic ac i i y 436
3.2.1 Pa ial oxida ion o me hane (POM) 437
The POM eac ion was ca ied ou on he ac i a ed (calcina ion ollowed by educ ion) 438
NiAl2O4-CP, NiAl2O4-D and 1%Rh/Al2O3 (used as e e ence) ca alys s, a 450, 550 and 439
650 ºC, by sequen ially inc easing he eac ion empe a u e wi h 100 °C in e als. The 440
in e se sequence was conduc ed by dec easing he eac ion empe a u e ( om 650 o 441
550 and 450 ºC) in o de o p obe he s abili y o he ca alys s. Fig. 8 shows he ob ained 442
19
esul s in e ms o CH4 con e sion and H2, CO and CO2 yields. The co esponding 443
equilib ium da a a e included as well (calcula ed ia he HSC Chemis y so wa e 444
package by he GIBBS p og am using he so-called Gibbs Ene gy Minimiza ion 445
Me hod). The epo ed da a e idenced ha he NiAl2O4-CP ca alys was ypically mo e 446
ac i e han he NiAl2O4-D. Fu he mo e, o e he co-p ecipi a ed sample me hane 447
con e sion as well as H2, CO and CO2 yields we e ai ly s able du ing 12.5 h on s eam 448
(Fig. 9). A 450 ºC he con e sion on his sample was a ound 25% and i inc eased o 449
each abou 36% a 550 ºC and 58% a 650 ºC. Likewise, he inc ease in he eac ion 450
empe a u e p omo ed he H2 yield, which a ained 0.56. Ca bon monoxide p oduc ion 451
also inc eased up o 0.44 a 650 ºC. I is no ewo hy ha a 650 ºC bo h con e sion and 452
yields achie ed wi h NiAl2O4-CP we e close o he alues ob ained wi h he comme cial 453
1%Rh/Al2O3 ca alys . As he pe o mance o he noble me al-based ca alys is 454
conside ed as e e ence in he POM eac ion [40-43], hese ca aly ic ea u es e idenced 455
he po en ial o NiAl2O4-CP as a p omising al e na i e ca alys . By con as , he ac i i y 456
appea ed o dec ease wi h ime on s eam in he case o NiAl2O4-D ca alys . I was 457
hough ha his di e ence in he ca aly ic ac i i y be ween he wo educed spinel 458
ca alys s was ela ed o he Ni pa icle size. Indeed, ou TEM esul s showed ha on he 459
NiAl2O4-CP ca alys me allic nickel was deposi ed wi h a smalle size (10.6 nm) 460
compa ed o he NiAl2O4-D (17.7 nm). 461
As shown in Fig. 9, me hane con e sion a 450 ºC g adually dec eased om 29% a he 462
beginning o he eac ion o 25% a e 12.5 h on s eam. Likewise, H2, CO and CO2
463
yields we e no s able wi h ime on line. Indeed, a sligh inc ease in ac i i y ollowed by 464
a s able pla eau was obse ed when he eac ion empe a u e dec eased which could be 465
a esul o an excess o ca bon o ma ion (Fig. 9). 466
20
Table 3 lis s he H2/CO and CO/CO2 a ios calcula ed o each eac ion empe a u e on 467
he es ed ca alys s. No e ha , on he wo es ed nickel ca alys s, he CO/CO2 a io 468
seemed o inc ease wi h eac ion empe a u e. Fo ins ance, a 450 ºC he CO/CO2 a io 469
on he NiAl2O4-CP was a ound 0.1 and i inc eased o each abou 3 a 650 ºC. A low 470
CO/CO2 alues (co esponding o ela i ely low empe a u es) CO disp opo iona ion 471
and/o me hane o al oxida ion eac ion may occu p oducing CO2 and ca bon and/o 472
H2O. On he o he hand, he H2/CO a io was in all he cases highe han 2 ( anging 473
be ween 2.2 and 2.5). I is widely accep ed ha he encapsula ion o he me al pa icles 474
by deposi ed ca bon does no occu i H2/CO o H2O/hyd oca bon a ios a e su icien ly 475
high [32]. Acco dingly, ou esul s showed, by compa ing he NiAl2O4-CP and 476
NiAl2O4-D ca alys s, ha he ca alys wi h he highes H2/CO a io (NiAl2O4-CP) did 477
no su e appa en deac i a ion. 478
Fig. 8/Fig. 9/Table 3 479
The mog a ime ic (no shown) and TPO-MS analyses (Fig. 10) pe o med in o de o 480
de e mine he amoun o ca bon accumula ed on he used ca alys s in POM esul ed in 481
deposi ed ca bon masses o 2.4 w .% on NiAl2O4-CP (close o 2 w % obse ed o he 482
comme cial hodium ca alys ) and 46.5 w .% on NiAl2O4-D (Table 3). Bo h TPO aces 483
consis ed o a CO2 p oduc ion majo peak a app oxima ely 670 ºC (Fig. 10). Thus, he 484
same ype o ca bonaceous species was p esen on he wo ca alys s. I could be 485
concluded, he e o e, ha he deac i a ion o NiAl2O4-D ca alys migh mainly occu as 486
a esul o he subs an ial o ma ion o coke. This was in ag eemen wi h XRD analysis 487
o he spen ca alys s which con i med he o ma ion o ca bon (g aphi e) on he wo 488
ca alys s. Specially, i was obse ed ha he p incipal peak (26.4º) was much mo e 489
in ense in he NiAl2O4-D di ac og am han ha o NiAl2O4-CP ca alys (Fig. 2). On 490
he o he hand, he Ni (200) (2θ = 51.6°) di ac ion line b oadening was used o 491
21
es ima e, by Sche e equa ion, he e olu ion o me allic Ni c ys alli e size a e ca aly ic 492
es (Table 1). I was ound ha he g ow h o Ni0 c ys alli es du ing POM eac ion on 493
NiAl2O4-D o ex ended pe iods o ime was signi ican (size es ima ed o be a ound 494
23 nm in he educed sample while i was a ound 35 nm a e ca aly ic es ). In 495
ag eemen wi h a p e ious s udy which epo ed ha , in he me hane e o ming 496
eac ions, he la ge Ni c ys alli es on he Ni ca alys a ou ed he o ma ion o g aphi ic 497
ca bon [44], we could conclude ha he same phenomenon occu ed in he case o ou 498
NiAl2O4-D ca alys . As s a ed p e iously, he educ ion o he Ni species deposi ed on 499
his sample p oduced la ge me allic Ni which was less esis an o sin e ing and coke 500
o ma ion. 501
Fig. 10 502
The XRD di ac og ams o he spen nickel alumina e ca alys s also e idenced he 503
p esence o NiO phase (Fig. 2). The NiO cha ac e is ic peaks we e much mo e in ense 504
on he NiAl2O4-CP di ac og am sugges ing ha he oxida ion o Ni species du ing he 505
POM eac ion was in luenced by hei in e ac ion wi h he suppo . Acco dingly, i 506
seemed ha he oxida ion o he smalle Ni pa icles, du ing POM eac ion, migh be 507
easie . Indeed, ou H2-TPR and TEM s udies indica ed ha mo e han wo dis inc 508
me allic nickel popula ions we e p esen on NiAl2O4-D in con as wi h he NiAl2O4-CP 509
ca alys wi h a na owe and mo e homogeneous size dis ibu ion (size = 10.6 nm). In 510
hei s udy on he mechanism o POM eac ion, Jin e al. [45] claimed ha du ing he 511
CH4/O2 eac ion o e Ni/Al2O3 ca alys s, Ni0 was i s oxidised o NiO, and he la e 512
was educed again by CH4 du ing he ansien p ocess. Since he educ ion o NiO by 513
CH4 is endo he mic and as he las s ep o ou ca aly ic expe imen s occu ed a 450 ºC 514
( ela i ely low empe a u e) he p esence o he cha ac e is ic peaks o NiO in ou XRD 515
di ac og ams o he used samples could be jus i ied. Fu he mo e, he p esence o he 516

22
NiO could explain he low CO/CO2 a ios ob ained a low empe a u es sugges ing ha 517
his ga e ise o o al oxida ion ins ead o e o ming eac ion. 518
On he o he hand, as desc ibed in p e ious wo ks he ca bon deposi ion may deac i a e 519
he ca alys ei he by co e ing o he ac i e si es and/o by po e blocking [32,44-46]. In 520
ou case, i was ound ha , a e ca aly ic es s, he po e size dis ibu ion o NiAl2O4-D 521
ca alys shi ed owa ds highe alues (8.1 nm o 10.7 nm) sugges ing he blocking o 522
he po es wi h small sizes by ca bon deposi ion (Table 1). In addi ion, i should be no ed 523
ha ca bon deposi ion on he NiAl2O4-CP and NiAl2O4-D ca alys s appea ed o 524
signi ican ly inc ease he su ace a ea (Table 1). Thus, on NiAl2O4-D he speci ic 525
su ace a ea inc eased om 48 o 57 m2 g-1 whe eas o e NiAl2O4-CP ca alys i 526
inc eased om 55 o 64 m2 g-1, sugges ing ha his migh be due o he simila po ous 527
na u e o he ca bon deposi ed on he wo ca alys s [46]. 528
On he o he hand, he chemical p ope ies o hei nea su ace could ha e an e ec on 529
he ca aly ic ac i i y and s abili y o NiAl2O4-D and NiAl2O4-CP ca alys s. Gene ally, 530
acidi y is conside ed o induce a nega i e impac on me hane e o ming beha iou by 531
ca alysing he coke o ma ion [32]. In ag eemen wi h ou NH3-TPD s udies, he su ace 532
o NiAl2O4-D bea s mo e acid si es han ha o NiAl2O4-CP ca alys . This di e ence in 533
he acid cha ac e could explain he ma kedly la ge o ma ion o coke on he su ace o 534
he NiAl2O4-D ca alys in compa ison wi h NiAl2O4-CP. 535
In addi ion, he in e sion deg ee o he o med spinel was ano he ac o o ake in o 536
conside a ion in o de o explain he di e en pe o mances o NiAl2O4-D and NiAl2O4-537
CP in POM eac ion. In hei s udy on he me hane d y e o ming eac ion o e nickel 538
alumina e ca alys s Ka hi ase e al. [26] concluded ha he in e se NiAl2O4 spinel 539
s uc u e posi i ely a ec ed he ca aly ic ac i i y compa ed o he no mal spinel. Ou 540
cha ac e iza ion esul s showed ha NiAl2O4-CP ended o be in he in e se 541
23
coo dina ion while he NiAl2O4-D was close o he no mal spinel phase. The obse ed 542
di e gence be ween NiAl2O4-D and NiAl2O4-CP migh in luence he ca aly ic 543
beha iou o hese ca alys s as well. 544
3.2.2. S eam e o ming o me hane (SRM) 545
Fig. 8 show CH4 con e sion and H2, CO and CO2 yields in he SRM eac ion o he 546
examined educed ca alys s as unc ion o empe a u e. In all cases me hane con e sion 547
as well as H2, CO and CO2 yields we e s able wi h ime on s eam. Mo eo e , he 548
NiAl2O4-CP and NiAl2O4-D ca alys s we e ma kedly mo e ac i e han 549
1%Rh/Al2O3 (80% and 53% s. 46% a 650 °C, espec i ely). The compa ison o he 550
pe o mance o he NiAl2O4-D ca alys in POM and SRM eac ions showed ha 551
eplacing oxygen wi h s eam esul ed in poo e me hane con e sion and CO yield while 552
i imp o ed he H2 p oduc ion. By con as , when compa ed o i s pe o mance in POM 553
eac ion, an inc ease in ac i i y as well as CO and H2 yields we e obse ed o e 554
NiA2O4-CP ca alys in SRM eac ion, especially a 550 ºC and 650 ºC. Fu he mo e, 555
among he h ee ca alys s he bes me hane con e sion and he la ges H2, CO and CO2 556
yields we e achie ed wi h NiAl2O4-CP a he h ee eac ion empe a u es. Thus, a 557
450 ºC he NiAl2O4-D and 1%Rh/Al2O3 ca alys s exhibi ed a poo pe o mance as hei 558
ac i i y did no exceed 10% ( he NiAl2O4-CP ca alys ga e a con e sion o 21% a his 559
empe a u e). I should be no ed ha he CO p oduc ion, a 450 ºC, was e y low o 560
negligible o e he h ee es ed ca alys s. Howe e , conside able yields o CO2 (0.18 561
o e NiAl2O4-CP and 0.07 o e NiAl2O4-D) and H2 (0.5 o e NiAl2O4-CP and 0.22 562
o e NiAl2O4-D) we e ob ained sugges ing he main occu ence o wa e gas shi 563
eac ion. On he e ec o he SRM eac ion empe a u e on he ca aly ic pe o mance o 564
he Ni ca alys s, we no ed ha highe empe a u es imp o ed con e sion which apidly 565
inc eased o each abou 80% and 53% o e NiAl2O4-CP and NiAl2O4-D espec i ely 566
24
(a 650 ºC). Likewise, i espec i e o he used ca alys bo h H2 and CO yields inc eased 567
as he SRM eac ion empe a u e inc eased. This end was mo e p onounced on he 568
NiAl2O4-CP ca alys which led o CO (0.47 a 650 ºC) and H2 (1.6 a 650 ºC) yields and 569
a H2/CO a io close o he he modynamic equilib ium. 570
On he o he hand, as expec ed, he ac i i y o he h ee ca alys s was no accompanied 571
by he ca bon deposi ion (Table 3) which migh be explained by adding wa e o he 572
eed wi h a high H2O/CH4 a io (a ound 3). The obse ed H2/CO a io was in all he 573
cases highe han 6. As no signi ican ca bon deposi ion was de ec ed (Table 3), one 574
could conclude ha his was he eason o he s abili y o he NiAl2O4-D and NiAl2O4-575
CP es ed ca alys s. No e ha , o bo h NiAl2O4-D and NiAl2O4-CP ca alys s, no 576
signi ican loss o hei su ace a ea was obse ed. XRD pa e ns o he NiAl2O4-D and 577
NiAl2O4-CP ca alys s eco ded a e he es s did no show any no iceable di e ence 578
when compa ed wi h ha o he eshly educed samples sugges ing ha hey did no 579
unde go any no iceable al e a ion o hei c ys alline s uc u es du ing he eac ion 580
(Fig. 2). Howe e , an app eciable inc ease o Ni pa icle size was no iced in he case o 581
NiAl2O4-D. Since his did no a ec i s ca aly ic s abili y he Ni pa icles g ow h was 582
supposed o apidly occu a he s a o he eac ion. No e ha , while he s able ca aly ic 583
beha iou could be e en ually masked by mass ans e limi a ions a 650 ºC, he ac is 584
ha a good s abili y was also no iced a lowe empe a u es (450 and 550 ºC). In 585
p inciple, he con ibu ion o mass ans e limi a ions o he obse ed ca aly ic 586
pe o mance could be conside ed negligible unde hese condi ions. In sum, i seems 587
ha he expe imen ally e o ming beha iou o he NiAl2O4-D ac ually was ha o a 588
educed spinel ca alys wi h a c ys alli e size o 47 nm (ins ead o 23 nm). This 589
beha iou could be ela ed wi h he na u e o he Ni species deposi ed and hei 590
in e ac ions wi h he ca ie . Ou cha ac e isa ion esul s showed ha he di e ence 591
25
be ween he wo NiAl2O4-CP and NiAl2O4-D ca alys s was ela ed o he ac ha he 592
educ ion o he Ni species p oduced monodispe sed ixed nickel on he i s one 593
whe eas i p oduced la ge pa icles o ee nickel on he second one. On he e ec o 594
his dis ibu ion we could conclude, hen, ha he g ow h o Ni pa icles in SRM 595
eac ion, obse ed exclusi ely on NiAl2O4-D ca alys , conce ned only he ee deposi ed 596
me allic Ni. 597
3.2.3. Oxida i e s eam e o ming o me hane (OSRM) 598
Finally he ca aly ic pe o mance in OSRM eac ion was also s udied and he esul s a e 599
collec ed in Fig. 8. A he h ee in es iga ed eac ion empe a u es he bes me hane 600
con e sion and he la ges H2 and CO yields, o e he Ni ca alys s, we e achie ed wi h 601
he NiAl2O4-CP ca alys . Mo eo e , compa ed o i s beha iou in POM eac ion, a 602
signi ican imp o emen o he ca aly ic ac i i y could be no ed; especially a 450 ºC 603
and 550 ºC eac ion empe a u es whe e i was e en mo e ac i e han he 1%Rh/Al2O3 604
ca alys . By con as , he NiAl2O4-D ca alys clea ly ga e he lowes me hane 605
con e sion. Fu he mo e, he BET su ace a ea o his sample dec eased om 48 o 606
37 m2 g-1 a e eac ion. Ne e heless, he wa e addi ion o he eed signi ican ly 607
imp o ed he ca aly ic s abili y o NiAl2O4-D wi h espec o he POM eac ion. Indeed, 608
he li e a u e epo ed ha he excess o wa e o oxygen o he eed cleans he me allic 609
su ace and imp o e he s abili y o he ca alys [47]. Acco dingly, ou es ima ion o 610
deposi ed coke, by he mog a ime ic and TPO-MS analysis, was less han 1%w . on 611
bo h NiAl2O4-D and NiAl2O4-CP ca alys s. On he o he hand, he combina ion o POM 612
and SRM gas mix u es seemed o inc ease he CO2 yield, which a ained 0.41 on he 613
NiAl2O4-CP ca alys a 650 ºC. Mo eo e , CO/CO2 a io did no exceed 0.5, which 614
could be explained by a high combus ion ac i i y and a low e o ming ac i i y [10]. 615
32
[43] T. B uno, A. Be e a, G. G oppi, M. Rode i, P. Fo za i, Ca al. Today 99 (2005) 762
89-98. 763
[44] Z.L. Zhang, X. E. Ve ykios, Ca al. Today 21 (1994) 589-595. 764
[45] R. Jin, Y. Chen, W. Li, W. Cui, Y. Ji, C. Yu, Y. Jiang, Appl. Ca al A 201 765
(2000) 71-80. 766
[46] S.Y. Foo, C.K. Cheng, T. Nguyen, E.M. Kennedy, B.Z. Dlugogo ski, A.A. 767
Adesina, Ca al. Commun. 26 (2012) 183-188. 768
[47] S.M. Lima, A.M. Sil a, L.O.O. Cos a, U.M. G aham, G. Jacobs, B.H. Da is, 769
L.V. Ma os, F. B. No onha, J. Ca al. 268 (2009) 268-281. 770
771

33
CAPTIONS FOR TABLES AND FIGURES 772
773
Table 1
Cha ac e isa ion da a o he syn hesised nickel alumina e ca alys s (calcined,
educed and used in POM, SRM and OSRM eac ions).
Table 2
Resul s om H2-TPR and XPS s udies o he calcined NiAl2O4-D and
NiAl2O4-CP samples.
Table 3
Values o H2/CO and CO/CO2 a ios and coke con en in POM, SRM and
OSRM eac ions o e NiAl2O4-D and NiAl2O4-CP ca alys s.
Fig. 1
Mesopo ous size dis ibu ion o (a) calcined and (b) educed NiAl2O4-D and
NiAl2O4-CP ca alys s.
Fig. 2
XRD pa e ns o NiAl2O4-D and NiAl2O4-CP ca alys s: (a) calcined, (b)
educed, and es ed in (c) POM, (d) SRM and (e) OSRM eac ions.
Fig. 3
H2-TPR p o iles o NiAl2O4-D and NiAl2O4-CP ca alys s.
Fig. 4
UV– is–NIR spec a o NiAl2O4-D and NiAl2O4-CP ca alys s.
Fig. 5
XPS spec a o Ni 2p3/2 egion (A) and O 1s egion (B) o NiAl2O4-D and
NiAl2O4-CP ca alys s.
Fig. 6
TEM images and Ni pa icle size dis ibu ion o educed NiAl2O4-D and
NiAl2O4-CP ca alys s.
Fig. 7
NH3-TPD pa e ns o (a) calcined and (b) educed NiAl2O4-D and NiAl2O4-
CP ca alys s.
Fig. 8
Me hane con e sion and H2, CO and CO2 yields o e educed NiAl2O4-CP
and NiAl2O4-D ca alys s e sus eac ion empe a u e in POM, SRM and
OSRM eac ions. Reac ions condi ions: 38400 cm3 CH4 g-1 h-1; W=0.125 g.
Gas mix u es: POM: 10%CH4/5%O2/N2, SRM: 10%CH4/30%H2O/N2 and
OSRM: 10%CH4/30%H2O/5%O2/N2. Da a co esponding o he comme cial
1%Rh/Al2O3 ca alys and he he modynamic equilib ium we e also included.
Fig. 9
E olu ion o CH4 con e sion and H2 and CO yield o e he educed NiAl2O4-
D and NiAl2O4-CP ca alys s wi h ime on s eam in POM eac ion a 450, 550
and 650 C. Reac ions condi ions: 38400 cm3 CH4 g-1 h-1; W=0.125 g. Gas
mix u e: 10%CH4/5%O2/N2.
Fig. 10
TPO-MS analysis o e spen NiAl2O4-D and NiAl2O4-CP ca alys s a e
POM eac ion.
774
34
775
Samples
Ni(1),
w .%
SBET,
m2 g-
1
Vp,
cm3 g-
1
dp, nm
Ni0
size, nm(2)
Ni0
size, nm(3)
mol(4)
NH3 g-
1
mol(4)
NH3 m-
2
Alumina
133
0.55
2.4
-
-


NiAl2O4-D
Calcined
33
55
0.14
7.5
-
-
272
4.95
Reduced
48
0.12
8.1
23
17.7
274
5.70
POM
57
0.12
10.7
35
-
-
-
SRM
47
0.16
11.2
47
-
-
-
OSRM
37
0.11
9.4
29
-
-
-
NiAl2O4-CP
Calcined
33
76
0.35
15.4
-
-
291
3.82
Reduced
55
0.32
19.2
11
10.6
240
4.38
POM
64
0.34
18.4
5
-
-
-
SRM
52
0.33
20.3
11
-
-
-
OSRM
52
0.32
17.7
7
-
-
-
(1) De e mined by WDXRF.
(2) Ni0 c ys alli es size de e mined by XRD.
(3) Ni0 c ys alli es size de e mined by TEM.
(4) To al acidi y de e mined by NH3-TPD.
776
Table 1 777
35
778
Ca alys
H2-TPR
XPS
Theo e ical
H2 up ake
mmol g-1
Expe imen al
H2 up ake
mmol g-1
Rela i e amoun o
educible Ni
species, %
Ni/Al
Ni 2p3/2
O 1s



Peak, eV
Ni, % in
he si es
Sa elli e,
eV
Peak, eV
NiAl2O4-D
5.7
5.8
33
18
49
1
854.1(1)
856.8(2)
61.7
38.3
861.3
528.5 (a)
531.1 (b)
532.6 (c)
NiAl2O4-CP
5.7
5.6
9
47
44
0.6
855.7 (2)
100
861.8
530.6 (b)
 educ ionpeak o NiO excess
 and (***) educ ion peaks o Ni2+ in nickel alumina e
(1) Ni2+ as NiO and (2) Ni2+ as NiAl2O4.
(a) O2− as NiO, (b) O2− as NiAl2O4 and (c) O2− as Al2O3
779
Table 2 780
781
782
783
784
785
786
36
Reac ion
Ca alys
H2/CO
CO/CO2
Coke, %(1)
450
550
650
450
550
650
-
POM
Equilib ium
21.3
6.0
2.7
0.19
1.06
5.86
-
NiAl2O4-D
5.5
3.5
2.2
0.24
0.57
3.00
46.5
NiAl2O4-CP
12.7
4.4
2.5
0.10
0.60
3.00
2.5
1%Rh/Al2O3
2.5
2.8
2.4
0.74
1.29
3.46
2
SRM
Equilib ium
44.9
11.9
6.7
0.10
0.45
1.09
-
NiAl2O4-D
118.6
19.2
8.5
0.05
0.34
1.03
<1
NiAl2O4-CP
68.5
13.3
6.9
0.08
0.51
1.42
<1
1%Rh/Al2O3
19.7
12.6
8.2
0.31
0.56
1.06
<1
OSRM
Equilib ium
36.3
12.1
7.3
0.07
0.28
0.56
-
NiAl2O4-D
43.1
20.1
10.3
0.05
0.16
0.39
<1
NiAl2O4-CP
55.9
20.1
9.4
0.04
0.18
0.50
<1
1%Rh/Al2O3
8.9
10.8
7.8
0.14
0.31
0.65
<1
Reac ions condi ions: 38400 cm3 CH4 g-1 h-1; W=0.125 g. Gas mix u es: POM: 10%CH4/5%O2/N2, SRM: 10%CH4/30%H2O/N2 and OSRM: 10%CH4/30%H2O/5%O2/N2. 787
(1) Deposi ed ca bon o used ca alys de e mined by TPO and TG analyses 788
789
790
Table 3791
37
792
0 5 10 15 20 25 30 35 40 45 50
0
5
10
15
20
25
30 NiAl2O4-CP
NiAl2O4-D
(a)
Vads, cm3 g-1
(a)
(b)
(b)
Po e diame e , nm
793
794
Fig. 1 795
796
797
798

38
10 20 30 40 50 60 70 80
(220)
(440)
Rela i e in ensi y, a. u.
NiO
Niº
NiAl2O4
-Al2O3
NiAl2O4-D
(c)
(e)
(d)
(b)
(a)
C g aphi e
Angle, 2
10 20 30 40 50 60 70 80
(e)
(d)
(c)
(b)
(a)
C g aphi e
NiAl2O4-CP
Rela i e in ensi y, a. u.
Angle, 2
799
Fig. 2 800
801
39
802
H2Consump ion, mmol g-1
Tempe a u e, ºC Time, min
200 400 600 800 20 30
Iso he mal hold
NiO
Iso he mal hold
NiAl2O4-CP Iso he mal hold
-NiO
-NiO
-NiO
(NiAl2O4)
NiAl2O4-D
H2Consump ion, mmol g-1
Tempe a u e, ºC Time, min
200 400 600 800 20 30
Iso he mal hold
NiO
Iso he mal hold
NiAl2O4-CP Iso he mal hold
-NiO
-NiO
-NiO
(NiAl2O4)
NiAl2O4-D
803
804
Fig. 3 805
806
40
807
300 600 900 1200 1500
NiO
NiAl2O4-D
NiAl2O4-CP
Ni2+ in Td
3 d-d ansi ion
o Ni2+ in Oh
Ni2+ in Oh
MLCT
Ni2+ in Td
d-d ansi ions o Ni2+
in unsuppo ed NiO
1 d-d ansi ion
o Ni2+ in Oh
Abso bance, a. u.
Wa elengh , nm
808
809
Fig. 4 810
811
41
812
875 870 865 860 855 850 845
In ensi y, a.u.
Binding ene gy, eV
In ensi y, a.u.
540 535 530 525 520
Binding ene gy, eV
NiAl2O4
NiO
Al2O3
NiAl2O4-D
NiAl2O4
NiO
Sa (Ni 2p3/2)
(A) (B)
NiAl2O4-CP
NiAl2O4-D
NiAl2O4-CP
875 870 865 860 855 850 845
In ensi y, a.u.
Binding ene gy, eV
In ensi y, a.u.
540 535 530 525 520
Binding ene gy, eV
NiAl2O4
NiO
Al2O3
NiAl2O4-D
NiAl2O4
NiO
Sa (Ni 2p3/2)
(A) (B)
NiAl2O4-CP
NiAl2O4-D
NiAl2O4-CP
813
814
Fig. 5 815
2
Abs ac 21
The ca aly ic pe o mance o bulk nickel alumina e ca alys s syn hesised by co-22
p ecipi a ion (NiAl2O4-CP) and co-dissolu ion (NiAl2O4-D) was examined o a ious 23
me hane e o ming eac ions, namely pa ial oxida ion, s eam e o ming and oxida i e 24
s eam e o ming. The calcined and educed spinels we e ho oughly cha ac e ised by a 25
wide numbe o analy ical echniques including N2 physiso p ion, XRD, UV– isible–26
NIR DRS, XPS, TEM, H2-TPR and NH3-TPD. The cha ac e isa ion esul s o he 27
calcined samples a 850 ºC showed ha nickel alumina e phase was mainly ob ained on 28
he NiAl2O4-CP nea -su ace oge he wi h a small NiO excess. By con as , a la ge 29
amoun o NiO phase was o med on NiAl2O4-D and deposi ed on he spinel su ace. 30
The educ ion a 850 ºC o he wo samples p oduced monodispe sed Ni pa icles (10.6 31
nm) in he NiAl2O4-CP while la ge me allic nickel (17.7 nm) was deposi ed on he 32
NiAl2O4-D. In he h ee in es iga ed e o ming eac ions he NiAl2O4-CP ca alys 33
p o ed o be highly ac i e, s able and esis an owa d ca bon deposi ion. Mo eo e , he 34
pe o mances o he NiAl2O4-CP ca alys appea ed o be gene ally compa able wi h ha 35
o a comme cial 1%Rh/Al2O3 ca alys . The di e ence in ca aly ic beha iou be ween 36
he wo nickel alumina es was ela ed o he Ni dispe sion, i s pa icle size and i s 37
capaci y o minimize he acid cha ac e o he alumina by co e ing a la ge pa o i s 38
su ace. 39
40
41
42
43
Keywo ds: Nickel alumina e, Co-p ecipi a ion, Co-dissolu ion, Ni C ys alli e size, 44
Me hane e o ming 45
46

3
1. In oduc ion 47
Me hane s eam e o ming (SRM) is he mos used echnology o p oducing syn hesis 48
gas (CO and H2) om na u al gas [1-5]. Mo eo e , his echnology is a p e e able 49
choice o p oducing clean and la ge hyd ogen yield. Howe e , because o he high 50
cos o he endo he mic cha ac e o SRM, he exo he mic me hane pa ial oxida ion 51
(POM) appea s as an al e na i e as i o e s some in e es ing ad an ages especially 52
when he pos e io use o he syngas s eam equi es a sui able H2/CO a io [6-8]. 53
Cu en ly indus ial acili ies a e equipped wi h sys ems ha o e he possibili y o 54
using POM and/o SRM echnologies. The combina ion o he POM and SRM s a egies 55
called oxida i e s eam e o ming (OSRM) has he ad an age o con olling he p ocess 56
hea and he dis ibu ion o he p oduc s by adjus ing he O/C mola a io [9-10]. 57
Ni-based ca alys s a e widely used o me hane e o ming eac ions due o hei high 58
ac i i y and hei compe i i e cos s compa ed o noble me als [11-13]. Alumina 59
suppo ed Ni sys ems ha e pa icula ly ecei ed conside able a en ion because o hei 60
p ac ical applica ions o a a ie y o e o ming eac ions [14-18]. Ne e heless, despi e 61
exhibi ing ad an ageous ca aly ic p ope ies, alumina lacks an adequa e he mal s abili y 62
which p o okes he sin e ing o suppo ed me als. Fu he mo e, he use o alumina as 63
suppo ma e ial has he d awback o apid deac i a ion o he ca alys due o ca bon 64
deposi ion on i s ac i e acid su ace [19-20]. Fo his eason, he in e es o ecen 65
in es iga ions has been ocused on he p epa a ion o o mula ions leading o a high 66
dispe sion o Ni species which can co e alumina su ace, hus minimising he nega i e 67
e ec o i s acid cha ac e . In his sense, he e ec o he addi ion o some alkali and 68
alkali ea h me als, used as chemical p omo e s, on he ac i i y and s abili y o 69
Ni/alumina ca alys s has been ex ensi ely in es iga ed. Typically hese addi i es help o 70
4
a oid ca bon deposi ion and enhance hei s abili y bu a he expense o a educ ion o 71
hei ac i i y due o he blocking o he mo e eac i e Ni si es [21,22]. 72
Wi hin he same objec i e ( he need o imp o e s abili y o Ni-based ca alys s) nickel 73
alumina e syn hesis is one o he ou es p oposed in he li e a u e o he design o a 74
p ecu so capable o p oduce, a e i s educ ion, me allic Ni p esen ing s ong 75
in e ac ion wi h alumina [14,15,23-25]. Fo so long, Ross e al. [25] had e idenced he 76
pa icipa ion o he educed su ace nickel alumina e si es in he s eam e o ming o 77
me hane o e Ni/alumina ca alys s. Since hen, a numbe o s udies dealing wi h he 78
p epa a ion o nickel alumina e as p ecu so o ac i e Ni me allic pa icles o me hane 79
e o ming eac ions a e a ailable in he li e a u e. A g ea majo i y o hese wo ks ha e 80
been de o ed o he in es iga ion o he in luence o p epa a ion me hods on he 81
s abili y and he ac i i y o he inal o med Ni s uc u es in e o ming eac ions 82
[14,15,24,26,27]. As s a ed in a ious e e ences, i is howe e complica ed o 83
syn hesise pu e o s oichiome ic nickel alumina e by means o con en ional me hods o 84
syn hesis. This is because o he o ma ion o NiO as an excess e en when hea ing a 85
e y high empe a u es (>1300 ºC) [28-30]. The e o e, many wo ks ha e examined he 86
impac o his pa icula i y on he ca alys s pe o mance [15,16,24,27,31]. 87
In ou p e ious s udy, we compa ed wo se ies o Ni/Al2O3 ca alys s wi h simila me al 88
con en p epa ed by co-imp egna ion, using solu ions con aining a mix u e o nickel 89
ace a e and aluminium ni a e o ob ain a s oichiome ic NiAl2O4 suppo ed on alumina, 90
and simple imp egna ion o nickel on alumina [15]. We demons a ed ha , in me hane 91
s eam e o ming, he co-imp egna ed ca alys was mo e ac i e and we a ibu ed his 92
beha iou o i s highe Ni–suppo in e ac ion. We also no iced ha ob aining he ac i e 93
me allic Ni, wi h small pa icle size, was con olled by he NiO/NiAl2O4 a io ini ially 94
p esen in he calcined samples. Achou i e al. [16] also s udied he in luence o he 95
5
p epa a ion me hod on he ca aly ic ac i i y in diesel s eam e o ming o wo alumina-96
suppo ed nickel alumina e ca alys s p epa ed by we -imp egna ion and co-p ecipi a ion. 97
They concluded ha he imp egna ed ca alys exhibi ed a highe ca aly ic pe o mance 98
and s abili y o e ime han he co-p ecipi a ed coun e pa . This di e en beha iou 99
was explained by he highe ca bon deposi ion endency shown by he co-p ecipi a ed 100
sample. 101
In his pape he in luence o he physico-chemical cha ac e is ics o wo bulk nickel 102
alumina e ca alys s, p epa ed by co-p ecipi a ion and co-dissolu ion me hods, on hei 103
ca aly ic beha iou in POM, SRM and OSRM eac ions is in es iga ed. To each his 104
a ge , an ex ensi e cha ac e isa ion has been ca ied ou by a wide numbe on analy ical 105
echniques including BET measu emen s, XRD, XPS, UV– isible–NIR DRS, TEM and 106
H2-TPR. Likewise, TPD o NH3, a well-known p obe molecule o acid si es, has been 107
used o cha ac e ise he su ace chemis y o he in es iga ed samples. A special 108
a en ion has been paid o he de e mina ion o he di e en nickel species o med a e 109
high- empe a u e calcina ion and subsequen high- empe a u e educ ion. In o de o 110
examine and compa e hei ca aly ic e iciency he wo p epa ed ca alys s we e 111
e alua ed in pa ial oxida ion, s eam e o ming and oxida i e s eam e o ming o 112
me hane. In e es ing conclusions a e d awn om he co ela ion be ween he 113
dis ibu ion o Ni ac i e species o he wo ca alys s and hei pe o mance in hese h ee 114
e o ming eac ions. 115
116
2. Expe imen al 117
2.1. Ca alys s p epa a ion 118
Bulk nickel alumina e samples we e syn hesised by co-p ecipi a ion (CP) and co-119
dissolu ion (D) me hods. The sample named NiAl2O4-D was syn hesised using wo 120
6
aqueous solu ions o Ni(CH3-COO)2·4H2O and Al(NO3)3·9H2O. Then hey we e mixed 121
(leading o a mix u e wi h he desi ed 1:2 Ni/Al mola a io), and e apo a ed on a ho 122
pla e (150 ºC). In he case o NiAl2O4-CP, aqueous ammonia was added o he mixed 123
aqueous solu ions o adjus he inal sui able pH (=8). Fo compa a i e pu poses NiO 124
was also p epa ed by simple calcina ion in ai o Ni(CH3-COO)2·4H2O. All he samples 125
we e d ied a 110 ºC o e nigh and hen calcined a 850 ºC in s a ic ai o 4 h a a 126
hea ing a e o 10 ºC min-1. Finally, pelle s o bulk (nickel alumina e o nickel oxide) 127
samples we e p epa ed by a p ocess o comp essing he powde s in o lakes in a 128
hyd aulic p ess (Specac), c ushing and sie ing (0.3-0.5 mm). 129
130
2.2. Ca alys cha ac e isa ion 131
The ca alys s we e cha ac e ised by N2 physiso p ion a -196 ºC, wa eleng h dispe si e 132
X- ay luo escence (WDXRF), X- ay di ac ion (XRD), ul a iole - isible-nea 133
in a ed di use e lec ance spec oscopy (DRS), X- ay pho oelec on spec oscopy 134
(XPS), empe a u e p og ammed educ ion wi h hyd ogen (H2-TPR) and empe a u e 135
p og ammed deso p ion o NH3 (NH3-TPD). The expe imen al de ails o each analy ical 136
echnique a e desc ibed elsewhe e [14,15]. Addi ionally, he mo phology and pa icle 137
size dis ibu ion o he nickel pa icles was examined by ansmission elec on 138
mic oscopy (TEM). P io o analysis, he samples we e dispe sed in absolu e e hanol 139
ul asonically o 30 min, and 10 cm3 o each sample we e hen placed on lexible ilm 140
(Pa a ilm® M). Glow-discha ged ca bon-coa ed coppe g ids we e in e ed on o he 141
d ople s o each sample. A e incuba ion o 1 min a oom empe a u e, he g ids we e 142
manually blo ed wi h il e pape ai -d ied. Digi ally eco ded 2D images o each 143
solu ion we e aken a oom empe a u e a a nominal magni ica ion o 80000 on a Jeol 144
JEM-1230 ansmission elec on mic oscope, wi h a LaB6 ilamen as he sou ce o 145
7
elec ons and ope a ed a 100 kV. Digi al images we e eco ded on an O ius SC1000 146
cooled slow-scan CCD came a, 4008×2672 pixels (GATAN), ob aining a inal pixel 147
size o 0.85 Å pixel-1. The pa icle size dis ibu ion was ob ained om he measu emen 148
o a leas 300 pa icles using ImageJ so wa e, and he a e age diame e was calcula ed 149
by dM =di·ni/ni, whe e ni is he numbe o he pa icles o diame e di. 150
On he o he hand, he amoun o ca bonaceous deposi s on he used ca alys was 151
de e mined by dynamic he mog a ime y using a Se a am Se sys E olu ion appa a us 152
unde a mosphe ic p essu e coupled o a P ei e P isma mass spec ome e (TPO-MS). 153
The mass loss and he sample empe a u e we e con inuously eco ded by a 154
compu e ised da a acquisi ion sys em. P e iously, he samples (20 mg) we e d ied om 155
oom empe a u e o 150 ºC. Then, he empe a u e was inc eased om 150 o 850 ºC a 156
a cons an hea ing a e o 5 ºC min-1. The oxidan s eam was 5%O2/He (50 cm3 min-1) 157
lowing downwa ds on o he cylind ical sample holde . 158
159
2.3. Ca aly ic es s 160
The h ee me hane e o ming eac ions we e s udied in a bench-scale ixed-bed eac o 161
ope a ed a a mosphe ic p essu e. The eac o was made o s ainless s eel wi h an 162
in e nal diame e o 9 mm and a heigh o 305 mm. P io o he eac ion he ca alys 163
(0.125 g) was dilu ed wi h ine qua z (0.875 g, 1-1.25 mm). The ca alys bed was 164
main ained in he eac o on a qua z wool plug. The empe a u e was measu ed by a 165
he mocouple placed be ween he pa icles o he ca alys . Be o e he eac ion he 166
NiAl2O4-D and NiAl2O4-CP ca alys s we e educed in si u wi h a mix u e o 167
5%H2/N2 a 850 °C o 2 h. Likewise, a 1%Rh/Al2O3 (Al a Aesa , 132 m2 g−1) 168
comme cial ca alys was educed a 700 ºC and i s ac i i y was used o compa a i e 169
pu poses in he h ee me hane e o ming eac ions. 170

8
Th ee di e en eed gas mix u es balanced wi h N2 (38400 cm3 CH4 g−1 h−1) we e used 171
in each e o ming eac ion as ollows: 172
i) 10% CH4 and 5% O2 in POM eac ion. 173
ii) 10% CH4 and 30% H2O in SRM eac ion. 174
iii) 10% CH4, 30% H2O and 5% O2 in OSRM eac ion. 175
The uns we e sequen ially ca ied ou by inc easing and dec easing he eac ion 176
empe a u e (450 ºC-550 ºC-650 ºC-550 ºC-450 ºC) wi h an accumula ed ime online o 177
abou 63 h. Ca aly ic ac i i y, p oduc yields and s abili y we e eco ded du ing 12.5 h 178
a each eac ion empe a u e. Feed and e luen s eams we e analysed online by a 179
Mic oGC (Agilen 3000) equipped wi h a TCD de ec o . Two columns, Molecula Sie e 180
5A and Plo U, we e used in a se ies/bypass a angemen o he comple e sepa a ion o 181
H2, N2, O2, CH4, CO and CO2. A cold ap a he ou le o he eac o was used o 182
condense ou any wa e om he p oduc gas s eam. On basis o he mola low a he 183
inle and ou le o he eac o , con e sion and p oduc yields we e calcula ed, acco ding 184
o he ollowing equa ions: 185
     
 
2
4
4
,% 100


ou ou
in
F CO F CO
X CH F CH
(1) 186
 
 
2
2
4
()2

ou
in
FH
YH F CH
(2) 187
 
 
4
()ou
in
F CO
Y CO F CH
(3) 188
 
 
2
2
4
()ou
in
F CO
Y CO F CH
(4) 189
190
191
9
3. Resul s and discussions 192
3.1 Cha ac e isa ion o he samples 193
3.1.1. N2-physo p ion (BET measu emen s) 194
N2 adso p ion a -196 ºC on NiAl2O4-D and NiAl2O4-CP showed ha he iso he ms (no 195
shown) we e cha ac e is ic o mesopo ous solids o ype IV acco ding o he IUPAC 196
classi ica ion. In he case o NiAl2O4-D he ni ogen deso p ion ga e ise o a hys e esis 197
loop, H2 ype, which was cha ac e is ic o diso de ed po ous ma e ials. Table 1 lis s he 198
da a ob ained om he analysis o he ex u al p ope ies o he calcined and educed 199
samples. The p epa ed NiAl2O4-CP spinel (76 m2 g-1) had a highe su ace a ea han 200
NiAl2O4-D (55 m2 g-1). This di e ence p obably esul ed om he p epa a ion me hod 201
and he co esponding p opo ion o NiO o med in each sample. Fig.1 shows he po e 202
sizes dis ibu ion o he NiAl2O4-D and NiAl2O4-CP ca alys s. The po e size 203
dis ibu ion ace o he NiAl2O4-D sample exhibi ed one maximum a 7 nm in he low 204
mesopo ous ange (<10 nm) whe eas he NiAl2O4-CP samples exhibi ed a peak cen ed 205
a 20 nm. A e educ ion a high empe a u e (850 ºC) bo h samples dec eased hei 206
o al su ace a ea (48 m2 g−1 o NiAl2O4–D and 55 m2 g−1 o NiAl2O4-CP); howe e i 207
did no in luence conside ably hei po e size dis ibu ion. 208
3.1.2. X- ay di ac ion (XRD) 209
The gene al o mula o s oichiome ic nickel alumina es (spinel s uc u e) is NiAl2O4. I 210
c ys allizes in he cubic sys em and belongs o Fd-3m space g oup [32-36]. Typically, 211
he amewo k o he “no mal” spinel s uc u es consis s o an ensemble o e ahed al 212
and oc ahed al coo dina ion occupied by bi alen (Ni2+) and i alen (Al3+) ca ions, 213
espec i ely [32,33]. Howe e , his dis ibu ion can change when he Ni2+ pa ially 214
adop s he oc ahed al si e while he e ahed al si e hos s he Al3+ oge he wi h Ni2+ 215
10
ions. This s uc u al lexibili y gene a es a amily o compounds o in e se spinel 216
s uc u e desc ibed as Ni1-xAlx[NixAl2-x]O4 (0 < x < 1) [32-36]. 217
The NiAl2O4-D and NiAl2O4-CP samples we e cha ac e ized by means o X- ay powde 218
di ac ion in o de o in es iga e hei s uc u al p ope ies. Fig. 2 compa es he 219
di ac og ams o he ca alys s be o e and a e educ ion a 850 ºC. The pa e ns o he 220
calcined NiAl2O4-D and NiAl2O4-CP e ealed he o ma ion o he spinel s uc u e 221
(JCPDS 78-1601). Fu he mo e, in he case o NiAl2O4-D ca alys addi ional peaks 222
associa ed wi h NiO s uc u e, a 43.5° and 63.1°, we e obse ed whe eas he 223
di ac og am o NiAl2O4-CP showed no lines o NiO. A mo e ca e ul analysis o he 224
pa e n o NiAl2O4-D e idenced ha he in ensi y a io o he peaks co esponding o 225
(220) and (440) e icula planes di e ed om ha o NiAl2O4-CP. This di e ence could 226
be explained by he ca ion dis ibu ion in he oc ahed al and e ahed al si es a ec ing 227
he in e sion deg ee o he spinel s uc u e. The di ac ion lines associa ed wi h he 228
(220) plane a e ela ed o he e ahed ally-coo dina ed ca ions while he di ac ion 229
signals belonging o he (440) plane a e a ibu ed o bo h e ahed ally and 230
oc ahed ally-coo dina ed ca ions [34-36]. F om ou expe imen al da a I(220)/I(440) 231
a ios we e es ima ed. Fo he NiAl2O4-CP ca alys his a io was 0.22, whe eas i 232
inc eased up o 0.33 o he NiAl2O4-D. The inc ease in I(220)/I(440) a io was also 233
obse ed by Wang e al. [35] in XRD pa e ns o a ious in e se spinel s uc u es. This 234
ea u e was assigned o he inc easing occupancy o he hea ie ca ions on he 235
e ahed al si es. We migh acco dingly conclude ha on he NiAl2O4-D ca alys nickel 236
was p e e en ially hos ed in e ahed al si es o he in e se spinel whe eas nickel ended 237
o occupy he oc ahed al coo dina ion in he case o NiAl2O4-CP sample p epa ed by 238
co-p ecipi a ion. 239
11
Fig. 2 also includes he X- ay di ac ion pa e ns o he bulk nickel alumina es educed 240
a 850 °C. A compa ison o he pa e ns be o e and a e educ ion con i med ha he 241
ans o ma ion o Ni2+ ions o he spinel amewo k in o me allic Ni (JCPDS 89-7128) 242
was comple e. Likewise, he o ma ion o alumina s uc u e (JCPDS 79-1558) ins ead o 243
he cha ac e is ic peaks a ibu ed o spinel s uc u e was no ed. Howe e , in he case o 244
he NiAl2O4-D sample, di ac ion peaks wi h a low in ensi y a ibu ed o NiO we e 245
also obse ed. This could be easonably explained by he su ace oom empe a u e 246
passi a ion o he ca alys [31]. Table 1 epo s he me allic Ni c ys alli e size, 247
calcula ed by Sche e equa ion, by using Ni (200) (2θ = 51.6°) di ac ion line 248
b oadening. The a e age size o Ni pa icles was ound o be a ound 11 nm on he 249
educed NiAl2O4-CP while la ge pa icles wi h an a e age size o abou 23 nm we e 250
de ec ed o he educed NiAl2O4-D. 251
In sum, he ob ained XRD esul s indica ed ha he p epa a ion me hod o nickel 252
alumina e had an impo an e ec on he excess o NiO and he Ni2+ ions dis ibu ion, 253
be ween e ahed al and oc ahed al si es in he spinel s uc u e. Mo eo e , he educ ion 254
o he NiAl2O4-CP sample p epa ed by co-p ecipi a ion me hod p oduced smalle Ni 255
pa icles in compa ison wi h he NiAl2O4-D ca alys . 256
3.1.3. Tempe a u e p og ammed educ ion (H2-TPR) 257
The TPR expe imen s we e pe o med in o de o de e mine he di e en Ni educible 258
species p esen in he wo p epa ed spinel ca alys s (Fig. 3). I was obse ed ha he 259
shape o he TPR aces depended on he p epa a ion me hod. The he mog am o he 260
NiAl2O4-D sample displayed a ypical educ ion spec um o a mix u e o wo Ni 261
species consis ing o ee NiO (-peak a 400 ºC) and nickel alumina es s uc u es (-262
peak a 600 ºC and -peak a 800 ºC) [14-15]. In he case o NiAl2O4-CP ca alys i was 263
no ed ha he cha ac e is ic educ ion peak a ibu ed o bulk NiO (a 400 ºC) was 264
18
empe a u e p omo ed he H2 yield, which a ained 0.56. Ca bon monoxide p oduc ion 415
also inc eased up o 0.44 a 650 ºC. I is no ewo hy ha a 650 ºC bo h con e sion and 416
yields achie ed wi h NiAl2O4-CP we e close o he alues ob ained wi h he comme cial 417
1%Rh/Al2O3 ca alys . As he pe o mance o he noble me al-based ca alys is 418
conside ed as e e ence in he POM eac ion [39-42], hese ca aly ic ea u es e idenced 419
he po en ial o NiAl2O4-CP as a p omising al e na i e ca alys . By con as , he ac i i y 420
appea ed o dec ease wi h ime on s eam in he case o NiAl2O4-D ca alys . As shown 421
in Fig. 9, me hane con e sion a 450 ºC g adually dec eased om 29% a he beginning 422
o he eac ion o 25% a e 12.5 h on s eam. Likewise, H2, CO and CO2 yields we e 423
no s able wi h ime on line. Indeed, a sligh inc ease in ac i i y ollowed by a s able 424
pla eau was obse ed when he eac ion empe a u e dec eased sugges ing ha an 425
ac i a ion o he ca alys occu ed wi h ime-on-s eam (Fig. 9). 426
Table 3 lis s he H2/CO and CO/CO2 a ios calcula ed o each eac ion empe a u e on 427
he es ed ca alys s. No e ha , on he wo es ed nickel ca alys s, he CO/CO2 a io 428
seemed o inc ease wi h eac ion empe a u e. Fo ins ance, a 450 ºC he CO/CO2 a io 429
on he NiAl2O4-CP was a ound 0.1 and i inc eased o each abou 3 a 650 ºC. A low 430
CO/CO2 alues (co esponding o ela i ely low empe a u es) CO disp opo iona ion 431
eac ion may occu p oducing CO2 and ca bon. On he o he hand, he H2/CO a io was 432
in all he cases highe han 2 ( anging be ween 2.2 and 2.5). I is widely accep ed ha 433
he encapsula ion o he me al pa icles by deposi ed ca bon does no occu i H2/CO o 434
H2O/hyd oca bon a ios a e su icien ly high [43]. Acco dingly, ou esul s showed, by 435
compa ing he NiAl2O4-CP and NiAl2O4-D ca alys s, ha he ca alys wi h he highes 436
H2/CO a io (NiAl2O4-CP) did no su e appa en deac i a ion. 437
The mog a ime ic (no shown) and TPO-MS analyses (Fig. 10) pe o med in o de o 438
de e mine he amoun o ca bon accumula ed on he used ca alys s in POM esul ed in 439

19
deposi ed ca bon masses o 2.4 w .% on NiAl2O4-CP (close o 2 w % obse ed o he 440
comme cial hodium ca alys ) and 46.5 w .% on NiAl2O4-D (Table 3). Bo h TPO aces 441
consis ed o a CO2 p oduc ion majo peak a app oxima ely 670 ºC (Fig. 10). Thus, he 442
same ype o ca bonaceous species was p esen on he wo ca alys s. I could be 443
concluded, he e o e, ha he deac i a ion o NiAl2O4-D ca alys migh mainly occu as 444
a esul o he subs an ial o ma ion o coke. This was in ag eemen wi h XRD analysis 445
o he spen ca alys s which con i med he o ma ion o ca bon (g aphi e) on he wo 446
ca alys s. Specially, i was obse ed ha he p incipal peak (26.4º) was much mo e 447
in ense in he NiAl2O4-D di ac og am han ha o NiAl2O4-CP ca alys (Fig. 2). On 448
he o he hand, he Ni (200) (2θ = 51.6°) di ac ion line b oadening was used o 449
es ima e, by Sche e equa ion, he e olu ion o me allic Ni c ys alli e size a e ca aly ic 450
es (Table 1). I was ound ha he g ow h o Ni0 c ys alli es du ing POM eac ion on 451
NiAl2O4-D o ex ended pe iods o ime was signi ican (size es ima ed o be a ound 452
23 nm in he educed sample while i was a ound 35 nm a e ca aly ic es ). In 453
ag eemen wi h a p e ious s udy which epo ed ha , in he me hane e o ming 454
eac ions, he la ge Ni c ys alli es on he Ni ca alys a ou ed he o ma ion o g aphi ic 455
ca bon [44], we could conclude ha he same phenomenon occu ed in he case o ou 456
NiAl2O4-D ca alys . As s a ed p e iously, he educ ion o he Ni species deposi ed on 457
his sample p oduced la ge me allic Ni which was less esis an o sin e ing and coke 458
o ma ion. 459
The XRD di ac og ams o he spen nickel alumina e ca alys s also e idenced he 460
p esence o NiO phase (Fig. 2). The NiO cha ac e is ic peaks we e much mo e in ense 461
on he NiAl2O4-CP di ac og am sugges ing ha he oxida ion o Ni species du ing he 462
POM eac ion was in luenced by hei in e ac ion wi h he suppo . Acco dingly, i 463
seemed ha he oxida ion o he smalle Ni pa icles, du ing POM eac ion, migh be 464
20
easie . Indeed, ou H2-TPR and TEM s udies indica ed ha mo e han wo dis inc 465
me allic nickel popula ions we e p esen on NiAl2O4-D in con as wi h he NiAl2O4-CP 466
ca alys wi h a na owe and mo e homogeneous size dis ibu ion (size = 10.6 nm). In 467
hei s udy on he mechanism o POM eac ion, Jin e al. [45] claimed ha du ing he 468
CH4/O2 eac ion o e Ni/Al2O3 ca alys s, Ni0 was i s oxidised o NiO, and he la e 469
was educed again by CH4 du ing he ansien p ocess. Since he educ ion o NiO by 470
CH4 is endo he mic and as he las s ep o ou ca aly ic expe imen s occu ed a 450 ºC 471
( ela i ely low empe a u e) he p esence o he cha ac e is ic peaks o NiO in ou XRD 472
di ac og ams o he used samples could be jus i ied. 473
On he o he hand, as desc ibed in p e ious wo ks he ca bon deposi ion may deac i a e 474
he ca alys ei he by co e ing o he ac i e si es and/o by po e blocking [43-46]. In ou 475
case, i was ound ha , a e ca aly ic es s, he po e size dis ibu ion o NiAl2O4-D 476
ca alys shi ed owa ds highe alues (8.1 nm o 10.7 nm) sugges ing he blocking o 477
he po es wi h small sizes by ca bon deposi ion (Table 1). In addi ion, i should be no ed 478
ha ca bon deposi ion on he NiAl2O4-CP and NiAl2O4-D ca alys s appea ed o 479
signi ican ly inc ease he su ace a ea (Table 1). Thus, on NiAl2O4-D he speci ic 480
su ace a ea inc eased om 48 o 57 m2 g-1 whe eas o e NiAl2O4-CP ca alys i 481
inc eased om 55 o 64 m2 g-1, sugges ing ha his migh be due o he simila po ous 482
na u e o he ca bon deposi ed on he wo ca alys s [46]. 483
On he o he hand, he chemical p ope ies o hei nea su ace could ha e an e ec on 484
he ca aly ic ac i i y and s abili y o NiAl2O4-D and NiAl2O4-CP ca alys s. Gene ally, 485
acidi y is conside ed o induce a nega i e impac on me hane e o ming beha iou by 486
ca alysing he coke o ma ion [43]. In ag eemen wi h ou NH3-TPD s udies, he su ace 487
o NiAl2O4-D bea s mo e acid si es han ha o NiAl2O4-CP ca alys . This di e ence in 488
21
he acid cha ac e could explain he ma kedly la ge o ma ion o coke on he su ace o 489
he NiAl2O4-D ca alys in compa ison wi h NiAl2O4-CP. 490
In addi ion, he in e sion deg ee o he o med spinel was ano he ac o o ake in o 491
conside a ion in o de o explain he di e en pe o mances o NiAl2O4-D and NiAl2O4-492
CP in POM eac ion. In hei s udy on he me hane d y e o ming eac ion o e nickel 493
alumina e ca alys s Ka hi ase e al. [26] concluded ha he in e se NiAl2O4 spinel 494
s uc u e posi i ely a ec ed he ca aly ic ac i i y compa ed o he no mal spinel. Ou 495
cha ac e iza ion esul s showed ha NiAl2O4-CP ended o be in he in e se 496
coo dina ion while he NiAl2O4-D was close o he no mal spinel phase. The obse ed 497
di e gence be ween NiAl2O4-D and NiAl2O4-CP migh in luence he ca aly ic 498
beha iou o hese ca alys s as well. 499
3.2.2. S eam e o ming o me hane (SRM) 500
Fig. 8 show CH4 con e sion and H2, CO and CO2 yields in he SRM eac ion o he 501
examined educed ca alys s as unc ion o empe a u e. In all cases me hane con e sion 502
as well as H2, CO and CO2 yields we e s able wi h ime on s eam. Mo eo e , he 503
NiAl2O4-CP and NiAl2O4-D ca alys s we e ma kedly mo e ac i e han 504
1%Rh/Al2O3 (80% and 53% s. 46% a 650 °C, espec i ely). The compa ison o he 505
pe o mance o he NiAl2O4-D ca alys in POM and SRM eac ions showed ha 506
eplacing oxygen wi h s eam esul ed in poo e me hane con e sion and CO yield while 507
i imp o ed he H2 p oduc ion. By con as , when compa ed o i s pe o mance in POM 508
eac ion, an inc ease in ac i i y as well as CO and H2 yields we e obse ed o e 509
NiA2O4-CP ca alys in SRM eac ion, especially a 550 ºC and 650 ºC. Fu he mo e, 510
among he h ee ca alys s he bes me hane con e sion and he la ges H2, CO and CO2 511
yields we e achie ed wi h NiAl2O4-CP a he h ee eac ion empe a u es. Thus, a 512
450 ºC he NiAl2O4-D and 1%Rh/Al2O3 ca alys s exhibi ed a poo pe o mance as hei 513
22
ac i i y did no exceed 10% ( he NiAl2O4-CP ca alys ga e a con e sion o 21% a his 514
empe a u e). I should be no ed ha he CO p oduc ion, a 450 ºC, was e y low o 515
negligible o e he h ee es ed ca alys s. Howe e , conside able yields o CO2 (0.18 516
o e NiAl2O4-CP and 0.07 o e NiAl2O4-D) and H2 (0.5 o e NiAl2O4-CP and 0.22 517
o e NiAl2O4-D) we e ob ained sugges ing he main occu ence o wa e gas shi 518
eac ion. On he e ec o he SRM eac ion empe a u e on he ca aly ic pe o mance o 519
he Ni ca alys s, we no ed ha highe empe a u es imp o ed con e sion which apidly 520
inc eased o each abou 80% and 53% o e NiAl2O4-CP and NiAl2O4-D espec i ely 521
(a 650 ºC). Likewise, i espec i e o he used ca alys bo h H2 and CO yields inc eased 522
as he SRM eac ion empe a u e inc eased. This end was mo e p onounced on he 523
NiAl2O4-CP ca alys which led o CO (0.47 a 650 ºC) and H2 (1.6 a 650 ºC) yields and 524
a H2/CO a io close o he he modynamic equilib ium. 525
On he o he hand, as expec ed, he ac i i y o he h ee ca alys s was no accompanied 526
by he ca bon deposi ion (Table 3) which migh be explained by adding wa e o he 527
eed wi h a high H2O/CH4 a io (a ound 3). The obse ed H2/CO a io was in all he 528
cases highe han 6. As no signi ican ca bon deposi ion was de ec ed (Table 3), one 529
could conclude ha his was he eason o he s abili y o he NiAl2O4-D and NiAl2O4-530
CP es ed ca alys s. No e ha , o bo h NiAl2O4-D and NiAl2O4-CP ca alys s, no 531
signi ican loss o hei su ace a ea was obse ed. XRD pa e ns o he NiAl2O4-D and 532
NiAl2O4-CP ca alys s eco ded a e he es s did no show any no iceable di e ence 533
when compa ed wi h ha o he eshly educed samples sugges ing ha hey did no 534
unde go any no iceable al e a ion o hei c ys alline s uc u es du ing he eac ion 535
(Fig. 2). Howe e , an app eciable inc ease o Ni pa icle size was no iced in he case o 536
NiAl2O4-D. Since his did no a ec i s ca aly ic s abili y he Ni pa icles g ow h was 537
supposed o apidly occu a he s a o he eac ion. This beha iou could be ela ed 538
23
wi h he na u e o he Ni species deposed and hei in e ac ions wi h he ca ie . Ou 539
cha ac e isa ion esul s showed ha he di e ence be ween he wo NiAl2O4-CP and 540
NiAl2O4-D ca alys s was ela ed o he ac ha he educ ion o he Ni species p oduced 541
monodispe sed ixed nickel on he i s one whe eas i p oduced la ge pa icles o ee 542
nickel on he second one. On he e ec o his dis ibu ion we could conclude, hen, ha 543
he g ow h o Ni pa icles in SRM eac ion, obse ed exclusi ely on NiAl2O4-D 544
ca alys , conce ned only he ee deposi ed me allic Ni. 545
3.2.3. Oxida i e s eam e o ming o me hane (OSRM) 546
Finally he ca aly ic pe o mance in OSRM eac ion was also s udied and he esul s a e 547
collec ed in Fig. 8. A he h ee in es iga ed eac ion empe a u es he bes me hane 548
con e sion and he la ges H2 and CO yields, o e he Ni ca alys s, we e achie ed wi h 549
he NiAl2O4-CP ca alys . Mo eo e , compa ed o i s beha iou in POM eac ion, a 550
signi ican imp o emen o he ca aly ic ac i i y could be no ed; especially a 450 ºC 551
and 550 ºC eac ion empe a u es whe e i was e en mo e ac i e han he 1%Rh/Al2O3 552
ca alys . By con as , he NiAl2O4-D ca alys clea ly ga e he lowes me hane 553
con e sion. Fu he mo e, he BET su ace a ea o his sample dec eased om 48 o 554
37 m2 g-1 a e eac ion. Ne e heless, he wa e addi ion o he eed signi ican ly 555
imp o ed he ca aly ic s abili y o NiAl2O4-D wi h espec o he POM eac ion. Indeed, 556
he li e a u e epo ed ha he excess o wa e o oxygen o he eed cleans he me allic 557
su ace and imp o e he s abili y o he ca alys [47]. Acco dingly, ou es ima ion o 558
deposi ed coke, by he mog a ime ic and TPO-MS analysis, was less han 1%w . on 559
bo h NiAl2O4-D and NiAl2O4-CP ca alys s. On he o he hand, he combina ion o POM 560
and SRM gas mix u es seemed o inc ease he CO2 yield, which a ained 0.41 on he 561
NiAl2O4-CP ca alys a 650 ºC. Mo eo e , CO/CO2 a io did no exceed 0.5, which 562
could be explained by a high combus ion ac i i y and a low e o ming ac i i y [10]. 563

24
Fig. 2 also shows he XRD pa e ns o he NiAl2O4-D and NiAl2O4-CP ca alys s used in 564
OSRM eac ion. The absence o 26.4º di ac ion peak indica ed ha , on bo h ca alys s, 565
no g aphi e ca bon deposi ion occu ed du ing he es s. By con as , changes in he Ni 566
c ys alli e size we e no ed. Fo he NiAl2O4-D ca alys he Ni pa icle size inc eased 567
om 22 up o 29 nm, whe eas o he NiAl2O4-CP i dec eased om 11 o 7 nm 568
(Table 1). Simila changes we e obse ed on he XRD pa e ns o he la e in POM 569
eac ion sugges ing ha a e-dis ibu ion o Ni ac i e phases migh ha e occu ed which 570
a ou ed i s ac i i y and s abili y. Howe e , on he NiAl2O4-D ca alys his e-571
dis ibu ion (inc eased Ni pa icle size) p o oked he decay o i s ac i i y. The XRD 572
di ac og ams o he used NiAl2O4-D and NiAl2O4-CP ca alys s in OSRM eac ion also 573
showed he p esence o in ense di ac ion peaks a ibu ed o he NiO phase (Fig. 2). A 574
simila beha iou was epo ed by Yoshida e al. [10] in hei s udy o OSR eac ion 575
o e Ni/α-Al2O3 and hey explained i by he oxida ion o he Ni species in he p esence 576
o gas-phase oxygen. The o ma ion o hese Ni oxidised species could explain he high 577
ac i i y o ou Ni ca alys s o me hane combus ion. 578
4. Conclusions 579
The beha iou o wo bulk nickel alumina e (NiAl2O4-CP and NiAl2O4-D) has been 580
in es iga ed in he pa ial oxida ion, s eam e o ming and oxida i e s eam e o ming o 581
me hane. These ca alys s ha e been p epa ed by co-p ecipi a ion and co-dissolu ion 582
me hods, espec i ely, and cha ac e ised by N2 physiso p ion, XRD, UV– isible–NIR 583
DRS, XPS, TEM, H2-TPR and NH3-TPD. 584
XRD, H2-TPR, UV– isible spec oscopy and XPS analyses show ha he wo ca alys s, 585
calcined a 850 ºC, con ain nickel alumina e as a majo phase oge he wi h NiO which 586
was conside ed as an excess. Mo eo e , i has been p o ed ha on he NiAl2O4-D 587
ca alys nickel is p e e en ially hos ed in e ahed al si es o he in e se spinel whe eas i 588
25
ends o occupy he oc ahed al coo dina ion in he case o NiAl2O4-CP sample. As 589
showed by H2-TPR, he in e ac ion o he esul ed NiO excess wi h he nickel alumina e 590
seems o depend on he p epa a ion me hod. Indeed, he educ ion o his NiO excess on 591
he NiAl2O4-D occu s a lowe empe a u es compa ed o he NiAl2O4-CP sugges ing 592
ha on he la e NiO has a s ong in e ac ion wi h nickel alumina e. TEM esul s 593
indica e ha on he NiAl2O4-CP ca alys he educ ion o he Ni species a 850 ºC 594
p oduces homogeneous and monodispe sed ixed nickel whe eas he e ogeneous and 595
la ge me allic ee nickel is de ec ed on he NiAl2O4-D ca alys . In addi ion, his 596
dis ibu ion has a ma ked e ec on he chemical p ope ies o he educed ca alys s 597
since he su ace o educed NiAl2O4-D con ains mo e acid si es han ha o educed 598
NiAl2O4-CP. 599
A clea ly di e en beha iou in he e o ming o me hane has been obse ed o e he 600
NiAl2O4-CP and NiAl2O4-D ca alys s wi h same Ni composi ion bu p epa ed by wo 601
di e en me hods. The esul s show ha he e o ming e iciency is highly dependen on 602
he ype o in e ac ion o he Ni ac i e phase wi h he ca ie . In he h ee es ed 603
e o ming eac ions he NiAl2O4-CP ca alys , p epa ed by co-p ecipi a ion me hod, has 604
p o ed o be highly ac i e and s able. Howe e , as a esul o he ca bon deposi ion 605
ound in POM eac ion, me hane con e sion, H2, CO and CO2 yields a e no s able wi h 606
ime o e he NiAl2O4-D ca alys . A low POM eac ion empe a u es (450 ºC and 607
550 ºC) CO disp opo iona ion eac ion occu s p oducing CO2 and ca bon. On he o he 608
hand, he H2/CO a io has esul ed in all he cases highe han 2. By using s eam ins ead 609
o oxygen no signi ican ca bon deposi ion has been de ec ed on NiAl2O4-D ca alys , 610
bu i leads o lowe me hane con e sion and CO yields while i imp o es he H2 611
p oduc ion. By con as , i has been obse ed ha his change o he composi ion o he 612
eac ion gas mix u e is bene icial in he case o he NiAl2O4-CP ca alys . On bo h nickel 613
26
alumina e ca alys s he main occu ence o wa e gas shi eac ion a low empe a u e is 614
e iden . The combina ion o POM and SRM gas mix u es seems o inc ease he 615
CO2 yield and o dec ease CO yields sugges ing ha he e a e high combus ion ac i i y 616
and low e o ming ac i i y. 617
Conce ning he beha iou o ac i e me allic Ni species laying on he ca alys s, he mos 618
impo an inding epo ed in his s udy is he ema kable s abili y o he educed 619
NiAl2O4-CP ca alys , which main ains i s Ni pa icle size du ing he e o ming 620
eac ions. Howe e , he p esence o ee me allic Ni (wi h la ge size) on he NiAl2O4-D 621
ca alys , which displayed an inc ease in i s pa icle size, p o okes a decay o i s ac i i y 622
and s abili y. 623
Acknowledgemen s 624
The au ho s wish o hank he inancial suppo o his wo k p o ided by he Spanish 625
Science and Inno a ion Minis y (CTQ2010-16752), he Basque Go e nmen 626
(PRE_2013_2_453, IT657-13) and he Uni e si y o The Basque Coun y (UFI 11/39). 627
Technical and human suppo om SGIke (XRD (A. La añaga), WDXRF (F.J. 628
Sangüesa), XPS (M.B. Sánchez) and UV- is-DRS (L.J. Ba olomé)) and CIC bioGUNE 629
(D. Gil and S. Delgado) is also g a e ully acknowledged. 630
5. Re e ences 631
[1] Y. Men, G. Kolb, R. Zap , M. O’Connell, A. Ziogas, Me hanol s eam e o ming 632
o e bime allic Pd-In/Al2O3 ca alys s in a mic os uc u ed eac o , Appl. Ca al. 633
A 380 (2010) 15-20. 634
[2] P. Engelha d , M. Maximini, F. Beckmann, M. B enne , In eg a ed uel cell 635
APU based on a compac s eam e o me o diesel and a PEMFC, In . J. 636
Hyd ogen Ene gy 37 (2012) 13470-13477. 637
27
[3] O. Pasdag, A. K asnicka, M. S e en, A. Heinzel, Highly in eg a ed s eam 638
e o ming uel p ocesso wi h condensing bu ne echnology o maximised 639
elec ical e iciency o CHP-PEMFC sys ems, Ene gy P ocedia 28 (2012) 57-640
65. 641
[4] M. O’Connell, G. Kolb, K.P. Schelhaas, J. Schue e , D. Tiemann, A. Ziogas, V. 642
Hessel, De elopmen and e alua ion o a mic o eac o o he e o ming o 643
diesel uel in he kW ange, In . J. Hyd ogen Ene g. 34 (2009) 6290-6303. 644
[5] J. Tho mann, L. Maie , P. P ei e , U. Kunz, O. Deu schmann, K. Schube , 645
S eam e o ming o hexadecane o e a Rh/CeO2 ca alys in mic ochannels: 646
expe imen al and nume ical in es iga ion, In . J. Hyd ogen Ene g. 34 (2009) 647
5108-5120. 648
[6] V.A. Tsipou ia i, Z. Zhang, X.E. Ve ykios, Ca aly ic pa ial oxida ion o 649
me hane o syn hesis gas o e Ni-based ca alys s: I. Ca alys pe o mance 650
cha ac e is ics, J. Ca al. 179 (1998) 283-291. 651
[7] P. Fe ei a-Apa icio, M.J. Beni o, J.L. Sanz, New ends in e o ming 652
echnologies: om hyd ogen indus ial plan s o mul i uel mic o e o me s, 653
Ca al. Re . Sci. Eng. 47 (2005) 491-588. 654
[8] Y. Song, H. Liu, S. Liu, D. He, Pa ial oxida ion o me hane o syngas o e 655
Ni/Al2O3 ca alys s p epa ed by a modi ied sol-gel me hod, Ene g. Fuel 23 656
(2009) 1925-1930. 657
[9] U.S. Amjad, A. Vi a, C. Galle i, L. Pino, S. Specchia, Compa a i e s udy on 658
s eam and oxida i e s eam e o ming o me hane wi h noble me al ca alys s, 659
Ind. Eng. Chem. Res. 52 (2013) 15428-15436. 660
34
776
Samples
Ni(1),
w .%
SBET,
m2 g-1
Vp,
cm3 g-1
dp,
nm
Ni0
size,
nm(2)
Ni0
size,
nm(3)
mol(4)
NH3 g-1
mol(4)
NH3 m-2
Alumina
133
0.55
2.4
-
-


NiAl2O4-D
Calcined
33
55
0.14
7.5
-
-
272
4.95
Reduced
48
0.12
8.1
23
17.7
274
5.70
POM
57
0.12
10.7
35
-
-
-
SRM
47
0.16
11.2
47
-
-
-
OSRM
37
0.11
9.4
29
-
-
-
NiAl2O4-CP
Calcined
33
76
0.35
15.4
-
-
291
3.82
Reduced
55
0.32
19.2
11
10.6
240
4.38
POM
64
0.34
18.4
5
-
-
-
SRM
52
0.33
20.3
11
-
-
-
OSRM
52
0.32
17.7
7
-
-
-
(1) De e mined by WDXRF.
(2) Ni0 c ys alli es size de e mined by XRD.
(3) Ni0 c ys alli es size de e mined by TEM.
(4) To al acidi y de e mined by NH3-TPD.
777
Table 1 778

35
779
Ca alys
H2-TPR
XPS
Rela i e amoun o
educible Ni
species, %
Ni/Al
Ni 2p3/2
O 1s



Peak, eV
Ni, % in
he si es
Sa elli e,
eV
Peak, eV
NiAl2O4-D
33
18
49
1
854.1(1)
856.8(2)
61.7
38.3
861.3
528.5 (a)
531.1 (b)
532.6 (c)
NiAl2O4-CP
9
47
44
0.6
855.7 (2)
100
861.8
530.6 (b)
 educ ionpeak o NiO excess
 and (***) educ ion peaks o Ni2+ in nickel alumina e
(1) Ni2+ as NiO and (2) Ni2+ as NiAl2O4.
(a) O2− as NiO, (b) O2− as NiAl2O4 and (c) O2− as Al2O3
780
Table 2 781
782
783
784
785
786
787
36
Reac ion
Ca alys
H2/CO
CO/CO2
Coke, %(1)
450
550
650
450
550
650
-
POM
Equilib ium
21.3
6.0
2.7
0.19
1.06
5.86
-
NiAl2O4-D
5.5
3.5
2.2
0.24
0.57
3.00
46.5
NiAl2O4-CP
12.7
4.4
2.5
0.10
0.60
3.00
2.5
1%Rh/Al2O3
2.5
2.8
2.4
0.74
1.29
3.46
2
SRM
Equilib ium
44.9
11.9
6.7
0.10
0.45
1.09
-
NiAl2O4-D
118.6
19.2
8.5
0.05
0.34
1.03
<1
NiAl2O4-CP
68.5
13.3
6.9
0.08
0.51
1.42
<1
1%Rh/Al2O3
19.7
12.6
8.2
0.31
0.56
1.06
<1
OSRM
Equilib ium
36.3
12.1
7.3
0.07
0.28
0.56
-
NiAl2O4-D
43.1
20.1
10.3
0.05
0.16
0.39
<1
NiAl2O4-CP
55.9
20.1
9.4
0.04
0.18
0.50
<1
1%Rh/Al2O3
8.9
10.8
7.8
0.14
0.31
0.65
<1
Reac ions condi ions: 38400 cm3 CH4 g-1 h-1; W=0.125 g. Gas mix u es: POM: 10%CH4/5%O2/N2, SRM: 10%CH4/30%H2O/N2 and OSRM: 10%CH4/30%H2O/5%O2/N2. 788 (1) Deposi ed ca bon o used ca alys de e mined by TPO and TG analyses 789
790
791
Table 3792
37
793
0 5 10 15 20 25 30 35 40 45 50
0
5
10
15
20
25
30 NiAl2O4-CP
NiAl2O4-D
(a)
Vads, cm3 g-1
(a)
(b)
(b)
Po e diame e , nm
794
795
Fig. 1 796
797
798
799
38
10 20 30 40 50 60 70 80
(220)
(440)
Rela i e in ensi y, a. u.
NiO
Niº
NiAl2O4
-Al2O3
NiAl2O4-D
(c)
(e)
(d)
(b)
(a)
C g aphi e
Angle, 2
10 20 30 40 50 60 70 80
(e)
(d)
(c)
(b)
(a)
C g aphi e
NiAl2O4-CP
Rela i e in ensi y, a. u.
Angle, 2
800
Fig. 2 801
802
39
803
H2Consump ion, mmol g-1
Tempe a u e, ºC Time, min
200 400 600 800 20 30
Iso he mal hold
NiO
Iso he mal hold
NiAl2O4-CP Iso he mal hold
-NiO
-NiO
-NiO
(NiAl2O4)
NiAl2O4-D
H2Consump ion, mmol g-1
Tempe a u e, ºC Time, min
200 400 600 800 20 30
Iso he mal hold
NiO
Iso he mal hold
NiAl2O4-CP Iso he mal hold
-NiO
-NiO
-NiO
(NiAl2O4)
NiAl2O4-D
804
805
Fig. 3 806
807

40
808
300 600 900 1200 1500
NiO
NiAl2O4-D
NiAl2O4-CP
Ni2+ in Td
3 d-d ansi ion
o Ni2+ in Oh
Ni2+ in Oh
MLCT
Ni2+ in Td
d-d ansi ions o Ni2+
in unsuppo ed NiO
1 d-d ansi ion
o Ni2+ in Oh
Abso bance, a. u.
Wa elengh , nm
809
810
Fig. 4 811
812
41
813
875 870 865 860 855 850 845
In ensi y, a.u.
Binding ene gy, eV
In ensi y, a.u.
540 535 530 525 520
Binding ene gy, eV
NiAl2O4
NiO
Al2O3
NiAl2O4-D
NiAl2O4
NiO
Sa (Ni 2p3/2)
(A) (B)
NiAl2O4-CP
NiAl2O4-D
NiAl2O4-CP
875 870 865 860 855 850 845
In ensi y, a.u.
Binding ene gy, eV
In ensi y, a.u.
540 535 530 525 520
Binding ene gy, eV
NiAl2O4
NiO
Al2O3
NiAl2O4-D
NiAl2O4
NiO
Sa (Ni 2p3/2)
(A) (B)
NiAl2O4-CP
NiAl2O4-D
NiAl2O4-CP
814
815
Fig. 5 816
42
010 20 30
0
2
4
6
8
F equency, %
A e age size o Ni0, nm
0
20
40
60
80
100
Accumula ed F equency, %
20 nm
NiAl2O4-CP
010 20 30
0
2
4
6
0
20
40
60
80
100
F equency, %
A e age size o Ni0, nm
Accumula ed F equency, %
20 nm
NiAl2O4-D
010 20 30
0
2
4
6
8
F equency, %
A e age size o Ni0, nm
0
20
40
60
80
100
Accumula ed F equency, %
010 20 30
0
2
4
6
8
F equency, %
A e age size o Ni0, nm
0
20
40
60
80
100
Accumula ed F equency, %
20 nm
NiAl2O4-CP
20 nm20 nm
NiAl2O4-CP
010 20 30
0
2
4
6
0
20
40
60
80
100
F equency, %
A e age size o Ni0, nm
Accumula ed F equency, %
010 20 30
0
2
4
6
0
20
40
60
80
100
F equency, %
A e age size o Ni0, nm
Accumula ed F equency, %
20 nm
NiAl2O4-D
20 nm20 nm
NiAl2O4-D
817
Fig. 6818
43
100 200 300 400 500 600 700 800
-Al2O3
NiAl2O4-D
(a)
(a)
(b)
(b)
340 ºC
367 ºC
256 ºC
348 ºC
323 ºC
Tempe a u e, ºC
dTG In e ed Signal, a.u.
NiAl2O4-CP
100 200 300 400 500 600 700 800
-Al2O3
NiAl2O4-D
(a)
(a)
(b)
(b)
340 ºC
367 ºC
256 ºC
348 ºC
323 ºC
Tempe a u e, ºC
dTG In e ed Signal, a.u.
NiAl2O4-CP
819
820
Fig. 7 821