In luence o Ca/P a io on he ca aly ic pe o mance o
Ni/hyd oxyapa i e samples in d y e o ming o me hane
Zouhai Boukhaa, Ma ía Pila Yes eb, Miguel Ángel Cauquib, Juan R. González-
Velascoa
(a)Chemical Technologies o En i onmen al Sus ainabili y G oup, Depa men o
Chemical Enginee ing, Facul y o Science and Technology, Uni e si y o he Basque
Coun y UPV/EHU, P.O. Box 644, E-48080 Bilbao, Spain
(b)Depa amen o de Ciencia de los Ma e iales e Ingenie ía Me alú gica y Química
Ino gánica, Facul y o Sciences, Uni e si y o Cadiz, Campus Río San Ped o s/n,
11510, Pue o Real, Cádiz, Spain
*Co esponding au ho : Zouhai Boukha
Phone: +34 946015502
Fax: +34 946013500
E-mail add ess: zouhai [email protected]
This is he accep ed manusc ip o he a icle ha appea ed in inal o m in Applied Ca alysis A: Gene al 580 : 34-45
(2019), which has been published in inal o m a h ps://doi.o g/10.1016/j.apca a.2019.04.034. © 2019 Else ie unde CC
BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/)
Abs ac
A se ies o Ni/hyd oxyapa i e samples p esen ing di e en Ca/P mola a ios we e
syn hesised o s udy he in luence o he hyd oxyapa i e suppo composi ion on hei
ca aly ic p ope ies in he d y e o ming o me hane. Ou esul s e eal ha a
p epa a ion s a ing om a sub-s oichiome ic composi ion (Ca/P < 1.67) ollowed by
he imp egna ion o Ni esul s in sui able p ope ies which gi e he highes ca aly ic
pe o mance compa ed wi h s oichiome ic (Ca/P = 1.67) and o e -s oichiome ic (Ca/P
= 1.73) composi ions, espec i ely.
The cha ac e isa ion o he in es iga ed Ni/HAP ma e ials shows ha hei ex u e,
su ace chemis y (acid/base) and he Ni species dis ibu ion a e mainly de i ed om
he s uc u al p ope ies o he used suppo . The ac i i y o he Ni/HAP samples in
DRM shows ha hei pe o mances ollow his end: Ni/HAP-D2 (Ca/P = 1.62) >
Ni/HAP-D1 (1.57) > Ni/HAP-S (1.67) > Ni/HAP-E (1.73). The supe io i y o he
sample wi h a Ca/P mola a io o 1.62 was explained by a sui able su ace chemis y
consis ing o an abundance o s ong acid si es and basic si es. While he o me ac as
ancho ing si es o Ni species he la e se e as CO2 chemiso p ion si es p oducing
in e media e species which in u n eac wi h deposi ed ca bon o o m CO. This
dis ibu ion oge he wi h i s imp o ed ex u al p ope ies lead o he deposi ion o
highly dispe sed, e icien and coke esis an Ni species.
Keywo ds: Hyd oxyapa i e, Ca/P mola a io, Ni dispe sion, acid/base p ope ies,
me hane d y e o ming.
1. In oduc ion
The choice o an app op ia e echnology o he p oduc ion o syn hesis gas (H2 and
CO) om na u al gas e o ming is based on i s end-use. In his sense, d y e o ming o
me hane (DRM) appea s o be a sui able s a egy o chemicals and uel p oduc ion
which equi e a H2/CO a io close o uni y, as in he case o Fishe -T opsch syn hesis
[1-4]. Fu he mo e, i s combina ion wi h o he e o ming p ocesses, such as s eam
e o ming o me hane (SRM), allows he adjus men o he esul ing H2/CO a io when
speci ic alues a e needed [2,3]. Likewise, o en i onmen al conce ns, DRM is an
a ac i e s a egy, ul illing one o he cons an challenges, because i allows he use o
g eenhouse gases (CH4 and CO2) as eac an s [1-4].
Nickel-based ca alys s a e widely used o DRM eac ion owing o hei ac i i y and
compe i i e cos compa ed o noble me als [1-5]. Despi e hei high ac i i y hey su e
om a apid deac i a ion, due o ca bon deposi ion, which is conside ed a majo
d awback [1-10]. Hence, he ca alys s o me hane d y e o ming should exhibi
adequa e p ope ies o endu e he se e e condi ions o he eac ion which is gene ally
ca ied ou a high empe a u es (> 600 ºC). Fo his pu pose, nume ous ca aly ic
o mula ions we e in es iga ed in o de o imp o e he sin e ing- esis ance o he Ni
pa icles and o educe he coke o ma ion [1, 4-10]. The na u e o he used suppo
seems o play a c ucial ole in con olling he dis ibu ion and hen he s abili y o he Ni
ac i e phases. In his sense, i would be essen ial o design Ni ca alys s ha exhibi
sui able me al-suppo in e ac ion in o de o p o ide highly dispe sed and esis an
ac i e phases. I is also well-known ha he su ace ca alys chemis y is a de e minan
ac o in o ien ing he p ocess owa ds ei he he desi able p oduc s o undesi able side
eac ions, such as me hane c acking (CH4 2
C + CO2) [4-7]. Howe e , a sui able dis ibu ion o acid and basic si es is s ill a ma e
o con o e sy. Fo ins ance, i was epo ed ha F addi ion o Ni/Al2O3 ca alys s
induces he o ma ion o Lewis acidic species which s abilise he me allic Ni pa icles
om high- empe a u e sin e ing by enhancing me al-suppo in e ac ion and educes
coke deposi ion unde DRM condi ions [4]. By con as , Bang e al. [5] linked he
excellen coke esis ance and s abili y o Ni/P-Al2O3 ca alys s in DRM eac ion o hei
weak acidi y. Likewise, se e al s udies epo ed ha inc easing he basic cha ac e o
he exposed su ace minimises he possibili y o ca bon o ma ion [3,6,9,10]. Fo
ins ance, Zhang e al. [6] ound an imp o emen o he ac i i y and s abili y o hei
Ni/Al2O3 ca alys s by in oducing small amoun s o La as p omo e . They a ibu ed he
obse ed beha iou o he inc eased numbe o medium-s eng h basic si es.
Since he pionee wo k by Boukha e al. [1] which demons a ed he p omising
beha iou o hyd oxyapa i e (Ca10(PO4)6(OH)2 : HAP) suppo ed Ni ca alys s in he
DRM eac ion, a numbe o s udies dealing wi h he use o hyd oxyapa i e as a DRM
ca alys suppo ha e been a ailable in he li e a u e [11-17]. Comme cial and/o
syn hesised ma e ials wi h a composi ion close o ha o s oichiome ic hyd oxyapa i e
(Ca/P = 1.67) a e by a he mos in es iga ed o DRM applica ion. These
in es iga ions specially conce n he e ec o he na u e o he ac i e phase, me al
loading, p omo e e ec , p epa a ion me hod and he expe imen al condi ions o he
eac ion, among o he s. Howe e , no special a en ion has been de o ed o he e ec o
he s oichiome y o he HAP suppo on he ca aly ic pe o mance o he suppo ed
ac i e phases in DRM. Acco ding o many epo s, a ying he Ca/P mola a io leads o
he o ma ion o addi ional phases and induces signi ican changes in he ex u al and
acid-base p ope ies o he hyd oxyapa i e [18,19]. Sil es e e al. [18] epo ed ha as a
esul o s uc u al ea angemen s Ca-de icien HAP (Ca/P < 1.67) ends o decompose
o s oichiome ic HAP and Ca3(PO4)2 whe eas o a Ca/P a io highe han 1.67 he
ma e ial should decompose o s oichiome ic apa i e and CaO. Tsuchida e al. [19] ha e
shown ha he highly selec i e syn hesis o 1-bu anol and 1,3-b adiene om e hanol
could be me ely They ound ha
p oduc selec i i y bo e a s ong co ela ion wi h he acid and basic p ope ies uned by
a ying Ca/P a io o he HAP ca alys s.
The p esen s udy is dealing wi h he in es iga ion on he in luence o he Ca/P mola
a io on he ex u al, s uc u al, su ace chemis y (acid/base) and ca aly ic beha iou in
DRM o a se ies o hyd oxyapa i e suppo ed nickel samples (Ni/HAP). Fo his
pu pose, hyd oxyapa i e suppo s wi h di e en Ca/P mola a ios ha e been
syn hesised. In e es ing conclusions will be p esen ed by co ela ing he e y ich
in o ma ion p o ided by a wide ba e y o echniques (including BET, XRD, TEM,
FTIR, NH3-TPD, CO2-TPD and TPO) and he ca aly ic pe o mance o he in es iga ed
samples. To he bes o ou knowledge he in luence o he composi ion, by a ying he
Ca/P a io, on he ca aly ic p ope ies o Ni/HAP in DRM has no been ye in es iga ed.
2. Expe imen al
Fou HAP samples p esen ing di e en s oichiome y we e syn hesized by p ecipi a ion
me hod, adding d op wise a calcium ni a e solu ion (MERCK) o a solu ion o
(NH4)2HPO4 (SIGMA-ALDRICH). In o de o ob ain di e en s oichiome ies he pH
o he p ecipi a ion was adjus ed, by adding ammonia solu ion, a 8.6, 9.3, 10 and 11.
The mix u e was, hen, s i ed a 80 °C o 16 h. A e il a ion, he eco e ed solid was
washed wi h dis illed wa e , d ied o e nigh a 120 °C and inally calcined in ai a
750 °C o 4 h. The p epa ed samples we e named HAP-S, HAP-D1, HAP-D2 and
HAP-
-de icien samples
-en iched sample (Ca/P > 1.67).
Fou Ni/HAP samples we e p epa ed by incipien we ness imp egna ion o he
syn hesised HAP suppo s, desc ibed abo e, om aqueous solu ions con aining equal
amoun o Ni ace a e e ahyd a e (SIGMA-ALDRICH, 98%) solu ion. The imp egna ed
samples we e d ied o e nigh a 120 ºC and, hen, calcined a 750 ºC o 4 h (wi h a
amp o 5 ºC min-1). The educ ion o he p epa ed samples was ca ied ou unde
lowing 5%H2/A (60 cm3 min-1) a 750 ºC o 2 h wi h a amp o 5 ºC min-1.
The N2 physiso p ion expe imen s we e pe o med on an au oma ic de ice
(Mic ome i ics, model TRISTAR II 3020). P io analysis he samples we e e acua ed a
300 °C unde ni ogen low o 12 h.
Fou ie ans o m in a ed spec oscopy (FTIR) spec a o he p epa ed samples we e
eco ded wi h a Ca y 600 Se ies FTIR appa a us using hin disks o samples (abou 3
w .%) dilu ed in KB .
The s uc u al p ope ies o he ba e suppo s and Ni-modi ied samples we e
in es iga ed by X- ay di ac ion (XRD). The XRD expe imen s we e conduc ed on a
-MPD X- = 1.5406 Å) and a X- ay
ube ope a ed a 40 kV and 40 mA. The samples we e scanned be ween 10° and 100°
a a scan a e o 0.13º s-1. The weigh ac ions o c ys alline phases in he as
calcined suppo s we e de e mined using a Rie eld p og am by ial and e o me hod.
This analysis conce ned only he samples showing, besides he hyd oxyapa i e s uc u e,
he p esence o CaO as addi ional phase . The ull p o ile e inemen o LaB6
was used o he ins umen al calib a ion. Fo he es ima ion o he a e age me allic Ni
pa icle size, by Sche e equa ion, he samples we e scanned be ween 43º and 46º wi h
a ela i ely slow a e (0.0074º s-1) in o de o imp o e he p ecision and he quali y o
he mos in ense peak (111) a 44.7º (± 0.1).
The mo phology and he Ni pa icle size dis ibu ions, o he samples educed a 750
ºC, we e in es iga ed by T ansmission Elec on mic oscopy (TEM). The co esponding
obse a ions we e pe o med using a JEOL 2010F mic oscope, wo king a 200 kV, wi h
a s uc u al esolu ion o 0.19 nm. Likewise, high-angle annula da k- ield scanning-
ansmission (HAADF-STEM) echnique was used o he obse a ions o he spen
ca alys s.
The Ni dispe sion alues we e also es ima ed by H2 chemiso p ion a 35 ºC using a
Mic ome i ics ASAP 2020 ins umen . P io o analysis, he samples we e educed in-
si u lowing 5%H2/A (60 cm3 min-1) a 750 ºC o 2 h. The ea e , he samples we e
pu ged unde high acuum a 750 ºC o 1 h and cooled o 35 ºC (du ing 3h). The H2
adso p ion measu emen s (a 35 ºC) consis ed o an iso he m in he p essu e ange o 0-
200 o . The Ni dispe sion was es ima ed assuming a uni y adso p ion s oichiome y
(H/Nis= 1).
Tempe a u e-p og ammed educ ion (H2-TPR) expe imen s we e conduc ed on a
Mic ome i ics Au oChem 2920 ins umen equipped wi h a TCD de ec o and coupled
o a MKS Ci us LM99 mass spec ome e . The samples we e p e- ea ed in a low o
5%O2/He a 750 °C o 30 min and hen cooled down o 50 ºC unde He low.
The ea e , he samples we e educed unde a 5%H2/A low (50 cm3 min-1) aising he
empe a u e om 50 ºC o 750 ºC wi h a amp o 10 ºC min-1.
Tempe a u e-p og ammed oxida ion (TPO) expe imen s we e ca ied ou in o de o
de e mine he amoun s and he na u e o he coke deposi ed o e he pos - eac ion
samples. The la e we e hea ed up o 900 ºC a 10 ºC min-1, in a 5%O2/He low (60 cm3
min-1). The CO2 signal (m/z = 44) was moni o ed online wi h a quad upole mass
spec ome e (P ei e , model The mos a GSD301T1).
The empe a u e p og ammed deso p ion o CO2 (CO2-TPD) expe imen s we e
conduc ed on a Mic ome i ics Au oChem 2920 ins umen coupled o a mass
spec ome e (MKS Ci us LM99). The p e- ea men o he samples consis ed o hei
educ ion a 750 ºC (2 h) in a low 5%H2/He and, hen, hei cooling o 50 °C in a low
o He. The adso p ion o CO2 was ca ied ou unde a low o 10%CO2/He (50 cm3 min-
1) o 30 min. The ea e , he samples we e ea ed wi h He (a 50 ºC) o 2 h and
hea ed up o 900 °C wi h a amp o 10 ºC min-1.
The acid p ope ies o he samples we e in es iga ed by empe a u e p og ammed
deso p ion o NH3 (NH3-TPD). The samples we e submi ed o he same p e- ea men
used o CO2-TPD. The adso p ion o NH3 was pe o med a 50 ºC unde a 10%NH3/He
low (50 cm3 min-1) o 30 min. Then, hey we e ea ed wi h He o 2 h and hea ed up
o 750 °C (10 ºC min-1). The NH3 signal (m/z = 15) was ollowed online by a MKS
Ci us LM99 mass spec ome e .
The me hane d y e o ming expe imen s we e pe o med in a qua z ixed-bed eac o
ope a ing a a mosphe ic p essu e. The ca alys s (50 mg) gen ly mixed (using a spa ula
in an aga e mo a ) wi h silicon ca bide (100 mg) we e p e- educed unde a 5%H2/A
low (60 cm3 min-1) a 750 ºC o 2 h; hen, e acua ed unde lowing He (60 cm3 min-1)
o 1 h. The eac ion mix u e was composed o 50%CH4 and 50%CO2 wi h a o al low
o 50 cm3 min-1. The expe imen s we e ca ied ou a 750 ºC o 24 h. The ca alys s
we e also es ed, unde mo e se e e condi ions, a ela i ely lowe empe a u e (600 ºC)
and mo e p olonged ime on s eam (65 h). The eac an s (CH4 and CO2) and he
p oduc s (H2 and CO) o he eac ion we e analysed by using a gas ch oma og aph
analyse (B uke 450-GC). The CH4 and CO2 con e sions we e calcula ed as ollows:
(1)
(2)
The he modynamic da a we e es ima ed by SimSci PRO II so wa e by using he Gibbs
Ene gy Minimiza ion Me hod.
3. Resul s and discussion
3.1. Cha ac e isa ion o he Ni/HAP samples
3.1.1. Tex u al cha ac e isa ion s udies
The N2 adso p ion/deso p ion s udies on he ba e suppo s and Ni/HAP samples show
ha hei iso he ms and hys e esis loops (H3) a e simila (Fig. S1 and Fig. S2,
espec i ely). Acco ding o many au ho s, he obse ed iso he ms a e cha ac e is ics o
ma e ials consis ing o agg ega ed pa icles and/o exhibi ing sli -shaped po es [20-21].
Fig. 1 displays he po e size dis ibu ion o all educed samples. The aces o he
HAP-D1 and Ni/HAP-D1 samples show an asymme ic and wide dis ibu ion peaked a
53 nm. By inc easing he Ca/P mola a io om 1.57 o 1.62 he dis ibu ion becomes
na owe and he obse ed peak signi ican ly shi s owa ds a lowe alue (33 nm). An
almos simila dis ibu ion could be obse ed o he s oichiome ic samples (HAP-S
and Ni/HAP-S). The HAP-E suppo (Ca/P = 1.73) p esen s a ela i ely wide
dis ibu ion, bu a e addi ion o Ni (Ni/HAP-E), he esul ing dis ibu ion appea s o be
qui e simila o ha o he Ni/HAP-S and Ni/HAP-D2 samples.
Table 1 esumes he expe imen al da a ex ac ed om he analysis o he esul ing
iso he ms. The speci ic su ace a eas o Ca-de icien and s oichiome ic suppo s (HAP-
D1, HAP-D2 and HAP-S) a e ound o be anged be ween 25-31 m2 g-1. The Ca-
en iched suppo (HAP-E), howe e , shows he lowes SBET alue which does no
Ni/HAP-D1, Ni/HAP-S and Ni/HAP-E ca alys s and he pa icle size dis ibu ion aces
o all analysed samples, espec i ely. The samples do no show signi ican di e ences
in mo phology (Fig. 5 and Fig. S5). In all cases sphe ical pa icles o me allic nickel a e
obse ed. Howe e , as shown in Fig. S6, he dis ibu ion o he Ni pa icle sizes seems
o depend on he used suppo . The Ca-de icien samples (Ni/HAP-D1 and Ni/HAP-D2)
p esen almos simila shape consis ing o a ela i ely na ow dis ibu ion peaked
a ound 11-15 nm. O e he s oichiome ic sample (Ni/HAP-S) he sizes o he de ec ed
Ni popula ion shi owa ds highe alues p esen ing a peak a 17-19 nm. By con as ,
he dis ibu ion o e he Ca-en iched sample (Ni/HAP-E) is signi ican ly much b oade
( anged be ween 15 and 55 nm) and appea s o be he e ogeneous compa ed o he o he
samples. Table 2 lis s he a e age size and he dispe sion o Ni pa icles. The Ni/HAP-
D2 exhibi s he smalles size (17 nm) ollowed by Ni/HAP-D1 (18.5 nm). Mo eo e ,
inc easing he Ca/P a io o each alues close o 1.67 (Ni/HAP-S) and 1.73 (Ni/HAP-E)
would imply a signi ican inc ease in he size o Ni pa icles (22 and 34.3 nm,
espec i ely). I is wo h ou lining ha , hough hey di e sligh ly om each o he , he
gene al end o hese obse ed alues is ully consis en wi h he XRD esul s,
discussed abo e.
3.1.5. H2 chemiso p ion s udy
Table 2 also epo s he Ni dispe sion and he Ni pa icle size o all educed ca alys s as
de e mined by H2 chemiso p ion. The objec i e o hese expe imen s is o de e mine
quan i a i ely he capaci y o he su ace Ni ac i e phases o chemiso b H2 molecule. As
can be deduced om Table 2, he gene al end o Ni pa icle size alues ma ches e y
well wi h hose es ima ed by TEM and XRD echniques. Howe e , he dispe sion da a
indica e ha he alues de e mined by H2 chemiso p ion a e in all cases lowe han hose
gi en by XRD and TEM. This sugges s ha a ac ion o he deposi ed Ni pa icles is
no accessible o he gas phase. Fo his eason, in he ollowing, he eac i i y o he
si es associa ed o Ni species will be co ela ed o he me allic su ace a ea es ima ed by
H2 chemiso p ion.
3.1.6. Acid-base p ope ies o he Ni/HAP ca alys s
3.1.6.1. Tempe a u e p og ammed deso p ion o NH3 (NH3-TPD)
The he mal s abili y o p e-adso bed NH3 on he acid si es o he educed HAP and
Ni/HAP ca alys s was s udied by means o NH3-TPD echniques (Fig. 6). Table 3 and
Table S2 summa ise he o al amoun s and he dis ibu ion o he acid si es, espec i ely,
calcula ed by in eg a ion o hei espec i e deso p ion peaks. Da a co esponding o he
ba e suppo s a e also included in o de o be used as e e ence.
As shown in Fig. 6, he aces o all analysed suppo s consis o wo deso p ion
domains, sugges ing he p esence o a leas wo adso p ion si es wi h di e en na u es.
The i s peak cen ed a ound 300-320 ºC is asc ibed o medium-s eng h acid si es on
he su ace o he solids whe eas he second ea u e (T > 600 ºC) could be assigned o
s ong acid si es. The la e was also epo ed by Lonyi e al. [34] in hei s udy on he
adso p ion o ammonia o e zeoli e ma e ials. They a ibu ed i s occu ence o a
deso p ion o NH3 om s ong Lewis acid si es.
In e es ingly, he Ca-de icien samples (HAP-D1 and HAP-D2) show simila su ace
densi y o acid si es (0.53-0.54 mol m-2) (Table 3) and exhibi almos iden ical
dis ibu ion, being composed o 66-68% o medium-s eng h acid si es (0.35-0.37 mol
m-2) and 32-34% o s ong acid si es (0.17-0.18 mol m-2) (Table S2). Fu he mo e, he
e ec o inc easing he Ca/P mola a io om 1.62 (HAP-D2) o 1.67 (HAP-S) would
mainly consis o an inc ease in he medium-s eng h acid si es ac ion ( om 66% o
80%, espec i ely) a he expense o he s ong acid si es ( om 34% o 20%,
espec i ely). As expec ed, a signi ican dec ease in he o e all densi y o adso p ion
si es is obse ed o he Ca-en iched sample (HAP-E) which does no exceed 0.46 mol
m-2. Sil es e e al. [18] a ibu ed he acidic p ope ies o hyd oxyapa i es o su ace
HPO42- and OH- acancies mainly occu ing on Ca-de icien samples, deno ed as Ca10-
z(HPO4)z(PO4)6-z(OH)2-z (0 < z 1) [23]. Mo eo e , hey linked he obse ed dec ease
in he numbe o acid si es wi h he Ca/P inc ease o he o ma ion o CaO species on
he hyd oxyapa i e su ace. Ne e heless, when compa ed o he o he s suppo s, he
HAP-E sample exhibi s ela i ely he highes densi y o s ong si es (0.22 mol m-2)
which ep esen 48% o he acid si es (Table S2).
The imp egna ion o nickel on o he s udied suppo s in luences signi ican ly he
amoun s and he dis ibu ion o he acid si es (Fig. 6, Table 3 and Table S2). Fo
ins ance, a e deposi ion o nickel on o HAP-D1 suppo he peak due o medium-
s eng h acid si es ends o spli in o wo peaks, ha ing maxima a 275 ºC and 380 ºC,
which implies he occu ence o new weak adso p ion si es. Simila obse a ion could
be no iced in he case o he o he Ni modi ied samples. Howe e , he posi ion o he
wo esul ing peaks seems o depend on he Ca/P mola a io. The low empe a u e
peak, o example, shi s om 275 ºC o he Ni/HAP-D1 o 260 ºC o Ni/HAP-D2, o
220 ºC o Ni/HAP-S and o 192 ºC o Ni/HAP-E.
On he o he hand, as can be deduced om Table 3, among all in es iga ed samples he
Ni/HAP-D1 bea s he la ges amoun s and he highes su ace densi y o acid si es (17.6
molNH3 g-1 and 0.73 molNH3 m-2, espec i ely). Mo eo e , depending on he na u e o
he suppo , he e ec o Ni addi ion on he amoun s and he dis ibu ion o acid si es
seem o p oceed ia di e en pa hways (Table S2). This would mainly consis o an
inc ease in he su ace densi y o acid si es o Ni/HAP-D1 ( om 0.54 o 0.73 molNH3
m-2) and Ni/HAP-E ( om 0.46 o 0.69 molNH3 m-2). By con as , he alues
co esponding o Ni/HAP-D2 (0.53 molNH3 m-2) and Ni/HAP-S (0.62 molNH3 m-2)
e eal ha he imp egna ion o Ni does no a ec he su ace acid si es densi y
measu ed on hei suppo s (HAP-D2 and Ni/HAP-S, espec i ely). I is wo h
men ioning ha , i espec i e o he Ni-modi ied sample an inc ease in he densi y o
s ong acid si es could be obse ed (compa ed wi h hei ba e suppo s) which becomes
almos cons an (0.24-0.26 molNH3 m-2). This implies ha hese s ong si es a e
associa ed wi h he e ec i e p edominance o su ace Ni species. In o he wo ds, we
claim ha hese s ong acid si es migh ac as ancho ing si es o s abilising Ni me als,
as epo ed in p e ious wo k [4]. In his since, a linea ela ionship be ween he me allic
su ace a ea, de e mined by H2 chemiso p ion, and he numbe o s ong acid si es could
be obse ed (Fig. S7).
3.1.6.2. Tempe a u e p og ammed deso p ion o CO2 (CO2-TPD)
The su ace basici y o he educed HAP and Ni/HAP samples has been cha ac e ised
by means o CO2-TPD echniques. Fig. 7 displays he co esponding deso p ion p o iles
a e adso p ion o CO2 a 50 ºC. Fou deso p ion egions associa ed wi h di e en
s eng hs can be obse ed on he CO2-TPD diag ams (Fig. 7). The samples show
deso p ion peaks, loca ed a low empe a u es (< 300 ºC), and asc ibed o weak basic
si es. Howe e , he peaks associa ed wi h medium-s eng h basic si es, occu ing in he
second empe a u e ange (300-550 ºC), can only be obse ed o s oichiome ic (HAP-
S and Ni/HAP-S) and Ca-en iched (HAP-E and Ni/HAP-E) samples. Thus, he
occu ence o such si es could be due o he o ma ion o CaO species on he
hyd oxyapa i e su ace [18]. This is in good ag eemen wi h ou FTIR and XRD da a,
discussed be o e, whe e a con ibu ion o CaO phase inhe en o he excess Ca has been
de ec ed in he educed HAP-E and HAP-S samples.
A high empe a u es (T > 550 ºC), i espec i e o he examined sample, he CO2
deso p ion p ocess gi es wo well esol ed peaks. The i s one (550-800 ºC) is
p obably due o s ong su ace basic. Acco ding o p e ious epo s hese si es a e
p incipally consis ing o su ace OH- species which s ongly in e ac wi h CO2
molecules [18,30]. The second ea u e (T > 800 ºC) could be asc ibed o he
decomposi ion o s uc u al ca bona e a he han o su ace species. In o de o e alua e
his hypo hesis a blank TPD expe imen was ca ied ou on he HAP-E ba e suppo
(Fig. S8). In his case he sample was submi ed o he same p e- ea men applied o
he CO2-TPD expe imen ( educ ion a 750 ºC o 2 h), bu i was no exposed o CO2
adso p ion. As expec ed, he co esponding TPD diag am shows ha no deso p ion peak
could be obse ed a empe a u es lowe han 800 ºC. Howe e , i e idences he
deso p ion o signi ican amoun s o CO2, e ained p obably in he hyd oxyapa i e
s uc u e, a high empe a u es (> 800 ºC). As s a ed p e iously, hese ca bona e species
a e loca ed in he hyd oxyapa i e amewo k by subs i u ion o PO43- ions.
Table 3 epo s he amoun s and he dis ibu ion o CO2 deso p ion si es, classi ied
acco ding o hei s eng h/na u e, de e mined by in eg a ion o he peaks cen ed in he
ou empe a u e egions (50-300 ºC, 300-550 ºC, 550-800 ºC and > 800 ºC). Fo
compa ison, da a co esponding o he ba e suppo s a e also included. As expec ed, he
o e all densi y o deso p ion si es inc eases wi h he Ca/P mola a io inc ease (2.3
molCO2 m-2, o Ni/HAP-D1, 2.9 molCO2 m-2, o Ni/HAP-D2, 4.6 molCO2 m-2, o
Ni/HAP-S, and 11.2 molCO2 m-2, o Ni/HAP-E). Likewise, in line wi h p e ious
indings [18,19], he densi y o he medium-s eng h basic si es, s ong basic si es and
s uc u al ca bona e inc ease sys ema ically as he Ca/P mola a io is inc eased. This
endency is explained by a s uc u al e olu ion whe e he hyd oxyapa i e su ace is
p og essi ely en iched by CaO, OH- and s uc u al ca bona e, espec i ely. In o de o
sepa a e he concep o his dis ibu ion, in he ollowing, he basic si es occu ing on
he HAP phase su ace will be called HAP basic si es whe eas hose ela ed o he CaO
phase will be named CaO basic si es.
3.2. Ca aly ic ac i i y in DRM
The in-si u educed Ni/HAP ca alys s we e assayed in he DRM eac ion a 750 ºC o
24 h. Fig. 8 (a, b and c) shows CH4 and CO2 con e sions and he esul ing H2/CO mola
a io, espec i ely. Table 4 epo s he ca aly ic ac i i y da a a 24 h and he coke
deposi ion amoun s (w .%), es ima ed by TPO. In addi ion, in o de o be used as
e e ence Ni ca alys s da a, ex ac ed om he li e a u e, a e also epo ed.
As shown in Fig. 8a he Ni/HAP-D2 sample exhibi s he highes ini ial con e sion o
me hane (80%) ollowed by NiHAP-D1 (78%). These alues a e signi ican ly lowe ed,
wi h he inc ease in he Ca/P a io, which become anged be ween 72 and 73% o e he
Ni/HAP-E and Ni/HAP-S, espec i ely. A simila end can be obse ed when he
compa ison is made on he basis o hei ini ial CO2 con e sion alues. I espec i e o
he es ed ca alys he la e gi es alues (82-89%) highe han hose o CH4 con e sion
(Fig. 8b). Mo eo e , he esul ing ini ial H2/CO alues egis e ed o e all ca alys s a e
lowe han 1 (0.77-0.80) (Fig. 8c). This could be explained by he occu ence o he
e e se wa e -gas shi eac ion, whe e a ac ion o p oduced H2 eac s wi h CO2
p oducing ca bon monoxide and wa e [35-46]. On he basis o hei pe o mances he
assayed ca alys s ollow his gene al end: Ni/HAP-D2 > Ni/HAP-D1 > Ni/HAP-S >
Ni/HAP-E. Then, i is clea ha he ac i i y is imp o ed o e he ca alys s p esen ing a
de iciency in calcium. Fu he mo e, excep o NiHAP-E, he ca alys s demons a e a
e y good s abili y, in he s udied ime on s eam. Fo ins ance, he Ni/HAP-D2 sample
exhibi s only 2.5% ac i i y loss a e 24 h eac ion ( om 80 o 78%). Howe e , he Ca-
en iched sample (Ni/HAP-E) su e s om deac i a ion o a la ge ex en (5.5%); o e
which me hane con e sion dec eases om 72 o 68%. On he o he hand, he TPO
esul s o he pos - eac ion ca alys s show a good esis ance agains coke deposi ion
which, in all cases, does no each 1% (Table 4). This is ac ually expec ed conside ing
he ela i ely high empe a u e o he eac ion (750 ºC) [31,36,37]. In pa allel, HAADF-
STEM analysis o he used samples show ha he e is no signi ican changes in he Ni
pa icle size when compa ed wi h he eshly educed samples. Mo eo e , images o
Ni/HAP-D2 (Fig. S9), Ni/HAP-S (Fig. S10) and Ni/HAP-E (Fig. S11) spen ca alys s
show ha he deposi ed ca bon is mainly ilamen -shaped which explain he high
esis ance o he ca alys s o deac i a ion. Ne e heless, as can be obse ed in Fig. 9
(co esponding o Ni/HAP-E spen sample) some analysed zones e idence he p esence
o o he o ms o ca bonaceous species which encapsula e Ni pa icles. Acco ding o
many epo s, he deposi ion o ilamen ous ca bon penalises a much lesse ex en he
ac i i y o he Ni ca alys s in DRM compa ed o hose encapsula ing he ac i e species
[13,38].
Fig. S12a and Fig. S12b displays he XRD pa e ns and he TPO diag ams, espec i ely,
eco ded o e he spen ca alys s. In con as o hei eshly educed coun e pa , he
di ac og ams o he es ed Ni/HAP-S and NiHAP-E samples e idence he absence o
he CaO phase -CPP phase becomes signi ican ly
mo e in ense (Fig. S12a) when compa ed o ha exhibi ed by he educed samples (Fig.
4). The e is a second di e ence wo h o ou lining. The used Ca-en iched sample
(Ni/HAP-E) p esen s addi ional in ense peak due o he o ma ion o a g aphi ic ca bon
phase (Fig. S12a). This esul is consis en wi h he ela i ely highes con e sion loss
(5.5%) obse ed o his sample and he encapsula ion o Ni pa icles by ca bonaceous
species de ec ed by HAADF. Ne e heless, he obse ed deac i a ion mainly occu s
du ing he i s hou o TOS a e wha he ca alys appea o be mo e s able (Fig. 8).
This beha iou is ypical o ca bon di usion in he Ni la ice which p oceeds un il i s
sa u a ion [39,46].
As can be deduced om Table 4 me hane and CO2 con e sions achie ed by he
Ni/HAP-D2 (78% and 86%, espec i ely) a e 24 h eac ion a e close o hose
co esponding o he he modynamic equilib ium (83.7% and 90.3%, espec i ely).
These esul s clea ly ou pe o m hose shown by Ni/alumina and Ni/CZ ca alys s,
epo ed in p e ious wo ks [5,35]. Likewise, i is wo h ou lining he supe io i y o ou
in es iga ed home-made and ee-p omo e ma e ials compa ed o a Ni-Co ca alys
suppo ed on a comme cial hyd oxyapa i e suppo [13]. E en hough, he la e was
es ed unde less se e e condi ions (Table 4).
Fig. 10a and Fig. 10b display he dependence o he ca aly ic ac i i y, a e 24 h
eac ion, on he me allic su ace a ea (measu ed by H2 chemiso p ion) and he numbe
o s ong acid si es, espec i ely. The epo ed da a e eal a linea inc ease in me hane
and CO2 con e sion, espec i ely, and H2/CO a io wi h inc easing he speci ic Ni
su ace a ea and he numbe o s ong acid si es, espec i ely. As discussed be o e, he
dis ibu ion o Ni on he ca alys su ace is linked o he e ec i e abundance o he
s ong acid si es. This ea u e is ma kedly ga he ed on he Ca-de icien samples. I
seems ha ancho ing ac i e me allic phase on hese si es induces a s ong me al-suppo
in e ac ion and, hen, a posi i e e ec on he obse ed ac i i y and s abili y o he
ca alys s. In a p e ious s udy [4] on he beha iou o F-modi ied Ni/Al2O3 ca alys s in
DRM, i was ound ha hei ac i i y and s abili y we e associa ed o he p esence o
s ong Lewis acid si es which s abilised he Ni pa icles om sin e ing h ough an
enhanced me al-suppo in e ac ion.
I is also wo h analysing he e ec o he basic si es on he ac i i y o he in es iga ed
ca alys s. Fig. 10c shows a ela ionship be ween he ac i i y and he numbe o su ace
HAP basic si es. I seems ha he abundance o he la e appea s o be bene icial o he
Ni/HAP ac i i y in DRM. In his sense, i is well-known ha su ace basici y leads o
inc eased adso p ion o CO2 p oducing in e media e species which in u n eac wi h
deposi ed ca bon o o m CO [39-41]. Howe e , no clea e ec , nei he posi i e no
nega i e, could be associa ed o he CaO species, de ec ed on he eshly educed
s oichiome ic (Ni/HAP-S) and he Ca-en iched (Ni/HAP-E) samples. As e ealed by
CO2-TPD hese species induce specially he o ma ion o su ace medium-s eng h basic
cen es. We claim ha hei p esence, a leas unde ou expe imen al condi ions, does
no a ec he ca aly ic p ope ies o he es ed ca alys s. Ou conclusion is in line wi h
p e ious s udies which claimed ha , despi e p o iding an ad an ageous basic cha ac e ,
modi ica ion o Ni ca alys s by CaO does no signi ican ly in luence hei ac i i y in
DRM [39,42].
In o de o examine he e ec o Ca/P on he capaci y o he ca alys s o endu e mo e
se e e condi ions, inducing an inc ease in he coke o ma ion [31,36,37], addi ional
expe imen s ha e been ca ied ou a ela i ely low empe a u e (600 ºC) and mo e
p olonged ime on s eam (65 h). Fo his s udy he ca alys s exhibi ing he lowes and
he highes Ca/P a io (1.57 and 1.73, espec i ely) ha e been assayed (Fig. 11). As
expec ed, he wo ca alys s exhibi lowe ini ial CH4 and CO2 con e sions compa ed o
hei espec i e alues shown a 750 ºC. The ini ial me hane con e sion becomes abou
59% o e Ni/HAP-D1 whe eas 40% is shown o e Ni/HAP-E. Likewise, he Ni/HAP-
D1 main ains i s supe io i y in e ms o i s ini ial CO2 con e sion (47% s. 33% o e
Ni/HAP-E). Ne e heless, he wo ca alys s seem o su e om a deac i a ion p ocess
in simila way. O e Ni/HAP-D1 ca alys he me hane con e sion dec eases quickly
du ing he i s se en hou s om 59% o 48%. The ea e , he deac i a ion a e appea s
o be lowe whe e he ca alys gi es a CH4 con e sion close o 42% a 65 h. Table S3
esumes he ob ained esul s on he wo es ed ca alys s. As s a ed be o e, he ex en o
deac i a ion occu ing on he wo samples is almos he same. A e 65 h eac ion hei
ac i i y dec eases by 28.2±0.5 %. The TPO analyses pe o med on he wo spen
ca alys s show ha hei esis ance agains coke o ma ion a e e y simila (coke
o ma ion a ound 7-8%) (Table S3) and he shapes o hei TPO aces (Fig. S13)
consis mainly o a CO2 o ma ion peak cen ed a 610±10 ºC. Mo eo e , XRD analyses
e idence he o ma ion o g aphi ic ca bon on he wo es ed samples. This sugges s ha
ca bonaceous species exhibi ing he same na u e a e deposi ed on he wo ca alys s.
These esul s con i m ha he ac i i y o he es ed ca alys s migh be associa ed o he
abundance o he same ac i e si es which su e om a simila deac i a ion p ocess.
4. Conclusions
The p esen s udy sheds ligh on he c ucial ole ha plays he composi ion o he
hyd oxyapa i e ma e ials when used as suppo o me allic Ni nanopa icles in hei
ca aly ic p ope ies in he DRM eac ion. I seems ha a p epa a ion s a ing om a sub-
s oichiome ic composi ion (Ca/P < 1.67) esul s in sui able p ope ies which gi e he
highes ca aly ic pe o mance compa ed wi h s oichiome ic (Ca/P = 1.67) and o e -
s oichiome ic (Ca/P = 1.73) composi ions, espec i ely.
We assume ha he ex u e, he su ace chemis y and he Ni species dis ibu ion shown
by he in es iga ed ma e ials a e mainly de i ed om he s uc u al p ope ies o he
suppo used. Indeed, he XRD da a showed ha a e hei calcina ions a 750 ºC he
[44
ca bonaceous deposi s ia he d y e o ming eac ion using ansi ion me al ca alys s,
Ca al. Today 253 (2015) 155-162.
[45] M.S. Aw, M. Zo ko, I.G. A.Pin a , P og ess in he Syn hesis o
Ca alys Suppo s: Syne gis ic E ec s o Nanocomposi es o A aining Long-Te m
S able Ac i i y in CH4-CO2 D y Re o ming, Ind. Eng. Chem. Res. 54 (2015) 3775-
3787.
[46] D.L. T imm, The Fo ma ion and Remo al o Coke om Nickel Ca alys , Ca al.
Re . 16 (1977) 155-189.
Acknowledgemen s
The inancial suppo o his wo k p o ided by Minis e io de Economía y
Compe i i idad (CTQ2015-73219-JIN (AEI/FEDER/UE) and MAT2017-87579-R),
Gobie no Vasco (GIC IT-657-13) and he Jun a de Andalucia (FQM-110 g oup) a e
g a e ully acknowledged. Likewise, he echnical suppo p o ided by SGIke
(UPV/EHU) is g a e ully acknowledged.
CAPTIONS FOR TABLES AND FIGURES
Table 1 Elemen al analyses and ex u al p ope ies da a o he p epa ed ca alys s
Table 2 H2-TPR, XRD, TEM and H2 chemiso p ion da a o he Ni/HAP ca alys s
Table 3 Acid and base p ope ies o he HAP and Ni/HAP samples as de e mined by
NH3-TPD and CO2-TPD, espec i ely
Table 4 DRM ca aly ic pe o mance (a 750 ºC) o he Ni/HAP ca alys s
. Compa ison
wi h Ni ca alys s epo ed in he li e a u e.
Figu e 1 Po e size dis ibu ion o he educed HAP ba e suppo s and Ni/HAP ca alys s
Figu e 2 FTIR spec a o he calcined (a) HAP suppo s and (b) Ni/HAP ca alys s
Figu e 3 H2-TPR diag ams o he HAP and Ni/HAP samples.
Figu e 4 XRD pa e ns o he (a) HAP suppo s and (b) Ni/HAP ca alys s: (c) samples
calcined a 750 ºC and ( ) samples educed a 750 ºC.
Figu e 5 TEM mic og aph o he educed Ni/HAP-D2 ca alys .
Figu e 6 NH3-TPD p o iles o he educed HAP ba e suppo s and Ni/HAP ca alys s
Figu e 7 CO2-TPD p o iles o he educed HAP ba e suppo s and Ni/HAP ca alys s.
Figu e 8 DRM ac i i y o e Ni/HAP ca alys s a 750 ºC: (a) CH4 con e sion, (b) CO2
con e sion and (c) H2/CO a io.
Figu e 9 HAADF-STEM images o he Ni/HAP-E spen ca alys (DRM a 750 ºC)
e idencing he encapsula ion o Ni pa icles
. Images (d), (e) and ( ) display he
colou maps o Ni ( ed) and C (g een).
Figu e 10 Dependence o DRM ac i i y on (a) me allic su ace a ea, (b) numbe o
s ong acid si es and (c) numbe o basic si es excluding hose due o he CaO
phase.
Figu e 11 DRM ac i i y o e Ni/HAP-D1 and Ni/HAP-E ca alys s a 600 ºC: (a) CH4
con e sion, (b) CO2 con e sion and (c) H2/CO a io.
Sample pH(a) (±0.1)
ICP
(b)
BET
(
c
)
Ca/P (±0.01)
Ni, w .% (±0.03)
SBET, m
2
g
-
1
Vp, cm
3
g
-
1
dp, nm
HAP-D1
8.6
1.57 0
25 0.28 49
HAP-D2
9.3
1.62 0
31 0.29 32
HAP-S
10
1.67 0
25 0.22 32
HAP-E
11
1.73 0
11 0.10 29
Ni/HAP-D1
-
1.57 3.41
24 0.28 44
Ni/HAP-D2
-
1.62 3.41
26 0.25 36
Ni/HAP-S - 1.67 3.41
17 0.16 32
Ni/HAP-E - 1.73 3.41
9 0.08 32
(a) pH o p ecipi a ion du ing he suppo s syn hesis.
(b) Elemen al analysis.
(c) Da a co esponding o he samples educed a 750 ºC
Table 1
Sample
H2-TPR
XRD
TEM
H2 chemiso p ion
H2/Ni
, %
, %
, % HAP, nm
Niº, nm
Disp, %
Niº, nm
Disp, %
Niº, nm Disp, %
Ni/HAP-D1
1.06 63.4 28.2 8.3
41 (40) 18 6.9
18.5 6.7
22.5 5.5
Ni/HAP-D2
1.04 49.4 41.7 8.9
34 (29) 15.2 8.2
17 7.3
18.3 6.8
Ni/HAP-S
1.05 70.4 29.6 0
44 (48) 33.1 3.8
22 5.7
35.5 3.5
Ni/HAP-E
1.03 61.7 38.3 0
51 (49) 36 3.5
34.3 3.6
42.8 2.9
Da a in b acke s co espond o he ba e suppo s.
Table 2
NH3-TPD
CO2-TPD
Sample
To al amoun o
acid si es,
molNH3 g-1
Su ace densi y
o acid si es,
molNH3 m-2
To al amoun o
deso bed CO2,
molCO2 g-1
Su ace densi y o
CO2
deso p ion si es,
molCO2 m-2
Dis ibu ion o CO
2
deso p ion si es, mol
CO2
m-2
Weak basic
si es(a)
Medium
-
s eng h
basic si es(b)
S ong basic
si es(c)
S uc u al
ca bona e(d)
HAP
-
D1
13.4
0.54
44.5
1.8
0.59
0
0.57
0.62
HAP
-
D2
16.5
0.53
92
3.0
0.78
0
1.14
1.04
HAP
-
S
15
0.60
102.4
4.1
0.67
0.28
1.71
1.43
HAP
-
E
5.1
0.46
97
8.8
1.15
1.42
2.94
3.31
Ni/HAP
-
D1
17.6
0.73
54.6
2.3
1.20
0
0.47
0.58
Ni/HAP
-
D2
13.7
0.53
75
2.9
1.15
0
0.86
0.86
Ni/HAP
-
S
10.6
0.62
78.6
4.6
0.92
0.68
1.41
1.61
Ni/HAP
-
E
6.2
0.69
100.8
11.2
1.12
2.87
2.53
4.69
(a) De e mined by in eg a ion o he CO2-TPD peak cen ed
(b) De e mined by in eg a ion o he CO2-
(c) De e mined by in eg a ion o he CO2-
(d) De e mined by in eg a ion o he CO2-TPD peak cen ed a T > 800 ºC.
Table 3
Ca alys
XCH4(a)
XCO2(a) H2/CO
Reac ion mix u e
CH
4
/CO
2
/N
2
GHSV,
cm3 h-1 g-1 TOS, h
Coke, % Re e ence
Ni/HAP-D1
75 83 0.78 50/50/0 60,000 24 0.6
This wo k
Ni/HAP-D2
78 86 0.79 50/50/0 60,000 24 0.8
Ni/HAP-S
70 80 0.75 50/50/0 60,000 24 0.7
Ni/HAP-E
67 78 0.72 50/50/0 60,000 24 0.9
Ni/P-Al2O3
63 61 0.86 40/40/20 60,000 20 8 [5]
Ni/CYSZ
66 78 0.70 50/50/0 60,000 24 7 [34]
Ni-Co/HAP
73 79 n.d. 20/20/60 15,882 24 n.d. [13]
Ni-Mg-Al-La
65 75 n.d. 50/50/0 24,000 10 n.d. [42]
NiCo/CeZ O2
78 84 0.84 50/50/0 12,000 80 0.3 [43]
Ni-Co/SiC-CeZ
74 80 0.75 50/50/0 12,000 23 0.5 [44]
(a) The ma gin o e o is a ound 1%.
Table 4
Figu e 1
0 20 40 60 80 100 120 140
160
0.00
0.05
0.10
0.15
0.20
0.25
0 20 40 60 80 100 120 140 160
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 20 40 60 80 100 120 140
160
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80 100 120 140 160
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Po e wid h, nm
Ni/HAP-E
HAP-E
Ni/HAP-D1
HAP-D1
Ni/HAP-D2
HAP-D2
Po e wid h, nm
Ni/HAP-S
HAP-S
Figu e 2
Figu e 3
Figu e 4
Figu e 11
0 10 20 30 40 50 60
0
10
20
30
40
50
60
70
80
90
0 1 2 3 4 5 6 7
20
40
60
80
(a)
Ni/HAP-D1
Ni/HAP-E
TOS, h
0 10 20 30 40 50 60
0
10
20
30
40
50
60
70
80
90
0 1 2 3 4 5 6 7
20
40
60
80
(b)
TOS, h
0 10 20 30 40 50 60
0.0
0.2
0.4
0.6
0.8
(c)
TOS, h
