1
Phospha e- ype suppo s o he design o WGS ca alys s
S. Na a o-Jaén*, F. Rome o-Sa ia, M.A. Cen eno, O. H. Laguna, J. A. Od iozola
Depa amen o de Química Ino gánica e Ins i u o de Ciencia de Ma e iales de Se illa,
Cen o mix o Uni e sidad de Se illa-CSIC, A . Amé ico Vespucio 49, 41092 Se ille,
Spain
*sna a [email protected]
Abs ac
The impo ance o wa e a ailabili y du ing he WGS eac ion has been ex ensi ely
epo ed. Thus, he sea ch o new suppo s able o in e ac wi h he wa e molecule is o
g ea impo ance. In his wo k, a se ies o phospha e- ype suppo s con aining Ce, Ca
and Ti ha e been s udied, demons a ing ha wa e in e ac ion wi h he suppo is closely
ela ed o he ex u al p ope ies, su ace composi ion and c ys al s uc u e o he solids.
Addi ionally, DRIFTS esul s showed ha di e en in e ac ion mechanisms wi h he wa e
molecule occu depending on he suppo . The sys em con aining Ca dissocia es he
wa e molecule and in e ac s wi h i ia he phospha e and Ca2+ ions. Howe e , he Ce
sys ems e ain wa e in i s molecula o m, which in e ac s wi h he solids ia hyd ogen
bonding wi h he phospha e g oups. On he o he hand, he Ti sys em expe iences a loss
o phospho ous, p esen ing a low deg ee o in e ac ion wi h he wa e molecule.
Addi ionally, he beha io o he suppo s wi h wa e has been success ully ela ed o he
WGS ca aly ic ac i i y o he co esponding phospha e-suppo ed P ca alys s.
Keywo ds: Wa e gas shi (WGS), P ca alys s, phospha e- ype suppo s, suppo
s uc u e, wa e a ailabili y.
1. In oduc ion
The wa e gas shi eac ion (WGS, CO + H2O ↔ CO2 + H2) is a well-known p ocess
implied, among o he s, in he hyd ogen clean-up p ocess o i s subsequen use as eed
2
o polyme elec oly e memb ane uel cells (PEMFCs) [1-3]. The ad an ages conce ning
he use o suppo ed noble me al ca alys s o his eac ion ha e mo i a ed a deep s udy
o hese ma e ials o e he yea s, ega ding bo h he employed me al phase and he
suppo [4-7]. A gene al conclusion de i ed om he nume ous s udies accomplished is
he syne gy exis ing be ween he ac i e phase and he suppo [8-11]. The la e is
in ol ed in he ac i a ion o he wa e molecule, gene ally ecognized as he a e-limi ing
s ep o he WGS eac ion [12-14]. Bo h educible and non- educible oxide- ype suppo s
ha e been ex ensi ely s udied o he WGS eac ion [5, 15-19]. Among hem, CeO2 has
been p oposed as an in e es ing candida e since i g ea ly enhances he wa e ac i a ion
a e h ough he in luence o i s elec onic p ope ies ( he edox pai Ce3+/Ce4+), di ec ly
ela ed o he p esence o oxygen acancies in he solid. Consequen ly, CeO2 has been
used bo h alone o as p omo e o Al2O3 suppo ed P ca alys s [11, 13]. González-
Cas año e al. [9] imp o ed he o mula ion o he CeO2/Al2O3 (CeAl) suppo by adding
Z and Fe dopan s, which p o oke a modi ica ion on he edox p ope ies o he suppo
and acili a es he wa e ac i a ion s ep. Addi ionally, he combina ion o Z and Fe
demons a ed o inhibi he ca bon deposi s o ma ion on he ca alys su ace.
A e wa ds, Ga cía-Moncada e al. [12, 20] in oduced he use o a physical mix u e o
p o onic conduc o s o a ypical P /CeAl ca alys , gene a ed by alio alen doping o
ansi ion o a e ea h me al oxides, which imp o es he WGS ca aly ic pe o mance
h ough a p o on conduc ion mechanism, p omo ing he wa e dissocia ion s ep.
Conside ing he a o emen ioned esul s, we e alua ed he use o CePO4 as suppo s o
P ca alys s, conside ing he p esence o ce ium in i s s uc u e, which could in luence
he elec onic p ope ies o he ma e ial, and i s p e ious use as p o onic conduc o in
uel cells. Addi ionally, he absence o he edox pai Ce3+/Ce4+ ypically desc ibed in
CeO2 has been poin ed ou in CePO4 [21]. The p o on anspo is gene ally desc ibed
on he basis o wo mechanisms: he “G o hus mechanism”, which implies p o ons
hopping and he “di usion mechanism”, which es ablishes he wa e molecule as p o on
ehicle [22, 23]. Thus, hese anspo p ope ies we e success ully adap ed o WGS
3
ca aly ic sys ems. On he o he hand, he main po en ial o phospha e- ype ma e ials lies
in he possibili y o uning hei chemical and s uc u al p ope ies and consequen ly, hei
ex u al and su ace p ope ies. In his sense, he ex u e o he suppo , especially i s
po osi y, in luences bo h he adso p ion and ion-exchange abili ies o he ma e ial [24-
26], whe eas he selec ion o he me al ca ion and he ino ganic anion becomes c ucial
o he e ec i e con ol o he su ace acid-base p ope ies [27]. Due o hese excep ional
unable p ope ies [21], an ex apola ion o hese esul s can be applied o ano he
phospha e- ype sys ems. Pa icula ly, i anium and calcium phospha e- ype sys ems
esul in e es ing, conside ing ha simila ly o CePO4, hey ha e been used p e iously
as ionic conduc o s in uel cells [28, 29]. Fu he mo e, he in oduc ion o ca ions di e en
om Ce in he phospha e s uc u e could in luence signi ican ly bo h he edox p ope ies
and he cha ge compensa ion e ec s on he suppo s, which could ha e an e ec on
hei beha io wi h ega d o he wa e molecule and, consequen ly, on he WGS ca aly ic
pe o mance. In addi ion, he calcium sys em has demons a ed o ac as an adequa e
suppo o he WGS eac ion, which has been a ibu ed o a supe io wa e ac i a ion
a e [30, 31].
In his wo k, a se ies o phospha e- ype ma e ials con aining di e en ca ions (Ce, Ca
and Ti) ha e been syn hesized and e alua ed as suppo s o 2 w . % P ca alys s. The
wa e a ailabili y in he suppo s and hei s uc u e a e co ela ed wi h he WGS ca aly ic
pe o mance.
2. Ma e ials and me hods
2.1. Suppo s and ca alys s syn hesis
A se ies o phospha e- ype suppo s con aining di e en ca ions (Ce, Ca and Ti) we e
p epa ed by a hyd o he mal me hod, wi h nominal mola a ios Ce/P 1:1, Ca/P 1.67:1
and Ti/P 1:1. The ob ained solids we e labelled as CeP, CaP and TiP espec i ely.
4
The syn hesis o CeP was ca ied ou by a hyd o he mal me hod p e iously epo ed [21,
32], and CaP was simila ly p epa ed. 50 ml o aqueous solu ions o phospho ic acid 85%
(Pan eac) 0.6 M and calcium ni a e e ahyd a e (VWR) 1 M we e mixed in a lask and
s i ed a 40 °C. Then, 50 ml o an aqueous solu ion o sodium ci a e dihyd a e (Sigma)
0.017 M was added and he pH was adjus ed o 10 by means o he addi ion o ammonia
30%. The esul an solu ion was main ained a 40 °C o 8 hou s unde s i ing and inally
ans e ed o a Te lon-lined au ocla e, in which he mix u e was aged a 100 °C o 8
hou s. The ma e ial was inally il e ed, washed wi h dis illed wa e and d ied o e nigh
a 100 °C. The solid CeP was di ided in wo ac ions and calcined a 400 and 600 °C,
denomina ed CeP400 and CeP600, whe eas he solid CaP was calcined a 350 °C.
The syn hesis o TiP was ca ied ou by means o an adap ed hyd o he mal me hod [33].
50 ml o a phospho ic acid 85% (Pan eac) aqueous solu ion 0.7 M we e mixed wi h 6·10-
4 moles o Plu onic® P123 Block Copolyme Su ac an (BASF) and main ained unde
s i ing un il comple e dissolu ion o he su ac an . 0.04 moles o C12H29O4Ti (Ald ich)
we e hen added and he esul an solu ion was s i ed o 2 hou s. The mix u e was
sealed in a Te lon-lined au ocla e and aged a 80 °C o 24 h. The ob ained p oduc was
il e ed, washed wi h wa e , d ied o e nigh a 60 °C and inally calcined a 600 °C in
o de o ensu e he comple e elimina ion o he su ac an .
The P ca alys s wi h a nominal me al con en o 2 w .% we e p epa ed by he we
imp egna ion me hod. Fo his pu pose, an aqueous solu ion o P (NH3)4(OH)2 0.053 M
was p epa ed, and he co esponding amoun o suppo was added. The mix u e was
main ained unde s i ing o 24 hou s and d ied a 60 °C. Finally, he ob ained ca alys s
we e calcined a 350 °C o 2 hou s.
5
2.2. Suppo s and ca alys s cha ac e iza ion
Powde X- ay di ac ion pa e ns we e collec ed on a Panaly ical X’PERT PRO
di ac ome e using CuKα adia ion (40 mA, 45 kV). The sample pa e ns we e eco ded
in he 10-90 °2θ ange wi h 0.05° s ep size and 300 s o s ep ime.
Chemical composi ion o he p epa ed samples was analyzed by Induc i ely Coupled
Plasma A omic Emission Spec ome y (ICP-AES) on a Ho iba Jobin Y on spec ome e ,
a e HF acid diges ion o he samples.
Tex u al p ope ies o he solids we e e alua ed by N2 adso p ion-deso p ion
expe imen s a liquid ni ogen empe a u e in a Mic ome i ics T iS a II 3020 appa a us.
P io o he expe imen , samples we e degassed a 250 °C o 2 hou s unde acuum.
T ansmission elec on mic oscopy (TEM) mic og aphs o he ca alys s we e ob ained in
a PHILIPS CM-200 ansmission elec on mic oscope, equipped wi h mic oanalysis
(EDS) wi h a minimum s ep size o 15 nm and 2.8 Å o maximum esolu ion be ween wo
poin s. Fo P pa icle size calcula ions, 150 pa icles we e coun ed in e e y mic og aph.
Adso p ion o wa e on he p epa ed suppo s was ollowed by in si u di use e lec ance
in a ed spec oscopy (DRIFTS) in a high- empe a u e eac ion chambe a ached on a
P aying Man is (Ha ick) DRIFTS op ical sys em wi h ZnSe windows. Spec a we e
collec ed by a The mo Nicole iS50 FT-IR spec ome e equipped wi h a liquid-ni ogen
cooled MCT de ec o , a a esolu ion o 4 cm-1 and an a e age o 64 scans. The ou le
gas composi ion was analyzed in an on-line connec ed PFEIFFER mass spec ome e
Vacuum P isma Plus con olled by he Quade a® so wa e. Fo e e y expe imen , 40 mg
o he solids we e inely g ound and loaded in he ca aly ic cell. The samples we e i s
ac i a ed a he calcina ion empe a u e unde a 10% H2/A low o 1 hou and submi ed
o a 10% H2O/A eed-s eam (50 ml·min-1) using a KNAUER Sma line 1050 HPLC pump
and a se ies o AALBORG mass- low con olle s. The expe imen s we e pe o med in

6
he ange o empe a u e be ween 150 and 350 °C. All he pipelines o he sys em we e
hea ed a 100 °C o a oid wa e condensa ion du ing he analysis.
2.3. Ca aly ic es
WGS eac ion was ca ied ou a a mosphe ic p essu e on a cylind ical Has elloy C
(nickel-ch omium-molibdenum) eac o (i.d. 17 mm). Fo e e y eac ion, 200 mg o he
powde ca alys (ø = 600-800 μm) we e dilu ed wi h SiC o gene a e a ca alys bed
olume o 6 cm3. Samples we e i s ly p e ea ed a 350 °C unde a 10% H2/N2 low wi h
a o al a e o 100 ml·min-1. The ca alys s we e hen submi ed o 100 ml·min-1 o a ypical
WGS eed-s eam (7 ol.% CO, 9 ol.% CO2, 50 ol.% H2, 30 ol.% H2O and 4 ol.% N2)
p epa ed om single gases in he labo a o y. Gases we e in oduced in he ca aly ic
sys em using a se ies o B onkho s ® mass low con olle s coupled o he eac ion uni .
Weigh hou ly space eloci y was in e e y case WHSV= 30 L·g·h-1. Reac an s and
p oduc s we e analyzed by gas ch oma og aphy in a VARIAN CP-4900 μGC equipped
wi h a ®Po apak Q column, a Mole-Sie e 5A and wo TCD de ec o s. Addi ionally, a
di ec eco ding o he CO2 ol.% a he eac o ou le was ollowed using a VAISALA
GMT220 se ies de ec o . CO con e sion was calcula ed acco ding o Eq. 1
XCO(%)=FCO in − FCO ou
FCO in
x100 (1)
Whe e FCO in and FCO ou a e he inle and ou le olume o CO in he eac o , espec i ely.
Fo sake o compa ison, low and medium empe a u e Cu-based comme cial ca alys s
(HiFuel W220 and HiFuel W230, Al a Aesa TM) we e also e alua ed.
3. Resul s and discussion
3.1. Cha ac e iza ion esul s
Table 1 summa izes he physicochemical p ope ies o he p epa ed solids. Speci ic
su ace a ea alues o he suppo s a e in he ange be ween 41 and 248 m2 g-1, being
ha o TiP signi ican ly highe compa ed o he o he p epa ed suppo s. Po e olume
7
alues a e comp ised be ween 0.32 and 0.62 cm3 g-1, co esponding he maximum alue
o TiP. A e P deposi ion and calcina ion, no signi ica i e changes on hese alues a e
obse ed, being simila o hose o he suppo s. Rega ding he ca ion/phospho ous
(C/P) mola a io o he suppo s, he alues ob ained a e hose expec ed om he
syn hesis p ocedu e excep o he solid TiP, which indica es ha a loss o phospho ous
has aken place in his solid.
Table 1. Physicochemical p ope ies o he suppo s and ca alys s.
Sample
SBET (m2 g-1) Vp (cm3 g-1) Dp (nm)
P mean
pa icle
size (nm)b
Me al
loading
(w %)
C/P a io
CeP400a
65
0.32
17.2
-
-
-
CeP600a
41
0.35
28.2
-
-
-
CaP
69
0.39
16.8
-
-
-
TiP
248
0.62
8.5
-
-
-
P /CeP400a
71
0.32
16.6
1
1.5
1
P /CeP600a
44
0.30
23.2
1.2
1.4
1
P /CaP
73
0.47
19.4
0.7
1.4
1.7
P /TiP
246
0.58
8.2
2.6
1.6
2.2
a P e iously epo ed in Re . [21]
b Calcula ed om TEM mic og aphs
X- ay di ac ion pa e ns o he p epa ed solids a e shown in Figu e 1. The suppo
CeP400 p esen he di ac ion lines co esponding o he habdophane- ype phase o
CePO4 (CePO4·0.53H2O), which is known o p esen s uc u al channels (JCPDS-ICDD
01-075-1880), whe eas he c ys allog aphic pa e n o he suppo CeP600 co esponds
o he monazi e- ype phase o he CePO4 (JCPDS-ICDD 01-077-0429), eme ging om
he dehyd a ion o he habdophane- ype phase and whe e he s uc u al channels a e
missed [34-36]. De ailed desc ip ions o hese s uc u es ha e been p e iously epo ed
[21]. The di ac ion pa e n o CaP co esponds o he hexagonal calcium hyd oxyapa i e
s uc u e (P63/m space g oup), wi h o mula Ca10(PO4)6(OH)2 (JCPDS-ICDD 01-074-
0566). Acco ding o he li e a u e [37, 38], he hyd oxyapa i e amewo k is cons i u ed
by he assembly o calcium ions and e ahed al phospha e g oups delimi ing wo ypes
o unconnec ed channels o ien ed along he c axis: one o hem has a diame e o 2.5 Å
8
and is bo de ed by calcium ions, and he second one possesses a diame e o
app oxima ely 3.5 Å and is bo de ed no only by calcium ions bu also by oxygen a oms.
These channels con ain he s uc u al OH g oups which balance he posi i e cha ge o
he compound. Con e sely o he p e ious suppo s, he suppo TiP seems o p esen
ei he a low deg ee o c ys allini y o a c ys alline na u e bu small c ys alli e size. The
amo phiza ion deg ee o TiP ha e been p e iously ela ed o he ime and empe a u e
condi ions o he hyd o he mal me hod employed du ing he syn hesis [39, 40].
Ne e heless, i is possible o dis inguish a b oad di ac ion line a 25.2 °2θ, ela ed o
he p esence o he ana ase phase o TiO2 (JCPDS-ICDD 00-002-0387). Al hough he
ana ase- u ile ansi ion phase akes place be ween 500 and 600 °C, he p esence o
phospho ous has demons a ed o inc ease he he mal s abili y o he ana ase phase
[41], which explains he p esence o his phase a e calcina ion a 600 °C. Addi ionally,
a di ac ion peak a 22.4 °2θ, co esponding o he mos in ense di ac ion line o TiP2O7
(JCPDS-ICDD 00-038-1468) is obse ed. An enla gemen o his egion is p esen ed in
he Supplemen a y ma e ial. Thus, his solid consis s in a mix u e o TiP2O7 and TiO2, in
good ag eemen wi h he loss o phospho ous induced by he calcina ion ea men
e ealed by chemical analysis. A e P deposi ion and calcina ion o he solids, no
di ac ion lines co esponding o P species a e obse ed, indica ing he small pa icle
size o he me allic pa icles in all he samples. Ne e heless, he ca alys P /TiP p esen s
an inc ease o he di ac ion peak a 25.2 °2θ, sugges ing ha he seg ega ion o ana ase
occu s a e P deposi ion and calcina ion.
9
Figu e 1. XRD pa e ns o he p epa ed solids.
Figu e 2 p esen s he N2 adso p ion/deso p ion iso he ms and he BJH po e size
dis ibu ion cu es o he suppo s a e calcina ion. Figu e 2a shows ha suppo s
con aining Ce and Ca p esen a p onounced adso p ion a p/p0 ≈ 0.8, indica ing ha
mul ilaye adso p ion s a s a high ela i e p essu e in hese solids, hus deno ing a g ea
mesopo e popula ion. Howe e , he adso p ion iso he m o he suppo TiP shows a
supe io adso p ion capaci y om p/p0 ≈ 0.5, hus poin ing ou ha his ma e ial exhibi s
a g ea e mic opo osi y ea u e. In addi ion, he middle sec ion o he cu e is less de ined
ha in he o he suppo s, which shows ha o ma ion o mono- and mul ilaye a e
o e lapped. Conside ing he hys e esis loop [42], bo h CeP suppo s p esen a H1
hys e esis, ypical o solids con aining uni o m channels, as in he suppo CeP400 o
cons i u ed by agg ega es o pa icles, as in CeP600. The solid CaP gi es ise o a H2
hys e esis, cha ac e is ic o solids con aining cylind ical channels wi h non-uni o m size
and shape, being consis en wi h he desc ip ion o he hyd oxyapa i e s uc u e in which
wo ypes o s uc u al channels ha e been desc ibed [38, 43]. The solid TiP p esen s a
H3 hys e esis, ypically ound in non- igid agg ega es o pa icles and hus, no
ep esen ing a eliable desc ip ion o he po osi y o he ma e ial [42, 44]. Acco ding o
he BJH po e size dis ibu ion de i ed om he iso he m (Fig. 2b), he mesopo e olume
dec eases in he o de : CaP > CeP400 > CeP600 > TiP. Al hough his dis ibu ion could
16
numbe o bands in his egion indica e di e en deg ee o dis o ion o he PO43-
e h aed a as a consequence o he phospha e g oup en i onmen in e e y solid [21, 51].
As in he CeP suppo s, an addi ional se o bands a ibu ed o o e ones and
combina ion bands o he phospha e g oup appea s in he 2100-2400 cm-1 ange [51].
Hyd oxyapa i es a e cha ac e ized by he subs i u ion o OH- o PO43- g oups by CO32-
g oups, gi ing ise o he so called ype A and ype B subs i u ion espec i ely [55].
The e o e, ib a ions co esponding o he CO32- g oup a e p esen . A- ype ca bona e
species in apa i e s uc u es a e cha ac e ized by abso p ion bands a 1545 and 1450
cm-1, co esponding o he asymme ic and symme ic s e ching espec i ely, and a
single a 880 cm-1, whe eas he B- ype ca bona e species a e expec ed a 1455, 1410
and 875 cm-1. In ou sample, wo in ense bands a 1417 and 1448 cm-1a e obse ed, as
well as a single a 872 cm-1, which allows o conclude ha a ype B subs i u ion has
aken place in his solid. This esul is in ag eemen wi h he p e iously desc ibed band
a 3535 cm-1, ela ed o he p esence o ca bona es.
4000 3500 3000 2500
Abso bance (a. u.)
Wa enumbe (cm-1)
TiP
CaP
CeP600
CeP400
3670
3670
3568
3535
2884
0.2
a)

17
Figu e 6. DRIFT spec a o he s udied suppo s a e he ac i a ion ea men unde 10%
H2/A , aken a 150 °C unde A a mosphe e, a) 4000-2500 and b) 1800-800 cm-1 spec al
egions.
Table 2. Assignmen o he suppo s ib a ion modes a e he ac i a ion ea men .
Solid
Band equency
(cm-1)
Assignmen
CeP400
CeP600
CaP
TiP
5319
5259
5345
-
Wa e combina ion bands
ν + δ(H2O)
CeP400
CeP600
CaP
TiP
3670
3670
3568
3535
3740 ( w)
OH s e ching
CeP400
2884
P-OH s e ching o acid o ophospha es
CeP and
CaP
TiP
2400-2100
-
O e one and combina ion bands o PO43-
ib a ions
CeP400
CeP600
CaP
TiP
1185
1185
1173
1230
P=O s e ching
CeP400
CeP600
CaP
TiP
1028
1090
1016-1083
-
P-O asymme ic s e ching
ν3 (P-O)
CeP and
CaP
960
P-O symme ic s e ching
ν1 (P-O)
CaP
1448, 1417, 872
CO32- ib a ions
The beha io o he su ace species in he p esence o wa e was analyzed h ough he
moni o ing o he wa e adso p ion expe imen s. Di e ence spec a o he suppo s
1800 1600 1400 1200 1000 800
TiP
CaP
CeP600
Abso bance (a. u.)
Wa enumbe (cm-1)
1185
1028
960
1185
1090
960
1450
1417
1173
960
872
1083
1083
1230
b) 0.2
CeP400
18
( aking he spec a a e he ac i a ion ea men as e e ence) a e p esen ed in igu e 7.
All he e alua ed suppo s p esen bands be ween 3500 and 2500 cm-1 cha ac e is ics
o adso bed molecula wa e . These bands a e pa icula ly weak in he suppo TiP,
which shows he low a ini y o wa e by his suppo and explains he low ac i i y o he
P /TiP ca alys on he WGS eac ion. Al hough his suppo p esen s he g ea es speci ic
su ace a ea alue, he loss o phospho ous and he absence o mesopo es in i s
s uc u e suppose a poo in e ac ion wi h he wa e molecules.
The spec a o CeP400 and CeP600 show he g ow h o bands a ca. 5300 cm-1
co esponding o wa e combina ion modes (ν + δ(H2O)), indica ing a g ea a ini y o bo h
solids wi h wa e in i s molecula o m. Addi ionally, hese suppo s p esen a band in his
egion a ela i ely low equency (⁓ 5200 cm-1), which poin s ou he p esence o H-
bonded H2O species [56], as well as a weak and b oad band abou 4000 cm-1, whe e
combina ion modes o he -OH g oups (ν + δ(OH)) appea . A nega i e band
co esponding o he -OH s e ching modes (3670 cm-1) is obse ed in he p esence o
wa e , hus hese -OH g oups a e pa icipa ing in he hyd ogen bonding. A he same
ime, a band a 3690 cm-1 appea s, a ibu ed o -OH g oups belonging o he wa e
molecule, since his band is con ibu ing o he combina ion bands a 5318 cm-1 and 5259
cm-1 o CeP400 and CeP600, espec i ely. The obse a ion o he H-O-H bending mode
(δH-O-H) egion allows o de e mine also he ype o in e ac ion be ween he wa e
molecule and he su ace. Since he wa e molecule is ampho e ic, i is able o in e ac
wi h bo h acidic su ace si es ( h ough he lone pai elec ons o he oxygen a om) and
wi h basic si es ( h ough he o ma ion o hyd ogen bonding). When he in e ac ion
occu s ia hyd ogen bonding, he δH-O-H mode shi s o highe wa enumbe s while he
in e ac ion ia he oxygen a om p oduces a shi o he δH-O-H mode owa ds lowe
wa enumbe s [56-58]. Acco ding o ha , he wa e in he solids CeP400 and CeP600
a e in e ac ing h ough hyd ogen bonding, due o he high H-O-H bending mode
equencies a 1625 and 1630 cm-1 espec i ely. CeP400 spec um shows also a band
19
a 1586 cm-1 as he empe a u e inc eases. Falk [58] ound a ela ionship be ween he
δH-O-H equency shi and he o mal cha ge o he ca ion wi h which he wa e molecules
we e in e ac ing. Fo he ca ion Ce3+ his alue would be ound a ca. 1532 cm-1, a alue
much lowe han ha ound in ou sample, hus his ib a ion mode has been assigned
o he wa e p esen in he s uc u al channels o he habdophane- ype phase o CePO4
[21]. Simul aneously o he g owing o he wa e - ela ed modes, he in ensi y o he
o e one and combina ion bands o he phospha e g oups (2100-2400 cm-1) dec ease in
bo h solids, which indica es ha he wa e molecule is in e ac ing wi h he phospha e
g oups. In CeP400 and CeP600, wa e seems o emain as molecula wa e in all he
empe a u e ange, poin ing ou ha CeP suppo s p esen a di usion mechanism,
meaning ha he p o on anspo is assis ed by he ansla ional dynamics o he wa e
molecule [23, 59].
The spec um o he suppo CaP p esen s also a combina ion band o he wa e modes
(5319 cm-1), bu i s in ensi y is lowe han ha o he suppo s CeP. In addi ion, a se ies
o g owing bands appea in he 3750-3600 cm-1 ange (inse igu e 7) when wa e is
adso bed, deno ing he gene a ion o new -OH g oups and hus sugges ing he abili y o
he hyd oxyapa i e o dissocia ing he wa e molecule, in con as o he beha io
showed by he CeP suppo s, whe e wa e species emain as molecula wa e [21]. As
occu ing in he solids CeP, a dec ease in in ensi y o he o e one and combina ion
bands o he phospha e g oup occu s, indica ing he pa icipa ion o he phospha e g oup
in he in e ac ion wi h wa e . Rega ding he δH-O-H egion, he spec um shows wo di e en
con ibu ions, one a 1642 cm-1, co esponding o he bending mode o hyd ogen-bonded
species and ano he one a 1568 cm-1. The a e age alue o he δH-O-H mode in e ac ing
wi h a ca ion wi h cha ge 2+ should appea a 1560 cm-1[58], close o he alue ound o
his mode in he CaP spec um. Consequen ly, his ib a ion could be assigned o ha o
he wa e molecules in e ac ing ia he lone elec on pai o oxygen wi h he Ca2+ ions o
he hyd oxyapa i e s uc u e. This band emains e en a 350 °C, exposing ha his
20
in e ac ion is s onge han ha ia hyd ogen bonding. This esul allows o conclude ha
a G o huss mechanism could ake place in he suppo CaP, i. e., he wa e molecules
show p onounced local dynamics bu hey s ay a hei si es, being he p o ons
ans e ed wi hin hyd ogen bonds om one molecule o ano he .
Liu e al. [60] ha e demons a ed ha p o on conduc ion p ope ies can be uned by
con olling he c ys al s uc u e, po ous ne wo k and chemical unc ionali y o he
conduc o solids, hus, he di e ences obse ed in he s udied phospha e- ype suppo s
can be asc ibed o he di e ences in hei c ys allog aphic s uc u e and po osi y.
Figu e 7. E olu ion o he di e ence spec a o he suppo s du ing he adso p ion o
10% H2O/A , om 150°C ( op) o 350°C (bo om).
5000 4000 3000 2000
5138
Abso bance (a. u.)
Wa enumbe (cm-1)
0.2
5319
3690
3500
3219
2884
1625
1616
1586
CeP400
5000 4000 3000 2000
Abso bance (a. u.)
Wa enumbe (cm-1)
0.2
5259
3690 3437
3189
1630
1593
CeP600
5000 4000 3000 2000
Abso bance (a. u.)
Wa enumbe (cm-1)
0.2
5319
3850
1642
1548
CaP
3800 3750 3700 3650 3600 3550 3500
5000 4000 3000 2000
Abso bance (a. u.)
Wa enumbe (cm-1)
0.2
TiP
21
The e olu ion o he ela i e in ensi y o he mass-on-cha ge (m/z) 18 du ing he wa e
adso p ion expe imen s, shown in Figu e 8, is in ag eemen wi h he esul s desc ibed
abo e. Wa e e en ion capaci y o he e alua ed suppo s dec eases in he o de : CaP
> CeP400 > CeP600 > TiP, which coincides wi h he end ollowed by he co esponding
ca alys s on he WGS eac ion. The suppo CaP shows an excellen wa e adso p ion
capaci y, being able o g ea ly e ain he wa e in he whole empe a u e ange e alua ed.
This ac can be a ibu ed o he in e ac ion o he wa e molecule no only wi h he
phospha e and -OH g oups bu also wi h he Ca2+ ions, as p o ed by DRIFTS. This esul
is in ag eemen wi h p e ious s udies, which ha e demons a ed ha calcium
hyd oxyapa i e in e ac s s ongly wi h he wa e molecule h ough elec os a ic/hyd ogen
bonding wi h he Ca2+ and phospha e ions o he apa i e su ace. Fu he mo e, he cha ge
densi y on he Ca2+ ions is mo e localized han on he phospha e g oups in he p esence
o wa e , p o oking a d ama ical dec ease on he wa e molecule mobili y in he p oximi y
o he apa i e su ace and hus a o ing he e en ion o wa e on he s uc u e e en a
high empe a u es [61-63]. This ac added o he wa e dissocia ion abili y o CaP allows
a supe io enhancemen o he WGS ca aly ic pe o mance. The suppo CeP400,
howe e , eaches a maximum adso p ion capaci y a 250 °C, which dec eases when his
empe a u e is exceeded. As p e iously epo ed [21], he hexagonal s uc u e o CePO4
has demons a ed a g ea in e ac ion abili y wi h he wa e molecule ha has been
a ibu ed o he in e ac ion o he la e wi h he phospha e g oups o he suppo and he
in luence o hei s uc u al -OH g oups. Since his suppo in e ac s wi h he wa e
molecule uniquely by hyd ogen bonding wi h he phospha e g oups o he suppo and i
is unable o dissocia e he wa e molecule, i seems easonable i s lowe ca aly ic
pe o mance. In addi ion, conside ing ha he suppo CaP con ains a g ea e po e
olume han CeP400, he in e ac ion o wa e wi h he channels o his solid is a o ed.
The suppo CeP600 shows a g ea a ini y o wa e , al hough i s adso p ion capaci y
s a s o dec ease a 200 °C and is lowe han ha o CeP400 in all he empe a u e
ange. In spi e o he in e ac ion o he su ace phospha e g oups o he monoclinic phase

22
o CePO4 wi h he wa e molecule, he absence o he s uc u al channels p esen in he
hexagonal phase gi es ise o a diminishing o he s eng h o his in e ac ion. Finally,
he suppo TiP p esen s a s ong adso p ion o wa e a 150 °C, which dec eases
d as ically while he empe a u e inc eases. This ac could be a ibu ed o he loss o
phospho ous in his suppo du ing he calcina ion p ocedu e and he subsequen
impossibili y o he wa e molecule o in e ac wi h he phospha e g oups. Thus, he wa e
e en ion capaci y o he suppo s is di ec ly ela ed wi h he ca aly ic end obse ed o
he P ca alys s in he WGS eac ion and wi h he selec i i y o he eac ion. These esul s
e idence he undamen al ole o he wa e a ailabili y o an enhanced WGS ca aly ic
pe o mance, demons a ing ha he WGS eac ion could be imp o ed h ough he
modula ion o he s uc u al cha ac e is ics o he employed phospha e- ype suppo s.
Figu e 8. E olu ion o he mass-on-cha ge (m/z) 18 as a unc ion o he empe a u e
du ing he adso p ion o 10% H2O/A .
4. Conclusions
A se ies o phospha e-based compounds con aining Ca, Ce and Ti ha e been used as
suppo s o P ca alys s and e alua ed in he WGS eac ion. Conside ing ha he
a ailabili y o wa e plays a undamen al ole in he WGS eac ion mechanism, he
s uc u e and wa e adso p ion capaci y o hese suppo s ha e been ela ed o he WGS
ca aly ic pe o mance. Resul s show ha ca aly ic ac i i y dec eases in he o de : P /CaP
23
> P /CeP400 > P /CeP600 > P /TiP. Suppo s con aining s uc u al channels, ha is CaP
and CeP400, a o he in e ac ion wi h wa e , al hough di e en mechanisms o
in e ac ion ha e been obse ed be ween bo h suppo s. CaP dissocia es he wa e
molecule, p esen ing he so-called “G o hus” mechanism. In addi ion, he wa e
in e ac ion akes place ia hyd ogen bonding wi h he phospha e g oups and ia s ong
elec os a ic in e ac ion wi h he Ca2+ ion, which explains he supe io ca aly ic ac i i y o
he P /CaP sys em. On he o he hand, CeP400 e ains wa e in i s molecula o m, hus
p esen ing a di usion mechanism. In his case, he wa e molecule is in e ac ing ia
hyd ogen bonding wi h he phospha e g oups bu no wi h he Ce3+ ion, and consequen ly
gi ing ise o a ela i e dec ease o he ca aly ic ac i i y o he ca alys . When channels
a e no p esen in he phospha e s uc u e, as ocu ing in he suppo CeP600, he
in e ac ion o wa e wi h he phospha e g oups is weakened and hus he ca aly ic ac i i y
dec eases. Finally, he suppo TiP expe ienced a loss o phospho ous du ing he
calcina ion p ocedu e, dec easing he su ace phospha e popula ion and consequen ly,
he in e ac ion wi h he wa e molecule and he WGS ca aly ic ac i i y. These esul s
expose ha phospha e- ype compounds cons i u e app op ia e suppo s o WGS
ca alys s, allowing o enhance hei in e ac ion wi h he wa e molecule du ing he
eac ion h ough he uning o hei su ace and s uc u al p ope ies.
5. Acknowledgmen s
The au ho s a e hank ul o he Spanish Minis y o Economy and Compe i i eness o
he inancial suppo p o ided h ough he P ojec ENE2015-66975-C3-2-R.
24
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