Ca alysis
Science &
Technology
PAPER
Ci e his: Ca al. Sci. Technol.,2024,
14,3514
Recei ed 20 h Feb ua y 2024,
Accep ed 6 h May 2024
DOI: 10.1039/d4cy00236a
sc.li/ca alysis
Moni o ing he in luence o s eam on highly-
ac i e hodium ca alys du ing he combined
e o ming o biogas by ansien and s eady-s a e
ope ando spec oscopic s udies
Vic o ia Ga cilaso, Rubén Blay-Roge , Mi iam González-Cas año,
Luis F. Bobadilla, Miguel A. Cen eno and José A. Od iozola *
The impac de i ed om inco po a ing wa e in o CH
4
/CO
2
biogas s eam o he gene a ion o syngas
was in es iga ed o e he Rh/MgAl
2
O
4
ca alys using ope ando s eady-s a e and ansien DRIFT
spec oscopy coupled wi h MS. The inco po a ion o s eam esul ed in imp o ed CH
4
con e sion a es and
a ained syngas s eams wi h highe H
2
/CO a ios. I was demons a ed ha in he p esence o s eam, he
gene a ion o CH
x
O species h ough he eac ion o CO*wi h ac i e *OH species is a o ed a he me al
suppo su ace. Besides, he enhanced esis ance deli e ed by wa e molecules owa ds deac i a ing he
coking phenomena was associa ed wi h easie ca bonaceous decomposi ion and he exposi ion o he e y
ac i e Rh (100) su aces o me hane decomposi ion. The Rh/MgAl
2
O
4
ca alys was demons a ed o be an
e ec i e ca alys o he p oduc ion o H
2
- ich syngas s eams. Mo e impo an ly, he insigh s epo ed
he ein p o ide new e idences ega ding he impac o s eam on biogas e o ming eac ions.
1. In oduc ion
O e he las se e al yea s, he p oduc ion o biogas om he
anae obic diges ion o biomass was es has a ac ed
conside able in e es as a enewable ca bon sou ce.
1,2
As biogas
is composed mainly o CH
4
and CO
2
, he p oduc ion o syngas
by he e o ming o biogas and he subsequen ans o ma ion
in o liquid uels ia Fische –T opsch syn hesis (FTS) is he mos
sui able p ocess and he mos cos -e ec i e om he indus ial
poin o iew.
3,4
Howe e , he chemical composi ion o aw
biogas depends on he ype o eeds ock, design o he diges e -
e men e and ope a ing condi ions.
5–7
Typically, biogas
con ains 60–75 ol% o me hane and 19–33 ol% o CO
2
along
wi h mois u e besides o he con aminan s such as sulphu
compounds, halogena ed compounds, ammonia, o ganic silicon
compounds (siloxanes), a s, and pa icula e ma e .
8–10
The
mos usual echnologies applied o emo e con aminan s om
aw biogas du ing he upg ada ion p ocess include memb ane
echnology, wa e sc ubbing, p essu e swing adso p ion (PSA),
c yogenic sepa a ion and biological echniques.
11
Appa en ly, he p esence o s eam in biogas is no a
d awback bu a majo ad an age o e o ming p ocesses.
12
Al hough he o ma ion o ca bon is he modynamically
limi ed a ypical e o ming empe a u es, he deposi ion o
coke on he ca alys su ace is una oidable in he d y
e o ming eac ion o me hane.
13
The o ma ion o ca bon
can be minimized due o he p esence o s eam, which
p o ides a highe oxidan le el h oughou he ca bon
gasi ica ion eac ion. Unlike he d y e o ming o model
biogas, in which he syngas ob ained is always p esen a a
H
2
/CO a io lowe han one, he p esence o wa e p oduces a
syngas mo e lexible by adjus ing he CO
2
/H
2
O a io o ma ch
he desi ed alue close o 2 equi ed o he downs eam
p oduc ion o he chemical alue-added ia Fische –T opsch
syn hesis.
14
In his scena io, he combina ion o d y
e o ming o me hane (DRM) and s eam e o ming o
me hane (SRM) has been explo ed ex ensi ely and i is known
as he bi- e o ming o me hane
15
o also he combined
e o ming o me hane.
16
This app oach has a ac ed g ea
in e es om he indus ial poin o iew and p o ides an
oppo uni y o ad anced esea ch owa d he
comme cializa ion o his echnology.
17,18
Ca alys s used o combined e o ming p ocesses a e usually
based on nickel, bu hei s abili y is limi ed by he me al
sin e ing and/o coke deposi ion p ocesses.
19,20
S oud e al.
21
p oposed an Sn–Ni/Ce–Al ca alys able o enhance bo h
me hane con e sion and ca aly ic s abili y in he p esence o
s eam in he bi- e o ming o me hane compa ed o he same
ca alys es ed unde d y- e o ming condi ions. On he o he
hand, MgO and Al
2
O
3
ha e been p obed as he mos p omising
3514 |Ca al. Sci. Technol., 2024, 14,3514–3523 This jou nal is © The Royal Socie y o Chemis y 2024
Depa amen o de Química Ino gánica and 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 illa, Spain. E-mail: [email p o ec ed]
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suppo s o he d y e o ming o me hane. I is well known
ha he inc eased basici y o he suppo a ou s he
adso p ion o CO
2
, acili a ing he gasi ica ion o ca bon o
o m CO, as Zhao e al.
22
epo ed in a ecen s udy. He ein, a
bime allic Ni–Co/MgAl
2
O
4
ca alys showed a high s abili y o e
mo e han 30 h unde e o ming a mosphe es wi hou e idence
o ca bon deposi s in he p esence o H
2
OandCO
2
.Ina
mic okine ic s udy, Geng e al.
23
a gued ha oxygen a oms
om s eam p omo ed me hane con e sion by swi ching he
eac ion pa h om CH*
3+*→CH*
2+H* o CH*
3+O*→CH*
2+
OH*whils es ic ing he gene a ion o C*species, which
cons ic ed he gene a ion o ca bon deposi s. Wi h mos o
he s udies ocusing on Ni ca alys s, he inco po a ion o
adequa e s eam concen a ions could inc ease he H
2
:CO
a ios and imp o e he ca alys s abili y by a ou ing he
gasi ica ion o ca bon deposi s.
24
In his sense, he la ge OH
concen a ions o e Ni ca alys s gene a ed in he p esence o
s eam we e p oposed o enhance he gasi ica ion o ca bon
deposi s gene a ing CHO*species.
25
S ill, deac i a ion issues
ela ed o me al sin e ing and he pa ial oxida ion o he ac i e
si es ha e also been epo ed.
26,27
Al hough hei limi ed a ailabili y and high cos es ain
hei use o la ge-scale applica ions, noble me als like P , Ru,
Pd and Rh a ain highe con e sions and majo coke esis ance
han ansi ion me als.
13
Among hem, Rh was demons a ed
o be he mos ac i e owa ds he con e sion o me hane when
suppo ed on CeO
2
/Al
2
O
3
.
13
O e di e en suppo s and
eac ion condi ions, Rh me al also exhibi ed he highes
u no e equency (TOF) compa ed o hei homologues based
on Ru and P .
28
Maes i e al.
29
p oposed a consis en
mic okine ic model on Rh ca alys ha poin ed ou he
mechanis ic analogies be ween s eam and d y e o ming. They
p oposed ha me hane consump ion p oceeds ia
decomposi ion and subsequen ca bon oxida ion by OH*
ollowing a sequen ial ou e (CH
4
→C*→CO*), and he ole
o ei he CO
2
o H
2
O is o p o ide he main oxidize (OH*).
Ros up-Nielsen and Bak Hansen
30
also obse ed ha eplacing
s eam by ca bon dioxide ha dly modi ied he e o ming
mechanism. Mo eo e , hey demons a ed ha Ru and Rh
ca alys s a e highly s able o ca bon- ee ope a ion, p esen ing
high e o ming a es combined wi h low coke o ma ion a es.
Ope ando di use e lec ance in a ed Fou ie ans o m
spec oscopy (DRIFTS) has been widely employed as a powe ul
ool o elucida e he eac ion mechanism and gain insigh s in o
he na u e o su ace in e media es. In a p e ious wo k,
31
we
s udied he ac i i y o he Rh/MgAl
2
O
4
ca alys owa d he biogas
e o ming eac ion using ansien and s eady-s a e DRIFTS
expe imen s, p oposing a eac ion mechanism in which he
dissocia i e adso p ion o CO
2
akes place ini ially and he ac i e
oxygen species o med on he me allic si es acili a es C–Hbond
ac i a ion. Al hough nume ous a icles ha e been published on
he d y e o ming o me hane eac ion on di e en suppo ed
Rh ca alys s, he e is no a clea and unambiguous explana ion
o he eal e ec o s eam on he na u e o he eac ion. The
sca ce s udies ocusing on he ole o s eam and OH su ace
g oups on he eac ion mechanism and cons i u ed
in e media es migh be associa ed wi h he low concen a ion
es ablished unde DRM a mosphe es.
32
Conside ing he impac
o la ge OH co e ages gene a ed in he p esence o s eam and
he s uc u e sensi i i y cha ac e o e o ming p ocesses,
33,34
es ablishing he ole o s eam o ca aly ic sys ems aside o Ni
me al is ex emely con enien . The p esen s udy con ibu es a
new pe spec i e o unde s and he e ec o s eam in he
ac i a ion p ocess o CO
2
and CH
4
du ing he e o ming o
biogas by means o ansien expe imen s and DRIFTS coupled
MS s udies o e he Rh/MgAl
2
O
4
ca alys . The in eg a ion o bo h
ansien and s eady-s a e ope ando expe imen s p o ides a
comp ehensi e ision o he ca alys su ace p ope ies, and
hei co ela ion wi h ca aly ic pe o mance is e y use ul o
achie ing he a ional design o ca alys s.
2. Expe imen al de ails
A ca alys based on 1 w % o Rh dispe sed on MgAl
2
O
4
suppo
was used in his s udy. The p epa a ion me hod and
cha ac e iza ion de ails o his ca alys ha e been desc ibed in
a p e ious wo k.
31
This ca alys was labelled as Rh/MgAl
2
O
4
.
Thebiogas e o ming eac ionwasca iedou ina
comme cial compu e ized mic oac i i y e e ence ca aly ic
eac o om PID Eng & Tech employing a s ainless-s eel eac o
wi h 9 mm in e nal diame e . A ixed-bed composed by 50 mg
o ca alys dilu ed in SiC (bo h sie ed in 100–200 μm) o ensu e
a good hea and mass ans e was placed a he cen e o he
eac o in di ec con ac wi h a he mocouple. Be o e ca alys
es ing, he sample was in si u educed in a low o 50% H
2
/N
2
a 750 °C o 3 h (100 mL min
−1
). Then, 50 mL min
−1
o ni ogen
was used o pu ge he eac o sys em o 30 min be o e eac ion
expe imen s a 750 °C. The eac ion was pe o med a 750 °C
and 1 ba , main aining a o al low a e o 100 mL min
−1
wi h a
weigh hou ly space eloci y (WHSV) equal o 200 L h
−1
g
−1
and
a ying amoun s o s eam. Reac an gas mix u es consis ing o
CH
4
,CO
2
, and N
2
wi h a CH
4
/CO
2
mola a io equal o 1.5 we e
in oduced using mass- low con olle s om B onkho s .
Va iable amoun s o H
2
O we e in oduced using a HPLC pump
(Gilbson Inc se ies) and e apo a ed o each concen a ions o
8, 12 and 17 ol% o s eam. In o de o main ain he space
eloci y and he pa ial p essu es o CO
2
and CH
4
, he low o
ni ogen was adjus ed as a unc ion o he s eam inco po a ed.
The e o ming es s we e pe o med unde iso he mal
condi ions, and e e y concen a ion o s eam was main ained
o 2 h o gua an ee s eady-s a e condi ions. The e luen gases
we e analysed on-line using a mic o-gas ch oma og aph (Va ian
4900 model) equipped wi h wo columns: Po aplo Q and
molecula sie e 5A. The con e sions o me hane and CO
2
we e
calcula ed on he basis o he di e ence be ween he inpu and
ou pu o me hane and CO
2
, espec i ely.
Ope ando DRIFTS-MS expe imen s we e pe o med in a high
empe a u e eac ion chambe (P aying Man is om Ha ick)
i ed wi h ZnSe windows and connec ed o a The mo Nicole
Nexus FTIR spec ome e equipped wi h a liquid N
2
-cooled
MCT de ec o . Spec a we e ob ained wi h a spec al esolu ion
o 4 cm
−1
and an a e age o 128 scans. The whole op ical pa h
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was con inuously pu ged wi h a low o wa e apou - and CO
2
-
ee ni ogen, and he backg ound spec um was collec ed
using an aluminium e lec i e mi o . Typically, abou 80 mg o
sample was pu in he DRIFTS cell o ensu e ha he gases pass
h ough he ca alys packed-bed mimicking he plug- low
condi ion o he eac ion. P io o he measu emen s, he
sample was ac i a ed in si u a 750 °C o 1hin50%H
2
/A a a
o al low a e o 50 mL min
−1
and hencooleddown o he
eac ion empe a u e by pu ging wi h A . The spec um o he
ac i a ed ca alys was used as he e e ence. The d y biogas
e o ming eac ion was pe o med by eeding in o he cell a gas
mix u e o 10 ol% CO
2
and 15 ol% CH
4
balanced in a gon
wi h a o al low a e o 50 mL min
−1
. Analogously, he bi-
e o ming eac ion o biogas was ca ied ou using a mix u e o
10 ol% o CO, 15 ol% CH
4
and 10 ol% o H
2
O in a gon wi h
a o al low a e o 50 mL min
−1
. Fo i , an adequa e amoun o
liquid wa e was ed con inuously using a HPLC pump and
apo ising he liquid in a homemade e apo a o . In bo h he
eac ions, he empe a u e was inc eased om 550 °C o750°C
wi h in e als o 50 °C, and each empe a u e was main ained
o 30 min o achie e s eady-s a e condi ions. Fo he ansien
expe imen s, he eed composi ion was empo a ily a ied
ollowing wo di e en sequences: 1) 10%CO
2
/A →A →
15%CH
4
/A , and 2) 10%CO
2
/10%H
2
O/A →10%H
2
O/A →
15%CH
4
/10%H
2
O/A . The di e en mix u es we e in oduced
unde iso he mal condi ions a 550 °Ce e y20min.A ou -way
al e was ins alled o swi ch he gas mix u es. The gas
composi ion exi ing he DRIFTS cell we e sen o a mass
spec ome e (PFEIFFER MS Vacuum P isma Plus) using a 1/
16-inch s ainless s eel capilla y ubing hea ed a 150 °C o
analysis o a oid condensa ion. The con inuous da a
acquisi ion o MS was eco ded using he Quade a so wa e,
ollowing mainly he signals m/z= 2, 15, 18, 28 and 44.
3. Resul s and discussion
3.1. E ec o s eam in he biogas e o ming eac ion
The e ec o di e en s eam concen a ions in he biogas
e o ming eac ion o e Rh/MgAl
2
O
4
ca alys was pe o med
unde iso he mal condi ions a 750 °C, eeding a CH
4
/CO
2
mola a io o 1.5. Fig. 1a shows ha he p esence o s eam
p omo es he me hane con e sion bu has a con a y e ec
on CO
2
con e sion. The gene al eac ion o bi- e o ming o
me hane can be ep esen ed by a combina ion o he s eam
e o ming and d y e o ming eac ions, as shown in he
ollowing equa ions.
D y e o ming o me hane: CH
4
+CO
2
→2CO + 2H
2
(1)
S eam e o ming o me hane: CH
4
+2H
2
O→CO + 3H
2
(2)
Bi- e o ming o me hane: 3CH
4
+CO
2
+2H
2
O→4CO + 8H
2
(3)
Addi ionally, a a ie y o di e en pa allel eac ions also
can occu , including he ollowing.
35
Wa e gas shi eac ion (WGS): CO + H
2
O↔CO
2
+H
2
(4)
Me hane decomposi ion: CH
4
↔C+2H
2
(5)
Boudoua d eac ion: CO
2
+C↔2CO (6)
Beggs eac ion: C + H
2
O↔H
2
+ CO (7)
CO
2
me hana ion: CO
2
+4H
2
→CH
4
+2H
2
O (8)
CO me hana ion: CO + 3H
2
→CH
4
+H
2
O (9)
The enhancemen o he CH
4
con e sion wi h he amoun
o wa e in he eed is mainly due o he pa icipa ion o he
s eam e o ming eac ion (eqn (2)), whe eas he lowe CO
2
con e sion is ela ed o he CO
2
p oduced by he WGS
eac ion (eqn (4)). An inc ease in he concen a ion o wa e
a ou s he WGS eac ion agains he d y e o ming o
me hane, esul ing in a nega i e CO
2
balance, as p edic ed
he by he modynamics.
15,21
Fu he mo e, Fig. 1b shows ha he H
2
/CO mola a io
inc eases om 1.1 o 1.7 by inc easing he ol% o wa e ,
which is s ic ly ela ed o he p oduc ion o hyd ogen and
he consump ion o CO by he WGS eac ion and he
gasi ica ion o ca bonaceous deposi s (eqn (7)). In a
collabo a ion wo k o ou g oup, S oud e al.
21
epo ed a
simila H
2
/CO a io o 1.6 o a eed o CH
4
/CO
2
/H
2
O = 1/1/1
(33 ol% o H
2
O) a 700 °C on a bime allic Ni–Sn suppo ed
ca alys . Mo eo e , i was demons a ed ha he in oduc ion
o wa e in he bi- e o ming eac ion a ou s he gasi ica ion
o ca bonaceous species deposi ed on he ca alys su ace.
Roh e al.
36
ound an op imal CO
2
:H
2
O:CH
4
eed a io o
ob ain he syngas wi h a H
2
/CO mola a io o 2 sui able o
Fische –T opsch syn hesis. The op imal concen a ion o
wa e epo ed by hese au ho s was 25 ol% o s eam o a
CH
4
/CO
2
a io o 2.5. In ano he p e ious wo k, we ound
Fig. 1 E ec o he s eam concen a ion on he pe o mance o Rh/
MgAl
2
O
4
in e ms o a) CO
2
and CH
4
con e sion, and b) H
2
/CO mola
a io du ing he biogas e o ming eac ion es ed unde WHSV = 200 L
h
−1
g
−1
,T= 750 °C and CH
4
/CO
2
mola a io = 1.5.
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ha an H
2
/CO a io close o 2 can be achie ed using a eed o
28 ol% o wa e and a CH
4
/CO
2
a io o 2.5 o e a Ni–Ru-
based ca alys . In he cu en wo k, he eac ion was
pe o med using a ypical biogas composi ion wi h a CH
4
/
CO
2
a io o 1.5 and concen a ions o wa e below 20 ol%.
Ne e heless, he endency obse ed o he H
2
/CO a io wi h
he wa e concen a ion (Fig. 1b) sugges s ha wa e
concen a ion o abou 8 o 18 ol% should esul in
indus ially ele an H
2
/CO mola a ios.
3.2. Ope ando DRIFTS-MS s udies
Ope ando DRIFTS-MS s udies we e pe o med o be e
unde s and he in luence o s eam and he in e media e
species o med du ing he d y e o ming and bi- e o ming o
biogas eac ion o e Rh/MgAl
2
O
4
ca alys . A comp ehensi e
ision o he eac ion mechanism p o ides a use ul guidance
o design an op imal ca alys in e ms o he ac i i y,
selec i i y, and s abili y.
3.2.1. In si u educ ion p e ea men . The sample was
educed in 50%H
2
/A a 750 °C o 1 h o clean he ca alys
su ace, ensu ing he comple e educ ion o Rh pa icles.
Fig. 2 gi es an accoun o he e olu ion o he DRIFT spec a
du ing he ac i a ion ea men as a unc ion o he
empe a u e. In he ini ial s age a RT, he esh sample
p esen s a cha ac e is ic band a 1647 cm
−1
assigned o he
de o ma ion mode o adso bed wa e on he su ace
37
and
se e al bands in he 1600–1300 cm
−1
spec al egion ha can
be a ibu ed o esidual ca bona e-like species o med by he
in e ac ion wi h he a mosphe ic CO
2
o he hyd oxyla ed
MgAl
2
O
4
su ace.
38
Hea ing up he sample in he educ ion low, he
physiso bed wa e disappea ed comple ely a empe a u es
nea 140 °C and on inc easing he empe a u e abo e 450 °C,
one band a 3720 cm
−1
appea ed, which is asc ibable o
“ ee”hyd oxyls, disclosing he comple e dehyd a ion o he
su ace by he elimina ion o he b oad band cen ed a ca.
3500 cm
−1
associa ed wi h he H-bonded hyd oxyls o weakly
adso bed wa e .
39
The e olu ion o he bands assigned o
ca bona e species du ing he p e ea men sugges s he
di e en he mal s abili y and eac i i y owa ds hyd ogen. I
is no ewo hy ha as he empe a u e inc eases abo e 200
°C, an al e a ion o he ca bona e ea u es is obse ed, and
new bands appea ed a 2010 and 1915 cm
−1
, which can be
assigned o Rh-ca bonyl species wi h di e en coo dina ion
geome ies, espec i ely.
40
I is well known ha he he mal
decomposi ion and/o educ ion o ca bona e species akes
place in he p esence o hyd ogen o e noble me als, leading
o he o ma ion o Rh-ca bonyl in e media e species and CO
gas a empe a u es o 200–300 °C.
41
Because Rh is an
op imal ac i e me al o he me hana ion eac ion, he
dissocia ion o CO on Rh pa icles o o m “C”species and
hei subsequen hyd ogena ion o o m me hane gas should
no be uled ou .
42
In his sense, he disappea ance o Rh-
ca bonyl species as he empe a u e is inc eased abo e 300
°C sugges s he possible me hana ion o adso bed CO.
Ne e heless, he low concen a ion o me hane o med is
oo low o be de ec ed using an IR spec ome e . When he
empe a u e eached 750 °C, all ca bona e species we e
emo ed, indica ing ha he p e ea men is e ec i e o
ob ain a clean su ace. We can conclude ha appa en ly he
ca alys su ace only con ains “ ee”hyd oxyls and Rh
me allic pa icles highly dispe sed a e educ ion
p e ea men o 1 h a 750 °C.
3.2.2. S eady-s a e d y and bi- e o ming o biogas. The
in luence o s eam was in es iga ed by ope ando DRIFTS-MS
on he Rh-based ca alys unde eac ion condi ions. Fo his
pu pose, 10 ol% s eam was selec ed and compa ed wi h he
Fig. 2 E olu ion o he DRIFT spec a wi h he empe a u e in he 4000–3100 and 2100–1300 cm
−1
egions du ing he educ ion p e ea men a
750 °C o 1 h in 50%H
2
/A o e he Rh ca alys .
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d y condi ions. Fig. 3 shows he e olu ion o he DRIFT
spec a wi h empe a u e in he absence (Fig. 3a) and
p esence (Fig. 3b) o wa e in he eac ion mix u e.
As shown Fig. 3a, a e in oducing he eac ing gas
mix u e o CO
2
and CH
4
ee o wa e a 550 °C, a p ominen
peak a 2360 cm
−1
associa ed wi h he asymme ic s e ching
(ν
as
)o CO
2
gas and wo bands a 3016 and 1303 cm
−1
associa ed wi h he o a ional ine s uc u e o me hane gas
appea .
43
The bands appea ing a 2030–2008 cm
−1
and 1920–
1860 cm
−1
indica e he o ma ion o linea ly adso bed CO
and b idged CO species on Rh si es, sugges ing ha CO
2
is
dissocia ed in o CO and “O”species.
40
Wi h inc easing
empe a u e, he appea ance o he cha ac e is ic Pb anch
be ween 2140 and 2200 cm
−1
o he gaseous CO ib a ional-
o a ional spec um is mo e app eciable, which indica es he
elease o CO gas.
43
Addi ionally, wo complex pai o bands
cen ed a 1595 and 1360 cm
−1
we e iden i ied a lowe
empe a u es. As he empe a u e inc eases, hese wo bands
e ol e oge he , gi ing a se o bands a 1630, 1535, 1377 and
1330 cm
−1
a 750 °C. These la e bands a e associa ed wi h
he p esence o esidual ca bonaceous species adso bed on
he suppo su ace.
44,45
Rega ding he assignmen o he
bands a 1595 and 1360 cm
−1
, some au ho s ha e assigned
he band a 1595 cm
−1
o he ν
as
(OCO) ib a ional mode o
he o ma e species. Ne e heless, he wo cha ac e is ic
bands o he o ma e species a 1395 and 1375 cm
−1
asc ibed
o CH de o ma ion and ν
s
(OCO), espec i ely, we e absen in
ou expe imen al da a (Fig. 3). Addi ionally, a band a 2900–
2800 cm
−1
asc ibed o he ν(C–H) s e ching o o ma e
should appea in he spec a. Al hough his egion is
o e lapped wi h he o a ional ine s uc u e o gaseous
me hane, he band ela ed o he C–H s e ching o o ma e
is ha dly obse able. The e o e, we canno a ibu e hese
bands o o ma e species. I is mo e easonable o assume
ha he bands a 1595–1360 cm
−1
a e a ibu able o
monoden a e ca bona es wi h Δν
3
o 100–200 cm
−1
o med on
he suppo 's basic si es.
46,47
These species a e he mally
labile ca bona es, which a e decomposed o educed as he
empe a u e inc eases.
In he p esence o wa e (Fig. 3b), he bands associa ed
wi h gaseous CO
2
and CH
4
we e also obse ed, al hough
impo an di e ences we e no iced in he adso bed species.
In he 1700–1200 cm
−1
spec al egion, i is wo h no icing
ha he bands a 1595 and 1360 cm
−1
anished apidly wi h
he empe a u e, whe eas he bands a 1630, 1535, 1377 and
1330 cm
−1
we e ha dly obse ed. This clea ly e eals ha
s eam a ou s he gasi ica ion/decomposi ion o
ca bonaceous species. On he o he hand, he p esence o CO
ca bonyl adso bed species o e Rh
0
si es cha ac e ized by he
bands a 2040–2028 cm
−1
and 1920–1860 cm
−1
, assigned o
CO adso bed linea ly and mul i-bonded ca bonyl species,
espec i ely, was also obse ed.
40,48
Thus, CO
2
dissocia ion
also akes place unde bi- e o ming condi ions. Ne e heless,
he in a ed spec a associa ed wi h he ca bonyl species we e
clea ly a ec ed by he eac ion a mosphe e.
The di e ences in he adso bed CO species can p o ide
in o ma ion abou he species and si es esponsible o he
ac i a ion o CO
2
and CH
4
in he p esence and absence o
s eam. In o de o assess he in luence o he s eam on he
CO adso bed species on Rh in de ail, Fig. 4 ocuses on he
di e en e olu ions no iced on inc easing he eac ion
empe a u e o he 2100–1800 cm
−1
egion. I can be no iced
ha he inc ease in he empe a u e om 450 °C o 750 °C
caused a sligh dec ease in he in ensi y o he CO adso p ion
peaks associa ed wi h linea ly adso bed CO wi h a
displacemen o lowe wa enumbe s om 2030 o 2008 cm
−1
in he absence o wa e (Fig. 4a), while in p esence o s eam,
he equi alen bands we e p og essi ely emo ed wi h he
empe a u e and displaced om 2048 o 2028 cm
−1
(Fig. 4b).
The ib a ional equency depends on he CO co e age, and
he highe equency obse ed a high CO co e ages is due o
he dipole–dipole coupling be ween he CO adso bed
molecules on neighbou ing si es. In he p esence o s eam,
dipole–dipole coupling accoun s o he edshi obse ed o
he ca bonyl bands due o he lowe CO co e age on Rh
pa icles when he empe a u e is inc eased. Ne e heless,
Fig. 3 E olu ion o DRIFT spec a unde he eac ion condi ions as a
unc ion o empe a u e on he educed sample. Spec a we e
eco ded in he eac ion mix u e wi h CH
4
/CO
2
= 1.5 in he absence o
wa e (a) and he p esence o 10 ol% o wa e (b).
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he displacemen o ca bonyl equencies obse ed unde d y
condi ions (Fig. 4a) canno be ela ed o dipole–dipole
coupling and CO co e age phenomena. Simul aneously, he
band a 2030 cm
−1
shi s o 2008 cm
−1
and p ese es i s
in ensi y, meaning ha he elec onic densi y on he hodium
si es inc eases unde hese condi ions. Acco dingly, he
o ma ion o ca bon species on he su ace mus be expec ed,
sugges ing ha CO adso bs on Rh si es close o ca bonaceous
deposi s ha inc ease he back-dona ion on he me al
su ace.
38,49–51
This obse a ion is cohe en conside ing ha
he absence o wa e led o he co e ing o Rh pa icles by
ca bonaceous deposi s, whe eas he wa e pa icipa es in he
ca bon gasi ica ion, cleaning he Rh su ace si es. On he
o he side, i is also no iced om Fig. 5a and b ha no shi
o he bands a ibu ed o b idge-bonded CO on Rh (1860
and 1920 cm
−1
) was obse ed wi h he inc ease in he
eac ion empe a u e. The band a 1860 cm
−1
has been
asc ibed o 1:2 CO–Rh species while he peak a 1920 cm
−1
is ela ed o a 3:2 s oichiome y.
52
These s oichiome ies
depend on he ype o he hodium en i onmen and clea ly
e eal ha he p esence o s eam is a ec ing he
coo dina ion o Rh si es. Acco ding o La alley e al.,
53
hese
wo bands a e ce ainly ela ed o CO adso p ion on wo
hodium aces such as Rh (100) and Rh (111). The mino
p esence o he band a 1920 cm
−1
unde d y condi ions may
be en a i ely explained by he blocking o b idged
adso p ion si es on he co esponding hodium ace. Wang
e al.
54
s udied CH
4
dehyd ogena ion on Rh (111), Rh (110)
and Rh (100) su aces using densi y unc ional heo y (DFT)
calcula ions. They ound ha CH
4
dehyd ogena ion on he
Rh (100) su ace is mo e a ou able in compa ison wi h ha
on Rh (111) and Rh (110) su aces. On his basis,
ca bonaceous deposi s o med in he absence o s eam could
p e e en ially supp ess CO-b idged adso p ion on Rh (100)
su ace si es.
Rega ding he e olu ion o he gas-phase analysed on-line
by MS in he absence and p esence o s eam (Fig. 5a and b),
subs an ial o ma ion o H
2
and CO was obse ed a
empe a u es abo e 450 °C du ing he d y e o ming and he
bi- e o ming o biogas eac ion on he Rh ca alys . As he
empe a u e inc eased, he con e sion o me hane is highe
and he signal o H
2
(m/z= 2) is inc eased. In he absence o
Fig. 4 E olu ion o he DRIFT spec a in he 2100–1800 cm
−1
egion wi h he empe a u e o he educed sample unde he eac ion condi ions
in he absence o wa e (a) and p esence o wa e (b).
Fig. 5 E olu ion o he gas-phase analysed on-line by MS as a unc ion o ime-on-s eam and empe a u e in he absence (a) and p esence (b) o
s eam: m/z=2(H
2
), m/z= 15 (CH
4
), m/z=18(H
2
O), m/z= 28 (CO) and m/z= 44 (CO
2
).
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wa e a 750 °C, he signi ican dec ease o me hane
con e sion and H
2
concen a ion was associa ed wi h ca alys
deac i a ion due o he o ma ion o ca bon deposi s o CO
species s ongly bonded o Rh pa icles. In ag eemen wi h
he ca aly ic ac i i y esul s, ope a ing in he p esence o
wa e esul ed in highe H
2
concen a ion and H
2
/CO a ios
close o 2. Rema kably, no signs o ca alys deac i a ion we e
no iced in he p esence o wa e , and ai ly s able hyd ogen
p oduc ion o he en i e empe a u e window was ob ained.
3.2.3. DRIFTS-MS ansien s udies. The kine ic esponses
o he su ace species analysed unde ansien condi ions a e
di e en om hose measu ed om s eady-s a e expe imen s
and p o ide di ec in o ma ion abou he na u e and co e age
o su ace in e media es. The ansien expe imen s we e
ca ied ou using di e en combina ions o eac an s o analyse
he e ec o s eam. Fi s ly, we examined he CO
2
ac i a ion
om gas s eams con aining 10%CO
2
/A , and hen we
moni o ed he e olu ion o he in e media es o med on
pu ging wi h A and subsequen ly by swi ching a gas s eam
con aining 15%CH
4
/A . In o de o s udy he impac o s eam,
we pe o med an analogous expe imen in which he eed
composi ion was empo a ily a ied ollowing he sequence 1)
10%CO
2
/10%H
2
O/A →2) 10%H
2
O/A →3) 15%CH
4
/10%H
2
O/
A . In bo h s udies, each s eam mix u e was main ained o 20
min unde iso he mal condi ions a 550 °C.
Fig. 6a and b display he e olu ion o he IR spec a
sequen ially eco ded by swi ching he h ee s eam e luen s
e e y 20 min in d y and we condi ions, espec i ely. As can
be no iced, unde CO
2
s eam, simila ea u es ela ed o
ca bonyl adso bed species and ca bona es we e obse ed
independen ly o he p esence o wa e . The p esence o bo h
linea (2020–2000 cm
−1
) and b idged (1920–1860 cm
−1
)
Fig. 6 E olu ion o he ansien DRIFT spec a obse ed a 550 °C unde he sequen ial swi ching o he eed gas in he absence and p esence o
s eam: a) 10%CO
2
/A →A →15%CH
4
/A , and b) 10%CO
2
/10%H
2
O/A →10%H
2
O/A →15%CH
4
/10%H
2
O/A .
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ca bonyls unde bo h s eam condi ions indica es ha CO
2
is
dissocia ed on he me allic si es and ha wa e plays no a
signi ican ole in he di ec dissocia ion o CO
2
. Upon
swi ching he CO
2
s eam o a pu ge s eam, signi ican
di e ences we e obse ed in he absence and p esence o
wa e . As shown in Fig. 6a, he IR bands a ibu ed o he
ca bona e species disappea ed almos comple ely, whe eas
he in ensi ies o he bands a ibu ed o he adso bed
ca bonyl species dec eased only pa ially du ing he A
pu ging s ep. Meanwhile, Fig. 6b e eals ha in he p esence
o s eam, he me allic ca bonyls and ca bona es disappea ed
e y as . Mo eo e , i is ema kable o no ice he nega i e
e olu ion o ca bonyls and he simul aneous eme gence o
wo bands a 1690 and 1420 cm
−1
. These wo bands a e
assigned o CO s e ching and CH bending in he CH
x
O
species.
55–57
These indings clea ly e eal ha CH
x
O species
a e eac ion in e media es a o ed in he p esence o s eam.
We can assume ha he gene a ion o CH
x
O species occu s
h ough he eac ion o CO*wi h he ac i e *OH species
o med likely a he me al suppo in e ace. Subsequen ly, by
swi ching he gas eed om pu ge o CH
4
s eam, he impac
o s eam is e en mo e e iden . F om Fig. 6a, i is e iden ha
he bands ela ed o ca bonyls and ca bona es comple ely
disappea whe eas esidual bands a 1580 and 1345 cm
−1
a e
de eloped. These ea u es a e asc ibed o ca bonaceous
species.
58,59
The obse ed nega i e band o ca bonyl could be
ela ed o he ans o ma ion o linea ly adso bed CO species
in o –COH adso bed species. The e o e, hese obse a ions
clea ly e eal ha he ca alys 's su ace is a co e ing o
accumula ed ca bon o med by me hane dissocia ion in he
absence o s eam. In con as , one can see in Fig. 6b ha
wi h he inco po a ion o CH
4
/H
2
O/A s eam, CO
2
and CO
gases we e eleased. Fu he mo e, bands asc ibed o ca bonyl
species adso bed o e Rh me al si es and bands a 1620,
1595, 1360 and 1330 cm
−1
asc ibed o ca bona es wi h
di e en coo dina ion geome y we e also de ec ed. The
o ma ion o CO and CO
2
indica e ha gasi ica ion and WGS
eac ions a e happening on he ca alys su ace.
Finally, Fig. 7a and b display he e olu ion o gaseous
p oduc s moni o ed by on-line mass spec ome y (MS). Ion
cu en s we e measu ed o he m/z alues o he H
2
O, CO
2
CH
4
,H
2
and CO and H
2
O. Fi s ly, i mus be no iced ha a
la ge amoun and s able p oduc ion o hyd ogen is achie ed
in he CH
4
/H
2
O/A s eam, whe eas in he absence o s eam,
he p oduc ion o hyd ogen decays apidly due o he
accumula ion o ca bonaceous deposi s, in line wi h he
abo e discussion.
3.3. Mechanis ic insigh s
The obse ed indings enable a ionalizing he impac o
s eam on he eac ion pa hway p oposed o he Rh/MgAl
2
O
4
ca alys in he absence and p esence o wa e (Fig. 8a and b).
Unde bo h eac ion condi ions, CO
2
and CH
4
we e
dissocia i ely adso bed o e he me al su ace. The esul ing
CH
x
species eac , ei he o e he me al su ace o a he
me al suppo in e ace, wi h close ac i e O*a oms,
p oducing CH
x
O species, which decompose o CO and H
2
.
S ill, ema kable di e ences conce ning he ype o su ace
species de eloped on he su ace and Rh–CO in e ac ions
we e obse ed. In ou p e ious wo k, we ound ha he
dissocia i e adso p ion o CO
2
o e Rh me al su aces was
associa ed wi h he pa ial oxida ion and e-dispe sion o Rh
a oms.
31
The lowe -coo dina ed Rh clus e we e p oposed o
p omo e he gene a ion o s able b idged ca bonyls h ough
he decomposi ion o uns able geminal ca bonyls. In ac , i
is gene ally accep ed ha he na u e and geome y o he
ac i e me al de e mines he dissocia ion o CH
4
and he
o e all ca alys pe o mance. He ein, we obse ed ha
b idged ca bonyl bands we e de eloped o e wo di e en Rh
Fig. 7 E olu ion o he gas-phase analysed on-line by MS as a unc ion o ime-on-s eam unde he sequen ial swi ching o he eed gas in he
absence a) and p esence o s eam b).
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su ace adso p ion si es. In his sense, he disappea ance o
s able b idged ca bonyl migh unde mine he ca alys
pe o mance by blocking he Rh ac i e si es h ough he
gene a ion o ca bon species ia Boudoua d's and me hane
c acking eac ions. The ca alys s' deac i a ion de i ed om
he coke deposi s gene a ed o e he Rh su ace in he
absence o wa e ag ees wi h he hyd ogen d op obse ed in
he MS analysis a highe empe a u es (Fig. 5a).
In he p esence o s eam, he absence o ca bonaceous
deposi s obse ed on he ca alys s su ace and he g ea e
CH
4
con e sion a es sugges ha wa e molecules acili a e
he decomposi ion o eac ion in e media es in o H
2
and CO.
F om he ope ando DRIFT-MS esul s, he hyd oxyla ed
su ace could imp o e he eac ion o he CO*species wi h
he nea by OH species, gene a ing CH
x
O species ha
subsequen ly decomposes in o he eac ion p oduc s.
Besides, he p esence o wa e should also p omo e he
o wa d WGS eac ion, emo ing CO species om he me al
su ace, he eby hinde ing coke gene a ion ia he
Boudoua d eac ion. Rema kably, he inco po a ion o wa e
molecules also cons ic ed he blocking o Rh ac i e si es. In
his sense, s eam could p omo e he exposi ion o Rh su ace
ac i e o me hane decomposi ion o some ex en and inhibi
he accumula ion o s able ca bonaceous species o e highly
dispe sed Rh si es. In all, he inco po a ion o wa e
molecules enhances he pe o mance o he Rh/MgAl
2
O
4
ca alys by p omo ing he gene a ion o CH
x
O species,
enhancing he decomposi ion o ca bonaceous species and
inhibi ing he accumula ion o ca bon deposi s o e he
ac i e Rh me al su aces.
4. Concluding ema ks
This wo k analyses he bene i s a ained by inco po a ing
wa e in o DRM s eam o e he Rh/MgAl
2
O
4
ca alys by
combining s eady-s a e and ansien ope ando DRIFT
coupled wi h MS spec ome y. The ca aly ic ac i i y esul s
e idenced he gene a ion o syngas wi h highe H
2
/CO a ios
because o he p omo ion o CH
4
con e sion and lowe CO
2
con e sions due o he impac o he o wa d WGS eac ion.
The inco po a ion o wa e a ec ed he cons i u ed Rh
nanopa icles and eac ion in e media es bo h in na u e and
concen a ion. Hence, he hyd oxyla ed su ace a ained in
he p esence o wa e enhanced he eac i i y o CH
x
species
wi h he ac i e O*species, he eby enhancing he gene a ion
o CH
x
O species, he p oposed majo eac ion in e media e
owa ds H
2
p oduc ion. Mo eo e , he p esence o s eam
p omo ed he decomposi ion o ca bonaceous species and
cons ic ed he coking phenomena o e he me al su ace,
hus enhancing he ca alys li e ime.
Con lic s o in e es
The e a e no con lic s o decla e.
Acknowledgemen s
D a. Mi iam González acknowledges o he EMERGIA
p og am o he Jun a de Andalucía by he EMC21_00052
con ac . The au ho s acknowledge inancial suppo om
Spanish Minis y o Science h ough he p ojec s NICER-
BIOFUELS (PLEC2021-008086) inanced by Nex Gene a ion
Eu opean Funds and SMART-FTS (PID2021-126876OB-I00).
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Fig. 8 P oposed eac ion pa hways in he absence a) and p esence o
s eam b).
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