Jou nal o CO2 U iliza ion 58 (2022) 101922
A ailable online 12 Feb ua y 2022
2212-9820/© 2022 The Au ho (s). Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-
nc-nd/4.0/).
Tuning basici y o dual unc ion ma e ials widens ope a ion empe a u e
window o e icien CO
2
adso p ion and hyd ogena ion o CH
4
Alejand o Be mejo-L´
opez, Be˜
na Pe eda-Ayo, Jon A. On ubia-Cal o, Jos´
e A. Gonz´
alez-Ma cos,
Juan R. Gonz´
alez-Velasco
*
Depa men o Chemical Enginee ing, Facul y o Science and Technology, Uni e sidad del País Vasco UPV/EHU, Ba io Sa iena, s/n, Leioa, Bizkaia 48940, Spain
ARTICLE INFO
Keywo ds:
CO
2
hyd ogena ion
In eg a ed CO
2
cap u e and u iliza ion
Me hana ion
Dual unc ion ma e ial
Tuning basici y
ABSTRACT
Mi iga ion o CO
2
emissions by in eg a ed CO
2
cap u e and u iliza ion (ICCU) is challenging. This wo k ocuses
on widening ope a ion empe a u e window o he hyd ogena ion o adso bed CO
2
o CH
4
. Fo his, a se o dual
unc ion ma e ials (DFMs) 4%Ru-x%Na
2
CO
3
-y%CaO/γ-Al
2
O
3
a e p epa ed. DFMs a e deeply cha ac e ized by N
2
adso p ion-deso p ion, XRD, H
2
chemiso p ion, TEM, H
2
-TPR and CO
2
-TPD. The ca aly ic beha io , in cycles o
CO
2
adso p ion and hyd ogena ion o CH
4
, is e alua ed and he empo al e olu ion o he concen a ion o e-
ac an s and p oduc s is analyzed. The p esence o bo h adso ben s in he DFMs imp o es u henium dispe sion
and he basici y is modula ed wi h he Na
2
CO
3
/CaO a io. Ru-8Na/8Ca imp o es me hane p oduc ion o e he
whole empe a u e window compa ed o DFMs based only on a unique adso ben . The bes esul s a e assigned o
he p omo ion o con ac be ween he ca bona es o medium s eng h wi h he me allic si es, which boos he CO
2
adso p ion and hyd ogena ion o CH
4
.
1. In oduc ion
The signi ican inc ease o CO
2
concen a ion in he a mosphe e,
om 280 ppm in 1760 o 410 ppm in 2020 [1], has become a se ious
global wa ming p oblem, esul ing in a se ies o se e e clima e and
en i onmen al changes [2]. The emissions o CO
2
a e mainly a ibu ed
o ossil uel consump ion [3]; in pa icula , he powe gene a ion sec o
is esponsible o mos CO
2
emissions, ollowed by indus ial and ans-
po a ion ehicles [4]. A his poin , he educ ion o CO
2
emissions in o
he a mosphe e is essen ial.
CO
2
cap u e echnologies p o ide a key pa hway o educing CO
2
emissions, especially conside ing ha ossil uels will con inue o play an
impo an ole in supplying global ene gy demand du ing he ene gy
ansi ion [5]. CO
2
cap u e and s o age (CCS) and di ec ai cap u e
(DAC) a e among hose echnologies. Reducing CO
2
emissions, espe-
cially o CO
2
in ensi e indus ies, would be impossible a sho - e m
wi hou CCS echnology. Howe e , he widesp ead deploymen o
hese echnologies will equi e signi ican cos s educ ion and pe o -
mance imp o emen s.
In ecen yea s, inc easing s udies ha e been ca ied ou on CO
2
cap u e and u iliza ion (CCU). A e seques a ion, CO
2
is co- eed in he
syn hesis o alue-added p oduc s, such as me hane, e hane, p opane,
syngas o liquid chemicals [1,6,7]. Mo e ecen ly, he in eg a ed CO
2
cap u e and u iliza ion echnology has been p oposed (ICCU), which
educes he cos o he o e all p ocess by elimina ing anspo a ion and
s o age o CO
2
[8]. ICCU achie es in si u CO
2
adso p ion and con e sion
using dual unc ion ma e ials (DFMs), which consis o CO
2
adso ben
and ca aly ic phases. Fi s , DFMs cap u e CO
2
om lue gas (4–14 ol%
CO
2
) o e ec i ely educe ca bon emissions. When he ca bon cap u e
p ocess is comple ed, he eed gas is swi ched o a educing enewable
agen o he con e sion o he adso bed CO
2
o syn he ic uels.
Depending on he educing agen , he composi ion o he DFM and
he ope a ing condi ions, he adso bed CO
2
can be ans o med in o
syngas o me hane [9]. The ans o ma ion in o syngas can ake place by
d y e o ming o me hane (ICCU-DRM) [10,11] o by e e se wa e gas
shi (ICCU-RWGS) [12,13]. On he o he hand, he ans o ma ion in o
me hane (Eq. 1) occu s by he o al hyd ogena ion o CO
2
(ICCU-me-
hana ion) [14–16]. I he con e sion o he cap u ed CO
2
is ca ied ou
using hyd ogen om enewable ene gies by means o elec olysis o
wa e , i would also be an e ec i e solu ion o s o e excess elec ical
ene gy in chemical p oduc s. The e o e, ICCU-me hana ion echnology
is a sus ainable echnology app oaching a closed ca bon cycle wi hou
ne CO
2
emissions o he a mosphe e. Besides, ICCU can add ess he
p oblem o in insic in e mi ency o enewable sou ces [17].
* Co esponding au ho .
E-mail add ess: [email p o ec ed] (J.R. Gonz´
alez-Velasco).
Con en s lis s a ailable a ScienceDi ec
Jou nal o CO2 U iliza ion
jou nal homepage: www.else ie .com/loca e/jcou
h ps://doi.o g/10.1016/j.jcou.2022.101922
Recei ed 28 Janua y 2022; Accep ed 1 Feb ua y 2022
Jou nal o CO2 U iliza ion 58 (2022) 101922
2
CO
2
+4H
2
⇆CH
4
+2H
2
O (1)
The i s ICCU-me hana ion wo k was published in 2015 [18] and
since hen, he numbe o publica ions is g owing exponen ially [9,
19–21]. DFMs a e commonly based on an alkaline o alkaline-ea h
compound, such as Na [22,23], Ca [18,24], Mg [16,25] o K [16,26].
Those elemen s a e used as CO
2
adso ben s, due o he a ini y o acid
CO
2
o bound o a basic elemen and o m he co esponding ca bona e.
On he o he hand, Ru [22,27], Ni [26,28] o Rh [29] a e used as ca alys
due o hei abili y o assis he CO
2
hyd ogena ion o CH
4
. Bo h phases
a e suppo ed on a high su ace a ea ca ie , being γ-Al
2
O
3
he mos
widely used. A ellano-T e i˜
no e al. [30] s udied di e en candida es as
ca ie ma e ials o he DFM applica ion: CeO
2
(high and low su ace
a ea), Na-Zeoli e-X, H-Mo deni e Zeoli e, SiC, SiO
2
and mixed oxides
(CeO
2
-Z O
2
, Z O
2
-Y
2
O
3
) which could enhance he ope a ion; al hough
hey concluded ha γ-Al
2
O
3
achie ed he mos p omising esul s among
hem. Some non-suppo ed ca alys s ha e been ecen ly p oposed [31].
In p e ious wo ks [22,24,28], i has been concluded ha he p es-
ence o Ca-based compounds as adso ben p o ides si es wi h high ba-
sici y s eng h in o he DFM and he e o e he CH
4
o ma ion is a o ed
a high empe a u es. On he o he hand, he p esence o Na-based
compounds, as adso ben , leads o he appea ance o medium s eng h
basic si es and he e o e he CH
4
o ma ion is a o ed a in e media e
ope a ing empe a u es.
On he o he hand, ecen ly Al-Mamoo i e al. [32] s udied he
de elopmen o Ca-based adso ben s doped wi h Na and K. The au ho s
concluded ha he addi ion o K and Na imp o ed he pe o mance o
CaO since hese ma e ials p esen ed high CO
2
adso p ion capaci y, as
kine ics, and good s abili y abo e 300 ◦C. Lee e al. [33] pe o med a
compa a i e s udy o adso p ion and egene a ion kine ics o con en-
ional and Na
2
CO
3
-doped CaO-adso ben s. In his s udy, he au ho s
concluded ha he addi ion o sodium ca bona e in o calcium adso ben
can imp o e he cyclic s abili y o CO
2
adso p ion wi h as kine ics.
In his wo k, he join p esence o Na and Ca in Ru-based DFMs is
s udied, which o ou knowledge has no been published o da e. I is
analyzed whe he i is possible o modula e he basici y o a DFM and
enhance he CH
4
p oduc ion in an ex ended empe a u e ange by
a ying he Na
2
CO
3
/CaO a io o a o al adso ben con en o 16%. Fo
his, a se o DFMs 4%Ru-x%Na
2
CO
3
-y%CaO/γ-Al
2
O
3
(x/y =16/0, 12/
4, 8/8, 4/12 and 0/16) a e p epa ed by he we ness imp egna ion
me hod. DFMs a e widely cha ac e ized and hei beha io is e alua ed
in cycles o CO
2
adso p ion and hyd ogena ion o CH
4
.
2. Expe imen al
2.1. Ca alys p epa a ion
All samples we e p epa ed by we ness imp egna ion. Fi s , app o-
p ia ed amoun o Ca(NO
3
)
2
⋅4H
2
O (Me ck) and/o Na
2
CO
3
(Riedel de-
Ha¨
en) was imp egna ed o e γ-Al
2
O
3
(Sain Gobain). The imp egna ed
powde was d ied a 120 ◦C o e nigh and hen calcined a 400 ◦C o 4 h
(1 ◦C min
-1
). A e wa ds, Ru(NO)(NO
3
)
2
(Sigma Ald ich) was imp eg-
na ed o e x%Na
2
CO
3
-y%CaO/γ-Al
2
O
3
(x/y =16/0, 12/4, 8/8, 4/12
and 0/16). A e d ying a 120 ◦C, he samples we e s abilized by
calcining again a 400 ◦C o 4 h (1 ◦C min
-1
). The nominal loading o
u henium was 4%.
2.2. X ay di ac ion (XRD)
X- ay di ac ion spec a we e ob ained in a Philips PW1710 di ac-
ome e . The samples we e inely g ound and we e subjec ed o Cu K
α
adia ion in a con inuous scan mode om 5◦ o 70◦2θ wi h 0.02 pe
second sampling in e al.
2.3. N
2
adso p ion-deso p ion
The N
2
adso p ion-deso p ion analysis we e ca ied ou a he ni-
ogen boiling empe a u e (−196 ◦C) using an au oma ed gas adso p-
ion analyse (T iS a II, Mic ome i ics). The samples we e p e-pu ged
wi h ni ogen o 10 h a 300 ◦C using Sma P ep degas sys em
(Mic ome i ics).
2.4. H
2
chemiso p ion
Ru henium dispe sion was de e mined using he H
2
chemiso p ion
me hod in a Mic ome i ics ASAP 2020 equipmen . P io o he expe i-
men s, he samples (0.2 g) we e educed wi h pu e H
2
o 2 h a 400 ◦C.
A e ha , he samples we e degasi ied a he same empe a u e o 90
min. Finally, H
2
was dossed o ob aining he adso p ion iso he m a
35 ◦C.
2.5. T ansmission elec on mic oscopy (TEM)
The mo phology o he samples was analysed by ansmission elec-
on mic oscopy (TEM) in a JEM-1400 Plus ins umen using a ol age o
100 kV. The educed samples we e dispe sed in dis illa ed wa e ul a-
sonically, and he solu ions we e hen d opped on coppe g ids coa ed
wi h lacey ca bon ilm. In addi ion, STEM measu emen s we e ca ied
ou a FEI Ti an Cubed G2 60–300 elec on mic oscope a 200 kV. The
ins umen is equipped wi h a high-b igh ness X-FEG Scho ky ield
emission elec on gun, a monoch oma o , CEOS GmbH sphe ical abe -
a ion (Cs) co ec o on he image side and a Supe -X EDX sys em unde
high annula da k ield (HAADF) de ec o o Z con as imaging in
STEM condi ions (came a leng h o 185 mm). The nominal size o he
elec on p obe used o STEM and EDX maps was 0.5 nm and he p obe
cu en 170 pÅ and he semicon e gence angle was 14 m ad. High-angle
annula da k- ield HAADF STEM images we e collec ed wi h an inne
de ec o adius o 63.5 m ad. The DFM we e dispe sed in e hanol ul-
asonically, and he solu ions we e hen d opped on coppe g ids coa ed
wi h lacey ca bon ilm.
2.6. Tempe a u e-p og ammed educ ion (H
2
-TPR)
The educibili y o he samples was in es iga ed by empe a u e
p og ammed educ ion (H
2
-TPR) in a Mic ome i ics Au oChem II equip-
men . The samples (0.1 g) was loaded in a qua z ube eac o and
p e ea ed a 350 ◦C o 15 min unde 5% O
2
/He (30 ml min
-1
) and hen
cooled down o 35 ◦C. The educing gas low was 30 ml min
-1
o 5% H
2
/
A and he empe a u e was inc eased om 30 ◦C o 950 ◦C wi h a
hea ing a e o 10 ◦C min
−1
. The wa e o med du ing educ ion was
apped using a cold ap and he hyd ogen consump ion was con inu-
ously moni o ed wi h a TCD de ec o .
2.7. Tempe a u e-p og ammed deso p ion (CO
2
-TPD)
The CO
2
-TPD expe imen s we e ca ied ou in a Mic ome i ics
Au oChem II equipmen . The samples (0.1 g) we e p e ea ed a 400 ◦C
unde 5% H
2
/A (50 ml min
-1
) o he comple e educ ion o he samples
be o e he expe imen and hen cooled down o 50 ◦C. Then, he samples
we e exposed o a gas s eam composed o 5% CO
2
/He (50 ml min
-1
) o
1 h a RT o sa u a e he ca alys wi h CO
2
. Subsequen ly, he samples
we e exposed o He (50 ml min
-1
) o 90 min o emo e he physically
adso bed CO
2
and inally hey we e hea ed om RT o 1000 ◦C a 10 ◦C
min
-1
and he CO
2
eleased was measu ed by mass spec ome y (HIDEN
ANALYTICAL HPR-20 EGA).
2.8. . Reac ion es s
Cycles o CO
2
adso p ion and hyd ogena ion o CH
4
we e ca ied ou
in a down- low s ainless s eel eac o . In each ac i i y es , 1 g o DFM
A. Be mejo-L´
opez e al.
Jou nal o CO2 U iliza ion 58 (2022) 101922
3
was loaded wi h a pa icle size be ween 0.3 and 0.5 mm. The ope a ing
empe a u e was measu ed h ough a he mocouple placed in he cen e
o he ca aly ic bed. P io o he analysis, he DFMs we e educed wi h a
s eam composed o 10% H
2
/A , p og essi ely inc easing he empe a-
u e om oom empe a u e o 400 ◦C, and inally he empe a u e was
main ained o 1 h. Then he DFM was cooled o 280 ◦C and he eac ion
empe a u e was a ied be ween 280 and 400 ◦C, wi h in e als o 30 ◦C.
In he adso p ion (s o age) pe iod, a s eam composed o 10% CO
2
/A
was ed o 1 min, ollowed by a pu ge wi h A o 2 min o emo e
weakly adso bed CO
2
and p e en he s eams om mixing. Nex , in he
hyd ogena ion (me hana ion) pe iod, a s eam composed o 10% H
2
/A
was ed o 2 min, ollowed by a pu ge wi h A o 1 min be o e s a ing
he adso p ion pe iod again. Se e al iso he mal cycles ha e been ca ied
ou a each empe a u e un il cycle- o-cycle s eady s a e is eached.
Th oughou he en i e expe imen , he o al low a e was se a 1200 ml
min
-1
. This low co esponds o a space eloci y o 45,000 h
-1
. The lue
gas composi ion was con inuously moni o ed o quan i a i e analysis o
CO
2
, CH
4
, CO and H
2
O wi h a Mul iGas 2030 FT-IR analyse .
The amoun o CO
2
s o ed was calcula ed om Eq. (2). Fo ha , he
amoun ha lea es he eac o was sub ac ed om he amoun ed. To
de e mine he amoun o CO
2
ed, he s eam om he eed sys em was
led di ec ly o he analyse . This p o ile co esponds o he ac ual CO
2
inpu ha was ed o he eac o .
STO.CO2(
μ
mol g−1)=1
W∫
0[Fin
CO2( ) − Fou
CO2( )]d (2)
On he o he hand, he CH
4
, CO and H
2
O p oduc ions we e calcu-
la ed om he ollowing exp essions:
YCH4(
μ
mol g−1)=1
W∫
0
Fou
CH4( )d (3)
YCO(
μ
mol g−1)=1
W∫
0
Fou
CO( )d (4)
YH2O(
μ
mol g−1)=1
W∫
0
Fou
H2O( )d (5)
CH
4
selec i i y is de e mined by ela ing he CH
4
and CO p oduc ions
since hey we e he only ca bon based p oduc s ha we e de ec ed:
SCH4(%) = YCH4
YCH4+YCO
×100 (6)
Finally, he ca bon balance check was ca ied ou om he ollowing
exp ession:
sCB(%) = (YCH4+YCO
STO.CO2−1)×100 (7)
3. Resul s and discussion
3.1. Tex u al p ope ies and phase iden i ica ion
Table 1 lis s he comple e o mula ion o he samples p epa ed in his
wo k and he nomencla u e used o e e hem. In addi ion, i summa-
izes he alues o speci ic su ace a ea (S
BET
), po e diame e (d
p
) and
po e olume (V
p
) o bo h he calcined and educed samples. The γ-Al
2
O
3
used has a speci ic su ace a ea o 218 m
2
g
-1
and wi h he inco po a ion
o 4% u henium he su ace a ea is educed o 204 m
2
g
-1
. This educ-
ion is mainly due o he dec ease in mesopo ous solid con en (γ-Al
2
O
3
).
On he o he hand, when 4% u henium and 16% adso ben a e inco -
po a ed, ei he Na
2
CO
3
, CaO o a combina ion o bo h, he su ace a ea
is educed o 105–113 m
2
g
-1
. The con ibu ion o alumina o he su ace
a ea o ully o mula ed DFMs would be 175 m
2
g
-1
depending on i s
composi ion (80% γ-Al
2
O
3
by weigh ), he e o e an addi ional phe-
nomenon is e idenced ha dec eases he speci ic su ace a ea. This
addi ional dec ease is assigned o he blocking o he smalle po es by
he p esence o he adso ben phase o phases.
On he o he hand, i he alues o he calcined samples a e compa ed
wi h he educed samples, i can be seen ha he e e ence sample only
wi h u henium (Ru- e ) sligh ly educes i s speci ic su ace a ea a e
he educ ion p e ea men . Howe e , o he DFMs he su ace a eas
inc ease o 122–131 m
2
g
-1
. As will be explained la e by XRD, peaks
belonging o ni ogenous compounds a e de ec ed in he calcined sam-
ples, which disappea a e he educ ion p e ea men . The e o e, he
inc ease in su ace a ea is assigned o he emo al o esidual ni a es
om he DFMs. In ag eemen wi h hese esul s, p e ious wo ks [22,24]
ha e de ec ed he p esence o ni ogenous compounds in he exhaus
gases o he H
2
-TPR expe imen s o simila DFMs.
The adso p ion and deso p ion iso he ms o N
2
a −196 ◦C o he
calcined and educed samples a e ype IV acco ding o he IUPAC clas-
si ica ion, which co esponds o mesopo ous solids. The iso he ms p e-
sen a hys e esis cycle H1 ha indica es he p esence o egula po es in
shape and size. Rep esen a i ely, Fig. S1a shows he iso he ms o he
calcined and educed Ru-8Na/8Ca DFM. I he iso he ms a e compa ed
o each o he , i can be seen how he educed DFM adso bs a highe
olume o N
2
i espec i e he ela i e p essu e. Consequen ly, he
exposed su ace a ea calcula ed by he BET me hod is highe o he
educed sample (118 m
2
g
-1
) han o he non- educed o simply
calcined one (111 m
2
g
-1
).
Alumina has a po e diame e o 107 Å and a po e olume o
0.604 cm
3
g
-1
. Wi h he addi ion o he adso ben phases oge he wi h
he me al, he po e diame e inc eases and he po e olume is educed.
This phenomenon con i ms he blocking o he smalle po es due o he
p esence o he adso ben phase o phases. On he o he hand, i he
alues o he calcined samples and he educed samples a e compa ed, i
can be seen how a e he educ ion p e ea men , bo h d
p
and V
p
in-
c ease. This again con i ms he p esence o esidual ni a es ha a e
he calcina ion s ep a e pa ially o o ally blocking some po es o he
alumina and ha a e elimina ed a e he educ ion p e ea men .
The po e size dis ibu ion o he calcined and educed samples has a
unimodal dis ibu ion cen e ed a 80–120 Å. Rep esen a i ely, Fig. S1b
shows he po e size dis ibu ion o he calcined and educed Ru-8Na/
8Ca DFM. The educ ion p e ea men shi s he dis ibu ion owa ds
Table 1
Nomencla u e and ex u al p ope ies o he calcined and educed samples.
Sample Nomencla u e S
BET
,
m
2
g
-1
d
p
,
Å
V
p
,
cm
3
g
-
1
S
BET
,
m
2
g
-1
d
p
,
Å
V
p
,
cm
3
g
-
1
calcined samples educed samples
γ-Al
2
O
3
Al
2
O
3
218 107 0.604 – – –
4%Ru/
γ-Al
2
O
3
Ru- e 204 104 0.546 199 108 0.549
4%Ru-
16%
Na/
γ-Al
2
O
3
Ru-16Na 105 126 0.341 128 135 0.444
4%Ru-
12%
Na-4%
Ca/
γ-Al
2
O
3
Ru-12Na/4Ca 113 123 0.359 125 130 0.416
4%Ru-
8%Na-
8%Ca/
γ-Al
2
O
3
Ru-8Na/8Ca 111 118 0.336 131 130 0.439
4%Ru-
4%Na-
12%
Ca/
γ-Al
2
O
3
Ru-4Na/12Ca 113 112 0.326 122 131 0.412
4%Ru-
16%
Ca/
γ-Al
2
O
3
Ru-16Ca 109 108 0.304 122 121 0.382
A. Be mejo-L´
opez e al.
Jou nal o CO2 U iliza ion 58 (2022) 101922
4
highe alues which, is assigned o he elimina ion o he esidual ni-
a es ha a e pa ially blocking he po es. On he o he hand, he a ea
unde he cu e inc eases in he educed DFM, indica ing a g ea e po e
olume, as obse ed in Table 1.
Fig. 1a shows he X- ay di ac ion spec a o he alumina, he
e e ence sample only wi h u henium (Ru- e ) and he calcined DFMs.
Alumina exhibi s a di ac ion p o ile wi h low in ensi y b oad peaks
cha ac e is ic o an amo phous solid. Wi h he addi ion o 4% Ru (Ru-
e ), h ee in ense peaks appea a 28.0, 35.1 and 54.2◦2θ, assignable o
RuO
2
. On he o he hand, wi h he join addi ion o he me al and he
adso ben phase o phases, he h ee peaks co esponding o RuO
2
a e
also de ec ed, howe e , addi ional peaks appea . In he DFM Ru-16Ca,
new peaks a e de ec ed a 11.1 and 18.9◦2θ belonging o Ca
6
Al
2
O
6
(-
NO
3
)
6
⋅xH
2
O and in all DFMs wi h sodium an addi ional peak appea s a
31.9◦2θ belonging o NaNO
3
.
The X- ay di ac ion spec a o he alumina, he e e ence sample
only wi h u henium (Ru- e ) and he DFMs a e he educ ion p e-
ea men a e shown in Fig. 1b. In he spec um o he Ru- e sample, he
peaks belonging o RuO
2
disappea and only a low in ensi y peak
belonging o me allic u henium was de ec ed a 44.0◦2θ. Cimino e al.
[27] o a educed Ru/γ-Al
2
O
3
ca alys also obse ed a simila beha io .
In he DFMs he peaks belonging o RuO
2
also disappea and he peak
co esponding o me allic u henium is de ec ed. On he o he hand, he
peaks belonging o ni ogenous species disappea and no addi ional
peaks belonging o he adso ben s a e dis inguished. The e o e, based on
he XRD spec a i is sugges ed ha he educ ion p e ea men is
adequa e o educe he u henium and decompose he esidual ni a es.
3.2. Ru henium dispe sion
The u henium dispe sion o he samples is de e mined by H
2
chemiso p ion conside ing a s oichiome y H/Ru =1 [34]. Table 2
shows he dispe sion alues (D
m
) o he e e ence sample only wi h
u henium (Ru- e ) and he p epa ed DFMs. The Ru- e sample has a
dispe sion o 11.5%, and in gene al, he dispe sion inc eases wi h he
addi ion o he adso ben o 19.6–24.8% wi h he excep ion o he DFM
Ru-16Ca, which p esen s a alue o 9.8%. In own p e ious wo k [22], we
s udied he e ec o he adso ben loading on DFMs based on u henium
and on CaO o Na
2
CO
3
as adso ben s. We concluded ha he p esence o
CaO penalizes he dispe sion o u henium while he p esence o Na
2
CO
3
p omo es i . The e o e, based on H
2
chemiso p ion esul s, in addi ion o
concluding ha he p esence o Na
2
CO
3
p omo es dispe sion, his p o-
mo ion inc eased by he join p esence o bo h adso ben s. No e ha he
highes dispe sion alues a e p esen ed by DFMs based on sodium and
calcium oge he . Speci ically, he highes dispe sion alue (24.8%) is
p esen ed by he DFM Ru-8Na/8Ca.
F om he dispe sion alues, an a e age pa icle size is es ima ed
conside ing sphe ical pa icles. Logically, he smalles pa icles a e
shown by he DFMs based on bo h adso ben s and he DFM Ru-8Na/8Ca
p esen s he smalles size o 5.4 nm.
Fig. 2 shows he TEM mic og aphs o he e e ence sample only wi h
u henium (Ru- e ) and he p epa ed DFMs. The da kes ci cula a eas
co espond o u henium pa icles due o he highe a omic numbe o
Ru wi h espec o o he elemen s in he DFM. Clea ly, o he Ru- e
sample and o he DFM Ru-16Ca, la ge pa icles a e obse ed
compa ed o he DFMs ha p esen sodium in hei composi ion. How-
e e , o he es o he DFMs, no signi ican di e ences a e obse ed
be ween hem in he mic og aphs. A his poin , in o de o make a mo e
exhaus i e compa ison, an a e age pa icle size is es ima ed by TEM.
Fo his, he pa icles a e conside ed o ha e a ci cula shape and a leas
100 pa icles we e coun ed o each sample and he es ima ed alues a e
Fig. 1. XRD di ac ion spec a o he alumina, he e e ence sample only wi h u henium (Ru- e ) and o he DFMs a e calcina ion (a) and a e he educ ion
p e ea men (b). Di ac ion peaks belonging o Ca
6
Al
2
O
6
(NO
3
)
6
⋅xH
2
O a e iden i ied wi h (+) and belonging o NaNO
3
wi h (o).
Table 2
Ru dispe sion and pa icle size es ima ed om H
2
chemiso p ion, TEM mic o-
g aphs and XRD spec a.
Sample D
m
, % d
c
(Ru-H
2
), nm d
c
(Ru-TEM), nm d
c
(Ru-XRD), nm
Ru- e 11.5 11.6 9.5 13.8
Ru-16Na 19.6 6.8 8.7 9.0
Ru-12Na/
4Ca
21.1 6.3 7.8 –
Ru-8Na/8Ca 24.8 5.4 6.4 –
Ru-4Na/
12Ca
20.4 6.5 7.0 –
Ru-16Ca 9.8 13.6 10.0 11.3
A. Be mejo-L´
opez e al.
Jou nal o CO2 U iliza ion 58 (2022) 101922
5
shown in Table 2. I is con i med ha he DFMs based on Na
2
CO
3
ha e
lowe sizes and mo e speci ically hose based on bo h adso ben s.
Fu he mo e, again he lowes alue (6.4 nm) is p esen ed by he DFM
Ru-8Na/8Ca.
Fig. 3 shows a HAADF image and EDX maps o Al, Na, Ca and Ru o
educed DFM Ru-8Na/8Ca. I is obse ed how bo h adso ben s (Na and
Ca) a e homogeneously dis ibu ed o e he alumina su ace. Howe e ,
u henium is in he o m o sphe ical pa icles as seen in TEM images
(Fig. 2). Addi ionally, he a e age pa icle size o Ru is de e mined om
he EDX map (Fig. 3e) and a alue o 5.5 nm is ob ained, simila o ha
ob ained by TEM images (6.4 nm) and by H
2
chemiso p ion (5.4 nm).
Finally, a c ys al size is also es ima ed om he u henium peak
(44.0◦2θ) o he di ac ion p o iles o Fig. 1b and is summa ized in
Table 2. This size can only be es ima ed o he sample Ru- e and o
DFMs based solely on calcium o sodium, since o DFMs based on bo h
adso ben s he peak is no well de ined. In gene al, simila ends a e
ob ained by he h ee echniques used, so i is concluded ha he p es-
ence o sodium p omo es he dispe sion o u henium and his p omo-
ion is g ea e when bo h adso ben s a e p esen .
3.3. P og ammed empe a u e echniques (H
2
-TPR and CO
2
-TPD)
The educibili y o he samples is e alua ed by empe a u e p o-
g ammed educ ion wi h H
2
. Fig. 4 shows he H
2
consump ion p o iles in
he H
2
-TPR expe imen s o he e e ence sample only wi h u henium
(Ru- e ) and he p epa ed DFMs. The Ru- e sample shows a hyd ogen
consump ion cen e ed a 100 ◦C, which is assigned o he educ ion o
RuO
2
, in line wi h he obse ed by XRD. This assignmen was con i med
by he H
2
/Ru a io de e mined which is close o 2. Wi h he inco po-
a ion o he adso ben phase o phases, H
2
consump ion shi s o highe
empe a u es and inc eases signi ican ly. The H
2
/Ru a io is be ween 12
and 15 o he i e DFMs s udied. This alue is much highe han he
alue o 2 ob ained by he e e ence sample. The e o e, an addi ional
phenomenon ha is consuming H
2
is e idenced.
In o de o collec mo e in o ma ion and cla i y he H
2
consump ion
pa e ns, he exhaus gas om he H
2
-TPR expe imen s was analysed by
mass spec ome y, as we al eady did in ou p e ious wo ks [22,24]. As
an example, Figu e S2 shows he e olu ion o NO, NH
3
and CH
4
du ing a
H
2
-TPR pe o med up o 400 ◦C wi h an iso he mal s ep o 1 h o he
Fig. 2. TEM mic og aphs o he e e ence sample only wi h u henium (Ru- e ) and o he DFMs wi h di e en Na
2
CO
3
/CaO a ios.
Fig. 3. STEM image (a) and EDX maps o Al (b), Na (c), Ca (d) and Ru (e) o educed DFM Ru-8Na/8Ca.
A. Be mejo-L´
opez e al.
Jou nal o CO2 U iliza ion 58 (2022) 101922
6
Ru-8Na/8Ca DFM. Ni ogen monoxide (NO) is he i s p oduc de ec ed.
The NO o ma ion is ela ed o he decomposi ion o esidual ni a es
belonging o he adso ben and noble me al p ecu so s ha ha e no
been comple ely decomposed du ing he calcina ion s ep.
The onse o NH
3
o ma ion is de ec ed a highe empe a u es. The
NH
3
o ma ion equi es he noble me al in i s me allic s a e, and
he e o e he NH
3
o ma ion can be used as an indi ec way o de e mine
he empe a u e a which u henium begins o educe. On he o he
hand, he CH
4
o ma ion is a ibu ed o he hyd ogena ion o he CO
2
adso bed in he samples, due o exposu e o he en i onmen be o e he
expe imen . The CH
4
o ma ion by hyd ogena ion o CO
2
equi es he
p esence o me allic u henium. The e o e, he s a ing empe a u e o
he de ec ion o CH
4
coincides wi h ha o NH
3
.
As a conclusion, he inc ease o he H
2
consump ion o he p epa ed
DFMs (wi h espec o he Ru- e sample) can be unequi ocally a ib-
u ed o he educ ion o esidual ni a es and he hyd ogena ion o
ca bona es. Fu he mo e, he p esence o esidual ni a es delays he
educ ion o u henium. As can be seen in Figu e S2, NO is de ec ed i s
and hen NH
3
. Since he s a o NH
3
p oduc ion is consis en wi h he
s a o u henium educ ion, i sugges ha he p esence o esidual
ni a es is delaying he educ ion o u henium. This was con i med wi h
an addi ional H
2
-TPR/O
2
-TPO/H
2
-TPR expe imen (no shown). The
absence o ni a es in he second H
2
-TPR and he shi o lowe em-
pe a u es om he i s o he second H
2
-TPR con i ms ha he p esence
o ni a es delays he educ ion o u henium.
Likewise, addi ional expe imen s a e ca ied ou by pe o ming an
iso he mal s ep a 400 ◦C o one hou . Figu e S3 shows he H
2
con-
sump ion du ing hese expe imen s. No H
2
consump ion abo e 400 ◦C is
de ec ed. These esul s demons a es ha u henium is comple ely
educed, con i ming ha he educ ion p e ea men chosen p io o he
ac i i y es s is adequa e.
The basici y o he samples is e alua ed by empe a u e p og ammed
deso p ion o CO
2
(CO
2
-TPD). Fig. 5a shows he e olu ion o he CO
2
signal measu ed by a mass spec ome e (m/e =44) as a unc ion o
empe a u e o alumina, he e e ence sample only wi h u henium (Ru-
e ) and he p epa ed DFMs. Alumina exhibi s a small deso p ion peak a
low empe a u e. The inco po a ion o 4% u henium (Ru- e ) does no
modi y he p o ile, also ob aining only a small deso p ion peak a low
empe a u e. Po a e al. [15] also did no obse e a modi ica ion o he
deso p ion p o ile wi h he inco po a ion o Ru in o alumina. This peak
is assigned o he decomposi ion o bica bona es ha esul om he
in e ac ion be ween CO
2
and he su ace hyd oxyl g oups o alumina
[35,36].
The inco po a ion o an adso ben phase o phases signi ican ly in-
c eases he amoun o CO
2
deso bed (Fig. 5a). A his poin , depending
on he deso p ion empe a u e, weak, medium and s ong basici y si es
a e dis inguished. Weak basic si es a e uns able and easily decomposed
below 250 ◦C. The medium basic si es a e decomposed be ween 250 and
700 ◦C while he s ong basic si es a e highly s able and decomposed a
empe a u es abo e 700 ◦C [37]. Po a e al. [16] analyzed by FTIR he
na u e o he CO
2
adso bed on Ru-K/Al
2
O
3
and Ru-Ba/Al
2
O
3
DMs. Bands
belonging o ca bona es adso bed on he adso ben phases a e only
obse ed. The au ho s did no obse e ypical su ace species o CO
2
adso p ion on he alumina suppo , so hey concluded ha he adso ben
phase co e s he su ace o he alumina.
Fig. 5b shows he con ibu ion in
μ
mol g
-1
o each ype o basici y o
DFMs. DFM Ru-16Na clea ly has he highes weak basici y and DFM Ru-
16Ca he highes s ong basici y. These esul s a e in ag eemen wi h
hose p e iously ob ained, in which he p esence o Na
2
CO
3
p omo es
he weak basici y and he p esence o CaO he s ong basici y [22,24].
Fig. 4. H
2
-TPR pa e ns o he DFMs wi h di e en Na
2
CO
3
/CaO a ios. Ru- e
sample is also included as e e ence.
Fig. 5. (a) CO
2
-TPD pa e ns o he DFMs wi h di e en Na
2
CO
3
/CaO a ios.
Al
2
O
3
and Ru- e samples is also included as e e ence. (b) E olu ion o he
weak, medium and s ong basici y in
μ
mol o CO
2
g
-1
o he DFMs wi h
di e en Na
2
CO
3
/CaO a ios.
A. Be mejo-L´
opez e al.
Jou nal o CO2 U iliza ion 58 (2022) 101922
7
Howe e , in DFMs based on bo h adso ben s, he medium basici y is
p edominan and, speci ically, he Ru-8Na/8Ca DFM is he one wi h he
highes medium basici y. The e o e, by adding Na
2
CO
3
and CaO
oge he , he basici y s eng h o DFMs can be modula ed. This aspec is
o special in e es gi en ha in he cyclical p ocess o CO
2
adso p ion
and hyd ogena ion o CH
4
, only he CO
2
ha can be bo h s o ed and
eleased, a a gi en empe a u e, is he one ha pa icipa es in he
ope a ion.
3.4. Ca aly ic ac i i y in successi e cycles o CO
2
adso p ion and
hyd ogena ion o CH
4
Fig. 6 shows he e olu ion o he concen a ions o CO
2
, CH
4
, H
2
O
and CO in a cycle o CO
2
adso p ion and hyd ogena ion o CH
4
o he
DFM Ru-8Na/8Ca ope a ing a 400 ◦C. No e ha he CO concen a ion is
e e ed o he igh o dina e axis in ppm. In addi ion, Fig. 6 also shows
he CO
2
concen a ion p o ile ed (g ay line). In he adso p ion pe iod, a
s eam wi h 10% CO
2
is ed o he eac o o 1 min, ollowed by a 2 min
pu ge. The hyd ogena ion pe iod hen begins by admi ing 10% H
2
o
he eac o o e 2 min and he cycle is ended wi h an addi ional 1 min
pu ge.
In he adso p ion pe iod, he CO
2
concen a ion p og esi ely in-
c eases o a alue o 10%, co esponding o ha o he eed. I he inpu
CO
2
signal ( ed line) is compa ed wi h he ou pu CO
2
signal (g ay line),
i can be seen ha du ing he i s momen s o he pe iod he adso p ion
is o al.
A his poin , i is impo an o ake in o accoun ha he decom-
posi ion empe a u e o Na
2
CO
3
is d as ically educed when suppo ed
on la ge su ace ma e ials, such as Al
2
O
3
. Nguyen e al. [38] obse ed by
TGA and XRD he decomposi ion o he bulk Na
2
CO
3
a 850 ◦C, while
he Na
2
CO
3
suppo ed on alumina began o decompose a 135 ◦C. In
ac , hey eco ded a negligible weigh loss o Na
2
CO
3
suppo ed in
alumina abo e 400 ◦C, e ealing comple e decomposi ion.
The e o e, whene e he decomposi ion o Na
2
CO
3
o "Na
2
O" species
occu s, he adso p ion o CO
2
in he sodium phase can be desc ibed as
he ca bona ion o an alkaline ea h compound [38–40]. Consequen ly,
as he DFM is composed o bo h adso ben s (Na and Ca), he adso p ion
o CO
2
du ing he i s momen s o he pe iod can occu h ough Eq. (8)
and Eq. (9):
CaO +CO
2
⇆ CaCO
3
(8)
Na
2
O +CO
2
⇆ Na
2
CO
3
(9)
A e ha , CO
2
begins o be de ec ed a he ou le ( ed line) and
su p isingly he H
2
O signal (blue line) inc eases. The p esence o CO
2
in
he exhaus gases indica es ha he adso p ion si es begin o sa u a e
un il eaching he alue o he eed (10%) a 0.5 min o he adso p ion
pe iod. The e o e, he 1 min du a ion o he adso p ion pe iod is su i-
cien o comple ely sa u a e he DFM. On he o he hand, as wa e is also
de ec ed, CO
2
is also adso bed on hyd a ed si es:
Ca(OH)
2
+CO
2
⇆ CaCO
3
+H
2
O (10)
2NaOH +CO
2
⇆ Na
2
CO
3
+H
2
O (11)
Consequen ly, CO
2
adso p ion can occu on he oxide si es (Eq. 8 and
Eq. 9) and on he hyd a ed si es (Eq. 10 and Eq. 11) and since a he
beginning o he pe iod, he adso p ion is o al and H
2
O is no de ec ed,
he oxide si es a e p oposed as mo e ac i e o he CO
2
adso p ion. In he
pu ge, he cu e belonging o he ou le CO
2
concen a ion ( ed line)
p esen s a highe alue compa ed o he inle one (g ay line). This ac
indica es ha pa o he weakly adso bed CO
2
is eleased du ing he
pu ge. A his poin , he amoun o CO
2
s o ed is de e mined om Eq. (2)
and esul s in 352 µmol g
-1
(Table 3).
Addi ionally, in he adso p ion pe iod a small CO p oduc ion is also
de ec ed. No e ha he scale is e e ed o he igh o dina e axis in
ppms. The CO p oduc ion is assigned o he pa ial hyd ogena ion o he
CO
2
ed by chemiso bed hyd ogen on he me al si es. The amoun o CO
p oduced and he amoun o H
2
O eleased a e de e mined by Eq. (4) and
Eq. (5) and he alues a e collec ed in Table 3.
In he hyd ogena ion pe iod (Fig. 6), an ins an aneous peak o CH
4
is
de ec ed, howe e he peak o H
2
O is delayed by i s adso p ion on he
basic si es o he adso ben . Consequen ly, in his pe iod he ca bona es
a e decomposed by he p esence o H
2
(Eq. 12 and Eq. 13), CO
2
is hy-
d ogena ed o CH
4
(Eq. 1) and pa o he wa e emains adso bed
o ming hyd oxides (Eq. 14 and Eq. 15).
S ep 1a: CaCO
3
⇆ CaO +CO
2
(12)
S ep 1b: Na
2
CO
3
⇆ Na
2
O +CO
2
(13)
S ep 2: CO
2
+4H
2
⇆ CH
4
+2H
2
O (1)
Fig. 6. CO
2
, H
2
O, CH
4
and CO concen a ion p o iles du ing one cycle o CO
2
adso p ion and hyd ogena ion o CH
4
o he DFM Ru-8Na/8Ca ope a ing a 400 ◦C.
Table 3
S o ed CO
2
and CH
4
, CO and H
2
O p oduc ions du ing he adso p ion and hy-
d ogena ion pe iods o he DFM Ru-8Na/8Ca ope a ing a 400 ◦C.
CO
2
s o age,
µmol g
-1
Y
CH4
, µmol
g
-1
Y
CO
, µmol
g
-1
Y
H2O
, µmol
g
-1
Adso p ion pe iod 352 – 14 183
Hyd ogena ion
pe iod
– 357 7 556
A. Be mejo-L´
opez e al.
Jou nal o CO2 U iliza ion 58 (2022) 101922
8
S ep 3a: CaO +H
2
O ⇆ Ca(OH)
2
(14)
S ep 3b: Na
2
O +H
2
O ⇆ 2NaOH (15)
The amoun s o CH
4
and H
2
O p oduced a e ob ained om Eq. (3) and
Eq. (5). Fu he mo e, in he hyd ogena ion pe iod, a small amoun o CO
is also p oduced, which is de e mined by Eq. (4). All alues a e sum-
ma ized in Table 3. Finally, selec i i y owa ds CH
4
is de e mined by Eq.
(6) and i is 98%.
To check he eliabili y o he da a, he e o wi h which he ca bon
balance is closed is de e mined. Fo his, Eq. (7) is used and he e o is
a ound 3%. On he o he hand, he o al amoun o wa e p oduced is
ela ed o he p oduc ion o CH
4
. This ela ionship esul s in 2.01, e y
close o he s oichiome y o he me hana ion eac ion (Eq. 1).
I is also wo h o no e a his poin ha he CO
2
adso p ion and
hyd ogena ion is a cyclic ope a ion. All he ca aly ic pa ame e s, i.e. CO
2
adso p ion, CH
4
p oduc ion and CO p oduc ion, a e calcula ed once
cycle- o-cycle s eady s a e is eached. As a cyclic ope a ion, his means
ha he s a e o he ca alys a a gi en poin depends on he p e ious
his o y. The e o e, a he end o he egene a ion pe iod some CO
2
will
emain adso bed. Consequen ly, a he beginning o he adso p ion
pe iod some basic si es will be al eady occupied by CO
2
. Thus, he
adso p ion capaci y o he DFM will be somewha limi ed by his ac .
Howe e , his does no a ec he ca bon balance. On he one hand, he
amoun o CO
2
s o ed in he DFM is calcula ed by Eq. (2) based on he
CO
2
concen a ion signal expe imen ally eco ded. In his calcula ion,
he CO
2
al eady p esen in he sample ( ha s ongly bonded which has
no been decomposed du ing he hyd ogena ion) is no quan i ied. Then,
CH
4
and CO o ma ion is quan i ied du ing he subsequen hyd ogena-
ion pe iod. On he o he hand, he ca bon balance ela es he amoun o
CO
2
adso bed du ing he adso p ion pe iod wi h espec o he amoun
o ca bon based p oduc s (CH
4
and CO) du ing he hyd ogena ion
pe iod.
Fig. 7 shows he e olu ion o CH
4
and CO p oduc ions o all DFMs in
he empe a u e ange 280–400 ◦C. These alues a e ob ained om
e olu ions simila o hose in Fig. 6 and in all cases, i is possible o close
he ca bon balance wi h an e o below ±5%. CH
4
p oduc ion (Fig. 7a)
shows di e en ends depending on each DFM. The DFM Ru-16Ca
shows an upwa d end wi h he ope a ing empe a u e; he e o e, i
p esen s he maximum p oduc ion (336 µmol g
-1
) a 400 ◦C. On he
o he hand, he DFM Ru-16Na ha dly shows a ia ion wi h empe a u e.
This DFM p oduces 310 µmol g
-1
a 310 ◦C, howe e , he p oduc ion
does no all below 281 µmol g
-1
in all he empe a u e ange s udied.
These opposi e endencies a e due o he di e en ypes o basici y
de e mined in he CO
2
-TPD expe imen s (Fig. 5a and b). In he DFM Ru-
16Ca he s ong basici y is p edominan , hus he inc ease in empe a-
u e can decompose a g ea e quan i y o CO
2
o be hyd ogena ed. On
he o he hand, he DFM Ru-16Na p esen s mainly weak basici y, so
ca bona es can decompose a signi ican ly lowe empe a u es and he
inc ease in ope a ing empe a u e limi s p oduc ion since he s abili y o
ca bona es is educed. Po a e al. [16] also epo ed a highe s abili y o
Ca ca bona es compa ed o Na ca bona es. E emo a e al. [41,42]
assigned he be e ca aly ic ac i i y o he enhanced basici y, which
imp o es he adso p ion and he ac i a ion o CO
2
.
The DFM Ru-12Na4Ca also shows li le a iabili y in CH
4
p oduc ion
wi h ope a ing empe a u e. In his occasion, he maximum p oduc ion
is a 340 ◦C and is 268 µmol g
-1
. Al hough he p esence o bo h adso -
ben s sligh ly imp o es he dispe sion and he medium basici y is p o-
mo ed, i is no possible o imp o e he CH
4
p oduc ion wi h his
o mula ion. On he o he hand, he DFM Ru-4Na12Ca shows an upwa d
end wi h empe a u e. A 280 ◦C i p oduces 234 µmol g
-1
and he
p oduc ion inc eases o 372 µmol g
-1
a 400 ◦C. This DFM p oduces a
highe amoun o CH
4
o e he en i e empe a u e ange compa ed o
he DFM Ru-16Ca and p oduces a highe amoun in he 340–400 ◦C
ange compa ed o he DFM Ru-16Na. Consequen ly, in a gene al way,
he p esence o bo h adso ben s imp o es CH
4
p oduc ion, especially a
highe ope a ing empe a u es.
Finally, he DFM Ru-8Na/8Ca no ably imp o es he CH
4
p oduc ion
in he ange 280–340 ◦C compa ed o he DFM Ru-4Na12Ca and he
maximum p oduc ion (364 µmol g
-1
) is a 370 ◦C. Fu he mo e, he DFM
Ru-8Na/8Ca exhibi s supe io p oduc ion ac oss he en i e empe a u e
ange compa ed o he DFMs Ru-16Na and Ru-16Ca. A his poin , he
DFM Ru-8Na/8Ca is p oposed as he mos ac i e o he CH
4
p oduc ion
in he empe a u e ange s udied. In he CO
2
-TPD expe imen s (Fig. 5b)
i has been obse ed ha he join inco po a ion o 8% Na
2
CO
3
and 8%
CaO signi ican ly inc eases he medium basici y. Likewise, he p esence
o bo h adso ben s also imp o es he dispe sion o u henium (Table 2).
These imp o emen s p omo e he con ac be ween he ca bona es o
medium s eng h wi h he me al si es and consequen ly he CO
2
adso p ion and hyd ogena ion o CH
4
. No e ha he maximum amoun
o CH
4
p oduced (364 µmol g
-1
) is less han he a e age basici y
(≈490 µmol g
-1
, Fig. 5b). As men ioned p e iously, only pa o he
ac i e adso p ion si es a e in ol ed in he eac ion.
Fig. 7b shows he e olu ion o CO p oduc ion o all DFMs in he
empe a u e ange 280–400 ◦C. All DFMs show he same end, CO
p oduc ion inc eases wi h ope a ing empe a u e. The inc ease in em-
pe a u e a o s he RWGS and consequen ly a g ea e amoun o CO is
p oduced. Analyzing Fig. 7b clea ly he DFM wi h only sodium (Ru-
16Na) p oduces much amoun o CO compa ed o he es o DFMs. In
ac , he selec i i y o CH
4
a 400 ◦C is educed o 92%. On he o he
hand, DFM Ru-16Ca p esen s a signi ican ly lowe CO p oduc ion,
showing high selec i i y o CH
4
e en a 400 ◦C (97%). P e ious s udies
in he li e a u e ha e concluded ha he addi ion o Ca o ca alys s o
CO
2
me hana ion imp o es he selec i i y o CH
4
by s eng hening he
Fig. 7. E olu ion o CH
4
(a) and CO (b) p oduc ions wi h empe a u e o he DFMs wi h di e en Na
2
CO
3
/CaO a ios.
A. Be mejo-L´
opez e al.
Jou nal o CO2 U iliza ion 58 (2022) 101922
9
CO
2
chemiso p ion, while he addi ion o Na a o s he o ma ion o CO
[43,44].
On he o he hand, i seems ha he join addi ion o bo h adso ben s
u he limi s CO p oduc ion. The Ru-8Na/8Ca as well as Ru-4Na/12Ca
DFMs show a selec i i y a 400 ◦C o 98%. The posi i e e ec on he
selec i i y o he p esence o Ca has a g ea e in luence han ha o Na.
In addi ion, as dispe sion is inc eased, mo e me al si es a e a ailable o
p omo e comple e hyd ogena ion o CO
2
.
Fig. 8 shows he empo al e olu ions o he CH
4
concen a ion du ing
he hyd ogena ion pe iod o all DFMs in he empe a u e ange
280–400 ◦C. F om hese e olu ions, he amoun o CH
4
p oduced shown
in Fig. 7a has been de e mined. The e olu ions o he di e en DFMs
show signi ican di e ences, especially a low ope a ing empe a u es. I
he CH
4
concen a ion e olu ions o DFMs based solely on a unique
adso ben a 280 and 310 ◦C a e compa ed, he maximum CH
4
con-
cen a ion is ound a ini ial imes o he Ru-16Ca DFM compa ed o he
Ru-16Na. The e o e, his sugges ha he CH
4
o ma ion a e is as e o
Ca han o Na when ope a ing a low empe a u es.
The CH
4
o ma ion o DFMs based on bo h adso ben s shows as CH
4
o ma ion a e in which mos o he o ma ion occu s in he i s minu e
o he hyd ogena ion pe iod. The e o e, he ime o he hyd ogena ion
pe iod could be sho ened o ob ain a highe con e sion o he H
2
ed
wi hou d as ically educing he CH
4
p oduc ion. In p e ious wo ks [45,
46], we modeled, simula ed and op imized he CO
2
adso p ion and
hyd ogena ion o CH
4
, which sugges ed he selec ion o mode a e hy-
d ogena ion imes because o a comp omise be ween he H
2
con e sion
and CH
4
p oduc ion. In his con ex , a as CH
4
o ma ion a e is c ucial
o he join op imiza ion o bo h pa ame e s.
Finally, he ob ained CH
4
p oduc ions a e compa ed wi h hose e-
po ed by o he au ho s. Table 4 lis s he CH
4
p oduc ions pe g am o
DFM and pe cycle (Y
CH4
) o he DFMs o his wo k oge he wi h o he
DFMs wi h simila o mula ion om he li e a u e. CH
4
p oduc ions a e
in a wide ange (47–1050 µmol g
-1
). Compa ison is no easy because o
di e en ope a ion pa ame e s as empe a u e, o al du a ion o he
cycle, o al low a e and H
2
concen a ion in he hyd ogena ion pe iod,
also shown in he Table 4. No e, o example, ha he du a ion o he
cycles pe o med in his wo k is signi ican ly lowe han in o he e-
po ed cycles. Ne e heless, in o de o ob ain compa able alues, we
es ablished a new ca aly ic pa ame e as he p oduc ion o CH
4
pe g am
o DFM and pe minu e (Y’
CH4
).
I can be seen in Table 4 ha Y’
CH4
o DFMs o his s udy a e no ably
highe han o he s in he li e a u e, in mos cases so highe as an o de o
magni ude. Also, no e ha in he cycles pe o med in his wo k, he e is
a pu ge o 2 min a e adso p ion and one minu e a e hyd ogena ion.
The e o e, o he 6 min o a cycle du a ion, 50% (3 min) co espond o
he pu ge pe iod. Lab-scale pu ge has been in oduced o p e en
me hane om being p oduced om non-adso bed CO
2
, bu in com-
me cial ope a ion i could be educed o e en elimina ed. This ac
would lead o a subs an ial inc ease in he p oduc ion o CH
4
pe g am o
DFM and ime uni (Y’
CH4
). The pu ge pe iods o he cycles ca ied ou
by o he au ho s accoun o less han 17% o he o al du a ion o he
cycle. The e o e, i can be concluded ha he DFMs o his wo k p esen
a highe CH
4
p oduc ion pe uni o ime compa ed o he ew s udies
epo ed in he li e a u e.
4. Conclusions
Ru-Na
2
CO
3
-CaO/Al
2
O
3
DFMs wi h di e en Na
2
CO
3
/CaO a ios
ha e been syn hesized o he CO
2
adso p ion and hyd ogena ion o
CH
4
. The addi ion o he me al and he adso ben single-phase o
Fig. 8. CH
4
concen a ion p o iles du ing he hyd ogena ion pe iod a di e en empe a u es o he DFMs wi h di e en Na
2
CO
3
/CaO a ios.
Table 4
CH
4
p oduc ions pe g am o DFM and pe cycle (Y
CH4
) and pe g am o DFM and
pe minu e (P
CH4
) oge he wi h he ope a ional condi ions o he DFMs o his
s udy and hose shown by o he au ho s.
DFM Y
CH4
,
μ
mol g
-1
Y’
CH4
,
μ
mol g
-1
min
-1
T,
◦C
cycle
,
min
Q
T
,
ml
min
-1
[H
2
],
%
Re
4%Ru-16%
Na/
Al
2
O
3
310 51.7 310 6 1200 10 This
wo k
4%Ru-12%
Na-4%
Ca/Al
2
O
3
268 44.7 340 6 1200 10 This
wo k
4%Ru-8%
Na-8%
Ca/Al
2
O
3
364 60.7 370 6 1200 10 This
wo k
4%Ru-4%
Na-12%
Ca/Al
2
O
3
372 62.0 400 6 1200 10 This
wo k
4%Ru-16%
Ca/
γ-Al
2
O
3
336 56.0 400 6 1200 10 This
wo k
5%Ru-10%
CaO/
Al
2
O
3
500 3.3 320 150 26 4 [27]
5%Ru-10%
Na
2
CO
3
/
Al
2
O
3
1050 7.0 320 150 26 4 [27]
5%Ru-10%
K
2
CO
3
/
Al
2
O
3
910 6.1 320 150 26 4 [27]
5%Ru-10%
Na
2
CO
3
/
Al
2
O
3
614 7.3 320 84 200 15 [28]
1%Ru-5%
K/Al
2
O
3
176 7.3 350 24 100 4 [14]
1%Ru-5%
Ca/Al
2
O
3
107 4.5 350 24 100 4 [14]
1%Ru-16%
Ba/Al
2
O
3
153 6.4 350 24 100 4 [14]
1%Ru-3%
Na/
Al
2
O
3
47 2.0 350 24 100 4 [14]
1%Ru-5%
Li/Al
2
O
3
340 11.3 293 30 33 10 [25]
A. Be mejo-L´
opez e al.
