No el windows o “sola commodi ies”:a
de ice o CO
2
educ ion using plasmonic
ca alys ac i a ion
Alexande Na a e e,*
a
Se gio Mu˜
noz,
a
Luis M. Sanz-Mo al,
a
Jue gen J. B andne ,
b
Pe e P ei e ,
b
´
Angel Ma ´
ın,
a
Roland Di meye
b
and Ma ´
ıa J. Coce o
a
Recei ed 12 h June 2015, Accep ed 2nd July 2015
DOI: 10.1039/c5 d00109a
A no el plasmonic eac o concep is p oposed and es ed o wo k as a isible ene gy
ha es ing de ice while allowing eac ions o ans o m CO
2
o be ca ied ou .
Pa icula ly he e e se wa e gas shi (RWGS) eac ion has been es ed as a means o
in oduce enewable ene gy in o he economy. The de elopmen o he new eac o
concep in ol ed he syn hesis o a new composi e capable o plasmonic ac i a ion
wi h ligh , he de elopmen o an imp egna ion me hod o c ea e a single ca alys
eac o en i y, and finally he assembly o a eac ion sys em o es he eac ion. The
composi e de eloped was based on a Cu/ZnO ca alys dispe sed in o anspa en
ae ogels. This allows efficien ligh ansmission and a high su ace a ea o he
ca alys . An effec i e ye simple imp egna ion me hod was de eloped ha allowed
in oduc ion o he composi es in o glass mic ochannels. The ac i a ion o he
eac ion was made using LEDs ha co e ed all he sides o he eac o allowing a high
powe deli e y. The esul s o he eac ion show a s able p ocess capable o low
empe a u e ans o ma ions.
In oduc ion
CO
2
as a enewable ene gy ec o
One o he mos indus ially p omising he e ogeneous ca aly ic p ocesses is
ca bon dioxide hyd ogena ion. In he p ocess o ca aly ic hyd ogena ion,
hyd ogen ob ained om ca bon neu al ene gy sou ces (e.g. wind o sola ) is
eac ed wi h CO
2
o ob ain p oduc s such as uels.
1
Thus, his p ocess se es a
double pu pose:  s ly, as chemical s o age o he su plus ene gy gene a ed by
uc ua ing enewable ene gies; and secondly, o educe emissions o CO
2
.
a
Uni e si y o Valladolid, Depa men o Chemical Enginee ing and En i onmen al, Technology, High P essu e
P ocesses G oup, Paseo P ado de la Magdalena s/n, 47005 Valladolid, Spain. E-mail: [email protected] a.es
b
Ins i u e o Mic o P ocess Enginee ing, Ka ls uhe Ins i u e o Technology, Eggens ein-Leopoldsha en,
Ge many
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Fa aday Discussions
Ci e his: Fa aday Discuss.,2015,183,249
PAPER
The success ul in oduc ion o CO
2
-use echnologies equi es plausible and
p o able p ocesses ha use efficien ly enewable ene gy. The mos abundan
and e enly dis ibu ed o such ene gies is he one p o ided by he Sun. I s o ed in
he o m o “sola commodi ies”such as me hanol o olens new oppo uni ies
o he use o CO
2
could be conside ed.
2
The p oduc ion o me hanol om CO
2
and hyd ogen in ol es he ollowing eac ions:
CO
2
+H
2
5CO + H
2
O; DH
0
¼+41.19 kJ mol
1
(1)
CO+2H
2
5CH
3
OH; DH
0
¼90.70 kJ mol
1
(2)
CO
2
+3H
2
5CH
3
OH + H
2
O; DH
0
¼49.51 kJ mol
1
(3)
In o de o cap u e ha ene gy in chemical bonds he e a e se e al use ul
me hods. F om a chemical poin o iew, hey can be g ouped in o pho oca aly ic,
he mal and elec ical ou es. In elec ical ou es, he sola ene gy is  s con-
e ed in o elec ici y and hen, he esul an elec ical ene gy is used in he
chemical ans o ma ion o CO
2
.
3–7
The he mal ou es concen a e sola adia-
ion and con ey ha ene gy di ec ly in o he eac o .
8–10
Pho oca aly ic CO
2
con e sion in ol es ei he wa e spli ing connec ed o a CO
2
educ ion eac ion,
o a p ocess combining bo h in one “single po ”.
11–13
Selec i e use o isual ligh wi h plasmon ca alys s
The su ace plasmon esonance (SPR) phenomenon is commonly ound in
me allic (o ca bon) nanos uc u es and allows he ange o he sola spec um
used on a gi en pho oinduced p ocess o be inc eased.
14
This effec is he esul o
he esponse o he conduc ion elec ons o he oscilla ions o he elec ic eld o
he ligh adia ion. Inc eased ene gy abso p ion by he elec ons is possible a
selec ed wa eleng hs unde he p ope pa icle size and shape o he nano-
pa icles o a gi en su ounding media (uid o ca alys ). This phenomenon
p oduces a high ligh concen a ion up o he poin ha a educ ion in he
amoun o semiconduc o o h ee o de s o magni ude o he same amoun o
ligh has been possible.
15
On he o he hand, inc eased ligh cap u e wi h SPR o
pho o he mal con e sions is leading o b eak h oughs in ene gy sys ems such as
sola collec o s.
16
This wo k explo es he educ ion o CO
2
o CO as a  s s ep in a sola -based
p ocess o p oduce me hanol. Thus, i is based on he e e se wa e gas shi
(RWGS) eac ion as desc ibed by eqn (1). Recen app oaches ha e used gold and
semiconduc o composi es o plasmonic enhancemen o he educ ion.
17,18
This p ocess is commonly ac i a ed in indus y using Cu/ZnO based ca a-
lys s.
19
He e, we ha e de eloped plasmonic ca aly ic composi es in mesopo ous
silica s uc u es (ae ogels). Fo his, we ha e used he plasmon- uneable Cu/ZnO
ca alys epo ed by Tan e al. (2013).
20
A plasmonic mic o eac o as a ligh ha es ing de ice
The efficiency o he chemical eac ions is no uled only by he ca aly ic ma e ial
bu also by he eac o congu a ion and hei mass and ene gy anspo cha -
ac e is ics. On many occasions, p omising ca aly ic ma e ials ha e ailed o each
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indus ial success due o he disconnec ion be ween he ca aly ic s uc u e and
he eac o -le el phenomena.
21,22
He e we p opose a no el concep o isual ene gy ha es ing: a plasmonic
mic o eac o de ice. I in eg a es a plasmon ca alys and eac o as one en i y wi h
a sole esponse o ligh (Fig. 1).
Mic o eac o s allow efficien ene gy and mass anspo while a e easily scal-
able (numbe ing up). Thus, he combina ion o efficien mic os uc u ed de ices
and di ec plasmonic abso p ion o sola ene gy by he ca alys would ep esen a
majo b eak h ough in he CO
2
-use eld.
In his wo k, a plasmon- uneable composi e is in eg a ed wi h a mic ochannel
based eac ion sys em unde isual LED illumina ion o he RWGS eac ion. This
in ol ed he syn hesis o he new composi e; he de elopmen o an imp egna ion
me hod o c ea e a ca alys - eac o en i y; and nally, he assembly o a eac ion
sys em o es he eac ion.
Me hods
Plasmo-ca aly ic composi e syn hesis
The chemicals used du ing his s age a e de ailed: zinc ace a e dihyd a e (>98%),
oleylamine (70%), e ame hyl o hosilica e (98%), ammonia (28–30%) and ie-
hyleneglycol (99%) we e pu chased om Sigma-Ald ich. E hyleneglycol (99.5%)
(Me ck) and coppe ace a e monohyd a e (99.9%) we e pu chased om Al a
Aesa . Me hanol (99.8%) was p o ided by Pan eac. All chemicals we e used
wi hou u he pu ica ion.
Syn hesis o he Cu/ZnO bime allic ca alys
This bime allic (Cu : ZnO, 1 : 2) ca alys was syn hesised ollowing he p ocedu e
p oposed by Tan e al. (2013).
20
B iey,  s ZnO nano ods we e p epa ed. Zinc ace a e (3 mmol) was added o
1.3 mmol o oleylamine in a wo necked ask. The oleylamine was no o high
pu i y (70%), equi ing a s ep whe e he eac an s we e degassed a 80 C o 45
Fig. 1 Concep o he plasmonic mic o eac o de ice.
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min unde a acuum a mosphe e. Then, he empe a u e was inc eased o 220 C
unde ni ogen pu ging. Du ing he hea ing p ocess, he solu ion u ned whi e
upon eaching 180 C. Ae 15 min o hea ing, he mix u e was cooled o a mo-
sphe ic empe a u e, washed wi h e hanol and cen i uged in o de o isola e he
p ecipi a e. I was washed wi h 6 ml o e hanol h ee imes o ensu e comple e
emo al o he eac an s o byp oduc s.
The ZnO nano ods hus p epa ed we e hen edispe sed in 20 ml o ie hy-
leneglycol by sonica ion o wo hou s, ollowed by s i ing unde oom condi-
ions o e nigh . E hyleneglycol (2 ml) was added o he ZnO dispe sion, and he
mix u e was degassed a oom empe a u e o 5 min be o e hea ing o 190 C.
Simul aneously, a second solu ion o coppe ace a e monohyd a e was p epa ed
by dissol ing in e hyleneglycol. This mix u e equi ed sonica ion in o de o
dissol e he coppe ace a e in he liquid. This mix u e was added o he ZnO
mix u e in a d opwise manne o e 10 min. Ae his, 5 mo e min hea ing a 190
C was allowed be o e he composi e was washed wi h isop opanol, cen i uged
o 15 minu es a 4500 pm (cen i uge Kubo a 5100, Japan) and isola ed om he
mix u e.
Syn hesis o he mesopo ous silica composi es
Ligh ansmission o he ca aly ic s uc u es is essen ial while enough su ace
a ea has o be p o ided in o de o ha e enough me al loading o cap u e ligh .
T anspa en ae ogels a e mesopo ous ma e ials combining high su ace a eas
and good ligh ansmission.
23
Ae ogels we e syn hesised ollowing he sol–gel ou e. The p ecu so o he
silica hyd ogel selec ed was e ame hyl o hosilica e (TMOS). The mola a io o
TMOS : CH
3
OH : H
2
O:NH
4
was 1 : 2.3 : 3.84 : 0.012.
Fi s ly, me hanol was used o dispe se he nanopa icles o med in he ca alys
syn hesis. Sonica ion (15 min) was applied o ensu e a good dispe sion o he
nanopa icles in he liquid phase. Me hanol wi h he pa icles and TMOS we e
mixed oge he . While his solu ion was s i ed, a second solu ion o ammonium
hyd oxide and wa e was p epa ed and s i ed. Ae a ew minu es o s i ing,
bo h solu ions we e mixed, and he gela ion p ocess began.
In his momen , he gela ion p ocess o he silica hyd ogel has s a ed, bu i is
s ill liquid o a ew minu es. This ime lapse, be o e gela ion, mus be used o
imp egna e he solu ion inside he mic ochannels o he mic o eac o s.
In eg a ion o he composi es and mic o eac o
In o de o ha e a single in eg a ed de ice i is necessa y o in eg a e ligh
ansmission and composi e ac i a ion in he same s uc u e. He e, we ha e
de eloped a me hod o in eg a e anspa en ae ogels in glass mic ochannels. The
me hod used o imp egna e he sol–gel ha showed he bes esul s was he
suc ion o he liquid wi h a sy inge, which was p e iously adap ed o he mic o-
eac o on i s op (Fig. 2). Wi h his me hod, placing he mic o eac o in a e ical
posi ion, i was e y easy o ll he mic ochannel placing he ip in he liquid.
Ae a sho ime, he gela ion p ocess nished and he hyd ogel o med had a
good adhe ence inside he mic ochannels.
I is wo h men ioning ha one o he mos impo an pa ame e s du ing his
s ep is he amoun o ammonia, as i ac s as a ca alys o he gela ion p ocess. I is
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necessa y o use an amoun o ammonia low enough ha allows enough ime o
imp egna e he hyd ogel inside he mic o eac o , because i mus be s ill a uid.
On he o he hand, he amoun o ammonia canno be e y low, because i he
gela ion p ocess is oo slow, he nanopa icles s a o p ecipi a e and hey will no
emain inside he silica ne . Finding an equilib ium be ween hese wo ac o s is
he key o achie ing good imp egna ion o he nanopa icles suppo ed in he
silica gels inside he mic o eac o s. The mic o eac o consis ed o a 0.5 mm ID
glass capilla y wi h an ex e nal diame e o 5 mm (Scho Du an, USA).
In o de o ensu e good adhe ence o he ae ogel o he walls o he mic o-
channels, i is necessa y o pe o m a p e ea men o he glass mic o eac o s
(15.0 cm) o clean he walls o he mic ochannels. Fo his cleaning p ocess, he
mos common op ion is o use a pi anha solu ion, which eac s iolen ly wi h
mos o ganic ma e ials. The solu ion used was a mix u e o sul u ic acid and
hyd ogen pe oxide ha can be p epa ed in diffe en p opo ions; he mos usual
being 4 : 1 in concen a ed sul u ic acid. Fo 15 o 30 minu es he ma e ial is
subme ged in he solu ion, hen emo ed, washed wi h plen y o Milli-Q wa e
and d ied ca e ully.
The emp y glass mic o eac o s we e pu inside a glass po , and he sul u ic
acid was  s added. Then, he hyd ogen pe oxide was added wi h ex eme ca e
because he eac ion is e y exo he mic; he empe a u e is suddenly inc eased
and some apo can be o med. Ae 20 minu es, he slides we e emo ed om
he pi anha solu ion, washed wi h Milli-Q wa e and d ied ca e ully.
Ae he in oduc ion o he nanopa icles in he sol–gel and i s in oduc ion
in o he mic o eac o s, hese we e pu in a essel wi h me hanol o aging. This
essel was ca e ully closed o a oid me hanol e apo a ion, and i was hea ed o 50
C in an o en. Wi h his p ocedu e he wa e con ained in he silica ne was
eplaced wi h me hanol, esul ing in alcogels.
Ae 24 hou s o hea ing, he alcogels we e d ied using supe c i ical ca bon
dioxide. The mic o eac o s we e pu in a high p essu e essel, and his essel was
lled comple ely wi h me hanol. Ca bon dioxide was in oduced slowly in he
essel o allow a good diffusion in o he me hanol. The p essu e was aised o 100
ba and he empe a u e o 40 C, abo e he c i ical poin o ca bon dioxide. Th ee
cycles o 45 minu es we e pe o med, enewing he ca bon dioxide be ween each
Fig. 2 Sy inge filling o he mic ochannels.
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cycle o comple e he d ying p ocess.
24
Ae his, silica ae ogels we e co ec ly
ob ained, keeping he adhe ence o he walls o he mic ochannels.
Composi e cha ac e iza ion
Scans o he bime allic ca alys s o check abso bance o isual ligh we e ca ied ou
using a UV- is spec ome e (UV 2550, Shimadzu). XRD analysis was ca ied ou
using a B uke Disco e D8 diff ac ome e . The po osi y measu emen s we e
ca ied ou using a Su ace A ea and Po osi y Analyze (ASAP 2020, Mic ome i ics).
P oo -o -concep se up
Once he Cu/ZnO based plasmonic composi es we e in eg a ed in o he glass
mic ochannels, a es o hese de ices was made. In o de o es he eac o
concep , a eac ion sys em had been buil ha included isual LED illumina-
ion and con ol o he eac ion empe a u e while a p ecise con ol o he ow
and p essu e was p o ided. A scheme o he expe imen al plan is p esen ed in
Fig. 3.
Hyd ogen and ca bon dioxide we e in oduced in he sys em, and hei ows
we e con olled wi h wo diffe en ow mass me e /con olle s (EL-Flow F-200,
B onkho s ) wi h anges om 0.02 o 1 ml min
1
.
Be o e he eac ion was ini ia ed, hyd ogen and ca bon dioxide we e mixed in a
3 : 1 p opo ion, and sen o he en while bo h ows we e s abilized.
When he ows we e co ec ly con olled, he mix u e o he gases mo ed o
he second pa o he se up. In his pa he gases we e hea ed, oge he wi h he
glass mic o eac o , in a gas ch oma og aphy o en (Agilen 7890).
A second en was used o ake ou he gases while he p essu e was
inc eased o 20 ba . P essu e was con olled by a p essu e me e /con olle
(EL-P ess se ies, B onkho s ). When he p essu e and ow we e s able
a 20 ba , he al e o ex ac ion was closed, and he o en and LEDs we e
u ned on.
Fig. 3 Schema ic flow diag am o he plan .
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Visual ligh s imula ion was p o ided by 36 LEDs (supe b igh , inspi ed LED)
su ounding he mic o eac o as shown in Fig. 4. In o al, hey p o ided he
equi alen o a nominal powe o 9780 W m
2
o whi e ligh .
The eac ion began and he p oduc s o he eac ion we e measu ed in a Mic o
Gas Ch oma og aph (CP-4900, Va ian) equipped wi h wo columns: a po aplo 10
m and a 5A molsie e. Be o e he mic o GC, he p essu e o he gas s eam was
educed o less han 5 ba .
Resul s and discussion
Composi e cha ac e iza ion
Ae he syn hesis UV- is scans we e ca ied ou in o de o check he abso p ion
o isual ligh om bo h he ZnO nano ods and he bime allic Cu/ZnO ca alys
(Fig. 5).
I can be seen ha he bime allic ca alys has an abso p ion peak a 498 nm.
This co esponds o abso p ion in he ange close o he g een colou .
25
The
anspa en ae ogels change and acqui e colou once he composi e is o med
(Fig. 6).
Fig. 4 LED ligh configu a ion. Le , de ail o he inne LED dis ibu ion. Righ , mic o-
eac o in he o en su ounded by he LEDs.
Fig. 5 Abso p ion spec a o ZnO (le ) and Cu/ZnO ( igh ).
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The XRD pa e n shows he p esence o ZnO and me allic coppe in he silica
amo phous s uc u e. The ZnO planes 100 (2q¼31.7), 002 (2q¼34.4), 101 (2q¼
36.1) and 110 (2q¼56.4) can be iden ied in Fig. 7. Coppe canno be seen due
o i s lowe p opo ion in he s uc u e.
BET su ace a ea and po e olume o he samples we e calcula ed om N
2
iso he ms. The adso p ion–deso p ion cu e shows a ype IV iso he m cu e
ypical o mesopo ous silica ae ogels
26
(Fig. 8). The BET su ace a ea is 945.8 m
2
g
1
which indica es ha he inclusion o he bime allic ca alys does no ha e a
signican inuence on he ex u al p ope ies o he ae ogel. The BJH po e
olume is equal o 2.29 cm
3
g
1
ein o cing ha he s uc u e is no affec ed.
Reac ion es o he concep
The ull powe o he LEDs was applied and he e olu ion o he compounds we e
ollowed. In o de o es he inuence o he main a iables o he p ocess,
changes in ow and empe a u e we e made du ing he eac ion (Fig. 9).
Fig. 6 Silica ae ogel be o e (le ) and a e ( igh ) ca alys imp egna ion.
Fig. 7 XRD pa e n o he plasmonic composi e.
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I could be obse ed ha he eac ion was s able a 50 C du ing mo e han 100
minu es. Then he empe a u e was inc eased a 70 C and es ed du ing he same
ime span. Finally, he ow was educed o hal o he ini ial condi ions. No
signican changes we e obse ed du ing almos 250 minu es a cons an ow,
which indica es he sui abili y o his sys em o es se e al ca alys loads unde
diffe en he modynamic and ow condi ions.
I is impo an o no e ha he a e age con e sion a e a highe ow is
simila o he one ob ained in o he wo ks a empe a u es a ound 200 C.
17
Thus, he in eg a ed plasmonic eac o concep p oposed he e opens new
a enues o couple low empe a u e sola collec o s and chemis y as a mean o
in oduce enewable ene gy in he economy, pa icula ly o he con e sion o
CO
2
.
Fig. 8 Adso p ion–deso p ion iso he ms o he composi es.
Fig. 9 E olu ion o he eac ion.
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