ca alys s
A icle
Reac o Design o CO2Pho o-Hyd ogena ion owa d
Sola Fuels unde Ambien Tempe a u e and P essu e
Chun-Ying Chen 1, Joseph Che-Chin Yu 1, Van-Huy Nguyen 2, Je ey Chi-Sheng Wu 1,*,
Wei-Hon Wang 3and Kamila Koˇcí 4
1Depa men o Chemical Enginee ing, Na ional Taiwan Uni e si y, Taipei 10617, Taiwan;
[email p o ec ed] (C.-Y.C.); [email p o ec ed] (J.C.-C.Y.)
2Facul y o Chemical and En i onmen al Enginee ing, Lac Hong Uni e si y, 812431,
No. 10 Huynh Van Nghe, Buu Long, Bien Hoa, Dong Nai, Vie Nam; [email p o ec ed]
3Chung-Shan Ins i u e o Science and Technology, Tao Yuan 32599, Taiwan; [email p o ec ed]
4Ins i u e o En i onmen al Technology, VŠB-Technical Uni e si y o Os a a, 17. lis opadu 15/2172,
708 33 Os a a-Po uba, Czech Republic; [email p o ec ed]
*Co espondence: [email p o ec ed]; Tel.: +886-2-23631994; Fax: +886-2-23623040
Academic Edi o : Rajend a S. Ghadwal
Recei ed: 27 Decembe 2016; Accep ed: 8 Feb ua y 2017; Published: 16 Feb ua y 2017
Abs ac :
Pho o-hyd ogena ion o ca bon dioxide (CO
2
) is a g een and p omising echnology and
has ecei ed much a en ion ecen ly. This echnique could con e sola ene gy unde ambien
empe a u e and p essu e in o desi able and sus ainable sola uels, such as me hanol (CH
3
OH),
me hane (CH
4
), and o mic acid (HCOOH). I is wo hwhile o men ion ha his di ec ion can no
only po en ially dep ess a mosphe ic CO
2
, bu also weaken dependence on ossil uel. He ein,
1 w % P /CuAlGaO
4
pho oca alys was success ully syn hesized and ully cha ac e ized by
ul a iole - isible ligh (UV- is) spec oscopy, X- ay di ac ion (XRD), Field emission scanning
elec on mic oscopy using ene gy dispe si e spec oscopy analysis (FE-SEM/EDS), ansmission
elec on mic oscopy (TEM), X- ay pho oelec on spec oscopy (XPS), and B unaue -Emme -Telle
(BET), espec i ely. Th ee kinds o expe imen al pho o-hyd ogena ion o CO
2
in he gas phase,
liquid phase, and gas-liquid phase, co espondingly, we e conduc ed unde di e en H
2
pa ial
p essu es. The ema kable esul has been obse ed in he gas-liquid phase. Addi ionally, inc easing
he pa ial p essu e o H
2
would enhance he yield o p oduc . Howe e , when an ex a amoun
o H
2
is supplied, i migh compe e wi h CO
2
o occupying he ac i e si es, esul ing in a nega i e
e ec on CO
2
pho o-hyd ogena ion. Fo liquid and gas-liquid phases, CH
3
OH is he majo p oduc .
Maximum o al hyd oca bons 8.302 µmol·g−1is achie ed in he gas-liquid phase.
Keywo ds: CO2 educ ion; P /CuAlGaO4; pho o-hyd ogena ion; pho oca alysis; sola uels
1. In oduc ion
Nowadays, i is impo an o de elop an e icien and e ec i e me hod o u ilizing ca bon dioxide
(CO
2
) g eenhouse gas. The CO
2
cap u ed om he a mosphe e will be s o ed in he g ound o he
ocean [
1
]. Ano he op ion, which is a mo e a ac i e me hod o CO
2
consump ion, is o con e i
in o use ul hyd oca bon uels di ec ly [
2
]. In indus y, CO
2
is widely used o Saba ie eac ion [
3
,
4
]
and e e se wa e -gas shi (RWGS) eac ion [
5
,
6
]. Rega ding he Saba ie eac ion, CO
2
eac s wi h
H
2
o o m CH
4
and H
2
O a 300–400
◦
C. As o RWGS eac ion, CO
2
i s ly eac s wi h H
2
o o m
CO and H
2
O; hen CO can be easily con e ed in o hyd oca bons by he Fische -T opsch eac ion.
Howe e , bo h eac ions equi e high empe a u e and high p essu e o educe CO
2
in o hyd oca bons.
Thus, i is ine i able o de elop ano he me hod o conduc CO2 educ ion.
Ca alys s 2017,7, 63; doi:10.3390/ca al7020063 www.mdpi.com/jou nal/ca alys s
Ca alys s 2017,7, 63 2 o 12
Thepossible CO
2
con e sion p ocesses, including biological [
7
], ca aly ic [
8
–
11
], pho oca aly ic [
12
–
14
],
and elec oca aly ic [
15
,
16
] con e sion a e summa ized in Table 1. Among hese p ocesses,
pho oca aly ic CO
2
educ ion is one o he mos p omising echnologies and has ecei ed much
a en ion ecen ly [
14
,
17
–
21
]. CO
2
as a C1- eeds ock could be educed o desi able and sus ainable
sola uels a ambien condi ions unde ligh i adia ion.
Table 1. Summa y o di e en p ocesses o CO2 educ ion.
Con e sion P ocess Main P oduc s Ad an ages Disad an ages Re .
Biological HCOOH: con e sion e iciency o
(22 ±9) ×10−2%Biological capabili y o
syn hesize liquid uels
Complica ed and
cumbe some biological
p ocesses [7]
Ca aly ic (He e ogeneous ca alysis)
CH
4
: 3.8
×
10
4µ
mol
·
h
−1·
g
−1
ca alys
High e iciency High empe a u e
equi emen
[8]
CH
3
OH: 8.8% o CO
2
con e sion wi h
he co esponded selec i i y o 63% [9]
CH3OH: 193.9 g·kgca −1·h−1[10]
Ca aly ic (Homogeneous ca alysis) CH3OH: 79% yield [11]
Pho oca aly ic (He e ogeneous ca alysis) CH4: 0.56 µmol h−1·g−1ca alys S o age o sola ene gy Low e iciency [12]
CH3OH: 4.6 µmol h−1·g−1ca alys [13]
Pho oca aly ic (Homogeneous ca alysis) CO: 12.66 h−1o TOF [14]
Elec oca aly ic Liquid uels Con e ing CO2di ec ly
o liquid uels
(long-chain molecule)
High ene gy ba ie
needs o e coming [15,16]
Ideally, pho oca aly ic CO
2
educ ion could p o ide an al e na i e and sus ainable pa hway o
p oducing desi ed hyd oca bon p oduc s om enewable ene gy and CO
2
. Howe e , i is no ed ha
hei e iciency is s ill ela i ely low. To enhance he pho oca aly ic pe o mances, se e al ac o s—such
as ligh ha es ing, loss o pho ons, p oduc sepa a ion, and cha ge ca ie ecombina ion—a e
conside ed in ou g oup o design and de eloped in a new pho o- eac o . In 2007, a ci cula ed
pho oca aly ic eac o was de eloped, which could p o ide a la ge speci ic su ace a ea and uni o mi y
o gas concen a ions in he eac o [
22
]. In 2008, he op ical- ibe pho o- eac o , which could deli e
ligh e icien ly and uni o mly o he su ace o a pho oca alys , was also success ully designed [
23
].
I is impo an o no e ha he e a e wo main sou ces o conduc CO
2
educ ion, H
2
O
(g)
and H
2(g)
.
In a p e ious s udy, Abbo e al. calcula ed a se ies o changes o en halpy (
∆
H
0
) and changes o
Gibbs ee ene gy (
∆
G
0
), espec i ely, o CO
2
educ ion o o m hyd oca bons [
24
]. The esul clea ly
shows ha adding H
2
la gely dec eases hese alues. Tha is, he CO
2
educing eac ion in which he
in ol ed H
2
equi es lowe ene gy and becomes mo e spon aneous. The e o e, in 2013, a no el win
eac o was success ully de eloped o hyd ogena e CO
2
in o CH
3
OH [
20
]. Howe e , i is no ed ha i s
e iciency is s ill ela i ely low.
In his s udy, 1 w % P /CuAlGaO
4
was p epa ed using a well-known solid-s a e usion and
pho o-deposi ion me hod, espec i ely. De ailed cha ac e iza ion o pho oca alys was conduc ed o
e eal i s s uc u e. Th ee kinds o eac o s—including gas phase eac o , liquid phase eac o , and
gas-liquid eac o —we e designed and employed o s udy he in luence o hyd ogen (H
2
) and ca bon
dioxide (CO
2
) on he pho o-hyd ogena ion o CO
2
. Addi ionally, a possible eac ion pa hway o CO
2
pho o-hyd ogena ion is also p oposed based on he knowledge o p oduc s p esen ed du ing he
pho oca aly ic eac ion.
2. Resul s and Discussion
2.1. Cha ac e iza ion o Pho oca alys s
All pho oca alys s we e ully cha ac e ized by se e al echniques. B unaue -Emme -Telle
(BET), ul a iole - isible ligh (UV- is) spec oscopy; X- ay di ac ion (XRD), ansmission elec on
mic oscopy (TEM), and Field emission scanning elec on mic oscopy (FE-SEM) using ene gy
dispe si e spec oscopy (EDS) analysis we e used o e eal hei s uc u e and su ace mo phology
Ca alys s 2017,7, 63 3 o 12
o pho oca alys s. X- ay pho oelec on spec oscopy (XPS) was used o e eal he chemical s a e o
he species.
Figu e 1displays he UV- is ligh abso p ion spec a o CuAlGaO
4
and 1 w % P /CuAlGaO
4
pho oca alys s, espec i ely. Clea ly, all he pho oca alys s ha e he abso p ion band in he ange o
250–670 nm, which is consis en wi h ha epo ed p e iously [
25
]. I no es ha loading o he P
nei he signi ican ly imp o es he ligh abso p ion no changes he band gap. A possible eason comes
om he ac ha P loading is e y small and highly dispe sed on he suppo ing su ace.
Ca alys s2017,7,633o 12
Figu e1displays heUV‐ isligh abso p ionspec ao CuAlGaO4and1w %P /CuAlGaO4
pho oca alys s, espec i ely.Clea ly,all hepho oca alys sha e heabso p ionbandin he angeo 
250–670nm,whichisconsis en wi h ha  epo edp e iously[25].I no es ha loadingo  heP 
nei he signi ican lyimp o es heligh abso p ionno changes hebandgap.Apossible eason
comes om he ac  ha P loadingis e ysmallandhighlydispe sedon hesuppo ingsu ace.

Figu e1.Ul a iole ‐ isibleligh (UV‐ is)spec a o CuAlGaO4and1w %P /CuAlGaO4
pho oca alys s.
Figu e2shows heXRDpa e nso CuAlGaO4,1w %P /CuAlGaO4pho oca alys sand hei 
s a ingma e ialso CuO,Al2O3,andGa2O3.Clea ly,nopeakscanbeassigned oei he CuO,Al2O3,
o Ga2O3,indica ing ha comple e eac iono  hep ecu so swasachie edbysolid‐s a e usiona
mix u eo Al2O3,Ga2O3,andZnOa 1125K o 12h.Theloadingo P ob iously e ains hes uc u al
ea u eso CuAlGaO4pho oca alys ,bu i sc ys alli esappea  obesligh lydec eased.Bo h
pho oca alys sha epa e nssimila  o ha o  hesingle‐phasespinel ypes uc u eo  heCuAlGaO4
(JCPDS ile,ca dNo.26‐0514).Howe e , he eisnono iceablec ys allinephaseobse edin heXRD
pa e no P /CuAlGaO4pho oca alys  ha canbea ibu ed oP elemen .Asdiscussedabo e, he
P loadingmigh be e ysmallandhighlydispe sedon hesuppo ingsu ace,whichisconsis en 
wi h heUV‐ isspec um.

Figu e2.X‐ aydi ac ion(XRD)pa e nso pho oca alys s:CuAlGaO4and1w %P /CuAlGaO4,in
compa edwi hs a ingma e ialso CuO,Al2O3,andGa2O3.
Figu e 1. Ul a iole - isible ligh (UV- is) spec a o CuAlGaO4and 1 w % P /CuAlGaO4pho oca alys s.
Figu e 2shows he XRD pa e ns o CuAlGaO
4
, 1 w % P /CuAlGaO
4
pho oca alys s and hei
s a ing ma e ials o CuO, Al
2
O
3
, and Ga
2
O
3
. Clea ly, no peaks can be assigned o ei he CuO, Al
2
O
3
,
o Ga
2
O
3
, indica ing ha comple e eac ion o he p ecu so s was achie ed by solid-s a e usion
a mix u e o Al
2
O
3
, Ga
2
O
3
, and ZnO a 1125 K o 12 h. The loading o P ob iously e ains he
s uc u al ea u es o CuAlGaO
4
pho oca alys , bu i s c ys alli es appea o be sligh ly dec eased. Bo h
pho oca alys s ha e pa e ns simila o ha o he single-phase spinel ype s uc u e o he CuAlGaO
4
(JCPDS ile, ca d No. 26-0514). Howe e , he e is no no iceable c ys alline phase obse ed in he XRD
pa e n o P /CuAlGaO
4
pho oca alys ha can be a ibu ed o P elemen . As discussed abo e, he P
loading migh be e y small and highly dispe sed on he suppo ing su ace, which is consis en wi h
he UV- is spec um.
Ca alys s2017,7,633o 12
Figu e1displays heUV‐ isligh abso p ionspec ao CuAlGaO4and1w %P /CuAlGaO4
pho oca alys s, espec i ely.Clea ly,all hepho oca alys sha e heabso p ionbandin he angeo 
250–670nm,whichisconsis en wi h ha  epo edp e iously[25].I no es ha loadingo  heP 
nei he signi ican lyimp o es heligh abso p ionno changes hebandgap.Apossible eason
comes om he ac  ha P loadingis e ysmallandhighlydispe sedon hesuppo ingsu ace.

Figu e1.Ul a iole ‐ isibleligh (UV‐ is)spec a o CuAlGaO4and1w %P /CuAlGaO4
pho oca alys s.
Figu e2shows heXRDpa e nso CuAlGaO4,1w %P /CuAlGaO4pho oca alys sand hei 
s a ingma e ialso CuO,Al2O3,andGa2O3.Clea ly,nopeakscanbeassigned oei he CuO,Al2O3,
o Ga2O3,indica ing ha comple e eac iono  hep ecu so swasachie edbysolid‐s a e usiona
mix u eo Al2O3,Ga2O3,andZnOa 1125K o 12h.Theloadingo P ob iously e ains hes uc u al
ea u eso CuAlGaO4pho oca alys ,bu i sc ys alli esappea  obesligh lydec eased.Bo h
pho oca alys sha epa e nssimila  o ha o  hesingle‐phasespinel ypes uc u eo  heCuAlGaO4
(JCPDS ile,ca dNo.26‐0514).Howe e , he eisnono iceablec ys allinephaseobse edin heXRD
pa e no P /CuAlGaO4pho oca alys  ha canbea ibu ed oP elemen .Asdiscussedabo e, he
P loadingmigh be e ysmallandhighlydispe sedon hesuppo ingsu ace,whichisconsis en 
wi h heUV‐ isspec um.

Figu e2.X‐ aydi ac ion(XRD)pa e nso pho oca alys s:CuAlGaO4and1w %P /CuAlGaO4,in
compa edwi hs a ingma e ialso CuO,Al2O3,andGa2O3.
Figu e 2.
X- ay di ac ion (XRD) pa e ns o pho oca alys s: CuAlGaO
4
and 1 w % P /CuAlGaO
4
,
in compa ed wi h s a ing ma e ials o CuO, Al2O3, and Ga2O3.
Ca alys s 2017,7, 63 4 o 12
Figu e 3shows he SEM images wi h co esponded elemen al spec a o CuAlGaO
4
and 1 w %
P /CuAlGaO
4
pho oca alys s, espec i ely. Fo CuAlGaO
4
pho oca alys , i has cube-like pa icles
and smoo h su aces. Mos impo an ly, loading o P on CuAlGaO
4
could no change he shape and
mo phology o he pho oca alys . Addi ionally, a uni o m dis ibu ion o he P pa icles was ound
wi h a na ow size ange o 4–20 nm.
Ca alys s2017,7,634o 12
Figu e3shows heSEMimageswi hco espondedelemen alspec ao CuAlGaO4and1w %
P /CuAlGaO4pho oca alys s, espec i ely.Fo CuAlGaO4pho oca alys ,i hascube‐likepa iclesand
smoo hsu aces.Mos impo an ly,loadingo P onCuAlGaO4couldno change heshapeand
mo phologyo  hepho oca alys .Addi ionally,auni o mdis ibu iono  heP pa icleswas ound
wi hana owsize angeo 4–20nm.

Figu e3.Scanningelec onmic oscopy(SEM)imageswi hco espondedelemen alspec ao (a)
CuAlGaO4and(b)1w %P /CuAlGaO4pho oca alys s.
Table2showselemen alanalysiso CuAlGaO4and1w %P /CuAlGaO4, espec i ely.I clea ly
poin sou  ha only1w %P /CuAlGaO4showed heP signal(Figu e3).On heo he hand, he
o he elemen alsignals(O,Al,Cu,Ga)we e e ysimila be weenCuAlGaO4and1w %
P /CuAlGaO4pho oca alys s.
Table2.Ene gydispe si espec oscopy(EDS)analysis o elemen composi ionso CuAlGaO4and
1w %P /CuAlGaO4pho oca alys s.
Elemen A omicRa io(%)
CuAlGaO4P /CuAlGaO4
O61.961.6
Al23.022.0
Cu12.813.0
Ga2.33.2
P N/D10.2
1N/D—No de ec ion.
Figu e4 e eals heTEMimages,whichwe econduc ed o u he unde s and hep esen ing
o P loadingon1w %P /CuAlGaO4pho oca alys .I hasse e alwell‐dispe sedP nanopa icles
wi hco espondedsizeo nea 4–20nma eloca edon hesu aceo P /CuAlGaO4pho oca alys .
Figu e 3.
Scanning elec on mic oscopy (SEM) images wi h co esponded elemen al spec a o
(a) CuAlGaO4and (b) 1 w % P /CuAlGaO4pho oca alys s.
Table 2shows elemen al analysis o CuAlGaO
4
and 1 w % P /CuAlGaO
4
, espec i ely. I clea ly
poin s ou ha only 1 w % P /CuAlGaO
4
showed he P signal (Figu e 3). On he o he hand,
he o he elemen al signals (O, Al, Cu, Ga) we e e y simila be ween CuAlGaO
4
and 1 w %
P /CuAlGaO4pho oca alys s.
Table 2.
Ene gy dispe si e spec oscopy (EDS) analysis o elemen composi ions o CuAlGaO
4
and
1 w % P /CuAlGaO4pho oca alys s.
Elemen A omic Ra io (%)
CuAlGaO4P /CuAlGaO4
O 61.9 61.6
Al 23.0 22.0
Cu 12.8 13.0
Ga 2.3 3.2
P N/D 10.2
1N/D—No de ec ion.
Figu e 4 e eals he TEM images, which we e conduc ed o u he unde s and he p esen ing o
P loading on 1 w % P /CuAlGaO
4
pho oca alys . I has se e al well-dispe sed P nanopa icles wi h
co esponded size o nea 4–20 nm a e loca ed on he su ace o P /CuAlGaO4pho oca alys .
Ca alys s 2017,7, 63 5 o 12
Ca alys s2017,7,635o 12

Figu e4.T ansmissionelec onmic oscopy(TEM)imageso pho oca alys s:(a)CuAlGaO4and(b)
1w %P /CuAlGaO4pho oca alys s.
XPSP 4 spec ao CuAlGaO4and1w %P /CuAlGaO4pho oca alys sa eshowninFigu e5.
Mos impo an ly, he ea e wopossiblepeaksa 73.6–74.1and77.0–77.4eVha ebeenobse ed o 
1w %P /CuAlGaO4pho oca alys ,whichmaybea ibu ed o4 7/2and4 5/2o P oxide,
espec i ely[26].

Figu e5.TheP 4 X‐ aypho oelec onspec oscopy(XPS)spec a o (a)CuAlGaO4and(b)1w %
P /CuAlGaO4pho oca alys s.
Las ly, heBETsu acea eao CuAlGaO4and1w %P /CuAlGaO4wasapp oxima ely0.65and
0.86m2∙g−1, espec i ely.Bo h alueswe eno  e yhighdue o hesolid‐s a e usionme hodwhich
ope a esa ahigh empe a u e,causing hepho oca alys agg ega ion.
2.2.Pho oca aly icCO2Reduc ionwi hGas,Liquid,Gas‐LiquidPhaseReac o s
In hiss udy,se e alblank es sinall ypeo  eac o s(gasphase,liquidphase,andgas‐liquid
phase eac o s)we econduc ed oanswe  heques ionwhe he  he o ma iono  he eac ion
p oduc scome ompho o‐hyd ogena ionandno  omCO2con amina ionso  hepho oca alys 
i sel .The esul o  es swasshowninTable3.The esul clea lyshows ha i CO2wasno in oduced
o he eac ingsys em,nop oduc wasgene a ede enwi h hep esenceo  hehyd ogen,
pho oca alys ,andligh i adia ion.I isimpo an  o emembe  ha  hepho oca alys syn hesis
p ocessdidno useanyca bon‐con ainingp ecu so .Thisblank es  esul is u he con i ma ion
ha  hepho oca alys i sel con ainsnea lynoca bon esidue.Table3alsoclea lyshows ha ,wi hou 
anyo  h eeessen ialelemen sinpe o ming hepho o‐hyd ogena ion eac ion,including(a)CO2,
(b)pho oca alys and(c)ligh sou ce,nop oduc couldbede ec ed.

Figu e 4.
T ansmission elec on mic oscopy (TEM) images o pho oca alys s: (
a
) CuAlGaO
4
and
(b) 1 w % P /CuAlGaO4pho oca alys s.
XPS P 4 spec a o CuAlGaO
4
and 1 w %P /CuAlGaO
4
pho oca alys s a e shown in Figu e 5.
Mos impo an ly, he e a e wo possible peaks a 73.6–74.1 and 77.0–77.4 eV ha e been obse ed
o 1 w % P /CuAlGaO
4
pho oca alys , which may be a ibu ed o 4
7/2
and 4
5/2
o P oxide,
espec i ely [26].
Ca alys s2017,7,635o 12

Figu e4.T ansmissionelec onmic oscopy(TEM)imageso pho oca alys s:(a)CuAlGaO4and(b)
1w %P /CuAlGaO4pho oca alys s.
XPSP 4 spec ao CuAlGaO4and1w %P /CuAlGaO4pho oca alys sa eshowninFigu e5.
Mos impo an ly, he ea e wopossiblepeaksa 73.6–74.1and77.0–77.4eVha ebeenobse ed o 
1w %P /CuAlGaO4pho oca alys ,whichmaybea ibu ed o4 7/2and4 5/2o P oxide,
espec i ely[26].

Figu e5.TheP 4 X‐ aypho oelec onspec oscopy(XPS)spec a o (a)CuAlGaO4and(b)1w %
P /CuAlGaO4pho oca alys s.
Las ly, heBETsu acea eao CuAlGaO4and1w %P /CuAlGaO4wasapp oxima ely0.65and
0.86m2∙g−1, espec i ely.Bo h alueswe eno  e yhighdue o hesolid‐s a e usionme hodwhich
ope a esa ahigh empe a u e,causing hepho oca alys agg ega ion.
2.2.Pho oca aly icCO2Reduc ionwi hGas,Liquid,Gas‐LiquidPhaseReac o s
In hiss udy,se e alblank es sinall ypeo  eac o s(gasphase,liquidphase,andgas‐liquid
phase eac o s)we econduc ed oanswe  heques ionwhe he  he o ma iono  he eac ion
p oduc scome ompho o‐hyd ogena ionandno  omCO2con amina ionso  hepho oca alys 
i sel .The esul o  es swasshowninTable3.The esul clea lyshows ha i CO2wasno in oduced
o he eac ingsys em,nop oduc wasgene a ede enwi h hep esenceo  hehyd ogen,
pho oca alys ,andligh i adia ion.I isimpo an  o emembe  ha  hepho oca alys syn hesis
p ocessdidno useanyca bon‐con ainingp ecu so .Thisblank es  esul is u he con i ma ion
ha  hepho oca alys i sel con ainsnea lynoca bon esidue.Table3alsoclea lyshows ha ,wi hou 
anyo  h eeessen ialelemen sinpe o ming hepho o‐hyd ogena ion eac ion,including(a)CO2,
(b)pho oca alys and(c)ligh sou ce,nop oduc couldbede ec ed.

Figu e 5.
The P 4 X- ay pho oelec on spec oscopy (XPS) spec a o (
a
) CuAlGaO
4
and (
b
) 1 w %
P /CuAlGaO4pho oca alys s.
Las ly, he BET su ace a ea o CuAlGaO
4
and 1 w % P /CuAlGaO
4
was app oxima ely 0.65 and
0.86 m
2·
g
−1
, espec i ely. Bo h alues we e no e y high due o he solid-s a e usion me hod which
ope a es a a high empe a u e, causing he pho oca alys agg ega ion.
2.2. Pho oca aly ic CO2Reduc ion wi h Gas, Liquid, Gas-Liquid Phase Reac o s
In his s udy, se e al blank es s in all ype o eac o s (gas phase, liquid phase, and gas-liquid
phase eac o s) we e conduc ed o answe he ques ion whe he he o ma ion o he eac ion
p oduc s come om pho o-hyd ogena ion and no om CO
2
con amina ions o he pho oca alys i sel .
The esul o es s was shown in Table 3. The esul clea ly shows ha i CO
2
was no in oduced o
he eac ing sys em, no p oduc was gene a ed e en wi h he p esence o he hyd ogen, pho oca alys ,
and ligh i adia ion. I is impo an o emembe ha he pho oca alys syn hesis p ocess did no use
any ca bon-con aining p ecu so . This blank es esul is u he con i ma ion ha he pho oca alys
i sel con ains nea ly no ca bon esidue. Table 3also clea ly shows ha , wi hou any o h ee essen ial
elemen s in pe o ming he pho o-hyd ogena ion eac ion, including (a) CO
2
, (b) pho oca alys and (c)
ligh sou ce, no p oduc could be de ec ed.

Ca alys s 2017,7, 63 6 o 12
Table 3. Summa y o blank es s o pho o-hyd ogena ion o CO2.
No. Type o
Reac o s
Expe imen al Condi ions Yield o P oduc s
(µmol·g−1)
CO2H2
(0.01 a m)
Pho oca alys
(1 w % P /CuAlGaO4)
Ligh
Sou ce
1Gas phase
eac o
X1O2O O BDL 3
2 O O X O BDL
3 O O O X BDL
4
Liquid phase
eac o
X O O O BDL
5 O O X O BDL
6 O O O X BDL
7Gas-liquid
phase
eac o
X O O O BDL
8 O O X O BDL
9 O O O X BDL
1
X—absen in he pho o eac o ;
2
O—p esen in he pho o eac o ;
3
BDL—below he de ec ion limi o
gas ch oma og aphy.
Fo CO
2
educ ion, he e a e se e al possible C
1
p oduc s, such as o mic acid, o maldehyde,
me hanol, and me hane. Because o he high de ec ion limi o o maldehyde by HPLC (3.3 ppm),
we used he Nash eagen [
27
] o eac wi h o maldehyde i s , hen analyzed he colo ed p oduc
diace yldihyd olu idine (DDL) by UV- is a 414 nm o quan i y he o maldehyde. The de ail was
desc ibed in he p e ious li e a u e [
27
,
28
]. E en he de ec ion limi o his me hod was 0.17
µ
mol
·
L
−1
,
which was abou h ee o de s lowe han HPLC, o maldehyde was no obse ed unde all in es iga ed
condi ions. Howe e , i is no ed ha al hough we did no de ec o maldehyde, some o he
o maldehyde migh also be o med on he su ace o he ca alys [
29
,
30
]. Fo he gas phase eac o ,
he e is also no o mic acid and me hanol could be de ec ed, and only me hane e olu ion would be
discussed in his case. The CH4yield unde gas phase condi ions was shown in Figu e 6.
Ca alys s2017,7,636o 12
Table3.Summa yo blank es s o pho o‐hyd ogena iono CO2.
No.Typeo Reac o s
Expe imen alCondi ions Yieldo P oduc s
(μmol∙g−1)
CO2H2
(0.01a m)
Pho oca alys 
(1w %P /CuAlGaO4)Ligh Sou ce
1
Gasphase eac o 
X1O2OOBDL3
2OOXOBDL
3OOOXBDL
4
Liquidphase eac o 
XOOOBDL
5OOXOBDL
6OOOXBDL
7
Gas‐liquidphase eac o 
XOOOBDL
8OOXOBDL
9OOOXBDL
1X—absen in hepho o eac o ;2O—p esen in hepho o eac o ;3BDL—below hede ec ionlimi o gasch oma og aphy.
Fo CO2 educ ion, he ea ese e alpossibleC1p oduc s,suchas o micacid, o maldehyde,
me hanol,andme hane.Becauseo  hehighde ec ionlimi o  o maldehydebyHPLC(3.3ppm),we
used heNash eagen [27] o eac wi h o maldehyde i s , henanalyzed hecolo edp oduc 
diace yldihyd olu idine(DDL)byUV‐ isa 414nm oquan i y he o maldehyde.Thede ailwas
desc ibedin hep e iousli e a u e[27,28].E en hede ec ionlimi o  hisme hodwas0.17μmol∙L−1,
whichwasabou  h eeo de slowe  hanHPLC, o maldehydewasno obse edunde all
in es iga edcondi ions.Howe e ,i isno ed ha al houghwedidno de ec  o maldehyde,someo 
he o maldehydemigh alsobe o medon hesu aceo  heca alys [29,30].Fo  hegasphase eac o ,
he eisalsono o micacidandme hanolcouldbede ec ed,andonlyme hanee olu ionwouldbe
discussedin hiscase.TheCH4yieldunde gasphasecondi ionswasshowninFigu e6.

Figu e6.The ime‐dependen yieldo CH4e olu iono e 1w %P /CuAlGaO4pho oca alys unde 
di e en H2(g)pa ialp essu esinagasphase eac o .Linesa eno basedonakine icmodelanda e
p esen ed o eye‐guidingonly.
Mos in e es ingly,H2playsanac i e oleonCH4yield.
CO2(g)+4H2(g)→CH4(g)+2H2O(l)
∆H0=−259.9kJ/mol;∆G0=−132.4kJ/mol(1)
I ise iden  ha  hecon e siono CO2in oCH4hasnega i e∆H0and∆G alues,meaning ha 
hep ocessisspon aneous,equilib ium a o able,andexo he mic.In hiss udy, heen halpy(∆H0)
andGibbs eeene gy(∆G0) aluesa 298K o se e alin e es ing eac ionswe ecalcula edbyAspen
[31].A  i s 2h, heini ial a eso CH4yieldwe e0.01,0.57,0.73,and0.40μmol∙g−1∙h−1 ha 
co espond oH2pa ialp essu e=0,0.01,0.1,0.2a m, espec i ely.I iswo hno ing ha inc easing
hepa ialp essu eo H2wouldenhance heyieldo  hep oduc .Among hecondi ionso H2pa ial
p essu e,0.1a mshows hebes pe o mance.Howe e ,anex asupplyo H2migh compe ewi h
CO2 o occupying heac i esi es,b inginganega i ee ec onCO2pho o‐hyd ogena ion.Asshown
Figu e 6.
The ime-dependen yield o CH
4
e olu ion o e 1 w % P /CuAlGaO
4
pho oca alys unde
di e en H2(g) pa ial p essu es in a gas phase eac o . Lines a e no based on a kine ic model and a e
p esen ed o eye-guiding only.
Mos in e es ingly, H2plays an ac i e ole on CH4yield.
CO2(g) + 4H2(g) →CH4 (g) + 2H2O(l)
∆H0=−259.9 kJ/mol; ∆G0=−132.4 kJ/mol (1)
I is e iden ha he con e sion o CO
2
in o CH
4
has nega i e
∆
H
0
and
∆
G alues, meaning
ha he p ocess is spon aneous, equilib ium a o able, and exo he mic. In his s udy, he en halpy
(
∆
H
0
) and Gibbs ee ene gy (
∆
G
0
) alues a 298 K o se e al in e es ing eac ions we e calcula ed by
Aspen [
31
]. A i s 2 h, he ini ial a es o CH
4
yield we e 0.01, 0.57, 0.73, and 0.40
µ
mol
·
g
−1·
h
−1
ha
Ca alys s 2017,7, 63 7 o 12
co espond o H
2
pa ial p essu e = 0, 0.01, 0.1, 0.2 a m, espec i ely. I is wo h no ing ha inc easing
he pa ial p essu e o H
2
would enhance he yield o he p oduc . Among he condi ions o H
2
pa ial
p essu e, 0.1 a m shows he bes pe o mance. Howe e , an ex a supply o H
2
migh compe e wi h
CO
2
o occupying he ac i e si es, b inging a nega i e e ec on CO
2
pho o-hyd ogena ion. As shown
in Figu e 6, CH
4
e olu ion became s able a e wo hou s in eac ion, implying ha he eac ions in he
sys em eached a dynamic equilib ium.
A compa a i e expe imen was also conduc ed in he liquid phase. The co ela ion be ween he
amoun o CH
4
, CH
3
OH, and HCOOH o med and he eac ion ime o he liquid phase a e shown in
Figu e 7.
Ca alys s2017,7,637o 12
inFigu e6,CH4e olu ionbecames ablea e  wohou sin eac ion,implying ha  he eac ionsin
hesys em eachedadynamicequilib ium.
Acompa a i eexpe imen wasalsoconduc edin heliquidphase.Theco ela ionbe ween he
amoun o CH4,CH3OH,andHCOOH o medand he eac ion ime o  heliquidphasea eshown
inFigu e7.

Figu e7.The ime‐dependen yieldo (a)CH4e olu ion,(b)CH3OH o ma ion,and(c)HCOOH
o ma iono e 1w %P /CuAlGaO4pho oca alys unde di e en H2(g)pa ialp essu esinaliquid
phase eac o .Linesa eno basedonakine icmodelanda ep esen ed o eye‐guidingonly.
Themainp oduc so  heliquidphase eac o we eCH4,CH3OH,andHCOOH, espec i ely.
CO2(g)+3H2(g)→CH3OH(l)+H2O(l)
∆H0=−137.8kJ/mol;∆G0=−10.7kJ/mol(2)
CO2(g)+H2(g)→HCOOH(l)
∆H0=−31.0kJ/mol;∆G0=+34.3kJ/mol(3)
I isclea  ha allexo he mic eac ionsexhibi nega i e∆H0 alues.Mo eo e ,many eac ions
a ealsoassocia edwi hnega i e∆G0 aluesand,asaconsequence, he eac ionwillbe
he modynamically a o able.Mos impo an ly,CH3OHwas ound obe hemajo p oduc unde 
cu en condi ion.This ac is ha CH3OH equi esonlysixpho oelec ons o  hepho o‐
hyd ogena iono CO2whileCH4needs oha eeigh pho oelec ons o  eac ion.Las bu no leas ,
anex aamoun o H2p o idedmayalsosupp ess heyieldo p oduc s,whichisconsis en wi h he
obse a ionin hegasphase.
Theco ela ionsbe ween heamoun o CH4,CH3OH,andHCOOH o medand he eac ion
ime o  hecombinedgas‐liquidphasea eshowninFigu e8.

Figu e8.The ime‐dependen yieldo (a)CH4e olu ion,(b)CH3OH o ma ion,and(c)HCOOH
o ma iono e 1w %P /CuAlGaO4pho oca alys unde di e en H2(g)pa ialp essu esingas‐liquid
phase eac o .Linesa eno basedonakine icmodelanda ep esen ed o eye‐guidingonly.
Figu e 7.
The ime-dependen yield o (
a
) CH
4
e olu ion, (
b
) CH
3
OH o ma ion, and (
c
) HCOOH
o ma ion o e 1 w % P /CuAlGaO4pho oca alys unde di e en H2(g) pa ial p essu es in a liquid
phase eac o . Lines a e no based on a kine ic model and a e p esen ed o eye-guiding only.
The main p oduc s o he liquid phase eac o we e CH4, CH3OH, and HCOOH, espec i ely.
CO2(g) + 3H2(g) →CH3OH(l) + H2O(l)
∆H0=−137.8 kJ/mol; ∆G0=−10.7 kJ/mol (2)
CO2(g) + H2(g) →HCOOH(l)
∆H0=−31.0 kJ/mol; ∆G0= +34.3 kJ/mol (3)
I is clea ha all exo he mic eac ions exhibi nega i e
∆
H
0
alues. Mo eo e , many eac ions a e
also associa ed wi h nega i e
∆
G
0
alues and, as a consequence, he eac ion will be he modynamically
a o able. Mos impo an ly, CH
3
OH was ound o be he majo p oduc unde cu en condi ion.
This ac is ha CH
3
OH equi es only six pho oelec ons o he pho o-hyd ogena ion o CO
2
while
CH
4
needs o ha e eigh pho oelec ons o eac ion. Las bu no leas , an ex a amoun o H
2
p o ided
may also supp ess he yield o p oduc s, which is consis en wi h he obse a ion in he gas phase.
The co ela ions be ween he amoun o CH
4
, CH
3
OH, and HCOOH o med and he eac ion
ime o he combined gas-liquid phase a e shown in Figu e 8.
Simila o he esul o he liquid phase, CH
3
OH s ill is he main p oduc o gas-liquid phase.
On he o he hand, he CH
4
yield has a endency o be supp essed a e conduc ing he expe imen
o wo hou s. I is possible ha an in e media e p oduc o CO
2
pho o- educ ion, CO, could be
gene a ed as he eac ion p oceeds (Equa ion (4)). Following is he compe ing eac ion pa hways o
pho o- educ ion o CO o CH
3
OH and HCOOH ia H
2
and H
2
O, espec i ely (Equa ions (5) and (6)).
This esul implies ha he p esence o pho oca alys in he gas phase will p omo e he con e ing o
CH4in o ano he compound, such as CH3OH o HCOOH, esul ing in a dec ease o CH4yield.
CO2(g) + CH4(g) →2CO(g) + 2H2(g)
∆H0=−247.5 kJ/mol; ∆G0= +170.8 kJ/mol (4)
Ca alys s 2017,7, 63 8 o 12
CO(g) + 2H2(g) →CH3OH(l)
∆H0=−131.6 kJ/mol; ∆G0=−29.9 kJ/mol (5)
CO(g) + H2O(g) →HCOOH(l)
∆H0=−24.8 kJ/mol; ∆G0= +15.1 kJ/mol (6)
In b ie summa y, he possible eac ion pa hways o CO
2
pho o-hyd ogena ion is illus a ed in
Scheme 1. This mechanism is p oposed based on he knowledge o p oduc s p esen ed du ing he
pho oca aly ic eac ion. Ou obse a ion indica es ha CH
4
is he only p oduc unde gas phase
condi ion. On he o he hand, CH
4
, CH
3
OH, and HCOOH a e ound in he liquid and gas-liquid phase
eac ions. Al hough we could no measu e an in e media e CO p oduc quan i a i ely, we expec
ha CO migh be gene a ed du ing he eac ion. In he gas-liquid phase eac o , he pho oca alys
is well packed and dispe sed on he qua z pla e in he gas phase. Hence, i migh p omo e he
ans o ma ion o CO in o CH
3
OH and HCOOH p oduc s. We do belie e ha conduc ing he
expe imen in di e en phases (such as gas, liquid, and gas-liquid phases) could no change he
mechanism o he pho o-hyd ogena ion o CO
2
. Howe e , he p esence o pho oca alys in di e en
phases migh accele a e di e en pa hways o he eac ion.
Ca alys s2017,7,637o 12
inFigu e6,CH4e olu ionbecames ablea e  wohou sin eac ion,implying ha  he eac ionsin
hesys em eachedadynamicequilib ium.
Acompa a i eexpe imen wasalsoconduc edin heliquidphase.Theco ela ionbe ween he
amoun o CH4,CH3OH,andHCOOH o medand he eac ion ime o  heliquidphasea eshown
inFigu e7.

Figu e7.The ime‐dependen yieldo (a)CH4e olu ion,(b)CH3OH o ma ion,and(c)HCOOH
o ma iono e 1w %P /CuAlGaO4pho oca alys unde di e en H2(g)pa ialp essu esinaliquid
phase eac o .Linesa eno basedonakine icmodelanda ep esen ed o eye‐guidingonly.
Themainp oduc so  heliquidphase eac o we eCH4,CH3OH,andHCOOH, espec i ely.
CO2(g)+3H2(g)→CH3OH(l)+H2O(l)
∆H0=−137.8kJ/mol;∆G0=−10.7kJ/mol(2)
CO2(g)+H2(g)→HCOOH(l)
∆H0=−31.0kJ/mol;∆G0=+34.3kJ/mol(3)
I isclea  ha allexo he mic eac ionsexhibi nega i e∆H0 alues.Mo eo e ,many eac ions
a ealsoassocia edwi hnega i e∆G0 aluesand,asaconsequence, he eac ionwillbe
he modynamically a o able.Mos impo an ly,CH3OHwas ound obe hemajo p oduc unde 
cu en condi ion.This ac is ha CH3OH equi esonlysixpho oelec ons o  hepho o‐
hyd ogena iono CO2whileCH4needs oha eeigh pho oelec ons o  eac ion.Las bu no leas ,
anex aamoun o H2p o idedmayalsosupp ess heyieldo p oduc s,whichisconsis en wi h he
obse a ionin hegasphase.
Theco ela ionsbe ween heamoun o CH4,CH3OH,andHCOOH o medand he eac ion
ime o  hecombinedgas‐liquidphasea eshowninFigu e8.

Figu e8.The ime‐dependen yieldo (a)CH4e olu ion,(b)CH3OH o ma ion,and(c)HCOOH
o ma iono e 1w %P /CuAlGaO4pho oca alys unde di e en H2(g)pa ialp essu esingas‐liquid
phase eac o .Linesa eno basedonakine icmodelanda ep esen ed o eye‐guidingonly.
Figu e 8.
The ime-dependen yield o (
a
) CH
4
e olu ion, (
b
) CH
3
OH o ma ion, and (
c
) HCOOH
o ma ion o e 1 w % P /CuAlGaO
4
pho oca alys unde di e en H
2(g)
pa ial p essu es in gas-liquid
phase eac o . Lines a e no based on a kine ic model and a e p esen ed o eye-guiding only.
Ca alys s2017,7,638o 12
Simila  o he esul o  heliquidphase,CH3OHs illis hemainp oduc  o gas‐liquidphase.
On heo he hand, heCH4yieldhasa endency obesupp esseda e conduc ing heexpe imen 
o  wohou s.I ispossible ha anin e media ep oduc o CO2pho o‐ educ ion,CO,couldbe
gene a edas he eac ionp oceeds(Equa ion(4)).Followingis hecompe ing eac ionpa hwayso 
pho o‐ educ iono CO oCH3OHandHCOOH iaH2andH2O, espec i ely(Equa ions(5)and(6)).
This esul implies ha  hep esenceo pho oca alys in hegasphasewillp omo e hecon e ingo 
CH4in oano he compound,suchasCH3OHo HCOOH, esul inginadec easeo CH4yield.
CO2(g)+CH4(g)→2CO(g)+2H2(g)
∆H0=−247.5kJ/mol;∆G0=+170.8kJ/mol(4)
CO(g)+2H2(g)→CH3OH(l)
∆H0=−131.6kJ/mol;∆G0=−29.9kJ/mol(5)
CO(g)+H2O(g)→HCOOH(l)
∆H0=−24.8kJ/mol;∆G0=+15.1kJ/mol(6)
Inb ie summa y, hepossible eac ionpa hwayso CO2pho o‐hyd ogena ionisillus a edin
Scheme1.Thismechanismisp oposedbasedon heknowledgeo p oduc sp esen eddu ing he
pho oca aly ic eac ion.Ou obse a ionindica es ha CH4is heonlyp oduc unde gasphase
condi ion.On heo he hand,CH4,CH3OH,andHCOOHa e oundin heliquidandgas‐liquid
phase eac ions.Al houghwecouldno measu eanin e media eCOp oduc quan i a i ely,we
expec  ha COmigh begene a eddu ing he eac ion.In hegas‐liquidphase eac o , he
pho oca alys iswellpackedanddispe sedon hequa zpla ein hegasphase.Hence,i migh 
p omo e he ans o ma iono COin oCH3OHandHCOOHp oduc s.Wedobelie e ha 
conduc ing heexpe imen indi e en phases(suchasgas,liquid,andgas‐liquidphases)couldno 
change hemechanismo  hepho o‐hyd ogena iono CO2.Howe e , hep esenceo pho oca alys 
indi e en phasesmigh accele a edi e en pa hwayso  he eac ion.
Scheme1.The eac ionpa hwaysdu ing hepho o‐hyd ogena iono CO2.
Table4summa izes hep oduc yieldsa  wohou sin h eekindso  eac o sunde di e en 
H2pa ialp essu es.Wesee ha H2playsanac i e olein hepho oca aly ic educ iono CO2.
Addi ionally,a ema kablesyne ge icac i i ywasclea lyobse edwhen heexpe imen was
conduc edingas‐liquidphaseunde 0.01a mo H2.Inmo ede ail, he o alp oduc sin hegasand
liquidphasea e0.400and0.243μmol∙g−1, espec i ely.In e es ingly,abou 8.302μmol∙g−1isachie ed
in hegas‐liquidphase.

Scheme 1. The eac ion pa hways du ing he pho o-hyd ogena ion o CO2.
Table 4summa izes he p oduc yields a wo hou s in h ee kinds o eac o s unde di e en
H
2
pa ial p essu es. We see ha H
2
plays an ac i e ole in he pho oca aly ic educ ion o CO
2
.
Addi ionally, a ema kable syne ge ic ac i i y was clea ly obse ed when he expe imen was
conduc ed in gas-liquid phase unde 0.01 a m o H
2
. In mo e de ail, he o al p oduc s in he gas
Ca alys s 2017,7, 63 9 o 12
and liquid phase a e 0.400 and 0.243
µ
mol
·
g
−1
, espec i ely. In e es ingly, abou 8.302
µ
mol
·
g
−1
is
achie ed in he gas-liquid phase.
Table 4.
Summa y o he p oduc yields a 2 h in h ee kinds o eac o s o e 1 w % P /CuAlGaO
4
unde di e en H2pa ial p essu es.
En y Phase H2Pa ial
P essu e (a m)
P oduc Yields (µmol·g−1)
CH4CH3OH HCOOH To al HCs 1
1
Gas
0.00 0.012 ±0.010 BDL 2BDL 0.012
2 0.01 0.400 ±0.100 BDL BDL 0.400
3 0.20 0.780 ±0.300 BDL BDL 0.780
4Liquid 0.00 0.010 ±0.010 0.235 ±0.100 BDL 0.245
5 0.01 0.149 ±0.050 BDL 0.094 ±0.045 0.243
6 0.20 0.112 ±0.030 0.340 ±0.200 BDL 0.452
7
Gas-Liquid
0.00 0.014 ±0.010 0.285 ±0.100 BDL 0.299
8 0.01 0.480 ±0.200 7.352 ±2.100 0.470 ±0.100 8.302
9 0.20 0.666 ±0.120 0.445 ±0.110 0.145 ±0.010 1.255
1
To al hyd oca bons yield = CH
4
yield + CH
3
OH yield + HCOOH yield);
2
BDL—below he de ec ion limi o
gas ch oma og aphy.
Las ly, he quan um e iciencies in h ee kinds o eac o s unde 0.01 a m o H
2
a e also calcula ed.
The highes quan um e iciency was in he gas-liquid phase eac o o 0.0011%, while he gas and
liquid phases we e abou 0.0001% and 0.0005%, espec i ely. The possible eason o highe quan um
e iciency in he gas-liquid phase eac o is ha bo h H
2
in he gas phase and he p o on in liquid phase
could be u ilized simul aneously, a o ding mo e chances o conduc CO2pho o- educ ion.
3. Ma e ials and Me hods
3.1. P epa a ion o Pho oca alys s
CuAlGaO
4
pho oca alys was i s ly p epa ed by solid-s a e usion me hod, which is men ioned
in p e ious s udies [
19
,
25
]. Fi s ly, coppe oxide (CuO, Showa, Tokyo, Japan), aluminum oxide
(Al
2
O
3
, Type A-5, Sigma-Ald ich, S . Louis, MO, USA), and
β
-gallium ioxide (
β
-Ga
2
O
3
,
≥
99.9%,
Sigma-Ald ich) powde s we e mixed in he mola a io o Cu/Al/Ga = 1:1:1 and pul e ized in a
mo a . Subsequen ly, he esul ing mix u e was calcined a 1150
◦
C o 12 h, and hen cooled o oom
empe a u e and u he pul e ized o ob ain a CuAlGaO4powde .
1 w % P /CuAlGaO
4
pho oca alys was p epa ed by he pho o-deposi ion me hod as desc ibed
in he same e e ence. The equi ed amoun o chlo opla inic acid hyd a e (H
2
P Cl
6·
xH
2
O,
≥
99.9%,
Sigma-Ald ich) solu ion was mixed wi h he as-p epa ed CuAlGaO
4
powde . He ein, he loading o
P on CuAlGaO
4
is 1 w %. The mixed solu ion was i adia ed by a UV sou ce (320–500 nm, EXFO
S1500, EXFO Inc., Quebec Ci y, QC, Canada) o 90 min o pe o m he pho o-deposi ion p ocess. A e
ha , he solid p oduc was cen i uged and washed wi h deionized wa e se e al imes. Finally, he
washed ma e ial was d ied a 80 ◦C o 8 h o ob ain 1 w % P /CuAlGaO4powde .
3.2. Cha ac e iza ion o Pho oca alys s
X- ay di ac ome e (XRD, Ul ima IV, Rigaku, Tokyo, Japan) equipped wi h Cu K
α
(1.5418 Å) was
used o e i y he c ys alline s uc u e o pho oca alys s. The UV- is di use e lec ance spec um o
he pho oca alys was ully eco ded o e he ange 300–800 nm by a Ca y 100 UV- isible spec ome e
(UV- is, Va ian Ca y 100, Agilen Technologies, San a Cla a, CA, USA). BaSO
4
was used as he
e lec ance s anda d. A ield emission scanning elec on mic oscope (FE-SEM, Nano SEM 230, FEI,
Hillsbo o, OR, USA) equipped wi h ene gy dispe si e spec oscopy (EDS) was used o di ec ly
e eal he p esence o he a omic elemen s in he pho oca alys s. T ansmission elec on mic oscopy