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Solar Energy 99 (2014) 55–66
Thermochemical CO2 splitting reaction with CexM1�xO2�d (M = Ti4+,Sn4+, Hf4+, Zr4+, La3+, Y3+ and Sm3+) solid solutions
Qingqing Jiang a,b, Guilin Zhou a,b, Zongxuan Jiang a, Can Li a,⇑
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy,
Dalian 116023, Chinab Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Received 25 July 2013; received in revised form 10 October 2013; accepted 20 October 2013Available online 26 November 2013
Communicated by: Associate Editor Michael Epstein
Abstract
This study deals with doping of CeO2 with different cations M (M = Ti4+, Sn4+, Hf4+, Zr4+, La3+, Y3+ and Sm3+) to improve thereaction activity in two step thermochemical CO2 splitting reaction. The results show that the addition of tetravalent cations M(M = Ti4+, Sn4+, Hf4+ and Zr4+) into CeO2 significantly enhances the O2 evolution activity. The O2 production at 1400 �C increasesas follows: CeO2 (2.5 ml/g) < Ce0.75Zr0.25O2 (6.5 ml/g) < Ce0.75Hf0.25O2 (7.2 ml/g) < Ce0.8Sn0.2O2 (11.2 ml/g) < Ce0.8Ti0.2O2 (13.2 ml/g). The corresponding CO production is increased from 4.5 ml/g for CeO2 up to 10.6 ml/g for Ce0.75Zr0.25O2 synthesized by solutioncombustion method. For Ce0.75Hf0.25O2 and Ce0.75Zr0.25O2 synthesized by hydrothermal method, steady-state CO and O2 productionwith a ratio of near 2 occurs simultaneously when the temperature exceeds 1100 �C and we define the phenomena as “one step thermo-chemical CO2 splitting reaction”. Further investigations for one step thermochemical CO2 splitting with both CeO2 and Ce0.75Zr0.25O2 ascatalysts and blank reaction tube as reference from 1200 �C to 1400 �C are also performed. 16.8 ml/h of CO was produced for 0.5 g ofCe0.75Zr0.25O2 at 1400 �C which indicates that one step reaction may be a promising way for CO2 reduction. For Ce0.8Ti0.2O2 and Ce0.8-
Sn0.2O2, although the O2 production is increased several times as compared to CeO2, the CO generation activity is still low due to theformation of Ce2Ti2O7 and Ce2Sn2O7 after the high temperature reduction reaction. The doping of trivalent cations M (M = La3+, Y3+
and Sm3+) into ceria has negative effect both on the O2 evolution activity and the CO production. The estimated activation energy for thereduction step of Ce0.75Zr0.25O2 is much lower than that of CeO2 and Ce0.85La0.15O2�d. The CO generation reaction of CeO2, Ce0.85-
La0.15O2�d and Ce0.75Zr0.25O2–C (synthesized by solid solution combustion method) is a surface limited reaction.� 2013 Elsevier Ltd. All rights reserved.
Keywords: Solar energy; CO2 reduction; Thermochemical splitting reaction; Ceria based solid solution; Kinetics
1. Introduction
The conversion of concentrated solar energy into chem-ical fuels from H2O and CO2 via two-step thermochemicalcycle has drawn much attention recently, because it is apromising option to produce long-term storable and trans-portable energy in the future and it is also a reasonable way
0038-092X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.solener.2013.10.021
⇑ Corresponding author. Tel.: +86 411 84379070; fax: +86 41184694447.
E-mail address: [email protected] (C. Li).
for CO2 reduction (Steinfeld, 2005; Pitz-Paal et al., 2011;Furler et al., 2012; Kodama et al., 2005, 2008; Gokonet al., 2009; Chueh and Haile, 2009; Chueh et al., 2010,2012). The two-step thermochemical splitting cycle withmetal oxide as redox material can be represented asfollows:
O2-releasing reaction:
MOox þ thermal energy ¼MOred þO2 ð1-1ÞCO-generation reaction:
MOred þ CO2 ¼MOox þ CO ð1-2Þ
56 Q. Jiang et al. / Solar Energy 99 (2014) 55–66
CeO2 has been regarded as a promising candidate forthe two step thermochemical splitting reaction because ofits excellent redox property. Abanades and Flament(2006) reported that CeO2 could achieve the two-step ther-mochemical H2O splitting reaction with rapid H2 genera-tion rate for the first time. Haile et al. (2009)demonstrated that H2, CO and CH4 can be produced fromH2O and CO2 with CeO2 as redox oxide. The main draw-back of CeO2 is that the reduction to nonstoichiometricCeO2�d below 1500 �C shows restricted chemical yield(Gal et al., 2011).
Many investigations showed that the addition of otherisovalent/aliovalent cations into CeO2 could noticeablyimprove its thermal stability and redox property (Reddyet al., 2007; Dutta et al., 2006; Baidya et al., 2009; Yanget al., 2010; Bueno-Lopez et al., 2005; Ma et al., 2010;Mamontov and Egami 2000). The isovalent cations fre-quently used were Ti4+, Zr4+, Hf4+, Sn4+, etc., and the alio-valent cations were Fe3+, Pr3+, La3+, Sm3+, Y3+ andothers. Several studies on the addition of dopants (Li2+,Mg2+, Ca2+, Mn2+, Fe2+, Ni2+, Cu2+, Zr4+, Hf4+, Sm3+,Y3+, La3+ and Pr3+) into ceria have already been carriedout for thermochemical H2O splitting reaction (Menget al. 2012; Gal and Abanades. 2012; Meng et al. 2011;Scheffe and Steinfeld 2012; Gal and Abanades 2011; Menget., 2012; Kanko et al. 2007). Most of these works focusedon the effect of different dopants on the H2 production.However, few publications gave the detailed kinetic analy-sis, especially the kinetic analysis for the reduction step(Tamaura et al., 2011, 2012). The correlation between thestructure of oxide and reaction performance was not wellunderstood.
This study deals with doping of ceria with different cat-ions to improve the reaction activity in two step thermo-chemical CO2 splitting reaction for CO production.Doping ceria with tetravalent cations (Ti4+, Sn4+, Hf4+
and Zr4+) improves the O2 evolution activity significantly.The corresponding CO production is increased from4.5 ml/g for CeO2 up to 10.6 ml/g for Ce0.75Zr0.25O2 and9.8 ml/g for Ce0.75Hf0.25O2. The estimated activation ener-gies for the reduction step agree well with the O2 evolutionactivity and the kinetic models for CO generation reactionexplain the main limitation of the reaction.
2. Experimental section
2.1. Synthesis of materials
CexM1�xO2�d (M = Hf4+, Zr4+, La3+, Sm3+ and Y3+,x = 0.75–0.95) solid solutions were synthesized by a hydro-thermal method according to a previous report with(NH4)2Ce(NO3)6, Zr(NO3)4�5H2O, HfCl4, La(NO3)�6H2O,Sm(NO3)3�6H2O, Y(NO3)3�6H2O, diethylenetriamine(DETA), and melamine as starting materials (Singh andHegde, 2008). Briefly, for the synthesis of CexM1�xO2�d
(M = La3+, Sm3+ and Y3+, x = 0.95–0.85), a certain ratioof nitrates were dissolved into deionized water under vigor-
ous stirring, and then DETA was added into the abovesolution. The resulting gel was transferred into a sealedTeflon autoclave and it was heated at 200 �C for 24 h ina hot air oven. The resulting precipitate was washed withdeionized water for several times. Finally, the sample wasdried at 120 �C overnight and calcined in air at 600 �Cfor 4 h. The preparation of CexHf1�xO2 and CexZr1�xO2
followed the similar procedure except for melamine wasused as complexing agent. The synthesized CexZr1�xO2
here was named CexZr1�xO2–H.CexTi1�xO2 (x = 0.8–0.9) was prepared with a coprecip-
itation method (Luo et al., 2001). Ti(OCH(CH3)2)4 andCe(NO3)3�6H2O were dissolved into ethanol and thenammonium hydroxide (25 wt.% NH3) was added into thesolution. The resulting precipitate was filtered and driedin air at 120 �C overnight and then it was calcined at650 �C for 4 h to yield CexTi1�xO2.
CexSn1�xO2 (x = 0.8–0.9) was prepared by the solutioncombustion method (Horlait et al., 2011). (NH4)2-
Ce(NO3)6, SnC2O4, and glycine in a certain mole ratio weredissolved in deionized water and then HNO3 was addedinto the solution to obtain a clear solution. The solutionwas transferred to preheated furnace kept at 500 �C. Afterdehydration, the whole mass catched fire and then Cex-
Sn1�xO2 was formed. Finally, the sample was calcined at600 �C for 4 h in the furnace. Ce0.75Zr0.25O2 was also pre-pared by the solution combustion method and it wasnamed as Ce0.75Zr0.25O2–C.
2.2. Characterizations of materials
The synthesized samples were characterized by X-raypowder diffraction (XRD) on a Rigaku D/Max-2500/PCpowder diffractometer. Each sample powder was scannedusing Cu Ka radiation with an operating voltage of40 kV and an operating current of 200 mA. The scan rateof 5 � min�1 was applied to record the XRD patterns inthe range of 20–80� at a step size of 0.02�.
The surface areas of the samples were taken with aMicromeritics ASAP 2000 adsorption analyzer. Ramanscattering spectra were recorded in back-scattering geome-try on an Acton Raman spectrometer equipped with theliquid nitrogen cooled CCD detector at a resolution of4 cm�1. A 532 nm semiconductor laser was used as theexcitation source with the power of 60 mW. UV Ramanspectra were collected on a home-made Raman spectro-graph system. The spectrograph is a triple-stage dispersedsubtractive spectrograph, in which the first two stages areused to cut off the Rayleigh line and the third one is usedto collect Raman spectra using a CCD detector. The scat-tered lights were collected by the ellipse collecting mirror ina back-scattering geometry and focused into the entranceof the Raman spectrograph. The Raman spectra wererecorded with a spectral resolution of 2 cm�1 with the laserexcitation at 325 nm from He–Cd Laser.
Elemental analyses were conducted on a Magix-2424X-ray fluorescence (XRF) spectrometer.
Fig. 1. XRD patterns of the synthesized samples and the samples afterCO2 splitting reactions. (a) CexM1�xO2: the synthesized samples; (b)CexM1�xO2: (c) the samples after CO2 splitting reactions.
Table 1Lattice constant, Crystallite size, BET surface area and Chemicalcomposition of CexM1�xO2�d (M = Ti4+, Sn4+, Hf4+, Zr4+, La3+, Y3+
and Sm3+).
Samples Latticeconstant(A)a
Crystallitesize (A)a
Surfacearea(m2 g�1)
ChemicalComposition(Ce:M)b
CeO2 5.425 133 41.2Ce0.75Hf0.25O2 5.344 120 89.4 78.4:21.6Ce0.75Zr0.25O2 5.348 110 99.6 76.9:22.3Ce0.8Ti0.2O2 5.371 124 40.3 82.4:17.6Ce0.8Sn0.2O2 5.360 134 36.2 81.5:18.5
Q. Jiang et al. / Solar Energy 99 (2014) 55–66 57
The morphology of the samples was examined by Scan-ning Electron Microscopy (SEM) images taken with aQuanta 200FEG scanning electron microscope.
Ce0.85La0.15O2�d 5.441 122 54.5 84.9:14.8Ce0.85Y0.15O2�d 5.412 137 44.8 88.8:11.2Ce0.85Sm0.15O2�d 5.433 131 66.4 84.0:16.0CeO2
c >1000 1.7Ce0.85La0.15O2�d
c >1000 2.7Ce0.75Hf0.25O2
c >1000 1.3Ce0.75Zr0.25O2–
Hc>1000 1.4
Ce0.75Zr0.25O2–Cc
>1000 2.1
a Lattice constant and crystallite size are calculated from XRD results.b Molar percentage is from XRF.c Samples after high temperature treatment.
2.3. Reaction activity test
The two-step thermochemical CO2 splitting reaction wascarried out at a laboratory scale in a fixed bed configura-tion. The vertical alumina tubular reactor was placed insidean electric furnace. The argon flow (purity 99.9996%) firstlypassed through a deoxidation tube to get rid of the residualO2 before it passed into the reactor. The O2 concentrationwas about 20 ppm in the reactor during the reaction.
For the O2-releasing experiment, the sample (0.5 g) washeated to 1200 �C with a 20 �C/min heating rate and then
to 1400 �C or 1500 �C with a 5 �C/min heating rate. Thetemperature plateau at 1400 �C or 1500 �C was maintainedfor 40 min while passing Ar at a flow rate of 100 ml/min.The O2 gas was analyzed with a gas chromatograph (Ali-gent 6890) equipped with a 5A molecular sieve columnand a TCD detector, which took gas sample at the reactoroutlet every ca. 2 min.
The reduced state of the oxide was maintained under theprotection of Ar with a flow rate of 500 ml/min before CO2
was exposing to the reactor. For the CO generation step,the electric furnace was maintained at a certain tempera-ture (600–1200 �C), and then CO2 with a flow rate of500 ml/min was injected to react with the oxygen-deficientmaterial. The CO gas product was analyzed with a gaschromatograph (Aligent 6890) equipped with a GDX-01column and a FID detector, which took gas sample atthe reactor outlet every ca. 1 or 2 min. Numerical integra-tion of the molar flow rate-time curves gave the totalamount of O2 and CO production.
The CO generation rate of CeO2 and Ce0.85La0.15O2�d
was fast and it could be finished within 1 min. In orderto solve the problem of gas sampling and obtain reliabledata, the experiments were carried out as follows: afterthe reduced CeO2 was exposing to CO2, a stopwatch wasused to record the data vs. time. For example, gas samplingwas done after CO2 was inleted for 10 s and thus the dataat 10 s was obtained. Repeat the experiment with sameredox oxide under the same experimental conditions exceptfor gas sampling at different time. Take CO generationreaction of CeO2 at 800 �C as an example, gas samplingwas carried out after CO2 was inleted for 10 s, 13 s, 15 s,17 s, 20 s, 25 s, 35 s, and 58 s, respectively. In other words,
Fig. 2. Raman spectra of (a) the synthesized samples at 532 nm, (b) thesynthesized samples at 325 nm, and (c) the samples after CO2 splittingreactions at 532 nm.
58 Q. Jiang et al. / Solar Energy 99 (2014) 55–66
in order to obtain the CO generation rate-time profile ofCeO2 at 800 �C, the experiment was repeated for 8 timesunder exactly same experimental conditions but gas sam-pling at different reaction times. To ensure the accuracyof the experimental results, each rate-time profile wasrepeated for 3 times. So the reaction rate was reliable forkinetic analysis. For Ce0.75Zr0.25O2 and Ce0.75Hf0.25O2, sev-eral initial point of the reaction rate was also obtained bythe same method. Many reactions for the same redox oxidehave been carried out to ensure the accuracy of the data.
For one step thermochemical CO2 splitting reaction,0.5 g of the oxide was heated to 1200 �C or 1300 �C or1400 �C with a 20 �C/min heating rate and then CO2 with
a flow rate of 500 ml/min was directly injected to the reac-tor. The steady-state CO and O2 production occurs simul-taneously with the oxide as catalyst.
The O2 evolution rate-time profiles with different heat-ing rate were performed as follows. 0.2 g of the Ce0.75
Zr0.25O2 was heated to 1000 �C with a 20 �C/min heatingrate and then to 1500 �C with a specified heating rate(5 �C/min or 10 �C/min or 20 �C/min). For CeO2 andCe0.85La0.15O2�d, 0.2 g sample was heated to 1000 �C witha 20 �C/min heating rate and then to 1500 �C with a10 �C/min heating rate.
3. Results and discussion
3.1. Structure of M-doped CeO2
The XRD patterns shown in Fig. 1 confirm that all theprepared oxides present a cubic fluorite structure similar tothat of CeO2 (JCPDF 81-0792). The lattice constants ofthese solid solutions are changed (Table 1) as comparedto CeO2 which confirm that the doped cations enter thecrystal lattice of CeO2 (Dacheux et al., 2011). ForCe0.75Zr0.25O2 and Ce0.85La0.15O2-d, the samples after hightemperature reactions are still in cubic fluorite structurewithout impurity phase. For CexHf1�xO2 samples afterreactions, there is no phase separation for Ce0.8Hf0.2O2,however, HfO2 is observed for Ce0.75Hf0.25O2. TheCe2Ti2O7 and Ce2Sn2O7 phases are formed after the hightemperature reduction for Ce0.8Ti0.2O2 and Ce0.8Sn0.2O2
(Kim et al., 2008; Huang et al., 2011).Fig. 2 shows the Raman spectra of samples with dif-
ferent excitation laser lines. The band at ca. 465 cm�1 isthe characteristic of the cubic fluorite structure. The shiftin the 465 cm�1 is attributed to the change in the M–Ovibration which accounts for the difference in the ionicradius of the dopants (Guo et al., 2011; Prasad et al.,2012). It also confirms that the doped cations enter thecrystal lattice of CeO2 which is in line with XRD result.The band at ca. 600 cm�1 is ascribed to the intrinsic oxy-gen vacancies which can be observed obviously in theRaman spectrum with excitation laser line at 325 nm.The ratio between the band intensities of 600–465 cm�1
has been related to the concentration of oxygen vacan-cies, and the higher the I600/I465 ratio, the higher theconcentration of oxygen vacancies (Guo et al., 2011).The addition of dopants into CeO2 induces large numberof oxygen vacancies. The fact that there are no bandsdue to La2O3 or ZrO2 phases after CO2 splitting reactionindicates that the phase structure of Ce0.75Zr0.25O2 andCe0.85La0.15O2�d is stable after high temperature reduc-tion reaction. For the Ce0.9Ti0.1O2 sample after CO2
splitting reaction, Raman bands appeared at 234 cm�1,368 cm�1 and 569 cm�1 are attributed to the compoundwith a monoclinic phase formed by substituting Ceatoms into the TiO2 lattice (Luo et al., 2011). New phaseis formed after high temperature reduction for Ce0.9Ti0.1O2
which agrees well with XRD patterns.
Fig. 3. The instantaneous O2 evolution rate-time profiles of (a) CeO2 and CexM1�xO2�d (M = Ti4+, Sn4+, Hf4+, Zr4+ and La4+) and (b) CexHf1�xO2
samples with different ratio of Hf4+ to Ce4+.
Table 2The O2 production of CeO2 and CexM1-xO2-d (M = Ti4+, Sn4+, Hf4+, Zr4+, La3+, Y3+ and Sm3+) at different temperatures.
O2 production in the work (ml/g) Ionic radius of dopants (10�10 m) O2 production in the ref. (ml/g)
1500 �C 1400 �C 1300 �C 1200 �C 1500 �C 1400 �C
CeO2 5.7 2.5 0.96 (Ce4+) 4.3 ± 0.3 2.0Ce0.8Ti0.2O2 13.2 0.66 (Ti4+)Ce0.8Sn0.2O2 11.2 0.69 (Sn4+)Ce0.75Hf0.25O2 10.2 7.2 4.7 3.0 0.78 (Hf4+)Ce0.75Zr0.25O2 9.2 6.5 3.5 2.3 0.84 (Zr4+) 6.7Ce0.9La0.1O2�d 4.5 2.0 1.17 (La3+) 2.1Ce0.85La0.15O2�d 3.9 1.5 1.17 (La3+)Ce0.85Sm0.15O2�d 3.8 1.4 1.08 (Sm3+) 3.0Ce0.85Y0.15O2�d 3.9 1.4 1.03 (Y3+)
Q. Jiang et al. / Solar Energy 99 (2014) 55–66 59
3.2. O2-evolution reaction
The samples were reduced at temperature up to 1400 �C(with a heating rate of 20 �C/min to 1200 �C and then5 �C/min to 1400 �C) under an argon flow of 100 ml/min.The argon flow (purity 99.9996%) firstly passed through adeoxidation tube to purge the residual O2 before it passedinto the reactor. The O2 concentration was about 20 ppmin the reactor during the reaction process.
Fig. 3(a) presents the O2 evolution rate-time profiles ofCexM1�xO2�d (M = Ti4+, Sn4+, Hf4+, Zr4+ and La3+)verse the temperature. The initial O2-releasing temperatureof tetravalent cations M (M = Hf4+, Zr4+ and Ti4+) dopedceria is greatly lowered (around 900 �C) as compared toCeO2 which starts reduction at about 1300 �C. The O2
evolution rate of CexM1�xO2 (M = Ti4+, Sn4+, Hf4+ and
Zr4+) is much higher than that of CeO2, especially forCe0.8Ti0.2O2 and Ce0.8Sn0.2O2. The O2 production forCexM1�xO2�d from 1200 �C to 1500 �C is listed in Table 2.Previous results are also presented in Table 2 as a compar-ison. Different synthesis method and different experimentalconditions induce some difference in the O2 evolutionactivity.
The results in Table 2 clearly suggest that the addition oftetravalent cations into ceria could improve the O2 produc-tion at a relatively lower temperature. The tetravalent cat-ion with a smaller ionic radius is more favorable for O2
production (Tamaura et al., 2011). Previous experimentaland theoretical studies showed that the addition of tetrava-lent cations with smaller ionic radius than Ce4+ decreasedboth the oxygen diffusivity barrier and the vacancy forma-tion energy (Andersson et al., 2007a,b; Shah et al., 2006;
0
20
40
60
CeO2CO generation step
400 ml.min-1.g-1
800 ml.min-1.g-1
1200 ml.min-1.g-1
Time / min
Reaction condition: 800(a)
0
20
40
60
80(b)
Time / min
CeO2CO generation step
700 800 900 1000 1100
0.0
0.5
1.0
1.5
2.0
2.5
Time / min
600 700 800 900 1000 1100 1200
Ce0.75Zr0.25O2-C - CO generation step
(c)
0.0
0.1
0.2
0.3
0.4
Time / min
Ce0.75Zr0.25O2-H CO generation step
1000 1100 1200
(d)
0.0
0.1
0.2
0.3
0.4
0.5
Time / min
1000 1100 1200
Ce0.75Hf0.25O2CO generation step
(e)
0
1
2
3
(f)
Ce0.8Ti0.2O2 CO generation step
Time / min
90010001100
0.0 0.5 1.0 1.5 2.0 0.4 0.8 1.2 1.6
0 10 20 30 0 20 40 60 80 100 120
0 20 40 60 80 100 120 140 0 1 2 3 4
0 1 2 3 4 50
4
8
12
16
Ce0.85La0.15O2-δ- CO generation
Time / min
700 800 900
(g)
Am
ount
of C
O e
volv
ed-1
g-1
ml m
inA
mou
nt o
f CO
evo
lved
-1 g
-1m
l min
Am
ount
of C
O e
volv
ed-1
g-1
ml m
in
Am
ount
of C
O e
volv
ed-1
g-1
ml m
inA
mou
nt o
f CO
evo
lved
-1 g
-1m
l min
Am
ount
of C
O e
volv
ed-1
g-1
ml m
in
Am
ount
of C
O e
volv
ed-1
g-1
ml m
in
Fig. 4. The instantaneous CO generation rate-time profiles (a) as a function of CO2 flow rate at 800 �C, (b–g) of CeO2 and CexM1�xO2�d (M = Zr4+,Hf4+, Ti4+ and La3+) from 600 �C to 1200 �C with CO2 flow rate of 500 ml/min.
60 Q. Jiang et al. / Solar Energy 99 (2014) 55–66
Nakayama and Martin, 2009). The oxygen diffusivitydetermines how fast the bulk ceria based material canexchange oxygen with the surrounding environment andthe low vacancy formation energy means high reducibilityat a lower reduction temperature. Consequently, the ceriadoped with tetravalent cations (Ti4+, Sn4+, Hf4+ andZr4+) with smaller ionic radius exhibit higher O2 evolutionactivity than CeO2.
However, the initial reduction temperature of Ce0.85
La0.15O2�d is even higher than that of CeO2 and the O2
evolution rate is slightly lower than CeO2. The doping oftrivalent cations M (M = La3+, Sm3+ and Y3+) into ceriahas negative effect on the activity of O2 evolution. TheO2 production of trivalent cations doped ceria is indepen-dent of their ionic radius. According to the previous stud-ies, the addition of trivalent cations M (M = La3+, Sm3+
and Y3+) into ceria creates large number of oxygen vacan-cies, therefore the useful oxygen non-stoichiometry d forM3+ doped ceria is lower than CeO2. In addition, dopingwith larger size M3+ increases the oxygen diffusivity barrier
Fig. 5. SEM images of (a) Ce0.75Zr0.25O2–C and (b) Ce0.75Zr0.25O2–H.
Table 3The CO production (ml/g) for CeO2 and CexM1�xO2�d (M = Ti, Hf, Zr and La) at different temperatures.
CO production for two step cycles (ml/g) The ratio of CO:O2 CO production for one step splitting reaction (ml/h)
700 �C 800 �C 900 �C 1000 �C 1100 �C 1400 �C
CeO2a 5.4 7.8 5.6 3.3 3.6 13.5
CeO2b 4.5 1.8
Ce0.75Zr0.25O2–Cb 7.5 10.0 10.6 9.4 10.1 1.6 16.8Ce0.75Zr0.25O2–Hb 2.7Ce0.75Hf0.25O2
b 2.5Ce0.8Ti0.2O2
b 1.2 0.8 0.4Ce0.85La0.15O2�d
a 5.6 6.1 5.1 1.6
Reaction conditions:a O2 evolution reaction was performed at 1500 �C for 40 min.b O2 evolution reaction was performed at 1400 �C for 40 min.
Fig. 6. One step CO2 splitting reaction with both CeO2 and Ce0.75Zr0.25O2 as catalysts and blank reaction tube as reference from 1200 �C to 1400 �C. CO2
with a flow rate of 500 ml/min was directly injected to the reactor. 0.5 g of CeO2 and Ce0.75Zr0.25O2 was utilized in the one step splitting reaction.
Q. Jiang et al. / Solar Energy 99 (2014) 55–66 61
which is also unfavorable for O2 evolution (Reddy et al.2010; Nakayama and Martin, 2009.).
Fig. 3(b) exhibits the O2 evolution rate-time profiles forCexHf1�xO2 samples with different ratios of Hf4+ to Ce4+.The O2 evolution activity increases with increasing theratio of Hf4+ to Ce4+, however, the XRD patterns showthat the phase separation occurs for Ce0.75Hf0.25O2 afterhigh temperature reaction.
3.3. CO generation reaction
The reduced state of the oxide was maintained under theprotection of Ar with a flow rate of 500 ml/min before CO2
was exposing to the reactor. The CO2 splitting reaction wasperformed from 600 �C to 1200 �C. For CeO2, the CO gen-eration rate and production is largely dependent on themolar ratio of CO2 to CeO2 as shown in Fig. 4(a). The
Fig. 7. Cycle performance of Ce0.75Zr0.25O2–C for 10 times. O2 evolutionwas performed at 1400 �C and CO generation reaction was performed at900 �C.
62 Q. Jiang et al. / Solar Energy 99 (2014) 55–66
CO generation rate increases significantly with increasingthe molar ratio of CO2 to CeO2. Fig. 4(b) indicates thatthe CO generation rate for CeO2 increases with tempera-ture from 700 �C to 900 �C and then it decreases from900 �C to 1100 �C. The CO generation rate for CeO2 is veryfast and the reaction can be finished within 1 min.
Fig. 4(c) and (d) shows the CO generation rate-time pro-files for Ce0.75Zr0.25O2 synthesized via solution combustion(Ce0.75Zr0.25O2–C) method and hydrothermal (Ce0.75
Zr0.25O2–H) method, respectively. It can be seen that theCO generation reaction of Ce0.75Zr0.25O2–C is quite differ-ent from Ce0.75Zr0.25O2–H. For Ce0.75Zr0.25O2–C, the reac-tion rate increases gradually from 600 �C to 800 �C andthen keeps stable from 800 �C to 1200 �C. For Ce0.75Zr0.25
O2–H, the CO generation rate is much lower than that ofCe0.75Zr0.25O2–C and it increases from 1000 �C to1200 �C gradually. The faster CO generation rate of Ce0.75
Zr0.25O2–C is mainly due to its porous structure, as shownin Fig. 5. As listed in Table 3, when the reduction reactionis performed at 1400 �C, the CO production is increasedfrom 4.5 ml/g for CeO2 up to 10.6 ml/g for Ce0.75Zr0.25
O2–C. The CO generation rate-time profiles of Ce0.75Hf0.25
O2 are very similar to those of Ce0.75Zr0.25O2–H. For eachprofile, the reaction rate decreases with the proceeding ofthe generation reaction and the peak tailing is long.
Fig. 8. O2 evolution rate of (a) Ce0.75Zr0.25O2–C under heating rate of 5 �C/mCe0.75Zr0.25O2–C under 10 �C/min heating rate.
Interestingly, as shown in Fig. 4(c) and (d), the CO gen-eration reaction rate for Ce0.75Zr0.25O2 and Ce0.75Hf0.25O2
does not return to baseline after a long period at 1100 �Cand the CO plateau becomes much stronger at 1200 �C.The similar phenomenon was recently reported by Scheffeet al. (2013) for H2O splitting reaction with cobalt ferrite-zirconia composite as redox oxide. They inferred that thereduction reaction and H2 production reaction occurredsimultaneously that was analogous to the water–gas shift(WGS) reaction. Therefore, the direct CO2 splittingreaction occurs with ceria based solid solutions as catalystswhen the temperature exceeds 1100 �C. We define the reac-tion as one step thermochemical CO2 splitting reaction.
Further investigations are performed with both CeO2
and Ce0.75Zr0.25O2 as catalysts and blank reaction tube asreference from 1200 �C to 1400 �C. The one step CO2 split-ting reaction operates as follows: CO2 with a flow rate of500 ml/min is injected to the reaction tube at a specifiedtemperature. The reaction performance is shown inFig. 6. The steady-state CO and O2 production with theratio of near 2 is observed for both catalysts and blankreaction tube. The CO production for Ce0.75Zr0.25O2 is lar-ger than that of CeO2 at 1200 �C and 1300 �C, however,when the reaction temperature increases to 1400 �C, theCO production of CeO2 becomes much higher than Ce0.75
Zr0.25O2. The activity of the blank tube may be due to thedirect thermal splitting of CO2 at high temperatures andthe redox oxide catalyst enhances the activity of the directCO2 splitting reaction. The CO production for 0.5 g Ce0.75
Zr0.25O2 at 1400 �C is 16.8 ml/h which indicates that directCO2 splitting reaction may be a promising way for solarthermal CO2 reduction reaction.
The Ce0.8Ti0.2O2 sample produces only 1.2 ml/g CO at900 �C in spite of high reduction yield because of the for-mation of a new species (Ce2Ti2O7) during the high temper-ature reduction. The Ce0.8Sn0.2O2 sample also exhibitspoor CO generation activity due to the formation of Ce2-
Sn2O7 phase.For Ce0.85La0.15O2�d, the CO generation rate-time pro-
files are similar to CeO2. Although the CO generation rateis lower than CeO2, it is still very fast comparing to Ce0.75
Zr0.25O2 and Ce0.75Hf0.25O2.
in, 10 �C/min and 20 �C/min, respectively, (b) CeO2, Ce0.85La0.15O2�d and
-4.8
-4.0
-3.2
-2.4
-1.6
L-F1: y = 5.635 - 1.493x; R2 = 0.998
H-F1: y = 24.114 - 4.488x; R2 = 0.960
ln k
1/T (x104 K-1) 1/T (x104 K-1)
1/T (x104 K-1) 1/T (x104 K-1)
1/T (x104 K-1) 1/T (x104 K-1)
L-F1 Linear fit of L-F1 H-F1 Linear fit of H-F1
(a)
-5
-4
-3
-2
-1
0
L-F2: y = 7.845 - 1.807x; R2 = 0.996
H-F2: y = 36.550 - 6.549x; R2 = 0.942
ln k
L-F2 Linear fit of L-F2 H-F2 Linear fit of H-F2
(b)
-5
-4
-3
-2
-1
CeO2
L-R2: y = 4.529 - 1.336x; R2 = 0.999
H-R2: y = 20.539 - 3.939x; R2 = 0.956
L-R2 Linear fit of L-R2 H-R2 Linear fit of H-R2
(c)
ln k
-5
-4
-3
-2
-1(d)
L-R3: y = 4.897-1.389x; R2 = 0.999
H-R3: y = 23.340-4.388x; R2 = 0.946
ln k
L-R3 Linear fit of L-R3 H-R3 Linear fit of H-R3
CeO2
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0R3 Linear fit of R3
(e)
ln k
y = 5.02106 - 1.38364x; R2 = 0.995
Ce0.75Zr0.25O2
6.0 6.4 6.8 7.2 6.0 6.4 6.8 7.2
5.6 6.0 6.4 6.8 7.2 5.6 6.0 6.4 6.8 7.2
6.0 6.4 6.8 7.2 7.6 5.6 5.8 6.0 6.2 6.4-5
-4
-3
-2
-1
L-R3: y = 5.902-1.549x; R2 = 0.999
H-R3: y = 33.918-6.218x; R2 = 0.944
ln k
L-R3 Linear fit of L-R3 H-R3 Linear fit of H-R3
Ce0.85La0.15O2-δ
(f)
Fig. 9. Arrhenius plot for the non-isothermal reduction of (a–f) CeO2; (e) Ce0.75Zr0.25O2-combustion; (f) Ce0.85La0.15O2�d.
Q. Jiang et al. / Solar Energy 99 (2014) 55–66 63
Cycle performance for Ce0.75Zr0.25O2-C was studiedwith the reduction temperature at 1400 �C and CO genera-tion reaction at 900 �C for 10 times. As shown in Fig. 7, theO2 and CO production decreases gradually with theincreasing of cycle number.
3.4. Kinetic analysis
The kinetic studies for the O2 evolution reaction wereperformed under non-isothermal experimental conditionwith a constant heating rate. According to the literature(Levenspiel, 1972; Galwey and Brown, 1999; Meng et al.,2012; Gal and Steinfeld, 2011; Schunk and steinfeld,2009; Francis et al., 2010, the model can be expressed asfollows:
lnðda=dT Þb
f ðaÞ
� �¼ ln A� Ea
RTð3-1Þ
Previous investigations (Tamaura et al., 2012) showedthat only reaction order and geometrical-contracting mod-els were appropriate for the O2 evolution step of ceriabased solid solutions, as illustrated below.
f ðaÞ ¼ ð1� aÞn ð3-2Þ
And a is defined as:
a ¼ Total O2ðtÞTotal O2
ð3-3Þ
The curve of ln ðda=dT Þbf ðaÞ
h iversus 1/T should be linear if
the hypothetical model is reasonable, and then activationenergy Ea can be calculated from slope of the line.
The O2 evolution rate-time profiles for Ce0.75Zr0.25O2
from 1000 �C to 1500 �C with different heating rate areshown in Fig. 8(a). A constant heating rate of b = 10 �C/minis chose for the following kinetic studies. The O2 evolution
0.0
0.4
0.8
1.2
1.6
(a)
F1
(dα
/dt)
/(dα
/dt) α
=0.5
(dα
/dt)
/(dα
/dt) α
=0.5
(dα
/dt)
/( dα
/dt) α
=0.5
(dα
/dt)
/(dα
/dt) α
=0.5
(dα
/dt)
/(dα
/dt) α
=0.3
(dα
/dt)
/( dα
/dt) α
=0.3
(dα
/dt)
/( dα
/dt) α
=0.3
Fractional Reaction α
CeO2 - CO generation - 800
0
1
2
3 Ce0.85La0.15O2-δ- CO generation-700
Fractional Reaction α
(b)F1
0.0
0.4
0.8
1.2
1.6
Fractional Reaction α
Ce0.85La0.15O2-δ- CO generation-800
(c)
F1
0.0
0.5
1.0
1.5
2.0 Ce0.85La0.15O2-δ- CO generation-900
Fractional Reaction α
(d)
F1
0
1
2
3
Fractional Reaction α
Ce0.75Zr0.25O2 - combustion - CO generation - 600
(e)
F2
0.0
0.5
1.0
1.5
2.0
2.5
F2
Fractional Reaction α
Ce0.75Zr0.25O2 - combustion - CO generation - 700
D1
(f)
0
2
4
6 Ce0.75Zr0.25O2 - combustion - CO generation - 800
Fractional Reaction α
F2
(g)
0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.00.0
0.5
1.0
1.5
2.0
2.5
D1
Ce0.75Zr0.25O2-H- CO generation -1000
F1
(dα
/dt)/
(dα
/dt) α
= 0.
5
Fractional Reaction α
(h)
Fig. 10. Normalized rate data compared to various solid state reaction models of the generation reaction with CeO2 and CexM1�xO2�d (M = La3+ andZr4+) at different temperatures.
2.0
2.4
2.8(a)
ln[(d
α/d
t)/(1
- α)]
CO generationCe0.85 La0.15 O2-δ
y = 6.99917- 0.50367xR2 = 0.92
8.4 8.8 9.2 9.6 10.0 10.4 9.0 9.5 10.0 10.5 11.0 11.5
-1.2
-0.8
-0.4
0.0
0.4Ce0.75Zr0.25O2 -combustion- CO generation
(b)
ln[(d
α/d
t )/(2
/ α)]
1/T (x104 K-1)1/T (x104 K-1)
y = 4.89931 - 0.50685xR2 = 0.82
Fig. 11. Arrhenius plots (a) between 700 �C and 900 �C for of Ce0.85La0.15O2�d, (0.2 < a < 1); (b) between 600 �C and 800 �C for CO2 splitting reaction ofCe0.75Zr0.25O2–C (0.3 < a < 0.9).
64 Q. Jiang et al. / Solar Energy 99 (2014) 55–66
rate-time profiles for CeO2 and Ce0.85La0.15O2-d withb = 10 �C/min within a temperature range of 1000–1500 �C are shown in Fig. 8(b). The experimental dataare processed according to (3-1). The linear regression forthe experimental data is shown in Fig. 9(a–f).
Fig. 9 exhibits the Arrhenius plot for the O2 evolutionreaction of CeO2, Ce0.85La0.15O2�d and Ce0.75Zr0.25O2.For CeO2, a deviation is observed whatever the modelsare used, therefore, the data are divided into two tempera-ture ranges. The low temperature range is from 1120 �C to1320 �C and the high temperature range is from 1320 �C to
1500 �C. As shown in Fig. 9(a–d), both reaction order (F1and F2) and geometrical-contracting (R2 and R3) modelsshow excellent linear fit. It is difficult to precisely determinethe actual reaction model for the reduction reaction ofCeO2. The estimated activation energy Ea for the low tem-perature range is about 115 kJ/mol and for the high tem-perature range is 365 kJ/mol as calculated from Fig. 9d(R3 model).
For Ce0.75Zr0.25O2, the geometrical-contracting modelR3 (n = 2/3) provides very good linear fit (R2 = 0.997)within the temperature range of 1000–1500 �C. The
Q. Jiang et al. / Solar Energy 99 (2014) 55–66 65
estimated activation energies Ea for the O2 evolution stepcalculated from the slope of the line is 115 kJ/mol.
For Ce0.85La0.15O2�d, the experimental data are alsodivided into two temperature ranges. One is from1280 �C to 1360 �C and the other one is from 1360 �C to1500 �C. The geometrical-contracting model R3 (n = 2/3)gives good linear fit (R2 = 0.999). The estimated activationenergy Ea is 129 kJ/mol from 1280 �C to 1360 �C and517 kJ/mol from 1360 �C to 1500 �C. The estimated activa-tion energies agree well with the O2 evolution activity.
The CO generation rate can be expressed as follows:
dadt¼ kf ðaÞ ð3-4Þ
For isothermal experiment, if the normalized rate dataagree well with a given kinetic model, (3-3) can beexpressed as (Gotor et al., 2000; Perez-Maqueda et al.,2002; Yang et al., 2009):
da=dtðda=dtÞa¼0:5
¼ f ðaÞf ðaÞa¼0:5
ð3-5Þ
The normalized rate data for CeO2 and CexM1�xO2�d
(M = La3+ and Zr4+) at different temperatures comparedto various solid state reaction models are shown inFig. 10. For CeO2, the data agree well with a first orderreaction model (F1) at 800 �C (Fig. 9(a)). It suggests thatthe reaction is mainly controlled by surface reaction with-out diffusion limitation. For Ce0.85La0.15O2�d, the experi-mental data are also represented by a first order reactionmodel (F1) from 700 �C to 900 �C. Consequently, the COgeneration rate-time profiles of Ce0.85La0.15O2�d are similarto CeO2.
For Ce0.75Zr0.25O2–C, the experimental data from600 �C to 1000 �C all agree well with a second order surfacereaction model (0.3 < a < 1.0). Inversely, the experimentaldata for Ce0.75Zr0.25O2–H at 1000 �C firstly agree well withthe first order reaction model (0.2 < a < 0.4) and then itchanges to the diffusion model (0.4 < a < 1.0). The differentkinetic models for Ce0.75Zr0.25O2–C and Ce0.75Zr0.25O2–Hinduce the difference in CO generation reaction rate.
As shown in Fig. 11, a linear dependence can beobtained between ln[(da/dt)/(1 � a)] and 1/T, and the acti-vation energies for the CO generation step can be esti-mated. The activation energy of the CO generation stepfor Ce0.85La0.15O2�d and Ce0.75Zr0.25O2-combustion is42 kJ/mol and 42 kJ/mol, respectively.
4. Conclusions
The activity of M (M = Ti4+, Sn4+, Hf4+, Zr4+, La3+,Y3+ and Sm3+) doped ceria in thermochemical CO2 split-ting reaction was investigated in detail. The addition oftetravalent cations (Ti4+, Sn4+, Hf4+ and Zr4+) intoCeO2 significantly improved the O2 evolution activityand the smaller the smaller ionic radius, the larger theO2 production. The corresponding CO production wasincreased from 4.5 ml/g for CeO2 up to 10.6 ml/g for
Ce0.75Zr0.25O2 synthesized by solution combustionmethod. For Ce0.75Hf0.25O2 and Ce0.75Zr0.25O2 synthe-sized by hydrothermal method, direct one step CO2 split-ting reaction was observed when the temperature exceeds1100 �C. Further investigations in one step thermochem-ical CO2 splitting showed that 16.8 ml/h CO was pro-duced at 1400 �C. For Ce0.8Ti0.2O2 and Ce0.8Sn0.2O2,although the O2 production was increased several timesas compared to CeO2, the CO generation activity wasstill low due to the formation of Ce2Ti2O7 and Ce2Sn2O7
after the high temperature reduction reaction. The dop-ing of trivalent cations M (M = La3+, Y3+ and Sm3+)into ceria had negative effect both on the O2 evolutionactivity and the CO production. The estimated activationenergy for the reduction step of Ce0.75Zr0.25O2 was muchlower than that of CeO2 and Ce0.85La0.15O2�d. The COgeneration reaction of CeO2, Ce0.85La0.15O2�d and Ce0.75-
Zr0.25O2–C was a surface limited reaction. Overall, thevalence, the ionic radii of the dopants, the structural sta-bility at high temperature and the synthesis method areall important factors for enhancing the activity of thetwo step thermochemical splitting reaction. Direct onestep CO2 splitting reaction may be a promising way forsolar thermal CO2 reduction reaction.
Acknowledgment
This work was financially supported by Solar EnergyAction Plan of Chinese Academy of Sciences (GrantKGCX2-YW-393-1).
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