Catalytic Function of IrOx in the Two-Step Thermochemical CO2

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Catalytic function of IrOx in two step thermochemical CO2 splitting reaction at high temperatures Qingqing Jiang, Zhenpan Chen, Jinhui Tong, Min Yang, Zongxuan Jiang, and Can Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01774 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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ACS Catalysis

Catalytic function of IrOx in two step thermochemical CO2 splitting reaction at high temperatures Qingqing Jiang a,† , Zhenpan Chen a,b,† , Jinhui Tong a, Min Yang a, 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, Liaoning 116023, R.P. China

b

University of the Chinese Academy of Sciences, Beijing 100049, China

† These authors contributed equally to this work.

ABSTRACT: Splitting CO2 into CO and O2 via two step thermochemical cycle by utilizing concentrated solar energy is a promising option for CO2 reduction. Herein, in order to enhance the solar to fuel energy conversion efficiency, IrOx as a catalyst is used to increase the fuel production rate of LaFeO3 based two step thermochemical CO2 splitting reaction. Compared with LaFeO3, 0.6 at. % IrOx as a catalyst that loaded on LaFeO3 almost can double the maximum CO release rate and increase the maximum O2 evolution rate to 1.5 times, when a small amount of IrOx (2.5 at. %) is doped into the structure of LaFeO3 through a solution combustion method, the initial CO generation rate can be increased by 5 times. This work demonstrates that the catalytic function is still necessary in two step thermochemical CO2 splitting reaction even at high temperatures. KEYWORDS: Two-step thermochemical cycle, CO2 reduction, IrOx catalyst, Ferrite-based oxides, Kinetics,

1. INTRODUCTION Efficient conversion of CO2 into chemical fuels has received much attention due to the significant rise in atmospheric carbon dioxide levels resulting from combustion of hydrocarbon fuels.1 Splitting CO2 into CO and O2 utilizing concentrated solar energy via two step thermochemical cycles is an attractive route to recycle into hydrocarbon fuels.2-7 In two-step CO2 thermochemical splitting cycle, the redox metal oxide (MO) is reduced at high temperatures with concentrated solar energy firstly (see equation (1)). And then, CO is produced by exposing the reduced metal oxide to CO2 at lower temperature (see equation (2)). Therefore, total reaction results the splitting reaction of CO2 to produce CO and O2 (see equation (3)). (Where the MOox and MOred represent the oxidized and reduced metal oxides, respectively.) MOox + solar energy = MOred + O2

(1)

MOred + CO2 = MOox + CO

(2)

CO2 + solar energy = CO + 1/2O2

(3)

To achieve high solar to fuel energy conversion efficiency, challenges are how to increase the reduction rate of MOox and the oxidation rate of MOred. Recent researches have shown that the chemical steps at the gassolid interface and diffusion process strongly influence the overall kinetics of fuel production.8-9 Several works devoted to improving the diffusion process by using three-dimensionally ordered macroporous (3DOM)

architecture materials, which results in great enhancement of the fuel production rate.10-12 However, the porous structure tends to aggregation and sintering after high temperature treatment. Haile et al. reported that the H2 production rate in two step thermochemical H2O splitting reaction could be increased by 3 folds with reduction temperature at 1500 oC after loading 2 wt. % Ru on CeO2.13 This result indicates that catalytic function is apparent for the chemical steps at the gas-solid interface even after high temperature treatment. Among candidates of redox materials, ferrite-based oxides show great potential but exhibit relatively slow reaction rates in two step thermochemical CO2 and H2O splitting reactions.14-16 So far, there is no report on catalytic function for ferrite-based oxides in two step thermochemical splitting reaction. In this work, the catalytic function of iridium has been investigated in two step thermochemical CO2 splitting reaction by using LaFeO3 as redox metal oxide (MO). The typical precious metal catalyst, IrOx, is employed to demonstrate if the thermochemical conversion of CO2 at high temperatures even over 1000 oC still require the catalytic function or not. Herein, a solution combustion method and a wet impregnation technique have been used to prepare IrOxcontaining LaFeO3 materials. It is possible to obtain materials with nanometer size crystallites and high-ionic dispersion of the noble metal ions by solution combustion method.17-18 Furthermore, doping iridium into LaFeO3 by solution combustion method may lead to more labile of lattice oxygen.19 LaFeO3 supported IrOx samples

ACS Paragon Plus Environment

ACS Catalysis synthesized by wet impregnation technique were tested as a comparison to bulk doped samples.

2.1 Physicochemical characterization XRD patterns shown in Fig. 1 indicate that all prepared LaFe1-xIrxO3 (x = 0.01, 0.025, 0.05 and 0.1) adopt orthorhombic perovskite-type structure. Diffraction lines corresponding to IrO2 are detected as x is increased to 0.05. The crystallite sizes are estimated to be < 60 nm using Scherrer’s equation, as presented in Table S1. The XRD patterns of the used catalysts are demonstrated in Fig. 2. As showed by the XRD, the obvious change has been observed after redox reaction at high temperatures. The SiO2 support partly reacts with the LaFe1-xIrxO3 in the reaction process at high temperatures and the mixture composed of SiO2, La2(Si2O7), Fe3-xSixO4 and Ir phases were formed under this condition. But, in the following redox cycles, the phase compositions of such mixture keep stable and the redox cycle is accomplished via the valence state variation of iron element. The specific surface areas of LaFe1-xIrxO3 samples synthesized by solution combustion method are larger than that of y at. % IrOx/LaFeO3 (y = 0.6, 2.5 and 7) synthesized by wet impregnation. However, the specific surface areas decrease to 1.6~1.8 m2.g-1 after a high temperature (1300 oC) treatment whatever the prepared methods used (Table S1). IrO2 LaFe0.9Ir0.1O3 LaFe0.95Ir0.05O3 LaFe0.975Ir0.025O3 LaFe0.99Ir0.01O3 LaFeO3

40

60

a: As-synthesized LaFe0.975Ir0.025O3 ♦ SiO2(Cristobalite) ∗ Fe Si O 3-x x 4 ♥ La2(Si2O7)

• Ir

♥ ♦ ♥ ♥

♥♦♥ ♦ ♥ ∗ ♥ ∗

♥ •

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2 Theta ( )

Fig. 2. XRD patterns of fresh (a) and used (b) LaFe0.975Ir0.025O3 sample.

2. RESULTS AND DISCUSSION

20

b: LaFe0.975Ir0.025O3/SiO2-after 6 cycles ♦

Intensity ( a.u.)

The reaction results indicate that catalytic function of IrOx is remarkable in two step thermochemical CO2 splitting reaction even after high temperature treatment. The initial CO generation rate of LaFeO3 could be increased by 5 folds after doping a small amount of IrOx (2.5 at. %) into LaFeO3. The enhancement of the CO production rate keeps relatively stable even after 10 reaction cycles at high temperatures. This work demonstrates that the catalytic function is still necessary and important to enhance solar thermochemical conversion efficiency even at high temperatures where most chemical reactions easily reach their thermodynamic equilibrium.

Intensity

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SEM image in Fig. 3(a) shows that the morphology of LaFe0.95Ir0.05O3 synthesized by solid solution combustion method is sponge-like porous structure, however, the samples after high temperature treatment are aggregated into large particles, see Fig. 3(b). Energy dispersive spectroscopy (EDS) shows that the molar ratio of Ir to La is consistent with that of the raw materials (Table S1). The EDX results also prove that IrOx phase are highly dispersed in LaFe1-xIrxO3 samples.

Fig. 3. SEM images of (a) as synthesized LaFe0.95Ir0.05O3 and (b) LaFe0.95Ir0.05O3 after CO2 splitting reaction test. Fig. 4 shows the asymmetrical X-ray photoelectron spectrum of Ir 4f levels, which has been proved to be an intrinsic property of Ir element in previous study.20 As shown by the XPS spectrum, the Ir 4f spectrum is partly overlaped with the Fe 3p spectrum, especially for the used catalyst. And, for the used catalyst, the binding energy of Ir/IrO2 4f7/2 and Ir/IrO2 4f5/2 are all ~1 ev lower than that of the fresh LaFe0.975Ir0.025O3, respectively. The spectrum displays two different valence states of surface iridium element (Ir4+ and Ir) both for fresh and used LaFe0.975Ir0.025O3. This indicates that a certain amount of IrOx (mixture of IrO2 and metal Ir) is remaining on the surface of the used catalyst. The appearance of Ir phase, for the used catalysts, in the XRD patterns (Fig. 2) is consistent with the XPS spectrum results.

80

o

2 Theta ( )

Fig. 1. XRD patterns of fresh LaFe1-xIrxO3 synthesized by solution combustion method.


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ACS Catalysis (a)

Fig. 5 shows the O2 evolution reaction results of LaFe1from 800 oC to 1300 oC (see details in Fig. S1). The initial O2 releasing temperature of LaFeO3 decreases after the addition of IrOx, and the higher the IrOx content, the lower the initial O2 releasing temperature. LaFe0.9Ir0.1O3 starts to release oxygen at temperatures as low as 800 oC, by contrast, the initial O2 releasing temperature of LaFeO3 is about 1240 oC. It is noteworthy that when x is increased to 0.1, the initial O2 evolution temperature is lowered by 400 oC. These results suggest that the addition of iridium greatly enhances the O2 evolution reaction possibly by weakening the lattice metal-oxygen bonds of LaFeO3.

Fe 3p

xIrxO3

Ir 4f7/2

IrO2 4f5/2

2+

Fe

68

66

64

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50

The enhancement of O2 evolution rate by catalytic function is not obvious as x varied from 0 to 0.025. By further increasing the x values to 0.05, the enhancement becomes much apparent. When x is increased to 0.1, the maximum O2 evolution rate is almost double that of LaFeO3. The O2 production is increased from 4.1 ml/gmaterial for LaFeO3 up to 8.3 ml/gmaterial for LaFe0.9Ir0.1O3 (Fig. S1 and Table S2).

Binding energy / eV (b)

Fe 3p

3+

Intensity / a.u.

Fe

Ir 4f5/2 IrO2 4f5/2

68

66

Ir 4f7/2

2+

Fe

IrO2 4f7/2

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Fig. 4. XPS spectrum of fresh (a) and used (b) LaFe0.975Ir0.025O3.

2.2 Catalytic function of IrOx for the O2 evolution reaction

1200 0.22 1100

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Maximum O2 evolution rate ml min

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Fig. 5. The maximum O2 evolution rate and the initial O2 evolution temperature for LaFe1-xIrxO3.

1300 O2 Temperature 1200

0.21 1100 0.18 1000

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o

-1 -1 g material

1300 O2

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Initial O2 evolution temperature / C

IrOx-containing perovskite oxides LaFe1-xIrxO3 and IrOx/LaFeO3 were prepared by solution combustion and wet impregnation methods, respectively. In order to enhance thermal stability of the samples, these IrOxcontaining LaFeO3 were mixed with SBA-15 in a weight ratio of 1:3 via mechanical milling before reaction test. All samples tested in the followed experiments were supported by SBA-15 and marked with simplified forms, otherwise, it is specially illustrated. The samples were heated from 800 oC to 1200 oC with a 20 oC /min heating rate and then to 1300 oC with a 5 oC /min heating rate. The temperature plateau at 1300 oC was maintained for 40 min while passing an argon flow at a rate of 100 ml/min.

As illustrated above, the catalytic function of IrOx in LaFe1-xIrxO3 for O2 evolution reaction becomes apparent until x value is increased to 0.05. Generally, there are three key steps for the O2 evolution reaction: the broken of M-O bond, the migration of oxygen ion from bulk to the surface and the O-O bond formation at the surface. As reported by the literature, incorporation of Pd element into B-site significantly improves the mobility of lattice oxygen and the reducibility of Fe-based perovskites.21 Similarly, the introduction of Ir ions into LaFeO3 may weaken the La-O and Fe-O bonds in some extent, therefore, the initial oxygen release temperature is lowered for Ir doped LaFeO3. It suggests that the kinetics enhancement of the O2 evolution reaction of LaFe1-xIrxO3, may be attributed to more labile of the lattice oxygen (especially for LaFe0.9Ir0.1O3) and surface catalytic function of IrOx. -1 -1 g material

IrO2 4f7/2

Ir 4f5/2

3+

Maximum O2 evolution rate ml min

Intensity / a.u.

Fe

Initial O2 evolution temperature / C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

800 0.12 0

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4

6

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Ir content (at.%)

Fig. 6. The maximum O2 evolution rate and the initial O2 evolution temperature for y at. % IrOx/LaFeO3. After loading 0.6 at. % IrOx on the surface of LaFeO3 by wet impregnation technique, the peak of the O2 evolution rate is increased from 0.14 ml⋅g-1material⋅min-1 up to 0.20 ml⋅g-1material⋅min-1, as shown in Fig. 6. By further increasing the percentage of IrOx, the peak of the O2 evolution rate continues to increase, but no longer obvious. The peak of O2 evolution rate for 7 at. % IrOx/LaFeO3 is about 0.22


ACS Catalysis

IrOx content in LaFe0.975Ir0.025O3 and 2.5 at. % IrOx/LaFeO3 are equal to each other, however, the latter exhibits much lower initial O2 evolution temperature and much higher O2 evolution rate than LaFe0.975Ir0.025O3 (Fig. S3). It indicates that the IrOx dispersed on LaFeO3 surface plays an important role in the O2 evolution reaction. The iridium oxide was reported as efficient catalyst for O2 evolution in photocatalysis and photo-electro-catalysis systems.22-23 Here, IrOx may catalyze the chemical reaction of oxygen evolution at the gas-solid interface possibly via promoting the O-O bond formation. Cycle performance of O2 evolution reaction for (a) 0.6 at. % IrOx/LaFeO3, (b) LaFe0.975Ir0.025O3 and (c) LaFe0.9Ir0.1O3 are shown in Fig. S4. It can be seen that for (a) and (b), the lower reduction onset temperature is repeatable, but for LaFe0.9Ir0.1O3, the lower reduction temperature is not repeatable for multiple cycles. However, the overall O2 production and peak O2 release rate decreased for the repeated cycles due to the decomposition of the perovskite structure.

2.3 Catalytic function of IrOx on the CO generation reaction

1.6

(a)

o

CO generation reaction at 1000 C

1.2 LaFeO3 LaFe0.99Ir0.01O3

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LaFe0.975Ir0.025O3

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-1 -1 maximum CO evolution rate ml min g material

The CO2 splitting reaction was performed by exposing the reduced oxides to CO2 (purity 99.999 %) with a flow rate of 500 ml/min. Fig. 7 exhibits the CO generation activity for LaFe1-xIrxO3 from 900 oC to 1100 oC. The initial CO generation rate can be increased by 2 times with x value as low as 0.01. By further increasing x value to 0.025,

the initial CO generation rate reaches 1.35 ml⋅g-1material⋅minwhich is 5 times higher than that of LaFeO3 (Fig. 7(a) and (b)). As x increases from 0 to 0.1, the peak CO generation rate reaches a maximum when the x value is 0.025, however, the maximum CO generation rate decreases when further increasing the iridium content. As shown in Fig. 7(c), the reaction temperature has a great influence on the catalytic activity. For all these LaFe1-xIrxO3 samples, the CO production rate reaches the maximum at 1000 oC. From the view of kinetics, the CO generation rate increases as the reaction temperature is increased. But from the point of thermodynamics, the CO generation reaction is an exothermic reaction,24-25 so higher reaction temperature is detrimental to its thermodynamic drive force. Furthermore, the reverse reaction is more favorable when the reaction temperature is above 1000 oC. As a compromise, the optimum reaction temperature is found to be 1000 oC. Namely, the IrOx exhibits the highest catalytic function for CO generation reaction at 1000 oC. 1

The introduction of micro-scale Ir dopant, especially lower than 2.5 at. %, leads to almost equal O2 release capacities but big difference in CO generation rate (Fig. 7 and Table S2). Therefore, for low iridium doping, the bulk iridium is not producing highly mobile oxygen ions thermodynamically, otherwise the O2 production will change obviously. On the other hand, The XPS results indicate that a certain amount of IrOx (mixture of IrO2 and metal Ir) is remaining on the surface both for fresh and used LaFe0.975Ir0.025O3 (Fig. 4). The evident catalytic effect on initial CO generation rate of LaFeO3 after doping a small amount of IrOx into LaFeO3 suggests that CO2 splitting is catalyzed by surface IrOx species, and the CO2 decomposition rate increases consequently.

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x values in LaFe1-xIrxO3

Time / min o


ml⋅g-1material⋅min-1. The initial O2 releasing temperature can also be gradually reduced with increasing the amount of loaded IrOx. The O2 production is increased from 4.1 ml/gmaterial for LaFeO3 to 6.2 ml/gmaterial for 7 at. % IrOx/LaFeO3 (Fig. S2 and Table S2).

-1 -1 CO generation rate ml min g material

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fig. 7. (a) The CO generation rate-time profiles at 1000 C for the first cycle, (b) the dependence of the maximum CO generation o rate on IrOx content at 1000 C, (c) the maximum CO generation rate at different temperatures of LaFe1-xIrxO3 (x = 0.01, 0.025, 0.05, 0.1).


ACS Catalysis may be result from decreased oxygen chemical potential on the solid surface followed by increased CO2 splitting reaction.

2.0 Cycle performance LaFe0.975Ir0.025O3 LaFeO3

0.8 -1 -1 g material

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CO evolution rate ml min

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CO generation reaction at 1000 C LaFeO3

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Fig. 8. The maximum CO generation rate as a function of cycle number for LaFe0.975Ir0.025O3.

The stability of the catalytic effect is an important parameter for long-term use under high temperature treatment. The cycle performance of LaFe0.975Ir0.025O3 was performed with the reduction temperature at 1300 oC and CO generation temperature at 1000 oC. The peak CO generation rate as a function of cycle numbers is shown in Fig. 8. It can be seen that catalytic function of IrOx on CO generation reaction keeps relatively stable even after 10 cycles. This indicates that the doped IrOx in LaFeO3 is still highly dispersed even after high temperature treatment at 1300 oC for many cycles. The CO and O2 production (ml/gmaterial) of LaFe0.975Ir0.025O3 for 10 cycles are shown in Table S3. It can be seen that the O2 production also keeps stable. Since the CO generation rate becomes very low after 30 min, therefore, we just do the CO generation reaction for 30 min and this leads to a part of CO loss. The CO production could be increased if the reaction time is prolonged. The collected CO produciton deviates from that of 2x O2 amount is caused by several reasons. First, small air leaks are inevitable in the system. And, a part of CO is undetected before the peak rate due to the inherent limit of GC detection method. Also, insufficient CO2 reduction time leads to CO loss. Fig. 9 exhibits the CO generation activity for y at. % IrOx/LaFeO3 (y = 0.6, 2.5 and 7) at 1000 oC and LaFe0.99Ir0.01O3 is presented here as a comparison. By loading 0.6 at. % IrOx on the surface of LaFeO3, the maximum CO generation rate of LaFeO3 is increased from 0.22 ml⋅g-1material⋅min-1 up to 0.41 ml⋅g-1material⋅min-1. The catalytic enhance function of IrOx in 0.6 at. % IrOx/LaFeO3 is observed but is much lower than that of LaFe0.99Ir0.01O3. However, the CO generation activity is largely decreased as further increasing the surface IrOx content (Fig. 9-a). These results indicate that extra IrOx on the surface of LaFeO3 lattice restrains the catalytic activity, which may be due to aggregation of IrOx particles followed by reduced CO2 activation sites and increased reverse reaction (CO + O = CO2) catalyzed by aggregated IrOx species. For 7 at. % IrOx loaded sample, the maximum CO release rate increased by further decreasing the CO2 splitting temperature to 900 0C (Fig. 9-b). This

-1 -1 g material

Cycle number

maximum CO evolution rate ml min

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(b)

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o

Fig. 9. (a) The CO generation rate-time profiles at 1000 C, (b) the dependence of the maximum CO generation rate on IrOx content of y at. % IrOx/LaFeO3 at different temperatures.

As illustrated above, for the CO generation reaction, IrOx in LaFe1-xIrxO3 exhibits much higher catalytic activity than that in IrOx/LaFeO3. The higher CO generation activity of LaFe1-xIrxO3 is mainly due to the high dispersion of IrOx species when it is synthesized by solution combustion method. The highly dispersed catalyst of IrOx can activate the CO2 and H2O molecules and also is helpful for the diffusion of oxygen ion into the metal oxide lattice. However, the surface loaded IrOx by wet impregnation technique tend to aggregate into larger particles. This may explain why different preparation routes lead to great differences in the catalytic activity. The CO generation activity is highly dependent on IrOx content and extra IrOx on the surface restrains the catalytic activity. If too much IrOx is loaded, the CO generation rate is even lower than that of LaFeO3. These results may be due to aggregation of IrOx particles and reverse reaction taking place on the surface of IrOx species. The CO production for the first 10 min is increased from 1.3 ml/gmaterial for LaFeO3 up to 2.9 ml/gmaterial for LaFe0.975Ir0.025O3 and 1.6 ml/gmaterial for 0.6 at. % IrOx/LaFeO3 (Fig. 10). As for CO2 conversion, for 1 g LaFe0.975Ir0.025O3 (redox between 1300 oC and 1000 oC), the maximum single pass CO2 conversion is about 1.1 % and the average single pass CO2 conversion is about 0.2 %. As a comparison, the maximum and average single pass CO2 conversion for 1 g CeO2 (redox between 1500 oC and 800 o C) is about 1.9 % and 1.0 %, respectively.2


ACS Catalysis CO production for the first 10 min CO production ml / gmaterial

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3

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1

La

O3 eO 3 eO 3 O3 aF aF 5 01 /L /L Ir 0 . .0 2 r x x 0 I O O Ir Ir 5 .9 9 .% Fe 0 t. % .9 7 at La Fe 0 7a 0.6 La

O3 Fe

Fig. 10. CO production for the first 10 min of LaFe1-xIrxO3 and y at. % IrOx/LaFeO3.

2.4 Possible catalytic process of the O2 and CO generation reaction For LaFe1-xIrxO3 samples synthesized by solution combustion method, when doping a small amount of IrOx, it is highly dispersed in the bulk. IrOx weakens the metal –oxygen (M-O) bonds and then the lattice oxygen becomes more labile. The reaction results of the initial O2 releasing temperature agree well with this hypothesis. By further increasing the content of IrOx, some of the catalyst is dispersed on the surface of the redox material. IrOx works in the chemical steps at the gas-solid interface

Page 6 of 10

which may catalyze the reaction of formation of O-O bond and further increase the O2 evolution rate. The reaction results of the O2 evolution rate also agree well with this hypothesis (Fig. 5 and Fig. 6). IrOx in LaFe1-xIrxO3 samples affects both the thermodynamics, (especially for high doped LaFe0.9Ir0.1O3), and the kinetics. For IrOx/LaFeO3 synthesized by wet impregnation technique, IrOx is dispersed on the surface of LaFeO3. The surface IrOx species catalyze the O2 formation. Therefore, the O2 evolution rate of 2.5 at. % IrOx/LaFeO3 is much higher than that of LaFe0.975Ir0.025O3 although the IrOx content in both LaFe0.975Ir0.025O3 and 2.5 at. % IrOx/LaFeO3 are equal to each other (Fig. S3). As for the CO2 splitting reation, the IrOx plays two roles in this process. On one hand, it activates the C-O bond of CO2 molecules and then increases its decomposition rate on the surface. On the other hand, it is helpful for the diffusion of oxygen species from surface into lattice. It may explain why catalytic function of IrOx in LaFe1-xIrxO3 is much higher than that in y at. % IrOx/LaFeO3. If too much IrOx is loaded on the surface of LaFeO3, aggregated IrOx particles are not in favor of CO2 decomposition but induce the congregation of oxygen species on the surface and then reverse reaction occurs. Therefore, extra IrOx on the surface of LaFeO3 lattice has negative effect on the reaction activity. Fig. 11 illustrates the possible catalytic process of O2 and CO generation reaction.

Fig. 11. The possible catalytic process of O2 evolution reaction and CO generation reaction for (a) LaFe1-xIrxO3 and (b) IrOx/LaFeO3.

2.5 Kinetic analysis The kinetic studies for the CO generation reaction were performed using master plot methods. Specifically, measured differential rate data have been compared to various functional forms of known solid state kinetics models. The CO generation rate can be expressed as follows:26-30

dα = kf (α ) dt

（3 -1）

Where α represents the reaction fraction and the kinetics model, f(α), is a function which is capable of describing the mechanism of a solid state reaction. For isothermal experiment, if the normalized rate data agree

well with a given kinetics model, equation (3-1) can be rearranged as:

dα / dt f (α ) = (dα dt )α =0.5 f (α )α =0.5

（3 - 2）

The normalized rate data for LaFe0.975Ir0.025O3 and LaFeO3 at different temperatures compared to various solid state reaction models are shown in Fig. 12(a-h). For LaFe0.975Ir0.025O3, the data agree well with a diffusion model (D1) at 850 oC (0.3 < α < 1.0) and then it changes to the second order reaction model (F2) from 900 oC to 1000 o C (0.3 < α < 1.0). For LaFeO3, the experimental data are represented by diffusion model (D1) from 850 oC to 900 oC and order reaction model (F1 and F2) at 950 oC and 1000 o C.


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LaFeO - CO generation - 850 ℃ 3

4

LaFe0.975Ir0.025O3

(dα /dt) / (dα /dt)


α =0.5

4

1.0

0.2

Fractional Reaction α

0.4

0.6

0.8

1.0


8

6

(b)

(f)

LaFeO - CO generation - 900 ℃ 3 α =0.5

CO generation - 900 ℃



α =0.5

LaFe0.975Ir0.025O3

6

4

F2

2

D1

4

2

0 0.0

0.2

0.4

0.6

0.8

0 0.0

1.0

0.2


1.0

α =0.5

CO generation - 950 ℃ 6

4

2

F2 3

2

1

F2

0 0.0

LaFeO - CO generation - 950 ℃ 3

4

LaFe0.975Ir0.025O3


α =0.5

0.8

(g)

8


0.6

5

(c)

F1

0

0.2

0.4

0.6

0.8

0.0

1.0

0.2


0.4

0.6

0.8

1.0


12

5

(d)

(h)

10

LaFeO3 - CO generation - 1000 ℃

4

8

α =0.5

LaFe0.975Ir0.025O3 CO generation - 1000 ℃


α =0.5

0.4


10


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ACS Catalysis

6 4 F2 2

F2

3

2

1

F1

0 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


Fig. 12. Normalized rate data compared to solid state reaction models of the CO generation reaction for LaFe0.975Ir0.025O3 and o o LaFeO3 from 850 C to 1000 C. F1 means first order reaction, F2 means second order reaction, D1 means first order diffusion reaction model. (The data for the analysis of kinetics models are the whole CO generation rate-time curves as shown in Fig. S5; o o o o a-d: CO generation data under (a) 850 C, (b) 900 C, (c) 950 C and (d) 1000 C for LaFe0.975Ir0.025O3; e-h: CO generation data o o o o under (e) 850 C, (f) 900 C, (g) 950 C and (h) 1000 C for LaFeO3; the dot symbols in the figures represent experimental data and the solid or dot lines represent proposed solid state reaction models.)


ACS Catalysis

-1

(b)

(a)

0 -2 LaFeO3

ln Rinitial

ln Rinitial

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-3

-4

Equation y = a + b*x Adj. R-Square 0.96825 B B

Intercept Slope

LaFe0.975Ir0.025O3

-1

-2

Equation

y = a + b*x

Adj. R-Square

Value Standard Error 16.67599 1.75244 -2.26735 0.20444

0.92431

B

Intercept

B

Slope

Value Standard Error 18.25165 2.75151 -2.2662

0.32099

-3

-5 8.0

8.4

8.8

9.2

8.0

8.4

8.8

9.2

4 -1 1/T(x10 K )

4 -1 1/T(x10 K )

o

o

Fig. 13. Arrhenius plots for (a) LaFeO3 and (b) for LaFe0.975Ir0.025O3 at temperatures from 800 C to 1000 C.

The activation energies for the CO generation step can be estimated from the linear obtained between ln[Rinitial] and 1/T, as shown in Fig. 13. The activation energy of the CO generation step at the temperatures from 800 oC to 1000 oC for LaFe0.975Ir0.025O3 is 190 KJ/mol and for LaFeO3 is 191 KJ/mol, respectively. The pre-exponential factor for LaFe0.975Ir0.025O3 is 8.4 × 107 g⋅mol-1⋅s-1 (F2 model), whereas the pre-exponential factor for LaFeO3 is only 1.7 × 107 g⋅mol-1⋅s-1 (F2 model). So, the faster initial kinetics for LaFe0.975Ir0.025O3 is due to an increase of the preexponential factor. The pre-factor is related to the amount of reaction active site as reported by the literature.31 It indicates that the reaction active site is increased after doping a small amount of IrOx into LaFeO3.

3. CONCLUSION IrOx catalyst is found to be efficient to enhance the fuel production rate in two step thermochemical CO2 splitting reaction with LaFeO3 as redox material. The maximum CO release rate is almost doubled and the maximun O2 evolution rate is increased to 1.5 times by loading 0.6 at. % IrOx on the surface of LaFeO3. When a small amount of IrOx (2.5 at. %) is doped into LaFeO3, the maximum CO generation rate is increased by 5 folds. It suggests that the catalyst on surface increases the chemical reaction rates at the gas-solid interface, and the catalyst in bulk leads to more labile of lattice oxygen and also promotes the diffusion of ionic oxygen. This work demonstrates that the presence of a small amount of catalyst can significantly enhance the thermochemical fuel production even at high temperatures over 1000 oC.

4. EXPERIMENTAL SECTION 4.1 Sample preparation 4.1.1 Solution combustion method

The LaFe1-xIrxO3 (x = 0.01, 0.025, 0.05 and 0.1) samples were prepared by a single step solution combustion method.19 In a typical preparation of LaFe0.99Ir0.01O3 sample, raw materials of La(NO3)3⋅6H2O, Fe(NO3)3⋅9H2O, H2IrCl6, and glycine were taken in the molar ratio of 1:0.99:0.01:3.3. These materials were dissolved in a minimum volume of water and then it was transferred to furnace kept at 500 oC. Then, the sample was calcined at 800 oC for 4 h. The preparation of other samples followed the similar procedure.

4.1.2 Wet Impregnation technique y at. % (y = 0.6, 2.5 and 7.0) IrOx was supported on LaFeO3 by wet impregnation technique using the corresponding precursors of H2IrCl6⋅6H2O.21 For the preparation of 0.6 at. % IrOx/LaFeO3, 1 g LaFeO3 synthesized by solution combustion method and 0.0113 g H2IrCl6⋅6H2O were added into the distilled water. After stirring for 1 h, water was evaporated under reduced pressure in a rotavapor and then the sample was dried overnight at 120 oC. At last, the sample was calcined at 500 o C for 2 h.

4.2 Characterization of samples The synthesized samples were characterized by X-ray powder diffraction (XRD) on a Rigaku D/Max-2500/PC powder diffractometer. Each sample powder was scanned using Cu Kα radiation with an operating voltage of 40 kV and an operating current of 200 mA. The scan rate of 5 ° min-1 was applied to record the XRD patterns in the range of 20 - 80 ° at a step size of 0.02 °. The specific surface areas of the samples were determined by a Micromeritics ASAP 2000 adsorption analyzer. The morphologies and particle sizes were examined by scanning electron microscopy (SEM) equiped with a Quanta 200 FEG scanning electron microscope. The X-ray photoelectron spectroscopy (XPS, Thermo ESCLAB 250Xi, a monochromatic Al Kα X-ray source) was used to determine the Ir 4f electrons binding energies and the


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ACS Catalysis

valence state of iridium, and all data were normalized to the C 1s peak (284.6 eV) for each sample.

4.3 Reaction activity test

(3) McDaniel, A. H.; Miller, E. C.; Arifin, D.; Ambrosini, A.; Coker, (4)

The two-step thermochemical CO2 splitting reaction was carried out in a fixed bed reactor at a laboratory scale. A vertical alumina tubular reactor was placed inside an electric furnace. The argon flow (purity 99.9996%) firstly passed through a deoxidation tube to get rid of the residual O2 before it passed into the reactor. Before reaction testing, these redox oxides were wellmixed with SBA-15 in the weight ratio of 1:3 by mechanical milling. For the O2-releasing experiment, 0.4 g mixed material was heated to 1200 oC at a 20 oC /min heating rate and then to 1300 oC at a 5 oC /min heating rate. The temperature plateau at 1300 oC was maintained for 40 min while passing Ar at a flow rate of 100 ml/min. The O2 gas was analyzed with a gas chromatograph (Aligent 6890) equipped with a 5 Å molecular sieve column and a TCD detector, taking the gas sample at the reactor outlet every ca. 2 min. The reduced state of the oxide was maintained under the protection with an Ar flow before CO2 was exposing to the reactor. For the CO generation step, the electric furnace was maintained at a given temperature (800 oC 1100 oC), and then CO2 (purity 99.999%) with a flow rate of 500 ml/min was injected into the reactor. The CO gas product was analyzed with a gas chromatograph (Aligent 6890) equipped with a GDX-102 column and a FID detector, taking the gas sample at the reactor outlet every ca. 1 or 2 min.

(5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

ASSOCIATED CONTENT

(20)

SUPPORTING INFORMATION

(21)

A detailed demonstration of physicochemical properties, the CO and O2 evolution rate-time plots, the repeated data of multiple cycles, and a reactivity comparison between bulk doped and surface loaded catalysts can be seen in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

(22) (23) (24) (25)

Corresponding Author

(26)

*E-mail: [email protected]

(27) (28)

Notes The authors declare no competing financial interest.

(29)

ACKNOWLEDGMENT

(30)

This work was financially supported by National Natural Science Foundation of China (No. 21373210) and DICP Fundamental Research Program for Clean Energy (No. DICP M201302).

(31)

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ACS Catalysis

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Graphical Abstract


Page 10 of 10

Catalytic Function of IrOx in the Two-Step Thermochemical CO2

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