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Effect of micro-structure and oxygen vacancy on the stability of (Zr-Ce)-additive CaO-based sorbent in CO2 adsorption
Journal of CO2 Utilization 19 (2017) 165–176
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Effect of micro-structure and oxygen vacancy on the stability of (Zr-Ce)additive CaO-based sorbent in CO2 adsorption Hongxia Guo, Jiaqi Feng, Yujun Zhao, Shengping Wang* , Xinbin Ma Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
A R T I C L E I N F O
Article history: Received 20 September 2016 Received in revised form 17 March 2017 Accepted 24 March 2017 Available online 4 April 2017 Keywords: (Zr-Ce)-additive Sol-gel CO2 capture CaO-based Oxygen vacancy
A B S T R A C T
This study describes the role of micro-structure and oxygen vacancy in CO2 adsorption over (Zr-Ce)additive CaO. The sorbent was prepared by sol-gel and subjected to characterization in terms of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). The sorbent (Ca/Zr/ Ce = 30:0.5:0.5) demonstrated an excellent CO2 capture capacity (0.65 gCO2/gsorbent) up to 35 cycles. The enhanced performance is attributed to well-dispersed Ce2Zr2O7 crystallite particle which prevents CaO crystallite growth and agglomeration. The presence of flake microstructure with porosity, nanosized particles, and uniform distribution of Ca and the inert components are essential to the adsorption of CO2. The existence of Ce4+ in the crystal phase of Ce2Zr2O7 may create oxygen vacancy, which favors the diffusion of CO2 and mobility of O2, further greatly facilitating the capture of CO2. The kinetics of CO2 adsorption was estimated by a double exponential method. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Concerns about climate change due to increasing CO2 concentration in the atmosphere require global action to reduce CO2 release. CO2 capture/storage represents a promising route for CO2 emission reduction. There are a number of materials (e.g., aminebased solvents, alkaline metal oxide-based sorbents, ionic liquids and membranes [1–9]) applicable for CO2 capture from fuel combustion and gasification. CaO-based sorbents have been identified as the most suitable candidates for large-scale CO2 capture at high temperatures in terms of high CO2 adsorption capacity, low material cost, fast CO2 carbonation/decarbonation kinetics and reversible carbonation/calcination process [10–12]. However, CaO-based sorbent suffers from rapid degradation, due to CaCO3 sintering during the high temperature regeneration, which is a disadvantage for scaling-up [13–18]. This drawback has driven researchers to look for efficient ways to enhance stability of CaO-based sorbents. Incorporation of refractory metals including Zr, Ce, Al, Mg, Cr, Si, Ti, Co, Y, Hf, Fe, W, and La into CaO, serves to resist sintering and suppress sorbent
* Corresponding author. E-mail address: [email protected] (S. Wang). http://dx.doi.org/10.1016/j.jcou.2017.03.015 2212-9820/© 2017 Elsevier Ltd. All rights reserved.
degradation [19–30]. CO2 capture by CaO involves CO2 adsorption (Eq. (1)), surface reaction with oxygen ions to CO32 (Eq. (2)) and final step to CaCO3 (Equation 3) [31]. CO2 (g) $ CO2 (ads)
(1)
CO2 (ads) + O2 ! CO32
(2)
CO32 + CaO ! CaCO3 + O2
(3) 2
The diffusion of CO2 and mobility of O are key to the capture of CO2 [32,33]. Therefore, more oxygen vacancies are favorable for CO2 capture. CeO2 is well known for oxygen mobility and vacancies generation, but limited by thermal stability at high temperature [34]. There is report that introduction of Zr to CeO2 produces nonstoichiometric oxide, promoting oxygen vacancy formation [34,35]. The motivated assumption is that (Zr-Ce) mixed oxides as an additive of the CO2 sorbent possess oxygen vacancy to provide a diffusion pathway for CO2 and facilitate O2 mobility. They present high oxygen mobility in CH4 oxidation and deNOx reaction [34,35]. In addition, (Zr-Ce) oxides can provide great thermal stability and resistance to sintering [36]. CO2 adsorption by (Zr-Ce)-additive CaO has not been studied to any significant level. We found two reports
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on application of (Zr-Ce)-additive Ca(OH)2 and CaO in CO2 capture [37,38]. However, the effect of adding (Zr-Ce) on improving the CO2 adsorption performance by facilitating the diffusion of CO2 and mobility of O2 in the sorbent has not been illustrated clearly. In this study, a series of (Zr-Ce)-additive CaO-based sorbents were prepared by a simple sol-gel method, aiming to synthesize a new regenerable and durable sorbent for CO2 capture with greater efficiency. The effect of micro-structure and oxygen vacancy on the CO2 adsorption performance of (Zr-Ce)-additive CaO-based sorbents was examined.
flow controllers. Temperature, imposed gas flow rate and weight of the sorbents as a function of time were recorded throughout the entire process. The carbonation/calcination process was repeated for 18 or 35 cycles. CO2 uptake capacity was calculated using the following equation: CO2 uptake ¼
mass of adsorbed CO2 100% mass of thesorbent
3. Results and discussion 2. Experimental 3.1. CO2 adsorption performance 2.1. Sorbent preparation (Zr-Ce)-additive CaO-based absorbents were prepared by solgel method using citric acid monohydrate (Tianjin Yuanli Chemical Co. Ltd., AR grade) as complexing agent [24]. Initially, an aqueous solution of calcium nitrate tetrahydrate, Ca (NO3)24H2O (Tianjin Yuanli Chemical Co. Ltd., AR grade), zirconium nitrate pentahydrate, Zr(NO3)45H2O (Tianjin Kermel Chemical Reagent Co. Ltd., AR grade), cerium nitrate hexahydrate, Ce(NO3)36H2O (Tianjin Kermel Chemical Reagent Co. Ltd., AR grade) and citric acid monohydrate with Zr/Ce = 1 and citric acid/ total metal cations = 2 was heated to 70 C under stirring for 3 h and a pale yellow gel obtained after superfluous water evaporation. The gel was dried at 100 C for 24 h generating a fluffy foam. The dried sample was crushed and calcined at 700 C (2 C/min) for 4 h. CaO was prepared using the same procedure and used as a reference. 2.2. Characterization XRD was recorded on a Rigaku D/max-2500 diffractometer using a Cu Ka (40 kV, 100 mA) radiation. The data was obtained over a 2u range of 5–85 with a scanning rate of 8 /min. Crystalline phases were identified by comparing the obtained patterns to the reference data from the International Centre for Diffraction Data (ICDD) files. The surface morphology and micro-structure of the sorbents was characterized using SEM (Hitachi S4800 field-emission microscope at 3.0 kV). The detailed structure was investigated through TEM (JEM2100F, accelerating voltage = 200 kV) and high-resolution transmission electron microscopy (HRTEM), respectively. The samples for analysis were prepared by ultrasonic dispersion in absolute ethanol and deposited on a carbon-Cu grid. XPS analysis was performed on a PEPHI-1600ESCA type X-ray photoelectronics spectrometer using Mg Ka radiation (Eb = 1253.3 eV). The charging of all the sorbents was calibrated by setting the binding energy of the adventitious carbon (C1s) at 285.0 eV. EPR measurements were carried out at 120 K with a Bruker A300 spectrometer operating in the X-band (9.41 GHz) and calibrated with a standard diphenylpicrylhydrazyl (DPPH) (g = 2.0036). 2.3. Performance measurements of sorbents The cyclic carbonation and calcination for CO2 capture testing was carried out in a thermogravimetricanalyzer (TGA, NETZSCH STA449F3) by monitoring mass change with time and temperature. The sample (ca. 10 mg) was placed in an alumina pan and heated to 600 C (10 C/min) in N2 (50 mL/min). The gas was switched to 50 v/v% CO2/N2 (100 mL/min) and maintained at 600 C for 45 min for carbonation. The sample was calcined at 700 C for 20 min in N2 (50 mL/min). The exact flow rates of the gases were set by mass
Zr and Ce content is critical in determining CO2 adsorption. We first investigated the effect of (Zr-Ce)-additive amount on CO2 capture. Different compositions of (Zr-Ce)-additive CaO-based sorbents were also expressed in terms of CaO/Ce2Zr2O7 = X/Y wt% for the sake of comprehension the ratio of the inert phase per composition, and the results of three samples with different molar ratios (Ca/Zr/Ce = 15:0.5:0.5, Ca/Zr/Ce = 30:0.5:0.5 and Ca/ Zr/Ce = 40:0.5:0.5) were 5.85/1, 11.70/1, and 15.60/1 wt%, respectively. As shown in Fig. 1(a), the performance tests showed that the sample (Ca/Zr/Ce = 40:0.5:0.5) with lower Zr and Ce content exhibited higher adsorption capacity in the first cycle, for which the theoretical maximum value of CO2 adsorption capacity was 0.74 gCO2/gsorbent, while it tended to deactivate from the second cycle onward. Although the sample (Ca/Zr/Ce = 15:0.5:0.5) did not show any sign of performance decay during 18 cycles, its overall capacity was much lower than that of the sorbent (Ca/Zr/ Ce = 30:0.5:0.5), meaning that the smaller the amount of free CaO available, the lower the adsorption performance [19]. At the same time, it could be seen that a slight increase of the CO2 capture capacity in the initial cycles due to the self-reactivation for the sorbent (Ca/Zr/Ce = 15:0.5:0.5) with a relatively large amount of inert material and low theoretical maximum value (0.67 gCO2/ gsorbent). This might be attributed to the structural rearrangement of particles and better dispersion of inert component among CaO particles [20]. It is noteworthy that there is an intimate relationship between the content of (Zr-Ce)-derivative inert material and the rate of reactive decay, which plays a role in improving the long-term performance of the sorbent. However, the initial CO2 uptake depends on the quantity of CaO sites, where higher content of Zr and Ce led to less CaO sites and decreased CO2 uptake. Consequently, as a balance, it is reasonable that the sorbent of Ca/Zr/Ce (30:0.5:0.5) exhibits the highest adsorption capacity and stability among various (ZrCe)-incorporated sorbents during 18 carbonation/calcination cycles. To study the long-term stability, the performance of (Zr-Ce)additive CaO-based sorbent (Ca/Zr/Ce = 30:0.5:0.5) in CO2 capture was examined in 35 cycles and compared with CaO. As presented in Fig. 1(b), pure CaO sorbent with high theoretical maximum value (0.786 gCO2/gsorbent) was inclined to deactivate onward. However, the (Zr-Ce)-additive CaO-based sorbent showed appreciably higher stability with an excellent CO2 capture capacity (0.65 gCO2/gsorbent) up to 35 cycles, which was close to the theoretical value (0.72 gCO2/gsorbent). 3.2. Sorbent characterization 3.2.1. XRD XRD patterns of fresh and used sorbents after 18 cycles are illustrated in Fig. 2, which can help to explain why the addition of (Zr-Ce) into the structure of pure CaO led to an increase in material stability. XRD analysis of fresh sorbents (Fig. 2(a)) revealed a
H. Guo et al. / Journal of CO2 Utilization 19 (2017) 165–176
CO2 uptake (gCO2 / gsorbent)
0.80
167
0.80
(a)
0.75
0.74
0.75
0.70
0.72
0.70
0.65
0.67
0.65
0.60
0.60
0.55
0.55
0.50
0.50
0.45
0.45
0.40
0.40
Ca/Zr/Ce=40:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=15:0.5:0.5
0.35
0.35 0.30
0.30 0
2
4
6
8
10
12
14
16
18
20
Number of cycles
CO2 uptake (gCO2 / gsorbent)
0.80
0.80
(b)
0.75
0.786
0.75
0.70
0.72
0.70
0.65
0.65
0.60
0.60
0.55
0.55
0.50
0.50
0.45
0.45
0.40
0.40
Ca/Zr/Ce=30:0.5:0.5 CaO
0.35
0.35
0.30
0.30 0
5
10
15
20
25
30
35
40
Number of cycles Fig. 1. CO2 adsorption performance: (a) various (Zr-Ce)-additive CaO-based sorbents, and (b) pure and (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce=30:0.5:0.5) during extended cyclic operation (Noted: Their respective theoretical maximum values are indicated by “—”.).
mixture of CaO, Ca(OH)2 and CaCO3. The formation of Ca(OH)2 and CaCO3 can be attributed to the reaction of CaO powder with moisture and/or CO2 in the cooling step of post calcination and/or storage. In addition to signals due to CaO, Ca(OH)2 and CaCO3, the incorporation of (Zr-Ce) to CaO resulted in formation of Ce2Zr2O7 with increased intensity at higher adding content. The generation of Ce2Zr2O7 attributed to the high calcination temperature that facilitates solid solution formation [39]. Correlated to CO2 adsorption performance (Fig. 1), the enhanced stability for the (Zr-Ce)-additive samples can be linked to the presence of Ce2Zr2O7 that prevents agglomeration of CaO particles. XRD patterns of the used sorbents (post 18 cycles testing) are displayed in Fig. 2(b). Obviously, the main crystallite phase of the used sorbents was still CaO. Ce2Zr2O7 crystallite phase was also maintained post-reaction. 3.2.2. SEM Representative SEM images of fresh and used sorbents are presented in Fig. 3. The fresh samples (Fig. 3(a-d)) showed a flake-
like morphology with porous microstructure. The particle size of CaO was smaller upon the addition of (Zr–Ce) and decreased with increasing the content. This result indicates Ce2Zr2O7 serves to suppress sintering and agglomeration of CaO particle that can contribute to higher CO2 uptake and stability observed over the (Zr-Ce)-additive samples. No significant changes in particle size and porosity was observed for the spent (Zr-Ce)-additive samples (Fig. 3(e–g)); while serious particle agglomeration and pore blockage occurred to the spent CaO (Fig. 3(h)), which accounts for the fast deactivation. 3.2.3. TEM Various fresh and used sorbents were examined by TEM/ HRTEM analysis in order to explore the structural features at the granular level. The representative TEM images for the fresh (ZrCe)-additive samples (Fig. 4) revealed a homogeneous dispersion of grains with a particle size range of 50–60 nm, which were wellknown to be more active owing to the relatively higher
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Fig. 2. XRD patterns of various (a) fresh materials and (b) used sorbents after 18 cycles.
concentration of coordinatively unsaturated sites on their surfaces. Furthermore, as indicated from literature, nanostructure enables reactions to operate in the reaction-controlled carbonation regime rather than in the diffusion-controlled one when employing sorbents produced with larger-particles [26]. A pattern of parallel high-contrast light and dark bands extending across the surfaces was displayed in a substantial number of crystals, which might be a diffraction phenomenon due to the presence of different phases in a surface layer or crystallographic orientation from that of the crystal underneath [19]. Interestingly, it was also found that the size of the grains decreased with increasing Zr and Ce concentration. For the used sorbents, severer agglomeration occurred to pure CaO compared with that in the (Zr-Ce)-additive samples, as shown in Fig. 4(e)–(h). Energy
disperse spectroscopy (EDS) mapping (attached to HRTEM) of the sorbent (Ca/Zr/Ce = 30:0.5:0.5) (presented in Fig. 5) reveals uniform distribution of Zr and Ce on CaO surface. This suggests a homogeneous dispersion of Zr and Ce oxides, which can contribute to a decrease in sintering and aggregation of CaO particles. The atomic lattice image with associated Fourier transforms in Fig. 6 demonstrates exposure of Ce2Zr2O7 (222) planes with an interplanar spacing of 0.31 nm calculated from the corresponding FFT pattern. 3.2.4. XPS XPS analysis provides information on the chemical/electronic state. Spectra of Ca2p (Fig. 7) for the fresh CaO exhibited a peak at BE (binding energy) = 346.7 ev for Ca2p3/2 with a satellite peak at
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169
Fig. 3. SEM images of various fresh materials and used sorbents after 18 ycles:(a)Ca/Zr/Ce=15:0.5:0.5, (b)Ca/Zr=30:0.5:0.5, (c) Ca/Zr=40:0.5:0.5, (d)CaO, (e)used Ca/Zr/ Ce=15:0.5:0.5, (f) used Ca/Zr=30:0.5:0.5, (g) used Ca/Zr/Ce=40:0.5:0.5, and (h)used CaO sorbents.
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Fig. 4. TEM images of various fresh materials and used sorbents after 18 cycles:(a)Ca/Zr/Ce=15:0.5:0.5, (b)Ca/Zr=30:0.5:0.5, (c) Ca/Zr=40:0.5:0.5, (d)CaO, (e)used Ca/Zr/ Ce=15:0.5:0.5, (f) used Ca/Zr=30:0.5:0.5, (g) used Ca/Zr/Ce=40:0.5:0.5, and (h) used CaO sorbents.
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171
Fig. 5. Energy disperse spectroscopy (EDS) mapping (attached to HRTEM) of the fresh sorbent ((Ca/Zr/Ce=30/0.5/0.5) blue spots, calcium; red spots, cerium; green spots, zirconium; white, oxygen.. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
BE = 350 ev for Ca2p1/2 that are characteristic of Ca2+ [40]. XPS profile of Zr3d (Fig. 8) is characteristic of peak at BE = 181.5 ev (Zr3d5/2) and BE = 184 ev (Zr3d3/2) corresponding to Zr4+ [41,42]. The spectra of Ce3d consist of up to ten peaks, where the peaks marked as u, u0, u', u”, u”' are connected to the spin-orbit split of Ce 3d3/2 with the peaks denoted v, v0, v', v”, v”' corresponding to Ce 3d5/2 [40,41]. The peaks labeled as u, u”, u”', v, v”, v”' are associated with Ce4+ ions while peaks denoted u0, u', v0, v', link to Ce3+ ions. As
depicted in Fig. 9, the intensity of the peak is not high due to the lower amount of the Ce-additive mixed phase in the sample. However, three main binding energy peaks could be observed, i.e. 882.5/889.0 eV ascribed to Ce4+3d5/2, and 899.3 eV assigned to Ce3+ 3d3/2. It indicates that both Ce3+ and Ce4+ exist in the sorbents. In order to maintain electric neutrality, oxygen vacancy can be formed because of the existence of Ce4+ and Zr4+ in the crystal phase of Ce2Zr2O7, with Ce3+ and Zr4+ in standard phase. They are in
Fig. 6. Atomic lattice image and inset Fourier transforms indexed to Ce2Zr2O7 phases for the fresh sorbent (Ca/Zr/Ce=30/0.5/0.5).
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Ca/Zr/Ce=15:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=40:0.5:0.5 CaO
Intensity(a.u.)
Ca2p
358
356
354
352
350
348
346
344
342
340
338
Binding energy(eV) Fig. 7. The XPS spectra of Ca2p of the fresh sorbents.
favor of the diffusion of CO2 and mobility of O2 according to the mechanism mentioned above. At the same time, a shift to higher BE of the core levels in Ca2p and Zr3d is clearly observed in the samples with increasing Zr and Ce concentration, indicating the electron cloud density decrease of Ca2+ and Zr4+, so it is easier for the (Zr-Ce)-additive sorbents to adsorb O2. There is also evidence that Ce2Zr2O7 exhibits a fast diffusion path for oxygen ions, but sluggish for Zr4+, which enhances CO2 uptake and stability [39]. On the other hand, as shown in Fig. 10, XPS spectra of O1s, a shift to higher BE was clearly found in the samples with an increased Zr and Ce concentration, indicating a connection between Zr and Ce, and the formation of Ce2Zr2O7. 3.2.5. EPR EPR measurement is an effective way of identifying the oxidation state of the transition metals [43,44]. As shown in Fig. 11, EPR spectra of the (Zr-Ce)-additive samples reveal formation of two types of oxygen-derived radicals, with cerium cation acting as chemisorption centers in some cases, according to
Zr3d
Intensity(a.u.)
Ca/Zr/Ce=15:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=40:0.5:0.5
188
186
184
182
180
178
176
174
172
Binding energy(eV) Fig. 8. The XPS spectra of Zr3d of various (Zr-Ce)-additive sorbents.
170
Fig. 9. The XPS spectra of Ce3d of the fresh sorbents.
H. Guo et al. / Journal of CO2 Utilization 19 (2017) 165–176
Ca/Zr/Ce=15:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=40:0.5:0.5 CaO
Intensity(a.u.)
O1s
540
538
536
534
532
530
528
526
524
522
520
Binding energy(eV) Fig. 10. The XPS spectra of O1 s of the fresh sorbents.
Ca/Zr/Ce=15:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=40:0.5:0.5 1.996
1.978
carbonation reaction in the critical point from the fast to the slow kinetics stages was large for both CaO and (Zr-Ce)-additive sample at the 1st cycle. CO2 uptake over pure CaO significantly declined relative to the (Zr-Ce)-additive sample that maintained stable at the 18th cycle. The extent of carbonation reaction during the fast reaction stage appeared a decrease at the 35th cycle as a result of sintering. To better understand the adsorption kinetics in practically relevant fast stage, the rates of pure and (Zr-Ce)-additive CaO in the first 2 min for the 1st and 18th cycles were calculated, and the results are illustrated in Fig. 12(b). It was observed that the reaction rate sharply went up a maximum at 0.25 min, followed by a decrease as carbonation proceeded. The maximum value of adsorption rate at the 1st cycle (r1,max), for the (Zr-Ce)-additive CaO was measurably lower than that of the pure CaO. r18,max (1.8 g/gmin) for the (Zr-Ce)-additive sample was close to r1,max, while pure CaO exhibited a significant drop (to ca. 1.3 g/gmin). The (Zr-Ce)-additive CaO showed a lower decrease in rate because of their more dispersed structure, smaller sized particle and the existence of more exposed active CaO. In addition, it could be seen that r35,max of the (Zr-Ce)-additive sample decreased due to thermal sintering after 35 carbonation/calcination cycles, but was still higher than that of pure CaO. 3.3.2. Adsorption kinetics estimate In order to study the influence of addition of (Zr-Ce) on the CO2 adsorption kinetics in the chemical and diffusion controlled stages, a double exponential method is used to extract the rate constants of pure and (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce = 30:0.5:0.5) in the 1st, 18th and 35th cycles. The double exponential method is an important fitting-method, similar to the other common exponential distribution. It is one of the most practical model and widely used in the field of scientific analysis. At the same time, it is an effective way of dealing with experimental data. Fitting the adsorption process behavior is one of the most important applications [46]. Following the double exponential method, the equation can be expressed as: y ¼ Aexpðk1 tÞ þ Bexpðk2 tÞ þ C
3370
3380
3390
3400
3410
Magnetic field(G) Fig. 11. The EPR spectra of (Zr-Ce)-additive samples.
previous research [43]. The first type corresponds to O2 anions (g = 1.996), which is associated with Ce4+ [40]. The second type is represented by a signal with a considerably broader line shape, being assigned to O anions (g = 1.978). The O anions are oxygen species that can be linked to Ce3+ [44]. These results indicate the existence of Ce4+ and Ce3+, which is in accordance with that obtained by XPS measurement. 3.3. CO2 adsorption kinetics 3.3.1. Adsorption kinetics analysis The variation of CO2 uptake with time and cycle numbers (1, 18 and 35) for pure and (Zr-Ce)-additive CaO (Ca/Zr/Ce = 30:0.5:0.5) is displayed in Fig. 12(a). The results showed chemical and diffusion controlled stages in the carbonation reaction, i.e. fast and slow reaction stages. Meanwhile, the extent of carbonation reaction during the first fast carbonation stage was confirmed according to the literature [45]. As shown in Table 1, the extent of
173
ð1Þ
where y is the CO2 uptake (gCO2/gsorbent), k1 and k2 are the rate constants of chemisorption and diffusion, respectively. A, B and C are the pre-exponential factors, and t is the reaction time. Regression analysis of the experimental data is presented in Fig. 13 and Table 2. The correlation parameters (R2) for the fitting are no lower than 0.99, which indicates a good correlation regression. Rate constant ratio of chemisorption to diffusion, k1/ k2, is greater than 1 at the 1st cycle for both pure and (Zr-Ce)additive CaO, implying that the control step is the diffusion of CO2 through a layer of CaCO3 to react with the unconverted CaO core. Therefore, the rate of surface chemical reaction is rapid and the higher adsorption capacity is obtained. At the 18th cycle, the value of k1/k2 is far less than 1 as to pure CaO, suggesting the ratedetermining step is surface reaction. Lower adsorption is measured due to sintering leading to decrease of active sites on the surface of pure CaO. No significant change was observed at the 18th cycle for the (Zr-Ce)-additive sample relative the 1st cycle. However, the value of k1/k2 (Table 2) is far lower than 1 for the (Zr-Ce)-additive sample at the 35th cycle, indicating that the rate-determining step shifted from diffusion of CO2 to surface reaction. 4. Conclusions This study develops an efficient (Zr-Ce)-additive CaO-based sorbent for CO2 capture. The principal findings can be summarized
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Fig. 12. Cyclic carbonation kinetics of pure and (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce=30:0.5:0.5): (a) adsorption performance at the 1st, 18th, and 35th cycles, and (b) adsorption rates in the first 2 min at the 1st, 18th, and 35th cycles.
as follows: (1) The (Zr-Ce)-additive sample delivered higher CO2 capture capacity (0.65 gCO2/gsorbent) up to 35 cycles with enhanced stability than that for CaO. (2) Ce2Zr2O7 crystallite phases as a
Table 1 Capacity comparison of the critical point for pure and (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce = 30:0.5:0.5). Number of cycles
CaO (gCO2/gsorbent)
Ca/Zr/Ce = 30:0.5:0.5 (gCO2/gsorbent)
1st 18th 35th
0.62 0.04 –
0.58 0.56 0.06
barrier between CaO particles served to prevent CaO crystallite growth and agglomeration. (3) Enhanced stability can be linked to porous flake microstructure, nano-scale particles, the uniform distribution of Ca and the inert components in the sorbent. (4) In order to maintain electric neutrality, oxygen vacancy existed in the crystal phase of Ce2Zr2O7, which was beneficial for the diffusion of CO2 and mobility of O2. The XPS results showed a shift to higher binding energy of the core levels in Ca2p and Zr3d by addition of (ZrCe), indicating the decrease of the electron cloud density of Ca2+ and Zr4+, which significantly enhanced the adsorption of O2. (5) The kinetics of CO2 adsorption was analyzed by a double exponential method.
H. Guo et al. / Journal of CO2 Utilization 19 (2017) 165–176
CO2 uptake (g CO2 / g sorbent)
0.7
175
(a)
0.6 0.5 0.4 0.3 0.2
1st, CaO ExpDec2 Fit of CaO, 1st 18th, CaO ExpDec2 Fit of CaO, 18th
0.1 0.0 0
10
20
30
40
Time(min)
CO2 uptake (g CO2 / g sorbent)
0.7
(b)
0.6 0.5 0.4 0.3
1st, Ca/Zr/Ce=30:0.5:0.5 ExpDec2 Fit of Ca/Zr/Ce=30:0.5:0.5, 1st 18th, Ca/Zr/Ce=30:0.5:0.5 ExpDec2 Fit of Ca/Zr/Ce=30:0.5:0.5, 18th 35th, Ca/Zr/Ce=30:0.5:0.5 ExpDec2 Fit of Ca/Zr/Ce=30:0.5:0.5, 35th
0.2 0.1 0.0 0
10
20
30
40
Time(min) Fig. 13. Comparison of the fitting and experimental results of the (a) pure and (b) (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce = 30:0.5:0.5).
Table 2 Rate constants of CO2 adsorption estimated by double exponential method. Sorbents
k1/s1
k2/s1
k1/k2
R2
CaO, 1st CaO, 18th Ca/Zr/Ce = 30:0.5:0.5, 1st Ca/Zr/Ce = 30:0.5:0.5, 18th Ca/Zr/Ce = 30:0.5:0.5, 35th
3.26520 0.02994 2.55912 1.22775 0.08975
0.05549 1.29676 0.15506 0.08388 1.42098
58.84363 0.02309 16.50392 14.63684 0.06316
0.99294 0.99216 0.99523 0.99 0.99
Acknowledgements Financial support by National Natural Science Foundation of China (NSFC) (Grant No. 21325626, U1462122, U1510203), the Program for New Century Excellent Talents in University (NCET-13-
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Effect of micro-structure and oxygen vacancy on the stability of (Zr-Ce)additive CaO-based sorbent in CO2 adsorption Hongxia Guo, Jiaqi Feng, Yujun Zhao, Shengping Wang* , Xinbin Ma Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
A R T I C L E I N F O
Article history: Received 20 September 2016 Received in revised form 17 March 2017 Accepted 24 March 2017 Available online 4 April 2017 Keywords: (Zr-Ce)-additive Sol-gel CO2 capture CaO-based Oxygen vacancy
A B S T R A C T
This study describes the role of micro-structure and oxygen vacancy in CO2 adsorption over (Zr-Ce)additive CaO. The sorbent was prepared by sol-gel and subjected to characterization in terms of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). The sorbent (Ca/Zr/ Ce = 30:0.5:0.5) demonstrated an excellent CO2 capture capacity (0.65 gCO2/gsorbent) up to 35 cycles. The enhanced performance is attributed to well-dispersed Ce2Zr2O7 crystallite particle which prevents CaO crystallite growth and agglomeration. The presence of flake microstructure with porosity, nanosized particles, and uniform distribution of Ca and the inert components are essential to the adsorption of CO2. The existence of Ce4+ in the crystal phase of Ce2Zr2O7 may create oxygen vacancy, which favors the diffusion of CO2 and mobility of O2, further greatly facilitating the capture of CO2. The kinetics of CO2 adsorption was estimated by a double exponential method. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Concerns about climate change due to increasing CO2 concentration in the atmosphere require global action to reduce CO2 release. CO2 capture/storage represents a promising route for CO2 emission reduction. There are a number of materials (e.g., aminebased solvents, alkaline metal oxide-based sorbents, ionic liquids and membranes [1–9]) applicable for CO2 capture from fuel combustion and gasification. CaO-based sorbents have been identified as the most suitable candidates for large-scale CO2 capture at high temperatures in terms of high CO2 adsorption capacity, low material cost, fast CO2 carbonation/decarbonation kinetics and reversible carbonation/calcination process [10–12]. However, CaO-based sorbent suffers from rapid degradation, due to CaCO3 sintering during the high temperature regeneration, which is a disadvantage for scaling-up [13–18]. This drawback has driven researchers to look for efficient ways to enhance stability of CaO-based sorbents. Incorporation of refractory metals including Zr, Ce, Al, Mg, Cr, Si, Ti, Co, Y, Hf, Fe, W, and La into CaO, serves to resist sintering and suppress sorbent
* Corresponding author. E-mail address: [email protected] (S. Wang). http://dx.doi.org/10.1016/j.jcou.2017.03.015 2212-9820/© 2017 Elsevier Ltd. All rights reserved.
degradation [19–30]. CO2 capture by CaO involves CO2 adsorption (Eq. (1)), surface reaction with oxygen ions to CO32 (Eq. (2)) and final step to CaCO3 (Equation 3) [31]. CO2 (g) $ CO2 (ads)
(1)
CO2 (ads) + O2 ! CO32
(2)
CO32 + CaO ! CaCO3 + O2
(3) 2
The diffusion of CO2 and mobility of O are key to the capture of CO2 [32,33]. Therefore, more oxygen vacancies are favorable for CO2 capture. CeO2 is well known for oxygen mobility and vacancies generation, but limited by thermal stability at high temperature [34]. There is report that introduction of Zr to CeO2 produces nonstoichiometric oxide, promoting oxygen vacancy formation [34,35]. The motivated assumption is that (Zr-Ce) mixed oxides as an additive of the CO2 sorbent possess oxygen vacancy to provide a diffusion pathway for CO2 and facilitate O2 mobility. They present high oxygen mobility in CH4 oxidation and deNOx reaction [34,35]. In addition, (Zr-Ce) oxides can provide great thermal stability and resistance to sintering [36]. CO2 adsorption by (Zr-Ce)-additive CaO has not been studied to any significant level. We found two reports
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on application of (Zr-Ce)-additive Ca(OH)2 and CaO in CO2 capture [37,38]. However, the effect of adding (Zr-Ce) on improving the CO2 adsorption performance by facilitating the diffusion of CO2 and mobility of O2 in the sorbent has not been illustrated clearly. In this study, a series of (Zr-Ce)-additive CaO-based sorbents were prepared by a simple sol-gel method, aiming to synthesize a new regenerable and durable sorbent for CO2 capture with greater efficiency. The effect of micro-structure and oxygen vacancy on the CO2 adsorption performance of (Zr-Ce)-additive CaO-based sorbents was examined.
flow controllers. Temperature, imposed gas flow rate and weight of the sorbents as a function of time were recorded throughout the entire process. The carbonation/calcination process was repeated for 18 or 35 cycles. CO2 uptake capacity was calculated using the following equation: CO2 uptake ¼
mass of adsorbed CO2 100% mass of thesorbent
3. Results and discussion 2. Experimental 3.1. CO2 adsorption performance 2.1. Sorbent preparation (Zr-Ce)-additive CaO-based absorbents were prepared by solgel method using citric acid monohydrate (Tianjin Yuanli Chemical Co. Ltd., AR grade) as complexing agent [24]. Initially, an aqueous solution of calcium nitrate tetrahydrate, Ca (NO3)24H2O (Tianjin Yuanli Chemical Co. Ltd., AR grade), zirconium nitrate pentahydrate, Zr(NO3)45H2O (Tianjin Kermel Chemical Reagent Co. Ltd., AR grade), cerium nitrate hexahydrate, Ce(NO3)36H2O (Tianjin Kermel Chemical Reagent Co. Ltd., AR grade) and citric acid monohydrate with Zr/Ce = 1 and citric acid/ total metal cations = 2 was heated to 70 C under stirring for 3 h and a pale yellow gel obtained after superfluous water evaporation. The gel was dried at 100 C for 24 h generating a fluffy foam. The dried sample was crushed and calcined at 700 C (2 C/min) for 4 h. CaO was prepared using the same procedure and used as a reference. 2.2. Characterization XRD was recorded on a Rigaku D/max-2500 diffractometer using a Cu Ka (40 kV, 100 mA) radiation. The data was obtained over a 2u range of 5–85 with a scanning rate of 8 /min. Crystalline phases were identified by comparing the obtained patterns to the reference data from the International Centre for Diffraction Data (ICDD) files. The surface morphology and micro-structure of the sorbents was characterized using SEM (Hitachi S4800 field-emission microscope at 3.0 kV). The detailed structure was investigated through TEM (JEM2100F, accelerating voltage = 200 kV) and high-resolution transmission electron microscopy (HRTEM), respectively. The samples for analysis were prepared by ultrasonic dispersion in absolute ethanol and deposited on a carbon-Cu grid. XPS analysis was performed on a PEPHI-1600ESCA type X-ray photoelectronics spectrometer using Mg Ka radiation (Eb = 1253.3 eV). The charging of all the sorbents was calibrated by setting the binding energy of the adventitious carbon (C1s) at 285.0 eV. EPR measurements were carried out at 120 K with a Bruker A300 spectrometer operating in the X-band (9.41 GHz) and calibrated with a standard diphenylpicrylhydrazyl (DPPH) (g = 2.0036). 2.3. Performance measurements of sorbents The cyclic carbonation and calcination for CO2 capture testing was carried out in a thermogravimetricanalyzer (TGA, NETZSCH STA449F3) by monitoring mass change with time and temperature. The sample (ca. 10 mg) was placed in an alumina pan and heated to 600 C (10 C/min) in N2 (50 mL/min). The gas was switched to 50 v/v% CO2/N2 (100 mL/min) and maintained at 600 C for 45 min for carbonation. The sample was calcined at 700 C for 20 min in N2 (50 mL/min). The exact flow rates of the gases were set by mass
Zr and Ce content is critical in determining CO2 adsorption. We first investigated the effect of (Zr-Ce)-additive amount on CO2 capture. Different compositions of (Zr-Ce)-additive CaO-based sorbents were also expressed in terms of CaO/Ce2Zr2O7 = X/Y wt% for the sake of comprehension the ratio of the inert phase per composition, and the results of three samples with different molar ratios (Ca/Zr/Ce = 15:0.5:0.5, Ca/Zr/Ce = 30:0.5:0.5 and Ca/ Zr/Ce = 40:0.5:0.5) were 5.85/1, 11.70/1, and 15.60/1 wt%, respectively. As shown in Fig. 1(a), the performance tests showed that the sample (Ca/Zr/Ce = 40:0.5:0.5) with lower Zr and Ce content exhibited higher adsorption capacity in the first cycle, for which the theoretical maximum value of CO2 adsorption capacity was 0.74 gCO2/gsorbent, while it tended to deactivate from the second cycle onward. Although the sample (Ca/Zr/Ce = 15:0.5:0.5) did not show any sign of performance decay during 18 cycles, its overall capacity was much lower than that of the sorbent (Ca/Zr/ Ce = 30:0.5:0.5), meaning that the smaller the amount of free CaO available, the lower the adsorption performance [19]. At the same time, it could be seen that a slight increase of the CO2 capture capacity in the initial cycles due to the self-reactivation for the sorbent (Ca/Zr/Ce = 15:0.5:0.5) with a relatively large amount of inert material and low theoretical maximum value (0.67 gCO2/ gsorbent). This might be attributed to the structural rearrangement of particles and better dispersion of inert component among CaO particles [20]. It is noteworthy that there is an intimate relationship between the content of (Zr-Ce)-derivative inert material and the rate of reactive decay, which plays a role in improving the long-term performance of the sorbent. However, the initial CO2 uptake depends on the quantity of CaO sites, where higher content of Zr and Ce led to less CaO sites and decreased CO2 uptake. Consequently, as a balance, it is reasonable that the sorbent of Ca/Zr/Ce (30:0.5:0.5) exhibits the highest adsorption capacity and stability among various (ZrCe)-incorporated sorbents during 18 carbonation/calcination cycles. To study the long-term stability, the performance of (Zr-Ce)additive CaO-based sorbent (Ca/Zr/Ce = 30:0.5:0.5) in CO2 capture was examined in 35 cycles and compared with CaO. As presented in Fig. 1(b), pure CaO sorbent with high theoretical maximum value (0.786 gCO2/gsorbent) was inclined to deactivate onward. However, the (Zr-Ce)-additive CaO-based sorbent showed appreciably higher stability with an excellent CO2 capture capacity (0.65 gCO2/gsorbent) up to 35 cycles, which was close to the theoretical value (0.72 gCO2/gsorbent). 3.2. Sorbent characterization 3.2.1. XRD XRD patterns of fresh and used sorbents after 18 cycles are illustrated in Fig. 2, which can help to explain why the addition of (Zr-Ce) into the structure of pure CaO led to an increase in material stability. XRD analysis of fresh sorbents (Fig. 2(a)) revealed a
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CO2 uptake (gCO2 / gsorbent)
0.80
167
0.80
(a)
0.75
0.74
0.75
0.70
0.72
0.70
0.65
0.67
0.65
0.60
0.60
0.55
0.55
0.50
0.50
0.45
0.45
0.40
0.40
Ca/Zr/Ce=40:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=15:0.5:0.5
0.35
0.35 0.30
0.30 0
2
4
6
8
10
12
14
16
18
20
Number of cycles
CO2 uptake (gCO2 / gsorbent)
0.80
0.80
(b)
0.75
0.786
0.75
0.70
0.72
0.70
0.65
0.65
0.60
0.60
0.55
0.55
0.50
0.50
0.45
0.45
0.40
0.40
Ca/Zr/Ce=30:0.5:0.5 CaO
0.35
0.35
0.30
0.30 0
5
10
15
20
25
30
35
40
Number of cycles Fig. 1. CO2 adsorption performance: (a) various (Zr-Ce)-additive CaO-based sorbents, and (b) pure and (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce=30:0.5:0.5) during extended cyclic operation (Noted: Their respective theoretical maximum values are indicated by “—”.).
mixture of CaO, Ca(OH)2 and CaCO3. The formation of Ca(OH)2 and CaCO3 can be attributed to the reaction of CaO powder with moisture and/or CO2 in the cooling step of post calcination and/or storage. In addition to signals due to CaO, Ca(OH)2 and CaCO3, the incorporation of (Zr-Ce) to CaO resulted in formation of Ce2Zr2O7 with increased intensity at higher adding content. The generation of Ce2Zr2O7 attributed to the high calcination temperature that facilitates solid solution formation [39]. Correlated to CO2 adsorption performance (Fig. 1), the enhanced stability for the (Zr-Ce)-additive samples can be linked to the presence of Ce2Zr2O7 that prevents agglomeration of CaO particles. XRD patterns of the used sorbents (post 18 cycles testing) are displayed in Fig. 2(b). Obviously, the main crystallite phase of the used sorbents was still CaO. Ce2Zr2O7 crystallite phase was also maintained post-reaction. 3.2.2. SEM Representative SEM images of fresh and used sorbents are presented in Fig. 3. The fresh samples (Fig. 3(a-d)) showed a flake-
like morphology with porous microstructure. The particle size of CaO was smaller upon the addition of (Zr–Ce) and decreased with increasing the content. This result indicates Ce2Zr2O7 serves to suppress sintering and agglomeration of CaO particle that can contribute to higher CO2 uptake and stability observed over the (Zr-Ce)-additive samples. No significant changes in particle size and porosity was observed for the spent (Zr-Ce)-additive samples (Fig. 3(e–g)); while serious particle agglomeration and pore blockage occurred to the spent CaO (Fig. 3(h)), which accounts for the fast deactivation. 3.2.3. TEM Various fresh and used sorbents were examined by TEM/ HRTEM analysis in order to explore the structural features at the granular level. The representative TEM images for the fresh (ZrCe)-additive samples (Fig. 4) revealed a homogeneous dispersion of grains with a particle size range of 50–60 nm, which were wellknown to be more active owing to the relatively higher
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Fig. 2. XRD patterns of various (a) fresh materials and (b) used sorbents after 18 cycles.
concentration of coordinatively unsaturated sites on their surfaces. Furthermore, as indicated from literature, nanostructure enables reactions to operate in the reaction-controlled carbonation regime rather than in the diffusion-controlled one when employing sorbents produced with larger-particles [26]. A pattern of parallel high-contrast light and dark bands extending across the surfaces was displayed in a substantial number of crystals, which might be a diffraction phenomenon due to the presence of different phases in a surface layer or crystallographic orientation from that of the crystal underneath [19]. Interestingly, it was also found that the size of the grains decreased with increasing Zr and Ce concentration. For the used sorbents, severer agglomeration occurred to pure CaO compared with that in the (Zr-Ce)-additive samples, as shown in Fig. 4(e)–(h). Energy
disperse spectroscopy (EDS) mapping (attached to HRTEM) of the sorbent (Ca/Zr/Ce = 30:0.5:0.5) (presented in Fig. 5) reveals uniform distribution of Zr and Ce on CaO surface. This suggests a homogeneous dispersion of Zr and Ce oxides, which can contribute to a decrease in sintering and aggregation of CaO particles. The atomic lattice image with associated Fourier transforms in Fig. 6 demonstrates exposure of Ce2Zr2O7 (222) planes with an interplanar spacing of 0.31 nm calculated from the corresponding FFT pattern. 3.2.4. XPS XPS analysis provides information on the chemical/electronic state. Spectra of Ca2p (Fig. 7) for the fresh CaO exhibited a peak at BE (binding energy) = 346.7 ev for Ca2p3/2 with a satellite peak at
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169
Fig. 3. SEM images of various fresh materials and used sorbents after 18 ycles:(a)Ca/Zr/Ce=15:0.5:0.5, (b)Ca/Zr=30:0.5:0.5, (c) Ca/Zr=40:0.5:0.5, (d)CaO, (e)used Ca/Zr/ Ce=15:0.5:0.5, (f) used Ca/Zr=30:0.5:0.5, (g) used Ca/Zr/Ce=40:0.5:0.5, and (h)used CaO sorbents.
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Fig. 4. TEM images of various fresh materials and used sorbents after 18 cycles:(a)Ca/Zr/Ce=15:0.5:0.5, (b)Ca/Zr=30:0.5:0.5, (c) Ca/Zr=40:0.5:0.5, (d)CaO, (e)used Ca/Zr/ Ce=15:0.5:0.5, (f) used Ca/Zr=30:0.5:0.5, (g) used Ca/Zr/Ce=40:0.5:0.5, and (h) used CaO sorbents.
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171
Fig. 5. Energy disperse spectroscopy (EDS) mapping (attached to HRTEM) of the fresh sorbent ((Ca/Zr/Ce=30/0.5/0.5) blue spots, calcium; red spots, cerium; green spots, zirconium; white, oxygen.. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
BE = 350 ev for Ca2p1/2 that are characteristic of Ca2+ [40]. XPS profile of Zr3d (Fig. 8) is characteristic of peak at BE = 181.5 ev (Zr3d5/2) and BE = 184 ev (Zr3d3/2) corresponding to Zr4+ [41,42]. The spectra of Ce3d consist of up to ten peaks, where the peaks marked as u, u0, u', u”, u”' are connected to the spin-orbit split of Ce 3d3/2 with the peaks denoted v, v0, v', v”, v”' corresponding to Ce 3d5/2 [40,41]. The peaks labeled as u, u”, u”', v, v”, v”' are associated with Ce4+ ions while peaks denoted u0, u', v0, v', link to Ce3+ ions. As
depicted in Fig. 9, the intensity of the peak is not high due to the lower amount of the Ce-additive mixed phase in the sample. However, three main binding energy peaks could be observed, i.e. 882.5/889.0 eV ascribed to Ce4+3d5/2, and 899.3 eV assigned to Ce3+ 3d3/2. It indicates that both Ce3+ and Ce4+ exist in the sorbents. In order to maintain electric neutrality, oxygen vacancy can be formed because of the existence of Ce4+ and Zr4+ in the crystal phase of Ce2Zr2O7, with Ce3+ and Zr4+ in standard phase. They are in
Fig. 6. Atomic lattice image and inset Fourier transforms indexed to Ce2Zr2O7 phases for the fresh sorbent (Ca/Zr/Ce=30/0.5/0.5).
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Ca/Zr/Ce=15:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=40:0.5:0.5 CaO
Intensity(a.u.)
Ca2p
358
356
354
352
350
348
346
344
342
340
338
Binding energy(eV) Fig. 7. The XPS spectra of Ca2p of the fresh sorbents.
favor of the diffusion of CO2 and mobility of O2 according to the mechanism mentioned above. At the same time, a shift to higher BE of the core levels in Ca2p and Zr3d is clearly observed in the samples with increasing Zr and Ce concentration, indicating the electron cloud density decrease of Ca2+ and Zr4+, so it is easier for the (Zr-Ce)-additive sorbents to adsorb O2. There is also evidence that Ce2Zr2O7 exhibits a fast diffusion path for oxygen ions, but sluggish for Zr4+, which enhances CO2 uptake and stability [39]. On the other hand, as shown in Fig. 10, XPS spectra of O1s, a shift to higher BE was clearly found in the samples with an increased Zr and Ce concentration, indicating a connection between Zr and Ce, and the formation of Ce2Zr2O7. 3.2.5. EPR EPR measurement is an effective way of identifying the oxidation state of the transition metals [43,44]. As shown in Fig. 11, EPR spectra of the (Zr-Ce)-additive samples reveal formation of two types of oxygen-derived radicals, with cerium cation acting as chemisorption centers in some cases, according to
Zr3d
Intensity(a.u.)
Ca/Zr/Ce=15:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=40:0.5:0.5
188
186
184
182
180
178
176
174
172
Binding energy(eV) Fig. 8. The XPS spectra of Zr3d of various (Zr-Ce)-additive sorbents.
170
Fig. 9. The XPS spectra of Ce3d of the fresh sorbents.
H. Guo et al. / Journal of CO2 Utilization 19 (2017) 165–176
Ca/Zr/Ce=15:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=40:0.5:0.5 CaO
Intensity(a.u.)
O1s
540
538
536
534
532
530
528
526
524
522
520
Binding energy(eV) Fig. 10. The XPS spectra of O1 s of the fresh sorbents.
Ca/Zr/Ce=15:0.5:0.5 Ca/Zr/Ce=30:0.5:0.5 Ca/Zr/Ce=40:0.5:0.5 1.996
1.978
carbonation reaction in the critical point from the fast to the slow kinetics stages was large for both CaO and (Zr-Ce)-additive sample at the 1st cycle. CO2 uptake over pure CaO significantly declined relative to the (Zr-Ce)-additive sample that maintained stable at the 18th cycle. The extent of carbonation reaction during the fast reaction stage appeared a decrease at the 35th cycle as a result of sintering. To better understand the adsorption kinetics in practically relevant fast stage, the rates of pure and (Zr-Ce)-additive CaO in the first 2 min for the 1st and 18th cycles were calculated, and the results are illustrated in Fig. 12(b). It was observed that the reaction rate sharply went up a maximum at 0.25 min, followed by a decrease as carbonation proceeded. The maximum value of adsorption rate at the 1st cycle (r1,max), for the (Zr-Ce)-additive CaO was measurably lower than that of the pure CaO. r18,max (1.8 g/gmin) for the (Zr-Ce)-additive sample was close to r1,max, while pure CaO exhibited a significant drop (to ca. 1.3 g/gmin). The (Zr-Ce)-additive CaO showed a lower decrease in rate because of their more dispersed structure, smaller sized particle and the existence of more exposed active CaO. In addition, it could be seen that r35,max of the (Zr-Ce)-additive sample decreased due to thermal sintering after 35 carbonation/calcination cycles, but was still higher than that of pure CaO. 3.3.2. Adsorption kinetics estimate In order to study the influence of addition of (Zr-Ce) on the CO2 adsorption kinetics in the chemical and diffusion controlled stages, a double exponential method is used to extract the rate constants of pure and (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce = 30:0.5:0.5) in the 1st, 18th and 35th cycles. The double exponential method is an important fitting-method, similar to the other common exponential distribution. It is one of the most practical model and widely used in the field of scientific analysis. At the same time, it is an effective way of dealing with experimental data. Fitting the adsorption process behavior is one of the most important applications [46]. Following the double exponential method, the equation can be expressed as: y ¼ Aexpðk1 tÞ þ Bexpðk2 tÞ þ C
3370
3380
3390
3400
3410
Magnetic field(G) Fig. 11. The EPR spectra of (Zr-Ce)-additive samples.
previous research [43]. The first type corresponds to O2 anions (g = 1.996), which is associated with Ce4+ [40]. The second type is represented by a signal with a considerably broader line shape, being assigned to O anions (g = 1.978). The O anions are oxygen species that can be linked to Ce3+ [44]. These results indicate the existence of Ce4+ and Ce3+, which is in accordance with that obtained by XPS measurement. 3.3. CO2 adsorption kinetics 3.3.1. Adsorption kinetics analysis The variation of CO2 uptake with time and cycle numbers (1, 18 and 35) for pure and (Zr-Ce)-additive CaO (Ca/Zr/Ce = 30:0.5:0.5) is displayed in Fig. 12(a). The results showed chemical and diffusion controlled stages in the carbonation reaction, i.e. fast and slow reaction stages. Meanwhile, the extent of carbonation reaction during the first fast carbonation stage was confirmed according to the literature [45]. As shown in Table 1, the extent of
173
ð1Þ
where y is the CO2 uptake (gCO2/gsorbent), k1 and k2 are the rate constants of chemisorption and diffusion, respectively. A, B and C are the pre-exponential factors, and t is the reaction time. Regression analysis of the experimental data is presented in Fig. 13 and Table 2. The correlation parameters (R2) for the fitting are no lower than 0.99, which indicates a good correlation regression. Rate constant ratio of chemisorption to diffusion, k1/ k2, is greater than 1 at the 1st cycle for both pure and (Zr-Ce)additive CaO, implying that the control step is the diffusion of CO2 through a layer of CaCO3 to react with the unconverted CaO core. Therefore, the rate of surface chemical reaction is rapid and the higher adsorption capacity is obtained. At the 18th cycle, the value of k1/k2 is far less than 1 as to pure CaO, suggesting the ratedetermining step is surface reaction. Lower adsorption is measured due to sintering leading to decrease of active sites on the surface of pure CaO. No significant change was observed at the 18th cycle for the (Zr-Ce)-additive sample relative the 1st cycle. However, the value of k1/k2 (Table 2) is far lower than 1 for the (Zr-Ce)-additive sample at the 35th cycle, indicating that the rate-determining step shifted from diffusion of CO2 to surface reaction. 4. Conclusions This study develops an efficient (Zr-Ce)-additive CaO-based sorbent for CO2 capture. The principal findings can be summarized
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Fig. 12. Cyclic carbonation kinetics of pure and (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce=30:0.5:0.5): (a) adsorption performance at the 1st, 18th, and 35th cycles, and (b) adsorption rates in the first 2 min at the 1st, 18th, and 35th cycles.
as follows: (1) The (Zr-Ce)-additive sample delivered higher CO2 capture capacity (0.65 gCO2/gsorbent) up to 35 cycles with enhanced stability than that for CaO. (2) Ce2Zr2O7 crystallite phases as a
Table 1 Capacity comparison of the critical point for pure and (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce = 30:0.5:0.5). Number of cycles
CaO (gCO2/gsorbent)
Ca/Zr/Ce = 30:0.5:0.5 (gCO2/gsorbent)
1st 18th 35th
0.62 0.04 –
0.58 0.56 0.06
barrier between CaO particles served to prevent CaO crystallite growth and agglomeration. (3) Enhanced stability can be linked to porous flake microstructure, nano-scale particles, the uniform distribution of Ca and the inert components in the sorbent. (4) In order to maintain electric neutrality, oxygen vacancy existed in the crystal phase of Ce2Zr2O7, which was beneficial for the diffusion of CO2 and mobility of O2. The XPS results showed a shift to higher binding energy of the core levels in Ca2p and Zr3d by addition of (ZrCe), indicating the decrease of the electron cloud density of Ca2+ and Zr4+, which significantly enhanced the adsorption of O2. (5) The kinetics of CO2 adsorption was analyzed by a double exponential method.
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CO2 uptake (g CO2 / g sorbent)
0.7
175
(a)
0.6 0.5 0.4 0.3 0.2
1st, CaO ExpDec2 Fit of CaO, 1st 18th, CaO ExpDec2 Fit of CaO, 18th
0.1 0.0 0
10
20
30
40
Time(min)
CO2 uptake (g CO2 / g sorbent)
0.7
(b)
0.6 0.5 0.4 0.3
1st, Ca/Zr/Ce=30:0.5:0.5 ExpDec2 Fit of Ca/Zr/Ce=30:0.5:0.5, 1st 18th, Ca/Zr/Ce=30:0.5:0.5 ExpDec2 Fit of Ca/Zr/Ce=30:0.5:0.5, 18th 35th, Ca/Zr/Ce=30:0.5:0.5 ExpDec2 Fit of Ca/Zr/Ce=30:0.5:0.5, 35th
0.2 0.1 0.0 0
10
20
30
40
Time(min) Fig. 13. Comparison of the fitting and experimental results of the (a) pure and (b) (Zr-Ce)-additive CaO-based sorbents (Ca/Zr/Ce = 30:0.5:0.5).
Table 2 Rate constants of CO2 adsorption estimated by double exponential method. Sorbents
k1/s1
k2/s1
k1/k2
R2
CaO, 1st CaO, 18th Ca/Zr/Ce = 30:0.5:0.5, 1st Ca/Zr/Ce = 30:0.5:0.5, 18th Ca/Zr/Ce = 30:0.5:0.5, 35th
3.26520 0.02994 2.55912 1.22775 0.08975
0.05549 1.29676 0.15506 0.08388 1.42098
58.84363 0.02309 16.50392 14.63684 0.06316
0.99294 0.99216 0.99523 0.99 0.99
Acknowledgements Financial support by National Natural Science Foundation of China (NSFC) (Grant No. 21325626, U1462122, U1510203), the Program for New Century Excellent Talents in University (NCET-13-
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