CO2 Sorption Durability of Zr-Modified Nano-CaO Sorbents with Cage

Research Article pubs.acs.org/journal/ascecg

CO2 Sorption Durability of Zr-Modified Nano-CaO Sorbents with Cage-like Hollow Sphere Structure Haoliang Ping and Sufang Wu* College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, People’s Republic of China S Supporting Information *

ABSTRACT: Novel Zr-modified nano-CaO sorbents with cage-like hollow sphere structure were prepared for increasing the durability of CO2 sorption via an ion precipitation method using carbon spheres as a template. A thorough study on the influence of the Ca/Zr molar ratio and the effect of cage-like structure on the CO2 sorption durability of the prepared sorbents was performed using thermogravimetric analysis. It was found that the optimum Ca/Zr molar ratio of 5 maintained the most favorable CO2 sorption stability, which maintained a CaO conversion of 76% after 30 cycles, whereas the CaO conversion of a nano-CaO cage-like hollow sphere sorbent without Zrmodification decayed to 20%. The combination of the Zr-modification and the cage-like structure demonstrates a synergistic effect in enhancing the CaO conversion and sorption stability due to the formation of CaZrO3 with a cagelike porous structure, which facilitated CO2 diffusion and minimized thermal sintering. Furthermore, a detailed comparison was made among the Zr-, Mgand Al-stabilizers. It was found that the Zr-stabilized sorbents exhibited better durability than that of Al- and Mg-stabilized sorbents due to the formation of CaZrO3 with a high Tammann temperature (TT). KEYWORDS: Carbon dioxide, Adsorbent, Cage-like structure, Nanocalcium oxide, Calcium looping, Carbon dioxide capture, Zr-modification

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INTRODUCTION

Incorporation of a sintering-resistant material into the CaO structure is a technique to enhance the cyclic durability of the sorbents. Improvements to CaO-based sorbents have been achieved by the addition of second-phase refractory “spacer” particles to inhibit sintering of the CaO particle matrix.18−29 Al2O3,18,19 TiO2,20−22 ZrO2,23−25 MgO26−28 and SiO229 are examples of second-phase refractory additives recently studied for enhancing the reactivity of CaO-based sorbents during calcium looping. However, these enhancement were very limited. For example, Radfarnia et al.24 produced Zr-modified CaO sorbents, and the best sample with a Ca/Zr molar ratio of 10:3.03 exhibited a drop in the molar conversion ratio from 32% to 24% after 15 cycles. Thus, substantial improvements remain possible for the CaO conversion and cyclic performance of CaO sorbents. To date, most CaO-based sorbents with refractory additives have been developed by “simple” techniques, such as mechanical mixing,28 wet-impregnation30 or coprecipitation.31,32 These simple techniques are not able to improve fundamentally the durability of the sorbents. The major issue associated with these techniques is that the key structural properties of the sorbents that significantly influence the overall CO2 uptake, such as pore-size distribution and morphology,

The generation of carbon dioxide in excess of 13 gigatonnes annually from the combustion of fossil fuels for the production of heat and electricity has been a major reason for the climate change and ocean acidification.1−3 Carbon capture and sequestration (CCS) technology has been proposed as an effective means to address these problems.4−6 Calcium oxide (CaO)-based materials as high-temperature CO2 sorbents have attracted tremendous attention due to their advantages of high theoretical sorption capacity and low cost.7−13 CO2 capture by calcium-oxide-based sorbents relies on the reversible calcination and carbonation reactions of CaO, and this process is commonly referred to as calcium looping.14 During calcium looping, calcium-oxide-based sorbents suffer from sustained high temperatures (600−950 °C). The major limitation of this technique is the sintering of sorbents, which causes a rapid decay in the reactivity of the sorbents during multiple calcination/carbonation cycles.15,16 For example, Abanades et al. studied the carbonation performance of a natural limestone during repeated calcination/carbonation cycles and found that the carbonation conversion of the CaO sorbent decreased to less than 20% after 30 cycles.15 This bottleneck hinders the applicability of calcium-oxide-based sorbents not only to CO2 capture of flue gas but also for enhanced hydrogen production through steam methane reforming, and energy storage systems through chemical heat pumps.17 © 2016 American Chemical Society

Received: October 30, 2015 Revised: March 2, 2016 Published: March 17, 2016 2047

DOI: 10.1021/acssuschemeng.5b01397 ACS Sustainable Chem. Eng. 2016, 4, 2047−2055

Research Article

ACS Sustainable Chemistry & Engineering

of starch,39 was added and was well dispersed into the solution by sonication for 10 min. The mixture was maintained at 80 °C for 6 h with stirring to ensure the hydrolysis of urea for Ca2+ and Zr4+ deposition on the surface of the carbon spheres. Subsequently, the products were washed with distilled water several times and then dried at 60 °C for 8 h before being calcined in a muffle furnace at 500 °C for 2 h and then calcined at 800 °C for 1 h under atmosphere. The asprepared cage-like hollow sphere sorbents were labeled as @CaZr-X, where @ and X represent the cage-like hollow sphere structure and the molar ratio of Ca/Zr, respectively. For example, @CaZr-15 represents the Zr-modified nano-CaO cage-like hollow sphere sorbent with a Ca/ Zr molar ratio of 15. For reference, nano-CaO-based cage-like hollow sphere sorbents using Mg and Al as second-phase additives were prepared via the ion precipitation method by replacing Zr(NO3)4·5H2O with Mg(NO3)2· 6H2O or Al(NO3)3·9H2O. The Ca/Mg and Ca/Al molar ratios were 5, and the sorbents were labeled as @CaMg-5 and @CaAl-5, respectively. In addition, the Zr-modified CaO sorbent without a cage-like hollow sphere structure and a Ca/Zr molar ratio of 5 was prepared using the urea hydrolysis coprecipitation method for reference and was marked as CaZr-5; pure nano-CaO cage-like hollow sphere sorbent without any modification was also prepared and was marked as @Ca. NanoCaO sorbents were obtained by precalcination of commercial nanoCaCO3 particles at 800 °C for 1 h. Characterization. The crystalline phases of the sorbent components were determined using X-ray diffraction (XRD, D/ MAX-RA, Rigaku, Japan) with nickel-filtered Cu Kα as the radiation source. The intensity data were collected over a 2θ range of 20−80° with a step size of 0.01° using a counting time of 1 s per point. The morphology of the carbon spheres and sorbents was characterized by field emission scanning electron microscopy (SEM, SU8010, Hitachi, Japan). The samples were placed on a double-sided conducting resin mounted on a sample holder, coated for 3 min using a gold semi-high-resolution coater and then observed at 3.0 kV. The distribution of the elements on the sorbent surface was characterized using energy dispersive spectrometry (EDS) at 26.0 kV. Specific surface area (BET) and desorption average pore diameter (BJH) analyses were conducted by nitrogen physisorption at −196 °C by means of a Micrometrics ASAP 2020 apparatus. Performance Tests of Sorbents. The CO2 sorption properties were tested using thermogravimetric analysis (TGA, Pyris1, Perkin− Elmer, America) by continuously monitoring and recording the change in the sample weight during CO2 sorption cycles. In a typical experiment, a small amount (∼2 mg) of the sorbents was placed in a platinum nacelle, heated to 800 °C and maintained for 10 min under high-purity nitrogen gas (N2) as a purge at 22 mL min−1. Then, the temperature was decreased to the carbonation temperature. The cyclic carbonations were conducted under the mix gas with a CO2 partial pressure of 0.02 MPa in N2 with a total flow rate of 22 mL min−1 at set temperatures (600, 650 or 700 °C) for 10 min. The cyclic calcinations were conducted at 800 °C in N2 for 10 min. The conversion of CaO, X, and the sorption capacity were used as indicators of CO2 sorption performance and were calculated according to the following equations:

cannot be easily tailored, and the molar conversion of CaO has often been unsatisfactory.23 Hence, it is necessary to design a new model to address these issues. Hollow spheres (HSs) with well-defined structures represent a special class of materials that have been widely used in applications, including catalysis, drug delivery, chromatography separation, chemical reactors, controlled release of various substances, and the protection of light-sensitive components.33 However, only a few studies on the preparation of CaO-based cage-like hollow spheres using the ion precipitation method and their application as CO2 sorbents have been reported.34−36 Because of the porous structure of hollow spheres, CO2 can diffuse easily during carbonation and sorption capacity will be enhanced.37 However, the preliminary studies have found that it is difficult for hollow spheres composed of pure nano-CaO to maintain their structure, and they will collapse during carbonation/calcination cycles because of the large molar volume change between calcium oxide and calcium carbonate, which then results the CO2 sorption durability decrease. Thus, as the former analysis, an inert refractory stabilizer should be introduced in order to increase the stability of the structure and CO2 sorption durability. ZrO2 has received attention due to the relatively high TT (1221 °C) among various stabilizers.38 Particularly, ZrO2 can react with CaO to form thermodynamically stable inert CaZrO3 binders, which is another reason for selecting ZrO2 as the stabilizer. In this study, novel Zr-modified nano-CaO sorbents with cage-like hollow sphere structure were prepared via an ion precipitation method using carbon spheres as a template to enhance the CO2 sorption durability. The structural characteristics and morphology of the novel cage-like hollow sphere sorbents were evaluated using emission scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Brunauer− Emmett−Teller (BET) and X-ray diffraction (XRD) measurements. A formatting mechanism of the novel sorbents was proposed. The influences of the synthesis parameter, i.e., Ca/Zr molar ratio, and operating parameter, i.e., the carbonation temperature, on the CO2 sorption performance of the prepared sorbents were evaluated using thermogravimetric analysis (TGA). The effect of the unique cage-like hollow sphere structure of the sorbents on the CO2 sorption performance was studied. Additionally, a series of Al- and Mg-stabilized nanoCaO cage-like hollow sphere sorbents was prepared for reference to investigate the effect of the added stabilizers on the cyclic CO2 sorption stability.

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EXPERIMENTAL SECTION

Reagents and Materials. Soluble starch from potato ((C6H10O5)n, AR) for template preparation, calcium nitrate tetrahydrate (Ca(NO3)2·4H2O, AR), zirconium nitrate pentahydrate (Zr(NO3)4·5H2O, AR), magnesium nitrate hexahydrate (Mg(NO3)2· 6H2O, AR), aluminum nitrate nonahydrate (Al(NO3)3·9H2O, AR), urea (CO(NH2)2, AR) and ethanol (C2H6O, AR) were obtained from Sinopharm Chemical Reagent Co., Ltd. Nano-CaCO3 (>95% purity) with a particle size of 80 nm (Hu Zhou Ling Hua Ltd. China) served as the nano-CaO sorbent precursor for reference. Preparation of Sorbents. Zr-modified nano-CaO cage-like hollow sphere sorbents were prepared via an ion precipitation method using carbon spheres as template. In a typical procedure, 20 mmol of Ca(NO3)2·4H2O and a certain amount of Zr(NO3)4·5H2O were dissolved in 100 mL of deionized water under stirring to form the different molar ratios of Ca/Zr ranging from 5−15, followed by dissolution of 80 mmol of urea to form a clear solution. Then, 2 g of carbon sphere templates, prepared by hydrothermal of carbonization

CaO conversionX =

sorption capacity =

m0 − m1 M × CaO × 100% m0 · a MCO2

(1)

m1 − m0 (gCO /gsorbent ) 2 m0

(2)

where m0 is the initial mass of the sorbents; m1 is the mass of the sorbent after carbonation, α is the molar ratio of CaO in the sorbents and MCO2 and MCaO is the molar mass of CO2 and CaO, respectively.

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RESULTS AND DISCUSSION Forming Mechanism and Characterization of the ZrModified Nano-CaO Cage-like Hollow Sphere Sorbents. The Zr-modified nano-CaO cage-like hollow sphere sorbents for CO2 capture reported in this study were prepared via an ion 2048


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°C (@CaZr-5 800 °C in Figure 2) and the nano-CaO sorbent as reference are presented in Figure 2. As shown in Figure 2, the crystallite phases of lime and calcium zirconate (CaZrO3) were detected in @CaZr-5 sorbent after calcination at 800 °C. The formation of CaZrO3 was in agreement with previous studies, reporting that CaO reacts with ZrO2 at calcination temperatures exceeding 700 °C as shown in eq 3.40 The formation of other Ca−Zr mixed oxides, e.g., CaZr4O941 and Ca6Zr19O44,42 was only reported for calcination temperatures exceeding 1000 °C and was not detected in our prepared sorbents. For the convenient understanding, a schematic representation of the formation mechanism of the nanoCaO/CaZrO3 cage-like hollow sphere sorbents is shown in Figure S1 (see the Supporting Information).

precipitation method. Carbon spheres were fabricated as templates via the hydrothermal reaction of starch. The Ca2+, Zr4+ cations were first adsorbed on the template surface due to the hydroxylation shell of the carbon sphere templates and were then deposited as CaCO3 and Zr(OH)4 by urea hydrolysis (Figure 1a). The as-prepared materials were calcined at 500 °C

CaO + ZrO2 = CaZrO3

(3)

Figure 3 shows the EDS image of the @CaZr-5 sorbent and the different colors representing the elements of calcium,

Figure 1. Scanning electron microscopy images of (a) carbon templates after Ca2+ and Zr4+ deposited; (b) hollow sphere obtained calcination at 500 °C; (c) the @CaZr-5 sorbent and (d) schematic diagram of the @CaZr-5 sorbent.

for 2 h to remove the carbon templates, after which the products (Figure 1b) were obtained as the precursor of the desired Zr-modified nano-CaO cage-like hollow sphere sorbents. As shown in Figure 1b, the carbon templates were removed completely after calcination at 500 °C, and the hollow sphere structure was obtained. However, the surface of the sorbent is still enclosed and without cage-like structure at this stage. What’s more, the XRD patterns shown in Figure 2

Figure 3. EDS image of the @CaZr-5 sorbent: green pots for calcium, blue pots for zirconium and purple pots for oxygen.

zirconium and oxygen. As shown in Figure 3, the @CaZr-5 sorbent possesses a cage-like hollow sphere structure, and the zirconium was evenly dispersed among the calcium, indicating a good separation of the CaO particles by CaZrO3 particles. In addition, it can be deduced that each cage-like hollow sphere structure was composed of evenly dispersed CaO and CaZrO3 particles, and the inert CaZrO3 acts as a hard framework to strengthen the entire structure. The element content of sample @CaZr-5 was analyzed by EDS, and the results are shown in Figure 4. As shown in Figure 4, the component (Ca, 45.60 wt %; Zr, 20.85 wt %), which equal a Ca/Zr molar ratio of 4.98, proved that the Ca2+ and Zr4+ ions was effectively precipitated on the carbon spheres and there was almost no loose of ions during synthesis and washing. Figure 5 shows the N2 adsorption/desorption isotherm and the pore-size distribution of the sample @CaZr-5. As shown in

Figure 2. XRD patterns of sample @CaZr-5 after calcination at 500 and 800 °C and commercial nano-CaO; (■) CaZrO3; (□) CaO; (●) ZrO2 and (▽) CaCO3.

confirmed that after calcination at 500 °C, the as-prepared hollow spheres (Figure 1b) were composed of CaCO3 and ZrO2 instead of the Zr-derivative (CaZrO3). Finally, calcination at 800 °C for 1 h led to the formation of the Zr-modified nanoCaO with cage-like hollow sphere structure, as shown in Figure 1c, and the particles composing the cage-like hollow sphere structure were approximately 70 nm. The as-prepared sorbents were composed of CaO and CaZrO3, see Figure 1d, which was confirmed in the XRD patterns of @CaZr-5 in Figure 2 (@ CaZr-5 500 °C). The XRD patterns of @CaZr-5 sorbent after calcination at 500 °C (@CaZr-5 500 °C in Figure 2) and 800 2049


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sorbent would decrease with the increase of the Ca/Zr molar ratio. To evaluate the influence of Ca/Zr molar ratio on the CO2 sorption performance of Zr-modified nano-CaO cage-like hollow sphere sorbents, several sorbent samples (@CaZr-5−@ CaZr-15) with different Ca/Zr molar ratios were synthesized and were compared with the nano-CaO sorbent. The CO2 sorption performance was investigated over 30 calcination/ carbonation cycles to evaluate the effect of Ca/Zr molar ratio on the CO2 sorption durability and the molar conversion of CaO. Recall that the Zr-derivative (CaZrO3) does not show considerable affinity for CO2 sorption; therefore, the CO2 uptake is based exclusively on the CaO content.43,44 Figure 6

Figure 6. CaO conversion of the Zr-modified nano-CaO cage-like hollow sphere sorbents and the nano-CaO sorbent (carbonation at 600 °C for 10 min with CO2 partial pressure of 0.02 MPa in N2, and calcination at 800 °C in pure N2 for 10 min).

Figure 4. Element content of sample @CaZr-5.

compares the CO2 sorption stability of the prepared Zrmodified nano-CaO cage-like hollow sphere sorbents with different molar ratios (@CaZr-5−@CaZr-15 and @Ca) and the nano-CaO sorbent. As shown in Figure 6, the CaO conversion of the nano-CaO sorbent and @Ca sorbent (pure nano-CaO with cage-like hollow sphere structure without any modification) strongly decreased with the number of cycles, whereas the cyclic stability of Zr-modified nano-CaO cage-like hollow sphere sorbents was significantly improved, which means CaZrO3 plays an important role in enhancing cyclic sorption durability. As shown in Figure 6, it was also found that the cyclic CO2 sorption stability was largely affected by the Ca/Zr molar ratio. For example, a Ca/Zr molar ratio of 15 (sample @CaZr-15 with the minimum CaZrO3 content) results in a significant reduction in the CaO conversion of the sorbent, whereas a Ca/ Zr molar ratio of 5 (sample @CaZr-5 with the maximum CaZrO3 content) exhibited a marked improvement in the cyclic sorption stability. The content of inert phase CaZrO3 would decrease with the increase of the Ca/Zr molar ratio. Thus, the inadequate dispersion of CaZrO3 within the CaO particles was not able to impede small-particle fusion or hinder the collapse of the cage-like hollow sphere structure under the hightemperature operation. It can be seen in Figure 7a that the cage-like hollow sphere structure of the @CaZr-15 sorbent disappeared after 30 calcination/carbonation cycles, which revealed the importance of the Ca/Zr molar ratio for maintaining this structure. As shown in Figure 6, although samples @CaZr-5 and @CaZr-10 present nearly the same CaO conversion in the initial cycle, the @CaZr-5 sorbent is more stable during the cycles and retains approximately 60% of the CaO conversion. This was due to the presence of more Zr

Figure 5. N 2 adsorption/desorption isotherm and pore-size distribution (inset) of @CaZr-5 sorbent.

Figure 5, the adsoption isotherm appears to be the mixed characteristics of type II and IV (IUPAC classification) and revealing the coexistence of nanoporous, mesoporous and macroprous structures in the sample @CaZr-5. The absence of an adsorption plateau at relative pressures near unity suggests the presence of a mainly macroporous structure, whereas the appearance of a narrow H3 hysteresis loop can denote the presence of minor mesopores, which is confirmed by the poresize distribution of the sample @CaZr-5 as shown in the inset of Figure 5. The BET surface area of the @CaZr-5 was 79.83 m2 g−1, which was higher than that of the conventional CaO (∼12 m2 g−1). High surface area has been reported to enhance the CO2 adsorption capacity and durability of the adsorbent.28 The larger BET surface area (79.83 m2 g−1) together with the meso-pore and macro-pore-size distributions suggested that assynthesized Zr-modified nano-CaO sorbents with cage-like hollow sphere structure would exhibit good CO2 adsorption properties. Influence of Ca/Zr Molar Ratio on the CO2 Sorption Performance. The content of inert phase CaZrO3 within the 2050


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which the sorption capacity decreased during the cycles, and the top and bottom line of each column represents the value of the sorption capacity of the 1st and the 30th cycle, respectively. It can be observed in Figure 8 that when the initial sorption capacity of the @CaZr-5 sorbent and that of the CaZr-5 sorbent are compared, the cage-like hollow sphere structure effectively enhanced the sorption capacity by approximately 24%. The morphology of the @CaZr-5 sorbent (Figure 1c), which was much different from the CaZr-5 sorbent, appeared as a cage-like hollow sphere structure with many pores that facilitated CO2 diffusion during the carbonation process. Thus, during the carbonation process, CO2 can easily diffuse into the inward portion of the sorbent and react with CaO in the interior and exterior walls of the cage-like hollow sphere sorbent. Therefore, the active adsorptive reaction sites of the cage-like hollow sphere sorbents were fully employed; therefore, the sorption capacity was enhanced. In addition, as shown in Figure 8, the @CaZr-5 sorbent was more stable than the CaZr-5 sorbent during the cycles. This is because the cage-like structure of the sorbents acts as a hard framework that can buffer against the local large volume change during the calcination/carbonation cycles and can alleviate the problem of pulverization and aggregation of the sorbents, hence improving the CO2 sorption durability. Influence of Carbonation Temperature on the CO2 Sorption Properties. Carbonation temperature is a key factor influencing the sorption kinetics. Fast kinetics of carbonation is generally a relevant matter in practical applications, particularly in the case of integrated carbonation-catalytic reaction systems, for example, the reactive sorption enhanced reforming (ReSER) process in fluidized bed carbonator reactors.45,46 Figure 9 demonstrates the weight change of the @CaZr-5

Figure 7. Scanning electron microscopy images of (a) the @CaZr-15 s and (b) the @CaZr-5 sorbent after 30 calcination/carbonation cycles.

stabilizer in the @CaZr-5 sorbent and the preservation of the integrated cage-like hollow sphere structure after repeated cycles (Figure 7b). Additionally, the CaO conversion of all the prepared Zr-modified nano-CaO cage-like hollow sphere sorbents was higher than that of the nano-CaO sorbent at the end of the 30th cycle. Therefore, the sample @CaZr-5 with a Ca/Zr molar ratio of 5 incorporating adequate Zr-derivative (CaZrO3) quantities within the framework of the CaO sorbent was able to hinder the fusion of small particles and prevent the porous cage-like hollow sphere structure from collapse, thereby increasing the cyclic sorption stability. The XRD characterization results of the @CaZr-5 sorbents, as shown in Figure 2, help to reveal why the incorporation of zirconium into the structure of pure CaO led to an increase in material stability. As illustrated in Figure 2, the incorporation of zirconium into the CaO results in the creation of a CaZrO3 phase that enhances the stability of the crystalline CaO lattice at elevated temperatures. This implies the complete reaction between ZrO2 and CaO to form CaZrO3 by the proposed synthesis method. The inert CaZrO3 binders can effectively separate the CaO particles and act as a hard physical framework to prevent the cage-like hollow sphere structure from deteriorating because of the high TT. Thus, a lower Ca/Zr molar ratio leads to the higher the CaZrO3 content in the sorbent, the better separation of the CaO particles and, simultaneously, the firmer the cage-like hollow sphere structure, and a better CO2 sorption performance is preserved. Influence of the Cage-like Structure on the CO2 Sorption Performance. To understand further the influence of the cage-like structure on the CO2 sorption performance, reference sample CaZr-5 without a cage-like structure was prepared and measured by TGA for 30 calcination/carbonation cycles. For a clear demonstration of the sorption capacity and the decay extent during the cycles, a floating column figure is inset in Figure 8. The column length represents the extent to

Figure 9. Weight change of the @CaZr-5 sorbent with temperature rise from 150 to 800 °C.

sorbent as the temperature increases from 150 to 800 °C using TGA. The black solid line represents the temperature increase, while the red dashed line represents the weight change of the @ CaZr-5 sorbent. As shown in Figure 9, there is an obvious inflection point of the sorbent weight change when the temperature increases to approximately 700 °C. This point is the initial decomposition point of the @CaZr-5 sorbent, and the corresponding temperature, 700 °C, represents the initial decomposition temperature of the @CaZr-5 sorbent. Therefore, 700 °C was chosen as the upper limit for carbonation in this study. Therefore, the effect of carbonation temperature, range of 600−700 °C, on the sorption rate of the @CaZr-5 sorbent was studied. As shown in Figure 10, the carbonation process is composed of a fast sorption stage and a slow sorption stage in which the components of nano-CaO react with CO2. It is

Figure 8. Sorption capacity as a function of the cycle numbers for the @CaZr-5 and the CaZr-5 sorbents (carbonation at 600 °C for 10 min with CO2 partial pressure of 0.02 MPa in N2, and calcination at 800 °C in pure N2 for 10 min). 2051


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conversion of CaO increased with increasing carbonation temperature because of the enhancement of the reaction rate at higher temperatures. When the carbonation temperature was 700 °C, the CaO conversion of the initial cycle reached 81%, and after 30 cycles, the value remained at 76%. This value was approximately two times higher than that of the Zr-stabilized sorbent prepared by Radfarnia et al.,24 which was with a Ca/Zr molar ratio of 3 and was carbonated at 700 °C for 15 cycles. Interestingly, the cyclic stability was strengthened when the sorbent was exposed to higher carbonation temperatures. For example, the CaO conversion of the @CaZr-5 sorbent decreased by approximately 20% at 600 °C at the end of the 30th calcination/carbonation cycle, whereas it surprisingly decreased by only 6% at 700 °C. This slight self-reactivation48 at temperatures higher than 600 °C may be attributable to the structural rearrangement of the particles, which prevents the collapse of the cage-like hollow sphere structure and particle sintering and enables better access of the CO2 molecules to the CaO active sites. Radfarnia et al.49 reported metal oxide (M: Al, Zr, Mg and Y)-stabilized calcium oxide CO2 sorbents with varied M/Ca molar ratio for multicycle operation. As shown in Figure 12,

Figure 10. CaO conversion and rate of the @CaZr-5 sorbent in during the first cycle at carbonation temperatures of 600, 650, and 700 °C, calcination at 800 °C for 10 min in pure N2.

believed that the first reaction step (fast sorption stage) is controlled by the chemisorption process, whereas the second step (slow sorption stage) is controlled by CO2 diffusion. The data presented in Figure 10 clearly show that the increase in carbonation temperature results in the extension of the fast reaction stage and enhances the maximal sorption rate, thus increasing the CaO conversion because of the increase in the chemisorption kinetics at elevated temperatures. For example, the maximal sorption rate (dX/dt) at a carbonation temperature of 600 °C is approximately 50 s−1, and the CaO conversion is 60%, whereas the maximal sorption rate increases to 70 s−1, and the CaO conversion reaches 78% when carbonation is conducted at 700 °C. In addition, the @CaZr5 sorbent demonstrated a good sorption rate. As shown in Figure 10, the maximum sorption rate appears in the first 0.5 to 1 min when CO2 gas was introduced. The fast sorption kinetics, which should be attributed to the highly active nanosized particles that compose the cage-like hollow sphere structure (see Figure 1c), is also important for realistic applications. For the short residence times in fluidized bed carbonator reactors,47 the Zr-modified nano-CaO cage-like hollow sphere sorbents are expected to reside within the order of a few minutes for reasonable values of bed inventories. The effect of carbonation temperature on the CaO conversion of the @CaZr-5 sorbent was tested at 600, 650, and 700 °C for 30 calcination/carbonation cyclic runs, and the results are shown in Figure 11. As expected, the CaO

Figure 12. CO2 sorption activity of samples in mild conditions: adsorption at 650 °C and 15 vol % CO2 and calcination at 750 °C and 100 vol % argon.49

these sorbents demonstrated different cyclic sorption durability; however, none of them was able to remain a relatively high CaO conversion (>70%) after 25 cycles. In contrast, the CaO conversion of sample @CaZr-5, even at a higher calcination temperature (800 °C), still remained 70% after 30 cycles. Influence of Stabilizers on the Cyclic CO2 Sorption Stability. To study the influence of stabilizers on the cyclic CO2 sorption stability, several nano-CaO-based cage-like hollow sphere sorbents with different types of stabilizers (@ CaM-5) were prepared for reference and were measured by TGA for 30 calcination/carbonation cycles under the same operating conditions. The experimental results are presented in Figure 13, which reveals that in the first cycle, the nano-CaO sorbent exhibits the highest sorption capacity of 0.539 g CO2/ gsorbent. However, the sorption capacity of the nano-CaO sorbents decreases rapidly with the increasing number of cycles, dropping to 0.158 g CO2/gsorbent at the 30th cycle. This decrease in sorption capacity over repeated cycles is due to the highly endothermic decarbonation process (800 °C) and the

Figure 11. Effect of carbonation temperature on the CaO conversion of the @CaZr-5 sorbent (carbonation at 600 °C for 10 min with CO2 partial pressure of 0.02 MPa in N2, and calcination at 800 °C in pure N2 for 10 min). 2052



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CONCLUSIONS In this study, Zr-modified nano-CaO with cage-like hollow sphere structure was prepared via an ion precipitation method using carbon spheres as a template with the primary goal of improving the CO2 sorption durability. XRD and EDS measurements showed that the Zr-derivative (CaZrO3) formed during the synthesis process was evenly dispersed among the CaO particles in a cage-like hollow sphere. The incorporation of adequate Zr-derivative (CaZrO3) quantities within the framework of the CaO sorbent effectively separated the CaO particles, thus preventing agglomeration, and enhanced the cyclic sorption stability. The cyclic sorption stability decreased by 6%, and the CaO conversion was maintained at approximately 76% after carbonation at 700 °C for 30 cycles, whereas the CaO conversion of pure nano-CaO cage-like hollow sphere sorbent without Zr-modification decayed to 20%. The combination of the cage-like structure and the Zrmodification demonstrates a synergistic effect in enhancing the CaO conversion and sorption stability due to the tough cage-like porous structure, which facilitated CO2 diffusion and minimized thermal sintering. In addition, it was found that the Zr-modified nano-CaO cage-like hollow sphere sorbents exhibited better CO2 sorption durability than those of Aland Mg-stabilized sorbents due to the relatively high TT of CaZrO3.

Figure 13. Sorption capacity as a function of the cycle numbers for the CaO-based CO2 sorbents with different kinds of metal stabilizers (carbonation at 600 °C for 10 min with CO2 partial pressure of 0.02 MPa in N2, and calcination at 800 °C in pure N2 for 10 min).

comparatively low TT of CaCO3 (533 °C), which contributed to the agglomeration of the monodispersed crystalline structure of the nano-CaO sorbent. Note that in Figure 13, all sorbents with metal stabilizers exhibited improved stability during the multiple calcination/ carbonation cycles compared with the nano-CaO sorbent. The @CaMg-5 sorbent obtained an initial sorption capacity of 0.326 g CO2/gsorbent and a sorption capacity at the 30th cycle of 0.146 g CO2/gsorbent. The @CaAl-5 sorbent performed slightly better than that of a @CaMg-5 sorbent but still had an evident disparity with the @CaZr-5 sorbent. Comparatively, the initial sorption capacity of the @CaZr-5 sorbent reached 0.308 g CO2/gsorbent and remained at 0.240 g CO2/gsorbent after 30 cycles. It can be observed in the inset of Figure 13 that the @ CaZr-5 sorbent revealed remarkable advantages over the sorbents doped with Mg and Al. Zhao23 and Kierzkowska7 noted that the effect of metal stabilizers added to the CaObased sorbents for stability improvement was related, to some extent, to the TT of the additives (see TT in Table 1).50 Thus, in

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* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01397. Diagram of the formation mechanism of Zr-modified nano-CaO cage-like hollow sphere sorbents (PDF).

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TTammann (°C)

mineral derivatives

TTammann (°C)

CaO Al2O3 ZrO2 TiO2 MgO

1170 900 1221 785 1150

CaCO3 Ca12Al14O33 CaZrO3 CaTiO3

533 725 1036 851

AUTHOR INFORMATION

Corresponding Author

*S. Wu. E-mail: [email protected]. Notes

Table 1. Tammann Temperatures (TTammann ≈ 0.5 Tmelting, Referring to the Temperatures at which Sintering Occurs) of the Materials50 oxides

ASSOCIATED CONTENT

S

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for finical supports from National Natural Science Foundation of China (Grant No. 21276234). REFERENCES

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addition to the effect of the cage-like hollow sphere structure, the excellent CO2 sorption stability of the @CaZr-5 sorbent during the cycles should also be attributed to the relatively high TT of CaZrO3, which is formed by the reaction between CaO and ZrO2, as indicated in the XRD results of the @CaZr-5 sorbent. These thermodynamically stable inert CaZrO3 binders effectively separated the CaO particles, acted as a hard physical framework to prevent deterioration of the cage-like hollow sphere structure, and inhibited the sintering of the CaO particles during the calcination/carbonation cycles (Figure 7b). 2053


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CO2 Sorption Durability of Zr-Modified Nano-CaO Sorbents with Cage

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