Influence of the Synthesis Method on the Structure and CO2

of Cincinnati, Cincinnati, Ohio 45221, United States. Energy Fuels , 2014, 28 (5), pp 3292–3299. DOI: 10.1021/ef402573u. Publication Date (Web):...
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Influence of the Synthesis Method on the Structure and CO2 Adsorption Properties of Ca/Zr Sorbents Gunugunuri K. Reddy, Steven Quillin, and Panagiotis Smirniotis* Chemical Engineering Program, School of Biomedical, Chemical, and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221, United States ABSTRACT: The influence of the synthesis method has been investigated on the structure and CO2 adsorption properties of Ca/Zr sorbents. For this purpose, Ca/Zr sorbents are synthesized using co-precipitation, sol−gel, and deposition−precipitation methods and compared to our previously reported Ca/Zr sorbent prepared by the flame spray pyrolysis method. Among the various sorbents, the sorbent synthesized by the sol−gel method exhibits excellent adsorption capacity and remarkable multicycle stability compared to the sorbent synthesized by the flame spray pyrolysis synthesis method. This interesting and rather useful behavior was a result of surface properties of the sol−gel-synthesized sorbents. X-ray diffraction measurements indicate that the CaO phase was dominant with respect to the CaZrO3 phase in the case of the Ca/Zr sorbent prepared by the deposition−precipitation method. The opposite trend was observed for the sorbents synthesized by the remaining methods. Temperature-programmed desorption measurements show that the basic properties of the sorbents depend upon the synthesis method adopted. O 1s X-ray photoelectron spectroscopy (XPS) spectra confirms the formation of the CaZrO3 phase in the prepared sorbents. Transmission electron microscopy (TEM) measurements show that the Ca/Zr sorbent synthesized by the sol−gel method exhibits smaller crystallites similar to the flame-spray-pyrolysis-synthesized sorbent. TEM−energy-dispersive spectrometry and XPS atomic ratios show that the sol−gel-synthesized Ca/Zr sorbent has more CaO particles over the surface compared to the Ca/Zr sorbent prepared by flame spray pyrolysis and is responsible for the better CO2 capture performance observed.

1. INTRODUCTION We are in an era where environmental concerns have become increasingly prevalent. An increase in the CO2 concentration is extensively regarded as an important factor for the global climate pattern shift.1 In 2009, the United States Environmental Protection Agency (U.S. EPA) reported that 18% of the worldwide CO2 emissions come from the U.S.A. alone and 95% of these emissions come from the combustion of fossil fuels from fire power plants.2 Because it has been confirmed that fossil fuels satisfy the majority of the U.S. energy needs in the future, the development of efficient and economic carbon capture technologies for fulfilling the global energy demand has become essential.3 CO2 capture via absorption using chemical solvents, such as rectisol, selexol, and amine, is the currently available technology in industries. However, many computational and theoretical studies show that, when we install a retrofit amine system in the power plant to capture 90% of the CO2 from flue gas, the plant efficiency will decrease by 30%.4,5 Other alternative technologies, such as adsorption or membranes, have been proposed and investigated.6−9 Among them, CO2 capture using a solid CaO sorbent at high temperatures has received tremendous attention in recent years.10−15 The use of a economic and regenerable calcium oxide sorbent is a capable technology because of several advantages, such as a high-temperature CO2 separation process, reductions in global energy requirements, and removal of the generation of liquid waste.14,15 In the CaO-based sorbent lopping, CO2 capture occurs via chemical reaction between CaO and CO2 and forms CaCO3 and subsequent thermal decarbonation in a separate vessel to regenerate CaO. The carbonation reaction is an exothermic reaction, and the © 2014 American Chemical Society

decarbonation reaction is an endothermic reaction. However, adsorption capacity and regeneration stability of pure CaO have suffered significantly during multi-cycle operation because of the decay of the surface area and increase in the particle size through sintering.16−18 This reduction in activity is due to the highly exothermic carbonation process, the increase in volume from CaO to CaCO3, and the lower Tammann temperature of CaCO3 (533 °C). To improve the carbonation capacity, several other processes, such as thermal pretreatment19 and steam regeneration,20,21 have been investigated. An alternative technique to improve the structural properties and sorbent multi-cycle stability of the CaO sorbent is the incorporation of temperature-resistant inert metal oxides, such as Al2O3, MgO, and ZrO2, into the sorbent structure.22−27 Among the various stabilizers, both ZrO2 and Al2O3 have received much attention because of the appropriate thermal stability of these compounds. In our previous study, we synthesized various Ca/Zr sorbents using the flame spray pyrolysis method by varying the ratio between Zr and Ca.28 Among the various sorbents, Ca/Zr sorbents with molar ratios of 10:5 and 10:6 exhibit excellent stability during multi-cycle operation. No deactivation was observed up to 1200 cycles of operation. Our X-ray diffraction (XRD) measurements show that there is a formation of the CaZrO3 perovskite phase in Ca/Zr sorbents and the amount of the perovskite phase increases with an increasing amount of Received: December 31, 2013 Revised: April 7, 2014 Published: April 7, 2014 3292

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ZrO2 doping.28 CaZrO3 stabilizes the CaO particles from sintering during multi-cycle operation. However, there is rapid decrease in the adsorption capacity with increasing the amount of Zr doping. The molar conversion decreases from 95 to 60% when we go from pure CaO to Ca/Zr (10:5) sorbents, respectively, and it is further decreased to 40% with increasing the Zr doping amount to 6 mol for every 10 mol of calcium. Our X-ray photoelectron spectroscopy (XPS) measurements show that an amorphous layer of zirconia formed on the surface of Ca with increasing the Zr doping amount.28 The zirconia amorphous layer inhibits the CO2 adsorption over the CaO particles. The present study mainly aimed at investigating the influence of the synthesis method on structural and surface properties of the Ca/Zr (10:5) sorbent. We aimed at developing a sorbent with high adsorption capacity and superior stability. For this purpose, we synthesized the Ca/Zr sorbent using four different methods, namely, co-precipitation, sol−gel, and deposition− precipitation, and flame spray pyrolysis synthesis methods. Carbonation−decarbonation multi-cycle experiments were performed over the synthesized sorbents. Among the various sorbents, the Ca/Zr sorbent synthesized by the sol−gel method exhibits higher adsorption capacity and remarkable multi-cycle stability. The fresh and spent sorbents were characterized using XRD, transmission electron microscopy (TEM), temperatureprogrammed desorption (TPD), and XPS techniques. Structural characterization measurements suggest that the perovskite CaZrO3 phase was formed along with CaO and CaCO3 in all of the samples. Surface characterization measurements show that sol−gel-synthesized Ca−Zr sorbent has a higher concentration of CaO particles over the surface compared to the flame-spraypyrolysis-synthesized Ca−Zr sorbent, which is responsible for the higher CO2 adsorption capacity.

monohydrate and Ca/Zr was maintained as 1. Then, the solution was vigorously stirred and heated at 80 °C. After some time, a viscous gel (pale yellow) was formed. Then, the gel was dried at 130 °C for 12 h. After drying, a low-density foam was obtained. Then, the foam was crushed and calcined at 800 °C for 5 h with a 2 °C/min heating rate. The reference CaO sorbent was also prepared by the same sol−gel method and calcined at 800 °C with 2 °C/min ramping. 2.1.4. Flame Spray Pyrolysis Method. Pure CaO and Ca/Zr (10:5) sorbents were also synthesized by the flame spray pyrolysis method, as described in our previous studies. The detailed synthesis procedure can be found in our earlier publications. Required amounts of calcium naphthenate (4% calcium in mineral spirits) and zirconyl(IV) 2ethylhexanoate (6% Zr in mineral spirit) were dissolved in xylene. The freshly prepared precursor solution was injected at predetermined rates by a syringe pump through the spray nozzle and dispersed by 5 L min−1 oxygen into a fine spray. The pressure drop of dispersion O2 was maintained at 1.5 bar at the nozzle tip. The combustion of the fine spray was initiated and maintained by the ignition of premixed fuel of 500 mL min−1 CH4 and 400 mL min−1 O2. The synthesized powders were collected on a glass fiber filter paper placed on a holder above the reactor with the aid of a vacuum pump. 2.2. Sorbent Characterization. 2.2.1. TPD. CO2-TPD measurements on various CaO sorbents were measured on a Micromeritics AutoChem 2910 instrument. Initially, the helium passed through the samples and heated to 800 °C at a ramp rate of 1 °C min−1. During the heating, we removed all of the impurities and atmospheric adsorbed CO2. Then, samples were bought down to 100 °C before CO2-TPD analysis. Then, 30 mL min−1 of 30 vol % CO2 in helium was passed through the samples for 1 h. Then, physisorbed CO2 was removed from the sorbent, by passing 30 mL min−1 helium for 3 h. The temperature was then increased to 800 °C with a ramping rate of 5 °C min−1 by recording the CO2 desorption patterns from the sorbent. 2.2.2. XRD. XRD measurements of synthesized CaO sorbents were measured on a Phillips X’pert diffractometer using Cu Kα (1.540 56 Å) as the radiation source. The data were measured over a 2θ range of 20−80° with a step size of 0.05° and a scanning rate of 1 s/point. Crystalline phases were identified by comparing the obtained patterns to the reference data from International Centre for Diffraction Data (ICDD) files. 2.2.3. TEM. All of the fresh and spent sorbents were investigated by TEM with a Philips CM 20 electron microscope. The samples were ultrasonically dispersed in ethanol and put into a carbon−Cu grid, and after evaporation of the ethanol, the particles that stayed on the walls of the carbon grid were examined. A 200 keV accelerating voltage was applied, with a LaB6 emission current and a point-to-point resolution of 0.27 nm. 2.2.4. XPS. XPS measurements of all of the sorbents were performed on a Pyris VG thermoscientific spectrometer using Al Kα (1486.6 eV) radiation as an excitation source. The charging of the sorbents was corrected by setting the binding energy of the adventitious carbon (C 1s) at 284.6 eV.30 Before the analysis, the samples were degassed under vacuum for 4 h. Then, the analysis was performed at room temperature and 10−8 Torr pressure. 2.3. Sorbent Performance Measurement. CO2 capture experiments were performed over all of the prepared sorbents with a PerkinElmer Pyris-1 thermogravimetric analyzer (TGA). A small amount of sorbent (ranging from 2 to 30 mg) was placed in a platinum boat. Each sample was preheated to 750 °C with a 10 °C/min ramp rate in the presence of helium before the carbonation−decarbonation experiments and then held at the same temperature for 30 min to remove all impurities and pre-adsorbed CO2. All of the carbonation− decarbonation were performed at 700 °C with 20 mL min−1 CO2 (99.5%, Wright Bros, Inc.) and 70 mL min−1 helium for 30 min alternatively. A thermal analysis gas station (PerkinElmer) was used to monitor the gas flows accurately. The sorbent weight and temperature as a function of time were recorded during the analysis. Molar conversion [XCaO (%)] of each sorbent was calculated by

2. EXPERIMENTAL SECTION 2.1. Sorbent Synthesis. 2.1.1. Co-precipitation. Both calcium and zirconium were precipitated together using NaOH as a precipitating agent. In a typical synthesis, the required amounts of Ca(NO3)2 and zirconium oxynitrate were dissolved individually in deionized water and the aqueous solutions were mixed. Diluted NaOH was added slowly dropwise to the mixture solutions and stirred vigorously until precipitation was complete (pH ≈ 10.5). The resultant precipitate gels were further aged overnight and filtered off. Then, filtered precipitate was dried in the oven at 80 °C for 12 h. Finally, the sorbent was calcined at 800 °C for 5 h in the presence of air with 2 °C/min ramping. Pure CaO was also prepared by following a similar precipitation method. 2.1.2. Deposition−Precipitation Method. Calcium oxide was deposited over zirconium oxide by precipitating calcium nitrate using NaOH as a precipitating agent. In a typical synthesis method, powdered ZrO2 was first dispersed in about 2000 mL of deionized water and stirred for 2 h. Calcium nitrate was dissolved in deionized water separately and mixed with the ZrO2-dispersed solution, and the whole mixture was diluted to 4000 mL with deionized water. Afterward, aqueous NaOH was added dropwise to the solution until precipitation was complete (pH ≈ 10.5) with vigorous stirring. The resultant precipitate was aged for 12 h and filtered off. The obtained filtered precipitate was dried in an oven at 80 °C for 12 h. Finally, the sorbent was calcined at 800 °C for 5 h in the presence of air with 2 °C/min ramping. 2.1.3. Sol−Gel Method. The Ca−Zr sorbent was also prepared using the sol−gel method using a modified Pechini procedure.29 Initially, the required amounts of calcium nitrate and zirconium oxynitrate were dissolved separately in deionized water and mixed together. Then, the required amount of citric acid monohydrate was added to the mixed solution. The molar ratio between citric acid 3293

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WCaO (%) 1 0.786 αCaO

described in our previous study,28 the Ca/Zr sorbent synthesized by the flame spray pyrolysis synthesis method exhibits excellent multi-cycle stability with a molar conversion of 60%. The sorbent did not activate at all for up to 1200 cycles. On the other hand, there is no improvement in the adsorption capacity and stability observed in the case of co-precipitated synthesized Ca/Zr compared to pure CaO synthesized by the precipitation method. The Ca/Zr sorbent synthesized by the co-precipitation method exhibits a molar conversion of 50% and started to deactivate from the second cycle onward. The deposition−precipitation-synthesized Ca/Zr sorbent exhibits a higher molar conversion (70%) compared to the coprecipitated Ca/Zr sorbent. However, this sorbent also started to deactivate from the second cycle onward. Hence, these results show that both co-precipitation and deposition− precipitation methods are not suitable methods for development of stable CO2 adsorption sorbents. On the other hand, a completely different behavior was observed for the Ca/Zr sorbent synthesized by the sol−gel method. Remarkably, the Ca/Zr sorbent synthesized by the sol−gel method shows a higher molar conversion (81%) compared to the Ca/Zr sorbent synthesized by the flame spray pyrolysis method and excellent stability until 1200 cycles. The sorbent did not deactivate at all for up to 1200 cycles. Hence, the CO2 adsorption results suggest that the Ca/Zr sorbent synthesized by the sol−gel method exhibits higher adsorption capacity compared to the flame spray pyrolysis method. On the other hand, both coprecipitation- and deposition−precipitation-synthesized sorbents started to deactivate from the second cycle onward. The interesting behavior of the sol−gel-synthesized Ca/Zr sorbent is explained using TPD, XRD, TEM, and XPS characterization techniques. 3.2. Sorbent Characterization. 3.2.1. TPD Measurements. The basic properties of the sorbents were evaluated using the CO2-TPD technique. The CO2-TPD profiles of various Ca/Zr sorbents synthesized by different synthesis techniques are presented in Figure 2. In general, basic sites are

(1)

where WCaO (%) is the change in the weight percentage of the sorbent during CO2 adsorption, 0.786 is CO2 stoichiometric by CaO, and αCaO is the CaO weight fraction in that particular sorbent. The initial weights of the various sorbents are as follows: coprecipitation Ca/Zr, 22.39 mg; sol−gel Ca/Zr, 7.656 mg; deposition− precipitation Ca/Zr, 24.89 mg; and flame Ca/Zr, 3.4 mg.

3. RESULTS AND DISCUSSION 3.1. CO2 Adsorption Performance. To investigate the influence of the synthesis method on the adsorption capacity and stability of the Ca/Zr (10:5) sorbents, multi-cyclic CO2 carbonation−decarbonation experiments have been performed over the sorbents synthesized by different methods in a TGA. All sorbents were pretreated at 750 °C in helium for 30 min before the carbonation−decarbonation analysis.31 Carbonation experiments were performed with 20 mL/min CO2 (99.5%), and decarbonation experiments were performed with 70 mL/ min helium at 700 °C. Table 1 shows the CO2 molar Table 1. CO2 Adsorption Molar Conversions (%) of Pure CaO Sorbents sorbent

molar conversion (%)

precipitation CaO sol−gel CaO flame spray pyrolysis CaO

47 51 95

conversions of pure CaO sorbents prepared by precipitation, sol−gel, and flame spray pyrolysis synthesis methods. Interestingly, all of the sorbents started to deactivate from the second cycle onward. Among the various sorbents, CaO synthesized by the flame spray pyrolysis method exhibits higher adsorption capacity compared to the sol−gel- and precipitationsynthesized CaO. Interestingly, both sol−gel- and precipitationsynthesized CaO sorbents exhibit similar adsorption capacities. These results suggest that the synthesis method has no influence on the adsorption stability of CaO sorbents. As reported in our previous studies,28,31 ZrO2 was selected as a structural stabilizer for its high Tammann temperature and refractory nature. Figure 1 shows the multi-cycle stability and adsorption molar conversions of various Ca/Zr (10:5) sorbents synthesized by co-precipitation, deposition−precipitation, sol− gel, and flame spray pyrolysis synthesis methods. As we

Figure 2. CO2-TPD profiles of pure CaO sorbents synthesized by different methods.

classified into three categories, namely, weak, medium, and strong basic sites. Weak basic sites give a peak below 300 °C in the CO2-TPD profile; medium basic sites give a peak between 300−600 °C; and stronger basic sites give a peak between 600 and 800 °C. As shown in Figure 2, the type and strength of basicity of the sorbent completely depend upon the synthesis method adopted. As described in our previous study,28 the flame-spray-pyrolysis-synthesized Ca/Zr sorbent exhibits a peak

Figure 1. CO2 adsorption profiles of various Ca/Zr sorbents synthesized. 3294

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Table 2. XPS Ca/Zr Atomic Ratios, TEM−EDS Ca/Zr Ratios, and Amount of CO2 Desorbed from CO2-TPD Profiles, CO2 Capture Capacity Per Gram of Sorbent sorbent

XPS Ca/Zr atomic ratio

TEM−EDS Ca/Zr ratio

amount of CO2 desorbed (μmol/g)

CO2 capture capacity per gram of sorbent (mg/g)

co-precipitation Ca/Zr deposition−precipitation Ca/Zr sol−gel CaO flame spray pyrolysis CaO

8.9 9.46 3.57 1.5

90.4:9.6 98:2 70.6:29.4 49.4:50.6

278 674 1363 1242

183 258 304 226

between 600 and 800 °C in the CO2-TPD profile, which is due to the desorption of CO2 from the stronger basic sites. As shown in Figure 2, the co-precipitated Ca/Zr sorbent exhibits a peak in the CO2-TPD profile in the temperature region of 400−500 °C because of the desorption of CO2 from medium basic sites. In addition to this peak, there was a peak started at around 750 °C and was not finished. This may be due to desorption of CO2 from strong basic sites. On the other hand, the Ca/Zr sorbent prepared by the deposition−precipitation method exhibits two peaks in the CO2-TPD profile: one in the temperature region of 350−450 °C and other one in the region of 600−750 °C. The first peak is due to the CO2 desorption from medium basic sites, and second is due to the stronger basic sites. The intensity of first peak is stronger than that of the second peak. The Ca/Zr sorbent synthesized by the sol−gel method also exhibits two peaks because of the medium and stronger basic sites. However, in this case, strong basic sites are dominating compared to the medium basic sites. The amount of CO2-desorbed values are presented in Table 2. Among the various sorbents, the Ca/Zr sorbent prepared by the sol−gel method exhibits more basicity compared to the other sorbents, which is in agreement with CO2 capture experiments. 3.2.2. XRD Patterns. The crystal structure and phase composition of the synthesized sorbents were investigated using the XRD technique. XRD patterns of the pure CaO sorbents prepared by precipitation, sol−gel, and flame spray pyrolysis methods are shown in Figure 3. As reported in our

CaCO3 easily compared to the precipitation- and sol−gelsynthesized CaO. This is due to the higher crystallite size of CaO in the case of precipitation- and sol−gel-synthesized samples, which inhibits the formation of CaCO3. The TEM results presented later in the study also support this observation. This is the reason why flame-spray-pyrolysissynthesized CaO exhibits higher CO2 adsorption capacity compared to the precipitation- and sol−gel-synthesized samples. The XRD patterns of the Ca/Zr sorbents are presented in Figure 4. As shown in Figure 4, the XRD pattern of the Ca/Zr

Figure 4. XRD patterns of Ca/Zr sorbents synthesized by different methods.

sorbent synthesized by the flame spray pyrolysis method exhibits reflections mainly at 31.7°, 45.4°, 55.8°, and 56.8°. These reflections are due to the formation of CaZrO3 (PDFICDD-00-035-0790). Along with these reflections, reflections because of CaCO3 were also observed. Interestingly, the reflections because of CaO were not observed. The Ca/Zr sorbent synthesized by the deposition−precipitation method mainly exhibits reflections because of CaO, along with CaCO3 and CaZrO3. However, the intensity of reflections because of CaO are much stronger compared to the reflections because of CaZrO3. This is expected. In the deposition−precipitation method, we precipitated Ca(NO3)2 over the surface of ZrO2, which lead to the minimum interaction between Ca and Zr compared to the other samples. The XRD patterns of coprecipitation- and sol−gel-synthesized Ca/Zr sorbents are similar. Both the samples exhibit reflections because of CaZrO3, CaCO3, and CaO. The reflections because of CaZrO3 are much stronger in these samples compared to deposition−precipitated sample. Interestingly, any of the samples did not exhibit reflections because of ZrO2. The absence of reflections because of ZrO2 in the XRD patterns shows the complete incorporation of Zr into the CaO crystal lattice during the synthesis. On the whole, the XRD measurements suggest that both co-precipitation- and sol−

Figure 3. XRD patterns of pure CaO sorbents synthesized by different methods.

previous study,28 the XRD pattern of pure CaO prepared by the flame spray pyrolysis method exhibited reflections mainly because of CaCO3 (PDF-ICDD-00-047-1743), along with traces of CaO (PDF-ICDD-00-004-0777).32 On the other hand, both precipitation- and sol−gel-synthesized CaO mainly exhibited reflections because of CaO. The reflections because of CaCO3 are very low in intensity. These results suggest that CaO synthesized by the flame spray pyrolysis method forms 3295

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gel-synthesized Ca/Zr sorbents form higher amounts of CaZrO3, such as in the case of the flame-spray-pyrolysissynthesized sample. As described in our earlier studies,28 CaZrO3 forms between CaO particles and contributes to the resistance of the sorbents toward sintering. 3.2.3. XPS. XPS measurements have been performed over the synthesized sorbents to investigate the elemental oxidation states and surface atomic ratios and environments. The X-ray photoelectron spectra of O 1s, Ca 2p, and Zr 3d peaks are presented in Figures 5 and 6. The corresponding binding

Figure 6. (a) Ca 2p XPS spectra of Ca/Zr sorbents and (b) Zr XPS spectra of Ca/Zr sorbents.

Table 3. O 1s, Ca 2p, and Zr 3d XPS Binding Energies of Various Ca/Zr Sorbents

Figure 5. O 1s XPS spectra of Ca/Zr sorbents.

energy values are presented in Table 3. As shown in Figure 5, the O 1s XPS spectra are broad and complicated because of the non-equivalence of surface O ions. The binding energy of the O 1s peak (Table 3) is dependent upon the synthesis method adopted. All of the samples, except Ca/Zr synthesized by the deposition−precipitation method, exhibit an additional peak at around 529 eV. As described in our previous studies, this peak arises from the oxygen atom of CaZrO3. Hence, O 1s XPS measurements agree well with the XRD measurements. The Ca 2p and Zr 3d XPS spectra of all of the synthesized sorbents are presented in panels a and b of Figure 6. As represented in Figure 6a, the Ca 2p XPS spectra of all of the samples show a peak at 348 eV and a satellite peak at 351 eV. This peak is due to the ionization of Ca 2p3/2 and Ca 2p1/2 electrons of the Ca2+ oxidation state. The Zr 3d spectra exhibit a photoelectron peak range between 183.2 and 184.6 eV and because of the presence of Zr4+ oxidation states. The binding energies of Ca 2p and Zr 3d peaks presented in Table 3 vary by varying the synthesis method. The Ca/Zr atomic ratios are presented in Table 2. As expected, the Ca/Zr sorbent synthesized by the deposition− precipitation method exhibits a higher Ca/Zr atomic ratio (9.46) compared to the other samples. Interestingly, the Ca/Zr

sample

O 1s

Zr 3d

Ca 2p

co-precipitation Ca−Zr sol−gel Ca−Zr deposition−precipitation Ca−Zr flame Ca−Zr

530.3 531.5 531 531.9

180.7 182.1 181.6 182.8

345.7 347.1 346.6 347.6

sorbent synthesized by the co-precipitation method also shows a higher atomic ratio of 8.9 eV, even though we precipitated both Ca and Zr simultaneously using NaOH. This is due to the difference in the precipitation constants of Ca and Zr. In general, Zr precipitates at around pH ∼ 4.5 and Ca precipitates at pH more than 9. Hence, zirconia precipitates first, and on top of zirconium hydroxide particles, calcium hydroxide particles precipitate. Both sol−gel- and flame-spray-pyrolysissynthesized Ca/Zr sorbents exhibit Ca/Zr atomic ratios of 3.57 and 1.5. These results suggest that sol−gel-synthesized Ca/Zr has more Ca particles over the surface compared to the sorbent synthesized by the flame spray pyrolysis method. This is the reason why the sol−gel-synthesized Ca/Zr sorbent exhibits higher CO2 adsorption capacity compared to the sorbent synthesized by the flame spray pyrolysis method. Even though the sorbents synthesized by co-precipitation and deposition− precipitation methods have very high Ca/Zr sorbents, they exhibit molar conversions of 50 and 71%, which are very low compared to that of the sol−gel method. This is due to the higher crystallite size. The TEM measurements present later in the study support this observation. 3296

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Figure 7. TEM images of fresh Ca/Zr sorbents synthesized by different methods: (a) co-precipitation, (b) deposition−precipitation, (c) sol−gel, and (d) flame spray pyrolysis methods.

3.2.4. TEM Measurements. The surface morphology and particle sizes of Ca particles are investigated using TEM measurements. The TEM images of all of the fresh sorbents are presented in Figure 7. The Ca/Zr sorbent synthesized by the flame spray pyrolysis method exhibits particle sizes of 10−12 nm. However, as shown in Figure 7, both co-precipitation- and deposition−precipitation-synthesized Ca/Zr sorbents exhibit crystallite sizes more than 100 nm. These results agree well with our previous studies. In our previous studies, also CaObased sorbents synthesized by the impregnation and precipitation methods exhibit much higher crystallite sizes compared to the sorbents synthesized by the flame spray pyrolysis method.32 On the other hand, the Ca/Zr sorbent synthesized by the sol−gel method exhibits much smaller crystallite sizes (10−20 nm) compared to the Ca/Zr sorbent synthesized by co-precipitation and deposition−precipitation methods. We also performed TEM measurements over pure CaO sorbents prepared by co-precipitation and sol−gel methods. The image of CaO synthesized by the co-precipitation method is presented in Figure 8. Interestingly, both CaO sorbents exhibit crystallite sizes more than 100 nm. These results suggest that zirconia stabilizes the Ca particles during the sol−gel synthesis against the sintering. Preparation of pure CaO by the sol−gel method yields particles with higher sizes. On the other hand, even though zirconia is able to form CaZrO3 with Ca during the synthesis by the co-precipitation method, however, zirconia is not able to stabilize the Ca particles against sintering. The Ca/Zr atomic ratios determined from TEM−energydispersive spectrometry (EDS) measurements are presented in

Figure 8. TEM image diffraction patterns of fresh CaO sorbents synthesized by the sol−gel method.

Table 2. The atomic ratios agree well with the XPS measurements. Both Ca/Zr sorbents prepared by the coprecipitation and deposition−precipitation methods exhibit very high Ca/Zr atomic ratios. Also, the sol−gel-synthesized Ca/Zr sorbent exhibits a higher Ca/Zr atomic ratio of 70:30 compared to the flame-spray-pyrolysis-synthesized Ca/Zr sorbent, which exhibits a Ca/Zr atomic ratio of 50:50. The TEM images of spent sorbents after CO2 adsorption are presented in Figure 9. The particle size of Ca/Zr sorbents 3297

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Figure 9. TEM images of spent Ca/Zr sorbents synthesized by different methods: (a) co-precipitation, (b) deposition−precipitation, (c) sol−gel, and (d) flame spray pyrolysis methods.

zirconium nitrate. However, we are using only 5 mol of zirconium for every 10 mol of calcium. Hence, our method is very techno-economical for real-time-integrated gasification combined cycle applications.

prepared by co-precipitation and deposition−precipitation further increased to 200 nm after CO2 adsorption. These results suggest that Zr did not stabilize the CaO particles against sintering during the synthesis and the CO2 adsorption. On the other hand, remarkably, there is no increase in the crystallite size observed in the case of the Ca/Zr sorbent synthesized by the sol−gel method even after 1200 cycles. The crystallite size remains between 10 and 20 nm. The role of the synthesis method on structural and CO2 adsorption properties of the Ca/Zr sorbent has been investigated. CO2 capture measurements show that the sol− gel-synthesized Ca/Zr sorbent exhibits higher adsorption capacity and similar stability as our previously reported Ca/ Zr sorbent synthesized by the flame spray pyrolysis method. XRD measurements suggest that all of the samples exhibit reflections because of CaZrO3. TPD measurements show that the basic properties of the sorbents depend upon the synthesis method adopted. TEM−EDS and XPS atomic ratios show that the sol−gel-synthesized Ca/Zr sorbent has more CaO particles compared to the Ca/Zr sorbent synthesized by the flame spray pyrolysis method and exhibit better CO2 capture performance. MacKenzie et al.33 reported a preliminary economic analysis of the Ca dry sorbent for the commercialized technologies. They showed that Ca-based sorbents have the potential to be an economically attractive option for CO2 capture. Also, we have used zirconium nitrate, calcium nitrate, and citric acid as precursors to synthesize Ca/Zr by the sol−gel method. On the other hand, we have used expensive organometallic precursors to synthesize Ca/Zr by the flame spray pyrolysis method. The only little expensive chemical in the sol−gel method is

4. CONCLUSION The Ca/Zr sorbent has been synthesized by four different methods, namely, co-precipitation, deposition−precipitation, sol−gel, and flame spray pyrolysis methods, and evaluated for high-temperature CO2 capture. Among the various sorbents, the Ca/Zr sorbent synthesized by the sol−gel method shows better adsorption capacity and similar stability as our previously reported flame-spray-pyrolysis-synthesized Ca/Zr sorbent. XRD measurements show that CaZrO3 formed in all of the samples during the synthesis. XRD measurements also suggest that the CaCO3 phase was dominant in the flame-spraypyrolysis-synthesized CaO samples, while the CaO phase was dominant in the pure CaO sorbents synthesized by precipitation and sol−gel methods. TPD measurements show that the Ca/Zr sorbent synthesized by the sol−gel method exhibits higher basicity compared to the other sorbents. O 1s XPS measurements agree well with the XRD measurements. TEM measurements show that the Ca/Zr sorbents synthesized by co-precipitation and deposition−precipitation methods exhibit particle sizes more than 100 nm. TEM measurements also show that the Ca/Zr sorbent synthesized by the sol−gel method exhibits much smaller particles (10−20 nm) compared to the sorbents synthesized by co-precipitation and deposition−precipitation methods. On the whole, the Ca/Zr sorbent synthesized by the sol−gel method has small crystallites and 3298

dx.doi.org/10.1021/ef402573u | Energy Fuels 2014, 28, 3292−3299

Energy & Fuels

Article

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more calcium particles on the surface and exhibited higher CO2 capture capacity and multi-cycle stability compared to the other sorbents.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 513-556-1474. Fax: 513-556-3473. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Rodica B. McCoy and Dr. Jacek Jasinski of the University of Louisville, Louisville, KY, for help with the XRD and XPS measurements.

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dx.doi.org/10.1021/ef402573u | Energy Fuels 2014, 28, 3292−3299