Facile Synthesis of Flame Spray Pyrolysisâderived Magnesium Oxide

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Applied Chemistry

Facile Synthesis of Flame Spray Pyrolysis–derived Magnesium Oxide Nanoparticles for CO2 Sorption: Effect of Precursors, Morphology and Structural Properties Thirupathi Boningari, Siva Nagi Reddy Inturi, Vasilios I. Manousiouthakis, and Panagiotis G. Smirniotis Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00188 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Industrial & Engineering Chemistry Research

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Facile Synthesis of Flame Spray Pyrolysis–derived Magnesium Oxide Nanoparticles for CO2 Sorption: Effect of Precursors, Morphology and Structural Properties

Thirupathi Boningari,† Siva Nagi Reddy Inturi,† Vasilios I. Manousiouthakis,‡ Panagiotis G. Smirniotis*†

†

Chemical Engineering Program, College of Engineering and Applied Science, University of Cincinnati, Cincinnati, Ohio 45221-0012, United States ‡

Chemical and Biomolecular Engineering Department, University of California, Los Angeles, California 90095-1592, United States

KEYWORDS: Magnesium oxide (MgO); Flame spray pyrolysis; Low-temperature; CO2 sorption

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

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ABSTRACT A series of MgO aerosol nanoparticles were prepared by using a flame aerosol method and examined for the CO2 sorption at low-temperatures. Our XRD results suggest the formation of high purity magnesium oxide (MgO) phase. The CO2 sorption was determined at temperatures ranging from 60 oC to 275 oC using thermogravimetric (TGA) analysis, where the results indicate that the increased CO2 sorption we observed is associated with the improved pore volume and surface area of the sorbent. The as-synthesized MgO-A sorbent exhibited the best CO2 sorption capacity (66.0 mg CO2/g sorbent) at 60 oC in comparison with all the sorbents tested. The CO2 uptake is predominantly controlled by the pore architecture as well as ultra-micropores. The uptake characteristics of selected MgO sorbents we synthesized are significantly higher than that of CaO based sorbents at low temperatures.


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1. Introduction The carbon dioxide level has been increased rapidly in the environment as a result of numerous anthropogenic activities. Combustion of fossil fuels, deforestation, human activities related to manufacturing, and chemical processing lead to the release of excessive amounts of CO2 into the atmosphere. The global CO2 emissions are 36.8 gigatons in 2017 and this number is expected to increase by 2% by the end of 2018. Hence, the excessive release of carbon dioxide (CO2) in the atmosphere is considered as the most important aspect for the observed global climate pattern.1, 2 In order to minimize the global warming several efforts focus on the large scale CO2 sorption and carbon sequestration (CCS) as the means to correct this serious environmental problem.3−5 Carbon sequestration (CCS) implicates longterm storing of CO2 by terrestrial storage and geologic storage methods. Other than CO2 removal and carbon sequestration (CCS) process, numerous alternative methods have been proposed for CO2 sorption and mitigation from flue gases, such as cryogenic separation and distillation, membrane separation, and CO2 absorption in aqueous solutions of alkanolamines.6 However, the above mentioned processes are associated with numerous challenges for industrial application due to the relatively low uptakes, high energy consumption, expensive procedures and specialized materials needed to operate. Accordingly, the development of inexpensive CO2 sorption processes is much desired. Among other techniques, CO2 sorption techniques by using solid metal oxide sorbents which can be regenerated has attracted much attention in recent years. For this purpose, several CO2 sorbents have been synthesized and investigated the CO2 sorption and separation over activated and porous carbons,7 lithium-based compounds, molecular sieves, zeolites,



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membranes8-10 metal−organic frameworks (MOFs),11 amine-related materials,12, 13 and metal oxides.14−16 The magnesium oxide (MgO) sorbent is a promising material for CO2 sorption at lowtemperatures predominantly due to its moderate basicity, abundance, low regeneration temperature, and being very inexpensive material.17 On the other hand, pristine magnesium oxide shows a low CO2 sorption capacity due to its relatively low surface area and pore volume. For this reason, several researchers tried to improve the CO2 sorption capability of magnesium oxide sorbents.18−24 Most of the magnesium oxide-based sorbents reported in the open literature were primarily synthesized by conventional synthesis such as sol-gel, coprecipitation, hydrothermal, and solvothermal methods. Conversely, synthesis parameters, procedures, and conditions intensely impact the surface texture, redox properties, structural properties, crystallite size, basicity, sorption capacity and pore structure of the materials. Flame Spray Pyrolysis (FSP) is an advanced, and inexpensive single-step method for synthesizing large (industrial) scale a variety of thermally stable, high surface area nanoparticles. In a traditional flame aerosol process, the liquid precursors are sprayed into the flame and the oxidation of the precursor results in the formation of the corresponding metal nanoparticles. Dispersion of the feed with high velocity gas spray jet in the main aerosol flame method leads to high yield, thermally stable, high surface area and welldefined chemical composition of nanoparticles as found both in lab experiments and large scale particle manufacturing.25, 26 The key objective of this study is to develop MgO-based nanosorbents by flame aerosol method and investigate the effect of CO2 capture capacity of different MgO sorbents synthesized from various precursors. The sorbent carbonation capacity is investigated at low


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temperatures by thermogravimetric analysis while the surface and structural properties of the sorbents are studied by various physicochemical characterizations. We have investigated the effect of various precursors for the synthesis of MgO sorbents to examine the CO2 sorption capacity and to identify the most effective sorbent. Among all the tested sorbents, the MgO sample synthesized from Mg(CH3COO)2 demonstrated relatively high CO2 sorption capacity at 60

o

C. Our thermogravimetric analysis showed that the CO2 sorption capacity

monotonically increased and reached a maximum (66.0 mg CO2/g sorbent) with increase in preheating temperatures up to 600 oC, while the carbonation capacity was dropped to some extent with further increase in the temperature.

2. Experimental 2.1.

Materials

The precursors used for synthesizing the materials for this study are magnesium ammonium chloride (MgNH4Cl3, P&B, purity N97%), magnesium acetate (Mg(C2H5O)2, Sigma-Aldrich, purity N98%), magnesium chloride (MgCl2, Sigma-Aldrich, anhydrous, ≥98%), magnesium nitrate (Mg(NO3)2, Sigma-Aldrich, anhydrous, ≥98%). The ethanol used as the fuel for the aerosol reactor was purchased from Sigma-Aldrich. Deionized water was obtained from a local supplier and used for all the synthesis experiment. 2.2.

Synthesis of MgO sorbents using Flame Spray Pyrolysis (FSP)

The preparation method for the magnesium oxide nanoparticles is elucidated comprehensively in our earlier studies.27, 28 In detail, ethanol is used as the solvent (fuel).



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The MgO sorbents were prepared by using various magnesium precursor solutions (Magnesium ammonium chloride, Magnesium acetate, Magnesium nitrate, and Magnesium chloride) with a molar ratio of 0.3M. During FSP, a syringe pump (Cole Parmer, 74900 series) was used to inject the metal precursor (liquid) through a spray nozzle at a flow rate of 3 mL/min, further it was isolated by a surrounding 5 L/min flow of O2. A supporting flame (premixed 1.0 L/min + O2/ 1.0 L/min CH4) surrounding the main flame was used for the combustion of the dispersed droplets. In order to make sure that the complete combustion of the reactants was achieved, oxygen sheath flow of 5 L/min was provided through a sinter metal ring (9/17 mm inner/outer diameter) adjacent to the supporting flame. All the required gas flows were controlled and monitored by calibrated mass flow controllers. Further, the obtained aerosol nanoparticles were deposited on a Whatman flat glass fiber filter (GF/A, 150 mm diameter) assisted by a vacuum pump. The flame-made MgO-based nanoparticles were scraped from the flat glass fiber filter and directly used as sorbents without any additional purifications or calcination. 2.3.

Sorbent Characterization The surface textural properties experiments were carried out by using a Micromeritics

ASAP 2010. All the sorbents were degassed at 300 oC under vacuum before analysis. The BET surface areas were determined from a 6-point N2 adsorption isotherm recorded in a relative partial pressure (p/p0) range 0.05 to 0.25. The pore size distribution measurements were obtained using the BJH method. The X-ray diffraction data were recorded over a 2θ range of 10–80o with a step size of 0.02o using a counting time of one second per point. The X-ray diffraction (XRD) profiles were obtained on a Phillips X’pert diffractometer using nickel-filtered CuKα (0.154 nm) as the radiation source. The crystalline phases of MgO


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nanoparticles were acknowledged through comparison with the reference data taken from the International Centre for Diffraction Data (ICDD) files. Transmission electron microscopy (TEM) was carried out with an electron microscope (Philips CM 20) where the accelerating voltage was 200 keV, with a LaB6 emission current and a point-to-point resolution of 0.27 nm. Prior to the examination, the as-synthesized MgO nanosorbents were sonically dispersed in iso-propyl alcohol and deposited on the carbon Cu grid. After the evaporation of the solvent, the MgO nanoparticles remaining on the carbon film grid were investigated. The CO2 sorption capacity was examined with a thermogravimertic analyzer (TGA, PerkinElmer Pyris™-1), thermal analysis gas station (TAGS, Perkin Elmer) and software of Pyris™ v3.8 (Perkin Elmer). The Pyris™-1 TGA microbalance functions as a high gain electromechanical servo system which permits the detection of weight changes as small as 0.1μg versus time. The shift and flow of CO2 (Wright Brothers, purity 99.5%) and ultra-high purity helium (Wright Brothers) were precisely maintained by thermal analysis gas station (TAGS). The thermal analysis gas station has four gas channels and can automatically switch between either of them as a function of the testing program. The carbonation and decarbonation experiments were programmed and operated batch wise including cooling and heating of the MgO nanoparticle sorbents and the switching of CO2 and helium gases. Initially, 4 to 8 mg amount of flame-made MgO sorbent was employed in a platinum sample pan followed by heating of the sample to the carbonation temperature at a ramp rate of 10 oC min-1. Then, the CO2 stream (30 vol. % CO2 balanced in helium) was introduced when the sample temperature reached the anticipated carbonation temperature. The CO2 sorption and the weight profile of the sample with respect to temperature and time were recorded and analyzed during the course of the experiment.



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3. Results and Discussion 3.1.

X-ray Diffraction (XRD) Studies

The X-ray powder diffraction profiles of the as-prepared MgO nanoparticles (MgO-N, MgOCl, MgO-A, and MgO-AC) using different precursors namely, Mg(NO3)2, MgCl2, Mg(CH3COO)2, MgNH4Cl3, respectively, are given in Figure 1A. The characteristic diffraction peaks at 2θ = 37.2°, 43.1°, 62.5°, 74.9°, and 78.7°) were observed for all the samples, which can be attributed to the (111), (200), (220), (311), and (222) planes of a single cubic phase of high purity MgO, respectively.29 These XRD patterns of the asprepared samples can be ascribed to JCPDS: 4–829 and indexed to an fm-3m (225) space group.30,

31

We have not observed any peaks related to pure hexagonal Mg(OH)2 [P m1

(164) space group] with JCPDS card no. 44-1482 (Figure 1A). The broadness of the diffraction peaks in MgO-A indicates the nano-crystalline nature of MgO. Conversely, the crystallinity nature of our MgO nanosorbents changes with respect to the metal precursor used for the preparation. In a similar manner, we also investigated the effect of molar concentration (0.2M−0.5M) of the precursor solutions using Mg(CH3OO)2 on the x-ray diffraction patterns (Figure 1B). As we can see from Figure 1B, the crystallinity has decreased with increase in the molarity of our magnesium precursor solution. The high fuel content within the droplets of the precursors in the reactor zone causes coagulation and sintering of the MgO nanoparticles and leads to the high crystallinity sorbents when 0.2M of precursor were used. For the relatively high concentration of the magnesium precursor (0.4M and 0.5M) under certain operating conditions of the aerosol reactor we also synthesized materials of high crystallinity.


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We have calculated the crystallite size, D; lattice parameter, a; inter planar spacing, d; number

of unit cells, n; and unit cell volume, V; in the MgO particles from the following relations (Table 1).

(Eq. 1) Where λ= x-rays wavelength, β = FWHM of the corresponding peak.

2d sin  = n

(Eq. 2)

The lattice parameter (a) for all the as-synthesized sorbents was calculated by considering the (200) plane in the XRD and the results are illustrated in Table 1. All the estimated lattice parameters of the samples we studied are in agreement with the standard data for the cubic crystal phase of MgO [JCPDS: 75-0447]. Rendering to the concept of surface energy,32 an increase or decrease in the lattice parameter and microstrain are anticipated by means of internal stress in the crystal lattice. The as-synthesized MgO-A sorbent illustrates higher lattice parameter (0.420 nm) compared to conventional MgO-aldrich sample (0.417 nm), thus indicating that the sample is subjected to a tensile stress in the plane parallel to the substrate surface.

V= a3

(Eq. 4) (Eq. 5)



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Other structural properties, such as the x-ray density, Dx; the dislocation density,  the microstrain,  the stacking fault, SF; and the stress,  were calculated using the below equations:

(Eq. 6) (Eq. 7) (Eq. 8) (Eq. 9) (Eq. 10)

Where (hkl)=Miller indices, M=weight of the molecule, a=lattice constant, N=Avogadro number (6.0221×1023), E=Young’s modulus of the sample or elastic constant and θ=Bragg’s angle. Young’s modulus of the magnesium oxide is ∼300 GPa.33,

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All the structural

properties and characterizations are shown in Table 1. In our present studies, the measured microstrain () along the (200) plane was positive, suggesting the existence of tensile stress over the surface of flame made MgO sample. It has been well established in the literature that the negative values of microstrain comes from the error analysis in the XRD, due to the nonsymmetrical nature of the corresponding diffraction peaks.35−37 The MgO-A sample exhibited very broad diffraction peaks among all the tested sorbents. The (200) plane in MgO-Acetate sample shifted to lower angles (43.019 o) compared to that of MgO-Aldrich


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sample (43.249 o) (Figure 1A). Among all the sorbents tested, MgO-A sample showed high values of stress () and microstrain () compared to other samples.

3.2.

BET specific surface area and pore volume

The MgO (MgO-N, MgO-Cl, MgO-A, and MgO-AC) nanoparticles were prepared by the aerosol synthesis technique using different precursors namely, Mg(NO3)2, MgCl2, Mg(CH3COO)2, MgNH4Cl3, respectively. The BET surface area, micropore volume, external surface area and pore size of the synthesized catalysts are summarized in Table 2. The specific surface area of the as-synthesized MgO (MgO-N, MgO-Cl, MgO-A, and MgOAC) nanoparticles obtained by N2 physisorption at liquid nitrogen temperature was found to be 13.5, 50.1, 177.2, and 117.8 m2 g-1, respectively (Table 2). Among all the prepared sorbents, MgO-A sample attained significant high BET surface area, pore volume, and micropore area compared to other sorbents. As one can see from Figure 2 and Table 2, the pore size distribution, the micropore volume and the external surface area play a vital role in CO2 sorption. We also examined the N2 adsorption/desorption isotherms for the assynthesized MgO-N, MgO-Cl, MgO-A, and MgO-AC nanoparticle sorbents (Figure 3). The adsorbed volume has been increased monotonically at a high relative pressure in all the sorbents, which can be ascribed to the nature of solid microporous (Figure 3).38 Our results on the micropore area and micropore volume of the MgO nanoparticles given in Table 2 also illustrate the same characteristics. Relatively high adsorption was observed in MgO-A (synthesized from Mg(CH3COO)2) followed by MgO-AC> MgO-Cl> MgO-N. The nitrogen adsorption-desorption isotherms of the as-prepared MgO nanoparticle sorbents can be attributed to Type II with an H3-type hysteresis in P/P0=0.8–1.0, and an H2-type hysteresis



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in the range of P/P0 = 0.2–0.8. The H3 hysteresis reveals the existence of slit type mesopores, and macropores.39,40 The CO2 capture materials with a large number of micropores are ideal for the sorption.41 Higher porosity in CO2 sorbents leads to increase storage capacity unless the pore size distribution tends towards macropores.

3.3.

Influence on the surface textural properties of MgO nanoparticles

The concentrations of the magnesium precursor solutions have substantial influence on surface morphology and textural properties.42−44 We have synthesized magnesium oxide nanosorbents by flame aerosol method using different concentrations (0.2–0.5 M) of MgO precursor solutions (Figure 4). Initially, with increasing molarity of the precursor solution both the pore volume and the specific surface area of the pristine MgO nanoparticle sorbents monotonically increased and reached the value of 0.823 cm3g−1 and 177.2 m2g−1, respectively, at 0.3 M of the precursor solution. For 0.4 M concentration of the MgO precursor solution, the pore volume and surface area decreased to some extent (0.626 cm3g−1 and 164 m2g−1, respectively). The pore volume and the surface area of MgO nanosorbents decreased drastically with the additional surge in the MgO precursor molarity. These results are consistent with the preparation of other materials synthesized by flame spray pyrolysis.4346

Thus, the flame-made sorbents for CO2 capture tests in the current studies were prepared

using 0.3 molar concentration of the precursor with a feed rate of 3 mL min−1 and oxygen dispersion gas equal to 5 L min−1.


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3.4. CO2 sorption over MgO nanoparticle sorbents prepared by using flame aerosol synthesis technique: Effect of the various precursors on the synthesis of MgO nanoparticles The CO2 sorption over CaO has limitations because the carbonation reaction is limited after the formation of monolayer (CaCO3) over the surface of CaO.

47, 48

Due to the high

regeneration (900−1000 °C) temperature of CaO, relatively large amount of energy will be required for the temperature shift operations between adsorption and desorption. Furthermore, the CO2 capture rates are relatively poor in the operating conditions (50−400 °C) of IGCC.49 In our earlier studies, we have observed rapid decline of the CO2 capture after several cycles due to continuous agglomeration of the sorbent.50 On the other hand, flue gas primarily comprises of carbon dioxide and nitrogen while the temperature range can vary from 27–77 oC in post-combustion environments. All these, introduce the need for an effective low temperature solid sorbent. In order to reduce the power utilization to a great extent, flame made MgO nanoparticles are preeminent for CO2 sorption technologies as a result of its low regenerating temperatures: T1 = 447 oC (pre-combustion) and T2 = 287 oC (post-combustion). Despite of the lower CO2 sorption capacity observed at relatively low temperatures, the MgO sorbents still attract significant attention due to low regeneration temperatures with less energy penalties, better CO2 sorption rates than CaO sorbents,51 and overall sorption during multiple cycles.51,

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Due to its low regeneration temperatures and basicity, MgO

sorbents can be effective materials for CO2 sorption.18 However, its applications are limited because of prolonged and complex synthesis procedures and the lack of high yield MgO product. Our one-pot rapid synthesis technique that produces highly stable magnesium oxide



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nanoparticle sorbents with elevated specific surface area and great thermal stability will be a great solution for the above mentioned problems. As we discussed in earlier sections, the synthesis of MgO nanoparticle sorbents by flame aerosol synthesis technique using different precursors affects the surface and structural properties such as morphology, lattice strain parameters, and oxygen storage capacity. The product particle size, crystallite size and surface areas can be precisely controlled by changing the concentrations of precursor solutions.25, 53 The effect of the precursor on the properties of the synthesized MgO sorbents is presented in Table 2. The specific surface area varied from 13.5 to 177.2 m2/g by changing the precursor type which indicates that it is essential to investigate various precursors for the synthesis of the most promising sorbent candidates. To investigate the potential application of the as-synthesized MgO nanoparticles their uptake for CO2 was investigated. For this set of experiments, every sample was preheated at 600 oC with a ramp rate of 10 oC min-1 under ultra-high purity (UHP, 99.999%) helium for 30 minutes. Each sorbent was carbonated and decarbonated at 60 oC with 20 mL/min of CO2 (Wright Brothers, purity 99.5%) and with 20 mL/min of ultra-high purity (UHP, 99.999%) helium for 30 minutes alternatively (Figure 5). As one can observe from figure 5, the sorbent synthesized with Mg(CH3COO)2 exhibited the highest CO2 sorption uptake (66.0 mg CO2/g sorbent) capacity among all the tested sorbents. Our results indicate that our as-synthesized MgO nanoparticles show improved CO2 sorption compared to some of the reported MgO materials.53-57 The high surface area and high amount of micropores of the MgO-A sorbent is the reason for high CO2 uptake. The higher porosity of the CO2 sorbents leads to increase storage capacity (Table 2). The CO2 uptake is predominantly controlled by the pore architecture as well as the extent of the ultra-micropores. In general,


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micropores comprise slit pore geometry and mesopores consist of cylindrical pore geometry. The affinity of linear CO2 molecules to lie flat on the sorbent surface favors the CO2 uptake in slit pore and cylindrical architectures rather than disordered pore structures.58 In addition, the creation of strong basic sites and strong basicity in the MgO-A sample is responsible for unidentate carbonates formation and there-by high CO2 sorption capacity. In continuation, we have investigated the influence of preheating temperatures on the CO2 sorption of the selected MgO nanoparticle sorbent (FSP-MgO-A), the CO2 sorption experiments have been conducted on the FSP-MgO-A sorbent in TGA at various preheating temperatures starting from 200-800 oC. The results acquired from the thermogravimetric analysis of flame-made magnesium oxide samples are shown in Figure 6. As one can observe from Figure 6, the CO2 adsorption capacity monotonically increased and reached maximum (66.0 mg CO2/g sorbent) with increase in preheating temperatures up to 600 oC, while the carbonation capacity was dropped to some extent with further increase in the temperature (Figure 6). Further, we have investigated the CO2 capture capacity over our best sorbent (MgO-A) with respect to various sorption temperatures (40, 60, 80, 100, 150, and 275 oC), the results attained from the thermogravimetric (TGA) analysis of our MgO sorbents are presented in Figure 7. The FSP-MgO-A sample demonstrated a maximum CO2 sorption at 60 oC (66 mg CO2/g sorbent) among all temperatures used. The CO2 sorption over MgO seems to be a surface-limited phenomenon, because the amount of CO2 captured per unit weight of MgO is very low. However, our experimental results illustrate that in a short span of time (30 minutes), the amount of adsorbed CO2 is 6.6 grams per 100 g of assynthesized MgO-A sorbent with a specific surface area of 177.2 m2/g and this quantity of CO2 is far higher with respect to the surface monolayer coverage. Interestingly, our results



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suggest that the CO2 capture capacity over our as-synthesized MgO nanoparticles are in good correlation with the microstrain as defined by equation 8 and stress in the crystal lattice as defined by equation 9 (Figure 8). Moreover, the CO2 capture update is also directly related with the micropore volume and external surface area (Figure 9).

4. Conclusions We have prepared a series of MgO nanosorbents by using a flame aerosol method and examined their CO2 sorption at low-temperatures as a function of the various metal precursors, and optimizing sorption temperatures. The CO2 sorption capacity of the assynthesized MgO nanoparticles was investigated with respect to the metal precursors, sorption, activation temperatures, micropore volume and surface area. Our XRD results suggest the formation of high purity magnesium oxides. The CO2 sorption capacity was measured from 60 oC to 275 oC by TGA analysis showed the enhancement of the CO2 sorption associated with elevated specific surface area, pore volume, and sorption capacity. CO2 sorption was determined at 60 oC using TGA analysis demonstrated relatively high CO2 sorption (66.0 mg CO2/g sorbent) capacity of MgO-A sample among all the tested sorbents. In particular, strong basic sites, micropore volume, high surface area, and the formation of unidentate carbonates released from low-coordination O2− anions play an important role in the CO2 capture of magnesium oxide at low temperatures compared to CaO which sorbs CO2 only at high-temperatures. AUTHOR INFORMATION

Corresponding Author


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* E-mail: [email protected] (Panagiotis G. Smirniotis); Tel.: (513) 556-1474. Fax: (513) 556-3473

Author Contributions The manuscript was written through contributions of all the authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.



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28. Inturi, S.I.; Boningari, T.; Suidan, M.; Smirniotis, P.G. Stabilization of cr in ti/si/cr ternary composites by aerosol flame spray-assisted synthesis for visible-light-driven photocatalysis. Ind. Eng. Chem. Res. 2016, 55, 11839−11849. 29. Sónia A.C.C.; Nina, B.; Alexey, P.; Pedro, B.T.; Lisete, S.G. F.; José, L.F. Gold nanoparticles supported on magnesium oxide for CO oxidation. Nanoscale Research Letters 2011, 6, 435. 30. Gu, F.; Chun, Z.L.; Hai, B.J. Combustion synthesis and photoluminescence of MgO : Eu3+ nanocrystals with Li+ addition. J. Cryst. Grow. 2006, 289, 400–404. 31. Devaraja P.B.; Avadhani, D.N.; Prashantha, S.C.; Nagabhushana, H.; Sharma, S.C.; Nagabhushana, B.M.; Nagaswarupa, H.P. MgO:Eu3+ red nanophosphor: Low temperature synthesis and photoluminescence properties. Spectrochim. Acta A: Molecular and Biomolecular Spectroscopy. 2014, 121, 46–52. 32. Chopra, K.L. Thin Film Phenomena, McGraw-Hill, New York, 1969, 191. 33. Devaraja, P.B; Avadhani, D.N; Nagabhushana, H; Prashantha, S.C; Sharma, S.C; Nagabhushana, B.M; Nagaswarupa, H.P.; Prasad, B.D. MgO:Dy3 + nanophosphor: Self ignition route, characterization and its photoluminescence properties. Mater. Charact. 2014, 97, 27–36. 34. Baudson, H.; Debucquoy, F.; Huger, M.; Gault, C.; Rigaud, M. Ultrasonic measurement of Young’s modulus MgO/C refractories at high temperature. J. Eur. Ceram. Soc. 1999, 19, 1895–1901. 35. Vidya, Y.S.; Anantharaju, K.S.; Nagabhushana, H.; Sharma, S.C.; Nagaswarupa, H.P.; Prashantha,

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Shivakumara,

C.

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tetragonal

ZrO2:

Eu3+

nanophosphors: Structural and photoluminescence studies. Spectrochim. Acta. A: Molecular and Biomolecular Spectroscopy. 2015, 135, 241–51,



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36. Suresh, R.; Ponnuswamy, V.; Mariappan, R. Effect of annealing temperature on the microstructural, optical and electrical properties of CeO2 nanoparticles by chemical precipitation method. Appl. Surf. Sci. 2013, 273, 457–64. 37. Lima, R.S.; Kucuk, A.; Berndt, C.C. Evaluation of microhardness and elastic modulus of thermally sprayed nanostructured zirconia coatings. Surf. Coat. Technol. 2001, 135, 166–72. 38. Orellana, F.; Los Reyes, J.P.D.; Urizar, S. Ru/SiO2 and Cu-Ru/SiO2 prepared by sol-gel: effect of pH and water amount. J. Chil. Chem. Soc. 2003, 48, N2. 39. Alcañiz-Monge, J.; Lozano-Castelló, D.; Cazorla-Amorós, D.; Linares-Solano, A. Fundamentals of methane adsorption in microporous carbons. Microporous and Mesoporous Materials 2009, 124, 110–116. 40. Li, W.-C.; Lu, A.-H.; Weidenthaler, C.; Schuth, F. Hard-templating pathway to create mesoporous magnesium oxide. Chem. Mater. 2004, 16, 5676–5681. 41. Yan, H.; Blanford, C.F.; Holland, B.T.; Smyrl, W.H.; Stein, A. General synthesis of periodic macroporous solids by templated salt precipitation and chemical conversion. Chem. Mater. 2000, 12, 1134–1141. 42. Song, S.A.; Jung, K.Y.; Park, S.B. Preparation of Y2O3 Particles by Flame Spray Pyrolysis with Emulsion. Langmuir 2009, 25, 3402–3406. 43. Mueller, R.; Mädler, L.; Pratsinis, S.E. Nanoparticle synthesis at high production rates by flame spray pyrolysis. Chem. Eng. Sci. 2003, 58, 1969–1976. 44. Chang, H.C.; Kim, S.J.; Jang, H.D.; Choi, J.W. Synthetic routes for titania nanoparticles in the flame spray pyrolysis. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2008, 313, 282–287.


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45. Pratsinis, S.E. Flame aerosol synthesis of ceramic powders. Progress in Energy and Combustion Science 1998, 24, 197. 46. Tok, A.I.Y.; Boey, F.Y.C.; Du, S.W.; Wong, B.K. Flame spray synthesis of ZrO2 nanoparticles using liquid precursors. Materials Science and Engineering B 2006, 130, 114. 47. Barker, R. The reactivity of calcium oxide towards carbon dioxide and its use for energy storage. J. Appl. Chem. Biotech. 1974, 24, 221–227. 48. Bhatia, S.K.; Perlmutter, D.D. Effect of the product layer on the kinetics of the CO2lime reaction. Am. Instit. Chem. Eng. J. 1983, 29, 79–86. 49. Wang, Q.; Luo, J.; Zhong, Z.; Borgna, A. CO2 Capture by Solid Adsorbents and Their Applications: Current Status and New Trends. Energy Environ. Sci. 2011, 4, 42. 50. Lu, H.; Reddy, E.P.; Smirniotis, P.G. Calcium Oxide Based Sorbents for Adsorption of CO2 at High Temperatures. Ind. Eng. Chem. Res. 2006, 45, 3944. 51. Hufton, J.R.; Mayorga, S.; Sircar, S. Sorption-Enhanced Reaction Process for Hydrogen Production. AIChE J. 1999, 45, 248. 52. Hanif, A.; Dasgupta, S.; Divekar, S.; Arya, A.; Garg, M.O.; Nanoti, A. A Study on High Temperature CO2 Capture by Improved Hydrotalcite Sorbents. Chem. Eng. J. 2014, 236, 91–99. 53. Rajesh, K.; Krishna, R.G.; Sotiris, E.P.; Smirniotis, P.G. Effect of Zirconia Doping on the Structure and Stability of CaO-Based Sorbents for CO2 Capture during Extended Operating Cycles. J. Phys. Chem. C 2011, 115, 24804–24812. 54. Young, -J.H.; Soo, -J.P., Facile Synthesis of MgOModified Carbon Adsorbents with Microwave- Assisted Methods: Effect of MgO Particles and Porosities on CO2 Capture, Scientific Reports 2017, 7, 5653.



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55. Zhongzheng, Z.; Jianyuan, L.; Jian, S.; Hui, W.; Wei, W.; Yuhan, S. Bimodal Mesoporous Carbon-Coated MgO Nanoparticles for CO2 Capture at Moderate Temperature Conditions, Ind. Eng. Chem. Res. 2016, 55, 7880−7887. 56. Lei, L.; Yong, .; Xia, W.; Feng, W.; Ning, Z.; Fukui, X.; Wei, W.; Yuhan, S.; CO2 Capture over K2CO3/MgO/Al2O3 Dry Sorbent in a Fluidized Bed, Energy Fuels 2011, 25, 3835– 3842. 57. Ward, S.M.; Braslaw, J.; Gealer, R.L. Carbon dioxide sorption studies on magnesium oxide, Thermochimica Acta, 1983, 64, 107-I 14. 58. Kumar, K.V.; Preuss, K.; Lu, L.; Guo, Z.X.; Titirici, M.M. Diffusion of H2, CO2, and Their Mixtures in the Porous Zirconium Based Metal−Organic Framework MIL-140A(Zr): Combination of Quasi-Elastic Neutron Scattering Measurements and Molecular Dynamics Simulations. J. Phys. Chem. C 2015, 119, 22310−22321.


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


25

Figure and Table captions Figure 1. (A) Powder X-ray diffraction (XRD) patterns of the synthesized MgO sorbents; (a) MgO-Cl (synthesized from MgCl2), (b) MgO-A (synthesized from Mg(CH3COO)2), (c) MgO-AC (synthesized from MgNH4Cl3), (d) MgO-N (synthesized from Mg(NO3)2), and (e) MgO-Al (purchased from Aldrich) sorbents. Figure 1. (B) Effect of the Mg(CH3COO)2 precursor molar concentration (0.2M−0.5M) on Powder X-ray diffraction (XRD) patterns of MgO nanoparticles. Figure 2. Pore size distribution of MgO-N, MgO-A, and MgO-AC samples, respectively. Figure 3. Nitrogen adsorption and desorption isotherms of the as-synthesized MgO sorbents. Figure 4. Effect of molar concentration (0.2M−0.5M) of the precursor Mg(CH3COO)2 solutions on pore volume and morphology of MgO nanoparticle sorbents. Figure 5. CO2 sorption uptake at 60 oC as a function of the MgO sorbent using flame spray pyrolysis technique synthesized with various magnesium precursors; MgO-N: Mg(NO3)2, MgO-Cl: MgCl2, MgO-A: Mg(CH3COO)2, and MgO-AC: MgNH4Cl3. Figure 6. Effect of preheating temperature on CO2 sorption at 60 oC over FSP-MgO-A sorbents; FSP-MgO: MgO synthesized by flame spray pyrolysis technique using Mg(CH3COO)2 precursor. Figure 7. Effect of the sorption temperature on CO2 capture over MgO-A sorbent synthesized by flame spray pyrolysis technique.



Page 26 of 38

26

Figure 8. Correlation between CO2 capture at 60 oC with the strain in the crystal lattice () and the stress () of the as-synthesized MgO sorbents by flame spray pyrolysis technique. Figure 9. Effect of micropore volume and external surface area on CO2 capture at 60 oC of the as-synthesized MgO sorbents by flame spray pyrolysis technique.

Table 1.

Calculated structural parameters of MgO nanoparticle sorbents prepared by adopting flame aerosol synthesis technique.

Table 2.

BET surface area, pore diameter and pore volume measurements of the sorbents

prepared by adopting flame aerosol synthesis technique with various precursors.


Page 27 of 38

(d)

(222)

(220)

(311)

0.5M

(111)

Intensity (a.u.)

(311)

(222)

(220) (111)

(e)

(b)(B)

MgO-Cl MgO-A MgO-AC MgO-N MgO-Al

(200)

(a) (A)

(200)

27

Intensity (a.u.)

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


0.4M 0.3M 0.2M

(c) (b) (a) 30

40

50

60

70

80

30

40

50

60

70

80

o

2 ( )

o

2 ( )

Figure 1. (A) Powder X-ray diffraction (XRD) patterns of the synthesized MgO sorbents; (a) MgO-Cl (synthesized from MgCl2), (b) MgO-A (synthesized from Mg(CH3COO)2), (c) MgO-AC (synthesized from MgNH4Cl3), (d) MgO-N (synthesized from Mg(NO3)2), and (e) MgO-Al (purchased from Aldrich) sorbents. Figure 1. (B) Effect of the Mg(CH3COO)2 precursor molar concentration (0.2M−0.5M) on Powder X-ray diffraction (XRD) patterns of MgO nanoparticles.



28

0.012

MgO-N MgO-A MgO-AC

0.010

Pore Volume (cm³/g·nm)

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

Page 28 of 38

0.008

0.006

0.004

0.002

0.000 0

10

20

30

40

50

60

70

80

Pore Width (nm)

Figure 2. Pore size distribution of MgO-N, MgO-A, and MgO-AC samples.


90

Page 29 of 38

29

600

Quantity Adsorbed (cm³/g STP)

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


FSP-MgO-N FSP-MgO-Cl FSP-MgO-A FSP-MgO-AC

500

400

300

200

100

0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po) Figure 3. Nitrogen adsorption and desorption isotherms of the as-synthesized MgO sorbents.



30

0.9 180

0.7

150

0.6

140 0.5 130 0.4

120 110

3

2

160

Pore volume (cm /g)

0.8

170

Surface area (m /g)

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

Page 30 of 38

0.3 0.20

0.25

0.30

0.35

0.40

0.45

0.50

Molar Concentration

Figure 4. Effect of molar concentration (0.2M−0.5M) of the precursor Mg(CH3COO)2 solutions on pore volume and morphology of MgO nanoparticle sorbents.


Page 31 of 38

31

80

MgO-A

(mg CO2/g sorbent)

70

CO2 capture capacity

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


60

50

MgO-N

MgO-Cl

40

30

20

MgO-AC 10

Mg (NO3)2

MgCl2

Mg(CH3COO)2

MgNH4Cl3

Mg precursor

Figure 5. CO2 sorption uptake at 60 oC as a function of the MgO sorbent using flame spray pyrolysis technique synthesized with various magnesium precursors; MgO-N: Mg(NO3)2, MgO-Cl: MgCl2, MgO-A: Mg(CH3COO)2, and MgO-AC: MgNH4Cl3.



32

70 FSP MgO-A 60 50

(mg CO2/g sorbent)


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

Page 32 of 38

40 30 20 10 0 200

300

400

500

600

700

800

o

Preheating Temperature ( C)

Figure 6. Effect of preheating temperature on CO2 sorption at 60 oC over FSP-MgO-A sorbents; FSP-MgO: MgO synthesized by flame spray pyrolysis technique using Mg(CH3COO)2 precursor.


Page 33 of 38

33

70 FSP MgO-A 60 50

(mg CO2/g sorbent)


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


40 30 20 10 0 50

100

150

200

250 o

Adsorption Temperature ( C)

Figure 7. Effect of the sorption temperature on CO2 capture over MgO-A sorbent synthesized by flame spray pyrolysis technique.



34

24 CO2 sorption

60

70

22

Microstrain Stress

20 18 16 40 14 30

12 10

20

9

50

Microstrain () *10

-2

60

50

40

30

8 10

Stress () *10 (Pa)

70

CO2 sorption (mg/g)

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

Page 34 of 38

20

6 MgO-N

MgO-Cl

MgO-A

MgO-AC

Sorbent Figure 8. Correlation between CO2 capture at 60 oC with the strain in the crystal lattice () and the stress () of the as-synthesized MgO sorbents by flame spray pyrolysis technique.


Page 35 of 38

35

0.9

Micropore Volume External Surface Area

0.8 0.7

50

0.6 0.5

40 0.4 30

0.3 0.2

20 0.1 10

160 140 120 100 80 60 40 20

0.0

MgO-N

MgO-Cl

External Surface Area (m²/g)

60

180

-2

CO2 sorption

Micropore Volume (*10 cm³/g STP)

70

CO2 sorption (mg/g)

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


0

MgO-A

MgO-AC

Sorbent Figure 9. Effect of micropore volume and external surface area on CO2 capture at 60 oC of the as-synthesized MgO sorbents by flame spray pyrolysis technique.



Page 36 of 38

36

Table 1.

Calculated structural parameters of MgO nanoparticle sorbents prepared by adopting flame aerosol synthesis technique.

Sorbent

MgO-Na MgOClb MgO-Ac

Average crystallite size (D, nm) 28.57

Inter planar spacing (d, nm) 0.21

Lattice Unit cell parameter volume (a, nm) (V)*10-26 (m3) 0.420 7.44

Number of unit cells (n)*10-27 2.21

X-ray density (Dx) *103 (gm−3) 3.59

Dislocation Microstrain Stress density () () *10-2 () *109 15 −3 *10 (gm ) (Pa)

Stacking fault (SF)

1.22

7.25

21.7

0.40

9.45

0.21

0.418

7.34

6.60

3.64

11.20

21.94

65.8

0.40

9.29

0.21

0.420

7.40

6.77

3.61

11.59

22.32

66.9

0.40

0.21

0.419

7.39

2.70

3.62

1.85

08.93

26.7

0.40

0.20

0.417

7.29

1.09

3.66

0.31

03.65

10.9

0.40

23.22 MgOACd MgO-Ale 56.68 a d

MgO-N – MgO prepared from Mg(NO3)2; b MgO-Cl – MgO prepared from MgCl2; c MgO-A – MgO prepared from Mg(CH3COO)2; MgO- AC – MgO prepared from MgNH4Cl3; e MgO-Al – MgO purchased from Aldrich.


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


37

Table 2. BET surface area, pore diameter and pore volume measurements of the sorbents prepared by adopting flame aerosol synthesis technique with various precursors. Catalyst

SBET (m2/g)

Average pore diameter (nm)

Micropore Volume (*10-2 cm³/g)

Micropore Area (m²/g)

External Surface Area (m²/g)

Pore volume (cm3/g)

MgO-N a

13.5

24.6

0.063

1.96

11.51

0.088

MgO-Cl b

50.1

7.2

0.236

6.76

44.6

0.099

MgO-A c

177.2

17.1

0.823

23.48

153.7

0.792

MgO-AC d

117.8

11.5

0.129

−

115.7

0.382

a

FSP-MgO-N: FSP-MgO synthesized from Mg(NO3)2; b FSP-MgO-Cl: FSP-MgO synthesized from MgCl2; c FSP-MgO-A: FSP-MgO synthesized from Mg(CH3COO)2; d FSP-MgO-AC: FSP-MgO synthesized from MgNH4Cl3 by using flame spray pyrolysis method.

37 ACS Paragon Plus Environment


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Table of Contents (TOC) Graphic

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Facile Synthesis of Flame Spray Pyrolysisâderived Magnesium Oxide

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