CaO Nanoparticles Coated by ZrO2 Layers for Enhanced CO2

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CaO nanoparticles coated by ZrO2 layers for enhanced CO2 cap-ture stability Kazi Saima Sultana, D. Trung Tran, John C. Walmsley, Magnus Rønning, and De Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00423 • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 18, 2015

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Industrial & Engineering Chemistry Research

CaO nanoparticles coated by ZrO2 layers for enhanced CO2 capture stability K. Saima Sultanaa, D. Trung Tranb, J. Charles Walmsleyc, M. Rønninga, D. Chena* Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Sem Sælands vei 4, NO-7491 Trondheim, Norway b Department of Physics, Sem Sælands vei 4, NO-7491 Trondheim, Norway c SINTEF Materials and Chemistry, Sem Sælands vei 4, NO-7491 Trondheim, Norway a

KEYWORDS. CaO CO2 sorbents, solgel, wet impregnation, surface coating, carbonation/decarbonation BRIEFS. Coating a ZrO2 layer on CaO nanoparticles significantly stabilized the solid CO2 sorbents in the presence of steam.

ABSTRACT. CaO is an attractive CO2 acceptor, due to its

good kinetics and high capture capacity, even under low CO2 concentration condition. The major problem associated with CaO acceptors is the loss of CO2 capture capacity during cyclic operations. The declining performance of the CaO acceptors is more pronounced in the presence of steam. In this paper we propose a method for fabrication of nano-CaO acceptors with a coated ZrO2 layer to improve the stability. The nano-CaO acceptor was prepared by thermaldecomposition method following a coating process by solgel, incipient wet impregnation or hydrolysis, respectively. The thermogravimetric analysis (TGA) of the cyclic process of CO2 carbonation/decarbonation revealed that the nanoCaO acceptors with sol-gel (CaO-SG) and wet impregnation (CaO-IM) coating have higher CO2 capture capacity and a longer life time than the uncoated ones. The capture capacity of CaO-IM declined after 14 cycles whereas CaO-SG remained stable upto 20 cycles. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectra were used to characterize the coated and uncoated nano-CaO structures. Combined characterization techniques showed that a CaZrO3 layer was formed around the nano-CaO particles by sol-gel coating. Multi-cycle carbonation/decarbonation processes were performed in absence and presence of steam in CO2 capture kinetic experiments as well as at the sorption enhanced reforming conditions, and CaO nanoparticles with surface CaZrO3 layer showed a promising cyclic stability. INTRODUCTION

CO2 capture by carbonation of solid materials has drawn significant attention to the academia since it provides a viable approach of capturing even at high temperatures and in the presence of steam. Although many materials could be utilized for CO2 capture by the carbonation reaction, calcium oxide is a potential candidate due to its high CO2 capture capacity and low cost.1-4 Nano-CaO particles have drawn increasing attention in terms of advantages in comparison to micro-CaO. These advantages include higher reactive sorption capacity, fast reaction rate, and an enhanced durability of the acceptor.5 However, the CO2 sorption capacity of CaO decreases significantly after few cycles of carbonation/decarbonation6-8 and, especially, deteriorates at a rapid rate in the presence of steam.9 This is caused by volumetric expansion and reduction of the acceptors. The rapid deactivation of CaO acceptors is typically governed by collapse of the pores during cycles of carbonation/decarbonation, and aggregation of particles due to existing molten phase of car-

bonate at high temperatures. Many approaches have been devoted to increase the acceptor stability. Among them using CaO based composites such as mixed oxides 10-12 and coating a oxide layer on nano-CaO particles13, 14 are prevalent strategy to enhance the cyclic stability. Nanocoating is performed either to cover a core material with a layer on the nanometer scale or to cover a nanoscale entity.15 Composite particles that contain an inner core covered by a shell (core-shell particles) exhibit significantly different properties from those of the core itself, thereby producing a material that generally has enhanced or specific properties due to the combined properties and/or structuring effects of the components. Electrostatic interactions, hydrogen bonding, and covalent bonding are some of the associating forces between the coating and the material being coated. Coating in solution is generally performed using either precursor molecules or preforming particles to form the layer. There are numerous coating methods such as sol−gel, 16-20 spray drying,21 impregnation, hydrolysis,22-24 deposition-precipitation,25 chemical vapour deposition.26 Nonetheless, the formation of a complete shell around the core particles was still hard to achieve. Different composite materials containing CaO have been fabricated to minimise the declination of CO2 sorption ability upon cyclic adsorption and desorption.12, 24, 27-31 Aihara developed a sample composed of CaTiO3 and CaO prepared by an alkoxide method.24 CO2 adsorption ability was improved in carbonation/decarbonation cycles. Wu et al. prepared CaTiO3-coated nano-CaO acceptor by forming Ti(OH)4 from the hydrolysis of titanium alkoxide in a nanoCaCO3 suspended solution.13 Yanase et al. coated on aluminosilicate foam and have found that CaO coated foam showed good adsorption ability and a long life time for the cyclic process of CO2 adsorption and desorption.14 Zhao et al.32 prepared CaO based sorbents by incorporating different inert materials (Ca2SiO4, Ca3Al2O6, CaTiO3, CaZrO3, and MgO) by citrate sol–gel method. Among them, CaO-based acceptor stabilized by CaZrO3 (29.1 wt %) had a stable CO2 capture capacity of 0.45 g-CO2/g-acceptor over 30 consecutive cycles. Radfarnia et al developed a CaO sorbent stabilized by Zr with improved thermal stability.33 They incorporate high temperature resistant calcium zirconate (CaZrO3) particles into the CaO structure retarding the sintering of CaO particles and increase the stability of the sorbent during cyclic carbonation/decarbonation. Radfarnia et al also developed new bifunctional sorbent- catalyst CaO-Zr/Ni (13, 18, and 20.5 wt % NiO) using the wet-mixing/sonication technique. They found that bifunctional sorbent–catalyst with

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20.5 wt% NiO loading showed superior activity and 92 % H2 was produced in sorption enhanced steam reforming (SESR).34 However, even though the different coating methods were applied to increase durability during carbonation/decarbonation cyclic runs, the stability of CaO-based CO2 acceptors is still too low. Moreover, the morphology of these coated composites have not been characterized, which depends significantly on the preparation method as shown later, making it difficult to assess the relationship between the structures and CO2 capture performance. The objective of the present work is to address the relationship between the composite structure and CO2 capture performance, with an aim of improving the stability and reversibility of the CaO-ZrO2 nano-acceptors. The morphology and structure of the composites were varied by different methods for coating a layer of ZrO2, such as sol-gel, incipient wet impregnation and hydrolysis. ZrO2 was selected as the external protect layer due to its high mechanical strength and high thermal stability. The acceptors were screened firstly by temperature programmed carbonation/decarbonation cycles. The stability of selected acceptors was examined in presence of CO2, steam as well as SESR conditions. Here we report the synthesis of high temperature CO2 acceptors based on CaO nanoparticles with a surface layer of CaZrO3 which was evidenced by combined techniques of SEM, TEM, Raman spectra, BET and XRD analyses. The CaZrO3 protecting layer suppresses significantly the solid reactions between CaO and thus the sintering of CaO, thus providing superior cyclic stability. EXPERIMENTAL Materials. Calcium nitrate tetra hydrate, Ca(NO3)2.4H2O (≥ 99 %, Aldrich), sodium hydroxide, NaOH (99 %, Merck), ethylene glycol, EG(≥ 99.5 %, Fluka), zirconium IV propoxide (70 wt% w/w in n-propanol , Aldrich), polyvinylpyrrolydine (K-30, Aldrich), zirconium oxychloride octahydrate, ZrOCl2.8H2O (≥ 99 %, Aldrich). Preparation of CaO Nanoparticle of CaO was prepared by thermal decomposition method. 118 g Ca(NO3)2. 4H2O was dissolved in ethylene glycol solution (250 cm3). 18 g NaOH was added into the mixture under vigorous stirring for 24 h. Here ethylene glycol act as solvent medium and NaOH as precipitant.35 The solution was then kept under static condition for about 5 h. The precipitate was filtered from solution and washed using deionized water. Hence, we obtained CaO nanoparticles. The nano-CaO were dried at 373 K for 1 day and calcined at 1023 K in the presence of air in a calcination oven for 3 h. Coating of nano CaO Solgel. The sol-gel procedure allows coating of templates with complex shapes on the micrometer to nanometer scale, which cannot be achieved from other commonly used coating methods. A major obstacle in preparing ZrO2 coating from zirconium alkoxides is the rapid hydrolysis and subsequent precipitation of colloidal zirconia upon water addition to Zr(OR)4-containing precursor solutions. Such a fast pre-

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cipitation causes difficulties in sol-gel coatings. To stabilize the Zr(OPr)4 precursor, acetic acid was used as a complexing agent to control the rapid hydrolysis.36, 37 This allows an increasing of the metallic atoms coordination by formation of acetate bridges and control alkoxide, Zr(OPr)4 hydrolysis. The sol-gel coating was carried out using zirconium (IV) n-propoxide, Zr(O-n-Pr)4 (70 wt% in propanol solution) as a precursor, acetic acid as an additive and propanol as solvent to prepare homogeneous sols. Molar ratio between CaO/ZrO2 = 10. At first, 3 g CaO-nanoparticles were dispersed in absolute alcohol (99 % ethanol) by ultrasonication for 30 min. 1 g of polyvinyl pyrrolidone (PVP) was added to the CaO suspension and was stirred with magnetic stirrer for 1 h. Here, PVP act as capping agent to cover the surface of CaO nanoparticles.38 Zr-n propoxide was diluted with npropanol (molar ratio: n-PrOH/Zr(O-n-Pr)4 = 20). Acetic acid [molar ratio, CH3COOH /Zr(O-n-Pr)4 = 4] was slowly added under vigorous stirring to the Zr(O-n-Pr)4 solution at room temperature. After that the solution was fed slowly into the flask containing CaO-nanoparticles suspension for 1 h. Deionized water [molar ratio, (H2O/Zr(O-n-Pr)4 = 4] was added drop wise and finally the solution was refluxed for 2 h at 353 K with continuous stirring. Thereafter, the coated sample was separated and washed with absolute alcohol (99 % ethanol) and dried at 373 K for 1 day. Finally coated sample was calcined at 1073 K in the presence of air for 3 h. The resulted samples were denoted as CaO-SG. Incipient wet impregnation. The incipient wet impregnation method is based on the sol-gel principle. Incipient wet impregnation method was employed to coat the zirconia layer on the surface of nano-CaO. 3 g of CaO nanoparticles (Molar ratio, CaO/ZrO2 = 10) were impregnated with 70 wt % zirconium (IV) n- propoxide solution in 1-propanol. During impregnation zirconium (IV) n-propoxide solution in propanol, forms a complex structure with the OH groups on the surface of CaO. The sample was kept at room temperature for drying. The impregnated coated sample was calcined in a static air at the rate of 4 K/min to 1073 K for 3 h. The resulted samples were denoted as CaO-IM. Hydrolysis. In this method, ZrOCl2·8H2O was employed as the precursor. Solid ZrOCl2·8H2O was first dissolved in 100 cm3 of deionized water to produce 0.2 mol/L solution under magnetic stirring. Then 2 g of nano-CaO (Molar ratio, CaO/ZrO2 = 5) were added into the aqueous solution. The solution was sonicated for about 1 h to get homogeneously dispersed solution. The stable aqueous suspension was then refluxed in a thermostatic oil bath at 363 K for 24 h. After completion of reaction, the resultant particles were filtered by vacuum filtration and washed with deionized water and then dried at room temperature for overnight. The product was calcined at 1073 K for 3 h. The resulted samples were denoted as CaO-HD. Characterizations and Testing X-ray Diffraction. All the resulted solid products were characterized by XRD to identify the phases and estimate the crystal sizes. X-ray diffraction patterns were recorded at room temperature on D8-focus X-ray diffactometer using CuK radiation (λ= 1.540 A). The X-ray tube voltage was set to 40 kV and the current to 50 mA. The scans were recorded in the 2θ range between 20 and 900 using a step size of 0.030.The diffractograms were compared with standards in a database (EVA) for phase identification.39


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Surface area and pore size. The measurements were carried out by Tri Star 3000 instrument. The samples were outgassed at 473 K for overnight prior to analysis. The specific surface area was calculated by Brunauer- Emmet-Teller (BET) equation.40 The total pore volume and pore size distribution were calculated based on Barrett-Joyner-Halenda (BJH) model.41 Scanning electron microscopy (SEM). SEM pictures were collected by using FE-SEM Zeiss Ultra 55LE scanning electron microscope. Secondary electron images were obtained using an accelerating voltage of 5 kV. Transmission electron microscopy (TEM). TEM was performed in a JEOL 2010F microscope at an acceleration voltage of 200 kV equipped with an Oxford Instruments INCA X-ray detector for energy dispersive spectroscopy (EDS). The samples for TEM were prepared by dispersing a small amount of powder in ethanol using an ultrasonic bath and placing a drop of the liquid on holey carbon film supported on Cu-grid. Conventional TEM images were recorded on a CCD camera. Raman spectroscopy. FT-Raman spectra were collected on a Horiba Yvon LabRAM HR800 spectrometer. The emission lines at 633 nm were focused on the sample with a 100x objective from a He-Ne laser and 325 nm were focused on the sample with a 40NUV objective. No filter was used during the measurement. Carbonation and decarbonation cycles in thermogravimetric analysis (TGA). TGA was carried out using a TG analyzer (Netzsch STA 449 F1). The kinetics and capacity of carbonation/ decarbonation of CO2 acceptors were measured by temperature programmed reactions with heating and cooling rates of 10 K/min and 50 K/min in a temperature range from 473 K to 1173 K. A standard condition at CO2 pressure of 0.8 bar for screening the acceptors using TGA in our laboratory was employed in the present work. The flow of CO2 during carbonation was set to 80 cm3/min. A total flow of 100 cm3/min was fed through the sample compartment by purging Ar. Decarbonation was performed in presence of pure Ar flow (100 cm3/min). The stability CO2 acceptors were tested by repeated multi-cycle carbonation/decarbonation reactions. Carbonation kinetics in CO2/H2O mixture in fixed bed reactor (FB). CO2 carbonation and decarbonation experiments were carried out in a stainless steel reactor of 1 cm diameter. A small amount of acceptor (0.3 g) was installed in the reactor. The reactor was placed into a furnace equipped with a temperature controller. Carbonation was carried out in presence of CO2, steam and N2 at 843 K. Online chemical analysis of CO2, N2 at the exit of the reactor was achieved by a gas chromatograph (Agilent 3000). N2 was used as the internal standard. The partial pressure of CO2 and H2O were 0.3 bar and 0.4 bar, respectively. Sorption enhanced steam methane reforming in fixed bed reactor (FB). Sorption enhanced steam methane reforming (SESMR) was performed in a stainless-steel fixed bed reactor with inner diameter of approximately 16 mm. The reforming reactions were carried out at 843 K and 1 bar, a steam-to carbon ratio of 3, and the acceptor-to-catalyst ratio of 5 ( 1g 40Ni-HTlc and 5 g CaO-SG). The flow of CH4 was 30 cm3/ min. A mixture of nickel hydrotalcite derived catalyst (40Ni-HTlc)42 and sol-gel coated CaO acceptor

(CaO-SG) was placed in the reactor. Particles sizes of both the acceptor and catalyst were within the range of 250-500 µm. The selection of 40Ni-HTlc catalyst with 40 wt% Ni loading here is based on our previous evaluation of Ni catalysts in the methane steam reforming activity where the 40Ni-HTlc catalyst presented the highest activity per weight of the catalyst.42 The Ni dispersion and surface area in 40NiHTlc were found 10.8 % and 28 m2 Ni/gcat respectively. It has been applied in SESMR previously able to generate high purity of hydrogen.11, 42, 43 Prior to reaction, the samples were reduced at 943 K for 10 h in H2/N2 = 100/100 cm3/min. A heating rate of 3 K/min was used to increase the temperature from ambient to 943 K. After reduction, the temperature was decreased to 843 K and the reactive gases were introduced in the reactor. Reaction was continued until the acceptor was saturated, which corresponds to the breakthrough of the CO2 concentration in the product stream. At this point, the CH4/steam mixture was switched to flow of N2 (100 cm3/min) only and the temperature was increased to 973 K. At the same time H2 flow of 20 cm3/min was fed to keep the Ni in metallic state. Multi-cycle experiments were carried out in order to study the stability of the process. The effluent gas was analyzed with Agilent 3000 micro gas chromatograph (GC) equipped with a Plot U and Molsieve molecular sieve column and the concentrations of H2, CO, CO2 and CH4 were normalized to 100 %. The CO2 uptake value for each cycle was calculated from the breakthrough curve for CO2 using following equation. The following equation was used to calculate the capacity for the cycles. t1

N CO2 = ∫ (FCO2 , SR − FCO2 , t )dt …………… (1) 0

Here, FCO2,SR = average value of CO2 flow rate during the course of the conventional steam reforming reaction, FCO2,t = CO2 flow rate at time t; t1 represents the time point where the CO2 content in the gas effluent did not increase any more after the breakthrough. RESULTS AND DISCUSSION The X-ray diffraction pattern of the bare and coated nanoCaO particles are shown in Figure 1. Nanoparticles coated by sol-gel and incipient wet impregnation show peaks for CaO, CaZrO3 and less prominent ZrO2 peak, respectively. It is consistent with the observation of Yi et al., which indicates that CaZrO3 is readily formed rather than remaining as separate phase when CaO and ZrO2 are exposed to elevated temperature.44 Figure 1 Figure 1. X-ray Diffraction (XRD) spectra of nano-CaO and coated CaO acceptors by sol-gel (CaO-SG), incipient wet impregnation (CaO-IM) and hydrolysis (CaO-HD).

In case of hydrolysis coating, no CaO peak is observed, CaZr4O9 pervoskite is found instead. This could be due to the formation of HCl upon dissolution of ZrOCl2·8H2O in deionized water 22 as described in Eq. (1):


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(2)

Figure 3. Pore size distributions of uncoated nano-CaO acceptor and coated nano CaO (CaO-SG, CaO-IM and CaO-HD).

The HCl formed in-situ dissolves CaO particles and gives Ca2+ ion in solution. The Ca2+ ions react with Zr(OH) 4 in reaction media and form CaZr4O9.

Table 1 Physical properties of coated and uncoated nano-CaO acceptor.

ZrOCl + 3H O → Zr (OH ) + 2 HCl 2 2 4

The crystal size of CaO calculated using Scherrer’s equation is about 42 nm for bare nano-CaO particles, 38 nm in the sol-gel coated composites and 40 nm in the incipient wet impregnation coated samples. The CaO crystal sizes in the coated composites are found to smaller than that the bare CaO nanoparticles, most likely due to the formation of CaZrO3 layer on the surface, as indicated by XRD spectra (Figure 1). The crystal sizes of nano-CaO in CaO-SG are smaller than in CaO-IM, which is consistent with a strong signal of CaZrO3 in CaO-SG than CaO-IM. The estimated CaO crystal sizes are in good agreement with the values measured by SEM micrographs (Figure 2). Figure 2 (a), ( b), (c), (d), (e), (f) Figure 2. SEM and TEM imaging of uncoated and coated nanoCaO. SEM image of uncoated nano-CaO (a), sol-gel coated (CaO-SG) (b), incipient wet impregnation coated (CaO-IM) (c) and hydrolysis (CaO-HD) (d), TEM images of CaO-IM (e) and CaO-SG (f).

SEM images in Figure 2b and 2c illustrate that the morphology of aggregates was changed by coating, and few particles are connected together possibly by a coating layer. The average particle size within the aggregates (Figure 2a-c) was in the range of 40-50 nm. The average sizes of coated samples are slightly larger than the uncoated CaO. However, it does not allow us to analyze precisely the difference in the average size between the coated and uncoated samples due to their irregular shapes. The SEM image (Figure 2a) reveals spherical like CaO particles, although the shape is not perfect. The SG and IM coating only partially distorts the particle shape, as shown in SEM (Figure 2b and 2c) and TEM (Figure 2e and 2f) images. However, hydrolysis coating changes completely the shape of the particles (Figure 2c), where the spherical shape disappears and very dense large crystals formed, due to a formation of new phase of CaZr4O9 as identified by XRD (Figure 1). Surface area, pore volume together with the crystal sizes of CaO of bare CaO particles and coated nano-CaO are summarized in Table 1. Surface area and the pore volume of nano-CaO increase after sol-gel and incipient wet impregnation coating. CaO-SG has higher surface area (24.7 m2/g) and pore volume (0.07 cm3/g) compared to CaO-IM (18.6 m2/g and 0.04 cm3/g). The surface area and pore volume of nanoCaO on coating by hydrolysis is very low (3.9 m2/g and 0.006 cm3/g), consistent with a dense structure shown in SEM images (Figure 2c). Pore size distributions of each samples measured by N2 adsorption are plotted in Figure 3. Large pore sizes in the pore size range between 15-40 nm are observed for CaO-SG and CaO-IM compared to nano-CaO. No micropores were detected in all the samples. The pore size distribution (Figure 3) suggests non porous structure of CaO nanaoaprticles and the large pores are generated between the nanoparticles, as also revealed in SEM (Figure 2a-c) and TEM (Figure 4e and 4f) images. Figure 3

Acceptor

Surface area

Pore volume

Pore size range

Crystal size CaO

(m2/g)

(cm3/g)

(nm)

(nm)

CaOnano

10.8

0.02

1.7- 46.3

42

CaO-SG

24.7

0.07

1.8- 68.3

38

CaO-IM

18.6

0.04

1.8- 57.8

40

CaO-HD

3.9

0.006

1.7- 44.5

-

•

The crystal sizes were measured by XRD

To study surface morphology and explore the structural features TEM studies were performed. The results obtained are presented in Figure 4(a-f). High-Angle-Annual-DarkField Scanning TEM (HAADF-STEM) images illustrated spherical-like particles with relatively uniform size distribution for nano-CaO particles (Figure 4a). The EDS-element mappings of Ca and Zr (Figure 4e-f) show that Ca and Zr are randomly distributed through the particles of CaO-SG. No separated CaO and ZrO2 particles were observed in the TEM investigation. It provides clear evidence that all the CaO nanoparticles were successfully coated by sol-gel method. In CaO-SG sample, the HAADF-STEM image with EDS linescanning represents two clusters of Ca and Zr-containing compounds (Figure 4c). Although the noise level for the EDS signal for element distribution was high, it can be observed that the ratio of Ca/Zr is higher than 1 at the center of the particles, while the Ca/Zr ratio is close to 1 at the edge. XRD (Figure 1) results indicated the presence of CaZrO3 in addition to CaO and ZrO2 in the CaO-SG sample. Therefore, TEM-EDS results possibly suggest a core-shell structure with CaZrO3 as the shell. However, TEM presented here could not conclusively proof the core-shell structure of the nanoparticles. Figure 4 (a), (b), (c), (d), (e) , (f) Figure 4 HAADF-STEM EDS analysis: (a and b) nano-CaO and the corresponding Ca map, respectively, (c and d) two clusters of CaO-SG with an EDS scanning line crossing from left to right and their corresponding EDS signals of calcium (red) and zirconium (cyan), (e and f) some clusters of CaO-IM and the corresponding EDS element maps of calcium (red) and zirconium (green), respectively.

The possible core-shell structure was further examined by the Raman spectroscopy study using two lasers in the near ultraviolet (325 nm) and visible range (633 nm). The use of these two lasers emitted at different wavelength may give surface and bulk information about the samples. UV-Raman spectroscopy is a surface region sensitive technique where the surface semiconductor materials adsorb strongly the UV light and suppress the penetration of the UV –light to the bulk,45-48 while visible Raman spectroscopy presents bulk


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properties. A comparison of the UV and visible Raman spectra provides information on species distribution. 45-48 For visible Raman of CaO-SG and CaO-IM samples (Figure 5a), the band near to 357 cm-1 is due to the vibration of the Ca-O bond in CaO, 714 cm-1 for ZrO2 and the bands at 211, 262, 284, 357, 438, 546 cm-1 correspond to well-known intense bands for CaZrO3.49, 50 Presence of CaO, ZrO2 and CaZrO3 phases confirmed from visible Raman spectra (Figure 5a) are in good agreement with the phases (CaO, ZrO2 and CaZrO3) observed in XRD pattern (Figure 1). Figure 5 (a) and Figure 5 (b) Figure 5 (a) Visible Raman results for coated samples by sol-gel (CaO-SG) and wet impregnation (CaO-IM). Spectrum recorded at 633 nm in the Raman shift interval between 200 and 900 cm-1. (b) UV-Raman spectra for CaO-SG and CaO-IM. Spectrum recorded in the range 200 and 900 cm-1(Laser excitation at 325 nm).

Different observation is found in near UV Raman spectra ((Figure 5b) comparing to visible Raman spectra for CaO-SG and CaO-IM. However, near UV-Raman for CaO-SG and CaO-IM samples indicate the presence of prominent peak at 284 cm-1 corresponding to CaZrO3.49 Raman band at 714 cm-1 in both samples correspond to ZrO2 phase. 46, 51 Therefore, absence of Raman bands for CaO in case of CaO-SG could be related to the formation of an oxide layer on CaO, forming a core-shell CaO@CaZrO3. Whereas in near UV-Raman of CaO-IM sample, bands corresponding to CaO (357 cm-1), CaZrO3 (284 cm-1) and ZrO2 (714 cm-1) are observed which indicates that CaZrO3 layer did not cover whole CaO nanoparticles and the incipient wet impregnation coating could not synthesize a perfect core-shell structure. Figure 6 (a), Figure 6 (b) and Figure 6 (c) Figure 6 Schematic representation of three different possible structures after coating. (a) Incipient wet impregnation (CaOIM), (b) Sol-gel (CaO-SG) and (c) Hydrolysis (CaO-HD).

Together with the SEM and TEM results, the comparison between the XRD, visible Raman and near UV-Raman spectra confirm that sol-gel, wet impregnated and hydrolysis coated samples resulted in three different structures, which are schematically represented in Figure 6. The relatively good core-shell like structured CaO@CaZrO3 nanoparticles was produced by sol-gel coating of CaO nanoparticles. In the present work, no porous CaO nanoparticles (NPs) were used as starting materials following coating by the sol-gel method. We have not observed separated CaO and ZrO2 particles in the coated sample, suggesting a successful coating procedure. The nominal mole ratio of Zr/Ca used here is low (0.1). It is expected that a thin layer of ZrO2 was coated on the CaO NPs, following a transformation to CaZrO3 layer through a solid reaction at the interface between the ZrO2 and CaO during the calcination at high temperatures. The maximum molar fraction of CaZrO3 in the whole NPs is about 10 %. Owing to the nature of the solid-state reaction between ZrO2 and CaO, CaZrO3 was formed preferably on the external surface of CaO NPs, thus generated core-shell like structure.

the detailed structure information will be discussed late. Next we will investigate the effects of the composite structures on the kinetic performance of CO2 capture and effects on cyclic stability will be addressed. Figure 7 Figure 7 Visible and UV- Raman results for coated sample (CaOSG) after cycling in the presence of steam. Spectrum recorded in the Raman shift interval between 200 and 4000 cm-1(Laser excitation for visible and near UV-Raman are 633 nm and 325 nm, respectively).

Multi-cycle performance at dry conditions. The reactivity and CO2 capture capacity of CaO acceptors were evaluated by measuring the weight gain as a function of temperature in the temperature programmed carbonation at constant CO2 pressure in the absence of steam. The increase in weight is caused by the carbonation of CaO, leading to the formation of CaCO3. A comparison of the multi-cycle sorption capacity of nano-CaO and coated CaO by sol-gel and by incipient wet impregnation is shown in Figure 8. None any capacity of CO2 capture is found on CaO-HD, indicating that CaZr4O9 is not active for carbonation reactions. Therefore, the CaO-HD sample was not included in the comparison. Figure 8 Figure 8 CO2 sorption capacities of nano-CaO acceptor before and after coating: Sol-gel (CaO-SG) and incipient wet impregnation (CaO-IM). Temperature programmed range = 473–1173 K, heating rate: 10 K/min, cooling rate: 50 K/min, carbonation: FCO2 = 80 cm3/min (Ar balance, PCO2 = 0.8 bar) and decarbonation: FAr = 100 cm3/min.

It has been observed from Figure 8 that sorption capacity of nano-CaO is initially high (0.70 g-CO2/ g-CaO) with 90 % conversion of CaO. The initial capacity follows an order: nano-CaO > CaO-IM > CaO-SG. The low initial CO2 capacity of the coated acceptors is a result of the formation of CaZrO3. A higher extent of CaZrO3 might be formed in CaO-SG than CaO-IM, consisting with the fact that CaO crystal size measured by XRD is smaller for CaO-SG than CaO-IM. The CO2 capacity of nano-CaO decreases concurrently with increasing cycle number, and the most significant decay in the capacity occurred in the first five cycles. After 20 cycles, the reactive sorption capacity of nano-CaO is much lower (0.29 g-CO2/gCaO) comparing to CaO-SG and CaO-IM (0.65 g-CO2/gacceptor and 0.60 g-CO2/g-acceptor, respectively). The stability of CaO-SG in multi-cycle carbonation/decarbonation is excellent, and no obvious deactivation is observed at dry conditions (Figure 8), while the stability of CaO-IM decreases from 14th cycle (0.64 g-CO2/g-acceptor) to 20th (0.60 gCO2/g-acceptor) but its stability as well as capacity is much higher than nano-CaO after 20th cycle. The better stability of CaO-SG than CaO-IM might be a result of the better protecting layer (CaZrO3) formed by sol-gel coating method. The formed protect layer suppressed solid reaction between CaO particles, which cause severe sintering of CaO particles without protecting layer during cyclic operation.24

The used sol-gel coated CaO@CaZrO3 after cyclic carbonation and calcination in the presence of steam was also examined by visible and UV-Raman spectroscopy. The spectra in Figure 7 shows the frequency range of 200-4000 cm-1, and


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Rates of weight change are plotted as a function of temperature in Figure 9 in the temperature programmed carbonation reaction on CaO-SG in TGA. Figure 9 shows that there is a negligible change in the rates of weight change for CaO-SG for different cycles. The rate of weight changes is almost same from 2nd to 20th cycle. It confirms excellent stability of CaO-SG in CO2 capture. Figure 9 Figure 9 Rate of weight change versus temperature for multicycle carbonation/decarbonation under dry condition of sol-gel coated CaO (CaO-SG).

Multi-cycle performance in CO2/H2O mixture: Steam has long been recognized to have great impact on the capture kinetics, regeneration property and on the stability of CO2 acceptors52-54 The CaO-SG acceptor, which was screened out as the most stable acceptor at the dry conditions, was selected for testing in multi-cycle of carbonation/decarbonation in the mixture of CO2 / H2O (PCO2 =0.3 bar and PH2O =0.4 bar), as well as sorption enhanced methane reforming in a fixed bed reactor. The uptake curves are presented in Figure 10 as a function of the carbonation time for the each cycle for CaO-SG sorbent at CO2 (PCO2 = 0.3 bar) and H2O pressure (PH2O = 0.4 bar). Figure 10 shows an increase in carbonation rates with increasing cycles up to 35 cycles of carbonation/decarbonation. However, it is difficult to draw a conclusion on the stability in terms of the CO2 capture capacity, since the acceptor did not reach complete saturation for the cycles with the low cycle numbers. Anyhow, the change in the capacity is relatively small. After 35 cycles the acceptor became stable and no obvious falling in the carbonation rate and capture capacity during 35 to 40 cycles. The low carbonation of CO2 is possibly limited by the diffusion rate of CO2 through the coated CaZrO3 layer to the CaO core. However, the concomitant increase in carbonation rate is found with increasing carbonation/decarbonation cycles from 1st to 40th cycle. This could be due to some structural changes of CaZrO3 film around nano-CaO nanoparticles during cyclic carbonation/decarbonation process which could lead to faster carbonation rate, but kept the stable capture capacity. The results reveal a promising hydrothermal stability of the CaO@CaZrO3 composites. Raman spectra of CaO-SG (Figure 7) after cyclic steam treatment prove that Ca(OH) 2 formed during the cycle in the presence of steam, Vibration modes 3535 cm-1 in visible region and at 3526 cm-1 in UV visible corresponds to a -OH-stretching mode for Ca (OH)2. 55The peak is very intense on the UV-Raman, indicating surface rich with Ca (OH) 2. Frequency in the range 700–900 cm-1 corresponds to bending of CO32-. The frequency between 10001400 cm-1 is related to CO32-, from CaCO3.55 It means that the CaCO3 are still remained mostly in the core of the material. Moreover, it suggests that the CaZrO3 layer could be broken down to a more open layer due to volume expansion and shrinking of the acceptor during the carbonation/decarbonation cycles. Therefore CaO become more accessible to CO2 in the gas phase, thus faster carbonation rates. At the surface CaO seems to react with steam to form Ca(OH)2. More open shell layer made Ca(OH)2 detectable by near UV-Raman spectroscopy. Figure 10

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Figure 10 Multi-cycle carbonation/decarbonation of sol-gel coated nano-CaO (CaO-SG) under wet condition. Carbonation: 843K, FCO2= 30 cm3/min, FN2= 30 cm3/min, FH2O= 19.6 g/h. PCO2 = 0.3 bar, PH2O = 0.4 bar. Decarbonation: 973 K, FN2 = 100 cm3/min.

It should be noted that the CO2 capture capacity and stability of the acceptor in the steam is not directly comparable to one at the dry conditions as shown in Figure 8, since the CO2 pressure and experimental conditions are very different. The lower capacity of the CO2 capture in the CO2/steam mixture is about 0.35 gCO2/g-acceptor, which is much lower than one at the dry conditions (0.65 g-CO2/g- acceptor). This is consistent with the relationship between the capacity and CO2 pressure reported previously, where the capacity increases with increase in CO2 pressure. H2 production with CO2 sorption in steam methane reforming (SESMR). SESMR has been carried out by using CaO-SG acceptor and 40Ni-HTlc catalyst. The H2 and CO2 evolution is plotted as a function of time on stream for different cycles (Figure 11a). It has been observed that the H2 concentrations in both pre- and post-breakthrough periods is constant (98 % and 78 %, respectively), regardless of the cycle number. The sorption enhancement is clearly demonstrated in the pre-breakthrough comparing to the postbreakthrough period where the acceptor becomes saturated. After saturation of the acceptor, it was regenerated at 973 K in the presence of N2 flow (100 cm3/min) along with small H2 flow (20 cm3/min) for 120 minutes. Later, the cycle was repeated several times to assess the stability of the process. The H2 and CO2 concentration remained similar in the consecutive cycles, but the pre-breakthrough period representing the occurrence of the SESMR reaction is increased from the first cycle to 20th cycle. The estimated CO2 capture capacities based on the CO2 breakthrough curves are plotted in Figure 11b and the CO2 capture capacity concomitantly increase with increasing cycle number. The phenomenon is very similar to the cyclic performance observed in the CO2/H2O mixture (see Figure 10). However, the initial CO2 capacity is much lower than one measured in the CO2 / H2O mixture. As we reported previously, the CO2 capture capacity increases significantly with increasing CO2 pressure in the mixture. It indicates that the effective conversion or effective utilization of acceptors depends on the CO2 pressure. Recently we have developed a new model of CO2 reacting with calcium oxide to properly describe the CO2 pressure dependency of the CO2 capture capacity, which will be reported elsewhere. Similar to the mechanism of the carbonation reaction of Li2ZrO3,56 three elementary reaction steps are involved in the carbonation reaction, namely CO2 reaction with surface O2- ions to form CO32-, reaction of CO32- with CaO at the interface to form CaCO3 and O2-, and count diffusion of diffusion of O2- and CO32- in the CaCO3 layer. The equilibrium of the reaction at the CaO interface determines the effective conversion of CaO in the carbonation reactions. The higher CO2 pressure in the gas phase could result in a higher concentration of CO32- at the interface, thus a higher CaO conversion. It explains well the observed CO2 capacity following a CO2 pressure order (0.8 bar at dry condition, 0.3 bar in CO2/H2O mixture and much lower in the SESMR conditions). The presence of CaZrO3 coated layer which limits CO2 to diffuse through the coating film formed around it. However, the kinetic behavior appears different in the CO2 capture


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kinetic experiments and in SESMR. The CO2 capture capacity increases only slightly, although the carbonation rate increases more significantly with increasing cycles in the CO2 capture experiments at CO2 pressure of 0.8 bar (Figure 9). The CO2 capacity increases almost linearly with cycle number in SESMR, but the capacity is still lower than one obtained in CO2 capture experiments. It could be a result of low CO2 pressure at the SESMR conditions and thus low carbonation rates. It seems that the CO2 capture capacity depends on the carbonation rates. The increased carbonation rate increases the CO2 capture capacity (Figure 11b). The capacity increases even after 20 cycles. Figure 11 (a) and Figure 11 (b) Figure 11 (a) The evolution of H2 and CO2 contents in the repeated cycles during SESMR reactions with CaO-SG. Conditions: 5 g acceptor, 1 g 40Ni-HTlc catalyst, T: 843 K, S/C: 3, FCH4 = 30 cm3/min. Regeneration: 973 K, FN2 = 100 cm3/min, H2 = 20 cm3/min. (b) CO2 capture capacity of CaO-SG ( ) in SESMR and 8 cycles for dolomite (▲)in sorption enhanced steam reforming of ethanol (SESRE) 57.

The CaO-SG performs totally different from the natural dolomite.57 The properties of the natural dolomite and its performance in sorption enhanced reforming of ethanol have been reported previously. Although it was tested in differnet reactions, the adsorption capacity are expected to be simialr for the differnet sorption enahnced reactions. For a comparison, the data for dolomite is also included in Figure 11b. The initial CO2 capture capacity of the natural dolomite is much higher, but deactivates very rapidly. It should be mentioned that the CaO-SG has not been optimized. Anyhow, the results clearly demonstrate that the core-shell structure could be a valuable design of CaO based acceptors for improving the stability. In addition, it should be noted that hydrogen was added during the regeneration of the acceptors to avoid the oxidation of 40Ni-HTlc catalyst by CO2 in the present work, to simply the experiments. In practice, it can be performed in a two sequential steps, namely regeneration and reduction in hydrogen containing gas mixture. We have recently reported that the reduction step can be eliminated by employing multifunctional Pd-Ni-Co catalysts.58, 59 CONCLUSIONS In order to improve the CO2 sorption capacity, nano-CaO acceptors prepared by thermal-decomposition were fabricated with coating by means of sol-gel, incipient wet impregnation and hydrolysis methods. Prepared acceptors were characterized using XRD, SEM, TEM and Raman spectra. The CO2 sorption capacity of both coated and uncoated nanoCaO acceptors was tested at both dry and wet dry conditions. The CO2 capture capacity of the nano-CaO decreases rapidly in the cyclic carbonation/decarbonation process. The cyclic stability of the nano-CaO acceptors is enhanced by coating. TGA of the cyclic CO2 carbonation/decarbonation indicate that the nano-CaO acceptors with sol-gel coatings and wet impregnation have higher CO2 capture capacity and a longer life time than the uncoated ones. Multi-cycle CO2 carbonation/decarbonation was also performed for the nano-CaO acceptors with sol-gel in the presence of steam and under sorption enhanced reforming condition. The CO2 capture capacity of CaO-SG increases with the cycle number and is dependent on the partial pressure of

CO2 as well as carbonation rates. The CO2 capture capacity increases with carbonation rates. The nano-CaO, coated by sol-gel showed improved sorption capacity compared to incipient wet impregnated samples. The capture capacity of CaO-IM declined after 14 cycles whereas CaO-SG remained stable up to 20 cycles. Structural characterization of the acceptors revealed that a core-shell CaO@CaZrO3 structure was formed by sol-gel coating. The stability of the acceptors is increased due to the formation of core-shell structure, which prevents the sintering of reactive CaO particles in the reaction and stabilizes the reversibility of the cyclic reaction. On the other hand, an uneven coating of the CaO surface by the incipient wet impregnation led to less stability. Coating with hydrolysis, the composite lost the CO2 capture capacity because of formation of pervoskite structure of CaZr4O9. Testing of sol-gel coated sample under wet condition and sorption enhanced steam reforming condition showed an increase in carbonation rate and capture capacity increases with cycle numbers due to the structural changes of CaZrO3 coated layers around nano-CaO. ACKNOWLEDGEMENTS The Norwegian Research Council (NFR) is acknowledged for financial support. Alexey Varonov is acknowledged supports in Raman-Spectroscopy measurements. NOTES Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Sem Sælands vei 4, NO-7491 Trondheim, (Norway), Email:[email protected] a

b Department of Physics, Norwegian University of Science and Technology (NTNU), Sem Sælands vei 4, NO-7491 Trondheim, (Norway) cSINTEF

Materials and Chemistry, NO-7491 Trondheim, Norway

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85x82mm (150 x 150 DPI)



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

100x98mm (150 x 150 DPI)


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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


79x80mm (150 x 150 DPI)



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

203x180mm (150 x 150 DPI)


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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


203x177mm (150 x 150 DPI)



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

79x100mm (150 x 150 DPI)


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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


86x65mm (150 x 150 DPI)



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

104x100mm (68 x 68 DPI)


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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


133x96mm (150 x 150 DPI)



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

145x125mm (150 x 150 DPI)


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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


203x161mm (150 x 150 DPI)



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

141x106mm (150 x 150 DPI)


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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


240x174mm (150 x 150 DPI)



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

240x174mm (150 x 150 DPI)


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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


102x88mm (144 x 144 DPI)


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