Modification of one-dimensional Ti[O.sub.2] nanotubes with CaO dopants for high C[O.sub.2] adsorption.
Recently, solid C[O.sub.2] adsorbents are used as an alternative and potentially less-energy-intensive separation technology. These C[O.sub.2] adsorbents can be utilized from ambient temperature up to 973 K by yielding less waste during cycle. In addition, their waste can be disposed of without undue environmental precautions as compared to liquid adsorbent . A variety of solid physical adsorbents have been considered for C[O.sub.2] capture including microporous and mesoporous materials (carbon-based sorbents, such as activated carbon and carbon molecular sieves, zeolites, and chemically modified mesoporous materials), metal oxides, and hydrotalcite-like compounds [2, 3]. These listed adsorbents usually can be classified into three types based on their sorption/desorption temperatures: (1) low temperature adsorbent: <473 K (carbon, zeolites, MOFs/ZIFs, alkali metal carbonates, and amine-based materials), (2) intermediate temperature adsorbent: 473-673 K (hydrotalcite-like compounds, HTLcs/layered double hydroxides, LDHs), and (3) high-temperature adsorbents: >673 K (calcium based and alkali ceramic). The summary of those adsorbents with their efficiency and operating parameters are shown in Table 1.
According to Table 1, CaO and alkali ceramics are promising candidates for C[O.sub.2] adsorption. Therefore, CaO-based adsorbent has gained great attention due to its great capability (11.6 mmol/g) as compared to other adsorbents in capturing C[O.sub.2] gases through cyclic carbonation-calcination reaction. In addition, CaO-based adsorbent has high reactivity with C[O.sub.2] gases, high capacity, and low material cost . The carbonation temperature for CaO-based adsorbents is between 873 and 973 K and their regeneration temperature is normally above 1223 K. The reversible reaction between CaO and C[O.sub.2] is
CaO (s) + C[O.sub.2] (g) [??] CaC[O.sub.3] (s)
[DELTA][H.sup.[degrees]/sub.873.15 K] = -171.2 kJ [mol.sup.-1] (for carbonation)
In this manner, nanocrystalline of CaO has been proven to be useful in the noncatalytic removal of C[O.sub.2] in [H.sub.2] production . The nanocrystalline of CaO with vacancies/defects, which often related to the presence of basic and acidic sites within their lattice. The structural defects normally are involved in the basic-acidic catalytic reactions. It is a well-known fact that CaO has highly reactive and strong basic sites because of the isolated [O.sup.2-] centers as well as weak residual OH groups which appear when mixed with the rare earths . However, these C[O.sub.2] adsorbents suffer severely from textural degradation during the sorption/desorption operations. These C[O.sub.2] adsorbents can only run several tens of cycles before any obvious degradation and are still far from practical applications . Instantly, the conversion of CaO decreased sharply from 70% in the first cycle to 20% in the eleventh cycle when tested in fluidized bed . The deactivation primarily results from the formation of thick layer structured from CaC[O.sub.3] surrounding the CaO, which severely hinders the diffusion of C[O.sub.2] gas to react with the inner core. Besides, it has also been reported that the adsorption capacity for CaO-based sorbents decays as a function of the sintering of CaO grain at high temperature and a certain loss in the porosity. When pores smaller than a critical value (e.g., 200 nm) are filled, the reaction gets much slower . Therefore, great efforts have to be made in order to further improve the cyclic stability of CaO-based sorbents.
One of the most promising solutions to improve the cyclic stability of CaO is controlling their architecture into one-dimensional nanomaterials. The main reason might be attributed to the fast reaction (chemical reaction) and the slow reaction (diffusion controlled) could be achieved during C[O.sub.2] adsorption. In this case, the diffusion of C[O.sub.2] into the particle interior to react with Ca dopants could be prevented and the whole C[O.sub.2] adsorption process could then be diffusion-controlled [2, 3]. Theoretically, the small particles size of sorbent (e.g., 30-50 nm) would perform better carbonation-calcination reaction, which allowed carbonation to take place at the rapid reaction-controlled regime. Another promising solution to improve cyclic stability is to incorporate high stability metal oxide (titanium dioxide) into CaO particles. The prevention of CaO oxidation during calcination stage could be expected. Therefore, detail investigation on one-dimensional CaO-Ti[O.sub.2] nanotubes for effective C[O.sub.2] adsorption will be discussed.
2. Experimental Procedure
One-dimensional Ti[O.sub.2] nanotube arrays were synthesized using a rapid-anodic oxidation electrochemical anodization technique. A high purity of Ti foil (99.6%, Strem Chemical, USA) with a thickness of 127 [micro]m was selected as substrate to grow Ti[O.sub.2] nanotubes. This process was conducted in a bath with electrolytes composed of ethylene glycol ([C.sub.2][H.sub.6][O.sub.2], >99.5%, Merck, USA), 5wt% ammonium fluoride (N[H.sub.4]F, 98%, Merck, USA), and 5wt% hydrogen peroxide ([H.sub.2][O.sub.2], 30% [H.sub.2][O.sub.2] and 70% [H.sub.2]O, J. T. Baker, USA) for 60 minutes at 60 V. This experimental condition was selected because it favors the formation of well-aligned Ti[O.sub.2] nanotube arrays [9, 10]. After the anodization process, as-anodized samples were cleaned using distilled water and dried under a nitrogen stream. CaO-Ti[O.sub.2] nanotubes were then prepared through wet impregnation technique using calcium nitrate tetrahydrate [(Ca(N[O.sub.3]).sub.2] x 4[H.sub.2]O, Merck, USA) as the precursor. This was an ex situ approach that was used to incorporate [Ca.sup.2+] ions into Ti[O.sub.2] nanotubes. Two different concentrations of calcium nitrate tetrahydrate solution (0.6, 1.2 M) were prepared at different reaction times (24, 48, 72 hours) in a water bath of 80[degrees]C. Subsequently, the samples were thermal-annealed at 673 K in an argon atmosphere for 4 h in order to produce crystalline Ti[O.sub.2] nanotubes.
The surface morphologies of the synthesized samples were observed through field emission scanning electron microscopy (FESEM) using a Zeiss SUPRA 35 VP, which is operated at a working distance of 1 mm and 5 kV. The energy dispersive X-ray spectroscopy (EDX) was applied to elemental analysis of the CaO-Ti[O.sub.2] nanotubes sorbents, which is equipped in the FESEM. The structural variations and phase determination for CaO-Ti[O.sub.2] nanotubes sorbents were determined using a Philips PW 1729 X-ray diffraction (XRD), which operated at 45 kV and 40 mV patterns. The thermogravimetric analysis (TGA) was used to investigate the C[O.sub.2] adsorption for CaO-Ti[O.sub.2] nanotubes sorbents (STA 6000, Perkin Elmer, USA). The steps included are N2 gas flow at a rate of 10[degrees]C/min from room temperature to 673 K and then holding for 30 min in C[O.sub.2] and finally cooling down to 573 Kby N2 gas. In the present study, carbonation-calcination reaction is set to be 673 K because nanotubular structure can be collapsed at high temperature (above 773 K) .
3. Results and Discussion
The surface morphologies of CaO-Ti[O.sub.2] nanotubes synthesized in 0.6M calcium nitrate solution for 24, 48, and 72 hours were subsequently observed via FESEM as presented in Figures 1(a) to 1(c), respectively. As shown in the FESEM images, the opening of the nanotubular structure showed aggregation of CaO species on wall surface of Ti[O.sub.2] nanotubes. The wall thickness of the nanotubes dramatically increased to 75 nm, which resulted in a narrow pore entrance for 24 hours reaction time (Figure 1(a)). Meanwhile, as the reaction time increased to 48 hours, the wall thickness of the nanotubes increased from about 75 nm to 100 nm (Figure 1(b)). With further increase of the reaction time to 72 hours, it was found that the nanotubes were covered with excess CaO species and clogged the pore entrance (Figure 1(c)). A rough, irregular, and corrugated surface was formed. Based on the FESEM images, it could be concluded that the appearance of Ti[O.sub.2] nanotubes was dependent on the reaction time in calcium nitrate solution. A narrow or blocked pore entrance of nanotubes was formed as increasing soaking period in the solution. Next, the average atomic percentage (at%) of the elements within CaO-Ti[O.sub.2] nanotubes was determined using EDX analysis. The numerical EDX analyses of the samples are listed in Table 2. As determined through EDX analysis, the average Ca contents of the nanotubes for 24, 48, and 72 hours were 1.01 at%, 3.67 at%, and 4.59 at%, respectively. The intensity of the Ca peak (3.69 keV) increased with increasing reaction time in calcium nitrate solution as presented in Figures 2(a) to 2(c). Another set of experiments was conducted to form CaO-Ti[O.sub.2] nanotubes in 1.2 M calcium nitrate solution for 24, 48, and 72 hours. All morphologies of the samples showed similar appearance of CaO-Ti[O.sub.2] nanotubes synthesized in 0.6 M calcium nitrate solution. The irregular CaO layer covered all of the Ti[O.sub.2] nanotubular structure and nanoporous structure arranged in a nonordered manner which could be observed in Figures 3(a) to 3(c). The chemical stoichiometry of the resultant samples was determined via EDX analysis as shown in Figures 4(a) to 4(c). A high Ca content of 9.78 at% was determined from those synthesized in 1.2 M calcium nitrate solution for 72 hours, indicating that the incorporation of the CaO became prominent with increasing the concentration of calcium nitrate solution. Based on the FESEM images and EDX analysis, the small [Ca.sup.2+] ions could be diffused into Ti[O.sub.2] nanotubes in the presence of lattice defects, especially nearby to the wall of nanotubes. In this case, the diffusion rate of [Ca.sup.2+] ions increased significantly when increasing the reaction time and concentration of precursor. However, the content of small [Ca.sup.2+] ions that diffused into the Ti[O.sub.2] lattice could reach a saturation condition and start to accumulate on the surface of nanotubes. The number of nucleation sites for [Ca.sup.2+] ions loaded on the wall surface of the nanotubes increased with longer reaction time and higher concentration of precursor, which produced nanotubes with thicker walls. The diffusion of the [Ca.sup.2+] ions formed Ca-O bonding with O-Ti-O bonding; thus, charge neutrality could be achieved.
In the present study, XRD analysis was used to determine the crystallographic structure and the changes in the phase structure of the CaO-Ti[O.sub.2] nanotubes synthesized in different reaction times and concentrations of precursor are presented in Figures 5 and 6. Numerous studies reported that heat treatment at about 400[degrees]C could transform the amorphous structure of Ti[O.sub.2] into the crystalline anatase phase. The obvious diffraction peaks from the XRD pattern attributed to the anatase phase (JCPDS no. 21-1272) were detected from the XRD patterns (Figure 5(a)). The diffraction peaks are allocated at 25.32[degrees], 37.84[degrees], 38.42[degrees], 48.02[degrees], 53.87[degrees], 55.09[degrees], 62.93[degrees], 70.65[degrees], and 76.23[degrees], which correspond to 101, 004, 112, 200, 105, 211, 204, 220, and 301 crystal planes for the anatase phase, respectively. Apparently, the incorporation of [Ca.sup.2+] ions into the lattice of Ti[O.sub.2] hindered the crystallization of Ti[O.sub.2], resulting in the peak intensity of the 101 peak at 25.32[degrees] decrease. The decrease in anatase phase is maybe due to the interruption of Ca atom, which diffused into Ti[O.sub.2] nanotubes and inhibited the formation of the anatase. The XRD pattern of the sample soaked in 1.2 M for 72 hours exhibits additional peaks 220 and 400 crystal planes at 54[degrees] and 80[degrees], corresponding to CaO phase. This indicates that crystalline CaO are formed once the concentration of Ca in Ti[O.sub.2] reaches a higher level. Next, the resultant anodized CaO-Ti[O.sub.2] nanotubes were used in the characterization of C[O.sub.2] adsorption using TGA analysis. The processing steps involved in TGA analysis are [N.sub.2] gas flow at a rate of 10[degrees]C/min from room temperature to 673 K and then holding for 30 min in C[O.sub.2] and finally cooling down to 573 K by [N.sub.2] gas. The TGA curves for 0.6 M of Ca and 1.2 M of Ca are shown in Figures 7 and 8, respectively, while the C[O.sub.2] adsorption capacity is summarized in Table 3. Based on the TGA analysis, it could be observed that all CaO-Ti[O.sub.2] samples showed their C[O.sub.2] adsorption capacity in the range of 3.3 mmol/g to 4.5 mmol/g. A maximum C[O.sub.2] adsorption capacity of up to 4.45 mmol/g was observed from the CaO-Ti[O.sub.2] nanotubes synthesized in 1.2 M of calcium nitrate solution for 48 hours. Basically, the C[O.sub.2] adsorption capacity based CaO-Ti[O.sub.2] sorbents used the following reaction: CaO + C[O.sub.2] [right arrow] CaC[O.sub.3]. In this case, the sorbent weight is increased significantly when C[O.sub.2] gas is applied to the TGA system, where all the weight added is C[O.sub.2] adsorbed. This reason clearly explains that C[O.sub.2] adsorption capacity is increased after carbonation process.
The present study demonstrated that one-dimensional CaO-Ti[O.sub.2] nanotubes sorbent was successfully formed using oxidation electrochemical anodization and wet impregnation techniques. All of the resultant CaO-Ti[O.sub.2] nanotubes sorbent exhibited promising C[O.sub.2] adsorption capacity in the range of 3.3 mmol/g to 4.5 mmol/g. It is shown that high active surface area of CaO-Ti[O.sub.2] nanotubes sorbent showed good stability during extended cyclic carbonation-calcination reaction.
Conflict of Interests
The author declares that there is no conflict of interests regarding the publication of this paper.
This research is supported by High Impact Research Chancellory Grant UM.C/625/1/HIR/228 (J55001-73873) from the University of Malaya. In addition, authors would like to thank University of Malaya for sponsoring this work under University of Malaya Research Grant (UMRG, no. RP0222012D).
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Chin Wei Lai
Nanotechnology & Catalysis Research Centre (NANOCAT), Institute of Postgraduate Studies (IPS), Universiti Malaya, 3rd Floor, Block A, 50603 Kuala Lumpur, Malaysia
Correspondence should be addressed to Chin Wei Lai; email@example.com
Received 30 January 2014; Revised 21 February 2014; Accepted 21 February 2014; Published 26 March 2014
Academic Editor: Tian-Yi Ma
TABLE 1: The different types of solid C[O.sub.2] adsorbents based on their sorption/desorption temperatures. Total capacity Temperature Adsorbent (mmol/g) (K) Low temperature adsorbents Amine based Amine-modified 4.5 348 mesoporous silica Amine-modified ordered 1.5 -- mesoporous silica (OMS) Amine-modified OMSs 0.2-1.4 348 Amine 4.6 348 tetraethylenepentamine (TEPA)/OMSs MOFs/ZIFs MOF-2 3.2 -- Carbon/activated carbon based Coal-based activated 0.3 323 carbon Porous carbon nitride 2.90 298 (CN) N-doped carbons 6.9 273 template from zeolite Activated carbon/ 3.75 [N.sub.2] Activated carbon/ 3.49 293 [H.sub.2] Activated carbon/ 3.22 N[H.sub.3] Activated carbon 1.5 303 Activated carbon 3.5 275 Carbon nanotubes (CNTs) 2.45 323 /3-aminopropyltriethoxysilane (APTS) Alumina based [gamma]-[Al.sub.2][O.sub.3] 0.31 295 Zeolite based Y-type zeolite/tetraethyl- 4.27 303-333 enepentamine (TEPA) Natural zeolite 2.05 Treated zeolite/[H.sub.3]P 1.95 5.49 298 [O.sub.4] Synthetic zeolite-5X Synthetic zeolite-13A 6.82 LilSX 0.17 NaLX 0.23 -- CaX 0.24 13X (NaX) 2.94 295 Magnesium based MgO/[Al.sub.2][O.sub.3] 1.36 333 Intermediate temperature adsorbents LDH Mg-Al-C[O.sub.3] LDHs 0.49 473 High temperature adsorbents Lithium orthosilicates [Li.sub.4]Si[O.sub.4] 6.14 773 Lithium zirconates [Li.sub.2]Zr[O.sub.3] 4.55 773 [Li.sub.2]Zr[O.sub.3] 4.50 823 [Li.sub.2]Zr[O.sub.3] 6.14 848 (nanocrystalline) Calcium oxide based CaO 2.3 923 CaO/[Al.sub.2][O.sub.3] 11.6 923 Cs/CaO 4.9 Rb/CaO 4.5 723 K/CaO 3.8 Na/CaO 3.1 CaC[O.sub.3] 1.5 573 CaO/[Al.sub.2][O.sub.3] 6.02 923 Nano CaO/[Al.sub.2][O.sub.3] 6.02 650 Pressure Adsorbent (atm) Low temperature adsorbents Amine based Amine-modified 1 mesoporous silica Amine-modified ordered -- mesoporous silica (OMS) Amine-modified OMSs 1 Amine 1 tetraethylenepentamine (TEPA)/OMSs MOFs/ZIFs MOF-2 41 Carbon/activated carbon based Coal-based activated 0.001 carbon Porous carbon nitride -- (CN) N-doped carbons 1 template from zeolite Activated carbon/ [N.sub.2] Activated carbon/ 1 [H.sub.2] Activated carbon/ N[H.sub.3] Activated carbon 40 Activated carbon 1 Carbon nanotubes (CNTs) -- /3-aminopropyltriethoxysilane (APTS) Alumina based [gamma]-[Al.sub.2][O.sub.3] -- Zeolite based Y-type zeolite/tetraethyl- enepentamine (TEPA) Natural zeolite Treated zeolite/[H.sub.3]P 0.1 [O.sub.4] Synthetic zeolite-5X Synthetic zeolite-13A LilSX NaLX 1 CaX 13X (NaX) -- Magnesium based MgO/[Al.sub.2][O.sub.3] 1 Intermediate temperature adsorbents LDH Mg-Al-C[O.sub.3] LDHs 0.05 High temperature adsorbents Lithium orthosilicates [Li.sub.4]Si[O.sub.4] 1 Lithium zirconates [Li.sub.2]Zr[O.sub.3] 1 [Li.sub.2]Zr[O.sub.3] 1 [Li.sub.2]Zr[O.sub.3] 1 (nanocrystalline) Calcium oxide based CaO -- CaO/[Al.sub.2][O.sub.3] -- Cs/CaO Rb/CaO 0.4 K/CaO Na/CaO CaC[O.sub.3] 1 CaO/[Al.sub.2][O.sub.3] 1 Nano CaO/[Al.sub.2][O.sub.3] -- Reference Adsorbent Low temperature adsorbents Amine based Amine-modified (Liu et al., 2010)  mesoporous silica Amine-modified ordered (Zelenak et al., 2008)  mesoporous silica (OMS) Amine-modified OMSs (Liu et al., 2010)  Amine (Liu etal., 2010)  tetraethylenepentamine (TEPA)/OMSs MOFs/ZIFs MOF-2 (Millward and Yaghi, 2005)  Carbon/activated carbon based Coal-based activated (Deng et al., 2011)  carbon Porous carbon nitride (Li et al., 2010)  (CN) N-doped carbons (Xia et al., 2011)  template from zeolite Activated carbon/ [N.sub.2] Activated carbon/ (Zhang et al., 2010)  [H.sub.2] Activated carbon/ N[H.sub.3] Activated carbon (Drage et al., 2009)  Activated carbon (Wang et al., 2008)  Carbon nanotubes (CNTs) (Su et al., 2011)  /3-aminopropyltriethoxysilane (APTS) Alumina based [gamma]-[Al.sub.2][O.sub.3] (Rege and Yang, 2001)  Zeolite based Y-type zeolite/tetraethyl- (Su et al., 2010)  enepentamine (TEPA) Natural zeolite Treated zeolite/[H.sub.3]P (Ertan and Cakicioglu-Ozkan, [O.sub.4] Synthetic zeolite-5X 2005)  Synthetic zeolite-13A LilSX NaLX (Brandani and Ruthven, 2004)  CaX 13X (NaX) (Rege and Yang, 2001)  Magnesium based MgO/[Al.sub.2][O.sub.3] (Li et al., 2010)  Intermediate temperature adsorbents LDH Mg-Al-C[O.sub.3] LDHs (Ram Reddy et al., 2006),  High temperature adsorbents Lithium orthosilicates [Li.sub.4]Si[O.sub.4] (Kato et al., 2002),  Lithium zirconates [Li.sub.2]Zr[O.sub.3] (Ida and Lin, 2003),  [Li.sub.2]Zr[O.sub.3] (Ochoa-Fernandez et al., 2005),  [Li.sub.2]Zr[O.sub.3] (Ochoa-Fernandez et al., 2006), (nanocrystalline)  Calcium oxide based CaO (Satrio et al., 2005),  CaO/[Al.sub.2][O.sub.3] (Li et al., 2006),  Cs/CaO Rb/CaO (Reddy and Smirniotis, 2004), K/CaO  Na/CaO CaC[O.sub.3] (Kuramoto et al., 2003),  CaO/[Al.sub.2][O.sub.3] (Wu et al., 2007),  Nano CaO/[Al.sub.2][O.sub.3] (Wu et al., 2008),  TABLE 2: EDX result of CaO-Ti[O.sub.2] with different soaking time in 0.6 M and 1.2 M calcium nitrate solution. Concentration of Element (at%)/ Ti O Ca calcium nitrate reaction time (h) solution (M) 0.6 24 46.07 52.92 01.01 48 38.88 57.45 03.67 72 42.30 53.11 04.59 1.2 24 36.44 60.66 02.90 48 32.64 60.64 06.64 72 28.83 61.10 09.78 TABLE 3: C[O.sub.2] adsorption capacity of the CaO-Ti[O.sub.2] samples. Concentration of calcium Soaking C[O.sub.2] adsorption nitrate solution (M) period capacity (mmol/g) (hours) 0.6 24 3.89 48 3.32 72 4.00 1.2 24 3.59 48 4.45 72 3.80
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|Title Annotation:||Research Article|
|Author:||Lai, Chin Wei|
|Publication:||International Journal of Photoenergy|
|Date:||Jan 1, 2014|
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