Modification of One-Dimensional TiO2 Nanotubes with CaO Dopants for High CO2 Adsorption

One-dimensional calcium oxide (CaO-) based titanium dioxide (TiO 2 ) nanotubes were successfully synthesized through a rapid electrochemical anodization and chemicalwet impregnation techniques. In this study, calciumnitrate solutionwas used as a calcium source precursor.The reaction time and concentration of calcium source on the formation ofCaO-TiO 2 nanotubeswere investigated using field emission microscopy, energy dispersion X-ray spectroscopy, and X-ray diffraction.The adsorption capacity of CO 2 was determined by thermal gravimetric analyzer. A maximum of 4.45mmol/g was achieved from the CaO-TiO 2 nanotubes (6.64 at% of Ca). The finding was attributed to the higher active surface area for CaO to adsorb more CO 2 gas and then formed CaCO 3 compound during cyclic carbonation-calcination reaction.


Introduction
Recently, solid CO 2 adsorbents are used as an alternative and potentially less-energy-intensive separation technology.These CO 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 [1].A variety of solid physical adsorbents have been considered for CO 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 CO 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 CO 2 gases through cyclic carbonation-calcination reaction.In addition, CaO-based adsorbent has high reactivity with CO 2 gases, high capacity, and low material cost [4].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 CO 2 is In this manner, nanocrystalline of CaO has been proven to be useful in the noncatalytic removal of CO 2 in H 2 production [5].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 wellknown fact that CaO has highly reactive and strong basic sites because of the isolated O 2− centers as well as weak 2 International Journal of Photoenergy  residual OH groups which appear when mixed with the rare earths [6].However, these CO 2 adsorbents suffer severely from textural degradation during the sorption/desorption operations.These CO 2 adsorbents can only run several tens of cycles before any obvious degradation and are still far from practical applications [3].Instantly, the conversion of CaO decreased sharply from 70% in the first cycle to 20% in the eleventh cycle when tested in fluidized bed [7].The deactivation primarily results from the formation of thick layer structured from CaCO 3 surrounding the CaO, which severely hinders the diffusion of CO 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 [8].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 CO 2 adsorption.In this case, the diffusion of CO 2 into the particle interior to react with Ca dopants could be prevented and the whole CO 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-TiO 2 nanotubes for effective CO 2 adsorption will be discussed.

Experimental Procedure
One-dimensional TiO 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 m was selected as substrate to grow TiO 2 nanotubes.This process was conducted in a bath with electrolytes composed of ethylene glycol (C 2 H 6 O 2 , >99.5%, Merck, USA), 5 wt% ammonium fluoride (NH 4 F, 98%, Merck, USA), and 5 wt% hydrogen peroxide (H 2 O 2 , 30% H 2 O 2 and 70% H 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 TiO 2 nanotube arrays [9,10].After the anodization process, as-anodized samples were cleaned using distilled water and dried under a nitrogen stream.CaO-TiO 2 nanotubes were then prepared through wet impregnation technique using calcium nitrate tetrahydrate (Ca(NO 3 ) 2 ⋅4H 2 O, Merck, USA) as the precursor.This was an ex situ approach that was used to incorporate Ca 2+ ions into TiO 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 ∘ C. Subsequently, the samples were thermalannealed at 673 K in an argon atmosphere for 4 h in order to produce crystalline TiO 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-TiO 2 nanotubes sorbents, which is equipped in the FESEM.The structural variations and phase determination for CaO-TiO 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 CO 2 adsorption for CaO-TiO 2 nanotubes sorbents (STA 6000, Perkin Elmer, USA).The steps included are N 2 gas flow at a rate of 10 ∘ C/min from room temperature to 673 K and then holding for 30 min in CO 2 and finally cooling down to 573 K by N 2 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) [11].

Results and Discussion
The surface morphologies of CaO-TiO 2 nanotubes synthesized in 0.6 M 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 TiO 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 TiO 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-TiO 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.Based on the FESEM images and EDX analysis, the small Ca 2+ ions could be diffused into TiO 2 nanotubes in the presence of lattice defects, especially nearby to the wall of nanotubes.In this case, the diffusion rate of Ca 2+ ions increased significantly when increasing the reaction time and concentration of precursor.However, the content of small Ca 2+ ions that diffused into the TiO 2 lattice could reach a saturation condition and start to accumulate on the surface of nanotubes.The number of nucleation sites for Ca 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 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-TiO 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 ∘ C could transform the amorphous structure of TiO 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   phase, respectively.Apparently, the incorporation of Ca 2+ ions into the lattice of TiO   1.2 M of calcium nitrate solution 48 hours.Basically, the CO 2 adsorption capacity based CaO-TiO 2 sorbents used the following CaO + CO 2 → CaCO .In case, the sorbent weight is increased significantly when CO 2 gas is applied to the TGA system, where all the weight added is CO 2 adsorbed.This reason clearly explains that CO 2 adsorption capacity is increased after carbonation process.

Conclusion
The present study demonstrated that one-dimensional CaO-TiO 2 nanotubes sorbent was successfully formed using oxidation electrochemical anodization and wet impregnation techniques.All of the resultant CaO-TiO 2 nanotubes sorbent exhibited promising CO 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-TiO 2 nanotubes sorbent showed good stability during extended cyclic carbonation-calcination reaction.

Figure 1 :
Figure 1: FESEM top view image of nanotubes subjected to wet impregnation of 0.6 M calcium nitrate solution for (a) 24 hours, (b) 48 hours, and (c) 72 hours and subsequent annealing at 673 K in Ar gas for 4 hours.

Figure 2 :
Figure 2: EDX spectra of nanotubes subjected to wet impregnation of 0.6 M calcium nitrate solution for (a) 24 hours, (b) 48 hours, and (c) 72 hours (all EDX spectrums are noneditable format based on our FESEM-EDX system).

Figure 3 :
Figure 3: FESEM top view image of nanotubes subjected to wet impregnation of 1.2 M calcium nitrate solution for (a) 24 hours, (b) 48 hours, and (c) 72 hours and subsequent annealing at 673 K in Ar gas for 4 hours.

Figure 4 :
Figure 4: EDX spectra of nanotubes subjected to wet impregnation of 1.2 M calcium nitrate solution for (a) 24 hours, (b) 48 hours, and (c) 72 hours (all EDX spectrums are noneditable format based on our FESEM-EDX system).

Figures 2 (
Figures 2(a) to 2(c).Another set of experiments was conducted to form CaO-TiO 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-TiO 2 nanotubes synthesized in 0.6 M calcium nitrate solution.The irregular CaO layer covered all of the TiO 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 2+ ions could be diffused into TiO 2 nanotubes in the presence of lattice defects, especially nearby to the wall of nanotubes.In this case, the diffusion rate of Ca 2+ ions increased significantly when increasing the reaction time and concentration of precursor.However, the content of small Ca 2+ ions that diffused into the TiO 2 lattice could reach a saturation condition and start to accumulate on the surface of nanotubes.The number of nucleation sites for Ca 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 2+ ions formed Ca-O bonding with O-Ti-O bonding; thus, charge neutrality could be achieved.

Figure 8 :
Figure 8: TGA curve of soaking in 1.2 M of Ca for different periods of time.

Table 1 :
The different types of solid CO 2 adsorbents based on their sorption/desorption temperatures.

Table 2 :
EDX result of CaO-TiO 2 with different soaking time in 0.6 M and 1.2 M calcium nitrate solution.
(a)).The diffraction peaks are allocated at 25.32 ∘ , 37.84 ∘ , 38.42 ∘ , 48.02 ∘ , 53.87 ∘ , 55.09 ∘ , 62.93 ∘ , 70.65 ∘ , and 76.23 ∘ , which correspond to 101, 004, 112, 200, 105, 211, 204, 220, and 301 crystal planes for the anatase 2 hindered the crystallization of TiO 2 , resulting in the peak intensity of the 101 peak at 25.32 ∘ decrease.The decrease in anatase phase is maybe due to the interruption of Ca atom, which diffused into TiO 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 ∘ and 80 ∘ , corresponding to CaO phase.This indicates that crystalline CaO are formed once the concentration of Ca in TiO 2 reaches a higher level.Next, the resultant anodized CaO-TiO 2 nanotubes were used in the characterization of CO 2 adsorption using TGA analysis.The processing steps involved in TGA analysis are N 2 gas flow at a rate of 10 ∘ C/min from room temperature to 673 K and then holding for 30 min in CO 2 and finally cooling down to 573 K by N 2 gas.The TGA curves for 0.6 M of Ca and 1.2 M of Ca are shown in Figures7 and 8, respectively, while the CO 2 adsorption capacity is summarized in Table3.Based on the TGA analysis, it could be observed that all CaO-TiO 2 samples showed their CO 2 adsorption capacity in the range of 3.3 mmol/g to 4.5 mmol/g.
A maximum CO 2 adsorption capacity of up to 4.45 mmol/g was observed from the CaO-TiO 2 nanotubes synthesized in

Table 3 :
CO 2 adsorption capacity of the CaO-TiO 2 samples.