Journal of Nanomaterials

Volume 2014 (2014), Article ID 876571, 6 pages

http://dx.doi.org/10.1155/2014/876571

Research Article

Anodic CaO-TiO₂ Nanotubes Composite Film for Low Temperature CO₂ Adsorption

Chin Wei Lai

Nanotechnology & Catalysis Research Centre (NANOCAT), Institute of Postgraduate Studies (IPS), Universiti Malaya, 3rd Floor, Block A, 50603 Kuala Lumpur, Malaysia

Received 12 January 2014; Revised 8 April 2014; Accepted 9 April 2014; Published 4 May 2014

Academic Editor: Sun-Hwa Yeon

Copyright © 2014 Chin Wei Lai. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A novel one-dimensional anodic CaO-TiO₂ nanotubes composite film was prepared using a rapid-anodic oxidation electrochemical anodization technique for low temperature CO₂ absorption application. This study aims to determine the optimum concentration of Ca(NO₃)₂·4H₂O used as the CaO precursor for loading CaO species on TiO₂ nanotubes. In this study, an optimum content of CaO on TiO₂ nanotubes (0.15 at% of Ca element) could enhance the CO₂ adsorption capacity up to 2.45 mmol/g at 400°C. This behavior was attributed to the large active surface area of CaO species were covered on the surface of TiO₂ nanotubes. The conversion of CaO into CaCO₃ could be achieved effectively for CO₂ absorption during the carbonate looping process.

1. Introduction

Nowadays, carbon dioxide (CO₂) has become the focus of attention as the primary greenhouse gas in the atmosphere which leads to the global warming [1]. The increasing of CO₂ emission in atmosphere is leading to global warming and climate change [1, 2]. The fossil fuels supply more than 98% of the world's energy needs and the combustion of the fossils fuels is one of the major sources of the greenhouse gas [3]. It is necessary to reduce the emission of this gas. Several options are available to reduce these CO₂ emissions, including substitution of nuclear power and natural gas for fossil fuels and coal, separating and capturing CO₂ prior to emission into atmosphere [4]. Nowadays, the CO₂ emission is almost 390 parts per million (ppm) which is above the safety limit of 350 ppm [2, 5]. Therefore, developments of carbon capturing work have become very significant option to stop and reduce the CO₂ emission. Malaysia as one of the ASEAN countries had agreed to reduce the CO₂ emission up to 40% by year of 2020. In fact, there are several advance technologies to capture CO₂, such as chemical (gas-liquid) adsorption, adsorption, cryogenic separation, membrane separation, and biological fixation [6]. Among all of these listed technologies, adsorption is a promising heterogeneous process that can separate CO₂ from the fuel gases of coal-fired power plants effectively [6, 7]. These CO₂ molecules can be captured on the surface of the sorbents effectively because of the interaction between sorbent and CO₂ gas [5]. In general, CO₂ adsorption process is very stable and has high cyclic capture capacities and low energy consumption for regeneration in comparison to aqueous systems. The adsorption kinetics depend on temperature, pressure, interaction energy between sorbent and CO₂ on the surface, and pore size or surface area of the adsorbents [2, 5, 6].

Nevertheless, the conventional CO₂ adsorbents used suffer severe degradation during their operations of sorption and desorption [8]. These conventional CO₂ adsorbents usually can only run several tens of cycles before showing obvious degradation. The deactivation primary resulted from the formation of thin CaCO₃ layer surrounding the CaO surface. Once a certain thickness of 20 nm CaCO₃ is reached, the diffusion of CO₂ will be hindered to react with CaO inner core [6, 8]. Thus, continuous efforts have been exerted to further improve the CaO texture and structure by designing its architecture in one-dimensional nanoscale for high efficiency in capturing the CO₂. Recent studies have indicated that one-dimensional nanotubular structure is able to provide a higher active surface area (inner and outer surface) for carbonate looping process [9–11]. However, the literature regarding the formation of one-dimensional CaO nanoarchitecture was limited. Thus, innovative new approaches and synthesis of a high quality one-dimensional CaO-TiO₂ nanoarchitecture are crucial for determining the potential of the material as efficient CO₂ adsorbents. The CaO-TiO₂ nanoarchitecture composite film is believed to have its own unique characteristics, such as high selectivity and adsorption capacity for CO₂, fast adsorption and desorption kinetics, stable cyclic adsorption capacity, and low energy needs for regeneration of pure CO₂ [2].

Herein, we report the formation of CaO-TiO₂ nanoarchitecture composite film using a simple electrochemical anodization method for high CO₂ adsorption capacity. The electrochemical anodization method is a compromising synthetic technique to grow the one-dimensional nanoarchitecture because of its low cost, mild conditions, and accurate process control [12]. Thus, the controlled growth of CaO-TiO₂ nanoarchitecture composite film is our innovation to further improve CO₂ adsorption capacity and adopted for large-scale industrial production. Meanwhile, the ultimate aim of the present work is to form well-aligned CaO-TiO₂ nanotubes composite film, which is able to perform CO₂ adsorption at low temperature of 400°C.

2. Experimental Procedure

The self-organized TiO₂ nanotubes film was synthesized from a rapid-anodic oxidation electrochemical anodization of a high purity Ti foil (99.6%, Strem Chemical, USA) with a thickness of 127 μm with surface area of about 10 cm². This process was conducted in a bath with electrolytes composed of ethylene glycol (C₂H₆O₂, >99.5%, Merck, USA), 5 wt% ammonium fluoride (NH₄F, 98%, Merck, USA), and 5 wt% hydrogen peroxide (H₂O₂, 30% H₂O₂ and 70% H₂O, J.T. Baker, USA), as well as different concentrations of calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O, Merck, USA). The concentrations of Ca(NO₃)₂·4H₂O were varied from 0.01 M up to 0.10 M. This process was carried out for 60 minutes at a constant potential of 60 V using a Keithley DC power supply. When the circuit was closed, external potential bias was generated and then the current moved from the positive terminal (platinum rod) to the negative terminal (Ti foil). Before synthesis, the distance between anode and cathode was fixed at 30 mm. After the anodization process, as-anodized samples were cleaned using distilled water and dried under a nitrogen (N₂) stream. The resultant samples were then thermally annealed at 400°C in argon atmosphere for 4 h.

The morphologies of the resultant samples were characterized using field emission scanning electron microscopy (FESEM, Zeiss SUPRA 35VP, Germany) operating at 5 kV. The elemental analysis, that is, atomic percentage of samples, can be determined by energy dispersion X-ray (EDX), which is equipped in the FESEM. The crystal structure and phase present in samples were determined using X-ray diffraction (XRD). The thermogravimetric analysis (TGA) was used to investigate the CO₂ adsorption using CaO-TiO₂ composite film. The CO₂ adsorption capacity could be identified through the carbonation/regeneration process via TGA curves (STA 6000, Perkin Elmer, USA). The carbonation process can be defined as CaO + CO₂ → CaCO₃ (400°C), whereas the regeneration process can be defined as CaCO₃ → CaO + CO₂ (300°C). A N₂ gas flow at a rate of 10°C/min from room temperature to 400°C, and then hold for 30 min in CO₂, finally cool down to 300°C by N₂ gas. The carbonation is set to be 400°C because TiO₂ nanotubes will collapse due to the effects of high temperature (above 500°C) and phase transition heat problems [13].

3. Results and Discussion

A preliminary experiment was carried out by adopting optimized laboratory conditions to grow TiO₂ nanotubes in C₂H₆O₂ electrolyte containing 5 wt% of H₂O₂ and 5 wt% of NH₄F. The electrochemical anodization voltage was kept at 60 V for 60 minutes and the pH of this electrolyte was maintained at 6.5. Based on our preliminary studies, this selected composition of electrolyte was favored to form the highly ordered and well-aligned nanotubular structure [14–16]. The top and cross-sectional view of pure TiO₂ nanotubes before incorporating with Ca(NO₃)₂·4H₂O dopants are illustrated in Figure 1(a). The vertical growth of TiO₂ nanotubular structure on Ti substrate could be observed clearly. The nanotubular structure has an average diameter of 110 nm and an average length of 10 μm. Then, the experimental works were continued by implementing the optimized condition to form CaO-TiO₂ nanotubes composite film with addition of different concentrations of Ca(NO₃)₂·4H₂O to electrolyte. In this case, the calcium ions (Ca²⁺) within the electrolyte were deposited on TiO₂ nanotubes. These Ca²⁺ ions were then converted into calcium oxide (CaO) for the formation of CaO-TiO₂ nanotubes, where dissolved Ca²⁺ ions reacted with oxygen species. The FESEM images of the synthesized CaO-TiO₂ nanotubes composite film with different concentrations of Ca(NO₃)₂·4H₂O are presented in Figures 1(b) to 1(d). All of these FESEM images showed that the appearance of the nanotubular structure was dependent on the concentration of Ca(NO₃)₂·4H₂O. The morphology of CaO-TiO₂ nanotubes composite film synthesized in 0.01 M presented similar appearance to the pure TiO₂ nanotubes (Figure 1(b)). This observation manifested that the small Ca²⁺ ions might be diffused into the lattice of TiO₂ significantly. Generally, amorphous TiO₂ has several defects such as oxygen-deficient defects, point defects (cationic vacancy), impurities, and microvoids to provide a better side for deposition of Ca²⁺ ions [17, 18]. Next, the average atomic percentage (at%) of the elements in the pure TiO₂ nanotubes was determined using EDX analysis. The numerical EDX analyses of the samples are listed in Table 1. The pure TiO₂ nanotubes were mainly composed of 35.37 at% Ti, 56.48 at% O, and 8.15 at% C. Notably, the incorporation of the carbon species into the nanotubular structure was identified from the pyrogeneration of organic electrolyte (ethylene glycol, C₃H₆O₂) during the electrochemical anodization stage [19]. As the concentration of Ca(NO₃)₂·4H₂O was increased to 0.05 M, most of the TiO₂ pore entrances were clogged with the CaO species as indicated in Figure 1(c). Moreover, a different surface morphology was observed for the sample synthesized in high concentration of 0.1 M Ca(NO₃)₂·4H₂O. All nanotubes formed were collapsed and completely clogged with excessive CaO species. Then, these excessive CaO species started to accumulate on the surface of TiO₂ nanotubes. This observation indicated that the content of Ca²⁺ ions that diffused into the lattice of TiO₂ reached saturation condition and started to form independent CaO layers on the surface of nanotubes. The presence of an additional peak of Ca at 3.69 KeV was identified from EDX spectra for the CaO-TiO₂ nanotubes composite film. The intensity of the Ca peak increased with increasing concentration of Ca(NO₃)₂·4H₂O as presented in Figure 2. The atomic percentages of the Ca element within CaO-TiO₂ composite film synthesized in 0.01 M, 0.05 M, and 0.1 M Ca(NO₃)₂·4H₂O were 0.15 at%, 0.25 at%, and 0.42 at%, respectively.

Table 1: Average elemental composition (at%) of pure TiO₂ nanotubes and CaO-TiO₂ composite film synthesized in different concentrations of Ca(NO₃)₂·4H₂O.

Element

Ca(NO₃)₂·4H₂O concentration
Ti (at%)
O (at%)
Ca (at%)
C (at%)

0
35.37
56.49
—
08.15

0.01
33.73
54.56
00.15
11.57

0.05
47.29
48.31
00.25
04.14

0.10
34.61
60.47
00.42
04.50

(a)

(b)

(c)

(d)

Figure 1: (a) Top view and cross-sectional view (inset) of FESEM images of pure TiO₂ nanotubes obtained; CaO-TiO₂ composite film synthesized in the electrolyte composed of different concentrations of Ca(NO₃)₂, (b) 0.01 M, (c) 0.05 M, and (d) 0.1 M.

(a)

(b)

(c)

(d)

Figure 2: EDX spectra of (a) pure TiO₂ and CaO-TiO₂ composite film with different concentrations of Ca(NO₃)₂, (b) 0.01 M, (c) 0.05 M, and (d) 0.10 M.

In the present study, XRD analysis was used to identify the crystallographic structure and the changes in the phase structure of CaO-TiO₂ composite film. The XRD patterns of pure TiO₂ nanotubes and CaO-TiO₂ composite film as a function of the concentration of Ca(NO₃)₂·4H₂O are shown in Figure 3. The obvious diffraction peaks from the XRD pattern are attributed to the anatase phase and titanium phase. Titanium phase is originated from the substrate. The diffraction peaks at 25.32°, 38.42°, 48.02°, and 55.09° correspond to (101), (112), (200), and (211) crystal plane for the anatase phase by referring to the JCPDS number 21-1272 which has tetragonal crystal system. The intensity XRD peaks of anatase phase decreased after depositing with CaO dopants. These results indicated that incorporation of Ca²⁺ ions hindered the crystallization of anatase TiO₂ significantly. Interestingly, no obvious CaO phases could be detected from the XRD patterns. There could be some possible reasons, such as insensitivity of XRD instrument to identify the small content of CaO (<1 at%) or formation of amorphous CaO layer on TiO₂ nanotubes [20].

Figure 3: XRD patterns of (a) pure TiO₂ nanotubes and CaO-TiO₂ composite film produced with different concentrations of Ca(NO₃)₂, (b) 0.01 M, (c) 0.05 M, and (d) 0.10 M (A: anatase phase; T: titanium phase).

Next, the resultant anodized CaO-TiO₂ composite film was used in the characterization of sorption CO₂ using TGA analysis. A N₂ gas flow at a rate of 10°C/min from room temperature (27°C up to 400°C), and then hold for 30 min in CO₂ atmosphere following by cooling down to 300°C by N₂ gas was carried out. In the present study, the carbonation stage was conducted at a lower temperature of 400°C due to the damage of nanotubular structure and phase transition at a higher temperature of 500°C [18]. The phase transition problem from anatase to rutile phase resulted in rapid growth of crystal size within the thin tube wall and eventually spoiled the nanotubular structure [21]. The TGA curves of the CaO-TiO₂ composite film with different concentrations of Ca(NO₃)₂·4H₂O are presented in Figure 4 and the CO₂ adsorption results are summarized in Table 2. It could be noticed that the CO₂ adsorption capacity is in the range from 1.89 to 2.45 mmol/g. The carbonation of CaO is through the following reaction: CaO + CO₂ → CaCO₃. This reaction is reversible. A schematic illustration of basic principal CO₂ absorption using anodized CaO-TiO₂ nanotubes composite film at operating temperature of 400°C is presented in Figure 5. In addition to that, it could be noticed that the CaO-TiO₂ nanotubes composite film (0.15 at%) synthesized in 0.01 M of Ca(NO₃)₂·4H₂O showed better CO₂ adsorption capacity of 2.45 mmol/g among the samples. This result inferred that the controlled concentration of Ca²⁺ ions within the anodization electrolyte for the formation of large active surface area of CaO-TiO₂ nanotubes is a crucial step in the improvement of CO₂ absorption. This finding might be attributed to the larger active surface area of CaO to absorb more CO₂ and simultaneous formation of CaCO₃. Interestingly, the sample synthesized at high concentration of 0.1 M Ca(NO₃)₂·4H₂O showed unusual patterns in TGA curve. The reason might be attributed to the fact that the nanotubular structure was collapsed and eventually formed a precipitate-like layer. Consequently, the CO₂ adsorption capacity was decreased significantly at the initial stage of carbonation process. The carbonation reaction was inhibited. This phenomenon resulted in lower weight percentage during the initial stage. However, it was found that the weight percentage was increased back at temperature of 100°C after 20 minutes of carbonation process. This result inferred that absorption of CO₂ molecules was started on anodic CaO-TiO₂ nanotubes composite film as the adsorbent weight was abruptly increased.

Table 2: CO₂ adsorption capacity of the anodized CaO-TiO₂ composite film.

Ca(NO₃)₂·4H₂O concentration
CO₂ adsorption capacity (mmol/g)

0.01 M
2.45

0.05 M
1.89

0.10 M
2.36

Figure 4: TGA curve of the anodized CaO-TiO₂ composite film with different concentrations of Ca(NO₃)₂.

Figure 5: Schematic illustration of basic principal CO₂ absorption using anodized CaO-TiO₂ nanotubes composite film at operating temperature of 400°C.

4. Conclusion

In summary, CaO-TiO₂ nanotubes composite film was successfully formed using rapid-anodic oxidation electrochemical anodization technique. The concentration of Ca(NO₃)₂·4H₂O played a critical role in the morphological control and content of CaO species loaded on TiO₂ nanotubes as well as CO₂ absorption ability. Well-aligned CaO-TiO₂ nanotubes composite film (0.15 at% of Ca element) enhanced the CO₂ absorption up to 2.45 mmol/g. The well distribution of CaO species throughout the TiO₂ nanotubular structure acted as an efficient CO₂ absorbent at 400°C.

Conflict of Interests

The author declares that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The author would like to thank University of Malaya for funding this research work under University of Malaya Research Grant (UMRG, RP022-2012D) and Fundamental Research Grant Scheme (FRGS, FP055-2013B).

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