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.
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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].
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.
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).