Abstract: The coprecipitation method was used to prepare Mg-Al Hydrotalcite-like compounds (HTlcs). The precipitated and calcined Mg-Al HTlcs were characterized using powder X-Ray Diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The XRD patterns for the materials indicated the presence of the hydrotalcite structure. The IR spectrum for calcined Mg-Al HTlcs with Mg/Al molar ratio of 2 showed reduction in water and CO2 characteristics due to their removal during calcination. Subsequent to the calcination, the materials were reduced using Temperature Programmed Reduction (TPR) in order to determine the H2 uptake. The H2 gas consumption was found to be very small and further modifications of the material synthesis are required to yield better results.
INTRODUCTION
Hydrogen has gained tremendous attention as a promising fuel which can be used to lower CO2 emissions and at the same time act as a replacement for the dwindling fossil fuels (Chen and Zhu, 2008). The many advantages of H2 include its possession of the highest heating value per weight among all chemical fuels apart from being regenerative and also environmentally friendly (Zuttel, 2004). The application of H2 as a fuel is mainly in H2 fuel cells with the highest potential market in the fuel cell vehicles (Walker, 2008).
Following its production, H2 must either be stored, distributed or both (Walker, 2008). The separation and direct storage of H2 after its production at high temperature may be able to save costs as the reactor system can be operated at lower temperatures (Rawadieh and Gomes, 2009). Compressed gas and liquid storage are the conventional methods of storing H2. Alternatively, H2 may also be stored as physically bound H2 where H2 gas is physisorbed to a high surface area adsorbent. A number of potential H2 adsorbents have been investigated such as metal hydrides (Chio et al., 2008) and also carbon materials (Xu et al., 2007). However, each of these materials comes with its own set of limitations and researches on other materials are necessary.
Another material that is worth investigating for the purpose of H2 storage is hydrotalcite-like compounds (HTlcs). The HTlcs is also known as Layered Double Hydroxides (LDHs) and can exist naturally or are synthetically produced (Kustrowski et al., 2005). The HTlcs are ionic and basic clays which are composed of brucite-like (Mg(OH)2) structure with trivalent cations substituting for divalent cations resulting in a layer charge. This positive charge is counterbalanced by the anions in the interlayer (Frost et al., 2005). The cations are located at the centers of octahedral sites of the hydroxide sheet. On the other hand, the vertex of the hydroxide sheet contains hydroxide ions where each -OH group is shared by three octahedral cations and points to the interlayer regions (Yong and Rodrigues, 2002). The general f ormula that can be used to represent HTlcs is [(M2+1-xM3+x(OH)2)x+. (An¯x/n.mH2O)x-], where M2+ = Mg2+,Ni2+, Zn2+, Cu2+, Mn2+ etc., M3+ = Al3+, Fe3+, Cr3+ etc., An-= CO2¯3, SO2¯4, NO¯3, Cl¯, OH¯ etc. (Yong et al., 2001). Meanwhile, x is the molar ratio of M3+/(M2++M3+) (Lwin et al., 2001) and the value of x is usually between 0.17 and 0.33 (Frost et al., 2005; Yong and Rodrigues, 2002). The stabilization of the structure is achieved by hydrogen bonds among interlayer water molecules, anions and the hydroxyl layers. Besides that, it is also stabilized by the electrostatic interactions between the layers and the anions (Han et al., 1997).
The Htlcs can possess more than two types of metal cations which makes it flexible for improved cation selection for various applications (Yu et al., 2006). This material is commonly applied in catalysis (Nieto et al., 1995), dye removal (Auxilio et al., 2009), adsorption of surfactants from aqueous solutions (Pavan et al., 2000) and also gas adsorption (Hutson and Attwood, 2008). Once calcined at temperatures exceeding 350°C, the HTlcs will produce mixed oxides which can be used for adsorption (Olsbye et al., 2002). Calcined HTlcs have been utilized in many applications including NOx reduction catalyst (Yu et al., 2006), transesterification catalyst (Zeng et al., 2008) and also high-temperature CO2 adsorption (Wang et al., 2008).
However, the use of HTlcs for high-temperature H2 storage has not been recorded in previous literature. Therefore, this study aims to develop an adsorbent derived from hydrotalcite-like compounds that can be used to store H2 at high-temperature. In this study, the coprecipitation method was used to synthesize the adsorbents. Additionally, the Mg/Al molar ratios used were 2 to 4 in order to see the effects of composition on the H2 adsorption. Powder XRD and FTIR were used to characterize the adsorbents. Meanwhile, the H2 adsorption was carried out using Temperature Programmed Reduction (TPR) and thermogravimetric analysis (TGA).
MATERIALS AND METHODS
Synthesis of Mg-Al hydrotalcite-like compounds: In this study, the Mg-Al HTlcs were synthesized by coprecipitation method with sodium carbonate (Na2CO3) as the precipitating agent. The experimental procedure for the HTlcs synthesis used is similar as the one in previous literature (Lwin et al., 2001). In the coprecipitation, three solutions with Mg/Al molar ratios of 2.0, 3.0 and 4.0 were prepared. Each solution contains 50±1g mixture of magnesium nitrate hexahydrate and aluminum nitrate nonahydrate. Next, each solution was added drop by drop into 250 mL sodium carbonate (0.5 M) solution. During the mixing, the solution was maintained at 313 K and subjected to vigorous stirring. The solution was continuously stirred for 15 min after precipitation. The precipitate was filtered and washed several times using distilled water to remove excess Na+ and NO3¯ ions and then dried overnight in an oven at 383 K.
Material characterization: Powder X-Ray Diffraction (XRD) was used to characterize the dried precipitate in order to determine the species present and their crystallinity. The crystallite sizes were calculated using the Debye-Scherrer equation as by Kannan (2004). Additionally, the synthesized materials were also characterized by Fourier transform infrared (FTIR) spectroscopy in KBr phase.
Hydrogen adsorption using TGA and TPR: In TGA experiments, the H2 adsorption by the materials was carried out according to a temperature profile which is similar to the one used in previous research (Lwin and Abdullah, 2009).
| Fig. 1: | The programmed temperature profile for hydrogen adsorption |
As shown in Fig. 1, the system is initially at room temperature (of about 30°C). Then the temperature is raised to 600°C under N2 flow, with a temperature increase of 10°C min-1. After 15 min at 600°C, the H2 flow will be introduced along with N2 into the system and the system is maintained at 600°C for another 10 min. Then, the temperature will be lowered to 400°C and the temperature is kept constant at this value for 10 min. Afterwards, the temperature will be lowered and kept constant for 10 min at other holding temperatures as shown in Fig. 1. Meanwhile, the materials will be calcined in air at 450°C for 3 h prior to the TPR experiment to provide mixed oxides for the following reduction process (Lwin and Abdullah, 2009). The TPR of the materials were conducted at 600°C.
RESULTS AND DISCUSSION
The materials were synthesized with Mg/Al molar ratio of 2 to 4 because the minimum M2+/M3+ molar ratio has to be 2:1 to avoid M3+(OH)6 octahedra sharing edges. According to the Lowenstein rule, the M3+(OH)6 octahedra sharing edges would be quite unstable due to electrical repulsion (Rives, 2002). Additionally, (Shen et al., 1994) stated that the M2+/M3+ molar ratio is normally from 1.5 to 4 in HTlcs.
Prior to the TPR experiments, the adsorbents were calcined in air at a temperature of 450°C for 3 h. The weights of the samples were reduced to about 40% of the initial weight before calcination. The weight reduction agreed with the findings by Rives (2002) since 15% of the total initial weights of carbonate-containing HTlcs consist of interlayer water.
| Fig. 2: | The XRD pattern for the Mg-Al HTlcs with Mg/Al molar ratio of (a) 2), (b) 3 (b) and (c) 4 |
| Fig. 3: | The XRD pattern for the Mg-Al HTlcs calcined at 450°C with Mg/Al molar ratio of (a) 2 and (b) 3 |
XRD analysis of synthesized materials: The powder XRD patterns of Mg-Al HTlcs are shown in Fig. 2a-c. Meanwhile Fig. 3a, b and 4 show the XRD pattern of calcined Mg-Al HTlcs for temperature of 450 and 600°C respectively. As shown in Fig. 2, the sharp peaks at 2θ angles of about 11, 22 and 35° corresponds to the (003), (006) and (009) crystal planes indicating crystalline layered structure. Furthermore, the broad and asymmetric peaks at lower 2θ angles of about 38 and 46° can be attributed to the (015) and (018) crystal planes which are characteristics of hydrotalcites (Yu et al., 2006). Additionally, the values of the d-spacing of the highest peak for all three materials are close to 7.6Å which indicates that the samples have hydrotalcite-like structure with carbonate anions in the interlayer (Rives, 2002).
It can be seen from Fig. 3 that the Mg-Al HTlcs with molar ratio of 2 was completely calcined and it has lost the hydrotalcite structure. On the other hand, the Mg-Al Htlcs with Mg-Al molar ratio of 3 still showed the peaks which correspond to the hydrotalcite structure indicating that the hydrotalcite structure is retained. This is also the same for Mg-Al HTlcs calcined at 600°C.
| Fig. 4: | The XRD pattern for the Mg-Al HTlcs calcined at 600°C, Mg/Al molar ratio (a) 2 and (b) 3 |
The preservation of the hydrotalcite structure after calcination has also been reported in earlier study (Labajos et al., 1992) and this may be caused by the rehydration of the materials during testing (Gao et al., 2008). Additionally, it can also be observed from Fig. 3 and 4 that in the calcined Mg-Al HTlcs there is a characteristic diffraction at 43 and 62°. These peaks can be attributed to the mixed Mg-Al oxides (Gao et al., 2008) especially MgO (Yu et al., 2006). This result is obtained as calcination of HTlcs will form mixed oxides (Rives, 2002).
Additionally, the XRD pattern for Mg-Al HTlcs calcined at 450°C showed a new peak at 2θ angle of 30°. The existence of the additional peak could be due to the formation of spinel phases (Yu et al., 2006). Meanwhile, the Mg-Al HTlcs with Mg-Al molar ratio of 3 showed new peaks at 2θ angles of 18 and 21°. These peaks can also be observed for both HTlcs calcined at 600°C and they can be attributed to gibbsite (Al2O3.3H2O).
Crystal size: The crystal size of the synthesized materials calculated using the Debye-Scherrer equation and for Mg-Al HTlcs with Mg-Al molar ratio of 2, 3 and 4 were 148, 152 and 125Å, respectively. The crystal sizes obtained were similar to the ones in literature (Kustrowski et al., 2005) where the coprecipitation method was also employed for material synthesis.
FTIR analysis of materials: The IR spectrum of the uncalcined samples presented in Fig. 5a-c has an intense broadband between 4000 and 2700 cm-1 which represents a superimposition of deformational vibrations of physically adsorbed water, vibrations of structural OH- groups, characteristic valent vibrations of OH…OH and/or CO32--OH¯ in hydrotalcite.
| Fig. 5: | IR spectrum of the Mg-Al HTlcs with Mg/Al molar ratio of (a) 2, (b) 3 and (c) 4 |
Besides that, this broadband may also represent characteristics stretching vibration of the Mg2+-OH- bond in Mg, Al-hydroxy-carbonate as suggested in (Parida and Das, 2000). Additionally, the band at around 1632 cm-1 may be assigned to the adsorbed interlayer water since this is the bending vibration for δ HOH. Meanwhile, the absorption band at 1383 cm-1 is attributed to the CO32¯ absorption and the impurities of NO3¯ which is probably due to the synthesis solution. Finally, the broadband at around 663 cm-1 was implied by Parida and Das (2000) as a superposition of the characteristic bonds of boehmite and hydrotalcite in this frequency interval.
On the other hand, upon calcination, there is a decrease in the intensity of water and carbonate characteristic peaks for Mg-Al HTlcs with Mg-Al molar ratio of 2 as shown in Fig. 6a-c. This is due to the removal of water and CO2 vapours during calcination at low temperature (Parida and Das, 2000). Besides that, it can be seen for Mg-Al HTlcs with Mg/Al molar ratio of 2 that the broadband at 663 cm-1 has disappeared confirming the disappearance of the hydrotalcite structure. However, the bands for NO3¯ is still present in the calcined samples as temperatures higher than 450°C are required to remove the interlayer nitrates (Kustrowski et al., 2005). The IR spectra of Mg-Al HTlcs with Mg-Al molar ratio of 3 and 4 showed that some weak bands exists in the 700-600 cm-1 wavenumber range which means that the hydrotalcite structure is still preserved.
Temperature-programmed reduction: The reduction profiles for the calcined Mg-Al HTlcs are shown in the Fig. 7a-c.
| Fig. 6: | IR spectrum of the calcined Mg-Al HTlcs with Mg/Al molar ratio of (a) 2, (b) 3 and (c) 4 |
| Fig. 7: | Reduction profile for Mg-Al HTlc with Mg/Al molar ratio of (a) 2, (b) 3 and (c) 4 |
The peak shifted towards lower time when the Mg content is higher. However, the amounts of consumed H2 decreased with increase in Mg content which are 3175.91765, 1395.65979 and 277.55466 mol g¯, respectively. The amount of gas uptake were very little as MgO is difficult to be reduced. Yu et al., 2006 reported that for calcined Mg-Al HTlcs or MgO, no reduction peaks were detected until 900°C.
CONCLUSION
The powder XRD analysis showed the presence of the hydrotalcite structure in the synthesized Mg-Al HTlcs. Additionally, the FTIR analysis gave IR spectrum which showed the characteristics of HTlcs. Besides that, it was also evident from the IR spectrum that calcination caused the removal of water and CO2 gas. The TPR experiments conducted on the materials showed that at 600ÉC only a small amount of gas is consumed.
The future work for this project will include modification of the synthesis steps with the addition of metals (eg: Li or Ni) into the adsorbent and also use of other calcination temperatures. This approach is going to be taken since addition of metal can make the material reducible (Yu et al., 2006) and it is anticipated that after prolonged exposure of the material to H2, the desired H2 uptake will occur. Subsequently, it is expected that the investigation of H2 adsorption of the adsorbents by using TGA can be conducted in the near future.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the support from Universiti Teknologi PETRONAS in carrying out this research.