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The use of silicon oxide extracted from rice husk ash as catalyst in magnesium hydrides (MgH2) prepared by mechanical alloying method

MgH2 is one of the most promising material for hydrogen storage application, since its reversible and able to store hydrogen in large amounts of 7.6wt%. However, to be applied as a hydrogen absorber material, this material still has a deficiency, its kinetic reaction is still very slow and its operational temperature is still high. In this study, nickel and silicon oxide catalysts were used to reduce the operational temperature of MgH2. The silicon oxide used in this study was extracted from rice husks ash. The composition of MgH2 +SiO2 5wt% + Ni 10wt% was prepared by mechanical alloying technique, by varying milling time 0, 1, 5, 10 and 15 hours. XRD results indicate a decrease in particle size with longer milling time. MgH2 was obtained as the main phase followed by Ni and SiO2. SEM results showed that the longer milling time, the more particle size is homogeneous. Furthermore, thermal investigation using DSC showed that the desorption temperature obtained was in the range of values of 334.62 °C to 347.85 °C. The addition of 5 wt% SiO2 and 10 wt% Ni have improved sorption temperature and kinetic of MgH2.


Introduction
MgH 2 has been considered as one of the most promising candidate as hydrogen storage material, because it has high hydrogen storage capacity, abundance in deposit, low cost and environmentally friendly. MgH 2 has high thermodynamic stability. This characteristic lead to a high temperature operation (>350 0 C) which is not good for hydrogen storage application. Another drawback of MgH 2 is its slow reaction kinetics and difficult activation. An ideal hydrogen storage material should cost-affordable, high hydrogen storage capacity, mild operation temperature, easy to absorb and de-absorb hydrogen, and long life span [1]. Some effort have been carried out to improve the properties of MgH 2 . Reducing the particle size and doping it with different kinds of catalysts [2,3] can partially eliminated the drawbacks of MgH 2 as hydrogen storage material. Nano scaling by using ball milling is one of effective method in optimizing the thermodynamics and kinetics of MgH 2 , because it can increase fresh surface, decreased diffusion distance for hydrogen, intimate contract between reactants, and condensed atom in the grain boundaries.
The effectiveness of metal oxide type silica (SiO 2 ) as catalyst for hydrogen storage have been investigated by Norton et al [4], Jalil et al [5,6] and Mustanir et al [7]. Jalil et al reported the progress of using 1 wt%, 3 wt% and 5 wt% nano-silica (SiO 2 ), extracted from local rice husk ash [5] and local beach sand [6], to improve the thermodynamics and kinetics of magnesium hydrides. Mustanir et al have added 2 wt% SiO 2 to MgH 2 . It was found that nano-silica reduce the hydrogen Furthermore, it has been reported that small amount of metals catalysts in nanometre scale can improve the kinetics of MgH 2 [8][9][10][11][12][13][14][15][16]. In this study we have used Ni and silica (SiO 2 ) which was extracted from rice husks ash (RHA) as catalyst in MgH 2 . The sample was prepared by intensive mechanical milling, using a planetary high energy ball milling technique. Using this doubles catalyst is an effort to improve sorption and kinetics properties of MgH 2 .
Then, the MgH 2 was catalysed with 5 wt% SiO 2 and 10 wt% Ni and prepared using a highenergy milling technique. The mixing process of the sample using planetary ball milling (Fritsch, P6) with a ball to powder ratio (BPR) of 10:1 and a speed of 350 rpm for 0 h, 1 h, 5 h, 10 h and 15 h. The composition phase of the samples was determined by using x-ray diffraction (XRD; Shimadzu D6000, Cu-Kα radiation λ = 1.54060 A). Morphological structures and thermal properties were observed by scanning electron microscopy (SEM; Philips, XL30) and Differential scanning calorimetric (DSC; Shimadzu, D50).

Results and Discussions
XRD patterns showed that the main phase was dominated by MgH 2 . The smaller intensity of the patterns was influenced by longer milling time (Figure 1). It showed that milling process have changed crystallite size of the sample. The calculation of crystallite size is summarized in the Table 1. Table 1 revealed that the crystal size is getting smaller when the milling time was extended, except for milling time about 15 h. There are the agglomeration phenomena during the milling process at the sample. This agglomeration occur due to the puncture and pressure given by the balls against the powder which cause the particle joint each other, and it part of mechanical alloying process. This agglomeration reduce surface area of magnesium contact with hydrogen, therefore it can reduce the sorption properties of MgH 2 [6].  Thermal observation, which was analyzed with DSC, is shown in Figure 2, and the resume can be seen in Table 2. We can observe that desorption temperature did not change significantly when crystallite size getting smaller, but desorption time is shorter ( Table 2). Addition 5 wt % SiO 2 , extracted from RHA, has improved the desorption temperature of pure MgH 2 that was in range of 409 0 C. Meanwhile, 10 wt% Ni has improved desorption time of MgH 2 , because of catalytic effect of Ni [10].   (Figure 3) showed that the longer milling time, the more particle size is homogeneous, and the particle size decrease with extending of milling time. However, the sample that milling for 15 h were agglomerated. These results agree with XRD result.

Conclusions
MgH 2 with double catalyst was successfully prepared using a high-energy ball milling technique. As a result, the addition of 5 wt% SiO 2 and 10 wt% Ni have improved sorption temperature and kinetic of MgH 2 . The addition of SiO 2 in the sample have decreased desorption temperature, meanwhile, the Ni catalyst decreased the desorption time. The Extending of milling time decrease crystallite size and increase the surface area and defect concentration.