Fourier-Transform Infrared Analysis of the Dehydration Mechanism of Mg(OH)₂ and Chemically Modified Mg(OH)₂

Abstract

Mg(OH) 2 is a medium-temperature chemical heat storage material. The addition of LiCl and/or LiOH to Mg(OH) 2 has been reported to promote the dehydration of Mg(OH) 2 . However, the mechanism underlying the effect of the addition of Li compounds on the dehydration of Mg(OH) 2 has not yet been elucidated. This study focused on the dehydration of pure Mg(OH) 2 and Mg(OH) 2 with added LiCl and/or LiOH. We analyzed the role of the Li compounds in the dehydration of Mg(OH) 2 using X-ray diffraction and Fourier-transform infrared (FT-IR) spectroscopy. Our FT-IR measurements assigned the peaks of the bulk OH groups of Mg(OH) 2 , which are not referred to in previous studies. FT-IR measurements revealed that the surface state of Mg(OH) 2 with added LiOH with a Mg(OH) 2 :LiOH mole rate of 100:20 was similar to that of MgO. This indicated that the addition of LiOH to Mg(OH) 2 affected the dehydration of the Mg(OH) 2 surface. Moreover, we confirmed that LiCl affected the dehydration of the bulk Mg(OH) 2 phase. Using these results, we proposed possible mechanisms for the effects of the addition of LiCl and/or LiOH on the dehydration of Mg(OH) 2 . Lastly, we discuss the reason for the Li compounds promoting the dehydration of Mg(OH) 2 .

Fourier-transform infrared and X-ray diffraction analyses of the hydration reaction of pure magnesium oxide and chemically modified magnesium oxide

“…2 shows the XRD patterns recorded for all the samples aer they had been dried at 120 C. The same gure has been presented in our previous report. 45 The gure presents the peaks corresponding to the brucite structure of Mg(OH) 2 (for all the samples). The peaks at 18.4 , 33.3 , 38.0 , 50.7 , 58.8 , 68.2 , and 72.0 were assigned to the Mg(OH) 2 brucite structure.…”

“…The absence of the peaks could also be attributed to the partial formation of the solid solution between LiCl and Mg(OH) 2 . 24,33,45 It was also difficult to detect an LiOH phase. We have previously reported similar diffraction patterns for LiOH (and LiOH$H 2 O).…”

“…We have previously reported similar diffraction patterns for LiOH (and LiOH$H 2 O). 32,33,45 Therefore, small amounts of LiOH or LiOH$H 2 O could be present on the sample surface. The partial conversion of LiOH to Li 2 CO 3 could be attributed to the adsorption of CO 2 during the preparation process.…”

“…3 shows the FT-IR spectral proles recorded for the samples aer they had been dried at 120 C. The same gure has been presented in our previous report. 45 The peak at 3696 cm À1 was assigned to the OH groups (that exhibited low coordination numbers) present on the surface edges and/or corner sites. [63][64][65][66]68 Knözinger et al 65 assigned it to the isolated mono-coordinated OH groups.…”

“…Subsequently, the water was evaporated under reduced pressure using a rotary evaporator operated at a temperature of 40 C. Finally, the samples were dried overnight at 120 C. All samples appeared white and were obtained in their powdered forms. 23,24,[32][33][34]45 LiCl-added Mg(OH) 2 with an Mg(OH) 2 : LiCl mole ratio of 100 : 10 (referred to as L10), LiOHadded Mg(OH) 2 with an Mg(OH) 2 : LiOH mole ratio of 100 : 20 (referred to as LO20), and LiCl and LiOH co-added Mg(OH) 2 with an Mg(OH) 2 : LiCl : LiOH mole ratio of 100 : 10 : 10 (referred to as L10/LO10) were prepared following this method. Mg(OH) 2 and MgO (99.9%, 0.05 mm, Wako Pure Chemical Industries, Ltd.), devoid of Li compounds, referred to as Mg(OH) 2 -W and MgO-W, respectively, were prepared following the same method.…”