In situ study on interactions between hydroxyl groups in kaolinite and re-adsorption water

The interactions between O–H groups in kaolinite and re-adsorption water is an important aspect that should be considered in the hydraulic fracturing method for the production of shale gas, because the external water adsorbed by kaolinite in shale would significantly affect the desorption of methane. In this study, the interactions were investigated via changing the amount of O–H groups and re-adsorption water in kaolinite by heating treatment and water re-adsorption. To overcome the overlap of IR vibration bands of the O–H functional groups in H2O and those in parent kaolinite, kaolinite samples with D2O re-adsorption were prepared by drying the H2O from raw kaolinite and soaking the dried kaolinite in D2O. The interactions between O–H groups in kaolinite and D2O molecules were investigated by in situ DRIFT and TG-MS. The results demonstrated that the vibration at 3670 ± 4 cm−1 in the DRIFT spectra could be due to the outer O–H groups of the octahedral sheet on the upper surface of the kaolinite microcrystal structure, rather than a type of inner-surface O–H group. All types of O–H groups, including the inner O–H groups in kaolinite, could be transformed into O–D groups after D2O re-adsorption at room temperature. The inner-surface O–H groups in kaolinite are the most preferred sites for D2O re-adsorption; thus, they would be the key factor for studying the effect of re-adsorption water on methane desorption. When the temperature increased from 100 °C to 300 °C, two layers of kaolinite slipped away from each other, resulting in the transformation of inner-surface O–H groups into outer O–H groups. Thus, the temperature range of 100 to 300 °C was suggested for the heat treatment of kaolinite to decrease the content of inner-surface O–H groups; thereby, the amount of re-adsorption water was reduced. However, to thoroughly remove the re-adsorption water, a temperature higher than 650 °C should be used.


Introduction
Water obviously affects the desorption behaviors of shale gases from clay minerals. [1][2][3][4] The hydraulic fracturing method could take external water into shale. 5,6 Thus, it would be of practical meaning to study the effect of the interaction between readsorption water and shale on shale gas behaviors. Kaolinite, as one of main components in clay minerals, interacts strongly with water through hydrogen bonds. 3,7-9 Ledoux et al. 10 summarized the four types of O-H groups in kaolinite, which include two types of outer O-H groups, located on the brokenedge site and on the upper surface of the kaolinite microcrystal structure; the inner-surface O-H groups, which exist on the surface of the octahedral sheets opposite the tetrahedral oxygen; and the inner O-H groups. Different O-H groups have different energies of bonding with water. The water behavior could be changed by changing the O-H groups. Therefore, a study on the interactions between O-H groups in kaolinite and water would provide fundamental understanding to control the interaction between water and kaolinite in shale, and thus to enhance the amount of methane desorbed from clays. 3,4,11 Previous studies were carried out using D 2 O to investigate the assignment of the O-H groups in kaolinite. 10,[12][13][14][15] However, scant reports use D 2 O instead of water to study the readsorption water behavior in kaolinite. Diffuse reectance infrared Fourier transform (DRIFT) has been widely used to observe the O-H and O-D group vibrations in kaolinite 16,17 and lignite. 18,19 In our previous studies, 18,19 in situ DRIFT and thermogravimetry coupled with mass spectrometry (TG-MS) were employed to research the heating process of lignite containing re-adsorption D 2 O. The interactions between hydrogen bonds in lignite and re-adsorption water were investigated in detail. In this study, the two instruments were also applied to research the heating process of kaolinite containing re-adsorption D 2 O. The changes in DRIFT spectra of O-H and O-D groups and the release of ion-related D during the heating process of kaolinite was systematically analyzed.
The exchange of hydrogen by deuterium results in a lower wavenumber of O-D bonds corresponding to the O-H groups. 12,13 Ledoux et al. 10 revealed that aer kaolinite was heated at 200 C and deuterated under vacuum, the intensity of three O-H peaks at 3695 cm À1 , 3670 cm À1 , and 3650 cm À1 decreased signicantly, whereas the 3620 cm À1 peak was not obviously affected. They assigned the former three peaks as inner-surface groups and the last peak as an inner O-H group. Mlchaelian et al. 14 reported that inner-surface O-H groups are more readily exchanged, so that aer deuteration, the peaks at 3651 cm À1 , 3669 cm À1 , and 3695 cm À1 easily shied to 2691 cm À1 , 2706 cm À1 , and 2725 cm À1 , respectively. However, according to the investigations of De Donato 13 and Rouxhet, 15 the inner O-H group was also exchanged with O-D aer natural and low deuteration in kaolinite at room temperature. Therefore, the exchange behaviors of inner O-H groups with O-D groups at room temperature were investigated in this paper. The 3697 cm À1 peak could be assigned to the inner-surface O-H groups oriented perpendicular to the octahedral sheet. 10,20 However, there are some conicts on the assignment of O-H groups at 3669 cm À1 and 3652 cm À1 . The two peaks were attributed to two inner-surface O-H groups approximately parallel to the octahedral sheet, according to the study of Kloprogge. 20 Farmer et al. 21 assigned the two peaks as the in-plane vibrations of the inner-surface O-H groups. However, Johansson et al. 22 reported that the peak at 3674 cm À1 could represent outer-surface O-H groups on the surface of the kaolinite. In this paper, the properties of the O-H groups at 3669 cm À1 and 3652 cm À1 were studied.
In this study, D 2 O instead of H 2 O was used to soak dried kaolinite. The DRIFT spectra of O-H and O-D bands and the release of ions of D, HD, OD/H 2 O and HOD during the heating of kaolinite containing re-adsorption D 2 O were detected by in situ DRIFT and TG-MS, respectively. Kaolinites heated at different temperatures were characterized by X-ray diffraction (XRD) and X-ray uorescence (XRF) to study the changes in crystal structure and elemental content of kaolinite during the heating process.

Preparation of samples
Raw kaolinite was purchased from Sigma-Aldrich (Irvine, UK). Its particle size was smaller than 74 mm. As samples need to be dried before XRF testing, the XRF analysis of dried kaolinite instead of that of raw kaolinite was analyzed, as shown in Table  1. As can be seen, the minor ionic impurities in the kaolinite were Fe, Ti, Ca, Na and K. The content of SO 3 indicated that alunite could exist in the raw kaolinite. 23 In order to minimize the interference of H 2 O on the DRIFT spectra of kaolinite, the raw kaolinite sample was initially dried at 110 C to remove H 2 O. The dried kaolinite was soaked in D 2 O (99.9% D, Aladdin Reagent) to prepare the kaolinite sample containing re-adsorption D 2 O. The soaking was carried out at room temperature for 7 days to ensure the full exchange of O-H groups with O-D groups. The samples of dried kaolinite and kaolinite containing re-adsorption D 2 O were denoted DK and K-D 2 O, respectively.
In order to analyze the changes in crystal structure and elemental content during the dehydroxylation of kaolinite, the DK sample was heated at the different temperatures of 500 C, 600 C and 650 C to prepare the DK-500, DK-600 and DK-650 samples, respectively, for XRD and XRF analyses.

Analyses of the interactions between hydroxyl groups in kaolinite and D 2 O
The interactions between hydroxyl groups in kaolinite and D 2 O were investigated using in situ DRIFT obtained using the Vertex 70 infrared spectrometer (Bruker Co. Ltd., Germany) and TG-MS apparatus (Setsys Evolution, SETARAM, France).
The DRIFT spectra of DK and K-D 2 O employed at 30 C and the in situ DRIFT spectra measured at 100 C, 200 C, 300 C, 400 C, 500 C, 600 C and 650 C of K-D 2 O during heating were recorded in the wavenumber range of 600-4000 cm À1 with the resolution of 4 cm À1 . The spectra of DRIFT and in situ DRIFT were measured with a Vertex 70 infrared spectrometer (Bruker Co. Ltd., Germany), and the reaction cell was the 0030-102 highpressure/high-temperature accessory (Pike Co. Ltd., USA) with ZnSe windows. The detailed parameters for the spectrometer and the measurement process can be found in our previous studies. 18,19 Specically, DK and K-D 2 O were fully ground for 30 min under N 2 protection before each run. About 30 mg of the ground DK and 40 mg of ground K-D 2 O were used for the tests of DRIFT and the in situ DRIFT, respectively. The spectra were recorded over the range of 600-4000 cm À1 at a resolution of 4 cm À1 .
In order to verify the ndings from in situ DRIFT, the D, HD, H 2 O/OD and HOD released during the heating of K-D 2 O were detected by the TG-MS apparatus. For each run, 20.0 AE 0.5 mg K-D 2 O sample was heated from 30 C to 700 C with the heating rate of 10 C min À1 using 100 mL min À1 high-purity He as the carrier gas. The test was repeated at least twice, and the detailed procedure can be found in the previous study. 8 During the TG-MS analysis, the TG and DTG curves could be obtained, and the D-containing substances released were simultaneously analyzed by MS.

Characterization of samples
To study the crystal structure change of kaolinite during dehydroxylation, the DK, DK-500, DK-600 and DK-650 samples were characterized using XRD equipment (Bruker D2 Phaser desktop X-ray diffractometer). The diffraction conditions were Cu Ka 1 /a 2 radiation, a 30 kV tube voltage and 10 mA tube current. The chemical compositions of DK, DK-500, DK-600 and DK-650 were analyzed by XRF (Bruker S8 TIGER) according to the method provided by ASTM D6349. 24 The pore structure of DK was measured by N 2 adsorption at À196 C (ASAP 2020, Micromeritics, USA). The calculation of specic surface area (S BET ) was done according to Brunauer Emmett Teller (BET) equation. The volume of total pores (V total ) and macropores (V macro ) were calculated using the BJH equation according to the adsorption curve.

Physical and chemical properties of kaolinite
As shown in Table 2, the S BET of kaolinite is 18.2 m 2 g À1 , which is between that of low-defect kaolinite (11.7 m 2 g À1 (ref. 25) or 16.1 m 2 g À1 ) 26 and that of high-defect kaolinite (23.5 m 2 g À1 , 23.1 m 2 g À1 or 22.4 m 2 g À1 ) 26 , indicating that the sample chosen in this study possesses both the properties of low-and highdefect kaolinite. The values of V total and V macro were 10.4 cm 3 g À1 and 6.2 cm 3 g À1 respectively, suggesting that pores in the kaolinite mainly existed as macropores. Water in kaolinite could exist in different forms. Water molecules could be conned in microplatelets or in contact with the central platelets of kaolinite. 27 Water in macropores is bulk (free) water, which possesses almost the same properties as pure water. 28 Thus, a high proportion of macropores in the kaolinite could imply that the impact of pores on the properties of readsorption water is lower than that of kaolinite possessing high micropore content.
The DRIFT spectrum of DK is displayed in Fig. 1. The narrow band around 3620 cm À1 (Fig. 1a) represents the inner O-H groups' vibration in kaolinite. 5 The peak at 3695 cm À1 is assigned to the inner-surface O-H groups oriented perpendicular to the octahedral sheet. 10,13,21 For the peaks at 3670 cm À1 and 3650 cm À1 , Farmer et al. 21 assigned the two peaks to the inplane vibrations of inner-surface O-H groups, whereas Johansson et al. 22 reported the peaks at 3674 cm À1 could represent outer surface O-H groups on the surface of the kaolinite. Ledoux et al. 10 pointed out two types of outer O-H groups in kaolinite. The two types of outer groups were located on the broken edge site and on the upper surface of kaolinite microcrystal structure. In this study, the peak at 3670 cm À1 is assigned to the outer surface O-H groups on the surface of kaolinite microcrystal structure, and the peak at 3650 cm À1 could be the other type of outer O-H groups. The reason for the assignment of the two peaks will be discussed below.
As displayed in Fig. 1b, the peak at 1110 cm À1 was assigned to the stretching mode of apical Si-O, and the band at 1010 cm À1 was caused by the stretching vibrations of Si-O-Si. 29,30 Several adsorption peaks appeared at lower wavenumbers: 950 cm À1 (surface Al-OH-Al bonds), 17,31 806 cm À1 (amorphous silica), 30 29 and 660-670 cm À1 (symmetric Al-O). 32,33 3.2 Interaction between re-adsorption water and hydroxyl groups of kaolinite 3.2.1 Effect of re-adsorbed water on hydrogen bonds in kaolinite. The DRIFT spectra of DK and K-D 2 O at 30 C are shown in Fig. 2. The signicant difference between the spectra of DK and K-D 2 O samples indicate that the water re-adsorption behaviors of kaolinite could be revealed through the isotopic substitution method. Compared to DK, there was a broad prominent band centered at approximately 2500 AE 10 cm À1 , which could be assigned to O-D vibration 18,34 (Fig. 2b). The intensities of peaks at 3695 cm À1 (inner-surface O-H group) and at 3620 cm À1 (inner O-H group) were weak for K-D 2 O (Fig. 2a).
The results indicate that there were interactions between readsorption D 2 O and O-H groups on the inner-surface sites and the inner sites of kaolinite. As shown in Fig. 2c, the peaks at 757 cm À1 (Si-O-Al) and 710 cm À1 (O-Si-O) shied to lower wavenumbers, whereas the peaks at 660-670 cm À1 (symmetric Al-O) shied to higher wavenumbers. According to the kaolinite structure, the interaction between re-adsorption D 2 O and inner O-H groups could affect the lower wavenumber transformation   Fig. 3. Fig. 3 shows the peaks around 757 cm À1 and 670 cm À1 related to Si-O-Al and Al-O, respectively, which gradually decreased as heating temperature increased from 30 C to 300 C, indicating that the kaolinite skeleton structure could be slightly affected at this stage. When the temperature was higher than 500 C, the intensity of the peaks around 1110 cm À1 , 1010 cm À1 , 950 cm À1 , 806 cm À1 and 710 cm À1 signicantly declined. This indicated that the crystal structure of kaolinite initially collapsed at 500 C. At 500 C, the peak around 655 cm À1 (Si-O-Si) 23,24 shied to 661 cm À1 , and a new peak at 636 cm À1 arising from [AlO 4/2 ] À (ref. 35) was found. It is reported that the rst decomposition stage of kaolinite occurs at around 500 C, during which water is eliminated and the octahedral sheet collapses. 36 This could be the reason for the appearance of a new peak arising from [AlO 4/2 ] À . When the   temperature was raised to 650 C, the bonds relating to Si and Al almost disappeared, indicating that heating to 650 C could obviously destroy the crystal structure of kaolinite.
As illustrated in Fig. 4a, the intensity of peaks in the range of 3750-3550 cm À1 corresponding to O-H vibration roughly decreased with increasing temperature. In order to quantitatively analyze the changes of the O-H groups, the region of O-H (3750 cm À1 to 3550 cm À1 ) vibration in in situ DRIFT spectra was deconvoluted into six Lorentzian peaks ( Fig. 4b and S1 †), because Lorentzian shape was reported very suitable for the deconvolution of DRIFT spectra of O-H groups in kaolinite. 37 For the bands, ve of them are due to O-H vibration, designated as follows: peak I around 3695 AE 2 cm À1 (inner-surface O-H groups), peak II around 3670 AE 4 cm À1 , peak III around 3652 AE 3 cm À1 , peak IV around 3620 AE 5 cm À1 (inner O-H group), and peak V around 3580 AE 5 cm À1 (O-H group in intercalated water). 17 The band at 3720 AE 5 cm À1 was probably caused by the presence of trace amounts of dickite. 37 Moreover, for the deconvolution, the initial half-widths for these bands were used according to the references 7,31 and our previous studies. 18,38,39 Peak areas of different O-H groups in the in situ DRIFT spectra obtained at different temperatures for the K-D 2 O sample are summarized in Table 3. When the heating temperature increased from 30 C to 100 C, the areas of the ve peaks remained almost unchanged, indicating that the O-H groups in kaolinite structure were intact at this stage. The peak V area signicantly decreased at 300 C, suggesting that the intercalated water was removed.
In situ DRIFT spectra of the O-D region are shown in Fig. 5. The intensity also declined with the increase of temperature. As can be seen from Fig. 5a, the bands around 2723 cm À1 , 2707 cm À1 , 2695 cm À1 , 2671 cm À1 , and 2610 cm À1 can be assigned to the vibrations of O-D groups. These O-D bonds have a one-to-one correspondence relationship with the bands of O-H vibration at 3695 cm À1 , 3670 cm À1 , 3650 cm À1 , 3620 cm À1 and 3580 cm À1 , respectively, which could be proved by the isotopic ratio of n (OH) /n (OD) reported from 1.352 to 1.374. 12,18,40 Also, based on the isotopic ratio of 3720/2751 ¼ 1.352, the O-D bands' vibration at 2751 cm À1 could be attributed to the D 2 O adsorbed on dickite. The bonds at 2660 cm À1 , 2574 cm À1 and 2540 cm À1 could be because of the imperfections in the kaolinite structure that were occupied by readsorption D 2 O. 40 As shown in Fig. 5a, the intensities of bonds at 2610 cm À1 disappeared at 300 C, indicating that the intercalated re-adsorption D 2 O could be removed at this stage. The peaks at 2751 cm À1 , 2660 cm À1 , 2574 cm À1 and 2540 cm À1 disappeared at 300 C, indicating that O-D groups bonded at these sites could also be removed. The areas of O-D vibration at the region of 2800 cm À1 and 2650 cm À1 have almost disappeared at 650 C, suggesting the majority of adsorption D 2 O could be removed at this temperature.
Similar to the analysis of O-H region, the O-D region (2800 cm À1 to 2650 cm À1 ) was also deconvoluted in the same way, as shown in Fig. 5b and S2 in ESI. † There were four peaks in Fig. 5b, peak I 0 at 2723 AE 3 cm À1 , peak II 0 at 2707 AE 3 cm À1 , peak III 0 at 2695 AE 3 cm À1 , and peak IV 0 at 2671 AE 2 cm À1 . The    Fig. 6. At 100 C, the area sequence for the different O-H groups in Fig. 6a is as follows: O-H groups corresponding to peak III > innersurface O-H groups (peak I) > O-H groups corresponding to peak II > inner O-H groups (peak IV), whereas the area sequence for different O-D groups in Fig. 6b followed the inner-surface O-D groups (peak I 0 ) > O-D groups corresponding to peak III > inner O-D groups (peak IV 0 ) > O-D groups corresponding to peak II 0 . The results indicated that the ability to re-adsorb water was different for different O-H groups, and the majority of readsorption D 2 O was initially adsorbed on the inner-surface sites of kaolinite. At 650 C, the inner-surface O-H groups (peak I) approximately disappeared, whereas the other three O-H groups still existed, indicating that the properties of O-H groups corresponding to peak II and III, and those of inner O-H groups (peak IV), were different from the properties of innersurface O-H groups. Peak II was reported to be a type of outer O-H group 10,22 or a type of inner-surface O-H group. 20,21 Thus, it was concluded that peak II could be assigned as an outer O-H group rather than an inner-surface O-H group.
As temperature increased from 100 C to 300 C, the area of inner-surface O-H groups (peak I) and that of inner-surface O-D groups (peak I 0 ) signicantly decreased, while the areas of peaks III and III 0 and peaks IV and IV 0 remained almost unchanged considering tting errors, whereas the peak II area slightly increased and the peak II 0 area at 200 C and 300 C became signicantly higher than that at 100 C. The area decrease of peaks I and I 0 and area increase of peaks II and II 0 indicated that peak I shied to peak II, and peak I 0 correspondingly changed into peak II 0 . Kristóf and Redaoui 41,42 reported that proton delocalization and predehydroxylation of kaolinite could occur between the temperatures of 30 C to 360 C. The results of Fig. 3 also proved that the kaolinite skeleton structure could be slightly affected as heating temperature increased from 30 C to 300 C, based on the changes of peak intensity at 757 cm À1 (Si-O-Al) and 670 cm À1 (Al-O). Thus, the transformation between peak I/I 0 and peak II/II 0 could be explained as the transformation of O-H/O-D groups on the inner-surface (peak I/I 0 ) into outer O-H/O-D groups of the octahedral sheet found on upper surface of the microcrystal structure (peak II/II 0 ). The transformation could be due to the two layers of kaolinite slipping away from each other.
From 300 C to 650 C, the areas of peaks I and I 0 continuously decreased, whereas the areas of peaks II, III and IV approximately kept unchanged until 400 C. At 500 C, the areas of peaks II and III dramatically decreased, indicating the initial  collapse of kaolinite crystal structure and the start of dehydroxylation. The areas of peaks II 0 and III 0 were also decreased at 500 C, suggesting that the outer O-D groups on the upper surface of kaolinite microcrystal structure and the O-D group's vibration at peak III 0 were also removed at the initial dehydroxylation stage. The areas of peak IV and IV 0 decreased at 600 C, suggesting that the inner O-H and O-D groups can be obviously removed during the dehydroxylation. At 650 C, the four O-D bonds almost disappeared, indicating that the readsorption D 2 O was approximately thoroughly removed at this temperature. The trace content of O-D bonds could be because of the inhomogeneous dehydroxylation. 43

Thermal decomposition behaviors of kaolinite
In order to further analyze the interaction of water with kaolinite, the D, HD, H 2 O/OD and HOD released during the heating process of K-D 2 O from 30 C to 700 C were measured by TG-MS. As shown in Fig. 7a, there were three weight loss steps in the TG curve. The rst weight loss appearing at 75 C ( Fig. 7b) was due to the evaporation of free adsorption D 2 O. The second weight loss happened from 150 C to 450 C, which could be assigned to the predehydration process, suggesting reorganization in the octahedral layer, because reorganization was reported to always accompany the release of the intercalated water in kaolinite. 44,45 The third step of weight loss, at 450 C to 650 C, was attributed to the dehydroxylation of kaolinite and the formation of metakaolinite.
As illustrated in Fig. 8 (30-150 C) corresponded to the release of free readsorption D 2 O (Fig. 8b). The HOD (Fig. 8a)   In order to investigate the changes in crystal structure and chemical composition of kaolinite during the dehydroxylation process, the DK, DK-500, DK-600, and DK-650 samples were analyzed by XRF and XRD.
The XRF results are shown in Table 5. For DK, DK-500 and DK-600 samples, the contents of major components in kaolinite (SiO 2 , Al 2 O 3 ) were approximately unchanged. However, for DK-650 sample, the content of Al 2 O 3 decreased, and that of SiO 2 increased. The result indicated that the dehydroxylation process was accompanied by the release of the central atoms from the octahedral, i.e., the removal of Al from the octahedral sheets. During the dehydroxylation process, kaolinite lost O-H groups and was transformed to metakaolinite. 23 Corresponding to the decrease in Al content, the concentration of Si slightly increased. As for the minor ionic impurities in kaolinite samples, the content of K 2 O, Na 2 O, and SO 3 showed decreasing trend when the heating temperature increased. The results could be due to the evaporation of alkali metals and the decomposition of alunite, which are found in raw kaolinite (see Table 1). As heating temperature increased to 650 C, the concentration of Fe 2 O 3 and CaO remained approximately unchanged, and that of TiO 2 slightly increased. The results demonstrated that the contents of minor ionic impurities (Fe 2 O 3 , CaO and TiO 2 ) did not decrease during the dehydroxylation process. The concentration changes of the three minor ionic impurities could also be attributed to the decrease of Al 2 O 3 content. Fig. 9 shows the XRD results. Two components, kaolinite and quartz, could be obviously found in the XRD patterns of kaolinite samples. The intensities of kaolinite peaks slightly decreased when temperature was higher than 500 C, indicating that the kaolinite crystal structure initially decomposed at the temperature. When the temperature increased to 600 C, the XRD pattern showed amorphous structure with partial crystalline peaks of kaolinite and quartz, suggesting the incomplete dehydroxylation and the decomposition of the crystal structure of kaolinite. At 650 C, the XRD pattern showed amorphous pattern with the trace crystalline peak of quartz, implying the amorphous structure of metakaolin. The diffraction angles for quartz and kaolinite corresponded to those found in the literature . 47,48 3.4 Mechanism of interaction between hydroxyl groups in kaolinite and re-adsorption water Based on the above discussion and the kaolinite structure reported in references, 10  groups would be transferred into outer O-H groups between 100 C and 300 C. Thus, the temperature range of 100-300 C was suggested for heat treatment of the kaolinite to decrease the content of inner-surface O-H groups, thereby reducing the amount of re-adsorption water. Moreover, if the kaolinite could be heated at 650 C, the effect of re-adsorption water on the desorption behaviors of methane would almost be eliminated.

Conclusions
The DRIFT vibration at 3670 AE 4 cm À1 could be due to the outer O-H groups of the octahedral sheet on the upper surface of kaolinite microcrystal structure rather than a type of innersurface O-H group. All types of O-H groups in kaolinite could be exchanged into O-D groups when soaking the dried kaolinite in D 2 O at room temperature. Most of the re-adsorption D 2 O was initially adsorbed on the inner-surface sites of kaolinite. As heating temperature increased from 100 C to 300 C, two layers of kaolinite slipped away from each other, resulting in the transformation of the inner-surface O-H/O-D groups into outer O-H/O-D groups. The inner-surface O-D groups, the outer O-D groups, and O-D group vibration at 2695 AE 3 cm À1 were gradually removed at 500 C, and the inner O-D groups were removed at 600 C, whereas all the O-D groups were approximately thoroughly removed at the temperature of 650 C.

Conflicts of interest
There are no conicts to declare.