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Investigation of monolayer dispersion of MoO3 supported on titanate nanotubes
Applied Surface Science 254 (2008) 1725–1729 www.elsevier.com/locate/apsusc
Investigation of monolayer dispersion of MoO3 supported on titanate nanotubes Wei Wang, Jingwei Zhang *, Huizhong Huang *, Zhishen Wu, Zhijun Zhang Key Laboratory for Special Functional Materials, Henan University, Kaifeng 475001, China Received 25 March 2007; received in revised form 15 July 2007; accepted 15 July 2007 Available online 21 July 2007
Abstract The monolayer dispersion of MoO3 supported on the surface of titanate nanotubes (TNT) were prepared by heating mechanical mixture of molybdate (HMA) and TNT. The result shows that MoO3 can disperse spontaneously onto the surface of TNT, and the dispersion capacity is ca. 27 mg MoO3/g TNT by X-ray diffraction (XRD). On the basis of thermogravimetric (TG) and X-ray photoelectron spectroscopy (XPS) analysis, it was found that the HMA as precursor could not decompose completely into MoO3 crystal on the surface of TNT around the threshold above decomposed temperature due to the strong interaction between HMA and the surface of TNT. # 2007 Elsevier B.V. All rights reserved. Keywords: Monolayer dispersion; Titanate nanotube; MoO3; XPS
1. Introduction It has been suggested that many salts and oxides can disperse spontaneously onto the surface of supports to form a monolayer or sub-monolayer at a temperature well below the melting point of the dispersates [1], this is a widespread phenomenon, because in this case the monolayer dispersion is a thermodynamically stable form. For each system, there is a monolayer dispersion capacity. When its loading is lower than the capacity, the salt or oxide will be in a monolayer state. While its loading exceeds the capacity, the surplus salt or oxide will remain as crystalline phase in the system together with its monolayer phase. Various explanations or models concerning the nature of the monolayer dispersion have been proposed [1–4]. The monolayer dispersion is meaningful in the performance of monolayer-type catalyst. Many supported catalysts or catalyst precursors are monolayer-dispersed system, for instance, Russell and Stokes reported that a maximum dehydrogenation activity of MoO3/Al2O3 catalyst occurred when the Al2O3 surface was covered completely by a monolayer of Mo(VI)-oxide [5]. Various combinations of Mo, Co, W, Vor Ni oxides supported on
* Corresponding authors. Tel.: +86 378 2852533; fax: +86 378 2852533. E-mail addresses: [email protected] (W. Wang), [email protected] (J. Zhang), [email protected] (H. Huang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.123
carriers used as catalyst precursors have been reported [6–8]. Among these oxides, supported MoO3 catalysts are important catalytic system in petroleum refining, chemical production, and pollution control industries; numerous studies have been carried out to analyze its function in hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrodemetalization (HDM) of petroleum and coal products. Generally, TiO2, g-Al2O3, ZrO2 as catalyst carriers were reported in quantity [9–12]. It has been found that crystalline MoO3 can disperse onto TiO2 and g-Al2O3 at temperatures of 350–360 8C below its melting point. The dispersion morphology and structure of MoO3 supported on various carriers have been thoroughly studied by various experimental techniques, such as XRD, XPS, temperature-programmed reduction, Raman and IR [13–26]. Among these carriers, the TiO2-derived titanate nanotubes (TNT) as carrier have never been reported. TNT have gradually received attention due to their one-dimensional nanostructures, uniform nanochannel, electronic conductivity, and larger specific surface area, which showed promise for applications such as photocatalysis, sensing, adsorbents, dyesensitized solar cells and mesoporous catalyst [27–29]. However, most work on TNT is the stage of synthesis process. In this contribution, titanate nanotubes (TNT) were selected as carrier for the first time. MoO3 supported on the surface of TNT were prepared by mechanical mixture of molybdate [(NH4)6Mo7O244H2O] (HMA) and TNT and heating method.
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2. Experimental Titanate nanotubes (TNT) were prepared using a chemical process similar to that described by Kasuga et al. [30]. A 100 ml of NaOH aqueous solution was placed in PTTE bottle. Five grams of TiO2 powder (P25, Degussa AG, Germany) was added to the above solution followed by refluxing with magnetic stirring for 48 h at 100 8C. After the reaction stopped, the precipitate was separated by filtration and washed with distilled water and ethanol until the pH value of the rinsing solution reached 7–8; then dried in an oven at 80 8C for more than 20 h, as-prepared TNT powders were obtained. A series of HMA/TNT samples were prepared at various HMA loadings (weight ratio of HMA to TNT at 0.010, 0.015, 0.020, 0.025, 0.030, 0.050, 0.100, 0.150, 0.200 and 0.250 (counted on MoO3 base)). Supported MoO3 samples were obtained by calcinations to decompose the supported HMA at 350 8C for 12 h in muffle. Transmission electron microscopic (TEM) images were taken on JEM 100CX-II electron microscope (JEOL, Japan), using 100 kV accelerating voltage (figure omitted here). X-ray diffraction (XRD) patterns were measured by X’Pert Pro X-ray diffractometer (Philips, Holland), using Cu Ka irradiation at a scan rate (2u) of 0.028 s1, the accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The thermogravimetric (TG) behavior of samples was performed on an Exstar 6000 TG/DSC analyzer (Seiko, Japan) at a heating rate of 10 8C/min, and in dynamic nitrogen. X-ray photoelectron spectra (XPS) were recorded with a Kratos AXIS Ultra spectrometer using monochromatized Al Ka (hn = 1486.6 eV) radiation as excitation source and hemispherical analyzer, connected to an eight-channel detector. The C 1s of adventitious carbon at 284.8 eV was used as an internal standard for the correction of charging shift. The X-ray source was operated at a power of 150 W (15 kV and 10 mA). The samples were analyzed at a take-off angle parallel to the surface. The electron spectrometer was operated in the fixed analyzer transmission (FAT) mode. The pressure of sample analysis chamber (SAC) during the data acquisition was kept below 108 Torr. First, survey spectra were collected across 1100–0 eV range at pass energy of 80 eV; then the spectra of core level were recorded at pass energy of 40 eV. To compensate for surface charging effects, the neutralization system patented was used. Analysis of spectra was conducted using the equipped software of the instrument. Quantitative analysis of spectra was carried out using integrated photoelectron peak area, and used the Kratos-supplied sensitivity factors.
Fig. 1. (a) XRD patterns of as-prepared TNT and calcined TNT at 350 8C for 12 h. (b) XPS survey spectra of TNT.
Ti atomic ratio of nanotube was ca.0.42 in TNT sample. Based on the detailed discussion about the formation mechanism and structure of TNT in our previous report [31], the structure of TNT is Na0.84H1.16Ti2O4(OH)2. The BET of calcined TNT is 217 m2/g. Fig. 2 shows XRD patterns of supported MoO3 on the surface of TNT at various HMA loadings. It was obviously seen that no crystalline was detected except TNT in lower HMA
3. Result and discussion XRD patterns of as-prepared TNT and calcined TNT were shown in Fig. 1(a). It was clearly seen that calcined TNT were essentially the same as the as-prepared, except the slight shift of d2 0 0 at 2u = 9.88. The change of d2 0 0 is related to the dehydration of interlayered OH group. Fig. 1(b) shows the XPS survey spectra of as-prepared TNT. Judging from the Na KLL peak, Na+ ion existed in sample. Na/
Fig. 2. XRD patterns of MoO3 supported on the surface of TNT at various HMA loadings (g/g) after calcinations: (a) 0.01; (b) 0.025; (c) 0.030; (d) 0.050; (e) 0.100; (f) 0.150; (g) 0.200; (h) 0.250.
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Fig. 4. TG curve of pure HMA. Fig. 3. XRD measurement of dispersion capacity of HMA supported on TNT after calcination at 350 8C.
loadings in Fig. 2, at 0.030 g/g loading (Fig. 2(c)), the slight diffraction peak (0 2 1) of crystalline MoO3 (JCPDS 76-1003) were detected, and then crystalline MoO3 diffraction peak intensity were stronger as the HMA loadings increasing. This indicated that the structure of HMA destroyed at lower loadings. When physically mixing the same loading of HMA and TNT without following calcinations, the crystalline diffraction peaks of HMA would appear (figure omitted here); this showed that the disappearance of diffraction peaks of HMA in lower loadings was due to the formation of monolayer dispersion of HMA, rather than the limit of XRD. The dispersion capacity of MoO3 on the surface of TNT can be measured by XRD quantitive phase analysis, which was applied to determine the relative amount of residual crystalline MoO3 in the samples. In the case of MoO3/TNT, the peak areas of reflections (0 2 1) of MoO3 and (0 2 0) of TNT were measured. The peak intensity ratio I MoO3 =I TNT represents the relative amount of residual crystalline MoO3 in the samples (Fig. 3). The dispersion capacity was 27 mg/g, which was not only quite smaller than theoretic value (0.31 g/g) according to monolayer closed-pack model [1], but also less than usual gAl2O3 carrier. This was possibly due to three reasons: first, the limit of TNT thermal stability led to the lower calcinations temperature and the shorter calcinations time; secondly, except the outer and inner surface, MoO3 was difficult to disperse into the adjacent layered space; lastly, on the basis of followed XPS
analysis, it was found that the HMA also dispersed onto the surface of TNT as sub-monolayer except MoO3. Fig. 4 illustrates the TG of pure HMA. It could be clearly seen that there were mainly three-weight loss peak in the temperature range of RT to 500 8C. The first weight loss was at about 130 8C; the second one was at the range of 200–240 8C; the last one was at 260–300 8C. At the 350–400 8C, there was a little weight loss, it was related possibly to the transformation of crystalline phase of MoO3. According to the TG analysis, the decomposed procedure of HMA was as follows: 130 C ðNH4 Þ6 Mo7 O24 4H2 O ! ðNH4 Þ4 Mo5 O17 þ NH3 " H2 O " 200240 C ðNH4 Þ4 Mo5 O17 ! ðNH4 Þ2 Mo4 O13 þ NH3 " H2 O " 260300 C ðNH4 Þ2 Mo4 O13 ! MoO3 þ NH3 " H2 O " XPS is a surface sensitive technique, which was employed to investigate surface structure of supported MoO3. The XPS spectra of Mo 3d and Mo 3p were shown in Fig. 5. The binding energy (BE) and the full-width-half-maxima (FWHM) of Mo 3d5/2 in pure HMA (Fig. 5(a)) and MoO3 (Fig. 5(b)) were 232.8 eV and 1.22, 233.1 eV and 1.20, respectively. As far as HMA and MoO3, the fact that it was difficult to distinguish the binding energy (BE) of Mo 3d [13,15,32,33]. So the XPS spectra of Mo 3p were investigated in the following experiment, the peaks position of Mo 3p3/2 and N 1s were also adjacent
Fig. 5. The XPS spectra of Mo 3d (left-hand) and Mo 3p (right-hand): (a) pure HMA; (b) pure MoO3.
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Fig. 6. The XPS spectra of Mo 3d and Mo 3p in various loadings (g/g): (a) 0.01; (b) 0.025; (c) 0.030; (d) 0.050; (e) 0.100; (f) 0.15.
(Fig. 5), but the peaks position of Mo 3p3/2 and N 1s was distinguished on the basis of the area ratio of p3/2 and p1/2 fixed at 2:1. Although the Mo 3p3/2 BE of pure HMA and MoO3 was near at ca. 398.8 eV, the BE of N 1s was 401.7 eV, the composition of sample was deduced by the existence of N 1s shape. Fig. 6 shows the XPS spectra of Mo 3d and Mo 3p in various HMA loadings. Clearly, the peak shape and BE of Mo 3p and Mo 3d spectra varied with HMA loadings. It was clearly seen that the N 1s peaks existed in lower loadings in Fig. 6(a)–(e); this shows that the HMA decomposed incompletely into MoO3 during calcinations, although HMA decomposed temperature was low (260 8C) as above mentioned. The reason that it was
probably the strong interaction between molybdate and the surface of TNT in lower loadings led to HMA decomposed incompletely. As HMA loadings increasing, around the dispersion capacity, the N 1s peak vanished, molybdate turned into MoO3 (Fig. 6(f)) in accord with XRD (Fig. 2(c)). Fig. 6 shows the change of BE and peak shape of Mo 3d with different HMA loadings. It was obviously seen that the peaks shape of Mo 3d were symmetric, and the FWHM was narrow in pure HMA and MoO3 samples (Fig. 5), which became asymmetric and wider after HMA supported on the surface of TNT in lower loading. The Mo 3d5/2 peak can be deconvoluted into two peaks at low loadings, and the peak shape of Mo 3d was asymmetric distinctly, which had a prominent shoulder on low binding energy of the principal peak; at higher loadings, the peak shape of Mo 3d tended to symmetry. A systematic study of Mo 3d BE values as a function of HMA loadings was summarized in Table 1. These differences in the XPS values indicated that the binding energies of Mo 3d5/2 were changing as one went from low loadings to high loadings. Table 1 shows that there is slight increase in the binding energies of the principle peak of Mo 3d3/ 2, but the shoulder peak had lower binding energies, which could be attributed to the strong interactions between HMA with the surface of TNT. The peak shape of Ti 2p for pure TNT was symmetric (Fig. 7), the binding energy of Ti 2p3/2 was 458.8 eV, were in agreement with the binding energy of Ti4+ reported by Wauthoz et al. [34], and the FWHM was narrow (1.20), which became slight asymmetric and wider after HMA supported on the surface of TNT (Fig. 7(a)–(e)). The Ti 2p3/2 peak can be deconvoluted into two peaks at low loadings, there was a new shoulder peak on high binding energy of the principal peak; at higher loadings, the peak shape of Ti 2p tended to symmetry (figure omitted). A systematic study of Ti 2p3/2 BE values as a function of HMA loadings was summarized in Table 1. These differences in the XPS values indicated that the binding energies of Ti 2p3/2 were changing as one went from low loadings to high loadings. Table 1 shows that there is slight increase in the binding energies of the principle peak of Ti 2p3/2, but the shoulder peak had higher binding energies, the difference value (D) of them is ca. 0.6. The deviation had been reported in the literature [35–37], which could be attributed to the strong interactions with the formation of Mo–O– Ti bond. Generally, salt-species as precursors can disperse on the surface of carrier as monolayer first, and then turned into
Table 1 BE and FWHM of Ti 2p3/2 and Mo 3d5/2 at different HMA loadings No.
1 2 3 4 5 6 7 8
HMA content (g/g)
0.010 0.025 0.030 0.050 0.100 0.150 0.200 0.250
Mo 3d5/2 BE (eV) and FWHM
Ti 2p3/2 BE (eV) and FWHM
Principle peak
Shoulder peak
Principle peak
Shoulder peak
232.1 232.0 232.1 232.1 232.2 232.2 232.3 232.4
231.0 230.9 231.0 230.9 None
458.4 458.8 458.2 458.6 458.7 458.8 458.7 458.9
459.0 459.3 458.8 459.2 None
(1.50) (1.51) (1.45) (1.33) (1.37) (1.38) (1.58) (1.57)
(1.16) (1.49) (1.54) (1.54)
(1.10) (1.14) (1.10) (1.10) (1.28) (1.22) (1.24) (1.22)
(1.03) (1.11) (1.09) (1.05)
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References
Fig. 7. The XPS spectra of Ti 2p of as-prepared TNT and various HMA loadings: (a) 0.01; (b) 0.025; (c) 0.030; (d) 0.050; (e) 0.100.
homological oxide. On the basis of the above analysis, the result indicates that in the process of calcinations the residual crystalline molybdate not only turns into molybdena, but also induce dispersion of the molybdena supported on the surface of TNT. The reason of this phenomenon is that the critical dispersion temperature of MoO3 on carriers was about 300 8C, and the Tammann temperature of MoO3 was relativity low (263 8C) [38]. Another, on the basis of XPS analysis the existence of N 1s peaks and the appearance of Mo 3d shoulder peaks in lower HMA loading showed that there was strong interaction between HMA and the surface of TNT, led to the HMA decomposed incompletely, part of which turned into MoO3. When the amount of HMA was around the dispersion capability of MoO3, the HMA could be decomposed completely. The appearance of Mo 3d shoulder peaks was probably due to the formation of Mo–O–Ti bond. Since Mo was more electronegativity in nature than Ti, when Ti–O–Ti bond was replaced by Mo–O–Ti bond, the balance of charge was broken around the O atom led to the rearrangement of charge. Because of the stronger electronegativity of Mo atom, the electron density increased in Mo–O bond led to the BE of Mo reduced. 4. Conclusions In conclusion, MoO3 can be sub-monolayer dispersed on the surface of TNT by heating mechanical mixture of HMA and TNT method. The dispersion capacity of MoO3 supported on TNT was 27 mg/g, less than usual g-Al2O3 carrier, but MoO3– TNT system’s catalytic activity in required further investigation. The surface morphology and structure of MoO3 and precursor on the surface of TNT were investigated exploringly. It seemed that TNT as catalyst carrier is feasible in theory so far. Another, owing to the structure’s distinctiveness of TNT, conventional monolayer dispersion mechanisms were difficult to explain the dispersion process of salts or oxides supported on TNT, this also require further investigation.
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Investigation of monolayer dispersion of MoO3 supported on titanate nanotubes Wei Wang, Jingwei Zhang *, Huizhong Huang *, Zhishen Wu, Zhijun Zhang Key Laboratory for Special Functional Materials, Henan University, Kaifeng 475001, China Received 25 March 2007; received in revised form 15 July 2007; accepted 15 July 2007 Available online 21 July 2007
Abstract The monolayer dispersion of MoO3 supported on the surface of titanate nanotubes (TNT) were prepared by heating mechanical mixture of molybdate (HMA) and TNT. The result shows that MoO3 can disperse spontaneously onto the surface of TNT, and the dispersion capacity is ca. 27 mg MoO3/g TNT by X-ray diffraction (XRD). On the basis of thermogravimetric (TG) and X-ray photoelectron spectroscopy (XPS) analysis, it was found that the HMA as precursor could not decompose completely into MoO3 crystal on the surface of TNT around the threshold above decomposed temperature due to the strong interaction between HMA and the surface of TNT. # 2007 Elsevier B.V. All rights reserved. Keywords: Monolayer dispersion; Titanate nanotube; MoO3; XPS
1. Introduction It has been suggested that many salts and oxides can disperse spontaneously onto the surface of supports to form a monolayer or sub-monolayer at a temperature well below the melting point of the dispersates [1], this is a widespread phenomenon, because in this case the monolayer dispersion is a thermodynamically stable form. For each system, there is a monolayer dispersion capacity. When its loading is lower than the capacity, the salt or oxide will be in a monolayer state. While its loading exceeds the capacity, the surplus salt or oxide will remain as crystalline phase in the system together with its monolayer phase. Various explanations or models concerning the nature of the monolayer dispersion have been proposed [1–4]. The monolayer dispersion is meaningful in the performance of monolayer-type catalyst. Many supported catalysts or catalyst precursors are monolayer-dispersed system, for instance, Russell and Stokes reported that a maximum dehydrogenation activity of MoO3/Al2O3 catalyst occurred when the Al2O3 surface was covered completely by a monolayer of Mo(VI)-oxide [5]. Various combinations of Mo, Co, W, Vor Ni oxides supported on
* Corresponding authors. Tel.: +86 378 2852533; fax: +86 378 2852533. E-mail addresses: [email protected] (W. Wang), [email protected] (J. Zhang), [email protected] (H. Huang). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.123
carriers used as catalyst precursors have been reported [6–8]. Among these oxides, supported MoO3 catalysts are important catalytic system in petroleum refining, chemical production, and pollution control industries; numerous studies have been carried out to analyze its function in hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrodemetalization (HDM) of petroleum and coal products. Generally, TiO2, g-Al2O3, ZrO2 as catalyst carriers were reported in quantity [9–12]. It has been found that crystalline MoO3 can disperse onto TiO2 and g-Al2O3 at temperatures of 350–360 8C below its melting point. The dispersion morphology and structure of MoO3 supported on various carriers have been thoroughly studied by various experimental techniques, such as XRD, XPS, temperature-programmed reduction, Raman and IR [13–26]. Among these carriers, the TiO2-derived titanate nanotubes (TNT) as carrier have never been reported. TNT have gradually received attention due to their one-dimensional nanostructures, uniform nanochannel, electronic conductivity, and larger specific surface area, which showed promise for applications such as photocatalysis, sensing, adsorbents, dyesensitized solar cells and mesoporous catalyst [27–29]. However, most work on TNT is the stage of synthesis process. In this contribution, titanate nanotubes (TNT) were selected as carrier for the first time. MoO3 supported on the surface of TNT were prepared by mechanical mixture of molybdate [(NH4)6Mo7O244H2O] (HMA) and TNT and heating method.
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2. Experimental Titanate nanotubes (TNT) were prepared using a chemical process similar to that described by Kasuga et al. [30]. A 100 ml of NaOH aqueous solution was placed in PTTE bottle. Five grams of TiO2 powder (P25, Degussa AG, Germany) was added to the above solution followed by refluxing with magnetic stirring for 48 h at 100 8C. After the reaction stopped, the precipitate was separated by filtration and washed with distilled water and ethanol until the pH value of the rinsing solution reached 7–8; then dried in an oven at 80 8C for more than 20 h, as-prepared TNT powders were obtained. A series of HMA/TNT samples were prepared at various HMA loadings (weight ratio of HMA to TNT at 0.010, 0.015, 0.020, 0.025, 0.030, 0.050, 0.100, 0.150, 0.200 and 0.250 (counted on MoO3 base)). Supported MoO3 samples were obtained by calcinations to decompose the supported HMA at 350 8C for 12 h in muffle. Transmission electron microscopic (TEM) images were taken on JEM 100CX-II electron microscope (JEOL, Japan), using 100 kV accelerating voltage (figure omitted here). X-ray diffraction (XRD) patterns were measured by X’Pert Pro X-ray diffractometer (Philips, Holland), using Cu Ka irradiation at a scan rate (2u) of 0.028 s1, the accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The thermogravimetric (TG) behavior of samples was performed on an Exstar 6000 TG/DSC analyzer (Seiko, Japan) at a heating rate of 10 8C/min, and in dynamic nitrogen. X-ray photoelectron spectra (XPS) were recorded with a Kratos AXIS Ultra spectrometer using monochromatized Al Ka (hn = 1486.6 eV) radiation as excitation source and hemispherical analyzer, connected to an eight-channel detector. The C 1s of adventitious carbon at 284.8 eV was used as an internal standard for the correction of charging shift. The X-ray source was operated at a power of 150 W (15 kV and 10 mA). The samples were analyzed at a take-off angle parallel to the surface. The electron spectrometer was operated in the fixed analyzer transmission (FAT) mode. The pressure of sample analysis chamber (SAC) during the data acquisition was kept below 108 Torr. First, survey spectra were collected across 1100–0 eV range at pass energy of 80 eV; then the spectra of core level were recorded at pass energy of 40 eV. To compensate for surface charging effects, the neutralization system patented was used. Analysis of spectra was conducted using the equipped software of the instrument. Quantitative analysis of spectra was carried out using integrated photoelectron peak area, and used the Kratos-supplied sensitivity factors.
Fig. 1. (a) XRD patterns of as-prepared TNT and calcined TNT at 350 8C for 12 h. (b) XPS survey spectra of TNT.
Ti atomic ratio of nanotube was ca.0.42 in TNT sample. Based on the detailed discussion about the formation mechanism and structure of TNT in our previous report [31], the structure of TNT is Na0.84H1.16Ti2O4(OH)2. The BET of calcined TNT is 217 m2/g. Fig. 2 shows XRD patterns of supported MoO3 on the surface of TNT at various HMA loadings. It was obviously seen that no crystalline was detected except TNT in lower HMA
3. Result and discussion XRD patterns of as-prepared TNT and calcined TNT were shown in Fig. 1(a). It was clearly seen that calcined TNT were essentially the same as the as-prepared, except the slight shift of d2 0 0 at 2u = 9.88. The change of d2 0 0 is related to the dehydration of interlayered OH group. Fig. 1(b) shows the XPS survey spectra of as-prepared TNT. Judging from the Na KLL peak, Na+ ion existed in sample. Na/
Fig. 2. XRD patterns of MoO3 supported on the surface of TNT at various HMA loadings (g/g) after calcinations: (a) 0.01; (b) 0.025; (c) 0.030; (d) 0.050; (e) 0.100; (f) 0.150; (g) 0.200; (h) 0.250.
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Fig. 4. TG curve of pure HMA. Fig. 3. XRD measurement of dispersion capacity of HMA supported on TNT after calcination at 350 8C.
loadings in Fig. 2, at 0.030 g/g loading (Fig. 2(c)), the slight diffraction peak (0 2 1) of crystalline MoO3 (JCPDS 76-1003) were detected, and then crystalline MoO3 diffraction peak intensity were stronger as the HMA loadings increasing. This indicated that the structure of HMA destroyed at lower loadings. When physically mixing the same loading of HMA and TNT without following calcinations, the crystalline diffraction peaks of HMA would appear (figure omitted here); this showed that the disappearance of diffraction peaks of HMA in lower loadings was due to the formation of monolayer dispersion of HMA, rather than the limit of XRD. The dispersion capacity of MoO3 on the surface of TNT can be measured by XRD quantitive phase analysis, which was applied to determine the relative amount of residual crystalline MoO3 in the samples. In the case of MoO3/TNT, the peak areas of reflections (0 2 1) of MoO3 and (0 2 0) of TNT were measured. The peak intensity ratio I MoO3 =I TNT represents the relative amount of residual crystalline MoO3 in the samples (Fig. 3). The dispersion capacity was 27 mg/g, which was not only quite smaller than theoretic value (0.31 g/g) according to monolayer closed-pack model [1], but also less than usual gAl2O3 carrier. This was possibly due to three reasons: first, the limit of TNT thermal stability led to the lower calcinations temperature and the shorter calcinations time; secondly, except the outer and inner surface, MoO3 was difficult to disperse into the adjacent layered space; lastly, on the basis of followed XPS
analysis, it was found that the HMA also dispersed onto the surface of TNT as sub-monolayer except MoO3. Fig. 4 illustrates the TG of pure HMA. It could be clearly seen that there were mainly three-weight loss peak in the temperature range of RT to 500 8C. The first weight loss was at about 130 8C; the second one was at the range of 200–240 8C; the last one was at 260–300 8C. At the 350–400 8C, there was a little weight loss, it was related possibly to the transformation of crystalline phase of MoO3. According to the TG analysis, the decomposed procedure of HMA was as follows: 130 C ðNH4 Þ6 Mo7 O24 4H2 O ! ðNH4 Þ4 Mo5 O17 þ NH3 " H2 O " 200240 C ðNH4 Þ4 Mo5 O17 ! ðNH4 Þ2 Mo4 O13 þ NH3 " H2 O " 260300 C ðNH4 Þ2 Mo4 O13 ! MoO3 þ NH3 " H2 O " XPS is a surface sensitive technique, which was employed to investigate surface structure of supported MoO3. The XPS spectra of Mo 3d and Mo 3p were shown in Fig. 5. The binding energy (BE) and the full-width-half-maxima (FWHM) of Mo 3d5/2 in pure HMA (Fig. 5(a)) and MoO3 (Fig. 5(b)) were 232.8 eV and 1.22, 233.1 eV and 1.20, respectively. As far as HMA and MoO3, the fact that it was difficult to distinguish the binding energy (BE) of Mo 3d [13,15,32,33]. So the XPS spectra of Mo 3p were investigated in the following experiment, the peaks position of Mo 3p3/2 and N 1s were also adjacent
Fig. 5. The XPS spectra of Mo 3d (left-hand) and Mo 3p (right-hand): (a) pure HMA; (b) pure MoO3.
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Fig. 6. The XPS spectra of Mo 3d and Mo 3p in various loadings (g/g): (a) 0.01; (b) 0.025; (c) 0.030; (d) 0.050; (e) 0.100; (f) 0.15.
(Fig. 5), but the peaks position of Mo 3p3/2 and N 1s was distinguished on the basis of the area ratio of p3/2 and p1/2 fixed at 2:1. Although the Mo 3p3/2 BE of pure HMA and MoO3 was near at ca. 398.8 eV, the BE of N 1s was 401.7 eV, the composition of sample was deduced by the existence of N 1s shape. Fig. 6 shows the XPS spectra of Mo 3d and Mo 3p in various HMA loadings. Clearly, the peak shape and BE of Mo 3p and Mo 3d spectra varied with HMA loadings. It was clearly seen that the N 1s peaks existed in lower loadings in Fig. 6(a)–(e); this shows that the HMA decomposed incompletely into MoO3 during calcinations, although HMA decomposed temperature was low (260 8C) as above mentioned. The reason that it was
probably the strong interaction between molybdate and the surface of TNT in lower loadings led to HMA decomposed incompletely. As HMA loadings increasing, around the dispersion capacity, the N 1s peak vanished, molybdate turned into MoO3 (Fig. 6(f)) in accord with XRD (Fig. 2(c)). Fig. 6 shows the change of BE and peak shape of Mo 3d with different HMA loadings. It was obviously seen that the peaks shape of Mo 3d were symmetric, and the FWHM was narrow in pure HMA and MoO3 samples (Fig. 5), which became asymmetric and wider after HMA supported on the surface of TNT in lower loading. The Mo 3d5/2 peak can be deconvoluted into two peaks at low loadings, and the peak shape of Mo 3d was asymmetric distinctly, which had a prominent shoulder on low binding energy of the principal peak; at higher loadings, the peak shape of Mo 3d tended to symmetry. A systematic study of Mo 3d BE values as a function of HMA loadings was summarized in Table 1. These differences in the XPS values indicated that the binding energies of Mo 3d5/2 were changing as one went from low loadings to high loadings. Table 1 shows that there is slight increase in the binding energies of the principle peak of Mo 3d3/ 2, but the shoulder peak had lower binding energies, which could be attributed to the strong interactions between HMA with the surface of TNT. The peak shape of Ti 2p for pure TNT was symmetric (Fig. 7), the binding energy of Ti 2p3/2 was 458.8 eV, were in agreement with the binding energy of Ti4+ reported by Wauthoz et al. [34], and the FWHM was narrow (1.20), which became slight asymmetric and wider after HMA supported on the surface of TNT (Fig. 7(a)–(e)). The Ti 2p3/2 peak can be deconvoluted into two peaks at low loadings, there was a new shoulder peak on high binding energy of the principal peak; at higher loadings, the peak shape of Ti 2p tended to symmetry (figure omitted). A systematic study of Ti 2p3/2 BE values as a function of HMA loadings was summarized in Table 1. These differences in the XPS values indicated that the binding energies of Ti 2p3/2 were changing as one went from low loadings to high loadings. Table 1 shows that there is slight increase in the binding energies of the principle peak of Ti 2p3/2, but the shoulder peak had higher binding energies, the difference value (D) of them is ca. 0.6. The deviation had been reported in the literature [35–37], which could be attributed to the strong interactions with the formation of Mo–O– Ti bond. Generally, salt-species as precursors can disperse on the surface of carrier as monolayer first, and then turned into
Table 1 BE and FWHM of Ti 2p3/2 and Mo 3d5/2 at different HMA loadings No.
1 2 3 4 5 6 7 8
HMA content (g/g)
0.010 0.025 0.030 0.050 0.100 0.150 0.200 0.250
Mo 3d5/2 BE (eV) and FWHM
Ti 2p3/2 BE (eV) and FWHM
Principle peak
Shoulder peak
Principle peak
Shoulder peak
232.1 232.0 232.1 232.1 232.2 232.2 232.3 232.4
231.0 230.9 231.0 230.9 None
458.4 458.8 458.2 458.6 458.7 458.8 458.7 458.9
459.0 459.3 458.8 459.2 None
(1.50) (1.51) (1.45) (1.33) (1.37) (1.38) (1.58) (1.57)
(1.16) (1.49) (1.54) (1.54)
(1.10) (1.14) (1.10) (1.10) (1.28) (1.22) (1.24) (1.22)
(1.03) (1.11) (1.09) (1.05)
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References
Fig. 7. The XPS spectra of Ti 2p of as-prepared TNT and various HMA loadings: (a) 0.01; (b) 0.025; (c) 0.030; (d) 0.050; (e) 0.100.
homological oxide. On the basis of the above analysis, the result indicates that in the process of calcinations the residual crystalline molybdate not only turns into molybdena, but also induce dispersion of the molybdena supported on the surface of TNT. The reason of this phenomenon is that the critical dispersion temperature of MoO3 on carriers was about 300 8C, and the Tammann temperature of MoO3 was relativity low (263 8C) [38]. Another, on the basis of XPS analysis the existence of N 1s peaks and the appearance of Mo 3d shoulder peaks in lower HMA loading showed that there was strong interaction between HMA and the surface of TNT, led to the HMA decomposed incompletely, part of which turned into MoO3. When the amount of HMA was around the dispersion capability of MoO3, the HMA could be decomposed completely. The appearance of Mo 3d shoulder peaks was probably due to the formation of Mo–O–Ti bond. Since Mo was more electronegativity in nature than Ti, when Ti–O–Ti bond was replaced by Mo–O–Ti bond, the balance of charge was broken around the O atom led to the rearrangement of charge. Because of the stronger electronegativity of Mo atom, the electron density increased in Mo–O bond led to the BE of Mo reduced. 4. Conclusions In conclusion, MoO3 can be sub-monolayer dispersed on the surface of TNT by heating mechanical mixture of HMA and TNT method. The dispersion capacity of MoO3 supported on TNT was 27 mg/g, less than usual g-Al2O3 carrier, but MoO3– TNT system’s catalytic activity in required further investigation. The surface morphology and structure of MoO3 and precursor on the surface of TNT were investigated exploringly. It seemed that TNT as catalyst carrier is feasible in theory so far. Another, owing to the structure’s distinctiveness of TNT, conventional monolayer dispersion mechanisms were difficult to explain the dispersion process of salts or oxides supported on TNT, this also require further investigation.
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