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Preparation and conductivity of decatungstomolybdovanado-germanic heteropoly acid supported on mesoporous silica SBA-15, SBA-16, MCM-41 and MCM-48
Recent Progress Progress in in Mesostructured Mesostructured Materials Materials Recent D. Zhao, S. S. Qiu, Qiu, Y. Y. Tang and and C. C. Yu Yu (Editors) (Editors) D. © 2007 2007 Elsevier Elsevier B.V. B.V. All All rights rights reserved. reserved. ©
847 847
Preparation and conductivity of decatungstomolybdovanado-germanic heteropoly acid supported on mesoporous silica SBA-15, SBA16, MCM-41 and MCM-48 Qingyin Wua*, Hongxiao Jina, Wenqi Fenga and Wenqin Pang a,b* "Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China 1 State Key Lab of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, P. R. China
1. Introduction Heteropoly acids (HPA) with Keggin structure have been attracted a lot of attention because of their high proton conductivity [1] and their potential application as solid electrolyte in hydrogen-oxygen fuel cells, electrochromic displays, desiccators, ¥t sensors at low temperature, solid modified electrodes, etc. [2, 3]. Mesoporous silica (MPS) materials are known as excellent supporter for HPAs, and have been extensively studied for their use of catalysts [4]. Because of the existence of large, uniform mesopore and the abundance of silanol (SiOH) groups, HPAs supported on mesoporous materials are also of solid highproton conductor, however, limited research papers were published in this field. Therefore, research on the conductivity of HPAs supported on MPS was significant. As a further study of our previous works which were mainly on decatungstomolybdovanadogermanic heteropoly acid and polyvinyl alcohol (PVA) doped decatungstomolybdovanadogermanic heteropoly acid [5, 6], we report here the preparation and conductivity of decatungstomolybdovanadogermanic heteropoly acid supported on mesoporous silica SBA-15, SBA-16, MCM-41 and MCM-48.
848
2. Experimental Section 2.1 Synthesis Decatungstomolybdovanadogermanic acid, H5GeWioMoV04o'21H20 was prepared according to the literature method [5]. SBA-15, SBA-16, MCM-41 and MCM-48 were synthesized according to the literature methods [7-9]. Preparation of MPS (MPS= SBA-15, SBA-16, MCM-41 and MCM-48) containing 75 wt% of H5GeW10MoVO40 (HPA/MPS): HPA (1.125 g) was dissolved in 20 ml of boiling water, then 0.375 g MPS were added to the solution with stirring. The stirring was continued for 2h and then kept static at 40°C over night, drying in vacuum oven. 2.1. Characterization IR spectra were recorded on a Nicolet Nexus 470 FT-IR spectrometer using KBr pellets and X-ray diffraction patterns were obtained with a Siemens D5005 diffractometer using Cu Ka radiation (A, = 0.15418 nm). The conductivity was determined by complex impedance spectroscopy using an M273 electrochemical impedance analyzer over the frequency range from 99.9 kHz to 12Hz at room temperature. The compound was pressed at 20 MPa into a compact pellet with 10.00 mm in diameter. The conductivity was calculated as o = (1/R) • (L/S), where L is the pellet thickness, and S is the area of the pellet. 3. Results and Discussion 3.1. Infrared Spectra Fig. 1 compares the infrared (IR) spectra of the H5GeWioMoV04o (HPA) and HPA/MPS. The Keggin structure of GeM1204o5" (M=W, Mo, or V) consists of one GeO4 tetrahedron surrounded by four M3O13 sets formed by three edgesharing octahedra. There are six characteristic bands in the IR spectrum of H5GeW10MoV04o: 978 cm"1, vas (M-Od); 877 cm"1, vas (M-Ob-M); 767 cm"1, vfl, (M-Oc-M); 816 cm"1, vas (Ge-Oa); 458 cm"1, 6(O-Ge-O), all of which correspond to the spectrum of the heteropoly complex of Keggin structure previously reported. In 3800-1200 cm"1 region the absorptions associated with OH modes (stretching v and bending S) are also present: 3434.7 cm"1, v(OHXieiS.Ocm"1, <5(H-O-H). In the spectra of HPA/MPS the characteristic bands associated with Keggin structure were also observed at 974±2, 886±3, 813±3, 774±3 and 462±1 cm" .
849 849
CO
2
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber(cm"1) Fig. 1 IR spectra of the HPA and HPA/MPS.
Generally, the M-Od stretching can be considered as pure vibration and is an increase function of the anion-anion interaction. The M-Od asymmetrical stretching frequency of HPA decreases from 978 to 974±2 cm"1 when doped with MPS. This is attributed to the weakening of anion-anion interactions of the electrostatic type. We assume that due to the influences of silica such as the lengthening of the anion-anion distances, the anion-anion interactions are weakened. The stretching involved Ob or Oc are different from M-Od stretching and they present some bending characteristics. This can be assumed from geometrical considerations. Because M-Ob-M and M-Oc-M are not pure and can not be free from bending characteristics, there is competition of the opposite effects. The electrostatic anion-anion interactions lead to an increase in the stretching frequencies, but they lead to a decrease in the bending vibrations. Moreover, perturbations due to water molecules and anion-cation interactions lead to a decrease in the frequencies of vibrations and can strengthen the decreasing effect of anion-anion interactions. As a result, we can see another five peaks of HPA had a little shift from higher wave number to lower wave number (i.e. v^ (M-Ob-M) from 885.2 to 886±3 cm'1) or otherwise lower wave number to higher wave number(/.e. vas (M-O c -M) from 771.4 to 774±3 cm"1). 3.2. X-ray Powder Diffraction XRD patterns of MPS show three well-resolved peaks at small angle rang (20=0.6-6°,) are indexed as mesoporous silicate. In each of the four ranges of 26 that are 7-10°, 16-22°, 25-30° and 33-38°, although the intensities are changed due to the influence of MPS, the characteristic diffraction peaks of HPA crystal
850
are still observed in pattern of HPA/MPS. Combined with IR spectra, we are sure that the Keggin anions exist in the HPA/MPS.
MCM-41
jl
/]
MCM-41
\
( l
,;,,
' MCM-48
Interisity
MCM-48
i \
SBA-15
SBA-15 SBA-16
SBA-16
/ i | i
1 2
3
4
26 / °
5
6
5
10
15
20
25
30
35 40
26 /
Fig.2 Low-angle (left) and wide-angle (right) XRD patterns of various GeMoWnV/MPS: MPS=MCM-41, MCM-48, SBA-15, SBA-16. The labels in the figure show the MPS type.
Comparing the four wide-angel XRD patterns in Fig. 2, we can find that HPA was dispersed better in SBA-15 than in the other three MPS, which might be important for conductivity of HPA/MPS. 3.3. Conductivity Conductivity is an important parameter. The conductivity of HPA/MPS at 20"C was as follows: 1.29xl(T3 S-cm"1, HPA/SBA-15; 5.14xl(r4 S-cm"1, HPA/SBA-16; 2.67xlO 4 S-cm'1, HPA/MCM-48; 2.25xl(r 4 Sxm- 1 , HPA/MCM41. The conductivity of HPA/SBA-15 and HPA/SBA-16 is bigger than pure H5GeWioMoV04o which is 3.58xl0' 4 Scm"1 at room temperature. Considering HPA supported on the mesoporous SBA-15/SBA-16, the protons could be come from: i) HPA itself; ii) hydrolysis of the water molecules attached to the HPA crystal and the surface of mesoporous SBA-15; iii) ionization of the surface silanol (Si-OH) groups. The high-proton conductivity of SBA-15/HPA and HPA/SBA-16 is because of that: firstly, the SBA-15/SBA-16 contains a large number of micropores and mesopores that well dispersed of 'liquid HPA' which can be utilized for fast proton transport could be expected; secondly, an abundance of mesopores and micropores in SBA-15/SBA-16 facilitates the adsorption of much water than pure HPA, thus the number of protons improved;
851
thirdly, a lot of silanol groups present in the surface of SBA-15/SBA-16 strongly interact with HPA anions (confirmed by IR) that ionization of the surface silanol groups are much easier than pure mesoporous silica itself. Proton conduction in solids is suggested to take place according to the Grotthuss mechanism or vehicle mechanism [10], all those facts contribute the slightly improvement of the conductivity, however further work should be done to prove how proton conduction in HPA/SBA-15 and HPA/SBA-16 is going on. 4. Conclusion Decatungstomolybdovanadogermanic heteropoly acid H5GeWioMoV040 with Keggin structure was introduced onto mesoporous silica SBA-15, SBA-16, MCM-41 and MCM-48 by incipient wetness method, respectively. The composite materials were characterized by IR and XRD to confirm the existence of Keggin structure. The protonic conductivity of the composite materials is also reported with the 75 wt.% heteropoly acids at 20 °C. HPA/SBA-15 showed highest proton conductivity among the four composite materials. The results indicated that GeW]0MoV/SBA-15 is a new excellent high-proton conductor. 5. Ackonwledgement The financial support from the National Natural Science Foundation of China under Grant No. 20271045, the Foundation of NSFC-RFBR under Grant No. 20511120009 and the Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University for this work is greatly appreciated. 6. References [1] [2] [3] [4] [5] [6] [7] [8]
X. G. Sang and Wu, Chem. Lett., 33 (2004) 1518. M. T. Pope and A. Muller, Angew. Chem. Int. Ed. Engl., 30 (1991) 34. X. G. Sang, Q. Y. Wu and W. Q. Pang, Mater. Chem. Phys., 82 (2003) 405. T. Okuhara, N. Mizuno and M. Misono, Appl. Catal. A, 222 (2001) 63. Q. Y. Wu and X. G. Sang, Mater. Res. Bull., 40 (2005) 405. W. Q. Feng, J. Q. Wang and Q. Y. Wu, Mater. Chem. Phys., 93 (2005) 31. H. X. Jin, Q.Y. Wu, P. Zhang and W. Q. Pang, Solid State Sci., 7 (2005) 333. D. Y. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc, 120 0998)6024. [9] H. X. Jin, Q.Y. Wu and W. Q. Pang, Mater. Lett., 58 (2004) 3657. [10] S. L. Zhao and Q. Y. Wu, Mater. Lett., 60 (2006) 2650.
847 847
Preparation and conductivity of decatungstomolybdovanado-germanic heteropoly acid supported on mesoporous silica SBA-15, SBA16, MCM-41 and MCM-48 Qingyin Wua*, Hongxiao Jina, Wenqi Fenga and Wenqin Pang a,b* "Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China 1 State Key Lab of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, P. R. China
1. Introduction Heteropoly acids (HPA) with Keggin structure have been attracted a lot of attention because of their high proton conductivity [1] and their potential application as solid electrolyte in hydrogen-oxygen fuel cells, electrochromic displays, desiccators, ¥t sensors at low temperature, solid modified electrodes, etc. [2, 3]. Mesoporous silica (MPS) materials are known as excellent supporter for HPAs, and have been extensively studied for their use of catalysts [4]. Because of the existence of large, uniform mesopore and the abundance of silanol (SiOH) groups, HPAs supported on mesoporous materials are also of solid highproton conductor, however, limited research papers were published in this field. Therefore, research on the conductivity of HPAs supported on MPS was significant. As a further study of our previous works which were mainly on decatungstomolybdovanadogermanic heteropoly acid and polyvinyl alcohol (PVA) doped decatungstomolybdovanadogermanic heteropoly acid [5, 6], we report here the preparation and conductivity of decatungstomolybdovanadogermanic heteropoly acid supported on mesoporous silica SBA-15, SBA-16, MCM-41 and MCM-48.
848
2. Experimental Section 2.1 Synthesis Decatungstomolybdovanadogermanic acid, H5GeWioMoV04o'21H20 was prepared according to the literature method [5]. SBA-15, SBA-16, MCM-41 and MCM-48 were synthesized according to the literature methods [7-9]. Preparation of MPS (MPS= SBA-15, SBA-16, MCM-41 and MCM-48) containing 75 wt% of H5GeW10MoVO40 (HPA/MPS): HPA (1.125 g) was dissolved in 20 ml of boiling water, then 0.375 g MPS were added to the solution with stirring. The stirring was continued for 2h and then kept static at 40°C over night, drying in vacuum oven. 2.1. Characterization IR spectra were recorded on a Nicolet Nexus 470 FT-IR spectrometer using KBr pellets and X-ray diffraction patterns were obtained with a Siemens D5005 diffractometer using Cu Ka radiation (A, = 0.15418 nm). The conductivity was determined by complex impedance spectroscopy using an M273 electrochemical impedance analyzer over the frequency range from 99.9 kHz to 12Hz at room temperature. The compound was pressed at 20 MPa into a compact pellet with 10.00 mm in diameter. The conductivity was calculated as o = (1/R) • (L/S), where L is the pellet thickness, and S is the area of the pellet. 3. Results and Discussion 3.1. Infrared Spectra Fig. 1 compares the infrared (IR) spectra of the H5GeWioMoV04o (HPA) and HPA/MPS. The Keggin structure of GeM1204o5" (M=W, Mo, or V) consists of one GeO4 tetrahedron surrounded by four M3O13 sets formed by three edgesharing octahedra. There are six characteristic bands in the IR spectrum of H5GeW10MoV04o: 978 cm"1, vas (M-Od); 877 cm"1, vas (M-Ob-M); 767 cm"1, vfl, (M-Oc-M); 816 cm"1, vas (Ge-Oa); 458 cm"1, 6(O-Ge-O), all of which correspond to the spectrum of the heteropoly complex of Keggin structure previously reported. In 3800-1200 cm"1 region the absorptions associated with OH modes (stretching v and bending S) are also present: 3434.7 cm"1, v(OHXieiS.Ocm"1, <5(H-O-H). In the spectra of HPA/MPS the characteristic bands associated with Keggin structure were also observed at 974±2, 886±3, 813±3, 774±3 and 462±1 cm" .
849 849
CO
2
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber(cm"1) Fig. 1 IR spectra of the HPA and HPA/MPS.
Generally, the M-Od stretching can be considered as pure vibration and is an increase function of the anion-anion interaction. The M-Od asymmetrical stretching frequency of HPA decreases from 978 to 974±2 cm"1 when doped with MPS. This is attributed to the weakening of anion-anion interactions of the electrostatic type. We assume that due to the influences of silica such as the lengthening of the anion-anion distances, the anion-anion interactions are weakened. The stretching involved Ob or Oc are different from M-Od stretching and they present some bending characteristics. This can be assumed from geometrical considerations. Because M-Ob-M and M-Oc-M are not pure and can not be free from bending characteristics, there is competition of the opposite effects. The electrostatic anion-anion interactions lead to an increase in the stretching frequencies, but they lead to a decrease in the bending vibrations. Moreover, perturbations due to water molecules and anion-cation interactions lead to a decrease in the frequencies of vibrations and can strengthen the decreasing effect of anion-anion interactions. As a result, we can see another five peaks of HPA had a little shift from higher wave number to lower wave number (i.e. v^ (M-Ob-M) from 885.2 to 886±3 cm'1) or otherwise lower wave number to higher wave number(/.e. vas (M-O c -M) from 771.4 to 774±3 cm"1). 3.2. X-ray Powder Diffraction XRD patterns of MPS show three well-resolved peaks at small angle rang (20=0.6-6°,) are indexed as mesoporous silicate. In each of the four ranges of 26 that are 7-10°, 16-22°, 25-30° and 33-38°, although the intensities are changed due to the influence of MPS, the characteristic diffraction peaks of HPA crystal
850
are still observed in pattern of HPA/MPS. Combined with IR spectra, we are sure that the Keggin anions exist in the HPA/MPS.
MCM-41
jl
/]
MCM-41
\
( l
,;,,
' MCM-48
Interisity
MCM-48
i \
SBA-15
SBA-15 SBA-16
SBA-16
/ i | i
1 2
3
4
26 / °
5
6
5
10
15
20
25
30
35 40
26 /
Fig.2 Low-angle (left) and wide-angle (right) XRD patterns of various GeMoWnV/MPS: MPS=MCM-41, MCM-48, SBA-15, SBA-16. The labels in the figure show the MPS type.
Comparing the four wide-angel XRD patterns in Fig. 2, we can find that HPA was dispersed better in SBA-15 than in the other three MPS, which might be important for conductivity of HPA/MPS. 3.3. Conductivity Conductivity is an important parameter. The conductivity of HPA/MPS at 20"C was as follows: 1.29xl(T3 S-cm"1, HPA/SBA-15; 5.14xl(r4 S-cm"1, HPA/SBA-16; 2.67xlO 4 S-cm'1, HPA/MCM-48; 2.25xl(r 4 Sxm- 1 , HPA/MCM41. The conductivity of HPA/SBA-15 and HPA/SBA-16 is bigger than pure H5GeWioMoV04o which is 3.58xl0' 4 Scm"1 at room temperature. Considering HPA supported on the mesoporous SBA-15/SBA-16, the protons could be come from: i) HPA itself; ii) hydrolysis of the water molecules attached to the HPA crystal and the surface of mesoporous SBA-15; iii) ionization of the surface silanol (Si-OH) groups. The high-proton conductivity of SBA-15/HPA and HPA/SBA-16 is because of that: firstly, the SBA-15/SBA-16 contains a large number of micropores and mesopores that well dispersed of 'liquid HPA' which can be utilized for fast proton transport could be expected; secondly, an abundance of mesopores and micropores in SBA-15/SBA-16 facilitates the adsorption of much water than pure HPA, thus the number of protons improved;
851
thirdly, a lot of silanol groups present in the surface of SBA-15/SBA-16 strongly interact with HPA anions (confirmed by IR) that ionization of the surface silanol groups are much easier than pure mesoporous silica itself. Proton conduction in solids is suggested to take place according to the Grotthuss mechanism or vehicle mechanism [10], all those facts contribute the slightly improvement of the conductivity, however further work should be done to prove how proton conduction in HPA/SBA-15 and HPA/SBA-16 is going on. 4. Conclusion Decatungstomolybdovanadogermanic heteropoly acid H5GeWioMoV040 with Keggin structure was introduced onto mesoporous silica SBA-15, SBA-16, MCM-41 and MCM-48 by incipient wetness method, respectively. The composite materials were characterized by IR and XRD to confirm the existence of Keggin structure. The protonic conductivity of the composite materials is also reported with the 75 wt.% heteropoly acids at 20 °C. HPA/SBA-15 showed highest proton conductivity among the four composite materials. The results indicated that GeW]0MoV/SBA-15 is a new excellent high-proton conductor. 5. Ackonwledgement The financial support from the National Natural Science Foundation of China under Grant No. 20271045, the Foundation of NSFC-RFBR under Grant No. 20511120009 and the Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University for this work is greatly appreciated. 6. References [1] [2] [3] [4] [5] [6] [7] [8]
X. G. Sang and Wu, Chem. Lett., 33 (2004) 1518. M. T. Pope and A. Muller, Angew. Chem. Int. Ed. Engl., 30 (1991) 34. X. G. Sang, Q. Y. Wu and W. Q. Pang, Mater. Chem. Phys., 82 (2003) 405. T. Okuhara, N. Mizuno and M. Misono, Appl. Catal. A, 222 (2001) 63. Q. Y. Wu and X. G. Sang, Mater. Res. Bull., 40 (2005) 405. W. Q. Feng, J. Q. Wang and Q. Y. Wu, Mater. Chem. Phys., 93 (2005) 31. H. X. Jin, Q.Y. Wu, P. Zhang and W. Q. Pang, Solid State Sci., 7 (2005) 333. D. Y. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc, 120 0998)6024. [9] H. X. Jin, Q.Y. Wu and W. Q. Pang, Mater. Lett., 58 (2004) 3657. [10] S. L. Zhao and Q. Y. Wu, Mater. Lett., 60 (2006) 2650.