Accepted Article Title: Identifying the Elusive Framework Niobium in NbS-1 Zeolite by UV Resonance Raman Authors: Yong Chen, Xinping Wang, and Lejian Zhang This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: ChemPhysChem 10.1002/cphc.201700873 Link to VoR: http://dx.doi.org/10.1002/cphc.201700873
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COMMUNICATION Identifying the Elusive Framework Niobium in NbS-1 Zeolite by UV Resonance Raman Yong Chen, Xinping Wang* and Lejian Zhang Abstract: It was found that bands at 739, 963 and 1107 cm –1 in resonant Raman spectra are characteristics of framework pentacoordinated NbV–OH species, and that band at 1336 cm –1 in UV Raman spectra excited by 320 nm is a sensitive detector to identify extraframework niobium species. The change of framework pentacoordinated NbV–OH species into Nb+ and NbO– species due to dehydration was definitively confirmed based on UV resonance Raman and UV/Vis results.
Niobium-containing zeolites have received increased attention due to their novel catalytic activity.[1–3] Their performance not only depends on the pore structure, but also on the nature of active sites,[3] however, the definite information is hardly obtained by the conventional spectroscopic techniques.[1,3,4] Especially, in the situation where the concentration of niobium is extremely low and various types of niobium species coexist. Thus, differentiating a specific niobium species from a complex molecular system generates a great challenge. Strong efforts have been made to characterize the framework niobium species in zeolites.[1,4–9,11,12] Ziolek and coworkers investigated the framework niobium in zeolites, e.g. NbMCM-41[5] and NbZSM-5,[6] by H2-TPR, FTIR of NO adsorption, and ESR techniques. They proposed that framework Nb+ and NbO– species detected in the zeolites should be generated from the penta-coordinated NbV–OH species that possibly exists in the zeolite before dehydration.[3,5,6] Whereas, the evidence for the penta-coordinated NbV–OH species in the zeolites was not straightforward, as the band at 966 cm–1 observed in IR spectra was confounded with that of silanol group.[3,5,7] Recently, Tielens and co-workers proposed that the framework NbV–OH species could be stabilized by the zeolite framework, based on their FTIR measurements and Density functional calculations.[8,9] The NbV–OH species in framework has been compared theoretically with that on silica support by DFT.[10] However, corresponding absorption of the NbV–OH species have not been detected by the authors,[8,9] and until now, it was not available in literature. Up to date, although niobium species has been extensively characterized by IR Raman spectroscopy,[1,4,11–15] the assignments for the niobium species seems to be quite confused due to the shortcoming of the characterization method. For instance, Wachs and co-workers have found that both isolated tetrahedral Nb=O species on Nb2O5/SiO2 and polymerized octahedral niobium species on the Nb 2O5/MOx (M=Al, Ti, Zr) generate bands in the region 980–988 cm–1.[11] The unspecific feature of niobium species in the IR Raman spectra was further confirmed later by the measurements of NbMCM-41[4,12] and Nb2O5•nH2O,[13] on which bands at ~990 cm–1 appeared. Recently, UV resonance Raman spectroscopy (UVRRS)
[*]
was successfully used for characterizing Ti, Fe and V species in zeolite framework,[16,17] whereas there are no literature studying Nb species by the technique. Herein, we reported definitive identification of framework niobium and isolated extraframework niobium species in the NbS-1 zeolite using UVRRS. In particular, we focused on exploration for the characteristic features of framework penta-coordinated NbV–OH species.
Figure. 1. UV/Vis spectra of as-synthesized samples: NbS-1(114), NbS1(220) and silicalite-1.
Figure 1 shows UV/Vis spectra of the as-synthesized silicalite-1 and NbS-1 samples. On the NbS-1(220) and NbS1(114), which contains niobium in Si/Nb molar ratio of 220 or 114, a strong absorption band due to niobium species appeared at 200 nm, while on the silicalite-1 the band did not appear. It can be deduced that the band is generated from framework niobium, as all kinds of extraframework niobium species only generate the bands at above 300 nm.[8,9,15,18] In principle, only the four kinds of niobium (V) species (a) ~ (d) shown in Figure 2 can be expected in the zeolite framework. Actually, the species
Yong Chen, Lejian Zhang, Prof. Dr. X. Wang State Key Laboratory of Fine Chemical, School of Environmental Science and Technology, Dalian University of Technology Dalian 116024 (P.R. China) E-mail: [email protected] Supporting information for this article is given via a link at the end of the document.
Figure. 2. Representation of the four different zeolite framework niobium (V) species (a), (b), (c), and (d) considered in the study.
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COMMUNICATION (a) and (b) were detected by Ziolek and co-workers with the UV/Vis spectroscopy, which appeared respectively at 250 and 228 nm on calcined NbSBA-15,[18] and at 270 and 217 nm on calcined NbMCF.[19] As for the species (c), although it has not been detected on the two zeolites, the authors supposed that the species (a) and (b) are generated from them,[5,6] via the following possible dehydration process:[5,6,8] 2Nb–OH site (c) → NbO– site (a) + Nb+ sits (b) + H2O (1) The acid-base lewis pair (NbO–/Nb+) has been described in detail by Tielens and co-workers.[10] As for the species (d) that has the same structure as that of the niobium species tetrahedrally coordinated to silica surface, [4,12] it should appear in the UV/Vis spectra with a band at 220 nm, as detected by Hoelderich and the Anilkumar on NbMCM-41.[7] Since only absorption peak at 200 nm being associated with niobium species appeared on our NbS-1(220), the presence of species (a), (b) and (d) can be excluded in the sample. It means that the absorption peak at 200 nm must be generated from species (c). To confirm the assignment, silicalite-1 and NbS-1 samples after calcination were measured by the UV/Vis spectroscopy. As shown in Figure 3, the absorption peak became a broad band in the range of 210–270 nm over the NbS-1(114) after calcination,
the range 980–988 cm–1 on the NbS-1(220) (insert), which is characteristic of terminal NbV=O bond (see Introduction),[4,11-15] therefore the species (d) can be excluded in the sample. For the two samples, bands at 378 and 804 cm –1 both appeared on them, which confirm that the NbS-1(220) has the same structure as that of silicalite-1. Different from the silicalite-1, the
Figure. 4. UV resonance Raman spectra of (a) silicalite-1 and (b) NbS-1(220) with excitation at 244 nm. The insert: enlarged portion of a and b.
Figure. 3. UV/Vis spectra of calcined samples: NbS-1(114), NbS-1(220) and silicalite-1.
which is quite similar to that described by Ziolek and co-workers about the species (a) and (b).[18,19] Clearly, the changes should be resulted from the dehydration of species (c). Similar phenomena have been observed by the group of Vinu, who found that the absorption peak on NbSBA-15 appeared at 200 nm before calcination, and that it was shifted to 205 nm on the sample after calcination.[20] It means that the absorption of the species (c) at 200 nm exclusively depends on the structure of the species, irrespective of the zeolite structure. Herein, it should be also noticed that the NbS-1(220) still presented the strong absorption peak at 200 nm even after calcination, which is quite different from NbS-1(114). The phenomena can be reasonably interpreted as that the NbS-1(220) has almost no this kind of species close to each other so as to be capable of dehydration [Eq. (1)], because of its quite low niobium concentration. Herein, it can be noticed that the weak absorption at ca. 240 nm, which appeared on all the as-synthesized samples (Figure 1), disappeared on the samples after calcination (Figure 3). The results confirm that the corresponding absorption arises from organic template. As for the conclusion that no niobium species (d) exists in the NbS-1(220), our following UVRRS provided more evidence. Figure 4 shows the UV Raman spectra of calcined NbS1(220) and silicalite-1 excited by 266 nm. No band appeared in
NbS-1(220) generated four new bands at 739, 925, 963, and 1107 cm–1 being associated with the framework niobium ions in the zeolite. The band at 925 cm–1 is associated with Si(–O–)/Si(– O–)2 functionality involving Nb–O–Si formation.[4] To assign the other bands, the Raman spectra of calcined NbS-1(220) excited respectively by 244, 266 and 320 nm were compared in Figure 5. For the band at 963 cm–1 appeared on the NbS-1(220), it was more intensive in the spectrum excited by 244 nm than that excited by 266 nm, and it disappeared in the spectrum excited by 320 nm. The results mean that the band possesses resonance-enhanced feature. Hence, it can be concluded that the band is not resulted from the silanol groups that require much higher energies for the O 2–Si4+ → O–Si3+ transition.[21] On the other hand, the band at 963 cm–1 did not appear on the calcined NbS-1(114) (Figure S1), and only niobium species (c) existed in the calcined NbS-1(220) in the UV/Vis spectra (Figure 3), therefore it can be known that the band at 963 cm–1 arises from niobium species (c). It is just consistent with the supposal of Ziolek and co-workers about the band at ~ 960 cm–1.[5] For the bands at 739 and 1107 cm–1, to the best of our knowledge, they have never been detected in FTIR or IR Raman spectra. They exhibited more obvious resonant property than 963 cm–1 band, hence they could be attributed to the totally symmetric stretching vibrations associated with the species (c) according to resonant Raman selection rules. [21] On the other hand, two bands at 975 and 1336 cm–1 appeared in the Raman spectrum excited by 320 nm, as shown in the insert of Figure 5. The band at 975 cm –1 is due to the Si– OH stretching in silanols,[4,21] while the band at 1336 cm–1, which was never detected in IR Raman spectra for niobium-containing zeolite, can be attributed to isolated NbO6 octahedra. Clearly, the band at 1336 cm–1 is the double-frequency band of 668 cm–1 arising from NbO6 octahedra[1] and it is independent of polymerized niobium species that generates band in the range 980–988 cm–1. The fact that the 668 cm–1 band did not appear in this spectrum should be resulted from strong fluorescence interference in the corresponding region.[16,17] Moving now to the UV/Vis spectra on the NbS-1(220), the band at 320 nm involving the NbO6 octahedra species (in the extraframework
This article is protected by copyright. All rights reserved.
10.1002/cphc.201700873
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COMMUNICATION positions)[3,7,18] was not detected because of the trace amount (see Figure 3). Interestingly, it still could be clearly identified by the double-frequency Raman band (at 1336 cm–1) due to its strong resonant Raman effect.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21277019 and 21777015). Keywords: Framework niobium • Identification • Trace analysis • UV resonance Raman spectroscopy • UV/Vis spectroscopy [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Figure. 5. UV resonance Raman spectra of NbS-1(220) with excitation at 244, 266, and 320 nm (insert), respectively.
In conclusion, framework penta-coordinated NbV–OH species in NbS-1 zeolite with Nb/Si molar ratio of 1/220 was synthesized and characterized by UVRRS combined with UV/Vis spectroscopy. It was demonstrated that bands at 739, 963 and 1107 cm–1 observed in the UV resonance Raman spectra are characteristics of the species in the framework, and hence the bands can be regarded as probes for identifying the species.
[11] [12] [13] [14] [15] [16] [17] [18]
Experimental Section Materials, methods, and further characterization can be found in the Supporting Information.
[19] [20] [21]
Acknowledgements
A. M. Prakash, L. Kevan, J. Am. Chem. Soc. 1998, 120, 13148–13155. D. M. Antonelli, J. Y. Ying, Angew. Chem. Int. Ed. Engl. 1996, 35(4), 426–430; Angew. Chem. 1996. 108, 461–464. M. Ziolek, I. Sobczak, Catal. Today 2017, 285, 211–225. X. Gao, I. E. Wachs, M. S. Wong, J. Y. Ying, J. Catal. 2001, 203, 18–24. M. Ziolek, I. Sobczak, A. Lewandowska, I. Nowak, P. Decyk, M. Renn, B. Jankowska, Catal. Today 2001, 70, 169–181. I. Sobczak, P. Decyk, M. Ziolek, M. Daturi, J. C. Lavalley, L. Kevan, A. M. Prakash, J. Catal. 2002, 207, 101–112. M. Anilkumar, W. F. Hoelderich, J. Catal. 2008, 260, 17–29. F. Tielens, T. Shishido, S. Dzwigaj, J. Phys. Chem. C 2010, 114, 3140– 3147. A. Wojtaszek, M. Ziolek, F. Tielens, J. Phys. Chem. C 2012, 116, 2462–2468. D. C. Tranca, A. Wojtaszek-Gurdak, M. Ziolek, F. Tielens, Phys. Chem. Chem. Phys., 2015, 17, 22402. L. J. Burcham, J. Datka, I. E. Wachs, J. Phys. Chem. B 1999, 103, 6015–6024. M. Anilkumar, W. F. Hoelderich, Appl. Catal., B 2015, 165, 87–93. K. Nakajima, Y. Baba, R. Noma, M. Kitano, J. N. Kondo, S. Hayashi, M. Hara, J. Am. Chem. Soc. 2011, 133, 4224–4227. A. Ramanathan, R. Maheswari, D. H. Barich, B. Subramaniam, Microporous Mesoporous Mater. 2014, 190, 240–247. I. D. Ivanchikova, N. V. Maksimchuk, I. Y. Skobelev, V. V. Kaichev, O. A. Kholdeeva, J. Catal. 2015, 332, 138–148. C. Li, J. Catal. 2003, 216, 203–212. F.-T. Fan, Z.-C. Feng, C. Li, Acc. Chem. Res. 2010, 43, 378-387. M. Trejda, A. Tuel, J. Kujawa, B. Kilos, M. Ziolek, Microporous Mesoporous Mater. 2008, 110, 271–278. K. Stawicka, A. E. Díaz-Álvarez, V. Calvino-Casilda, M. Trejda, M. A. Bañares, M. Ziolek, J. Phys. Chem. C 2016, 120, 16699–16711. P. Srinivasu, C. Anand, S. Alam, K. Ariga, S. B. Halligudi, V. V. Balasubramanian, A. Vinu, J. Phys. Chem. C 2008, 112, 10130–10140. G. Ricchiardi, A. Damin, S. Bordiga, C. Lamberti, G. Spano, F. Rivetti, A. Zecchina, J. Am. Chem. Soc. 2001, 123, 11409–11419.
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COMMUNICATION COMMUNICATION Yong Chen, Xinping Wang* and Lejian Zhang
((Insert TOC Graphic here)) Page No. – Page No.
Fingerprints of the Nb silicalite: The band at 200 nm in UV/Vis spectrum and the bands at 739, 963 and 1107 cm–1 in UV resonance Raman spectrum excited by 244 nm are selective and sensitive probes for identifying the framework pentacoordinated NbV–OH species in zeolites.
Identifying the Elusive Framework Niobium in NbS-1 Zeolite by UV Resonance Raman
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COMMUNICATION Identifying the Elusive Framework Niobium in NbS-1 Zeolite by UV Resonance Raman Yong Chen, Xinping Wang* and Lejian Zhang Abstract: It was found that bands at 739, 963 and 1107 cm –1 in resonant Raman spectra are characteristics of framework pentacoordinated NbV–OH species, and that band at 1336 cm –1 in UV Raman spectra excited by 320 nm is a sensitive detector to identify extraframework niobium species. The change of framework pentacoordinated NbV–OH species into Nb+ and NbO– species due to dehydration was definitively confirmed based on UV resonance Raman and UV/Vis results.
Niobium-containing zeolites have received increased attention due to their novel catalytic activity.[1–3] Their performance not only depends on the pore structure, but also on the nature of active sites,[3] however, the definite information is hardly obtained by the conventional spectroscopic techniques.[1,3,4] Especially, in the situation where the concentration of niobium is extremely low and various types of niobium species coexist. Thus, differentiating a specific niobium species from a complex molecular system generates a great challenge. Strong efforts have been made to characterize the framework niobium species in zeolites.[1,4–9,11,12] Ziolek and coworkers investigated the framework niobium in zeolites, e.g. NbMCM-41[5] and NbZSM-5,[6] by H2-TPR, FTIR of NO adsorption, and ESR techniques. They proposed that framework Nb+ and NbO– species detected in the zeolites should be generated from the penta-coordinated NbV–OH species that possibly exists in the zeolite before dehydration.[3,5,6] Whereas, the evidence for the penta-coordinated NbV–OH species in the zeolites was not straightforward, as the band at 966 cm–1 observed in IR spectra was confounded with that of silanol group.[3,5,7] Recently, Tielens and co-workers proposed that the framework NbV–OH species could be stabilized by the zeolite framework, based on their FTIR measurements and Density functional calculations.[8,9] The NbV–OH species in framework has been compared theoretically with that on silica support by DFT.[10] However, corresponding absorption of the NbV–OH species have not been detected by the authors,[8,9] and until now, it was not available in literature. Up to date, although niobium species has been extensively characterized by IR Raman spectroscopy,[1,4,11–15] the assignments for the niobium species seems to be quite confused due to the shortcoming of the characterization method. For instance, Wachs and co-workers have found that both isolated tetrahedral Nb=O species on Nb2O5/SiO2 and polymerized octahedral niobium species on the Nb 2O5/MOx (M=Al, Ti, Zr) generate bands in the region 980–988 cm–1.[11] The unspecific feature of niobium species in the IR Raman spectra was further confirmed later by the measurements of NbMCM-41[4,12] and Nb2O5•nH2O,[13] on which bands at ~990 cm–1 appeared. Recently, UV resonance Raman spectroscopy (UVRRS)
[*]
was successfully used for characterizing Ti, Fe and V species in zeolite framework,[16,17] whereas there are no literature studying Nb species by the technique. Herein, we reported definitive identification of framework niobium and isolated extraframework niobium species in the NbS-1 zeolite using UVRRS. In particular, we focused on exploration for the characteristic features of framework penta-coordinated NbV–OH species.
Figure. 1. UV/Vis spectra of as-synthesized samples: NbS-1(114), NbS1(220) and silicalite-1.
Figure 1 shows UV/Vis spectra of the as-synthesized silicalite-1 and NbS-1 samples. On the NbS-1(220) and NbS1(114), which contains niobium in Si/Nb molar ratio of 220 or 114, a strong absorption band due to niobium species appeared at 200 nm, while on the silicalite-1 the band did not appear. It can be deduced that the band is generated from framework niobium, as all kinds of extraframework niobium species only generate the bands at above 300 nm.[8,9,15,18] In principle, only the four kinds of niobium (V) species (a) ~ (d) shown in Figure 2 can be expected in the zeolite framework. Actually, the species
Yong Chen, Lejian Zhang, Prof. Dr. X. Wang State Key Laboratory of Fine Chemical, School of Environmental Science and Technology, Dalian University of Technology Dalian 116024 (P.R. China) E-mail: [email protected] Supporting information for this article is given via a link at the end of the document.
Figure. 2. Representation of the four different zeolite framework niobium (V) species (a), (b), (c), and (d) considered in the study.
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COMMUNICATION (a) and (b) were detected by Ziolek and co-workers with the UV/Vis spectroscopy, which appeared respectively at 250 and 228 nm on calcined NbSBA-15,[18] and at 270 and 217 nm on calcined NbMCF.[19] As for the species (c), although it has not been detected on the two zeolites, the authors supposed that the species (a) and (b) are generated from them,[5,6] via the following possible dehydration process:[5,6,8] 2Nb–OH site (c) → NbO– site (a) + Nb+ sits (b) + H2O (1) The acid-base lewis pair (NbO–/Nb+) has been described in detail by Tielens and co-workers.[10] As for the species (d) that has the same structure as that of the niobium species tetrahedrally coordinated to silica surface, [4,12] it should appear in the UV/Vis spectra with a band at 220 nm, as detected by Hoelderich and the Anilkumar on NbMCM-41.[7] Since only absorption peak at 200 nm being associated with niobium species appeared on our NbS-1(220), the presence of species (a), (b) and (d) can be excluded in the sample. It means that the absorption peak at 200 nm must be generated from species (c). To confirm the assignment, silicalite-1 and NbS-1 samples after calcination were measured by the UV/Vis spectroscopy. As shown in Figure 3, the absorption peak became a broad band in the range of 210–270 nm over the NbS-1(114) after calcination,
the range 980–988 cm–1 on the NbS-1(220) (insert), which is characteristic of terminal NbV=O bond (see Introduction),[4,11-15] therefore the species (d) can be excluded in the sample. For the two samples, bands at 378 and 804 cm –1 both appeared on them, which confirm that the NbS-1(220) has the same structure as that of silicalite-1. Different from the silicalite-1, the
Figure. 4. UV resonance Raman spectra of (a) silicalite-1 and (b) NbS-1(220) with excitation at 244 nm. The insert: enlarged portion of a and b.
Figure. 3. UV/Vis spectra of calcined samples: NbS-1(114), NbS-1(220) and silicalite-1.
which is quite similar to that described by Ziolek and co-workers about the species (a) and (b).[18,19] Clearly, the changes should be resulted from the dehydration of species (c). Similar phenomena have been observed by the group of Vinu, who found that the absorption peak on NbSBA-15 appeared at 200 nm before calcination, and that it was shifted to 205 nm on the sample after calcination.[20] It means that the absorption of the species (c) at 200 nm exclusively depends on the structure of the species, irrespective of the zeolite structure. Herein, it should be also noticed that the NbS-1(220) still presented the strong absorption peak at 200 nm even after calcination, which is quite different from NbS-1(114). The phenomena can be reasonably interpreted as that the NbS-1(220) has almost no this kind of species close to each other so as to be capable of dehydration [Eq. (1)], because of its quite low niobium concentration. Herein, it can be noticed that the weak absorption at ca. 240 nm, which appeared on all the as-synthesized samples (Figure 1), disappeared on the samples after calcination (Figure 3). The results confirm that the corresponding absorption arises from organic template. As for the conclusion that no niobium species (d) exists in the NbS-1(220), our following UVRRS provided more evidence. Figure 4 shows the UV Raman spectra of calcined NbS1(220) and silicalite-1 excited by 266 nm. No band appeared in
NbS-1(220) generated four new bands at 739, 925, 963, and 1107 cm–1 being associated with the framework niobium ions in the zeolite. The band at 925 cm–1 is associated with Si(–O–)/Si(– O–)2 functionality involving Nb–O–Si formation.[4] To assign the other bands, the Raman spectra of calcined NbS-1(220) excited respectively by 244, 266 and 320 nm were compared in Figure 5. For the band at 963 cm–1 appeared on the NbS-1(220), it was more intensive in the spectrum excited by 244 nm than that excited by 266 nm, and it disappeared in the spectrum excited by 320 nm. The results mean that the band possesses resonance-enhanced feature. Hence, it can be concluded that the band is not resulted from the silanol groups that require much higher energies for the O 2–Si4+ → O–Si3+ transition.[21] On the other hand, the band at 963 cm–1 did not appear on the calcined NbS-1(114) (Figure S1), and only niobium species (c) existed in the calcined NbS-1(220) in the UV/Vis spectra (Figure 3), therefore it can be known that the band at 963 cm–1 arises from niobium species (c). It is just consistent with the supposal of Ziolek and co-workers about the band at ~ 960 cm–1.[5] For the bands at 739 and 1107 cm–1, to the best of our knowledge, they have never been detected in FTIR or IR Raman spectra. They exhibited more obvious resonant property than 963 cm–1 band, hence they could be attributed to the totally symmetric stretching vibrations associated with the species (c) according to resonant Raman selection rules. [21] On the other hand, two bands at 975 and 1336 cm–1 appeared in the Raman spectrum excited by 320 nm, as shown in the insert of Figure 5. The band at 975 cm –1 is due to the Si– OH stretching in silanols,[4,21] while the band at 1336 cm–1, which was never detected in IR Raman spectra for niobium-containing zeolite, can be attributed to isolated NbO6 octahedra. Clearly, the band at 1336 cm–1 is the double-frequency band of 668 cm–1 arising from NbO6 octahedra[1] and it is independent of polymerized niobium species that generates band in the range 980–988 cm–1. The fact that the 668 cm–1 band did not appear in this spectrum should be resulted from strong fluorescence interference in the corresponding region.[16,17] Moving now to the UV/Vis spectra on the NbS-1(220), the band at 320 nm involving the NbO6 octahedra species (in the extraframework
This article is protected by copyright. All rights reserved.
10.1002/cphc.201700873
ChemPhysChem
COMMUNICATION positions)[3,7,18] was not detected because of the trace amount (see Figure 3). Interestingly, it still could be clearly identified by the double-frequency Raman band (at 1336 cm–1) due to its strong resonant Raman effect.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21277019 and 21777015). Keywords: Framework niobium • Identification • Trace analysis • UV resonance Raman spectroscopy • UV/Vis spectroscopy [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Figure. 5. UV resonance Raman spectra of NbS-1(220) with excitation at 244, 266, and 320 nm (insert), respectively.
In conclusion, framework penta-coordinated NbV–OH species in NbS-1 zeolite with Nb/Si molar ratio of 1/220 was synthesized and characterized by UVRRS combined with UV/Vis spectroscopy. It was demonstrated that bands at 739, 963 and 1107 cm–1 observed in the UV resonance Raman spectra are characteristics of the species in the framework, and hence the bands can be regarded as probes for identifying the species.
[11] [12] [13] [14] [15] [16] [17] [18]
Experimental Section Materials, methods, and further characterization can be found in the Supporting Information.
[19] [20] [21]
Acknowledgements
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