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Nanoscale characterization of acid properties of heteropolyacids by scanning tunneling microscopy and tunneling spectroscopy
Applied Catalysis A: General 194 –195 (2000) 129–136
Nanoscale characterization of acid properties of heteropolyacids by scanning tunneling microscopy and tunneling spectroscopy Mahmoud S. Kaba a , Mark A. Barteau a , Wha Young Lee b , In Kyu Song c,∗ a
Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark DE 19716, USA b Department of Chemical Engineering, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-742, South Korea c Department of Industrial Chemistry, Kangnung National University, Kangnung, Kangwondo 210-702, South Korea Received 23 March 1999; received in revised form 24 May 1999; accepted 8 June 1999
Abstract Nanoscale characterization of the acid properties of H3 PMo12−x Wx O40 (x = 0, 3, 6, 9, 12) was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy. Pyridine binding with the acid sites of heteropolyacids (HPAs) was reflected in both the STM images and FT-IR spectra of these materials. All the HPAs investigated formed well-ordered monolayer arrays on highly oriented pyrolytic graphite (HOPG) before and after pyridine adsorption. Exposure to pyridine increased the lattice constants of the two-dimensional HPA arrays by ca. 6 Å. Exposure to pyridine also shifted the negative differential resistance (NDR) peak voltages of HPAs to less negative values in the tunneling spectroscopy measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with acid properties of HPAs for the first time, suggesting that tunneling spectra measured by STM could serve to fingerprint acid properties of HPAs. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Heteropolyacid; Acid property; Scanning tunneling microscopy; Pyridine interaction; Negative differential resistance
1. Introduction Heteropolyacids (HPAs) such as H3 PMo12 O40 and H3 PW12 O40 are inorganic acids, but at the same time can act as oxidizing agents [1–3]. Many are highly soluble in polar solvents such as water, alcohols, and amines, but insoluble in some non-polar solvents such as benzene and olefins [4–6]. The solubility of these compounds in turn is closely related to their ability to adsorb various reactants. Non-polar molecules are adsorbed on the surface of bulk HPAs, while most polar molecules are mainly adsorbed in the bulk by forming ∗ Corresponding author. Tel.: +82-391-640-2404; fax: +82-391-640-2244. E-mail address: [email protected] (I.K. Song).
a ‘pseudo-liquid phase’ [7]. This characteristic leads to two typical catalytic reaction-types involving HPAs; namely, surface-type reaction and bulk-type reaction. It is well known that the acid and redox properties of HPAs can be modified by exchanging protons with metal cations and/or by substituting the heteroatom or framework metal ions in the heteropolyanion [8–10]. Commercial processes utilizing HPAs as acid catalysts include production of t-butanol [11] and polytetramethyleneglycolether (PTMG) [12]. The effects of counter cation and framework transition-metal cation substitutions on the acid properties of HPAs have been reported in the literature [13–15]. We have previously reported STM images of HPAs on graphite surfaces and have shown that HPAs form two-dimensional monolayer arrays [16–22]. These
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compounds exhibit distinctive negative differential resistance (NDR) behavior at specific applied voltages in their spatially resolved tunneling spectra [16]. It was observed that the NDR peak voltages could be related to the electronic properties of HPAs, and are influenced by the identity of the counter cations [18] and adsorbed organic molecules [19]. We have previously reported STM images of pyridine-exposed H3 PMo12 O40 arrays on graphite and have shown that the NDR peak voltage of these arrays shifted to a less negative bias upon pyridine adsorption [19]. In this work, nanoscale characterization of the acid properties of heteropolyacids of the family H3 PMo12−x Wx O40 was carried out by STM in order to compare NDR behavior with the acid properties of these compounds. The HPAs were deposited on a highly oriented pyrolytic graphite surface in order to obtain images, as well as tunneling spectra, by STM before and after pyridine adsorption. FT-IR experiments were performed to examine the acid–base interaction of these HPA monolayers with pyridine. It was found that NDR peak positions of HPA monolayers measured by STM could be correlated with acid properties of HPAs fingerprinted by FT-IR. 2. Experimental 2.1. HPA preparation and deposition HPAs of the type H3 PMo12−x Wx O40 (x = 0, 3, 6, 9, 12) were examined as acid catalysts in this work. H3 PMo12 O40 and H3 PW12 O40 were obtained from Aldrich. H3 PMo9 W3 O40 , H3 PMo6 W6 O40 and H3 PMo3 W9 O40 were prepared according to literature methods [23]. All HPAs were calcined at 300◦ C for 2 h to remove the water of crystallization in order to prepare solutions of consistent concentration. Approximately 0.01 M aqueous solutions of each sample were prepared. A drop of solution was deposited on freshly cleaved HOPG and allowed to dry in air for ca. 45 min at room temperature for STM imaging. Pyridine exposure was carried out by placing a drop of pyridine on the previously deposited HPA layer and drying in air for ca. 1 h at room temperature. Reversibly adsorbed pyridine molecules were then removed by evacuating the sample at ca. 25 millitorr for 1 h at room temperature prior to the STM measurement.
2.2. STM image and tunneling spectroscopy STM images were obtained in air using a Topometrix TMX 2010 instrument. Mechanically formed Pt/Ir (90/10) tips were used as probes. Scanning was done in the constant current mode at a positive sample bias of 100 mV and tunneling current of 1–2 nA. Tunneling spectra were measured in air and in a glove box filled with N2 . Both Topometrix TMX 2010 and LK Technologies LK-1000 STM were used to confirm consistency and reproducibility of tunneling spectra. All STM images presented in this report are unfiltered, and the reported periodicities (lattice constant) represent average values determined by performing two-dimensional Fast Fourier Transform (FFT) analyses on at least three images for each sample. Each image was acquired using a different tip; the tips were first calibrated by imaging bare HOPG to confirm the standard periodicity of HOPG (2.46 Å). Several tunneling spectra were then taken on the bare graphite section of the surface to ensure the stability of the tip and the reproducibility of the tunneling spectrum of HOPG. Once these had been established, the ‘good’ tip would be moved to the HPA-covered section to image and obtain tunneling spectra of the HPA sample. To measure a tunneling spectrum, the sample bias was ramped from −2 to +2 V with respect to the tip and the tunneling current was monitored. The voltage axis in the tunneling spectrum represents the potential applied to the sample relative to that of the tip. 2.3. IR spectroscopy A drop of HPA solution was deposited on the diamond probe of a Total Internal Reflectance Fourier Transform Infrared spectrometer (Applied Systems, ReactIR 1000) and allowed to dry in air for ca. 45 min at room temperature. IR spectra of the fresh samples were then recorded under ambient conditions. This step was further extended for the pyridine adsorption experiments by putting a drop of pyridine on the previously deposited HPA layer and allowing it also to dry in air for ca. 1 h at room temperature. Reversibly adsorbed pyridine molecules were then removed by evacuating the sample for 1 h at room temperature prior to performing FT-IR measurements on the pyridine-exposed HPAs.
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Fig. 1. STM images and schematic representations of the unit cells of (a) fresh H3 PW12 O40 and (b) fresh H3 PMo12 O40 monolayers.
3. Results and discussion 3.1. Self-assembled HPA arrays The primary structure (Keggin structure) of the heteropolyanion is relatively stable and insensitive to its surroundings. The three-dimensional array of solid-state HPAs comprising heteropolyanions, protons, cations, water, and/or organics is called the secondary structure [24]. Unlike the primary structure, the secondary structure is very labile and may change in different environments by either increasing or decreasing the interstitial space between heteropolyanions. Fig. 1(a) shows the STM image and schematic representation of the unit cell of H3 PW12 O40 deposited on HOPG. This image indicates the formation of an ordered HPA array on the graphite surface. The periodicity is 11.7 ± 0.1 Å and is in good agreement
with lattice constants obtained by STM [16–22] and X-ray crystallography [25–27]. The unit cell constructed on the basis of the lattice constant determined from 2D FFT shows that the arrays of H3 PW12 O40 have a nearly square symmetry (α = 79.5◦ ). Fig. 1(b) shows the STM image and schematic representation of unit cell of H3 PMo12 O40 deposited on HOPG. This image also shows the formation of an ordered array on graphite with approximately square symmetry (α = 84.9◦ ). Its periodicity is 10.8 ± 0.3 Å. The periodicities measured for two-dimensional arrays of H3 PMo12−x Wx O40 are summarized in Table 1. All HPAs examined in this work formed well-ordered arrays on graphite over scan areas of at least 200 × 200 Å. No systematic differences were observed in the periodicities of HPA arrays with variation of the W/Mo content of the framework. The surface arrays of H3 PMo12−x Wx O40 were quite
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Table 1 Periodicities of STM images of H3 PMo12−x Wx O40 Heteropolyacid
Periodicity (Å)
Included angle (degree)
H3 PW12 040 H3 PMo3 W9 O40 H3 PMo6 W6 O40 H3 PMo9 W3 O40 H3 PMo12 O40
11.7 ± 0.1 10.8 ± 0.2 10.6 ± 0.1 11.1 ± 0.1 10.8 ± 0.3
79.5 69.8 77.9 72.3 84.9
homogeneous. A small variation of periodicity among HPA samples did not come from the inhomogeneity of the sample but from STM imaging measurement. It is noteworthy that the tip structure may change electronically during or between STM imaging and may drift slightly in a small scan area. The molecular dimensions of H3 PMo12−x Wx O40 species were consistent with the reported values of 10–12 Å by STM [16–22] and X-ray crystallography [25–27].
3.2. Rearrangement of ordered arrays after pyridine adsorption A three-dimensional crystal structure of pyridineadsorbed H3 PW12 O40 array was reported [28]. In the three-dimensional array, it was reported that the pyridine molecules were paired around H+ forming (C5 H5 N)–H+ –(NC5 H5 ) cations. Fig. 2(a) shows the two-dimensional STM image of H3 PW12 O40 after pyridine exposure. The two-dimensional ordered arrays exhibit a periodicity of 18.2 ± 0.4 Å. Unlike the fresh H3 PW12 O40 arrays (α = 79.5◦ ), pyridine-exposed H3 PW12 O40 forms roughly hexagonal arrays (α = 59.1◦ ). As shown in Fig. 2(b), pyridine-exposed H3 PMo12 O40 also forms well ordered two-dimensional arrays having a periodicity of 16.5 ± 0.2 Å. It also has hexagonal-like arrays (α = 53.1◦ ) unlike the rectangular arrays of fresh
Fig. 2. STM images and schematic representations of the unit cells of (a) pyridine-exposed H3 PW12 O40 and (b) pyridine-exposed H3 PMo12 O40 monolayers.
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H3 PMo12 O40 (α = 84.9◦ ). The increase of periodicities of pyridine-exposed HPA arrays in both cases is ca. 6 Å; roughly the molecular size of pyridine. These results demonstrate that pyridine molecules are bound at the acid sites in the interstitial spaces between polyanions in the two-dimensional surface arrays, in a similar manner to that in three-dimensional solids. These results also indicate that two-dimensional HPA arrays were rearranged and expanded upon pyridine exposure. A model for pyridine-exposed H3 PMo12 O40 arrays on HOPG has previously been proposed in which pyridinium ions occupy bridging sites between heteropolyanions [19]. However, as in previous STM images of HPAs, the pyridinium ions, like other charge-compensating cations in these arrays, are not imaged directly. 3.3. Tunneling spectroscopy I–V spectra of the H3 PW12 O40 array on HOPG taken at two different positions, denoted as Site I and Site II in Fig. 1(a), are shown in Fig. 3(a). Site II exhibits the typical I–V spectrum of freshly cleaved HOPG and Site I exhibits the characteristic I–V spectrum of H3 PW12 O40 . The latter spectrum shows negative differential resistance (NDR) behavior at −1.20 V where dI/dV is negative in this region, as shown in Fig. 3(b). The NDR peak voltage was defined as the voltage at which the maximum current was observed in this region. The difference in I–V spectra of the two sites indicates that the array of H3 PW12 O40 on HOPG is a monolayer. The striking NDR behavior of HPAs measured by STM may be closely related to the electronic properties of these materials. Fig. 4 shows the trend of NDR peak voltage of H3 PMo12−x Wx O40 with framework metal substitution. The NDR peak shifts to less negative applied voltage with the increase of Mo substitution into the Keggin ion framework. The HPA images in this work were obtained at positive sample biases with respect to the tip. This means that electrons flow from tip to sample in the imaging mode of operation. NDR behavior in the tunneling spectra of HPAs is observed at negative sample biases, i.e. when electrons tunnel from sample to tip. We have observed that the NDR behavior of HPAs is strongly related to their electronic properties [18,20,22]. Previous results have shown that the NDR peak appeared at less negative applied voltage when the protons of the
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HPA were replaced by more electronegative cations such as Cu2+ ; the NDR peaks shifted to higher negative voltages when the protons were replaced by less electronegative cations such as Cs+1 . Thus it is expected that NDR behavior of HPA monolayers will be affected by the replacement of protons with pyridinium ions in the interstitial space between polyanions in these arrays. 3.4. Pyridine interaction with HPAs Fig. 5 shows the tunneling spectra of H3 PW12 O40 and H3 PMo12 O40 taken before and after pyridine exposure. The NDR peak of H3 PW12 O40 appears at −1.2 V before pyridine adsorption, whereas it appears at −0.65 V after pyridine adsorption. Introduction of electron-rich pyridine molecules into H3 PW12 O40 arrays leads to an NDR voltage shift of 0.55 V. The NDR peak voltage of H3 PMo12 O40 shifts from −0.95 to −0.75 V after pyridine adsorption. The shift of NDR peak voltage of HPAs to less negative applied voltages after pyridine adsorption was attributed to the replacement of protons with pyridium ions. The effect on the NDR position is comparable to that obtained by exchanging more electronegative counter cations for protons [18]. The interaction of pyridine with H3 PW12 O40 was confirmed by FT-IR, as shown in Fig. 6. The P–O, W = O (terminal oxygen), and W–O–W (corner sharing and edge-sharing) bands of fresh H3 PW12 O40 appeared at 1073, 973, 903 and 779 cm−1 , respectively. Pyridinium ions can be identified by the band at 1540 cm−1 [24]. The characteristic chemisorbed pyridium ion was detected after the removal of physisorbed pyridine from HPAs by evacuation. The P–O, W=O (terminal oxygen), and W–O–W (corner sharing and edge-sharing) bands of pyridine-exposed H3 PW12 O40 appeared at 1042, 948, 888 and 787 cm−1 , respectively. The shifts of IR bands of pyridine-exposed H3 PW12 O40 were attributed to the interaction of the cationic species, pyridinium ions, with the heteropolyanions. 3.5. Correlation between NDR shift and acid property of H3 PMo12−x Wx O40 We have shown that NDR peak voltages of HPAs were strongly affected by the pyridine binding to the
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Fig. 3. Tunneling spectra of H3 PW12 O40 taken at Sites I and II in Fig. 1 (a): (a) I–V spectra and (b) dI/dV spectra.
acid sites of the HPAs. We consider below whether the magnitudes of the NDR peak shifts of different HPAs measured before and after pyridine adsorption may serve as a fingerprint of their acid properties. This is still controversial as for the acid sites of solid-state HPAs [29–31]. Lee et al. [30] reported from IR and NMR investigations that the protons are located on the bridging oxygens of HPAs. In a recent paper, however, it was reported from 17 O NMR investigations of the proton sites in dehydrated HPAs
that the predominant protonation sites are the terminal M(metal)=O (oxygen) atoms [31]. In the FT-IR analyses of pyridine-exposed HPAs shown in Fig. 6, the quantification of bridging oxygen bands was not simple. Thus the adsorption band corresponding to the M=O stretch might be expected to track the extent of proton transfer upon interaction with basic molecules such as pyridine. We measured the IR intensity of pyridium ion at 1540 cm−1 of the pyridine-exposed H3 PMo12−x Wx O40 and calculated the intensity ratio
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Fig. 4. NDR peak voltage of fresh H3 PMo12−x Wx O40 vs. tungsten content (x).
of I(1540 cm−1 )/I(M=O) bands in an attempt to track the acid properties of H3 PMo12−x Wx O40 . The NDR shifts of H3 PMo12−x Wx O40 were also measured before and after pyridine adsorption. Fig. 7 shows the correlation between the I(1540 cm−1 )/I(M=O) ratio and NDR peak shift of H3 PMo12−x Wx O40 . It is noteworthy that the acid amounts of H3 PMo12−x Wx O40 measured by FT-IR can be directly correlated with NDR peak voltage shifts. H3 PW12 O40 , which exhibits the largest I(1540 cm−1 )/I(M=O) ratio upon pyridine exposure, experiences the largest NDR peak shift. Thus we suggest that NDR voltage shifts of HPAs measured by STM can be used to track the acid property of bulk HPAs. This is the first example showing how to estimate the bulk acid properties of HPAs from the properties of nanostructured HPA monolayers.
Fig. 5. Tunneling spectra of (a) H3 PW12 O40 and (b) H3 PMo12 O40 taken before and after pyridine adsorption.
4. Conclusions Nanoscale characterization of the acid properties of H3 PMo12−x Wx O40 was carried out by STM and tunneling spectroscopy. Pyridine adsorption onto the acid sites of HPAs was confirmed by STM and FT-IR. All HPAs formed well-ordered two-dimensional arrays on graphite before and after pyridine adsorption. The introduction of cationic pyridinium ions into the HPA arrays resulted in the expansion of the arrays and shifts of their NDR peak voltages to less negative
Fig. 6. IR spectra of H3 PW12 O40 before and after pyridine adsorption.
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Fig. 7. Correlation between NDR peak voltage shifts upon pyridine exposure of H3 PMo12−x Wx O40 and acidic properties (I(1540 cm−1 )/I(M=O)) determined from FT-IR spectra after pyridine exposure.
values. The NDR shift of HPAs obtained before and after pyridine adsorption was correlated with the acid property of the HPAs for the first time. It was confirmed that H3 PW12 O40 , the member of this series having the highest acid amounts, experienced the largest NDR peak shift upon pyridine adsorption. Thus the NDR shift measured with the STM may serve as a fingerprint for the acid property of bulk HPAs. Acknowledgements We acknowledge support from the National Science Foundation (Grant CTS 9410965). In Kyu Song acknowledges support from Korea Science and Engineering Foundation (981-1101-001-2) for this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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[11] A. Aoshima, S. Yamamatsu, T. Yamaguchi, Nippon Kagaku Kaishi, 1987 976. [12] A. Aoshima, S. Tonomura, S. Yamamatsu, Polym. Adv. Tech. 2 (1990) 127. [13] H. Hayashi, J.B. Moffat, J. Catal. 81 (1983) 61. [14] T. Baba, J. Sakai, H. Watanabe, Y. Ono, Bull. Chem. Soc. Jpn. 55 (1982) 2555. [15] Y. Izumi, R. Hasabe, K. Urabe, J. Catal. 84 (1983) 402. [16] B.A. Watson, M.A. Barteau, L. Haggerty, A.M. Lenhoff, R.S. Weber, Langmuir 8 (1992) 1145. [17] I.K. Song, M.S. Kaba, G. Coulston, K. Kourtakis, M.A. Barteau, Chem. Mater. 8 (1996) 2352. [18] M.S. Kaba, I.K. Song, M.A. Barteau, J. Phys. Chem. 100 (1996) 19577. [19] I.K. Song, M.S. Kaba, M.A. Barteau, J. Phys. Chem. 100 (1996) 17528. [20] M.S. Kaba, I.K. Song, M.A. Barteau, J. Vac. Sci. Tech. A15 (1997) 1299. [21] M.S. Kaba, I.K. Song, D. Duncan, C.L. Hill, M.A. Barteau, Inorg. Chem. 37 (1998) 398. [22] I.K. Song, M.S. Kaba, M.A. Barteau, W.Y. Lee, Catal. Today 44 (1998) 285. [23] M. Misono, N. Mizuno, K. Katamura, A. Kasai, Y. Konishi, K. Sakata, T. Okuhara, Y. Yoneda, Bull. Chem. Soc. Jpn. 55 (1982) 400. [24] M. Misono, Catal. Rev. -Sci. Eng. 29 (1987) 269. [25] H. Hayashi, J.B. Moffat, J. Catal. 77 (1982) 473. [26] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, New York, 1983. [27] T. Okuhara, T. Nishimura, H. Watanabe, M. Misono, J. Mol. Catal. 74 (1992) 247. [28] M. Hashimoto, M. Misono, Acta Cryst. C50 (1994) 231. [29] T. Okuhara, N. Mizuno, M. Misono, Advan. Catal. 41 (1996) 113. [30] K.Y. Lee, N. Mizuno, T. Okuhara, M. Misono, Bull. Chem. Soc. Jpn. 62 (1989) 1731. [31] I.V. Kozhevnikov, A. Sinnema, H. van Bekkum, Catal. Lett. 34 (1995) 213.
Nanoscale characterization of acid properties of heteropolyacids by scanning tunneling microscopy and tunneling spectroscopy Mahmoud S. Kaba a , Mark A. Barteau a , Wha Young Lee b , In Kyu Song c,∗ a
Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark DE 19716, USA b Department of Chemical Engineering, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-742, South Korea c Department of Industrial Chemistry, Kangnung National University, Kangnung, Kangwondo 210-702, South Korea Received 23 March 1999; received in revised form 24 May 1999; accepted 8 June 1999
Abstract Nanoscale characterization of the acid properties of H3 PMo12−x Wx O40 (x = 0, 3, 6, 9, 12) was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy. Pyridine binding with the acid sites of heteropolyacids (HPAs) was reflected in both the STM images and FT-IR spectra of these materials. All the HPAs investigated formed well-ordered monolayer arrays on highly oriented pyrolytic graphite (HOPG) before and after pyridine adsorption. Exposure to pyridine increased the lattice constants of the two-dimensional HPA arrays by ca. 6 Å. Exposure to pyridine also shifted the negative differential resistance (NDR) peak voltages of HPAs to less negative values in the tunneling spectroscopy measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with acid properties of HPAs for the first time, suggesting that tunneling spectra measured by STM could serve to fingerprint acid properties of HPAs. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Heteropolyacid; Acid property; Scanning tunneling microscopy; Pyridine interaction; Negative differential resistance
1. Introduction Heteropolyacids (HPAs) such as H3 PMo12 O40 and H3 PW12 O40 are inorganic acids, but at the same time can act as oxidizing agents [1–3]. Many are highly soluble in polar solvents such as water, alcohols, and amines, but insoluble in some non-polar solvents such as benzene and olefins [4–6]. The solubility of these compounds in turn is closely related to their ability to adsorb various reactants. Non-polar molecules are adsorbed on the surface of bulk HPAs, while most polar molecules are mainly adsorbed in the bulk by forming ∗ Corresponding author. Tel.: +82-391-640-2404; fax: +82-391-640-2244. E-mail address: [email protected] (I.K. Song).
a ‘pseudo-liquid phase’ [7]. This characteristic leads to two typical catalytic reaction-types involving HPAs; namely, surface-type reaction and bulk-type reaction. It is well known that the acid and redox properties of HPAs can be modified by exchanging protons with metal cations and/or by substituting the heteroatom or framework metal ions in the heteropolyanion [8–10]. Commercial processes utilizing HPAs as acid catalysts include production of t-butanol [11] and polytetramethyleneglycolether (PTMG) [12]. The effects of counter cation and framework transition-metal cation substitutions on the acid properties of HPAs have been reported in the literature [13–15]. We have previously reported STM images of HPAs on graphite surfaces and have shown that HPAs form two-dimensional monolayer arrays [16–22]. These
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compounds exhibit distinctive negative differential resistance (NDR) behavior at specific applied voltages in their spatially resolved tunneling spectra [16]. It was observed that the NDR peak voltages could be related to the electronic properties of HPAs, and are influenced by the identity of the counter cations [18] and adsorbed organic molecules [19]. We have previously reported STM images of pyridine-exposed H3 PMo12 O40 arrays on graphite and have shown that the NDR peak voltage of these arrays shifted to a less negative bias upon pyridine adsorption [19]. In this work, nanoscale characterization of the acid properties of heteropolyacids of the family H3 PMo12−x Wx O40 was carried out by STM in order to compare NDR behavior with the acid properties of these compounds. The HPAs were deposited on a highly oriented pyrolytic graphite surface in order to obtain images, as well as tunneling spectra, by STM before and after pyridine adsorption. FT-IR experiments were performed to examine the acid–base interaction of these HPA monolayers with pyridine. It was found that NDR peak positions of HPA monolayers measured by STM could be correlated with acid properties of HPAs fingerprinted by FT-IR. 2. Experimental 2.1. HPA preparation and deposition HPAs of the type H3 PMo12−x Wx O40 (x = 0, 3, 6, 9, 12) were examined as acid catalysts in this work. H3 PMo12 O40 and H3 PW12 O40 were obtained from Aldrich. H3 PMo9 W3 O40 , H3 PMo6 W6 O40 and H3 PMo3 W9 O40 were prepared according to literature methods [23]. All HPAs were calcined at 300◦ C for 2 h to remove the water of crystallization in order to prepare solutions of consistent concentration. Approximately 0.01 M aqueous solutions of each sample were prepared. A drop of solution was deposited on freshly cleaved HOPG and allowed to dry in air for ca. 45 min at room temperature for STM imaging. Pyridine exposure was carried out by placing a drop of pyridine on the previously deposited HPA layer and drying in air for ca. 1 h at room temperature. Reversibly adsorbed pyridine molecules were then removed by evacuating the sample at ca. 25 millitorr for 1 h at room temperature prior to the STM measurement.
2.2. STM image and tunneling spectroscopy STM images were obtained in air using a Topometrix TMX 2010 instrument. Mechanically formed Pt/Ir (90/10) tips were used as probes. Scanning was done in the constant current mode at a positive sample bias of 100 mV and tunneling current of 1–2 nA. Tunneling spectra were measured in air and in a glove box filled with N2 . Both Topometrix TMX 2010 and LK Technologies LK-1000 STM were used to confirm consistency and reproducibility of tunneling spectra. All STM images presented in this report are unfiltered, and the reported periodicities (lattice constant) represent average values determined by performing two-dimensional Fast Fourier Transform (FFT) analyses on at least three images for each sample. Each image was acquired using a different tip; the tips were first calibrated by imaging bare HOPG to confirm the standard periodicity of HOPG (2.46 Å). Several tunneling spectra were then taken on the bare graphite section of the surface to ensure the stability of the tip and the reproducibility of the tunneling spectrum of HOPG. Once these had been established, the ‘good’ tip would be moved to the HPA-covered section to image and obtain tunneling spectra of the HPA sample. To measure a tunneling spectrum, the sample bias was ramped from −2 to +2 V with respect to the tip and the tunneling current was monitored. The voltage axis in the tunneling spectrum represents the potential applied to the sample relative to that of the tip. 2.3. IR spectroscopy A drop of HPA solution was deposited on the diamond probe of a Total Internal Reflectance Fourier Transform Infrared spectrometer (Applied Systems, ReactIR 1000) and allowed to dry in air for ca. 45 min at room temperature. IR spectra of the fresh samples were then recorded under ambient conditions. This step was further extended for the pyridine adsorption experiments by putting a drop of pyridine on the previously deposited HPA layer and allowing it also to dry in air for ca. 1 h at room temperature. Reversibly adsorbed pyridine molecules were then removed by evacuating the sample for 1 h at room temperature prior to performing FT-IR measurements on the pyridine-exposed HPAs.
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Fig. 1. STM images and schematic representations of the unit cells of (a) fresh H3 PW12 O40 and (b) fresh H3 PMo12 O40 monolayers.
3. Results and discussion 3.1. Self-assembled HPA arrays The primary structure (Keggin structure) of the heteropolyanion is relatively stable and insensitive to its surroundings. The three-dimensional array of solid-state HPAs comprising heteropolyanions, protons, cations, water, and/or organics is called the secondary structure [24]. Unlike the primary structure, the secondary structure is very labile and may change in different environments by either increasing or decreasing the interstitial space between heteropolyanions. Fig. 1(a) shows the STM image and schematic representation of the unit cell of H3 PW12 O40 deposited on HOPG. This image indicates the formation of an ordered HPA array on the graphite surface. The periodicity is 11.7 ± 0.1 Å and is in good agreement
with lattice constants obtained by STM [16–22] and X-ray crystallography [25–27]. The unit cell constructed on the basis of the lattice constant determined from 2D FFT shows that the arrays of H3 PW12 O40 have a nearly square symmetry (α = 79.5◦ ). Fig. 1(b) shows the STM image and schematic representation of unit cell of H3 PMo12 O40 deposited on HOPG. This image also shows the formation of an ordered array on graphite with approximately square symmetry (α = 84.9◦ ). Its periodicity is 10.8 ± 0.3 Å. The periodicities measured for two-dimensional arrays of H3 PMo12−x Wx O40 are summarized in Table 1. All HPAs examined in this work formed well-ordered arrays on graphite over scan areas of at least 200 × 200 Å. No systematic differences were observed in the periodicities of HPA arrays with variation of the W/Mo content of the framework. The surface arrays of H3 PMo12−x Wx O40 were quite
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Table 1 Periodicities of STM images of H3 PMo12−x Wx O40 Heteropolyacid
Periodicity (Å)
Included angle (degree)
H3 PW12 040 H3 PMo3 W9 O40 H3 PMo6 W6 O40 H3 PMo9 W3 O40 H3 PMo12 O40
11.7 ± 0.1 10.8 ± 0.2 10.6 ± 0.1 11.1 ± 0.1 10.8 ± 0.3
79.5 69.8 77.9 72.3 84.9
homogeneous. A small variation of periodicity among HPA samples did not come from the inhomogeneity of the sample but from STM imaging measurement. It is noteworthy that the tip structure may change electronically during or between STM imaging and may drift slightly in a small scan area. The molecular dimensions of H3 PMo12−x Wx O40 species were consistent with the reported values of 10–12 Å by STM [16–22] and X-ray crystallography [25–27].
3.2. Rearrangement of ordered arrays after pyridine adsorption A three-dimensional crystal structure of pyridineadsorbed H3 PW12 O40 array was reported [28]. In the three-dimensional array, it was reported that the pyridine molecules were paired around H+ forming (C5 H5 N)–H+ –(NC5 H5 ) cations. Fig. 2(a) shows the two-dimensional STM image of H3 PW12 O40 after pyridine exposure. The two-dimensional ordered arrays exhibit a periodicity of 18.2 ± 0.4 Å. Unlike the fresh H3 PW12 O40 arrays (α = 79.5◦ ), pyridine-exposed H3 PW12 O40 forms roughly hexagonal arrays (α = 59.1◦ ). As shown in Fig. 2(b), pyridine-exposed H3 PMo12 O40 also forms well ordered two-dimensional arrays having a periodicity of 16.5 ± 0.2 Å. It also has hexagonal-like arrays (α = 53.1◦ ) unlike the rectangular arrays of fresh
Fig. 2. STM images and schematic representations of the unit cells of (a) pyridine-exposed H3 PW12 O40 and (b) pyridine-exposed H3 PMo12 O40 monolayers.
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H3 PMo12 O40 (α = 84.9◦ ). The increase of periodicities of pyridine-exposed HPA arrays in both cases is ca. 6 Å; roughly the molecular size of pyridine. These results demonstrate that pyridine molecules are bound at the acid sites in the interstitial spaces between polyanions in the two-dimensional surface arrays, in a similar manner to that in three-dimensional solids. These results also indicate that two-dimensional HPA arrays were rearranged and expanded upon pyridine exposure. A model for pyridine-exposed H3 PMo12 O40 arrays on HOPG has previously been proposed in which pyridinium ions occupy bridging sites between heteropolyanions [19]. However, as in previous STM images of HPAs, the pyridinium ions, like other charge-compensating cations in these arrays, are not imaged directly. 3.3. Tunneling spectroscopy I–V spectra of the H3 PW12 O40 array on HOPG taken at two different positions, denoted as Site I and Site II in Fig. 1(a), are shown in Fig. 3(a). Site II exhibits the typical I–V spectrum of freshly cleaved HOPG and Site I exhibits the characteristic I–V spectrum of H3 PW12 O40 . The latter spectrum shows negative differential resistance (NDR) behavior at −1.20 V where dI/dV is negative in this region, as shown in Fig. 3(b). The NDR peak voltage was defined as the voltage at which the maximum current was observed in this region. The difference in I–V spectra of the two sites indicates that the array of H3 PW12 O40 on HOPG is a monolayer. The striking NDR behavior of HPAs measured by STM may be closely related to the electronic properties of these materials. Fig. 4 shows the trend of NDR peak voltage of H3 PMo12−x Wx O40 with framework metal substitution. The NDR peak shifts to less negative applied voltage with the increase of Mo substitution into the Keggin ion framework. The HPA images in this work were obtained at positive sample biases with respect to the tip. This means that electrons flow from tip to sample in the imaging mode of operation. NDR behavior in the tunneling spectra of HPAs is observed at negative sample biases, i.e. when electrons tunnel from sample to tip. We have observed that the NDR behavior of HPAs is strongly related to their electronic properties [18,20,22]. Previous results have shown that the NDR peak appeared at less negative applied voltage when the protons of the
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HPA were replaced by more electronegative cations such as Cu2+ ; the NDR peaks shifted to higher negative voltages when the protons were replaced by less electronegative cations such as Cs+1 . Thus it is expected that NDR behavior of HPA monolayers will be affected by the replacement of protons with pyridinium ions in the interstitial space between polyanions in these arrays. 3.4. Pyridine interaction with HPAs Fig. 5 shows the tunneling spectra of H3 PW12 O40 and H3 PMo12 O40 taken before and after pyridine exposure. The NDR peak of H3 PW12 O40 appears at −1.2 V before pyridine adsorption, whereas it appears at −0.65 V after pyridine adsorption. Introduction of electron-rich pyridine molecules into H3 PW12 O40 arrays leads to an NDR voltage shift of 0.55 V. The NDR peak voltage of H3 PMo12 O40 shifts from −0.95 to −0.75 V after pyridine adsorption. The shift of NDR peak voltage of HPAs to less negative applied voltages after pyridine adsorption was attributed to the replacement of protons with pyridium ions. The effect on the NDR position is comparable to that obtained by exchanging more electronegative counter cations for protons [18]. The interaction of pyridine with H3 PW12 O40 was confirmed by FT-IR, as shown in Fig. 6. The P–O, W = O (terminal oxygen), and W–O–W (corner sharing and edge-sharing) bands of fresh H3 PW12 O40 appeared at 1073, 973, 903 and 779 cm−1 , respectively. Pyridinium ions can be identified by the band at 1540 cm−1 [24]. The characteristic chemisorbed pyridium ion was detected after the removal of physisorbed pyridine from HPAs by evacuation. The P–O, W=O (terminal oxygen), and W–O–W (corner sharing and edge-sharing) bands of pyridine-exposed H3 PW12 O40 appeared at 1042, 948, 888 and 787 cm−1 , respectively. The shifts of IR bands of pyridine-exposed H3 PW12 O40 were attributed to the interaction of the cationic species, pyridinium ions, with the heteropolyanions. 3.5. Correlation between NDR shift and acid property of H3 PMo12−x Wx O40 We have shown that NDR peak voltages of HPAs were strongly affected by the pyridine binding to the
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Fig. 3. Tunneling spectra of H3 PW12 O40 taken at Sites I and II in Fig. 1 (a): (a) I–V spectra and (b) dI/dV spectra.
acid sites of the HPAs. We consider below whether the magnitudes of the NDR peak shifts of different HPAs measured before and after pyridine adsorption may serve as a fingerprint of their acid properties. This is still controversial as for the acid sites of solid-state HPAs [29–31]. Lee et al. [30] reported from IR and NMR investigations that the protons are located on the bridging oxygens of HPAs. In a recent paper, however, it was reported from 17 O NMR investigations of the proton sites in dehydrated HPAs
that the predominant protonation sites are the terminal M(metal)=O (oxygen) atoms [31]. In the FT-IR analyses of pyridine-exposed HPAs shown in Fig. 6, the quantification of bridging oxygen bands was not simple. Thus the adsorption band corresponding to the M=O stretch might be expected to track the extent of proton transfer upon interaction with basic molecules such as pyridine. We measured the IR intensity of pyridium ion at 1540 cm−1 of the pyridine-exposed H3 PMo12−x Wx O40 and calculated the intensity ratio
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Fig. 4. NDR peak voltage of fresh H3 PMo12−x Wx O40 vs. tungsten content (x).
of I(1540 cm−1 )/I(M=O) bands in an attempt to track the acid properties of H3 PMo12−x Wx O40 . The NDR shifts of H3 PMo12−x Wx O40 were also measured before and after pyridine adsorption. Fig. 7 shows the correlation between the I(1540 cm−1 )/I(M=O) ratio and NDR peak shift of H3 PMo12−x Wx O40 . It is noteworthy that the acid amounts of H3 PMo12−x Wx O40 measured by FT-IR can be directly correlated with NDR peak voltage shifts. H3 PW12 O40 , which exhibits the largest I(1540 cm−1 )/I(M=O) ratio upon pyridine exposure, experiences the largest NDR peak shift. Thus we suggest that NDR voltage shifts of HPAs measured by STM can be used to track the acid property of bulk HPAs. This is the first example showing how to estimate the bulk acid properties of HPAs from the properties of nanostructured HPA monolayers.
Fig. 5. Tunneling spectra of (a) H3 PW12 O40 and (b) H3 PMo12 O40 taken before and after pyridine adsorption.
4. Conclusions Nanoscale characterization of the acid properties of H3 PMo12−x Wx O40 was carried out by STM and tunneling spectroscopy. Pyridine adsorption onto the acid sites of HPAs was confirmed by STM and FT-IR. All HPAs formed well-ordered two-dimensional arrays on graphite before and after pyridine adsorption. The introduction of cationic pyridinium ions into the HPA arrays resulted in the expansion of the arrays and shifts of their NDR peak voltages to less negative
Fig. 6. IR spectra of H3 PW12 O40 before and after pyridine adsorption.
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Fig. 7. Correlation between NDR peak voltage shifts upon pyridine exposure of H3 PMo12−x Wx O40 and acidic properties (I(1540 cm−1 )/I(M=O)) determined from FT-IR spectra after pyridine exposure.
values. The NDR shift of HPAs obtained before and after pyridine adsorption was correlated with the acid property of the HPAs for the first time. It was confirmed that H3 PW12 O40 , the member of this series having the highest acid amounts, experienced the largest NDR peak shift upon pyridine adsorption. Thus the NDR shift measured with the STM may serve as a fingerprint for the acid property of bulk HPAs. Acknowledgements We acknowledge support from the National Science Foundation (Grant CTS 9410965). In Kyu Song acknowledges support from Korea Science and Engineering Foundation (981-1101-001-2) for this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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