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Identification of Surface Sites on Monoclinic WO3 Powders by Infrared Spectroscopy Sofian M. Kanan,† Zhixiang Lu,†,‡ Juliet K. Cox,†,§ George Bernhardt,† and Carl P. Tripp*,†,‡ Laboratory for Surface Science & Technology and Department of Chemistry, University of Maine, Orono, Maine 04469 Received September 11, 2001. In Final Form: December 4, 2001 The dehydroxylation/dehydration and Lewis acidity of the surface of monoclinic tungsten oxide (m-WO3) powder as a function of evacuation temperature was investigated by infrared spectroscopy. At room temperature, the m-WO3 surface contains both isolated and hydrogen bonded hydroxyl groups along with equal amounts of strongly and weakly adsorbed layers of water. Most of the surface hydroxyl groups and the weakly adsorbed water layer are eliminated by evacuation at room temperature. The strongly adsorbed water is removed by evacuation above 200 °C. Adsorption of D2O shows that the surface hydroxyl groups and adsorbed water are accessible and easily exchanged. However, the removal of the strongly held water is not related to the number of Lewis or Brønsted acid sites on the surface. While there is little change in the amount of adsorbed water between a room temperature sample and a sample evacuated at 150 °C, pyridine adsorption shows that there is a corresponding 50% reduction in the number of Lewis acid sites. Furthermore, the strongly held water is eliminated by evacuation between 200 and 400 °C, whereas there is little change in the number of Brønsted or Lewis acid sites. The changes in Lewis/Brønsted acidity are not related to the dehydration but rather attributed to reduction of the oxide due to removal of lattice oxygen.
Introduction There is much interest in developing monoclinic tungsten oxide (m-WO3) based sensors for low level (ppb) detection of gaseous compounds. Detection is based on the oxidation of gaseous molecules on the surface of m-WO3, which leads to a change in the conductivity of the oxide. In a typical operating environment, the sensor will be exposed to a gas stream containing a myriad of compounds. Under these conditions it is difficult to decipher the complicated molecular chemistry that leads to a change in conductivity in m-WO3 based sensors. Thus, the ability to identify and monitor surface species formed in gas-phase reactions on m-WO3 surfaces would aid the development of this technology. While infrared spectroscopy is a powerful technique for identifying surface sites and surface reactions on metal oxide particles, its application to surface studies on m-WO3 is difficult because high surface area m-WO3 particles are not commercially available. Commercial m-WO3 particles are typically 1-5 µm in diameter with surface areas of 1-2 m2/g.1 The low surface area coupled with the high scattering associated with particles of this size renders them unusable for surface infrared studies.2-4 Recently, we have developed synthetic methods for preparing m-WO3 particles that are about 20 nm in diameter.1 In brief, hydrated amorphous WO3 particles are formed by the condensation of H2WO4 in the presence of oxalate and * Corresponding author: Phone: 207-581-2235. Fax: 207-5812255. E-mail:
[email protected]. † Laboratory for Surface Science & Technology. ‡ Department of Chemistry. § Current address: Sun Chemical Ink (GPI), Carlstadt, NJ 07072. (1) Lu, Z.; Kanan, S. M.; Tripp, C. P. J. Mater. Chem., in press. (2) Paul, J.-L.; Lassegues, J. C. J. Solid State Chem. 1993, 106, 357. (3) Gotic, M.; Ivanda, M.; Popovic, S.; Music, S. Mater. Sci. Eng. B 2000, 77, 193. (4) Daniel, M. F.; Desbat, B.; Lassegues, J. C.; Gerand, B.; Figlarz, M. J. Solid State Chem. 1987, 67, 235.
acetate ligands or in the presence of emulsion-stabilized water droplets. The hydrated WO3 particles are washed, dried, and converted to the monoclinic form by calcination at 500 °C. These particles are approximately 20 nm diameter and are suitable for surface infrared studies because they exhibit low scattering of the infrared beam and have high surface areas of about 45 m2/g (BET N2). Before embarking on gas-phase adsorption studies with these high surface area particles, there is a need to first identify the reactive groups present on the m-WO3 surface. This is important because the degree of surface hydroxylation/hydration, and hence the number of exposed Lewis acid and base sites on metal oxides, can be altered by evacuation at elevated temperature. As a consequence, evacuation temperature plays a critical role in defining the surface reactions and surface species formed in subsequent reactions of gaseous molecules with the metal oxide surface. While there are numerous infrared studies devoted to monitoring the dehydroxylation/dehydration behavior as a function of temperature on many oxides,5 we are not aware of similar studies on monoclinic WO3 (m-WO3) powders. Typically, the identification of surface sites on metal oxide powders is largely based on the ability to detect infrared bands due to various hydroxyl groups (both from adsorbed water and surface M-OH modes) and using the position of bands of adsorbed molecules such as pyridine to probe Lewis acid/Brønsted acid site densities.6-8 Bands due to adsorbed water on WO3‚2H2O and NH3 adsorption on sputtered tungsten oxide films have been detected,4,9 (5) Morrow, B. A. In Spectroscopic Analysis of Heterogeneous Catalysts, Part A: Methods of Surface Analysis; Fierro, J. L. G., Ed.; Elsevier: Amsterdam, 1990. (6) Gabrusenoks, J. SPIE 1997, 2968, 192. (7) Kung, M. C.; Kung, H. H. Catal. Rev.sSci. Eng. 1985, 27, 425. (8) Busca, G. Catal. Today 1998, 41, 191. (9) Agrawal, A.; Habibi, H. Thin Solid Films 1989, 169, 257.
10.1021/la011428u CCC: $22.00 © 2002 American Chemical Society Published on Web 01/26/2002
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Figure 1. Infrared spectra of the m-WO3 powder recorded in air at room temperature (a) and after evacuation at various temperatures: (b) room temperature; (c) 150 °C; (d) 300 °C; (e) 400 °C.
but these studies were on amorphous WO3 based material before crystallization into the stable monoclinic form. Experimental Section Pyridine and D2O were purchased from Aldrich and were transferred to evacuable glass bulbs using standard freeze-thaw methods. The nanosized m-WO3 powders were prepared in our laboratory.1 Two methods were used to record IR spectra. We used diffuse reflectance (DRIFT) on the neat powder in order to detect the relatively weak IR bands due to adsorbed water and surface hydroxyl groups in the dehydroxylation/dehydration experiments. Although it was possible to record transmission spectra using self-supporting disks, we found that this method was not practical because the disks were fragile and therefore difficult to fabricate for routine use. DRIFT spectra were recorded using a Harrick preying mantis equipped with an environmental chamber on neat m-WO3 powders. A DRIFT reference spectrum was recorded using KBr powder. The adsorption of pyridine was recorded in transmission mode using a thin film of m-WO3 powder dispersed on a KBr window. Details of the thin film technique are described elsewhere.10 The thin film technique enabled detection of bands due to adsorbed species across the entire infrared region. This was important for detection of bands that lie in the same region as the strong W-O bulk modes (1300-970 cm-1). In the thin film experiments, difference spectra are plotted with the reference spectrum recorded using the thin film m-WO3 just prior to adding the pyridine vapor. The difference spectrum was recorded after addition of the pyridine at room temperature for 5 min, followed by evacuation for 2 min. Positive bands are due to bonds that formed on the surface, and negative bands represent bond removal from the surface. All spectra were recorded on a Bomem MB-155S FTIR with a liquid N2 cooled MCT detector. Typically 200 scans were coadded at a resolution of 4 cm-1. Standard vacuum line techniques were used to expose gaseous reagents to the m-WO3 powder.
Results and Discussion Dehydration of m-WO3. Figure 1 shows the DRIFT infrared spectra of the m-WO3 sample recorded in air at room temperature (Figure 1a) and evacuated at various temperatures (Figure 1b-e). The region below 1300 cm-1 (10) Tripp, C. P.; Hair, M. L. Langmuir 1991, 7, 923.
is not shown because it is opaque because of the presence of strong W-O bulk modes. Bands located at 2063 and 1854 cm-1 do not change with evacuation and are therefore assigned to various overtone and combination W-O bulk modes. The region between 3800 and 3000 cm-1 contains bands due to both surface WOH and adsorbed water. Since this section is devoted to dehydration and not dehydroxylation, we first describe the trends observed for the H2O bending mode located at 1630 cm-1 because this band is unique to the presence of adsorbed water on the surface. By monitoring the change in intensity of the bending mode at 1630 cm-1 it is possible to determine the degree of dehydration of the m-WO3 with evacuation temperature. This curve is shown in Figure 2, and the ordinate value is calculated from the integrated peak areas at 1630 cm-1 ratiod to the m-WO3 bulk mode at 2063 cm-1. The shape of the curve in Figure 2 is very similar to the weight loss curve in a thermal programmed desorption experiment on amorphous hydrated tungsten hydroxy oxide (WOx(OH)y‚H2O) films.9,11 On the amorphous material, there is a large initial decrease in weight that levels off at about 200 °C followed by a second smaller decrease in weight between 300 and 350 °C. Both weight losses were due to elimination of water. It was concluded that the initial weight loss below 200 °C was due to removal of adsorbed molecular water and that the second decrease in weight beginning at 300 °C was from condensation of adjacent surface WOH groups (see structure I).
In our case, the data shown in Figure 2 are derived from the bending mode of water, and thus, the second decrease between 200 and 400 °C is due to water removal and not WOH condensation. This shows that there are two types of adsorbed water on the surface. Most of the adsorbed water is weakly bound to the surface and (11) Schlotter, P.; Pickelmann, L. J. Electron. Mater. 1982, 11, 207.
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Figure 2. Desorption of water from the m-WO3 surface as a function of evacuation temperature.
Figure 3. Infrared spectra of (a) m-WO3 powder and (b) that after addition of D2O vapor for 15 min at room temperature followed by evacuation for 2 min.
eliminated with evacuation at room temperature. A second type of adsorbed water remains on the surface and is eliminated only after evacuation at 400 °C. One possible explanation for the presence of adsorbed water above 200 °C is that it is caged in the microporous structure of the oxide. Evidence supporting a caged hypothesis is provided by the sharp band at 2345 cm-1, which appears in all curves in Figure 1. This band at 2345 cm-1 is due to weakly adsorbed CO2, and this band persists even with evacuation at 400 °C. The band at 2345 cm-1 is not observed when the m-WO3 sol-gel powders were synthesized in the absence of oxalate/acetate ligands or surfactants. A possible explanation is that the water and CO2 produced from the combustion of the organic oxalic/ acetic acid ligands or surfactants become trapped during annealing of the m-WO3 powders at 500 °C. Additional experiments show that the contribution due to trapped water is small and cannot account for the loss
of water between 200 and 400 °C. If the elimination of water above 200 °C was due to inaccessible or trapped water, then a sample evacuated at 400 °C should not readily hydrate upon exposure to water vapor or exchange with addition of D2O vapor at room temperature. However, we find that m-WO3 that has been first evacuated at 400 °C completely rehydrates upon exposure to air at room temperature. More convincing evidence is provided by D2O exchange experiments at room temperature. Parts a and b of Figure 3 show the infrared spectra of the m-WO3 thin film before and after the exposure to D2O for 15 min at room temperature, respectively. There is almost a complete disappearance of the OH modes in the 3600-3200 cm-1 region along with the H2O band at 1630-1610 cm-1, and this is accompanied by the appearance of infrared bands at 2710, 2539, and 2266 cm-1. The D2O bending mode located at 1178 cm-1 is not observed because it is below the cutoff frequency of the DRIFT spectrum shown in
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Figure 3b. This clearly shows that the D2O easily accesses all the water/WOH groups at room temperature. Only a weak band at 3460 cm-1 remains, showing that only a very small fraction of the adsorbed water could be due to entrapped water from the combustion of the organic oxalate/acetate ligands or surfactants remaining from the sol-gel synthesis. This trapped water cannot account for the loss of water observed between 200 and 400 °C in curve 2. Therefore, there is a second, more strongly bound, water layer on the WO3 surface. There is also spectroscopic evidence supporting both strong and weakly bound water on the surface. The bending mode of adsorbed water is located at 1630 cm-1; this band shifts to 1610 cm-1 upon evacuation at room temperature. A band at 1630 cm-1 is attributed to a weakly held hydrogen bonded water layer similar to that found on oxides such as silica.12 The second more strongly adsorbed water has a band at lower frequency (i.e. 1610 cm-1), suggesting that it is adsorbed through the oxygen atom of the water via coordination with Lewis sites (structure II). In support of this assignment, it is noted
that a shift to lower frequency in the H2O bending mode has been observed at 1609 cm-1 for hydrated WO3‚1/3H2O, and this was assigned to water bonded through the oxygen atom of the water molecule.4 The existence of a strongly adsorbed water that is resistant to removal by evacuation at elevated temperature is not unique to WO3. Adsorbed water is also detected on anitase powder at 300 °C.13 It is noted that the OH asymmetric stretching mode of the strongly adsorbed water is weaker in intensity than the bending mode at 1610 cm-1. This is unexpected, as the OH stretching modes of water are typically twice as intense as the corresponding bending mode. While the origin of this difference remains unclear, one possible explanation is that it is related to a change in the relative dipole moment associated with each vibrational mode arising from a specific orientation of water at the surface. Assuming an adsorbed species as shown in structure II, the dipole moment associated with the asymmetric stretching modes would be in the plane parallel to the surface whereas the corresponding bending mode would have the dipole moment perpendicular to the surface. Dehydroxylation of m-WO3. The OH stretching region contains a sharp band at 3698 cm-1 and two broad bands centered at 3460 and 3220 cm-1. The change in intensity of the 3460 cm-1 band differs from those for the 3698 and 3220 cm-1 bands and mirrors the change in intensity of the 1630 cm-1 band. Therefore, the band at 3460 cm-1 is the OH stretching mode of weakly adsorbed water on the surface. On oxides such as alumina, titania, and silica, isolated surface hydroxyl groups produce sharp bands in the 3750-3600 cm-1 region and hydrogen bonded hydroxyl groups give rise to broad bands between 3400 and 3000 cm-1.5 On the basis of this trend, we assign the band at 3698 cm-1 to isolated WOH and the broad band centered at 3220 cm-1 to hydrogen bonded WOH groups. (12) Tripp, C. P.; Combes, J. R. Langmuir 1998, 14, 7348. (13) Tripp, C. P. Spectroscopic Studies of Selected Reactions on Oxides. Ph.D. Thesis, University of Ottawa, 1988.
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The isolated WOH band is completely eliminated, and the hydrogen bonded WOH groups show a large decrease upon evacuation at room temperature (see Figure 1). While the level of dehydroxylation with ambient evacuation is large when compared to that for alumina or titania, it is noted that m-WO3 is more easily reduced with evacuation or heating than the other oxides. Furthermore, this is consistent with the rapid exchange of the WOH groups with D2O at room temperature. Other oxides such as alumina, titania, and silica usually require multiple exposures of D2O vapor at elevated temperatures to achieve high exchange levels of the surface OH groups.14,15 The reduction of m-WO3 is most often associated with the loss of lattice oxygen, resulting in a change in conductivity and optical transparency of the oxide.16,17 However, with the high surface sol-gel particles, a significant fraction of the loss of oxygen is associated with removal of surface hydroxyl groups. It is noted that this does not preclude the loss of lattice oxygen in the sol-gel particles at higher evacuation temperatures, as we do observe a decrease in the overall transparency of the oxide, which increases with evacuation temperature.18,19 By expanding the ordinate scales of the curves in Figure 1 (not shown), at evacuation temperatures between ambient and 400 °C there is a continual decrease in the weak broad band between 3500 and 3100 cm-1 with a higher degree of erosion on the low-frequency side. The gradual decrease with evacuation on the low-frequency side of the broad band between 3500 and 3100 cm-1 at temperatures between 150 and 400 °C shows that further reduction in surface OH groups arises from condensation of the strongest H-bonded WOH groups. The remaining broad band near 3500 cm-1 is most likely due to inaccessible or trapped H2O/OH groups. In the exchange experiment with D2O we find a small but detectable broad band at 3500 cm-1 that is resistant to exchange with D2O. Adsorption of Pyridine. Adsorption on metal oxide surfaces occurs primarily with the surface hydroxyl groups, the adsorbed water layer, and Lewis acid/base sites. On oxides such as titania and alumina, dehydroxylation leads to the creation of coordinatively unsaturated Lewis acid and base sites.5
Pyridine is a common probe molecule to monitor the adsorption sites on metal oxide surfaces.7,8,20,21 Adsorbed pyridine shows different and characteristic infrared bands depending on whether the pyridine forms hydrogen bonds (III), forms a pyridinium ion via proton transfer with the acidic surface hydroxyl groups (IV), or is coordinated to a Lewis acid site (V). The three types of bonding are shown in Chart 1. (14) Bartlett, J. R.; Gazeau, D.; Zemb, T.; Woolfrey, J. L. Langmuir 1998, 14, 3538. (15) Kanan, S. M.; Tripp, C. P. Langmuir 2001, 17, 2213. (16) Gutie´rrez-Alejandre, A.; Ramı´rez, J.; Busca, G. Langmuir 1998, 14, 630. (17) Ozer, N.; Lampert, C. M. Thin Solid Films 1999, 349, 205. (18) Guinneton, F.; Valmalette, J.; Gavarri, J. R. Opt. Mater. 2000, 15, 111. (19) Guinneton, F.; Sauques, L.; Valmalette, J. C.; Cros, F.; Gavarri, J. R. J. Phys. Chem. Solids 2001, 62, 1229. (20) Lavalley, J. C. Catal. Today 1996, 27, 377. (21) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497.
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Figure 4. Infrared spectra of pyridine adsorbed at room temperature on m-WO3 evacuated at (a) room temperature, (b) 150 °C, and (c) 400 °C. Chart 1
Clearly the number and type of sites will vary with the degree of dehydroxylation/dehydration of the surface. Given the dehydration behavior shown in Figure 2, we selected m-WO3 surfaces evacuated at room temperature, 150 °C, and 400 °C for the adsorbed pyridine studies. The spectra are shown in Figure 4. For all three samples an excess quantity of pyridine vapor was added for 5 min at room temperature, followed by evacuation for 2 min. The spectrum obtained for pyridine addition to m-WO3 evacuated at room temperature (Figure 4a) produces bands at 1609, 1575, 1485, and 1445 cm-1 characteristic of coordination to Lewis acid sites.22,23 Figure 4a also shows a weak band at 1530 cm-1 assigned to the pyridinium ion, indicating that a few WOH groups are acidic in nature. For a m-WO3 sample evacuated at 150 °C the amount of adsorbed pyridine with Lewis acid sites is 50% lower than that obtained with the room temperature evacuated sample while the number of Brønsted coordinated species (band at 1530 cm-1) remains constant. On both the room temperature and 150 °C samples, the adsorbed pyridine does not displace the strongly adsorbed water on the surface, as there is no change in intensity of the H2O bending mode at 1610 cm-1. Furthermore, in comparing curves 4b and 4c, there is a small decrease in pyridine adsorption on Lewis acid sites and a small increase on Brønsted acid sites in a sample that was evacuated at 400 (22) Jones, P.; Hockey, J. A. J. Chem. Soc., Faraday Trans. 1 1971, 2669. (23) Kataoka, T.; Dumesic, J. A. J. Catal 1988, 112, 66.
°C relative to a sample evacuated at 150 °C. There is no change in the number of Brønsted acid sites with evacuation at the three temperatures, and this trend is consistent with the presence of small quantities of hydrated amorphous WO3. Some residual amorphous hydrated WO3 is most likely present in the sample because a hydrated WO3 is formed in the sol-gel process and this is then converted to m-WO3 with heating at temperatures above 500 °C. The adsorption of pyridine shows that the major change is the number of Lewis acid sites, and this occurs between ambient temperature and 150 °C. We recall that there is little change in the amount of strongly bound water between ambient temperature and 150 °C evacuation and that the adsorbed water is eliminated by evacuation at 400 °C. Therefore, there is little connection between removal of bound H2O and the number of Lewis sites available for adsorption of pyridine. It is likely that the decrease in Lewis acidity is due to the reduction of the oxide through oxygen removal. We find that evacuation at elevated temperature reduces the transmission of the m-WO3, and this has been attributed to the creation of vacancies in the WO3 conduction band arising from the extraction of lattice oxygen.16 It is noted that XPS measurements on an annealed tungsten oxide film in a vacuum have shown a lowering of oxidation states from W6+ to W5+ and W4+ at higher temperatures.24 A similar trend was obtained for XPS spectra of the WO3 powders used in this study. A room temperature evacuated sample shows peaks only due to the presence of W6+, whereas samples evacuated at 150 and 400 °C show additional bands due to W5+ and W4+. Therefore, the observed reduction in the oxidation state of the tungsten atom from W6+ to W5+ and W4+ reduces the Lewis acidity of the surface and this is consistent with the spectral data obtained from pyridine adsorption studies. The removal of the strongly bound water occurs at sites that are not responsible for the reduction of the oxide, as we find that a sample evacuated at 400 °C is rehydrated to the same level as an original sample in air. (24) Leftheriotis, G.; Papaefthimiou, S.; Yianoulis, P.; Siokou, A. Thin Solid Films 2001, 384, 298.
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Conclusion The surface of m-WO3 contains both free and hydrogen bonded hydroxyl groups as well as a layer of weakly and strongly adsorbed water. Both the weakly adsorbed water and hydroxyl groups are easily removed by evacuation at room temperature. While evacuation at 150 °C does not remove the strongly bound water layer, it does lead to a 50% reduction in the number of Lewis acid adsorption sites. This is attributed to reduction of the surface due to removal of bulk oxygen. With evacuation between 200
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and 400 °C the strongly bound water layer is removed but the number of surface Lewis and Brønsted acid adsorption sites remains essentially unchanged. Acknowledgment. This work was supported by the Department of the Navy, Naval Surface Warfare Center, Dohlgren Division, Grant N00178-1-9002. The authors would like to thank Professor Bob Lad (at the University of Maine) for his many helpful discussions. LA011428U
