Pyridine Used as a Probe for Internal Br nsted Acid Sites in Pyrochlore Antimony (V) Oxide: An Infrared Spectroscopy Study EDILSON V. BENVENUTTI, YOSHITAKA GUSHIKEM, and CELSO U. DAVANZO* Instituto de Qufmica, UNICAMP, CP 6154, 13081, Campinas, SP, Brazil Brsnsted acid sites inside the 0.3-nm-diameter cavities of pyrochlore antimony (V) oxide were detected with the use of pyridine as a probe molecule, and following the appearance of pyridinium ions by infrared spectroscopy (8a and 19b modes). The pyridine molecule is larger than the cavities' windows and can only bind acid sites when the oxide is strongly hydrated. This observation indicates that the protons are free to migrate to the surface. Index Headings: Infrared; Surface analysis; Pyrochlore; Protons. INTRODUCTION The investigation of the acid properties on solid sur- faces with the use of infrared spectroscopy, with pyridine as a probe molecule, has been extensively used for a long time. t-3 There are two main advantages to this approach: metallic oxides in which the metal atom is in a high oxidation state can be probed, and it is possible to dif- ferentiate Lewis and Br~nsted acid sites. It is expected that, in order to respond to an acid site, probe molecules have to be able not only to react with these sites but also to have free access to them. This second requirement depends on the dimensions of the probe molecule when it is supposed to react with internal or hindered acid sites. The molecular diameter of the pyridine molecule can be calculated from bond angles and bond lengths; 4 it is ~0.40 nm. When the van der Waals radii of the atoms are used instead, an even larger molecular diameter is obtained--0.59 nm) For this reason, the detection of acid sites of solids within cavities that have windows smaller than 0.40 nm should be impossible. The aim of this work is to show that in some special cases, under certain con- ditions, even pyridine can be used to probe acid sites. Crystalline hydrated antimony (V) oxide (CAO) was cho- sen, because its pyrochlore structure (spatial group Fd3m) 6~ has cavities with ~0.3-nm windows (Fig. 1). EXPERIMENTAL The cell used for the infrared measurements has been described elsewhere. 9 It was connected to a greaseless vacuum line, where a pressure as good as 10-2 Pa was obtained. Pyridine (Merck) was dried over a 0.4-nm mo- lecular sieve and submitted to various freeze/pump/thaw cycles. Infrared spectra were taken with an FT-IR Per- kin-Elmer Model 1600, with 4-cm -1 resolution and a maximum of 256 scans. Received 28 April 1992. * Author to whom correspondence should be sent. It was necessary to dilute the CAO powder using fumed silica in order to obtain a proper transmittance. Forty milligrams of CAO/silica (1:1) were pressed under a pres- sure of 6 MPa. The samples were treated at temperatures up to 300°C. At this stage the infrared spectra were ob- tained; and subsequently the oxide was exposed to 1.6 kPa of pyridine. After that, the vapor was pumped off and the disk was heated in vacuum at various temper- atures. CAO was obtained from the hydrolysis of antimony pentachloride, as described in the literature. ~° X-ray diffractograms were collected in a Shimadzu Model XD-3A diffractometer with the use of Cu Ka ra- diation, scanning at 2 0 min -~. The BET surface areas of fumed silica and CAO were obtained in a Micromeritics Flow Sorb 2300 instrument and found to be 144 and 20 m 2 g-t, respectively. RESULTS AND DISCUSSION X-ray diffraction of CAO sample and of two pellets diluted to a 50% level in fumed silica, before and after thermal treatment at 300°C in the infrared cell, were measured; the unit cell dimensions obtained are shown in Table I. The 1.03-nm value for the a vector of the unit cell is in good agreement with the literature, 7,s.lt,12 indi- cating that the thermal treatment causes no structural changes. The adsorption of pyridine on pure fumed silica was studied to gain understanding of the behavior of CAO diluted on silica. The results are presented in Fig. 2. Figure 2a is the infrared spectrum of the fumed silica exposed to 1.6 kPa of pyridine and pumped off, where the two bands at 1596 and 1444 cm -1 are seen. Further pumping (10 -2 Pa) at 100°C (Fig. 2b) causes these bands to vanish. Spectrum 2c is the difference spectrum (2b - 2a), showing the band at 3740 cm -t that is due to free silanol groups, indicating that these groups interact with pyridine by hydrogen bonding, t,13 Figure 3 shows the results of pyridine adsorption on a CAO (diluted) sample treated at 300°C under vacuum (10 -2 Pa) for two hours. Figure 3a is the spectrum of the sample/pyridine system. Bands at 1596 and 1444 cm -1 are due to pyridine interactions with the free silanol groups of silica, as shown in Fig. 2a. Bands at 1580 and 1438 cm -~, which disappeared after pumping off at room temperature (Fig. 3b), were assigned to physically ad- sorbed pyridine, the so-called liquid-like pyridine. 2 Fig- ure 3c is the spectrum of 3b but with the addition of 1.3 Pa of water vapor. A band at 1547 cm -~ can be clearly observed and corresponds to the 19b mode of the pyri- 1474 Volume 46, Number 1 0, 1992 0oo3-7o28/92/461o-147452.0o/o APPLIED SPECTROSCOPY © 1992 Society for Applied Spectroscopy