Study of the Acid Site Structure on the WO₃/ZrO₂ Surface

Abstract

ABSTRACT:The IR and UV-vis diffuse reflectance spectra of mesoporous WO 3 /ZrO 2 are discussed and a scheme proposed for the acid site structure on the WO 3 /ZrO 2 surface. The high Brönsted acidity of WO 3 /ZrO 2 (H 0 ≤ −11.38) can be explained by the formation of large zirconium polytungstate species such as on the WO 3 /ZrO 2 surface.

Superacids Based on Zirconium Dioxide

“…The reason for the appearance of strong acidity in tungstate-containing ZrO 2 remains unclear in many cases and continues to be discussed in the literature [34,36,39,[42][43][44][45]. Most of the results from study of the active centers on the WO 3 /ZrO 2 surface were obtained using electronic and IR spectroscopy.…”

“…The IR spectra also contain bands for the W=O and W-O stretching vibrations in the region of 1200-400 cm -1 [34,45,46]. The bands at 1080 and 980 cm -1 belong to the W=O vibrations.…”

“…The bands at 1080 and 980 cm -1 belong to the W=O vibrations. Absorption at 960 and 980 cm -1 is characteristic of polytungstate anions W 6 O 20 (OH) 5 The Brönsted and Lewis acidity of the tungstate-containing zirconium dioxide shows up in the IR spectra of chemisorbed pyridine [34,45,47], acetonitrile [45], and carbon monoxide [46]. The B centers on the surface of WO 3 /ZrO 2 are stable up to 500°C [45].…”

“…Absorption at 960 and 980 cm -1 is characteristic of polytungstate anions W 6 O 20 (OH) 5 The Brönsted and Lewis acidity of the tungstate-containing zirconium dioxide shows up in the IR spectra of chemisorbed pyridine [34,45,47], acetonitrile [45], and carbon monoxide [46]. The B centers on the surface of WO 3 /ZrO 2 are stable up to 500°C [45]. In the IR spectra of pyridine adsorbed on mesoporous WO 3 /ZrO 2 previously evacuated at 700°C the band at 1550 cm -1 is absent, and only the bands at 1610 and 1440 cm -1 characteristic of the L centers are observed [45].…”

“…The B centers on the surface of WO 3 /ZrO 2 are stable up to 500°C [45]. In the IR spectra of pyridine adsorbed on mesoporous WO 3 /ZrO 2 previously evacuated at 700°C the band at 1550 cm -1 is absent, and only the bands at 1610 and 1440 cm -1 characteristic of the L centers are observed [45]. The Lewis centers appear in the IR spectra of adsorbed CD 3 CN in the form of two absorption bands at 2304 and 2348 cm -1 [45].…”