DEVELOPMENT AND SUPPRESSION OF SURFACE-ACIDITY ON MONOCLINIC ZIRCONIA - A SPECTROSCOPIC AND CALORIMETRIC INVESTIGATION

The surface Lewis acidity of pure monoclinic Zr02 activated at three different temperatures (670, 870, and 1070 K) has been studied from a quantitative and energetic point of view by means of adsorption of CO at room temperature, coupling IR spectroscopy, and adsorption microcalorimetry. Two different CO adspecies are formed at the surface, whose spectral position, heat of adsorption, and relative population depend on the extent of surface dehydroxylation: a high-frequency (CO)H species (2192-2195 cm-1, 65-73 kJ mol-1, 0.18-0.24 CO molecule nm-2 on Zr02 outgassed at 670 and 870 K, respectively) and a low-frequency (CO)L species (2184-2187 cm-1, 44-50 kJ mol-1, 0.50-0.80 CO molecule nm'2 on Zr02 outgassed at 670 ana 870 K, respectively). At 1070 K, the onset of the sintering causes a marked decrease in the population of the high-frequency species but does not modify the acidic strength of the two sites. The change of acidity with the extent of surface dehydroxylation has also been checked by adsorbing CO during a stepwise rehydration of the surface obtained by irreversible adsorption of H20. Following reconstruction of the surface-hydroxylated layer, the activity toward CO progressively declines and is eventually suppressed. There is a linear dependence of the decrease of the activity toward CO on the amount of preadsorbed water: four molecules of H20 irreversibly held at the surface cause the elimination of one acidic site. Both spectroscopic and energetic data indicate that the rehydration pattern is far from being the plain reverse of the dehydration one. © 1990, American Chemical Society. All rights reserved.