Study of zirconia microporous structure
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Study of Zirconia Microporous Structure V. Yu. Gavrilov Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia ReceivedDecember23, 1998
Abstract--The microporous structure of zirconium dioxide obtained by precipitation at different pH and different times of gel aging is studied by the physical adsorption of nitrogen, oxygen, and molecular hydrogen. It is shown that the ratio between the supermicropore surface areas measured by the adsorption of 02 and N2 as a function of the supermicropore volume previously revealed for SnO2 is also retained for zirconia (the surface area of supermicropores measured by 02 adsorption is larger than that measured by N2 adsorption). Possible reasons for this phenomenon are discussed. The conclusions on the factors responsible for the formation of the zirconia microporous structure are drawn.
Zirconia is widely used as a selective adsorbent, a support a number of catalysts for high-temperature processes and selective conversions, and an ion-exchanger with controlled adsorption properties [1-5]. Amorphous metals supported on ZrO2 exhibit high activity in the low-temperature oxidation of CO [6]. The formation and retention of microporous structure is rather typical for zirconia obtained by precipitation, which is most often used now, especially for its low-temperature synthesis. Evidently, this affects the behavior of materials obtained in the majority of the processes. Therefore, studies of the properties of such xerogels by traditional adsorption methods with the use of conventional adsorbates and by other physical and chemical methods are of particular interest [7, 8]. The use of new experimental procedures for adsorption analysis, including the use of nontraditional adsorbates, leads to rather unexpected results. The results of our studies of tin dioxide xerogels [9] provide an example. The goal of the present paper is to study the microporous structure of zirconia obtained by precipitation by comparison of data on the low-temperature (77.4 K) physical adsorption of N 2, 02, and H2.
water, extruded as granules, and dried in air and then in a drying oven at 150~ for 12-14 h. The samples of ZrO2 synthesized at 80~ in the pH range from 4 to 7 were chosen for the study. Our previous investigations [10] showed that, in this range of pH, xerogels with the largest volume of micropores were formed, whereas the volumes of meso- and macropores were quite small. In addition, the microtexture of several xerogels obtained at pH 4 and aged for a certain time (up to 22 h) in intermiceUar medium was examined. The adsorption isotherms of N 2, 02, and H Eat 77.4 K were measured with an automatic Digisorb-2600 Micrometrics instrument of the volumetric type. The samples were trained in a vacuum at 180~ for 6 h. To exclude a possible irreproducibility of textural parameters, the isotherms for different adsorbates were measured using the same weighed portions of samples for each set of samples. The adsorption isotherms of nitrogen and oxygen vapors w
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