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  • Author or Editor: A. Auroux x
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The acidity and basicity of about twenty simple oxides mainly used in catalysis and ceramic materials were investigated by adsorption microcalorimetry of basic and acidic gas probe molecules such as ammonia and carbon dioxide. The determination of the number, strength and site energy distribution of both acidic and basic sites in all the samples led to an improved description of the surfaces. The results allowed us to classify our samples in three main groups in relation to their acidic, basic or amphoteric character. Their acid-base behaviour was tentatively correlated to more general electronic and redox data given in the literature such as the electronegativity.

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The acid-base character of vanadium pentoxide, V2O5/SiO2 and V2O5/γ-Al2O3 catalysts has been investigated by adsorption of ammonia and sulphur dioxide using microcalorimetry. By depositing vanadium oxide on silica; new surface sites are formed which present more acid strength than bulk vanadium pentoxide and pure silica. Alumina-supported vanadium catalysts can be regarded as acidic monolayers VOx. Sulphur dioxide was found to be selective for uncovered alumina.

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Thermally stable mesoporous aluminophosphates (AIPO) and silicoaluminophosphates (SAPO) were prepared at room temperature in the presence of a cationic surfactant and an organic base. These materials possess high surface areas and regular mesopores of approximately 35 Å diameter. By contrast to microporous crystalline aluminophosphate molecular sieves, mesoporous compounds are amorphous and characterized by Al/P ratios greater than 1. These particularities are responsible for a strong Lewis acidity, as evidenced by ammonia adsorption microcalorimetry. Mesoporous materials are more acidic than the microporous analogues and the amount of strong acid sites increases with the silicon content.

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Alumina-boria catalysts were prepared by impregnation of porous and non porous aluminas with various amounts of boron oxide. A calorimetric investigation of their acidity was performed by gaseous ammonia adsorption. The differential heat evolved decreases when the amount of boria on alumina increases while the corresponding number of acid sites, as determined by volumetry, increases with the amount of boron oxide. The thermal behaviour and the stability of the catalysts, when dehydration occurs, were studied by differential scanning calorimetry linked to thermogravimetry.

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Acidity and basicity of alumina-boria catalysts supported on porous or non-porous alumina have been studied by adsorption microcalorimetry of probe molecules (ammonia, pyridine and sulphur dioxide). Despite decreasing in initial heats, the total acidity as determined by ammonia adsorption increased in number and strength as a function of percentage of boron oxide. Ammonia, as a strong base, was shown to cover all types sites from strong to weak acid sites. Pyridine, as a weaker probe, was shown to dose only the stronger sites of the samples which stay nearly constant after B2O3 coverage approaching the monolayer. The basic sites of the amphoteric alumina support are neutralized by 10 wt% of boron oxide on non-porous alumina and 20 wt% of B2O3 on porous alumina. The catalytic activity for partial oxidation of ethane increased with acidity and reached a maximum constant value above 20 wt% of boron oxide.

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In this work we report about the characterization of the acidic and redox properties of four different commercial tungstated zirconia catalysts with W loadings of about 12–13 mass%. The samples have been characterized in terms of their micro-structural and surface properties by BET, X-ray diffraction, temperature programmed reduction, elemental chemical analysis and adsorption microcalorimetry of NH3. Improved acidity has been detected upon addition of WO3 to zirconia and differences between the samples were pointed out thanks to the results obtained by the complementary physico-chemical techniques used in this study.

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The surface properties of gallium oxide and tin dioxide supported on alumina or titania have been studied by adsorption microcalorimetry. The differential heats of adsorption of various pollutant adsorbates such as sulfur dioxide, nitrogen monoxide, nitrogen dioxide and also ammonia were measured on these catalytic surfaces. NH3, SO2, NO2 are strongly adsorbed while NO is only physisorbed. The supported Ga2O3 samples show a slight decrease in acidity as probed by ammonia adsorption, compared to alumina or titania. The addition of SnO2 decreases the number of strong acid sites but creates a few weak and medium strength acid sites on alumina and does not modify the acidity of titania. In all cases, the basicity, probed by SO2 adsorption, is very strongly affected by the deposition of Ga2O3 or SnO2. The differential heats of NO2 adsorption remain nearly constant on all samples. The heats of adsorption are discussed as a function of the coverage and of the amount of guest oxide.

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The surface properties of supported gallium oxide catalysts prepared by impregnation of various supports (γ-Al2O3, SiO2, TiO2, ZrO2) were investigated by adsorption microcalorimetry, using ammonia and water as probe molecules. In the case of acidic supports (γ-Al2O3, ZrO2, TiO2), the acidic character of supported gallium catalysts always decreased in comparison with gallium-free supports; on very weakly acidic SiO2, new acidic centers were created when depositing Ga2O3. The addition of gallium oxide decreased the hydrophilic properties of alumina, titania and zirconia, but increased the amount of water adsorbed on silica. The catalytic performances in the selective catalytic reduction of NO by C2H4 in excess oxygenwere in the order Ga/Al2O3>Ga/TiO2>Ga/ZrO2>>Ga/SiO2. This order is more related to the quality of the dispersion of Ga2O3 on the support than to the global acidity of the solids.

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