Acid-base Properties Of Oxides Research Articles

Illustration of Selected Functional Inorganic Materials Fabricated by Mechanochemical Approaches

In mechanochemistry, there are various approaches that bene t the engineering of solids and, additionally, they can be considered as green processes [1 7]. These are mainly three categories: #A Direct mechanosyntheses between the reagents depending upon the nature of the desired product with the use of: a) SHS reactions, b) acid-base properties of oxides; #B Modi cation of properties via high-energy milling by: a) appropriate substitution of various cations in sublattices of complex oxides e.g. with perovskite structure, b) the use of the presence of impurities from the reactor and balls in order to improve the usable features of a product; #C Activation of solids by co-milling process by: a) supporting of catalytic precursor on carrier, b and c) mechanical treatment of reactants before or after conventional synthesis process of catalysts. The practical examples are described below.

Wettability of Metallic Oxides—Application to Stainless Stell Acid-Base Properties

The Brönsted acid-base properties of oxides of metallic systems are investigated by contact angle measurements of water droplets for different pH. Theoretical analysis based on the electrical and chemical variations of the solid/liquid interfacial tension allows to understand the experimental results observed on model oxide surfaces as SiO2 and Al2O3. Similar wetting measurements are performed on the passive films of a model Fe-Cr-Si alloy and 304 stainless steel. Significant correlations are observed between the contact angle-pH curve and the oxide composition of the passive films deduced from XPS analysis.

Evaluation of the acid-base surface properties of several oxides and supported metal catalysts by means of model reactions

Acid-base properties of oxides (Al 2O 3, SiO 2, ZrO 2, CeO 2, MgO, SiO 2Al 2O 3 and CeO 2Al 2O 3) were investigated by means of model reactions: 3,3-dimethylbut-1-ene isomerization (33DMB1), methylene cyclohexane isomerization (MECH), cyclohexanol conversion (CHOL) and CO 2 chemisorption at room temperature. The effect of acid (Cl −, SO 2− 4) and basic (K +) promoters on certain oxides (Al 2O 3, ZrO 2) was also studied. Surface acidity was evaluated by means of 33DMB1 or MECH isomerization and of CHOL dehydration into cyclohexene. CO 2 chemisorption as well as the cyclohexanone to cyclohexene ratio in CHOL conversion were used to measure the surface basicity of the solids. Except for very few cases, all the tests gave coherent results which led to well-defined scales of acidity and of basicity. Contrary to what could be observed with the conventional isopropanol test, the CHOL test proves to be little sensitive to the redox sites of the oxides. The presence of metals can create significant perturbations in these acid-base tests, except in the case of 33DMB1 isomerization. The latter reaction is not catalyzed by Pt and Rh surfaces, which allows to measure (in the presence of these metals) the acid properties of the support and to investigate the changes in surface acidity, resulting from the metal impregnations (metal → support electronic effect, presence of anions,…).

Oxide/aqueous solution interfaces, interplay of surface chemistry and electrocatalysis

A number of insoluble, conducting transition metal oxides have acquired primary importance in electrochemical technology. So-called activated electrodes are prepared by depositing oxide layers on “inert” supports (Ti, Ta, Nb, etc.) by thermal decomposition of suitable precursors. The resulting oxides are non-stoichiometric, the temperature and procedure of preparation being crucial variables. Separation of true catalytic effects from the apparent ones due to geometric factors is needed for a proper understanding of the factors governing the catalytic activity. The acid-base properties of oxides in contact with aqueous solutions depend intimately on the surface structure. Being intensive properties they are not affected by geometric factors. Any variation of these properties with temperature and procedure of oxide preparation can thus be associated with modifications in the catalytic properties. This idea is illustrated by a number of examples for the oxides of highest technological interest: RuO 2 (primary active component of the so-called ‘Dimensionally Stable Anodes’), IrO 2, IrO 2 + RuO 2, Co 30 4, NiCo 2O 4.