Acid-base properties of oxides

An acid is a substance which donates a proton to, or accepts a pair of electrons from, another substrate which is classified as being a 

When an oxide is in contact with aqueous media the distinction between oxide, hydrated oxide, and hydroxide (especially when 

According to this view the acidity of the hydrous material should decrease, as is known for solution species,39 with decreasing charge on the central metal ion. Eventually a nondissociated hydrous layer, or possibly even a basic, i.e., cationic, film (in which protons are attracted in to neutralize coordinated hydroxide ions, or the latter are simply lost by desorption), will be attained at low central metal cation charge values. 

The dissociation constant for the complex anion, whose activity may be represented as ac, is given by the expression 

P being the ionic product of water. Apart from demonstrating that potential/pH values greater than zero (RHE scale) are of thermodynamic significance, this approach provides a simple interpretation 

From a thermodynamic viewpoint, oxide electrodes are frequently regarded as metal/insoluble salt electrodes in which the activity of the metal ion is modified by interaction with the ligands, which in this case are the OH ions. Thus, reaction (6) may be regarded as a combination of the following  

This means that a proton is most readily produced by the molecule with the largest number of attached oxygen atoms (HCIO4). 

This type of behavior is also observed for hydrated metal ions. Earlier in this chapter we saw that highly charged metal ions such as AP produce acidic solutions. The acidity of the water molecules attached to the metal ion is increased by the attraction of electrons to the positive metal ion  

The greater the charge on the metal ion, the more acidic the hydrated ion becomes. 

A compound containing the H—O—X group will produce an acidic solution in water if the O—X bond is strong and covalent. If the O—X bond is ionic, the compound will produce a basic solution in water. 

We can use these principles to explain the acid-base behavior of oxides when they are dissolved in water. For example, when a covalent oxide such as sulfur trioxide is dissolved in water, an acidic solution results because sulfuric acid is formed  

The structure of H2SO4 is shown in the margin. In this case, the strong, covalent 0—S bonds remain intact and the H—O bonds break to produce protons. Other common covalent oxides that react with water to form acidic solutions are sulfur dioxide, carbon dioxide, and nitrogen dioxide, as shown by the following reactions  

when a covalent oxide dissolves in water, an acidic solution forms. These oxides are called acidic oxides. 

On the other hand, when an ionic oxide dissolves in water, a basic solution results, as shown by the following reactions  

These reactions can be explained by recognizing that the oxide ion has a high affinity for protons and reacts with water to produce hydroxide ions  

Normal oxides of a particular formula type tend to become less acidic, and more basic in descending a Group. Thus, in Group II, BeO is amphoteric and has very low solubility in water, but dissolves in both acids and bases. The other Group II monoxides, however, are basic, and yield basic solutions when added to water for example 

The full change from acidic to basic is best seen in Group III. The oxide B2O3 is acidic, AI2O3 and Ga203 are amphoteric, and In203 and TI2O3 are basic. 

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The aim of this study is to develop model reaction for the characterization of the acidity and basicity of various transition aluminas, the experimental conditions being close to that for catalysis use. Among various model reactions, the transformation of cyclopentanol and cyclohexanone mixture was chosen for this work. Indeed, this reaction was well known for estimating simultaneously the acid-base properties of oxide catalysts [1], Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. The global reaction scheme is shown in Figure 1. 

In other words, state what you think happens to the acid-base properties of oxides as you go across a period and down a group. Make a quick sketch of the periodic table to illustrate this trend. Flow would you describe the acid-base properties of the metalloids (Use your knowledge of the physical properties of the metalloids to help you make your inference.)... 

Kireev V.A. Acid-base properties of oxides. Zhurn, Fiz. Khim. 1964 38 1881-92. 

Tthe reactions between LiOH and Li2COi with different manganese oxides can be viewed as acid-base interactions. According to Lewis classification, lithium compounds are bases whereas manganese oxides are acids of different strength. Acid-base properties of oxides significantly depend on the oxidation degree of cation. The acidity increases in the raw... 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

IV. Study of the Acid-Base Properties of Oxide Surfaces... 

Most of the recent adsorption literature has emphasized the importance of the acid-base properties of oxide surfaces when explaining or estimating their sorption behavior. However, Sveijensky (1993) has shown that log values for the adsorption of a specific cation by multiple mineral sorbents are a simple linear function of l/e, where e is the dielectric constant of each mineral. He has used this approach to estimate CC and TL model K"" values for the adsorption of up to 18 cations on 7 oxide and silicate mineral surfaces. 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

Cane, A., Roger, R, and Varinot, C., Study of acid/base properties of oxide, oxide glass, and glass-ceramic surfaces,/. Colloid Interf. Sci., 154, 174, 1992. 

The acidity of materials can be controlled by mixing oxides with different properties and in various ratios. If phase separation does not occur, substitution of cations of the minor component oxide in the host oxide lattice results in bulk phases of true mixed oxides. Another method used to modify the acid-base properties of oxide... 

SECTION 16.11 Acid-Base Properties of Oxides and Hydroxides... 

In this chapter, a brief and incomplete overview of the acid-base properties of oxide materials used in catalysis and adsorption technologies has been done. While the most general and fundamental aspects of solid acidity and basicity are quite well established and unanimously recognized, many questions are still opened in several particular systems of practical interest. Some of them have been underlined earlier. The most classical experimental techniques briefly described here allow to have a quite complete picture of the materials under study, but still the knowledge needs to be refined in many cases. Theoretical and computational methods are in rapid progress and allow now to go deeper in the understanding of the experimental results. 

Lima, E., Charles de Menorval, L., Tichit, D., Lasperas, M., Graffin, P., and Fajula, F. Characterization of acid-base properties of oxide surfaces by C CP/MAS NMR using adsorption of nitromethane. J. Phys. Chem. B 2003,107, 4070-4073. 

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