Metal oxides acid-base properties

Ingo, G.M., Riccucci, C., and Chiozzini, G., Origin of gas porosity in gold-based alloys cast in calcium sulfate-bonded investment and influence of metal oxide acid-base properties on calcium sulfate thermal stabihty, J. Am. Ceram. Soc., 84, 1839, 2001. 

We expected to control the direction of OTM reaction over NiO by sur ce modification, namely making use of the interaction between NiO and other conq>onents to beget a synergistic effect. In this paper, two completely different behaviors of the oxidative transformation of methane were performed over the nickel-based catalysts because of the different modifications by alkali metal oxide and rare earth metal oxide and the different interactions between nickel and supports. Furthermore, the two completely different reactions were related with the acid-base properties of catalysts and the states of nickel present. 

The field of surface-mediated synthesis of metal carbonyl clusters has developed briskly in recent years [4-6], although many organometallic chemists still seem to be unfamiliar with the methods or consider themselves ill-equipped to carry them out. In a typical synthesis, a metal salt or an organometallic precursor is brought from solution or the gas phase onto a high-area porous metal oxide, and then gas-phase reactants are brought in contact with the sample to cause conversion of the surface species into the desired products. In these syntheses, characteristics such as the acid-base properties of the support influence fhe chemisfry, much as a solvenf or coreactant influences fhe chemisfry in a convenfional synfhesis. An advanfage of... 

As discussed in the previous section, metal oxides have both acidic and basic properties. The acid-base properties of metal oxides have led to many interesting catalytic reactions. Catalytic reactions such as H2-D2 exchange, hydrogenation, isomerization, dehydrogenation, dehydrohalo-genation, and benzylation can be considered as examples of acid-base catalysis reactions.31-36 These reactions will be briefly discussed in the following section. The remarkable properties of MgO as a catalyst have been well documented in the literature and we shall discuss some of these unique catalytic properties. 

Describe the acid-base properties of metal oxides. 

The metallic properties increase down any column and towards the left in any row on the periodic table. One important metallic property is that metal oxides are base anhydrides. A base anhydride will produce a base in water. These are not oxidation-reduction reactions. Many metal oxides are too insoluble for them to produce any significant amount of base. However, most metal oxides, even those that are not soluble in water, will behave as bases to acids. A few metal oxides, and their hydroxides, are amphoteric. Amphoteric means they may behave either as a base or as an acid. Amphoterism is important for aluminum, beryllium, and zinc. Complications occur whenever the oxidation number of the metal exceeds +4 as in the oxides that tend to be acidic. 

Studies on the nature of the interaction between the dispersed metal oxide species and the support have shown that their catalytic behavior and their acid-base properties are strongly affected by the inductive effect of the metal ions in the solids [187,188]. It has also been established in the literature that the support influences... 

The snrface acid-base properties of supported oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. As shown, acidic and basic sites are often present simultaneously on solid surfaces. The knowledge of the detailed amphoteric character of supported metal oxides is of extreme interest due to the possibility of using them as catalysts in different reactions in which acidity governs the reaction mechanism. 

When immersed in aqueous solutions, the surfaces of metal oxides are expected to be hydroxylated (see Section 3.1). However, metal oxides may also react with aqueous solutions leading to dissolution, which can effectively be thought of as mass transfer from the solid to aqueous phase. The rate and extent of dissolution reactions depend on a number of factors, including solution pH, acid-base properties of oxo groups on the metal oxide surface, types of ligands present in solution, metal... 

The interaction between selective metal oxides and molecules to be oxidized is, of course, based on electron-accepting and electron-donating properties, respectively. In this way, Mo6+, Vs+, etc. act as electron acceptors and molecules with 7r-bonds as donors. Ai et al. [5—12] have drawn attention to the fact that this can also be described by acid—base properties. An electron donor molecule like butene is a basic entity interacting with acidic sites on the catalyst. Hence it follows that activity and selectivity depend on the relative acidity and basicity. Mo03, for example, is an acidic oxide, while Bi203 is a basic oxide. Different compositions Bi Mo have different acidities. The rate of oxidation depends on the number of acid sites (=acidity) and the acid strength, viz. 

Solid metal sulphates and phosphates also exhibit acid—base properties their acid strength is lower than that of silica—alumina but they are stronger acids than some oxide catalysts [5]. Correlation of activity with electronegativity of cations has been obtained for several reactions [9, 50,51],... 

The acid-base properties and the ionic-covalent character of an element s oxide depend on both the element s position in the periodic table and its oxidation number. As Figure 14.6 shows, both the acidic character and the covalent character of an oxide increase across the periodic table from the active metals on the left to the electronegative nonmetals on the right. In the third row, for example, Na20... 

Oxygen forms ionic oxides, such as Li20 and MgO, with active metals, and covalent oxides, such as P4OK) and SO3, with nonmetals. Oxides can also be classified according to their acid-base properties. Basic oxides are ionic, and acidic oxides are covalent. Amphoteric oxides, such as AI2O3, exhibit both acidic and basic properties. 

Oxides exhibit similar trends. In the third row, for example, Na20 and MgO are typical high-melting, ionic solids, and P4O10, S03, and C1207 are volatile, covalent, molecular compounds (Section 14.9). The metallic or nonmetallic character of an oxide also affects its acid-base properties. Na20 and MgO are basic, for example,... 

As noted, for example by van der Wiele and van den Berg,4 the interaction between the molecule to be oxidized and the catalyst is based on the electron-donating and -accepting properties and can be described in terms of the acid-base properties. Activity and selectivity are thus expected to depend on the relative acidity and basicity. Thus Boreskov et al.80 observed a clear correlation between the acidity of a V205/Ti02 catalyst modified with different alkali metals and the selectivity for 0-xylene oxidation. The most acidic site was assumed to be responsible for the destructive oxidation. Thus the selectivity increased with the basicity, i.e., with the atomic number of the alkali metal. 

Carbon is a prominent catalyst support material as it allows the anchoring of metal particles on a substrate which docs not exhibit solid acid-base properties. Carbon is finally a catalyst in its own right, enabling the activation of oxygen and chlorine for selective oxidation, chlorination, and dechlorination reactions. 

The metallic character of the elements in the periodic table, and the acid-base properties of their oxides, show a distinct trend across periods and down groups. Infer what this trend is. 

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