Metal oxide binary, surface acidity

In addition to the aluminosilicate systems, a wide variety of other binary metal oxides exhibit surface acidities and acid-catalyzed activities that are greater when the oxides are chemically combined than would be the case for a physical mixture of the same oxides. The existence of this type of synergism is most easily explained in terms of Pauling s electrostatic valence rules (135). In a recent paper, Tanabe et al. (136) have extended such rules to account for the generation of acidity in 26 binary oxides. They make two postulates ... 

Metal oxides, 31 78-79, 89, 102, 123, 157-158, 191, 32 199-121 see also Amorphous metal oxides Sulfate-supported metal oxides specific oxides adsorbed oxygen on, 27 196-198 binary, surface acidity, 27 136-138 catalytic etching, 41 390-396 coordination number, 27 136 electrocatalysts, 40 127-128 Fe3(CO)i2 reaction with, 38 311-314 Lewis acid-treated, 37 169-170 multiply-valent metals, electrocatalytic oxidations, 40 154-157 superacids by, 37 201-204 surface acidity, methods for determining, 27 121... 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

Corrosion of the material used is another factor that limits the selection of the electrocatalyst. The electrochemical corrosion of pure noble metals is not as important as in the case of binary or ternary alloys in strong acid or alkaline solutions, since these catalysts are widely used in electrochemical reactors. In the case of anodic bulk electrolysis, noble metal alloys used in electrocatalysis mainly contain noble metal oxides to make the oxidation mechanism more favorable for complete electron transfer. The corrosion problem that occurs from this type of catalyst is the auto-corrosion of the electrode surface instead of the electrode/electrolyte solution interface degradation. The problem of corrosion is considered in detail in Chapter 22. 

Adsorption microcalorimetry is the measure of the heat of adsorption evolved when dosing measured small amounts of a vapor probe on a surface. Cardona-Martinez and Dumesic [36] summarized the results obtained for oxides, zeolite, and metal catalysts before 1992. Summaries of the application of these techniques to gas-solid interactions and heterogeneous catalysis have been published recently [37-39]. As done by Auroux and Gervasini [40] for a number of binary metal oxides, calorimetric studies of the acidity and basicity are mostly performed using ammonia as an acidity probe and carbon dioxide as a basicity probe [41]. 

As discussed in a previous review [9], acidity and basicity of metal oxides are basically linked to the nature of the element involved, whose valency and atomic size are the main factors generating both the bulk structure and the surface chemistry. This relation is summarized in Table 9.9 for binary oxides and Table 9.10 for mixed oxides. However, the particular preparation of any compound has its own properties in relation to morphology and purity/impurity, which arise from the particular preparation method. In the following case studies, we will take into consideration also some of these aspects. 

K. Tanabe, T. Sumiyoshi, K. Shibata, T. Kiyoura and J. Kitagawa, A new hypothesis regarding the surface acidity of binary metal oxides, Bull. Chem. Soc. Japan, 47, 1064—1066 (1974). 

Bulk and supported mixed oxide compositions, from binary metal oxides to quaternary metal oxides, consist in general of large crystalline phases possessing low surface area values (typically from 1 to 10m g ). Examples of oxides of this type of catalytic relevance are V-Nb-0, Mo-Nb-O, Co-Ti-0, Ni-Ti-0, etc. The acid-base properties of mixed metal oxides have been found to change with the nature of the constituents and their relative concentrations, preparation (co-precipitation and sol-gel synthesis among are the most popular methods used), and pre-treatments procedures. Appropriately choosing the mentioned variables, mixed oxides can be used to prepare catalysts with the desired-acid-base characteristics. 

From practical and theoretical points of view concerning binary metal oxides, it is interesting to And oxide combinations having well defined and mnable acid or basic properties. On a catalytic oxide surface, the acid or basic sites can be either too strong causing some irreversible adsorption of the substrate species or the sites can be too weak to activate the substrate species. Therefore, the possibility to regulate the acid-base strength, besides the acid site amount of the oxide surfaces, appears a necessary tool for catalytic purposes. 

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