Binary oxides surface acidic properties

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)... 

As already noted, the most relevant aspects of the chemical reactivity of the higher binary rare earth oxides, i.e, ceria, praseodymia and terbia, deal with their redox properties. Typical acid-base processes, like hydration and carbonation phenomena, very important for the sesquioxides, have much less significance in the case of the higher oxides. Except for praseodymia [188,189], the information available about the intemction of the higher binary oxides, particularly ceria, with CO2 and H2O, under the usual experimental conditions, suggests the occurrence of purely surface processes [190]. Accordingly, this section of the chapter will be mainly devoted to the redox chemistry of the higher rare earth oxides. 

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. 

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. 

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. 

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