Calcination catalytic character

The above work has demonstrated that not all double metal hydroxides display purely basic character upon calcination. Basic character in some cases appears due to the formation of single oxides. The active sites for ketone formation have been proposed to be the surface O-anion and the hydroxyl anion. The role of tetrahedral Al in the Ni-Al and Mg-Al systems is not clear however, it does appear to contribute to the formation of basic sites at least for the Mg-Al system. Structural considerations have showed that Mg-Al and Ni-Al hydroxides behave differently upon calcination and therefore generate different types of active sites. In this paper a preliminary picture is presented of the coordination of the cations, particularly Al, and their effect on catalytic selectivity. 

XRD analyses of mked oxide samples showed that spinel structures or mbctures of spinel phases and binary oxides occurred at a calcination temperature of 500 °C. However, at such low temperatures an amorphous character of part of the sample is very likely. The crystalline part of the samples consists of small particles of about 5 nm in size according to TEM studies. These oxides showed interesting catalytic activities in CO-oxidation and are currently a topic of intense investigation. 

The spectra of the surface hydroxyl groups ofa-Al203 powders vary sig-nificandy from sample to sample (246—249). The adsorption of basic probe molecules such as pyridines or CO typically reveals Lewis acid sites of medium strength, whereas very strongly acidic sites, which are observed for transition aluminas, are absent (186,247—251). However, the observed sites depend on the calcination conditions, the dehydration conditions apphed prior to adsorption, and on the surface area of the sample some of the surface properties of the original transition alumina may be retained (252). The catalytic activity ofa-Al203 for acid-catalyzed reactions is usually low (207) in fact, even a basic character has been documented (206). 

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