Molybdena techniques

Infrared spectra of chemisorbed NO. Chemisorption of NO on partially reduced molybdena based catalysts has been shown to be a usefull technique in evaluating M0O3 exposures on such a catalyst surface (see, e. g. 16)). However, the application of this technique to a series of catalysts which differ in M0O3 loading, or even in preparation method, requires a carefull control of the degree of reduction. As NO appears to be chemisorbed mainly at Mo sites, the catalysts must be quantitatively reduced according to the reaction ... 

The molybdena catalyst probably represents one of the best examples of a catalyst which has been studied by a wide variety of techniques. This, in itself, is a good gauge of the complexity of the catalyst. 

Table I lists the major characterization techniques which have been applied to the molybdena catalyst. They may be grouped into two broad categories nonspectroscopic and spectroscopic methods. Space does not permit a full discussion of the theory, experimental techniques, or interpretation of results of these techniques—we give here only the author s interpretations of their results. The reader is referred to any number of standard texts or reviews on the specific technique for a more complete description.

The individual techniques used to characterize molybdena catalysts are now considered. Table II presents a listing of articles concerning the characterization of molybdena catalysts. Unless otherwise specified, we implicitly refer to Mo and/or Co supported on an activated alumina, commonly y-AlaOs. Most work has been done on the calcined (oxidized) state of the catalyst because of ease of sample handling. Reduced and sulfided catalysts are more difficult to work with since for meaningful results, exposure of these samples to air or moisture should be rigorously avoided. Therefore, sample transfer or special in situ treatment facilities must be provided. 

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

This technique measures the optical spectrum of light which is diffusely scattered off the catalyst sample. Absorption frequencies are characteristic of certain arrangements of molecules and their environment. Even pure compounds give rather broad diffuse spectra, and catalysts show even broader spectra. Hence, results are only semiquantitative at best. In regards to molybdena catalysts, the information derived with this technique is the coordination environments of Mo and Co in the catalyst. 

In summary, the IR results are in agreement with other techniques and in particular provide evidence for Mo-Al surface interactions. Also, the finding of terminal and bridged Mo-0 species agrees with the picture of a monolayer of molybdena on the alumina surface. 

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