Multiple "/" in catalysts

What does it mean if a catalyst has multiple "/" symbols, such as $\ce{Mn/Fe/Al2O3}$? Does it mean either $\ce{Mn}$ or $\ce{Fe}$, or does it mean both are supported on $\ce{Al2O3}$?




There should be only one "/" symbol. When more are used, it should be unambiguous from the context what is the carrier, e.g. in your case it would be more appropriate to use a dash:


$$\ce{Mn-Fe/Al2O3}$$


From section [*1.3 Composition, structure and texture of catalysts* of 1975 IUPAC recommendations for heterogeneous catalysis](https://www.degruyter.com/downloadpdf/j/pac.1976.46.issue-1/pac197646010071/pac197646010071.pdf#page=8) [[1](https://doi.org/10.1351/pac197646010071), p. 79] (emphasis mine):



> 
> *Support.* In multiphase catalysts, the active catalytic material is often present as the minor component dispersed upon a support sometimes called a carrier. The support 
>  may be catalytically inert but it may contribute to the overall catalytic activity. Certain bifunctional catalysts (§1.2.8) constitute an extreme example of this. **In naming 
>  such a catalyst, the active component should be listed first, the support second and the two words or phrases should be separated by a solidus, for example, platinum/silica or 
>  platinum/silica-alumina.** The solidus is sometimes replaced by the word "on", for example, platinum on alumina. 
> 
> 
> *Promoter.* In some cases, a relatively small quantity of one or more substances, the promoter or promoters, when added to a catalyst improves the activity, the selectivity, or the useful lifetime of the catalyst. In general, a promoter may either augment a desired reaction or suppress an undesired one. **There is no formal system of nomenclature 
>  for designating promoted catalysts.** One may, however, for example, employ the phrase "iron promoted with alumina and potassium oxide".
> 
> 
> 


References
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1. Burwell, R. L. Manual of Symbols and Terminology for Physicochemical Quantities and Units. Appendix II. Heterogeneous Catalysis; *Pure Appl. Chem.* 46, 71, **1976**. <https://doi.org/10.1351/pac197646010071> (Open Access)


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