Reforming with Bimetallic Catalysts

The essential application of LEISS analysis concerns supported bimetallic catalysts. For these solids, XPS analysis can no longer be considered as a surface analysis. The sizes of the particles present on the surface of these catalysts are around a nanometre, similar to the mean free path of the photoelectrons that determines the thickness probed by XPS. Certain pairs of elements do not lend themselves to LEISS analysis. This is the case, for example, with Pt-Rc for which the mass difference of the two metals is small compared to the resolution. One example of a pair that lends itself to LEISS analysis is provided by a study of Pt-Sn reforming catalysts. LEISS is ten times more sensitive for platinum than for tin. Despite this, an intensity of the tin peak at least comparable to that of platinum can be observed on a catalyst with a Pl/Sn ratio of three (Fig. 6.5), This study can be used to show a surface segregation of tin that is dilTicult to detect by XPS because of the small size of the platinum particles (2 nm). 

The understanding of the interaction of S with bimetallic surfaces is a critical issue in two important areas of heterogeneous catalysis. On one hand, hydrocarbon reforming catalysts that combine noble and late-transition metals are very sensitive to sulphur poisoning [6,7]. For commercial reasons, there is a clear need to increase the lifetime of this type of catalysts. On the other hand. Mo- and W-based bimetallic catalysts are frequently used for hydrodesulphurization (HDS) processes in oil refineries [4,5,7,8]. In order to improve the quality of fuels and oil-derived feedstocks there is a general desire to enhance the activity of HDS catalysts. These facts have motivated many studies investigating the adsorption of S on well-defined bimetallic surfaces prepared by the deposition of a metal (Co, Ni, Cu, Ag, Au, Zn, A1 or Sn) onto a single-crystal face of anodier metal (Mo, Ru, Pt, W or Re) [9-29]. 

It is found that Mode E behaves similarly to the zeolite free Pt-Re/Al203 Both catalysts have a relatively high proportion of isomer products which could be formed over the metal surface via a bond-shift mechanism [8]. Isomers are formed by doublebond isomerization and skeletal isomerization reactions at both the acid sites of the alumina support and the metal sites. The later provides a dehydrogenation-hydrogenation function and the acid sites an isomeiization function for the olefins to dehydrogenate from paraffins over the metal function, since it is known that olefin isomerization proceeds much quicker than the respective paraffin isomerization [8]. On the other hand, branched paraffins are less easily cracked than linear ones [10]. Therefore, once isomers are formed over conventional reforming catalysts, they are likely to be the final products. Evidently, the isomerization of paraffin requires the metal function in the bimetallic catalyst, and so does the paraffin aromatization. This can also explain the obseiwed decrease in the isomers and aromatics production with time-on-Hne since it is well- known that coke preferentially deposits on a metal surface first [14]. 

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