Spillover-backspillover Phenomena

It is now well established that spillover-backspillover phenomena play an important role in numerous catalytic systems. It is worth reminding that the effect of strong-metal-support interactions (SMSI), which was discovered by Tauster74 and attracted the intense interest of the catalytic community for the least a decade75 was eventually shown to be due to backspillover of ionic species from the Ti02 support onto the supported metal surfaces. 

Can one use STM to study spillover/backspillover phenomena and to confirm the origin of electrochemical promotion The answer is positive and the experimental setup used for the first demonstration of electrochemically controlled spillover/backspillover between a catalyst-electrode (Pt) and a solid electrolyte (p"-Al203) is shown in Figure 5.48.78,79 A polished Pt(lll) single crystal (lOmmxlOmmxlmm) was mounted on an appropriately carved polycrystalline p"-Al203 sample (20mmx20mmx3mm). 

Spillover-backspillover phenomena can take place over enormous ( mm) atomic distances. 

The use of X-ray photoelectron spectroscopy (XPS) has provided conclusive evidence that spillover/backspillover phenomena are real and that electrochemically controlled backspillover of oxide ions, O, is the origin of electrochemical promotion. The first XPS investigation of Ag electrodes on YSZ under electrochemical 0 pumping conditions was published in 1983 [36] and provided strong evidence for the creation of backspillover oxide ions on Ag (01s at 529.2 eV) upon positive current application. These results were confirmed by Gopel and coworkers who used XPS, UPS and EELS to study AgA SZ catalyst surfaces under electrochemical bias conditions [35]. 

This study, in addition to explaining NEMCA with Na" -conducting solid electrolytes, provided the first STM confirmation of spillover/backspillover phenomena. 

Electrochemical promotion of catalysis, similarly to usual (chemical) promotion and to metal-support interactions in heterogeneous catalysis, is related to spillover-backspillover phenomena. The latter can be described as the mobility of adsorbed species from one phase on which they easily adsorb (donor) to another phase where they do not directly adsorb (acceptor). By this mechanism a seemingly inert material can acquire catalytic activity. Spillover may lead to an improvement of catalytic activity or selectivity and also to an increase in lifetime of the catalyst. 

Promotion, electrochemical promotion and metal-support interactions are three, at a first glance, independent phenomena which can affect catalyst activity and selectivity in a dramatic manner. In Chapter 5 we established the (functional) similarities and (operational) differences of promotion and electrochemical promotion. In this chapter we established again the functional similarities and only operational differences of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports. It is therefore clear that promotion, electrochemical promotion and metal-support interactions on ion-conducting and mixed-conducting supports are three different facets of the same phenomenon. They are all three linked via the phenomenon of spillover-backspillover. And they are all three due to the same underlying cause The interaction of adsorbed reactants and intermediates with an effective double layer formed by promoting species at the metal/gas interface (Fig. 11.2). 

The model of electrochemical promotion regards the phenomenon as catalysis in presence of an electrically controlled double layer formed by spillover-backspillover mechanism at the gas-exposed catalyst surface. This shows strong analogy with catalyst-support interactions... 

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