Spillover definition

A difference between the two systems is that in NEMCA experiments the spillover-backspillover rate can be accurately measured and controlled by simply measuring the imposed current or potential. Another difference is that in electrochemical promotion experiments backspillover provides a promoting species, not an active site, to the catalyst surface. This latter difference can however be accommodated by a broader definition of the active site . 

Figure 7.9. Schematic representation of the density of states N(E) in the conduction band of two transition metal electrodes (W and R) and of the definitions of work function O, chemical potential of electrons p, electrochemical potential of electrons or Fermi level p, surface potential x, Galvani (or inner) potential (p and Volta (or outer) potential for the catalyst (W) and for the reference electrode (R). The measured potential difference UWr is by definition the difference in p q>, p and p are spatially uniform O and can vary locally on the metal surfaces 21 the T terms are equal, see Fig. 5.18, for the case of fast spillover, in which case they also vanish for an overall neutral cell Reprinted with permission from The Electrochemical Society.

Scanning tunneling microscopy, STM ordered adlattices, 264 oxygen adlattices, 261 platinum, 261 sodium adlattices, 262 spillover-backspillover, 259 Self-consistent field, 269 Selectivity definition, 17... 

First, it is necessary to define spillover. During the recent international symposium a definition was proposed and affirmed by the congress (JO). 

This article has attempted to put spillover in perspective. At this time, there seem to be more questions than definitive conclusions. As with any newly discovered phenomenon, spillover has been used to explain a variety of phenomena on heterogeneous solids. Even if limited to interphase phenomena the picture is clouded. The influence is well documented, but the extent of the relative influence has been studied in less quantitative terms. Further, it... 

One female and one male patient had hyperparathyroidism with elevated serum alkaline phosphatase activities and extensive bone changes characteristic of generalized osteitis fibrosa cystica. In both instances, the serum acid phosphatase activity of the serum fell to normal values after removal of the parathyroid adenoma despite transitorily increased serum alkaline phosphatase activity. The fifth patient was a female with osteopetrosis involving the major part of the skeleton. The serum acid phosphatase was 8.7 K.A. units, the highest in the control series— yet the serum alkaline phosphatase was within normal limits. It would appear, therefore, that some patients with skeletal disease may have a slight but definitely elevated serum acid phosphatase activity, at least as determined by the Gutman method (GIO, G14), which cannot be explained by concurrent prostatic carcinoma or by a spillover of alkaline phosphatase activity to a pH of 5.0. 

The definition of promotion suggested above excludes cases where the support itself provides an essential service (e.g., in bifunctional and spillover catalysis) or where the promotion occurs unintentionally. Carbon deposition can however sometimes result in a selectivity improvement and of course a species may act at the same time as both an electronic and a selectivity promoter. For these reasons it is not always easy to classify what a particular promoter does, or how it works. 

According to the Catalytic Cycle (Figure 5.1), an active centre is a place where the reactants are adsorbed adjacently, ready to react. We may therefore attempt a working definition of an active centre as a location on the surface of a catalyst where reactants may be adsorbed in the right way and with the best strengths to give the desired products efficiently. It is not necessary for the reactants to adsorb directly onto the active centre one or both may adsorb elsewhere and then diffuse to the active point this is what happens in spillover catalysis. There are without doubt cases where two reactants do not adsorb in competition on the same type of site each has its own requirement, and it may be more closely restricted for one reactant than for the other. The active centre will then contain two sites of different character, on which each reactant is adsorbed non-competitively. Kinetic analysis should reveal when this situation arises. 

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