Spillover

Structural control systems must be designed using rather small-scale models but are applied in the real structure with a theoretically infinite number of eigenmodes. Unwanted interaction or energy flow from the control system to neglected but excitable structural modes may occur and lead to loss of performance or even instability. This effect is known as spillover [17]. 

The notion of spillover is important with respect to neglected structural modes. Other modelling errors include parametric uncertainties, which are more difficult to model and may have a substantial impact on the stability and performance of the closed-loop system. 

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In a phase-to-phase fault, however, the system will be composed of two balanced systems, one with positive sequence and the other with negative sequence components. The phasors of these two systems individually will add up to zero, and once again, as in the above case, there will be no residual quantities through the neutral or the ground circuit, except for the transient and spillover quantities. 

Avoid any spillovers, leakage or vaporization of liquid or disposal of a waste unit. One can use Araldite, solder or any other means to stop a leakage. If this is not possible, collect carefully all the chemical PCB and carefully dispose it of. 

Volume 77 New Aspects of Spillover Effect in Catalysis. For Development of Highly Active... 

Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20,1993... 

Proceedings of the 7th International Symposium, Cancun, Mexico, October 5-8,1997 edited by C.H. Bartholomew and G.A. Fuentes Volume 112 Spillover and Migration of Surface Species on Catalysts. 

Proceedings of the 4th International Conference on Spillover, Dalian, China, September 15-18,1997 edited by Can Li and Qin Xin... 

The reader must have already identified some of the basic concepts which play a key role in understanding the electrochemical activation of heterogeneous catalysis catalysis, electrocatalysis, promotion, electrochemical promotion, spillover, backspillover. It is therefore quite important to define these terms unambiguously so that their meaning is clearly determined throughout this book. 

L. Ploense, M. Salazar, B. Gurau, and E.S. Smotkin, Proton Spillover Promoted Isomerization of n-Butylenes on Pt-black Cathodes/Nafion 117, JACS119, 11550-11551(1997). 

C.G. Vayenas, R.M. Lambert, S. Ladas, S. Bebelis, S. Neophytides, M.S. Tikhov, N.C. Filkin, M. Makri, D. Tsiplakides, C. Cavalca, and K. Besocke, Direct STM, XPS and TPD observation of spillover phenomena over mm distances on metal catalyst films interfaced with solid electrolytes, Stud. Surf. Sci. Catal. 112, 39-47 (1997). 

The effect of spillover plays an important role in heterogeneous catalysis and was extensively studied during recent years. It was first noticed in the 1950s by Kuriacose.62 Work in this area has been reviewed by Teichner63 and by Conner et al.64... 

The spillover effect can be described as the mobility of sorbed species from one phase on which they easily adsorb (donor) to another phase where they do not directly adsorb (acceptor). In this way a seemingly inert material can acquire catalytic activity. In some cases, the acceptor can remain active even after separation from the donor. Also, quite often, as shown by Delmon and coworkers,65 67 simple mechanical mixing of the donor and acceptor phases is sufficient for spillover to occur and influence catalytic kinetics leading to a Remote Control mechanism, a term first introduced by Delmon.65 Spillover may lead, not only to an improvement of catalytic activity and selectivity but also to an increase in lifetime and regenerability of catalysts. 

The effect of spillover was observed for different species such as H,68 O69 n,70 NO64 or CO.69 Most of the research has been carried out with hydrogen spillover. 

The simplest example of oxygen spillover is found in the adsorption of oxygen on carbon. The spillover oxygen migrates from the basal carbon (donor) to carbon atoms exposed at steps between layers of the graphite surface, where it reacts with the edge carbons (acceptor).71 In this case the donor and acceptor phase consist of the same material with different surface properties. 

Examples of reverse spillover (or backspillover) are the dehydrogenation of isopentane and cyclohexane on active carbon. Deposition of a transition metal on the active carbon accelerates the recombination of H to H2 due to a reverse spillover or backspillover effect.72... 

In cases of spillover in heterogeneous catalysis the usual kinetic models can no longer be applied in a direct way. The creation of new surface sites or... 

Equations (3.16) and (3.17) describe the dissociative adsorption and, recombination of oxygen on a donor D. The transfer between the donor D and acceptor A is described by eq. (3.18). The spillover oxygen (O) is a mobile species which is present on the acceptor surface without being associated with a particular surface site. The mobile spillover species can interact with a particular surface site B forming an active site C (eq. 3.19). Eq. (3.20) represents the deactivation of the active site C by interaction with a reactant E. 

Figure 3.5. Schematic representation of the spillover mechanism described in reactions (3.16) to (3.20).

The rate of creation of active sites fo) on the acceptor is proportional to the fraction of the acceptor covered with spillover oxygen (0soa) and to the fraction of inactive Mo03 surface sites (1-a)... 

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. 

Also the similarity between the remote control spillover mechanism of Fig. 3.5 and the mechanism of electrochemical promotion (NEMCA) already outlined in Figure 1.4c and thoroughly proven in Chapter 5, should be noted. In electrochemical promotion the solid electrolyte is the donor phase and the conductive catalyst is the acceptor phase, using Delmon s terminology. 

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 . 

Thermodynamics and Kinetics of Spillover-Backspillover Between a Solid Electrolyte and a Metal Catalyst-Electrode... 

Based on the preceding discussion on spillover-backspillover and in anticipation of Chapter 4 it is worth to briefly examine the thermodynamic driving force for ion (e.g. O2 ) backspillover between a solid electrolyte (e.g. YSZ) and the gas-exposed surface of a metal (e.g. Pt) electrode. 

Figure 3.6. Spatial variation of the electrochemical potential, jl02-, of O2 in YSZ and on a metal electrode surface under conditions of spillover (broken lines A and B) and when equilibrium has been established. In case (A) surface diffusion on the metal surface is rate limiting while in case (B) the backspillover process is controlled by the rate, I/nF, of generation of the backspillover species at the three-phase-boundaries. This is the case most frequently encountered in electrochemical promotion (NEMCA) experiments as shown in Chapter 4.

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