Spillover phenomenon

The promotion by reduced ceria could be due to a spillover phenomenon of O species from metal to support. In fact, this is not sufficient to explain all the results of Mullins and Overbury. An exposure of the Rh/CeOx surface to water leads to a re-oxidation accompanied by a hydroxylation of the support while the metal surface is left unchanged. In fact, it seems that preferential orientation of Rh surface on reduced ceria may also explain the specific role of CeOx surface. This is consistent with the fact that NO dissociation occurs at lower temperatures on Rh (110) and on Rh (100) than on Rh (111) [83,84],... 

On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffin [2], hydrocracking of hydrocarbons [3] or methanol conversion to hydrocarbon [4] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [5]. Recently Masai et al. [6] and Steinberg et al. [7] showed that the physical mixtures of protonated zeolite and R/AI2O3 showed high hydrocracking activities of paraffins and skeletal isomerization to some extent. 

Platinum remains more active than rhenimn even in the presence of CPE and CPD, which confirms that the metals play a dual role in the formation of coke dehydrogenation giving coke precursors (non operating here since CPE and CPD are already present in the reactant) and consolidation of the coke deposited on the support by continuous elimination of hydrogen via a reverse spillover phenomenon. It is clear that Pt remains more effective than Re in coke consolidation. TPO profiles on Re (Fig.4) show a small... 

The high and stable catalytic activity of Pd-hybrid catalyst in hydrogen atmosphere should be attributed the proton (H+so) formed in the spillover phenomenon lioni Pd to H-ZSM-5. 

The presence of the substrate, even if it does not directly take part in the reaction, can have a profound influence on the reaction kinetics. Molecules physisorbed on the substrate are, besides the gas phase molecules, a source of reactants for the metal particles. This second source of reactants depends on particle size and density, as we will see in Sect. 3.1. This phenomenon [22-24] is called reverse-spillover in reference to the well-known spillover phenomenon, which represents the diffusion of adsorbed species from the metal particles to the support [25]. 

In this paper, pyridine adsorption was utilized to investigate the hydrogen spillover phenomenon from metal to acidic centers of zeolite by observing hydrogenation of chemisorbed pyridine on B or L acid sites of USY zeolite using hybrid catalyst composed of USY zeolite and Pt/SiOo. To give insight into spill-over process in the hybrid catalyst system, isomerization of n-pentane, which is one of the typical add catalyzed reaction, was also studied. 

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). 

Since then Electrochemical Promotion of Catalysis has been proven to be a general phenomenon at the interface of Catalysis and Electrochemistry. More than seventeen groups around the world have made important contributions in this area and this number is reasonably expected to grow further as the phenomenon of electrochemical promotion has very recently been found, as analyzed in this book, to be intimately related not only to chemical (classical) promotion and spillover, but also to the heart of industrial catalysis, i.e. metal-support interactions of classical supported catalysts. 

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

Fig. 11 represents macroscopically the flux of spillover oxygen O which can either reoxidise a prereduced surface, or reoxidise continuously the active sites which have been accidentally reduced (the second case is chosen in the representation of fig. 11, with the black patches corresponding to active sites, but the phenomenon is identical in both cases). It is clear that the more is present in the mixture, the more O o can flow to M0O3, and... 

A different phenomenon has also been detected in tellurium-containing mixtures used in the oxidation of isobutene to methacrolein. The addition of a-St>204 inhibits sintering (35). Te02 appears as a weak acceptor (36). A hypothesis, still to be confirmed, is that the inhibition of sintering has to do with a spillover of oxygen from a-Sb204 to Te02. 

The phenomenon of spillover was first noticed in the 1950s (2) when it was observed by Kuriacose that the decomposition of GeH4 on a Ge film was... 

Reviews of the publications have appeared in 1973 (7), 1980 (8), and recently from a historical perspective in 1983 (9). The focus of this article is to present our current understanding of the phenomenon and articulate the unknown aspects of spillover. 

We recognize that the phenomenon of spillover is still controversial, and yet the potential implications are far reaching. Therefore, we will take this opportunity to advance some hypotheses concerning the phenomena associated with spillover and their consequences. 

Finally, oxygen spillover has also been advocated in typically inorganic reactions. In an earlier work by Batley and Ekstrom (155), the phenomenon of spillover was not mentioned. Instead, the authors referred to a topochemi-cal heterogeneous catalysis for the reaction of UF4 with 02. This, however, can be understood through the spillover of oxygen. The reaction... 

It has been shown in the previous sections that the addition of small amounts of a transition metal to various metal oxides lowers the temperature required for their reduction by H2. This phenomenon has been attributed to hydrogen spillover. It follows that a partial reduction of the host oxide can induce or modify the catalytic activity of the host material. 

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... 

Conner, Pajonk, and Teichner discuss the once mysterious phenomenon of molecular spillover, or migration of hydrogen and other molecular species across boundaries of heterogeneous compositional regions of solids. Its implications extend to many aspects of solid-state science and practices. 

On the other hand, the transport of hydrogen across the surface from an activation centre (or phase) to another is an extensively studied phenomenon hydrogen spillover [e.g. 6,7]). To asses the potential influence of hydrogen, application of the hydrogen spillover model seems to be a suitable tool to close the explanation gap between the experimental results obtained from hydrocarbon conversion in hydrogen and nitrogen, respectively. 

The support plays an important effect in the adsorption kinetics of CO on supported clusters. Indeed CO physisorbed on the support is captured by surface diffusion on the periphery of the metal clusters where it becomes chemisorbed. The role of a precursor state played by CO adsorbed on the support is a rather general phenomenon. It has been observed first on Pd/mica [173] then on Pd/alumina [174,175], on Pd/MgO [176], on Pd/silica [177], and on Rh/alumina [178]. This effect has been theoretically modeled assuming the clusters are distributed on a regular lattice [179] and more recently on a random distribution of clusters [180]. The basic features of this phenomenon are the following. One can define around each cluster a capture zone of width Xg, where is the mean diffusion length of a CO molecule on the support. Each molecule physisorbed in the capture zone will be chemisorbed (via surface diffusion) on the metal cluster. When the temperature decreases, Xg increases, then the capture zone increases to the point where the capture zones overlap. Thus the adsorption rate increases when temperature decreases before the overlap of the capture zones that occurs earlier when the density of clusters increases. Another interesting feature is that the adsorption flux increases when cluster size decreases. It is worth mentioning that this effect (often called reverse spillover) can increase the adsorption rate by a factor of 10. We later see the consequences for catalytic reactions. 

---