Spillover hydrogen phenomenon

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. 

In the last 15 years, our work has shown that spillover hydrogen Hso or spillover oxygen Oso, can react with catalyst surfaces to create new sites, or transform catalytic sites poorly active or active in other reactions, to sites very selective for given transformations (2-4). We call this phenomenon the Remote Control (RC), because the phase which dissociates H2 or O2 acts as a sort of "control room" sending a message (Hso or Oso) which modifies the way the "chemical plant", namely the active site, operates. 

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. 

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. 

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. 

Surface diffusion as a solid-phase process is conceptually similar to those discussed above, except that it involves the movement of heteroatoms rather than carbon atoms. Oxygen and hydrogen are of special interest, because of the widely documented importance of their spillover on the carbon surface. Indeed, the phenomenon of spillover was first observed on the surface of a channel carbon black [238]. 

In recent years a number of studies have been devoted to the phenomenon of" spillover", which is defined as the migration of adsorbed species from one solid phase where it is easily adsorbed, onto another solid phase in contact with the first, where it is not directly adsorbed. One can find many experimental evidences of hydrogen spillover in IR spectroscopy, often being combined with H-D exchange tl-3]. 

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. 

The phenomenon that spiltover hydrogen hydrogenated the pyridine adsorbed on acidic sites of USY can be schematically expressed as a model shown in Rg. 3. The gas phase hydrogen is first dissociated on metal sites and then spillover to SiO support first, then cross the interface between Pt/Si02 and USY and finally move to acidic sites of USY zeolite through surface migration. Here, pyridine molecules bonded on USY acid sites acted as an acceptor for spiltover hydrogen. 

Since the pioneering work of Kuriakose and Taylor on hydrogen spillover, and of Bond on oxygen spillover, several hundreds of papers directly connected to this phenomenon and probably several thousands suggesting spillover effects to explain results, were published in the literature. We will now examine those papers which are directly concerned with the measurement of relative or absolute coefEcients of surface diffusion. 

Little interest has been shown in the phenomenon in recent years, although the role of hydrogen spillover in controlling carbon deposition during petroleum reforming, or even in the reforming process itself,has been canvassed. This early work may therefore not have been entirely in vain. 

Hydrogen spillover is a well-documented phenomenon in the catalysis literature. Hydrogen spillover is defined as the dissociative chemisorption of hydrogen on metal nanoparticles, and subsequent migration of hydrogen atoms onto adjacent surfaces of a receptor via spillover and surface diffusion [8,57,58] (Figure 6.6). The comprehensive review by Coimer and... 

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