Catalytic spillover

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. 

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. 

B. Delmon, and G.F. Froment, Remote Control of Catalytic Sites by Spillover Species ... 

O.A. Mar ina, V.A. Sobyanin, V.D. Belyaev, and V.N. Parmon, The effect of electrochemical pumping of oxygen on catalytic behaviour of metal electrodes in methane oxidation, in New Aspects of Spillover Effect in Catalysis for Development of Highly Active Catalysts, Stud. Surf. Sci. Catal. 77 (T. Inui, K. Fujimoto, T. Uchijima,... 

As shown on Figure 9.1 when the circuit is opened (I = 0) the catalyst potential starts increasing but the reaction rate stays constant. This is different from the behaviour observed with O2 conducting solid electrolytes and is due to the fact that the spillover oxygen anions can react with the fuel (e.g. C2H4, CO), albeit at a slow rate, whereas Na(Pt) can be scavenged from the surface only by electrochemical means.1 Thus, as shown on Fig. 9.1, when the potentiostat is used to impose the initial catalyst potential, U r =-430 mV, then the catalytic rate is restored within 100-150 s to its initial value, since Na(Pt) is now pumped electrochemically as Na+ back into the P"-A1203 lattice. 

The addition of a spillover proton to an adsorbed alkene to yield a secondary carbonium ion followed by abstraction of a proton from the C3 carbon would yield both isomers of 2-butene. The estimated faradaic efficiencies show that each electromigrated proton causes up to 28 molecules of butene to undergo isomerization. This catalytic step is for intermediate potentials much faster than the consumption of the proton by the electrochemical reduction of butene to butane. However, the reduction of butene to butane becomes significant at lower potentials, i.e., less than 0.1V, with a concomitant inhibition of the isomerization process, as manifest in Fig. 9.31 by the appearance of the maxima of the cis- and tram-butene formation rates. 

B. Delmon, and G.F. Froment, Remote Control of Catalytic Sites by Spillover Species A Chemical Reaction Engineering Approach, Catal. Rev.-Sci. Eng 38(1), 69-100 (1996). 

As shown in Figure 12.4 this finely dispersed Pt catalyst can be electrochemically promoted with p values on the order of 3 and A values on the order of 103. The implication is that oxide ions, O2", generated or consumed via polarization at the Au/YSZ/gas three-phase-boundaries migrate (backspillover or spillover) on the gas exposed Au electrode surface and reach the finely dispersed Pt catalyst thereby promoting its catalytic activity. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

This interpretation of the experimental data is supported by the differences observed in the deactivation patterns and carbon contents after test, since one notorious effect of Hjp is the capacity to diminish the deactivation caused by coke deposition on the active sites [21,22]. This is supposed to be due to a reaction with the coke precursors, very likely a hydrogenolysis. In pure silica-aluminas, where no source of spillover is present, no special protection against deactivation should be observed. Indeed, the silica-aluminas lose most of their activity (about 80%) before reaching the steady-state and present the highest carbon contents after catalytic test. On the other hand, in the case of the mechanical mixtures, where spillover hydrogen is continuously produced by the CoMo/Si02 phase and can migrate to the silica-alumina surface, the predicted protection effect is noticed. The relative losses of activity are much lower... 

It was found in the 1960s that disperse platinum catalyst supported by certain oxides will in a number of cases be more active than a similar catalyst supported by carbon black or other carbon carrier. At platinum deposits on a mixed carrier of WO3 and carbon black, hydrogen oxidation is markedly accelerated in acidic solutions (Hobbs and Tseung, 1966). This could be due to a partial spillover of hydrogen from platinum to the oxide and formation of a tungsten bronze, H WOj (0 < a < 1), which according to certain data has fair catalytic properties. 

Isocamphane, 20 281 Isocyanate reactions, 13 393 species, spillover of, 34 41 2 formation in catalytic exhaust-gas reaction, 34 41... 

According to long-lasting experimental efforts, the use of alloy catalysts that contain a less noble metal whose oxide exhibits low solubilities in acid electrolytes—in particular Sn and Bi are effective in this respect—enhance the catalytic activity of platinum. The rationale of this effect has been that the oxide of the nonnoble component at close atomic distance from the Pt surface atoms supplies by spillover the oxygen that is necessary to oxidize the adsorbed CO species. Today research and development turn more to Ru and lr or Rh, the more easily oxidizable platinum metals as alloying metals that seem to be at least as efficient as Bi and Sn and are certainly more stable than those in acidic environments—in particular if the anode potential becomes more anodic in cases of poor supply of fuel (158). The Pt-Ru anode exhibits a sizeable higher oxidation current for methanol and for adsorbed hydrogen than the Pt electrode, indication that a smaller part of the Pt electrode surface is blocked by CO adsorption. Still the catalytic activity is too low because the onset of the anodic peak of methanol oxidation is at a... 

Determination of the Structure of Catalyst Supports by Spectroscopy with Particular Reference to Spillover and Hydrogen Diffusion. - The adsorption of gas at a metal and the subsequent diffusion of that gas or some of its atoms onto the surface of a support is known as spillover. The process is a critical step in a number of catalytic reactions and it can be exploited in the... 

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