Oxygen 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. 

B. Luerssen, S. Gunther, H. Marbach, M. Kiskinova, J. Janek, and R. Imbihl, Photoelectron spectromicroscopy of electrochemically induced oxygen spillover at the Pt/YSZ interface, Chem. Phys. Lett. 316, 331-335 (2000). 

Figure 3.25 A sequence of STM images taken at 673 K of oxygen spillover from Pd clusters in an ambient pressure of 5 x 10-8 mbar of 02 (doubled before image (f)).The total exposures to oxygen from image (a)-(f) were (in units of L) 114, 178, 237, 282, 344, and 531. The spillover begins with the formation of one layer...

Oxygen spillover was also advocated for reactions other than simple oxidations. Tascon et al. (151) studied the transformation of formamides to nitriles at 275°C on BiPMo catalysts prepared by a physcial mixture of powders of Mo03 and BiP04 ... 

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... 

More recently, Chadwick and Christie reported evidence for oxygen spillover in the oxidation of lead monolayers on copper (756). Polycrystalline copper or single crystal (111) or (210) surfaces were covered by less than a monolayer of lead. 02 reacts with Pb to give PbO. If a complete monolayer of Pb was formed on Cu, the rate of oxidation of Pb decreased. Now H2S reacts with Cu but not with Pb. If H2S was adsorbed at saturation on Cu containing less than a monolayer of Pb (which does not adsorb H2S), the oxidation of Pb again decreased. These results were explained by the spillover of oxygen from Cu to Pb (and its oxidation into PbO). Oxygen was therefore adsorbed on a free surface of Cu, not covered either by Pb or by H2S, and then spilled over to Pb. 

To sum up this section, it appears that some new and unforeseen catalytic properties may be induced on refractory oxides by hydrogen spillover, and to a lesser extent by oxygen spillover. If the activator, like Pt/Al203 catalyst, is not separated after the spillover activation from the activated oxide, these new properties could be masked by the catalytic activity of Pt/Al203. 

We believe that supports could play a more important role in the oxidation of light alkanes than it did in allylic oxidation. But this role will be complex, and include better dispersion of the active phase, stabilisation of the selective phase, control of oxido-reduction, and/or facilitation of oxygen spillover. 

The observed shift of the measured bistability region of the Pt/ceria sample compared to the Pt/silica sample can qualitatively be explained by oxygen spillover between ceria and Pt. This can be understood by considering that for Pt/ceria O2 may adsorb on ceria and diffuse to the Pt-ceria boundary and react with CO on the Pt particles, even if the Pt particle is covered by CO. The spillover channel thus extends the gas mixing (/ ) regime, where a high reaction rate can be maintained. The data shown in Fig. 4.33 is particularly... 

In the past years, many independent investigators have shown strong evidences of the existence of oxygen spillover and of the positive role played by this species in improving the performance of catalysts. In the latter case, these authors often alluded to a "spillover effect" rather than a "remote control". These two different terms nevertheless reflect the same concept [11-15]. 

The hgure also shows that loading the carbon with hnely dispersed palladium has a further accelerating effect on surface oxidation, which is most pronounced with the carbon black. This can be explained by dissociative adsorption of O2 molecules on the metal surface and oxygen spillover. Platinum had a similar effect. Palladium (200 p.mol/g) was deposited on the carbons by incipient wetness impregnation followed by H2 reduction at 523 K the dispersion was about 25% with Norit and 15% with Corax 3. 

Figure 4. Schematic representation of the mechanism of oxygen spillover in a purely catalytic system. Symbols as in Eq. (7).

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. 

Considering the overall mechanism of VOC oxidation, it could be agreed that Pt addition slows down deactivation of the catalyst in by speeding up the final oxidation step, if this step is CO oxidation as proposed in this study. As also stated in literature, oxygen spillover could be an important step in the removal of the surface deposited carbon causing deactivation [9]. 

High sensitivity of the maximal /34(f) value of the ratio between the rate heteroexchange on the oxide surface and the diffusion coefficient helped to determine the kinetic characteristics of the spillover step between Pt and the support. Although the characteristic time of the oxygen spillover from Pt to the support is less than 10 s at and below 650°C, the rate of spillover is determined by the surface diffusion with a characteristic time (/JsuRF / Aurf) of ca. 2 s (at 650 C). The rate of surface diffusion prominently increases with temperature. As a consequence, at 850°C, the rate of spillover is determined by the diffusion into the bulk, which depends weakly on temperature. 

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