Oxide support effect

Khodakov, A., Olthof, B., BeU, A.T. and Iglesia, E. (1999) Structure and catalytic properties of supported vanadium oxides support effects on oxidative dehydrogenation reactions. Journal of Catalysis,... 

Khodakov, A., Olthof, B., Bell, A., etal. (1999). Structure and Catalytic Properties of Supported Vanadium Oxides Support Effects on Oxidative Dehydrogenation Reactions, J. Catal., 181, pp. 205-216. 

Le Bars, J., Auroux, A., Forissier, M., and Vedrine, J. C. Active sites of YfiJg-AiPj catalysts in the oxidative dehydrogenation of ethane. J. Catal. 162,250-259 (19%). Khodakov, A., Olthof, B., Bell, A. T., and Iglesia, E. Structure and catalytic properties of supported vanadium oxides Support effects on oxidative dehydrogenation reactions. J. Catal. 181,205-216 (1999). 

In any case, it is interesting to note that catalytic efficacy has been observed with nano- or mesoporous gold sponges [99-101, 145] suggesting that neither a discrete particle nor an oxide support is actually a fundamental requirement for catalysis. An alternative mechanism invokes the nanoscale structural effect noted in Section 7.2.2, and proposes that the catalytic effect of nanoscale gold structures is simply due to the presence of a large proportion of lowly-coordinated surface atoms, which would have their own, local electronic configurations suitable for the reaction to be catalyzed [34, 49,146] A recent and readily available study by Hvolbaek et al. [4] summarizes the support for this alternate view. 

Figure 31. Effect of metal oxide supports in glucose oxidation over gold catalysts [98].

In addition, it sustains CO electro-oxidation at relatively low overpotential, and there are crystal face dependences for both the ORR and CO oxidation. Since Au is also a system that exhibits both particle size and support effects in heterogeneous catalysis, it provides an interesting model system for smdying such effects in electrocatalysis. 

Baker WS, Pietron JJ, Teliska ME, et al. 2006. Enhanced oxygen reduction activity in acid by tin-oxide supported Au nanoparticle catalysts. J Electrochem Soc 153 A1702-A1707. Blizanac BB, Lucas CA, Gallagher ME, et al. 2004a. Anion adsorption, CO oxidation, and oxygen reduction reaction on a Au(lOO) surface The pH effect. J Phys Chem B 108 625-634. 

Except Ru (not usable in TWC because of the volatility of its oxide [68]), the most active metal is the rhodium. This has been largely confirmed by further studies so that Rh may be considered as a key-component of TWC for NO reduction [69,70], As far as Pd is concerned, it seems that the active site is composed of Pd"+ —Pd° pairs, which may explain the higher activity of Pd in N0+C0+02 mixture (T5( 200°C) [71]. A detailed kinetic study by Pande and Bell on Rh catalysts has evidenced a significant support effect [72], The kinetic data were represented by a conventional power law expression ... 

As we have seen in the previous chapter, the apparent topography and corrugation of thin oxide films as imaged by STM may vary drastically as a function of the sample bias. This will of course play an important role in the determination of cluster sizes with STM, which will be discussed in the following section. The determination of the size of the metallic nanoparticles on oxide films is a crucial issue in the investigation of model catalysts since the reactivity of the particles may be closely related to their size. Therefore, the investigation of reactions on model catalysts calls for a precise determination of the particle size. If the sizes of the metal particles on an oxidic support are measured by STM, two different effects, which distort the size measurement, have to be taken into account. 

Rhenium is one of the oxophilic atoms effective for oxidation reactions. ReOx species are likely to have chemical interaction with various oxide supports and exhibit unique catalytic properties that cannot be observed on monomeric rhenium oxides. A new active six-membered octahedral Re cluster in zeolite pores (H-ZSM-5 [HZ]) is produced from inactive [Re04] monomers in situ under selective propene oxidation to acrolein (C3H6+02 - CH2=CHCH0+H20) in the presence of ammonia that is not involved in the reaction equation [16], The cluster is transformed back to the original inactive monomer in the absence ammonia. Note that coexistence of spectator NH3 is indispensable for the selective oxidation. 

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