Hydrocarbon activation, over

We previously undertook a study of hydrocarbon activation over different transition metal based oxide catalysts mainly in relation to their total oxidation [7-9]. We proposed that hydrocarbons are activated at their weakest C-H bond on high-oxidation-state transition metal cationic centers with the formation of alkoxy species [7,8]. In the case of propene and 1-butene we suggested that the primary surface intermediates are allyl-oxy species [7,8]. 

Kondratenko, E., Perez-Ramrrez, J. (2006). Importance of the Lifetime of Oxygen Species Cenerated by N2O Decomposition for Hydrocarbon Activation over Fe-Silicahte, Appl. Catal. B, 64, pp. 35-41. 

Combined Application Hydrocarbon Activation over Pd/Ce02... 

The effect of lead on hydrocarbon oxidation over noble metal catalysts has been variously presented as a relation between catalyst activity and lead content in gasoline (19, 22), or of lead supplied to the engine (31, 32), etc. The most meaningful correlation is between hydrocarbon activity and lead deposit on the catalyst. Two examples of such correlations, one for laboratory-tested samples and the other for fleet-tested catalysts, will follow. 

In hydrocarbon conversion over zeolite catalysts, the formation and retention of heavy products (carbonaceous compounds often called coke ) is the main cause of catalyst deactivation. 5X 77 XI1 These carbonaceous compounds may poison or block the access of reactant molecules to the active sites. Moreover, their removal, carried out through oxidation treatment at high temperature, often causes a decrease in the number of accessible acid sites due to, e.g., zeolite dealumination or sintering of supported metals. 

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. 

Three aspects of the performance of supported catalysts are also discussed in this Chapter. With the development of techniques, as outlined above, for the characterization of supported metal catalysts, it seems timely to survey studies of crystallite size effect/structure sensitivity with special reference to the possible intrusion of adventitious factors (Section 5). Recently there has been considerable interest in the existence of (chemical) metal-support interactions and their significance for chemisorption and catalytic activity/ selectivity (Section 6). Finally, supported bimetallic catalysts are discussed for various reactions not involving hydrocarbons (hydrocarbon reactions over alloys and bimetallic catalysts have already been reviewed in this Series with respect to both basic research and technical applications ). References to earlier reviews (including some on techniques) that complement material in this Chapter are given in the appropriate sections. It might be useful, however, to note here some topics not discussed that also form part of the vast subject of supported metal and bimetallic catalysts and for which recent reviews are available, viz, spillover, catalyst deactivation, the growth and... 

Oil spills and coal mining command considerable attention from the media because they are often large scale and visually very dramatic. Nothing seems worse than a mass of toxic crude oil and tarry hydrocarbons smeared over the natural habitats of some foreshore or the sight of strip mining operations. As a result, there is a massive public response and a frenzy of activity by agencies, community groups, and politicians. 

Noble metals and hybrid of noble metal- and base metal-supported catalysts have also been developed in order to minimize the carbon formation and sulfur tolerance in the steam reforming of liquid hydrocarbons. Idemitsu Co., Ltd. in Japan has reported Ru-based naphtha reforming catalyst (ISR-7G) for the steam reforming of kerosene,134 whose chemical composition is more or less similar to that of JP-8 jet fuel as described in Table 2.13. The catalyst exhibited stable activity over 12,000h with 100% kerosene conversion and stable outlet gas composition. The catalyst was expected to meet the target lifetime of about 40,000 h, which corresponds to 10 years on the presupposition that the fuel cell would be operated in daytime only. 

As described in Section 21.3.2, the H2 addition promotes the oxidation of hydrocarbon to surface oxygenates and the oxidation of NO to nitrates. It is clear, that the co-presence of protons and reduced Ag species is indispensable for reductive oxidation of 02 to yield reactive oxygen species (02 ). This clearly explains why CO is not effective as a co-reductant and why the formation of Ag clusters is not sufficient to improve the catalytic activity. Over 0.5 wt.% Ag/Al203, showing no activity for NO reduction, monomeric Ag+ species are not reduced to clusters, and 02 is not produced because of the absence of reduced Ag species. The co-presence of CO leads to the formation of Ag clusters, though the condition is not sufficient because of the absence of protons. The co-presence of both the dissociated hydrogen as acidic proton and Ag cluster are indispensable for the 02 activation. 

Studies of the activity and selectivity of tin-antimony oxides for the oxidation of propylene to acrolein have been reviewed by Hucknall (6), and more recent investigations of hydrocarbon oxidation over the catalyst have been described by Higgins and Hayden (7). In this article only those studies that are directly relevant to the fundamental properties of the catalyst will be reexamined. 

Hydrocarbon synthesis over initially clean transition metal surfaces leads to the rapid buildup of an overlayer of carbidic carbon, often concurrent with an initial rise in the catalytic activity. 

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