Mechanism direct partial oxidation

The two mechanisms proposed to account for the partial oxidation of methane to syngas may be dedgnated as the IPO (Indirect Partial Oxidation) mechanism and the DPO (Direct Partial Oxidation) mechanism. The IPO mechanism was proposed by Prette et al [16] and Lunsford et al [12]. They think CO and H2 are the products of indirect reaction, the overall reaction of the POM reaction is composed of three different reactions... 

Two mechanisms have been proposed for the POM reaction (i) The Combustion and Reforming Reactions mechanism (CRR). In this, the methane is combusted in the absence of oxygen in the first part of the catalytic bed, producing CO2 and H2O. Along the rest of the bed, and after total oxygen conversion, the remaining methane is converted to CO + H2 by SMR and CO2 reforming (reaction (2)). (ii) The Direct Partial Oxidation mechanism (DPO). CO + H2 is produced directly from methane by recombination of CHX and O species at the surface of the catalysts. 

Two reaction mechanisms do exist in literature for partial oxidation. One of them proposes that the reaction starts with catalytic combustion followed by reactions of lower rate, namely, steam reforming, CO2 reforming, and WGS. This mechanism is supported by the fact that water is found as primary product of partial oxidation and autothermal reforming in many cases. The other mechanism proposes direct partial oxidation at very short residence times. The reaction is significantly faster than steam reforming and usually performed in the difiiusion-limited regime. 

Catalysts for partial oxidation consist mostly of a noble metal supported on porous ceramic monoliths [113]. Two mechanisms have been proposed the direct partial oxidation mechanism, where hydrocarbon fuel and oxygen are adsorbed and react directly on the catalyst surface to give H2 and CO, and the combustion reforming mechanism, which assumes that the reaction is initiated near the entrance of... 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

A so-called direct pathway involving a more weakly adsorbed perhaps even partially dissolved intermediate. Likely candidates for such intermediates are formaldehyde and formic acid. The oxidation mechanism of formic acid is discussed in Section 6.3. The idea is that the formation of a strongly adsorbed intermediate is circumvented in the direct pathway, though in practice this has appeared difficult to achieve (the dashed line in Fig. 6.1). Section 6.4 will discuss this in more detail in relation to the overall reaction mechanism for methanol oxidation. 

A simplified scheme of the dual pathway electrochemical methanol oxidation on Pt resulting from recent advances in the understanding of the reaction mechanism [Cao et al., 2005 Housmans et al, 2006] is shown in Fig. 15.10. The term dual pathway encompasses two reaction routes one ( indirect ) occurring via the intermediate formation of COads. and the other ( direct ) proceeding through partial oxidation products such as formaldehyde. 

Neutral square coplanar complexes of divalent transition metal ions and monoanionic chelate or dianionic tetrachelate ligands have been widely studied. Columnar stack structures are common but electrical conductivities in the metal atom chain direction are very low and the temperature dependence is that of a semiconductor or insulator. However, many of these compounds have been shown to undergo partial oxidation when heated with iodine or sometimes bromine. The resulting crystals exhibit high conductivities occasionally with a metallic-type temperature dependence. The electron transport mechanism may be located either on predominantly metal orbitals, predominantly ligand re-orbitals and occasionally on both metal and ligand orbitals. Recent review articles deal with the structures and properties of this class of compound in detail.89 90 12... 

In conclusion we find that on both catalysts epoxidation proceeds similarly via a non-concerted process. Concerted mechanisms in which O adds across the C=C bond, directly producing epoxide, have been ruled out. In both pathways a C-Ag bond is formed and further broken. The key implications are that both surfaces are active for epoxidation. This partial oxidation reaction is hence shown to take place over a wide O coverage regime characterizing two types of oxygen species. 

In summary, the available experimental evidence suggests that an adsorbed form of molecular oxygen is involved in partial oxidation while lattice oxygen is required for carbon dioxide production. This proposed mechanism is directly opposed to that generally accepted for propene oxidation over mixed oxide catalysts such as bismuth molybdate. In this case, lattice oxygen is responsible for acrolein formation while adsorbed oxygen results in complete combustion. This means that the fully oxidized phase is the selective catalyst while the reaction is first order with respect to alkene. 

The combination of reactions (2) and (5) may be considered as a scheme for direct methane oxidation to synthesis gas (CO -f H2). Similar reactions may determine the high efficiency of mixed catalysts containing Ni and rare-earth oxides for the partial oxidation of methane to synthesis gas [9]. This mechanism does not require a preliminary total oxidation of methane followed by its reforming with CO2 and/or water which was considered as the main route for synthesis gas formation [10,11]... 

Talcing our earlier work into account, we proposed a mechanism for the NOx reduction by C .dUOM, which can be described by a simplitled reaction scheme, shown in Scheme 2. The reaction starts with the fonnaiioii of both ad.sorbed nitrates via NO oxidation by (). and enolic species and acetate via the partial oxidation of C dCOH over Ag/AI O . The reaction between the two kinds of adsorbed species then leads to the fonnalion of NCO directly, or via organo-nitrogcn compounds (such as R-ONO and R-NO ). which is widely accepted in the studies of the S( R of NOx 15.22.43.46.66). Subsetiuenily. NCO reacts w ith NO - O and nitrates to yield N,. It should be pt>inted out that the acetate formed by the reaction of C H.OII O alsi> reads toward NO O to produce NC O. However, as a result of its low adiviiy, this parallel reaction dt>es not play an important role in the fonnation of NCO. 

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