Hydrocarbons oxidation over perovskites

In recent years, much attention has been focused on hydrocarbons total oxidation over mixed oxides. It was reported that perovskite type oxides remarkably oxidise carbon monoxide, light alkanes and also methane at low temperatures [1]. However, the major obstacles to the successful application of these materials in a large scale are both then-low resistance to sulphur poisoning and also their scarce BET surface area which is often linked to the catalytic activity. For this, development of more active catalysts has become a challenge to be overcome. Many attempts have been made to develop new preparation methods to improve... 

Catalysts with perovskitic structure guarantee a good compromise between stability and activity and have a relatively low cost, so they can constitute a valid alternative to supported noble metals, with particular reference to the reactions of partial or total oxidation of hydrocarbons (catalytic combustion). The traditional route used to synthesize perovskites was first introduced by Delmon and co-workers in the late nineteen sixties [1]. It enables to obtain i) mixed oxides over a wide range of composition ii) good control of the stoichiometry iii) an excellent interspersion of the elements in the final product iv) very small grain size materials. 

Very little work has been reported concerning the hydrogenation of CO2 over perovskite oxides. The most comprehensive work to date has been reported by Ulla et al. (1987) and Marcos et al. (1987). They studied the CO2 + H2 reaction over La xMxCo03 (M = Sr, Th). This system is expected to yield hydrocarbons and no oxygenates. They used XRD, XPS and H2 chemisorption to characterize the different solids that were almost always prereduced in H2. They worked below atmospheric pressure in a recirculation system, H2 CO2 = 4 1, and at 553 K. In fact, they used CO2 +H2 as a test reaction to characterize the evolution of the different solids following hydrogen reduction. 

There is no single mechanism that describes adequately the Idnetics of oxidation of various hydrocarbons such as alkanes, alkenes, aromatics, and so on over perovskites, but various reaction mechanisms and reaction orders applied for different molecules and catalysts. Nevertheless, there are some general rules. 

Solid oxide catalysts such as hexaaluminates and perovskites, in which an active metal catalyst is incorporated into a coke-resistant lattice, are effective for liquid hydrocarbon reforming due to their thermal stability over a broad-range of temperature. However, sulfur tolerance of those materials has yet to be demonstrated. 

Yamazoe and Teraoka (1990) summarized the results from several researchers who reported rates for oxidation of hydrocarbons over Co- and Fe-perovskite-type oxides that tend to be maximum at smaller x values (0.1-0.4) than Mn perovskites (0.6-0.8). Figure 16 shows the amount of desorbed oxygen and the catalytic activity, expressed in terms of the temperature at which the conversion of C4H10 was 50% (T5o%), as a function of the Sr content, x in Lai vSr[Coo.4Fe(j(-)03. The amount of desorbed O2 increased monotonically with La substitution up to x = 0.8 while T50% had a maximum at x = 0.2, in agreement with the results mentioned above. 

Reducible metal oxides, which are able to provide their lattice oxygen for methane activation, are potential catalytic materials for alternating feeding of CH4 and O2. Mn- and Co-based perovskites were identified as effective catalysts for such OCM operation [24]. At best, the yield of C2 hydrocarbons of 20% with the corresponding selectivity of 73% was obtained at 1073 K over S1C0O3 doped with the oxides or hydroxides of K and Na applying a 1.5-min cycle of methane. However, the catalyst productivity remained still low. 

The selective catalytic reduction of NO by hydrocarbons over metal oxides, such us perovkite-type oxides, has attracted much attention nowadays as promising alternatives to noble metal-supported catalysts. A representative TWC reaction for the NO reduction by hydrocarbons is the NO + CsHe, which is widely investigated in several perovskites (Table 25.5). 

Taking into account the Mars-van Krevelen mechanism, any enhanced oxygen vacancy densities can improve the oxidation activities of an oxide-based catalyst Perovskite-based materials can act as catalysts for NO SCR by H2 or hydrocarbons [72,73] or simultaneously reduce NO in the presence of PM under lean conditions [74-77]. The major drawback of such high-temperature crystal oxides is their low surface area, for example, <2-3 m /g. However, over the last years improved preparation methods and compositional control had a significant effect on materials features and performance of the perovskite-type catalysts [75,78-85] as illustrated in Figure 26.3. However, the performance of perovskite-based catalysts becomes remarkable when noble metal coexist, either as supported or as dopant or even in the form of a solid solution [15,17,81-86]. 

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