Total Oxidation of Light Hydrocarbons

Ethane represents one of the less investigated light hydrocarbons for total oxidation, at least comparatively to methane. Substituted Lai eSr tFe03 perovskites [36] and substituted halo- Lai eSr tFe03 5Xo. (X = F, Cl) perovskites [37] were reported to convert ethane at temperatures higher than 400 °C with a maximum in selectivity in CO2 of 60%. For both perovskites, ethene was the only by-product. Also, the substitution of La by K in Lai eK eMn03 perovskites decreased the activity for ethane combustion and allowed the formation of ethylene [38,39]. These studies also point out that the bulkier cation in the A position perovskite structure plays a role in determining the catalytic performance. 

The catalytic data also indicated a strong dependence of the low hydrocarbons conversion on the catalyst elemental composition. For the Lai, s Fe03 g perovskites, a progressive Fe20s enrichment of the surface was evidenced by XPS characterization when decreasing the lanthanum content of the solid. Such additional undesirable surface iron oxide enrichment induced an inhibiting effect on the catalytic activity. As a result, the most efficient lanthanum iron-based perov-sldte was the stoichiometric LaFe03 mixed oxide [40]. 

Total oxidation of other light hydrocarbons reported YFeOs, and LaFeOs catalysts also revealed the importance of the network stability [41]. The superior activity of LaFeOs compared to that of YFeOs was explained by the contribution of La that stabilizes the perovskite crystal structure, thus promoting a partial oxidation or reduction of Fe cations allowing a fraction of oxygen ions to be removed from/or incorporated back to the lattice. On the contrary, the presence of Y in the perovskite structure does not permit an intensive conversion of iron ions between the different oxidation states that corresponded in a lower activity. 

Propane represents another low hydrocarbon investigated as VOC since it is being released in increasing amounts due to the increase in liquefied petroleum gas usage as a fuel. Some of the propane in these fuels is not combusted during the main reaction and, therefore, its catalytic total oxidation is very important to prevent its release into the atmosphere. To achieve this, several metal oxides have been prepared using specific procedures and investigated. 

Deep oxidation of propane was also obtained using ordered cobalt oxides prepared by a nanocasting route [31]. However, the role of the ordered structure in the catalytic performance does not seem to be beneficial. The enhanced catalytic activity, almost similar to that of the catalysts prepared by combustion of organic acids [42], has been explained in terms of both the high and low surface areas. The presence of oxygen vacancies, directly related to the high reducibihty of the ordered cobalt oxide species, was also very important. 

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