The total oxidation of two representative VOCs, propane and toluene, has been studied using mesoporous-Fe 2 O 3 catalysts. Different preparation methods have been followed leading to mesoporous materials with different characteristics. Whilst a mesoporous catalyst formed by aggregation of nanocrystals has been produced by soft chemistry using oxalic acid as precipitating agent, a mesoporous material with crystalline walls have been prepared by a nanocasting route using a hard template. These catalysts have been characterized by several physicochemical techniques: XRD, N 2 adsorption, TPR, XPS, TEM, HR-TEM, SAED and EDX. Among the different Fe 2 O 3 catalysts synthesized differences not only in the surface area and morphology have been observed but also in the lattice parameter, in the concentration of oxygen defects for VOCs adsorption and in the reducibility. In the case of the toluene oxidation it has been observed that the catalytic activity is highest for the catalysts prepared by a nanocasting route, which presents a very high surface area of 208 m 2 g-1. Conversely, for propane oxidation the most active catalyst resulted to be the mesoporous nanocrystalline catalyst formed by aggregation. In this case, a direct relationship between reducibility and catalytic activity normalized per surface area has been observed. The differences between toluene and propane oxidation can be tentatively ascribed to different reaction mechanisms to be accounted for.

Catalysts consisting of NiO diluted in high surface area TiO2 can be as efficient in the oxidative dehydrogenation of ethane as the most selective NiO-promoted catalysts reported previously in the literature. By selecting the titania matrix and the NiO loading, yields to ethylene over 40% have been obtained. In the present article, three different titanium oxides (TiO2) have been employed as supports or diluters of nickel oxide and have been tested in the oxidative dehydrogenation of ethane to ethylene. All TiO2 used present anatase as the main crystalline phase and different surface areas of 11, 55 and 85 m 2 g-1. It has been observed that by selecting an appropriate nickel loading and the titanium oxide extremely high selectivity towards ethylene can be obtained. Thus, nickel oxide supported on TiO2 with high surface areas (i.e. 55 and 85 m 2 g-1) have resulted to give the best catalytic performance although the optimal nickel loading is different for each case. The optimal catalyst has been obtained for NiO-loadings up to 5-10 theoretical monolayers regardless of the TiO2 employed. Free TiO2 is inactive whereas unsupported NiO is active and unselective (forming mainly carbon dioxide) and, therefore, unmodified NiO particles have to be avoided in order to obtain the optimal catalytic performance. The use of low surface area titania (11 m 2 g-1) have led to the lowest selectivity to olefin due to the presence of an excess of free NiO particles.

No abstract

Several CeO 2 and CuO-CeO 2 catalysts were prepared using different methods, i.e., a homogeneous precipitation with urea, a nanocasting route using CMK-3 carbon as a hard template and a sol-gel process using Poly(methyl methacrylate) (PMMA) polymer as a soft template, and tested in the total oxidation of propane. The catalysts were characterized by a number of physicochemical techniques (XRD, N 2 adsorption, TPR, XPS, Raman spectroscopy) showing distinct characteristics. For each series, Cu-Ce-O catalysts with low Cu-loadings (5 wt % CuO) showed the highest activity, higher than those samples either without copper or with high Cu-loading (13 wt % CuO). The incorporation of copper leads to an increase of the concentration of bulk defects but if the Cu-loading is too high the surface area drastically falls. The highest activity in the total oxidation of propane was achieved by Cu-containing ceria catalysts synthesized using a polymer as a template, as this method yields high surface area materials. The surface area and the number of bulk/sub-surface defects of the ceria seem to be the main properties determining the catalytic activity.

Porous Clays Heterostructures (PCH) from natural pillared clays (bentonite with a high proportion of montmorillonite) have been used as supports of iron oxide for two reactions of environmental interest: i) the elimination of toluene (a representative compound of one of the most toxic subsets of volatile organic compounds, aromatics) by total oxidation and ii) the selective oxidation of H 2 S to elemental sulfur. For both reactions these catalysts have resulted to be remarkably more efficient than similar catalysts prepared using conventional silica as a support. Thus, in the total oxidation of toluene it has been observed that the catalytic activity obtained using siliceous PCH is two orders of magnitude higher than that with conventional silica. The catalytic activity has shown to be dependant of the capacity of the support for dispersing iron oxide in a way that the higher the dispersion of iron oxide on the surface of the support, the higher is the activity. In the case of the selective oxidation of H 2 S to S both higher catalytic activity and higher selectivity to S have been observed using siliceous porous clays heterostructures than using conventional silica. Highly dispersed FeOx species have been shown as highly selective towards elemental sulfur whereas more aggregated FeOx species favour the formation of sulphur oxides decreasing the selectivity to S. Analyses of the surface by XPS have shown the predominance of sulfate species in the catalysts presenting low selectivity to elemental sulfur.