The oxidative coupling of methane (OCM) to C 2 hydrocarbons (C 2 H 4 and C 2 H 6 ) was examined on Mn/SiO 2 and sodium salt-modified Mn/SiO 2 catalysts containing different oxo anions, i.e., WO 4 2-, MoO 4 2-, SO 4 2-, PO 4 3-, P 2 O 7 4-, CO 3 2-, and SiO 3 2-. The catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, laser Raman spectroscopy, and temperature-programmed reduction with H 2 . The structural and catalytic properties of the catalysts largely depend on the presence of the Na + ions and the identity of the oxo anions of the salts. Mn/SiO 2 consisted of Mn 3 O 4 and amorphous SiO 2 phases. The addition of the sodium salts to Mn/SiO 2 led to the transformation of amorphous SiO 2 exclusively to R-cristobalite, and the concurrent oxidation of Mn 3 O 4 to different Mn species. Mn 2 O 3 was the predominant species as the salts contained oxo anions of WO 4 2-, MoO 4 2-, SO 4 2-, PO 4 3-, and P 2 O 7 4-, whereas the basic sodium salts of CO 3 2and SiO 3 2led to the preferential formation of Mn 4+ species. These effects on the formation of the Mn species were demonstrated indeed to require the coexistence of Na + and the oxo anions. Compared to Mn/SiO 2 , the sodium salt-modified samples with the formation of Mn 2 O 3 showed much higher reducibility, activities, and selectivities to C 2 products. However, the samples with the formation of Mn 4+ species exhibited very low OCM activities, as a result of the strong basicity of Na 2 CO 3 and Na 2 SiO 3 inhibiting the partial reduction of the Mn 4+ species. The observed effects on the structures and catalytic performances suggest that Mn 2 O 3 species act as the active sites responsible for the methane activation, which may provide the rationale for the design of new efficient catalysts for the OCM reaction.

A flame synthesis method has been used to prepare nanosized, high-surface-area Cu-Ce-O, Ni-Ce-O, and Fe-Ce-O catalysts from aqueous solutions of metal acetate precursors. The particles were formed by vaporization of the precursors followed by reaction and then gas to particle conversion. The specific surface areas of the synthesized powders ranged from 127 to 163 m(2)/g. High-resolution transmission electron microscope imaging showed that the particle diameters for the ceria materials are in the range of 3-10 nm, and a thin layer of amorphous material was observed on the surface of the particles. The presence and surface enrichment of the transition-metal oxides (CuO, NiO, and Fe(2)O(3)) on the ceria particles were detected using X-ray photoelectron spectroscopy. Electron energy-loss spectroscopic studies suggest the formation of a core-shell structure in the as-prepared particles. Extended X-ray absorption fine structure studies suggest that the dopants in all M-Ce-O systems are almost isostructural with their oxide counterparts, indicating the doping materials form separate oxide phases (CuO, Fe(2)O(3), NiO) within the host matrix (CeO(2)). Etching results confirm that most of the transition-metal oxides are present on the surface of CeO(2), easily dissolved by nitric acid. The performance of the flame-synthesized catalysts was examined toward water-gas shift (WGS) activity for fuel processing applications. The WGS activity of metal ceria catalysts decreases in the order Cu-Ce-O > Ni-Ce-O > Fe-Ce-O > CeO(2) with a feed mixture having a hydrogen to carbon monoxide (H(2)/CO) ratio of 1. There was no methane formation for these catalysts under the tested conditions.

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