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Alumina-supported PdO is largely accepted as an effective catalyst for the methane combustion reaction. In this study, we investigated the influence of the addition of CeO 2 to the PdO catalysts supported on -Al 2 O 3 on their microstructures by XAFS spectroscopy and X-ray diffraction technique (XRD).We obtained the radial structure functions (RSF) from Fourier transform of Pd K-edge EXAFS spectra of both fresh and as-tested catalysts for the PdO and PdO-CeO 2 system on alumina, respectively. A single peak, which is assigned to the nearest neighbor Pd-Pd bonds in Pd metal, was seen after the catalytic activity test at 573 K under a methane-oxygen stream. This is consistent with the fact that PdO in both the catalysts are reduced to metallic Pd at 573 K. In the fresh PdO-CeO 2 catalyst, the second-and third-neighbor peak intensities in the RSF were reduced to about one-half, compared with those in the fresh CeO 2 -free catalyst, while these catalysts showed the same peak intensities for the nearest neighbor RSF. These results suggest that CeO 2 induces disordering of the outer shells of Pd scattering atoms in the PdO catalyst. It can be presumed that CeO 2 improves the dispersiveness of PdO particles over the support and depresses the aggregation among the particles.

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Iron oxides were prepared by air oxidation of suspension obtained by mixing solutions of NaOH and FeSO4•7H2O. The ratio R was defined by mixing molar ratio of cations in the respective solutions (Na/Fe). Goethite (α-FeOOH) was mainly obtained in the range of R ≤ 1.9 (acidic region) and R ≥ 2.6 (basic region). Methane oxidation over the iron oxides started at as low temperature as [Formula: see text], at which the goethite changed to a hematite (α- Fe 2 O 3). The conversion ratio was low by the acidic goethite, in spite of its low crystallinity and high specific surface area. The basic goethite showed high activity for methane oxidation and the activity decreased grddually with increasing R. The low activity by the acidic goethite is attributable to the existence of some more percents of residual sulfate ion. And the sulfate ions disturbed the arrangement of oxide ions in the crystal structure of hematite.