XAFS and XRD Studies of PdOCeO2 Catalysts on Al2O3

Abstract

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.

Direct Detection of a Phase Change in PdO/CeO2 Supported on χ-Al2O3 by Means of in situ High-Temperature Measurements of XRD and FTIR

“…A role of CeO2 is to suppress the crystal growth of PdO in the catalyst upon elevating the temperature, and it is tentatively concluded that the high catalytic activity at R = 1 comes from the small crystallite size of PdO as well as the largest surface area, as has been reported before. 5 Figure 2 illustrates the XP spectra of Pd in two catalysts of R = 0 (a) and R = 1 (b) before and after reducing in a methane atmosphere at 300˚C; two peaks due to the emission of photoelectron from the inner-core orbitals, 3d3/2 and 3d5/2, of the Pd atom, were observed. A significant shift of these two peaks by as much as 1.9 -2.9 eV to the lower binding energy side was observed in common after these catalysts were treated in a reducing atmosphere of methane at 300˚C, although an unshifted peak is still observed as a shoulder in Fig.…”

“…4,5 In a small volume of an aqueous solution of Ce(NO3)3·6H2O, a weighed portion of Al(OH)3 (gibbsite) was suspended and admixed by vigorous stirring for 30 min. An aqueous solution of Pd((NO3)2 was added to the mixture in various atomic ratios of R = Ce/Pd.…”

“…4,5 A small portion (0.5 g) of the catalyst was loaded in the reactor through which the mixed reactive gas and helium gas were passed when the pressures of methane and oxygen were 3.4 kPa and 20.3 kPa, respectively, and the total volume of the gas used in the test of catalytic activity was 60 mL min -1 (under an ordinary condition) with a space velocity of 7200 h -1 . The gas components were analyzed by an on-stream gas chromatography (Shimadzu GC-8APT) equipped with an integrating TC detector (Shimadzu C-R6A).…”

“…We have reported that the Pd metal phase is observed in the X-ray absorption fine structure (XAFS) after treating the catalysts in an O2-deficient atmosphere at high temperature, and have suggested that a phase change between PdO and Pd takes place in the PdO/CeO2 catalysts. 4,5 In this work, in situ hightemperature measurements of XRD (in situ HTXRD) of the PdO/CeO2 catalysts and FT-IR (in situ HTFT-IR) of PdO microcrystals were made in order to verify the phase change in the temperature range where the catalysts work. The in situ high-temperature measurements allow us to have an accurate insight into the thermal change of the catalytic-active PdO under the condition of operation in which PdO catalyzes methane oxidation.…”

“…1,2 It has been reported that PdO microcrystals supported on χ-Al2O3 show a high catalytic activity in methane oxidation below 300˚C when CeO2 coexists altogether with PdO on the support (PdO/CeO2 catalysts). [3][4][5] Thus, PdO/CeO2 catalysts have the capability to ignite combustion at moderate temperatures as well as other advantages; colorless and transparent to visible light, which enables the catalyst to be utilized in various fields. We have reported that the Pd metal phase is observed in the X-ray absorption fine structure (XAFS) after treating the catalysts in an O2-deficient atmosphere at high temperature, and have suggested that a phase change between PdO and Pd takes place in the PdO/CeO2 catalysts.…”