No abstract

The effect of crystallinity on the photoreduction of cerium oxide (Ce4+ to Ce3+) during X-ray photoelectron spectroscopy (XPS) analysis of CeO2 and Ce/Al2O3 catalysts has been determined. Cerium oxide samples were prepared by calcining cerium(IV) methoxyethoxide in air at different temperatures. The structure of the resulting metal oxides was determined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The oxide obtained from calcination at 200 °C consisted primarily of amorphous cerium oxide, while the material produced at 750 °C contained large CeO2 crystallites. XPS Ce 3d spectra obtained for the samples showed that the amorphous cerium oxide sample was reduced more extensively than the crystalline material during XPS analysis. Ce/Al2O3 catalysts (5.1 wt % CeO2) were prepared by incipient wetness impregnation using ammonium cerium(IV) nitrate and cerium(IV) methoxyethoxide precursors. XRD and XPS analyses of the Ce/Al2O3 catalyst derived from the nitrate precursor (designated CeN) indicated that most of the cerium was present as poorly dispersed CeO2 crystallites. The catalyst prepared using the alkoxide precursor (designated CeA) also contained poorly dispersed cerium oxide; however, XRD data suggested that very little of the cerium oxide phase was crystalline. The XPS Ce 3d spectrum obtained for the CeN catalyst was typical of Ce(IV) oxide while the spectrum collected for the CeA catalyst was similar to a Ce(III) compound. These results have been attributed to enhanced photoreduction of the amorphous cerium oxide present in the CeA catalyst.

X-ray photoelectron spectroscopy (ESCA or XPS), X-ray diffraction (XRD), Raman spectroscopy, H2 chemisorption, and gravimetric analysis have been used to characterize three series of La/Al303 and CoLa/Al203 catalysts. CoLa/Al2G3 catalysts were prepared by two methods: impregnation of La first followed by Co (designated "CoLay") and impregnation of Co first followed by La (designated "LayCo"). The information obtained from surface and bulk characterization has been compared with CO hydrogenation activity and selectivity of the supported Co/A1203 catalysts. For CoLay catalysts with low La loadings (La/Al atomic ratio <0.026), the presence of La had little effect on the structure or CO hydrogenation activity. However, the selectivity to higher hydrocarbons and olefinic products increased with increasing La content. For CoLay catalysts with higher La loadings, Co304 is suppressed in favor of an amorphous dispersed La-Co mixed oxide. ESCA and H2 chemisorption indicated higher dispersion of the metallic cobalt phase for high La loadings. The turnover frequency (TOF) for CO hydrogenation decreased dramatically for high La loadings. This has been correlated to the decrease in the amount of Co304 present in the La-rich catalysts. Catalysts prepared by reverse impregnation (LayCo) showed little evidence of La-Co interaction. No significant variation in reducibility or cobalt metal dispersion was observed. Lanthanum addition had little effect on the TOF for CO hydrogenation or the selectivity to olefinic products and higher hydrocarbons.

X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) have been used to characterize a series of Cu/Cr/Al2O3 catalysts prepared by stepwise incipient wetness impregnation of first chromium followed by copper (designated “CuCry”). The copper loading was held constant at 8 wt % CuO, and chromium loadings were varied from 0 to 20 wt % Cr2O3. The information obtained from surface and bulk characterization has been correlated with the CO oxidation activity of the catalysts. XPS and XRD results of analogous Cry indicated that the Cr dispersion decreased and the concentration of Cr3+ species increased with increasing Cr content. The decrease in Cu dispersion of CuCry with increasing Cr content has been attributed to the formation of large crystalline CuO and CuCr2O4. Copper addition decreased the Cr dispersion by reacting selectively with a dispersed Cr3+ species to form CuCr2O4 species. However, the Cu addition did not affect the Cr oxidation state distribution compared to that of Cry. For low Cr loading CuCry catalysts (Cr/Al ≤ 0.027), the CO oxidation activity increased with increasing Cr content due to the formation of crystalline CuO on the Cr-modified alumina. This has been attributed to the inhibition of Cu ion diffusion into alumina lattice vacancies by highly dispersed chromium species. The CuCry catalyst of Cr/Al = 0.054 showed the highest CO oxidation activity due to the formation of CuCr2O4 which was more active than the CuO phase. For Cr-rich catalysts (Cr/Al ≥ 0.080), the decrease in CO oxidation activity has been ascribed to the encapsulation of the active site with Cr2O3 species.

No abstract