Actual constitution of the mixed oxide promoter in a Rh/Ce_1−xPr_xO_2−y/Al₂O₃ catalyst. Evolution throughout the preparation steps

Abstract

The evolution of the ceria-praseodymia mixed oxide promoter throughout the successive steps involved in the preparation of a 3% Rh/25% Ce 0.8 Pr 0.2 O 2−x /Al 2 O 3 -3.5% SiO 2 catalyst is studied with the help of X-ray diffraction (XRD), spatially resolved electron energy loss spectroscopy (EELS), XPS, and thermal programed desorption (TPD) techniques. Incipient wetness impregnation has been used to obtain the appropriate loading of this promoter. As revealed by XRD, EELS, and XPS data, the alumina-supported CeO 2 -PrO 2−x mixed oxide sample consisted of a bimodal distribution of the particles of the promoter. XRD shows two fluorite-like phases: cerium-rich particles with a mean crystallite size greater than 6 nm and smaller praseodymium-rich crystals, about 3.5 nm size. Surface Ce/Pr ratio obtained by XPS was consistent with a higher dispersion of the praseodymium-rich oxide particles. The final Rh catalyst was prepared in a second step, by impregnation of the CePrO x /Al 2 O 3 system with an acidic solution of Rh(NO 3 ) 3 . The low pH of this solution is responsible for further modifications of the lanthanide oxides distribution. Thus, there is a redispersion of the praseodymium-rich phase, which is evidenced by EELS and XPS data. Likewise, CO 2 desorption from samples exposed to the atmosphere is consistent with an enhanced basicity of the sample due to the increase of Pr 3+ content at the surface of the catalyst. The observed changes on the nanostructure of the mixed oxide promoter can be attributable to a partial dissolution of the oxide, selectively leaching Pr 3+ cations from the lattice, which are finally deposited as small particles after the drying steps of the preparation.

Improved Oxidase Mimetic Activity by Praseodymium Incorporation into Ceria Nanocubes

“…These results are in good agreement with previous data in the literature. 29,30,39,40 Concerning the major lanthanide, cerium, the XPS results (Figure 5b) indicate that this element is mainly present as Ce 4+ on the surface of the pure and Pr-modified CeO 2 NCs. Thus, the fraction of Ce 3+ on the outermost surface layers of CeO 2 NC amounts only to 2%, but it is below the detection limit on the other Pr-modified CeO 2 NC samples with different Pr concentrations.…”

Solid state reactions of nanocrystalline Ce0.5Yb0.5O1.75 mixed oxide with high surface area silica in oxidizing and reducing atmosphere

“…al. [35] who showed, using analytical STEM, that in the sample prepared by co-impregnation of alumina support with a mixture of Ce-and Pr-nitrate solutions particles with broad range of compositions were present, in spite of occurrence of the symmetrical reflections in XRD pattern. Smaller content of Yb in ceria and existence of highly dispersed Yb at the [32].…”

“…For Ce 1À x Tb x O 2À x/2 and Ce 1À x Pr x O 2À x/2 supported on an alumina formation of CeAlO 3 , TbAlO 3 or PrAlO 3 was observed in a reducing atmosphere at temperatures over 900 1C [33][34][35]. It is worth to emphasize that the mixed oxides used in these works were prepared by co-impregnation of the support with a solution of lanthanide nitrates what rises a question on their homogeneity [35].…”

X‐Ray Photoelectron Spectroscopy in Analysis of SurfacesUpdate based on the original article by Steffen Oswald,Encyclopedia of Analytical Chemistry, © 2000, John Wiley & Sons, Ltd.

“…(228,266 -268) To reach high catalytical activity, high surface fraction is necessary; thus, most catalysts are used in form of nanoparticles (Section 4.5.2). Up to now, most of the XPS studies are still on noble metal (269,270) and oxide (271,272) catalysts; however, also in catalysis, there is a trend to overcome the pressure gap (273 -276) by special equipment and using synchrotron radiation (Sections 5.5 and 5.6).…”