Modification by lanthanide (La, Ce) promotion of catalytic properties of palladium: Characterization of the catalysts

Abstract

catalysts modiÐed by cerium or lanthanum oxide were prepared according to a standard Pd/c-Al 2 O 3 procedure. The nature of the interaction between the metallic particles and the promoted support was addressed. The e †ects of changing the following parameters were studied : (i) precursor palladium salt (chloride or nitrate), (ii) content of lanthanide (from the unpromoted catalyst to the pure lanthanide oxide) and (iii) nature of the lanthanide (La or Ce). This paper focuses on the study of the transition metal, whereas the lanthanide redox properties have been reported in an earlier paper (J. Phys. Chem., 1988Chem., , 92, 2561. Both structural [transmission electron microscopy (TEM), and extended X-ray absorption Ðne structure (EXAFS) at the Pd K edge] and spectroscopic investigations (XPS at the Pd 3d core level) emphasize the role of the precursor palladium salt. When derived from a palladium chloride precursor and after a standard treatment including calcination and reduction, the chloride ion is quantitatively trapped by the support and the metalÈsupport interaction is strong. This is thought to be due to the preferential localization of the chlorine atoms in bridge positions between the metal and the lanthanide. When exposed to an air atmosphere, the palladium atoms at the interface are partially chlorided and cerium is in the ]4 oxidation state. After a mild treatment at 150 ¡C, palladium is fully reduced, except when the cerium content is within 1È5% where the H 2 reduction is only partial. Cerium is converted into the trivalent oxidation state either as a chlorinated or an oxychlorinated form. It is thought that this metalÈsupport interaction at the interface, which does not exist in the nitrate precursor catalysts, might explain the catalytic behaviour of such catalysts by the creation of new active sites or a promoting e †ect (J. Mol. Catal. A, in the press). The strength of this interaction depends on the cation of the support at the interface (Ce3`[ La3`[ Al3`) and is stronger for a cerium content between 1 and 5%.

Effect of lanthanum on the catalytic properties of PtSn/γ-Al2O3 bimetallic catalysts prepared by successive impregnation and controlled surface reaction

“…A similar result was observed with PtSn/γ -Al 2 O 3 -La catalysts prepared by coimpregnation [15]. It has been reported that lanthanides have the ability to retain chlorine on the surface [19,20].…”

A simple synthesis of catalytically active, high surface area ceria aerogels

“…The ratio of the band areas due to metallic palladium as compared to the sum of the band areas due to metallic and oxidized palladium is 0.44, suggesting that approximately 44% of palladium present is in metallic form. It must be noted that although palladium is reduced by hydrogen at 200°C, upon exposure to air, the palladium can be re-oxidized [34]. This likely occurred for this system, as samples were stored under ambient conditions after treatment.…”

Electrodeposition of a Au-Dy2O3 Composite Solid Oxide Fuel Cell Catalyst from Eutectic Urea/Choline Chloride Ionic Liquid

“…The interfacial polarization resistance R p of the bare gold electrode used as anode resulted 4,220 Ω·cm [4]. This catalytic behaviour might be explained by an high H 2 consumption creating new active sites through hydrogen or oxygen spillover [4,29]. Even more evident effect was noticed by evaluating the catalytic activity of the novel material cathodic catalyst.…”