Palladium catalysts in the selective hydrogenation of hexa-1,5-diene and hexa-1,3-diene in the liquid phase. Effect of tin and silver addition Part 1. Preparation and characterization: from the precursor species to the final phases

Abstract

Palladium, palladiumÈtin and palladiumÈsilver catalysts, supported on two alumina samples with di †erent acidity, have been prepared by di †usional impregnation for use in the selective hydrogenation of hexa-1,5diene and hexa-1,3-diene in the liquid phase. UVÈVISÈNIR (transmission and di †use reÑectance) studies of the palladium starting solutions and of the solids obtained during all preparation stages (impregnation, drying, calcination) show that the nature of the supported precursor species depends on the acidity of the alumina. Palladium tetrachloropalladate (strongly bound anionic species) are predominant on the more acidic alumina while the more basic one favours the formation of weakly bound neutral complexes through exchange of chloride ligands with hydroxy groups and water. The nature of these species inÑuences not only the particle size of monometallic Pd samples (the stronger the interaction, the smaller the metal particle size), but also the nature of the Ðnal bimetallic PdÈAg catalysts ; a PdÈAg solid solution is observed on the bimetallic catalyst prepared by interaction of a cationic Ag precursor with supported whereas interaction with neutral PdCl 4 2~, Pd species leads to separate metal phases. The interaction of Sn precursor solutions with supported PdII species induces the formation of heteronuclear species which decompose upon drying ; or phases Pd 2 Sn Pd 3 Sn are detected on the Ðnal catalysts, depending on the initial Pd/Sn ratio and on the nature of the Sn precursor solvent (using ethanol favours the formation of Pd 3 Sn).

Pd supported catalyst for gas-phase 1,2-dichloroethane abatement: Efficiency and high selectivity towards oxygenated products

“…Moreover, the Pd(0.5)/HAP spectrum show a well-resolved band at 475 nm and a shoulder at 325 nm, ascribed to allowed d-d transitions. It is worth mentioning that this set of the four UVvisible bands were also observed by Sales et al [38] in their PdCl 2 solution spectrum and they assigned them to the (PdCl 4 ) 2complexes in D 4h symmetry. The formation of the latter on the used Pd(0.5)/HAP sample can reasonably explain its colour change from brown-green to orange.…”

“…On the other hand, due to their p-type semiconductor character, the particle sizes of the PdO species could be correlated with the width of their band gaps [38][39]. For example, narrow band gap indicates the presence of relatively large particles (> 20 nm) as in the case of bulk PdO whose absorption threshold lies in the NIR, 0.6-0.8 eV (1550-2070 nm) [38].…”

“…On the other hand, due to their p-type semiconductor character, the particle sizes of the PdO species could be correlated with the width of their band gaps [38][39]. For example, narrow band gap indicates the presence of relatively large particles (> 20 nm) as in the case of bulk PdO whose absorption threshold lies in the NIR, 0.6-0.8 eV (1550-2070 nm) [38]. For all the Pd(x)/HAP catalysts the band gap lies in the visible domain, 1.9-2.1 eV (590-650 nm), pointing out that, in good agreement with the TEM results, the PdO species are deposited as nanosized particles (< 20 nm).…”

Preparation and Investigation of Pd and Bimetallic Pd-Sn Nanocrystals on γ-Al2O3

“…However, among all methods, chemical impregnation of powder supports with metallic precursors is certainly the most used one to obtain supported (bi)metallic catalysts. As such, they have been extensively used to obtain PdSn catalysts either by using inorganic tin precursor such as stannic chloride or stannous chloride and palladium chloride or nitrate [42][43][44][45][46][47][48][49][50][51][52] or organic precursors such as acetylacetonates [53][54][55][56][57][58] which result from the association of acetylacetone, one of the most commonly used organic oxygen ligands with donor atoms, with Pd(II) yielding a very stable chelate complex Pd(acac) 2 . The main advantage of the former is the simplicity associated to (at least for most of them) their solubility in water.…”