Synthesis of highly dispersed palladium alumina supported particles: Influence of the particle surface density on physico-chemical properties

Abstract

Complexation with nitrite ions in aqueous medium was used to prepare highly dispersed Pd8 particles supported on alumina. It has been possible to obtain catalysts with different Pd loading keeping the particle size lower than 1 nm as shown by HRTEM, chemisorption, or extended X-ray absorption fine structure (EXAFS) characterizations, and as a consequence with different particle surface density. We observed that electronic properties measured by IR(CO) or X-ray photoelectron spectroscopy (XPS) measurements are strongly influenced by this last parameter and that the origin of these variations can be related to the different strengths of the interactions of the palladium precursor with the alumina surface sites as shown by low temperature CO adsorption followed by FTIR spectroscopy. On increasing Pd loading, saturation of unsaturated Lewis surface sites follows their acid strength: the more acidic ones are the first to react as germination sites, followed by the medium and finally the weaker sites. These different interactions may explain the variation of electronic properties of such small reduced particles which appears to be very sensitive to the chemical nature of their initial germination site on the oxide support. On the other hand, no such effects were observed on changing the type of alumina (d or g) support, in agreement with the principle of the synthesis method which tends to limit strong interaction with the support. Comparison with Pd8 particles with the same particle size and particle surface density but prepared with the well known grafting of acetylacetonate precursor shows marked differences in terms of electronic properties followed by CO FTIR. This result illustrates the possibility to prepare highly dispersed Pd particles with different physico-chemical properties. #

Effect of alloy composition on dispersion stability and catalytic activity for NO oxidation over alumina-supported Pt–Pd catalysts

“…IR spectra of CO adsorbed to saturation coverage (at 50°C) on selected catalysts before aging are shown in figure 1 (at constant relative absorbance but offset for clarity). The spectra from pure Pt and pure Pd catalysts are quite distinct, and agree well in resolution and spectral features to reported data for similarly loaded alumina-supported Pt and Pd catalysts [13][14][15]. As expected, the spectrum from a physical mixture of equal amounts of these two catalysts closely resembles the superposition of their individual spectra.…”

Influences of Pd precursors and preparation method on the catalytic performances of Pd-only close-coupled catalysts

“…Here, the amount and nature of washcoating material for these samples are almost the same, so the interaction of palladium precursor with the support may be mainly created by synthesis process and post-thermaltreatment. The key parameters that affect the interaction between the support surface and the precious metals precursors are the point of zero charge (PZC) of the support, the pH of the aqueous solution, the nature of the metallic complex and atmosphere of thermal treatment [21][22][23]. Zero potential measurements have been used to study the surface charge of alumina as a function of pH [24,25], and alumina surface is positively charged at pH < 8, zero charged at pH 8-9 and negatively charged at pH > 9.…”

A mini-review on air-stable organometallic Lewis acids: synthesis, characterization, and catalytic application in organic synthesis

“…82 We used the metallocene Lewis acids catalysts 3a, 3b, 2a and 2b in the allylation and Mukaiyama aldol reactions, and observed high catalytic activity (Table 4). 34,38,40 We assessed the metallocene Lewis acids catalysts 3a, 3b, 2a and 2b in the allylation and Mukaiyama aldol reactions of aldehydes (14) with nucleophiles such as tetraallyltin and ketene silyl acetals, and observed high catalytic activity (Table 4). 34,38,40 As expected, the reactions resulted in good yields of homoallyl alcohols (15a-15h) and b-hydroxy ester derivatives (16a-16h) in CH 3 CN or Et 2 O.…”