Preparation of Supported Pd Catalysts: From the Pd Precursor Solution to the Deposited Pd²⁺ Phase

Abstract

The preparation by the deposition-precipitation method (using Na(2)PdCl(4) as a palladium precursor and Na(2)CO(3) as a basic agent) of Pd catalysts supported on gamma-Al(2)O(3) and on two different types of active carbons has been followed by several techniques (UV-vis, EXAFS, XRPD, and TPR). This work consists of four successive parts: the investigation of (i) the palladium precursor liquid solution (in the absence of substrate), (ii) the solid precipitated phase (in the absence of substrate), (iii) the precipitated Pd(2+)-phase on the supports as a function of Pd loading from 0.5 to 5.0 wt % (i.e., the final catalyst for debenzylation reactions), and (iv) the Pd(0)-phase formed upon reduction in H(2) atmosphere at 393 K. A time/pH-dependent UV-vis experiment indicates that Pd(2+) is present in the mother solution mainly as PdCl(2)(H(2)O)(2)] and [PdCl(H(2)O)(3)](+). Upon progressive addition of NaOH (3.0 < pH < approximately 3.8), the concentration of the two complexes is almost constant and then they rapidly disappear because of the precipitation of an amorphous aggregation of Pd(2+)-polynuclearhydroxo complexes. This phase represents a model material for the active supported phase. Thermal treatments at increasing temperature of this phase cause progressive water loss and resulted in a progressive increase in crystallinity typical of a defective PdO-like phase. The EXAFS spectrum of the final catalysts has been found to be intermediate between that of the unsupported amorphous Pd(2+)-polynuclearhydroxo complexes and that of the PdO-like phase. Independent of the support, EXAFS was not able to evidence any fraction of reduced metallic Pd, meaning that all Pd is in the 2+ oxidation state within the sensitivity of the technique (a few percent). Analogously, independent of the support, XRPD was not able to detect the presence of any crystalline supported phase. The Pd local environment of the as-precipitated samples changes slightly as a function of Pd loading from 0.5 to 2.0 wt %: at higher loadings, no further modification has been observed. After reduction in an H(2) atmosphere, two trends have been observed: (i) the dispersion of Pd nanoparticles tends to decrease with increasing Pd concentration, less significantly on Al(2)O(3)-supported samples and more significantly on carbon-supported ones and (ii) the dispersion depends on the carrier following the sequence Al(2)O(3) >> Cp > Cw according to the increasing palladium-support interaction strength.

Room-Temperature CO Oxidation Catalyst: Low-Temperature Metal–Support Interaction between Platinum Nanoparticles and Nanosized Ceria

“…From our experience, the possible presence of other atoms such as H would not affect the EXAFS signal and are thus not relevant in this study. 5 Different FMS cluster sizes were tested. We find convergence for an FMS cluster size of 5 Å.…”

Formation and growth of palladium nanoparticles inside porous poly(4-vinyl-pyridine) monitored by operando techniques: The role of different reducing agents

“…Feature 2 grows as soon as feature 1 starts to change. The final intensity of feature 2 (0.02 eV −1 ) is much smaller with respect to that observed for palladium metal foil (0.04 eV −1 ) [21], as expected for nanoparticles, due to the relevant fraction of low-coordination surface sites compared to the total amount of metal sites [50][51][52]. Complementary information regarding the palladium particle size can be obtained by the analysis of SAXS data.…”

Chemical Synthesis and Characterization of Pd/SiO2: The Effect of Chemical Reagent

“…For this purpose, the SiO 2 -supported complex was dispersed in 60 mL PEG and placed in a high-pressure reactor BR-200 (Berghof, Eningen, Germany). The prepared suspension was heated under stirring to 180 • C and held for 2 h. The aliquots of the reaction mixture at different time intervals (i.e., at the start and after 15,30,45,60,90, 120 min at 180 • C and after cooling) were extracted for UV-Vis measurements (vide infra). After cooling, the prepared powder was washed with distilled water and dried.…”

“…As a general procedure, porous support such as amorphous carbons [20][21][22][23][24][25], alumina, silica, or other mesoporous oxides [19,[26][27][28][29] is precipitated by palladium precursor containing palladium in its ionic phase [30] with subsequent reduction to Pd 0 and formation of palladium NPs [31]. Size and shape distributions of the resulting NPs depend on different parameters, such as type of support [32], temperature, and duration of reduction [33].…”

“…The role of the reducing agent has also been studied [34]. In particular, reduction in H 2 atmosphere can lead to the formation of particles 1 to 10 nm in size [30,33]. Another approach is to use a reducing agent in liquid form.…”