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

The preparation by deposition-precipitation method (using Na2PdCl4 as palladium precursor and Na2CO3 as basic agent) of Pd catalysts supported on gamma-Al2O3 and on two different types of active carbons has been followed by several techniques (UV-Vis, EXAFS, XRPD, TPR). This work consists in three successive parts. Investigation of: (i) the palladium precursor liquid solution (in absence of substrate); (ii) the solid precipitated phase (in absence of substrate); (iii) the precipitated phase on the supports as a function of Pd loading from 0.5 to 5.0 wt% (i.e. the final catalyst). A time/pH dependent UV-Vis experiment indicates that Pd2+ is present in the mother solution mainly as PdCl2(H2O)2] and [PdCl(H2O)3]+. Upon progressive addition of NaOH (3.0 < pH < ~3.8), the concentration of the two complexes is almost constant, then they rapidly disappear due to precipitation of an amorphous aggregation of Pd2+-polynuclearhydroxo-complexes. This phase represents a model material for the active supported phase. Thermal treatments at increasing temperature of this phase cause a 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 Pd2+-polynuclearhydroxo-complexes and that of the PdO-like phase. Independently from the support, EXAFS was not able to evidence any fraction of reduced metallic Pd, meaning that all Pd is in 2+ oxidation state within the sensitivity of the technique (few %). Analogously, independently from 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 the Pd loading from 0.5 to 2.0 wt%: at higher loadings no further modification has been observed. After reduction in H2 atmosphere two trends have been observed: (i) the dispersion of Pd nanoparticles tends to decrease with increasing Pd concentration, less significantly on Al2O3 supported samples, and more on carbon supported ones, and (ii) the dispersion depends on the carrier following the sequence Al2O3 >> Cp > Cw, according to the increasing palladium-support interaction strength.