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.