The preparation of Pd/SiO2 catalysts by chemical vapor deposition in a fluidized-bed reactor Original Research Article

Palladium catalysts supported on silica were prepared by chemical vapor deposition using allylcyclopentadienylpalladium [Pd(allyl)Cp] as precursor (MOCVD) in a fluidized-bed reactor. The CVD of Pd(allyl)Cp was either carried out in one step by adding H2 as reducing agent to the inert carrier gas, or in two steps using first N2 as carrier gas resulting in the dissociative chemisorption of Pd(allyl)Cp on SiO2, and then adding H2 to obtain supported metallic Pd particles. The Pd/SiO2 catalysts were characterized by inductively coupled plasma–optical emission spectroscopy (ICP–OES), X-ray powder diffraction (XRD), chemisorption, temperature-programmed desorption (TPD)/temperature-programmed reduction (TPR), and XPS. The catalytic activity was assessed by hydrogenating cyclooctene in a fixed-bed microreactor. The results show that the catalysts produced by the two-step method had a significantly higher metal dispersion ranging from 0.9 to 1.0 than those prepared by the one-step method with dispersions of 0.2–0.3. The one-step method is assumed to lead to larger Pd particles due to the immediate formation of metallic Pd clusters catalyzing the reductive decomposition of Pd(allyl)Cp. In the two-step preparation the precursor is first adsorbed uniformly on the support dissociating off the cyclopentadienyl ligand, and then reduced yielding Pd particles with diameters in the range from 1 to 2 nm. The catalytic activity of the two-step MOCVD catalysts was very high in the hydrogenation reaction using stoichiometric amounts of H2. Although some deactivation was observed after 1 h of time on stream, the conversion remained much higher than that of any sample prepared by the one-step method. Thus, the two-step MOCVD of Pd(allyl)Cp in a fluidized-bed reactor is considered a highly suitable method for preparing hydrogenation catalysts with very high Pd dispersion in a well-controlled and solvent-free way.