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

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 H-2 as reducing agent to the inert carrier gas, or in two steps using first N-2 as carrier gas resulting in the dissociative chemisorption of Pd(allyl)Cp on SiO2, and then adding H-2 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 I'd 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 I to 2 nm. The catalytic activity of the two-step MOCVD catalysts was very high in the hydrogenation reaction using stoichiometric amounts of H-2. Although some deactivation was observed after I 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. (C) 2003 Elsevier Science B.V. All rights reserved.