CHARACTERIZATION OF PD AND PD-PB ALPHA-AL2O3 CATALYSTS - A TPR-TPD STUDY/

Low loaded (< 0.5 wt.-%) palladium on alpha-Al2O3 catalysts were prepa red using palladium acetylacetonate as a chemical precursor. Temperatu re programmed reduction studies (TPR) indicate that in the initial oxi dized state a fraction of palladium exhibits a strong interaction with the alumina support. Hydrogen treatments remove the interaction allow ing the formation of palladium particles that upon reoxidation stabili ze the palladium as PdO1.4. Bimetallic Pd-Pb catalysts were obtained b y reacting the palladium catalysts with tetra-n-butyl lead. Hydrogen a nd carbon monoxide desorption studies (TPD) have shown that progressiv e lead addition suppresses the dissociative adsorption of hydrogen as well as the multiple coordinated forms of carbon monoxide. For hydroge n, the TPD spectra supports the existence of an ensemble effect upon a lloying. On the other hand the increased adsorption strength of multip le coordinated carbon monoxide suggested the presence of a ligand effe ct. Another important effect of lead on the TPD of carbon monoxide is the inhibition of carbon dioxide formation. The role of lead in improv ing the selectivity of alkynes hydrogenation reactions is discussed.