Tailoring silica-alumina-supported Pt-Pd as poison-tolerant catalyst for aromatics hydrogenation

The tailoring of the physicochemical and catalytic properties of mono- and bimetallic Pt-Pd catalysts supported on amorphous silica-alumina was studied. Electron-energy-loss spectroscopy and extended X-ray absorption fine structure analyses indicated that bimetallic Pt-Pd and relatively large monometallic Pd particles were formed, whereas the X-ray absorption near edge structure provided direct evidence for the electronic deficiency of the Pt atoms. The heterogeneous distribution of metal particles was also shown by high-resolution transmission electron microscopy. The average structure of the bimetallic particles (Pt-rich core and Pd-rich shell) and the presence of Pd particles led to surface Pd enrichment, which was independently shown by IR spectra of adsorbed CO. The specific metal distribution, average size, and surface composition of the Pt-Pd particles depend to a large extent on the metal precursors. In the presence of NH₃ ligands, Pt-Pd particles with a fairly homogeneous bulk and surface metal distribution were formed. Also, high Lewis acid site concentration of the carrier leads to more homogeneous bimetallic particles. All catalysts were active for the hydrogenation of tetralin in the absence and presence of quinoline and dibenzothiophene (DBT). Monometallic Pt catalysts had the highest hydrogenation activity in poison-free and quinoline-containing feed. When DBT was present, bimetallic Pt-Pd catalysts with the most homogenous metal distribution showed the highest activity. The higher resistance of bimetallic catalysts toward sulfur poisoning compared to their monometallic Pt counterparts results from the weakened metal-sulfur bond on the electron-deficient Pt atoms. Thus, increasing the fraction of electron-deficient Pt on the surface of the bimetallic clusters increases the efficiency of the catalyst in the presence of sulfur-containing compounds.