Bimetallic Pt-Pd/silica-alumina hydrotreating catalysts. Part I: Physicochemical Characterization

Supported Pt–Pd catalysts were synthesized using the respective tetrammonium complexes as precursor salts. The materials used as carriers were silica and amorphous silica–alumina (ASA) with Al$_2$O$_3$/SiO$_2$ wt.% ratio of 55/45, 20/80, and 5/95. The concentration of Lewis acid sites increased with the alumina content of support, whereas the concentration of Brønsted acid sites reached a maximum at 20 wt.% of alumina. According to the chemical analysis, the content of Pt and Pd was close to 0.3 and 0.5 wt.%, respectively, keeping the Pd/Pt molar ratio of 3. The average particle size slightly varied from 1.4 to 1.8 nm (TEM). The analysis of the EXAFS and CO adsorption IR characterization revealed that the general morphology of the bimetallic clusters was Pt-rich core and Pd-enriched surface. The Pt atoms in the bimetallic clusters were electron deficient due to electron transfer to Pd (XANES). Furthermore, monometallic Pd clusters coexist with those Pt–Pd particles. The proportion of surface Pt increased (from 13.9% to 16.9%) with increasing alumina content in the support. The catalyst supported on ASA(55/45) exhibited the highest coverage of Pt. The varying surface metal composition is attributed to different interaction strengths between the metal precursors and the supports during the preparation steps