EXAFS study on the sulfidation behavior of Pd, Pt and Pd–Pt catalysts supported on amorphous silica and high-silica USY zeolite

The sulfur tolerance of monometallic Pd, Pt and bimetallic Pd–Pt catalysts supported on slightly acidic ultra-stable Y (USY) zeolite (SiO₂/Al₂O₃ = 390) and on non-acidic silica, having mesopores with a pore diameter of 3 or 10 nm, were investigated using the CO adsorption method and the extended X-ray adsorption fine structure (EXAFS) method. Well-dispersed noble metal particles supported on USY zeolite and silica with an average pore diameter of 3 nm showed high surface sulfur tolerance and high catalytic hydrogenation activity, although bulk phase sulfidation simultaneously occurred. The synergistic effects of sulfur tolerance were significant in the bimetallic Pd–Pt particles supported on USY zeolite and silica with an average pore diameter of 3 nm. On the other hand, on silica with an average pore diameter of 10 nm, the surface sulfur tolerance of low dispersed noble metals was the lowest, although its bulk phase sulfur tolerance was the highest. The Pd K-edge and Pt L_III-edge EXAFS spectra indicated a strong interaction between the well-dispersed noble metal particles and the supports of the USY zeolite and silica with an average pore diameter of 3 nm. This distorted structure may increase the sulfur tolerance of noble metals, though some surface and bulk phase sulfidation simultaneously occurred.