Hydrogen adsorption sites studied by carbon monoxide adsorption to explain the hydrogenation activity of benzene on Pd and Pt catalysts

FT-IR of CO has been employed to study the relations between hydrogen adsorption and benzene hydrogenation on Pd/SiO2 and Pt/SiO2 catalysts. Three IR bands of CO stretching: 2085 (L1 band), 1984 (B1 band) and 1948 cm−1 (B2 band), have been observed on Pd/SiO2. Hydrogenation of benzene is dominated by hydrogen adsorption on B1 sites (CO band at 1984 cm−1) of Pd/SiO2, because benzene and/or the benzene ring of acetophenone can preferentially cover on the B1 sites to hinder hydrogen adsorption, leading to poor catalytic activity for hydrogenation of benzene. When hydrogen is pre-covered on Pd/SiO2 to occupy the B1 sites, it will cause intense benzene hydrogenation. Pre-adsorbed acetophenone on Pt/SiO2 surface presents a surprising phenomenon that adsorbed acetophenone does not impede CO adsorption. We propose that adsorption of hydrogen is independent of the exposure of pre-adsorbed acetophenone, and therefore Pt/SiO2 catalyst can offer high catalytic activity for hydrogenation of benzene.