Influence of Zn on the characteristics and catalytic behavior of TiO2-supported Pt catalysts

Vapor-phase hydrogenation of toluene and selective hydrogenation of crotonaldehyde have been performed over Pt/TiO2 and PtZn/TiO2 after low (473 K) and high (773 K) temperature reduction. The catalysts were characterized by H2 chemisorption, X-ray photoelectron spectroscopy (XPS), and Auger spectroscopy (AES) after each reduction treatment. Infrared spectra of CO alone and followed by the introduction of crotonaldehyde have been recorded for both catalysts after reduction at 473, 673, and 773 K. The presence of zinc and an increase in the reduction temperature strongly decreased the catalytic activity for toluene hydrogenation, but a different effect was observed for crotonaldehyde hydrogenation. After reduction at low temperature, the bimetallic catalyst was somewhat less active than the monometallic catalyst, but the initial activity of the former was greatly increased after reduction at 773 K, whereas the Pt/TiO2 catalyst reduced at this high temperature was almost inactive. The selectivity toward the hydrogenation of the carbonyl bond exhibited a strong dependence on the presence of zinc and on the reduction temperature. Monometallic Pt/TiO2 reduced at low temperature showed the maximum selectivity (up to 75%), although with a continuous decrease with time on stream. The bimetallic PtZn/TiO2 catalyst, reduced at 773 K, showed a more stable selectivity to crotyl alcohol, centered at about 30%.