5 wt% Pt deposited on two different supports made of 3 and 1/6 equivalent monolayers of ZnCl2 deposited on silica (termed Pt30 and Pt2) were prepared and tested in the selective hydrogenation of crotonaldehyde. Pt-based catalysts were prepared starting from tetraammine platinum nitrate as a metallic salt precursor. These catalysts showed very different catalytic behaviors in terms of the activity and the selectivity during time on stream. Pt2 was more active than Pt30 but less selective toward the formation of crotyl alcohol. On the Pt30 catalyst, the selectivity to the formation of crotyl alcohol increased with the reduction temperature, whereas the activity did not change significantly. When the catalyst was reduced at 400 degrees C, the selectivity to crotyl alcohol was > 80%. It behaved similarly to Pt/ZnO catalysts prepared from hexachloroplatinic acid precursor [F. Ammari, J. Lamotte, R. Touroude, J. Catal. 221 (2004) 32]. In contrast, the Pt2 sample showed a decrease in the selectivity to crotyl alcohol and an increase in the activity with increasing reduction temperature. The catalysts were extensively characterized by BET, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and Fourier transform infrared pyridine adsorption. The higher selectivity of Pt30 is explained by a synergetic effect of Zn and chlorine. The greater amount of chlorine present on the catalyst surface led to an increase in the Lewis acidity of the support; moreover, the formation of PtZn alloy was observed. In agreement with our previous findings, we propose that modification of the electronic properties of platinum both from alloying to Zn and from the Lewis acidity of the support changes the adsorption mode of crotonaldehyde by favoring the binding of terminal oxygen of the molecule to the electronically modified platinum atoms. (c) 2005 Elsevier Inc. All rights reserved.