Crotonaldehyde hydrogenation over bimetallic Pt-Sn catalysts supported on pregraphitized carbon black. Effect of the Sn/Pt atomic ratio

Three bimetallic Pt-Sn/C catalysts have been prepared by successive impregnation of pregraphitized carbon black with an aqueous solutions of hexachloroplatinic acid and tin(II) chloride. One monometallic Pt/C sample was also prepared and studied for comparison. All catalysts were characterized by hydrogen and carbon monoxide chemisorption at room temperature and X-ray photoelectron spectroscopy and their behaviour in the gas phase hydrogenation of crotonaldehyde, at atmospheric pressure, determined. The amount of surface platinum is greatly reduced by the addition of tin, as deduced from chemisorption measurements and XPS. Both Pt-0 and Pt-II are detected by XPS in the fresh bimetallic catalysts; after reduction in flowing hydrogen at 623 K platinum is completely reduced to the metallic state and, although a high proportion of tin remains in an oxidized state, a relatively important amount is reduced to Sn-0, this allowing the possibility of Pt-Sn alloys formation. The catalytic activity in the gas phase hydrogenation of crotonaldehyde is greatly improved by the presence of tin, in spite of the fact that the amount of surface platinum is reduced. Tin has also a very important effect on the selectivity towards the hydrogenation of the C=O bond, increasing the production of crotyl alcohol in respect to the hydrogenation of the C=C bond that would lead to the production of butyraldehyde. The observed results are explained on the basis of a promoting effect of oxidized tin species for the hydrogenation of the C=O group, whereas the formation of a Pt-Sn alloy or the dilution of surface platinum by metallic tin would hinder the hydrogenation of the olefinic C=C bond.