Bimetallic Pt-Sn catalysts supported on activated carbon - I. The effects of support modification and impregnation strategy

A commercial activated carbon (AC) was used as a support either after washing with HCl or after further oxidation with air or HNO3. The supports were characterized by N-2 adsorption, TPD and SEM. The results indicated the drastic change in physical and surface chemical properties of activated carbons due to the oxidation treatments. Three sets of catalysts were prepared on these supports. In each set, monometallic Pt/AC and bimetallic Pt-Sn/AC catalysts were prepared. Bimetallic catalysts were prepared either by coimpregnation or by sequential impregnation in which the Sn precursor was introduced first. In all catalysts, the Pt loading was kept fixed as 1 wt.%. Two levels of Sn loading, 0.25 and 0.50 wt.%, were applied in bimetallic samples. The catalysts were characterized by H-2 adsorption, TPR, SEM-EDX, XRD, XPS and tested in a structure insensitive reaction (benzene hydrogenation). The results indicated the pronounced changes in catalytic properties depending on the abundance of surface oxide groups of the supports and Pt-Sn interaction. Coimpregnation favored Pt-Sn interaction and alloy formation. The type of alloy formed was affected by the surface chemistry of the activated carbon; oxidized samples favored the formation of Pt3Sn. XPS results indicated the migration of Pt from surface to interior sites which is driven by decomposition of oxygen bearing anchoring sites during the reduction. The migration was stabilized up to a point by the introduction of Sn as the second metallic phase and, as a consequence, by the Pt-Sn interaction. The interaction between metallic phases and alloy formation led to striking decreases in benzene hydrogenation activity, which indicates the lower amount of active Pt sites available. Very high H-2 adsorption capacities of bimetallic samples compared to their benzene hydrogenation activities point out to the H-2 adsorption capacities of the Pt-Sn alloys formed. (C) 2000 Elsevier Science B.V All rights reserved.