Formation of multilayered tin organometallic surface species. Preparation of new type of supported Sn-Pt catalysts

In this study, a new aspect of anchoring of tin organic moieties onto platinum is described and discussed. The new approach resulted in substantial increase of the Sn/Pt ratio in tin modified Pt/SiO2 catalysts. In the earlier approach, adsorbed hydrogen was exclusively used for tin anchoring, resulting in a monolayer of tin organic moieties at the top of platinum. In the new approach, a large excess of either tin tetraethyl or hydrogen was used in the tin-anchoring reaction. The presence of coadsorbents, such as oxygen, also led to a substantial increase of the amount of tin anchored. When the primary formed SnR3 surface entities were partially decomposed, the coordinatively formed unsaturated -SnR2 and -SnR surface species provided additional anchoring sites for the next layer of SnR4. In the presence of adsorbed oxygen, additional new types of landing sites were created to anchor SnR4 in the neighborhood of platinum. The above approach resulted in Sn-Pt/SiO2 catalysts with exclusive tin-platinum interaction and an Sn/Pt ratio ca. 2. Results obtained in this study also reveal that the formation of surface organometallic moieties takes place in a stepwise way, e.g, the buildup of tin organic moieties occurs layer-by-layer. The supported Sn-Pt bimetallic entities formed showed both high activity and selectivity in the hydrogenation of crotonaldehyde to crotylalcohol. (C) 1998 Elsevier Science B.V. All rights reserved.