Surface organometallic chemistry on metals: Formation of a stable ≡ Sn(n-C₄H₉) fragment as a precursor of surface alloy obtained by stepwise hydrogenolysis of Sn(n-C₄H₉)₄ on a platinum particle supported on silica

Selective hydrogenolysis of Sn(n-C₄H₉)₄ on a Pt/SiO₂ catalyst has been carried out at various temperatures and coverages of the metallic surface to prepare via surface organometallic chemistry a well-defined class of bimetallic catalysts. The stoichiometry and kinetics of the reaction was followed by the careful analysis of reagents and products, including extraction of unreacted reagents, and elemental analysis of the samples. The various surface species formed were characterized by electron microscopy (CTEM and TEM EDAX) and EXAFS analysis. Possible structures of the surface organometallic fragments were considered using molecular modeling. At 50 °C, the hydrogenolysis reaction occurs selectively on the platinum surface with exclusive evolution of n-butane. There is first formation of a Sn(n-C₄H₉)₃ fragment grafted on the platinum particle which undergoes a stepwise cleavage of two tin - carbon σ-bonds to form a stable Pt-Sn(n-C₄H₉) fragment. Regardless of the reaction time, surface coverage, or loading, the number of grafted butyl fragments per platinum is never greater than unity, that is to say that when Sn(n-C₄H₉)₃ is formed the platinum coverage by tin is 0.3 whereas when Sn(n-C₄H₉) is formed the platinum coverage is closer to 1. It is therefore suggested that the surface composition is governed by the bulkiness of the alkyl chains which are 'close packed' on the surface, At 100 °C, the reaction takes place both on the platinum and the silica surface. On the platinum surface, the same fragments (namely Sn(n-C₄H₉)₃, Sn(n-C₄H₉)₂, and Sn(n-C₄H₉)) were identified, but simultaneously on the silica surface, the well-described ≡SiOSn(n-C₄H₉), species was also formed. Thermal treatment under hydrogen of Pt(s)-Sn(n-C₄H₉) lead to alkyl-free tin atoms which are located at the periphery of the particle as evidenced by Sn K edge EXAFS (Pt-Sn distance of 2.75 Å with a coordination number of ca. ₄). Even if the organotin fragments are grafted with a coverage of unity, after their complete hydrogenolysis at 300 °C, about 40% of the platinum is still accessible to H₂ chemisorption. This could be explained by the increase of the particle diameter (+0.5 Å) which prevents a close packing of the tin atoms around the particle and leaves some platinum atoms still accessible to the hydrogen. After treatment of the catalyst at higher temperatures, typically 500 °C, the structure of the catalyst is slightly changed since the tin atoms migrate into the first monolayer of the particle, as evidenced by a significant increase of the tin coordination number (ca. 4.4-5.6) as determined by EXAFS. Hypothetical surface structures have been proposed on the basis of molecular modeling of platinum particles covered by various surface organotin fragments.