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

Abstract

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)3 species was also formed. Thermal treatment under hydrogen of Pts−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.

Synthesis and Site Structure of a Replica Platinum−Carbon Composite Formed Utilizing Ordered Mesopores of Aluminum-MCM-41 for Catalysis in Fuel Cells

“…The XANES spectra tuned to the Pt L 1 peak top for replica Pt-C/Nafion both in air and in H 2 exclusively detected metallic Pt sites ( Figure 5B(e), (f)). The spectra resembled well theoretical spectrum B(g) generated for the surface Pt site on the (111) face of a cuboctahedral Pt 38 cluster.…”

“…The mean size corresponds to a total of 66 atoms in one particle. 38 Most of the Pt particles are within the pore size of Al-MCM-41 (2.7 nm) ( Figure 2C). The replica C is not clear in TEM images probably because the axis of replica Pt-C may randomly distribute, and the diameter of the rod/tube-like structure should be less than 3 nm dispersed over amorphous C coated on Cu mesh.…”

“…Among the Pt atoms, N(Pt-Pt) values of 6, 8, and 24 atoms were 12, 9, and 6, respectively. 38 The central Pt atom in the (111) surface (N ) 9) was considered as a representative Pt site for calculations ( Figure 1) compared to an average N of 7.6 for all of the Pt sites. A central Pt atom in the (111) surface was in vacuum or at the interface with 177 atoms of three layers of graphite [R(C-C) ) 0.1418 nm].…”

“…The average N value for Pt-Pt bonds by EXAFS (6.1; Table 1) suggested even smaller Pt nanoparticles. 38 This discrepancy is due to the contribution of N Pt-O or N Pt-C . Pt-C-Al-MCM-41 and replica Pt-C samples both measured in air provided N Pt-O values of 2.1-2.2, including the undividable contribution of N Pt-C (Table 1) because the exterior of these Pt nanoparticles should be oxidized and/or carburized.…”

The role of Sn in Pt–Sn/CeO2 catalysts for the complete oxidation of ethanol

“…Several controversial views concerning the exact nature of Pt-Sn interaction were proposed, probably due to different preparation techniques and treatment conditions [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The way that Sn modifies the catalytic behavior of Pt can vary from bulk alloying or surface decoration to electronic interaction.…”