Oxidation of Sn at the Cluster–Support Interface: Sn and Pt–Sn Clusters on TiO₂(110)

Abstract

The growth of Sn and Pt–Sn clusters on TiO2(110) has been studied by scanning tunneling microscopy, X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and density functional theory (DFT). At low Sn coverages (0.02 ML), single-layer high clusters of SnO x are formed with a narrow size distribution and uniform spatial distribution. XPS experiments indicate that these clusters consist of oxidized Sn, and the corresponding reduction in the TiO2 substrate is observed. At higher Sn coverages, the surface is still dominated by two-dimensional clusters of SnO x , but larger three-dimensional clusters of metallic Sn also appear. As the Sn coverage is increased, the number of three-dimensional clusters increases, and the ratio of Sn/SnO x increases, suggesting that SnO x and reduced TiO x form at the cluster–support interface. When Pt is deposited on top of the Sn/SnO x clusters, the relatively mobile Pt atoms diffuse across the TiO2 surface and become incorporated into existing Sn/SnO x clusters. Furthermore, the addition of Pt to the Sn/SnO x clusters causes the reduction of SnO x to metallic Sn and the oxidation of Ti3+ to Ti4+; this behavior is attributed to the formation of Pt–Sn alloy clusters, which results in the diffusion of Sn away from the interface with the TiO2 support. In contrast, when Sn is deposited on an equal coverage of Pt clusters, new Sn/SnO x clusters are formed that coexist with Pt–Sn clusters. However, the surfaces of both Pt on Sn and Sn on Pt clusters are Sn-rich due to the lower surface free energy of Sn compared to Pt. DFT calculations demonstrate that M–TiO2 bonding is favored over M–M bonding for M = Sn, unlike for transition metals such as M = Pt, Au, Ni, and Co. Furthermore, the substantial charge transfer from Sn to TiO2 leads to dipole–dipole repulsion of Sn atoms that prevents agglomeration into the larger clusters that are observed for the mid-late transition metals. DFT studies also confirm that the addition of Pt to a Sn cluster results in strong Pt–Sn bond formation and diminished Sn–O interactions.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

“…The metal–support interaction also has a profound influence on the electronic structures of bimetallic nanoclusters because of the charge transfer between the nanoclusters and the support. For example, after deposition of Pt atoms on SnOx/TiO 2 surface, in which the SnOx species exist as single-layer structures, SnOx is partially transformed into metallic Sn, giving to the formation of Pt–Sn bonding . The electron transfer from Sn species to the TiO 2 support causes the dipole–dipole repulsion of Sn atoms and the sintering of the PtSn clusters is therefore suppressed.…”

“…For example, after deposition of Pt atoms on SnOx/TiO 2 surface, in which the SnOx species exist as single-layer structures, SnOx is partially transformed into metallic Sn, giving to the formation of Pt−Sn bonding. 38 The electron transfer from Sn species to the TiO 2 support causes the dipole−dipole repulsion of Sn atoms and the sintering of the PtSn clusters is therefore suppressed. When the support is changed from inorganic to organic carrier (e.g., polymers, metal−organic frameworks), the electron-transfer from the functional groups to the bimetallic nanoclusters will be largely different, although the electronic features of the polymer-encapsulated and MOFencapsulated bimetallic nanoclusters have not been systematically compared with those supported on conventional inorganic carriers.…”

Silica-Confined Pt3Sn Clusters for Propane Dehydrogenation

“…The binding energy at 485.1 eV evidenced Sn 0 while that at 486.8 eV referred to Sn 2+ / Sn 4+ . 41,42 The surface Pt/Sn ratio, estimated from the total XPS peak area, was 3.6 for Pt 0.8 Sn 0.2 @SiO 2 and 1.0 for Pt 0.6 Sn 0.4 @SiO 2 .…”

The Al-Containing Silicates Modified with Organic Ligands and SnO2 Nanoparticles for Catalytic Baeyer-Villiger Oxidation and Aerobic Carboxylation of Carbonyl Compounds

“…It is interesting to observe the chemical state of Sn on the surface of C3, which may show additional information on the catalytic component of C3. The C3 showed two peaks at 495.8 eV and 487.3 eV on the Sn 3d region ( Figure 4 ), representing binding energies of Sn 3d 3/2 and 3d 5/2 photoelectrons, respectively, corresponding to the Sn 4+ that is contained in the SnO 2 phase [ 35 ]. In comparison with previously reported data, the binding energy of Sn 3d 5/2 photoelectron of Sn 2+ was found to be 485.8 eV, and that of metallic Sn was to be 485.3 eV [ 36 ].…”