Understanding the Growth, Chemical Activity, and Cluster–Support Interactions for Pt–Re Bimetallic Clusters on TiO₂(110)

Abstract

The growth and chemical activity of Re, Pt, and Pt−Re bimetallic clusters supported on TiO 2 (110) have been studied. Pure Re clusters interact strongly with the titania support, resulting in the reduction of the titania surface, and the Re clusters also appear to be partially covered by TiO x at Re coverages as high as 13 ML. Bimetallic clusters can be grown from sequential deposition of Pt and Re in either order at high metal coverages (3.7 ML), where the number of initial nucleation sites is large; in contrast, at lower coverages (0.24 ML), pure Re clusters coexist with Pt−Re clusters for Re deposited on Pt due to the higher nucleation density of Re compared with Pt. The surface composition of the high coverage Pt on Re clusters is ∼100% Pt, but the Re on Pt clusters contain both Pt and Re at the surface after diffusion of some fraction of Re atoms in the bulk. The lower surface free energy of Pt compared to Re makes it thermodynamically favorable for Pt to remain at the surface when Pt is deposited on Re, whereas Re atoms deposited on the Pt clusters will diffuse into the clusters. Isotopic labeling experiments that incorporate 18 O into the titania lattice demonstrate that lattice oxygen participates in both CO oxidation on the Pt on Re bimetallic clusters and recombination of carbon and oxygen to form CO on the Re-containing clusters.

Reshaping, Intermixing, and Coarsening for Metallic Nanocrystals: Nonequilibrium Statistical Mechanical and Coarse-Grained Modeling

“…From this perspective, one expects that 153,176,235 layer ~ L exp[-/(kBT)] hk and eq ~ exp[-/(kBT)] hk L 2 for L << L and Lc. (77) Consequently, in this regime * = 2 consistent with results from the analysis for metal (111) surfaces. A more detailed and rigorous analysis of behavior in both regimes follows from a more comprehensive analysis of evolution in configuration space based on the appropriate master equations.…”

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

“…Ti­(2p), Sn­(3d), and O­(1s) peaks were fit with Gaussian–Lorentzian (GL) line shapes with values for a full width at half-maximum (fwhm) of 1.7–1.8, 1.9–2.2, and 1.5–1.7 eV, respectively. These parameters are consistent with values in the literature and data previously collected with this XPS system . Based on previously published work, Sn­(3d 5/2 ) spectra were fit with two GL peaks with fwhms of 1.9–2.2 eV ,− and O­(1s) spectra were fit by two GL components with fwhms of 1.5–1.7 eV. − …”

“…These parameters are consistent with values in the literature and data previously collected with this XPS system. 42 Based on previously published work, Sn(3d 5/2 ) spectra were fit with two GL peaks with fwhms of 1.9−2.2 eV 19,54−56 and O(1s) spectra were fit by two GL components with fwhms of 1.5−1.7 eV. 57−59 LEIS experiments were conducted with 600 eV He + ions, and collection parameters were chosen to minimize beam damage during data collection.…”

“…Experiments were carried out in two ultrahigh vacuum chambers that have been described in detail elsewhere. The first chamber has a base pressure of 1 × 10 –10 Torr , and is equipped with a variable-temperature scanning tunneling microscope (Omicron VT-25), a single-channel hemispherical analyzer (Omicron EA125) for XPS and LEIS spectroscopy, optics for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES, Omicron Spec 3), a quadrupole mass spectrometer (Leybold-Inficon, Transpector 2), and a load lock chamber for rapid introduction of samples and STM tips. The second chamber , has a base pressure of 1 × 10 –9 Torr and houses a multichannel hemispherical analyzer (Omicron EA2000 SPHERA) for XPS studies, a custom-designed microreactor, a quadrupole mass spectrometer (Stanford Research Systems, RGA 300), and a load lock chamber.…”

Bimetallic Pt–Re Nanoporous Networks: Synthesis, Characterization, and Catalytic Reactivity

“…Thus, Pt enrichment was solely detected in the top layers of the BNN and correlated to the lower surface free energy of Pt compared to Re, which makes it thermodynamically favorable for Pt to remain at the surface when annealed, whereas Re atoms will diffuse to the bulk. 35 Similarly, it was previously demonstrated that the composition of the top atomic layers in bimetallic nanostructures can be widely modified following their exposure to oxidizing and reducing conditions. [36][37] The changes in the composition of the top layers (~3 nm) are eventually balanced by changes in the bulk composition.…”