We have synthesized model hydrophobic silicone thin films on gold surfaces by a two-step covalent grafting procedure. An amino-functionalized gold surface reacts with monoepoxy-terminated polydimethylsiloxane (PDMS) via a click reaction, resulting in a covalently attached nanoscale thin film of PDMS, and the click chemistry synthesis route provides great selectivity, reproducibility, and stability in the resulting model hydrophobic silicone thin films. The asymmetric interaction forces between the PDMS thin films and mica surfaces were measured with the surface forces apparatus in aqueous sodium chloride solutions. At an acidic pH of 3, attractive interactions are measured, resulting in instabilities during both approach (jump-in) and separation (jump-out from adhesive contact). Quantitative analysis of the results indicates that the DerjaguinÀLandauÀVerweyÀOverbeek theory alone, i.e., the combination of electrostatic repulsion and van der Waals attraction, cannot fully describe the measured forces and that the additional measured adhesion is likely due to hydrophobic interactions. The surface interactions are highly pH-dependent, and a basic pH of 10 results in fully repulsive interactions at all distances, due to repulsive electrostatic and steric-hydration interactions, indicating that the PDMS is negatively charged at high pH. We describe an interaction potential with a parameter, known as the Hydra parameter, that can account for the extra attraction (low pH) due to hydrophobicity as well as the extra repulsion (high pH) due to hydrophilic (steric-hydration) interactions. The interaction potential is general and provides a quantitative measure of interfacial hydrophobicity/hydrophilicity for any set of interacting surfaces in aqueous solution.

Combining advantages of homogeneous and heterogeneous catalysis by incorporating active species on a solid support is often an effective strategy for improving overall catalyst performance, although the influences of the support are generally challenging to establish, especially at a molecular level. Here, we report the local compositions, and structures of platinum species incorporated into covalent triazine framework (Pt-CTF) materials, a solid analogue of the molecular Periana catalyst, Pt(bpym)Cl₂, both of which are active for the selective oxidation of methane in the presence of concentrated sulfuric acid. By using a combination of solid-state 195Pt nuclear magnetic resonance (NMR) spectroscopy, aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), important similarities and differences are observed between the Pt-CTF and Periana catalysts, which are likely related to their respective macroscopic reaction properties. In particular, wide-line solid-state 195Pt NMR spectra enable direct measurement, identification, and quantification of distinct platinum species in as-synthesized and used Pt-CTF catalysts. The results indicate that locally ordered and disordered Pt sites are present in as-synthesized Pt-CTF, with the former being similar to one of the two crystallographically distinct Pt sites in crystalline Pt(bpym)Cl₂. A distribution of relatively disordered Pt moieties is also present in the used catalyst, among which are the principal active sites. Similarly XAS shows good agreement between the measured data of Pt-CTF and a theoretical model based on Pt(bpym)Cl₂. Analyses of the absorption spectra of Pt-CTF used for methane oxidation suggests ligand exchange, as predicted for the molecular catalyst. XPS analyses of Pt(bpym)Cl₂, Pt-CTF, as well as the unmodified ligands, further corroborate platinum coordination by pyridinic N atoms. Aberration-corrected high-angle annular dark-field STEM proves that Pt atoms are distributed within Pt-CTF before and after catalysis. The overall results establish the close similarities of Pt-CTF and the molecular Periana catalyst Pt(bpym)Cl₂, along with differences that account for their respective properties

We demonstrate a preparative method which produces highly-monodisperse Pt-nanoparticles of tunable size without the external addition of seed particles. Hexachloroplatinic acid is dosed slowly to an ethylene glycol solution at 120 ˚C and reduced in the presence of a stabilizing polymer poly-Nvinylpyrollidone (PVP). Slow addition of the Pt-salt first will first lead to the formation of nuclei (seeds) which then grow further to produce larger particles of any desired size between 3 and 8nm. The amount of added hexachloroplatinic acid precursor controls the size of the final nanoparticle product. TEM was used to determine size and morphology and to confirm the crystalline nature of the nanoparticles. Good reproducibility of the technique was demonstrated. Above 7nm, the particle shape and morphology changes suddenly indicating a change in the deposition selectivity of the Pt-precursor from (100) towards (111) crystal faces and breaking up of larger particles into smaller entities.Preparation of size-tunable, highly monodisperse PVP-

Pt nanoparticles (NPs) with submonolayer Ag coverage were synthesized and catalytically tested to investigate the effects of surface Ag and NP shape on C 2 H 2 hydrogenation activity and selectivity. Various NP shapes (cubes, cuboctahedra, and octahedra) in the 6−8 nm range were synthesized using colloidal methods with PVP (polyvinylpyrrolidone) as a capping ligand and Ag + for structure direction. Solid-state 13 C NMR of PVP−NP interactions, as well as electrochemical measurements of oxygen evolution reaction (OER) activity, revealed that submonolayer levels of Ag significantly modify both the physical and electronic structure of the Pt NP surface. Octahedral NP catalysts with 0.5 monolayers of Ag were found to be highly selective for C 2 H 2 -to-C 2 H 4 hydrogenation (C 2 H 4 /C 2 H 6 >8) at reaction rates comparable to total hydrogenation on pure Pt. Traditionally prepared PtAg catalysts synthesized via incipient wetness showed analogously high selectivities but lower site time yields compared to NPs. In contrast, total hydrogenation was observed for Pt, Pd, and PdAg catalysts under the H 2 -rich reaction conditions used. Surface Ag, rather than nanoparticle shape or faceting, was found to be responsible for high selectivity in the PtAg case. This work demonstrates that bimetallic systems with inherent phase segregation (e.g., PtAg) offer a unique route toward thermally stable surface compositions that can promote high activity and selectivity over a large operating window.