Supported Platinum Nanoparticles Catalyzed Carbon-Carbon Bond Cleavage of Polyolefins: Role of the Oxide Support Acidity.

Supported platinum nanoparticle catalysts are known to convert polyolefins to high-quality liquid hydrocarbons using hydrogen under relatively mild conditions. To date, few studies using platinum grafted onto various metal oxide (MxOy) supports have been undertaken to understand the role of the acidity of the oxide support in the carbon-carbon bond cleavage of polyethylene under consistent catalytic conditions. Specifically, two Pt/MxOy catalysts (MxOy = SrTiO3 and SiO2-Al2O3; Al = 3.0 wt %, target Pt loading 2 wt % Pt ∼1.5 nm), under identical catalytic polyethylene hydrogenolysis conditions (T = 300 °C, P(H2) = 170 psi, t = 24 h; Mw = ∼3,800 g/mol, Mn = ∼1,100 g/mol, D = 3.45, Nbranch/100C = 1.0), yielded a narrow distribution of hydrocarbons with molecular weights in the range of lubricants (Mw = < 600 g/mol; Mn < 400 g/mol; D = 1.5). While Pt/SrTiO3 formed saturated hydrocarbons with negligible branching, Pt/SiO2-Al2O3 formed partially unsaturated hydrocarbons (<1 mol % alkenes and ∼4 mol % alkyl aromatics) with increased branch density (Nbranch/100C = 5.5). Further investigations suggest evidence for a competitive hydrocracking mechanism occurring alongside hydrogenolysis, stemming from the increased acidity of Pt/SiO2-Al2O3 compared to Pt/SrTiO3. Additionally, the products of these polymer deconstruction reactions were found to be independent of the polyethylene feedstock, allowing the potential to upcycle polyethylenes with various properties into a value-added product.

Adsorption and Thermal Decomposition of Triphenyl Bismuth on Silicon (001).

We investigate the adsorption and thermal decomposition of triphenyl bismuth (TPB) on the silicon (001) surface using atomic-resolution scanning tunneling microscopy, synchrotron-based X-ray photoelectron spectroscopy, and density functional theory calculations. Our results show that the adsorption of TPB at room temperature creates both bismuth-silicon and phenyl-silicon bonds. Annealing above room temperature leads to increased chemical interactions between the phenyl groups and the silicon surface, followed by phenyl detachment and bismuth subsurface migration. The thermal decomposition of the carbon fragments leads to the formation of silicon carbide at the surface. This chemical understanding of the process allows for controlled bismuth introduction into the near surface of silicon and opens pathways for ultra-shallow doping approaches.

Structural and reactive evolution of oxidatively grafted Pd catalysts on MnO2 for the low-temperature oxidation of CO.

Isolated Pd atoms supported on high surface area MnO2, prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) oxidation of CO (7.7 kPa O2, 2.6 kPa CO) with results of in situ/operando and ex situ spectroscopic characterization signifying a synergistic role of Pd and MnO2 in facilitating redox turnovers.

Formation of silica-supported platinum nanoparticles as a function of preparation conditions and boron impregnation.

Preparation of supported metal nanoparticles for catalytic applications often relies on an assumption that the initially prepared wet-impregnated support material is covered with approximately a monolayer of adsorbed species that are shaped into the target nanoparticulate material with a desired size distribution by utilizing appropriate post-treatments that often include calcination and reduction schemes. Here, the formation and evolution of surface nanoparticles were investigated for wet-chemistry deposition of platinum from trimethyl(methylcyclopentadienyl)platinum (IV) precursor onto flat silica supports to interrogate the factors influencing the initial stages of nanoparticle formation. The deposition was performed on silicon-based substrates, including hydroxylated silica (SiO2) and boron-impregnated hydroxylated silica (B/SiO2) surfaces. The deposition resulted in the immediate formation of Pt-containing nanoparticles, as confirmed by atomic force microscopy and x-ray photoelectron spectroscopy. The prepared substrates were later reduced at 550 °C under H2 gas environment. This reduction procedure resulted in the formation of metallic Pt particles. The reactivity of the precursor and dispersion of Pt nanoparticles on the OH-terminated silica surface were compared to those on the B-impregnated surface. The size distribution of the resulting nanoparticles as a function of surface preparation was evaluated, and density functional theory calculations were used to explain the differences between the two types of surfaces investigated.