Single-crystal metal oxides and supported metal nanoclusters as model catalyst sytems

We have investigated the morphology and electronic structure of two basic classes of systems: metal oxide surfaces that catalyze the formation of environmentally persistent free radicals (EPFRs) from aromatic precursors, and Au and Cu nanoparticles that may be suitable catalysts for the catalytic oxidation of CO or hydrogenation of CO2. First, we examine the adsorption behavior of phenol on rutile TiO2(110) and ultrathin films of alumina prepared on a NiAl(110) substrate. Electron paramagnetic resonance studies show that exposure of both γ-alumina and titania powder to phenol at 250°C results in the formation of persistent phenoxyl radicals. EELS studies of phenol dosed on single-crystal titania and alumina show that phenol adsorbed at elevated temperature demonstrates a significantly narrower HOMO-LUMO gap than molecular phenol in the gas phase or physisorbed molecular phenol. Ultraviolet photoelectron spectroscopy shows direct evidence of charge transfer from high-temperature adsorbed phenol to electronic states of TiO2(110) usually associated with the accumulation of charge at surface oxygen vacancies, providing direct evidence of a frequently-hypothesized radical formation mechanism. Second, we deposit and characterize Au nanoparticles on the self-assembled hexagonal boron nitride “nanomesh” prepared on a Rh(111) substrate. STM studies show that at all levels of coverage, Au clusters almost always remain confined to the nanomesh “pores” and are restricted in size to \u3c 3 nm diameter. XPS studies suggest that the resulting Au clusters are negatively charged, and for \u3c 1 ML Au coverage, the electronic properties of most of the clusters formed are dominated by final-state effects that arise from the reduced dimensionality of the smallest clusters (one or two Au layers). A similar morphology for Au deposited on ZnO(10-10) has been previously observed; however, we find that the Au-ZnO interaction instead results in positively charged clusters. Cu on ZnO(10-10) grows as three-dimensional clusters even at very small coverage and shows positive charging similar to Au. It is clear that catalytically relevant properties of supported metal nanoclusters are strongly influenced by interactions with the support, even if the cluster morphology is identical for particles on various different substrates

Cold heteromolecular dipolar collisions

We present the first experimental observation of cold collisions between two different species of neutral polar molecules, each prepared in a single internal quantum state. Combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling allows the enhancement of molecular interaction time by 10$^5$. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND$_3$ at a mean collision energy of 3.6 cm$^{-1}$ (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from \emph{ab initio} potential energy surfaces are in excellent agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a $^2\Pi$ radical and a closed-shell polyatomic molecule.Comment: 7 pages, 5 figure

Sulfur monoxide thermal release from an anthracene-based precursor, spectroscopic identification, and transfer reactivity

Sulfur monoxide (SO) is a highly reactive molecule and thus, eludes bulk isolation. We report here on synthesis and reactivity of a molecular precursor for SO generation, namely 7-sulfin-ylamino-7-azadibenzonorbornadiene (1). This compound has been shown to fragment readily driven by dinitrogen expulsion and anthracene formation on heating in the solid state and in solution, releasing SO at mild temperatures (<100 ◦C). The generated SO was detected in the gas phase by MS and rotational spectroscopy. In solution, 1 allows for SO transfer to organic molecules as well as transition metal complexes. Keywords: microwave spectroscopy; reactive intermediate; molecular precursor; astrochemistry; sulfur monoxid

Reviewer Acknowledgements 2019

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