Desorption kinetics from a surface derived from direct imaging of the adsorbate layer

There are numerous indications that adsorbed particles on a surface do not desorb statistically, but that their spatial distribution is important. Evidence almost exclusively comes from temperature-programmed desorption, the standard method for measuring desorption rates. However, this method, as a kinetics experiment, cannot uniquely prove an atomic mechanism. Here we report a low-energy electron microscopy investigation in which a surface is microscopically imaged while simultaneously temperature-programmed desorption is recorded. The data show that during desorption of oxygen molecules from a silver single crystal surface, islands of oxygen atoms are present. By correlating the microscopy and the kinetics data, a model is derived that includes the shapes of the islands and assumes that the oxygen molecules desorb from the island edges. The model quantitatively reproduces the complex desorption kinetics, confirming that desorption is affected by islands and that the often used mean-field treatment is inappropriate

Desorption kinetics from a surface derived from direct imaging of the adsorbate layer

There are numerous indications that adsorbed particles on a surface do not desorb statistically, but that their spatial distribution is important. Evidence almost exclusively comes from temperature-programmed desorption, the standard method for measuring desorption rates. However, this method, as a kinetics experiment, cannot uniquely prove an atomic mechanism. Here we report a low-energy electron microscopy investigation in which a surface is microscopically imaged while simultaneously temperature-programmed desorption is recorded. The data show that during desorption of oxygen molecules from a silver single crystal surface, islands of oxygen atoms are present. By correlating the microscopy and the kinetics data, a model is derived that includes the shapes of the islands and assumes that the oxygen molecules desorb from the island edges. The model quantitatively reproduces the complex desorption kinetics, confirming that desorption is affected by islands and that the often used mean-field treatment is inappropriate

The selective species in ethylene epoxidation on silver

Silver s unique ability to selectively oxidize ethylene to ethylene oxide under an oxygen atmosphere has long been known. Today it is the foundation of ethylene oxide manufacturing. Yet, the mechanism of selective epoxide production is unknown. Here we use a combination of ultrahigh vacuum and in situ experimental methods along with theory to show that the only species that has been shown to produce ethylene oxide, the so called electrophilic oxygen appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed SO4. This adsorbate is part of a 2D Ag SO4 phase, where the nonstoichiometric surface variant, with a formally S V species, facilitates selective transfer of an oxygen atom to ethylene. Our results demonstrate the significant and surprising impact of a trace impurity on a well studied heterogeneously catalyzed reactio

Oxidation of Ethylene on Oxygen Reconstructed Silver Surfaces

We report on theoretical and experimental studies of the reactivity of ethylene with oxygen in two well-known oxygen induced surface reconstructions on silver, the p(2 × 1) reconstruction on the Ag(110) surface and the p(4 × 4) reconstruction on the Ag(111) surface. Density functional theory calculations demonstrate that ethylene can react with oxygen on both surfaces to form an oxametallacycle that can decompose into either ethylene oxide or a CO₂ precursor, acetaldehyde. The activation energy associated with acetaldehyde formation is predicted to be 0.4 eV lower than that associated with epoxide formation on both surfaces, though we find lower barriers for all elementary steps on the p(4 × 4) reconstruction due to its unique structural dynamics. Our calculations predict these dynamics make the p(4 × 4) reconstruction active in acetaldehyde formation at room temperature. Experiments performed by exposing the p(4 × 4) reconstruction to ethylene at room temperature support this finding with CO₂, the only carbonaceous product formed during temperature-programmed desorption. Our results unambiguously demonstrate that, alone, these oxygen reconstructions are not selective in ethylene epoxidation on silver

LEED-I(V) Structure Analysis of the (7 × √3)rect SO₄ Phase on Ag(111): Precursor to the Active Species of the Ag-Catalyzed Ethylene Epoxidation

According to a recently proposed mechanism, the silver-catalyzed industrial synthesis of ethylene oxide (EO) involves adsorbed SO4. The O atoms that are added to the ethylene molecules to give EO originate from SO4, which may solve the long-standing question about the active oxygen species in this reaction. Here, we report a low-energy electron diffraction structure analysis of an ordered phase of SO4 on the Ag(111) surface, forming a (7 × √3)rect structure and containing the oxygen species that before had been spectroscopically identified on the active catalyst. Using I(V) data from a low-energy electron microscope and an input model from density functional theory, the complex structure could be solved. It contains SO4 moieties on a reconstructed Ag(111) surface in which all four O atoms bind to Ag atoms. In the proposed ethylene epoxide reaction model, the structure represents the parent phase from which the active SO4 phase is formed by a lifting of the reconstruction

Iron Doped Nickel Oxide Nanocrystals as Highly Efficient Electrocatalysts for Alkaline Water Splitting

Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in <i>tert</i>-butanol leads to the formation of ultrasmall crystalline and highly dispersible Fe_<i>x</i>Ni_1–<i>x</i>O nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5–3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous <i>ca</i>. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe_0.1Ni_0.9O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s^–1 at the overpotential of 300 mV. The current density of 10 mA cm^–2 is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec^–1. The extremely high catalytic activity, facile preparation, and low cost of the single crystalline Fe_<i>x</i>Ni_1–<i>x</i>O nanoparticles make them very promising catalysts for the oxygen evolution reaction