Despite the industrial importance of copper oxides, the nature of the (100) surface of Cu 2 O has remained poorly understood. The surface has previously been subject to several theoretical and experimental studies, but has until now not been investigated by atomically resolved microscopy or high-resolution photoelectron spectroscopy. Here we determine the atomic structure and electronic properties of Cu 2 O(100) by a combination of multiple experimental techniques and simulations within the framework of density functional theory (DFT). Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) characterized the three ordered surface structures found. From DFT calculations, the structures are found to be energetically ordered as (3,0;1,1), c(2 × 2), and (1 × 1) under ultrahigh vacuum conditions. Increased oxygen pressures induce the formation of an oxygen terminated (1 × 1) surface structure. The most common termination of Cu 2 O(100) has previously been described by a (3√2 × √2)R45°unit cell exhibiting a LEED pattern with several missing spots. Through atomically resolved STM, we show that this structure instead is described by the matrix (3,0;1,1). Both simulated STM images and calculated photoemission core level shifts compare favorably with the experimental results.
An
attractive solution to mitigate tars and also to decompose lighter
hydrocarbons in biomass gasification is secondary catalytic reforming,
converting hydrocarbons to useful permanent gases. Albeit that it
has been in use for a long time in fossil feedstock catalytic steam
reforming, understanding of the catalytic processes is still limited.
Naphthalene is typically present in the biomass gasification gas and
to further understand the elementary steps of naphthalene transformation,
we investigated the temperature dependent naphthalene adsorption,
dehydrogenation and passivation on Ni(111). TPD (temperature-programmed
desorption) and STM (scanning tunneling microscopy) in ultrahigh vacuum
environment from 110 K up to 780 K, combined with DFT (density functional
theory) were used in the study. Room temperature adsorption results
in a flat naphthalene monolayer. DFT favors the dibridge[7] geometry
but the potential energy surface is rather smooth and other adsorption
geometries may coexist. DFT also reveals a pronounced dearomatization
and charge transfer from the adsorbed molecule into the nickel surface.
Dehydrogenation occurs in two steps, with two desorption peaks at
approximately 450 and 600 K. The first step is due to partial dehydrogenation
generating active hydrocarbon species that at higher temperatures
migrates over the surface forming graphene. The graphene formation
is accompanied by desorption of hydrogen in the high temperature TPD
peak. The formation of graphene effectively passivates the surface
both for hydrogen adsorption and naphthalene dissociation. In conclusion,
the obtained results on the model naphthalene and Ni(111) system,
provides insight into elementary steps of naphthalene adsorption,
dehydrogenation, and carbon passivation, which may serve as a good
starting point for rational design, development and optimization of
the Ni catalyst surface, as well as process conditions, for the aromatic
hydrocarbon reforming process.
Surface coated magnetite nanoparticles (Fe3O4 NPs) with 3-mercaptopropionic acid were immobilized on amidoximated polyacrilonitrile (APAN) nanofibers using electrospinning followed by crosslinking.
The temperature dependent dehydrogenation of naphthalene on Ni(111) has been investigated using vibrational sum-frequency generation spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory with the aim of discerning the reaction mechanism and the intermediates on the surface. At 110 K, multiple layers of naphthalene adsorb on Ni(111); the first layer is a flat lying chemisorbed monolayer, whereas the next layer(s) consist of physisorbed naphthalene. The aromaticity of the carbon rings in the first layer is reduced due to bonding to the surface Ni-atoms. Heating at 200 K causes desorption of the multilayers. At 360 K, the chemisorbed naphthalene monolayer starts dehydrogenating and the geometry of the molecules changes as the dehydrogenated carbon atoms coordinate to the nickel surface; thus, the molecule tilts with respect to the surface, recovering some of its original aromaticity. This effect peaks at 400 K and coincides with hydrogen desorption. Increasing the temperature leads to further dehydrogenation and production of H 2 gas, as well as the formation of carbidic and graphitic surface carbon.
Reduction
of Cu2O by hydrogen is a common preparation
step for heterogeneous catalysts; however, a detailed understanding
of the atomic reaction pathways is still lacking. Here, we investigate
the interaction of atomic hydrogen with the Cu2O(100):(3,0;1,1)
and Cu2O(111):(√3 × √3)R30° surfaces using scanning tunneling microscopy (STM), low-energy
electron diffraction, temperature-programmed desorption (TPD), and
X-ray photoelectron spectroscopy (XPS). The experimental results are
compared to density functional theory simulations. At 300 K, we identify
the most favorable adsorption site on the Cu2O(100) surface:
hydrogen atoms bind to an oxygen site located at the base of the atomic
rows intrinsic to the (3,0;1,1) surface. The resulting hydroxyl group
subsequently migrates to a nearby Cu trimer site. TPD analysis identifies
H2 as the principal desorption product. These observations
imply that H2 is formed through a disproportionation reaction
of surface hydroxyl groups. The interaction of H with the (111) surface
is more complex, including coordination to both Cu+ and
OCUS sites. STM and XPS analyses reveal the formation of
metallic copper clusters on the Cu2O surfaces after cycles
of hydrogen exposure and annealing. The interaction of the Cu clusters
with the substrate is notably different for the two surface terminations
studied: after annealing, the Cu clusters coalesce on the (100) termination,
and the (3,0;1,1) reconstruction is partially recovered. Clusters
formed on the (111) surface are less prone to coalescence, and the
(√3 × √3)R30° reconstruction
was not recovered by heat treatment, indicating a weaker Cu cluster
to support interaction on the (100) surface.
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