7 research outputs found
Graphene-Supported Pd Nanoclusters Probed by Carbon Monoxide Adsorption
The
adsorption of CO on graphene-supported Pd nanoparticles was studied
in situ with high-resolution synchrotron-based X-ray photoelectron
spectroscopy. At 150 K, CO adsorbs mainly in bridge and 3-fold-hollow
sites. The nanoparticles are considered as a mixture of low-index
facets. The variation of the amount of deposited Pd revealed identical
CO adsorption behavior for all investigated cases, confirming a similar
average cluster size over a wide range of Pd coverages. The desorption
characteristics were studied with temperature-programmed XPS. The
observed desorption maxima at 230 and 430 K are in good agreement
with temperature-programmed desorption data on stepped Pd single crystals.
At 500 K, CO is completely desorbed from the Pd clusters. The adsorption
and desorption of CO are found to be not fully reversible as the Pd
particles undergo restructuring upon heating
Graphene-Templated Growth of Pd Nanoclusters
Graphene grown on Rh(111) was used
as a template for the growth
of Pd nanoclusters. Using high-resolution synchrotron radiation-based
X-ray photoelectron spectroscopy, we studied the deposition of Pd
on corrugated graphene in situ. From the XP spectra, we deduce a cluster-by-cluster
growth mode. The formation of clusters with 3 nm diameter was confirmed
by low-temperature scanning tunneling microscopy measurements. The
investigation of the thermal stability of the Pd particles showed
three characteristic temperature regimes: Up to 550 K restructuring
of the particles takes place, between 550 and 750 K the clusters coalesce
into larger agglomerates, and finally between 750 and 900 K Pd intercalates
between the graphene layer and the Rh surface
Unraveling the Effect of Rh Isolation on Shallow d States of GalliumāRhodium Alloys
In this study, we report the electronic and chemical
structure
of supported GaRh alloys as model systems for the active phase in
supported catalytically active liquid metal solutions (SCALMS). We
prepared a series of galliumārhodium samples with different
Rh contents and tracked the evolution of the sample topography and
surface electronic structure via photoemission spectroscopy in combination
with ab initio calculations and electron microscopy. Our results reveal
a characteristic shift of the Rh 3d core levels and narrowing and
shifting of the Rh 4d derived band with decreasing Rh content. Calculations
show that these spectroscopic observations can be explained by the
coexistence of isolated Rh atoms in random GaRh alloys and GaRh intermetallic
compounds (IMCs). These results contribute to an enhancement of the
fundamental understanding of the electronic surface structure of GaRh
alloys, which is crucially required for apprehending and thus further
exploiting the improved catalytic activity of GaRh SCALMS
Photochemical Energy Storage and Electrochemically Triggered Energy Release in the NorbornadieneāQuadricyclane System: UVĀ Photochemistry and IR Spectroelectrochemistry in a Combined Experiment
The
two valence isomers norbornadiene (NBD) and quadricyclane (QC)
enable solar energy storage in a single molecule system. We present
a new photoelectrochemical infrared reflection absorption spectroscopy
(PEC-IRRAS) experiment, which allows monitoring of the complete energy
storage and release cycle by in situ vibrational spectroscopy. Both
processes were investigated, the photochemical conversion from NBD
to QC using the photosensitizer 4,4ā²-bisĀ(dimethylamino)Ābenzophenone
(Michlerās ketone, MK) and the electrochemically triggered
cycloreversion from QC to NBD. Photochemical conversion was obtained
with characteristic conversion times on the order of 500 ms. All experiments
were performed under full potential control in a thin-layer configuration
with a Pt(111) working electrode. The vibrational spectra of NBD,
QC, and MK were analyzed in the fingerprint region, permitting quantitative
analysis of the spectroscopic data. We determined selectivities for
both the photochemical conversion and the electrochemical cycloreversion
and identified the critical steps that limit the reversibility of
the storage cycle
Graphene on Ni(111): Coexistence of Different Surface Structures
A combined high-resolution X-ray photoelectron spectroscopy (HR-XPS) and ab initio density functional theory study on graphene on Ni(111) shows the coexistence of two structures, a bridge-top and a top-fcc structure, that have almost identical energies according to DFT calculations. Consequently, both geometries are detected simultaneously on the Ni(111) surface by HR-XPS, while their relative fractions depend on minor surface defect concentrations (pinning sites). The two structures are identified due to their different core level shifts that are in line with DFT calculations
Carbon Dioxide Capture by an Amine Functionalized Ionic Liquid: Fundamental Differences of Surface and Bulk Behavior
Carbon
dioxide (CO<sub>2</sub>) absorption by the amine-functionalized
ionic liquid (IL) dihydroxyethyldimethylammonium taurinate at 310 K was studied
using surface-
and bulk-sensitive experimental techniques. From near-ambient pressure
X-ray photoelectron spectroscopy at 0.9 mbar CO<sub>2</sub>, the amount
of captured CO<sub>2</sub> per mole of
IL in the near-surface region is quantified to ā¼0.58 mol, with
ā¼0.15 mol in form of carbamate dianions and ā¼0.43 mol
in form of carbamic acid. From isothermal uptake experiments combined
with infrared spectroscopy, CO<sub>2</sub> is found to be bound in
the bulk as carbamate (with nominally 0.5 mol of CO<sub>2</sub> bound
per 1 mol of IL) up to ā¼2.5 bar CO<sub>2</sub>, and as carbamic
acid (with nominally 1 mol CO<sub>2</sub> bound per 1 mol IL) at higher
pressures. We attribute the fact that at low pressures carbamic acid
is the dominating species in the near-surface region, while only carbamate
is formed in the bulk, to differences in solvation in the outermost
IL layers as compared to the bulk situation
Size and Structure Effects Controlling the Stability of the Liquid Organic Hydrogen Carrier Dodecahydroā<i>N</i>āethylcarbazole during Dehydrogenation over Pt Model Catalysts
Hydrogen
can be stored conveniently using so-called liquid organic
hydrogen carriers (LOHCs), for example, <i>N</i>-ethylcarbazole
(NEC), which can be reversibly hydrogenated to dodecahydro-<i>N</i>-ethylcarbazole (H<sub>12</sub>-NEC). In this study, we
focus on the dealkylation of H<sub>12</sub>-NEC, an undesired side
reaction, which competes with dehydrogenation. The structural sensivity
of dealkylation was studied by high-resolution X-ray photoelectron
spectroscopy (HR-XPS) on Al<sub>2</sub>O<sub>3</sub>-supported Pt
model catalysts and Pt(111) single crystals. We show that the morphology
of the Pt deposit strongly influences LOHC degradation via CāN
bond breakage. On smaller, defect-rich Pt particles, the onset of
dealkylation is shifted by 90 K to lower temperatures as compared
to large, well-shaped particles and well-ordered Pt(111). We attribute
these effects to a reduced activation barrier for CāN bond
breakage at low-coordinated Pt sites, which are abundant on small
Pt aggregates but are rare on large particles and single crystal surfaces