6 research outputs found
Anisotropy of Chemical Bonding in Semifluorinated Graphite C<sub>2</sub>F Revealed with Angle-Resolved X‑ray Absorption Spectroscopy
Highly oriented pyrolytic graphite characterized by a low misorientation of crystallites is fluorinated using a gaseous mixture of BrF<sub>3</sub> with Br<sub>2</sub> at room temperature. The golden-colored product, easily delaminating into micrometer-size transparent flakes, is an intercalation compound where Br<sub>2</sub> molecules are hosted between fluorinated graphene layers of approximate C<sub>2</sub>F composition. To unravel the chemical bonding in semifluorinated graphite, we apply angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and quantum-chemical modeling. The strong angular dependence of the CK and FK edge NEXAFS spectra on the incident radiation indicates that room-temperature-produced graphite fluoride is a highly anisotropic material, where half of the carbon atoms are covalently bonded with fluorine, while the rest of the carbon atoms preserve π electrons. Comparison of the experimental CK edge spectrum with theoretical spectra plotted for C<sub>2</sub>F models reveals that fluorine atoms are more likely to form chains. This conclusion agrees with the atomic force microscopy observation of a chain-like pattern on the surface of graphite fluoride layers
Heat-Induced Dip of Optical Limiting Threshold in Carbon Nanotube Aqueous Suspension
Carbon
nanotube (CNT) suspensions possess nonlinear light scattering,
which leads to a manifestation of the optical limiting (OL) phenomenon.
Here we present the results of research into the temperature influence
on the OL of multiwalled CNT suspensions. The experiments were carried
out with a stable aqueous suspension of chemically treated CNTs synthesized
by graphite arc evaporation. The OL study was performed by a z-scan
technique using a 532 nm single-mode nanosecond laser. An increase
in the suspension temperature by 17 °C from 23 to 40 °C
was found to result in 3 times reduction of the OL threshold intensity
of the suspension. Upon further heating, the OL threshold remains
constant. The analysis of the results obtained is presented. Our experimental
findings allow one to reveal the optimal temperature range for an
optical limiter based on aqueous CNT suspension
Nanometer-Sized MoS<sub>2</sub> Clusters on Graphene Flakes for Catalytic Formic Acid Decomposition
MoS<sub>2</sub> was deposited on graphene flakes via decomposition
of MoS<sub>3</sub> in vacuum at different temperatures (500–800
°C). The materials obtained were tested for catalytic formic
acid decomposition, giving mainly hydrogen and carbon dioxide. According
to atom-resolved transmission electron microscopy study, a considerable
amount of MoS<sub>2</sub> clusters with a mean size of 1 nm was formed
on the graphene surface at 500 °C. Simulation of the structure
of a cluster revealed the presence of Mo-edge atoms. Raising the preparation
temperature up to 800 °C led to agglomeration of MoS<sub>2</sub> clusters and formation of thin crystalline MoS<sub>2</sub> particles
20–30 nm in size. The sample enriched with the MoS<sub>2</sub> clusters showed 6 times higher catalytic activity at 160 °C
than the sample with the crystalline MoS<sub>2</sub> particles. This
demonstrates that the observed nanometer-sized MoS<sub>2</sub> clusters
are responsible for catalysis
Pd Clusters Supported on Amorphous, Low-Porosity Carbon Spheres for Hydrogen Production from Formic Acid
Amorphous, low-porosity carbon spheres
on the order of a few micrometers in size were prepared by carbonization
of squalane (C<sub>30</sub>H<sub>62</sub>) in supercritical CO<sub>2</sub> at 823 K. The spheres were characterized and used as catalysts’
supports for Pd. Near-edge X-ray absorption fine structure studies
of the spheres revealed sp<sup>2</sup> and sp<sup>3</sup> hybridized
carbon. To activate carbons for interaction with a metal precursor,
often oxidative treatment of a support is needed. We showed that boiling
of the obtained spheres in 28 wt % HNO<sub>3</sub> did not affect
the shape and bulk structure of the spheres, but led to creation of
a considerable amount of surface oxygen-containing functional groups
and increase of the content of sp<sup>2</sup> hybridized carbon on
the surface. This carbon was seen by scanning transmission electron
microscopy in the form of waving graphene flakes. The H/C atomic ratio
in the spheres was relatively high (0.4) and did not change with the
HNO<sub>3</sub> treatment. Palladium was deposited by impregnation
with Pd acetate followed by reduction in H<sub>2</sub>. This gave
uniform Pd clusters with a size of 2–4 nm. The Pd supported
on the original C spheres showed 2–3 times higher catalytic
activity in vapor phase formic acid decomposition and higher selectivity
for H<sub>2</sub> formation (98–99%) than those for the catalyst
based on the HNO<sub>3</sub> treated spheres. Using of such low-porosity
spheres as a catalyst support should prevent mass transfer limitations
for fast catalytic reactions
Single isolated Pd2+ cations supported on N-Doped carbon as active sites for hydrogen production from formic acid decomposition
Single-site heterogeneous catalysis with isolated Pd atoms was reported earlier, mainly for oxidation reactions and for Pd catalysts supported on oxide surfaces. In the present work, we show that single Pd atoms on nitrogen-functionalized mesoporous carbon, observed by aberration-corrected scanning transmission electron microscopy (ac STEM), contribute significantly to the catalytic activity for hydrogen production from vapor-phase formic acid decomposition, providing an increase by 2-3 times in comparison to Pd catalysts supported on nitrogen-free carbon or unsupported Pd powder. Some gain in selectivity was also achieved. According to X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) studies after ex situ reduction in hydrogen at 573 K, these species exist in a Pd2+ state coordinated by nitrogen species of the support. Extended density functional theory (DFT) calculations confirm that an isolated Pd atom can be the active site for the reaction, giving decomposition of the formic acid molecule into an adsorbed hydrogen atom and a carboxyl fragment, but only if it is coordinated by a pair of pyridinic-type nitrogen atoms located on the open edge of the graphene sheet. Hence, the role of the N-doping of the carbon support is the formation and stabilization of the new active Pd sites. A long-term experiment performed for more than 30 h on stream indicated an excellent stability of these Pd species in the reaction
Single Isolated Pd<sup>2+</sup> Cations Supported on N‑Doped Carbon as Active Sites for Hydrogen Production from Formic Acid Decomposition
Single-site heterogeneous catalysis
with isolated Pd atoms was
reported earlier, mainly for oxidation reactions and for Pd catalysts
supported on oxide surfaces. In the present work, we show that single
Pd atoms on nitrogen-functionalized mesoporous carbon, observed by
aberration-corrected scanning transmission electron microscopy (ac
STEM), contribute significantly to the catalytic activity for hydrogen
production from vapor-phase formic acid decomposition, providing an
increase by 2–3 times in comparison to Pd catalysts supported
on nitrogen-free carbon or unsupported Pd powder. Some gain in selectivity
was also achieved. According to X-ray photoelectron spectroscopy (XPS)
and near-edge X-ray absorption fine structure (NEXAFS) studies after
ex situ reduction in hydrogen at 573 K, these species exist in a Pd<sup>2+</sup> state coordinated by nitrogen species of the support. Extended
density functional theory (DFT) calculations confirm that an isolated
Pd atom can be the active site for the reaction, giving decomposition
of the formic acid molecule into an adsorbed hydrogen atom and a carboxyl
fragment, but only if it is coordinated by a pair of pyridinic-type
nitrogen atoms located on the open edge of the graphene sheet. Hence,
the role of the N-doping of the carbon support is the formation and
stabilization of the new active Pd sites. A long-term experiment performed
for more than 30 h on stream indicated an excellent stability of these
Pd species in the reaction