Anisotropy of Chemical Bonding in Semifluorinated Graphite C₂F 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 BrF₃ with Br₂ at room temperature. The golden-colored product, easily delaminating into micrometer-size transparent flakes, is an intercalation compound where Br₂ molecules are hosted between fluorinated graphene layers of approximate C₂F 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 C₂F 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 MoS₂ Clusters on Graphene Flakes for Catalytic Formic Acid Decomposition

MoS₂ was deposited on graphene flakes via decomposition of MoS₃ 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 MoS₂ 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 MoS₂ clusters and formation of thin crystalline MoS₂ particles 20–30 nm in size. The sample enriched with the MoS₂ clusters showed 6 times higher catalytic activity at 160 °C than the sample with the crystalline MoS₂ particles. This demonstrates that the observed nanometer-sized MoS₂ 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 (C₃₀H₆₂) in supercritical CO₂ 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 sp² and sp³ 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 % HNO₃ 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 sp² 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 HNO₃ treatment. Palladium was deposited by impregnation with Pd acetate followed by reduction in H₂. 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 H₂ formation (98–99%) than those for the catalyst based on the HNO₃ 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 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