NiFe and CoFe nanocatalysts supported on highly dispersed alumina-silica: Structure, surface properties, and performance in CO2 methanation.

The hydrogenation of CO2 to CH4 has gained considerable interest in terms of sustainable energy and environmental mitigation. In this regard, the present work aims to investigate the adsorptive concentration and CO2 methanation performance over CoFe and NiFe bimetallic catalysts supported on fumed alumina-silica SA96 support at 170-450 °C and under atmospheric pressure. The catalysts were prepared by wet impregnation method, subjected to calcination and further reduced with hydrogen, and their performance in CO2 methanation was investigated in a hydrogen-rich 2%CO2-55%H2-43%He gas mixture. In this study, we describe the crystal and mesoporous structures of the prepared catalysts by in-situ XRD and ex-situ nitrogen adsorption, evaluate the NiFe and CoFe metal surface states before and after catalysis by XPS, visualize the surface morphology by SEM, estimate the catalytic activity by gas chromatography, and investigate the adsorbed surface species, showing the presence of *HCOO/*HCO and *CO intermediates, determine two possible pathways of CH4 formation on the studied catalysts by temperature-programmed desorption mass spectrometry, and correlate the structural and surface properties with high CO2 conversions up to 100% and methanation selectivities up to 72%. The latter is related to changes in the elemental chemical states and surface composition of CoFe and NiFe nanocatalysts induced by treatment under reaction conditions, and the surface reconstruction during catalysis transfers the part of active 3d transition metals into the pores of the SA96 support. Our thorough characterization study with complementary techniques allowed us to conclude that this high activity is related to the formation of catalytically active Ni/Ni3Fe and Co/CoFeOx nanoscale crystallites under H2 reduction and their maintenance under CO2 methanation conditions. The successfully applied combination of CO2 chemisorption and thermodesorption techniques demonstrates the ability to adsorb the CO2 molecules by supported NiFe and CoFe nanocatalysts and the pure alumina-silica SA96 support.

Fluorine and Bromine Dual-Doped Nanoporous Carbons: Preparation and Surface Chemistry Studies.

A novel method for the concurrent introduction of fluorine and bromine into the surface of nanoporous activated carbon (NAC) is evaluated. According to the method, the preheated NAC was treated with 1,2-dibromotetrafluoroethane at elevated temperatures (400-800 °C). Potentiometric and elemental analysis, nitrogen adsorption-desorption, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and 19F solid-state NMR were used to study the NAC microstructure and changes in surface chemistry. The specific modification temperature was found to have a decisive influence on the resulting halogen content of the NAC surface. About 1.5 mmol g-1 of bromine and only 0.5 mmol g-1 of fluorine are chemisorbed on the NAC surface when dual-doped at 400 °C. The fluorination efficiency increases dramatically to 1.84-2.22 mmol g-1 when the process temperature is increased to 500-700 °C. Under the same conditions, the bromination efficiency unexpectedly decreases to 0.66-1.32 mmol g-1. Since halogen-containing groups undergo significant thermal decomposition around 800 °C, the overall halogenation efficiency decreases, accordingly. Both the volume and surface area of the micropores decrease moderately when halogen-containing groups are introduced into the carbon surface layer. Fluorine and bromine are unevenly distributed in the porous structure of the dual-doped NACs, and the outer surface is more halogen-rich than the inner surface of the micropores. XPS and 19F solid-state NMR revealed the selective formation of CF2 groups in the NAC surface layer independent of the temperature. In contrast, the percentage of semi-ionic fluorine in the form of CF groups directly bonded to the π-electron system of the carbon matrix increases significantly with temperature.

Surface chemistry and catalytic activity in H2O2 decomposition of pyrolytically fluoralkylated activated carbons.

According to the proposed pyrolytic method, granular activated carbon (AC) Norit 830 W was functionalized by thermal treatment of AC in hydrofluorocarbon (HFC) gases, pentafluoroethane and 1,1,1,2-tetrafluoroethane, at 400-800 °C. This method does not require activation by plasma and photons. Chemical and elemental analysis showed that the pyrolytic treatment provides a loading of 2.95 mmol (5.6 wt%) of fluorine per gram of AC. Nitrogen adsorption measurements indicated that the microporous structure contracted when AC was treated with HFC at temperatures above 400 °C. Thermogravimetry, Fourier transform infrared spectroscopy (FTIR) with attenuated total reflectance (ATR), and X-ray photoelectron spectroscopy (XPS) demonstrated the evolution of oxygen-containing and fluorine-containing groups to more thermostable groups with treatment temperature. The fluorine-containing groups grafted at high temperature, above 600 °C exhibited the highest thermal stability up to 1250 °C in dry argon. From the data of XPS and solid-state 19F nuclear magnetic resonance spectroscopy data, the grafted fluorine exists in several types of grafted F-containing groups, the HFC residues. By changing the thermal regime of fluorination, the composition of fluorine-containing groups on a carbon surface can be regulated. Isolated fluoroalkyl groups can be grafted at temperatures of 400-500 °C, while at 600 °C and above, the semi-ionic fluorine groups increase significantly. The hydrophobized surface demonstrated the ability to effectively decompose H2O2 in methanol solutions.

Surface Response of Brominated Carbon Media on Laser and Thermal Excitation: Optical and Thermal Analysis Study.

The present study is objected to develop an analytical remote optical diagnostics of the functionalized carbons surface. Carbon composites with up to 1 mmol g-1 of irreversibly adsorbed bromine were produced by the room temperature plasma treatment of an activated carbon fabric (ACF) derived from polyacrylonitrile textile. The brominated ACF (BrACF) was studied by elastic optical scattering indicatrix analysis at wavelength 532 nm. The obtained data were interpreted within results of the thermogravimetric analysis, X-ray photoelectron spectroscopy and temperature programmed desorption mass spectrometry. The bromination dramatically reduces the microporosity producing practically non-porous material, while the incorporated into the micropores bromine induces the dielectric and structural impact on surface polarizability and conductivity due to the charging effect. We have found that the elastic optical scattering in proper solid angles in the forward and the backward hemispheres is sensitive to the kind of the bromine bonding, e.g., physical adsorption or chemisorption, and the bromination level, respectively, that can be utilized for the express remote fabrication control of the nanoscale carbons with given interfaces.