A comparative study on the effect of carbon fillers on electrical and thermal conductivity of a cyanate ester resin

Carbon fillers including multi-walled carbon nanotubes (MWCNTs), carbon black (CB) and graphite were introduced in a cyanate ester (CE) resin, respectively. The effects of the fillers on the electrical and thermal conductivity of the resin were measured and analyzed based on the microscopic observations. MWCNTs, CB and graphite exhibited percolation threshold at 0.1 wt%, 0.5 wt% and 10 wt%, respectively. The maximal electrical conductivity of the composites was 1.08 S/cm, 9.94 × 10−3 S/cm and 1.70 × 10−5 S/cm. MWCNTs showed the best enhancement on the electrical conductivity. The thermal behavior of the composites was analyzed by calorimetry method. Incorporation of MWCNTs, CB and graphite increased the thermal conductivity of CE resin by 90%, 15% and 92%, respectively. Theoretical models were introduced to correlate the thermal conductivity of the CE/MWCNTs composite. The interfacial thermal resistance between CE resin and MWCNTs was 8 × 10−8 m2K/W and the straightness ratio was 0.2. The MWCNTs were seriously entangled and agglomerated. Simulation results revealed that thermal conductivity of the CE/MWCNTs composites can be substantially elevated by increasing the straightness ratio and/or filler content of MWCNTs

Active Site Ensembles Enabled C–C Coupling of CO₂ and CH₄ for Acetone Production

The production of acetone through ketonization of acetic acid is both energy intensive and environmentally detrimental, as it emits equimolecular CO₂. Herein, a novel approach for acetone production from CO₂ and CH₄ was proposed through consecutive C–C coupling based on an extensive density functional theory computational study. To realize the consecutive C–C coupling, CeO₂-based catalyst doped with Zn site ensemble was constructed. Direct coupling of CH₃* with the acetate species, formed from C–C coupling of CO₂ and stabilized CH₃*, could be achieved to produce acetone. The results show that carbon chain growth is possible on the active site ensembles constructed on a CeO₂-based catalyst. The coupling of the acetate species with the methyl moiety follows a nucleophilic addition mechanism and has apparent activation energies of 0.25 and 0.14 eV on O_vac (oxygen vacancy) and neutral surfaces, respectively. This process could potentially replace the traditional acetone production technology based on ketonization of acetic acid

Single Pd Atom–In₂O₃ Catalyzes Production of CH₃CH₂OH from Atom-Economic C–C Coupling of HCHO and CH₄

Using methane as a reagent to synthesize high-value chemicals and high-energy density fuels through C–C coupling has attracted intense attention in recent decades, as it avoids completely breaking all C–H bonds in CH4. In the present study, we demonstrated that the coupling of HCHO with the CH3 species from CH4 activation to produce ethanol can be accomplished on the single Pd atom–In2O3 catalyst based on the results of density functional theory (DFT) calculations. The results show that the supported single Pd atom stabilizes the CH3 species following the activation of one C–H bond of CH4, while HCHO adsorbs on the neighboring In site. Facile C–C coupling of HCHO with the methyl species is achieved with an activation barrier of 0.56 eV. We further examined the C–C coupling on other single metal atoms, including Ni, Rh, Pt, and Ag, supported on In2O3 by following a similar pathway and found that a balance of the three key steps for ethanol formation, i.e., CH4 activation, C–C coupling, and ethoxy hydrogenation, was achieved on Pd/In2O3. Taking the production of acetaldehyde and ethylene on the Pd/In2O3 catalyst into consideration, the DFT-based microkinetic analysis indicates that ethanol is the dominant product on the Pd/In2O3 catalyst. The facile C–C coupling between HCHO and dissociated CH4 makes formaldehyde a potential C1 source in the conversion and utilization of methane through an energy- and atom-efficient process

Direct C–C Coupling of CO₂ and the Methyl Group from CH₄ Activation through Facile Insertion of CO₂ into Zn–CH₃ σ‑Bond

Conversion of CO₂ and CH₄ to value-added products will contribute to alleviating the green-house gas effect but is a challenge both scientifically and practically. Stabilization of the methyl group through CH₄ activation and facile CO₂ insertion ensure the realization of C–C coupling. In the present study, we demonstrate the ready C–C coupling reaction on a Zn-doped ceria catalyst. The detailed mechanism of this direct C–C coupling reaction was examined based on the results from density functional theory calculations. The results show that the Zn dopant stabilizes the methyl group by forming a Zn–C bond, thus hindering subsequent dehydrogenation of CH₄. CO₂ can be inserted into the Zn–C bond in an activated bent configuration, with the transition state in the form of a three-centered Zn–C–C moiety and an activation barrier of 0.51 eV. The C–C coupling reaction resulted in the acetate species, which could desorb as acetic acid by combining with a surface proton. The formation of acetic acid from CO₂ and CH₄ is a reaction with 100% atom economy, and the implementation of the reaction on a heterogeneous catalyst is of great importance to the utilization of the greenhouse gases. We tested other possible dopants including Al, Ga, Cd, In, and Ni and found a positive correlation between the activation barrier of C–C coupling and the electronegativity of the dopant, although C–H bond activation is likely the dominant reaction on the Ni-doped ceria catalyst

Size Dependence of Vapor Phase Hydrodeoxygenation of <i>m</i>‑Cresol on Ni/SiO₂ Catalysts

Understanding the effect of metal particle size on the reactions during hydrodeoxygenation of phenolics is of great importance for rational design of a catalyst for selective control of a desirable reaction. To this end, vapor phase hydrodeoxygenation of <i>m</i>-cresol was studied over 5% Ni/SiO₂ catalysts with varying Ni particle sizes (2–22 nm) at 300 °C and 1 atm H₂. The Ni particle sizes were confirmed by several characterization techniques, and the varying surface concentration of terrace, step, and corner sites with Ni particle sizes was verified by H₂ temperature-programmed desorption. Decreasing the Ni particle size from 22 to 2 nm improves the intrinsic reaction rate by 24 times and the turnover frequency (TOF) by 3 times. The TOFs for toluene and methylcyclohexanone/methylcyclohexanol formation increase by 6 and 4 times, respectively, while the TOF for CH₄ formation decreases by 3/4, indicating that smaller particles with more defect sites (step and corner) favor deoxygenation and hydrogenation while larger particles with more terrace sites favor C–C hydrogenolysis. Density functional theory study shows that the barrier for direct dehydroxylation of phenol on Ni(111), Ni(211), and defected Ni(211) decreases from 175.6 to 145.6 and then to 120.5 kJ/mol. The results indicate that a highly coordinatively unsaturated surface Ni site is responsible for C–O cleavage through facile adsorption and stabilization of −OH in the transition state, thus facilitating deoxygenation toward toluene. Our results indicate that tuning the metal particle size is an effective approach to control reactions during hydrodeoxygenation

Titania-Modified Silver Electrocatalyst for Selective CO₂ Reduction to CH₃OH and CH₄ from DFT Study

Electrochemical reduction of CO₂ to produce useful fuels and chemicals is one of the attractive means to reuse CO₂. Herein, we constructed a (TiO₂)₃/Ag­(110) model electrocatalyst and examined CO₂ reduction pathways. Our results show that the interface between oxide and supporting Ag provides the active sites for CO₂ adsorption and activation. These active sites enable the electron transfer to the adsorbed CO₂. In this setup, Ag acts as an electron donor, partially reducing the supported (TiO₂)₃ and supplies the needed electrons to the adsorbed CO₂. Once CO₂* is formed at the interface, the subsequent hydrogenation steps take place sequentially. Our results further indicate that the dominating pathway to produce CH₃OH is via the H₂COOH* intermediate following the formation of HCOO*. The formation of H₂COOH* with a free energy of 0.47 eV is the potential-limiting step. Furthermore, protonating H₂COOH* followed by dehydration to CH₃O* and hydrogenation of CH₃O* leads to CH₄ formation. The COOH* pathway may converge to the same H₂COOH* intermediate instead of forming CO*. These results demonstrated the benefit of metal supported metal oxides as electrocatalysts to produce CH₃OH or CH₄ from electrochemical reduction of CO₂