Carbon nano-onions (CNOs) were successfully synthesized by employing the flame pyrolysis (FP) method, using flaxseed oil as a carbon source. The alcohol reduction method was used to prepare Pd/CNOs and Pd-Sn/CNOs electro-catalysts, with ethylene glycol as the solvent and reduction agent. The metal-nanoparticles were supported on the CNO surface without adjusting the pH of the solution. High-resolution transmission electron microscopy (HRTEM) images reveal CNOs with concentric graphite ring morphology, and also PdSn nanoparticles supported on the CNOs. X-ray diffractometry (XRD) patterns confirm that CNOs are amorphous and show the characteristic diffraction peaks of Pd. There is a shifting of Pd diffraction peaks to lower angles upon the addition of Sn compared to Pd/CNOs. X-ray photoelectron spectroscopy (XPS) results also confirm the doping of Pd with Sn to form a PdSn alloy. Fourier transform infrared spectroscopy (FTIR) displays oxygen, hydroxyl, carboxyl, and carbonyl, which facilitates the dispersion of Pd and Sn nanoparticles. Raman spectrum displays two prominent peaks of carbonaceous materials which correspond to the D and G bands. The Pd-Sn/CNOs electro-catalyst demonstrates improved electro-oxidation of methanol and ethanol performance compared to Pd/CNOs and commercial Pd/C electro-catalysts under alkaline conditions.
Carbon nanofibers (CNFs) supported by Pd and Pd-Sn electro-catalysts were prepared by the chemical reduction method using ethylene glycol as the reducing agent. Their physicochemical characteristics were studied using high resolution-transmission electron microscopy (HR-TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Bruanaer-Emmett-Teller (BET) analysis. FTIR revealed that oxygen, hydroxyl, carboxylic and carbonyl functional groups facilitated the dispersion of Pd and Sn nanoparticles. The doping of Pd with Sn to generate PdSn alloy was also confirmed by XPS data. The amorphous nature of CNFs was confirmed by XRD patterns which exhibited the Pd diffraction peaks. When Sn was added to Pd/CNFs, the diffraction peaks moved to lower angles. HRTEM images revealed that the CNFs with cylindrical shape-like morphology and also Pd-Sn nanoparticles dispersed on carbon support. The catalytic activity and stability towards alcohol electro-oxidation in alkaline medium at room temperature was evaluated using cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). The obtained Pd-Sn/CNFs electro-catalyst exhibited a better electro-catalytic activity than Pd/CNFs and Pd/C electro-catalysts for both methanol and ethanol oxidation. The improvement of the electrochemical performance was associated with the synergistic effect via the addition of Sn which modified the Pd atom arrangement, thereby promoting oxidation through a dehydrogenation pathway. Furthermore, SnO2 generates abundant OH species which helps with increasing the rate of the oxidative removal of carbon monoxide (CO) intermediates from Pd sites.
Pd/C, Pd/CNFs and PdÀ Ru/CNFs nanocomposite materials were utilized as anode nanocatalysts in lowtemperature alkaline direct alcohol fuel cells. The palladium based nanocatalysts performance and stability were firmly relying upon the attributes of the carbon nanofibers (CNFs). CNFs were successfully synthesized employing a chemical vapour deposition method. The nanocatalysts were synthesized by dispersing Pd and PdÀ Ru nanoparticles onto the CNFs surface using alcohol reduction method. The physical properties of the synthesized nanocatalysts were explored utilizing several techniques such as transmission electron microscope (TEM), scanning electron microscope-Energy dispersive x-ray (SEM-EDX), X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and confirmed successful synthesis of Pd/C, Pd/ CNFs and PdÀ Ru/CNFs nanocomposite. TEM showed that Pd and Ru nanoparticles were uniformly dispersed on the CNFs support surface. ICP-OES determined the palladium and ruthenium concentration in Pd/C, Pd/CNFs and PdÀ Ru/CNFs nanocatalysts to be Pd (7.67 %), Pd (7.74 %), Pd (7.82 %) and Ru (3.22 %) respectively. The three prepared nanocatalysts were evaluated by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) in the evaluation of ethanol and methanol oxidation reactions. CV, CA and EIS experiments of PdÀ Ru/CNFs nanocatalyst displayed superior activity towards alcohol oxidation reaction in alkaline conditions than Pd/CNFs and commercial Pd/C nanocatalysts.
This study reports on the synthesis of carbon nano-onions (CNOs; ca. d ≤ 55 nm) and nitrogen-doped CNOs (N-CNOs) using a facile pyrolysis method and ex-situ doping of the CNOs. Elemental analysis of the N-CNOs revealed that their nitrogen content depended on the ammonia flow rate. Analysis of the N-CNOs revealed that they all exhibited structural defects. After the successful synthesis of CNOs and N-CNOs, polyvinylpyrrolidone (PVP):CNOs/N-CNOs:MnO2-nanorods (MONRs) composites were prepared and used as active sensing materials. In every case, the PVP polymer was used to stabilize the MONRs for acetone detection at 25 °C. The chemi-resistive gas sensors that showed the highest acetone sensitivity (pS = 2.0 × 10−4 ppm−1 ) was fabricated using a pristine CNOs (pCNOs) based composite. However, the N-CNOs based sensor (a1.5S) presented the lowest acetone limit of detection (LoD) at 1.2 ppm. The study implicated the effect of the nitrogen and oxygen content of the CNOs surfaces on the acetone detection. Thus, a higher sensitivity with lower LoD was observed at room temperature using the pCNOs based sensor, when compared to earlier literature reports.
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