PtRu/carbon hybrid materials prepared by hydrothermal carbonization as electrocatalysts for methanol electrooxidation

PtRu/carbon hybrid materials were prepared by hydrothermal carbonization using starch as carbon source and reducing agent and H₂PtCl₆.6H₂O and RuCl₃.xH₂O as metal sources of the carbonization process. The materials were prepared in the following conditions: without pH adjustment, in the absence and in the presence of tetrapropylammonium chloride, and adjusting the pH using potassium hydroxide or tetrapropylammonium hydroxide. The as-synthesized materials were further treated under argon atmosphere at 900 °C and characterized by energy dispersive X-ray spectroscopy, thermogravimetric analysis, BET isotherm, Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscope, and cyclic voltammetry. The electrooxidation of methanol was studied by chronoamperometry. The addition of tetrapropylammonium ion promoted an increase in surface area and total pore volume while the alkaline medium favored smaller particle sizes. The material prepared using tetrapropylammonium hydroxide showed the best electroactivity for methanol electrooxidation compared to others obtained materials.     

High catalytic performance of Pt nanoparticles on plasma treated carbon nanotubes for electrooxidation of ethanol in a basic solution

Nanosized Pt particles deposited on plasma treated multi-walled carbon nanotubes have been used in electrocatalytic oxidation of ethanol in a basic solution. These Pt nanoparticles have very narrow size distribution and exhibit significant higher catalytic activities, higher Pt utilization efficiency (93.77%) and improved durability in comparison to the commercial available Johnson Matthey Pt/C catalyst.     

Amino acid adsorption on single-walled carbon nanotubes

We investigate and discuss the adsorption of a few amino acids on (3,3) carbon nanotubes and on graphite sheets through calculations within density functional theory. Results show weak binding of the biomolecules on both substrates, but through generally favourable adsorption pathways. Zwitterion adsorption through the charged amine and carboxylate groups are bound stronger to the nanotube surface in comparison to their nonionic counterparts, as well as on histidine, phenylalanine, and cysteine side chain groups fixed in specific orientations. Binding strengths on graphite suggest dissimilar trends for amino acid interactions with increasing nanotube diameter.     

Rare earth cuprates as electrocatalysts for methanol oxidation

A series of rare earth cuprates with overall composition Ln_2−xM_xCu_1−yM_y′O_4−δ (where Ln=La and Nd; M=Sr, Ca and Ba; M′=Ru and Sb: 0.0≤x≤0.4 and y=0.1) have been tested as anode electrocatalysts for methanol oxidation. The evaluation of electrode kinetic parameters was made galvanostatically. The catalyst characterization was carried out by specific conductivity measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Iodometry. These materials exhibit significant activity for methanol oxidation at higher potentials. The linear correlation between Cu(3+) content and methanol oxidation activity suggests that the active sites for adsorption of methanol is Cu(3+). The methanol oxidation onset potential depends on the ease of Cu(2+)→Cu(3+) oxidation reaction. These materials show better tolerance towards the poisoning by the intermediates of methanol oxidation compared to that of conventional noble metal electrocatalysts (supported and bulk). The lattice oxygen in these oxides could be considered as active oxygen to remove CO intermediates of methanol oxidation reaction.     

Metallic nanoparticles on plasma treated carbon nanotubes: Nanohybrids

Multi-wall carbon nanotubes (MWCNTs) were decorated with metal clusters by thermal evaporation. Transmission electron microscopy (TEM) shows that the nature and extent of metal coverage can be varied by plasma treating the MWCNT surface. The metal clusters on oxygen plasma treated arc-discharge MWCNTs have a more dense distribution than the clusters evaporated on as-synthesized arc-discharge MWCNTs. In contrast, the plasma treatment did not affect the cluster distribution on CVD MWCNTs. Analyses of the valence band and the core levels by X-ray photoelectron spectroscopy suggest poor charge transfer between gold clusters and MWCNTs; on the contrary suggest good charge transfer between Ni clusters and MWCNTs.     

Vacuum vapour deposition of phenylphosphonic acid on amorphous alumina

A study of the growth of phenylphosphonic acid (PPOA) on amorphous alumina thin films on top of polycrystalline aluminium foil is presented. The self-assembled monolayer (SAM) of the acid was grown in ultra high vacuum via vapour phase deposition, which allows us to investigate the molecular growth process in situ by photoelectron spectroscopy. The acid adlayer was deposited on the alumina surface at 300 K from an evaporator consisting of a ceramic tube heated by a tungsten wire. On adsorption of the PPOA acid on the alumina surface the anchoring properties of the phosphonic group lead to the formation of an ordered layer with an outward phenyl ring group and a phosphonate interface. The C 1s, P 2s, P 2p, Al 2p and O 1s core level spectra were analysed as a function of the deposition time. The acid adsorption occurs in two ranges: formation of an initial monolayer phase, followed by saturation coverage where the P 2s peak intensity is constant with deposition time. It was confirmed that the phenylphosphonic acid molecules react with the alumina surface forming P-O-Al bonds. It was shown that the SAM formation is driven by the evolution of acid molecular coverage on the alumina surface. The results on the vapour deposited PPOA SAM monolayer on the amorphous alumina surface are consistent with previous findings for a number of phosphonic acids SAMs prepared through immersion of the alumina surfaces in an acid solution.     

Nanoparticulate TiO2-promoted PtRu/C catalyst for methanol oxidation

To improve the electrocatalytic properties of PtRu/C in methanol electrooxidation, nanoparticulate TiO₂-promoted PtRu/C catalysts were prepared by directly mixing TiO₂ nanoparticles with PtRu/C. Using cyclic voltammetry, it was found that the addition of 10 wt% TiO₂ nanoparticles can effectively improve the electrocatalytic activity and stability of the catalyst during methanol electrooxidation. The value of the apparent activation energy (E _a) for TiO₂-PtRu/C was lower than that for pure PtRu/C at a potential range from 0.45 to 0.60 V. A synergistic effect between PtRu and TiO₂ nanoparticles is likely to facilitate the removal of CO-like intermediates from the surface of PtRu catalyst and reduce the poisoning of the PtRu catalysts during methanol electrooxidation. Therefore, we conclude that the direct introduction of TiO₂ nanoparticles into PtRu/C catalysts offers an improved facile method to enhance the electrocatalytic performance of PtRu/C catalyst in methanol electrooxidation.     

Improve the field emission uniformity of carbon nanotubes treated by ball-milling process

Carbon nanotubes (CNTs) deposited by chemical vapor deposition (CVD) were treated by ball milling. The morphologies and field emission properties of the treated CNTs depending on milling time were studied. The emission turn-on field is increased, and the field emission current density is reduced, when the milling time increased from 0.5 to 3 h. The as-deposited long CNTs were cut to short CNTs (∼1 h) and micro-particles (>1 h) with increasing of the milling time. It is found that the optimized milling time is 0.5-1 h, the treated CNTs showed excellent field emission properties, such as low turn-on field, high emission current density and uniform luminescence spots distribution.     

Adsorption structure of phenylphosphonic acid on an alumina surface

The adsorption structure of phenylphosphonic acid (PPOA) on an alumina surface was investigated as a function of exposure and temperature using infrared reflected absorption spectroscopy (IRAS) and a Kelvin probe. The alumina surface was held at room temperature during deposition. At monolayer adsorption, PO and Phenyl-P bands are observed, which indicate the creation of POAl bonds. The aromatic ring plane is positioned perpendicular to the surface, i.e. deprotonated PPOA stands vertically on the surface. At multilayer adsorption, PO and POH bands appear in the spectra. The multilayer PPOA film starts to desorb at 400 K. From 400 to 700 K, the IRAS spectra are similar to the monolayer spectrum, indicating that the multilayer structure reverses to the monolayer-like PPOA adlayer by heating. The acid molecules start to desorb at 700 K.     

Evaluation of residual iron in carbon nanotubes purified by acid treatments

A detailed analysis by X-ray photoelectron spectroscopy was carried out on multi-walled carbon nanotube (MWCNT) surfaces after non-oxidative and oxidative purification treatments in liquid-phase. The MWCNT were produced by pyrolysis of camphor and ferrocene, that provides a high yield but with high iron contamination (∼15% wt). The elimination and/or oxidation of iron nanoparticles were monitored by Fe2p and O1s core level. Oxygen-based functional groups attachment was also investigated by C1s fitting. The effectiveness of each treatment in iron removal was evaluated by thermogravimetric analysis (TGA) and atomic absorption spectroscopy (AAS). The integrity of the MWCNT structure was verified by Raman spectroscopy (RS) and transmission electron microscopy (TEM). A purity degree higher than 98% was achieved only with non-oxidative treatments using sonification process.     

Interaction between methanol and single-walled carbon nanotubes: Density functional theory study

Density functional calculations have been performed to investigate the dependence of methanol interaction with the side walls of single-walled carbon nanotubes (SWCNTs) on the nanotube's type, curvature and chirality. The author's results show that methanol prefers to be physically adsorbed on semiconducting CNTs in comparison with the metallic one. It was found that the binding energy of methanol is increased for adsorption on larger-diameter nanotubes. Furthermore, we find that when a methanol molecule was adsorbed on higher chiral angle nanotubes the binding energy was increased. The study of the electronic structures and Mulliken analysis indicate that the methanol and CNT are interacting rather weakly, consistent with recent experimental observation.     

Synthesis of monodisperse palladium nanocubes and their catalytic activity for methanol electrooxidation

The single crystalline palladium nanocubes with an average size of 7 nm were prepared in the presence of poly (vinyl pyrrolidone) (PVP) and KBr using the polyol method. The as-prepared Pd nanocubes were highly uniform in both size and shape. The ordered packing structures including monolayer and multilayer can be fabricated via the rate-controlled evaporation of solution solvent. The electrochemical catalytic activity of these Pd nanocubes towards methanol oxidation was found to be higher than that of spherical Pd nanoparticles of similar size.     

Preparation of highly dispersed Pt-SnOx nanoparticles supported on multi-walled carbon nanotubes for methanol oxidation

To maximize the utilization of catalysts and thereby reduce the high price, a new strategy was developed to prepare highly dispersed Pt-SnO_x nanoparticles supported on 8-Hydroxyquinoline (HQ) functionalized multi-walled carbon nanotubes (MWCNTs). HQ functionalized MWCNTs (HQ-MWCNTs) provide an ideal support for improving the utilization of platinum-based catalysts, and the introduction of SnO_x to the catalyst prevents the CO poisoning effectively. The as-prepared catalysts are characterized by Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It is found that the HQ functionalization process preserves the integrity and electronic structure of MWCNTs, and the resulting Pt-SnO_x particles are well dispersed on the HQ-MWCNTs with an average diameter of ca. 2.2 nm. Based on the electrochemical properties characterized by cyclic voltammetry and chronoamperometry, the Pt-SnO_x/HQ-MWCNTs catalyst displays better electrocatalytic activity and stability for the methanol oxidation. It is worth mentioning that the forward peak current density of Pt-SnO_x/HQ-MWCNTs catalyst is ca. 1.9 times of that of JM commercial 20% Pt/C catalyst, which makes it the preferable catalyst for direct methanol fuel cells.     

Functional multi-walled carbon nanotube/polyaniline composite films as supports of platinum for formic acid electrooxidation

Embedding of carbon nanotubes in conducting polymeric matrices for various nanocomposites material is now a popular area. In this article, a concise chemical method has been described for the preparation of homogeneous nanocomposite of multi-walled carbon nanotube (MWNT)/polyaniline (PANI) by electrochemical codeposition. For this we functionalized the MWNTs via the diazotization reaction. This helped to disperse the nanotubes in aniline. The composite films were dispersed Pt by electrodeposition technique. The presence of MWNTs and platinum in the composite films was confirmed by XRD analysis and transmission electron microscopy (TEM). Four-point probe investigations revealed that the MWNT/PANI composite films exhibited a good conductivity. Cyclic voltammograms (CV) showed that Pt-modified MWNT/PANI composite films perform higher electrocatalytic activity and better long-term stability than Pt-modified pure PANI film toward formic acid oxidation. The results imply that the MWNT/PANI composite films as a promising support material improves the electrocatalytic activity for formic acid oxidation greatly.     

Aspect ratio control of acid modified multiwalled carbon nanotubes

This study examined the effects of acid treatments on the length of multiwalled carbon nanotubes (MWCNTs) as well as the influence of the aspect ratio of the MWCNTs on the electrical percolation threshold. In particular, the length distribution, intensity ratio of the G and D Raman peaks, BET surface area, and maximum decomposition temperatures of the MWCNTs were investigated. The MWCNTs showed different electrical percolation thresholds depending on the aspect ratio. The higher aspect ratio MWCNTs had lower electrical percolation thresholds than those with smaller ratios due to their ease of contact with other MWCNTs. In terms of the electrical behavior of a MWCNT film, many more short MWCNTs were necessary to reach the electrical percolation threshold than long MWCNTs. These results demonstrate the need to control the aspect ratio of MWCNTs in order for them to be used efficiently in electrical applications.     

Chemisorption of hydrogen by carbon nanotubes

Chemisorption of hydrogen by carbon nanotubes (CNTs) is studied by thermodynamics and kinetics methods. Expressions are derived for the adsorption isotherm and desorption kinetics. Methods for determining chemisorption parameters are developed. The partial free energy of binding of hydrogen with a CNT (3.6 eV) is determined. It is shown that residual products of synthesis are removed from CNTs as a result of prolonged annealing at high temperatures. The capacity of a CNT relative to chemisorbed hydrogen is esti mated at 4 mass %.     

Effects of acid treatment duration and sulfuric acid molarity on purification of multi-walled carbon nanotubes

Multi-walled carbon nanotubes were synthesized using a Fe-Ni bimetallic catalyst supported by MgO using thermal chemical vapor deposition. Purification processes to remove unwanted carbon structures and other metallic impurities were carried out by boiling in sulfuric acid solution. Various analytical techniques such as TGA/DSC, Raman spectroscopy, SEM, HRTEM and EDAX were employed to investigate the morphology, graphitization and quality of the carbon nanotubes. The obtained results reveal the molarity of sulfuric acid and immersed time of the carbon nanotubes in the acid solution is very effective at purifying multi-walled carbon nanotubes. It was also found that 5 M concentration of boiling sulfuric acid for a 3 h treatment duration led to the highest removal of the impurities with the least destructive effect. Moreover, it was observed that acid treatment results in decreasing of CNTs’ diameter.     

An XANES study on the modification of single‐walled carbon nanotubes by nitric acid

X‐ray absorption near‐edge structure (XANES) spectroscopy has been applied to identify the modification process of single‐walled carbon nanotubes (SWCNTs) treated by nitric acid. The carboxyl groups created by the nitric acid treatment have been found to be formed on both the carbonaceous fragments and the side walls of SWCNTs. The carbonaceous fragments could be removed by a following washing treatment with sodium hydroxide. XANES spectra indicate that carbonaceous fragments are the result of the synthesis process and/or of the nitric acid treatment. Tube walls of SWCNTs are weakly oxidized by the nitric acid treatment although, after removing carbonaceous fragments, a direct oxidation process of SWCNTs is observed. Experimental data address the removal of carbonaceous fragments on SWCNTs as an efficient method for side‐wall modification of a SWCNT.     

Electrooxidation of formic acid catalyzed by Pd Nanoparticles supported on multi-walled carbon nanotubes with sodium oxalate

This paper repots a highly catalytic palladium nanoparticle catalyst dispersed on the purified multi-walled carbon nanotubes (P-MWCNTs) for the electrooxidation of formic acid, in which sodium oxalate is employed as both a dispersant and a coordination agent. The nanostructured catalysts have been characterized by X-ray diffraction technique and transmission electron microscopy. It is found that the as-prepared face-centered cubic crystal Pd nanoparticles are uniformly dispersed on the surface of MWCNTs with an average particle size of 5.6 nm. Fourier transform infrared spectroscopy and thermogravimetric analysis revel that sodium oxalate is a tractable ligand with the aid of a suitable solution. Cyclic voltammetry and chronoamperometry tests demonstrate that the obtained Pd/P-MWCNT catalyst from typical experiment has better catalytic activity and stability for formic acid electrooxidation than acid-oxidation treatment MWCNT (AO-MWCNT)-supported Pd catalyst from the control experiment. Therefore, the as-prepared Pd/P-MWCNTs would be a potential candidate as an anode electrocatalyst in direct formic acid fuel cells.