Pt and Pt–Ru nanoparticles decorated polypyrrole/multiwalled carbon nanotubes and their catalytic activity towards methanol oxidation

Conducting polymer composite films comprised of polypyrrole (PPy) and multiwalled carbon nanotubes (MWCNTs) [PPy–CNT] were synthesized by in situ polymerization of pyrrole on carbon nanotubes in 0.1 M HCl containing (NH₄)S₂O₈ as oxidizing agent over a temperature range of 0–5 °C. Pt nanoparticles are deposited on PPy–CNT composite films by chemical reduction of H₂PtCl₆ using HCHO as reducing agent at pH = 11 [Pt/PPy–CNT]. The presence of MWCNTs leads to higher activity, which might be due to the increase of electrochemically accessible surface areas, electronic conductivity and easier charge-transfer at polymer/electrolyte interfaces allowing higher dispersion and utilization of the deposited Pt nanoparticles. A comparative investigation was carried out using Pt–Ru nanoparticles decorated PPy–CNT composites. Cyclic voltammetry demonstrated that the synthesized Pt–Ru/PPy–CNT catalysts exhibited higher catalytic activity for methanol oxidation than Pt/PPy–CNT catalyst. Such kinds of Pt and Pt–Ru particles deposited on PPy–CNT composite polymer films exhibit excellent catalytic activity and stability towards methanol oxidation, which indicates that the composite films is more promising support material for fuel cell applications.     

Effect of Co doping on catalytic activity of small Pt clusters

Platinum is the most widely used catalyst in fuel cell electrodes. Designing improved catalysts with low or no platinum content is one of the grand challenges in fuel cell research. Here, we investigate electronic structures of Pt(4) and Pt(3)Co clusters and report a comparative study of adsorption of H(2), O(2), and CO molecules on the two clusters using density functional theory. The adsorption studies show that H(2) undergoes dissociative chemisorption on the tetrahedral clusters in head on and side on approaches at Pt centers. O(2) dissociation occurs primarily in three and four center coordinations and CO prefers to adsorb on Pt or Co atop atoms. The adsorption energy of O(2) is found to be higher for the Co doped cluster. For CO, the Pt atop orientation is preferred for both Pt(4) and Pt(3)Co tetrahedral clusters. Adsorption of CO molecule on tetrahedral Pt(3)Co in side on approach leads to isomerization to planar rhombus geometry. An analysis of Hirshfeld charge distribution shows that the clusters become more polarized after adsorption of the molecules.     

Efficient anchorage of Pd nanoparticles on carbon nanotubes as a catalyst for hydrazine oxidation

A simple method is devised to deposit well-dispersed Pd nanoparticles on multi-walled carbon nanotubes (CNTs). Pd nanoparticles (1–3 nm) prepared in ethanol were transferred to toluene solution and modified by organic molecule benzyl mercaptan which acts as a cross linker between Pd nanoparticles and CNTs. The morphology and structure of the resulting Pd/CNT nanocomposite were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results show that Pd nanoparticles were highly dispersed and effectively anchored on CNTs. The excellent electrocatalytic activity of the Pd/CNT nanocomposite for the oxidation of hydrazine was demonstrated by cyclic voltammetry.     

Atomic-scale deformation in N-doped carbon nanotubes

We present the N-doping induced atomic-scale structural deformation in N-doped carbon nanotubes by using density functional theory calculations. For substitutional N-doped nanotube clusters, the N dopant with an excess electron lone pair exhibits the high negative charge, and the homogeneously distributed dopants enlarge the tube diameter in both zigzag and armchair cases. On the other hand, in pyridine-like N-doped ones, the concentrated N atoms result in a positively curved graphene layer and, thus, can be responsible for tube wall roughness and the formation of interlinked structures.     

Synthesis and stabilization of subnanometric gold oxide nanoparticles on multiwalled carbon nanotubes and their catalytic activity

Small gold nanoclusters in a very narrow size distribution (1.1 ± 0.5 nm) have been stabilized onto multiwalled carbon nanotubes (MWCNT). Theoretical studies supported by XPS and (16)O(2)/(18)O(2) isotopic exchange experiments have shown that, on small gold nanoparticles (0.9-1.5 nm), dissociation of molecular O(2) and formation of a surface oxide-like layer is energetically favorable and occurs at room temperature, while O(2) recombination and desorption involves a larger activation barrier. CO titration experiments and theoretical studies demonstrate that the reactivity of the oxidized particles toward CO does not only depend on particle size but also on oxygen coverage. The oxidation-reduction process described is reversible, and the oxidized nanoparticles are active in the epoxidation of styrene with air.     

Nitrogen-doped coatings on carbon nanotubes and their stabilizing effect on Pt nanoparticles

A homogeneous coating of nitrogen-doped carbon on carbon nanotubes is performed using ionic liquids. The N-doped material is employed as a support for nanoparticles. Electrochemical degradation behavior is monitored in situ and compared to an unmodified material. The strongly enhanced stability is explained on the basis of a Pt-nitrogen interaction.     

Biomimetic mineralization of vertical N-doped carbon nanotubes

Biomimetic mineralization of vertical N-doped carbon nanotubes is demonstrated as a straightforward route for carbon-based mineral nanocomposites. The N-doped sites along the carbon nanotube backbone play the role of nucleation sites for mineralization.     

Decoration of multi-walled carbon nanotubes with NiO nanoparticles and investigation on their catalytic activity to synthesize pyrimidinone heterocycles

A supported carbon material is shown to be a highly efficient, eco-friendly and recyclable solid acid catalyst for the Biginelli reaction of aldehyde, β-ketoester and urea or thiourea under microwave irradiation in the absence of solvent. This method offers significant advantages such as efficiency, the excellent yield, avoidance of the organic solvents, mild reaction conditions, easy separation and simple operation. In addition, because of employing microwave as heating source and reducing use of organic solvents, this novel method emerges as a green-approach leading to less harmful residues. Furthermore, a mechanism was proposed to rationalize the reaction and the role of NiO–MWCNTs was also investigated in these transformations.     

Highly dispersed Pt catalysts on single-walled carbon nanotubes and their role in methanol oxidation

Well-dispersed Pt catalysts with very high utilization efficiencies for fuel cell reactions have been prepared by ethylene glycol reduction on polymer-wrapped single-walled carbon nanotubes (SWCNTs). By wrapping the SWCNTs in a polymer such as polystyrene sulfonate, we are able to break up the nanotube bundles to achieve better dispersion. These polymer-wrapped SWCNTs with platinum nanoparticles deposited on them show very high electrochemically active surface areas. The increase in utilization efficiencies for platinum catalysts on these SWCNT supports can be attributed to the increased surface areas and the well-dispersed nature of the carbon support and catalyst. The catalyst dispersion facilitates diffusion of reactant species which in turn results in higher methanol oxidation currents and more positive potentials for oxygen reduction.     

Tuning the electrocatalytic activity of Pt nanoparticles on carbon nanotubes via surface functionalization

Polyelectrolytes with various characteristic functional groups as interlinkers to anchor Pt nanoparticles were used to functionalize carbon nanotubes (CNTs) as Pt electrocatalyst support. It was found that polyanions (poly(styrenesulfonic acid) (PSS), and poly(acrylic acid sodium) (PAA)) have a beneficial effect on methanol electrooxidation on Pt nanoparticles supported on carbon nanotubes via modifying their electronic structure through charge transfer from polyanions to Pt sites and supply of oxygen-containing species. The increased electron density around Pt sites by the charge transfer from polyanions would cause partial filling of Pt 5d-bands, resulting in the downshift of d-band center and weaker chemisorption with oxygen-containing species (e.g. CO_ad). The weakened chemisorption of CO on Pt nanoparticles would promote the methanol electrooxidation. On the contrary, polycations would have an opposite effect on the electronic structure and chemisorption properties of Pt nanoparticles.     

CVD growth of N-doped carbon nanotubes on silicon substrates and its mechanism

In the present study, we report the chemical vapor deposition (CVD) of nitrogen-doped (N-doped) aligned carbon nanotubes on a silicon (Si) substrate using ferrocene (Fe(C5H5)2) as catalyst and acetonitrile (CH3CN) as the carbon source. The effect of experimental conditions such as temperature, gaseous environment, and substrates on the structure and morphology of N-doped carbon nanotubes arrays is reported. From XPS and EELS data, it was found that the nitrogen content of the nanotubes could be determined over a wide range, from 1.9% to 12%, by adding the addition of hydrogen (H2) to the reaction system. It was also shown by SEM that N-doped carbon nanotube arrays could be produced on Si and SiO2 substrates at suitable temperatures, although at different growth rates. Using these concentrations, it was possible to produce three-dimensional (3D) carbon nanotubes architectures on predetermined Si/SiO2 patterns. The mechanism underlying the effect of nitrogen containing carbon sources on nanotube formation was explored using X-ray photoelectron spectroscopy (XPS).     

Facile synthesis of N-doped carbon nanotubes grafted on N-doped carbon nanosheets co-encapsulating cobalt and molybdenum carbide nanoparticles for efficient methanol oxidation

Developing efficient electrocatalysts based on inexpensive and environment protection materials for methanol oxidation reaction (MOR) is crucial to new generation renewable energy storage and conversion processes, but it remains a major challenge. In our current research, N-doped carbon nanotubes grafted on N-doped carbon nanosheets co-encapsulating cobalt and Mo₂C nanoparticles (Co-Mo₂C/NCNT@NCN) are designed and synthesized by simple mixing and one-step annealing approach. After depositing Pt nanoparticles, Pt/Co-Mo₂C/NCNT@NCN exhibits superior catalytic activity and stability for MOR in acid medium; more precisely, it has a commendable mass activity of 842 mA/mg_Pt, which is 2.1 times of Pt/C-JM. And it is worth mentioning that its anti-CO poisoning ability and stability have also a visible improvement; the enhanced catalytic performance mainly benefits from the rich N doping, excellent structural features, and the synergistic effect of Co and Mo₂C. Therefore, this study opens up a new direction for designing and developing a favorable active multicomponent catalyst for MOR.     

Controlled deposition of Pt on Au nanorods and their catalytic activity towards formic acid oxidation

Platinum submonolayer decorated gold nanorods with controlled coverage were prepared by the addition of Au nanorods into the growth solution of Pt in the presence of NH₂OH · HCl as the growth agent. The properties of Au nanorods decorated by Pt submonolayer were investigated by various techniques including transimission electron microscopy, X-ray diffraction, and electrochemical methods. The Pt decorated Au nanorods on carbon black showed significantly higher activity on formic acid electrooxidation than the conventional Pt/C catalysts. They showed different reaction path of formic acid electrooxidation by suppressing the formation of poisoning intermediate CO.     

氮掺杂碳纳米管负载的Ru和Pd催化剂上异丙醇的转化(英文)

Ru and Pd (2 wt%) loaded on pure and on Ndoped carbon nanotubes (NCNTs) were prepared and tested using the isopropyl alcohol decomposition reaction as probe reaction. The presence of nitrogen functionalities (pyridinic, pyrrolic, and quaternary nitrogen) on the nitrogen doped support induced a higher metal dispersion: Pd/NCNT (1.8 nm) Pd/CNT (4.9 nm), and Ru/NCNT (2.4 nm) Ru/CNT (3.0 nm). The catalytic activity of the supports was determined first. Isopropyl alcohol conversion produces acetone on CNTs while on NCNTs it led to both dehydration and dehydrogenation products. At 210 °C and in the presence of air, the isopropyl alcohol conversion was higher on the NCNTs (25%) than on the CNTs (11%). The Pd loaded catalysts were more active and more selective than the Ru ones. At 115 °C, the Pd catalysts were 100% selective towards acetone for a conversion of 100%, whereas the Ru catalysts led to dehydration and dehydrogenation products. The nitrogen doping induced the appearance of redox properties when oxygen is present in the reaction mixture.     

Reactivity of chemisorbed oxygen atoms and their catalytic consequences during CH4-O2 catalysis on supported Pt clusters

Kinetic and isotopic data and density functional theory treatments provide evidence for the elementary steps and the active site requirements involved in the four distinct kinetic regimes observed during CH(4) oxidation reactions using O(2), H(2)O, or CO(2) as oxidants on Pt clusters. These four regimes exhibit distinct rate equations because of the involvement of different kinetically relevant steps, predominant adsorbed species, and rate and equilibrium constants for different elementary steps. Transitions among regimes occur as chemisorbed oxygen (O*) coverages change on Pt clusters. O* coverages are given, in turn, by a virtual O(2) pressure, which represents the pressure that would give the prevalent steady-state O* coverages if their adsorption-desorption equilibrium was maintained. The virtual O(2) pressure acts as a surrogate for oxygen chemical potentials at catalytic surfaces and reflects the kinetic coupling between C-H and O═O activation steps. O* coverages and virtual pressures depend on O(2) pressure when O(2) activation is equilibrated and on O(2)/CH(4) ratios when this step becomes irreversible as a result of fast scavenging of O* by CH(4)-derived intermediates. In three of these kinetic regimes, C-H bond activation is the sole kinetically relevant step, but occurs on different active sites, which evolve from oxygen-oxygen (O*-O*), to oxygen-oxygen vacancy (O*-*), and to vacancy-vacancy (*-*) site pairs as O* coverages decrease. On O*-saturated cluster surfaces, O*-O* site pairs activate C-H bonds in CH(4) via homolytic hydrogen abstraction steps that form CH(3) groups with significant radical character and weak interactions with the surface at the transition state. In this regime, rates depend linearly on CH(4) pressure but are independent of O(2) pressure. The observed normal CH(4)/CD(4) kinetic isotope effects are consistent with the kinetic-relevance of C-H bond activation; identical (16)O(2)-(18)O(2) isotopic exchange rates in the presence or absence of CH(4) show that O(2) activation steps are quasi-equilibrated during catalysis. Measured and DFT-derived C-H bond activation barriers are large, because of the weak stabilization of the CH(3) fragments at transition states, but are compensated by the high entropy of these radical-like species. Turnover rates in this regime decrease with increasing Pt dispersion, because low-coordination exposed Pt atoms on small clusters bind O* more strongly than those that reside at low-index facets on large clusters, thus making O* less effective in H-abstraction. As vacancies (*, also exposed Pt atoms) become available on O*-covered surfaces, O*-* site pairs activate C-H bonds via concerted oxidative addition and H-abstraction in transition states effectively stabilized by CH(3) interactions with the vacancies, which lead to much higher turnover rates than on O*-O* pairs. In this regime, O(2) activation becomes irreversible, because fast C-H bond activation steps scavenge O* as it forms. Thus, O* coverages are set by the prevalent O(2)/CH(4) ratios instead of the O(2) pressures. CH(4)/CD(4) kinetic isotope effects are much larger for turnovers mediated by O*-* than by O*-O* site pairs, because C-H (and C-D) activation steps are required to form the * sites involved in C-H bond activation. Turnover rates for CH(4)-O(2) reactions mediated by O*-* pairs decrease with increasing Pt dispersion, as in the case of O*-O* active structures, because stronger O* binding on small clusters leads not only to less reactive O* atoms, but also to lower vacancy concentrations at cluster surfaces. As O(2)/CH(4) ratios and O* coverages become smaller, O(2) activation on bare Pt clusters becomes the sole kinetically relevant step; turnover rates are proportional to O(2) pressures and independent of CH(4) pressure and no CH(4)/CD(4) kinetic isotope effects are observed. In this regime, turnover rates become nearly independent of Pt dispersion, because the O(2) activation step is essentially barrierless. In the absence of O(2), alternate weaker oxidants, such as H(2)O or CO(2), lead to a final kinetic regime in which C-H bond dissociation on *-* pairs at bare cluster surfaces limit CH(4) conversion rates. Rates become first-order in CH(4) and independent of coreactant and normal CH(4)/CD(4) kinetic isotope effects are observed. In this case, turnover rates increase with increasing dispersion, because low-coordination Pt atoms stabilize the C-H bond activation transition states more effectively via stronger binding to CH(3) and H fragments. These findings and their mechanistic interpretations are consistent with all rate and isotopic data and with theoretical estimates of activation barriers and of cluster size effects on transition states. They serve to demonstrate the essential role of the coverage and reactivity of chemisorbed oxygen in determining the type and effectiveness of surface structures in CH(4) oxidation reactions using O(2), H(2)O, or CO(2) as oxidants, as well as the diversity of rate dependencies, activation energies and entropies, and cluster size effects that prevail in these reactions. These results also show how theory and experiments can unravel complex surface chemistries on realistic catalysts under practical conditions and provide through the resulting mechanistic insights specific predictions for the effects of cluster size and surface coordination on turnover rates, the trends and magnitude of which depend sensitively on the nature of the predominant adsorbed intermediates and the kinetically relevant steps.     

Decorating catalytic palladium nanoparticles on carbon nanotubes in supercritical carbon dioxide

Hydrogen reduction of a Pd(II)-beta-diketone precursor in supercritical carbon dioxide produces palladium nanoparticles on multi-walled carbon nanotubes that exhibit promising catalytic properties for hydrogenation of olefins in carbon dioxide as well as electro-reduction of oxygen in fuel cell applications.     

Fabrication of different silica nanotubes and examination of their catalytic activity in organic solvents

Silica nanotubes (SNTs) with different geometrical architectures and comparable pore sizes were fabricated inside the one-dimensional (1D) channels of anodic alumina membrane (AAM) using different nonionic Brij template surfactants. Experimental results revealed the formation of SNTs with ordered 1D, two-dimensional (2D), three-dimensional (3D), and wormlike mesostructures. Cytochrome C (Cyt C) was chosen as a hemoprotein to be immobilized inside the SNTs. Under the same starting Cyt C concentration, the amount immobilized was found to be dependent on the SNT mesostructure. Unlike native Cyt C, which lost its activity while interacting in organic solvents, Cyt C entrapped within silica nanotube hybrid membrane (SNM) showed remarkable catalytic conversion activity in organic solvents, with wormlike mesostructure being favorable over ordered mesostructures. This could be ascribed to facile access to active centers of Cyt C due to the random orientations of wormlike SNTs.     

Synthesis of double-walled carbon nanotubes by catalytic chemical vapor deposition and their field emission properties

Double-walled carbon nanotubes (DWCNTs) were synthesized by catalytic chemical vapor deposition using Fe-Mo/MgO as a catalyst at 1000 degrees C under the mixture of methane and hydrogen gas. The nanotubes were purified by acid but were not damaged. Thermogravimetric analysis revealed the purity of the tubes to be about 90%. The high-resolution transmission electron microscopy image showed that DWCNTs have inner tube diameters of 1.4-2.6 nm and outer tube diameters of 2.3-3.4 nm. We observed radial breathing modes in Raman spectra, which are related to the diameter of inner nanotubes. The purified DWCNTs were mixed with organic vehicles and glass frit, and then they were screen-printed on glass substrate coated with indium tin oxide. The field emission properties of the screen-printed DWCNT films were examined by varying the amount of glass frit ingredient within the DWCNT paste. The results showed that DWCNT emitters had good emission properties such as turn-on field of 1.33-1.78 V/microm and high brightness. When the applied anode voltage was gradually increased, current density and brightness became saturated. We also observed DWCNTs adsorbed on the anode plate; they were DWCNTs peeled off from the cathode plate for field emission measurement.     

Co-and N-doped carbon nanotubes with hierarchical pores derived from metal-organic nanotubes for oxygen reduction reaction

Biomolecules with a broad range of structure and heteroatom-containing groups offer a great opportunity for rational design of promising electrocatalysts via versatile chemistry.In this study,uniform folic acid-Co nanotubes(FA-Co NTs) were hydrothermally prepared as sacrificial templates for highly porous Co and N co-doped carbon nanotubes(Co-N/CNTs) with well-controlled size and morphology.The formation mechanism of FA-Co NTs was investigated and FA-Co-hydrazine coordination interaction together with the H-bond interaction between FA molecules was characterized to be the driving force for growth of one-dimensional nanotubes.Such distinct metal-ligand interaction afforded the resultant CNTs rich Co-N_x sites,hierarchically porous structure and Co nanoparticle-embedded conductive network,thus an overall good electrocatalytic activity for oxygen reduction.Electrochemical tests showed that Co-N/CNTs-900 promoted an efficient 4 e ORR process with an onset potential of 0.908 V vs.RHE,a limiting current density of 5.66 mA cm^-2 at 0.6 V and a H_2 O_2 yield lower than 5%,comparable to that of 20%Pt/C catalyst.Moreover,the catalyst revealed very high stability upon continuous operation and remarkable tolerance to methanol.