Showing papers by "Wei Ding published in 2013"

Space-confinement-induced synthesis of pyridinic- and pyrrolic-nitrogen-doped graphene for the catalysis of oxygen reduction

Abstract: The development of high-performance and low-cost catalytic materials for the oxygen reduction reaction (ORR) has been a major challenge for the large-scale application of fuel cells. Currently, platinum and platinum-based alloys are the most efficient ORR catalysts in fuel-cell cathodes; however, they cannot meet the demand for the widespread commercialization of fuel cells because of the scarcity of platinum. Thus, the ongoing search for platinum-free catalysts for the ORR has attracted much attention. Graphene, single-layer sheets of sp-hybridized carbon atoms, has attracted tremendous attention and research interest. The abundance of free-flowing p electrons in carbon materials composed of sp-hybridized carbon atoms makes these materials potential catalysts for reactions that require electrons, such as the ORR. However, these p electrons are too inert to be used directly in the ORR. In N-doped electron-rich carbon nanostructures, carbon p electrons have been shown to be activated through conjugation with lone-pair electrons from N dopants; thus, O2 molecules are reduced on the positively charged C atoms that neighbor N atoms. Recently, Hu and co-workers found that as long as the electroneutrality of the sp-hybridized carbon atoms is broken and charged sites that favor O2 adsorption are created, these materials will be transformed into active metal-free ORR electrocatalysts regardless of whether the dopants are electron-rich (e.g., N) or electrondeficient (e.g., B). Nitrogen-doped carbon (NC) materials are considered to be promising catalysts because of their acceptable ORR activity, low cost, good durability, and environmental friendliness. However, their ORR activity is less competitive, especially in acidic media. Relative to commercial Pt/C, the difference in the half-wave potential for ORR is within 25 mV in alkaline electrolytes but is greater than 200 mV in acidic electrolytes. The activity of NC materials can be enhanced through efficient N doping with sufficient active species that favor ORR and through an increase in electrical conductivity. The annealing of graphitized carbon materials, such as carbon nanotubes and microporous carbon black, in NH3 leads to insufficient substitution of nitrogen because of the well-ordered structure of the host materials. Alternatively, the direct pyrolysis of nitrogen-containing hydrocarbons or polymers produces NC materials with good incorporation of nitrogen. However, suitable pyrolysis temperatures are difficult to pinpoint; without optimization, temperatures that are excessively low or excessively high lead to low electronic conductivity or a remarkable loss of active N species, respectively. Recently, mesoporous-alumina-assisted and silica-template-assisted nitrogen incorporation, which can preserve a high content of N in synthesized NC materials, have been reported. However the activities of the resulting NC materials in the ORR were still significantly lower than that of Pt/C, even when the N content was as high as 10.7 atm%. Among three types of N atoms, that is, pyridinic, pyrrolic, and quaternary N, only the pyridinic and pyrrolic forms, which have planar structures, have been proven to be active in the ORR. In contrast, quaternary N atoms, which possess a 3D structure, are not active in the ORR. The low electrical conductivity of NC materials with quaternary N atoms results from the interruption of their p–p conjugation by the 3D structure and is thought to be predominantly responsible for the poor catalysis. Therefore, the synthesis of NC materials with more planar pyridinic and pyrrolic N atoms and fewer quaternary N atoms is important for the preparation of ORR-active catalysts. Herein, we present a novel strategy for the selective synthesis of pyridinicand pyrrolic-nitrogen-doped graphene (NG) by the use of layered montmorillonite (MMT) as a quasi-closed flat nanoreactor, which is open only along the perimeter to enable the entrance of aniline (AN) monomer molecules. The flat MMT nanoreactor, which is less than 1 nm thick, extensively constrains the formation of quaternary N because of its 3D structure but facilitates the formation of pyridinic and pyrrolic N. Nitrogen is well-known to be incorporated into quaternary N in tetrahedral sp hybridization but incorporated into pyridinic and pyrrolic N in planar sp hybridization. The confinement effect of MMT ensures that N is incorporated into the structure and that the graphitization is successful without significant loss of N species. Furthermore, planar pyridinic and pyrrolic N can be [*] Dr. W. Ding, Prof. Z.-D. Wei, Dr. S.-G. Chen, Dr. X.-Q. Qi, Dr. T. Yang, Dr. S. F. Alvi, Dr. L. Li The State Key Laboratory of Power Transmission Equipment and System Security and New Technology, College of Chemistry and Chemical Engineering, Chongqing University Shapingba 174, Chongqing (China) E-mail: zdwei@cqu.edu.cn

Anchoring Effect of Exfoliated-Montmorillonite-Supported Pd Catalyst for the Oxygen Reduction Reaction

Abstract: We report a combined experimental and computational study on exfoliated montmorillonite (ex-MMT) nanoplatelet supported palladium catalysts. The experimental results revealed that the Pd/ex-MMT catalysts were more stable than Pd/C catalysts in an acidic environment. The results from the density functional theory (DFT) calculations revealed that the O atoms from the AlO6 octahedra in ex-MMT act as anchoring sites for Pd nanoparticles (NPs) that are strongly bound to the ex-MMT support. The partial density of state calculations indicated that the Pd-d states and the O(AlO6)-p states have similar energy. Therefore, it is very easy for electrons to transfer between these states and to form Pd–O(AlO6) bonds. In addition, the defective or destroyed SiO4 tetrahedra can be restructured directly by the O atoms from O2 molecules or indirectly by the intermediates from the reduction of O2. The DFT calculations also revealed that the defects in the ex-MMT supports provide anchor sites for the Pd catalysts to tightly ...

Sn and Sb co-doped RuTi oxides supported on TiO2 nanotubes anode for selectivity toward electrocatalytic chlorine evolution

Abstract: The (Ru0.3Ti0.34Sn0.3Sb0.06)O2–TiO2 nanotubes (TNTs) anode has been prepared via anodization, deposition, and annealing. X-ray diffraction, field-emission scanning electron microscopy, cyclic voltammetry, and linear scanning voltammetry were used to scrutinize the electrodes and the electrochemical activity. The results indicate that highly ordered TNTs with large specific surface area could be implanted with active metal oxides. The catalyst firmly binds with the TNTs and enhances the electrochemical stability of the electrode. It displays high over-potential for oxygen evolution reaction. Accordingly, the constructed (Ru0.3Ti0.34Sn0.3Sb0.06)O2–TNTs anode exhibits a greater potential difference (ΔE) between the evolutions of oxygen and chlorine than that exhibited by the traditional dimensionally stable anode, which is beneficial for improving the selectivity toward chlorine evolution reaction. This superior performance is explained in terms of the surface properties and geometric structure of coated catalyst, as well as the electrochemical selectivity ascribed by the addition of tin and antimony species.

Pd-induced Pt(IV) reduction to form Pd@Pt/CNT core@shell catalyst for a more complete oxygen reduction

Abstract: We describe a facile and controllable process for preparing Pd@Pt/CNT core@shell catalysts for the oxygen reduction reaction (ORR) via Pd-induced Pt(IV) reduction on Pd/CNT. The mass-specific activity for the ORR of the Pd@Pt/CNT catalysts is 7–9 times higher than that of the state-of-the-art Pt/C catalysts, but the yield of H2O2, a harmful species for the stability of catalysts, of the former is only 14.1% of that of the latter. The reason for the enhanced activity and the lower H2O2 yield on the Pd@Pt/CNT catalysts was studied by DFT calculations.

On the Capacity Region of Broadcast Packet Erasure Relay Networks With Feedback

Abstract: We derive a new outer bound on the capacity region of broadcast traffic in multiple input broadcast packet erasure channels with feedback, and extend this outer bound to packet erasure relay networks with feedback. We show the tightness of the outer bound for various classes of networks. An important engineering implication of this work is that for network coding schemes for parallel broadcast channels, the `xor' packets should be sent over correlated broadcast subchannels.