A rational design of bimetallic PdAu nanoflowers as efficient catalysts for methanol oxidation reaction

Methanol fuel cells have been intensively developed as clean and high-efficiency energy conversion system due to their high efficiency and low emission of pollutants. Here, we developed a simple aqueous synthetic method to prepare bimetallic Pd Au nanoflowers catalysts for methanol oxidation reaction(MOR) in alkaline environment. Their composition can be directly tuned by changing the ratio between Pd and Au precursors. Compared with commercial Pd/C catalyst, all of the Pd Au nanoflowers catalysts show the enhanced catalytic activity and durability. In particular, the Pd Au nanoflowers specific activity reached 0.72 m A/cm2, which is 14 times that of commercial Pd/C catalyst. The superior MOR activity could be attributed to the unique porous structure and the shift of the d-band center of Pd.     

Surface voltammetric dealloying investigation on PdCu/C electrocatalysts toward ethanol oxidation in alkaline media

Voltammetric dealloying is employed here to investigate the correlations between catalytic performance and surface composition and structure, taking ethanol oxidation reaction (EOR) on Pd-Cu alloy surface as a case study. First, home-made PdCu/C with a mean particle size of ca. 3.11?±?0.6 nm is dealloyed by repetitive potential cycling in 0.5 M H₂SO₄. With dealloying cycles rising, the Cu component is gradually leached out and the corresponding Pd/Cu atomic ratio gradually increases from ca. 2.1 to 4.0; meanwhile, SEM images display that Pd-rich porous shell is formed due to dealloying-induced surface structural rearrangement, being verified by the appearance of ear-like peaks at ??0.015 V (vs. SCE) in CVs collected in 0.5 M H₂SO₄; furthermore, XPS spectra indicate that core-level binding energies of Pd 3d_5/2 first positively shift to 336.1 eV and then oppositively move down to 334.9 eV, indicating that the d-band center of Pd composition is modulated by the dealloying treatment. Moreover, the voltammetric peak current densities for EOR follow the order of PdCu/C-DA15?>?as-prepared PdCu/C ??>?PdCu/C-DA30 ? commercial Pd/C ? PdCu/C-DA75, due to the modest downshift of Pd d-band center resulted by charge transfer and surface atomic rearrangement. In addition, the EOR durability gradually decays with the continuous loss of Cu, indicating that electro-oxidation of surface species also follows the so-called bi-functional mechanism. This work might provide some new insights into the catalysis enhancement by tuning the surface/interfacial structure of catalysts.

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Graphical abstract The voltammetric peak current densities for ethanol oxidation on home-made PdCu/C catalysts gradually decrease with the dealloying cycles rising, suggesting that the surface voltammetric dealloyment could effectively modulate the surface composition and structure, so as to tune the catalytic performance.

     

Electro-oxidation of ethanol using PtSnRh/C electrocatalysts prepared by an alcohol-reduction process

PtRh/C (90:10), PtRh/C (50:50), PtSn/C (50:50), and PtSnRh/C (50:40:10) electrocatalysts were prepared by an alcohol-reduction process using ethylene glycol as solvent and reduction agent and Vulcan Carbon XC72 as supports. The electrocatalysts were characterized by energy-dispersive X-ray analysis, X-ray diffraction, and transmission electron microscopy. The electro-oxidation of ethanol was studied by cyclic voltammetry chronoamperometry at room temperature and on a single cell of a direct ethanol fuel cell at 100 °C. Cyclic voltammetry and chronoamperometry experiments showed that PtSnRh/C and PtSn/C electrocatalysts have similar performance for ethanol oxidation at room temperature, while the activity of PtRh/C electrocatalysts was very low. At 100 °C on a single cell, PtSnRh/C showed superior performance compared to PtSn/C and PtRh/C electrocatalysts.     

Electro-oxidation of ethylene glycol on PtSn/C and PtSnNi/C electrocatalysts

A PtSn/C electrocatalyst with a Pt–Sn molar ratio of 50:50 and A PtSnNi/C electrocatalyst with a Pt–Sn–Ni molar ratio of 50:40:10 were prepared by alcohol-reduction process using ethylene glycol as solvent and reducing agent. The electrocatalysts were characterized by energy dispersive X-ray, X-ray diffraction, and cyclic voltammetry. The electro-oxidation of ethylene glycol was studied by cyclic voltammetry and chronoamperometry using the thin porous coating technique. PtSnNi/C electrocatalyst showed a superior performance compared to PtSn/C electrocatalysts in the potential range of interest for a direct ethylene glycol fuel cell.     

A comparative study on electrocatalytic performance of PtAu/C and PtRu/C nanoparticles for methanol oxidation reaction

We present a comparative study on the electrocatalytic performance of PtRu/C and PtAu/C nanoparticles for methanol oxidation reaction. The PtRu/C nanoparticles are commercially available, while the PtAu/C nanoparticles were prepared using the conventional sodium borohydride reduction method. The particle size, surface morphology, and crystallinity of the two samples were characterized using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The electrocatalytic activity for MOR and the long-term durability of the PtAu/C and PtRu/C were carefully investigated. The results showed that the PtAu/C exhibited much higher activity for MOR and improved long-term durability in comparison with the PtRu/C. The catalysts before and after accelerated potential cycling test (APCT) were characterized by X-ray photoelectron spectroscopy (XPS). It was found that Ru dissolved and escaped completely from the electrode meanwhile the majority of Au remained after APCT.

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Graphical abstract The synthesized PtAu/C exhibits higher electrocatalytic activity of methanol oxidation and improved durability compared to the commercial PtRu/C

     

Improved Pt/C electrocatalysts for methanol oxidation prepared by different reducing agents and surfactants and DFT studies on it

In this work, PtCl₄ as precursor; sodium borohydride (Cat I), hydrazinium hydroxide (Cat II), and formaldehyde (Cat III) as reducing agents; and 1-heptanamine (a), N-methyl-1-heptanamine (b), and N,N-dimethyl-1-heptanamine (c) as surfactants were used to prepare platinum nanoparticles which were then dispersed on carbon XC-72 for use as catalysts in the methanol oxidation reaction. XRD and TEM results indicate that the platinum has a face-centered cubic structure and is found as small and agglomerated particles in different shapes, sizes, and densities. Cat I comprises small (~?5 nm) cubic and formless agglomerated (~?20–~?300 nm) particles, Cat II is composed of small (~?5 nm) and a significant number of quite dense spherical agglomerated (~?20–~?150 nm) particles, and Cat III contains large number of small (~?5 nm) and a small number of spherical, less dense, and agglomerated (~?20–~?200 nm) particles. XPS data shows that the platinum exists in two different oxidation states Pt(0) (~?64.5–~?69.6%) and Pt(IV) (~?35.5–~?30.4%), and platinum surface also contains OH, H₂O, C–O, C=O, and carbon. DFT and FTIR show that the surfactants decompose to form partially crystalline carbon. Electrochemical studies reveal that performance order of the catalysts towards the methanol oxidation reaction is Cat II < Cat I < Cat III, and that Cat IIIc has the highest performance, which is 2.23 times larger than E-TEK catalysts. It was found that the performance of the catalysts depends on the kind of surfactant, reducing agent, electrochemical surface area, percent platinum utility, roughness factor, and I_f/I_r ratio.

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Graphical abstract ?

     

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.     

Effect of Ni proportion on the performance of proton exchange membrane fuel cells using PtNi/C electrocatalysts

PtNi/C electrocatalysts were synthesised by borohydride method on functionalised carbon support. Energy-dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy and both cyclic and linear voltammetry were employed to characterise the composition, crystalline structure, morphology and catalytic properties of the PtNi/C electrocatalysts. Different Ni proportions in the PtNi/C electrocatalysts were evaluated in the cathode or anode in a H₂/air proton exchange membrane fuel cells (PEMFC) by polarisation curves. PtNi particles uniformly dispersed with different proportions of metals obtained. The increase of Ni proportion in the electrocatalyst led to materials with higher mass activity values toward the oxygen reduction reaction and a greater electrochemical-active surface area. PtNi/C electrocatalysts in the cathode presented higher mass activity values at high potential in the PEMFC. The best PEMFC performance was obtained with PtNi 13 at.% Ni (cathode) and Pt/C (anode) relative to the Pt/C (cathode and anode) with identical Pt loadings. PtNi/C electrocatalysts in PEMFC may be used as an alternative to Pt/C electrocatalyst.     

Preparation of PtSn/C electrocatalysts using citric acid as reducing agent for direct ethanol fuel cell (DEFC)

PtSn/C electrocatalysts (Pt:Sn atomic ratios of 50:50 and 60:40) were prepared using citric acid as reducing agent, and the pH of the reaction medium was varied by the addition of OH⁻ ions. The obtained electrocatalysts were characterized by energy dispersive X-ray analysis, X-ray diffraction, and transmission electron microscopy. The electrocatalysts were tested on the direct ethanol fuel cell (DEFC) at 90 °C. The obtained PtSn/C electrocatalysts showed the presence of a face-centered cubic, Pt, and SnO₂ phases. In DEFC studies, the PtSn/C electrocatalysts showed a superior performance compared to a commercial PtSn/C and Pt/C electrocatalysts from E-TEK.     

The effect of gold on platinum oxidation in homogeneous Au-Pt electrocatalysts

Oxidation of Au-Pt thin films was carried out in ambient air at room temperature and characterized by X-ray photoelectron spectroscopy. The homogeneous films were prepared by RF co-sputtering with concentrations varying from Au₉Pt₉₁ to Au₈₉Pt₁₁ and compared to pure Pt and Au thin films. Spectral deconvolution of the Au 4f and Pt 4f core levels revealed linear peak shifts for both the Au-Au and Pt-Pt bonding components as a function of alloy mixture and metallic component peak asymmetry that remained constant for all alloy stoichiometries. The predominant oxidation products were PtO and PtO₂ and were characterized by stable core level binding energies for all films. A gradual decline in the Pt-O_x products and corresponding levels of elemental oxygen was observed with increasing Au content but was similar in proportion to the metallic Pt components. Based on these results, variations in Pt oxide phases and/or concentration do not appear to contribute to enhanced electrocatalytic activity for oxygen reduction observed for the intermediate alloy stoichiometries.     

Electrooxidation of ethanol using Pt rare earth–C electrocatalysts prepared by an alcohol reduction process

Pt rare earth–C electrocatalysts (rare earth = La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm, and Lu) were prepared by an alcohol reduction process using ethylene glycol as reduction agent and solvent and Vulcan XC 72 as support. The electrocatalysts were characterized by energy-dispersive X-ray analysis, X-ray diffraction (XRD), and cyclic voltammetry. The electrooxidation of ethanol was studied in acid medium by cyclic voltammetry and chronoamperometry using thin porous coating technique. The XRD patterns indicate that all electrocatalysts present the face-centered cubic structure of Pt and the presence of rare earth hydroxides. All electrocatalysts prepared by this methodology showed superior performance for ethanol electrooxidation at room temperature compared to Pt–C.     

Ab initio evaluation of primary cyclo-hexane oxidation reaction rates

Naphthenes are chemical species that are always present in liquid hydrocarbon fuels and their pyrolysis and oxidation can play an important role in real liquid fuel combustion. In spite of its practical relevance, the chemical kinetics of naphthene pyrolysis and oxidation is not yet thoroughly investigated and there is not a general agreement on the role and rate of several elementary reactions involved. In this paper, the kinetics of the pyrolysis and oxidation of a simple naphthene, namely cyclo-hexane, has been investigated through detailed kinetic modeling. Ab initio calculations were performed to estimate the kinetic parameters of some primary reactions following the oxygen attack to the cyclo-hexane radical. In fact, due to the complex behavior induced by the ring structure of cyclo-hexane, such data were difficult to determine through thermo-chemical methods. Density functional theory (B3LYP/6-31g(d, p)) was adopted to determine structure and vibrational frequencies of transition states and reaction intermediates, while energies were evaluated using the G2MP2 approach. The kinetic parameters of the investigated primary reactions were then introduced in a general detailed kinetic model consisting of elementary reactions whose kinetic constants were taken from the literature. The so obtained kinetic model was used to simulate ignition delay times and species concentrations measured in various experiments reported in the literature. The agreement between experimental data and theoretical predictions shows the validity of the chosen approach and supports the correctness of the proposed kinetic model.     

Ion-beam formation of the active surface of methanol oxidation electrocatalysts on the tantalum substrates

The active surfaces of electrocatalysts are formed by the ion-beam-assisted deposition of one of the rare-earth metals and platinum onto tantalum substrates from a neutral vapor fraction and the vacuum-arc discharge plasma of a pulsed ion source. Deposited metal ions are used as agents to aid deposition. The composition and microstructure of the formed surface layers are investigated via scanning electron microscopy, electron probe microanalysis, electron backscatter diffraction, and Rutherford backscattering spectrometry. Electrocatalytic activity in the electrochemical oxidation of methanol, which underlies the operation of low-temperature fuel cells, is investigated via cyclic voltamperometry.     

Electrochemical water splitting using nano-zeolite Y supported tungsten oxide electrocatalysts

Zeolites are often used as supports for metals and metal oxides because of their well-defined microporous structure and high surface area. In this study, nano-zeolite Y (50–150 nm range) and micro-zeolite Y (500–800 nm range) were loaded with WO₃, by impregnating the zeolite support with ammonium metatungstate and thermally decomposing the salt thereafter. Two different loadings of WO₃ were studied, 3 wt.% and 5 wt.% with respect to the overall catalyst. The prepared catalysts were characterized for their morphology, structure, and surface areas through scanning electron microscope (SEM), XRD, and BET. They were further compared for their electrocatalytic activity for hydrogen evolution reaction (HER) in 0.5 M H₂SO₄. On comparing the bare micro-zeolite particles with the nano-form, the nano-zeolite Y showed higher currents with comparable overpotentials and lower Tafel slope of 62.36 mV/dec. WO₃ loading brought about a change in the electrocatalytic properties of the catalyst. The overpotentials and Tafel slopes were observed to decrease with zeolite-3 wt.% WO₃. The smallest overpotential of 60 mV and Tafel slope of 31.9 mV/dec was registered for nano-zeolite with 3 wt.% WO₃, while the micro-zeolite gave an overpotential of 370 mV and a Tafel slope of 98.1 mV/dec. It was concluded that even with the same metal oxide loading, nano-zeolite showed superior performance, which is attributed to its size and hence easier escape of hydrogen bubbles from the catalyst.     

Electrochemical performance of LiFePO4/C synthesized by solid state reaction using different lithium and iron sources

LiFePO₄/C cathode materials were prepared from different lithium and iron sources, using glucose as the carbon source and the reducing agent, via a solid state reaction. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), galvanostatic charge-discharge test and cyclic voltammetry (CV). The results showed that the LiFePO₄/C is olivine-type phase, and composed of relatively large particles of about 400 nm and some nano-sized particles, which favor the electronic conductivity. The LiFePO₄/C cathode material synthesized from Li₂CO₃ and Fe₂O₃ had the smallest particles and the highest uniformity. It delivered the capacity of 145.8 mA h/g at 0.2 C, and had good reversibility and high capacity retention. The precursor of LiFePO₄/C was characterized by thermogravimetry (TG) to discuss the crystallization formation mechanism of LiFePO₄.     

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.     

Ultrasound-assisted transformation from waste biomass to efficient carbon-based metal-free pH-universal oxygen reduction reaction electrocatalysts

Efficient carbon-based nitrogen-doped electrocatalysts derived from waste biomass are regarded as a promising alternative to noble metal catalysts for oxygen reduction reaction (ORR), which is crucial to fuel cell performance. Here, coconut palm leaves are employed as the carbon source and a series of nitrogen-doped porous carbons were prepared by virtue of a facile and mild ultrasound-assisted method. The obtained carbon material (ANDC-900-10) conveys excellent pH-universal catalytic activity with onset potentials (E_onset) of 1.01, 0.91 and 0.84 V vs. RHE, half-wave potentials (E_1/2) of 0.87, 0.74 and 0.66 V vs. RHE and limiting current densities (J_L) of 5.50, 5.45 and 4.97 mA cm⁻² in alkaline, neutral and acidic electrolytes, respectively, prevailing over the commercial Pt/C catalyst and, what's more, ANDC-900-10 displays preeminent methanol crossover resistance and long-term stability in the broad pH range (0–13), thanks to its abundant hierarchical nanopores as well as effective nitrogen doping with high-density pyridinic-N and graphitic-N. This work provides sonochemical insight for underpinning the eco-friendly approach to rationally designing versatile metal-free carbon-based catalysts toward the ORR at various pH levels.     

Alpha particles from the reaction12C +12C at 28.7 MeV/nucleon

A classical dynamical alpha-cluster model has been developed and applied in order to get inclusive energy spectra of alpha particles produced in the collision of¹²C +¹²C at the beam energy 28.7 MeV/A. Results of the calculations are compared with experimental data. The shapes of the experimental energy spectra and the absolute normalization at forward angles are approximately described without any free parameters. The model makes it possible to distinguish alpha particles originating from the compound system and from direct processes. The spectra at forward angles are dominated by projectile fragmentation processes. The cross section at larger angles is overestimated, which is partially due to emission of particles other than alpha particles in central collisions. The evaporation Hauser-Feshbach model predicts that alpha particles emitted from the compound nucleus constitute less than 26% of all emitted particles.     

Production of neutral mesons in the reaction12C+12C at 2 GeV/u

A systematic study of neutral meson production in heavy-ion reactions at 2 GeV/u was started with a¹²C+¹²C experiment using the photon spectrometer TAPS. Special emphasis is put on the possible observation of the-meson in a heavy-ion reaction exploiting the decay channel ⁰.Presented at the International School-Workshop Relativistic Heavy-Ion Physics, Prague (Czech Republic), 19–23 September 1994.