The particle size effect of N-doped mesoporous carbons as oxygen reduction reaction catalysts for PEMFC

The particle size effect of N-doped mesoporous carbon was investigated for ORR activity in acid condition and for issue of a mass transfer and gas diffusion in PEMFCs. As for a non-Pt ORR catalyst, nitrogen (N)-doped ordered mesoporous carbons (OMCs) with a various particle sizes with the range of the average 20, 45 and 75 μm were synthesized by the precursor of polyaniline for the N/C species, and a mesoporous silica template was used for the physical structure for preparation of nitrogen doped OMCs. The N-doped mesoporous carbons are promoted by a transition metal (Fe) to improve catalytic activity for ORR in PEMFCs. All the prepared carbons were characterized by via scanning electron microscopy (SEM), and to evaluate the activities of synthesized doped carbons, linear sweep was recorded in an acidic solution to compare the ORR catalytic activities values for the use in the PEMFC system. The surface area and pore volume were increased as the particles decreased, which was effective for the mass transfer of the reactant for higher activity at the limiting current regions.     

Electrode kinetics of oxygen reduction on oxide-free platinum electrodes

The cathodic V log i curves for O₂ reduction on oxide-free Pt electrodes were determined in oxygen-saturated purified acid (HClO₄, pH 1–3) and alkaline (NaOH, pH 13) solutions. It was found that ∂V/∂ log i = −2·3RT/F, ∂V/∂ log p_O2 = −2·3RT/F and ∂V/∂ pH = −100 mV. Coverage of electrodes by oxygen-containing species was determined by superimposing a constant cathodic current over the working electrode. Intermediate values of coverages were found to be nearly linearly dependent on the potential in the Tafel region.The kinetics mechanism of O₂ reduction on oxide-free Pt electrode are different from that reported for the oxide-covered electrode. Reaction mechanism are discussed in terms of Langmuir and Temkin adsorption isotherms. It is suggested that under Temkin conditions the rate-determining step in the over-all four-electron process is the first electron transfer.     

The effect of surface coverage by adsorbed oxygen on the kinetics of oxygen reduction at oxide free platinum

The kinetics of oxygen reduction was studied at prereduced platinum electrodes in oxygen saturated perchloric acid solutions using potentiostatic transients and steady-state techniques. The steady-state experiments show that the reduction at potentials cathodic to 1·0 V is characterized by ∂V/∂ log i = −60, and ∂V/∂ pH = −100m V. These parameters are compatible with the previously proposed mechanism in which the first charge transfer step is rate determining under Temkin conditions of adsorption by reaction intermediates. In transients too (when the electrode potential was increased from a low level where the coverage with oxygen is virtually zero to any potential between 0·8 and 1·0 V) the initial current i_i, is controlled by the first charge transfer step but now under Langmuirian conditions of adsorption. This accounts for the observed slope ∂V/∂ log i_i of −120 mV. From the steady state and the initial currents the change in the heat of adsorption of reaction intermediates with coverage is found to be 20 kcal/equiv.     

Electrode kinetics of oxygen reduction on platinum in trifluoromethanesulphonic acid

The kinetics of oxygen reduction at Pt in trifluoromethanesulphonic acid (TFMSA) (0.05–6.0 M) and in 1.0 M TFMSA with addition of small concentrations of phosphoric acid (0.003–0.1 M) was investigated using therrde technique. In TFMSA, the oxygen reduction current on the oxide-covered Pt was found to be smaller than that on the oxide-free Pt surface. This result is consistent with the greater amonut of hydrogen peroxide produced on the oxide-covered Pt. A reaction order of one-half with respect to the oxygen concentration for the oxygen reduction reaction was obtained from the ring-disc data. Addition of increasing amounts of phosphoric acid to TFMSA resulted in a progressive decrease in the oxygen reduction current and in an increase of the reaction order with respect to oxygen. On the basis of these experimental results the reaction mechanism proposed for the oxygen reduction of Pt in TFMSA is the fast dissociative adsorption of oxygen, followed by the slow electron transfer step.     

Effect of platinum particle size and catalyst support on the platinum catalyzed selective oxidation of carbohydrates

The effect of the platinum particle diameter and the catalyst support was investigated for the aqueous phase selective oxidation of methyl α-D-glucopyranoside, octyl α-D-glucopyranoside and α-cyclodextrin with molecular oxygen. No platinum particle size effect was observed in the platinum particle diameter range of 1.4 to 3.0 nm. The structure-insensitivity is attributed to the high degree of surface coverage by oxygen. The rate of deactivation due to over-oxidation of the catalyst increases to a small extent with decreasing platinum particle diameter. The effect of the catalyst support is much larger than the platinum particle size effect. The turnover frequency for platinum on activated carbon and carbon fibrils supported catalyst is 3-7 times higher than for graphite as the catalyst support. The support effect is attributed to the hydrophilic character of the support. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of polybutadiene nanoparticles by emulsion polymerization: The effect of electrolyte and initiator type on particle size and reaction kinetics

Emulsion polymerization of the butadiene (Bu) was performed in the presence of disproportionate potassium rosinate (DPR) as anionic emulsifier, potassium hydroxide (KOH), and potassium carbonate (K₂CO₃) as electrolytes, and three different initiators including potassium persulfate (KPS), 2,2^′-azobisisobutyronitrile (AIBN) or 4,4′-azobis(4-cyanovaleric acid) (ACVA, also known as VAZO) at 70 °C. Latexes were prepared with a solid content of about 30 wt%. The particle size and its distribution were measured by dynamic light scattering (DLS) analysis, while the polymerization conversion was determined gravimetrically at different time intervals. Results on the emulsion polymerization of Bu in the presence of KOH and K₂CO₃ co-electrolytes showed that adding KOH to the reaction media decreases the polymerization rate. Positive effect of co-electrolytes on the control over polybutadiene latex (PBL) particles size and its distribution was also confirmed, where K₂CO₃ played roles as electrolyte and pH buffer and KOH served double roles as electrolyte and alkaline supplier of the reaction media. Complete solubility of the AIBN in Bu resulted in higher rate of polymerization in the presence of AIBN in comparison to other initiators, i.e., VAZO or KPS. The results showed that initiator type plays a significant role on the formation of PBL nanoparticles and kinetics of the polymerization. The kinetic studies revealed that emulsion polymerization of Bu follows case 1 (i.e., \(\bar{n}\) ?0.5, where \(\bar{n}\) indicates average number of the propagating chains per particle) of the Smith-Ewart kinetics.     

Electrolyte effects on oxygen reduction kinetics at platinum: A rotating ring-disc electrode analysis

A comparative study of the electrode kinetics of oxygen reduction of platinum in perchloric, phosphoric, sulfuric, trifluoromethanesulfonic acids (all at pH = 0) and in potassium hydroxide (pH = 14) was made at 25°C using rotatating ring-disc electrode techniques. The platinum electrode was first characterised in these electrolytes using the cyclic voltammetric method. The results showed that in the potential region from 0.8 to 0.6 V/rhe, the kinetics of oxygen reduction in these electrolytes decreases in the order KOH > H₂SO₄ ～ CF₃SO₃H > H₃PO₄ > HClO₄. This order of activity is reflected in the effects of the electrolytes, in respect to specific adsorption of anions, on the platinum oxide formation reaction. The role of anion adsorption is also apparent in the dependence of the rate constant for oxygen reduction to water or to hydrogen peroxide and of hydrogen peroxide reduction to water on potential. The superior behavior of oxygen reduction in KOH is due to minimal adsorption of the OH^? ion. The more complex adsorption behavior of the oxyanions in the investigated acid electrolytes than that of simple anions like the halide ions presents difficulties in drawing detailed correlations between oxygen reduction kinetics and adsorption behavior of oxyanions of platinum.     

Effects of pore structure in nitrogen functionalized mesoporous carbon on oxygen reduction reaction activity of platinum nanoparticles

Nitrogen-doped mesoporous carbons (NMCs) with ordered to disordered pore structures were fabricated on SBA-15 modified with different concentrations of tetraethyl orthosilicate using pyrrole as a carbon source. The carbonization temperature of NMCs was maintained at 800 °C so that the amount and type of nitrogen functionalities were constant. Pt nanoparticles (NPs) were deposited onto NMCs using a modified polyol process. N₂ adsorption isotherms, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to characterize NMCs and Pt NPs. A significant shift in binding energy was found in Pt NPs deposited on NMC with disordered pore structure compared to Pt NPs deposited on NMC with ordered pore structure. Pt NPs deposited on NMC with the disordered pore structure had the highest intrinsic oxygen reduction reaction (ORR) activity among the Pt/NMC catalysts. It showed that the interaction between Pt NPs and NMCs could be modulated for enhancement of the ORR activity of Pt NPs by changing the pore structure of NMCs.     

粒度对赤泥直接还原动力学的影响

为了得出粒度对赤泥在煤基条件下等速率加热过程中的直接还原反应的转化过程及反应动力学的影响,采用热分析技术、扫描电镜等手段研究了赤泥在直接还原过程中的热行为和不同条件下焙烧产物中铁矿物的存在形式等。实验结果发现,煤基条件下的直接还原反应主要分为赤泥脱水和铁氧化物还原两个阶段。通过计算得出了这几种粒度的赤泥的还原度,同时运用Coats-Redfern积分法推断出了试样在各个阶段的最概然机理函数和活化能、频率因子等反应动力学参数,并且用Flynn-Wall-Ozawa法对计算结果进行了验证。     

Highly durable platelet carbon nanofiber-supported platinum catalysts for the oxygen reduction reaction

We report the preparation and characterization of highly durable platinum catalysts supported on platelet-structure carbon nanofibers (Pt/p-CNFs) for the oxygen reduction reaction. The p-CNFs were prepared by liquid phase carbonization of polyvinyl chloride using a porous anodic alumina template at 600 °C; their degree of graphitization was increased by the subsequent heat treatment at higher temperatures of up to 1400 °C. The platinum nanoparticles with ∼3 nm diameter were deposited more uniformly on the p-CNFs compared with those on the commercial Ketjen black (KB). The catalytic activity and durability of the Pt/p-CNFs for the oxygen reduction reaction (ORR) in H₂SO₄ solution were improved by increasing the heat-treatment temperature of p-CNFs. The durability of the Pt/p-CNFs was much higher than that of Pt/KB; in particular, a loss of less than 10% was observed in the ORR activity of Pt/p-CNF heat-treated at 1400 °C after potential cycling from 0.5 to 1.5 V vs. RHE for 200 cycles in an argon-saturated H₂SO₄ aqueous solution.     

The influence of platinum crystallite size on the electrochemical reduction of oxygen in phosphoric acid

An experimental program was conducted to investigate the catalytic activity of platinum black and platinum supported on carbon for the electrochemical reduction of oxygen in 99 wt% phosphoric acid at 177°C as a function of platinum surface area. The activity of platinum was found to approximately double as the surface area of platinum was decreased from 80 to 10 m²/g. The Tafel slope was found to be approximately equal to 2.3 R/F on the higher surface area catalysts and greater than 2.3 RT/F on the lower surface area catalysts.     

Immobilization of dendrimer-encapsulated platinum nanoparticles on pretreated carbon-fiber surfaces and their application for oxygen reduction

This paper reports the successful preparation of catalytic electrodes based on carbon-fiber paper modified with dendrimer-encapsulated platinum nanoparticles. The metallic nanoparticles were first synthesized from solution within generation-four hydroxyl-terminated PAMAM dendrimers, which serve as a carrier for their subsequent immobilization on the solid substrates. The carbon-fiber surfaces were activated by means of three alternative anodic pretreatments and then loaded with the dendrimer-metal nanocomposites by cycling of the carbon-electrode potential. The degree of oxidation of the carbon surface affects the anchoring of the dendritic material, the coverage of which is indicated by the electroactive area of the encapsulated platinum. The modified carbon-fiber surfaces pretreated by cyclic polarization are found to be electrocatalytic for the oxygen reduction reaction, presenting a good exchange-current density at low platinum loading.     

Surface coverage of Pt atoms on PtCo nanoparticles and catalytic kinetics for oxygen reduction

The surface coverage of Pt atoms on PtCo nanoparticles and its effect on catalytic kinetics for oxygen reduction were investigated. The PtCo nanoparticles with different surface coverage of Pt atoms were synthesized with various methods, including normal chemical method, microemulsion synthesis, and ultrasound-assisted microemulsion. A model of Pt atoms filling into a spherical nanoparticle was proposed to explain the relationship of surface metal atoms and nanoparticle size. The catalytic activity of the PtCo nano-particles is highly dependent on the synthetic methods, even if they have the same chemical composition. The PtCo nano-particles synthesized with ultrasound-assisted microemulsion showed the highest activity, which is attributed to an increase of active surface coverage of Pt atoms on the metal nanoparticles. The rate of oxygen reduction at 0.5 V (vs. SCE) catalyzed by the PtCo synthesized with ultrasound-assisted micro-emulsion was about four times higher than that of the PtCo synthesized with normal chemical method. As demonstrated with rotating-ring disk electrode measurement, the PtCo nano-particles can catalyze oxygen 4-electron reduction to water without intermediate H₂O₂ detected.     

Synthesis and characterizations of Ni-NiO nanoparticles on PDDA-modified graphene for oxygen reduction reaction

We are presenting our recent research results about the Ni-NiO nanoparticles on poly-(diallyldimethylammonium chloride)-modified graphene sheet (Ni-NiO/PDDA-G) nanocomposites prepared by the hydrothermal method at 90°C for 24 h. The Ni-NiO nanoparticles on PDDA-modified graphene sheets are measured by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED) pattern for exploring the structural evidence to apply in the electrochemical catalysts. The size of Ni-NiO nanoparticles is around 5 nm based on TEM observations. The X-ray diffraction (XRD) results show the Ni in the (012), (110), (110), (200), and (220) crystalline orientations, respectively. Moreover, the crystalline peaks of NiO are found in (111) and (220). The thermal gravimetric analysis (TGA) result represents the loading content of the Ni metal which is about 34.82 wt%. The electron spectroscopy for chemical analysis/X-ray photoelectron spectroscopy (ESCA/XPS) reveals the Ni⁰ to Ni^II ratio in metal phase. The electrochemical studies with Ni-NiO/PDDA-G in 0.5 M aqueous H₂SO₄ were studied for oxygen reduction reaction (ORR).     

Microstructure effect of carbon nanofiber on electrocatalytic oxygen reduction reaction

Carbon nanofibers (CNFs) with controlled microstructures, i.e. platelet CNF (p-CNF), fish-bone CNF (f-CNF) and tube CNF (t-CNF), are synthesized, and their behaviors in electrocatalytic oxygen reduction reaction (ORR) in acid media are investigated in this paper. The physico-chemical properties of the CNFs are characterized by high resolution transmission electron microscope (HRTEM), N₂ adsorption–desorption and Raman spectrum. Cyclic voltammetry experiments show that the CNFs have higher ORR activities than graphite. The p-CNF, which has the highest ratio of edge atoms to basal atoms, demonstrates the most positive ORR onset potential and ORR peak potential. The f-CNF, which has the largest amounts of ORR active sites, exhibits the highest ORR peak current. The t-CNF demonstrates the most negative ORR onset potential, negative ORR peak potential, and the least ORR peak current, which is a result of the fewest catalytic active sites. Furthermore, the microstructures of CNFs can impact the reaction process. The ORR on p-CNF or f-CNF is controlled by diffusion, while the ORR on t-CNF is jointly controlled by surface reaction and diffusion.     

Open circuit reduction of chemisorbed oxygen on platinum

The open circuit reduction of chemisorbed oxygen on a platinum electrode in the presence of a dissolved depolarizer, such as formic acid, is shown to proceed by a purely electrochemical mechanism. It is unnecessary to invoke an initial purely chemical reduction. The concept of such a reduction is not supported by the measured rate of reaction.     

A cooperative effect between support and the heterogenised metalloporphyrins on electrocatalytic oxygen reduction

A new general method to heterogenise porphyrins and different metalloporphyrins (Co, Fe) on silica, mesoporous MCM-41 and delaminated zeolites ITQ-2 and ITQ-6 yields stable materials, which act as active catalysts for the electrochemical oxygen reduction, without catalyst desorption from the electrode.     

The conversion of methane with silica-supported platinum catalysts: the effect of catalyst preparation method and platinum particle size

The particle size distribution of platinum in silica prepared by the complexing agent-assisted sol–gel method and impregnation method and also in MCM-41 has been compared and its influence on the product distribution in the non-oxidative dehydrogenation of methane has been investigated. The sol–gel method gives a narrow range of platinum particle size distribution compared to the impregnation method. It was found that as the particle size increases, the selectivity for the higher hydrocarbons increases though the yield decreases.     

Heat treatment effect of Pt-V/C and Pt/C on the kinetics of the oxygen reduction reaction in acid media

This work studies the heat treatment effect of carbon-dispersed platinum and platinum-vanadium alloys on the kinetics of the oxygen reduction reaction (ORR) in acid medium. The catalyst powders were subjected to heat treatments at three temperatures for 1 h. The electronic and structural features of the materials were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The XANES results for the oxidized state composites showed an increase of the Pt 5d band occupancy with increased heat treatment temperature for the Pt/C catalyst, while no changes were noted for Pt-V/C for the same treatments. The electrochemical characteristics for the ORR were investigated by cyclic voltammetry and state-state polarization measurements. The results showed that the ORR takes place by the multi-electronic charge transfer process, following a four electron mechanism. The kinetics of the ORR was evaluated using Tafel diagrams. It was observed that the ORR activity of the Pt/C and Pt-V/C is enhanced with the increase of the heat treatment temperature. The catalytic activity of the materials was analyzed in terms of the electronic and structural properties of Pt in the metallic particles.