Open-circuit reduction of oxygen coverage on Pt by organic substances: III. Mixed kinetics. Complete reduction of the coverage

At medium degrees of coverage the rates of the chemical and electrochemical oxidation of an organic substance are comparable. The mathematical analysis in this case predicts the appearance of one or more inflections in the coverage-time and potential-time curves. The number of inflections depending on the value of the transfer coefficient is examined. It is shown that up to α=2/3 one inflection may practically appear. The degree of coverage, the potentials and the slopes of the corresponding curves at the inflection are found. The relationships are given in a form which allows their easy experimental verification and their application in the determination of the kinetic parameters.The dependence of the time for the complete reduction of the oxygen coverage on the concentration of the organic substance, on the initial degree of coverage, on the potential and the time for the potentiostatic formation of the coverage and on the temperature is examined as well. The relationships derived are in agreement with the experimental results available in the literature.     

Open-circuit reduction of oxygen coverage on Pt by organic substances: I. Mechanism and kinetics

A mechanism for the open-circuit reduction of oxygen coverage on Pt by organic reducing agents is suggested. It includes a chemical and an electrochemical oxidation of the organic substance and an electrochemical removal and formation of the coverage. The rate of the electrochemical oxidation of the reducing agent is assumed to be determined by its Temkin type adsorption on the free electrode surface. General kinetic equations are derived. The effect of various factors on the form of the curves described by these equations is examined. It is demonstrated that with suitably chosen kinetic parameters a very good coincidence of the calculated and experimental curves can be achieved.     

Open-circuit reduction of oxygen coverage on Pt by organic substances: IV. Experimental verification of the dependences derived

The equations derived in the previous three parts are experimentally verified using propionaldehyde as a model compound. The kinetic parameters of the process are found. As the consistency of the experimental results with the predicted relationships is quite good it seems to be justified to conclude that the overall process of the open-circuit reduction of the oxygen coverage on Pt by organic substances follows the reaction mechanism suggested and is well described by the general kinetic equations derived.     

Oxygen reduction on polycobaltprotoporphyrin film: II. pH effect and the mechanism of oxygen reduction

The oxygen reduction kinetics on polycobaltprotoporphyrin (PCoPP) film has been examined with the rotating disc electrode technique in O₂-saturated solution of various pH. PCoPP film promotes the oxygen reduction process via two electrons to produce peroxide over wide pH range. When pH value of solution decreases, catalytic activity of PCoPP film increases. Possible catalytic mechanisms have been proposed for oxygen reduction.     

Boosting Pt oxygen reduction reaction activity by tuning the tin oxide support

The influence of the dopant element on the physicochemical properties of the SnO₂ support and on the oxygen reduction reaction (ORR) activity of the supported Pt nanoparticles was systematically studied. The dopant element and concentration significantly influence the SnO₂ crystal structure and the electrical conductivity, as well as the ORR activity in 0.1 M HClO₄ of the supported Pt catalysts. ORR activities significantly exceeding that of Pt deposited on carbon were observed for Pt nanoparticles supported on Nb- and W-doped SnO₂. No linear correlation between the support conductivity and the ORR activity was observed, suggesting that other factors, such as (electronic) metal–support interactions, could play a role in the ORR activity of Pt catalysts supported on metal oxides.     

Enhancement of the electrocatalytic activity of Pt nanoparticles in oxygen reduction by chlorophenyl functionalization

Chlorophenyl-stabilized platinum nanoparticles (1.85 nm) exhibited electrocatalytic activity for oxygen reduction up to 3 times higher than that of commercial Pt/C catalysts. Similar enhancement was observed with naked Pt/C functionalized by the same chlorophenyl fragments, suggesting the important role of organic capping ligands in the manipulation of nanoparticle electrocatalytic performance.     

Studies on oxygen determination in organic microanalysis. II. On the microanalysis of oxygen by decrease in the quantity of anhydro-iodic acid

Summary Oxygen in organic compounds is determined by means of the decrease in weight of anhydro-iodic acid. Anhydro-iodic acid is stable in air and can be used for a long time. Satisfactory analyses are obtained.

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Zusammenfassung Sauerstoff kann in organischen Verbindungen durch Messung des Gewichtsverlustes der Anhydrojodsäure bestimmt werden. Diese ist an der Luft beständig und bleibt lange Zeit verwendungsfähig. Zufriedenstellende Analysenergebnisse werden erhalten.

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Résumé Le dosage de l'oxygène dans les composés organiques est effectué par la détermination de la diminution de poids de l'acide anhydroiodique. L'acide anhydroiodique est stable à l'air et peut être employé pendant des temps très longs. Des résultats satisfaisants ont été obtenus par les auteurs.

     

Effect of organic solvents on the reduction of oxygen in water

The effect of solvents on the reduction of oxygen was investigated by pulse voltammetry. The established differences in the effect of the investigated solvents on certain stages of the reduction of oxygen are determined to a significant degree by their concentration in the system. Suggestions about the mechanism of the toxic action of the solvents in biosystems are made. Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, 1 Murmanskaya ul., Kiev-94. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 1, pp. 11–13, January–February, 1998.     

Effect of particle size on the kinetics of the electrocatalytic oxygen reduction reaction catalyzed by Pt dendrimer-encapsulated nanoparticles

Platinum dendrimer-encapsulated nanoparticles (DENs) containing an average of 55, 100, 147, 200, and 240 atoms were prepared within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers. These DENs were immobilized on glassy carbon electrodes, and the effect of particle size on the kinetics of the oxygen reduction reaction (ORR) was quantitatively evaluated using rotating disk voltammetry. The total areas of the Pt DENs were determined by electrochemical CO stripping and hydrogen desorption, and the results were found to be in reasonable agreement with calculated values. The largest particles exhibited the highest specific activities for the ORR.     

Use of aluminum containers in the determination of oxygen in organic substances by elemental microanalysis

The possibility of using cheap aluminum containers in oxygen elemental microanalysis of organic substances was studied. The absolute values of oxygen retention displayed by aluminum containers, as compared with silver ones, for CH(N)O(S)(Br) substances are shown. The magnitude of retention due an aluminum container is reproducible under given conditions. The precision and accuracy of the results of oxygen determination in CH(N)O compounds pyrolyzed in aluminum containers at 980 °C do not differ from the precision and accuracy of the results obtained with the use of silver containers.     

Active sites engineering of Pt/CNT oxygen reduction catalysts by atomic layer deposition

Understanding carbon-supported Pt-catalyzed oxygen reduction reaction(ORR)from the perspective of the active sites is of fundamental and practical importance.In this study,three differently sized carbon nanotube-supported Pt nanoparticles(Pt/CNT)are prepared by both atomic layer deposition(ALD)and impregnation methods.The performances of the catalysts toward the ORR in acidic media are comparatively studied to probe the effects of the sizes of the Pt nanoparticles together with their distributions,electronic properties,and local environments.The ALD-Pt/CNT catalysts show much higher ORR activity and selectivity than the impregnation-Pt/CNT catalysts.This outstanding ORR performance is ascribed to the well-controlled Pt particle sizes and distributions,desirable Pt^04f binding energy,and the Cl-free Pt surfaces based on the electrocatalytic measurements,catalyst characterizations,and model calculations.The insights reported here could guide the rational design and fine-tuning of carbon-supported Pt catalysts for the ORR.     

Mono and bi-metallic electrocatalysts of Pt and Ag for oxygen reduction reaction synthesized by sonication

Nanoparticles of Ag, Pt and Pt–Ag were synthesized using ultrasonic irradiation with no consecutive thermal treatment to catalyze the oxygen reduction reaction. Metal nanoparticles are supported on carbon substrate. The synthesized materials were characterized by XRD, TEM, and cyclic and lineal voltammetry techniques. The kinetic formation of the metallic nanoparticles in solution was followed using UV–vis spectroscopy. The metal particles have crystalline structure and particle size with < 10 nm in size and in the form of spherical agglomerates. Ag/C exhibits lower electrochemical activity and stability for the ORR compared to Pt/C and Pt–Ag/C in acid medium. The mass and specific activity results demonstrate that the synthesized bimetallic sample exhibits 1.5 and 5 times greater electrochemical activities for the ORR compared to the commercial sample.     

Kinetics of Cu(II) reduction by natural organic matter

The kinetics of Cu(II) reduction by Suwannee River fulvic acid (SRFA) at concentrations from 0.25 to 8 mg L(-1) have been investigated in 2 mM NaHCO(3) and 0.7 M NaCl at pH 8.0. In the absence of oxygen, SRFA reduced Cu(II) to Cu(I) in a biphasic manner, with initial rapid formation of Cu(I) followed by a much slower increase in Cu(I) concentration over time. When present, oxygen only had a noticeable effect on Cu(I) concentrations in the second phase of the reduction process and at high [SRFA]. In both the absence and presence of oxygen, the rate of Cu(I) generation increased with increasing [SRFA]. At 8 mg L(-1) [SRFA], nearly 75% of the 0.4 μM Cu(II) initially present was reduced to Cu(I) after 20 min, although the yield of Cu(I) relative to [SRFA] decreased at [SRFA] > 1 mg L(-1). Two plausible kinetic modeling approaches were found to satisfactorily describe the experimental data over a range of [SRFA]. Despite some uncertainty as to which approach is correct, common features of both approaches were complexation of Cu(II) by SRFA and reduction of Cu(II) by two different electron donor groups within SRFA: a relatively labile electron donor (with a concentration of 1.1 × 10(-4) equiv of e(-) (g of SRFA)(-1)) that reduced Cu(II) relatively rapidly and a less labile donor (with a concentration of 3.1 × 10(-4) equiv of e(-) (g of SRFA)(-1)) that reduced Cu(II) more slowly.     

甲醇浓度对Pt/C催化剂氧还原活性的影响

采用半池考察了Pt/C催化剂在含不同浓度甲醇的0.5mol/L硫酸中的氧还原活性(ORR).研究发现,当甲醇浓度为0.1mol/L时,Pt/C催化剂的ORR活性最高,在催化层上热压商品NafionNRE-212膜后也出现同样趋势.线性扫描伏安曲线显示,压膜前后的Pt/C催化剂的ORR活性在含0.1mol/L甲醇的0.5mol/L硫酸中几乎没有变化.电化学阻抗谱结果表明,在该溶液中,Nafion膜的电阻比在其它电解液中低,这可能是导致Pt/C催化剂ORR活性提高的主要原因.有必要关注Nafion膜的这一异常性质并通过特殊设计后用于电池堆,以提高燃料电池性能.     

Automation in organic analysis II. Simultaneous microdetermination of oxygen and nitrogen

A simple, rapid, and accurate method of simultaneous determination of oxygen and nitrogen in organic compounds, containing (in addition to C, H, and O) Cl, Br, and I, is described. The principle of the method consists in the pyrolytic decomposition of the analyzed compound in a current of hydrogen at 1000–1100 °C and the conducting of the hydropyrolytic product through layers of nickel and platinized carbon (50% Pt) heated at 1050 °C. On this filling the nitrogen-containing products are transformed to elementary nitrogen, the oxygen-containing products are transformed to carbon monoxide, the halogens to hydrohalogenides, and methane is decomposed. After removing hydrohalogenides by soda asbestos, carbon monoxide and nitrogen are separated on a chromatographic column filled with Porapak Q, and determined with a catharometer. The whole analysis, including the weighing of the sample, requires 25 min. Sulfur interferes with the determination.     

Polarographic analysis of the reduction waves of Pt(II)-SATSC complex

A polarographic study of the Pt(II)-salicylaldehyde thiosemicarbazone complex in sodium perchlorate as supporting electrolyte is described. In addition to the reduction wave of the complex, a catalytic hydrogen wave is also recorded. The characteristics of this catalytic hydrogen wave are studied under different experimental conditions such as varying acid/complex/supporting electrolyte/surfactant concentrations and also with mercury droptime. Based on these, a probable mechanism for the electrode process has been postulated. Presented at the 12th Annual Symposium in Chemistry held at the Indian Institute of Technology, Madras in March 1987.     

Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction

Electrocatalysis of oxygen reduction using Pt nanoparticles supported on functionalized graphene sheets (FGSs) was studied. FGSs were prepared by thermal expansion of graphite oxide. Pt nanoparticles with average diameter of 2 nm were uniformly loaded on FGSs by impregnation methods. Pt-FGS showed a higher electrochemical surface area and oxygen reduction activity with improved stability as compared with the commercial catalyst. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical characterization suggest that the improved performance of Pt-FGS can be attributed to smaller particle size and less aggregation of Pt nanoparticles on the functionalized graphene sheets.