O. A. Khazova

Carbon nanotubes as a support for Pt-and Pt-Ru-catalysts of reactions proceeding in fuel cells

Comparative study of two types of single-wall carbon nanotubes and standard carbon black Vulcan XC-72 as supports for catalysts of reactions proceeding in fuel cells is carried out. The nanotubes were prepared by arc method; they differed in the degree of their purifying from amorphous carbon and metal impurities. The structure and hydrophobic-hydrophilic properties of these carbon supports are studied by etalon porosimetry. The effect of the supports’ specific surface area on the deposited catalyst particles size and specific surface area is studied. The catalysts (Pt-Ru and Pt) were deposited from aqueous solutions of their salts. Platinum was also deposited by thermal decomposition of ethoxy clusters. It is shown that in methanol oxidation reaction at the Pt-Ru catalysts the current values per unit true surface area do not depend on the support nature, provided the catalyst loading is equal and the particle size is similar. When oxygen is reduced at platinum deposited onto purified nanotubes and the carbon black Vulcan XC-72, specific kinetic currents also are close to each other. It is shown that the degree of nanotubes purification and their structure affect the kinetics of this reaction significantly.

Tolerant-to-methanol cathodic electrocatalysts based on organometallic clusters

A new approach to the fabrication of catalytic systems based on hetero-and homometal-chalcogenide clusters of Pt-M-X type (M: Fe, Mn; X: S, Se, Te), which provides the reproducibility of catalyst composition and uniform distribution of catalyst over the carbon support, is proposed. Thus obtained catalysts are characterized using the XRD, TEM, and EDAX methods. The electrocatalytic activity of these systems in the oxygen reduction reaction, the role of the nature of chalcogenide atom and the atom of the second metal, which is the platinum partner, and the electrochemical behavior of nonplatinum chalcogenide systems are studied.

Nanostructured catalysts for cathodes of oxygen-hydrogen fuel cells

Bimetallic catalysts platinum-cobalt, platinum-chromium, and platinum-tungsten, deposited onto highly dispersed carbon black from complex cluster-type compounds of corresponding metals with a 1: 1 atomic ratio of metals are developed. The catalysts are characterized by methods of x-ray diffraction analysis and energy dispersive analysis of x-rays. The procedure involving use of a thin-film rotating disk electrode is employed to probe kinetic parameters of the oxygen reduction reaction on the catalysts developed. The investigated binary catalysts exhibit specific electrochemical characteristics that are not inferior and, in some cases, are superior to the characteristics intrinsic to the commercial platinum catalyst E-TEK, when tested in the composition of a gas-diffusion electrode under conditions that are close to real conditions in which cathodes of oxygen-hydrogen fuel cells operate.

Thin-film rotating disk electrode as a tool for comparing the activity of catalysts in the hydrogen oxidation reaction

A thin-film disperse rotating disk electrode is used to study the hydrogen oxidation reaction on platinum catalysts E-TEK with different purification degrees and on disperse palladium catalysts obtained from colloid solutions of organometallic complex precursors with subsequent thermal decomposition in an inert atmosphere or in hydrogen at diverse temperatures. Kinetic currents of the hydrogen oxidation reaction on these catalysts are determined at a potential of 0.025 V. The obtained values of currents may be utilized for performing a comparative estimation of the activity of various catalysts and the degree of their purification from the precursors or accidental impurities.

CO oxidation at platinum-molybdenum electrodes

CO oxidation at Pt-Mo electrodes prepared by different procedures is studied. When CO is oxidized from its saturated solutions at Mo-containing electrodes, catalysis is observed at lower potentials (<0.4 V (RHE)); inhibiting, at higher potentials (>0.7 V). When adsorbed CO is oxidized in supporting electrolyte, no oxidation current is observed at lower potentials; the current observed on platinum at higher potentials (>0.7 V) is also lowered in the presence of molybdenum. Depending on the preparation procedure, catalysts with different phase structure were obtained, namely, as the Pt and Mo separate phases, alloys with the platinum-type crystal lattice, or mixed amorphous deposits. The catalyst phase structure did not affect the general picture of observed processes; however, it had influence on the magnitude of the catalytic and inhibiting effects. The presence of crystallinity made the catalyst more stable against oxidation. Amorphous structures were unstable during the potential cycling and the catalyst storage.

The carbon nanotubes-polyaniline composites and their effect on catalytic properties of deposited catalysts

Composites of functionalized single-wall carbon nanotubes and polyaniline are deposited onto electrodes by in situ electrochemical polymerization. Their electrochemical behavior and differential capacitance are studied by cyclic voltammetry, electrochemical impedance spectroscopy, and chronovoltamperometry. The differential capacitance of the composite electrode exceeds that of pure polyaniline film deposited onto electrode, which can be explained by the nanotubes’ loosening effect on the polyaniline structure. The composite-electrode capacitance is as large as 1000 F g−1 or higher. Thus obtained composite films were used as a support for deposited platinum-ruthenium catalyst. The Pt-Ru structure and catalytic properties in the methanol oxidation reaction are studied. It is shown that the specific current of methanol oxidation at Pt-Ru is larger by a factor of 7–15 than those measured when pure polyaniline, pure carbon nanotubes, or standard Vulcan XC-72 carbon black are used as supports. It is found that the catalytic activity is the same for all studied supports, provided the current is reduced to the unit of Pt-Ru true surface area. Thus, the observed large catalytic effect is associated with the structure and high dispersivity of the electrodeposited metals incorporated to the single-wall carbon nanotubes-polyaniline composite.

Pt/Pd/C catalysts with ultra low platinum content for oxygen reduction reaction

The activity of Pt/Pd/C ETEK catalysts of the core-shell type with an ultralow content of platinum (0.5–15 μg cm−2) based on a commercial palladium catalyst is shown to exceed the activity of commercial Pt/C ETEK catalysts in the oxygen reduction reaction. The activity sharply increases with the decrease in the platinum content down to values corresponding to monolayer and submonolayer of platinum on palladium. This dependence wasn’t observed for the same amounts of platinum deposited on the carbon support Vulcan XC-72. This makes it possible to conclude that the most probable factor responsible for the high catalytic activity of Pt/Pd/C ETEK is the effect of palladium on the electronic properties of platinum rather than the effect of structural modification of the platinum deposit induced by the decrease in the platinum amount deposited on a foreign metal or a carbon support.

Ultradisperse catalytic layers supported by nanotubes and poly(diallyldimethylammonium)chloride polymer

A catalytic system consisting of carbon nanotubes, poly(diallyldimethylammonium)chloride, and a very thin layer of platinum or platinum-ruthenium is assembled layer-by-layer (LbL) on a glassy carbon (GC) electrode. Deposits of platinum metals are studied by electrochemical methods, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Such catalyst layers are shown to exhibit much higher activity in the methanol oxidation reaction as compared with commercial and electroplated catalysts. The currents compared are calculated per the surface area of deposited metals determined with respect to hydrogen adsorption.

Effect of the functionalizing of carbon nanotubes on the electrodeposited catalysts’ structure and catalytic properties

The structure and hydrophilic-hydrophobic properties of functionalized single-wall carbon nanotubes are studied by the standard porosimetry method. It is shown that the functionalized nanotubes have highly hydrophilic surface; at that the summary surface area measured “by octane” decreased, as a result of the functionalizing, due to the blocking of the nanotubes’ inner channels by the functional groups located at the nanotubes’ ends. The nanotubes’ capacitive properties are studied; their charging-discharging curves appeared being highly reversible, unlike those of other carbonaceous materials. Catalytic properties of the functionalized nanotubes are studied, with particular tendency toward their using as a carrier of platinum catalysts for the methanol oxidation and oxygen electroreduction reactions. When minor amounts (5–10 µg cm−2) of platinum or platinum-ruthenium alloy are deposited onto the nanotubes’ hydrophilic surface, uniform layer of the catalyst is formed, with specific surface area up to 150–300 m2 g−1; high current of the methanol oxidation or oxygen electroreduction is observed at these catalysts. When the catalyst deposit mass increased, its specific surface area decreased, as well as the specific current of the reactions occurring thereon. When the current is related to the electrochemically active unit surface, the catalytic activity is nearly the same both for different catalyst mass deposited onto the nanotubes and the same catalyst mass at different carbonaceous carriers.

Electron microscopy study of a Pt-Pd bimetallic structure formation on soot for catalytic systems

Bimetallic structures—Pt nanoparticles distributed on the surface of a Pd catalyst on Vulcan XC-72 soot—have been investigated. Several series of bimetallic catalysts with various contents of platinum have been obtained by pulsed electrochemical deposition. Their structure is studied using analytical high resolution transmission electron microscopy with a correction of aberrations. The data allows us to present a scheme of evolution in the bimetallic catalyst structure caused by a change in the ratio of metals Pt : Pd. For the ratio Pt : Pd = 1 : 25, the electrochemical measurements of catalytic activity have shown an extremely high current of oxygen reduction.