Mariya A. Kazakova

Investigation of defectiveness of multiwalled carbon nanotubes produced with Fe–Co catalysts of different composition

Abstract. We have performed a study of CVD multiwalled carbon nanotubes (MWCNTs) produced with Fe–Co catalysts with a variable ratio of active metals. The Raman data were considered in combination with the temperature dependence of MWCNT conductivity. The data analysis is based on the point that the value of I2D/ID ratio correlates with the graphene fragment size. The fragments are considered as building blocks of MWCNTs. We showed that MWCNT defectiveness depends on the ratio of bimetallic active components in the Fe–Co catalyst. Thus, the ratio of I2D/ID increases and the D-mode intensity decreases while the Fe content in the catalyst increases. This also points to the reduction of defect number in the bigger graphene fragments. These results correlate with the data on conductivity temperature dependence. Namely, the increase of Fe content in the active component of the Fe–Co catalyst results in the increase of charge carrier concentration, which, in turn, indicates a decrease in MWCNT defectiveness.

Bifunctional Oxygen Reduction/Oxygen Evolution Activity of Mixed Fe/Co Oxide Nanoparticles with Variable Fe/Co Ratios Supported on Multiwalled Carbon Nanotubes

A facile strategy is reported for the synthesis of Fe/Co mixed metal oxide nanoparticles supported on, and embedded inside, high purity oxidized multiwalled carbon nanotubes (MWCNTs) of narrow diameter distribution as effective bifunctional catalysts able to reversibly drive the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline solutions. Variation of the Fe/Co ratio resulted in a pronounced trend in the bifunctional ORR/OER activity. Controlled synthesis and in-depth characterization enabled the identification of an optimal Fe/Co composition, which afforded a low OER/OER reversible overvoltage of only 0.831 V, taking the OER at 10 mA cm-2 and the ORR at -1 mA cm-2 . Importantly, the optimal catalyst with a Fe/Co ratio of 2:3 exhibited very promising long-term stability with no evident change in the potential for both the ORR and the OER after 400 charge/discharge (OER/ORR) cycles at 15 mA cm-2 in 6 m KOH. Moreover, detailed investigation of the structure, size, and phase composition of the mixed Fe/Co oxide nanoparticles, as well as their localization (inside of or on the surface of the MWCNTs) revealed insight of the possible contribution of the individual catalyst components and their synergistic interaction in the catalysis.

Aldose to ketose interconversion: galactose and arabinose isomerization over heterogeneous catalysts

Isomerization of glucose, galactose and arabinose to corresponding keto-sugars was studied in the present work over a range of heterogeneous catalysts. Magnesium aluminates with different ratios between oxides resulting in materials with a Mg/Al ratio from 0.2 to 0.9 were prepared, characterized and evaluated in terms of their catalytic behavior. The catalyst with a Mg/Al ratio close to hydrotalcites was the most efficient considering activity, selectivity and stability. The sugar structure was shown to have a minor influence on catalytic activity and selectivity.