Lidiya S. Kibis

Ruthenium Clusters on Carbon Nanofibers for Formic Acid Decomposition: Effect of Doping the Support with Nitrogen

The catalytic properties of 1 wt % Ru catalysts with the same mean Ru cluster size of 1.4–1.5 nm supported on herringbone‐type carbon nanofibers with different N contents were compared for H2 production from formic acid decomposition. The Ru catalyst on the support with 6.8 wt % N gave a 1.5–2 times higher activity for the dehydrogenation reaction (CO2, H2) than the catalyst on the undoped support. The activity in the dehydration reaction (CO, H2O) was the same. As a result, the selectivity to H2 increased significantly from 83 to 92 % with N‐doping, and the activation energies for both reactions were close (55–58 kJ mol−1). The improvement could be explained by the presence of Ru clusters stabilized by pyridinic N located on the open edges of the external surface of the carbon nanofibers. This N may activate formic acid by the formation of an adduct (>NH+HCOO−) followed by its dehydrogenation on the adjacent Ru clusters.

Palladium Nanoparticles Supported on Nitrogen‐Doped Carbon Nanofibers: Synthesis, Microstructure, Catalytic Properties, and Self‐Sustained Oscillation Phenomena in Carbon Monoxide Oxidation

The oxidation of CO over Pd nanoparticles supported on carbon nanofibers (CNFs) and N‐doped carbon nanofibers (N‐CNFs) has been studied. Investigation by scanning transmission electron microscopy together with electron energy‐loss spectroscopy revealed that Pd nanoparticles are located on the N‐CNFs surface patches that have a high concentration of N atoms. The N‐doping of CNFs was shown to change the electric conductivity of N‐CNFs and redox properties of Pd, which thus determines the self‐oscillatory behavior of the catalysts during CO oxidation, the type of oscillations, and the conditions of their generation. Mechanisms that underlie the effect of N in N‐CNFs on the electronic state of Pd as well as the occurrence of two types of oscillation mechanisms—the known redox mechanism and the mechanism related to Pd intercalation into graphene layers—are discussed.

Highly Oxidized Platinum Nanoparticles Prepared through Radio‐Frequency Sputtering: Thermal Stability and Reaction Probability towards CO

Platinum-oxide nanoparticles were prepared through the radio-frequency (RF) discharge sputtering of a Pt electrode in an oxygen atmosphere. The structure, particles size, electronic properties, and surface composition of the RF-sputtered particles were studied by using transmission electron microscopy and X-ray photoelectron spectroscopy. The application of the RF discharge method resulted in the formation of highly oxidized Pt(4+) species that were stable under ultrahigh vacuum conditions up to 100 °C, indicating the capability of Pt(4+) -O species to play an important role in the oxidation catalysis under real conditions. The thermal stability and reaction probability of Pt(4+) oxide species were analyzed and compared with those of Pt(2+) species. The reaction probability of PtO2 nanoparticles at 90 °C was found to be about ten times higher than that of PtO-like structures.

A correlation between structural changes in a Ni-Cu catalyst during decomposition of ethylene/ammonia mixture and properties of nitrogen-doped carbon nanofibers

Abstract Changes of a 65Ni25Cu10Al 2 O 3 catalyst consisting of Ni-enriched and Cu-enriched alloys were investigated in the bulk and on the surface during the growth of nitrogen-doped carbon nanofibers (N-CNFs) by decomposition of a 50%C 2 H 4 /50%NH 3 mixture using in situ X-ray diffraction (XRD) analysis, ex situ X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. It was shown that N-CNF growth at 450–650 °C is accompanied by dissolution of carbon and nitrogen in the Ni-enriched alloy, whereas Cu-enriched alloy remains inactive. A correlation between nickel and copper surface concentrations and properties of N-CNFs in relation to the nitrogen content was found. It was demonstrated that phase composition of the catalyst during N-CNF growth determines the type of N-CNFs structure.

Reactivity and thermal stability of oxidized copper clusters on the tantalum(V) oxide surface

Oxidized copper clusters 2–5 nm in size have been obtained by RF discharge sputtering of a copper wire in an oxygen atmosphere. Isolated CuO clusters or, at long deposition times, their agglomerates form on the support. The thermal stability of the oxidized nanoparticles in a vacuum and their reactivity toward CO in relation to the deposition time have been investigated by X-ray photoelectron spectroscopy. The asprepared clusters show low reactivity (10−7−10−9), but their activation by reduction and subsequent reoxidation in an oxygen medium raises their reactivity to ∼10−5. This is due to the appearance of weakly charged oxygen species on the surface. The reactivity of the CuO clusters has been compared to the reactivity of earlier studied nanosized copper oxide model objects.

Observation of the superstructural diffraction peak in the nitrogen doped carbon nanotubes: Simulation of the structure

ABSTRACT A superstructural peak at ∼12° in X-ray diffraction patterns of nitrogen-doped carbon nanotubes compared to the undoped carbon nanotubes was observed and assigned to the formation of spatially ordered defects. The simulation of the N-CNT structure using the graphitic g-C3N4 phase and turbostratic ordering made it possible to propose a new model of the spatially ordered defects in the N-CNT layer, which consist of clusters of carbon vacancies and pyridine-like nitrogen. A correlation between this type of defects and electrical conductivity of the N-CNTs is defined.

Comparison of growth mechanisms of undoped and nitrogen-doped carbon nanofibers on nickel-containing catalysts

Abstract The growth mechanisms of carbon nanofibers on Ni catalysts and nitrogen-doped carbon nanofibers on Ni and Ni-Cu catalysts were studied. The growth of both types of nanofibers was found to occur by a mechanism that included the formation of surface non-stoichiometric nickel carbide followed by the dissolution and diffusion of carbon, or carbon and nitrogen into the bulk of the catalyst particles.

Synthesis and catalytic activity of porous blocked Ag/SiO2 composites in low-temperature carbon monoxide oxidation

The Ag/SiO2 composites were synthesized based on porous blocked silica with a pore size of 30–50 nm and a specific surface area of 99 m2/g. Silver particles were introduced into the pores of the support by its impregnation with a solution of an ammonium complex of silver followed by reduction with hydrogen. The liquid-phase reduction of silver ions in pores was performed in the absence of stabilizing agents with the use of ethylene glycol (a polyol method) or formamide as a reducing agent. The methods used in the preparation of composites made it possible to vary the particle size of silver. The greatest size that is almost comparable with the pore size was achieved with the use of formamide. The catalytic activity of the Ag/SiO2 composites was studied in the reaction of CO oxidation. It was found that the catalysts obtained upon the reduction of Ag+ ions by formamide exhibited considerable low-temperature activity. A necessary condition for the manifestation of low-temperature activity is redox treatment, in the course of which the particle size of silver considerably decreases.

Intermetallic compounds as potential alternatives to noble metals in heterogeneous catalysis: The partial hydrogenation of butadiene on alpha-Al4Cu9(110)

Recently, non‐noble intermetallic compounds have shown promising catalytic performances in the partial hydrogenation of alkynes and alkenes. In this work, the properties of γ‐Al4Cu9(1 1 0) toward the gas‐phase hydrogenation of butadiene were investigated at total pressures of 2–20 mbar and temperatures of 110–180 °C. The model catalyst is active and 100 % selective to butenes. Moreover, although less active than Al13Fe4(0 1 0), which was evaluated previously for the same reaction, it is more selective and more stable. The combination of catalytic tests with pre‐ and postreaction Auger electron spectroscopy measurements and comparative tests with Cu(1 1 0) shows that Cu governs the reaction on γ‐Al4Cu9(1 1 0). However, the lower activity of the (more Cu‐rich) sputtered Al4Cu9 surface with respect to the annealed one and the differences between Al4Cu9 and Cu surfaces in terms of butene isomer distribution, butene conversion kinetics, and sensitivity to poisons, demonstrate the unique character of the intermetallic compound.

Application of RF discharge in oxygen to create highly oxidized metal layers

ABSTRACT The radio frequency (RF) discharge in an oxygen atmosphere was used to produce metal oxide films. The high efficiency of the RF discharge technique for metal oxidation at room temperature was demonstrated for gold, silver and copper foils. Oxide films up to 10 nm in thickness were obtained. The produced oxide films were studied by X-ray photoelectron spectroscopy (XPS). The XPS data showed the formation of oxidised species: Cu2+, Ag1+ and Au3+. Analysis of the oxygen species was performed using O1s spectra. For copper and silver foils, the formation of additional oxygen species apart from oxygen in the structure of oxides was shown. The reaction probability toward oxidation of carbon monoxide (CO) was estimated for all oxidised layers. It was established that gold and silver oxide films interacted with CO at room temperature, while cupric oxide showed high activity at temperature >353 K.