Elena M. Slavinskaya

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.

Metal–support interaction in Pd/CeO2 model catalysts for CO oxidation: from pulsed laser-ablated nanoparticles to highly active state of the catalyst

Palladium and cerium oxide nanoparticles obtained by pulsed laser ablation (PLA) in liquid (water or ethanol) have been used as nanostructured precursors for the synthesis of composite Pd/CeO2 catalysts. The initial mixture of Pd and CeO2 nanoparticles does not show catalytic activity at temperatures lower than 100 °C. It has been found that the composites prepared by PLA in alcohol are easily activated by calcination in air at 450–600 °C, demonstrating a high level of activity at room temperature. Application of XRD, TEM and XPS reveals that laser ablation in water leads to the formation of large and well-crystallized nanoparticles of palladium and CeO2, whereas ablation in alcohol results in the formation of much smaller PdCx nanoparticles. The activation of the composites takes place due to the strong Pd–ceria interaction which occurs more easily for highly dispersed defective particles obtained in alcohol. Such an interaction implies the introduction of palladium ions into the ceria lattice with the formation of a mixed phase of PdxCe1−xO2−x−δ solid solution at the contact spaces of palladium and cerium oxide nanoparticles. TPR-CO and XPS data show clearly that on the surface of the PdxCe1−xO2−x−δ solid solution the oxidized PdOx(s)/Pd–O–Ce(s) clusters are formed. These clusters are composed of highly reactive oxygen which is responsible for the high level of catalytic activity in LTO CO.

Effect of preparation procedure on the properties of CeO2

The effect of preparation procedure on the physicochemical and catalytic properties of CeO2 was studied. Differences in the electronic and structural characteristics of CeO2 depending on preparation procedure and treatment temperature were found using X-ray diffraction analysis, transmission electron microscopy, UV-visible electronic spectroscopy, and X-ray photoelectron spectroscopy. With the use of the temperature-programmed reaction with CO, it was demonstrated that CeO2 samples with a high concentration of point defects—oxygen vacancies caused by the presence of Ce3+—were characterized by an increased mobility of bulk oxygen. The samples of CeO2 with a high concentration of structural defects—micropores of size 1–2 nm and stepwise vicinal faces in crystallites—exhibited a high catalytic activity in the reaction of CO oxidation.

Catalytic and capacity properties of nanocomposites based on cobalt oxide and nitrogen-doped carbon nanofibers

Abstract The nanocomposites based on cobalt oxide and nitrogen-doped carbon nanofibers (N-CNFs) with cobalt oxide contents of 10–90 wt% were examined as catalysts in the CO oxidation and supercapacity electrodes. Depending on Co 3 O 4 content, such nanocomposites have different morphologies of cobalt oxide nanoparticles, distributions over the bulk, and ratios of Co 3+ /Co 2+ cations. The 90%Co 3 C 4 -N-CNFs nanocomposite showed the best activity because of the increased concentration of defects in N-CNFs. The capacitance of electrodes containing 10% Co 3 c 4 -N-CNFs was 95 F/g, which is 1.7 times higher than electrodes made from N-CNFs.

Synthesis and physicochemical characterization of palladium-cerium oxide catalysts for the low-temperature oxidation of carbon monoxide

The effect of CeO2 preparation procedure on the electronic and structural states of the active component of Pd/CeO2 catalysts and their activity in the low-temperature reaction of CO oxidation was studied. The following two nonequivalent states of palladium were detected in the catalysts having low-temperature activity using XPS and IR spectroscopy: Pd0(Pdδ+) as the constituent of a palladium-reduced interaction phase and Pd2+ as the constituent of a palladium-oxidized interaction phase PdxCeO2 −δ. It was found that the procedure used for preparing a CeO2 support considerably affected the formation of these phases and quantitative ratios between them. It was demonstrated that the palladium-oxidized interaction phase was responsible for low-temperature activity, whereas the palladium-reduced interaction phase was responsible for activity in the region of medium and high temperatures.

Transformation of a Pt-CeO2 Mechanical Mixture of Pulsed-Laser-Ablated Nanoparticles to a Highly Active Catalyst for Carbon Monoxide Oxidation

The pulsed laser ablation (PLA) in alcohol and water media was employed to prepare Pt and CeO2 PLA‐nanoparticles of different sizes and degrees of defectiveness. Interactions of metallic platinum and ceria particles were studied using the thermal activation of Pt–CeO2 mechanical mixtures in the CO+O2 reaction medium or O2 atmosphere. The thermal activation resulted in oxidized Pt2+/Pt4+ states of platinum in the surface solid solutions PtCeOx and/or PtOx clusters. Catalysts formed after calcination of the PLA‐ablated Pt–CeO2 mixtures in oxygen at 450–600 °C revealed CO conversion at very low temperatures up to 70 % depending on the conditions of PLA particles preparation and thermal activation of Pt–CeO2 mechanical mixture.

Highly active and durable Pd/Fe2O3 catalysts for wet CO oxidation under ambient conditions

Pd/Fe2O3(FeOOH) catalysts were prepared in different ways: T – traditional incipient wetness impregnation (IWI) from a solution of palladium nitrate, D – modification of the support surface by dimethylformamide (DMF) prior to IWI, and DF – variant D followed by treatment with a sodium formate solution. These catalysts have been tested for CO oxidation under isothermal conditions at 20 °C in the presence and absence of water vapor and characterized by XRD, TEM, XPS, H2-reduction and adsorption methods. The Pd(T)/Fe2O3 catalyst is highly active in CO oxidation at room temperature under “dry” conditions but is deactivated in the presence of water vapor. The Pd(D)/Fe2O3 catalyst is inactive in low-temperature CO oxidation, whereas Pd(DF)/Fe2O3(FeOOH) catalysts are characterized by high activity at room temperature and ambient humidity. The main state of palladium in the Pd(T)/Fe2O3 catalyst without pretreatment with DMF is in nitrate complexes, where it can be readily reduced to form clusters ∼1.5 nm in size. In the case of Pd(D)/Fe2O3, palladium interacts with dimethylformamide forming complexes which cannot be reduced by hydrogen at room temperature. It is proposed that palladium clusters are located within the interdomain boundaries of the hydrophobic support in (0.5–1.0)% Pd(DF)/Fe2O3 active catalysts. These (0.5–1.0)% Pd(DF)/Fe2O3 catalysts were active towards CO oxidation at ambient temperature and humidity for several hours.

Steam reforming of methane over Ni-substituted Sr hexaaluminates

Abstract Ni-substituted Sr-aluminates Sr1-xNixAl11.2+xNi0.8-xO19-δ (x = 0; 0.2; 0.4; 0.8) obtained by a precipitation method and calcined at 1200°C have been characterized by different physicochemical techniques and their catalytic properties have been tested in steam reformation of methane. It has been shown that substitution of Al3+ and/or Sr2+ by Ni2+ in the aluminate structure results in changes of phase composition, specific surface area, and reducibility of samples. It has been established that the samples are not completely reduced in the temperature range of 30-900°C. The Sr1-xNixAl11.2+xNi0.8-xO19-δ (x = 0; 0.2; 0.4) catalysts are active and stable in the steam reforming of methane at 700oC: residual amount of methane is (1.1±1.0) vol.%, while the Sr1-xNixAl11.2+xNi0.8-xO19-δ (x = 0.8) sample is rapidly deactivated by coking.

The influence of CuO dispersion on catalytic properties in the CO oxidation: a comparative studies in two types of catalytic reactors

The catalytic properties of copper(II) oxide powders were studied in the CO oxidation reaction by using plug‐flow and flow‐circulation reactors. Data obtained from different catalytic experiments were in good agreement with each other. The specific catalytic rate [molecules cm−2 s−1] increased by a factor of approximately four if the surface area of the CuO powder was reduced from 90 to 8 m2 g−1. A further decrease in the CuO surface area to 1 m2 g−1 resulted in the decrease of the specific CO oxidation rate by more than 20‐fold. These results indicate that CO oxidation is sensitive to the structure of the copper(II) oxide catalyst. The structural sensitivity of CuO powders was discussed in terms of its defect structure and particle morphology.

Enhanced Thermal Stability of Pd/Ce–Sn–O Catalysts for CO Oxidation Prepared by Plasma-Arc Synthesis

The plasma-arc (PA) method was applied for the highly efficient synthesis of Pd/Ce–Sn–O catalysts for CO oxidation. Using the PA sputtering of a graphite electrode together with Pd, Ce and Sn metallic components in inert atmosphere, a PdCeSnC composite was obtained. After the subsequent calcination in oxygen over the temperature range of 600–1000 °C, the initial composites were transformed into active catalysts of CO oxidation at low temperatures (LTO CO). Catalytic testing showed that these PA-prepared Pd/Ce–Sn–O catalysts were characterized by unusually high thermal stability. The catalysts demonstrated the excellent LTO CO performance after calcination at 1000 °C. According to the XRD and HRTEM observations, the Pd/Ce–Sn–O catalysts can be described as heterogeneous structures consisting of small CeO2 and SnO2 particles that interact with each other, forming extended grain boundaries and a composite structure. The TPR-CO and XPS methods detected highly dispersed Pd species in the active catalysts, namely Pd2+ in the lattice of ceria (a Pd-ceria solid solution) and the PdOx clusters on the surface. Deactivation of the Pd/Ce–Sn–O is governed by decomposition of the Pd-ceria solid solution accompanied by the sintering of the PdOx clusters and formation of the metallic and oxide palladium nanoparticles. Oxygen species with high mobility in the Pd/Ce–Sn–O catalyst were detected by a TPR-CO method. The amount of the highly mobile oxygen species is in five times higher for the Pd/Ce–Sn–O catalyst then for the Pd/CeO2 sample. Promising perspectives of the plasma-arc application for catalyst the synthesis of with improved properties are discussed.