Andrey I. Boronin

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

The catalytic properties of 1u2005wtu2009% Ru catalysts with the same mean Ru cluster size of 1.4–1.5u2005nm 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 6.8u2005wtu2009% N gave a 1.5–2u2005times 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 92u2009% with N‐doping, and the activation energies for both reactions were close (55–58u2005kJu2009mol−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.

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.

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.

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 600–1000u2009°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 1000u2009°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.