Physicochemical investigation of nanopowders prepared by laser ablation of crystalline silicon in water

In this work, an investigation of SiOx nanopowders synthesised by the laser ablation of crystalline silicon in water, evaporation of solvent and heat treatment is presented. The material is obtained for the first time by laser ablation in large quantities, which allows the use of a variety of physicochemical methods. The chemical composition and structure of the nanopowders were analysed before and after calcination in air and/or argon from 200 to 1000 °С. The particles prepared by laser ablation of a silicon target in water contain an amorphous and/or crystalline silicon core and a silicon oxide shell. Calcination results in the disappearance of crystalline silicon reflections in diffraction patterns and the appearance of oxygen vacancies in the silica. These materials are potentially important for such applications as optics, sensors and catalysis

Physicochemical investigation of nanopowders prepared by laser ablation of crystalline silicon in water

In this work, an investigation of SiOx nanopowders synthesised by the laser ablation of crystalline silicon in water, evaporation of solvent and heat treatment is presented. The material is obtained for the first time by laser ablation in large quantities, which allows the use of a variety of physicochemical methods. The chemical composition and structure of the nanopowders were analysed before and after calcination in air and/or argon from 200 to 1000 °С. The particles prepared by laser ablation of a silicon target in water contain an amorphous and/or crystalline silicon core and a silicon oxide shell. Calcination results in the disappearance of crystalline silicon reflections in diffraction patterns and the appearance of oxygen vacancies in the silica. These materials are potentially important for such applications as optics, sensors and catalysis

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

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

Unraveling the low-temperature activity of Rh-CeO2 catalysts in CO oxidation: probing the local structure and Red-Ox transformation of Rh3+ species

The local structure of the active sites is one of the key aspects of establishing the nature of the catalytic activity of the systems. In this work, a detailed structural investigation of the Rh-CeO2 catalysts prepared by the co-precipitation method was carried out. The application of a variety of physicochemical methods such as XRD, Raman spectroscopy, XPS, TEM, TPR-H2, and XAS revealed the presence of highly dispersed Rh3+ species in the catalysts: Rh3+ single ions and RhOx clusters. The substitution of Ce4+ ions by Rh3+ species, which provided a strong distortion of the CeO2 lattice, is shown. XAS data ensured the refinement of the Rh local structure. It was shown that single Rh3+ sites located next to each other can merge the formation of RhOx clusters with Rh local environment close to the one in Rh2O3 and CeRh2O5 oxides. The distortion of the CeO2 lattice around single and cluster rhodium species had a beneficial effect on the catalytic activity of the samples in low-temperature CO oxidation (LTO-CO). TEM, XAS, and in situ XRD data allowed establishing the structural transformations of the catalysts under Red-Ox treatments. The reduction treatment led to Rhn metallic cluster formation localized on defects of the reduced CeO2−δ. The reduced sample demonstrated efficient CO conversion at 0 °C. However, this system was not stable: its contact with air led to ceria reoxidation and partial reoxidation of Rh to highly dispersed Rh3+ species at room temperature, while heating in an oxidizing atmosphere resulted in the complete reoxidation of metallic rhodium species. The results of the work shed light on the structural aspects of the reversibility of the Rh-CeO2 catalysts based on the highly dispersed Rh3+ species under treatment in the reaction conditions

NiCuMo-SiO2 catalyst for pyrolysis oil upgrading: model acidic treatment study

The main reasons of catalysts deactivation in hydro-processing pyrolysis liquids are by coke deposition, poisoning by bio-oil impurities (S, N, K, Cl, etc.), leaching of catalyst components, structural degradation in the presence of H2O, and sintering. The deactivation of catalysts by the acidity of the pyrolysis liquid is a specific concern, and this deactivation mechanism was studied by treating newly developed NiCuMo-SiO2 catalysts in 1 M acetic acid water solution (pH = 2-3). The activity of the acid-treated catalysts was subsequently investigated in the hydrodeoxygenation of gaseous propionic acid, in a tubular reactor at 225 degrees C with n-hexane and n-octane serving as diluent and internal standard, respectively. The samples treated by acid at different times (15-360 min) were characterized by X-ray diffraction (XRD), high resolution transition electron microscopy (HRTEM), X-ray fluorescence (XRF), CO chemisorption, N-2 physical adsorption, and X-ray photoelectron spectroscopy (XPS). XRF and HRTEM studies together with the residual mass of catalyst pointed out at gradual leaching of catalyst components. Among the catalyst components, dissolution of nickel was the most pronounced, while molybdenum content decreased to a lesser extent. This is due to the formation of more acid stable molybdenum blues. The amount of copper decreased only slightly, due its higher electrochemical potential. Oxidation of metallic species Cu and Ni is shown to obtain Cu2O, NiO and Ni(OH)(2)-like phases. Interestingly, the acidic treatment resulted in increasing active surface of the catalyst, nevertheless, the catalyst activity in propionic acid conversion irreversibly decreased in time by the acetic acid treatment due to loss of the active components (substantially nickel)