Pt/CeO2 and Pt/CeSnOx catalysts for low-temperature CO oxidation prepared by plasma-arc technique

We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeOx and Pt/CeSnOx for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnOx and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt2+ and Pt4+ ions in the catalyst Pt/CeOx, whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnOx. Insertion of Sn ions into the Pt/CeOx lattice destabilizes/reduces Pt4+ cations in the Pt/CeSnOx catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce3+ ions. Our DFT calculations corroborate destabilization of Pt4+ ions by incorporation of cationic Sn in Pt/CeOx. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions

States of Pt/CeO2 catalysts for CO oxidation below room temperature.

CO molecules can be efficiently oxidized over Pt/CeO2 catalysts, but the stability and reactivity of different states of Pt in the catalysts are still unclear. Here we combine experimental and computational methods to characterize Pt/CeO2 catalysts subjected to reductive and oxidative pre-treatments and exposed to CO oxidation reaction conditions. Particles of metallic Pt, known to be catalytically active at elevated temperature, are shown to be precursors for the formation, under operando conditions, of more stable PtOx particles that enable CO oxidation below room temperature. These PtOx particles are also similarly stable to - but more active than - atomically dispersed Pt2+ species. The results and approaches presented in this study illustrate the complex response of catalytic materials to reaction conditions and pave the way for future efforts to improve Pt/CeO2 and similar catalysts using dedicated pre-treatment strategies

Influence of Titania Synthesized by Pulsed Laser Ablation on the State of Platinum during Ammonia Oxidation

A set of physicochemical methods, including X-ray photoelectron spectroscopy (XPS), X-ray diffraction, electron microscopy and X-ray absorption spectroscopy, was applied to study Pt/TiO$_{2}$ catalysts prepared by impregnation using a commercial TiO$_{2}$-P25 support and a support produced by pulsed laser ablation in liquid (PLA). The Pt/TiO$_{2}$-PLA catalysts showed increased thermal stability due to the localization of the highly dispersed platinum species at the intercrystalline boundaries of the support particles. In contrast, the Pt/TiO$_{2}$-P25 catalysts were characterized by uniform distribution of the Pt species over the support. Analysis of Pt4f XP spectra shows that oxidized Pt$^{2+}$ and Pt$^{4+}$ species are formed in the Pt/TiO$_{2}$-P25 catalysts, while the platinum oxidation state in the Pt/TiO$_{2}$-PLA catalysts is lower due to stronger interaction of the active component with the support due to strong interaction via Pt-O-Ti bonds. The Pt4f XP spectra of the samples after reaction show Pt$^{2+}$ and metallic platinum, which is the catalytically active species. The study of the catalytic properties in ammonia oxidation showed that, unlike the catalysts prepared with a commercial support, the Pt/TiO$_{2}$-PLA samples show higher stability during catalysis and significantly higher selectivity to N$_{2}$ in a wide temperature range of 200–400 °C

Pt/CeO2 and Pt/CeSnOx Catalysts for Low-Temperature CO Oxidation Prepared by Plasma-Arc Technique

We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeOx and Pt/CeSnOx for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnOx and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt2+ and Pt4+ ions in the catalyst Pt/CeOx, whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnOx. Insertion of Sn ions into the Pt/CeOx lattice destabilizes/reduces Pt4+ cations in the Pt/CeSnOx catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce3+ ions. Our DFT calculations corroborate destabilization of Pt4+ ions by incorporation of cationic Sn in Pt/CeOx. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions

In situ probing of Pt/TiO2_{2} activity in low-temperature ammonia oxidation

The improvement of the low-temperature activity of the supported platinum catalysts in selective ammonia oxidation to nitrogen is still a challenging task. The recent developments in in situ/operando characterization techniques allows to bring new insight into the properties of the systems in correlation with their catalytic activity. In this work, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and operando X-ray absorption spectroscopy (XAS) techniques were applied to study Pt/TiO$_{2}$ catalysts in ammonia oxidation (NH$_{3}$ + O$_{2}$ reaction). Several synthesis methods were used to obtain samples with different size of Pt particles, oxidation state of Pt, and morphology of the support. Metal platinum particles on titania prepared by pulsed laser ablation in liquids exhibited the highest activity at lower temperatures with the temperature of 50% conversion of NH$_{3}$ being 150 °C. The low-temperature activity of the catalysts synthesized by impregnation can be improved by the reductive pretreatment. NAP-XPS and operando XANES data do not show formation of PtO$_{x}$ surface layers or PtO/PtO$_{2}$ oxides during NH$_{3}$ + O$_{2}$ reaction. Despite the differences in the oxidation state of platinum in the as-prepared catalysts, their treatment in the reaction mixture results in the formation of metallic platinum particles, which can serve as centers for stabilization of the adsorbed oxygen species. Stabilization of the bulk platinum oxide structures in the Pt/TiO$_{2}$ catalysts seems to be less favorable due to the metal–support interaction

Society, State, Nation and the People in the Democratic South Africa: Two Decades of Illusions in The Practice of Public Administration, Development Planning and Management

The purpose of this article is to provide a conceptual argument in that as South Africa transcended beyond its democratic dispensation, an opportunity was missed in the process leading to transition to define a society, state, nation and the people for sustaining the democratic founding for purposes of public administration practice, development planning and management. That is done by critically portraying South Africa as a society, state, nation and locating the people for governance purposes within a democratic founding. It is argued that attempts are made to rewrite the history of the country with a view of bolstering its societal status, nation, state and the people without a profound context. South Africa has become what it is today due to its history that remains its defining factor if it has to locate its society, state, nation and the people. Having lost that opportunity during transition, governance has become so unwieldy in that those assigned with authority in the governance landscape, tend to confuse the roles of society, nation, state and the people and that eventually strain the fragile democracy by distorting the facts and the role of constitutional apparatus that are instrumental to the country’s democratic founding. The conclusion is rather pessimistic in that as long as these issues are not properly located within the governance landscape; the democratic dispensation remains vulnerable for demise just like other democracies within the African continent with the potential of the middle class hijacking it from the vulnerable poor majority being the people that public administration practice has to serve. DOI: 10.5901/mjss.2015.v6n2s1p61

Peculiarities of structure and morphology of copper-cerium nanopowders produced by laser ablation

Copper-cerium nanopowders CuOx–CeO2 (mass ratio Cu:Ce = 6:100) are prepared by mixing the dispersions of the copper and cerium oxides produced by the method of pulse laser ablation (PLA) in liquid, followed by drying. The initial dispersions of copper oxides were prepared by the method of PLA of a metal copper target in distilled water or 1% hydrogen peroxide solution, and those of cerium oxide – by PLA of metal cerium in distilled water. It is shown that ablation of copper in water and water solution of peroxide is followed by the formation of copper oxide particles of different morphologies and compositions (structure). It is established that no crystal phases of copper oxides are formed in the copper-cerium nanopowders produced from separate dispersions. Given this approach to forming copper-cerium nanoparticles, the oxidized copper is distributed in the form of a thin layer on the CeO2 surface, which is demonstrated by the results of investigation of these particles by the methods of high-resolution transmission electron microscopy and X-ray diffraction. The formation of a Cu–O–Ce interface at the interphase boundary gives rise to the formation of defects on the CeO2 surface, which is confirmed by the Raman spectroscopy. An investigation of the composition and electronic structure of the surface of CuOx nanoparticles and CuOx–CeO2 nanopowders performed by the method of X-ray photoelectronic spectroscopy reveals the presence of copper in the form of a combination of Cu (I) and Cu (II) with the prevailing contribution from a single-valence state for CuOx–CeO2 nanopowders, which could have resulted from the interaction between CuOx and CeO2 particles

Nitrogen Doped Carbon Nanotubes and Nanofibers for Green Hydrogen Production: Similarities in the Nature of Nitrogen Species, Metal–Nitrogen Interaction, and Catalytic Properties

The effect of nitrogen doped bamboo-like carbon nanotubes (N–CNTs) on the properties of supported platinum (0.2 and 1 wt %) catalysts in formic acid decomposition for hydrogen production was studied. It was shown that both impregnation and homogeneous precipitation routes led to the formation of electron-deficient platinum stabilized by pyridinic nitrogen sites of the N–CNTs. The electron-deficient platinum species strongly enhanced the activity and selectivity of the Pt/N–CNTs catalysts when compared to the catalysts containing mainly metallic platinum nanoparticles. A comparison of bamboo-like N–CNTs and herring-bone nitrogen doped carbon nanofibers (N–CNFs) as the catalyst support allowed us to conclude that the catalytic properties of supported platinum are determined by its locally one-type interaction with pyridinic nitrogen sites of the N–CNTs or N–CNFs irrespective of substantial structural differences between nanotubes and nanofibers