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

A study of Pt/al2O3 nanocomposites obtained by pulsed laser ablation to be used as catalysts of oxidation reactions

Pulsed laser ablation (PLA) in liquids is an effective high-energy method for the synthesis of functional nanomaterials. In the present work, a nanocomposite catalyst Pt/Al2O3(PLA) is prepared by mixing solutions of platinum and aluminium nanodispersions obtained by the PLA method in alcohol and water, respectively. After being dried out, the obtained nanocomposite is thermally treated in air at 400 °C and 550 °C. It is shown by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction that platinum in the composition of the calcined samples stabilizes on the Al2O3 surface in the form of metal nanoparticles. The main crystal structure of η-Al2O3 is determined and impurity phases of Al(OH)3 hydroxide and metallic aluminium are revealed using X-ray powder diffraction (XRPD). The Pt/Al2O3 nanocomposite samples obtained by the PLA method are found to be highly prospective for the use in reactions of catalytic oxidation of CO and NH3. The Pt/Al2O3(PLA) nanocomposites are compared with the Pt/Al2O3 (IMP) catalyst synthesized by the method of solution chemistry. The Pt/Al2O3(IMP) sample containing highly dispersed platinum nanoparticles (1–2 nm) on the γ-Al2O3 surface has a lower T50 value (188 °C) in the reaction of CO oxidation that the PLA catalyst (T50 = 198 °C). At the same time, in the reaction of NH3 oxidation, the PLA catalyst is more active (T50 = 167 °C) than the IMP sample (T50 = 180 °C). The observed regularities are discussed in terms of the dispersion and the oxidation depth of platinum particles in the composition of Pt/Al2O3 catalysts

XPS Study of Nanostructured Rhodium Oxide Film Comprising Rh⁴⁺ Species

Studies of highly oxidized rhodium species as potential active sites in catalytic oxidation reactions are of great interest. In this work, we investigated the properties of highly oxidized nanostructured rhodium film prepared by radio frequency discharge in an oxygen atmosphere. The charge states of Rh in RhO_<i>x</i> particles, their thermal stability, and reactivity toward CO were analyzed in comparison with the properties of thermally prepared Rh₂O₃ oxide. The formation of Rh⁴⁺ species in a composition of Rh⁴⁺/Rh³⁺ oxyhydroxide structures was shown to take place in plasma-synthesized films. The highly oxidized rhodium species was stable up to 150 °C and demonstrated reactivity in a CO oxidation reaction at 100 °C. The reoxidation of a partially reduced Rh/RhO_<i>x</i> film was observed at 100 °C under treatment with molecular O₂. However, Rh⁴⁺ species were not recovered under such conditions

Highly Oxidized Gold Nanoparticles: In Situ Synthesis, Electronic Properties, and Reaction Probability Toward CO Oxidation

Highly oxidized gold nanoparticles prepared by RF-discharge under an oxygen atmosphere were studied by X-ray photoelectron spectroscopy and transmission electron microscopy depending on the particle size. A surface-like gold oxide was found in the case of small nanoparticles (1–2 nm) obtained at the first steps of deposition. With an increase of the particle size up to 5 nm, a bulklike gold oxide was formed. The O 1s spectra exhibited an oxygen peak at binding energy <i>E</i>_b = 529.4 eV for the surface-like oxide and <i>E</i>_b = 530.7 eV for the bulklike gold-oxide. The reaction probability of oxidized gold nanoparticles was examined in the reaction of CO oxidation at room temperature. The surface-like gold oxide interacted with CO with a high reaction probability of approximately 0.005, while CO interaction with the bulklike oxide was characterized by an induction period with lower reaction probability (0.001). The mechanisms of the interaction of oxidized gold nanoparticles with CO depending on its size are discussed

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 CeO2d. The reduced sample demonstrated efficient CO conversion at 0 1C. 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

Co-doped ZnS clusters and nanostructures produced by pulsed laser ablation

In this work, we synthesize Co-doped ZnS nanocrystals by pulsed laser ablation and deposition and investigate the composition and dynamics of the plasma produced in the ablation by time-of-flight mass spectrometry. The first experimental observation of Co-substituted ZnS clusters in the ablation plasma is reported; three series of cationic bimetallic clusters Zn n-mComSn +, Znn-mCo mSn+1 +, and Znn-mCo mSn-1 + are identified upon 1064 nm laser ablation of targets of Co (2%) in ZnS. Neutral clusters produced in the ablation are also observed by F2 laser (λ = 157 nm) postionization. Deposits are collected at the same laser ablation wavelength and fluence at which the above bimetallic clusters are observed in the plasma. Analysis by high-resolution transmission electron microscopy and energy-dispersive X-ray spectrometry obtains that the deposits are composed of Zn0.96Co 0.04S nanocrystals with local structure showing epitaxial growth between the zinc blende and wurtzite politypes. The deposits show paramagnetic behavior, but no magnetic ordering is observed. The results constitute a first step to help to elucidate the participation of cluster assembly processes in the synthesis of Co-doped ZnS nanomaterials by pulsed laser ablation and deposition. © 2013 American Chemical Society.Financial support was provided by the Spanish MINECO (CTQ2010-15680), Russian RFBR and SBRAS (10-03-00441 and Integration Project No. 134), and CSIC-RFBR (2008RU0069/09-02-91291). M.L.A. and M.J. wish to thank the CSIC and ESF (European Social Fund) for contracts under the JAE Fellowship Programme.Peer Reviewe

Redox and Catalytic Properties of Rh_<i>x</i>Ce_1–<i>x</i>O_2−δ Solid Solution

In this work, a detailed study of the redox properties of solid solution Rh_<i>x</i>Ce_1–<i>x</i>O_2−δ in correlation with its catalytic activity in CO oxidation reaction was carried out. The ex situ X-ray photoelectron spectroscopy technique was applied to follow the charging states of the elements on the surface during the redox treatments at a temperature range of 25–450 °C. The results were compared with the data of temperature-programmed reduction by CO. The dissolution of rhodium in the ceria bulk considerably increased the mobility of CeO₂ lattice oxygen, with redox transitions Ce⁴⁺ ↔ Ce³⁺ and Rh³⁺ ↔ Rh_<i>n</i>^δ+ observed already at low temperatures (below 150 °C). The reduced rhodium clusters (Rh_<i>n</i>^δ+) formed during the reduction treatment significantly improved the catalytic activity of the Rh_<i>x</i>Ce_1–<i>x</i>O_2−δ solid solution. The small size of the rhodium clusters (Rh_<i>n</i>^δ+) and high defectiveness of the fluorite phase provided the reversibility of Rh_<i>n</i>^δ+/CeO₂ ↔ Rh_<i>x</i>Ce_1–<i>x</i>O_2−δ transitions upon redox treatment, resulting in the high reproducibility of the CO conversion curves in the temperature-programmed reaction CO + O₂. The homogeneous solid solution was stable up to 800 °C. Above this temperature, the CeO₂ volume was depleted of Rh³⁺ ions because of their partial segregation into the surface and/or subsurface layers with the formation of Rh₂O₃. For these inhomogeneous samples, the oxygen mobility was considerably lower, while the redox transitions, Ce⁴⁺ ↔ Ce³⁺ and Rh³⁺ ↔ Rh_<i>n</i>^δ+, required higher temperatures