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

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

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

Influence of Y doping on catalytic activity of CeO2, MnOx, and CeMnOx catalysts for selective catalytic reduction of NO by NH3

Novel yttrium-doped CeO2, MnOx, and CeMnOx composites are investigated as catalysts for low-temperature NH3-SCR. The study involves the preparation of unmodified oxide supports using a citrate method followed by modification with Y (2 wt.%) using two approaches, including the one-pot citrate method and incipient wetness impregnation of undoped oxides. The NH3-SCR reaction is studied in a fixed-bed quartz reactor to test the ability of the prepared catalysts in NO reduction. The gas reaction mixture consists of 800 ppm NO, 800 ppm NH3, 10 vol.% O2, and He as a balance gas at a WHSV of 25,000 mL g−1 h−1. The results indicate that undoped CeMnOx mixed oxide exhibits significantly higher deNOx performance compared with undoped and Y-doped MnOx and CeO2 catalysts. Indeed, yttrium presence in CeMnOx promotes the competitive NH3-SCO reaction, reducing the amount of NH3 available for NO reduction and lowering the catalyst activity. Furthermore, the physical-chemical properties of the prepared catalysts are studied using nitrogen adsorption/desorption, XRD, Raman spectroscopy, temperature-programmed reduction with hydrogen, and temperature-programmed desorption of ammonia. This study presents a promising approach to enhancing the performance of NH3-SCR catalysts at low temperatures that can have significant implications for reducing NO emissions

Structural insight into strong Pt-CeO 2 interaction: from single Pt atoms to PtOx clusters

Pt-CeO2 nanocomposites were obtained by coprecipitation, varying the Pt loading over a wide range of 1-30 wt %. The samples were calcined in air at 450-1000 °C. The Pt-CeO2 nanocomposites were investigated by a set of structural (X-ray diffraction, extended X-ray absorption fine structure (EXAFS), pair distribution function (PDF), and transmission electron microscopy) and spectroscopic (X-ray photoelectron spectroscopy and Raman spectroscopy) methods. Over the whole range of Pt loading, the main species were Pt2+ and Pt4+. They were localized either in a single-atom state or in the form of PtOx clusters on the ceria surface. The joint PDF and EXAFS modeling based on the combination of [Pt2+O4] single-atom and Pt3O4 structural fragments allowed us to propose the local structure of the PtOx clusters. The formation of such surface structures is associated with a distorted ceria surface on the Pt-CeO2 nanocomposites. We assume that the close arrangement of platinum ions in the PtOx clusters could be responsible for the effective redox properties of the samples

Effects of the Carbon Support Doping with Nitrogen for the Hydrogen Production from Formic Acid over Ni Catalysts

Porous nitrogen-doped and nitrogen-free carbon materials possessing high specific surface areas (400–1000 m2 g−1) were used for deposition of Ni by impregnation with nickel acetate followed by reduction. The nitrogen-doped materials synthesized by decomposition of acetonitrile at 973, 1073, and 1173 K did not differ much in the total content of incorporated nitrogen (4–5 at%), but differed in the ratio of the chemical forms of nitrogen. An X-ray photoelectron spectroscopy study showed that the rise in the synthesis temperature led to a strong growth of the content of graphitic nitrogen on the support accompanied by a reduction of the content of pyrrolic nitrogen. The content of pyridinic nitrogen did not change significantly. The prepared nickel catalysts supported on nitrogen-doped carbons showed by a factor of up to two higher conversion of formic acid as compared to that of the nickel catalyst supported on the nitrogen-free carbon. This was related to stabilization of Ni in the state of single Ni2+ cations or a few atoms clusters by the pyridinic nitrogen sites. The nitrogen-doped nickel catalysts possessed a high stability in the reaction at least within 5 h and a high selectivity to hydrogen (97%)

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

Pd/CeO2-SnO2 catalysts with varying tin content: Promotion of catalytic properties and structure modification

1%Pd/CeO2-SnO2 catalysts with varying Ce/Sn ratio were synthesized by counter-precipitation followed by calcination in a wide temperature range. The catalysts with Ce/Sn < 3/1 possess high thermal stability after calcination up to 1000 ◦C while maintaining low-temperature activity in CO oxidation. The PdOx clusters serving as active centers in CO oxidation are modified by Sn upon calcination. High tin content (Ce/Sn = 1/3) provides the activity of the catalysts in CH4 oxidation due to stabilization of PdO nanoparticles in the form of core@shell PdO@(CeO2 + SnO2) structures. Formation of the nanoheterophase structure upon calcination plays a key role in the stabilization of Pd-active centers of different types