Labeled 15NO Study on N2 and N2O Formation Over Pt–Ba/Al2O3 NSR Catalysts

Mechanistic aspects involved in the formation of N2 and of N2O during the reduction of NO, stored nitrites and stored nitrates in the presence of NO are investigated in this work by means of isotopic labeling experiments over a model PtBa/Al2O3 NSR catalyst. The reduction of gaseous labeled NO with unlabelled NH3 leads to the formation of N2O at low temperature (below 180 °C), and of N2 at high temperature. All N2 possible isotopes are observed, whereas only labeled molecules have been detected in the case of N2O. Hence the formation of nitrous oxide involves undissociated NO molecules, whereas that of N2 can be explained on the basis of the statistical coupling of 15N- and 14N-adatoms on Pt. However, due to a slight excess of the mixed 15N14N isotope, a SCR-like pathway likely operates as well. The reduction of the stored labelled nitrates is very selective to N2 and all isotopes are observed, confirming the occurrence of the recombination pathway. However also in this case a SCR-like pathway likely occurs and this explains the abundance of the 14N15N species. When the reduction of the stored nitrates is carried out in the presence of NO, this species is preferentially reduced pointing out the higher reactivity of gaseous NO if compared to the nitrates

Thermal stability of linear siloxanes and their mixtures

The working fluid thermal stability is one of the crucial features of an effective organic Rankine cycle. Hexamethyldisiloxane (MM -C6H18OSi2) and octamethyltrisiloxane (MDM -C8H24O2Si3) are siloxane fluids currently exploited in high temperature organic Rankine cycles. However, data about their thermal stability are scarce or absent in literature. This manuscript presents a study of their behavior and decomposition at operating temperatures in the range 270 - 420 degrees C. The assessment of thermal stability can be performed with several methods, which are either based on pressure anomalous variation in isothermal stresses or on the deviation of the saturation curves experimentally obtained before and after the fluid is thermally stressed. An enhanced method is proposed here, based on chemical analysis of both vapor and liquid phases of the sample before and after it is subjected to thermal stress. A comparison of the pre-and post-stress vapor-liquid equilibrium curve complements the analysis. Results proved a higher stability for MM than for MDM. Moreover, due to the current interest in applying mixtures in organic Rankine cycles, an equimolar mixture of MM and MDM was also tested, which exhibit a behavior that appears to be different from the simple superimposition of pure fluid ones

Mechanistic investigation of the reduction of NOx over Pt-and Rh-based LNT catalysts

The influence of the noble metals (Pt vs. Rh) on the NOx storage reduction performances of lean NOx trap catalysts is here investigated by transient micro-reactor flow experiments. The study indicates a different behavior during the storage in that the Rh-based catalyst showed higher storage capacity at high temperature as compared to the Pt-containing sample, while the opposite is seen at low temperatures. It is suggested that the higher storage capacity of the Rh-containing sample at high temperature is related to the higher dispersion of Rh as compared to Pt, while the lower storage capacity of Rh-Ba/Al2O3 at low temperature is related to its poor oxidizing properties. The noble metals also affect the catalyst behavior upon reduction of the stored NOx, by decreasing the threshold temperature for the reduction of the stored NOx. The Pt-based catalyst promotes the reduction of the adsorbed NOx at lower temperatures if compared to the Rh-containing sample, due to its superior reducibility. However, Rh-based material shows higher reactivity in the NH3 decomposition significantly enhancing N2 selectivity. Moreover, formation of small amounts of N2O is observed on both Pt- and Rh-based catalyst samples only during the reduction of highly reactive NOx stored at 150 °C, where NOx is likely in the form of nitrites

Sinergies between organic-matrix combustion synthesis and Pt-promotion on the performances of Co-based FT catalysts

Ninety years after its discovery, Fischer–Tropsch Synthesis (FTS) is recently receiveing new interest as the key-process to convert synthesis gas derived from natural gas, coal and biomass, into high quality transportation fuels. While most of the original FTS processes relied on high-temperature bulky Fe-based catalysts, the attention today has shifted toward low-temperature Co-based supported catalysts, which exhibit higher activity and selectiv-ity. Due to the constant turnover frequency of cobalt crystallites bigger than 6 nm, the activity of cobalt catalyst linearly depends on the number of metallic active sites located on the support. In turn, the number of Co-metal centers is determined by the Co particle size (i.e. cobalt dispersions), the Co loading, and the Co reduction degree. Understanding the role of these parameters and controlling such parameters during the synthesis rep-resent one of the most significant challenge in view of the development of highly active catalysts to be used within intensified FTS reactors. The synthesis of highly active catalysts has been recently reported by both academic and industrial research groups. With respect to conventional supported FTS catalysts, which are prepared by incipient wetness impregnation, highly active catalysts have been obtained either by adding PGM as promoters or by adopting new synthesis routes, such as the organic matrix combustion, OM. PGM have been found able to favor cobalt reduction, thus increasing the fraction of cobalt which takes part to the reaction. OM, which consists in the addition of an organic material to the precursors-containing solution and in the combus-tion/decomposition of such compound during a fast calcination step, has been found effective in the synthesis of small and stable (with respect to sintering) Co-crystallites, at least in the case of SiO2 supported catalysts. How-ever, small crystallites are known to be much more resilient to reduction then bigger Co-aggregates. In this work, we investigated for the first time the synergies between the OM method, using urea as fuel, and the adoption of PGM, platinum in particular, as reduction promoter

Design and commissioning of a thermal stability test-rig for mixtures as working fluids for ORC applications

Abstract A novel test-rig for studying the thermal stability of mixtures as working fluids for ORC applications was designed and commissioned at the Laboratory of Compressible-fluid dynamics for Renewable Energy Applications (CREA) of Politecnico di Milano, in collaboration with the University of Brescia. The set-up is a standard one, in which a vessel containing the fluid under scrutiny is placed in a vertical oven for ~ 100 hours at a constant temperature T = Tstress. During the test, the pressure P is monitored to detect thermal decomposition of the fluid. After the test, the vessel is placed in a controlled thermal bath, where the pressure is measured at different value of the temperature T, with

Experimental evidence of planar channeling in a periodically bent crystal

The usage of a Crystalline Undulator (CU) has been identified as a promising solution for generating powerful and monochromatic $\gamma$-rays. A CU was fabricated at SSL through the grooving method, i.e., by the manufacturing of a series of periodical grooves on the major surfaces of a crystal. The CU was extensively characterized both morphologically via optical interferometry at SSL and structurally via X-ray diffraction at ESRF. Then, it was finally tested for channeling with a 400 GeV/c proton beam at CERN. The experimental results were compared to Monte Carlo simulations. Evidence of planar channeling in the CU was firmly observed. Finally, the emission spectrum of the positron beam interacting with the CU was simulated for possible usage in currently existing facilities

Steering efficiency of a ultrarelativistic proton beam in a thin bent crystal

Crystals with small thickness along the beam exhibit top performance for steering particle beams through planar channeling. For such crystals, the effect of nuclear dechanneling plays an important role because it affects their efficiency. We addressed the problem through experimental work carried out with 400 GeV/c protons at fixed-target facilities of CERN-SPS. The dependence of efficiency vs. curvature radius has been investigated and compared favourably to the results of modeling. A realistic estimate of the performance of a crystal designed for LHC energy including nuclear dechanneling has been achieved

Gamma-radiation as a Signature of Ultra Peripheral Ion Collisions at LHC energies

We study the peripheral ion collisions at LHC energies in which a nucleus is excited to the discrete state and then emits $\gamma$-rays. Large nuclear Lorenz factor allows to observe the high energy photons up to a few ten GeV and in the region of angles of a few hundred micro-radians around the beam direction. These photons can be used for tagging the events with particle production in the central rapidity region in the ultra-peripheral collisions. For that it is necessary to have an electromagnetic detector in front of the zero degree calorimeter in the LHC experiments.Comment: 14 pages, 6 Postscript figure

Single decay-lepton angular distributions in polarized e+e- --> t tbar and simple angular asymmetries as a measure of CP-violating top dipole couplings

In the presence of an electric dipole coupling of t-tbar to a photon, and an analogous "weak" dipole coupling to the Z, CP violation in the process e+e- --> t tbar results in modified polarization of the top and anti-top. This polarization can be analyzed by studying the angular distributions of decay charged leptons when the top or anti-top decays leptonically. Analytic expressions are presented for these distributions when either t or tbar decays leptonically, including O(alpha_s) QCD corrections in the soft-gluon approximation. The angular distributions are insensitive to anomalous interactions in top decay. Two types of simple CP-violating polar-angle asymmetries and two azimuthal asymmetries, which do not need the full reconstruction of the t or tbar, are studied. Independent 90% CL limits that may be obtained on the real and imaginary parts of the electric and weak dipole couplings at a linear collider operating at sqrt{s}=500 GeV with integrated luminosity 500 fb^{-1} and also at sqrt{s}=1000 GeV with integrated luminosity 1000 fb^{-1} have been evaluated. The effect of longitudinal electron and/or positron beam polarizations has been included.Comment: 26 pages, latex, figures included as latex files, version appearing in Pramana - Journal of Physics, minor corrections and note added in proo