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•Fe and Sn modify the Pt metallic phases on CN-P and CV supports in a different way.•On both supports prevail Fe ionic species which produce mainly a geometric effect.•Sn reduced ...species prevail on the bimetallic surfaces producing mainly an electronic effect.•PtFe catalysts with high Fe loadings show very high selectivities to UA.•Sn/Pt molar ratio <1 in PtSn catalysts is enough to obtain very high selectivities to UA.
In this paper, a study on the selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols (UA) by using two series of PtSn and PtFe catalysts with different metallic loadings and supported on carbon nanotubes and carbon Vulcan is reported. The catalysts were prepared by conventional impregnation using the corresponding metallic precursors, H2PtCl6, SnCl2, and FeCl3. Once reduced under hydrogen flow, the supported catalysts were characterized by temperature-programmed reduction (TPR), H2 chemisorption, test reactions of the metallic phase, and X-ray photoelectron spectroscopy (XPS). Hydrogenation results show that the addition of a second metal to Pt leads to important modification of the selectivity, although the highest selectivities to UA are reached with very different Fe or Sn loadings. The performance of the catalysts in the citral hydrogenation was related to the characteristics of the bimetallic phase. It was found that the Fe addition to Pt catalysts results in a typical behavior such as for an usual ionic promoter in contact with the active metal, reaching high selectivities to UA as the Fe loading and the Fe ionic species percentage increases, while for PtSn catalysts, a high selectivity to UA was found only with a very low amount of Sn ionic species in contact with a very high percentage of Sn reduced species. The best selectivity to UA (about 98%) was found for PtSn(1wt%)/CN-P catalyst treated with N2 at high temperature.
The new setup of the CODALEMA experiment installed at the Radio Observatory in Nançay, France, is described. It includes broadband active dipole antennas and an extended and upgraded particle ...detector array. The latter gives access to the air shower energy, allowing us to compute the efficiency of the radio array as a function of energy. We also observe a large asymmetry in counting rates between showers coming from the North and the South in spite of the symmetry of the detector. The observed asymmetry can be interpreted as a signature of the geomagnetic origin of the air shower radio emission. A simple linear dependence of the electric field with respect to
v
∧
B is used which reproduces the angular dependencies of the number of radio events and their electric polarity.
•Ga and In modify the Pt metallic phases on CN-P and CV supports in a different way.•Ga ionic and Ga0 species prevail on both supports producing mainly a geometric effect.•In0 species prevail on the ...bimetallic surfaces producing mainly an electronic effect.•PtGa catalysts with high Ga loadings show high selectivities to unsaturated alcohols.•In/Pt<1 in PtIn catalysts leads to very high selectivities to unsaturated alcohols.
In this paper, bimetallic PtGa and PtIn catalysts supported on multiwall carbon nanotubes and carbon Vulcan were used to study the hydrogenation of citral (α,β-unsaturated aldehyde) in liquid phase to produce nerol and geraniol (unsaturated alcohols, UA). The catalysts were prepared with different metallic loadings by conventional impregnation. All the catalysts contained a Pt loading of 5wt%. Once reduced under hydrogen flow, the supported catalysts were characterized by test reactions of the metallic phase, H2 chemisorption, temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Hydrogenation results showed that the addition of a second metal to Pt leads to important modifications of the selectivity to UA. The highest selectivities to UA were reached with different promoter/Pt atomic ratios for Ga and In. The catalyst performances in the citral hydrogenation were related to the characteristics of each supported bimetallic phase. It was found that the PtGa catalysts have a typical behavior of an ionic promoter together with a contribution of the Ga reduced species, reaching high selectivities to UA. On the other hand, PtIn catalysts with a metallic phase composed mainly by zerovalent In species in contact with the metallic phase, also showed high activities and selectivities to UA. The best selectivity value to UA (about 97%) was found for PtIn(2.5wt%)/CN-P catalyst with an excellent activity.
Pt/carbon nanotubes were developed to study the influence of the support surface composition on the catalytic performance in the selective hydrogenation of citral to the corresponding unsaturated ...alcohols (UAs). The Fe content in these catalysts and a low amount of oxygenated groups on the support surface have a fundamental role over the selectivity. Display omitted
► Oxidative treatments develop different oxygenated groups on the carbon nanotubes. ► Thermal treatments reduce the oxygenated groups and induce sintering of Pt. ► Platinum dispersion depends on the nature and amount of oxygenated groups. ► The Pt particle size is important for citral selective hydrogenation. ► The Fe content has a fundamental role on the selectivity to unsaturated alcohols.
Pt-based catalysts supported on multi-wall carbon nanotubes were developed to study the influence of the support surface composition on the catalytic performance in the selective hydrogenation of citral to the corresponding unsaturated alcohols. Supports with different oxidative and thermal treatments were characterized and used to prepare Pt and PtFe catalysts. Three factors seem to be important to obtain an adequate catalytic phase: the size of the Pt metallic particles, the interaction of the Pt particles with the support, and the presence of a second metal as promoter. The results indicated that the Fe content in these catalysts and a low amount of oxygenated groups on the support surface, which produce an optimized Pt particle size, have a fundamental role over the selectivity of the reaction to unsaturated alcohols. In fact, the combination of these effects in the Pt/CN–N2700 catalyst gives a very high selectivity (about 96%) to unsaturated alcohols.
Abstract
A new analysis of the data set from the Pierre Auger Observatory provides evidence for anisotropy in the arrival directions of ultra-high-energy cosmic rays on an intermediate angular scale, ...which is indicative of excess arrivals from strong, nearby sources. The data consist of 5514 events above
with zenith angles up to 80° recorded before 2017 April 30. Sky models have been created for two distinct populations of extragalactic gamma-ray emitters: active galactic nuclei from the second catalog of hard
Fermi
-LAT sources (2FHL) and starburst galaxies from a sample that was examined with
Fermi
-LAT. Flux-limited samples, which include all types of galaxies from the
Swift
-BAT and 2MASS surveys, have been investigated for comparison. The sky model of cosmic-ray density constructed using each catalog has two free parameters, the fraction of events correlating with astrophysical objects, and an angular scale characterizing the clustering of cosmic rays around extragalactic sources. A maximum-likelihood ratio test is used to evaluate the best values of these parameters and to quantify the strength of each model by contrast with isotropy. It is found that the starburst model fits the data better than the hypothesis of isotropy with a statistical significance of 4.0
σ
, the highest value of the test statistic being for energies above
. The three alternative models are favored against isotropy with 2.7
σ
–3.2
σ
significance. The origin of the indicated deviation from isotropy is examined and prospects for more sensitive future studies are discussed.
We present a detailed study of the large-scale anisotropies of cosmic rays with energies above 4 EeV measured using the Pierre Auger Observatory. For the energy bins 4, 8 EeV and E ≥ 8 EeV, the most ...significant signal is a dipolar modulation in R.A. at energies above 8 EeV, as previously reported. In this paper we further scrutinize the highest-energy bin by splitting it into three energy ranges. We find that the amplitude of the dipole increases with energy above 4 EeV. The growth can be fitted with a power law with index β = 0.79 0.19. The directions of the dipoles are consistent with an extragalactic origin of these anisotropies at all the energies considered. Additionally, we have estimated the quadrupolar components of the anisotropy: they are not statistically significant. We discuss the results in the context of the predictions from different models for the distribution of ultrahigh-energy sources and cosmic magnetic fields.
We present a new method for probing the hadronic interaction models at ultrahigh energy and extracting details about mass composition. This is done using the time profiles of the signals recorded ...with the water-Cherenkov detectors of the Pierre Auger Observatory. The profiles arise from a mix of the muon and electromagnetic components of air showers. Using the risetimes of the recorded signals, we define a new parameter, which we use to compare our observations with predictions from simulations. We find, first, inconsistencies between our data and predictions over a greater energy range and with substantially more events than in previous studies. Second, by calibrating the new parameter with fluorescence measurements from observations made at the Auger Observatory, we can infer the depth of shower maximum Xmax for a sample of over 81,000 events extending from 0.3 to over 100 EeV. Above 30 EeV, the sample is nearly 14 times larger than what is currently available from fluorescence measurements and extending the covered energy range by half a decade. The energy dependence of ⟨Xmax⟩ is compared to simulations and interpreted in terms of the mean of the logarithmic mass. We find good agreement with previous work and extend the measurement of the mean depth of shower maximum to greater energies than before, reducing significantly the statistical uncertainty associated with the inferences about mass composition.
This paper studies the preparation of Pt-based bimetallic (PtFe, PtSn, PtIn and PtGa) catalysts supported on carbon nanotubes for the citral hydrogenation by deposition–reduction in liquid phase and ...their comparison with the conventional impregnation method (CI). When conventionally impregnated, the selectivity to unsaturated alcohols (UA) highly increased from the monometallic to the bimetallic samples, whereas for liquid phase reduced ones, the selectivity to UA was rather similar for both Pt and bimetallic samples except for PtFe ones and for the PtSn catalyst treated treated with N
2
. The results were explained taken into account the intimate interaction achieved between the metals according to the preparation method and the formation of Pt–promotor complexes in the impregnating solution.
Graphical Abstract
We present a combined fit of a simple astrophysical model of UHECR sources to both the energy spectrum and mass composition data measured by the Pierre Auger Observatory. The fit has been performed ...for energies above $5 \cdot 10^{18}$ eV, i.e.~the region of the all-particle spectrum above the so-called "ankle" feature. The astrophysical model we adopted consists of identical sources uniformly distributed in a comoving volume, where nuclei are accelerated through a rigidity-dependent mechanism. The fit results suggest sources characterized by relatively low maximum injection energies, hard spectra and heavy chemical composition. We also show that uncertainties about physical quantities relevant to UHECR propagation and shower development have a non-negligible impact on the fit results.
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