Using transition metal tetraphosphonates as precursor of phosphorus-containing electrocatalysts

Coordination polymers (CPs) are widely studied due to their applicability in many fields. Among them, metal phosphonates (MPs) are attractive materials due to their versatile structural diversity and functionality, with interesting properties as proton conductors and electrocatalyst precursors. In this work, we report the synthesis and crystal structures of several MPs derived from the combination of hexamethylenediamine-N,N,N’,N’-tetrakis(methylenephosphonic acid) (HDTMP) with different transition metals (M2+= Mn, Fe, Co, and Ni). The resulting solids, M[(HO3PCH2)2N(CH2)6N(CH2PO3H)2]·2H2O, show pillared-layered structures with capabilities of ammonia adsorption (Co2+ and Ni2+ derivatives). The ammonia-containing solids are crystalline, with a composition M[(HO3PCH2)2N(CH2)6N(CH2PO3H)2(H2O)2](NH3)4(H2O)12. The catalytic activities toward Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR) and Hydrogen Evolution Reaction (HER) of the corresponding (5% H2-Ar)-pyrolyzed materials, as well as the crystal structure of non-pyrolyzed precursor solids, will be discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Structural Characterization of Transition-Metal Carboxyethanephosphonates as Precursors of Electrocatalysts

Phosphorus-containing coordination polymers (CPs) are considered as one of the most promising precursors of materials alternative to the highly expensive commercial catalysts (Pt, Ru and Ir) used in polymeric fuel cells and electrolysers. Among them, metal phosphonates, a subtype of coordination polymers, are attractive precursors due to their great chemical and structural diversity. Indeed, the possibility of containing in their compositions electrocatalytically active transition metals in combination with other elements (N, C, P, S, etc…) may enhance the electrochemical properties of the resulting materials after pyrolytic treatments. In this work, we report the synthesis and crystal structures of several transition-metal phosphonates derived from the ligand phosphonopropionic acid (PPA). Solids with compositions AxB1-x[O3PCH2CH2COOH]2·nH2O (A2+ and B2+= Mn, Fe, Co, and Ni; 0≤ x≤ 2 and n= 0, 2) were prepared by thermal-drying or mechano-assisted synthesis and, depending on the metal cation and water content, different layered frameworks were obtained, which have been solved from laboratory powder X-ray diffraction data. All obtained solids were pyrolyzed under 5%H2-Ar atmosphere, at different temperatures, in order to obtain transition-metal phosphides (MxP) which have been studied as possible electrocatalysts toward Oxygen Evolution Reaction (OER), Oxygen Reduction Reaction (ORR) and Hydrogen Evolution Reaction (HER). Correlations between the crystalline structures of the precursor phases with the electrocatalytic activity of the pyrolyzed derivatives will be discussed.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Proyectos nacionales MAT2016-77648-R y PID2019-110249RB-I0

The exposure of the hybrid detector of the Pierre Auger Observatory

The Pierre Auger Observatory is a detector for ultra-high energy cosmic rays. It consists of a surface array to measure secondary particles at ground level and a fluorescence detector to measure the development of air showers in the atmosphere above the array. The "hybrid" detection mode combines the information from the two subsystems. We describe the determination of the hybrid exposure for events observed by the fluorescence telescopes in coincidence with at least one water-Cherenkov detector of the surface array. A detailed knowledge of the time dependence of the detection operations is crucial for an accurate evaluation of the exposure. We discuss the relevance of monitoring data collected during operations, such as the status of the fluorescence detector, background light and atmospheric conditions, that are used in both simulation and reconstruction.Comment: Paper accepted by Astroparticle Physic

Atmospheric effects on extensive air showers observed with the Surface Detector of the Pierre Auger Observatory

Atmospheric parameters, such as pressure (P), temperature (T) and density, affect the development of extensive air showers initiated by energetic cosmic rays. We have studied the impact of atmospheric variations on extensive air showers by means of the surface detector of the Pierre Auger Observatory. The rate of events shows a ~10% seasonal modulation and ~2% diurnal one. We find that the observed behaviour is explained by a model including the effects associated with the variations of pressure and density. The former affects the longitudinal development of air showers while the latter influences the Moliere radius and hence the lateral distribution of the shower particles. The model is validated with full simulations of extensive air showers using atmospheric profiles measured at the site of the Pierre Auger Observatory.Comment: 24 pages, 9 figures, accepted for publication in Astroparticle Physic

The Fluorescence Detector of the Pierre Auger Observatory

The Pierre Auger Observatory is a hybrid detector for ultra-high energy cosmic rays. It combines a surface array to measure secondary particles at ground level together with a fluorescence detector to measure the development of air showers in the atmosphere above the array. The fluorescence detector comprises 24 large telescopes specialized for measuring the nitrogen fluorescence caused by charged particles of cosmic ray air showers. In this paper we describe the components of the fluorescence detector including its optical system, the design of the camera, the electronics, and the systems for relative and absolute calibration. We also discuss the operation and the monitoring of the detector. Finally, we evaluate the detector performance and precision of shower reconstructions.Comment: 53 pages. Submitted to Nuclear Instruments and Methods in Physics Research Section

Anisotropy studies around the galactic centre at EeV energies with the Auger Observatory

Data from the Pierre Auger Observatory are analyzed to search for anisotropies near the direction of the Galactic Centre at EeV energies. The exposure of the surface array in this part of the sky is already significantly larger than that of the fore-runner experiments. Our results do not support previous findings of localized excesses in the AGASA and SUGAR data. We set an upper bound on a point-like flux of cosmic rays arriving from the Galactic Centre which excludes several scenarios predicting sources of EeV neutrons from Sagittarius $A$. Also the events detected simultaneously by the surface and fluorescence detectors (the `hybrid' data set), which have better pointing accuracy but are less numerous than those of the surface array alone, do not show any significant localized excess from this direction.Comment: Matches published versio

Operations of and Future Plans for the Pierre Auger Observatory

Technical reports on operations and features of the Pierre Auger Observatory, including ongoing and planned enhancements and the status of the future northern hemisphere portion of the Observatory. Contributions to the 31st International Cosmic Ray Conference, Lodz, Poland, July 2009.Comment: Contributions to the 31st ICRC, Lodz, Poland, July 200

Measurement of the Depth of Maximum of Extensive Air Showers above 10^18 eV

We describe the measurement of the depth of maximum, Xmax, of the longitudinal development of air showers induced by cosmic rays. Almost four thousand events above 10^18 eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of (106 +35/-21) g/cm^2/decade below 10^(18.24 +/- 0.05) eV and (24 +/- 3) g/cm^2/decade above this energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm^2. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.Comment: Accepted for publication by PR

Evidence for a mixed mass composition at the `ankle' in the cosmic-ray spectrum

We report a first measurement for ultra-high energy cosmic rays of the correlation between the depth of shower maximum and the signal in the water Cherenkov stations of air-showers registered simultaneously by the fluorescence and the surface detectors of the Pierre Auger Observatory. Such a correlation measurement is a unique feature of a hybrid air-shower observatory with sensitivity to both the electromagnetic and muonic components. It allows an accurate determination of the spread of primary masses in the cosmic-ray flux. Up till now, constraints on the spread of primary masses have been dominated by systematic uncertainties. The present correlation measurement is not affected by systematics in the measurement of the depth of shower maximum or the signal in the water Cherenkov stations. The analysis relies on general characteristics of air showers and is thus robust also with respect to uncertainties in hadronic event generators. The observed correlation in the energy range around the `ankle' at $\lg(E/{\rm eV})=18.5-19.0$ differs significantly from expectations for pure primary cosmic-ray compositions. A light composition made up of proton and helium only is equally inconsistent with observations. The data are explained well by a mixed composition including nuclei with mass $A > 4$. Scenarios such as the proton dip model, with almost pure compositions, are thus disfavoured as the sole explanation of the ultrahigh-energy cosmic-ray flux at Earth.Comment: Published version. Added journal reference and DOI. Added Report Numbe