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Insight towards the role of ceria-based supports for reverse water gas shift reaction over RuFe nanoparticles
No full textUtilization of CO2 through the reverse water gas shift (RWGS) reaction is a promising solution in managing greenhouse gas emissions. Here, we used the RWGS reaction to evaluate the catalytic conversion of CO2 over Ru-Fe nanoparticles supported on samarium-doped ceria (SDC) support. Catalysts of different RuxFe100-x compositions (x\u202f=\u202f100, 80, 45, 20, 0\u202fat.%) have been studied under steady-state conditions in a packed bed reactor in the temperature range 300\u2013800\u202f\ub0C to determine their catalytic activity and selectivity towards CO. The metal-support interaction (MSI) effect for SDC was evaluated to determine its promotional behavior for the RWGS reaction. The catalyst was characterized using TEM, TGA, and ICP-ES techniques. Among all investigated catalysts, Ru45Fe55/SDC (2\u202fwt.%) displayed the overall best activity and CO selectivity. A stability test of 100\u202fh at 650\u202f\ub0C confirmed an excellent stability of the Ru45Fe55/SDC catalyst. Overall, the use of Ru45Fe55/SDC (2\u202fwt.%) is a promising catalyst in the utilization of CO2, reaching a maximum CO yield of 3c47.5% at 800\u202f\ub0C and 100% CO selectivity above 500\u202f\ub0C. Furthermore, Ru45Fe55 (2\u202fwt.%) nanoparticles were deposited on un-doped CeO2 and doped ceria: Gd-CeO2, Y2O3-CeO2, as well as yttria-stabilized zirconia (YSZ) and carbon supports. Contrary to carbon support, all catalysts containing oxygen conducting-ceramic supports displayed 100% selectivity to CO at temperatures above 600\u202f\ub0C, which can be attributed to the synergistic relationship between Ru-Fe nanoparticles being promoted through the MSI and thermally induced migration of promoting ionic species (O\u3b4 12) from oxygen conducting ceramics to the nanoparticles.Peer reviewed: YesNRC publication: Ye
Intensity-corrected Herschel observations of nearby isolated low-mass clouds
No full textWe present intensity-corrected Herschel maps at 100, 160, 250, 350, and 500 \u3bcm for 56 isolated low-mass clouds. We determine the zero-point corrections for Herschel Photodetector Array Camera and Spectrometer (PACS) and Spectral Photometric Imaging Receiver (SPIRE) maps from the Herschel Science Archive (HSA) using Planck data. Since these HSA maps are small, we cannot correct them using typical methods. Here we introduce a technique to measure the zero-point corrections for small Herschel maps. We use radial profiles to identify offsets between the observed HSA intensities and the expected intensities from Planck. Most clouds have reliable offset measurements with this technique. In addition, we find that roughly half of the clouds have underestimated HSA-SPIRE intensities in their outer envelopes relative to Planck, even though the HSA-SPIRE maps were previously zero-point corrected. Using our technique, we produce corrected Herschel intensity maps for all 56 clouds and determine their line-of-sight average dust temperatures and optical depths from modified blackbody fits. The clouds have typical temperatures of ~14\u201320 K and optical depths of ~10 125\u201310 123. Across the whole sample, we find an anticorrelation between temperature and optical depth. We also find lower temperatures than what was measured in previous Herschel studies, which subtracted out a background level from their intensity maps to circumvent the zero-point correction. Accurate Herschel observations of clouds are key to obtaining accurate density and temperature profiles. To make such future analyses possible, intensity-corrected maps for all 56 clouds are publicly available in the electronic version.Peer reviewed: YesNRC publication: Ye
Interpreting and propagating the uncertainty of the standard atomic weights (IUPAC Technical Report)
No full textIn 2009, the Commission on Isotopic Abundances and Atomic Weights (CIAAW) of the International Union of Pure and Applied Chemistry (IUPAC) introduced the interval notation to express the standard atomic weights of elements whose isotopic composition varies significantly in nature. However, it has become apparent that additional guidance would be helpful on how representative values should be derived from these intervals, and on how the associated uncertainty should be characterized and propagated to cognate quantities, such as relative molecular masses. The assignment of suitable probability distributions to the atomic weight intervals is consistent with the CIAAW\u2019s goal of emphasizing the variability of the atomic weight values in nature. These distributions, however, are not intended to reflect the natural variability of the abundances of the different isotopes in the earth\u2019s crust or in any other environment. Rather, they convey states of knowledge about the elemental composition of \u201cnormal\u201d materials generally, or about specific classes of such materials. In the absence of detailed knowledge about the isotopic composition of a material, or when such details may safely be ignored, the probability distribution assigned to the standard atomic weight intervals may be taken as rectangular (or, uniform). This modeling choice is a reasonable and convenient default choice when a representative value of the atomic weight, and associated uncertainty, are needed in calculations involving atomic and relative molecular masses. When information about the provenance of the material, or other information about the isotopic composition needs to be taken into account, then this distribution may be non-uniform. We present several examples of how the probability distribution of an atomic weight or relative molecular mass may be characterized, and also how it may be used to evaluate the associated uncertainty.Peer reviewed: YesNRC publication: Ye
Ion-ion dynamic structure factor, acoustic modes, and equation of state of two-temperature warm dense aluminum
No full textThe ion-ion dynamical structure factor and the equation of state of warm dense aluminum in a two-temperature quasiequilibrium state, with the electron temperature higher than the ion temperature, are investigated using molecular-dynamics simulations based on ion-ion pair potentials constructed from a neutral pseudoatom model. Such pair potentials based on density functional theory are parameter-free and depend directly on the electron temperature and indirectly on the ion temperature, enabling efficient computation of two-temperature properties. Comparison with ab initio simulations and with other average-atom calculations for equilibrium aluminum shows good agreement, justifying a study of quasiequilibrium situations. Analyzing the van Hove function, we find that ion-ion correlations vanish in a time significantly smaller than the electron-ion relaxation time so that dynamical properties have a physical meaning for the quasiequilibrium state. A significant increase in the speed of sound is predicted from the modification of the dispersion relation of the ion acoustic mode as the electron temperature is increased. The two-temperature equation of state including the free energy, internal energy, and pressure is also presented.Peer reviewed: YesNRC publication: Ye
Methylammonium cation dynamics in methylammonium lead halide perovskites: a solid-state NMR perspective
No full textIn light of the intense recent interest in the methylammonium lead halides, CH3NH3PbX3 (X = Cl, Br, and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via 2H, 14N, and 207Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, \u3c42, for the C\u2013N axis of the iodide salt is 0.85 \ub1 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic \u2194 tetragonal phase transition; however, 2H and 14N NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic \u2194 tetragonal phase transition temperature is approached, the six sites are nearly equally populated. Below the tetragonal \u2194 orthorhombic phase transition, 2H NMR line shapes indicate that the C\u2013N axis is essentially frozen.Peer reviewed: YesNRC publication: Ye
Orbital control of photocurrents in large area all-carbon molecular junctions
No full textPhotocurrents generated by illumination of carbon-based molecular junctions were investigated as diagnostics of how molecular structure and orbital energies control electronic behavior. Oligomers of eight aromatic molecules covalently bonded to an electron-beam deposited carbon surface were formed by electrochemical reduction of diazonium reagents, with layer thicknesses in the range of 5\u201312 nm. Illumination through either the top or bottom partially transparent electrodes produced both an open circuit potential (OCP) and a photocurrent (PC), and the polarity and spectrum of the photocurrent depended directly on the relative positions of the frontier orbitals and the electrode Fermi level (EF). Electron donors with relatively high HOMO energies yielded positive OCP and PC, and electron acceptors with LUMO energies closer to EF than the HOMO energy produced negative OCP and PC. In all cases, the PC spectrum and the absorption spectrum of the oligomer in the molecular junction had very similar shapes and wavelength maxima. Asymmetry of electronic coupling at the top and bottom electrodes due to differences in bonding and contact area cause an internal potential gradient which controls PC and OCP polarities. The results provide a direct indication of which orbital energies are closest to EF and also indicate that transport in molecular junctions thicker than 5 nm is controlled by the difference in energy of the HOMO and LUMO orbitals.Peer reviewed: YesNRC publication: Ye
OSSOS. IX. Two objects in Neptune's 9:1 resonance: implications for resonance sticking in the scattering population
No full textWe discuss the detection in the Outer Solar System Origins Survey (OSSOS) of two objects in Neptune's distant 9:1 mean motion resonance at semimajor axis a 48 130 au. Both objects are securely resonant on 10 Myr timescales, with one securely in the 9:1 resonance's leading asymmetric libration island and the other in either the symmetric or trailing asymmetric island. These objects are the largest semimajor axis objects with secure resonant classifications, and their detection in a carefully characterized survey allows for the first robust resonance population estimate beyond 100 au. The detection of these objects implies a 9:1 resonance population of 1.1
7 104 objects with H r < 8.66 (D gsim 100 km) on similar orbits (95% confidence range of ~(0.4\u20133)
7 104). Integrations over 4 Gyr of an ensemble of clones spanning these objects' orbit-fit uncertainties reveal that they both have median resonance occupation timescales of ~1 Gyr. These timescales are consistent with the hypothesis that these objects originate in the scattering population but became transiently stuck to Neptune's 9:1 resonance within the last ~1 Gyr of solar system evolution. Based on simulations of a model of the current scattering population, we estimate the expected resonance sticking population in the 9:1 resonance to be 1000\u20134500 objects with H r < 8.66; this is marginally consistent with the OSSOS 9:1 population estimate. We conclude that resonance sticking is a plausible explanation for the observed 9:1 population, but we also discuss the possibility of a primordial 9:1 population, which would have interesting implications for the Kuiper Belt's dynamical history.Peer reviewed: YesNRC publication: Ye
Screening of cyclic imine and paralytic shellfish toxins in isolates of the genus Alexandrium (Dinophyceae) from Atlantic Canada
No full textThe dinoflagellate genus Alexandrium Halim has frequently been associated with harmful algal blooms. Although a number of species from this genus are known to produce paralytic shellfish toxins (PST) and/or cyclic imines (CI), studies on comprehensive toxin profiling using techniques capable of detecting the full range of PST and CI analogues are limited. Isolates of Alexandrium spp. from Atlantic Canada were analyzed by targeted and untargeted liquid chromatography-tandem mass spectrometry (LC\u2013MS). Results showed a number of distinct profiles and wide ranging cell quotas of PST and spirolides (SPX) in both A. catenella (Whedon & Kofoid) Balech and A. ostenfedii (Paulsen) Balech & Tangen. The concentration of PST in A. catenella ranged from 0.0029 to 54 fmol cell 121 with the major components being C2 and GTX4. In addition, putative PST metabolites were confirmed for the first time in A. catenella by high resolution MS/MS. By comparison, A. ostenfeldii isolates showed much lower concentrations of PST (<LOD to 2 fmol cell 121) and high total levels of SPX (14 to 43 fmol cell 121). The SPX profile of the A. ostenfeldii strains mainly included 13-desmethyl SPX-C, SPX-C and 20-methyl SPX-G, with low levels of other SPX and gymnodimine-like analogues detected by untargeted \u2013high-resolution LC-MS. This work demonstrates the importance of using screening methods capable of detecting the full suite of PST and CI compounds when analyzing Alexandrium isolates for toxin production and adds further complexity to the known toxin profiles of this genus.Peer reviewed: YesNRC publication: Ye
A photometric study of globular clusters observed by the APOGEE survey
No full textIn this paper, we describe the photometric and spectroscopic properties of multiple populations in seven northern globular clusters. In this study, we employ precise ground-based photometry from the private collection of Stetson, space photometry from the Hubble Space Telescope (HST), literature abundances of Na and O, and Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey abundances for Mg, Al, C, and N. Multiple populations are identified by their position in the CU, B, I 12Vpseudo colour\u2013magnitude diagram (pseudo-CMD) and confirmed with their chemical composition determined using abundances. We confirm the expectation from previous studies that the red giant branches (RGBs) in all seven clusters are split and the different branches have different chemical compositions. The Mg\u2013Al anticorrelations were well explored by the APOGEE and Gaia-ESO surveys for most globular clusters, some clusters showing bimodal distributions, while others continuous distributions. Even though the structure (i.e. bimodal versus continuous) of Mg\u2013Al can greatly vary, the Al-rich and Al-poor populations do not seem to have very different photometric properties, agreeing with theoretical calculations. There is no one-to-one correspondence between the Mg\u2013Al anticorrelation shape (bimodal versus continuous) and the structure of the RGB seen in the HST pseudo-CMDs, with the HST photometric information usually implying more complex formation/evolution histories than the spectroscopic ones. We report on finding two second-generation horizontal branch (HB) stars in M5, and five second-generation asymptotic giant branch (AGB) stars in M92, which is the most metal-poor cluster to date in which second-generation AGB stars have been observed.Peer reviewed: YesNRC publication: Ye
A UV-to-NIR study of molecular gas in the dust cavity around RY Lupi
No full textWe present a study of molecular gas in the inner disk (r < 20au) around RY Lupi, with spectra from HST-COS, HST-STIS, and VLT-CRIRES. We model the radial distribution of flux from hot gas in a surface layer between r = 0.1\u201310 au, as traced by Ly\u3b1-pumped H2. The result shows H2 emission originating in a ring centered at ~3 au that declines within r < 0.1 au, which is consistent with the behavior of disks with dust cavities. An analysis of the H2 line shapes shows that a two-component Gaussian profile $(FWHM mathrm,H2=105 +- 15 km s ^-1; FWHM mathrm,H2=43 +- 13 km s^-1) is statistically preferred to a single-component Gaussian. We interpret this as tentative evidence for gas emitting from radially separated disk regions ({r broad,H2} ~0.4 +- 0.1 au; {r narrow,H2} ~ 3 +- 2 au). The 4.7 \u3bcm 12CO emission lines are also well fit by two-component profiles ({r broad,CO} =0.4 +- 0.1 au; {r narrow,CO} =15 +- 2 au). We combine these results with 10 \u3bcm observations to form a picture of gapped structure within the mm-imaged dust cavity, providing the first such overview of the inner regions of a young disk. The HST SED of RY Lupi is available online for use in modeling efforts.Peer reviewed: YesNRC publication: Ye