Au@Pt Dendrimer Encapsulated Nanoparticles As Model Electrocatalysts for Comparison of Experiment and Theory

In this paper we report the electrochemical synthesis of core@shell dendrimer-encapsulated nanoparticles (DENs) consisting of cores containing 147 Au atoms (Au-147) and Pt shells having similar to 54 or similar to 102 atoms (Au-147@Pt-n (n = 54 or 102)). The significance of this work arises from the correlation of the experimentally determined structural and electrocatalytic properties of these particles with density functional theory (DFT) calculations. Specifically, we describe an experimental and theoretical study of Pb underpotential deposition (UPD) on Au-147 DENs, the structure of both Au-147@Pb-n and Au-147@Pt-n DENs, and the activity of these DENs for the oxygen reduction reaction (ORR). DFT calculations show that Pb binding is stronger on the (100) facets of Au as compared to (111), and the calculated deposition and stripping potentials are consistent with those measured experimentally. Galvanic exchange is used to replace the surface Pb atoms with Pt, and a surface distortion is found for Au-147@Pt-n particles using molecular dynamics simulations in which the Pt-covered (100) facets shear into (111) diamond structures. DFT calculations of oxygen binding show that the distorted surfaces are the most active for the ORR, and that their activity is similar regardless of the Pt coverage. These calculations are consistent with rotating ring-disk voltammetry measurements.Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U. S. Department of Energy DE-FG02-09ER16090Robert A. Welch Foundation F-0032, F-1601Institute of Computational and Engineering Sciences at UT-AustinChemistr

Pyrite geochemistry across the Betze-Post deposit, northern Carlin Trend, Nevada

Petrographic, geochemical, and statistical analyses show that stratigraphy and structural controls promoted ore stage fluid flow. For example, ore stage pyrites occur within brecciated rocks and small-scale fractures within the Wispy Member of the Popovich Formation. Ore stage pyrites have trace element signatures that suggest that the fluid was evolving both temporally and spatially. The associated ore stage trace elements include: Au, Cu, Hg, Tl, As, and Sb. Sb has the highest concentration at North Betze and Betze, suggesting that the J-B series faults transported ore fluids that were chemically distinct. As has the highest concentration along the southern part of the deposit and along the Post fault. Trace elements in pyrite decrease in concentration from the ore stage to the late-ore stage; Pre-ore stage sulfur isotope values of multiple morphologies of pyrite overlap with values characteristic of sedimentary sulfur sources and delta 34S ranges from -15.2 to 52.3‰. However, one type of pre-ore pyrite is interpreted to have derived from the Goldstrike stock. The ore stage pyrite analyses exhibit a more narrow delta34S range of -0.8 to 4.2‰, and are consistent with a magmatic origin; however, the values also overlap with a sedimentary source. The late-ore stage pyrite delta34S signatures extend to higher values of 5.9 to 15.5‰, which suggests that the magmatic or sedimentary ore sulfur source had been diluted by a second sulfur source with a higher sulfur isotope signature, perhaps of sedimentary origin

Hydrogen Absorption Properties of Metal-Ethylene Complexes

Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a single ethylene molecule can form stable complexes with light transition metals (TM) such as Ti and the resulting TMn-ethylene complex can absorb up to ~12 and 14 wt % hydrogen for n=1 and 2, respectively. Here we extend this study to include a large number of other metals and different isomeric structures. We obtained interesting results for light metals such as Li. The ethylene molecule is able to complex with two Li atoms with a binding energy of 0.7 eV/Li which then binds up to two H2 molecules per Li with a binding energy of 0.24 eV/H2 and absorption capacity of 16 wt %, a record high value reported so far. The stability of the proposed metal-ethylene complexes was tested by extensive calculations such as normal-mode analysis, finite temperature first-principles molecular dynamics (MD) simulations, and reaction path calculations. The phonon and MD simulations indicate that the proposed structures are stable up to 500 K. The reaction path calculations indicate about 1 eV activation barrier for the TM2-ethylene complex to transform into a possible lower energy configuration where the ethylene molecule is dissociated. Importantly, no matter which isometric configuration the TM2-ethylene complex possesses, the TM atoms are able to bind multiple hydrogen molecules with suitable binding energy for room temperature storage. These results suggest that co-deposition of ethylene with a suitable precursor of TM or Li into nanopores of light-weight host materials may be a very promising route to discovering new materials with high-capacity hydrogen absorption properties