Emergence of Antiferromagnetic Correlation in LiTi2-xVxO4 via 7Li NMR

We report 7Li NMR studies of V-substitution effects on spinel oxide superconductor LiTi2O4 (Tc = 13.4 K). In LiTi2-xVxO4 (x = 0-0.4), the V substitution for the Ti site suppressed the relative volume fraction of superconductivity faster than Tc. From the observation of fairly homogeneous enhancement in a 7Li nuclear spin-lattice relaxation rate, we conclude that the V substitution changes electron correlation effects through electron carrier doping from quarter electron filling 3d0.5 to 3d1.5 and then the antiferromagnetic correlation emerges.Comment: 4 pages, 7 figure

Methane Activation at the Pd/CeO₂ Interface

We combine density functional theory (DFT) and reactive force-field (ReaxFF) simulations to assess the stability and activity of unique catalytic sites at the interface between Pd clusters and a CeO₂ support. ReaxFF-based Grand Canonical Monte Carlo (GC-MC) simulations provide insight into the oxide structure at the Pd/CeO₂ interface. Surface models derived with GC-MC are employed in reactive molecular dynamics (RMD) simulations, which demonstrate that methane lightoff rapidly occurs when there is Pd mixing in the CeO₂ lattice. DFT investigations, utilizing models inspired by GC-MC/RMD, demonstrate that Pd⁴⁺ states are stabilized in PdO_<i>x</i> clusters partially embedded in the CeO₂ lattice, and that such sites yield low methane activation barriers. The integrated DFT/ReaxFF methodology employed here demonstrates a combined quantum/classical workflow that can be extended to examine emergent behavior in other oxide-supported metal catalysts

A ReaxFF Investigation of Hydride Formation in Palladium Nanoclusters via Monte Carlo and Molecular Dynamics Simulations

Palladium can readily dissociate and absorb hydrogen from the gas phase, making it applicable in hydrogen storage devices, separation membranes, and hydrogenation catalysts. To investigate hydrogen transport properties in Pd on the atomic scale, we derived a ReaxFF interaction potential for Pd/H from an extensive set of quantum data for both bulk and surface properties. Using this potential, we employed a recently developed hybrid grand canonical-Monte Carlo/molecular dynamics (GC-MC/MD) method to derive theoretical hydrogen absorption isotherms in Pd bulk crystals and nanoclusters for hydrogen pressures ranging from 10^–1 atm to 10^–14 atm, and at temperatures ranging from 300 to 500 K. Analysis of the equilibrated cluster structures reveals the contributing roles of surface, subsurface, and bulk regions during the size-dependent transition between the solid solution α phase and the hydride β phase. Additionally, MD simulations of the dissociative adsorption of hydrogen from the gas phase were conducted to assess size-dependent kinetics of hydride formation in Pd clusters. Hydrogen diffusion coefficients, apparent diffusion barriers, and pre-exponential factors were derived from MD simulations of hydrogen diffusion in bulk Pd. Both the thermodynamic results of the GC-MC/MD method and the kinetic results of the MD simulations are in agreement with experimental values reported in the literature, thus validating the Pd/H interaction potential, and demonstrating the capability of the GC–MC and MD methods for modeling the complex and dynamic phase behavior of hydrogen in Pd bulk and clusters

Role of Site Stability in Methane Activation on Pd_<i>x</i>Ce_1<i>–x</i>O_δ Surfaces

Doped metal oxide catalysts can be optimized by identifying dopant metal/host oxide combinations that exhibit synergistic interactions not present in the parent systems. This is exemplified by Pd_<i>x</i>Ce_1–<i>x</i>O_δ mixed oxides that yield methane oxidation rates unobtainable by the separate systems. Here we demonstrate that rapid C–H activation on Pd_<i>x</i>Ce_1–<i>x</i>O_δ catalysts can be attributed to emergent behavior of the doped oxide enabling Pd⁴⁺ ↔ Pd²⁺ transitions not evident in catalysts featuring a PdO_<i>x</i> active phase. Pd_<i>x</i>Ce_1–<i>x</i>O_δ surfaces activate methane through hydrogen abstraction over Pd⁴⁺ surface states, in contrast to the σ-complex activation route favored over PdO_<i>x</i> surfaces. The stability of the active Pd⁴⁺ state is dependent on temperature and oxygen pressure during catalytic operation, and as such we combine reaction kinetics and thermodynamic stability arguments from density functional theory (DFT) calculations to derive the apparent methane activation barrier. This accounts for varying conditions affecting the stability of the Pd⁴⁺ state, demonstrating that active Pd⁴⁺ sites are metastable. These states form under the reaction environment and offer lower methane activation barriers in comparison to Pd²⁺ states. The Pd⁴⁺ state is stabilized by the incorporation of Pd in the fluorite lattice structure of CeO₂, which in turn provides unique methane activation chemistry from the Pd_<i>x</i>Ce_<i>x</i>–1O_δ mixture. We generalize these results over (<i>T,P</i>) space by deriving phase boundaries demarcating regions where each Pd surface oxidation state is thermodynamically stable or kinetically active. The approach presented here can be readily extended to other systems, providing a method for assessing the interplay between site activity and stability on catalytic surfaces

Molecular Dynamics Investigation of the Effects of Tip–Substrate Interactions during Nanoindentation

Nanoindentation in molecular dynamics (MD) simulations typically uses highly idealized indenter tip models. Such tips usually consist of either a single sphere or a collection of atoms, both of which are purely repulsive in their interactions with the substrate. It is also assumed that there is no environmental or substrate contamination, nor is there a surface oxide layer. In this work we examine the effects of these assumptions by comparing detailed MD simulations utilizing varying interaction potentials against both experimental atomic force microscopy observations and calculations using density functional theory. Specifically, we examine the effect of a tip–substrate interaction on the indenter under clean, hydrogenated, and oxidized conditions. We find that under clean or oxidized conditions (where we include oxygen on the nickel surface to mimic a passivating NiO layer) there is a substantial material transfer from the substrate to the tip. This material (Ni atoms) remains adsorbed on the tip upon retraction. However, the presence of hydrogen on the diamond tip drastically reduces, or even altogether eliminates, this material transfer, therefore having an effect much larger than that of a contaminating oxide layer