Joerg C. Neuefeind

A New Molybdenum Nitride Catalyst with Rhombohedral MoS2 Structure for Hydrogenation Applications

Nitrogen-rich transition-metal nitrides hold great promise to be the next-generation catalysts for clean and renewable energy applications. However, incorporation of nitrogen into the crystalline lattices of transition metals is thermodynamically unfavorable at atmospheric pressure; most of the known transition metal nitrides are nitrogen-deficient with molar ratios of N:metal less than a unity. In this work, we have formulated a high-pressure route for the synthesis of a nitrogen-rich molybdenum nitride through a solid-state ion-exchange reaction. The newly discovered nitride, 3R-MoN2, adopts a rhombohedral R3m structure, isotypic with MoS2. This new nitride exhibits catalytic activities that are three times more active than the traditional catalyst MoS2 for the hydrodesulfurization of dibenzothiophene and more than twice as high in the selectivity to hydrogenation. The nitride is also catalytically active in sour methanation of syngas with >80% CO and H2 conversion at 723 K. Our formulated route for the synthesis of 3R-MoN2 is at a moderate pressure of 3.5 GPa and, thus, is feasible for industrial-scale catalyst production.

Probing disorder in isometric pyrochlore and related complex oxides

There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales). In particular, compounds with the isometric pyrochlore structure, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization, fast ion conduction, and thermal barrier coatings.

Acoustic levitator for structure measurements on low temperature liquid droplets

A single-axis acoustic levitator was constructed and used to levitate liquid and solid drops of 1-3 mm in diameter at temperatures in the range -40 to +40 degrees C. The levitator comprised (i) two acoustic transducers mounted on a rigid vertical support that was bolted to an optical breadboard, (ii) an acoustic power supply that controlled acoustic intensity, relative phase of the drive to the transducers, and could modulate the acoustic forces at frequencies up to 1 kHz, (iii) a video camera, and (iv) a system for providing a stream of controlled temperature gas flow over the sample. The acoustic transducers were operated at their resonant frequency of approximately 22 kHz and could produce sound pressure levels of up to 160 dB. The force applied by the acoustic field could be modulated to excite oscillations in the sample. Sample temperature was controlled using a modified Cryostream Plus and measured using thermocouples and an infrared thermal imager. The levitator was installed at x-ray beamline 11 ID-C at the Advanced Photon Source and used to investigate the structure of supercooled liquids.

Effect of poling on nanodomains and nanoscale structure in A-site disordered lead-free piezoelectric Na0.5Bi0.5TiO3–BaTiO3

This paper establishes the nanoscale poling mechanism operating in A-site disordered lead-free piezoelectric ceramics. Nanoscale domain maps and quantitative structural changes were obtained by deploying high-resolution transmission electron microscopy, dielectric spectroscopy, Raman spectroscopy and neutron scattering. Based on these results a microscopic model is proposed that provides insight into the E-field induced structural transformation. The stripe-like nanodomains in the unpoled system transformed into lamellar tetragonal domains with a reduced degree of displacement disorder during poling. It is proposed that the synergic effect of change in octahedral tilt disorder and cation displacement disorder leads to this transformation under an E-field. The criterion for achieving superior functional response includes stabilization of the long range order and reduction in the tilt disorder through compositional adjustments. Understanding of the poling mechanism in lead-free piezoelectric materials has been mostly limited to the behavior of domains under an applied field. However, this work provides an in-depth understanding of the changes in the local structure along with domain morphology under an applied field.

More accurate X-ray scattering data of deeply supercooled bulk liquid water

Deeply supercooled water droplets held containerless in an acoustic levitator are investigated with high-energy X-ray scattering. The temperature dependence of the X-ray structure function is found to be nonlinear. Comparison with two popular computer models reveals that structural changes are predicted too abrupt by the TIP5P-E model, while the rate of change predicted by TIP4P-Ew is in much better agreement with experiment. The abrupt structural changes, predicted by the TIP5P-E model to occur in the temperature range between 260 and 240 K as water approaches the homogeneous nucleation limit, are unrealistic. Both models underestimate the distance between neighbouring oxygen atoms and overestimate the sharpness of the OO distance distribution.

Nearest-neighbor coordination and chemical ordering in multicomponent bulk metallic glasses

The authors report complementary use of high-energy x-ray and neutron diffraction to probe the local atomic structure in a Zr-based bulk metallic glass. By analyzing the partial coordination numbers, the authors demonstrate the presence of multiple types of solute-centered clusters in the multicomponent glass and efficient packing of the amorphous structure at atomic scale. The authors’ findings provide a basis for understanding how local structures change during phase transformation and mechanical deformation of multicomponent amorphous alloys.

The structure of molten ZnCl2

Abstract High energy photon diffraction measurements on molten ZnCl2 have been carried out in the temperature range 623–853 K. The momentum transfer range covered is 0.15 A−1

Local Structure and Short-Range Order in a NiCoCr Solid Solution Alloy

Multielement solid solution alloys are intrinsically disordered on the atomic scale, and many of their advanced properties originate from the local structural characteristics. The local structure of a NiCoCr solid solution alloy is measured with x-ray or neutron total scattering and extended x-ray absorption fine structure (EXAFS) techniques. The atomic pair distribution function analysis does not exhibit an observable structural distortion. However, an EXAFS analysis suggests that the Cr atoms are favorably bonded with Ni and Co in the solid solution alloys. This short-range order (SRO) may make an important contribution to the low values of the electrical and thermal conductivities of the Cr-alloyed solid solutions. In addition, an EXAFS analysis of Ni ion irradiated samples reveals that the degree of SRO in NiCoCr alloys is enhanced after irradiation.

Structure of molten CaSiO3: neutron diffraction isotope substitution with aerodynamic levitation and molecular dynamics study.

We have performed neutron diffraction isotopic substitution experiments on aerodynamically levitated droplets of CaSiO(3), to directly extract intermediate and local structural information on the Ca environment. The results show a substantial broadening of the first Ca-O peak in the pair distribution function of the melt compared to the glass, which comprises primarily of 6- and 7-fold coordinated Ca-polyhedra. The broadening can be explained by a redistribution of Ca-O bond lengths, especially toward longer distances in the liquid. The first order neutron difference function provides a test of recent molecular dynamics simulations and supports the MD model which contains short chains or channels of edge shared Ca-octahedra in the liquid state. It is suggested that the polymerization of Ca-polyhedra is responsible for the fragile viscosity behavior of the melt and the glass forming ability in CaSiO(3).

Structural water engaged disordered vanadium oxide nanosheets for high capacity aqueous potassium-ion storage

Aqueous electrochemical energy storage devices using potassium-ions as charge carriers are attractive due to their superior safety, lower cost and excellent transport properties compared to other alkali ions. However, the accommodation of potassium-ions with satisfactory capacity and cyclability is difficult because the large ionic radius of potassium-ions causes structural distortion and instabilities even in layered electrodes. Here we report that water induces structural rearrangements of the vanadium-oxygen octahedra and enhances stability of the highly disordered potassium-intercalated vanadium oxide nanosheets. The vanadium oxide nanosheets engaged by structural water achieves high capacity (183 mAh g−1 in half-cells at a scan rate of 5 mV s−1, corresponding to 0.89 charge per vanadium) and excellent cyclability (62.5 mAh g−1 in full cells after 5,000 cycles at 10 C). The promotional effects of structural water on the disordered vanadium oxide nanosheets will contribute to the exploration of disordered structures from earth-abundant elements for electrochemical energy storage.