Robert J. Colby

Coexistence of Weak Ferromagnetism and Polar Lattice Distortion in Epitaxial NiTiO3 thin films of the LiNbO3-Type Structure

The authors report the magnetic and structural characteristics of epitaxial NiTiO3 films grown by pulsed laser deposition that are isostructural with acentric LiNbO3 (space group R3c). Optical second harmonic generation and magnetometry demonstrate lattice polarization at room temperature and weak ferromagnetism below 250 K, respectively. These results appear to be consistent with earlier predictions from first-principles calculations of the coexistence of ferroelectricity and weak ferromagnetism in a series of transition metal titanates crystallizing in the LiNbO3 structure. This acentric form of NiTiO3 is believed to be one of the rare examples of ferroelectrics exhibiting weak ferromagnetism generated by a Dzyaloshinskii–Moriya interaction.

A method for measuring the local gas pressure within a gas-flow stage in situ in the transmission electron microscope.

Environmental transmission electron microscopy (TEM) has enabled in situ experiments in a gaseous environment with high resolution imaging and spectroscopy. Addressing scientific challenges in areas such as catalysis, corrosion, and geochemistry can require pressures much higher than the ∼20 mbar achievable with a differentially pumped environmental TEM. Gas flow stages, in which the environment is contained between two semi-transparent thin membrane windows, have been demonstrated at pressures of several atmospheres. However, the relationship between the pressure at the sample and the pressure drop across the system is not clear for some geometries. We demonstrate a method for measuring the gas pressure at the sample by measuring the ratio of elastic to inelastic scattering and the defocus of the pair of thin windows. This method requires two energy filtered high-resolution TEM images that can be performed during an ongoing experiment, at the region of interest. The approach is demonstrated to measure greater than atmosphere pressures of N2 gas using a commercially available gas-flow stage. This technique provides a means to ensure reproducible sample pressures between different experiments, and even between very differently designed gas-flow stages.

Rh-Based Mixed Alcohol Synthesis Catalysts: Characterization and Computational Report

The U.S. Department of Energy is conducting a program focused on developing a process for the conversion of biomass to bio-based fuels and co-products. Biomass-derived syngas is converted thermochemically within a temperature range of 240 to 330°C and at elevated pressure (e.g., 1200 psig) over a catalyst. Ethanol is the desired reaction product, although other side compounds are produced, including C3 to C5 alcohols; higher (i.e., greater than C1) oxygenates such as methyl acetate, ethyl acetate, acetic acid and acetaldehyde; and higher hydrocarbon gases such as methane, ethane/ethene, propane/propene, etc. Saturated hydrocarbon gases (especially methane) are undesirable because they represent a diminished yield of carbon to the desired ethanol product and represent compounds that must be steam reformed at high energy cost to reproduce CO and H2. Ethanol produced by the thermochemical reaction of syngas could be separated and blended directly with gasoline to produce a liquid transportation fuel. Additionally, higher oxygenates and unsaturated hydrocarbon side products such as olefins also could be further processed to liquid fuels. The goal of the current project is the development of a Rh-based catalyst with high activity and selectivity to C2+ oxygenates. This report chronicles an effort to characterize numerous supports and catalysts to identify particular traits that could be correlated with the most active and/or selective catalysts. Carbon and silica supports and catalysts were analyzed. Generally, analyses provided guidance in the selection of acceptable catalyst supports. For example, supports with high surface areas due to a high number of micropores were generally found to be poor at producing oxygenates, possibly because of mass transfer limitations of the products formed out of the micropores. To probe fundamental aspects of the complicated reaction network of CO with H2, a computational/ theoretical investigation using quantum mechanical and ab initio molecular dynamics calculations was initiated in 2009. Computational investigations were performed first to elucidate understanding of the nature of the catalytically active site. Thermodynamic calculations revealed that Mn likely exists as a metallic alloy with Rh in Rh-rich environments under reducing conditions at the temperatures of interest. After determining that reduced Rh-Mn alloy metal clusters were in a reduced state, the activation energy barriers of numerous transition state species on the catalytically active metal particles were calculated to compute the activation barriers of several reaction pathways that are possible on the catalyst surface. Comparison of calculations with a Rh nanoparticle versus a Rh-Mn nanoparticle revealed that the presence of Mn enabled the reaction pathway of CH with CO to form an adsorbed CHCO species, which was a precursor to C2+ oxygenates. The presence of Mn did not have a significant effect on the rate of CH4 production. Ir was observed during empirical catalyst screening experiments to improve the activity and selectivity of Rh-Mn catalysts. Thus, the addition of Ir to the Rh-Mn nanoparticles also was probed computationally. Simulations of Rh-Mn-Ir nanoparticles revealed that, with sufficient Ir concentrations, the Rh, Mn and Ir presumably would be well mixed within a nanoparticle. Activation barriers were calculated for Rh-Mn-Ir nanoparticles for several C-, H-, and O-containing transitional species on the nanoparticle surface. It was found that the presence of Ir opened yet another reactive pathway whereby HCO is formed and may undergo insertion with CHx surface moieties. The reaction pathway opened by the presence of Ir is in addition to the CO + CH pathway opened by the presence of Mn. Similar to Mn, the presence of Ir was not found to not affect the rate of CH4 production.

Estimating the Local Gas Pressure in a Gas Flow Cell Stage In Situ using Electron Energy Loss Spectroscopy

The development of environmental transmission electron microscopy (TEM) has enabled in situ experiments in a gaseous environment with high resolution imaging and spectroscopy for many systems. Many important systems, in such areas as catalysis and geochemistry, require much higher pressures than the ~20 mbar achievable with a differentially pumped, dedicated environmental TEM. Gas flow stages, in which the gaseous environment is contained between two thin membrane windows (typically SiN), have been demonstrated at reported pressures of up to several atmospheres (Fig. 1a,b). While the potential to work at realistic pressures is attractive, the design of many current gas flow stages is such that the pressure at the sample cannot necessarily be directly inferred from the pressure differential across the entire system. The flow rate at the sample can depend sensitively upon parameters such as the spacing between the SiN windows, which will vary (often significantly) each time the cell is sealed. Furthermore, the window spacing and pressure are interdependent in a typical two-window gas cell design. The flow rate of a gas through the cell—the critical measure for reactions in an in situ experiment— should be sensitively dependent upon both the channel size and the pressure.

Level Set Method for Tip Shape Evolution Simulation for Atom Probe Tomography

Tomography Jie Bao, Zhijie Xu, Robert Colby, Suntharampillai Thevuthasan, Arun Devaraj 1 Nuclear Science Division, Pacific Northwest National Laboratory, Richland, WA, USA; 2 Computational Mathematics Division, Pacific Northwest National Laboratory, Richland, WA, USA; 3 Environmental Molecular sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, USA; 4 Qatar Energy and Environmental Research Institute, Qatar, UAE

The Role of Cation Intermixing, Interfacial Chemistry, and Oxygen Deficiency in Understanding the Properties of the LaFeO 3 /SrTiO 3 (100) Interface

The properties of oxide perovskites vary broadly for different cations and stoichiometries, and interfaces between dissimilar oxides can have entirely novel characteristics not seen in the bulk of either material. The anomalous conducting interface between LaAlO3 and SrTiO3(001) has received the most attention, and numerous hypothetical explanations have been posited related to resolution of the polar discontinuity at the interface, including cation inter-diffusion, interfacially-reduced B-site cations, local oxygen deficiency, and various combinations thereof [1–3]. The growing body of research indicates a diverse interplay of these effects in various polar/non-polar perovskite oxide interfaces—possibly specific to each materials system—and a coherent approach to predicting the properties of such interfaces remains elusive.