N-Alkyldinaphthocarbazoles, Azaheptacenes, for Solution-Processed Organic Field-Effect Transistors
Abstract not provided
Pho, Toan V↗
Engineering topics
Kurzman, Joshua A
Publications and source records attributed to Kurzman, Joshua A.
C-H Bond Activation by Pd-substituted CeO 2 : Substituted Ions versus Reduced Species
Substituted metal oxides containing ionic species have been attracting a great deal of attention because of their potential ability to reduce the usage of precious metals in heterogeneous catalysts. We investigate Pd-substituted CeO 2 for C-H bond activation reactions including the partial oxidation and dry reforming of CH 4 . This catalyst has been previously studied for CO oxidation, NO x reduction, and the water-gas shift reaction. Pd-substituted CeO 2 , Ce 1-x Pd x O 2-δ , was prepared as a powder with high surface area and a hollow sphere morphology using ultrasonic spray pyrolysis. The catalysts were extensively characterized using synchrotron X-ray diffraction and other techniques, confirming phase pure samples up to 10 mol % Pd substitution. Ce 0.95 Pd 0.05 O 2-δ was found to be active for partial oxidation of CH 4 around 500 °C and higher. Our studies, including postcatalytic synchrotron diffraction, suggest that the single-phase Ce 1-x Pd x O 2-δ material is not the active species and that catalysis occurs instead over the reduced two-phase Pd 0 /CeO 2 . This observation has been further confirmed by verifying the activity of the reduced Pd 0 /CeO 2 catalysts for ethylene hydrogenation, a reaction that is known to require Pd 0 .
36 MATERIALS SCIENCE↗
Neutron diffraction study of La[subscript 4]LiAuO[subscript 8]: Understanding Au[superscript 3+] in an oxide environment
Owing to gold's oxophobicity, its oxide chemistry is rather limited, and elevated oxygen pressures are usually required to prepare ternary and quaternary oxide compounds with gold ions. The Au{sup 3+} oxide, La{sub 4}LiAuO{sub 8}, is remarkable both because it can be prepared at ambient pressure in air, and because of its unusual stability toward thermal decomposition and reduction. The structure of La{sub 4}LiAuO{sub 8} was established by Pietzuch et al. using single crystal X-ray diffraction [1]. The compound adopts an ordered modification of the Nd{sub 2}CuO{sub 4} structure, containing two-dimensional sheets in which AuO{sub 4} square planes are separated from one another by LiO{sub 4} square planes. In light of the meager X-ray scattering factors of Li and O, relative to La and Au, we report here a neutron powder diffraction study of La{sub 4}LiAuO{sub 8}, definitively confirming the structure. To our knowledge, this is the first reported neutron diffraction study of any stoichiometric oxide compound of gold. X-N maps, which make use of nuclear positions obtained from Rietveld refinement of time-of-flight neutron diffraction data and electron densities obtained from synchrotron X-ray powder diffraction data, point to the highly covalent nature of the Au-O bonding in La{sub 4}LiAuO{sub 8}. This is in good agreement with charge densities and Bader charges obtained from full density functional relaxation of the structure.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Cobalt Coordination and Clustering in [alpha]-Co(OH)[subscript 2] Revealed by Synchrotron X-ray Total Scattering
Structures of layered metal hydroxides are not well described by traditional crystallography. Total scattering from a synthesis-controlled subset of these materials, as described here, reveals that different cobalt coordination polyhedra cluster within each layer on short length scales, offering new insights and approaches for understanding the properties of these and related layered materials. Structures related to that of brucite [Mg(OH){sub 2}] are ubiquitous in the mineral world and offer a variety of useful functions ranging from catalysis and ion-exchange to sequestration and energy transduction, including applications in batteries. However, it has been difficult to resolve the atomic structure of these layered compounds because interlayer disorder disrupts the long-range periodicity necessary for diffraction-based structure determination. For this reason, traditional unit-cell-based descriptions have remained inaccurate. Here we apply, for the first time to such layered hydroxides, synchrotron X-ray total scattering methods - analyzing both the Bragg and diffuse components - to resolve the intralayer structure of three different {alpha}-cobalt hydroxides, revealing the nature and distribution of metal site coordination. The different compounds with incorporated chloride ions have been prepared with kinetic control of hydrolysis to yield different ratios of octahedrally and tetrahedrally coordinated cobalt ions within the layers, as confirmed by total scattering. Real-space analyses indicate local clustering of polyhedra within the layers, manifested in the weighted average of different ordered phases with fixed fractions of tetrahedrally coordinated cobalt sites. These results, hidden from an averaged unit-cell description, reveal new structural characteristics that are essential to understanding the origin of fundamental material properties such as color, anion exchange capacity, and magnetic behavior. Our results also provide further insights into the detailed mechanisms of aqueous hydrolysis chemistry of hydrated metal salts. We emphasize the power of the methods used here for establishing structure-property correlations in functional materials with related layered structures.
43 PARTICLE ACCELERATORS↗
La[subscript 4]LiAuO[subscript 8] and La[subscript 2]BaPdO[subscript 5]: Comparing Two Highly Stable d[superscript 8] Square-Planar Oxides
La{sub 4}LiAuO{sub 8} and La{sub 2}BaPdO{sub 5}, two previously known oxides, are presented as model compounds for examining the role of isolated and immobilized Au{sup 3+} and Pd{sup 2+} ions in heterogeneous catalysis. Structural characterization, stability, surface composition, and electronic structure of these compounds are presented. These are examined in studies ranging from synchrotron X-ray scattering, including pair distribution function (PDF) and maximum entropy method (MEM) analysis, to density functional calculations of the electronic structures. The exceptional stability displayed by these compounds as verified by thermogravimetric analysis can be attributed to the presence of covalent Au-O and Pd-O interactions revealed in MEM studies, which suggests a criterion for stabilizing these highly oxophobic transition metals in oxide environments. Catalytic testing of the two compounds as heterogeneous catalysts in the oxidation of CO to CO{sub 2} are presented. La{sub 2}BaPdO{sub 5} appears to be an effective catalyst for CO oxidation, despite the low surface area of the oxide being used. This is the first time that a fully ordered (rather than doped) Pd{sup 2+} oxide had been used to catalyze CO oxidation. La{sub 4}LiAuO{sub 8} on the other hand, is much less effective at catalyzing CO oxidation. Differences in the reactivities of the two compounds are discussed with respect to differences in their density functional electronic structures.
43 PARTICLE ACCELERATORS↗