Douglas J. Buttrey

Multifunctionality of active centers in (amm)oxidation catalysts: from Bi-Mo-Ox to Mo-V-Nb-(Te, Sb)-Ox

Catalytic centers in selective (allylic) oxidation and ammoxidation catalysts are multimetallic and multifunctional. In the historically important bismuth molybdates, used for propylene (amm)oxidation, they are composed of (Bi3+)(Mo6+)2 complexes in which the Bi3+ site is associated with the α-H abstraction and the (Mo6+)2 site with the propylene chemisorption and O or NH insertion. An updated reaction mechanism is presented. In the Mo–V–Nb–Te–Ox systems, three crystalline phases (orthorhombic Mo7.5V1.5NbTeO29, pseudohexagonal Mo6Te2VO20, and monoclinic TeMo5O16) were identified, with the orthorhombic phase being the most important one for propane (amm)oxidation. Its active centers contain all necessary key catalytic elements (2V5+/Mo6+, 1V4+/Mo5+, 2Mo6+/Mo5+, 2Te4+) for this reaction wherein a V5+ surface site (V5+ = O ↔ 4+V•–O•) is associated with paraffin activation, a Te4+ site with α-H abstraction once the olefin has formed, and a (Mo6+)2 site with the NH insertion. Four Nb5+ centers, each surrounded by five molybdenum octahedra, stabilize and structurally isolate the catalytically active centers from each other (site isolation), thereby leading to high selectivity of the desired acrylonitrile product. A detailed reaction mechanism of propane ammoxidation to acrylonitrile is proposed. Combinatorial methodology identified the nominal composition Mo0.6V0.187Te0.14Nb0.085Ox for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo7.5V1.5NbTeO29, 40% Mo6Te2VO20, and trace TeMo5O16, functions with a combination of compositional pinning of the optimum orthorhombic Mo7.5V1.5±xNb1±yTe1±zO29±δ phase and symbiotic mop-up of olefin intermediates through phase cooperation. Under mild reaction conditions, a single optimum orthorhombic composition might suffice as the catalyst; under demanding conditions this symbiosis is additionally required. Improvements in catalyst performance could be attained by further optimization of the elemental distributions at the active catalytic center of Mo7.5V1.5NbTeO29, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.

Structural aspects of the M1 and M2 phases in MoVNbTeO propane ammoxidation catalysts

Abstract The structures of M1 and M2 in MoVNbTeO propane ammoxidation catalysts have been solved using a combination of TEM, neutron powder diffraction, and synchrotron X-ray powder diffraction. The unit cell of M1 is Pba2 (No. 32) with a = 21.134(2) Å, b = 26.658(2) Å, c = 4.0146(3) Å and Z = 4. The formula unit is Mo7.8V1.2NbTe0.937O28.9. The unit cell of M2 is Pmm2 (No. 25) with a = 12.6294(6) Å, b = 7.29156(30) Å, c = 4.02010(7) Å and Z = 4. The formula unit is Mo4.31V1.36Te1.81Nb0.33O19.81. Tellurium sites in hexagonal channels of both phases are displaced toward vanadium-occupied framework sites, whereas Te in the heptagonal channel of M1 is near the channel center. The chemical topology resulting from oxidation states and Madelung site potentials presents active moieties for the ammoxidation of propane in M1 and propene in M2. EPR confirmed the presence of V4+ and possibly Mo5+ in M1 and V4+ in M2.

Structural Characterization of the Orthorhombic Phase M1 in MoVNbTeO Propane Ammoxidation Catalyst

The structure of the orthorhombic phase in the MoVNbTeO propane ammoxidation catalyst system has been characterized and refined using a combination of TEM, synchrotron X-ray powder diffraction (S-XPD), and neutron powder diffraction (NPD). This phase, designated as M1 by Ushikubo et al. [1], crystallizes in the orthorhombic space group Pba2 (No. 32) with a = 21.134(2) Å, b = 26.658(2) Å, and c = 4.0146(3) Å. The formula unit is Mo7.5V1.5NbTeO29. Bond valence sum calculations indicate the presence of d1 metal sites neighbored by d0 metal sites. The d1 sites are occupied by a distribution of Mo5+ and V4+, whereas the d0 sites are occupied by a distribution of Mo6+ and V5+. Out-of-center distortions in d0 octahedra are consistent with the second-order Jahn–Teller effect and lattice effects. We argue that the V5+–O–V4+/Mo5+ moieties adjacent to Te4+ and Mo6+ sites in the [001] terminal plane provide a spatially isolated active site at which the selective ammoxidation of propane occurs.

Crystal structure and semiconductor-metal transition of the quasi-two-dimensional transition metal oxide, La2NiO4

Mesures de diffraction RX, calorimetrie differentielle a balayage, RPE, susceptibilite magnetique et conductivite electrique, entre 10 et 300 K, sur des poudres et monocristaux La 2 NiO 4

Oxygen excess in layered lanthanide nickelates

Density measurements on large single-crystal specimens of La2NiO4+δ and Pr2NiO4+δ show that oxygen nonstoichiometry arises from the presence of excess lattice oxygen. X-ray photoelectron spectra as well as X-ray absorption edge studies provide no evidence for the existence of Ni3+ in these oxygen-excess nickelates under the conditions of the measurements. Transmission electron microscopy has revealed a novel type of exsolution process of the stoichiometric phase out of nonstoichiometric La2NiO4 during heating in CO2 at 870 K for 3 h. An interpretation of the results in terms of the existence of peroxide species within the conducting layers is proposed.

A direct carbon fuel cell with a molten antimony anode

The direct utilization of carbonaceous fuels is examined in a solid oxide fuel cell (SOFC) with a molten Sb anode at 973 K. It is demonstrated that the anode operates by oxidation of metallic Sb at the electrolyte interface, with the resulting Sb2O3 being reduced by the fuel in a separate step. Although the Nernst Potential for the Sb-Sb2O3 mixture is only 0.75 V, the electrode resistance associated with molten Sb is very low, approximately 0.06 Ωcm2, so that power densities greater than 350 mW cm−2 were achieved with an electrolyte-supported cell made from Sc-stabilized zirconia (ScSZ). Temperature programmed reaction measurements of Sb2O3 with sugar char, rice starch, carbon black, and graphite showed that the Sb2O3 is readily reduced by a range of carbonaceous solids at typical SOFC operating conditions. Finally, stable operation with a power density of 300 mW cm−2 at a potential of 0.5 V is demonstrated for operation on sugar char.

Magnetic properties of quasi-two-dimensional La2NiO4

Magnetic susceptibility studies on single crystals of nearly stoichiometric La2NiO4 with the applied field both parallel and perpendicular to the c axis show a transition at 204 K below which two-dimensional canted antiferromagnetic order seems to exist. This oxide also undergoes a transition from isotropic to anisotropic susceptibility near 100 and 250 K.

An orthorhombic Mo3VOx catalyst most active for oxidative dehydrogenation of ethane among related complex metal oxides

Four distinct structural types (orthorhombic, trigonal, tetragonal and amorphous) of Mo3VOx catalyst were each synthesized by a hydrothermal method as a single phase, characterized structurally and tested for oxidative dehydrogenation of ethane. A common structural feature of the catalysts is that the materials are a layer-type structure and constructed with pentagonal {Mo6O21} units. The arrangement of the pentagonal units can form heptagonal channels to create different structural features. The orthorhombic Mo3VOx catalyst has microporosity due to the open heptagonal channels adsorbing nitrogen molecules and showed the highest activity for the reaction among four distinct catalysts. Furthermore, this phase appeared to be most active, currently, compared to other complex metal oxide catalysts reported. An observed positive relation between the microporosity and the oxidation activity suggests that the catalytic oxidation takes place at the heptagonal channels.

Phase equilibrium study of the YBaNiO system and structural characterization of the new quasi-one-dimensional oxide Y2BaNiO5

Phase equilibria in the Y{sub 2}O{sub 3}-BaO-NiO system were studied in the temperature range 1,000-1,350{degree}C in air. In addition to previously reported compositions, the new phase Y{sub 2}BaNiO{sub 5} was identified. Conditions for synthesis and stability of Y{sub 2}BaNiO{sub 5} are reported, as well as the dependence of oxygen nonstoichiometry and lattice parameters on conditions of preparation. Single crystals were obtained by rf induction melting and the structure was refined by four circle x-ray diffraction. Y{sub 2}BaNiO{sub 5} is isomorphous with Nd{sub 2}BaNiO{sub 5} and consists of isolated, highly compressed chains of corner-shared NiO{sub 6} octahedra.

Neutron-diffraction study of stripe order in La{sub 2}NiO{sub 4+{delta}} with {delta}= (2) /(15)

We report a detailed neutron scattering study of the ordering of spins and holes in oxygen-doped La(2)NiO(4.133). The single-crystal sample exhibits the same oxygen-interstitial order but better defined charge-stripe order than that studied previously in crystals with d = 0.125. In particular, charge order is observed up to a temperature at least twice that of the magnetic transition, T_m = 110.5 K. On cooling through T_m, the wave vector \epsilon, equal to half the charge-stripe density within an NiO(2) layer, jumps discontinuously from 1/3 to 0.2944. It continues to decrease with further cooling, showing several lock-in transitions on the way down to low temperature. To explain the observed lock-ins, a model is proposed in which each charge stripe is centered on either a row of Ni or a row of O ions. The model is shown to be consistent with the l-dependence of the magnetic peak intensities and with the relative intensities of the higher-order magnetic satellites. Analysis of the latter also provides evidence that the magnetic domain walls (charge stripes) are relatively narrow. In combination with a recent study of magnetic-field-induced effects, we find that the charge stripes are all O-centered at T>T_m, with a shift towards Ni centering at T<T_m. Inferences concerning the competing interactions responsible for the the temperature dependence of \epsilon and the localization of charge within the stripes are discussed.