Potassium Ferrites Formation in Promoted Hematite Catalysts for Dehydrogenation. Thermal and structural analyses

K-promoted hematite catalysts for ethylbenzene dehydrogenation were studied by thermal analysis (TG/DTG) and high-temperature XRD. The formation of potassium ferrite (K₂Fe₂O₄), considered to be a catalytically active phase, was observed during calcination of the Ce-promoted catalysts. A linear correlation of the catalytic activity and the temperature of potassium ferrites formation was found. This revised version was published online in July 2006 with corrections to the Cover Date.     

Equilibrium Statistical Predictions for Baroclinic Vortices: The Role of Angular Momentum

We develop a point-vortex equilibrium statistical model for baroclinic quasigeostrophic vortices within the context of a two-layer quasigeostrophic fluid that evolves in all of space. Angular momentum, which follows from the rotational symmetry of the unbounded domain, is the key conserved quantity, introducing a length scale that confines the most probable states of the statistical theory. We apply the theory as a model of localized convection in a preconditioned gyre. To illustrate this application, the preconditioned cyclonic, largely barotropic gyres are modeled as „zero inverse temperature” states, which are explicit solutions to the mean-field equations with a Gaussian probability distribution of vortices. Convection is modeled by a cloud of point-vortex hetons – purely baroclinic arrangements of point vortices, cyclonic above and anticyclonic below – which capture the short-term, geostrophically balanced response to strong surface cooling. Numerical heton studies (Legg and Marshall, 1993, 1998) have shown that a preexisting barotropic rim current can suppress baroclinic instability and confine anomalies of potential vorticity and temperature introduced by the cold-air outbreak. Here, we demonstrate that the lateral extent of the most probable states of the statistical theory are constrained by the angular momentum. Without resolution of the detailed dynamics, the equilibrium statistical theory predicts that baroclinic instability is suppressed for preconditioned flows with potential vorticity of the same sign in each layer provided that the strength of convective overturning does not change the sign of potential vorticity in one of the layers. This result agrees with detailed simulations (Legg and Marshall, 1998) and supports the potential use of these statistical theories as parametrizations for crude closure. Received 12 April 2000 and accepted 21 September 2000     

A mathematical framework for stochastic climate models

There has been a recent burst of activity in the atmosphere‐ocean sciences community in utilizing stable linear Langevin stochastic models for the unresolved degrees of freedom in stochastic climate prediction. Here a systematic mathematical strategy for stochastic climate modeling is developed, and some of the new phenomena in the resulting equations for the climate variables alone are explored. The new phenomena include the emergence of both unstable linear Langevin stochastic models for the climate mean variables and the need to incorporate both suitable nonlinear effects and multiplicative noise in stochastic models under appropriate circumstances. All of these phenomena are derived from a systematic self‐consistent mathematical framework for eliminating the unresolved stochastic modes that is mathematically rigorous in a suitable asymptotic limit. The theory is illustrated for general quadratically nonlinear equations where the explicit nature of the stochastic climate modeling procedure can be elucidated. The feasibility of the approach is demonstrated for the truncated equations for barotropic flow with topography. Explicit concrete examples with the new phenomena are presented for the stochastically forced three‐mode interaction equations. The conjecture of Smith and Waleffe [Phys. Fluids 11 (1999), 1608–1622] for stochastically forced three‐wave resonant equations in a suitable regime of damping and forcing is solved as a byproduct of the approach. Examples of idealized climate models arising from the highly inhomogeneous equilibrium statistical mechanics for geophysical flows are also utilized to demonstrate self‐consistency of the mathematical approach with the predictions of equilibrium statistical mechanics. In particular, for these examples, the reduced stochastic modeling procedure for the climate variables alone is designed to reproduce both the climate mean and the energy spectrum of the climate variables. © 2001 John Wiley & Sons, Inc.     

A simple justification of the singular limit for equatorial shallow‐water dynamics

The equatorial shallow‐water equations at low Froude number form a symmetric hyperbolic system with large variable‐coefficient terms. Although such systems are not covered by the classical Klainerman‐Majda theory of singular limits, the first two authors recently proved that solutions exist uniformly and converge to the solutions of the long‐wave equations as the height and Froude number tend to 0. Their proof exploits the special structure of the equations by expanding solutions in series of parabolic cylinder functions. A simpler proof of a slight generalization is presented here in the spirit of the classical theory. © 2008 Wiley Periodicals, Inc.     

Poly(methyl methacrylate) composites prepared by in situ polymerization using organophillic nano-to-submicrometer zinc oxide particles

Nano- and submicrometer zinc(II) oxide particles were synthesized by the polyol method and were used for the preparation of ZnO/poly(methyl methacrylate) (ZnO/PMMA) composite materials by the chain polymerization of methyl methacrylate (MMA) in bulk. ZnO particles with an organophilic surface layer were homogeneously dispersed in the PMMA matrix. Very low concentrations (0.1 wt.%) of nano zinc oxide absorbed over 98% of UV light as determined by UV-vis spectroscopy. Nano zinc oxide (75 nm) increased the initial decomposition temperature of the PMMA matrix by 30-40 °C at concentrations of 0.1% and above. This was explained by the changes in the termination mechanism of MMA polymerization resulting in a reduced concentration of vinylidene chain ends. Nano ZnO also increased the MMA polymerization reaction rate and reduced the activation energy. Submicrometer ZnO showed lower UV absorption, thermal stabilization and no influence on the reaction kinetics indicating that average particle size is of vital importance for the properties of PMMA nanocomposites and for MMA polymerization.     

Electrochemical studies of the lateral diffusion of TEMPO in the aqueous liquid/vapor interfacial region

Surface partitioning and lateral mobility of TEMPO (2,2,6,6-tetramethyl-1-piperidynyloxy free radical) in the aqueous liquid/gas interfacial region were investigated electrochemically with 100 nm wide, 1.0 cm long microband electrodes positioned at the air/water interface. For redox active amphiphiles such as TEMPO, the electrochemical current is the sum of the surface and solution components representing the diffusive transport of TEMPO in both domains as well as the dynamics of equilibration at the air/water interface. Interpretation of the recorded current-voltage curves was aided by a FEMLAB simulation code developed to analyze transport processes in this class of systems. TEMPO and TEMPO(+) partition constants (K(T), K(T)+) and solution diffusivities (D(sol), equal for the two species) were obtained experimentally yielding K(T) = 5.0 +/- 0.7 x 10(2) M(-1), K(T+) = 41 +/- 3 M(-1), and D(sol) = 7.7 +/- 0.35 x 10(-6) cm(2)/s. In view of the low value of K(T)+, TEMPO(+) was assumed not to partition to the air/water interface. We further assumed that the desorption rate constants (k(des)) of both TEMPO and TEMPO(+) were the same. Good fits between the recorded and simulated cyclic voltammograms were obtained using two correlated, adjustable parameters, k(des) and the TEMPO lateral, surface diffusion constant (D(surf)). Detailed analysis of the behavior of this class of systems was obtained for a broad range of D(surf) and k(des) values. In addition, a calibration curve of k(des) versus D(surf) was obtained. Assuming that TEMPO k(des) is in a likely range of 10-100 s(-1), its lateral diffusion constant is in the range of 7.9-3.6 x 10(-5) cm(2)/s. In view of our earlier work (Wu, D. G.; Malec, A. D.; Head-Gordon, M.; Majda, M. J. Am. Chem. Soc. 2005, 127, 4490-4496) showing that at the air/water interface TEMPO is unimmersed, and that its interactions with water are limited to hydrogen bonding with one or two water molecules, D(surf) can be related to the viscosity of the aqueous interfacial region.     

Thermal decomposition of polycrystalline [Ni(NH3)6](NO3)2

The thermal decompositions of polycrystalline samples of [Ni(NH₃)₆](NO₃)₂ were studied by thermogravimetric analysis with simultaneous gaseous products of the decomposition identified by a quadruple mass spectrometer. Two measurements were made for samples placed in alumina crucibles, heated from 303 K up to 773 K in the flow (80 cm³ min^?1) of Ar 6.0 and He 5.0, at a constant heating rate of 10 K min^?1. Thermal decomposition process undergoes two main stages. First, the deamination of [Ni(NH₃)₆](NO₃)₂ to [Ni(NH₃)₂](NO₃)₂ occurs in four steps, and 4NH₃ molecules per formula unit are liberated. Then, decomposition of survivor [Ni(NH₃)₂](NO₃)₂ undergoes directly to the final decomposition products: NiO_1+x, N₂, O₂, nitrogen oxides and H₂O, without the formation of a stable Ni(NO₃)₂, because of the autocatalytic effect of the formed NiO_1+x. Obtained results were compared both with those published by us earlier, by Farhadi and Roostaei-Zaniyani later and also with the results published by Rejitha et al. quite recently. In contradiction to these last ones, in the first and second cases agreement between the results was obtained.     

Electrochemistry of TEMPO: an assessment of the water diffusion constant in the aqueous liquid/vapor interfacial region

TEMPO, 2,2,6,6-tetramethylpiperidnyl-1-oxy, is a weak surfactant exhibiting a reversible redox activity: a one-electron oxidation to its oxonium cation. In the course of our earlier work (ref. Wu et al. in J Am Chem Soc 127:4490–4496, 2005; Glandut et al. in J Phys Chem B 110:6101–6109, 2006; Glandut et al. in Langmuir 22:10697–10704, 2006), we developed a full understanding of TEMPO’s electrochemistry at line microband electrodes. In these experiments TEMPO diffuses to the line electrode residing in the plane of the air/water interface in two coupled media, bulk aqueous phase with D?=?7.7?×?10^?6 cm²/s and along the 2D air/water interface with at least an order of magnitude greater surface diffusion constant, D _surf. The magnitude of the TEMPO oxidation current depends jointly on D _surf and on the rate of surface partitioning expressed by the desorption rate constant, k _des. The population of TEMPO partitioned to the air/water interface is largely unsolvated and couples to the aqueous solution by hydrogen bonding to predominately one water molecule. Our experimental methodology allows us to simultaneously determine D _surf and k _des by recording TEMPO voltammetric curves with line microband and barrier microband electrodes. In this report, we present a new methodology of producing and characterizing barrier microband electrodes using vapor-deposited SiO and introduce additional measures such as aspiration of the air/water interface designed to substantially reduce if not eliminate negative error due to surface impurities. These investigations generated a more accurate value of D _surf of 1.0?±?0.3?×?10^?4 cm²/s which we discuss in terms of the dynamic properties of water in the air/water interfacial region.