Interaction of water with titania: implications for high-temperature gas sensing

High-temperature gas sensors based on semiconducting metal oxides show potential for optimization of combustion processes, resulting in efficient energy use and minimization of emissions. Such metal oxides can function as gas sensors because of the reaction of the sensing gas (e.g., CO) with ionosorbed oxygen species on the oxide surface with the resulting increase in conductivity. A limitation of metal oxide sensors is their difficulty of distinguishing between different gases. Designing selectivity into sensors necessitates a better understanding of the chemistry of gas-solid interactions at high temperatures. In this paper, we have used in situ infrared spectroscopy to monitor the dehydration of a hydrated anatase surface up to 600 degrees C and also to examine the hydration/dehydration of anatase held at 400 degrees C. When the O-H stretching region (3000-3800 cm(-1)) was primarily focused on, it was found that water loss from the titania surface proceeded at lower temperatures (<200 degrees C) through desorption, whereas at higher temperatures, water dissociation to terminal (approximately 3710 cm(-1)) and bridged (approximately 3660 cm(-1)) hydroxyl groups was noted. With a further increase in temperature to 600 degrees C, the bridged hydroxyl groups disappeared faster than the terminal ones. The electrical resistance of anatase at 600 degrees C was measured in the presence of moist gas streams and resulted in an increase in conductivity in the presence of water. In situ vibrational spectroscopy indicated a temporal correlation between the appearance of the bridging hydroxyl group and the change in electrical resistance. Several possible mechanisms are discussed. The chemical reaction of water with anatase at high temperatures necessitates that water be removed from the gas stream to avoid interference. A strategy involving the use of a hydrophobic microporous filter that can reject water and let gases such as CO pass unimpeded is examined. Successful use of such a concept has been demonstrated with a silicalite filter using moist CO gas streams.     

Photochemical studies with a zeolite Y membrane formed via secondary growth

Zeolite membranes offer an attractive feature in design of artificial photosynthetic assemblies because they can keep the charge separated species apart, yet allow for communication between the two sides via the intervening zeolite channels and cages. However, such a strategy would require that the zeolite membrane be pinhole-free. In an earlier paper (J. Phys. Chem. B 106, 11898 (2002)), we have shown that nanocrystalline zeolite Y can be spin-coated on porous substrates to form membranes, and the pinholes can be plugged by a photoresist. Such membranes do not allow manipulation at higher temperatures or even in solution chemistry because of the thermal instability and solubility characteristics of the photoresist. In this paper, we report on a zeolite Y membrane made by the secondary growth method. The growth process leads to minimal pinholes as determined by leak tests. The sturdiness of the membranes allows for manipulation and we have shown that organic reactions can be carried out in the zeolite pores. Charge transport through the membrane by an electron hopping process mediated via self-exchange of ion-exchanged bipyridinium ions are demonstrated. The electron transfer was initiated using a photochemical Ru(bpy)²⁺₃-EDTA system.     

Trapped modes in a channel containing three layers of fluids and a submerged cylinder

The problem of existence of trapped waves in fluids due to a cylinder is investigated for the hydrodynamic set-up which involves a horizontal channel of infinite length and depth and of finite width containing three layers of incompressible fluids of different constant densities. The set-up also contains a cylinder which is impermeable, fully immersed in the bottom (lower-most) fluid layer of infinite depth, and extends across the channel with its generators perpendicular to the side walls of the channel. When the ratios of the densities of the adjacent fluids differ from unity by sufficiently small quantities, the underlying mathematical problem reduces to a generalized nonlinear eigenvalue problem involving a cubic polynomial-cum-operator equation. The perturbation analysis of this eigenvalue problem suggests existence of three distinct modes with different frequencies: one of the order of one persisting at the free surface, and the other two of the order of the density ratio (except for modulo one) persisting at the two internal interfaces. The correlation between these results for the three-layer case and very recent numerical results of other authors in the two-layer case has also been addressed. Received: March 3, 2005     

Elastic properties of the RIV-RIII rotator phases of alkanes

A model free energy has been constructed to describe the R_IV-R_III rotator phase transition in alkanes in terms of the elastic strains and order parameter. The conditions for the R_IV-R_III phase transition are discussed. From the free energy, the order parameter and the elastic strains are determined. The model free energy describes the first or second order character of the R_IV-R_III transition depending on the strength of the coupling. The elastic properties in the vicinity of the R_IV-R_III transition are discussed on the basis of a free energy expansion. The temperature dependence of the elastic constants is calculated on both sides of the transition. The coupling between the order parameter and elastic stains is shown to have a crucial influence on the phase behavior and the order of the transition.     

Two stable methods with numerical experiments for solving the backward heat equation

This paper presents results of some numerical experiments on the backward heat equation. Two quasi-reversibility techniques, explicit filtering and structural perturbation, to regularize the ill-posed backward heat equation have been used. In each of these techniques, two numerical methods, namely Euler and Crank-Nicolson (CN), have been used to advance the solution in time.Crank-Nicolson method is very counter-intuitive for solving the backward heat equation because the dispersion relation of the scheme for the backward heat equation has a singularity (unbounded growth) for a particular wave whose finite wave number depends on the numerical parameters. In comparison, the Euler method shows only catastrophic growth of relatively much shorter waves. Strikingly we find that use of smart filtering techniques with the CN method can give as good a result, if not better, as with the Euler method which is discussed in the main text. Performance of these regularization methods using these numerical schemes have been exemplified.     

Gravitational Collapse with Dark Energy and Dark Matter in Hořava-Lifshitz Gravity

In this work, the collapsing process of a spherically symmetric star, made of dust cloud, is studied in Ho?ava Lifshitz gravity in the background of Chaplygin gas dark energy. Two different classes of Chaplygin gas, namely, New variable modified Chaplygin gas and generalized cosmic Chaplygin gas are considered for the collapse study. Graphs are drawn to characterize the nature and to determine the possible outcome of gravitational collapse. A comparative study is done between the collapsing process in the two different dark energy models. It is found that for open and closed universe, collapse proceeds with an increase in black hole mass, the only constraint being that, relatively smaller values of Λ has to be considered in comparison to λ. But in case of flat universe, possibility of the star undergoing a collapse in highly unlikely. Moreover it is seen that the most favourable environment for collapse is achieved when a combination of dark energy and dark matter is considered, both in the presence and absence of interaction. Finally, it is to be seen that, contrary to our expectations, the presence of dark energy does not really hinder the collapsing process in case of Ho?ava-Lifshitz gravity.     

Removal of Organic Compounds Found in the Wastewater through Electrochemical Advanced Oxidation Processes: A Review

Russian Journal of Electrochemistry - Existence of an organic compounds, especially aromatic compounds in wastewater is observed as an emerging environmental problem because of their harmful...     

Selection Principle of Optimal Profiles for Immiscible Multi-Fluid Hele-Shaw Flows and Stabilization

In this paper, we discuss a previously unknown selection principle of optimal viscous configurations for immiscible multi-fluid Hele-Shaw flows that have emerged from numerical experiments on three- and four-layer flows. Moreover, numerical investigation on four-layer flows shows evidence of four-layer systems which are almost completely stabilizing. Simple physical mechanisms that explain both of these findings are discussed.