Experimental observations of isolated surface steady-state branches

In this paper we report infrared spectroscopic observations of isolated surface steady-state branches during CO oxidation over $\text{Pt}\text{γ-Al}{}_2\text{O}{}_3$ catalysts. These are branches that cannot be reached by continuous smooth changes of the experimentally controlled variables, but only through global perturbations of the catalytic reaction system.     

Reaction rate oscillations during CO oxidation over Pt/γ-Al2O3; experimental observations and mechanistic causes

In this paper we present experimental observations of reaction rate oscillations during CO oxidation over Pt/γ-Al₂O₃ at atmospheric pressures. Based on our experimental observations and prior experimental literature on this reaction we propose a mechanistic scheme, which we believe explains in qualitative terms the oscillatory behavior exhibited by this catalytic reaction system. This mechanistic scheme involves a Langmuir-Hinshelwood reaction between adsorbed CO and oxygen and a slow formation and reduction step of an inactive surface oxide species. Our experimental observations and mechanistic ideas, furthermore, support the existence of multiple time scale phenomena for this catalytic reaction system, an idea originally suggested by Chang and Aluko.     

A novel sacrificial interlayer-based method for the preparation of silicon carbide membranes

A novel method is presented based on the use of sacrificial interlayers for the preparation of nanoporous silicon carbide membranes. It involves periodic and alternate coatings of polystyrene sacrificial interlayers and silicon carbide pre-ceramic layers on the top of slip-casted tubular silicon carbide supports. Membranes prepared by this technique exhibit single gas ideal separation factors of helium and hydrogen over argon in the ranges 176–465 and 101–258, respectively, with permeances that are typically two to three times higher than those of silicon carbide membranes prepared previously by the more conventional techniques. Mixed-gas experiments with the same membranes indicate separation factors as high as 117 for an equimolar H₂/CH₄ mixture. We speculate that the improved membrane characteristics are due to the sacrificial interlayers filling the pores in the underlying structure and preventing their blockage by the pre-ceramic polymer. The new method has good promise for application to the preparation of a variety of other inorganic microporous membranes.     

Atomistic simulation of nanoporous layered double hydroxide materials and their properties. I. Structural modeling

An atomistic model of layered double hydroxides, an important class of nanoporous materials, is presented. These materials have wide applications, ranging from adsorbents for gases and liquid ions to nanoporous membranes and catalysts. They consist of two types of metallic cations that are accommodated by a close-packed configuration of OH- and other anions in a positively charged brucitelike layer. Water and various anions are distributed in the interlayer space for charge compensation. A modified form of the consistent-valence force field, together with energy minimization and molecular dynamics simulations, is utilized for developing an atomistic model of the materials. To test the accuracy of the model, we compare the vibrational frequencies, x-ray diffraction patterns, and the basal spacing of the material, computed using the atomistic model, with our experimental data over a wide range of temperature. Good agreement is found between the computed and measured quantities.     

Molecular dynamics simulations of transport and separation of carbon dioxide-alkane mixtures in carbon nanopores

The configurational-bias Monte Carlo method, which is used for efficient generation of molecular models of n-alkane chains, is combined for the first time with the dual control-volume grand-canonical molecular-dynamics simulation, which has been developed for studying transport of molecules in pores under an external potential gradient, to investigate transport and separation of binary mixtures of n-alkanes, as well as mixtures of CO2 and n-alkanes, in carbon nanopores. The effect of various factors, such as the temperature of the system, the composition of the mixture, and the pore size, on the separation of the mixtures is investigated. We also report the preliminary results of an experimental study of transport and separation of some of the same mixtures in a carbon molecular-sieve membrane with comparable pore sizes. The results indicate that, for the mixtures considered in this paper, even in very small carbon nanopores the energetic effects still play a dominant role in the transport and separation properties of the mixtures, whereas in a real membrane they are dominated by the membrane's morphological characteristics. As a result, for the mixtures considered, a single pore may be a grossly inadequate model of a real membrane, and hence one must resort to three-dimensional molecular pore network models of the membrane.     

Nonlinear optical spin Hall effect and long-range spin transport in polariton lasers

We report on the experimental observation of the nonlinear analogue of the optical spin Hall effect under highly nonresonant circularly polarized excitation of an exciton-polariton condensate in a GaAs/AlGaAs microcavity. The circularly polarized polariton condensates propagate over macroscopic distances, while the collective condensate spins coherently precess around an effective magnetic field in the sample plane performing up to four complete revolutions.     

Atomistic simulation of nanoporous layered double hydroxide materials and their properties. II. Adsorption and diffusion

Nanoporous layered double hydroxide (LDH) materials have wide applications, ranging from being good adsorbents for gases (particularly CO(2)) and liquid ions to membranes and catalysts. They also have applications in medicine, environmental remediation, and electrochemistry. Their general chemical composition is [M(1-x)(II)M(x)(III)(OH(-))(2)](x+)[X(nm)(m-)nH(2)O], where M represents a metallic cation (of valence II or III), and X(nm)(m-) is an m-valence inorganic, or heteropolyacid, or organic anion. We study diffusion and adsorption of CO(2) in a particular LDH with M(II)=Mg, M(III)=Al, and x approximately = 0.71, using an atomistic model developed based on energy minimization and molecular dynamics simulations, together with a modified form of the consistent-valence force field. The adsorption isotherms and self-diffusivity of CO(2) in the material are computed over a range of temperature, using molecular simulations. The computed diffusivities are within one order of magnitude of the measured ones at lower temperatures, while agreeing well with the data at high temperatures. The measured and computed adsorption isotherms agree at low loadings, but differ by about 25% at high loadings. Possible reasons for the differences between the computed properties and the experimental data are discussed, and a model for improving the accuracy of the computed properties is suggested. Also studied are the material's hydration and swelling properties. As water molecules are added to the pore space, the LDH material swells to some extent, with the hydration energy exhibiting interesting variations with the number of the water molecules added. The implications of the results are discussed.     

Molecular dynamics simulation of diffusion and sorption of water in conducting polyaniline

Energy minimization and molecular dynamics simulations are used to develop, for the first time, atomistic models of HCl- and HBr-doped conducting polyanilines, in order to study diffusion and adsorption of water vapor in the polymers. Various morphological properties of the polymers are computed, including their pair correlation functions that are found to be in good agreement with the experimental data, and their accessible free volumes. Also computed are the sorption isotherms and effective self-diffusivity of water vapor in the polymers. The computed sorption isotherms are in quantitative agreement with the experimental data, while the diffusivities are within an order of magnitude of the data. The reasons for the differences between the computed and measured diffusivities are discussed.