Environmental catalysis

Catalysis plays a major worldwide role in improving our atmosphere and reducing pollution. Through the next decade, catalysis will continue to provide attractive means to remove undesirable pollutants. This review provides a summary of examples of where catalysis is used to improve our environment as well as new and emerging catalytic solutions. Over twelve major types of pollutants are discussed with a focus on commercially applied processes using catalysis as a solution. In addition, a number of specific catalysis opportunities are discussed.     

Effectiveness of cardenolides as feeding deterrents toPeromyscus mice

I compared the feeding responses of five species ofPeromyscus mice (aztecus, polionotus, melanotis, leucopus, andmaniculatus) to three bitter-tasting cardenolides (ouabain, digoxin, and digitoxin) that differ greatly in lipophilic character.Peromyscus, like other muroid rodents, are unusual in that they can ingest relatively large amounts of cardenolides without adverse physiologic effects. In experiment 1, I determined avoidance thresholds for the three cardenolides with 48 hr, two-choice tests. Mice exhibited large interspecific differences in avoidance threshold, and the interspecific ranking of the thresholds (maniculatus=leucopus >melanotis >polionotus >aztecus) was the same for each of the cardenolides. In experiment 2, I reevaluated the avoidance thresholds, but this time monitored the pattern of intake (i.e., bout lengths) during initial feeding encounters with cardenolidelaced diets. For each cardenolide, mice were subjected to three tests. In test 1, they received a control diet; in test 2, a diet containing the cardenolide at a concentration 1 log, unit below the avoidance threshold (as determined in experiment 1); and in test 3, a diet containing the cardenolide at the avoidance threshold concentration. Results were similar across all species and cardenolide types: Bout lengths in tests 1 and 2 were statistically equal, whereas those in test 3 were significantly shorter than those in test 1. The rapid rejection of cardenolide-laced diets in test 3 is consistent with a preingestive (i.e., gustatory) mechanism underlying the avoidance thresholds. I conclude (1) thatPeromyscus species differ substantially in taste sensitivity to cardenolides and that these differences may influence each species' respective ability to eat cardenolide-laced insects; and (2) that a species' relative taste sensitivity to one cardenolide predicts its sensitivity to other cardenolides.     

High-performance liquid chromatography of human milk triacylglycerols and gas chromatography of component fatty acids

Human milk triacylglycerols were separated by high-performance liquid chromatography. A 5-μ Supelcosil LC-18 column (Supelco, Inc., Bellefonte, PA) was used with acetone/acetonitrile (64∶36, vol/vol) as mobile phase. Triacylglycerols were tentatively identified based on theoretical carbon number and relative retention time. Despite changes resulting from dietary fat variation, the major component triacylglycerols were those composed of palmitic, oleic and linoleic acids. Triacylglycerols with palmitic, stearic and oleic acids were present as minor components. Fatty acids were quantified by gas chromatography relative to an internal standard. Ratios of n−6/n−3 fatty acids were found to be high than previously reported. Based on a paper presented at the Symposium on Milk Lipids held at the AOCS Annual Meeting, Baltimore, MD, April 1990.     

The structural consequences of exchanging tryptophan and tyrosine residues in B.stearothermophilus lactate dehydrogenase

A mutant Bacillus stearothermophilus lactate dehydrogenase hasbeen prepared in which all three tryptophan residues in thewild-type enzyme have been replaced by tyrosines. In addition,a tyrosine residue has been mutated to a tryptophan, which actsas a fluorescence probe to monitor protein folding. The mutantenzyme crystallizes in the same crystal form as the wild-type.The crystal structure of the mutant has been determined at 2.8Å resolution. Solution studies have suggested that thereis little effect upon the mutant enzyme as judged by its kineticproperties. Comparison of the crystal structures of the mutantand wild-type enzymes confirms this conclusion, and revealsthat alterations in structure in the region of these mutationsare of a similar magnitude to those observed throughout thestructure, and are not significant when compared with the errorsin atomic positions expected for a structure at this resolution.     

Volatility of Simulated High-Level Nuclear Waste Glass by Thermogravimetric Analysis

The volatilities of simulated, high-level nuclear waste glasses have been measured using thermogravimetric analysis (TGA). These volatilities were measured in the region of the glass transition temperature ( T _g) of the waste glasses, which is between 450° and 500°C. These data were obtained because the Waste Acceptance Preliminary Specifications require that no foreign materials be released into the canistered waste form upon heating of the canister to this glass transition temperature. In fact, all of the waste glass samples studied actually exhibited a net weight gain upon heating. This weight gain was shown to be due to oxygen uptake through oxidation of FeO. Powdered glass samples did show a small weight loss which was smaller in magnitude than the weight gain and was associated with water desorption. No true volatility was detected to the level of sensitivity (0.01 wt%) of the TGA instrument. This converts to a sensitivity of 330 μ g/m² of glass surface and a corresponding minimum value of 41 mg of volatiles for each Defense Waste Processing Facility conistered waste form. TGA experiments carried out at higher temperatures (800°C) revealed that organic concentrations in the waste glasses are less than 0.01 wt%. Thus, these results demonstrate that the Defense Waste Processing Facility will be able to comply with the Waste Acceptance Preliminary Specifications on the exclusion of foreign materials from the canistered waste forms, after exposure to T _g.     

SiC-Whisker-Reinforced Glass-Ceramic Composites: Interfaces and Properties

Different types of SiC whiskers were incorporated into lithium aluminosilicate (LAS) and calcium aluminosilicate (CAS) glass-ceramic matrices. Interfaces in these composites were characterized using Auger spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and the observations were correlated with measurements of fracture toughness and strength. The chemistry and morphology of the resulting interfaces affected the composite strength and toughness and controlled the mode of crack propagation. Certain types of SiC whiskers were characterized by a carbon-rich near-surface chemistry that became more carbon rich after composite fabrication. In these materials, the flexural strength at 20°C increased by up to 400% and the fracture toughness increased by up to 500%. Crack propagation modes were characterized by crack deflection, whisker–matrix debonding, and crack bridging. In contrast, SiC whiskers with stoichiometric near-surface chemistry generally did not form carbon-rich interfaces during composite fabrication, resulting in composites with low strength and fracture toughness.     

The effect of chemical composition on the miscibility of styrene/acrylonitrile/fumaronitrile terpolymers with styrene/acrylonitrile copolymers

Blends were prepared of styrene–acrylonitrile–fumaronitrile (SANFN) terpolymers with styrene–acrylonitrile (SAN) copolymers and of SANFN terpolymers with SANFN terpolymers of different compositions. Miscibility was determined by differential scanning calorimetry. For three SANFN terpolymer compositions containing 11–17% fumaronitrile, a range of miscible SAN copolymers was defined. The miscibility can be predicted by using a Flory–Huggins-type mean-field approach.     

A local theory of heating in cross-ply carbon fiber thermoplastic composites by magnetic induction

For joining and repair of continuous fiber thermoplastic composites, induction heating has been viewed a strong candidate. Induction heating employs an applied alternating magnetic field, which induces a rotational emf in a grid of conductive carbon fibers, which are then used to carry resulting currents. In continuous carbon fiber crossply composites the available paths for “eddy current” loops are along the network of conductive carbon fibers. For this to occur, an electrical transfer must take place between crossing fibers in adjacent plies. Tests involving variable thicknesses of interply neat film layers have been performed to provide insight into the mechanisms taking place. These tests indicate that the primary mechanism for heating in such laminates is dielectric losses in the polymeric region between fibers in adjacent planes that form the conductive loop. Therefore, heating is not uniform in such composites despite a uniform magnetic flux. Heating patterns were viewed using liquid crystal materials and E-type thermocouples. Several factors leading to nonhomogeneous thermal distributions have been considered, including current density effects, internal emf cancellation, and rotational field effects. Global and local considerations are addressed, a localized model is proposed, and the corresponding theory is developed qualifying the early results. Additional testing has supported the theory.     

The effect of cyclic stress on the physical properties of a poly(dimethylsiloxane) elastomer

The dynamic creep behavior of a filled poly(dimethylsiloxane) elastomer was studied under cyclic stress. The stress level was chosen such that the increase in the internal temperature was small and that microcracks were not observed. This work has demonstrated that cyclic stress in combination with high temperature accelerates the degradation of the elastomer. The results suggest that because of the applied force, breaks in the load-bearing chains of the network occur. These breaks, while relieving the mechanical stress, create highly reactive ionic fragments. It is believed that because of the subsequent reactions of the ionic fragments, changes in the specific gravity, storage modulus, effective crosslink density, and length of the sample (creep) are observed. The observed decrease in the storage modulus is thought to occur because of the reaction of the ionic fragments with moisture, which results in the formation of silanol chain ends that reduce the effective crosslink density. The results also show that contrary to the prediction of the Boltzmann's Superposition Principle, the rate of creep is greatly enhanced when the sample is subjected to a sinusoidally varying dynamic load as compared to a comparable static load. The polymer weight loss was found to be linear with time and strongly dependent on the level of applied dynamic and static force. In addition, the weight loss and rate of creep were also found to be strongly dependent upon temperature.     

Nanograin magnetoresistive manganite coatings for EMI shielding against directed energy pulses

Two magnetoresistive manganites, La_0.83Sr_0.17MnO₃ and La_0.7Sr_0.3MnO₃, are synthesized by the environmentally friendly “deposition by aqueous acetate solution (DAAS)” technique. The manganite film has a grain size of 100 nm, and can be processed as thinly as 0.03 μm per layer, while the powder form has a crystallite size of 40 nm. These magnetoresistive materials are shown to be effective and inexpensive electromagnetic interference (EMI) shield for the extremely low frequency (ELF) EM fields. The electrical resistance of manganites is very sensitive to external influences, such as temperature and electromagnetic fields. Both permeability (μ) and conductivity (σ) of manganites tend to increase with increasing applied magnetic field. The manganites have been shown to “react” to field increases in a way that is particularly useful for shielding EMI field fluctuations (e.g., due to current or voltage spikes).

The manganite properties, e.g., crystal structure, film morphology, radiation absorption and reflection, electrical resistivity, and magnetization, etc., have been measured. The ceramic manganites have a metal–insulator transition at 300 K or higher, and are suitable for a room temperature operation. A thin film (approx. 0.1 μm) of La_0.83Sr_0.17MnO₃ was fabricated on a quartz tube or refractory ceramic fiber blanket. Using this thin manganite film, the EMI shielding effectiveness for the measured E-field attenuation is similar to that of a 25 μm thickness of copper tube, aluminum foil, and silver–nickel particle-dispersed paper. The silver–nickel impregnated paper has an EMI shielding effectiveness of 35 dB at 10 kHz, and 15 dB at 60 Hz (or frequency above 1 MHz). The ceramic manganites are chemically inert, thermally stable, and mechanically flexible. They provide low cost EMI shielding against directed energy pulses and may serve as a “signature reduction” barrier.