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1.
The catalytic treatment of motor vehicle exhaust gas has been a feature of all passenger cars in the USA since the 1975 model year. It has since been adopted in all the developed countries as the principal means of con-trolling the emissions from vehicles powered by Otto-cycle internal com-bustion engines. These include passenger cars, light trucks, and heavy-duty trucks. Catalysts have been developed for motorcycles and even smaller power plants. The so-called three-way catalyst (TWC) has been in use since the 1979 model year. The name reflects the simultaneous treatment by this catalyst of the two reducing pollutants, carbon monoxide and uncombusted hydrocarbons, and the oxidizing pollutant, oxides of nitrogen. 相似文献
2.
A flow cell was used for the in situ ESR monitoring of the state and reactivity of chromium ions in Cr-ZSM-5. Calcination of Cr(NO3)3/NH4-ZSM-5 in air at 500°C is accompanied by migration of chromium ions inside the zeolitic channels and stabilization ofisolated Cr5+ cations near lattice A13+ ions. Calcination of Cr-ZSM-5 at 750°C leads to a gradual disappearance of the isolated Cr5+ cations and formation of -Cr2O3 microcrystals. All the Cr5+ cations are accessible to gas-phase molecules: O2 strongly broadens the dipole-dipole signal; H2O sorption increases the local crystal field symmetry; admission of CCl4 results in a small change of the Cr5+ local coordination; strongly stabilized complexes on Cr-ZSM-5 are observed upon sorption of either NO or NO2. The sorption of C2H6 on Cr-ZSM-5 at 20°C is accompanied by a gradual reduction of the Cr5+ sites. At 500°C in [C3H6 + O2 + He] flow, even at a large excess of oxidant, the reduction of a noticeable part of Cr5+ ions takes place. At 400°C, in the same gas mixtures, a deeper reduction of Cr5+ occurs. Closer to stoichiometric conditions, in a [C3H6 + NO + He] flow with 120% excess of oxidant the Cr5+ is completely reduced at 500°C. The oxidation of propene is accompanied by coke deposition on the surface of the catalyst. The implications of the results are discussed. 相似文献
3.
Alkaline earths strongly catalyze steam gasification of graphite. The catalytic effect increases in the order Ca < Sr < Ba. Reaction rates of graphite impregnated with 0.3 at.% Ba are 500 times larger than those of pure graphite. Catalysis is due to the participation of additional reaction sites, not to a lowering of the true activation energy. It follows from kinetic data, electron microscopic observations, and theoretical considerations that the efficiency of the catalysts is closely related to their ability to coat a carbon surface and to continue contact with carbon atoms emerging from the bulk. In the presence of sulfur, severe, but not complete, catalyst deactivation occurs. This poisoning is to a large degree reversible. 相似文献
4.
The catalytic activity and selectivity of three PdO-MoO3/-Al2O3 catalysts containing about 2% Pd and 2% Mo were studied for the reduction of NO by h2 in the presence of varying amounts of oxygen at temperatures from 50 to 550 °C. The results are compared with those for PdO/-Al2O3, PdO-MoO3/-Al2O3 containing 2% Pd and 20% Mo, and a commercial Pt-Rh catalyst. In the absence of oxygen, the conversion of NO to N2 and N2O is higher on the three catalysts than it is on PdO/-Al2O3 at 500 and 550 °C. In the presence of oxygen, the yields of N2 and N2O are generally lower on two of the PdO-MoO3/-Al2O3 catalysts than on PdO/-Al2O3. 相似文献
5.
Analysis is conducted for slender beams with a varying cross-section under large non-linear elastic deformation. A thickness
variation function is derived to achieve optimal - constant maximum bending stress distribution along the beam for inclined
end load of arbitrary direction. Closed form solutions are derived for the large deflections that correspond to the various
loading conditions. The analysis is repeated for a beam with optimally varying width (for arbitrary end force) and the width
variation function is also determined. 相似文献
6.
In metal-free ZSM-5, the decrease in the concentration of Bronsted and Lewis acid sites parallels the increase in the SiO2/Al2O3 ratio. In the presence of residual moisture, the adsorption of small alkanes does not markedly depend on the SiO2/Al2O3 ratio in the range from 30 to 150. The acidity is more important for the adsorption of alkenes (propene) which oligomerize following adsorption. The addition of 5 wt.% of La does not significantly affect the strength of the surface acid sites but does decrease their concentration. It causes a transformation, in a part, of the Bronsted acid sites. These new centers could perhaps be assigned to LaO(OH) or La(OH)2+ species which may be formed on the surface in La-containing ZSM-5. La could exchange only the strongest acidic sites without inducing large changes in their total concentration. La addition has a very small effect on the adsorption of small hydrocarbons. The exchange of the protons by Cu2+ decreases the concentration of Bronsted acid sites. At a sufficient loading of the ZSM-5 by Cu2+, i.e. at an atomic ratio Cu/Al > 1.0, the exchange can almost totally suppress the Bronsted acidity. Another effect of the ion exchange is the suppression of the adsorption capacity of small hydrocarbons due to lowered surface area and internal pore volume. The lowered available internal pore volume also limits the extent of the oligomerization of propene. 相似文献
7.
Effect of oxidants on microalgal flocculation 总被引:5,自引:0,他引:5
The effects of chlorine, ozone and chlorine dioxide on Scenedesmus sp. cultures were studied. Algal cell viability and chlorophyll concentration decreased, and the concentration of dissolved organic substances increased with increasing applied oxidant concentration. Pretreatment with chlorine dioxide (1, 3 or 5 mg l−1) or ozone (2.6, 4.6 or 8.1 mg l−1) on algal cultures enhanced algal flocculation with alum, while prechlorination with 10 or 20 mg l−1 increased the required dosage of alum by 15%. Scanning electron micrographs of oxidized cells revealed drastically adverse effects upon the cell surface architecture: in addition to the oxidation of noncellular organic materials, the oxidants damaged both cell surface structures and intracellular components. A model explaining the effects of the different oxidants on microalgal flocculation is suggested. 相似文献
8.
Shelef Karni; Diamond Gary M.; Diamond Guy S.; Liddle Howard A. 《Canadian Metallurgical Quarterly》2005,73(4):689
In this study, the authors examined the relation between adolescent and parent therapeutic alliances and treatment outcome among 65 substance-abusing adolescents receiving multidimensional family therapy. Observer ratings of parent alliance predicted premature termination from treatment. Observer ratings, but not self-report, of adolescent alliance predicted adolescents' substance abuse and dependency symptoms at posttreatment, as well as days of cannabis use at 3-month follow-up. The association between adolescent alliance and substance abuse and dependency symptoms at posttreatment was moderated by the strength of the parent alliance. Results reveal the unique and interactive effects of the 2 alliances on treatment outcome and emphasize the need for a systemic and well-articulated approach to developing and maintaining the multiple alliances inherent to family therapy. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
9.
On the Nova Laser at LLNL, we have recently demonstrated many of the key elements required for assuring that the next proposed laser, the National Ignition Facility (NIF) will reach ignition. In particular, we have achieved a drive of 300 eV in laser heated hohlraums; have shown good understanding and control of symmetry in hohlruams; created large NIF-Scale plasmas with plasma and irradiation conditions relevant to NIF targets that showed low levels of plasma instabilities; demonstrated a good understanding of hydrodynamic instability and subsequent pusher/fuel mix in implosions by means of spectroscopic tracers; and performed integrated implosion experiments that have performed well even under stringent convergences of order 25, which is well into the NIF ignition target regime. 相似文献
10.
Decomposition of nitric oxide and its reduction by CO over superconducting and related cuprate catalysts 总被引:1,自引:0,他引:1
Catalytic activities of five non-conducting and three superconducting cuprates were measured for the decomposition of NO and the reduction of NO by CO. The concentration of the reactants and the space velocities approximate the conditions of automotive catalysts. In contrast to earlier results, obtained at 20 to 30 times higher partial pressures of NO and 20 to 100 times lower space velocities, none of the catalysts, including five perovskite-like cuprates, showed significant activity for the decomposition of NO at reaction temperatures up to 700 °C. All catalysts were fairly active for the reduction of NO. At temperatures above about 400 °C on the perovskite-like cuprates YBa2Cu3O7-x
and Ba2CuO3.5-x
, the rates for NO reduction were higher than on CuO. All catalysts lost activity for NO reduction in the presence of oxygen (oxidizing conditions). 相似文献