Why Rhodium in Automotive Three-Way Catalysts?

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.     

In situ ESR of CrH-ZSM-5 up to 500°C in flowing gas containing O2, H2O,CCl4, NO,NO2, and C3H6

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.     

Catalytic steam gasification of graphite: Effects of calcium,strontium, and barium with and without sulfur

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.     

Reduction of NO by H2 on PdO-MoO3/γ-Al2O3 of low molybdena loading

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.     

Adsorption of hydrocarbons by ZSM-5 zeolites with different SiO2/Al2O3 ratios: a combined FTIR and gravimetric study

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.     

Effect of oxidants on microalgal flocculation

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.     

Adolescent and Parent Alliance and Treatment Outcome in Multidimensional Family Therapy.

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)     

Decomposition of nitric oxide and its reduction by CO over superconducting and related cuprate catalysts

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 YBa₂Cu₃O_7-x and Ba₂CuO_3.5-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).     

Nitric Oxide: Surface Reactions and Removal from Auto Exhaust

In the last 10 years an extensive research and development effort has been made to implement the catalytic removal of noxious constituents from automotive exhaust. This effort is finally coming to fruition as this paper is going into print. Vehicles sold in the United States in 1975 will be, in a significant proportion, equipped with catalytic devices. The incorporation of a miniature chemical conversion plant into millions of moving objects operating under widely varying conditions, while being required to assure reliable performance for extended usage periods, has called for the combined skills of scientists, engineers, and designers in several industries. Even so, at present only the catalytic oxidation of carbon monoxide and hydrocarbons has reached the implementation stage. The catalytic removal of nitrogen oxides has turned out to be a still more difficult task. This review will summarize very briefly the present state-of-art of the catalytic devices for the treatment of NO, emissions, but its larger part will be concerned with the scientific background underlying the operation of the devices themselves.     

Effects of calcium,strontium, and barium as catalysts and sulphur scavengers in the steam gasification of coal chars

Effects of alkaline earths on the steam gasification of two coal chars, one of low and one of high intrinsic reactivity, were evaluated gravimetrically. The chars were derived from coal powders which had been impregnated with Ca, Sr, or Ba, pelletized, and pyrolysed in nitrogen. The additives increased the gasification rates in the order Ca < Sr < Ba. It follows from reaction kinetics that catalysis is caused by a large increase in the density of reaction sites, not by a lowering of the true activation energy. As shown by electron micrographs and elemental maps obtained by X-ray analysis, the strong catalytic effect is closely associated with the ability of the alkaline-earth species to spread over the char and to preserve contact with a freshly formed carbon surface as the gasification proceeds. The alkaline-earth catalysts are severely poisoned by hydrogen sulphide or sulphur dioxide from an external source.