Kathleen C. Taylor

Nitric Oxide Catalysis in Automotive Exhaust Systems

Abstract This review covers the literature through 1991 on nitric oxide catalysis as applied to automobile exhaust systems. Attention is given to the threeway catalyst system which simultaneously promotes the reduction of nitrogen oxides and the oxidation of carbon monoxide and hydrocarbons. These systems have been used on most passenger cars in the United States since 1982. Prior to 1980, emission control catalysts were oxidation catalysts, and reduction in exhaust nitric oxide was achieved using engine modifications (i.e., exhaust gas recirculation). This review focuses on catalytic control of NO, for gasoline-fueled vehicles (not diesels and alternate fuels) and primarily on developments reported since 1982. The term NO, refers to both NO and NOz. The reader is referred to an earlier publication by the author for a general review of automobile catalytic converters [1] and to a review by Egelhoff [2] on the nitric oxide literature through 1980. The recent literature on NO, reduction in lean exhaust is c...

Automobile Catalytic Converters

Publisher Summary This chapter reveals that the automobile catalytic converters have been used in the U.S. since 1974 (1975 model year vehicles) to meet regulated emission standards. In the early 1970s, the use of catalysts for automobile exhaust emission control represented totally new catalyst technology. The development of this technology is an environmental success story. Environmental and political issues came together to force technology development. Use of the catalytic converter for controlling emissions freed up constraints on engine parameter settings. Initially catalytic converters were used to control just carbon monoxide and hydrocarbons. The nitrogen oxide standard of 3.1 g/mile and later 2 g/mile could be met by using exhaust gas recirculation which leads to the formation of less nitrogen oxide in the engine. The catalysts used through 1980 were oxidation catalysts containing the noble metals platinum and palladium. A typical catalyst used by General Motors contained 0.05 oz t noble metal per converter, with a 5/2 ratio of platinum to palladium. New platinum mines were opened in South Africa to supply noble metals for these catalysts. A development which paved the way for the catalytic converter was the removal of lead from gasoline. General Motors had successfully argued for the availability of unleaded gasoline which was necessary to prevent contamination of the catalyst.

Determination of ruthenium surface areas by hydrogen and oxygen chemisorption

Abstract The feasibility of using oxygen adsorption at ambient temperatures and using hydrogenoxygen titration at 100 °C to measure the specific surface area of alumina supported ruthenium catalysts has been evaluated by comparison with hydrogen adsorption data. All three methods give agreement for crystallite sizes greater than 40 ± 10 A but not for small particles. Above 40 A, the oxygen technique is fast and gives linear isotherms down to pressures of a few torricellis. The hydrogen-oxygen titration has the same crystallite size requirements as the oxygen adsorption techniques.

Selective reduction of nitric oxide over noble metals

Abstract Laboratory studies of the partitioning of CO between NO and O 2 under net lean conditions over the noble metals Ir, Rh, Pt, and Pd showed that over Ir alone is the CO + NO reaction favored over the CO + O 2 reaction. Furthermore, Ir exhibited a particle size dependence such that small Ir crystallites were more effective for reducing NO than large particles. Space velocity experiments showed that NO and O 2 react simultaneously with CO over Ir. NO reduction is positive order in NO concentration under the conditions studied. The NO reaction is enhanced by CO (CO is not inhibiting) but inhibited by competitive adsorption of oxygen. The conversion of oxygen was determined by CO availability and was slightly retarded by NO. A reaction mechanism is proposed for the reduction of NO in the presence of excess O 2 at 550 °C. We hypothesize that a different site requirement for NO and O 2 adsorption may be important in determining the selectivity of the catalyst when the number of vacant adsorption sites is limited. Ir differs from Rh in that Ir is able to maintain high NO conversion at elevated temperatures in the presence of excess oxygen. Rh, on the other hand, promotes NO reduction at lower temperatures than does Ir. Rh is expected to make a greater contribution to both CO oxidation and NO reduction during catalyst warmup than Ir. Iridium can be lost from the catalyst under our reaction conditions, presumably via volatilization.

Platinum and palladium addition to supported rhodium catalysts for automotive emission control

Abstract Simultaneous catalytic control of the three major pollutants in automobile exhaust requires both reduction of nitric oxide and oxidation of carbon monoxide and hydrocarbons. A supported rhodium catalyst with low loading (0.002 wt% Rh) was found to have good activity for converting nitric oxide to nitrogen in a laboratory feedstream, but its oxidation activity was inadequate. Addition of platinum or palladium improved the oxidation activity. However, it was found that, compared to rhodium alone, these two-metal combinations (Pt-Rh or PdRh) form more ammonia under reducing conditions and decrease nitric oxide conversion under oxidizing conditions. Both of these undesired effects were circumvented by depositing the added metals on separate support beads. The rhodium catalyst was located in the front of the catalyst bed and was followed by a second layer of pellets containing the platinum or palladium. Evaluations in engine exhaust remain to be done, since laboratory results are not conclusive indicators of actual emission-control performance.

The catalytic reduction of nitric oxide over supported ruthenium catalysts

The activity of supported ruthenium catalysts for reducing NO to N2 in an exhaust-like feedstream has been examined in laboratory experiments. The rate and temperature of NO removal is largely dependent on the NO inlet concentration and independent of the concentration of reducing agents in the system. The selectivity for nitrogen formation, however, is dependent on the concentration of the reducing agents CO and H2 as well as the concentration of NO. No evidence was found for an ammonia intermediate in the conversion of nitric oxide to elemental nitrogen over ruthenium. The high selectivity of ruthenium for the NO to N2 conversion is explained and compared with the behavior of platinum and palladium catalysts.

The dual state behavior of supported noble metal catalysts

In earlier NO reduction studies, difficulties in reproducing results with ruthenium catalysts were traced to some rather unusual pretreatment effects which were able to generate two states for ruthenium catalysts which differed in activity characteristics. This dual state behavior has subsequently been observed for other noble metal catalysts in a variety of reactions. The effect is most dramatic for the water-gas shift reaction, but was also observed for the CO-H2 reaction, ammonia decomposition, and nitric oxide reduction. It was found that sulfur contamination eliminated the effect for ruthenium but not for platinum and palladium catalysts. Chemisorption measurements have shown that the effect is not a simple function of metal loading or dispersion. A mechanism involving surface reconstruction and/or metal-support interaction is proposed.

Sulfur Storage on Automotive Catalysts

Laboratory study results are reported on the catalytic conversion of SO/sub 2/ over supported noble metal catalysts. The dependence of product distribution on catalyst pretreatment with SO/sub 2/ was determined. Moreover, the effect of catalyst pretreatment and operating temperature, gas hourly space velocity (GHSV), oxygen concentration, catalyst composition, and H/sub 2/SO/sub 4/ formation were examined. It was found that sulfur storage on pelleted catalysts may dominate effects of operating conditions and catalyst composition. When the catalyst sulfur content is low, much of the SO/sub 3/ formed is stored in the catalyst. Sulfur storage may indeed provide an effective means of lowering sulfuric acid emissions as long as the sulfur content of the catalyst is kept low by appropriate release mechanisms. It is noted that some of the other exhaust constituents may play a part in the results obtained with auto exhaust systems.

Automobile exhaust emission control by catalysts

The past year has seen a wealth of published articles from around the world related to automobile exhaust emission control. The applied automotive catalysis work continues in response to the need for robust systems that comply with new, stricter regulations for the control of nitric oxides (NOx), hydrocarbons and carbon monoxide under an expanded range of driving conditions that includes higher speeds and air conditioner operation. The search also continues for catalysts that effectively reduce nitric oxide levels in lean feed streams. Although promising results continually emerge, no catalysts have surfaced that meet all the desired requirements for lean burn gasoline or diesel engines. In the absence of effective lean NOx catalysts, NOx sorption systems are emerging as the alternative method for lean NOx control in gasoline engines.

Exhaust catalysts: appropriate conditions for comparing platinum and base metal.

There are fundamental differences in the behavior of alumina-supported samples of a platinum and a copper-chromium catalyst for oxidation of carbon monoxide in a simulated automotive exhaust stream. Ignoring such differences can result in inappropriate comparisons between oxidation catalysts for automotive application.