I SPECIAL REPORT
ENVIRONMEOTAL CATALYSTS Robert J. Farrauto, Ronald M. Heck, and Barry K. Speronello, Engelhard Corp. ecently enacted federal and state laws to improve air quality, especially the 1990 a m e n d m e n t s to the Clean Air Act, have set the air pollution control agenda for the 1990s. Among other things, these laws define limits for harmful emissions from industry, transportation, power generation, and other sources. The new requirements, which become increasingly restrictive through the 1990s and beyond, are expected to prompt strong growth for environmental catalysts, which in 1989 accounted for more than a third of the $1.9 billion catalyst market in the U.S. Most of the growth will come from new or improved products that reduce such air pollutants as carbon monoxide; volatile organic compounds (VOCs); nitrogen oxides, which react photochemically with VOCs to form ground-level ozone and also contribute to acid rain formation; and particulates. In all, the 1990 amendments to the Clean Air Act set limits for 189 toxic air pollutants. Carbon monoxide is a partial combustion product of many fuels. VOCs are emitted by most industrial processes. Nitrogen oxides (which generally refer to nitric oxide and nitrogen dioxide) form when nitrogen and oxygen react in power plants or combustion engines at temperatures above 1500 °C (most power plants have peak temperatures of 2500 °C or more), in burning nitrogen-containing fuels, or in various chemical operations. Particulates e n c o m p a s s a broad range of materials derived from diesel engines, coalfired boilers, and coal-fired power plants. Another category of regulated air pollutants, sulfur oxides, which, like nitrogen oxides, contribute to acid rain formation, form when sulfur compounds in fuel burn, especially in boilers and power plants. Abatement of sulfur oxides, which is accomplished primarily by traditional noncatalytic processes such as scrubbing, is not addressed here, except as it relates to diesel truck emissions. The new clean-air mandates, by pushing pollution control methods well beyond what they can do now, are stimulating research in many areas, especially catalysis. This report describes catalytic approaches being developed and
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SEPTEMBER 7, 1992 C&EN
their potential to help achieve the lower pollution limits set by law. Catalysts have been used in the U.S. to control automotive emissions since 1975 and gaseous emissions from industrial facilities since the 1940s. Now, the new regulations are pressing for wider applications and better catalytic performance. For example, the new rules will require: • Automobile catalytic converters to last 100,000 miles (versus the current 50,000) and to reduce pollutant emissions even further. • Many diesel trucks and buses to be equipped with "aftertreatment" devices, such as flow-through catalysts and soot filters, to control particulates and nitrogen oxides. • Catalytic oxidation of VOCs from industrial exhausts containing certain constituents, such as halogen-containing compounds, that need special treatment. • Use of oxygenated and reformulated gasolines in heavily polluted urban areas, which will alter the refinery flow scheme, especially in catalytic units. In addition to making 100,000-mile catalytic converters mandatory for all new automobiles after 1996, the Clean Air Act amendments (contingent on tier 2 standards to be set by the Environmental Protection Agency), reduce nonmethane hydrocarbon (NMHC) emissions to a maximum of 0.125 g per mile by 2004 (down from 0.41 g per mile in 1991), carbon monoxide to 1.7 g per mile (down from 3.4 g in 1991), and nitrogen oxides to 0.2 g per mile (down from 1.0 g). California has set even more stringent regulations, which 11 Northeast and Middle Atlantic states have said they will adopt. In California, for example, N M H C emissions must be reduced to 0.075 g per mile by 2000 for 96% of all passenger cars. By 2003, 15% of these must have emissions no greater than 0.04 g per mile, and 10% must emit no N M H C s at all.
Auto emissions control Automotive exhaust emissions are controlled by catalytic converters located in the exhaust system so that all exhaust gases pass through them. Converters using oxidation catalysts were introduced in 1975 in response to the original Clean Air Act of 1970. They convert the carbon monoxide
ulate the catalytic surface so that it can handle larger quantities of catalyst poisons and to incorporate more catalytic sites, redistributed within the washcoat to make them more accessible to exhaust molecules. Researchers are also trying to reduce the amounts of rhodium and platinum in three way catalysts and increase the proportion of the less expensive palladium. Altering the composition of the alumina washcoat by including various Microactivity test reactors forfluidcatalytic cracking catalysts are used for bench-scalenonprecious-metal oxides, such as oxides of barium, cerium, and lanthaevaluation of catalyst effectiveness num, is being looked at to promote catalyst activity and stabilize precious-metal dispersion. The redesign of catalysts and washand hydrocarbons produced by incomplete fuel combustion coats is helped by current trends to use computer controls into carbon dioxide and water. Three-way automobile conand cleaner fuels in autos, which make exhaust composition verters, adopted widely in 1981 to meet the federal 1-g-permore predictable. mile nitrogen oxides standard, catalyze these oxidation re actions and simultaneously reduce nitrogen oxides. ThreeElectrically heating the catalyst requires the honeycomb way automobile converters have been used in all new monolith that supports it to be made not of a ceramic, but vehicles since 1981, either alone or in combination with a of a metal that can withstand temperatures up to 1000 °C. A conventional oxidation catalyst. resistance heater warms the unit during, or just before, startup. In two studies, one by Volvo and Emitec (a German Catalytic materials in automobile converters are generalmanufacturer of metal substrates) and the other by W. R. ly supported on a ceramic honeycomb monolith. The honGrace, Southwest Research Institute (SRI), and the Califoreycomb, made of cordierite, contains 300 to 400 square nia Air Resources Board, this approach has demonstrated channels per sq in, each coated with an activated alumina that it can reduce cold-start NMHC emissions enough to allayer called the washcoat. Platinum, palladium, and rhodilow vehicles to meet the emissions test standard. However, um catalysts are highly dispersed on the washcoat. Threethe heater requires 4 to 5 kW of additional power, and it's way catalysts operate near the stoichiometric air-fuel comnot known how well the catalyzed washcoat will adhere to bustion ratio and at exhaust temperatures of normally 400 the metal substrate over repeated heating cycles. NGK, a to 600 °C. Supplementary oxidation catalysts are sometimes Japanese ceramic company, and Corning each have deused for additional control of carbon monoxide or hydrosigned systems in which a catalyzed metal substrate is carbon emissions. placed before the main catalyst and electrically preheated to Today's catalytic converters don't reach their minimum raise the inlet temperature to the main catalyst. These devicoperating temperature until about 100 seconds after coldes are still in the preliminary stage of development. start of an automobile. During this warmup period, more Moving the main converter nearer the engine manifold, than 50% of NMHCs and carbon monoxide generated in the where temperature is higher, would also heat the converter emissions test cycle defined by the Clean Air Act pass unre faster. But this move also would expose the catalyst to conacted. New catalysts are being designed to work faster, tinuously higher temperatures during use (950 to 1000 °C), thereby shortening the cold-start time and reducing the iniwhich would put it under greater stress. In 1991, a team tial emissions of NMHCs and carbon monoxide. Strategies from Johnson Matthey Corp., led by Robert J. Brisley, found to do this include catalyst formulations that work at lower that between 780 and 960 °C, platinum-rhodium catalysts temperatures, electrically heated catalysts, converters that have better thermal resistance than palladium-rhodium catcan be placed nearer the engine in the manifold, prelimialysts. However, increasing the proportion of palladium to nary or starter catalysts, and storage of hydrocarbons until rhodium in the latter produced a catalyst that performed as the catalyst is at operating temperature. well as the platinum-rhodium compound. Research by sevEfforts to redesign catalyst formulations involve both cateral major catalyst manufacturers on thermally stable washalyst and washcoat. One thrust of this research is to manipSEPTEMBER 7,1992 C&EN 35
SPECIAL REPORT coats finds that adding oxides of lanthanum, barium, or cerium stabilizes the alumina and thus reduces sintering and loss of internal surface area. A starter catalyst placed near the exhaust manifold also can be coupled with a primary catalytic converter in the conventional location. In 1990, a Ford Motor Co. team led by Ronald G. Hurley looked at such heated and unheated minicatalysts coupled with a downstream converter. The heated version was superior in reducing carbon monoxide and hydrocarbons both in cold-start emissions and over the entire emissions test cycle. In hydrocarbon storage, a unit upstream from the converter prevents hydrocarbons from passing unreacted through the converter by adsorbing them when the catalyst is cold and releasing them to the catalyst when it has reached operating temperature. Toyota received a U.S. patent in 1991 for a zeolite (mordenite) adsorbent for hydrocarbon storage. An SRI consortium as well as individual catalyst companies, including Engelhard, is exploring the feasibility of using molecular sieves as a storage medium. Catalyst longevity is also a key issue if converters are to operate for 100,000 miles. The switch to unleaded gasolines in the U.S. has eliminated catalyst deactivation caused by lead. Other contaminants in fuel and engine oil still present problems, however, especially phosphorus, zinc, and sulfur. Phosphorus and zinc compounds such as zinc dialkyl dithiophosphate in lubricating oil (a wear-retardant additive) reduce catalyst performance because they deposit on the washcoat surface and form an amorphous glaze, which keeps exhaust molecules from reaching catalytic sites within the washcoat. Phosphate deposition has been partially controlled by raising the ratio of alkaline-earth metal (an additive that neutralizes acid buildup in engine oil) to phosphorus. Although use of methylcyclopentadienyl manganese tricarbonyl (MMT), an antiknock compound, is not allowed in the U.S., it is added to gasoline in Canada. U.S.-owned cars
driven in Canada will use this gasoline, which can interfere with catalyst performance. A study by Ford Motor's Hurley found that MMT decreases activity by blocking catalyzed washcoat pores and increases both operating temperature ^nd the time it takes to reach it. Hurley also showed that even at low levels of MMT, catalyst performance deteriorates, particularly for hydrocarbon oxidation. (A request to EPA by Ethyl Corp. to use MMT in the U.S. at a lower concentration than in Canada was recently denied.) If converters are moved close to the engine, their higher operating temperatures may increase nitrogen oxides emissions because catalytic reduction of these oxides declines at high temperatures. If the temperature exceeds 800 °C, as it can in a manifold catalyst, rhodium, the primary nitrogen oxides control agent in three-way catalysts, tends to react with the washcoat and deactivate it. More effective use of cerium oxide and other washcoat nonprecious-metal oxides may limit rhodium's interactions and promote durability and activity. Research on stabilizing rhodium and preventing its interaction with support materials has been under way at major catalyst suppliers and automotive companies for several years but continues to be a challenge. To improve fuel economy, engine manufacturers have tried raising air-to-fuel ratios from the current stoichiometric ratio of between 14.6 and 14.7 to between 18 and 21. Unfortunately, current three-way catalysts cannot reduce nitrogen oxides to nitrogen in the more highly oxidizing environment created when the proportion of air is increased. Research has been under way for many years to find viable catalysts that decompose nitrogen oxides in oxidizing environments. These catalysts are called lean nitrogen oxides reduction catalysts because the ratio of fuel to air is "lean." Pioneering work in 1981 at Nagasaki University led by Masakazu Iwamoto and coworkers and at Toyota showed that nitrogen can be decomposed to nitrogen and oxygen in the presence of excess oxygen using a copper-containing zeolite catalyst called Cu/ZSM-5. In 1991, Iwamoto, then at
Cutaway view of Allied-Signal automotive catalytic converter (left) shows interior of ceramic honeycomb monolith. The honeycomb (enlarged,right)contains 300 to 400 square channels per sq in, each coated with a porous, high-surface-area laye as activated alumina, on which precious-metal catalysts are dispersed. Carbon monoxide and hydrocarbons in exhaust gases which must pass through the channels, are catalytically converted to carbon dioxide and water 36
SEPTEMBER 7,1992 C&EN
Hokkaido University, and coworkers there showed that although this catalyst is severely poisoned by sulfur dioxide, the use of propylene or other olefins as reductants (still in the presence of excess oxygen) increases nitrogen oxides conversion and decreases sensitivity to sulfur dioxide. However, the catalyst's effectiveness is inhibited, and slowly and irreversibly attenuated, by the water vapor always present in auto exhaust. In 1991, Makoto N. Mizuno and Kouzou Kondo at the University of Tokyo found that ZSM-5 zeolite containing substituted rare earth ions such as cerium show promise as lean nitrogen oxides reduction catalysts. Mordenite, acidexchanged zeolite Y, Al203-supported cobalt, and solid acid catalysts also show reducing activity in lean conditions, based on recent work by Hideaki Hameda and coworkers at the National Chemistry Laboratory in Ibaraki, Japan. In the same time period, Yi Li and W. Keith Hall at the University of Pittsburgh surveyed a number of catalysts for nitrogen oxides reduction and also concluded that Cu/ ZSM-5 is the most active. A year earlier, Wolfgang Held and coworkers at Volkswagen had found that urea could be - used as a reductant with Cu/ZSM-5 to reduce nitrogen oxides. But catalytic activity is too low to be useful. A lean nitrogen oxides reduction system must be integrated with the engine so the exhaust stream will have the type and amount of hydrocarbons needed to reduce these oxides at the optimum temperature for the particular hydrocarbon. For example, propane is effective at 500 °C with Cu/ZSM-5 catalyst, but is ineffective at lower temperature. In contrast, ethylene reduces nitrogen oxides at 160 to 200 °C. Despite substantial work on lean nitrogen oxides catalysts in the past decade, a viable technology has not yet been developed. This area will undoubtedly continue to receive a great deal of attention. Alternative fuels are another area of active study. Fuels such as compressed natural gas, liquid petroleum gas, methanol, and ethanol are attractive alternatives to gasoline because they are potentially less polluting, although they still produce gaseous emissions. Engines that burn them could be run lean with an oxidation catalyst or they may be able to use a conventional three-way catalyst to meet the new standards for carbon monoxide, hydrocarbon, and nitrogen oxides emissions. Vehicle and catalyst systems have been developed for fuels that contain 85% methanol, and prototype vehicles with catalysts that operate with dual fuels of gasoline and neat methanol are under development.
Air-fuel ratio determines efficiency of three-way catalytic converter % efficiency 100
14.3
14.4
14.5
14.6
14.7
14.8
14.9
Air-fuel ratio
Oxidation of carbon monoxide and hydrocarbons to carbon dioxide and water takes place in automobile converters via heterogeneous catalysis. The reactants diffuse to and are adsorbed on the catalyst's active sites (primarily platinum and rhodium deposited on alumina), where a reaction intermediate forms. This intermediate is converted to the final products, which must desorb and diffuse into the exhaust stream, making the sites available to more reactants. This sequence of events occurs in a few thousandths of a second. Three-way automobile converters reduce nitrogen oxides to nitrogen, in addition to catalytically oxidizing carbon monoxide and hydrocarbons. Careful control of the engine air-fuel ratio, which determines the ratio of air to fuel entering the converter, allows the oxidation and reduction reactions to proceed simultaneously with efficiencies of 80% or greater for both reactions. The diagram shows the air-fuel ratios at which such efficiencies are possible. More air causes reduction of nitrogen oxides to fall off sharply. More fuel decreases the efficiency of the oxidation reaction. Control of the air-fuel ratio is achieved by means of an oxygen sensor in the exhaust stream. In current three-way catalysts, the stoichiometric ratio of air to fuel lies between 14.6 and 14.7
Controlling truck emissions In a multistep process that began in 1988, when a particulate standard of 0.6 g per brake-horsepower hour and a 10.7 g per bhp-hour nitrogen oxides standard went into effect, the Clean Air Act began to tighten the limits for nitrogen oxides and particulate emissions from truck diesel engines (Brake horsepower is a measure of engine power developed. Particulate concentration is expressed as grams emitted divided by horsepower generated in an hour.) By 1994, particulate levels from these exhausts must be reduced from the present limit of 0.25 g per bhp-hour to 0.1 g per bhp-hour. Emission limits for nitrogen oxides were decreased to 5 g per bhp-hour in 1991 and will be reduced to 4 g per bhp-hour by 1998. Particulates are primarily soot (dry carbonaceous materi-
al), liquid hydrocarbons, and sulfates. Liquid hydrocarbons, also called the soluble organic fraction, consist of unburned or partially burned fuel or lubricating oil. They may condense on the walls of the exhaust system, form aerosols, or adsorb on soot. Sulfur trioxide and sulfur dioxide form when the sulfur in diesel fuel burns in the engine. Sulfur trioxide rapidly forms sulfate particles as the exhaust cools and reacts either with water in the exhaust or metal oxides in the exhaust system. Even though sulfur in diesel fuel will have to drop to 0.05% by weight in 1994 from the current 0.15 to 0.3% by weight, significant amounts of sulfur oxides still will be present in the exhaust. Because engine advances alone probably will not be SEPTEMBER 7,1992 C&EN 37
SPECIAL REPORT enough for exhaust emissions to meet 1994 standards for many trucks, exhaust aftertreatment will be needed for soot, soluble organic fraction, and sulfate abatement. Research has focused on filters to remove dry soot or soot on which soluble organic fraction is adsorbed, on oxidation catalysts to remove some of the soluble organic fraction, and on preventing sulfate formation. Soot filters or traps usually use a porous ceramic to trap 60 to 90% of particulates in the exhaust. As soot builds up in the filter, resistance to flow increases. When this resistance becomes too great, the filter must be regenerated by heating to burn off the soot. Regeneration is costly and complex, requiring two filter beds so trapping can continue during regeneration, a control package, and an energy source. Soot traps are currently being evaluated in diesel buses in many U.S. and European cities. Traps were seen as a promising pollution control option in the late 1980s, but their complexity, questionable durability, inorganic ash buildup, and high cost may make them impractical for meeting the 1994 standards. Engine modifications have reduced particulate emissions from engines for heavy-duty trucks (above 33,000 lb) enough that they can meet 1994 standards without aftertreatment. Medium-sized trucks (19,500 to 33,000 lb) will most likely use oxidation catalysts to remove an adequate fraction of soluble organic fraction. Manufacturers can adjust engines to produce less dry soot by increasing the soluble organic fraction, which can be treated catalytically. Diesel oxidation catalysts typically cut the particulate load 30 to 40% by oxidizing 50 to 80% of the soluble organic fraction. They also eliminate 30 to 80% of gaseous hydrocarbons and 40 to 90% of carbon monoxide, as well as the exhaust's pungent odor. They do not act on nitrogen oxides and have little effect on dry soot. Diesel catalysts use a metal or ceramic honeycomb monolith support, and look similar to automobile converters. But unlike the catalytic converters for gasoline engines, which
Engelhard diesel-engine testing facility 38 SEPTEMBER 7,1992 C&EN
treat only gases, diesel converters also cope with liquids and solids, including significant amounts of inorganic ash. The catalysts usually consist of either platinum or palladium dispersed on a high-surface-area carrier like aluminum oxide or silicon oxide. They must function between 200 and 600 °C and contend with sulfur and lubricating oil contaminants, such as phosphorus, zinc, magnesium, and calcium, that can shorten the life of the catalysts. The Clean Air Act amendments require truck emission control devices to operate for up to 290,000 miles, depending on engine size, by 1994—nearly three times the mileage for auto catalysts. Lifetimes of diesel oxidation catalysts could be extended by developing low-ash lubricating oils; reducing fuel sulfur; and by modifying the catalyst itself to change its surface chemistry, pore structure, surface area, or other properties. Degussa, Engelhard, and Nippon Shokubai Kagaku Kogyo, in Japan, all plan to commercialize diesel catalysts that can handle large accumulations of oil contaminants in time to meet the 1994 standards. Diesel oxidation catalysts oxidize the sulfur dioxide in the exhaust to sulfur trioxide, which forms sulfate particles that add to the tailpipe particulate emissions. Catalysts that selectively oxidize hydrocarbons and carbon monoxide but do not convert sulfur dioxide to sulfur trioxide would reduce sulfate formation. Although details on oxidation catalyst composition are usually proprietary, a 1990 U.S. patent issued to Degussa describes a catalyst containing platinum, vanadium pentoxide, and titanium dioxide. Makoto Horiuchi and coworkers at Nippon Shokubai Kagaku Kogyo reported in 1990 that catalysts with platinum on aluminum oxide or combinations of noble and base metals on aluminum oxide remove the soluble organic fraction efficiently. Alumina may not be the best washcoat material for diesel oxidation catalysts because it takes up sulfate compounds at low temperatures, causing a decline in catalytic activity. Horiuchi's team developed a proprietary washcoat, on which palladium is deposited, that produces and stores less sulfate than alumina. Patents issued to Douglas Ball at A. C. Rochester, a division of General Motors Corp. located in Flint, Mich., state that a silicon dioxide washcoat deters sulfate storage. Diesel oxidation catalysts can complement diesel-engine technology for new trucks by allowing engine builders to focus on reducing nitrogen oxides formation in the engine, knowing they can deal with excess hydrocarbon particulates via catalytic aftertreatment. Such nitrogen oxides control will become critical later in the decade. Significant impetus toward this approach has come from the California Air Resources Board, which will institute a joint nitrogen oxidesgaseous hydrocarbons limit of 3.9 g per bhp-hour in 1995, the year after the Clean Air Act requires particulate emissions to be reduced to 0.1 g per bhp-hour. The California standard's
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ÎUPON! SEPTEMBER 7,1992 C&EN 39
SPECIAL
REPORT
Catalytic incinerator exhibits three modes of operation Rate of reaction
Region C (thermal combustion)
Region Β (mass transfer)
Region A (kinetics) Temperature
The rate at which conversion of reactants takes place within a catalytic incinerator is governed by three fac tors: the speed at which the catalyst can convert reactants to products, the rate of diffusion of reactants to the sur face of the catalyst, and temperature. In region A (the initial operating temperature), the reaction is governed by the speed of the catalyst, or the kinetics of the reac tion. As temperature is increased, the reaction rate in creases exponentially until it is as fast as the rate of dif fusion of reactants to the catalyst surface. Now the limit ing factor is mass transfer of the reactants (region B). Most current catalytic incinerators operate in region B, between 300 and 350 °C. Catalytic reactions in general follow this pattern. Thermal incineration (region C) takes place when the temperature is high enough—usu ally about 1000 °C—that homogeneous (thermal) reac tions occur in the gas phase
nitrogen oxides emission limit is lower than the 4 g per bhp-hour limit required by the Clean Air Act by 1998. These standards will likely be met by a combination of new en gine technology, catalytic lean nitrogen oxides aftertreatment, and alternate fuels. Engines could be changed by recirculating part of the ex haust gas back to the engine or altering injection timing to reduce nitrogen oxides formation. But these measures may increase particulates levels so much that, even if catalysts remove all the soluble organic fraction, the dry soot alone would still put the engine out of compliance. One way to solve this problem may be to merge the oxidation catalyst with a particulate trap. Catalyzed traps can remove solid and liquid particulates at efficiencies greater than 90% while abating carbon mon oxide and gaseous hydrocarbons. A catalyzed trap placed 40
SEPTEMBER 7,1992 C&EN
near the engine exhaust manifold would be heated to be tween 350 and 400 °C, hot enough to combust soluble or ganic fraction and soot on the catalyst, and opening the possibility of a self-regenerating unit. The ideal catalyzed trap would be simple, reliable, economical; impose a mini mum fuel penalty; and produce a minimum of sulfate. It also would function well in the presence of oil additives and withstand high-temperature exhaust spikes that might crack or melt it. Catalyzed traps were evaluated for diesel mining vehicles in 1986 as part of a joint program of Cana da's Ontario Research Foundation and the U.S. Department of the Interior. The test results suggest that these systems can be self-regenerating. Alternative fuels could also reduce particulates and other pollutants for diesel engines. Compressed natural gas and methanol are beginning to be used in city buses in Denver, Seattle, New York, and Stockholm. Even though these fuels are cleaner, catalysts are still needed to lower fuel hydrocar bon, nitrogen oxides, and carbon monoxide emissions. Be fore alternative fuels can be widely used in trucks, howev er, issues of fuel cost and availability must be overcome. And, although lean nitrogen oxides diesel catalysts offer a direct approach to meeting nitrogen oxides emissions re quirements of 1998, they present the same difficulties in diesel trucks as in automobiles.
Control of NO^ for stationary sources As the major generator of nitrogen oxides, stationary sources far outweigh cars and trucks in yearly tonnage of nitrogen oxides emissions. Stationary sources include boil ers, incinerators, process heaters, combustion turbines, and reciprocating (piston) engines that burn fuels ranging from coal, oil, and natural gas to wood and solid waste. Concen trations of nitrogen oxides in their exhausts range from less than 50 ppm in gas-fired boilers and turbines to more than 500 ppm in solid-fueled boilers and incinerators. Stationary sources such as plants that manufacture nitric acid, adipic acid, fertilizer, and nuclear fuel can produce exhaust gases that have nitrogen oxides concentrations of up to 70,000 ppm. Nitrogen oxides contribute to ground-level ozone forma tion by reacting photochemically with VOCs. In a report is sued last December, the National Research Council con cluded that in areas where the ratio of VOCs to nitrogen oxides is greater than 20, controlling nitrogen oxides emis sions is probably a more effective way to reduce ozone for mation than controlling hydrocarbons or carbon monoxide. Nitrogen oxides are removed most efficiently by selective catalytic reduction (SCR), widely used in Japan and Europe. The process adds ammonia to an oxygen-containing ex haust stream that passes over the catalyst. SCR selectively reduces nitrogen oxides with an efficiency of 80 to 95%, de pending on the nature of the feed. Performance depends on catalyst activity, the ratio of gas flow rate to catalyst vol ume, and the ratio of ammonia to nitrogen oxides in the ex haust. Most SCR catalysts are supported on a ceramic or metal lic honeycomb or are directly extruded as a honeycomb. There are three types: • Platinum catalysts, primarily developed in the 1960s by Engelhard, that function between 230 and 290 °C. • Base-metal catalysts, initially developed in Japan by
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comb catalyst. As catalytic reactions take place down the honeycomb channel, the gas becomes hot enough for noncatalytic thermal combustion to occur. No nitrogen oxides are produced, and the reactions are so efficient that the fuel is burned completely to carbon dioxide and water. Such adiabatic catalytic combustion works well with stationary combustion turbines, where catalyst outlet temperatures of 1300 to 1400 °C are needed for efficient gas turbine power generation. The technology is based on a process developed by William Pfefferle at Engelhard Catalyst / in the mid-1970s. At that time, nitrogen oxHeat exchanger Polluted air or ides emissions were not regulated, and process exhaust honeycomb and catalyst materials that could withstand the high-temperature environment of a gas turbine had yet to be In catalytic converters that destroy volatile organic compounds (VOCs), a burner and a preheat section raise the temperature of the incoming developed. Gas turbine companies are polluted air or process exhaust gas to the optimum temperature for now showing interest in this process. ComVOC oxidation by the catalyst. To improve efficiency, these converters panies including Engelhard, Toshiba, ICI, often include a heat exchanger to transfer energy from the gases exiting and Catalytica, and the Electric Power Rethe reactor to the incoming exhaust search Institute and Gas Research Institute are working independently, and, in some cases together, to resolve problems of activity and durability with catalyst materials. A system of honeycombs and catalysts for natural gas Sumitomo Chemical in the late 1970s, that function between combustion may be field-tested by the end of 1993. 300 and 450 °C. They usually consist of vanadium pentoxide on a titanium dioxide support and are the predominant Volatile organics and hydrocarbons SCR catalyst in use today. Manufacturing processes often involve organic solvents, • Newer zeolite catalysts, developed by Norton Corp. feedstocks, or decomposition products that generate VOCs. and Engelhard, that work between 350 and 600 °C. The Clean Air Act seeks to reduce emissions of these comThe SCR process demands extremely tight control of the pounds in 96 metropolitan areas by requiring businesses to ammonia-nitrogen oxides ratio. Ongoing research aims at install "reasonably available control technology" by May giving the process greater latitude, which will make SCR 31, 1995. An estimated 40,000 facilities, including printers, easier to operate and less costly. It also seeks to improve bakeries, and chemical plants, will be affected. tolerance to contaminants, especially to inorganic oxides VOCs in exhausts can be removed by incineration, consuch as sodium oxide, potassium monoxide, arsenic trioxdensation, scrubbing, or adsorption. Incineration, the most ide, and phosphorous pentoxide. often used method, destroys these compounds by burning Another approach, called nonselective catalytic reduction, them at temperatures greater than 1000 °C or by oxidation uses hydrocarbons, hydrogen, or carbon monoxide as reat temperatures between 300 and 350 °C over a catalyst (catducing agents when exhaust oxygen concentration is low, alytic incineration). Thermal (noncatalytic) incinerators are as it is in rich-burn reciprocating engines (where it is less widely used even though their operating temperatures than 1%), and in nitric acid plants (2 to 3%). An oxygen make their fuel costs relatively high. Their high-temsensor in the exhaust stream signals the air-fuel delivery perature operation also causes other problems: They need system to adjust the air-fuel ratio so there is just enough reexotic high-temperature materials; they produce nitrogen ducing agent present to react with all the oxygen and nitrooxides; and they sometimes yield undesirable by-products, gen oxides. The exhaust passes over a precious-metal honsuch as dioxins from chlorinated materials. Their higher eycomb catalyst where free oxygen is converted to carbon fuel costs often make them less desirable than catalytic indioxide and water by reaction with hydrocarbons, hydrocinerators for treating the fairly dilute gases typical of VOC gen, and carbon monoxide. Nitrogen oxides react with eiemissions. ther excess hydrogen or carbon monoxide to form nitrogen, Even though catalytic incineration has been in use for water, and carbon dioxide. In exhausts from nitric acid more than a decade and is a proven method for reducing plants, the oxidation reaction is followed by a reduction reVOC concentrations to the levels mandated by the Clean action that converts nitrogen oxides to nitrogen. Air Act, research continues to increase its effectiveness and The best way to control a pollutant is to prevent its forto extend its use to more demanding applications, such as mation in the first place. Experimental technology in the treatment of industrial exhausts containing halogenated U.S. and Japan combines catalytic and thermal methods to compounds. burn fuels without forming nitrogen oxides. Mixtures of VOC catalysts usually are supported on a ceramic honeynatural gas or some other fuel and air that are outside the comb structure, although companies such as Camet use range of flame-forming compositions pass through a honey-
Typical catalytic converter for volatile organic compounds
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metal substrates in place of traditional ceramic supports. A typical catalyst contains 0.3% platinum by weight, with rhodium sometimes added as a promoter. When the exhaust stream is relatively clean (poison-free), pelleted and basemetal catalysts such as copper, cobalt, manganese, and chromic oxide are also effective. VOC catalysts lose their activity when char and particulates in the gas cover catalytic sites or plug catalyst pores, or when contaminants interact with the catalyst. Activity generally can be regained by thermal, physical, or chemical regeneration. Thermal regeneration burns off char and other organic compounds, whereas physical treatment, such as purging with compressed air, removes dust and other particulates. Chemical treatment uses mild acids, bases, or chelating solutions to remove contaminants. With periodic regeneration, these catalysts can be used for up to 12 years, or even longer in specific applications. Precious-metal VOC catalysts can destroy chlorinated hydrocarbons, but increased amounts of catalyst and higher temperatures are needed to overcome chloride poisoning. Henry M. Shaw and his students at the New Jersey Institute of Technology find that catalysts that use platinum and palladium supported on aluminum oxide require temperatures greater than 450 °C to destroy chlorinated hydrocarbons, such as those typically used as degreasing agents. Improved zeolite catalysts are being developed by Howard Green, at the University of Akron, to oxidize these hydrocarbons at temperatures in the range of about 300 to 350 °C. Several companies have commercialized new VOC catalysts. Allied-Signal has reported success in catalytically oxidizing chlorinated hydrocarbons, but has not released details. Its patent cites platinum, vanadium pentoxide, and titanium dioxide as major catalyst constituents. ARI International uses a chromic oxide catalyst in a fluidized bed to eliminate chlorinated hydrocarbons, but this system releases fine particles of chromium that may pose an environmental problem. Dedert-Topsoe manufactures a proprietary commercial metal oxide catalyst for destroying chlorinated materials, and KSE has developed an experimental proprietary catalyst for the same purpose. In 1990, Lynette A. Dibble and Gregory B. Raupp of Arizona State University reported photochemical oxidation of trichloroethylene to hydrochloric acid and carbon dioxide using titanium dioxide catalysts irradiated with ultraviolet
Regeneration prolongs lifetime of VOC catalysts % of volatile organic compounds removed Spent catalyst Source of VOC emissions
Wire enameling Semiconductor coating Can coating Aluminum coating Paint bake oven Phthalic anhydride
Fresh catalyst
Before cleaning
After cleaning
86% 86
78% 71
86% 80
86 86 95 95
65 70 71 77
85 86 93 92
Note: Figures are for chemical regeneratior1
Attainable catalyst lifetime (years)
3 6 7 5 4 12
Englehard researchers discuss process control strategies for pilot plant tests on environmental catalysts
light. But more work is needed to determine the commercial feasibility of this process. Although chlorinated hydrocarbons in wastewater are usually removed by air-stripping (air is forced through the wastewater to take up the hydrocarbons), a competitive approach is to treat them directly in the liquid phase. A catalytic hydrodechlorination method that yields chloride-free hydrocarbons and hydrochloric acid is being developed by Suphan Kovenklioglu at Stevens Institute of Technology in collaboration with Engelhard. Kovenklioglu and coworkers use precious-metal catalysts supported on carbon carriers to gain high conversion efficiencies and selectivities at room temperature in both batch and trickle-bed reactors. Process scaleup is under way. David T. Allen and Bijon Hagh at the University of California, Los Angeles, have used nickel and molybdenum on an aluminum oxide support for high-efficiency destruction of chlorinated hydrocarbons. However, the system requires temperatures up to 375 °C and pressures above 1500 psig. E. G. Baker and coworkers at Battelle Pacific Northwest Laboratories, Richland, Wash., are also using nickel catalysts, at 350 to 450 °C and 3000 to 4000 psig. And Thomas N. Kalnes and Robert B. James of Allied-Signal have developed a proprietary industrial catalytic process for hydrotreating polychlorinated biphenyl dielectric liquids and halogenated petrochemicals.
Carbon monoxide Carbon monoxide is emitted by gas turbine power plants, reciprocating engines, and-coal-fired boilers and heaters. Gas turbine plants, unlike the other carbon monoxide SEPTEMBER 7,1992 C&EN
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sources, would produce little of this gas if control of nitro gen oxides weren't needed. Because carbon monoxide is easier to control than the nitrogen oxides, gas turbine oper ators inject steam or water into the combustion chamber to lower the temperature. This reduces the amount of nitrogen oxides produced but raises carbon monoxide levels. Carbon monoxide can be controlled by a precious-metal oxidation catalyst on a ceramic or metal honeycomb. The catalyst promotes reaction of the gas with oxygen to form carbon dioxide at efficiencies that can exceed 95%. This technology has been installed in the exhausts of gas tur bines and reciprocating engines, and it also is finding grow ing acceptance with oil-fired boilers. Powdered platinumon-alumina catalysts are used in the regenerators of fluid catalytic cracking units in petroleum refineries to combust carbon monoxide. Carbon monoxide oxidation catalyst technology is now broadening to applications that require better catalyst dura bility, such as the combustion of heavy oil, coal, municipal solid waste, and wood. Research is under way to help cope with particulates and contaminants, such as fly ash and lu bricating oil, in gases generated by these fuels.
New gasoline blends The 1990 amendments to the Clean Air Act define two new types of gasoline—oxygenated gasoline, for use during the winter months in urban areas with excessive levels of carbon monoxide, and reformulated gasoline, for use in ar eas with excessive ozone pollution. Oxygenated gasoline, which must have an oxygen content of at least 2.7% by weight, will be required by November 1992 in 39 metropol itan areas with severe carbon monoxide pollution. Reformulated gasolines, designed to reduce emissions of volatile and toxic organic compounds, must be used in a to tal of 97 ozone nonattainment areas starting January 1995. EPA is expected to issue initial specifications for these gas olines early next year. To make oxygenated gasolines, refiners are blending me thyl tert-butyl ether (MTBE), tert-amyl methyl ether, and ethanol into the gasoline pool. Supplies of isobutylene, a raw material for MTBE, are expected to fall far short of this added demand. Building enough refineries to produce iso butylene from τι-butane in sufficient quantities to meet clean-air requirements would cost an estimated $10 billion to $30 billion. Fortunately, fluid catalytic cracking (FCC) units in gas-oil refineries can be used to make isobutylene and other light olefins from C4 and larger size cracking products and so hold promise for reducing capital expendi ture. Current commercial FCC technologies can boost refiner ies' production of light olefins only about 10 to 30%. Exist ing FCC technologies that use ZSM-5 zeolite additives in-
Reprints of this C&EN special report are available in black and white at $10 per copy. For orders of 50 or more, subtract 30% from the total order cost. On orders of $50 or less, please send check or money order with request. Send orders to: Distribution, Room 210, American Chemical Society, 1155—16th St., N.W., Washington, D.C. 20036. SEPTEMBER 7,1992 C&EN
crease the yield of C 3 and C4 olefins to the same extent, but do so at the expense of gasoline yield. New FCC catalysts are needed that maximize production of isobutylene and isoamylene, a terf-amyl methyl ether precursor. Both Engelhard and W. R. Grace have an nounced programs to develop catalysts that double or triple isobutylene yields, and Engelhard has begun to offer such catalysts commercially. The new clean air legislation has thrown many areas of catalytic research into a period of creative ferment. New or improved catalysts will be part of the solutions for many of the challenges posed by meeting this law throughout the 1990s and beyond for the chemical, petrochemical, and au tomotive industries. Π
Robert ]. Farrauto is a principal scien tist in Engelhard's research and devel opment department. He is involved in all aspects of catalysis for the environ mental, chemical, and petroleum indus tries. Farrauto has authored 35 papers in catalysis and holds 15 U.S. patents. An adjunct professor of chemical engi neering at Stevens Institute of Technol ogy, Hoboken, N.J., he is also affiliated with the New Jersey Institute of Tech nology, located in Newark. Fanauto re ceived a bachelor's degree in chemistry from Manhattan College, Riverdale, N.Y., and a Ph.D. in chemistry from Rensselaer Polytech nic Institute. Ronald M. Heck, a research and devel opment group leader, is responsible for testing and evaluating Engelhard's en vironmental catalysts, including those for automobiles and stationary sources. Among others, he has worked on cata lysts for reducing nitrogen oxides, for ozone abatement and abatement of vola tile organic compounds, and for diesel exhausts. The author of more than 30 papers and recipient of eight U.S. pat ents, Heck received both bachelor's and Ph.D. degrees in chemical engineering from the University of Maryland. Barry K. Speronello, a principal devel opment scientist, works in the environ mental catalyst section of Engelhard's research and development department, developing catalytic processes to control emissions of nitrogen oxides, carbon monoxide, and hydrocarbons from sta tionary sources. Before this assignment, Speronello managed Engelhard's petro leum catalyst research and development section, which focused on catalytic cracking. The author received bachelor's, master's, and Ph.D. degrees in ceramic engineering from Rutgers University.
