Catalysts for Automobile Emission Control

Preparation of Catalysts, edited by B. Delmon, P.A. Jacobs and G. Poncelet o 1976, Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands CATALYSTS FOR AUTOMOBILE EMISSION CONTROL JOE W. HIGHTOWER Department of Chemical Engineering; Rice University; Houston, Texas 77001 (U.S.A.) SUMMARY The use of catalytic converters to control pollutants in automobile exhausts is the newest and second largest catalyst application. Although implementation in the U.S. has been brought about largely by governmental regulations, the attendant benefits

of improved air quality, relative simplicity, and minimal fuel economy loss make it likely that catalytic converters will be a standard feature on most U.S. cars for the next 10 years.

When

supported on either alumina pellets or ceramic monoliths of Cordierite covered with,an alumina wash coat, the noble metals Pt, Pd, (and possibly Rh) provide the active material for oxidation of HC and CO. Reduction catalysts for control of NO,, in a dual bed system are less well established but may possibly include the base metals Co, Cu, Ni, and Cr. The single bed 3-way catalyst system is perhaps the most promising approach for controlling all three pollutants simultaneously; catalysts may include supported Rh and possibly Ir. Availability of noble metals poses a potential problem. Use of monolithic catalysts directly inside the engine to promote flameless combustion represe.its another way to control all the pollutants and at the same time avoid formation of sulfuric acid that plagues the present oxidation catalysts. €i IS'i'd;',ICALBACKGROUND

The newest and second largest (in terms of pounds of material) application of heterogeneous catalysis is for automobile exhaust purification. Although used for many years to control emissions of vehicles operated in restricted environmental areas (forklift trucks, mining equipment, etc. ),(l) such devices were first installed on gcneral purpose automobiles exactly one year ago in the U.S. P.eaction to their use has been varied and strong with L o 2 proponents and assailants using arguments based on environmental quality, fuel economy, cost, simplicity, and reliability.

The

catalysts do indeed denrease the emissions of hydrocarbons and CO, hut they have the added problem of producing sulfur trioxide (which becomes sulfuric acif!). By "uncoupling" engine performance and emission control, the converters allow the engine to be tuned f o r better fuel economy and drivability, but they require use of

616 unleaded, lower octane gasoline that results in lower conpression, less efficient engines. Installation of catalytic converters is expensive and complicated, but proponents claim that this is the least expensive and simplest way to produce cars with relatively clean exhausts. Political decisions have had an overwhelming effect on development of catalytic converters. It all started over 15 years ago when the California legislature enacted a law that would require all new cars to be fitted with emission control equipment as soon as two such devices that met their standards could be perfected.( 2 ) The potential market being substantial, considerable effort was expended on both catalytic and non-catalytic approaches to the problem. Three catalytic devices were actually ~ertified!~) but the unavailability of unleaded fuel coupled with the fact that the proposed standards could be met by minor engine modificztions and carburetion recalibration to lean mixtures stifled further development of the catalytic devices for several years. The next political decision that affected catalyst development was the U . S .

Federal Clean Air Act of 197C1.(~)This law required a

90% reduction of HC and CO by 1975 and of NOx by 1976 on all new cars for 50,000 miles. Since some progress had already been made by the manufacturers in reducing emissions by about a factor of 2 since the mid 1960's, the law in fact required about a 95% decrease in emissions when compared with the "dirtiest" automobiles; the technology simply did not exist at that time to meet these requirements on mass produced cars. Even though each of these deadlines for compliance could be legally delayed for one year (providing it could be demonstrated that the standards could not be met after a "good faith" effort had been exerted), this is the first time that legislation has attempted to enforce implementation of a technology that had not been perfected. Because of the Oil Embargo, the Clean Air Act was amended by the Energy Supply and Environmental Coordination Act in 1974 (5) to postpone until 1977 (or 1978 as allowed by the Environmental Protection Agency Administrator Russell E. Train") ) enforcement of the statutory standards and established some less stringent interim standards. The California Air Control Board has established even more stringent interim controls that apply to new cars sold in that state, and they have enforced these standards recently by fining the Chrysler Corporation almost $ 0 . 4 ~ 1 0for ~ failure to comply. All the standards are now being re-considered

617

by both executive and legislative branches of government, very likely the statutory HC and CO standards will be postponed at least until 1981, and there is a strong possibility that the Federa1 NOx standard will be permanently relaxed.( 7 ) When automobile emissions were first being regulated, it was suggested that a tailpipe "concentration" standard be established for each pollutant. Of course this was unrealistic, since installation of an air pump could produce whatever dilution was desired. It was then suggested that a standard based on "mass emissions" per vehicle distance traveled be established, and this is the basis for all current regulations. Since the emissions vary widely for a given car depending on the mode of operation, it is essential that some standard test be established; the Federal Test Procedure(4) now used in the U.S. is the "Constant-Volume Sampling, Cold-Hot Start" (CVS-CH) test. With a car mounted on a chassis dynamometer and put through a complex, well-defined 11.5 mile (41.3 min. including a 10 min. shutdown) driving cycle, constant volume samples of the diluted exhaust are collected sequentially in 3 bags for analysis by non-dispersive infrared (for CO and C02), flame ionization (for HC), and chemiluminescence (for NO,, x 3 l ) techniques. Table I summarizes some of the observed and/or allowable emissions according to the various regulations. Table I

-

It is

Exhaust Emissions from U.S. Cars FTP Emissions, g/mile HC

Pre-Control Cars, before 1968

co

*Ox-

17.0

125.0

6.0

Present 49 state stds., until 1978 Present California standards

1.5 0.9

15.0 9.0

3.1 2.0

Fed. Clean Air Statutory Requirements

0.41

3.4

0.4

apparent that progress has been made in decreasing emissions, but even the present California standards are sill between a factor of 2 to 5 away from the statutory federal requirements. Technology now exists through use of catalysts to meet the statutory requirements for HC and CO, but there are some unpleasant side effects that decrease the desirability of meeting these standards at the present time. the TJO,

On the other hand, technology for meeting

statutory standard for 50,000 miles has not been demon-

strated, and it is doubtful that such can be developed without imposing a substantial fuel economy penalty. It is in this area where research needs to be concentrated. Japan also has laws that will require oxidation catalysts on

618

many cars by the end of this year and.NOx reduction catalysts

by

the end of 1 9 7 8 . ( 8 ) SYSTEM CHARACTERISTICS Before it is meaningful to discuss the details of catalyst preparation, let us first examine some of the system characteristics and a few of the demands that are placed on catalytic converters. In the first place, a primary characteristic is transience.( 3 ) Even when the fully warmed up car is operated at a constant speed, the exhaust gas pulsates as the contents of each cylinder are dumped into the exhaust manifold.

More importantly, as driving

modes change, the catalyst must be expected to perform under a wide variety of temperature, space velocity, and mechanical shock conditions. It must also be resistant to poisons that may occasionally contact the catalyst from the fuel, oil, or air.

In gen-

eral, it must be able to withstand considerable mistreatment in the hands of a public that has not been educated to appreciate the sensitivities of catalytic materials. This is a far cry from the usual mode of operation for catalytic reactors in chemical plants or petroleum refineries where the keyword is stability. Basically, two different environments are required to purify the exhaust gases. For HC and CO control, an oxidation catalyst in a fuel-lean atmosphere must be used. For NOx removal, a reduction catalyzt operating in a fuel-rich atmosphere is employed to effect reduction by CO, H2, and/or HC. Actually, the most straightforward way of removing NO is by simple decomposition X into N 2 and 02, a reaction that is thermodynamically favored except at very high temperatures. However, to date no effective catalyst for this reaction has been found. To accommodate these different environments, it was very early proposed that two different catalyst beds be used in series, the first operating in a reducing atmosphere for :iOx control and the second in an oxidizing atmosphere for HC/CO control, as shown in Fig. 1. The engine would be operated fuel rich to produce a

I

Engine ’ (tuned to

run rich) Fig. 1.

Dual Catalyst Exhaust Control Scheme

reducing atmosphere in the first reactor, and an air pump driven by the fan belt would inject air at point 2 to provide an oxidi-

619

zing atmosphere in the second reactor. While such a scheme would in theory accomplish the objectives, it has some problems. First, if the mixture is too far on the rich side of stoichiometric in the first bed, some NH3 is formed due to reaction of H2 (produced by the water-gas-shift reaction) with the NO. The PJH3, while itself not particularly harmful (already there is a significant amount of NH3 in the atmosphere), would be converted back into NO in the oxidizing atmosphere in the second reactor. Thus, the net NO conversion would be significantly decreased. Secondly, operating the car in a fuel-rich mode at all times will cause a substantial fuel economy penalty, an ecologically undesirable alternative. Third, the first catalyst that will become effective (or reach its "light-off'' temperature where 5 0 % conversion occurs) is the one for NOx control, whereas the oxidation catalyst (being further down stream) will heat more slowly. Unfortunately, it is oxidation activity that is more needed early in the driving cycle beginning from a cold start, because the reducing atmosphere caused by functioning of the choke creates large amounts of CO and HC initially. NO emX

issions do not become important until the engine becomes hot. To avoid this situation, it has been suggested that air be injected into the first reactor at point 1 (Fig. 1) to use it as an oxidation catalyst until the system becomes hot, at which time the air is switched to point 2 for normal operation. This will sacrifice PJO

X

control for ashorttime initially, but it will gain by in-

creasing the HC/CO control since a large fraction of the HC and CO collected during the Federal Test Procedure comes from "Bag 1" which includes the cold start part of the driving cycle.

In ad-

dition to being more complex by involving a switching valve V, such an approach demands extreme versatility of the first catalyst by requiring both oxidation and reduction activity. Fourth, introduction of two catalyst beds doubles the pressure drop in the system, a situation that can cause accelerated engine wear and decreased performance. Finally, the excessive air injected into the oxidation reactor will maximize the formation of sulfuric acid, since the equilibrium formation of S O 3 from SO2 and air is clearly dependent on the partial pressure of the brium conversion curves in Fig. 2 .( 9 )

O2

as seen by the equili-

Another scheme that has been suggested is a single catalyst bed to effect removal of all three pollutants simultaneously. This "3-way'' catalyst approach stems from the observation that for

620 mixtures very near stoichiome-

t r i c (A/F r a t i o a b o u t 1 4 . 7 pounds air/pound f u e l ) convers i o n o f all t h r e e p o l l u t a n t s

i s h i g h ( F i g . 3a).

However, i f

o n e s h i f t s m o r e t h a n 2 0 . 1 A/F u n i t s away from t h a t p o i n t i n e i t h e r d i r e c t i o n , c o n v e r s i o n of o n e o r more of t h e components f a l l s off substantially.

Dur-

i n g normal d r i v i n g modes, t h e wider range than t h i s .

For ex-

a m p l e , d u r i n g medium c r u i s e t h e A/F

r a t i o normally i s on t h e

900

600

500

c a r b u r e t i o n v a r i e s o v e r a much

Temperature, OC Fig. 2 . Equilibrium conversion of SO2 to SO3 at one atm. total pressure for various o2 concentrations. ( 9 )

l e a n s i d e a t about 1 6 , whereas

i t may d r o p a s l o w a s 1 2 f o r maximum power d u r i n g r a p i d acceleration.

Thus i t i s o b v i o u s

t h a t s u b s t a n t i a l changes i n engine control w i l l be required

t o s t a y w i t h i n t h e f 3 . 1 A/F "window!'

Such c o n t r o l c a n p r o -

b a b l y o n l y be a c h i e v e d t h r o u g h u s e of a n o x y g e n s e n s o r and a feedback system t o m a i n t a i n t h e oxygen p a r t i a l p r e s s u r e a t exa c t l y t h e correct l e v e l .

:ligh

temperature s o l i d e l e c t r o l y t e s , such a s zirconium d i o x i d e , can

----

catalyst

be used, a s they develop l a r g e v o l t a q e s when t h e oxygen p a r t i a l pressure approaches zero. AII example of how t h c p o t e n t i a l

chanyes w i t h t h e A/F shown i n ? i g .

3b.

ratio is

&I v)

i:

m

I f a sensor

were p l a c e d n e a r t h e c a t a l y s t bed, i t s o u t p u t v o l t a g e can be f e d i n t o a s m a l l computer t h a t

w i l l e i t h e r i n c r e a s e o r dec r e a s e t h e A/F r a t i o i n t h e

14.0

14.5

15.0

Air/Fuel Ratio Fig. 3. Performance of 3-way catalyst and oxygen sensor as a f of the carburetion A / F ratio.

Y';6Fi0"

621

carburetor as needed.

As with the dual catalyst system, t.?c 3 m y

approach is not without its problems. First, the A/F tolerance is extremely limited. Second, the sensor does not become effective until it is hot, a problem it shares with the catalyst.

Third,

there is a delay between what the catalyst "sees" and the action that the computer dictates at the carburetor. Fourth, both the catalyst "window" and the sensor signal may shift as the system ages, as indicated by the dashed curves in Figs. 3a,b. If the two curves do not shift in concert, it is possible that the device may begin controlling at a point far removed from the catalyst "window!' On the positive side, such a system has a lower pressure drop, requires less catalyst, and minimizes the NH3 and SO problems that plague the dual catalyst approach. Considerable 3 catalytic research needs to be done to increase the width of the

effective window and to stabilize the system against shifts due to aging. The scheme that is currently being used in U.S. cars involves simply an oxidation catalyst to control HC and CO either with or without addition of air pumps. Some degree of NOx control is achieved with exhaust gas recirculation (EGR) which minimizes formation of NOx in the engine by decreasing the combustion temperature. A large fixed recirculation ratio will result in a substantial fuel economy penalty, although a "proportional" recirculation ratio that varies with driving mode can almost eliminate the penalty.

In the best case, EGR can be expected to reduce the

NOx emissions to no less than 1.0 g/mile, a value far in excess of the U.S. statutory limits. Other engine forms, such as stratified charge and diesel, also have limiting NOx emissions in the same range.(I1) It thus appears that catalytic converters will be required if the statutory 0 . 4 g/mile NO, standard is ultimately to be enforced. CATALYTIC CONVERTER GEOMETRY The location and physical geometry of the catalytic converters play extremely important roles in determining their overall performance. If located very near the exhaust manifold for rapid heating, the converters may be subject to overheating that can irreversibly damage the catalyst. Misfiring cylinders or missing sparkplug wires are common malfunctions that cause overheating. In most configurations the converters are located either under the front seat or just ahead of the front floor panel.

Although at least six different physical forms of catalysts have been proposed, only the first two are currently in use. These include catalyst pellets (used by GM, American Motors, etc.), ceramic monoliths (placed on Ford and Chrysler products, as well as several non-U.S. cars), layered expanded metal screens (tested by Questor (12)) , coiled wire mesh (made by Gould(13) ) , aluminacoated wire strands (synthesized by Texaco), and metal sponges (proposed by Clyde Engineering(14) ) The "pelleted" catalysts are in the form of extrudates, spherical particles, or cylindrical pellets about 1/8" in diameter. The most popular converter is the "frying pan" configuration developed by GM which has a volume of 260 in3 (just over 4 liters); another version used on some smaller cars has 160 in3 capacity. The catalyst particles are held in a thin bed between two screens that are almost horizontal, but the exhaust gases enter the converter at one end, flow downward through the bed, and are collected below the bottom screen for exit at the other end. The device is so designed as to give a uniform flow through the entire bed, and the thin bed minimizes the pressure drop. The converter has the capability of being refilled (if necessary) with fresh catalyst through a hole in its side without removal from the car. The large converter has a bed density of about 0 . 6 5 and holds a total of about 2.5 kg of catalyst. The monolithic catalysts are single pieces of ceramic material that have parallel channels running the length of the device. The length varies from about 3 to 6 inches, the diameter is 4 to 6 inches (although some are oblong), and there are usually between 10 and 20 channels per inch. The channel geometry can be triangular, square, hexagonal, or sinusoidal depending on the manufacturer. The volume of the monolithic converters is usually 4 to 4 that of the particulate converter, and the open structure minimizes back-pressure effects for flow rates that can approach 500 SCFM (or give a space velocity of 200,000 hr-') . The monolithic catalysts suffer from being difficult (if not impossible) to replace without complete removal of the converter from the car. They are also more subject to thermal stress cracking than are the pelleted catalysts. Both the expanded metal screens and the wire mesh configurations begin with a coated metal film between 3 and 30 mils thick. The film is perforated with slots running perpendicular to the film length, and the film is stretched to give an open structure that can be either layered and welded ihto the converter (thick

.

623

films, Questor) or wound together and inserted as a cylinarical cartridge into a converter (thin films, Gould). The metal sponge is made by frothing molten metal and quenching the froth into a material that has as much as 90% random void spaces. OXIDATION CATALYST FORMULATIONS In all cases the active catalytic components for both oxidation and reduction are noble metals or base transition metals and their oxides.

These components are mounted on appropriate supports

(either ceramic pellets, ceramic monoliths, or metal films). While there is general agreement about the chemical formulation of the oxidation catalysts, considerable work remains to be done on the physical characteristics of these devices. Pt and Pd (and sometimes Rh) are the only active ingredients for oxidation catalysts that have proved durable for application in automobile exhausts, and in most cases a loading of about 0.06 In the case of pellets, the entire support is 2 generally gamma alumina (100-200 m /g) combined with a "stabilizer"

oz/car is required.

such as MgO, Ce02, Na20, ZnO, Ti02, etc. to increase high temperature stability and decrease shrinkage. The noble metals are added either by a batch impregnation technique where the pellets are immersed in an aqueous solution of H2PtC16 and PdC12 or by a continuous flow method where the pre-formed pellets are "sprayed" with a solution containing these chemicals. For best results the noble metal should be maintained near the outer surface of the pellets since crystallites buried deep inside are prevented from effective participation by diffusion resistance. However, some of the metal should be at least a bit below the external surface to protect it from poisons such as lead and phosphorous that may periodically find their way into the fuel or lubricants. Wei (15) has contrasted the poisoning resistance of the "egg yolk" catalyst with the reaction availability of the "egg shell" catalyst; a bit of both seems to be optimal with none of the noble metal deposited deep in the center of the pellets. There has been considerable speculation about the exact nature of the chemistry that is involved in making the supported catalysts.

Maatman(16) has postulated that chloroplatinic acid reacts

with alumina to form a tetravalent PtC14 surface species and releases Al+3 ions into the solution. It is known that the acidity of the impregnating solution has a great effect on the depth to which the Pt penetrates the pellets.( I 7 ) Once impregnated, the metals are reduced to zero valent metal atoms, and treatment with

624

H2S helps "fix" the noble metal in such a way as to minimiic tallite growth. The ceramic material used in the monolithic oxidation catalysts is now almost exclusively cordierite (2Mg0.2A12O3.5SiO2) chosen mainly because of its very low thermal coefficient of expansion,(18) an absolutely essential feature to avoid stress cracking under the large thermal gradients that can occur during operation. Spodumene has been largely discarded as a support material because of deactivation of the noble metals by the lithium. Most suppliers now use extrusion processes to prepare the monoliths, although the earliest versions (I8) were made from layered (or wound) corrugated cardboard-like material containing powdered ceramic that could be calcined to remove the paper binder and fix the geometry. There are about 300 channels/in2 in the best configurations. The cera2 mic has an extremely low surface area (<0.1 m /g) , and a thin "wash coat" of stabilized gamma alumina (the exact stabilizers used are proprietary, but they probably are not alkaline earth oxides) is then deposited on the insides of the channels to serve as a support for the noble metal. This wash coat is usually applied as a slurry that is forced through the channels, and the excess is removed by compressed air. In some cases the noble metal is then impregnated on the wash coat from an aqueous solution, but in other cases they are mixed directly with the wash coat slurry before its application. If the wash coat is too thick or is improperly applied, it has a tendency to flake off the ceramic support. In any case, the final material is calcined and the crystallites "fixed" by treatment with H2S. In all monolithic catalyst beds, the flow is essentially laminar with Reynolds number less than 500.( 3 ) Thus, the most effective catalysts should have very narrow, long channels. However, if the overall device is too long with very narrow pores, the pressure drop can become significant. In some very nice modeling studies Hegedus (19) has shown that channels of elongated rectangular cross section are preferable to other geometries if one does not consider mechanical or thermal stress arguments. Proper selection of the channel geometry and the channel size in this mass transfer limited regime will allow significant savings in the size and weight of monoliths at constant catalytic performance. Much additional research is needed in this area. Other types of support that have higher melting temperatures than cordierite have a l s o been considered. Silicon nitride appears (sr;.s-

625

to be an excellent candidate, but sintering of noble metals deposited on its surface is quite rapid. Zirconia is another attractive alternative to alumina in certain base metal formulations. Base metals cannot compete

100

with noble metals for several c

reasons. First, the light-off characteristics for the base

4 10

metals are usually much less desirable than for the

u

noble metals, as shown in Fig. 4. To obtain these data, a stream of synthetic exhaust is passed through a laboratory reactor containing a small amount of catalyst.

As the

-

Noble Metal Catalyst

!-l

$

50

CI

L:

a

'

!-l

a 0-

150

----

Base Metal Catalyst 350

250 Temperature, O C

temperature is slowly increased, Fig. 4. Activity comparison of base the conversion O f co and HC is periodically measured. Noble

metal and noble metal oxidation catalysts in a laboratory reactor; lighto f f curves.

metal catalysts typically give curves that increase rapidly at a moderate temperature to near 100% conversion, whereas the base mptal catalysts require much higher temperatures to approach 100% conversion. Secondly, the conversion over base metals is a sensitive function of space velocity, whereas conversion over noble metals is relatively insensitive to changes in the flow rate (a desirable feature since the flow characteristics vary so widely under various driving conditions). Third, the noble metals are not poisoned by sulfur in the fuel, whereas the base metals form surface sulfates and become deactivated until the decomposition temperature for the sulfate is reached. This is generally above the normal range of desirable operating temperatures.

Fourth, the

base metals have a tendency to react chemically with the support to form inactive spinals, such as N i A 1 2 0 4 , but this can be mini-

mized by proper choice of the transition metal/support combination. Fifth, the base retals are at least a factor of 100 to 1000 times less active per unit weight than are the noble metals, and this requires a considerable quantity of the former for acceptable performance. Such larcje amounts can lead to a decrease of the surface area by plugging the pores of the support. Finally, the base metals follow essentially first order kinetics for CO oxidation, whereas over Pt the order is almost inverse first order in CO concentration. This means that as the partial pressure of CO is

626

decreased, the oxidation rate increases, a factor that makes catalysts containing Pt exceedingly active for removal of small amounts of that pollutant. Both noble metal and base metal oxidation catalysts are subject to "permanent" poisoning by lead, although in general base metals (presumably due to the larger quantity of "active" ingredient) have a higher tolerance than noble metals for lead compounds. The noble metal catalysts are also "temporarily" poisoned by halide scavengers, especially ethylene dibromide in lead "motor mix"; however, the activity returns immediately when the halide is removed. Phosphorous permanently poisons both types of catalysts presumably through formation of very stable surface phosphates. Lead can be at least partially removed from poisoned catalysts and the activity partially regenerated by solvents such as acetic acid. However, such a process requires removing the converter from the automobile. REDUCTION CATALYST FORMULATION No catalyst has been found that will decompose NO effectively in an oxidizing atmosphere probably because oxygen poisons the surface. A catalyst that would cause formation of NO- species on the surface might have a chance of working, however. Most NO removal research has centered on finding active and stable catalysts for reduction by CO, H 2 , and/or HC. The problem is more complex than that for oxidation since a selectivity factor is involved. NH3 can be formed if the mixture is too rich, and in some cases N 2 0 is formed in significant amounts. It has been suggested that one of the primary routes by which NO is reduced is through ammonia as an intermediate according to Eq. ( 1 ) .( 2 0 ) Cu, Pt, and NO

+

H 2 -NH3

1

-N2

2

+ H2

(1)

Pd are good catalysts for reaction 1, but they do not catalyze NH3 decomposition effectively. On the other hand, Ni is a good catalyst for the NH3 decomposition reaction 2. Thus, a combination of the noble metals with Ni, or a mixture of Cu and Ni (e.g. the alloy "Monel:' which has a Cu/Ni ratio of 30/70),are good catalysts for this reaction. Considerable work has been done by Amoco, Exxon, and Gould on the Monel systems, and similar material is still being seriously considered as a catalyst for NO reduction in cars. One problem stems from its susceptibility to physical deterioration during cycling from oxidizing to reducing atmospheres, and for this reason it should probably be used with some other metals.

627

It does, however, have the advantage of being used either as sc!.:supporting homogeneous alloy chips (or wires) or in a form that can be supported on ceramic or metal supports. By far the most effective catalyst for NO reduction is Ru.( 2 1 ) It is so active that only a very small amount of the metal is required (perhaps as little as 10% of the Pt in oxidation catalysts) and it does not form significant amounts of NH3 even under extremely rich conditions. Unfortunately, the metal can form the volatile oxides Ru03-Ru04 under oxidizing conditions, and this leads to its removal from the catalyst. The Ru can be "stabilized" through formation of ruthenates ( 2 2 ) such as LaRuOj, BaRu03, or MgRu03, the latter being formed by impregnation of MgO with an aqueous solution of RuC13. Although there is some dispute about this point, it appears that even the "stabilization" techniques are not sufficient to guarantee that Ru will not be depleted from the catalyst in actual operation. It is therefore doubtful that Ru can be used.for this purpose since its oxides are quite toxic. One of the very earliest catalysts used for NO reduction was copper chromite.(23' While this material has good initial activity, it is subject to poisoning and physical deterioration. More realistically, mixtures of Cu, Co, Nil and Cr have been electrodeposited onto thin metal foils by Gould to form active expanded metal mesh catalysts.(13) This material is quite a good reduction catalyst, but it also suffers physical deterioration (called "green rot:' a concentrating of Cr at the grain boundaries that leads to flaking of the electrodeposited material) when subjected to rapid oxidation-reduction cycling. To avoid such cycling, Gould has suggested installing a third catalyst, another oxidation bed called a "getter", just upstream of the reduction catalyst to remove excess oxygen through reaction with CO and HC.

Addition of this

third catalyst allows the engine to be operated very near stoichiometric (just slightly reducing), a factor that minimizes the fuel economy penalty associated with rich operation.

All the catalysts thus far described require a reducing atmosphere (no excess 0 2 ) . However, just recently workers at Exxon (24) reported that Ir supported on AIZOj can selectively catalyze the 130-CO reduction reaction even in presence of excess oxygen. Much more work will be required to determine if this effect can be naintained in a system that is more realistic than the laboratory reactor and synthetic gas mixture used in their tests.

628

THREE-WAY CATALYST FORMULATIONS Since the 3-way catalyst system is designed to operate near stoichiometric, formation of NH3 is not usually a problem.

Fur-

thermore, absence of excess oxygen minimizes the formation of SO 3Finally, the possibility of using only a single catalyst bed makes this system extremely attractive.

The very narrow A / F range for

effective performance is the only factor that detracts from this approach. The ordinary Pt/Pd oxidation catalysts simply do not have a sufficiently wide window to be prime candidates. However, catalysts containing Rh have a 90% conversion window of about L0.3 A/F units, and additional research is underway to stretch this window even more. Perhaps addition of some Ir will help extend the window on the oxidizing side. If the window can be widened, if may be possible to use such a system without an oxygen sensor and feedback loop by properly designing the carburetor. There is optimism that this can be achieved, and it would greatly simplify the emission control systems, although the availability of Rh will probably be a significant problem. Another possible 3-way catalyst was recently patented by an inventor at du Pont ( 2 5 ) who claims that his material is active for both oxidation and reduction and, more importantly, is not poisongd by lead in the fuel!

It is a perovskite-type AB203 material with

Ru or Pt substituted into 1-20% of the B sites.

The remainder of

the B sites contain cobalt, and the A sites are occupied by a mixture of lanthanide ions. in automobiles.

The catalyst is currently being tested

One characteristic is that it operates at con-

siderably higher temperatures than do present oxidation catalysts. AVAILABILITY OF CATALYST MATERIALS The catalysts typically contain from about 0.03 to 0.16 oz/car (average about 0.06 oz/car) of noble metal, and the weight ratio of Pt to Pd is usually in the range of 66/34 to 71/29. One manufacturer uses Pt and Rh in a ratio of 93/7. Based on a total of 8x106 cars with oxidation catalytic converters sold each year in the U.S., this amounts to half a million 02. of noble metal required, not including another 1 0 % or so kept for replacement parts. Such an amount represents a sizable increase in the consumption of these two noble metals which have averaged over the last 5 years about 1.3 and 3.7 million oz. annually in the U.S. and worldwide, respective 1y. (26) To cover this new usage, world production has been increased at least 20%, and an even larger increase will be

629

required when noble metal oxidation and/or reduction catalysts are installed on cars used outside the U.S. The noble metals come primarily from three countries: the Soviet Union, South Africa, and Canada. Table I1 shows the approxTable I1

- Distribution of Noble Metals in Ores, Weight %("

Noble Metal P1atinum Palladium Iridium Rhodium Ru thenium Osmium Total oz ( x ~ O - of ~) Pt+Pd produced/year (not incl. auto cats.)

Canada 43.4 42.9 2.2 3.0 8.5

U.S.S.R. 30 60 2

2 6

---

--

375

2004

26)

Africa 64.02 25.61 0.64 3.20 6.40 0.13

So.

1230 --

imate percentage of each noble metal in the ores from various producing countries. It is interesting to note that the Pt/Pd ratio most commonly used (71/29) is almost the same as that of the ore from the South African mines, the principal source of the noble metals used in the U . S . Although there have been some tests that have indicated this to be the most effective formulation for oxidation catalysts, it appears to me that the natural abundance ratio in the ore is in fact the overriding consideration in determing the composition: This kind of constraint should always be kept in mind and accommodated whenever possible in designing catalysts so as not to cause an imbalance in raw materials supply. Assuming an average of 4 pounds of catalyst per car, the total weight of oxidation catalysts in this year's cars is about 15x106 kg. This is about 10% as much FCC (27) sold annually in the U . S . and represents the second largest catalyst application in terms of weight of catalyst produced. In sales, the auto exhaust catalysts may well outrank all other forms of catalysts. At the present rate of worldwide usage (about 4 . 2 ~ 1 0oz/yr ~ including catalysts on only U . S . cars), it appears that the 394 x106 oz of reserve Pt and Pd (26) should last about 100 years. This assumes no recycle, a factor that could substantially decrease However, in my the demand for new metal after a few years.('*) opinion it seems unlikely that recycle will ever exceed 50% due to the dilute mixture on the support (about 0.05 wt% noble metals). Since the total value of noble metals in the catalysts averages about $lO/car, at present prices there is little financial incen-

630 tive to recover the used metal. The use of alumina in automobile emission control catalysts should have no noticeable effect on the supply of this material since it will amount to less than 1% of the total now used. HEALTH EFFECTS OF CATALYTIC CONVERTERS When functioning properly, the catalytic converters can greatly decrease emissions of"HC, CO, and NOx. The oxidation catalysts are particularly effective for removing polynuclear aromatics that are known carcinogens as well as olefins and aldehydes that contribute significantly to smog formation.( 2 9 ) Moreover, they require use of unleaded gasoline which will effect a decrease of the harmful lead burden in the environment and indirectly decrease the particulate emissions that are normally associated with lead. On the other hand, use of catalytic converters also has some potentially harmful environmental side effects. The potential always exists that through continued vibration and mechanical shocks some of the catalyst (particularly the pelleted variety) will become attrited and dumped into the environment. If these airborne particles are in the respirable range (less than 1 u), they can be taken into the deep recesses of the lungs.(30) While the oxide support would probably have little effect, the small noble metal particles on the support could conceivably have some physiological activity. It is known, for example, that certain soluble Pt complexes (e.9. cis-dichloro-diamino-Pt(I1)) are extremely effective chemotherapy drugs for inhibiting certain forms of tumors.(31) Also, some soluble Pt salts can cause "platinosis" in susceptible

people.( 3 2 ) Fortunately, there is no evidence that any of these compounds are emitted from the catalytic systems, although care must be taken in the plants where the catalysts are made or handled to assure that employees are not harmed. Pre-shrinking of the pellets, addition of hardening agents, and proper design of the catalyst bed to minimize particle mobility have essentially eliminated the problem of attrition. Some concern has also been expressed about the possible solubilization of the noble metals by microorganisms that might attack the converters in "junked" cars.

Such solubilized material could

become dissolved in water, carried to farmland, and incorporated into the food chain where it could conceivably become concentrated in the higher life forms. Recently such solubilization by microorganisms has been demonstrated, but it is doubtful that the quantities of material are enough to pose any serious problem!33)

631

Another troublesome problem is associated with the sulfur in the fuel. Normally the sulfur is emitted from the exhaust of cars as SO2 which is slowly (over a period of hours to days depending on the atmospheric conditions) converted into SO3 and becomes sulfate particulate matter. The oxidation catalysts, however, convert part of the SO to SO3 before it leaves the car, and some mathematical models ' 3 4 ) have predicted that this could produce 3 atmospheric concentrations of sulfuric acid as high as 800 ug/m along roadways under "worst case" conditions. Health effect studies indicate that 24 hour exposure to concentrations as low as 10 ug/m3 are sufficient to produce harmful effects among people with respiratory or cardiovascular insufficiencies. The mathematical models used, however, do not account for adsorption of the SO3 on the pavement.or for reaction with NH3 or metallic oxides in the air to form sulfates which are considerably less toxic than sulfuric acid. When these factors are included in the model,(35) the maximum "worst case" concentrations of H2S04 decrease to about 3 30 ug/m , and this would be expected to occur only under extremely unrealistic conditions. While the sulfur from automobiles accounts for less than 1% of the total sulfur emitted into the atmosphere by man-made sources, its concentration along busy highways, in suburban shopping center parking lots, and in city street canyons sandwiched between large buildings is a source of concern. Based on the effects predicted by the mathematical models, it is possible that regulations may be imposed in the U.S. to limit the amount of SO3 that can be emitted over a given driving distance.( 6 ) Some studies have indicated that Rh catalysts provide HC/CO oxidation without producing as much SO3 as do Pt-Pd catalysts, although the observations may have reflected only differences in the ways the two catalysts were prepared instead of any chemical differences. It comes as no surprise to catalytic chemists that Pt is a good catalyst for SO2 oxidation. In fact, the use of Pt as a catalyst for this reaction was the basis of the very first patent ever issued in catalysis.(36) The actual amounts of SO3 emitted depend on the type of catalyst used, the temperature, and the oxygen concentration (see Fig. 2) . Pelleted catalysts "store" large quantities of sulfate presumably as A 1 2 ( S 0 4 ) 3 when the catalysts are cold, but this can be "dumped" when the decomposition temperature of the sulfate is reached (about 77OOC). Monolithic oxidation catalysts have far less storage capacity than do the

632 pelleted catalysts mainly because of the considerably lower' amounts of alumina present in the wash coat. Another potential problem is associated with sulfur in a reducing atmosphere. With excess reductant, the sulfur is reduced over the noble metal catalysts to H2S, a substance that is many times more toxic than CO or S O z .

The presence of H S has 2 been noted by several consumers, and it is usually indicative of a maladjusted carburetor, a stuck choke, or a disconnected or faulty air pump. H 2 S can also be noticed during a cold start if the oxidation catalyst reaches its "light-off'' temperature before the choke has been released. To avoid these problems, it may be necessary to decrease the fuel sulfur level below O.Olwt% (it is not about 0.03 wt8) and/or install traps containing perhaps CaO or CaCO to collect the 3

sulfur as sulfates.( 3 7 ) Finally, it has recently been reported ( 3 8 ) that HCN can be formed under certain conditions when synthetic exhaust gases are passed over oxidation catalysts, but fortunately the presence of water vapor seems to poison this reaction completely. While most of these adverse toxicological effects are not significant, one must always test any potential catalyst for release of harmful substances into the environment under either normal or abnormal driving conditions. In some quarters (e.g. national parks and petroleum refineries) concern has been expressed about the possibility of heat generated by the converters starting grass fires or causing explosions. To my knowledge no such cases have been conclusively shown, and recent tests at UOP have indicated that the external temperature of their converters (even when intentionally operated under abnormal conditions that gave very high temperatures) is well below the flash point of dried grass. Actually, the temperature within the catalytic converter is a very sensitive probe of engine performance, as a misfiring cylinder will cause a significant catalyst temperature increase. Someone has suggested that catalytic converters can serve as "rectal thermometers" to test engine performance as cars come off the assembly lines and thereby decrease the number of so-called "lemons" produced. OTHER CATALYTIC APPLICATIONS IN AUTOMOBILES Although not employed for its catalytic activity, the first high surface area material used in automobiles was a charcoal trap to decrease evaporation of fuel.

Such devices are commonly used

633

today to meet fuel evaporation regulations. Two other catalyst uses have been proposed for use in controlling automobile emissions, but in contrast with the auto exhaust converters, these devices operate on the fuel either before it reaches the carburetor or within the combustion chamber. Mobil researchers ( 3 9 ) have proposed installing a steam reforming catalyst in the fuel line before the carburetor to increase the octane rating of the fuel and thereby decrease the amount of refining that is necessary at the petroleum refinery. More recently there has been a flurry of activity(40) involving catalyst initiated flameless combustion for use on gas turbihes and also in reciprocating gasoline engines. In the latter approach a thin layer of very stable monolithic catalyst is placed directly above the piston within the combustion chamber. The mixture can be extremely lean, total point point

and yet the presence of the catalyst causes essentially combustion (41) at a temperature significantly lower than the where significant amounts of NOx are formed but above the where SO3 is thermodynamically stable. Apparently the

chemical material is not terribly critical since the reaction rate exceeds that in the external mass transfer limited regime.(42) Such an approach would be almost ideal since it would result in extremely low emissions of all pollutants. With the potential benefits of such a system, I would strongly recommend that considerable research effort be invested in this direction. COMPARISON OF THERMAL AND CATALYTIC REACTORS Except for stratified charge or diesel engines, some form of external converter is required to meet the present strict regulations for automobile emissions in the U . S . Thermal converters are effective for oxidation, but they require extremely high temperatures, as shown in Fig. 5. ( 4 3 ) Since a weli-

.oo

I

/- 1' 1 1 I I

/

4

w

tuned, efficient engine

----

emits exhaust at temperatures below 6 O O 0 C , signi-

-

Catalytic Converter Thermal

ficant de-tuning is required to elevate the temperature to about 8OO0C required to convert 9 0 % of the CO in a manifold thermal reactor. This imposes an enormous

100

300

500

700

900

Temperature, OC Fig. 5. Comparison of performance of a noble metal catalytic system with thermal reactor for HC/CO oxidation. (43)

634

fuel economy penalty (as much as 2 5 % ) , and materials problams at these temperatures can be severe. An advantage of this s y s e m , however, is the possibility of using high compression engines with leaded fuel. On the other hand, since the catalytic systems operate well below the normal exhaust temperatures, their use effectively "de-couples" the emission control from the engine performance and allows the engine to be tuned to peak efficiency and performance, albeit at a lower compression ratio than convenient with a non-catalytic system. Another difference is that CO conversion is the more difficult reaction in thermal reactors, whereas noble metal oxidation catalysts have more difficulty converting HC than CO. Thus, catalytic systems should be developed to oxidize HC, and the CO will automatically be taken care of. Though the debate about the merits and demerits of catalytic systems continues, it appears to me that catalysts represent the best of all short term approaches to improve the quality of emissions from automobiles. I predict that catalytic systems will continue to be used on most U.S. cars for at least the next 10 years and that such devices will become common on automobiles in Japan and Western Europe by the end of this decade. I would hope that base metal catalysts can be developed to replace the noble metals, as this application seems like a very costly way to consume these precious metals. ACKNOWLEDGMENTS Most of the information discussed herein has been obtained through investigations undertaken on behalf of various panels charged by the National Research Council of the U . S . I am grateful to colleagues who have served with me in various capacities on these panels; they include John B. Butt, Robert L. Burwell, Jr., Vladimir Haensel, David F. O l l i s , Henry Wise, and James Wei. Also, I would like to acknowledge considerable assistance from Richard L. Klimisch from General Motors and Mordecai Shelef from Ford. REFERENCES 1. E. J. Houdry, Adv. Catal. 9, 499 (1957). 2. J. A. Maga, Adv. Environ. Sci. Tech. 2, 57 (1971). 3 . J. Wei, Adv. Catal.

4,

57 (1975). 35, No. 219, 17288, Nov. 10, 1970;

36, No. 128, 12652, J u l y 2, 1971. Public Law 93-319, 88, Stat. 258 (1974). R. E. Train, Fed. Reg. 40,NO. 51, 11900, March 14, 1975. J. W. Hightower, House Subcom. on Env. and Atm. testimony, July 8, 1975. J. R. Fedor, personal communication, September 1 2 , 1975.

4. Public Law 91-604, Fed. Reg. 5. 6. 7. 8.

635 9. Engelhard Industries information, June 10, 1974. 10. Nissan Company information, May 23, 1974. 11. NRC Committee on Motor Vehicle Emissions Report, Nov. 1974. 12. D. R. Bentley and D. J. Schweibold, Nat. Petro. Ref Assoc., Houston, 1973. 13. J. R. Fedor, C. H. Lee, and M. P. Makowski, AIChE, New Orleans, 1973. 14. R. Clyde, Clyde Eng., P.O. 430820, So. Miami, Florida, Aug 1, 1975. 15. J. Wei, 167 Nat. ACS Meeting, L o s Angeles, Aug. 1, 1974. 16. R. W. Maatman, P. Mahaffy, P. Hoekstra, and C. Addink, J. Catal. 2, 105 (1971). 17. J. F. Roth and T. E. Reichard, J. Res. Inst. Catal. (Hokkaido) 85 (1972). 18. American Lava Corp., Technical Bulletin 721. 19. L. L. Hegedus, 166 Nat. ACS Meeting, Chicago, Aug. 1973. 20. R. L. Klimisch and K. C. Taylor, Calif. Catal. SOC. Meeting, Pasadena, 1973. 21. T. P. Kobylinski, B. W. Taylor, and J. E. Young, SAE paper 74250, Feb. 1974. 22. M. Shelef and H. S. Gandhi, Pt Metals Rev. Is, 2 (1974); Ind. Eng. Chem. Prod. Res. Dev. 2 (1972). 23. J. R. Roth and R. C. Doerr, Ind. and Eng. Chem. 53, 293 (1961). 24. S. J. Tauster and L. L. Murrell, 170 Nat. ACS Meeting, Chicago, Aug. 1975. 25. A. Lauder, U.S. Patent 3,897,367, July 29, 1975. 26. R. A. Mayer, W. L. Prehen, and D. E. Johnson, report prepared for EPA, April 15, 1974. 27. J. P. Rupert, Baroid Div., NL Ind., private com., Sept. 1975. 28. D. J. Kusler, U.S. Dept. of Interior, Bu. Mines, June 29, 1972. 29. NRC Report on Air Quality and Automobile Emission Control, Sept. 1973. 30. W. D. Balgord, Science 180, 1168 (1973). 31. B. Rosenberg, Naturwissenschaften 60, 399 (1973). (in English) 32. A. E. Roberts, Arch. Ind. Hyg. and Occ. Med. 4, 549 (1951); J. Pepys, C.A. Pickering, and E. G. Hughes, Clin. Allergy 2, 391 (1972). 33. J. M. Wood, Science 183,1049 (1974). 34. EPA Issue Paper, Jan. 11, 1974. 35. L. C. Doelp, Air Prod. and Chern. report, April 10, 1975. 36. P. Phillips, British Patent 6,096 (1831). 37. E. L. Holt, K. C. Bachman, W. R. Leppard, and E. E. Wigg, SAE paper 750,683, Houston, 1975. 38. R. J. H. Voorhoeve, Bell Labs, 170 Nat. ACS Meeting, Chicago, Aug., 1975. 39. N. Y. Chen and S. J. Lucki, 167 Nat. ACS Meeting, Los Angeles, April, 1974. 40. W. C. Pfefferle, Belgium Patent 814,752, NOV. 8, 1974. 41. F. B. Wampler, D. W. Clark, and F. A. Gaines, Combustion & Emissions Research, General Motors, Indianapolis, Ind., October 1974. 42. W. C. Pfefferle, Engelhard, U.S. Patent 3,846,679, Nov. 12, 1974. 43. L. E. Furlong, E. L. Holt, and L. S. Burnstein, 167 Nat. ACS Meeting, Los Angeles, April, 1974.

s,

11,

636 DISCUSSION

V.

FATTORE : 1) The emission limit in 1978 for HC will be 0 . 4 1

g/mile, as far as it is known. I have no information on the pollution caused by the filling up of tank-cars at gasoline stations. In fact, filling a tank with, for instance, 24 1 of fuel the same amount of gas will go from the tank to the atmosphere,that is around 80 g of HC. With 24 1 (roughly 6 U.S. gallons) an american car will run for 80-100 miles. This is about 0.8-1 g/mile or about three times the HC from the exhaust gases ! Do you have in the U . S . any prescription on this matter ? 2 ) After one year from the introduction of catalytic mufflers, is it possible to say whi&of the two systems adopted, pellets or monoliths, offers better performances and higher confidence in maintainance in different conditions of car utilisation ? J.W. HIGHTOWER : 1) Unquestionably,HC evaporation during fuel

tank filling is a factor that must be considered in dealing with the overall problem of automobile emissions. It would not surprise me if the absolute amounts of evaporated materials are the same order of magnitude as hydrocarbons allowed to escape from the exhaust system according to the statutory HC standards (0.41 g/mile)

.

2 ) Both pellets and monolithic catalyst systems have advantages

and disadvantages. While I have no absolute scientific basis on which to answer your question, my own personal opinion is that monolithic systems (due mainly to their smaller size,lower pressure drop, and lesser sulfate storage) are slightly favored.