Aspects of Automotive Catalyst Preparation, Performance and Durability

A. Crucq and A. Frennet (Editors), Catalysis and Automotive Pollution Control 1987 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

117

©

ASPECTS OF AUTOMOTIVE CATALYST PREPARATION, PERFORMANCE AND DURABILITY B. J. COOPER, W. D. J. EVANS

2

and B. HARRISON

3

lJohnson Matthey PIc, Catalytic Systems Division, 456 Devon Park Drive, Wayne, PA 19087 (United States of America) 2Johnson Matthey PIc, Catalytic Systems Division, Orchard Road, Royston, Hertfordshire SG8 5HE (United Kingdom) 3Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG49NH (United Kingdom)

ABSTRACT The development of legislative controls on petrol engined passenger cars in the USA and Western Europe is reviewed. The application of catalytic control strategies to these requirements is discussed. The principle components of modern exhaust emission control catalysts are identified. They comprise (a) a ceramic substrate, (b) a high surface area wash coat, (c) base metal promoters and/or stabilisers and (d) platinum group metals either singly or in combination. The functional role of these components is discussed and their interaction reviewed from the materials technology standpoint. Aspects of catalyst performance and durability influenced by preparation factors are discussed with particular reference to factors (b), (c) and (d). LEGISLATIVE REVIEW The increasingly urban nature of industrialised society has resulted in deterioration of air quality and generated political pressure for control of atmospheric pollution.

Many states have introduced measures to reduce emissions

including latterly those from vehicle sources. During the early 1940's significant environmental problems were occurring with increasing frequencies in the Los Angeles basin.

In the early 1950's the smog

problem was related (ref. 1) to photochemical interaction of nitrogen oxides (NOx), hydrocarbons (HC) and oxygen.

Surveys established that a high proportion of man

made emissions in that locality were derived from the motor vehicle. These conclusions, supported by numerous studies, provoked intensive research into methods of emission control.

Notable contributors were Eugene Houdry who, in

1949, invented a form of the ceramic monolith now in almost universal use and the Inter Industries Emission Control Programme led by Ford and Mobil which, during the 1960' s, defined the emissions control system which would be required to meet severe regulations. Political

pressures

derived

from

an

increasingly

powerful

and

vocal

environmental lobby culminated in 1970 in the US Clean Air Act (ref. 2), which

118

included progressively more stringent regulations covering inter alia, emissions of CO, HC and NOx.

This targetted a reduction of approximately 90% in emissions

relative to an uncontrolled average late 1960 model year vehicle. Features of the legislation were introduction of lead free gasoline in 1974, a requirement for emissions control systems to be effective for at least 50,000 miles, and

the

definition

of

a

test

cycle

and

procedure

to

measure

emissions.

Intervention of international fuel crises during the 1970's caused some easing of the timetable and emissions limits, the historical development being summarised in Table 1. TABLE 1 Development of U.S. Federal Emissions Regulations Model Year 1970 1975 1980 1981 1983

CO

HC

NOx (g/mile)

34.0 15.0 7.0 7.0 3.4

4.1 1.5 0.41 0.41 0.41

4.0 3.1 2.1 1.0 1.0

The increasing stringency of the limits required progressive introduction of catalytic control strategies beginning in 1975. Subsequent to introduction of this legislation, standards of similar severity (involving a different test procedure) were introduced rapidly in Japan.

More

recently Australia (from January 1986) has adopted the US 1975 Federal limits. Universally, the solution to emissions control from motor vehicles for the US market has included a platinum group metal catalyst.

This has created, over a 12

year period, the largest single application for catalysts and certainly the largest application of platinum group metals (Fig. 1) (ref. 3). The complex political development of Europe relative to the US and Japan has resulted in a different and more fragmented approach to the problem of control of emissions from motor vehicles (refs. 4,5).

European nations under the auspices of

the United Nations Economic Commission for Europe (ECE) have developed a unique test cycle (ECE R15-04), sampling and measurement protocol.

Although the sampling and

measuring protocols are now similar to the US Federal Test Procedure (FTP-75) the driving cycle is radically different.

Thus, for the ECE-15 test, maximum and

average speeds are 50 and 18.7 km/hr respectively with approximately 31% at idle. This simulates city driving in congested conditions.

In contrast, the FTP-75

simulates urban driving, typical of that in the Los Angeles basin.

Maximum and

average speeds are 91 and 34 km/hr respectively, with 18.4% at idle.

119

RHODIUM DEMAND IN THE WESTERN WORLD 1985

Glass

Electrical

6%

Chemical 18%

9%

PLATINUM DEMAND IN THE WESTERN WORLD 1985

Petroleum 1%

Total Demand = 250,000 oz

Fig. 1

Glass 5%

Electrical 7%

Total Demand =2,810,000 oz

Rhodium and Platinum useage by major application.

As in the USA, limits were progressively lowered and refinements made to the test procedure (ref. 6).

However, in the USA a single standard applies to all

passenger vehicles whereas in Europe standards have traditionally been related to vehicle inertia weight. Currently regulation ECE R15-04 is in force (ref. 7) and has been adopted by the European Economic Community

(EEC) and

by most

other European States.

The

requirements of this regulation are lax relative to contemporary US and Japanese limits. By 1984, after several years of gradual reductions in emission levels, the political climate, notably in West Germany, favoured a much more rapid change.

The

West German proposals required introduction of three way catalysts and necessitated use of unleaded fuel.

After a lengthy period of debate, a compromise solution was

developed by the 'EEC' which substantially diluted the original proposals.

The

'final proposals' (ref. 8) were published in June 1985 and entail progressive introduction of standards (Table 2.) As a separate issue it had already been agreed that unleaded fuel should be made available throughout the community from 1989.

This date may be anticipated and the

fuel specification will be 95 RON, O.013g/litre lead (max.) The directive resulting from these proposals will be based upon the concept of optional harmonisation.

It will permit, but not require, Member States to adopt

national legislation in line with it. There remains considerable controversy surrounding the 'final proposals'. There is strong polarisation with respect to identification of Phase 2 standards for small vehicles targetted for January 1st 1987.

120 TABLE 2 Final Proposals for European Common Market Automobile Emission Control Standards Date of Introduction

Engine Capacity

Emissions, g/test

New Models

All New Cars

CO

(HC+NOx)

NOx

Oct. 1988 Oct. 1991

Oct. 1989 Oct. 1993

25 30

6.5 8

3.5

Oct. 1990 Oct. 1992

Oct. 1991 Oct. 1993

15 6 45 To be decided by 1987

Over 2 li tres 1.4 - 2 litres Less than 1. 4L Stage 1 Stage 2

Over one year after publication of the 'final proposals' there remains no immediate likelihood of ratification.

Nevertheless, West Germany has taken the

lead in promoting National Standards supported, during a voluntary introductory period, by significant fiscal measures.

In contrast, UK, France and Italy are not

expected to adopt or make the proposals mandatory for some time. The schism within the EEC is mirrored by further divisions reflecting the wide range of national interests of non-member states.

Thus, Sweden has announced a

proposal to adopt US 1983 standards from 1989. The dis pari t y between emission test procedures, allowable tail pipe emissions and local market conditions conspire to prevent a universal solution to world wide certification of any given vehicle.

Consequently, even though a basic vehicle may

be utilised in several markets, there are generally significant differences in subsystems to cope with, e.g. different emissions constraints.

In consequence,

vehicles of European manufacture may be produced in several specifications.

Thus,

models may be produced to Japanese specification involving an oxidation catalyst, to US specification involving TWC and to a range of European specification involving no catalyst at all.

This substantially magnifies the capital and human resources

required to maintain a broad market presence. CONTROL STRATEGIES The emissions from conventional spark ignition engines are strongly dependent on air:fuel (A/F) ratio.

No single operating regime exists within which levels of

emissions of all pollutants is low. In practical terms this has constrained the development of only three basic control strategies (refs 9,10) in the context of stringent legislation.

These are

all based upon application of supported platinum group metal catalysts.

The

strategies are: (1)

Removal of HC and CO by use of an oxidation catalyst (COC) generally containing

121 Pd or Pt/Pd with other means of reducing NOx emissions, e.g. exhaust gas recirculation.

This strategy normally entails a slightly lean tune and

secondary air injection.

The extent of NOx reduction is determined by

driveability considerations,

limiting

applicability

to

less

demanding

requirements. (2)

A combination of sequential reduction of NOx, over what is essentially a three way catalyst (TWC), followed by oxidation of residual CO and HC over a COC after injection of secondary air.

This procedure requires a rich tune to provide

the necessary net reducing atmosphere in the first catalyst, has an adverse impact on fuel economy and is not likely to be favoured in the European Context. (3)

Removal of pollutants by use of a TWC.

This can be achieved using a Pt/Rh

formulation but only if the engine management system controls the fuelling closely at the stoichiometric point (A/F : 14.7: 1;

A:

1).

Current European

practise for US models is unique in utilising only the single bed TWC and electronic multipoint fuel injection, under oxygen sensor control, for implementation of this strategy. These strategies as applied in the USA market, which can be implemented by a variety of routes, were recently reviewed by Duleep (ref. 10).

A strong trend

towards the single bed TWC operating under closed loop control of electronic fuel injection was noted. Strategies for the emerging European market have been reviewed recently by Evans et al (ref. 11). A significant benefit of a lean fuelling strategy is improved fuel economy. This has motivated intensive research into lean burn technology involving reliable operation at high air:fuel ratios typically in excess of 20:1 (refs 12,13).

A

corollary of such operation would be substantially reduced NOx emissions, (ref. 14) albeit at higher NOx levels than a comparable vehicle fitted with a TWC, but at the expense of an increase in HC.

Operation of conventional engines at high air/fuel

ratios is limited by onset of pre-ignition, rapid torque fluctuations, fast deterioration of the engine and poor driveability. Thus far it has not been demonstrated that acceptable driveability can be achieved for a vehicle operating at 20-22:1 A/F other than by a very high level of equipment, i.e. total electronic closed loop management with multipoint fuel injection.

However, even at that level, it is not possible to achieve severe

legislation limits without provision of a COC to remove hydrocarbon species (ref. 13).

Nevertheless, it is evident that substantial progress has been achieved and

that in the European context a fourth control strategy is potentially available for mid-range vehicles.

122 CATALYSTS FOR AUTOMOTIVE APPLICATION Catalyst technology was developed in the mid period of this century for chemical process operations.

In such applications the catalyst is generally sited

in a fixed bed reactor and after commissioning operates in a more or less steady state mode for a long period of time.

Furthermore, space considerations are

normally a minor factor in the design of the catalyst and reactor; space velocities are generally quite low with large catalyst volumes being employed.

Economic

considerations associated with selecti vi ty and yield generally dictate tight control of space velocity, temperatures and protection of the bed from poisons. Addi tionally complex reac tors,

often wi th recycle or interbed cooling, are

practical solutions to maintaining the required yields. The situation in a motor vehicle could not be more different.

The duty of the

automoti ve catalyst comprises a series of 'commissionings' followed by opera tion in a highly perturbed fashion.

In the USA, the mandatory cold start and 50,000 mile

durability requirement demands operation of the catalyst at low temperatures. During actual operation the catalyst would be subjected to extremes of gas flow and temperature with large variations in concentration of pollutants over the loadspeed envelope of the vehicle. In the emerging European market the situation is even more complex.

Thus,

vehicles are generally much smaller but average and maximum speeds are higher. However, the lower speed test cycle and consequent cooler exhaust gas temperature requires high catalyst activity at low temperatures.

Consequently the operating

temperature requirement is even broader than that for the US market (ref. 11). In addition to the highly non steady state operation, uncontrolled poisoning is a major threat to the catalyst. phosphorus and zinc (refs. 15-18).

The principal poisons are lead, sulphur, The latter two species are generally derived

from lubricating oil, principally from the anti-scuff agent ZDDP.

Very few

examples of significant catalyst deterioration in service have been reported due to Zn/P poisoning (ref. 19). Lead and sulphur are derived from the fuel and there is a complex equilibrium dependent

upon

temperatures

and

absorption/desorption of these poisons.

gas

composition

controlling

the

In the case of lead, extended trials have

demonstrated the feasi bili ty (ref. 20) of successful operation of oxidation catalysts on leaded fuel.

However, it has been noted that in the decade since

introduction of lead-free fuel in the USA, residual lead levels have fallen dramatically.

In that market, where leaded and unleaded fuels are both available,

incidents of poisoning reflect contamination of distribution deliberate misfuelling (refs. 21,22).

equipment

or

Sulphur may also be derived from lube oil

but its impact in the sense of poisoning is low on PGM catalysts.

Interaction with

catalyst components can, however, influence secondary/unregulated emissions of

sulphur bearing species such as sulphate (refs. 23-26). A further major difference with respect to chemical process reactors is the critical need to achieve low pressure drop to minimise power

losses.

This

requirement conflicts to a large extent with those for high activity, Le. good heat and mass transfer. In the early phases of the emerging market, the dominant technology for achieving the total requirement derived from conventional fixed bed pelleted catalyst technology, albeit with special high aspect ratio beds to minimise power losses.

However, widespread use was made of an alternative technology based on a

multicellular

ceramic

substrate or monolith

(ref.

27).

Due to persistent

durability problems with pellet bed reactors the monolithic support catalyst has become the dominant technology accounting for perhaps 95% of all new vehicle systems. The monolith has strong, porous, thin walls supporting an array of parallel channels presenting a high geometric surface area.

The high open area and

structure promote laminar flow, limiting pressure drop at high flow rate.

Use of a

low expansion body based upon Cordierite provides a high degree of thermal shock and strength while offering a high maximum operating temperature. Major advances in ceramic extrusion technology and processing have enabled substantial advances in product quality.

In consequence a wide range of shapes,

sizes and cell dimensions are available (ref. 27). Although ceramic monolith based catalysts dominate the global market, there has been significant interest in Europe latterly in metallic monoliths (refs. 2830).

The reduced wall thickness offers specific advantages in conversion in

applications where space is at a premium or ceramic based solutions are not possible.

Several major applications now exist (ref. 31) but presently cost

factors remain a major determinant in favour of ceramics. However, it is not possible to achieve the combination of strength and thermal shock resistance required for a ceramic monolith together with the high specific surface area required for catalysis.

This surface area is applied to the monolith,

generally in the form of an aqueous suspension of a highly porous material - the wash coat.

Its characteristics, along with those of the underlying support, have a key

role in determining the activity and durability of the catalyst system. Accordingly the key first stage of manufacturing a monolithic type catalyst is formulation of the wash coat and uniform application over the internal surface of the monolith.

Although commercial processes are proprietary with little detail

available, the coating is generally fixed, by calcination, at elevated temperature. The second key activity is application of precious metals and promoters, for economic reasons generally from solution or dispersion.

After drying, reduction

or calcination processes are used to fix the precious metal.

In principle. the

124 precious metal may be included with the wash coat. The catalytic species of current automoti ve catalysts are balanced mixtures of precious metals and promoters selected, as discussed previously, on the basis of application.

Precious metals are favoured due to high catalytic activity and

selectivity, particularly at low temperatures (as experienced with cold start tests).

Additionally their supported dispersions are relatively stable at high

temperatures and exhibit good resistance to poisoning. The idealised requirements of the three chemical constituents of the catalyst must be met in a manner which allows economic manufacture by routes compatible with mass production.

Subsequent sections are concerned with each of the three key

components (wash coat, base metal promoters and precious metals) and examine the influence of preparation on performance. WASH COAT An autocatalyst wash coat must provide a high, stable surface area upon which crystallites of precious metals and promoters can be dispersed.

The overall

stability of the catalyst is to a large extent dependent upon that of the wash coat in terms of surface area and adhesion. Washcoats generally comprise mixtures of stabilisers, promoters and alumina. Alumina forms the bulk of the wash coat, frequently in excess of 90%, and accordingly its stability is crucial.

Preparation of wash coats is proprietary but generally

involves formation of a high solids

dispersion

of

activated alumina.

Such

dispersions are generally produced by milling or use of high shear mixers. Addi tions of dispersing agents, e t c , , are necessary to provide the surface tension and flow properties required to allow penetration of a 400 cpsi monolith and achieve uniform coating of cell walls. Choice of alumina precursor has a significant impact on stability of surface area (ref. 32).

This is illustrated in Fig. 2 for activated aluminas derived from

Boehmite and Gibbsite, the two major industrial raw materials commonly available. It is readily apparent that activated aluminas derived from boehmite are the most

thermally

stable

in

the

principle

temperature

ranges

of

interest.

Additionally •. transitional aluminas derived from gibbsite undergo major reordering of the lattice at lower temperatures than

¥' alumina with

significant implications

for shrinkage as well as the surface area changes noted above. The inherent stability of aluminas can be further improved by addition of other oxides (ref. 32).

Base metals can act as promoters and in an ideal si tua tion would

fulfill a dual role.

Fig. 3 shows the change in surface areas for boehmite derived

activated aluminas as a function of temperature.

It may be seen that addition of

barium retards phase transformation and consequent loss of surface area to well above 10000C.

125

Gibbsite X:

a

X~

Boe'hmite

200 Surface Area m'g 100

o

400

600 800 1000 Temperature

c

1200

Fig. 2. Surface area thermal stability and phase transformations for transitional aluminas derived from Gibbsite and Boehmite.

100

40

20

o '--.,jL---,_ _,.--_r--r-.,.-' Fresh 750 1000 Temperature C

Fig. 3.

1200

Thermal stability of surface area of ~ alumina - metal oxide mixtures.

The benefits of such improved stability in terms of catalyst performance is illustrated in Fig. 4 for unstabilised and barium stabilised palladium and rhodium catalysts after ageing under the specified conditions. improved performance is achieved.

In each case significantly

126

Pretreatment

Test Hrs 300

950 C/ 1% 0,/10% H,O' lHour Pert 1.00 «\) 1.00 Hz

100,-----'--"'-'-....:...:'-'--;;::-::-'-"1

c 80' o 'iii Q; 60 > s: 40

'iii

Q; 60

> c

8 40

8 if!

JCO HC

___-----,I--}~-

80

t:

o

if!

20'

20

(A)

0.96

0.98

1.00

1.02

1.04

0.96

Equivalence Ratio (r.)

0.98

1.00

1.02

1.04

Equivalence Ratio (Al

__ (1) Pd/AI ,0 3

-

NOx

(B)

__ (1) Rh/AI,03 (2) Rh/Ba/AI,03

(2) Pd/Ba/AI,03

Fig. 4. Static engine based selectivity test showing the. influence of barium stabiliser on catalyst performance (A) for palladium based catalysts after 300 hrs engine ageing (800 0C max, ) and (B) for Rh based catalysts after hydrothermal ageing for 1 hr at 950 0C. We must now consider a complex series of trade-offs that are involved in the application of the wash coat to the substrate. are as follows.

In simple terms the considerations

The wash coat provides the means for a highly dispersed catalytic

material to maintain a high surface area.

Therefore, for a given loading of

catalytic material, a higher quantity of wash coat will result in a more stable dispersion.

This is because, over the higher total surface area present, there

will be fewer next neighbour interactions between the precious metal components. Therefore, coalescence sintering will be reduced.

100 c:

80

0

'iii 60 i >

c: 0

U


40

20 0

--

//~~~

.- / >..-

0.96

100 c:

80

0

'iii

--I

ICO NOx~ (Al HC ......

0.98

In addition to this effect, the

1.00

Lambda Value

1.02

i

60

0

40


20 ..:.--

>

c:

o

0

0.96

'"

~ O

(6)

0.98

NOx~

- -

HC .....'

1.00

1.02

Lambda Value

Fig. 5. Static engine based selectivity test showing the influence of wash coat loading on the performance of a 5:1 Pt/Rh TWC after ageing for 200 hrs on an 8 mode cycle (peak temperature 850°C, 3mgL-llead) Catalyst A contains 68 percent by weight of the wash coat deposited on Catalyst B.

127 washcoat acts as a poison sink and the higher the surface area of wash coat present, the better the catalyst will resist the effects of poisons.

The effects of wash

coat loading on catalyst activity are illustrated in figure 5. Clearly the activity of the catalyst with a high wash coat loading and therefore higher surface area is better. Fig. 6 shows the activity pattern for a series of catalysts, differing solely in wash coat loading, after thermal pretreatment in a wet oxidising gas and subsequent 150 hours engine ageing in a perturbed ageing cycle.

CO and NOx

conversion shows a significant dependence upon wash coat loading in this test.

100 c

o

80

o ()

70

'"

!mHC

I

90

"iii

~

I3 co

I~NOX

60

50

1111 x

1.20X

1.42X

1.51X

Relative Washcoat loading

Fig. 6. Static engine test data showing the effect of wash coat loading on conversion at ~ = 0.995 in a selectivity test after extended ageing (150 hrs). In addition to surface area stability, the wash coat must maintain good adhesion

to

the monolith at high loadings over the operating envelope.

In

principle, this can be achieved by increasing the solids content of the dispersion or repeated coatings.

However, close process control must be exerted over the

application process which otherwise becomes a source of adhesion problems.

Thus,

packing of solid particles during removal of occluded water by drying may provoke shrinkage cracks. loss of wash coat.

Thermal cycling during processing may provoke delamination and Prevention of premature failure due to these mechanisms

requires tight control over all aspects of wash coat preparation and application. Retention of high activity during service is critically dependent upon maintaining integrity of the wash coat/monolith bond.

However, even initially

well bonded coatings can be susceptible to deterioration due to frequent, rapid, high temperature cycling.

The

influence of

thermal ageing at

initially highly adherent coating is shown in Fig.

7.

such changes.

0C

on an

Severe shrinkage has

occurred due to major changes in surface area and the alumina phase. may be overcome by inclusion of phase stabilisers (Fig. 8)

1350

whi~h

This problem

defer and reduce

128

Fig. 7. Optical micrograph of wash coat after sintering at high temperature showing severe shrinkage.

Fig. 8. Optical micrograph of stable wash coat after high temperature exposure showing freedom from shrinkage cracking.

Benefits derived from these improvements may be seen from comparison of the hydrocarbon breakthrough for two otherwise identical catalyst systems after 350 hours operation at temperatures up to 800 fuel containing 3mgL

-1

0C

(for 80% of the time) when exposed to

lead (Table 3).

TABLE 3 Effect of Washcoat Type on the Durability of Pt/Pd Catalysts for Hydrocarbon Oxidation % Unconverted Hydrocarbon at 25 Hrs at 355 Hrs Coating A (Figure 7)

13

18

Coating B (Figure 8)

11

14

Coating A (Fig. 7) shows approximately twice the rate of deterioration of that for B (Figure 8). In addition to the specific features relating to activity and catalyst durability, it is critical that the wash coat does not adversely impact upon the

129 overall performance of the monolith. During normal service the monolith support is subjected to frequent thermal cycling.

Typically, exhaust gas temperature can reach several hundreds of degrees

celsius in less than a minute from a cold start.

In most converter designs the flow

distribution is non uniform with flow concentrated over the central region.

This,

coupled with highly exothermic reactions, results in development of strong axial and radial thermal gradients.

Radial gradients due to the relatively cool outer

skin are accentuated in the increasingly favoured non-cylindrical type converter. These rapidly fluctuating temperature gradients may induce a catastrophic failure of the ceramic as a result of thermal shock.

Low expansion bodies have demonstrated

ability to resist thermal shock during service life in the USA but such problems would be expected to be more severe in Europe due to different, more severe, driving patterns and a growing tendency to move the catalyst nearer to the exhaust manifold. Such problems can, however, be overcome by careful design of catalyst, converter and exhaust train (ref. 33). Fatigue type studies of thermally induced failures of ceramic monoliths have been

the

subject

of

intensive

investigation

(refs.

34,35).

However,

the

statistical nature of brittle fracture and the difficult nature of the property measurements has provoked development of a number of empirical tests.

The most

useful of these is the burner type test in which the unit is heated rapidly from room temperature to a predetermined high temperature and subsequently rapidly cooled by shutting off the fuel.

After a fixed number of cycles the unit is removed and

examined visually and accoustically for fracture. higher

temperature

until

failure

is

If unbroken, it is retested at a

experienced.

characteristic of the thermal shock resistance.

This

temperature

is

As with all strength tests of

brittle materials, it is essential that a statistically significant sample is taken as a measure of the mean property and dispersion. The thermal shock characteristics as determined by a burner type test for raw monolith and various types of coated catalyst are shown in Fig. 9.

It may be seen

that a coating of washcoat to early formulations resulted in a marked degradation in failure temperatures to a barely acceptable level.

This is attributed to the large

-6

differential in coefficient of thermal expansion of cordierite (10 x 10 alumina (60 x 10

-6

0

/ C) and

0

/ C) resulting in thermal stresses at the monolith/wash coat

interface. One method of preventing such interaction is precoating the monolith (ref. 36) with an organic material which is subsequently removed during calcination (to fix the wash coat).

The effectiveness of such processes, which have been widely

practised for several years, is shown in Fig. 9 where the differential is reduced to oC. 30/40

130

o

en

e

~

CIJ

Ol CIJ

o

1000

Min. Spec. Value

IIllI Pre-treat Cat.

D

m

Substrate

II

'74 Catalyst

New Catalyst

Q.

E CIJ

I-

...CIJ:;,

800

C\J

LL.

c: C\J

CIJ

4 x 6 inch

:E

4.66 x 6 inch

SUBSTRATE ICATALYST SIZE

Fig. 9. Mean thermal shock failure temperature (burner test, minimum 15 units) for 400 cell ceramic monoliths and catalysts of various types. However, there are inherent disadvantages due to additional raw materials and extra process costs.

Furthermore process control is more difficult and the total

wash coat deposit feasible on a unit basis is much reduced.

In consequence this

provides an artificial and undesirable limitation on activity, durability and poison resistance.

In response to these limitations, a new process has been

developed which minimises surface interactions without resort to precoats.

The

data shown in Fig. 9 indicate that this technology enables the benefits of stabilised high wash coat levels to be achieved without adverse impact on thermal shock characteristics. BASE METAL PROMOTERS/STABILISERS The critical role of Rh in the performance of single-bed three-way catalysts and its extreme sensitivity to deactivation by exposure to high temperature lean operation, dictates that any new catalyst development must address the issues of Rh performance and stability.

Rh deactivation in three-way catalysts, after exposure

to high temperature lean ageing has been attributed to a strong Rh/Al (Ref. 38).

interaction Z03 Additional work (Refs. 39,40) has shown this interaction can be

eliminated, with substantial improvements in thermal stability, by supporting the Rh on zirconia.

Unfortunately,

the incorporation of Rh/ZrO

Z

into three-way

catalysts requires complex manufacturing methods which are not suitable for high speed production. Rh/Al

An alternative approach is suggested by work that indicates

interaction may occur preferentially at the grain boundaries of the

Z03 support (ref. 41).

We have thus chosen to incorporate a stabilizer into the alumina

support system designed to preferentially block this interaction. Although these results showed that substantial stabilization can be achieved they also demonstrated the major problem of utilization of single-bed three-way catalysts Extensive

CO and NOx performance around the stoichiometric air/fuel ratio. testing

of

base

metal

improvement in performance.

stabilisers

failed

to

secure

the

desired

However, incorporation of base metal promoters in

three-way catalysts has been shown to improve CO and NOx performance in the region of the stoichiometric air/fuel ratio.

The two mos t widely used and studied promoters

are nickel and cerium (refs. 42-46).

Their influence at equivalent total promoter

loading is shown in Fig. 10.

Conversion 0.02 wt% Rh Etf.ciency

-_.... Ce Promoted - - Unpromoted - - - NilCe Promoted

(%)

100

co

80 60 40

...~ NOx

20

.96

.98

1.00

1.02

1.04

Equivalence Ratio

Fig. 10. Performance of unpromoted, Ni/Ce and Ce only promoted 0.02 wt. % Rh 0C catalysts after ageing at 980 in 1% 02' 10% 02 atmosphere for 1 hr. A substantial increase in performance, particularly in the stoichiometric region, is noted for both promoted systems. shows superior stability.

In that respect the ceria only system

This, at least in part, can be attributed to the reaction

of nickel and alumina to form nickel aluminate (ref. 42) at elevated temperatures. That effect, and increasing concern wi th regard to environmental impact of nickel, has resulted in a trend away from use of that element. The

activity/stability relationships of such catalysts has been further

explored by synthetic gas studies using a reactor system

cy~ling

between rich and

lean conditions as shown in Table 4. Under lean condi tions the promoter type and loading has very li ttle impact on performance or thermal stability.

Under rich conditions the promoter type and

loading affects both fresh performance and thermal stability.

Substitution of

cerium-only for nickel/cerium results in a dramatic improvement in fresh CO performance wi th a further more modest improvement seen from an increase in cerium

132 loading.

After thermal ageing the "arne performance trend" are obs er ved .

However.

only the high cerium ca t a Ly s t doe" not show a large drop in performance in comparison to the fre"h "tate. TABLE 4 Tr ans I en t performance of f r esh and aged ca t a l ys r s (0.24% Pt/0.05% Rh) under lean and rich conditions. Temperature 400 oC. GHSV 100,000 hr- l• gas compo s Lt Lon s - base mix of 1200 ppm HC (C 3H6). 500 ppm NO, 14.0% COZ' 0.17% HZ and 10% HZO pI us either rich "pike 2.0% CO, 0.5% O2 for 4 s ec . or lean "pike 0.5% CO, 2.0% O2 for 10 sec. Balance

N2·

Lean Spike (% conver"ion)

Rich Spike (% conver"ion)

HC

CO

NOx

HC

CO

NOx

Ce/Ni Promoter

Fresh Aged'"

98 96

89 86

34 30

88 72

51 24

50 46

Ce Promoter

Fresh Aged'"

100 95

89 87

39 32

86 84

71 46

54 48

ZX Ce Promoter

Fresh Aged'"

99 96

89 87

37 34

86 81

76 74

54 50

"'750 oC

I

10% H20

I

Air

I

5 hrs.

The origin of this large effect on CO performance has been explored by measuring the rich spike CO performance wi th and without H20 present. CO conversions under rich condi tions, after hydrothermal ageing at 900 0C in 1% oxygen for four hours are shown in Table 5. TABLE 5 Performance of hydrothermally aged 0.16 wt% Pt/0.03 wt% Rh catalysts containing ceria promoter in the presence and absence of water vapour. (Conditions otherwise as shown in Table 4). CO Conversion (%) with H2O

CO Conversion (%) without H2O

IX Ce Pr omot ar

54

49

2X Ce Promoter

64

49

6X Ce Promoter

70

49

133

°

Variation in cerium promoter level has no effect on CO performance when H is Z With HZO present in the feedgas CO performance is

absent from the feedgas stream.

higher and increases with increased cerium loading.

This is consistent with an

enhancement of the water-gas shift reaction upon addi tion of. cerium to Pt/Rh threeway catalysts.

This enhanced performance is at least partially transient in nature

with CO conversions dropping below 50% under steady state conditions. These results show that a Pt/Rh catalyst system, based upon a stabilized alumina wash coat designed to minimize the adverse effects of strong Rh/Al

Z03 interactions and a high cerium promoter level for enhanced CO performance and stability, should result in significantly improved three-way catalyst performance

and durability. This conclusion was confirmed by separate static engine ageing of replicate catalyst units under stoichiometric, lean and high temperature lean conditions. Data for the first two conditions (entailing a maximum temperature of 760 of the cycle) are similar;

that for lean ageing is shown in Fig. ll(A).

only catalyst shows enhanced stability in the stoichiometric region.

0C

for 17%

The ceriaData for the

much more severe high temperature lean cycle is shown in Fig. ll(B).

Conversion

Conversion Efficiency (%) 100

0.16 wt% PtlO.03 wt% Rh

y (% Efficiencr-:....;. ....;. )---------------, 0.16 wt% PtlO.03 wt% Rh 100

CO HC

80

80

60

60

40

40

20

20

(A)

O ...........,....-.--..,.....--r"-~...,....-,...._..,....._.,.----' .96

.98 1.00 1.02 Equivalence Ratio

1.04

o

(B)

.96

.98 1.00 1.02 Equivalence Ratio

Fig. 11. Performance of high Ce promoted (solid lines) and mixed Ni/Ce promoted (broken lines) Pt/Rh Catalysts after lean ageing at (A) 760 0C and (B) 1050 0C peak temperatures. This cycle, which involved lean excursions (0.3% excess oxygen), provokes much greater deterioration of the catalyst.

However, the high ceria system shows

superior stability relative to the mixed promoter system. PRECIOUS METAL COMPONENT In the design of an automotive exhaust catalyst the method of precious metal incorporation plays an important role in the activity, selectivity, durability and cost effectiveness of the system.

In addition, the support material, together with

appropriate stabilisers and promoters, can playa significant role in determining

134 the precious metal location, dispersion and activity. these has been mentioned above.

The contribution of some of

This section examines the deposition of precious

metals with particular reference

to

those

presently most

commonly found

in

automotive catalysts namely platinum and rhodium. There are a number of possible methods of deposition of the metals onto support materials;

these

include

impregnation,

absorption

or

precipitation with the support and vapour deposition.

ion

exchange,

co-

Vapour deposition is not

practical on economic grounds and co-precipitation, often used for the preparation of base metal catalysts, cannot be used because of the problems of recycling and recovery. or

ion

Thus precious metal catalysts are usually prepared by the impregnation

exchange

of

metal

salts

onto

the

support

materials.

A schematic

representation of the ion exchange process is shown below. Ion Exchange of Metal Salt onto Support I

Cationic exchange

S-OH+

+

I

S-OC+

C+

I

I

S-A

Anionic exchange C+

S

_

+

H+

+

(OH)

2+ 2+ ' Pd(NH 3)4 ' [Rh(NH 3)SClj 222PtCl 6 ' PdCl 4 ,RhCl 6 Pt(NH 3)4

2+

support surface

High pH promotes cation exchange, low pH promotes anion exchange.

As the pH is

lowered in a cation exchange regime, interaction between precious metal and the support decreases until the process can be considered a simple impregnation. same

process

occurs

as

the

pH is

raised

under

anion

exchange

The

condi t Lons .

Impregnation is considered a pore wetting process only, the salt being deposited on the support as the solvent is removed by drying.

This has the advantage that the

salt solution is not selectively depleted in precious metal during a continuous process.

If there are ion exchange processes,

depletion does occur and the

solution requires frequent monitoring and metal replenishment.

Ion exchange does,

however, have the advantage of the potential for selective metal placement whilst impregnation generally gives a uniform dispersion. The firing stage, following ion exchange or impregnation of the precious metal, is an important one in the catalyst preparation.

Depending upon temperature

and atmosphere the precursor salt decomposes to ei ther the metal or an oxide.

The

effects that can be achieved are illustrated in figures 12(A) and (B) where decomposition products, particle size and the light-off temperature (for carbon monoxide) are plotted against firing temperature for salts of platinum and rhodium.

135 The results shown in Figure 12(A) are for platinum deposited on alumina via the precursor platinum (II) tetrammine chloride.

Apparently some CO oxidation occurs

even on the undecomposed precursor, although this may be due to CO enhanced reduction of the salt.

As the firing temperature is increased the precursor goes

through several stages of decomposition, during which the CO oxidation light-off temperature also increases.

The most noticeable effect, however, is the sharp

increase in particle size and light-off temperature when the precursor is fully decomposed to the metal. o

Hence, platinum, which does not have an oxide phase stable

above 400 C, sinters rapidly as the metal and the oxidation kinetics (which are negative order for CO over platinum) come into play.

TGA

TGA

r--t--?f--.:r"'-------,

Result I----,~+",.".,.....,~-:;....;:~---{

Result !-:L:+~~;--------i

330

300

300~

o

tlIl

'0

250L

s:

Ol

:.J (Al

200

200 400 600 800 1000 Firing Temperature (OC)

L..-_,-----,..----._.....,._.....,._-:-/:200 200 400 600 300 1000 1200 Firing Temperature (OC)

Fig. 12. Curves showing correlation between metal crystallite size, light off temperature for CO oxidation and calcination temperature and composition for alumina supported catalysts prepared from (a) platinum tetrammine (chloride) and (b) Claus' salt (1%Rh/A1 203) In contrast, when Claus' salt ( [Rh(NH ) is used as the precursor for 3\CljC1 2 rhodium, the initial decomposition product upon calcination is rhodium metal which retains a relatively low particle size (Fig. 12(B)).

As the temperature is

increased rhodium is converted to rhodium (III) oxide and particle growth increases markedly.

Thus, rhodium sinters as the oxide and a parallel, although not entirely

coincident, increase occurs in CO oxidation light-off temperature. Thus far, only one precursor of each of the precious metals has been discussed in the context of the calcination process.

In practice, a number of precursors are

available and these can play a major role in determining metal location and dispersion (ref. 47).

The effect of precursor on rhodium dispersion on alumina is

shown in Table 6 where the absorption of NO is used as a measure, of dispersion. The multiple absorption of NO on rhodium is characteristic of the highly

136 dispersed metal (refs. 37,48) and has also been observed for CO, 0z and HZ (refs. 49,50).

The ratio of NO to Rh would not normally be expected to be greater than Z.

TABLE 6 The effect of precursor on Rhodium Dispersion (1% Rh on alumina) Precursor

NO/Rh

[Rh(NH3)5CljClz

0.81

[Rh(Cl)6](NH 4)3

0.96

Rhodium nitrate

1. 54

Rhodium sulphate

1.78

The dispersion of a precious metal on a support material is also strongly dependent on the metal loading and the atmosphere in which the catalyst is fired. These effects are illustrated in figure 13 where NO uptake is plotted against rhodium loading on alumina for catalysts prepared from Claus' salt and rhodium chloride.

For

each

precursor,

three

firing

hydrogen/nitrogen and air, were investigated. precursors is immediately apparent.

atmospheres,

i.e.

nitrogen,

A major difference between the two

The catalyst prepared from Claus' salt does

not show a progressive increase in NO uptake above a critical rhodium loading.

This

can be related to the relatively low solubility of Claus' salt compared to rhodium chloride.

At higher concentrations, the former crystallises, or sinters as the

salt, during the drying process prior to firing.

,..:

w

~ ';"

1.0

E co

..

(;

E

0.1

~

.2

.

~

:; E

"

0.01

s:

- - - N2

o

oz

.... Air

0.001 L0.01

--~----.J

0.1

1.0

Rh loading...u mole m- 2 (B.E.T.)

10

Fig. 13. Effect of concentration on rhodium dispersion using (A) [Rh(NH3)5CljCIZ and (B) Rh C13 as precursors.

137

A second difference between the two is the behaviour when the catalysts are fired in air.

Claus' salt initially decomposes to rhodium metal but in the presence

of air is converted to the oxide which sinters rapidly.

Thus a worse dispersion of

rhodium is observed when Claus' salt is fired in air than when it is fired in nitrogen or hydrogen/nitrogen.

In the case of rhodium chloride a superior overall rhodium

dispersion is achieved and air firing is not so detrimental to dispersion as it is for the ammine complex.

These observations can again be explained in terms of the

decomposition chemistry of the precursor.

Newkirk and McKee (ref. 51) have studied

the decomposition of rhodium chloride, both unsupported and supported on alumina, in a hydrogen atmosphere. o

The salt is reduced to the metal at temperatures below

200 C and, in the case of the supported material, the hydrogen chloride evolved is strongly adsorbed by alumina and is not released until temperatures in excess of 600

0C.

The decomposition of rhodium chloride in air is slow and produces lower

chlorides or oxychlorides which retard the sintering process.

Nitrogen firing is

also likely to produce a lower chloride content. The role of the support material in determining the activity and selectivity of precious metal catalysts is critical and there is now a significant literature on metal support interactions.

The effect may be

considering alumina and ceria as support phases.

illustrated

support interaction was investigated by firing 1%Rh/AI range of temperatures (table 7).

for

rhodium by

In the case of alumina the metal Z03

samples in air over a

TABLE 7 The effect of alumina phase and ageing (8 hr s in air at the specified temperature) on rhodium dispersion (1%Rh/A1 Z03 ex [Rh(NH3)5CljC1Z) ALUMINA PHASE Gamma

Delta

Theta

AGEING TEMP.

°c

NO/Rh

450

0.86

650

0.42

850

0.00

450

0.74

650

0.40

850

0.00

450

0.33

650

0.Z8

850

0.00

138

The rhodium dispersion becomes progressively worse on the higher temperature and, therefore, lower surface area alumina phases. the ageing temperature of each Rh/ Al

NO uptake also falls sharply as

The lower NO uptake can Z03 be explained partially by rhodium sintering (as the oxide) and also by a metal support interaction (Ref. 36).

phase is increased.

The interaction is less for the high temperature,

less reactive alumina phases but even here NO absorption is not measurable after ageing

at

850

0C.

The

rhodium/alumina

interaction

is

also

observed

when

temperature programmed reduction (TPR) is performed (Fig. l4(A) and (B). 2.8

r - - - . , . . - - - - - - - - - , 18.

;:-2.32

4.3

~

'"

::l

B

~

:e -:Cl.64 ". .:<

~1.36 e

'" e" .88 ,..

't>

:r

.4 L-_,....-_,....-_,....-_,....-----'

200

400

600

800

200 400 600 800 1000 Temperature Deg. Celsius

Temperature Deg. Celsius

Fig. 14. Temperature programmed reduction traces for (A) 1% Rh/AI Z03 and (B) 1% Rh/CeOZ catalysts. Rhodium begins to reduce at relatively low temperatures but the reduction peak o

shows a very long tail and reduction is not complete until 800 C.

In contrast, when

rhodium is supported on ceria the metal support interaction is weaker and reduction is complete by 250

0C,

the other peak in this system being assigned to the partial

reduction of ceria itself (Fig. 14(B)).

Thus,

in preparing precious metal

catalysts, careful attention must be paid to the choice of the support material since this strongly influences activity, selectivity and durability. In addi tion to individual precious metal/ support interactions, those between metals themselves must also be considered.

Thus, it has been established that Pt

and Rh can form alloys, surface enrichment of which, with oxidised Rh species, is adverse to high activity (ref. 52).

Thus, preparative methods must target

carefully the juxtaposition of all key components for optimum performance and durabili t y , CONCLUDING REMARKS High performance automotive emission control catalysts are a combination of the compromises required by the sometimes opposing requirements of their highly

139

dynamic operating environment. emission control.

In consequence there is no universal solution to

Choice of support, chemical componen t s and careful control over

interactions is crucial to activity and durability. Current generation systems achieve high activity and stability by combination of stabilisers/promoters, controlled dispersion and targetting of precious metal components to optimise metal support interactions.

Over the 12 years of vehicle

application thus far accumulated, substantial improvements have been achieved in performance, reflecting extensive investment in Research and Development.

Over

that relatively short period this has established automotive applications as the largest single application of heterogeneous catalysts and the principal consumer of platinum group metals. During that interval, the scientific basis of heterogeneous catalysis has advanced substantially.

New and improved techniques, e s g , temperature programmed

methods such as TPR and TPO, EXAFS, etc. have become more readily available and have been/are being applied more widely, together with metal-supported interactions.

establi~hed

tools to examine

Such techniques have proved of immense value in a

sector previously dominated by empirical techniques which nevertheless remain of great importance.

Although much has been achieved there remain major challenges

from established markets (USA, Japan), large emerging markets (Europe, Australia, 'Korea) and potential markets in developing countries such as Brazil.

Notable among

them are the economic and strategic requirements to reduce the absolute and relative proportions

of

precious metals

without compromising performance.

Although

significant progress has been achieved, it is evident that such increasingly demanding requirements can be met

only as a result of improved scientific

understanding of these complex interactions. ACKNOWLEDGEMENT The data reviewed in this paper is a selection from that of many workers in the Research and

Development Laboratories of

Johnson Matthey world wide.

The

particular contribution of Drs. T. Truex and P. N. Hawker in preparation of this review is gratefully acknowledged. Figures 4, 7, 9, 10 and 11 and Tables 4 and 5 are published by kind permission of SAE from paper SAE 850128 (ref. 46). Figure 13 and Tables 6 and 7 are reproduced by kind permission of Kodansha Lt d , , Tokyo, from Proceedings of 7th Int. Congo Cat. 1980 (ref. 47). REFERENCES 1. 2. 3.

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140 4.

W. Berg, Evolution of Motor Vehicle Emission Control Legislation Leading to the Catalyst Car?, SAE 850384. 5. M. P. Walsh, Global Trends in Motor Vehicle Air Pollution Control, SAE 850383. 6. C. de Boer and J. A. Jeyes, The Interaction of Fuel Economy and Emission Control in Europe - A Literature Study, Paper G422/84, The Institution of Mechanical Engineers, 1984. 7. Anon., Addendum 14: Regulation No. 15 Geneva: United National Economic Commission for Europe, 1958, Revision No.3, 1981. 8. Anon , , Commission of the European Communi ties, Proposal for Amendment of ECE Directives in the Lead Content of Petrol and Motor Vehicle Emissions. Comm (85), 288 Final, 19th June 1985. 9. - G. J. K. Acres and B. J. Cooper, Automobile Emission Control Systems, Platinum Metals Review, 16(3) (1972) 74. 10. K. G. Duleep, Future Automotive Emission Control and Strategy, SAE 841244. 11. W. D. J. Evans and A. J. J. Wilkins, Catalytic Emission Control Strategies for Europe, Sci. Total Environ., In Press. 12. S. Matsushita, T. Inoue, K. Wakanishi, N. Kato and N. Kobayashi, Development of the Toyota Lean Combustion System, SAE 850044. 13. L. C. van Beckhoven, R. C. Rijkboer and P. van Slaten, Air Pollution by Road Traffic - Problems and Solutions in the European Context, SAE 850387. 14. Y. Kimbara, K. Shinoda, H. Koide and N. Kobayashi, NOx Reduction is Compatible with Fuel Economy Through Toyota's Lean Combustion System, SAE 851210. 15. W. B. Williamson, H. S. Gandhi, M. E. Heyde and G. A. Zawaki, Deactivation of Three Way Catalysts by Fuel Contaminants - Lead, Phosphorous and Sulphur, SAE 79094. 16. R. H. Hammerle and Y. B. Graves, Lead Accumulation on Automotive, SAE 830270. 17. B. Harrison, J. R. Taylor, A. F. Diwell and A. Salathiel, Lead Species in Vehicle Exhaust: A Thermodynamic Approach to Lead Tolerant Catalyst Design, SAE 830268. 18. B. J. Cooper, B. Harrison, E. Shutt and 1. Lichtenstein, The Role of Rhodium in Platinum/Rhodium Catalysts for Carbon Monoxide/Hydrocarbon/Nitrogen Oxides (NOx) and Sulphate Emission Control - The Influence of Oxygen on Catalyst Performance, SAE 770367. 19. W. B. Williamson, J. Perry, R. L. Gross, H. S. Gandhi and R. E. Beason. Catalyst Deactivation due to Glaze Formation from Oil Derived Phosphorous and Zinc, SAE 841406. 20. A. F. Diwell and B. Harrison, Car Exhaust Catalyst for Europe, Platinum Metals Review 25(4) (1981) pp 142-151. 21. B. D. McNutt, D. Elliot and R. Dalla, Patterns of Vehicle Misfuelling in 1981 and 1982, SAE 841345. 22. R. B. Michael, Misfuelling Emissions of Three Way Catalyst Vehicles, SAE 841354. 23. W. R. Pierson, R. H. Hammerle and J. T. Kummer, Sulfuric Acid Aerosol Emissions from Catalyst Equipped Cars, SAE 740287. 24. B. J. Cooper, E. Shutt and P. Oeser, Sulphate Emissions from Automobile Exhaust, Platinum Metals Review, 20 (2)(1976) 20. 25. C. M. Urban and R. J. Garbe, Exhaust Emissions from Malfunctioning Three Way Catalyst Equipped Automobiles, SAE 800511. 26. L. R. Smith and F. M. Black, Characterisation of Exhaust Emissions from Passenger Cars Equipped with Three Way Catalyst Systems, SAE 800822. 27. J. S. Howitt, Thin Wall Ceramics as Monolithic Catalyst Supports, SAE 800082. 28. C. A. Dulieu, W. D. J. Evans, R. J. Larbey, A. M. Verrall, A. J. J. Wilkins and J. H. Pavey, Metal Supported Catalysts for Automotive Applications, SAE 770299. 29. A. S. Pratt and J. A. Cairns, Noble Metal Catalysts on Metallic Substrates, Platinum Metals Review 21(3) (1977) pp 2-11. 30. M. Nonnenmann, Metal Supports for Exhaust Gas Catalysts, SAE 850131. 31. H. Schuster, J. Abthoff and C. Noller, Concept of Catalytic Control for Europe, SAE 852095.

141 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

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