Catalysts for Nitrogen Oxides
`Control under Lean Burn Conditions
`THE OPPORTUNITY FOR NEW TECHNOLOGY TO
`COMPLEMENT PLATINUM GROUP METAL AUTOCATALY STS
`By T. J. Truex
`Johnson Matthey Technology Centre
`R. A. Searles
`Johnson Matthey Catalytic Systems Division, Royston
`and D. C. Sun
`Johnson Matthey Catalytic Systems Division, Wayne
`
`Regulations to control the exhaust emissions from motor vehicles are be-
`ing adopted by more and more countries around the world, and in future
`more stringent regulations will be introduced, particularly in the U.S.A.
`and Europe. This, together with the need to show good pollution control
`under real-world driving conditions, has led to the widespread introduc-
`tion of closed-loop, three-way catalysts based on the use of platinum
`group metal technology. “he increasing concern about emissions of car-
`bon dioxide, as well as the three traditional pollutants, offers an oppor-
`tunity for catalyst technology to control nitrogen oxides from both fuel
`eficient lean burn petrol engines and from diesel engines, thus com-
`plementing the use of platinum group metals catalysts to control nitrogen
`oxides and other emissions. This paper reviews the development of “lean-
`NOx” technology based on the use of zeolite supported catalysts; it
`highlights the promise shown and the shortcomings still to be overcome.
`
`Over the last 25 years the motor vehicle has
`increasingly become a cause for concern on en-
`vironmental issues. This initially led to the con-
`trol of carbon monoxide, because of
`the
`potential build-up of this toxic gas in congested
`city centres, and of hydrocarbons and nitrogen
`oxides, both precursors of the photochemical
`smog and low level ozone prevalent in some
`regions, particularly in the Los Angeles basin.
`Now the motor vehicle is identified as the
`contributor of around 15 per cent of carbon
`dioxide emissions. Carbon dioxide is the main
`“greenhouse” gas contributing to perhaps 50
`per cent of the predicted global warming and is
`of course the inevitable product of burning
`carbon-containing fossil fuels.
`
`The twin goals of low and efficient fuel use
`and minimum emissions are increasingly being
`addressed by research in both the motor and
`the catalyst industries of the world.
`The Lean Burn Engine
`In various prototype forms the lean burn
`engine has been around for nearly 25 years.
`However, successful and widespread usage of
`this engine has been restricted by increasingly
`strict control on the level of pollutants emitted
`under the full range of engine operating condi-
`tions (1). Recent developments have included
`the evolution of lean operating two-stroke
`engines (Figure 1).
`Lean burn operation involves the burning of
`
`Platinum Metals Rev., 1992, 36, (l), 2-11
`
`2
`
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`
`

`
`Fig. 1 Among lean burn engines
`under development that would
`benefit from using lean-nitrogen
`oxides catalysts is
`the Orbital
`2-stroke engine, shown here
`
`fuel with an excess of air, in ratios up to 24
`parts of air to one part of fuel. Under these con-
`ditions nitrogen oxides and carbon monoxide
`emissions are at a minimum, but hydrocarbons
`can rise at the onset of unstable combustion, as
`can be seen in Figure 2. Engine design to in-
`crease the swirl of the aidfuel charge can in-
`crease the air:fuel ratio at which misfire starts
`
`and minimise, but not prevent, hydrocarbon
`emissions.
`The main reason that lean burn engines have
`not so far had widespread acceptance has been
`that the power output from an engine falls as
`the fuelling moves to leaner operation. This
`means that to meet driver expectations of per-
`formance and drivability a rich fuel setting is
`
`LEAN
`
`1
`
`ENGINE
`
`1
`
`I
`
`Hydrocarbons
`
`\ \
`
`10.1
`
`1i:l
`
`
`.
`.
`14:l (4.7.1
`
`16:l
`
`20 :l
`
`22:l
`
`24:l
`
`ie:i
`A I R : FUEL RATIO, by weight
`Fig. 2 These four curves indicate trends which apply to all petrol engines. Under lean burn
`operation fuel is burnt with an excess of air at ratios of up to 24 parts of air to 1 part of
`fuel, and it can be seen that nitrogen oxides and carbon oxides emissions are at a minimum,
`but that hydrocarbon emissions can rise at the start of unstable combustion. When there
`is excess fuel, the engine power is high but high levels of hydrocarbons and carbon monox-
`ide are emitted. Peak combustion temperatures occur just on the lean side of the
`stoichiometric composition and result in the highest emissions of nitrogen oxides
`
`Platinum Metals Rev., 1992, 36, (1)
`
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`
`Table I
`European Regulations
`
`I
`I Pollutants
`Carbon monoxide
`Hydrocarbons + nitrogen oxides
`Particulates/diesel
`
`I
`I
`
`1992 Standards in gramslkm
`
`I
`
`Type approval
`
`2 . 7 2
`0 . 9 7
`0.14
`
`I
`
`Production
`
`3.16
`1.13
`0 . 1 8
`
`On 1/7/92 applies to new models; on 31/12/92 applies to all new registrations
`
`provided for acceleration, high speed cruising
`and hill climbing, thus causing nitrogen oxides
`emissions to increase.
`The Diesel Engine
`The only true lean burn engine in widespread
`use is the diesel engine. The diesel engine stays
`in the lean operating region under all engine
`conditions. The petrol engine is throttled on
`the air intake, and ultimate power from a given
`engine is limited by the amount of air that the
`engine can “breathe”. Conversely the diesel
`engine is unthrottled and its power output is
`determined by the amount of fuel that is in-
`jected into the combustion chamber. The max-
`imum fuel input level is controlled by the onset
`of unacceptable levels of smoke or particulate
`formation. To limit particulate emissions to ac-
`ceptable or legislated levels it is necessary for
`the diesel engine always to operate in the lean
`region (2). This means that the diesel engine
`has a significantly lower power output than a
`petrol engine of the same capacity.
`Three-Way Catalyst Operation
`Conventional three-way catalysts, based on
`the use of combinations of platinum group
`metals - platinum, palladium and rhodium - can
`convert over 90 per cent of the three main
`pollutants carbon monoxide, hydrocarbons and
`nitrogen oxides (3). They do this by the exhaust
`gas being controlled by an airfuel ratio (or
`lambda)
`sensor
`around
`the
`so-called
`
`stoichiometric point at which neither air nor
`fuel is in excess at the intake to the engine; for
`a typical petrol composition this is at a ratio of
`14.7 parts of air to 1 part of petrol.
`Under lean conditions the three-way catalyst
`will act as an oxidation catalyst controlling car-
`bon monoxide and hydrocarbon emissions, but
`the conversion of the nitrogen oxides emissions
`falls to very low levels.
`Limitations to Lean Operation
`as Legislation Tightens
`The introduction of lean bum engines is
`limited by a number of key factors. The Euro-
`pean driving cycle, during which the emissions
`from motor cars are measured against the
`legislated
`levels, has been changed. The
`original City Test Cycle was based on inner city
`driving in congested traffic with a top speed of
`50 k.p.h. (31 m.p.h.) and an average speed of
`18.8 k.p.h. (11.7 m.p.h.). Under these condi-
`tions a typical car might need to use only 15 per
`cent of its maximum available power, so that
`lean operation would be possible throughout
`the cycle. However, the realisation that a major
`contribution to regional and global pollution is
`made by motor vehicles operating at high
`speeds on highways led to the addition of the
`Extra Urban Driving Cycle (EUDC), which in-
`cludes speeds up to 120 k.p.h. (75 m.p.h.) and
`needs more power than the City Test Cycle.
`This causes greater nitrogen oxides emissions.
`The new European Community directive,
`
`Platinum Metals Rev., 1992, 36, (1)
`
`4
`
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`
`

`
`4
`
`Fig. 3 The U.S. Federal Stan-
`dards include a timetable for
`the progressive reduction of
`2
`E 3
`pollutant emissions in exhaust
`E
`gas:
`hydrocarbon reduction 9
`c
`5
`from 0.41 glmile in 1991, to
`0.125 glmile in 2004
`VI
`L"
`carbon monoxide 5 1
`reduction from 3.4 glmile in
`1991 to 1.7 glmile in 2004
`nitrogen oxides reduc-
`tion from 1 .O glmile in 199 1
`to 0.2 glmile in 2004
`
`1991
`
`1994 to 1996
`(40%)
`llW%)
`YEARS
`
`2004
`
`published on 30th August 1991 (4), sets max-
`imum pollution levels for all sizes of motor cars
`(Table I) and is based on the combined City and
`EUDC cycles. The standards will necessitate
`the use of closed loop, three-way catalysts on all
`new models sold from 1st July 1992 and on all
`new cars registered for sale after 31st December
`1992.
`In the U.S. new, more demanding Federal
`and Californian standards have been set. The
`former will reduce the allowed hydrocarbon
`
`emissions by 40 per cent and nitrogen oxides
`emissions by 60 per cent, by 1996 (Figure 3).
`Californian standards call for increasingly lower
`and lower emissions, culminating in a require-
`ment for all motor vehicle manufacturers to in-
`clude 10 per cent of zero emissions vehicles in
`their fleets by 2003 (Figure 4 and Table 11).
`The Californian standards are also expected to
`be adopted by 13 states in north eastern
`U.S.A., which together account for nearly 40
`per cent of U.S. car sales.
`
`100
`
`8 0
`
`1 c 8 60
`
`L P)
`
`c'
`
`4 0
`1 LL
`
`20
`
`1994
`
`1995
`
`1996
`
`1997
`
`1999
`1998
`YEARS
`
`2 0 0 0
`
`2001
`
`2002
`
`2003
`
`Fig. 4 Californian standards give a timetable for lower emissions and call for motor vehicle
`manufacturers to include 10 per cent of zero emission vehicles in their fleets by 2003
`transitional low emission vehicles
`zero emission vehieles
`1993 standards
`ultra low emission vehicles
`
`a low emission vehicles 0 1991 standards
`
`Platinum Metals Rev., 1992, 36, (1)
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`
`Table II
`Californian Standards
`
`Emission limits, gramshile
`
`Hydrocarbons
`
`Carbon monoxide
`
`Nitrogen oxides
`
`0.39
`0.25
`0.125
`0.075
`0.04
`0
`
`7.0
`3.4
`3.4
`3.4
`1.7
`0
`
`0.4
`0.4
`0.4
`0.2
`0.2
`0
`
`I
`
`Year and standard
`
`1991
`1993
`1994 (TLEV)
`1997 (LEV)
`1997 (ULEV)
`1998 (ZEV)
`
`The European Commission will propose
`tougher standards for the European Communi-
`ty by the end of 1992, for agreement during
`1993 and with implementation expected in
`1996. It is anticipated that these will be similar
`to the new U.S. Federal standards.
`These increasing restrictions on nitrogen ox-
`ides emissions and the inclusion of real-world
`driving conditions mitigates against the use of
`lean burn engines, unless the emissions of
`nitrogen oxides can be limited in the engine or
`controlled externally.
`Removal of Nitric Oxide under
`Lean Operation
`Nitric oxide is thermodynamically unstable
`relative to nitrogen and oxygen under the full
`range of exhaust gas stoichiometries and
`temperatures encountered in internal combus-
`tion engines (5). A number of catalysts were
`studied during the 1970s, including platinum
`group metals and metal oxides (6) and some
`were found to decompose nitric oxide, but none
`of these had sufficiently high activity to be of
`practical importance. In their reduced states,
`these catalysts are rapidly oxidised by nitric ox-
`ide, with release of nitrogen. Oxygen is retained
`on the catalyst surface, however, inhibiting fur-
`ther nitric oxide adsorption and decomposition.
`Reducing agents are required to remove this
`surface oxygen and regenerate catalyst activity.
`Selective catalytic reduction using ammonia as
`
`the reducing agent has been utilised for the
`removal of nitric oxide from industrial boilers
`and gas turbines under conditions of excess ox-
`ygen (7). Careful stoichiometric control of the
`ammonia must be maintained to assure efficient
`nitric oxide removal without emission of
`surplus ammonia. For transportation applica-
`tions this process is not practical because of the
`problems associated with the storage of am-
`monia, and controlling ammonia injection
`under transient conditions. An active and
`durable nitric oxide decomposition catalyst, or
`a selective reduction catalyst utilising reducing
`species present in the engine exhaust stream
`would be a major breakthrough for the control
`of nitric oxide in transportation applications.
`Recent literature reports and work conducted
`by Johnson Matthey now indicate that progress
`is being made towards developing these catalyst
`technologies employing platinum group metals.
`
`New Nitric Oxide Decomposition
`Catalysts
`Copper-exchanged zeolites have high activity
`for the catalytic decomposition of nitric oxides
`according to Iwamoto and co-workers (8). A
`number of zeolite systems were investigated in-
`cluding Mordenite, Ferrierite, L-type and
`ZSM-5, with the Cu-ZSM-5 system showing
`the highest activities (9, 10). Using gas mix-
`tures of 0.5-2.1 per cent nitric oxide in helium,
`with gas hourly space velocities of 10-80,000
`
`Platinum Metals Rev., 1992, 36, (1)
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`
`per hour, nitric oxide conversions of 13 to 60
`per cent were reported. Maximum conversions
`to nitrogen were observed at about 5OO0C,
`with the degree of decomposition decreasing at
`higher temperatures. This is apparently due to
`a change in mechanism, as opposed to catalyst
`degradation, as the same conversions were
`observed with decreasing catalyst temperature
`after high temperature operation. Water and
`carbon monoxide additions to the gas mixture
`did not greatly influence activity. The addition
`of oxygen resulted in a decrease in nitric oxide
`decomposition activity, although it is claimed
`that this effect is dependent upon the level of
`copper exchange in the zeolite, with high levels
`of exchange resulting in less oxygen inhibition.
`The addition of sulphur dioxide was found to
`poison catalyst activity completely in the 400 to
`6OOOC temperature range, with restoration of
`activity being achieved after
`the high
`temperature desorption of sulphur.
`Detailed kinetic studies of nitric oxide
`decomposition over the Cu-ZSM-5 system have
`been reported by Li and Hall (11, 12). Their
`work has shown that the reaction is first order
`in nitric oxide pressure and inhibited by ox-
`ygen. The kinetics can be described in the
`Langmuir-Hinshelwood form with the inhibi-
`tion being half-order in oxygen pressure. As
`with Iwamoto and co-workers, they observed
`that the nitrogen formed was significantly less
`than the equivalent nitric oxide which disap-
`peared. This discrepancy was accounted for by
`nitrogen dioxide which appears in the products
`as a result of homogeneous reaction of product
`oxygen with undecomposed nitric oxide
`downstream of the catalyst. The redox capacity
`of Cu-ZSM-5 was found to be near 0.5 O:Cu,
`that is le-. In particular, it was shown that ox-
`ygen could be desorbed isothermally from the
`catalyst surface upon reducing the oxygen par-
`tial pressure in the gas stream and that Cu' *
`was reduced to Cu+ I during the desorption of
`oxygen. Since it is generally believed that the
`rate of removal of strongly adsorbed product
`oxygen from the surface limits the nitric oxide
`decomposition on most catalysts, the con-
`tinuous desorption of oxygen from the active
`
`sites of the Cu-ZSM-5 catalyst during steady-
`state reaction is felt to be the key to their high
`sustained activity.
`Although the above results represent a
`significant advancement in the development of
`catalysts for nitric oxide decomposition, major
`improvements will be required before such
`systems can be used to control nitric oxide
`emissions from combustion sources. Nitric ox-
`ide concentrations in exhausts of internal com-
`bustion engines are generally <3000 ppm,
`significantly lower than those used in the work
`reported above. Tests in our laboratories at
`these concentrations with 5 per cent oxygen
`present and gas hourly space velocities of
`20,000 per hour show little nitric oxide conver-
`sion. In addition, the almost complete poison-
`ing of activity by sulphur dioxide in the
`reaction gas mixture would preclude use with
`present petroleum-derived fuels.
`Selective Catalytic Reduction of
`Nitric Oxide by Hydrocarbons
`Another significant, and potentially more
`practical, breakthrough has been the reports by
`Iwamoto (10, 13, 14), Hamada (15, 16, 17) and
`Held (1 8) and co-workers demonstrating the
`selective catalytic reduction of nitric oxide by
`hydrocarbons in the presence of excess oxygen.
`Hamada and co-workers have studied nitric ox-
`ide reduction over solid acid (19, transition
`metal promoted alumina and silica (17), and H-
`form zeolites (16). Some of their results using
`C,H, as the selective reducing agent are sum-
`marked in Table 111. Alumina, titania and zir-
`conia all showed modest activity for the
`selective reduction of nitric oxide with C, H, .
`The addition of transition metals to alumina
`resulted in substantially higher activity, with
`cobalt and iron presenting the highest conver-
`sions. Interestingly, the observation was made
`that a correlation exists between cobalt
`aluminate formation and nitric oxide reduction
`activity. Of
`the platinum group metals,
`platinum exhibited the highest activity. As the
`results presented in Table 111 show, the H-form
`zeolites were the most active catalysts tested by
`these workers. In all these studies nitric oxide
`
`Platinum Metals Rev., 1992, 36, (1)
`
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`

`
`Table 111
`Selective Reduction of Nitric Oxide over Solid Acid (15),
`Transition Metal Promoted Alumina (1 7) and Zeolite (16) Catalysts
`
`Nitric oxide conversion to nitrogen, per cent
`
`at 3OOOC
`< 0.4
`0.5
`0.8
`-
`4.2
`14.0
`19.0
`4.5
`13.0
`9.0
`12.0
`13.0
`49.0
`58.0
`9.0
`
`at 4OOOC
`< 0.6
`13.0
`4.8
`23.0
`49.0
`23.0
`35.0
`31 .O
`13.0
`28.0
`5.6
`8.9
`59.0
`65.0
`19.0
`
`at 5OOOC
`< 1 . 1
`32.0
`8.9
`20.0
`29.0
`11.0
`17.0
`40.0
`7.4
`12.0
`5.4
`8.5
`38.0
`48.0
`26.0
`
`Catalyst
`
`(blue)
`(green)
`(orange)
`(green)
`(brown)
`
`3
`
`
`
`SiO,
`4
`0
`TiO,
`TiO,
`ZrO,
`ZrO,
`2% Co/AI,O,
`2% Cu/AI,O,
`2% Fe/AI,O,
`2% Ni/AI,O,
`2% Mn/AI,O,
`0.5% Pt/AI,O,
`0.5% Pd/AI,O,
`0.5% Rh/AI,O,
`H-ZSM-5
`H-Mordenite
`HY
`
`~
`
`1000 ppm NO, 329 ppm C,H.. 10% 0, at 3.7 LlHG MHSV Whg mass hourly space velocity1
`
`conversion to nitrogen was observed to go
`through a maximum with increasing reaction
`temperature. In the region where significant
`nitrogen was formed carbon monoxide, in addi-
`tion to carbon dioxide, was found in the pro-
`duct gas stream. At higher temperatures where
`nitrogen formation decreases, the hydrocarbon
`was completely oxidised to carbon dioxide. It
`was also found that the temperature of max-
`imum nitric oxide reduction to nitrogen was
`dependent upon the hydrocarbon species, with
`maximum
`conversion
`occurring
`at
`a
`temperature about 100°C lower when C,H,
`was used as reductant, instead of C,H,.
`A number of ion-exchanged zeolites have
`been examined for their selective nitric oxide
`reduction activity by hydrocarbons (10, 13, 14).
`Using a gas mixture containing 1000 ppm nitric
`oxide, 250 ppm C,H,, and 2 per cent oxygen
`at a mass hourly space velocity of 18
`litresihour/gram,
`the order of
`activity
`(temperature for maximum nitric oxide reduc-
`tion to nitrogen) was copper (250°C)<cobalt
`
`(350°C)<H (400°C)<silver (450-600°C)<zinc
`(6OOOC). Maximum conversions to nitrogen
`were 30-40 per cent and were relatively in-
`dependent of the cation. Activities of various
`copper ion-exchanged zeolites were (conversion
`to nitrogen at 25OOC): Cu-ZSM-5 (31%)>cu-
`(26%) - Cu-L-type
`Mordenite
`(25%)>cu-
`Ferrierite (23%). The effect of space velocity on
`conversion was studied for Cu-ZSM-5, H-
`ZSM-5, and alumina catalysts. Conversion over
`alumina dropped rapidly at gas hourly space
`velocities greater than about 5000 per hour, H-
`ZSM-5 conversions showed a modest decline in
`the range of 5-20,000 per hour with a rapid fall
`at higher space velocities, and conversion over
`Cu-ZSM-5 was stable up to 48,000 per hour
`with a fall at higher values. These results show
`that transition metal ion-exchanged zeolites can
`have substantially higher activity than the H-
`form systems, with
`the copper-exchanged
`catalysts showing most promise.
`Using laboratory gas mixtures more typical of
`engine exhaust, together with actual engine
`
`Platinum Metals Rev., 1992, 36, (1)
`
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`

`
`I
`
`I
`
`Table I V
`Selective Nitric Oxide Reduction over Copper-Mordenite and Copper-ZSM-5
`
`Catalyst
`
`Cu-Mordenite
`
`CU-ZSM-5
`
`Nitric oxide conversion to nitrogen, per cent
`
`HC
`(ppm as C , )
`
`C,H, (1001
`C,H, (700)
`C,H, (2500)
`C,H, (5000)
`
`C,H, (700)
`C,H, (2500)
`C,H, (5000)
`C,H, (2500)
`CH, (250)
`
`200OC
`
`3OOOC
`
`4OOOC
`
`5OOOC
`
`6OOOC
`
`0
`3
`4
`3
`
`-
`-
`-
`0
`0
`
`3
`15
`38
`58
`
`12
`16
`12
`0
`0
`
`2
`13
`40
`55
`
`30
`50
`68
`15
`0
`
`2
`12
`33
`45
`
`23
`47
`58
`43
`0
`
`2
`2
`6
`8
`
`12
`20
`40
`37
`7
`
`250 ppm NO. HC ias indicatedl. 5% 0,. 1 3 % CO,, and 10% H,O in N, at 20,000ih gas hourly space velocity
`
`evaluation, Held and co-workers (18) have
`reported results on a number of transition metal
`ion-exchanged zeolites. Cu-Mordenite; Cr-, Fe-,
`Mn-, V-, Cu-, Co-, Ni-, and Ag-Y zeolite; Cu-
`X zeolite; and Ir-, Pt-, Rh-, Ni-, Co-, and Cu-
`ZSM-5 were all reported to have some activity
`for selective reduction of nitric oxide by
`hydrocarbons in the presence of excess oxygen.
`The Cu ion-exchanged systems showed the
`highest activities of those tested. They also
`showed that there was an inhibition of activity
`when water was added to the reaction gas mix-
`ture. Studies of the two most active catalyst
`systems, Cu-ZSM-5 and Cu-Mordenite, with a
`gas mixture containing 1000 ppm nitric oxide,
`400 ppm C,H,, and 1.5 per cent oxygen at
`35OOC and a gas hourly space velocity of
`13,000 per hour showed that nitric oxide con-
`versions over Cu-Mordenite dropped from 37
`to 17 per cent with the inclusion of 10 per cent
`water vapour. Under the same conditions, the
`more hydrophobic Cu-ZSM-5 catalyst showed a
`drop in nitric oxide conversion from 50 to 37
`per
`cent. A
`conventional
`autocatalyst
`monolithic substrate was coated with Cu-
`ZSM-5 for engine evaluation. Under steady-
`state conditions at about 4OOOC
`inlet
`temperature and a gas hourly space velocity of
`
`approximately 15,000 per hour, nitrogen oxides
`conversions of 35 to 45 per cent were observed
`at air:fuel ratios of 17.5-19: 1. A correlation
`exists between nitrogen oxides conversion and
`the hydrocarbonxitrogen oxides ratio in the
`exhaust gas mixtllre, with the highest nitrogen
`oxides conversions occurring at the highest
`hydrocarb0n:nitrogen oxides ratios.
`In our laboratories, monolithic substrates
`have been coated with Cu-Mordenite and Cu-
`ZSM-5 catalysts for laboratory flow reactor and
`engine evaluation. Laboratory flow reactor
`results are summarised in Table IV. It should
`be noted that under these conditions no s i d i -
`cant nitric oxide conversion was observed in the
`absence of hydrocarbons, indicating that direct
`decomposition was not contributing to nitric
`oxide conversion. As was found by other
`workers, nitric oxide conversion went through
`a maximum as a function of temperature. Car-
`bon monoxide formation was observed during
`the onset of nitric oxide conversion. Complete
`conversion of hydrocarbon to carbon dioxide
`was observed at higher temperatures associated
`with declining nitric oxide conversion. These
`results show that zeolite type, the hydrocar-
`bon:nitric oxide ratio, and the hydrocarbon
`species all have a significant effect on nitric
`
`Platinum Metals Rev., 1992, 36, (1)
`
`9
`
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`

`
`oxide reduction activity. Consistent with other
`reports, the Cu-ZSM-5 catalyst gave higher
`conversions than the Cu-Mordenite catalyst.
`Maximum conversions increased with increas-
`ing hydrocarbon concentrations (increasing
`HC:NO ratio). The use of C,H, instead of
`C,H, as reductant resulted in an increase in
`temperature for maximum nitric oxide conver-
`sion from - 4OOOC to - 5OOOC. The use of CH,
`resulted in minimal nitric oxide reduction at
`temperatures up to 600OC. The dependence on
`hydrocarbon
`species
`and
`concentration,
`together with the observation that carbon
`monoxide formation occurs simultaneously
`with nitric oxide reduction,
`implies that
`hydrocarbon partial oxidation or decomposition
`products
`are
`involved
`in
`the
`reaction
`mechanism. In separate experiments we have
`exposed a Cu-ZSM-5 catalyst first
`to a
`C,H,/O, mixture and subsequently
`to a
`NO/O, mixture. Immediately upon switching
`to the NO/O, mixture, significant nitric oxide
`conversion is observed which decays with time,
`implying that hydrocarbon-containing species
`deposited on the catalyst surface are involved in
`the reaction, and are being depleted with time.
`We have also shown that carbon monoxide and
`hydrogen are much
`less effective
`than
`hydrocarbon for selective nitric oxide reduction
`over these catalyst systems. Iwamoto has
`classified reductants into two groups, selective
`(C,H,, C3H,, C,H,, C,H,) and non-selective
`(H2, COY CH, and C2H,) based upon similar
`observations (10). His group has also in-
`vestigated the role of oxygen, showing that up
`to about 2 per cent oxygen in the reactant
`stream strongly activates nitric oxide reduction
`with a slight inhibition observed at higher con-
`centrations. Our studies have shown similar ef-
`fects with the added observation that exposure
`of a Cu-Mordenite catalyst to nitric oxide and
`C H , in the absence of oxygen resulted in a
`reddish colour change to the catalyst, implying
`the presence of reduced metallic copper. This
`occurs even when the reactant gas mixture is
`net oxidising and may indicate that the impor-
`tance of oxygen is in maintaining copper in an
`active oxidation state.
`
`At present there is very little information
`available on catalyst durability but, as ex-
`pected¶ there are indications that the zeolite
`systems would be limited to temperatures less
`than 600°C to avoid thermal degradation. In-
`itial resistance to sulphur dioxide poisoning has
`been
`indicated (14) although longer term
`poisoning effects remain to be assessed.
`Future Work and Requirements
`for a Practical Emission Catalyst
`A much better understanding of
`the
`mechanism(s) involved in the selective nitric
`oxide reduction reaction with hydrocarbons
`must be obtained as a basis for development of
`improved catalyst systems. These studies,
`together with engine evaluations, must address
`a number of possible limitations of present
`technology for vehicle applications. Some of
`* Thermal stability of the present Cu-zeolite
`these limitations include:
`
`systems
`t Effects of potential poisons, particularly
`sulphur compounds
`
`* The relatively narrow range of temperatures
`* The effects of cyclic engine operation, par-
`
`and low space velocities associated with op-
`timum performance
`
`ticularly the effects of varying emission rates of
`hydrocarbons versus nitrogen oxides on catalyst
`function.
`The importance of these last two effects is
`demonstrated by the work of Held and co-
`workers (18). They tested Cu-ZSM-5 catalysts
`on a vehicle under the transient FTP cycle.
`Nitrogen oxides conversions of about 15 per
`cent were obtained during cold-start (bag 1)
`and about 30 per cent during hot-start (bag 3)
`tests. The lower conversions were ascribed to
`low catalyst temperatures during cold-start and
`the generally higher space velocities incurred in
`the FTP during the hot-start test. It was also
`noted that under acceleration conditions where
`significant nitrogen oxides are formed, the
`nitrogen oxides:hydrocarbon ratio was un-
`favourable for nitrogen oxides reduction. The
`requirement for high hydrocarb0n:nitrogen ox-
`ides ratios was found to be a major problem in
`
`Platinum Metals Rev., 1992, 36, (1)
`
`10
`
`BASF-2005.009
`
`

`
`studies using a diesel engine. Diesel engines
`have inherently low hydrocarbon emissions and
`injection of hydrocarbons into the exhaust
`stream was necessary to obtain significant
`nitrogen oxides conversion.
`Finally, a complete automotive emission con-
`trol catalyst system must have adequate
`hydrocarbon and carbon monoxide activity in
`addition to nitrogen oxides removal. The
`catalyst systems described above show deficien-
`cies in hydrocarbons and, particularly, carbon
`monoxide activity which would require the use
`
`of a dual function catalyst system. The first
`component would be the selective nitrogen ox- 1
`ides reduction catalyst, followed by a conven-
`tional platinum group metal catalyst for
`hydrocarbons and carbon monoxide removal.
`Such concepts are already described in the pa-
`tent literature. It is obvious that a practical
`selective nitrogen oxides reduction catalyst for
`automotive applications is some time away, but
`the exciting results obtained in the past several
`years and the high level of activity in this area
`are reasons for encouragement.
`
`References
`9 M. Iwamoto, H. Yahiro and K. Tanda, “Suc-
`1 R. A. Searles, Platinum Metals Rev., 1988, 32,
`( 3 ~ 123
`cessful Design of Catalysts”, ed. T. h i ,
`Amsterdam, 1988, pp. 219-226
`2 B. J. Cooper and S. A. Roth, Platinum Metals
`10 M. Iwamotoand H. Hamada, Catal. Today, 1991,
`Rev., 1991, 35, (4), 178
`10, 57
`3 B. Harrison, B. J. Cooper and A. J. J. Wilkins,
`11 Y. Li and W. K. Hall, 3. Phys. Chem., 1990,94,
`Platinum Metals Rev., 1981, 25, (l), 14
`6145
`4 Off. 3., 30th August 1991, 34, L242
`12 Y. Li and W. K. Hall,3. Catal., 1991, 129, 202
`5 B. Harrison, M. Wyatt and K. G . &ugh,
`13 S. %to, Y. Yu-u, H. Yahiro, N. W o a n d M.
`“Catalysis”, Vol. 5, R. Sot. Chem., London,
`Iwamoto, ~ p p l . Catal., 1991, 70, ~1
`1982, pp. 127-171
`14 M. Iwamoto, H. Yahiro, S. Shundo, Y. Yu-u and
`6 J. W. Hightower and D. A. VanLeirsburg, in
`Appl. Catal., 1991, 69, L15
`N. &no,
`‘‘The Catalytic Chemistry of Ninogen a i d e s ” ,
`15 Y. Kintaichi, H. Hamada, M. Tabata, M. Sasaki
`ed. R. L. Klimisch and J. G. Larson, 1975,
`and T. Ito, Catal. Lett., 1990, 6, 239
`Plenum Press, New York, p. 63
`16 H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito and
`7 B. Harrison, A. F. Diwell and M. Wyatt,
`M. Tabata, Appl. Catal., 1990, 64, L1
`17 Op. cir. (Ref. 16), 1991, 75, L1
`Platinum Metals Rev., 1985, 29, (2), 5
`18 W. Held, A. Konig, T. Richter and L. Ruppe,
`8 M. Iwamoto, S. Yokoo, K. Sakai and S. Kagawa,
`3. Chem. SOC., Faraday Trans., 1981, 77, 1629
`SAE Paper No. 900496, 1990
`Rhodium-Iron Resistance Thermometer
`Fused-Silica Coil Frame”, Cryogenics, 1991,
`For some twenty years the rhodium-iron
`31, (lo), 869-873). The use of fused silica
`resistance thermometer has been regarded as
`one of the most reliable for low temperature
`enables the sensing element to be annealed at
`temperatures above 600OC. The four lead wires
`measurement, and in various forms has been
`used from millikelvin regions up to room
`and the protective sheath are made of platinum.
`temperature. Although it is known that the an-
`The influence of annealing temperatures bet-
`ween 700 and 900OC upon the resistance of the
`nealing treatment is one of the most important
`factors controlling the thermometric properties
`thermometer has been investigated, and a
`of such thermometers, only limited information
`calibration method proposed for cryogenic use
`is available about their stability when the
`of the thermometers.
`It is concluded that an annealing temperature
`rhodium-iron (mole fraction 0.5 per cent) is in
`the form of wire.
`of 8OOOC is required to remove the strain pro-
`A resent communication from the National
`duced in the wire by coiling; thermometers an-
`Research Laboratory of Metrology, Japan,
`nealed at or above this temperature have similar
`reports on the effects of annealing on a new
`temperature-resistance
`characteristics
`and,
`type of rhodium-iron thermometer, in which
`after calibrating the deviation from a reference
`the 50 pm diameter wire is wound bifilarly
`function at only three calibration points, can be
`around a cross-shaped frame machined from
`used with an accuracy better than 0.5 mK over
`fused silica. (0. Tamura and H. Sakurai,
`the range 4.2 to 25 K. Self-heating effects were
`“Rhodium-Iron Resistance Thermometer with
`found to be of a reasonable magnitude.
`
`Platinum Metals Rev., 1992, 36, (1)
`
`11
`
`BASF-2005.010