1 Catalytic Reduction of Oxides of Nitrogen Emissions in Auto Exhaust Gas
Catalysts for the Control of Automotive Pollutants Downloaded from pubs.acs.org by 179.61.200.187 on 09/29/18. For personal use only.
W. A . M A N N I O N , K . A Y K A N , J. G. C O H N , C. E . T H O M P S O N , and J. J. M O O N E Y Engelhard Industries Division, Engelhard Minerals and Chemicals Corp., Menlo Park, Edison, N.J. 08817
This report discusses the progress made in developing catalysts for use in the control of nitrogen oxides from automotive exhaust. A stabilized supported ruthenium catalyst for use as the NO reduction bed in a dual-bed system was evaluated in laboratory bench tests and on an engine dynamometer with lead sterile and unleaded fuel. Catalyst exposure to high-temperature lean operation on an engine dynamometer revealed that the catalyst is not completely stable to such an environment. Initial evaluation of a three -way conversion (TWC) catalyst demonstrated that good conversion of carbon monoxide, hydrocarbons, and nitrogen oxides is maintained after exposure to elevated temperatures under oxidizing conditions using unleaded fuel. x
/^\xides of nitrogen are important components of photochemical smog which have an adverse effect on health ( J ) . A major source of nitrogen oxides is the exhaust gas from internal combustion engines; this was recognized by the Clean A i r A c t of 1970 which requires effective control of such emissions. Some reduction i n the output of nitrogen oxides by automobiles has been effected through a reduction in the peak temperature reached during combustion by diluting the cylinder charge with exhaust gases (exhaust gas recirculation) (2). This technique is of only limited effectiveness before driveability and fuel economy are affected too severely. Catalysts NO# Reduction Catalyst. The use of catalysts to control emissions from automobiles has been investigated for a long time (3). However, 1
2
C A T A L Y S T S
F O R T H E C O N T R O L
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Figure 1.
2
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HC NO
Schematic of bench scale NO catalyst screening unit x
only lately has attention been focused on catalytic means of controlling oxides of nitrogen. In the dual-bed method, the internal combustion engine is operated net fuel rich, and a catalyst is used to promote the reduction of oxides of nitrogen to nitrogen with minimum ammonia for mation. A i r is then injected downstream of the reduction catalyst to make the stream net fuel lean, and the second bed catalyzes the oxidation of carbon monoxide and hydrocarbons (as well as any ammonia formed in the first b e d ) . O n warm-up, air is injected upstream of the reduction catalyst to control quickly the carbon monoxide and hydrocarbons by reaction i n the manifold or over the reduction catalyst. The reduction catalyst, therefore, must be stable to repeated exposure to an oxidizing environment at high temperatures. A number of recent papers deal with the selection of a reduction catalyst (4,5,6,7) and the durability prob lems that must be overcome (8, 9, 10, 11, 12). This paper reports on development of a supported ruthenium catalyst, stabilized by alloy forma-
1.
MANNioN E T A L .
Catalytic Reduction of Oxides
Operating Conditions of N C v Bench Scale Test Unit"
Table I. Gases
Amount, vol %
Analytical Method*
0.2 0.03 13.5 13.5 balance
NDIR FID
Constant NO HC (3C H + 2C H ) C0 H 0 N 2
4
3
8
2
2 2
Amount (vol %) at Nominal A/F Variable o CO H 2
2
18.5
14.5
14.7
14.9
0.1 3.0 1.0
0.8 1.5 0.5
0.8 1.0 0.33
0.8 0.7 0.23
Ratio
0
15.1 1.0 0.5 0.17
diffusion-cond. cell NDIR —
T e m p e r a t u r e : v a r i a b l e t o 6 5 0 ° C ; V H S V : v a r i a b l e t o 200,000/hr. N D I R : non-dispersive I R ; F I D : flame i o n i z a t i o n detector. Stoichiometric n o m i n a l A / F is 14.65. I g n i t i o n tests were r u n a t a constant n o m i n a l A / F ratio. 0
6 c
tion against ruthenium loss under oxidizing conditions, for use as the reduction catalyst in a dual-bed system. Three-Way Conversion (TWC) Catalyst. The dual-bed method requires net fuel-rich operation of the engine and therefore lower fuel economy. Interest has heightened recently i n a technique to control a l l three pollutants—carbon monoxide, hydrocarbon, and nitrogen oxides— by maintaining the air/fuel ( A / F ) at near-stoichiometric ratios and by converting all pollutants over a single catalyst. A sensor is used to control the partial pressure of oxygen i n the exhaust stream b y feedback to a carburetion or fuel injection system (13). The increased cost of the sensor-carburetor system w i l l be compensated for by better fuel economy, Table II. Test 1
2
Fuel Lead sterile
