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Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 226-231
Exhaust Gas Catalytic Reduction of Nitrogen Oxides over NiFe204-NiCr204Solid Solutions Ph. Courty," B. Raynal, B. Rebours, M. Prigent, and A. Sugier Research Laboratories, Institut Frangais du Pgtrole and Soci6t6 Franpaise des Produits pour Catalyse: PROCA TALYSE, B.P. 3 7 1, 92506 Rueil-Malmaison Cgdex, France
This report deals with the catalytic reduction of nitrogen oxides in exhaust gases, over mixed NiFe,-xCrx04oxides in a bulk state or deposited on a pelletized a-alumina carrier. Initial activity tests (laboratory or motor-bench tests) indicate a better activity and selectivity for the ternary Fe2O3-Cr2O3-NiOoxide, when compared with the three binary combinations (FeCr, FeNi, and CrNi oxides). The effects of composition variations on catalytic properties of the ternary mixed oxide were studied. The mechanism of aging was investigated for the NiFe,,,Cr,,,O, composition by TGA, X-ray diffraction, and activity tests after aging; it was concluded that the aging is probably due to chemical reactions between the a-alumina carrier and the nickel contained in the ternary oxide. This reaction is enhanced when the engine exhaust gases contain the most residual oxygen and can be prevented by adding precious metals to the base oxide formula.
Introduction A great deal of research has been done on the catalytic purification of exhaust gases from internal combustion engines. One of the solutions proposed (Shelef, 1975; Wei, 1975; Hightower, 1975; Montarnal, 1976; Prigent et al., 1977) consists of treating this exhaust through two successive reactors. The first one works with a slightly reducing gas mixture and removes nitrogen oxides by chemical reduction; the unburned products (CO and hydrocarbons) are then catalytically oxidized in the second reactor by means of an intermediate injection of air. The catalytic reduction of nitric oxide (NO) by the hydrogen contained in exhaust gases produces mainly molecular nitrogen or ammonia. The formation of ammonia must be prevented because this molecule is largely transformed back into NO, by reoxidation in the second reactor. The catalyst to be used in the first reactor must then be highly selective with regard to the reduction of NO into N2. As the operating conditions are especially severe ( T , "C up to 900, GHSV = lo4 to lo5 h-l), the catalyst concerned must also resist thermal and chemical aging resulting from the sintering of the active phase and phase carrier interactions. Some results obtained with a family of catalysts made of mixed nickel-base oxides (Courty; 1971) are described in this paper. Catalyst Preparation A chemically pure alumina carrier having the following characteristics was used: shape, pellets 2.4 to 4 mm in size; grain specific gravity, 1.41 g/cm3; total pore volume, 45.6 cm3/100 g; specific surface area, 8.5 m2 g-l. The catalysts were produced by impregnating this carrier with aqueous solutions of iron, chromium, and nickel salts, complexed with citric acid so as to prevent any segregation of the different metallic ions and to selectively produce the desired mixed oxide. The active phase content was generally about 10 wt % . After being impregnated, the catalysts were oven-dried at 200 "C for 2 h and then subjected to thermal decomposition a t 500 "C for 2 h (catalyst N). A portion of each sample was also thermally aged by air-calcination for 24 h a t 870 "C (catalyst W). A bulk NiFe2,3Cr4,304mixed oxide composition producing optimum catalytic activity was also prepared by means of the citric complexing method. The finely divided powder produced by the thermal decomposition of the
Table I catalyst volume catalyst bed length reactor @ i.d. reactor length GHSV
20 m L 20 cm 11.3 mm 700 mm 20 000 h - I
Composition of Synthetic Gas Mixture NO 0.2%
H* CO 0,
H2O N2
1% 2% 0.3%
3.3% 93.2%
mixed oxide precursor was first agglomerated, dried, and ground. It was then pelletized into cylinders (4 = H = 4 mm) which were subjected to a thermal treatment at 500 "C for 2 h. The catalyst thus produced had a specific surface area of 55 m2 g-' but did not present a very high resistance to sintering; after 24 h at 870 "C, its specific surface area was reduced to 14.7 m2 g-l. For the sake of comparison, the performances of various catalysts containing only Fe-Ni, Cr-Ni, or Cr-Fe are given below. These catalysts were prepared in exactly the same way as the Fe-Cr-Ni catalysts by impregnation on the same carrier A. Test Conditions and Analytical Procedure-Results Laboratory Tests. Table I lists the pertinent specifications and synthetic gas mixture compositions. Engine Tests. Engine: 4 cylinders, with fuel injection; equivalence ratio set to 1.05-1.10. The operational equivalence ratio of an engine, R , is defined as (mass of fuel)/(mass of air used) R= (mass of fuel)/(mass of stoichiometric air) Fuel: unleaded gasoline, sulfur content: 0.011%. The exhaust gases (23 Nm3/h) were passed through a 1-L bed of catalyst (axial catalytic muffler with 4 = 100 mm and height = 127 mm); the resulting GHSV was 22 OOO to 24 OOO h-'. The mean composition of the exhaust gases is given in Table 11. Analytical Procedure. Reactants and Reaction Products. The CO and C 0 2 contents were measured by nondispersive IR analyzers, NO by chemiluminescence
0196-4321/80/1219-0226$01.00/00 1980 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980 227 Table 11. Mean Composition of Exhaust Gases
____
co
1.8-2.3% 0.18-0.25% 0.05-0.08%a 0.55-0.75% 0.3% 12% 73% 12%
NO unburned hydrcicarbons H, 0,
Hi 0 N*
co,
a
Equivalent hexane .-
1
-
_-
-.-__
~rmsfwmea fraction
7
1 T 4 C Gas inlet
--.-A
~
700
Figure 2. Gross NO conversion (engine test bench) of some (Ni, Fe, Cr) mixed oxides.
can be obtained (curve d) from curves a and c. Here it increases monotonically as a function of temperature. The following relations link ?NO, rN2, 1 N H 3 , and S N O . T N ~= / /
/8-. 3w
4w
T,'C __L 500
800
Ga rid
1
700
Figure 1. Initial activity (engine test bench) of the mixed oxide phase NiFe2,,Cr,,,0,, 10% wt on alumina carrier A.
with 03,and unburned hydrocarbons by flame ionization detection. NH, was chemically titrated by reaction with HzS04. N 2 0 formation was also measured by chromatographic analysis, but the amounts detected were negligibly small. These results will not be reported here. Catalysts. The tiextural properties of the catalysts (specific surface area, porosity, bulk density, etc.) were determined by conventional methods (porosimetry, sorptometry, etc.). The X-ray diffraction diagrams presented here were made with i i Philips diffractometer (Anticathode Cu K a ) . Results. Figure 1gives typical curves for initial activity as a function of temperature, measured on an engine test bench with a mixed iron, chromium, nickel oxide catalyst. Gross NO conversion (into N2 + NH,, curve a) is plotted directly by comparing the NO contents measured a t the reactor inlet and outlet.
Ammonia yield, lN]
