Znd. Eng. Chem. Res. 1996,34, 1929-1932
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Ruthenium Perovskite Catalysts for Lean NO, Automotive Emission Control R. Bradowt Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695
D. Jovanovi6,* S. Petrovi6, 2.Jovanovi6, and A. Terlecki-BariEevi6 Department of Catalysis and Chemical Engineering, Institute of Chemistry, Technology and Metallurgy, Njegozeva 12, 11000 Belgrade, FR Yugoslavia
Perovskite-type catalyst samples La,Srl-,Ru,Cr~-,O3 0, = 0.7; 0.025 Ix 5 0.100), synthesized by the ceramic method, were used in activity measurements in the simultaneous conversion of CO, hydrocarbons (HC), and NO, in the pulse-flame catalyst testing (PFCT) system and synthetic dry gas mixtures. In the presence of water vapor, in the reaction mixture (PFCT system), a n unusually high level of CO oxidation and a higher concentration of H2 detected in the outlet reactor gases, compared to the inlet reactor gases, can be ascribed to a water gas shift reaction. The generated hydrogen, present on the catalyst surface in its dissociated form, could be responsible for the higher NO, conversion under net oxidizing (lean) conditions, which favors ruthenium synthesized catalysts as three-way auto exhaust catalysts. 1. Introduction
In comparison with other noble metals and transition metal oxides, ruthenium has the unique ability to selectively catalyze the reduction of NO, by hydrogen and CO toward the formation of nitrogen rather than ammonia. This property makes metallic ruthenium a desirable catalyst for the control of NO, in automotive exhaust gases (Taylor, 1975; Shelef and Gandhi, 1972; Taylor, 1984). The applicability of Ru catalysts to exhaust gas purification is limited by the formation of volatile and toxic oxides, RuO3 and Ru04, under oxidizing conditions (Bell and Tagami, 1963). Several procedures have been proposed for stabilizing Ru in an oxidizing atmosphere. One among them is the incorporation of Ru in the matrix of perovskite-type oxides. Studies on slightly decreased losses of Ru when incorporated in the form of ruthenates, MeRuO3, have been reported (Kobylinsky et al., 1974; Shelef and Gandhi, 1974a; Gandhi et al., 1975), but this is still unsatisfactory for the use of Ru in oxidizing conditions. Another matter of particular interest for the use of Rucontaining perovskites in exhaust gas purification is their resistance to sulfur poisoning, especially under reducing conditions. Therefore, it is obviously necessary to select a perovskite matrix with a low susceptibility to SO, for Ru incorporation, but data reported on this topic are rare. In our recent studies on the effect of sulfur oxides on the activity of monophase perovskites of LaMeO3 and La,Srl-,CrO3 (Me = Co, Cu, and Cr; y = 0.7) type in CO and hydrocarbon (HC) oxidation and NO, reduction (JovanoviC et al., 1991) it has been shown that only LaCrO3 and Lao.7Sr0.3Cr03maintain an almost unchanged initial activity in the presence of sulfur oxides.
* Author to whom correspondence should be addressed. t Deceased.
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By partial substitution of Cr for Ru in the Lao.7Sro.3CrO3 matrix, a series of Lao.7Sr0.3Ru,Crl-,03 (0.025 5 x 5 0.100) perovskites was synthesized (JovanoviC et al., 1994). SEM-XRF analyses confirmed that the incorporation of Ru in a perovskite matrix prevents the volatilization and agglomeration of Ru particles up to 1270 "C in air (Terlecki-BariEeviC et al., 1991). The activity of synthesized perovskites in the simultaneous conversion of carbon monoxide, hydrocarbons, and nitrogen oxides under conditions of controlled gas composition near the stoichiometric ratio of oxidizing to reducing agents showed an unexpectedly high conversion of NO, in comparison with Pt-Rh catalysts under net oxidizing conditions (JovanoviC et al., 1991). The objective of the present study was to follow the influence of water, present in the reaction mixture, on the simultaneous oxidation of CO and HC and reduction of NO, on synthesized ruthenium perovskites.
2. Experimental Methods 2.1. Catalysts. Perovskite catalysts Lao.7Sro.3Ru,Crl-,O3 with 0.025 I x I 0.100, synthesized by the ceramic method, were used (JovanoviC et al., 1994). Surface areas of all synthesized catalysts were about 1.0 m2/g. In the following figures all synthesized ruthenium perovskite catalysts are denoted according t o the appropriate mole fractions, i.e., Ruo.025presents Lao.7Sro.3Ruo.o2~Cro,g~503, etc. The X-ray diffraction (XRD)analysis (Philips diffkactometerPW 1710)showed that all samples have the perovskite structure with a minor presence of the SrCrOs phase. The amount of metals determined by the X-ray fluorescence (XRF) analysis (System Cambera, Model 73333) of samples calcinated at 1200 "C corresponded to the stoichiometric values of the proposed chemical composition (JovanoviC et al., 1994). The XPS (X-ray photoelectron spectros-
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copy) measurements were recorded on a VG Escalab I1 spectrometer using Mg& radiation (1253.6 eV) at a pressure of about 4 x Torr (1Torr = 133.322 Pa) and a 150" spherical sector analyzer ensuring resolution of the Ag 3ds12 line at 0.9 eV. Quantification was achieved by multiplexing all observable elemental core levels and applying a cross-section correction to the integrated intensities. 2.2. The Three-way Activity Measurements. The pulse-flame catalyst testing (PFCT) system, which has been described previously (Otto et al., 1974;PetroviC et al., 1982; JovanoviC et al., 1984; Terlecki-BariEeviC et al., 19841, was used for three-way activity. This system simultaneously promotes catalytic measurements of the reduction of nitrogen oxides and the oxidation of carbon monoxide and hydrocarbons. The investigation of the activity of ruthenium perovskites was performed in a gas mixture obtained by pulseflame combustion (PFCT system) of 2,2,4-trimethylpentane (isooctane)containing about 11vol % H2O and also in a dry synthetic gas mixture. The inlet concentration of the pollutants after isooctane combustion was kept constant during a single run with variations less than 10%. The reactor inlet and outlet concentrations of H2, CO, 0 2 , N2, and HC were analyzed with an on-line gas chromatograph Shimadzu, Model GC-14A/C-R5A. Nitrogen oxides (NO,) of inlet and outlet gases from the catalytic reactor were analyzed with an on-line NO, analyzer Thermo Electron Corp., Model 44. The main composition of the wet exhaust (PFCT system), depending of the run, varied within the following ranges: 2.02.5% CO, 0.6-0.8% H2,150-200 ppm NO,, 1400-1500 ppm C1 HC, 1.2-2.2% 0 2 , and the rest N2 and C02. Concentrations of the components in the synthetic gas mixture were adjusted to the average pulse-flame exhaust. The redox potential ( R )(Gandhi et al., 19761,directly related to the AIF ratio, was used as the measure of the exhaust stoichiometry. The redox potential values represent the ratio of the sum of the equivalent reducing agent concentrations (CO, H2, and HC) to the sum of the oxidizing agent concentrations ( 0 2 and NO,), and it was adjusted t o the desired value by adding oxygen at the inlet of the catalytic reactor, as required by the following expression:
where CCO,CH*,CHC,Co2,and CNO,denote the concentrations (vol %) of CO, H2, HC, 0 2 , and NO,, respectively. The hydrocarbon (HC) concentration is calculated on methane (C1). In the synthetic gas mixture, hydrocarbons are represented as propylene (&He), whose concentration is recalculated to CI. Thus R = 1 represents a stoichiometric gas mixture, R > 1 represents an overall reducing gas mixture, and R < 1 an overall oxidizing gas mixture. All three-way runs were carried out at a space velocity of about 33 000 h-l and a temperature of 550 "C, using 4.6-4.9 g of the catalysts of 1.08-1.50 mm particle size. The temperature of 550 "C was chosen as the average temperature in automotive catalytic converters. 3. Results and Discussions
The activity of La0,7Sro,3Ru,Cr1-,0~(0.025 I x I 0.100) perovskites in the oxidation of CO and HC in feed
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redox potential (R) Figure 2. (a, top) HC,, conversion vs redox potential (PFCT system). Legend: 0,Ru 0.025; 0, Ru 0.050; A , Ru 0.075; Is, Ru 0.100. (b, bottom) C3Hs conversion vs redox potential (dry system). Legend: 0, Ru 0.025; 0 , Ru 0.050; A , Ru 0.075; Is, Ru 0.100.
streams from the PFCT system obtained by the combustion of isooctane and in synthetic dry gas mixtures, as a function of the redox potential (R), is given in Figures 1 and 2, respectively. The significantly higher conversion of CO under net reducing conditions (R > 1) observed from the PFCT system (a wet gas mixture with about 11 vol % H2O) from Figure l a , with respect to those observed in synthetic dry gas mixture (Figure lb), could be attributed to the contribution of the water gas shift
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redox potential (R) Figure 3. Comparison between inlet and outlet H2 concentration vs redox potential for PFCT system and dry system. Legend: 0, inlet concentration of H2 (PFCT system); 0, outlet concentration of Ha (PFCT System); W, inlet concentration of H2 (dry system); 0, outlet concentration of Ha (dry system).
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redox potential (R) Figure 4. (a, top) NO; conversion vs redox potential (PFCT system). Legend: 0, Ru 0.025; 0,Ru 0.050; A, Ru 0.075; a, Ru 0.100. (b, bottom) NO, conversion vs redox potential (dry system). Legend: 0, Ru 0.025; 0,Ru 0.050; A, Ru 0.075; a, Ru 0.100.
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(WGS: CO H20 CO2 H2) reaction to the overall conversion of CO. The conversion of unsaturated hydrocarbons ,(Figure 2a) in the PFCT system and the conversion of propylene in synthetic dry gas mixtures (Figure 2b) on Lao.7Sro.3RuxSr1-,03 samples, with x > 0.025, exceeded 80% in the entire range of the redox potentials. The fact that the presence of water vapor has only a slight effect on the conversion of HC implies that the steam-reforming reaction (HC H20 CO2 H2) proceeds much slower than the total catalytic oxidation of hydrocarbons. It appears that the main source of hydrogen is the WGS reaction. The total HC conversion on ruthenium perovskite catalysts is apparently low (less than 50%)as a result of the high saturated hydrocarbons fraction (about 63 ~ 0 1 %in ) the total HC flame emission on the PFCT system (Jovanovid et al., 1991). The results obtained in the kinetic regime also show an increasing HC oxidation activity with an increasing content of
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ruthenium in the perovskite. The atomic concentration of Ru on surface layers of the samples, based on XPS data, increases proportionally with the increase of its concentration in the bulk. The binding energy for Ru~d (464.3 eV) is virtually the same in all samples, and it is consistent with that of R u + ~ (Petrovid, 1995). Hence, the oxidation activity could be attributed to Ru in an ionic form, but the participation of adsorbed oxygen and lattice oxygen from the perovskite matrix should not be excluded (Seiyama, 1992). Differences in catalytic activities for 0.050 Ix I0.100 are very small because of diffusion limitation at the applied temperature of 550 "C. Figure 3 shows a comparison between the H2 inlet and Ha outlet concentrations at different redox potentials in the two investigated systems, i.e., in synthetic dry gas mixtures and in the PFCT system. In the case of the PFCT system, under highly reducing conditions up to R > 1.5, the outlet hydrogen concentration significantly exceeds the hydrogen inlet concentration, indicating that the rate of H2 formation is considerably faster than its consumption in the oxidation process and in the reduction of NO,. The amount of Ha generated by the WGS reaction decreases at lower R, where CO is largely consumed in direct oxidation. Also, with increasing partial pressure of oxygen (lower R),the rate of H2 oxidation is increased leading to its complete disappearance near to the stoichiometric condition in the synthetic gas feed stream. However, some hydrogen is detected under stoichiometric and net oxidizing conditions in the outlet from the PFCT system. This observation suggests that the WGS reaction still proceeds, which could be supported by the somewhat higher conversions of CO in comparison with those observed in synthetic gas mixtures at the corresponding R values. Hydrogen generated by the WGS reaction is present on the surface of the catalysts in its dissociated form before being desorbed (Shelef and Gandhi, 197413). This surface hydrogen could be the reason for the higher conversion of NO, under net oxidizing conditions, obtained in the PFCT system on all Lao.7Sro.3Ru,Crl-,03 perovskites (Figure 4a) with respect to those obtained in dry gas mixture (Figure 4b). On the basis of published data (Shelef and Gandhi, 1974b), the rate of NH3 formation on ruthenium catalysts decreases with increasing temperature. At the temperature of 550 "C (the temperature applied in our experiments) ammonia is formed only to a minor extent. However, the participation of hydrogen in the reduction of NO, in the competitive system where both CO and Ha are present increases with increasing temperature. According to Shelef end Gandhi (197413) this is particularly pronounced in cases where the WGS reaction proceeds. Therefore, we assume that under the experimental conditions applied in this investigation, the formation of ammonia on ruthenium perovskites has only a marginal effect on NO, conversion. 4. Conclusion
The published data (Shelef and Gandhi, 197413)favor Ru as a three-way auto exhaust catalyst due to its high NO, activity and low ammonia formation. In addition, according to our experimental results, ruthenium in the Lao.7Sro.3Ru,Crl-,03 (0.050 Ix I 0.100) perovskites catalyzes the WGS reaction which plays an important role in the high conversion of CO under net reducing conditions. The hydrogen generated by the WGS reac-
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tion could be the reason for the extended NO, reduction under net oxidizing (lean) conditions.
Acknowledgment This work was supported by the US-Yugoslav Joint Board on Scientific and Technological Cooperation (Grant EPA 885).
Nomenclature AIF = air-fuel ratio PFCT = pulse-flame catalyst testing R = redox potential SEM-XRF = scanning electron microscopy-X-ray fluorescence XPS = X-ray photoelectron spectroscopy XRD = X-ray diffraction XRF = X-ray fluorescence WGS = water gas shift Literature Cited Bell W. E.; Tagami M. High-Temperature Chemistry of the Ruthenium-Oxygen System. J . Phys. Chem. 1963, 67, 24322436. Gandhi, H. S.; Stepien, H. K.; Shelef, M. Durability Testing of Stabilized Ru-containing Catalysts. SAE Pap. 1975,No. 750177, 9 PP. Gandhi, H. S.; Piken, A. G.; Shelef, M.; Delosh, R. G. Evaluation of Three-way Catalysts. SAE Pap. 1976, No. 760201, 10 pp. JovanoviC, D.; Terlecki-BariEeviC,A.; PetroviC, K. Modified Apparatus for the Simulation of Engine Exhaust Emissions. Goriva Maziua 1984,23, 33-38. Jovanovid, D.; Dondur, V.; Terlecki-BariEeviC, A.; GrbiC, B. Threeway Activity and Sulfur Tolerance of Single Phase Perovskites. Studies in Surface Science and Catalysis, “Catalysis and Automotive Pollution Control-CAPoC ZI”, Crucq, A.; Frennet, A,, Eds.; Elsevier: Amsterdam, 1991; Vol. 71, pp 371-379. JovanoviC, D.; Terlecki-BariEeviC, A.; GrbiC, B.; Bradow, R. Ruthenium-Containing Perovskite Materials, Catalysts and Methods. US.Patent 5,318,937, June 7, 1994.
Kobylinsky, T. P.; Taylor, B. W.; Young, J. E. Stabilized Ruthenium Catalysts for Nitrogen Oxide (NO,) Reduction. SAE Pup. 1974, No. 740250, 7 pp. Otto, K.; Dalla Betta, R. A.; Yao, H. C. A Laboratory Method for the Simulation of Automobile Exhaust and Studies of Catalyst Poisoning. J . Air Pollut. Control Ass?c. 1974,24, 596-600. PetroviC, K.; Putanov, P.; JovanoviC, Z.; Terlecki-BariEeviC, A.; JovanoviC, D. EPA Project “Reduction of CO and HC Auto Emissions”. Final Report EPA-JF2-570-6, 1982. PetroviC, S. The Role of Ru in the Perovskite-Type Catalysts in the Oxido-Reduction Processes. Master Dissertation, PMF Faculty of Physical Chemistry, Belgrade, 1995. Seiyama, T. Total Oxidation of Hydrocarbons on Perovskite Oxides. Catal. Rev. Sci. Eng. 1992, 34, 281-300. Shelef, M.; Gandhi, H. S. Ammonia Formation in Catalytic Reduction of Nitric Oxide by Molecular Hydrogen. 11. Noble metal catalysts, Znd. Eng. Chem. Prod. Res. Dev. 1972,11,393396. Shelef, M.; Gandhi, H. S. Reduction of Nitric Oxide in Automobile Emissions. Stabilization of Catalysts Containing Ruthenium. Platinum Met. Rev. 1974a, 18, 2-14. Shelef, M.; Gandhi, H. S. Ammonia Formation in the Catalytic Reduction of Nitric Oxide. 111. The Role of Water Gas Shift, Reduction by Hydrocarbons, and Steam Reforming, Znd. Eng. Chem. Prod. Res. Dev. 1974b, 13, 80-85. Taylor, K. C. The Catalytic Chemistry ofNitrogen Oxides;Klimisch, R. L., Larson, J. G., Eds.; Plenum: New York, 1975. Taylor, K. C. Automobile Catalytic Converters; Springer-Verlag: Berlin, 1984. Terlecki-BariEeviC, A.; JovanoviC, D.; PetroviC, K. Catalysts Efficiency Investigation in the Laboratory Conditions. Motori Motorna Vozila 1984, X-56/57,310-317. Terlecki-BariEeviC, A.; JovanoviC, D.; GrbiC, B.; PetroviC, S. EPA Project “Perovskite-Catalysts for Exhaust Gas Purification”. Annual Progress Report EPA-JF-885, (1991).
Received for review October 20, 1994 Accepted March 6, 1995 @
IE940607M Abstract published in Advance ACS Abstracts, April 15, 1995. @
