J. Phys. Chem. 1990, 94, 6464-6467
6464
Eliminating m and n between eqs B1, B2, and B4a yields
- 18 I4 2 b - Z )
I36
037)
Since y and z represent integer coordination numbers, they must be equal to satisfy eq B7. The significance of this in reaction 18 is that one initially coordinated HA transfers its proton and provides the charge compensating anion on reduction of TH+. Substituting y = z into eq B1 yields n-m=-l
(B8)
This relation tells us that one water molecule is expelled from the film on reduction. Furthermore, by combining eqs 82, B6, and B8 and recalling that n cannot be negative
or m = 2 and y = l We can now write eq 20 as TH+A-&H,O)(HA)X + 2H3O+, + 2e = TH42'(A-)2,d(
(B9b)
+ 3Hz0, (B10)
where X may be either acetic acid (eq B9a) or water (eq B9b). Note that had we assumed a coordination number larger than 3, say 3 + K, then "X" would simply be replaced by UXK+ln.In other words, the model is blind to species not participating in the overall redox process. Registry No. Polythionine, 87257-37-2; acetic acid, 64-1 9-7.
Enhanced Oxygen Storage Capacity of Cerium Oxides in CeO2/La2O3/AI2O, Containing Precious Metals Takeshi Miki, Takao Ogawa, Masaaki Haneda, Noriyoshi Kakuta, Akifumi Ueno,* Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi 440, Japan
Syuji Tateishi, Shinji Matsuura, and Masayasu Sat0 Cataler Industrial Co., Chihama. Daitocho, Ogasagun, Sizuoka 437- 14, Japan (Received: January 22, 1990)
Effects of the addition of precious metals (PM; Pt, Rh) on Ce02/A1203and CeO2/LaZO3/Al2O3were confirmed to enhance the oxygen storage capacities (OSC). Increments in the OSC of the added CeO2/LazO3/AI2O3catalysts were much greater than those in the OSC of the PM added CeO2/Al2O3.The enhanced OSC is ascribed to the interaction between PM and a Ce02-La203solid solution formed during the catalyst preparation. No enhancements in the OSC were observed on physical mixing of CeO2/La2O3/AI2O3and Pt-Rh/A1203, although the composition ratio of PM:Ce02:La203is the same as that in the PM added Ce02/La203/A1203.This indicates that the intimate contacts between the precious metals and Ce02particles dispersed on A1203are essential for the enhanced OSC of cerium oxides.
Cerium oxide has often been dispersed in an automotive exhaust catalyst to prevent an active alumina, the catalyst carrier, from the thermal sintering.I The role of cerium oxide is not only the thermal stabilization of the active alumina but the extension of the "window" of the air/fuel ratio where the catalysts can work to reduce NO, and to oxidize C O and C,H, simultaneously.z Because of the low redox potential between Ce3+ and Ce4+ (1.7 V),3 CeOz dominates in the oxidative atmosphere, while in the reducing circumstances CezO3 becomes predominant: 2Ce0, F? Ce2O3 + '/202. Thus, according to a cyclic rich-lean composition fluctuation in the automotive exhaust gas, the cerium oxides can either provide oxygen for the oxidation of CO and C,H, or remove oxygen from the gas phase for the reduction of NO,.4 The amounts of oxygen reversibly provided in and removed from the gas phase are called "OSC" of ceria.s An enhanced OSC of ceria in CeO2/AI2O3has been reported when precious metals such as Pt, Pd, and Rh are highly dispersed on the catalyst.6 The enhanced OSC has been attributed to an
increase in the dispersion of ceria on alumina, caused by the PM addition. Recently, the interaction between precious metals and ceria in PM/CeO2/AI2O3catalysts has been studied by XPS to demonstrate that the precious metals reversibly facilitate the reaction CeA103 F? Ce02 A1203.6,7Jin et aI.* are of the opinion that a lattice oxygen and, conversely, an oxygen vacancy in a ceria crystallite play a substantial role in the oxidation of C O and the reduction of C 0 2 over Pt/Ce02 catalyst. Dissolution of La3+ ions into CeOz lattice to form a La202-Ce02 solid solution has been reported by Zintl and C r ~ a t t o . Miyoshi ~ et a1.I0 have also found the dissolution of La3+ ions into C e 0 2 lattice to be responsible for the enhanced activity of a PM/CeOz/La2O3/AlZO3catalyst for the simultaneous abatements of NO, CO, and C,H, at low temperatures.Il They have concluded that the enhanced activities are attributed to an accelerated diffusion of oxygen ions in the ceria-lanthana solid solution. In the present work, PM/CeO2/(La2O3)/AI2O3catalysts were prepared by two different methods. One is a subsequent impregnation of alumina with an aqueous solution containing Ce4+
( I ) Yu-Yao, Y. F.; Kummer, J. T. J . Caral. 1987, 106, 307. Su,E. C.; Rothchild, W. G. J . Catal. 1986, 99, 506. Su, E. C.; Montreuil, C. N.; Rothchild, W . G. Appl. Caral. 1985, 17, 75. (2) Herz, R. K. Ind. Eng. Chem. Prod. Res. Dew. 1981, 20, 451; 1983, 22, 387. Gandhi, H. S.: Piken, A. G.;Stepien, H . K.: Shelef, M.; Delosh, R. G.; Heyde, M. E. SAE Preprint 770196, Detroit, MI, 1977. Kim, G. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 267. ( 3 ) Kagaku-Binran,Kisohen II; Japanese Chemical Society, Ed.; Maruzen: Tokyo, 1984; p 11-476 (in Japanese). (4) Loof, P.; Kasemo, B.; Keck, K.-E. J . Catal. 1989, 118, 339. (5) Yao, H . C.: Yu-Yao, Y. F. J . Catal. 1984, 86, 254.
(6) Shyu, J . Z.; Otto, K. J . Catal. 1989, / I S , 16. Shyu, J. Z.; Otto, K.; Watkins, W. L. H.; Graham, G.W.; Belitz, R. K. J . Caral. 1988, 114, 23. (7) Mizuno, M.; Berjoan, R.; Coutures, J. P.: Ferox, M.; Yogyo-KyokaiShi 1975, 83, SO. (8) Jin, T.: Okuhara, T.; Mains, G.J.; White, J. M. J . Phys. Chem. 1987, 91, 3310. (9) Zintl, E.; Croatto, U . Z . Anorg. Allg. Chem. 1939, 242, 79. (IO) Miyoshi, N.; Matsumoto, S.; Kimura, M.; Muraki, H. Preprint in Meeting (A) of Catalysis Society of Japan, 4E07: 1987 (in Japanese). ( 1 !) Ozawa, M.; Kimura, M.; Miyoshi, N.; Matsumoto, S. Preprint in Meeting (A) of Catalysis Society of Japan, 4B222; 1988 (in Japanese).
Introduction
+
0022-3654/90/2094-6464$02.50/0 0 1990 American Chemical Society
Oxygen Storage Capacity of Cerium Oxides
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6465
I
0 2
0.1
L.in1
Lan t 11 a n a con t (',i 1
Figure 1. Change in the lattice constant of Ce02 in Ce02/La203/A1203 catalyst with the change in La203content. La203contents are expressed in terms of La203/(La203 + Ce02).
i.in,i
0.3
0 1
I1 i
c o . i i t in!
Effects of additional La203upon the OSC of Ce02/A1203and PM/CeO2/AI2O3catalysts. The OSC of both catalysts were observed at 500 ( 0 ,0),700 (aO ) , and 900 OC (A,A). The solid marks are for PM/CeO2/AI2O3and open marks for Ce02/AI,03 catalysts. Figure 2.
ions and then with that containing both Pt2+and Rh3+ ions. The other method is a mixing of CeOz/(La2O3)/AI2O3with PM/AI2O3 so that the PM:CeO2:(La2O3) ratio is same as the ratio in the impregnated catalyst. Study on the differences in the OSC of these catalysts would lead us to the discussion of actual interactions between the precious metals and Ce0, or Ce02-La203, since intimate contacts between PM and CeO, or Ce02-Laz03 are expected in the former but not in the latter. Experimental Section 1 . Cafalyst Preparation. Alumina powder (168 m2/g of BET surface area) was immersed in an aqueous solution of cerium or cerium-lanthanum mixed nitrates, followed by drying and calcining at 500 OC in air. The catalysts, thus prepared, contained 20 wt % lanthanide oxides (CeO, + La203)with the Laz03/(Ce02 + La2O3) ratios of 0,O. 1,0.25,0.49, and 1.O. The catalyst powders were immersed again in an aqueous solution of H4PtC16-RhC13 mixture, followed by drying and calcining at 500 OC for 4 h. Pt and Rh loadings on the catalysts prepared here were 1.0 and 0.2 wt %, respectively. The catalysts thus produced are called PM dispersed (or added) catalysts below. A Pt (1 .O wt %)-Rh (0.2 wt %)/A1203 catalyst was also prepared by immersing the alumina powder in an aqueous solution of H4PtC16-RhC13 mixture. This catalyst was physically mixed with the CeO2/AI2O3and/or the Ce0,/Laz03/A1203 with 1:l weight ratio so that the composition ratio of PM:Ce02:(La203) is the same as that in the PM added catalyst. The formation of solid solution between CeO, and Laz03 in the CeO2/LazO3/AI2O3catalysts was measured by X-ray diffraction spectroscopy, monitoring the shifts of diffraction peak assigned to CeO, (31 1). 2. Estimation of OSC. The OSC of the catalyst was estimated following Yao and Yu-Yao.S Briefly, a certain amount of the catalyst (200 mg) was placed in a quartz reactor, followed by heating at 500 OC in flowing He. Then, 1 vol % 0 2 / H e was iteratively pulsed at a desired temperature of 500, 700, or 900 OC, until no more loss of 0, injected was detected. When the oxidation of the catalyst was completed, cyclic pulses of 5 mL of 1 vol % Oz/He and 10 mL of 1 vol 7% H,/He were consecutively injected into the catalyst, until an apparent equilibrium breakthrough for both gases was established. Several times of the experimental runs were carried out for one sample to determine the OSC of the catalyst. 3. XPS Measurements. XPS (X-ray photoelectron spectroscopy) spectra were recorded on a VG Scientific ESCA Lab-5 Mark I I spectrometer with monochromatized Mg Ka X-rays. All the samples were exposed to air, while they were transferred to
Illll,llli:
l
