The Journal of
Physical Chemistry VOLUME 97, NUMBER 8, FEBRUARY 25,1993
Q Copyright 1993 by the American Chemical Society
LETTERS Effects of Magnetic Field on Catalytic Activity of CO Oxidation and 0 2 Adsorption over Pei-yan Lin,' Jian Peng, Bi-hui Hou, and Yi-lu Fu Department of Modern Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, Anhui, People's Republic of China Received: July 30, 1992; In Final Form: December 29, 1992
Paramagnetism was exhibited on the samples Lno.+ro.3Mn03 (Ln = Ce, Pr, Nd, Sm, Gd,Tb, Dy, Ho,and Er), and ferromagnetism was presented on the La0.7Sr0.3MnO3at room temperature. In an external magnetic field (40 and 80 mT), the increase of CO conversion percent was found at 200 and 300 OC separately, except on the sample containing Ce. When x2I3X SA (x is the specific susceptibility and SA the specific area) is plotted against A.4 (A.4 is the increment of adsorption oxygen in 80 mT magnetic field at 200 "C), AA increased as x2I3 X SA increased.
Introduction Theeffects of the external magnetic field on chemical reactions, such as photochemical reaction,l-3 polymeri~ation,~isotopic enrichment,s and electrochemicalreactions? have been extensively studied. In general, as for the reaction molecules which have radicals containing unpaired electrons, their magnetic moments were not zero and were influenced by an external magnetic field. It was also said that for the reaction molecules having zero-field splitting (such as triplet state molecules) by the magnetic field, a change of system entropy was observed. This could result in a change of the rate of reaction. But the effects of the magnetic field on the heterogeneous catalytic reaction of CO oxidation, which did not belong to radical mechanism, have been rarely studied. In addition, the chemisorption characteristics were closely related to the magnetism of the catalyst itself. Because the d holes of transition metals could interact strongly with oxygen, this interaction has a profound influence on surface magneti~m,~J as well as on catalytic oxidation a ~ t i v i t y . ~ It was of interest to study the effects of an external magnetic field on 0 2 adsorption and CO oxidation activity over paramagnetic catalysts which possess d states or f states. In this paper, the results of a preliminary study of the magnetism of composite metal oxides, the effects of external magnetic field on CO conversion percent, and the oxygen adsorption are presented and discussed. The composition of the catalysts studied was Ln0.70022-3654/93/2097-1471$04.00/0
Sro.3Mn03, which contains both d states (manganese ions) and f states (rare earth ions; Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho,and Er). Experimental Work The samples were prepared in the conventional way of coprecipitation. Calcination was carried out at 1100 OC for 2 h. XRD (X-ray diffraction) patterns showed that all the samples, except C Q . ~ S ~ ~ . ~belong M~O to the ~ , perovskite type structure.I0 The structure of CQ,$ro,jMnO3 was mixed single metal oxides. The specific surface areas were determined by means of a SP-03 BET surface meter in a flowing system. The specific susceptibility was measured by means of a Model 9500 vibration sample magnetometer at 20 OC. EPR (electronic paramagnet resonance) spectrawere measured by means of an ER-2OOD-SRC type EPR meter with an X-band frequency (0.978 mT) corresponding to a resonant magnetic field of 0.347914 T f 0.018 mT. CO conversion percent on catalysts was determined in a flow microreactor in zero magnetic field and in 40 f 5 and 80 f 5 mT of magnetic field. The reaction conditions were adopted with 4 0 6 0 mesh samples of 200 mg, SV = 4600 mL/(g h), and the reactantsof 1.l%CO,3.5%02,and therestN2. Themeasurement of errors of CO conversion percent was f0.596. The amount of oxygen adsorption was measured in a flowing system in zero magnetic field and 80 mT magnetic field separately 0 1993 American Chemical Society
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1472 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993
TABLE I: Specific Susceptibility ( x ) of Samples' sample x (4r X 10-'/kg) P2 (room temp)" Ceo 7Sr0 3MnOl Pro $30 3MnO3 Ndo 7Sro 3MnO3 Smo $30 3MnO3 Gdo7Sr03MnO3 Tbo 7Sr03Mn03 DYO7% 3MnO3 Hoo 7Sr0 N n O 3 Ex0 $30 3Mn03 (1
0.171 1.010 1.640 0.865 0.781 1450 1.560 1.560 1.210
5.8 12.3 12.3 2.3 64.0 90.3 112.3 108.2 90.3
g,,,
2.709 2.021 2.765 1.947 2.057 2.069
P is the effective Bohr magneton for related rare earth ions.
at 200 OC. Prior to adsorption with oxygen, helium gas (CIG, 99.999%) was passed over the sample at 500 OC for 2 h, and then a known volume of 02 was injected onto the sample repeatedly at the same temperature, until the area of the peaks did not change. The adsorption amount A was calculated from the total reduction of peak areas with measurement errors of A f 0.3 ccmol/g.
Result9 Data for the specific susceptibility x and the Lande factor g in EPR are listed in Table I ( x = AM/", where M is the magnetization, H the external magnetic field, and m the mass of samples). It was found that x was a constant at different H, except for the sample of Lao 7Sro 3MnO3. It was also found that x responded to the square of the Bohr magnetons of rare earth ions," except for the Smo7Sr03Mn03. Although the "chemical environment" of rare earth ions in Lno 7Sr03Mn03was different from the single rare earth ions, g data varied only a little. Both the data of x and g were identified from the samples of Lno 7Sro ,Mn03 (Ln = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er) that exhibited paramagnetism at room temperature. It is interesting that a small hysteresis loop was revealed on the sample of Lao7Sro3Mn03. The remaining magnetism was equal to 9.28 X lo3/ (kg T), and the coercive force was 12.0mT. The x varied with external magnetic field (H= 81 mT, x = 215 X 103/(kg T), H =600mT,x = 5.79 X 103/(kgT)). Thespinstatesofmanganese ions and their strong exchange interactions with neighboring oxygen might be responsible for the parasitic ferromagnetism of Lao7Sro ,Mn03. But it still needs to be studied why Lao7Sro 3MnO, showed ferromagnetismandothen showed paramagnetism. CO conversion percent in a magnetic field of 40 and 80 mT at 200 and 300 OC is shown in Figure 1 and Figure 2,respectively. The fluctuating pattern of CO conversion percent was revealed and corresponds to the increase of atomic weight of rare earths. Under zero magnetic field, the catalytic activity was better on the samples containing Ho, Nd, and Tb at 200 OC, while at 300 OC the catalytic activity was better on the samples containing Ce, Tb, and Gd. It is observed that a large increaseof CO conversion percent was presented on the samples which contained Ho,Nd, and Tb in external magnetic field at 200 OC. On the sample Tbo 7Sro 3Mn03,the CO conversion percent increased about 14% in a magnetic field of 80 mT at 200 and 300 OC. There was almost no change of CO conversion percent on the sample CQ 7Sio3Mn03in an external magnetic field. The reason might be due to the structure of mixed simple oxides of this sample. But the CO conversion percent obviously increases with the change of reaction temperature from 200 to 300 OC. It was considered that its lattice oxygen may be more active than that in other samples at 300 OC.12 In general, the main reaction mechanism of CO oxidation was the Eley-Rideal mechanism-the adsorbed oxygen species (02-, 0-) reacted with CO in the gas phase at low temperature (-200 "C) on perovskite catalysts containing manganese ions." Thus, the determination of adsorbed oxygen was more important than thatofadsorbedcoat 200OC. Basedon theaboveconsiderations, the amount of adsorption oxygen in nonmagnetic field and 80
a?
t
8
I
I
I
I
1
I
I
I
1
Pr Nd Sm Cd Tb Dy Ho Er Sample8 &.~Srt,.~MnOs(Ln=rareearths) Figure 1. CO conversion percent in magnetic field of 40 and 80 mT at 200 O C : case A, nonmagnetic field; case B, in magnetic field of 40 mT: case C,in magnetic field of 80 mT.
La
Ce
100 I I
* Case B Case A
-E-
f
8 t 01
La
I
Ce
4
Pr
I
1
I
Nd Sm Cd
I
Tb
,
I
Dy
Ho
Er
earths) Figure 2. CO conversion percent in magnetic field of 40 and 80 mT at 300 "C: case A, nonmagnetic field; case B, in magnetic field of 40 mT; case C,in magnetic field of 80 mT. Samples h.,Srt,.&Xb(Ln=rare
mT of magnetic field at 200 OC was measured and listed in Table 11. It was indicated that the increment of adsorption oxygen (AA) increased to some extent for the samples except the samples containing La and Ce. The remaining magnetism still existed on used ferromagnetic La0,7Sro.~MnO3,even in a nonexternal magnetic field. Since the increment of adsorption oxygen (AA) is related to the surface magnetism ( ~ ~ 2 1 and 3 ) specific surface area (SA) of the samples at 200 OC, an exponential relation between x213 X SA and AA should be found, if x213 X SA is plotted against AA. It was shown that as x 2 I 3X SA increased, AA increased too. While the two separate linear relations crossed, at seven 4f electronsthey emerged as shown in Figure 3. According to Hund's
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The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1473
TABLE Ik Adso tion Amount of Oxygen over Samples h . f i r O . s M d ) B at 00 O C
P
sample
specific surf. area (m2/a) 4.3 3.1 3 .O 2.5 3.4 2.6 6.8 5.2 4.1 5.6
-13-
adsorption amount of oxygen (Mmol/g) absence of magnetic field magnetic field of 80 mT 33.1 24.5 23.5 19.7 25.6 24.6 45.8 31.9 31.5 28.5
33.4 25.1 26.5 24.4 29.5 26.6 53.2 36.3 34.6 32.1
rule, when the rare earth ions possess electrons 6 7 in 4f orbitals (Ln = Ce, Gd, Pr, Sm, and Nd), all the 4f electrons are unpaired and they are in parallel spin states. However, when the rare earth ions possess 4f electrons >7 (Ln = Tb, Dy, Ho,and Er), both unpaired and paired 4f electrons exist. The paired electrons are in antiparallel spin states. Therefore, the effects of the "magnetic environment" might be quite different between those two groups of samples.
ConClImioa The conversion percent of CO oxidation and the amount of 02 adsorption over the Lno,7Sro.3Mn03catalysts increased to some extent except for the sample containingCe in an external magnetic field at 200 and 300 OC. The samples with Ln = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er exhibit paramagnetism, and the sample with Ln = La showed ferromagnetism at room temperature. When x*I3X SA is plotted against M,two separate linear relations are found.
Acknowledgment. This work was supported by the National Science Foundation of China.
References and Notes ( I ) Hata, N. Bull. Chem. Soc. Jpn. 1986, 59, 2723. (2) Tanimoto, Y.; Udagawa, H.; Itoh, M. J . Phys. Chem. 1983,87,724.
:
eleclronss7 in4f orbits
+ : e1ectrons>i'in4torbits
h
.i -0
/ 2
4
6
8
10
xY3xSA Figure 3. xZ/' X SA-AA relations for samples Ln0.7Sro.3MnO1. x is the spxificsusceptibilityat 20 "C,SAthesptcificarcas,andAA theincrement of adsorption oxygen at 80 mT magnetic field at 200 O C . (3) Ferraudi, G.; Arguello, G. A.; Frik, M. E. J. Phys. Chem. 1987, 91, 64. (4) Turro, N. J.; Chow, M. F.; Chung, C. J.; Tung, C. H. J. Am. Chem. SOC.1983, 105, 1572. (5) Turro,N. J.; Chow, M. F.; Chung, C. J.; Kracutler, B. J. Am. Chem. Soc. 1981, 103, 3886. (6) Fahidy, T. 2.J . Electrochem. Soc. 1983, 130, 297. (7) Johnson, P.D.; Clarke, A.; Brooke, N. B. Phys. Reo. Lori. 1988,61, 2257. (8) Schonhense, G.; Donath, M.; Kolac, V.;Dose, V. Surf. Sci. 1988. 206, L888. (9) Lin, P.Y.; Chen, Y.; Yu,S. M.; Fu, Y. L.; Mizuno, N.; Misono, M. J. Catal. (Chin.) 1991, 12, 193. (10) Yu, M.; Lin, P. Y. Chem. J. Chin. Unio. 1986, 7, 634. (1 1) Kittel, C. Introduction of Solid Stare Physics; John Wiley 8 Sons: New York, 1976. (12) Harrison, B.; Biwell, A. F.; Hallet, C. Platinum Mer. Reo. 1988,32, 73. (13) Fu, Y. L.; Lin, P.Y. J . Catal. (Chin.) 1B2, 3, 822.
