-U292 PREPRRRTION RUO CHAkRRCTERIZRTION OF COII)/ZR02 SOLID /SOLUTION(U) BROW UNIV PROVIDENCE RI DEPT OF CHEMISTRY

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OFFICE OF NAVAL RESEARCH

CONTRACT N00014-86-K-0234

TECHNICAL REPORT No. 9

Preparation and Characterization of Co(II)/ZrO2 Solid Solution

by "0N Ping Wu, Robert Kershaw, Kirby Dwight and Aaron Wold000Prepared for Publication

in the

Materials Research Bulletin pELECTE:

Brown UniversityDepartment of Chemistry,Providence, RI 02912

October 27, 1987

Reproduction in whole or in part is permitted forany purpose of the United States Government

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11 TIT..E ;ncituoe Security Classilication)PREPARATION AND CHARACTERIZATION OF Co(II)/ZrO2 SOLID SOLUTION

12 PC;SO\AL AUTHOR(S)

!3a. 7YPE ~~13b. TiME COVERED 1(- 6 f R ,1Jg ,,orLa) 5PC O Nff3i&. I FROM ____TO___

16 SuPPLEIENTARY NOTATION

ACCEPTED FOR PUBLICATION IN MATERIALS RESEARCH BULLETIN

17 COSAri CODES 18 SUBj"CT TERMS (Continue on reverse itf necessary and identity by block number)

F;ELD GROUP SUB-GROUP

19 ABSTRACT (Continue on reverse f'necessary and identity by block number)

Samples of cubicZrO2 containing up to almost 10 atomic percent cobalt wereprepared by codecomposition of the nitrate. Magnetic susceptibility measurementsconfirmed the limit of solubility and the presence of cobalt as Co(II). Thereduction of the cobalt inserted in cubic zirconia took place at a considerablyhigher temperature than bulk cobalt oxide.

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PREPARATION AND CHARACTERIZATION OF Co(II)/ZrO2 SOLID SOLUTION

byPing Wu, Robert Kershaw, Kirby Dwight and Aaron Wold*

Department of Chemistry, Brown UniversityProvidence, RI 02912 . .

*Address all correspondence

ABSTRACT /Samples of cubicjZrO 2 containing up to almost 10 atomic percentcobalt were prepared by codecomposition of the nitrate. Magneticsusceptibility measurements confirmed the limit of solubility and thepresence of the cobalt as Co(Il). The reduction of the cobalt insertedin cubic zirconia took place at a considerably higher temperature thanbulk cobalt oxide.MATERIALS INDEX: Cobalt stabilized cubic ZrO 2.

Introduction

"Cubic ZrO 2 samples containing some transition metal oxides including rhodiumoxide (1), iron oxide (2), chromium oxide (3) and nickel oxide (4) were studiedpreviously in this laboratory. Their structures, magnetic properties and thestabilities towards reduction have been studied. There is little published dataconcerning the properties of the members in the cobalt oxide-zirconium oxidesystem. Structural studies of the supported Co(II)/ZrO2 catalysts were carriedout by Bettman and Yao (5, 6). Their samples were prepared by incipient wetnessmethods and the crystal structure of the products indicated the presence ofmonoclinic ZrO 2. Previous work has indicated that the presence of monoclinic

>hZrO2 is characteristic of an absence of reaction between thn ZrO 2 support andthe catalyst.

-It is the purpose of this work to investigate the formation of solidsolution between ZrO 2 and cobalt oxide and to study the magnetic properties ofsuch a system. In addition, the stability towards reduction of cubic ZrO2samples containing various concentrations of reacted cobalt oxide will bedetermined.

Experimental

Bulk cobalt oxide samples were prepared by decomposition of Co(N0 3)3 "91120.The nitrate was dissolved in water and dried at 150°C for 12 hours. The sampleswere then ground and decomposed at temperatures ranging from 500°C to 800°C.X-ray analysis of the final products indicated the formation of Co304.

4 %

Zirconium oxide samples, containing various percentages of cobalt, were preparedby dissolving the desired quantity of analyzed cobalt nitrate Co(N03)3 "9H20 withappropriate amounts of ZrO(N03)2 . Two ml of water were added for each millimoleof total nitrates. The solution was then dried at 150*C for 12 hours and theproduct was ground and heated at 500*C for 24 hours. Some samples were thensubsequently heated at elevated temperatures between 500-8000 C. In order toascertain the temperatures for complete decomposition of the nitrates,preliminary decomposition experiments were carried out in a Cahn system 113thermal balance.

X-ray powder diffraction patterns of the samples were obtained using aPhilips diffractometer and monochromated high intensity CuKal radiation (X =1.5405A). For qualitative identification of the phases present, the diffractionpatterns were taken in the range 120 < 20 < 80° with a scan rate of 10 20/minand a chart speed of 30 inches/hr. The scan rate used to obtain x-ray patternsfor calculation of cell parameters was 0.25 20/min with a chart speed of 30in/hr. Cell parameters were obtained from a least squares refinement of thedata with the aid of a computer program which corrected for the systematicexperimental errors.

Magnetic susceptibilities were measured using a Faraday balance at a fieldstrength of 10.4 kOe. Honda-Owens (field dependency) plots were also made andall magnetic susceptibility data were corrected for core diamagnetism. Magneticsusceptibility measurements were made from liquid nitrogen temperature to 315K.

emperature programmed reductions were carried out in a thermal balanceequipped with a magnet (7). The weight of the sample was determined using aCahn electrobalance (model RG) alternately in a magnetic field gradient andwithout the magnetic field gradient. The temperature was measured by a type Sthermocouple which was positioned just below the sample. This techniquecombines magnetic measurements with thermogravimetric analysis and is verysensitive to the appearance and growth of a magnetic phase during the reaction(7). An 85%Ar/lS%H 2 mixture was predried by P205 and passed at a rate of 60ml/min into the TGA balance. The samples were heated at 500C per hour.

Results and Discussion

Bulk samples of cobalt oxide were prepared by the decomposition ofCo(NO3)3 -9H20 at temperatures ranging from 500*C to 800*C. X-ray diffractionanalysis of the products indicated the presence of a single phase, namely Co304.In order to analyze any deviation from stoichiometry, temperature programmedreduction was carried out by thermogravimetric analysis. The weight change inthe process of T.P.R occurs between 2300 and 3200 C. The total observed weightchange is 26.6%, whereas the calculated weight change of the reduction of Co304to cobalt metal is 26.5%. These results indicate that the composition of bulkcobalt oxide prepared under these conditions is Co304.

Samples of the cobalt-zirconium oxide system were prepared by thecodecomposition of Co(N03 )3 .9120 and ZrO(N03)2. X-ray analyses of productscontaining varying compositions are given in Table 1. All of the productsreported in Table 1 were prepared at 500 0C.

Decomposition of pure zirconyl nitrate resulted in the formation oftetragonal ZrO 2 containing a small quantity of monoclinic ZrO 2. For the samplesprepared by codecomposition of the nitrates, x-ray analysis indicated that ZrO,crystallized with a cubic structure even when only 5 atomic percent of cobaltwas introduced into the ZrO2 . This is consistent with previous reports (8) thatthe stabilization of cubic ZrO2 requires the presence of a solid solution withcobalt oxide. Products containing up to 10 atomic percent of cobalt showed noevidence of bulk cobalt oxide in the x-ray diffraction patterns. However, itcan be seen from Table I that there is a decrease in the cell parameter of thecubic ZrO2 phase which is consistent with an increase in the cobalt content of

TABLE 1

IDENTIFICATION OF PHASES FORMED IN THE Co(II)/ZrO2 SYSTEM

Composition (at% Co) Phase(s) X-ray Parameters

a c/aZrO 2 = 0 Tetragonal ZrO 2 5.081(3) 1.02

plus small amountamount monoclinic ZrO2

Co/(Co+Zr) = 5 Cubic ZrO2 5.086(3)

Co/(Co+Zr) 1 10 Cubic ZrO 2 5.072(3)

Co/(Co+Zr) = 15 Cubic ZrO2 + Co304

this phase. When attempts were made to prepare cubic zirconium oxide containing15 atomic percent cobalt, bulk Co304 was evident in the x-ray diffractionpatterns. The limit of solubility of cobalt in ZrO 2 is therefore below 15percent.

A sample of ZrO 2 containing 5 atomic percent cobalt was heated to 6000. 7000and 800*C. The cubic zirconium oxide remains stable at 600*C but at 700'C linesof Co0 4 and tetragonal ZrO2 appear in the diffraction patterns of te products.Finally the monoclinic structure is obtained as the temperature is raised from7000 to 800*C.

The reciprocal magnetic susceptibility versus temperature data for bulkCo304 (i.e. Co(II)[Co(III) 2]04) is given in Fig. 1. The sample measured has amoment of 4.8 BM per Co(II), since low-spin Co(III) is diamagnetic. This isconsistent with the value reported in the literature (9). Strong spin-orbitcoupling is a characteristic of Co(II), which usually does not show a spin-onlymoment.

Magnetic measurements were also made on zirconium oxide samples containing 5and 10 atomic percent cobalt. These measurements were made as functions of both

L.'~ Ze J& If ,(2%

CoaO0.20

C

0.15

0.10

0 .05

0. 0j,,,

0 100 200 300 400

Temperature (K)

Fig. 1. Reciprocal magnetic susceptibility versus temperature datafor bulk Co304 heated at 500°C for 24 hours.

field and temperature. All samples showed paramagnetic behavior and have nofield dependency at either room temperature or liquid nitrogen temperature. Theresults of the magnetic measurements are plotted in Fig. 2 as reciprocalsusceptibility versus temperature. The measured paramagnetic moment for thesample containing S atomic percent cobalt is 4.8 BM and hence all of the cobaltpresent is Co(II). The measured value for the paramagnetic moment per cobaltion of the sample containing 10 atomic percent cobalt is 4.4 BM which mayindicate the coexistence of a small quantity of bulk Co304 . Additional evidenceto substantiate these results follow from the TPR studies.

The temperature programmed reduction studies were carried out in a85%Ar/15%H 2 atmosphere using a thermomagnetic balance. The onset of reductionwas detected by the difference in sample weight with and without a magneticfield gradient and arises from the ferromagnetism of cobalt. In the case ofbulk Co304, cobalt metal begins to form at 270 0C (Fig. 3), and the sample with 5atomic percent cobalt in ZrO2 begins to reduce at S40

0C. The stability ofcobalt oxide towards reduction is therefore greatly increased by forming solidsolution with zirconium oxide.

TPR of the sample containing 10 atomic percent cobalt indicated that therewere two kinds of cobalt present in the sample (Fig. 3). Most of the cobalt wasreduced above 5000C whereas a small amount of the cobalt began to reduce at280*C. Therefore most of the cobalt has been inserted into the ZrO2 but a smallamount remains as bulk cobalt oxide. This is consistent with the observeddecrease of the paramagnetic moment for the 10 atomic percent sample compared to

Co(II)/ZrO20.20 *

0.15 'Satz0

0

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0

-X 0.05

0.000 100 200 300 400

Temperature (K)

Fig. 2. Reciprocal magnetic susceptibility vs temperature for5 and 10 at% Co(II)/ZrO2 heated at 500°C for 24 hours.

Magnetic TPR in 85%Ar/15%H2

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Fig. 3. Variation with temperature of the weight change caused by

~the application of a magnetic field gradient during thetemperature programmed reduction of bulk C0304, 5 and 10atoo, Co(ITl)ZrOi in SSt ArllC')ll2 .

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that obtained for the 5 atomic percent sample. The maximum solubility of cobaltoxide in zirconium oxide is therefore determined to be slightly lower than 10atomic percent. It appears that codecomposition of the nitrates results inphases which contain cobalt (up to almost 10 atomic percent cobalt) as anintegral part of the phase. However, it is only at 15 atomic percent that thepresence of bulk cobalt oxide appears in the x-ray diffraction pattern. This isin contrast to the usual method of preparation, i.e. incipient wetness, in whichthe cobalt oxide is dispersed as a second phase on the ZrO 2.

Acknowledgments

This research was partially supported by the Office of Naval Research and bythe Exxon Education Fund. The authors also which to acknowledge the support ofthe National Science Foundation, Grant No. DMR 820 3667, for the partial supportof K. Dwight and the use of the Materials Research Laboratory at BrownUniversity which is funded by the National Science Foundation.

References

1. Y-C. Zhang, K. Dwight and A. Wold, Mat. Res. Bull., 21(7), 853 (1986).

2. S. Davison, R. Kershaw, K. Dwight and A. Wold, To be published in J. Sol.St. Chem.

3. P. Wu, R. Kershaw, K. Dwight and A. Wold. To be published in J. Mat. Sci.Lett.

4. K. E. Smith, R. Kershaw, K. Dwight and A. Wold. To be published inMat. Res. Bull.

5. M. Bettman and H.C. Yao, "Materials Science Research" Vol. 10 Sintering and

Catalysis. Ed. G. C. Kuczynski, New York 1975, page 165.

6. H. C. Yao and M. Bettman, J. of Catal., 41, 349 (1976).

7. M. Schwartz, R. Kershaw, K. Dwight and A. Wold, Mat. Res. Bull., 22(5),609 (1987).

8. R. Collongues and J. Stocker, Compt. Rend. 246, 3641 (1958).

9. R. Perthel and H. Jahn, Phys. Stat. Sol. 5, 563 (1964).

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