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OFFICE OF NAVAL RESEARCH

CONTRACT NOOO 14 86K0234

TECHNICAL REPORT NO. 4

AN EXAMINATION OF THE RELATIVE STABILITIES OF

MgxNil-xO AND NiO ON SPHEROCARB

by

Michael Schwartz, Robert Kershaw, Kirby Dwight and Aaron Wold

C'%J Prepared for publication

in

MATERIALS RESEARCH BULLETIN

Brown University

Department of ChemistryProvidence, RI 02912 .= ,TB7

January 13, 1987

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AN EXAMINATION OF THE RELATIVE STABILITIES OF mgxNil_x O AND NiO ON SPHEROCARB

12. ScArTHOR Kershaw, K. Dwight, A. Wold

3a. T'.Pc OF RVPOR 1i3b. TiME COVERED 114 DATE OF REPORT (Year, Month. Da)I AE COUNT

16 echnicai FROM TO 716 SU-PLEMENTARY NOTATION

SUBMITTED FOR PUBLICATION IN MATERIALS RESEARCH BULLETIN

17 COSATI CODES 18 SUBJECT TERMS (Continue on reverse if necessary and ident:fy by block number)

F.ELD IGROuP SUB-GROUP INTERACTIONS: NiO on MgO

1(BST ACT Continue on reyers if necessary and ideiQratfy block number) Carbon were studied using" The '--,eductions of Mg XNii1 xo and1Ald dispersed on Spherocarbahe o _" ed. he It was found that the NiO was greatly

coms ned magne l-thermogravmerc n thgj .but the carbon support did not stabilizestabilized by SOT ~ 4olution formation wi th n , . .. . .... . .

• the NiO towards H2-) duction. -

- I-

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All.0 r' SEC IY CLASS.,CAT N OF TZ . P_-t

' Accesion ForNTIS CRA&IDTIC TAB rUnannounced

Justification

By ........ . .

DIt: ibutionAvailability Codes

AN EXAMINATION OF THE RELATIVE STABILITIES OF Dist Avail andlorMgxNil_xO AND NiO ON SPHEROCARB

pecial

by AIMichael Schwartz, Robert Kershaw, Kirby Dwight and Aa: " it

Department of Chemistry, Brown UniversityProvidence, RI 02912

ABSTRACTThe H2 reductions of MgxNil_xO and NiO dispersed on SpherocarbCarbon were studied using a combined magnetic-thermogravimetrictechnique. It was found that the NiO was greatly stabilized by solidsolution formation with MgO, but the carbon support did not stabilizethe NiO towards H2 reduction.MATERIALS INDEX: MgxNil-xO, NiO/Spherocarb, Thermomagnetic,

Thermogravimetric

Introduction

It has been shown (1) that stabilization of hexagonal iron oxide on rutileTio 2 only occurs if ternary phases such as Fe2TiO 4 or FeTiO 3 are formed under areducing atmosphere. The stabilization of a transition metal oxide can also beachieved by the formation of a solid solution without a change of crystalstructure. A simple example of such solid solution formation is the systemMgxNil_xO where all of the members crystallize with the rock salt structure.

The system MgxNil_xO was chosen for this study because of the ease ofreduction of Ni(II) to metallic nickel in a hydrogen atmosphere. Furthermore,the Curie point of nickel, 358 0C, makes it convenient to study the magneticproperties as the reduction proceeds as a function of temperature. In addition,the solid solution MgxNij_xO is an ideal system to study for several reasons.First, MgO is not reduced by hydrogen up to 2500'C (2), and therefore any weightchanges can be attributed solely to the reduction of NiO. Second, the magneticproperties of NiO-MgO solid solutions are readily understood. They have beenreported to be paramagnetic at low nickel oxide concentrations (3) andantiferromagnetic at higher nickel oxide concentrations (4). Finally, therehave been no reports of other nickel-magnesium oxides which would interlere withthe interpretation of the experimental results.

A recent paper by Gallagher et al. concerns a study of the H2 reduction ofNiO using thermogravimetric and evolved gas analyses (5). For NiO with lowsurface areas, 1.0 m2/g, and using pure H2 as the reductant, they found thatinitial reduction began at approximately 250 0C, depending upon the rate ofheating. Their results were consistent with other reports and presented a clearpicture of the temperature dependence of the reduction of NiO. A secondrelevant paper described the effect that doping small amounts of MgO into NiO

.o. C

..... had upon the rate of H2 reduction of NiO (6). Small amounts of MgO, 1.5% and7.1%, greatly decreased the rate of reduction at constant high temperatures.However, the study did not correlate the magnetic properties of the phases

formed on reduction with the increased stabilization of the NiO.

It was the purpose of this study to investigate this correlation and toi compare the stabilization of nickel oxide in a solid solution containing MgO

with a sample of NiO dispersed on Spherocarb Carbon where there appear to be nointeractions present (7).

IExperimentalThe starting materials were Mg(N03)2 "6H20, Ni(N03)2"6H 20 (Fisher Certified

Reagents) and Spherocarb particles (Analab, GCA-012). MgxNil-xO solid solutionswere prepared by the codecomposition of the nitrates. Typically, theappropriate amount of the metal nitrates were dissolved in distilled H20, 2 mlof H20 per gram of starting material. This solution was then dried 12 hr at150*C. The resulting solid was ground and then heated in a porcelain cruciblein air for 24 hr at 600'C. A temperature of 600*C was chosen as the preparationtemperature in order to ensure complete decomposition of the dried magnesiumnitrate precursor. X-ray powder diffraction patterns showed the resultingproducts to be single phased with the rock salt structure. NiO was alsoprepared by the same procedure.

The samples of NiO dispersed on Spherocarb were prepared according to themethod of Kim et al. (7) using Ni(NO3)2-6H20 (Fisher Certified Reagent) as thesource of Ni. The dried Ni nitrate/C precursor was heated at 450*C in a wet N2atmosphere in order to prevent reduction. The resulting product consisted ofNiO as determined by x-ray powder diffraction. No Ni metal was detected in thesamples by either x-ray powder diffraction or magnetic measurements. A loadingof 10 atomic percent of Ni was used.

The thermogravimetric balance used in this study combines magneticmeasurements with thermogravimetric analysis. As the reaction proceeds, theweight of the sample can be determined alternately in a magnetic field gradientand without the magnetic field gradient. This allows for the constantmonitoring of the appearance and growth of a magnetic phase in conjunction withweight changes associated with the reaction. The new phases can be identifiedon the basis of their Curie temperatures. Although x-ray powder diffraction canbe used to identify new phases, this technique is more useful because itmonitors the reaction as it occurs. Magnetic measurements are also much moresensitive than x-ray powder diffraction. Therefore, new information onreactions can be gained using this combined technique.

A schematic diagram of the apparatus used for the combined magneticand thermogravimetric analyses is shown in Fig. 1. The magnet is mountedon a shaft which is connected to a motor through a crank. The magnet movesfrom the vertical position (sample out of the field) up to the horizontalposition (sample in the field) once a minute. The sample is positioned so thatit sits in the m-,ximum field gradient. The weight of the sample was determinedusing a Cahn electrobalance (model RG). The temperature was measured by a type

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FURNACE

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TO MOTOR

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Fig. ISchematic Diagram of Magnetic-Thermogravimetric Balance

Typical sample weights were 35-40 mg. 85%Ar/15%H 2 was used for thereductions. The gas was dried by passing through a P205 column. The flow ratewas 30 cm3/min and the samples were heated at 500C per hour.

X-ray powder diffraction patterns were taken with a Philips diffractometerusing copper radiation (A = 1.5405 A) and a single crystal graphitemonochromator. The scan rate was 10 20 per minute with a chart speed of 30in/hr. For cell constant determination, the scan rate was 1/4' 20 per minute.

Results and Discussion

Samples having the composition MgxNil_xO (x = 0.1-0.3) were prepared bydouble decomposition of the nitrates. The minimum temperature for completedecomposition of the nitrates was established from TGA data. X-ray analysisindicated that all members of the system crystallized with the rock saltstructure. The variation in the cell parameters vs. nickel content is shown inFig. 2. A comparison of the stability of bulk nickel oxide was made with thecompositions in the system MgxNij_xO. The TGA results obtained under an85%Ar/15%11 2 atmosphere are shown in Fig. 3. In order to avoid discrepancie- dueto kinetic effects, samples of at least 35-40 mg were used for thc stability

LiMg9 N i _×

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Fig. 2Plot of Cubic Cell Parameter vs. Composition Parameter x

M9 ×Ni 1×O Reduction110

. 100 ..A........ ***...................0000000 0

",'A A AAA

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A - x=O.080 A x=O. 1

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200 300 400 500 600

Temperature ('C)

Fig. 3Thermogravimetric Results for If2 Reduction of NgxNil1 xO

x.O

MgNi 0 Reduction

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v 80 A A A X=0. Ia AA * x=O. 2

A

A4-)A

CUAN 60

A

A A

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200 300 400 500 600

Temperature (°C)

Fig. 4Thermomagnetic Results for H2 Reduction of MgxNil-xO

NiO/C Reduction100 . I ' K I K * ,

xx

x

80 -

x x

CU 60

rx

0

rKLi 403

aU> x ognetization

20 * Weight of NiO

-20

200 250 300 350 400

Temperoture ('C)

Fig. 5

Thermogravimetric and Thermomagnetic Results for 112 Reduction of NiO/C

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determinations. Whereas bulk NiO begins to reduce at 255C, samples with the

composition MgxNil-xO (x = 0.1-0.3) begin to reduce at temperatures between 312- 365'C. However, the initial weight loss is so small that it is not apparenton the scale of Fig. 3. The rate of temperature increase was 50C per hour.

Thermomagnetic studies (Fig. 4) were carried out simultaneously with the TGA

experiments using the thermomagnetic balance previously described. Reduction ofbulk NiO with 85%Ar/15%H 2 resulted in the formation of metallic nickel which was

detected at 255 0C. This is the same temperature at which TGA results indicatereduction of NiO. For samples containing approximately 30 mg of metallicnickel, the Curie point was found to be 358 0 C. This agrees with the reportedCurie point of metallic nickel. For the compositions x = 0.1 and 0.2, magneticstudies clearly indicate the formation of nickel at temperatures whichcorrespond to those reported ab -,e where the barely detectable weight lossesbegan. It is evident that t' -Iomagnetic studies are far more sensitive indetermining the onset of . For x = 0.3, the temperature required for

the formation of nickel above the Curie temperature. It can be seen that

appreciable stabilizatio. of nickel occurs when only a small fraction ofmagnesium is substituted into the rock salt structure.

The degree of stabilization of nickel towards reduction in the systemMgxNil-xO (x = 0.1-0.3) was compared to nickel dispersed on Spherocarb carbonwhere no solid solution occurs. The magnetic TGA results for the reduction in85%Ar/15%H 2 of NiO dispersed on carbon are shown in Fig. 5. Both the formationof the magnetic phase and weight loss were first detected at 225C. Acomparison of these results with those obtained for reduction of bulk nickeloxide and nickel-magnesium oxides indicate that there is a lowering of thereduction temperature of nickel oxide. Furthermore, it is clear that there isno stabilization of NiO when it is supported on Spherocarb.

Conclusion N

MgO, which interacts with NiO through solid solution formation, greatlystabilizes the NiO. When NiO is dispersed on C, no stabilization is observed.

These results also show the utility of the combined magnetic-TGA experiment.Moreover, the barely detectable weight loss is concident with the pronounced

appearance of a magnetic phase.

Acknowledgments

This work was supported in part by the Eastman Kodak Company and the Officeof Naval Research. In addition, acknowledgment is made to the National ScienceFoundation for the partial support of K. Dwight. The authors also express their

appreciation for the use of Brown University's Materials Research Laboratory

which is supported by the National Science Foundation.

/ -

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References

1. J. Yu, R. Kershaw, K. Dwight and A. Wold. Submitted for publication inJ. Sol. State Chem.

2. E. Newberry and J. N. Pring, Proc. Royal Soc. London A92, 276 (1916).

3. A. Cimino, M. LoJacono, P. Porta and M. Valigi, Z. Phys. Chem. (Weisbaden)55, 14 (1967).

4. 0. Evrard, J. Francois and J. M. Lecuine, Rev. Chim. Miner. 9, 463(1972).

5. P. K. Gallagher, E. M. Gyorgy and W. R. Jones, J. Thermal Anal. 23,185 (1982).

6. M. H. Tikkanen, B. 0. Rosell and W. Wiberg, Acta Chem. Scand. 17, 513(1963).

7. K. Kim, R. Kershaw, K. Dwight, A. Wold and K. Colle, Mat. Res. Bull. 17,591 (1932).

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