Chemistry of Zirconium Dioxide X-Ray Diffraction Studies G . L. CLARK AND D. H. REYNOLDS University of Illinois, Urbana, Ill.
When ZrOC12.H20 is heated at 110" C., it loses its water of hydration and becomes amorphous. (Between 300" and 500" C. it is transformed to zirconia.) The tetragonal modification of zirconia is formed at 500" C. and is converted t o monoclinic baddeleyite above 600" C. When zirconyl hydroxide is ignited a t 650" C.,it first forms tetragonal zirconia which is then converted irreversibly to the monoclinic form. Complete conversion requires considerable time a t that temperature. When zirconyl hydroxide is ignited a t 500" C., tetragonal zirconia is formed which shows no tendHE chemistry of zirconium dioxide has attracted great interest because of its possibilities as a refractory and its possible importance as an opacifying pigment in ceramic products, particularly enamels. Because of its relative abundance and consequent low cost compared with other opacifiers now used, it is desirable to find means of overcoming the drawbacks which prevent its general use. The purpose of this paper is to present some observations on the thermal behavior of zirconium dioxide and other zirconium compounds, which may point the way to their more effective use in chemical industry. Cohn and Tolksdorf (5)showed by means of x-ray diffraction studies and dilatometric measurements that the failure of zirconia-opacified enamels could be caused by the many polymorphic transitions of which the substance is capable a t elevated temperatures and the relatively great volume changes accompanying the transformations. A study of the influence of the environment upon these allotropic changes was not reported. Cohn (4)lists the forms of purezirconium dioxide as follows:
T
Modifioatioo
C baddeleyite
B AdAi Glassy
Crystal System Monoclinic Tetragonal Trigonal
.......
Unit Cell a-5.21.b-5,.26, e-5.37, 8-80°32 a-5.07,c-5 16 a-3.60,c/a-1.633
.
.
.
Modification C is stable between room temperature and 1000" C., and is obtained by slowly cooling pure zirconia from temperatures between 600" and 1900" C. Form B appears a t 1000" C. and is reported to be preserved to room temperature by extremely rapid quenching. A2 is formed by heating pure zirconia for a long time at 1900° C. or above. The bransition of B to A2 is irreversible. Az changes to AI, another trigonal form, reversibly a t 625" C.
ency to change to monoclinic even upon long heating at this temperature. The low-temperature tetragonal form of zirconia is probably identical in structure with t h e form which is stable between 1000" and 1900" C. The slow conversion rate of the low form may be due to its extremely small crystal size. Silica exhibits some solubility in tetragonal zirconia. The dissolved silica stabilizes the tetragonal form to raise t h e transition temperature from 600" t o above 1000" C. The silica-zirconia solution is converted to zirconium silicate (zircon) a t 1460" C. Goldschmidt (6) obtained a tetragonal form by heating zircony1 salts below 500" C. This form is said to be metastable and has been assumed to be identical with the B form of Cohn's classification. Van Arkel (1) reported a cubic form as being stable a t 1400" C. and above, which could be preserved to room temperature by the presence of small amounts of magnesia and calcium oxide. This form was found by Cohn and Tolksdorf to exist only in the presence of those metallic oxides. Their dilatometric measurements suggested that a compound of the type ZrOz-2 RO was formed. De Boer (9) suggested that the regular forms of zirconium dioxide belong to a magnesium compound, 2Mg0.3Zr02(Mg2Zr30s). Some recent x-ray studies on the system zirconia-magnesia show ( 7 ) that less than 3 per cent magnesia will give rise to the cubic form, which will absorb magnesia in solid solution to the extent of about 28 per cent, with a minimum a0 value at the composition corresponding to MgZZr308. In the present investigation it was desired: (a) to study the thermal behavior of zirconyl chloride and zirconyl hydroxide, ( b ) to differentiate between the low-temperature tetragonal modification of zirconia and that formed a t 1000° C., and (c) to determine the influence of an acid environment upon the thermal transitions of zirconium dioxide.
Specimens and X-Ray Technic The samples used %ere prepared from ZrOC12.HZ0, which was Wrified by several recrystallizations from hydrochloric acid solution. For one set of experiments the airdried crystals were used.
INDUSTRIAL AND ENGINEERING CHEMISTRY
712
VOL. 29, NO. 6
FIGURE 1. EFFECTOF HEATING ON ZIRCONYLCELORIDE A
A. B. C. D. E.
Air-dry orystals
Dried st lloo c.
I2 hours at 3 W 0 C. 12 hours at 500" 12 hours st BOOo
C. C.
P. 12 hours at 7000 C .
8
c
0
Zirconyl hydroxide was produced by precipitation froni hot acid s o l u t i o n with excess ammonium hydroxide. The filtered and washed residue was dried at 110" C. The loss upon ignition of the dried residue was 13.0 per cent, iiidicating eirconyl lrvdroxide of a Iiieh deeree oi purity. Mixtures of z i r c o n i a and silica were formed by enprecipitation of sirconyl hydroxide and silicic acid. 'Two samples were prepared, one containing 1 3 per cent and the other 30 per cent silica; the latter c o n t a i n e d silica and zirconia in a l m o s t e q u imolecu 1a r proport i o n s . T h e gel o b I
I.
FIGWE 2. EFFECT OF HEATING A T 650- c. UPON ZIR-
C O N HYDROXIDE ~ ~ A. Unheated B. 15minvtes C. aominutes D . 1 hour E.
P.
7 hours
24 liouis
tained w a s was 11e d and dried at 110" C . Efforts to obtain the €3 form of zirconia by heating in an oxygen-gas flame and quenching in w a t e r m e t with failure. The subseqnont x - r a y p a t t e r n ; s h o w e d lines for the C' form only. Theinstantane ous transition of R to C may he significant when compared with tho slo\,reaction rates obtained for the transition from the Ion: teinperat,ure t e t r a g o n a l forin to the C mndificstion.
JUNE, 1937
INDUSTRIAL AND ENGISEERING CHEMISTRY
given in more detail later. The accompanying chemical and physical changes w e r e s t u d i e d by x-ray diffraction patterns. The x-ray technic u s e d w a s the p o w d e r " w e d g e " method p r e v i o u s l y described (S). Sufficient patterns to illustrate the results of this investigation are suhmitted as Figures 1 to 4.
"13
A
B
c
Thermal Behavior of Zirconyl Chloride and Hydroxide I n Figure 1 x-ray patterns representing sircony1 chloride heat.ed at different temperatures are reproduced. Pattern 18 ia for the air-dried crystals of
D
E
ZrOC1~WsO. Figure IC s h o n s t h e effect of heating the same specimen at 300" C. for 12 hours. The material is still amorphous, but the halos are more pronounced t h a n in the previous pattern, which may indicate formation of crvstal nuclei. For Firmre 1D the same sample w a s heated at jODa C . for 12 hours. The halos of t h e p r e i' l' o u s patterns h a v e r e s o l r e d t h e m s e l v e s into a line pattern. T h k pattern is identified w i t h t h e l o n temperature t e t r a g o n a l form of z i r c o n i u m diA. Unheated ouide reported by GoldB . 30minutes c. 1 hour s r h m i d t . T h e sarnde D. 2lioiiis s h o w e d b u t a trece of chlorine upon analysis, indicating almost complete transformation of the chloride to the dioxide. Figure 1E represents tlie same specimen keated at 600" C. for 12 hours. The principal lines are those of the C or innnoclinic modification of zirconia. Faint lines are present, far the tetragonal form, indicating t.hat its conversioii to the C forni is slow or incomplete at GOO" C . Pattern 1F, obtained after heating the sample 12 ho1ir.i at 700" C., shows that, the conversion to the C forin is conplete. In Figure 2 the patterns obtained from zirconyl hydroxide heated for various time intervals at 650" C., are reproduced. Pattern 2 8 shows the starting material to be aniorphous. This pattern is practically identical with patt.erns 1B and E . Figure ZB,made aft.er 15 minutes of heating, shorn the presence of the low tetragonal form of ,zirconia and the absence of the C fwni. In Figure ZC, after heating 30 minutes, the nionocliiiic iorm has made its appearance, but t.he low tet,ragoiial modificat,ion is still predominant.
P
0
11
B. 18 hours F . 44 llouia 0. PZ hours If. Caloiuin Ruoride (thorite)
111 pattern 2D, after one hour, t.he C forni overshadows the tetragonal, and in ZE,after 7 hours, the tetragonal Iorm has almost disappeared. The tetragonal form had vanished conipletely after 24 hours, as illdieabed in pattern 2F. The slow rate of transformat,ion indicated here has not been reported previously, and is apparently incongruent with the iristantaneous inversion rate for the I3 to C transition observed in the quenching experiments. In a similar series heated a t 500" C., the tetragonal modification persisted, even after 92 Iiourz, ~ i t hno indicatioii of tlie formation nf the C fonn. Patterns for this series are shown in Figure 3. The orily noticeable change in the patterns upnn continued heating is the gradual sharpening of the interferences, resulting in clearer resolution of close together lines (most apparent in lines 7 and 8 in pattern 3G). This indicates a slow growth and gradual perfection of the crystals, although aft,er 92 hours of heating they are still subli~icroscopic. Since the low tetragonal inotlification of zirconia cannot be obtairied tjy slow cooling of the C form, the transition
.
VOL. 29, NO. 6
IKDT~STRI4L 4RD ENGINEERIKG CHEMISFR1
i14
Since high tetragonal zirconia was not obtained by the authors, it was necessary to match the pattern for the low form against data recorded in the literature for the former. To obtain accurate lattice spacings for the latter, the specimen of pattern 36 was mixed with powdered sodium chloride, and patterns were registered using a semicircular cassette with filtered radiation from a copper anode x-ray tube. The lattice constants are shown in Table I, compared with data presented by Ruff and Ehert (8) for the high form. The very close agreement in the spacings and line densities indicates that the high and low tetragonal forms of zirconium dioxide are identical. The slow rate of conversion of the assumed low form to monoclinic must then be tied up with the extremely small particle size of the crystals formed by the thermal decomposition bf zirconyl compounds. Since this work is based upon samples of very pure zirconia, the suggestion by de Boer that the regular forms of zirconia represent a compound with magnesia is not subtantiated.
Effect of Silica upon 600" C. Transition
FlouaE 4. EFFECTOF
SILICA ON THE
600'
c. T a m s r T l o N OF ~ I R c o N I A
a t 600" C. is irreversible, &s is the 1900" C. transformation. Both tetragonal forms may then he termed metastable.
Comparison of High- and Low-Tetragonal Zirconia The similarity of the tetragonal zirconia arrangement to that of calcium fluoride is seen by comparison with the fluorite pattern shown in Figure 3 H . This observation was first made by Goldschmidt, and repeated by Cohn and Tolksdorf for the high fornl. TABLE1. LATTICECONSTANTS
FOR
TETRAWNAL ZlRCONIFM
DIOXIDE
de 2.943 2.678 2.537
dh
h kl
2 . e44 2,580 2.535
111
002
2w 202
{;:%
ZLO
D"
fib
vs
~
W
vs
TABLE11. LA~PICE PATTERNS OF TKE ZlRCoNU-&LICA SOLUTION D 6kl IObsvd.1 2.962 111 Sb 2.624 002 F 2.561 200 W 1.824 202 V8 1.801 220 M 1.567 113 W 1.536 311 s 1.481 222 w 1.301 0434 W 4W 0 * Line dsmitiea for pure VS = verystrong: S d
2; 1
i
showing the material to be tetragonal although the temperature was a t least 200' C. a b o v e t.t.e transition point for pure zirconia. Calculations based upon calibrated patterns of the sample of Figure 4C show a. 7 5.090 1.and c/a = 1.025 as compared with a. = 5.070 A. and c/a = 1.081 for pure sirconia. This, coupled with the complete nonappearance of silica lines, indicates that some type of rather stable solid solution of silica in tetragonal zirconia exists. The data are shown in Table 11. The solubility of silica must be rather limited, as indicated by the relatively small change in lattice parameters, and by the identical appearance of patterns 4B and C', whose samples contained 13 and 30 per cent silica, respectively. The relative packing siees of silicon and zirconium are quite different. Therefore, a random substitution of silicon for zirconium would not be expected; but iI such were the rase, a highly distorted lattice should result. It may be
M
1.548 1.551 113 M 1.5332 1.531 311 vs 1.471 3.472 222 M 1.288 1.290 004 W 1.267 1.268 400 M 1.172 1.173 313 W 1.184 1.163 38 1 M 1.147 1.150 240 W I . 139 1.047 1.048 224 W 1.040 1.038 422 M M 0 . 993 0.992 W 115 W W 0 981 n 882 333 M -Data for low tetras.one.1 ZrOl. b Dsta for bigb tetraeons1 210s. Vs verystruw; S strong: M = medium: 11' = resk; F
{;:;E
The zirconia-silica mixtures were heated in a platinum resistance furnace a t temperatures of 800", 1000", and 1460" C., for 2-hour periods. The changes produced are indicated in Figure 4. The oven-dried specimens gave the pattern of Figure 4.4, which s h o w s t h e s a m p l e s to be amorphous. Two hours a t 800" C. eave the snecimens
.
~.
ioint.
i
!)
d'
VS W
w
M 78 M
vs M
D
SOLID
d 1.181
hkl
IObsvd.)
dn
i:ik
133 331
W 0
204
402 420
$1 W
W M W M (difference)
1:i35 1.055
224
1.043 1.m
422 I15 333
w M B 0
W M
W ... M M tetragonal 2 x 0 ~(R,uE and Ebert). strong; M medium: W rn wussk: F -faint.
i
W
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
JUNE, 1937
that the variations between observed and expected line densities are significant and indicative of a distorted lattice. It is apparent that most of the silica remains amorphous, and that the small amount dissolved exerts a keying action to prevent the normal transition of tetragonal zirconia to the C modification. For Figure 4 0 the sample of pattern 4C was heated 2 hours at 1000° C. The tetragonal form of crystal is still predominant, with undisplaced lines, but faint new lines show some other change to be taking place. The additional lines are probably from one of the forms of tridymite but were not positively identified. The degree of crystal growth of the tetragonal zirconia a t 1000" C. is apparent in the increased sharpness of the interferences. Some sintering took place upon heating. Figure 4E shows the result of heating the same specimen for 2 hours a t 1460" C. The specimen was sintered to a hard mass and had to be pulverized before a powder pattern could be registered. The identity of the material is shown by comparison with the pattern for naturally occurring zirconium silicate (zircon) shown in Figure 4 F . That this compound should be formed in a sintered condition 1000" C. below its melting point is remarkable. The explanation may lie in the fact that the temperature was near the tridymite-cristobalite transition point, a t which the silica would be more reactive chemically than at lower temperatures. These results, together with the observations recorded in the introduction concerning the influence of basic oxides on the transitions of zirconia, indicate that dilatometric and other measurements made on the pure oxide may be useless
715
in predicting the behavior of zirconia in an enamel, or to account for the failures observed. Its reactions in an acidic or basic environment are quite different from those for the pure material. Further investigation of the nature of these changes, study of reaction rates, and possibly the determination of the complete phase diagram for zirconiasilica, zirconia-magnesia, zirconia-calcium oxide, and the three-component systems must precede any extended general use of zirconia as an enamel opacifier.
Acknowledgment The authors wish to express their thanks to J. W. Haslam, of the Chemistry Department, University of Illinois, for the preparation of the pure zirconyl chloride used in this study, and to B. W. King of the Ceramics Department for igniting the zirconia-silica mixtures.
Literature Cited (1) Arkel, van, Physica, 4, 286 (1924). (2) Boer, de, Foofe-notes, 3, No. 2 (1930). (3) Clark and Reynolds, IND.ENO.CHEM.,Anal. Ed., 8, 36 (1936). (4)Cohn, preprint of paper presented at 68th general meeting of Electrochem. Soc., Oct., 1935. (5) Cohn and Tolksdorf, 2.physilc. Chem., B8, 351 (1930). (6) Goldschmidt, Nuturwissenschaften, 14, 477 (1926) (7) Herold, Paul, private communication. (8) Ruff and Ebert, 2. anorg. Chem., 180,19 (1929). RECEIVED January 18, 1937. This paper is part of a dissertation presented ~
by D. H. Reynolds in June, 1936, to the Graduate School of the University of Illinois in partial fulfillment of the requirements for the degree of doctor of philosophy.
Preparation of an Active Cobalt-Copper Catalyst for
THE WATER-GAS SHIFT REACTION H. H. STORCH AND I. I. PINKEL, U. S. Bureau of Mines Experiment Station, Pittsburgh, Pa.
OBALT catalysts containing small amounts of copper have been shown (2) to be very active a t 300' to 325" C. in catalyzing the water-gas shift reaction: GO HzO = Con HZ The methods of preparation given in the literature involve fusion of the metal oxides in an oxyhydrogen flame or coprecipitation as hydrated oxides from solutions of the nitrates. The former is a somewhat expensive method for practical work, and the latter does not yield a mechanically stable product. One of the authors (H. H. Storch) accidentally discovered that, if a mixture of cobalt carbonate and small amounts (5 to 25 per cent) of copper oxide is heated rapidly (in about 3 minutes) from room temperature to between 900' and 1100' C. (or higher), a sintered, pumice-like mass of granules is obtained. This product has excellent mechanical properties that permit prolonged use without appreciable spalling. It is also a very active catalyst when properly reduced and subsequently protected from sulfur poisoning. I n order to produce hydrogen for use in an experimental coalhydrogenation plant, water gas was made, starting with Pittsburgh natural gas and steam. This water gas contained about 68 per cent hydrogen, 23 carbon monoxide, 1.6 carbon dioxide, 5.0 nitrogen, 2.0 methane, and 0.5 per cent oxygen. This product, along with four volumes of steam, was passed through an 8-mesh cobalt-copper catalyst at 310' C. at a space velocity of 3000 per hour (based on dry water-gas volume, not including the steam) for more than 100 hours without any sign of deterioration in activity. The off-gas was analyzed every 5 hours and the average analysis was as follows: 73.8 per cent hydrogen, 0.3 carbon monoxide, 18.7 carbon dioxide; the remainder was nitrogen plus methane. No separate analysis for methane was made, but it is apparent from the data that the amount of carbon monoxide converted to methane was not more than about 1 per cent of the original water gas.
C
+
+
Water gas made from Pittsburgh natural gas and steam may be used without any further purification, the sulfur content being sufficiently low so that no sign of poisoning is observable in 100 hours of operation. In order to study the regeneration of a poisoned specimen of this catalyst, it was exposed to steam containing traces of hydrogen sulfide. The catalyst lost its activity rapidly (in about 2 hours at 3000 space velocities per hour). It could, however, be readily regenerated by heating to 900" C. and passing a stream of air over it. Subsequent reduction and use with water gas-steam mixtures yielded an activity identical with the original catalyst. Slower heating of the mixture of cobalt carbonate and copper oxide results in a powder that is not suitable for the rapid gas flow demanded by 3000 space velocities per hour. This rather curious behavior cannot be readily explained. The melting points of both cobaltous oxide and cupric oxide are probably too high t o account for the sintering upon heating to about 1050' C. Furthermore, it is difficult to undersband why slow heating should not sinter a mixture of the two oxides, whereas rapid heating apparently does accomplish this. Perhaps the formation of intermediate com ounds (CoO),~(CoCOs) as postulated by Krustinsons (1) res& in a sufficiently low meflting compound to yield the observed sintering phenomenon. It is conceivable that such intermediary compounds may decom ose below their melting points, and hence rapid heating woulcff result in sintering, whereas a slow rate of heating would give ample time for complete decomposition to occur before the melting point is reached.
Literature Cited (1) Krustinsons, J., 2. Elektrochern.,39,936 (1933). ( 2 ) White, E. C., and Schulta, J. F., IND.ENQ.CHEX.,26, 95 (1934). RECBIVEID January 21, 1937. Published by permission of the Director, U. 6 . Bureau of Mines. (Not subject t o copyrighc.)
