THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in
VOLUME60
U. 6. Patent Office)
(0Copyright, 1956,by the American Chemical Society)
JULY 23, 1956
NUMBER 7
THE ELECTRICAL CONDUCTIVITY CHANGE CAUSED BY THE CHERIISORPTIOI?; OF HYDROGEN O N ZnO, Zn0.Cr203 AND ZnO.MoO.{ BY YUTAKA KUBOKAWA AND OSAMUTOYAMA Department of Applied Chemistry, Naniwa University, Sakai, Japan Received February 9,1966
The electrical conductivity change caused by the chemisorption of hydrogen and the rate of chemisorption have been measured with ZnO (not sintered), ZnO (sintered), ZnO (evaporated), ZnO 1 mole yo A1@3, ZnO 1 mole yqLizO, ZnO Crz03 and Zn0.Mo03. I n all cases examined, the chemisorption of hydrogen caused an increase in conductivity which was reversible. Chemisorption rate curves show in most cases two types of chemisorption, one predominating a t high temperatures and the other at low temperatures. It is concluded from comparison with the conductivity curves that the chemisorption of high temperature type is solely responsible for the observed increase in conductivity. The analysis of the conductivity curves gives about 25 kcal./mole as the activation energy of chemisorption of this type, nearly constant for all adsorbents.
+
*
Introduction Taylor and his co-workers found in their earlier work' two types of chemisorption of hydrogen on zinc oxide which, lately, they have confirmed by Observing the desorption-readsorption phenomellaZ a t varied temperatures. Nevertheless, the natures of these two types of chemisorption are not yet clear. The present study was undertaken to obtaill any clue to the problem which might be foulld by measuring the electrical conductivity change of the adsorbent caused by chemisorption; the approach along this line, e.g., in the work by Garner, e' On copper Oxide, appeared to be promising* The rate Of chemisorption Of hydrogen, as as the electrical conductivity change by chemisorption, has been measured on zinc oxide, zinc oxidechromic oxide, zinc oxide-mo~ybdenum oxide and zinc oxide containing lithium and aluminum oxides which are known t o vary the electrical conductivity of zinc oxide. This paper presents the summarized results. Detailed accoullts of the work will be given elsewhere.4 Experimental Materials. Zinc Oxide.-Zirlc oxalate was precipitated from the solutions of ammonium oxalate and zinc nitrnte. (1) H. S. Taylor and D. V. Sickman, J . Am. Chem. SOC.,'54, 602 (1932); H.S. Taylor and C. 0. Strother, ibid., 66, 586 (1934). (2) H. 8. Taylor and S. C. Liang, ibid., 69, 1306 (1947). (3) W. E. Garner, T. J. Gray and F. 8. Stone, Dims. Faraday Soc., 8,246 (1950): PTOC.R o y . Soc. (London),A191, 314 (1949). (4) For a preliminary work carried o u t on zino oxide alone, cJ. Y. Kubokawa and 0. Toyama, BulE. A ' a n i u ~Gniv., A2, 103 (1954).
+
The precipitate was washed, filtered and dried a t llOo,converted to the oxide by heating in air at 400" and sintered 5200 for 5 hours. Zinc Oxide (Sintered).-Pure ZnO described above (before sintering a t 520") was sintered a t 900" for 4 hours. Zinc Oxide (Evaporated) .-Zinc was evaporated on a glass plate and oxidized as dencribed by E. pt~ollwo.6 zno + 1 mole % ~ 1 and~ zno 0 +~ 1 mole % J,~,o.-A solution containing a desired amount of aluminum or lithium nitrate was impregnated with pure zinc oxide described Shove (before sintering at 520"), dried a t 110" and sintered at 530" for 5 hours. ZnO-CrZO,.-This was prepared from zinc nitrate and ammonium chromate by the method described by Taylor and Strother.' ZnO-Moo3.-The preparation from ammonium paramolybdate and zinc nitrate was the same as that described by Taylor and Ogden,6 except that zinc paramolybdate was decomposed in air a t 150-200° in this study. Gases.-A 30% solution of potassium hydroxide was electrolyzed to obtain hydrogen, which was purified by passing through palladium asbestos and phosphorus pentoxide. Kitrogen was prepared by the thermal decompos~t~on of sodium azide, and purified by passing through phosphorus pentoxide. Pure helium was obtained from commercial sources, and was used without further purification. Procedures.-Each specimen of the adsorbent for electrical conductivity measurement was, exclusive of evaporated film, shaped by compression into a cylindrical form 10 mm. long and 5 mm. in diameter. The use of too strong a pressure in shaping wa.8 avoided in order to yevent the adsorbent conditions from being very different gom those in chemisorption rate measurements; the apparent density of the specimens used was of the order of 20-30% of the bulk. The specimen wa8 held between two platinum electrode8 ( 5 ) E. Mollivo, Ann. Physik, [GI 8,230 (1948). (0) H. S. Taylor and G. Ogden, Trans. Faradag Soc., SO, 1178
(1934).
833
YUTAKA KUBOKAWA AND OSAMUTOYAMA
834
and put in a holder with a spring which secured the rigid contact between the specimen and the electrodes. The whole entity was placed in a glass vessel which was connected with the vacuum system and manometer. In the case of an evaporated film, the nickel evaporated only on both ends of the glass plate prior to the evaporation of zinc served as electrodes. Conductivity was measured either by means of a Wheatstone bridge or by reading the current intensity at constant voltage of direct current. Temperature rise of the specimen was avoided by keeping the current through the specimen below 10 ma. In this range Ohm's law was found to be approximately obeyed. Preceding a series of conductivity measurements on a particular specimen, alternating evacuation and exposure to hydrogen a t 400" were repeated, until reproducible results were obtained. The rate of chemisorption was investigated with an ordinary apparatus of constant volume. Except in the case of ZnO.MoO3, pressure differences were read with a cathetometer to *0.02 mm. Hg, corresponding to the accuracy of the amount adsorbed, hO.01 cc. The adsorbents were subjected to a pretreatment similar to that in the abovementioned conductivity measurements. Dead space of the adsorption apparatus was measured either with nitrogen or helium in the usual manner. I n both conductivity and chemisorption rate measurements the whole system was evacuated a t 420' for 3 hours between each run. An oilbath was used for temperatures up to 200' and an electric furnace for higher temperatures, the constancy of temperature being fl'. Surface area of the adsorbent was determined by the B.E.T. method using nitrogen as an adsorbate.
4*5
Vol. 60
I r'
40 60 80 100 Minutes. Fig. 2.-Conductivity-time curves for ZnO (sintered); pressure, 65-60 mm.
0
20
Results and Discussion For all the adsorbents examined in this study the conductivity change on the admission of hydrogen was reversible; by evacuating the vessel a t 420" after each run, the conductivity was always brought back nearly to the original value. This indicates that the observed changes in conductivity were not due to the reduction of oxides by hydrogen but to the chemisorption of hydrogen on these adsorbents. The results of conductivity measurements are shown in Figs. 1-4. The conductivity change was insignificant below a specified temperature,
I
0
20 30 Minutes. Fig. 3.-Conductivity-time curves for ZnOCr2O3; pressure, 30 mm. The initial conductivity before the admission of hydrogen was as follows: 1.0 X 10-8 0-l a t 217O, 0.4 X 10-8Q-l a t 184" and 1.5 X 10-8 S2-l a t 146". The data a t 146" refer to a sample of different treatment.
30 40 50 60 70 Minutes. Fig. 1.-Conductivity-time curves for ZnO; pressure, 32-30 mm. 0
10
20
e.g., 80" for zinc oxide and 110" for zinc oxide (sintered), while a t higher temperatures the conductivity increased steadily for all the adsorbents with a rate increasing with temperature as seen in the figures.
10
The rate of chemisorption was measured a t various temperatures and pressures on all the adsorbents used for conductivity measurements, exclusive of zinc oxide (evaporated). Some of the results obtained are given in Figs. 5-7 to show the relation of chemisorption to conductivity change. The kinetics in detail are to be given elsewhere. On zinc oxide (sintered), as shown in Fig. 5, the rate and amount of hydrogen uptake decreased up to l l O o , but increased again a t higher temperatures. This is to be compared with the results of conductivity measurements on the same adsorbent given in Fig. 2 , where the conductivity increase was recognized a t temperatures higher than 110". It may therefore be concluded that two different types of hydrogen chemisorption exist on zinc oxide (sintered) : one with alower heat of adsorption appearing a t lower temperatures and the other with a higher
t .i
July, 1956
CHEMISORPTION OF HYDROGEN ON ZINC OXIDE
835
2.0 22 h 3
& 1.5
. 20
v
5
h
d
:
1.0
Q
ui4
0.5
16
"li'
I
0
10
20 30 40 50 60 Minutes. Fig. 4.-Conductivity-time curves for ZnO.MoOa; pressure, 18 mm. The initial conductiviy before the admi!Q-1 at, 265 , sion of hydrogen was as, follows: 5.81 X 4.40 X f2-l a t 235 and 2.89 X Q - I a t 201
.
.
h
Id"
-
n
A-
I
.
.
.
.
.
.
.
1 . .
0
.
.
I
.
,
,
.
I
.
.
.
.
>
.
.
.
100 150 Minutes. Fig. 6.-Rate of adsorption on ZnOCrz08; initial pressure, 64-60 mm.; surface area, 28 m.*/g.; weight of adsorbent, 12.37 g.
8 o'c.
170.C.
60 80 100 120 iMinutes. Fig. 5.-Rate of adsorption on ZnO (sintered); initial pressure, 34-30 mm.; surface area, 1.3 m.2/g.; weight of adsorbent, 60.21 g. 0
20
c
12
40
50
I
u 20
40
heat of adsorption and with a higher activation energy predominating at higher temperatures. It may be further concluded that chemisorption of the high temperature type is responsible for an increase in conductivity, while that of the low temperature type bears little effect on conductivity. For zinc oxide not sintered the adsorption rate curves are complex and the two types of chemisorption are not clearly discriminated. It is most probable, however, that the same correlation between conductivity and high temperature chemisorption exists in this case too, for a marked increase in conductivity appeared, as shown in Fig. 1, only above 80°, whereas the chemisorption extended to a much lower temperature range, in accordance with the results previously obtained by other w0rkers.l Two types of chemisorption of hydrogen are also recognized in Fig. 6 , which shows the adsorption rate curves obtained on ZnOCrzOs. The ap-
10
0
30 40 50 60 70 80 Minutes. Fig. 7.-Rate of adsorption on ZnO.MoOs; initial pressure, 55 cm.; surface area, 13 m.a/g.; weight of adsorbent, 21.83 g.
10
20
parent activation energy of adsorption computed from required times for a definite amount of adsorption was about 7 kcal. in the range 0-80", 13-16 kcal. above 140°, and was negligibly small between 80 and 110°.7 Figure 3 shows that conductivity increased on this adsorbent a t temperatures higher than 140'. Hence the correlation between the conductivity and the high temperature chemisorption here again is evident. (7)Taylor and Strother investigated the hydrogen adsorption on the same adsorbent with considerably different results from the present work; the adsorption rate monotonously increased with temperature up to 200°, with an activation energy of 3-4 kcsl. in the temperature range 80-200°. The reason for the difference is not yet clear.
YUTAKAKUBOKAWA AND OSAMU TOYAMA
836
On ZnO.MoOo hydrogen chemisorption was recognized only at temperatures higher than 180". Rate curves a t 225-300" are shown in Fig. 7. Conductivity increase was also observed in the same temperature range as shown in Fig. 4. Hence the correspondence between the chemisorption and the conductivity change is very simple and clear in this case. Presumably, the observed chemisorption consisted almost solely of the high temperature type in this case, the contribution of the low temperature type being negligibly small. For this adsorbent, the Zeldovich equationa was applicable to the earlier stages of the chemisorption rate curves. The activation energy calculated from the initial rates determined by this equation was found to be 28 kcal. Except in the last-mentioned case of Zn0.Mo03, the activation energy of chemisorption responsible for the conductivity change cannot be directly obtained from the chemisorption rate curves, since both types of chemisorption are overlapping in them. The activation energies may, however, be estimated from the conductivity measurements in the following manner. We consider here, for simplicity, only the initial stage of the conductivity change. Denoting the initial rate of the conductivity increase and that of the chemisorption by (dK/dt)oand (dn/dQo, respectively, we have (dn/dl)o
E
Vol. GO
evaluated from the temperature dependence of the conductivity in the evacuated state. The results thus obtained are given in Table I, and show a roughly constant value of E for all adsorbents, in spite of the marked dispersion in El as well as Ez. The value of E = 25 kcal. for ZnO.MoOa in Table I is checked by the approximate agreement with 28 kcal. obtained directly from chemisorption rate measurements on the same adsorbent. As another test on the reliability of the activation energies obtained above, some experiments were carried out a t various pressures of hydrogen, with the results given in Table 11, which show but little pressure effect.
e
TABLE I" VALUESOF Ez, E1 A N D E FOR VARIOUSSPECIMENS El
(kcctl./ mole)
Adsorbent
EI (kcal./ mole)
E, (kocLl./ mole)
1.7 24.3 ZnO 26.0 0.1 25.7 ZnO (sintered) 25.8 0.1 23.9 ZnO (evaporated) 24.0 24.8 0.3 24.5 ZnO 1 mole yo ALOa 6.5 24.5 ZnO 1 mole % LizO 31.0 5.2 24.8 ZnO.MoOl 30.0 The activation energy of conductivity for ZnOCrz03 was found to be so much influenced by hydrogen chemisorption that reliable values of E1 and, consequently, of E were not obtained for this adsorbent.
+ +
(dn/dK)o X (dK/dt)o
The temperature dependence of (dK/dn)o may be approximately represented by
VALUESOF E
(dK/dn)o = const. exp ( - E l / R T )
The activation energy of chemisorption E is therefore given by E = E2
- El
where E2is the activation energy of the conductivity increase in the initial stage. Actually, except in the case of ZnOCr20r, the conductivity-time curves were well represented by the parabolic rate law K = K~ k d i in the earlier stages and Ez was determined from the temperature dependence of k 2 since dK/dt = l/z k 2 / A ~ .El was approximately
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(8) Ya. Zeldovioh, Acta Physicochim. (U.R.S.S.), 1, No. 3/4, 449 (1934); H. A. Taylor and N. Thon, J . A m . Chem. Soc., 74, 4109 (1952).
FOR
TABLE I1 EXPERIMENTS AT VARIOUS PRESSURE
Adsorbent
ZnO ZnO
+ 1 mole yoA1208
Pressure, mm.
E (kcal.)mole)
10-12 30-33 18-20 36-39
25.0 24.3 24.4 24.5
Further study must be made to elucidate the nature of chemisorption responsible for the conductivity change as well as the effect of added substances. Acknowledgments.-The authors wish to express their best thanks to Prof. H. Kawamura of Osaka City University for valuable advice and discussion in this work. Thanks are also due to Mr. S. Taniguchi for his assistance in the experiments.
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