May, 1956
ELECTRICAL CONDUCTIVITY AND CATALYTIC ACTIVITY OF ZINC OXIDE
559
ELECTRICAL CONDUCTIVITY AND CATALYTIC ACTIVITY OF ZINC OXIDE' BY LOUISF. HECKELSBERG, ALFREDCLARKAND GRANTC. BAILEY Phillips Petroleum Company, Research Division, Bartlesville, Oklahoma Received August 16, 1966
Zinc oxides were investigated experimentally both as to electrical conductivity, a t 200 to 550°, and as to catalytic activity for hydrogen-deuterium exchange, at -10 to 320". Lattice defects were produced by heating the various zinc oxides in a selected gas (hydrogen, nitrogen or oxygen) or by incorporating as an impurity the oxide of a metal different from zinc in valence (aluminum, lithium). The results indicate that treatment of the zinc oxides so as to increase the number of lattice defects, increases their electrical conductivity and catalytic activity.
Zinc oxide, which is an n-type semi-conductor, anhydrous magnesium perchlorate at room temperature; nitrogen (oxygen content found, 6 p. m.), Air Rcduction increases in electrical conductivity when the partial Co.'s prepurified, further purified by grierite, Ascarite and pressure of oxygen in contact with it is decreased.2 anhydrous magnesium perchlorate; oxygen, USP Linde, According to the concept of lattice defects in solid purified by Drierite and Ascarite; deuterium, Stuart Oxysemi-conductors, the zinc oxide, on losing oxygen gen Co.'s 99.5%, purified as was the hydrogen. and Procedure.-The apparatus and procedure atoms, contains excess zinc atoms, which dissociate forApparatus hydrogen-deuterium exchange have been described.' into zinc ions and quasi-free electrons. Increasing The rate constants were calculated with the assumption the amount of excess zinc atoms increases the num- that the reaction Hz + Dz k~ 2 H D is first order. The conber of quasi-free electrons, thereby increasing the version was given by the ratio of the hydrogen deuteride concentration in the product, as found by a mass specelectrical conductivity. trometer, to the equilibrium concentration. The concentration of lattice defects can be varThe electrical-conductivity apparatus, which was deied as by heating the zinc oxide in a selected gas or signed for easy assembling and disassembling, had two by incorporating as an impurity in the zinc oxide vertically aligned shaft-like stainless steel electrodes in a housing of a Pyrex glass tube ending in female taper joints. the oxide of a metal differing from zinc in ~ a l e n c e . ~The contact end of each electrode was a welded-on platAlumina as an impurity increases the electrical con- inum disk, 12 mm. in diameter. The lower end of the lower electrode was screwed into a rigidly held brass plug fitting ductivity; lithia decreases it. Although an investigation of zinc oxide contain- the lower end of the housing. The upper end of the upper was attached through a spring-and-screw device ing alumina or lithia as a catalyst for the hydrogen- electrode to a brass plug fitting the upper end of the housing. A gas deuterium exchange has been reported15measure- inlet and a gas outlet were provided near the upper and ments of the electrical conductivity were not in- lower ends of the housing, respectively. The central part of the apparatus was heated by an electric furnace. The cluded. was controlled by a Tagliabue Celectray conIn the present work, zinc oxides with different temperature troller actuated by a thermocouple with its junction outside concentrations of lattice defects were investigated the housing at a level between the two platinum disks. both as to electrical conductivity and as to cata- Another thermocouple, with similarly located junction, lytic activity for the hydrogen-deuterium exchange, measured the temperature. The apparatus was charged, assembled and used as folto see if these two properties could be related. lows: A paste of zinc oxide particles finer than 120-mesh in water was spread in a layer on the platinum disk of the lower Experimental electrode, which was then mounted in the furnace. The Materials.-Zinc oxide I, of relatively high surface Area, was prepared by precipitating and thermally decomposing zinc oxalate,B using zinc nitrate and ammonium oxalate (J. T. Baker labeled C.P.). Its BET surface area was 21 sq. m./g.' Zinc oxide 11, of relatively low surface area, was prepared by thermally decomposing zinc nitrate. Zinc oxides 11-AI and 11-Li were prepared similarly from aqueous solutions of zinc nitrate plus aluminum nitrate and lithium nitrate, respectively. (Their surface areas, by the BET method and by microscopic examination, were in the range 0.01-0.1 sq. m./g.) These three catalysts were heated in flowing dry air a t 800' for 60 hours. Zinc oxide 11-A1 contained,.as determined analytically, 0.3 mole % alumina; zinc oxide 11-Li contained 0.03 mole % lithia. The following gases were used: hydrogen, commercial electrolytic, purified by platinized silica gel a t 300" and (1) Presented in part at the Joint Symposium on Mechanisms of Homogeneous and Heterogeneous Hydrocarbon Reactions at the Kansas City meeting of the American Chemical Society, March 29 t o April 1, 1954. (2) H . H . v. Baumbach and C. Wagner, 2.physik. Chem.. 22B, 199 (1933). (3) A. H . Wilson, Pro2. R o y . So.. ( L o n d o n ) ,A133, 458 (1941); A134, 277 (1931). (4) C . Wagner, J. Chem. P h y s . , 18, 6 2 (1950); E. J. Verwey, P. W.
Haaymon, P. C. Roweijne and G . W. Osterbout, Phillips Research Reports, 6, 173 (1950). ( 5 ) E. Molinari and G . Parravano, J . Am. Chem. SOC.,76, 5283 (1953). ( 6 ) H. S. Taylor and D. V . Sickman, ibid., 64, GO2 (1932).
housing was lowered into position, and its top was closed with a Pyrex glass plug. A downward flow of about 10 cc./ min. of the gas selected for the heat treatment was established. The furnace was heated to the desired heat treatment temperature. At the end of the heat treatment, the glass plug was replaced by the brass plug with the upper electrode well retracted. The screw was turned to lower the upper electrode into contact with the zinc oxide layer, which had dried to a soft, porous cake. The turning was continued until the brass plug was pushed up, with a perceptible "pop," within the upper end of the housing. The weight of the upper electrode, spring-and-screw device, and brass plug now rested on the zinc oxide layer, exerting a pressure of approximately 300 g./sq. em. Ten additional turns were made to ensure a contact of good reproducibility. The gaa, if hydrogen or oxygen, was now replaced by nitrogen, which removed adsorbed hydrogen or oxygen; and the rate wiis adjusted to 5 cc./min., which was considered sufficient t o prevent leakage of air into the apparatus. In a typical run the temperature was increased to 560", and the heating current was switched off. The apparatus cooled a t a rate that declined from 9"/min. a t first to 1'/min. at 260". As cooling occurred, the electrical conductivity was measured at 10" intervals, with the initial measurement being made a t 550". The measurement was maclc by momentary application of a direct current voltage selected in the range of 1.5-135 volts and measuring the resulting current with a microammeter or with a galvanometer. The ( 7 ) V . C. F. Holm and R. W. Blue, Ind. Eng. Chern., 43, 501 (1951).
polarity was reversed after each measurement. At the end of a series of measurements, the apparatus was disassembled, and the thickness of the soft tablet of zinc oxide, which was in the range of 0.5-2.0 mm., was measured with a micrometer caliper. The specific conductivity was calculated by dividing the roduct of the thickness and the current by the product o r t h e contact area and the applied voltage. A fresh sample of zinc oxide was used for each series of measurements, and at least two series of measurements were made for each set of heat treatment conditions. Adsorbed Hydrogen.-Adsorbed hydrogen was found to increase the electrical conductivity of zinc oxide enormously; for example, at 100" the electrical conductivity in hydrogen was 108 times as great as in nitrogen or helium. Instances of changes in electrical conductivity because of adsorbed hydrogen have been reported previously.8 The amount of hydrogen adsorbed at various temperatures by zinc oxide prepared similarly to zinc oxide I from Merck Reagent Grade chemicals was found bv the volumetric method to be as follows; in cc. (STP)/g.:" 0", 0.21; looo, 0.19; 200°, 0.17; 300",0.14; 400°, 0.09; 500", 0.03. The adsorbed hydrogen multiplied the experimental difficulty of obtaining reproducible measurements of the electrical conductivity so greatly that only a single measurement believed to be reliable could be obtained for a given sample of zinc oxide in hydrogen. Seven such measurements made in the range of 30-100" gave values of -4.8 to -3.0 for the logarithm of the s ecific conductivity. When plotted against the reciprocaf of the absolute temperature these values gave a straight line that indicated an activation energy of 11.3 kcal ./mole. In contrast, no difficulty was experienced when measurements were made in helium with another ortion of the zinc oxide; in the range of 120-500" the rogarithm of the specific conductivity, when similarly plotted, determined a straight line from -10.2 to -5.8, and the activation energy had the much larger value of 18.8 kcal./ mole. This observation of increase in activation energy appears to be consistent with the idea that the Fermi level rises with an increase in the number of defects, in this case, adsorbed hydrogen. -5
Vol. 60
L. F. HECKELSBERG, A. CLARKAND G. C. BAILEY
560
Results and Discussions Heat-treated Zinc Oxide.-Three portions of zinc oxide I were modified by heating for 16 hours in different flowing gases to obtain relatively high, intermediate, and low concentrations of lattice defects, at the following temperatures: 400" in hydrogen; 500" in nitrogen (4 hours) ; 500" in oxygen. After the heat treatment, the electrical conductivity of each of these heat-treated zinc oxides was measured at 10" intervals as the zinc oxide cooled in a slow current of nitrogen. The results are given in Fig. 1 in the form of curves for the logarithm of the specific conductivity plotted against the reciprocal of the absolute temperature. (The curves were obtained by the method of least squares from at least two sets of measurements for a given oxide. The plotted points are for only one set of measurements.) The relative positions of the three curves show that the hydrogen-treated zinc oxide, with the highest concentration of lattice defects, had the highest electrical conductivity; the nitrogen-treated zinc oxide, with an intermediate concentration of lattice defects, had an intermediate conductivity; and the oxygen-treated zinc oxide, with the lowest concentration of lattice defects, had the lowest conductivity. These heat-treated zinc oxides were investigated for catalytic activity for hydrogen-deuterium exchange. The results are given in Fig. 2 in the form
I (N2 AT 25')
--*-e -Y U I
1
HYDROGEN TREATED
z
v
--7
z
=
I-
v
NITROGEN TREATED
n
z
8
2 -8 LL
M A OXYGEN TREATED
b. 0
0
3 -9
I
I
1.3
1.4
I
I
I
I
1.7
1.8
I
I
2.0
2.4
\
I
2.8 I / T x I$.
I
I
3.2
3.6
1.9
Fig. 2.-Relationship of reaction rate constants with temperature for zinc oxide for difiercnt heat treatments.
Fig. 1.-Relationship of specific conductivity with temperature for zinc oxide after different heat treatments.
of curves for the reaction rate constant plotted logarithmically against the reciprocal of the absolute temperature. The three curves are in the same relative order as the corresponding curves of Fig. 1, indicating that the catalytic activity was greater the greater the electrical conductivity or the greater the concentration of lattice defects. Because of the following circumstance, the curve in Fig. 2 for the oxygen-treated zinc oxide is in fact
1.2
1.5 1.6 VTXIO~.
In the major part of the investigation, adsorbed hydrogen was preliminarily removed from the zinc oxide by flushing with nitrogen. (8) J. 8. Anderson and M. C. Morton, Proc. Roy. Soc. (London), A184, 83 (1945); P. B. Weisz, C. D. Prater and K. D. Rittenhouse, J . Chem. Phys. 21, 2236 (1953); L. F. Heckelsbcrg, G. C. Bailey and A. Clark, J . Am. Chem. Soc., 77, 1373 (1955).
May, 1956
ELECTRICAL CONDUCTIVITY AND CATALYTIC ACTIVITYOF ZINCOXIDE
for zinc oxide reduced to various undetermined degrees. Since oxygen poisons the hydrogen-deuterium exchange, the oxygen-treated zinc oxide was preliminarily flushed with nitrogen at room temperature for 5 minutes and with the reactant mixture of hydrogen and deuterium at the reaction temperature for 30 seconds, whereupon some reduction occurred. Further reduction occurred during the runs although it was minimized by limiting the runs to 5 minutes each; the second of two successive samples of product collected for analysis always differed from the first by containing more hydrogen deuteride, indicating progressive increase in catalytic activity. A new sample of zinc oxide was used for each run. The curvature of the curve indicates that the extent of reduction increased with increase in reaction temperature. The energy of activation for the hydrogen-deuterium exchange, calculated from the slope of the curve for the hydrogen-treated zinc oxide in Fig. 2, was 6.5 kcal./mole, in fair agreement with literature value^.^^^ Impurity-containing Zinc Oxide.-In Figs. 3 and 4 are given the results for electrical conductivity and for catalytic activity of zinc oxides, 11, 11-A1 and 11-Li. The relative positions of the curves in Fig. 3 show, in agreement with the literature, that the electrical conductivity of zinc oxide was increased by alumina and was decreased by lithia. The changes caused by the impurities were relatively much smaller for the oxygen-treated samples than for the hydrogen-treated ones, indicating that relatively fewer stoichiometrically excess metal atoms were present. The fact that part of the curve for the oxygen-treated zinc oxide containing alumina is below the curve for oxygen-treated pure zinc oxide is attributed to experimental difficulties. Some of the curyes in Fig. 3 change slope at about 300-400". The relatively steep slopes above the inflections indicate qn activation energy of the order of 23 kcal./mole. Since this value is about onehalf of the energy gap between the valence and the conduction band of zinc oxide, 46 kcal./mole,'O it appears that the conductivity in this temperature region may be intrinsic conductivity. The less steep slopes below the inflections may be related to the impurity aluminum atoms and/or excess zinc atoms. Similar inflection to a lesser extent may be noted in the curves in Fig. 1. The slopes of the curves of Fig. 3 decrease in the order of passage from the oxides of low electrical conductivity to oxides of high electrical conductivity, reflecting the fact that the Fermi level moves up with increase in the concentration of electrondonors (aluminum atoms or excess zinc atoms). For lithium, which in comparison with zinc behaves relatively like an electron acceptor, the Fermi level moves down, increasing the slope and the activation energy. Figure 4 shows that alumina in the zinc oxide increased the catalytic activity for hydrogen-deuterium exchange, and that lithia decreased it. Two batches of hydrogen-treated zinc oxide containing 0.03 mole yolithia showed low catalytic activity; the experimental points were of poor repro(9) E. A. Smith and H. 8. Taylor, J . Am. Chem. Soc., 60,362 (1938). (IO) C. Kittle, "Introduction to Solid State Physics," John Wiley and Sons, Intl., New York, N.Y.,1953,p. 276.
56 1
-H2 TREATED
0
\ I
I
I
I
