1907
CATALYTIC ACTIVITY OF COLD-WORKED AND QUENCHED GOLD
Catalytic Activity of Cold-Worked and Quenched Gold for the Decomposition of Hydrogen Peroxide by Shozo Kishimoto* and Migaki Nishioka Chembtry Department, Faculty of Science, Kobe University, Rokkodai, -Vada-ku, Kobe, J a p a n
(Received January 28, 1972)
Publication costs borne completely by T h e Journal of Physical Chemistry
Changes in hardness and thermoelectric force were observed for cold-worked gold in the range of annealing temperature 100-200" (Td),where dislocations disappear. A sudden decrease in the catalytic activity for the decomposition of hydrogen peroxide was found to take place in the same range ( T d ) . Consequently,it is concluded that the surface terminations of dislocations of cold-worked gold play the part of active sites for the reaction. An increase in the vacancy concentration was observed after quenching from 800-1000" to 0". On the other hand, the quenched catalysts were inactive for the reaction. I t is suggested that the presence of surface emergent point defects of gold has no effect on the catalytic activity.
Much effort has been paid to study the relationship between surface emergent defects in solids and their catalytic activity. Uhara and coworkers1-3 have shown that lattice defects (point defects and dislocations) play an important role for various catalytic reactions in the case of cold-worked metals, in view of the fact that the thermal deactivation temperature was approximately the same as the disappearance temperature of defects during annealing. Eley and n4acMahon14 studying the effect of annealing on the catalytic activity of copper for the decomposition of hydrogen peroxide, showed that the dependence of activity on annealing temperature is similar to the result obtained by Uhara and coworkers.l However, they pointed out that the concentration of surface emergent defects has no effect on their activity and that each crystallographic plane has its own activity. Keating, Rozner, and Youngblood5 showed that the increase in the activity of platinum plate for the decomposition of hydrogen peroxide is apparently due to deformation by the detailed metallurgical characterization since the possible effects of preferred orientation and increased surface area of coldworked metals proved to be negligible. On the other hand, little has been known about the catalytic behavior of quenched metals. It is well known that vacancies are generated during quenching. I n the study of the catalytic decomposition of formic acid on gold powder, Bhalita and Taylors found the generation of high activity after quenching and attributed it to surface defects. Duell and Robertson' showed a similar effect on flashing metals a t temperature near the melting point. The purpose of this study was to establish the role of defect structure of cold-worked and quenched gold on the decomposition of hydrogen peroxide and on some physical properties. The type of defects and the recovery temperature ranges are markedly affected by the presence of impurities as well as by the nature and
degree of working. Consequently, it is desirable to employ the same specimens for the measurements of both catalytic activities and physical properties. The measurement of thermoelectric force and hardness are most convenient to this purpose.
Experimental Section The pure hydrogen peroxide used was obtained by vacuum distillation of 30% commercial hydrogen peroxide at 30 mm pressure after removing the contained stabilizers by means of adsorption on stannic acid. The rate of the decomposition of 3Oj, solution was determined volumetrically with a modified Warburg apparatus a t 30 f 0.1". Well-annealed gold (99.9% or higher purity) was rolled to a constant degree (84% compression) at room temperature. These rolled specimens (50 X 0.17 X 0.005 cm) mere used as the catalyst (surface area approximately 17.5 emz). The annealing of specimens was carried out in an electric furnace for 1 hr a t various temperatures. The quenched samples were obtained in the following manner. The specimens were suspended in a vertical furnace for 1 hr at different temperatures, then allowed to fall into water a t 0". The micro-hardness ( H ) was determined with a micro-Vickers hardness tester. The thermoelectric (1) I. Uhara, S. Yanagimoto, K. Tani, G. Adachi, and S. Teratani, J . Phys. Chem., 66, 2691 (1962). (2) I. Uhara, S. Kishimoto, T. Hikino, Y . Kageyama, H. Hamada, and Y. Numata, ibid., 67, 996 (1963). (3) I. Uhara, S. Kishimoto, Y . Yoshida, and T. Hikino, ibid., 69, 880 (1965). (4) D. D. Eley and D. M. MacMahon, J. Catal., 14, 193 (1969). (5) K. B. Keating, A. G. Roaner, and J. L. Youngblood, ibid., 4, 608 (1965). (6) M. A. Bhakta and H. A. Taylor, J . Chem. Phys., 44, 1264 (1966). (7) M. J. Duell and A. J. B. Robertson, Trans. Faraday Soc., 57, 1416 (1961).
T h e Journal of Physical Chemistry, Vol. 76, N o . IS, 1979
1908
SHOZO KISHINOTO AND MIGAKINIBHIOKA
I 0
I
1 ANNEALING
Figure 1. Thermoelectric force ( E )and hardness ( H ) of cold-worked gold as functions of annealing temperature.
TEMF! ('C)
Figure 2. Catalytic activity of cold-worked gold as a function of annealing temperature.
force ( E ) was measured by means of the method previously described.*
Results and Discussion As Figure 1 shows, the changes in E and H of coldworked gold are considerable in the range of about 100-200" (Td). In comparison with Clarebrough and coworkers' r e ~ u l t s these , ~ ~ ~changes ~ are attributed to the disappearance of dislocations. The dependence of the catalytic activity on annealing temperature is shown in Figure 2. The sudden change observed in the range 100-200" corresponds to T d in Figure 1. This result is similar to the case of cold-worked silver catalysts obtained by Uhara and coworkers for the decomposition of hydrogen peroxide. We may conclude analogously that the active sites of gold catalyst for the reaction are the surface terminations of dislocations. It can be pointed out that the other possible factors affecting the catalytic activity are decrease in surface area and change in preferred orientation due to annealing. Kabe, Mizuno, and Yasumorill found that the roughness factors of palladium foil were only 2.0-1.0 in the range of annealing, 150-800". As a clue to elucidate this problem, it certainly seems desirable to obtain inforniation about the catalytic behavior of single crystals and to study the influence of defects. On quenching, the whole or part of the vacancies can be trapped in the lattice, and the presence of these nonequilibriurn vacancies gives rise to the increase in E , as expressed by AE = A exp(--Er/T,), where A is a constant, E f the formation energy of a vacancy, and T , the quenching temperature. Ef is obtained from Figure 3A; Ef = 1.0 eV for the range T , = 1100-1300"I-L A4sFigure 3B shows, the values of N after quenching were the same as that of well-annealed specimens. It is concluded that the generation of dislocations does not occur during quenching. Making a comparison between the present results and other i n v e ~ t i g a t i o n s , l these *~~~ The Journal of Physical Chemistry, Vol. 76, iVo. I S , IQ7g
\{A)
I
Figure 3. Logarithmic plot of thermoelectric increment (A), hardness (B), and catalytic activity (C) of quenched gold as functions of the reciprocal of quenching temperature ( T4).
phenomena can be at,tributed to the formation of vacancies. On the other hand, the quenched catalysts are inactive for the reaction as shown in Figure 3C; that is, the catalytic activity does not depend on T,. It is suggested that surface emergent point defects of quenched gold have no effect on the catalytic activity, contrary to dislocations produced by cold-working.
-Acknowledgment, The authors wish to thank professor I. Uhara for reading the manuscript and for helpful discussions. (8) S.Kishimoto, J . Phys. Chem., 66, 2694 (1962). (9) L. M. Clarebrough, M. E. Hargreaves, and M. H. Loretto, Phil. Mag., 6 , 115 (1962). (10) "Recovery and Recrystallization of Metals," Commonwealth Scientific and Industrial Research Organization, Australia, 1963. (11) T. Kabe, T. R'lizuno, and I. Yasumori, Bull. Chem. SOC.Jap., 40, 2047 (1967).
(12) J. E. Bauerle and J.
S.Koehler, Phys. Rev., 6 ,
107 (1957).
(13) T. Broom and R. K. Ham, "Vacancies and Other Point Defects in LMetals and Alloys," The Institute of Metals, London, 1958.
