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section it is concluded that the square-well model using the above-described perturbation technique for obtaining the pair correlation function provides a good
first-order theory of the transport properties of simple liquids. I n the future we hope to apply this model to several simple liquids.
Dislocations as Active Centers of Catalysis and Chemical Action in Silver
by Ituro Uhara, Shozo Kishimoto, Yasuko Yoshida, and Tadashi Hikino Chemistry Department, Faculty of Science, Kobe University, Rokkodai, Nada-ku, Kobe, J a p a n (Received September 1, 1964)
On annealing cold-worked Ag, decrease in hardness and in thermoelectric force with increase in temperature is observed in the temperature range 200-350" ( T D ) ,where dislocations disappear. Parallel studies on catalytic activities for (A) decomposition of HzOz, (B) oxidation of ethanol, and (C) decomposition of HCOzH showed that activities decrease on annealing a t nearly the same temperature range ( T A ) . Chemical activity was demonstrated autoradiographically to anneal out in a similar way. I t was concluded that active centers of both catalysis and chemical action in Ag are the surface terminations of dislocations.
Introduction On annealing cold-worked metals, vacancies and dislocations disappear a t two temperature ranges, TV and T D , respectively. These ranges can be observed separately if the degree of cold-working is slight, as shown in the case of Ni.1,2 These phenomena are traced by measuring the changes in various physical properties on annealing. Uhara and his co-workers found that the thermal deactivation temperature (TA) of Cu catalyst (in fact, cold-worked pieces of wire) is approximately the same ~ that catalytic activities of slightly coldas T D and worked S i for many reactions decrease in two steps a t temperature ranges corresponding to TV and T D , respeetively, on annealing and estimated that the active centers in Cu catalyst are the terminations of dislocations at the metallic surface and those in Ni catalyst are both point defects and dislocations a t the surface. In order to determine the structure of active centers in metallic catalysts by means of this ~ n e t h o d ,the ~ use of slightly cold-worked specimens is necessary to avoid overlapping of TV and TD and The Journal of Physical Chemistry
theoretical complexity due to the interaction of lattice defects of high concentration. Ordinary catalysts as reduced metals are not appropriate for the same reaSince the value of T D varies considerably with the degree of cold-working (or the degree of distortion of the crystal lattice) and the purity of the specimen, we must employ an identical specimen or at least a specimen of identical material cold-worked equally for the comparison of T A and TD. TV and T D can be most conveniently determined by measuring hardness ( H ) , thermoelectric force ( E ),5 and electric resistivity ( p ) of ribbon or wire with a small quantity of the specimen. Clarebrough, et ~ l . observed , ~ heat (1) L. M . Clarebrough, M. E. Hargreaves, and G. W. West, Proc. Roy. SOC.(London), A232, 252 (1955): Phil. M a g . , 1, 528 (1956). (2) I. Uhara, S. Yanagimoto, K. Tani, and G. Adachi, Nature, 192, 867 (1961); I. Uhara, S. Yanagimoto, K. Tani, G. Adachi, and S. Teratani, J . Phys. Chem., 66, 2691 (1962).
(3) I. Uhara, S. Kishimoto, T. Hikino, Y. Kageyama, H. Hamada, and Y. Numata, ibid., 67, 996 (1963). (4) Besides, determination of Tv and T D of powder specimens is very difficult. (5)
s. Kishimoto, ibid., 66, 2694 (1962);
67, 1161 (1963).
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evolution and changes in p , 111 density, aiid in H at 110-210" on annealing a 7 5 7 , compressed Ag specimen of 99.987, purity arid attributed theiii to the disappearance of dislocations. Since point defects generated in h g by slight cold-working disappear at around -150 and -40",7 they are out of the question as far as catalysis at a higher temperature is concerned. Decrease in H with increase in teiiiperature of a specimen quenched from 600" was reported to occur a t 250" ,8 These data indicate a remarkable difference in values of T D existing ainorig individual specimens.
Experimental Ag (99.9570pure) was rolled to a constant degree
H
VHN I20
1
zot
(goy6compression) to plate or ribbon at room teniperature and annealed in S:, for 1 hr. at various tenipera00 0 tures. H was iileasured with a micro-T'ickers hardness 200 400 Annealing t e m p , O C tester aiid E was measured potentioiiietrically against well-annealed wire (Figure 1). Figure 1. Hardness ( H ) and thermoelectric force ( E ) of coldThe theriiial deactivation temperature ( T A ) was norked Ag as functions of annealing temperatures. measured for I he following three catalytic reactions and one cheinicttl reaction of Ag. ( A ) Decompositzon of H,O:,. A 0.10% solution was deconiposed by rolled plate of apparent area 2.1 c i ~ i . ~ in a Warburg apparatus at 20" (Figure 2 A ) . ( B ) Oxzdatzon of Ethanol. Rolled plate of apparent area 2.0 ~ i i iwas . ~ used as catalyst after being washed with dilute H S 0 3 arid SH,. Ethanol mas purified by adding 2,4-dinitrophenylhydrazineto reiiiove carbonyl conipounds. Air (1 1.) was bubbled through ethanol at a constant teiiiperature and over the catalyst kept at 280 f. 2" in 40 min. The acetaldehyde formed was caught in rold water, then was analyzed colorimetrically at 470 iiip after being treated with an alkaline solution of 2,4-dinitrophenylhydrazine(Figure 2B). I t was ascertained that dehydrogenation does not take place under this experiiiiental condition. ( C ) Decomposztzon of HC02H (+ H2 C02). A ribbon of 100 mi.* area was eiiiployed after annealing in H2 for 1 lir. at various temperatures. HCOzH Annealing temp., O C was purified by means of repeated vacuuiii distillation Figure 2. Catalytic activities of cold-worked Ag as after standing over B& The reaction rate at 220" functions of annealing temperat,ures: A, decomposition was determined statically by iiieasuring the pressure of H202( 0 ) ; volume (pl. a t STP) of 0 2 evolved in 1 hr.; increase (the nitial pressure being 20-30 iiiiii.). I t B, oxidation of ethanol ( A ) ; fraction of CHZCHO formed is reproducible arid of zero order, the initial value (%); C, decomposition of HCOlH ( 0 ) : relative activity. being used for he comparison of activities (Figure 2C). (D) Conibziiafzon of Ag wzth S Vapor. Thin plates ( T D ) and the catalytic activities for reactions A, B , 9 are exposed to saturated S vapor (at 50" for 30 niin.) and C decrease on annealing iiiainly at 260-340", containing radioactive 36S (ca. 1 mc./iiig. of S) and studied autoradiographically (3-month exposure). (6) L. 31. Clarebrough, M .E. Hargreaves, and 31. H. Loretto. Phil. 1
I
+
Results and Discussion As seen in Figures 1 arid 2, changes in H and in E of the specimens eiiiployed are remarkable at 200-350"
Mug., 7, 115 (1962). (7) J. -4.Manintveld, Suture, 169, 623 (1952). (8) International Critical Tables, Vol. 2 , Xational Research Council, Yew \-ark, N. \-., 1927, p , 478.
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ca. 300", and 260-370" ( T A )respectively. , The change of surface area of plates used as catalysts on annealing is considered to be negligible, because Uhara2 found that Cu powder has almost the same adsorption capacity for methylene blue even after it is annealed at a temperature higher than T D and loses most of the catalytic activity, and the behavior of Ag is very similar to that of Cu in both annealing and catalysis. We may be able to conclude in the same way as for Cu that T A is approximately the same as T Dand that the active centers of Ag catalysts are the surface terminations of dislocations. According to Sosnousky,lo a single crystal of Ag which was bombarded with argon ion can decompose HCOzH catalytically and its activity is of the same order as a cold-worked one. Since the activity remains even after 18 hr. annealing a t 250", he concluded that the active centers are not point defects but dislocations, in accordance with our results (C). In A and B most of the activities are lost with the disappearancae of dislocations, therefore the normal surface is proved to be practically inactive. In C, on the other hand, ca. 60% of the activity remains even when dislocations are annealed out (decomposition on glass surface or in the gaseous phase is negligible). This means that the normal surface is fairly active for this reaction. I n the case of industrial oxidation of methanol the temperature of metallic catalysts is much higher (500-900") , and contribution of active centers niay be negligible and the reaction may take place rather uniformly at the surface of catalysts. New Active Center Generated at Higher Temperatures than TD. On annealing nietals at T Dor a t higher temperatures most of the dislocations disappear. Although a small fraction of them survives their catalytic activities do not seem to be significant. I n Figure 2, however, we see enhanced activity of specimens which were heated at temperatures much higher than TD. This tendency is frequently observed in some nietals such as P t , Pd, and Ni, too, for certain catalytic reactions. Since there is no indication of concurrent change in physical properties of bulk metal as shown for E in Figure 1 (density and p do not change either
The Jocunal of Physical Chemistry
I. UHARA, S. KISHIMOTO, Y. YOSHIDA, A N D T. HIKINO
as are well known), this might be attributed to some surface phenomena. Researches on this line are now in progress. Active Centers of Chemical Reaction. Chemical activities of the surface termination of dislocations are well known from the formation of etch pits. In our case (D), Ag plates with and without dislocations were exposed to S vapor containing %. Whether dislocations are chemically more active than the normal plane or not can be decided by comparing the blackness (B) of the autoradiograph as follows: ( a ) rolled plate (dense gray autoradiograph), ( p ) plate rolled and annealed for 1 hr. a t 250" (almost transparent), and ( y ) plate rolled and annealed for 1 hr. at 350" (transparent). B may be originated from (X) van der Waals adsorption of S on Ag, (Y) chemisorption of S on Ag, and (Z) surface compound formation. The negligibly small values of B for specinlens p and y indicate that none of X, Y, and Z took place appreciably in these annealed specimens. Besides, the decrease in the surface area of the plates is probably negligible as described previously and so X cannot be responsible for the large difference in B between a and p or y. Consequently, the large B value of a that contains dislocations may be concluded to have been caused by Y or Z. At any rate, the combination of A4gand S is chemical, being probably the initial stage of the chemical reaction, and the existence of dislocation is indispensable to it a t least at 50". It niay be concluded, therefore, that chemical action takes place preferentially a t the surface terniinations of dislocations.
Acknowledgment. The authors express their hearty thanks to Professor T. Iida for her kind advice and to Mr. S. Taniguchi for his assistance. (9) As for the data of B at 100 and 200°, the specimens must have undergone annealing corresponding to 280" to some extent during the catalytic reaction since the rate was measured at this temperature. But the perfect coincidence of the data with that at 280' demonstrates the nonexistence of any change in the catalyst a t 100-280°, as long as the disappearance of dislocations is not too fast. (10) H. M.C. Sosnousky, J . P h y s . Chem. Solids, 10, 304 (1959).
