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The adsorption of hydrogen on nickel catalysts. II. Sorption isobars from 20°K. to room temperature
THE ADSORPTION OF HYDROGEN ON NICKEL CATALYSTS. II. SORPTION ISOBARS FROM 20°K. TO ROOM TEMPERATURE O. Beeck, J. W. Givens and A. W. Ritchie Shell Development Company, Emeryville, California Received November 9, 1949
INTRODUCTION The first paper of this series (1) gives an account of studies of the adsorption of hydrogen on evaporated nickel films in which the effect of sintering the films before adsorption takes place was investigated. In this work it was found that the ratio of slowly sorbed hydrogen to rapidly adsorbed hydrogen increases with the sintering temperature of the film and it was shown that the slow "activated adsorption" of hydrogen on nickel, which has been previously observed by other investigators, was actually slow sorption of hydrogen into the interior of the metal structure rather than adsorption. This Work further showed that sites occupied by hydrogen that is slowly sorbed into the interior of the metal structure are not accessible to ehemisorption of carbon monoxide or van der Waals' adsorption of krypton, nor are they accessible to ethylene. It was concluded that the very fast chemisorption of hydrogen at liquid nitrogen temperature is a true measure of the surface available for ehemisorption of carbon monoxide, for van der Waals' adsorption of krypton, and for surface reactions such as the hydrogenation of ethylene. It is the purpose of this paper to present the results of measurement of sorption isobars from 20°I~. to room temperature This work was undertaken to elucidate further the nature of adsorption by metal films and,to obtain further confirmation of the results obtained in the first paper. It will be seen that the conclusions from the low temperature results are in good agreement with those from the earlier work. As a result a clear picture of the sorption process on evaporated metal films has been obtained. EXPERIMENTAL The evaporated metal films were made by the technique previously described (2). The films used in these experiments were evaporated in a high vacuum (10-5 ram.) and were deposited on the surface of a cylindrical adsorption vessel kept at 0°C. by an ice-bath. The films were subsequently 141
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sintered at either 23°C. or 200°C. All adsorption measurements were carried out in a Collins helium cryostat (3). This apparatus is admirably suited for carrying out a wide variety of measurements over the temperature range from 1.5°K. to room temperature. In this work the adsorption vessel containing the deposited film was inserted into a brass cylinder on which was wound a heating coil of manganin wire and to which was soldered the copper-constantan thermocouple used for measuring the temperature. This thermocouple, designated Copper-Constantan Thermocouple No. 1, consists of five parallel No. 30 glass covered especially selected constantan wires soldered to one No. 24 thermocouple grade copper wire. This thermocouple was calibrated against a helium gas thermometer and against the boiling point of helium at one atmosphere. All thermocouple potentials were measured by means of a Wenner potentiometer. In carrying out the adsorption measurements, the temperature of the vessel was regulated manually by balancing the heat input to the brass cylinder against the cooling rate produced by the cryostat engines. In this way the adsorption vessel could be brought to a n y predetermined temperature and held there with a variation of only 0.1°K. RESULTS
In Fig. 1 are shown adsorption isobars at 0.1 mm. hydrogen pressure for hydrogen on evaporated nickel films. The solid curves represent the isobars for increasing and decreasing temperature. With increasing temperature (the part between 20 and 80°K. will be discussed later), sorption increases rapidly between 80 and 170°K. While initial sorption at each point, after raising the temperature, was fairly rapid, it was followed by a very slow sorption, and true equilibrium values were not obtained in the time of about 30 rain. allowed for equilibration. It is possible, therefore, that the maximum at about 170°K. would have Shifted to lower temperatures if long enough waiting periods had been used. I n the descending branch of the isobar, true equilibrium was obtained very rapidly and, as indicated by the squares, this isobar could be traced exactly when again increasing the temperature, without any intermediate outgassing of the system. When the system was pumped to high vacuum for 15 min. at 80°K. the subsequently measured isobar starting at 20°K. with increasing temperature exactly retraced the isobar obtained initially on the branch of decreasing temperatures, assuming identity of the two curves at the temperature of pumping. After the system was pumped to high vacuum for 15 min. at 175°K., the isobar for increasing temperature showed a pronounced minimum and maximum, the amount taken up at the 80°K. minimum agreeing well with the amount which was taken up in the first isobar when 175°K. had been reached on the branch of descending temperatures, again assuming identity of the two curves at the temperature
HYDROGEN ADSORPTION. II
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of pumping. After pumping the system to high vgcuum for 15 min. at 296°K., the subsequently measured isobar for increasing temperature (starting again at 20°K.) showed at its minimum value an uptake practitally identical with the uptake found at 296°K. on the ascending branch of the very first isobar. As in the two previous cases, identity of the two curves at 296°K. is assumed. These facts are of interest because they show that the amount of hydrogen which can be removed from the system by pumping to high vacuum for 15 rain. at 175°K. and 296°K. is being resorbed almost completely in the lower temperature region between 20 ~nd 80°K. More important, however, is the fact that the rise of the initial isobar with decreasing temperature down to about 80°K. must also be partly due to absorption. Further proof of this will be given in the next paragraph. 0 V [3 X • "~" "0-
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FIO. 1. Sorption isobar of hydrogen at 0.1 ram. on evaporated nickel fihn sintered at 23°C. (To obtain the ordinate in molecules X 10-is/100 rag., divide by 9,07.)
Figure 2 shows the ascending and descending branch of a hydrogen isobar at 0.1 ram. pressure on a nickel film previously sintered at 200°C. If the 80°K. point of the ascending isobar in this figure is compared with the 80°K. point of the ascending isobar in Fig. 1 (after converting the ordinate in Fig. 1 to molecules X 10-18/100 rag. of nickel film as indicated below the figure), it is noted that the initial hydrogen adsorption at 80°K. is nine times larger per weight unit for the film sintered at 23 ° than for the film sintered at 200°C. At the same time it is seen that the difference of hydrogen sorption between the descending and ascending branch of the
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isobar at 80°K. is the same for both types of films and, thus, evidently a weight effect. Furthermore, the ratio of total sorption at 80°K to that at room temperature on the decreasing branch of the two isobars is the same for both types of films, indicating that the relative decrease of total sorption with temperature is independent of surface. Figure 2 shows clearly that the major portion of the hydrogen sorption at room temperature of a film sintered at 200°C. is absorption into the interior of the structure. It is interesting to note that the total van der Waals' adsorption as given by the differences of sorption between 20 ° and 80°K. is practically the same for the ascending and the descending part of the isobar. Furthermore, the numerical value of the van der Waals' adsorptio n for the film sintered at 200°C. is seen to be approximately 4 times lower than the van der Waals' adsorption observed on a film sintered at 23°C. as shown by Figs. 1 and 2. The reason that the ratio of the van der Waals' adsorp4.0
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FIG. 2. Sorption isobar of hydrogen at 0,i ram. on
evaporated nickel film sintered at 200°C. tion in the two types of film is not equal to the ratio of the surfaces as measured by the adsorption at 80°K. in the ascending part of the isobars of the two types of film will become clear after the following discussion of Fig. 3, which represents an isobar on the same type of film in the region between 20 and 120°K. All points of this isobar with increasing or decreasing temperature are numbered in the sequence in which they were measured. It is seen that, starting with point No. 1 at 20°K. and proceeding to 50°K., the isobar can be traced back to point No. 3 at 20°K., which coincides with point No. 1. Ascending again with the temperature, point No. falls on the same curve and, after obtaining point No. 5, the isobar can be traced back and forth to point No. 9. Since the isobar flattens out completely at 80°K. with the value of 7.2 X 1018molecules/100 mg. it can
145
HYDROGEN ADSORPTION. II
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Fie. 3. Sorption isobar between 20°K. and 120°K. of hydrogen at 0.1 ram.
on evaporated nickel film sintered at 23°C. be conclu~led that the total adsorption of hydrogen at 20°~2. consists of 7.2 X I0 Is molecules/lO0 mg. of nickel of chemisorption and of 6.0 X 1018 molecules/lO0 mg. van der Waals' adsorption.
Discussion Measurements made in these laboratories (3a) have shown the heat of adsorption of hydrogen at liquid air temperature to be 30,000 cal./mole for the sparsely covered surface decreasing to 18,000 cal./mole for the completely covered surface. These values are in complete agreement with those obtained at room temperature. Since the isobar in Fig. 3, starting at 20°K. can be retraced back and forth to 80°K., it is evident that the chemisorption observed at 80°I<. must already have occurred at 20°I4. By tracing the isobar further to point No. 10, it is seen that a small amount of sorption into the interior of the structure has taken place. After initiation of this process, which is activated by the energy supplied by raising the temperature from 80°K. to 120°K., it is interesting to note that this process will continue, even at an increasing rate, upon lowering the temperature again to point No. 11. Upon tracing the isobar down to
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20°K., it is found that the total sorption value of point No. 13 is higher (within the limits of experimental error) by an amount corresponding to the absorption that has taken place during this time. Upon retracing the isobar back through points 14, 15, and 16, more gas is absorbed and the process could presumably have been continued until the total absorption would have been identical with that of the descending branch in Fig. 1, the absolute value at 80°K. being higher by the difference in adsorption of the two films. These measurements show clearly that after the absorption process has once been started by raising the temperature from 80 ° to 120°K., it will continue even at lower temperatures. It should be pointed out that several repetitions of this experiment have shown that absorption was sometimes initiated at temperatures as low as 70 ° to 80°K., if sufficient time were allowed. The increase observed with lower temperatures seems to be due to the higher surface concentration of molecules in the van der WaMs' adsorption layer. Once the formation of the "hydride phase" was initiated at lower temperatures, the growth of this new phase, even at temperatures below the initiation temperature, is probably due to the assistance of nuclei of the new phase that have opened up the passage into the interior. It is concluded that this process is an exothermic type of absorption or solution, possibly the formation of a hydride. In comparing Figs. 2 and 3 it is seen that the van der Waals' adsorption at 20°K. of an unsintered film as measured by the difference of the ordinates of points 1 and 5 in Fig. 3 is five times as large as that of the sintered film in Fig. 2 (difference between adsorption at 20°K. and 80°K.), while the chemisorption is thirteen times larger for the unsintered than the sintered film. This discrepancy may possibly be explained by the increased capillary condensation in the sintered film. Additional studies at very low temperatures are necessary to clarify this point. It is of interest to note that in the measurement of the hydrogen absorption on the ascending isobar of Fig. 1, a sudden desorption of hydrogen took place, after raising the temperature, followed by a slow sorption that exceeded the initial desorption. This phenomenon is identical to that reported by Taylor and Chou-Shou Liang (4) and earlier by Frankenburger and Messner (4a) and suggests that their experiments may deserve reexamination in the light of the findings on nickel, inasmuch as it is possible that Taylor and Chou-Shou Liang may have interpreted an activated absorption process as activated adsorption in the absence of further proof. From the nature of the isobars, it is clear that the absorption process must be exothermic and that, like adsorption, it is reversible at higher temperature. The heat of absorption must be considerably lower than the heat of adsorption, inasmuch as the absorption process, which is definitely of the activated type with a measurable temperature-dependent rate, is more readily reversible at high temperatures than the adsorption process. An interesting side light on the nature of the absorption process was ob-
HYDROGEN ADSORPTION. II
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rained through the measurement of sorption isobars on palladium. The adsorption on palladium at liquid nitrogen temperature was shown (through rates of hydrogenation measurements and by CO adsorption) to be comparable to the adsorption on the other metal films investigated, although the initial fast hydrogen adsorption is always followed by a slow sorption that seems to continue indefinitely. When the temperature is raised, the uptake of hydrogen increases in the neighborhood of - 100°C. to an amount which is approximately equal to the absorption of one hydrogen atom per atom of palladium. This absorption decreases again rapidly when increasing the temperture to room temperature and more slowly when increasing the temperature further to 100°C., at which point the sorption again is low (about four times larger than that obtained at - 196°C.). The 1 : 1 ratio of hydrogen atoms to palladium leaves no doubt that hydride formation is involved, and reports in the literature (5) have indeed shown that low temperature solution of hydrogen in palladium leads first to a hydride of cubic structure and that raising of the temperature causes this hydride to lose hydrogen with the formation of a hexagonal hydride with a palladium-hydrogen ratio of 3 : 1. These reports show also that upon raising the temperature still higher the hydride decomposes again. In the case of nickel, the number of hydrogen atoms in solution is too small to be detected by diffraction methods. SUMMARY
1. Sorption isobars for hydrogen at 0.1 mm. on evaporated nickel films have been obtained from 20°K. to room temperature by aid of a Collins helium cryostat. 2. Slow activated sorption, which is interpreted as sorption of the hydrogen into the metal structure, has been observed in confirmation of the results previously reported. 3. Slow sorption or "hydride phase" formation is exothermic and after initiation by raising the temperature to about 120°K. will continue at much lower temperatures. This effect is probably due to the assistance of nuclei of the new phase that have opened up passage into the interior. REFERENCES 1. BEECK, O., t~ITCHIE, A. W., AND WHEELER, A., J. Colloid Sci. 3, 505 (1948). 2. BEECK, 0., SMITH~A. E., AN-DWHEELER, A., Proc. Roy. Soc. (London) A177, 62 (1940). 3. COLLINS,S. C., Rev. Sci. Instruments 18, 157 (1947). 3a. BEECh:, 0., Rev. Modern Phys. 17, 61 (1945). 4. TAYLOR,H. S., AND CHou-SHov LING, J. Am. Chem. Soc. 69, 1306 (1947). 4a. MESSNER, a., AND FRANKENBURGER; W., Z. physik. Chem. Bodenstein Festband 593 (1931). 5. MICHEL, A., et al., Bull. soc. chim. [5] 12, 336 (1945); OWEN, E. A., AND JONES, J. I., Proc. Phys. Soe. (London) 49, 603 (1937). MICHEL, A., AND GALLISSOT, M., Compt. rend. 208, 434 (1939).
INTRODUCTION The first paper of this series (1) gives an account of studies of the adsorption of hydrogen on evaporated nickel films in which the effect of sintering the films before adsorption takes place was investigated. In this work it was found that the ratio of slowly sorbed hydrogen to rapidly adsorbed hydrogen increases with the sintering temperature of the film and it was shown that the slow "activated adsorption" of hydrogen on nickel, which has been previously observed by other investigators, was actually slow sorption of hydrogen into the interior of the metal structure rather than adsorption. This Work further showed that sites occupied by hydrogen that is slowly sorbed into the interior of the metal structure are not accessible to ehemisorption of carbon monoxide or van der Waals' adsorption of krypton, nor are they accessible to ethylene. It was concluded that the very fast chemisorption of hydrogen at liquid nitrogen temperature is a true measure of the surface available for ehemisorption of carbon monoxide, for van der Waals' adsorption of krypton, and for surface reactions such as the hydrogenation of ethylene. It is the purpose of this paper to present the results of measurement of sorption isobars from 20°I~. to room temperature This work was undertaken to elucidate further the nature of adsorption by metal films and,to obtain further confirmation of the results obtained in the first paper. It will be seen that the conclusions from the low temperature results are in good agreement with those from the earlier work. As a result a clear picture of the sorption process on evaporated metal films has been obtained. EXPERIMENTAL The evaporated metal films were made by the technique previously described (2). The films used in these experiments were evaporated in a high vacuum (10-5 ram.) and were deposited on the surface of a cylindrical adsorption vessel kept at 0°C. by an ice-bath. The films were subsequently 141
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sintered at either 23°C. or 200°C. All adsorption measurements were carried out in a Collins helium cryostat (3). This apparatus is admirably suited for carrying out a wide variety of measurements over the temperature range from 1.5°K. to room temperature. In this work the adsorption vessel containing the deposited film was inserted into a brass cylinder on which was wound a heating coil of manganin wire and to which was soldered the copper-constantan thermocouple used for measuring the temperature. This thermocouple, designated Copper-Constantan Thermocouple No. 1, consists of five parallel No. 30 glass covered especially selected constantan wires soldered to one No. 24 thermocouple grade copper wire. This thermocouple was calibrated against a helium gas thermometer and against the boiling point of helium at one atmosphere. All thermocouple potentials were measured by means of a Wenner potentiometer. In carrying out the adsorption measurements, the temperature of the vessel was regulated manually by balancing the heat input to the brass cylinder against the cooling rate produced by the cryostat engines. In this way the adsorption vessel could be brought to a n y predetermined temperature and held there with a variation of only 0.1°K. RESULTS
In Fig. 1 are shown adsorption isobars at 0.1 mm. hydrogen pressure for hydrogen on evaporated nickel films. The solid curves represent the isobars for increasing and decreasing temperature. With increasing temperature (the part between 20 and 80°K. will be discussed later), sorption increases rapidly between 80 and 170°K. While initial sorption at each point, after raising the temperature, was fairly rapid, it was followed by a very slow sorption, and true equilibrium values were not obtained in the time of about 30 rain. allowed for equilibration. It is possible, therefore, that the maximum at about 170°K. would have Shifted to lower temperatures if long enough waiting periods had been used. I n the descending branch of the isobar, true equilibrium was obtained very rapidly and, as indicated by the squares, this isobar could be traced exactly when again increasing the temperature, without any intermediate outgassing of the system. When the system was pumped to high vacuum for 15 min. at 80°K. the subsequently measured isobar starting at 20°K. with increasing temperature exactly retraced the isobar obtained initially on the branch of decreasing temperatures, assuming identity of the two curves at the temperature of pumping. After the system was pumped to high vacuum for 15 min. at 175°K., the isobar for increasing temperature showed a pronounced minimum and maximum, the amount taken up at the 80°K. minimum agreeing well with the amount which was taken up in the first isobar when 175°K. had been reached on the branch of descending temperatures, again assuming identity of the two curves at the temperature
HYDROGEN ADSORPTION. II
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of pumping. After pumping the system to high vgcuum for 15 min. at 296°K., the subsequently measured isobar for increasing temperature (starting again at 20°K.) showed at its minimum value an uptake practitally identical with the uptake found at 296°K. on the ascending branch of the very first isobar. As in the two previous cases, identity of the two curves at 296°K. is assumed. These facts are of interest because they show that the amount of hydrogen which can be removed from the system by pumping to high vacuum for 15 rain. at 175°K. and 296°K. is being resorbed almost completely in the lower temperature region between 20 ~nd 80°K. More important, however, is the fact that the rise of the initial isobar with decreasing temperature down to about 80°K. must also be partly due to absorption. Further proof of this will be given in the next paragraph. 0 V [3 X • "~" "0-
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FIO. 1. Sorption isobar of hydrogen at 0.1 ram. on evaporated nickel fihn sintered at 23°C. (To obtain the ordinate in molecules X 10-is/100 rag., divide by 9,07.)
Figure 2 shows the ascending and descending branch of a hydrogen isobar at 0.1 ram. pressure on a nickel film previously sintered at 200°C. If the 80°K. point of the ascending isobar in this figure is compared with the 80°K. point of the ascending isobar in Fig. 1 (after converting the ordinate in Fig. 1 to molecules X 10-18/100 rag. of nickel film as indicated below the figure), it is noted that the initial hydrogen adsorption at 80°K. is nine times larger per weight unit for the film sintered at 23 ° than for the film sintered at 200°C. At the same time it is seen that the difference of hydrogen sorption between the descending and ascending branch of the
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isobar at 80°K. is the same for both types of films and, thus, evidently a weight effect. Furthermore, the ratio of total sorption at 80°K to that at room temperature on the decreasing branch of the two isobars is the same for both types of films, indicating that the relative decrease of total sorption with temperature is independent of surface. Figure 2 shows clearly that the major portion of the hydrogen sorption at room temperature of a film sintered at 200°C. is absorption into the interior of the structure. It is interesting to note that the total van der Waals' adsorption as given by the differences of sorption between 20 ° and 80°K. is practically the same for the ascending and the descending part of the isobar. Furthermore, the numerical value of the van der Waals' adsorptio n for the film sintered at 200°C. is seen to be approximately 4 times lower than the van der Waals' adsorption observed on a film sintered at 23°C. as shown by Figs. 1 and 2. The reason that the ratio of the van der Waals' adsorp4.0
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FIG. 2. Sorption isobar of hydrogen at 0,i ram. on
evaporated nickel film sintered at 200°C. tion in the two types of film is not equal to the ratio of the surfaces as measured by the adsorption at 80°K. in the ascending part of the isobars of the two types of film will become clear after the following discussion of Fig. 3, which represents an isobar on the same type of film in the region between 20 and 120°K. All points of this isobar with increasing or decreasing temperature are numbered in the sequence in which they were measured. It is seen that, starting with point No. 1 at 20°K. and proceeding to 50°K., the isobar can be traced back to point No. 3 at 20°K., which coincides with point No. 1. Ascending again with the temperature, point No. falls on the same curve and, after obtaining point No. 5, the isobar can be traced back and forth to point No. 9. Since the isobar flattens out completely at 80°K. with the value of 7.2 X 1018molecules/100 mg. it can
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HYDROGEN ADSORPTION. II
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Fie. 3. Sorption isobar between 20°K. and 120°K. of hydrogen at 0.1 ram.
on evaporated nickel film sintered at 23°C. be conclu~led that the total adsorption of hydrogen at 20°~2. consists of 7.2 X I0 Is molecules/lO0 mg. of nickel of chemisorption and of 6.0 X 1018 molecules/lO0 mg. van der Waals' adsorption.
Discussion Measurements made in these laboratories (3a) have shown the heat of adsorption of hydrogen at liquid air temperature to be 30,000 cal./mole for the sparsely covered surface decreasing to 18,000 cal./mole for the completely covered surface. These values are in complete agreement with those obtained at room temperature. Since the isobar in Fig. 3, starting at 20°K. can be retraced back and forth to 80°K., it is evident that the chemisorption observed at 80°I<. must already have occurred at 20°I4. By tracing the isobar further to point No. 10, it is seen that a small amount of sorption into the interior of the structure has taken place. After initiation of this process, which is activated by the energy supplied by raising the temperature from 80°K. to 120°K., it is interesting to note that this process will continue, even at an increasing rate, upon lowering the temperature again to point No. 11. Upon tracing the isobar down to
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20°K., it is found that the total sorption value of point No. 13 is higher (within the limits of experimental error) by an amount corresponding to the absorption that has taken place during this time. Upon retracing the isobar back through points 14, 15, and 16, more gas is absorbed and the process could presumably have been continued until the total absorption would have been identical with that of the descending branch in Fig. 1, the absolute value at 80°K. being higher by the difference in adsorption of the two films. These measurements show clearly that after the absorption process has once been started by raising the temperature from 80 ° to 120°K., it will continue even at lower temperatures. It should be pointed out that several repetitions of this experiment have shown that absorption was sometimes initiated at temperatures as low as 70 ° to 80°K., if sufficient time were allowed. The increase observed with lower temperatures seems to be due to the higher surface concentration of molecules in the van der WaMs' adsorption layer. Once the formation of the "hydride phase" was initiated at lower temperatures, the growth of this new phase, even at temperatures below the initiation temperature, is probably due to the assistance of nuclei of the new phase that have opened up the passage into the interior. It is concluded that this process is an exothermic type of absorption or solution, possibly the formation of a hydride. In comparing Figs. 2 and 3 it is seen that the van der Waals' adsorption at 20°K. of an unsintered film as measured by the difference of the ordinates of points 1 and 5 in Fig. 3 is five times as large as that of the sintered film in Fig. 2 (difference between adsorption at 20°K. and 80°K.), while the chemisorption is thirteen times larger for the unsintered than the sintered film. This discrepancy may possibly be explained by the increased capillary condensation in the sintered film. Additional studies at very low temperatures are necessary to clarify this point. It is of interest to note that in the measurement of the hydrogen absorption on the ascending isobar of Fig. 1, a sudden desorption of hydrogen took place, after raising the temperature, followed by a slow sorption that exceeded the initial desorption. This phenomenon is identical to that reported by Taylor and Chou-Shou Liang (4) and earlier by Frankenburger and Messner (4a) and suggests that their experiments may deserve reexamination in the light of the findings on nickel, inasmuch as it is possible that Taylor and Chou-Shou Liang may have interpreted an activated absorption process as activated adsorption in the absence of further proof. From the nature of the isobars, it is clear that the absorption process must be exothermic and that, like adsorption, it is reversible at higher temperature. The heat of absorption must be considerably lower than the heat of adsorption, inasmuch as the absorption process, which is definitely of the activated type with a measurable temperature-dependent rate, is more readily reversible at high temperatures than the adsorption process. An interesting side light on the nature of the absorption process was ob-
HYDROGEN ADSORPTION. II
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rained through the measurement of sorption isobars on palladium. The adsorption on palladium at liquid nitrogen temperature was shown (through rates of hydrogenation measurements and by CO adsorption) to be comparable to the adsorption on the other metal films investigated, although the initial fast hydrogen adsorption is always followed by a slow sorption that seems to continue indefinitely. When the temperature is raised, the uptake of hydrogen increases in the neighborhood of - 100°C. to an amount which is approximately equal to the absorption of one hydrogen atom per atom of palladium. This absorption decreases again rapidly when increasing the temperture to room temperature and more slowly when increasing the temperature further to 100°C., at which point the sorption again is low (about four times larger than that obtained at - 196°C.). The 1 : 1 ratio of hydrogen atoms to palladium leaves no doubt that hydride formation is involved, and reports in the literature (5) have indeed shown that low temperature solution of hydrogen in palladium leads first to a hydride of cubic structure and that raising of the temperature causes this hydride to lose hydrogen with the formation of a hexagonal hydride with a palladium-hydrogen ratio of 3 : 1. These reports show also that upon raising the temperature still higher the hydride decomposes again. In the case of nickel, the number of hydrogen atoms in solution is too small to be detected by diffraction methods. SUMMARY
1. Sorption isobars for hydrogen at 0.1 mm. on evaporated nickel films have been obtained from 20°K. to room temperature by aid of a Collins helium cryostat. 2. Slow activated sorption, which is interpreted as sorption of the hydrogen into the metal structure, has been observed in confirmation of the results previously reported. 3. Slow sorption or "hydride phase" formation is exothermic and after initiation by raising the temperature to about 120°K. will continue at much lower temperatures. This effect is probably due to the assistance of nuclei of the new phase that have opened up passage into the interior. REFERENCES 1. BEECK, O., t~ITCHIE, A. W., AND WHEELER, A., J. Colloid Sci. 3, 505 (1948). 2. BEECK, 0., SMITH~A. E., AN-DWHEELER, A., Proc. Roy. Soc. (London) A177, 62 (1940). 3. COLLINS,S. C., Rev. Sci. Instruments 18, 157 (1947). 3a. BEECh:, 0., Rev. Modern Phys. 17, 61 (1945). 4. TAYLOR,H. S., AND CHou-SHov LING, J. Am. Chem. Soc. 69, 1306 (1947). 4a. MESSNER, a., AND FRANKENBURGER; W., Z. physik. Chem. Bodenstein Festband 593 (1931). 5. MICHEL, A., et al., Bull. soc. chim. [5] 12, 336 (1945); OWEN, E. A., AND JONES, J. I., Proc. Phys. Soe. (London) 49, 603 (1937). MICHEL, A., AND GALLISSOT, M., Compt. rend. 208, 434 (1939).