1618
C O M M U N I C A T I O N S T O THE E D I T O R Carbon-14 Tracer S t u d y of Active Sites o n Cold-Worked Palladium Catalyst
Sir: We have found that the catalytic activity of cold-worked palladium foil shows a marked change with annealing; a drastic decrease in the initial rate of acetylene hydrogenation occurs when the foil which was subjected to successive rolling in one direction a t room temperature is annealed under vacuum a t temperatures between 200 and 300", but the rate partly recovers after similar treatments above 600", Further detailed information about the nature of active sites on the palladium foil was obtained from a study of adsorbed acetylene labeled with carbon-14. A glass reaction vessel was incorporated with a G.M. counter, which was similar to that used by Thomson, et a1.l A catalyst foil of cold-worked palladium (99,99% pure), mounted on a glass boat, was annealed at various temperatures under vacuum below mm pressure and reduced with hydrogen a t 150" and then brought beneath the mica window of the counter in order to measure the amount of adsorbed acetylene-C14. The temperature of the system was maintained at 27" throughout the study. First, acetylene-C14 was adsorbed on the palladium mm. After foil at an equilibrium pressure of 5 X acetylene-C14 in the gas phase was pumped out, a reaction mixture of nonradioactive acetylene and hydrogen, 1:2 in molar ratio, was admitted to the vessel a t a total pressure of 20 mm. It was ascertained, from a separate kinetic study using gas chromatographic analysis, that the main product was ethylene and that the initial rate of ethylene formation was expressed by vi = kPhP,-0~4"-0~6in the range of hydrogen pressures (Ph)from 10 to 40 mm and acetylene pressures (Pa) from 5 to 20 mm ( k , constant). The pressure decrease which corresponded to formation of ethylene was followed with a glass Bourdon gauge. The counter was operated to detect the change in the surface concentration of radioactive acetylene. An example of results is shown in Figure 1. Figure 1 shows a sharp decrease in the surface concentration of radioactive acetylene due to evacuation, followed by a slower decrease concurrent with the reaction of the nonradioactive species. A second run yielded the same initial surface concentration after evacuation, followed by a very rapid decrease to the lowest level previously reached. The figure also shows the reaction rate per unit area, which increases to a steady reaction rate level as rapidly as the attainment of the steady surface concentration of radioactive The Journal of Physical Chemistry
acetylene. When the reaction rate attains a steady value, a definite amount of radioactive species still remains on the surface. This remaining acetylene-Cl4 was partly removed by reduction with hydrogen a t 150". We can, therefore, classify adsorbed acetylene into several species, such as : A, which desorbs on evacuation prior to the reaction; B, which is removed from the surface during the reaction; C, which is removed by reduction with hydrogen at 150"; and D, which remains on the surface after the reduction. This procedure was applied to the rolled foil which was annealed under vacuum a t various temperatures up to 800". The results are summarized in Figure 2. Changes in specific activities per unit geometric area and per molecuIe of species B with annealing are also shown in Figure 3. I n the case of catalyst annealed a t 800", it was found that the adsorbed species, except A, do not exchange with gaseous acetylene and are removed in a hydrogen atmosphere to the same extent as in the case of the hydrogenation reaction. These results lead to the conclusion that active sites for hydrogenation correspond to only those sites which can be occupied with species I3 and that there are a t least two kinds of sites, I and 11, with different activities; the I sites exist only below 300"
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Figure 1. Change in the amount of adsorbed a~ety1ene-C'~ and reaction rate: 1, first run; 2, second run after evacuation. (1) D. Cormack, S.
(1966).
J. Thomson, and G. Webb, J . Catal.,
5, 224
1619
COMMUNICATIONS TO THE EDITOR
Since the change in the activity does not correspond
t o that in surface area2 and the activity which was generated by argon ion bombardment (500 V) on the
200
400
800
600
Annealing temperature
t t
Figure 2 . Effect of annealing. Changes in fractions of adsorbed acetylene: A, the amount of adsorbed acetylene-C14 which desorbs on evacuation; B, that removed from the surface during the reaction; C, that removed by reduction with hydrogen a t 150'; D, that which remains on the surface after reduction.
r-----l
foil annealed a t 800" shows a similar change with annealing in a grease-free system, the observed change can be explained by neither sintering nor poisoning due to contaminants. When palladium foil is subjected to rolling at room temperature, the (110) plane appears in parallel with the rolled surface and crystal imperfections involving vacancies are formed. The vacancies disappear when metal is annealed a t temperature from 200 to 300". This temperature region corresponds to that of a drastic decrease of the activity of the hydrogenation. On the other hand, the (110) plane vanishes at annealing near 600" and the (111) plane is exposed gradually a t higher temperature than 600". Therefore the present results strongly suggest that a close correlation exists between the structure of the I sites and lattice imperfections, such as vacancies, and another correlation' exists between the I1 sites and lattice planes or boundaries which are preferentially developed during recrystallization. Detailed studies on the nature of these active sites are in progress by means of infrared spectroscopy and ion bombardment techniques. (2) The roughness factor, defined as the ratio of observed surface area to geometric surface area, was 1.4 on palladium foil annealed at 2 0 0 O . 1.2 at 300°. 1.8 at 400O. 1.5 at 600O. and 1.1 at 800O: see T. Kabe, T. Mizuno, and I. Yasurnori, Bull.' Chem. SOC.Jap., 40, 2047 (1967).
Y. INOUE I. YASUMORI
DEPARTMENT OF CHEMISTRY
TOKYOINSTITUTE OF TECHNOLOGY TOKYO, JAPAN OOKAYAMA, MEQURO, RECEIVED JUNE7, 1968
The Electric Moment of Dichlorobis(pyridine)cobalt(II)
Complex
Sir: Examination of the literature shows a paucity of Annealing temperature
("C)
Figure 3. Changes in specific activities per unit geometric area ( 0 )and per molecule of species B ( 0 ) .
and are more active than the I1 sites. The recovery of activity by the treatment above 600" is due to the increase in number of the I1 sites. The effect of carbon mon~xide-C'~as a poison on active sites was also examined. It was shown that the activity due to the I1 sites decreases linearly with the admount of adsorbed CO and k destroyed completely a t about 80% surface coverage while that due to the I sites is not affected by adsorbed CO up to 20% surface coverage.
data on the electric moments of cobalt(I1) halide complexes and no data are available for their pyridine analogs. This results from their insolubility in a number of nonpolar solvents. The CopyzClz complex is appreciably soluble in nitromethane, and this coupled with the fact that nitromethane has been shown to be "unreactive" toward the compound' led to this study. Dielectric constants .of purified nitromethane and solutions of concentrations up to 0.1 M in CopyzCla were measured at 23 and 30" using bridge methods. The solution densities and indices of refraction were determined in the usual manner. Dielectric constants obtained from bridge measurements between 5 and 30 (1) H. C. A. King, E. Koros, and S. M. Nelson, J. Chem. Soc., 5449
(1963).
Volume 75,Number 5
Mag 1960
