COMMUNICATIONB TO THE EDITOR
594
A H , = D(CFrH) - D(H-CI), we have D(CFaH) = 106.3 kcal mole-’ after correction to 298°K. This agrees well with D(CF3-H) = 106.0 kcal mole-’
1070, 1030, 1025, 1008, 988, 969, 958, 933, 923, 912, 900, 891, 882, 877, 871, 867, 861. It should also be noted that evidence of the 2060-cm-‘ band obfrom our bromination work and we believe that the served and reported early2 was found in the studies average value of D(CFrH) = 106.2 kcal mole-’ is of the nickel films reported above, and the background accurate to *0.5 kea1 mole-’. data in this region were also free of structure. Further studies of carbon monoxide on evaporated metal CHEMISTRY DEPARTMENT J. C. AMPHLETT UNIVERSITY COLLEGE J. W. COOMBER films are in progress. CATHAYS PARK CARDIFF,GREATBRITAIN
E. WHITTLE
RECEIVED OCTOBER 29, 1965
Infrared Studies of Carbon Monoxide Chemisorbed on Metallic Surfaces
Sir: Relatively recent papers by Garland, Lord, and Troiano’ presented a new method of forming metal films evaporated in carbon monoxide onto the windows of the infrared cell. They report a new band at 1620 cm-’ which was not reported previously for supported nickel surfaces and was not reported by Pickering and Eckstrom2 for bulk evaporated nickel films. Our work on infrared studies of carbon monoxide chemisorbed on nickel and on rhodium evaporated films using the multiple-reflection technique2 reveals the possibility of a very weak band at approximately 1620 cm-l for carbon monoxide on nickel and several very weak absorption bands in the region from 1350 to 750 cm-l; many of these bands are coincident for both nickel and rhodium. The experimental method used by Pickering and Eckstrom makes the unequivocal assignment of absorption bands in the water region difficulta and, in addition, the band at 1620 cm-l is very weak, appearing as a shoulder on a water line; in the region from 1350 to 750 cm-I several bands may be assigned, and even though they are very weak they are nevertheless “real.” The assignment of many bands to carbon monoxide chemisorbed on nickel and on rhodium confirms the complicated nature of “surface complexes” and complicates the theoretical interpretation. The number of assignments for hydrogen chemisorbed on evaporated rhodium films2 was large and our later studies have indicated there are more bands. Also, our studies of deuterium chemisorbed on rhodium films are showing approximately as many bands as for hydrogen. The “strongest” of the bands we have observed in this region for carbon monoxide are as follows: on nickel (cm-l): 1195, 1069, 1056, 1036, 1028, 1017, 1011, 1004, 999, 981, 964, 948, 933, 927, 920, 906, 896, 891, 883, 875, 864, 856, 775; on rhodium (cm-I): The Journal
of
Physical Chemistry
Acknowledgment, The infrared studies of chemisorbed molecules at the University of Kentucky are supported in part by the United States Atomic Energy Commission Contract No. AT-(40-1)-2948. (1) C. W. Garland, R. C. Lord, and P. F. Troiano, J . Phys. Chem., 69, 1188, 1195 (1965). Also, for the purposes of this Communica-
tion these papers should be referred to for literature references and review of studies in this field. (2) H.L. Pickering and H. C. Eckstrom, ibid., 63, 512 (1959). (3) Water spectra appears in their calculations because of very small changes in water vapor content in the “comparison cell” after the taking of the “background data,” upon which all calculations are based. It should be emphasized that no water vapor is present in the cell used for the chemisorption studies and any contamination is very unlikely because of outgassing procedures and vacuums maintained during film preparation.
DBPARTMENT OF CHEMISTRY UNIVERSITY OF KENTUCKY LEXINGTON, KENTUCKY
HARTLEYC. ECKSTROM
RECEIVED NOVEMBER 5, 1965
The Change of the Rate-Determining Step of the Ammonia Decomposition over an Ammonia Synthetic Iron Catalyst Sir: Two mechanisms have been proposed on the ammonia decomposition over the doubly promoted iron catalysts. Thus, on the one hand, Temkin and Pyzhev’ proposed the rate equation on the basis of the desorption of adsorbed nitrogen as the rate-determining step which could explain many of the experimental resu1tsa2 On the other hand, the dehydrogenation of NH3(a), NH2(a), or “(a) was proposed as the ratedetermining step.3 Here the (a)’s signify the adsorbed state. This disagreement has frequently been considered as due to differences in the catalysts used. _ _
~
(1) M. I. Temkin and V. Pyahev, Acta Physicochim. U.R.S.S., 12, 327 (1940). (2) W. G. Frankenburg, “Catalysis,” Vol. 111, P. H. Emmett, Ed., Reinhold Publishing Corp., New York, N. I-.,1955, p 171; C. Bokhoven, C. van Heerden, R. Westrik, and P. Zwietering, ibid., p 265; A. Nielsen, Advan. Catalysis, 5, 1 (1953). (3) S. Enomoto and J. Horiuti, Proc. Japan Acad., 28, 493, 499 (1952); J . Res. Inst. Catalysis, Hokkaido Univ., 2 , 87 (1952); J. Horiuti and I. Toyoshima, ibid., 5, 120 (1957); 6, 68 (1958); J. Horiuti and N. Takezawa, Chid., 8, 170 (1961).
