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).
COMMUNICATIONS TO THE EDITOR
595
5 479OC
401
1
6 0.04
I
0.06
1
1
l
I
0.08 0.1
PNH?, atm.
1
0.2
I
0.4
In this Communication, it is suggested from recent studies on the dependence of the rate of ammonia decomposition on the ammonia, hydrogen, and nitrogen pressures that the mechanism depends on the experimental conditions even on a given catalyst. The decomposition of ammonia over an ammonia synthetic catalyst (4.72% alumina, 0.31% potassium oxide, 0.05% silica) was performed in a flow system at 1 atm by a differential reactor. The catalyst was reduced at 600" for 50 hr in a stream of well-purified hydrogen at a flow rate of 500 cc (STP)/min. A mixture of purified hydrogen, ammonia, and helium or nitrogen was led over the catalyst. The undecomposed ammonia was determined by absorbing it in a sulfuric acid solution. Helium was used as diluent. The rate of decomposition was estimated from the difference between the inflow and outflow rates of ammonia. In order to determine the dependence of the decomposition rate on the ammonia pressure, the temperature of the catalyst, the hydrogen inflow, and the total inflow rate were held constant and the ammonia inflow was varied. The dependence of the rate on the hydrogen or nitrogen pressures was obtained in a similar manner. These results are plotted in Figures 1 and 2. From these figures, the reaction order with respect to the ammonia or hydrogen pressure could be obtained as long as the per cent decomposition of ammonia was small. A slight inhibitive effect of nitrogen on the rate was observed above 479" with a reaction order of
479OC
8t-
I
Figure 1. Rate of ammonia decomposition us. partial pressure of ammonia: 424' (10 g of catalyst); total flow, 890 cc (STP)/min; hydrogen flow, 398 cc (STP)/min; 479" (2 g of catalyst); total flow, 1090 cc (STP)/min; hydrogen flow, 668 cc (STP)/min; helium as diluent. Number in the figure indicates the number of the experimental points observed in different runs.
40t
0.2
0.4
Pa,, atm.
0.6
0.8
1.0
Figure 2. Rate of ammonia decomposition us. partial pressure of hydrogen: 424" (10 g of catalyst); total flow, 890 cc (STP)/min; ammonia flow, 99.2 cc (STP)/min; 479' (2 g of catalyst); total flow, 1090 cc (STP)/min; ammonia flow, 99 cc (STP)/min; helium as diluent. Numbers in the figure indicate the number of the experimental points observed in different runs.
less than 0.1. This was not observed below 479". From those results, the rate can be expressed approximately as
v
= kl(P"8/PH21'6)o'4*(at 420') ;
v = k2(PNH,/PH:*6)0*'6 (at 479") where P N H etc. ~ represent ammonia pressure etc., and kl and kz are the rate constants. The rate expression at lower temperatures is clearly explained by the theory proposed by Temkin and Pyzhev, while the different expression valid at higher temperatures suggests that the rate-determining step is then the dehydrogenation of adsorbed amino radical NH2(a). These results and the kinetic expressions indicate that the rate-determining step of ammonia decomposition changes from the desorption of adsorbed nitrogen to the dehydrogenation of the adsorbed amino radical NH2(a) with the reaction temperature. Details will be published elsewhere.
Acknowledgment. The authors are grateful to Professor Emeritus J. Horiuti for his encouragement in this work and also the Ministry of Education for the grant for the scientific research. RESEARCH INSTITUTE FOR CATALYSIS HOKXAIDO UNIVERSITY SAPPORO, JAPAN RECEIVED NOVBMBER 8, 1965
N. TAKEZAWA I. TOYOSHIMA
Volume 70, Number 8 February 1066
