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R. SHEETS AND G . BLYHOLDER
The anomalous dependence of Tm upon e can be explained by a model which considers the diffusion of carbon through the bulk to be important. Smithells and Ransley (SR) lo studied the diffusion of carbon, oxygen, and carbon monoxide through nickel, and they concIuded that C and 0 atoms diffuse readily while CO diffuses only very slowly if at all. SR observed that CO desorption originated from dissolved oxygen and carbon and that the diffusion of carbon was the ratelimiting step in the desorption process. The results of our experiment are well explained by the conclusions of SR. Assuming the carbon waa distributed uniformly throughout the target, there was enough carbon in the bulk to desorb 10'6 molecules
cm-2 of CO as many aa lo4 times (baaed on a 0.02% carbon impurity concentration). After each oxygen exposure a t room temperature, the oxygen is localized at or near the surface and, upon heating the target at 7.6"K sec-', the carbon diffuses to the surface and reacts with oxygen to form carbon monoxide which desorbs. Tm increases with 0 because the diffusion of carbon is the ratecontrolling step in the oxidation process. Larger oxygen coverages lead to the consump tion of larger amounts of carbon, and the latter requires additional time; thus a larger T,. (10) C. J. Smithells and C. E. Ransley, Proc. Roy. Soc., Ser.A , 155, 195 (1936).
Concerning the Isotope Effect in the Decomposition of Ammonia on Tungsten Surfaces
by R. Sheets and G. Blyholder* Departmen! of Chemistry, Universay of Arkansas, FwetteviUe, Arhnsaa
Publicatwn costs borne completely by The Journal of Phy&
76701 (Received July 1 , 1071)
Chemietry
It has been stated in the literature that NHa decomposition on W being 1.6 times faster than NDa precludes Nz desorption being the rate-determining step as has been suggested from kinetic measurements. Having found ND3 is B better electron donor than NH3 we prwnt a simple molecular orbital model of bonding to the surface which suggests the isotope effect can be explained as a &secondary isotope effect which is compatible with Nz desorption being rate limiting.
Although thermal decomposition of ammonia on tungsten surfaces has been studied for nearly half a century, the identity of the rate-determining step has not yet been established. Early investigations by Jungers and Taylor' and by Barre? of the rates of decomposition of ammonia and deuterioammonia on tungsten showed that, while both rates were nearly zero order with respect to ammonia and hydrogen, ammonia decomposed 1.6 times as fast as deuterioammonia. This isotope effect seemed to imply that the breaking of a N-H or N-D bond is involved in the rate-determining step. A more recent study of ammonia decomposition has led T a m a r ~to~conclude ~ that decomposition occurs through consecutive reactions comprising nitride layer formation to produce Hz, followed by the ratelimiting decomposition of the nitride layers to give N2. A recent paper by Matsushita and Hamenab supports the conclusion that nitrogen desorption is the limiting step although Szostak and Germer' and Ozaki, Taylor, The Journal of Physical Chemistry, Vol. 76, No. 7, 1978
and Boudart6 have maintained that the observed, zeroorder decomposition of ammonia on tungsten, unaffected by Nz or H2,when taken together with the isotope effect, is not explicable in terms of the theory which predicts that N2 desorption is rate limiting. They argue that the zero order means the surface is completely covered with N atoms, and thus the rate should be equal to Nz desorption and no isotope effect should be observed. Peng and Dawsone on the basis of desorbed gas analysis state that the surface contains W2NsH and claim that N-H bond breaking is involved (1) J. Jungers and H. S. Taylor, J . Amer. C h m . Soc., 57, 679 (1935). (2) R. Barrer, Trans. Faraday SOC.,32,490 (1936). (3) (a) K. Tamaru, ibid., 57, 1410 (1961). (b) K. Matsushita and R . S. Hansen, J . Phys. Chem., 52,4877 (1970). (4) J. May, R. Saostak and L. Germer, Surface Sei., 15,37 (1969). (5) A. Oaaki, H . 8.Taylor, and M. Boudart, Proc. Roy. SOC.,Ser. A , 258, 47 (1980).
(6) Y. K. Peng and P. T . Dawson, J . Chem. Phya., 54, 950 (1971).
ISOTOPE EFFECT IN THE DECOMPOSITION OF AMMONIA in the rate-determining step of NH3 decomposition. However establishing the ratio of W, N, and H atoms on the surface in no way establishes that a N-H bond even exists on the surface and so their conclusion that N-H bond breaking is rate determining cannot be a necessary conclusion from their data. We believe the isotope effect can be explained as a P-secondary isotope effect, a process which does not involve the breaking of a K-H or N-D bond, and thus the observed isotope effect and a zero-order SH, decomposition with N2 desorption rate limiting can be compatible. The causes of P-isotope effects, which have had magnitudes around 1 . 5 , ' ~are ~ not well understood. Halevi and coworkerss~loconcluded that the CD3 group is a better electron donor than the CHI group and that in their work this electron donation could account for most of the /3 effect. We have found" that NDS is similarly a better electron donor than NH3. Spectra of chemisorbed CO show one or more strong bands in the 1800-2100-cm-' region which are attributed to C 4 stretching vibrations.12 It is known12*13 that simultaneous chemisorption of electron-donating species causes a shift in the positions of these bands and a theoretical explanation of these shifts has been given by Blyh01der.l~ We found that when co and were On iron or nickel films, the co bands were shifted in the direction (to lower energy) and by an amount which indicated that the chemisorbed ammonia species were donating to the surface. The spectra indicated that the chemisorbed ammonia was undissociated. Similar experiments with ND3 showed shifts in the same direction but of larger magnitude. The conclusion followed that ND3 is a better electron donor than NH3. Since N 2 with co7the same molecular orbital picture which applies to CO also applies to N2.
97 1 In this picture the highest partially filled molecular orbital, which is bonding for the metal-adsorbate bond would receive more charge when an electron-donating group is coadsorbed. Desorption of ?\T2 would therefore be slower from a surface with coadsorbed ND3 than from one with coadsorbed "3. Thus, desorption of N2 as the slow step in ammonia decomposition would have a @-isotope effect. Furthermore, the P-isotope hypothesis does not require exact knowledge of what the coadsorbed species is, whether ND,, S D 2 , or XD, as long as such species are better electron donors than the corresponding NH3, NH2, or NH species. As Schwabl5 has pointed out, in gas-solid catalysis it is difficult to draw definite conclusions as to the mechanism from the magnitude of the observed isotope effect. Xeverthcless, we feel that the explanation we propose brings harmony to isotope and kinetic measurement which had previously appeared to be inconsistent.
Acknowledgment. This investigation was supported in part by Research Grant No. 00818-02 from the Air Pollution Control Office, Environmental Protection Agency. We thank the National Science Foundation for financial support to R. Sheets. (7) The discussion of primary and secondary isotope effects taken from J. March, "Advanced Organic Chemistry," McGraw-Hill. New York. x* l9689 2 ~ ~ 1 6 , (8) K.T.Leffek, J. A. Lewellyn, and R. E. Robertson, Can. J . Chem., 38,2171 (1960). (9) E. A. Halevi, M. Nussim, and A. Ron, J. Chem. Soc., 866 (1963). E, A, ~ d and M ~ .~ ~ ~i%d., i 876 ~ (1963). ~ i ~ (11) R.sheets, ph.D. Dhefiation, University of Arkansas, 1970. (12) L. H.Little, "Infrared Spectra of Adsorbed Species," Academic New N. y . lg6'*~ (13) C. R. Guerra, J. ColloidInterjace Sei., 29, 229 (1969). (14) G. Blyholder, J . Phys. C h a . , 68,2772 (1964). (15) G. M. Schwab and A. M. Watsen, Trans. Faraday Soc., 60, 1833 (1964). y.1
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The Journal of Physical Chemistry, Vol. 76, N o . 7, 1978
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