135
NlCATlONS TO THE EDITOR
On the lsotsps Effect in the Decomposition of Ammonia on Tungsten Surfaces
Sir: I t was recently suggested by Sheets and Blyholderl that the observed isotope effect in the decomposition of ammonia on tungsten surfaces could be understood as a P-secondar:y isotope effect, allowing nitrogen desorption to be rate limiting. Their conclusion that this interpretation brings harmony to isotope293 and kinetic455 measurements i s optimistic ;and i.t is based on a very selective and incorrect use of the available data. In the kinetic measurements referred to4,h the authors use the term nitrogen desorption rate limiting in the sense that nitrogen desorption alone occurs in the rate-limiting step. The prime evidence for t.his c o d u s i o n is that the surface intermedi,ate contains no hydrogen. It is assumed that the surface is saturated with nitr'ogen and that hydrogen desorption is extremely fast a t temperatures below those required for nitrogen desorption. These conclusions, far from being resolved. with the isotope effect by the model of Sheets and Rlyholder are completely a t variance with it. No such secondary isotope effect i s possible in a surface devoid of hydrogen (in the absence of a carbon skeleton the term /? secondary is confusing and will not be adopted). Furthermore, this conflict no longer exists. Thermal desorption mass spectrometry studies697 of the interaction of arnmonia with tungsten surfaces have shown that a t steady state during catalytic decomposition, except at very low reactant pressure, the rsurface is saturated with a species (17 species) which does contain hydrogen. Possible reasons for a misinterpretation of Tamaru's experiments4 have been presented.8 Matsushita and Hansen's experiments5 were carried out a t too low a temperature and reactant pressure to permit fcrmmtion of the hydrogen-containing 7 species. Sheets and Blyholder have been selective in their use of the thermal desorption mass spectrometric data. These experiments are rather more than desorbed gas analysis and contain more information than overall surface stoichiometry. Let bus surnmarise this information. In 1966, it was pointed out9 that the desorption rate of p nitrogen (at the time, the only known surface species of nitrogen on tungsten stable a t catalytic temperatures) was more than lo7 times slower than nitrogen production during ammonia decomposition on tungsten surfaces. Clearly a process, or processes, must be identified which allows this faster desorption of nitrogen. Since that time two additional surface species have been identified, 6 nitrogen5x7 and the 7 species.6,7 The formation and decomposition reactions which can occur can be summarized in the following way (where Ws denotes a surface tungsten atom) B
2"'
.iNH3(g)
+
WzsN(p)
+ YZIiIZ(g)
(1)
While the kinetic parameters for these reactions are known, for the present purposes the relative rates will be adequate and these are (1) > (3) > > ( 5 ) and ( 2 ) C < < (4) < (6). It can be seen that as the surface density of nitrogen increases the rate of nitrogen desorption increases, reactions 2 , 4, and 6, but also the rate of formation of the adlayer decreases, reactions 1, 3, and 5 . For any given reaction conditions, uiz., reactant pressure and catalyst temperature, a steady state will be attained in which the rate of formation and removal of the surface intermediates will be equal. This determines the nature of the surface intermediate. Reaction 2 is too slow t u be part of the decomposition mechanism. Since the rate of reaction 5 is dependent on the reactant pressure, we anticipate two regimes for the decomposition mechanism determined by competition between reactions 4 and 5 . At high reactant pressure reaction 6 should be rate limiting and we anticipate a hydrogen isotope effect, whereas a t very low reactant pressure reaction 4 should be rare limiting and the isotope effect should disappear. The observed kinetic parameters for reactions 3-6 suggest that the transition pressure should be in the vicinity of 10-1 Torr a t 1000°K; this is currently under investigation. Reaction 6 is unlikely to be an elementary step. The desorption of N2 and H are closely related a t the molecular level since the desorption peaks are simultaneous and first order. However, the critical question of whether on the time scale of molecular vibrations hydrogen desorption precedes or follows nitrogen desorption is clearly not answered by such experiments. An answer must await, a t least, an unambiguous determination of the structure of the adlayer. Two structures have been proposed,7 ( a ) WN2H.WN and (b) WN2.WNI-J, botli of which preserve the N-H bond of the reactant. Sheets and Blyholderl question this assumption, which is swrprismg since it is a requirement of their model. It is known that N-H bond breaking becomes increasangly difficult as the density of the adlayer increase^.^ Structures a and b impiy the following pairs of elementary steps for reaction 6 WNzH*WN
WNz-WN
4
+ M(g)
(Gal
(1) R. Sheets and G. Blyholder. J. Phys. Chem.. 76,970 (1972) (2) J. C. Jungers and H. S. Taylor. J . Amer. Chem. SOC., 57, 679 (1 935). 13) R. M . Barrer, Trans. FaradaySoc.. 32,490 (1936). (4) K.Tarnaru. Trans. FaradaySoc., 57, 1410 (1961). (5) K. Matsushita and R. S, Hansen, J. Chem. Phys., 5 2 , 4877 (1970j (6) P. T. Dawson and Y. K. Peng, J. Chem. Phys,, 52, 7014 (1970). (7) Y . K . Peng and P. T. Dawson, J. Chem. Phys., 54, 950 (1971). (8) P. 1.Dawson and R. S. Hansen, J. Chem. Phys., 48,623 (1968). (9) P. T. Dawson and R. S.Hansen, J. Chem. Phys., 45, 3148 (1966). The Journal of Physicai Chemistry. VO/.77, No. 7 . 7973
t 36
C O M M U N I C A T I O N S TO T H E EDITOR
WNz.WI\JH-W*WHN +Nz(g) W-WNH ---* WzN
(6b)
+ H(g)
and a primary or secondary nature, respectively, for the isotope effect. Both possibilities adequately account for the thermal desorption results. Molecular nitrogen, a nitrogen,lo is weakly chemisorbed, desorbing a t much lower temperatures than the 7 species, viz., 400°K compared with 900°K. Thus in (6a) its desorption would immediately follow that of H. In (6b), desorption of Nz would free adjacent surface tungsten atoms for formation of transition states in breaking the N-H bond of the imide group and H desorption would follow immediately. However, the relative instability of adsorbed Nz suggests that its desorption wouid not be rate limiting and it seems unlikely that an inductive effect from an adjacent NH (or ND) group would lead to a stabilization of the required magnitude. In the alterriatiire model (6a) the presence of an N-H bond in the K 2 species, Le., N-NH, forces a lower bond order fclr the "4 bond, stronger bonding to the surface and a species desorbing a t higher temperature as observed. The behavior of coadsorbed films of CO and NH3 on iron and nickel fiims reported by Sheets and Blyholder has little in common with the adlayer during ammonia decomposition on tungsten. NH3 and ND3 contain a lone electron pair and one would anticipate that chemisorbed NH3 would bond. to a transition metal surface with accompanying electron donation to the metal. This would result in a dipole alrra,y which would lower the work funcThis was confirmed in 1968 in field ts of Dawson and Hansen;B chemisorbed NI-13 changes ithe work function of a tungsten surface by amounts varying from -1.0 to -1.6 eV depending on crystallographic orientation. At catalytic temperatures undissociated ammonia adsorption does not occur. Hydrogen desorption accompanies ammonia adsorption in the formation of both d ni.trogen (3) and the 1) species ( 5 ) . The work functions of the 6 and adlayers have recently been measured in this lebcratory and are found to be 4.70 and 5.00 eV, respectiveby. Since the emitting regions in both cases correspond to those emitting for the clean surface ,)' we can conclude tha.t the 6 and 7 (work function '4.501 e% species change the work function by +0.20 and +0.50 eV, respectively. Thus, whatever the structure of the 7 adlayer, whether it contains NH groups, N-NH groups, or neither, the incorporation of hydrogen into the surface via reaction 5 has the effect of withdrawing electrons from the tungsten. The electron donation picture of Sheets and Blyholder is unsat,isfactory. Finally we ace concerned as to the magnitude anticipaeed for a secondary isotope effect for this reaction. By implication,1 an observed isotope effect of 1.62,3compared with a quoterl maximum of 1.5 for P-secondary isotope effect@ lends credence to the secondary isotope mechanism. It should. be pointed out that secondary isotope effects are generally much smaller than this and there is no evidence to suggest that such a large value would be appropriate for the effect under discussion, e . g . , reaction 6b. More import ani still the maximum P-secondary isotope effect of 1.5 is for room temperature whereas that observed in the decomposition of NH3 and ND3 on tungsten surfaces is for a te1n:perature of 1000°K. The exact temperature dependence of secondary isotope effects is imposThe Journal o f Fhysicnl Chemistry, Vol. 77, No. 1 , 7973
sible to predict but one would anticipate that, depending on the relative magnitudes of the vibrational frequencies and temperature, the magnitude of the isotope effect varies exponentially as either 1/T or l/'P, or both if the reacting system contains high- and low -frequency vibrations.12J3 In the latter case anomalous temperature dependencies (maxima, minima, and crossovers) are possible if the high- and low-frequency contributions to the isotope effect are opposite in sign.13 In this case, however, the overall effect is expected to be smaller and in any case over a large temperature range the magnitude of the isotope effect will still diminish. It has been pointed out13 that such anomalies are expected to be even less frequent in kinetic isotope effects than in the equilibrium isotope effects considered. The existing evidence suggests that secondary isotope effects would not have sufficient magnitude at 1000°K to account for the observed effect. No such problems arise with the primary isotope effect interpretation. The differences in the zero point energy for the six normal vibrational modes of NH3 and ND3 vary from 575 to 2600 cal mol-1. Without a detailed knowledge of the reaction trajectory and potential surface it is uncertain what magnitude a primary isotope effect would attain but it would be expected to lie between these limits. In order to account for NH3 decomposition occurring 1.6 times faster than ND3 at 1000°K a difference in activation energy of 940 cal mol-1 is required. This does indeed fall within the limits expected for a full primary isotope effect and provides strong evidence for this mechanism. In conclusion, the identification of a hydrogen-containing 7 species during the catalytic decomposition of NH3 on tungsten surfaces has removed any problems created by the existence of a hydrogen .isotope effect. However, there is still considerable speculation as to the nature of the isotope effect. Nevertheless, its magnitude suggests that it is a primary isotope effect. Conversely, this strengthens those structural models which imply a primary isotope effect. (10) (11) (12) (13)
G. Ehrlich, J. Chem. Phys., 34, 29 (1961). J. March, Advan. Org. Chem., 6, 216 (1968). M. Wolfsberg and M. J. Stern, Pure Appl. Chem., 8 , 225 (1964). M. J. Stern, W . Spindel, and E. U. Monse, J. Chem. Phys., 48, 2908 (1968).
Department of Chemistry and institute for Materials Research McMaster University Hamilton, Ontario, Canada
P. T. Dawson* Y. K. Peng
Received May 15, 1972
Mass Spectrometric Determination of the Dissociation Energies of AIC2, Al2C2, and AIAuCZ1 Publication costs assisted by the National Aeronautics and Space Administra tion
Sir: As part of a study of the thermodynamics of aluminum-containing molecules2a and as a continuation of our studies on the vaporization of metal carbides,Zb we have Presented in part at the Twentieth Annual Conference on Mass Spectrometry and Allied Topics, ASMS-ASTM E-14, Dallas, Tex., June 4-9, 1972. (a) C. A. Stearns and F. J. Kohl, High Temp. Sci., in press; (b) C. A. Stearns and F. J. Kohl, J. Chern. Phys., 54, 5180 (1971); 5 4 , 1414 (1971); J. Phys. Chem., 74, 2714 (1970); High Temp. Sci., 2, 274 (1970): J. Chem. Phys., 5 2 , 6310 (1970).
