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).
COMMUNlCATIONS TO THE EDITOR
I37
TABLE I: Ions Qbserveid in the AI-Au-C System Appearance potential, eV Relative intensity (2147 K, 30 eV)
Ion
Not measured.
2082 2113 2147 2171 21 62
* Reference 5.
Present work
Literature
1.oo 1.7 X 2.4 x 10-5 2.5 x 10-5 1.3 X 6.4 x 10-5 -7 x 10-2 3.4 x 10-1 -10-6 2.0 x 10-6
Standard 14.0 f 1.0 9.3 zk 1.0 5.4 f 1.0
5.984b
a
9.5 f 0.3
Reference 6.
a 8.0 f 0.5 Standard
a a
56.58 56.51 56.46 56.48
31 1.6 301.8 319.4 321.4 321.5 Av 315.1 f 7.6(f21)b
Alntg) f X ( S ) = A12Cz(g)
2051 2074 2085 2059 2082 2113 2149 21 71 21 62
0.21W-c 0.386 0.423 0.5:37 0.333 0.492' 0 346 0.106 0.1 2 6
35.38 35.41 35.43 35.39 35.42 35.47 35.52 35.55 35.54 Av
2113 2147 21 71 21 62
64.0 58.1 57.0 51.7 60.5 55.1 62.1 72.8 71.6 61.4 A= 6 . 7 ( f 2 0 ) b
APAu(g) f 2C(s) = AIAUCZ(g) - 5 325a,d 49.50
-5.332 -5.191 -5.247
49.57 49.61 49.60
9.2,C9.7d
AI AIC2, (AIC?)
Ab AI&?, A12C2 AIzC;. Au AlAu AIAuC?, A I A u C ~ AIAuC2 AU2
References 7 and 8
Al(g) 4- ~ C ( S=): AICa(g) -4.858a 56.65
-4 5061 -4.8 1 ti - 4.784 - 4.8 18
9.22b
7.6 f 0.3
Parent species
320.0 325.6 323.5 324.4 Av 323.4 f 2.1 (A=20)b
a K, = (/(AlG*)/!(bcl+)) (u(AI)/u(AICZI ) ~ o ~ \ . ( Y ( A I + ) / Y ( A I C Z( + 7 /) ) a ( C ) ) 2 . The c'oss sections for AI and C were taken from Mann (ref 9). ( u ( A i ) / o ( A l C ~ )= ) 0.6246. 'The y terms were assumed to cancel and the activity of graphite was assumed to be 1. Overall estimated uncertaintv.
identified anG measured the dissociation energies of the molecules A.162, A12C2, and AlAuC2. Chupka, et al.,3 have previously identified AlzC2 but were uncertain with regard to the existence of other aluminum-carbon molecules. The mass spectrometer and experimental procedure used in the present study have been described adequately elsewhere.2.4 'The aluminum-carbon-containing molecules were observed with the mass spectrometer by vaporizing a mixture of '41 and Au from a graphite liner in a tantalum
Knudsen cell. The ions reported in Table 15-8 were observed when the sample was heated to temperatures above 2000 K. The appearance potentials (AP) measured for AlC2+, Alzt, A12C2+, AlAu+, and Auz+ indicated that these were parent molecular ions whose neutral precursors originated in the Knudsen cell. A1C+ is probably a fragment ion from AlCz because of its high AP of 14 eV. We assume that AlAuCZ+ is a parent molecular ion while the origin of the low-intensity ,A12C+ and AIAuC+ are uncertain. Ion intensity measurements were made for the ions of interest over the temperature range of 2051 to 2171 K. The pressure calibration independent reactions involving AlC2, A12C2, and AlAuC2 are listed in Table 119 along with the logarithms of the equilibrium constants, freeenergy function changes, and third-law heats of reaction a t each experimental temperature. The values o f the thermodynamic functions for Al(g) and C(s) were taken from the JANAF tables;lO for Al&) and AlAu(g) other recently reportedZa values were used. For AlCZ(g) a linear asymmetric AI-C-C structure and for Al&(g) a linear symmetric A1-C-C-A1 structure were chosen by analogy to other metal dicarbides.Zb The A1-C interatomic distance was estimated as 1.61 A, consistent with the choice of Chupka, et a1.,3 while the C-Cdistance of 1.31 6, was taken from C2(g).ll The fundamentai vibrational frequencies were calculated according to the valence force formulation.12 The stretching force constant for the AI-C was assumed to be 5.0 X 102 N/m3 and the C-C force constant of 9.25 x 102 N / m was taken from Cz(g).ll The calculated frequencies in cm-z for AlCz are 755, 503(2), and 1751 arid for A1262, 1842, 492, 1011, 321(2), and 109(2). For AlAuCZ(g) a linear AI-C-6-Au W . A. Chupka, J. Berkowitz, C. F Giese, and M. G. Inghrani, J. Phys. Chem., 6 2 , 61 1 (1958). C. A. Stearns and F. J. Kohl, NASA Tech. Note, No. D-5027 (1969): No. D-5646 (1970). R. W . Kiser, "Introduction to Mass Spectrometry," Prentice-Hall Englewood Cliffs, N. J,, 1965, Appendix IV. K. A. Gingerich, d . Chem. Phys., 50, 5426 (1969). K. A . Gingerich, d. Chem. Phys., 54, 2646 (1971). K. A. Gingerich and H. C. Finkbeiner, J . (:hem. Phys., 52, 2956 (1970). J. B. Mann, J. Chem. Phys., 46, 1646 (1957). D. R. Stull, Ed., "JANAF Thermocheniical Tables," Dow Chemical Company, Midland, Mich. G. Herzberg, "Moiecular Spectra and Molecular Structure. Vol. I . Spectra of Diatomic Moiecules," 2nd ed, Van Nos?rand, New York, N. Y . , 1950, Appendix. G. Herzberg, "Molecular Spectra and Molecular Structure. Vol. 1 1 . Infrared and Raman Spectra of Polyatomic Molecules," Van Nostrand. New York, N. y., 1945. The Journai of Physicai Chemisfry, Vo!. 77, No. 7 , 7973
138
COMMUNICATIONS TO THE EDITOR
structure was assumed and the Au-C stretching force con- monia complex1 of zeolite 4A, four sorption sites containstant was taken as equal to the A1-C force constant. The ing 8, 4, 8, and 12 NH3 molecules, respectively, were Au-C interatomic distance of 1.86 A was obtained by found. The most favorable site a t lesser loadings, or the comparison of the vdues from AuSi and AuB with the Pt, site selected by the first molecules to enter each unit cell, Rh, Ru, and l:r carbides, silicides, and borides.13-15 The was not determined. Accordingly, the heat of sorption negative of the free-energy functions in JK-1 mol-1 a t which might be calculated from the initial slope of the 2000, 2100, and 2200 K are 284.79, 287.40, and 289.90 for sorption isotherm1 cannot be associated with a particular ALCZ(gj; 347.1Y7 350.93, and 354.52 for A12C~(g); and site. To resolve this issue, the eight ammonia (per Pm3m 379.55, 383.32, and 386.94 for AlAuCz(g). unit cell) complex was prepared and studied by crystalloThe atomiza.tion energies for the molecules were calcu- graphic procedures similar to those previously employed.1 A single crystal of zeolite 4A, a cube 0.070 mm on an l a t d by combining the third-law heats of the reactions listed in Table I1 with the heat of formation of C(g), edge, was dehydrated a t 350" and 10-6 Torr for 24 hr, and was then exposed to zeolitically dried ammonia gas a t 28" A&.f0 = 709.5 i 1.9 kJ mol-l,l@the dissociation energy and a t a pressure of 12 Torr for 30 hr. The crystal in its of Ai&), DO"= 249,8 i 14 kJ mol-1.?2a and the dissociaglass capillary was then sealed off from the vacuum systion energy of AIAu(g), Ilo" = 322.2 f 6 kJ molk1:16 tem and studied without exposure to the atmosphere. The Do,,l,,,,o(AIC2) = 1104 I 21 kJ mol--I, DO,atomso(AlpCp)= 1507 f 25 kJ m01-I~ and D O , ~ ~ ~ ~ ~ " ( =A 1418 ~AU fC 21~ ) cell constant based on a least-squares treatment of 15 inkJ K I O I - ~The ~ atomization energy of A12C2(g) is in good tense reflections is 12.289(5) A. Of the intensities observed, 137 unique reflections were significant at the 3a agreement with the ,value calculated from the experimenlevel and these were used throughout. tal results of Chupka, et d . , 3 of 1556 f 42 kJ mol-l. The initial structural parameters used in least-squares The measurement of the dissociation energies of AlCz refinement were those previously found1 for Na, Si, Al, and A1262 is an extension of previous work in this laboraand 0 atoms in the 32 ammonia zeolite 4A complex. The tory on the determination of the stabilities of the transitwelfth sodium ion, found in the structure of dehydrated tion metal dicnrbides and comparison of metal-carbide 4A2 was included and refined well. The error indices a t and -oxide bond energies.2b Again we see that dicarbide convergence with this model were R1 = 0.067 and R2 = molecules are formed which are analogous to the stable 0.076, and the goodness of fit was 0.92. The final paramemonoxides A10 and ,4120. For most metal dicarbide moleters are given in Table I, and a tabulation of structure cules it is found that the dissociation energy of the M-Cz bond is 40-130 kJ mol-l less than that of the correspond- factors is available.3 A difference Fourier synthesis was prepared and refineing M-0 bond. The AI-C bond in AlCz is an apparent exments were attempted a t the positions of several small ception to this empirical rule because E(A1-C2) of 514.2 i peaks found there, even though this function appeared to 21 is slightly highor than Do"(A1Qj of 502 f 15 kJ be particularly featureless. The positions for nitrogen m 0 l - ~ . 1On ~ the other hand, each A1-C bond in A& of atoms as determined in the filled ammonia complex1 were 459 kJ mol-' i s 63 kJ mol-1 lower in energy than the subjected to least-squares refinement a t occupancies corAI-8 bonds in Al@(g) and follows the trend for most responding to four nitrogen atoms per equipoint, Positions metal dicar'r)ides. off threefold axes were also considered and their refineFor the AlAuC2 niolecule we have assumed the strucment was attempted. In no case was a site located which ture Ai-CX-Aan. If the energies of the A1-C and 6-C satisfied the elementary crystallographic criteria of statisbonds are subtractedi from the experimentally determined tically significant occupancy and lowered error functions. atomization energy, the balance (365 kJ mol-1) may repIt appears that even a t a loading of as few as eight amresent the energy for an Au-I: bond. This value is consismonia molecules per unit cell the sorbed molecules are tent with an upper limit (375 kJ mol-1) determined14 for not predominantly found a t one kind of sorption site. PerDo" of the unobserved. AuC molecule. haps all three kinds of Na+ compete favorably a t room (13) A. VanderAuwera-Mahieu, R , Peeters, N. S. Mclntyre, and J. temperature for NH3 association, all at sites where further Drowart, 'rrans. Faraday Soc., 66, 809 (1970). (14) A. VanderAbwera-Mahteu and J. Drowart, Chem. Phys. Lett., 1, hydrogen bonding can occur to framework oxygen atoms. 311 (1967). This result is consistent with the complete absence of any (15) pi. S. Mcinfyre, A. VanderAuwera-Mahieu, and J. Drowart, Trans. Faraday Soc., 64, 30116 (1968). indication of plateauing or unevenness in the sorption iso(16) K. A. Gingerick, J. QystalGrowth, 9, 31 (1971). therm;l even the first derivative decreases entirely regu(17) M Farber, K. D. Srivastava, and 0 . M, Uy, J. Chem. Soc., Faraday larly as a function of ammonia content. Trans. 1, 68, 2,49 (1972). The zeolite framework and cation positions have altered Nafionai Aeronautics arid Carl A. Stearns slightly upon the introduction of eight ammonia moleSpace A dniinistratiorl Fred J. Kohl* cules, confirming that sorption has indeed occurred (see Lewis Research Center Table 11). The geometries of the fully ammoniated and Cieveiand, Ohio 44 7 35 fully hydrated 4A structures are very s i n ~ i l a r ,and ~ , ~may Rsctaived September 18, 7972 be referred to as the relaxed conformation, the conformation of zeolite 4A a t its synthesis. Eight ammonia molehie Study of the Structure led Ammonia Sorption Complex of Zlealite 4A Publication costs assistecl by The National Institute of Health
Sir: In the cr:qstal structure of the nearly filled 32 amThe Journal of ,PhpicaI Chemistry, Vol. 77, No. I , 1973
(1) R. Y. Yanagida and K. Seff, J. Phys, Chem.. 78,2597 (1972). (2) R . Y . Yanagida and K. Seff, J. Phys. Chem., in press. (3) Listings of the observed and calculated structure factors for both structures wili appear following these pages in the microfiim edition of this volume of the journai. Single copies may be obtained from the Business Operations Office, Books and Journals Division, American Chemical Society, 1155 Sixteenth St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-73-138. (4) V. Gramlich and W. M , Meier. Z. Kristallogr , 133. 134 (1971).
