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Thermal Decomposition of N2O over ZnO: Kinetic Isotope Effects Study Peter Zˇ emva, Antonija Lesar, Ivan Kobal,* and Marjan Senegacˇnik J. Stefan Institute, P.O. Box 3000, 1001 Ljubljana, Slovenia Received August 7, 2000. In Final Form: December 5, 2000 The kinectic parameters were measured for the catalytic decomposition of nitrous oxide on ZnO powder between 40 and 60 kPa and the temperature range 725-835 K. Under studied conditions the reaction was found to be first order in N2O. An Arrhenius fit to the rate constants results in an apparent activation energy of 120 ( 4 kJ mol-1. Only the temperature-dependent nitrogen kinetic isotope effects were experimentally obtained, while the oxygen kinetic isotope effects were perturbed by 18O exchanged between N2O and ZnO. Theoretical interpretation of KIEs was evaluated according to Bigeleisen’s formalism. From comparison of calculated results with the experimental data both a cis ONNO and a fork-type NNOO models are acceptable as transition state of the reaction.
1. Introduction Nitrous oxide (N2O) is widely used as a source of oxygen in laboratory and industry processes1-5 but is at the same time a notorious greenhouse gas produced by mobile and stationary devices.6,7 It appears as an intermediate in NO decomposition,8-14 and great effort is made to minimize its production during operation of the three-way automotive catalysts.15-19 A peculiar decomposition of N2O with sharply inclined N2 desorption, observed on Pd(110)11 and Si(100),20-22 is interesting for experimentalists and theoreticians. A great interest for this reaction is evident in * To whom correspondence should be sent. Tel. no.: +386 1 477 3580. Fax no.: +386 1 477 3811. E-mail:
[email protected]. (1) Ward, M. B.; Lin, M. J.; Lunsford, J. H. J. Catal. 1977, 50, 306. (2) Iwasawa, Y.; Nakamura, T.; Takamatsu, K.; Ogasawara, S. J. Chem. Soc., Faraday Trans. I 1980, 76, 939. (3) Nakamura, M.; Mitsuhashi, H.; Takezawa, N. J. Catal. 1992, 138, 686. (4) Yamamoto, H.; Chu, H. Y.; Xu, M.; Shi, C.; Lunsford, H. J. J. Catal. 1993, 142, 325. (5) Duma, V.; Ho¨nicke, D. J. Catal. 2000, 191, 93. (6) Lox, E. S. J.; Engler, B. H. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkampf, J., Eds.; VCH Verlagsgesellschaft: Weinheim, 1997; Vol. 4, p 1633. (7) Janssen, F. J. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkampf, J., Eds.; VCH Verlagsgesellschaft: Weinheim, 1997; Vol. 4, p 1559. (8) Permana, H.; Ng, K. Y. S.; Peden, C. H. F.; Schmieg, S. J.; Lambert, D. K.; Belton, D. N. J. Catal. 1996, 164, 194. (9) Kortlu¨ke, O.; von Niessen, W. Surf. Sci. 1998, 401, 185. (10) Sharpe, R. G.; Bowker, M. Surf. Sci. 1996, 360, 21. (11) Ohno, Y.; Kimura, K.; Bi, M.; Matsushima, T. J. Chem. Phys. 1999, 110, 8221. (12) Ohno, Y.; Kobal, I.; Kimura, K.; Horino, H.; Matsushima, T. Catal. Catal. 1999, 41, 421. (13) Kobal, I.; Kimura, K.; Ohno, Y.; Matsushima, T. Surf. Sci. 2000, 445, 472. (14) Kobal, I.; Matsushima, T. Trends Chem. Phys. 1999, 7, 169. (15) Taylor, C. K. Catal. Rev. Sci. Eng. 1993, 35, 457. (16) Paˆrvulescu, V.; Grange, P.; Delmon, B. Catal. Today 1998, 46, 233. (17) Kasˇpar, J.; de Leitenburg, C.; Fornasiero, P.; Trovarelli, A.; Graziani, M. J. Catal. 1994, 146, 136. (18) Ranga Rao, G.; Fornasiero, P.; di Monte, R.; Kasˇpar, J.; Vlaic, G.; Balducci, G.; Meriani, S.; Gubitosa, G.; Cremona, A.; Graziani, M. J. Catal. 1996, 162, 1. (19) Fornasiero, P.; Ranga Rao, G.; Kasˇpar, J.; Erario, F. L.; Graziani, M. J. Catal. 1998, 175, 269. (20) Lee, J.; Kato, H.; Sawabe, K.; Matsumoto, Y. Chem. Phys. Lett. 1995, 240, 417. (21) Kubo, T.; Ema, T.; Alti, A.; Aruga, T.; Takagawa, N.; Nishijima, M. Surf. Sci. 1997, 382, 214. (22) Kato, H.; Lee, J.; Sawabe, K.; Matsumoto, Y. Surf. Sci. 2000, 445, 209.
a recent review paper 23 which compiled N2O decomposition over Pt and Au, over about 80 pure oxides, over more than 30 mixed oxides (solid solutions, spinels, perovskites, and ex-hydrotalcites), about 15 metal and metal oxides supported on alumina, silica, and zirconia, and a large number of zeolites. In our work we focus on the n-type semiconductor ZnO which, though only with moderate catalytic activity, has a noticeable place among oxides.24-34 In general, in the thermal decomposition of N2O over oxides, the kinetics are +1 (or slightly less due to oxygen inhibition) order in N2O and -1/2 order in O2,23 although on regenerated ZnO, inhibition by oxygen was not observed.29,30 N2O molecules are adsorbed to the surface Zn2+ cations through oxygen atoms.34 In photocatalytic decomposition, adsorption is accompanied by charge transfer,29,32 resulting in bent N2O- adspecies.35,36 On the contrary, charge transfer was not observed in thermal decomposition, in which dissociation of neutral N2O adspecies is considered as the rate-limiting step of the reaction mechanism.29,30 Despite extensive research of this subject, little is known about the properties of the transition state of the rate-limiting step. As part of our program to investigate transition state, we have studied nitrogen and oxygen kinetic isotope effects in this reaction, this approach having already been shown to be successful in understanding homogeneous N2O decomposition,37 homogeneous N2O decomposition catalyzed by bromine38 (23) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Appl. Catal., B 1996, 9, 25. (24) Hauffe, K.; Schlosser, E.-G. Z. Elektrochem. 1957, 61, 506. (25) Winter, E. R. S. Adv. Catal. Relat. Subj. 1958, 10, 196. (26) Schulz, W. D.; Scheve, J. Z. Anorg. Allg. Chem. 1969, 366, 231. (27) Iyengar, R. D.;. Rao, V. V. S.; Zettlemoyer, A. C. Surf. Sci. 1969, 13, 251. (28) Winter, E. R. S. J. Catal. 1970, 19, 32. (29) Tanaka, K.; Blyholder, G. Chem. Commun. 1970, 1130 (30) Tanaka, K.; Blyholder, G. J. Phys. Chem. 1971, 75, 1037. (31) Cunningham, J.; Kelly, J. J.; Penny, A. L. J. Phys. Chem. 1970, 74, 1992. (32) Cunningham, J.; Kelly, J. J.; Penny, A. L. J. Phys. Chem. 1971, 75, 617. (33) Dupont-Pavlovsky, N.; Caralp, F. J. Catal. 1977, 46, 426. (34) Hussain, G.; Rahman, M. M.; Sheppard, N. Spectrochim. Acta 1991, 47A, 1525. (35) Harcourt, R. D.; Hall, N. Theochem 1995, 342, 59. (36) Wang, F.; Harcourt, R. D. J. Phys. Chem. A 2000, 104, 1304. (37) Zielin´ski, M. C-13, C-14 and O-18 kinetic Isotope Effects in Some Chemical Reactions. Habilitation Thesis, University of Warsaw, Poland, 1966.
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and chlorine,39 and heterogeneous N2O decomposition over MgO.40 The following isotopic reactions are involved in the thermal decomposition of N2O 14
k0
N14N16O 98 14N14N + 1/216O2 k1
15
N14N16O 98 15N14N + 1/216O2
14
N15N16O 98 14N15N + 1/216O2
14
N14N18O 98 14N14N + 1/218O2
k2
k3
where ki is the rate constant of individual overall reaction. The kinetic isotope effect, i, is defined as
i ) 1 -
ki k0
Two 15N kinetic isotope effects can be measured for this reaction where 1 and 2 are secondary and primary ones, respectively. In our experiment the N2O of natural isotopic abundance was used; thus only the sum of 1 + 2 was obtained. The determination of 3 is unreliable due to 18O exchange between N2O and ZnO confirmed by experiment. 2. Experimental Section 2.1. Apparatus. The Pyrex glass vacuum system identical with that in our previous work was used40 and is described elsewhere.38 It is comprised of 10 dm3 vessels for storing gases, traps for condensing and purifying gases, a capillary mercury manometer to record pressure during kinetic studies, a To¨pler mercury pump to transfer gases and measure their pressure in the reaction vessel, and a 220 cm3 cylindrical Pyrex glass or quartz reaction vessel (40 mm diameter). The reaction temperature was controlled within (1 K by an electric cylindrical kanthal furnace. 2.2. Materials. Gases. Pharmaceutical N2O (Lek Ljubljana, Slovenia) was purified by passing through a trap filled with KOH pellets, followed by several vacuum sublimations between 195 K (CHCl3-CCl4 1:1 mixture in liquid nitrogen) and 77 K (liquid nitrogen) and then stored in a 10-dm3 vessel. Oxygen from a commercial cylinder (Tovarna Dusˇika Rusˇe, Slovenia) was liquefied and distilled in a vacuum. The middle portion only of the distillate was stored in a 10-dm3 vessel. Catalyst. Commercial ZnO of p.a. quality grade and specific surface area of 2.8 m2 g-1 (Kemika, Zagreb, Croatia) was used. A dense slurry, prepared by suspending catalyst powder in water, was poured into the reaction vessel, and a uniform layer of catalyst at the inner surface of the vessel was obtained once the water had evaporated. The vessel was then kept at 825 K and evacuated for 6-8 h, reaching a pressure below 10 mPa. The catalyst was stabilized by several decompositions of about 2 mmol of N2O at 775 K. 2.3. Procedures. 2.3.1. Kinetic Runs. For the kinetic study, the total pressure of the reaction mixture was measured in the reaction vessel with 2 g of catalyst by means of a capillary U-tube mercury manometer. Most (38) Lesar, A.; Senegacˇnik, M. J. Chem. Phys. 1993, 99, 187. (39) Lesar, A.; Hodosˇcˇek, M.; Senegacˇnik, M. J. Chem. Phys. 1996, 105, 917. (40) Zˇ emva, P.; Lesar, A.; Senegacˇnik, M.; Kobal, I. Phys. Chem. Chem. Phys. 2000, 2, 3319.
experiments at 725, 775, and 825 K were run to over 90% decomposition of N2O. Before each experiment, the reaction vessel with ZnO catalyst was kept for an hour at the temperature of the kinetic experiment and evacuated to a pressure below 10 mPa. For some experiments, the catalyst was additionally exposed to oxygen for 2-20 min at 5-7 kPa at the temperature of the kinetic experiment. N2O was introduced into the reaction vessel at room temperature to a pressure of 40-60 kPa. Initially, N2O was kept condensed in a pocket on the vessel neck outside the furnace. It was evaporated when the temperature of the vessel reached the reaction temperature and the total pressure started to be monitored. 2.3.2. Oxygen Isotopic Exchange. The isotopic exchange of oxygen between N2O and ZnO was checked by using an ZnO layer enriched in 18O, prepared by exposing ZnO to 75 mg of water, 15.6% enriched in 18O, for 12 h at 775 K. N2O was then introduced and decomposed on the Zn18O, following the same procedure as applied for kinetic isotope effect (KIE) determinations described in the section below. 2.3.3. KIE Determinations. Nitrogen kinetic isotope effects were determined at 725, 735, 775, 785, 805, and 835 K. For the last two temperatures 0.7 g was used, otherwise 2 g of ZnO was used. Prior to each experiment, the reaction vessel with ZnO catalyst was evacuated to a pressure below 10 mPa and kept for an hour at the temperature of the experiment. N2O was introduced at room temperature to a pressure of 40-60 kPa. After 7090% N2O decomposition, the reaction was stopped by removing the furnace and the reaction gas mixture was pumped back and forth by means of the To¨pler mercury pump through a trap cooled with liquid nitrogen in order to freeze out residual N2O. Nitrogen and oxygen were then pumped away, and the N2O was purified by repeated sublimation between 195 and 77 K (see above). From the initial P0N2O and final pressure PfN2O of N2O, the extent of reaction, f, was calculated from the expression
f)1-
PfN2O P0N2O
N2O was then left overnight over KOH pellets in order to remove any traces of NO2, sealed into a glass ampule, and stored for isotopic analysis. Isotopic mass ratios
R1 ) [15N14N16O]/[14N14N16O] R2 ) [14N15N16O]/[14N14N16O] and
R3 ) [14N14N18O]/[14N14N16O] in the initial N2O (at f ) 0, denoted below by 0) and in the residual N2O (at f ) f, denoted by f) were measured using a Nier-McKinney type 60° double collector mass spectrometer.38 Both 15N14N16O and 14N15N16O species contribute to the mass of 45, leading to the sum of the nitrogen effects determination, but not their individual values. In the initial N2O, [45/44] and [46/44] isotopic ratios were (7.982 ( 0.008) × 10-3 and (2.190 ( 0.005) × 10-3, respectively, while in the residual N2O these ratios were around (depending on the extent of reaction), for instance, 8.140 × 10-3 and 2.380 × 10-3, respectively. Kinetic isotope effects were calculated from the extent of reaction f and
Thermal Decomposition of N2O over ZnO
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Figure 1. Kinetics of N2O decomposition on ZnO (2 g, 2.8 m2 g-1): P and P∞ are total pressures at time t and after complete N2O decomposition, respectively. Table 1. Rate Constants for N2O Decomposition over ZnO as a Function of Temperature 107k /cm s-1
at 725 K
at 775 K
at 825 K
1.5 ( 0.2
5.3 ( 1.2
18 ( 2
the enrichment factors S, defined as38,41,42
( )
S
R1f + R2f 45 ) 0 44 R1 + R20
( )
S
R3f 46 ) 0 44 R3
The following formulas were applied:
( )
45 -1 44 KIE( N) ) 100(1 + 2) ) 200 log(1 - f) S
15
( )
46 44 KIE(18O) ) 1003 ) -100 log(1 - f) log S
Figure 2. Arrhenius plot of log(k/cm s-1) vs 1/T for decomposition of N2O on ZnO.
activation energy of 120 ( 4 kJ mol-1. The values for ZnO in the literature range from 147 to 248 kJ mol-1 26,28,30 and depend significantly on sample preparation.30 Pretreatment of catalyst by oxygen did not affect the reaction rate if the treatment time was less than 2 min, but the rate was decreased by about 15% after a 20-min pretreatment. 3.2. Oxygen Kinetic Isotope Effects. N2O decomposition over Zn18O at 735 K up to extent of reaction f ) 0.32 resulted in an 18O enrichment factor S of 12.42, caused however by the oxygen exchange reaction between N2O and ZnO. This exchange interferes with pure kinetic isotopic fractionation, and oxygen kinetic isotope effects could not be determined. 3.3. Nitrogen Kinetic Isotope Effects. Table 2 summarizes the mean values of the nitrogen kinetic isotope effects at different reaction temperatures. The temperature dependence will be shown in Figure 4. 3.4. Calculations of Kinetic Isotope Effects. Theoretical interpretation of isotopic ratios of harmonic rate constants were calculated by applying Bigeleisen’s equation, developed within the absolute rate theory38,41,42
3. Results and Discussion 3.1. Kinetics. The results of selected kinetics study experiments are shown in Figure 1, where log(P∞/3(P∞ P)) as equivalence to log(P0N2O/PN2O) was plotted versus reaction time. P0N2O and PN2O are the N2O partial pressures at t ) 0 and t ) t, respectively, while P and P∞ denote pressures of the reaction gas mixture at t ) t and t ) ∞. Due to the evaporation of N2O, data are not reliable in the first minute. Subsequently, they fit well to the first-order rate equation
dPN2O dt
) -kPN2O
A V
where k is the rate constant of the overall reaction (cm s-1), A is the catalyst surface area (cm2), and V is the volume (cm3) of the reaction vessel. This confirms firstorder kinetics in N2O. No inhibition due to oxygen was observed, in agreement with the literature findings.28,29 Rate constants of three to five replicate experiments at the same temperature did not differ by more than 1015%, and their average values are given in Table 1. The Arrhenius plot, shown in Figure 2, results in an apparent (41) Bigeleisen, J.; Wolfsberg, M. Adv. Chem. Phys. 1958, 1, 15. (42) van Hook, W. A. In Isotope Effects in Chemical Reactions; Collins, C. J., Bowman, N. S., Eds.; Van Nostrand-Reinhold: New York, 1970; p 1.
k0 ki
)
νL0q 3n-6 uij sinh(u0j/2) 3nq-7 u0jq sinh(uijq/2) νLiq
∏ j)1 u
0j
sinh(uij/2)
∏ j)1
uijq sinh(u0jq/2)
in which q denotes the transition state. The first product in the rate constant ratio includes all the isotopic frequencies of the reactant, and the second one includes the real frequencies of the transition state. νL is the frequency of the normal mode belonging to the reaction coordinate, and u ) hcω/kBT (ω is the wavenumber in cm-1, h is Planck’s constant, kB is Boltzmann’s constant, c is the speed of light, and T is the temperature). Isotopic normal frequencies for the reactant N2O molecule are available in the literature43 but for the transition state were obtained by solving Wilson’s FG matrix equation44,45
GFL ) LΛ in which G is the Wilson matrix of the kinetic energy, F is the force-constant matrix, L is the eigenvector matrix, and Λ is a diagonal matrix of eigenvalues λii ) 4π2νi2 with νi being the frequency of the ith normal vibration. (43) Bigeleisen, J.; Friedman, L. J. Chem. Phys. 1950, 18, 1656. (44) Guns, P. Vibrating Molecule; Chapman & Hall: London, 1971. (45) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1965.
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Table 2. Mean Values of Experimental Nitrogen Kinetic Isotope Effects, KIE(15N), for N2O Decomposition on ZnO as a Function of Temperature KIE(15N)
725 K
735 K
775 K
785 K
805 K
835 K
2.15 ( 0.05
2.20 ( 0.05
2.15 ( 0.05
2.05 ( 0.08
2.00 ( 0.10
2.05 ( 0.10
Figure 3. NNO bent transition state: ranges of acceptable values of (FD, Fd) pairs for selected values of R and FR, FDd ) +(FDFd)1/2.
Four models of transition state were applied in kinetic isotope effect calculations. They are similar to those in our previous studies of decomposition of N2O over MgO,40 i.e., linear and bent NNO, a planar ONNO with cis arrangement of atoms, and a planar fork-type NNOO. The KIE(15N) and KIE(18O) values of the particular transition state model were calculated from Bigeleisen’s equation for 765, 775, and 875 K and compared graphically46,47 with the experimental values. NNO Linear Transition State. As for the decomposition of N2O on MgO,40 this model did not lead to agreement with experiment. NNO Bent Transition State. Definitions of internal coordinates are shown in Figure 3. The parameters were varied in the following ranges: FD, 500-2600 N m-1; Fd, 50-1200 N m-1; FR, 50-250 N m-1 (all in steps of 50 or 100 N m-1, 100 N m-1 ) 1 mdyn Å-1); R 100-170° (in steps of 10°). To ensure νL ) 0, the condition FDd ) +(FDFd)1/2 was fulfilled, i.e., the asymmetric reaction coordinate, with a simultaneous strengthening of the N-N bond and weakening of the N-O bond which causes dissociation of NNO. The regions of acceptable values of (FD, Fd) pairs for various FR and selected R are shown in Figure 3. With increasing R, the required values of FD are greater and those of Fd smaller. The situation is similar to that observed on MgO,40 only the FD values are slightly higher there. For one of the acceptable combinations of parameter values from Figure 3, the isotopic frequencies of the transition state and the calculated KIE values are shown in Table 3. The agreement of the calculated KIE(15N) with the experimental values is illustrated in Figure 4. Calculated KIE(18O) values are included though they could not be determined experimentally. All the acceptable regions in Figure 3 reveal a productlike transition state. Thus, the force constant of the N-N bond is close to the value of 2300-2400 N m-1 in N2 molecule48 and much higher than the value of 1800-1900 N m-1 in N2O.48-50 To have FR values low, higher FD values in Figure 3 are preferred. A similar situation was found (46) Ogrinc, N.; Kobal, I.; Senegacˇnik, M. J. Phys. Chem. A 1997, 101, 7236. (47) Kobal, I.; Burghaus, U.; Senegacˇnik, M.; Ogrinc, N. Langmuir 1999, 15, 5825. (48) Csa´sza´r, A. G. J. Phys. Chem. A 1994, 98, 8823. (49) Martin, J. L. M.; Taylor, P. R.; Lee, T. J. Chem. Phys. Lett. 1993, 205, 535. (50) Yan, G.; Xian, H.; Xie, D. Chem. Phys. Lett. 1997, 271, 157.
Figure 4. Experimental (points with error bars) KIE and calculated values (lines) for the NNO bent transition state with R ) 120°, FD ) 2200 N m-1 (D ) 110 pm), Fd ) 50 N m-1 (d ) 203 pm), FR ) 150 N m-1, FDd ) +(FDFd)1/2. Table 3. Isotopic Frequencies (wavenumbers) and Calculated 15N and 18O Kinetic Isotope Effects for the NNO Bent Transition State: r ) 120°, FD ) 2200 N m-1 (D ) 110 pm), Fd ) 50 N m-1 (d ) 203 pm), Fr ) 150 N m-1, FDd ) +(FDFd)1/2 ω1q/cm-1 ω2q/cm-1 T/Kf k1/k0 k2/k0 k3/k0 KIE(15N) KIE(18O)
14N14N16O
15N14N16O
14N15N16O
14N14N18O
2250.9 579.0 625 1.00117 0.97421 0.93883 2.46 6.11
2211.6 571.4 675 1.00075 0.97617 0.94210 2.31 5.79
2216.0 568.8 725 1.00039 0.97780 0.94480 2.18 5.52
2249.6 576.5 775 1.00039 0.97921 0.94707 2.08 5.29
for bent NNO transition states on MgO40 and for the planar NNOCl and NNOBr transition states in N2O decomposition catalyzed by chlorine39 and bromine.51 The force constant of the N-O bond is much lower than the value of about 1200 N m-1 in N2O.48-50 A similarly weak N-O bond was also found for bent transition states in decomposition on MgO.40 From comparison of the above FD and Fd values with those in coordinated N2O complexes,52 it can be concluded that adsorption of N2O to ZnO surface via the O atom34 is favored. ONNO Cis Transition State. Definitions of internal coordinates are shown in Figure 5. The torsional internal coordinate τ is defined by a twisting angle between two d bonds around the D bond. Although one originates from N2O and the other one from the surface oxide, both O atoms are considered to be equivalent.53,54 This equivalence is adopted in order to explain isotopic exchange of oxygen between N2O and ZnO (surface O2- is isotopically labeled) 18
O2- + NN16O T 18ONN16O2- T 16O2- + NN18O
To obtain νL ) 0, the condition FDd ) +(FDFd/2)1/2 must be fulfilled, i.e., an asymmetric reaction coordinate with a (51) Lesar, A.; Hodsˇcˇek, M. J. Chem. Phys. 1998, 109, 9410. (52) Diamantis, A. A.; Sparrow, G. J. J. Colloid Interface Sci. 1974, 47, 455. (53) Harcourt, R. D. Chem. Phys. Lett. 1994, 218, 175. (54) Ge, Q.; Brown, W. A.; Sharma, R. K.; King, D. A. J. Chem. Phys. 1999, 110, 12082.
Thermal Decomposition of N2O over ZnO
Figure 5. ONNO cis transition state: ranges of acceptable values of (FD, Fd) pairs for three temperatures: τ ) 0°, R ) 120°, Fτ ) 50 N m-1, FR ) 60 N m-1, FDd ) +(FDFd/2)1/2.
Figure 6. NNOO fork transition state: ranges of acceptable values of (FD, Fd) pairs for three temperatures: γ ) 0°, R ) 120°, Fγ ) 50 N m-1, FR ) 60 N m-1, Fdd ) +Fd.
simultaneous strengthening of the N-N bond and weakening of the N-O bond will lead to formation of N2 and O2.55 We kept τ and R fixed at 0° and 120°, respectively, and Fτ and FR fixed at 50 and 60 N m-1, respectively, while scanning stretching force constants over the same ranges as for the NNO bent transition state above. From Figure 5 the acceptable ranges of values of (FD, Fd) pairs for the three selected temperatures can be evaluated. The same pair values ought to be operative over the whole temperature range studied. As is evident, this appears only within a very limited region (marked with a dashed circle in the figure) around FD ) 2150 N m-1 (D ) 110 pm56,57) and Fd ) 100 N m-1 (d ) 180 pm 56,57). The N-N bond is much shorter than 221 pm and the N-O bond much longer than 120 pm in NO dimers on platinum.54 NNOO Fork Transition State. Definitions of the internal coordinates are shown in Figure 6. Out-of-plane bending is defined by the angle γ of the D bond toward the molecular plane. Keeping γ ) 0°, R ) 120°, Fγ ) 50 N m-1, and FR ) 60 N m-1, the stretching force constants were scanned over the same ranges as for the NNO bent transition state above. To achieve νL ) 0, the condition Fdd ) (Fd was imposed, thus including both d internal (55) Vincent, M. A.; Hillier, I. H.; Salsi, L. Phys. Chem. Chem. Phys. 2000, 2, 707. (56) Thomas, T. H.; Ladd, J. A.; Jones, I. P.; Orville-Thomas, W. J. J. Mol. Struct. 1969, 3, 49. (57) Ladd, J. A.; Orville-Thomas, W. J. Spectrochim. Acta 1969, 22, 919.
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Figure 7. NNOO fork transition state: ranges of acceptable values of (FD, Fd) pairs for three temperatures: γ ) 0°, R ) 120°, Fγ ) 50 N m-1, FR ) 60 N m-1, Fdd ) -Fd.
coordinates into the asymmetric (+) and symmetric (-) reaction coordinates. Results with the asymmetric reaction coordinate are shown in Figure 6. The dashed region contains values of (FD, Fd) pairs which reproduce well the experimental KIE(15N). Here, the FD values are lower and the Fd values higher than those in the bent NNO transition state discussed above. Dissociation of the NNOO transition state is split into two steps,58 (i) NNOO f NNO + O and (ii) NNO f N2 + O, and proceeds via a bent NNO intermediate. This is still in accord with the general reaction mechanism, and in addition, the equivalence of both O atoms can explain isotopic exchange of oxygen between N2O and ZnO. The fork type geometry points out that the adsorbing N2O molecule may also interact with the ZnO surface via its central N atom58,59 and not necessarily via its terminal N or O atoms.34 The results for the Fdd ) -Fd case are presented in Figure 7. There is no common region for any of the three temperatures, so this transition state cannot be accepted. Isotopic exchange of oxygen between N2O and the surface was observed on both MgO and ZnO. Although the fork-type NNOO transition state with two equivalent O atoms can explain the exchange well, it is interesting that on MgO the symmetric reaction coordinate is operative, but on ZnO the asymmetric reaction coordinate is operative. 4. Conclusions The decomposition of N2O on ZnO in the temperature range from 725 to 825 K and at initial N2O pressures between 40 and 60 kPa obeyed first-order kinetics in N2O and oxygen inhibition was not observed. The activation energy was 120 ( 4 kJ mol-1. Due to isotopic exchange of oxygen between N2O and ZnO, oxygen kinetic isotope effects could not be determined. Nitrogen kinetic isotope effects, as the sum of the primary and secondary effects, were obtained experimentally and interpreted by applying the Bigeleisen approach. Linear and bent NNO, cis ONNO, and fork-type NNOO transition states were considered. A linear NNO transition state does not reproduce the experimental data. In contrast, a bent NNO transition state with an asymmetric reaction coordinate was sucessfully found. Although a large range of values of parameters (cf. Figure 3) give agreement with experiment, this transition state cannot explain the isotopic exchange (58) Dehnbostel, C. P.; Ludviksson, A.; Huang, C.; Ja¨nsch, H. J.; Martin, R. M. Surf. Sci. 1992, 265, 305. (59) Burda, J. V. Chem. Phys. 1998, 230, 13.
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of oxygen between N2O and ZnO and, therefore, can only be accepted with hesitation. On the other hand, oxygen exchange can be explained well by a cis ONNO or a forktype NNOO transition state, both with an asymmetric reaction coordinate. For the former case, the acceptable range of parameter values is very limited (cf. Figure 5) compared to the latter (cf. Figure 6). We may conclude that N2O interacts with the surface either by its terminal N atom to form a cis ONNO or by its central N atom to form a fork-type NNOO transition state. Whether the former dissociates directly into products or whether the
latter via a bent NNO intermediate cannot be resolved on the basis of the present study alone. We expect that the results of our ab initio quantum chemical calculations will help to answer this question. Acknowledgment. This study was funded by the Ministry of Science and Technology of Slovenia. The authors thank Ms. Lilijana Per for the graphics. LA001131G
