7942
J. Phys. Chem. 1993, 97, 7942-7946
Kinetics of Neutral Transition-Metal Atoms in the Gas Phase: Oxidation Reactions of Ti(a3F) from 300 to 600 K Mark L. Campbell' Chemistry Department, United States Naval Academy, Annapolis, Maryland 21 402
Roy E. McCleant Chemistry Division, Code 61 10, Naval Research Laboratory, Washington, DC 20375 Received: January 28, 1993; In Final Form: April 16, 1993
Gas-phase kinetics are reported for the reactions of Ti(a3F) with 0 2 , N20, NO, CO,,S02, and NO2 from 300 to 600 K. Titanium atoms were produced by the photolysis of T i c 4 at 248 nm and were detected by laserinduced fluorescence. Arrhenius expressions obtained for these reactions at a buffer gas pressure of 20 Torr exp(-(11.6 f 0.8 kJ/mol)/RT) cm3 s-l, k(N2O) = (1.74 f 0.44) X are k(O2) = (1.69 f 0.41) X exp(-(14.3 f 0.9 kJ/mol)/RTcm3 s-l, k(N0) = 3.28 f 0.69) X exp(-(3.62 f 0.71 kJ/mol)/RT) cm3 s-l, k(CO2) = (7.0 f 1.6) X exp(-(14.9 f 0.8 kJ/mol)/RT) cm3 s-l, and k(SO2) = (1.70 f 0.33) X exp(-(2.66 f 0.64 kJ/mol)/RT) cm3 s-l. The rate constant of Ti with NO2 was found to be temperature independent from 300 to 500 K with a value of (9 f 4) X cm3 s-l. Quoted uncertainities are f2u. With the exception of Ti 0 2 , all reactions were investigated as a function of pressure. Only the reactions of Ti with NO and C02 were found to depend on the argon buffer gas pressure. Termolecular rate constants at 300 Kweredeterminedtobe(5.8 f 2.6) X 10-31cm6s-1and(3.5 f 1.0) X 10-32cm6s-1forNOandCO2,respectively.
+
Introduction
Ti(a3F)
The gas-phase chemistry of transition-metal (TM) species has recently received considerable attention.l-I0 Of particular interest is the influence of the ground and low-lying excited electronic states on the reactivity of TM's in oxidation reactions. For example, it is well established that for reactions involving the first-row transition-metal cations with molecular hydrogen, the electronconfigurationand spin multiplicityhave a definite effect on the selectivity and reactivity of these species." Furthermore, Fontijn and co-workers*J2have found correlations between the sum of the ionizationpotential and s-p promotion energy of various metals with the rate constants in reactions involving these metals with nitrous oxide. It has been suggested that the reactivity of TM's may be correlated with the energy of the lowest lying s1d-l electronic state, where n is the number of valence electron^.^^^^^ For example, in the association reactions of the 3d metal atoms (Cr-Cu) with 02, it was found that TM atoms with electron configurations having a sld*l ground state or low-lying sld*l excited state were reactive whereas other atoms were not.6 This difference is presumably due to the energy barrier associated with the s,d*, sld-1 promotion energy since it is the dd-1 states which are expected to be correlated to the lowest lying states of the p r o d ~ c t s . 5Ritter ~ ~ and Weisshaar3 support an electron-transfer model for the abstraction reactions of Sc, Ti, and V, a shorter rangeversion of the harpoon mechanism used to explain the large cross sections for atom abstraction from high electron affinity molecules by low ionizationpotential metal atoms. Experiments performed in this laboratory? however, found no correlation between the rate constants/activation energies and the electron affinities of the oxidants for reactions involving vanadium. In an effort to further understandthedynamics of TM oxidation reactions, we report here a kinetic study of the oxidation of groundstate titanium atoms (a3F) in the temperature range 295-600 K. All reactions were studied by monitoring the disappearance of titanium atoms. The reactions studied are
-
NRC/NRL Postdoctoral Research Associate.
Ti(a3F)
+ 0,
+ N,O
-
+ NO Ti(a3F) + C 0 2 Ti(a3F)
Ti(a3F)
+ SO,
Ti(a3F)
+ NO2
products
(1)
products
(2)
products
(3)
products
(4)
products
(5)
-
products
(6)
Reactions 1-3 have previously been studied at room temperature ~ n l y , and ~ . ~no studies of reactions 4-6 have been reported. By obtaining Arrhenius parameters for these reactions, we distinguish steric factors and energy barrier effects. A comparison of these parameters with those reported for vanadium will help us further understand the relationshipbetween the reactivity and the energy of the lowest lying sldrclconfiguration. In addition, we compare Arrhenius parameters as a function of oxidant to determine the effect the electron affinity of the oxidant has on the reactivity. Experimental Section Apparatw md Procedure. Pseudo-first-order kinetic experiments ([Ti]
.-ln 4-
C
a
4-
E
- 2 0
.->
-ma 4-
K
1
.. . .
I
0
20
10
0
30
40
Time (ps)
Figure 1. Typical decay profile of the Ti(a3Fz)+ NzO reaction. T = 297 K, P t d = 20.3 Torr, P(N20) = 2.74 Torr. The points are experimentaldata; the solid line is an exponential fit which yields r1= 0.0662 p r 1 . 0.1I
o.os/
c0
0.01
7" 1
0.5
1.5
2
2.5
I
Pressure (Torr) Figure 2. Plot of the pseudo-first-orderdisappearance rate constants for Ti(a3Fz) as a function of N20 pressure at 297, 425, and 598 K. Experiments were performed at =20 Torr total pressure. The symbols are the measured decay constants, and the solid lines are the results of linear least-squares fits to the data. The point at 2.74 Torr at 297 K is that obtained from Figure 1.
Figure 3 is an Arrhenius plot of the data in Table I. The solid lines through the data are weighted least-squares linear fits to the Arrhenius expression. Empirical fits obtained are
k(O2) = (1.69 f 0.41) X 10-'oexp(-(11.6 f 0.8 kJ/mol)/RT) k(N20) = (1.74 f 0.44) X 10-'oexp(-(14.3 f 0.9 kJ/mol)/RT) k(N0) = (3.28 f 0.69)
X
lo-" exp(-(3.62 f 0.71 kJ/mol)/RT)
k(C02) = (7.0 f 1.6) X lo-" exp(-(14.9 f 0.8 kJ/mol)/RT) k(S0,) = (1.70 f 0.33) x lo-'' exp(-(2.66 f 0.64 kJ/mol)/RT)
Campbell and McClean
7944 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993
TABLE I: Measured Reaction Rate Constants for Ti(a3F) + Oxidant (PtOt = 20 Torr) oxidant T (K) 298 323 349 394 419 453 490 507 547 597 295 326 346 377 396 456 507 557 599 295 299 319 346 386 396 434 456 484 507 518 554 597 295 297 353 360 399 442 450 476 509 519 548 599 298 349 394 425 453 490 507 550 598 298 352 397 458 490 507 510
-
k f 20 (cm3 s-1) (1.60 f 0.25) X 1&l2 (2.43 f 0.42) X 1&l2 (2.94 f 0.54) X 10-12 (4.26 f 0.68) X 10-'2 (6.1 f 1.1) X 10-12 (8.9 f 1.4) X 10-Iz (9.2 f 1.5) X 10-12 (1.17 f 0.18) X (1.28 f 0.20) X (1.62 f 0.32) X (1.58 f 0.27) X l@l3 (3.04 f 0.51) X (3.55 f 0.55) X l&13 (6.3 f 1.0) x 10-13 (7.3 f 1.1) x 10-13 (1.29 f 0.20) X 1 V 2 (1.90 f 0.29) X lW2 (2.99 f 0.45) X 10-12 (3.48 f 0.53) X 1W2 (6.09 f 0.97) X 1&I1 (6.08 f 0.93) X 10-" (6.5 f 1.0) X (6.32 f 0.97) X (7.0 f 1.1) X 10-11 (7.5 f 1.2) x 10-11 (7.5 f 1.1) x 10-11 (7.6 f 1.1) X 10-" (8.7 f 1.4) X 1WI (9.5 f 1.5) X 10-" (9.6 f 1.5) X 10-l1 (1.04 f 0.16) X (1.03 f 0.17) X 1&Io (7.3 f 1.2) x 10-12 (7.4 f 1.2) x 10-12 (9.9 f 1.6) X (1.04 f 0.16) X 10-" (1.11 f 0.17) X 10-" (1.15 f 0.18) X (1.28 f 0.20) X 10-" (1.31 f 0.21) X 10-" (1.34 f 0.21) X l t l l (1.46 f 0.22) X 10-11 (1.48 f 0.23) X (1.59 0.31) X 10-l1 (5.9 i 1.1) x 10-13 (1.36 f 0.22) X l&Iz (2.06 f 0.34) X 10-12 (2.78 f 0.44) X 10-12 (3.59 f 0.54) X (4.75 f 0.73) X 1 W 2 (5.53 f 0.84) X (8.4 f 1.3) X 1W2 (1.12 f 0.17) X (8.1 f 1.2) X 1 0 - I l (1.26 f 0.22) X 1W0 (8.1 f 1.3) X 10-l1 (9.7 f 1.6) X (1.06 f 0.17) X l&Io (6.3 f 1.0) X 10-Io (6.9 i 1.1) X 1O-Io
where k( r ) is in units of cm3 s-1 and the uncertainties represent f2u. The rate constant for Ti reacting with NO2 was found to be temperature independent from 300 to 500 K with a value of (9f 4) X cm3 s-l. The measurements for this reaction were particularly difficult due to the difficulty in detecting the titanium in thepresenceofN02. Weattribute this difficulty to theapparent reaction of the precursor Tic14 with the NOz. Attempts were made to observe this reaction at higher temperatures, but these proved unsuccessful due to our inability to detect the Ti LIF signal at these higher temperatures. The importance of termolecular processes was investigated for each of the reactants other than 0 2 by determiningthe bimolecular rate constant as a function of argon buffer gas pressure. Oxygen
1 t
-*
1.6
2.0
1
-=
2.4
1
2.8
3.2
1000/T(K-')
+ oxidant reactions. Experiments were performed at =20 Torr total pressure. Symbols are experimentaldata points; the solid lines are weighted least-squares fits. Uncertainities are f2u.
Figure 3. Arrhenius plots for the Ti(a3F2)
120
c
0 7
7 40
-
N
Y
20
-
0 -
0
50 100 150 Total Pressure (Torr)
200
Figure 4. Plot ofthe measured bimolecular rateconstants for thercaction of Ti + NO as a function of pressure at 300 K. The symbols are the measured rate constants, and the solid line is the weighted linear leastsquares fit to the data. Uncertainties are f l u .
was not investigated since previous experiments indicated termolecular processes were negligible for this reaction.6 The reactions with S02, N20, and NO2 were found to be independent of pressure from 10 to 160, 10 to 200, and 10 to 105 Torr, respectively. The higher pressure for each reactant represents the maximum pressure using this reactor design for which a LIF intensity with sufficient signal-to-noiseratio could be adequately detected. The reactions with N O and COz were found to be slightly pressure dependent. Plots of the second-order rate constants for the removal of Ti by NO and COz with variation in total (Ar reactant) buffer gas pressure at 300 K are shown in Figures 4 and 5 . The linearity of the plots indicatesthe reactions are in their third-order kinetic regimes. Assuming a LindemannHinshelwood type mechanism and low-pressurelimit, the observed second-order rate constants are given by
+
k2nd,01m kbi + k3rd[M1 where kbi is the bimolecular component and [MI = [Ar] + [oxidant] ([Ar] >> [oxidant]). Thus, we obtain k3rd from the limiting slope of the linear least-squares lines. The derived rate constants are (5.8 f 2.6) X lO-3l cm6s-l and (3.5 f 1.0) X 10-32 cm6 s-l for NO and COz, respectively.
Oxidation Reactions of Ti(a3F) from 300 to 600 K I
The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 7945
'
I
TABLE IIk Activation Energies' (in kJ mol-') for the Reactions of Sc, Ti, and V with NO, 0 2 , COS and NzO oxidant SCPD)~ Ti(a3Fd V(a4Fs/2) NO 18.4 3.62 f 0.71 1.67 f 0.79' 0 2
r
N20
In
co2
m
Eo
a
m
19.6 114
11.6 f 0.8 14.3 k 0.9 14.9 f 0.8
9.04 f 0.7Y
116b 12.1 f 0.6C
Uncertainties are f 2 u . b Reference 3. Reference 4.
r
71.0
t
0.01,
,
0
,
,
40
,
80
,
,
,
,
j
160
120
Total Pressure (Torr) Figure 5. Plot of the measured bimolecular rate constants for the reaction of Ti + C02 as a function of pressure at 300 K. The symbols are the measured rate constants, and the solid line is the weighted linear leastsquares fit to the data. Uncertainties are f l u .
TABLE Ik Thermodynamics,*Arrhenius Parameters, and Electron Affinities for Oxidants Studied AHC EA E. oxidant (kJ mol-') A (cm3 s-1) P (kJ mol-1) (ev) COz NO N2O 0 2
SO2
NO2
(7.0 f 1.6) X 10-I' (3.28 0.69) X 1 0 - I ' (1.74 f 0.44) X 10-lo (1.69 0.41) X 10-lo (1.70 f 0.33) X 10-lo (9 f 4) X 10-l'
-136.2 -36.8 -501.3 -170.1 -117.4 -362.0
* *
0.26 14.9 f 0.8 4.6d 0.13 3.62 f 0.71 0.026e 0.63 14.3 f 0.9 0.22, 0.67 11.6 f 0.8 0.451e 0.63 2.66 f 0.64 1.1078 0.5 =O 2.30h
-
Uncertainties are f2u. For the reaction Ti@) + XO(g) TiO(g) at 298.15 K. cEnthalpies are from ref 14. dReference 15. Reference 16. /Reference 17. 8 Reference 18. * Reference 19.
+ X(g)
Discussion The room temperature rate constants reported here for the reaction of Ti with 0 2 and NO agree to within the combined error estimates with those reported elsewhere.316 Our measured rateconstant for the reaction of Ti with NzO a t room temperature is 50% higher than that reported previ~usly,~ although the two measurements may be consistent within the absolute accuracy of the experimental methods. The reactions of titanium with 0 2 , N20, S02, and NO2 are pressure independent, indicating an atom abstraction reaction to form TiO. All of these abstraction reactions are exothermic as can be seen in Table 11. The slight pressure dependence in the reactions of Ti with C02 and NO indicates a contribution from a termolecular association channel. Association reactions have previously been observed for the reactions of CO2 with A12C-22 and N O with CraS For the Ti reactions with C02 and NO the rate constant extrapolated to zero buffer gas pressure is approximately equal to the measured rate constant a t 20 Torr. Therefore, the contribution from the association channel to the reaction rate a t 20 Torr is negligible, and our reported temperature-dependent rate constants are meaningful. The positive activation energies for these exothermic reactions implies an abstraction of an oxygen atom to form TiO. The small activation energy for the nitric oxide reaction indicates the abstraction may go through a short-lived intermediate? Ti
+ NO
-
kz
ki
TiNO
+
Ti0+N
E, = AH, + E,
k-i
where A H 1 is the enthalpy change for the formation of the TiNO intermediate and E2 is the activation energy for the decomposition
of the intermediate to yield the T i 0 product. In this case, Eo is small because A H 1 is negative (exothermic), and the magnitudes of AHl and E2 are approximately equal. Table I1 lists the Armehius parameters for the reactions studied along with the adiabatic electron affinities of each of the oxidants. All of the frequency factors are less than the collision rate, implying the presence of steric effects in these reactions. The steric factor, P, defined as the ratio of the measured frequency factor to the calculated collision rate at room temperature is shown in Table 11. An electron-transfer model implies a relationship between the electron affinity of the oxidant and the activation energy. With the exception of NO, a general trend is observed in which the oxidant with the higher electron affinity has the smaller activation energy, the expected result for an electron-transfer mechanism. NO, with its relatively small electron affinity, has a small activation energy. This observation may be due to the reaction going through an intermediate as described previously. Table I11 lists activation energies for the reactions of Sc, Ti, and V with several oxidants. The activation energies for Scare upper bounds and are most likely smaller than that shown because of steric factors; cf. Table I1 and ref 4. The ionization potentials of Sc, Ti, and V are 6.54,6.82, and 6.74 eV, r e s p e c t i ~ e l y Therefore, .~~ E,(Ti) > E,(V) > E,(Sc) is expected and is generally observed. (The reactions of T M + N O are special cases.) Thus, our results do support an electron-transfer mechanism whereby charge separation occurs in the course of Ti (and Sc, V) abstracting an oxygen atom from the oxidant; i.e., the reactant goes through a polar transition state. Another model which has been applied to oxidation reactions of TM's is based on a crossing from the ground s2dn-2 state to the first excited s1d6I state. Such a crossing seems feasible for the oxidation reactions of Sc, Ti, and V atoms because the T M monoxide correlates with the sldn-l state of the T M at0m.25-2~ For such a mechanism, the activation energy would depend on the barrier for the avoided crossing. The promotion to the lowest excited state of Sc, Ti, and V are 11 520,6557, and 21 12 cm-1, r e s p e c t i ~ e l y .From ~ ~ Table I11 we see that for a given oxidant, Ti has a higher activation energy than V. However, theactivation energies for Sc which are smaller than the activation energies for Ti (and possibly V) in at least two cases suggest that promotion energy may not be the only factor involved in the dynamics of these reactions. A larger kinetic data base with more T M atoms and oxidants along with theoretical calculations would be helpful in determining the effect of the s*d*l state. Recently, a resonance interaction model has been developed that correlates the sum of the ionization potentials and s-p promotion energies to activation energies for reactions of metal atoms with N20.8J2 The s-p promotion energies for Sc, Ti, and V are 1.943, 1.969, and 2.029 eV, respectively. Application of this resonance treatment to the activation energies in Table I11 yields results similar to the IP vs E, correlations. It would be interesting to test this resonance model with a large number of reactions of metal atoms with various oxidants. Summary
We have measured reaction rates and Arrhenius parameters for the reactions of ground-state titanium atoms with 02, N20, CO2, NO, SOz, and NO2 over the temperature range 300-600
1946 The Journal of Physical Chemistry, Vol. 97, No. 30, 1993 K. In each case abstractionof 0 atoms is the predominant reaction channel. The activation energies for the reactions are small, less than 15 M/mol. The pattern of rate constants suggests that an electron transfer from the metal atom to the oxidant may be the key mechanistic step. Comparison of the activation energies of Ti andV reactionswith thesame oxidants indicate the promotion energy to the lowest sldn-1 state may influence the reactivity of the transition metal. Measurements of activation energies for ground and excited states of other transition-metal atoms would be useful to explore our understanding of these reactions.
Acknowledgment. M.L.C. gratefully acknowledges salary support through the Naval Research Laboratory and the Naval Reserve Science and Technology Program of the Office of Naval Research. Funding of this study was provided by the Office of Naval Research. References and Notes (1) Vinckier, C.; Corthouts, J.; De Jaegere, S.J . Chem. Soc., Faraday Trans. 2 1988,84, 1951. (21 Ritter, D.: Weisshaar. J. C. J . Phvs. Chem. 1989, 93, 1576. (3) Ritter; D.; Weisshaar; J. C. J . Phis. Chem. 1990, 94; 4907. (4) McClean, R. E.; Pasternack, L. J . Phys. Chem. 1992, 96, 9828. (5) Parnis, J. M.;Mitchell, S. A,; Hackett, P. A. J . Phys. Chem. 1990, 94, 8152. (6) Brown, C. E.; Mitchell, S. A,; Hackett, P. A. J . Phys. Chem. 1991, 95, 1062. (7) Mitchell, S.A.; Hackett, P. A. J. Chem. Phys. 1990, 93, 7822. ( 8 ) Narayan, A. S.;Futerko, P. M.; Fontijn, A. J. Phys. Chem. 1992, 96, 290.
Campbell and McClean (9) Akhmadov, U. S.;Zaslonko, I. S.;Smimov,V. N.Kinet. Katal. 1988, 299
Raiche, G. A.; Belbruno, J. J. Chem. Phys. Lett. 1987, 134, 341. Elkind, J. L.; Armentrout, P. B. J . Phys. Chem. 1987, 91, 2037. Futerko, P. M.; Fontijn, A. J. Chem. Phys. 1991, 95, 8065. Campbell, M. L.; McClean, R. E.; Garland, N. L.; Nelson, H. H. Chem. Phys. k r t . 1992, 194, 187. (14) Chase, M. W., Jr.; Davies, C. A,; Downey, J. R., Jr.; Frurip, D.J.; McDonald, R. A,; Syverud, A. N. J. Phys. Chem. Ref. Data 1985,14,Suppl. (10) (11) (12) (13)
1.
(15) Compton, R. N.; Reinhardt, P. W.; Cooper, C. D. J . Chem. Phys. 1975,63, 3821. (16) Travers, M.J.; Cowlcs, D. C.;Ellison, G. B. Chem. Phys.Lert. 1989, 164, 449. (17) Hopper, D. G.; Wahl, A. C.; Wu, R. L. C.; Tiernan, T. 0.J . Chem. Phys. 1976,65, 5474. (18) Nimlos, M. R.; Ellison, G. B. J . Phys. Chem. 1986,90, 2574. (19) Chowdhury, S.;Heinis, T.; Grimsrud, E. P.; Kebarle, P. J. Phys. Chem. 1986, 90,2747. (20) McQuaid, M.; Woodward, J. R.; Gole, J. L. J. Phys. Chem. 1988, 92, 252. (21) Parnis, J. M.; Mitchell, S. A.; Hackett, P. A. Chem. Phys. Leu. 1988, 151, 485. (22) Garland, N. L.; Douglass,C. H.; Nelson,H. H. J. Phys. Chem. 1992, 96, 8390. (23) Fontijn, A. Combust. Sci. Technol. 1986, 50, 151. (24) Moore,C. E. Atomic Energy Levels as Derived from the Analysis
Natl. Bur. of Optical Spectra, Vol. I; Narl. Stand. Ref. Data Ser. (US., Stand.) 1971, NSRDS-NBS35. (25) Senneaal, J. M.;Schamp, J. Chem. Phys. 1987,114, 37. (26) Bauschlicher, C. W., Jr.; Langhoff, S . R. J. Chem. Phys. 1986,85, 5936. (27) Jeung, G. H.; Koutecky, J. J . Chem. Phys. 1988,88, 3747.
